Thin filament mutations: developing an integrative approach to a complex disorder

Natural history and clinical outcomes of patients with hypertrophic cardiomyopathy from thin filament mutations

Hypertrophic cardiomyopathy (HCM) due to thick filament variants is more common; however, HCM due to thin filament variants (HCM-Thin) may be associated with a more malignant phenotype with an increased risk of sudden cardiac death. The aim of this study was to review all the published cases of HCM-Thin to better understand the natural history and clinical outcomes of this disease. A literature review of HCM-Thin identified 21 studies with a total of 177 patients that were suitable for analysis. There were three outcomes of interest, which included a heart failure composite, a ventricular arrhythmia composite and a heart failure and arrhythmia composite outcome. Kaplan-Meier (KM) survival analyses for freedom from each of the abovementioned composite outcomes were completed for the entire cohort and stratified by age of onset and sarcomeric variant. The heart failure composite occurred in 24 (13.6%) patients, the ventricular arrhythmia composite occurred in 30 patients (16.9%) and the combined heart failure and arrhythmia composite occurred in 50 patients (28.2%). In regard to left ventricular ejection fraction (LVEF), the majority of patients were preserved (LVEF > 50%) compared with mildly reduced (LVEF 41%-50%) and reduced (LVEF ≤ 40%) (respectively 26.6% vs. 0.6% vs. 3.4%). The median maximal left ventricular wall thickness (LVWT) was 19.0 mm [interquartile range (IQR) 5.3]. Only 10.7% of the cohort had evidence of left ventricular outflow tract (LVOT) obstruction. Those with paediatric-onset HCM had earlier onset and were at higher risk for each endpoint than their adult counterparts. When stratified by genetic variant, patients with TNNI3 and TPM1 were at a higher risk of the heart failure composite endpoint and the combined heart failure and arrhythmia composite endpoint in comparison with those with the other genetic variants. HCM-Thin is associated with significant morbidity and mortality, with a high arrhythmia burden despite low rates of cardiac obstruction and mild hypertrophy. The paediatric onset of disease and certain sarcomeric variants appear to be associated with a worse prognosis than their adult-onset and other sarcomeric variant counterparts. HCM-Thin seems to have a distinct phenotype, which may require a different management approach.

Myosin's powerstroke transitions define atomic scale movement of cardiac thin filament tropomyosin

Dynamic interactions between the myosin motor head on thick filaments and the actin molecular track on thin filaments drive the myosin-crossbridge cycle that powers muscle contraction. The process is initiated by Ca2+ and the opening of troponin-tropomyosin-blocked myosin-binding sites on actin. The ensuing recruitment of myosin heads and their transformation from pre-powerstroke to post-powerstroke conformation on actin produce the force required for contraction. Cryo-EM-based atomic models confirm that during this process, tropomyosin occupies three different average positions on actin. Tropomyosin pivoting on actin away from a TnI-imposed myosin-blocking position accounts for part of the Ca2+ activation observed. However, the structure of tropomyosin on thin filaments that follows pre-powerstroke myosin binding and its translocation during myosin's pre-powerstroke to post-powerstroke transition remains unresolved. Here, we approach this transition computationally in silico. We used the myosin helix-loop-helix motif as an anchor to dock models of pre-powerstroke cardiac myosin to the cleft between neighboring actin subunits along cardiac thin filaments. We then performed targeted molecular dynamics simulations of the transition between pre- and post-powerstroke conformations on actin in the presence of cardiac troponin-tropomyosin. These simulations show Arg 369 and Glu 370 on the tip of myosin Loop-4 encountering identically charged residues on tropomyosin. The charge repulsion between residues causes tropomyosin translocation across actin, thus accounting for the final regulatory step in the activation of the thin filament, and, in turn, facilitating myosin movement along the filament. We suggest that during muscle activity, myosin-induced tropomyosin movement is likely to result in unencumbered myosin head interactions on actin at low-energy cost.

Lipid metabolism drives allele-specific early-stage hypertrophic cardiomyopathy

Hypertrophic cardiomyopathy (HCM) results from pathogenic variants in sarcomeric protein genes, that increase myocyte energy demand and lead to cardiac hypertrophy. But it is unknown whether a common metabolic trait underlies the cardiac phenotype at early disease stage. This study characterized two HCM mouse models (R92W-TnT, R403Q-MyHC) that demonstrate differences in mitochondrial function at early disease stage. Using a combination of cardiac phenotyping, transcriptomics, mass spectrometry-based metabolomics and computational modeling, we discovered allele-specific differences in cardiac structure/function and metabolic changes. TnT-mutant hearts had impaired energy substrate metabolism and increased phospholipid remodeling compared to MyHC-mutants. TnT-mutants showed increased incorporation of saturated fatty acid residues into ceramides, cardiolipin, and increased lipid peroxidation, that could underlie allele-specific differences in mitochondrial function and cardiomyopathy.

Novel Mutation Glu98Lys in Cardiac Tropomyosin Alters Its Structure and Impairs Myocardial Relaxation

We characterized a novel genetic variant c.292G > A (p.E98K) in the TPM1 gene encoding cardiac tropomyosin 1.1 isoform (Tpm1.1), found in a proband with a phenotype of complex cardiomyopathy with conduction dysfunction and slow progressive neuromuscular involvement. To understand the molecular mechanism by which this mutation impairs cardiac function, we produced recombinant Tpm1.1 carrying an E98K substitution and studied how this substitution affects the structure of the Tpm1.1 molecule and its functional properties. The results showed that the E98K substitution in the N-terminal part of the Tpm molecule significantly destabilizes the C-terminal part of Tpm, thus indicating a long-distance destabilizing effect of the substitution on the Tpm coiled-coil structure. The E98K substitution did not noticeably affect Tpm's affinity for F-actin but significantly impaired Tpm's regulatory properties. It increased the Ca2+ sensitivity of the sliding velocity of regulated thin filaments over cardiac myosin in an in vitro motility assay and caused an incomplete block of the thin filament sliding at low Ca2+ concentrations. The incomplete motility block in the absence of Ca2+ can be explained by the loosening of the Tpm interaction with troponin I (TnI), thus increasing Tpm mobility on the surface of an actin filament that partially unlocks the myosin binding sites. This hypothesis is supported by the molecular dynamics (MD) simulation that showed that the E98 Tpm residue is involved in hydrogen bonding with the C-terminal part of TnI. Thus, the results allowed us to explain the mechanism by which the E98K Tpm mutation impairs sarcomeric function and myocardial relaxation.

An Alternative Mechanism of Subcellular Iron Uptake Deficiency in Cardiomyocytes

BACKGROUND

Systemic defects in intestinal iron absorption, circulation, and retention cause iron deficiency in 50% of patients with heart failure. Defective subcellular iron uptake mechanisms that are independent of systemic absorption are incompletely understood. The main intracellular route for iron uptake in cardiomyocytes is clathrin-mediated endocytosis.

METHODS

We investigated subcellular iron uptake mechanisms in patient-derived and CRISPR/Cas-edited induced pluripotent stem cell-derived cardiomyocytes as well as patient-derived heart tissue. We used an integrated platform of DIA-MA (mass spectrometry data-independent acquisition)-based proteomics and signaling pathway interrogation. We employed a genetic induced pluripotent stem cell model of 2 inherited mutations (TnT [troponin T]-R141W and TPM1 [tropomyosin 1]-L185F) that lead to dilated cardiomyopathy (DCM), a frequent cause of heart failure, to study the underlying molecular dysfunctions of DCM mutations.

RESULTS

We identified a druggable molecular pathomechanism of impaired subcellular iron deficiency that is independent of systemic iron metabolism. Clathrin-mediated endocytosis defects as well as impaired endosome distribution and cargo transfer were identified as a basis for subcellular iron deficiency in DCM-induced pluripotent stem cell-derived cardiomyocytes. The clathrin-mediated endocytosis defects were also confirmed in the hearts of patients with DCM with end-stage heart failure. Correction of the TPM1-L185F mutation in DCM patient-derived induced pluripotent stem cells, treatment with a peptide, Rho activator II, or iron supplementation rescued the molecular disease pathway and recovered contractility. Phenocopying the effects of the TPM1-L185F mutation into WT induced pluripotent stem cell-derived cardiomyocytes could be ameliorated by iron supplementation.

CONCLUSIONS

Our findings suggest that impaired endocytosis and cargo transport resulting in subcellular iron deficiency could be a relevant pathomechanism for patients with DCM carrying inherited mutations. Insight into this molecular mechanism may contribute to the development of treatment strategies and risk management in heart failure.

Thin filament regulation of cardiac muscle power output: Implications for targets to improve human failing hearts

The heart's pumping capacity is determined by myofilament power generation. Power is work done per unit time and measured as the product of force and velocity. At a sarcomere level, these contractile properties are linked to the number of attached cross-bridges and their cycling rate, and many signaling pathways modulate one or both factors. We previously showed that power is increased in rodent permeabilized cardiac myocytes following PKA-mediated phosphorylation of myofibrillar proteins. The current study found that that PKA increased power by ∼30% in permeabilized cardiac myocyte preparations (n = 8) from human failing hearts. To address myofilament molecular specificity of PKA effects, mechanical properties were measured in rat permeabilized slow-twitch skeletal muscle fibers before and after exchange of endogenous slow skeletal troponin with recombinant human Tn complex that contains cardiac (c)TnT, cTnC and either wildtype (WT) cTnI or pseudo-phosphorylated cTnI at sites Ser23/24Asp, Tyr26Glu, or the combinatorial Ser23/24Asp and Tyr26Glu. We found that cTnI Ser23/24Asp, Tyr26Glu, and combinatorial Ser23/24Asp and Tyr26Glu were sufficient to increase power by ∼20%. Next, we determined whether pseudo-phosphorylated cTnI at Ser23/24 was sufficient to increase power in cardiac myocytes from human failing hearts. Following cTn exchange that included cTnI Ser23/24Asp, power output increased ∼20% in permeabilized cardiac myocyte preparations (n = 6) from the left ventricle of human failing hearts. These results implicate cTnI N-terminal phosphorylation as a molecular regulator of myocyte power and could serve as a regional target for small molecule therapy to unmask myocyte power reserve capacity in human failing hearts.

De Novo Asp219Val Mutation in Cardiac Tropomyosin Associated with Hypertrophic Cardiomyopathy

Hypertrophic cardiomyopathy (HCM), caused by mutations in thin filament proteins, manifests as moderate cardiac hypertrophy and is associated with sudden cardiac death (SCD). We identified a new de novo variant, c.656A>T (p.D219V), in the TPM1 gene encoding cardiac tropomyosin 1.1 (Tpm) in a young SCD victim with post-mortem-diagnosed HCM. We produced recombinant D219V Tpm1.1 and studied its structural and functional properties using various biochemical and biophysical methods. The D219V mutation did not affect the Tpm affinity for F-actin but increased the thermal stability of the Tpm molecule and Tpm-F-actin complex. The D219V mutation significantly increased the Ca2+ sensitivity of the sliding velocity of thin filaments over cardiac myosin in an in vitro motility assay and impaired the inhibition of the filament sliding at low Ca2+ concentration. The molecular dynamics (MD) simulation provided insight into a possible molecular mechanism of the effect of the mutation that is most likely a cause of the weakening of the Tpm interaction with actin in the "closed" state and so makes it an easier transition to the “open” state. The changes in the Ca2+ regulation of the actin-myosin interaction characteristic of genetic HCM suggest that the mutation is likely pathogenic.

The effect of Mg2+ on Ca2+ binding to cardiac troponin C in hypertrophic cardiomyopathy associated TNNC1 variants

Cardiac troponin C (cTnC) is the critical Ca2+ -sensing component of the troponin complex. Binding of Ca2+ to cTnC triggers a cascade of conformational changes within the myofilament that culminate in force production. Hypertrophic cardiomyopathy (HCM)-associated TNNC1 variants generally induce a greater degree and duration of Ca2+ binding, which may underly the hypertrophic phenotype. Regulation of contraction has long been thought to occur exclusively through Ca2+ binding to site II of cTnC. However, work by several groups including ours suggest that Mg2+ , which is several orders of magnitude more abundant in the cell than Ca2+ , may compete for binding to the same cTnC regulatory site. We previously used isothermal titration calorimetry (ITC) to demonstrate that physiological concentrations of Mg2+ may decrease site II Ca2+ -binding in both N-terminal and full-length cTnC. Here, we explore the binding of Ca2+ and Mg2+ to cTnC harbouring a series of TNNC1 variants thought to be causal in HCM. ITC and thermodynamic integration (TI) simulations show that A8V, L29Q and A31S elevate the affinity for both Ca2+ and Mg2+ . Further, L48Q, Q50R and C84Y that are adjacent to the EF hand binding motif of site II have a more significant effect on affinity and the thermodynamics of the binding interaction. To the best of our knowledge, this work is the first to explore the role of Mg2+ in modifying the Ca2+ affinity of cTnC mutations linked to HCM. Our results indicate a physiologically significant role for cellular Mg2+ both at baseline and when elevated on modifying the Ca2+ binding properties of cTnC and the subsequent conformational changes which precede cardiac contraction.

Restrictive cardiomyopathy: definition and diagnosis

Restrictive cardiomyopathy (RCM) is a heterogeneous group of diseases characterized by restrictive left ventricular pathophysiology, i.e. a rapid rise in ventricular pressure with only small increases in filling volume due to increased myocardial stiffness. More precisely, the defining feature of RCM is the coexistence of persistent restrictive pathophysiology, diastolic dysfunction, non-dilated ventricles, and atrial dilatation, regardless of ventricular wall thickness and systolic function. Beyond this shared haemodynamic hallmark, the phenotypic spectrum of RCM is wide. The disorders manifesting as RCM may be classified according to four main disease mechanisms: (i) interstitial fibrosis and intrinsic myocardial dysfunction, (ii) infiltration of extracellular spaces, (iii) accumulation of storage material within cardiomyocytes, or (iv) endomyocardial fibrosis. Many disorders do not show restrictive pathophysiology throughout their natural history, but only at an initial stage (with an evolution towards a hypokinetic and dilated phenotype) or at a terminal stage (often progressing from a hypertrophic phenotype). Furthermore, elements of both hypertrophic and restrictive phenotypes may coexist in some patients, making the classification challenge. Restrictive pathophysiology can be demonstrated by cardiac catheterization or Doppler echocardiography. The specific conditions may usually be diagnosed based on clinical data, 12-lead electrocardiogram, echocardiography, nuclear medicine, or cardiovascular magnetic resonance, but further investigations may be needed, up to endomyocardial biopsy and genetic evaluation. The spectrum of therapies is also wide and heterogeneous, but disease-modifying treatments are available only for cardiac amyloidosis and, partially, for iron overload cardiomyopathy.

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.

Low expression of the K280N TNNT2 mutation is sufficient to increase basal myofilament activation in human hypertrophy cardiomyopathy

Background

Hypertrophic cardiomyopathy (HCM) is an autosomal dominant genetic disorder with patients typically showing heterozygous inheritance of a pathogenic variant in a gene encoding a contractile protein. Here, we study the contractile effects of a rare homozygous mutation using explanted tissue and human-induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) to gain insight into how the balance between mutant and WT protein expression affects cardiomyocyte function.

Methods

Force measurements were performed in cardiomyocytes isolated from a HCM patient carrying a homozygous troponin T mutation (cTnT-K280N) and healthy donors. To discriminate between mutation-mediated and phosphorylation-related effects on Ca2+-sensitivity, cardiomyocytes were treated with alkaline phosphatase (AP) or protein kinase A (PKA). Troponin exchange experiments characterized the relation between mutant levels and myofilament function. To define mutation-mediated effects on Ca2+-dynamics we used CRISPR/Cas9 to generate hiPSC-CMs harbouring heterozygous and homozygous TnT-K280N mutations. Ca2+-transient and cell shortening experiments compared these lines against isogenic controls.

Results

Myofilament Ca2+-sensitivity was higher in homozygous cTnT-K280N cardiomyocytes and was not corrected by AP- and PKA-treatment. In cTnT-K280N cells exchanged with cTnT-WT, a low level (14%) of cTnT-K280N mutation elevated Ca2+-sensitivity. Similarly, exchange of donor cells with 45 ± 2% cTnT-K280N increased Ca2+-sensitivity and was not corrected by PKA. cTnT-K280N hiPSC-CMs show elevated diastolic Ca2+ and increases in cell shortening. Impaired cardiomyocyte relaxation was only evident in homozygous cTnT-K280N hiPSC-CMs.

Conclusions

The cTnT-K280N mutation increases myofilament Ca2+-sensitivity, elevates diastolic Ca2+, enhances contractility and impairs cellular relaxation. A low level (14%) of the cTnT-K280N sensitizes myofilaments to Ca2+, a universal finding of human HCM.

Arrhythmogenic Cardiomyopathy: Exercise Pitfalls, Role of Connexin-43, and Moving beyond Antiarrhythmics

Arrhythmogenic Cardiomyopathy (ACM), a Mendelian disorder that can affect both left and right ventricles, is most often associated with pathogenic desmosomal variants that can lead to fibrofatty replacement of the myocardium, a pathological hallmark of this disease. Current therapies are aimed to prevent the worsening of disease phenotypes and sudden cardiac death (SCD). Despite the use of implantable cardioverter defibrillators (ICDs) there is no present therapy that would mitigate the loss in electrical signal and propagation by these fibrofatty barriers. Recent studies have shown the influence of forced vs. voluntary exercise in a variety of healthy and diseased mice; more specifically, that exercised mice show increased Connexin-43 (Cx43) expression levels. Fascinatingly, increased Cx43 expression ameliorated the abnormal electrical signal conduction in the myocardium of diseased mice. These findings point to a major translational pitfall in current therapeutics for ACM patients, who are advised to completely cease exercising and already demonstrate reduced Cx43 levels at the myocyte intercalated disc. Considering cardiac dysfunction in ACM arises from the loss of cardiomyocytes and electrical signal conduction abnormalities, an increase in Cx43 expression-promoted by low to moderate intensity exercise and/or gene therapy-could very well improve cardiac function in ACM patients.

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.

Computational and biophysical determination of pathogenicity of variants of unknown significance in cardiac thin filament

Point mutations within sarcomeric proteins have been associated with altered function and cardiomyopathy development. Difficulties remain, however, in establishing the pathogenic potential of individual mutations, often limiting the use of genotype in management of affected families. To directly address this challenge, we utilized our all-atom computational model of the human full cardiac thin filament (CTF) to predict how sequence substitutions in CTF proteins might affect structure and dynamics on an atomistic level. Utilizing molecular dynamics calculations, we simulated 21 well-defined genetic pathogenic cardiac troponin T and tropomyosin variants to establish a baseline of pathogenic changes induced in computational observables. Computational results were verified via differential scanning calorimetry on a subset of variants to develop an experimental correlation. Calculations were performed on 9 independent variants of unknown significance (VUS), and results were compared with pathogenic variants to identify high-resolution pathogenic signatures. Results for VUS were compared with the baseline set to determine induced structural and dynamic changes, and potential variant reclassifications were proposed. This unbiased, high-resolution computational methodology can provide unique structural and dynamic information that can be incorporated into existing analyses to facilitate classification both for de novo variants and those where established approaches have provided conflicting information.

Beneficial effects of cardiomyopathy-associated genetic variants on physical performance: a hypothesis-generating scoping review

Background Genetic variants associated with cardiomyopathies (CMPs) are prevalent in the general population. In young athletes, CMPs account for roughly a quarter of sudden cardiac death, with further unexplained clustering in specific sports. Consequently, most CMPs form a contra-indication for competitive sports. We hypothesized that genetic variants might (paradoxically) improve physical performance early in life while impairing cardiac function later in life. Methods Systematic PubMed search to investigate whether genetic variants in genes associated with CMPs could be related to beneficial performance phenotypes. Summary In a limited number of studies (n=6), 2860 individuals/ subjects with genetic variants were able to outperform those without said variants, as measured by running speed (~38 m/min in heterozygous (HET) mice, n=6 vs ~32 m/min in wild type (WT) mice, n=7, P=0.004) and distance (966±169 km HET mice vs 561±144 km WT mice, P=0.0035, n=10), elite athlete status in endurance athletes (n=1672, P=1.43*10-8), maximal oxygen uptake in elite athletes (absolute difference not provided, n=32, P=0.005), maximal oxygen uptake in unrelated individuals (n=473, P=0.0025), personal records in highly trained marathon runners (2:26:28±0:06:23 min HET, n=32 vs. 2:28:53±0:05:50 min without polymorphism, n=108, P=0.020), and peripheral muscle force contraction in patients following a cardiac rehabilitation programme (absolute values not provided, n=260). Key message Beneficial effects in genetic variants associated with CMPs could hypothetically play a role in the selection of young athletes, consequently explaining the prevalence of such genetic variants in athletes and the general population.

Loss of crossbridge inhibition drives pathological cardiac hypertrophy in patients harboring the TPM1 E192K mutation

Hypertrophic cardiomyopathy (HCM) is an inherited disorder caused primarily by mutations to thick and thinfilament proteins. Although thin filament mutations are less prevalent than their oft-studied thick filament counterparts, they are frequently associated with severe patient phenotypes and can offer important insight into fundamental disease mechanisms. We have performed a detailed study of tropomyosin (TPM1) E192K, a variant of uncertain significance associated with HCM. Molecular dynamics revealed that E192K results in a more flexible TPM1 molecule, which could affect its ability to regulate crossbridges. In vitro motility assays of regulated actin filaments containing TPM1 E192K showed an overall loss of Ca2+ sensitivity. To understand these effects, we used multiscale computational models that suggested a subtle phenotype in which E192K leads to an inability to completely inhibit actin-myosin crossbridge activity at low Ca2+. To assess the physiological impact of the mutation, we generated patient-derived engineered heart tissues expressing E192K. These tissues showed disease features similar to those of the patients, including cellular hypertrophy, hypercontractility, and diastolic dysfunction. We hypothesized that excess residual crossbridge activity could be triggering cellular hypertrophy, even if the overall Ca2+ sensitivity was reduced by E192K. To test this hypothesis, the cardiac myosin-specific inhibitor mavacamten was applied to patient-derived engineered heart tissues for 4 d followed by 24 h of washout. Chronic mavacamten treatment abolished contractile differences between control and TPM1 E192K engineered heart tissues and reversed hypertrophy in cardiomyocytes. These results suggest that the TPM1 E192K mutation triggers cardiomyocyte hypertrophy by permitting excess residual crossbridge activity. These studies also provide direct evidence that myosin inhibition by mavacamten can counteract the hypertrophic effects of mutant tropomyosin.

Integration of Cardiac Actin Mutants Causing Hypertrophic (p.A295S) and Dilated Cardiomyopathy (p.R312H and p.E361G) into Cellular Structures

The human mutant cardiac α-actins p.A295S or p.R312H and p.E361G, correlated with hypertrophic or dilated cardiomyopathy, respectively, were expressed by the baculovirus/Sf21 insect cell system and purified to homogeneity. The purified cardiac actins maintained their native state but showed differences in Ca²⁺-sensitivity to stimulate the myosin-subfragment1 ATPase. Here we analyzed the interactions of these c-actins with actin-binding and -modifying proteins implicated in cardiomyocyte differentiation. We demonstrate that Arp2/3 complex and the formin mDia3 stimulated the polymerization rate and extent of the c-actins, albeit to different degrees. In addition, we tested the effect of the MICAL-1 monooxygenase, which modifies the supramolecular actin organization during development and adaptive processes. MICAL-1 oxidized these c-actin variants and induced their de-polymerization, albeit at different rates. Transfection experiments using MDCK cells demonstrated the preferable incorporation of wild type and p.A295S c-actins into their microfilament system but of p.R312H and p.E361G actins into the submembranous actin network. Transduction of neonatal rat cardiomyocytes with adenoviral constructs coding HA-tagged c-actin variants showed their incorporation into microfilaments after one day in culture and thereafter into thin filaments of nascent sarcomeric structures at their plus ends (Z-lines) except the p.E361G mutant, which preferentially incorporated at the minus ends.

Computational Methods Elucidate Consequences of Mutations and Post-translational Modifications on Troponin I Effective Concentration to Troponin C

Ca²⁺ binding to cardiac troponin C (cTnC) causes a conformational shift that exposes a hydrophobic patch (cTnC_HP) for binding of the cTnI switch peptide (cTnI_SP), ultimately resulting in contraction of the heart. The inhibitory peptide (cTnI_IP), attached at the N-terminal end of the cTnI_SP, serves as a tether for the cTnI_SP to the rest of the troponin complex. Due to this tethered nature, the cTnI_SP remains within proximity of the hydrophobic patch region, resulting in the cTnC_HP experiencing an elevated "effective concentration" of the cTnI_SP. Mutations to the cTnI_IP region have been hypothesized to cause disease by affecting the ability of the cTnI_SP to "find" the hydrophobic patch, resulting in alterations to the heart's ability to contract normally. We tested this hypothesis using molecular dynamics (MD) simulations of the troponin complex using a model that contained all three subunits of troponin: C, I, and T. We developed methods that allowed us to quantitatively measure the effective concentration of the cTnI_SP from the simulations. A significant reduction in the cTnI_SP effective concentration was observed when the cTnI_IP was removed from the system, showcasing the importance of a tethered cTnI_SP. Through accelerated MD methods, we proposed the minimum effective concentration of a tethered cTnI_SP to be approximately 21 mM. Modification of the cTnI_IP via PKC-mediated phosphorylation of T143 was shown to significantly increase the estimated effective concentration of cTnI_SP, help the cTnI_SP find the cTnC_HP more effectively, and maintain the relative shape of the cTnI_IP when compared to the native model. All of these data indicate that pT143 may be able to help promote binding of cTnI_SP to the cTnC_HP. We then tested six mutations within the cTnI_IP region that are known cTnC Ca²⁺-sensitizing mutations and have been linked with cardiomyopathy. We did not observe a significant reduction in the effective concentration upon the introduction of these mutations; however, we did observe increased variability in the flexibility and dynamics of the cTnI_IP region when compared to native. Our observations led us to hypothesize that the mechanism by which these cardiomyopathic mutations affect Ca²⁺ sensitivity is by altering the off rate of cTnI_SP from the hydrophobic patch.

Molecular dynamics provides new insights into the mechanism of calcium signal transduction and interdomain interactions in cardiac troponin

Understanding the regulation of cardiac muscle contraction at a molecular level is crucial for the development of therapeutics for heart conditions. Despite the availability of atomic structures of the protein components of cardiac muscle thin filaments, detailed insights into their dynamics and response to calcium are yet to be fully depicted. In this study, we used molecular dynamics simulations of the core domains of the cardiac muscle protein troponin to characterize the equilibrium dynamics of its calcium-bound and calcium-free forms, with a focus on elements of cardiac muscle contraction activation and deactivation, that is, calcium binding to the cardiac troponin Ca²⁺ -binding subunit (TnC) and the release of the switch region of the troponin inhibitory subunit (TnI) from TnC. The process of calcium binding to the TnC binding site is described as a three-step process commencing with calcium capture by the binding site residues, followed by cooperative residue interplay bringing the calcium ion to the binding site, and finally, calcium-water exchange. Furthermore, we uncovered a set of TnC-TnI interdomain interactions that are critical for TnC N-lobe hydrophobic pocket dynamics. Absence of these interactions allows the closure of the TnC N-lobe hydrophobic pocket while the TnI switch region remains expelled, whereas if the interactions are maintained, the hydrophobic pocket remains open. Modification of these interactions may fine-tune the ability of the TnC N-lobe hydrophobic pocket to close or remain open, modulate cardiac contractility and present potential therapy-relevant targets.

The spatial distribution of thin filament activation influences force development and myosin activity in computational models of muscle contraction

Striated muscle contraction is initiated by Ca²⁺ binding to, and activating, thin filament regulatory units (RU) within the sarcomere, which then allows myosin cross-bridges from the opposing thick filament to bind actin and generate force. The amount of overlap between the filaments dictates how many potential cross-bridges are capable of binding, and thus how force is generated by the sarcomere. Myopathies and atrophy can impair muscle function by limiting cross-bridge interactions between the filaments, which can occur when the length of the thin filament is reduced or when RU function is disrupted. To investigate how variations in thin filament length and RU density affect ensemble cross-bridge behavior and force production, we simulated muscle contraction using a spatially explicit computational model of the half-sarcomere. Thin filament RUs were disabled either uniformly from the pointed end of the filament (to model shorter thin filament length) or randomly throughout the length of the half-sarcomere. Both uniform and random RU 'knockout' schemes decreased overall force generation during maximal and submaximal activation. The random knockout scheme also led to decreased calcium sensitivity and cooperativity of the force-pCa relationship. We also found that the rate of force development slowed with the random RU knockout, compared to the uniform RU knockout or conditions of normal RU activation. These findings imply that the relationship between RU density and force production within the sarcomere involves more complex coordination than simply the raw number of RUs available for myosin cross-bridge binding, and that the spatial pattern in which activatable RU are distributed throughout the sarcomere influences the dynamics of force production.

C-terminal troponin-I residues trap tropomyosin in the muscle thin filament blocked-state

Tropomyosin and troponin regulate muscle contraction by participating in a macromolecular scale steric-mechanism to control myosin-crossbridge - actin interactions and consequently contraction. At low-Ca²⁺, the C-terminal 30% of troponin subunit-I (TnI) is proposed to trap tropomyosin in a position on thin filaments that sterically interferes with myosin-binding, thus causing muscle relaxation. In contrast, at high-Ca²⁺, inhibition is released after the C-terminal domains dissociate from F-actin-tropomyosin as its component switch-peptide domain binds to the N-lobe of troponin-C (TnC). Recent, paradigm-shifting, cryo-EM reconstructions by the Namba group have revealed density attributed to TnI along cardiac muscle thin filaments at both low- and high-Ca²⁺ concentration. Modeling the reconstructions showed expected high-Ca²⁺ hydrophobic interactions of the TnI switch-peptide and TnC. However, under low-Ca²⁺ conditions, sparse interactions of TnI and tropomyosin, and in particular juxtaposition of non-polar switch-peptide residues and charged tropomyosin amino acids in the published model seem difficult to reconcile with an expected steric-blocking conformation. This anomaly is likely due to inaccurate fitting of tropomyosin into the cryo-EM volume. In the current study, the low-Ca²⁺ cryo-EM volume was fitted with a more accurate tropomyosin model and representation of cardiac TnI. Our results show that at low-Ca²⁺ a cluster of hydrophobic residues at the TnI switch-peptide and adjacent H₄ helix (Ala149, Ala151, Met 154, Leu159, Gly160, Ala161, Ala163, Leu167, Leu169, Ala171, Leu173) draw-in tropomyosin surface residues (Ile143, Ile146, Ala151, Ile154), presumably attracting the entire tropomyosin cable to its myosin-blocking position on actin. The modeling confirms that neighboring TnI "inhibitory domain" residues (Arg145, Arg148) bind to thin filaments at actin residue Asp25, as previously suggested. ClusPro docking of TnI residues 137-184 to actin-tropomyosin, including the TnI inhibitory-domain, switch-peptide and Helix H₄, verified the modeled configuration. Our residue-to-residue contact-mapping of the TnI-tropomyosin association lends itself to experimental validation and functional localization of disease-bearing mutations.

Cardiomyopathic troponin mutations predominantly occur at its interface with actin and tropomyosin

Reversible Ca2+ binding to troponin is the primary on-off switch of the contractile apparatus of striated muscles, including the heart. Dominant missense mutations in human cardiac troponin genes are among the causes of hypertrophic cardiomyopathy (HCM) and dilated cardiomyopathy. Structural understanding of troponin action has recently advanced considerably via electron microscopy and molecular dynamics studies of the thin filament. As a result, it is now possible to examine cardiomyopathy-inducing troponin mutations in thin-filament structural context, and from that to seek new insight into pathogenesis and into the troponin regulatory mechanism. We compiled from consortium reports a representative set of troponin mutation sites whose pathogenicity was determined using standardized clinical genetics criteria. Another set of sites, apparently tolerant of amino acid substitutions, was compiled from the gnomAD v2 database. Pathogenic substitutions occurred predominantly in the areas of troponin that contact actin or tropomyosin, including, but not limited to, two regions of newly proposed structure and long-known implication in cardiomyopathy: the C-terminal third of troponin I and a part of the troponin T N terminus. The pathogenic mutations were located in troponin regions that prevent contraction under low Ca2+ concentration conditions. These regions contribute to Ca2+-regulated steric hindrance of myosin by the combined effects of troponin and tropomyosin. Loss-of-function mutations within these parts of troponin result in loss of inhibition, consistent with the hypercontractile phenotype characteristic of HCM. Notably, pathogenic mutations are absent in our dataset from the Ca2+-binding, activation-producing troponin C (TnC) N-lobe, which controls contraction by a multi-faceted mechanism. Apparently benign mutations are also diminished in the TnC N-lobe, suggesting mutations are poorly tolerated in that critical domain.

The effect of variable troponin C mutation thin filament incorporation on cardiac muscle twitch contractions

One of the complexities of understanding the pathology of familial forms of cardiac diseases is the level of mutation incorporation in sarcomeres. Computational models of the sarcomere that are spatially explicit offer an approach to study aspects of mutational incorporation into myofilaments that are more challenging to get at experimentally. We studied two well characterized mutations of cardiac TnC, L48Q and I61Q, that decrease or increase the release rate of Ca²⁺ from cTnC, k_-Ca, resulting in HCM and DCM respectively [1]. Expression of these mutations in transgenic mice was used to provide experimental data for incorporation of 30 and 50% (respectively) into sarcomeres. Here we demonstrate that fixed length twitch contractions of trabeculae from mice containing mutant differ from WT; L48Q trabeculae have slower relaxation while I61Q trabeculae have markedly reduced peak tension. Using our multiscale modelling approach [2] we were able to describe the tension transients of WT mouse myocardium. Tension transients for the mutant cTnCs were simulated with changes in k_-Ca, measured experimentally for each cTnC mutant in whole troponin complex, a change in the affinity of cTnC for cTnI, and a reduction in the number of detached crossbridges available for binding. A major advantage of the multiscale explicit 3-D model is that it predicts the effects of variable mutation incorporation, and the effects of variations in mutation distribution within thin filaments in sarcomeres. Such effects are currently impossible to explore experimentally. We explored random and clustered distributions of mutant cTnCs in thin filaments, as well as distributions of individual thin filaments with only WT or mutant cTnCs present. The effects of variable amounts of incorporation and non-random distribution of mutant cTnCs are more marked for I61Q than L48Q cTnC. We conclude that this approach can be effective for study on mutations in multiple proteins of the sarcomere. SUMMARY: A challenge in experimental studies of diseases is accounting for the effect of variable mutation incorporation into myofilaments. Here we use a spatially explicit computational approach, informed by experimental data from transgenic mice expressing one of two mutations in cardiac Troponin C that increase or decrease calcium sensitivity. We demonstrate that the model can accurately describe twitch contractions for the data and go on to explore the effect of variable mutant incorporation and localization on simulated cardiac muscle twitches.

Mechanosignaling pathways alter muscle structure and function by post-translational modification of existing sarcomeric proteins to optimize energy usage

A transduced mechanical signal arriving at its destination in muscle alters sarcomeric structure and function. A major question addressed is how muscle mass and tension generation are optimized to match actual performance demands so that little energy is wasted. Three cases for improved energy efficiency are examined: the troponin complex for tuning force production, control of the myosin heads in a resting state, and the Z-disc proteins for sarcomere assembly. On arrival, the regulation of protein complexes is often controlled by post-translational modification (PTM), of which the most common are phosphorylation by kinases, deacetylation by histone deacetylases and ubiquitination by E3 ligases. Another branch of signals acts not through peptide covalent bonding but via ligand interactions (e.g. Ca²⁺ and phosphoinositide binding). The myosin head and the regulation of its binding to actin by the troponin complex is the best and earliest example of signal destinations that modify myofibrillar contractility. PTMs in the troponin complex regulate both the efficiency of the contractile function to match physiologic demand for work, and muscle mass via protein degradation. The regulation of sarcomere assembly by integration of incoming signaling pathways causing the same PTMs or ligand binding are discussed in response to mechanical loading and unloading by the Z-disc proteins CapZ, α-actinin, telethonin, titin N-termini, and others. Many human mutations that lead to cardiomyopathy and heart disease occur in the proteins discussed above, which often occur at their PTM or ligand binding sites.

A comprehensive guide to genetic variants and post-translational modifications of cardiac troponin C

Familial cardiomyopathy is an inherited disease that affects the structure and function of heart muscle and has an extreme range of phenotypes. Among the millions of affected individuals, patients with hypertrophic (HCM), dilated (DCM), or left ventricular non-compaction (LVNC) cardiomyopathy can experience morphologic changes of the heart which lead to sudden death in the most detrimental cases. TNNC1, the gene that codes for cardiac troponin C (cTnC), is a sarcomere gene associated with cardiomyopathies in which probands exhibit young age of presentation and high death, transplant or ventricular fibrillation events relative to TNNT2 and TNNI3 probands. Using GnomAD, ClinVar, UniProt and PhosphoSitePlus databases and published literature, an extensive list to date of identified genetic variants in TNNC1 and post-translational modifications (PTMs) in cTnC was compiled. Additionally, a recent cryo-EM structure of the cardiac thin filament regulatory unit was used to localize each functionally studied amino acid variant and each PTM (acetylation, glycation, s-nitrosylation, phosphorylation) in the structure of cTnC. TNNC1 has a large number of variants (> 100) relative to other genes of the same transcript size. Surprisingly, the mapped variant amino acids and PTMs are distributed throughout the cTnC structure. While many cardiomyopathy-associated variants are localized in α-helical regions of cTnC, this was not statistically significant χ² (p = 0.72). Exploring the variants in TNNC1 and PTMs of cTnC in the contexts of cardiomyopathy association, physiological modulation and potential non-canonical roles provides insights into the normal function of cTnC along with the many facets of TNNC1 as a cardiomyopathic gene.

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

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

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

The effect of tropomyosin variants on cardiomyocyte function and structure that underlie different clinical cardiomyopathy phenotypes

Background - Variants within the alpha-tropomyosin gene (TPM1) cause dominantly inherited cardiomyopathies, including dilated (DCM), hypertrophic (HCM) and restrictive (RCM) cardiomyopathy. Here we investigated whether TPM1 variants observed in DCM and HCM patients affect cardiomyocyte physiology differently. Methods - We identified a large family with DCM carrying a recently identified TPM1 gene variant (T201M) and a child with RCM with compound heterozygote TPM1 variants (E62Q and M281T) whose family members carrying single variants show diastolic dysfunction and HCM. The effects of TPM1 variants (T201M, E62Q or M281T) and of a plasmid containing both the E62Q and M281T variants on single-cell Ca²⁺ transients (CaT) in HL-1 cardiomyocytes were studied. To define toxic threshold levels, we performed dose-dependent transfection of TPM1 variants. In addition, cardiomyocyte structure was studied in human cardiac biopsies with TPM1 variants. Results - Overexpression of TPM1 variants led to time-dependent progressive deterioration of CaT, with the smallest effect seen for E62Q and larger and similar effects seen for the T201M and M281T variants. Overexpression of E62Q/M281T did not exacerbate the effects seen with overexpression of a single TPM1 variant. T201M (DCM) replaced endogenous tropomyosin dose-dependently, while M281T (HCM) did not. Human cardiac biopsies with TPM1 variants revealed loss of sarcomeric structures. Conclusion - All TPM1 variants result in reduced cardiomyocyte CaT amplitudes and loss of sarcomeric structures. These effects may underlie pathophysiology of different cardiomyopathy phenotypes.

TNNT2 mutations in the tropomyosin binding region of TNT1 disrupt its role in contractile inhibition and stimulate cardiac dysfunction

Muscle contraction is regulated by the movement of end-to-end-linked troponin-tropomyosin complexes over the thin filament surface, which uncovers or blocks myosin binding sites along F-actin. The N-terminal half of troponin T (TnT), TNT1, independently promotes tropomyosin-based, steric inhibition of acto-myosin associations, in vitro. Recent structural models additionally suggest TNT1 may restrain the uniform, regulatory translocation of tropomyosin. Therefore, TnT potentially contributes to striated muscle relaxation; however, the in vivo functional relevance and molecular basis of this noncanonical role remain unclear. Impaired relaxation is a hallmark of hypertrophic and restrictive cardiomyopathies (HCM and RCM). Investigating the effects of cardiomyopathy-causing mutations could help clarify TNT1's enigmatic inhibitory property. We tested the hypothesis that coupling of TNT1 with tropomyosin's end-to-end overlap region helps anchor tropomyosin to an inhibitory position on F-actin, where it deters myosin binding at rest, and that, correspondingly, cross-bridge cycling is defectively suppressed under diastolic/low Ca2+ conditions in the presence of HCM/RCM lesions. The impact of TNT1 mutations on Drosophila cardiac performance, rat myofibrillar and cardiomyocyte properties, and human TNT1's propensity to inhibit myosin-driven, F-actin-tropomyosin motility were evaluated. Our data collectively demonstrate that removing conserved, charged residues in TNT1's tropomyosin-binding domain impairs TnT's contribution to inhibitory tropomyosin positioning and relaxation. Thus, TNT1 may modulate acto-myosin activity by optimizing F-actin-tropomyosin interfacial contacts and by binding to actin, which restrict tropomyosin's movement to activating configurations. HCM/RCM mutations, therefore, highlight TNT1's essential role in contractile regulation by diminishing its tropomyosin-anchoring effects, potentially serving as the initial trigger of pathology in our animal models and humans.

A role for actin flexibility in thin filament-mediated contractile regulation and myopathy

Striated muscle contraction is regulated by the translocation of troponin-tropomyosin strands over the thin filament surface. Relaxation relies partly on highly-favorable, conformation-dependent electrostatic contacts between actin and tropomyosin, which position tropomyosin such that it impedes actomyosin associations. Impaired relaxation and hypercontractile properties are hallmarks of various muscle disorders. The α-cardiac actin M305L hypertrophic cardiomyopathy-causing mutation lies near residues that help confine tropomyosin to an inhibitory position along thin filaments. Here, we investigate M305L actin in vivo, in vitro, and in silico to resolve emergent pathological properties and disease mechanisms. Our data suggest the mutation reduces actin flexibility and distorts the actin-tropomyosin electrostatic energy landscape that, in muscle, result in aberrant contractile inhibition and excessive force. Thus, actin flexibility may be required to establish and maintain interfacial contacts with tropomyosin as well as facilitate its movement over distinct actin surface features and is, therefore, likely necessary for proper regulation of contraction.

The α-cardiac actin M305L hypertrophic cardiomyopathy-causing mutation is located near residues that help confine tropomyosin to an inhibitory position along thin filaments. Here the authors assessed M305L actin in vivo, in vitro, and in silico to characterize emergent pathological properties and define the mechanistic basis of disease.

Increased Myocardial Oxygen Consumption Precedes Contractile Dysfunction in Hypertrophic Cardiomyopathy Caused by Pathogenic TNNT2 Gene Variants

Background

Hypertrophic cardiomyopathy is caused by pathogenic sarcomere gene variants. Individuals with a thin‐filament variant present with milder hypertrophy than carriers of thick‐filament variants, although prognosis is poorer. Herein, we defined if decreased energetic status of the heart is an early pathomechanism in TNNT2 (troponin T gene) variant carriers.

Methods and Results

Fourteen individuals with TNNT2 variants (genotype positive), without left ventricular hypertrophy (G+/LVH−; n=6) and with LVH (G+/LVH+; n=8) and 14 healthy controls were included. All participants underwent cardiac magnetic resonance and [¹¹C]‐acetate positron emission tomography imaging to assess LV myocardial oxygen consumption, contractile parameters and myocardial external efficiency. Cardiac efficiency was significantly reduced compared with controls in G+/LVH− and G+/LVH+. Lower myocardial external efficiency in G+/LVH− is explained by higher global and regional oxygen consumption compared with controls without changes in contractile parameters. Reduced myocardial external efficiency in G+/LVH+ is explained by the increase in LV mass and higher oxygen consumption. Septal oxygen consumption was significantly lower in G+/LVH+ compared with G+/LVH−. Although LV ejection fraction was higher in G+/LVH+, both systolic and diastolic strain parameters were lower compared with controls, which was most evident in the hypertrophied septal wall.

Conclusions

Using cardiac magnetic resonance and [¹¹C]‐acetate positron emission tomography imaging, we show that G+/LVH− have an initial increase in oxygen consumption preceding contractile dysfunction and cardiac hypertrophy, followed by a decline in oxygen consumption in G+/LVH+. This suggests that high oxygen consumption and reduced myocardial external efficiency characterize the early gene variant–mediated disease mechanisms that may be used for early diagnosis and development of preventive treatments.

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

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

Docking Troponin T onto the Tropomyosin Overlapping Domain of Thin Filaments

Complete description of thin filament conformational transitions accompanying muscle regulation requires ready access to atomic structures of actin-bound tropomyosin-troponin. To date, several molecular-docking protocols have been employed to identify troponin interactions on actin-tropomyosin because high-resolution experimentally determined structures of filament-associated troponin are not available. However, previously published all-atom models of the thin filament show chain separation and corruption of components during our molecular dynamics simulations of the models, implying artifactual subunit organization, possibly due to incorporation of unorthodox tropomyosin-TnT crystal structures and complex FRET measurements during model construction. For example, the recent Williams et al. (2016) atomistic model of the thin filament displays a paucity of salt bridges and hydrophobic complementarity between the TnT tail (TnT1) and tropomyosin, which is difficult to reconcile with the high, 20 nM K_d binding of TnT onto tropomyosin. Indeed, our molecular dynamics simulations show the TnT1 component in their model partially dissociates from tropomyosin in under 100 ns, whereas actin-tropomyosin and TnT1 models themselves remain intact. We therefore revisited computational work aiming to improve TnT1-thin filament models by employing unbiased docking methodologies, which test billions of trial rotations and translations of TnT1 over three-dimensional grids covering end-to-end bonded tropomyosin alone or tropomyosin on F-actin. We limited conformational searches to the association of well-characterized TnT1 helical domains and either isolated tropomyosin or actin-tropomyosin yet avoided docking TnT domains that lack known or predicted structure. The docking programs PIPER and ClusPro were used, followed by interaction energy optimization and extensive molecular dynamics. TnT1 docked to either side of isolated tropomyosin but uniquely onto one location of actin-bound tropomyosin. The antiparallel interaction with tropomyosin contained abundant salt bridges and intimately integrated hydrophobic networks joining TnT1 and the tropomyosin N-/C-terminal overlapping domain. The TnT1-tropomyosin linkage yields well-defined molecular crevices. Interaction energy measurements strongly favor this TnT1-tropomyosin design over previously proposed models.

Dilated cardiomyopathy mutation (R174W) in troponin T attenuates the length-mediated increase in cross-bridge recruitment and myofilament Ca2+ sensitivity

Alterations in length-dependent activation (LDA) may constitute a mechanism by which cardiomyopathy mutations lead to deleterious phenotypes and compromised heart function, because LDA underlies the molecular basis by which the heart tunes myocardial force production on a beat-to-beat basis (Frank-Starling mechanism). In this study, we investigated the effect of DCM-linked mutation (R173W) in human cardiac troponin T (TnT) on myofilament LDA. R173W mutation is associated with left ventricular dilatation and systolic dysfunction and is found in multiple families. R173W mutation is in the central region (residues 80-180) of TnT, which is known to be important for myofilament cooperativity and cross-bridge (XB) recruitment. Steady-state and dynamic contractile parameters were measured in detergent-skinned guinea pig left ventricular muscle fibers reconstituted with recombinant guinea pig wild-type TnT (TnT_WT) or mutant TnT (TnT_R174W; guinea pig analog of human R173W mutation) at two different sarcomere lengths (SL): short (1.9 µm) and long (2.3 µm). TnT_R174W decreased pCa₅₀ (-log [Ca²⁺]_free required for half-maximal activation) to a greater extent at long than at short SL; for example, pCa₅₀ decreased by 0.12 pCa units at long SL and by 0.06 pCa units at short SL. Differential changes in pCa₅₀ at short and long SL attenuated the SL-dependent increase in myofilament Ca²⁺ sensitivity (ΔpCa₅₀) in TnT_R174W fibers; ΔpCa₅₀ was 0.10 units in TnT_WT fibers but only 0.04 units in TnT_R174W fibers. Furthermore, TnT_R174W blunted the SL-dependent increase in the magnitude of XB recruitment. Our observations suggest that the R173W mutation in human cardiac TnT may impair Frank-Starling mechanism.NEW & NOTEWORTHY This work characterizes the effect of dilated cardiomyopathy mutation in cardiac troponin T (TnT_R174W) on myofilament length-dependent activation. TnT_R174W attenuates the length-dependent increase in cross-bridge recruitment and myofilament Ca²⁺ sensitivity.

Proof of Principle that Molecular Modeling Followed by a Biophysical Experiment Can Develop Small Molecules that Restore Function to the Cardiac Thin Filament in the Presence of Cardiomyopathic Mutations

This article reports a coupled computational experimental approach to design small molecules aimed at targeting genetic cardiomyopathies. We begin with a fully atomistic model of the cardiac thin filament. To this we dock molecules using accepted computational drug binding methodologies. The candidates are screened for their ability to repair alterations in biophysical properties caused by mutation. Hypertrophic and dilated cardiomyopathies caused by mutation are initially biophysical in nature, and the approach we take is to correct the biophysical insult prior to irreversible cardiac damage. Candidate molecules are then tested experimentally for both binding and biophysical properties. This is a proof of concept study-eventually candidate molecules will be tested in transgenic animal models of genetic (sarcomeric) cardiomyopathies.

The actin 'A-triad's' role in contractile regulation in health and disease

Striated muscle contraction is regulated by Ca²⁺ -dependent modulation of myosin cross-bridge binding to F-actin by the thin filament troponin (Tn)-tropomyosin (Tm) complex. In the absence of Ca²⁺ , Tn binds to actin and constrains Tm to an azimuthal location where it sterically occludes myosin binding sites along the thin filament surface. This limits force production and promotes muscle relaxation. In addition to Tn-actin interactions, inhibitory Tm positioning requires associations between other thin filament constituents. For example, the actin 'A-triad', composed of residues K326, K328 and R147, forms numerous, highly favourable electrostatic contacts with Tm that are critical for establishing its inhibitory azimuthal binding position. Here, we review recent findings, including the identification and interrogation of modifications within and proximal to the A-triad that are associated with disease and/or altered muscle behaviour, which highlight the surface feature's role in F-actin-Tm interactions and contractile regulation.

Moving beyond simple answers to complex disorders in sarcomeric cardiomyopathies: the role of integrated systems

The classic clinical definition of hypertrophic cardiomyopathy (HCM) as originally described by Teare is deceptively simple, "left ventricular hypertrophy in the absence of any identifiable cause." Longitudinal studies, however, including a seminal study performed by Frank and Braunwald in 1968, clearly described the disorder much as we know it today, a complex, progressive, and highly variable cardiomyopathy affecting ~ 1/500 individuals worldwide. Subsequent genetic linkage studies in the early 1990s identified mutations in virtually all of the protein components of the cardiac sarcomere as the primary molecular cause of HCM. In addition, a substantial proportion of inherited dilated cardiomyopathy (DCM) has also been linked to sarcomeric protein mutations. Despite our deep understanding of the overall function of the sarcomere as the primary driver of cardiac contractility, the ability to use genotype in patient management remains elusive. A persistent challenge in the field from both the biophysical and clinical standpoints is how to rigorously link high-resolution protein dynamics and mechanics to the long-term cardiovascular remodeling process that characterizes these complex disorders. In this review, we will explore the depth of the problem from both the standpoint of a multi-subunit, highly conserved and dynamic "machine" to the resultant clinical and structural human phenotype with an emphasis on new, integrative approaches that can be widely applied to identify both novel disease mechanisms and new therapeutic targets for these primary biophysical disorders of the cardiac sarcomere.

A new twist on tropomyosin binding to actin filaments: perspectives on thin filament function, assembly and biomechanics

Tropomyosin, best known for its role in the steric regulation of muscle contraction, polymerizes head-to-tail to form cables localized along the length of both muscle and non-muscle actin-based thin filaments. In skeletal and cardiac muscles, tropomyosin, under the control of troponin and myosin, moves in a cooperative manner between blocked, closed and open positions on filaments, thereby masking and exposing actin-binding sites necessary for myosin crossbridge head interactions. While the coiled-coil signature of tropomyosin appears to be simple, closer inspection reveals surprising structural complexity required to perform its role in steric regulation. For example, component α-helices of coiled coils are typically zippered together along a continuous core hydrophobic stripe. Tropomyosin, however, contains a number of anomalous, functionally controversial, core amino acid residues. We argue that the atypical residues at this interface, including clusters of alanines and a charged aspartate, are required for preshaping tropomyosin to readily fit to the surface of the actin filament, but do so without compromising tropomyosin rigidity once the filament is assembled. Indeed, persistence length measurements of tropomyosin are characteristic of a semi-rigid cable, in this case conducive to cooperative movement on thin filaments. In addition, we also maintain that tropomyosin displays largely unrecognized and residue-specific torsional variance, which is involved in optimizing contacts between actin and tropomyosin on the assembled thin filament. Corresponding twist-induced stiffness may also enhance cooperative translocation of tropomyosin across actin filaments. We conclude that anomalous core residues of tropomyosin facilitate thin filament regulatory behavior in a multifaceted way.

Structural and functional impact of troponin C-mediated Ca2+ sensitization on myofilament lattice spacing and cross-bridge mechanics in mouse cardiac muscle

Acto-myosin cross-bridge kinetics are important for beat-to-beat regulation of cardiac contractility; however, physiological and pathophysiological mechanisms for regulation of contractile kinetics are incompletely understood. Here we explored whether thin filament-mediated Ca²⁺ sensitization influences cross-bridge kinetics in permeabilized, osmotically compressed cardiac muscle preparations. We used a murine model of hypertrophic cardiomyopathy (HCM) harboring a cardiac troponin C (cTnC) Ca²⁺-sensitizing mutation, Ala8Val in the regulatory N-domain. We also treated wild-type murine muscle with bepridil, a cTnC-targeting Ca²⁺ sensitizer. Our findings suggest that both methods of increasing myofilament Ca²⁺ sensitivity increase cross-bridge cycling rate measured by the rate of tension redevelopment (k_TR); force per cross-bridge was also enhanced as measured by sinusoidal stiffness and I_1,1/I_1,0 ratio from X-ray diffraction. Computational modeling suggests that Ca²⁺ sensitization through this cTnC mutation or bepridil accelerates k_TR primarily by promoting faster cross-bridge detachment. To elucidate if myofilament structural rearrangements are associated with changes in k_TR, we used small angle X-ray diffraction to simultaneously measure myofilament lattice spacing and isometric force during steady-state Ca²⁺ activations. Within in vivo lattice dimensions, lattice spacing and steady-state isometric force increased significantly at submaximal activation. We conclude that the cTnC N-domain controls force by modulating both the number and rate of cycling cross-bridges, and that the both methods of Ca²⁺ sensitization may act through stabilization of cTnC's D-helix. Furthermore, we propose that the transient expansion of the myofilament lattice during Ca²⁺ activation may be an additional factor that could increase the rate of cross-bridge cycling in cardiac muscle. These findings may have implications for the pathophysiology of HCM.

The genetics of hypertrophic cardiomyopathy

Hypertrophic cardiomyopathy (HCM) is most commonly transmitted as an autosomal dominant trait, caused by mutations in genes encoding cardiac sarcomere proteins^1–3. Other inheritable causes of the disease include mutations in genes coding for proteins important in calcium handling or that form part of the cytoskeleton^4–6. At present, the primary clinical role of genetic testing in HCM is to facilitate familial screening to allow the identification of individuals at risk of developing the disease^7,8. It is also used to diagnose genocopies, such as lysosomal^9–11 and glycogen storage disease which have different treatment strategies, rates of disease progression and prognosis^12–14. The role of genetic testing in predicting prognosis is limited at present, but emerging data suggest that knowledge of the genetic basis of disease will assume an important role in disease stratification^15–17 and offer potential targets for disease-modifying therapy in the near future¹⁸.

Unraveling obscurins in heart disease

Obscurins, expressed from the single OBSCN gene, are a family of giant, modular, cytoskeletal proteins that play key structural and regulatory roles in striated muscles. They were first implicated in the development of heart disease in 2007 when two missense mutations were found in a patient diagnosed with hypertrophic cardiomyopathy (HCM). Since then, the discovery of over a dozen missense, frameshift, and splicing mutations that are linked to various forms of cardiomyopathy, including HCM, dilated cardiomyopathy (DCM), and left ventricular non-compaction (LVNC), has highlighted OBSCN as a potential disease-causing gene. At this time, the functional consequences of the identified mutations remain largely elusive, and much work has yet to be done to characterize the disease mechanisms of pathological OBSCN variants. Herein, we describe the OBSCN mutations known to date, discuss their potential impact on disease development, and provide future directions in order to better understand the involvement of obscurins in heart disease.

Mechanistic complexity of contractile dysfunction in hypertrophic cardiomyopathy

Reflections on recent work providing mechanistic insight into the pathological effects of a cardiac troponin T mutation.

The TNNI3 Arg192His mutation in a 13-year-old girl with left ventricular noncompaction

Left ventricular noncompaction (LVNC) is a distinct cardiomyopathy that is morphologically characterized by a two-layered myocardium, numerous prominent trabeculations, and deep intertrabecular recesses communicating with the left ventricular cavity. We present a case report regarding the identification of a new mutation in TNNI3 in a patient with LVNC using next-generation sequencing. A 13-year-old girl who had no family history of cardiac disease was hospitalized with dyspnea after exercise and electrocardiographic abnormalities during a school screening. Based on her clinical features, she was diagnosed with LVNC. Via genetic analysis, a TNNI3 heterozygous missense variant was identified in the proband. Although mutations in TNNI3 have been reported in patients with hypertrophic cardiomyopathy and restrictive cardiomyopathy, this is the first report of a mutation in this gene in a patient with LVNC. <Learning objective: We identified a variant in TNNI3 in a patient with isolated left ventricular noncompaction using next-generation sequencing (NGS). Mutations in TNNI3 have been reported in patients with hypertrophic cardiomyopathy and restrictive cardiomyopathy. The use of NGS also results in the identification of multiple genetic variants of unknown significance to the investigated disease.>.

Actin-associated proteins and cardiomyopathy-the 'unknown' beyond troponin and tropomyosin

It has been known for several decades that mutations in genes that encode for proteins involved in the control of actomyosin interactions such as the troponin complex, tropomyosin and MYBP-C and thus regulate contraction can lead to hereditary hypertrophic cardiomyopathy. In recent years, it has become apparent that actin-binding proteins not directly involved in the regulation of contraction also can exhibit changed expression levels, show altered subcellular localisation or bear mutations that might lead to hereditary cardiomyopathies. The aim of this review is to look beyond the troponin/tropomyosin mechanism and to give an overview of the different types of actin-associated proteins and their potential roles in cardiomyocytes. It will then discuss recent findings relevant to their involvement in heart disease.

Distortion of the Actin A-Triad Results in Contractile Disinhibition and Cardiomyopathy

Striated muscle contraction is regulated by the movement of tropomyosin over the thin filament surface, which blocks or exposes myosin binding sites on actin. Findings suggest that electrostatic contacts, particularly those between K326, K328, and R147 on actin and tropomyosin, establish an energetically favorable F-actin-tropomyosin configuration, with tropomyosin positioned in a location that impedes actomyosin associations and promotes relaxation. Here, we provide data that directly support a vital role for these actin residues, termed the A-triad, in tropomyosin positioning in intact functioning muscle. By examining the effects of an A295S α-cardiac actin hypertrophic cardiomyopathy-causing mutation, over a range of increasingly complex in silico, in vitro, and in vivo Drosophila muscle models, we propose that subtle A-triad-tropomyosin perturbation can destabilize thin filament regulation, which leads to hypercontractility and triggers disease. Our efforts increase understanding of basic thin filament biology and help unravel the mechanistic basis of a complex cardiac disorder.

The continuing evolution of cardiac troponin I biomarker analysis: from protein to proteoform

INTRODUCTION

The troponin complex consists of three proteins that fundamentally couple excitation with contraction. Circulating cardiac-specific Troponin I (cTnI) serves as diagnostic biomarker tools for risk stratification of acute coronary syndromes and acute myocardial infarction (MI). Within the heart, cTnI oscillates between inactive and active conformations to either block or disinhibit actinomyosin formation. This molecular mechanism is fine-tuned through extensive protein modifications whose profiles are maladaptively altered with co-morbidities including hypertrophic cardiomyopathy, diabetes, and heart failure. Technological advances in analytical platforms over the last decade enable routine baseline cTnI analysis in patients without cardiovascular complications, and hold potential to expand cTnI readouts that include modified cTnI proteoforms. Areas covered: This review covers the current state, advances, and prospects of analytical platforms that now enable routine baseline cTnI analysis in patients. In parallel, improved mass spectrometry instrumentation and workflows already reveal an array of modified cTnI proteoforms with promising diagnostic implications. Expert commentary: New analytical capabilities provide clinicians and researchers with an opportunity to address important questions surrounding circulating cTnI in the improved diagnosis of specific patient cohorts. These techniques also hold considerable promise for new predictive and prescriptive applications for individualized profiling and improve patient care.

Modulation of the picosecond dynamics of troponin by the cardiomyopathy-causing mutation K247R of troponin T observed by quasielastic neutron scattering

Troponin (Tn), consisting of three subunits (TnC, TnI, and TnT), regulates cardiac muscle contraction in a Ca²⁺-dependent manner. Various point mutations of human cardiac Tn are known to cause familial hypertrophic cardiomyopathy due to aberration of the regulatory function. In this study, we investigated the effects of one of these mutations, K247R of TnT, on the picosecond dynamics of the Tn core domain (Tn-CD), consisting of TnC, TnI and TnT2 (183-288 residues of TnT), by carrying out the quasielastic neutron scattering measurements on the reconstituted Tn-CD containing either the wild-type TnT2 (wtTn-CD) or the mutant TnT2 (K247R-Tn-CD) in the absence and presence of Ca²⁺. It was found that Ca²⁺-binding to the wtTn-CD decreases the residence time of atomic motions in the Tn-CD with slight changes in amplitudes, suggesting that the regulatory function mainly requires modulation of frequency of atomic motions. On the other hand, the K247R-Tn-CD shows different dynamic behavior from that of the wtTn-CD both in the absence and presence of Ca²⁺. In particular, the K247R-Tn-CD exhibits a larger amplitude than the wtTn-CD in the presence of Ca²⁺, suggesting that the mutant can explore larger conformational space than the wild-type. This increased flexibility should be relevant to the functional aberration of this mutant.

Clinically Divergent Mutation Effects on the Structure and Function of the Human Cardiac Tropomyosin Overlap

The progression of genetically inherited cardiomyopathies from an altered protein structure to clinical presentation of disease is not well understood. One of the main roadblocks to mechanistic insight remains a lack of high-resolution structural information about multiprotein complexes within the cardiac sarcomere. One example is the tropomyosin (Tm) overlap region of the thin filament that is crucial for the function of the cardiac sarcomere. To address this central question, we devised coupled experimental and computational modalities to characterize the baseline function and structure of the Tm overlap, as well as the effects of mutations causing divergent patterns of ventricular remodeling on both structure and function. Because the Tm overlap contributes to the cooperativity of myofilament activation, we hypothesized that mutations that enhance the interactions between overlap proteins result in more cooperativity, and conversely, those that weaken interaction between these elements lower cooperativity. Our results suggest that the Tm overlap region is affected differentially by dilated cardiomyopathy-associated Tm D230N and hypertrophic cardiomyopathy-associated human cardiac troponin T (cTnT) R92L. The Tm D230N mutation compacts the Tm overlap region, increasing the cooperativity of the Tm filament, contributing to a dilated cardiomyopathy phenotype. The cTnT R92L mutation causes weakened interactions closer to the N-terminal end of the overlap, resulting in decreased cooperativity. These studies demonstrate that mutations with differential phenotypes exert opposite effects on the Tm-Tn overlap, and that these effects can be directly correlated to a molecular level understanding of the structure and dynamics of the component proteins.

Overview of the Muscle Cytoskeleton

Cardiac and skeletal striated muscles are intricately designed machines responsible for muscle contraction. Coordination of the basic contractile unit, the sarcomere, and the complex cytoskeletal networks are critical for contractile activity. The sarcomere is comprised of precisely organized individual filament systems that include thin (actin), thick (myosin), titin, and nebulin. Connecting the sarcomere to other organelles (e.g., mitochondria and nucleus) and serving as the scaffold to maintain cellular integrity are the intermediate filaments. The costamere, on the other hand, tethers the sarcomere to the cell membrane. Unique structures like the intercalated disc in cardiac muscle and the myotendinous junction in skeletal muscle help synchronize and transmit force. Intense investigation has been done on many of the proteins that make up these cytoskeletal assemblies. Yet the details of their function and how they interconnect have just started to be elucidated. A vast number of human myopathies are contributed to mutations in muscle proteins; thus understanding their basic function provides a mechanistic understanding of muscle disorders. In this review, we highlight the components of striated muscle with respect to their interactions, signaling pathways, functions, and connections to disease. © 2017 American Physiological Society. Compr Physiol 7:891-944, 2017.