Sarcomere-based genetic enhancement of systolic cardiac function in a murine model of dilated cardiomyopathy

The D75N and P161S Mutations in the C0-C2 Fragment of cMyBP-C Associated with Hypertrophic Cardiomyopathy Disturb the Thin Filament Activation, Nucleotide Exchange in Myosin, and Actin-Myosin Interaction

About half of the mutations that lead to hypertrophic cardiomyopathy (HCM) occur in the MYBPC3 gene. However, the molecular mechanisms of pathogenicity of point mutations in cardiac myosin-binding protein C (cMyBP-C) remain poorly understood. In this study, we examined the effects of the D75N and P161S substitutions in the C0 and C1 domains of cMyBP-C on the structural and functional properties of the C0-C1-m-C2 fragment (C0-C2). Differential scanning calorimetry revealed that these mutations disorder the tertiary structure of the C0-C2 molecule. Functionally, the D75N mutation reduced the maximum sliding velocity of regulated thin filaments in an in vitro motility assay, while the P161S mutation increased it. Both mutations significantly reduced the calcium sensitivity of the actin-myosin interaction and impaired thin filament activation by cross-bridges. D75N and P161S C0-C2 fragments substantially decreased the sliding velocity of the F-actin-tropomyosin filament. ADP dose-dependently reduced filament sliding velocity in the presence of WT and P161S fragments, but the velocity remained unchanged with the D75N fragment. We suppose that the D75N mutation alters nucleotide exchange kinetics by decreasing ADP affinity to the ATPase pocket and slowing the myosin cycle. Our molecular dynamics simulations mean that the D75N mutation affects myosin S1 function. Both mutations impair cardiac contractility by disrupting thin filament activation. The results offer new insights into the HCM pathogenesis caused by missense mutations in N-terminal domains of cMyBP-C, highlighting the distinct effects of D75N and P161S mutations on cardiac contractile function.

The W792R HCM missense mutation in the C6 domain of cardiac myosin binding protein-C increases contractility in neonatal mouse myocardium

Missense mutations in cardiac myosin binding protein C (cMyBP-C) are known to cause hypertrophic cardiomyopathy (HCM). The W792R mutation in the C6 domain of cMyBP-C causes severe, early onset HCM in humans, yet its impact on the function of cMyBP-C and the mechanism through which it causes disease remain unknown. To fully characterize the effect of the W792R mutation on cardiac morphology and function in vivo, we generated a murine knock-in model. We crossed heterozygous W792RWR mice to produce homozygous mutant W792RRR, heterozygous W792RWR, and control W792RWW mice. W792RRR mice present with cardiac hypertrophy, myofibrillar disarray and fibrosis by postnatal day 10 (PND10), and do not survive past PND21. Full-length cMyBP-C is present at similar levels in W792RWW, W792RWR and W792RRR mice and is properly incorporated into the sarcomere. Heterozygous W792RWR mice displayed normal heart morphology and contractility. Permeabilized myocardium from PND10 W792RRR mice showed increased Ca2+ sensitivity, accelerated cross-bridge cycling kinetics, decreased cooperativity in the activation of force, and increased expression of hypertrophy-related genes. In silico modeling suggests that the W792R mutation destabilizes the fold of the C6 domain and increases torsion in the C5-C7 region, possibly impacting regulatory interactions of cMyBP-C with myosin and actin. Based on the data presented here, we propose a model in which mutant W792R cMyBP-C preferentially forms Ca2+ sensitizing interactions with actin, rather than inhibitory interactions with myosin. The W792R-cMyBP-C mouse model provides mechanistic insights into the pathology of this mutation and may provide a mechanism by which other central domain missense mutations in cMyBP-C may alter contractility, leading to HCM.

Torsional and strain dysfunction precede overt heart failure in a mouse model of dilated cardiomyopathy pathogenesis

Detailed assessments of whole heart mechanics are crucial for understanding the consequences of sarcomere perturbations that lead to cardiomyopathy in mice. Echocardiography offers an accessible and cost-effective method of obtaining metrics of cardiac function, but the most routine imaging and analysis protocols might not identify subtle mechanical deficiencies. This study aims to use advanced echocardiography imaging and analysis techniques to identify previously unappreciated mechanical deficiencies in a mouse model of dilated cardiomyopathy (DCM) before the onset of overt systolic heart failure (HF). Mice lacking muscle LIM protein expression (MLP-/-) were used to model DCM-linked HF pathogenesis. Left ventricular (LV) function of MLP-/- and wild-type (WT) controls were studied at 3, 6, and 10 wk of age using conventional and four-dimensional (4-D) echocardiography, followed by speckle-tracking analysis to assess torsional and strain mechanics. Mice were also studied with RNA-seq. Although 3-wk-old MLP-/- mice showed normal LV ejection fraction (LVEF), these mice displayed abnormal torsional and strain mechanics alongside reduced β-adrenergic reserve. Transcriptome analysis showed that these defects preceded most molecular markers of HF. However, these markers became upregulated as MLP-/- mice aged and developed overt systolic dysfunction. These findings indicate that subtle deficiencies in LV mechanics, undetected by LVEF and conventional molecular markers, may act as pathogenic stimuli in DCM-linked HF. Using these analyses in future studies will further help connect in vitro measurements of the sarcomere function to whole heart function.NEW & NOTEWORTHY A detailed study of how perturbations to sarcomere proteins impact whole heart mechanics in mouse models is a major yet challenging step in furthering our understanding of cardiovascular pathophysiology. This study uses advanced echocardiographic imaging and analysis techniques to reveal previously unappreciated subclinical whole heart mechanical defects in a mouse model of cardiomyopathy. In doing so, it offers an accessible set of measurements for future studies to use when connecting sarcomere and whole heart function.

The contribution of N-terminal truncated cMyBPC to in vivo cardiac function

Cardiac myosin binding protein C (cMyBPC) is an 11-domain sarcomeric protein (C0-C10) integral to cardiac muscle regulation. In vitro studies have demonstrated potential functional roles for regions beyond the N-terminus. However, the in vivo contributions of these domains are mostly unknown. Therefore, we examined the in vivo consequences of expression of N-terminal truncated cMyBPC (C3C10). Neonatal cMyBPC-/- mice were injected with AAV9-full length (FL), C3C10 cMyBPC, or saline, and echocardiography was performed 6 wk after injection. We then isolated skinned myocardium from virus-treated hearts and performed mechanical experiments. Our results show that expression of C3C10 cMyBPC in cMyBPC-/- mice resulted in a 28% increase in systolic ejection fraction compared to saline-injected cMyBPC-/- mice and a 25% decrease in left ventricle mass-to-body weight ratio. However, unlike expression of FL cMyBPC, there was no prolongation of ejection time compared to saline-injected mice. In vitro mechanical experiments demonstrated that functional improvements in cMyBPC-/- mice expressing C3C10 were primarily due to a 35% reduction in the rate of cross-bridge recruitment at submaximal Ca2+ concentrations when compared to hearts from saline-injected cMyBPC-/- mice. However, unlike the expression of FL cMyBPC, there was no change in the rate of cross-bridge detachment when compared to saline-injected mice. Our data demonstrate that regions of cMyBPC beyond the N-terminus are important for in vivo cardiac function, and have divergent effects on cross-bridge behavior. Elucidating the molecular mechanisms of cMyBPC region-specific function could allow for development of targeted approaches to manipulate specific aspects of cardiac contractile function.

Personalized Medicine Approach in a DCM Patient with LMNA Mutation Reveals Dysregulation of mTOR Signaling

Background: Mutations in the Lamin A/C (LMNA) gene are responsible for about 6% of all familial dilated cardiomyopathy (DCM) cases which tend to present at a young age and follow a fulminant course. Methods: We report a 47-year-old DCM patient with severely impaired left ventricular ejection fraction and NYHA functional class IV despite optimal heart failure treatment. Whole-exome sequencing revealed an LMNA E161K missense mutation as the pathogenetic cause for DCM in this patient. We generated a patient-specific LMNA-knock in (LMNA-KI) in vitro model using mES cells. Results: Beta adrenergic stimulation of cardiomyocytes derived from LMNA-KI mES cells resulted in augmented mTOR signaling and increased dysregulation of action potentials, which could be effectively prevented by the mTOR-inhibitor rapamycin. A cardiac biopsy confirmed strong activation of the mTOR-signaling pathway in the patient. An off-label treatment with oral rapamycin was initiated and resulted in an improvement in left ventricular ejection fraction (27.8% to 44.5%), NT-BNP (8120 ng/L to 2210 ng/L) and NYHA functional class. Conclusion: We have successfully generated the first in vitro model to recapitulate a patient-specific LMNA E161K mutation which leads to a severe form of DCM. The model may serve as a template for individualized and specific treatment of heart failure.

The Sliding Filament Theory Since Andrew Huxley: Multiscale and Multidisciplinary Muscle Research

Two groundbreaking papers published in 1954 laid out the theory of the mechanism of muscle contraction based on force-generating interactions between myofilaments in the sarcomere that cause filaments to slide past one another during muscle contraction. The succeeding decades of research in muscle physiology have revealed a unifying interest: to understand the multiscale processes-from atom to organ-that govern muscle function. Such an understanding would have profound consequences for a vast array of applications, from developing new biomimetic technologies to treating heart disease. However, connecting structural and functional properties that are relevant at one spatiotemporal scale to those that are relevant at other scales remains a great challenge. Through a lens of multiscale dynamics, we review in this article current and historical research in muscle physiology sparked by the sliding filament theory.

cMyBPC phosphorylation modulates the effect of omecamtiv mecarbil on myocardial force generation

Omecamtiv mecarbil (OM), a direct myosin motor activator, is currently being tested as a therapeutic replacement for conventional inotropes in heart failure (HF) patients. It is known that HF patients exhibit dysregulated β-adrenergic signaling and decreased cardiac myosin-binding protein C (cMyBPC) phosphorylation, a critical modulator of myocardial force generation. However, the functional effects of OM in conditions of altered cMyBPC phosphorylation have not been established. Here, we tested the effects of OM on force generation and cross-bridge (XB) kinetics using murine myocardial preparations isolated from wild-type (WT) hearts and from hearts expressing S273A, S282A, and S302A substitutions (SA) in the M domain, between the C1 and C2 domains of cMyBPC, which cannot be phosphorylated. At submaximal Ca2+ activations, OM-mediated force enhancements were less pronounced in SA than in WT myocardial preparations. Additionally, SA myocardial preparations lacked the dose-dependent increases in force that were observed in WT myocardial preparations. Following OM incubation, the basal differences in the rate of XB detachment (krel) between WT and SA myocardial preparations were abolished, suggesting that OM differentially affects the XB behavior when cMyBPC phosphorylation is reduced. Similarly, in myocardial preparations pretreated with protein kinase A to phosphorylate cMyBPC, incubation with OM significantly slowed krel in both the WT and SA myocardial preparations. Collectively, our data suggest there is a strong interplay between the effects of OM and XB behavior, such that it effectively uncouples the sarcomere from cMyBPC phosphorylation levels. Our findings imply that OM may significantly alter the in vivo cardiac response to β-adrenergic stimulation.

Modulating the tension-time integral of the cardiac twitch prevents dilated cardiomyopathy in murine hearts

Dilated cardiomyopathy (DCM) is often associated with sarcomere protein mutations that confer reduced myofilament tension–generating capacity. We demonstrated that cardiac twitch tension-time integrals can be targeted and tuned to prevent DCM remodeling in hearts with contractile dysfunction. We employed a transgenic murine model of DCM caused by the D230N-tropomyosin (Tm) mutation and designed a sarcomere-based intervention specifically targeting the twitch tension-time integral of D230N-Tm hearts using multiscale computational models of intramolecular and intermolecular interactions in the thin filament and cell-level contractile simulations. Our models predicted that increasing the calcium sensitivity of thin filament activation using the cardiac troponin C (cTnC) variant L48Q can sufficiently augment twitch tension-time integrals of D230N-Tm hearts. Indeed, cardiac muscle isolated from double-transgenic hearts expressing D230N-Tm and L48Q cTnC had increased calcium sensitivity of tension development and increased twitch tension-time integrals compared with preparations from hearts with D230N-Tm alone. Longitudinal echocardiographic measurements revealed that DTG hearts retained normal cardiac morphology and function, whereas D230N-Tm hearts developed progressive DCM. We present a computational and experimental framework for targeting molecular mechanisms governing the twitch tension of cardiomyopathic hearts to counteract putative mechanical drivers of adverse remodeling and open possibilities for tension-based treatments of genetic cardiomyopathies.

Tuning the molecular mechanisms that govern the twitch tension of cardiomyopathic hearts counteracts mechanical drivers of adverse remodeling.

AAV9 gene transfer of cMyBPC N-terminal domains ameliorates cardiomyopathy in cMyBPC-deficient mice

Decreased cardiac myosin-binding protein C (cMyBPC) expression due to inheritable mutations is thought to contribute to the hypertrophic cardiomyopathy (HCM) phenotype, suggesting that increasing cMyBPC content is of therapeutic benefit. In vitro assays show that cMyBPC N-terminal domains (NTDs) contain structural elements necessary and sufficient to modulate actomyosin interactions, but it is unknown if they can regulate in vivo myocardial function. To test whether NTDs can recapitulate the effects of full-length (FL) cMyBPC in rescuing cardiac function in a cMyBPC-null mouse model of HCM, we assessed the efficacy of AAV9 gene transfer of a cMyBPC NTD that contained domains C0C2 and compared its therapeutic potential with AAV9-FL gene replacement. AAV9 vectors were administered systemically at neonatal day 1, when early-onset disease phenotypes begin to manifest. A comprehensive analysis of in vivo and in vitro function was performed following cMyBPC gene transfer. Our results show that a systemic injection of AAV9-C0C2 significantly improved cardiac function (e.g., 52.24 ± 1.69 ejection fraction in the C0C2-treated group compared with 40.07 ± 1.97 in the control cMyBPC^–/– group, P < 0.05) and reduced the histopathologic signs of cardiomyopathy. Furthermore, C0C2 significantly slowed and normalized the accelerated cross-bridge kinetics found in cMyBPC^–/– control myocardium, as evidenced by a 32.41% decrease in the rate of cross-bridge detachment (k_rel). Results indicate that C0C2 can rescue biomechanical defects of cMyBPC deficiency and that the NTD may be a target region for therapeutic myofilament kinetic manipulation.

Cardiac function improves following AAV9-mediated delivery of the C0C2 domains of cardiac myosin-binding protein C in a mouse model of hypertrophic cardiomyopathy.

Strategies for targeting the cardiac sarcomere: avenues for novel drug discovery

Introduction: Heart failure remains one of the largest clinical challenges in the United States. Researchers have continually searched for more effective heart failure treatments that target the cardiac sarcomere but have found few successes despite numerous expensive cardiovascular clinical trials. Among many reasons, the high failure rate of cardiovascular clinical trials may be partly due to incomplete characterization of a drug candidate's complex interaction with cardiac physiology.Areas covered: In this review, the authors address the issue of preclinical cardiovascular studies of sarcomere-targeting heart failure therapies. The authors consider inherent tradeoffs made between mechanistic transparency and physiological fidelity for several relevant preclinical techniques at the atomic, molecular, heart muscle fiber, whole heart, and whole-organism levels. Thus, the authors suggest a comprehensive, bottom-up approach to preclinical cardiovascular studies that fosters scientific rigor and hypothesis-driven drug discovery.Expert opinion: In the authors' opinion, the implementation of hypothesis-driven drug discovery practices, such as the bottom-up approach to preclinical cardiovascular studies, will be imperative for the successful development of novel heart failure treatments. However, additional changes to clinical definitions of heart failure and current drug discovery culture must accompany the bottom-up approach to maximize the effectiveness of hypothesis-driven drug discovery.

Deletion of Enigma Homologue from the Z-disc slows tension development kinetics in mouse myocardium

Enigma Homologue (ENH) is a component of the Z-disc, a structure that anchors actin filaments in the contractile unit of muscle, the sarcomere. Cardiac-specific ablation of ENH protein expression causes contractile dysfunction that ultimately culminates in dilated cardiomyopathy. However, whether ENH is involved in the regulation of myocardial contractility is unknown. To determine if ENH is required for the mechanical activity of cardiac muscle, we analyze muscle mechanics of isolated trabeculae from the hearts of ENH ^+/+ and ENH ^-/- mice. We detected no differences in steady-state mechanical properties but show that when muscle fibers are allowed to relax and then are restretched, the rate at which tension redevelops is depressed in ENH ^-/- mouse myocardium relative to that in ENH ^+/+ myocardium. SDS-PAGE analysis demonstrated that the expression of β-myosin heavy chain is increased in ENH ^-/- mouse myocardium, which could partially, but not completely, account for the depression in tension redevelopment kinetics. Using top-down proteomics analysis, we found that the expression of other thin/thick filament regulatory proteins is unaltered, although the phosphorylation of a cardiac troponin T isoform, cardiac troponin I, and myosin regulatory light chain is decreased in ENH ^-/- mouse myocardium. Nevertheless, these alterations are very small and thus insufficient to explain slowed tension redevelopment kinetics in ENH ^-/- mouse myocardium. These data suggest that the ENH protein influences tension redevelopment kinetics in mouse myocardium, possibly by affecting cross-bridge cycling kinetics. Previous studies also indicate that ablation of specific Z-disc proteins in myocardium slows contraction kinetics, which could also be a contributing factor in this study.

The HCM-causing Y235S cMyBPC mutation accelerates contractile function by altering C1 domain structure

Mutations in cardiac myosin binding protein C (cMyBPC) are a major cause of hypertrophic cardiomyopathy (HCM). In particular, a single amino acid substitution of tyrosine to serine at residue 237 in humans (residue 235 in mice) has been linked to HCM with strong disease association. Although cMyBPC truncations, deletions and insertions, and frame shift mutations have been studied, relatively little is known about the functional consequences of missense mutations in cMyBPC. In this study, we characterized the functional and structural effects of the HCM-causing Y235S mutation by performing mechanical experiments and molecular dynamics simulations (MDS). cMyBPC null mouse myocardium was virally transfected with wild-type (WT) or Y235S cMyBPC (KO^Y235S). We found that Y235S cMyBPC was properly expressed and incorporated into the cardiac sarcomere, suggesting that the mechanism of disease of the Y235S mutation is not haploinsufficiency or poison peptides. Mechanical experiments in detergent-skinned myocardium isolated from KO^Y235S hearts revealed hypercontractile behavior compared to KO^WT hearts, evidenced by accelerated cross-bridge kinetics and increased Ca²⁺ sensitivity of force generation. In addition, MDS revealed that the Y235S mutation causes alterations in important intramolecular interactions, surface conformations, and electrostatic potential of the C1 domain of cMyBPC. Our combined in vitro and in silico data suggest that the Y235S mutation directly disrupts internal and surface properties of the C1 domain of cMyBPC, which potentially alters its ligand-binding interactions. These molecular changes may underlie the mechanism for hypercontractile cross-bridge behavior, which ultimately results in the development of cardiac hypertrophy and in vivo cardiac dysfunction.