Myofibril relaxation with mavacamten is modified by cMyBP-c.

Abstract

cMyBP-C regulates contractile function through interactions with both thin and thick filaments. Mutations in cMyBP-C are linked to hypertrophic cardiomyopathy (HCM) which is characterized by impaired relaxation. We previously showed that the HCM L348P mutation increased binding affinity of cMyBP-C for actin and significantly prolonged cardiac relaxation in transgenic mice. Here we used single myofibril mechanics and our SpyC “cut and paste” system to study relaxation at high resolution. We first measured relaxation kinetics following ablation of the N-terminal part of cMyBP-C (C0-C7) and its replacement with recombinant C0-C7 domains. We also applied 1 μM mavacamten (Mava), a myosin inhibitor, during relaxation to determine whether effects of Mava depend on cMyBP-C. Results showed that acute ablation of N-terminal C0-C7 of cMyBP-C had no effect on activation kinetics at saturating Ca2+, but slightly increased the fast exponential phase of relaxation. In general, 1 μM Mava slowed the rate of force development and improved relaxation, in particular Mava accelerated the isometric linear phase of myofibril relaxation. Next, we measured effects of the L348P mutation as well as a shorter recombinant protein containing only the C0-C2 domains. Both proteins prolonged the time of isometric linear phase of relaxation. Relaxation in the presence of Mava reduced a new level of passive force relative to baseline in the absence of Mava and the magnitude of the reduction was greater in the presence of the L348P mutation. Interestingly, this effect was blunted in myofibrils in the absence of cMyBP-C or in the presence of C0-C2. We propose that effects of Mava depend on the presence of full-length cMyBP-C and HCM mutations may modulate these effects.