Synaptic junction formation is initiated between axons and dendrites by intercellular adhesion molecules, which through bound intracellular effectors, nucleate the reciprocal recruitment of proteins required for presynaptic neurotransmitter release and postsynaptic neurotransmitter reception. Junctions analogous to adherens junctions are formed during synaptogenesis. Cadherins are intercellular adhesion molecules central to the formation of adherens junctions in epithelial cells. Studies on cadherin function in neurons have demonstrated a role for cadherins and their associated catenins in regulating synapse number and maturation of both pre- and post-synaptic membranes. Expression studies have found that cadherin isoforms are not expressed ubiquitously, but are localized to distinct neural circuits, suggesting that cadherins may play a role in synaptic specificity. Based upon studies in epithelial cells, adherens junctions are thought to be initiated by nectin interaction in trans followed by cadherin recruitment. Nectins rely upon their interaction with the PDZ domain of afadin to recruit and activate cadherins. Through this same PDZ domain, afadin can bind other cell surface receptors, which have an established role in synaptogenesis. I am proposing to examine the role of afadin-mediated cell surface receptor's association with cadherins on synaptogenesis, by examining the affect of loss of afadin expression. Loss of this molecular link will likely block cadherin association with several cell surface receptor families. As loss of afadin in cultured epithelial cells has been shown to lead to a loss of nectin and cadherin adhesion, we expect a severe reduction in synapse formation and/or stabilization. As previous analysis has not satisfactorily demonstrated how afadin associates with alpha-catenin and p120-catenin, additional analysis will be directed at examining the domains of afadin that binds these catenins. I will examine the role of afadin association with these catenins, by determining the ability of these mutants to rescue the afadin null phenotype in vitro. Finally, the effect of loss of afadin on signaling initiated by these other cell surface receptors will be determined. Proper nervous system functioning requires the ordered synaptic connection of disparate brain regions between seemingly unrelated cells. While much research has documented the molecules required for guiding axons to the right brain regions, we are interested in determining how axons then connect with the proper neuron and on the proper part of the neuron. This proposal is studying the associations between different classes of adhesion proteins and the role of this association in creating and modifying synapses, which is likely to play a role in generating synaptic specificity.
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