CONDITIONING NERVE CRUSH ACCELERATES CYTOSKELETAL PROTEIN-TRANSPORT IN SPROUTS THAT FORM AFTER A SUBSEQUENT CRUSH

To examine the relationship between axonal outgrowth and the delivery of cytoskeletal proteins to the growing axon tip, outgrowth was accelerated by using a conditioning nerve crush. Because slow component b (SCb) of axonal transport is the most rapid vehicle for carrying cytoskeletal proteins to the axon tip, the rate of SCb was measured in conditioned vs. sham-conditioned sprouts. In young Sprague-Dawley rats, the conditioning crush was made to sciatic nerve branches at the knee; 14 days later, the test crush was made where the L4 and L5 spinal nerves join to form the sciatic nerve in the flank. Newly synthesized proteins were labeled in motor neurons by injecting S-35-methionine into the lumbar spinal cord 7 days before the test crush. The wave of pulse-labeled SCb proteins reached the crush by the time it was made and subsequently entered sprouts. The nerve was removed and sectioned for SDS-PAGE and fluorography 4-12 days after the crush. Tubulins, neurofilament proteins, and representative "cytomatrix" proteins (actin, calmodulin, and putative microtubule-associated proteins) were removed from gels for liquid scintillation counting. Labeled SCb proteins entered sprouts without first accumulating in parent axon stumps, presumably because sprouts begin to grow within hours after axotomy. The peak of SCb moved 11% faster in conditioned than in sham-conditioned sprouts: 3.0 vs. 2.7 mm/d (p < 0.05). To confirm that sprouts elongate more rapidly when a test crush is preceded by a conditioning crush, outgrowth distances were measured in a separate group of rats by labeling fast axonal transport with H-3-proline 24 hours before nerve retrieval. The rate of outgrowth was 17% faster in conditioned than in sham-conditioned sprouts: 5.13 vs. 4.38 mm/d (p < 0.01). The increase in outgrowth rate correlated with the increase in SCb rate, suggesting that outgrowth is a function of cytoskeletal protein transport.