Functional Characterization of Na+/Ca2+ Exchangers in Caenorhabditis elegans Open Access
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Na+/Ca2+ exchangers are low affinity/high capacity transporters that mediate Ca2+ extrusion by coupling Ca2+ efflux to the influx of Na+ ions. Their primary function is to maintain Ca2+ homeostasis in cells of all organisms and they play a particularly important role in excitable cells that experience transient Ca2+ fluxes. While their functions have been studied extensively in muscle cells, much is still unknown about their contributions to the nervous system. Data suggests that Na+/Ca2+ exchangers play a key role in neuronal processes such as memory formation, learning, oligodendrocyte differentiation and axon guidance. They are also implicated in pathologies such as Alzheimer’s Disease, Parkinson’s Disease, Multiple Sclerosis and Epilepsy. While they are implicated in critical neuronal processes, a clear understanding of their mechanism remains unknown. This dissertation examines the role of Na+/Ca2+ exchangers in the invertebrate model organism Caenorhabditis elegans. There are ten identified Na+/Ca2+ exchanger genes in C. elegans (labeled ncx-1 to ncx-10). Data presented here is the first comprehensive description of their genetics and function in C. elegans. The expression pattern of all 10 Na+/Ca2+ exchanger genes is described and their phylogeny is examined comparatively across humans and flies. Analysis of ncx-2 and ncx-8 mutants shows important roles for Na+/Ca2+ exchanger genes in egg-laying, lipid storage and longevity, suggesting a role in diverse biological functions for Na+/Ca2+ exchangers in C. elegans. The function of an NCLX type Na+/Ca2+ exchanger NCX-9 is also detailed comprehensively. Analysis of ncx-9 mutants shows that NCX-9 is required for asymmetrical axon guidance choices made by the DD and VD GABAergic motor neuron circuit. Pathway analysis shows that NCX-9 regulates asymmetric circuit patterning through RAC-dependent UNC-6/Netrin signaling and LON-2/Glypican Heparan Sulfate signaling. In vitro analysis of NCX-9 physiology in HEK cells shows that NCX-9 is a mitochondrial Na+/Ca2+ exchanger, similar to NCLX, which is its homolog in humans.