Cardiac Myosin Binding Protein-C Phosphorylation, Contractile Function and Cardioprotection

Abstract

Cardiac myosin binding protein-C (cMyBP-C) is a sarcomeric thick filament assembly protein with regulatory functions in the heart. The cMyBP-C protein differs from the skeletal isoform in that it has a small insertion near the carboxyl terminus that contains 3 phosphorylatable serines, Ser-273, −282 and −302. While the precise functional correlates of cMyBP-C phosphorylation remain obscure, we do know that cMyBP-C is targeted by multiple kinases, such as PKA, PKC, RSK, PKD, CaMKII and PKG, suggesting that it plays a vital role in cardiac signaling. We previously reported that cMyBP-C phosphorylation is essential for normal heart function and that Ser-282 phosphorylation is critical for the subsequent phosphorylation of Ser-302 and normal cardiac function. However, the role of Ser-282 cMyBP-C phosphorylation in cardiac function, particularly as it affects contractile properties and sarcomere organization, is unclear. Therefore, to better understand the mechanisms and significance of cMyBP-C phosphorylation, we established several transgenic mouse models to determine the necessity and sufficiency of Ser-282 phosphorylation for normal cardiac function. Our findings suggested that cMyBP-C phosphorylation at Ser-282 is essential for normal cardiac function and that dephosphorylation at this site accelerates cMyBP-C degradation and cleavage of a 40 kDa fragment. During MI, we showed that cMyBP-C is extensively fragmented when dephosphorylated and that such fragmentation correlates well with contractile dysfunction and heart failure. Meanwhile, we also established that the release of cMyBP-C in the blood post-MI could be a potential diagnostic biomarker for MI. Overall, these studies show that Ser-282 phosphorylation is a critical determinant of Ser-302 phosphorylation and that cMyBP-C dephosphorylation accelerates its degradation and release into the circulation. In conclusion, we provide strong evidence that cMyBP-C phosphorylation directly affects the heart's contractile properties, sarcomere organization and cardioprotection.