Axonal transport of the cytoskeleton in regenerating motor neurons: constancy and change

We have examined slow axonal transport in regenerating motor neurons of the rat sciatic nerve. Using SDS-PAGE (sodium dodecyl sulfate-polyacrylamide gel electrophoresis) we previously found that the slow component is the vehicle for the axonal cytoskeletal proteins, i.e. the neurofilament triplet proteins, tubulin and actin. When these proteins are pulse-labeled by injecting [3H]- or [35S]-amino acids into the spinal cord, they are transported distally in the nerve as two distinguishable waves of radioactivity, SCa and SCb. In normal motor neurons, the neurofilament triplet proteins and the tubulin are transported in SCa at an average velocity of 1.7 mm/day; the less heavily labeled SCb which moves at 2-5 mm/day is the primary vehicle for actin. We now find that during regeneration the velocity of SCa is unchanged in the region of the axon between the cell body and the lesion, but the amount of labeled neurofilament triplet and associated tubulin transported in the axon is decreased in neurons which had been labeled 20 days post-lesion. In contrast, the labeling of the slowly transported proteins moving ahead of the neurofilament triplet is greater in regenerating nerves than in controls. On the basis of our findings, we propose that in motor axons the normal supply of cytoskeletal protein, which is continuously transported in the slow component, is sufficient to support regeneration. Nevertheless, the neuron cell body can alter the supply of these cytoskeletal proteins so as to enhance its regenerative capacity.