VPA inhibits myo‐inositol phosphate synthase activity by inhibition of Gsk3 (LB150)

Abstract

Valproic acid (VPA) has been used for decades for the treatment of bipolar disorder. Yet, the therapeutic mechanism of VPA is not understood, hindering the development of more effective drugs. VPA causes inositol depletion in the human brain cells and in yeast cells. A similar degree of inositol depletion was observed in the yeast gsk3Δ mutant, in which the four glycogen synthase kinase 3 (GSK3) homologs were deleted including MCK1, MRK1, MDS1 and YGK3. VPA was also shown to inhibit Gsk3 in mammalian cells. These findings suggest that Gsk3 may play a role in inositol metabolism, and that VPA may cause inositol depletion by inhibiting Gsk3. The rate limiting enzyme in yeast inositol de novo synthesis is myo‐inositol phosphate synthase (MIPS). VPA treatment has been shown to cause a 50% decrease in MIPS activity. However, VPA did not inhibit MIPS in gsk3Δ cells, suggesting that VPA‐induced inositol depletion occurs by inhibition of Gsk3, leading to decreased MIPS activity. Of the four GSK3 homologs, we show that MCK1 is required for optimal inositol synthesis. mck1Δ is the only GSK3 single mutant that exhibited multiple features of inositol depletion, including inositol‐dependent growth, decreased intracellular inositol and decreased MIPS activity. VPA‐induced inhibition of MIPS was not observed in mck1Δ, indicating that Mck1 is required by VPA to inhibit MIPS. To verify that MCK1 is the sole GSK3 gene involved in regulating inositol metabolism, a triple mutant (mrk1Δmds1Δygk3Δ) was constructed. The triple mutant exhibited normal growth in inositol‐free media and increased intracellular inositol compared to WT, indicating that inositol depletion observed in gsk3Δ does not result from deletion of MRK1, MDS1 or YGK3. In conclusion, MCK1 is the yeast GSK3 homolog required for optimal inositol synthesis and VPA‐induced inhibition of MIPS is Mck1 dependent.Grant Funding Source: Supported by NIH‐DK081367 & Wayne State University Enhancement Award for Doctoral Student Travel