Nerve growth factor stimulates GAP-43 expression in PC12 cell clones independently of neurite outgrowth

Expression of the growth associated protein GAP-43 (B-50, F1, neuromodulin) increases with the onset of neuronal development as seen by the growth of axons. To investigate the relationship of the signaling events leading to GAP-43 expression and neurite outgrowth, we examined PC12 clones with different phenotypes. Three clones, PC12-N09, PC12-N15, and PC12-N21, responded to NGF with increased expression of GAP-43, but only two clones, PC12-N15 and PC12-N21, responded with growth of neurites. Similar increases in expression of GAP-43 were obtained when these clones were exposed to the phorbol ester PMA. Thus, NGF and PMA induced GAP-43 expression in PC12-N09 cells in the absence of neurite outgrowth. In contrast, all three clones, were able to respond to forskolin (FOR) by initiation of long neurites which had synaptophysin in the growth cones, but showed only low levels of GAP-43. Combined stimulation of PC12-N09 cells with FOR and PMA both initiated neurites and increased expression of GAP-43 as seen in normal PC12 cells. These results show that PC12-N09 cells, in response to either NGF or PMA, can express GAP-43, but without neurite outgrowth, and that all the PC12 clones were also able to respond to FOR with increased neurite outgrowth in the presence of low levels of GAP-43. The dissociation of GAP-43 expression and growth of neurites observed in PC12-N09 cells suggests that signaling mechanisms can independently regulate GAP-43 expression and neurite outgrowth during neuronal differentiation.