Copine1 C2 domains have a critical calcium-independent role in the neuronal differentiation of hippocampal progenitor HiB5 cells

Copine1 (CPNE1) has tandem C2 domains and an A domain and is known as a calcium-dependent membrane-binding protein that regulates signal transduction and membrane trafficking. We previously demonstrated that CPNE1 directly induces neuronal differentiation via Akt phosphorylation in the hippocampal progenitor cell line, HiB5. To determine which region of CPNE1 is related to HiB5 cell neurite outgrowth, we constructed several mutants. Our results show that over-expression of each C2 domain of CPNE1 increased neurite outgrowth and expression of the neuronal marker protein neurofilament (NF). Even though protein localization of the calcium binding-deficient mutant of CPNE1 was not affected by ionomycin, this mutant increased neurite outgrowth and NF expression in HiB5 cells. Furthermore, Akt phosphorylation was increased by over-expression of the calcium binding-deficient CPNE1 mutant. These results suggest that neither cellular calcium levels nor the localization of CPNE1 affect its function in neuronal differentiation. Collectively, our findings indicating that the C2 domains of CPNE1 play a calcium-independent role in regulating the neuronal differentiation of HiB5 cells.