As its long-term goal, the Sadayappan Lab aims to delineate the role of myosin binding protein- C (MyBP- C) structure, regulation and function in striated muscles of both cardiac and skeletal tissues. MyBP-C is localized in the inner two-thirds of the A band, the so-called C zone. Importantly, in humans, mutations in the cardiac MyBP-C gene are associated with familial hypertrophic and dilated cardiomyopathy, accounting for over 40% of all mutations linked to cardiomyopathies. To prevent the development of either hypertrophic or dilated cardiomyopathy at the early stage, the causal defect in the gene encoding cMyBP-C must be elucidated. MyBP-C binds to titin, myosin, actin and tropomyosin filaments as a transverse filament protein connecting both thick and thin filaments in the sarcomere. MyBP-C has three paralogs encoded by three distinct genes: fast-skeletal, slow-skeletal and cardiac. The cardiac paralog differs from the skeletal paralog by having an extra Ig domain at the N-terminus (C0), three phosphorylation sites within the MyBP-C motif region and a module (C5) that has an inserted loop of 28 residues. However, the differential roles of these paralogs in striated muscles remain unclear. Importantly, studies have shown that the cardiac isoform undergoes several different post- translational modifications, such as phosphorylation, glutathionylation, acetylation, carboxylation, O-GlcNAcylation and Citrullination, indicating that cMyBP-C is a regulatory protein and a central target of sarcomeric signaling. Some of these modifications alter MyBP-C proteolysis during muscle injury. Exactly how these modifications alter sarcomere regulation, structure and function is still unclear. Therefore, Sadayappan is committed to exploring the regulation of MyBP-C in various preclinical and clinical conditions using in vitro, ex vivo and in vivo approaches.

• » cMyBP-C regulation, structure and function in health and disease: cMyBP-C is a140-kDa sarcomeric thick filament protein that is necessary to regulate sarcomere structure and function in the heart. Based on our most recent studies, it can be considered a trans-filament protein because of its ability to connect both thick (myosin S2) and thin filament (actin and -tropomyosin) proteins via its amino terminal region. The long-term objective of the lab is to understand the functional consequences of cMyBP-C on heart function. In particular, the ongoing studies are focused on determining the specific role(s) of the amino terminal-region of cMyBP-C in regulating myosin (S2 region), sarcomere structure and function at the cardiac sarcomeric and whole-heart levels, leading to the development of potential cardioprotective therapeutic approaches to improve cardiac function in heart failure.
• » fMyBP-C and contractility in heart failure: Among its three isoforms, cMyBP-C is exclusively expressed in heart, and remaining two paralogs, slow skeletal and fast skeletal myosin binding protein-Cs (sMyBP-C and fMyBP-C) are skeletal muscle specific. However, several recent reports showed a significant increase of fMyBP-C protein expression in heart and failure and dilated cardiomyopathy, which confers possible compensatory role of fMyBP-C in heart failure. Pilot study from our lab further showed that fMyBP-C is capable of decreasing force generation in cardiac muscle fiber. These findings raise the expectation of therapeutic targets to reduce fMyBP-C expression and to improve cardiac function during the development of heart failure, but the regulatory and physiological role of fMyBP-C in the cardiomyocyte have not been studied yet. The ongoing studies will elucidate the molecular mechanism underlying fMyBP-C as a regulator of cardiac contractility, both in vivo, using novel mouse models, and in vitro using various biochemical and biophysical assays.
• » Molecular mechanism of hypertrophic cardiomyopathy: Over the past two decades, the Sadayappan Lab has made significant contributions towards understanding the development of HCM. This has comprised a variety of clinical and animal strategies, including poison polypeptide, haploinsufficiency, inflammation and oxidative stress. The long-term objective is to understand the pathogenesis of MYBPC3 genetic variants (MYBPC3 Δ25bp ) known to cause contractile dysfunction and HCM in South Asian descendants. Secondly, in collaboration with Dr. Kranias, the lab is engaged to determine comorbidity of histidine-rich Ca 2+ -binding protein variant (HRC S96A ) and MYBPC3 Δ25bp variant in the pathogenesis of diastolic dysfunction, HCM, arrhythmias and sudden cardiac death using knock-in mouse models (single and double variants). In particular, the present studies are focused on determining the molecular mechanism underlying the pathogenicity of MYBPC3 and MYH7 mutations using human iPSC-CMs, organoids and mouse models, leading to the discovery of cardioprotective agents to prevent or ameliorate HCM and heart failure.
• » Slow and fast skeletal MyBP-C structure and function: The myosin binding protein-C (MyBP-C) family is a group of sarcomeric proteins important for striated muscle structure and function. Comprising approximately 2% of the myofilament mass, MyBP-C has important roles in both contraction and relaxation. Three paralogs of MyBP-C are encoded by separate genes with distinct expression profiles in striated muscle. In mammals, cMyBP- C is limited to the heart (MYBPC3), and it is the most extensively studied owing to its involvement in cardiomyopathies. However, the roles of two skeletal paralogs, slow (MYBPC1) and fast (MYBPC2), in muscle biology remain poorly characterized. Nonetheless, both have been recently implicated in the development of skeletal myopathies. This calls for a better understanding of their function in the pathophysiology of distal arthrogryposis. Our ongoing studies are focused on determining the regulation, structure, and function of slow and fast MyBP-C in skeletal muscle by using in vitro and in vivo studies using various knockout and knock-in mouse models.
• » Autoimmune mechanisms in post-myocardial injury: Chronic inflammation following myocardial infarction (MI) has a detrimental effect on reperfusion, myocardial remodeling and cardiac function, and it is increasingly associated with high morbidity and mortality with a 50% 5-year mortality rate. The activation of macrophages and T-cells correlates with several clinical conditions and disease prognosis of heart failure and sudden cardiac death. Inflammation is often triggered by cardiac muscle proteins released into the blood following injury. cMyBP-C is very sensitive to degradation post-ischemia-reperfusion injury. Both full- length cMyBP-C and its N’-terminal fragments are released into the blood, predominantly a key 40 kDa N’-terminal protein that serves as an early biomarker of myocardial infarction. cMyBP-C proteolysis induces autoantibodies in mice and patients with cardiomyopathy and myocardial infarction. However, the immunological, inflammatory and pathophysiological roles of cMyBP-C remain unknown.
• » Improving heart failure with preserved ejection fraction: Heart failure is the number one killer worldwide with heart failure with preserved ejection fraction (HFpEF, heart failure symptoms with normal systolic, but depressed diastolic function) comprising ~50% of heart failure cases. Type 2 diabetes (T2D) is a major risk factor for abnormal diastolic function, an early predictor of HFpEF. The presence of both HFpEF and T2D is associated with strikingly increased morbidity and mortality. However, currently, no effective pathophysiology- specific treatment is available for patients with HFpEF apart from general supportive care. Given the epidemic of obesity, HFpEF will become an even more prevalent highly morbid disease unless an effective treatment is developed. Therefore, ongoing studies in the lab are aimed at defining the molecular mechanism(s) underlying the development of HFpEF, particularly in the setting of T2D.