Modifications in the cytoskeleton transported by slow component A during axonal regeneration.

We studied the modifications occurring in the parent cytoskeleton carried by SCa (the slower of the two slow axonal transport subcomponents) after peripheral nerve crush. The proteins transported in rat sciatic motor axons were radiolabelled by injecting [35S]methionine into the ventral horn of the spinal cord, and the nerve was crushed so as to entrap only the proteins transported by SCa along the parent axon. Two weeks after the crush, the regenerating nerve was removed and the distributions of the polymerized and unpolymerized radiolabelled cytoskeletal proteins were compared with those in normal, non-regenerating nerves. We found that in the parent axons, most of the radioactive neurofilaments were arrested by the crush, but microtubules, soluble tubulin, insoluble and soluble actin were normally transported. Thus, when the resulting cytoskeleton transported by SCa entered the daughter axon, it was enriched in microtubules and actin, and partially depleted of neurofilaments. This cytoskeleton contained larger proportions of soluble tubulin and insoluble actin than the parent cytoskeleton, but retained its coordinated progression and transport velocity, suggesting that after axotomy, the main destiny of the parent cytoskeleton carried by SCa is to become the equivalent cytoskeleton in the daughter axons.