Roles for Cardiac MyBP-C in Maintaining Myofilament Lattice Rigidity and Prolonging Myosin Cross-Bridge Lifetime

Abstract

We investigated the influence of cardiac myosin binding protein-C (cMyBP-C) and its constitutively unphosphorylated status on the radial and longitudinal stiffnesses of the myofilament lattice in chemically skinned myocardial strips of the following mouse models: nontransgenic (NTG), effective null for cMyBP-C (t/t), wild-type cMyBP-C expressed into t/t (WT(t/t)), and constitutively unphosphorylated cMyBP-C (AllP-(t/t)). We found that the absence of cMyBP-C in the t/t and the unphosphorylated cMyBP-C in the AllP-(t/t) resulted in a compressible cardiac myofilament lattice induced by rigor not observed in the NTG and WT(t/t). These results suggest that the presence and phosphorylation of the N-terminus of cMyBP-C provides structural support and radial rigidity to the myofilament lattice. Examination of myofilament longitudinal stiffness under rigor conditions demonstrated a significant reduction in cross-bridge-dependent stiffness in the t/t compared with NTG controls, but not in the AllP-(t/t) compared with WT(t/t) controls. The absence of cMyBP-C in the t/t and the unphosphorylated cMyBP-C in the AllP-(t/t) both resulted in a shorter myosin cross-bridge lifetime when myosin isoform was controlled. These data collectively suggest that cMyBP-C provides radial rigidity to the myofilament lattice through the N-terminus, and that disruption of the phosphorylation of cMyBP-C is sufficient to abolish this structural role of the N-terminus and shorten cross-bridge lifetime. Although the presence of cMyBP-C also provides longitudinal rigidity, phosphorylation of the N-terminus is not necessary to maintain longitudinal rigidity of the lattice, in contrast to radial rigidity.

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

“…This result is especially impressive given that mouse tissue lacking MyBP-C used by Stelzer et al 29 also possessed a higher fraction of the slower β-MyHC compared with controls, yet demonstrated faster responses. We have shown that myosin cross-bridge detachment rate is inhibited by MyBP-C 30 ; the inhibition is more potent in β-MyHC than in α-MyHC. 31 Importantly, phosphorylation of MyBP-C by PKA removes its normal inhibition of the rate of force redevelopment 30 and would likewise be expected to enhance acto-myosin cross-bridge detachment rate and myofilament-dependent relaxation.…”

“…We have shown that myosin cross-bridge detachment rate is inhibited by MyBP-C 30 ; the inhibition is more potent in β-MyHC than in α-MyHC. 31 Importantly, phosphorylation of MyBP-C by PKA removes its normal inhibition of the rate of force redevelopment 30 and would likewise be expected to enhance acto-myosin cross-bridge detachment rate and myofilament-dependent relaxation.…”

Impact of the Myosin Modulator Mavacamten on Force Generation and Cross‐Bridge Behavior in a Murine Model of Hypercontractility

“…This effect is likely because of the fact that the presence of cMyBPC imparts an elastic and viscous load on the myosin heads, which acts to slow force relaxation and that absence of cMyBPC leads to a more compliant XB and possibly a greater degree of radial compliance of the sarcomeric lattice55 in the KO myocardium, resulting in an abbreviated XB “ on ” time. Interestingly, our data also show that Myk461 incubation significantly slowed k rel in the KO myocardium but not in the WT myocardium, effectively normalizing the basal differences in k rel between KO and WT groups (Table 3; Figure 5), suggesting a slowing in the strain‐induced XB detachment from actin in the KO myocardium.…”

Effects of Protein Kinase A on the Phosphorylation Status and Transverse Stiffness of Cardiac Myofibrils

“…However, subsequent PKA treatment significantly increased the phosphorylation of TnI (1.8-fold), titin (1.7-fold), and MyBP-C (1.5-fold). Therefore, phosphorylation of titin, a major modulator of the passive longitudinal stiffness, as well as TnI and MyBP-C, might be involved in muscle relaxation by causing a decrease in the transverse stiffness of myocardial fibers in response to b-AR stimulation under normal physiological conditions (11,12).…”