Abstract
Thin filament hypertrophic cardiomyopathy (HCM) mutations increase myofilament Ca2+ sensitivity and alter Ca2+ handling and buffering. The myosin inhibitor mavacamten reverses the increased contractility caused by HCM thick filament mutations, and we here test its effect on HCM thin filament mutations where the mode of action is not known. Mavacamten (250 nM) partially reversed the increased Ca2+ sensitivity caused by HCM mutations Cardiac troponin T (cTnT) R92Q, and cardiac troponin I (cTnI) R145G in in vitro ATPase assays. The effect of mavacamten was also analyzed in cardiomyocyte models of cTnT R92Q and cTnI R145G containing cytoplasmic and myofilament specific Ca2+ sensors. While mavacamten rescued the hypercontracted basal sarcomere length, the reduced fractional shortening did not improve with mavacamten. Both mutations caused an increase in peak systolic Ca2+ detected at the myofilament, and this was completely rescued by 250 nM mavacamten. Systolic Ca2+ detected by the cytoplasmic sensor was also reduced by mavacamten treatment, although only R145G increased cytoplasmic Ca2+. There was also a reversal of Ca2+ decay time prolongation caused by both mutations at the myofilament but not in the cytoplasm. We thus show that mavacamten reverses some of the Ca2+-sensitive molecular and cellular changes caused by the HCM mutations, particularly altered Ca2+ flux at the myofilament. The reduction of peak systolic Ca2+ as a consequence of mavacamten treatment represents a novel mechanism by which the compound is able to reduce contractility, working synergistically with its direct effect on the myosin motor.NEW & NOTEWORTHY Mavacamten, a myosin inhibitor, is currently in phase-3 clinical trials as a pharmacotherapy for hypertrophic cardiomyopathy (HCM). Its efficacy in HCM caused by mutations in thin filament proteins is not known. We show in reductionist and cellular models that mavacamten can rescue the effects of thin filament mutations on calcium sensitivity and calcium handling although it only partially rescues the contractile cellular phenotype and, in some cases, exacerbates the effect of the mutation.
Highlights
Hypertrophic cardiomyopathy (HCM) is the most common inherited cardiac disease, globally affecting 1 in 500 people [10]
This study aims to assess this in a pairwise manner for the first time by testing the direct effect of mavacamten on HCM mutations [cardiac troponin T R92Q, and cardiac troponin I R145G] in the reductionist in vitro actomyosin ATPase assay and in an appropriate cellular model of transduced guinea pig cardiomyocytes [17]
To assess the efficacy of mavacamten on the Ca2ϩ regulation of actomyosin ATPase activity, we measured in vitro skeletal muscle myosin subfragment-1 (S-1) ATPase activity activated by reconstituted thin filaments containing either WT recombinant human troponin subunits or those containing the HCM causing mutations cardiac troponin T (cTnT) R92Q or cardiac troponin I (cTnI) R145G (Fig. 1)
Summary
Hypertrophic cardiomyopathy (HCM) is the most common inherited cardiac disease, globally affecting 1 in 500 people [10]. We have recently developed novel Ca2ϩ probes that report relative [Ca2ϩ] in distinct cytoplasmic and myofilament-specific pools [17]; using these in wild-type adult left ventricular cardiomyocytes, we have shown that mavacamten decreases the [Ca2ϩ] in the cytoplasm and at the myofilament in addition to increasing the Ca2ϩ release rate from the myofilament. This suggests that mavacamten might qualitatively reverse the altered Ca2ϩ handling observed in thin filament Ca2ϩ-sensitizing mutations [13].
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