The Functional Consequences of Hypertrophic Cardiomyopathy Troponin C Mutations in the Regulation of Slow Skeletal Muscle Contraction: The Protective Role of Slow Skeletal Troponin I

Abstract

Mutations in cardiac troponin C (cTnC) have been linked to hypertrophic cardiomyopathy (HCM) in humans. The cTnC subunit of the troponin complex is expressed in both slow skeletal and cardiac muscle. The cTnC HCM mutations A8V, C84Y and D145E increase the Ca2+-sensitivity of contraction in cardiac muscle; however, their functional consequences in slow skeletal muscle regulation are unknown. Here we investigated the Ca2+-sensitivity of TnC-extracted rabbit soleus skinned fibers reconstituted with either WT or HCM mutant cTnC. Surprisingly, the slow skeletal skinned fibers extinguished the Ca2+-sensitization typical of cardiac fibers for A8V (pCa50 = 6.01) and D145E (pCa50 = 6.05), while for C84Y (pCa50 = 6.33) the Ca2+-sensitization was maintained compared to WT (pCa50 = 6.00). To determine whether slow skeletal TnI (ssTnI) alone was responsible for the protective effect of incorporating the A8V and D145E mutants into the slow skeletal myofilaments, cardiac skinned fibers were reconstituted with a hybrid cardiac troponin complex containing cTnT, ssTnI and cTnC-WT or -HCM mutants. The presence of ssTnI in cardiac fibers partially protected against Ca2+-sensitization by the cTnC mutants in cardiac skinned fibers, with the exception of C84Y. MgATPase activity of slow or cardiac myofibrils replaced with exogenous HCM cTnC mutants were also affected. The absence of increased Ca2+-sensitivity of contraction arising from the A8V and D145E in the soleus muscle suggests that the aberrant effects of cardiomyopathic cTnCs may be tissue-specific, and ssTnI is an important partner that partially protects the slow muscle from a deleterious effect of the HCM cTnC mutants. Supported by NIH HL103840 (JRP).