In vivo cardiac myosin binding protein C gene transfer rescues myofilament contractile dysfunction in cardiac myosin binding protein C null mice

Gene Therapy in Cardiology: Is a Cure for Hypertrophic Cardiomyopathy on the Horizon?

Hypertrophic cardiomyopathy (HCM) is the most common genetic cardiomyopathy worldwide, affecting approximately 1 in 500 individuals. Current therapeutic interventions include lifestyle optimisation, medications, septal reduction therapies, and, rarely, cardiac transplantation. Advances in our understanding of disease-causing genetic variants in HCM and their associated molecular mechanisms have led to the potential for targeted therapeutics and implementation of precision and personalised medicine. Results from preclinical research are promising and raise the question of whether cure of some subtypes of HCM may be possible in the future. This review provides an overview of current genetic therapy platforms, including 1) genome editing, 2) gene replacement, 3) allelic-specific silencing, and 4) signalling pathway modulation. The current applicability of each of these platforms within the paradigm of HCM is examined, with updates on current and emerging trials in each domain. Barriers and limitations within the current landscape are also highlighted. Despite recent advances, translation of genetic therapy for HCM to clinical practice is still in early development. In realising the promises of genetic HCM therapies, ethical and equitable access to safe gene therapy must be prioritised.

Bringing into focus the central domains C3-C6 of myosin binding protein C

Myosin binding protein C (MyBPC) is a multi-domain protein with each region having a distinct functional role in muscle contraction. The central domains of MyBPC have often been overlooked due to their unclear roles. However, recent research shows promise in understanding their potential structural and regulatory functions. Understanding the central region of MyBPC is important because it may have specialized function that can be used as drug targets or for disease-specific therapies. In this review, we provide a brief overview of the evolution of our understanding of the central domains of MyBPC in regard to its domain structures, arrangement and dynamics, interaction partners, hypothesized functions, disease-causing mutations, and post-translational modifications. We highlight key research studies that have helped advance our understanding of the central region. Lastly, we discuss gaps in our current understanding and potential avenues to further research and discovery.

Characterization of heterozygous and homozygous mouse models with the most common hypertrophic cardiomyopathy mutation MYBPC3c.2373InsG in the Netherlands

Hypertrophic cardiomyopathy (HCM) is frequently caused by mutations in the cardiac myosin binding protein-C (cMyBP-C) encoding gene MYBPC3. In the Netherlands, approximately 25% of patients carry the MYBPC3c.2373InsG founder mutation. Most patients are heterozygous (MYBPC3+/InsG) and have highly variable phenotypic expression, whereas homozygous (MYBPC3InsG/InsG) patients have severe HCM at a young age. To improve understanding of disease progression and genotype-phenotype relationship based on the hallmarks of human HCM, we characterized mice with CRISPR/Cas9-induced heterozygous and homozygous mutations. At 18-28 weeks of age, we assessed the cardiac phenotype of Mybpc3+/InsG and Mybpc3InsG/InsG mice with echocardiography, and performed histological analyses. Cytoskeletal proteins and cardiomyocyte contractility of 3-4 week old and 18-28 week old Mybpc3c.2373InsG mice were compared to wild-type (WT) mice. Expectedly, knock-in of Mybpc3c.2373InsG resulted in the absence of cMyBP-C and our 18-28 week old homozygous Mybpc3c.2373InsG model developed cardiac hypertrophy and severe left ventricular systolic and diastolic dysfunction, whereas HCM was not evident in Mybpc3+/InsG mice. Mybpc3InsG/InsG cardiomyocytes also presented with slowed contraction-relaxation kinetics, to a greater extent in 18-28 week old mice, partially due to increased levels of detyrosinated tubulin and desmin, and reduced cardiac troponin I (cTnI) phosphorylation. Impaired cardiomyocyte contraction-relaxation kinetics were successfully normalized in 18-28 week old Mybpc3InsG/InsG cardiomyocytes by combining detyrosination inhibitor parthenolide and β-adrenergic receptor agonist isoproterenol. Both the 3-4 week old and 18-28 week old Mybpc3InsG/InsG models recapitulate HCM, with a severe phenotype present in the 18-28 week old model.

Hypertrophic cardiomyopathy in purpose-bred cats with the A31P mutation in cardiac myosin binding protein-C

We sought to establish a large animal model of inherited hypertrophic cardiomyopathy (HCM) with sufficient disease severity and early penetrance for identification of novel therapeutic strategies. HCM is the most common inherited cardiac disorder affecting 1 in 250-500 people, yet few therapies for its treatment or prevention are available. A research colony of purpose-bred cats carrying the A31P mutation in MYBPC3 was founded using sperm from a single heterozygous male cat. Cardiac function in four generations was assessed by periodic echocardiography and measurement of blood biomarkers. Results showed that HCM penetrance was age-dependent, and that penetrance occurred earlier and was more severe in successive generations, especially in homozygotes. Homozygosity was also associated with progression from preclinical to clinical disease. A31P homozygous cats represent a heritable model of HCM with early disease penetrance and a severe phenotype necessary for interventional studies aimed at altering disease progression. The occurrence of a more severe phenotype in later generations of cats, and the occasional occurrence of HCM in wildtype cats suggests the presence of at least one gene modifier or a second causal variant in this research colony that exacerbates the HCM phenotype when inherited in combination with the A31P mutation.

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.

Ioxynil and diethylstilbestrol increase the risks of cardiovascular and thyroid dysfunction in zebrafish

Endocrine disruption results from exposure to chemicals that alter the function of the endocrine system in animals. Chronic 60 days of exposure to a low dose (0.1 μM) of ioxynil (IOX) or diethylstilbestrol (DES) via food was used to determine the effects of these chemicals on the physiology of the heart and thyroid follicles in juvenile zebrafish. Immunofluorescence analysis and subsequent 3D morphometric analysis of the zebrafish heart revealed that chronic exposure to IOX induced ventricle deformation and significant volume increase (p < 0.001). DES exposure caused a change in ventricle morphology, but volume was unaffected. Alongside, it was found that DES exposure upregulated endothelial related genes (angptl1b, mhc1lia, mybpc2a, ptgir, notch1b and vwf) involved in vascular homeostasis. Both IOX and DES exposure caused a change in thyroid follicle morphology. Notably, in IOX exposed juveniles, thyroid follicle hypertrophy was observed; and in DES-exposed fish, an enlarged thyroid field was present. In summary, chronic exposure of juvenile zebrafish to IOX and DES affected the heart and the thyroid. Given that both chemicals are able to change the morphology of the thyroid it indicates that they behave as endocrine disruptive chemicals (EDCs). Heart function dynamically changes thyroid morphology, and function and hence it is likely that the observed cardiac effects of IOX and DES are the source of altered thyroid status in these fish.

Thin filament cardiomyopathies: A review of genetics, disease mechanisms, and emerging therapeutics

All muscle contraction occurs due to the cyclical interaction between sarcomeric thin and thick filament proteins within the myocyte. The thin filament consists of the proteins actin, tropomyosin, Troponin C, Troponin I, and Troponin T. Mutations in these proteins can result in various forms of cardiomyopathy, including hypertrophic, restrictive, and dilated phenotypes and account for as many as 30% of all cases of inherited cardiomyopathy. There is significant evidence that thin filament mutations contribute to dysregulation of Ca2+ within the sarcomere and may have a distinct pathomechanism of disease from cardiomyopathy associated with thick filament mutations. A number of distinct clinical findings appear to be correlated with thin-filament mutations: greater degrees of restrictive cardiomyopathy and relatively less left ventricular (LV) hypertrophy and LV outflow tract obstruction than that seen with thick filament mutations, increased morbidity associated with heart failure, increased arrhythmia burden and potentially higher mortality. Most therapies that improve outcomes in heart failure blunt the neurohormonal pathways involved in cardiac remodeling, while most therapies for hypertrophic cardiomyopathy involve use of negative inotropes to reduce LV hypertrophy or septal reduction therapies to reduce LV outflow tract obstruction. None of these therapies directly address the underlying sarcomeric dysfunction associated with thin-filament mutations. With mounting evidence that thin filament cardiomyopathies occur through a distinct mechanism, there is need for therapies targeting the unique, underlying mechanisms tailored for each patient depending on a given mutation.

The mechanics of the heart: zooming in on hypertrophic cardiomyopathy and cMyBP-C

Hypertrophic cardiomyopathy (HCM), a disease characterized by cardiac muscle hypertrophy and hypercontractility, is the most frequently inherited disorder of the heart. HCM is mainly caused by variants in genes encoding proteins of the sarcomere, the basic contractile unit of cardiomyocytes. The most frequently mutated among them is MYBPC3, which encodes cardiac myosin-binding protein C (cMyBP-C), a key regulator of sarcomere contraction. In this review, we summarize clinical and genetic aspects of HCM and provide updated information on the function of the healthy and HCM sarcomere, as well as on emerging therapeutic options targeting sarcomere mechanical activity. Building on what is known about cMyBP-C activity, we examine different pathogenicity drivers by which MYBPC3 variants can cause disease, focussing on protein haploinsufficiency as a common pathomechanism also in nontruncating variants. Finally, we discuss recent evidence correlating altered cMyBP-C mechanical properties with HCM development.

Gene therapy for inherited arrhythmias

Inherited arrhythmias are disorders caused by one or more genetic mutations that increase the risk of arrhythmia, which result in life-long risk of sudden death. These mutations either primarily perturb electrophysiological homeostasis (e.g. long QT syndrome and catecholaminergic polymorphic ventricular tachycardia), cause structural disease that is closely associated with severe arrhythmias (e.g. hypertrophic cardiomyopathy), or cause a high propensity for arrhythmia in combination with altered myocardial structure and function (e.g. arrhythmogenic cardiomyopathy). Currently available therapies offer incomplete protection from arrhythmia and fail to alter disease progression. Recent studies suggest that gene therapies may provide potent, molecularly targeted options for at least a subset of inherited arrhythmias. Here, we provide an overview of gene therapy strategies, and review recent studies on gene therapies for catecholaminergic polymorphic ventricular tachycardia and hypertrophic cardiomyopathy caused by MYBPC3 mutations.

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.

Genetic, clinical, molecular, and pathogenic aspects of the South Asian-specific polymorphic MYBPC3Δ25bp variant

Hypertrophic cardiomyopathy (HCM) is a cardiac genetic disease characterized by ventricular enlargement, diastolic dysfunction, and increased risk for sudden cardiac death. Sarcomeric genetic defects are the predominant known cause of HCM. In particular, mutations in the myosin-binding protein C gene (MYBPC3) are associated with ~ 40% of all HCM cases in which a genetic basis has been established. A decade ago, our group reported a 25-base pair deletion in intron 32 of MYBPC3 (MYBPC3Δ25bp) that is uniquely prevalent in South Asians and is associated with autosomal dominant cardiomyopathy. Although our studies suggest that this deletion results in left ventricular dysfunction, cardiomyopathies, and heart failure, the precise mechanism by which this variant predisposes to heart disease remains unclear. Increasingly appreciated, however, is the contribution of secondary risk factors, additional mutations, and lifestyle choices in augmenting or modifying the HCM phenotype in MYBPC3Δ25bp carriers. Therefore, the goal of this review article is to summarize the current research dedicated to understanding the molecular pathophysiology of HCM in South Asians with the MYBPC3Δ25bp variant. An emphasis is to review the latest techniques currently applied to explore the MYBPC3Δ25bp pathogenesis and to provide a foundation for developing new diagnostic strategies and advances in therapeutics.

Phenotypic variation and targeted therapy of hypertrophic cardiomyopathy using genetic animal models

Hypertrophic cardiomyopathy (HCM) has a variable clinical presentation due to the diversity of causative genetic mutations. Animal models allow in vivo study of genotypic expression through non-invasive imaging, pathologic sampling, and force analysis. This review focuses on the spontaneous and induced mutations in various animal models affecting mainly sarcomere proteins. The sarcomere is comprised of thick (myosin) filaments and related proteins including myosin heavy chain and myosin binding protein-C; thin (actin) filament proteins and their associated regulators including tropomyosin, troponin I, troponin C, and troponin T. The regulatory milieu including transcription factors and cell signaling also play a significant role. Animal models provide a layered approach of understanding beginning with the causative mutation as a foundation. The functional consequences of protein energy utilization and calcium sensitivity in vivo and ex vivo can be studied. Beyond pathophysiologic disruption of sarcomere function, these models demonstrate the clinical sequalae of diastolic dysfunction, heart failure, and arrhythmogenic death. Through this cascade of understanding the mutation followed by their functional significance, targeted therapies have been developed and are briefly discussed.

Novel Therapies for Prevention and Early Treatment of Cardiomyopathies

Heritable cardiomyopathies are a class of heart diseases caused by variations in a number of genetic loci. Genetic variants on one allele lead to either a degraded protein, which causes a haploinsufficiency of that protein, or a nonfunctioning protein that subverts the molecular system within which the protein works. Over years, both of these mechanisms eventually lead to diseased heart tissue and symptoms of a failing heart. Most cardiomyopathy treatments repurpose heart failure drugs to manage these symptoms and avoid adverse outcomes. There are few therapies that correct the underlying pathogenic genetic or molecular mechanism. This review will reflect on this unmet clinical need in genetic cardiomyopathies and consider a variety of therapies that address the mechanism of disease rather than patient symptoms. These therapies are genetic, targeting a defective gene or transcript, or ameliorating a genetic insufficiency. However, there are also a number of small molecules under exploration that modulate downstream faulty protein products affected in cardiomyopathies.

Recovery of left ventricular function following in vivo reexpression of cardiac myosin binding protein C

The loss of cardiac myosin binding protein C (cMyBP-C) results in left ventricular dilation, cardiac hypertrophy, and impaired ventricular function in both constitutive and conditional cMyBP-C knockout (MYBPC3 null) mice. It remains unclear whether the structural and functional phenotypes expressed in the MYBPC3 null mouse are reversible, which is an important question, since reduced expression of cMyBP-C is an important cause of hypertrophic cardiomyopathy in humans. To investigate this question, we generated a cardiac-specific transgenic mouse model using a Tet-Off inducible system to permit the controlled expression of WT cMyBP-C on the MYBPC3 null background. Functional Tet-Off mice expressing WT cMyBP-C (FT-WT) were generated by crossing tetracycline transactivator mice with responder mice carrying the WT cMyBP-C transgene. Prior to dietary doxycycline administration, cMyBP-C was expressed at normal levels in FT-WT myocardium, which exhibited similar levels of steady-state force and in vivo left ventricular function as WT mice. Introduction of dietary doxycycline for four weeks resulted in a partial knockdown of cMyBP-C expression and commensurate impairment of systolic and diastolic function to levels approaching those observed in MYBPC 3 null mice. Subsequent withdrawal of doxycycline from the diet resulted in the reexpression of cMyBP-C to levels comparable to those observed in WT mice, along with near-complete recovery of in vivo ventricular function. These results show that the cardiac phenotypes associated with MYBPC3 null mice are reversible. Our work also validates the use of the Tet-Off inducible system as a means to study the mechanisms underlying hypertrophic cardiomyopathy.

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.

Allelic imbalance and haploinsufficiency in MYBPC3-linked hypertrophic cardiomyopathy

Mutations in cardiac myosin binding protein C (MYBPC3) represent the most frequent cause of familial hypertrophic cardiomyopathy (HCM), making up approximately 50% of identified HCM mutations. MYBPC3 is distinct among other sarcomere genes associated with HCM in that truncating mutations make up the vast majority, whereas nontruncating mutations predominant in other sarcomere genes. Several studies using myocardial tissue from HCM patients have found reduced abundance of wild-type MYBPC3 compared to control hearts, suggesting haploinsufficiency of full-length MYBPC3. Further, decreased mutant versus wild-type mRNA and lack of truncated mutant MYBPC3 protein has been demonstrated, highlighting the presence of allelic imbalance. In this review, we will begin by introducing allelic imbalance and haploinsufficiency, highlighting the broad role each plays within the spectrum of human disease. We will subsequently focus on the roles allelic imbalance and haploinsufficiency play within MYBPC3-linked HCM. Finally, we will explore the implications of these findings on future directions of HCM research. An improved understanding of allelic imbalance and haploinsufficiency may help us better understand genotype-phenotype relationships in HCM and develop novel targeted therapies, providing exciting future research opportunities.

Transcriptome Analysis of Cardiac Hypertrophic Growth in MYBPC3-Null Mice Suggests Early Responders in Hypertrophic Remodeling

Rationale: With a prevalence of 1 in 200 individuals, hypertrophic cardiomyopathy (HCM) is thought to be the most common genetic cardiac disease, with potential outcomes that include severe hypertrophy, heart failure, and sudden cardiac death (SCD). Though much research has furthered our understanding of how HCM-causing mutations in genes such as cardiac myosin-binding protein C (MYBPC3) impair contractile function, it remains unclear how such dysfunction leads to hypertrophy and/or arrhythmias, which comprise the HCM phenotype. Identification of early response mediators could provide rational therapeutic targets to reduce disease severity. Our goal was to differentiate physiologic and pathophysiologic hypertrophic growth responses and identify early genetic mediators in the development of cardiomegaly in the cardiac myosin-binding protein C-null (cMyBP-C^-/-) mouse model of HCM.

Methods and Results: We performed microarray analysis on left ventricles of wild-type (WT) and cMyBPC^-/- mice (n = 7 each) at postnatal day (PND) 1 and PND 9, before and after the appearance of an overt HCM phenotype. Applying the criteria of ≥2-fold change, we identified genes whose change was exclusive to pathophysiologic growth (n = 61), physiologic growth (n = 30), and genes whose expression changed ≥2-fold in both WT and cMyBP-C^-/- hearts (n = 130). Furthermore, we identified genes that were dysregulated in PND1 cMyBP-C^-/- hearts prior to hypertrophy, including genes in mechanosensing pathways and potassium channels linked to arrhythmias. One gene of interest, Xirp2, and its protein product, are regulated during growth but also show early, robust prehypertrophic upregulation in cMyBP-C^-/- hearts. Additionally, the transcription factor Zbtb16 also shows prehypertrophic upregulation at both gene and protein levels.

Conclusion: Our transcriptome analysis generated a comprehensive data set comparing physiologic vs. hypertrophic growth in mice lacking cMyBP-C. It highlights the importance of extracellular matrix pathways in hypertrophic growth and early dysregulation of potassium channels. Prehypertrophic upregulation of Xirp2 in cMyBP-C^-/- hearts supports a growing body of evidence suggesting Xirp2 has the capacity to elicit both hypertrophy and arrhythmias in HCM. Dysregulation of Xirp2, as well as Zbtb16, along with other genes associated with mechanosensing regions of the cardiomyocyte implicate stress-sensing in these regions as a potentially important early response in HCM.

Impact of the Myosin Modulator Mavacamten on Force Generation and Cross-Bridge Behavior in a Murine Model of Hypercontractility

Background Recent studies suggest that mavacamten (Myk461), a small myosin-binding molecule, decreases hypercontractility in myocardium expressing hypertrophic cardiomyopathy-causing missense mutations in myosin heavy chain. However, the predominant feature of most mutations in cardiac myosin binding protein-C ( cMyBPC ) that cause hypertrophic cardiomyopathy is reduced total cMyBPC expression, and the impact of Myk461 on cMyBPC -deficient myocardium is currently unknown. Methods and Results We measured the impact of Myk461 on steady-state and dynamic cross-bridge ( XB ) behavior in detergent-skinned mouse wild-type myocardium and myocardium lacking cMyBPC (knockout (KO)). KO myocardium exhibited hypercontractile XB behavior as indicated by significant accelerations in rates of XB detachment (krel) and recruitment (kdf) at submaximal Ca2+ activations. Incubation of KO and wild-type myocardium with Myk461 resulted in a dose-dependent force depression, and this impact was more pronounced at low Ca2+ activations. Interestingly, Myk461-induced force depressions were less pronounced in KO myocardium, especially at low Ca2+ activations, which may be because of increased acto-myosin XB formation and potential disruption of super-relaxed XB s in KO myocardium. Additionally, Myk461 slowed krel in KO myocardium but not in wild-type myocardium, indicating increased XB " on" time. Furthermore, the greater degree of Myk461-induced slowing in kdf and reduction in XB recruitment magnitude in KO myocardium normalized the XB behavior back to wild-type levels. Conclusions This is the first study to demonstrate that Myk461-induced force depressions are modulated by cMyBPC expression levels in the sarcomere, and emphasizes that clinical use of Myk461 may need to be optimized based on the molecular trigger that underlies the hypertrophic cardiomyopathy phenotype.

Regulation of TLR4 expression mediates the attenuating effect of erythropoietin on inflammation and myocardial fibrosis in rat heart

The mechanism underlying the anti-inflammatory or antifibrotic activity of erythropoietin (EPO) in myocardial fibrosis (MF) remains elusive. In the current study, abdominal aortic constriction (AAC) was performed on rats and EPO and/or Toll-like receptor (TLR)4 were overexpressed in rat hearts through intramyocardial administration of lentivirus expressing the EPO and TLR4 genes. Hematoxylin and eosin staining and Masson's trichrome staining were performed on tissue sections from rat hearts for histopathological examination. ELISA was used to determine the levels of inflammatory mediators in serum. Gene expression levels were determined by quantitative polymerase chain reaction analysis and protein expression levels were determined by western blot analysis and immunofluorescence staining. The results indicated that EPO overexpression improved MF in rat hearts, by inhibiting the release of transforming growth factor (TGF)-β1, tumor necrosis factor (TNF)-α, interleukin (IL)-6, IL-1β, IL-17A, matrix metalloproteinase (MMP)-9 and MMP-2. Moreover, EPO overexpression suppressed the expression of TLR4, while promoting phosphoinositide 3-kinase (PI3K) and phosphorylated AKT serine/threonine kinase 1 (Akt) expression levels. However, the beneficial effects of EPO were attenuated by overexpression of TLR4. In addition, inhibition of PI3K/Akt signaling activity by treatment with LY294002 markedly reversed the protective effect of EPO on the AAC-induced MF. Taken together, the present study demonstrated that EPO may have a critical role against MF by activating PI3K/Akt signaling and by down-regulating TLR4 expression, thereby inhibiting the release of TGF-β1, TNF-α, IL-6, IL-1β, IL-17A, MMP-9 and MMP-2. These findings suggest that the PI3K/Akt/TLR4 signaling pathway is associated with the anti-inflammatory effects of EPO and may play a role in attenuating AAC-induced MF.

Alterations in sarcomere function modify the hyperplastic to hypertrophic transition phase of mammalian cardiomyocyte development

It remains unclear how perturbations in cardiomyocyte sarcomere function alter postnatal heart development. We utilized murine models that allowed manipulation of cardiac myosin-binding protein C (MYBPC3) expression at critical stages of cardiac ontogeny to study the response of the postnatal heart to disrupted sarcomere function. We discovered that the hyperplastic to hypertrophic transition phase of mammalian heart development was altered in mice lacking MYBPC3 and this was the critical period for subsequent development of cardiomyopathy. Specifically, MYBPC3-null hearts developed evidence of increased cardiomyocyte endoreplication, which was accompanied by enhanced expression of cell cycle stimulatory cyclins and increased phosphorylation of retinoblastoma protein. Interestingly, this response was self-limited at later developmental time points by an upregulation of the cyclin-dependent kinase inhibitor p21. These results provide valuable insights into how alterations in sarcomere protein function modify postnatal heart development and highlight the potential for targeting cell cycle regulatory pathways to counteract cardiomyopathic stimuli.

Cardiac Myosin Binding Protein-C Phosphorylation Modulates Myofilament Length-Dependent Activation

Cardiac myosin binding protein-C (cMyBP-C) phosphorylation is an important regulator of contractile function, however, its contributions to length-dependent changes in cross-bridge (XB) kinetics is unknown. Therefore, we performed mechanical experiments to quantify contractile function in detergent-skinned ventricular preparations isolated from wild-type (WT) hearts, and hearts expressing non-phosphorylatable cMyBP-C [Ser to Ala substitutions at residues Ser273, Ser282, and Ser302 (i.e., 3SA)], at sarcomere length (SL) 1.9 μm or 2.1μm, prior and following protein kinase A (PKA) treatment. Steady-state force generation measurements revealed a blunting in the length-dependent increase in myofilament Ca(2+)-sensitivity of force generation (pCa50) following an increase in SL in 3SA skinned myocardium compared to WT skinned myocardium. Dynamic XB behavior was assessed at submaximal Ca(2+)-activations by imposing an acute rapid stretch of 2% of initial muscle length, and measuring both the magnitudes and rates of resultant phases of force decay due to strain-induced XB detachment and delayed force rise due to recruitment of additional XBs with increased SL (i.e., stretch activation). The magnitude (P2) and rate of XB detachment (k rel) following stretch was significantly reduced in 3SA skinned myocardium compared to WT skinned myocardium at short and long SL, and prior to and following PKA treatment. Furthermore, the length-dependent acceleration of k rel due to decreased SL that was observed in WT skinned myocardium was abolished in 3SA skinned myocardium. PKA treatment accelerated the rate of XB recruitment (k df) following stretch at both SL's in WT but not in 3SA skinned myocardium. The amplitude of the enhancement in force generation above initial pre-stretch steady-state levels (P3) was not different between WT and 3SA skinned myocardium at any condition measured. However, the magnitude of the entire delayed force phase which can dip below initial pre-stretch steady-state levels (Pdf) was significantly lower in 3SA skinned myocardium under all conditions, in part due to a reduced magnitude of XB detachment (P2) in 3SA skinned myocardium compared to WT skinned myocardium. These findings demonstrate that cMyBP-C phospho-ablation regulates SL- and PKA-mediated effects on XB kinetics in the myocardium, which would be expected to contribute to the regulation of the Frank-Starling mechanism.

Inherited cardiomyopathies--Novel therapies

Cardiomyopathies arising due to a single gene defect represent various pathways that evoke adverse remodeling and cardiac dysfunction. While the gene therapy approach is slowly evolving and has not yet reached clinical "prime time" and gene correction approaches are applicable at the bench but not at the bedside, major advances are being made with molecular and drug therapies. This review summarizes the contemporary drugs introduced or being tested to help manage these unique disorders bearing a major impact on the quality of life and survival of the affected individuals. The restoration of the RNA reading frame facilitates the expression of partly functional protein to salvage or alleviate the disease phenotype. Chaperones are used to prevent the degradation of abnormal but still functional proteins, while other molecules are given for pathogen silencing, to prevent aggregation or to enhance clearance of protein deposits. The absence of protein may be managed by viral gene delivery or protein therapy. Enzyme replacement therapy is already a clinical reality for a series of metabolic diseases. The progress in molecular biology, based on the knowledge of the gene defect, helps generate small molecules and pharmaceuticals targeting the key events occurring in the malfunctioning element of the sick organ. Cumulatively, these tools augment the existing armamentarium of phenotype oriented symptomatic and evidence-based therapies for patients with inherited cardiomyopathies.

Cardiac myosin binding protein C regulates postnatal myocyte cytokinesis

Homozygous cardiac myosin binding protein C-deficient (Mybpc(t/t)) mice develop dramatic cardiac dilation shortly after birth; heart size increases almost twofold. We have investigated the mechanism of cardiac enlargement in these hearts. Throughout embryogenesis myocytes undergo cell division while maintaining the capacity to pump blood by rapidly disassembling and reforming myofibrillar components of the sarcomere throughout cell cycle progression. Shortly after birth, myocyte cell division ceases. Cardiac MYBPC is a thick filament protein that regulates sarcomere organization and rigidity. We demonstrate that many Mybpc(t/t) myocytes undergo an additional round of cell division within 10 d postbirth compared with their wild-type counterparts, leading to increased numbers of mononuclear myocytes. Short-hairpin RNA knockdown of Mybpc3 mRNA in wild-type mice similarly extended the postnatal window of myocyte proliferation. However, adult Mybpc(t/t) myocytes are unable to fully regenerate the myocardium after injury. MYBPC has unexpected inhibitory functions during postnatal myocyte cytokinesis and cell cycle progression. We suggest that human patients with homozygous MYBPC3-null mutations develop dilated cardiomyopathy, coupled with myocyte hyperplasia (increased cell number), as observed in Mybpc(t/t) mice. Human patients, with heterozygous truncating MYBPC3 mutations, like mice with similar mutations, have hypertrophic cardiomyopathy. However, the mechanism leading to hypertrophic cardiomyopathy in heterozygous MYBPC3(+/-) individuals is myocyte hypertrophy (increased cell size), whereas the mechanism leading to cardiac dilation in homozygous Mybpc3(-/-) mice is primarily myocyte hyperplasia.

Epigenetic Therapy for the Treatment of Hypertension-Induced Cardiac Hypertrophy and Fibrosis

BACKGROUND

The development of heart failure is associated with changes in the size, shape, and structure of the heart that has a negative impact on cardiac function. These pathological changes involve excessive extracellular matrix deposition within the myocardial interstitium and myocyte hypertrophy. Alterations in fibroblast phenotype and myocyte activity are associated with reprogramming of gene transcriptional profiles that likely requires epigenetic alterations in chromatin structure. The aim of our work was to investigate the potential of a currently licensed anticancer epigenetic modifier as a treatment option for cardiac diseases associated with hypertension-induced cardiac hypertrophy and fibrosis.

METHODS AND RESULTS

The effects of DNA methylation inhibition with 5-azacytidine (5-aza) were examined in a human primary fibroblast cell line and in a spontaneously hypertensive rat (SHR) model. The results from this work allude to novel in vivo antifibrotic and antihypertrophic actions of 5-aza. Administration of the DNA methylation inhibitor significantly improved several echocardiographic parameters associated with hypertrophy and diastolic dysfunction. Myocardial collagen levels and myocyte size were reduced in 5-aza-treated SHRs. These findings are supported by beneficial in vitro effects in cardiac fibroblasts. Collagen I, collagen III, and α-smooth muscle actin were reduced in a human ventricular cardiac fibroblast cell line treated with 5-aza.

CONCLUSION

These findings suggest a role for epigenetic modifications in contributing to the profibrotic and hypertrophic changes evident during disease progression. Therapeutic intervention with 5-aza demonstrated favorable effects highlighting the potential use of this epigenetic modifier as a treatment option for cardiac pathologies associated with hypertrophy and fibrosis.

Molecular effects of the myosin activator omecamtiv mecarbil on contractile properties of skinned myocardium lacking cardiac myosin binding protein-C

Decreased expression of cardiac myosin binding protein-C (cMyBP-C) in the myocardium is thought to be a contributing factor to hypertrophic cardiomyopathy in humans, and the initial molecular defect is likely abnormal cross-bridge (XB) function which leads to impaired force generation, decreased contractile performance, and hypertrophy in vivo. The myosin activator omecamtiv mecarbil (OM) is a pharmacological drug that specifically targets the myosin XB and recent evidence suggests that OM induces a significant decrease in the in vivo motility velocity and an increase in the XB duty cycle. Thus, the molecular effects of OM maybe beneficial in improving contractile function in skinned myocardium lacking cMyBP-C because absence of cMyBP-C in the sarcomere accelerates XB kinetics and enhances XB turnover rate, which presumably reduces contractile efficiency. Therefore, parameters of XB function were measured in skinned myocardium lacking cMyBP-C prior to and following OM incubation. We measured ktr, the rate of force redevelopment as an index of XB transition from both the weakly- to strongly-bound state and from the strongly- to weakly-bound states and performed stretch activation experiments to measure the rates of XB detachment (krel) and XB recruitment (kdf) in detergent-skinned ventricular preparations isolated from hearts of wild-type (WT) and cMyBP-C knockout (KO) mice. Samples from donor human hearts were also used to assess the effects of OM in cardiac muscle expressing a slow β-myosin heavy chain (β-MHC). Incubation of skinned myocardium with OM produced large enhancements in steady-state force generation which were most pronounced at low levels of [Ca(2+)] activations, suggesting that OM cooperatively recruits additional XB's into force generating states. Despite a large increase in steady-state force generation following OM incubation, parallel accelerations in XB kinetics as measured by ktr were not observed, and there was a significant OM-induced decrease in krel which was more pronounced in the KO skinned myocardium compared to WT skinned myocardium (58% in WT vs. 76% in KO at pCa 6.1), such that baseline differences in krel between KO and WT skinned myocardium were no longer apparent following OM-incubation. A significant decrease in the kdf was also observed following OM incubation in all groups, which may be related to the increase in the number of cooperatively recruited XB's at low Ca(2+)-activations which slows the overall rate of force generation. Our results indicate that OM may be a useful pharmacological approach to normalize hypercontractile XB kinetics in myocardium with decreased cMyBP-C expression due to its molecular effects on XB behavior.

Targeting the sarcomere to correct muscle function

Various human diseases can disrupt the balance between muscle contraction and relaxation. Sarcomeric modulators can be used to readjust this balance either indirectly by intervening in signalling pathways or directly through interaction with the muscle proteins that control contraction. Such agents represent a novel approach to treating any condition in which striated muscle function is compromised, including heart failure, cardiomyopathies, skeletal myopathies and a wide range of neuromuscular conditions. Here, we review agents that modulate the mechanical function of the sarcomere, focusing on emerging compounds that target myosin or the troponin complex.

Gene transfer into cardiac myocytes

Traditional methods for DNA transfection are often inefficient and toxic for terminally differentiated cells, such as cardiac myocytes. Vector-based gene transfer is an efficient approach for introducing exogenous cDNA into these types of primary cell cultures. In this chapter, separate protocols for adult rat cardiac myocyte isolation and gene transfer with recombinant adenovirus are provided and are routinely utilized for studying the effects of sarcomeric proteins on myofilament function.

Length-dependent changes in contractile dynamics are blunted due to cardiac myosin binding protein-C ablation

Enhanced cardiac contractile function with increased sarcomere length (SL) is, in part, mediated by a decrease in the radial distance between myosin heads and actin. The radial disposition of myosin heads relative to actin is modulated by cardiac myosin binding protein-C (cMyBP-C), suggesting that cMyBP-C contributes to the length-dependent activation (LDA) in the myocardium. However, the precise roles of cMyBP-C in modulating cardiac LDA are unclear. To determine the impact of cMyBP-C on LDA, we measured isometric force, myofilament Ca²⁺-sensitivity (pCa₅₀) and length-dependent changes in kinetic parameters of cross-bridge (XB) relaxation (k_rel), and recruitment (k_df) due to rapid stretch, as well as the rate of force redevelopment (k_tr) in response to a large slack-restretch maneuver in skinned ventricular multicellular preparations isolated from the hearts of wild-type (WT) and cMyBP-C knockout (KO) mice, at SL's 1.9 μm or 2.1 μm. Our results show that maximal force was not significantly different between KO and WT preparations but length-dependent increase in pCa₅₀ was attenuated in the KO preparations. pCa₅₀ was not significantly different between WT and KO preparations at long SL (5.82 ± 0.02 in WT vs. 5.87 ± 0.02 in KO), whereas pCa₅₀ was significantly different between WT and KO preparations at short SL (5.71 ± 0.02 in WT vs. 5.80 ± 0.01 in KO; p < 0.05). The k_tr, measured at half-maximal Ca²⁺-activation, was significantly accelerated at short SL in WT preparations (8.74 ± 0.56 s⁻¹ at 1.9 μm vs. 5.71 ± 0.40 s⁻¹ at 2.1 μm, p < 0.05). Furthermore, k_rel and k_df were accelerated by 32% and 50%, respectively at short SL in WT preparations. In contrast, k_tr was not altered by changes in SL in KO preparations (8.03 ± 0.54 s⁻¹ at 1.9 μm vs. 8.90 ± 0.37 s⁻¹ at 2.1 μm). Similarly, KO preparations did not exhibit length-dependent changes in k_rel and k_df. Collectively, our data implicate cMyBP-C as an important regulator of LDA via its impact on dynamic XB behavior due to changes in SL.

The contribution of cardiac myosin binding protein-c Ser282 phosphorylation to the rate of force generation and in vivo cardiac contractility

Cardiac myosin binding protein-C phosphorylation plays an important role in modulating cardiac muscle function and accelerating contraction. It has been proposed that Ser282 phosphorylation may serve as a critical molecular switch that regulates the phosphorylation of neighbouring Ser273 and Ser302 residues, and thereby govern myofilament contractile acceleration in response to protein kinase A (PKA). Therefore, to determine the regulatory roles of Ser282 we generated a transgenic (TG) mouse model expressing cardiac myosin binding protein-C with a non-phosphorylatable Ser282 (i.e. serine to alanine substitution, TG(S282A)). Myofibrils isolated from TG(S282A) hearts displayed robust PKA-mediated phosphorylation of Ser273 and Ser302, and the increase in phosphorylation was identical to TG wild-type (TG(WT)) controls. No signs of pathological cardiac hypertrophy were detected in TG(S282A) hearts by either histological examination of cardiac sections or echocardiography. Baseline fractional shortening, ejection fraction, isovolumic relaxation time, rate of pressure development and rate of relaxation (τ) were unaltered in TG(S282A) mice. However, the increase in cardiac contractility as well as the acceleration of pressure development observed in response to β-adrenergic stimulation was attenuated in TG(S282A) mice. In agreement with our in vivo data, in vitro force measurements revealed that PKA-mediated acceleration of cross-bridge kinetics in TG(S282A) myocardium was significantly attenuated compared to TG(WT) myocardium. Taken together, our data suggest that while Ser282 phosphorylation does not regulate the phosphorylation of neighbouring Ser residues and basal cardiac function, full acceleration of cross-bridge kinetics and left ventricular pressure development cannot be achieved in its absence.

MYBPC3 in hypertrophic cardiomyopathy: from mutation identification to RNA-based correction

Mutations in MYBPC3 gene, encoding cardiac myosin-binding protein C (cMyBP-C), frequently cause hypertrophic cardiomyopathy (HCM), which affects 0.2 % of the general population. This myocardial autosomal-dominant disorder is the leading cause of sudden cardiac death particularly in young athletes. The current pharmacological and surgical treatments of HCM focus on symptoms relief, but do not address the cause of the disease. With the development of novel strategies targeting the endogenous mutation, causal HCM therapy is now possible. This review will discuss the current knowledge on HCM from the identification of MYBPC3 gene mutations to potential RNA-based correction.

Cardiac myosin binding protein-C: a novel sarcomeric target for gene therapy

Through its ability to interact with both the thick and thin filament proteins within the sarcomere, cardiac myosin binding protein-C (cMyBP-C) regulates the contractile properties of the myocardium. The central regulatory role of cMyBP-C in heart function is emphasized by the fact that a large proportion of inherited hypertrophic cardiomyopathy cases in humans are caused by mutations in cMyBP-C. The primary dysfunction in cMyBP-C-related cardiomyopathies is likely to be abnormal myofilament contractile function; however, currently, there are no effective therapies for ameliorating these contractile defects. Thus, there is a compelling need to design novel therapies to restore normal contractile function in cMyBP-C-related cardiomyopathies. To this end, concepts gleaned from various structural, functional, and biochemical studies can now be utilized to engineer cMyBP-C proteins that, when incorporated into the sarcomere, can significantly improve contractile function. In this review, we discuss the rationale for cMyBP-C-based gene therapies that can be utilized to treat contractile dysfunction in inherited and acquired cardiomyopathies.

Impaired contractile function due to decreased cardiac myosin binding protein C content in the sarcomere

Mutations in cardiac myosin binding protein C (MyBP-C) are a common cause of familial hypertrophic cardiomyopathy (FHC). The majority of MyBP-C mutations are expected to reduce MyBP-C expression; however, the consequences of MyBP-C deficiency on the regulation of myofilament function, Ca²⁺ homeostasis, and in vivo cardiac function are unknown. To elucidate the effects of decreased MyBP-C expression on cardiac function, we employed MyBP-C heterozygous null (MyBP-C+/-) mice presenting decreases in MyBP-C expression (32%) similar to those of FHC patients carrying MyBP-C mutations. The levels of MyBP-C phosphorylation were reduced 53% in MyBP-C+/- hearts compared with wild-type hearts. Skinned myocardium isolated from MyBP-C+/- hearts displayed decreased cross-bridge stiffness at half-maximal Ca²⁺ activations, increased steady-state force generation, and accelerated rates of cross-bridge recruitment at low Ca²⁺ activations (<15% and <25% of maximum, respectively). Protein kinase A treatment abolished basal differences in rates of cross-bridge recruitment between MyBP-C+/- and wild-type myocardium. Intact ventricular myocytes from MyBP-C+/- hearts displayed abnormal sarcomere shortening but unchanged Ca²⁺ transient kinetics. Despite a lack of left ventricular hypertrophy, MyBP-C+/- hearts exhibited elevated end-diastolic pressure and decreased peak rate of LV pressure rise, which was normalized following dobutamine infusion. Furthermore, electrocardiogram recordings in conscious MyBP-C+/- mice revealed prolonged QRS and QT intervals, which are known risk factors for cardiac arrhythmia. Collectively, our data show that reduced MyBP-C expression and phosphorylation in the sarcomere result in myofilament dysfunction, contributing to contractile dysfunction that precedes compensatory adaptations in Ca²⁺ handling, and chamber remodeling. Perturbations in mechanical and electrical activity in MyBP-C+/- mice could increase their susceptibility to cardiac dysfunction and arrhythmia.