Interplay Between the Effects of Dilated Cardiomyopathy Mutation (R206L) and the Protein Kinase C Phosphomimic (T204E) of Rat Cardiac Troponin T Are Differently Modulated by α‐ and β‐Myosin Heavy Chain Isoforms

BACKGROUND: We hypothesized that the functional effects of R206L—a rat analog of the dilated cardiomyopathy (DCM) mutation R205L in human cardiac troponin T (TnT)—were differently modulated by myosin heavy chain (MHC) isoforms and T204E, a protein kinase C (PKC) phosphomimic of TnT. Our hypothesis was based on two observations: (1) α‐ and β‐MHC differentially influence the functional effects of TnT; and (2) PKC isoforms capable of phosphorylating TnT are upregulated in failing human hearts. METHODS AND RESULTS: We generated 4 recombinant TnT variants: wild type; R206L; T204E; and R206L+T204E. Functional effects of the TnT variants were tested in cardiac muscle fibers (minimum 14 per group) from normal (α‐MHC) and propylthiouracil‐treated rats (β‐MHC) using steady‐state and dynamic contractile measurements. Notably, in α‐MHC fibers, Ca(2+)‐activated maximal tension was attenuated by R206L (≈32%), T204E (≈63%), and R206L+T204E (≈64%). In β‐MHC fibers, maximal tension was unaffected by R206L, but was attenuated by T204E (≈33%) and R206L+T204E (≈40%). Thus, β‐MHC differentially counteracted the attenuating effects of the TnT variants on tension. However, in β‐MHC fibers, R206L+T204E attenuated tension to a greater extent when compared to T204E alone. In β‐MHC fibers, R206L+T204E attenuated the magnitude of the length‐mediated recruitment of new cross‐bridges (≈28%), suggesting that the Frank‐Starling mechanism was impaired. CONCLUSIONS: Our findings are the first (to our knowledge) to demonstrate that the functional effects of a DCM‐linked TnT mutation are not only modulated by MHC isoforms, but also by the pathology‐associated post‐translational modifications of TnT.