Cardiomyopathies represent a heterogeneous group of diseases of the heart muscle that often lead to progressive heart failure with high morbidity and mortality. In a significant and increasing percentage of the patient population, cardiomyopathies have been associated with hereditary mutations in genes encoding critical cellular components that make up the cytoarchitecture of cardiac muscle cells, or cardiomyocytes. While specific mutations have been linked to different classes of cardiomyopathies, it is however not well understood how these mutations cause cytostructural abnormalities that ultimately lead to dysfunction of cardiomyocytes. To gain insights into the pathogenesis of inherited cardiomyopathies, we focus in this thesis on a particular set of mutations in the cardiac cytoskeleton and desmosomes that are associated with dilated and arrhythmogenic cardiomyopathies, and probe their pathogenic mechanisms using cardiomyocytes derived from human induced pluripotent stem cells and bioengineered culture platforms. In part one, we describe the mechanical and molecular basis for the assembly of sarcomeres, the fundamental contractile units within cardiomyocytes, and reveal how mutations in titin (TTN) abolish this process by disrupting cell-matrix interaction and impairing diastolic force generation, a hallmark of dilated cardiomyopathy. In the second part of this thesis, we reveal that plakophilin-2 (PKP2) mutations that are associated with arrhythmogenic cardiomyopathy lead to impaired systolic function by destabilizing cell-cell junctions and in turn disrupting sarcomere stability and organization. Together, our studies establish a deeper understanding of how cell-matrix and cell-cell interactions contribute to the organization and function of cardiomyocytes and how disruption of these interactions by pathogenic mutations lead to cardiac dysfunction. / 2022-05-18T00:00:00Z

Cardiac muscle contraction is responsible for pumping blood throughout the body. The cyclical, ATP-hydrolysis dependent interaction of the myosin motor protein with filamentous actin drives muscle contraction. During this process the α-helical neck region of myosin acts as a lever arm, transmitting contractile force between thick and thin filaments by amplifying small conformational changes in the myosin motor domain. The resulting relative displacement of thick and thin filaments causes muscle shortening. The regulatory light chain (RLC) of myosin mechanically supports the lever arm by binding to the myosin heavy chain neck region; this is a crucial interaction in maintaining myosin's ability to produce force and motion. We investigated the role of N-terminal modifications of the RLC in modulating actomyosin contractility at the molecular level. Phosphorylation of the RLC is a naturally occurring post-translational modification of the RLC N-terminus that is important for cardiac function and has been shown to enhance contractility at the cellular level. In contrast, genetic mutations of the RLC that lead to familial hypertrophic cardiomyopathy (FHC) disrupt cardiac function and trigger remodeling of the cardiac muscle structure. We studied two FHC-linked mutations, N47K and R58Q, located in the N-terminus of the RLC in close proximity to the phosphorylation site. Using in vitro motility assays we examined how RLC modifications affect the mechanochemical properties of cardiac β-myosin. We found that the FHC mutations reduced myosin force and power generation, in contrast to RLC phosphorylation which increased myosin force and power for WT and mutant myosins. Phosphorylation of mutant RLC resulted in a restoration of the mutation-induced decreases in contractility to WT dephosphorylated levels. These results point to RLC phosphorylation as a general mechanism to increase force production of the individual myosin motor and as a potential target to ameliorate the fundamental contractile FHC-induced phenotype.

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