MYBPC3, encoding cardiac myosin binding protein-C (cMyBP-C), is the most mutated gene known to cause hypertrophic cardiomyopathy (HCM). However, since little is known about the underlying etiology, additional in vitro studies are crucial to defining the underlying molecular mechanisms. Accordingly, this study aimed to investigate the molecular mechanisms underlying the pathogenesis of HCM associated with a polymorphic variant (D389V) in MYBPC3 by using isogenic human-induced pluripotent stem cell (hiPSC)-derived cardiac organoids (hCOs). The hiPSC-derived cardiomyocytes (hiPSC-CMs) and hCOs were generated from human subjects to define the molecular, cellular, functional, and energetic changes caused by the MYBPC3^D389V variant, which is associated with increased fractional shortening and highly prevalent in South Asian descendants. Recombinant C0-C2, N’ region of cMyBP-C (wild-type and D389V), and myosin S2 proteins were also utilized to perform binding and motility assays in vitro. Confocal and electron microscopic analyses of hCOs generated from noncarriers (NC) and carriers of the MYBPC3^D389V variant revealed the presence of highly organized sarcomeres. Furthermore, functional experiments showed hypercontractility, faster calcium cycling, and faster contractile kinetics in hCOs expressing MYBPC3^D389V than NC hCOs. Interestingly, significantly increased cMyBP-C phosphorylation in MYBPC3^D389V hCOs was observed, but without changes in total protein levels, in addition to higher oxidative stress and lower mitochondrial membrane potential (ΔΨm). Next, spatial mapping revealed the presence of endothelial cells, fibroblasts, macrophages, immune cells, and cardiomyocytes in the hCOs. The hypercontractile function was significantly improved after the treatment of the myosin inhibitor mavacamten (CAMZYOS^®) in MYBPC3^D389V hCOs. Lastly, various vitro binding assays revealed a significant loss of affinity in the presence of MYBPC3^D389V with myosin S2 region as a likely mechanism for hypercontraction. Conceptually, we showed the feasibility of assessing the functional and molecular mechanisms of HCM using highly translatable hCOs through pragmatic experiments that led to determining the MYBPC3^D389V hypercontractile phenotype, which was rescued by the administration of a myosin inhibitor.

Background: Skeletal muscle plays a vital role in voluntary motion and locomotion. Fast-twitch muscle fibers are characterized by their rapid contraction kinetics, high force generation capacity, and a distinct gene expression profile compared to slow-twitch fibers. Skeletal myosin binding protein-C (MyBP-C) paralogs, slow (sMyBP-C) and fast (fMyBP-C), interact with myosin and actin filaments within sarcomeres to modulate force development during contraction. These paralogs are differentially expressed in muscle fibers, with fMyBP-C predominantly expressed in the fast-twitch fibers. However, the role of fMyBP-C in skeletal muscle disease states and aging remains poorly understood. This study employs mouse models with fMyBP-C ablation to investigate its significance in skeletal muscle physiology. Methods: Adult skeletal muscle samples aged 2∼7 months from male and female wild-type, db/db, MDX, ECC injury model, were used to determine the differential expression of fMyBP-C. Next, Mybpc2 knockout (C2 ^-/- ) young (3∼5 months) and old (22 months) male mice were used to define the role of fMyBP-C in aging. Western immunoblotting was employed to analyze the expression of fMyBP-C and sMyBP-C and the phosphorylation status of sMyBP-C. The impact of C2 ^-/- and aging on the fiber type, size, and number as well as general muscle structure was assessed by immunohistochemistry and electron microscopy. The functional effect of C2 ^-/- and aging was measured in terms of in vivo and ex vivo muscle force generation. Lastly, RNA sequencing was performed to identify the molecular pathways dysregulated in the C2 ^-/- mediated muscle dysfunction in young and old mice. Results: fMyBP-C was significantly reduced with a modest compensatory upregulation of sMyBP-C in the diseased fast-twitch muscles. fMyBP-C has a significantly higher expression in the male skeletal muscles compared to females. Further studies using young male C2 ^-/- mice showed a significant reduction in isometric tetanic force generation and relaxation rate, fiber type switching, atrophy, and altered gene expressions related to muscle function and metabolism compared to wild-type mice. Similarly, compared to their wild-type counterparts, aged male C2 ^-/- mice display significant deficits in muscle strength, endurance, and survival rate, accompanied by changes in muscle fiber size and molecular signaling pathways critical for muscle homeostasis. Conclusion: fMyBP-C is an important regulator of muscle function and homeostasis in young and aged male fast-twitch muscle fibers. The absence of fMyBP-C aggravates the effect of aging on muscle structure and function. fMyBP-C has the potential to be a therapeutic target to modulate muscle wasting caused by aging and disease.