RLC phosphorylation amplifies Ca²⁺ sensitivity of force in myocardium from cMyBP-C knockout mice

Hypertrophic cardiomyopathy (HCM) is the leading genetic cause of heart disease. The heart comprises several proteins that work together to properly facilitate force production and pump blood throughout the body. Cardiac myosin binding protein-C (cMyBP-C) is a thick-filament protein, and mutations in cMyBP-C are frequently linked with clinical cases of HCM. Within the sarcomere, the N-terminus of cMyBP-C likely interacts with the myosin regulatory light chain (RLC); RLC is a subunit of myosin located within the myosin neck region that modulates contractile dynamics via its phosphorylation state. Phosphorylation of RLC is thought to influence myosin head position along the thick-filament backbone, making it more favorable to bind the thin filament of actin and facilitate force production. However, little is known about how these two proteins interact. We tested the effects of RLC phosphorylation on Ca²⁺-regulated contractility using biomechanical assays on skinned papillary muscle strips isolated from cMyBP-C KO mice and WT mice. RLC phosphorylation increased Ca²⁺ sensitivity of contraction (i.e., pCa₅₀) from 5.80 ± 0.02 to 5.95 ± 0.03 in WT strips, whereas RLC phosphorylation increased Ca²⁺ sensitivity of contraction from 5.86 ± 0.02 to 6.15 ± 0.03 in cMyBP-C KO strips. These data suggest that the effects of RLC phosphorylation on Ca²⁺ sensitivity of contraction are amplified when cMyBP-C is absent from the sarcomere. This implies that cMyBP-C and RLC act in concert to regulate contractility in healthy hearts, and mutations to these proteins that lead to HCM (or a loss of phosphorylation with disease progression) may disrupt important interactions between these thick-filament regulatory proteins.