Cooperativity in the Ca2+ regulation of muscle contraction

High-speed AFM reveals subsecond dynamics of cardiac thin filaments upon Ca2+ activation and heavy meromyosin binding

The advent of high-speed atomic force microscopy (HS-AFM) changed the field of biology considerably. HS-AFM is the only method where in situ dynamics of biological samples and imaging can be coupled with a spatial resolution of 1 to 5 nm in the horizontal direction. Unlike electron or cryo-electron microscopy, HS-AFM does not require fixation or freezing of the samples, and has the ability to derive kinetic parameters by recording the live movements of single-molecule dynamics. In this paper, we used HS-AFM to investigate directly the mechanisms of cardiac muscle activation. We visualized the muscle regulatory tropomyosin–troponin complex movements during activation by calcium or myosin (motor that drives contraction), and the structural transitions that happen during these events.

High-speed atomic force microscopy (HS-AFM) can be used to study dynamic processes with real-time imaging of molecules within 1- to 5-nm spatial resolution. In the current study, we evaluated the 3-state model of activation of cardiac thin filaments (cTFs) isolated as a complex and deposited on a mica-supported lipid bilayer. We studied this complex for dynamic conformational changes 1) at low and high [Ca²⁺] (pCa 9.0 and 4.5), and 2) upon myosin binding to the cTF in the nucleotide-free state or in the presence of ATP. HS-AFM was used to directly visualize the tropomyosin–troponin complex and Ca²⁺-induced tropomyosin movements accompanied by structural transitions of actin monomers within cTFs. Our data show that cTFs at relaxing or activating conditions are not ultimately in a blocked or activated state, respectively, but rather the combination of states with a prevalence that is dependent on the [Ca²⁺] and the presence of weakly or strongly bound myosin. The weakly and strongly bound myosin induce similar changes in the structure of cTFs as confirmed by the local dynamical displacement of individual tropomyosin strands in the center of a regulatory unit of cTF at the relaxed and activation conditions. The displacement of tropomyosin at the relaxed conditions had never been visualized directly and explains the ability of myosin binding to TF at the relaxed conditions. Based on the ratios of nonactivated and activated segments within cTFs, we proposed a mechanism of tropomyosin switching from different states that includes both weakly and strongly bound myosin.

Tropomyosin Structure, Function, and Interactions: A Dynamic Regulator

Tropomyosin is the archetypal-coiled coil, yet studies of its structure and function have proven it to be a dynamic regulator of actin filament function in muscle and non-muscle cells. Here we review aspects of its structure that deviate from canonical leucine zipper coiled coils that allow tropomyosin to bind to actin, regulate myosin, and interact directly and indirectly with actin-binding proteins. Four genes encode tropomyosins in vertebrates, with additional diversity that results from alternate promoters and alternatively spliced exons. At the same time that periodic motifs for binding actin and regulating myosin are conserved, isoform-specific domains allow for specific interaction with myosins and actin filament regulatory proteins, including troponin. Tropomyosin can be viewed as a universal regulator of the actin cytoskeleton that specifies actin filaments for cellular and intracellular functions.

Analysis of the molecular pathogenesis of cardiomyopathy-causing cTnT mutants I79N, ΔE96, and ΔK210

Three troponin T (TnT) mutants that cause hypertrophic, restrictive, and dilated cardiomyopathy (I79N, ΔE96, and ΔK210, respectively), were examined using the thin-filament extraction/reconstitution technique. Effects of Ca(2+), ATP, phosphate, and ADP concentrations on force and its transients were studied at 25°C. Maximal Ca(2+) tension (THC) and Ca(2+)-activatable tension (Tact), respectively, were similar among I79N, ΔE96, and WT, whereas ΔK210 led to a significantly lower THC (∼20% less) and Tact (∼25% less) than did WT. In pCa solution containing 8 mM Pi and ionic strength adjusted to 200 mM, the Ca(2+) sensitivity (pCa50) of I79N (5.63 ± 0.02) and ΔE96 (5.60 ± 0.03) was significantly greater than that of WT (5.45 ± 0.04), but the pCa50 of ΔK210 (5.54 ± 0.04) remained similar to that of WT. Five equilibrium constants were deduced using sinusoidal analysis. All three mutants showed significantly lower K0 (ADP association constant) and larger K4 (equilibrium constant of force generation step) relative to the corresponding values for WT. I79N and ΔK210 were associated with a K2 (equilibrium constant of cross-bridge detachment step) significantly lower than that of ΔE96 and WT. These results demonstrated that at pCa 4.66, the force/cross-bridge is ∼18% less in I79N and ∼41% less in ΔK210 than that in WT. These results indicate that the molecular pathogenesis of the cardiac TnT mutation-related cardiomyopathies is different for each mutation.

Equilibrium self-association of tropomyosin

It has recently been reported that tropomyosin exists exclusively as a dimer in physiological salt conditions. It is shown in the present work using analytical ultracentrifugation that, on the contrary, tropomyosin is in equilibrium between monomer, dimer and tetramer with a weak tendency to dimerize and tetramerize. Such a finding has consequences for the assembly of the tropomyosin-actin complex.

Myosin light chain kinase and the role of myosin light chain phosphorylation in skeletal muscle

Skeletal muscle myosin light chain kinase (skMLCK) is a dedicated Ca(2+)/calmodulin-dependent serine-threonine protein kinase that phosphorylates the regulatory light chain (RLC) of sarcomeric myosin. It is expressed from the MYLK2 gene specifically in skeletal muscle fibers with most abundance in fast contracting muscles. Biochemically, activation occurs with Ca(2+) binding to calmodulin forming a (Ca(2+))(4)•calmodulin complex sufficient for activation with a diffusion limited, stoichiometric binding and displacement of a regulatory segment from skMLCK catalytic core. The N-terminal sequence of RLC then extends through the exposed catalytic cleft for Ser15 phosphorylation. Removal of Ca(2+) results in the slow dissociation of calmodulin and inactivation of skMLCK. Combined biochemical properties provide unique features for the physiological responsiveness of RLC phosphorylation, including (1) rapid activation of MLCK by Ca(2+)/calmodulin, (2) limiting kinase activity so phosphorylation is slower than contraction, (3) slow MLCK inactivation after relaxation and (4) much greater kinase activity relative to myosin light chain phosphatase (MLCP). SkMLCK phosphorylation of myosin RLC modulates mechanical aspects of vertebrate skeletal muscle function. In permeabilized skeletal muscle fibers, phosphorylation-mediated alterations in myosin structure increase the rate of force-generation by myosin cross bridges to increase Ca(2+)-sensitivity of the contractile apparatus. Stimulation-induced increases in RLC phosphorylation in intact muscle produces isometric and concentric force potentiation to enhance dynamic aspects of muscle work and power in unfatigued or fatigued muscle. Moreover, RLC phosphorylation-mediated enhancements may interact with neural strategies for human skeletal muscle activation to ameliorate either central or peripheral aspects of fatigue.

Myosin light-chain phosphorylation and potentiation of dynamic function in mouse fast muscle

The intent of this study was to determine if the stimulation-induced increase or "potentiation" of dynamic function of mouse extensor digitorum longus muscle (in vitro 25°C) during work cycles is graded to myosin regulatory light-chain (RLC) phosphorylation. To do this, concentric force and muscle work output during sinusoidal length changes were determined before (unpotentiated) and after (potentiated) the application of conditioning stimuli (CS) producing incremental elevations in RLC phosphorylation from rest. Sine wave excursion was from 1.09 to 0.91 of L (o) with a period of 142 ms; stimulating muscles to twitch and generate force during these cycles produced plots of force × displacement termed work loops. Stimulation at 2.5-, 5.0-, and 100-Hz elevated RLC phosphorylation from 0.16±0.02 (rest) to 0.29±0.03, 0.45±0.02 and 0.56±0.02 mol phos per mole RLC, respectively (n= 6-7, P<0.05). These CS potentiated mean concentric force (at all lengths) to 1.14±0.02, 1.26±0.04 and 1.41±0.06 of pre-stimulus, control levels (all n= 5-7, P<0.05) while work was increased to 1.07±0.02, 1.17±0.02 and 1.34±0.03 of controls, respectively. In a No CS condition that did not elevate RLC phosphorylation, neither mean concentric force nor work was altered. Thus, strong correlations between RLC phosphorylation and mean concentric force and work support the hypothesis that this molecular mechanism modulates muscle power output. No length-dependence for concentric force potentiation was observed in any condition, an outcome suggesting that interactions between instantaneous variations in muscle length and shortening velocity during work cycles modulates the potentiation response.

Insights into modulation of calcium signaling by magnesium in calmodulin, troponin C and related EF-hand proteins

The Ca(2+)-binding helix-loop-helix structural motif called "EF-hand" is a common building block of a large family of proteins that function as intracellular Ca(2+)-receptors. These proteins respond specifically to micromolar concentrations of Ca(2+) in the presence of ~1000-fold excess of the chemically similar divalent cation Mg(2+). The intracellular free Mg(2+) concentration is tightly controlled in a narrow range of 0.5-1.0mM, which at the resting Ca(2+) levels is sufficient to fully or partially saturate the Ca(2+)-binding sites of many EF-hand proteins. Thus, to convey Ca(2+) signals, EF-hand proteins must respond differently to Ca(2+) than to Mg(2+). In this review the structural aspects of Mg(2+) binding to EF-hand proteins are considered and interpreted in light of the recently proposed two-step Ca(2+)-binding mechanism (Grabarek, Z., J. Mol. Biol., 2005, 346, 1351). It is proposed that, due to stereochemical constraints imposed by the two-EF-hand domain structure, the smaller Mg(2+) ion cannot engage the ligands of an EF-hand in the same way as Ca(2+) and defaults to stabilizing the apo-like conformation of the EF-hand. It is proposed that Mg(2+) plays an active role in the Ca(2+)-dependent regulation of cellular processes by stabilizing the "off state" of some EF-hand proteins, thereby facilitating switching off their respective target enzymes at the resting Ca(2+) levels. Therefore, some pathological conditions attributed to Mg(2+) deficiency might be related to excessive activation of underlying Ca(2+)-regulated cellular processes. This article is part of a Special Issue entitled: 11th European Symposium on Calcium.

Gene transfer, expression, and sarcomeric incorporation of a headless myosin molecule in cardiac myocytes: evidence for a reserve in myofilament motor function

The purpose of this study was to implement a living myocyte in vitro model system to test whether a motor domain-deleted headless myosin construct could be incorporated into the sarcomere and affect contractility. To this end we used gene transfer to express a "headless" myosin heavy chain (headless-MHC) in complement with the native full-length myosin motors in the cardiac sarcomere. An NH2-terminal Flag epitope was used for unique detection of the motor domain-deleted headless-MHC. Total MHC content (i.e., headless-MHC+endogenous MHC) remained constant, while expression of the headless-MHC in transduced myocytes increased from 24 to 72 h after gene transfer until values leveled off at 96 h after gene transfer, at which time the headless-MHC comprised ∼20% of total MHC. Moreover, immunofluorescence labeling and confocal imaging confirmed expression and demonstrated incorporation of the headless-MHC in the A band of the cardiac sarcomere. Functional measurements in intact myocytes showed that headless-MHC modestly reduced amplitude of dynamic twitch contractions compared with controls (P<0.05). In chemically permeabilized myocytes, maximum steady-state isometric force and the tension-pCa relationship were unaltered by the headless-MHC. These data suggest that headless-MHC can express to 20% of total myosin and incorporate into the sarcomere yet have modest to no effects on dynamic and steady-state contractile function. This would indicate a degree of functional tolerance in the sarcomere for nonfunctional myosin molecules.

Tropomyosin period 3 is essential for enhancement of isometric tension in thin filament-reconstituted bovine myocardium

Tropomyosin (Tm) consists of 7 quasiequivalent repeats known as "periods," and its specific function may be associated with these periods. To test the hypothesis that either period 2 or 3 promotes force generation by inducing a positive allosteric effect on actin, we reconstituted the thin filament with mutant Tm in which either period 2 (Delta2Tm) or period 3 (Delta3Tm) was deleted. We then studied: isometric tension, stiffness, 6 kinetic constants, and the pCa-tension relationship. N-terminal acetylation of Tm did not cause any differences. The isometric tension in Delta2Tm remained unchanged, and was reduced to approximately 60% in Delta3Tm. Although the kinetic constants underwent small changes, the occupancy of strongly attached cross-bridges was not much different. The Hill factor (cooperativity) did not differ significantly between Delta2Tm (1.79 +/- 0.19) and the control (1.73 +/- 0.21), or Delta3Tm (1.35 +/- 0.22) and the control. In contrast, pCa(50) decreased slightly in Delta2Tm (5.11 +/- 0.07), and increased significantly in Delta3Tm (5.57 +/- 0.09) compared to the control (5.28 +/- 0.04). These results demonstrate that, when ions are present at physiological concentrations in the muscle fiber system, period 3 (but not period 2) is essential for the positive allosteric effect that enhances the interaction between actin and myosin, and increases isometric force of each cross-bridge.

Malignant familial hypertrophic cardiomyopathy D166V mutation in the ventricular myosin regulatory light chain causes profound effects in skinned and intact papillary muscle fibers from transgenic mice

Transgenic (Tg) mice expressing approximately 95% of the D166V (aspartic acid to valine) mutation in the ventricular myosin regulatory light chain (RLC) shown to cause a malignant familial hypertrophic cardiomyopathy (FHC) phenotype were generated, and the skinned and intact papillary muscle fibers from the Tg-D166V mice were examined using a Guth muscle research system. A large increase in the Ca(2+) sensitivity of force and ATPase (Delta pCa(50)>0.25) and a significant decrease in maximal force and ATPase were observed in skinned muscle fibers from Tg-D166V mice compared with control mice. The cross-bridge dissociation rate g was dramatically decreased, whereas the energy cost (ATPase/force) was slightly increased in Tg-D166V fibers compared with controls. The calculated average force per D166V cross-bridge was also reduced. Intact papillary muscle data demonstrated prolonged force transients with no change in calcium transients in Tg-D166V fibers compared with control fibers. Histopathological examination revealed fibrotic lesions in the hearts of the older D166V mice. Our results suggest that a charge effect of the D166V mutation and/or a mutation-dependent decrease in RLC phosphorylation could initiate the slower kinetics of the D166V cross-bridges and ultimately affect the regulation of cardiac muscle contraction. Profound cellular changes observed in Tg-D166V myocardium when placed in vivo could trigger a series of pathological responses and result in poor prognosis for D166V-positive patients.

Tropomodulin binds two tropomyosins: a novel model for actin filament capping

Tropomodulin, a tropomyosin-binding protein, caps the slow-growing (pointed) end of the actin filament regulating its dynamics. Tropomodulin, therefore, is important for determining cell morphology, cell movement, and muscle contraction. For the first time we show that one tropomodulin molecule simultaneously binds two tropomyosin molecules in a cooperative manner. On the basis of the tropomodulin solution structure and predicted secondary structure, we introduced a series of point mutations in regions important for tropomyosin binding and actin capping. Capping activity of these mutants was assayed by measuring actin polymerization using pyrene fluorescence. Using direct methods (circular dichroism and native gel electrophoresis) for detecting tropomodulin/tropomyosin binding, we localized the second tropomyosin-binding site to residues 109-144. Despite previous reports that the second binding site is for erythrocyte tropomyosin only, we found that both short nonmuscle and long muscle alpha-tropomyosins bind there as well, though with different affinities. We propose a model for actin capping where one tropomodulin molecule can bind to two tropomyosin molecules at the pointed end.

Dynamics of crossbridge-mediated activation in the heart

Both intracellular calcium and strongly bound crossbridges contribute to thin filament activation in the heart, but the magnitude and the duration of the effects due to crossbridges are not well characterized. In this study, crossbridge attachment was altered in tetanized ferret papillary muscles and changes in the rate constant for the recovery of force (k (TR)) and unloaded shortening velocity (V (U)) were measured to track thin filament activation. k (TR) decreased as the time the muscles spent at low levels of crossbridge attachment (shortening deactivation) increased (0.02 s=17.9+/-2.3 s(-1), 0.32 s=3.3+/-0.4 s(-1); half-time=0.052 s; P<0.05). Furthermore, the deactivation was reversible and k (TR) recovered when muscles were allowed to regenerate force isometrically during the same tetanus. V (U) also decreased when the preceding load was lower (isometric load, V (U)=1.93+/-0.26 muscle lengths/s (ML/s); zero load, V (U)=0.93+/-0.14 ML/s, P<0.05) and as the length of time the muscle spent unloaded increased (>60% decline after 0.3 s). In addition, V (U) recovered when the muscle was allowed to regenerate force isometrically. These results indicate that crossbridge attachment increases thin filament activation as reflected in measurements of V (U) and k (TR). This 'extra' activation by crossbridges appears to be a dynamic process that decays during unloaded shortening and redevelops during isometric contraction.

Effect of the Structure of the N Terminus of Tropomyosin on Tropomodulin Function

Tropomodulins (Tmod) bind to the N terminus of tropomyosin and cap the pointed end of actin filaments. Tropomyosin alone also inhibits the rate of actin depolymerization at the pointed end of filaments. Here we have defined 1) the structural requirements of the N terminus of tropomyosin important for regulating the pointed end alone and with erythrocyte Tmod (Tmod1), and 2) the Tmod1 subdomains required for binding to tropomyosin and for regulating the pointed end. Changes in pyrene-actin fluorescence during polymerization and depolymerization were measured with actin filaments blocked at the barbed end with gelsolin. Three tropomyosin isoforms differently influence pointed end dynamics. Recombinant TM5a, a short non-muscle alpha-tropomyosin, inhibited depolymerization. Recombinant (unacetylated) TM2 and N-acetylated striated muscle TM (stTM), long alpha-tropomyosin isoforms with the same N-terminal sequence, different from TM5a, also inhibited depolymerization but were less effective than TM5a. All blocked the pointed end with Tmod1 in the order of effectiveness TM5a >stTM >TM2, showing the importance of the N-terminal sequence and modification. Tmod1-(1-344), lacking the C-terminal 15 residues, did not nucleate polymerization but blocked the pointed end with all three tropomyosin isoforms as does a shorter fragment, Tmod1-(1-92), lacking the C-terminal "capping" domain though higher concentrations were required. An even shorter fragment, Tmod1-(1-48), bound tropomyosin but did not influence actin filament elongation. Tropomyosin-Tmod may function to locally regulate cytoskeletal dynamics in cells by stabilizing actin filaments.