Perturbed glial scaffold formation precedes axon tract malformation in Drosophila mutants

The drosophila T-box transcription factor midline functions within Insulin/Akt and c-Jun-N terminal kinase stress-reactive signaling pathways to regulate interommatial bristle formation and cell survival

We recently reported that the T-box transcription factor midline (mid) functions within the Notch-Delta signaling pathway to specify sensory organ precursor (SOP) cell fates in early-staged pupal eye imaginal discs and to suppress apoptosis (Das et al.). From genetic and allelic modifier screens, we now report that mid interacts with genes downstream of the insulin receptor(InR)/Akt, c-Jun-N-terminal kinase (JNK) and Notch signaling pathways to regulate interommatidial bristle (IOB) formation and cell survival. One of the most significant mid-interacting genes identified from the modifier screen is dFOXO, a transcription factor exhibiting a nucleocytoplasmic subcellular distribution pattern. In common with dFOXO, we show that Mid exhibits a nucleocytoplasmic distribution pattern within WT third-instar larval (3(o)L) tissue homogenates. Because dFOXO is a stress-responsive factor, we assayed the effects of either oxidative or metabolic stress responses on modifying the mid mutant phenotype which is characterized by a 50% loss of IOBs within the adult compound eye. While metabolic starvation stress does not affect the mid mutant phenotype, either 1 mM paraquat or 20% coconut oil, oxidative stress inducers, partially suppresses the mid mutant phenotype resulting in a significant recovery of IOBs. Another significant mid-interacting gene we identified is groucho (gro). Mid and Gro are predicted to act as corepressors of the enhancer-of-split gene complex downstream of Notch. Immunolabeling WT and dFOXO null 3(o)L eye-antennal imaginal discs with anti-Mid and anti-Engrailed (En) antibodies indicate that dFOXO is required to activate Mid and En expression within photoreceptor neurons of the eye disc. Taken together, these studies show that Mid and dFOXO serve as critical effectors of cell fate specification and survival within integrated Notch, InR/dAkt, and JNK signaling pathways during 3(o)L and pupal eye imaginal disc development.

Glial cell adhesive molecule unzipped mediates axon guidance in Drosophila

Axon guidance needs help from the glial cell system during embryogenesis. In the Drosophila embryonic central nervous system (CNS), longitudinal glia (LG) have been implicated in axon guidance but the mechanism remains unclear. We identified the protein encoded by the Drosophila gene unzipped (uzip) as a novel cell adhesion molecule (CAM). Uzip expressed in Drosophila S2 cells triggered cell aggregation through homophilic binding. In the embryonic CNS, Uzip was mainly produced by the LG but was also located at axons, which is consistent with the secretion of Uzip expressed in cultured cells. Although uzip mutants displayed no axonal defect, loss of uzip enhanced the axonal defects in the mutant of N-cadherin (CadN) and the Wnt gene family member wnt5. Overexpression of uzip could rescue the phenotype in the CadNuzip(D43) mutant. Thus, Uzip is a novel CAM from the LG regulating axon guidance.

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.

Midline governs axon pathfinding by coordinating expression of two major guidance systems

Formation of the neural network requires concerted action of multiple axon guidance systems. How neurons orchestrate expression of multiple guidance genes is poorly understood. Here, we show that Drosophila T-box protein Midline controls expression of genes encoding components of two major guidance systems: Frazzled, ROBO, and Slit. In midline mutant, expression of all these molecules are reduced, resulting in severe axon guidance defects, whereas misexpression of Midline induces their expression. Midline is present on the promoter regions of these genes, indicating that Midline controls transcription directly. We propose that Midline controls axon pathfinding through coordinating the two guidance systems.

Development of the lymphatic vascular system: a mystery unravels

The blood vascular and the lymphatic system play complementary roles in tissue perfusion and fluid reabsorption. Despite its critical role in mediating tissue fluid homeostasis, intestinal lipid absorption, and the immune response, the lymphatic system has not received as much attention as the blood vascular system, largely due to a lack of lymphatic-specific markers and to the dearth of knowledge about the molecular regulation of lymphatic development and function. A series of recent landmark studies now significantly has advanced our understanding of the lymphatic system. Based upon the discovery and characterization of lymphatic-specific growth factors, receptors, and transcriptional regulators, the mystery of lymphatic vascular system development begins to be unraveled. The successful isolation and cultivation of blood vascular and lymphatic endothelial cells has enabled comparative molecular and cellular analyses of these two genetically and developmentally closely related cell lineages. Moreover, studies of several genetic mouse models have set the framework for a new molecular model of embryonic lymphatic vascular development and have identified molecular pathways whose mutational inactivation leads to human diseases associated with lymphedema. Although these rapid advances already have led to development of the first lymphatic-targeted molecular therapies, there still remain many unanswered questions regarding almost every aspect of lymphatic vascular biology, making the lymphatic system a highly exciting and rewarding field of study.

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.

Molecular mechanisms in lymphangiogenesis: model systems and implications in human disease

The basic science and development of therapies targeting the blood vascular system has enjoyed much focus due to the knowledge of the molecular mechanisms behind its development and roles in disease. However, the closely associated lymphatic system, while also being responsible for a number of serious and debilitating diseases, has not garnered as much attention due to the lack of specific molecular markers, thereby limiting this field to no more than descriptive analysis. Within the past decade, great strides have been taken to identify a number of molecular signatures unique to the lymphatic system. To this end, the timeline for lymphatic development has now been redefined at the molecular level, and diseases associated with lymphatics now have a molecular basis. With this knowledge, the current modes of treatment for disease such as lymphedema, lymphangiomas, and metastatic progression can now be augmented with potential molecular therapies that have currently been tested in a number of animal models. Much like the therapeutics that have been associated with vasculogenesis and angiogenesis, manipulation of the molecular pathways that define lymphatic development may lead to better clinical outcomes associated with developmental defects and disease.

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.

Interactive nervous system development: control of cell survival in Drosophila

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

Programmed transformations in neuroblast gene expression during Drosophila CNS lineage development

During Drosophila embryonic CNS development, the sequential neuroblast (NB) expression of four proteins, Hunchback (Hb), Pou-homeodomain proteins 1 and 2 (referred to collectively as Pdm), and Castor (Cas), identifies a transcription factor network regulating the temporal development of all ganglia. The Zn-finger proteins Hb and Cas, acting as repressors, confine Pdm expression to a narrow intermediate temporal window; this results in the generation of three panneural domains whose cellular constituents are marked by expression of Hb, Pdm, or Cas (R. Kambadur et al., 1998, Genes Dev. 12, 246-260). Seeking to identify the cellular mechanisms that generate these expression compartments, we studied the lineage development of isolated NBs in culture. We found that the Hb, Pdm, and Cas expression domains are generated by transitions in NB gene expression that are followed by gene product perdurance within sequentially produced sublineages. Our results also indicate that following Cas expression, many CNS NBs continue their asymmetric divisions generating additional progeny, which can be identified by the expression of the bHLH transcription factor Grainyhead (Gh). Gh appears to be a terminal embryonic CNS lineage marker. Taken together, these studies indicate that once NBs initiate lineage development, no additional signaling between NBs and the neuroectoderm and/or mesoderm is required to trigger the temporal progression of Hb --> Pdm --> Cas --> Gh expression during NB outgrowth.

Developmental dynamics of peripheral glia in Drosophila melanogaster

To study the roles of peripheral glia in nervous system development, a thorough characterization of wild type glial development must first be performed. We present a developmental profile of peripheral glia in Drosophila melanogaster that includes glial genesis, developmental morphology, the establishment of transient cellular contacts, migration patterns, and the extent of nerve wrapping in the embryonic and larval stages. In early embryonic development, immature peripheral glia that are born in the CNS seem to be intermediate targets for neurites that are migrating into the periphery. During migration to the PNS, peripheral glia follow the routes of pioneer neurons. The glia preferentially adhere to sensory axonal projections, extending cytoplasmic processes along them such that by the end of embryogenesis peripheral glial coverage of the sensory system is complete. In contrast, significant lengths of motor branch termini are unsheathed in the mature embryo. During larval stages however, peripheral glia further extend and elaborate their cytoplasmic processes until they often reach to the neuromuscular junction. Throughout the embryonic and larval developmental stages, we have also observed a number of similarities of peripheral glia to vertebrate Schwann cells and astrocytes. Peripheral glia seem to have dynamic and diverse roles and their similarities to vertebrate glia suggest that Drosophila may serve as a powerful tool for analysis of glial roles in PNS development in the future.

Glia dictate pioneer axon trajectories in the Drosophila embryonic CNS

Whereas considerable progress has been made in understanding the molecular mechanisms of axon guidance across the midline, it is still unclear how the axonal trajectories of longitudinal pioneer neurons, which never cross the midline, are established. Here we show that longitudinal glia of the embryonic Drosophila CNS direct formation of pioneer axon pathways. By ablation and analysis of glial cells missing mutants, we demonstrate that glia are required for two kinds of processes. Firstly, glia are required for growth cone guidance, although this requirement is not absolute. We show that the route of extending growth cones is rich in neuronal cell bodies and glia, and also in long processes from both these cell types. Interactions between neurons, glia and their long processes orient extending growth cones. Secondly, glia direct the fasciculation and defasciculation of axons, which pattern the pioneer pathways. Together these events are essential for the selective fasciculation of follower axons along the longitudinal pathways.

Pan-neural Prospero terminates cell proliferation during Drosophila neurogenesis

Organogenesis requires coordination between developmental specific regulators and genes governing cell proliferation. Here we show thatDrosophila prospero encodes a critical regulator of the transition from mitotically active cells to terminal differentiated neurons. Loss of pros results in aberrant expression of multiple cell-cycle regulatory genes and ectopic mitotic activity. In contrast, ectopic pros expression causes transcriptional suppression of multiple cell-cycle regulatory genes and premature termination of cell division. pros activity, hence, provides a critical regulatory link between neuronal lineage development and transcriptional regulation of cell cycle regulatory genes.

Glia development in the embryonic CNS of Drosophila

The major axon tracts in the embryonic CNS of Drosophila are organized in a simple, ladder like pattern. Each neuromere contains two commissures which connect the contra-lateral hemi-neuromeres and two longitudinal connectives which connect the different neuromeres along the anterior-posterior axis. The formation of these axon tracts occurs in close association with different glial cells. Loss of specific glial cells within the CNS leads to predictable defects in the organization of the CNS axon pattern. To unravel the genes underlying CNS glia development, we have conducted a saturating F2 EMS mutagenesis, screening for mutations, which disrupt axon pattern in the embryonic nervous system. We found a large number of mutations that lead to phenotypes indicative for glia defects. The analysis of the genes identified, show that glial cell differentiation requires the function of two independent regulatory pathways.

Prospero is a panneural transcription factor that modulates homeodomain protein activity

The panneural protein Prospero is required for proper differentiation of neuronal lineages and proper expression of several genes in the nervous system of Drosophila. Prospero is an evolutionarily conserved, homeodomain-related protein with dual subcellular localization. Here we show that Prospero is a sequence-specific DNA-binding protein with novel sequence preferences that can act as a transcription factor. In this role, Prospero can interact with homeodomain proteins to differentially modulate their DNA-binding properties. The relevance of functional interactions between Prospero and homeodomain proteins is supported by the observation that Prospero, together with the homeodomain protein Deformed, is required for proper regulation of a Deformed-dependent neural-specific transcriptional enhancer. We have localized the DNA-binding and homeodomain protein-interacting activities of Prospero to its highly conserved C-terminal region, and we have shown that the two regulatory capacities are independent.

Postembryonic development of the midline glia in the CNS of Drosophila: proliferation, programmed cell death, and endocrine regulation

The development of Drosophila midline glia during larval and pupal stages was characterized by localizing beta-gal expression in enhancer trap lines, as well as with BrdU incorporation and pulse-chase experiments. At hatching about 40 to 50 glial cells are present along the midline of the ventral nerve cord (2 to 3 dorsal and 1 to 2 ventral cells per neuromere). The cells proliferate during the third larval instar and spread dorsoventrally within the midline, increasing in number to about 230 or more (around 20 cells per neuromere). Cell divisions cease shortly after pupariation, and the cells persist for the first half of pupal life with no apparent changes in numbers or positions. Between 50 and 80% of metamorphosis, however, virtually all of the midline glia undergo programmed cell death. Tissue culture experiments indicate that the peak of ecdysteroids occurring at pupariation is required for the cessation of proliferation of midline glia and their subsequent degeneration. Midline glia in central nervous systems (CNS) cultured with low or no ecdysteroids survive and continue to divide, whereas they cease proliferating and later degenerate with high ecdysteroids levels. The midline glial may play a role during CNS metamorphosis similar to that of their progenitors in the embryo, in stabilizing outgrowing neurites that cross or run along the midline.

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.

Genetic analysis of stomatogastric nervous system development in Drosophila using enhancer trap lines

The stomatogastric nervous system (SNS) of Drosophila melanogaster is a small, simply organized neural circuitry which innervates the anterior enteric system. It is responsible for regulating the passage of food through the pharynx and esophagus and into the midgut. Here we show that the development of the SNS is amenable to genetic dissection. We screened lines from a P-element mutagenesis, selecting those with lacZ reporter gene expression and/or a phenotype in the SNS, associated glia, and garland cells. We report a collection of expression patterns and mutant phenotypes among lines found to have a mutation in genes required for the establishment of the larval SNS. Our results indicate that SNS development depends on pattern organizer genes including components of the Ras/Raf pathway.

Electron microscopic analysis of Drosophila midline glia during embryogenesis and larval development using beta-galactosidase expression as endogenous cell marker

To thoroughly study developmental problems it is often desirable to identify specific cells at the resolution of the electron microscope (TEM). Specific antibodies, and immunogold and other antibody labelling techniques can be successfully used with the TEM. But for these techniques to be successful there must be substantial adjustments for each antibody and tissue analyzed. To develop a more generally applicable labelling method we took advantage of the enhancer trap technique in Drosophila. Enhancer trap fly strains show cell- and/or tissue-specific beta-galactosidase expression which can be visualized by a simple X-gal staining procedure. To combine the power of the enhancer trap approach with electron microscopy, we have improved the fixation and staining conditions, which allow detection of X-gal crystals (by TEM) and thus provide precise information on ultrastructural morphology. We have tested our technique using the well-known midline glial cells and examined these cells between late embryonic and pupal developmental stages. The four embryonic midline glial cells found in each neuromere reside ventrally and dorsally to the midline of the neuropile and are closely associated with unpaired neurons, major commissures, and other types of glial cells. During larval and pupal life dramatic cell growth and endomitotic nuclear replication occur in midline glial cells. By the end of larval life, the giant midline glial cells fragment to give rise to a variable number of small midline glial cells. Here we show that the combination of transmission electron microscopy with cytochemical detection of beta-galactosidase expression represents a promising and valuable tool for the study of the morphology and development of specific cell types.

Migration of glial cells into retinal axon target field in Drosophila melanogaster

During the development of the Drosophila visual system, photoreceptor (retinal) axons (R axons) project retino-topically to their targets in the optic lobes. The establishment of this precise pattern of connections does not depend on interactions between adjacent axon bundles, suggesting that R axons rely on environmental signals for proper pathfinding. Glial cells that are located along the R-axon trajectory are likely candidates to provide guidance cues for R-axon navigation. This study defines the origin of lamina glia (L glia), and demonstrates that L glia migrate into the lamina over a considerable distance. Glia are located in positions at which the R axons make critical growth choices. In the absence of cues from the eye, several classes of glia migrate to their final positions within the optic lobe anlage and begin to differentiate. Our results are consistent with a role for the glia in providing guidance cues to the R axons.

Glide directs glial fate commitment and cell fate switch between neurones and glia

Glial cells constitute the second component of the nervous system and are important during neuronal development. In this paper we describe a gene, glial cell deficient, (glide), that is necessary for glial cell fate commitment in Drosophila melanogaster. Mutations at the glide locus prevent glial cell determination in the embryonic central and peripheral nervous system. Moreover, we show that the absence of glial cells is the consequence of a cell fate switch from glia to neurones. This suggests the existence of a multipotent precursor cells in the nervous system. glide mutants also display defects in axonal navigation, which confirms and extends previous results indicating a role for glial cells in these processes.

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

Each abdominal neuromere of a Drosophila embryo contains about 60 glial cells [Klämbt C, Goodman CS (1991): Glia 4:205-213; Ito et al. (1995): Roux's Arch Dev Biol, 204:284-307]. Among these, the midline and longitudinal glia are described to some detail. The midline glia are located dorsally in the nerve cord ensheathing the two segmental commissures. They are required for the proper establishment of commissures. The longitudinal glia, the A and B glia, and the segment boundary cells (SBC) are covering the longitudinal connectives. The longitudinal glia prefigure longitudinal axon paths and appear capable of regulating the expression of neuronal antigens. In the following we summarize the knowledge on the function of these glial cells.

Regulatory interactions during early neurogenesis in Drosophila

During neurogenesis in Drosophila, ectodermal cells are endowed with the capacity to become neuronal precursors. Following their selection, these cells initiate neuronal lineage development and differentiation. The processes of neuronal precursor specification and neuronal lineage development require the activities of several groups of genes functioning in a complex, hierarchical regulatory network. Whereas the proneural genes promote neurogenic potential, neurogenic genes restrict the acquisition of this identity to a subset of ectodermal cells. Following their selection, these cells express the pan neural neuronal precursor genes and a set of neuronal lineage identity genes. While lineage identity genes allow the various lineages to acquire specific identities, neuronal precursor genes presumably regulate functional and developmental characteristics common to all neuronal precursor cells.

Targeted ablation of glia disrupts axon tract formation in the Drosophila CNS

Glial cells are thought to play a role in growth cone guidance, both in insects and in vertebrates. In the developing central nervous system of the Drosophila embryo, the interface glia form a scaffold prior to the extension of the first pioneer growth cones. Growing axons appear to contact the glial scaffold as the axon tracts are established. We have used a novel technique for targeted cell ablation to kill the interface glia and thus to test their role in establishment of the embryonic axon tracts. We show that ablation of the interface glia early in development leads to a complete loss of the longitudinal axon tracts. Ablation of the glia later in embryonic development results in defects comprising weakening and loss of axon fascicles within the connectives. We conclude that the interface glia are required first for growth cone guidance in the formation of the longitudinal axon tracts in the Drosophila embryo and then either to direct the follower growth cones, or to maintain the longitudinal axon tracts.

glial cells missing: a binary switch between neuronal and glial determination in Drosophila

In the Drosophila CNS, both neurons and glial are derived from neuroblasts. We have identified a gene, glial cells missing (gcm), that encodes a novel nuclear protein expressed transiently in early glial cells. Its mutation causes presumptive glial cells to differentiate into neurons, whereas its ectopic expression forces virtually all CNS cells to become glial cells. Thus, gcm functions as a binary switch that turns on glial fate while inhibiting default neuronal fate of the neuroblasts and their progeny. Similar results are also obtained in the PNS. Analyses of the mutant revealed that "pioneer neurons" can find correct pathways without glial cells and that neurons and glia have a common molecular basis for individual identity.

Macrophages and glia participate in the removal of apoptotic neurons from the Drosophila embryonic nervous system

Cell death in the Drosophila embryonic central nervous system (CNS) proceeds by apoptosis, which is revealed ultrastructurally by nuclear condensation, shrinkage of cytoplasmic volume, and preservation of intracellular organelles. Apoptotic cells do not accumulate in the CNS but are continuously removed and engulfed by phagocytic haemocytes. To determine whether embryonic glia can function as phagocytes, we studied serial electronic microscopic sections of the Drosophila CNS. Apoptotic cells in the nervous system are engulfed by a variety of glia including midline glia, interface (or longitudinal tract) glia, and nerve root glia. However, the majority of apoptotic cells in the CNS are engulfed by subperineurial glia in a fashion similar to the microglia of the vertebrate CNS. A close proximity between macrophages and subperineurial glia suggests that glia may transfer apoptotic profiles to the macrophages. Embryos affected by the maternal-effect mutation Bicaudal-D have no macrophages. In the absence of macrophages, most apoptotic cells are retained at the outer surfaces of the CNS, and subperineurial glia contain an abundance of apoptotic cells. Some apoptotic cells are expelled from the CNS, which suggests that the removal of apoptotic cells can occur in the absence of macrophages. The number of subperineurial glia is unaffected by changes in the rate of neuronal apoptosis.

Expression of glial fibrillary acidic protein and its relation to tract formation in embryonic zebrafish (Danio rerio)

To address possible roles of glial cells during axon outgrowth in the vertebrate central nervous system, we investigated the appearance and distribution of the glial-specific intermediate filament, glial fibrillary acidic protein (GFAP), during early embryogenesis of the zebrafish (Danio rerio). Immunopositive cells first appear at 15 hours, which is at the time of, or slightly before, the first axon outgrowth in the brain. Immunopositive processes are not initially present in a pattern that prefigures the location of the first tracts but rather are distributed widely as endfeet adjacent to the pia, overlying most of the surface of the brain with the exception of the dorsal and ventral midline. The first evidence for a specific association of immunopositive cells with the developing tracts is observed at 24 hours in the hindbrain, where immunopositive processes border axons in the medial longitudinal fasciculus. By 48 hours, immunopositive processes have disappeared from most of the subpial lamina and are found exclusively in association with tracts and commissures in three forms: endfeet, radially oriented processes, and tangentially oriented processes parallel to axons. This last form is particularly prominent in the transverse plane of the hindbrain, where they define the boundaries between rhombomeres. These results suggest that glial cells contribute to the development and organization of the central nervous system by supporting early axon outgrowth in the subpial lamina and by forming boundaries around tracts and between neuromeres. The results are discussed in relation to previous results on neuron-glia interactions and possible roles of glial cells in axonal guidance.

Distribution, classification, and development ofDrosophila glial cells in the late embryonic and early larval ventral nerve cord

To facilitate the investigation of glial development inDrosophila, we present a detailed description of theDrosophila glial cells in the ventral nerve cord. A GAL4 enhancer-trap screen for glial-specific expression was performed. Using UAS-lacZ and UAS-kinesin-lacZ as reporter constructs, we describe the distribution and morphology of the identified glial cells in the fully differentiated ventral nerve cord of first-instar larvae just after hatching. The three-dimensional structure of the glial network was reconstructed using a computer. Using the strains with consistent GAL4 expression during late embryogenesis, we traced back the development of the identified cells to provide a glial map at embryonic stage 16. We identify typically 60 (54-64) glial cells per abdominal neuromere both in embryos and early larvae. They are divided into six subtypes under three categories: surface-associated glia (16-18 subperineurial glial cells and 6-8 channel glial cells), cortex-associated glia (6-8 cell body glial cells), and neuropile-associated glia (8-10 nerve root glial cells, 14-16 interface glial cells, and 3-4 midline glial cells). The proposed glial classification system is discussed in comparison with previous insect glial classifications.

Apoptosis of the midline glia during Drosophila embryogenesis: a correlation with axon contact

We have examined cell death within lineages in the midline of Drosophila embryos. Approximately 50% of cells within the anterior, middle and posterior midline glial (MGA, MGM and MGP) lineages died by apoptosis after separation of the commissural axon tracts. Glial apoptosis is blocked in embryos deficient for reaper, where greater than wild-type numbers of midline glia (MG) are present after stage 12. Quantitative studies revealed that MG death followed a consistent temporal pattern during embryogenesis. Apoptotic MG were expelled from the central nervous system and were subsequently engulfed by phagocytic haemocytes. MGA and MGM survival was apparently dependent upon proper axonal contact. In embryos mutant for the commissureless gene, a decrease in axon-glia contact correlated with a decrease in MGA and MGM survival and accelerated the time course of MG death. In embryos mutant for the slit gene, MGA and MGM maintained contact with longitudinally and contralaterally projecting axons and MG survival was comparable to that in wild-type embryos. The initial number of MG within individual ventral nerve cord segments was increased by ectopic expression of the rhomboid gene, without changing axon number. Extra MGA and MGM were eliminated from the ventral nerve cord by apoptosis to restore wild-type numbers of midline glia. Ectopic rhomboid expression also shifted MGA and MGM cell death to an earlier stage of embryogenesis. One possible explanation is that axon-glia contact or communication promotes survival of the MG and that MG death may result from a competition for available axon contact.

The homeobox gene repo is required for the differentiation and maintenance of glia function in the embryonic nervous system of Drosophila melanogaster

We describe the cloning, expression and phenotypic characterisation of repo, a gene from Drosophila melanogaster that is essential for the differentiation and maintenance of glia function. It is not, however, required for the initial determination of glial cells. In the embryo, the gene, which encodes a homeodomain protein, is expressed exclusively in all developing glia and closely related cells in both the central and peripheral nervous systems. The only observed exceptions in the CNS are the midline glia derived from the mesectoderm and two of three segmental nerve root glial cells. Using a polyclonal antibody we traced the spatial and temporal pattern of the protein expression in detail. Embryos homozygous for null alleles of the protein exhibit late developmental defects in the nervous system, including a reduction in the number of glial cells, disrupted fasciculation of axons, and the inhibition of ventral nerve cord condensation. The expression of an early glial-specific marker is unaffected in such homozygotes. By contrast, the expression of late glial-specific markers is either substantially reduced or absent. The specificity of expression is also observed in the locust Schistocerca gregaria and is thus evolutionarily conserved.

RK2, a glial-specific homeodomain protein required for embryonic nerve cord condensation and viability in Drosophila

We report the identification of RK2, a glial-specific homeodomain protein. RK2 is localized to the nucleus of virtually all embryonic and imaginal glial cells, with the exception of midline glia. Embryos mutant for the gene encoding RK2 are embryonic lethal but normal for early gliogenesis (birth, initial divisions and migration of glia) and axonogenesis (neuronal pathfinding and fasciculation). However, later in development, there are significantly fewer longitudinal glia that are spatially disorganized; in addition, there is a slight disorganization of axon fascicles and a defective nerve cord condensation. This suggests that RK2 is not required for early glial determination, but rather for aspects of glial differentiation or function that are required for embryonic viability.

The Ets transcription factors encoded by the Drosophila gene pointed direct glial cell differentiation in the embryonic CNS

The Drosophila gene pointed (pnt) encodes two putative transcription factors (P1 and P2) of the Ets family, which in the embryonic CNS are found exclusively in glial cells. Loss of pnt function leads to poorly differentiated glial cells and a marked decrease in the expression of the neuronal antigen 22C10 in the MP2 neurons, which are known to interact intimately with the pntP1-expressing longitudinal glial cells. Ectopic expression of pntP1 RNA forces additional CNS cells to enter the glial differentiation pathway. Interestingly, the additional glial-like cells are often flanked by cells that ectopically express the neuronal antigen 22C10. Therefore, both the pnt loss-of-function as well as the gain-of-function phenotype suggest that glial cells are able to induce 22C10 expression on neighboring neurons. This was further verified by cell transplantation experiments. Thus, pnt is not only required but also sufficient for several aspects of glial differentiation.

Mesectodermal cell fate analysis in Drosophila midline mutants

We have used enhancer traps and antibodies as markers of cell identity to assess the relative contribution of individual mesectodermal cell (MEC) lineages to CNS midline morphogenesis in four mutations that disrupt commissure formation in Drosophila. The absence of commissures, leading to longitudinal tract collapse, was seen in embryos mutant for the genes single-minded and slit. MEC lineages did not survive in single-minded mutant embryos, in contrast to the survival of all MEC lineages in slit mutant embryos. The midline glial cells were displaced and appeared ultrastructurally normal in slit mutant embryos, yet the presence of the MG was not sufficient to generate commissures. Commissure formation requires correct MEC cytoarchitecture, dependent upon slit activity. In fused commissure mutants (rhomboid and Star) neuron number was reduced in the ventral unpaired median neuron (VUM) lineage and the median neuroblast lineage before commissure formation (stage 12). Subsequent to these neuronal defects, the midline glia died by apoptosis (stage 13). Commissure fusion and glial apoptosis may be triggered by the earlier perturbations in MEC neuronal lineages. These studies establish when the respective activities of each gene are required for the development of each MEC lineage.