PgmNr W4098: Neurons and glia cooperate in assembly of the embryonic C. elegans nerve ring.
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Authors:
G. Rapti; S. Shaham


Institutes
The Rockefeller University, New York, NY.


Keyword: Other ( axon guidance )

Abstract:

    Central nervous system (CNS) assembly involves multiple cell-recognition events, including axon guidance and fasciculation. Despite extensive studies, how a connected brain emerges from these interactions remains enigmatic. Glia can act as guideposts for axon navigation, yet a detailed molecular understanding of this process is not currently at hand. To determine how pioneer neurons and glia cooperate to form a CNS, we are studying the assembly of the C. elegans CNS-like neuropil, the nerve ring (NR). The NR consists of ~170 axons and is enveloped and infiltrated by processes of four astrocyte-like CEPsh glia. CEPsh ablation does not result in neuronal death, allowing detailed investigation of glial roles in CNS assembly.
    From a screen of fluorescent reporters expressed in subsets of embryonic neurons or glia we determined that the NR is populated in an orderly manner, with CEPsh extending processes early on, in conjunction with a small set of axons. Ablation studies reveal these pioneers are essential for NR entry insertion of diverse axons. We performed a forward genetic screen for mutants with NR entry defects resembling those in glia-ablated animals. In addition to known guidance factors, we isolated a new mutant that carries lesions in two genes, encoding a GTPase regulator and a pro-hormone convertase. Interestingly, each lesion alone results in only mild NR defects (5-15%); yet combining both lesions blocks 70% of NR entry of each of multiple neuron types. Importantly, full rescue of the double-mutant phenotype is achieved by combined expression of either gene in both pioneer neurons and glia. The mutant lesions result in defective axon guidance of pioneer neurons, they genetically interact with at least six known guidance pathways and they specifically affect intracellular trafficking of key guidance proteins. The synergistic nature of the double mutant, allowed us to perform screens for a novel class of axon-guidance mutants that function redundantly. We isolated over 20 such mutants, at least 14 of which appear to be in previously unknown genes. Our studies provide an inroad for studying CNS assembly in the light of neuron-glia communication and the complex redundancies of guidance pathways. Taken together, our findings suggest pivotal roles for glia in C. elegans CNS formation. This is highly reminiscent of roles of radial glia in the vertebrate brain. Furthermore, the mammalian homologs of the genes defective in our double mutant have known axon guidance roles in vertebrates, thus our studies will likely reveal conserved mechanisms promoting CNS assembly.