Regulation of neurite outgrowth mediated by localized phosphorylation of protein translational factor eEF2 in growth cones.

Nerve growth cones contain mRNA and its translational machinery and thereby synthesize protein locally. The regulatory mechanisms in the growth cone, however, remain largely unknown. We previously found that the calcium entry-induced increase of phosphorylation of eukaryotic elongation factor-2 (eEF2), a key component of mRNA translation, within growth cones showed growth arrest of neurites. Because dephosphorylated eEF2 and phosphorylated eEF2 are known to promote and inhibit mRNA translation, respectively, the data led to the hypothesis that eEF2-mediating mRNA translation may regulate neurite outgrowth. Here, we validated the hypothesis by using a chromophore-assisted light inactivation (CALI) technique to examine the roles of localized eEF2 and eEF2 kinase (EF2K), a specific calcium calmodulin-dependent enzyme for eEF2 phosphorylation, in advancing growth cones of cultured chick dorsal root ganglion (DRG) neurons. The phosphorylated eEF2 was weakly distributed in advancing growth cones, whereas eEF2 phosphorylation was increased by extracellular adenosine triphosphate (ATP)-evoked calcium transient through P2 purinoceptors in growth cones and resulted in growth arrest of neurites. The increase of eEF2 phosphorylation within growth cones by inhibition of protein phosphatase 2A known to dephosphorylate eEF2 also showed growth arrest of neurites. CALI of eEF2 within growth cones resulted in retardation of neurite outgrowth, whereas CALI of EF2K enhanced neurite outgrowth temporally. Moreover, CALI of EF2K abolished the ATP-induced retardation of neurite outgrowth. These findings suggest that an eEF2 phosphorylation state localized to the growth cone regulates neurite outgrowth.