Distinct Molecular Regulation of Glycogen Synthase Kinase-3α Isozyme Controlled by Its N-terminal Region FUNCTIONAL ROLE IN CALCIUM/CALPAIN SIGNALING

Glycogen synthase kinase-3 (GSK-3) is expressed as two isozymes alpha and beta. They share high similarity in their catalytic domains but differ in their N- and C-terminal regions, with GSK-3 alpha having an extended glycine-rich N terminus. Here, we undertook live cell imaging combined with molecular and bioinformatic studies to understand the distinct functions of the GSK-3 isozymes focusing on GSK-3 alpha N-terminal region. We found that unlike GSK-3 beta, which shuttles between the nucleus and cytoplasm, GSK-3 alpha was excluded from the nucleus. Deletion of the N-terminal region of GSK-3 alpha resulted in nuclear localization, and treatment with leptomycin B resulted in GSK-3 alpha accumulation in the nucleus. GSK-3 alpha rapidly accumulated in the nucleus in response to calcium or serum deprivation, and accumulation was strongly inhibited by the calpain inhibitor calpeptin. This nuclear accumulation was not mediated by cleavage of the N-terminal region or phosphorylation of GSK-3 alpha. Rather, we show that calcium-induced GSK-3 alpha nuclear accumulation was governed by GSK-3 alpha binding with as yet unknown calpain-sensitive protein or proteins; this binding was mediated by the N-terminal region. Bioinformatic and experimental analyses indicated that nuclear exclusion of GSK-3 alpha was likely an exclusive characteristic of mammalian GSK-3 alpha. Finally, we show that nuclear localization of GSK-3 alpha reduced the nuclear pool of beta-catenin and its target cyclin D1. Taken together, these data suggest that the N-terminal region of GSK-3 alpha is responsible for its nuclear exclusion and that binding with a calcium/calpain-sensitive product enables GSK-3 alpha nuclear retention. We further uncovered a novel link between calcium and nuclear GSK-3 alpha-mediated inhibition of the canonical Wnt/beta-catenin pathway.