A commonality of many of the diseases which lead to heart failure is a derangement in normal mechanosensing by the cardiac muscle cell. Despite extensive investigation, the precise mechanisms by which the cardiomyocyte senses mechanical changes, transmits these changes into the cell, and alters myocyte function, is poorly understood. Further, how the heart is maintained in a compensated state despite mechanical challenges, and what alters this state and transitions the myocardium towards failure, is not known. Two areas of the cardiomyocyte that appear critical for proper mechanotransduction are the costamere and caveolae. Key proteins of these areas are the integrins and caveolins. Integrins are a large family of cell- surface adhesive receptors that are also signaling molecules in virtually all cells. Caveolae are flask-shaped invaginations in the plasma membrane of cells, including cardiac myocytes. These structures have been linked to numerous cellular functions including endocytosis, establishment of a signaling microdomain and lipid metabolism. Caveolin protein expression has been found essential for caveolae formation. Importantly, mutations in members of both these families have been linked to skeletal and cardiac myopathies. Our preliminary data show that b1 and Caveolin-3 co-localize in the adult myocyte and that these proteins interact with each, as shown by immunoprecipitation experiments. Importantly, we have produced mice which can be used for loss and gain of function approaches to assess the role of these proteins in the myocyte, and whole heart. These include b1 cardiac-specific knockout (KO) (b1cKO) and Caveolin-3KO, as well as transgenic mice in which various integrin subunits and Caveolin-3 expression are increased in a cardiac myocyte specific manner. Using these mouse models we have also shown that in the Caveolin-3KO, costameric localization of b1 integrin is lost and that its distribution in cellular sub-fractions is altered. Likewise in the b1cKO, the sub-cellular arrangement of Caveolin-3 is changed. Further, loss of b1 integrin as well as Caveolin-3 from the cardiac myocyte can ultimately lead to heart failure. In contrast, we have begun to show that increased expression of these proteins in myocytes can protect. Based on this data, the overall hypothesis for this proposal is that: b1 integrins and caveolin-3 have cooperative properties as mechanotransducers and membrane stabilizing proteins in the cardiac myocyte. A corollary that follows from this is that together, these proteins will have important roles in the process of cardiac adaptations to mechanical load and may modify how the heart transitions from a compensated state towards heart failure. To investigate this, 3 aims are proposed: 1) Determine the cooperative roles of b1 integrins and caveolin-3 in mechanotransductive responses of the myocardium by use of Caveolin-3 null and b1 integrin cardiac myocyte specific knockout mice alone or in combination. 2) Define the interactions which direct cooperative mechanical signaling of cardiomyocyte beta-1 integrin and caveolin-3. 3) Determine how single or dual overexpression of caveolin-3 and/ or ab1 integrins alters mechanical responses, and delays or protects from cardiac failure in the face of hypertrophic induction using mouse models. Clinical Significance: Heart failure of varied causes is found in a large number of VA patients, necessitating frequent hospitalizations and attention to outpatient care. Identification of root causes of cardiac dysfunction and importantly, studies which could lead to novel therapeutics of this common disease, are essential, and will be the focus of this proposal.
This proposal aims to advance our understanding of how the heart responds to excessive mechanical loads, a process that frequently can lead to heart dysfunction. Studies in the grant will focus on 2 protein families, integrins and caveolins, which we propose act together as important modulators of this process. We will explore how these proteins function in the heart and if they may be used as a novel therapy to prevent heart failure. A large number of Veteran patients have diseases such as high blood pressure or valvular diseases, which can lead to increased cardiac stress and subsequent heart failure. This proposal will explore the scientific basis for responses of the heart to stress and also test if a novel therapy might be developed to prevent heart malfunction.