Heart failure (HF) is a large and growing U.S. health problem with multifactorial origins, rare treatments of its root cause, and high 5-year mortality after diagnosis. Derangements in myocardial mechanosensing may lead to HF, yet how this process occurs in the cardiac myocyte (CM) remains poorly understood. Mechanotransduction is complex, requiring conversion of mechanical perturbations to biochemical and functional endpoints. In the CM it involves connections from the sarcolemma to the nucleus and requires a group of proteins in the costamere, including integrins (ITGs). ITGs act as adhesive receptors and signaling molecules, joining the extracellular matrix (ECM) to the cytoskeleton. My lab has studied the function of CM ITGs in transducing mechanical signals, and has shown that reduced expression and / or mutation of ITGs can cause cardiomyopathies. ITGs are bi-directional receptors. When they bind ECM ligands, activate downstream kinases and reorganize the cytoskeleton, it is termed outside-in signaling. In contrast, when intracellular events are triggered by receptors or pathways, unrelated to ITGs, they can produce conformational alterations in the ITG receptors leading to changes in their ligand (ECM) binding affinity, termed inside-out signaling. Increased ITG- ECM affinity has also been termed ITG activation. Two key CM costameric proteins which join to ITGs are Talin (Tln) and Kindlin (Kln). Both Tln and Kln have been suggested to be important for ITG activation from general work in cells such as fibroblasts or platelets, but their mechanistic function has not been studied previously in CMs. Two Tln isoforms, Tln1 and Tln2, are produced. In addition, there are 3 Kln forms, with the dominant CM Kln protein being Kln2. Tln1, Tln2 and Kln2 are all expressed in the human CM and Tln1 is significantly upregulated in human dilated cardiomyopathy. To begin study of Tln and Kln in CMs, we have generated knockout (KO) mouse models including Tln1 CM-specific KO, Tln2KO, and Kln2 CM-specific KO. Using these models we formulated our overall hypotheses that Tln and Kln2 proteins provide unique roles in cardiac form and function, specifically regulating ITG activation and transmission of mechanical signals within the CM, and that altered expression of Tln may protect the stressed myocardium. We will test this global hypothesis with 3 aims: 1) evaluate the hypotheses that Talin-1 and Talin-2 are essential for myocyte and whole heart function, mechanical signaling and integrin activation. 2) Critically examine the hypothesis that expression of Kindlin-2 is required for normal myocardial development and function of the mature heart. 3) Examine how Talin can be cardioprotective against hemodynamic stress. The proposal is significant since it will advance our understanding of the molecular basis of cardiomyopathies, allow a greater understanding of CM mechanical signaling and with this lead to potential novel HF therapies.
This proposal aims to understand how talin and kindlin proteins function in the cardiomyocyte and specifically how they regulate cell surface receptors termed integrins. This is essential since hemodynamic stress in the heart is communicated in part through integrins, and if this system is deranged, dysfunction of the myocardium can result. Understanding more about this mechanical signaling process could potentially lead to future novel therapies for treating heart failure.
