Glycogen Synthase Kinase-3 Is Activated in Neuronal Cells by G 12 and G 13 by Rho-Independent and Rho-Dependent Mechanisms

Glycogen synthase kinase-3 (GSK-3) was generally considered a constitutively active enzyme, only regulated by inhibition. Here we describe that GSK-3 is activated by lysophosphatidic acid (LPA) during neurite retraction in rat cerebellar granule neurons. GSK-3 activation correlates with an increase in GSK-3 tyrosine phosphorylation. In addition, LPA induces a GSK-3mediated hyperphosphorylation of the microtubule-associated protein tau. Inhibition of GSK-3 by lithium partially blocks neurite retraction, indicating that GSK-3 activation is important but not essential for the neurite retraction progress. GSK-3 activation by LPA in cerebellar granule neurons is neither downstream of G i nor downstream of G q/phospholipase C, suggesting that it is downstream of G 12/13. Overexpression of constitutively active G 12 (G 12QL) and G 13 (G 13QL) in Neuro2a cells induces upregulation of GSK-3 activity. Furthermore, overexpression of constitutively active RhoA (RhoAV14) also activates GSK-3 However, the activation of GSK-3 by G 13 is blocked by coexpression with C3 transferase, whereas C3 does not block GSK-3 activation by G 12. Thus, we demonstrate that GSK-3 is activated by both G 12 and G 13 in neuronal cells. However, GSK-3 activation by G 13 is Rho-mediated, whereas GSK-3 activation by G 12 is Rho-independent. The results presented here imply the existence of a previously unknown mechanism of GSK-3 activation by G 12/13 subunits.