CMyBP-C Phosphorylation Modulates Sarcomere Length-Dependent Changes in Cardiac Muscle Contractile Function

Abstract

It is well-established that cardiac myosin binding protein-C (cMyBP-C) phosphorylation enhances acto-myosin interactions and also impacts cross-bridge (XB) cycling rates, mechanisms which are known to affect sarcomere length (SL)-dependent contractile function. However, the precise role of cMyBP-C phosphorylation in modulating SL-dependent changes is unknown. Therefore, we measured isometric force, myofilament Ca2+-sensitivity (pCa50), SL-dependent changes in the rates of XB detachment (krel), recruitment (kdf), and the amplitude of the delayed force development (Pdf) in detergent-skinned ventricular preparations isolated from hearts of wild-type (WT) mice and from mice expressing non-phosphorylatable cMyBP-C (Ser to Ala substitutions at residues Ser273, Ser282, and Ser302 (i.e., 3SA)), at SL 1.9µm or 2.1µm both at baseline and following protein kinase A (PKA) treatment. No significant differences in maximal force and pCa50 were noted between WT and 3SA at either SL, at baseline and following PKA treatment. Importantly, however, krel was significantly slower in the 3SA compared to WT at both long (∼40%) and short (∼53%) SL at baseline, and slowed krel in 3SA myocardium was even more pronounced following PKA treatment (krel was ∼135% and ∼67% slower in compared to WT at long and short SL, respectively). No differences in kdf were observed between the WT and 3SA at either SL at baseline, however, PKA treatment accelerated kdf by ∼26% and ∼38% at long and short SL in WT myocardium but not in 3SA myocardium. Moreover, at baseline, Pdf amplitude was significantly lower in 3SA myocardium compared to WT myocardium at either SL, and PKA treatment enhanced Pdf amplitude at both SL in WT but not in 3SA, demonstrating that cMyBP-C phosphorylation modulates the magnitude and rate of XB recruitment. Collectively, our data indicate that cMyBP-C phosphorylation is a key regulator of SL-dependent contractile function.