Ca2+ sensitivity changes in skinned myocardial fibers induced by myosin–actin crossbridge-independent sarcomere stretch: Role of N-domain of MyBP-C

Sarcomere length-dependent activation (LDA) is the key cellular mechanism underlying the Frank-Starling law of the heart, in which sarcomere stretch leads to increased Ca²⁺ sensitivity of myofilament and force of contraction. Despite its key role in both normal and pathological states, the precise mechanisms underlying LDA remain unclear but are thought to involve multiple interactions among sarcomere proteins, including troponin of the thin filament, myosin, titin and myosin binding protein C (MyBP-C). Our previous study with permeabilized rat cardiac fibers demonstrated that the mechanism underlying the increase in Ca²⁺ sensitivity of thin filament induced by sarcomere stretch may involve sarcomere length (SL)-induced interactions between troponin and weakly bound, disordered relaxed state (DRX) myosin heads in diastole, rather than strong myosin-actin crossbridge interactions. In this study we investigated the role of the N-domains of MyBP-C in this newly discovered mechanism. To examine the potential role of the N-domain of MyBP-C in SL-induced myosin-troponin interactions, skinned myocardial fibers from a transgenic ΔN-MyBP-C rat with deleted N-terminal C0-C2 domains and a non-transgenic rat were reconstituted with troponin containing wild-type cTnT, cTnC(13C/51C)_AEDANS-DDPM and mutant ΔSP-cTnI or wild-type cTnI. Because the switching peptide (SP) of ΔS-cTnI is replaced by a nonfunctional peptide linker, force-generating actin-myosin crossbridge interactions of the reconstituted skinned fibers with mutant ΔSP-cTnI are inhibited regardless of the presence of Ca²⁺. This approach allowed us to examine the sensitivity of troponin/thin filament to Ca²⁺ binding in response to sarcomere stretch by monitoring Ca²⁺-induced changes in fluorescence resonance energy transfer (FRET) between AEDANS and DDPM attached to the N-domain of cTnC in the presence/absence of myosin-actin crossbridge interaction with or without deletion of C0-C2 domains of MyBP-C. Our measurements of SL-induced changes in muscle fiber mechanics and FRET Ca²⁺ sensitivities provide strong evidence that both the weakly bound myosin heads and the N-terminus of MyBP-C are critical for SL to activate troponin in the diastolic state. A model based on the results is proposed for the mechanism underlying LDA of myofilament.