Inhibition of GSK3β is a common event in neuroprotection by different survival factors

Depolarizing concentrations of potassium (HK, 25 mM), cyclic AMP elevating agents and analogs (cAMP), insulin-like growth factor-1 (IGF-1), or lithium can maintain the survival of cultured rat cerebellar granule neurons (CGNs). We investigated the possibility that the signal transduction pathways utilized by these four survival factors converge in regulating a common molecular target. We targeted the regulation of the kinase GSK3β as the critical event in the survival directed by the four survival factors. We found that treatment of CGNs with HK, the cAMP-elevating agent forskolin, IGF-1, and lithium resulted in phosphorylation of GSK3β at serine-9 and thus its inactivation. Furthermore, pharmacological inhibition of core components in the survival signaling cascades initiated by HK, forskolin, IGF-1, and lithium causes apoptosis and activation of GSK3β accompanies this death. Finally, we examined the pharmacological inhibitors of GSK3β, GSK3 inhibitor I, TDZD-8, and SB-415286, for their ability to prevent low potassium (LK)-induced apoptosis. Although previous reports demonstrate inhibition of GSK3β in in vitro kinase assays with GSK3 inhibitor I and TDZD-8, we were unable to detect inhibition of GSK3β in neuronal cultures treated with these compounds and thus no protection from LK-induced apoptosis. SB-415286 on the other hand, was able to rescue CGNs from cell death. Taken together, we conclude that regulation of GSK3β is a critical convergence event in the promotion of CGN survival by different factors.