A muscle-epidermis-glia signaling axis sustains synaptic specificity during allometric growth in Caenorhabditis elegans

Age-progressive interplay of HSP-proteostasis, ECM-cell junctions and biomechanics ensures C. elegans astroglial architecture

Tissue integrity is sensitive to temperature, tension, age, and is sustained throughout life by adaptive cell-autonomous or extrinsic mechanisms. Safeguarding the remarkably-complex architectures of neurons and glia ensures age-dependent integrity of functional circuits. Here, we report mechanisms sustaining the integrity of C. elegans CEPsh astrocyte-like glia. We combine large-scale genetics with manipulation of genes, cells, and their environment, quantitative imaging of cellular/ subcellular features, tissue material properties and extracellular matrix (ECM). We identify mutants with age-progressive, environment-dependent defects in glial architecture, consequent disruption of neuronal architecture, and abnormal aging. Functional loss of epithelial Hsp70/Hsc70-cochaperone BAG2 causes ECM disruption, altered tissue biomechanics, and hypersensitivity of glia to environmental temperature and mechanics. Glial-cell junctions ensure epithelia-ECM-CEPsh glia association. Modifying glial junctions or ECM mechanics safeguards glial integrity against disrupted BAG2-proteostasis. Overall, we present a finely-regulated interplay of proteostasis-ECM and cell junctions with conserved components that ensures age-progressive robustness of glial architecture.

Glia Development and Function in the Nematode Caenorhabditis elegans

The nematode Caenorhabditis elegans is a powerful experimental setting for uncovering fundamental tenets of nervous system organization and function. Its nearly invariant and simple anatomy, coupled with a plethora of methodologies for interrogating single-gene functions at single-cell resolution in vivo, have led to exciting discoveries in glial cell biology and mechanisms of glia-neuron interactions. Findings over the last two decades reinforce the idea that insights from C. elegans can inform our understanding of glial operating principles in other species. Here, we summarize the current state-of-the-art, and describe mechanistic insights that have emerged from a concerted effort to understand C. elegans glia. The remarkable acceleration in the pace of discovery in recent years paints a portrait of striking molecular complexity, exquisite specificity, and functional heterogeneity among glia. Glial cells affect nearly every aspect of nervous system development and function, from generating neurons, to promoting neurite formation, to animal behavior, and to whole-animal traits, including longevity. We discuss emerging questions where C. elegans is poised to fill critical knowledge gaps in our understanding of glia biology.

Cellular and Molecular Mechanisms Underlying Synaptic Subcellular Specificity

Chemical synapses are essential for neuronal information storage and relay. The synaptic signal received or sent from spatially distinct subcellular compartments often generates different outcomes due to the distance or physical property difference. Therefore, the final output of postsynaptic neurons is determined not only by the type and intensity of synaptic inputs but also by the synaptic subcellular location. How synaptic subcellular specificity is determined has long been the focus of study in the neurodevelopment field. Genetic studies from invertebrates such as Caenorhabditis elegans (C. elegans) have uncovered important molecular and cellular mechanisms required for subcellular specificity. Interestingly, similar molecular mechanisms were found in the mammalian cerebellum, hippocampus, and cerebral cortex. This review summarizes the comprehensive advances in the cellular and molecular mechanisms underlying synaptic subcellular specificity, focusing on studies from C. elegans and rodents.

OGT-1 regulates synaptic assembly through the insulin signaling pathway

The formation and maintenance of synapses are precisely regulated, and the misregulation often leads to neurodevelopmental or neurodegenerative disorders. Besides intrinsic genetically encoded signaling pathways, synaptic structure and function are also regulated by extrinsic factors, such as nutrients. O-GlcNAc transferase (OGT), a nutrient sensor, is abundant in the nervous system and required for synaptic plasticity, learning, and memory. However, whether OGT is involved in synaptic development and the mechanism underlying the process are largely unknown. In this study, we found that OGT-1, the OGT homolog in C. elegans, regulates the presynaptic assembly in AIY interneurons. The insulin receptor DAF-2 acts upstream of OGT-1 to promote the presynaptic assembly by positively regulating the expression of ogt-1. This insulin-OGT-1 axis functions most likely by regulating neuronal activity. In this study, we elucidated a novel mechanism for synaptic development, and provided a potential link between synaptic development and insulin-related neurological disorders.

Loss of the Extracellular Matrix Protein DIG-1 Causes Glial Fragmentation, Dendrite Breakage, and Dendrite Extension Defects

The extracellular matrix (ECM) guides and constrains the shape of the nervous system. In C. elegans, DIG-1 is a giant ECM component that is required for fasciculation of sensory dendrites during development and for maintenance of axon positions throughout life. We identified four novel alleles of dig-1 in three independent screens for mutants affecting disparate aspects of neuronal and glial morphogenesis. First, we find that disruption of DIG-1 causes fragmentation of the amphid sheath glial cell in larvae and young adults. Second, it causes severing of the BAG sensory dendrite from its terminus at the nose tip, apparently due to breakage of the dendrite as animals reach adulthood. Third, it causes embryonic defects in dendrite fasciculation in inner labial (IL2) sensory neurons, as previously reported, as well as rare defects in IL2 dendrite extension that are enhanced by loss of the apical ECM component DYF-7, suggesting that apical and basolateral ECM contribute separately to dendrite extension. Our results highlight novel roles for DIG-1 in maintaining the cellular integrity of neurons and glia, possibly by creating a barrier between structures in the nervous system.

Glia-derived temporal signals orchestrate neurogenesis in the Drosophila mushroom body

Intrinsic mechanisms such as temporal series of transcription factors orchestrate neurogenesis from a limited number of neural progenitors in the brain. Extrinsic regulations, however, remain largely unexplored. Here we describe a two-step glia-derived signal that regulates neurogenesis in the Drosophila mushroom body (MB). In a temporal manner, glial-specific ubiquitin ligase dSmurf activates non-cell-autonomous Hedgehog signaling propagation by targeting the receptor Patched to suppress and promote the exit of MB neuroblast (NB) proliferation, thereby specifying the correct α/β cell number without affecting differentiation. Independent of NB proliferation, dSmurf also stabilizes the expression of the cell-adhesion molecule Fasciclin II (FasII) via its WW domains and regulates FasII homophilic interaction between glia and MB axons to refine α/β-lobe integrity. Our findings provide insights into how extrinsic glia-to-neuron communication coordinates with NB proliferation capacity to regulate MB neurogenesis; glial proteostasis is likely a generalized mechanism in orchestrating neurogenesis.

Establishment and Maintenance of Neural Circuit Architecture

The ability to sense the world, process information, and navigate the environment depends on the assembly and continuous function of neural circuits in the brain. Within the past two decades, new technologies have rapidly advanced our understanding of how neural circuits are wired during development and how they are stably maintained, often for years. Electron microscopy reconstructions of model organism connectomes have provided a map of the stereotyped (and variable) connections in the brain; advanced light microscopy techniques have enabled direct observation of the cellular dynamics that underlie circuit construction and maintenance; transcriptomic and proteomic surveys of both developing and mature neurons have provided insights into the molecular and genetic programs governing circuit establishment and maintenance; and advanced genetic techniques have allowed for high-throughput discovery of wiring regulators. These tools have empowered scientists to rapidly generate and test hypotheses about how circuits establish and maintain connectivity. Thus, the set of principles governing circuit formation and maintenance have been expanded. These principles are discussed in this review.

Temperature regulates synaptic subcellular specificity mediated by inhibitory glutamate signaling

Environmental factors such as temperature affect neuronal activity and development. However, it remains unknown whether and how they affect synaptic subcellular specificity. Here, using the nematode Caenorhabditis elegans AIY interneurons as a model, we found that high cultivation temperature robustly induces defects in synaptic subcellular specificity through glutamatergic neurotransmission. Furthermore, we determined that the functional glutamate is mainly released by the ASH sensory neurons and sensed by two conserved inhibitory glutamate-gated chloride channels GLC-3 and GLC-4 in AIY. Our work not only presents a novel neurotransmission-dependent mechanism underlying the synaptic subcellular specificity, but also provides a potential mechanistic insight into high-temperature-induced neurological defects.