Aberrant Cardiac Muscle Mechanics in a Hypertrophic Cardiomyopathy Troponin T ILE79ASN Transgenic Mouse

Abstract

The missense mutation Ile79Asn in human cardiac troponin T (HcTnT-I79N) has been associated with phenotypic outcomes of hypertrophic cardiomyopathy (HCM), arrhythmias, and sudden cardiac death. Previous characterization of an HcTnT-I79N transgenic mouse, which expresses 52% of the mutant protein, demonstrated increased Ca2+-sensitivity of isometric force. Little else is known about any changes in cardiac muscle mechanics caused by the mutation. This study investigates cross-bridge kinetics of skinned papillary muscle bundles (isometric force, sinusoidal stiffness and rate of tension redevelopment, kTR), post-translational modifications (PTM) in sarcomeric proteins, and in vivo cardiac function. For fiber mechanics and PTM analysis, cardiac muscle was isolated from the left ventricle of non-transgenic wild-type (WT) mice or transgenic mice bearing the HcTnT-I79N mutation. Sarcomere length (SL) was set at pCa8 using HeNe laser diffraction after the attachment points of each skinned fiber bundle were fixed with glutaraldehyde to minimize end compliance. Sinusoidal stiffness (0.2% PTP length oscillation) and kTR were obtained after recording steady-state isometric tension. As previously reported, the HcTnT-I79N mutation led to increased Ca2+-sensitivity of isometric force. Length-dependent activation at the longer SL (2.1 vs 1.9 µm) resulted in increased Ca2+-sensitivity of isometric force and sinusoidal stiffness and slower kTR for both WT and HcTnT-I79N. However, HcTnT-I79N exhibited slightly faster kTR and greater sinusoidal stiffness at sub-maximum Ca2+-activation (∼40% of maximal isometric force) compared to WT regardless of SL. These data indicate that the HCM HcTnT-I79N mutation, and SL, markedly influence the dynamics of individual regulatory units as well as the kinetics of cross-bridge cycling. Next steps include echocardiographic, sarcomeric protein phosphorylation and further fiber mechanic analyses. NIH-HL128683.