GSK3β regulates the parasympathetic response in the heart via the control of sterol response element binding protein: A new therapeutic target for cardiac autonomic neuropathy

Glycogen synthase kinase 3β (GSK3β), whose activity is inhibited by insulin via AktPI3K mediated phosphorylation, has recently been implicated in the pathogenesis of diabetes, cancers and neurodegenerative diseases. Diabetic autonomic neuropathy is a major complication of diabetes mellitus which is associated with parasympathetic dysfunction of the heart. Parasympathetic stimulation of the heart involves acetylcholine (Ach) activation of the G protein-coupled K+ channel, (GIRK1)2/(GIRK4)2, which mediates an Ach-gated K+ current (IKACh) resulting in hyperpolarization of the myocyte membrane and a decrease in beat rate. We have previously demonstrated that Type 1 diabetic Akita mice develop parasympathetic dysfunction associated with impaired IKAch in atrial myocytes from these mice and that Sterol response element binding protein (SREBP) regulates both IKAch and the expression of GRIK1. GSK3β has been shown to regulate the turnover of SREBP-1. Here we demonstrate that increased GSK3β in the hypoinsulinemic Type I diabetic mouse heart regulates the parasympathetic response via the control of SREBP-1. Western blot analysis of atrial extracts from atria of Aktia mice demonstrate a 50% decrease in the level of GIRK4 and pGSK3β and a marked decrease in SREBP-1 compared to WT. Insulin treatment of Aktia mice increased levels of GIRK4, SREBP-1 and pGSK3β to levels seen in WT mice. The PI3 kinase inhibitor LY294002 reversed the effect of insulin stimulation on GIRK4, SREBP-1 and pGSK3β expression. Expression of DN-SREBP-1 in atrial myocytes reversed insulin stimulation of GIRK4 expression. Treatment of atrial myocytes with the GSK3β inhibitor Kenpaullone or adenoviral expression of a DA-GSK3β decreased the expression of both SREBP-1 and GIRK4. Finally Kenpaullone treatment increased IKAch in atrial myocytes 2.5±0.5 (n=15, p<0.001) fold. These data strongly support the conclusion that GSK3β regulates the parasympathetic response in the heart via an effect of SREBP-1 on GIRK4 expression and that the hypoinsulinemia in the type I diabetic mouse might result in parasympathetic dysfunction via an effect of increased GSK3β on SREBP-1.