Glial cell development in the Drosophila embryo

The astrocyte-enriched gene deathstar plays a crucial role in the development, locomotion, and lifespan of D. melanogaster

The Drosophila melanogaster brain is a complex organ with various cell types, orchestrating the development, physiology, and behaviors of the fly. While each cell type in Drosophila brain is known to express a unique gene set, their complete genetic profile is still unknown. Advances in the RNA sequencing techniques at single-cell resolution facilitate identifying novel cell type markers and/or re-examining the specificity of the available ones. In this study, exploiting a single-cell RNA sequencing data of Drosophila optic lobe, we categorized the cells based on their expression pattern for known markers, then the genes with enriched expression in astrocytes were identified. CG11000 was identified as a gene with a comparable expression profile to the Eaat1 gene, an astrocyte marker, in every individual cell inside the Drosophila optic lobe and midbrain, as well as in the entire Drosophila brain throughout its development. Consistent with our bioinformatics data, immunostaining of the brains dissected from transgenic adult flies showed co-expression of CG11000 with Eaat1 in a set of single cells corresponding to the astrocytes in the Drosophila brain. Physiologically, inhibiting CG11000 through RNA interference disrupted the normal development of male D. melanogaster, while having no impact on females. Expression suppression of CG11000 in adult flies led to decreased locomotion activity and also shortened lifespan specifically in astrocytes, indicating the gene's significance in astrocytes. We designated this gene as 'deathstar' due to its crucial role in maintaining the star-like shape of glial cells, astrocytes, throughout their development into adult stage.

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.

Non-autonomous regulation of neurogenesis by extrinsic cues: a Drosophila perspective

The formation of a functional circuitry in the central nervous system (CNS) requires the correct number and subtypes of neural cells. In the developing brain, neural stem cells (NSCs) self-renew while giving rise to progenitors that in turn generate differentiated progeny. As such, the size and the diversity of cells that make up the functional CNS depend on the proliferative properties of NSCs. In the fruit fly Drosophila, where the process of neurogenesis has been extensively investigated, extrinsic factors such as the microenvironment of NSCs, nutrients, oxygen levels and systemic signals have been identified as regulators of NSC proliferation. Here, we review decades of work that explores how extrinsic signals non-autonomously regulate key NSC characteristics such as quiescence, proliferation and termination in the fly.

The transcriptional factor Apt regulates neuroblast differentiation through activating CycE expression

In Drosophila, the thoracic neuroblast 6-4 (NB6-4T) divides asymmetrically into a glial precursor and a neuronal precursor, while the abdominal neuroblast 6-4 (NB6-4A) divides symmetrically to produce two glial cells. The underlying mechanism by which NB6-4T and NB6-4A undergo distinct differentiation is still elusive. Here, we find that the transcription factor Apontic (Apt) exclusively expresses in NB6-4T cells and is involved in regulating NB6-4T differentiation. Loss of Apt results in neuronal precursor loss. Epistasis analysis shows that Apt controls NB6-4T differentiation through activating CycE expression. On the other hand, Gcm suppresses Apt expression in the NB6-4A cell, thus inhibiting CycE expression. Taken together, our findings reveal a Gcm-Apt-CycE axis that regulates neuroblast and glia cell differentiation.

A Unique Class of Neural Progenitors in the Drosophila Optic Lobe Generates Both Migrating Neurons and Glia

How neuronal and glial fates are specified from neural precursor cells is an important question for developmental neurobiologists. We address this question in the Drosophila optic lobe, composed of the lamina, medulla, and lobula complex. We show that two gliogenic regions posterior to the prospective lamina also produce lamina wide-field (Lawf) neurons, which share common progenitors with lamina glia. These progenitors express neither canonical neuroblast nor lamina precursor cell markers. They bifurcate into two sub-lineages in response to Notch signaling, generating lamina glia or Lawf neurons, respectively. The newly born glia and Lawfs then migrate tangentially over substantial distances to reach their target tissue. Thus, Lawf neurogenesis, which includes a common origin with glia, as well as neuronal migration, resembles several aspects of vertebrate neurogenesis.

Intrinsic dorsoventral patterning and extrinsic EGFR signaling genes control glial cell development in the Drosophila nervous system

Dorsoventral patterning and epidermal growth factor receptor (EGFR) signaling genes are essential for determining neural identity and differentiation of the Drosophila nervous system. Their role in glial cell development in the Drosophila nervous system is not clearly established. Our study demonstrated that the dorsoventral patterning genes, vnd, ind, and msh, are intrinsically essential for the proper expression of a master glial cell regulator, gcm, and a differentiation gene, repo, in the lateral glia. In addition, we showed that esg is particularly required for their expression in the peripheral glia. These results indicate that the dorsoventral patterning and EGFR signaling genes are essential for identity determination and differentiation of the lateral glia by regulating proper expression of gcm and repo in the lateral glia from the early glial development. In contrast, overexpression of vnd, msh, spi, and Egfr genes repressed the expression of Repo in the ventral neuroectoderm, indicating that maintenance of correct columnar identity along the dorsoventral axis by proper expression of these genes is essential for restrictive formation of glial precursor cells in the lateral neuroectoderm. Therefore, the dorsoventral patterning and EGFR signaling genes play essential roles in correct identity determination and differentiation of lateral glia in the Drosophila nervous system.

Dye coupling and immunostaining of astrocyte-like glia following intracellular injection of fluorochromes in brain slices of the grasshopper, Schistocerca gregaria

Injection of fluorochromes such as Alexa Fluor(®) 568 into single cells in brain slices reveals a network of dye-coupled cells to be associated with the central complex. Subsequent immunolabeling shows these cells to be repo positive/glutamine synthetase positive/horseradish peroxidase negative, thus identifying them as astrocyte-like glia. Dye coupling fails in the presence of n-heptanol indicating that dye spreads from cell to cell via gap junctions. A cellular network of dye-coupled, astrocyte-like, glia surrounds and infiltrates developing central complex neuropils. Intracellular dye injection techniques complement current molecular approaches in analyzing the functional properties of such networks.

A cellular network of dye-coupled glia associated with the embryonic central complex in the grasshopper Schistocerca gregaria

The central complex of the grasshopper (Schistocerca gregaria) brain comprises a modular set of neuropils, which develops after mid-embryogenesis and is functional on hatching. Early in embryogenesis, Repo-positive glia cells are found intermingled among the commissures of the midbrain, but then redistribute as central complex modules become established and, by the end of embryogenesis, envelop all midbrain neuropils. The predominant glia associated with the central body during embryogenesis are glutamine synthetase-/Repo-positive astrocyte-like glia, which direct extensive processes (gliopodia) into and around midbrain neuropils. We used intracellular dye injection in brain slices to ascertain whether such glia are dye-coupled into a communicating cellular network during embryogenesis. Intracellular staining of individual cells located at any one of four sites around the central body revealed a population of dye-coupled cells whose number and spatial distribution were stereotypic for each site and comparable at both 70 and 100% of embryogenesis. Subsequent immunolabeling confirmed these dye-coupled cells to be astrocyte-like glia. The addition of n-heptanol to the bathing saline prevented all dye coupling, consistent with gap junctions linking the glia surrounding the central body. Since dye coupling also occurred in the absence of direct intersomal contacts, it might additionally involve the extensive array of gliopodia, which develop after glia are arrayed around the central body. Collating the data from all injection sites suggests that the developing central body is surrounded by a network of dye-coupled glia, which we speculate may function as a positioning system for the developing neuropils of the central complex.

Drosophila melanogaster as a model system for human brain cancers

Glioblastomas (GBM), the most common primary brain tumors, infiltrate the brain, grow rapidly, and are refractory to current therapies. Signature genetic lesions in glioblastomas include mutation of the epidermal growth factor receptor tyrosine kinase (EGFR) receptor tyrosine kinase and activating mutations in components of the PI-3 kinase (PI3K) pathway. Despite years of study, how these pathways specifically regulate glial pathogenesis is unclear. To address the genetic and cellular origins of this disease, a novel Drosophila GBM model has been developed in which glial progenitor cells give rise to proliferative and invasive neoplastic cells that create transplantable tumors in response to constitutive co-activation of the EGFR-Ras and PI3K pathways. Standing with a rich literature demonstrating the direct relevance of Drosophila to studies on human cancer, neurological disease, and neurodevelopment, this model represents a robust cell-type specific Drosophila neurological disease model in which malignant cells are created by mutations in genetic pathways thought to be driving forces in a homologous human disease. Using lineage analysis and cell-type specific markers, neoplastic glial cells were found to originate from committed glial progenitor cells, rather than from multipotent neuroblasts. Genetic analyses demonstrated that EGFR-Ras and PI3K induce fly glial neoplasia through activation of a combinatorial genetic network composed, in part, of other genetic pathways also commonly mutated in human glioblastomas. In the future, large-scale forward genetic screens with this model may reveal new insights into the origins and treatments of human glioblastoma.

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.

Recognition, presence, and survival of locust central nervous glia in situ and in vitro

Insect glial cells serve functions for the formation, maintenance, and performance of the central nervous system in ways similar to their vertebrate counterparts. Characterization of physiological mechanisms that underlie the roles of glia in invertebrates is largely incomplete, partly due to the lack of markers that universally label all types of glia throughout all developmental stages in various species. Studies on primary cell cultures from brains of Locusta migratoria demonstrated that the absence of anti-HRP immunoreactivity, which has previously been used to identify glial cells in undissociated brains, can also serve as a reliable glial marker in vitro, but only in combination with a viability test. As cytoplasmic membranes of cultured cells are prone to degradation when they lose viability, only cells that are both anti-HRP immunonegative and viable should be regarded as glial cells, whereas the lack of anti-HRP immunoreactivity alone is not sufficient. Cell viability can be assessed by the pattern of nuclear staining with DAPI (4',6-diamidino-2-phenylindole), a convenient, sensitive labeling method that can be used in combination with other immunocytochemical cellular markers. We determined the glia-to-neuron ratio in central brains of fourth nymphal stage of Locusta migratoria to be 1:2 both in situ and in dissociated primary cell cultures. Analysis of primary cell cultures revealed a progressive reduction of glial cells and indicated that dead cells detach from the substrate and vanish from the analysis. Such changes in the composition of cell cultures should be considered in future physiological studies on cell cultures from insect nervous systems.

The transcription factor, glial cell missing 2, is involved in differentiation and functional regulation of H+-ATPase-rich cells in zebrafish (Danio rerio)

H(+)-ATPase-rich (HR) cells in zebrafish are known to be involved in acid secretion and Na(+) uptake mechanisms in zebrafish gills/skin; however, little is known about how HR cells are functionally regulated. In the present work, we studied the roles of Drosophila glial cell missing (gcm), a cell fate-related transcription factor, in the differentiation and functional regulation of zebrafish HR cells. Zebrafish gcm2 (zgcm2) was found to begin expression in zebrafish embryos at 10 h postfertilization (hpf), and to be extensively expressed in gills but only mildly so in eyes, heart, muscles, and testes. By whole mount in situ hybridization, zgcm2 mRNA signals were found in a group of cells on the zebrafish yolk sac surface initially in the tail bud stage (10 hpf); they had disappeared at 36 hpf and thereafter appeared again in the gill region from 48 hpf. Double fluorescence in situ hybridization further demonstrated specific colocalization of zgcm2 mRNA in HR cells in zebrafish embryos. Knockdown of zgcm2 with a specific morpholino oligonucleotide caused the complete disappearance of HR cells with a concomitant decrease in H(+) activity at the apical surface of HR cells, but it did not affect the occurrence of Na(+)-K(+)-ATPase-rich cells. A decrease in the H(+)-ATPase subunit A (zatp6v1a) expression and no change in zgcm2 expression in zebrafish gills were seen from 12 h to 3 days after transfer to acidic fresh water, but a compensatory stimulation in the expressions of both genes appeared 4 days post-transfer. In conclusion, functional regulation of HR cells is probably achieved by enhancing cell differentiation via zGCM2 activation.

Organization and postembryonic development of glial cells in the adult central brain of Drosophila

Glial cells exist throughout the nervous system, and play essential roles in various aspects of neural development and function. Distinct types of glia may govern diverse glial functions. To determine the roles of glia requires systematic characterization of glia diversity and development. In the adult Drosophila central brain, we identify five different types of glia based on its location, morphology, marker expression, and development. Perineurial and subperineurial glia reside in two separate single-cell layers on the brain surface, cortex glia form a glial mesh in the brain cortex where neuronal cell bodies reside, while ensheathing and astrocyte-like glia enwrap and infiltrate into neuropils, respectively. Clonal analysis reveals that distinct glial types derive from different precursors, and that most adult perineurial, ensheathing, and astrocyte-like glia are produced after embryogenesis. Notably, perineurial glial cells are made locally on the brain surface without the involvement of gcm (glial cell missing). In contrast, the widespread ensheathing and astrocyte-like glia derive from specific brain regions in a gcm-dependent manner. This study documents glia diversity in the adult fly brain and demonstrates involvement of different developmental programs in the derivation of distinct types of glia. It lays an essential foundation for studying glia development and function in the Drosophila brain.

Glial ensheathment of peripheral axons in Drosophila

The ensheathment of neurons and their axons creates an ion-sensitive microenvironment that allows rapid conduction of nerve impulses. One of the fundamental questions about axonal ensheathment is how insulating glial cells wrap around axons. The mechanisms that underlie insulation of axons in invertebrates and vertebrates are not fully understood. In the present article we address cellular aspects of axonal ensheathment in Drosophila by taking advantage of glial mutants that illustrate a range of phenotypic defects including ensheathment of axons. From the findings of these mutant studies, we summarize that loss of glial cells, defects in glial membrane wrapping, failure of glial migration, and loss of specialized ladderlike septate junctions between ensheathing glial membranes result in axon-glial functional defects. These studies provide a broad perspective on glial ensheathment of axons in Drosophila and key insights into the anatomical and cellular aspects of axonal insulation. Given the powerful genetic approaches available in Drosophila, the axonal ensheathment process can be dissected in great detail to reveal the fundamental principles of ensheathment. These observations will be relevant to understanding the very similar processes in vertebrates, where defects in glial cell functions lead to devastating neurological diseases.

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.

Tracheal development in the Drosophila brain is constrained by glial cells

The Drosophila brain is tracheated by the cerebral trachea, a branch of the first segmental trachea of the embryo. During larval stages the cerebral trachea splits into several main (primary) branches that grow around the neuropile, forming a perineuropilar tracheal plexus (PNP) at the neuropile surface. Five primary tracheal branches whose spatial relationship to brain compartments is relatively invariant can be distinguished, although the exact trajectories and branching pattern of the brain tracheae are surprisingly variable. Immunohistochemical and electron microscopic studies demonstrate that all brain tracheae grow in direct contact with the glial cell processes that surround the neuropile. To investigate the effect of glia on tracheal development, embryos and larvae lacking glial cells as a result of a genetic mutation or a directed ablation were analyzed. In these animals, the tracheal branching pattern was highly abnormal. In particular, the number of secondary branches entering the central neuropile was increased. Wild-type larvae possess only two central tracheae, typically associated with the mushroom body and the antennocerebral tract. In larvae lacking glial cells, six to ten tracheal branches penetrate the neuropile in a variable pattern. This finding indicates that glia-derived signals constrained tracheal growth in the Drosophila brain and restrict the number of branches entering the neuropile.

Glia-neuron interactions in the nervous system of Caenorhabditis elegans

A century and a half after first being described, glia are beginning to reveal their intricate and important roles in nervous system development and function. Recent studies in the nematode Caenorhabditis elegans suggest that this invertebrate will provide important insight into these roles. Studies of C. elegans have revealed a connection between glial ensheathment of neurons and tubulogenesis, have uncovered glial roles in neurite growth, navigation, and function, and have demonstrated roles for glia and glia-like cells in synapse formation and function. Given the conservation of basic anatomical, functional and molecular features of the nervous systems between C. elegans and vertebrates, these recent advances are likely to be informative in describing nervous system assembly and function in all organisms possessing a nervous system.

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

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

Constructing circuits: neurogenesis and migration in the developing neocortex

Our knowledge of the proliferation, migration, and differentiation of neurons has changed dramatically over the last 10 years. Whereas traditionally it was thought that glial and neuronal cells were separate cell lines with different lineages, we now know that this is not true. Radial glia are a type of neural stem cell that generate excitatory pyramidal neurons directly through asymmetric cell division in the ventricular zone (VZ) of the telencephalon and indirectly through the symmetric division of daughter intermediate precursor cells that divide in the subventricular zone (SVZ). Moreover, pyramidal neurons, once thought to migrate only along radial guide fibers to the developing layers of the cortex, have been shown to proceed through four distinct stages of migration during which they change shape, direction, and speed. Gamma-aminobutyric acid (GABAergic) inhibitory interneurons, on the other hand, are generated not in the cortex, but in the medial ganglionic eminence and migrate tangentially to their final cortical destinations. Evidence suggests that GABA activation may play a role in coordinating the generation and migration of both pyramidal and interneuron populations. At the end of neurogenesis, radial glial cells translocate to the cortex and transform into astrocytes. Although they do not actively divide in the adult brain, astrocytes may retain the potential to generate new neurons. These new findings have increased our understanding of the mechanisms underlying certain developmental disorders and, in doing so, reveal potentially useful modes of therapeutic intervention.

Transcriptional control of glial cell development in Drosophila

Neurons and glia are generated from multipotent neural progenitors. In Drosophila, the transcriptional regulation of glial vs. neuronal fates is controlled by the expression of the transcription factor encoded by the glial cells missing gene (gcm) in multiple neural lineages. The cis-regulatory control of gcm transcription serves as a nodal point to translate a complex array of spatially and temporally regulated transcription factors in distinct neural lineages into glial-specific expression. Gcm acts synergistically with several downstream transcription factors to initiate and maintain glial-specific gene expression. The identification of a large set of glial-specific genes through the application of computational and whole genome tools provides the opportunity to analyze the transcriptional regulation of glial cell development at the genomic level in a relatively simple genetic model system.

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.

Quantitative trait loci affecting starvation resistance in Drosophila melanogaster

The ability to withstand periods of scarce food resources is an important fitness trait. Starvation resistance is a quantitative trait controlled by multiple interacting genes and exhibits considerable genetic variation in natural populations. This genetic variation could be maintained in the face of strong selection due to a trade-off in resource allocation between reproductive activity and individual survival. Knowledge of the genes affecting starvation tolerance and the subset of genes that affect variation in starvation resistance in natural populations would enable us to evaluate this hypothesis from a quantitative genetic perspective. We screened 933 co-isogenic P-element insertion lines to identify candidate genes affecting starvation tolerance. A total of 383 P-element insertions induced highly significant and often sex-specific mutational variance in starvation resistance. We also used deficiency complementation mapping followed by complementation to mutations to identify 12 genes contributing to variation in starvation resistance between two wild-type strains. The genes we identified are involved in oogenesis, metabolism, and feeding behaviors, indicating a possible link to reproduction and survival. However, we also found genes with cell fate specification and cell proliferation phenotypes, which implies that resource allocation during development and at the cellular level may also influence the phenotypic response to starvation.

Glia–Neuron Interactions in Nervous System Function and Development

Nervous systems are generally composed of two cell types-neurons and glia. Early studies of neurons revealed that these cells can conduct electrical currents, immediately implying that they have roles in the relay of information throughout the nervous system. Roles for glia have, until recently, remained obscure. The importance of glia in regulating neuronal survival had been long recognized. However, this trophic support function has hampered attempts to address additional, more active functions of these cells in the nervous system. In this chapter, recent efforts to reveal some of these additional functions are described. Evidence supporting a role for glia in synaptic development and activity is presented, as well as experiments suggesting glial guidance of neuronal migration and process outgrowth. Roles for glia in influencing the electrical activity of neurons are also discussed. Finally, an exciting system is described for studying glial cells in the nematode C. elegans, in which recent studies suggest that glia are not required for neuronal viability.

Signaling in glial development: differentiation migration and axon guidance

Glial cells have diverse functions that are necessary for the proper development and function of complex nervous systems. During development, a variety of reciprocal signaling interactions between glia and neurons dictate all parts of nervous system development. Glia may provide attractive, repulsive, or contact-mediated cues to steer neuronal growth cones and ensure that neurons find their appropriate synaptic targets. In fact, both neurons and glia may act as migrational substrates for one another at different times during development. Also, the exchange of trophic signals between glia and neurons is essential for the proper bundling, fasciculation, and ensheathement of axons as well as the differentiation and survival of both cell types. The growing number of links between glial malfunction and human disease has generated great interest in glial biology. Because of its relative simplicity and the many molecular genetic tools available, Drosophila is an excellent model organism for studying glial development. This review will outline the roles of glia and their interactions with neurons in the embryonic nervous system of the fly.Key words: glia, axon guidance, migration, EGF receptor.

Basic helix-loop-helix proteins expressed during early embryonic organogenesis

The basic helix-loop-helix proteins form a special group of transcription factors unique for the eukaryotic organisms. They are crucial for the embryonic development of many fundamental organ systems such as muscle, heart, central nervous system, hematopoiteic system, and many others. They are very flexible in terms of regulating transcription in that they can either promote or repress transcription, and do so in many different ways. Basic helix-loop-helix proteins can form homo- or heterodimers with other members of the group, and are subject to post-transcriptional modifications. In this review, an overview of basic helix-loop-helix protein classification, biochemical function, and examples of past and recent advances in our understanding of embryonic development are presented, with emphasis on the vertebrate muscle, heart, brain, and eye.

Impacts of a new transcription factor family: mammalian GCM proteins in health and disease

GCM proteins constitute a small transcription factor family with a DNA-binding domain exhibiting a novel fold composed of two subdomains rigidly held together by coordination of one of two structural zinc cations. In all known cases, GCM proteins exert the role of master regulators: the prototypical family member determines gliogenesis in Drosophila melanogaster, whereas mammalian GCM proteins orchestrate divergent aspects of development and physiology in placenta, kidney, thymus, and parathyroid gland. Recent data point to an involvement of GCM proteins in different pathological contexts, such as preeclampsia, hyper- or hypoparathyroidism, and parathyroid gland tumors.

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.

A hidden program in Drosophila peripheral neurogenesis revealed: fundamental principles underlying sensory organ diversity

How is cell fate diversity reliably achieved during development? Insect sensory organs have been a favorable model system for investigating this question for over 100 years. They are constructed using defined cell lineages that generate a maximum of cell diversity with a minimum number of cell divisions, and display tremendous variety in their morphologies, constituent cell types, and functions. An unexpected realization of the past 5 years is that very diverse sensory organs in Drosophila are produced by astonishingly similar cell lineages, and that their diversity can be largely attributed to only a small repertoire of developmental processes. These include changes in terminal cell differentiation, cell death, cell proliferation, cell recruitment, cell-cell interactions, and asymmetric segregation of cell fate determinants during mitosis. We propose that most Drosophila sensory organs are built from an archetypal lineage, and we speculate about how this stereotyped pattern of cell divisions may have been built during evolution.

Quantitative Trait Loci Affecting Starvation Resistance in Drosophila melanogaster

The ability to withstand periods of scarce food resources is an important fitness trait. Starvation resistance is a quantitative trait controlled by multiple interacting genes and exhibits considerable genetic variation in natural populations. This genetic variation could be maintained in the face of strong selection due to a trade-off in resource allocation between reproductive activity and individual survival. Knowledge of the genes affecting starvation tolerance and the subset of genes that affect variation in starvation resistance in natural populations would enable us to evaluate this hypothesis from a quantitative genetic perspective. We screened 933 co-isogenic P-element insertion lines to identify candidate genes affecting starvation tolerance. A total of 383 P-element insertions induced highly significant and often sex-specific mutational variance in starvation resistance. We also used deficiency complementation mapping followed by complementation to mutations to identify 12 genes contributing to variation in starvation resistance between two wild-type strains. The genes we identified are involved in oogenesis, metabolism, and feeding behaviors, indicating a possible link to reproduction and survival. However, we also found genes with cell fate specification and cell proliferation phenotypes, which implies that resource allocation during development and at the cellular level may also influence the phenotypic response to starvation.

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.

Unwrapping glial biology: Gcm target genes regulating glial development, diversification, and function

Glia are the most abundant cell type in the mammalian brain. They regulate neuronal development and function, CNS immune surveillance, and stem cell biology, yet we know surprisingly little about glia in any organism. Here we identify over 40 new Drosophila glial genes. We use glial cells missing (gcm) mutants and misexpression to verify they are Gcm regulated in vivo. Many genes show unique spatiotemporal responsiveness to Gcm in the CNS, and thus glial subtype diversification requires spatially or temporally restricted Gcm cofactors. These genes provide insights into glial biology: we show unc-5 (a repulsive netrin receptor) orients glial migrations and the draper gene mediates glial engulfment of apoptotic neurons and larval locomotion. Many identified Drosophila glial genes have homologs expressed in mammalian glia, revealing conserved molecular features of glial cells. 80% of these Drosophila glial genes have mammalian homologs; these are now excellent candidates for regulating human glial development, function, or disease.

Neurons from radial glia: the consequences of asymmetric inheritance

Recent work suggests that radial glial cells represent many, if not most, of the neuronal progenitors in the developing cortex. Asymmetric cell division of radial glia results in the self-renewal of the radial glial cell and the birth of a neuron. Among the proteins that direct cell fate in Drosophila melanogaster that have known mammalian homologs, Numb is the best candidate to have a similar function in radial glia. During asymmetric divisions of radial glial cells, the basal cell may inherit the radial glial fibre, while the apical cell sequesters the majority of the Numb protein. We suggest two models that make opposite predictions as to whether the radial glia or nascent neuron inherit the radial glial fiber or the majority of the Numb protein.

The glial cell undergoes apoptosis in the microchaete lineage of Drosophila

Apoptosis plays a major role in vertebrate and invertebrate development. The adult Drosophila thoracic microchaete is a mechanosensory organ whose development has been extensively studied as a model of how cell division and cell determination intermingle. This sensory organ arises from a cell lineage that produces a glial cell and four other cells that form the organ. In this study, using an in vivo approach as well as fixed material, we show that the glial cell undergoes nucleus fragmentation shortly after birth. Fragmentation was blocked after overexpression of the caspase inhibitor p35 or removal of the pro-apoptotic genes reaper, hid and grim, showing that the glial cell undergoes apoptosis. Moreover, it seems that fragments are eliminated from the epithelium by mobile macrophages. Forcing survival of the glial cells induces precocious axonal outgrowth but does not affect final axonal patterning and connectivity. However, under these conditions, glial cells do not fragment but leave the epithelium by a mechanism that is reminiscent of cell competition. Finally, we present evidences showing that glial cells are committed to apoptosis independently of gcm and prospero expression. We suggest that apoptosis is triggered by a cell autonomous mechanism.

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.

Role of glia-derived cholesterol in synaptogenesis: new revelations in the synapse-glia affair

Brain development and function relies on the exchange of signals between neurons and glial cells. Here we review a series of recent studies on cultures of purified retinal ganglion cells (RGCs) that point to a new role of glial cells in the formation and plasticity of synaptic connections. The results suggest that neurons must import glia-derived cholesterol via lipoproteins to form numerous and efficient synaptic connections. This finding may explain why throughout the central nervous system (CNS) the main phase of synaptogenesis starts synchronously after glia differentiation and why astrocytes produce apolipoprotein E (apoE) and cholesterol-containing lipoproteins. Experimental tests of these hypotheses may further our understanding of the cholesterol metabolism in the brain and may help to explain neurologic symptoms resulting from defective cholesterol and lipoprotein metabolism.

Genetic models meet trophic mechanisms: EGF family members are gliatrophins in Drosophila

Trophic survival mechanisms are crucial for the determination of cell numbers in the developing vertebrate nervous system, but important neurotrophic factor families such as the neurotrophins have not yet been found in either Drosophila or C. elegans. Two independent studies on distinct glial populations in Drosophila have now shown that their survival is regulated by EGF family members secreted by adjacent neurons. Fly genetics thus promises new insights on trophic signaling mechanisms and confirms that trophic regulation of cell survival is an evolutionarily ancient mechanism for building the nervous system.