The C-Terminal End Peptide of Troponin I as a Myofilament Ca2+Desensitizer

Abstract

The C-terminal end segment of troponin I (TnI) is a highly conserved structure. Mutation in this segment impairs cardiac muscle relaxation. This segment is a Ca2+-regulated allosteric structure in troponin complex and binds tropomyosin in low Ca2+state, indicating a role in the inhibitory activity of TnI during muscle relaxation. The C-terminal end peptide forms a conserved epitope recognized by a monoclonal antibody. We characterized the C-terminal 27 amino acid peptide of human cardiac TnI (HcTnI-C27) for its effect on modulating muscle contractility. Biologically or chemically synthesized HcTnI-C27 free peptide in physiological solution retains the native epitope recognized by the anti-TnI C-terminus monoclonal antibody. HcTnI-C27 peptide also retains the binding affinity for tropomyosin as that residing in intact TnI. A restrictive cardiomyopathy mutation inHcTnI-C27 peptide (R192H) abolishes its bindings to the anti-TnI C-terminus monoclonal antibody and tropomyosin, demonstrating a pathogenic destruction with loss of function. Contractility studies using skinned muscle preparations demonstrated that addition of free HcTnI-C27 peptide reduces Ca2+-sensitivity of myofibrils at high Ca2+state without decreasing the maximum force production. The results indicate that the C-terminal end segment of TnI is a regulatory element of troponin, which retains the native configuration in the form of free peptide to confer a physiological effect on myofilament Ca2+-desensitization. Without negative inotropic impact, this short peptide may be developed into a novel reagent to selectively facilitate cardiac muscle relaxation in the activated state as a potential treatment for diastolic dysfunction and heart failure. This notion is supported by previous findings that a conformational modulation of the T subunit of troponin (TnT) to add a TnI C-terminus-like element in the contractile machinery of transgenic mouse cardiac muscle selectively prolongs ventricular end systolic ejection time to increase stroke volume without increase in pressure development.