TNNT2 mutations in the tropomyosin binding region of TNT1 disrupt its role in contractile inhibition and stimulate cardiac dysfunction.

Proceedings of the National Academy of Sciences of the United States of America 2020 Aug 04

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Muscle contraction is regulated by the movement of end-to-end-linked troponin-tropomyosin complexes over the thin filament surface, which uncovers or blocks myosin binding sites along F-actin. The N-terminal half of troponin T (TnT), TNT1, independently promotes tropomyosin-based, steric inhibition of acto-myosin associations, in vitro. Recent structural models additionally suggest TNT1 may restrain the uniform, regulatory translocation of tropomyosin. Therefore, TnT potentially contributes to striated muscle relaxation; however, the in vivo functional relevance and molecular basis of this noncanonical role remain unclear. Impaired relaxation is a hallmark of hypertrophic and restrictive cardiomyopathies (HCM and RCM). Investigating the effects of cardiomyopathy-causing mutations could help clarify TNT1's enigmatic inhibitory property. We tested the hypothesis that coupling of TNT1 with tropomyosin's end-to-end overlap region helps anchor tropomyosin to an inhibitory position on F-actin, where it deters myosin binding at rest, and that, correspondingly, cross-bridge cycling is defectively suppressed under diastolic/low Ca2+ conditions in the presence of HCM/RCM lesions. The impact of TNT1 mutations on Drosophila cardiac performance, rat myofibrillar and cardiomyocyte properties, and human TNT1's propensity to inhibit myosin-driven, F-actin-tropomyosin motility were evaluated. Our data collectively demonstrate that removing conserved, charged residues in TNT1's tropomyosin-binding domain impairs TnT's contribution to inhibitory tropomyosin positioning and relaxation. Thus, TNT1 may modulate acto-myosin activity by optimizing F-actin-tropomyosin interfacial contacts and by binding to actin, which restrict tropomyosin's movement to activating configurations. HCM/RCM mutations, therefore, highlight TNT1's essential role in contractile regulation by diminishing its tropomyosin-anchoring effects, potentially serving as the initial trigger of pathology in our animal models and humans.

Citation

Aditi Madan, Meera C Viswanathan, Kathleen C Woulfe, William Schmidt, Agnes Sidor, Ting Liu, Tran H Nguyen, Bosco Trinh, Cortney Wilson, Sineej Madathil, Georg Vogler, Brian O'Rourke, Brandon J Biesiadecki, Larry S Tobacman, Anthony Cammarato. TNNT2 mutations in the tropomyosin binding region of TNT1 disrupt its role in contractile inhibition and stimulate cardiac dysfunction. Proceedings of the National Academy of Sciences of the United States of America. 2020 Aug 04;117(31):18822-18831