The establishment of specific axon connections depends on the interaction of growth cone receptors with positive and negative guidance cues in the neuronal environment. T-cadherin (Ranscht and Dours-Zimmermann, Neuron 7:391-402) is distributed on subpopulations of growing axons and delineates specific regions in the pathway of growing commissural and motor axons. Preliminary functional studies suggest that T-cadherin acts as an inhibitor of neurite growth. The current proposal aims to extend knowledge about the distribution of T-cadherin to the CNS and to experimentally define the function and mechanism of T-cadherin in axon growth and pathway selection using in vitro assays. First, we will study T-cadherin's spatial and temporal distribution in the developing chick brain and specifically focus on the expression pattern during early neurogenesis. Second, we will determine the effect of T-cadherin substrates on neurite growth and growth cone recognition in vitro. Different neuron populations will be tested for their responses to T-cadherin using both uniform T-cadherin substrates and choice assays. In both assays, growth cone interactions with T-cadherin will be compared to those with neurite-promoting N-cadherin. Possible effects will be neutralized with specific antibodies, recombinant proteins and antisense oligonucleotides. Third, to study the mechanism of T- cadherin-induced growth cone responses, we will determine if T-cadherin- elicited growth cone responses involve homophilic or heterophilic recognition. We will determine if T-cadherin-induced growth cone interaction induce signal transduction events in the growth cone and obtain evidence for or against the participation of G-proteins, calcium-signaling and phosphorylation events. Fourth, we will define the responsible molecular domains that regulate T-cadherin-induced growth cone avoidance and N-cadherin-simulated neurite growth. We will test mutated T-cadherin molecule, in which selected regions of T-cadherin are eliminated or replaced with the corresponding region of N-cadherin and vice versa, in neurite growth and choice assays.
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