Glial cells mediate target layer selection of retinal axons in the developing visual system of Drosophila

Development of a Polymeric Pharmacological Nanocarrier System as a Potential Therapy for Spinocerebellar Ataxia Type 7

Spinocerebellar ataxia type 7 (SCA7) is an autosomal-dominant inherited disease characterized by progressive ataxia and retinal degeneration. SCA7 belongs to a group of neurodegenerative diseases caused by an expanded CAG repeat in the disease-causing gene, resulting in aberrant polyglutamine (polyQ) protein synthesis. PolyQ ataxin-7 is prone to aggregate in intracellular inclusions, perturbing cellular processes leading to neuronal death in specific regions of the central nervous system (CNS). Currently, there is no treatment for SCA7; however, a promising approach successfully applied to other polyQ diseases involves the clearance of polyQ protein aggregates through pharmacological activation of autophagy. Nonetheless, the blood-brain barrier (BBB) poses a challenge for delivering drugs to the CNS, limiting treatment effectiveness. This study aimed to develop a polymeric nanocarrier system to deliver therapeutic agents across the BBB into the CNS. We prepared poly(lactic-co-glycolic acid) nanoparticles (NPs) modified with Poloxamer188 and loaded with rapamycin to enable NPs to activate autophagy. We demonstrated that these rapamycin-loaded NPs were successfully taken up by neuronal and glial cells, demonstrating high biocompatibility without adverse effects. Remarkably, rapamycin-loaded NPs effectively cleared mutant ataxin-7 aggregates in a SCA7 glial cell model, highlighting their potential as a therapeutic approach to fight SCA7 and other polyQ diseases.

Regulation of axon pathfinding by astroglia across genetic model organisms

Glia and neurons are intimately associated throughout bilaterian nervous systems, and were early proposed to interact for patterning circuit assembly. The investigations of circuit formation progressed from early hypotheses of intermediate guideposts and a "glia blueprint", to recent genetic and cell manipulations, and visualizations in vivo. An array of molecular factors are implicated in axon pathfinding but their number appears small relatively to circuit complexity. Comprehending this circuit complexity requires to identify unknown factors and dissect molecular topographies. Glia contribute to both aspects and certain studies provide molecular and functional insights into these contributions. Here, I survey glial roles in guiding axon navigation in vivo, emphasizing analogies, differences and open questions across major genetic models. I highlight studies pioneering the topic, and dissect recent findings that further advance our current molecular understanding. Circuits of the vertebrate forebrain, visual system and neural tube in zebrafish, mouse and chick, the Drosophila ventral cord and the C. elegans brain-like neuropil emerge as major contexts to study glial cell functions in axon navigation. I present astroglial cell types in these models, and their molecular and cellular interactions that drive axon guidance. I underline shared principles across models, conceptual or technical complications, and open questions that await investigation. Glia of the radial-astrocyte lineage, emerge as regulators of axon pathfinding, often employing common molecular factors across models. Yet this survey also highlights different involvements of glia in embryonic navigation or pioneer axon pathfinding, and unknowns in the molecular underpinnings of glial cell functions. Future cellular and molecular investigations should complete the comprehensive view of glial roles in circuit assembly.

Ets-1 transcription factor regulates glial cell regeneration and function in planarians

Glia play multifaceted roles in nervous systems in response to injury. Depending on the species, extent of injury and glial cell type in question, glia can help or hinder the regeneration of neurons. Studying glia in the context of successful regeneration could reveal features of pro-regenerative glia that could be exploited for new human therapies. Planarian flatworms completely regenerate their nervous systems after injury - including glia - and thus provide a strong model system for exploring glia in the context of regeneration. Here, we report that planarian glia regenerate after neurons, and that neurons are required for correct glial numbers and localization during regeneration. We also identify the planarian transcription factor-encoding gene ets-1 as a key regulator of glial cell maintenance and regeneration. Using ets-1 (RNAi) to perturb glia, we show that glial loss is associated with altered neuronal gene expression, impeded animal movement and impaired nervous system architecture - particularly within the neuropil. Importantly, our work reveals the inter-relationships of glia and neurons in the context of robust neural regeneration.

An Atlas of the Developing Drosophila Visual System Glia and Subcellular mRNA Localization of Transcripts in Single Cells

Glial cells are essential for proper nervous system development and function. To understand glial development and function, we comprehensively annotated glial cells in a single-cell mRNA-sequencing (scRNAseq) atlas of the developing Drosophila visual system. This allowed us to study their developmental trajectories, from larval to adult stages, and to understand how specific types of glia diversify during development. For example, neuropil glia that are initially transcriptionally similar in larvae, split into ensheathing and astrocyte-like glia during pupal stages. Other glial types, such as chiasm glia change gradually during development without splitting into two cell types. The analysis of scRNA-seq allowed us to discover that the transcriptome of glial cell bodies can be distinguished from that of their broken processes. The processes contain distinct enriched mRNAs that were validated in vivo. Therefore, we have identified most glial types in the developing optic lobe and devised a computational approach to identify mRNA species that are localized to cell bodies or cellular processes.

Neural specification, targeting, and circuit formation during visual system assembly

Like other sensory systems, the visual system is topographically organized: Its sensory neurons, the photoreceptors, and their targets maintain point-to-point correspondence in physical space, forming a retinotopic map. The iterative wiring of circuits in the visual system conveniently facilitates the study of its development. Over the past few decades, experiments in Drosophila have shed light on the principles that guide the specification and connectivity of visual system neurons. In this review, we describe the main findings unearthed by the study of the Drosophila visual system and compare them with similar events in mammals. We focus on how temporal and spatial patterning generates diverse cell types, how guidance molecules distribute the axons and dendrites of neurons within the correct target regions, how vertebrates and invertebrates generate their retinotopic map, and the molecules and mechanisms required for neuronal migration. We suggest that basic principles used to wire the fly visual system are broadly applicable to other systems and highlight its importance as a model to study nervous system development.

Drosophila USP22/nonstop polarizes the actin cytoskeleton during collective border cell migration

Polarization of the actin cytoskeleton is vital for the collective migration of cells in vivo. During invasive border cell migration in Drosophila, actin polarization is directly controlled by the Hippo signaling complex, which resides at contacts between border cells in the cluster. Here, we identify, in a genetic screen for deubiquitinating enzymes involved in border cell migration, an essential role for nonstop/USP22 in the expression of Hippo pathway components expanded and merlin. Loss of nonstop function consequently leads to a redistribution of F-actin and the polarity determinant Crumbs, loss of polarized actin protrusions, and tumbling of the border cell cluster. Nonstop is a component of the Spt-Ada-Gcn5-acetyltransferase (SAGA) transcriptional coactivator complex, but SAGA’s histone acetyltransferase module, which does not bind to expanded or merlin, is dispensable for migration. Taken together, our results uncover novel roles for SAGA-independent nonstop/USP22 in collective cell migration, which may help guide studies in other systems where USP22 is necessary for cell motility and invasion.

The RNA-Binding Protein SBR (Dm NXF1) Is Required for the Constitution of Medulla Boundaries in Drosophila melanogaster Optic Lobes

Drosophila melanogaster sbr (small bristles) is an orthologue of the Nxf1 (nuclear export factor 1) genes in different Opisthokonta. The known function of Nxf1 genes is the export of various mRNAs from the nucleus to the cytoplasm. The cytoplasmic localization of the SBR protein indicates that the nuclear export function is not the only function of this gene in Drosophila. RNA-binding protein SBR enriches the nucleus and cytoplasm of specific neurons and glial cells. In sbr12 mutant males, the disturbance of medulla boundaries correlates with the defects of photoreceptor axons pathfinding, axon bundle individualization, and developmental neurodegeneration. RNA-binding protein SBR participates in processes allowing axons to reach and identify their targets.

More Than Mortar: Glia as Architects of Nervous System Development and Disease

Glial cells are an essential component of the nervous system of vertebrates and invertebrates. In the human brain, glia are as numerous as neurons, yet the importance of glia to nearly every aspect of nervous system development has only been expounded over the last several decades. Glia are now known to regulate neural specification, synaptogenesis, synapse function, and even broad circuit function. Given their ubiquity, it is not surprising that the contribution of glia to neuronal disease pathogenesis is a growing area of research. In this review, we will summarize the accumulated evidence of glial participation in several distinct phases of nervous system development and organization-neural specification, circuit wiring, and circuit function. Finally, we will highlight how these early developmental roles of glia contribute to nervous system dysfunction in neurodevelopmental and neurodegenerative disorders.

An Immobilization Technique for Long-Term Time-Lapse Imaging of Explanted Drosophila Tissues

Time-lapse imaging is an essential tool to study dynamic biological processes that cannot be discerned from fixed samples alone. However, imaging cell- and tissue-level processes in intact animals poses numerous challenges if the organism is opaque and/or motile. Explant cultures of intact tissues circumvent some of these challenges, but sample drift remains a considerable obstacle. We employed a simple yet effective technique to immobilize tissues in medium-bathed agarose. We applied this technique to study multiple Drosophila tissues from first-instar larvae to adult stages in various orientations and with no evidence of anisotropic pressure or stress damage. Using this method, we were able to image fine features for up to 18 h and make novel observations. Specifically, we report that fibers characteristic of quiescent neuroblasts are inherited by their basal daughters during reactivation; that the lamina in the developing visual system is assembled roughly 2-3 columns at a time; that lamina glia positions are dynamic during development; and that the nuclear envelopes of adult testis cyst stem cells do not break down completely during mitosis. In all, we demonstrate that our protocol is well-suited for tissue immobilization and long-term live imaging, enabling new insights into tissue and cell dynamics in Drosophila.

Function and regulation of the Spt-Ada-Gcn5-Acetyltransferase (SAGA) deubiquitinase module

The Spt-Ada-Gcn5 Acetyltransferase (SAGA) chromatin modifying complex is a critical regulator of gene expression and is highly conserved across species. Subunits of SAGA arrange into discrete modules with lysine aceyltransferase and deubiquitinase activities housed separately. Mutation of the SAGA deubiquitinase module can lead to substantial biological misfunction and diseases such as cancer, neurodegeneration, and blindness. Here, we review the structure and functions of the SAGA deubiquitinase module and regulatory mechanisms acting to control these.

The Gcn5 complexes in Drosophila as a model for metazoa

The histone acetyltransferase Gcn5 is conserved throughout eukaryotes where it functions as part of large multi-subunit transcriptional coactivator complexes that stimulate gene expression. Here, we describe how studies in the model insect Drosophila melanogaster have provided insight into the essential roles played by Gcn5 in the development of multicellular organisms. We outline the composition and activity of the four different Gcn5 complexes in Drosophila: the Spt-Ada-Gcn5 Acetyltransferase (SAGA), Ada2a-containing (ATAC), Ada2/Gcn5/Ada3 transcription activator (ADA), and Chiffon Histone Acetyltransferase (CHAT) complexes. Whereas the SAGA and ADA complexes are also present in the yeast Saccharomyces cerevisiae, ATAC has only been identified in other metazoa such as humans, and the CHAT complex appears to be unique to insects. Each of these Gcn5 complexes is nucleated by unique Ada2 homologs or splice isoforms that share conserved N-terminal domains, and differ only in their C-terminal domains. We describe the common and specialized developmental functions of each Gcn5 complex based on phenotypic analysis of mutant flies. In addition, we outline how gene expression studies in mutant flies have shed light on the different biological roles of each complex. Together, these studies highlight the key role that Drosophila has played in understanding the expanded biological function of Gcn5 in multicellular eukaryotes.

Gene regulatory networks during the development of the Drosophila visual system

The Drosophila visual system integrates input from 800 ommatidia and extracts different features in stereotypically connected optic ganglia. The development of the Drosophila visual system is controlled by gene regulatory networks that control the number of precursor cells, generate neuronal diversity by integrating spatial and temporal information, coordinate the timing of retinal and optic lobe cell differentiation, and determine distinct synaptic targets of each cell type. In this chapter, we describe the known gene regulatory networks involved in the development of the different parts of the visual system and explore general components in these gene networks. Finally, we discuss the advantages of the fly visual system as a model for gene regulatory network discovery in the era of single-cell transcriptomics.

Loss of zebrafish Ataxin-7, a SAGA subunit responsible for SCA7 retinopathy, causes ocular coloboma and malformation of photoreceptors

Polyglutamine (polyQ) expansion in Ataxin-7 (ATXN7) results in spinocerebellar ataxia type 7 (SCA7) and causes visual impairment. SCA7 photoreceptors progressively lose their outer segments (OSs), a structure essential for their visual function. ATXN7 is a subunit of the transcriptional coactivator Spt-Ada-Gcn5 Acetyltransferase complex, implicated in the development of the visual system in flies. To determine the function of ATXN7 in the vertebrate eye, we have inactivated ATXN7 in zebrafish. While ATXN7 depletion in flies led to gross retinal degeneration, in zebrafish, it primarily results in ocular coloboma, a structural malformation responsible for pediatric visual impairment in humans. ATXN7 inactivation leads to elevated Hedgehog signaling in the forebrain, causing an alteration of proximo-distal patterning of the optic vesicle during early eye development and coloboma. At later developmental stages, malformations of photoreceptors due to incomplete formation of their OSs are observed and correlate with altered expression of crx, a key transcription factor involved in the formation of photoreceptor OS. Therefore, we propose that a primary toxic effect of polyQ expansion is the alteration of ATXN7 function in the daily renewal of OS in SCA7. Together, our data indicate that ATXN7 plays an essential role in vertebrate eye morphogenesis and photoreceptor differentiation, and its loss of function may contribute to the development of human coloboma.

Molecular Targets and Therapeutic Strategies in Spinocerebellar Ataxia Type 7

Spinocerebellar ataxia type 7 (SCA7) is a rare autosomal dominant neurodegenerative disorder characterized by progressive neuronal loss in the cerebellum, brainstem, and retina, leading to cerebellar ataxia and blindness as major symptoms. SCA7 is due to the expansion of a CAG triplet repeat that is translated into a polyglutamine tract in ATXN7. Larger SCA7 expansions are associated with earlier onset of symptoms and more severe and rapid disease progression. Here, we summarize the pathological and genetic aspects of SCA7, compile the current knowledge about ATXN7 functions, and then focus on recent advances in understanding the pathogenesis and in developing biomarkers and therapeutic strategies. ATXN7 is a bona fide subunit of the multiprotein SAGA complex, a transcriptional coactivator harboring chromatin remodeling activities, and plays a role in the differentiation of photoreceptors and Purkinje neurons, two highly vulnerable neuronal cell types in SCA7. Polyglutamine expansion in ATXN7 causes its misfolding and intranuclear accumulation, leading to changes in interactions with native partners and/or partners sequestration in insoluble nuclear inclusions. Studies of cellular and animal models of SCA7 have been crucial to unveil pathomechanistic aspects of the disease, including gene deregulation, mitochondrial and metabolic dysfunctions, cell and non-cell autonomous protein toxicity, loss of neuronal identity, and cell death mechanisms. However, a better understanding of the principal molecular mechanisms by which mutant ATXN7 elicits neurotoxicity, and how interconnected pathogenic cascades lead to neurodegeneration is needed for the development of effective therapies. At present, therapeutic strategies using nucleic acid-based molecules to silence mutant ATXN7 gene expression are under development for SCA7.

Drosophila ClC-a is required in glia of the stem cell niche for proper neurogenesis and wiring of neural circuits

Glial cells form part of the neural stem cell niche and express a wide variety of ion channels; however, the contribution of these channels to nervous system development is poorly understood. We explored the function of the Drosophila ClC‐a chloride channel, since its mammalian ortholog CLCN2 is expressed in glial cells, and defective channel function results in leukodystrophies, which in humans are accompanied by cognitive impairment. We found that ClC‐a was expressed in the niche in cortex glia, which are closely associated with neurogenic tissues. Characterization of loss‐of‐function ClC‐a mutants revealed that these animals had smaller brains and widespread wiring defects. We showed that ClC‐a is required in cortex glia for neurogenesis in neuroepithelia and neuroblasts, and identified defects in a neuroblast lineage that generates guidepost glial cells essential for photoreceptor axon guidance. We propose that glia‐mediated ionic homeostasis could nonautonomously affect neurogenesis, and consequently, the correct assembly of neural circuits.

Ataxin-7 and Non-stop coordinate SCAR protein levels, subcellular localization, and actin cytoskeleton organization

Atxn7, a subunit of SAGA chromatin remodeling complex, is subject to polyglutamine expansion at the amino terminus, causing spinocerebellar ataxia type 7 (SCA7), a progressive retinal and neurodegenerative disease. Within SAGA, the Atxn7 amino terminus anchors Non-stop, a deubiquitinase, to the complex. To understand the scope of Atxn7-dependent regulation of Non-stop, substrates of the deubiquitinase were sought. This revealed Non-stop, dissociated from Atxn7, interacts with Arp2/3 and WAVE regulatory complexes (WRC), which control actin cytoskeleton assembly. There, Non-stop countered polyubiquitination and proteasomal degradation of WRC subunit SCAR. Dependent on conserved WRC interacting receptor sequences (WIRS), Non-stop augmentation increased protein levels, and directed subcellular localization, of SCAR, decreasing cell area and number of protrusions. In vivo, heterozygous mutation of SCAR did not significantly rescue knockdown of Atxn7, but heterozygous mutation of Atxn7 rescued haploinsufficiency of SCAR.

Drosophila Tet Is Expressed in Midline Glia and Is Required for Proper Axonal Development

Ten-Eleven Translocation (TET) proteins are important epigenetic regulators that play a key role in development and are frequently deregulated in cancer. Drosophila melanogaster has a single homologous Tet gene (dTet) that is highly expressed in the central nervous system during development. Here, we examined the expression pattern of dTet in the third instar larval CNS and discovered its presence in a specific set of glia cells: midline glia (MG). Moreover, dTet knockdown resulted in significant lethality, locomotor dysfunction, and alterations in axon patterning in the larval ventral nerve cord. Molecular analyses on dTet knockdown larvae showed a downregulation in genes involved in axon guidance and reduced expression of the axon guidance cue Slit. Our findings point toward a potential role for dTet in midline glial function, specifically the regulation of axon patterning during neurodevelopment.

Netrin Signaling Defines the Regional Border in the Drosophila Visual Center

The brain consists of distinct domains defined by sharp borders. So far, the mechanisms of compartmentalization of developing tissues include cell adhesion, cell repulsion, and cortical tension. These mechanisms are tightly related to molecular machineries at the cell membrane. However, we and others demonstrated that Slit, a chemorepellent, is required to establish the borders in the fly brain. Here, we demonstrate that Netrin, a classic guidance molecule, is also involved in the compartmental subdivision in the fly brain. In Netrin mutants, many cells are intermingled with cells from the adjacent ganglia penetrating the ganglion borders, resulting in disorganized compartmental subdivisions. How do these guidance molecules regulate the compartmentalization? Our mathematical model demonstrates that a simple combination of known guidance properties of Slit and Netrin is sufficient to explain their roles in boundary formation. Our results suggest that Netrin indeed regulates boundary formation in combination with Slit in vivo.

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Netrin regulates boundary formation in combination with Slit in the fly brain

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Dual Netrin functions as attractant and repellent explain boundary formation

Temporal and spatial order of photoreceptor and glia projections into optic lobe in Drosophila

Photoreceptor (PR) axons project from the retina to the optic lobe in brain and form a precise retinotopic map in the Drosophila visual system. Yet the role of retinal basal glia in the retinotopic map formation is not previously known. We examined the formation of the retinotopic map by marking single PR pairs and following their axonal projections. In addition to confirming previous studies that the spatial information is preserved from the retina to the optic stalk and then to the optic lamina, we found that the young PR R3/4 axons transiently overshoot and then retract to their final destination, the lamina plexus. We then examined the process of wrapping glia (WG) membrane extension in the eye disc and showed that the WG membrane extensions also follow the retinotopic map. We show that the WG is important for the proper spatial distribution of PR axons in the optic stalk and lamina, suggesting an active role of wrapping glia in the retinotopic map formation.

The role of SAGA coactivator complex in snRNA transcription

The general snRNA gene transcription apparatus has been extensively studied. However, the role of coactivators in this process is far from being clearly understood. Here, we have demonstrated that the Drosophila SAGA complex interacts with the PBP complex, the key component of the snRNA gene transcription apparatus, and is present at the promoter regions of the snRNA genes transcribed by both the RNA polymerase II and RNA polymerase III (U6 snRNA). We show that SAGA interacts with the Brf1 transcription factor, which is a part of the RNA polymerase III transcription apparatus and is present at promoters of a number of Pol III-transcribed genes. Mutations inactivating several SAGA subunit genes resulted in reduced snRNA levels in adult flies, indicating that SAGA is indeed the transcriptional coactivator for the snRNA genes. The transcription of the Pol II and Pol III-transcribed U genes was reduced by mutations in all tested SAGA complex subunits. Therefore, the transcription of the Pol II and Pol III-transcribed U genes was reduced by the mutations in the deubiquitinase module, as well as in the acetyltransferase module of the SAGA, indicating that the whole complex is essential for their transcription. Therefore, the SAGA complex activates snRNA genes suggesting its wide involvement in the regulation of gene transcription, and consequently, in the maintenance of cellular homeostasis.

Ubiquitin-specific peptidase 22 functions and its involvement in disease

Deubiquitylases remove ubiquitin moieties from different substrates to regulate protein activity and cell homeostasis. Since this posttranslational modification plays a role in several different cellular functions, its deregulation has been associated with different pathologies. Aberrant expression of the Ubiquitin-Specific Peptidase 22 (USP22) has been associated with poor cancer prognosis and neurological disorders. However, little is known about USP22 role in these pathologies or in normal physiology. This review summarizes the current knowledge about USP22 function from yeast to human and provides an overview of the possible mechanisms by which USP22 is emerging as a potential oncogene.

Enzymatic modules of the SAGA chromatin-modifying complex play distinct roles in Drosophila gene expression and development

In this study, Li et al. demonstrate that the two enzymatic modules of the Drosophila Spt–Ada–Gcn5–acetyltransferase (SAGA) chromatin-modifying complex are differently required in oogenesis. Their findings demonstrate that loss of the histone acetyltransferase (HAT) activity blocks oogenesis, while loss of H2B deubiquitinase (DUB) activity does not, suggesting that the DUB module has functions within SAGA as well as independent functions.

The Spt–Ada–Gcn5–acetyltransferase (SAGA) chromatin-modifying complex is a transcriptional coactivator that contains four different modules of subunits. The intact SAGA complex has been well characterized for its function in transcription regulation and development. However, little is known about the roles of individual modules within SAGA and whether they have any SAGA-independent functions. Here we demonstrate that the two enzymatic modules of Drosophila SAGA are differently required in oogenesis. Loss of the histone acetyltransferase (HAT) activity blocks oogenesis, while loss of the H2B deubiquitinase (DUB) activity does not. However, the DUB module regulates a subset of genes in early embryogenesis, and loss of the DUB subunits causes defects in embryogenesis. ChIP-seq (chromatin immunoprecipitation [ChIP] combined with high-throughput sequencing) analysis revealed that both the DUB and HAT modules bind most SAGA target genes even though many of these targets do not require the DUB module for expression. Furthermore, we found that the DUB module can bind to chromatin and regulate transcription independently of the HAT module. Our results suggest that the DUB module has functions within SAGA and independent functions.

Transcriptome Profiling Identifies Multiplexin as a Target of SAGA Deubiquitinase Activity in Glia Required for Precise Axon Guidance During Drosophila Visual Development

The Spt-Ada-Gcn5 Acetyltransferase (SAGA) complex is a transcriptional coactivator with histone acetylase and deubiquitinase activities that plays an important role in visual development and function. In Drosophila melanogaster, four SAGA subunits are required for the deubiquitination of monoubiquitinated histone H2B (ubH2B): Nonstop, Sgf11, E(y)2, and Ataxin 7. Mutations that disrupt SAGA deubiquitinase activity cause defects in neuronal connectivity in the developing Drosophila visual system. In addition, mutations in SAGA result in the human progressive visual disorder spinocerebellar ataxia type 7 (SCA7). Glial cells play a crucial role in both the neuronal connectivity defect in nonstop and sgf11 flies, and in the retinal degeneration observed in SCA7 patients. Thus, we sought to identify the gene targets of SAGA deubiquitinase activity in glia in the Drosophila larval central nervous system. To do this, we enriched glia from wild-type, nonstop, and sgf11 larval optic lobes using affinity-purification of KASH-GFP tagged nuclei, and then examined each transcriptome using RNA-seq. Our analysis showed that SAGA deubiquitinase activity is required for proper expression of 16% of actively transcribed genes in glia, especially genes involved in proteasome function, protein folding and axon guidance. We further show that the SAGA deubiquitinase-activated gene Multiplexin (Mp) is required in glia for proper photoreceptor axon targeting. Mutations in the human ortholog of Mp, COL18A1, have been identified in a family with a SCA7-like progressive visual disorder, suggesting that defects in the expression of this gene in SCA7 patients could play a role in the retinal degeneration that is unique to this ataxia.

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.

X-exome sequencing of 405 unresolved families identifies seven novel intellectual disability genes

X-linked intellectual disability (XLID) is a clinically and genetically heterogeneous disorder. During the past two decades in excess of 100 X-chromosome ID genes have been identified. Yet, a large number of families mapping to the X-chromosome remained unresolved suggesting that more XLID genes or loci are yet to be identified. Here, we have investigated 405 unresolved families with XLID. We employed massively parallel sequencing of all X-chromosome exons in the index males. The majority of these males were previously tested negative for copy number variations and for mutations in a subset of known XLID genes by Sanger sequencing. In total, 745 X-chromosomal genes were screened. After stringent filtering, a total of 1297 non-recurrent exonic variants remained for prioritization. Co-segregation analysis of potential clinically relevant changes revealed that 80 families (20%) carried pathogenic variants in established XLID genes. In 19 families, we detected likely causative protein truncating and missense variants in 7 novel and validated XLID genes (CLCN4, CNKSR2, FRMPD4, KLHL15, LAS1L, RLIM and USP27X) and potentially deleterious variants in 2 novel candidate XLID genes (CDK16 and TAF1). We show that the CLCN4 and CNKSR2 variants impair protein functions as indicated by electrophysiological studies and altered differentiation of cultured primary neurons from Clcn4(-/-) mice or after mRNA knock-down. The newly identified and candidate XLID proteins belong to pathways and networks with established roles in cognitive function and intellectual disability in particular. We suggest that systematic sequencing of all X-chromosomal genes in a cohort of patients with genetic evidence for X-chromosome locus involvement may resolve up to 58% of Fragile X-negative cases.

Invadolysin acts genetically via the SAGA complex to modulate chromosome structure

Identification of components essential to chromosome structure and behaviour remains a vibrant area of study. We have previously shown that invadolysin is essential in Drosophila, with roles in cell division and cell migration. Mitotic chromosomes are hypercondensed in length, but display an aberrant fuzzy appearance. We additionally demonstrated that in human cells, invadolysin is localized on the surface of lipid droplets, organelles that store not only triglycerides and sterols but also free histones H2A, H2Av and H2B. Is there a link between the storage of histones in lipid droplets and the aberrantly structured chromosomes of invadolysin mutants? We have identified a genetic interaction between invadolysin and nonstop, the de-ubiquitinating protease component of the SAGA (Spt-Ada-Gcn5-acetyltransferase) chromatin-remodelling complex. invadolysin and nonstop mutants exhibit phenotypic similarities in terms of chromosome structure in both diploid and polyploid cells. Furthermore, IX-14¹/not¹ transheterozygous animals accumulate mono-ubiquitinated histone H2B (ubH2B) and histone H3 tri-methylated at lysine 4 (H3K4me3). Whole mount immunostaining of IX-14¹/not¹ transheterozygous salivary glands revealed that ubH2B accumulates surprisingly in the cytoplasm, rather than the nucleus. Over-expression of the Bre1 ubiquitin ligase phenocopies the effects of mutating either the invadolysin or nonstop genes. Intriguingly, nonstop and mutants of other SAGA subunits (gcn5, ada2b and sgf11) all suppress an invadolysin-induced rough eye phenotype. We conclude that the abnormal chromosome phenotype of invadolysin mutants is likely the result of disrupting the histone modification cycle, as accumulation of ubH2B and H3K4me3 is observed. We further suggest that the mislocalization of ubH2B to the cytoplasm has additional consequences on downstream components essential for chromosome behaviour. We therefore propose that invadolysin plays a crucial role in chromosome organization via its interaction with the SAGA complex.

Drosophila PS2 and PS3 integrins play distinct roles in retinal photoreceptors-glia interactions

Cellular migration and differentiation are important developmental processes that require dynamic cellular adhesion. Integrins are heterodimeric transmembrane receptors that play key roles in adhesion plasticity. Here, we explore the developing visual system of Drosophila to study the roles of integrin heterodimers in glia development. Our data show that αPS2 is essential for retinal glia migration from the brain into the eye disc and that glial cells have a role in the maintenance of the fenestrated membrane (Laminin-rich ECM layer) in the disc. Interestingly, the absence of glial cells in the eye disc did not affect the targeting of retinal axons to the optic stalk. In contrast, αPS3 is not required for retinal glia migration, but together with Talin, it functions in glial cells to allow photoreceptor axons to target the optic stalk. Thus, we present evidence that αPS2 and αPS3 integrin have different and specific functions in the development of retinal glia.

The deubiquitinase Leon/USP5 regulates ubiquitin homeostasis during Drosophila development

Ubiquitination and the reverse process deubiquitination regulate protein stability and function during animal development. The Drosophila USP5 homolog Leon functions as other family members of unconventional deubiquitinases, disassembling free, substrate-unconjugated polyubiquitin chains to replenish the pool of mono-ubiquitin, and maintaining cellular ubiquitin homeostasis. However, the significance of Leon/USP5 in animal development is still unexplored. In this study, we generated leon mutants to show that Leon is essential for animal viability and tissue integrity during development. Both free and substrate-conjugated polyubiquitin chains accumulate in leon mutants, suggesting that abnormal ubiquitin homeostasis caused tissue disorder and lethality in leon mutants. Further analysis of protein expression profiles in leon mutants shows that the levels of all proteasomal subunits were elevated. Also, proteasomal enzymatic activities were elevated in leon mutants. However, proteasomal degradation of ubiquitinated substrates was impaired. Thus, aberrant ubiquitin homeostasis in leon mutants disrupts normal proteasomal degradation, which is compensated by elevating the levels of proteasomal subunits and activities. Ultimately, the failure to fully compensate the dysfunctional proteasome in leon mutants leads to animal lethality and tissue disorder.

The evolution and development of neural superposition

Visual systems have a rich history as model systems for the discovery and understanding of basic principles underlying neuronal connectivity. The compound eyes of insects consist of up to thousands of small unit eyes that are connected by photoreceptor axons to set up a visual map in the brain. The photoreceptor axon terminals thereby represent neighboring points seen in the environment in neighboring synaptic units in the brain. Neural superposition is a special case of such a wiring principle, where photoreceptors from different unit eyes that receive the same input converge upon the same synaptic units in the brain. This wiring principle is remarkable, because each photoreceptor in a single unit eye receives different input and each individual axon, among thousands others in the brain, must be sorted together with those few axons that have the same input. Key aspects of neural superposition have been described as early as 1907. Since then neuroscientists, evolutionary and developmental biologists have been fascinated by how such a complicated wiring principle could evolve, how it is genetically encoded, and how it is developmentally realized. In this review article, we will discuss current ideas about the evolutionary origin and developmental program of neural superposition. Our goal is to identify in what way the special case of neural superposition can help us answer more general questions about the evolution and development of genetically “hard-wired” synaptic connectivity in the brain.

A multipotent transit-amplifying neuroblast lineage in the central brain gives rise to optic lobe glial cells in Drosophila

The neurons and glial cells of the Drosophila brain are generated by neural stem cell-like progenitors during two developmental phases, one short embryonic phase and one more prolonged postembryonic phase. Like the bulk of the adult-specific neurons, most of glial cells found in the adult central brain are generated postembryonically. Five of the neural stem cell-like progenitors that give rise to glial cells during postembryonic brain development have been identified as type II neuroglioblasts that generate neural and glial progeny through transient amplifying INPs. Here we identify DL1 as a novel multipotent neuroglial progenitor in the central brain and show that this type II neuroblast not only gives rise to neurons that innervate the central complex but also to glial cells that contribute exclusively to the optic lobe. Immediately following their generation in the central brain during the second half of larval development, these DL1 lineage-derived glia migrate into the developing optic lobe, where they differentiate into three identified types of optic lobe glial cells, inner chiasm glia, outer chiasm glia and cortex glia. Taken together, these findings reveal an unexpected central brain origin of optic lobe glial cells and central complex interneurons from one and the same type II neuroglioblast.

Brain-specific-homeobox is required for the specification of neuronal types in the Drosophila optic lobe

The Drosophila optic lobe comprises a wide variety of neurons forming laminar and columnar structures similar to the mammalian brain. The Drosophila optic lobe may provide an excellent model to investigate various processes of brain development. However, it is poorly understood how neuronal specification is regulated in the optic lobe to form a complicated structure. Here we show that the Brain-specific-homeobox (Bsh) protein, which is expressed in the lamina and medulla ganglia, is involved in specifying neuronal identity. Bsh is expressed in L4 and L5 lamina neurons and in Mi1 medulla neurons. Analyses of loss-of-function and gain-of-function clones suggest that Bsh is required and largely sufficient for Mi1 specification in the medulla and L4 specification in the lamina. Additionally, Bsh is at least required for L5 specification. In the absence of Bsh, L5 is transformed into glial cells.

Neurodegeneration in a Drosophila model for the function of TMCC2, an amyloid protein precursor-interacting and apolipoprotein E-binding protein

We previously identified TMCC2 as a protein that interacted differentially with normal versus Alzheimer's disease-risk forms of both apolipoprotein E (apoE) and the amyloid protein precursor (APP). We hypothesized that disrupted function of TMCC2 would affect neurodegeneration. To test this hypothesis, we investigated the Drosophila orthologue of TMCC2, that we have named Dementin. We showed that Dementin interacts genetically both with human APP and its Drosophila orthologue, the APP-like protein (APPL). Ectopic expression of Dementin in Drosophila rescued developmental and behavioral defects caused by expression of human APP. Both a hypomorphic lethal mutation in the dementin gene (dmtn¹) and RNAi for Dementin caused the accumulation of fragments derived from APPL. We found that Dementin was required for normal development of the brain, and that glial Dementin was required for development of the Drosophila medulla neuropil. Expression of wild-type Dementin in either the neurons or glia of dmtn¹ flies rescued developmental lethality. Adult dmtn¹ flies rescued by expression of wild-type Dementin in glia, i.e. whose neurons expressed only dmtn¹, showed pathological features resembling early onset Alzheimer's disease, accumulation of abnormal APPL metabolites, synaptic pathology, mis-localized microtubule-binding proteins, neurodegeneration, and early death.

A Serrate-Notch-Canoe complex mediates glial-neuroepithelial cell interactions essential during Drosophila optic lobe development

It is firmly established that neuron-glia interactions are fundamental across species for the correct establishment of a functional brain. Here, we found that the glia of the Drosophila larval brain display an essential non-autonomous role during the development of the optic lobe. The optic lobe develops from neuroepithelial cells that proliferate by dividing symmetrically until they switch to asymmetric/differentiative divisions generating neuroblasts. The proneural gene lethal of scute (l'sc) is transiently activated by the Epidermal Growth Factor Receptor (EGFR)/Ras signal transduction pathway at the leading edge of a proneural wave that sweeps from medial to lateral neuroepithelium promoting this switch. This process is tightly regulated by the tissue-autonomous function within the neuroepithelium of multiple signaling pathways, including EGFR/Ras and Notch. This study shows that the Notch ligand Serrate (Ser) is expressed in the glia and it forms a complex in vivo with Notch and Canoe, which colocalize at the adherens junctions of neuroepithelial cells. This complex is crucial for glial-neuroepithelial cell interactions during optic lobe development. Ser is tissue-autonomously required in the glia where it activates Notch to regulate its proliferation, and non-autonomously in the neuroepithelium where Ser induces Notch signaling to avoid the premature activation of the EGFR/Ras pathway and hence of L'sc. Interestingly, different Notch activity reporters showed very different expression patterns in the glia and in the neuroepithelium, suggesting the existence of tissue-specific factors that promote the expression of particular Notch target genes or/and a reporter response dependent on different thresholds of Notch signaling.

Postembryonic development of astrocyte-like glia of the central complex in the grasshopper Schistocerca gregaria

Central complex modules in the postembryonic brain of the grasshopper Schistocerca gregaria are enveloped by Repo-positive/glutamine-synthetase-positive astrocyte-like glia. Such cells constitute Rind-Neuropil Interface glia. We have investigated the postembryonic development of these glia and their anatomical relationship to axons originating from the w, x, y, z tract system of the pars intercerebralis. Based on glutamine synthetase immunolabeling, we have identified four morphological types of cells: bipolar type 1 glia delimit the central body but only innervate its neuropil superficially; monopolar type 2 glia have a more columnar morphology and direct numerous gliopodia into the neuropil where they arborize extensively; monopolar type 3 glia are found predominantly in the region between the noduli and the central body and have a dendritic morphology and their gliopodia project deeply into the central body neuropil where they arborize extensively; multipolar type 4 glia link the central body neuropil with neighboring neuropils of the protocerebrum. These glia occupy type-specific distributions around the central body. Their gliopodia develop late in embryogenesis, elongate and generally become denser during subsequent postembryonic development. Gliopodia from putatively type 3 glia within the central body have been shown to lie closely apposed to individual axons of identified columnar fiber bundles from the w, x, y, z tract system of the central complex. This anatomical association might offer a substrate for neuron/glia interactions mediating postembryonic maturation of the central complex.

Roles for the ubiquitin-proteasome pathway in protein quality control and signaling in the retina: implications in the pathogenesis of age-related macular degeneration

The accumulation of damaged or postsynthetically modified proteins and dysregulation of inflammatory responses and angiogenesis in the retina/RPE are thought be etiologically related to formation of drusen and choroidal neovascularization (CNV), hallmarks of age-related macular degeneration (AMD). The ubiquitin-proteasome pathway (UPP) plays crucial roles in protein quality control, cell cycle control and signal transduction. Selective degradation of aberrant proteins by the UPP is essential for timely removal of potentially cytotoxic damaged or otherwise abnormal proteins. Proper function of the UPP is thought to be required for cellular function. In contrast, age--or stress induced--impairment the UPP or insufficient UPP capacity may contribute to the accumulation of abnormal proteins, cytotoxicity in the retina, and AMD. Crucial roles for the UPP in eye development, regulation of signal transduction, and antioxidant responses are also established. Insufficient UPP capacity in retina and RPE can result in dysregulation of signal transduction, abnormal inflammatory responses and CNV. There are also interactions between the UPP and lysosomal proteolytic pathways (LPPs). Means that modulate the proteolytic capacity are making their way into new generation of pharmacotherapies for delaying age-related diseases and may augment the benefits of adequate nutrition, with regard to diminishing the burden of AMD.

Gcm proteins function in the developing nervous system

A fundamental issue during nervous system development is how individual cells are formed from the undefined precursors. Differentiated neurons and glia, two major cell types mediating neuronal function, are acquired from immature precursors via a series of explicit controls exerted by transcription factors such as proteins in the family of Glial cells missing (Gcm). In mammals, Gcm proteins are involved in placenta and parathyroid gland development, whereas in the invertebrate organism Drosophila, Gcm proteins act as fate determinants for glial cell fate, regulate neural stem cell (NSC) induction and conversion, and promote glial proliferation. In particular, Gcm protein levels are carefully tuned for Drosophila gliogenesis and their stability is under precise control via the ubiquitin-proteasome system (UPS). Here we summarize recent advances on Gcm proteins function. In addition to describe various features of Gcm protein family, the significance of their functions in the developing nervous system is also discussed.

Visual circuit assembly in Drosophila

Both insect and vertebrate visual circuits are organized into orderly arrays of columnar and layered synaptic units that correspond to the array of photoreceptors in the eye. Recent genetic studies in Drosophila have yielded insights into the molecular and cellular mechanisms that pattern the layers and columns and establish specific connections within the synaptic units. A sequence of inductive events and complex cellular interactions coordinates the assembly of visual circuits. Photoreceptor-derived ligands, such as hedgehog and Jelly-Belly, induce target development and expression of specific adhesion molecules, which in turn serve as guidance cues for photoreceptor axons. Afferents are directed to specific layers by adhesive afferent-target interactions mediated by leucine-rich repeat proteins and cadherins, which are restricted spatially and/or modulated dynamically. Afferents are restricted to their topographically appropriate columns by repulsive interactions between afferents and by autocrine activin signaling. Finally, Dscam-mediated repulsive interactions between target neuron dendrites ensure appropriate combinations of postsynaptic elements at synapses. Essentially, all these Drosophila molecules have vertebrate homologs, some of which are known to carry out analogous functions. Thus, the studies of Drosophila visual circuit development would shed light on neural circuit assembly in general.

bantam is required for optic lobe development and glial cell proliferation

microRNAs (miRNAs) are small, conserved, non-coding RNAs that contribute to the control of many different cellular processes, including cell fate specification and growth control. Drosophila bantam, a conserved miRNA, is involved in several functions, such as stimulating proliferation and inhibiting apoptosis in the wing disc. Here, we reported the detailed expression pattern of bantam in the developing optic lobe, and demonstrated a new, essential role in promoting proliferation of mitotic cells in the optic lobe, including stem cells and differentiated glial cells. Changes in bantam levels autonomously affected glial cell number and distribution, and non-autonomously affected photoreceptor neuron axon projection patterns. Furthermore, we showed that bantam promotes the proliferation of mitotically active glial cells and affects their distribution, largely through down regulation of the T-box transcription factor, optomotor-blind (omb, Flybase, bifid). Expression of omb can rescue the bantam phenotype, and restore the normal glial cell number and proper glial cell positioning in most Drosophila brains. These results suggest that bantam is critical for maintaining the stem cell pools in the outer proliferation center and glial precursor cell regions of the optic lobe, and that its expression in glial cells is crucial for their proliferation and distribution.

Role of the ubiquitin-proteasome in protein quality control and signaling: implication in the pathogenesis of eye diseases

The ubiquitin-proteasome pathway (UPP) plays important roles in many cellular functions, such as protein quality control, cell cycle control, and signal transduction. The selective degradation of aberrant proteins by the UPP is essential for the timely removal of potential cytotoxic damaged or otherwise abnormal proteins. Conversely, accumulation of the cytotoxic abnormal proteins in eye tissues is etiologically associated with many age-related eye diseases such as retina degeneration, cataract, and certain types of glaucoma. Age- or stress-induced impairment or overburdening of the UPP appears to contribute to the accumulation of abnormal proteins in eye tissues. Cell cycle and signal transduction are regulated by the conditional UPP-dependent degradation of the regulators of these processes. Impairment or overburdening of the UPP could also result in dysregulation of cell cycle control and signal transduction. The consequences of the improper cell cycle and signal transduction include defects in ocular development, wound healing, angiogenesis, or inflammatory responses. Methods that enhance or preserve UPP function or reduce its burden may be useful strategies for preventing age-related eye diseases.

Flickin' the ubiquitin switch: the role of H2B ubiquitylation in development

The reversible ubiquitylation of histone H2B has long been implicated in transcriptional activation and gene silencing. However, many questions regarding its regulation and effects on chromatin structure remain unanswered. In addition, while several studies have uncovered an involvement of this modification in the control of certain developmental processes, a more general understanding of its requirement is lacking. Herein, we present a broad overview of the pathways known to be regulated by H2B ubiquitylation, while drawing parallels between findings in disparate organisms, in order to facilitate continued delineation of its spatiotemporal role in development. Finally, we integrate the findings of recent studies into how H2B ubiquitylation affects chromatin, and cast an eye over emerging areas for future research.

Membrane-targeted WAVE mediates photoreceptor axon targeting in the absence of the WAVE complex in Drosophila

A tight spatial-temporal coordination of F-actin dynamics is crucial for a large variety of cellular processes that shape cells. The Abelson interactor (Abi) has a conserved role in Arp2/3-dependent actin polymerization, regulating Wiskott-Aldrich syndrome protein (WASP) and WASP family verprolin-homologous protein (WAVE). In this paper, we report that Abi exerts nonautonomous control of photoreceptor axon targeting in the Drosophila visual system through WAVE. In abi mutants, WAVE is unstable but restored by reexpression of Abi, confirming that Abi controls the integrity of the WAVE complex in vivo. Remarkably, expression of a membrane-tethered WAVE protein rescues the axonal projection defects of abi mutants in the absence of the other subunits of the WAVE complex, whereas cytoplasmic WAVE only slightly affects the abi mutant phenotype. Thus complex formation not only stabilizes WAVE, but also provides further membrane-recruiting signals, resulting in an activation of WAVE.

Astrocyte-like glia associated with the embryonic development of the central complex in the grasshopper Schistocerca gregaria

In this study we employed the expression of the astrocyte-specific enzyme glutamine synthetase, in addition to the glia-specific marker Repo, to characterize glia cell types associated with the embryonic development of the central complex in the grasshopper Schistocerca gregaria. Double labeling experiments reveal that all glutamine synthetase-positive cells associated with the central complex are also Repo-positive and horseradish peroxidase-negative, confirming they are glia. Early in embryogenesis, prior to development of the central complex, glia form a continuous population extending from the pars intercerebralis into the region of the commissural fascicles. Subsequently, these glia redisperse to envelop each of the modules of the central complex. No glial somata are found within the central complex neuropils themselves. Since glutamine synthetase is expressed cortically in glia, it allows their processes as well as their soma locations to be visualized. Single cell reconstructions reveal one population of glia as directing extensive ensheathing processes around central complex neuropils such as the central body, while another population projects columnar-like arborizations within the central body. Such arborizations are only seen in central complex modules after their neuroarchitecture has been established suggesting that the glial arborizations project onto a prior scaffold of neurons or tracheae.

Roles of glial cells in neural circuit formation: insights from research in insects

Investigators over the years have noted many striking similarities in the structural organization and function of neural circuits in higher invertebrates and vertebrates. In more recent years, the discovery of similarities in the cellular and molecular mechanisms that guide development of these circuits has driven a revolution in our understanding of neural development. Cellular mechanisms discovered to underlie axon pathfinding in grasshoppers have guided productive studies in mammals. Genes discovered to play key roles in the patterning of the fruitfly's central nervous system have subsequently been found to play key roles in mice. The diversity of invertebrate species offers to investigators numerous opportunities to conduct experiments that are harder or impossible to do in vertebrate species, but that are likely to shed light on mechanisms at play in developing vertebrate nervous systems. These experiments elucidate the broad suite of cellular and molecular interactions that have the potential to influence neural circuit formation across species. Here we focus on what is known about roles for glial cells in some of the important steps in neural circuit formation in experimentally advantageous insect species. These steps include axon pathfinding and matching to targets, dendritic patterning, and the sculpting of synaptic neuropils. A consistent theme is that glial cells interact with neurons in two-way, reciprocal interactions. We emphasize the impact of studies performed in insects and explore how insect nervous systems might best be exploited next as scientists seek to understand in yet deeper detail the full repertory of functions of glia in development.