Abstract 1447: The Dilated Cardiomyopathy Gly 159 Asp Troponin C Mutation Abolishes the Effect of Troponin I PKA Phosphorylation in Skinned Trabeculae
Contraction of cardiac muscle is regulated by the thin filament through Ca2+ binding to the troponin C subunit of the troponin (Tn) complex, composed of troponin I (TnI), troponin T and troponin C (TnC). In vivo, cardiac contractility is increased by β-adrenergic stimulation. At the level of the thin filament, β-adrenergic stimulation results from the PKA phosphorylation of TnI at Ser-23 and Ser-24. Recently the Gly 159 to Asp (G159D) mutation of TnC was identified in a family with a history of dilated cardiomyopathy. To investigate the effect of the TnC(G159D) mutation on myofilament function, we exchanged recombinant whole human cardiac Tn complex containing wild type (WT) or TnC(G159D) into skinned rat trabeculae. Exchanged trabeculae were exposed to various Ca2+ concentrations and the force measured. Trabeculae exchanged with Tn containing TnC(G159D) exhibited a slight decrease in maximal force (Fmax) and cooperativity (Hill coefficient) without a shift in Ca2+ sensitivity (pCa50) compared to trabeculae exchanged with Tn containing TnC(WT). Introducing PKA pseudo-phosphorylation (S23D/S24D) of TnI into the Tn complex with TnC(WT) resulted in decreased Fmax and pCa50 as expected. Surprisingly, in trabeculae exchanged with Tn complex containing both TnC(G159D) and TnI(S23D/S24D), the effects of TnI phosphorylation were abolished and these trabeculae exhibited Fmax and hill coefficient similar to that of WT with a slightly increased pCa50. These results demonstrate that the G159D mutation of TnC only slightly alters force but completely eliminates the effect of TnI PKA phosphorylation in cardiac trabeculae. The abolishment of the PKA effect in Tn with TnC(G159D) and not the slight decrease in force of the G159D alone may be critical to alter contractility in vivo and causal to the development of cardiomyopathy.