Abstract 16614: Myosin Light Chain Phosphorylation is Critical for Adaptation to Cardiac Stress
Background: The dynamics of cardiac contraction and relaxation are fundamentally related to the dynamics of actin-myosin interactions, which are regulated by a range of factors. Phosphorylation of cardiac myosin light chain 2 (MLC2v), bound to myosin at the head-rod junction, facilitates actin-myosin interactions and enhances contractility. We previously identified an enzyme that predominantly phosphorylates MLC2v in cardiomyocytes, cardiac-MLCK (cMLCK); yet the role(s) played by cMLCK in regulating cardiac function under physiological and pathological conditions remain to be determined.
Methods and Results: Mice were subjected to transverse aortic constriction (TAC) and swim training, and we performed functional (MRI and echocardiogram), cellular (cell shortening and intracellular free calcium) and proteomic and molecular analyses. One week of TAC in wild-type mice reduced phosphorylated(p)-MLC2v levels by ∼40% and cMLCK levels by ∼85%. In contrast, 4 weeks of swim training induced pMLC2v by ∼40% and cMLCK protein by ∼75% in wild-type mice. To examine potential roles of cMLCK and pMLC2v in hypertrophic stress, we generated Mylk3/cMLCK gene-targeted mice and transgenic (TG) mice overexpressing cMLCK in cardiomyocytes. Pressure-overload for 3 months led to severe heart failure in Mylk3-/- mice (%fractional shortening [FS], 21.4% in +/+ vs. 9.5% in -/-, n=7 and 5), but not in mice with cMLCK overexpression in which cMLCK protein synthesis exceeded degradation (%FS, 19.2% in non-transgenic [NTG] vs. 31.7% in TG, n=6 and 5). Swim exercise increased mortality (0% in +/+ vs. 25% in -/-, n=11 vs. 12) and reduced cardiac contraction in Mylk3-/- mice (%FS, 31.0% in +/+ vs. 19.2% in -/-, n=11 and 9). We also found that the reduction in cMLCK protein during pressure-overload was attenuated by inhibition of ubiquitin-proteasome protein degradation systems using MG132 and/or lactacystine (1 mg/kg/day).
Conclusion: The results of our study suggest that accelerated cMLCK-protein turnover by the ubiquitin-proteasome system represents a novel mechanism that underlies the transition to heart failure in a setting of pressure overload. In addition, the induction of cMLCK is critical for the beneficial cardiac adaptive response to exercise stress.
- © 2012 by American Heart Association, Inc.