Title: The Effects of Arrhythmogenic Right Ventricular Cardiomyopathy-Causing Proteins on the Mechanical and Signaling Properties of Cardiac Myocytes
Description: Arrhythmogenic right ventricular cardiomyopathy (ARVC) is characterized by a high incidence of lethal ventricular arrhythmias, fibrofatty replacement of myocardium, and can account for up to 20% of sudden cardiac death (SCD) cases in the young. Typically involving autosomal dominant transmission, germline mutations in genes encoding desmosomal proteins have been identified as a cause of ARVC, although the pathogenesis of the disease is still unclear. While early detection and treatment can provide a normal life expectancy for the majority of patients, with less than 10% progressing to overt right ventricular failure, low genetic penetrance and epigenetic modifiers (such as endurance exercise) can make the condition difficult to diagnose. Addressing this clinical challenge requires a better understanding of the defective molecular mechanisms that underlie the disease. To that end, the goal of this dissertation is to provide insight into the effects of ARVC-causing mutant proteins on the mechanical and signaling properties of cardiac myocytes.

Using elastography and histological techniques, we begin by characterizing the structural and mechanical properties of the native right ventricular myocardium, particularly the right ventricular apex (RVA). Because the RVA is a key site for development of arrhythmias and a potential pacing target, a careful characterization of its structure and mechanical properties are essential for understanding its role in cardiac physiology. In the first section of this dissertation, we perform a systematic analysis of the structural features and mechanical strains in the heart, focusing on the RVA region. More than half of ARVC patients exhibit one or more mutations in genes encoding desmosomal proteins. This has led many investigators to suggest that ARVC is a "disease of the desmosome" in which defective cell-cell adhesion plays a critical pathogenic role, although direct evidence for this hypothesis is lacking. To gain greater insights into potential mechanisms by which desmosomal mutations cause ARVC, we next characterize biomechanical properties and responses to shear stress (motivated by our results in the previous section) in neonatal rat ventricular myocytes expressing two distinct mutant forms of the desmosomal protein plakoglobin which have been linked to ARVC in patients.

We show that ARVC-causing mutations in plakoglobin lead to altered cellular distribution of plakoglobin, without alterations in cell mechanical properties or certain early signaling pathways. The identification of defective molecular mechanisms that are common across ARVC-patients remains a strategic area of research. Specifically, recent studies have investigated the mechanistic basis for different ARVC-causing mutations in hopes of identifying common defects in a signaling pathway - information that could be used to develop diagnostic tests or identify therapeutic targets. In the last section of this dissertation, we investigate the effects of mutant plakophilin-2 expression, and repeat key experiments performed in the previous section to identify common defects in mechanical and signaling properties. We identify a common, underlying defect in ARVC pathogenesis. Specifically, we show that disease-causing mutations across different desmosomal proteins can cause the cell to respond abnormally to mechanical shear stress with respect to plakoglobin trafficking.