patched

DEVELOPMENTAL BIOLOGY

Embryonic

Patched RNA is first detected during cellularization throughout the cortex except for a dorsal anterior region and around the posterior pole, including pole cells. By stage 8, PatchedmRNA is expressed in broad stripes of segmental periodicity in the posterior part of the parasegment compartment. These later split into two stripes per segment primordium (Hooper, 1989). Additional regions of expression include ectodermally derived hindgut and the labrum of the head. Late embryos express ptc in internal structures like esophagus, foregut, visceral mesoderm and hindgut. In the head region, parts of the mandibular, maxillary and labial buds [Image] are stained, as well as the hyperpharyngeal lobe and pharynx (Capdevila, 1994a).

During neurogenesis, the transmembrane protein Patched promotes a wingless-mediated specification of a neuronal precursor cell, NB4-2. Wg, secreted by row 5 cells promotes wingless expression in adjacent row 4 cells; Wg in turn represses gooseberry. Novel interactions of these genes with engrailed and invected during neurogenesis have been uncovered. While in row 4 cells Ptc represses gsb and wg, in row 5 cells en/inv relieve Ptc repression of gsb by a non-autonomous mechanism that does not involve hedgehog. The non-autonomous mechanism originates in Row 6/7 cells where en/inv engender hedgehog and another unknown secreted signal which acts in turn on adjacent row 5 cells to heighten wingless, and consequently, the expression of gooseberry. This differential regulation of gsb leads to the specification of NB5-3 and NB4-2 identities to two distinct neuroblasts. The row 5, NB5-3, neuroblasts are specified by high levels of gsb, expressed autonomously in row 5. The fate of row 4, NB4.2, requires an absence of gooseberry, assured by Patched repression and Wingless signaling from adjacent row 5 cells. The uncoupling of the ptc-gsb regulatory circuit by hedgehog and the unknown secreted signal from row 6/7 cells enables gsb to promote Wg expression in row 5 cells. These results suggest that the en/inv->ptc->gsb->wg pathway uncovered here and the hh->wg are distinct pathways that function to maintain the wild-type level of Wg. These results also indicate that Hh is not the only ligand for Ptc and similarly, that Ptc is not the only receptor for Hh (Bhat, 1997).

The genital disc consists of three primordia: moving from anterior to posterior they are the female genital primordium, the male genital primordium and the anal primordia. Only one of the two genital primordia develops, depending on the individual's sex, whereas the anal primordium develops in both sexes. It is proposed here that the genital disc, which is of ventral origin, is organized in a manner similar to the antennal and leg discs: the expression domains of decapentaplegic and wingless are mostly complementary and abut engrailed expression. An analysis was made of the roles of the genes hedgehog, patched, dpp and wg in the development of the three primordia that form the genital disc. The morphogenetic alterations produced by ectopic expression of hh mimic a lack of ptc function. Both genetic conditions cause derepression of dpp and wg. Ectopic expression of either of these genes causes non-autonomous duplications and/or reductions of genital and anal structures. Some of these alterations are explained by the mutual repression of wg and dpp. In the development of the genital disc, the functional relationships between these genes seem to be analogous to those described for leg and antennal discs: dpp and wg are induced in the anterior compartment by Hh protein blocking the repressive effect of Ptc, and the mutual repression of dpp and wg restrict one another to their respective domains. It may be concluded that dpp and wg act as general organizers for development of the genital disc (Sánchez, 1997).

The development of the genital primordia is based on two processes: cell proliferation and sexual differentiation. Cell proliferation refers to the capacity of each genital primordium to grow or to be kept in the repressed state. Sexual differentiation refers to the type of adult structure formed by each genital primordium. It is proposed that the control of cell proliferation in the male and female genitalia requires the concerted action of Abd-B and doublesex, either directly or indirectly, through the expression of the genes dpp and wingless. Thus, in female genital discs, the repressed male primordium does not express dpp whereas the repressed female primordium of the male genital discs expresses a reduced level of dpp. This reduced level seems to be insufficient to stimulate cell growth. In contrast, when strong dpp levels are obtained in the repressed female primordium of male discs, repressed female primordia overproliferate in mutants for patched or costal-2, as well as in the discs where uniform ectopic expreession of hedgehog is produced. The genes dpp and wg, however, do not participate in the sexual differentiation process, which depends on sexual cytodifferentiation genes. Thus the growth of repressed female primordia of the patched mutant male discs would give rise to no adult female genital structures since the genetic sex is male (Sánchez, 1997 and references).

Larval

Patched is involved in the formation of the boundary between anterior and posterior compartments of the wing disc (Johnson, 1995).

Very little information is available about gene expression during the larval period, a developmental interval critical to the formation of the adult. To what extent does gene expression during this period resemble that in the embryonic stages, and how does gene expression during the larval period contribute to segment polarity in the adult? In fact, all the genes expressed during embryonic segment polarity also play a similar role in the formation of the adult. Cells destined to form the cuticle of the adult abdomen are present as clusters of small, non-dividing diploid cells (the anterior dorsal, posterior dorsal and ventral histoblast nests) located at stereotyped postions in the larval epidermis. These cells, just as do their embryonic counterparts, express engrailed, hedgehog, wingless, patched, cubitus interruptus and sloppy paired in a stereotyped manner dependent on their positions within each segment. Each segment is subdivided into an anterior (A) and posterior (P) compartment, distinguished by activity of the selector gene engrailed (en) in P but not A compartment cells. The ventral epidermis of each abdominal segment forms a flexible cuticle, the pleura, with a small plate of sclerotised cuticle, the sternite, centered on the ventral midline. The pleura is covered with a uniform lawn of hairs, all pointed posteriorly, whereas the sternite contains a stereotyped pattern of bristles. Posterior compartments are to a large degree devoid of hairs and bristles, while the sternite cuticle of the A compartment consists of an anterior-to posterior progression of six types of cuticle distinguished by ornamentation and pigmentation. Just anterior to the posterior compartment, A6 is unpigmented, with hairs and none of the larger ornaments called bristles. A5 is darkly pigmented with hairs and bristles of large size. A4 and A3 are darkly and lightly pigmented respectively with moderately sized hairs and bristles. A2 is lightly pigmented with hairs, and A1, adjacent to the next more anteriorly located "posterior" compartment is unpigmented without hairs (Struhl, 1997a).

Hedgehog (Hh), a protein secreted by engrailed expressing P compartment cells, spreads into each A compartment across the anterior and the posterior boundaries to form opposing concentration gradients that organize cell pattern and polarity. Anteriorly and posteriorly situated cells within the A compartment respond in distinct ways to Hh: they express different combinations of genes and form different cell types. patched is expressed at both boundaries. patched is expressed in a graded fashion within each stripe, just anterior to each P compartment. ci peaks at high level in those cells abutting Hh- secreting cells of the P compartment and declines progressively in cells further away. wingless is also expressed in this domain and sloppy paired is expressed in the same region as wingless. decapentaplegic is expressed only in the ventral pleura in those A compartment cells neighboring P compartment cells within the same segment. dpp is not expressed immediately behind posterior compartments (Struhl, 1997).

Cell death-induced regeneration in wing imaginal discs requires JNK signalling

Regeneration and tissue repair allow damaged or lost body parts to be replaced. After injury or fragmentation of Drosophila imaginal discs, regeneration leads to the development of normal adult structures. This process is likely to involve a combination of cell rearrangement and compensatory proliferation. However, the detailed mechanisms underlying these processes are poorly understood. A system was established to allow temporally restricted induction of cell death in situ. Using Gal4/Gal80 and UAS-rpr constructs, targeted ablation of a region of the disc could be performed and regeneration monitored without the requirement for microsurgical manipulation. Using a ptc-Gal4 construct to drive rpr expression in the wing disc resulted in a stripe of dead cells in the anterior compartment flanking the anteroposterior boundary, whereas a sal-Gal4 driver generated a dead domain that includes both anterior and posterior cells. Under these conditions, regenerated tissues were derived from the damaged compartment, suggesting that compartment restrictions are preserved during regeneration. These studies reveal that during regeneration the live cells bordering the domain in which cell death was induced first display cytoskeletal reorganisation and apical-to-basal closure of the epithelium. Then, proliferation begins locally in the vicinity of the wound and later more extensively in the affected compartment. Finally, regeneration of genetically ablated tissue was shown to require JNK activity. During cell death-induced regeneration, the JNK pathway is activated at the leading edges of healing tissue and not in the apoptotic cells, and is required for the regulation of healing and regenerative growth (Bergantiños, 2010).

Two main conclusions can be drawn from this work: (1) that genetically induced regeneration entails compartment-specific proliferation; and (2), that this type of regeneration requires JNK signalling for early regeneration events (Bergantiños, 2010).

This study established that the proliferation response to ptc>rpr induction is concentrated in the A compartment and consists of two activities: a local and a compartment-associated response. The local proliferation response resembles the activity of blastemas, a feature found in discs after fragmentation and implantation. The late compartment-restricted proliferation could be indicative of a reutilisation of developmental programs. The entire A compartment responds to the lack of the original ptc region by reactivating proliferation in order to achieve the final organ size. Thus, it is concluded that genetically induced regenerating discs restore the overall organ size by activation of proliferation, not only near the wound, as in fragmented and implanted discs, but also in the whole affected compartment. Thus, it is believed that the local proliferation is a fast and early response to the lost structures and that the later compartment-associated proliferation is a response to adjust the size of the tissue (Bergantiños, 2010).

ptc and sal were selected because of the precise removal of cells and also because they enabled testing whether both A and P compartments are involved in regeneration. The results suggest that when the A compartment is damaged (ptc>rpr), the P compartment only responds to the injury by sealing the gap that separates it from the A compartment through the generation of F-actin-rich cell extensions. These are projected to anchor the extensions from the cells at the edge of the A compartment as they proceed towards recovery of the intact cell sheet. In this situation, the regenerated tissue is derived exclusively from the A compartment. By contrast, when cells from both the A and P compartments are killed (sal>rpr), proliferation increases in both compartments. The boundaries between compartments are rapidly re-established after injury and prevent cells from crossing into adjacent compartments. Thus, boundaries are respected and compartments act as units of growth during regeneration (Bergantiños, 2010).

Following genetic ablation driven by either the ptc or sal drivers, healing starts at the DV boundary and spreads laterally towards the proximal regions, which are the last to close the wound. Cells at the DV boundary are arrested in G1-S, through a mechanism based on Notch and wg signalling. These arrested cells are the first to respond to healing and drive the cytoskeletal machinery for tissue reorganisation. This is consistent with the idea that the requirements for cell proliferation and for cell shape changes that occur during normal fly and vertebrate development and wound repair place incompatible demands on the cytoskeletal machinery of the cell. Another issue to be considered is that the DV boundary is the first zone of closure for F-actin extensions. This is reminiscent of Drosophila embryonic dorsal closure and wound repair, in which matching filopodia on both sides of the opening are recognised by the code of segment polarity genes in each parasegment. In addition, mechanical forces may be involved in tissue reorganisation. Stretching forces could be altered upon the induction of cell death, and they could have an important role in mounting a quick healing response. For example, mechanical forces, which have been proposed to act in the developing wing disc and compress the tissue through the central region, could stretch it towards the DV border after ablation of the ptc domain. Thus, either by matching affinities or by stretching forces, wound repair spreads from the apical DV border to basal and proximal domains (Bergantiños, 2010).

It has been shown in the Drosophila wing disc that massive loss of cells after irradiation gives rise to apparently normal adult wings as a result of compensatory proliferation driven by surviving cells. Experiments involving irradiation or induction of apoptosis in a p35 background have suggested that this compensatory proliferation is controlled by signals, including JNK, emerging from cells that have entered apoptosis, and that cell-death regulators, such as p53 and the caspase Dronc (Nedd2-like caspase), function as regulators of compensatory proliferation and blastema formation in the surviving cells. By contrast, the results show that proliferation is compartment specific and occurs independently of the dead tissue following targeted ablation. Two observations strongly support this interpretation. First, puc expression, as a marker of JNK activity, is concentrated in a narrow strip of apical cells, suggesting that JNK signalling is activated in the leading edges during wound closure. This again resembles other repair mechanisms described not only in imaginal discs, but also in other healing tissues, and reiterates epithelial fusion events observed in embryogenesis. Second, perturbation of the JNK pathway within the dying domain has no effect on either healing or regeneration. Even the early peak of localised mitosis near the wound and the later A-compartment-associated mitoses are present when UAS-bsk^DN and UAS-puc are driven in the dying domain. Effects on healing and regeneration are found only in hep mutant backgrounds, when JNK is impaired in the whole epithelium and not only in the dead domain. This requirement for the JNK pathway at the edges of the wound has also been found in studies of microsurgically induced regeneration. Cell lineage analysis of puc-expressing cells near the wound has shown that puc sets the limits of a blastema and that puc derivatives are able to reconstitute most of the missing tissue (Bergantiños, 2010).

Finally, whether JNK is required for healing alone or also functions as a signal for proliferation remains an open issue. Rapid local proliferation is affected in unhealed hep heterozygotes. Also, sal^PE>rpr wing regeneration cannot be achieved after 10 hours induction in a hep^r75 background. Reduced proliferation could be due to a lack of healing or to loss of JNK activity. The possibility canot be ruled out that the JNK cascade, through the active AP-1 (Kayak and Jun-related antigen -- FlyBase) transcription factor complex, targets not only genes required for healing and epithelial fusion, but also those required for regenerative growth. In mammals, inhibition of the JNK pathway or lack of c-Jun results in eyelid-closure defects and also impairs proliferation by targeting Egfr transcription. Reconstruction of normal pattern and size might also require multiple signals. It has recently been found that regenerative growth induced by cell death requires Wnt/Wg signalling to increase dMyc stability, suggesting the involvement of other signalling pathways and also cell competition. It is very likely that an integrated network of signals and cell behaviours is necessary to reconstitute the damaged tissue (Bergantiños, 2010).

Taken together, these results suggest a model for cell-induced regeneration that includes two phases. The first, which occurs near the wound edges, involves JNK activity and is important for healing and rapid local proliferation. The second involves proliferation to compensate for the lost tissue and is extended throughout the damaged compartment. As in normal development, the regenerative growth that occurs in this second phase requires the reconstitution of morphogenetic signals that drive proliferation (Bergantiños, 2010).

The PI3K class III complex promotes axon pruning by downregulating a Ptc-derived signal via endosome-lysosomal degradation

Developmental axon pruning is essential for wiring the mature nervous system, but its regulation remains poorly understood. This study shows that the endosomal-lysosomal pathway regulates developmental pruning of Drosophila mushroom body γ neurons. The UV radiation resistance-associated gene (Uvrag) functions together with all core components of the phosphatidylinositol 3-kinase class III (PI3K-cIII) complex to promote pruning via the endocytic pathway. By studying several PI(3)P binding proteins, this study found that Hrs, a subunit of the ESCRT-0 complex, required for multivesicular body (MVB) maturation, is essential for normal pruning progression. Thus, the existence of an inhibitory signal that needs to be downregulated is hypothesized. Finally, the data suggest that the Hedgehog receptor, Patched, is the source of this inhibitory signal likely functioning in a Smo-independent manner. Taken together, this in vivo study demonstrates that the PI3K-cIII complex is essential for downregulating Patched via the endosomal-lysosomal pathway to execute axon pruning (Issman-Zecharya, 2014).

Pupal

The cuticle of the adult abdomen of Drosophila is produced by nests of imaginal histoblasts, which proliferate and migrate during metamorphosis to replace the polyploid larval epidermal cells. In this report, a detailed description is presented of the expression of four key patterning genes, engrailed (en), hedgehog (hh), patched (ptc), and optomotor-blind (omb), in abdominal histoblasts during the first 42 h after pupariation, a period in which the adult pattern is established. In addition, the expression is described of the homeotic genes Ultrabithorax, abdominal-A, and Abdominal-B, which specify the fates of adult abdominal segments. The results indicate that abdominal segments develop in isolation from one another during early pupal stages, and that some patterning events are independent of hh, wg, and dpp signaling. Pattern and polarity in a large anterior portion of the segment are specified without input from Hh, and evidence is presented that abdominal tergites possess an underlying symmetric pattern upon which patterning by Hh is superimposed. The signals responsible for this underlying symmetry remain to be identified (Kopp, 2002).

The dorsal cuticle of a typical abdominal segment contains a stereotyped sequence of pattern elements. At the anterior edge of each segment is the acrotergite, a narrow strip of naked sclerotized cuticle (a1). The remainder of the tergite is covered by trichomes, and can be subdivided into four regions. From anterior to posterior these regions are: a lightly pigmented region with no bristles (a2 fate); a lightly pigmented region that contains two to three rows of microchaetes (a3); a darkly pigmented region with one to two rows of microchaetes (a4); and a darkly pigmented region with a single row of macrochaetes (a5). The tergite is followed by the unpigmented posterior hairy zone (PHZ), which is composed of both anterior (a6) and posterior (p3) compartment cells. All trichomes and bristles in the segment are oriented uniformly from anterior to posterior. Finally, at the posterior edge of the segment is a zone of thin, naked intersegmental membrane (ISM), which can be subdivided into anterior smooth (p2) and posterior crinkled (p1) regions (Kopp, 2002).

The adult abdominal pattern is established in the first 2 days of pupal development, concurrent with the proliferation and migration of histoblasts and the destruction of the larval epidermal cells (LECs.) The spatial and temporal evolution of en, hh, ptc, and omb expression is followed during this critical period. The cuticle of each abdominal hemisegment is formed by three major histoblast nests. The anterior dorsal nest (aDHN) is composed of anterior compartment histoblasts and produces the tergite and part of the PHZ (a1-a6), whereas the posterior dorsal nest (pDHN) is composed of posterior compartment cells and produces the intersegmental membrane and the remainder of the PHZ (p1-p3). The ventral histoblast nest, which produces the sternite and pleura, contains both anterior and posterior compartment cells. en, hh, ptc, and omb are expressed in similar patterns in dorsal and ventral histoblasts, and the description is limited to the dorsal abdomen (Kopp, 2002).

en-lacZ and hh-lacZ are expressed throughout the pDHN, but are not expressed in the aDHN. hh-lacZ is expressed in a gradient within the pDHN, with expression highest at the anterior edge. A similar gradient can be detected in understained preparations of en-lacZ. ptc-lacZ expression is present in only a few cells at the posterior edge of the aDHN. omb-GAL4 expression is seen in the posterior of the aDHN and the anterior of the pDHN. omb-GAL4 expression is highest near the compartment boundary and decreases symmetrically in both anterior and posterior directions. By 20-24 h APF, the aDHN and pDHN fuse to form a combined dorsal histoblast nest (DHN). The gradients of en-lacZ and hh-lacZ expression within the posterior compartment become more pronounced at this stage. ptc-lacZ is expressed in a narrow stripe in the middle of the DHN, which is presumably located just anterior to the compartment boundary. The posterior border of this stripe is sharply defined, whereas a short gradient forms in the anterior direction; no ptc-lacZ expression can be detected at the anterior edge of the DHN at this time. omb-GAL4 is expressed in a wide, double-sided gradient in the middle of the DHN. Double labeling for ß-galactosidase and En protein in omb-GAL4²/UAS-lacZ pupae shows that omb-GAL4 is expressed in both compartments (Kopp, 2002).

At ~30 h APF, the DHN of consecutive segments begin to merge. Contact occurs as the border cells, a specialized row of LECs located at the posterior edge of each segment, are lost. At this stage, expression of en-lacZ and hh-lacZ is still highest at the compartment boundary, and lowest at the posterior edge of the segment. At high magnification, a clear gradient of En protein can be seen at this stage on a cell-by-cell basis. The ptc-lacZ stripe in the middle of the segment widens somewhat, but retains a sharp posterior limit. As the border cells are eliminated and histoblasts of consecutive segments come into contact, cells at the anterior edge of each segment activate ptc-lacZ. Activation occurs only where border cells have been lost; no expression of ptc-lacZ is detected posterior to persisting border cells. This pattern strongly suggests that the border cells insulate anterior histoblasts from the Hh protein secreted by the posterior compartment cells of the preceding segment. Consistent with such a role, the border cells do not express hh transcript, although they do express En. omb-GAL4 continues to be expressed in a symmetric, double-sided gradient at this stage (Kopp, 2002).

By 40-42 h APF, the border cells, which are the last LECs to be replaced by histoblasts, have been eliminated and segmental fusion has been completed. en-lacZ and hh-lacZ are upregulated at the posterior edge of the segment at this time, and soon the expression of both genes becomes uniform within the posterior compartment. For a short time, En levels are highest in cells at both edges of the posterior compartment, and lower in the middle cells, suggesting that en expression is upregulated by contact of anterior and posterior compartment histoblasts. In addition to the main ptc-lacZ stripe, a weak second stripe develops at the anterior edge of the segment. omb-GAL4 expression becomes asymmetric, with a well-defined posterior and graded anterior boundaries; based on the positions of muscle insertion points, most or all of omb-GAL4 expression at this stage is in the anterior compartment (Kopp, 2002).

To test whether Hh signaling is required for ptc and omb expression, homozygous hh^ts2 individuals were grown at 29°C for 48 h prior to dissection. Under these conditions, ptc-lacZ expression was completely eliminated at all stages. However, the effect on omb-GAL4 expression was different, depending on the stage of development. In early pupae, the symmetric expression of omb-GAL4 about the compartment boundary was only slightly reduced, while expression in the LECs appeared normal. In contrast, the later asymmetric expression of omb-GAL4 in the anterior compartment was virtually eliminated. No change was seen in the expression of en-lacZ or En protein in hh^ts2 pupae raised at 29°C, suggesting that the gradients of en expression in the posterior compartment are established independently of Hh function (Kopp, 2002).

Ectopic expression of en causes transformation of anterior compartment structures to posterior compartment identity, and produces a mirror-symmetric double-posterior pattern (p1-p2-p3-p3-p2-p1). This phenotype is seen in the en gain-of-function en mutant, which causes near-ubiquitous expression of en in the pupal abdomen and in T155-GAL4/UAS-en heterozygotes. Examination of En-expressing clones in otherwise wild-type flies reveals that the line of symmetry lies within the anterior compartment. En-expressing cells located posterior to this line orient to the posterior, whereas En-expressing cells located anterior to it orient to the anterior. This effect of En on cell fate and polarity is strictly cell autonomous. Whether Hh signaling plays a role in the symmetric polarization of en-expressing cells has been tested. No activation of en-lacZ is seen in the anterior compartment of gain of function en heterozygotes, although sporadic activation of hh-lacZ and hh transcript is observed. However, it is difficult to see how such variable activation of hh could be responsible for the highly regular mirror-symmetric cuticular pattern produced. ptc-lacZ expression is reduced at both edges of the anterior compartment in gain of function en, consistent with repression of ptc by En. omb-GAL4 expression appears unchanged relative to wild type (Koop, 2002).

Patterning axon targeting of olfactory receptor neurons by coupled hedgehog signaling at two distinct steps

Evidence is presented for a coupled two-step action of Hedgehog signaling in patterning axon targeting of Drosophila olfactory receptor neurons (ORNs). In the first step, differential Hedgehog pathway activity in peripheral sensory organ precursors creates ORN populations with different levels of the Patched receptor. Different Patched levels in ORNs then determine axonal responsiveness to target-derived Hedgehog in the brain: only ORN axons that do not express high levels of Patched are responsive to and require a second step of Hedgehog signaling for target selection. Hedgehog signaling in the imaginal sensory organ precursors thus confers differential ORN responsiveness to Hedgehog-mediated axon targeting in the brain. This mechanism contributes to the spatial coordination of ORN cell bodies in the periphery and their glomerular targets in the brain. Such coupled two-step signaling may be more generally used to coordinate other spatially and temporally segregated developmental events (Chou, 2010).

The central finding of this study is the coupled two-step action of Hedgehog in patterning ORN axon targeting. In the first step, differential Hh pathway activity in peripheral sensory organ precursors in larva and early pupa creates ORN populations with different levels of the Patched receptor. These Patched levels in ORNs then determine axonal responsiveness to target-derived Hh in the brain in the second step: only ORN axons that do not express high levels of Ptc are responsive to and require a second-step of Hh signaling for proper target selection. Multiple lines of evidence support this model. First, genetic loss-of-function studies indicate that ORNs fall into two groups based on their autonomous requirement for Smo, a classic Hh pathway component, as well as Ihog, a recently discovered positive receptor component for Hh. Second, Smo/Ihog-dependence for axon targeting coincides with Ptc levels for all 21 classes examined (11 high-Ptc and 10 low-Ptc). Third, knockdown of Hh from brain neurons only affects the targeting of low-Ptc ORN classes, with similar mistargeting preferences as compared to loss of Smo or Ihog in ORNs. Fourth, overexpression of Ptc in ORNs preferentially affects targeting of low-Ptc classes, whereas loss of Ptc in ORNs only affects targeting of high-Ptc classes. Fifth and perhaps most telling, loss of Hh in the antenna and maxillary palp preferentially affects targeting of high-Ptc classes; these mistargeting defects can be suppressed by Ptc overexpression. This result supports two important predictions of the model: Hh from the periphery is not directly required for axon targeting, at least for low-Ptc classes, but is required for the initiation and maintenance of high levels of Ptc in high-Ptc classes. Removing Hh from the periphery results in loss of Ptc expression in high-Ptc ORNs, which lifts Smo inhibition and causes axon mistargeting similar to loss of Ptc. Brain-derived Hh, by contrast, is required for low-Ptc classes but should not be read by at least 6 high-Ptc classes (Chou, 2010).

Vertebrate Sonic hedgehog has been proposed to act locally as an axon guidance cue whose action is dependent on the classic Hh pathway component Smo and the Robo related protein Boc, an Ihog homolog. The finding that Drosophila Hh also plays a role in ORN axon targeting that is dependent on Smo and Ihog suggests an evolutionarily conserved function of Hh in regulating axon development. A recent in vitro study supports the idea that Shh acts directly as an axon guidance cue in a rapid, transcription-independent manner (Yam, 2009). In the fly olfactory system, low-Ptc ORN classes originate from the En- and Hh-producing compartment, which are exposed to their own Hh yet do not show a transcriptional response. This is likely because ci expression is repressed by En. Brain-derived Hh may thus also act locally in axon targeting, as reported in vitro for Shh (Chou, 2010).

The data do not distinguish whether Hh acts instructively as an axon guidance cue, or permissively to modulate activities of other axon guidance receptors. The primary argument against an instructive model is the lack of spatial patterns of Hh proteins in the antennal lobe to account for the spatial distribution of glomerular targets of low- and high-Ptc ORN classes. This does not rule out the instructive model, however, as Hh activity can be modulated post-translationally such that the spatial distribution of Hh activity may differ from Hh protein levels. Alternatively, a permissive model for Hh action on ORN axons is also possible. For instance, Hh may regulate the cAMP/PKA pathway, which can in turn modulate axon guidance signaling. Indeed, it has recently been shown that Shh can modulate axon responsiveness to Semaphorins at the midline of the vertebrate spinal cord (Chou, 2010).

Whatever the downstream effector, the coupled two-step mechanism uncovered in this study can be used to coordinate cell body positions of ORNs in the sensory organs and their glomerular targets in the brain. The data indicate that it is essential both for ORN classes that depend on brain-derived Hh for axon targeting to express low levels of Ptc, and at least a subset of ORN axons that do not respond to brain-derived Hh for axon targeting to express high levels of Ptc, in order to ensure their targeting fidelity. Ptc expression levels thus create a code to diversify ORN classes according to their cell body positions in the sensory organ. Indeed, mistargeting of low-Ptc ORNs in the absence of Smo shows a significant preference for glomeruli that are normally high-Ptc ORN targets. This switch of axon target is by no means complete, suggesting that Hh signaling works together with other mechanisms to ensure axon targeting fidelity. It has previously been shown that transcription factors Atonal and Amos divide the ORN classes largely according to sensillar groups, which might regulate coarse correspondence of ORN cell body positions in periphery and their target glomeruli in the antennal lobe. The Notch system also diversifies ORN classes within each sensillum. This analysis indicates that Hh/Ptc demarcation of ORN cell bodies and their glomerular targets does not coincide precisely with the sensillar groups or with the Notch system, suggesting that the Hh system acts to diversify ORN classes independently, and likely at a step in between large sensillar group specification by Atonal/Amos and finer level diversification within each sensillum by the Notch system (Chou, 2010).

Hh was previously shown to coordinate the development of sensory neurons and their targets in the Drosophila visual system: Hh made in photoreceptors is transported down their axons to trigger neurogenesis of target laminar neurons. The olfactory system is constructed differently: target PNs are born and create a spatial pattern with their dendrites before ORN axon arrival. Consistent with this idea, Hh signaling is not required for PN development. Despite this fundamental difference from the visual system, Hh signaling is also used, but in a novel manner, to coordinate the ORN cell body position in the sensory organ with the glomerular map in the brain. Hh signaling in the periphery creates populations of ORNs with different Ptc levels such that cells that respond to the Hh signal in the first round are incapable of responding in the second round. Such a coupled two-step mechanism may be generally used for a single signaling pathway to coordinate spatially and/or temporally separate developmental events. Signal-induced expression of a positive or negative pathway component during an early phase of signaling could serve as a time-delayed cellular memory to specify responses at a later stage by rendering cells sensitive or insensitive to a second round of signaling (Chou, 2010).

Home page: The Interactive Fly © 1997 Thomas B. Brody, Ph.D.

The Interactive Fly resides on the
Society for Developmental Biology's Web server.