Ablation of Cardiac Myosin Binding Protein-C Accelerates Contractile Kinetics in the Absence of Hypertrophic Remodeling in Engineered Cardiac Tissue

Abstract

Truncation mutations in cardiac myosin binding protein-C (cMyBP-C) are prevalent causes of hypertrophic cardiomyopathy (HCM). Mounting evidence suggest that truncated cMyBP-C causes HCM through haploinsufficiency. Mouse models in which cMyBP-C is ablated (KO), or truncated, show varying degrees of systolic and diastolic dysfunction and alterations in contractile kinetics. Phenotypic variation observed in these models may, however, be related to secondary adaptations and hypertrophic remodeling, rather than the direct effect of cMyBP-C depletion.In order to study the effects of cMyBP-C ablation on contractility in the absence of hypertrophic remodeling, we produced engineered cardiac tissue (ECT) from wild type (WT) and (KO) neonatal mouse cardiomyocytes.Expression levels of several hypertrophic markers were similar in WT and KO ECT, indicating a lack of hypertrophic remodeling. Contraction (50.0±1.1ms vs. 42.9±1.9ms; p=0.002) and relaxation (41.3±0.8ms vs. 29.2±1.7; p<0.001) times were significantly shorter in KO than WT ECT. This difference was more pronounced during relaxation, indicating a possible disconnect in contraction-relaxation coupling. In order to investigate this possibility, we studied the 1st order derivative (dFdt) of the force tracings. We found no significant difference in +dFdMax/-dFdtMax ratios (0.38±0.02 vs. 0.43±0.03; p=0.178), indicating that cMyBP-C ablation does not directly affect contraction-relaxation coupling. Elapsed times between +dFdtMax and peak developed force (FMax) (27.3±0.8ms vs. 21.7±1.5ms; p=0.002), as well as between FMax and -dFdtMax (36.5±1.6 vs. 18.4±2.1ms; p<0.001) was, however, significantly shorter.These findings indicate that cMyBP-C is involved in regulation of contractile velocity and that its ablation shortens the period of sustained force production. Our data indicates that previous reports of impaired relaxation and diastolic dysfunction due to cMyBP-C KO depletion in mice and humans may be caused by hypertrophic remodeling, rather than directly by cMyBP-C ablation/haploinsufficiency.