An Internal Domain of Beta Tropomyosin Increases Myofilament Calcium Sensitivity

Abstract

Table of ContentsTropomyosin (TM) is involved in calcium mediated muscle contraction and relaxation in the heart. Striated muscle alpha TM is the major isoform expressed in the heart. Expression of striated muscle beta TM in murine myocardium results in a decreased rate of relaxation and increased myofilament calcium sensitivity. Replacing the carboxyl terminus (amino acids 258-284) of alpha TM for beta TM (a troponin-T [TnT] binding region) results in decreased rates of contraction and relaxation in the heart and decreased myofilament calcium sensitivity. We hypothesized that the putative internal TnT binding domain (amino acids 175-190) of beta TM may be responsible for the increased myofilament calcium sensitivity observed when the entire beta TM is expressed in the heart. To test this hypothesis, we generated transgenic mice that express a chimeric TM containing beta TM amino acids 175-190 in the backbone of alpha TM (amino acids 1-174 and 191-284). These mice express 16% - 57% chimeric TM, and they do not develop cardiac hypertrophy or any other morphological changes. Physiological analysis shows these hearts exhibit systolic and diastolic dysfunction and a positive response to isoproterenol. Skinned fiber bundle analyses show a significant increase in myofilament calcium sensitivity. Biophysical studies demonstrate that the exchanged amino acids do not influence the flexibility of TM. This is the first study to demonstrate that a specific domain within TM can increase calcium sensitivity of the thin filament. Further, these results enhance the understanding of why TM mutations associated with familial hypertrophic cardiomyopathy also demonstrate increased myofilament sensitivity to calcium.