Glia Development and Function in the Nematode Caenorhabditis elegans

Abstract

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.

Figure 1.

Subtypes and anatomy of Caenorhabditis elegans glia. (A) A schematic representation of each glia type in the head, (A′) hermaphrodite tail, (A′′) and male tail. Anterior and posterior deirid glia (ADEsh/so, PDEsh/so) are not depicted. Glia–neuron associations are magnified in B–E as follows: amphid glia (B), IL socket glia (C ), cephalic sensilla (E), and cephalic posterior membrane sheaths enveloping the brain neuropil (D). (B–B′): (B) Amphid sensilla schematic showing AMso–AMsh sense organ glia forming a channel lumen associating with dendrite tips, neuron-receptive endings (NREs) that traverse the glial channel (ASE, ASH, ADL), and embedded NREs (AFD and AWA/B/C neurons). (B′) Cross section of bilateral AMsh glia–AWC pairs, (C ) ILso glia interact with NREs of different neurons (URX, IL, BAG) at distinct contact sites, with only IL neurons traversing a channel made by the glia. (D) Schematic of CEPsh glial processes (green) ensheathing different axon commissures. (D′) A schematic cross-section view of the brain neuropil shows the relative location of axon commissures with CEPsh and GLR glia. (D′′) Glial processes also infiltrate between neuron processes in the neuropil. (D′′′) Electron micrograph showing the CEPsh glia–ALA neuron–AVE neuron tripartite synapse. (E) Schematic of the cephalic sensilla of a male animal, noting relative localization and glia–neuron contacts of the sex-shared CEP neuron and male-specific CEM neuron. Sensory NREs in B, C, and E are depicted without dendrites and cell bodies for simplicity. Neurons in D are depicted without dendrites for simplicity. (Schematics in A, B, and D are reprinted from Singhvi and Shaham 2019 with permission from the author. Electron microscope [EM] image in D′′′ is adapted with permission from White et al. 1986. Schematic in C is based on data in White et al. 1986, Ward et al. 1975, and Cebul et al. 2020. Panel E is based on data in Wang et al. 2015 and Sulston et al. 1980.)

Figure 2.

Glial cell development and morphogenesis. (A) Schematic depicting apical–basal polarity of AMsh and AMso glia at the amphid channel lumen. (B–B′′′) Diagram and images of AMsh glia in wild-type (B) and different mutant backgrounds (B′–B′′′). AMsh glia anterior processes are collapsed in dyf-7 and dex-1 mutant (B′), glial cell size is enlarged in eas-1 mutant (B′′) and glial cell body migration is disrupted in sax-3 mutant (B′′′) animals.(C–C′) Diagram and images of AMsh glia in wild-type (C ) and daf-6 (C′) mutant animals showing aberrant sensory compartment lumen in mutants, which impedes ADF-NRE from accessing the outside environment. (D–D′) Diagram and image of CEPsh and pioneer/follower neurons growing into the neuropil for brain assembly in the embryo (D) and larvae (D′). Scale bar: 10 μm. Micrograph shows CEPsh glia process guiding pioneer axon processes. (E, E′) Schematic of AMsh glia and AWC neuron remodeling in non-dauer (E) and post-dauer animals (E′). (AMsh) Amphid sheath glia, (AMso) amphid socket glia, (CEPsh) cephalic sheath glia, (NRE) neuronal receptive ending. (Fluorescence images as follows: B,B′ reprinted from Heiman and Shaham 2009 with permission; B′′ from Zhang et al. 2020b, reprinted under the terms of the Creative Commons CC BY 4.0 License; C,C′′ from Qu et al. 2020, reprinted under the terms of the Creative Commons Attribution License; D,D′ from Rapti et al. 2017, reprinted with permission from the author. Schematics from Singhvi and Shaham 2019, reprinted with permission from the author.)

Figure 3.

Caenorhabditis elegans glial cells in neuronal generation and morphogenesis. (A–A′) Schematic of AMso glia in males in L3 (A) and L4 (A′) larval stages showing its cell division to generate the MCM neuron. (B) Schematic of the male PHso glia transdifferentiating into a PHD neuron during developmental L3–L4-adult transition stages. (C–C′) Diagram and image of AFD-NRE in wild-type animals with intact AMsh glial ensheathment (C), and in kcc-3 mutant animals (C′). (D–D′) Schematic (D) and image (D′) of bilateral AMsh glia–AFD. AFD-NRE staining (top arrow) is also seen as punctate fragments in the AMsh glia cell body (bottom arrow) on the side with AFD neurons present and lost in the AMsh glial cell body on the side with AFD neuron ablated. (E–E′′) Pruning by AMsh glia regulates AFD-NRE shape. Reduced pruning (ced-10 mutants) causes elongated AFD-NRE, and excess pruning (overexpress CED-10 in AMsh) causes shorter AFD-NRE. (F–F′) RME synapses localized to a specific region of the neurite in wild-type (F) are misrouted along the neurite processes in GLR innexin mutant animals (F′). (G–G′′′) Diagram and image of CEPsh glial posterior membrane sheaths directing pioneer and follower neuronal axons (G, G′), and their misdirection in glia-ablated-animals (G′–G′′′). (H–H′) Diagram and image of glial cell and axon processes in the brain neuropil of wild-type animals (H), which become truncated or misguided in kpc-1; chin-1 double mutant animals with abnormal trafficking in CEPsh glial cells, leading to reduced brain neuropil size. (I–I′′) Schematic of epithelia and AIY neuron synapses within CEPsh glia posterior process zone. Densities of AIY synapses apposing specific CEPsh glia posterior membrane sheath process regions in wild-type animals (I, I′) are decreased in animals defective for glia-secreted UNC-6/Netrin (I′) and ectopically positioned in cima-1 animals with aberrant CEPsh posterior sheath processes. (Panel C is reprinted, with permission, from Singhvi et al. 2016; D, E reprinted from Raiders et al. 2021a under the terms of the Creative Commons Attribution License; G,H reprinted from Rapti et al. 2017 with permission from the author. Schematics adapted from Singhvi and Shaham 2019 with permission from the author.)

Figure 4.

Caenorhabditis elegans glial functions in animal behavior. (A–A′′) Schematic (A), representative micrograph (A′) (scale bar, 20 μm), and Ca²⁺ transient quantification (A′′) in AMsh glia and ASH neuron upon two pulses of isoamylalcohol (IAA) stimulation. (B–B′′) Schematic (B) depicting the region where CEPsh posterior processes contact AVA and RIM neurons. Representative traces of spontaneous glutamate (light, measured by iGluSnFR) and calcium (darkline, measured by GCaMP) dynamics near the AVA neuron in wild-type (B′) and glt-1 mutant (B′′) animals. (Images in A are reprinted with permission from Duan et al. 2020. Images in B are reprinted from Katz et al. 2019 under a Creative Commons Attribution 4.0 International License.)