Thin filament incorporation of an engineered cardiac troponin C variant (L48Q) enhances contractility in intact cardiomyocytes from healthy and infarcted hearts

Physiology of intracellular calcium buffering

Calcium signaling underlies much of physiology. Almost all the Ca2+ in the cytoplasm is bound to buffers, with typically only ∼1% being freely ionized at resting levels in most cells. Physiological Ca2+ buffers include small molecules and proteins, and experimentally Ca2+ indicators will also buffer calcium. The chemistry of interactions between Ca2+ and buffers determines the extent and speed of Ca2+ binding. The physiological effects of Ca2+ buffers are determined by the kinetics with which they bind Ca2+ and their mobility within the cell. The degree of buffering depends on factors such as the affinity for Ca2+, the Ca2+ concentration, and whether Ca2+ ions bind cooperatively. Buffering affects both the amplitude and time course of cytoplasmic Ca2+ signals as well as changes of Ca2+ concentration in organelles. It can also facilitate Ca2+ diffusion inside the cell. Ca2+ buffering affects synaptic transmission, muscle contraction, Ca2+ transport across epithelia, and the killing of bacteria. Saturation of buffers leads to synaptic facilitation and tetanic contraction in skeletal muscle and may play a role in inotropy in the heart. This review focuses on the link between buffer chemistry and function and how Ca2+ buffering affects normal physiology and the consequences of changes in disease. As well as summarizing what is known, we point out the many areas where further work is required.

Mouse Models of Cardiomyopathies Caused by Mutations in Troponin C

Cardiac muscle contraction is regulated via Ca2+ exchange with the hetero-trimeric troponin complex located on the thin filament. Binding of Ca2+ to cardiac troponin C, a Ca2+ sensing subunit within the troponin complex, results in a series of conformational re-arrangements among the thin filament components, leading to an increase in the formation of actomyosin cross-bridges and muscle contraction. Ultimately, a decline in intracellular Ca2+ leads to the dissociation of Ca2+ from troponin C, inhibiting cross-bridge cycling and initiating muscle relaxation. Therefore, troponin C plays a crucial role in the regulation of cardiac muscle contraction and relaxation. Naturally occurring and engineered mutations in troponin C can lead to altered interactions among components of the thin filament and to aberrant Ca2+ binding and exchange with the thin filament. Mutations in troponin C have been associated with various forms of cardiac disease, including hypertrophic, restrictive, dilated, and left ventricular noncompaction cardiomyopathies. Despite progress made to date, more information from human studies, biophysical characterizations, and animal models is required for a clearer understanding of disease drivers that lead to cardiomyopathies. The unique use of engineered cardiac troponin C with the L48Q mutation that had been thoroughly characterized and genetically introduced into mouse myocardium clearly demonstrates that Ca2+ sensitization in and of itself should not necessarily be considered a disease driver. This opens the door for small molecule and protein engineering strategies to help boost impaired systolic function. On the other hand, the engineered troponin C mutants (I61Q and D73N), genetically introduced into mouse myocardium, demonstrate that Ca2+ desensitization under basal conditions may be a driving factor for dilated cardiomyopathy. In addition to enhancing our knowledge of molecular mechanisms that trigger hypertrophy, dilation, morbidity, and mortality, these cardiomyopathy mouse models could be used to test novel treatment strategies for cardiovascular diseases. In this review, we will discuss (1) the various ways mutations in cardiac troponin C might lead to disease; (2) relevant data on mutations in cardiac troponin C linked to human disease, and (3) all currently existing mouse models containing cardiac troponin C mutations (disease-associated and engineered).

Machine learning meets Monte Carlo methods for models of muscle's molecular machinery to classify mutations

The timing and magnitude of force generation by a muscle depend on complex interactions in a compliant, contractile filament lattice. Perturbations in these interactions can result in cardiac muscle diseases. In this study, we address the fundamental challenge of connecting the temporal features of cardiac twitches to underlying rate constants and their perturbations associated with genetic cardiomyopathies. Current state-of-the-art metrics for characterizing the mechanical consequence of cardiac muscle disease do not utilize information embedded in the complete time course of twitch force. We pair dimension reduction techniques and machine learning methods to classify underlying perturbations that shape the timing of twitch force. To do this, we created a large twitch dataset using a spatially explicit Monte Carlo model of muscle contraction. Uniquely, we modified the rate constants of this model in line with mouse models of cardiac muscle disease and varied mutation penetrance. Ultimately, the results of this study show that machine learning models combined with biologically informed dimension reduction techniques can yield excellent classification accuracy of underlying muscle perturbations.

Computational Exploration and Characterization of Potential Calcium Sensitizing Mutations in Cardiac Troponin C

Calcium-dependent heart muscle contraction is regulated by the cardiac troponin protein complex (cTn) and specifically by the N-terminal domain of its calcium binding subunit (cNTnC). cNTnC contains one calcium binding site (site II), and altered calcium binding in this site has been studied for decades. It has been previously shown that cNTnC mutants, which increase calcium sensitization may have therapeutic benefits, such as restoring cardiac muscle contractility and functionality post-myocardial infarction events. Here, we computationally characterized eight mutations for their potential effects on calcium binding affinity in site II of cNTnC. We utilized two distinct methods to estimate calcium binding: adaptive steered molecular dynamics (ASMD) and thermodynamic integration (TI). We observed a sensitizing trend for all mutations based on the employed ASMD methodology. The TI results showed excellent agreement with experimentally known calcium binding affinities in wild-type cNTnC. Based on the TI results, five mutants were predicted to increase calcium sensitivity in site II. This study presents an interesting comparison of the two computational methods, which have both been shown to be valuable tools in characterizing the impacts of calcium sensitivity in mutant cNTnC systems.

Adaptative Steered Molecular Dynamics Study of Mutagenesis Effects on Calcium Affinity in the Regulatory Domain of Cardiac Troponin C

Calcium-dependent cardiac muscle contraction is regulated by the protein complex troponin (cTn) and specifically by the regulatory N-terminal domain (N-cTnC) which contains one active Ca²⁺ binding site (site II). It has been previously shown that cardiac muscle contractility and functionality is affected by mutations in N-cTnC which alter calcium binding affinity. Here, we describe the application of adaptive steered molecular dynamics to characterize the influence of N-cTnC mutations on site II calcium binding affinity. We observed the correct trends for all of the studied calcium sensitizing and desensitizing mutants, in conjunction with loop II perturbations. Additionally, the potential of mean force accuracy was shown to increase substantially with increasingly slower speeds and using fewer trajectories. This study presents a novel approach to computationally estimate the Ca²⁺ binding affinity of N-cTnC structures and is a valuable potential tool to support the design and characterization of novel mutations with potential therapeutic benefits.

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.

The Positively Charged C-Terminal Region of Human Skeletal Troponin T Retards Activation and Decreases Calcium Sensitivity

Calcium binding to troponin C (TnC) activates striated muscle contraction by removing TnI (troponin I) from its inhibitory site on actin. Troponin T (TnT) links TnI with tropomyosin, causing tropomyosin to move from an inhibitory position on actin to an activating position. Positive charges within the C-terminal region of human cardiac TnT limit Ca²⁺ activation. We now show that the positively charged region of TnT has an even larger impact on skeletal muscle regulation. We prepared one variant of human skeletal TnT that had the C-terminal 16 residues truncated (Δ16) and another with an added C-terminal Cys residue and Ala substituted for the last 6 basic residues (251C-HAHA). Both mutants reduced (based on S1 binding kinetics) or eliminated (based on acrylodan-tropomyosin fluorescence) the first inactive state of actin at <10 nM free Ca²⁺. 251C-HAHA-TnT and Δ16-TnT mutants greatly increased ATPase activation at 0.2 mM Ca²⁺, even without high-affinity cross-bridge binding. They also shifted the force-pCa curve of muscle fibers to lower Ca²⁺ by 0.8-1.2 pCa units (the larger shift for 251C-HAHA-TnT). Shifts in force-pCa were maintained in the presence of para-aminoblebbistatin. The effects of modification of the C-terminal region of TnT on the kinetics of S1 binding to actin were somewhat different from those observed earlier with the cardiac analogue. In general, the C-terminal region of human skeletal TnT is critical to regulation, just as it is in the cardiac system, and is a potential target for modulating activity.

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.

Sarcomere length-dependent effects on Ca2+-troponin regulation in myocardium expressing compliant titin

Increases in sarcomere length cause enhanced force generation in cardiomyocytes by an unknown mechanism. Li et al. reveal that titin-based passive tension contributes to length-dependent activation of myofilaments and that tightly bound myosin–actin cross-bridges are associated with this effect.

Cardiac performance is tightly regulated at the cardiomyocyte level by sarcomere length, such that increases in sarcomere length lead to sharply enhanced force generation at the same Ca²⁺ concentration. Length-dependent activation of myofilaments involves dynamic and complex interactions between a multitude of thick- and thin-filament components. Among these components, troponin, myosin, and the giant protein titin are likely to be key players, but the mechanism by which these proteins are functionally linked has been elusive. Here, we investigate this link in the mouse myocardium using in situ FRET techniques. Our objective was to monitor how length-dependent Ca²⁺-induced conformational changes in the N domain of cardiac troponin C (cTnC) are modulated by myosin–actin cross-bridge (XB) interactions and increased titin compliance. We reconstitute FRET donor- and acceptor-modified cTnC(13C/51C)AEDANS-DDPM into chemically skinned myocardial fibers from wild-type and RBM20-deletion mice. The Ca²⁺-induced conformational changes in cTnC are quantified and characterized using time-resolved FRET measurements as XB state and sarcomere length are varied. The RBM20-deficient mouse expresses a more compliant N2BA titin isoform, leading to reduced passive tension in the myocardium. This provides a molecular tool to investigate how altered titin-based passive tension affects Ca²⁺-troponin regulation in response to mechanical stretch. In wild-type myocardium, we observe a direct association of sarcomere length–dependent enhancement of troponin regulation with both Ca²⁺ activation and strongly bound XB states. In comparison, measurements from titin RBM20-deficient animals show blunted sarcomere length–dependent effects. These results suggest that titin-based passive tension contributes to sarcomere length–dependent Ca²⁺-troponin regulation. We also conclude that strong XB binding plays an important role in linking the modulatory effect of titin compliance to Ca²⁺-troponin regulation of the myocardium.

Molecular Dynamics and Umbrella Sampling Simulations Elucidate Differences in Troponin C Isoform and Mutant Hydrophobic Patch Exposure

Troponin C (TnC) facilitates muscle contraction through calcium-binding within its N-terminal region (NTnC). As previously observed using molecular dynamics (MD) simulations, this calcium-binding event leads to an increase in the dynamics of helices lining a hydrophobic patch on TnC. Simulation times of multiple microseconds were required to even see a partial opening of the hydrophobic patch, limiting the ability to thoroughly and quantitatively investigate these rare events. Here we describe the application of umbrella sampling to probe the TnC hydrophobic patch opening in a more targeted and quantitative fashion. Umbrella sampling was utilized to investigate the differences in the free energy of opening between cardiac (cTnC) and fast skeletal TnC (sTnC). We found that, in agreement with previous reports, holo (calcium-bound) sTnC had a lower free energy of opening compared with holo cTnC. Additionally, differences in the free energy of opening of hypertrophic (HCM) and dilated cardiomyopathy (DCM) cTnC systems were investigated. MD simulations and umbrella sampling revealed a lower free energy of opening for the HCM mutations A8V and A31S, as well as the calcium-sensitizing mutation L48Q. The DCM mutations, Y5H, Q50R, and E59D/D75Y, all exhibited a higher free energy of opening. An umbrella sampling simulation of cTnI-bound holo cTnC exhibited the lowest free energy in the open configuration, in agreement with experimental data. In conclusion, this study presents a novel and successful protocol for applying umbrella sampling simulations to quantitatively study the molecular basis of muscle contraction and proposes a mechanism by which HCM and DCM-associated mutations influence contraction.

Early sensitization of myofilaments to Ca2+ prevents genetically linked dilated cardiomyopathy in mice

Background

Dilated cardiomoypathies (DCM) are a heterogeneous group of inherited and acquired diseases characterized by decreased contractility and enlargement of cardiac chambers and a major cause of morbidity and mortality. Mice with Glu54Lys mutation in α-tropomyosin (Tm54) demonstrate typical DCM phenotype with reduced myofilament Ca2+ sensitivity. We tested the hypothesis that early sensitization of the myofilaments to Ca2+ in DCM can prevent the DCM phenotype.

Methods and results

To sensitize Tm54 myofilaments, we used a genetic approach and crossbred Tm54 mice with mice expressing slow skeletal troponin I (ssTnI) that sensitizes myofilaments to Ca2+. Four groups of mice were used: non-transgenic (NTG), Tm54, ssTnI and Tm54/ssTnI (DTG). Systolic function was significantly reduced in the Tm54 mice compared to NTG, but restored in DTG mice. Tm54 mice also showed increased diastolic LV dimensions and HW/BW ratios, when compared to NTG, which were improved in the DTG group. β-myosin heavy chain expression was increased in the Tm54 animals compared to NTG and was partially restored in DTG group. Analysis by 2D-DIGE indicated a significant decrease in two phosphorylated spots of cardiac troponin I (cTnI) in the DTG animals compared to NTG and Tm54. Analysis by 2D-DIGE also indicated no significant changes in troponin T, regulatory light chain, myosin binding protein C and tropomyosin phosphorylation.

Conclusion

Our data indicate that decreased myofilament Ca2+ sensitivity is an essential element in the pathophysiology of thin filament linked DCM. Sensitization of myofilaments to Ca2+ in the early stage of DCM may be a useful therapeutic strategy in thin filament linked DCM.

Cardiac Myosin Activation with Gene Therapy Produces Sustained Inotropic Effects and May Treat Heart Failure with Reduced Ejection Fraction

Chronic inotropic therapy is effective for the treatment of heart failure with reduced ejection fraction, but has been limited by adverse long-term safety profiles, development of tolerance, and the need for chronic parenteral administration. A safe and convenient therapeutic agent that produces sustained inotropic effects could improve symptoms, functional capacity, and quality of life. Small amounts of 2-deoxy-adenosine triphosphate (dATP) activate cardiac myosin leading to enhanced contractility in normal and failing heart muscle. Cardiac myosin activation triggers faster myosin crossbridge cycling with greater force generation during each contraction. This paper describes the rationale and results of a translational medicine effort to increase dATP levels using a gene therapy strategy to deliver and upregulate ribonucleotide reductase (R1R2), the enzyme responsible for dATP synthesis, selectively in cardiomyocytes. In small and large animal models of heart failure, a single dose of this gene therapy has led to sustained inotropic effects with a benign safety profile. Further animal studies are appropriate with the goal of testing this agent in patients with heart failure.

Biology of the cardiac myocyte in heart disease

Cardiac hypertrophy is a major risk factor for heart failure, and it has been shown that this increase in size occurs at the level of the cardiac myocyte. Cardiac myocyte model systems have been developed to study this process. Here we focus on cell culture tools, including primary cells, immortalized cell lines, human stem cells, and their morphological and molecular responses to pathological stimuli. For each cell type, we discuss commonly used methods for inducing hypertrophy, markers of pathological hypertrophy, advantages for each model, and disadvantages to using a particular cell type over other in vitro model systems. Where applicable, we discuss how each system is used to model human disease and how these models may be applicable to current drug therapeutic strategies. Finally, we discuss the increasing use of biomaterials to mimic healthy and diseased hearts and how these matrices can contribute to in vitro model systems of cardiac cell biology.

Changes in the dynamics of the cardiac troponin C molecule explain the effects of Ca2+-sensitizing mutations

Cardiac troponin C (cTnC) is the regulatory protein that initiates cardiac contraction in response to Ca²⁺ TnC binding Ca²⁺ initiates a cascade of protein-protein interactions that begins with the opening of the N-terminal domain of cTnC, followed by cTnC binding the troponin I switch peptide (TnI_SW). We have evaluated, through isothermal titration calorimetry and molecular-dynamics simulation, the effect of several clinically relevant mutations (A8V, L29Q, A31S, L48Q, Q50R, and C84Y) on the Ca²⁺ affinity, structural dynamics, and calculated interaction strengths between cTnC and each of Ca²⁺ and TnI_SW Surprisingly the Ca²⁺ affinity measured by isothermal titration calorimetry was only significantly affected by half of these mutations including L48Q, which had a 10-fold higher affinity than WT, and the Q50R and C84Y mutants, each of which had affinities 3-fold higher than wild type. This suggests that Ca²⁺ affinity of the N-terminal domain of cTnC in isolation is insufficient to explain the pathogenicity of these mutations. Molecular-dynamics simulation was used to evaluate the effects of these mutations on Ca²⁺ binding, structural dynamics, and TnI interaction independently. Many of the mutations had a pronounced effect on the balance between the open and closed conformations of the TnC molecule, which provides an indirect mechanism for their pathogenic properties. Our data demonstrate that the structural dynamics of the cTnC molecule are key in determining myofilament Ca²⁺ sensitivity. Our data further suggest that modulation of the structural dynamics is the underlying molecular mechanism for many disease mutations that are far from the regulatory Ca²⁺-binding site of cTnC.

Translation of Cardiac Myosin Activation with 2-deoxy-ATP to Treat Heart Failure via an Experimental Ribonucleotide Reductase-Based Gene Therapy

Despite recent advances, chronic heart failure remains a significant and growing unmet medical need, reaching epidemic proportions carrying substantial morbidity, mortality, and costs. A safe and convenient therapeutic agent that produces sustained inotropic effects could ameliorate symptoms and improve functional capacity and quality of life. The authors discovered that small amounts of 2-deoxy-ATP (dATP) activate cardiac myosin leading to enhanced contractility in normal and failing heart muscle. Cardiac myosin activation triggers faster myosin cross-bridge cycling with greater force generation during each contraction. They describe the rationale and results of a translational medicine effort to increase dATP levels using a gene therapy strategy that up-regulates ribonucleotide reductase, the rate-limiting enzyme for dATP synthesis, selectively in cardiomyocytes. In small and large animal models of heart failure, a single dose of this gene therapy has led to sustained inotropic effects with no toxicity or safety concerns identified to date. Further animal studies are being conducted with the goal of testing this agent in patients with heart failure.

Nanotopography-Induced Structural Anisotropy and Sarcomere Development in Human Cardiomyocytes Derived from Induced Pluripotent Stem Cells

Understanding the phenotypic development of human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) is a prerequisite to advancing regenerative cardiac therapy, disease modeling, and drug screening applications. Lack of consistent hiPSC-CM in vitro data can be largely attributed to the inability of conventional culture methods to mimic the structural, biochemical, and mechanical aspects of the myocardial niche accurately. Here, we present a nanogrid culture array comprised of nanogrooved topographies, with groove widths ranging from 350 to 2000 nm, to study the effect of different nanoscale structures on the structural development of hiPSC-CMs in vitro. Nanotopographies were designed to have a biomimetic interface, based on observations of the oriented myocardial extracellular matrix (ECM) fibers found in vivo. Nanotopographic substrates were integrated with a self-assembling chimeric peptide containing the Arg-Gly-Asp (RGD) cell adhesion motif. Using this platform, cell adhesion to peptide-coated substrates was found to be comparable to that of conventional fibronectin-coated surfaces. Cardiomyocyte organization and structural development were found to be dependent on the nanotopographical feature size in a biphasic manner, with improved development achieved on grooves in the 700-1000 nm range. These findings highlight the capability of surface-functionalized, bioinspired substrates to influence cardiomyocyte development, and the capacity for such platforms to serve as a versatile assay for investigating the role of topographical guidance cues on cell behavior. Such substrates could potentially create more physiologically relevant in vitro cardiac tissues for future drug screening and disease modeling studies.

Sarcomere length dependent effects on the interaction between cTnC and cTnI in skinned papillary muscle strips

Sarcomere length dependent activation (LDA) of myocardial force development is the cellular basis underlying the Frank-Starling law of the heart, but it is still elusive how the sarcomeres detect the length changes and convert them into altered activation of thin filament. In this study we investigated how the C-domain of cardiac troponin I (cTnI) functionally and structurally responds to the comprehensive effects of the Ca(2+), crossbridge, and sarcomere length of chemically skinned myocardial preparations. Using our in situ technique which allows for simultaneous measurements of time-resolved FRET and mechanical force of the skinned myocardial preparations, we measured changes in the FRET distance between cTnI(167C) and cTnC(89C), labeled with FRET donor and acceptor, respectively, as a function of [Ca(2+)], crossbridge state and sarcomere length of the skinned muscle preparations. Our results show that [Ca(2+)], cross-bridge feedback and sarcomere length have different effects on the structural transition of the C-domain cTnI. In particular, the interplay between crossbridges and sarcomere length has significant impacts on the functional structural change of the C-domain of cTnI in the relaxed state. These novel observations suggest the importance of the C-domain of cTnI and the dynamic and complex interplay between various components of myofilament in the LDA mechanism.

Cardiac troponin structure-function and the influence of hypertrophic cardiomyopathy associated mutations on modulation of contractility

Cardiac troponin (cTn) acts as a pivotal regulator of muscle contraction and relaxation and is composed of three distinct subunits (cTnC: a highly conserved Ca(2+) binding subunit, cTnI: an actomyosin ATPase inhibitory subunit, and cTnT: a tropomyosin binding subunit). In this mini-review, we briefly summarize the structure-function relationship of cTn and its subunits, its modulation by PKA-mediated phosphorylation of cTnI, and what is known about how these properties are altered by hypertrophic cardiomyopathy (HCM) associated mutations of cTnI. This includes recent work using computational modeling approaches to understand the atomic-based structural level basis of disease-associated mutations. We propose a viewpoint that it is alteration of cTnC-cTnI interaction (rather than the Ca(2+) binding properties of cTn) per se that disrupt the ability of PKA-mediated phosphorylation at cTnI Ser-23/24 to alter contraction and relaxation in at least some HCM-associated mutations. The combination of state of the art biophysical approaches can provide new insight on the structure-function mechanisms of contractile dysfunction resulting cTnI mutations and exciting new avenues for the diagnosis, prevention, and even treatment of heart diseases.

A Tension-Based Model Distinguishes Hypertrophic versus Dilated Cardiomyopathy

The heart either hypertrophies or dilates in response to familial mutations in genes encoding sarcomeric proteins, which are responsible for contraction and pumping. These mutations typically alter calcium-dependent tension generation within the sarcomeres, but how this translates into the spectrum of hypertrophic versus dilated cardiomyopathy is unknown. By generating a series of cardiac-specific mouse models that permit the systematic tuning of sarcomeric tension generation and calcium fluxing, we identify a significant relationship between the magnitude of tension developed over time and heart growth. When formulated into a computational model, the integral of myofilament tension development predicts hypertrophic and dilated cardiomyopathies in mice associated with essentially any sarcomeric gene mutations, but also accurately predicts human cardiac phenotypes from data generated in induced-pluripotent-stem-cell-derived myocytes from familial cardiomyopathy patients. This tension-based model also has the potential to inform pharmacologic treatment options in cardiomyopathy patients.

Rationally engineered Troponin C modulates in vivo cardiac function and performance in health and disease

Treatment for heart disease, the leading cause of death in the world, has progressed little for several decades. Here we develop a protein engineering approach to directly tune in vivo cardiac contractility by tailoring the ability of the heart to respond to the Ca(2+) signal. Promisingly, our smartly formulated Ca(2+)-sensitizing TnC (L48Q) enhances heart function without any adverse effects that are commonly observed with positive inotropes. In a myocardial infarction (MI) model of heart failure, expression of TnC L48Q before the MI preserves cardiac function and performance. Moreover, expression of TnC L48Q after the MI therapeutically enhances cardiac function and performance, without compromising survival. We demonstrate engineering TnC can specifically and precisely modulate cardiac contractility that when combined with gene therapy can be employed as a therapeutic strategy for heart disease.

Designing proteins to combat disease: Cardiac troponin C as an example

Throughout history, muscle research has led to numerous scientific breakthroughs that have brought insight to a more general understanding of all biological processes. Potentially one of the most influential discoveries was the role of the second messenger calcium and its myriad of handling and sensing systems that mechanistically control muscle contraction. In this review we will briefly discuss the significance of calcium as a universal second messenger along with some of the most common calcium binding motifs in proteins, focusing on the EF-hand. We will also describe some of our approaches to rationally design calcium binding proteins to palliate, or potentially even cure cardiovascular disease. Considering not all failing hearts have the same etiology, genetic background and co-morbidities, personalized therapies will need to be developed. We predict designer proteins will open doors for unprecedented personalized, and potentially, even generalized medicines as gene therapy or protein delivery techniques come to fruition.

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

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

The embryonic myosin R672C mutation that underlies Freeman-Sheldon syndrome impairs cross-bridge detachment and cycling in adult skeletal muscle

Distal arthrogryposis is the most common known heritable cause of congenital contractures (e.g. clubfoot) and results from mutations in genes that encode proteins of the contractile complex of skeletal muscle cells. Mutations are most frequently found in MYH3 and are predicted to impair the function of embryonic myosin. We measured the contractile properties of individual skeletal muscle cells and the activation and relaxation kinetics of isolated myofibrils from two adult individuals with an R672C substitution in embryonic myosin and distal arthrogryposis syndrome 2A (DA2A) or Freeman-Sheldon syndrome. In R672C-containing muscle cells, we observed reduced specific force, a prolonged time to relaxation and incomplete relaxation (elevated residual force). In R672C-containing muscle myofibrils, the initial, slower phase of relaxation had a longer duration and slower rate, and time to complete relaxation was greatly prolonged. These observations can be collectively explained by a small subpopulation of myosin cross-bridges with greatly reduced detachment kinetics, resulting in a slower and less complete deactivation of thin filaments at the end of contractions. These findings have important implications for selecting and testing directed therapeutic options for persons with DA2A and perhaps congenital contractures in general.