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

Experience-dependent serotonergic signaling in glia regulates targeted synapse elimination

The optimization of brain circuit connectivity based on initial environmental input occurs during critical periods characterized by sensory experience-dependent, temporally restricted, and transiently reversible synapse elimination. This precise, targeted synaptic pruning mechanism is mediated by glial phagocytosis. Serotonin signaling has prominent, foundational roles in the brain, but functions in glia, or in experience-dependent brain circuit synaptic connectivity remodeling, have been relatively unknown. Here, we discover that serotonergic signaling between glia is essential for olfactory experience-dependent synaptic glomerulus pruning restricted to a well-defined Drosophila critical period. We find that experience-dependent serotonin signaling is restricted to the critical period, with both (1) serotonin production and (2) 5-HT2A receptors specifically in glia, but not neurons, absolutely required for targeted synaptic glomerulus pruning. We discover that glial 5-HT2A receptor signaling limits the experience-dependent synaptic connectivity pruning in the critical period and that conditional reexpression of 5-HT2A receptors within adult glia reestablishes "critical period-like" experience-dependent synaptic glomerulus pruning at maturity. These results reveal an essential requirement for glial serotonergic signaling mediated by 5-HT2A receptors for experience-dependent synapse elimination.

Neurobiology of egg-laying behavior in Drosophila: neural control of the female reproductive system

Egg-laying is one of the key aspects of female reproductive behavior in insects. Egg-laying has been studied since the dawn of Drosophila melanogaster as a model organism. The female's internal state, hormones, and external factors, such as nutrition, light, and social environment, affect egg-laying output. However, only recently, neurobiological features of egg-laying behavior have been studied in detail. fruitless and doublesex, two key players in the sex determination pathway, have become focal points in identifying neurons of reproductive significance in both central and peripheral nervous systems. The reproductive tract and external terminalia house sensory neurons that carry the sensory information of egg maturation, mating and egg-laying. These sensory signals include the presence of male accessory gland products and mechanical stimuli. The abdominal neuromere houses neurons that receive information from the reproductive tract, including sex peptide abdominal ganglion neurons (SAGs), and send their information to the brain. In the brain, neuronal groups like aDNs and pC1 clusters modulate egg-laying decision-making, and other neurons like oviINs and oviDNs are necessary for egg-laying itself. Lastly, motor neurons involved in egg-laying, which are mostly octopaminergic, reside in the abdominal neuromere and orchestrate the muscle movements required for laying the egg. Egg-laying neuronal control is important in various evolutionary processes like cryptic female choice, and using different Drosophila species can provide intriguing avenues for the future of the field.

Actin-related protein 6 facilitates proneural protein-induced gene activation for rapid neural differentiation

Neurogenesis is initiated by basic helix-loop-helix proneural proteins. Here, we show that Actin-related protein 6 (Arp6), a core component of the H2A.Z exchange complex SWR1, interacts with proneural proteins and is crucial for efficient onset of proneural protein target gene expression. Arp6 mutants exhibit reduced transcription in sensory organ precursors (SOPs) downstream of the proneural protein patterning event. This leads to retarded differentiation and division of SOPs and smaller sensory organs. These phenotypes are also observed in proneural gene hypomorphic mutants. Proneural protein expression is not reduced in Arp6 mutants. Enhanced proneural gene expression fails to rescue retarded differentiation in Arp6 mutants, suggesting that Arp6 acts downstream of or in parallel with proneural proteins. H2A.Z mutants display Arp6-like retardation in SOPs. Transcriptomic analyses demonstrate that loss of Arp6 and H2A.Z preferentially decreases expression of proneural protein-activated genes. H2A.Z enrichment in nucleosomes around the transcription start site before neurogenesis correlates highly with greater activation of proneural protein target genes by H2A.Z. We propose that upon proneural protein binding to E-box sites, H2A.Z incorporation around the transcription start site allows rapid and efficient activation of target genes, promoting rapid neural differentiation.

Single-cell Senseless protein analysis reveals metastable states during the transition to a sensory organ fate

Cell fate decisions can be envisioned as bifurcating dynamical systems, and the decision that Drosophila cells make during sensory organ differentiation has been described as such. We extended these studies by focusing on the Senseless protein which orchestrates sensory cell fate transitions. Wing cells contain intermediate Senseless numbers before their fate transition, after which they express much greater numbers of Senseless molecules as they differentiate. However, the dynamics are inconsistent with it being a simple bistable system. Cells with intermediate Senseless are best modeled as residing in four discrete states, each with a distinct protein number and occupying a specific region of the tissue. Although the states are stable over time, the number of molecules in each state vary with time. The fold change in molecule number between adjacent states is invariant and robust to absolute protein number variation. Thus, cells transitioning to sensory fates exhibit metastability with relativistic properties.

ASC proneural factors are necessary for chromatin remodeling during neuroectodermal to neuroblast fate transition to ensure the timely initiation of the neural stem cell program

Background

In both Drosophila and mammals, the achaete-scute (ASC/ASCL) proneural bHLH transcription factors are expressed in the developing central and peripheral nervous systems, where they function during specification and maintenance of the neural stem cells in opposition to Notch signaling. In addition to their role in nervous system development, ASC transcription factors are oncogenic and exhibit chromatin reprogramming activity; however, the impact of ASC on chromatin dynamics during neural stem cell generation remains elusive. Here, we investigate the chromatin changes accompanying neural commitment using an integrative genetics and genomics methodology.

Results

We found that ASC factors bind equally strongly to two distinct classes of cis-regulatory elements: open regions remodeled earlier during maternal to zygotic transition by Zelda and less accessible, Zelda-independent regions. Both classes of cis-elements exhibit enhanced chromatin accessibility during neural specification and correlate with transcriptional regulation of genes involved in a variety of biological processes necessary for neuroblast function/homeostasis. We identified an ASC-Notch regulated TF network that includes likely prime regulators of neuroblast function. Using a cohort of ASC target genes, we report that ASC null neuroblasts are defectively specified, remaining initially stalled, unable to divide, and lacking expression of many proneural targets. When mutant neuroblasts eventually start proliferating, they produce compromised progeny. Reporter lines driven by proneural-bound enhancers display ASC dependency, suggesting that the partial neuroblast identity seen in the absence of ASC genes is likely driven by other, proneural-independent, cis-elements. Neuroblast impairment and the late differentiation defects of ASC mutants are corrected by ectodermal induction of individual ASC genes but not by individual members of the TF network downstream of ASC. However, in wild-type embryos, the induction of individual members of this network induces CNS hyperplasia, suggesting that they synergize with the activating function of ASC to consolidate the chromatin dynamics that promote neural specification.

Conclusions

We demonstrate that ASC proneural transcription factors are indispensable for the timely initiation of the neural stem cell program at the chromatin level by regulating a large number of enhancers in the vicinity of neural genes. This early chromatin remodeling is crucial for both neuroblast homeostasis as well as future progeny fidelity.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-022-01300-8.

The wing imaginal disc

The Drosophila wing imaginal disc is a tissue of undifferentiated cells that are precursors of the wing and most of the notum of the adult fly. The wing disc first forms during embryogenesis from a cluster of ∼30 cells located in the second thoracic segment, which invaginate to form a sac-like structure. They undergo extensive proliferation during larval stages to form a mature larval wing disc of ∼35,000 cells. During this time, distinct cell fates are assigned to different regions, and the wing disc develops a complex morphology. Finally, during pupal stages the wing disc undergoes morphogenetic processes and then differentiates to form the adult wing and notum. While the bulk of the wing disc comprises epithelial cells, it also includes neurons and glia, and is associated with tracheal cells and muscle precursor cells. The relative simplicity and accessibility of the wing disc, combined with the wealth of genetic tools available in Drosophila, have combined to make it a premier system for identifying genes and deciphering systems that play crucial roles in animal development. Studies in wing imaginal discs have made key contributions to many areas of biology, including tissue patterning, signal transduction, growth control, regeneration, planar cell polarity, morphogenesis, and tissue mechanics.

Arf6 is necessary for senseless expression in response to wingless signalling during Drosophila wing development

Wnt signalling is a core pathway involved in a wide range of developmental processes throughout the metazoa. In vitro studies have suggested that the small GTP binding protein Arf6 regulates upstream steps of Wnt transduction, by promoting the phosphorylation of the Wnt co-receptor, LRP6, and the release of β-catenin from the adherens junctions. To assess the relevance of these previous findings in vivo, we analysed the consequence of the absence of Arf6 activity on Drosophila wing patterning, a developmental model of Wnt/Wingless signalling. We observed a dominant loss of wing margin bristles and Senseless expression in Arf6 mutant flies, phenotypes characteristic of a defect in high level Wingless signalling. In contrast to previous findings, we show that Arf6 is required downstream of Armadillo/β-catenin stabilisation in Wingless signal transduction. Our data suggest that Arf6 modulates the activity of a downstream nuclear regulator of Pangolin activity in order to control the induction of high level Wingless signalling. Our findings represent a novel regulatory role for Arf6 in Wingless signalling.

Cost-precision trade-off relation determines the optimal morphogen gradient for accurate biological pattern formation

Spatial boundaries formed during animal development originate from the pre-patterning of tissues by signaling molecules, called morphogens. The accuracy of boundary location is limited by the fluctuations of morphogen concentration that thresholds the expression level of target gene. Producing more morphogen molecules, which gives rise to smaller relative fluctuations, would better serve to shape more precise target boundaries; however, it incurs more thermodynamic cost. In the classical diffusion-depletion model of morphogen profile formation, the morphogen molecules synthesized from a local source display an exponentially decaying concentration profile with a characteristic length λ. Our theory suggests that in order to attain a precise profile with the minimal cost, λ should be roughly half the distance to the target boundary position from the source. Remarkably, we find that the profiles of morphogens that pattern the Drosophila embryo and wing imaginal disk are formed with nearly optimal λ. Our finding underscores the cost-effectiveness of precise morphogen profile formation in Drosophila development.

A unique histone 3 lysine 14 chromatin signature underlies tissue-specific gene regulation

Organismal development and cell differentiation critically depend on chromatin state transitions. However, certain developmentally regulated genes lack histone 3 lysine 9 and 27 acetylation (H3K9ac and H3K27ac, respectively) and histone 3 lysine 4 (H3K4) methylation, histone modifications common to most active genes. Here we describe a chromatin state featuring unique histone 3 lysine 14 acetylation (H3K14ac) peaks in key tissue-specific genes in Drosophila and human cells. Replacing H3K14 in Drosophila demonstrates that H3K14 is essential for expression of genes devoid of canonical histone modifications in the embryonic gut and larval wing imaginal disc, causing lethality and defective wing patterning. We find that the SWI/SNF protein Brahma (Brm) recognizes H3K14ac, that brm acts in the same genetic pathway as H3K14R, and that chromatin accessibility at H3K14ac-unique genes is decreased in H3K14R mutants. Our results show that acetylation of a single lysine is essential at genes devoid of canonical histone marks and uncover an important requirement for H3K14 in tissue-specific gene regulation.

Single-cell visualization of mir-9a and Senseless co-expression during Drosophila melanogaster embryonic and larval peripheral nervous system development

The Drosophila melanogaster peripheral nervous system (PNS) comprises the sensory organs that allow the fly to detect environmental factors such as temperature and pressure. PNS development is a highly specified process where each sensilla originates from a single sensory organ precursor (SOP) cell. One of the major genetic orchestrators of PNS development is Senseless, which encodes a zinc finger transcription factor (Sens). Sens is both necessary and sufficient for SOP differentiation. Senseless expression and SOP number are regulated by the microRNA miR-9a. However, the reciprocal dynamics of Senseless and miR-9a are still obscure. By coupling single-molecule FISH with immunofluorescence, we are able to visualize transcription of the mir-9a locus and expression of Sens simultaneously. During embryogenesis, we show that the expression of mir-9a in SOP cells is rapidly lost as Senseless expression increases. However, this mutually exclusive expression pattern is not observed in the third instar imaginal wing disc, where some Senseless-expressing cells show active sites of mir-9a transcription. These data challenge and extend previous models of Senseless regulation and show complex co-expression dynamics between mir-9a and Senseless. The differences in this dynamic relationship between embryonic and larval PNS development suggest a possible switch in miR-9a function. Our work brings single-cell resolution to the understanding of dynamic regulation of PNS development by Senseless and miR-9a.

Crucial role of framework with cytoskeletal actin filaments for shaping microstructure of footpad setae in the ladybird beetle, Harmonia axyridis

Insects that can walk on smooth surfaces have specialized structures, footpads, on their legs. Footpads play an important role in adhesion to the substrate surface. Although the morphology and function of footpads have been studied, the mechanism of their formation is still elusive. In the ladybird beetle (Harmonia axyridis), hairy footpads are present on the first and second tarsal segments of the legs. The footpads are covered with hundreds of hairs, i.e. setae, whose tips consist of four types: pointed, lanceolate, spatular, and discoidal. We examined the formation of the footpad during the pupal stage using immuno-staining and fluorescent-conjugated phalloidin staining. We found that a seta was composed of a shaft and a socket and some setae were accompanied by a neuron. By the mid-pupal stages, the shaft cells elongated to form a setal structure. Cytoskeletal actin bundles ramified to create a framework for the setal tip structure of the cells. We examined the effects of the application of cytochalasin D, which inhibits actin polymerization, on the formation of footpad setal structures. The results showed that the setal tips were deformed by the inhibition of actin polymerization. Our observations reveal that cytoskeletal actin filaments are involved in shaping the setae.

The Wg and Dpp morphogens regulate gene expression by modulating the frequency of transcriptional bursts

Morphogen signaling contributes to the patterned spatiotemporal expression of genes during development. One mode of regulation of signaling-responsive genes is at the level of transcription. Single-cell quantitative studies of transcription have revealed that transcription occurs intermittently, in bursts. Although the effects of many gene regulatory mechanisms on transcriptional bursting have been studied, it remains unclear how morphogen gradients affect this dynamic property of downstream genes. Here we have adapted single molecule fluorescence in situ hybridization (smFISH) for use in the Drosophila wing imaginal disc in order to measure nascent and mature mRNA of genes downstream of the Wg and Dpp morphogen gradients. We compared our experimental results with predictions from stochastic models of transcription, which indicated that the transcription levels of these genes appear to share a common method of control via burst frequency modulation. Our data help further elucidate the link between developmental gene regulatory mechanisms and transcriptional bursting.

Ordered patterning of the sensory system is susceptible to stochastic features of gene expression

Sensory neuron numbers and positions are precisely organized to accurately map environmental signals in the brain. This precision emerges from biochemical processes within and between cells that are inherently stochastic. We investigated impact of stochastic gene expression on pattern formation, focusing on senseless (sens), a key determinant of sensory fate in Drosophila. Perturbing microRNA regulation or genomic location of sens produced distinct noise signatures. Noise was greatly enhanced when both sens alleles were present in homologous loci such that each allele was regulated in trans by the other allele. This led to disordered patterning. In contrast, loss of microRNA repression of sens increased protein abundance but not sensory pattern disorder. This suggests that gene expression stochasticity is a critical feature that must be constrained during development to allow rapid yet accurate cell fate resolution.

Extramacrochaetae promotes branch and bouton number via the sequestration of daughterless in the cytoplasm of neurons

The Class I basic helix-loop-helix (bHLH) proteins are highly conserved transcription factors that are ubiquitously expressed. A wealth of literature on Class I bHLH proteins has shown that these proteins must homodimerize or heterodimerize with tissue-specific HLH proteins in order to bind DNA at E-box consensus sequences to control tissue-specific transcription. Due to its ubiquitous expression, Class I bHLH proteins are also extensively regulated posttranslationally, mostly through dimerization. Previously, we reported that in addition to its role in promoting neurogenesis, the Class I bHLH protein daughterless also functions in mature neurons to restrict axon branching and synapse number. Here, we show that part of the molecular logic that specifies how daughterless functions in neurogenesis is also conserved in neurons. We show that the Type V HLH protein extramacrochaetae (Emc) binds to and represses daughterless function by sequestering daughterless to the cytoplasm. This work provides initial insights into the mechanisms underlying the function of daughterless and Emc in neurons while providing a novel understanding of how Emc functions to restrict daughterless activity within the cell.

Exocyst-mediated apical Wg secretion activates signaling in the Drosophila wing epithelium

Wnt proteins are secreted signaling factors that regulate cell fate specification and patterning decisions throughout the animal kingdom. In the Drosophila wing epithelium, Wingless (Wg, the homolog of Wnt1) is secreted from a narrow strip of cells at the dorsal-ventral boundary. However, the route of Wg secretion in polarized epithelial cells remains poorly understood and key proteins involved in this process are still unknown. Here, we performed an in vivo RNAi screen and identified members of the exocyst complex to be required for apical but not basolateral Wg secretion. Specifically blocking the apical Wg secretion leads to reduced downstream signaling. Using an in vivo ‘temporal-rescue’ assay, our results further indicate that apically secreted Wg activates target genes that require high signaling activity. In conclusion, our results demonstrate that the exocyst is required for an apical route of Wg secretion from polarized wing epithelial cells.

Regulation of Wnt signaling and the production of Wnt ligands is crucial for proper development and homeostasis, as dysregulation leads to developmental defects and diseases such as cancer. This study addresses the question of how functional Wnt ligands are secreted by epithelial cells. By using the polarized epithelium of the developing Drosophila wing as a model system to study Wnt/Wg secretion, the authors performed a large-scale RNAi screen and identified proteins of the exocyst complex to be required for Wnt signaling. The study shows that exocyst complex preferentially regulates apical secretion of Wg proteins. Taken together, this study identifies routes and regulators for secretion of signaling-active Wnt proteins from polarized epithelial cells.

Repressive Gene Regulation Synchronizes Development with Cellular Metabolism

Metabolic conditions affect the developmental tempo of animals. Developmental gene regulatory networks (GRNs) must therefore synchronize their dynamics with a variable timescale. We find that layered repression of genes couples GRN output with variable metabolism. When repressors of transcription or mRNA and protein stability are lost, fewer errors in Drosophila development occur when metabolism is lowered. We demonstrate the universality of this phenomenon by eliminating the entire microRNA family of repressors and find that development to maturity can be largely rescued when metabolism is reduced. Using a mathematical model that replicates GRN dynamics, we find that lowering metabolism suppresses the emergence of developmental errors by curtailing the influence of auxiliary repressors on GRN output. We experimentally show that gene expression dynamics are less affected by loss of repressors when metabolism is reduced. Thus, layered repression provides robustness through error suppression and may provide an evolutionary route to a shorter reproductive cycle.

ADAMTS Sol narae cleaves extracellular Wingless to generate a novel active form that regulates cell proliferation in Drosophila

Wnt/ Wingless (Wg) is essential for embryonic development and adult homeostasis in all metazoans, but the mechanisms by which secreted Wnt/Wg is processed remain largely unknown. A Drosophila Sol narae (Sona) is a member of ADisintegrin And Metalloprotease with ThromboSpondin motif (ADAMTS) family, and positively regulates Wg signaling by promoting Wg secretion. Here we report that Sona and Wg are secreted by both conventional Golgi and exosomal transports, and Sona cleaves extracellular Wg at the two specific sites, leading to the generation of N-terminal domain (NTD) and C-terminal domain (CTD) fragments. The cleaved forms of extracellular Wg were detected in the extracellular region of fly wing discs, and its level was substantially reduced in sona mutants. Transient overexpression of Wg-CTD increased wing size while prolonged overexpression caused lethality and developmental defects. In contrast, Wg-NTD did not induce any phenotype. Moreover, the wing defects and lethality induced by sona RNAi were considerably rescued by Wg-CTD, indicating that a main function of extracellular Sona is the generation of Wg-CTD. Wg-CTD stabilized cytoplasmic Armadillo (Arm) and had genetic interactions with components of canonical Wg signaling. Wg-CTD also induced Wg downstream targets such as Distal-less (Dll) and Vestigial (Vg). Most importantly, Cyclin D (Cyc D) was induced by Wg-CTD but not by full-length Wg. Because Sona also induces Cyc D in a cell non-autonomous manner, Wg-CTD generated by Sona in the extracellular region activates a subset of Wg signaling whose major function is the regulation of cell proliferation.

Destruction complex dynamics: Wnt/β-catenin signaling alters Axin-GSK3β interactions in vivo

The central regulator of the Wnt/β-catenin pathway is the Axin/APC/GSK3β destruction complex (DC), which, under unstimulated conditions, targets cytoplasmic β-catenin for degradation. How Wnt activation inhibits the DC to permit β-catenin-dependent signaling remains controversial, in part because the DC and its regulation have never been observed in vivo Using bimolecular fluorescence complementation (BiFC) methods, we have now analyzed the activity of the DC under near-physiological conditions in Drosophila By focusing on well-established patterns of Wnt/Wg signaling in the developing Drosophila wing, we have defined the sequence of events by which activated Wnt receptors induce a conformational change within the DC, resulting in modified Axin-GSK3β interactions that prevent β-catenin degradation. Surprisingly, the nucleus is surrounded by active DCs, which principally control the degradation of β-catenin and thereby nuclear access. These DCs are inactivated and removed upon Wnt signal transduction. These results suggest a novel mechanistic model for dynamic Wnt signal transduction in vivo.

Transcriptional and post-transcriptional regulation of extra macrochaetae during Drosophila adult peripheral neurogenesis

Regulation of the Drosophila ID protein Extra macrochaetae (Emc) is important because reduced Emc levels have been proposed to favor proneural gene activity and thereby define a prepattern for neurogenesis. Recent studies suggest a major role for post-translational control of Emc levels. To further define the mechanisms of Emc regulation, we identified two redundant cis-regulatory regions by germline transformation-rescue experiments that make use of new molecularly-defined emc mutants. We distinguished the mechanisms by which Daughterless (Da) regulated Emc expression, finding post-translational regulation in most tissues, and additional transcriptional regulation in the eye imaginal disc posterior to the morphogenetic furrow. Dpp and Hh signaling pathways repressed Emc transcriptionally and post-translationally within the morphogenetic furrow of the eye disc, whereas Wg signaling repressed Emc expression at the anterior margin of the wing imaginal disc. Although the emc 3' UTR is potentially regulatory, no effect of miRNA pathways on Emc protein levels was discernible. Our work supports recent evidence that post-transcriptional mechanisms contribute more to regulation of Emc protein levels than transcriptional mechanisms do.

Using Drosophila to study mechanisms of hereditary hearing loss

Johnston's organ - the hearing organ of Drosophila - has a very different structure and morphology to that of the hearing organs of vertebrates. Nevertheless, it is becoming clear that vertebrate and invertebrate auditory organs share many physiological, molecular and genetic similarities. Here, we compare the molecular and cellular features of hearing organs in Drosophila with those of vertebrates, and discuss recent evidence concerning the functional conservation of Usher proteins between flies and mammals. Mutations in Usher genes cause Usher syndrome, the leading cause of human deafness and blindness. In Drosophila, some Usher syndrome proteins appear to physically interact in protein complexes that are similar to those described in mammals. This functional conservation highlights a rational role for Drosophila as a model for studying hearing, and for investigating the evolution of auditory organs, with the aim of advancing our understanding of the genes that regulate human hearing and the pathogenic mechanisms that lead to deafness.

WNK Kinases in Development and Disease

WNK (With-No-Lysine (K)) kinases are serine-threonine kinases characterized by an atypical placement of a catalytic lysine within the kinase domain. Mutations in human WNK1 or WNK4 cause an autosomal dominant syndrome of hypertension and hyperkalemia, reflecting the fact that WNK kinases are critical regulators of renal ion transport processes. Here, the role of WNKs in the regulation of ion transport processes in vertebrate and invertebrate renal function, cellular and organismal osmoregulation, and cell migration and cerebral edema will be reviewed, along with emerging literature demonstrating roles for WNKs in cardiovascular and neural development, Wnt signaling, and cancer. Conserved roles for these kinases across phyla are emphasized.

A re-examination of the selection of the sensory organ precursor of the bristle sensilla of Drosophila melanogaster

The bristle sensillum of the imago of Drosophila is made of four cells that arise from a sensory organ precursor cell (SOP). This SOP is selected within proneural clusters (PNC) through a mechanism that involves Notch signalling. PNCs are defined through the expression domains of the proneural genes, whose activities enables cells to become SOPs. They encode tissue specific bHLH proteins that form functional heterodimers with the bHLH protein Daughterless (Da). In the prevailing lateral inhibition model for SOP selection, a transcriptional feedback loop that involves the Notch pathway amplifies small differences of proneural activity between cells of the PNC. As a result only one or two cells accumulate sufficient proneural activity to adopt the SOP fate. Most of the experiments that sustained the prevailing lateral inhibition model were performed a decade ago. We here re-examined the selection process using recently available reagents. Our data suggest a different picture of SOP selection. They indicate that a band-like region of proneural activity exists. In this proneural band the activity of the Notch pathway is required in combination with Emc to define the PNCs. We found a sub-group in the PNCs from which a pre-selected SOP arises. Our data indicate that most imaginal disc cells are able to adopt a proneural state from which they can progress to become SOPs. They further show that bristle formation can occur in the absence of the proneural genes if the function of emc is abolished. These results suggest that the tissue specific proneural proteins of Drosophila have a similar function as in the vertebrates, which is to determine the time of emergence and position of the SOP and to stabilise the proneural state.

The sensory organ precursor cell (SOP) that forms the mechanosensory bristles of the adult PNS of Drosophila is a paradigm to study neural precursor determination. The current model states that the SOP is selected in proneural clusters (PNCs) defined through the expression of the proneural genes. The selection occurs through lateral inhibition mediated by the Notch signalling pathway. The SOP is pre-selected by differential expression of Extramacrochaetae (Emc), the only member of the Id proteins in Drosophila, which inactivates the proneural factors. We have re-examined the selection process using novel markers and mutants. Our data suggest a different picture of SOP selection. We discovered a band–like region of varying proneural activity where the peaks constitute the proneural clusters. Within the PNC, a subgroup exists from which the SOP arises. The Notch pathway has two distinct functions in the subgroup and in the rest of the band. We show that so far one unappreciated essential role of the proneural genes is the neutralisation of the activity of Emc. Our data suggest that the selection of the SOP is more similar to neural selection in vertebrates than previously anticipated.

The COP9 signalosome converts temporal hormone signaling to spatial restriction on neural competence

During development, neural competence is conferred and maintained by integrating spatial and temporal regulations. The Drosophila sensory bristles that detect mechanical and chemical stimulations are arranged in stereotypical positions. The anterior wing margin (AWM) is arrayed with neuron-innervated sensory bristles, while posterior wing margin (PWM) bristles are non-innervated. We found that the COP9 signalosome (CSN) suppresses the neural competence of non-innervated bristles at the PWM. In CSN mutants, PWM bristles are transformed into neuron-innervated, which is attributed to sustained expression of the neural-determining factor Senseless (Sens). The CSN suppresses Sens through repression of the ecdysone signaling target gene broad (br) that encodes the BR-Z1 transcription factor to activate sens expression. Strikingly, CSN suppression of BR-Z1 is initiated at the prepupa-to-pupa transition, leading to Sens downregulation, and termination of the neural competence of PWM bristles. The role of ecdysone signaling to repress br after the prepupa-to-pupa transition is distinct from its conventional role in activation, and requires CSN deneddylating activity and multiple cullins, the major substrates of deneddylation. Several CSN subunits physically associate with ecdysone receptors to represses br at the transcriptional level. We propose a model in which nuclear hormone receptors cooperate with the deneddylation machinery to temporally shutdown downstream target gene expression, conferring a spatial restriction on neural competence at the PWM.

A critical step in building a functional nervous system is to generate neurons at the appropriate locations. Neural competence is acquired at the precursor stage with the expression of specific transcription factors. One such critical factor is Senseless (Sens), as precursors lacking Sens fail to develop to neurons. Here we describe the critical role of protein complex COP9 signalosome (CSN) that regulates Sens expression by integrating temporal and spatial information. This was studied in developing Drosophila wing tissues, in which the anterior wing margin develops neuron-innervated bristles, while the posterior wing margin develops non-innervated bristles. The CSN complex is required for the anterior-posterior difference in spatial patterning of neuron formation, and posterior cells lacking CSN develop innervated bristles like anterior cells. CSN accomplishes this by transforming the temporal hormonal ecdysone signaling from activation to repression of downstream target BR-Z1. As BR-Z1 itself is a transcription activator, repression of BR-Z1 in turn leads to repression of Sens in posterior wing margin, eventually terminating the neural competence. Repression of BR-Z1 expression requires the interaction between the CSN complex and the ecdysone receptors. Our results suggest a novel CSN-mediated regulation that converts temporal hormone signaling to the patterning of neurons at the right place.

Carrier of Wingless (Cow), a secreted heparan sulfate proteoglycan, promotes extracellular transport of Wingless

Morphogens are signaling molecules that regulate growth and patterning during development by forming a gradient and activating different target genes at different concentrations. The extracellular distribution of morphogens is tightly regulated, with the Drosophila morphogen Wingless (Wg) relying on Dally-like (Dlp) and transcytosis for its distribution. However, in the absence of Dlp or endocytic activity, Wg can still move across cells along the apical (Ap) surface. We identified a novel secreted heparan sulfate proteoglycan (HSPG) that binds to Wg and promotes its extracellular distribution by increasing Wg mobility, which was thus named Carrier of Wg (Cow). Cow promotes the Ap transport of Wg, independent of Dlp and endocytosis, and this function addresses a previous gap in the understanding of Wg movement. This is the first example of a diffusible HSPG acting as a carrier to promote the extracellular movement of a morphogen.

Inhibitors of endocytosis prevent Wnt/Wingless signalling by reducing the level of basal β-catenin/Armadillo

A key step in the canonical Wnt signalling pathway is the inhibition of GSK3β, which results in the accumulation of nuclear β-catenin (also known as CTNNB1), and hence regulation of target genes. Evidence suggests that endocytosis is required for signalling, yet its role and the molecular understanding remains unclear. A recent and controversial model suggests that endocytosis contributes to Wnt signalling by causing the sequestration of the ligand-receptor complex, including LRP6 and GSK3 to multivesicular bodies (MVBs), thus preventing GSK3β from accessing β-catenin. Here, we use specific inhibitors (Dynasore and Dyngo-4a) to confirm the essential role of endocytosis in Wnt/Wingless signalling in human and Drosophila cells. However, we find no evidence that, in Drosophila cells or wing imaginal discs, LRP6/Arrow traffics to MVBs or that MVBs are required for Wnt/Wingless signalling. Moreover, we show that activation of signalling through chemical blockade of GSK3β is prevented by endocytosis inhibitors, suggesting that endocytosis impacts on Wnt/Wingless signalling downstream of the ligand-receptor complex. We propose that, through an unknown mechanism, endocytosis boosts the resting pool of β-catenin upon which GSK3β normally acts.

Modulating and measuring Wingless signalling

The main Wnt ligand of Drosophila activates a conserved canonical signalling pathway to regulate a plethora of cellular activities during development, regeneration and nervous system function. Here I first describe experimental means of measuring and modulating Wingless signalling in Drosophila cell culture. Various reporters have been devised by placing TCF-binding sites or DNA fragments from known target genes upstream of luciferase-coding sequences. Signalling can be activated in cells by addition of Wingless conditioned medium, treatment with a chemical inhibitor of Shaggy/GSK3 or transfection with a plasmid encoding activated Armadillo (Drosophila β-catenin). Measuring Wingless signalling in intact tissue is somewhat more challenging than in cell culture. Synthetic transgenic reporters have been devised but further improvements are needed to achieve sensitive responsiveness to Wingless at all times and places. As an alternative, gene traps in frizzled3 and notum/wingful, two context-independent endogenous targets, can be used as reporters. It is hoped that further modification of these loci could lead to more versatile and sensitive means of detecting signalling. Many genetic tools are available to trigger ectopic signalling or prevent endogenous signalling. These mostly rely on RNAi-producing transgenes or the generation of mutant patches by mitotic recombination. New developments in genome engineering are opening further means of manipulating the components of Wingless signalling with exquisite temporal and spatial precision.

Patterning and growth control by membrane-tethered Wingless

Wnts are evolutionarily conserved secreted signalling proteins that, in various developmental contexts, spread from their site of synthesis to form a gradient and activate target-gene expression at a distance. However, the requirement for Wnts to spread has never been directly tested. Here we used genome engineering to replace the endogenous wingless gene, which encodes the main Drosophila Wnt, with one that expresses a membrane-tethered form of the protein. Surprisingly, the resulting flies were viable and produced normally patterned appendages of nearly the right size, albeit with a delay. We show that, in the prospective wing, prolonged wingless transcription followed by memory of earlier signalling allows persistent expression of relevant target genes. We suggest therefore that the spread of Wingless is dispensable for patterning and growth even though it probably contributes to increasing cell proliferation.

Immunohistochemical tools and techniques to visualize Notch in Drosophila melanogaster

The ability to accurately visualize proteins in Drosophila tissues is critical for studying their abundance and localization relative to the morphology of cells during tissue development and homeostasis. Here we describe the procedure to visualize Notch localization in whole-mount preparations of several Drosophila organs using confocal microscopy. The use of monoclonal antibodies directed to distinct portions of Notch allows one to follow the fate of the receptor during constitutive and inductive processes. The protocol described here can be used to co-label with antibodies recognizing markers of subcellular compartments in wild-type as well as mutant tissues.

Essential roles of Da transactivation domains in neurogenesis and in E(spl)-mediated repression

E proteins are a special class of basic helix-loop-helix (bHLH) proteins that heterodimerize with many bHLH activators to regulate developmental decisions, such as myogenesis and neurogenesis. Daughterless (Da) is the sole E protein in Drosophila and is ubiquitously expressed. We have characterized two transcription activation domains (TADs) in Da, called activation domain 1 (AD1) and loop-helix (LH), and have evaluated their roles in promoting peripheral neurogenesis. In this context, Da heterodimerizes with proneural proteins, such as Scute (Sc), which is dynamically expressed and also contributes a TAD. We found that either one of the Da TADs in the Da/Sc complex is sufficient to promote neurogenesis, whereas the Sc TAD is incapable of doing so. Besides its transcriptional activation role, the Da AD1 domain serves as an interaction platform for E(spl) proteins, bHLH-Orange family repressors which antagonize Da/Sc function. We show that the E(spl) Orange domain is needed for this interaction and strongly contributes to the antiproneural activity of E(spl) proteins. We present a mechanistic model on the interplay of these bHLH factors in the context of neural fate assignment.

The dREAM/Myb-MuvB complex and Grim are key regulators of the programmed death of neural precursor cells at the Drosophila posterior wing margin

Successful development of a multicellular organism depends on the finely tuned orchestration of cell proliferation, differentiation and apoptosis from embryogenesis through adulthood. The MYB-gene family encodes sequence-specific DNA-binding transcription factors that have been implicated in the regulation of both normal and neoplastic growth. The Drosophila Myb protein, DMyb (and vertebrate B-Myb protein), has been shown to be part of the dREAM/MMB complex, a large multi-subunit complex, which in addition to four Myb-interacting proteins including Mip130, contains repressive E2F and pRB proteins. This complex has been implicated in the regulation of DNA replication within the context of chorion gene amplification and transcriptional regulation of a wide array of genes. Detailed phenotypic analysis of mutations in the Drosophila myb gene, Dm myb, has revealed a previously undiscovered function for the dREAM/MMB complex in regulating programmed cell death (PCD). In cooperation with the pro-apoptotic protein Grim and dREAM/MMB, DMyb promotes the PCD of specified sensory organ precursor daughter cells in at least two different settings in the peripheral nervous system: the pIIIb precursor of the neuron and sheath cells in the posterior wing margin and the glial cell in the thoracic microchaete lineage. Unlike previously analyzed settings, in which the main role of DMyb has been to antagonize the activities of other dREAM/MMB complex members, it appears to be the critical effector in promoting PCD. The finding that Dm myb and grim are both involved in regulating PCD in two distinct settings suggests that these two genes may often work together to mediate PCD.

Wg signaling via Zw3 and mad restricts self-renewal of sensory organ precursor cells in Drosophila

It is well known that the Dpp signal transducer Mad is activated by phosphorylation at its carboxy-terminus. The role of phosphorylation on other regions of Mad is not as well understood. Here we report that the phosphorylation of Mad in the linker region by the Wg antagonist Zw3 (homolog of vertebrate Gsk3-β) regulates the development of sensory organs in the anterior-dorsal quadrant of the wing. Proneural expression of Mad-RNA interference (RNAi) or a Mad transgene with its Zw3/Gsk3-β phosphorylation sites mutated (MGM) generated wings with ectopic sensilla and chemosensory bristle duplications. Studies with pMad-Gsk (an antibody specific to Zw3/Gsk3-β-phosphorylated Mad) in larval wing disks revealed that this phosphorylation event is Wg dependent (via an unconventional mechanism), is restricted to anterior-dorsal sensory organ precursors (SOP) expressing Senseless (Sens), and is always co-expressed with the mitotic marker phospho-histone3. Quantitative analysis in both Mad-RNAi and MGM larval wing disks revealed a significant increase in the number of Sens SOP. We conclude that the phosphorylation of Mad by Zw3 functions to prevent the self-renewal of Sens SOP, perhaps facilitating their differentiation via asymmetric division. The conservation of Zw3/Gsk3-β phosphorylation sites in vertebrate homologs of Mad (Smads) suggests that this pathway, the first transforming growth factor β-independent role for any Smad protein, may be widely utilized for regulating mitosis during development.

Cell death and selective adhesion reorganize the dorsoventral boundary for zigzag patterning of Drosophila wing margin hairs

Animal tissues and organs are comprised of several types of cells, which are often arranged in a well-ordered pattern. The posterior part of the Drosophila wing margin is covered with a double row of long hairs, which are equally and alternately derived from the dorsal and ventral sides of the wing, exhibiting a zigzag pattern in the lateral view. How this geometrically regular pattern is formed has not been fully understood. In this study, we show that this zigzag pattern is created by rearrangement of wing margin cells along the dorsoventral boundary flanked by the double row of hair cells during metamorphosis. This cell rearrangement is induced by selective apoptosis of wing margin cells that are spatially separated from hair cells. As a result of apoptosis, the remaining wing margin cells are rearranged in a well-ordered manner, which shapes corrugated lateral sides of both dorsal and ventral edges to interlock them for zigzag patterning. We further show that the corrugated topology of the wing edges is achieved by cell-type specific expression and localization of four kinds of NEPH1/nephrin family proteins through heterophilic adhesion between wing margin cells and hair cells. Homophilic E-cadherin adhesion is also required for attachment of the corrugated dorsoventral edges. Taken together, our results demonstrate that sequential coordination of apoptosis and epithelial architecture with selective adhesion creates the zigzag hair alignment. This may be a common mechanism for geometrically ordered repetitive packing of several types of cells in similarly patterned developmental fields such as the mammalian organ of Corti.

The function and evolution of Wnt genes in arthropods

Wnt signalling is required for a wide range of developmental processes, from cleavage to patterning and cell migration. There are 13 subfamilies of Wnt ligand genes and this diverse repertoire appeared very early in metazoan evolution. In this review, we first summarise the known Wnt gene repertoire in various arthropods. Insects appear to have lost several Wnt subfamilies, either generally, such as Wnt3, or in lineage specific patterns, for example, the loss of Wnt7 in Anopheles. In Drosophila and Acyrthosiphon, only seven and six Wnt subfamilies are represented, respectively; however, the finding of nine Wnt genes in Tribolium suggests that arthropods had a larger repertoire ancestrally. We then discuss what is currently known about the expression and developmental function of Wnt ligands in Drosophila and other insects in comparison to other arthropods, such as the spiders Achaearanea and Cupiennius. We conclude that studies of Wnt genes have given us much insight into the developmental roles of some of these ligands. However, given the frequent loss of Wnt genes in insects and the derived development of Drosophila, further studies of these important genes are required in a broader range of arthropods to fully understand their developmental function and evolution.

Transcriptional control of stem cell maintenance in the Drosophila intestine

Adult stem cells maintain tissue homeostasis by controlling the proper balance of stem cell self-renewal and differentiation. The adult midgut of Drosophila contains multipotent intestinal stem cells (ISCs) that self-renew and produce differentiated progeny. Control of ISC identity and maintenance is poorly understood. Here we find that transcriptional repression of Notch target genes by a Hairless-Suppressor of Hairless complex is required for ISC maintenance, and identify genes of the Enhancer of split complex [E(spl)-C] as the major targets of this repression. In addition, we find that the bHLH transcription factor Daughterless is essential to maintain ISC identity and that bHLH binding sites promote ISC-specific enhancer activity. We propose that Daughterless-dependent bHLH activity is important for the ISC fate and that E(spl)-C factors inhibit this activity to promote differentiation.

Apical constriction and invagination downstream of the canonical Wnt signaling pathway require Rho1 and Myosin II

The tumor suppressor Adenomatous polyposis coli (APC) is a negative regulator of Wnt signaling and functions in cytoskeletal organization. Disruption of human APC in colonic epithelia initiates benign polyps that progress to carcinoma following additional mutations. The early events of polyposis are poorly understood, as is the role of canonical Wnt signaling in normal epithelial architecture and morphogenesis. To determine the consequences of complete loss of APC in a model epithelium, we generated APC2 APC1 double null clones in the Drosophila wing imaginal disc. APC loss leads to segregation, apical constriction, and invagination that result from transcriptional activation of canonical Wnt signaling. Further, we show that Wnt-dependent changes in cell fate can be decoupled from Wnt-dependent changes in cell shape. Wnt activation is reported to upregulate DE-cadherin in wing discs, and elevated DE-cadherin is thought to promote apical constriction. We find that apical constriction and invagination of APC null tissue are independent of DE-cadherin elevation, but are dependent on Myosin II activity. Further, we show that disruption of Rho1 suppresses apical constriction and invagination in APC null cells. Our data suggest a novel link between canonical Wnt signaling and epithelial structure that requires activation of the Rho1 pathway and Myosin II.

Wingless signaling and the control of cell shape in Drosophila wing imaginal discs

The control of cell morphology is important for shaping animals during development. Here we address the role of the Wnt/Wingless signal transduction pathway and two of its target genes, vestigial and shotgun (encoding E-cadherin), in controlling the columnar shape of Drosophila wing disc cells. We show that clones of cells mutant for arrow (encoding an essential component of the Wingless signal transduction pathway), vestigial or shotgun undergo profound cell shape changes and are extruded towards the basal side of the epithelium. Compartment-wide expression of a dominant-negative form of the Wingless transducer T-cell factor (TCF/Pangolin), or double-stranded RNA targeting vestigial or shotgun, leads to abnormally short cells throughout this region, indicating that these genes act cell autonomously to maintain normal columnar cell shape. Conversely, overexpression of Wingless, a constitutively-active form of the Wingless transducer beta-catenin/Armadillo, or Vestigial, results in precocious cell elongation. Co-expression of Vestigial partially suppresses the abnormal cell shape induced by dominant-negative TCF. We conclude that Wingless signal transduction plays a cell-autonomous role in promoting and maintaining the columnar shape of wing disc cells. Furthermore, our data suggest that Wingless controls cell shape, in part, through maintaining vestigial expression.

A twist of insight - the role of Twist-family bHLH factors in development

Members of the Twist-family of bHLH proteins play a pivotal role in a number of essential developmental programs. Twist-family bHLH proteins function by dimerizing with other bHLH members and binding to cis- regulatory elements, called E-boxes. While Twist-family members may simply exhibit a preference in terms of high-affinity binding partners, a complex, multilevel cascade of regulation creates a dynamic role for these bHLH proteins. We summarize in this review information on each Twist-family member concerning expression pattern, function, regulation, downstream targets, and interactions with other bHLH proteins. Additionally, we focus on the phospho-regulatory mechanisms that tightly control posttranslational modification of Twist-family member bHLH proteins.

Ero1L, a thiol oxidase, is required for Notch signaling through cysteine bridge formation of the Lin12-Notch repeats in Drosophila melanogaster

Notch-mediated cell–cell communication regulates numerous developmental processes and cell fate decisions. Through a mosaic genetic screen in Drosophila melanogaster, we identified a role in Notch signaling for a conserved thiol oxidase, endoplasmic reticulum (ER) oxidoreductin 1–like (Ero1L). Although Ero1L is reported to play a widespread role in protein folding in yeast, in flies Ero1L mutant clones show specific defects in lateral inhibition and inductive signaling, two characteristic processes regulated by Notch signaling. Ero1L mutant cells accumulate high levels of Notch protein in the ER and induce the unfolded protein response, suggesting that Notch is misfolded and fails to be exported from the ER. Biochemical assays demonstrate that Ero1L is required for formation of disulfide bonds of three Lin12-Notch repeats (LNRs) present in the extracellular domain of Notch. These LNRs are unique to the Notch family of proteins. Therefore, we have uncovered an unexpected requirement for Ero1L in the maturation of the Notch receptor.

Context dependence of proneural bHLH proteins

A key point in neural development is the commitment of progenitor cells to a specific neural fate. In all animals studied, proneural proteins - transcription factors of the basic helix-loop-helix (bHLH) family - are central to this process. The function of these factors is strongly influenced by the spatial and temporal context in which they are expressed. It is important to understand the molecular mechanisms by which developmental context interacts with and modifies the intrinsic functions and properties of the proneural proteins. Recent insights have been obtained in Drosophila and vertebrates from analysis of how bHLH proteins interact with other transcription factors to regulate target genes.

Klumpfuss is involved in the determination of sensory organ precursors in Drosophila

The neural precursor cells (sensory organ precursor cell (SOP)) of the external sense organs of Drosophila melanogaster arise from proneural clusters, which are defined through the expression pattern of proneural genes such as the genes of the achaete-scute complex (AS-C). The activities of these genes enable each cell within a cluster to become the SOP. A selection process mediated by the Notch signalling pathway and Extramacrochaetae selects a defined number of cells within the proneural cluster to realise the SOP fate, while it redirects the rest to the epidermoblast fate. Here we report a new function required for SOP determination mediated by the zinc finger transcription factor Klumpfuss (Klu). Klu participates in a novel mechanism that appears to regulate the expression as well as the activity of the proneural proteins. Our analysis indicates that Klu is a repressor of transcription, which acts via a double-negative loop to promote SOP formation: it suppresses the expression of an unidentified antagonist of proneural activity. We present a detailed structure function analysis that identifies functionally important domains within Klu.

Specificity of Atonal and Scute bHLH factors: analysis of cognate E box binding sites and the influence of Senseless

The question of how proneural bHLH transcription factors recognize and regulate their target genes is still relatively poorly understood. We previously showed that Scute (Sc) and Atonal (Ato) target genes have different cognate E box motifs, suggesting that specific DNA interactions contribute to differences in their target gene specificity. Here we show that Sc and Ato proteins (in combination with Daughterless) can activate reporter gene expression via their cognate E boxes in a non-neuronal cell culture system, suggesting that the proteins have strong intrinsic abilities to recognize different E box motifs in the absence of specialized cofactors. Functional comparison of E boxes from several target genes and site-directed mutagenesis of E box motifs suggests that specificity and activity require further sequence elements flanking both sides of the previously identified E box motifs. Moreover, the proneural cofactor, Senseless, can augment the function of Sc and Ato on their cognate E boxes and therefore may contribute to proneural specificity.

HLH-3 is a C. elegans Achaete/Scute protein required for differentiation of the hermaphrodite-specific motor neurons

Basic helix-loop-helix (bHLH) proteins of the Achaete/Scute (Ac/Sc) family are required for neurogenesis in both Drosophila and vertebrates. These transcription factors are commonly referred to as 'proneural' factors, as they promote neural fate in many contexts. Although Ac/Sc proteins have been studied in Hydra, jellyfish, many insects, and several vertebrates, the role of these proteins in Caenorhabditis elegans neurogenesis is relatively uncharacterized. The C. elegans genome consists of three Ac/Sc genes, previously identified as hlh-3, hlh-6, and hlh-14. Here, we characterize the role of hlh-3 in nervous system development. Although hlh-3 appears to be expressed in all neural precursors, we find that hlh-3 null mutants have a mostly functional nervous system. However, these mutants are egg-laying defective, resulting from a block in differentiation of the HSN motor neurons. Detectable HSNs have misdirected axon projection, which appears to result from a lack of netrin signaling within the HSNs. Thus, our findings suggest a novel link between Ac/Sc bHLH proteins and the expression of genes required for proper interpretation of axon guidance cues. Lastly, based on sequence identity, expression pattern, and a role in neural differentiation, hlh-3 is most likely an ortholog of Drosophila asense.

Neurogenin 2 has an essential role in development of the dentate gyrus

The dentate gyrus (DG) of the hippocampus has a central role in learning and memory in adult rodents. The DG is generated soon after birth, although new neurons continue to be generated in the DG throughout life. The proneural factors Mash1 (Ascl1) and neurogenin 2 (Ngn2) are expressed during formation of the DG but their role in the development of this structure has not yet been addressed. Here, we show that Ngn2 is essential for the development of the DG. Ngn2 mutant mice have fewer DG progenitors and these cells present defects in neuronal differentiation. By contrast, the DG is normal in Mash1 mutant mice at birth, and loss of both Mash1 and Ngn2 does not aggravate the defect observed in Ngn2 single mutants. These data establish a unique role of Ngn2 in DG neurogenesis during development and raise the possibility that Ngn2 has a similar function in adult neurogenesis.

How Drosophila melanogaster Forms its Mechanoreceptors

A strictly determined number of external sensory organs, macrochaetes, acting as mechanoreceptors, are orderly located on drosophila head and body. Totally, they form the bristle pattern, which is a species-specific characteristic of drosophila.Each mechanoreceptor comprises four specialized cells derived from the single sensory organ precursor (SOP) cell. The conserved bristle pattern combined with a comparatively simple structure of each mechanosensory organ makes macrochaetes a convenient model for studying the formation spatial structures with a fixed number of elements at certain positions and the mechanism underlying cell differentiation.The macrochaete morphogenesis consists of three stages. At the first stage, the proneural clusters segregate from the massive of ectodermal cells of the wing imaginal disc. At the second stage, the SOP cell is determined and its position in the cluster is specified. At the third stage, the SOP cell undergoes two asymmetric divisions, and the daughter cells differentiate into the components of mechanoreceptor: shaft, socket, bipolar neuron, and sheath.The critical factor determining the neural pathway of cell development is the content of proneural proteins, products of the achaete-scute (AS-C) gene complex, reaching its maximum in the SOP cell.The experimental data on the main genes and their products involved in the control of bristle pattern formation are systematized. The roles of achaete-scute complex, EGFR and Notch signaling pathways, and selector genes in these processes are considered. An integral scheme describing the functioning of the system controlling macrochaete development in D. melanogaster is proposed based on analysis of literature data.

Senseless functions as a molecular switch for color photoreceptor differentiation in Drosophila

A major question in development is how different specialized cell types arise from a common progenitor. In the adult Drosophila compound eye, color discrimination is achieved by UV-, blue- and green-sensitive photoreceptors (PRs). These different PR subsets arise from neuronal precursors called R7 and R8 cells. Recent studies have demonstrated that R7-based UV-sensitive PRs require the repression of R8-based blue/green-sensitive PR characteristics to properly develop. This repression is mediated by the transcription factor Prospero (Pros). Here, we report that Senseless (Sens), a Drosophila ortholog of the vertebrate Gfi1 transcription factor, plays an opposing role to Pros by both negatively regulating R7-based features and positively enforcing R8-based features during terminal differentiation. In addition, we demonstrate that Pros and Sens function together with the transcription factor Orthodenticle (Otd) to oppositely regulate R7 and R8 PR Rhodopsin gene expression in vitro. These data show that sens, previously shown to be essential for neuronal specification, also controls differentiation of specific neuronal subtypes in the retina. Interestingly, Pros has recently been shown to function as a tumor suppressor, whereas Gfi1 is a well-characterized oncogene. Thus, we propose that sens/pros antagonism is important for regulating many biological processes.