Regulation of semaphorin III/collapsin-1 gene expression during peripheral nerve regeneration.

The competence of neurons to regenerate depends on their ability to initiate a program of gene expression supporting growth and on the growth-permissive properties of glial cells in the distal stump of the injured nerve. Most studies on intrinsic molecular mechanisms governing peripheral nerve regeneration have focussed on the lesion-induced expression of proteins promoting growth cone motility, neurite extension, and adhesion. However, little is known about the expression of intrinsic chemorepulsive proteins and their receptors, after peripheral nerve injury and during nerve regeneration. Here we report the effect of peripheral nerve injury on the expression of the genes encoding sema III/coll-1 and its receptor neuropilin-1, which are known to be expressed in adult sensory and/or motor neurons. We have shown that peripheral nerve crush or transection results in a decline in sema III/coll-1 mRNA expression in injured spinal and facial motor neurons. This decline was paralleled by an induction in the expression of the growth-associated protein B-50/GAP-43. As sema III/coll-1 returned to normal levels following nerve crush, B-50/GAP-43 returned to precrush levels. Thus, the decline in sema III/coll-1 mRNA coincided with sensory and motor neuron regeneration. A sustained decline in sema III/coll-1 mRNA expression was found when regeneration was blocked by nerve transection and ligation. No changes were observed in neuropilin-1 mRNA levels after injury to sensory and motor neurons, suggesting that regenerating peripheral neurons continue to be sensitive to sema III/coll-1. Therefore we propose that a decreased expression of sema III/coll-1, one of the major ligands for neuropilin-1, during peripheral nerve regeneration is an important molecular event that is part of the adaptive response related to the success of regenerative neurite outgrowth occurring following peripheral nerve injury.