AXON-SCHWANN CELL INTERACTION IN DEGENERATING AND REGENERATING PERIPHERAL NERVE (MYELIN, PHOSPHORUS, PROTEIN)

Abstract

Severence of a peripheral nerve stimulates a characteristic sequence of events in the distal stump, including the dissolution of axons and myelin and the proliferation of Schwann cells within their basal lamina. The first part of this thesis employs the cat tibial nerve to examine the relationship between the spatio-temporal pattern of Schwann cell mitosis, loss of the structural and functional properties of axolemma, synthesis of P(,0), the major myelin glycoprotein, and the clearance of morphological myelin.;Induction of S phase was measured by determining the uptake of ('3)H thymidine into trichloroacetic acid (TCA) precipitates following a 3 hour in vitro incubation in Krebs-Ringers buffer containing ('3)H thymidine. Nerve transection stimulated a monophasic increase in ('3)H thymidine uptake that peaked at 4 days post-transection throughout an 80mm length of distal stump. Light microscope autoradiography revealed prominent incorporation into Schwann cells of myelinated fibers. Schwann cell S phase occurred prior to the earliest changes in the myelin sheath. Treatment of distal stumps with Mitomycin C at the time of nerve transection greatly reduced thymidine uptake and the clearance of myelin debris, but not the course or timing of axonal degeneration or the onset of ovoid formation.;Nerve transection also produced dramatic changes in the intrafascicular binding of ('3)H STX which binds to voltage-sensitive sodium channels. STX binding fell precipitously to 20% of normal at 4 days post-transection, concurrent with the peak of ('3)H thymidine uptake. These levels of binding were maintained for periods of up to 70 days demonstrating that STX binds to structures other than axons in denervated distal stumps. Prior treatment with Mitomycin C delayed loss of intrafascicular STX binding. In conclusion, these studies suggest: (a) Schwann cells divide more or less contemporaneously throughout the distal stump; (b) changes in axons rather than myelin are likely to stimulate the Schwann cell to divide; (c) mitosis regulates other events during Wallerian degeneration, including myelin degeneration and the clearance of sodium channels from nodal axolemma.;Distal stumps denervated for 0,3,5,7 or 16 days were excised, incubated in ('3)H tryptophan in vitro for 3h, homogenized and run on a 10% SDS polyacrylamide gel. Radioactivity along the gel was determined. The radioactive peak corresponding to P(,0) has declined precipitously by 7 and 16 days post transection. At 16 days, a P(,0) Coomassie Blue-stained band is absent from the gels. (Abstract shortened with permission of author.).