The N-Terminal Mutations of cMyBP-C Affect Calcium Regulation, Kinetics, and Force of Muscle Contraction

Abstract

The cardiac myosin binding protein-C (cMyBP-C) regulates cross-bridge formation and controls the duration of systole and diastole at the whole heart level. As known, mutations in cMyBP-C increase the cross-bridge number and rate of their cycling, hypercontractility, and myocardial hypertrophy. We investigated the effects of the mutations D75N and P161S of cMyBP-C related to hypertrophic cardiomyopathy on the mechanism of force generation in isolated slow skeletal muscle fibers. The mutation D75N slowed the kinetics of force development but did not affect the relaxation rate. The mutation P161S slowed both the relaxation and force development. The mutation D75N increased the calcium sensitivity of force, and the mutation P161S decreased it. The mutation D75N decreased the maximal isometric tension and increased the tension and stiffness at low calcium. Both mutations studied disrupt the calcium regulation of contractile force and affect the kinetics of its development and thus may impair cardiac diastolic function and cause myocardial hypertrophy.

Keywords: actin-myosin interaction; cardiac myosin binding protein C; fiber force kinetics; fiber stiffness; hypertrophic cardiomyopathy mutations; slow skeletal muscle fiber; tension recovery.

Figure 1

Normalized calcium dependences of slow muscle fiber tension (a) and stiffness (b) with WT, P161S, and D75N fragments of cMyBP-C. Symbols show experimental points, and lines are their approximation by the Hill Equation (1). Experimental points are shown as mean ± SD.

Figure 2

Tension redevelopment (lower panel) in muscle fiber after a run of Protocol 1 at pCa 5.5 (solid lines) and pCa 6.8 (dashed lines) in the control and in the presence of the WT C0-C2 fragment of cMyBP-C. The left tension scale relates to pCa 5.5, and the right one is for pCa 6.8. The upper panel shows changes in the sarcomere length ΔSL in nm per half-sarcomere.

Figure 3

Example of the tension transient in the muscle fiber on the run of Protocol 2 in the control (black line) and in the presence of the P161S C0-C2 fragment (red line). The upper panel shows changes in sarcomere length in nm per half-sarcomere.

Figure 4

Typical tension response of a slow fiber to a release-stretch protocol at the saturating calcium concentration (pCa 5.5). Changes in sarcomere length ΔSL in nm per half-sarcomere are shown in the upper panel. The tension response is fitted by an exponential function (red dots).