Hendrikus L Granzier

Familial hypertrophic cardiomyopathy (HCM) is a disease of the sarcomere and may lead to hypertrophic, dilated, restrictive, and/or arrhythmogenic cardiomyopathy, congestive heart failure, or sudden cardiac death. We hypothesized that hearts from transgenic HCM mice harboring a mutant myosin heavy chain increase the energetic cost of contraction in a sex-specific manner. To do this, we assessed Ca(2+) sensitivity of tension and crossbridge kinetics in demembranated cardiac trabeculas from male and female wild-type (WT) and HCM hearts at an early time point (2 mo of age). We found a significant effect of sex on Ca(2+) sensitivity such that male, but not female, HCM mice displayed a decrease in Ca(2+) sensitivity compared with WT counterparts. The HCM transgene and sex significantly impacted the rate of force redevelopment by a rapid release-restretch protocol and tension cost by the ATPase-tension relationship. In each of these measures, HCM male trabeculas displayed a gain-of-function when compared with WT counterparts. In addition, cardiac remodeling measured by echocardiography, histology, morphometry, and posttranslational modifications demonstrated sex- and HCM-specific effects. In conclusion, female and male HCM mice display sex dimorphic crossbridge kinetics accompanied by sex- and HCM-dependent cardiac remodeling at the morphometric, histological, and cellular level.

This review covers recent developments in the titin field. Most recent reviews have discussed titin's role in cardiac function: here we will mainly focus on skeletal muscle, and discuss recent advances in the understanding of titin's role in skeletal muscle function and disease.

Right ventricular (RV) diastolic function is impaired in patients with pulmonary arterial hypertension (PAH). Our previous study showed that elevated cardiomyocyte stiffness and myofilament Ca(2+) sensitivity underlie diastolic dysfunction in PAH. This study investigates protein modifications contributing to cellular diastolic dysfunction in PAH.

Previous work suggested that altered Ca(2+) homeostasis might contribute to dysfunction of nebulin-free muscle, as gene expression analysis revealed that the sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA)-inhibitor sarcolipin (SLN) is up-regulated >70-fold in nebulin knockout mice, and here we tested this proposal. We investigated SLN protein expression in nebulin-free and wild-type skeletal muscle, as well as expression of other Ca(2+)-handling proteins. Ca(2+) uptake capacity was determined in isolated sarcoplasmic reticulum vesicles and in intact myofibers by measuring Ca(2+) transients. Muscle contractile performance was determined in skinned muscle activated with exogenous Ca(2+), as well as in electrically stimulated intact muscle. We found profound up-regulation of SLN protein in nebulin-free skeletal muscle, whereas expression of other Ca(2+)-handling proteins was not (calsequestrin and phospholamban) or was minimally (SERCA) affected. Speed of Ca(2+) uptake was >3-fold decreased in sarcoplasmic reticulum vesicles isolated from nebulin-free muscle as well as in nebulin-free intact myofibers. Ca(2+)-activated stress in skinned muscle and stress produced by intact nebulin-free muscle were reduced to a similar extent compared with wild type. Half-relaxation time was significantly longer in nebulin-free compared with wild-type muscle. Thus, the present study demonstrates for the first time that nebulin might also be involved in physiological Ca(2+) handling of the SR-myofibrillar system.

Arginylation is a posttranslational modification that plays a global role in mammals. Mice lacking the enzyme arginyltransferase in skeletal muscles exhibit reduced contractile forces that have been linked to a reduction in myosin cross-bridge formation. The role of arginylation in passive skeletal myofibril forces has never been investigated. In this study, we used single sarcomere and myofibril measurements and observed that lack of arginylation leads to a pronounced reduction in passive forces in skeletal muscles. Mass spectrometry indicated that skeletal muscle titin, the protein primarily linked to passive force generation, is arginylated on five sites located within the A band, an important area for protein-protein interactions. We propose a mechanism for passive force regulation by arginylation through modulation of protein-protein binding between the titin molecule and the thick filament. Key points are as follows: 1) active and passive forces were decreased in myofibrils and single sarcomeres isolated from muscles lacking arginyl-tRNA-protein transferase (ATE1). 2) Mass spectrometry revealed five sites for arginylation within titin molecules. All sites are located within the A-band portion of titin, an important region for protein-protein interactions. 3) Our data suggest that arginylation of titin is required for proper passive force development in skeletal muscles.