Wallerian degeneration, the process of axonal breakdown and clearance of axonal and myelln debris, follows focal transection of an axon from mechanical, thermal, or Ischemic causes. The pathologically similar process of distally predominant Wallerian-like degeneration characterizes many heritable metabolic, toxic and Inflammatory disorders of nerve. Teleologically, Wallerian degeneration serves the purpose of clearing the distal stump and preparing the way for regeneration. The pivotal event Initiating most of the sequence of Wallerlan degeneration Is the breakdown of the axon and an early event in axonal breakdown is granular disintegration of the axonal cytoskeleton (GDC). Previous reports and our preliminary data suggest that GDC Is the consequence of limited cleavage of neurofilaments and other cytoskeletal proteins by the ion-sensitive protease calpain. We have shown that axonal survival can be markedly prolonged by preventing entry of extracellular calcium and by cellpermeant calpain blockers. The goals of this project are to understand the mechanism of axonal degeneration as Initiated by transection of nerve fibers, to develop methods for preventing or retarding axonal degeneration, and to manipulate the production of factors that promote nerve regeneration, such as nerve growth factor (NGF) so that retarding axonal degeneration need not prevent axonal regeneration. To accomplish these goals we will complete a definitive analysis of the spatiotemporal sequence of Wallerian degeneration use short term organ cultures of mature nerve segments in vitro to dissect the mechanisms of GDC, and apply these results to means of retarding Wallerian degeneration in vivo. Finally we will use combined immunocytochemistry and in situ hybridization to examine the phenotypic changes and neurotrophin production of target cells in the skin. In this system we will determine whether nerve growth factor production can be dissociated from axonal breakdown. The results will provide new insights into a fundamental and prevalent pathologic process in nervous system disease, and they should identify ways of protecting nerve fibers and of maintaining function in monophasic inflammatory or toxic disorders of nerve.
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