Deletion of 1- 43 Amino Acids from the N-terminus of Myosin Essential Light Chain in Transgenic Mice Decreases Force Production by Reducing the Number of Myosin Cross-Bridges

Abstract

The N-terminus of the myosin essential light chain (ELC) has been shown to regulate myosin motor function by binding to actin and thus influencing the cross-bridge cycling kinetics. However, the role of the N-terminus of ELC in the Ca2+-regulation and the muscle length-dependent aspects of cardiac contractile function are still to be determined. In this study, we used transgenic (Tg) mouse hearts expressing the human ventricular ELC wild-type (Tg-WT) or a 43 amino acid deletion mutant of the human ventricular ELC (Tg-Δ43) (Kazmierczak et al., JMB 387, 706-725, 2009). We simultaneously measured force and ATPase activity at various levels of Ca2+ -activation under isometric conditions and the length-dependent force responses during constant activation in detergent-skinned papillary muscle fibers from the Tg-Δ43 and Tg-WT mice. As was shown by Kazmierczak et al., force was significantly lower in fibers from Tg-Δ43 mice, which was comparable to the decrease in ATPase activity, resulting in a tension cost that was not significantly different from Tg-WT fibers. However, our current study on fibers from 5-month old female Tg-Δ43 mice shows a small but significantly higher Ca2+ sensitivity when compared to Tg-WT mice. In addition, the relationship between the maximal tension and stiffness demonstrates that the average force production per cross-bridge is not altered in Tg-Δ43 fibers. Our findings demonstrate that the reduction in the Ca2+ -activated force in Tg-Δ43 fibers is most likely due to a decreased number of force-bearing myosin cross-bridges. Further studies are focused on gaining a better understanding of the functional significance of the N-terminus of cardiac ELC in mediating mechanisms of contractile activation.Supported by NIH-(MC) and NIH-HL071778 (DSC).