Characterization of BASP1-mediated neurite outgrowth

The brain acid-soluble protein BASP1 (CAP-23, NAP-22) belongs to the family of growth-associated proteins, which also includes GAP-43, a protein recently shown to regulate neural cell adhesion molecule (NCAM)-mediated neurite outgrowth. Here, the effects of BASP1 overexpression were investigated in PC12E2 cells and primary hippocampal neurons. BASP1 overexpression stimulated neurite outgrowth in both cell types. The effects of BASP1 and trans-homophilic NCAM interactions were additive, and BASP1-induced neurite outgrowth was not inhibited by ectopic expression of cytoplasmic NCAM domains. Furthermore, inhibition of signaling via the fibroblast growth factor receptor, Src-family nonreceptor tyrosine kinases, protein kinase C, or GSK3beta, and expression of constructs of the cytoskeletal proteins spectrin and tau inhibited NCAM- but not BASP1-induced neurite outgrowth. Expression of BASP1 mutated at the serine-5 phosphorylation site stimulated neurite outgrowth to a degree comparable to that observed in response to overexpression of wild-type BASP1, whereas expression of BASP1 mutated at the myristoylation site at glycine-1 completely abrogated the stimulatory effects of the protein on neurite outgrowth. Finally, coexpression experiments with dominant negative and wild-type versions of GAP-43 and BASP1 demonstrated that the two proteins could substitute for each other with respect to induction of NCAM-independent neurite outgrowth, whereas BASP1 was unable to replace the stimulatory effect of GAP-43 on NCAM-mediated neurite outgrowth. These observations demonstrate that BASP1 and GAP-43 have overlapping, but not identical, functions in relation to neurite outgrowth and indicate that the main function of BASP1 is to regulate the organization and morphology of the plasma membrane.