Nature versus nurture: asymmetric cell divisions in Drosophila bristle development

Novel Genome-Engineered H Alleles Differentially Affect Lateral Inhibition and Cell Dichotomy Processes during Bristle Organ Development

Hairless (H) encodes the major antagonist in the Notch signaling pathway, which governs cellular differentiation of various tissues in Drosophila. By binding to the Notch signal transducer Suppressor of Hairless (Su(H)), H assembles repressor complexes onto Notch target genes. Using genome engineering, three new H alleles, HFA, HLLAA and HWA were generated and a phenotypic series was established by several parameters, reflecting the residual H-Su(H) binding capacity. Occasionally, homozygous HWA flies develop to adulthood. They were compared with the likewise semi-viable HNN allele affecting H-Su(H) nuclear entry. The H homozygotes were short-lived, sterile and flightless, yet showed largely normal expression of several mitochondrial genes. Typical for H mutants, both HWA and HNN homozygous alleles displayed strong defects in wing venation and mechano-sensory bristle development. Strikingly, however, HWA displayed only a loss of bristles, whereas bristle organs of HNN flies showed a complete shaft-to-socket transformation. Apparently, the impact of HWA is restricted to lateral inhibition, whereas that of HNN also affects the respective cell type specification. Notably, reduction in Su(H) gene dosage only suppressed the HNN bristle phenotype, but amplified that of HWA. We interpret these differences as to the role of H regarding Su(H) stability and availability.

Co-option of epidermal cells enables touch sensing

The epidermis is equipped with specialized mechanosensory organs that enable the detection of tactile stimuli. Here, by examining the differentiation of the tactile bristles, mechanosensory organs decorating the Drosophila adult epidermis, we show that neighbouring epidermal cells are essential for touch perception. Each mechanosensory bristle signals to the surrounding epidermis to co-opt a single epidermal cell, which we named the F-Cell. Once specified, the F-Cell adopts a specialized morphology to ensheath each bristle. Functional assays reveal that adult mechanosensory bristles require association with the epidermal F-Cell for touch sensing. Our findings underscore the importance of resident epidermal cells in the assembly of functional touch-sensitive organs.

Genetic and Molecular Interactions between HΔCT, a Novel Allele of the Notch Antagonist Hairless, and the Histone Chaperone Asf1 in Drosophila melanogaster

Cellular differentiation relies on the highly conserved Notch signaling pathway. Notch activity induces gene expression changes that are highly sensitive to chromatin landscape. We address Notch gene regulation using Drosophila as a model, focusing on the genetic and molecular interactions between the Notch antagonist Hairless and the histone chaperone Asf1. Earlier work implied that Asf1 promotes the silencing of Notch target genes via Hairless (H). Here, we generate a novel HΔCT allele by genome engineering. Phenotypically, HΔCT behaves as a Hairless gain of function allele in several developmental contexts, indicating that the conserved CT domain of H has an attenuator role under native biological contexts. Using several independent methods to assay protein-protein interactions, we define the sequences of the CT domain that are involved in Hairless-Asf1 binding. Based on previous models, where Asf1 promotes Notch repression via Hairless, a loss of Asf1 binding should reduce Hairless repressive activity. However, tissue-specific Asf1 overexpression phenotypes are increased, not rescued, in the HΔCT background. Counterintuitively, Hairless protein binding mitigates the repressive activity of Asf1 in the context of eye development. These findings highlight the complex connections of Notch repressors and chromatin modulators during Notch target-gene regulation and open the avenue for further investigations.

Wingless and Archipelago, a fly E3 ubiquitin ligase and a homolog of human tumor suppressor FBW7, show an antagonistic relationship in wing development

BACKGROUND

Archipelago (Ago) is a Drosophila homolog of mammalian F-box and WD repeat domain-containing 7 (FBW7, also known as FBXW7). In previous studies, FBW7 has been addressed as a tumor suppressor mediating ubiquitin-dependent proteolysis of several oncogenic proteins. Ubiquitination is a type of protein modification that directs protein for degradation as well as sorting. The level of beta-catenin (β-cat), an intracellular signal transducer in Wnt signaling pathway, is reduced upon overexpression of FBW7 in human cancer cell lines. Loss of function mutations in FBW7 and overactive Wnt signaling have been reported to be responsible for human cancers.

RESULTS

We found that Ago is important for the formation of shafts in chemosensory bristles at wing margin. This loss of shaft phenotype by knockdown of ago was rescued by knockdown of wingless (wg) whereas wing notching phenotype by knockdown of wg was rescued by knockdown of ago, establishing an antagonistic relationship between ago and wg. In line with this finding, knockdown of ago increased the level of Armadillo (Arm), a homolog of β-cat, in Drosophila tissue. Furthermore, knockdown of ago increased the level of Distal-less (Dll) and extracellular Wg in wing discs. In S2 cells, the amount of secreted Wg was increased by knockdown of Ago but decreased by Ago overexpression. Therefore, Ago plays a previously unidentified role in the inhibition of Wg secretion. Ago-overexpressing clones in wing discs exhibited accumulation of Wg in endoplasmic reticulum (ER), suggesting that Ago prevents Wg protein from moving to Golgi from ER.

CONCLUSIONS

We concluded that Ago plays dual roles in inhibiting Wg signaling. First, Ago decreases the level of Arm, by which Wg signaling is downregulated in Wg-responding cells. Second, Ago decreases the level of extracellular Wg by inhibiting movement of Wg from ER to Golgi in Wg-producing cells.

Complex genetic interactions of novel Suppressor of Hairless alleles deficient in co-repressor binding

Throughout the animal kingdom, the Notch signalling pathway allows cells to acquire diversified cell fates. Notch signals are translated into activation of Notch target genes by CSL transcription factors. In the absence of Notch signals, CSL together with co-repressors functions as a transcriptional repressor. In Drosophila, repression of Notch target genes involves the CSL homologue Suppressor of Hairless (Su(H)) and the Notch (N) antagonist Hairless (H) that together form a repressor complex. Guided by crystal structure, three mutations Su(H)LL, Su(H)LLF and Su(H)LLL were generated that specifically affect interactions with the repressor H, and were introduced into the endogenous Su(H) locus by gene engineering. In contrast to the wild type isoform, these Su(H) mutants are incapable of repressor complex formation. Accordingly, Notch signalling activity is dramatically elevated in the homozygotes, resembling complete absence of H activity. It was noted, however, that heterozygotes do not display a dominant H loss of function phenotype. In this work we addressed genetic interactions the three H-binding deficient Su(H) mutants display in combination with H and N null alleles. We included a null mutant of Delta (Dl), encoding the ligand of the Notch receptor, as well as of Su(H) itself in our genetic analyses. H, N or Dl mutations cause dominant wing phenotypes that are sensitive to gene dose of the others. Moreover, H heterozygotes lack bristle organs and develop bristle sockets instead of shafts. The latter phenotype is suppressed by Su(H) null alleles but not by H-binding deficient Su(H) alleles which we attribute to the socket cell specific activity of Su(H). Modification of the dominant wing phenotypes of either H, N or Dl, however, suggested some lack of repressor activity in the Su(H) null allele and likewise in the H-binding deficient Su(H) alleles. Overall, Su(H) mutants are recessive perhaps reflecting self-adjusting availability of Su(H) protein.

The protein phosphatase 4 complex promotes the Notch pathway and wingless transcription

The Wnt/Wingless (Wg) pathway controls cell fate specification, tissue differentiation and organ development across organisms. Using an in vivo RNAi screen to identify novel kinase and phosphatase regulators of the Wg pathway, we identified subunits of the serine threonine phosphatase Protein Phosphatase 4 (PP4). Knockdown of the catalytic and regulatory subunits of PP4 cause reductions in the Wg pathway targets Senseless and Distal-less. We find that PP4 regulates the Wg pathway by controlling Notch-driven wg transcription. Genetic interaction experiments identified that PP4 likely promotes Notch signaling within the nucleus of the Notch-receiving cell. Although the PP4 complex is implicated in various cellular processes, its role in the regulation of Wg and Notch pathways was previously uncharacterized. Our study identifies a novel role of PP4 in regulating Notch pathway, resulting in aberrations in Notch-mediated transcriptional regulation of the Wingless ligand. Furthermore, we show that PP4 regulates proliferation independent of its interaction with Notch.

Summary: The protein phosphatase 4 complex promotes Notch signaling and target gene expression during Drosophila wing development.

Regulation of Notch Signaling by an Evolutionary Conserved DEAD Box RNA Helicase, Maheshvara in Drosophila melanogaster

Notch signaling is an evolutionary conserved process that influences cell fate determination, cell proliferation, and cell death in a context-dependent manner. Notch signaling is fine-tuned at multiple levels and misregulation of Notch has been implicated in a variety of human diseases. We have characterized maheshvara (mahe), a novel gene in Drosophila melanogaster that encodes a putative DEAD box protein that is highly conserved across taxa and belongs to the largest group of RNA helicase. A dynamic pattern of mahe expression along with the maternal accumulation of its transcripts is seen during early stages of embryogenesis. In addition, a strong expression is also seen in the developing nervous system. Ectopic expression of mahe in a wide range of tissues during development results in a variety of defects, many of which resemble a typical Notch loss-of-function phenotype. We illustrate that ectopic expression of mahe in the wing imaginal discs leads to loss of Notch targets, Cut and Wingless. Interestingly, Notch protein levels are also lowered, whereas no obvious change is seen in the levels of Notch transcripts. In addition, mahe overexpression can significantly rescue ectopic Notch-mediated proliferation of eye tissue. Further, we illustrate that mahe genetically interacts with Notch and its cytoplasmic regulator deltex in trans-heterozygous combination. Coexpression of Deltex and Mahe at the dorso-ventral boundary results in a wing-nicking phenotype and a more pronounced loss of Notch target Cut. Taken together we report identification of a novel evolutionary conserved RNA helicase mahe, which plays a vital role in regulation of Notch signaling.

The diaphanous gene of Drosophila interacts antagonistically with multiple wing hairs and plays a key role in wing hair morphogenesis

The Drosophila wing is covered by an array of distally pointing hairs that has served as a key model system for studying planar cell polarity (PCP). The adult cuticular hairs are formed in the pupae from cell extensions that contain extensive actin filaments and microtubules. The importance of the actin cytoskeleton for hair growth and morphogenesis is clear from the wide range of phenotypes seen in mutations in well-known actin regulators. Formin proteins promote the formation of long actin filaments of the sort thought to be important for hair growth. We report here that the formin encoding diaphanous (dia) gene plays a key role in hair morphogenesis. Both loss of function mutations and the expression of a constitutively active Dia led to cells forming both morphologically abnormal hairs and multiple hairs. The conserved frizzled (fz)/starry night (stan) PCP pathway functions to restrict hair initiation and activation of the cytoskeleton to the distal most part of wing cells. It also ensures the formation of a single hair per cell. Our data suggest that the localized inhibition of Dia activity may be part of this mechanism. We found the expression of constitutively active Dia greatly expands the region for activation of the cytoskeleton and that dia functions antagonistically with multiple wing hairs (mwh), the most downstream member of the fz/stan pathway. Further we established that purified fragments of Dia and Mwh could be co-immunoprecipitated suggesting the genetic interaction could reflect a direct physical interaction.

Pox neuro control of cell lineages that give rise to larval poly-innervated external sensory organs in Drosophila

The Pox neuro (Poxn) gene of Drosophila plays a crucial role in the development of poly-innervated external sensory (p-es) organs. However, how Poxn exerts this role has remained elusive. In this study, we have analyzed the cell lineages of all larval p-es organs, namely of the kölbchen, papilla 6, and hair 3. Surprisingly, these lineages are distinct from any previously reported cell lineages of sensory organs. Unlike the well-established lineage of mono-innervated external sensory (m-es) organs and a previously proposed model of the p-es lineage, we demonstrate that all wild-type p-es lineages exhibit the following features: the secondary precursor, pIIa, gives rise to all three support cells-socket, shaft, and sheath, whereas the other secondary precursor, pIIb, is neuronal and gives rise to all neurons. We further show that in one of the p-es lineages, that of papilla 6, one cell undergoes apoptosis. By contrast in Poxn null mutants, all p-es lineages have a reduced number of cells and their pattern of cell divisions is changed to that of an m-es organ, with the exception of a lineage in a minority of mutant kölbchen that retains a second bipolar neuron. Indeed, the role of Poxn in p-es lineages is consistent with the specification of the developmental potential of secondary precursors and the regulation of cell division but not apoptosis.

Gain of function notch phenotypes associated with ectopic expression of the Su(H) C-terminal domain illustrate separability of Notch and hairless-mediated activities

The Notch signaling pathway is instrumental for cell fate decisions. Signals from the Notch receptor are transduced by CSL-type DNA-binding proteins. In Drosophila, this protein is named Suppressor of Hairless [Su(H)]. Together with the intracellular domain of the activated Notch receptor ICN, Su(H) assembles a transcriptional activator complex on Notch target genes. Hairless acts as the major antagonist of the Notch signaling pathway in Drosophila by means of the formation of a repressor complex together with Su(H) and several co-repressors. Su(H) is characterized by three domains, the N-terminal domain NTD, the beta-trefoil domain BTD and the C-terminal domain CTD. NTD and BTD bind to the DNA, whereas BTD and CTD bind to ICN. Hairless binds to the CTD, however, to sites different from ICN. In this work, we have addressed the question of competition and availability of Su(H) for ICN and Hairless binding in vivo. To this end, we overexpressed the CTD during fly development. We observed a strong activation of Notch signaling processes in various tissues, which may be explained by an interference of CTD with Hairless corepressor activity. Accordingly, a combined overexpression of CTD together with Hairless ameliorated the effects, unlike Su(H) which strongly enhances repression when overexpressed concomitantly with Hairless. Interestingly, in the combined overexpression CTD accumulated in the nucleus together with Hairless, whereas it is predominantly cytoplasmic on its own.

Insensitive is a corepressor for Suppressor of Hairless and regulates Notch signalling during neural development

The Notch intracellular domain functions as a co-activator for the DNA-binding protein Suppressor of Hairless (Su(H)) to mediate myriad cell fate decisions. Notch pathway activity is balanced by transcriptional repression, mediated by Su(H) in concert with its Drosophila corepressor Hairless. We demonstrate that the Drosophila neural BEN-solo protein Insensitive (Insv) is a nuclear factor that inhibits Notch signalling during multiple peripheral nervous system cell fate decisions. Endogenous Insv was particularly critical when repressor activity of Su(H) was compromised. Reciprocally, ectopic Insv generated several Notch loss-of-function phenotypes, repressed most Notch targets in the E(spl)-C, and opposed Notch-mediated activation of an E(spl)m3-luc reporter. A direct role for Insv in transcriptional repression was indicated by binding of Insv to Su(H), and by strong chromatin immunoprecipitation of endogenous Insv to most E(spl)-C loci. Strikingly, ectopic Insv fully rescued sensory organ precursors in Hairless null clones, indicating that Insv can antagonize Notch independently of Hairless. These data shed first light on the in vivo function for a BEN-solo protein as an Su(H) corepressor in the Notch pathway regulating neural development.

Discrete regulatory regions control early and late expression of D-Pax2 during external sensory organ development

The transcription factor D-Pax2 is required for the correct differentiation of several cell types in Drosophila sensory systems. While the regulation of its expression in the developing eye has been well studied, little is known about the mechanisms by which the dynamic pattern of D-Pax2 expression in the external sensory organs is achieved. Here we demonstrate that early activation of D-Pax2 in the sensory organ lineage and its maintenance in the trichogen and thecogen cells are governed by separate enhancers. Furthermore, the initial activation is controlled in part by proneural proteins whereas the later maintenance expression is regulated by a positive feedback loop.

Notch regulates numb: integration of conditional and autonomous cell fate specification

The Notch cell-cell signaling pathway is used extensively in cell fate specification during metazoan development. In many cell lineages, the conditional role of Notch signaling is integrated with the autonomous action of the Numb protein, a Notch pathway antagonist. During Drosophila sensory bristle development, precursor cells segregate Numb asymmetrically to one of their progeny cells, rendering it unresponsive to reciprocal Notch signaling between the two daughters. This ensures that one daughter adopts a Notch-independent, and the other a Notch-dependent, cell fate. In a genome-wide survey for potential Notch pathway targets, the second intron of the numb gene was found to contain a statistically significant cluster of binding sites for Suppressor of Hairless, the transducing transcription factor for the pathway. We show that this region contains a Notch-responsive cis-regulatory module that directs numb transcription in the pIIa and pIIIb cells of the bristle lineage. These are the two precursor cells that do not inherit Numb, yet must make Numb to segregate to one daughter during their own division. Our findings reveal a new mechanism by which conditional and autonomous modes of fate specification are integrated within cell lineages.

Concomitant requirement for Notch and Jak/Stat signaling during neuro-epithelial differentiation in the Drosophila optic lobe

The optic lobe forms a prominent compartment of the Drosophila adult brain that processes visual input from the compound eye. Neurons of the optic lobe are produced during the larval period from two neuroepithelial layers called the outer and inner optic anlage (OOA, IOA). In the early larva, the optic anlagen grow as epithelia by symmetric cell division. Subsequently, neuroepithelial cells (NE) convert into neuroblasts (NB) in a tightly regulated spatio-temporal progression that starts at the edges of the epithelia and gradually move towards its centers. Neuroblasts divide at a much faster pace in an asymmetric mode, producing lineages of neurons that populate the different parts of the optic lobe. In this paper we have reconstructed the complex morphogenesis of the optic lobe during the larval period, and established a role for the Notch and Jak/Stat signaling pathways during the NE-NB conversion. After an early phase of complete overlap in the OOA, signaling activities sort out such that Jak/Stat is active in the lateral OOA which gives rise to the lamina, and Notch remains in the medial cells that form the medulla. During the third instar, a wave front of enhanced Notch activity progressing over the OOA from medial to lateral controls the gradual NE-NB conversion. Neuroepithelial cells at the medial edge of the OOA, shortly prior to becoming neuroblasts, express high levels of Delta, which activates the Notch pathway and thereby maintains the OOA in an epithelial state. Loss of Notch signaling, as well as Jak/Stat signaling, results in a premature NE-NB conversion of the OOA, which in turn has severe effects on optic lobe patterning. Our findings present the Drosophila optic lobe as a useful model to analyze the key signaling mechanisms controlling transitions of progenitor cells from symmetric (growth) to asymmetric (differentiative) divisions.

Drosophila microRNAs 263a/b confer robustness during development by protecting nascent sense organs from apoptosis

miR-263a/b are members of a conserved family of microRNAs that are expressed in peripheral sense organs across the animal kingdom. Here we present evidence that miR-263a and miR-263b play a role in protecting Drosophila mechanosensory bristles from apoptosis by down-regulating the pro-apoptotic gene head involution defective. Both microRNAs are expressed in the bristle progenitors, and despite a difference in their seed sequence, they share this key common target. In miR-263a and miR-263b deletion mutants, loss of bristles appears to be sporadic, suggesting that the role of the microRNAs may be to ensure robustness of the patterning process by promoting survival of these functionally specified cells. In the context of the retina, this mechanism ensures that the interommatidial bristles are protected during the developmentally programmed wave of cell death that prunes excess cells in order to refine the pattern of the pupal retina.

Partial co-option of the appendage patterning pathway in the development of abdominal appendages in the sepsid fly Themira biloba

The abdominal appendages on male Themira biloba (Diptera: Sepsidae) are complex novel structures used during mating. These abdominal appendages superficially resemble the serially homologous insect appendages in that they have a joint and a short segment that can be rotated. Non-genital appendages do not occur in adult pterygote insects, so these abdominal appendages are novel structures with no obvious ancestry. We investigated whether the genes that pattern the serially homologous insect appendages have been co-opted to pattern these novel abdominal appendages. Immunohistochemistry was used to determine the expression patterns of the genes extradenticle (exd), Distal-less (Dll), engrailed (en), Notch, and the Bithorax Complex in the appendages of T. biloba during pupation. The expression patterns of Exd, En, and Notch were consistent with the hypothesis that a portion of the patterning pathway that establishes the coxopodite has been co-opted to pattern the developing abdominal appendages. However, Dll was only expressed in the bristles of the developing appendages and not the proximal-distal axis of the appendage itself. The lack of Dll expression indicates the absence of a distal domain of the appendage suggesting that sepsid abdominal appendages only use genes that normally pattern the base of segmental appendages.

Asymmetric cell divisions and asymmetric cell fates

The regulation of self-renewal, cell diversity, and differentiation can occur by modulating symmetric and asymmetric cell divisions. Remarkably, asymmetric cell divisions can arise through multiple processes in which molecules in the cytoplasm and nucleus, as well as template "immortal" DNA strands, can segregate to one daughter cell during cell division. Explaining how these events direct distinct daughter cell fates is a major challenge to understanding how the organism is assembled and maintained for a lifetime. Numerous technical issues that are associated with assessing how distinct cell fates are executed in vivo have resulted in divergent interpretations of experimental findings. This review addresses some of these points and considers different developmental model systems that attempt to investigate how cell fate decisions are determined, as well as the molecules that guide these choices.

Two themes on the assembly of the Drosophila eye

Cells are sequentially recruited during formation of the Drosophila compound eye. A few simple rules are reiteratively utilized to control successive steps of eye assembly. Two themes emerge: the interplay between cell signaling and competence determines diversity of cell types and selective cell adhesion determines spatial patterns of cells. Cell signaling through competence creates signaling relays, which sequentially trigger differentiation of all cell types. Selective cell adhesion, on the other hand, provides forces to drive cells into energy-favored spatial configurations. Organ formation is nevertheless a complex process. The complexity lies in the spatial, temporal, and quantitative precision of gene expression. Many challenging questions remain.

New roles for Notch in tuberous sclerosis

Tuberous sclerosis complex (TSC) is a dominantly inherited disease that is characterized by the growth of multiple benign tumors that are often difficult to treat. TSC is caused by mutations that inactivate the TSC1 or TSC2 genes, which normally function to inhibit activation of mammalian target of rapamycin signaling. In this issue of the JCI, two studies reported by Karbowniczek et al. and Ma et al. link TSC inactivation with activated Notch signaling (see the related articles beginning on pages 93 and 103, respectively). Using a variety of approaches, both studies show that inactivation of TSC leads to Notch1 activation. Furthermore, studies in tumor cells suggest that inhibiting Notch slows growth of the tumor cells. Although much remains to be learned about the precise mechanisms by which TSC loss leads to Notch activation, the newly identified link of TSC to Notch provides the rationale for testing Notch inhibitors in TSC-associated tumors.

Sanpodo: a context-dependent activator and inhibitor of Notch signaling during asymmetric divisions

Asymmetric cell divisions generate sibling cells of distinct fates ('A', 'B') and constitute a fundamental mechanism that creates cell-type diversity in multicellular organisms. Antagonistic interactions between the Notch pathway and the intrinsic cell-fate determinant Numb appear to regulate asymmetric divisions in flies and vertebrates. During these divisions, productive Notch signaling requires sanpodo, which encodes a novel transmembrane protein. Here, we demonstrate that Drosophila sanpodo plays a dual role to regulate Notch signaling during asymmetric divisions - amplifying Notch signaling in the absence of Numb in the 'A' daughter cell and inhibiting Notch signaling in the presence of Numb in the 'B' daughter cell. In so doing, sanpodo ensures the asymmetry in Notch signaling levels necessary for the acquisition of distinct fates by the two daughter cells. These findings answer long-standing questions about the restricted ability of Numb and Sanpodo to inhibit and to promote, respectively, Notch signaling during asymmetric divisions.

On the roles of Notch, Delta, kuzbanian, and inscuteable during the development of Drosophila embryonic neuroblast lineages

The generation of cellular diversity in the nervous system involves the mechanism of asymmetric cell division. Besides an array of molecules, including the Par protein cassette, a heterotrimeric G protein signalling complex, Inscuteable plays a major role in controlling asymmetric cell division, which ultimately leads to differential activation of the Notch signalling pathway and correct specification of the two daughter cells. In this context, Notch is required to be active in one sibling and inactive in the other. Here, we investigated the requirement of genes previously known to play key roles in sibling cell fate specification such as members of the Notch signalling pathway, e.g., Notch (N), Delta (Dl), and kuzbanian (kuz) and a crucial regulator of asymmetric cell division, inscuteable (insc) throughout lineage progression of 4 neuroblasts (NB1-1, MP2, NB4-2, and NB7-1). Notch-mediated cell fate specification defects were cell-autonomous and were observed in all neuroblast lineages even in cells born from late ganglion mother cells (GMC) within the lineages. We also show that Dl functions non-autonomously during NB lineage progression and clonal cells do not require Dl from within the clone. This suggests that within a NB lineage Dl is dispensable for sibling cell fate specification. Furthermore, we provide evidence that kuz is involved in sibling cell fate specification in the central nervous system. It is cell-autonomously required in the same postmitotic cells which also depend on Notch function. This indicates that KUZ is required to facilitate a functional Notch signal in the Notch-dependent cell for correct cell fate specification. Finally, we show that three neuroblast lineages (NB1-1, NB4-2, and NB7-1) require insc function for sibling cell fate specification in cells born from early GMCs whereas insc is not required in cells born from later GMCs of the same lineages. Thus, there is differential requirement for insc for cell fate specification depending on the stage of lineage progression of NBs.

Complex interplay of three transcription factors in controlling the tormogen differentiation program of Drosophila mechanoreceptors

We have investigated the expression and function of the Sox15 transcription factor during the development of the external mechanosensory organs of Drosophila. We find that Sox15 is expressed specifically in the socket cell, and have identified the transcriptional cis-regulatory module that controls this activity. We show that Suppressor of Hairless [Su(H)] and the POU-domain factor Ventral veins lacking (Vvl) bind conserved sites in this enhancer and provide critical regulatory input. In particular, we find that Vvl contributes to the activation of the enhancer following relief of Su(H)-mediated default repression by the Notch signaling event that specifies the socket cell fate. Loss of Sox15 gene activity was found to severely impair the electrophysiological function of mechanosensory organs, due to both cell-autonomous and cell-non-autonomous effects on the differentiation of post-mitotic cells in the bristle lineage. Lastly, we find that simultaneous loss of both Sox15 and the autoregulatory activity of Su(H) reveals an important role for these factors in inhibiting transcription of the Pax family gene shaven in the socket cell, which serves to prevent inappropriate expression of the shaft differentiation program. Our results indicate that the later phases of socket cell differentiation are controlled by multiple transcription factors in a collaborative, and not hierarchical, manner.

Caenorhabditis elegans num-1 negatively regulates endocytic recycling

Much of the material taken into cells by endocytosis is rapidly returned to the plasma membrane by the endocytic recycling pathway. Although recycling is vital for the correct localization of cell membrane receptors and lipids, the molecular mechanisms that regulate recycling are only partially understood. Here we show that in Caenorhabditis elegans endocytic recycling is inhibited by NUM-1A, the nematode Numb homolog. NUM-1AGFP fusion protein is localized to the baso-lateral surfaces of many polarized epithelial cells, including the hypodermis and the intestine. We show that increased NUM-1A levels cause morphological defects in these cells similar to those caused by loss-of-function mutations in rme-1, a positive regulator of recycling in both C. elegans and mammals. We describe the isolation of worms lacking num-1A activity and show that, consistent with a model in which NUM-1A negatively regulates recycling in the intestine, loss of num-1A function bypasses the requirement for RME-1. Genetic epistasis analysis with rab-10, which is required at an early part of the recycling pathway, suggests that loss of num-1A function does not affect the uptake of material by endocytosis but rather inhibits baso-lateral recycling downstream of rab-10.

LvNumb works synergistically with Notch signaling to specify non-skeletal mesoderm cells in the sea urchin embryo

Activation of the Notch signaling pathway segregates the non-skeletogenic mesoderm (NSM) from the endomesoderm during sea urchin embryo development. Subsequently, Notch signaling helps specify the four subpopulations of NSM, and influences endoderm specification. To gain further insight into how the Notch signaling pathway is regulated during these cell specification events, we identified a sea urchin homologue of Numb (LvNumb). Previous work in other model systems showed that Numb functions as a Notch signaling pathway antagonist, possibly by mediating the endocytosis of other key Notch interacting proteins. In this study, we show that the vegetal endomesoderm expresses lvnumb during the blastula and gastrula stages, and that the protein is localized to the presumptive NSM. Injections of lvnumb mRNA and antisense morpholinos demonstrate that LvNumb is necessary for the specification of mesodermal cell types, including pigment cells, blastocoelar cells and muscle cells. Functional analysis of the N-terminal PTB domain and the C-terminal PRR domain of LvNumb shows that the PTB domain, but not the PRR domain, is sufficient to recapitulate the demonstrable function of full-length LvNumb. Experiments show that LvNumb requires an active Notch signal to function during NSM specification and that LvNumb functions in the cells responding to Delta and not in the cells presenting the Delta ligand. Furthermore, injection of mRNA encoding the intracellular domain of Notch rescues the LvNumb morpholino phenotype, suggesting that the constitutive intracellular Notch signal overcomes, or bypasses, the absence of Numb during NSM specification.

Frizzled/PCP-dependent asymmetric neuralized expression determines R3/R4 fates in the Drosophila eye

Planar cell polarity (PCP) is a common feature in many epithelia, reflected in cellular organization within the plane of an epithelium. In the Drosophila eye, Frizzled (Fz)/PCP signaling induces cell-fate specification of the R3/R4 photoreceptors through regulation of Notch activation in R4. Except for Dl upregulation in R3, the mechanism of how Fz/PCP signaling regulates Notch in this context is not understood. We demonstrate that the E3-ubiquitin ligase Neuralized (Neur), required for Dl-N signaling, is asymmetrically expressed within the R3/R4 pair. It is required in R3, where it is also upregulated in a Fz/PCP-dependent manner. As is the case for Dl, N activity in R4 further represses neur expression, thus, reinforcing the asymmetry. We demonstrate that Neur asymmetry is instructive in correct R3/R4 specification. Our data indicate that Fz/PCP-dependent Neur expression in R3 ensures the proper directionality of Dl-N signaling during R3/R4 specification.

Senseless and Daughterless confer neuronal identity to epithelial cells in the Drosophila wing margin

The basic helix-loop-helix (bHLH) proneural proteins Achaete and Scute cooperate with the class I bHLH protein Daughterless to specify the precursors of most sensory bristles in Drosophila. However, the mechanosensory bristles at the Drosophila wing margin have been reported to be unaffected by mutations that remove Achaete and Scute function. Indeed, the proneural gene(s) for these organs is not known. Here, we show that the zinc-finger transcription factor Senseless, together with Daughterless, plays the proneural role for the wing margin mechanosensory precursors, whereas Achaete and Scute are required for the survival of the mechanosensory neuron and support cells in these lineages. We provide evidence that Senseless and Daughterless physically interact and synergize in vivo and in transcription assays. Gain-of-function studies indicate that Senseless and Daughterless are sufficient to generate thoracic sensory organs (SOs) in the absence of achaete-scute gene complex function. However, analysis of senseless loss-of-function clones in the thorax implicates Senseless not in the primary SO precursor (pI) selection, but in the specification of pI progeny. Therefore, although Senseless and bHLH proneural proteins are employed during the development of all Drosophila bristles, they play fundamentally different roles in different subtypes of these organs. Our data indicate that transcription factors other than bHLH proteins can also perform the proneural function in the Drosophila peripheral nervous system.

Epithelial differentiation in pancreatic development and neoplasia: new niches for nestin and Notch

Changes in epithelial differentiation represent a characteristic early feature of human pancreatic cancer. Recent work suggests that many of these changes may reflect a pathologic recapitulation of developmental events. Reflecting this principle, metaplastic and neoplastic pancreatic epithelium appear to share many features in common with embryonic pancreatic epithelium, including reactivation of the Notch signaling pathway. In this review, we summarize recent studies involving regulation of epithelial differentiation in both embryonic and adult pancreas and highlight the role of Notch in regulating an exocrine progenitor pool.

Notch in lung development and lung cancer

Although data regarding the role of the Notch pathway in human lung cancer are still limited, fetal lung developmental studies suggest that Notch signaling plays a critical role in regulating airway epithelial development. The moderate hypotrophic phenotype of lungs from animals bearing a Hes1 mutation, and the expression of Notch components in the distal lung bud during branching morphogenesis, together suggest that Notch may play a role in normal lung growth, especially in Clara cell precursors. Non-small cell lung cancers, including adenocarcinoma, appear to actively utilize this conserved developmental pathway. Pharmacologic inhibition of the Notch pathway is a potential experimental approach to lung cancer treatment.

tantalus, a potential link between Notch signalling and chromatin-remodelling complexes

The tantalus (tan) gene encodes a protein that interacts specifically with the Polycomb/trithorax group protein Additional sex combs (ASX). Both loss-of-function and gain-of-function mutations in tan cause tissue-specific defects in the eyes, wing veins and bristles of adult flies. As these defects are also typical for components of the Notch (N) signalling pathway, we wished to determine if TAN interacts with this pathway. Through careful examination of ectopic tan phenotypes, we find that TAN specifically disrupts all three major processes associated with the N signalling pathway (boundary formation, lateral inhibition, and lineage decisions). Furthermore, ectopic tan expression abolishes expression of two N target genes, wingless (wg) and cut, at the dorsal-ventral boundary of the wing. An interaction between tan and N was also observed using a genetic assay that previously detected interactions between tan and Asx. The previously observed ability of TAN to move between the cytoplasm and nucleus, and to associate with DNA, provides a potential mechanism for TAN to respond to N signalling.

Control of central synaptic specificity in insect sensory neurons

Synaptic specificity is the culmination of several processes, beginning with the establishment of neuronal subtype identity, followed by navigation of the axon to the correct subdivision of neuropil, and finally, the cell-cell recognition of appropriate synaptic partners. In this review we summarize the work on sensory neurons in crickets, cockroaches, moths, and fruit flies that establishes some of the principles and molecular mechanisms involved in the control of synaptic specificity. The identity of a sensory neuron is controlled by combinatorial expression of transcription factors, the products of patterning and proneural genes. In the nervous system, sensory axon projections are anatomically segregated according to modality, stimulus quality, and cell-body position. A variety of cell-surface and intracellular signaling molecules are used to achieve this. Synaptic target recognition is also controlled by transcription factors such as Engrailed and may be, in part, mediated by cadherin-like molecules.

A tissue specific cytochrome P450 required for the structure and function of Drosophila sensory organs

Cytochrome P450s have generally been acknowledged as broadly tuned detoxifying enzymes. However, emerging evidence argues P450s have an integral role in cell signaling and developmental processes, via their metabolism of retinoic acid, arachidonic acid, steroids, and other cellular ligands. To study the morphogenesis of Drosophila sensory organs, we examined mutants with impaired mechanosensation and discovered one, nompH, encodes the cytochrome P450 CYP303a1. We now report the characterization of nompH, a mutant defective in the function of peripheral chemo- and mechanoreceptor cells, and demonstrate CYP303a1 is essential for the development and structure of external sensory organs which mediate the reception of vital mechanosensory and chemosensory stimuli. Notably this P450 is expressed only in sensory bristles, localizing in the apical region of the socket cell. The wide diversity of the P450 family and the growing number of P450s with developmental phenotypes suggests the exquisite tissue and subcellular specificity of CYP303a1 illustrates an important aspect of P450 function; namely, a strategy to process critical developmental signals in a tissue- and cell-specific manner.

Notch inhibits Ptf1 function and acinar cell differentiation in developing mouse and zebrafish pancreas

Notch signaling regulates cell fate decisions in a variety of adult and embryonic tissues, and represents a characteristic feature of exocrine pancreatic cancer. In developing mouse pancreas, targeted inactivation of Notch pathway components has defined a role for Notch in regulating early endocrine differentiation, but has been less informative with respect to a possible role for Notch in regulating subsequent exocrine differentiation events. Here, we show that activated Notch and Notch target genes actively repress completion of an acinar cell differentiation program in developing mouse and zebrafish pancreas. In developing mouse pancreas, the Notch target gene Hes1 is co-expressed with Ptf1-P48 in exocrine precursor cells, but not in differentiated amylase-positive acinar cells. Using lentiviral delivery systems to induce ectopic Notch pathway activation in explant cultures of E10.5 mouse dorsal pancreatic buds, we found that both Hes1 and Notch1-IC repress acinar cell differentiation, but not Ptf1-P48 expression, in a cell-autonomous manner. Ectopic Notch activation also delays acinar cell differentiation in developing zebrafish pancreas. Further evidence of a role for endogenous Notch in regulating exocrine pancreatic differentiation was provided by examination of zebrafish embryos with homozygous mindbomb mutations, in which Notch signaling is disrupted. mindbomb-deficient embryos display accelerated differentiation of exocrine pancreas relative to wild-type clutchmate controls. A similar phenotype was induced by expression of a dominant-negative Suppressor of Hairless [Su(H)] construct, confirming that Notch actively represses acinar cell differentiation during zebrafish pancreatic development. Using transient transfection assays involving a Ptf1-responsive reporter gene, we further demonstrate that Notch and Notch/Su(H) target genes directly inhibit Ptf1 activity, independent of changes in expression of Ptf1 component proteins. These results define a normal inhibitory role for Notch in the regulation of exocrine pancreatic differentiation.

A hidden program in Drosophila peripheral neurogenesis revealed: fundamental principles underlying sensory organ diversity

How is cell fate diversity reliably achieved during development? Insect sensory organs have been a favorable model system for investigating this question for over 100 years. They are constructed using defined cell lineages that generate a maximum of cell diversity with a minimum number of cell divisions, and display tremendous variety in their morphologies, constituent cell types, and functions. An unexpected realization of the past 5 years is that very diverse sensory organs in Drosophila are produced by astonishingly similar cell lineages, and that their diversity can be largely attributed to only a small repertoire of developmental processes. These include changes in terminal cell differentiation, cell death, cell proliferation, cell recruitment, cell-cell interactions, and asymmetric segregation of cell fate determinants during mitosis. We propose that most Drosophila sensory organs are built from an archetypal lineage, and we speculate about how this stereotyped pattern of cell divisions may have been built during evolution.

Notch signaling: control of cell communication and cell fate

Notch is a transmembrane receptor that mediates local cell-cell communication and coordinates a signaling cascade present in all animal species studied to date. Notch signaling is used widely to determine cell fates and to regulate pattern formation; its dysfunction results in a tremendous variety of developmental defects and adult pathologies. This primer describes the mechanism of Notch signal transduction and how it is used to control the formation of biological patterns.

Notch and Presenilin: regulated intramembrane proteolysis links development and degeneration

Intensive studies of three proteins--Presenilin, Notch, and the amyloid precursor protein (APP)--have led to the recognition of a direct intersection between early development and late-life neurodegeneration. Notch signaling mediates many different intercellular communication events that are essential for determining the fates of neural and nonneural cells during development and in the adult. The Notch receptor acts in a core pathway as a membrane-bound transcription factor that is released to the nucleus by a two-step cleavage mechanism called regulated intramembrane proteolysis (RIP). The second cleavage is effected by Presenilin, an unusual polytopic aspartyl protease that apparently cleaves Notch and numerous other single-transmembrane substrates within the lipid bilayer. Another Presenilin substrate, APP, releases the amyloid ss-protein that can accumulate over time in limbic and association cortices and help initiate Alzheimer's disease. Elucidating the detailed mechanism of Presenilin processing of membrane proteins is important for understanding diverse signal transduction pathways and potentially for treating and preventing Alzheimer's disease.

Numb inhibits membrane localization of Sanpodo, a four-pass transmembrane protein, to promote asymmetric divisions in Drosophila

Cellular diversity is a fundamental characteristic of complex organisms, and the Drosophila CNS has proved an informative paradigm for understanding the mechanisms that create cellular diversity. One such mechanism is the asymmetric localization of Numb to ensure that sibling cells respond differently to the extrinsic Notch signal and, thus, adopt distinct fates (A and B). Here we focus on the only genes known to function specifically to regulate Notch-dependent asymmetric divisions: sanpodo and numb. We demonstrate that sanpodo, which specifies the Notch-dependent fate (A), encodes a four-pass transmembrane protein that localizes to the cell membrane in the A cell and physically interacts with the Notch receptor. We also show that Numb, which inhibits Notch signaling to specify the default fate (B), physically associates with Sanpodo and inhibits Sanpodo membrane localization in the B cell. Our findings suggest a model in which Numb inhibits Notch signaling through the regulation of Sanpodo membrane localization.

Critical role of active repression by E2F and Rb proteins in endoreplication during Drosophila development

E2F transcription factors can activate or actively repress transcription of their target genes. The role of active repression during normal development has not been analyzed in detail. dE2F1(su89) is a novel allele of dE2F1 that disrupts dE2F1's association with RBF [the Drosophila retinoblastoma protein (Rb) homolog] but retains its transcription activation function. Interestingly, the dE2F1(su89) mutant, which has E2F activation by dE2F1(su89) and active repression by dE2F2, is viable and fertile with no gross developmental defects. In contrast, complete removal of active repression in de2f2;dE2F1(su89) mutants results in severe developmental defects in tissues with extensive endocycles but not in tissues derived from mitotic cycles. We show that the endoreplication defect resulted from a failure to downregulate the level of cyclin E during the gap phase of the endocycling cells. Importantly, reducing the gene dosage of cyclin E partially suppressed all the phenotypes associated with the endoreplication defect. These observations point to an important role for E2F-Rb complexes in the downregulation of cyclin E during the gap phase of endocycling cells in Drosophila development.

Default repression and Notch signaling: Hairless acts as an adaptor to recruit the corepressors Groucho and dCtBP to Suppressor of Hairless

The DNA-binding transcription factor Suppressor of Hairless [Su(H)] functions as an activator during Notch (N) pathway signaling, but can act as a repressor in the absence of signaling. Hairless (H), a novel Drosophila protein, binds to Su(H) and has been proposed to antagonize N signaling by inhibiting DNA binding by Su(H). Here we show that, in vitro, H directly binds two corepressor proteins, Groucho (Gro) and dCtBP. Reduction of gro or dCtBP function enhances H mutant phenotypes and suppresses N phenotypes in the adult mechanosensory bristle. This activity of gro is surprising, because it is directed oppositely to its traditionally defined role as a neurogenic gene. We find that Su(H)-H complexes can bind to DNA with high efficiency in vitro. Furthermore, a H-VP16 fusion protein causes dominant-negative phenotypes in vivo, a result consistent with the proposal that H functions in transcriptional repression. Taken together, our findings indicate that "default repression" of N pathway target genes by an unusual adaptor/corepressor complex is essential for proper cell fate specification during Drosophila peripheral nervous system development.

Numb: "Adapting" notch for endocytosis

During sensory organ precursor divisions in Drosophila, the numb gene product segregates asymmetrically into one of the two daughter cells, to which it confers a specific fate by inhibiting Notch signaling. In this issue of Developmental Cell, Berdnik et al. show that Numb recruits alpha-Adaptin and that this physical interaction plays a role in downregulating Notch, presumably by stimulating endocytosis of Notch.

Context-dependent utilization of Notch activity in Drosophila glial determination

During Drosophila neurogenesis, glial differentiation depends on the expression of glial cells missing (gcm). Understanding how glial fate is achieved thus requires knowledge of the temporal and spatial control mechanisms directing gcm expression. A recent report showed that in the adult bristle lineage, gcm expression is negatively regulated by Notch signaling (Van De Bor, V. and Giangrande, A. (2001). Development128, 1381-1390). Here we show that the effect of Notch activation on gliogenesis is context-dependent. In the dorsal bipolar dendritic (dbd) sensory lineage in the embryonic peripheral nervous system (PNS), asymmetric cell division of the dbd precursor produces a neuron and a glial cell, where gcm expression is activated in the glial daughter. Within the dbd lineage, Notch is specifically activated in one of the daughter cells and is required for gcm expression and a glial fate. Thus Notch activity has opposite consequences on gcm expression in two PNS lineages. Ectopic Notch activation can direct gliogenesis in a subset of embryonic PNS lineages, suggesting that Notch-dependent gliogenesis is supported in certain developmental contexts. We present evidence that POU-domain protein Nubbin/PDM-1 is one of the factors that provide such context.

A comparative study of odorant binding protein genes: differential expression of the PBP1-GOBP2 gene cluster inManduca sexta(Lepidoptera) and the organization of OBP genes inDrosophila melanogaster(Diptera)

Insects discriminate odors using sensory organs called olfactory sensilla, which display a wide range of phenotypes. Sensilla express ensembles of proteins, including odorant binding proteins (OBPs), olfactory receptors (ORs) and odor degrading enzymes (ODEs); odors are thought to be transported to ORs by OBPs and subsequently degraded by ODEs. These proteins belong to multigene families. The unique combinatorial expression of specific members of each of these gene families determines, in part, the phenotype of a sensillum and what odors it can detect. Furthermore, OBPs, ORs and ODEs are expressed in different cell types, suggesting the need for cell–cell communication to coordinate their expression. This report examines the OBP gene family. In Manduca sexta, the genes encoding PBP1Msex and GOBP2Msex are sequenced, shown to be adjacent to one another, and characterized together with OBP gene structures of other lepidoptera and Drosophila melanogaster. Expression of PBP1Msex, GOBP1Msex and GOBP2Msex is characterized in adult male and female antenna and in larval antenna and maxilla. The genomic organization of 25 D. melanogaster OBPs are characterized with respect to gene locus, gene cluster, amino acid sequence similarity, exon conservation and proximity to OR loci, and their sequences are compared with 14 M. sexta OBPs. Sensilla serve as portals of important behavioral information, and genes supporting sensilla function are presumably under significant evolutionary selective pressures. This study provides a basis for studying the evolution of the OBP gene family, the regulatory mechanisms governing the coordinated expression of OBPs, ORs and ODEs, and the processes that determine specific sensillum phenotypes.

A misexpression study examining dorsal thorax formation in Drosophila melanogaster

We studied thorax formation in Drosophila melanogaster using a misexpression screen with EP lines and thoracic Gal4 drivers that provide a genetically sensitized background. We identified 191 interacting lines showing alterations of thoracic bristles (number and/or location), thorax and scutellum malformations, lethality, or suppression of the thoracic phenotype used in the screen. We analyzed these lines and showed that known genes with different functional roles (selector, prepattern, proneural, cell cycle regulation, lineage restriction, signaling pathways, transcriptional control, and chromatin organization) are among the modifier lines. A few lines have previously been identified in thorax formation, but others, such as chromatin-remodeling complex genes, are novel. However, most of the interacting loci are uncharacterized, providing a wealth of new genetic data. We also describe one such novel line, poco pelo (ppo), where both misexpression and loss-of-function phenotypes are similar: loss of bristles and scutellum malformation.

Requirement of Notch 1 and its ligand jagged 2 expressions for spermatogenesis in rat and human testes

It has already been demonstrated that the Notch signaling system is essential for gametogenesis in the adult germ line of Caenorhabditis elegans. However, the role of the Notch signaling system in mammalian spermatogenesis has not been well investigated. Recently, it has been revealed that this signaling system is expressed in the mammalian testis by showing coexpression of Jagged 2 and its receptor, Notch 1, is consistent with Notch 1 being a cognate receptor for Jagged 2 in the mammalian testis. Therefore, we investigated expressions of messenger RNAs of Notch 1 and Jagged 2 in the testicular tissues of developing Sprague-Dawley rats by reverse transcription-polymerase chain reaction and Northern blot analysis, expressions of their proteins in the testicular tissues of developing rats, fertile human controls and infertile human patients with maturation arrest by immunohistochemistry, and effects of antibodies to this system by culturing rat testicular tissues with these antibodies. Transcripts of Notch 1 and Jagged 2 in the rat testis were positive throughout the examined period; these intensities became higher at day 13 after birth, coincidence with the formation of spermatocytes, and peaked at day 19 after birth. Expressions of Notch 1 and Jagged 2 were recognized at first in the perinuclear regions of spermatocytes in the rat testis as a round structure at day 19 after birth and thereafter in further differentiated germ cells as meiosis proceeded. In the adult rat testis, positive staining was present as a round structure in spermatocytes, as a typical horseshoe-shaped structure in round spermatids, and as a covering structure spreading around the nucleus of elongated spermatids, but not in spermatozoa. Notch 1 was recognized in the vacuole of the Golgi complex of primary spermatocytes and the acrosome of elongated spermatids with electron microscopy. When rat testicular tissues were cultured with anti-Notch 1 or anti-Jagged 2 antibody, round and elongated spermatids decreased after 5 and 7 days of culture, respectively, and disappeared at around 9 and 12 days of culture, respectively, with shrinkage of the diameter of seminiferous tubules. Spermatocytes, however, increased after 11 days of culture. Expressions of both proteins have been detected in the testicular tissues of human fertile controls as in the rat testicular tissues. However, Notch 1 expression has not been detected in testicular tissues of 11 patients with maturation arrest, whereas Jagged 2 expression has been recognized in all of them. In conclusion, the results presented in this study offer the possibility that Notch 1/Jagged 2 signaling system plays an important role for male germ cells to differentiate or at least to survive in the rat testis and fails to express in the testis of spermatogenic maturation arrest patients.

Lineage diversity in the Drosophila nervous system

The detailed descriptions of cellular lineages in the Drosophila nervous system have provided the foundations for an in-depth genetic analysis of the mechanisms that regulate fate decisions at every cell cycle.

Neurogenesis in the spiderCupiennius salei

To uncover similarities and differences in neurogenesis in arthropod groups, we have studied the ventral neuroectoderm of the spider Cupiennius salei (Chelicerata, Aranea, Ctenidae). We found that invaginating cell groups arose sequentially, at stereotyped positions in each hemisegment and in separate waves, comparable with the generation of neuroblasts in Drosophila. However, we found no evidence for proliferating stem cells that would be comparable with the neuroblasts. Instead, the whole group of invaginating cells was directly recruited to the nervous system. The invagination process is comparable with Drosophila, with the cells attaining a bottle-shaped form with the nuclei moving inwards, while actin-rich cell processes remain initially connected to the surface of the epithelium. This general pattern is also found in another spider, Pholcus phalangioides, and appears thus to be conserved at least among the Araneae. We have identified two basic helix-loop-helix encoding genes – CsASH1 and CsASH2 – that share sequence similarities with proneural genes from other species. Functional analysis of the genes by double-stranded RNA interference revealed that CsASH1 was required for the formation of the invagination sites and the process of invagination itself, whereas CsASH2 seemed to be required for the differentiation of the cells into neurones. Our results suggest that the basic processes of neurogenesis, as well as proneural gene function is conserved among arthropods, apart of the lack of neuroblast-like stem cells in spiders.