Cytoskeleton Coordination in Neuronal Morphogenesis

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DESCRIPTION (provided by applicant): A characteristic feature of neuronal morphogenesis is the formation of long, branched membrane extensions, termed neurites, which seek out and form highly selective connections between neurons. Our long-term objective is to understand how cytoskeletal networks specify complex changes in membrane dynamics required for neurite extension, and how signaling pathways regulate this behavior. Changes in actin and microtubule cytoskeleton assembly and organization are important in neurite outgrowth, and cadherins and the protein adenomatous polyposis coli (APC) are well placed to play key roles in coordinating these changes. Cadherin cell-cell adhesion proteins are involved in axonal path finding, the initiation of contacts between neurite extensions, and synapse function. Cadherins are linked to the actin cytoskeleton by beta- and alpha-catenin, and this complex is essential for cadherin function in adhesion. APC also binds cytoplasmic beta-catenin and targets beta-catenin for degradation thereby regulating another function of beta-catenin in controlling gene expression in response to Wnt signaling. APC protein binds to microtubules, stimulates microtubule assembly and bundling in vitro, accumulates in protein clusters at the plus-end of microtubules at the tips of epithelial and neuronal membrane extensions, and is enriched in axonal growth cones and at synapses. Binding of beta-catenin to APC in microtubule-associated clusters at the plasma membrane decreases membrane outgrowth indicating another function for beta-catenin in regulating cellular morphogenesis through APC. Our working hypothesis is that microtubule-associated APC clusters at the cortex locally regulate cytoskeletal re-organization during neurite outgrowth and contact formation, and that APC clusters are in turn regulated by Wnt signaling and signals from the cadherin complex at the cell surface. This hypothesis will be tested: 1). Identify and characterize proteins in cortical APC clusters, and determine their role in APC cluster formation; 2). Characterize the function of membrane-attached APC clusters in microtubule assembly and formation of neurite extensions; and 3). Identify and characterize signaling pathways from Wnt and cadherins that regulate APC cluster function. The significance of these proposed studies is they will provide novel understanding of how cytoskeletal restructuring is controlled in locally defined, subcellular domains in order to specify neurite outgrowth and cell-cell contacts in response to extracellular signals.