Thin-filament regulation of force redevelopment kinetics in rabbit skeletal muscle fibres

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.

Fluorescent Protein-Based Ca2+ Sensor Reveals Global, Divalent Cation-Dependent Conformational Changes in Cardiac Troponin C

Cardiac troponin C (cTnC) is a key effector in cardiac muscle excitation-contraction coupling as the Ca²⁺ sensing subunit responsible for controlling contraction. In this study, we generated several FRET sensors for divalent cations based on cTnC flanked by a donor fluorescent protein (CFP) and an acceptor fluorescent protein (YFP). The sensors report Ca²⁺ and Mg²⁺ binding, and relay global structural information about the structural relationship between cTnC’s N- and C-domains. The sensors were first characterized using end point titrations to decipher the response to Ca²⁺ binding in the presence or absence of Mg²⁺. The sensor that exhibited the largest responses in end point titrations, CTV-TnC, (Cerulean, TnC, and Venus) was characterized more extensively. Most of the divalent cation-dependent FRET signal originates from the high affinity C-terminal EF hands. CTV-TnC reconstitutes into skinned fiber preparations indicating proper assembly of troponin complex, with only ~0.2 pCa unit rightward shift of Ca²⁺-sensitive force development compared to WT-cTnC. Affinity of CTV-TnC for divalent cations is in agreement with known values for WT-cTnC. Analytical ultracentrifugation indicates that CTV-TnC undergoes compaction as divalent cations bind. C-terminal sites induce ion-specific (Ca²⁺ versus Mg²⁺) conformational changes in cTnC. Our data also provide support for the presence of additional, non-EF-hand sites on cTnC for Mg²⁺ binding. In conclusion, we successfully generated a novel FRET-Ca²⁺ sensor based on full length cTnC with a variety of cellular applications. Our sensor reveals global structural information about cTnC upon divalent cation binding.

Dissecting the role of the myofilament in diaphragm dysfunction during the development of heart failure in mice

Dyspnea and reduced exercise capacity, caused, in part, by respiratory muscle dysfunction, are common symptoms in patients with heart failure (HF). However, the etiology of diaphragmatic dysfunction has not been identified. To investigate the effects of HF on diaphragmatic function, models of HF were surgically induced in CD-1 mice by transverse aortic constriction (TAC) and acute myocardial infarction (AMI), respectively. Assessment of myocardial function, isolated diaphragmatic strip function, myofilament force-pCa relationship, and phosphorylation status of myofilament proteins was performed at either 2 or 18 wk postsurgery. Echocardiography and invasive hemodynamics revealed development of HF by 18 wk postsurgery in both models. In vitro diaphragmatic force production was preserved in all groups while morphometric analysis revealed diaphragmatic atrophy and fibrosis in 18 wk TAC and AMI groups. Isometric force-pCa measurements of myofilament preparations revealed reduced Ca(2+) sensitivity of force generation and force generation at half-maximum and maximum Ca(2+) activation in 18 wk TAC. The rate of force redevelopment (ktr) was reduced in all HF groups at high levels of Ca(2+) activation. Finally, there were significant changes in the myofilament phosphorylation status of the 18 wk TAC group. This includes a decrease in the phosphorylation of troponin T, desmin, myosin light chain (MLC) 1, and MLC 2 as well as a shift in myosin isoforms. These results indicate that there are multiple changes in diaphragmatic myofilament function, which are specific to the type and stage of HF and occur before overt impairment of in vitro force production.

Cell-based delivery of dATP via gap junctions enhances cardiac contractility

The transplantation of human pluripotent stem cell-derived cardiomyocytes (hPSC-CMs) is a promising strategy to treat myocardial infarction and reverse heart failure, but to date the contractile benefit in most studies remains modest. We have previously shown that the nucleotide 2-deoxyadenosine triphosphate (dATP) can substitute for ATP as the energy substrate for cardiac myosin, and increasing cellular dATP content by globally overexpressing ribonucleotide reductase (R1R2) can dramatically enhance cardiac contractility. Because dATP is a small molecule, we hypothesized that it would diffuse readily between cells via gap junctions and enhance the contractility of neighboring coupled wild type cells. To test this hypothesis, we performed studies with the goals of (1) validating gap junction-mediated dATP transfer in vitro and (2) investigating the use of R1R2-overexpressing hPSC-CMs in vivo as a novel strategy to increase cardiac function. We first performed intracellular dye transfer studies using dATP conjugated to fluorescein and demonstrated rapid gap junction-mediated transfer between cardiomyocytes. We then cocultured wild type cardiomyocytes with either cardiomyocytes or fibroblasts overexpressing R1R2 and saw more than a twofold increase in the extent and rate of contraction of wild type cardiomyocytes. Finally, we transplanted hPSC-CMs overexpressing R1R2 into healthy uninjured rat hearts and noted an increase in fractional shortening from 41±4% to 53±5% just five days after cell transplantation. These findings demonstrate that dATP is an inotropic factor that spreads between cells via gap junctions. Our data suggest that transplantation of dATP-producing hPSC-CMs could significantly increase the effectiveness of cardiac cell therapy.

Variation in isometric force after active shortening and lengthening and their mechanisms: a review

Introduction The isometric force history dependence of skeletal muscle has been studied along the last one hundred years. Several theories have been formulated to explain and establish the causes of the phenomenon, but not successfully, as they have not been fully accepted and demonstrated, and much controversy on such a subject still remains. Objective To present a systematic literature review on the dynamics of the mechanisms of force depression and force enhancement after active shortening and lengthening, respectively, identifying the key variables involved in the phenomenon, and to date to present the main theories and hypothesis developed trying to explaining it. Method The procedure of literature searching complied the major databases, including articles either, those which directly investigated the phenomena of force depression and force enhancement or those which presented possible causes and mechanisms associated with their respective events, from the earliest studies published until the year of 2010. Results 97 references were found according to the criteria used. Conclusion Based on this review, it is suggested that the theory of stress inhibition of actin-myosin cross-bridges is that better explain the phenomenon of force depression. Whereas regarding the force enhancement phenomenon, one theory have been well accepted, the increased number of actin-myosin cross-bridges in strong binding state influenced by the recruitment of passive elastic components, which hole is attributed to the titin filament.

Diaphragm and ventilatory dysfunction during cancer cachexia

Cancer cachexia is characterized by a continuous loss of locomotor skeletal muscle mass, which causes profound muscle weakness. If this atrophy and weakness also occurs in diaphragm muscle, it could lead to respiratory failure, which is a major cause of death in patients with cancer. Thus, the purpose of the current study was to determine whether colon-26 (C-26) cancer cachexia causes diaphragm muscle fiber atrophy and weakness and compromises ventilation. All diaphragm muscle fiber types were significantly atrophied in C-26 mice compared to controls, and the atrophy-related genes, atrogin-1 and MuRF1, significantly increased. Maximum isometric specific force of diaphragm strips, absolute maximal calcium activated force, and maximal specific calcium-activated force of permeabilized diaphragm fibers were all significantly decreased in C-26 mice compared to controls. Further, isotonic contractile properties of the diaphragm were affected to an even greater extent than isometric function. Ventilation measurements demonstrated that C-26 mice have a significantly lower tidal volume compared to controls under basal conditions and, unlike control mice, an inability to increase breathing frequency, tidal volume, and, thus, minute ventilation in response to a respiratory challenge. These data demonstrate that C-26 cancer cachexia causes profound respiratory muscle atrophy and weakness and ventilatory dysfunction.

Fast skeletal muscle troponin activation increases force of mouse fast skeletal muscle and ameliorates weakness due to nebulin-deficiency

The effect of the fast skeletal muscle troponin activator, CK-2066260, on calcium-induced force development was studied in skinned fast skeletal muscle fibers from wildtype (WT) and nebulin deficient (NEB KO) mice. Nebulin is a sarcomeric protein that when absent (NEB KO mouse) or present at low levels (nemaline myopathy (NM) patients with NEB mutations) causes muscle weakness. We studied the effect of fast skeletal troponin activation on WT muscle and tested whether it might be a therapeutic mechanism to increase muscle strength in nebulin deficient muscle. We measured tension-pCa relations with and without added CK-2066260. Maximal active tension in NEB KO tibialis cranialis fibers in the absence of CK-2066260 was ∼60% less than in WT fibers, consistent with earlier work. CK-2066260 shifted the tension-calcium relationship leftwards, with the largest relative increase (up to 8-fold) at low to intermediate calcium levels. This was a general effect that was present in both WT and NEB KO fiber bundles. At pCa levels above ∼6.0 (i.e., calcium concentrations <1 µM), CK-2066260 increased tension of NEB KO fibers to beyond that of WT fibers. Crossbridge cycling kinetics were studied by measuring k(tr) (rate constant of force redevelopment following a rapid shortening/restretch). CK-2066260 greatly increased k(tr) at submaximal activation levels in both WT and NEB KO fiber bundles. We also studied the sarcomere length (SL) dependence of the CK-2066260 effect (SL 2.1 µm and 2.6 µm) and found that in the NEB KO fibers, CK-2066260 had a larger effect on calcium sensitivity at the long SL. We conclude that fast skeletal muscle troponin activation increases force at submaximal activation in both wildtype and NEB KO fiber bundles and, importantly, that this troponin activation is a potential therapeutic mechanism for increasing force in NM and other skeletal muscle diseases with loss of muscle strength.

Slowed Dynamics of Thin Filament Regulatory Units Reduces Ca2+-Sensitivity of Cardiac Biomechanical Function

Actomyosin kinetics in both skinned skeletal muscle fibers at maximum Ca²⁺-activation and unregulated in vitro motility assays are modulated by solvent microviscosity in a manner consistent with a diffusion limited process. Viscosity might also influence cardiac thin filament Ca²⁺-regulatory protein dynamics. In vitro motility assays were conducted using thin filaments reconstituted with recombinant human cardiac troponin and tropomyosin; solvent microviscosity was varied by addition of sucrose or glucose. At saturating Ca²⁺, filament sliding speed (s) was inversely proportional to viscosity. Ca²⁺-sensitivity (pCa₅₀ ) of s decreased markedly with elevated viscosity (η/η₀ ≥ ~1.3). For comparison with unloaded motility assays, steady-state isometric force (F) and kinetics of isometric tension redevelopment (k_TR ) were measured in single, permeabilized porcine cardiomyocytes when viscosity surrounding the myofilaments was altered. Maximum Ca²⁺-activated F changed little for sucrose ≤ 0.3 M (η/η₀ ~1.4) or glucose ≤ 0.875 M (η/η₀ ~1.66), but decreased at higher concentrations. Sucrose (0.3 M) or glucose (0.875 M) decreased pCa₅₀ for F. k_TR at saturating Ca²⁺ decreased steeply and monotonically with increased viscosity but there was little effect on k_TR at sub-maximum Ca²⁺. Modeling indicates that increased solutes affect dynamics of cardiac muscle Ca²⁺-regulatory proteins to a much greater extent than actomyosin cross-bridge cycling.

Significance of troponin dynamics for Ca2+-mediated regulation of contraction and inherited cardiomyopathy

Ca(2+) dissociation from troponin causes cessation of muscle contraction by incompletely understood structural mechanisms. To investigate this process, regulatory site Ca(2+) binding in the NH(2)-lobe of subunit troponin C (TnC) was abolished by mutagenesis, and effects on cardiac troponin dynamics were mapped by hydrogen-deuterium exchange (HDX)-MS. The findings demonstrate the interrelationships among troponin's detailed dynamics, troponin's regulatory actions, and the pathogenesis of cardiomyopathy linked to troponin mutations. Ca(2+) slowed HDX up to 2 orders of magnitude within the NH(2)-lobe and the NH(2)-lobe-associated TnI switch helix, implying that Ca(2+) greatly stabilizes this troponin regulatory region. HDX of the TnI COOH terminus indicated that its known role in regulation involves a partially folded rather than unfolded structure in the absence of Ca(2+) and actin. Ca(2+)-triggered stabilization extended beyond the known direct regulatory regions: to the start of the nearby TnI helix 1 and to the COOH terminus of the TnT-TnI coiled-coil. Ca(2+) destabilized rather than stabilized specific TnI segments within the coiled-coil and destabilized a region not previously implicated in Ca(2+)-mediated regulation: the coiled-coil's NH(2)-terminal base plus the preceding TnI loop with which the base interacts. Cardiomyopathy-linked mutations clustered almost entirely within influentially dynamic regions of troponin, and many sites were Ca(2+)-sensitive. Overall, the findings demonstrate highly selective effects of regulatory site Ca(2+), including opposite changes in protein dynamics at opposite ends of the troponin core domain. Ca(2+) release triggers an intramolecular switching mechanism that propagates extensively within the extended troponin structure, suggests specific movements of the TnI inhibitory regions, and prominently involves troponin's dynamic features.

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.

Tropomyosin flexural rigidity and single ca(2+) regulatory unit dynamics: implications for cooperative regulation of cardiac muscle contraction and cardiomyocyte hypertrophy

Striated muscle contraction is regulated by dynamic and cooperative interactions among Ca²⁺, troponin, and tropomyosin on the thin filament. While Ca²⁺ regulation has been extensively studied, little is known about the dynamics of individual regulatory units and structural changes of individual tropomyosin molecules in relation to their mechanical properties, and how these factors are altered by cardiomyopathy mutations in the Ca²⁺ regulatory proteins. In this hypothesis paper, we explore how various experimental and analytical approaches could broaden our understanding of the cooperative regulation of cardiac contraction in health and disease.

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.

Prolonged space flight-induced alterations in the structure and function of human skeletal muscle fibres

The primary goal of this study was to determine the effects of prolonged space flight (180 days) on the structure and function of slow and fast fibres in human skeletal muscle. Biopsies were obtained from the gastrocnemius and soleus muscles of nine International Space Station crew members 45 days pre- and on landing day (R+0) post-flight. The main findings were that prolonged weightlessness produced substantial loss of fibre mass, force and power with the hierarchy of the effects being soleus type I > soleus type II > gastrocnemius type I > gastrocnemius type II. Structurally, the quantitatively most important adaptation was fibre atrophy, which averaged 20% in the soleus type I fibres (98 to 79 μm diameter). Atrophy was the main contributor to the loss of peak force (P(0)), which for the soleus type I fibre declined 35% from 0.86 to 0.56 mN. The percentage decrease in fibre diameter was correlated with the initial pre-flight fibre size (r = 0.87), inversely with the amount of treadmill running (r = 0.68), and was associated with an increase in thin filament density (r = 0.92). The latter correlated with reduced maximal velocity (V(0)) (r = 0.51), and is likely to have contributed to the 21 and 18% decline in V(0) in the soleus and gastrocnemius type I fibres. Peak power was depressed in all fibre types with the greatest loss (55%) in the soleus. An obvious conclusion is that the exercise countermeasures employed were incapable of providing the high intensity needed to adequately protect fibre and muscle mass, and that the crew's ability to perform strenuous exercise might be seriously compromised. Our results highlight the need to study new exercise programmes on the ISS that employ high resistance and contractions over a wide range of motion to mimic the range occurring in Earth's 1 g environment.

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+).

Functional and evolutionary relationships of troponin C

Striated muscle contraction is initiated when, following membrane depolarization, Ca(2+) binds to the low-affinity Ca(2+) binding sites of troponin C (TnC). The Ca(2+) activation of this protein results in a rearrangement of the components (troponin I, troponin T, and tropomyosin) of the thin filament, resulting in increased interaction between actin and myosin and the formation of cross bridges. The functional properties of this protein are therefore critical in determining the active properties of striated muscle. To date there are 61 known TnCs that have been cloned from 41 vertebrate and invertebrate species. In vertebrate species there are also distinct fast skeletal muscle and cardiac TnC proteins. While there is relatively high conservation of the amino acid sequence of TnC homologs between species and tissue types, there is wide variation in the functional properties of these proteins. To date there has been extensive study of the structure and function of this protein and how differences in these translate into the functional properties of muscles. The purpose of this work is to integrate these studies of TnC with phylogenetic analysis to investigate how changes in the sequence and function of this protein, integrate with the evolution of striated muscle.

Modulation of the rate of cardiac muscle contraction by troponin C constructs with various calcium binding affinities

We investigated whether changing thin filament Ca2+sensitivity alters the rate of contraction, either during normal cross-bridge cycling or when cross-bridge cycling is increased by inorganic phosphate (Pi). We increased or decreased Ca2+sensitivity of force production by incorporating into rat skinned cardiac trabeculae the troponin C (TnC) mutants V44QTnCF27Wand F20QTnCF27W. The rate of isometric contraction was assessed as the rate of force redevelopment ( ktr) after a rapid release and restretch to the original length of the muscle. Both in the absence of added Piand in the presence of 2.5 mM added Pi1) Ca2+sensitivity of ktrwas increased by V44QTnCF27Wand decreased by F20QTnCF27Wcompared with control TnCF27W; 2) ktrat submaximal Ca2+activation was significantly faster for V44QTnCF27Wand slower for F20QTnCF27Wcompared with control TnCF27W; 3) at maximum Ca2+activation, ktrvalues were similar for control TnCF27W, V44QTnCF27W, and F20QTnCF27W; and 4) ktrexhibited a linear dependence on force that was indistinguishable for all TnCs. In the presence of 2.5 mM Pi, ktrwas faster at all pCa values compared with the values for no added Pifor TnCF27W, V44QTnCF27W, and F20QTnCF27W. This study suggests that TnC Ca2+binding properties modulate the rate of cardiac muscle contraction at submaximal levels of Ca2+activation. This result has physiological relevance considering that, on a beat-to-beat basis, the heart contracts at submaximal Ca2+activation.

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

We studied how enhanced skeletal troponin C (sTnC) Ca2+-binding affinity affects cooperative thin filament activation and contraction in single demembranated rabbit psoas fibres. Three sTnC mutants were created and incorporated into skeletal troponin (sTn) for measurement of Ca2+ dissociation, resulting in the following order of rates: wild-type (WT) sTnC-sTn>sTnC(F27W)-sTn>M80Q sTnC-sTn>M80Q sTnCF27W-sTn. Reconstitution of sTnC-extracted fibres increased Ca2+ sensitivity of steady-state force (pCa(50)) by 0.08 for M80Q sTnC, 0.15 for sTnCF27W and 0.32 for M80Q sTnCF27W with minimal loss of slope (nH, degree of cooperativity). Near-neighbour thin filament regulatory unit (RU) interactions were reduced in fibres by incorporating mixtures of WT or mutant sTnC and D28A, D64A sTnC (xxsTnC) that does not bind Ca2+ at N-terminal sites. Reconstitution with sTnC: xxsTnC mixtures to 20% of pre-exchanged maximal force reduced pCa50 by 0.35 for sTnC: xxsTnC, 0.25 for M80Q sTnC: xxsTnC, and 0.10 for M80Q sTnCF27W: xxsTnC. It is interesting that pCa50 increased by approximately 0.1 for M80Q sTnC and approximately 0.3 for M80Q sTnCF27W when near-neighbour RU interactions were reduced; these values are similar in magnitude to those for fibres reconstituted with 100% mutant sTnC. After reconstitution with sTnC: xxsTnC mixtures, nH decreased to a similar value for all mutant sTnCs. Altered sTnC Ca2+-binding properties (M80Q sTnCF27W) did not affect strong crossbridge inhibition by 2,3-butanedione monoxime when near-neighbour thin filament RU interactions were reduced. Together these results suggest increased sTnC Ca2+ affinity strongly influences Ca2+ sensitivity of steady-state force without affecting near-neighbour thin filament RU cooperative activation or the relative contribution of crossbridges versus Ca2+ to thin filament activation.

The effects of force inhibition by sodium vanadate on cross-bridge binding, force redevelopment, and Ca2+ activation in cardiac muscle

Strongly bound, force-generating myosin cross-bridges play an important role as allosteric activators of cardiac thin filaments. Sodium vanadate (Vi) is a phosphate analog that inhibits force by preventing cross-bridge transition into force-producing states. This study characterizes the mechanical state of cross-bridges with bound Vi as a tool to examine the contribution of cross-bridges to cardiac contractile activation. The K(i) of force inhibition by Vi was approximately 40 microM. Sinusoidal stiffness was inhibited with Vi, although to a lesser extent than force. We used chord stiffness measurements to monitor Vi-induced changes in cross-bridge attachment/detachment kinetics at saturating [Ca(2+)]. Vi decreased chord stiffness at the fastest rates of stretch, whereas at slow rates chord stiffness actually increased. This suggests a shift in cross-bridge population toward low force states with very slow attachment/detachment kinetics. Low angle x-ray diffraction measurements indicate that with Vi cross-bridge mass shifted away from thin filaments, implying decreased cross-bridge/thin filament interaction. The combined x-ray and mechanical data suggest at least two cross-bridge populations with Vi; one characteristic of normal cycling cross-bridges, and a population of weak-binding cross-bridges with bound Vi and slow attachment/detachment kinetics. The Ca(2+) sensitivity of force (pCa(50)) and force redevelopment kinetics (k(TR)) were measured to study the effects of Vi on contractile activation. When maximal force was inhibited by 40% with Vi pCa(50) decreased, but greater force inhibition at higher [Vi] did not further alter pCa(50). In contrast, the Ca(2+) sensitivity of k(TR) was unaffected by Vi. Interestingly, when force was inhibited by Vi k(TR) increased at submaximal levels of Ca(2+)-activated force. Additionally, k(TR) is faster at saturating Ca(2+) at [Vi] that inhibit force by > approximately 70%. The effects of Vi on k(TR) imply that k(TR) is determined not only by the intrinsic properties of the cross-bridge cycle, but also by cross-bridge contribution to thin filament activation.

Investigation of thin filament near-neighbour regulatory unit interactions during force development in skinned cardiac and skeletal muscle

Ca(2+)-dependent activation of striated muscle involves cooperative interactions of cross-bridges and thin filament regulatory proteins. We investigated how interactions between individual structural regulatory units (RUs; 1 tropomyosin, 1 troponin, 7 actins) influence the level and rate of demembranated (skinned) cardiac muscle force development by exchanging native cardiac troponin (cTn) with different ratio mixtures of wild-type (WT) cTn and cTn containing WT cardiac troponin T/I + cardiac troponin C (cTnC) D65A (a site II inactive cTnC mutant). Maximal Ca(2+)-activated force (F(max)) increased in less than a linear manner with WT cTn. This contrasts with results we obtained previously in skeletal fibres (using sTnC D28A, D65A) where F(max) increased in a greater than linear manner with WT sTnC, and suggests that Ca(2+) binding to each functional Tn activates < 7 actins of a structural regulatory unit in cardiac muscle and > 7 actins in skeletal muscle. The Ca(2+) sensitivity of force and rate of force redevelopment (k(tr)) was leftward shifted by 0.1-0.2 -log [Ca(2+)] (pCa) units as WT cTn content was increased, but the slope of the force-pCa relation and maximal k(tr) were unaffected by loss of near-neighbour RU interactions. Cross-bridge inhibition (with butanedione monoxime) or augmentation (with 2 deoxy-ATP) had no greater effect in cardiac muscle with disruption of near-neighbour RU interactions, in contrast to skeletal muscle fibres where the effect was enhanced. The rate of Ca(2+) dissociation was found to be > 2-fold faster from whole cardiac Tn compared with skeletal Tn. Together the data suggest that in cardiac (as opposed to skeletal) muscle, Ca(2+) binding to individual Tn complexes is insufficient to completely activate their corresponding RUs, making thin filament activation level more dependent on concomitant Ca(2+) binding at neighbouring Tn sites and/or crossbridge feedback effects on Ca(2+) binding affinity.