Influence of enhanced troponin C Ca2+-binding affinity on cooperative thin filament activation in rabbit skeletal muscle

The force of the myosin motor sets cooperativity in thin filament activation of skeletal muscles

Contraction of striated muscle is regulated by a dual mechanism involving both thin, actin-containing filament and thick, myosin-containing filament. Thin filament is activated by Ca2+ binding to troponin, leading to tropomyosin displacement that exposes actin sites for interaction with myosin motors, extending from the neighbouring stress-activated thick filaments. Motor attachment to actin contributes to spreading activation along the thin filament, through a cooperative mechanism, still unclear, that determines the slope of the sigmoidal relation between isometric force and pCa (−log[Ca2+]), estimated by Hill coefficient nH. We use sarcomere-level mechanics in demembranated fibres of rabbit skeletal muscle activated by Ca2+ at different temperatures (12–35 °C) to show that nH depends on the motor force at constant number of attached motors. The definition of the role of motor force provides fundamental constraints for modelling the dynamics of thin filament activation and defining the action of small molecules as possible therapeutic tools.

Hypertrophic cardiomyopathy: Mutations to mechanisms to therapies

Hypertrophic cardiomyopathy (HCM) affects more than 1 in 500 people in the general population with an extensive burden of morbidity in the form of arrhythmia, heart failure, and sudden death. More than 25 years since the discovery of the genetic underpinnings of HCM, the field has unveiled significant insights into the primary effects of these genetic mutations, especially for the myosin heavy chain gene, which is one of the most commonly mutated genes. Our group has studied the molecular effects of HCM mutations on human β-cardiac myosin heavy chain using state-of-the-art biochemical and biophysical tools for the past 10 years, combining insights from clinical genetics and structural analyses of cardiac myosin. The overarching hypothesis is that HCM-causing mutations in sarcomere proteins cause hypercontractility at the sarcomere level, and we have shown that an increase in the number of myosin molecules available for interaction with actin is a primary driver. Recently, two pharmaceutical companies have developed small molecule inhibitors of human cardiac myosin to counteract the molecular consequences of HCM pathogenesis. One of these inhibitors (mavacamten) has recently been approved by the FDA after completing a successful phase III trial in HCM patients, and the other (aficamten) is currently being evaluated in a phase III trial. Myosin inhibitors will be the first class of medication used to treat HCM that has both robust clinical trial evidence of efficacy and that targets the fundamental mechanism of HCM pathogenesis. The success of myosin inhibitors in HCM opens the door to finding other new drugs that target the sarcomere directly, as we learn more about the genetics and fundamental mechanisms of this disease.

Structural determinants of muscle thin filament cooperativity

End-to-end connections between adjacent tropomyosin molecules along the muscle thin filament allow long-range conformational rearrangement of the multicomponent filament structure. This process is influenced by Ca(2+) and the troponin regulatory complexes, as well as by myosin crossbridge heads that bind to and activate the filament. Access of myosin crossbridges onto actin is gated by tropomyosin, and in the case of striated muscle filaments, troponin acts as a gatekeeper. The resulting tropomyosin-troponin-myosin on-off switching mechanism that controls muscle contractility is a complex cooperative and dynamic system with highly nonlinear behavior. Here, we review key information that leads us to view tropomyosin as central to the communication pathway that coordinates the multifaceted effectors that modulate and tune striated muscle contraction. We posit that an understanding of this communication pathway provides a framework for more in-depth mechanistic characterization of myopathy-associated mutational perturbations currently under investigation by many research groups.

The length-tension curve in muscle depends on lattice spacing

Classic interpretations of the striated muscle length-tension curve focus on how force varies with overlap of thin (actin) and thick (myosin) filaments. New models of sarcomere geometry and experiments with skinned synchronous insect flight muscle suggest that changes in the radial distance between the actin and myosin filaments, the filament lattice spacing, are responsible for between 20% and 50% of the change in force seen between sarcomere lengths of 1.4 and 3.4 µm. Thus, lattice spacing is a significant force regulator, increasing the slope of muscle's force-length dependence.

Contractility and kinetics of human fetal and human adult skeletal muscle

Little is known about the contraction and relaxation properties of fetal skeletal muscle, and measurements thus far have been made with non-human mammalian muscle. Data on human fetal skeletal muscle contraction are lacking, and there are no published reports on the kinetics of either fetal or adult human skeletal muscle myofibrils. Understanding the contractile properties of human fetal muscle would be valuable in understanding muscle development and a variety of muscle diseases that are associated with mutations in fetal muscle sarcomere proteins. Therefore, we characterised the contractile properties of developing human fetal skeletal muscle and compared them to adult human skeletal muscle and rabbit psoas muscle. Electron micrographs showed human fetal muscle sarcomeres are not fully formed but myofibril formation is visible. Isolated myofibril mechanical measurements revealed much lower specific force, and slower rates of isometric force development, slow phase relaxation, and fast phase relaxation in human fetal when compared to human adult skeletal muscle. The duration of slow phase relaxation was also significantly longer compared to both adult groups, but was similarly affected by elevated ADP. F-actin sliding on human fetal skeletal myosin coated surfaces in in vitro motility (IVM) assays was much slower compared with adult rabbit skeletal myosin, though the Km(app) (apparent (fitted) Michaelis-Menten constant) of F-actin speed with ATP titration suggests a greater affinity of human fetal myosin for nucleotide binding. Replacing ATP with 2 deoxy-ATP (dATP) increased F-actin speed for both groups by a similar amount. Titrations of ADP into IVM assays produced a similar inhibitory affect for both groups, suggesting ADP binding may be similar, at least under low load. Together, our results suggest slower but similar mechanisms of myosin chemomechanical transduction for human fetal muscle that may also be limited by immature myofilament structure.

Structural and functional consequences of cardiac troponin C L57Q and I61Q Ca(2+)-desensitizing variants

Two cTnC variants, L57Q and I61Q, both of which are located on helix C within the N domain of cTnC, were originally reported in the skeletal muscle system [Tikunova, Davis, J. Biol. Chem. 279 (2004) 35341-35352], as the analogous L58Q and I62Q sTnC, and demonstrated a decreased Ca(2+) binding affinity. Here, we provide detailed characterization of structure-function relationships for these two cTnC variants, to determine if they behave differently in the cardiac system and as a framework for determining similarities and differences with other cTnC mutations that have been associated with DCM. We have used an integrative approach to study the structure and function of these cTnC variants both in solution and in silico, to understand how the L57Q and I61Q mutations influence Ca(2+) binding at site II, the subsequent effects on the interaction with cTnI, and the structural changes which are associated with these changes. Steady-state and stopped flow fluorescence spectroscopy confirmed that a decrease in Ca(2+) affinity for recombinant cTnC and cTn complexes containing the L57Q or I61Q variants. The L57Q variant was intermediate between WT and I61Q cTnC and also did not significantly alter cTnC-cTnI interaction in the absence of Ca(2+), but did decrease the interaction in the presence of Ca(2+). In contrast, I61Q decreased the cTnC-cTnI interaction in both the absence and presence of Ca(2+). This difference in the absence of Ca(2+) suggests a greater structural change in cNTnC may occur with the I61Q mutation than the L57Q mutation. MD simulations revealed that the decreased Ca(2+) binding induced by I61Q may result from destabilization of the Ca(2+) binding site through interruption of intra-molecular interactions when residue 61 forms new hydrogen bonds with G70 on the Ca(2+) binding loop. The experimentally observed interruption of the cTnC-cTnI interaction caused by L57Q or I61Q is due to the disruption of key hydrophobic interactions between helices B and C in cNTnC. This study provides a molecular basis of how single mutations in the C helix of cTnC can reduce Ca(2+) binding affinity and cTnC-cTnI interaction, which may provide useful insights for a better understanding of cardiomyopathies and future gene-based therapies.

Filament compliance influences cooperative activation of thin filaments and the dynamics of force production in skeletal muscle

Striated muscle contraction is a highly cooperative process initiated by Ca²⁺ binding to the troponin complex, which leads to tropomyosin movement and myosin cross-bridge (XB) formation along thin filaments. Experimental and computational studies suggest skeletal muscle fiber activation is greatly augmented by cooperative interactions between neighboring thin filament regulatory units (RU-RU cooperativity; 1 RU = 7 actin monomers+1 troponin complex+1 tropomyosin molecule). XB binding can also amplify thin filament activation through interactions with RUs (XB-RU cooperativity). Because these interactions occur with a temporal order, they can be considered kinetic forms of cooperativity. Our previous spatially-explicit models illustrated that mechanical forms of cooperativity also exist, arising from XB-induced XB binding (XB-XB cooperativity). These mechanical and kinetic forms of cooperativity are likely coordinated during muscle contraction, but the relative contribution from each of these mechanisms is difficult to separate experimentally. To investigate these contributions we built a multi-filament model of the half sarcomere, allowing RU activation kinetics to vary with the state of neighboring RUs or XBs. Simulations suggest Ca²⁺ binding to troponin activates a thin filament distance spanning 9 to 11 actins and coupled RU-RU interactions dominate the cooperative force response in skeletal muscle, consistent with measurements from rabbit psoas fibers. XB binding was critical for stabilizing thin filament activation, particularly at submaximal Ca²⁺ levels, even though XB-RU cooperativity amplified force less than RU-RU cooperativity. Similar to previous studies, XB-XB cooperativity scaled inversely with lattice stiffness, leading to slower rates of force development as stiffness decreased. Including RU-RU and XB-RU cooperativity in this model resulted in the novel prediction that the force-[Ca²⁺] relationship can vary due to filament and XB compliance. Simulations also suggest kinetic forms of cooperativity occur rapidly and dominate early to get activation, while mechanical forms of cooperativity act more slowly, augmenting XB binding as force continues to develop.

In striated muscle myosin binds to actin and converts chemical energy from ATP hydrolysis into force, work, and power. Myosin cross-bridge binding is regulated by Ca²⁺ and the thin filament proteins troponin and tropomyosin. Cooperative interactions between actin, myosin, troponin, and tropomyosin greatly influence spatial and kinetic properties of thin filament activation, thereby affecting muscle mechanics and contractility. Such cooperative interactions are complex and individual contributions from the different contractile and regulatory proteins are difficult to separate experimentally. However, a few theoretical models have explored interactions between the spatial, kinetic, and mechanical processes that affect cooperative cross-bridge binding to actin. Building on our prior spatially-explicit computational models, we investigated the relative contributions of thin filament regulatory proteins and cross-bridges to cooperatively amplify skeletal muscle force production. We find that Ca²⁺-dependent contraction in skeletal muscle is dominated by neighboring regulatory protein interactions along the thin filament, while cross-bridge binding is critical for maintaining or stabilizing thin filament activation as force develops. Moreover, we reveal that variations in filament and cross-bridge stiffness can alter Ca²⁺-sensitivity and cooperativity of skeletal muscle force production. In conclusion, these simulations show that multiple cooperative mechanisms combine to produce physiological force responses measured from muscle cells.

Assessment of contractility in intact ventricular cardiomyocytes using the dimensionless 'Frank-Starling Gain' index

This paper briefly recapitulates the Frank–Starling law of the heart, reviews approaches to establishing diastolic and systolic force–length behaviour in intact isolated cardiomyocytes, and introduces a dimensionless index called ‘Frank–Starling Gain’, calculated as the ratio of slopes of end-systolic and end-diastolic force–length relations. The benefits and limitations of this index are illustrated on the example of regional differences in Guinea pig intact ventricular cardiomyocyte mechanics. Potential applicability of the Frank–Starling Gain for the comparison of cell contractility changes upon stretch will be discussed in the context of intra- and inter-individual variability of cardiomyocyte properties.

Calcium binding kinetics of troponin C strongly modulate cooperative activation and tension kinetics in cardiac muscle

Tension development and relaxation in cardiac muscle are regulated at the thin filament via Ca(2+) binding to cardiac troponin C (cTnC) and strong cross-bridge binding. However, the influence of cTnC Ca(2+)-binding properties on these processes in the organized structure of cardiac sarcomeres is not well-understood and likely differs from skeletal muscle. To study this we generated single amino acid variants of cTnC with altered Ca(2+) dissociation rates (k(off)), as measured in whole troponin (cTn) complex by stopped-flow spectroscopy (I61Q cTn>WT cTn>L48Q cTn), and exchanged them into cardiac myofibrils and demembranated trabeculae. In myofibrils at saturating Ca(2+), L48Q cTnC did not affect maximum tension (T(max)), thin filament activation (k(ACT)) and tension development (k(TR)) rates, or the rates of relaxation, but increased duration of slow phase relaxation. In contrast, I61Q cTnC reduced T(max), k(ACT) and k(TR) by 40-65% with little change in relaxation. Interestingly, k(ACT) was less than k(TR) with I61Q cTnC, and this difference increased with addition of inorganic phosphate, suggesting that reduced cTnC Ca(2+)-affinity can limit thin filament activation kinetics. Trabeculae exchanged with I61Q cTn had reduced T(max), Ca(2+) sensitivity of tension (pCa(50)), and slope (n(H)) of tension-pCa, while L48Q cTn increased pCa(50) and reduced n(H). Increased cross-bridge cycling with 2-deoxy-ATP increased pCa(50) with WT or L48Q cTn, but not I61Q cTn. We discuss the implications of these results for understanding the role of cTn Ca(2+)-binding properties on the magnitude and rate of tension development and relaxation in cardiac muscle.

Effect of Ca2+ binding properties of troponin C on rate of skeletal muscle force redevelopment

To investigate effects of altering troponin (Tn)C Ca(2+) binding properties on rate of skeletal muscle contraction, we generated three mutant TnCs with increased or decreased Ca(2+) sensitivities. Ca(2+) binding properties of the regulatory domain of TnC within the Tn complex were characterized by following the fluorescence of an IAANS probe attached onto the endogenous Cys(99) residue of TnC. Compared with IAANS-labeled wild-type Tn complex, V43QTnC, T70DTnC, and I60QTnC exhibited ∼1.9-fold higher, ∼5.0-fold lower, and ∼52-fold lower Ca(2+) sensitivity, respectively, and ∼3.6-fold slower, ∼5.7-fold faster, and ∼21-fold faster Ca(2+) dissociation rate (k(off)), respectively. On the basis of K(d) and k(off), these results suggest that the Ca(2+) association rate to the Tn complex decreased ∼2-fold for I60QTnC and V43QTnC. Constructs were reconstituted into single-skinned rabbit psoas fibers to assess Ca(2+) dependence of force development and rate of force redevelopment (k(tr)) at 15°C, resulting in sensitization of both force and k(tr) to Ca(2+) for V43QTnC, whereas T70DTnC and I60QTnC desensitized force and k(tr) to Ca(2+), I60QTnC causing a greater desensitization. In addition, T70DTnC and I60QTnC depressed both maximal force (F(max)) and maximal k(tr). Although V43QTnC and I60QTnC had drastically different effects on Ca(2+) binding properties of TnC, they both exhibited decreases in cooperativity of force production and elevated k(tr) at force levels <30%F(max) vs. wild-type TnC. However, at matched force levels >30%F(max) k(tr) was similar for all TnC constructs. These results suggest that the TnC mutants primarily affected k(tr) through modulating the level of thin filament activation and not by altering intrinsic cross-bridge cycling properties. To corroborate this, NEM-S1, a non-force-generating cross-bridge analog that activates the thin filament, fully recovered maximal k(tr) for I60QTnC at low Ca(2+) concentration. Thus TnC mutants with altered Ca(2+) binding properties can control the rate of contraction by modulating thin filament activation without directly affecting intrinsic cross-bridge cycling rates.

Myofilament length dependent activation

The Frank-Starling law of the heart describes the interrelationship between end-diastolic volume and cardiac ejection volume, a regulatory system that operates on a beat-to-beat basis. The main cellular mechanism that underlies this phenomenon is an increase in the responsiveness of cardiac myofilaments to activating Ca(2+) ions at a longer sarcomere length, commonly referred to as myofilament length-dependent activation. This review focuses on what molecular mechanisms may underlie myofilament length dependency. Specifically, the roles of inter-filament spacing, thick and thin filament based regulation, as well as sarcomeric regulatory proteins are discussed. Although the "Frank-Starling law of the heart" constitutes a fundamental cardiac property that has been appreciated for well over a century, it is still not known in muscle how the contractile apparatus transduces the information concerning sarcomere length to modulate ventricular pressure development.

Thin filament Ca2+ binding properties and regulatory unit interactions alter kinetics of tension development and relaxation in rabbit skeletal muscle

The influence of Ca(2+) binding properties of individual troponin versus cooperative regulatory unit interactions along thin filaments on the rate tension develops and declines was examined in demembranated rabbit psoas fibres and isolated myofibrils. Native skeletal troponin C (sTnC) was replaced with sTnC mutants having altered Ca(2+) dissociation rates (k(off)) or with mixtures of sTnC and D28A, D64A sTnC, that does not bind Ca(2+) at sites I and II (xxsTnC), to reduce near-neighbour regulatory unit (RU) interactions. At saturating Ca(2+), the rate of tension redevelopment (k(TR)) was not altered for fibres containing sTnC mutants with decreased k(off) or mixtures of sTnC:xxsTnC. We examined the influence of k(off) on maximal activation and relaxation in myofibrils because they allow rapid and large changes in [Ca(2+)]. In myofibrils with M80Q sTnC(F27W) (decreased k(off)), maximal tension, activation rate (k(ACT)), k(TR) and rates of relaxation were not altered. With I60Q sTnC(F27W) (increased k(off)), maximal tension, k(ACT) and k(TR) decreased, with no change in relaxation rates. Surprisingly, the duration of the slow phase of relaxation increased or decreased with decreased or increased k(off), respectively. For all sTnC reconstitution conditions, Ca(2+) dependence of k(TR) in fibres showed Ca(2+) sensitivity of k(TR) (pCa(50)) shifted parallel to tension and low-Ca(2+) k(TR) was elevated. Together the data suggest the Ca(2+)-dependent rate of tension development and the duration (but not rate) of relaxation can be greatly influenced by k(off) of sTnC. This influence of sTnC binding kinetics occurs primarily within individual RUs, with only minor contributions of RU interactions at low Ca(2+).

Molecular and functional characterization of novel hypertrophic cardiomyopathy susceptibility mutations in TNNC1-encoded troponin C

Hypertrophic Cardiomyopathy (HCM) is a common primary cardiac disorder defined by a hypertrophied left ventricle, is one of the main causes of sudden death in young athletes, and has been associated with mutations in most sarcomeric proteins (tropomyosin, troponin T and I, and actin, etc.). Many of these mutations appear to affect the functional properties of cardiac troponin C (cTnC), i.e., by increasing the Ca(2+)-sensitivity of contraction, a hallmark of HCM, yet surprisingly, prior to this report, cTnC had not been classified as a HCM-susceptibility gene. In this study, we show that mutations occurring in the human cTnC (HcTnC) gene (TNNC1) have the same prevalence (~0.4%) as well established HCM-susceptibility genes that encode other sarcomeric proteins. Comprehensive open reading frame/splice site mutation analysis of TNNC1 performed on 1025 unrelated HCM patients enrolled over the last 10 years revealed novel missense mutations in TNNC1: A8V, C84Y, E134D, and D145E. Functional studies with these recombinant HcTnC HCM mutations showed increased Ca(2+) sensitivity of force development (A8V, C84Y and D145E) and force recovery (A8V and D145E). These results are consistent with the HCM functional phenotypes seen with other sarcomeric-HCM mutations (E134D showed no changes in these parameters). This is the largest cohort analysis of TNNC1 in HCM that details the discovery of at least three novel HCM-associated mutations and more strongly links TNNC1 to HCM along with functional evidence that supports a central role for its involvement in the disease. This study may help to further define TNNC1 as an HCM-susceptibility gene, a classification that has already been established for the other members of the troponin complex.

Intramolecular interactions in the N-domain of cardiac troponin C are important determinants of calcium sensitivity of force development

Myocardial contraction is initiated when Ca2+ binds to site II of cardiac troponin C. This 12-residue EF-hand loop (NH2-DEDGSGTVDFDE-COOH) contains six residues (bold) that coordinate Ca2+ binding and six residues that do not appear to influence Ca2+ binding directly. We have introduced six single-cysteine substitutions (italics) within site II of cTnC to investigate whether these residues are essential for Ca2+ binding affinity in isolation and Ca2+ sensitivity of force development in single muscle fibers. Ca2+ binding properties of mutant proteins were examined in solution and after substitution into rat skinned soleus fibers. Except for the serine mutation, cysteine substitution had no effect on Ca2+ binding on cTnC in solution. However, as part of the myofilament, the threonine mutation reduced Ca2+ sensitivity while the phenylalanine mutation increased Ca2+ sensitivity. Analysis of the available crystal and NMR structures reveals specific structural mechanisms for these effects.

Familial hypertrophic cardiomyopathy-related cardiac troponin C mutation L29Q affects Ca2+ binding and myofilament contractility

The cardiac troponin C (cTnC) mutation, L29Q, has been found in a patient with familial hypertrophic cardiomyopathy. We previously showed that L29, together with neighboring residues, Asp2, Val28, and Gly30, plays an important role in determining the Ca(2+) affinity of site II, the regulatory site of mammalian cardiac troponin C (McTnC). Here we report on the Ca(2+) binding characteristics of L29Q McTnC and D2N/V28I/L29Q/G30D McTnC (NIQD) utilizing the Phe(27) --> Trp (F27W) substitution, allowing one to monitor Ca(2+) binding and release. We also studied the effect of these mutants on Ca(2+) activation of force generation in single mouse cardiac myocytes using cTnC replacement, together with sarcomere length (SL) dependence. The Ca(2+)-binding affinity of site II of L29Q McTnC(F27W) and NIQD McTnC(F27W) was approximately 1.3- and approximately 1.9-fold higher, respectively, than that of McTnC(F27W). The Ca(2+) disassociation rate from site II of L29Q McTnC(F27W) and NIQD McTnC(F27W) was not significantly different than that of control (McTnC(F27W)). However, the rate of Ca(2+) binding to site II was higher in L29Q McTnC(F27W) and NIQD McTnC(F27W) relative to control (approximately 1.5-fold and approximately 2.0-fold respectively). The Ca(2+) sensitivity of force generation was significantly higher in myocytes reconstituted with L29Q McTnC (approximately 1.4-fold) and NIQD McTnC (approximately 2-fold) compared with those reconstituted with McTnC. Interestingly, the change in Ca(2+) sensitivity of force generation in response to an SL change (1.9, 2.1, and 2.3 mum) was significantly reduced in myocytes containing L29Q McTnC or NIQD McTnC. These results demonstrate that the L29Q mutation enhances the Ca(2+)-binding characteristics of cTnC and that when incorporated into cardiac myocytes, this mutant alters myocyte contractility.