Abstract 15076: Altered Trafficking of Junctional Proteins Mediated by SAP97 in Arrhythmogenic Cardiomyopathy
We have shown that expression of 2057del2 plakoglobin, known to cause arrhythmogenic cardiomyopathy (AC) in patients, leads to ~75% reductions in INa and IK1 current densities in zebrafish ventricular myocytes. Trafficking of Nav1.5 (INa) and Kir2.1 (IK1) to the cell surface is coordinately regulated by the MAGUK protein SAP97. To determine if altered SAP97 distribution plays a role in AC, we immunostained neonatal rat ventricular myocytes expressing 2057del2 plakoglobin and observed dramatic loss of both SAP97 and Nav1.5 immunoreactive signals. However, immunoblotting showed no apparent change in total cellular content of these proteins, thus strongly suggesting a trafficking defect. The normal pattern of SAP97 and Nav1.5 immunostaining was restored completely by incubating cells in SB216763, a small molecule shown previously to rescue the heart failure phenotype, reduce mortality and restore normal cell electrophysiology in the 2057del2 plakoglobin model of AC in fish. Knock-down of SAP97 expression using shRNA in normal rat myocytes prevented junctional localization of Nav1.5 as expected, but also reduced cell surface localization of plakoglobin (but not other desmosomal proteins including desmoplakin and plakophilin-2). These observations in fish and rat models suggest that altered forward trafficking of Nav1.5, mediated by SAP97, is an important disease mechanism in AC. The unexpected finding that forward trafficking of plakoglobin is also regulated by SAP97 suggests a central role for SAP97 in the normal localization of critical cell-cell junction proteins and implicates abnormal SAP97 distribution in both electrophysiological abnormalities and altered Wnt signaling dependent on abnormal localization of plakoglobin in AC. To determine if abnormal SAP97 distribution occurs in patients, we immunostained myocardium from 11 patients with AC including 8 with desmosomal gene mutations. SAP97 signal was markedly reduced in all AC samples but not in myocardium from patients with end-stage ischemic, dilated or hypertrophic cardiomyopathies (n=5 for each). Taken together, these observations show that altered SAP97-mediated trafficking is critical in the disease pathway in AC, and identify SAP97 as a potential new biomarker in AC.
- © 2013 by American Heart Association, Inc.