The cell surface molecules NCAM and NILE/L1 facilitate cellular adherence. It has been postulated they play a role in axonal guidance. Both sets of molecules are expressed on neurons, growing axons, and their growth cones. We plan to focus on the distribution and regulation of these molecules at the cellular level using rat brain tissue. Our preliminary work indicates a strong heterogeneity between cells from different parts of the brain. We will test the hypothesis that, rather than being controlled globally, NCAM and NILE are regulated at the cellular/subcellular level, and that cells at different stages of growth and different parts of the central nervous system differ in their expression of these molecules. The distribution of these molecules in membranes of neurons will be studied in CNS cryoultrathin tissue sections and primary tissue culture. The local regulation of NILE/L1 and different forms of NCAM will be examined in different membrane domains of neurons including cell bodies, axons, dendrites and associated growth cones with colloidal gold immunocytochemistry and scanning and transmission electron microscopy. The homophilic binding of NCAM to other NCAM molecules is reduced by the polysialic acid on the molecule; the degree of NCAM polysialation diminishes during development. We will compare the regulation of polysialic acid in neuron cultures under different conditions of cell interaction. Although it is clear that the expression of these molecules undergoes radical change during development, what mechanism causes this is not clear. We postulate it may involve cellular activity, and will compare the role of cell age, cell density, and cell activity in the expression of both molecules. Previous work showing a dramatic response of these molecules to axon injury has been done primarily in the peripheral nervous system. We will focus on the response of central nervous system neurons in vivo and in vitro to axon injury. How axons find their way to their correct postsynaptic target is crucial during development, and similarly is critical in axonal outgrowth after brain or spinal cord injury. The type, density, and expression of these molecules on and around a growing axon may facilitate or hinder correct guidance after injury, and therefore merits close examination from a cellular and molecular viewpoint with great clinical relevance.
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