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Putative role of innexins in neuronal arbor formation


Gap junctions have been implicated in a multitude of developmental phenomena, most recently gaining attention for their central role in neurodevelopment. Here, I explore an unusual function that gap junctions may serve based on investigations of the medicinal leech CNS. A pair of anterior pagoda (AP) neurons, found in 20 segmental ganglia, develops mirrored morphology throughout embryogenesis. Each AP extends secondary neurites which interact with those of their contralateral and ipsilateral segmental homologs. During this 5 to 8 day period, functional gap junctions link these cells cytoplasmically as demonstrated through imaging and dye- and electrical coupling. Normally following this period of gap junction formation, these branches degenerate and recede establishing their adult AP morphology. However, if an AP is ablated, these branches can overtake the vacated territory and persist into adulthood. Indeed, subsequent ablation studies have established that AP homologs normally inhibit each others' neurite growth. What is the molecular nature of homologous inhibition? I propose that innexin gap junctions mediate the inhibitory interaction. Invertebrate gap junctions are composed of hexameric hemichannels of the protein innexin and there are 14 innexin genes found within the leech genome. In this study, I have used single cell RNAi techniques for two innexins, Hm-inx1 and Hm-inx6, in order to functionally remove gap junctions in individual AP neurons. Gap junction knock- down experiments may disrupt AP homolog communication and prevent inhibition. Should this be the case, gap junctions have a novel developmental role; direct cell to cell mediation of an inhibitory neural growth signal.

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