What's new in edition 65 August 2012 Gene sites new with this edition

The Interactive Fly was first released July/August 1996, with updates provided at approximately one month intervals, through September 1997 (edition 13). Updating quarterly started with edition 14. With edition 40, the Interactive Fly began to schedule updates three times a year: fall, winter and spring.

Gene sites new with this edition of the Interactive Fly:

Adenosine deaminase-related growth factor A

Maintenance of a hematopoietic progenitor population requires extensive interaction with cells within a microenvironment or niche (see Hematopoetic progenitor maintenance in the Drosophila blood system). In the Drosophila hematopoietic organ, niche-derived Hedgehog signaling maintains the progenitor population. This study shows that the hematopoietic progenitors also require a signal mediated by Adenosine deaminase growth factor A (Adgf-A) arising from differentiating cells that regulates extracellular levels of adenosine. The adenosine signal opposes the effects of Hedgehog signaling within the hematopoietic progenitor cells and the magnitude of the adenosine signal is kept in check by the level of Adgf-A secreted from differentiating cells. These findings reveal signals arising from differentiating cells that are required for maintaining progenitor cell quiescence and that function with the niche-derived signal in maintaining the progenitor state. Similar homeostatic mechanisms are likely to be utilized in other systems that maintain relatively large numbers of progenitors that are not all in direct contact with the cells of the niche (Mondal, 2011).

Crimpy

The BMP pathway is essential for scaling of the presynaptic motoneuron arbor to the postsynaptic muscle cell at the Drosophila neuromuscular junction (NMJ). Genetic analyses indicate that the muscle is the BMP-sending cell and the motoneuron is the BMP-receiving cell. Nevertheless, it is unclear how this directionality is established, since Glass bottom boat (Gbb), the known BMP ligand, is active in motoneurons. This study demonstrates that crimpy (cmpy) limits neuronal Gbb activity to permit appropriate regulation of NMJ growth. cmpy was identified in a screen for motoneuron-expressed genes and encodes a single-pass transmembrane protein with sequence homology to vertebrate Cysteine-rich transmembrane BMP regulator 1 (Crim1). A targeted deletion of the cmpy locus was generated, and loss-of-function mutants were found to exhibit excessive NMJ growth. In accordance with its expression profile, tissue-specific rescue experiments indicate that cmpy functions neuronally. The overgrowth in cmpy mutants depends on the activity of the BMP type II receptor Wishful thinking, arguing that Cmpy acts in the BMP pathway upstream of receptor activation and raising the possibility that it inhibits Gbb activity in motoneurons. Indeed, the cmpy mutant phenotype is strongly suppressed by RNAi-mediated knockdown of Gbb in motoneurons. Furthermore, Cmpy physically interacts with the Gbb precursor protein, arguing that Cmpy binds Gbb prior to the secretion of mature ligand. These studies demonstrate that Cmpy restrains Gbb activity in motoneurons. A model is presented whereby this inhibition permits the muscle-derived Gbb pool to predominate at the NMJ, thus establishing the retrograde directionality of the pro-growth BMP pathway (James, 2011).

Crinkled

The specification of the body plan in vertebrates and invertebrates is controlled by a variety of cell signaling pathways, but how signaling output is translated into morphogenesis is an ongoing question. This study describes genetic interactions between the Wingless (Wg) signaling pathway and a nonmuscle myosin heavy chain, encoded by the crinkled (ck) locus in Drosophila. In a screen for mutations that modify wg loss-of-function phenotypes, multiple independent alleles of ck were isolated. These ck mutations dramatically alter the morphology of the hook-shaped denticles that decorate the ventral surface of the wg mutant larval cuticle. In an otherwise wild-type background, ck mutations do not significantly alter denticle morphology, suggesting a specific interaction with Wg-mediated aspects of epidermal patterning. This study shows that changing the level of Wg activity changes the structure of actin bundles during denticle formation in ck mutants. It was further found that regulation of the Wg target gene, shaven-baby (svb), and of its transcriptional targets, miniature (m) and forked (f), modulates this ck-dependent process. It is concluded that Ck acts in concert with Wg targets to orchestrate the proper shaping of denticles in the Drosophila embryonic epidermis (Bejsovec, 2012).

Cutoff

In a broad range of organisms, Piwi-interacting RNAs (piRNAs) have emerged as core components of a surveillance system that protects the genome by silencing transposable and repetitive elements. A vast proportion of piRNAs is produced from discrete genomic loci, termed piRNA clusters, which are generally embedded in heterochromatic regions. The molecular mechanisms and the factors that govern their expression are largely unknown. This study shows that Cutoff (Cuff), a Drosophila protein related to the yeast transcription termination factor Rai1, is essential for piRNA production in germline tissues. Cuff accumulates at centromeric/pericentromeric positions in germ-cell nuclei and strongly colocalizes with the major heterochromatic domains. Remarkably, Cuff is enriched at the dual-strand piRNA cluster 1/42AB and is likely to be involved in regulation of transcript levels of similar loci dispersed in the genome. Consistent with this observation, Cuff physically interacts with the Heterochromatin Protein 1 (HP1) variant Rhino (Rhi). These results reveal a link between Cuff activity, heterochromatin assembly and piRNA cluster expression, which is critical for stem-cell and germ-cell development in Drosophila (Pane, 2011).

Cyclin G

Morphological consistency in metazoans is remarkable given the pervasive occurrence of genetic variation, environmental effects, and developmental noise. Developmental stability, the ability to reduce developmental noise, is a fundamental property of multicellular organisms, yet its genetic bases remains elusive. Imperfect bilateral symmetry, or fluctuating asymmetry, is commonly used to estimate developmental stability. It was observed that Drosophila overexpressing Cyclin G (CycG) exhibit wing asymmetry clearly detectable by sight. Quantification of wing size and shape using geometric morphometrics reveals that this asymmetry is a genuine-but extreme-fluctuating asymmetry. Overexpression of CycG indeed leads to a 40-fold increase of wing fluctuating asymmetry, which is an unprecedented effect, for any organ and in any animal model, either in wild populations or mutants. This asymmetry effect is not restricted to wings, since femur length is affected as well. Inactivating CycG by RNAi also induces fluctuating asymmetry but to a lesser extent. Investigating the cellular bases of the phenotypic effects of CycG deregulation, it was found that misregulation of cell size is predominant in asymmetric flies. In particular, the tight negative correlation between cell size and cell number observed in wild-type flies is impaired when CycG is upregulated. These results highlight the role of CycG in the control of developmental stability in D. melanogaster. Furthermore, they show that wing developmental stability is normally ensured via compensatory processes between cell growth and cell proliferation. The possible role of CycG as a hub in a genetic network that controls developmental stability is discussed (Debat, 2011).

Golden goal

During Drosophila visual system development, photoreceptor (R) axons choose their correct paths and targets in a step-wise fashion. R axons with different identities make specific pathfinding decisions at different stages during development. This study shows that the transmembrane protein Golden goal (Gogo), which is dynamically expressed in all R neurons and localizes predominantly to growth cones, is required in two distinct steps of R8 photoreceptor axon pathfinding: Gogo regulates axon-axon interactions and axon-target interactions in R8 photoreceptor axons. gogo loss-of-function and gain-of-function phenotypes suggest that Gogo mediates repulsive axon-axon interaction between R8 axons to maintain their proper spacing, and it promotes axon-target recognition at the temporary layer to enable R8 axons to enter their correct target columns in the medulla. From detailed structure-function experiments, it is proposed that Gogo functions as a receptor that binds an unidentified ligand through its conserved extracellular domain (Tomasi, 2008).

G protein β-subunit 13F

The function of an organ relies on its form, which in turn depends on the individual shapes of the cells that create it and the interactions between them. Despite remarkable progress in the field of developmental biology, how cells collaborate to make a tissue remains an unsolved mystery. To investigate the mechanisms that determine organ structure, this work studied the cells that form the dorsal appendages (DAs) of the Drosophila eggshell. These cells consist of two differentially patterned subtypes: roof cells, which form the outward-facing roof of the lumen, and floor cells, which dive underneath the roof cells to seal off the floor of the tube. This paper presents three lines of evidence that reveal a further stratification of the DA-forming epithelium. Laser ablation of only a few cells in the anterior of the region causes a disproportionately severe shortening of the appendage. Genetic alteration through the twin peaks allele of tramtrack⁶⁹ (ttk^twk), a female-sterile mutation that leads to severely shortened DAs, causes no such shortening when removed from a majority of the DA-forming cells, but rather, produces short appendages only when removed from cells in the very anterior of the tube-forming tissue. Additionally it was shown that heterotrimeric G-protein function is required for DA morphogenesis. Like TTK⁶⁹, Gβ13F is not required in all DA-forming follicle cells but only in the floor and leading roof cells. The different phenotypes that result from removal of Gβ13F from each region demonstrate a striking division of function between different DA-forming cells. Gβ mutant floor cells are unable to control the width of the appendage while Gβ mutant leading roof cells fail to direct the elongation of the appendage and the convergent-extension of the roof-cell population (Boyle, 2010).

Histone H2A

Polycomb group (PcG) proteins are transcriptional repressors that control processes ranging from the maintenance of cell fate decisions and stem cell pluripotency in animals to the control of flowering time in plants. In Drosophila, genetic studies identified more than 15 different PcG proteins that are required to repress homeotic (HOX) and other developmental regulator genes in cells where they must stay inactive. Biochemical analyses established that these PcG proteins exist in distinct multiprotein complexes that bind to and modify chromatin of target genes. Among those, Polycomb repressive complex 1 (PRC1) and the related dRing-associated factors (dRAF) complex contain an E3 ligase activity for monoubiquitination of histone H2A. This study shows that the uncharacterized Drosophila PcG gene calypso encodes the ubiquitin carboxy-terminal hydrolase BAP1. Biochemically purified Calypso exists in a complex with the PcG protein ASX, and this complex, named Polycomb repressive deubiquitinase (PR-DUB), is bound at PcG target genes in Drosophila. Reconstituted recombinant Drosophila and human PR-DUB complexes remove monoubiquitin from H2A but not from H2B in nucleosomes. Drosophila mutants lacking PR-DUB show a strong increase in the levels of monoubiquitinated H2A. A mutation that disrupts the catalytic activity of Calypso, or absence of the ASX subunit abolishes H2A deubiquitination in vitro and HOX gene repression in vivo. Polycomb gene silencing may thus entail a dynamic balance between H2A ubiquitination by PRC1 and dRAF, and H2A deubiquitination by PR-DUB (Scheuermann, 2010; full text of article).

Histone H2B

Histone post-translational modifications play an important role in regulating chromatin structure and gene expression in vivo. Extensive studies investigated the post-translational modifications of the core histones H3 and H4 or the linker histone H1. Much less is known on the regulation of H2A and H2B modifications. This study shows that a major modification of H2B in Drosophila is the methylation of the N-terminal proline, which increases during fly development. Experiments performed in cultured cells revealed higher levels of H2B methylation when cells are dense, regardless of their cell cycle distribution. dNTMT was identified as the enzyme responsible for H2B methylation. It was also found that the level of N-terminal methylation is regulated by dART8, an arginine methyltransferase that physically interacts with dNTMT and asymmetrically methylates Histone H3 arginine 2 (H3R2). These results demonstrate the existence of a complex containing two methyltransferases enzymes that negatively influence each other's activity (Villar-Garea, 2012).

Histone gene-specific Epigenetic Repressor in late S phase

Cell cycle-dependent expression of canonical histone proteins enables newly synthesized DNA to be integrated into chromatin in replicating cells. However, the molecular basis of cell cycle-dependency in the switching of histone gene regulation remains to be uncovered. This study reports the identification and biochemical characterization of a molecular switcher, HERS (histone gene-specific epigenetic repressor in late S phase), for nucleosomal core histone gene inactivation in Drosophila. HERS protein is phosphorylated by a cyclin-dependent kinase (Cdk) at the end of S-phase. Phosphorylated HERS binds to histone gene regulatory regions and anchors HP1 and Su(var)3-9 to induce chromatin inactivation through histone H3 lysine 9 methylation. These findings illustrate a salient molecular switch linking epigenetic gene silencing to cell cycle-dependent histone production (Ito, 2012).

HLH54F

HLH54F, the Drosophila ortholog of the vertebrate basic helix-loop-helix domain-encoding genes capsulin and musculin, is expressed in the founder cells and developing muscle fibers of the longitudinal midgut muscles. These cells descend from the posterior-most portion of the mesoderm, termed the caudal visceral mesoderm (CVM), and migrate onto the trunk visceral mesoderm prior to undergoing myoblast fusion and muscle fiber formation. HLH54F expression in the CVM is regulated by a combination of terminal patterning genes and snail. HLH54F mutations were generated and this gene was shown to be crucial for the specification, migration and survival of the CVM cells and the longitudinal midgut muscle founders. HLH54F mutant embryos, larvae, and adults lack all longitudinal midgut muscles, which causes defects in gut morphology and integrity. The function of HLH54F as a direct activator of gene expression is exemplified by analysis of a CVM-specific enhancer from the Dorsocross locus, which requires combined inputs from HLH54F and Biniou in a feed-forward fashion. It is concluded that HLH54F is the earliest specific regulator of CVM development and that it plays a pivotal role in all major aspects of development and differentiation of this largely twist-independent population of mesodermal cells (Ismat, 2010).

Modigliani

Several proteins have been identified that protect Drosophila telomeres from fusion events. They include UbcD1, HP1, HOAP, the components of the Mre11-Rad50-Nbs (MRN) complex, the ATM kinase, and the putative transcription factor Woc. Of these proteins, only HOAP has been shown to localize specifically at telomeres. This study shows that the modigliani gene encodes a protein (Moi) that is enriched only at telomeres, colocalizes and physically interacts with HOAP, and is required to prevent telomeric fusions. Moi is encoded by the bicistronic CG31241 locus. This locus produces a single transcript that contains 2 ORFs that specify different essential functions. One of these ORFs encodes the 20-kDa Moi protein. The other encodes a 60-kDa protein homologous to RNA methyltransferases that is not required for telomere protection (Drosophila Tat-like). Moi and HOAP share several properties with the components of shelterin, the protein complex that protects human telomeres. HOAP and Moi are not evolutionarily conserved unlike the other proteins implicated in Drosophila telomere protection. Similarly, none of the shelterin subunits is conserved in Drosophila, while most human nonshelterin proteins have Drosophila homologues. This suggests that the HOAP-Moi complex, name in this study 'terminin,' plays a specific role in the DNA sequence-independent assembly of Drosophila telomeres. It is speculated that this complex is functionally analogous to shelterin, which binds chromosome ends in a sequence-dependent manner (Raffa, 2009).

Neuropilin and tolloid-like (Neto)

Neurotransmitter receptor recruitment at postsynaptic specializations is key in synaptogenesis, since this step confers functionality to the nascent synapse. The Drosophila neuromuscular junction (NMJ) is a glutamatergic synapse, similar in composition and function to mammalian central synapses. Various mechanisms regulating the extent of postsynaptic ionotropic glutamate receptor (iGluR) clustering have been described, but none are known to be essential for the initial localization and clustering of iGluRs at postsynaptic densities (PSDs). This study identified and characterized the Drosophila neto (neuropilin and tolloid-like) as an essential gene required for clustering of iGluRs (GluRIIA, GluRIIB, and GluRIIC) at the NMJ. Neto colocalizes with the iGluRs at the PSDs in puncta juxtaposing the active zones. neto loss-of-function phenotypes parallel the loss-of-function defects described for iGluRs. The defects in neto mutants are effectively rescued by muscle-specific expression of neto transgenes. Neto clustering at the Drosophila NMJ coincides with and is dependent on iGluRs. These studies reveal that Drosophila Neto is a novel, essential component of the iGluR complexes and is required for iGluR clustering, organization of PSDs, and synapse functionality (Kim, 2012).

No-on-and-no-off transient C

A systematic Drosophila forward genetic screen for photoreceptor synaptic transmission mutants identified no-on-and-no-off transient C (nonC) based on loss of retinal synaptic responses to light stimulation. The cloned gene encodes phosphatidylinositol-3-kinase-like kinase (PIKK) Smg1, a regulatory kinase of the nonsense-mediated decay (NMD) pathway. The Smg proteins act in an mRNA quality control surveillance mechanism to selectively degrade transcripts containing premature stop codons, thereby preventing the translation of truncated proteins with dominant-negative or deleterious gain-of-function activities. At the neuromuscular junction (NMJ) synapse, an extended allelic series of Smg1 mutants show impaired structural architecture, with decreased terminal arbor size, branching and synaptic bouton number. Functionally, loss of Smg1 results in a ~50% reduction in basal neurotransmission strength, as well as progressive transmission fatigue and greatly impaired synaptic vesicle recycling during high-frequency stimulation. Mutation of other NMD pathways genes (Upf2 and Smg6) similarly impairs neurotransmission and synaptic vesicle cycling. These findings suggest that the NMD pathway acts to regulate proper mRNA translation to safeguard synapse morphology and maintain the efficacy of synaptic function (Long, 2010).

Tachykinin-like receptor at 99D

The insulin-signaling pathway is evolutionarily conserved in animals and regulates growth, reproduction, metabolic homeostasis, stress resistance and life span. In Drosophila seven insulin-like peptides (DILP1-7) are known, some of which are produced in the brain, others in fat body or intestine. This study shows that DILP5 is expressed in principal cells of the renal tubules of Drosophila and affects survival at stress. Renal (Malpighian) tubules regulate water and ion homeostasis, but also play roles in immune responses and oxidative stress. This study investigated the control of DILP5 signaling in the renal tubules by Drosophila tachykinin peptide (DTK) and its receptor DTKR during desiccative, nutritional and oxidative stress. The DILP5 levels in principal cells of the tubules are affected by stress and manipulations of DTKR expression in the same cells. Targeted knockdown of DTKR, DILP5 and the insulin receptor dInR in principal cells or mutation of Dilp5 resulted in increased survival at either stress, whereas over-expression of these components produced the opposite phenotype. Thus, stress seems to induce hormonal release of DTK that acts on the renal tubules to regulate DILP5 signaling. Manipulations of S6 kinase and superoxide dismutase (SOD2) in principal cells also affect survival at stress, suggesting that DILP5 acts locally on tubules, possibly in oxidative stress regulation. These findings are the first to demonstrate DILP signaling originating in the renal tubules and that this signaling is under control of stress-induced release of peptide hormone (Söderberg, 2011).

Zinc-finger protein

How a cell decides to self-renew or differentiate is a critical issue in stem cell and cancer biology. Atypical protein kinase C (aPKC) promotes self-renewal of Drosophila larval brain neural stem cells, neuroblasts. However, it is unclear how aPKC cortical polarity and protein levels are regulated. This study identified a zinc-finger protein, Zif, which is required for the expression and asymmetric localization of aPKC. aPKC displays ectopic cortical localization with upregulated protein levels in dividing zif mutant neuroblasts, leading to neuroblast overproliferation. Zif was shown to be a transcription factor that directly represses aPKC transcription. It was further shown that Zif is phosphorylated by aPKC both in vitro and in vivo. Phosphorylation of Zif by aPKC excludes it from the nucleus, leading to Zif inactivation in neuroblasts. Thus, reciprocal repression between Zif and aPKC act as a critical regulatory mechanism for establishing cell polarity and controlling neuroblast self-renewal (Chang, 2010).

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