The Drosophila segment polarity gene patched is involved in a position-signalling mechanism in imaginal discs

Global methylation in 'Valencia' orange seedlings associated with rootstocks and Huanglongbing

Citrus farming is one of the main activities that contributed to the Brazilian trade balance, with citrus seedling being the most important input in the formation of orchards to guarantee high productivity and fruit quality, which fundamentally depends on the chosen genetics. The present study aimed to analyze the existence of epigenetic variability in 'Valencia' orange plants on rootstocks, associated or not with HLB, through the quantification of the global methylation of its genome, in order to support works on genetic improvement and crop production. For this purpose, this work was carried out in greenhouse in a completely randomized experimental design, with 5 treatments and 6 replicates per treatment, each seedling being considered a replicate, namely: T1 = "Valencia" orange grafted onto "Rangpur" lemon, inoculated with HLB; T2 = "Valencia" orange grafted onto "Swingle" citrumelo, inoculated with HLB; T3 = "Valencia" orange grafted onto "Rangpur" lemon, without HLB inoculation ; T4 = "Valencia" orange grafted onto "Swingle" citrumelo, without HLB inoculation ; T5 = "Valencia" orange in free standing. The DNA was extracted from leaves and the ELISA test (Enzyme-Linked Immunosorbent Assay) was carried out, based on the use of receptors sensitive to 5-mC., to measure the relative quantification of global methylation between genomic orange DNAs . Since the control treatment (T5) consists of "Valencia" orange in free standing, it could be inferred that both the normal grafting technique in the seedling formation process and the inoculation of buds infected with HLB are external factors capable of changing the methylation pattern in the evaluated plants, including the DNA demethylation process, causing an adaptive response in association with the expression of genes previously silenced by genome methylation.

Slik maintains tissue homeostasis by preventing JNK-mediated apoptosis

BACKGROUND

The c-Jun N-terminal kinase (JNK) pathway is an evolutionarily conserved regulator of cell death, which is essential for coordinating tissue homeostasis. In this study, we have characterized the Drosophila Ste20-like kinase Slik as a novel modulator of JNK pathway-mediated apoptotic cell death.

RESULTS

First, ectopic JNK signaling-triggered cell death is enhanced by slik depletion whereas suppressed by Slik overexpression. Second, loss of slik activates JNK signaling, which results in enhanced apoptosis and impaired tissue homeostasis. In addition, genetic epistasis analysis suggests that Slik acts upstream of or in parallel to Hep to regulate JNK-mediated apoptotic cell death. Moreover, Slik is necessary and sufficient for preventing physiologic JNK signaling-mediated cell death in development. Furthermore, introduction of STK10, the human ortholog of Slik, into Drosophila restores slik depletion-induced cell death and compromised tissue homeostasis. Lastly, knockdown of STK10 in human cancer cells also leads to JNK activation, which is cancelled by expression of Slik.

CONCLUSIONS

This study has uncovered an evolutionarily conserved role of Slik/STK10 in blocking JNK signaling, which is required for cell death inhibition and tissue homeostasis maintenance in development.

Generation of LexA enhancer-trap lines in Drosophila by an international scholastic network

Conditional gene regulation in Drosophila through binary expression systems like the LexA-LexAop system provides a superb tool for investigating gene and tissue function. To increase the availability of defined LexA enhancer trap insertions, we present molecular, genetic, and tissue expression studies of 301 novel Stan-X LexA enhancer traps derived from mobilization of the index SX4 line. This includes insertions into distinct loci on the X, II, and III chromosomes that were not previously associated with enhancer traps or targeted LexA constructs, an insertion into ptc, and seventeen insertions into natural transposons. A subset of enhancer traps was expressed in CNS neurons known to produce and secrete insulin, an essential regulator of growth, development, and metabolism. Fly lines described here were generated and characterized through studies by students and teachers in an international network of genetics classes at public, independent high schools, and universities serving a diversity of students, including those underrepresented in science. Thus, a unique partnership between secondary schools and university-based programs has produced and characterized novel resources in Drosophila, establishing instructional paradigms devoted to unscripted experimental science.

Hedgehog signaling

Hedgehog (Hh) proteins constitute one family of a small number of secreted signaling proteins that together regulate multiple aspects of animal development, tissue homeostasis and regeneration. Originally uncovered through genetic analyses in Drosophila, their subsequent discovery in vertebrates has provided a paradigm for the role of morphogens in positional specification. Most strikingly, the Sonic hedgehog protein was shown to mediate the activity of two classic embryonic organizing centers in vertebrates and subsequent studies have implicated it and its paralogs in a myriad of processes. Moreover, dysfunction of the signaling pathway has been shown to underlie numerous human congenital abnormalities and diseases, especially certain types of cancer. This review focusses on the genetic studies that uncovered the key components of the Hh signaling system and the subsequent, biochemical, cell and structural biology analyses of their functions. These studies have revealed several novel processes and principles, shedding new light on the cellular and molecular mechanisms underlying cell-cell communication. Notable amongst these are the involvement of cholesterol both in modifying the Hh proteins and in activating its transduction pathway, the role of cytonemes, filipodia-like extensions, in conveying Hh signals between cells; and the central importance of the Primary Cilium as a cellular compartment within which the components of the signaling pathway are sequestered and interact.

Regulation of growth and cell fate during tissue regeneration by the two SWI/SNF chromatin-remodeling complexes of Drosophila

To regenerate, damaged tissue must heal the wound, regrow to the proper size, replace the correct cell types, and return to the normal gene-expression program. However, the mechanisms that temporally and spatially control the activation or repression of important genes during regeneration are not fully understood. To determine the role that chromatin modifiers play in regulating gene expression after tissue damage, we induced ablation in Drosophila melanogaster imaginal wing discs, and screened for chromatin regulators that are required for epithelial tissue regeneration. Here, we show that many of these genes are indeed important for promoting or constraining regeneration. Specifically, the two SWI/SNF chromatin-remodeling complexes play distinct roles in regulating different aspects of regeneration. The PBAP complex regulates regenerative growth and developmental timing, and is required for the expression of JNK signaling targets and the growth promoter Myc. By contrast, the BAP complex ensures correct patterning and cell fate by stabilizing the expression of the posterior gene engrailed. Thus, both SWI/SNF complexes are essential for proper gene expression during tissue regeneration, but they play distinct roles in regulating growth and cell fate.

Competitive coordination of the dual roles of the Hedgehog co-receptor in homophilic adhesion and signal reception

Hedgehog (Hh) signaling patterns embryonic tissues and contributes to homeostasis in adults. In Drosophila, Hh transport and signaling are thought to occur along a specialized class of actin-rich filopodia, termed cytonemes. Here, we report that Interference hedgehog (Ihog) not only forms a Hh receptor complex with Patched to mediate intracellular signaling, but Ihog also engages in trans-homophilic binding leading to cytoneme stabilization in a manner independent of its role as the Hh receptor. Both functions of Ihog (trans-homophilic binding for cytoneme stabilization and Hh binding for ligand sensing) involve a heparin-binding site on the first fibronectin repeat of the extracellular domain. Thus, the Ihog-Ihog interaction and the Hh-Ihog interaction cannot occur simultaneously for a single Ihog molecule. By combining experimental data and mathematical modeling, we determined that Hh-Ihog heterophilic interaction dominates and Hh can disrupt and displace Ihog molecules involved in trans-homophilic binding. Consequently, we proposed that the weaker Ihog-Ihog trans interaction promotes and stabilizes direct membrane contacts along cytonemes and that, as the cytoneme encounters secreted Hh ligands, the ligands trigger release of Ihog from trans Ihog-Ihog complex enabling transport or internalization of the Hh ligand-Ihog-Patched -receptor complex. Thus, the seemingly incompatible functions of Ihog in homophilic adhesion and ligand binding cooperate to assist Hh transport and reception along the cytonemes.

Dynamic Notch signalling regulates neural stem cell state progression in the Drosophila optic lobe

Background

Neural stem cells generate all of the neurons and glial cells in the central nervous system, both during development and in the adult to maintain homeostasis. In the Drosophila optic lobe, neuroepithelial cells progress through two transient progenitor states, PI and PII, before transforming into neuroblasts. Here we analyse the role of Notch signalling in the transition from neuroepithelial cells to neuroblasts.

Results

We observed dynamic regulation of Notch signalling: strong activity in PI progenitors, low signalling in PII progenitors, and increased activity after neuroblast transformation. Ectopic expression of the Notch ligand Delta induced the formation of ectopic PI progenitors. Interestingly, we show that the E3 ubiquitin ligase, Neuralized, regulates Delta levels and Notch signalling activity at the transition zone. We demonstrate that the proneural transcription factor, Lethal of scute, is essential to induce expression of Neuralized and promote the transition from the PI progenitor to the PII progenitor state.

Conclusions

Our results show dynamic regulation of Notch signalling activity in the transition from neuroepithelial cells to neuroblasts. We propose a model in which Lethal of scute activates Notch signalling in a non-cell autonomous manner by regulating the expression of Neuralized, thereby promoting the progression between different neural stem cell states.

Electronic supplementary material

The online version of this article (10.1186/s13064-018-0123-8) contains supplementary material, which is available to authorized users.

Opposing Action of Hedgehog and Insulin Signaling Balances Proliferation and Autophagy to Determine Follicle Stem Cell Lifespan

Egg production declines with age in many species, a process linked with stem cell loss. Diet-dependent signaling has emerged as critical for stem cell maintenance during aging. Follicle stem cells (FSCs) in the Drosophila ovary are exquisitely responsive to diet-induced signals including Hedgehog (Hh) and insulin-IGF signaling (IIS), entering quiescence in the absence of nutrients and initiating proliferation rapidly upon feeding. Although highly proliferative FSCs generally exhibit an extended lifespan, we find that constitutive Hh signaling drives FSC loss and premature sterility despite high proliferative rates. This occurs due to Hh-mediated induction of autophagy in FSCs via a Ptc-dependent, Smo-independent mechanism. Hh-dependent autophagy increases during aging, triggering FSC loss and consequent reproductive arrest. IIS is necessary and sufficient to suppress Hh-induced autophagy, promoting a stable proliferative state. These results suggest that opposing action of diet-responsive IIS and Hh signals determine reproductive lifespan by modulating the proliferation-autophagy balance in FSCs during aging.

Persistence of RNAi-Mediated Knockdown in Drosophila Complicates Mosaic Analysis Yet Enables Highly Sensitive Lineage Tracing

RNA interference (RNAi) has emerged as a powerful way of reducing gene function in Drosophila melanogaster tissues. By expressing synthetic short hairpin RNAs (shRNAs) using the Gal4/UAS system, knockdown is efficiently achieved in specific tissues or in clones of marked cells. Here we show that knockdown by shRNAs is so potent and persistent that even transient exposure of cells to shRNAs can reduce gene function in their descendants. When using the FLP-out Gal4 method, in some instances we observed unmarked “shadow RNAi” clones adjacent to Gal4-expressing clones, which may have resulted from brief Gal4 expression following recombination but prior to cell division. Similarly, Gal4 driver lines with dynamic expression patterns can generate shadow RNAi cells after their activity has ceased in those cells. Importantly, these effects can lead to erroneous conclusions regarding the cell autonomy of knockdown phenotypes. We have investigated the basis of this phenomenon and suggested experimental designs for eliminating ambiguities in interpretation. We have also exploited the persistence of shRNA-mediated knockdown to design a sensitive lineage-tracing method, i-TRACE, which is capable of detecting even low levels of past reporter expression. Using i-TRACE, we demonstrate transient infidelities in the expression of some cell-identity markers near compartment boundaries in the wing imaginal disc.

Ihog and Boi elicit Hh signaling via Ptc but do not aid Ptc in sequestering the Hh ligand

Hedgehog (Hh) proteins are secreted molecules essential for tissue development in vertebrates and invertebrates. Hh reception via the 12-pass transmembrane protein Patched (Ptc) elicits intracellular signaling through Smoothened (Smo). Hh binding to Ptc is also proposed to sequester the ligand, limiting its spatial range of activity. In Drosophila, Interference hedgehog (Ihog) and Brother of ihog (Boi) are two conserved and redundant transmembrane proteins that are essential for Hh pathway activation. How Ihog and Boi activate signaling in response to Hh remains unknown; each can bind both Hh and Ptc and so it has been proposed that they are essential for both Hh reception and sequestration. Using genetic epistasis we established here that Ihog and Boi, and their orthologs in mice, act upstream or at the level of Ptc to allow Hh signal transduction. In the Drosophila developing wing model we found that it is through Hh pathway activation that Ihog and Boi maintain the boundary between the anterior and posterior compartments. We dissociated the contributions of Ptc from those of Ihog/Boi and, surprisingly, found that cells expressing Ptc can retain and sequester the Hh ligand without Ihog and Boi, but that Ihog and Boi cannot do so without Ptc. Together, these results reinforce the central role for Ptc in Hh binding in vivo and demonstrate that, although Ihog and Boi are dispensable for Hh sequestration, they are essential for pathway activation because they allow Hh to inhibit Ptc and thereby relieve its repression of Smo.

Wing tips: The wing disc as a platform for studying Hedgehog signaling

Hedgehog (Hh) signal transduction is necessary for the development of most mammalian tissues and can go awry and cause birth defects or cancer. Hh signaling was initially described in Drosophila, and much of what we know today about mammalian Hh signaling was directly guided by discoveries in the fly. Indeed, Hh signaling is a wonderful example of the use of non-vertebrate model organisms to make basic discoveries that lead to new disease treatment. The first pharmaceutical to treat hyperactive Hh signaling in Basal Cell Carcinoma was released in 2012, approximately 30 years after the isolation of Hh mutants in Drosophila. The study of Hh signaling has been greatly facilitated by the imaginal wing disc, a tissue with terrific experimental advantages. Studies using the wing disc have led to an understanding of Hh ligand processing, packaging into particles for transmission, secretion, reception, signal transduction, target gene activation, and tissue patterning. Here we describe the imaginal wing disc, how Hh patterns this tissue, and provide methods to use wing discs to study Hh signaling in Drosophila. The tools and approaches we highlight form the cornerstone of research efforts in many laboratories that use Drosophila to study Hh signaling, and are essential for ongoing discoveries.

A dynamic population of stromal cells contributes to the follicle stem cell niche in the Drosophila ovary

Epithelial stem cells are maintained within niches that promote self-renewal by providing signals that specify the stem cell fate. In the Drosophila ovary, epithelial follicle stem cells (FSCs) reside in niches at the anterior tip of the tissue and support continuous growth of the ovarian follicle epithelium. Here, we demonstrate that a neighboring dynamic population of stromal cells, called escort cells, are FSC niche cells. We show that escort cells produce both Wingless and Hedgehog ligands for the FSC lineage, and that Wingless signaling is specific for the FSC niche whereas Hedgehog signaling is active in both FSCs and daughter cells. In addition, we show that multiple escort cells simultaneously encapsulate germ cell cysts and contact FSCs. Thus, FSCs are maintained in a dynamic niche by a non-dedicated population of niche cells.

Microarray comparison of anterior and posterior Drosophila wing imaginal disc cells identifies novel wing genes

Signaling between cells in the anterior (A) and posterior (P) compartments directs Drosophila wing disc development and is dependent on expression of the homeodomain transcription factor Engrailed (En) in P cells. Downstream of en, posteriorly expressed Hedgehog (Hh) protein signals across the A/P border to establish a developmental organizer that directs pattern formation and growth throughout the wing primordium. Here we extend investigations of the processes downstream of en by using expression array analysis to compare A and P cells. A total of 102 candidate genes were identified that express differentially in the A and P compartments; four were characterized: Stubble (Sb) expression is restricted to A cells due to repression by en. CG15905, CG16884; CG10200/hase und igel (hui) are expressed in A cells downstream of Hh signaling; and RNA interference for hui, Stubble, and CG16884 revealed that each is essential to wing development.

Human mitochondrial disease-like symptoms caused by a reduced tRNA aminoacylation activity in flies

The translation of genes encoded in the mitochondrial genome requires specific machinery that functions in the organelle. Among the many mutations linked to human disease that affect mitochondrial translation, several are localized to nuclear genes coding for mitochondrial aminoacyl-transfer RNA synthetases. The molecular significance of these mutations is poorly understood, but it is expected to be similar to that of the mutations affecting mitochondrial transfer RNAs. To better understand the molecular features of diseases caused by these mutations, and to improve their diagnosis and therapeutics, we have constructed a Drosophila melanogaster model disrupting the mitochondrial seryl-tRNA synthetase by RNA interference. At the molecular level, the knockdown generates a reduction in transfer RNA serylation, which correlates with the severity of the phenotype observed. The silencing compromises viability, longevity, motility and tissue development. At the cellular level, the knockdown alters mitochondrial morphology, biogenesis and function, and induces lactic acidosis and reactive oxygen species accumulation. We report that administration of antioxidant compounds has a palliative effect of some of these phenotypes. In conclusion, the fly model generated in this work reproduces typical characteristics of pathologies caused by mutations in the mitochondrial aminoacylation system, and can be useful to assess therapeutic approaches.

Genetic interaction screens identify a role for hedgehog signaling in Drosophila border cell migration

BACKGROUND

Cell motility is essential for embryonic development and physiological processes such as the immune response, but also contributes to pathological conditions such as tumor progression and inflammation. However, our understanding of the mechanisms underlying migratory processes is incomplete. Drosophila border cells provide a powerful genetic model to identify the roles of genes that contribute to cell migration.

RESULTS

Members of the Hedgehog signaling pathway were uncovered in two independent screens for interactions with the small GTPase Rac and the polarity protein Par-1 in border cell migration. Consistent with a role in migration, multiple Hh signaling components were enriched in the migratory border cells. Interference with Hh signaling by several different methods resulted in incomplete cell migration. Moreover, the polarized distribution of E-Cadherin and a marker of tyrosine kinase activity were altered when Hh signaling was disrupted. Conservation of Hh-Rac and Hh-Par-1 signaling was illustrated in the wing, in which Hh-dependent phenotypes were enhanced by loss of Rac or par-1.

CONCLUSIONS

We identified a pathway by which Hh signaling connects to Rac and Par-1 in cell migration. These results further highlight the importance of modifier screens in the identification of new genes that function in developmental pathways.

Involvement of the conserved Hox gene Antennapedia in the development and evolution of a novel trait

BACKGROUND

Hox proteins specify segment identity during embryogenesis and have typical associated expression patterns. Changes in embryonic expression and activity of Hox genes were crucial in the evolution of animal body plans, but their role in the post-embryonic development of lineage-specific traits remains largely unexplored. Here, we focus on the insect Hox genes Ultrabithorax (Ubx) and Antennapedia (Antp), and implicate the latter in the formation and diversification of novel, butterfly-specific wing patterns.

RESULTS

First, we describe a conserved pattern of Ubx expression and a novel pattern of Antp expression in wing discs of Bicyclus anynana butterflies. The discrete, reiterated domains of Antp contrast with the typical expression of Hox genes in single continuous regions in arthropod embryos. Second, we show that this pattern is associated with the establishment of the organizing centres of eyespots. Antp upregulation is the earliest event in organizer development described to date, and in contrast to all genes implicated in eyespot formation, is exclusive to those centres. Third, our comparative analysis of gene expression across nymphalids reveals unexpected differences in organizer determination.

CONCLUSIONS

We show that the Antp's recruitment for the formation of novel traits in butterfly wing discs involved the evolution of new expression domains, and is restricted to a particular lineage. This study contributes novel insights into the evolution of Antp expression, as well as into the genetic mechanisms underlying morphological diversification. Our results also underscore how a wider representation of morphological and phylogenetic diversity is essential in evolutionary developmental biology.

Regulation of smoothened by Drosophila G-protein-coupled receptor kinases

The Hedgehog (Hh) signaling pathway plays a conserved and essential role in regulating development and homeostasis of numerous tissues. Cytoplasmic signaling is initiated by Smoothened (Smo), a G-protein-coupled receptor (GPCR) family member, whose levels and activity are regulated by the Hh receptor Patched (Ptc). In response to Hh binding to Ptc, Ptc-mediated repression of Smo is relieved, leading to Smo activation, surface accumulation, and downstream signaling. We find that downregulation of Drosophila Smo protein in Hh-responding imaginal disc cells is dependent on the activity of G-protein-coupled receptor kinase 2 (Gprk2). By analyzing gain- and null loss-of-function phenotypes, we provide evidence that Gprk2 promotes Smo internalization subsequent to its activation, most likely by direct phosphorylation. Ptc-dependent regulation of Smo accumulation is normal in gprk2 mutants, indicating that Gprk2 and Ptc downregulate Smo by different mechanisms. Finally, we show that both Drosophila G-protein-coupled receptor kinase orthologues, Gprk1 and Gprk2, act in a partially redundant manner to promote Hh signaling. Our results suggest that Smo is regulated by distinct Ptc-dependent and Gprk2-dependent trafficking mechanisms in vivo, analogous to constitutive and activity-dependent regulation of GPCRs. G-protein-coupled receptor kinase activity is also important for efficient downstream signaling.

Tow (Target of Wingless), a novel repressor of the Hedgehog pathway in Drosophila

Hedgehog (Hh) signalling plays a crucial role in the development and patterning of many tissues in both vertebrates and invertebrates. Aberrations in this pathway lead to severe developmental defects and cancer. Hh signal transduction in receiving cells is a well studied phenomenon; however questions still remain concerning the mechanism of repression of the pathway activator Smoothened (Smo) in the absence of Hh. Here we describe a novel repressor of the Hh pathway, Target of Wingless (Tow). Tow represents the Drosophila homolog of a conserved uncharacterised protein family. We show that Tow acts in Hh receiving cells, where its overexpression represses all levels of Hh signalling, and that this repression occurs upstream or at the level of Smo and downstream of the Hh receptor Patched (Ptc). In addition, we find that like Ptc, overexpression of Tow causes an accumulation of lipophorin in the wing disc. We demonstrate that loss of tow enhances different ptc alleles in a similar manner to another pathway repressor, Suppressor of Fused (SuFu), possibly through mediating Ptc dependant lipophorin internalisation. Combined, these results demonstrate that Tow is an important novel regulator of the Hh pathway in the wing imaginal disc, and may shed light on the mechanism of Ptc repression of Smo.

Wing vein patterning in Drosophila and the analysis of intercellular signaling

The positioning and elaboration of ectodermal veins in the wing of Drosophila melanogaster rely on widely utilized developmental signals, including those mediated by EGF, BMP, Hedgehog, Notch, and Wnt. Analysis of vein patterning mutants, using the molecular and genetic mosaic techniques available in Drosophila, has provided important insights into how a combination of short-range and long-range signaling can pattern a simple epidermal tissue. Moreover, venation has become a powerful system for isolating and analyzing novel components in these signaling pathways. I here review the basic events of vein patterning and give examples of how changes in venation have been used to identify important features of cell signaling pathways.

Independent roles of Drosophila Moesin in imaginal disc morphogenesis and hedgehog signalling

The three ERM proteins (Ezrin, Radixin and Moesin) form a conserved family required in many developmental processes involving regulation of the cytoskeleton. In general, the molecular function of ERM proteins is to link specific membrane proteins to the actin cytoskeleton. In Drosophila, loss of moesin (moe) activity causes incorrect localisation of maternal determinants during oogenesis, failures in rhabdomere differentiation in the eye and alterations of epithelial integrity in the wing imaginal disc. Some aspects of Drosophila Moe are related to the activity of the small GTPase RhoA, because the reduction of RhoA activity corrects many phenotypes of moe mutant embryos and imaginal discs. We have analysed the phenotype of moesin loss-of-function alleles in the wing disc and adult wing, and studied the effects of reduced Moesin activity on signalling mediated by the Notch, Decapentaplegic, Wingless and Hedgehog pathways. We found that reductions in Moesin levels in the wing disc cause the formation of wing-tissue vesicles and large thickenings of the vein L3, corresponding to breakdowns of epithelial continuity in the wing base and modifications of Hedgehog signalling in the wing blade, respectively. We did not observe any effect on signalling pathways other than Hedgehog, indicating that the moe defects in epithelial integrity have not generalised effects on cell signalling. The effects of moe mutants on Hedgehog signalling depend on the correct gene-dose of rhoA, suggesting that the requirements for Moesin in disc morphogenesis and Hh signalling in the wing disc are mediated by its regulation of RhoA activity. The mechanism linking Moesin activity with RhoA function and Hedgehog signalling remains to be elucidated.

Functional analysis in Drosophila indicates that the NBCCS/PTCH1 mutation G509V results in activation of smoothened through a dominant-negative mechanism

Mutations in the human homolog of the patched gene are associated with the developmental (and cancer predisposition) condition Nevoid Basal Cell Carcinoma Syndrome (NBCCS), as well as with sporadic basal cell carcinomas. Most mutations that have been identified in the germline of NBCCS patients are truncating or frameshift mutations, with amino acid substitutions rarely found. We show that a missense mutation in the sterol-sensing domain G509V acts as a dominant negative when assayed in vivo in Drosophila. Ectopic expression of a Drosophila patched transgene, carrying the analogous mutation to G509V, causes ectopic activation of Hedgehog target genes and ectopic membrane stabilisation of Smoothened. The G509V transgene behaves in a manner similar, except in its subcellular distribution, to a C-terminal truncation that has been characterised previously as a dominant-negative protein. G509V exhibits vesicular localisation identical to the wild-type protein, but the C-terminal truncated Patched molecule is localised predominantly to the plasma membrane. This finding suggests that dominant-negative function can be conferred by interruption of different aspects of Patched protein behaviour. Another mutation at the same residue, G509R, did not exhibit dominant-negative activity, suggesting that simple removal of the glycine at 509 is not sufficient to impart dominant-negative function.

Eyeless collaborates with Hedgehog and Decapentaplegic signaling in Drosophila eye induction

eyeless (ey) is a key regulator of the eye development pathway in Drosophila. Ectopic expression of ey can induce the expression of several eye-specification genes (eya, so, and dac) and induce eye formation in multiple locations on the body. However, ey does not induce eye formation everywhere where it is ectopically expressed, suggesting that EY needs to collaborate with additional factors for eye induction. We examined ectopic eye induction by EY in the wing disc and found that eye induction was spatially restricted to the posterior compartment and the anterior-posterior (A/P) compartmental border, suggesting a requirement for both HH and DPP signaling. Although EY in the anterior compartment induced dpp and dac, these were not sufficient for eye induction. Coexpression experiments show that EY needs to collaborate with high level of HH and DPP to induce ectopic eye formation. Ectopic eye formation also requires the activation of an eye-specific enhancer of the endogenous hh gene.

Requirements of high levels of Hedgehog signaling activity for medial-region cell fate determination in Drosophila legs: identification of pxb, a putative Hedgehog signaling attenuator gene repressed along the anterior-posterior compartment boundary

We show that high levels of Hedgehog signaling activity are essential for medial-region patterning in Drosophila legs. In mid-to-late third instar leg discs, high levels of Hedgehog signals repress the transcription of pxb, a newly identified gene encoding a transmembrane protein expressed specifically in the anterior compartment. Misexpression experiments indicate that Pxb may serve as a Hedgehog signaling attenuator capable of acting prior to Hedgehog-Patched interactions, suggesting that Hedgehog signaling in leg discs includes a pxb-repression-mediated positive feedback loop. RNA interference and clonal analysis show that neither Wingless nor Decapentaplegic signaling is required for pxb repression but high levels of Wingless signaling activity are essential for patterning in the leg ventral medial region.

Conservation of the expression of Dll, en, and wg in the eye-antennal imaginal disc of stalk-eyed flies

We studied the developmental basis of exaggerated eye span in two species of stalk-eyed flies (Cyrtodiopsis dalmanni and Sphyracephala beccarn). These flies have eyes laterally displaced at the end of eyestalks, and males have greatly exaggerated eye span, which they use as a sexual display. To investigate eye span development we have compared eye-antennal disc morphology and the expression of three key regulator genes of Drosophila head development, Distal-less (Dll), engrailed (en), and wingless (wg), in the stalk-eyed flies and Drosophila. We found great similarity in the basic division of the disc into anterior-antennal and posterior-eye portions and in the general patterning of Dll, en, and wg. Unexpectedly, our results showed that although the eye and antenna are adjacent in adult stalk-eyed flies, their primordia are physically separated by the presence of an intervening region between the anterior and posterior portions of the disc. This region is absent from Drosophila eye-antennal discs. We chose two stalk-eyed fly species that differed in the degree of eye-stalk exaggeration but surprisingly we found no corresponding difference in the size of the en-wg expression domains that mark the boundaries of the dorsal head capsule primordia. In summary, our expression data establish the regional identity of the eye-antennal disc and provide a framework from which to address the developmental genetics of hypercephaly.

Mutations in the sterol-sensing domain of Patched suggest a role for vesicular trafficking in Smoothened regulation

The tumor suppressor gene patched (ptc) encodes an approximately 140 kDa polytopic transmembrane protein [1-3] [corrected] that binds members of the Hedgehog (Hh) family of signaling proteins [4-6] [corrected] and regulates the activity of Smoothened (Smo), a G protein-coupled receptor-like protein essential for Hh signal transduction [7-9] [corrected]. Ptc contains a sterol-sensing domain (SSD) [10, 11] [corrected], a motif found in proteins implicated in the intracellular trafficking of cholesterol [12] [corrected], and/or other cargoes [13-15] [corrected]. Cholesterol plays a critical role in Hedgehog (Hh) signaling by facilitating the regulated secretion and sequestration of the Hh protein [16] [corrected], to which it is covalently coupled. In addition, cholesterol synthesis inhibitors block the ability of cells to respond to Hh [18, 19] [corrected], and this finding points to an additional requirement for the lipid in regulating downstream components of the Hh signaling pathway. Although the SSD of Ptc has been linked to both the sequestration of, and the cellular response to Hh [16, 20, 21] [corrected], definitive evidence for its function has so far been lacking. Here we describe the identification and characterization of two missense mutations in the SSD of Drosophila Ptc; strikingly, while both mutations abolish Smo repression, neither affects the ability of Ptc to interact with Hh. We speculate that Ptc may control Smo activity by regulating an intracellular trafficking process dependent upon the integrity of the SSD.

Drosophila phosphoinositide-dependent kinase-1 regulates apoptosis and growth via the phosphoinositide 3-kinase-dependent signaling pathway

Phosphoinositide-dependent kinase-1 (PDK-1) is a central mediator of the cell signaling between phosphoinositide 3-kinase (PI3K) and various intracellular serine/threonine kinases including Akt/protein kinase B (PKB), p70 S6 kinases, and protein kinase C. Recent studies with cell transfection experiments have implied that PDK-1 may be involved in various cell functions including cell growth and apoptosis. However, despite its pivotal role in cellular signalings, the in vivo functions of PDK-1 in a multicellular system have rarely been investigated. Here, we have isolated Drosophila PDK-1 (dPDK-1) mutants and characterized the in vivo roles of the kinase. Drosophila deficient in the dPDK-1 gene exhibited lethality and an apoptotic phenotype in the embryonic stage. Conversely, overexpression of dPDK-1 increased cell and organ size in a Drosophila PI3K-dependent manner. dPDK-1 not only could activate Drosophila Akt/PKB (Dakt1), but also substitute the in vivo functions of its mammalian ortholog to activate Akt/PKB. This functional interaction between dPDK-1 and Dakt1 was further confirmed through genetic analyses in Drosophila. On the other hand, cAMP-dependent protein kinase, which has been proposed as a possible target of dPDK-1, did not interact with dPDK-1. In conclusion, our findings provide direct evidence that dPDK-1 regulates cell growth and apoptosis during Drosophila development via the PI3K-dependent signaling pathway and demonstrate our Drosophila system to be a powerful tool for elucidating the in vivo functions and targets of PDK-1.

An absolute requirement for Cubitus interruptus in Hedgehog signaling

Hedgehog (Hh) proteins play diverse organizing roles in animal development by regulating gene expression in responding cells. Several components of the Hh signal transduction pathway have been identified, yet their precise role in mediating the various outputs of the pathway is still poorly understood. The Gli homolog Cubitus interruptus (Ci) is involved in controlling the transcription of Drosophila Hh target genes and thus represents the most downstream component known in this pathway. We address the question of whether the Hh pathway is distally branched or, in other words, whether the regulation of Ci activity is the sole output of Hh signaling. Putative Ci-independent branches of Hh signaling are explored by analyzing the behavior of cells that lack Ci but have undergone maximal activation of the Hh transduction pathway due to the removal of Patched (Ptc). The analysis of target gene expression and morphogenetic read-outs of Hh in embryonic, larval and adult stages indicates that Ci is absolutely required for all examined aspects of Hh outputs. We interpret this as evidence against the existence of Ci-independent branches in the Hh signal transduction pathway and propose that most cases of apparent Ci/Gli-independent Hh output can be attributed to the derepression of target gene expression in the absence of Ci/Gli repressor function.

Distinct signals generate repeating striped pattern in the embryonic parasegment

How repeating striped patterns arise across cellular fields is unclear. To address this we examined the repeating pattern of Stripe (Sr) expression across the parasegment (PS) in Drosophila. This pattern is generated in two steps. First, the ligands Hedgehog (Hh) and Wingless (Wg) subdivide the PS into smaller territories. Second, the ligands Hh, Spitz (Spi), and Wg each emanate from a specific territory and induce Sr expression in an adjacent territory. We also show that the width of Sr expression is determined by signaling strength. Finally, an enhancer trap in the sr gene detects the response to Spi and Wg, but not to Hh, implying the existence of separable control elements in the sr gene. Thus, a distinct inductive event is used to initiate each element of the repeating striped pattern.

A gain-of-function mutant of patched dissects different responses to the hedgehog gradient

The Hedgehog (Hh) signal has an inductive role during Drosophila development. Patched is part of the Hedgehog-receptor complex and shows a repressive function on the signaling cascade, which is alleviated in the presence of Hh. Herein, we identify the first dominant gain-of-function allele of patched, Confused (patched(Con)). Analysis of the patched(Con) allele led us to uncover novel features of the reception and function of the Hh signal. At least three different regions of gene expression were identified and a gradient of cell affinities was established in response to Hh. A new state of Cubitus interruptus activity responsible for the activation of araucan and caupolican genes of the iroquois complex, independent of Fused kinase function, was shown. In the disc, patched(Con) behaved like fused mutants and was rescued by Suppressor of fused mutations. However, fused mutants are embryonic lethal while patched(Con) is not, suggesting that Patched could interpret Hedgehog signaling differently in the embryo and in the adult.

A screen for dominant modifiers of ro(Dom), a mutation that disrupts morphogenetic furrow progression in Drosophila, identifies groucho and hairless as regulators of atonal expression

ro(Dom) is a dominant allele of rough (ro) that results in reduced eye size due to premature arrest in morphogenetic furrow (MF) progression. We found that the ro(Dom) stop-furrow phenotype was sensitive to the dosage of genes known to affect retinal differentiation, in particular members of the hedgehog (hh) signaling cascade. We demonstrate that ro(Dom) interferes with Hh's ability to induce the retina-specific proneural gene atonal (ato) in the MF and that normal eye size can be restored by providing excess Ato protein. We used ro(Dom) as a sensitive genetic background in which to identify mutations that affect hh signal transduction or regulation of ato expression. In addition to mutations in several unknown loci, we recovered multiple alleles of groucho (gro) and Hairless (H). Analysis of their phenotypes in somatic clones suggests that both normally act to restrict neuronal cell fate in the retina, although they control different aspects of ato's complex expression pattern.

The combgap locus encodes a zinc-finger protein that regulates cubitus interruptus during limb development in Drosophila melanogaster

The combgap locus, first described by C. B. Bridges in 1925, is a gene required for proper anteroposterior pattern formation in the limbs of Drosophila melanogaster. The development of the anteroposterior axis of fly limbs is initiated by hedgehog signaling from cells of the posterior half to cells of the anterior half of the limb primordium. Hedgehog signaling requires the anterior-specific expression of the gene cubitus interruptus to establish posterior-specific hedgehog secretion and anterior-specific competence to respond to hedgehog. We have cloned combgap and find that it encodes a chromosomal protein with 11 C(2)H(2) zinc fingers. Limb defects found in combgap mutants consist of either loss or duplication of pattern elements in the anteroposterior axis and can be explained through the inappropriate expression of cubitus interruptus and its downstream target genes. In combgap mutants, cubitus interruptus is ectopically expressed in the posterior compartments of wing imaginal discs and is downregulated in the anterior compartment of legs, wings and antennae. We are able to rescue anterior compartment combgap phenotypes by expressing additional cubitus interruptus using the Gal4/UAS system. Dominant alleles of cubitus interruptus, which result in posterior expression, phenocopy combgap posterior compartment phenotypes. Finally, we find that the combgap protein binds to polytene chromosomes at many sites including the cubitus interruptus locus, suggesting that it could be a direct regulator of cubitus interruptus transcription.

In vivo functions of the patched protein: requirement of the C terminus for target gene inactivation but not Hedgehog sequestration

The membrane protein Patched (Ptc) is a key regulator of Hedgehog (Hh) signaling in development and is mutated in human tumors. Ptc opposes Hh-induced gene transcription and sequesters Hh protein. To dissect these functions, we tested partially deleted forms of Ptc in Drosophila. Deletion of either half of Ptc abolishes all function while coexpression of the halves restores nearly full activity. Deletion of the final 156 residues of Ptc permits Hh sequestration but abolishes inhibition of Hh targets. This deletion has dominant-negative activity, promoting target gene activation in a ligand-independent manner. We observe little or no association of full-length or partially deleted Ptc with the membrane protein Smoothened in Drosophila cultured cells.

Drawing lines in the Drosophila wing: initiation of wing vein development

It has been proposed that wing veins in Drosophila form at boundaries between discrete sectors of cells that subdivide the anterior-posterior axis of the developing wing primordium. Recently, analysis of events underlying initiation of vein formation suggests that there is a general developmental mechanism for drawing lines between adjacent domains of cells, which is referred to as 'for-export-only-signaling'. In this model, cells in one domain produce a short range signal to which they cannot respond. As a consequence of this constraint, cells lying in a narrow line immediately outside the signal-producing domain are the only cells that can respond to the signal by activating expression of vein-promoting genes.

hedgehog and wing development in Drosophila: a morphogen at work?

In Drosophila, Hedgehog (Hh) is a key regulator of limb development and activates decapentaplegic (dpp), a gene encoding a TGFbeta-related factor that controls growth and patterning of the limbs. During wing development, Hh also has morphogen-like Dpp-independent functions, controlling the morphogenesis of the central part of the wing through the activation of the evolutionarily conserved transcription factors encoded by the iroquois and collier genes. The ways in which Hh forms an activity gradient to lay the basis of patterning of the adult wing are described here. As the signal transduction pathway of Hh is strongly conserved during evolution and human Hh may be implicated in congenital diseases and cancers, these observations provide important advances which may help in understanding the function of Hh proteins in normal and pathological development and tumourigenesis in humans.

Activation of knot (kn) specifies the 3–4 intervein region in the Drosophila wing

Hedgehog (Hh) plays an important role in Drosophila wing patterning by inducing expression of Dpp, which serves to organize the wing globally across the A-P axis. We show here how Hh signalling also plays a direct role in patterning the medial wing through the activation of the Hh-target gene, knot (kn). kn is expressed in Hh-responsive cells near the A-P compartment boundary, where its expression is dependent on fu, a component of Hh signalling. kn is required for the proper positioning of veins 3 and 4 and to prevent ectopic venation between them. Furthermore, the expansion anteriorly of the normal kn expression domain causes an associated anterior shift in the position of vein 3 in the resultant wing. Ectopic expression of kn elsewhere in the wing imaginal disc results in the failure to properly activate the vein initiation genes, rho and Dl. Expression of the gene encoding the EGF-receptor (EGFR), which is required for vein initiation and subsequent differentiation, is normally depressed in the 3–4 intervein region. This downregulation of EGFR in the medial portion of the imaginal disc is dependent on kn activity and ectopic expression of kn inactivates EGFR elsewhere in the wing primordium. We propose kn expression in Hh-responsive cells of the wing blade anlagen during the late third instar creates a zone of cells in the medial wing in which vein primordia cannot be induced. The primordia for veins 3 and 4 are laid down adjacent to the kn-imposed vein-free zone, presumably by a signalling factor (such as Vn) also synthesized in the medial region of the wing.

Overexpression of ptc1 inhibits induction of Shh target genes and prevents normal patterning in the neural tube

Patched (Ptc) is a human tumor suppressor protein and a candidate receptor for Hedgehog (Hh) proteins, which regulate growth and patterning in embryos. Ptc represses expression of Hh target genes such as Gli1 and ptc1 itself. Localized secretion of Hh appears to induce transcription of target genes in specific patterns by binding to Ptc and preventing it from functioning in recipient cells. People who are heterozygous for PTC1 exhibit a range of developmental defects, suggesting that some genes are inappropriately expressed when there is not enough Ptc protein. To test the idea that a balance between Hh and Ptc activities is essential for normal development, we overexpressed Ptc in the neural tube. We find that excess Ptc is sufficient to inhibit expression of Gli1 and ptc1, suggesting that Sonic hedgehog (Shh) cannot signal effectively. This leads to partial dorsalization of the neural tube and a wide spectrum of neural defects, ranging from embryonic lethality to hydrocephaly.

The COE transcription factor Collier is a mediator of short-range Hedgehog-induced patterning of the Drosophila wing

BACKGROUND

The secreted Hedgehog (Hh) proteins have been implicated as mediators of positional information in vertebrates and invertebrates. A gradient of Hh activity contributes to antero-posterior (A/P) patterning of the fly wing. In addition to inducing localised expression of Decapentaplegic (Dpp), which in turn relays patterning cues at long range, Hh directly patterns the central region of the wing.

RESULTS

We show that short-range, dose-dependent Hh activity is mediated by activation of the transcription factor Collier (Col). In the absence of col activity, longitudinal veins 3 and 4 (L3 and L4) are apposed and the central intervein is missing. Hh expression induces col expression in a narrow stripe of cells along the A/P boundary through a dual-input mechanism: inhibition of proteolysis of Cubitus-interruptus (Ci) and activation of the Fused (Fu) kinase. Col, in cooperation with Ci, controls the formation of the central intervein by activating the expression of blistered (bs), which encodes the Drosophila serum response factor (D-SRF), the activity of which is required for the adoption and maintenance of the intervein cell fate. Furthermore, col is allelic to knot, a gene involved in the formation of the central part of the wing. This finding completes our understanding of the sectorial organisation of the Drosophila wing.

CONCLUSIONS

Col, the Drosophila member of the COE family (Col/Olf-1/EBF) of non-basic, helix-loop-helix (HLH)-containing transcription factors, is a mediator of the short-range organising activity of Hh in the Drosophila wing. Our results support the idea that Hh controls target gene expression in a concentration-dependent manner and highlight the importance of the Fu kinase in this differential regulation. The high degree of evolutionary conservation of the COE proteins and the diversity of developmental processes controlled by Hh signalling raises the possibility that the specific genetic interactions depicted here may also operate in vertebrates.

Recruitment of a hedgehog regulatory circuit in butterfly eyespot evolution

The origin of new morphological characters is a long-standing problem in evolutionary biology. Novelties arise through changes in development, but the nature of these changes is largely unknown. In butterflies, eyespots have evolved as new pattern elements that develop from special organizers called foci. Formation of these foci is associated with novel expression patterns of the Hedgehog signaling protein, its receptor Patched, the transcription factor Cubitus interruptus, and the engrailed target gene that break the conserved compartmental restrictions on this regulatory circuit in insect wings. Redeployment of preexisting regulatory circuits may be a general mechanism underlying the evolution of novelties.

The roles of the homeobox genes aristaless and Distal-less in patterning the legs and wings of Drosophila

In the leg and wing imaginal discs of Drosophila, the expression domains of the homeobox genes aristaless (al) and Distal-less (Dll) are defined by the secreted signaling molecules Wingless (Wg) and Decapentaplegic (Dpp). Here, the roles played by al and Dll in patterning the legs and wings have been investigated through loss of function studies. In the developing leg, al is expressed at the presumptive tip and a molecularly defined null allele of al reveals that its only function in patterning the leg appears to be to direct the growth and differentiation of the structures at the tip. In contrast, Dll has previously been shown to be required for the development of all of the leg more distal than the coxa. Dll protein can be detected in a central domain in leg discs throughout most of larval development, and in mature discs this domain corresponds to the distal-most region of the leg, the tarsus and the distal tibia. Clonal analysis reveals that late in development these are the only regions in which Dll function is required. However, earlier in development Dll is required in more proximal regions of the leg suggesting it is expressed at high levels in these cells early in development but not later. This reveals a correlation between a temporal requirement for Dll and position along the proximodistal axis; how this may relate to the generation of the P/D axis is discussed. Dll is required in the distal regions of the leg for the expression of tarsal-specific genes including al and bric-a-brac. Dll mutant cells in the leg sort out from wild-type cells suggesting one function of Dll here is to control adhesive properties of cells. Dll is also required for the normal development of the wing, primarily for the differentiation of the wing margin.

Modulation of Hedgehog target gene expression by the Fused serine-threonine kinase in wing imaginal discs

The Fused (Fu) serine-threonine kinase and the Suppressor of fused (Su(fu)) product are part of the Hedgehog (Hh) signaling pathway both in embryos and in imaginal discs. In wing imaginal discs, the Hh signal induces Cubitus interruptus (Ci) accumulation and activates patched (ptc) and decapentaplegic (dpp) expression along the anterior/posterior (A/P) boundary. In this paper, we have examined the role of the Fu and Su(fu) proteins in the regulation of Hh target gene expression in wing imaginal discs, by using different classes of fu alleles and an amorphic Su(fu) mutation. We show that, at the A/P boundary, Fu kinase activity is involved in the maintenance of high ptc expression and in the induction of late anterior engrailed (en) expression. These combined effects can account for the modulation of Ci accumulation and for the precise localization of the Dpp morphogen stripe. In contrast, in more anterior cells which do not receive Hh signal, we show that Fu plays a role independent of its kinase function in the regulation of Ci accumulation. In these cells, Fu may be involved in the stabilization of a large protein complex which is probably responsible for the regulation of Ci cleavage and/or targeting to nucleus. We propose that the Fused function is necessary for the activation of full-length Ci and counteracts the negative Su(fu) effect on the pathway, leading to en, ptc and dpp expression.

Boundaries in the Drosophila wing imaginal disc organize vein-specific genetic programs

Previous studies have suggested that vein primordia in Drosophila form at boundaries along the A/P axis between discrete sectors of the larval wing imaginal disc. Genes involved in initiating vein development during the third larval instar are expressed either in narrow stripes corresponding to vein primordia or in broader ‘provein’ domains consisting of cells competent to become veins. In addition, genes specifying the alternative intervein cell fate are expressed in complementary intervein regions. The regulatory relationships between genes expressed in narrow vein primordia, in broad provein stripes and in interveins remains unknown, however. In this manuscript, we provide additional evidence for veins forming in narrow stripes at borders of A/P sectors. These experiments further suggest that narrow vein primordia produce secondary short-range signal(s), which activate expression of provein genes in a broad pattern in neighboring cells. We also show that crossregulatory interactions among genes expressed in veins, proveins and interveins contribute to establishing the vein-versus-intervein pattern, and that control of gene expression in vein and intervein regions must be considered on a stripe-by-stripe basis. Finally, we present evidence for a second set of vein-inducing boundaries lying between veins, which we refer to as paravein boundaries. We propose that veins develop at both vein and paravein boundaries in more ‘primitive’ insects, which have up to twice the number of veins present in Drosophila. We present a model in which different A/P boundaries organize vein-specific genetic programs to govern the development of individual veins.

The knirps and knirps-related genes organize development of the second wing vein in Drosophila

The neighboring homologous knirps (kni) and knirps-related (knrl) genes in Drosophila encode transcription factors in the steroid hormone receptor superfamily. During early embryogenesis, kni functions as a gap gene to control expression of segmentation genes within the abdominal region of the embryo. In this study, we present evidence that kni and knrl link A/P positional information in larval wing imaginal discs to morphogenesis of the second longitudinal wing vein (L2). We show that kni and knrl are expressed in similar narrow stripes corresponding to the position of the L2 primordium. The kni and knrl L2 stripes abut the anterior border of the broad central expression domain of the Dpp target gene spalt major (salm). We provide evidence that radius incompletus (ri), a well-known viable mutant lacking the L2 vein, is a regulatory mutant of the kni/knrl locus. In ri mutant wing discs, kni and knrl fail to be expressed in the L2 primordium. In addition, the positions of molecular breakpoints in the kni/knrl locus indicate that the ri function is provided by cis-acting sequences upstream of the kni transcription unit. Epistasis tests reveal that the kni/knrl locus functions downstream of spalt major (salm) and upstream of genes required to initiate vein-versus-intervein differentiation. Mis-expression experiments suggest that kni and knrl expressing cells inhibit neighboring cells from becoming vein cells. Finally, kni and knrl are likely to refine the L2 position by positively auto-regulating their own expression and by providing negative feedback to repress salm expression. We propose a model in which the combined activities of kni and knrl organize development of the L2 vein in the appropriate position.

Signals transmitted along retinal axons in Drosophila: Hedgehog signal reception and the cell circuitry of lamina cartridge assembly

The arrival of retinal axons in the brain of Drosophila triggers the assembly of glial and neuronal precursors into a ‘neurocrystalline’ array of lamina synaptic ‘cartridges’. Hedgehog, a secreted protein, is an inductive signal delivered by retinal axons for the initial steps of lamina differentiation. In the development of many tissues, Hedgehog acts in a signal relay cascade via the induction of secondary secreted factors. Here we show that lamina neuronal precursors respond directly to Hedgehog signal reception by entering S-phase, a step that is controlled by the Hedgehog-dependent transcriptional regulator Cubitus interruptus. The terminal differentiation of neuronal precursors and the migration and differentiation of glia appear to be controlled by other retinal axon-mediated signals. Thus retinal axons impose a program of developmental events on their postsynaptic field utilizing distinct signals for different precursor populations.

slimb coordinates wg and dpp expression in the dorsal-ventral and anterior-posterior axes during limb development

In the Drosophila leg disc, wingless (wg) and decapentaplegic (dpp) are expressed in a ventral-anterior and dorsal-anterior stripe of cells, respectively. This pattern of expression is essential for proper limb development. While the Hedgehog (Hh) pathway regulates dpp and wg expression in the anterior-posterior (A/P) axis, mechanisms specifying their expression in the dorsal-ventral (D/V) axis are not well understood. We present evidence that slimb mutant clones in the disc deregulate wg and dpp expression in the D/V axis. This suggests for the first time that their expression in the D/V axis is actively regulated during imaginal disc development. Furthermore, slimb is unique in that it also deregulates wg and dpp in the A/P axis. The misexpression phenotypes of slimb- clones indicate that the regulation of wg and dpp expression is coordinated in both axes, and that slimb plays an essential role in integrating A/P and D/V signals for proper patterning during development. Our genetic analysis further reveals that slimb intersects the A/P pathway upstream of smoothened (smo).

Interactions between Wingless and DFz2 during Drosophila wing development

Drosophila Wingless (Wg) is a secreted signaling protein of the Wnt family. Mutations in the wg gene disrupt the patterning of embryonic segments and their adult derivatives. Wg protein has been shown in cell culture to functionally interact with DFz2, a receptor that is structurally related to the tissue polarity protein Frizzled (Fz). However, it has not been determined if DFz2 functions in the Wg signaling pathway during fly development. Here we demonstrate that overexpression of DFz2 increases Wg-dependent signaling to induce ectopic margin bristle formation in developing Drosophila wings. Overexpression of a truncated form of DFz2 acts in a dominant-negative manner to block Wg signaling at the wing margin, and this block is rescued by co-expression of full-length DFz2 but not full-length Fz. Our results suggest that DFz2 and not Fz acts in the Wg signaling pathway for wing margin development. However, a truncated form of Fz also blocks Wg signaling in embryo and wing margin development, and the truncated form of DFz2 affects ommatidial polarity during eye development. These observations suggest that a single dominant-negative form of Fz or DFz2 can block more than one type of Wnt signaling pathway and imply that truncated proteins of the Fz family lose some aspect of signaling specificity.