The glial cell undergoes apoptosis in the microchaete lineage of Drosophila

A single-cell atlas of the sexually dimorphic Drosophila foreleg and its sensory organs during development

To respond to the world around them, animals rely on the input of a network of sensory organs distributed throughout the body. Distinct classes of sensory organs are specialized for the detection of specific stimuli such as strain, pressure, or taste. The features that underlie this specialization relate both to the neurons that innervate sensory organs and the accessory cells they comprise. To understand the genetic basis of this diversity of cell types, both within and between sensory organs, we performed single-cell RNA sequencing on the first tarsal segment of the male Drosophila melanogaster foreleg during pupal development. This tissue displays a wide variety of functionally and structurally distinct sensory organs, including campaniform sensilla, mechanosensory bristles, and chemosensory taste bristles, as well as the sex comb, a recently evolved male-specific structure. In this study, we characterize the cellular landscape in which the sensory organs reside, identify a novel cell type that contributes to the construction of the neural lamella, and resolve the transcriptomic differences among support cells within and between sensory organs. We identify the genes that distinguish between mechanosensory and chemosensory neurons, resolve a combinatorial transcription factor code that defines 4 distinct classes of gustatory neurons and several types of mechanosensory neurons, and match the expression of sensory receptor genes to specific neuron classes. Collectively, our work identifies core genetic features of a variety of sensory organs and provides a rich, annotated resource for studying their development and function.

Cell-intrinsic and -extrinsic roles of the ESCRT-III subunit Shrub in abscission of Drosophila sensory organ precursors

Although the molecular mechanisms governing abscission of isolated cells have largely been elucidated, those underlying the abscission of epithelial progenitors surrounded by epidermal cells (ECs), connected via cellular junctions, remain largely unexplored. Here, we investigated the remodeling of the paracellular diffusion barrier ensured by septate junctions (SJs) during cytokinesis of Drosophila sensory organ precursors (SOPs). We found that SOP cytokinesis involves the coordinated, polarized assembly and remodeling of SJs in the dividing cell and its neighbors, which remain connected to the former via membrane protrusions pointing towards the SOP midbody. SJ assembly and midbody basal displacement occur faster in SOPs than in ECs, leading to quicker disentanglement of neighboring cell membrane protrusions prior to midbody release. As reported in isolated cells, the endosomal sorting complex required for the transport-III component Shrub/CHMP4B is recruited at the midbody and cell-autonomously regulates abscission. In addition, Shrub is recruited to membrane protrusions and is required for SJ integrity, and alteration of SJ integrity leads to premature abscission. Our study uncovers cell-intrinsic and -extrinsic functions of Shrub in coordinating remodeling of the SJs and SOP abscission.

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.

Cortical Cyclin A controls spindle orientation during asymmetric cell divisions in Drosophila

The coordination between cell proliferation and cell polarity is crucial to orient the asymmetric cell divisions to generate cell diversity in epithelia. In many instances, the Frizzled/Dishevelled planar cell polarity pathway is involved in mitotic spindle orientation, but how this is spatially and temporally coordinated with cell cycle progression has remained elusive. Using Drosophila sensory organ precursor cells as a model system, we show that Cyclin A, the main Cyclin driving the transition to M-phase of the cell cycle, is recruited to the apical-posterior cortex in prophase by the Frizzled/Dishevelled complex. This cortically localized Cyclin A then regulates the orientation of the division by recruiting Mud, a homologue of NuMA, the well-known spindle-associated protein. The observed non-canonical subcellular localization of Cyclin A reveals this mitotic factor as a direct link between cell proliferation, cell polarity and spindle orientation.

The Frizzled/Dishevelled planar cell polarity pathway is involved in mitotic spindle orientation, but how this is coordinated with the cell cycle is unclear. Here, the authors show with Drosophila sensory organ precursor cells that Cyclin A is recruited in prophase by Frizzled/Dishevelled, regulating division orientation.

A neural progenitor mitotic wave is required for asynchronous axon outgrowth and morphology

Spatiotemporal mechanisms generating neural diversity are fundamental for understanding neural processes. Here, we investigated how neural diversity arises from neurons coming from identical progenitors. In the dorsal thorax of Drosophila, rows of mechanosensory organs originate from the division of sensory organ progenitor (SOPs). We show that in each row of the notum, an anteromedial located central SOP divides first, then neighbouring SOPs divide, and so on. This centrifugal wave of mitoses depends on cell-cell inhibitory interactions mediated by SOP cytoplasmic protrusions and Scabrous, a secreted protein interacting with the Delta/Notch complex. Furthermore, when this mitotic wave was reduced, axonal growth was more synchronous, axonal terminals had a complex branching pattern and fly behaviour was impaired. We show that the temporal order of progenitor divisions influences the birth order of sensory neurons, axon branching and impact on grooming behaviour. These data support the idea that developmental timing controls axon wiring neural diversity.

Programmed cell death reshapes the central nervous system during metamorphosis in insects

Metamorphosis is fascinating and dramatic stage of postembryonic development in insects [1]. The most prominent metamorphic changes seen in holometabolous insects involve destruction of most larval structures and concomitant generation of adult ones. Such diverse cellular events are orchestrated by ecdysone. The central nervous system (CNS) is also extensively remodeled to process new sensory inputs; to coordinate new types of locomotion; and to perform higher-order decision making [2]. Programmed cell death (PCD) is an integral part of the metamorphic development. It eliminates obsolete larval tissues and extra cells that are generated from the morphogenesis of adult tissues. In the CNS, PCD of selected neurons and glial cells as well as reshaping of persistent larval cells are essential for establishing the adult CNS. In this review, we summarize the ecdysone signaling, and then molecular and cellular events associated with PCD primarily in the metamorphosing CNS of Drosophila melanogaster.

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 pleiotropic effects of Innexin genes expressed in Drosophila glia encompass wing chemosensory sensilla

The neuroanatomy of Drosophila wing chemosensilla and the analysis of their sensory organ precursor cell lineage have demonstrated that they are surprisingly related to taste perception. The microarchitecture of wing bristles limits the use of electrophysiology methods to investigate wing chemosensory mechanisms. However, by monitoring the fluorescence of the complex calcium/GCaMP, calcium flux triggered upon tastant stimulation was observed within sensilla aligned along the wing anterior nerve. This string of fluorescent puncta was impaired in wings of Innexin 2 (Inx2) mutant flies; although it is unclear whether the Innexin proteins act at the level of the wing imaginal disc, adult wing and/or at both levels. Glial cells known to shelter Innexin(s) expression have no documented role in adult chemosensory sensilla. Our data suggest that Innexin(s) are likely required for the maturation of functional wing chemosensilla in adulthood. The unexpected presence of most Innexin transcripts in adult wing RNAseq data set argues for the expression of Innexin proteins in the larval imaginal wing disc that are continued in wing chemosensilla at adulthood. OPEN PRACTICES: This article has earned an Open Data badge for making publicly available the digitally-shareable data necessary to reproduce the reported results. The data is available as supporting materials and includes the electronic lab notebook. Learn more about the Open Practices badges from the Center for Open Science: https://osf.io/tvyxz/wiki.

Shaping of Drosophila Neural Cell Lineages Through Coordination of Cell Proliferation and Cell Fate by the BTB-ZF Transcription Factor Tramtrack-69

Cell diversity in multicellular organisms relies on coordination between cell proliferation and the acquisition of cell identity. The equilibrium between these two processes is essential to assure the correct number of determined cells at a given time at a given place. Using genetic approaches and correlative microscopy, we show that Tramtrack-69 (Ttk69, a Broad-complex, Tramtrack and Bric-à-brac - Zinc Finger (BTB-ZF) transcription factor ortholog of the human promyelocytic leukemia zinc finger factor) plays an essential role in controlling this balance. In the Drosophila bristle cell lineage, which produces the external sensory organs composed by a neuron and accessory cells, we show that ttk69 loss-of-function leads to supplementary neural-type cells at the expense of accessory cells. Our data indicate that Ttk69 (1) promotes cell cycle exit of newborn terminal cells by downregulating CycE, the principal cyclin involved in S-phase entry, and (2) regulates cell-fate acquisition and terminal differentiation, by downregulating the expression of hamlet and upregulating that of Suppressor of Hairless, two transcription factors involved in neural-fate acquisition and accessory cell differentiation, respectively. Thus, Ttk69 plays a central role in shaping neural cell lineages by integrating molecular mechanisms that regulate progenitor cell cycle exit and cell-fate commitment.

Evolutionary variation in neural gene expression in the developing sense organs of the crustacean Daphnia magna

Arthropods have numerous sense organs, which are adapted to their habitat. While some sense organs are similar in structure and function in all arthropod groups, structural differences in functionally related sense organs have been described, as well as the absence of particular sense organ subtypes in individual arthropod groups. Here we address the question of how the diverse structures of arthropod sense organs have evolved by analysing the underlying molecular developmental processes in a crustacean, an arthropod group that has been neglected so far. We have investigated the development of four types of chemo- and mechanosensory sense organs in the branchiopod Daphnia magna (Cladocera) that either cannot be found in arthropods other than crustaceans or represent adaptations to an aquatic environment. The formation of the sensory organ precursors shows greater similarity to the arthropod taxa Chelicerata and Myriapoda than to the more closely related insects. All analysed sense organ types co-express the proneural genes ASH and atonal regardless of their structure and function. In contrast, in Drosophila melanogaster, ASH and atonal expression does not overlap and the genes confer different sense organ subtype identities. We performed experimental co-expression studies in D. melanogaster and found that the combinatorial expression of ato and ASH can change the external structure of sense organs. Our results indicate a central role for ASH and Atonal family members in the emergence of structural variations in arthropod sense organs.

Escargot and Scratch regulate neural commitment by antagonizing Notch activity in Drosophila sensory organs

During Notch (N)-mediated binary cell fate decisions, cells adopt two different fates according to the levels of N pathway activation: an Noff-dependent or an Non-dependent fate. How cells maintain these N activity levels over time remains largely unknown. We address this question in the cell lineage that gives rise to the Drosophila mechanosensory organs. In this lineage a primary precursor cell undergoes a stereotyped sequence of oriented asymmetric cell divisions and transits through two neural precursor states before acquiring a neuron identity. Using a combination of genetic and cell biology strategies, we show that Escargot and Scratch, two transcription factors belonging to the Snail superfamily, maintain Noff neural commitment by directly blocking the transcription of N target genes. We propose that Snail factors act by displacing proneural transcription activators from DNA binding sites. As such, Snail factors maintain the Noff state in neural precursor cells by buffering any ectopic variation in the level of N activity. Since Escargot and Scratch orthologs are present in other precursor cells, our findings are fundamental for understanding precursor cell fate acquisition in other systems.

G2 phase arrest prevents bristle progenitor self-renewal and synchronizes cell division with cell fate differentiation

Developmentally regulated cell cycle arrest is a fundamental feature of neurogenesis, whose significance is poorly understood. During Drosophila sensory organ (SO) development, primary progenitor (pI) cells arrest in G2 phase for precisely defined periods. Upon re-entering the cell cycle in response to developmental signals, these G2-arrested precursor cells divide and generate specialized neuronal and non-neuronal cells. To study how G2 phase arrest affects SO lineage specification, we forced pI cells to divide prematurely. This produced SOs with normal neuronal lineages but supernumerary non-neuronal cell types because prematurely dividing pI cells generate a secondary pI cell that produces a complete SO and an external precursor cell that undergoes amplification divisions. pI cells are therefore able to undergo self-renewal before transit to a terminal mode of division. Regulation of G2 phase arrest thus serves a dual role in SO development: preventing progenitor self-renewal and synchronizing cell division with developmental signals. Cell cycle arrest in G2 phase temporally coordinates the precursor cell proliferation potential with terminal cell fate determination to ensure formation of organs with a normal set of sensory cells.

Detection of Cell Death in Drosophila Tissues

Drosophila has served as a particularly attractive model to study cell death due to the vast array of tools for genetic manipulation under defined spatial and temporal conditions in vivo as well as in cultured cells. These genetic methods have been well supplemented by enzymatic assays and a panel of antibodies recognizing cell death markers. This chapter discusses reporters, mutants, and assays used by various laboratories to study cell death in the context of development and in response to external insults.

Asymmetric cell division in the Drosophila bristle lineage: from the polarization of sensory organ precursor cells to Notch-mediated binary fate decision

Asymmetric cell division (ACD) is a simple and evolutionary conserved process whereby a mother divides to generate two daughter cells with distinct developmental potentials. This process can generate cell fate diversity during development. Fate asymmetry may result from the unequal segregation of molecules and/or organelles between the two daughter cells. Here, I will review how fate asymmetry is regulated in the sensory bristle lineage in Drosophila and focus on the molecular mechanisms underlying ACD of the sensory organ precursor cells (SOPs). WIREs Dev Biol 2015, 4:299–309. doi: 10.1002/wdev.175

For further resources related to this article, please visit theWIREs website.

Conflict of interest: The author has declared no conflicts of interest for this article.

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.

The extracellular matrix protein artichoke is required for integrity of ciliated mechanosensory and chemosensory organs in Drosophila embryos

Sensory cilia are often encapsulated by an extracellular matrix (ECM). In Caenorhabditis elegans, Drosophila melanogaster, and vertebrates, this ECM is thought to be directly involved in ciliary mechanosensing by coupling external forces to the ciliary membrane. Drosophila mechano- and chemosensory cilia are both associated with an ECM, indicating that the ECM may have additional roles that go beyond mechanosensory cilium function. Here, we identify Artichoke (ATK), an evolutionarily conserved leucine-rich repeat ECM protein that is required for normal morphogenesis and function of ciliated sensilla in Drosophila. atk is transiently expressed in accessory cells in all ciliated sensory organs during their late embryonic development. Antibody stainings show ATK protein in the ECM that surrounds sensory cilia. Loss of ATK protein in atk null mutants leads to cilium deformation and disorientation in chordotonal organs, apparently without uncoupling the cilia from the ECM, and consequently to locomotion defects. Moreover, impaired chemotaxis in atk mutant larvae suggests that, based on ATK protein localization, the ECM is also crucial for the correct assembly of chemosensory receptors. In addition to defining a novel ECM component, our findings show the importance of ECM integrity for the proper morphogenesis of ciliated organs in different sensory modalities.

Proneural proteins Achaete and Scute associate with nuclear actin to promote formation of external sensory organs

Basic helix-loop-helix (bHLH) proneural proteins promote neurogenesis through transcriptional regulation. Although much is known about the tissue-specific regulation of proneural gene expression, how proneural proteins interact with transcriptional machinery to activate downstream target genes is less clear. Drosophila proneural proteins Achaete (Ac) and Scute (Sc) induce external sensory organ formation by activating neural precursor gene expression. Through co-immunoprecipitation and mass spectrometric analyses, we found that nuclear but not cytoplasmic actin associated with the Ac and Sc proteins in Drosophila S2 cells. Daughterless (Da), the common heterodimeric partner of Drosophila bHLH proteins, was observed to associate with nuclear actin through proneural proteins. A yeast two-hybrid assay revealed that the binding specificity between actin and Ac or Sc was conserved in yeast nuclei without the presence of additional Drosophila factors. We further show that actin is required in external sensory organ formation. Reduction in actin gene activity impaired proneural-protein-dependent expression of the neural precursor genes, as well as formation of neural precursors. Furthermore, increased nuclear actin levels, obtained by expression of nucleus-localized actin, elevated Ac-Da-dependent gene transcription as well as Ac-mediated external sensory organ formation. Taken together, our in vivo and in vitro observations suggest a novel link for actin in proneural-protein-mediated transcriptional activation and neural precursor differentiation.

Regulation of centrosome movements by Numb and the Collapsin Response Mediator Protein during Drosophila sensory progenitor asymmetric division

Asymmetric cell division generates cell fate diversity during development and adult life. Recent findings have demonstrated that during stem cell divisions, the movement of centrosomes is asymmetric in prophase and that such asymmetry participates in mitotic spindle orientation and cell polarization. Here, we have investigated the dynamics of centrosomes during Drosophila sensory organ precursor asymmetric divisions and find that centrosome movements are asymmetric during cytokinesis. We demonstrate that centrosome movements are controlled by the cell fate determinant Numb, which does not act via its classical effectors, Sanpodo and α-Adaptin, but via the Collapsin Response Mediator Protein (CRMP). Furthermore, we find that CRMP is necessary for efficient Notch signalling and that it regulates the duration of the pericentriolar accumulation of Rab11-positive endosomes, through which the Notch ligand, Delta is recycled. Our work characterizes an additional mode of asymmetric centrosome movement during asymmetric divisions and suggests a model whereby the asymmetry in centrosome movements participates in differential Notch activation to regulate cell fate specification.

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.

Endocytosis and control of Notch signaling

The Notch signaling pathway controls patterning and cell fate decisions during development in metazoans, and is associated with human diseases such as cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL) and certain cancers. Studies over the last several years have revealed sophisticated regulation of both the membrane-bound Notch receptor and its ligands by vesicle trafficking. This is perhaps most evident in neural progenitor cells in Drosophila, which divide asymmetrically to segregate Numb, an endocytic adaptor protein that acts as a Notch pathway inhibitor, to one daughter cell. Here, we discuss recent findings addressing how receptor and ligand trafficking to specific membrane compartments control activation of the Notch pathway in asymmetrically dividing cells and other tissues.

Dynamics of endoreplication during Drosophila posterior scutellar macrochaete development

Endoreplication is a variant type of DNA replication, consisting only of alternating G1 and S phases. Many types of Drosophila tissues undergo endoreplication. However, the timing and the extent to which a single endocycling macrochaete undergoes temporally programmed endoreplication during development are unclear. Here, we focused on the dynamics of endoreplication during posterior scutellar (pSC) macrochaete development. Quantitative analyses of C values in shaft cells and socket cells revealed a gradual rise from 8C and 4C at 8 hours after pupal formation (APF) to 72C and 24C at 29 hours APF, respectively. The validity of the values was further confirmed by the measurement of DNA content with a confocal laser microscope. BrdU incorporation assays demonstrated that shaft cells undergo four rounds of endoreplication from 18 to 29.5 hours APF. In contrast, socket cells undergo two rounds of endoreplication during the same period. Statistical analyses showed that the theoretical C values, based on BrdU assays, nearly coincide with the actually measured C values in socket cells, but not in shaft cells after 22 hours APF. These analyses suggest that socket cells undergo two rounds of endoreplication. However, the mechanism of endoreplication in the shaft cells may change from 22 hours APF, suggesting the possibility that shaft cells undergo two or four rounds of endoreplication during the periods. We also found that the timing of endoreplication differs, depending on the type of macrochaete. Moreover, endocycling in shaft cells of both the left and right sides of pSC bristle lineages occurs in the same pattern, indicating that the process is synchronized for specific types of macrochaete. Our findings suggest that endocycling in macrochaete cell lineages can be a model for understanding mechanisms of endoreplication at the single-cell level.

Drosophila neural progenitor polarity and asymmetric division

In the Drosophila embryonic central nervous system, the neural precursor cells called neuroblasts undergo a number of asymmetric divisions along the apical-basal axis to give rise to different daughter cells of distinct fates. This review summarizes recent progress in understanding the mechanisms of these asymmetric cell divisions. We discuss proteins that are localized at distinct domains of cortex in the neuroblasts and their role in generating asymmetry. We also review uniformly cortical localized factors and actin cytoskeleton-associated motor proteins with regard to their potential role to serve as a link between distinct cortical domains in the neuroblasts. In this review, asymmetric divisions of sensory organ precursor and larval neuroblasts are also briefly discussed.

New tools for the analysis of glial cell biology in Drosophila

Because of its genetic, molecular, and behavioral tractability, Drosophila has emerged as a powerful model system for studying molecular and cellular mechanisms underlying the development and function of nervous systems. The Drosophila nervous system has fewer neurons and exhibits a lower glia:neuron ratio than is seen in vertebrate nervous systems. Despite the simplicity of the Drosophila nervous system, glial organization in flies is as sophisticated as it is in vertebrates. Furthermore, fly glial cells play vital roles in neural development and behavior. In addition, powerful genetic tools are continuously being created to explore cell function in vivo. In taking advantage of these features, the fly nervous system serves as an excellent model system to study general aspects of glial cell development and function in vivo. In this article, we review and discuss advanced genetic tools that are potentially useful for understanding glial cell biology in Drosophila.

The Fz-Dsh planar cell polarity pathway induces oriented cell division via Mud/NuMA in Drosophila and zebrafish

The Frizzled receptor and Dishevelled effector regulate mitotic spindle orientation in both vertebrates and invertebrates, but how Dishevelled orients the mitotic spindle is unknown. Using the Drosophila S2 cell "induced polarity" system, we find that Dishevelled cortical polarity is sufficient to orient the spindle and that Dishevelled's DEP domain mediates this function. This domain binds a C-terminal domain of Mud (the Drosophila NuMA ortholog), and Mud is required for Dishevelled-mediated spindle orientation. In Drosophila, Frizzled-Dishevelled planar cell polarity (PCP) orients the sensory organ precursor (pI) spindle along the anterior-posterior axis. We show that Dishevelled and Mud colocalize at the posterior cortex of pI, Mud localization at the posterior cortex requires Dsh, and Mud loss-of-function randomizes spindle orientation. During zebrafish gastrulation, the Wnt11-Frizzled-Dishevelled PCP pathway orients spindles along the animal-vegetal axis, and reducing NuMA levels disrupts spindle orientation. Overall, we describe a Frizzled-Dishevelled-NuMA pathway that orients division from Drosophila to vertebrates.

grim promotes programmed cell death of Drosophila microchaete glial cells

The Inhibitor of apoptosis (IAP) antagonists Reaper (Rpr), Grim and Hid are central regulators of developmental apoptosis in Drosophila. Ectopic expression of each is sufficient to trigger apoptosis, and hid and rpr have been shown to be important for programmed cell death (PCD). To investigate the role for grim in PCD, a grim null mutant was generated. grim was not a key proapoptotic gene for embryonic PCD, confirming that grim cooperates with rpr and hid in embryogenesis. In contrast, PCD of glial cells in the microchaete lineage required grim, identifying a death process dependent upon endogenous grim. Grim associates with mitochondria and has been shown to activate a mitochondrial death pathway distinct from IAP antagonization; therefore, the Drosophila bcl-2 genes buffy and debcl were investigated for genetic interaction with grim. Loss of buffy led to microchaete glial cell survival and suppressed death in the eye induced by ectopic Grim. This is the first example of a developmental PCD process influenced by buffy, and places buffy in a proapoptotic role. PCD of microchaete glial cells represents an exceptional opportunity to study the mitochondrial proapoptotic process induced by Grim.

Temporal regulation of Drosophila IAP1 determines caspase functions in sensory organ development

Caspase activation is regulated by the turnover of E3 ubiquitin ligase, DIAP1, and depends on cell type and maturity.

The caspases comprise a family of cysteine proteases that function in various cellular processes, including apoptosis. However, how the balance is struck between the caspases’ role in cell death and their nonapoptotic functions is unclear. To address this issue, we monitored the protein turnover of an endogenous caspase inhibitor, Drosophila IAP1 (DIAP1). DIAP1 is an E3 ubiquitin ligase that promotes the ubiquitination of caspases and thereby prevents caspase activation. For this study, we developed a fluorescent probe to monitor DIAP1 turnover in the external sensory organ precursor (SOP) lineage of living Drosophila. The SOP divides asymmetrically to make the shaft, socket, and sheath cells, and the neuron that comprise each sensory organ. We found that the quantity of DIAP1 changed dramatically depending on the cell type and maturity, and that the temporal regulation of DIAP1 turnover determines whether caspases function nonapoptotically in cellular morphogenesis or cause cell death.

Notch activity in the nervous system: to switch or not switch?

The Notch pathway is instrumental for cell fate diversification during development. Pioneer studies conducted in Drosophila and more recent work performed in vertebrates have shown that in the nervous system, Notch is reiteratively employed when cells choose between two alternative fates, a process referred to as a binary fate decision. While the early (neural versus epidermal) fate decisions mainly involve an inhibitory effect of Notch on the neural fate, late fate decisions (choice between different subtypes of neural cells) have been proposed to involve a binary switch activity whereby Notch would be instructive for one fate and inhibitory for the other. We re-examine this binary switch model in light of two recent findings made in the vertebrate nervous system. First, in the zebrafish epiphysis, Notch is required to resolve a mixed identity through the inhibition of one specific fate. Second, in the murine telencephalon, Notch regulates the competence of neural progenitors to respond to the JAK/STAT pathway, thereby allowing for the induction of an astrocyte fate. In neither case is Notch instructive for the alternative fate, but rather cooperates with another signalling pathway to coordinate binary fate choices. We also review current knowledge on the molecular cascades acting downstream of Notch in the context of neural subtype diversification, a crucial issue if one is to determine Notch function as an instructive, permissive or inhibitory signal in the various cellular contexts where it is implicated. Finally, we speculate as to how such a 'non-switch' activity could contribute to the expansion of neuronal subtype diversity.

Notch and Prospero repress proliferation following cyclin E overexpression in the Drosophila bristle lineage

Understanding the mechanisms that coordinate cell proliferation, cell cycle arrest, and cell differentiation is essential to address the problem of how “normal” versus pathological developmental processes take place. In the bristle lineage of the adult fly, we have tested the capacity of post-mitotic cells to re-enter the cell cycle in response to the overexpression of cyclin E. We show that only terminal cells in which the identity is independent of Notch pathway undergo extra divisions after CycE overexpression. Our analysis shows that the responsiveness of cells to forced proliferation depends on both Prospero, a fate determinant, and on the level of Notch pathway activity. Our results demonstrate that the terminal quiescent state and differentiation are regulated by two parallel mechanisms acting simultaneously on fate acquisition and cell cycle progression.

Despite substantial progress that has been made, we still know little about how single precursor cells undergo a limited number of cell divisions before arrest. Discovering the mechanisms by which terminal cells maintain cell division arrest is essential for understanding “normal” development, as well as the origin of pathological deregulations. Using the bristle cell lineage, a model system widely employed to analye cell identity acquisition, we observed that only two out of four terminal cells in this lineage are unable to re-enter the cell cycle and proliferate. Our study shows that in these cells, cell division arrest is maintained by the action of the transcription factor Prospero and the signalling pathway Notch. Since both of these factors also control cell identity in this lineage, this finding demonstrates that common elements acting simultaneously and in parallel regulate the terminal quiescent state and differentiation. This system provides a unique animal model in which to understand how the mechanisms involved in cell fate acquisition and those controlling cell division intermingle to produce cell lineages resulting in terminal cells in the right number and at the right place and time.

Sequoia regulates cell fate decisions in the external sensory organs of adult Drosophila

The adult Drosophila external sensory organ (ESO), comprising the hair, socket, neuron, sheath and glia cells, arises through the asymmetric division of sensory organ precursor cells (SOPs). In a mosaic screen designed to identify new components in ESO development, we isolated mutations in sequoia, which encodes a putative zinc-finger transcription factor that has previously been shown to have a role in dendritogenesis. Here, we show that adult clones mutant for seq exhibit a loss of hair cells and a gain of socket cells. We propose that the seq mutant phenotype arises, in part, owing to the loss of several crucial transcription factors known to be important in peripheral nervous system development such as D-Pax2, Prospero and Hamlet. Thus, Sequoia is a new upstream regulator of genes that orchestrates cell fate specification during development of the adult ESO lineage.

The role of Notch signalling and numb function in mechanosensory organ formation in the spider Cupiennius salei

In the spider Cupiennius salei the mechanosensory organs of the legs are generated from epithelial sensory precursor groups which are specified by elevated levels of the achaete-scute homologues CsASH1 and CsASH2. Neural precursors delaminate from the groups and occupy positions basal and proximal to the accessory cells which remain in the epithelium. Here we analyse the role of Notch signalling and numb function in the development of the mechanosensory organs of the spider. We show that Notch signalling is required for several processes: the selection of the sensory precursor groups, the maintenance of undifferentiated sensory precursors, the binary cell fate decision between accessory and neural fate and the differentiation of sensory neurons. Our data suggest that Numb antagonises Notch signalling in the neural precursors, which results in activation of the neural cell fate determinant Prospero and delamination of the neural precursors from the epithelium. Prospero is expressed de novo in sensory neural precursors and we assume that the expression of the gene is regulated by the Notch to Numb ratio within the sensory precursors. Interestingly, the spider numb RNAi phenotype resembles the numb/numblike loss of function phenotypes in the mammalian nervous system, indicating that the interaction between Notch signalling and Numb might play a similar role in both systems.

S-phase favours notch cell responsiveness in the Drosophila bristle lineage

We have studied cell sensitivity to Notch pathway signalling throughout the cell cycle. As model system, we used the Drosophila bristle lineage where at each division N plays a crucial role in fate determination. Using in vivo imaging, we followed this lineage and activated the N-pathway at different moments of the secondary precursor cell cycle. We show that cells are more susceptible to respond to N-signalling during the S-phase. Thus, the period of heightened sensitivity coincided with the period of the S-phase. More importantly, modifications of S-phase temporality induced corresponding changes in the period of the cell's reactivity to N-activation. Moreover, S-phase abolition was correlated with a decrease in the expression of tramtrack, a downstream N-target gene. Finally, N cell responsiveness was modified after changes in chromatin packaging. We suggest that high-order chromatin structures associated with the S-phase create favourable conditions that increase the efficiency of the transcriptional machinery with respect to N-target genes.

Negative-feedback regulation of proneural proteins controls the timing of neural precursor division

Neurogenesis requires precise control of cell specification and division. In Drosophila, the timing of cell division of the sensory organ precursor (SOP) is under strict temporal control. But how the timing of mitotic entry is determined remains poorly understood. Here, we present evidence that the timing of the G2-M transition is determined by when proneural proteins are degraded from SOPs. This process requires the E3 ubiquitin ligase complex, including the RING protein Sina and the adaptor Phyl. In phyl mutants, proneural proteins accumulate, causing delay or arrest in the G2-M transition. The G2-M defect in phyl mutants is rescued by reducing the ac and sc gene doses. Misexpression of phyl downregulates proneural protein levels in a sina-dependent manner. Phyl directly associates with proneural proteins to act as a bridge between proneural proteins and Sina. As phyl is a direct transcriptional target of Ac and Sc, our data suggest that, in addition to mediating cell cycle arrest, proneural protein initiates a negative-feedback regulation to time the mitotic entry of neural precursors.

The abdomen of Drosophila: does planar cell polarity orient the neurons of mechanosensory bristles?

Background

In the adult abdomen of Drosophila, the shafts of mechanosensory bristles point consistently from anterior to posterior. This is an example of planar cell polarity (PCP); some genes responsible for PCP have been identified. Each adult bristle is made by a clone of four cells, including the neuron that innervates it, but little is known as to how far the formation or positions of these cells depends on PCP. The neurons include a single dendrite and an axon; it is not known whether the orientation of these processes is influenced by PCP.

Results

We describe the development of the abdominal mechanosensory bristles in detail. The division of the precursor cell gives two daughters, one (pIIa) divides to give rise to the bristle shaft and socket cell and the other (pIIb) generates the neuron, the sheath and the fifth cell. Although the bristles and their associated shaft and socket cells are consistently oriented, the positioning and behaviour of the neuron, the sheath and the fifth cell, as well as the orientation of the axons and the dendritic paths, depend on location. For example, in the anterior zone of the segment, the axons grow posteriorly, while in the posterior zone, they grow anteriorly. Manipulating the PCP genes can reverse bristle orientation, change the path taken by the dendrite and the position of the cell body of the neuron. However, the paths taken by the axon are not affected.

Conclusion

PCP genes, such as starry night and dachsous orient the bristles and position the neuronal cell body and affect the shape of the dendrites. However, these PCP genes do not appear to change the paths followed by the sensory axons, which must, therefore, be polarised by other factors.

Analysis of four achaete-scute homologs in Bombyx mori reveals new viewpoints of the evolution and functions of this gene family

Background

achaete-scute complexe (AS-C) has been widely studied at genetic, developmental and evolutional levels. Genes of this family encode proteins containing a highly conserved bHLH domain, which take part in the regulation of the development of central nervous system and peripheral nervous system. Many AS-C homologs have been isolated from various vertebrates and invertebrates. Also, AS-C genes are duplicated during the evolution of Diptera. Functions besides neural development controlling have also been found in Drosophila AS-C genes.

Results

We cloned four achaete-scute homologs (ASH) from the lepidopteran model organism Bombyx mori, including three proneural genes and one neural precursor gene. Proteins encoded by them contained the characteristic bHLH domain and the three proneural ones were also found to have the C-terminal conserved motif. These genes regulated promoter activity through the Class A E-boxes in vitro. Though both Bm-ASH and Drosophila AS-C have four members, they are not in one by one corresponding relationships. Results of RT-PCR and real-time PCR showed that Bm-ASH genes were expressed in different larval tissues, and had well-regulated expressional profiles during the development of embryo and wing/wing disc.

Conclusion

There are four achaete-scute homologs in Bombyx mori, the second insect having four AS-C genes so far, and these genes have multiple functions in silkworm life cycle. AS-C gene duplication in insects occurs after or parallel to, but not before the taxonomic order formation during evolution.

Mechanotransduction and auditory transduction in Drosophila

Insects are utterly reliant on sensory mechanotransduction, the process of converting physical stimuli into neuronal receptor potentials. The senses of proprioception, touch, and hearing are involved in almost every aspect of an adult insect's complex behavioral repertoire and are mediated by a diverse array of specialized sensilla and sensory neurons. The physiology and morphology of several of these have been described in detail; genetic approaches in Drosophila, combining behavioral screens and sensory electrophysiology with forward and reverse genetic techniques, have now revealed specific proteins involved in their differentiation and operation. These include three different TRP superfamily ion channels that are required for transduction in tactile bristles, chordotonal stretch receptors, and polymodal nociceptors. Transduction also depends on the normal differentiation and mechanical integrity of the modified cilia that form the neuronal sensory endings, the accessory structures that transmit stimuli to them and, in bristles, a specialized receptor lymph and transepithelial potential. Flies hear near-field sounds with a vibration-sensitive, antennal chordotonal organ. Biomechanical analyses of wild-type antennae reveal non-linear, active mechanical properties that increase their sensitivity to weak stimuli. The effects of mechanosensory and ciliary mutations on antennal mechanics show that the sensory cilia are the active motor elements and indicate distinct roles for TRPN and TRPV channels in auditory transduction and amplification.

Role of proneural genes in the formation of the larval olfactory organ of Drosophila

In this paper, we address the role of proneural genes in the formation of the dorsal organ in the Drosophila larva. This organ is an intricate compound comprising the multineuronal dome-the exclusive larval olfactory organ-and a number of mostly gustatory sensilla. We first determine the numbers of neurons and of the different types of accessory cells in the dorsal organ. From these data, we conclude that the dorsal organ derives from 14 sensory organ precursor cells. Seven of them appear to give rise to the dome, which therefore may be composed of seven fused sensilla, whereas the other precursors produce the remaining sensilla of the dorsal organ. By a loss-of-function approach, we then analyze the role of atonal, amos, and the achaete-scute complex (AS-C), which in the adult are the exclusive proneural genes required for chemosensory organ specification. We show that atonal and amos are necessary and sufficient in a complementary way for four and three of the sensory organ precursors of the dome, respectively. AS-C, on the other hand, is implicated in specifying the non-olfactory sensilla, partially in cooperation with atonal and/or amos. Similar links for these proneural genes with olfactory and gustatory function have been established in the adult fly. However, such conserved gene function is not trivial, given that adult and larval chemosensory organs are anatomically very different and that the development of adult olfactory sensilla involves cell recruitment, which is unlikely to play a role in dome formation.

ADAPTIVE ROLES OF PROGRAMMED CELL DEATH DURING NERVOUS SYSTEM DEVELOPMENT

The programmed cell death (PCD) of developing cells is considered an essential adaptive process that evolved to serve diverse roles. We review the putative adaptive functions of PCD in the animal kingdom with a major focus on PCD in the developing nervous system. Considerable evidence is consistent with the role of PCD in events ranging from neurulation and synaptogenesis to the elimination of adult-generated CNS cells. The remarkable recent progress in our understanding of the genetic regulation of PCD has made it possible to perturb (inhibit) PCD and determine the possible repercussions for nervous system development and function. Although still in their infancy, these studies have so far revealed few striking behavioral or functional phenotypes.

Drosophila Ric-8 regulates Galphai cortical localization to promote Galphai-dependent planar orientation of the mitotic spindle during asymmetric cell division

Localization and activation of heterotrimeric G proteins have a crucial role during asymmetric cell division. The asymmetric division of the Drosophila sensory precursor cell (pl) is polarized along the antero-posterior axis by Frizzled signalling and, during this division, activation of Galphai depends on Partner of Inscuteable (Pins). We establish here that Ric-8, which belongs to a family of guanine nucleotide-exchange factors for Galphai, regulates cortical localization of the subunits Galphai and Gbeta13F. Ric-8, Galphai and Pins are not necessary for the control of the anteroposterior orientation of the mitotic spindle during pl cell division downstream of Frizzled signalling, but they are required for maintainance of the spindle within the plane of the epithelium. On the contrary, Frizzled signalling orients the spindle along the antero-posterior axis but also tilts it along the apico-basal axis. Thus, Frizzled and heterotrimeric G-protein signalling act in opposition to ensure that the spindle aligns both in the plane of the epithelium and along the tissue polarity axis.

glial cells missing and gcm2 cell autonomously regulate both glial and neuronal development in the visual system of Drosophila

The transcription factors Glial cells missing (Gcm) and Gcm2 are known to play a crucial role in promoting glial-cell differentiation during Drosophila embryogenesis. Our findings reveal a central function for gcm genes in regulating neuronal development in the postembryonic visual system. We demonstrate that Gcm and Gcm2 are expressed in both glial and neuronal precursors within the optic lobe. Removal of gcm and gcm2 function shows that the two genes act redundantly and are required for the formation of a subset of glial cells. They also cell-autonomously control the differentiation and proliferation of specific neurons. We show that the transcriptional regulator Dachshund acts downstream of gcm genes and is required to make lamina precursor cells and lamina neurons competent for neuronal differentiation through regulation of epidermal growth factor receptor levels. Our findings further suggest that gcm genes regulate neurogenesis through collaboration with the Hedgehog-signaling pathway.

Evidence that nervy, the Drosophila homolog of ETO/MTG8, promotes mechanosensory organ development by enhancing Notch signaling

In the imaginal tissue of developing fruit flies, achaete (ac) and scute (sc) expression defines a group of neurally-competent cells called the proneural cluster (PNC). From the PNC, a single cell, the sensory organ precursor (SOP), is selected as the adult mechanosensory organ precursor. The SOP expresses high levels of ac and sc and sends a strong Delta (Dl) signal, which activates the Notch (N) receptor in neighboring cells, preventing them from also adopting a neural fate. Previous work has determined how ac and sc expression in the PNC and SOP is regulated, but less is known about SOP-specific factors that promote SOP fate. Here, we describe the role of nervy (nvy), the Drosophila homolog of the mammalian proto-oncogene ETO, in mechanosensory organ formation. Nvy is specifically expressed in the SOP, where it interacts with the Ac and Sc DNA binding partner Daughterless (Da) and affects the expression of Ac and Sc targets. nvy loss- and gain-of-function experiments suggest that nvy reinforces, but is not absolutely required for, the SOP fate. We propose a model in which nvy acts downstream of ac and sc to promote the SOP fate by transiently strengthening the Dl signal emanating from the SOP.

Cullin-5 plays multiple roles in cell fate specification and synapse formation during Drosophila development

We describe a developmental analysis of Drosophila Cullin-5 (Cul-5) identified from the genome sequence on the basis of its high degree of homology to vertebrate and worm sequences. The gene is expressed in a restricted manner in ectodermal cells throughout development suggesting pleiotropic functions. We decided to examine the phenotypes of Cul-5 aberrations in two well-studied developmental systems: the neuromuscular junction (NMJ) and the developing sensory organ. Alteration of Cul-5 levels in motoneurons results in an increase in bouton number at the NMJ. The cells of a sensory organ on the adult notum arise from a single progenitor cell by regulated cell division. Aberrations in Cul-5 affect different steps in the lineage consistent with a role in cell fate determination, proliferation, and death. Such phenotypes highlight the multiple cellular processes in which Cul-5 can participate.

Cell cycle and cell-fate determination in Drosophila neural cell lineages

"Normal" development requires a finely tuned equilibrium between cell differentiation and cell proliferation. Important issues in development include whether the cell cycle controls the cell-fate determination and whether cell identity in turn regulates cell-cycle progression. Although, these issues are of general biological relevance, stereotyped Drosophila neural lineages are particularly suited to address these questions and have provided insights into the links between cell-cycle progression and cell-fate specification.

Lethal Giant Larvae Controls the Localization of Notch-Signaling Regulators Numb, Neuralized, and Sanpodo in Drosophila Sensory-Organ Precursor Cells

Asymmetric distribution of fate determinants is a fundamental mechanism underlying the acquisition of distinct cell fates during asymmetric division. In Drosophila neuroblasts, the apical DmPar6/DaPKC complex inhibits Lethal giant larvae (Lgl) to promote the basal localization of fate determinants. In contrast, in the sensory precursor (pI) cells that divide asymmetrically with a planar polarity, Lgl inhibits Notch signaling in the anterior pI daughter cell, pIIb, by a yet-unknown mechanism. We show here that Lgl promotes the cortical recruitment of Partner of Numb (Pon) and regulates the asymmetric distribution of the fate determinants Numb and Neuralized during the pI cell division. Analysis of Pon-GFP and Histone2B-mRFP distribution in two-color movies confirmed that Lgl regulates Pon localization. Moreover, posterior DaPKC restricts Lgl function to the anterior cortex at mitosis. Thus, Lgl functions similarly in neuroblasts and in pI cells. We also show that Lgl promotes the acquisition of the pIIb cell fate by inhibiting the plasma membrane localization of Sanpodo and thereby preventing the activation of Notch signaling in the anterior pI daughter cell. Thus, Lgl regulates cell fate by controlling Pon cortical localization, asymmetric localization of Numb and Neuralized, and plasma-membrane localization of Sandopo.

Cell cycle diversity involves differential regulation of Cyclin E activity in the Drosophila bristle cell lineage

In the Drosophila bristle lineage, five differentiated cells arise from a precursor cell after a rapid sequence of asymmetric cell divisions (one every 2 hours). We show that, in mitotic cells, this rapid cadence of cell divisions is associated with cell cycles essentially devoid of the G1-phase. This feature is due to the expression of Cyclin E that precedes each cell division, and the differential expression of the S-transition negative regulator, Dacapo. Thus, apart from endocycles (G/S), which occurred in two out of five terminal cells, two other cell cycles coexist in this lineage: (1) an atypical cell cycle (S/G2/M), in which the S-phase is initiated during the preceding telophase; and (2) a canonical cell cycle (G1/S/G2/M) with a brief G1 phase. These two types of cell cycle result from either the absence or very transient expression of Dap, respectively. Finally, we show that the fate determinant factor, Tramtrack, downregulates Cyclin E expression and is probably involved in the exit of the cells from the cell cycle.