Overexpression of c-src and n-src in the developing Xenopus retina differentially impairs axonogenesis.

To compare the roles of the nonreceptor tyrosine kinase c-src and its neuronal splice form n-src in developing neurons, Xenopus retinal precursors were transfected in vivo with c-src, n-src, or constitutively active mutants. Axonogenesis of retinal ganglion cells was markedly impaired by the expression of constitutively active c-src and only mildly affected by the expression of constitutively active n-src. This differential phenotype could not be accounted for by raised levels of intracellular tyrosine phosphorylation alone because the average anti-phosphotyrosine staining intensity of retinal neurons expressing mutant n-src was almost twofold greater than that of neurons expressing mutant c-src. The expression of either constitutively active isoform inhibited photoreceptor differentiation by 72% but did not influence other cell fates. These results suggest that c-src and n-src have both overlapping and distinct activities in differentiating retinal neurons.

Expression and herbimycin A-sensitive localization of pp125FAK in retinal growth cones.

Proper elongation of Xenopus retinal ganglion cell (RGC) axons in the optic tract during development requires intact functioning of beta 1 integrin and tyrosine kinase (TK) activity. The cytoplasmic TK pp125FAK can directly associate with and become activated by beta 1 integrin in cultured fibroblasts. Here we demonstrate the presence of pp125FAK in the developing retina and in cultured retinal neurities, growth cones and filopodia. We show that pp125FAK immunoprecipitated from neural tissue is phosphorylated, and we compare the pattern of FAK and phosphotyrosine immunolabeling. Finally, we show that the localization of pp125FAK in filopodia depends upon TK activity sensitive to the TK inhibitor herbimycin A. These results indicate a role for pp125FAK in signaling the growth of retinal axons.