UC San Diego

Adult animals exhibit an amazing array of behaviors controlled by an exquisitely complex nervous system. Axon pathfinding is an essential component to the development of a fully functioning nervous system. Axon pathfinding cues are molecules in the tissue surrounding growing axons that instruct directionality of axonal outgrowth leading to their proper trajectory from cell body to target. Cell adhesion molecules are one class of proteins that function as contact attractant or repellent pathfinding cues. This dissertation examines the contribution to axon pathfinding of T-cadherin, a member of the cadherin family of adhesion molecules. Both in vitro and in vivo, T-cadherin is found to be a contact repellant pathfinding cue. Not only is T- cadherin a cue in the environment, it signals to T- cadherin on growth cones in a homotypic manner. Soluble T- cadherin collapses growth cones of motor neuron explants. T-cadherin substrates inhibit neurite outgrowth of wildtype--but not knock-out--spinal neurons. T-cadherin expressed on motor neurons and in posterior sclerotome of chicken embryos directs the outgrowth of motor neurons exclusively through the anterior sclerotome. T-cadherin's properties of homophilic adhesion and inhibition of neurite outgrowth require dimerization, and both functions can be blocked by a single point mutation disrupting this dimer formation as predicted by structural studies. The presence of the pro-domain of T-cadherin also appears to disrupt normal T-cadherin function. T-cadherin acts in a dominant negative manner over N-cadherin function in several in vitro assays. T-cadherin co-expression negates N-cadherin induced cellular morphology, causes abnormal cell aggregation and segregation, and abrogates strong N- cadherin homophilic adhesion. T-cadherin may have dominance by inducing the cleavage of N-cadherin. When T- and N-cadherin are coexpressed in the same cell, a portion of the N-cadherin is detected as a smaller species than when expressed alone. Thus, T-cadherin is shown to be a homotypic repulsive axon pathfinding cue, which may be functioning though an interaction with other cadherins. This interaction may explain T-cadherin's signaling abilities--in spite of being GPI-anchored--by modulating the signal transduction abilities that normally induce neurite outgrowth as a function of the other cadherin's interactions