Abstract 3853: Ventricular Myosin Light Chain-2 Phosphorylation (S14A/S15A) Mutant Knock-in Mice Display Novel Defects in Myocardial Torsion and Dilated Cardiomyopathy
Phosphorylation of ventricular myosin light chain 2 (MLC2v) has been implicated in regulating various aspects of muscle function in the heart, including cardiac torsion and contractile function. Mutations in MLC2v phosphorylation have been identified in patients with hypertrophic cardiomyopathy and dephosphorylation of MLC2v has been reported in hearts of patients with dilated cardiomyopathy, suggesting that loss of MLC2v phosphorylation is linked to loss of muscle function and development of cardiac disease. To understand the functional significance of endogenous MLC2v phosphorylation in vivo and to determine whether elimination of endogenous MLC2v phosphorylation is sufficient to induce cardiomyopathy, single (S15A) and double (S14A/S15A) MLC2v phosphorylation mutant knock-in mice were generated. Mutation of the single S15A MLC2v phosphorylation site was not sufficient to eliminate phosphorylation and instead augmented the ratio of phosphorylated to non-phosphorylated MLC2v in mouse hearts in vivo, suggesting redundancy amongst MLC2v phosphorylation sites in vivo. Consequently, these mutant mice did not exhibit any basal defects in cardiac morphology and function as well as survival. In contrast, double mutation of MLC2v phosphorylation sites S14A/S15A was sufficient and necessary to eliminate baseline endogenous cardiac MLC2v phosphorylation as well as result in dilated cardiomyopathy and premature death. The effects on cardiac morphology/function were not associated with early changes in the upregulation of classical hypertrophic markers but were associated with early defects in cardiac torsion that were predicted through computational modeling and subsequently detected in vivo. Interestingly, ventricles from both single (S15A) and double (S14A/S15A) MLC2v mutant mice displayed increased baseline levels of both total and phosphorylated cardiac troponin I, highlighting a potential adaptive response to a structurally defective myofilament. These studies altogether highlight the importance of MLC2v phosphorylation and its spatial gradient in normal ventricular function and disease in vivo as well as provide a model system to assess the effects of loss of MLC2v phosphorylation in cardiac function and disease.