Clonal analysis of Drosophila embryonic neuroblasts: neural cell types, axon projections and muscle targets

The Drosophila transmembrane protein Fear-of-intimacy controls glial cell migration

Development of complex organs depends on intensive cell-cell interactions, which help coordinate movements of many cell types. In a genetic screen aimed to identify genes controlling midline glia migration in the Drosophila nervous system, we have identified mutations in the gene kastchen. Here we show that during embryogenesis kastchen is also required for the normal migration of longitudinal and peripheral glial cells. During larval development, kastchen non-cell autonomously affects the migration of the subretinal glia into the eye disc. During embryonic development, kastchen not only affects glial cell migration but also controls the migration of muscle cells toward their attachment sites. In all cases, kastchen apparently functions in terminating or restricting cell migration. We identified the molecular nature of the gene by performing transgenic rescue experiments and by sequence analysis of mutant alleles. Kastchen corresponds to the recently described gene fear-of-intimacy (foi) that was identified in screen for genes affecting germ cell migration, suggesting that Foi-Kastchen is more generally involved in regulating cell migration. It encodes a member of an eight-transmembrane domain protein family of putative Zinc transporters or proteases. We determined the topology of the Foi protein by using antisera against luminal and intracellular domains of the protein and provide evidence that it does not act as a Zinc transporter. Genetic evidence suggests that one of the functions of foi may be associated with hedgehog signaling.

SPECIFICATION OF TEMPORAL IDENTITY IN THE DEVELOPING NERVOUS SYSTEM

The nervous system of higher organisms exhibits extraordinary cellular diversity owing to complex spatial and temporal patterning mechanisms. The role of spatial patterning in generating neuronal diversity is well known; here we discuss how neural progenitors change over time to contribute to cell diversity within the central nervous system (CNS). We focus on five model systems: the vertebrate retina, cortex, hindbrain, spinal cord, and Drosophila neuroblasts. For each, we address the following questions: Do multipotent progenitors generate different neuronal cell types in an invariant order? Do changes in progenitor-intrinsic factors or progenitor-extrinsic signals regulate temporal identity (i.e., the sequence of neuronal cell types produced)? What is the mechanism that regulates temporal identity transitions; i.e., what triggers the switch from one temporal identity to the next? By applying the same criteria to analyze each model system, we try to highlight common themes, point out unique attributes of each system, and identify directions for future research.

Necessity and redundancy of guidepost cells in the embryonic Drosophila CNS

Guidepost cells are specific cellular cues in the embryonic environment utilized by axonal growth cones in pathfinding decisions. In the embryonic Drosophila CNS the RP motor axons make stereotypic pathways choices involving distinct cellular contacts: (i) extension across the midline via contact with the axon and cell body of the homologous contralateral RP motoneuron, (ii) extension down the contralateral longitudinal connective (CLC) through contact with connective axons and longitudinal glia, and (iii) growth into the intersegmental nerve (ISN) through contact with ISN axons and the segmental boundary glial cell (SBC). We have now ablated putative guidepost cells in each of the CNS pathway subsections and uncovered their impact on subsequent RP motor axon pathfinding. Removal of the longitudinal glia or the SBC did not adversely affect pathfinding. This suggests that the motor axons either utilized the alternative axonal substrates, or could still make filopodial contact with the next pathway section's cues. In contrast, RP motor axons did require contact with the axon and soma of their contralateral RP homologue. Absence of this neuronal substrate frequently impeded RP axon outgrowth, suggesting that the next cues were beyond filopodial reach. Together these are the first direct ablations of putative guidepost cells in the CNS of this model system, and have uncovered both pathfinding robustness and susceptibility by RP axons in the absence of specific contacts.

The ladybird homeobox genes are essential for the specification of a subpopulation of neural cells

In Drosophila, neurons and glial cells are produced by neural precursor cells called neuroblasts (NBs), which can be individually identified. Each NB generates a characteristic cell lineage specified by a precise spatiotemporal control of gene expression within the NB and its progeny. Here we show that the homeobox genes ladybird early and ladybird late are expressed in subsets of cells deriving from neuroblasts NB 5-3 and NB 5-6 and are essential for their correct development. Our analysis revealed that ladybird in Drosophila, like their vertebrate orthologous Lbx1 genes, play an important role in cell fate specification processes. Among those cells that express ladybird are NB 5-6-derived glial cells. In ladybird loss-of-function mutants, the NB 5-6-derived exit glial cells are absent while overexpression of these genes leads to supernumerary glial cells of this type. Furthermore, aberrant glial cell positioning and aberrant spacing of axonal fascicles in the nerve roots observed in embryos with altered ladybird function suggest that the ladybird genes might also control directed cell movements and cell-cell interactions within the developing Drosophila ventral nerve cord.

The Drosophila RCC1 homolog, Bj1, regulates nucleocytoplasmic transport and neural differentiation during Drosophila development

The Bj1 gene encodes the Drosophila homolog of RCC1, the guanine-nucleotide exchange factor for RanGTPase. Here, we provide the first phenotypic characterization of a RCC1 homolog in a developmental model system. We identified Bj1 (dRCC1) in a genetic screen to identify mutations that alter central nervous system development. We find that zygotic dRCC1 mutant embryos exhibit specific defects in the development and differentiation of lateral CNS neurons although cell division and the cell cycle appear grossly normal. dRCC1 mutant nerve cords contain abnormally large cells with compartmentalized nuclei and exhibit increased transcription in the lateral CNS. As RCC1 is an important component of the nucleocytoplasmic transport machinery, we find that dRCC1 function is required for nuclear import of nuclear localization signal sequence (NLS)-carrying cargo molecules. Finally, we show that dRCC1 is required for cell proliferation and/or survival during germline, eye and wing development and that dRCC1 appears to facilitate apoptosis.

Turning back the clock on neural progenitors

Drosophila neural progenitor cells, or neuroblasts, alter their transcriptional profile over time to produce different neural cell types. A recent paper by Pearson and Doe shows that older neuroblasts can be reprogrammed to behave like young neuroblasts, and to produce early neural cell types, simply by expressing the transcription factor, Hunchback. The authors show that competence to respond to Hunchback diminishes over time. Manipulating neural progenitors in this way may have important implications for therapeutic uses of neural stem cells.

Development of a polar morphology by identified embryonic motoneurons

Motoneuron morphology arises through the coordinated growth of the motor axon and dendrites. In the Drosophila embryo the RP motoneurons have a contralaterally-extended motor axon, ipsilateral dendrites that extend a short distance in the ipsilateral connective, and a tuft of short dendrites in the contralateral connective. In the present study mechanical and genetic manipulations were utilized to test if (i) the ipsilateral dendrites can develop an axon morphology, (ii) the presence of the contralateral motor axon suppresses the development of an axon-like morphology by the ipsilateral dendrites and (iii) whether establishment of a contralateral motor axon can be genetically suppressed. It was found that an ipsilateral motor axon could develop-but only at the expense of the contralateral motor axon. Axotomy could overturn the normal polarity of the RP motoneurons in favor of the development of an ipsilateral motor axon, and this reversed morphology was also observed when the motor axon could not extend across the midline in the commissureless mutant. These findings show that the RP motoneurons have the plasticity for an alternative polarity, but that the extension of an ipsilateral axon is normally suppressed by the presence of the contralateral axon. The RP motoneurons now represent a genetically amenable in vivo system for analyzing the basis of polarity formation in neurons.

Transcriptional control of glial and blood cell development in Drosophila: cis-regulatory elements of glial cells missing

In Drosophila, glial cell differentiation requires the expression of glial cells missing (gcm) in multiple neural cell lineages, where gcm acts as a binary switch for glial vs. neuronal fate. Thus, the primary event controlling gliogenesis in neural progenitors is the transcription of gcm. In addition, gcm is also required for the differentiation of macrophages, and is expressed in the hemocyte lineage. This dual role of gcm in glial cell and blood cell development underscores the need for the precise temporal and spatial regulation of gcm transcription. To understand how gcm transcription is regulated, we have undertaken an analysis of the cis-regulatory DNA elements of gcm using lacZ reporter activity in transgenic embryos, testing the activity of approximately 35 kilobases of DNA from the gcm locus. We have identified several distinct DNA regions that promote most of the elements of gcm expression. These include elements for general neural expression, gcm-independent and gcm-dependent glial-specific expression, as well as early and late hemocyte expression. We show that expression of an abdominal glial-specific element is dependent on the homeotic gene abdominal-A. Our results indicate that gcm transcription is controlled by a combination of general and lineage-specific elements, positive autoregulation, and neuronal repression.

Mesothoracic ventral unpaired median (mesVUM) neurons in the blowfly Calliphora erythrocephala

The study describes five ventral unpaired median neurons in the mesothoracic neuromere of the fused thoracic ganglion of Calliphora identified by biocytin staining (mesVUM neurons). The group comprises four efferent neurons and one interneuron which are characterized by a common soma cluster in the ventral midline of the neuromere, bifurcating primary neurites and bilaterally symmetrical arborizations. Respective soma clusters of not-yet-identified VUM neurons were also found in the prothoracic, metathoracic, and abdominal neuromeres. The efferent mesVUM neurons are associated with the flight system. Their main arborizations are located in the mesothoracic wing neuropil and their bilateral axons terminate at the flight control muscles, the flight starter muscles, the flight power muscles, or at myocuticular junctions of the latter. In contrast, an association of the interneuron with a particular functional system is not apparent. The arborizations of the neuron are intersegmental and invade all thoracic neuromeres. A further difference between the two types of neurons regards their somatic action potentials, which are overshooting in the efferent neurons and strongly attenuated in the interneuron. Immunocytochemical stainings revealed four clusters of octopamine-immunoreactive (OA-IR) somata in the thoracic ganglion, which reside in the same positions as the VUM somata. We regard this as strong evidence that all groups of VUM neurons contain OA-IR cells and that, in particular, the identified efferent mesVUM neurons are OA-IR. Our results demonstrate that the mesVUM neurons of Calliphora have similar morphological, electrophysiological, and presumably also immunocytochemical characteristics as the unpaired median neurons of other insects.

Embryonic origins of a motor system: motor dendrites form a myotopic map in Drosophila

The organisational principles of locomotor networks are less well understood than those of many sensory systems, where in-growing axon terminals form a central map of peripheral characteristics. Using the neuromuscular system of the Drosophila embryo as a model and retrograde tracing and genetic methods, we have uncovered principles underlying the organisation of the motor system. We find that dendritic arbors of motor neurons, rather than their cell bodies, are partitioned into domains to form a myotopic map, which represents centrally the distribution of body wall muscles peripherally. While muscles are segmental, the myotopic map is parasegmental in organisation. It forms by an active process of dendritic growth independent of the presence of target muscles, proper differentiation of glial cells, or (in its initial partitioning) competitive interactions between adjacent dendritic domains. The arrangement of motor neuron dendrites into a myotopic map represents a first layer of organisation in the motor system. This is likely to be mirrored, at least in part, by endings of higher-order neurons from central pattern-generating circuits, which converge onto the motor neuron dendrites. These findings will greatly simplify the task of understanding how a locomotor system is assembled. Our results suggest that the cues that organise the myotopic map may be laid down early in development as the embryo subdivides into parasegmental units.

Functional characterization of Drosophila sialyltransferase

Sialylation is an important carbohydrate modification of glycoconjugates in the deuterostome lineage of animals. By contrast, the evidence for sialylation in protostomes has been scarce and somewhat controversial. In the present study, we characterize a Drosophila sialyltransferase gene, thus providing experimental evidence for the presence of sialylation in protostomes. This gene encodes a functional alpha2-6-sialyltransferase (SiaT) that is closely related to the vertebrate ST6Gal sialyltransferase family, indicating an ancient evolutionary origin for this family. Characterization of recombinant, purified Drosophila SiaT revealed a novel acceptor specificity as it exhibits highest activity toward GalNAcbeta1-4GlcNAc carbohydrate structures at the non-reducing termini of oligosaccharides and glycoprotein glycans. Oligosaccharides are preferred over glycoproteins as acceptors, and no activity toward glycolipid acceptors was detected. Recombinant Drosophila SiaT expressed in cultured insect cells possesses in vivo and in vitro autosialylation activity toward beta-linked GalNAc termini of its own N-linked glycans, thus representing the first example of a sialylated insect glycoconjugate. In situ hybridization revealed that Drosophila SiaT is expressed during embryonic development in a tissue- and stage-specific fashion, with elevated expression in a subset of cells within the central nervous system. The identification of a SiaT in Drosophila provides a new evolutionary perspective for considering the diverse functions of sialylation and, through the powerful genetic tools available in this system, a means of elucidating functions for sialylation in protostomes.

The control of cell number during central nervous system development in flies and mice

Growth is confined within a size that is normal for each species, revealing that somehow an organism 'knows' when this size has been reached. Within a species, growth is also variable, but despite this, proportion and structure are maintained. Perhaps, the key element in the control of size is the control of cell number. Here we review current knowledge on the mechanisms controlling cell number in the nervous system of vertebrates and flies. During growth, clonal expansion is confined, the number of progeny cells is balanced through the control of cell survival and cell proliferation and excess cells are eliminated by apoptosis. Simultaneously, organ architecture emerges and as neurons become active they also influence growth. The interactive control of cell number provides developmental plasticity to nervous system development. Many findings are common between flies and mice, other aspects have been studied more in one organism than the other and there are also aspects that are unique to either organism. Although cell number control has long been studied in the nervous system, analogous mechanisms are likely to operate during the growth of other organs and organisms.

Specification of motoneuron fate inDrosophila: Integration of positive and negative transcription factor inputs by a minimaleve enhancer

We are interested in the mechanisms that generate neuronal diversity within the Drosophila central nervous system (CNS), and in particular in the development of a single identified motoneuron called RP2. Expression of the homeodomain transcription factor Even-skipped (Eve) is required for RP2 to establish proper connectivity with its muscle target. Here we investigate the mechanisms by which eve is specifically expressed within the RP2 motoneuron lineage. Within the NB4-2 lineage, expression of eve first occurs in the precursor of RP2, called GMC4-2a. We identify a small 500 base pair eve enhancer that mediates eve expression in GMC4-2a. We show that four different transcription factors (Prospero, Huckebein, Fushi tarazu, and Pdm1) are all expressed in GMC4-2a, and are required to activate eve via this minimal enhancer, and that one transcription factor (Klumpfuss) represses eve expression via this element. All four positively acting transcription factors act independently, regulating eve but not each other. Thus, the eve enhancer integrates multiple positive and negative transcription factor inputs to restrict eve expression to a single precursor cell (GMC4-2a) and its RP2 motoneuron progeny.

Regulation of neuroblast competence in Drosophila

Individual neural progenitors generate different cell types in a reproducible order in the retina, cerebral cortex and probably in the spinal cord. It is unknown how neural progenitors change over time to generate different cell types. It has been proposed that progenitors undergo progressive restriction or transit through distinct competence states; however, the underlying molecular mechanisms remain unclear. Here we investigate neural progenitor competence and temporal identity using an in vivo genetic system--Drosophila neuroblasts--where the Hunchback transcription factor is necessary and sufficient to specify early-born cell types. We show that neuroblasts gradually lose competence to generate early-born fates in response to Hunchback, similar to progressive restriction models, and that competence to acquire early-born fates is present in mitotic precursors but is lost in post-mitotic neurons. These results match those observed in vertebrate systems, and establish Drosophila neuroblasts as a model system for the molecular genetic analysis of neural progenitor competence and plasticity.

Dissecting Drosophila embryonic brain development using photoactivated gene expression

The Drosophila brain is generated by a complex series of morphogenetic movements. To better understand brain development and to provide a guide for experimental manipulation of brain progenitors, we created a fate map using photoactivated gene expression to mark cells originating within specific mitotic domains and time-lapse microscopy to dynamically monitor their progeny. We show that mitotic domains 1, 5, and 9 give rise to discrete cell populations within specific regions of the brain. Two novel observations were that the antennal sensory system, composed of four disparate cell clusters, arose from mitotic domain 5 and that mitotic domain B produced glial cells, while neurons were produced from mitotic domains 1, 5, and 9. Time-lapse analysis of marked cells showed complex mitotic and migratory patterns for cells derived from these mitotic domains. Photoactivated gene expression was also used either to kill, to induce ectopic divisions, or to alter cell fate. This revealed that deficits were not repopulated, while ectopic cells were removed and extra glia were tolerated.

Control of developmental timing by micrornas and their targets

In Caenorhabditis elegans the timing of many developmental events is regulated by heterochronic genes. Such genes orchestrate the timing of cell divisions and fates appropriate for the developmental stage of an organism. Analyses of heterochronic mutations in the nematode C. elegans have revealed a genetic pathway that controls the timing of post-embryonic cell divisions and fates. Two of the genes in this pathway encode small regulatory RNAs. The 22 nucleotide (nt) RNAs downregulate the expression of protein-coding mRNAs of target heterochronic genes. Analogous variations in the timing of appearance of particular features have been noted among closely related species, suggesting that such explicit control of developmental timing may not be exclusive to C. elegans. In fact, some of the genes that globally pattern the temporal progression of C. elegans development, including one of the tiny RNA genes, are conserved and temporally regulated across much of animal phylogeny, suggesting that the molecular mechanisms of temporal control are ancient and universal. A very large family of tiny RNA genes called microRNAs, which are similar in structure to the heterochronic regulatory RNAs, have been detected in diverse animal species and are likely to be present in most metazoans. Functions of the newly discovered microRNAs are not yet known. Other examples of temporal programs during growth include the exquisitely choreographed temporal sequences of developmental fates in neurogenesis in Drosophila and the sequential programs of epidermal coloration in insect wing patterning. An interesting possibility is that microRNAs mediate transitions on a variety of time scales to pattern the activities of particular target protein-coding genes and in turn generate sets of cells over a period of time. Plasticity in these microRNA genes or their targets may lead to changes in relative developmental timing between related species, or heterochronic change. Instead of inventing new gene functions, even subtle changes in temporal expression of pre-existing control genes can result in speciation by altering the time at which they function.

Targeted expression of tetanus toxin reveals sets of neurons involved in larval locomotion in Drosophila

The Drosophila larva is widely used for studies of neuronal development and function, yet little is known about the neuronal basis of locomotion in this model organism. Drosophila larvae crawl over a plain substrate by performing repetitive waves of forward peristalsis alternated by brief episodes of head swinging and turning. To identify sets of central and peripheral neurons required for the spatial or temporal pattern of larval locomotion, we blocked neurotransmitter release from defined populations of neurons by targeted expression of tetanus toxin light chain (TeTxLC) with the GAL4/UAS system. One hundred fifty GAL4 lines were crossed to a UAS-TeTxLC strain and a motion-analysis system was used to identify larvae with abnormal movement patterns. Five lines were selected that show discrete locomotor defects (i.e., increased turning and pausing) and these defects are correlated with diverse sets of central neurons. One line, 4C-GAL4, caused an unusual circling behavior that is correlated with approximately 200 neurons, including dopaminergic and peptidergic interneurons. Expression of TeTxLC in all dopaminergic and serotonergic but not in peptidergic neurons, caused turning deficits that are similar to those of 4C-GAL4/TeTxLC larvae. The results presented here provide a basis for future genetic studies of motor control in the Drosophila larva.

Transcriptional regulation of glial cell specification

Neuronal differentiation relies on proneural factors that also integrate positional information and contribute to the specification of the neuronal type. The molecular pathway triggering glial specification is not understood yet. In Drosophila, all lateral glial precursors and glial-promoting activity have been identified, which provides us with a unique opportunity to dissect the regulatory pathways controlling glial differentiation and specification. Although glial lineages are very heterogeneous with respect to position, time of differentiation, and lineage tree, they all express and require two homologous genes, glial cell deficient/glial cell missing (glide/gcm) and glide2, that act in concert, with glide/gcm constituting the major glial-promoting factor. Here, we show that glial specification resides in glide/gcm transcriptional regulation. The glide/gcm promoter contains lineage-specific elements as well as quantitative and turmoil elements scattered throughout several kilobases. Interestingly, there is no correlation between a specific regulatory element and the type of glial lineage. Thus, the glial-promoting factor acts as a naive switch-on button that triggers gliogenesis in response to multiple pathways converging onto its promoter. Both negative and positive regulation are required to control glide/gcm expression, indicating that gliogenesis is actively repressed in some neural lineages.

Genetic control of Drosophila nerve cord development

The Drosophila ventral nerve cord has been a central model system for studying the molecular genetic mechanisms that control CNS development. Studies show that the generation of neural diversity is a multistep process initiated by the patterning and segmentation of the neuroectoderm. These events act together with the process of lateral inhibition to generate precursor cells (neuroblasts) with specific identities, distinguished by the expression of unique combinations of regulatory genes. The expression of these genes in a given neuroblast restricts the fate of its progeny, by activating specific combinations of downstream genes. These genes in turn specify the identity of any given postmitotic cell, which is evident by its cellular morphology and choice of neurotransmitter.

Apical Complex Genes Control Mitotic Spindle Geometry and Relative Size of Daughter Cells in Drosophila Neuroblast and pI Asymmetric Divisions

Drosophila neuroblast asymmetric divisions generate two daughters of unequal size and fate. A complex of apically localized molecules mediates basal localization of cell fate determinants and apicobasal orientation of the mitotic spindle, but how daughter cell size is controlled remains unclear. Here we show that mitotic spindle geometry and unequal daughter cell size are controlled by two parallel pathways (Bazooka/DaPKC and Pins/G alpha i) within the apical complex. While the localized activity of either pathway alone is sufficient to mediate the generation of an asymmetric mitotic spindle and unequal size neuroblast daughters, loss of both pathways results in symmetric divisions. In sensory organ precursors, Bazooka/DaPKC and Pins/G alpha i localize to opposite sides of the cortex and function in opposition to generate a symmetric spindle.

Comm sorts robo to control axon guidance at the Drosophila midline

Axon growth across the Drosophila midline requires Comm to downregulate Robo, the receptor for the midline repellent Slit. We show here that comm is required in neurons, not in midline cells as previously thought, and that it is expressed specifically and transiently in commissural neurons. Comm acts as a sorting receptor for Robo, diverting it from the synthetic to the late endocytic pathway. A conserved cytoplasmic LPSY motif is required for endosomal sorting of Comm in vitro and for Comm to downregulate Robo and promote midline crossing in vivo. Axon traffic at the CNS midline is thus controlled by the intracellular trafficking of the Robo guidance receptor, which in turn depends on the precisely regulated expression of the Comm sorting receptor.

glide/gcm: at the crossroads between neurons and glia

Neurons and glia are generated by multipotent precursors. Recent studies indicate that the choice between the two fates depends on the combined activity of extracellular influences and factors that respond to precise spatial and temporal cues. Drosophila provides a simple genetic model to study the cellular and molecular mechanisms controlling fate choice, mode of precursor division and generation of cell diversity. Moreover, all glial precursors and glial-promoting activities have been identified in Drosophila, which provides us with a unique opportunity to dissect regulatory pathways controlling glial differentiation and specification.

Cell lineage specification in the nervous system

Nervous system development requires the specification of numerous neural stem cells. Subsequently these stem cells divide in a spatially and temporally controlled manner to generate the diverse cell types found in the different layers of the nervous system. Lineage specification is brought about by transcriptional regulators, which often act as transcriptional repressors.

Asymmetric division of Drosophila neural progenitors

Asymmetric division is a fundamental mechanism for generating cellular diversity. Studies on Drosophila neural progenitors have provided valuable insight into how evolutionarily conserved protein cassettes may be differentially deployed in different developmental contexts to mediate asymmetric divisions. Recent findings also suggest possible mechanisms by which the processes of cell-cycle progression, neuronal lineage development and asymmetric divisions may be integrated.

Inscuteable-independent apicobasally oriented asymmetric divisions in the Drosophila embryonic CNS

Inscuteable is the founding member of a protein complex localised to the apical cortex of Drosophila neural progenitors that controls their asymmetric division. Aspects of asymmetric divisions of all identified apicobasally oriented neural progenitors characterised to date, in both the central and peripheral nervous systems, require inscuteable. Here we examine the generality of this requirement. We show that many identified neuroblast lineages, in fact, do not require inscuteable for normal morphological development. To elucidate the requirements for apicobasal asymmetric divisions in a context where inscuteable is not essential, we focused on the MP2 > dMP2 + vMP2 division. We show that for MP2 divisions, asymmetric localisation and segregation of Numb and the specification of distinct dMP2 and vMP2 identities require bazooka but not inscuteable. We conclude that inscuteable is not required for all apicobasally oriented asymmetric divisions and that, in some cellular contexts, bazooka can mediate apicobasal asymmetric divisions without inscuteable.

Drosophila homeodomain protein dHb9 directs neuronal fate via crossrepressive and cell-nonautonomous mechanisms

Here we present the identification and characterization of dHb9, the Drosophila homolog of vertebrate Hb9, which encodes a factor central to motorneuron (MN) development. We show that dHb9 regulates neuronal fate by restricting expression of Lim3 and Even-skipped (Eve), two homeodomain (HD) proteins required for development of distinct neuronal classes. Also, dHb9 and Lim3 are activated independently of each other in a virtually identical population of ventrally and laterally projecting MNs. Surprisingly, dHb9 represses Lim3 cell nonautonomously in a subset of dorsally projecting MNs, revealing a novel role for intercellular signaling in the establishment of neuronal fate in Drosophila. Lastly, we provide evidence that dHb9 and Eve regulate each other's expression through Groucho-dependent crossrepression. This mutually antagonistic relationship bears similarity to the crossrepressive relationships between pairs of HD proteins that pattern the vertebrate neural tube.

Interactive nervous system development: control of cell survival in Drosophila

The non-autonomous control of cell survival has long been thought to be a mechanism of adjusting cell populations in the vertebrate nervous system, enabling connectivity and myelination to produce a functional brain. Despite cellular evidence that analogous mechanisms occur in invertebrates, scepticism has long reigned over whether they operate in model organisms such as Drosophila. This has led to speculation that there are inherent differences between the development and evolution of simple brains and the brains of vertebrates. The great paradox has, until recently, been the absence of molecular evidence of trophic factors in Drosophila. Recent data have finally shown that EGFR (epidermal-growth-factor receptor) ligands function in the Drosophila CNS to maintain glial survival. Trophic interactions are, thus, a general mechanism of nervous system development.

Membrane targeting and asymmetric localization of Drosophila partner of inscuteable are discrete steps controlled by distinct regions of the protein

Asymmetric division of neural progenitors is a key mechanism by which neuronal diversity in the Drosophila central nervous system is generated. The distinct fates of the daughter cells derived from these divisions are achieved through preferential segregation of the cell fate determinants Prospero and Numb to one of the two daughters. This is achieved by coordinating apical and basal mitotic spindle orientation with the basal cortical localization of the cell fate determinants during mitosis. A complex of apically localized proteins, including Inscuteable (Insc), Partner of Inscuteable (Pins), Bazooka (Baz), DmPar-6, DaPKC, and G alpha i, is required to mediate and coordinate basal protein localization with mitotic spindle orientation. Pins, a molecule which directly interacts with Insc, is a key component required for the integrity of this complex; in the absence of Pins, other components become mislocalized or destabilized, and basal protein localization and mitotic spindle orientation are defective. Here we define the functional domains of Pins. We show that the C-terminal region containing the G alpha i binding GoLoco motifs is necessary and sufficient for targeting to the neuroblast cortex, which appears to be a prerequisite for apical localization of Pins. The N-terminal tetratricopeptide repeat-containing region of Pins is required for two processes; TPR repeats 1 to 3 plus the C-terminal region are required for apical localization but are insufficient to recruit Insc to the apical cortex, whereas TPR repeats 1 to 7 plus C-terminal Pins can perform both functions. Hence, the abilities of Pins to cortically localize, to apically localize, and to restore Insc apical localization are all separable, and all three capabilities are necessary to mediate asymmetric division. Moreover, the need for N-terminal Pins can be obviated by fusing a minimal Insc functional domain with the C-terminal region of Pins; this chimeric molecule is apically localized and can fulfill the functions of both Insc and Pins.

Precocious expression of the Glide/Gcm glial-promoting factor in Drosophila induces neurogenesis

Neurons and glial cells depend on similar developmental pathways and often originate from common precursors; however, the differentiation of one or the other cell type depends on the activation of cell-specific pathways. In Drosophila, the differentiation of glial cells depends on a transcription factor, Glide/Gcm. This glial-promoting factor is both necessary and sufficient to induce the central and peripheral glial fates at the expense of the neuronal fate. In a screen for mutations affecting the adult peripheral nervous system, we have found a dominant mutation inducing supernumerary sensory organs. Surprisingly, this mutation is allelic to glide/gcm and induces precocious glide/gcm expression, which, in turn, activates the proneural genes. As a consequence, sensory organs are induced. Thus, temporal misregulation of the Glide/Gcm glial-promoting factor reveals a novel potential for this cell fate determinant. At the molecular level, this implies unpredicted features of the glide/gcm pathway. These findings also emphasize the requirement for both spatial and temporal glide/gcm regulation to achieve proper cell specification within the nervous system.

Pattern of expression of engrailed in relation to gamma-aminobutyric acid immunoreactivity in the central nervous system of the adult grasshopper

Engrailed (En) protein expression in neurons of the mesothoracic and metathoracic ganglia of the adult grasshopper, Schistocerca americana, was examined by immunohistochemistry. Each neuromere had a dorsally located cluster of En-positive neurons within the dorsal unpaired median (DUM) group, comprising one cluster in the mesothoracic ganglion (T2) and four clusters in the metathoracic ganglion, one for each component neuromere (T3, A1, A2, A3). Ventrally, En-positive neurons occurred in the posterior one-third of each neuromere. In T2 and T3, three ventral groups of neurons were labeled bilaterally. In the abdominal neuromeres, many fewer ventral neurons were En-positive. These also were bilaterally symmetrical, but did not occur in patterns that allowed assignment of homology with the T2 and T3 groups. Altogether, En-positive neurons comprised roughly 10% of the ganglionic populations. In the bilateral groups, as in the DUM groups, En expression was restricted to interneurons, consistent with the suggestion that En expression contributes to some aspect of interneuronal phenotype. En-positive neurons in the DUM groups also expressed gamma-aminobutyric acid (GABA) immunoreactivity. Further study showed that all neurons in one En-positive bilateral group and some neurons in another bilateral group were GABA immunoreactive, but that neurons in a third bilateral group were En-positive only. Additionally, several discrete clusters of neurons were GABA-immunoreactive but En-negative. A provisional morphological scheme is presented, which relates the several neuronal clusters to their likely neuroblasts of origin, as a basis for further research into the composition of neuronal lineages.

Axon guidance of Drosophila SNb motoneurons depends on the cooperative action of muscular Krüppel and neuronal capricious activities

The body wall musculature of the Drosophila larva consists of a stereotyped pattern of 30 muscles per abdominal hemisegment which are innervated by about 40 distinct motoneurons. Proper innervation by motoneurons is established during late embryogenesis. Guidance of motor axons to specific muscles requires appropriate pathfinding decisions as they follow their pathways within the central nervous system and on the surface of muscles. Once the appropriate targets are reached, stable synaptic contacts between motoneurons and muscles are formed. Recent studies revealed a number of molecular components required for proper motor axon pathfinding and demonstrated specific roles in fasciculation/defasciculation events, a key process in the formation of discrete motoneuron pathways. The gene capricious (caps), which encodes a cell-surface protein, functions as a recognition molecule in motor axon guidance, regulating the formation of the selective connections between the SNb-derived motoneuron RP5 and muscle 12. Here we show that Krüppel (Kr), best known as a segmentation gene of the gap class, functionally interacts with caps in establishing the proper axonal pathway of SNb including the RP5 axons. The results suggest that the transcription factor Krüppel participates in proper control of cell-surface molecules which are necessary for the SNb neurons to navigate in a caps-dependent manner within the array of the ventral longitudinal target muscles.

Glial cell development in the Drosophila embryo

Glial cells play a central role in the development and function of complex nervous systems. Drosophila is an excellent model organism for the study of mechanisms underlying neural development, and recent attention has been focused on the differentiation and function of glial cells. We now have a nearly complete description of glial cell organization in the embryo, which enables a systematic genetic analysis of glial cell development. Most glia arise from neural stem cells that originate in the neurogenic ectoderm. The bifurcation of glial and neuronal fates is under the control of the glial promoting factor glial cells missing. Differentiation is propagated through the regulation of several transcription factors. Genes have been discovered affecting the terminal differentiation of glia, including the promotion glial-neuronal interactions and the formation of the blood-nerve barrier. Other roles of glia are being explored, including their requirement for axon guidance, neuronal survival, and signaling.

Drosophila neuroblasts sequentially express transcription factors which specify the temporal identity of their neuronal progeny

Neural precursors often generate distinct cell types in a specific order, but the intrinsic or extrinsic cues regulating the timing of cell fate specification are poorly understood. Here we show that Drosophila neural precursors (neuroblasts) sequentially express the transcription factors Hunchback --> Krüppel --> Pdm --> Castor, with differentiated progeny maintaining the transcription factor profile present at their birth. Hunchback is necessary and sufficient for first-born cell fates, whereas Krüppel is necessary and sufficient for second-born cell fates; this is observed in multiple lineages and is independent of the cell type involved. We propose that Hunchback and Krüppel control early-born temporal identity in neuroblast cell lineages.

Lineage diversity in the Drosophila nervous system

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

Correlated patterns of neuron differentiation and Hox gene expression in the hindbrain: a comparative analysis

Hindbrain neurons are organized into coherent subpopulations with characteristic projection patterns and functions. Many of these serve vital functions that have been conserved throughout the vertebrate radiation, but diversification to modified or highly specialized functions has also occurred. The differentiation of identifiable neuron groups in specific spatial domains must involve the regional expression of determinants within the hindbrain neuroepithelium. The Hox genes are involved in longitudinal regionalization of the neural tube, and their expression patterns in the hindbrain are closely related to the rhombomeres which partition the hindbrain into morphogenetic units. Hox gene expression also exhibits conserved patterning as well as phylogenetic variation. One plausible mechanism that may have contributed to evolutionary diversification in hindbrain neuron populations is therefore the emergence of species-specific differences in Hox gene expression. This article presents a comparative overview of the regional patterning of selected Hox genes and hindbrain neuron populations in several embryologically important species. Although tantalizing correlations exist, the relationship between Hox genes and neuronal patterning is complex, and complicated by dynamic features in each. Much more comparative and developmental data must be obtained before the link between Hox gene expression and hindbrain neuron patterning can be elucidated satisfactorily in an evolutionary context.

Neurons and glia in the midline of the higher crustacean Orchestia cavimana are generated via an invariant cell lineage that comprises a median neuroblast and glial progenitors

Midline cells are a common feature of both insects and crustaceans. Midline cells in the insects Schistocerca americana and Drosophila melanogaster have been shown to give rise to pairs of either neurons or glial cells (midline precursor) as well as to repeatedly generate neurons (median neuroblast) or both neurons and glia (median neuroglioblast). This study addresses midline cell lineages in a higher crustacean, the amphipod Orchestia cavimana. In vivo labeling of single midline cells shows that the resulting cell lineage is invariant and that these cells act as progenitors for sets of three glial precursors and one median neuroblast. The progeny are restricted to parasegmental units. The glial precursors give rise to three pairs of glial cells; two of them enwrap the commissures. The median neuroblast gives rise to about 10 cells that differentiate into 3 classes of neurons. The presence of median neuroblasts is also shown for another higher crustacean, the isopod Porcellio scaber using BrdU labeling. This is the first study to analyze the cell lineage of crustacean neurons generated by early ectodermal precursors. A comparison with those of insects demonstrates both conservation and change during the evolution of arthropods.

A novel mode of asymmetric division identifies the fly neuroglioblast 6-4T

Asymmetric cell divisions and segregation of fate determinants are crucial events in the generation of cell diversity. Fly neuroblasts, the precursors that self-reproduce and generate neurons, represent a clear example of asymmetrically dividing cells. Less is known about how neurons and glial cells are generated by multipotent precursors. Flies provide the ideal model system to study this process. Indeed, neuroglioblasts (NGBs) can be specifically identified and have been shown to require the glide/gcm fate determinant to produce glial cells, which otherwise would become neurons. Here, we follow the division of a specific NGB (NGB6-4T), which produces a neuroblast (NB) and a glioblast (GB). We show that, to generate the glioblast, glide/gcm RNA becomes progressively unequally distributed during NGB division and preferentially segregates. Subsequently, a GB-specific factor is required to maintain glide/gcm expression. Both processes are necessary for gliogenesis, showing that the glial vs. neuronal fate choice is a two-step process. This feature, together with glide/gcm subcellular RNA distribution and the behavior of the NGB mitotic apparatus identify a novel type of division generating cell diversity.

Glial cell development inDrosophila

In the Drosophila central nervous system (CNS) about 10% of the cells are of glial nature. A set of molecular markers has allowed unraveling a number of genes controlling glial cell fate determination as well as genes required for glial cell differentiation. Here we focus on the embryonic CNS glia and review the recent progress in the field.

Notch signaling represses the glial fate in fly PNS

By using gain-of-function mutations it has been proposed that vertebrate Notch promotes the glial fate. We show in vivo that glial cells are produced at the expense of neurons in the peripheral nervous system of flies lacking Notch and that constitutively activated Notch produces the opposite phenotype. Notch acts as a genetic switch between neuronal and glial fates by negatively regulating glial cell deficient/glial cells missing, the gene required in the glial precursor to induce gliogenesis. Moreover, Notch represses neurogenesis or gliogenesis, depending on the sensory organ type. Numb, which is asymmetrically localized in the multipotent cell that produces the glial precursor, induces glial cells at the expense of neurons. Thus, a cell-autonomous mechanism inhibits Notch signaling.

A requirement for Notch in the genesis of a subset of glial cells in the Drosophila embryonic central nervous system which arise through asymmetric divisions

In the Drosophila central nervous system (CNS) glial cells are known to be generated from glioblasts, which produce exclusively glia or neuroglioblasts that bifurcate to produce both neuronal and glial sublineages. We show that the genesis of a subset of glial cells, the subperineurial glia (SPGs), involves a new mechanism and requires Notch. We demonstrate that the SPGs share direct sibling relationships with neurones and are the products of asymmetric divisions. This mechanism of specifying glial cell fates within the CNS is novel and provides further insight into regulatory interactions leading to glial cell fate determination. Furthermore, we show that Notch signalling positively regulates glial cells missing (gcm) expression in the context of SPG development.

sidestep encodes a target-derived attractant essential for motor axon guidance in Drosophila

At specific choice points in the periphery, subsets of motor axons defasciculate from other axons in the motor nerves and steer into their muscle target regions. Using a large-scale genetic screen in Drosophila, we identified the sidestep (side) gene as essential for motor axons to leave the motor nerves and enter their muscle targets. side encodes a target-derived transmembrane protein (Side) that is a novel member of the immunoglobulin superfamily (IgSF). Side is expressed on embryonic muscles during the period when motor axons leave their nerves and extend onto these muscles. In side mutant embryos, motor axons fail to extend onto muscles and instead continue to extend along their motor nerves. Ectopic expression of Side results in extensive and prolonged motor axon contact with inappropriate tissues expressing Side.

A gain-of-function screen for genes controlling motor axon guidance and synaptogenesis in Drosophila

BACKGROUND

The neuromuscular system of the Drosophila larva contains a small number of identified motor neurons that make genetically defined synaptic connections with muscle fibers. We drove high-level expression of genes in these motor neurons by crossing 2293 GAL4-driven EP element lines with known insertion site sequences to lines containing a pan-neuronal GAL4 source and UAS-green fluorescent protein elements. This allowed visualization of every synapse in the neuromuscular system in live larvae.

RESULTS

We identified 114 EPs that generate axon guidance and/or synaptogenesis phenotypes in F1 EP x driver larvae. Analysis of genomic regions adjacent to these EPs defined 76 genes that exhibit neuromuscular gain-of-function phenotypes. Forty-one of these (known genes) have published mutant alleles; the other 35 (new genes) have not yet been characterized genetically. To assess the roles of the known genes, we surveyed published data on their phenotypes and expression patterns. We also examined loss-of-function mutants ourselves, identifying new guidance and synaptogenesis phenotypes for eight genes. At least three quarters of the known genes are important for nervous system development and/or function in wild-type flies.

CONCLUSIONS

Known genes, new genes, and a set of previously analyzed genes with phenotypes in the Adh region display similar patterns of homology to sequences in other species and have equivalent EST representations. We infer from these results that most new genes will also have nervous system loss-of-function phenotypes. The proteins encoded by the 76 identified genes include GTPase regulators, vesicle trafficking proteins, kinases, and RNA binding proteins.

Dopamine-synthesizing neurons include the putative H-cell homologue in the moth Manduca sexta

The catecholamine dopamine (DA) plays a fundamental role in the regulation of behavior and neurodevelopment across animal species. Uncovering the embryonic origins of neurons that express DA opens a path for a deeper understanding of how DA expression is regulated and, in turn, how DA regulates the activities of the nervous system. In a well-established insect model, Manduca sexta, we identified the putative homologue of the embryonic grasshopper "H-cell" using intracellular techniques, laser scanning confocal microscopy, and immunohistochemistry. In both species, this neuron possesses four axons and has central projections resembling the letter H. The H-cell in grasshoppers is known to be derived from the midline precursor 3 cell (MP3) and to pioneer the pathways of the longitudinal connectives; in Drosophila, the H-cell is also known to be derived from MP3. In the current study, we demonstrate that the Manduca H-cell is immunoreactive to antibodies raised against DA and its rate-limiting synthetic enzyme, tyrosine hydroxylase (TH). In larvae and adults, one DA/TH-immunoreactive (-ir) H-cell per ganglion is present. In embryos, individual ganglia contain a single midline TH-ir cell body positioned along side its putative sibling. Such observations are consistent with the known secondary transformation (in grasshoppers) of only one of the two MP3 progeny during early development. Although a hallmark feature of invertebrate neurons is the fairly stereotypical position of neuronal somata, we found that the H-cell somata can "flip-flop" by 180 degrees between an anterior and posterior position. This variability appears to be random and is not restricted to any particular ganglion. Curiously, what is segment-specific is the absence of the DA/TH-ir H-cell in the metathoracic (T3) ganglion as well as the unique structure of the H-cell in the subesophageal ganglion. Because this is the first immunohistochemical study of DA neurons in Manduca, we have provided the distribution pattern and morphologies of dopaminergic neurons, in addition to the H-cells, within the ventral nerve cord during development.

Complex genetic interactions among four receptor tyrosine phosphatases regulate axon guidance in Drosophila

Four receptor-linked protein tyrosine phosphatases are selectively expressed on central nervous system axons in the Drosophila embryo. Published data show that three of these (DLAR, DPTP69D, DPTP99A) regulate motor axon guidance decisions during embryonic development. Here we examine the role of the fourth neural phosphatase, DPTP10D, by analyzing double-, triple-, and quadruple-mutant embryos lacking all possible combinations of the phosphatases. This analysis shows that all four phosphatases participate in guidance of interneuronal axons within the longitudinal tracts of the central nervous system. In the neuromuscular system, DPTP10D works together with the other three phosphatases to facilitate outgrowth and bifurcation of the SNa nerve, but acts in opposition to the others in regulating extension of ISN motor axons past intermediate targets. Our results provide evidence for three kinds of genetic interactions among the neural tyrosine phosphatases: partial redundancy, competition, and collaboration.

Single-cell analysis of Drosophila larval neuromuscular synapses

The neuromuscular system of Drosophila has been widely used in studies on synaptic development. In the embryo, the cellular components of this model system are well established, with uniquely identified motoneurons displaying specific connectivity with distinct muscles. Such knowledge is essential to analyzing axon guidance and synaptic matching mechanisms with single-cell resolution. In contrast, to date the cellular identities of the larval neuromuscular synapses are hardly established. It is not known whether synaptic connections seen in the embryo persist, nor is it known how individual motor endings may differentiate through the larval stages. In this study, we combine single-cell dye labeling of individual synaptic boutons and counterstaining of the entire nervous system to characterize the synaptic partners and bouton differentiation of the 30 motoneuron axons from four nerve branches (ISN, SNa, SNb, and SNd). We also show the cell body locations of 4 larval motoneurons (RP3, RP5, V, and MN13-Ib) and the types of innervation they develop. Our observations support the following: (1) Only 1 motoneuron axon of a given bouton type innervates a single muscle, while up to 4 motoneuron axons of different bouton types can innervate the same muscle. (2) The type of boutons which each motoneuron axon forms is likely influenced by cell-autonomous factors. The data offer a basis for studying the properties of synaptic differentiation, maintenance, and plasticity with a high cellular resolution.

The midline glia of Drosophila: a molecular genetic model for the developmental functions of glia

The Midline Glia of Drosophila are required for nervous system morphogenesis and midline axon guidance during embryogenesis. In origin, gene expression and function, this lineage is analogous to the floorplate of the vertebrate neural tube. The expression or function of over 50 genes, summarised here, has been linked to the Midline Glia. Like the floorplate, the cells which generate the Midline Glia lineage, the mesectoderm, are determined by the interaction of ectoderm and mesoderm during gastrulation. Determination and differentiation of the Midline Glia involves the Drosophila EGF, Notch and segment polarity signaling pathways, as well as twelve identified transcription factors. The Midline Glia lineage has two phases of cell proliferation and of programmed cell death. During embryogenesis, the EGF receptor pathway signaling and Wrapper protein both function to suppress apoptosis only in those MG which are appropriately positioned to separate and ensheath midline axonal commissures. Apoptosis during metamorphosis is regulated by the insect steroid, Ecdysone. The Midline Glia participate in both the attraction of axonal growth cones towards the midline, as well as repulsion of growth cones from the midline. Midline axon guidance requires the Drosophila orthologs of vertebrate genes expressed in the floorplate, which perform the same function. Genetic and molecular evidence of the interaction of attractive (Netrin) and repellent (Slit) signaling is reviewed and summarised in a model. The Midline Glia participate also in the generation of extracellular matrix and in trophic interactions with axons. Genetic evidence for these functions is reviewed.

Surprises from Drosophila: genetic mechanisms of synaptic development and plasticity

Drosophila are excellent models for the study of synaptic development and plasticity, thanks to the availability and applicability of a wide variety of powerful molecular, genetic, and cell-biology techniques. Three decades of study have led to an intimate understanding of the sequence of events leading to a functional and plastic synapse, yet many of the molecular mechanisms underlying these events are still poorly understood. Here, we provide a review of synaptogenesis at the Drosophila glutamatergic neuromuscular junction (NMJ). Next, we discuss the role of two proteins that forward genetic screens in Drosophila have revealed to play crucial-and completely unexpected-roles in NMJ development and plasticity: the origin of replication complex protein Latheo, and the enzyme glutamate decarboxylase. The requirement for these proteins at the NMJ highlights the fact that synaptic development and plasticity involves intense inter- and intracellular signaling about which we know almost nothing.