The structure of the native cardiac thin filament at systolic Ca2+ levels

Every heartbeat relies on cyclical interactions between myosin thick and actin thin filaments orchestrated by rising and falling Ca2+ levels. Thin filaments are comprised of two actin strands, each harboring equally separated troponin complexes, which bind Ca2+ to move tropomyosin cables away from the myosin binding sites and, thus, activate systolic contraction. Recently, structures of thin filaments obtained at low (pCa ∼9) or high (pCa ∼3) Ca2+ levels revealed the transition between the Ca2+-free and Ca2+-bound states. However, in working cardiac muscle, Ca2+ levels fluctuate at intermediate values between pCa ∼6 and pCa ∼7. The structure of the thin filament at physiological Ca2+ levels is unknown. We used cryoelectron microscopy and statistical analysis to reveal the structure of the cardiac thin filament at systolic pCa = 5.8. We show that the two strands of the thin filament consist of a mixture of regulatory units, which are composed of Ca2+-free, Ca2+-bound, or mixed (e.g., Ca2+ free on one side and Ca2+ bound on the other side) troponin complexes. We traced troponin complex conformations along and across individual thin filaments to directly determine the structural composition of the cardiac native thin filament at systolic Ca2+ levels. We demonstrate that the two thin filament strands are activated stochastically with short-range cooperativity evident only on one of the two strands. Our findings suggest a mechanism by which cardiac muscle is regulated by narrow range Ca2+ fluctuations.

Analysis of the causes of discrepancies in troponin I concentrations as measured by ARCHITECT High-Sensitive Troponin I ST and STACIA CLEIA cTnI

Background Recently developed reagents for the highly sensitive measurement of cardiac troponin I are useful for early diagnosis of acute coronary syndrome. However, differences in measured values between these new reagents and previously used reagents have not been well studied. In this study, we aimed to compare the values between ARCHITECT High-Sensitive Troponin I ST (newly developed reagents), ARCHITECT Troponin I ST and STACIA CLEIA cardiac troponin I (two previously developed reagent kits). Methods Gel filtration high-performance liquid chromatography was used to analyse the causes of differences in measured values. Results The measured values differed between ARCHITECT High-Sensitive Troponin I ST and STACIA CLEIA cardiac troponin I reagents (r = 0.82). Cross-reactivity tests using plasma with added skeletal-muscle troponin I resulted in higher reactivity (2.17-3.03%) for the STACIA CLEIA cardiac troponin I reagents compared with that for the ARCHITECT High-Sensitive Troponin I ST reagents (less than 0.014%). In addition, analysis of three representative samples using gel filtration high-performance liquid chromatography revealed reagent-specific differences in the reactivity against each cardiac troponin I complex; this could explain the differences in values observed for some of the samples. Conclusion The newly developed ARCHITECT High-Sensitive Troponin I ST reagents were not affected by the presence of skeletal-muscle troponin I in the blood and may be useful for routine examinations.

Fast skeletal muscle troponin activator in the dy2J muscular dystrophy model

INTRODUCTION

Tirasemtiv is a novel small molecule activator of the fast skeletal muscle troponin complex that produces sensitization of the sarcomere to calcium. Tirasemtiv is currently in Phase II clinical trials for neuromuscular disease.

METHODS

We conducted a blinded, randomized, placebo-controlled preclinical study of the effect of tirasemtiv on forearm grip strength, endurance, respiratory physiology, and muscle pathology in adequate sample sizes of the Lama2(dy-2J) mouse model of congenital muscular dystrophy.

RESULTS

Mice receiving a high dose of tirasemtiv had significantly higher muscle fiber cross-sectional area and respiratory response to CO₂ stimulation at 16 weeks than mice on low dose or placebo. There were no changes in muscle pathology, serum creatine kinase, strength, endurance, or respiration following long-term treatment.

CONCLUSIONS

We conclude that tirasemtiv influences the structure of the skeletal muscle fiber in this model of muscular dystrophy but does not impact muscle function, as evaluated in this study.