The Troponin C G159D Mutation Blunts Myofilament Desensitization Induced by Troponin I Ser23/24 Phosphorylation

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Abstract
Striated muscle contraction is regulated by the binding of Ca2+ to the N-terminal regulatory lobe of the cardiac troponin C (cTnC) subunit in the troponin complex. In the heart, β-adrenergic stimulation induces protein kinase A phosphorylation of cardiac troponin I (cTnI) at Ser23/24 to alter the interaction of cTnI with cTnC in the troponin complex and is critical to the regulation of cardiac contractility. We investigated the effect of the dilated cardiomyopathy linked cTnC Gly159 to Asp (cTnC-G159D) mutation on the development of Ca2+-dependent tension and ATPase rate in whole troponin-exchanged skinned rat trabeculae. Even though this mutation is located in the C-terminal lobe of cTnC, the G159D mutation was demonstrated to depress ATPase activation and filament sliding in vitro. The effects of this mutation within the cardiac myofilament are unknown. Our results demonstrate that the cTnC-G159D mutation by itself does not alter the myofilament response to Ca2+ in the cardiac muscle lattice. However, in the presence of cTnI phosphorylated at Ser23/24, which reduced Ca2+ sensitivity and enhanced cross-bridge cycling in controls, cTnC-G159D specifically blunted the phosphorylation induced decrease in Ca2+-sensitive tension development without altering cross-bridge cycling. Measurements in purified troponin confirmed that this cTnC-G159D blunting of myofilament desensitization results from altered Ca2+-binding to cTnC. Our results provide novel evidence that modification of the cTnC-cTnI interaction has distinct effects on troponin Ca2+-binding and cross-bridge kinetics to suggest a novel role for thin filament mutations in the modulation of myofilament function through β-adrenergic signaling as well as the development of cardiomyopathy.