Regulation of glycogen synthase kinase-3

grantor: University of TorontoGlycogen synthase kinase-3 (GSK-3) represents a family of protein serine/threonine protein kinases implicated in cellular regulation. GSK-3 family members participate in a number of signal transduction pathways that regulate cell growth, differentiation and development. The high degree of conservation between GSK-3 proteins, as well as their ubiquitous representation, indicates a fundamental role of this kinase family in regulation of cellular processes. The presented studies are aimed at elucidating different aspects of regulation of GSK-3. A mechanism of inhibition of GSK-3 in intact cells by phosphorylation of an N-terminal serine is revealed. N-terminal phosphorylation might represent a common mechanism of negative regulation of GSK-3 enzymes in vivo, since phosphorylation of a homologous serine in several other GSK-3 family members is also inhibitory. Stimulation of cells by the agonists of the ras signalling pathway downregulates GSK-3 via N-terminal phosphorylation by p90rsk-1, implicating GSK-3 as one of the targets of this important cellular regulatory cascade. During development, exposure of eukaryotic cells to lithium ions causes marked effects on cellular fate and organization, similar to the effects of inhibition or disruption of GSK-3. Results presented here demonstrate that lithium specifically inhibits GSK-3 family members in vitro and in intact cells, implying that this may be the dominant cellular action of the ion. The effect of lithium on GSK-3 is reversible in vitro indicating a direct mechanism of inactivation. Furthermore, lithium treatment of cells mimics Wingless/Wnt signalling, consistent with its role in downregulation of GSK-3. Using a novel method for the analysis of activity of protein kinases in intact Drosophila embryos, it is shown that the Drosophila GSK-3 homologue is inhibited in response to the Wingless signal. A cotransfection approach in mammalian cells indicates that overexpression of another genetically identified component of Wingless signalling upstream of GSK-3, Dishevelled, does not affect GSK-3 activity. The implications of novel modes of GSK-3 regulation on its cellular functions are discussed.Ph.D