Ecdysone receptor

DEVELOPMENTAL BIOLOGY (part 2/2)

Larval and Pupal (part 2)

In larvae of the hawkmoth, Manduca sexta, accessory planta retractor (APR) motoneurons undergo a segment-specific pattern of programmed cell death at pupation. APR death is triggered hormonally by the prepupal peak of the ecdysteroid 20-hydroxyecdysone (20-HE). APRs were removed from the nervous system before the prepupal peak and placed in low density cell culture. Physiological levels of 20-HE trigger the same segment-specific pattern of APR death in vitro as seen in vivo. The presence or absence of contact with other cells does not influence the response of APRs to 20-HE. The death of APRs in culture is characterized by fragmentation or rounding up of the cll body and fragmentation of the neurites. These findings suggest that intrinsic segmental identity regulates whether these motoneurons live or die when exposed to a steroid hormone during development. Most Manduca motoneurons including the APRs express ecdysteroid receptors during the prepupal peak, consistent with the possibility of direct hormone action (Streichert, 1997).

One system that has proven amenable for the study of programmed cell death is the intersegmental muscle (ISM) of the tobacco hawkmoth Manduca sexta. These giant muscle cells are used during the eclosion (emergence) behavior of the adultmoth, and then die during the subsequent 30 h. For the ISMs, the trigger for PCD is a decline in the circulating titer of the insect molting hormone, 20-hydroxyecdysone (20-HE). During cell death there are rapid decreases in both the myofibrillar sensitivity to intracellular calcium and the resulting force of fiber contraction. The ability of the ISMs to undergo PCD requires the repression and activation of specific genes. Two of the repressed genes encode actin and myosin. One of the upregulated presumptive cell-death genes encodes polyubiquitin, which appears to play a critical role in the rapid proteolysis that accompanies ISM death (Schwartz, 1992).

The developmentally programmed cell death of abdominal intersegmental muscles in the tobacco hawk-moth Manduca sexta is coincident with a 10-fold induction of the polyubiquitin gene as a hormonally regulated event. The induction of polyubiquitin mRNA is accompanied by a proportional increase in total ubiquitin polypeptide. Ubiquitin conjugate pools increase 10-fold at eclosion, during which loss of muscle protein mass is maximum. A smaller but measurable increase in ubiquitin conjugates is observed earlier in pupal development coincident with a modestly enhanced degradation of myofibrillar proteins. Accumulation of ubiquitin conjugates is accompanied by induction in the pathway for polypeptide ligation, including the activating enzyme (E1), several carrier protein (E2) isoforms, and ubiquitin:protein isopeptide ligase (E3). Both accumulation of ubiquitin polypeptide and the enzymes of the conjugation pathway are subject to regulation by declining titers of the insect molting hormone 20-hydroxyecdysone, which signals onset of programmed cell death in the intersegmental muscles. Thus, programmed cell death within the intersegmental muscles is accomplished in part by stimulation of the ubiquitin-mediated degradative pathway through a coordinated induction of ubiquitin and the enzymes responsible for its conjugation to yield proteolytic intermediates. This suggests enzymes required for ubiquitin conjugation may represent additional genes recruited for developmentally programmed death (Haas, 1995).

Ecdysteroids regulate the remodeling of the dorsal external oblique 1 (DEO1) muscle during metamorphosis in Manduca sexta. The temporal and spatial patterning of the A and B1 isoforms of the ecdysone receptor (EcR) within muscle DEO1 corresponds to the developmental fates of the fibers. Using antibodies directed to specific isoforms of EcR, it has been shown that the expression of various EcR isoforms in myonuclei differ among the five fibers of DEO1 and correspond to the developmental response of the muscle to the changing steroid titers and to the pattern of innervation. Muscle degeneration and apoptosis of myonuclei in all fibers are correlated with the expression of only EcR-A just before pupal ecdysis and then with the expression of low levels of both EcR-A and EcR-B1 shortly after pupation. Only the first fiber of muscle DEO1 participates in the regrowth of the adult muscle, and only this fiber shows an upregulation of EcR-B1 that is evident at 3 d after pupal ecdysis. Denervation of the muscle prevents both the upregulation of EcR-B1 and myoblast proliferation. It is concluded that the developmental fate of muscle DEO1 during metamorphosis is orchestrated by interactions between rising and falling ecdysteroid titers, the pattern of expression of EcR isoforms by the muscle, and interactions with other cells in the local environment (Hegstrom, 1998).

Proliferation zones in each optic lobe for EcR expression begin in the late second instar, expressing only the B1 isoform. Levels peak 24 hours prior to pupariation but by pupariation the optic lobe is devoid of EcR-B1. This expression may be related to the unique development of the optic lobe during larval life, correlated with development of the compound eye. During this period ingrowing retinal axons induce proliferation in the lamina. The gradient of axon ingrowth that ends about 10 hours after pupariation sets up in the lamina a corresponding gradient of differentiation that is evident through at least the next 30-40 hours (Truman, 1994).

Cell proliferation within the optic lobe anlagen is dependent on ecdysteroids during metamorphosis of the moth Manduca sexta. Cultured tissues were used to show that ecdysteroids must be maintained above a sharp threshold concentration to sustain proliferation. Proliferation can be turned on and off repeatedly simply by shifting the ecdysteroid concentration to levels above or below this threshold. In subthreshold hormone, cells arrest in the G2 phase of the cell cycle. Ecdysteroid control of proliferation is distinguished from differentiative and maturational responses to ecdysteroids by requiring tonic exposure to the hormone and lower levels of 20-hydroxyecdysone, and by being sensitive to either 20-hydroxyecdysone or its precursor, ecdysone. These characteristics allow optic lobe development to be divided into two ecdysteroid-dependent phases. Initially, moderate levels of ecdysteroid stimulate proliferation. Later, high levels of 20-hydroxyecdysone trigger a wave of apoptosis within the anlage that marks completion of its proliferative phase (Champlin, 1998a).

The optic lobe is composed of three ganglia, the lamina, medulla and lobula. The neurons of the optic ganglia are progeny of neuroblasts in the optic lobe anlage (OA). In Manduca sexta, the neuroblasts are arranged in a double-banded bracelet that comprises the inner and outer OA. During early larval stages, expansion of the neuroblast population occurs by symmetric cell divisions. In the final larval instar, the neuroblasts switch to the asymmetric divisions that lead to neuron production. The smaller daughter of each asymmetric division, the ganglion mother cell (GMC), divides to produce neurons. GMCs of the inner OA produce the neurons of the lobula. The medial margin of the outer OA is composed of the neuroblasts and GMCs that produce the neurons of the medulla (medulla precursor cells, MPC), while the lateral margin is composed of the neuroblasts and GMCs that produce the neurons of the lamina (lamina precursor cells, LPC) (Champlin, 1998a).

Proliferation of the LPC has been shown to be regulated by incoming retinal afferents. Transection of the optic nerve in Manduca also leads to disruption of LPC proliferation. Communication via retinal afferents provides a coordinating link between the developing retina and production of lamina neurons. Ecdysteroids, by contrast, control production of neurons throughout the optic lobe. The precursors for the medullar and lobular neurons both require ecdysteroids for proliferation. The optic nerve is severed in cultures making it difficult to determine if the same is true for the LPCs. Despite these conditions, arrested LPCs still enter mitosis in response to 20E. It appears that neural precursors throughout the optic lobe have a similar ecdysone-dependent G2 checkpoint. The failure of LPCs in cultured brains to incorporate BUdR suggests that 20E allows the cells to divide but they then arrest in G1 in the absence of innervation. This interpretation is consistent with data from Drosophila that LPCs arrest in G1 in mutants lacking proper retinal innervation. Thus, the cell cycle of LPCs in Manduca may have two developmentally regulated control points, stimulation by retinal afferents being required for progression through G1 and stimulation by ecdysteroids being required for progression through G2 (Champlin, 1998a).

MPCs are found to proliferate in an ecdysteroid-dependent manner, even in small pieces of the outer OA, clearly indicating that other long-range signals are not required for these cells. Cells throughout the OA, including the MPCs, express the ecdysteroid receptor; therefore, these cells could be responding directly to ecdysteroids. An important issue is whether the ecdysteroid-dependent proliferation of the MPC is a direct response to the steroid or due to short-range signals from neighboring cells. Evidence for communication between surrounding glia and neuroblasts is seen for the protein product of the anachronism locus, which is secreted by nearby glial cells and inhibits proliferation of optic lobe neuroblasts in Drosophila. The fact that the MPCs appear to respond to ecdysteroid as a unit suggests that there may be coordinating signals within the OA of Manduca (Champlin, 1998a).

Development of a multicellular organism requires precise coordination of cell division and cell type determination. The selector homeoprotein Even skipped (Eve) plays a very specific role in determining cell identity in the Drosophila embryo, both during segmentation and in neuronal development. However, studies of gene expression in eve mutant embryos suggest that eve regulates the embryonic expression of the vast majority of genes. Genetic interaction and phenotypic analysis is presented showing that eve functions in the trol pathway to regulate the onset of neuroblast division in the larval CNS. Surprisingly, Eve is not detected in the regulated neuroblasts, and culture experiments reveal that Eve is required in the body, not the CNS. Furthermore, the effect of an eve mutation can be rescued both in vivo and in culture by the hormone ecdysone. These results suggest that eve is required to produce a trans-acting factor that stimulates cell division in the larval brain (Park, 2001).

Several genes have been identified that affect neuroblast proliferation, including anachronism (ana), terribly reduced optic lobes (trol) and eve. trol was originally identified in a genetic screen for abnormal larval brain morphology that was due to defective patterns of neuroblast proliferation in the larval brain. Mutations in trol cause a dramatic decrease in the reactivation of proliferation from mitotic quiescence. Recent studies suggest that trol may regulate this reactivation of neuroblast proliferation by stimulating the G1/S transition through upregulation of Cyclin E (CycE) expression. Several studies on trol and ana have led to the hypothesis that trol is required to overcome the repression of neuroblast cell division imposed by ana. eve was identified in a screen for enhancers of a hypomorphic allele trol. Mutations in eve enhanced both the trol proliferation phenotype and the associated lethality, indicating that eve may regulate transcription of cell cycle genes in the trol pathway (Park, 2001).

Analysis of explants has shown that ecdysone enables activation of neuroblast division and can substitute for larval extract. Furthermore, addition of ecdysone does not rescue the proliferation phenotype of cultured trol mutant brains, implying that ecdysone acts upstream of trol. Thus, ecdysone can overcome the lack of eve-induced activity in extracts of mutant flies. Interestingly, almost complete rescue is obtained when animals are fed ecdysone from 16-20 hours posthatching, indicating that the time between ecdysone action and S phase entry is at most four hours (Park, 2001).

The genetic interaction between eve and trol has all the characteristics expected for two components of a common pathway: (1) the eve;trol interaction is not allele specific and the known functional domains of Eve are implicated in the interaction; (2) the strength of the interaction mirrors the strength of the eve allele in segmentation; (3) eve mutants themselves have the predicted proliferation phenotype; and (4) neuroblasts arrested in trol;eve double heterozygotes can be rescued by expression of CycE, as can the neuroblasts arrested in a strong trol mutant. The latter is especially revealing, as induction of CycE expression in trol mutants results in the activation of cell division only in the number of neuroblasts appropriate to the developmental stage of the induction. That is, not all mitotically quiescent neuroblasts are arrested at the same cell cycle phase, and the extent to which CycE is a limiting factor is developmentally controlled. Therefore, as in embryonic segmentation and determination of neuronal identity, eve appears to function in a specific genetic pathway to affect the behavior of specific cells at specific times (Park, 2001).

However, Eve is not detectable in regulated neuroblasts at any time during first instar. Furthermore, eve function is not required within the larval CNS, but is required within the larval body from which extracts are prepared. Moreover, low levels (10%-20%) of extract made from eve plus animals will not support activation of neuroblast division while higher concentrations will. This concentration dependence indicates that eve does not inhibit production of a trans-acting proliferation repressor that is produced at higher levels in a eve mutant, since dilution of such a repressor would allow neuroblast division at lower rather than higher extract concentrations. These results strongly suggest that eve function is required for the production of a trans-acting factor that stimulates neuroblast division (Park, 2001).

Is ecdysone the trans-acting factor produced in response to eve? Ecdysone can rescue eve-dependent proliferation defects both in vivo and in vitro, but not the proliferation defect of trol mutants in vitro. This suggests that ecdysone acts upstream of trol, as would be expected if it is the eve-dependent trans-acting signal, and trol acts within the receiving cells. However, while the ecdysone receptor has been detected in a few neurosecretory cells of the first instar CNS, it has not been detected in neuroblasts. This may indicate that only a few high-affinity receptors are required to transduce the ecdysone signal, or that ecdysone acts indirectly through the products of the neurosecretory cells. However, as Eve is not detectable in the neurosecretory cells in wild-type brain lobes, it is unlikely that the added ecdysone rescues mutant animals by compensating for a loss of Eve activity in those cells. In each of these cases, eve could be acting through ecdysone production. Alternatively, ecdysone may act through a parallel pathway to that stimulated by an (unknown) eve-dependent signal. While the relationship between eve and ecdysone is not yet clear, it seems likely that eve is required for the production of an organismal-level trans-acting signal that is specifically required to stimulate larval neuroblast proliferation (Park, 2001).

The eye primordium of the moth, Manduca sexta, shows two different developmental responses to ecdysteroids depending on the concentration to which it is exposed. Tonic exposure to moderate levels of 20-hydroxyecdysone (20E) or its precursor, ecdysone, are required for progression of the morphogenetic furrow across the primordium. Proliferation, cell-type specification and organization of immature ommatidial clusters occur in conjunction with furrow progression. These events can be reversibly started or stopped in cultured primordia simply by adjusting the levels of ecdysteroid either above or below a critical threshold concentration. In contrast, high levels of 20E cause maturation of the photoreceptors and the support cells that comprise the ommatidia. Ommatidial maturation normally occurs after the furrow has crossed the primordium, but premature exposure to high levels of 20E at any time causes precocious maturation. In such cases, the furrow arrests irreversibly and cells behind the furrow produce a well-formed, but miniature, eye (Champlin, 1998b).

To sustain furrow progression in culture, the concentration of 20E had to be maintained between 1.2x10 -7 M and 2x10 -6 M. Outside of this range, little or no BUdR incorporation is found in cells flanking the furrow, even when the labeling period is extended through 24 hours. In contrast, when eye primordia were cultured with 20E concentrations within the 'proliferative' range, a high number of S phase and M phase cells, comparable to that observed in freshly dissected primordia are detected flanking the furrow. Their frequency appears to be fairly constant regardless of whether primordia are cultured at the high or low end of the proliferative range. To confirm that the furrow was indeed advancing in culture, primordia were exposed to a second pulse of BUdR 13 hours after the first. The distance between cells labeled by the two pulses of BUdR is similar regardless of whether primordia are cultured in 20E concentrations at the low or high end of the proliferative range, showing that the rate of furrow progression is independent of steroid concentration within this range. Both ends of the proliferative range show sharp thresholds beyond which the furrow is arrested. The type of arrest, though, is different for the lower and upper ends of the range. The concentration causing 50% arrest (ED50) for the lower threshold is about 1.2x10 -7 M concentration of 20E; this is similar for eye primordia isolated either from P+1 or P+4 day animals. Furrow progression stops in concentrations of 20E below this threshold, but resumes any time the 20E concentration is shifted up into the proliferative range. In contrast, the furrow arrest caused by concentrations of 20E above the proliferative range is not reversed when the concentration of 20E is shifted down into the proliferative range, even though the furrow has traversed only a portion of the eye primordium (Champlin, 1998b).

Ecdysone, the precursor for 20E, is widely regarded as an inactive prohormone. However, it was noted that the furrow continues advancing early in the pupal stage when ecdysone is essentially the only ecdysteroid detected in the blood. Ecdysone is also able to maintain furrow progression in culture at concentrations above a threshold of 6x10 -7 M, a concentration about fivefold higher than the threshold for 20E. However, in contrast with 20E, high concentrations of ecdysone do not cause furrow arrest even at the highest levels tested (Champlin, 1998).

Interruption of furrow progression during diapause (overwintering) Manduca larvae can be programmed by short-day length to enter a diapause state shortly after they pupate. Animals destined to diapause are indistinguishable from those programmed by long-day length for continuous development until P+2 days, and the morphogenetic furrow continues advancing through this time. Prothoracicotropic hormone, the neuropeptide that stimulates ecdysteroid synthesis, fails to be released in short-day animals on P+2 days: the ecdysteroid titer drops to low levels, and diapause ensues. By day P+3, furrow progression has ceased in short-day animals at a point about one-third of the way across the primordium. The morphogenetic furrow then remains arrested during the months that pupae are in diapause even though the animals may remain at the normal rearing temperature. Diapausing pupae can be induced to resume development at any time by injection of physiological doses of ecdysteroid. In such animals, incorporation of BUdR in cells flanking the furrow is detected within 12 hours of injection. The concentration of 20E needed to stimulate resumption of furrow progression in eye primordia isolated from pupae in diapause is identical to that needed to maintain furrow progression in primordia isolated from long-day animals (Champlin, 1998b).

Precocious and catastrophic metamorphosis occurs throughout animals treated with high levels of 20E, suggesting that ecdysteroids control development of other tissues in a manner similar to the eye. The threshold concentrations of 20E required for furrow progression versus ommatidial maturation differ by about 17-fold, with maturation requiring higher levels. This capacity to regulate distinct phases of development by different concentrations of a single hormone is probably achieved by differential sensitivity of target gene promoters to induction by the hormone-bound receptor(s). Expression of early genes was examined in eye primordia that were cultured with different levels of 20E in the presence of anisomycin, an inhibitor of protein synthesis. Different early genes are induced at concentrations that correspond to the thresholds for furrow progression and for ommatidial maturation, respectively. For example, MHR3 is the Manduca homolog of the Drosophila nuclear receptor DHR3, an early gene that plays an integral role in maturational responses to ecdysteroid. The induction of MHR3 mRNA requires high concentrations of 20E in the range needed to trigger maturation. In contrast, direct upregulation of the ecdysone receptor mRNA occurs at lower 20E concentrations, in the range that maintains furrow progression (Champlin, 1998b).

Ecdysteroids regulate insect metamorphosis through the edysone receptor complex, a heterodimeric nuclear receptor consisting of the Ecdysone receptor (EcR) and its partner Ultraspiracle (Usp). Differentiation in the Drosophila ovary at metamorphosis correlates with colocalization of Usp and the EcR-A isoform in all but one of eight oocytic mesoderm-derived somatic cell types. The eight oocytic mesoderm-derived somatic cell types consist of apical cells, terminal filament cells, cap cells, the epithelial sheath, the inner germarial sheath cells, the follicle cells, the basal stalk and the oviduct. The first recognizable event of ovarian differentiation is the formation of the terminal filaments (TFs), a process of convergent extension that begins at around 12 hours after ecdysis to the third instar, and continues throughout the remainder of the final larval stage. At pupariation (the onset of the larval-pupal transition), all of the approximately 21 TF stacks have formed, and the location of these stacks prefigures the positions of the mature ovarioles (the functional units of the ovary). During TF differentiation, three additional cell types are present: germ cells (in the central region of the ovary); apical cells (anterior to the germ cells), and basal somatic cells (posterior to the germcells). At pupariation, a subset of the apical cells, the epithelial sheath cells, have begun to surround each terminal filament, and will ultimately separate the ovarioles. The epithelial sheath and the other apical cells are collectively referred to as anterior somatic cells. At pupariation, an additional somatic cell type is distinguishable: the cap cells occupy a position between the TF cells and the germ cells (Hodin, 1998).

By 12 hours after pupariation (12 h AP), the germ cells have begun to form cysts and the epithelial sheaths are continuing to advance in an apical to basal progression. Adjacent to the germ cells reside inner germarial sheath cells, which line the interior lateral edges of the germarium (the birth place of the egg chambers). At 12 h AP, the basal somatic cells remain in a predifferentiative state. By 18 h AP, follicle cells have begun to surround the first egg chambers. At 24 h AP, the epithelial sheaths have separated the ovaries into ovarioles and the basal somatic cells begin to differentiate into the basal stalks and the anterior oviduct (cells at the extreme posterior of the ovary that will eventually fuse with the genital disc-derived oviduct). By 42 h AP, the ovaries and oviducts have fused, and the basal stalks are present (Hodin, 1998).

The exceptional cell type, that is, the one in which EcR cannot be detected, is the larval TF cell, in which only Usp is detectable during cell differentiation. The onset of TF formation is accompanied by the onset of expression in the presumptive TF cells of the Bric-a-brac protein, which is required for proper TF formation. In cells destined to form the basal stalks and anterior oviduct, Usp colocalizes with what appears to be the EcR-B2 isoform. BrdU incorporation in pupal ovaries correlates with ecdysteroid levels, suggesting that ecysteroids regulate proliferation in the ovary, presumably via the EcR-Usp heterodimeric receptor. EcR-A is first detected at approximately 12 h after ecdysis into the third instar, in the somatic cells of the ovary but not in the forming TF cells. The EcR-A isoform is abundant only in the anterior cells (Hodin, 1998).

Flies heterozygous for a deletion of the EcR gene exhibit several defects in ovarian morphogenesis, including a heterochronic delay in the onset of terminal filament differentiation. In such mutants there is a significant increase in the number of TF cells formed (identified by Bric-a-brac expression), but there are fewer TF stacks. Flies heterozygous for a strong usp allele exhibit accelerated TF differentiation. Flies simultaneously heterozygous for both EcR and usp have additional phenotypes, including several heterochronic shifts, delayed initiation and completion of terminal filament morphogenesis and delayed ovarian differentiation during the first day of metamorphosis. Thus usp3 heterozygotes exhibit accelerated TF formation; the Df EcR heterozygotes show delayed TF formation, and the EcR-usp double heterozygotes are delayed both in the onset and in the completion of TF formation. Terminal filament morphogenesis is severely disrupted in homozygous usp clones. These results demonstrate that proper expression of the Ecdysone receptor complex is required to maintain the normal progression and timing of the events of ovarian differentiation in Drosophila. These findings are discussed in the context of a developmental and evolutionary role for the Ecdysone receptor complex in regulating the timing of ovarian differentiation in dipteran insects. It is concluded that heterochronic shifts in ovarian differentiation have apparently been accomplished by uncoupling the process of ovarian differentiation from tissue differentiation in the rest of the animals and that alterations in timing of expression of the EcR complex may reside in a more general mechanism by which heterochronic changes in the differentiation of individual tissues have been accomplished in insect evolution (Hodin, 1998).

Drosophila motor neuron retraction during metamorphosis is mediated by inputs from TGF-beta/BMP signaling and orphan nuclear receptors

Larval motor neurons remodel during Drosophila neuro-muscular junction dismantling at metamorphosis. This study describes the motor neuron retraction as opposed to degeneration based on the early disappearance of β-Spectrin and the continuing presence of Tubulin. By blocking cell dynamics with a dominant-negative form of Dynamin, this study shows that phagocytes have a key role in this process. Importantly, the presence of peripheral glial cells is shown close to the neuro-muscular junction that retracts before the motor neuron. In muscle, expression of EcR-B1 encoding the steroid hormone receptor required for postsynaptic dismantling, is under the control of the ftz-f1/Hr39 orphan nuclear receptor pathway but not the TGF-β signaling pathway. In the motor neuron, activation of EcR-B1 expression by the two parallel pathways (TGF-β signaling and nuclear receptor) triggers axon retraction. This study interrupted TGF-β signaling in motor neurons using expression of dominate negative Wishful thinking. It is proposed that a signal from a TGF-β family ligand is produced by the dismantling muscle (postsynapse compartment) and received by the motor neuron (presynaptic compartment) resulting in motor neuron retraction. The requirement of the two pathways in the motor neuron provides a molecular explanation for the instructive role of the postsynapse degradation on motor neuron retraction. This mechanism insures the temporality of the two processes and prevents motor neuron pruning before postsynaptic degradation (Boulanger, 2012).

It is a general feature of maturing brains, both in vertebrates and in invertebrates, that neural circuits are remodeled as the brain acquires new functions. In holometabolous insects, the difference in lifestyle is particularly apparent between the larval and the adult stages. These insects possess two distinct nervous systems at the larval and adult stages. A class of neurons is likely to function in both the larval and the adult nervous systems. The neuronal remodeling occurring during this developmental period is expected to be necessary for the normal functioning of the new circuits (Boulanger, 2012).

The pruning of an axon can involve a retraction of the axonal process, its degeneration or both a retraction and degeneration. The MB γ axon is pruned through a local degeneration mechanism. In contrast, axons may retract their cellular processes from distal to proximal in the absence of fragmentation and this mechanism is called retraction. Interestingly, the two mechanisms can occur sequentially in the same neuron, as in the case of the dendrites of the da neurons, where branches degenerate and the remnant distal tips retract (Boulanger, 2012).

This study provides evidence that the motor neuron innervating larval muscle 4 (NMJ 4) is pruned predominantly through a retraction mechanism. The first morphological indication of motor neuron retraction is the absence of fragmentation observed with anti-HRP staining at the level of the presynapse in all the developmental stages analyzed, together with a decrease in perimeter size observed after 2 h APF. The continuity of this HRP staining is in contrast to the pronounced interruptions between blebs observed with an antibody against mCD8 in γ axons. A molecular indication of motor neuron retraction in these studies is the fact that β-Spectrin disappears at the synapse 5 h APF, before motor neuron pruning takes place. Indeed, it has been shown using an RNA interference approach that loss of presynaptic β-Spectrin leads to presynaptic retraction and synapse elimination at the NMJ during larval stages. The modifications of the microtubule morphology that were observed, such as an increase in microtubule thickness and withdrawal, provide additional evidence of axonal retraction during NMJ remodeling. Finally, a strong argument in favor of a motor neuron retraction mechanism is the fact that Tubulin is present at the NMJ throughout all stages of axonal pruning at the start of metamorphosis (0-7 h APF). This stands in clear contrast to the abolition of Tubulin expression observed before the first signs of γ axon degeneration. It is also interesting to note that the motor neuron retraction observed in this study at metamorphosis and at larval stages are morphologically different. During metamorphosis, retraction bulbs or postsynaptic footprints, which have been reported at larval stages, were never visualized. The fact that the postsynapse dismantles at metamorphosis before motor neuron retraction might explain these discrepancies. Worth noting is the mechanistic correlation between accelerated debris shedding observed here for NMJ pruning at the start of metamorphosis and axosome shedding occurring during vertebrate motor neuron retraction (Boulanger, 2012).

In vertebrates, glia play an essential role in the developmental elimination of motor neurons. In Drosophila, the role of glia in sculpting the developing nervous system is becoming more apparent. Clear examples of a role for engulfing glial cells in axon pruning are well documented during the MB γ axon degeneration at metamorphosis. Also, glia are required for clearance of severed axons of the adult brain. A distinct protective role of glia has been recently discovered during the patterning of dorsal longitudinal muscles by motor neurobranches. This study describes the presence of glia processes close to the end of the pupal NMJ. The observations suggest that the glial extensions retract at 5 h APF, just before motor neuron retraction is observed. When the glial dynamic is blocked, the NMJ dismantling might be also blocked. It is hypothesized that during development in larvae and early pupae, glial processes have a protective role and aid in the maintenance of the NMJ. Then, between 2 and 5 h APF, glial retraction would be a necessary initial step that allows NMJ dismantling. In accordance with this hypothesis, glia play a protective role in the maintenance of NMJ during pruning of second order motor neuron branches 31 h APF (Boulanger, 2012).

Disruption of shi function specifically in glial cells results in an unpruned mushroom body γ neuron phenotype and prevents glial cell infiltration into the mushroom body (Awasaki, 2004). One can note that at the NMJ the role of the glia is proposed to be essentially opposite from its role in MB γ axons pruning but in both cases blocking the glia dynamics results in a similar blocking of the pruning process (Boulanger, 2012).

In vertebrates, phagocytes are recruited to the injured nerve where they clear, by engulfment, degenerating axons. In Drosophila, phagocytic blood cells engulf neuronal debris during elimination of da sensory neurons. This study shows that blocking phagocyte dynamics with shi produces a strong blockade of the NMJ dismantling process. One possibility is that phagocytes attack and phagocytose the postsynaptic material, a process blocked by compromising shi function resulting in postsynaptic protection. In accordance, it has been shown that phagocytes attack not only the da dendrites to be pruned, but also the epidermal cells that are the substrate of these dendrites (Boulanger, 2012).

During NMJ dismantling, the muscle has an instructive role for motor neuron retraction. In all the situations where postsynapse dismantling is blocked, the corresponding presynaptic motor neuron retraction is also blocked. Therefore, it is sufficient to propose that both glial cells and phagocytes affect only the postsynaptic compartment. Nevertheless, one cannot rule out that these two cell types both act directly at the pre and at the postsynapse (Boulanger, 2012).

ECR-B1 is highly expressed and/or required for pruning in remodeling neurons of the CNS. MB γ neurons and antennal lobe projection neurons remodeling require both the same TGF-β signaling to upregulate EcR-B1. In the MBs only neurons destined to remodel show an upregulation of EcR-B1. At least two independent pathways insure EcR-B1 differential expression. The TGF-β pathway and the nuclear receptor pathway are thought to provide the necessary cell specificity of EcR-B1 transcriptional activation. This study shows that in the motor neuron pruning these two pathways are also necessary to activate EcR-B1. Noteworthy, showing an analogous requirement of ftz-f1/Hr39 pathway in two different remodeling neuronal systems unravels the fundamental importance of this newly described pathway (Boulanger, 2012).

The following model is proposed for the sequential events that are occurring during NMJ dismantling at early metamorphosis. First, EcR-B1 is expressed in the muscle under the control of FTZ-F1. FTZ-F1 activates EcR-B1 and represses Hr39. This repression is compulsory for EcR-B1 activation. Importantly, TGF-β/BMP signaling does not appear to be required for EcR-B1 activation in this tissue, however, a result of EcR-B1 activation in the muscle would be the production of a secreted TGF-β family ligand. Then, this secreted TGF-β family ligand reaches the appropriate receptors and activates the TGF-β signaling in the motor neuron. Finally, TGF-β signaling in association with the nuclear receptor pathway activates EcR-B1 expression resulting in motor neuron retraction. Since glial cells and phagocytes are required for the dismantling process, it is possible that a TGF-β/BMP family ligand(s) be produced by one or both of these cell types and not by the postsynaptic compartment. Noteworthy, a recent study shows that glia secrete myoglianin, a TGF-β ligand, to instruct developmental neural remodeling in Drosophila MBs (Awasaki, 2011). Nevertheless, one can note that the requirement of the two pathways in the motor neuron provides a simple molecular explanation of the instructive role of postsynapse degradation on motor neuron retraction. This mechanism insures the temporality of the two processes and prevents motor neuron pruning before postsynaptic degradation. It was proposed that in the MBs, the association of these two pathways provides the cell (spatial) specificity of pruning. In this paper, this association is proposed to provide the temporal specificity of the events. Future studies will be necessary to understand how EcR-B1 controls the production of a TGF-β/BMP ligand(s) in the muscle, the reception of this signal by the motor neuron and the ultimate response by the motor neuron to initiate retraction. These steps will be necessary to unravel the molecular mechanisms underlying the NMJ dismantling process and related phenomenon in vertebrate NMJ development and disease. Interestingly, it appears that TGF-β ligands on the one hand are positive regulators of synaptic growth during larval development and on the other hand, they are positive regulators of synaptic retraction, at the onset of metamorphosis. In both situations signaling provides a permissive role, sending a signal from the target tissue to the neuron. The consequence of this signal would be dependent on developmental timing thus, on a change in context (Boulanger, 2012).

Adults

In many sexually mature insects, egg production and oviposition are regulated as consequence of copulation. . Sex-Peptide (SP) is a 36-amino-acid peptide synthesized in the accessory glands of Drosophila melanogaster males and is transferred to the female during copulation. Sex-Peptide stimulates vitellogenic oocyte progression through a putative control point at about stage 9 of oogenesis. Application of the juvenile hormone (JH) analog methoprene mimics the Sex-Peptide-mediated stimulation of vitellogenic oocyte progression in sexually mature virgin females. Apoptosis is induced by 20-hydroxyecdysone in nurse cells of stage 9 egg chambers at physiological concentrations [10(-7) M]. 20-Hydroxyecdysone thus acts as an antagonist of early vitellogenic oocyte development. However, simultaneous application of JH analog protects early vitellogenic oocytes from 20-hydroxyecdysone-induced resorption. These results suggest that the balance of these hormones in the hemolymph regulates whether oocytes will progress through the control point at stage 9 or undergo apoptosis. These data are further supported by a molecular analysis of the regulation of yolk protein synthesis and uptake into the ovary by the two hormones. It is concluded that JH is a downstream component in the Sex-Peptide response cascade and acts by stimulating vitellogenic oocyte progression and inhibiting apoptosis. Since juvenile hormone analogue does not elicit increased oviposition and reduced receptivity, Sex-Peptide must have an additional, separate effect on these two postmating responses (Soller, 1999).

SP stimulates JH biosynthesis in corpus allatum complexes isolated from sexually mature virgin females in vitro. Consistent with this finding, JH application stimulates progression of oocytes through the control point at stage 9, involving an increased uptake of from the hemolymph and an increased synthesis of yolk proteins in the ovary. JH also protects early vitellogenic oocytes from ecdysone-mediated resorption. Thus, after mating, ecdysone-mediated oocyte resorption in virgins is relieved due to the increase of JH levels. The corpus allatum is likely to be a target organ for SP action. Since application of JH neither induces a reduction in receptivity nor elicits complex behavioral change, neuronal tissues have to be considered further targets of SP (Soller, 1999 and references).

The similarity of Taiman to steroid hormone receptor coactivators suggests that Tai might interact with one or more steroid hormone receptors. The only known steroid hormone in Drosophila is ecdysone, and the ovary is a major site of ecdysone synthesis, which peaks at stage 9. The functional ecdysone receptor is a heterodimer composed of Ultraspiracle (Usp), which is the fly retinoid X receptor (RXR) homolog, and the Ecdysone receptor. To determine whether the ecdysone receptor complex would be a good candidate for interaction with Tai, expression of ecdysone receptor subunits in egg chambers was examined using antibodies against Usp, EcR-A, and EcR-B. EcR-A and EcR-B are distinct isoforms of the EcR subunit, which are generated by alternative splicing. Usp, EcR-A, and EcR-B colocalize with Tai protein in migrating border cells; Usp and EcR-A are expressed generally, in both follicle cells and nurse cells (Bai, 2000).

These observations raise the possibility that the timing of border cell migration might be controlled by ecdysone. To test whether border cell migration is responsive to hormone, the effects of injecting hormone into female flies were examined. It was not expected that increasing the hormone concentration alone would be sufficient to cause precocious border cell migration because expression of the slbo gene and its targets are independently required for migration. Therefore, slbo was precociously expressed using transgenic flies carrying a heat-inducible slbo transgene, followed by injection of hormone. Border cell migration was assayed in stage 8 egg chambers dissected from flies treated with heat shock and hormone, and compared to control flies treated with heat shock and ethanol, or with hormone in the absence of heat shock. Precocious border cell migration was observed in 20% of egg chambers that were treated with both heat shock and hormone but not in controls. The observed effects are consistent with a role for ecdysone in regulating the timing of border cell migration (Bai, 2000).

If the rising ecdysone level at stage 9 is required to stimulate border cell migration, then reducing the ecdysone level should cause a delay in border cell migration. The ecdysoneless mutant ecd¹ is temperature sensitive for production of ecdysone. Females homozygous for ecd¹ are sterile when held at the nonpermissive temperature for 5 days, and egg chambers in these flies arrest development at stage 8 and subsequently degenerate. Border cells fail to develop in these arrested egg chambers. However, when ecd¹ mutants are held at the nonpermissive temperature for 2 days, some stage 10 egg chambers develop, in which border cells differentiate and express Slbo protein. Greater than 50% of these egg chambers exhibit delayed border cell migration (Bai, 2000).

Since the effects on border cell migration of increasing or decreasing ecdysone levels could have been indirect, whether there is a cell autonomous requirement for the ecdysone receptor in border cells was tested. The EcR locus is proximal to available FRT insertion sites, preventing mosaic analysis. Therefore, the analysis was carried out using mutations in usp. Border cells that were homozygous mutant for a null allele of usp exhibit inhibition of border cell migration, but no obvious defects in other follicle cells (Bai, 2000).

To assess whether Tai and the ecdysone receptor are likely to associate in a complex in vivo, Tai expression was examined in third instar larvae. Antibodies against Tai react specifically with the salivary gland nuclei, as well as other larval tissues. Polytene chromosome spreads were stained with antibodies against Tai and Usp proteins in a double labeling experiment. Anti-Tai antibody labels specific loci on the polytene chromosomes. Moreover, Usp and Tai proteins colocalize precisely. Since previous experiments have shown that Usp and EcR colocalize as a complex on polytene chromosomes, these results indicated that Tai colocalizes with the functional Ecdysone receptor complex at specific target sites (Bai, 2000).

Whether expression of Tai can enhance ecdysone receptor-dependent transcriptional activation in EcR-293 mammalian cells was tested. These cells respond to hormone, either ecdysone or an analog known as ponasterone, with a substantial increase in transcriptional activation of genes placed under the control of a cis-acting sequence known as an E/GRE. Transcriptional activation was tested in cells expressing varying amounts of Tai in transient transfection assays. Tai expression increases transcriptional activation up to 5-fold, in a dose-dependent manner, specifically in the presence of hormone (Bai, 2000).

Furthermore, a GST-fusion protein containing the region of Tai protein containing the LXXLL motifs predicted to interact with EcR (residues 1028 to 1235 of Tai) associates with in vitro translated EcR in a ligand-dependent manner. The same fusion protein does not associate detectably with Usp alone. However, in the presence of EcR and ligand, the Tai-GST fusion protein is able to coprecipitate Usp. Taken together, these results suggest that Tai is a bona fide ecdysone receptor coactivator (Bai, 2000).

Thus, Tai appears to be a coactivator of the p160 class based not only on amino acid sequence similarity and overall domain structure, but based also on its in vivo colocalization with EcR, its direct, ligand-dependent binding to EcR, and its ability to potentiate hormone-dependent transcription in cultured cells. The homology of Tai to SRC proteins suggests that Tai might interact with a steroid hormone receptor. Although there are more than 20 genes in Drosophila that code for proteins related to nuclear hormone receptors, ecdysone is the only known steroid hormone. Since SRC proteins require the presence of a ligand in order to interact with receptors, the ecdysone receptor seems like the best candidate partner for Tai. The colocalization of Tai protein with the ecdysone receptor complex at specific chromosomal loci in third instar larva, the direct and ligand-dependent binding of Tai to EcR in vitro, and the ability of Tai to potentiate the ecdysone response in cell culture lend substantial support to this proposal (Bai, 2000).

The ligand-dependent interaction of Tai with the ecdysone receptor suggests that ecdysone regulates border cell migration. The strongest evidence in support of this is that border cells lacking Usp are unable to migrate. Consistent with this observation, numerous unfertilized eggs were produced from females lacking usp function. Moreover usp is required specifically in somatic cells for production of a fertilizable egg. Defects in border cell migration are known to lead to the production of unfertilized eggs. Whether EcR loss of function mutations affect border cell migration could not be examined. This is because the EcR locus, at 42A, is proximal to available FRT insertions, making it impossible to make FLP-mediated mosaic clones. The frequency of X-ray induced mitotic clones is too low to be useful, and marking such clones is problematic. A temperature-sensitive allele of EcR exists and flies at the nonpermissive temperature exhibit a variety of defects in oogenesis, including arrest prior to border cell migration. Even though it was not possible to assess the effect of EcR mutations specifically in the border cells, the observations that hormone injections can lead to precocious border cell migration and that reduced ecdysone levels can lead to delayed migration provide additional support for the hormonal control of migration (Bai, 2000).

The rise in ecdysone after eclosion, specifically in females, occurs in response to adequate nutrition. In the absence of a rich diet, yolk protein synthesis is inhibited and oogenesis does not progress. Yolk protein synthesis can be restored in the absence of a rich diet by applying ecdysone or juvenile hormone (JH) to cultured ovaries. Recent studies indicate that functional ecdysone receptors are required in the germline for progression of oogenesis through vitellogenesis, the stages during which yolk is taken up by the oocyte. In summary, then, adequate nutrition appears to lead to elevated hormone levels, which in turn stimulate yolk protein synthesis and uptake, and progression of oogenesis beyond stage 8. Together with the results reported here, these findings suggest that a rising ecdysone titer coordinates a variety of events that occur in early vitellogenic egg chambers, including border cell migration (Bai, 2000).

Nutritional status affects 20-hydroxyecdysone concentration and progression of oogenesis in Drosophila

Drosophila egg production depends upon the nutrition available to females. When food is in short supply, oogenesis is arrested and apoptosis of the nurse cells is induced at mid-oogenesis via a mechanism that is probably controlled by ecdysteroid hormone. Expression of some ecdysone-response genes is correlated with apoptosis of egg chambers. Moreover, ecdysteroid injection and application of juvenile hormone respectively induces and suppresses the apoptosis. In this study, an investigation was carried out to see which tissues show increases in the concentration of ecdysteroids under nutritional shortage to begin to link together nutrient intake, hormone regulation and the choice between egg development or apoptosis made within egg chambers. Ecdysteroid levels in the whole body, ovaries and haemolymph samples were measured by RIA, and it was found that the concentration of ecdysteroid increased in all samples. This contributes to the idea that nutritional shortage leads to a rapid high ecdysteroid concentration within the fly and that the high concentration induces apoptosis. Low concentrations of ecdysteroid are essential for normal oogenesis. It is suggested there is threshold concentration in the egg chambers and that apoptosis at mid-oogenesis is induced when the ecdysteroid levels exceed the threshold. Starvation causes the ovary to retain the ecdysteroid it produces, thus enabling individual egg chambers to undergo apoptosis and thus control the number of eggs produced in relation to food intake (Terashima, 2005).

The prothoracic glands, which are the principal source of ecdysone in the immature stages, are no longer present in adults. The egg chambers produce ecdysone, which, at least in some insects, accumulates in the oocyte. In the fat body, ecdysone is converted to 20E, the active hormone, and shade, which encodes 20-hydroxylase for converting ecdysone to 20E, is expressed in nurse cells and follicle cells in the ovary and fat body (Terashima, 2005).

Ecdysteroid synthesis is affected by the nutritional status of the female, and ecdysteroids affect oogenesis in many insects. Egg production in mosquitoes is triggered by a blood meal. The digested products of the blood meal stimulate the brain to secrete egg development neurosecretory hormone (EDNH), which is also known as ovarian ecdysteroidogenic hormone (OEH). EDNH stimulates the ovary to synthesize ecdysteroids, which instruct the fat body cells to make vitellogenin for the oocytes. Vitellogenin is critical for egg production, thus without the blood meal there is no vitellogenin and no eggs, so to produce mature eggs ecdysteroids are essential. In contrast, nutritional shortage induces an increase in ecdysteroid concentration in Drosophila females, ecdysteroid concentration increases in Drosophila whole body, haemolymph and ovaries during starvation. Feeding suppresses the high ecdysteroid concentration that is induced by nutritional shortage (Terashima, 2005).

Under starvation, apoptosis of nurse cells in stage-8 and -9 egg chambers is induced; 20E injection into the females under adequate nutrition also induces the apoptosis and JHA treatment of females under nutritional shortage suppresses this apoptosis. Presumably high ecdysteroid concentrations in the haemolymph and/or the ovary, which are induced by starvation, may induce the apoptosis of nurse cells in stage-8 and -9 egg chambers. However, ecdysteroid is indispensable to produce mature eggs in Drosophila. Oogenesis in ecd-1 mutants is arrested at mid-oogenesis, and germline clones of EcR mutations led to developmental arrest; egg chambers degenerated during mid-oogenesis in Drosophila. Presumably, there is an ecdysteroid threshold for inducing apoptosis of nurse cells at stages 8 and 9 and ecdysteroids induce normal development when below the threshold concentration and induce apoptosis of nurse cells at stages 8 and 9 when over the threshold. Starvation induces an increase in ecdysteroid concentration to above the threshold level in the haemolymph and the ovary through activation of the ecdysone synthesis pathway in the egg chamber. Ecdysteroid secretion from the ovary decreased following nutritional shortage. Thus, ecdysteroid secretion from the fat body or other ecdysteroid-synthesizing tissues must be stimulated to induce the high ecdysteroid concentration observed in haemolymph (Terashima, 2005).

JHA suppresses the high ecdysteroid concentration that is induced by starvation. JH and JHA suppress ecdysone synthesis/secretion from the prothoracic glands in larvae of Maduca sexta. It is likely that JHA suppression decreases the high ecdysteroid concentration in the ovary that induces apoptosis of nurse cells in stage-8 and -9 egg chambers under starvation, and therefore JHA treatment retains minimal ecdysteroid levels needed for inducing normal oogenesis (Terashima, 2005).

There is a developmental checkpoint at stage 8 of oogenesis. YP synthesis commences at stage 8 and YP is accumulated during development into mature eggs. Drosophila egg chambers normally transit through stages 8 and 9 during a 6-h period, but starvation induced an accumulation of stage-8 and -9 egg chambers in Drosophila oogenesis. The number of stage-8 egg chambers is increased during a 5-12-h period after starvation starts, but the number of stage-9 egg chambers does not increase for 0-12 h after starvation started. This means that oogenesis progresses from stage 7 to 8, but does not progress from stage 8 to 9 and then to 10 under nutritional shortage. When 20E is injected into the fed flies, the accumulation of stage-8 chambers is not seen; therefore this arrest of oogenesis at stage 8 was not caused by the increasing 20E concentration in haemolymph and ovary. Perhaps starvation signals induce the arrest of oogenesis at stages 8 and 9 directly, or they could inhibit YP uptake. Some nutrient- and stress-response genes exhibit different expression patterns in the ovaries of females under adequate nutrition and starvation. It is suggested that the genes which respond directly to stress and nutrients interact with the ecdysone-synthesis pathway, resulting in the induction of apoptosis of nurse cells in stage-8 and -9 egg chambers through activation of BR-C Z2, Z3 and E75A expression in the follicle cells. Other genes could have altered their expression levels, so as to arrest oogenesis at stages 8 and 9 and to check the developmental status of the egg chamber. As a result, the decision is made to develop into a mature egg or undergo apoptosis at stages 8 and 9. The arrest in the progression of oogenesis at stages 8 and 9 is independent of increasing ecdysteroid levels (Terashima, 2005).

Starvation signals are needed to activate a number of pathways to adjust the rate of egg production in Drosophila. These pathways could be classified into two groups: one to stimulate ecdysone synthesis in the follicle cells and/or nurse cells to activate the apoptosis pathway, including BR-C Z2, Z3 and E75A expression in the follicle cells, and another one to interact with and participate in the developmental checkpoint, giving rise to an arrest in oogenesis at stage 8 under nutritional shortage. A possible scheme is presented for the regulation of oogenesis related to nutrition in Drosophila. It is likely that starvation signals from the gut activate ecdysteroid synthesis in the ovary in Drosophila under starvation. Ecdysteroid is then accumulated in the egg chamber by decreasing 20E secretion from the ovary, and the fat body secretes 20E to haemolymph. It is suggested that there are two thresholds of 20E concentration in Drosophila ovary -- one is the concentration for normal oogenesis and the other is the concentration for inducing apoptosis -- and that starvation elevates the ecdysone levels in some egg chambers over the threshold that leads to apoptosis (Terashima, 2005).

Control in time and space: Tramtrack69 cooperates with Notch and Ecdysone to repress ectopic fate and shape changes during Drosophila egg chamber maturation

Organ morphogenesis requires cooperation between cells, which determine their course of action based upon location within a tissue. Just as important, cells must synchronize their activities, which requires awareness of developmental time. To understand how cells coordinate behaviors in time and space, Drosophila egg chamber development was analyzed. The transcription factor Tramtrack69 (TTK69) was found to control the fates and shapes of all columnar follicle cells by integrating temporal and spatial information, restricting characteristic changes in morphology and expression that occur at stage 10B to appropriate domains. TTK69 is required again later in oogenesis: it controls the volume of the dorsal-appendage (DA) tubes by promoting apical re-expansion and lateral shortening of DA-forming follicle cells. TTK69 and Notch were shown to compete to repress each other's expression, and a local Ecdysone signal is required to shift the balance in favor of TTK69. It is hypothesized that TTK69 then cooperates with spatially restricted co-factors to define appropriate responses to a globally available (but as yet unidentified) temporal signal that initiates the S10B transformations (Boyle, 2009).

This paper has demonstrated that TTK69 plays a central role in regulating the behavior of follicle cells during mid-to-late oogenesis. At S10 and again at S12, TTK69 integrates temporal and spatial information to coordinate the behaviors of subpopulations of follicle cells (Boyle, 2009).

Elsewhere in development, TTK69 acts as a determinant of cell fate. Best studied is its role in binary cell-fate decisions downstream of N-mediated lateral inhibition, such as in asymmetric sensory organ precursor (SOP) cell division, where N becomes active in one of two daughter cells, leading to TTK69 expression and repression of neural determinants. TTK69 also acts downstream of N to promote endocycle entry at S6, and in the transition from endocycle to chorion gene amplification at S10. Recent studies demonstrate a role for TTK69 in morphogenesis. ttk^twk, an allele of ttk69 that does not disrupt patterning, prevents DA elongation, and TTK69 is required for proper cell shape and fate during tracheal morphogenesis (Boyle, 2009).

This study advances understanding of TTK69 significantly by reporting four novel mechanisms of TTK69 action. (1) TTK69 controls DA tube volume by inducing apical expansion and lateral shortening, independent of basal extension. (2) Loss of TTK69 in mid-oogenesis causes cells to adopt inappropriate fates without inducing a binary cell-fate switch. (3) A surprising, mutually repressive relationship was found between TTK69 and N that is modulated by Ecdysone receptor activity. (4) TTK coordinates the response to temporal and spatial signals and thereby ensures the fidelity of egg chamber maturation (Boyle, 2009).

Although apical constriction has been well studied, less is known about apical re-expansion and the mechanisms that determine final apical size. Surprisingly, ttk^twk reveals independent control of apical and basal surfaces. Although DA-forming cells elongate, the tube extends only when roof-cell apices expand (Boyle, 2009).

The ttk^1e11 allele deletes a portion of the TTK69 zinc finger. Although long regarded as null, The possibility is considered that some truncated protein, expressed even at undetectable levels, might cause gain-of-function effects. Several lines of evidence support the argument against this possibility. First, heterozygous cells, the majority in the ttk^1e11 mosaics, show no phenotype; thus, any gain-of-function effect would be recessive, a very rare occurrence. Furthermore, N-CA phenocopies ttk^1e11 and causes TTK69 downregulation, independently demonstrating that TTK69 reduction produces the ttk^1e11 phenotypes (Boyle, 2009).

ttk^1e11 caused dramatic shape and fate transformations in all columnar follicle cells, leading to cells with highly constricted apices, elevated E-cadherin, and altered Broad (BR) levels. These changes did not result from constitutive activation of BR or cell-shape determinants, as S10A and earlier egg chambers did not exhibit any of these phenotypes. Thus, TTK69 regulates diverse processes at S10B. Unlike its role in SOP cell division, TTK69 does not switch cells from one type to another. Rather, ttk^1e11 cells take on aspects of many S10B follicle cell subtypes, not all of which are normally exhibited by a single cell (e.g. high basal E-cadherin and apical constriction) (Boyle, 2009).

Ecdysone signaling was required to flip the bistable relationship between N and TTK69 toward TTK69 at S10. Inactivating Ecdysone receptor via a dominant-negative construct largely mirrored ttk^1e11. Additional phenotypes, such as failure to generate stretch cells, were probably due to earlier requirements for EcR function, processes that do not involve TTK69. Interestingly, TTK69 levels were sometimes elevated in the cytoplasm and excluded from the nuclei of EcR-DN-expressing cells, indicating that Ecdysone may promote nuclear localization of TTK69. This mechanism could provide more rapid and reversible control over TTK69 activity than transcriptional regulation. The mutually repressive relationship between TTK69 and N could accelerate this change, once triggered (Boyle, 2009).

How does Ecdysone signaling produce this change? Autocrine signaling within the germline could induce downregulation of Delta, thereby reducing N activity. At the same time, Ecdysone signaling to the follicle cells could stabilize TTK69 in the nucleus, modulating expression of target genes that control N degradation. Alternatively, as activated N can block Ecdysone signaling at S10B, decreased Delta activity could allow Ecdysone receptor function in the follicle cells, indirectly activating TTK69. Regardless of the exact mechanism, these relationships would act as a positive feedback loop to ensure a rapid and reliable switch between the two states (Boyle, 2009).

The transition from S10A to S10B is marked by the adoption of specific follicle-cell fates in positions specified by EGF and DPP signaling. Strikingly, many of the observed phenotypes appeared at S10B but not S10A, revealing roles for TTK69, N and EcR in regulating temporal maturation to S10B. Thus, fundamental differences exist between S10A and S10B follicle cells. It is hypothesized that a signal experienced by all follicle cells induces this change. Importantly this signal cannot be a combination of EGF and DPP, as these signals are spatially restricted to the dorsal anterior at this stage, and ttk^1e11, UAS-N-CA and UAS-EcR-DN have equivalent effects in all columnar follicle cells (Boyle, 2009).

What is this signal? Probably, several factors contribute. Ecdysone itself is an interesting candidate. While ttk^1e11 mutant and N-CA-expressing cells were apically constricted after S10B, EcR-DN-expressing cells more closely resembled S10A cells (smaller overall with smaller nuclei). This distinction could indicate that Ecdysone is required to progress beyond S10A. EcR-DN-expressing cells, however, do not perfectly resemble S10A cells. Differences could be due to dominant effects of EcR-DN complexes, which could strongly repress EcR targets rather than simply failing to activate them. Another potential input at this time could be prostaglandin signaling, which induces nurse-cell dumping. Finally, the egg chamber grows during this period and overall egg chamber nutrition could determine the timing of the S10A-S10B transition. Insulin signaling is required earlier for vitellogenesis and may continue to monitor growth during this period (Boyle, 2009).

How does TTK69 integrate this temporal signal with the spatial pattern to achieve appropriate responses at the correct time and place? ttk^1e11 cells adopt aspects of multiple cell types. In wild type, each cell performs a subset of behaviors, repressing inappropriate responses in a ttk69-dependent fashion. For example, roof cells need TTK69 to prevent BR downregulation, whereas main body cells need TTK69 to prevent apical constriction. How does TTK69 repress different processes in different cells? Spatial information from DPP and EGF must contribute to TTK69 activity. TTK69 expression, however, although dynamic and variable, has no consistent spatial pattern and is required in all columnar follicle cells (Boyle, 2009).

A conceptual solution is to imagine TTK69 working in concert with two co-factors that are spatially restricted by EGF and DPP signaling: one co-factor is expressed in a U\M pattern, all follicle cells except the T using the system described previously (see Yakoby, 2008), and one is present in a U\R pattern (all follicle cells except the roof). When combined with TTK69, the U\M factor would repress basal E-cadherin, N expression and the early clearance of BR that normally occurs in the T. In combination with TTK69, the U\R factor would repress apical constriction and BR upregulation, which normally occur in the roof. Removal of TTK69 causes a failure to repress all of these behaviors, leading to the observed phenotypes, with moderate BR levels as a result of both up- and downregulation. These co-factors could take the form of BTB transcription factors that dimerize with TTK69. Alternatively, they could modify TTK69 by phosphorylation or mono-ubiquitylation or affect expression of other genes that alter the response to TTK69 function (Boyle, 2009).

In conclusion, this study has identified several novel functions for TTK69. At S10, presumably in collaboration with region-specific interacting proteins, it facilitates spatially correct responses to a uniform temporal signal; its function at S12 coordinates a return to a cuboidal cell shape. The regulatory network by which TTK69, N and Ecdysone receptor control the progression of egg chambers from mid- to late oogenesis could serve as a simple model to explain how a bistable system flips its state to regulate the temporal progression of development (Boyle, 2009).

The ecdysone receptor signalling regulates microvilli formation in follicular epithelial cells

Epithelial morphogenesis contributes greatly to the development and homeostasis of the organs and body parts. This study analysed the consequences of impaired Ecdysone receptor (EcR) signalling in the Drosophila follicular epithelium. Besides governing cell growth, the three EcR isoforms act redundantly in controlling follicle cell positioning. Flattening of the microvilli and an aberrant actin cytoskeleton arise from defective EcR signalling in follicle cells, and these defects impact on the organisation of the oocyte membrane. This signalling was found to govern a complex molecular network since its impairment affects key molecules as atypical protein kinase C and activated Moesin. Interestingly, the activity of the transcription factor Tramtrack69 isoform is required for microvilli and their actin core morphogenesis as well as for follicle cell positioning. In conclusion, these findings provide evidence of novel roles for EcR signalling and Tramtrack69 transcription factor in controlling stage-specific differentiation events that take place in the follicular epithelium (Romani, 2015).

Fine-tuning of secondary arbor development: the effects of the ecdysone receptor on the adult neuronal lineages of the Drosophila thoracic CNS

The adult central nervous system (CNS) of Drosophila is largely composed of relatively homogenous neuronal classes born during larval life. These adult-specific neuron lineages send out initial projections and then arrest development until metamorphosis, when intense sprouting occurs to establish the massive synaptic connections necessary for the behavior and function of the adult fly. This study identified and characterized specific lineages in the adult CNS and describes their secondary branch patterns. Because prior studies show that the outgrowth of incumbent remodeling neurons in the CNS is highly dependent on the ecdysone pathway, this study investigated the role of ecdysone in the development of the adult-specific neuronal lineages using a dominant-negative construct of the ecdysone receptor (EcR-DN). When EcR-DN was expressed in clones of the adult-specific lineages, neuroblasts persisted longer, but no alteration was seen in the initial projections of the lineages. Defects were observed in secondary arbors of adult neurons, including clumping and cohesion of fine branches, misrouting, smaller arbors and some defasciculation. The defects varied across the multiple neuron lineages in both appearance and severity. These results indicate that the ecdysone receptor complex influences the fine-tuning of connectivity between neuronal circuits, in conjunction with other factors driving outgrowth and synaptic partnering (Brown, 2009).

The majority of the adult CNS of Drosophila is composed of adult-specific neurons born during larval life. Proper neuronal connectivity and function of the nervous system rely on both regulation of neurogenesis to produce correct neuron number, and establishment of appropriate synaptic connections by these neurons. In the thorax, postembryonic neurogenesis is initiated by the reactivation of NBs in the late first instar larva, and terminates within 24 hours after pupariation. This reactivation of quiescent NBs requires growth of the first instar larva. Although 20E has been implicated in the regulation of NB cell cycle speed, it is not sufficient to initiate NB divisions in starved larvae, and the signals that terminate NB division are unknown. NBs could be predetermined to stop cycling after a specific number of divisions, or an external cue could trigger termination. Previous studies in Manduca sexta showed that transplantation of ganglia from fourth instar larvae into wandering fifth instar larvae resulted in continuing neurogenesis in the implant well after neurogenesis had terminated in the host. This suggests that young NBs cannot be shut off early by the host's metamorphic signals. Temperature-sensitive ecdysone-mutant larvae that wander permanently rather than undergoing pupariation have NBs that cycle more slowly, but apparently stop divisions in the same spatiotemporal pattern as in normal metamorphosing flies. Additionally, Drosophila larval CNSs cultured without 20E divide at a slower rate, but stop dividing when approximately the correct number of neurons has been produced (Brown, 2009).

In this study, blocking the ecdysone signal through the expression of EcR-DN resulted in lineage clones with NBs that persisted longer than usual. In 20% of clones expressing EcR-DN, NBs were observed at 24 hours APF in animals raised at 29°C, whereas no NBs were seen in control animals at the same stage. The developmental state of these pupae corresponds to a developmental time of approximately 29 hours APF at 25°C, well past the time when neurogenesis ceases in the ventral CNS. However, the number of neurons in clones expressing EcR-DN was not grossly increased . Thus, delaying death in the thoracic NBs through expression of EcR-DN either fails to extend the neurogenic period, or extends it for just a short time, resulting in clones with only slightly more neurons. Taken together with previous studies examining 20E and NB termination, these data suggest that the number of NB divisions might be approximated through endogenous mechanisms, but that the precise timing of NB termination might be fine-tuned through 20E signaling (Brown, 2009).

After an adult-specific neuron is born in the larva, it sends out an initial process before arresting its development. Each neuron born from the same neuroblast sends its projection along the same path as previously born neurons from the same sibling cluster, or hemilineage. If a lineage has more than one neurite bundle (for example, lineage 6 projects across both the pD and the pI commissures), one of the two neurons born from a GMC projects to one initial target, while its sibling projects to the other. Typically, the projections from a hemilineage make contact with the neurite bundle from a different hemilineage, possibly setting up future synaptic connections. Before metamorphosis, these contacts are limited and the neuropil remains partitioned into domains allotted to each hemilineage. This paper describes the final adult arbor morphology of seven of the 25 thoracic lineages. After metamorphosis was complete, all lineages examined showed intense sprouting and elaboration to form finely branched terminal and interstitial arbors, many located within the leg or flight neuropils. The mature neurons within a given hemilineage typically shared similar domains of dendritic and axonal arborization. There are exceptions, though, such as in lineages 7 and 18, in which a few neurons in a hemilineage took a trajectory that differed from that of the rest of the group. The peripheral projections of lineage 15 defasciculated and extended over the femoral and tibial muscles of the leg, forming neuromuscular junctions throughout. Overall, the early partitioning of the immature neuropils by the hemilineages is dissolved during metamorphosis as complex arbors develop, overlapping with those from other lineages to establish the final pattern of synaptic connections (Brown, 2009).

Previously, the role of EcR in Drosophila neuronal development was examined by expressing EcR-DN in remodeling neurons. When ecdysone signaling through EcR is blocked, pruning is slow and incomplete in both peripherally and centrally located neurons. In the neurosecretory Tv neurons, filipodial activity, axonal sprouting and growth is also limited, resulting in a much reduced, misshapen axonal arbor. By comparison, expression of EcR-DN in the adult-specific neurons examined in this study resulted in less severe phenotypes, with no defects in initial projections or in ability to sprout secondary arbors. Initial projections are established before pupariation when EcR levels are low, so it is not surprising that EcR-DN expression does not affect this initial phase of growth. However, because all lineages examined still showed some secondary sprouting despite expression of EcR-DN, sprouting initiation might be prompted by signals other than the rising ecdysone titer, such as by cell-cell interactions. The lineage arbors exist within a complex neuropil in the vicinity of future synaptic partners, making dependence on local cues likely. By contrast, the Tv neuron axon arbors are embedded within the basal lamina on the dorsal surface of the CNS, away from other neurons, thereby increasing their reliance on hormonal signals rather than on local cues to initiate growth. The increased specific synaptic connectivity among the interneuron lineages might result in a decreased reliance on circulating factors, and place greater importance on the surrounding environment to correctly pattern the complicated neural circuitry of the CNS. Interestingly, EcR modulation of growth but not sprouting is also seen in the dendritic arborizing (da) sensory neurons during larval growth. The elimination of ecdysone signaling in da neurons via usp² or EcR-DN MARCM clones causes reduced numbers of branches, but growth is still initiated. The reduction in complexity but not the footprint of the da neurons might reflect their establishment of an early dendritic scaffold to tile the body wall, followed by elaboration of higher order branches. Unlike the da neurons in the larva, it was found that blocking EcR signaling during metamorphosis reduces the overall arbor area in some of the lineages (Brown, 2009).

Expression of EcR-DN in the adult neuron lineages resulted in varying phenotypes amongst lineages, ranging from normal in appearance to having reduced and clumped arbors. This variability was not correlated with the expression level of EcR-DN within the MARCM clone. Also, the abnormalities were likely not to be due to adding more EcR to the system, but rather to the suppression of ecdysone signaling by the dominant-negative receptor. This conclusion is based on expressing wild-type EcR-A or EcR-B pan-neuronally using the elav driver through metamorphosis and finding no effect on behavior or the size or structure of the adult CNS. However, expression of EcR-DN with the same driver resulted in flies that were unable to eclose, their CNSs were reduced in size, and a few immature neurotactin-positive bundles were still evident (Brown, 2009).

Lineages 15 and 24 were by far the most affected, with both the central and peripheral projections of lineage 15 showing drastically reduced and compacted arbors. Cell-cell cohesion factors often function in bundling neurites together; misexpression of these factors could contribute to the compacted arbor phenotype. The expression of neurotactin, shown to be a cell adhesion molecule involved in axon guidance was examined, but no difference was found between control clones and those expressing EcR-DN. Another factor that might affect the size and density of arbors is the extent and quality of filopodia during sprouting and outgrowth. Unfortunately, the location of the motoneuron dendritic arbors within the central neuropil negatively impacted use of live imaging techniques, so the state of filopodial activity could not be ascertained (Brown, 2009).

In contrast to the motor lineages, the interneuron lineages were less severely affected by expression of EcR-DN. Lineages 6, 7 and 18 had moderate defects, whereas lineage 9 was mildly affected by expression of EcR-DN and lineage 2 cells expressing EcR-DN were indistinguishable from controls. Why are some lineages more affected by expression of EcR-DN than others? Interpretation of the effects of EcR-DN on the adult-specific interneuron lineages is hindered by the lack of information on their neural function. However, if synaptic partners specific to each interneuron lineage help to direct arbor formation, variability between different lineages could be expected owing to differences in local cues. The extent of input from local cues might modulate the developmental response of a particular lineage to ecdysone, leading to variability across the lineages. Not only does the disruption of EcR signaling give different phenotypes across the complement of neuronal lineages, but the consistent lack of visible phenotypes in some lineages suggests that the extent of the effects of EcR on fine-tuning the connectivity of a neuron within the CNS might depend on the identity of its synaptic partners and the timing of their connections. The reasons for the differential effects seen with EcR-DN expression may become clear as the circuitry and functions of the CNS neuron lineages are revealed (Brown, 2009).

How might altering the morphology of the Drosophila adult lineages through expression of EcR-DN impact the functionality of the nervous system? Reduced or compacted secondary arbors could result in either absent, decreased or misaligned connections with synaptic partners, or changes in autonomous neuron function. In vertebrates, small changes in arbor morphology and size can have substantial effects on neuronal processing, even if the correct synaptic partners are established. Pyramidal neurons have been shown to have non-linear operations based on morphology, and branch points and varicosities can affect signal propagation in axons. Wiring optimization also predicts that axons and dendrites have specific dimensions to minimize metabolic material, signaling delay and space costs to the cell, while keeping a fixed functionality. This means that a reduction in the secondary arbor of a neuron lineage might impair its processing ability. However, given the current lack of specific labeling or drivers for individual neuron lineages, the particular functions of the Drosophila CNS lineages are still unknown, complicating any testing of functional impairment (Brown, 2009).

Glia instruct developmental neuronal remodeling through TGF-β signaling

Glia secrete myoglianin, a TGF-β ligand, to instruct developmental neural remodeling in Drosophila. Glial myoglianin upregulates neuronal expression of an ecdysone nuclear receptor that triggers neurite remodeling following the late-larval ecdysone peak. Thus glia orchestrate developmental neural remodeling not only by engulfment of unwanted neurites but also by enabling neuron remodeling (Awasaki, 2011).

To establish and refine functional neural circuits, neurons alter connections as the organism matures. In Drosophila, larval brain neural circuits are remodeled into adult ones during metamorphosis. Neurons forming functional larval neural circuits prune their neural projections by local degeneration in early metamorphosis and re-extend their neurites to form the adult-specific neural circuits. This phenomenon requires activation of TGF-β signaling in the remodeling neurons. TGF-β signaling upregulates expression of the B1 isoform of the ecdysone receptor (EcR-B1) at the late larval stage. The pruning of larval projections is then triggered by the steroid molting hormone ecdysone (Awasaki, 2011).

The activin-β gene (Actβ), which encodes a Drosophila activin/TGF-β family molecule, is expressed in the developing larval brain. Temporal inhibition of Actβ with its dominant-negative form or double-stranded RNA (dsRNA) partially suppresses the expression of EcR-B1 in the wandering larvae. In a previous study, it was proposed that Actβ is a candidate ligand for TGF-β signaling in neuronal remodeling. However, the recently isolated Actβ null mutant, Actβ^ed80 had no developmental defects in the remodeling of the mushroom body γ neurons. This observation excludes Actβ as a principal ligand for TGF-β–dependent remodeling of mushroom body neurons. In addition, the Actβ null mutant grows normally until the pharate adult stage, contrasting the embryonic lethality that results from the ubiquitous induction of the dominant-negative form or dsRNA of Actβ. These contradictory phenomena suggest that off-target effects occur when Actβ is suppressed with dominant-negative proteins or RNA interference (Awasaki, 2011).

Notably, myoglianin (myo), which encodes another Drosophila TGF-β ligand (Lo, 1999), is temporally expressed in the brain of third instar larvae. Although no myo transcripts could be detected in the brain of early larvae, intense signals for myo transcripts were seen in subsets of glial cells in the cortex and inner regions of the central brain after the mid third instar larval stage. myo is selectively expressed in two subtypes of larval glial cells: the larval cortex and astrocyte-like glial cells. The cortex glia surround the cell body of each mature neuron and the astrocyte-like glia infiltrate into brain neuropile. The glial processes of both types are in the vicinity of, if not directly contacting, the larval mushroom body γ neurons (Awasaki, 2011).

To determine whether myo governs mushroom body remodeling, the glial expression of myo was silenced by targeted RNAi. dsRNA or microRNA (miRNA) against myo was selectively expressed in glia using the pan-glial GAL4 driver repo-GAL4. myo transcripts were no longer detectable after induction of myo dsRNA in pan-glial cells. The pruning and re-extension of mushroom body γ axons were examined by immunostaining with antibody to Fasciclin 2 (Fas2). In wild-type animals, the perpendicular γ axonal branches in the larval mushroom body lobes are completely pruned by 18 h after puparium formation (APF). γ neurons subsequently re-extended axons horizontally to form the midline-projecting γ lobe in adult brains. This remodeling was blocked by pan-glial induction of myo RNAi. The perpendicular axonal branches of γ neurons persisted through early metamorphosis, and the abnormally retained larval neurites coexisted with the α/β lobes in the adult mushroom bodies that failed to remodel. Direct visualization of mushroom body γ neurons validated the above observations with antibody to Fas2. The myo-silenced brains, including their glial network, were otherwise grossly normal. These observations indicate that a loss of myo expression in glia has no detectable effect on glial cells but adversely affects mushroom body remodeling (Awasaki, 2011).

myo was knocked down using glial subtype–specific drivers. Notably, only cortex glia–specific silencing could marginally block mushroom body remodeling and elicit mild mushroom body lobe defects in about 60% of adult mushroom bodies. However, silencing myo in both larval cortex and astrocyte-like glia fully recapitulated the mushroom body remodeling defects caused by the pan-glial induction of myo RNAi. These findings indicate that myo from two glial sources acts redundantly to govern mushroom body remodeling (Awasaki, 2011).

Next, to determine whether glial-derived myo is required for upregulation of EcR-B1 in remodeling mushroom body γ neurons, EcR-B1 expression in late-larval mushroom bodies in wild-type larvae was compared with expression in myo RNAi larvae. Although the characteristic pattern of EcR-B1 enrichment was detected in wild-type larvae, no such enrichment was detected in myo RNAi-expressing larvae. For example, the strong nuclear signal of EcR-B1 in the mushroom body γ neurons was no longer discernible. When EcR-B1 expression was selectively restored in the mushroom body γ neurons of animals expressing myo RNAi in glia, no defect in mushroom body remodeling could be detected. These findings suggest that the neuronal phenotypes resulting from glial myo RNAi can be effectively rescued by neuronal induction of EcR-B1. These results indicate that the glia-derived Myo instructs mushroom body remodeling via upregulation of neuronal EcR-B1 (Awasaki, 2011).

Remodeling of larval olfactory projection neurons is under the control of the same TGF-β and ecdysone signaling as the mushroom body γ neurons. The loss of glial myo blocked EcR-B1 expression and neurite remodeling of projection neurons, and the remodeling defect was substantially rescued by projection neuron–specific induction of transgenic EcR-B1. These results suggest that glia-derived Myo acts broadly to pattern neuronal remodeling via upregulation of EcR-B1 (Awasaki, 2011).

Using an FRT-mediated inter-chromosomal recombination technique, a deletion mutant, myo^Δ1 was generated. Organisms homozygous for myo^Δ1 showed no developmental delay through the third instar larval stage. However, mutant larvae prepupate on the surface of or in the food and are developmentally arrested before head inversion. In the myo mutant pupae (0 h APF) or prepupae (2–6 h APF), no enhancement of EcR-B1 expression was detected in the clustered cell bodies of mushroom body γ neurons. When myo expression was restored using myo-GAL4 to drive UAS-myo, the myo mutants grew into pharate or eclosed adults. These animals showed enriched EcR-B1 in the larval brain and they underwent normal mushroom body remodeling. In contrast, when myo expression was restored with myo-GAL4, but excluded in glia by using repo-GAL80 to selectively block GAL4 function in all glial cells, no enrichment of EcR-B1 was detected in the late larval or prepupal brains, and no evidence was found of neuronal remodeling in the pharate adults. These results with myo null mutants substantiate the notion that myo expression in glia governs neuronal remodeling via upregulation of neuronal EcR-B1 expression (Awasaki, 2011).

Does Myo activate TGF-β signaling through the Baboon (Babo) receptor that, contrasting with Myo, acts cell-autonomously to enable neuronal remodeling? There are three Babo isoforms with different ligand-binding domains. Babo-A has been implicated in governing neuron remodeling. To determine whether Myo activity requires Babo-A, their relationship was examined by epistasis. Ubiquitous expression of Myo induced larval lethality. If Myo signals through Babo-A, silencing babo-A should suppress the Myo-induced larval lethality. Attempts were made to deplete specific Babo isoforms by miRNAs to isoform-specific exons. Targeted induction of babo-A miRNA, but not babo-B or babo-C miRNA, effectively blocked mushroom body remodeling. When such isoform-specific miRNAs were co-induced with the myo transgene, only miRNA against babo-A potently suppressed the larval lethality that resulted from ectopic Myo expression. These epistasis results provide in vivo evidence that Myo and Babo-A act in a linear pathway to upregulate EcR-B1 and enable neuronal remodeling (Awasaki, 2011).

Remodeling of larval neurons occurs promptly as the larvae cease activity and become pupae. The tight temporal control of this developmental neuronal remodeling requires a timely induction of the EcR-B1 in these neurons. Three pathways, including TGF-β signaling, the cohesin protein complex and the FTZ-F1 nuclear receptor, are essential for the late-larval upregulation of EcR-B1. The nature and source of the TGF-β signaling become increasingly clear with the finding that Myo, in addition to Babo and dSmad2, is indispensable for the upregulation of EcR-B1. Myo can bind with the Babo/Wit receptor complex in culture (Lee-Hoeflich, 2005). Notably, Myo is produced by glia and is required in glia for neuronal expression of EcR-B1. Namely, glial cells directly instructed the neural remodeling through secretion of Myo. Glial cells further participate in the execution of neuronal remodeling through facilitation of neurite pruning by engulfment of the unwanted neuronal processes. Thus, glial cells orchestrate developmental neural remodeling and may have active roles in dynamically modulating mature neuronal connections (Awasaki, 2011).

Ecdysone-dependent and ecdysone-independent programmed cell death in the developing optic lobe of Drosophila

The adult optic lobe of Drosophila develops from the primordium during metamorphosis from mid-3rd larval stage to adult. Many cells die during development of the optic lobe with a peak of the number of dying cells at 24 h after puparium formation (h APF). Dying cells were observed in spatio-temporal specific clusters. This study analyzed the function of a component of the insect steroid hormone receptor, EcR, in this cell death. Expression patterns of two EcR isoforms, EcR-A and EcR-B1, were examined in the optic lobe. Expression of each isoform altered during development in isoform-specific manner. EcR-B1 was not expressed in optic lobe neurons from 0 to 6h APF, but was expressed between 9 and 48 h APF and then disappeared by 60 h APF. In each cortex, its expression was stronger in older glia-ensheathed neurons than in younger ones. EcR-B1 was also expressed in some types of glia. EcR-A was expressed in optic lobe neurons and many types of glia from 0 to 60 h APF in a different pattern from EcR-B1. Then, EcR function were genetically analyzed in the optic lobe cell death. At 0 h APF, the optic lobe cell death was independent of any EcR isoforms. In contrast, EcR-B1 was required for most optic lobe cell death after 24 h APF. It was suggested that cell death cell-autonomously required EcR-B1 expressed after puparium formation. betaFTZ-F1 was also involved in cell death in many dying-cell clusters, but not in some of them at 24 h APF. Altogether, the optic lobe cell death occurred in ecdysone-independent manner at prepupal stage and ecdysone-dependent manner after 24 h APF. The acquisition of ecdysone-dependence was not directly correlated with the initiation or increase of EcR-B1 expression (Hara, 2013).

This study analyzed the requirement of ecdysone in the optic lobe cell death. The role of ecdysone in cell death during metamorphosis has been examined in the salivary gland, larval midgut, and two types of neurons in the VNC, vCrz neurons and RP2 neurons. In the salivary gland, ecdysone triggers cell death in vitro, and the cell death required some components of ecdysone cascade, BR-C, E93, E74 and βFTZ-F1. In the midgut, cell death was induced by injection of ecdysone, and required BR-C and E93. For the cell death in vCrz neurons and RP2 neurons, ecdysone requirement was shown using EcR mutants. Among these tissues, a requirement for EcR isoforms was addressed only in the vCrz neurons and RP2 neurons. In the vCrz neurons, the cell death occurred in EcR-A or EcR-B1 mutants, but not in EcR-B1 and EcR-B2 mutant, indicating that EcR-B2 is required for this cell death. In RP2 neurons, cell death did not require EcR-A, but EcR-B1, EcR-B2 or both. Here, it was shown that the optic lobe cell death included ecdysone-independent and dependent tissues. The ecdysone-dependent cell death required EcR-B1 (Hara, 2013).

The number of dying cells in the optic lobe of EcR-B1 mutant animals at 24 and 36 h APF was much smaller than that in wild-type animals, but the number in EcR-A mutant animals was not. This finding showed that cell death in the optic lobe at these stages required EcR-B1, but not EcR-A (Hara, 2013).

Dying cells were examined in the following structures; LAD, lamina anterior dying cells; LPD, lamina posterior dying cells; LUD, lamina underlying dying cells; MALD, medulla anterolateral dying cells; MAMD, medulla anteromedial dying cells; MCD, medulla cortex dying cells; MCLD, medulla cortex lateral dying cells; MCMD, medulla cortex medial dying cells; MLBD, medulla-lamina boundary dying cells; MPLD, medulla posterolateral dying cells; PMCD, posterior medulla cortex dying cells; PMD, posteromedial dying cells; T/C, dying cells in the T/C region; LopD, dying cells in the lobula plate cortex; MMC, abnormal dying cells in the medial side of the medulla cortex; MLopD, abnormal dying cells in the medial side of the lobula plate cortex. The dependence on EcR-B1 was common among all dying cells in all clusters, except the MCMD. At 24 h APF, dying cells in the LAD, LPD, LUD, MCLD and T/C region were absent in optic lobes of most EcR-B1 mutants. In these mutants at 36 h APF, dying cells in the LUD, MLBD, MPLD and the lobula plate cortex were not evident. Similarly, at 48 h APF, MPLDs were absent from the EcR-B1 mutants. These results indicated that cell death in most clusters required EcR-B1 at stages after 24 h APF. It could not be determined whether death of MCMD was dependent on EcR-B1 because it was not clear whether the abnormal dying cells included MCMD in the medial side of the medulla cortex in EcR-B1 mutants (Hara, 2013).

In EcR-B1 mutant, a significant number of dying cells was constantly observed after 24 h APF. However, this fact does not mean that the loss of EcR-B1 delayed the timing of cell death. From 24 to 72 h APF, dying cells were mostly located in the medial side of the medulla cortex in EcR-B1 mutants and they did not include those in the clusters which would have been normally observed from 24 to 48 h APF, i.e., the LAD, LPD, LUD, MCLD, MPLD, MLBD, and dying cells in the T/C region and the lobula plate cortex. This fact strongly suggests that the optic lobe cell death was not delayed but suppressed by EcR-B1 mutation (Hara, 2013).

In some EcR-B1 mutants, enormous dying cells were observed at 72 h APF at positions where cell death would have normally occurred: the LAD, MCLD, MPLD, MLBD and T/C region. This suggests that delayed cell death can be induced at normal position without EcR-B1 at 72 h APF in these samples. It is known that experimental suppression of cell death can lead a delayed cell death by another complementally cell death mechanism. Therefore, it is possible that a complementary mechanism was induced in these samples (Hara, 2013).

It has been shown that a cell death initiator caspase, Dronc, had a EcRE in its promoter and EcR-B1 could induce Dronc expression. Indeed, the optic lobe cell death was suppressed in Dronc mutant in a preliminary experiment. Altogether, it is most likely that EcR-B1 directly controls cell death and consistently induces the death at right time in the optic lobe (Hara, 2013).

In this study, requirement of EcR-B2 function was not suggested. When the functions of all EcR isoforms were inhibited after puparium formation, the number of dying cells was larger than that in EcR-B1 mutant at 24 h APF (638.6 versus 363.8). If EcR-B2 was required for the optic lobe cell death, the number would have been less than that in the mutant. However, a function of EcR-B2 in the ecdysone-dependent cell death can still not be entirely excluded since there was a possibility that RNAi was insufficient to entirely inhibit EcR function in this experimental condition (Hara, 2013).

The number of dying cells in the optic lobes of EcR-A and EcR-B1 mutants was the same as that in wild-type animals at 0 h APF. When distribution of dying cells was examined, all clusters that were present in wild-type animals (specifically the LAD, MALD, MCD, MAMD, PMCD and dying cells in the T/C region) were also observed in the mutants. These results strongly indicated the cell death at 0 h APF was independent of both EcR-A and EcR-B1. EcR-B2 was also not required for the cell death because concurrent knockdown of all EcR isoforms via expression of hs-EcRi-11 resulted in no reduction in the number of dying cells at 0 h APF (Hara, 2013).

The above argument is relevant only for the zygotic not with maternal EcR. The contribution of maternal EcR should be tested. The result of the heat shock-inducible EcR RNAi denied the contribution of maternal EcR mRNA of all EcR isoforms. As with maternal proteins, there was no detectable EcR-B1 in any cluster or region at 0 h APF. In contrast, EcR-A was weakly expressed in all cluster regions, and these is no information about EcR-B2. Therefore, possible roles of maternal EcR-A and EcR-B2 protein cannot be excluded. However, there are no published reports of a requirement for maternal EcR proteins during metamorphosis. Taken together, these findings indicated that cell death in the optic lobe at 0 h APF is independent of any EcR. Furthermore, it seems likely this cell death is also independent of ecdysone because the number of dying cells gradually increased from 0 to 6 h APF rather than decreased, but the ecdysone titer rapidly drops and is very low during this period (Hara, 2013).

The period around 12 h APF may be a transient period when the ecdysone-dependence of cell death changes. In many EcR-B1 mutant optic lobes, the number of dying cells was the same as that in the wild type. In contrast, the number was reduced in a few mutant optic lobes and dying cells were absent in many of the clusters. These findings indicated that most of the cell deaths in many optic lobes was independent of EcR-B1, but some had become EcR-B1-dependent (Hara, 2013).

There is no previous report on ecdysone-independent cell death during metamorphosis. The ecdysone-independent cell death was limited to the early phase of metamorphosis in the optic lobe. However, this timing does not necessarily indicate that all cell death is independent of ecdysone because the larval midgut and vCrz neurons die ecdysone-dependently during this period. Therefore, ecdysone independence is a unique feature of the cell death that occurs in the optic lobe. There has been no report that the cell death that occurs during embryogenesis and larval development depends on ecdysone. Hence, it is proposed that the same mechanisms that mediate cell death during embryogenesis or larval development work for cell death during the early phase of optic lobe development (Hara, 2013).

Based on findings from many previous studies, every cell death that occurred during metamorphosis was part of the degeneration of a larval tissue and was dependent on ecdysone. These finding are understandable because ecdysone orchestrates the entire developmental process of metamorphosis. However, cell death within the optic lobe was independent of ecdysone during an early phase and then this cell death became ecdysone dependent later. This unique feature of the optic lobe cell death may be due to the fact that cell death in the optic lobe takes place during metamorphosis and is simultaneously involved in the organogenesis. So two cell death mechanisms, i.e., an organogenesis-accompanied (ecdysone-independent) mechanism and a metamorphosis-accompanied (ecdysone-dependent) mechanism may have evolved to cooperate during the optic lobe development (Hara, 2013).

The expression pattern of EcR-A and EcR-B1 was examined in this study. Expression of each isoform altered during development in isoform specific manner. However, there was no direct relationship between EcR-B1 expression and the emergence of the cell death. At 0 h APF, when cells die independent of ecdysone, EcR-B1 was not expressed in any region with clusters of dying cells. In contrast, EcR-B1 was expressed in all regions with clusters of dying cells at 12 h APF, although cell death, at this stage, was, for the most part, ecdysone independent in all clusters, except PMD. Thus, there was a temporal gap between EcR-B1 expression and ecdysone-dependent cell death. This indicates that the expression of EcR-B1 was not a direct cause that shifted cell death from an ecdysone-independent to an ecdysone-dependent one. EcR-B1 expression would be one of the requisites to make cells competent to undergo ecdysone-dependent cell death at a later time point and another mechanism following EcR-B1 expression would be required for the shift (Hara, 2013).

Although cell death in the optic lobe after 24 h APF required EcR-B1, the level of EcR-B1 expression varied among cluster regions during this period. For example, at 24 h APF, EcR-B1 was expressed weakly in the anterior region of the lamina cortex where LAD was located. On the lateral side of the medulla cortex where MCLD were present, EcR-B1 was expressed moderately. EcR-B1 was strongly expressed in the T/C region where many dying cells were present. The expression levels also varied among cluster regions at 36 and 48 h APF. All these findings indicate that the death decision, even for the ecdysone-dependent cell death, was not simply related to high EcR-B1 levels. This decision would be made within specific context of each cluster (Hara, 2013).

EcR-B1 expression was correlated with glial ensheathment in the lamina cortex, medulla cortex and T/C region. In these regions, newly-born neurons derived from the OOA or IOA compose pre-ensheathed domains. As development proceeds, they become to be surrounded by glial membrane and compose ensheathed domains of mature neurons. In particular, the ensheathed domain in lamina cortex corresponds to a region with columnar structures. In the lamina cortex, EcR-B1 was weakly expressed in the pre-ensheathed domain, while strongly expressed in the ensheathed domain. In the medulla cortex and T/C region, it was not expressed in the pre-ensheathed domains, but expressed in the ensheathed domains. These facts suggest a possibility that the glial ensheathment promotes or initiates EcR-B1 expression in the process of neuronal differentiation in these regions. This possibility is supported by the fact that EcR-B1 expression became stronger after the ensheathment as development proceeded (Hara, 2013).

With regard to cell death, the LAD, MCLD and MALD were always located near the border of the pre-ensheathed and ensheathed domains. Therefore, cell death may be linked to the entry of glial membrane in these clusters. Since this positional relationship was observed from 0 to 24 h APF, the glial ensheathement and ecdysone signaling via EcR-B1 may cooperate to induce cell death in the clusters after 12 h APF, when cell death become dependent on EcR-B1 (Hara, 2013).

Ecdysone signaling at metamorphosis triggers apoptosis of Drosophila abdominal muscles

One of the most dramatic examples of programmed cell death occurs during Drosophila metamorphosis, when most of the larval tissues are destroyed in a process termed histolysis. Much of the understanding of this process comes from analyses of salivary gland and midgut cell death. In contrast, relatively little is known about the degradation of the larval musculature. This study analyzed the programmed destruction of the abdominal dorsal exterior oblique muscle (DEOM) which occurs during the first 24h of metamorphosis. Ecdysone signaling through Ecdysone receptor isoform B1 is required cell autonomously for the muscle death. Furthermore, the orphan nuclear receptor FTZ-F1, opposed by another nuclear receptor, HR39, plays a critical role in the timing of DEOM histolysis. Unlike the histolysis of salivary gland and midgut, abdominal muscle death occurs by apoptosis, and does not require autophagy. Thus, there is no set rule as to the role of autophagy and apoptosis during Drosophila histolysis (Zirin, 2013)

There are three different isoforms of the EcR gene, EcR A, EcR B1, and EcR B2, each sharing the same DNA binding and ligand binding domains, but with a unique amino terminus. The different temporal and spatial expression patterns of EcR A and EcR B isoforms are thought to reflect their distinct functions during development. EcR B1 is expressed primarily in larval cells that are destined for histolysis, while EcR A is expressed primarily in imaginal tissues destined for differentiation into adult structures. Thus the response of salivary glands and midgut to ecdysone during metamorphosis is dependent on EcR B1. Nonetheless, EcR A mutants also have a defect in salivary gland histolysis, suggesting that the isoform might also contribute to this process. Furthermore, some neurons in the ventral nerve cord and brain that strongly express EcR A undergo apoptosis in response to ecdysone soon after eclosion, suggesting that ecdysone induced PCD is not strictly a function of EcR B1 signaling (Zirin, 2013)

This study examined expression of both EcR A and EcR B1 isoforms and found that only EcR B1 was detectable in the dorsal internal oblique muscles (DIOMs) and DEOMs during pupariation. Consistent with its expression pattern, knockdown of EcR B1 specifically in the muscle inhibited DEOM histolysis. The inhibition achieved with the EcR B1 isoform RNAi was not as strong as with RNAi targeting all isoforms, or with overexpression of the dominant negative EcR B1. This could be due to either differences in the efficiency of knockdown or to a role for EcR B2 in DEOM degradation. Taken together these data strongly supports the view that, like in salivary glands and midgut, ecdysone signals through EcR B1 to induce cell death in abdominal muscles. However, given that both DIOMs and DEOMs express EcR B1 at the same time, the presence of the receptor is not sufficient to explain why only the latter muscles are degraded. Another important player in the timing of salivary glands cell death is the orphan nuclear hormone receptor gene ftz fl, which is transcribed midway through prepupal development, when the ecdysone titer is relatively low. FTZ F1 has been hypothesized as a competence factor, directing the subsequent genetic responses to the ecdysone pulse at the prepupal/pupal transition. Thus FTZ F1 is required for the induction of salivary gland histolysis. In contrast, the midgut does not express ftz fl prior to its earlier ecdysone induced cell death, indicating that FTZ F1 is not required for all histolysis during Drosophila metamorphosis. Despite the fact that the timing of muscle histolysis is similar to that of the midgut, the function of FTZ F1 was more like in the salivary gland, as FTZ F1 was observed in the DEOMs starting at ~ 5 h APF, prior to caspase activation at 8 h APF. Furthermore, ftz fl was required for proper muscle histolysis, as knockdown cell autonomously delayed caspase activation and death in the DEOMs (Zirin, 2013)

These results raised the possibility that the presence of FTZ F1 in the DEOMs, but not in the DIOMs, determined the different response of these muscles to ecdysone. However, overexpression of FTZ F1 in the OJOMs, while causing severe muscle degeneration, was unable to induce caspase activity or cell death. Nor could the presence of HR39 in the DIOMs account for the different response of the muscles to ecdysone. Although a reciprocity of HR39 and FTZ F1 expression was observed in the DIOMs and DEOMs, Hr39 mutant DJOMs still persist through metamorphosis. It is concluded that FTZ F1 and HR39 expression determine the timing of the abdominal muscle response to ecdysone but that these factors do not change the nature of the response (Zirin, 2013)

It was recently shown that EcR B1 expression is regulated by FTZ F1 and HR39 in mushroom body neurons and abdominal motor neurons during metamorphosis. The current observation that EcR B1 promotes muscle degeneration is consistent with the finding that EcR B1 promotes post synaptic dismantling in the motor neuron, and supports the notion that muscle degeneration is instructive on motor neuron retraction. However, it was show that even though both ftz f1 and EcR B1 are essential for the proper histolysis of DEOMs, there was no change in EcR B1 staining upon ftz fl knockdown as was observed in the mushroom body system. This suggests that changes observed in the muscle synapse due to ftz fl knockdown are not the result of a downstream effect on EcR B1 expression in the muscle cell. Thus, the regulatory relationship between FTZ F1, HR39 and EcR B1 in early pupal abdominal muscles is distinct from the relationship reported in neurons (Zirin, 2013)

This suggests that there is an additional unknown factor whose expression dictates the fate of the OJOMs or DEOMs. This factor is unlikely to be either of the nuclear proteins EAST or Chromator (Chro ), which were previously identified as having opposing effects on the destruction of the abdominal DEOMs during metamorphosis. Breakdown of DEOMs was incomplete in Chro mutants, and promoted in east mutants, leading to the proposal that Chro activates and EAST inhibits tissue destruction and remodeling. However, neither east nor chro alleles cause histolysis of the DIOMs, nor do they alter caspase activation in either DEOMs or DIOMs. Rather these genes may affect the timing of muscle histolysis through a function downstream of PCD induction. It is proposed that there must be an additional factor present in the DIOMs which inhibits EcRB1 signaling from inducing PCD, or alternatively, a factor present in the DEOMs, which permits EcRB1 to activate a death program. The identification of this factor will be a focus of future studies (Zirin, 2013)

The previously reported cleaved caspase 3 staining in the DEOMs is the only data addressing the nature of abdominal muscle PCD prior to this study. This study addressed whether muscle histolysis is apoptotic, autophagic or some combination of both. During salivary gland histolysis, several autophagy related genes (ATGs) are upregulated, and mutations or knockdown of these ATGs specifically in the salivary gland inhibit the destruction of the tissue. Caspase activation also occurs in the histolyzing salivary glands, but overexpression of the caspase inhibitor p35 only partially blocks salivary gland degradation. Simultaneous inhibition of both autophagy and apoptosis in the salivary gland produces the strongest inhibition of death, suggesting that both pathways contribute to histolysis of this tissue). In contrast to salivary gland histolysis, midgut histolysis requires autophagy but not caspase activity. Mutations or knockdown of ATGs inhibit midgut death, but p35 expression has no effect. Based on these two model systems, it appears that there is no set rule as to the role of autophagy and apoptosis during Drosophila histolysis (Zirin, 2013)

These data serves to further highlight how distinctive PCD for each of the tissues undergoing histolysis. DEOMs stain positive for cleaved caspase 3, consistent with previous reports. TUNEL positive staining and chromatin condensation was also observed in the DEOMs at 8 h APF, both markers of apoptosis. Importantly, it was possible to suppress DEOM degradation by overexpression of the pan caspase inhibitor p35, indicating that unlike the midgut, muscle histolysis is apoptotic. To determine whether the muscle PCD was autophagic in nature, the DEOMs were examined by EM. Although some autophagic vesicles were observed in the dying muscles, they were not abundant, nor were GFP Atg8 localization to autophagosomes examined by confocal microscopy. Several essential components of the autophagic machinery were knocked down, and no effect was observed on the timing or extent of DEOM histolysis. Although knockdown efficiency is always a concern with RNAi experiments. each of the transgenes was able to strongly inhibit autophagosome formation in larval muscles. It can be said therefore with confidence that autophagy is not required for DEOM PCD, putting the abdominal muscle in the unique category of non autophagic histolysis. In future studies it will be interesting to compare muscles, salivary gland, and midgut to determine why each tissue has a distinctive type of PCD (Zirin, 2013)

Dynamic feedback circuits function as a switch for shaping a maturation-inducing steroid pulse in Drosophila

Steroid hormones trigger the onset of sexual maturation in animals by initiating genetic response programs that are determined by steroid pulse frequency, amplitude and duration. Although steroid pulses coordinate growth and timing of maturation during development, the mechanisms generating these pulses are not known. This study shows that the ecdysone steroid pulse that drives the juvenile-adult transition in Drosophila is determined by feedback circuits in the prothoracic gland (PG), the major steroid-producing tissue of insect larvae. These circuits coordinate the activation and repression of hormone synthesis, the two key parameters determining pulse shape (amplitude and duration). Ecdysone has a positive-feedback effect on the PG, rapidly amplifying its own synthesis to trigger pupariation as the onset of maturation. During the prepupal stage, a negative-feedback signal ensures the decline in ecdysone levels required to produce a temporal steroid pulse that drives developmental progression to adulthood. The feedback circuits rely on a developmental switch in the expression of Broad isoforms that transcriptionally activate or silence components in the ecdysone biosynthetic pathway. Remarkably, this study shows that the same well-defined genetic program that stimulates a systemic downstream response to ecdysone is also utilized upstream to set the duration and amplitude of the ecdysone pulse. Activation of this switch-like mechanism ensures a rapid, self-limiting PG response that functions in producing steroid oscillations that can guide the decision to terminate growth and promote maturation (Moeller, 2013).

The feed-forward loop described in this study provides an explanation for a number of previous observations. These studies have indicated that ecdysone can modulate PG steroidogenic activity and that PG cells undergo autonomous activation under long-term culture conditions. Interestingly, autonomous activation is prevented by juvenile hormone (JH), which inhibits br expression. During the last larval instar of holometabolous insects, a drop in JH levels eventually leads to the production of a high-level ecdysone pulse that triggers metamorphosis, although the mechanism underlying this is poorly understood. Since the decline of JH is permissive for br expression, the fact that Br promotes PG steroidogenic activity is likely to explain how the drop in JH results in the production of a high-level ecdysone pulse initiating metamorphosis. Thus, these data provide a link between JH and ecdysone that might explain how the presence of JH prevents metamorphosis (Moeller, 2013).

The observations clearly show that positive feedback is required for the transcriptional upregulation of phm, dib and sad, all of which encode enzymes that act at late steps in the ecdysone biosynthetic pathway. By contrast, EcR and Br activity are not necessary for the normal activity of spok, which is involved in an earlier step in the pathway and whose transcription is regulated by Molting defective, a factor that is not involved in the regulation of the other identified biosynthetic enzymes. In addition, in contrast to the other ecdysone biosynthetic enzymes, Spok is also likely to be regulated at the level of translation and phosphorylation in response to PTTH signaling. Furthermore, expression of torso is not EcR and Br dependent, consistent with levels of torso not being synchronized with the ecdysone peaks. Together with the current results demonstrating that the feedback is required downstream of Ras in the PG, this shows that the feed-forward loop functions downstream of PTTH to amplify the signal and not for endowing the PG with competence to respond to PTTH (Moeller, 2013).

The current findings raise an important issue that challenges the classical view that ecdysone released from the PG is converted to its more active metabolite 20-hydroxyecdysone (20E) in peripheral target tissues, where it interacts with EcR. Although 20E may travel back and inform the PG, a more direct route would be that ecdysone produced by the PG acts on the gland itself or that the PG produces small amounts of 20E that control the activity of the gland. Consistent with these possibilities, it was observed that reduced expression of shade, which encodes the enzyme that converts ecdysone to 20E, in the PG leads to a developmental arrest in the larval stages and that all three Drosophila EcR isoforms can induce transcription in response to ecdysone. Interestingly, recent reports have demonstrated the essential function of E75, DHR3 (Hr46), βFTZ-F1 and DHR4 in regulating the production of ecdysone in the PG. Although nitric oxide and PTTH regulate the activity of some of these factors, these signals alone are unlikely to explain the regulation of their function in the PG. Based on the current results, an obvious possibility is that EcR controls the expression of these classical ecdysone-inducible genes in the PG. Extensive studies on these ecdysone target genes have led to the elucidation of an early response network for steroid hormone action and the molecular characterization of the genetic architecture underlying the cellular responses to steroids. Surprisingly, this study shows that this genetic program that guides the downstream cellular decisions in response to regulatory ecdysone pulses is utilized upstream to shape the pulse by setting its duration and amplitude. Thus, the same genetic components are used for coordinating the production and reception of the steroid signals that drive directional developmental progression (Moeller, 2013).

Previous experiments demonstrated that ecdysone, produced by the PG, induces an inactivation enzyme responsible for clearance of circulating ecdysone. This study shows that termination of the pulse requires negative feedback that represses PG steroid production activity in coordination with peripheral clearance. How does ecdysone stimulate and repress biosynthesis in the PG through EcR? The results show that EcR induces different Br isoforms, forming circuits that either increase or inhibit the activity of the biosynthetic pathway by regulating the levels of the enzymatic components. Br is required specifically for the juvenile-adult transition and is expressed during the last instar. This study shows that the appearance of Br in the PG requires EcR and correlates with the ecdysone peak. The positive effect of EcR on ecdysone biosynthesis is mediated largely through Br-Z4, which has previously been shown to induce transcription of Npc1a, which encodes a key cellular component required in the PG for the delivery of cholesterol as a substrate for steroid synthesis. Together, this suggests that ecdysone-mediated positive feedback coordinates increased substrate delivery with upregulation of the biosynthetic machinery in order to produce the maturation-inducing ecdysone pulse. Conversely, the Br-Z1 isoform inhibits ecdysone synthesis, forming a negative feedback that is important for the decline of the ecdysone titer during the prepupal stage. Thus, the temporal control of these circuits relies on a dynamic switch in the PG from Br-Z4 to Br-Z1. A similar switch has been found in the imaginal discs, where Br-Z4 rapidly accumulates in response to ecdysone and then disappears several hours later when Br-Z1 is upregulated. It has been suggested that the switch from Br-Z4 to Br-Z1 is regulated at the level of alternative splicing of br transcripts. The data suggest that the switch is a hard-wired genetic timing mechanism rather than being dependent on ecdysone concentrations. This switching might also occur at the enhancer level through competition of binding to overlapping Br-Z1/Z4 regulatory sites, as found in the phm promoter. Importantly, coupling a negative with a positive feedback through a common regulatory site ensures a self-limiting response by preventing 'run away' synthesis that would otherwise result from positive-feedback amplification alone (Moeller, 2013).

In conclusion, this study shows that the maturation-inducing ecdysone pulse is shaped by an autonomous feed-forward and feedback circuitry within the endocrine tissue that modulates the rate of hormone synthesis. The coupling of these feedback circuits ensures rapid, self-limiting hormone production that translates neuropeptide signaling into a regulatory steroid pulse which functions as a switch to drive developmental progression (Moeller, 2013).

Ligand-independent requirements of steroid receptors EcR and USP for cell survival

The steroid hormone ecdysone 20-hydroxyecdysone (20E) binds the heterodimer EcR/USP nuclear receptor to regulate target genes that elicit proliferation, cell death and differentiation during insect development. This study shows that the prothoracic gland (PG), the major steroid-producing organ of insect larvae, requires EcR and USP to survive in a critical period previous to metamorphosis, and that this requirement is 20E-independent. The cell death induced by the downregulation of these receptors involves the activation of the JNK-encoding basket gene and it can be rescued by upregulating EcR isoforms which are unable to respond to 20E. Also, while PG cell death prevents ecdysone production, blocking hormone synthesis or secretion in normal PG does not lead to cell death, demonstrating further the ecdysone-independent nature of the receptor-deprivation cell death. In contrast to PG cells, wing disc or salivary glands cells do not require these receptors for survival, revealing their cell and developmental time specificity. For exploring the potential use of this feature of steroid receptors in cancer, tumor overgrowth induced by altered yorkie signaling was assayed. This overgrowth is suppressed by EcR downregulation in PG, but not in wing disc, cells. The mechanism of all these cell death features is based on the transcriptional regulation of reaper. These novel and context-dependent functional properties for EcR and USP receptors may help to understand the heterogeneous responses to steroid-based therapies in human pathologies (Mansilla, 2015).

Ecdysone signaling opposes epidermal growth factor signaling in regulating cyst differentiation in the male gonad of Drosophila melanogaster

The development of stem cell daughters into the differentiated state normally requires a cascade of proliferation and differentiation steps that are typically regulated by external signals. The germline cells of most animals, in specific, are associated with somatic support cells and depend on them for normal development. In the male gonad of Drosophila melanogaster, germline cells are completely enclosed by cytoplasmic extensions of somatic cyst cells, and these cysts form a functional unit. Signaling from the germline to the cyst cells via the Epidermal Growth Factor Receptor (EGFR) is required for germline enclosure and has been proposed to provide a temporal signature promoting early steps of differentiation. A temperature-sensitive allele of the EGFR ligand Spitz (Spi) provides a powerful tool for probing the function of the EGRF pathway in this context and for identifying other pathways regulating cyst differentiation via genetic interaction studies. Using this tool, this study showed that signaling via the Ecdysone Receptor (EcR), a known regulator of developmental timing during larval and pupal development, opposes EGF signaling in testes. In spi mutant animals, reducing either Ecdysone synthesis or the expression of Ecdysone signal transducers or targets in the cyst cells resulted in a rescue of cyst formation and cyst differentiation. Despite of this striking effect in the spi mutant background and the expression of EcR signaling components within the cyst cells, activity of the EcR pathway appears to be dispensable in a wildtype background. It is proposed that EcR signaling modulates the effects of EGFR signaling by promoting an undifferentiated state in early stage cyst cells (Qian, 2014).

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