Role of Sterols and Insulin in Cardiac Autonomic Response

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DESCRIPTION (provided by applicant: Diabetic Autonomic Neuropathy is a severe complication of diabetes. More than 50% of patients with a 10-year history of diabetes demonstrate an impaired response of the heart to parasympathetic stimulation and a resulting sympathovagal imbalance. Furthermore, there is a marked increase in the incidence of sudden death in diabetics which may be associated, at least in part, with a decrease in parasympathetic responsiveness of the heart. Parasympathetic stimulation of the heart involves acetylcholine binding to M2 muscarinic receptor, and activation of the G protein-activated inward rectifier K+ channel, (GIRK1)2/(GIRK4)2 which is responsible for IKACh, the hyperpolarizing K+ current that causes a hyperpolarization of the cardiac membrane and a decrease in heart rate. Sterol regulatory element binding proteins (SREBPs) are the transcription factors that regulate genes involved in fatty acid and cholesterol synthesis. Glycogen synthase kinase 3beta (GSK3beta), which was found to be a key regulatory component of the insulin-signaling pathway, is constitutively active and is inhibited by Akt-mediated phosphorylation in response to insulin. It has been suggested that GSK3beta plays a role in the ubiquitination and degradation of SREBPs. Insulin has been shown to increase SREBP-1 levels. We previously demonstrated that the Akita diabetic mouse, which has a point mutation in the pro-insulin gene (ins2), demonstrates a markedly decreased response to parasympathetic stimulation of the heart. Using this mouse we previously showed that G1i2 and GIRK1 are up-regulated by SREBP-1 and that the hypoinsulinemia in the diabetic mouse resulted in decreased expression of GIRK1 and a decrease in IKACh. Adenoviral expression of SREPBP-1 reversed this impairment of IKACh in atrial myocytes from the Akita mouse. Akt/GSK3beta are important mediators of the metabolic effects of insulin. In this application will test 4 major hypotheses: 1) that GSK3beta activity in the diabetic heart is increased and that this is associated with decreased levels of SREBP, GIRK1 and GIRK4. 2) that GSK3 regulates the expression of GIRK1 and GIRK4 at the level of transcription via an effect on the turnover of SRBP-1, 3) that chemical inhibitors of GSK3 and overexpression of a DN-GSK3 reverse the impairment of IKACh in atrial myocytes from Akita mice and stimulates the expression of GIRK1 and GIRK4 in atrial myocytes form Akita mice and that treatment of mice with an inhibitor of GSK3 reverses parasympathetic dysfunction in these mice and 4) that crossing a mouse with a conditional cardiac specific KO of GSK3 with the Akita type I diabetic mouse protects the mouse from developing parasympathetic dysfunction and that this effect is reversed by the expression of a DA-GSK3 in the atria of these mice. These studies should not only identify a new pathway and potential therapeutic target for the pathogenesis and treatment of diabetic autonomic neuropathy, but test the efficacy of GSK3 inhibitors in the treatment and prevention of this devastating complication of diabetes.