Interactive nervous system development: control of cell survival in Drosophila

Expression of the Human Serotonin 5-HT7 Receptor Rescues Phenotype Profile and Restores Dysregulated Biomarkers in a Drosophila melanogaster Glioma Model

Gliomas are the most common primary brain tumors in adults. Significant progress has been made in recent years in identifying the molecular alterations involved in gliomas. Among them, an amplification/overexpression of the EGFR (Epidermal Growth Factor Receptor) proto-oncogene and its associated signaling pathways have been widely described. However, current treatments remain ineffective for glioblastomas, the most severe forms. Thus, the identification of other pharmacological targets could open new therapeutic avenues. We used a glioma model in Drosophila melanogaster that results from the overexpression of constitutively active forms of EGFR and PI3K specifically in glial cells. We observed hyperproliferation of glial cells that leads to an increase in brain size and lethality at the third instar larval stage. After expression of the human serotonin 5-HT7 receptor in this glioma model, we observed a decrease in larval lethality associated with the presence of surviving adults and a return to a normal morphology of brain for some Drosophila. Those phenotypic changes are accompanied by the normalization of certain metabolic biomarkers measured by High-Resolution Magic Angle Spinning NMR (HR-MAS NMR). The 5-HT7R expression in glioma also restores some epigenetic modifications and characteristic markers of the signaling pathways associated with tumor growth. This study demonstrates the role of the serotonin 5-HT7 receptor as a tumor suppressor gene which is in agreement with transcriptomic analysis obtained on human glioblastomas.

Genetic regulation and function of epidermal growth factor receptor signalling in patterning of the embryonic Drosophila brain

The specification of distinct neural cell types in central nervous system development crucially depends on positional cues conferred to neural stem cells in the neuroectoderm. Here, we investigate the regulation and function of the epidermal growth factor receptor (EGFR) signalling pathway in early development of the Drosophila brain. We find that localized EGFR signalling in the brain neuroectoderm relies on a neuromere-specific deployment of activating (Spitz, Vein) and inhibiting (Argos) ligands. Activated EGFR controls the spatially restricted expression of all dorsoventral (DV) patterning genes in a gene- and neuromere-specific manner. Further, we reveal a novel role of DV genes—ventral nervous system defective (vnd), intermediate neuroblast defective (ind), Nkx6—in regulating the expression of vein and argos, which feed back on EGFR, indicating that EGFR signalling stands not strictly atop the DV patterning genes. Within this network of genetic interactions, Vnd acts as a positive EGFR feedback regulator. Further, we show that EGFR signalling becomes dependent on single-minded-expressing midline cells in the posterior brain (tritocerebrum), but remains midline-independent in the anterior brain (deuto- and protocerebrum). Finally, we demonstrate that activated EGFR controls the proper formation of brain neuroblasts by regulating the number, survival and proneural gene expression of neuroectodermal progenitor cells. These data demonstrate that EGFR signalling is crucially important for patterning and early neurogenesis of the brain.

Taking advantage of neural development to treat glioblastoma

Glioblastoma (GBM) is by far the most common and most malignant primary adult brain tumor (World Health Organization grade IV), containing a fraction of stem-like cells that are highly tumorigenic and multipotent. Recent research has revealed that GBM stem-like cells play important roles in GBM pathogenesis. GBM is thought to arise from genetic anomalies in glial development. Over the past decade, a wide range of studies have shown that several signaling pathways involved in neural development, including basic helix-loop-helix, Wnt-β-catenin, bone morphogenetic proteins-Smads, epidermal growth factor-epidermal growth factor receptor, and Notch, play important roles in GBM pathogenesis. In this review, we highlight the significance of these pathways in the context of developing treatments for GBM. Extrapolating knowledge and concepts from neural development will have significant implications for designing better strategies with which to treat GBM.

The functional organisation of glia in the adult brain of Drosophila and other insects

This review annotates and categorises the glia of adult Drosophila and other model insects and analyses the developmental origins of these in the Drosophila optic lobe. The functions of glia in the adult vary depending upon their sub-type and location in the brain. The task of annotating glia is essentially complete only for the glia of the fly's lamina, which comprise: two types of surface glia-the pseudocartridge and fenestrated glia; two types of cortex glia-the distal and proximal satellite glia; and two types of neuropile glia-the epithelial and marginal glia. We advocate that the term subretinal glia, as used to refer to both pseudocartridge and fenestrated glia, be abandoned. Other neuropiles contain similar glial subtypes, but other than the antennal lobes these have not been described in detail. Surface glia form the blood brain barrier, regulating the flow of substances into and out of the nervous system, both for the brain as a whole and the optic neuropiles in particular. Cortex glia provide a second level of barrier, wrapping axon fascicles and isolating neuronal cell bodies both from neighbouring brain regions and from their underlying neuropiles. Neuropile glia can be generated in the adult and a subtype, ensheathing glia, are responsible for cleaning up cellular debris during Wallerian degeneration. Both the neuropile ensheathing and astrocyte-like glia may be involved in clearing neurotransmitters from the extracellular space, thus modifying the levels of histamine, glutamate and possibly dopamine at the synapse to ultimately affect behaviour.

A functional role for EGFR signaling in myelination and remyelination

Cellular strategies for oligodendrocyte regeneration and remyelination involve characterizing endogenous neural progenitors that are capable of generating oligodendrocytes during normal development and after demyelination, and identifying the molecular signals that enhance oligodendrogenesis from these progenitors. Using both gain- and loss-of-function approaches, we explored the role of epidermal growth factor receptor (EGFR) signaling in adult myelin repair and in oligodendrogenesis. We show that 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNP) promoter-driven overexpression of human EGFR (hEGFR) accelerated remyelination and functional recovery following focal demyelination of mouse corpus callosum. Lesion repopulation by Cspg4+ (also known as NG2) Ascl1+ (also known as Mash1) Olig2+ progenitors and functional remyelination were accelerated in CNP-hEGFR mice compared with wild-type mice. EGFR overexpression in subventricular zone (SVZ) and corpus callosum during early postnatal development also expanded this NG2+Mash1+Olig2+ progenitor population and promoted SVZ-to-lesion migration, enhancing oligodendrocyte generation and axonal myelination. Analysis of hypomorphic EGFR-mutant mice confirmed that EGFR signaling regulates oligodendrogenesis and remyelination by NG2+Mash1+Olig2+ progenitors. EGFR targeting holds promise for enhancing oligodendrocyte regeneration and myelin repair.

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.

Epidermal Growth Factor Receptor–Mediated Signal Transduction in the Development and Therapy of Gliomas

The epidermal growth factor receptor (EGFR) and its ligands figure prominently in the biology of gliomas, the most common tumors of the central nervous system (CNS). Although their histologic classification seems to be straightforward, these tumors constitute a heterogeneous class of related neoplasms. They are associated with a variety of molecular abnormalities affecting signal transduction, transcription factors, apoptosis, angiogensesis, and the extracellular matrix. Under normal conditions, these same interacting factors drive CNS growth and development. We are now recognizing the diverse molecular genetic heterogeneity that underlies tumors classified histologically into three distinct grades. This recognition is leading to new therapeutic strategies targeted directly at specific molecular subtypes. In this article, we will review the role of EGFR and related molecular pathways in the genesis of the normal CNS and their relationship to glial tumorigenesis. We will discuss barriers to effective treatment as they relate to anatomic specialization of the CNS. We will also consider the ways in which specific EGFR alterations common to glioma reflect outcomes following treatment with targeted therapies, all with an eye towards applying this understanding to improved patient outcomes.

Building Complex Brains – Missing Pieces in an Evolutionary Puzzle

The mechanisms underlying evolution of complex nervous systems are not well understood. In recent years there have been a number of attempts to correlate specific gene families or evolutionary processes with increased brain complexity in the vertebrate lineage. Candidates for evocation of complexity include genes involved in regulating brain size, such as neurotrophic factors or microcephaly-related genes; or wider evolutionary processes, such as accelerated evolution of brain-expressed genes or enhanced RNA splicing or editing events in primates. An inherent weakness of these studies is that they are correlative by nature, and almost exclusively focused on the mammalian and specifically the primate lineage. Another problem with genomic analyses is that it is difficult to identify functionally similar yet non-homologous molecules such as different families of cysteine-rich neurotrophic factors in different phyla. As long as comprehensive experimental studies of these questions are not feasible, additional perspectives for evolutionary and genomic studies will be very helpful. Cephalopod mollusks represent the most complex nervous systems outside the vertebrate lineage, thus we suggest that genome sequencing of different mollusk models will provide useful insights into the evolution of complex brains.

Origin and evolution of the Trk family of neurotrophic receptors

Among the numerous tyrosine kinase receptors, those belonging to the Trk family are distinctively involved in the development of complex traits within the vertebrate nervous system. Until recently, the lack of a proper Nt/Trk system in invertebrates has lead to the belief that they were a vertebrate innovation. Recent data, however, have challenged the field, and proved that bona fide Trk receptors do exist in invertebrates. Here, we review and discuss the evolutionary history of the Trk receptor family, and draw a comprehensive scenario that situates the origin of the Nt/Trk signalling prior to the origin of vertebrates. Probably, a ProtoTrk receptor was invented by means of domain and exon shuffling from pieces of ancient genes, generating the unique combination of domains found in extant Trk receptors. It is suggestive to propose that subtle protein mutations, gene duplications, and co-options in particular territories of a primitive Nt/Trk system were instrumental to the development of a complex vertebrate nervous system.

DrosophilaRET contains an active tyrosine kinase and elicits neurotrophic activities in mammalian cells

The RET receptor tyrosine kinase controls kidney organogenesis and development of subpopulations of enteric and sensory neurons in different vertebrate species, including humans, rodents, chicken and zebrafish. RET is activated by binding to a ligand complex formed by a member of the glial cell line-derived neurotrophic factor (GDNF) family of neurotrophic factors bound to its cognate GFRalpha GPI-linked co-receptor. Despite the absence of GDNF or GFRalpha molecules in the Drosophila genome, a RET orthologue (dRET) has recently been described in this organism and shown to be expressed in subpopulations of cells of the excretory, digestive and nervous systems, thus resembling the expression pattern of RET in vertebrates. In this study, we report on the initial biochemical and functional characterization of the dRET protein in cell culture systems. Full-length dRET could be produced in mammalian and insect cells. Similar to its human counterpart (hRET), overexpression of dRET resulted in its ligand-independent tyrosine phosphorylation, indicating that it bears an active tyrosine kinase. Unlike hRET, however, the extracellular domain of dRET was unable to interact with mammalian GDNF and GFRalpha1. Self association between dRET molecules could neither be detected, indicating that dRET is incapable of mediating cell adhesion by homophilic interactions. A chimeric molecule comprising the extracellular domain of hRET and the kinase domain of dRET was constructed and used to probe ligand-mediated downstream activities of the dRET kinase in PC12 cells. GDNF stimulation of cells transfected with the hRET/dRET chimera resulted in neurite outgrowth comparable to that obtained after transfection of wild-type hRET. These results indicate significant conservation between the biological effects elicited by the human and Drosophila RET kinases, and suggest functions for dRET in neuronal differentiation in the fly.

Expanded polyglutamine peptides disrupt EGF receptor signaling and glutamate transporter expression in Drosophila

Huntington's disease (HD) is a late onset heritable neurodegenerative disorder caused by expansion of a polyglutamine (polyQ) sequence in the protein huntingtin (Htt). Transgenic models in mice have suggested that the motor and cognitive deficits associated to this disease are triggered by extended neuronal and possibly glial dysfunction, whereas neuronal death occurs late and selectively. Here, we provide in vivo evidence that expanded polyQ peptides antagonize epidermal growth factor receptor (EGFR) signaling in Drosophila glia. We targeted the expression of the polyQ-containing domain of Htt or an extended polyQ peptide alone in a subset of Drosophila glial cells, where the only fly glutamate transporter, dEAAT1, is detected. This resulted in formation of nuclear inclusions, progressive decrease in dEAAT1 transcription and shortened adult lifespan, but no significant glial cell death. We observed that brain expression of dEAAT1 is normally sustained by the EGFR-Ras-extracellular signal-regulated kinase (ERK) signaling pathway, suggesting that polyQ could act by antagonizing this pathway. We found that the presence of polyQ peptides indeed abolished dEAAT1 upregulation by constitutively active EGFR and potently inhibited EGFR-mediated ERK activation in fly glial cells. Long polyQ also limited the effect of activated EGFR on Drosophila eye development. Our results further indicate that the polyQ acts at an upstream step in the pathway, situated between EGFR and ERK activation. This suggests that disruption of EGFR signaling and ensuing glial cell dysfunction could play a direct role in the pathogenesis of HD and other polyQ diseases in humans.

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.

Neurons and glia: team players in axon guidance

In the developing nervous system, growth cones follow specific trajectories to reach their target area and ultimately connect with their correct postsynaptic partners. This review focuses on studies in both Drosophila and vertebrates to highlight that mutual interactions between neurons and glia are essential in forming specific neuronal connections. Glia signal to neurons to direct pathfinding and targeting of axons, as well as to stabilize and refine axonal branches within the target area. Equally, neurons provide crucial information to glia, supporting their migration and correct positioning.

Time-lapse and cell ablation reveal the role of cell interactions in fly glia migration and proliferation

Migration and proliferation have been mostly explored in culture systems or fixed preparations. We present a simple genetic model, the chains of glia moving along fly wing nerves, to follow such dynamic processes by time-lapse in the whole animal. We show that glia undergo extensive cytoskeleton and mitotic apparatus rearrangements during division and migration. Single cell labelling identifies different glia: pioneers with high filopodial,exploratory, activity and, less active followers. In combination with time-lapse, altering this cellular environment by genetic means or cell ablation has allowed to us define the role of specific cell-cell interactions. First, neurone-glia interactions are not necessary for glia motility but do affect the direction of migration. Second, repulsive interactions between glia control the extent of movement. Finally, autonomous cues control proliferation.

Programmed cell death and context dependent activation of the EGF pathway regulate gliogenesis in the Drosophila olfactory system

In the Drosophila antenna, sensory lineages selected by the basic helix-loop-helix transcription factor Atonal are gliogenic while those specified by the related protein Amos are not. What are the mechanisms that cause the two lineages to act differentially? We found that ectopic expression of the Baculovirus inhibitor of apoptosis protein (p35) rescues glial cells from the Amos-derived lineages, suggesting that precursors are removed by programmed cell death. In the wildtype, glial precursors express the extracellular-signal regulated kinase transiently, and antagonism of Epidermal Growth Factor (EGF) pathway signaling compromises their development. We suggest that all sensory lineages on the antenna are competent to produce glia but only those specified by Atonal respond to EGF signaling and survive. These results underscore the importance of developmental context of cell lineages in their responses to non-autonomous signaling in the choice between survival and death.

Neurotrophic activities of trk receptors conserved over 600 million years of evolution

The trk family of receptor tyrosine kinases is crucial for neuronal survival in the vertebrate nervous system, however both C. elegans and Drosophila lack genes encoding trks or their ligands. The only invertebrate representative of this gene family identified to date is Ltrk from the mollusk Lymnaea. Did trophic functions of trk receptors originate early in evolution, or were they an innovation of the vertebrates? Here we show that the Ltrk gene conserves a similar exon/intron order as mammalian trk genes in the region encoding defined extracellular motifs, including one exon encoding a putative variant immunoglobulin-like domain. Chimeric receptors containing the intracellular and transmembrane domains of Ltrk undergo ligand-induced autophosphorylation followed by MAP kinase activation in transfected cells. The chimeras are internalized similarly to TrkA in PC12 cells, and their stimulation leads to differentiation and neurite extension. Knock-down of endogenous Ltrk expression compromises outgrowth and survival of Lymnaea neurons cultured in CNS-conditioned medium. Thus, Ltrk is required for neuronal survival, suggesting that trophic activities of the trk receptor family originated before the divergence of molluscan and vertebrate lineages approximately 600 million years ago.

The PDGF/VEGF Receptor Controls Blood Cell Survival in Drosophila

The Drosophila PDGF/VEGF receptor (PVR) has known functions in the guidance of cell migration. We now demonstrate that during embryonic hematopoiesis, PVR has a role in the control of antiapoptotic cell survival. In Pvr mutants, a large fraction of the embryonic hemocyte population undergoes apoptosis, and the remaining blood cells cannibalistically phagocytose their dying peers. Consequently, total hemocyte numbers drop dramatically during embryogenesis, and large aggregates of engorged macrophages carrying multiple apoptotic corpses form. Hemocyte-specific expression of the pan-caspase inhibitor p35 in Pvr mutants eliminates hemocyte aggregates and restores blood cell counts and morphology. Additional rescue experiments suggest involvement of the Ras pathway in PVR-mediated blood cell survival. In cell culture, we demonstrate that PVR directly controls survival of a hemocyte cell line. This function of PVR shows striking conservation with mammalian hematopoiesis and establishes Drosophila as a model to study hematopoietic cell survival in development and disease.

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

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