Expression of GAP-43, a rapidly transported growth-associated protein, and class II beta tubulin, a slowly transported cytoskeletal protein, are coordinated in regenerating neurons.

GAP-43 is a membrane-associated phosphoprotein enriched in elongating axons (Meiri et al., 1986; Skene et al., 1986). After an axon has been interrupted by cutting or crushing a nerve (axotomy), the portion of the axon disconnected from the cell body (distal stump) degenerates and is replaced by the outgrowth (elongation) of regenerating sprouts arising from the proximal stump. Previous studies have shown that increased amounts of pulse-labeled GAP-43 undergo fast axonal transport in regenerating neurons (Benowitz et al., 1981; Skene and Willard, 1981 a, b). Using hybridization with a cloned cDNA probe, I now show that mRNA levels for GAP-43 increase in lumbar sensory neurons of rat after regeneration is initiated by crushing the sciatic nerve; the relatively high levels of GAP-43 mRNA in regenerating neurons are comparable to those in the developing neurons of 5-d-old animals. I further demonstrate that the induction of GAP-43 expression in regenerating neurons coincides temporally with an increase in mRNA levels for class II beta tubulin (Hoffman and Cleveland, 1988), suggesting that the expression of these proteins is closely coordinated during axonal elongation.