Function and Mechanism of the Intercalated Disc Protein XinB in Cardiomyocyte Proliferation and Cardiac Regeneration

ABSTRACT Cardiovascular diseases remain the leading cause of death in humans, yet the molecular mechanisms underlying these devastating conditions have not been fully elucidated. Cardiac disease is especially common in the elderly, and as the global population ages their elevated incidence will pose a serious healthcare challenge. An important structure in heart muscle cells is the intercalated disc (ICD), which mediates the coordination of the cardiac syncytium. It functions by connecting neighboring cardiomyocytes, thereby maintaining the functional integrity of this syncytium; this is crucial to the proper contraction of the heart. Although many reports demonstrate the importance of ICDs in the organization of the myocardium, relatively little is known about how these cell-to-cell junctions transmit information between cardiac muscle cells to modulate gene expression and cardiac function. The Xin-repeat containing adaptor proteins Xinα and Xinβ, also called XIRP1 and XIRP2 respectively, were first discovered by the PI. These two proteins are located in the ICD of adult cardiomyocytes and interact with various adherens junction proteins including N-cadherin and β-catenin, supporting an essential role for them in the formation/maintenance of this structure. They also play important roles during early cardiac development and in the pathogenesis of heart disease. However, the role of the Xin proteins remains poorly studied and their specific cellular and molecular functions are largely unknown. Our recent studies of the hearts of Xin knock-out (KO) mice have identified defects in development associated with impaired cardiomyocyte proliferation. Our studies further demonstrated a physical and genetic interaction between Xin and NF2, a component of the important Hippo/YAP pathway. The Hippo-YAP pathway is a highly conserved cellular regulatory network that has been previously implicated in multiple developmental systems and disease, including the heart; however, the mechanisms of its action remain unclear and a link to the ICD is a novel and exciting new discovery. Therefore, we have designed two integrative Specific Aims to test the mechanism by which the ICD protein Xin mediates cardiomyocyte proliferation, maturation, and regeneration. For the first Aim, we will investigate the interaction between Xin and the Hippo/YAP pathway. We will study how Xin regulates YAP activity and how the interaction between Xin and Hippo-YAP signaling regulates cardiac function and regeneration. For the second Aim, we will study how YAP/Tead1 regulates Xin transcription and test our hypothesis that Xin-YAP cross-regulation is crucial to cardiac gene expression and heart regeneration. The studies proposed here will reveal novel molecular mechanisms by which the important pathophysiological Hippo/YAP signal is modulated by the ICD protein Xin in the heart. The molecules defined in this study will become targets for therapeutic intervention in the treatment of cardiac diseases.