alpha 2-chimaerin, a Cdc42/Rac1 regulator, is selectively expressed in therat embryonic nervous system and is involved in neuritogenesis in N1E-115 neuroblastoma cells

Neuronal differentiation involves Pac and Cdc42 GTPases. alpha -Chimaerin, a Rac/Cdc42 regulator, occurs as alpha1- and alternatively spliced Src homo logy 2 (SH2) domain-containing alpha2-isoforms. alpha2-chimaerin mRNA was h ighly expressed in the rat embryonic nervous system, especially in early po stmitotic neurons. alpha1-chimaerin mRNA was undetectable before embryonic day 16.5. Adult alpha2-chimaerin mRNA was restricted to neurons within spec ific brain regions, with highest expression in the entorhinal cortex. alpha 2-chimaerin protein localized to neuronal perikarya, dendrites, and axons. The overall pattern of alpha2-chimaerin mRNA expression resembles that of c yclin-dependent kinase regulator p35 (CDK5/p35) which participates in neuro nal differentiation and with which chimaerin interacts. To determine whethe r a2-chimaerin may have a role in neuronal differentiation and the relevanc e of the SH2 domain, the morphological effects of both chimaerin isoforms w ere investigated in N1E-115 neuroblastoma cells. When plated on poly-lysine , transient alpha2-chimaerin but not alpha1-chimaerin transfectants formed neurites. Permanent alpha2-chimaerin transfectants generated neurites wheth er or not they were stimulated by serum starvation, and many cells were enl arged. Permanent alpha1-chimaerin transfectants displayed numerous microspi kes and contained F-actin clusters, a Cdc42-phenotype, but generated few ne urites. In neuroblastoma cells, alpha2-chimaerin was predominantly soluble with some being membrane-associated, whereas alpha1-chimaerin was absent fr om the cytosol, being membrane- and cytoskeleton-associated, paralleling th eir subcellular distribution in brain. Transient transfection with alpha2-c himaerin mutated in the SH2 domain (N94H) generated an alpha1-chimaerin-lik e phenotype, protein partitioned in the particulate fraction, and in NGF-st imulated pheochromocytoma cell line 12 (PC12) cells, neurite formation was inhibited. These results indicate a role for alpha2-chimaerin in morphologi cal differentiation for which its SH2 domain is vital.