Reactive oxygen species (ROS) contribute to the etiology of multiple muscle-related diseases. There is emerging evidence that cellular stress can lead to destabilization of sarcomeres, the contractile unit of muscle. However, it is incompletely understood how cellular stress induces structural destabilization of sarcomeres. Here we report that glutathionylation of SMYD2 contributes to a loss of myofibril integrity and degradation of sarcomeric proteins mediated by MMP-2 and calpain 1. We used a clickable glutathione approach in a cardiomyocyte cell line and found selective glutathionylation of SMYD2 at Cys13. Biochemical analysis demonstrated that SMYD2 upon oxidation or glutathionylation at Cys13 loses its interaction with Hsp90 and N2A, a domain of titin. Upon dissociation from SMYD2, N2A or titin is degraded by activated MMP-2, suggesting a protective role of SMYD2 in sarcomere stability. Taken together, our results support that SMYD2 glutathionylation is a novel molecular mechanism by which ROS contribute to sarcomere destabilization.

A sedentary lifestyle is associated with increased cardiovascular risk factors and reduced cardiac compliance when compared to a lifestyle that includes exercise training. Exercise training increases cardiac compliance in humans, but the mechanisms underlying this improvement are unknown. A major determinant of cardiac compliance is the compliance of the giant elastic protein titin. Experimentally reducing titin compliance in animal models reduces exercise tolerance, but it is not known whether sedentary versus chronic exercise conditions cause differences in titin isoform content. We hypothesized that sedentary conditions would be associated with a reduction in the content of the longer, more compliant N2BA isoform relative to the stiffer N2B isoform (yielding a reduced N2BA:N2B ratio) compared to age-matched exercising controls. We obtained left ventricles from 16-week old rats housed for 12 weeks in standard (sedentary) or voluntary running wheel (exercised) housing. The N2BA:N2B ratio was decreased in the hearts of sedentary versus active rats (p = 0.041). Gene expression of a titin mRNA splicing factor, RNA Binding Motif 20 protein (RBM20), correlated negatively with N2BA:N2B ratios (p = 0.006, r = −0.449), but was not different between groups, suggesting that RBM20 may be regulated post-transcriptionally. Total phosphorylation of cardiac titin was not different between the active and sedentary groups. This study is the first to demonstrate that sedentary rats exhibit reduced cardiac titin N2BA:N2B isoform ratios, which implies reduced cardiac compliance. These data suggest that a lack of exercise (running wheel) reduces cardiac compliance and that exercise itself increases cardiac compliance.

Background Approximately 1 in 6 adolescents report regular binge alcohol consumption, and we hypothesize it affects heart growth during this period. Methods and Results Adolescent, genetically diverse, male Wistar rats were gavaged with water or ethanol once per day for 6 days. In vivo structure and function were assessed before and after exposure. Binge alcohol exposure in adolescence significantly impaired normal cardiac growth but did not affect whole‐body growth during adolescence, therefore this pathology was specific to the heart. Binge rats also exhibited signs of accelerated pathological growth (concentric cellular hypertrophy and thickening of the myocardial wall), suggesting a global reorientation from physiologic to pathologic growth. Binge rats compensated for their smaller filling volumes by increasing systolic function and sympathetic stimulation. Consequently, binge alcohol exposure increased PKA (protein kinase A) phosphorylation of troponin I, inducing myofilament calcium desensitization. Binge alcohol also impaired in vivo relaxation and increased titin‐based cellular stiffness due to titin phosphorylation by PKCα (protein kinase C α). Mechanistically, alcohol inhibited extracellular signal‐related kinase activity, a nodal signaling kinase activating physiology hypertrophy. Thus, binge alcohol exposure depressed genes involved in growth. These cardiac structural alterations from binge alcohol exposure persisted through adolescence even after cessation of ethanol exposure. Conclusions Alcohol negatively impacts function in the adult heart, but the adolescent heart is substantially more sensitive to its effects. This difference is likely because adolescent binge alcohol impedes the normal rapid physiological growth and reorients it towards pathological hypertrophy. Many adolescents regularly binge alcohol, and here we report a novel pathological consequence as well as mechanisms involved.

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mutations have been characterized in the context of cardiomyopathy pathogenesis, the precise role of individual proteins in regulating length dependence of force remains unclear. Here, we used previously characterized point mutations of regulatory proteins to probe the thin filament and elucidate the role of tropomyosin in modulating the length dependence of cardiac contractility. Twitch amplitude was measured at short ($2.0mm) and long ($2.3mm) sarcomere lengths (SL) of intact cardiac trabeculae from hearts of a transgenic murine model containing a dilated cardiomyopathy-associated Tpm mutation (D230N; denoted Tpm D230N ). Trabeculae were mounted between a force transducer and length-controlling motor, perfused with oxygenated physiological solution (30 C), and electrically stimulated at 1 Hz. At short SL, twitch force between wild-type (WT) and Tpm D230N trabeculae were not significantly different (2454 versus 1653 mN/mm 2 , respectively). At long SL, WT trabeculae produced significantly higher twitch force compared to Tpm D230N (5156 versus 2753 mN/mm 2 , respectively), demonstrating reduced length-dependent augmentation of contractility in Tpm D230N trabeculae. We hypothesized that this is due to reduced azimuthal displacement of Tpm D230N along the thin filament, limiting effects of cross-bridge-mediated cooperative activation at longer SL. Thus, we incorporated an engineered calcium-sensitizing troponin C mutation (L48Q, denoted TnC L48Q ) to aid thin filament activation by developing a Tpm D230N /TnC L48Q double mutant murine model. Twitch forces in intact trabeculae from Tpm D230N /TnC L48Q mice were increased compared to Tpm D230N and were not significantly different than WT at short and long SL (3054 and 4656 mN/mm 2 , respectively). Our results suggest that tropomyosin plays a unique role in modulating the length dependence of contractility in cardiac muscle. 1561-PosWe study the regulation of cardiac contractility by using SAXS-USAXS at the ID02-beamline of the European Synchrotron (ESRF, Grenoble, France) on intact trabeculae isolated from the rat ventricle to record both the nanometerscale X-ray signals from the contractile proteins along the thin (actin) and thick (myosin) filaments and the changes of the sarcomere length (SL). Previously we demonstrated that in diastole (external [Ca 2þ ] 2.5 mM, 27 C) most of the myosin motors are in the off-state (unavailable for actin binding and ATP hydrolysis), packed into helical tracks with 43-nm periodicity on the surface of the thick filament, and that the fraction of myosin motors leaving the off-state during the twitch depends on the load through a rapid positive feedback based on thick-filament mechano-sensing (Reconditi et al. PNAS 114:3240, 2017). This regulatory mechanism occurs downstream with respect to the Ca 2þ -dependent thin-filament activation which controls cardiac contractility via the intracellular [Ca 2þ ] and the Ca 2þ -sensitivity of the filaments. Here we tested the interdependency of the two regulatory mechanisms by recording the X-r...