Date Published: April 12, 2019
Publisher: Public Library of Science
Author(s): Yali Zhang, Charles M. Welzig, Marian Haburcak, Bo Wang, Mark Aronovitz, Robert M. Blanton, Ho-Jin Park, Thomas Force, Sami Noujaim, Jonas B. Galper, Andrea Caporali.
Type 1 diabetic Akita mice develop severe cardiac parasympathetic dysfunction that we have previously demonstrated is due at least in part to an abnormality in the response of the end organ to parasympathetic stimulation. Specifically, we had shown that hypoinsulinemia in the diabetic heart results in attenuation of the G-protein coupled inward rectifying K channel (GIRK) which mediates the negative chronotropic response to parasympathetic stimulation due at least in part to decreased expression of the GIRK1 and GIRK4 subunits of the channel. We further demonstrated that the expression of GIRK1 and GIRK4 is under the control of the Sterol Regulatory element Binding Protein (SREBP-1), which is also decreased in response to hypoinsulinemia. Finally, given that hyperactivity of Glycogen Synthase Kinase (GSK)3β, had been demonstrated in the diabetic heart, we demonstrated that treatment of Akita mice with Li+, an inhibitor of GSK3β, increased parasympathetic responsiveness and SREBP-1 levels consistent with the conclusion that GSK3β might regulate IKACh via an effect on SREBP-1. However, inhibitor studies were complicated by lack of specificity for GSK3β. Here we generated an Akita mouse with cardiac specific inducible knockout of GSK3β. Using this mouse, we demonstrate that attenuation of GSK3β expression is associated with an increase in parasympathetic responsiveness measured as an increase in the heart rate response to atropine from 17.3 ± 3.5% (n = 8) prior to 41.2 ± 5.4% (n = 8, P = 0.017), an increase in the duration of carbamylcholine mediated bradycardia from 8.43 ± 1.60 min (n = 7) to 12.71 ± 2.26 min (n = 7, P = 0.028) and an increase in HRV as measured by an increase in the high frequency fraction from 40.78 ± 3.86% to 65.04 ± 5.64 (n = 10, P = 0.005). Furthermore, patch clamp measurements demonstrated a 3-fold increase in acetylcholine stimulated peak IKACh in atrial myocytes from GSK3β deficiency mice compared with control. Finally, western blot analysis of atrial extracts from knockout mice demonstrated increased levels of SREBP-1, GIRK1 and GIRK4 compared with control. Taken together with our prior observations, these data establish a role of increased GSK3β activity in the pathogenesis of parasympathetic dysfunction in type 1 diabetes via the regulation of IKACh and GIRK1/4 expression.
Diabetic Autonomic Neuropathy (DAN) is a major complication of diabetes mellitus and has been associated with a marked increase in the incidence of sudden death in diabetics [1, 2]. Sudden out of hospital cardiac death and arrhythmic death in the setting of acute MI continue to be major public health problems . The incidence of sudden death was 2.9 times higher in diabetic compared with non-diabetic patients in which ventricular tachycardia (VT) was documented in at least half these patients near the time of collapse . Risk factors for sudden death include clinical manifestations of parasympathetic dysfunction, such as a decreased high frequency (HF) component of heart rate variability (HRV) and increased dispersion of QT intervals . More than half of the patients with diabetes for 10 years or more have an impaired response of the heart to parasympathetic stimulation characterized by a reduction in the HF component of HRV. Furthermore, studies of type 1 diabetics who die suddenly in their sleep, “dead in bed syndrome”, suggested that HRV analysis of diabetic patients who lack clinical evidence of autonomic neuropathy often demonstrate decreased parasympathetic tone . Hence, decreased HRV is an important risk factor for arrhythmia and sudden death in diabetics.
The onset of Cardiac Autonomic Neuropathy has been attributed to vagal nerve dysfunction. However, a number of animal models for diabetes including an alloxan treated rabbit model demonstrating impaired baroreceptor mediated bradycardia, a streptozotocin treated rat model and our more recent studies in the Akita type 1 diabetic mouse support the conclusion that parasympathetic dysfunction might also be associated at least in part with a decrease in the ability of the end organ to respond to parasympathetic stimulation [6, 7, 25, 26]. Specifically, studies in our laboratory demonstrated that parasympathetic dysfunction is at least in part due to decreased expression of proteins which mediate the response of the heart to parasympathetic stimulation including the M2 muscarinic receptors, the G-protein Gαi2 and the structural components of the IKACh ion channel, GIRK1 and GIRK4 in atrial myocytes . We further demonstrated that the transcription factor SREBP-1 which regulates GIRK1 and GIRK4 at the level of transcription is also decreased in the Akita mouse.