Role of Cardiac Myosin Binding Protein C in Sustaining Left Ventricular Systolic Stiffening

Abstract

Abstract-Despite advances in the molecular biology of cardiac myosin binding protein-C (cMyBP-C), little is understood about its precise role in muscle contraction, particularly in the intact heart. We tested the hypothesis that cMyBP-C is central to the time course and magnitude of left ventricular systolic elastance (chamber stiffening), and assessed mechanisms for this influence in intact hearts, trabeculae, and skinned fibers from wild-type (ϩ/ϩ) and homozygous truncated cMyBP-C (t/t) male mice. cMyBP-C protein was not detected by gel electrophoresis or Western blot in t/t myocardium. cMyBP-C t/t ventricles displayed reduced peak elastance, but more strikingly a marked abbreviation of the systolic elastance time course, which peaked earlier (27.6Ϯ2.1 ms) than in ϩ/ϩ controls (47.8Ϯ1.6 ms). Control hearts reached only 42Ϯ4% of maximum elastance at the onset of ejection, with substantial further stiffening during ejection. This contrasted to t/t mutants, which reached 77Ϯ3% of peak elastance before ejection of peak. These unusual findings were not observed in alternative models involving severe cardiomyopathy, but were recapitulated in a cMyBP-C null mouse. The abbreviated elastance time course and lower peak were consistent with earlier time-to-peak trabecular tension, increased unloaded shortening velocity in t/t skinned muscle strips, and dramatically reduced myofilament stiffness at diastolic calcium concentrations. These results provide novel insights into the role of cMyBP-C in myocardial systolic mechanics. Abnormal sarcomere shortening velocity and abbreviated muscle stiffening may underlie development of cardiac dysfunction associated with deficient incorporation of cMyBP-C.

Cardiac myosin binding protein-C modulates actomyosin binding and kinetics in the in vitro motility assay

“…These results are consistent with previous published experiments in muscle fibers where an increase in the unloaded shortening velocity was demonstrated with either chemical extraction [41] or transgenic deletion of cMyBP-C [13,42]. Compared to the skinned fiber studies, the motility assay allowed us to characterize the effect of cMyBP-C on unloaded shortening over the full physiologic calcium concentration range.…”

“…Studies in muscle fibers where cMyBP-C has been either chemically extracted [12] or transgenically deleted [13] have defined a role of cMyBP-C in decreasing cooperative activation and unloaded shortening speed. This has lead to the hypothesis that cMyBP-C may function as a tether restricting the interaction of myosin with actin.…”

“…For example, the effect of cMyBP-C on half maximal calcium activated tension (i.e. calcium sensitivity) has been reported to be decreased, unchanged or increased in fibers depending on the experimental approach [12][13][14][15][16][17].…”

Kinetics of cardiac muscle contraction and relaxation are linked and determined by properties of the cardiac sarcomere

“…Although the SD was nearly 10%, this was mostly due to muscle-to-muscle variation, as within one muscle the variation was significantly less, typically only 1-2%. Only one strain was Ͼ1 SD removed from the overall mean; mice with truncated MyBP-C relaxed significantly slower than they contracted (29), with a ratio of only 0.61 (P Ͻ 0.01). However, in this strain, none of the perturbations (length, frequency, or ␤-stimulation) changed this coupling parameter, indicating indeed that this is related to a structural entity, residing downstream from Ca 2ϩ handling within the sarcomere.…”

“…The majority of data presented in this article are derived from muscle studies published in the past 5 yr by our laboratory with or without the help of collaborating laboratories (18,19,21,23,29,36), with the remaining experiments having been conducted for this study specifically. However, the specific data in this article were collected during those studies but were not analyzed or presented in/for those studies.…”