Glia development in the embryonic CNS of Drosophila

Astrocyte identity: evolutionary perspectives on astrocyte functions and heterogeneity

The development of new animal models, in vivo isolation approaches, and improvements in genome-wide RNA expression methods have greatly propelled molecular profiling of astrocytes and the characterization of astrocyte heterogeneity in the central nervous system (CNS). Several recent reviews have comprehensively discussed the molecular and functional diversity of mammalian astrocytes. In this brief review, we emphasize interspecies comparisons and an evolutionary perspective regarding the astro(glia) of vertebrates and invertebrates which are similar in form and function. This analysis has revealed conserved astrocyte transcriptomes in the fly, mouse and human. We also offer opinions about the pattern and origin of astrocyte heterogeneity in the CNS.

Lessons from Drosophila Pointed, an ETS family transcription factor and key nuclear effector of the RTK signaling pathway

The ETS family of transcription factors are evolutionarily conserved throughout the metazoan lineage and are critical for regulating cellular processes such as proliferation, differentiation, apoptosis, angiogenesis, and migration. All members have an ETS DNA binding domain, while a subset also has a protein-protein interaction domain called the SAM domain. Pointed (Pnt), an ETS transcriptional activator functions downstream of the receptor tyrosine kinase (RTK) signaling pathway to regulate diverse processes during the development of Drosophila. This review highlights the indispensable role that Pnt plays in regulating normal development and how continued investigation into its function and regulation will provide key mechanistic insight into understanding why the de-regulation of its vertebrate orthologs, ETS1 and ETS2 results in cancer.

Glia: from 'just glue' to essential players in complex nervous systems: a comparative view from flies to mammals

In the last years, glial cells have emerged as central players in the development and function of complex nervous systems. Therefore, the concept of glial cells has evolved from simple supporting cells to essential actors. The molecular mechanisms that govern glial functions are evolutionarily conserved from Drosophila to mammals, highlighting genetic similarities between these groups, as well as the great potential of Drosophila research for the understanding of human CNS. These similarities would imply a common phylogenetic origin of glia, even though there is a controversy at this point. This review addresses the existing literature on the evolutionary origin of glia and discusses whether or not insect and mammalian glia are homologous or analogous. Besides, this manuscript summarizes the main glial functions in the CNS and underscores the evolutionarily conserved molecular mechanisms between Drosophila and mammals. Finally, I also consider the current nomenclature and classification of glial cells to highlight the need for a consensus agreement and I propose an alternative nomenclature based on function that unifies Drosophila and mammalian glial types.

Genomic mapping of chromatin proteins by using Daminv modification of an FLP-dependent DamID approach

To identify interactions of chromatin proteins with the genome of the cell type of interest that is a part of heterologous tissues and organs of Drosophila, an FLP-dependent DamID approach was recently developed [4], which does not require sorting of cells or nuclei. Here, a modification of this approach, Dam^inv, is described. The modified approach was validated by generating the binding pattern of the LAM protein, a component of the inner membrane of the nuclear envelope, with the genome of glial cells of the Drosophila larval central brains.

The glia of the adult Drosophila nervous system

Glia play crucial roles in the development and homeostasis of the nervous system. While the GLIA in the Drosophila embryo have been well characterized, their study in the adult nervous system has been limited. Here, we present a detailed description of the glia in the adult nervous system, based on the analysis of some 500 glial drivers we identified within a collection of synthetic GAL4 lines. We find that glia make up ∼10% of the cells in the nervous system and envelop all compartments of neurons (soma, dendrites, axons) as well as the nervous system as a whole. Our morphological analysis suggests a set of simple rules governing the morphogenesis of glia and their interactions with other cells. All glial subtypes minimize contact with their glial neighbors but maximize their contact with neurons and adapt their macromorphology and micromorphology to the neuronal entities they envelop. Finally, glial cells show no obvious spatial organization or registration with neuronal entities. Our detailed description of all glial subtypes and their regional specializations, together with the powerful genetic toolkit we provide, will facilitate the functional analysis of glia in the mature nervous system. GLIA 2017 GLIA 2017;65:606–638

New insights in the clockwork mechanism regulating lineage specification: Lessons from the Drosophila nervous system

BACKGROUND

Powerful transcription factors called fate determinants induce robust differentiation programs in multipotent cells and trigger lineage specification. These factors guarantee the differentiation of specific tissues/organs/cells at the right place and the right moment to form a fully functional organism. Fate determinants are activated by temporal, positional, epigenetic, and post-transcriptional cues, hence integrating complex and dynamic developmental networks. In turn, they activate specific transcriptional/epigenetic programs that secure novel molecular landscapes.

RESULTS

In this review, we use the Drosophila Gcm glial determinant as a model to discuss the mechanisms that allow lineage specification in the nervous system. The dynamic regulation of Gcm via interlocked loops has recently emerged as a key event in the establishment of stable identity. Gcm induces gliogenesis while triggering its own extinction, thus preventing the appearance of metastable states and neoplastic processes.

CONCLUSIONS

Using simple animal models that allow in vivo manipulations provides a key tool to disentangle the complex regulation of cell fate determinants.

Morphological identification and development of neurite in Drosophila ventral nerve cord neuropil

In Drosophila, ventral nerve cord (VNC) occupies most of the larval central nervous system (CNS). However, there is little literature elaborating upon the specific types and growth of neurites as defined by their structural appearance in Drosophila larval VNC neuropil. Here we report the ultrastructural development of different types VNC neurites in ten selected time points in embryonic and larval stages utilizing transmission electron microscopy. There are four types of axonal neurites as classified by the type of vesicular content: clear vesicle (CV) neurites have clear vesicles and some T-bar structures; Dense-core vesicle (DV) neurites have dense-core vesicles and without T-bar structures; Mixed vesicle (MV) neurites have mixed vesicles and some T-bar structures; Large vesicle (LV) neurites are dominated by large, translucent spherical vesicles but rarely display T-bar structures. We found dramatic remodeling in CV neurites which can be divided into five developmental phases. The neurite is vacuolated in primary (P) phase, they have mitochondria, microtubules or big dark vesicles in the second (S) phase, and they contain immature synaptic features in the third (T) phase. The subsequent bifurcate (B) phase appears to undergo major remodeling with the appearance of the bifurcation or dendritic growth. In the final mature (M) phase, high density of commensurate synaptic vesicles are distributed around T-bar structures. There are four kinds of morphological elaboration of the CV_I neurite sub-types. First, new neurite produces at the end of axon. Second, new neurite bubbles along the axon. Third, the preexisting neurite buds and develops into several neurites. The last, the bundled axons form irregularly shape neurites. Most CV_I neurites in M phase have about 1.5–3 µm diameter, they could be suitable to analyze their morphology and subcellular localization of specific proteins by light microscopy, and they could serve as a potential model in CNS in vivo development.

Lineage specification in the fly nervous system and evolutionary implications

Over the last decades, it has become clear that glia are multifunctional and plastic cells endowed with key regulatory roles. They control the response to developmental and/or pathological signals, thereby affecting neural proliferation, remodeling, survival, and regeneration. It is, therefore, important to understand the biology of these cells and the molecular mechanisms controlling their development/activity. The fly community has made major breakthroughs by characterizing the bases of gliogenesis and function. Here we describe the regulation and the role of the fly glial determinant. Then, we discuss the impact of the determinant in cell plasticity and differentiation. Finally, we address the conservation of this pathway across evolution.

Glial cells in the central nervous system of earthworm, Eisenia fetida

Glial elements in the central nervous system of Eisenia fetida were studied at light- and electron microscopic level. Cells were characterized with the aid of toluidine blue, Glial Fibrillary Acidic Protein (GFAP), S100 staining. We identified neurilemmal-, subneurilemmal-, supporting-nutrifying- and myelinsheath forming glial cells. Both neuronal and non-neuronal elements are S100-immunoreactive in the CNS. Among glial cells neurilemmal and subneurilemmal cells are S100-immunopositive. With the antibody against the S100 protein one band is visible at 15 kDa. GFA P-immunopositive supporting-nutrifying glial cells are localized around neurons and they often appear as cells with many vacuoles. GFA P-positive cell bodies of elongated neurilemmal glial cells are also visible. Western blot analysis shows a single 57 kDa GFA P immunoreactive band in the Eisenia sample. At ultrastructural level contacts between neuronal and glial cells are recognizable. Glial cell bodies and their filopodia contain a granular and vesicular system. Close contacts between neuronal cell membranes and glial filopodia create a special environment for material transport. Vesicles budding off glial cell granules move towards the cell membranes, probably emptying their content with kiss and run exocytosis. The secreted compounds in return may help neuronal survival, provide nutrition, and filopodia may also support neuronal terminals.

Axonal ensheathment and intercellular barrier formation in Drosophila

Glial cells are critical players in every major aspect of nervous system development, function, and disease. Other than their traditional supportive role, glial cells perform a variety of important functions such as myelination, synapse formation and plasticity, and establishment of blood-brain and blood-nerve barriers in the nervous system. Recent studies highlight the striking functional similarities between Drosophila and vertebrate glia. In both systems, glial cells play an essential role in neural ensheathment thereby isolating the nervous system and help to create a local ionic microenvironment for conduction of nerve impulses. Here, we review the anatomical aspects and the molecular players that underlie ensheathment during different stages of nervous system development in Drosophila and how these processes lead to the organization of neuroglial junctions. We also discuss some key aspects of the invertebrate axonal ensheathment and junctional organization with that of vertebrate myelination and axon-glial interactions. Finally, we highlight the importance of intercellular junctions in barrier formation in various cellular contexts in Drosophila. We speculate that unraveling the genetic and molecular mechanisms of ensheathment across species might provide key insights into human myelin-related disorders and help in designing therapeutic interventions.

Enhanced sensitivity of midline glial cells to apoptosis is achieved by HOW(L)-dependent repression of Diap1

The selective sensitivity of cells to programmed cell death (PCD) depends on the positive and negative death-inducing signals that converge into the apoptotic pathway. In Drosophila, the midline glial (MG) cells undergo selective death during development. Here, we show that the long isoform of the RNA-binding protein Held Out Wing (HOW(L)) is essential for enhancing the sensitivity of the MG cells to PCD. In how mutant embryos, the number of MG cells was elevated. This phenotype could be rescued by midline expression of the HOW(L) repressor isoform. In how mutant embryos, the levels of the caspase inhibitor of apoptosis, Diap1 were elevated, in parallel to reduction in the levels of activated caspase. Similarly, reducing the levels of HOW in S2 cells led to elevation of Diap1, whereas over expression of HOW(L) promoted reduction of Diap1 protein as well as mRNA levels. Importantly, deletion of the two HOW binding sites from diap1 3'UTR abrogated HOW-dependent repression of Diap1, suggesting that HOW represses diap1 by binding to its 3'UTR. These results suggest that HOW(L) enhances the sensitivity of MG cells to apoptotic signals by reducing the levels of diap1 in these cells in, demonstrating a novel mode of regulation of PCD at the mRNA level.

CNS midline cells influence the division and survival of lateral glia in the Drosophila nervous system

Central nervous system (CNS) midline cells are essential for identity determination and differentiation of neurons in the Drosophila nervous system. It is not clear, however, whether CNS midline cells are also involved in the development of lateral glial cells. The roles of CNS midline cells in lateral glia development were elucidated using general markers for lateral glia, such as glial cell missing and reverse polarity, and specific enhancer trap lines labeling the longitudinal, A, B, medial cell body, peripheral, and exit glia. We found that CNS midline cells were necessary for the proper expression of glial cell missing, reverse polarity, and other lateral glia markers only during the later stages of development, suggesting that they are not required for initial identity determination. Instead, CNS midline cells appear to be necessary for proper division and survival of lateral glia. CNS midline cells were also required for proper positioning of three exit glia at the junction of segmental and intersegmental nerves, as well as some peripheral glia along motor and sensory axon pathways. This study demonstrated that CNS midline cells are extrinsically required for the proper division, migration, and survival of various classes of lateral glia from the ventral neuroectoderm.

Glutamatergic synapses of Drosophila neuromuscular junctions: a high-resolution model for the analysis of experience-dependent potentiation

The glutamatergic synapses of developing neuromuscular junctions (NMJ) of Drosophila larvae are readily accessible, morphologically simple, and physiologically well-characterized. They therefore have a long and highly successful tradition as a model system for the discovery of genetic and molecular mechanisms of target recognition, synaptogenesis, NMJ development, and synaptic plasticity. However, since the development and the activity-dependent refinement of NMJs are concurrent processes, they cannot easily be separated by the widely applied genetic manipulations that mostly have chronic effects. Recent studies have therefore begun systematically to incorporate larval foraging behavior into the physiological and genetic analysis of NMJ function in order to analyze potential experience-dependent changes of glutamatergic transmission. These studies have revealed that recent crawling experience is a potent modulator of glutamatergic transmission at NMJs, because high crawling activities result after an initial lag-phase in several subsequent phases of experience-dependent synaptic potentiation. Depending on the time window of occurrence, four distinct phases of experience-dependent potentiation have been defined. These phases of potentiation can be followed from their initial induction (phase-I) up to the morphological consolidation (phase-III/IV) of previously established functional changes (phase-II). This therefore establishes, for the first time, a temporal hierarchy of mechanisms involved in the use-dependent modification of glutamatergic synapses.

GAL4 enhancer trap targeting of the Drosophila sex determination gene fruitless

The fru4 allele of the sex determination gene fruitless is induced by insertion of a P[lacZ,ry+] enhancer trap element. This insert also acts to disrupt expression of the fru P1 promoter derived male-specific proteins, consequently impairing male courtship behavior. fru4 maps less than 2 kb upstream of the fru P3 promoter, whose function is essential for viability. We replaced this insert with a GAL4 element, P[GAL4,w+], recovering two lines with insertions in opposite orientations at the locus, one of which demonstrated fru-specific mutant phenotypes. Reporter expression of these lines recapitulated that of P3- and P4-derived proteins which, when correlated with a developmental and tissue specific survey of fru promoters' activities, uncovered a previously unsuspected complexity of fru regulation. These novel fru alleles provide the tools for manipulation of fru-expressing cells, allowing the consequent effects to be related back to specific fru functions and the regulatory units controlling these activities.

DPP signaling controls development of the lamina glia required for retinal axon targeting in the visual system of Drosophila

The Drosophila visual system consists of the compound eyes and the optic ganglia in the brain. Among the eight photoreceptor (R) neurons, axons from the R1-R6 neurons stop between two layers of glial cells in the lamina, the most superficial ganglion in the optic lobe. Although it has been suggested that the lamina glia serve as intermediate targets of R axons, little is known about the mechanisms by which these cells develop. We show that DPP signaling plays a key role in this process. dpp is expressed at the margin of the lamina target region, where glial precursors reside. The generation of clones mutant for Medea, the DPP signal transducer, or inhibition of DPP signaling in this region resulted in defects in R neuron projection patterns and in the lamina morphology, which was caused by defects in the differentiation of the lamina glial cells. glial cells missing/glial cells deficient (gcm; also known as glide) is expressed shortly after glia precursors start to differentiate and migrate. Its expression depends on DPP; gcm is reduced or absent in dpp mutants or Medea clones, and ectopic activation of DPP signaling induces ectopic expression of gcm and REPO. In addition, R axon projections and lamina glia development were impaired by the expression of a dominant-negative form of gcm, suggesting that gcm indeed controls the differentiation of lamina glial cells. These results suggest that DPP signaling mediates the maturation of the lamina glia required for the correct R axon projection pattern by controlling the expression of gcm.

Neuron-glia interaction in the insect nervous system

In all complex organisms, glial cells are pivotal for neuronal development and function. Insects are characterized by having only a small number of these cells, which nevertheless display a remarkable molecular diversity. An intricate relationship between neurons and glia is initially required for glial migration and during axonal patterning. Recent data suggest that in organisms such as Drosophila, a prime role of glial cells lies in setting boundaries to guide and constrain axonal growth.

Regulation of Latent Sensory Hair Cell Precursors by Glia in the Zebrafish Lateral Line

The lateral line is a placodally derived mechanosensory organ in anamniotes that detects the movement of water. In zebrafish embryos, a migrating primordium deposits seven to nine clusters of sensory hair cells, or neuromasts, at intervals along the trunk. Postembryonically, neuromasts continue to be added. We show that some secondary neuromasts arise from a pool of latent precursors that are deposited by the primordium between primary neuromasts. Interneuromast cells lie adjacent to the lateral line nerve and associated glia. These cells remain quiescent while they are juxtaposed with the glia; however, when they move away from the nerve they increase proliferation and form neuromasts. If glia are manually removed or genetically ablated by mutations in cls/sox10, hypersensitive (hps), or rowgain (rog), neuromasts precociously differentiate. Transplantation of wt glia into mutants rescues the appropriate temporal differentiation of interneuromast cells. Our studies reveal a role for glia in regulating sensory hair cell precursors.

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.

Identification and Functional Analysis of the Drosophila Gene loco

In contrast to vertebrates, the fruit fly Drosophila melanogaster contains only a small number of regulator of G-protein signaling (RGS) domain genes. This article reviews current knowledge on these genes. Although the fruit fly is particularly amenable to genetic analysis and manipulation, not much is known about the functions and mechanisms of action. The best-studied RGS gene in Drosophila is loco, a member of the D/R12 subfamily. The four different protein isoforms all contain RGS, GoLoco, and RBD domains. This article describes the identification and functional analyses of loco in the Drosophila system and discusses some mechanistic models that may underlie loco function.

The dead ringer/retained transcriptional regulatory gene is required for positioning of the longitudinal glia in the Drosophila embryonic CNS

The Drosophila dead ringer (dri, also known as retained, retn) gene encodes a nuclear protein with a conserved DNA-binding domain termed the ARID (AT-rich interaction domain). We show here that dri is expressed in a subset of longitudinal glia in the Drosophila embryonic central nervous system and that dri forms part of the transcriptional regulatory cascade required for normal development of these cells. Analysis of mutant embryos revealed a role for dri in formation of the normal embryonic CNS. Longitudinal glia arise normally in dri mutant embryos, but they fail to migrate to their final destinations. Disruption of the spatial organization of the dri-expressing longitudinal glia accounts for the mild defects in axon fasciculation observed in the mutant embryos. Consistent with the late phenotypes observed, expression of the glial cells missing (gcm) and reversed polarity (repo) genes was found to be normal in dri mutant embryos. However, from stage 15 of embryogenesis, expression of locomotion defects (loco) and prospero (pros) was found to be missing in a subset of LG. This suggests that loco and pros are targets of DRI transcriptional activation in some LG. We conclude that dri is an important regulator of the late development of longitudinal glia.

Regulation of glial cell number and differentiation by ecdysone and Fos signaling

In the midline glia of the embryonic ventral nerve cord of Drosophila, differentiation as well as the subsequent regulation of cell number is under the control of EGF-receptor signaling. During pupal stages apoptosis of all midline glial cells is initiated by ecdysone signaling. In a genetic screen we have identified mutations in disembodied, rippchen, spook, shade, shadow, shroud and tramtrack that all share a number of phenotypic traits, including defects in cuticle differentiation and nervous system development. Some of these genes were previously placed in the so-called 'Halloween-group' and were shown to affect ecdysone synthesis during embryogenesis. Here we demonstrate that the Halloween mutations not only affect glial differentiation but also lead to an increase in the number of midline glial cells, suggesting that during embryogenesis ecdysone signaling is required to adjust glial cell number similar to pupal stages. Finally we isolated a P-element-induced mutation of shroud, which controls the expression of ecdysone inducible genes. The P-element insertion occurs in one of the promoters of the Drosophila fos gene for which we present a yet undescribed complex genomic organization. The recently described kayak alleles affect only one of the six different Fos isoforms. This work for the first time links ecydsone signaling to Fos function and shows that during embryonic and pupal stages similar developmental mechanisms control midline glia survival.

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.

The fruitless gene is required for the proper formation of axonal tracts in the embryonic central nervous system of Drosophila

The fruitless (fru) gene in Drosophila melanogaster is a multifunctional gene that has sex-specific functions in the regulation of male sexual behavior and sex-nonspecific functions affecting adult viability and external morphology. While much attention has focused on fru's sex-specific roles, less is known about its sex-nonspecific functions. We have examined fru's sex-nonspecific role in embryonic neural development. fru transcripts from sex-nonspecific promoters are expressed beginning at the earliest stages of neurogenesis, and Fru proteins are present in both neurons and glia. In embryos that lack most or all fru function, FasII- and BP102-positive axons have defasciculation defects and grow along abnormal pathways in the CNS. These defects in axonal projections in fru mutants were rescued by the expression of specific UAS-fru transgenes under the control of a pan-neuronal scabrous-GAL4 driver. Our results suggest that one of fru's sex-nonspecific roles is to regulate the pathfinding ability of axons in the embryonic CNS.

Terminal glial differentiation involves regulated expression of the excitatory amino acid transporters in the Drosophila embryonic CNS

The Drosophila excitatory amino acid transporters dEAAT1 and dEAAT2 are nervous-specific transmembrane proteins that mediate the high affinity uptake of L-glutamate or aspartate into cells. Here, we demonstrate by colocalization studies that both genes are expressed in discrete and partially overlapping subsets of differentiated glia and not in neurons in the embryonic central nervous system (CNS). We show that expression of these transporters is disrupted in mutant embryos deficient for the glial fate genes glial cells missing (gcm) and reversed polarity (repo). Conversely, ectopic expression of gcm in neuroblasts, which forces all nerve cells to adopt a glial fate, induces an ubiquitous expression of both EAAT genes in the nervous system. We also detected the dEAAT transcripts in the midline glia in late embryos and dEAAT2 in a few peripheral neurons in head sensory organs. Our results show that glia play a major role in excitatory amino acid transport in the Drosophila CNS and that regulated expression of the dEAAT genes contributes to generate the functional diversity of glial cells during embryonic development.

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.

Gliogenesis inDrosophila: genome-wide analysis of downstream genes ofglial cells missingin the embryonic nervous system

In Drosophila, the glial cells missing (gcm) gene encodes a transcription factor that controls the determination of glial versus neuronal fate. In gcm mutants, presumptive glial cells are transformed into neurons and, conversely, when gcm is ectopically misexpressed, presumptive neurons become glia. Although gcm is thought to initiate glial cell development through its action on downstream genes that execute the glial differentiation program, little is known about the identity of these genes. To identify gcm downstream genes in a comprehensive manner, we used genome-wide oligonucleotide arrays to analyze differential gene expression in wild-type embryos versus embryos in which gcm is misexpressed throughout the neuroectoderm. Transcripts were analyzed at two defined temporal windows during embryogenesis. During the first period of initial gcm action on determination of glial cell precursors, over 400 genes were differentially regulated. Among these are numerous genes that encode other transcription factors, which underscores the master regulatory role of gcm in gliogenesis. During a second later period, when glial cells had already differentiated, over 1200 genes were differentially regulated. Most of these genes, including many genes for chromatin remodeling factors and cell cycle regulators, were not differentially expressed at the early stage, indicating that the genetic control of glial fate determination is largely different from that involved in maintenance of differentiated cells. At both stages, glial-specific genes were upregulated and neuron-specific genes were downregulated, supporting a model whereby gcm promotes glial development by activating glial genes, while simultaneously repressing neuronal genes. In addition, at both stages, numerous genes that were not previously known to be involved in glial development were differentially regulated and, thus, identified as potential new downstream targets of gcm. For a subset of the differentially regulated genes, tissue-specific in vivo expression data were obtained that confirmed the transcript profiling results. This first genome-wide analysis of gene expression events downstream of a key developmental transcription factor presents a novel level of insight into the repertoire of genes that initiate and maintain cell fate choices in CNS development.

Key interactions between neurons and glial cells during neural development in insects

Nervous system function is entirely dependent on the intricate and precise pattern of connections made by individual neurons. Much of the insightful research into mechanisms underlying the development of this pattern of connections has been done in insect nervous systems. Studies of developmental mechanisms have revealed critical interactions between neurons and glia, the non-neuronal cells of the nervous system. Glial cells provide trophic support for neurons, act as struts for migrating neurons and growing axons, form boundaries that restrict neuritic growth, and have reciprocal interactions with neurons that govern specification of cell fate and axonal pathfinding. The molecular mechanisms underlying these interactions are beginning to be understood. Because many of the cellular and molecular mechanisms underlying neural development appear to be common across disparate insect species, and even between insects and vertebrates, studies in developing insect nervous systems are elucidating mechanisms likely to be of broad significance.

Temporal control of glial cell migration in the Drosophila eye requires gilgamesh, hedgehog, and eye specification genes

In the Drosophila visual system, photoreceptor neurons (R cells) extend axons towards glial cells located at the posterior edge of the eye disc. In gilgamesh (gish) mutants, glial cells invade anterior regions of the eye disc prior to R cell differentiation and R cell axons extend anteriorly along these cells. gish encodes casein kinase Igamma. gish, sine oculis, eyeless, and hedgehog (hh) act in the posterior region of the eye disc to prevent precocious glial cell migration. Targeted expression of Hh in this region rescues the gish phenotype, though the glial cells do not require the canonical Hh signaling pathway to respond. We propose that the spatiotemporal control of glial cell migration plays a critical role in determining the directionality of R cell axon outgrowth.

Glia in development, function, and neurodegeneration of the adult insect brain

Glial cells have long been viewed as a passive framework for neurons but in the meanwhile were shown to play a much more active role in brain function and development. Several reviews have described the function of glia in the insect embryo. The focus of this review is the role of glial cells in the development and function of the normal and diseased adult brain. In different insect species, a considerable variety of central nervous system glia has been described indicating adaptation to different functional requirements. In the development of the adult visual and olfactory system, glial cells guide incoming axons acting as intermediate targets. Glia are part of the insect blood-brain barrier, provide nourishment for neurons, and help to regulate the extracellular concentration of ions and neurotransmitters. To fulfill these tasks insect glial cells, like vertebrate glia, interact with each other and with neurons, thus influencing neural activity. The examples presented suggest that crosstalk between all brain cells is necessary not only to develop and maintain the complex insect brain but also to endow it with the capacity to respond and adapt to the changing environment.

Survival and glial fate acquisition of neural crest cells are regulated by an interplay between the transcription factor Sox10 and extrinsic combinatorial signaling

The transcription factor Sox10 is required for proper development of various neural crest-derived cell types. Several lineages including melanocytes, autonomic and enteric neurons, and all subtypes of peripheral glia are missing in mice homozygous for Sox10 mutations. Moreover, haploinsufficiency of Sox10 results in neural crest defects that cause Waardenburg/Hirschsprung disease in humans. We provide evidence that the cellular basis to these phenotypes is likely to be a requirement for Sox10 by neural crest stem cells before lineage segregation. Cell death is increased in undifferentiated, postmigratory neural crest cells that lack Sox10, suggesting a role of Sox10 in the survival of neural crest cells. This function is mediated by neuregulin, which acts as a survival signal for postmigratory neural crest cells in a Sox10-dependent manner. Furthermore, Sox10 is required for glial fate acquisition, as the surviving mutant neural crest cells are unable to adopt a glial fate when challenged with different gliogenic conditions. In Sox10 heterozygous mutant neural crest cells, survival appears to be normal, while fate specifications are drastically affected. Thereby, the fate chosen by a mutant neural crest cell is context dependent. Our data indicate that combinatorial signaling by Sox10, extracellular factors such as neuregulin 1, and local cell-cell interactions is involved in fine-tuning lineage decisions by neural crest stem cells. Failures in fate decision processes might thus contribute to the etiology of Waardenburg/Hirschsprung disease.

Glial control of neuronal development

Reciprocal interactions between differentiating glial cells and neurons define the course of nervous system development even before the point at which these two cell types become definitively recognizable. Glial cells control the survival of associated neurons in both Drosophila and mammals, but this control is dependent on the prior neuronal triggering of glial cell fate commitment and trophic factor expression. In mammals, the growth factor neuregulin-1 and its receptors of the ErbB family play crucial roles in both events. Similarly, early differentiating neurons and their associated glia rely on reciprocal signaling to establish the basic axon scaffolds from which neuronal connections evolve. The importance of this interactive signaling is illustrated by the action of glial transcription factors and of glial axon guidance cues such as netrin and slit, which together regulate the commissural crossing of pioneer axons at the neural midline. In these and related events, the defining principle is one of mutually reinforced and mutually dependent signaling that occurs in a network of developing neurons and glia.

Human medulloblastoma cell line DEV is a potent tool to screen for factors influencing differentiation of neural stem cells

The aim of our study was to investigate whether a human neural cell line could be used as a reliable screening tool to examine the functional conservation, in humans, of transcription factors involved in neuronal or glial specification in other species. Gain-of-function experiments were performed on DEV cells, a cell line derived from a human medulloblastoma. Genes encoding nine different transcription factors were tested for their influence on the process of specification of human DEV cells towards a neuronal or glial fate. In a first series of experiments, DEV cells were transfected with murine genes encoding transcription factors known to be involved in the neuronal differentiation cascade. Neurogenins-1, -2, and -3; Mash-1; and NeuroD increased the differentiation of DEV cells towards a neuronal phenotype by a factor of 2-3.5. In a second series of experiments, we tested transcription factors involved in invertebrate glial specification. In the embryonic Drosophila CNS, the development of most glial cells depends on the master regulatory gene glial cell missing (gcm). Expression of gcm in DEV cells induced a twofold increase of astrocytic and a sixfold increase of oligodendroglial cell types. Interestingly, expression of tramtrack69, which is required in all Drosophila glial cells, resulted in a fourfold increase of only the oligodendrocyte phenotype. Expression of the related tramtrack88 protein, which is not expressed in the fly glia, or the C. elegans lin26 protein showed no effect. These results show that the Drosophila transcription factor genes tested can conserve their function upon transfection into the human DEV cells, qualifying this cell line as a screening tool to analyze the mechanisms of neuronal and glial specification.

Cell type-specific expression of fasciclin II isoforms reveals neuronal-glial interactions during peripheral nerve growth

During the formation of the insect peripheral nervous system (PNS), the cell adhesion receptor fasciclin II has been shown to play a prominent role in axonal fasciculation and synapse formation during motor neuron outgrowth. In the moth Manduca, fasciclin II (MFas II) is expressed both as a transmembrane isoform (TM-MFas II) and a glycosyl phosphatidylinositol-linked isoform (GPI-MFas II). By using RNA and antibody probes, we have shown that these two isoforms are expressed in nonoverlapping patterns: TM-MFas II is expressed exclusively by neurons and becomes localized to their most motile regions, while GPI-MFas II is expressed primarily by the glial cells that ensheath the peripheral nerves. This cell-type specificity of expression allowed us to monitor the nature of neuronal-glial interactions during PNS development. The outgrowth of TM-MFas II-positive axons in many regions preceded the arrival of GPI-MFas II-expressing glial processes that enwrapped them. In a few key locations, however, GPI-MFas II-positive glial cells differentiated before the arrival of the first axons and prefigured their subsequent trajectories. Prior inhibition of GPI-MFas II expression disrupted the subsequent outgrowth of axons at these locations but not elsewhere in the PNS. Our results suggest that the two isoforms of MFas II play distinct roles with respect to cellular motility and nerve formation.

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.

The EGF receptor defines domains of cell cycle progression and survival to regulate cell number in the developing Drosophila eye

The number of cells in developing organs must be controlled spatially by extracellular signals. Our results show how cell number can be regulated by cell interactions controlling proliferation and survival in local neighborhoods in the case of the Drosophila compound eye. Intercellular signals act during the second mitotic wave, a cell cycle that generates a pool of uncommitted cells used for most ommatidial fates. We find that G1/S progression to start the cell cycle requires EGF receptor inactivity. EGF receptor activation is then required for progression from G2 to M phase of the same cells, and also prevents apoptosis. EGF receptor activation depends on short-range signals from five-cell preclusters of photoreceptor neurons not participating in the second mitotic wave. Through proliferation and survival control, such signals couple the total number of uncommitted cells being generated to the neural patterning of the retina.

A role for the helix-loop-helix protein Id2 in the control of oligodendrocyte development

Compared to neurons, the intracellular mechanisms that control glial differentiation are still poorly understood. We show here that oligodendrocyte lineage cells express the helix-loop-helix proteins Mash1 and Id2. Although Mash1 has been found to regulate neuronal development, we found that in the absence of Mash1 oligodendrocyte differentiation occurs normally. In contrast, we found that overexpression of Id2 powerfully inhibits oligodendrocyte differentiation, that Id2 normally translocates out of the nucleus at the onset of differentiation, and that absence of Id2 induces premature oligodendrocyte differentiation in vitro. These findings demonstrate that Id2 is a component of the intracellular mechanism that times oligodendrocyte differentiation and point to the existence of an as yet unidentified MyoD-like bHLH protein necessary for oligodendrocyte differentiation.

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.

Neuregulin, a factor with many functions in the life of a schwann cell

The signalling system comprising the ligand Neuregulin-1, and its receptors, ErbB2 and ErbB3, plays multiple and important roles in glial development. These include functions in early development of neural crest cells, in expansion of the Schwann cell precursor pool and in myelination. Neuregulin is one of the crucial axon-derived signals that influence development of Schwann cells. These are specialized cells that ensheath peripheral axons and provide electrical insulation. Schwann cells have also long been implicated in providing more than a simple ensheathing function. Compelling evidence for this has emerged from the analysis of mice lacking these cells, resulting from a non-functional or compromised Neuregulin signalling system. They serve as a model to study glia-nerve interactions in vivo and indicate that Schwann cells provide important neurotrophic signals, and also cues that regulate perineurium development and nerve fasciculation.

Mechanisms underlying olfactory neuronal connectivity in Drosophila-the atonal lineage organizes the periphery while sensory neurons and glia pattern the olfactory lobe

Patterning of the antennal lobe of adult Drosophila occurs through a complex interaction between sensory neurons, glia, and central neurons of larval and adult origin. Neurons from the olfactory sense organs are organized into distinct fascicles lined by glial cells. The glia originate from one of the three types of sensory lineages-specified by the proneural gene atonal. Gain-of-function as well as loss-of-function analysis validates a role for cells of the Atonal lineage in the ordered fasciculation of sensory neurons. Upon entry of the antennal nerve to central regions, sensory neurons at first remain closely associated with central glia which lie around the periphery of the lobe anlage. Coincident with the arrival of sensory neurons into the brain, glial precursors undergo mitosis and neural precursors expressing Dachshund appear around the lobe. Sensory neurons and glial cells project into the lobe at around the same time and are likely to coordinate the correct localization of different glomeruli. The influence of sensory neurons on the development of the olfactory lobe could serve to match and lock peripheral and central properties important for the generation of olfactory behavior.

Neuronal and glial cell biology

Here, we review progress in our understanding of neuronal and glial cell biology during the past ten years, with an emphasis on glial cell fate specification, apoptosis, the cytoskeleton, neuronal polarity, synaptic vesicle recycling and targeting, regulation of the cytoskeleton by extracellular signals, and neuron-glia interactions.

rax, Hes1, and notch1 promote the formation of Müller glia by postnatal retinal progenitor cells

We are interested in the mechanisms of glial cell development in the vertebrate central nervous system. We have identified genes that can direct the formation of glia in the retina. rax, a homeobox gene, Hes1, a basic helix-loop-helix gene, and notch1, a transmembrane receptor gene, are expressed in retinal progenitor cells, downregulated in differentiated neurons, and expressed in Müller glia. Retroviral transduction of any of these genes resulted in expression of glial markers. In contrast, misexpression of a dominant-negative Hes1 gene reduced the number of glia. Cotransfection of rax with reporter constructs containing the Hes1 or notch1 regulatory regions led to the upregulation of reporter transcription. These data suggest a regulatory heirarchy that controls the formation of glia at the expense of neurons.