Drosophila ß(Heavy)-Spectrin is required in polarized ensheathing glia that form a diffusion-barrier around the neuropil

Apical cell expansion maintained by Dusky-like establishes a scaffold for corneal lens morphogenesis

The Drosophila corneal lens is entirely composed of chitin and other apical extracellular matrix components, and it is not known how it acquires the biconvex shape that enables it to focus light onto the retina. We show here that the zona pellucida domain-containing protein Dusky-like is essential for normal corneal lens morphogenesis. Dusky-like transiently localizes to the expanded apical surfaces of the corneal lens-secreting cells and prevents them from undergoing apical constriction and apicobasal contraction. Dusky-like also controls the arrangement of two other zona pellucida domain proteins, Dumpy and Piopio, external to the developing corneal lens. Loss of either dusky-like or dumpy delays chitin accumulation and disrupts the outer surface of the corneal lens. We find that artificially inducing apical constriction by activating myosin contraction is sufficient to similarly alter chitin deposition and corneal lens morphology. These results demonstrate the importance of cell shape in controlling the morphogenesis of overlying apical extracellular matrix structures such as the corneal lens.

Epithelial UNC-23 limits mechanical stress to maintain glia-neuron architecture in C. elegans

For an organ to maintain correct architecture and function, its diverse cellular components must coordinate their size and shape. Although cell-intrinsic mechanisms driving homotypic cell-cell coordination are known, it is unclear how cell shape is regulated across heterotypic cells. We find that epithelial cells maintain the shape of neighboring sense-organ glia-neuron units in adult Caenorhabditis elegans (C. elegans). Hsp co-chaperone UNC-23/BAG2 prevents epithelial cell shape from deforming, and its loss causes head epithelia to stretch aberrantly during animal movement. In the sense-organ glia, amphid sheath (AMsh), this causes progressive fibroblast growth factor receptor (FGFR)-dependent disruption of the glial apical cytoskeleton. Resultant glial cell shape alteration causes concomitant shape change in glia-associated neuron endings. Epithelial UNC-23 maintenance of glia-neuron shape is specific both spatially, within a defined anatomical zone, and temporally, in a developmentally critical period. As all molecular components uncovered are broadly conserved across central and peripheral nervous systems, we posit that epithelia may similarly regulate glia-neuron architecture cross-species.

The Drosophila blood-brain barrier invades the nervous system in a GPCR-dependent manner

The blood-brain barrier (BBB) represents a crucial interface between the circulatory system and the brain. In Drosophila melanogaster, the BBB is composed of perineurial and subperineurial glial cells. The perineurial glial cells are small mitotically active cells forming the outermost layer of the nervous system and are engaged in nutrient uptake. The subperineurial glial cells form occluding septate junctions to prevent paracellular diffusion of macromolecules into the nervous system. To address whether the subperineurial glia just form a simple barrier or whether they establish specific contacts with both the perineurial glial cells and inner central nervous system (CNS) cells, we undertook a detailed morphological analysis. Using genetically encoded markers alongside with high-resolution laser scanning confocal microscopy and transmission electron microscopy, we identified thin cell processes extending into the perineurial layer and into the CNS cortex. Interestingly, long cell processes were observed reaching the glia ensheathing the neuropil of the central brain. GFP reconstitution experiments highlighted multiple regions of membrane contacts between subperineurial and ensheathing glia. Furthermore, we identify the G-protein-coupled receptor (GPCR) Moody as negative regulator of the growth of subperineurial cell processes. Loss of moody triggered a massive overgrowth of subperineurial cell processes into the CNS cortex and, moreover, affected the polarized localization of the xenobiotic transporter Mdr65. Finally, we found that GPCR signaling, but not septate junction formation, is responsible for controlling membrane overgrowth. Our findings support the notion that the Drosophila BBB is able to bridge the communication gap between circulation and synaptic regions of the brain by long cell processes.

Neuron cilia restrain glial KCC-3 to a microdomain to regulate multisensory processing

Glia interact with multiple neurons, but it is unclear whether their interactions with each neuron are different. Our interrogation at single-cell resolution reveals that a single glial cell exhibits specificity in its interactions with different contacting neurons. Briefly, C. elegans amphid sheath (AMsh) glia apical-like domains contact 12 neuron-endings. At these ad-neuronal membranes, AMsh glia localize the K/Cl transporter KCC-3 to a microdomain exclusively around the thermosensory AFD neuron to regulate its properties. Glial KCC-3 is transported to ad-neuronal regions, where distal cilia of non-AFD glia-associated chemosensory neurons constrain it to a microdomain at AFD-contacting glial membranes. Aberrant KCC-3 localization impacts both thermosensory (AFD) and chemosensory (non-AFD) neuron properties. Thus, neurons can interact non-synaptically through a shared glial cell by regulating microdomain localization of its cues. As AMsh and glia across species compartmentalize multiple cues like KCC-3, we posit that this may be a broadly conserved glial mechanism that modulates information processing across multimodal circuits.

Apical cell expansion maintained by Dusky-like establishes a scaffold for corneal lens morphogenesis

The biconvex shape of the Drosophila corneal lens, which enables it to focus light onto the retina, arises by organized assembly of chitin and other apical extracellular matrix components. We show here that the Zona Pellucida domain-containing protein Dusky-like is essential for normal corneal lens morphogenesis. Dusky-like transiently localizes to the expanded apical surfaces of the corneal lens-secreting cells, and in its absence, these cells undergo apical constriction and apicobasal contraction. Dusky-like also controls the arrangement of two other Zona Pellucida-domain proteins, Dumpy and Piopio, external to the developing corneal lens. Loss of either dusky-like or dumpy delays chitin accumulation and disrupts the outer surface of the corneal lens. Artificially inducing apical constriction with constitutively active Myosin light chain kinase is sufficient to similarly alter chitin deposition and corneal lens morphology. These results demonstrate the importance of cell shape for the morphogenesis of overlying apical extracellular matrix structures.

Glia as Functional Barriers and Signaling Intermediaries

Glia play a crucial role in providing metabolic support to neurons across different species. To do so, glial cells isolate distinct neuronal compartments from systemic signals and selectively transport specific metabolites and ions to support neuronal development and facilitate neuronal function. Because of their function as barriers, glial cells occupy privileged positions within the nervous system and have also evolved to serve as signaling intermediaries in various contexts. The fruit fly, Drosophila melanogaster, has significantly contributed to our understanding of glial barrier development and function. In this review, we will explore the formation of the glial sheath, blood-brain barrier, and nerve barrier, as well as the significance of glia-extracellular matrix interactions in barrier formation. Additionally, we will delve into the role of glia as signaling intermediaries in regulating nervous system development, function, and response to injury.

Homemade: building the structure of the neurogenic niche

Neural stem/progenitor cells live in an intricate cellular environment, the neurogenic niche, which supports their function and enables neurogenesis. The niche is made of a diversity of cell types, including neurons, glia and the vasculature, which are able to signal to and are structurally organised around neural stem/progenitor cells. While the focus has been on how individual cell types signal to and influence the behaviour of neural stem/progenitor cells, very little is actually known on how the niche is assembled during development from multiple cellular origins, and on the role of the resulting topology on these cells. This review proposes to draw a state-of-the art picture of this emerging field of research, with the aim to expose our knowledge on niche architecture and formation from different animal models (mouse, zebrafish and fruit fly). We will span its multiple aspects, from the existence and importance of local, adhesive interactions to the potential emergence of larger-scale topological properties through the careful assembly of diverse cellular and acellular components.

Signaling Pathways Controlling Axonal Wrapping in Drosophila

The rapid transmission of action potentials is an important ability that enables efficient communication within the nervous system. Glial cells influence conduction velocity along axons by regulating the radial axonal diameter, providing electrical insulation as well as affecting the distribution of voltage-gated ion channels. Differentiation of these wrapping glial cells requires a complex set of neuron-glia interactions involving three basic mechanistic features. The glia must recognize the axon, grow around it, and eventually arrest its growth to form single or multiple axon wraps. This likely depends on the integration of numerous evolutionary conserved signaling and adhesion systems. Here, we summarize the mechanisms and underlying signaling pathways that control glial wrapping in Drosophila and compare those to the mechanisms that control glial differentiation in mammals. This analysis shows that Drosophila is a beneficial model to study the development of even complex structures like myelin.

Description of trunk neural crest migration and peripheral nervous system formation in the Egyptian cobra Naja haje haje

The neural crest is a stem cell population that forms in the neurectoderm of all vertebrates and gives rise to a diverse set of cells such as sensory neurons, Schwann cells and melanocytes. Neural crest development in snakes is still poorly understood. From the point of view of evolutionary and comparative anatomy is an interesting topic given the unique anatomy of snakes. The aim of the study was to characterize how trunk neural crest cells (TNCC) migrate in the developing elapid snake Naja haje haje and consequently, look at the beginnings of development of neural crest derived sensory ganglia (DRG) and spinal nerves. We found that trunk neural crest and DRG development in Naja haje haje is like what has been described in other vertebrates and the colubrid snake strengthening our knowledge on the conserved mechanisms of neural crest development across species. Here we use the marker HNK1 to follow the migratory behavior of TNCC in the elapid snake Naja haje haje through stages 1-6 (1-9 days postoviposition). We observed that the TNCC of both snake species migrate through the rostral portion of the somite, a pattern also conserved in birds and mammals. The development of cobra peripheral nervous system, using neuronal and glial markers, showed the presence of spectrin in Schwann cell precursors and of axonal plexus along the length of the cobra embryos. In conclusion, cobra embryos show strong conserved patterns in TNCC and PNS development among vertebrates.

Competition between myosin II and βH-spectrin regulates cytoskeletal tension

Spectrins are membrane cytoskeletal proteins generally thought to function as heterotetramers comprising two α-spectrins and two β-spectrins. They influence cell shape and Hippo signaling, but the mechanism by which they influence Hippo signaling has remained unclear. We have investigated the role and regulation of the Drosophila β-heavy spectrin (β_H-spectrin, encoded by the karst gene) in wing imaginal discs. Our results establish that β_H-spectrin regulates Hippo signaling through the Jub biomechanical pathway due to its influence on cytoskeletal tension. While we find that α-spectrin also regulates Hippo signaling through Jub, unexpectedly, we find that β_H-spectrin localizes and functions independently of α-spectrin. Instead, β_H-spectrin co-localizes with and reciprocally regulates and is regulated by myosin. In vivo and in vitro experiments support a model in which β_H-spectrin and myosin directly compete for binding to apical F-actin. This competition can explain the influence of β_H-spectrin on cytoskeletal tension and myosin accumulation. It also provides new insight into how β_H-spectrin participates in ratcheting mechanisms associated with cell shape change.

Apical-basal polarity of the spectrin cytoskeleton in the C. elegans vulva

The C. elegans vulva is a polarized epithelial tube that has been studied extensively as a model for cell-cell signaling, cell fate specification, and tubulogenesis. Here we used endogenous fusions to show that the spectrin cytoskeleton is polarized in this organ, with conventional beta-spectrin ( UNC-70 ) found only at basolateral membranes and beta heavy spectrin ( SMA-1 ) found only at apical membranes. The sole alpha-spectrin ( SPC-1 ) is present at both locations but requires SMA-1 for its apical localization. Thus, beta spectrins are excellent markers for vulva cell membranes and polarity.

A cold and quiet brain: mechanisms of insect CNS arrest at low temperatures

Exposure to cold causes insects to enter a chill coma at species-specific temperatures and such temperature sensitivity contributes to geographic distribution and phenology. Coma results from abrupt spreading depolarization (SD) of neural tissue in the integrative centers of the CNS. SD abolishes neuronal signaling and the operation of neural circuits, like an off switch for the CNS. Turning off the CNS by allowing ion gradients to collapse will conserve energy and may offset negative consequences of temporary immobility. SD is modified by prior experience via rapid cold hardening (RCH) or cold acclimation which alter properties of K_v channels, Na⁺/K⁺-ATPase and Na⁺/K⁺/2Cl^- cotransporter. The stress hormone octopamine mediates RCH. Future progress depends on developing a more complete understanding of ion homeostasis in and of the insect CNS.

Drosophila glia take shape to sculpt the nervous system

The importance of glial cells has become increasingly apparent over the past 20 years, yet compared to neurons we still know relatively little about these essential cells. Most critical glial cell functions are conserved in Drosophila glia, often using the same key molecular players as their vertebrate counterparts. The relative simplicity of the Drosophila nervous system, combined with a vast array of powerful genetic tools, allows us to further dissect the molecular composition and functional roles of glia in ways that would be much more cumbersome or not possible in higher vertebrate systems. Importantly, Drosophila genetics allow for in vivo manipulation, and their transparent body wall enables in vivo imaging of glia in intact animals throughout early development. Here we discuss recent advances in Drosophila glial development detailing how these cells take on their mature morphologies and interact with neurons to perform their important functional roles in the nervous system.

Neuron cilia constrain glial regulators to microdomains around distal neurons

Each glia interacts with multiple neurons, but the fundamental logic of whether it interacts with all equally remains unclear. We find that a single sense-organ glia modulates different contacting neurons distinctly. To do so, it partitions regulatory cues into molecular microdomains at specific neuron contact-sites, at its delimited apical membrane. For one glial cue, K/Cl transporter KCC-3, microdomain-localization occurs through a two-step, neuron-dependent process. First, KCC-3 shuttles to glial apical membranes. Second, some contacting neuron cilia repel it, rendering it microdomain-localized around one distal neuron-ending. KCC-3 localization tracks animal aging, and while apical localization is sufficient for contacting neuron function, microdomain-restriction is required for distal neuron properties. Finally, we find the glia regulates its microdomains largely independently. Together, this uncovers that glia modulate cross-modal sensor processing by compartmentalizing regulatory cues into microdomains. Glia across species contact multiple neurons and localize disease-relevant cues like KCC-3. Thus, analogous compartmentalization may broadly drive how glia regulate information processing across neural circuits.

Molecular sensing of mechano- and ligand-dependent adhesion GPCR dissociation

Adhesion G-protein-coupled receptors (aGPCRs) bear notable similarity to Notch proteins1, a class of surface receptors poised for mechano-proteolytic activation2-4, including an evolutionarily conserved mechanism of cleavage5-8. However, so far there is no unifying explanation for why aGPCRs are autoproteolytically processed. Here we introduce a genetically encoded sensor system to detect the dissociation events of aGPCR heterodimers into their constituent N-terminal and C-terminal fragments (NTFs and CTFs, respectively). An NTF release sensor (NRS) of the neural latrophilin-type aGPCR Cirl (ADGRL)9-11, from Drosophila melanogaster, is stimulated by mechanical force. Cirl-NRS activation indicates that receptor dissociation occurs in neurons and cortex glial cells. The release of NTFs from cortex glial cells requires trans-interaction between Cirl and its ligand, the Toll-like receptor Tollo (Toll-8)12, on neural progenitor cells, whereas expressing Cirl and Tollo in cis suppresses dissociation of the aGPCR. This interaction is necessary to control the size of the neuroblast pool in the central nervous system. We conclude that receptor autoproteolysis enables non-cell-autonomous activities of aGPCRs, and that the dissociation of aGPCRs is controlled by their ligand expression profile and by mechanical force. The NRS system will be helpful in elucidating the physiological roles and signal modulators of aGPCRs, which constitute a large untapped reservoir of drug targets for cardiovascular, immune, neuropsychiatric and neoplastic diseases13.

A role for glia in cellular and systemic metabolism: insights from the fly

Excitability and synaptic transmission make neurons high-energy consumers. However, neurons do not store carbohydrates or lipids. Instead, they need support cells to fuel their metabolic demands. This role is assumed by glia, both in vertebrates and invertebrates. Many questions remain regarding the coupling between neuronal activity and energy demand on the one hand, and nutrient supply by glia on the other hand. Here, we review recent advances showing that fly glia, similar to their role in vertebrates, fuel neurons in times of high energetic demand, such as during memory formation and long-term storage. Vertebrate glia also play a role in the modulation of neurons, their communication, and behavior, including food search and feeding. We discuss recent literature pointing to similar roles of fly glia in behavior and metabolism.

Quantifying Glial-Glial Tiling Using Automated Image Analysis in Drosophila

Not only do glia form close associations with neurons throughout the central nervous system (CNS), but glial cells also interact closely with other glial cells. As these cells mature, they undergo a phenomenon known as glial tiling, where they grow to abut one another, often without invading each other’s boundaries. Glial tiling occurs throughout the animal kingdom, from fruit flies to humans; however, not much is known about the glial-glial interactions that lead to and maintain this tiling. Drosophila provide a strong model to investigate glial-glial tiling, where tiling occurs both among individual glial cells of the same subtype, as well as between those of different subtypes. Furthermore, the spatial segregation of the CNS allows for the unique ability to visualize and manipulate inter-subtype interactions. Previous work in Drosophila has suggested an interaction between cortex glia and astrocytes, where astrocytes cross the normal neuropil-cortex boundary in response to dysfunctional cortex glia. Here, we further explore this interaction by implementing an automated pipeline to more fully characterize this astrocyte-cortex glial relationship. By quantifying and correlating the extent of cortex glial dysfunction and aberrant astrocyte infiltration using automated analysis, we maximize the size of the quantified dataset to reveal subtle patterns in astrocyte-cortex glial interactions. We provide a guide for creating and validating a fully-automated image analysis pipeline for exploring these interactions, and implement this pipeline to describe a significant correlation between cortex glial dysfunction and aberrant astrocyte infiltration, as well as demonstrate variations in their relationship across different regions of the CNS.

Glial Tiling in the Insect Nervous System

The Drosophila nervous system comprises a small number of well characterized glial cell classes. The outer surface of the central nervous system (CNS) is protected by a glial derived blood-brain barrier generated by perineurial and subperineurial glia. All neural stem cells and all neurons are engulfed by cortex glial cells. The inner neuropil region, that harbors all synapses and dendrites, is covered by ensheathing glia and infiltrated by astrocyte-like glial cells. All these glial cells show a tiled organization with an often remarkable plasticity where glial cells of one cell type invade the territory of the neighboring glial cell type upon its ablation. Here, we summarize the different glial tiling patterns and based on the different modes of cell-cell contacts we hypothesize that different molecular mechanisms underlie tiling of the different glial cell types.

Redundant functions of the SLC5A transporters Rumpel, Bumpel, and Kumpel in ensheathing glial cells

Neuronal processing is energy demanding, and relies on sugar metabolism. To nurture the Drosophila nervous system, the blood-brain barrier forming glial cells take up trehalose from the hemolymph and then distribute the metabolic products further to all neurons. This function is provided by glucose and lactate transporters of the solute carrier (SLC) 5A family. Here we identified three SLC5A genes that are specifically expressed in overlapping sets of CNS glial cells, rumpel, bumpel and kumpel. We generated mutants in all genes and all mutants are viable and fertile, lacking discernible phenotypes. Loss of rumpel causes subtle locomotor phenotypes and flies display increased daytime sleep. In addition, in bumpel kumpel double mutants, and to an even greater extent in rumpel bumpel kumpel triple mutants, oogenesis is disrupted at the onset of the vitollegenic phase. This indicates a partially redundant functions between these genes. Rescue experiments exploring this effect indicate that oogenesis can be affected by CNS glial cells. Moreover, expression of heterologous mammalian SLC5A transporters, with known transport properties, suggest that Bumpel and/or Kumpel transport glucose or lactate. Overall, our results imply a redundancy in SLC5A nutrient sensing functions in Drosophila glial cells, affecting ovarian development and behavior.