Rescuing Myopathic Phenotype of Abnormal Cardiac Troponin T with a Single Amino Acid Substitution in Cardiac Troponin I

Abstract

Troponin T (TnT) and troponin I (TnI) are two subunits of the troponin complex. We previously found a single amino acid substitution in the TnT-binding helix of cardiac TnI (cTnI) in wild turkey hearts that expressed abnormally spliced myopathic cardiac TnT (cTnT) (Biesiadecki et al., JBC 279:13825-32, 2004). To test the potential role of this cTnI modification in rescuing a cTnT abnormality, we developed transgenic mice expressing the cTnI variant (K118C) with or without a deletion of endogenous cTnI gene to mimic homozygote and heterozygote wild turkeys. Double and triple transgenic mice were created to combine the cTnI-K118C allele with an allele encoding the abnormally spliced cTnT (exon 7 deletion). Functional analysis in ex vivo working hearts found that cTnI-K118C had no destructive effect on cardiac muscle and baseline heart function but was able to rescue the decreases in cardiac function caused by cTnT exon 7 deletion. Further characterizations showed that cTnI-K118C significantly blunted the inotropic response of cardiac muscle to β-adrenergic stimulation whereas the PKA-dependent phosphorylation of cTnI was unchanged. These data indicate that TnI-TnT interaction is a critical link in Ca2+ signaling and β-adrenergic regulation of myocardial contraction, providing a novel target for the treatment of heart failure.