Tropomyosin-Kappa Alters Cardiac Dynamics in a Mouse Heart Model

Abstract

Tropomyosin-kappa (TPM1κ), a novel TM isoform, is exclusively expressed in the human heart. Alternative splicing of the α-TM gene generates TPM1κ, in which the skeletal muscle exon 2b is replaced by the smooth muscle exon 2a. We previously reported that TPM1κ expression was increased in the hearts of patients with chronic dilated cardiomyopathy (DCM). To understand how the TPM1κ isoform affects cardiac dynamics, we generated transgenic (TG) mice expressing TPM1κ in the myocardium. Most of the native TM (90%) is replaced by TPM1κ. In situ cardiac dynamics were determined by echocardiographic analysis. Results demonstrated that the TG hearts exhibited a diastolic dysfunction associated with a dilation of the left ventricle compared with the non transgenic (NTG) controls. We also compared force-pCa relations in detergent extracted (skinned) fiber bundles isolated from NTG and TG-TPM1κ hearts at sarcomere lengths (SL) 1.9 μm and 2.3µm. Our data demonstrated a significant decrease in the Ca2+ sensitivity of the myofilaments from TG hearts at both SL compared to NTG ones. There was an equal leftward shift of the force-pCa curves from both NTG and TG hearts when SL was increased to 2.3 µm. In addition, NEM-S1, a mimic of strongly bound, rigor cross-bridges, was not able to induce activation of TG myofilaments to the same extent as in the NTG controls. To determine whether isoform switching affects sarcomeric protein phosphorylation, we performed two-dimensional difference in gel electrophoresis (2D-DIGE) and Western blots using TM, TPM1κ, and Serine-283P specific antibodies. We observed an increase in the total phosphorylation of TPM1κ compared with that of α-TM. MLC2, TnI, and TnT phosphorylation was not significantly affected. Our results demonstrate that the increased cardiac expression of TPM1κ alters cardiac dynamics in a similar way to DCM-linked point mutations of TM.