Comparison of two murine models of familial hypertrophic cardiomyopathy

Hypertrophic cardiomyopathy in MYBPC3 carriers in aging

Hypertrophic cardiomyopathy (HCM) is characterized by abnormal thickening of the myocardium, leading to arrhythmias, heart failure, and elevated risk of sudden cardiac death, particularly among the young. This inherited disease is predominantly caused by mutations in sarcomeric genes, among which those in the cardiac myosin binding protein-C3 (MYBPC3) gene are major contributors. HCM associated with MYBPC3 mutations usually presents in the elderly and ranges from asymptomatic to symptomatic forms, affecting numerous cardiac functions and presenting significant health risks with a spectrum of clinical manifestations. Regulation of MYBPC3 expression involves various transcriptional and translational mechanisms, yet the destiny of mutant MYBPC3 mRNA and protein in late-onset HCM remains unclear. Pathogenesis related to MYBPC3 mutations includes nonsense-mediated decay, alternative splicing, and ubiquitin-proteasome system events, leading to allelic imbalance and haploinsufficiency. Aging further exacerbates the severity of HCM in carriers of MYBPC3 mutations. Advancements in high-throughput omics techniques have identified crucial molecular events and regulatory disruptions in cardiomyocytes expressing MYBPC3 variants. This review assesses the pathogenic mechanisms that promote late-onset HCM through the lens of transcriptional, post-transcriptional, and post-translational modulation of MYBPC3, underscoring its significance in HCM across carriers. The review also evaluates the influence of aging on these processes and MYBPC3 levels during HCM pathogenesis in the elderly. While pinpointing targets for novel medical interventions to conserve cardiac function remains challenging, the emergence of personalized omics offers promising avenues for future HCM treatments, particularly for late-onset cases.

Inhibition of miR-199a-3p in a murine hypertrophic cardiomyopathy (HCM) model attenuates fibrotic remodeling

Background

Hypertrophic cardiomyopathy (HCM) is an autosomal dominant genetic disorder, characterized by cardiomyocyte hypertrophy, cardiomyocyte disarray and fibrosis, which has a prevalence of ∼1: 200-500 and predisposes individuals to heart failure and sudden death. The mechanisms through which diverse HCM-causing mutations cause cardiac dysfunction remain mostly unknown and their identification may reveal new therapeutic avenues. MicroRNAs (miRNAs) have emerged as critical regulators of gene expression and disease phenotype in various pathologies. We explored whether miRNAs could play a role in HCM pathogenesis and offer potential therapeutic targets.

Methods and results

Using high-throughput miRNA expression profiling and qPCR analysis in two distinct mouse models of HCM, we found that miR-199a-3p expression levels are upregulated in mutant mice compared to age- and treatment-matched wild-type mice. We also found that miR-199a-3p expression is enriched in cardiac non-myocytes compared to cardiomyocytes. When we expressed miR-199a-3p mimic in cultured murine primary cardiac fibroblasts and analyzed the conditioned media by proteomics, we found that several extracellular matrix (ECM) proteins (e.g., TSP2, FBLN3, COL11A1, LYOX) were differentially secreted (data are available via ProteomeXchange with identifier PXD042904). We confirmed our proteomics findings by qPCR analysis of selected mRNAs and demonstrated that miR-199a-3p mimic expression in cardiac fibroblasts drives upregulation of ECM gene expression, including Tsp2, Fbln3, Pcoc1, Col1a1 and Col3a1. To examine the role of miR-199a-3p in vivo, we inhibited its function using lock-nucleic acid (LNA)-based inhibitors (antimiR-199a-3p) in an HCM mouse model. Our results revealed that progression of cardiac fibrosis is attenuated when miR-199a-3p function is inhibited in mild-to-moderate HCM. Finally, guided by computational target prediction algorithms, we identified mRNAs Cd151 and Itga3 as direct targets of miR-199a-3p and have shown that miR-199a-3p mimic expression negatively regulates AKT activation in cardiac fibroblasts.

Conclusions

Altogether, our results suggest that miR-199a-3p may contribute to cardiac fibrosis in HCM through its actions in cardiac fibroblasts. Thus, inhibition of miR-199a-3p in mild-to-moderate HCM may offer therapeutic benefit in combination with complementary approaches that target the primary defect in cardiac myocytes.

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.

cMyBP-C ablation in human engineered cardiac tissue causes progressive Ca2+-handling abnormalities

Truncation mutations in cardiac myosin binding protein C (cMyBP-C) are common causes of hypertrophic cardiomyopathy (HCM). Heterozygous carriers present with classical HCM, while homozygous carriers present with early onset HCM that rapidly progress to heart failure. We used CRISPR-Cas9 to introduce heterozygous (cMyBP-C+/-) and homozygous (cMyBP-C-/-) frame-shift mutations into MYBPC3 in human iPSCs. Cardiomyocytes derived from these isogenic lines were used to generate cardiac micropatterns and engineered cardiac tissue constructs (ECTs) that were characterized for contractile function, Ca2+-handling, and Ca2+-sensitivity. While heterozygous frame shifts did not alter cMyBP-C protein levels in 2-D cardiomyocytes, cMyBP-C+/- ECTs were haploinsufficient. cMyBP-C-/- cardiac micropatterns produced increased strain with normal Ca2+-handling. After 2 wk of culture in ECT, contractile function was similar between the three genotypes; however, Ca2+-release was slower in the setting of reduced or absent cMyBP-C. At 6 wk in ECT culture, the Ca2+-handling abnormalities became more pronounced in both cMyBP-C+/- and cMyBP-C-/- ECTs, and force production became severely depressed in cMyBP-C-/- ECTs. RNA-seq analysis revealed enrichment of differentially expressed hypertrophic, sarcomeric, Ca2+-handling, and metabolic genes in cMyBP-C+/- and cMyBP-C-/- ECTs. Our data suggest a progressive phenotype caused by cMyBP-C haploinsufficiency and ablation that initially is hypercontractile, but progresses to hypocontractility with impaired relaxation. The severity of the phenotype correlates with the amount of cMyBP-C present, with more severe earlier phenotypes observed in cMyBP-C-/- than cMyBP-C+/- ECTs. We propose that while the primary effect of cMyBP-C haploinsufficiency or ablation may relate to myosin crossbridge orientation, the observed contractile phenotype is Ca2+-mediated.

Modeling hypertrophic cardiomyopathy with human cardiomyocytes derived from induced pluripotent stem cells

One of the obstacles in studying the pathogenesis of hypertrophic cardiomyopathy (HCM) is the poor availability of myocardial tissue samples at the early stages of disease development. This has been addressed by the advent of induced pluripotent stem cells (iPSCs), which allow us to differentiate patient-derived iPSCs into cardiomyocytes (iPSC-CMs) in vitro. In this review, we summarize different approaches to establishing iPSC models and the application of genome editing techniques in iPSC. Because iPSC-CMs cultured at the present stage are immature in structure and function, researchers have attempted several methods to mature iPSC-CMs, such as prolonged culture duration, and mechanical and electrical stimulation. Currently, many researchers have established iPSC-CM models of HCM and employed diverse methods for performing measurements of cellular morphology, contractility, electrophysiological property, calcium handling, mitochondrial function, and metabolism. Here, we review published results in humans to date within the growing field of iPSC-CM models of HCM. Although there is no unified consensus, preliminary results suggest that this approach to modeling disease would provide important insights into our understanding of HCM pathogenesis and facilitate drug development and safety testing.

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.

Electrophysiological and Structural Remodeling of the Atria in a Mouse Model of Troponin-I Mutation Linked Hypertrophic Cardiomyopathy: Implications for Atrial Fibrillation

Hypertrophic cardiomyopathy (HCM) is an inherited cardiac disorder affecting one in 500 of the general population. Atrial fibrillation (AF) is the most common arrhythmia in patients with HCM. We sought to characterize the atrial electrophysiological and structural substrate in young and aging Gly203Ser cardiac troponin-I transgenic (HCM) mice. At 30 weeks and 50 weeks of age (n = 6 per strain each group), the left atrium was excised and placed on a multi-electrode array (MEA) for electrophysiological study; subsequent histological analyses and plasma samples were analyzed for biomarkers of extracellular matrix remodeling and cell adhesion and inflammation. Wild-type mice of matched ages were included as controls. Young HCM mice demonstrated significantly shortened atrial action potential duration (APD), increased conduction heterogeneity index (CHI), increased myocyte size, and increased interstitial fibrosis without changes in effective refractory periods (ERP), conduction velocity (CV), inflammatory infiltrates, or circulating markers of extracellular matrix remodeling and inflammation. Aging HCM mice demonstrated aggravated changes in atria electrophysiology and structural remodeling as well as increased circulating matrix metalloproteinases (MMP)-2, MMP-3, and VCAM-1 levels. This model of HCM demonstrates an underlying atrial substrate that progresses with age and may in part be responsible for the greater propensity for AF in HCM.

Improving characterization of hypertrophy-induced murine cardiac dysfunction using four-dimensional ultrasound-derived strain mapping

Mouse models of cardiac disease have become essential tools in the study of pathological mechanisms, but the small size of rodents makes it challenging to quantify heart function with noninvasive imaging. Building off recent developments in high-frequency four-dimensional ultrasound (4DUS) imaging, we have applied this technology to study cardiac dysfunction progression in a murine model of metabolic cardiomyopathy. Cardiac knockout of carnitine palmitoyltransferase 2 (Cpt2^M-/-) in mice hinders cardiomyocyte bioenergetic metabolism of long-chain fatty acids, and leads to progressive cardiac hypertrophy and heart failure. The proposed analysis provides a standardized approach to measure localized wall kinematics and simultaneously extracts metrics of global cardiac function, LV morphometry, regional circumferential strain, and regional longitudinal strain from an interpolated 4-D mesh of the endo- and epicardial boundaries. Comparison of metric changes due to aging suggests that circumferential strain at the base and longitudinal strain along the posterior wall are most sensitive to disease progression. We further introduce a novel hybrid strain index (HSI) that incorporates information from these two regions and may have greater utility to characterize disease progression relative to other extracted metrics. Potential applications to additional disease models are discussed that could further demonstrate the utility of metrics derived from 4DUS imaging and strain mapping.NEW & NOTEWORTHY High-frequency four-dimensional ultrasound can be used in conjunction with standardized analysis procedures to simultaneously extract left-ventricular global function, morphometry, and regional strain metrics. Furthermore, a novel hybrid strain index (HSI) formula demonstrates greater performance compared with all other metrics in characterizing disease progression in a model of metabolic cardiomyopathy.

Machine Learning Driven Contouring of High-Frequency Four-Dimensional Cardiac Ultrasound Data

Automatic boundary detection of 4D ultrasound (4DUS) cardiac data is a promising yet challenging application at the intersection of machine learning and medicine. Using recently developed murine 4DUS cardiac imaging data, we demonstrate here a set of three machine learning models that predict left ventricular wall kinematics along both the endo- and epi-cardial boundaries. Each model is fundamentally built on three key features: (1) the projection of raw US data to a lower dimensional subspace, (2) a smoothing spline basis across time, and (3) a strategic parameterization of the left ventricular boundaries. Model 1 is constructed such that boundary predictions are based on individual short-axis images, regardless of their relative position in the ventricle. Model 2 simultaneously incorporates parallel short-axis image data into their predictions. Model 3 builds on the multi-slice approach of model 2, but assists predictions with a single ground-truth position at end-diastole. To assess the performance of each model, Monte Carlo cross validation was used to assess the performance of each model on unseen data. For predicting the radial distance of the endocardium, models 1, 2, and 3 yielded average R2 values of 0.41, 0.49, and 0.71, respectively. Monte Carlo simulations of the endocardial wall showed significantly closer predictions when using model 2 versus model 1 at a rate of 48.67%, and using model 3 versus model 2 at a rate of 83.50%. These finding suggest that a machine learning approach where multi-slice data are simultaneously used as input and predictions are aided by a single user input yields the most robust performance. Subsequently, we explore the how metrics of cardiac kinematics compare between ground-truth contours and predicted boundaries. We observed negligible deviations from ground-truth when using predicted boundaries alone, except in the case of early diastolic strain rate, providing confidence for the use of such machine learning models for rapid and reliable assessments of murine cardiac function. To our knowledge, this is the first application of machine learning to murine left ventricular 4DUS data. Future work will be needed to strengthen both model performance and applicability to different cardiac disease models.

Preventative therapeutic approaches for hypertrophic cardiomyopathy

Sarcomeric gene mutations are associated with the development of hypertrophic cardiomyopathy (HCM). Current drug therapeutics for HCM patients are effective in relieving symptoms, but do not prevent or reverse disease progression. Moreover, due to heterogeneity in the clinical manifestations of the disease, patients experience variable outcomes in response to therapeutics. Mechanistically, alterations in calcium handling, sarcomeric disorganization, energy metabolism and contractility participate in HCM disease progression. While some similarities exist, each mutation appears to lead to mutation‐specific pathophysiology. Furthermore, these alterations may precede or proceed development of the pathology. This review assesses the efficacy of HCM therapeutics from studies performed in animal models of HCM and human clinical trials. Evidence suggests that a preventative rather than corrective therapeutic approach may be more efficacious in the treatment of HCM. In addition, a clear understanding of mutation‐specific mechanisms may assist in informing the most effective therapeutic mode of action.

Comparative analysis of functional assay evidence use by ClinGen Variant Curation Expert Panels

BACKGROUND

The 2015 American College of Medical Genetics and Genomics (ACMG) and the Association for Molecular Pathology (AMP) guidelines for clinical sequence variant interpretation state that "well-established" functional studies can be used as evidence in variant classification. These guidelines articulated key attributes of functional data, including that assays should reflect the biological environment and be analytically sound; however, details of how to evaluate these attributes were left to expert judgment. The Clinical Genome Resource (ClinGen) designates Variant Curation Expert Panels (VCEPs) in specific disease areas to make gene-centric specifications to the ACMG/AMP guidelines, including more specific definitions of appropriate functional assays. We set out to evaluate the existing VCEP guidelines for functional assays.

METHODS

We evaluated the functional criteria (PS3/BS3) of six VCEPs (CDH1, Hearing Loss, Inherited Cardiomyopathy-MYH7, PAH, PTEN, RASopathy). We then established criteria for evaluating functional studies based on disease mechanism, general class of assay, and the characteristics of specific assay instances described in the primary literature. Using these criteria, we extensively curated assay instances cited by each VCEP in their pilot variant classification to analyze VCEP recommendations and their use in the interpretation of functional studies.

RESULTS

Unsurprisingly, our analysis highlighted the breadth of VCEP-approved assays, reflecting the diversity of disease mechanisms among VCEPs. We also noted substantial variability between VCEPs in the method used to select these assays and in the approach used to specify strength modifications, as well as differences in suggested validation parameters. Importantly, we observed discrepancies between the parameters VCEPs specified as required for approved assay instances and the fulfillment of these requirements in the individual assays cited in pilot variant interpretation.

CONCLUSIONS

Interpretation of the intricacies of functional assays often requires expert-level knowledge of the gene and disease, and current VCEP recommendations for functional assay evidence are a useful tool to improve the accessibility of functional data by providing a starting point for curators to identify approved functional assays and key metrics. However, our analysis suggests that further guidance is needed to standardize this process and ensure consistency in the application of functional evidence.

SarcTrack

RATIONALE

Human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) in combination with CRISPR/Cas9 genome editing provide unparalleled opportunities to study cardiac biology and disease. However, sarcomeres, the fundamental units of myocyte contraction, are immature and nonlinear in hiPSC-CMs, which technically challenge accurate functional interrogation of contractile parameters in beating cells. Furthermore, existing analysis methods are relatively low-throughput, indirectly assess contractility, or only assess well-aligned sarcomeres found in mature cardiac tissues.

OBJECTIVE

We aimed to develop an analysis platform that directly, rapidly, and automatically tracks sarcomeres in beating cardiomyocytes. The platform should assess sarcomere content, contraction and relaxation parameters, and beat rate.

METHODS AND RESULTS

We developed SarcTrack, a MatLab software that monitors fluorescently tagged sarcomeres in hiPSC-CMs. The algorithm determines sarcomere content, sarcomere length, and returns rates of sarcomere contraction and relaxation. By rapid measurement of hundreds of sarcomeres in each hiPSC-CM, SarcTrack provides large data sets for robust statistical analyses of multiple contractile parameters. We validated SarcTrack by analyzing drug-treated hiPSC-CMs, confirming the contractility effects of compounds that directly activate (CK-1827452) or inhibit (MYK-461) myosin molecules or indirectly alter contractility (verapamil and propranolol). SarcTrack analysis of hiPSC-CMs carrying a heterozygous truncation variant in the myosin-binding protein C ( MYBPC3) gene, which causes hypertrophic cardiomyopathy, recapitulated seminal disease phenotypes including cardiac hypercontractility and diminished relaxation, abnormalities that normalized with MYK-461 treatment.

CONCLUSIONS

SarcTrack provides a direct and efficient method to quantitatively assess sarcomere function. By improving existing contractility analysis methods and overcoming technical challenges associated with functional evaluation of hiPSC-CMs, SarcTrack enhances translational prospects for sarcomere-regulating therapeutics and accelerates interrogation of human cardiac genetic variants.

Ablation of the calpain-targeted site in cardiac myosin binding protein-C is cardioprotective during ischemia-reperfusion injury

Cardiac myosin binding protein-C (cMyBP-C) phosphorylation is essential for normal heart function and protects the heart from ischemia-reperfusion (I/R) injury. It is known that protein kinase-A (PKA)-mediated phosphorylation of cMyBP-C prevents I/R-dependent proteolysis, whereas dephosphorylation of cMyBP-C at PKA sites correlates with its degradation. While sites on cMyBP-C associated with phosphorylation and proteolysis co-localize, the mechanisms that link cMyBP-C phosphorylation and proteolysis during cardioprotection are not well understood. Therefore, we aimed to determine if abrogation of cMyBP-C proteolysis in association with calpain, a calcium-activated protease, confers cardioprotection during I/R injury. Calpain is activated in both human ischemic heart samples and ischemic mouse myocardium where cMyBP-C is dephosphorylated and undergoes proteolysis. Moreover, cMyBP-C is a substrate for calpain proteolysis and cleaved by calpain at residues 272-TSLAGAGRR-280, a domain termed as the calpain-target site (CTS). Cardiac-specific transgenic (Tg) mice in which the CTS motif was ablated were bred into a cMyBP-C null background. These Tg mice were conclusively shown to possess a normal basal structure and function by analysis of histology, electron microscopy, immunofluorescence microscopy, Q-space MRI of tissue architecture, echocardiography, and hemodynamics. However, the genetic ablation of the CTS motif conferred resistance to calpain-mediated proteolysis of cMyBP-C. Following I/R injury, the loss of the CTS reduced infarct size compared to non-transgenic controls. Collectively, these findings demonstrate the physiological significance of calpain-targeted cMyBP-C proteolysis and provide a rationale for studying inhibition of calpain-mediated proteolysis of cMyBP-C as a therapeutic target for cardioprotection.

Impaired calcium handling and mitochondrial metabolic dysfunction as early markers of hypertrophic cardiomyopathy

Hypertrophic cardiomyopathy (HCM) is a primary myocardial disorder, characterised by myocyte remodeling, disorganisation of sarcomeric proteins, impaired energy metabolism and altered cardiac contractility. Gene mutations encoding cardiac contractile proteins account for 60% of HCM aetiology. Current drug therapy including L-type calcium channel antagonists, are used to manage symptoms in patients with overt HCM, but no treatment exists that can reverse or prevent the cardiomyopathy. Design of effective drug therapy will require a clear understanding of the early pathophysiological mechanisms of the disease. Numerous studies have investigated specific aspects of HCM pathophysiology. This review brings these findings together, in order to develop a holistic understanding of the early pathophysiological mechanisms of the disease. We focus on gene mutations in cardiac myosin binding protein-C, β-cardiac myosin heavy chain, cardiac troponin I, and cardiac troponin T, that comprise the majority of all HCM sarcomeric gene mutations. We find that although some similarities exist, each mutation leads to mutation-specific alterations in calcium handling, myofilament calcium sensitivity and mitochondrial metabolic function. This may contribute to the observed clinical phenotypic variability in sarcomeric-related HCM. An understanding of early mutation-specific mechanisms of the disease may provide useful markers of disease progression, and inform therapeutic design.

Hypertrophic cardiomyopathy mutations in MYBPC3 dysregulate myosin

The mechanisms by which truncating mutations in MYBPC3 (encoding cardiac myosin-binding protein C; cMyBPC) or myosin missense mutations cause hypercontractility and poor relaxation in hypertrophic cardiomyopathy (HCM) are incompletely understood. Using genetic and biochemical approaches, we explored how depletion of cMyBPC altered sarcomere function. We demonstrated that stepwise loss of cMyBPC resulted in reciprocal augmentation of myosin contractility. Direct attenuation of myosin function, via a damaging missense variant (F764L) that causes dilated cardiomyopathy (DCM), normalized the increased contractility from cMyBPC depletion. Depletion of cMyBPC also altered dynamic myosin conformations during relaxation, enhancing the myosin state that enables ATP hydrolysis and thin filament interactions while reducing the super relaxed conformation associated with energy conservation. MYK-461, a pharmacologic inhibitor of myosin ATPase, rescued relaxation deficits and restored normal contractility in mouse and human cardiomyocytes with MYBPC3 mutations. These data define dosage-dependent effects of cMyBPC on myosin that occur across the cardiac cycle as the pathophysiologic mechanisms by which MYBPC3 truncations cause HCM. Therapeutic strategies to attenuate cMyBPC activity may rescue depressed cardiac contractility in patients with DCM, whereas inhibiting myosin by MYK-461 should benefit the substantial proportion of patients with HCM with MYBPC3 mutations.

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.

Cardiomyopathy phenotypes in human-induced pluripotent stem cell-derived cardiomyocytes-a systematic review

Human-induced pluripotent stem cells (hiPSC) can be differentiated to cardiomyocytes at high efficiency and are increasingly used to study cardiac disease in a human context. This review evaluated 38 studies on hypertrophic (HCM) and dilated cardiomyopathy (DCM) of different genetic causes asking to which extent published data allow the definition of an in vitro HCM/DCM hiPSC-CM phenotype. The data are put in context with the prevailing hypotheses on HCM/DCM dysfunction and pathophysiology. Relatively consistent findings in HCM not reported in DCM were larger cell size (156 ± 85%, n = 15), more nuclear localization of nuclear factor of activated T cells (NFAT; 175 ± 65%, n = 3), and higher β-myosin heavy chain gene expression levels (500 ± 547%, n = 8) than respective controls. Conversely, DCM lines showed consistently less force development than controls (47 ± 23%, n = 9), while HCM forces scattered without clear trend. Both HCM and DCM lines often showed sarcomere disorganization, higher NPPA/NPPB expression levels, and arrhythmic beating behaviour. The data have to be taken with the caveat that reporting frequencies of the various parameters (e.g. cell size, NFAT expression) differ widely between HCM and DCM lines, in which data scatter is large and that only 9/38 studies used isogenic controls. Taken together, the current data provide interesting suggestions for disease-specific phenotypes in HCM/DCM hiPSC-CM but indicate that the field is still in its early days. Systematic, quantitative comparisons and robust, high content assays are warranted to advance the field.

Dilated cardiomyopathy myosin mutants have reduced force-generating capacity

Dilated cardiomyopathy (DCM) and hypertrophic cardiomyopathy (HCM) can cause arrhythmias, heart failure, and cardiac death. Here, we functionally characterized the motor domains of five DCM-causing mutations in human β-cardiac myosin. Kinetic analyses of the individual events in the ATPase cycle revealed that each mutation alters different steps in this cycle. For example, different mutations gave enhanced or reduced rate constants of ATP binding, ATP hydrolysis, or ADP release or exhibited altered ATP, ADP, or actin affinity. Local effects dominated, no common pattern accounted for the similar mutant phenotype, and there was no distinct set of changes that distinguished DCM mutations from previously analyzed HCM myosin mutations. That said, using our data to model the complete ATPase contraction cycle revealed additional critical insights. Four of the DCM mutations lowered the duty ratio (the ATPase cycle portion when myosin strongly binds actin) because of reduced occupancy of the force-holding A·M·D complex in the steady state. Under load, the A·M·D state is predicted to increase owing to a reduced rate constant for ADP release, and this effect was blunted for all five DCM mutations. We observed the opposite effects for two HCM mutations, namely R403Q and R453C. Moreover, the analysis predicted more economical use of ATP by the DCM mutants than by WT and the HCM mutants. Our findings indicate that DCM mutants have a deficit in force generation and force-holding capacity due to the reduced occupancy of the force-holding state.

Early remodeling of repolarizing K+ currents in the αMHC403/+ mouse model of familial hypertrophic cardiomyopathy

Familial hypertrophic cardiomyopathy (HCM), linked to mutations in myosin, myosin-binding proteins and other sarcolemmal proteins, is associated with increased risk of life threatening ventricular arrhythmias, and a number of animal models have been developed to facilitate analysis of disease progression and mechanisms. In the experiments here, we use the αMHC^403/+ mouse line in which one αMHC allele harbors a common HCM mutation (in βMHC, Arg403 Gln). Here, we demonstrate marked prolongation of QT intervals in young adult (10-12week) male αMHC^403/+ mice, well in advance of the onset of measurable left ventricular hypertrophy. Electrophysiological recordings from myocytes isolated from the interventricular septum of these animals revealed significantly (P<0.001) lower peak repolarizing voltage-gated K⁺ (Kv) current (I_K,peak) amplitudes, compared with cells isolated from wild type (WT) littermate controls. Analysis of Kv current waveforms revealed that the amplitudes of the inactivating components of the total outward Kv current, I_to,f, I_to,s and I_K,slow, were significantly lower in αMHC^403/+, compared with WT, septum cells, whereas I_ss amplitudes were similar. The amplitudes/densities of I_K,peak and I_K,slow were also lower in αMHC^403/+, compared with WT, LV wall and LV apex myocytes, whereas I_to,f was attenuated in αMHC^403/+ LV wall, but not LV apex, cells. These regional differences in the remodeling of repolarizing Kv currents in the αMHC^403/+ mice would be expected to increase the dispersion of ventricular repolarization and be proarrhythmic. Quantitative RT-PCR analysis revealed reductions in the expression of transcripts encoding several K⁺ channel subunits in the interventricular septum, LV free wall and LV apex of (10-12week) αMHC^403/+ mice, although this transcriptional remodeling was not correlated with the observed decreases in K⁺ current amplitudes.

Cardiac inflammation in genetic dilated cardiomyopathy caused by MYBPC3 mutation

Cardiomyopathies are a leading cause of heart failure and are often caused by mutations in sarcomeric genes, resulting in contractile dysfunction and cellular damage. This may stimulate the production of a robust proinflammatory response. To determine whether myocardial inflammation is associated with cardiac dysfunction in dilated cardiomyopathy (DCM) caused by MYBPC3 mutation, we used the well-characterized cMyBP-C(t/t) mouse model of DCM at 3months of age. Compared to wild type (WT) mice, DCM mice exhibited significantly decreased fractional shortening (36.4±2% vs. 15.5±1.0%, p<0.0001) and significantly increased spleen weight (5.3±0.3 vs. 7.2±0.4mg/mm, p=0.002). Intriguingly, flow cytometry analysis revealed a significant increase in total (CD45+CD11b+Ly6C-MHCII+F480+) macrophages (6.5±1.4% vs. 14.8±1.4%, p=0.002) and classically activated (CD45+CD11b+Ly6C-MHCII+F480+CD206-) proinflammatory (M1) macrophages (3.4±0.8% vs. 10.3±1.2%, p=0.0009) in DCM hearts as compared with WT hearts. These results were further confirmed by immunofluorescence analysis of heart tissue sections. Splenic red pulp (CD11b+Ly6C+MHCIIlowF480hi) macrophages were significantly elevated (1.3±0.1% vs. 2.4±0.1%, p=0.0001) in DCM compared to WT animals. Serum cytokine analysis in DCM animals exhibited a significant increase (0.65±0.2 vs. 2.175±0.5pg/mL, p=0.02) in interleukin (IL)-6 compared to WT animals. Furthermore, RNA-seq analysis revealed the upregulation of inflammatory pathways in the DCM hearts. Together, these data indicate a robust proinflammatory response in DCM hearts, likely in response to cellular damage triggered by MYBPC3 mutation and resultant contractile dysfunction.

The genetic basis of hypertrophic cardiomyopathy in cats and humans

Mutations in genes that encode for muscle sarcomeric proteins have been identified in humans and two breeds of domestic cats with hypertrophic cardiomyopathy (HCM). This article reviews the history, genetics, and pathogenesis of HCM in the two species in order to give veterinarians a perspective on the genetics of HCM. Hypertrophic cardiomyopathy in people is a genetic disease that has been called a disease of the sarcomere because the preponderance of mutations identified that cause HCM are in genes that encode for sarcomeric proteins (Maron and Maron, 2013). Sarcomeres are the basic contractile units of muscle and thus sarcomeric proteins are responsible for the strength, speed, and extent of muscle contraction. In people with HCM, the two most common genes affected by HCM mutations are the myosin heavy chain gene (MYH7), the gene that encodes for the motor protein β-myosin heavy chain (the sarcomeric protein that splits ATP to generate force), and the cardiac myosin binding protein-C gene (MYBPC3), a gene that encodes for the closely related structural and regulatory protein, cardiac myosin binding protein-C (cMyBP-C). To date, the two mutations linked to HCM in domestic cats (one each in Maine Coon and Ragdoll breeds) also occur in MYBPC3 (Meurs et al., 2005, 2007). This is a review of the genetics of HCM in both humans and domestic cats that focuses on the aspects of human genetics that are germane to veterinarians and on all aspects of feline HCM genetics.

Slowed atrial and atrioventricular conduction and depressed HRV in a murine model of hypertrophic cardiomyopathy

Hypertrophic cardiomyopathy (HCM) is a common heritable cardiac disorder with diverse clinical outcomes including sudden death, heart failure, and stroke. Depressed heart rate variability (HRV), a measure of cardiac autonomic regulation, has been shown to predict mortality in patients with cardiovascular disease. Cardiac autonomic remodelling in animal models of HCM are not well characterised. This study analysed Gly203Ser cardiac troponin-I transgenic (TG) male mice previously demonstrated to develop hallmarks of HCM by age 21 weeks. 33 mice aged 30 and 50 weeks underwent continuous electrocardiogram (ECG) recording for 30 min under anaesthesia. TG mice demonstrated prolonged P-wave duration (P < 0.001) and PR intervals (P < 0.001) compared to controls. Additionally, TG mice demonstrated depressed standard deviation of RR intervals (SDRR; P < 0.01), coefficient of variation of RR intervals (CVRR; P < 0.001) and standard deviation of heart rate (SDHR; P < 0.001) compared to controls. Additionally, total power was significantly reduced in TG mice (P < 0.05). No significant age-related difference in either strain was observed in ECG or HRV parameters. Mice with HCM developed slowed atrial and atrioventricular conduction and depressed HRV. These changes were conserved with increasing age. This finding may be indicative of atrial and ventricular hypertrophy or dysfunction, and perhaps an indication of worse clinical outcome in heart failure progression in HCM patients.

The Role of the L-Type Ca2+ Channel in Altered Metabolic Activity in a Murine Model of Hypertrophic Cardiomyopathy

Heterozygous mice (αMHC^403/+) expressing the human disease-causing mutation Arg403Gln exhibit cardinal features of hypertrophic cardiomyopathy (HCM) including hypertrophy, myocyte disarray, and increased myocardial fibrosis. Treatment of αMHC^403/+mice with the L-type calcium channel (I_Ca-L) antagonist diltiazem has been shown to decrease left ventricular anterior wall thickness, cardiac myocyte hypertrophy, disarray, and fibrosis. However, the role of the I_Ca-L in the development of HCM is not known. In addition to maintaining cardiac excitation and contraction in myocytes, the I_Ca-L also regulates mitochondrial function through transmission of movement of I_Ca-L via cytoskeletal proteins to mitochondrial voltage-dependent anion channel. Here, the authors investigated the role of I_Ca-L in regulating mitochondrial function in αMHC^403/+mice. Whole-cell patch clamp studies showed that I_Ca-L current inactivation kinetics were significantly increased in αMHC^403/+cardiac myocytes, but that current density and channel expression were similar to wild-type cardiac myocytes. Activation of I_Ca-L caused a significantly greater increase in mitochondrial membrane potential and metabolic activity in αMHC^403/+. These increases were attenuated with I_Ca-L antagonists and following F-actin or β-tubulin depolymerization. The authors observed increased levels of fibroblast growth factor-21 in αMHC^403/+mice, and altered mitochondrial DNA copy number consistent with altered mitochondrial activity and the development of cardiomyopathy. These studies suggest that the Arg403Gln mutation leads to altered functional communication between I_Ca-L and mitochondria that is associated with increased metabolic activity, which may contribute to the development of cardiomyopathy. I_Ca-L antagonists may be effective in reducing the cardiomyopathy in HCM by altering metabolic activity.

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Heterozygous mice (αMHC^403/+) expressing the human hypertrophic cardiomyopathy (HCM) disease causing mutation Arg403Gln exhibit cardinal features of HCM.

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This study investigated the role of L-type Ca2+ channel (ICa-L) in regulating mitochondrial function in Arg403Gln (αMHC^403/+) mice.

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Activation of ICa-L in αMHC^403/+mice caused a significantly greater increase in mitochondrial membrane potential and metabolic activity when compared to wild-type mice.

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Increases in mitochondrial membrane potential and metabolic activity were attenuated with ICa-L antagonists and when F-actin or β-tubulin were depolymerized.

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ICa-L antagonists may be effective in reducing the cardiomyopathy in HCM by altering metabolic activity.

Contractile Defect Caused by Mutation in MYBPC3 Revealed under Conditions Optimized for Human PSC-Cardiomyocyte Function

Maximizing baseline function of human pluripotent stem cell-derived cardiomyocytes (hPSC-CMs) is essential for their effective application in models of cardiac toxicity and disease. Here, we aimed to identify factors that would promote an adequate level of function to permit robust single-cell contractility measurements in a human induced pluripotent stem cell (hiPSC) model of hypertrophic cardiomyopathy (HCM). A simple screen revealed the collaborative effects of thyroid hormone, IGF-1 and the glucocorticoid analog dexamethasone on the electrophysiology, bioenergetics, and contractile force generation of hPSC-CMs. In this optimized condition, hiPSC-CMs with mutations in MYBPC3, a gene encoding myosin-binding protein C, which, when mutated, causes HCM, showed significantly lower contractile force generation than controls. This was recapitulated by direct knockdown of MYBPC3 in control hPSC-CMs, supporting a mechanism of haploinsufficiency. Modeling this disease in vitro using human cells is an important step toward identifying therapeutic interventions for HCM.

Oxidative Stress in Dilated Cardiomyopathy Caused by MYBPC3 Mutation

Cardiomyopathies can result from mutations in genes encoding sarcomere proteins including MYBPC3, which encodes cardiac myosin binding protein-C (cMyBP-C). However, whether oxidative stress is augmented due to contractile dysfunction and cardiomyocyte damage in MYBPC3-mutated cardiomyopathies has not been elucidated. To determine whether oxidative stress markers were elevated in MYBPC3-mutated cardiomyopathies, a previously characterized 3-month-old mouse model of dilated cardiomyopathy (DCM) expressing a homozygous MYBPC3 mutation (cMyBP-C((t/t))) was used, compared to wild-type (WT) mice. Echocardiography confirmed decreased percentage of fractional shortening in DCM versus WT hearts. Histopathological analysis indicated a significant increase in myocardial disarray and fibrosis while the second harmonic generation imaging revealed disorganized sarcomeric structure and myocyte damage in DCM hearts when compared to WT hearts. Intriguingly, DCM mouse heart homogenates had decreased glutathione (GSH/GSSG) ratio and increased protein carbonyl and lipid malondialdehyde content compared to WT heart homogenates, consistent with elevated oxidative stress. Importantly, a similar result was observed in human cardiomyopathy heart homogenate samples. These results were further supported by reduced signals for mitochondrial semiquinone radicals and Fe-S clusters in DCM mouse hearts measured using electron paramagnetic resonance spectroscopy. In conclusion, we demonstrate elevated oxidative stress in MYPBC3-mutated DCM mice, which may exacerbate the development of heart failure.

Haploinsufficiency of MYBPC3 exacerbates the development of hypertrophic cardiomyopathy in heterozygous mice

Mutations in MYBPC3, the gene encoding cardiac myosin binding protein-C (cMyBP-C), account for ~40% of hypertrophic cardiomyopathy (HCM) cases. Most pathological MYBPC3 mutations encode truncated protein products not found in tissue. Reduced protein levels occur in symptomatic heterozygous human HCM carriers, suggesting haploinsufficiency as an underlying mechanism of disease. However, we do not know if reduced cMyBP-C content results from, or initiates the development of HCM. In previous studies, heterozygous (HET) mice with a MYBPC3 C'-terminal truncation mutation and normal cMyBP-C levels show altered contractile function prior to any overt hypertrophy. Therefore, this study aimed to test whether haploinsufficiency occurs, with decreased cMyBP-C content, following cardiac stress and whether the functional impairment in HET MYBPC3 hearts leads to worsened disease progression. To address these questions, transverse aortic constriction (TAC) was performed on three-month-old wild-type (WT) and HET MYBPC3-truncation mutant mice and then characterized at 4 and 12weeks post-surgery. HET-TAC mice showed increased hypertrophy and reduced ejection fraction compared to WT-TAC mice. At 4weeks post-surgery, HET myofilaments showed significantly reduced cMyBP-C content. Functionally, HET-TAC cardiomyocytes showed impaired force generation, higher Ca(2+) sensitivity, and blunted length-dependent increase in force generation. RNA sequencing revealed several differentially regulated genes between HET and WT groups, including regulators of remodeling and hypertrophic response. Collectively, these results demonstrate that haploinsufficiency occurs in HET MYBPC3 mutant carriers following stress, causing, in turn, reduced cMyBP-C content and exacerbating the development of dysfunction at myofilament and whole-heart levels.

Endothelin-1 induces myofibrillar disarray and contractile vector variability in hypertrophic cardiomyopathy-induced pluripotent stem cell-derived cardiomyocytes

BACKGROUND

Despite the accumulating genetic and molecular investigations into hypertrophic cardiomyopathy (HCM), it remains unclear how this condition develops and worsens pathologically and clinically in terms of the genetic-environmental interactions. Establishing a human disease model for HCM would help to elucidate these disease mechanisms; however, cardiomyocytes from patients are not easily obtained for basic research. Patient-specific induced pluripotent stem cells (iPSCs) potentially hold much promise for deciphering the pathogenesis of HCM. The purpose of this study is to elucidate the interactions between genetic backgrounds and environmental factors involved in the disease progression of HCM.

METHODS AND RESULTS

We generated iPSCs from 3 patients with HCM and 3 healthy control subjects, and cardiomyocytes were differentiated. The HCM pathological phenotypes were characterized based on morphological properties and high-speed video imaging. The differences between control and HCM iPSC-derived cardiomyocytes were mild under baseline conditions in pathological features. To identify candidate disease-promoting environmental factors, the cardiomyocytes were stimulated by several cardiomyocyte hypertrophy-promoting factors. Interestingly, endothelin-1 strongly induced pathological phenotypes such as cardiomyocyte hypertrophy and intracellular myofibrillar disarray in the HCM iPSC-derived cardiomyocytes. We then reproduced these phenotypes in neonatal cardiomyocytes from the heterozygous Mybpc3-targeted knock in mice. High-speed video imaging with motion vector prediction depicted physiological contractile dynamics in the iPSC-derived cardiomyocytes, which revealed that self-beating HCM iPSC-derived single cardiomyocytes stimulated by endothelin-1 showed variable contractile directions.

CONCLUSIONS

Interactions between the patient's genetic backgrounds and the environmental factor endothelin-1 promote the HCM pathological phenotype and contractile variability in the HCM iPSC-derived cardiomyocytes.

Age-related changes in familial hypertrophic cardiomyopathy phenotype in transgenic mice and humans

β-myosin heavy chain mutations are the most frequently identified basis for hypertrophic cardiomyopathy (HCM). A transgenic mouse model (αMHC(403)) has been extensively used to study various mechanistic aspects of HCM. There is general skepticism whether mouse and human disease features are similar. Herein we compare morphologic and functional characteristics, and disease evolution, in a transgenic mouse and a single family with a MHC mutation. Ten male αMHC(403) transgenic mice (at t-5 weeks, -12 weeks, and -24 weeks) and 10 HCM patients from the same family with a β-myosin heavy chain mutation were enrolled. Morphometric, conventional echocardiographic, tissue Doppler and strain analytic characteristics of transgenic mice and HCM patients were assessed. Ten male transgenic mice (αMHC(403)) were examined at ages -5 weeks, -12 weeks, and -24 weeks. In the transgenic mice, aging was associated with a significant increase in septal (0.59±0.06 vs. 0.64±0.05 vs. 0.69±0.11 mm, P<0.01) and anterior wall thickness (0.58±0.1 vs. 0.62±0.07 vs. 0.80±0.16 mm, P<0.001), which was coincident with a significant decrease in circumferential strain (-22%±4% vs. -20%±3% vs. -19%±3%, P=0.03), global longitudinal strain (-19%±3% vs. -17%±2% vs. -16%±3%, P=0.001) and E/A ratio (1.9±0.3 vs. 1.7±0.3 vs. 1.4±0.3, P=0.01). The HCM patients were classified into 1st generation (n=6; mean age 53±6 years), and 2nd generation (n=4; mean age 32±8 years). Septal thickness (2.2±0.9 vs. 1.4±0.1 cm, P<0.05), left atrial (LA) volume (62±16 vs. 41±5 mL, P=0.03), E/A ratio (0.77±0.21 vs. 1.1±0.1, P=0.01), E/e' ratio (25±10 vs. 12±2, P=0.03), global left ventricular (LV) strain (-14%±3% vs. -20%±3%, P=0.01) and global LV early diastolic strain rate (0.76±0.17 s(-1) vs. 1.3±0.2 s-1, P=0.01) were significantly worse in the older generation. In β-myosin heavy chain mutations, transgenic mice and humans have similar progression in morphologic and functional abnormalities. The αMHC(403) transgenic mouse model closely recapitulates human disease.

Sarcomere mutation-specific expression patterns in human hypertrophic cardiomyopathy

BACKGROUND

Heterozygous mutations in sarcomere genes in hypertrophic cardiomyopathy (HCM) are proposed to exert their effect through gain of function for missense mutations or loss of function for truncating mutations. However, allelic expression from individual mutations has not been sufficiently characterized to support this exclusive distinction in human HCM.

METHODS AND RESULTS

Sarcomere transcript and protein levels were analyzed in septal myectomy and transplant specimens from 46 genotyped HCM patients with or without sarcomere gene mutations and 10 control hearts. For truncating mutations in MYBPC3, the average ratio of mutant:wild-type transcripts was ≈1:5, in contrast to ≈1:1 for all sarcomere missense mutations, confirming that nonsense transcripts are uniquely unstable. However, total MYBPC3 mRNA was significantly increased by 9-fold in HCM samples with MYBPC3 mutations compared with control hearts and with HCM samples without sarcomere gene mutations. Full-length MYBPC3 protein content was not different between MYBPC3 mutant HCM and control samples, and no truncated proteins were detected. By absolute quantification of abundance with multiple reaction monitoring, stoichiometric ratios of mutant sarcomere proteins relative to wild type were strikingly variable in a mutation-specific manner, with the fraction of mutant protein ranging from 30% to 84%.

CONCLUSIONS

These results challenge the concept that haploinsufficiency is a unifying mechanism for HCM caused by MYBPC3 truncating mutations. The range of allelic imbalance for several missense sarcomere mutations suggests that certain mutant proteins may be more or less stable or incorporate more or less efficiently into the sarcomere than wild-type proteins. These mutation-specific properties may distinctly influence disease phenotypes.

Sexual dimorphic response to exercise in hypertrophic cardiomyopathy-associated MYBPC3-targeted knock-in mice

Hypertrophic cardiomyopathy (HCM), the most common genetic cardiac disorder, is frequently caused by mutations in MYBPC3, encoding cardiac myosin-binding protein C (cMyBP-C). Moreover, HCM is the leading cause of sudden cardiac death (SCD) in young athletes. Interestingly, SCD is more likely to occur in male than in female athletes. However, the pathophysiological mechanisms leading to sex-specific differences are poorly understood. Therefore, we studied the effect of sex and exercise on functional properties of the heart and sarcomeres in mice carrying a MYBPC3 point mutation (G > A transition in exon 6) associated with human HCM. Echocardiography followed by isometric force measurements in left ventricular (LV) membrane-permeabilized cardiomyocytes was performed in wild-type (WT) and heterozygous (HET) knock-in mice of both sex (N = 5 per group) in sedentary mice and mice that underwent an 8-week voluntary wheel-running exercise protocol. Isometric force measurements in single cardiomyocytes revealed a lower maximal force generation (F max) of the sarcomeres in male sedentary HET (13.0 ± 1.1 kN/m(2)) compared to corresponding WT (18.4 ± 1.8 kN/m(2)) male mice. Exercise induced a higher F max in HET male mice, while it did not affect HET females. Interestingly, a low cardiac troponin I bisphosphorylation, increased myofilament Ca(2+)-sensitivity, and LV hypertrophy were particularly observed in exercised HET females. In conclusion, in sedentary animals, contractile differences are seen between male and female HET mice. Male and female HET hearts adapted differently to a voluntary exercise protocol, indicating that physiological stimuli elicit a sexually dimorphic cardiac response in heterozygous MYBPC3-targeted knock-in mice.

Contractile dysfunction in a mouse model expressing a heterozygous MYBPC3 mutation associated with hypertrophic cardiomyopathy

The etiology of hypertrophic cardiomyopathy (HCM) has been ascribed to mutations in genes encoding sarcomere proteins. In particular, mutations in MYBPC3, a gene which encodes cardiac myosin binding protein-C (cMyBP-C), have been implicated in over one third of HCM cases. Of these mutations, 70% are predicted to result in C'-truncated protein products, which are undetectable in tissue samples. Heterozygous carriers of these truncation mutations exhibit varying penetrance of HCM, with symptoms often occurring later in life. We hypothesize that heterozygous carriers of MYBPC3 mutations, while seemingly asymptomatic, have subtle functional impairments that precede the development of overt HCM. This study compared heterozygous (+/t) knock-in MYBPC3 truncation mutation mice with wild-type (+/+) littermates to determine whether functional alterations occur at the whole-heart or single-cell level before the onset of hypertrophy. The +/t mice show ∼40% reduction in MYBPC3 transcription, but no changes in cMyBP-C level, phosphorylation status, or cardiac morphology. Nonetheless, +/t mice show significantly decreased maximal force development at sarcomere lengths of 1.9 μm (+/t 68.5 ± 4.1 mN/mm(2) vs. +/+ 82.2 ± 3.2) and 2.3 μm (+/t 79.2 ± 3.1 mN/mm(2) vs. +/+ 95.5 ± 2.4). In addition, heterozygous mice show significant reductions in vivo in the early/after (E/A) (+/t 1.74 ± 0.12 vs. +/+ 2.58 ± 0.43) and E'/A' (+/t 1.18 ± 0.05 vs. +/+ 1.52 ± 0.15) ratios, indicating diastolic dysfunction. These results suggest that seemingly asymptomatic heterozygous MYBPC3 carriers do suffer impairments that may presage the onset of HCM.

MYBPC3's alternate ending: consequences and therapeutic implications of a highly prevalent 25 bp deletion mutation

Hypertrophic cardiomyopathy (HCM) is the most common form of inherited cardiac disease and the leading cause of sudden cardiac death in young people. HCM is caused by mutations in genes encoding contractile proteins. Cardiac myosin binding protein-C (cMyBP-C) is a thick filament contractile protein that regulates sarcomere organization and cardiac contractility. About 200 different mutations in the cMyBP-C gene (MYBPC3) have thus far been reported as causing HCM. Among them, a 25 base pair deletion in the branch point of intron 32 of MYBPC3 is widespread, particularly affecting people of South Asian descent, with 4% of this population carrying the mutation. This polymorphic mutation results in skipping of exon 33 and a reading frame shift, which, in turn, replaces the last 65 amino acids of the C-terminal C10 domain of cMyBP-C with a novel sequence of 58 residues (cMyBP-C(C10mut)). Carriers of the 25 base pair deletion mutation are at increased risk of developing cardiomyopathy and heart failure. Because of the high prevalence of this mutation in certain populations, genetic screening of at-risk groups might be beneficial. Scientifically, the functional consequences of C-terminal mutations and the precise mechanisms leading to HCM should be defined using induced pluripotent stem cells and engineered heart tissue in vitro or mouse models in vivo. Most importantly, therapeutic strategies that include pharmacology, gene repair, and gene therapy should be developed to prevent the adverse clinical effects of cMyBP-C(C10mut). This review article aims to examine the effects of cMyBP-C(C10mut) on cardiac function, emphasizing the need for the development of genetic testing and expanded therapeutic strategies.

Inhibition of connexin 43 in cardiac muscle during intense physical exercise

Endurance training is accompanied by important adaptations in both cardiovascular and autonomic nervous systems. Previous works have shown that the main component of gap junctions in the ventricular myocardium (connexin 43 (Cx43) can be regulated by adrenergic stimulus. On the other hand, training raises vagal and decreases sympathetic tone, while augmenting myocardial sensitivity to sympathetic stimulation during exercise. We therefore evaluated the regulation of Cx43 expression by sympathetic tone during exercise in trained and sedentary mice. Training induced an increase in the protein level of Cx43 by 45-70% under resting conditions. The expression of Cx43 was inhibited in trained but not in untrained mice in response to a 60 min exercise bout. Normal basal expression was restored after 60 min of resting. Cx43 reached a minimum that was not different from basal expression in untrained mice. In accordance, electrocardiography and action potential analysis did not reveal major electrophysiological implications for the drop in Cx43 abundance in trained-exercise mice. We prevented Cx43 inhibition using propranolol, and observed increased basal mRNA levels of β-adrenergic receptors without significant changes in the ratio β1 to β2. In conclusion, we showed that Cx43 expression is transiently inhibited by β-adrenergic stimulus in trained mice during acute exercise.

Cardiac-specific over-expression of epidermal growth factor receptor 2 (ErbB2) induces pro-survival pathways and hypertrophic cardiomyopathy in mice

BACKGROUND

Emerging evidence shows that ErbB2 signaling has a critical role in cardiomyocyte physiology, based mainly on findings that blocking ErbB2 for cancer therapy is toxic to cardiac cells. However, consequences of high levels of ErbB2 activity in the heart have not been previously explored.

METHODOLOGY/PRINCIPAL FINDINGS

We investigated consequences of cardiac-restricted over-expression of ErbB2 in two novel lines of transgenic mice. Both lines develop striking concentric cardiac hypertrophy, without heart failure or decreased life span. ErbB2 transgenic mice display electrocardiographic characteristics similar to those found in patients with Hypertrophic Cardiomyopathy, with susceptibility to adrenergic-induced arrhythmias. The hypertrophic hearts, which are 2-3 times larger than those of control littermates, express increased atrial natriuretic peptide and β-myosin heavy chain mRNA, consistent with a hypertrophic phenotype. Cardiomyocytes in these hearts are significantly larger than wild type cardiomyocytes, with enlarged nuclei and distinctive myocardial disarray. Interestingly, the over-expression of ErbB2 induces a concurrent up-regulation of multiple proteins associated with this signaling pathway, including EGFR, ErbB3, ErbB4, PI3K subunits p110 and p85, bcl-2 and multiple protective heat shock proteins. Additionally, ErbB2 up-regulation leads to an anti-apoptotic shift in the ratio of bcl-xS/xL in the heart. Finally, ErbB2 over-expression results in increased activation of the translation machinery involving S6, 4E-BP1 and eIF4E. The dependence of this hypertrophic phenotype on ErbB family signaling is confirmed by reduction in heart mass and cardiomyocyte size, and inactivation of pro-hypertrophic signaling in transgenic animals treated with the ErbB1/2 inhibitor, lapatinib.

CONCLUSIONS/SIGNIFICANCE

These studies are the first to demonstrate that increased ErbB2 over-expression in the heart can activate protective signaling pathways and induce a phenotype consistent with Hypertrophic Cardiomyopathy. Furthermore, our work suggests that in the situation where ErbB2 signaling contributes to cardiac hypertrophy, inhibition of this pathway may reverse this process.

Dissociation of structural and functional phenotypes in cardiac myosin-binding protein C conditional knockout mice

BACKGROUND

Cardiac myosin-binding protein C (cMyBP-C) is a sarcomeric protein that dynamically regulates thick-filament structure and function. In constitutive cMyBP-C knockout (cMyBP-C(-/-)) mice, loss of cMyBP-C has been linked to left ventricular dilation, cardiac hypertrophy, and systolic and diastolic dysfunction, although the pathogenesis of these phenotypes remains unclear.

METHODS AND RESULTS

We generated cMyBP-C conditional knockout (cMyBP-C-cKO) mice expressing floxed cMyBP-C alleles and a tamoxifen-inducible Cre-recombinase fused to 2 mutated estrogen receptors to study the onset and progression of structural and functional phenotypes caused by the loss of cMyBP-C. In adult cMyBP-C-cKO mice, knockdown of cMyBP-C over a 2-month period resulted in a corresponding impairment of diastolic function and a concomitant abbreviation of systolic ejection, although contractile function was largely preserved. No significant changes in cardiac structure or morphology were immediately evident; however, mild hypertrophy developed after near-complete knockdown of cMyBP-C. In response to pressure overload induced by transaortic constriction, cMyBP-C-cKO mice treated with tamoxifen also developed greater cardiac hypertrophy, left ventricular dilation, and reduced contractile function.

CONCLUSIONS

These results indicate that myocardial dysfunction is largely caused by the removal of cMyBP-C and occurs before the onset of cytoarchitectural remodeling in tamoxifen-treated cMyBP-C-cKO myocardium. Moreover, near ablation of cMyBP-C in adult myocardium primarily leads to the development of hypertrophic cardiomyopathy in contrast to the dilated phenotype evident in cMyBP-C(-/-) mice, which highlights the importance of additional factors such as loading stress in determining the expression and progression of cMyBP-C-associated cardiomyopathy.

Cell-intrinsic functional effects of the α-cardiac myosin Arg-403-Gln mutation in familial hypertrophic cardiomyopathy

Human familial hypertrophic cardiomyopathy is the most common Mendelian cardiovascular disease worldwide. Among the most severe presentations of the disease are those in families heterozygous for the mutation R403Q in β-cardiac myosin. Mice heterozygous for this mutation in the α-cardiac myosin isoform display typical familial hypertrophic cardiomyopathy pathology. Here, we study cardiomyocytes from heterozygous 403/+ mice. The effects of the R403Q mutation on force-generating capabilities and dynamics of cardiomyocytes were investigated using a dual carbon nanofiber technique to measure single-cell parameters. We demonstrate the Frank-Starling effect at the single cardiomyocyte level by showing that cell stretch causes an increase in amplitude of contraction. Mutant 403/+ cardiomyocytes exhibit higher end-diastolic and end-systolic stiffness than +/+ cardiomyocytes, whereas active force generation capabilities remain unchanged. Additionally, 403/+ cardiomyocytes show slowed relaxation dynamics. These phenotypes are consistent with increased end-diastolic and end-systolic chamber elastance, as well as diastolic dysfunction seen at the level of the whole heart. Our results show that these functional effects of the R403Q mutation are cell-intrinsic, a property that may be a general phenomenon in familial hypertrophic cardiomyopathy.

Cardiac myosin binding protein-C: redefining its structure and function

Mutations of cardiac myosin binding protein-C (cMyBP-C) are inherited by an estimated 60 million people worldwide, and the protein is the target of several kinases. Recent evidence further suggests that cMyBP-C mutations alter Ca(2+) transients, leading to electrophysiological dysfunction. Thus, while the importance of studying this cardiac sarcomere protein is clear, preliminary data in the literature have raised many questions. Therefore, in this article, we propose to review the structure and function of cMyBP-C with particular respect to the role(s) in cardiac contractility and whether its release into the circulatory system is a potential biomarker of myocardial infarction. We also discuss future directions and experimental designs that may lead to expanding the role(s) of cMyBP-C in the heart. In conclusion, we suggest that cMyBP-C is a regulatory protein that could offer a broad clinical utility in maintaining normal cardiac function.

Increased myofilament Ca2+ sensitivity and diastolic dysfunction as early consequences of Mybpc3 mutation in heterozygous knock-in mice

Hypertrophic cardiomyopathy (HCM) is frequently caused by mutations in MYBPC3 encoding cardiac myosin-binding protein C (cMyBP-C). The mechanisms leading from gene mutations to the HCM phenotype remain incompletely understood, partially because current mouse models of HCM do not faithfully reflect the human situation and early hypertrophy confounds the interpretation of functional alterations. The goal of this study was to evaluate whether myofilament Ca(2+) sensitization and diastolic dysfunction are associated or precede the development of left ventricular hypertrophy (LVH) in HCM. We evaluated the function of skinned and intact cardiac myocytes, as well as the intact heart in a recently developed Mybpc3-targeted knock-in mouse model carrying a point mutation frequently associated with HCM. Compared to wild-type, 10-week old homozygous knock-in mice exhibited i) higher myofilament Ca(2+) sensitivity in skinned ventricular trabeculae, ii) lower diastolic sarcomere length, and faster Ca(2+) transient decay in intact myocytes, and iii) LVH, reduced fractional shortening, lower E/A and E'/A', and higher E/E' ratios by echocardiography and Doppler analysis, suggesting systolic and diastolic dysfunction. In contrast, heterozygous knock-in mice, which mimic the human HCM situation, did not exhibit LVH or systolic dysfunction, but exhibited higher myofilament Ca(2+) sensitivity, faster Ca(2+) transient decay, and diastolic dysfunction. These data demonstrate that myofilament Ca(2+) sensitization and diastolic dysfunction are early phenotypic consequences of Mybpc3 mutations independent of LVH. The accelerated Ca(2+) transients point to compensatory mechanisms directed towards normalization of relaxation. We propose that HCM is a model for diastolic heart failure and this mouse model could be valuable in studying mechanisms and treatment modalities.

Cardiac myosin binding protein C insufficiency leads to early onset of mechanical dysfunction

BACKGROUND

Decreased expression of cardiac myosin binding protein C (cMyBPC) as a result of genetic mutations may contribute to the development of hypertrophic cardiomyopathy (HCM); however, the mechanisms that link cMyBPC expression and HCM development, especially contractile dysfunction, remain unclear.

METHODS AND RESULTS

We evaluated cardiac mechanical function in vitro and in vivo in young mice (8-10 weeks of age) carrying no functional cMyBPC alleles (cMyBPC(-/-)) or 1 functional cMyBPC allele (cMyBPC(±)). Skinned myocardium isolated from cMyBPC(-/-) hearts displayed significant accelerations in stretch activation cross-bridge kinetics. Cardiac MRI studies revealed severely depressed in vivo left ventricular (LV) magnitude and rates of LV wall strain and torsion compared with wild-type (WT) mice. Heterozygous cMyBPC(±) hearts expressed 23±5% less cMyBPC than WT hearts but did not display overt hypertrophy. Skinned myocardium isolated from cMyBPC(±) hearts displayed small accelerations in the rate of stretch induced cross-bridge recruitment. MRI measurements revealed reductions in LV torsion and circumferential strain, as well reduced circumferential strain rates in early systole and diastole.

CONCLUSIONS

Modest decreases in cMyBPC expression in the mouse heart result in early-onset subtle changes in cross-bridge kinetics and in vivo LV mechanical function, which could contribute to the development of HCM later in life.

Genetic variations of β-MYH7 in hypertrophic cardiomyopathy and dilated cardiomyopathy

CONTEXT:

Hypertrophic cardiomyopathy (HCM) is known to be manifested by mutations in 12 sarcomeric genes and dilated cardiomyopathy (DCM) is known to manifest due to cytoskeletal mutations. Studies have revealed that sarcomeric mutations can also lead to DCM. Therefore, in the present study, we have made an attempt to compare and analyze the genetic variations of beta-myosin heavy chain gene (β-MYH7), which are interestingly found to be common in both HCM and DCM. The underlying pathophysiological mechanism leading to two different phenotypes has been discussed in this study. Till date, about 186 and 73 different mutations have been reported in HCM and DCM, respectively, with respect to this gene.

AIM:

The screening of β-MYH7 gene in both HCM and DCM has revealed some common genetic variations. The aim of the present study is to understand the pathophysiological mechanism underlying the manifestation of two different phenotypes.

MATERIALS AND METHODS:

100 controls, 95 HCM and 97 DCM samples were collected. Genomic DNA was extracted following rapid nonenzymatic method as described by Lahiri and Nurnberger (1991), and the extracted DNA was later subjected to polymerase chain reaction (PCR) based single stranded conformation polymorphism (SSCP) analysis to identify single nucleotide polymorphism (SNP)s/mutations associated with the diseased phenotypes.

RESULTS AND CONCLUSION:

Similar variations were observed in β-MYH7 exons 7, 12, 19 and 20 in both HCM and DCM. This could be attributed to impaired energy compromise, or to dose effect of the mutant protein, or to even environmental factors/modifier gene effects wherein an HCM could progress to a DCM phenotype affecting both right and left ventricles, leading to heart failure.

In the thick of it: HCM-causing mutations in myosin binding proteins of the thick filament

In the 20 years since the discovery of the first mutation linked to familial hypertrophic cardiomyopathy (HCM), an astonishing number of mutations affecting numerous sarcomeric proteins have been described. Among the most prevalent of these are mutations that affect thick filament binding proteins, including the myosin essential and regulatory light chains and cardiac myosin binding protein (cMyBP)-C. However, despite the frequency with which myosin binding proteins, especially cMyBP-C, have been linked to inherited cardiomyopathies, the functional consequences of mutations in these proteins and the mechanisms by which they cause disease are still only partly understood. The purpose of this review is to summarize the known disease-causing mutations that affect the major thick filament binding proteins and to relate these mutations to protein function. Conclusions emphasize the impact that discovery of HCM-causing mutations has had on fueling insights into the basic biology of thick filament proteins and reinforce the idea that myosin binding proteins are dynamic regulators of the activation state of the thick filament that contribute to the speed and force of myosin-driven muscle contraction. Additional work is still needed to determine the mechanisms by which individual mutations induce hypertrophic phenotypes.

Screening mutations in myosin binding protein C3 gene in a cohort of patients with Hypertrophic Cardiomyopathy

Background

MyBPC3 mutations are amongst the most frequent causes of hypertrophic cardiomyopathy, however, its prevalence varies between populations. They have been associated with mild and late onset disease expression. Our objectives were to establish the prevalence of MyBPC3 mutations and determine their associated clinical characteristics in our patients.

Methods

Screening by Single Strand Conformation Polymorphisms (SSCP) and sequencing of the fragments with abnormal motility of the MyBPC3 gene in 130 unrelated consecutive HCM index cases. Genotype-Phenotype correlation studies were done in positive families.

Results

16 mutations were found in 20 index cases (15%): 5 novel [D75N, V471E, Q327fs, IVS6+5G>A (homozygous), and IVS11-9G>A] and 11 previously described [A216T, R495W, R502Q (2 families), E542Q (3 families), T957S, R1022P (2 families), E1179K, K504del, K600fs, P955fs and IVS29+5G>A]. Maximum wall thickness and age at time of diagnosis were similar to patients with MYH7 mutations [25(7) vs. 27(8), p = 0.16], [46(16) vs. 44(19), p = 0.9].

Conclusions

Mutations in MyBPC3 are present in 15% of our hypertrophic cardiomyopathy families. Severe hypertrophy and early expression are compatible with the presence of MyBPC3 mutations. The genetic diagnosis not only allows avoiding clinical follow up of non carriers but it opens new possibilities that includes: to take preventive clinical decisions in mutation carriers than have not developed the disease yet, the establishment of genotype-phenotype relationship, and to establish a genetic diagnosis routine in patients with familial HCM.

Differential protein expression profiling of myocardial tissue in a mouse model of hypertrophic cardiomyopathy

Hypertrophic cardiomyopathy (HCM) is a genetic disorder caused by mutations in genes encoding sarcomere proteins. The mechanisms involved in the development of cardiac hypertrophy and heart failure remain poorly understood. Global proteomic profiling was used to study the cardiac proteome of mice predisposed to developing HCM. Hearts from three groups of mice (n=3 hearts per group) were studied: non-transgenic (NTG) and cardiac-specific transgenic models over-expressing either the normal (TnI(WT)) or a mutant cardiac troponin I gene (Gly203Ser; TnI(G203S)). Two-dimensional gel electrophoresis (2-DE) coupled with tandem mass spectrometry was used to identify proteins. Image analysis was performed using Progenesis SameSpots. A total of 34 proteins with at least a twofold change in the TnI(G203S) mouse model were identified. Alterations were detected in components involved in energy production, Ca(2+) handling, and cardiomyocyte structure. Expression level changes in cytoskeletal and contractile proteins were well represented in the study, including the intermediate filament protein desmin, which was further investigated in two additional physiological and pathological settings, i.e., exercise treatment, and severe heart failure in a novel double-mutant TnI-203/MHC-403 model of HCM. This study highlights the potential role of tissue proteomic profiling for mapping proteins, which may be critical in cardiac dysfunction and progression to heart failure in HCM.

Role of animal models in HCM research

Hypertrophic cardiomyopathy (HCM) is a complex cardiovascular genetic disorder characterized by marked clinical and genetic heterogeneity. Major advances have been made in the clinical characterization of patients with HCM and in identifying causative gene mutations. However, many questions remain regarding the underlying disease mechanisms. Furthermore, in a disease where no pharmacological treatments currently exists which can either prevent or cause regression of disease, processes to identify novel therapies are the crucial next steps. Animal models of HCM have already proved to be universally useful in confirming gene causation and dissecting out key molecular pathways involved in the development of HCM and its sequelae, including heart failure and sudden death. These findings have led to studies in animal models investigating novel therapeutic approaches in HCM, specifically targeting the development and progression of cardiac hypertrophy, fibrosis, and heart failure. This review will provide a brief summary of some of the key animal models of HCM and how these models have been utilized to understand disease mechanisms and to investigate new potential therapies. Ongoing studies using animal models of HCM will lead to a greater understanding of disease pathogenesis and will facilitate the translation of these findings to improved clinical outcomes in HCM patients.

Expression patterns of cardiac myofilament proteins: genomic and protein analysis of surgical myectomy tissue from patients with obstructive hypertrophic cardiomyopathy

BACKGROUND

Mutations in myofilament proteins, most commonly MYBPC3-encoded myosin-binding protein C and MYH7-encoded beta-myosin heavy chain, can cause hypertrophic cardiomyopathy (HCM). Despite significant advances in structure-function relationships pertaining to the cardiac sarcomere, there is limited knowledge of how a mutation leads to clinical HCM. We, therefore, set out to study expression and localization of myofilament proteins in left ventricular tissue of patients with HCM.

METHODS AND RESULTS

Frozen surgical myectomy specimens from 47 patients with HCM were examined and genotyped for mutations involving 8 myofilament-encoding genes. Myofilament protein levels were quantified by Western blotting with localization graded from immunohistochemical staining of tissue sections. Overall, 25 of 47 (53%) patients had myofilament-HCM, including 12 with MYBPC3-HCM and 9 with MYH7-HCM. As compared with healthy heart tissue, levels of myofilament proteins were increased in patients manifesting a mutation in either gene. Patients with a frameshift mutation predicted to truncate MYBPC3 exhibited marked disturbances in protein localization as compared with missense mutations in either MYBPC3 or MYH7.

CONCLUSIONS

In this first expression study in human HCM tissue, increased myofilament protein levels in patients with either MYBPC3- or MYH7-mediated HCM suggest a poison peptide mechanism. Specifically, the mechanism of dysfunction may vary according to the genetic subgroup suggested by a distinctly abnormal distribution of myofilament proteins in patients manifesting a truncation mutation in MYBPC3.

Use of mouse models for the analysis of human disease

The first half of this unit discusses the two main approaches to developing genetically engineered mouse models of human disease (i.e., transgenesis and gene targeting). The experimental steps for each method are discussed. Potential pitfalls, advantages, and limitations are also addressed. The second half of the unit describes two mouse models of hypertrophic cardiomyopathy, which illustrate the value of studying such models of human disease, both in terms of understanding disease pathogenesis as well as potential new therapeutic avenues.

Impact of multiple gene mutations in determining the severity of cardiomyopathy and heart failure

1. Familial hypertrophic cardiomyopathy (FHC) is a primary cardiac disorder characterized by myocardial hypertrophy that demonstrates substantial diversity in both genetic causes and clinical manifestations. 2. Clinical heterogeneity can be explained by the causative gene (at least 13 have been identified to date), the position of the amino acid residue affected by a mutation within the protein (over 450 mutations have been reported to date) and modifying genetic and environmental factors. 3. Multiple mutations are found in up to 5% of human FHC cases, who typically present with a more severe phenotype compared with single-mutation carriers (i.e. earlier onset of disease, greater left ventricular hypertrophy and a higher incidence of sudden cardiac death events). 4. Multiple mutations usually involve MYH7, MYBPC3 and, to a lesser extent, TNNI2, reflecting the higher contribution of mutations in these genes to FHC. 5. Multiple-mutation mouse models appear to mimic the human multiple-mutation phenotype and, thus, will help improve our understanding of disease pathogenesis. The models provide a tool for future studies of disease mechanisms and signalling pathways in FHC and its sequelae (i.e. heart failure and sudden death), thereby allowing identification of novel targets for potential therapies and disease prevention strategies.

Exposure to inhaled particulate matter impairs cardiac function in senescent mice

Daily exposure to particulate matter (PM) is known to adversely affect cardiac function and is also known to be exaggerated with senescence. This study tests the hypothesis that cardiac function is uniquely altered by PM exposure in senescent mice. A mechanism for PM-induced cardiac effects is also postulated by examining the activity of nitric oxide synthase (NOS) and the generation of reactive oxygen species (ROS) in heart tissue. Echocardiography is performed in awake 18- and 28-mo-old mice at baseline and immediately following 3-h exposures to either filtered air or carbon black (CB; approximately 400 microg/m3) on 4 days. At 28 mo, left ventricular diameter at end-systole and end-diastole is significantly (P < 0.05) elevated, and fractional shortening is significantly reduced (49 +/- 3% vs. 56 +/- 3%) with CB exposure. In vivo hemodynamic measurements at 28 mo also demonstrate significant (P < 0.05) reductions in ejection fraction and increases in right ventricular and pulmonary vascular pressures following CB exposure. Functional changes at 28 mo are associated with increased ROS production as suggested by enhanced luminol activity. This elevated ROS production with aging and CB exposure is attributable to NOS uncoupling. Measurements of natriuretic peptide (atrial and brain) transcription and matrix metalloproteinase (MMP2 and MMP9) activity in heart tissue are significantly (P < 0.05) amplified with senescence and exposure to CB, pointing to increased cardiac stress and remodeling. These results demonstrate that acute PM exposure reduces cardiac contractility in senescent mice, and this decline in function is associated with increased ROS production linked to NOS uncoupling.