Cardiac Myosin Binding Protein-C Is Essential for Thick-Filament Stability and Flexural Rigidity

Abstract

Using atomic force microscopy, we examined the contribution of cardiac myosin binding protein-C (cMyBP-C) to thick-filament length and flexural rigidity. Native thick filaments were isolated from the hearts of transgenic mice bearing a truncation mutation of cMyBP-C (t/t) that results in no detectable cMyBP-C and from age-matched wild-type controls (+/+). Atomic force microscopy images of these filaments were evaluated with an automated analysis algorithm that identified filament position and shape. The t/t thick-filament length (1.48 +/- 0.02 microm) was significantly (P < 0.01) shorter than +/+ (1.56 +/- 0.02 microm). This 5%-shorter thick-filament length in the t/t was reflected in 4% significantly shorter sarcomere lengths of relaxed isolated cardiomyocytes of the t/t (1.97 +/- 0.01 microm) compared to +/+ (2.05 +/- 0.01 microm). To determine if cMyBP-C contributes to the mechanical properties of thick filaments, we used statistical polymer chain mechanics to calculate a per-filament-specific persistence length, an index of flexural rigidity directly proportional to Young's modulus. Thick-filament-specific persistence length in the t/t (373 +/- 62 microm) was significantly lower than in +/+ (639 +/- 101 microm). Accordingly, Young's modulus of t/t thick filaments was approximately 60% of +/+. These results provide what we consider a new understanding for the critical role of cMyBP-C in defining normal cardiac output by sustaining force and muscle stiffness.

Flightin Is Necessary for Length Determination, Structural Integrity, and Large Bending Stiffness of Insect Flight Muscle Thick Filaments

“…The absence of a bare zone is not uncommon and is likely due to the presence of residual M-line proteins that remain bound to the filament after calpain digestion. 10 Because it was difficult to distinguish filament fragments from entire filaments that have no noticeable bare zone and tapered end, we generated 95% confidence intervals around the data sets to eliminate fragments that would skew the filament length distribution to be smaller than actual. The filament length distribution for 3-to 5-day-old wild-type adult IFM (Fig.…”

“…This increase in thick filament compliance is likely to contribute to the reduced left ventricular systolic elastance that characterizes the hypertrophic heart of t/t mice. 10,11 These combined observations reveal how the bulk properties of muscle are influenced by the nanomechanical properties of its underlying components.…”

“…10 Equation (1) (see Materials and Methods) was used to compute the persistence length of filaments that are energetically equilibrated onto a substrate using a twodimensional equilibration model. 22 In order to statistically compare persistence lengths between groups, we first calculated a per-filament persistence length [referred to as specific persistence length (SPL); see Materials and Methods].…”

“…We showed in a previous study that myosin binding protein C (MyBP-C), a protein that binds to the myosin rod, contributes to the stiffness of mouse cardiac thick filaments. 10 Cardiac thick filaments lacking MyBP-C, isolated from t/t transgenic mice, are more compliant than cardiac thick filaments isolated from wild-type mice. This increase in thick filament compliance is likely to contribute to the reduced left ventricular systolic elastance that characterizes the hypertrophic heart of t/t mice.…”

Updating the Physiology and Pathophysiology of Cardiac Myosin-Binding Protein-C

“…Its C-terminus binds to the myosin rod and titin and accounts for about half of thick filament longitudinal rigidity. 17,18 The placement of the N-terminus within the intact sarcomere is not as well delineated. It is capable of binding to both actin and the myosin S2 segment on the thick filament.…”