Abstract 17911: Acute Cardiac Dysfunction by Adult-onset Cardiac Myosin Light Chain Kinase Ablation

Abstract

Introduction: Under sustained pressure overload, myocytes that initially remodel to become short and thick, eventually become elongated and maladaptive with little force for contraction. Mechanisms underlying this transition are largely unknown. We previously demonstrated that 1 week of transverse aortic constriction in wild-type mice reduced cardiac myosin light chain kinase (cMLCK) levels by ~85%. In cultured cardiomyocytes, cMLCK knockdown led to sarcomere disorganization. Germline Mylk3 (encoding cMLCK) knockout mice demonstrated moderate heart failure. Hypothesis: Acute cMLCK reduction is causative for reduced contractility and remodeling of cardiomyocytes during the transition from compensated to decompensated hypertrophy. Methods: To mimic acute cMLCK reduction, adult inducible Mylk3 gene knockout mice, Mylk3flox/flox/merCremer, were generated and compared to 2 control mice (Mylk3flox/flox and Mylk3+/+/merCremer) following tamoxifen injection (50 mg/kg/day, 2 consecutive days). We assessed cardiac function (MRI, echocardiogram) in addition to histopathologic (tissue sectioning, immunostaining, and EM), cellular (cell size and cell shortening and intracellular free Ca2+) and molecular analyses. Results: Reduction of cMLCK proteins was evident at day 4 following tamoxifen-injection in inducible Mylk3 knockout mice. At day 7, inducible Mylk3 knockout but not control mice demonstrated heart failure [%FS, 28.0% in flox/flox (n=6), 27.7% in +/+/merCremer (n=6), 19.8% in flox/flox/merCremer (n=8)]. Analysis of inducible Mylk3 knockout mice revealed unique, severely convoluted cell morphology; isolated cardiomyocytes were elongated and thinner with disorganized sarcomere and reduced contractility (%FS in sarcomere length, control 5.1% vs. inducible knockout 2.4%). Adenovirus encoding cMLCK enhanced organized sarcomere structures; this was not prevented by co-infection of adenovirus unphosphorylatable MLC2v(Ser14/15Ala) mutants incorporated into sarcomere. Conclusion: Results of acute cMLCK reduction in a novel mouse model suggest that cMLCK plays a pivotal role in transition from compensated to decompensated hypertrophy via sarcomere disorganization likely independent of MLC2 phosphorylation.