The expression of B-50/GAP-43 in Schwann cells is upregulated in degenerating peripheral nerve stumps following nerve injury

We have detected mRNA for B-50 (GAP-43, pp46, F1, neuromodulin), which was originally believed to be a neuron-specific protein, in non-neuronal cells in the rat sciatic nerve. In control rats, the level of B-50 mRNA in sciatic nerve tissue was much lower than in dorsal root ganglia. Following nerve crush or transection, the expression of B-50 mRNA in the distal nerve stump increased dramatically between 1 and 2 days post-injury. The B-50 mRNA levels in the distal stump of crushed nerves remained elevated for up to 4 weeks and subsequently returned to control levels after 7 weeks. In contrast, after nerve transection B-50 mRNA levels in the distal nerve portion continued to increase up to 7 weeks post-lesion. No changes in the levels of the B-50 transcript were observed in the proximal portion of either crush-lesioned or transected sciatic nerves. In situ hybridization demonstrated B-50 mRNA associated with Schwann cells in the distal nerve stump. The observation that Schwann cells are capable of producing B-50 mRNA was confirmed by Northern blot analysis of total RNA isolated from primary Schwann cell cultures. Taken together, these data show the expression of B-50 mRNA by Schwann cells and the up-regulation of B-50 mRNA in reactive Schwann cells upon loss of axonal contact.