Development and function of embryonic central nervous system glial cells in Drosophila

Molecular profiling of invertebrate glia

Caenorhabditis elegans and Drosophila melanogaster are powerful experimental models for uncovering fundamental tenets of nervous system organization and function. Findings over the last two decades show that molecular and cellular features are broadly conserved between invertebrates and vertebrates, indicating that insights derived from invertebrate models can broadly inform our understanding of glial operating principles across diverse species. In recent years, these model systems have led to exciting discoveries in glial biology and mechanisms of glia-neuron interactions. Here, we summarize studies that have applied current state-of-the-art "-omics" techniques to C. elegans and D. melanogaster glia. Coupled with the remarkable acceleration in the pace of mechanistic studies of glia biology in recent years, these indicate that invertebrate glia also exhibit striking molecular complexity, specificity, and heterogeneity. We provide an overview of these studies and discuss their implications as well as emerging questions where C. elegans and D. melanogaster are well-poised to fill critical knowledge gaps in our understanding of glial biology.

Glial expression of DmMANF is required for the regulation of activity, sleep and circadian rhythms in the visual system of Drosophila melanogaster

DmMANF, Drosophila melanogaster mesencephalic astrocyte-derived neurotrophic factor (DmMANF) is an evolutionarily conserved orthologue of mammalian MANF. This neurotrophic factor exerts many functions in the Drosophila brain, particularly those dependent on glial cells. As we found in our earlier study, downregulation of DmMANF in glia induces degeneration of glial cells in the first optic neuropil (lamina) where DmMANF abundance is especially high. In the present study, we observed that changes in the level of DmMANF in two types of glia, astrocyte-like glia (AlGl) and ensheathing glia (EnGl), affect activity and sleep of flies. Interestingly, a proper level of DmMANF in AlGl seems to be important in guiding processes of pigment dispersing factor (PDF)-expressing clock neurons. This is supported by our finding that DmMANF overexpression in AlGl leads to structural changes in the architecture of the PDF-positive arborization in the brain. Finally, we detected that DmMANF also affects rhythms in glia themselves, as circadian oscillations in expression of the catalytic α subunit of the sodium pump in the lamina epithelial glia were abolished after DmMANF silencing. DmMANF expressed in AlGl and EnGl seems to affect the activity of neurons leading to changes in behaviour.

On Variations in the Level of PER in Glial Clocks of Drosophila Optic Lobe and Its Negative Regulation by PDF Signaling

We show that the level of the core protein of the circadian clock Period (PER) expressed by glial peripheral oscillators depends on their location in the Drosophila optic lobe. It appears to be controlled by the ventral lateral neurons (LNvs) that release the circadian neurotransmitter Pigment Dispersing Factor (PDF). We demonstrate that glial cells of the distal medulla neuropil (dMnGl) that lie in the vicinity of the PDF-releasing terminals of the LNvs possess receptors for PDF (PDFRs) and express PER at significantly higher level than other types of glia. Surprisingly, the amplitude of PER molecular oscillations in dMnGl is increased twofold in PDF-free environment, that is in Pdf⁰ mutants. The Pdf⁰ mutants also reveal an increased level of glia-specific protein REPO in dMnGl. The photoreceptors of the compound eye (R-cells) of the PDF-null flies, on the other hand, exhibit de-synchrony of PER molecular oscillations, which manifests itself as increased variability of PER-specific immunofluorescence among the R-cells. Moreover, the daily pattern of expression of the presynaptic protein Bruchpilot (BRP) in the lamina terminals of the R-cells is changed in Pdf⁰ mutant. Considering that PDFRs are also expressed by the marginal glia of the lamina that surround the R-cell terminals, the LNv pacemakers appear to be the likely modulators of molecular cycling in the peripheral clocks of both the glial cells and the photoreceptors of the compound eye. Consequently, some form of PDF-based coupling of the glial clocks and the photoreceptors of the eye with the central LNv pacemakers must be operational.

Expression of the Drosophila homeobox gene, Distal-less, supports an ancestral role in neural development

BACKGROUND

Distal-less (Dll) encodes a homeodomain transcription factor expressed in developing appendages of organisms throughout metazoan phylogeny. Based on earlier observations in the limbless nematode Caenorhabditis elegans and the primitive chordate amphioxus, it was proposed that Dll had an ancestral function in nervous system development. Consistent with this hypothesis, Dll is necessary for the development of both peripheral and central components of the Drosophila olfactory system. Furthermore, vertebrate homologs of Dll, the Dlx genes, play critical roles in mammalian brain development.

RESULTS

Using fluorescent immunohistochemistry of fixed samples and multiphoton microscopy of living Drosophila embryos, we show that Dll is expressed in the embryonic, larval and adult central nervous system and peripheral nervous system (PNS) in embryonic and larval neurons, brain and ventral nerve cord glia, as well as in PNS structures associated with chemosensation. In adult flies, Dll expression is expressed in the optic lobes, central brain regions and the antennal lobes.

CONCLUSIONS

Characterization of Dll expression in the developing nervous system supports a role of Dll in neural development and function and establishes an important basis for determining the specific functional roles of Dll in Drosophila development and for comparative studies of Drosophila Dll functions with those of its vertebrate counterparts.

Drosophila cortex and neuropile glia influence secondary axon tract growth, pathfinding, and fasciculation in the developing larval brain

Glial cells play important roles in the developing brain during axon fasciculation, growth cone guidance, and neuron survival. In the Drosophila brain, three main classes of glia have been identified including surface, cortex, and neuropile glia. While surface glia ensheaths the brain and is involved in the formation of the blood-brain-barrier and the control of neuroblast proliferation, the range of functions for cortex and neuropile glia is less well understood. In this study, we use the nirvana2-GAL4 driver to visualize the association of cortex and neuropile glia with axon tracts formed by different brain lineages and selectively eliminate these glial populations via induced apoptosis. The larval central brain consists of approximately 100 lineages. Each lineage forms a cohesive axon bundle, the secondary axon tract (SAT). While entering and traversing the brain neuropile, SATs interact in a characteristic way with glial cells. Some SATs are completely invested with glial processes; others show no particular association with glia, and most fall somewhere in between these extremes. Our results demonstrate that the elimination of glia results in abnormalities in SAT fasciculation and trajectory. The most prevalent phenotype is truncation or misguidance of axon tracts, or abnormal fasciculation of tracts that normally form separate pathways. Importantly, the degree of glial association with a given lineage is positively correlated with the severity of the phenotype resulting from glial ablation. Previous studies have focused on the embryonic nerve cord or adult-specific compartments to establish the role of glia. Our study provides, for the first time, an analysis of glial function in the brain during axon formation and growth in larval development.

The scoop on the fly brain: glial engulfment functions in Drosophila

Glial cells provide support and protection for neurons in the embryonic and adult brain, mediated in part through the phagocytic activity of glia. Glial cells engulf apoptotic cells and pruned neurites from the developing nervous system, and also clear degenerating neuronal debris from the adult brain after neural trauma. Studies indicate that Drosophila melanogaster is an ideal model system to elucidate the mechanisms of engulfment by glia. The recent studies reviewed here show that many features of glial engulfment are conserved across species and argue that work in Drosophila will provide valuable cellular and molecular insight into glial engulfment activity in mammals.

The Drosophila Wnt5 protein mediates selective axon fasciculation in the embryonic central nervous system

The decision of whether and where to cross the midline, an evolutionarily conserved line of bilateral symmetry in the central nervous system, is the first task for many newly extending axons. We show that Wnt5, a member of the conserved Wnt secreted glycoprotein family, is required for the formation of the anterior of the two midline-crossing commissures present in each Drosophila hemisegment. Initial path finding of pioneering neurons across the midline in both commissures is normal in wnt5 mutant embryos; however, the subsequent separation of the early midline-crossing axons into two distinct commissures does not occur. The majority of the follower axons that normally cross the midline in the anterior commissure fail to do so, remaining tightly associated near their cell bodies, or projecting inappropriately across the midline in between the commissures. The lateral and intermediate longitudinal pathways also fail to form correctly, similarly reflecting earlier failures in pathway defasciculation. Panneural expression of Wnt5 in a wnt5 mutant background rescues both the commissural and longitudinal defects. We show that the Wnt5 protein is predominantly present on posterior commissural axons and at a low level on the anterior commissure and longitudinal projections. Finally, we demonstrate that transcriptional repression of wnt5 in AC neurons by the recently described Wnt5 receptor, Derailed, contributes to this largely posterior commissural localization of Wnt5 protein.

Midline lineages in grasshopper produce neuronal siblings with asymmetric expression of Engrailed

The median neuroblast lineage of grasshopper has provided a model for the development of differing neuronal types within the insect central nervous system. According to the prevailing model, neurons of different types are produced in sequence. Contrary to this, we show that each ganglion mother cell from the median neuroblast produces two neurons of asymmetric type: one is Engrailed positive (of interneuronal fate); and one is Engrailed negative (of efferent fate). The mature neuronal population, however, results from differential neuronal death. This yields many interneurons and relatively few efferent neurons. Also contrary to previous reports, we find no evidence for glial production by the median neuroblast. We discuss evidence that neuronal lineages typically produce asymmetric progeny, an outcome that has important developmental and evolutionary implications.

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.

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.

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.

Migration and function of glia in the developing Drosophila eye

Although glial cells have been implicated widely in the formation of axon tracts in both insects and vertebrates, their specific function appears to be context-dependent, ranging from providing essential guidance cues to playing a merely facilitory role. Here we examine the role of the retinal basal glia (RBG) in photoreceptor axon guidance in Drosophila. The RBG originate in the optic stalk and have been thought to migrate into the eye disc along photoreceptor axons, thus precluding any role in axon guidance. Here we show the following. (1) The RBG can, in fact, migrate into the eye disc even in the absence of photoreceptor axons in the optic stalk; they also migrate to ectopic patches of differentiating photoreceptors without axons providing a continuous physical substratum. This suggests that glial cells are attracted into the eye disc not through haptotaxis along established axons, but through another mechanism, possibly chemotaxis. (2) If no glial cells are present in the eye disc, photoreceptor axons are able to grow and direct their growth posteriorly as in wild type, but are unable to enter the optic stalk. This indicates that the RBG have a crucial role in axon guidance, but not in axonal outgrowth per se. (3) A few glia close to the entry of the optic stalk suffice to guide the axons into the stalk, suggesting that glia instruct axons by local interaction.

loco encodes an RGS protein required for Drosophila glial differentiation

In Drosophila, glial cell development depends on the gene glial cells missing (gcm). gcm activates the expression of other transcription factors such as pointed and repo, which control subsequent glial differentiation. In order to better understand glial cell differentiation, we have screened for genes whose expression in glial cells depends on the activity of pointed. Using an enhancer trap approach, we have identified loco as such a gene. loco is expressed in most lateral CNS glial cells throughout development. Embryos lacking loco function have an normal overall morphology, but fail to hatch. Ultrastructural analysis of homozygous mutant loco embryos reveals a severe glial cell differentiation defect. Mutant glial cells fail to properly ensheath longitudinal axon tracts and do not form the normal glial-glial cell contacts, resulting in a disruption of the blood-brain barrier. Hypomorphic loco alleles were isolated following an EMS mutagenesis. Rare escapers eclose which show impaired locomotor capabilities. loco encodes the first two known Drosophila members of the family of Regulators of G-protein signalling (RGS) proteins, known to interact with the alpha subunits of G-proteins. loco specifically interacts with the Drosophila alphai-subunit. Strikingly, the interaction is not confined to the RGS domain. This interaction and the coexpression of LOCO and Galphai suggests a function of G-protein signalling for glial cell development.

Positive autoregulation of the glial promoting factor glide/gcm

Fly gliogenesis depends on the glial-cell-deficient/glial-cell-missing (glide/gcm) transcription factor. glide/gcm expression is necessary and sufficient to induce the glial fate within and outside the nervous system, indicating that the activity of this gene must be tightly regulated. The current model is that glide/gcm activates the glial fate by inducing the expression of glial-specific genes that are required to maintain such a fate. Previous observations on the null glide/gcmN7-4 allele evoked the possibility that another role of glide/gcm might be to maintain and/or amplify its own expression. Here we show that glide/gcm does positively autoregulate in vitro and in vivo, and that the glide/gcmN7-4 protein is not able to do so. We thereby provide the first direct evidence of both a target and a regulator of glide/gcm. Our data also demonstrate that glide/gcm transcription is regulated at two distinct steps: initiation, which is glide/gcm-independent, and maintenance, which requires glide/gcm. Interestingly, we have found that autoregulation requires the activity of additional cell-specific cofactors. The present results suggest transcriptional autoregulation is a mechanism for glial fate induction.

Wrapper, a novel member of the Ig superfamily, is expressed by midline glia and is required for them to ensheath commissural axons in Drosophila

The midline glia are specialized, nonneuronal cells at the midline of the Drosophila central nervous system (CNS). During development, the midline glia provide guidance cues for extending axons. At the same time, they migrate and help separate the two axon commissures. They then wrap around and ensheath the commissural axons. In many segments, a few of the glia do not enwrap the axons, and these cells die. The wrapper gene encodes a novel member of the immunoglobulin (Ig) superfamily. Wrapper protein is expressed specifically on the surface of midline glia. In wrapper mutant embryos, the midline glia express their normal guidance cues and migrate normally. However, they do not ensheath the commissural axons, and as a result, the glia die. In the absence of Wrapper, the two axon commissures are not properly separated.

glide/gcm is expressed and required in the scavenger cell lineage

Glial cell differentiation in Drosophila melanogaster requires the activity of glide/gcm (glial cell deficient/glial cell missing). The role of this gene is to direct the cell fate switch between neurons and glial cells by activating the glial developmental program in multipotent precursor cells of the nervous system. In this paper, we show that glide/gcm is also expressed and required in the lineage of hemocytes/macrophages, scavenger cells that phagocytose cells undergoing programmed cell death. In addition, we show that, as for glial cells, glide/gcm plays an instructive role in hemocyte differentiation. Interestingly, it has been shown that in the development of the fly adult nervous system the role of scavenger cells is played by glial cells. These data and our findings on the dual role of glide/gcm indicate that glial cells and hemocytes/macrophages are functionally and molecularly related.

Glia, neurons, and axon pathfinding during optic chiasm development

The importance of vision in the behavior of animals, from invertebrates to primates, has led to a good deal of interest in how projection neurons in the retina make specific connections with targets in the brain. Recent research has focused on the cellular interactions occurring between retinal ganglion cell (RGC) axons and specific glial and neuronal populations in the embryonic brain during formation of the mouse optic chiasm. These interactions appear to be involved both in determining the position of the optic chiasm on the ventral diencephalon (presumptive hypothalamus) and in ipsilateral and contralateral RGC axon pathfinding, development events fundamental to binocular vision in the adult animal.

Genetic analysis of axon pattern formation in the embryonic CNS of Drosophila

The major axon tracts in the embryonic CNS of Drosophila are organised in a simple, ladder-like pattern. Each neuromere contains two commissures which connect the contra-lateral sides and two longitudinal connectives which connect the different neuromeres along the anterior-posterior axis. The commissures form in close association with only few cells located at the CNS midline. The formation of longitudinal connectives depends in part on the presence of specific lateral glial cells. To unravel the genes underlying the formation of the embryonic CNS axon pattern, we conducted a saturating F2 EMS mutagenesis, screening for mutations, which disrupt this process. The analyses of the identified mutations lead to a simple sequential model on axon pattern formation in embryonic CNS.

Glial development in the Drosophila CNS requires concomitant activation of glial and repression of neuronal differentiation genes

Two classes of glial cells are found in the embryonic Drosophila CNS, midline glial cells and lateral glial cells. Midline glial development is triggered by EGF-receptor signalling, whereas lateral glial development is controlled by the gcm gene. Subsequent glial cell differentiation depends partly on the pointed gene. Here we describe a novel component required for all CNS glia development. The tramtrack gene encodes two zinc-finger proteins, one of which, ttkp69, is expressed in all non-neuronal CNS cells. We show that ttkp69 is downstream of gcm and can repress neuronal differentiation. Double mutant analysis and coexpression experiments indicate that glial cell differentiation may depend on a dual process, requiring the activation of glial differentiation by pointed and the concomitant repression of neuronal development by tramtrack.

Control of midline glia development in the embryonic Drosophila CNS

The midline glial cells are required for correct formation of the axonal pattern in the embryonic ventral nerve cord of Drosophila. Initially, six midline cells form an equivalence group with the capacity to develop as glial cells. By the end of embryonic development three to four cells are singled out as midline glial cells. Midline glia development occurs in two steps, both of which depend on the activation of the Drosophila EGF-receptor homolog and subsequent ras1/raf-mediated signal transduction. Nuclear targets of this signalling cascade are the ETS domain transcription factors pointedP2 and yan. In the midline glia pointedP2 in turn activates the transcription of argos, which encodes a diffusible negative regulator of EGF-receptor signalling.

Control of midline glia development in the embryonic Drosophila CNS

The midline glial cells are required for correct formation of the axonal pattern in the embryonic ventral nerve cord of Drosophila. Initially, six midline cells form an equivalence group with the capacity to develop as glial cells. By the end of embryonic development three to four cells are singled out as midline glial cells. Midline glia development occurs in two steps, both of which depend on the activation of the Drosophila EGF-receptor homolog and subsequent ras1/raf-mediated signal transduction. Nuclear targets of this signalling cascade are the ETS domain transcription factors pointedP2 and yan. In the midline glia pointedP2 in turn activates the transcription of argos, which encodes a diffusible negative regulator of EGF-receptor signalling.

Glial cells in insect ganglia

Glial cells associated with elements of central neuropils in several insect species were studied with conventional light and electron microscopical techniques, the Golgi procedure, and a combination of the latter with electron microscopy. Different types of cells located in the layer of cells covering the neuropil were found to send complex arborizations into synaptic neuropils. These arborizations grow in clusters that seem to represent discrete compartments circumscribing groups of synaptic terminals. The thinnest glial processes are found deep in the neuropil and consist of compact membrane leaflets lacking cell organelles and with reduced amounts of cytoplasmic matrix. Some of these glial processes also invest neuropil tracheoles in a manner reminiscent of the association between astrocyte end-feet and blood capillaries in the central nervous system of mammals. Other glial cells reside completely in the neuropil, where they enwrap fiber bundles in a similar manner to oligodendrocytes in the central nervous system of mammals.