Dendrites with specialized glial attachments develop by retrograde extension using SAX-7 and GRDN-1

Abstract

Dendrites develop elaborate morphologies in concert with surrounding glia, but the molecules that coordinate dendrite and glial morphogenesis are mostly unknown. C. elegans offers a powerful model for identifying such factors. Previous work in this system examined dendrites and glia that develop within epithelia, similar to mammalian sense organs. Here, we focus on the neurons BAG and URX, which are not part of an epithelium but instead form membranous attachments to a single glial cell at the nose, reminiscent of dendrite-glia contacts in the mammalian brain. We show that these dendrites develop by retrograde extension, in which the nascent dendrite endings anchor to the presumptive nose and then extend by stretch during embryo elongation. Using forward genetic screens, we find that dendrite development requires the adhesion protein SAX-7/L1CAM and the cytoplasmic protein GRDN-1/CCDC88C to anchor dendrite endings at the nose. SAX-7 acts in neurons and glia, while GRDN-1 acts in glia to nonautonomously promote dendrite extension. Thus, this work shows how glial factors can help to shape dendrites, and identifies a novel molecular mechanism for dendrite growth by retrograde extension.C. elegans offers several advantages. Each cell can be uniquely identified and has a remarkably stereotyped shape and set of cell-cell contacts; neuron-and glia-specific promoters allow the targeted expression of transgenes in single cells to visualize and manipulate specific neuron-glia contacts in intact living animals; and its powerful genetics permit unbiased screens to identify factors that control neuron-glia interactions. Additionally, C. elegans glia share a number of important features with mammalian glia (Mizeracka and Heiman, 2015). This system Retrograde extension by SAX-7 and GRDN-1 4 thus offers a powerful genetic model for dissecting dendrite-glia contacts. Here, we turned our attention to a previously overlooked class of dendrite-glia contacts whose elaborate, tightly apposed membranous structures are reminiscent of the highly coordinated membranous contacts found between astrocytes and dendritic spines at mammalian synapses.Using super-resolution microscopy, we find that the dendrites of two neuron types, URX and BAG, form specialized attachments with a single glial partner. Using forward genetics, we identify two conserved molecules, SAX-7 and GRDN-1, that are required for the development of these dendrites. SAX-7 is a homolog of the mammalian neuron-glia adhesion molecule L1CAM, and GRDN-1 is a homolog of proteins involved in cell polarity and cytoskeleton organization.Each of these homologs is expressed in the mammalian brain and causes developmental brain disorders when disrupted (Jouet et al., 1994;Vits et al., 1994; Ekici et al., 2010). Using embryonic time-course imaging, we find that URX and BAG dendrites develop by retrograde extension, a type of outgrowth in which the dendrites anchor to their targets and then extend by stretch. SAX-7 and GRDN-1 help to anchor dendrite endings at the nose during embryo elong...

Lineage-specific control of convergent differentiation by a Forkhead repressor

“…We obtained the reference allele for this gene, the deletion strain ev505 , and characterized sense organ perturbations in detail (Nash et al, 2000). In previous work, we identified the gene grl-18 , which encodes for a poorly characterized protein that contains a ground-like domain, as well as col-53 and col-177 , which both encode collagens, as highly specific markers for ILso glia (Cebul et al, 2020; Fung et al, 2020). Using these markers, we find that 100% of unc-130 mutants lost expression of all three known ILso markers ( grl-18 pro, col-53 pro, and col-177 pro) specifically in one or both dorsal ILso glia (ILsoD), but not in lateral (ILsoL/R) or ventral (ILsoV) glia (Fig.…”

“…We focused on the development of the IL socket glia which are six-fold radially symmetric, such that there is a dorsal, lateral, and ventral pair of cells (ILsoD, ILsoL/R, ILsoV, respectively) (Mizeracka & Heiman, 2015). We recently identified specific markers for ILso glia and determined a role for them during dendrite extension of associated sensory neurons (Cebul et al, 2020). ILso glia develop via convergent differentiation - all three pairs of cells arise from distinct lineages that diverge at early stages of embryogenesis (Sulston et al, 1983).…”

Neuronal post-developmentally acting SAX-7S/L1CAM can function as cleaved fragments to maintain neuronal architecture inC. elegans

“…Indeed, transgenic wild-type copies of sax-7S (+) expressed pan-neuronally (P unc-14::sax-7S, Prab-3::sax-7S ) rescue sax-7 mutant defects including head ganglia disorganization ( Fig. 2A ), PVQ axon flip-over, AIY and AVK neuronal soma displacement (Pocock et al, 2008), AIY soma position and branching (Diaz-Balzac et al, 2015), AFD neuronal soma position (Sasakura et al, 2005) and PVD length or defasiculation (Ramirez-Suarez et al, 2019), as well as neuronal SAX-7 expression rescues dendrite retrograde extension (Cebul et al, 2020). Interestingly, we observed that sfGFP::SAX-7S expression levels vary among specific neurons in a given animal, and these neuron-specific differences appear to be reproducible across animals.…”

“…sax-7 /L1CAM is well known to contribute to the development of distinct neurons in C. elegans . It is involved in dendrite development and axon guidance (Cebul et al, 2020; Chen et al, 2019; Diaz-Balzac et al, 2015; Diaz-Balzac et al, 2016; Dong et al, 2013; Heiman and Pallanck, 2011; Ramirez-Suarez et al, 2019; Salzberg et al, 2013; Schafer and Frotscher, 2012; Sherry et al, 2020; Yip and Heiman, 2018; Zhao et al, 1998; Zhu et al, 2017). In flies and mammals, homologues of sax-7 function in neuronal migration, axon guidance, and synaptogenesis (Bieber et al, 1989; Godenschwege et al, 2006; Hall and Bieber, 1997; Rougon and Hobert, 2003; Sonderegger et al, 1998).…”

Schwann cell development: From neural crest to myelin sheath

“…Additionally, knockdown of a chromatin remodeler Drosophila homolog of the human alpha‐thalassemia and mental retardation X‐linked protein (dATRX) prevents somatodendritic ensheathment of class IV dendritic arborization neurons during development which then changes dendrite density and decreases regenerative potential (Yadav et al, 2019). A recent study in Caenorhabditis elegans identified that the cytoplasmic expression of glial protein girdin homolog ( grdn‐1 ), whose mammalian homolog is crucial for proper brain development, is required non‐autonomously to anchor and stretch dendrites retrograde from their targets in the nose during embryo elongation (Cebul, McLachlan, & Heiman, 2020), supporting an early role of glia in regulating dendrite morphology.…”