Kinetics of cardiac myosin isoforms in mouse myocardium are affected differently by presence of myosin binding protein-C

We tested whether cardiac myosin binding protein-C (cMyBP-C) affects myosin cross-bridge kinetics in the two cardiac myosin heavy chain (MyHC) isoforms. Mice lacking cMyBP-C (t/t) and transgenic controls (WTt/t)\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$( {\text{WT}}^{\text{t/t}} )$$\end{document} were fed l-thyroxine (T4) to induce 90/10 % expression of α/β-MyHC. Non-transgenic (NTG) and t/t mice were fed 6-n-propyl-2-thiouracil (PTU) to induce 100 % expression of β-MyHC. Ca2+-activated, chemically-skinned myocardium underwent length perturbation analysis with varying [MgATP] to estimate the MgADP release rate k-ADP\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\left( {k_{ - ADP} } \right)$$\end{document} and MgATP binding rate k+ATP\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\left( {k_{ + ATP} } \right)$$\end{document}. Values for k-ADP\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$k_{ - ADP}$$\end{document} were not significantly different between t/tT4\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{t/t}}_{\text{T4}}$$\end{document} (102.2 ± 7.0 s−1) and WTT4t/t\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{WT}}^{\text{t/t}}_{\text{T4}}$$\end{document} (91.3 ± 8.9 s−1), but k+ATP\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$k_{ + ATP}$$\end{document} was lower in t/tT4\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{t/t}}_{\text{T4}}$$\end{document} (165.9 ± 12.5 mM−1 s−1) compared to WTT4t/t\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{WT}}^{\text{t/t}}_{\text{T4}}$$\end{document} (298.6 ± 15.7 mM−1 s−1, P < 0.01). In myocardium expressing β-MyHC, values for k-ADP\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$k_{ - ADP}$$\end{document} were higher in t/tPTU\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{t/t}}_{\text{PTU}}$$\end{document} (24.8 ± 1.0 s−1) compared to NTGPTU\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{NTG}}_{\text{PTU}}$$\end{document} (15.6 ± 1.3 s−1, P < 0.01), and k+ATP\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$k_{ + ATP}$$\end{document} was not different. At saturating [MgATP], myosin detachment rate approximates k-ADP\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$k_{ - ADP}$$\end{document}, and detachment rate decreased as sarcomere length (SL) was increased in both t/tT4\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{t/t}}_{\text{T4}}$$\end{document} and WTT4t/t\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{WT}}^{\text{t/t}}_{\text{T4}}$$\end{document} with similar sensitivities to SL. In myocardium expressing β-MyHC, detachment rate decreased more as SL increased in t/tPTU\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{t/t}}_{\text{PTU}}$$\end{document} (21.5 ± 1.3 s−1 at 2.2 μm and 13.3 ± 0.9 s−1 at 3.3 μm) compared to NTGPTU\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\text{NTG}}_{\text{PTU}}$$\end{document} (15.8 ± 0.3 s−1 at 2.2 μm and 10.9 ± 0.3 s−1 at 3.3 μm) as detected by repeated-measures ANOVA (P < 0.01). These findings suggest that cMyBP-C reduces MgADP release rate for β-MyHC, but not for α-MyHC, even as the number of cMyBP-C that overlap with the thin filament is reduced to zero. Therefore, cMyBP-C appears to affect β-MyHC kinetics independent of its interaction with the thin filament.