Glia are essential for sensory organ function in C. elegans

Molecular profiling of invertebrate glia

Caenorhabditis elegans and Drosophila melanogaster are powerful experimental models for uncovering fundamental tenets of nervous system organization and function. Findings over the last two decades show that molecular and cellular features are broadly conserved between invertebrates and vertebrates, indicating that insights derived from invertebrate models can broadly inform our understanding of glial operating principles across diverse species. In recent years, these model systems have led to exciting discoveries in glial biology and mechanisms of glia-neuron interactions. Here, we summarize studies that have applied current state-of-the-art "-omics" techniques to C. elegans and D. melanogaster glia. Coupled with the remarkable acceleration in the pace of mechanistic studies of glia biology in recent years, these indicate that invertebrate glia also exhibit striking molecular complexity, specificity, and heterogeneity. We provide an overview of these studies and discuss their implications as well as emerging questions where C. elegans and D. melanogaster are well-poised to fill critical knowledge gaps in our understanding of glial biology.

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.

The AFD-expressed SRTX-1 GPCR does not contribute to AFD thermosensory functions

Temperature experience-regulated gene expression changes have been shown to underlie long-term adaptation of the response threshold of the AFD thermosensory neuron pair, and contribute to thermotaxis behavioral plasticity in C. elegans . We previously showed that the SRTX-1 GPCR is expressed primarily in AFD and is localized to their sensory endings. Here we find that SRTX-1 levels are regulated by the animal's temperature experience. However, loss or overexpression of srtx-1 does not affect thermotaxis behaviors or examined temperature-evoked calcium responses in AFD. Our observations suggest that SRTX-1 may modulate AFD responses and behavior under defined temperature conditions, or in response to specific environmental stimuli.

Neurogenesis in Caenorhabditis elegans

Animals rely on their nervous systems to process sensory inputs, integrate these with internal signals, and produce behavioral outputs. This is enabled by the highly specialized morphologies and functions of neurons. Neuronal cells share multiple structural and physiological features, but they also come in a large diversity of types or classes that give the nervous system its broad range of functions and plasticity. This diversity, first recognized over a century ago, spurred classification efforts based on morphology, function, and molecular criteria. Caenorhabditis elegans, with its precisely mapped nervous system at the anatomical level, an extensive molecular description of most of its neurons, and its genetic amenability, has been a prime model for understanding how neurons develop and diversify at a mechanistic level. Here, we review the gene regulatory mechanisms driving neurogenesis and the diversification of neuron classes and subclasses in C. elegans. We discuss our current understanding of the specification of neuronal progenitors and their differentiation in terms of the transcription factors involved and ensuing changes in gene expression and chromatin landscape. The central theme that has emerged is that the identity of a neuron is defined by modules of gene batteries that are under control of parallel yet interconnected regulatory mechanisms. We focus on how, to achieve these terminal identities, cells integrate information along their developmental lineages. Moreover, we discuss how neurons are diversified postembryonically in a time-, genetic sex-, and activity-dependent manner. Finally, we discuss how the understanding of neuronal development can provide insights into the evolution of neuronal diversity.

Specialized structure and function of the apical extracellular matrix at sense organs

Apical extracellular matrix (aECM) covers every surface of the body and exhibits tissue-specific structures that carry out specialized functions. This is particularly striking at sense organs, where aECM forms the interface between sensory neurons and the environment, and thus plays critical roles in how sensory stimuli are received. Here, we review the extraordinary adaptations of aECM across sense organs and discuss how differences in protein composition and matrix structure assist in sensing mechanical forces (tactile hairs, campaniform sensilla, and the tectorial membrane of the cochlea); tastes and smells (uniporous gustatory sensilla and multiporous olfactory sensilla in insects, and salivary and olfactory mucus in vertebrates); and light (cuticle-derived lenses in arthropods and mollusks). We summarize the power of using C. elegans, in which defined sense organs associate with distinct aECM, as a model for understanding the tissue-specific structural and functional specializations of aECM. Finally, we synthesize results from recent studies in C. elegans and Drosophila into a conceptual framework for aECM patterning, including mechanisms that involve transient cellular or matrix scaffolds, mechanical pulling or pushing forces, and localized secretion or endocytosis.

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.

Glial KCNQ K+ channels control neuronal output by regulating GABA release from glia in C. elegans

KCNQs are voltage-gated K+ channels that control neuronal excitability and are mutated in epilepsy and autism spectrum disorder (ASD). KCNQs have been extensively studied in neurons, but their function in glia is unknown. Using voltage, calcium, and GABA imaging, optogenetics, and behavioral assays, we show here for the first time in Caenorhabditis elegans (C. elegans) that glial KCNQ channels control neuronal excitability by mediating GABA release from glia via regulation of the function of L-type voltage-gated Ca2+ channels. Further, we show that human KCNQ channels have the same role when expressed in nematode glia, underscoring conservation of function across species. Finally, we show that pathogenic loss-of-function and gain-of-function human KCNQ2 mutations alter glia-to-neuron GABA signaling in distinct ways and that the KCNQ channel opener retigabine exerts rescuing effects. This work identifies glial KCNQ channels as key regulators of neuronal excitability via control of GABA release from glia.

Dendrite morphogenesis in Caenorhabditis elegans

Since the days of Ramón y Cajal, the vast diversity of neuronal and particularly dendrite morphology has been used to catalog neurons into different classes. Dendrite morphology varies greatly and reflects the different functions performed by different types of neurons. Significant progress has been made in our understanding of how dendrites form and the molecular factors and forces that shape these often elaborately sculpted structures. Here, we review work in the nematode Caenorhabditis elegans that has shed light on the developmental mechanisms that mediate dendrite morphogenesis with a focus on studies investigating ciliated sensory neurons and the highly elaborated dendritic trees of somatosensory neurons. These studies, which combine time-lapse imaging, genetics, and biochemistry, reveal an intricate network of factors that function both intrinsically in dendrites and extrinsically from surrounding tissues. Therefore, dendrite morphogenesis is the result of multiple tissue interactions, which ultimately determine the shape of dendritic arbors.

Zebrafish and nematodes as whole organism models to measure developmental neurotoxicity

Despite the growing epidemiological evidence of an association between toxin exposure and developmental neurotoxicity (DNT), systematic testing of DNT is not mandatory in international regulations for admission of pharmaceuticals or industrial chemicals. However, to date around 200 compounds, ranging from pesticides, pharmaceuticals and industrial chemicals, have been tested for DNT in the current OECD test guidelines (TG-443 or TG-426). There are calls for the development of new approach methodologies (NAMs) for DNT, which has resulted in a DNT testing battery using in vitro human cell-based assays. These assays provide a means to elucidate the molecular mechanisms of toxicity in humans which is lacking in animal-based toxicity tests. However, cell-based assays do not represent all steps of the complex process leading to DNT. Validated models with a multi-organ network of pathways that interact at the molecular, cellular and tissue level at very specific timepoints in a life cycle are currently missing. Consequently, whole model organisms are being developed to screen for, and causally link, new molecular targets of DNT compounds and how they affect whole brain development and neurobehavioral endpoints. Given the practical and ethical restraints associated with vertebrate testing, lower animal models that qualify as 3 R (reduce, refine and replace) models, including the nematode (Caenorhabditis elegans) and the zebrafish (Danio rerio) will prove particularly valuable for unravelling toxicity pathways leading to DNT. Although not as complex as the human brain, these 3 R-models develop a complete functioning brain with numerous neurodevelopmental processes overlapping with human brain development. Importantly, the main signalling pathways relating to (neuro)development, metabolism and growth are highly conserved in these models. We propose the use of whole model organisms specifically zebrafish and C. elegans for DNT relevant endpoints.

Phasic/tonic glial GABA differentially transduce for olfactory adaptation and neuronal aging

Phasic (fast) and tonic (sustained) inhibition of γ-aminobutyric acid (GABA) are fundamental for regulating day-to-day activities, neuronal excitability, and plasticity. However, the mechanisms and physiological functions of glial GABA transductions remain poorly understood. Here, we report that the AMsh glia in Caenorhabditis elegans exhibit both phasic and tonic GABAergic signaling, which distinctively regulate olfactory adaptation and neuronal aging. Through genetic screening, we find that GABA permeates through bestrophin-9/-13/-14 anion channels from AMsh glia, which primarily activate the metabolic GABAB receptor GBB-1 in the neighboring ASH sensory neurons. This tonic action of glial GABA regulates the age-associated changes of ASH neurons and olfactory responses via a conserved signaling pathway, inducing neuroprotection. In addition, the calcium-evoked, vesicular glial GABA release acts upon the ionotropic GABAA receptor LGC-38 in ASH neurons to regulate olfactory adaptation. These findings underscore the fundamental significance of glial GABA in maintaining healthy aging and neuronal stability.

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.

LET-381/FoxF and its target UNC-30/Pitx2 specify and maintain the molecular identity of C. elegans mesodermal glia that regulate motor behavior

While most glial cell types in the central nervous system (CNS) arise from neuroectodermal progenitors, some, like microglia, are mesodermally derived. To understand mesodermal glia development and function, we investigated C. elegans GLR glia, which envelop the brain neuropil and separate it from the circulatory system cavity. Transcriptome analysis shows that GLR glia combine astrocytic and endothelial characteristics, which are relegated to separate cell types in vertebrates. Combined fate acquisition is orchestrated by LET-381/FoxF, a fate-specification/maintenance transcription factor also expressed in glia and endothelia of other animals. Among LET-381/FoxF targets, the UNC-30/Pitx2 transcription factor controls GLR glia morphology and represses alternative mesodermal fates. LET-381 and UNC-30 co-expression in naive cells is sufficient for GLR glia gene expression. GLR glia inactivation by ablation or let-381 mutation disrupts locomotory behavior and promotes salt-induced paralysis, suggesting brain-neuropil activity dysregulation. Our studies uncover mechanisms of mesodermal glia development and show that like neuronal differentiation, glia differentiation requires autoregulatory terminal selector genes that define and maintain the glial fate.

Cell non-autonomous signaling through the conserved C. elegans glycopeptide hormone receptor FSHR-1 regulates cholinergic neurotransmission

Modulation of neurotransmission is key for organismal responses to varying physiological contexts such as during infection, injury, or other stresses, as well as in learning and memory and for sensory adaptation. Roles for cell autonomous neuromodulatory mechanisms in these processes have been well described. The importance of cell non-autonomous pathways for inter-tissue signaling, such as gut-to-brain or glia-to-neuron, has emerged more recently, but the cellular mechanisms mediating such regulation remain comparatively unexplored. Glycoproteins and their G protein-coupled receptors (GPCRs) are well-established orchestrators of multi-tissue signaling events that govern diverse physiological processes through both cell-autonomous and cell non-autonomous regulation. Here, we show that follicle stimulating hormone receptor, FSHR-1, the sole Caenorhabditis elegans ortholog of mammalian glycoprotein hormone GPCRs, is important for cell non-autonomous modulation of synaptic transmission. Inhibition of fshr-1 expression reduces muscle contraction and leads to synaptic vesicle accumulation in cholinergic motor neurons. The neuromuscular and locomotor defects in fshr-1 loss-of-function mutants are associated with an underlying accumulation of synaptic vesicles, build-up of the synaptic vesicle priming factor UNC-10/RIM, and decreased synaptic vesicle release from cholinergic motor neurons. Restoration of FSHR-1 to the intestine is sufficient to restore neuromuscular activity and synaptic vesicle localization to fshr-1- deficient animals. Intestine-specific knockdown of FSHR-1 reduces neuromuscular function, indicating FSHR-1 is both necessary and sufficient in the intestine for its neuromuscular effects. Re-expression of FSHR-1 in other sites of endogenous expression, including glial cells and neurons, also restored some neuromuscular deficits, indicating potential cross-tissue regulation from these tissues as well. Genetic interaction studies provide evidence that downstream effectors gsa-1 / Gα S , acy-1 /adenylyl cyclase and sphk-1/ sphingosine kinase and glycoprotein hormone subunit orthologs, GPLA-1/GPA2 and GPLB-1/GPB5, are important for FSHR-1 modulation of the NMJ. Together, our results demonstrate that FSHR-1 modulation directs inter-tissue signaling systems, which promote synaptic vesicle release at neuromuscular synapses.

Heat Shock Proteins Function as Signaling Molecules to Mediate Neuron-Glia Communication During Aging

The nervous system is primarily composed of neurons and glia, and the communication between them plays profound roles in regulating the development and function of the brain. Neuron-glia signal transduction is known to be mediated by secreted or juxtacrine signals through ligand-receptor interactions on the cell membrane. Here, we report a novel mechanism for neuron-glia signal transduction, wherein neurons transmit proteins to glia through extracellular vesicles, activating glial signaling pathways. We find that in the amphid sensory organ of Caenorhabditis elegans, different sensory neurons exhibit varying aging rates. This discrepancy in aging is governed by the crosstalk between neurons and glia. We demonstrate that early-aged neurons can transmit heat shock proteins (HSP) to glia via extracellular vesicles. These neuronal HSPs activate the IRE1-XBP1 pathway, further increasing their expression in glia, forming a positive feedback loop. Ultimately, the activation of the IRE1-XBP-1 pathway leads to the transcriptional regulation of chondroitin synthases to protect glia-embedded neurons from aging-associated functional decline. Therefore, our studies unveil a novel mechanism for neuron-glia communication in the nervous system and provide new insights into our understanding of brain aging.

Evolution of Astrocyte-Neuron Interactions Across Species

Proper functioning of the central nervous system depends on various tightly regulated phenomena, among which astrocyte-neuron interactions are of critical importance. Various studies across the species have highlighted the diverse yet crucial roles of astrocytes in regulating the nervous system development and functions. In simpler organisms like worms or insects, astrocyte-like cells govern basic functions such as structural support to neurons or regulation of extracellular ions. As the species complexity increases, so does the functional and morphological complexity of astrocytes. For example, in fish and amphibians, these cells are involved in synaptic development and ion homeostasis, while in reptiles and birds, astrocytes regulate synaptic transmission and plasticity and are reported to be involved in complex behaviors. Other species like those belonging to mammals and, in particular, primates have a heterogeneous population of astrocytes, exhibiting region-specific functional properties. In primates, these cells are responsible for proper synaptic transmission, neurotransmitter release and metabolism, and higher cognitive functions like learning, memory, or information processing. This chapter highlights the well-established and somewhat conserved roles of astrocytes and astrocyte-neuron interactions across the evolution of both invertebrates and vertebrates.

Glia in Invertebrate Models: Insights from Caenorhabditis elegans

Glial cells modulate brain development, function, and health across all bilaterian animals, and studies in the past two decades have made rapid strides to uncover the underlying molecular mechanisms of glial functions. The nervous system of the invertebrate genetic model Caenorhabditis elegans (C. elegans) has small cell numbers with invariant lineages, mapped connectome, easy genetic manipulation, and a short lifespan, and the animal is also optically transparent. These characteristics are revealing C. elegans to be a powerful experimental platform for studying glial biology. This chapter discusses studies in C. elegans that add to our understanding of how glia modulate adult neural functions, and thereby animal behaviors, as well as emerging evidence of their roles as autonomous sensory cells. The rapid molecular and cellular advancements in understanding C. elegans glia in recent years underscore the utility of this model in studies of glial biology. We conclude with a perspective on future research avenues for C. elegans glia that may readily contribute molecular mechanistic insights into glial functions in the nervous system.

Glia detect and mount a protective response to loss of dendrite substructure integrity in C. elegans

Neurons have elaborate structures that determine their connectivity and functions. Changes in neuronal structure accompany learning and memory formation and are hallmarks of neurological disease. Here we show that glia monitor dendrite structure and respond to dendrite perturbation. In C. elegans mutants with defective sensory-organ dendrite cilia, adjacent glia accumulate extracellular matrix-laden vesicles, secrete excess matrix around cilia, alter gene expression, and change their secreted protein repertoire. Inducible cilia disruption reveals that this response is acute. DGS-1, a 7-transmembrane domain neuronal protein, and FIG-1, a multifunctional thrombospondin-domain glial protein, are required for glial detection of cilia integrity, and exhibit mutually-dependent localization to and around cilia, respectively. While inhibiting glial secretion disrupts dendritic cilia properties, hyperactivating the glial response protects against dendrite damage. Our studies uncover a homeostatic protective dendrite-glia interaction and suggest that similar signaling occurs at other sensory structures and at synapses, which resemble sensory organs in architecture and molecules.

LET-381/FoxF and UNC-30/Pitx2 control the development of C. elegans mesodermal glia that regulate motor behavior

While most CNS glia arise from neuroectodermal progenitors, some, like microglia, are mesodermally derived. To understand mesodermal glia development and function, we investigated C. elegans GLR glia, which ensheath the brain neuropil and separate it from the circulatory-system cavity. Transcriptome analysis suggests GLR glia merge astrocytic and endothelial characteristics relegated to separate cell types in vertebrates. Combined fate acquisition is orchestrated by LET-381/FoxF, a fate-specification/maintenance transcription factor expressed in glia and endothelia of other animals. Among LET-381/FoxF targets, UNC-30/Pitx2 transcription factor controls GLR glia morphology and represses alternative mesodermal fates. LET-381 and UNC-30 co-expression in naïve cells is sufficient for GLR glia gene expression. GLR glia inactivation by ablation or let-381 mutation disrupts locomotory behavior and induces salt hypersensitivity, suggesting brain-neuropil activity dysregulation. Our studies uncover mechanisms of mesodermal glia development and show that like neurons, glia differentiation requires autoregulatory terminal selector genes that define and maintain the glial fate.

Molecular heterogeneity of C. elegans glia across sexes

A comprehensive description of nervous system function, and sex dimorphism within, is incomplete without clear assessment of the diversity of its component cell types, neurons and glia. C. elegans has an invariant nervous system with the first mapped connectome of a multicellular organism and single-cell atlas of component neurons. Here we present single nuclear RNA-seq evaluation of glia across the entire adult C. elegans nervous system, including both sexes. Machine learning models enabled us to identify both sex-shared and sex-specific glia and glial subclasses. We have identified and validated molecular markers in silico and in vivo for these molecular subcategories. Comparative analytics also reveals previously unappreciated molecular heterogeneity in anatomically identical glia between and within sexes, indicating consequent functional heterogeneity. Furthermore, our datasets reveal that while adult C. elegans glia express neuropeptide genes, they lack the canonical unc-31/CAPS-dependent dense core vesicle release machinery. Thus, glia employ alternate neuromodulator processing mechanisms. Overall, this molecular atlas, available at www.wormglia.org, reveals rich insights into heterogeneity and sex dimorphism in glia across the entire nervous system of an adult animal.

Glial regulators of ions and solutes required for specific chemosensory functions in Caenorhabditis elegans

Glia and accessory cells regulate the microenvironment around neurons and primary sensory cells. However, the impact of specific glial regulators of ions and solutes on functionally diverse primary cells is poorly understood. Here, we systemically investigate the requirement of ion channels and transporters enriched in Caenorhabditis elegans Amsh glia for the function of chemosensory neurons. Although Amsh glia ablated worms show reduced function of ASH, AWC, AWA, and ASE neurons, we show that the loss of glial enriched ion channels and transporters impacts these neurons differently, with nociceptor ASH being the most affected. Furthermore, our analysis underscores the importance of K+, Cl-, and nucleoside homeostasis in the Amphid sensory organ and uncovers the contribution of glial genes implicated in neurological disorders. Our findings build a unique fingerprint of each glial enriched ion channel and transporter and may provide insights into the function of supporting cells of mammalian sensory organs.

Neuron-glia interaction at the receptor level affects olfactory perception in adult Drosophila

Some types of glia play an active role in neuronal signaling by modifying their activity although little is known about their role in sensory information signaling at the receptor level. In this research, we report a functional role for the glia that surround the soma of the olfactory receptor neurons (OSNs) in adult Drosophila. Specific genetic modifications have been targeted to this cell type to obtain live individuals who are tested for olfactory preference and display changes both increasing and reducing sensitivity. A closer look at the antenna by Ca²⁺ imaging shows that odor activates the OSNs, which subsequently produce an opposite and smaller effect in the glia that partially counterbalances neuronal activation. Therefore, these glia may play a dual role in preventing excessive activation of the OSNs at high odorant concentrations and tuning the chemosensory window for the individual according to the network structure in the receptor organ.

cGMP dynamics that underlies thermosensation in temperature-sensing neuron regulates thermotaxis behavior in C. elegans

Living organisms including bacteria, plants and animals sense ambient temperature so that they can avoid noxious temperature or adapt to new environmental temperature. A nematode C. elegans can sense innocuous temperature, and navigate themselves towards memorize past cultivation temperature (Tc) of their preference. For this thermotaxis, AFD thermosensory neuron is pivotal, which stereotypically responds to warming by increasing intracellular Ca2+ level in a manner dependent on the remembered past Tc. We aimed to reveal how AFD encodes the information of temperature into neural activities. cGMP synthesis in AFD is crucial for thermosensation in AFD and thermotaxis behavior. Here we characterized the dynamic change of cGMP level in AFD by imaging animals expressing a fluorescence resonance energy transfer (FRET)-based cGMP probe specifically in AFD and found that cGMP dynamically responded to both warming and cooling in a manner dependent on past Tc. Moreover, we characterized mutant animals that lack guanylyl cyclases (GCYs) or phosphodiesterases (PDEs), which synthesize and hydrolyze cGMP, respectively, and uncovered how GCYs and PDEs contribute to cGMP and Ca2+ dynamics in AFD and to thermotaxis behavior.

Evolution of neuroglia

The evolution of the nervous system progressed through cellular diversification and specialization of functions. Conceptually, the nervous system is composed of electrically excitable neuronal networks connected by chemical synapses and nonexcitable glial cells that provide for homeostasis and defense. The evolution of neuroglia began with the emergence of the centralized nervous system and proceeded through a continuous increase in their complexity. In the primate brain, especially in the brain of humans, the astrocyte lineage is exceedingly complex, with the emergence of new types of astroglial cells possibly involved in interlayer communication and integration.

Evolution of astrocytes: From invertebrates to vertebrates

The central nervous system (CNS) shows incredible diversity across evolution at the anatomical, cellular, molecular, and functional levels. Over the past decades, neuronal cell number and heterogeneity, together with differences in the number and types of neuro-active substances, axonal conduction, velocity, and modes of synaptic transmission, have been rigorously investigated in comparative neuroscience studies. However, astrocytes, a specific type of glial cell in the CNS, play pivotal roles in regulating these features and thus are crucial for the brain's development and evolution. While special attention has been paid to mammalian astrocytes, we still do not have a clear definition of what an astrocyte is from a broader evolutionary perspective, and there are very few studies on astroglia-like structures across all vertebrates. Here, I elucidate what we know thus far about astrocytes and astrocyte-like cells across vertebrates. This information expands our understanding of how astrocytes evolved to become more complex and extremely specialized cells in mammals and how they are relevant to the structure and function of the vertebrate brain.

The Voltage-Gated Calcium Channel EGL-19 Acts on Glia to Drive Olfactory Adaptation

Calcium channelopathies have been strongly linked to cardiovascular, muscular, neurological and psychiatric disorders. The voltage-gated calcium channels (VGCC) are vital transducers of membrane potential changes to facilitate the dynamics of calcium ions and release of neurotransmitter. Whether these channels function in the glial cell to mediate calcium variations and regulate behavioral outputs, is poorly understood. Our results showed that odorant and mechanical stimuli evoked robust calcium increases in the amphid sheath (AMsh) glia from C. elegans, which were largely dependent on the L-Type VGCC EGL-19. Moreover, EGL-19 modulates the morphologies of both ASH sensory neurons and AMsh glia. Tissue-specific knock-down of EGL-19 in AMsh glia regulated sensory adaptability of ASH neurons and promoted olfactory adaptation. Our results reveal a novel role of glial L-Type VGCC EGL-19 on olfaction, lead to improved understanding of the functions of VGCCs in sensory transduction.

Structural diversity in a stereotypic organelle - Sensory cilia of Caenorhabditis elegans

Sensory cilia, an ancient organelle, displays a high degree of conservation in its structure and functioning. Sensory cilia also fulfill a wide range of sensory functions, from sensing environmental signals (light, sound, chemicals, and mechanical forces) to interpreting intercellular developmental signals. One way they appear to fulfill these diverse and specialized roles is by adopting a variety of shapes and sizes. We are only beginning to document and appreciate this complexity. Here in this review, using the varied and specialized cilia found on Caenorhabditis elegans sensory neurons, I highlight some of the most obvious examples of this structural diversity and the underlying mechanisms if known. Such structural diversity appears to arise from the modulation of deeply conserved molecular pathways and also from cell- and species-specific mechanisms. Studying these ciliary specializations will thus provide for a comprehensive understanding of ciliary biology and might uncover understudied aspects of ciliary disease biology.

Invited review: Unearthing the mechanisms of age-related neurodegenerative disease using Caenorhabditis elegans

As human life expectancy increases, neurodegenerative diseases present a growing public health threat, for which there are currently few effective treatments. There is an urgent need to understand the molecular and genetic underpinnings of these disorders so new therapeutic targets can be identified. Here we present the argument that the simple nematode worm Caenorhabditis elegans is a powerful tool to rapidly study neurodegenerative disorders due to their short lifespan and vast array of genetic tools, which can be combined with characterization of conserved neuronal processes and behavior orthologous to those disrupted in human disease. We review how pre-existing C. elegans models provide insight into human neurological disease as well as an overview of current tools available to study neurodegenerative diseases in the worm, with an emphasis on genetics and behavior. We also discuss open questions that C. elegans may be particularly well suited for in future studies and how worms will be a valuable preclinical model to better understand these devastating neurological disorders.

Open Frontiers in Neural Cell Type Investigations; Lessons From Caenorhabditis elegans and Beyond, Toward a Multimodal Integration

Nervous system cells, the building blocks of circuits, have been studied with ever-progressing resolution, yet neural circuits appear still resistant to schemes of reductionist classification. Due to their sheer numbers, complexity and diversity, their systematic study requires concrete classifications that can serve reduced dimensionality, reproducibility, and information integration. Conventional hierarchical schemes transformed through the history of neuroscience by prioritizing criteria of morphology, (electro)physiological activity, molecular content, and circuit function, influenced by prevailing methodologies of the time. Since the molecular biology revolution and the recent advents in transcriptomics, molecular profiling gains ground toward the classification of neurons and glial cell types. Yet, transcriptomics entails technical challenges and more importantly uncovers unforeseen spatiotemporal heterogeneity, in complex and simpler nervous systems. Cells change states dynamically in space and time, in response to stimuli or throughout their developmental trajectory. Mapping cell type and state heterogeneity uncovers uncharted terrains in neurons and especially in glial cell biology, that remains understudied in many aspects. Examining neurons and glial cells from the perspectives of molecular neuroscience, physiology, development and evolution highlights the advantage of multifaceted classification schemes. Among the amalgam of models contributing to neuroscience research, Caenorhabditis elegans combines nervous system anatomy, lineage, connectivity and molecular content, all mapped at single-cell resolution, and can provide valuable insights for the workflow and challenges of the multimodal integration of cell type features. This review reflects on concepts and practices of neuron and glial cells classification and how research, in C. elegans and beyond, guides nervous system experimentation through integrated multidimensional schemes. It highlights underlying principles, emerging themes, and open frontiers in the study of nervous system development, regulatory logic and evolution. It proposes unified platforms to allow integrated annotation of large-scale datasets, gene-function studies, published or unpublished findings and community feedback. Neuroscience is moving fast toward interdisciplinary, high-throughput approaches for combined mapping of the morphology, physiology, connectivity, molecular function, and the integration of information in multifaceted schemes. A closer look in mapped neural circuits and understudied terrains offers insights for the best implementation of these approaches.

Impairing one sensory modality enhances another by reconfiguring peptidergic signalling in Caenorhabditis elegans

Animals that lose one sensory modality often show augmented responses to other sensory inputs. The mechanisms underpinning this cross-modal plasticity are poorly understood. We probe such mechanisms by performing a forward genetic screen for mutants with enhanced O₂ perception in Caenorhabditis elegans. Multiple mutants exhibiting increased O₂ responsiveness concomitantly show defects in other sensory responses. One mutant, qui-1, defective in a conserved NACHT/WD40 protein, abolishes pheromone-evoked Ca²⁺ responses in the ADL pheromone-sensing neurons. At the same time, ADL responsiveness to pre-synaptic input from O₂-sensing neurons is heightened in qui-1, and other sensory defective mutants, resulting in enhanced neurosecretion although not increased Ca²⁺ responses. Expressing qui-1 selectively in ADL rescues both the qui-1 ADL neurosecretory phenotype and enhanced escape from 21% O₂. Profiling ADL neurons in qui-1 mutants highlights extensive changes in gene expression, notably of many neuropeptide receptors. We show that elevated ADL expression of the conserved neuropeptide receptor NPR-22 is necessary for enhanced ADL neurosecretion in qui-1 mutants, and is sufficient to confer increased ADL neurosecretion in control animals. Sensory loss can thus confer cross-modal plasticity by changing the peptidergic connectome.

A glial ClC Cl- channel mediates nose touch responses in C. elegans

In touch receptors, glia and accessory cells play a key role in mechanosensation. However, the mechanisms underlying such regulation are poorly understood. We show, for the first time, that the chloride channel CLH-1 is needed in glia of C. elegans nose touch receptors for touch responses and for regulation of excitability. Using in vivo Ca²⁺ and Cl^- imaging, behavioral assays, and combined genetic and pharmacological manipulations, we show that CLH-1 mediates Cl^- flux needed for glial GABA inhibition of ASH sensory neuron function and for regulation of cyclic AMP levels in ASH neurons. Finally, we show that the rat ClC-2 channel rescues the clh-1 nose-touch-insensitive phenotype, underscoring conservation of function across species. Our work identifies a glial Cl^- channel as a novel regulator of touch sensitivity. We propose that glial CLH-1 regulates the interplay between Ca²⁺ and cAMP signaling in ASH neurons to control the sensitivity of the worm's nose touch receptors.

The ClC Cl- channel CLH-1 mediates HCO3 - efflux from the amphid sheath glia in C. elegans

Cellular function is regulated by the concentration of intracellular and extracellular ions, including pH. Ion channels and transporters that mediate the flux/transport of protons and bicarbonate (HCO3 -) are the chief regulators of pH. In the nervous system, due to their high electrical activity, neurons tend to produce and excrete large amounts of acids. On the contrary, glial cells have been proposed to be key contributors of pH buffering. We published that the Cl-/HCO3 - permeable channel CLH-1 mediates intracellular pH buffering of C. elegans Amphid sheath (AMsh) glia at baseline. We show here that, under physiological conditions, clh-1 knock out worms show reduced HCO3 - extrusion from AMsh glia, suggesting that CLH-1 may help prevent cellular alkalinization. This function becomes even more apparent when animals are grown on plates enriched with HCO3 -. We speculate that CLH-1 might function as a regulator of extracellular pH.

Nuclear hormone receptors promote gut and glia detoxifying enzyme induction and protect C. elegans from the mold P. brevicompactum

Animals encounter microorganisms in their habitats, adapting physiology and behavior accordingly. The nematode Caenorhabditis elegans is found in microbe-rich environments; however, its responses to fungi are not extensively studied. Here, we describe interactions of C. elegans and Penicillium brevicompactum, an ecologically relevant mold. Transcriptome studies reveal that co-culture upregulates stress response genes, including xenobiotic-metabolizing enzymes (XMEs), in C. elegans intestine and AMsh glial cells. The nuclear hormone receptors (NHRs) NHR-45 and NHR-156 are induction regulators, and mutants that cannot induce XMEs in the intestine when exposed to P. brevicompactum experience mitochondrial stress and exhibit developmental defects. Different C. elegans wild isolates harbor sequence polymorphisms in nhr-156, resulting in phenotypic diversity in AMsh glia responses to microbe exposure. We propose that P. brevicompactum mitochondria-targeting mycotoxins are deactivated by intestinal detoxification, allowing tolerance to moldy environments. Our studies support the idea that C. elegans NHRs may be regulated by environmental cues.

Lineage-specific control of convergent differentiation by a Forkhead repressor

During convergent differentiation, multiple developmental lineages produce a highly similar or identical cell type. However, few molecular players that drive convergent differentiation are known. Here, we show that the C. elegans Forkhead transcription factor UNC-130 is required in only one of three convergent lineages that produce the same glial cell type. UNC-130 acts transiently as a repressor in progenitors and newly-born terminal cells to allow the proper specification of cells related by lineage rather than by cell type or function. Specification defects correlate with UNC-130:DNA binding, and UNC-130 can be functionally replaced by its human homolog, the neural crest lineage determinant FoxD3. We propose that, in contrast to terminal selectors that activate cell type-specific transcriptional programs in terminally differentiating cells, UNC-130 acts early and specifically in one convergent lineage to produce a cell type that also arises from molecularly distinct progenitors in other lineages.

Ectocytosis prevents accumulation of ciliary cargo in C. elegans sensory neurons

Cilia are sensory organelles protruding from cell surfaces. Release of extracellular vesicles (EVs) from cilia was previously observed in mammals, Chlamydomonas, and in male Caenorhabditis elegans. Using the EV marker TSP-6 (an ortholog of mammalian CD9) and other ciliary receptors, we show that EVs are formed from ciliated sensory neurons in C. elegans hermaphrodites. Release of EVs is observed from two ciliary locations: the cilia tip and/or periciliary membrane compartment (PCMC). Outward budding of EVs from the cilia tip leads to their release into the environment. EVs' budding from the PCMC is concomitantly phagocytosed by the associated glial cells. To maintain cilia composition, a tight regulation of cargo import and removal is achieved by the action of intra-flagellar transport (IFT). Unbalanced IFT due to cargo overexpression or mutations in the IFT machinery leads to local accumulation of ciliary proteins. Disposal of excess ciliary proteins via EVs reduces their local accumulation and exports them to the environment and/or to the glia associated to these ciliated neurons. We suggest that EV budding from cilia subcompartments acts as a safeguard mechanism to remove deleterious excess of ciliary material.

Charging Up the Periphery: Glial Ionic Regulation in Sensory Perception

The peripheral nervous system (PNS) receives diverse sensory stimuli from the environment and transmits this information to the central nervous system (CNS) for subsequent processing. Thus, proper functions of cells in peripheral sense organs are a critical gate-keeper to generating appropriate animal sensory behaviors, and indeed their dysfunction tracks sensory deficits, sensorineural disorders, and aging. Like the CNS, the PNS comprises two major cell types, neurons (or sensory cells) and glia (or glia-like supporting neuroepithelial cells). One classic function of PNS glia is to modulate the ionic concentration around associated sensory cells. Here, we review current knowledge of how non-myelinating support cell glia of the PNS regulate the ionic milieu around sensory cell endings across species and systems. Molecular studies reviewed here suggest that, rather than being a passive homeostatic response, glial ionic regulation may in fact actively modulate sensory perception, implying that PNS glia may be active contributors to sensorineural information processing. This is reminiscent of emerging studies suggesting analogous roles for CNS glia in modulating neural circuit processing. We therefore suggest that deeper molecular mechanistic investigations into critical PNS glial functions like ionic regulation are essential to comprehensively understand sensorineural health, disease, and aging.

A perspective on C. elegans neurodevelopment: from early visionaries to a booming neuroscience research

The formation of the nervous system and its striking complexity is a remarkable feat of development. C. elegans served as a unique model to dissect the molecular events in neurodevelopment, from its early visionaries to the current booming neuroscience community. Soon after being introduced as a model, C. elegans was mapped at the level of genes, cells, and synapses, providing the first metazoan with a complete cell lineage, sequenced genome, and connectome. Here, I summarize mechanisms underlying C. elegans neurodevelopment, from the generation and diversification of neural components to their navigation and connectivity. I point out recent noteworthy findings in the fields of glia biology, sex dimorphism and plasticity in neurodevelopment, highlighting how current research connects back to the pioneering studies by Brenner, Sulston and colleagues. Multifaceted investigations in model organisms, connecting genes to cell function and behavior, expand our mechanistic understanding of neurodevelopment while allowing us to formulate emerging questions for future discoveries.

DYF-4 regulates patched-related/DAF-6-mediated sensory compartment formation in C. elegans

Coordination of neurite extension with surrounding glia development is critical for neuronal function, but the underlying molecular mechanisms remain poorly understood. Through a genome-wide mutagenesis screen in C. elegans, we identified dyf-4 and daf-6 as two mutants sharing similar defects in dendrite extension. DAF-6 encodes a glia-specific patched-related membrane protein that plays vital roles in glial morphogenesis. We cloned dyf-4 and found that DYF-4 encodes a glia-secreted protein. Further investigations revealed that DYF-4 interacts with DAF-6 and functions in a same pathway as DAF-6 to regulate sensory compartment formation. Furthermore, we demonstrated that reported glial suppressors of daf-6 could also restore dendrite elongation and ciliogenesis in both dyf-4 and daf-6 mutants. Collectively, our data reveal that DYF-4 is a regulator for DAF-6 which promotes the proper formation of the glial channel and indirectly affects neurite extension and ciliogenesis.

In C. elegans sensory organ, the ciliated neuronal endings are wrapped in a luminal channel formed by glial cells, forming a specialized sensory compartment critical for sensory activity. Coordination of glial channel formation, dendritic tip anchoring and ciliogenesis are critical for the formation of a functional sensory compartment. DAF-6, a patched-related glial membrane protein, was reported to play an important role in glial channel morphogenesis, but the molecular function and regulatory mechanism of DAF-6 remain poorly understood. Here, we found that DYF-4, a glia-secreted protein, interacts and colocalizes with DAF-6, and functions in a same pathway as DAF-6 to regulate sensory compartment formation. We propose that DYF-4 is a novel regulator for DAF-6 to control sensory compartment formation.

Inferences of glia-mediated control in Caenorhabditis elegans

Astrocytes modulate synaptic transmission; yet, it remains unclear how glia influence complex behaviors. Here, we explore the effects of Caenorhabditis elegans astrocyte-like cephalic glia (CEPglia ) and the glia-specific bHLH transcription factor HLH-17 on mating behavior and the defecation motor program (DMP). In C. elegans, male mating has been explicitly described through the male tail circuit and is characterized by coordination of multiple independent behaviors to ensure that copulation is achieved. Furthermore, the sex-specific male mating circuitry shares similar components with the DMP, which is complex and rhythmic, and requires a fixed sequence of behaviors to be activated periodically. We found that loss of CEPglia reduced persistence in executing mating behaviors and hindered copulation, while males that lacked HLH-17 demonstrated repetitive prodding behavior that increased the time spent in mating but did not hinder copulation. During the DMP, we found that posterior body wall contractions (pBocs) and enteric muscle contractions (EMCs) were differentially affected by loss of HLH-17 or CEPglia in males and hermaphrodites. pBocs and EMCs required HLH-17 activity in both sexes, whereas loss of CEPglia alone did not affect DMP in males. Our data suggest that CEPglia mediate complex behaviors by signaling to the GABAergic DVB neuron, and that HLH-17 activity influences those discrete steps within those behaviors. Collectively, these data provide evidence of glia as a link in cooperative regulation of complex and rhythmic behavior that, in C. elegans links circuitry in the head and the tail.

Vitamin B12 Regulates Glial Migration and Synapse Formation through Isoform-Specific Control of PTP-3/LAR PRTP Expression

Vitamin B12 is known to play critical roles during the development and aging of the brain, and vitamin B12 deficiency has been linked to neurodevelopmental and degenerative disorders. However, the underlying molecular mechanisms of how vitamin B12 affects the development and maintenance of the nervous system are still unclear. Here, we report that vitamin B12 can regulate glial migration and synapse formation through control of isoform-specific expression of PTP-3/LAR PRTP (leukocyte-common antigen-related receptor-type tyrosine-protein phosphatase). We found the uptake of diet-supplied vitamin B12 in the intestine to be critical for the expression of a long isoform of PTP-3 (PTP-3A) in neuronal and glial cells. The expression of PTP-3A cell autonomously regulates glial migration and synapse formation through interaction with an extracellular matrix protein NID-1/nidogen 1. Together, our findings demonstrate that isoform-specific regulation of PTP-3/ LAR PRTP expression is a key molecular mechanism that mediates vitamin-B12-dependent neuronal and glial development.

Glia actively sculpt sensory neurons by controlled phagocytosis to tune animal behavior

Glia in the central nervous system engulf neuron fragments to remodel synapses and recycle photoreceptor outer segments. Whether glia passively clear shed neuronal debris or actively prune neuron fragments is unknown. How pruning of single-neuron endings impacts animal behavior is also unclear. Here, we report our discovery of glia-directed neuron pruning in Caenorhabditis elegans. Adult C. elegans AMsh glia engulf sensory endings of the AFD thermosensory neuron by repurposing components of the conserved apoptotic corpse phagocytosis machinery. The phosphatidylserine (PS) flippase TAT-1/ATP8A functions with glial PS-receptor PSR-1/PSR and PAT-2/α-integrin to initiate engulfment. This activates glial CED-10/Rac1 GTPase through the ternary GEF complex of CED-2/CrkII, CED-5/DOCK180, CED-12/ELMO. Execution of phagocytosis uses the actin-remodeler WSP-1/nWASp. This process dynamically tracks AFD activity and is regulated by temperature, the AFD sensory input. Importantly, glial CED-10 levels regulate engulfment rates downstream of neuron activity, and engulfment-defective mutants exhibit altered AFD-ending shape and thermosensory behavior. Our findings reveal a molecular pathway underlying glia-dependent engulfment in a peripheral sense-organ and demonstrate that glia actively engulf neuron fragments, with profound consequences on neuron shape and animal sensory behavior.

Neurons are tree-shaped cells that receive information through endings connected to neighbouring cells or the environment. Controlling the size, number and location of these endings is necessary to ensure that circuits of neurons get precisely the right amount of input from their surroundings.

Glial cells form a large portion of the nervous system, and they are tasked with supporting, cleaning and protecting neurons. In humans, part of their duties is to ‘eat’ (or prune) unnecessary neuron endings. In fact, this role is so important that defects in glial pruning are associated with conditions such as Alzheimer’s disease. Yet it is still unknown how pruning takes place, and in particular whether it is the neuron or the glial cell that initiates the process.

To investigate this question, Raiders et al. enlisted the common laboratory animal Caenorhabditis elegans, a tiny worm with a simple nervous system where each neuron has been meticulously mapped out. First, the experiments showed that glial cells in C. elegans actually prune the endings of sensory neurons. Focusing on a single glia-neuron pair then revealed that the glial cell could trim the endings of a living neuron by redeploying the same molecular machinery it uses to clear dead cell debris. Compared to this debris-clearing activity, however, the glial cell takes a more nuanced approach to pruning: specifically, it can adjust the amount of trimming based on the activity load of the neuron.

When Raiders et al. disrupted the glial pruning for a single temperature-sensing neuron, the worm lost its normal temperature preferences; this demonstrated how the pruning activity of a single glial cell can be linked to behavior.

Taken together the experiments showcase how C. elegans can be used to study glial pruning. Further work using this model could help to understand how disease emerges when glial cells cannot perform their role, and to spot the genetic factors that put certain individuals at increased risk for neurological and sensory disorders.

Maintenance of protein homeostasis in glia extends lifespan in C. elegans

Mounting evidence support that glia play a key role in organismal ageing. However, the mechanisms by which glia impact ageing are not understood. One of the processes that has significant impact on the rate of ageing is the unfolded protein response. The more robust the UPR, the more the organism can counteract the effect of environmental and genetic stressors. However, how decline of cellular UPR translates into organismal ageing and eventual death is not fully understood. Here we discuss recent findings highlighting that neuropeptides released by glia act long distance to regulate ageing in C. elegans. Taking advantage of the short lifespan and the genetic amenability of this organism, the endoplasmic reticulum unfolded protein responses (UPR^ER) can be activated in C. elegans glia. This leads to cell-nonautonomous activation of the UPR^ER in the intestine. Activation of intestinal UPR^ER requires the function of genes involved in neuropeptide processing and release, suggesting that neuropeptides signal from glia to the intestine to regulate ER stress response. Importantly, the cell-nonautonomous activation of UPR^ER leads to extension of lifespan. Taken together, these data suggest that environmental and genetic factors that impact the response of glia to stress have the potential to influence organismal ageing. Further research on the specific neuropeptides involved should cast new light on the mechanism of ageing and may suggest novel anti-ageing therapies.

C. elegans as a model to study glial development

Glia make up roughly half of all cells in the mammalian nervous system and play a major part in nervous system development, function, and disease. Although research in the past few decades has shed light on their morphological and functional diversity, there is still much to be known about key aspects of their development such as the generation of glial diversity and the factors governing proper morphogenesis. Glia of the nematode C. elegans possess many developmental and morphological similarities with their vertebrate counterparts and can potentially be used as a model to understand certain aspects of glial biology owing to advantages such as its genetic tractability and fully mapped cell lineage. In this review, we summarize recent progress in our understanding of genetic pathways that regulate glial development in C. elegans and discuss how some of these findings may be conserved.

Engulfed by Glia: Glial Pruning in Development, Function, and Injury across Species

Phagocytic activity of glial cells is essential for proper nervous system sculpting, maintenance of circuitry, and long-term brain health. Glial engulfment of apoptotic cells and superfluous connections ensures that neuronal connections are appropriately refined, while clearance of damaged projections and neurotoxic proteins in the mature brain protects against inflammatory insults. Comparative work across species and cell types in recent years highlights the striking conservation of pathways that govern glial engulfment. Many signaling cascades used during developmental pruning are re-employed in the mature brain to "fine tune" synaptic architecture and even clear neuronal debris following traumatic events. Moreover, the neuron-glia signaling events required to trigger and perform phagocytic responses are impressively conserved between invertebrates and vertebrates. This review offers a compare-and-contrast portrayal of recent findings that underscore the value of investigating glial engulfment mechanisms in a wide range of species and contexts.

Roles of the ClC chloride channel CLH-1 in food-associated salt chemotaxis behavior of C. elegans

The ability of animals to process dynamic sensory information facilitates foraging in an ever-changing environment. However, molecular and neural mechanisms underlying such ability remain elusive. The ClC anion channels/transporters play a pivotal role in cellular ion homeostasis across all phyla. Here, we find a ClC chloride channel is involved in salt concentration chemotaxis of Caenorhabditis elegans. Genetic screening identified two altered-function mutations of clh-1 that disrupt experience-dependent salt chemotaxis. Using genetically encoded fluorescent sensors, we demonstrate that CLH-1 contributes to regulation of intracellular anion and calcium dynamics of salt-sensing neuron, ASER. The mutant CLH-1 reduced responsiveness of ASER to salt stimuli in terms of both temporal resolution and intensity, which disrupted navigation strategies for approaching preferred salt concentrations. Furthermore, other ClC genes appeared to act redundantly in salt chemotaxis. These findings provide insights into the regulatory mechanism of neuronal responsivity by ClCs that contribute to modulation of navigation behavior.

C. elegans Glia Are Bona Fide Odorant Receptor Cells

There is increasing evidence that supporting/glial cells in sensory systems function in sensory transduction. Duan et al. demonstrate that the glial cells of the C. elegans amphid apparatus serve as odorant receptor cells and regulate neuronal output and behavior.

Behaviorally consequential astrocytic regulation of neural circuits

Astrocytes are a large and diverse population of morphologically complex cells that exist throughout nervous systems of multiple species. Progress over the last two decades has shown that astrocytes mediate developmental, physiological, and pathological processes. However, a long-standing open question is how astrocytes regulate neural circuits in ways that are behaviorally consequential. In this regard, we summarize recent studies using Caenorhabditis elegans, Drosophila melanogaster, Danio rerio, and Mus musculus. The data reveal diverse astrocyte mechanisms operating in seconds or much longer timescales within neural circuits and shaping multiple behavioral outputs. We also refer to human diseases that have a known primary astrocytic basis. We suggest that including astrocytes in mechanistic, theoretical, and computational studies of neural circuits provides new perspectives to understand behavior, its regulation, and its disease-related manifestations.

Regulation of glial size by eicosapentaenoic acid through a novel Golgi apparatus mechanism

Coordination of cell growth is essential for the development of the brain, but the molecular mechanisms underlying the regulation of glial and neuronal size are poorly understood. To investigate the mechanisms involved in glial size regulation, we used Caenorhabditis elegans amphid sheath (AMsh) glia as a model and show that a conserved cis-Golgi membrane protein eas-1/GOLT1B negatively regulates glial growth. We found that eas-1 inhibits a conserved E3 ubiquitin ligase rnf-145/RNF145, which, in turn, promotes nuclear activation of sbp-1/ SREBP, a key regulator of sterol and fatty acid synthesis, to restrict cell growth. At early developmental stages, rnf-145 in the cis-Golgi network inhibits sbp-1 activation to promote the growth of glia, and when animals reach the adult stage, this inhibition is released through an eas-1-dependent shuttling of rnf-145 from the cis-Golgi to the trans-Golgi network to stop glial growth. Furthermore, we identified long-chain polyunsaturated fatty acids (LC-PUFAs), especially eicosapentaenoic acid (EPA), as downstream products of the eas-1-rnf-145-sbp-1 pathway that functions to prevent the overgrowth of glia. Together, our findings reveal a novel and potentially conserved mechanism underlying glial size control.

The molecular mechanisms underlying the regulation of glial and neuronal size are poorly understood. This study in nematodes reveals eicosapentaenoic acid as the downstream product of a pathway that functions to prevent the overgrowth of glia, suggesting a novel and potentially conserved mechanism underlying glial size control.

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.

Robo functions as an attractive cue for glial migration through SYG-1/Neph

As one of the most-studied receptors, Robo plays functions in many biological processes, and its functions highly depend on Slit, the ligand of Robo. Here we uncover a Slit-independent role of Robo in glial migration and show that neurons can release an extracellular fragment of Robo upon cleavage to attract glia during migration in Caenorhabditis elegans. Furthermore, we identified the conserved cell adhesion molecule SYG-1/Neph as a receptor for the cleaved extracellular Robo fragment to mediate glial migration and SYG-1/Neph functions through regulation of the WAVE complex. Our studies reveal a previously unknown Slit-independent function and regulatory mechanism of Robo and show that the cleaved extracellular fragment of Robo can function as a ligand for SYG-1/Neph to guide glial migration. As Robo, the cleaved region of Robo, and SYG-1/Neph are all highly conserved across the animal kingdom, our findings may present a conserved Slit-independent Robo mechanism during brain development.