Familial hypertrophic cardiomyopathy (HCM) is the most common inherited cardiac disease and is typically caused by mutations in genes encoding sarcomeric proteins that regulate cardiac contractility. HCM manifestations include left ventricular hypertrophy and heart failure, arrythmias, and sudden cardiac death. The mechanotransduction mechanism by which dysregulated sarcomeric force production is sensed and leads to pathological remodeling remains poorly understood in HCM, thereby inhibiting the efficient development of new therapeutics. Our discovery was based on insights from a severe phenotype of an individual with HCM and a second genetic alteration in a sarcomeric mechano-sensing protein. We effectively derived cardiomyocytes from patient-specific induced pluripotent stem cells (iPSC-CMs) and developed robust engineered heart tissues (EHTs) by seeding iPSC-CMs into a laser-cut scaffold possessing native cardiac fiber alignment, for studying human cardiac mechanobiology at both cellular and tissue levels. Coupled with computational modeling for muscle contraction and rescue of disease phenotype via gene editing and pharmacological interventions, we have identified a new mechanotransduction pathway in HCM. Enhanced actomyosin crossbridge formation caused by sarcomeric mutations in cardiac myosin heavy chain (MYH7) led to increased force generation, which when coupled with slower twitch relaxation, destabilized the muscle LIM protein (MLP) stretch-sensing complex at the Z-disc. Subsequent reduction in the sarcomeric MLP level caused disinhibition of calcineurin–nuclear factor of activated T-cells (NFAT) signaling, which promoted cardiac hypertrophy. By mitigating enhanced actomyosin crossbridge formation through either genetic or pharmacological means, we alleviated stress at the Z-disc, preventing the development of hypertrophy associated with sarcomeric mutations. This proposal will dissect the roles of systolic and diastolic Z-disc stress in modulating the MLP mechanosensory complex and elucidate the molecular mechanisms that mediate the repression of calcineurin/NFAT by MLP as well as MLP protein degradation by stretch-sensing. We have recently developed a new bioreactor that can expose EHTs to precisely prescribed afterloads, so we can test the hypothesis that higher systolic forces produced by crossbridges under higher afterloads destabilize MLP at the Z-disc and activate hypertrophic signaling during systole. Additionally, EHTs will be subjected to culture under conditions of either constant length or diastolic stretch to mimic ventricular filling. After repeated stretching, EHTs will be examined for hypertrophic signaling. We will unravel mechanistic insights into how saromeric MLP is degraded in response to Z-disc stress. In addition, we will dissect molecular mechanisms by which MLP inhibits calcineurin/NFAT hypertrophic responses in systole and diastole. Elucidation of the molecular mechanisms of a common sarcomeric contraction/MLP/calcineurin mechanotransduction pathway will help to design novel strategies for a wide spectrum of heart failure patients potentially through stabilizing the Z-disk MLP mechanosensory complex.

Existing studies suggest that enhanced and prolonged myofilament force generation may be a major physiological cause underlying hypertrophic cardiomyopathy (HCM), but the mechano-signaling cascades through which the heightened sarcomeric force generation is sensed at the sarcomere and leads to abnormal nuclear hypertrophic signaling remains poorly understood. Patient-specific induced pluripotent stem cell-derived cardiomyocytes (iPSC- CMs) and functional engineered heart tissues (EHTs) have provided a robust human cardiomyocyte-based system for studying the mechanobiology of HCM. This project details our current and future efforts to extensively investigate a novel biomechanical mechanism illuminating how dysregulated sarcomeric force production is sensed and leads to pathological signaling, remodeling, and hypertrophic responses, which will establish the foundation for developing innovative mechanism-based treatments for HCM that stabilize the Z-disc mechanosensory complex.