Inhibition of neuronal surface proteases decreases the requirement for GAP-43 in neurite outgrowth.

Intracellular delivery of anti-GAP IgG inhibits the elaboration of neurites by NB2a/d1 cells. However, recent studies indicate that the extent of inhibition is minimized when cells were cultured on poly-L-lysine-coated or laminin-coated versus uncoated plates, suggesting that the role of GAP-43 in neuritogenesis may be specifically related to membrane adhesiveness. We therefore examined the influence of inhibition of thrombin, the neuronal surface protease that restricts neurite outgrowth, on GAP-43-dependent neurite outgrowth. Treatment of cells with the specific thrombin inhibitor hirudin in the presence of serum induced a similar percentage of neurite outgrowth as was observed following serum withdrawal. However, while neurite outgrowth induced by serum deprivation of cells was reduced following intracellular delivery of anti-GAP IgG, neurite outgrowth induced by hirudin treatment of cells was not. That inhibition of neuronal surface protease activity overcomes the inhibition of neurite outgrowth following intracellular delivery of anti-GAP IgG provides further evidence that the role that GAP-43 plays in neuritogenesis is related to membrane adhesiveness.