Isolation, electron microscopic imaging, and 3-D visualization of native cardiac thin myofilaments

Mechanical interaction of myosin and native thin filament in the disused rat soleus muscle

The disuse of skeletal limb muscles occurs in a variety of conditions, yet our comprehension of the molecular mechanisms involved in adaptation to disuse remains incomplete. We studied the mechanical characteristics of actin-myosin interaction using an in vitro motility assay and isoform composition of myosin heavy and light chains by dint of SDS-PAGE in soleus muscle of both control and hindlimb-unloaded rats. 14 days of hindlimb unloading led to the increased maximum sliding velocity of actin, reconstituted, and native thin filaments over rat soleus muscle myosin by 24 %, 19 %, and 20 %, respectively. The calcium sensitivity of the "pCa-velocity" relationship decreased. There was a 26 % increase in fast myosin heavy chain IIa (MHC IIa), a 22 % increase in fast myosin light chain 2 (MLC 2f), and a 13 % increase in fast MLC 1f content. The content of MLC 1s/v, typical for slow skeletal muscles and cardiac ventricles did not change. At the same time, MLC 1s, typical only for slow skeletal muscles, disappeared. The maximum velocity of soleus muscle native thin filaments was 24 % higher compared to control ones sliding over the same rabbit myosin. Therefore, both myosin and native thin filament kinetics could influence the mechanical characteristics of the soleus muscle. Additionally, the MLC 1s and MLC 1s/v ratio may contribute to the mechanical characteristics of slow skeletal muscle, along with MHC, MLC 2, and MLC 1 slow/fast isoforms ratio.

The inter-chamber differences in the contractile function between left and right atrial cardiomyocytes in atrial fibrillation in rats

Introduction

The left and right atria (LA, RA) work under different mechanical and metabolic environments that may cause an intrinsic inter-chamber diversity in structure and functional properties between atrial cardiomyocytes (CM) in norm and provoke their different responsiveness to pathological conditions. In this study, we assessed a LA vs. RA difference in CM contractility in paroxysmal atrial fibrillation (AF) and underlying mechanisms.

Methods

We investigated the contractile function of single isolated CM from LA and RA using a 7-day acetylcholine (ACh)-CaCl2 AF model in rats. We compared auxotonic force, sarcomere length dynamics, cytosolic calcium ([Ca2+]i) transients, intracellular ROS and NO production in LA and RA CM, and analyzed the phosphorylation levels of contractile proteins and actin-myosin interaction using an in vitro motility assay.

Results

AF resulted in more prominent structural and functional changes in LA myocardium, reducing sarcomere shortening amplitude, and velocity of sarcomere relengthening in mechanically non-loaded LA CM, which was associated with the increased ROS production, decreased NO production, reduced myofibrillar content, and decreased phosphorylation of cardiac myosin binding protein C and troponin I. However, in mechanically loaded CM, AF depressed the auxotonic force amplitude and kinetics in RA CM, while force characteristics were preserved in LA CM.

Discussion

Thus, inter-atrial differences are increased in paroxysmal AF and affected by the mechanical load that may contribute to the maintenance and progression of AF.

Myosin with hypertrophic cardiac mutation R712L has a decreased working stroke which is rescued by omecamtiv mecarbil

Hypertrophic cardiomyopathies (HCMs) are the leading cause of acute cardiac failure in young individuals. Over 300 mutations throughout β-cardiac myosin, including in the motor domain, are associated with HCM. A β-cardiac myosin motor mutation (R712L) leads to a severe form of HCM. Actin-gliding motility of R712L-myosin is inhibited, despite near-normal ATPase kinetics. By optical trapping, the working stroke of R712L-myosin was decreased 4-fold, but actin-attachment durations were normal. A prevalent hypothesis that HCM mutants are hypercontractile is thus not universal. R712 is adjacent to the binding site of the heart failure drug omecamtiv mecarbil (OM). OM suppresses the working stroke of normal β-cardiac myosin, but remarkably, OM rescues the R712L-myosin working stroke. Using a flow chamber to interrogate a single molecule during buffer exchange, we found OM rescue to be reversible. Thus, the R712L mutation uncouples lever arm rotation from ATPase activity and this inhibition is rescued by OM.

Differing effects of estrogen deficiency on the contractile function of atrial and ventricular myocardium

Estrogen deficiency has a significant influence on the excitation-contraction coupling in the ventricular myocardium but its impact on the atrial contractile function has not been studied. We have compared the effects of estrogen deficiency on the contractility and cytosolic Ca²⁺ transient of single cardiomyocytes isolated from the left atrium (LA) and the left ventricle (LV) of rats subjected to ovariectomy (OVX) or sham surgery (Sham). The characteristics of actin-myosin interaction were studied in an in vitro motility assay. We found that OVX decreased the contractility of LV single cardiomyocytes but increased that of LA myocytes. The disturbance of ventricular mechanical function may be explained by the acceleration of Ca²⁺ transient and reduced Ca²⁺ sensitivity of the actin-myosin interaction. The augmentation of LA contractility may be explained by accelerated cross-bridge kinetics and increased end-diastolic sarcomere length, which may lead to elevated tension in atrial cells due to the Frank-Starling mechanism.

Structural and functional effects of myosin-binding protein-C phosphorylation in heart muscle are not mimicked by serine-to-aspartate substitutions

Myosin-binding protein-C (cMyBP-C) is a key regulator of contractility in heart muscle, and its regulatory function is controlled in turn by phosphorylation of multiple serines in its m-domain. The structural and functional effects of m-domain phosphorylation have often been inferred from those of the corresponding serine-to-aspartate (Ser–Asp) substitutions, in both in vivo and in vitro studies. Here, using a combination of in vitro binding assays and in situ structural and functional assays in ventricular trabeculae of rat heart and the expressed C1mC2 region of cMyBP-C, containing the m-domain flanked by domains C1 and C2, we tested whether these substitutions do in fact mimic the effects of phosphorylation. In situ changes in thin and thick filament structure were determined from changes in polarized fluorescence from bifunctional probes attached to troponin C or myosin regulatory light chain, respectively. We show that both the action of exogenous C1mC2 to activate contraction in the absence of calcium and the accompanying change in thin filament structure are abolished by tris-phosphorylation of the m-domain, but unaffected by the corresponding Ser–Asp substitutions. The latter produced an intermediate change in thick filament structure. Both tris-phosphorylation and Ser–Asp substitutions abolished the interaction between C1mC2 and myosin sub-fragment 2 (myosin S2) in vitro, but yielded different effects on thin filament binding. These results suggest that some previous inferences from the effects of Ser–Asp substitutions in cMyBP-C should be reconsidered and that the distinct effects of tris-phosphorylation and Ser–Asp substitutions on cMyBP-C may provide a useful basis for future studies.

Omecamtiv Mecarbil modulates the kinetic and motile properties of porcine β-cardiac myosin

We determined the effect of Omecamtiv Mecarbil, a novel allosteric effector of cardiac muscle myosin, on the kinetic and "in vitro" motility properties of the porcine ventricular heavy meromyosin (PV-HMM). Omecamtiv Mecarbil increases the equilibrium constant of the hydrolysis step (M-ATP ⇄ M-ADP-Pi) from 2.4 to 6 as determined by quench flow, but the maximal rates of both the hydrolysis step and tryptophan fluorescence increase are unchanged by the drug. OM also increases the amplitude of the fast phase of phosphate dissociation (AM-ADP-Pi → AM-ADP + Pi) that is associated with force production in muscle by 4-fold. These results suggest a mechanism in which hydrolysis of M-ATP to M-ADP-Pi occurs both before and after the recovery stroke, but rapid acceleration of phosphate dissociation by actin occurs only on post-recovery stroke A-M-ADP-Pi. One of the more dramatic effects of OM on PV-HMM is a 14-fold decrease in the unloaded shortening velocity measured by the in vitro motility assay. The increase in flux through phosphate dissociation and the unchanged rate of ADP dissociation (AM-ADP → AM + ADP) by the drug produce a higher duty ratio motor in which a larger fraction of myosin heads are strongly bound to actin filaments. The increased internal load produced by a larger fraction of strongly attached crossbridges explains the reduced rate of in vitro motility velocity in the presence of OM and predicts that the drug will produce slower and stronger contraction of cardiac muscle.

Electron Microscopy: From 2D to 3D Images with Special Reference to Muscle

This is a brief and necessarily very sketchy presentation of the evolution in electron microscopy (EM) imaging that was driven by the necessity of extracting 3-D views from the essentially 2-D images produced by the electron beam. The lens design of standard transmission electron microscope has not been greatly altered since its inception. However, technical advances in specimen preparation, image collection and analysis gradually induced an astounding progression over a period of about 50 years. From the early images that redefined tissues, cell and cell organelles at the sub-micron level, to the current nano-resolution reconstructions of organelles and proteins the step is very large. The review is written by an investigator who has followed the field for many years, but often from the sidelines, and with great wonder. Her interest in muscle ultrastructure colors the writing. More specific detailed reviews are presented in this issue.

Three-dimensional organization of troponin on cardiac muscle thin filaments in the relaxed state

Muscle contraction is regulated by troponin-tropomyosin, which blocks and unblocks myosin binding sites on actin. To elucidate this regulatory mechanism, the three-dimensional organization of troponin and tropomyosin on the thin filament must be determined. Although tropomyosin is well defined in electron microscopy helical reconstructions of thin filaments, troponin density is mostly lost. Here, we determined troponin organization on native relaxed cardiac muscle thin filaments by applying single particle reconstruction procedures to negatively stained specimens. Multiple reference models led to the same final structure, indicating absence of model bias in the procedure. The new reconstructions clearly showed F-actin, tropomyosin, and troponin densities. At the 25 Å resolution achieved, troponin was considerably better defined than in previous reconstructions. The troponin density closely resembled the shape of troponin crystallographic structures, facilitating detailed interpretation of the electron microscopy density map. The orientation of troponin-T and the troponin core domain established troponin polarity. Density attributable to the troponin-I mobile regulatory domain was positioned where it could hold tropomyosin in its blocking position on actin, thus suggesting the underlying structural basis of thin filament regulation. Our previous understanding of thin filament regulation had been limited to known movements of tropomyosin that sterically block and unblock myosin binding sites on actin. We now show how troponin, the Ca(2+) sensor, may control these movements, ultimately determining whether muscle contracts or relaxes.

Modulation of thin filament activation of myosin ATP hydrolysis by N-terminal domains of cardiac myosin binding protein-C

We have used enzyme kinetics to investigate the molecular mechanism by which the N-terminal domains of human and mouse cardiac MyBP-C (C0C1, C1C2, and C0C2) affect the activation of myosin ATP hydrolysis by F-actin and by native porcine thin filaments. N-Terminal domains of cMyBP-C inhibit the activation of myosin-S1 ATPase by F-actin. However, mouse and human C1C2 and C0C2 produce biphasic activating and inhibitory effects on the activation of myosin ATP hydrolysis by native cardiac thin filaments. Low ratios of MyBP-C N-terminal domains to thin filaments activate myosin-S1 ATP hydrolysis, but higher ratios inhibit ATP hydrolysis, as is observed with F-actin alone. These data suggest that low concentrations of C1C2 and C0C2 activate thin filaments by a mechanism similar to that of rigor myosin-S1, whereas higher concentrations inhibit the ATPase rate by competing with myosin-S1-ADP-P_i for binding to actin and thin filaments. In contrast to C0C2 and C1C2, the activating effects of the C0C1 domain are species-dependent: human C0C1 activates actomyosin-S1 ATPase rates, but mouse C0C1 does not produce significant activation or inhibition. Phosphorylation of serine residues in the m-linker between the C1 and C2 domains by protein kinase-A decreases the activation of thin filaments by huC0C2 at pC_a > 8 but has little effect on the activation mechanism at pC_a = 4. In sarcomeres, the low ratio of cMyBP-C to actin is expected to favor the activating effects of cMyBP-C while minimizing inhibition produced by competition with myosin heads.

Myosin-binding protein C displaces tropomyosin to activate cardiac thin filaments and governs their speed by an independent mechanism

Myosin-binding protein C (MyBP-C) is an accessory protein of striated muscle thick filaments and a modulator of cardiac muscle contraction. Defects in the cardiac isoform, cMyBP-C, cause heart disease. cMyBP-C includes 11 Ig- and fibronectin-like domains and a cMyBP-C-specific motif. In vitro studies show that in addition to binding to the thick filament via its C-terminal region, cMyBP-C can also interact with actin via its N-terminal domains, modulating thin filament motility. Structural observations of F-actin decorated with N-terminal fragments of cMyBP-C suggest that cMyBP-C binds to actin close to the low Ca(2+) binding site of tropomyosin. This suggests that cMyBP-C might modulate thin filament activity by interfering with tropomyosin regulatory movements on actin. To determine directly whether cMyBP-C binding affects tropomyosin position, we have used electron microscopy and in vitro motility assays to study the structural and functional effects of N-terminal fragments binding to thin filaments. 3D reconstructions suggest that under low Ca(2+) conditions, cMyBP-C displaces tropomyosin toward its high Ca(2+) position, and that this movement corresponds to thin filament activation in the motility assay. At high Ca(2+), cMyBP-C had little effect on tropomyosin position and caused slowing of thin filament sliding. Unexpectedly, a shorter N-terminal fragment did not displace tropomyosin or activate the thin filament at low Ca(2+) but slowed thin filament sliding as much as the larger fragments. These results suggest that cMyBP-C may both modulate thin filament activity, by physically displacing tropomyosin from its low Ca(2+) position on actin, and govern contractile speed by an independent molecular mechanism.

Mechanism of regulation of native cardiac muscle thin filaments by rigor cardiac myosin-S1 and calcium

We have studied the mechanism of activation of native cardiac thin filaments by calcium and rigor myosin. The acceleration of the rate of 2'-deoxy-3'-O-(N-methylanthraniloyl)ADP (mdADP) dissociation from cardiac myosin-S1-mdADP-P(i) and cardiac myosin-S1-mdADP by native cardiac muscle thin filaments was measured using double mixing stopped-flow fluorescence. Relative to inhibited thin filaments (no bound calcium or rigor S1), fully activated thin filaments (with both calcium and rigor-S1 bound) increase the rate of product dissociation from the physiologically important pre-power stroke myosin-mdADP-P(i) by a factor of ∼75. This can be compared with only an ∼6-fold increase in the rate of nucleotide diphosphate dissociation from nonphysiological myosin-mdADP by the fully activated thin filaments relative to the fully inhibited thin filaments. These results show that physiological levels of regulation are not only dependent on the state of the thin filament but also on the conformation of the myosin. Less than 2-fold regulation is due to a change in affinity of myosin-ADP-P(i) for thin filaments such as would be expected by a simple "steric blocking" of the myosin-binding site of the thin filament by tropomyosin. Although maximal activation requires both calcium and rigor myosin-S1 bound to the cardiac filament, association with a single ligand produces ∼70% maximal activation. This can be contrasted with skeletal thin filaments in which calcium alone only activated the rate of product dissociation ∼20% of maximum, and rigor myosin produces ∼30% maximal activation.

The myosin-binding protein C motif binds to F-actin in a phosphorylation-sensitive manner

Cardiac myosin-binding protein C (cMyBP-C) is a regulatory protein expressed in cardiac sarcomeres that is known to interact with myosin, titin, and actin. cMyBP-C modulates actomyosin interactions in a phosphorylation-dependent way, but it is unclear whether interactions with myosin, titin, or actin are required for these effects. Here we show using cosedimentation binding assays, that the 4 N-terminal domains of murine cMyBP-C (i.e. C0-C1-m-C2) bind to F-actin with a dissociation constant (K(d)) of approximately 10 microm and a molar binding ratio (B(max)) near 1.0, indicating 1:1 (mol/mol) binding to actin. Electron microscopy and light scattering analyses show that these domains cross-link F-actin filaments, implying multiple sites of interaction with actin. Phosphorylation of the MyBP-C regulatory motif, or m-domain, reduced binding to actin (reduced B(max)) and eliminated actin cross-linking. These results suggest that the N terminus of cMyBP-C interacts with F-actin through multiple distinct binding sites and that binding at one or more sites is reduced by phosphorylation. Reversible interactions with actin could contribute to effects of cMyBP-C to increase cross-bridge cycling.

The self-assembly, elasticity, and dynamics of cardiac thin filaments

Solutions of intact cardiac thin filaments were examined with transmission electron microscopy, dynamic light scattering (DLS), and particle-tracking microrheology. The filaments self-assembled in solution with a bell-shaped distribution of contour lengths that contained a population of filaments of much greater length than the in vivo sarcomere size ( approximately 1 mum) due to a one-dimensional annealing process. Dynamic semiflexible modes were found in DLS measurements at fast timescales (12.5 ns-0.0001 s). The bending modulus of the fibers is found to be in the range 4.5-16 x 10(-27) Jm and is weakly dependent on calcium concentration (with Ca2+ > or = without Ca2+). Good quantitative agreement was found for the values of the fiber diameter calculated from transmission electron microscopy and from the initial decay of DLS correlation functions: 9.9 nm and 9.7 nm with and without Ca2+, respectively. In contrast, at slower timescales and high polymer concentrations, microrheology indicates that the cardiac filaments act as short rods in solution according to the predictions of the Doi-Edwards chopsticks model (viscosity, eta approximately c(3), where c is the polymer concentration). This differs from the semiflexible behavior of long synthetic actin filaments at comparable polymer concentrations and timescales (elastic shear modulus, G' approximately c(1.4), tightly entangled) and is due to the relative ratio of the contour lengths ( approximately 30). The scaling dependence of the elastic shear modulus on the frequency (omega) for cardiac thin filaments is G' approximately omega(3/4 +/- 0.03), which is thought to arise from flexural modes of the filaments.

The functional domains of human ventricular myosin light chain 1

The biological functions of the myosin light chain 1 (LC1) have not been clearly elucidated yet. In this work we cloned and expressed N- and C- terminal fragments of human ventricular LC1 (HVLC1) containing amino acid residues 1-98 and 99-195 and two parts, NN and NC of N fragment in GST-fusion forms, respectively. Using GST pull-down assay, the direct binding experiments of LC1 with rat cardiac G-actin, F-actin and thin filaments, as well as rat cardiac myosin heavy chain (RCMHC) have been performed. Furthermore, the recombinant complexes of rat myosin S1 with N- and C-fragments, as well as the whole molecular of HVLC1 were generated. The results suggested that both binding sites of HVLC1 for actin and myosin heavy chain are positioned in its N-terminal fragment, which may contain several actin-binding sites in tandem. The polymerization of G-actin, the tropomyosin and troponin molecules located in the thin filaments do not hinder the binding of N-terminal fragment of HVLC1 with actin and thin filaments in vitro. The recombinant complex of rat cardiac myosin S1 (RCMS1) with N fragment of HVLC1 greatly decreased actin-activated Mg(2+)-ATPase activity for lack of C fragment. We conclude that the N-fragment is the binding domain of human ventricular LC1, whereas the C-fragment serves as a functional domain, which may be more involved in the modulation of the actin-activated ATPase activity of myosin.

Imbalance between tissue inhibitor of metalloproteinase-4 and matrix metalloproteinases during acute myocardial [correction of myoctardial] ischemia-reperfusion injury

BACKGROUND

We have previously reported that matrix metalloproteinase-2 (MMP-2) contributes to myocardial ischemia-reperfusion injury by degradation of troponin I, a regulatory element of the contractile proteins. MMP activities are also tightly regulated by tissue inhibitors of metalloproteinase (TIMPs). The change in TIMPs during acute myocardial ischemia-reperfusion injury is not clear.

METHODS AND RESULTS

Isolated rat hearts were perfused either aerobically for 75 minutes or subjected to 15, 20, or 25 minutes of global, no-flow ischemia followed by 30 minutes of aerobic reperfusion. During reperfusion after ischemia, there was a rapid, enhanced release of TIMP-4, the most abundant TIMP in the heart, into the coronary effluent, as shown both by reverse zymography and Western blot. There was a negative correlation between the recovery of cardiac mechanical function and the release of TIMP-4 during reperfusion in hearts subjected to different durations of ischemia. Immunogold electron microscopy revealed a close association of TIMP-4 with the sarcomeres in aerobically perfused hearts. Moreover, TIMP-4 was present only in thin myofilaments prepared from aerobically perfused hearts but not in ischemic-reperfused hearts. An enhanced MMP activity was shown in ischemic-reperfused hearts by in situ zymography.

CONCLUSIONS

Loss of TIMP-4 from the cardiac myocyte leads to an increase in net myocardial MMP activity that contributes to acute myocardial stunning injury.

Intracellular action of matrix metalloproteinase-2 accounts for acute myocardial ischemia and reperfusion injury

BACKGROUND

Matrix metalloproteinases are best recognized for their ability to degrade the extracellular matrix in both physiological and pathological conditions. However, recent findings indicate that some of them are also involved in mediating acute processes such as platelet aggregation and vascular tone. The acute contractile defect of the heart after ischemia-reperfusion may involve the proteolytic degradation of the thin filament protein troponin I; however, the protease responsible for this remains obscure.

METHODS AND RESULTS

Here we report that matrix metalloproteinase-2 is colocalized with troponin I within the thin myofilaments of cardiomyocytes in ischemic-reperfused hearts and that troponin I is a novel intracellular target for proteolytic cleavage by matrix metalloproteinase-2. Inhibition of matrix metalloproteinase-2 activity prevented ischemia-reperfusion-induced troponin I degradation and improved the recovery of mechanical function of the heart.

CONCLUSIONS

These data reveal for the first time a novel molecular mechanism by which matrix metalloproteinase-2 causes acute myocardial dysfunction after ischemia-reperfusion-injury and that matrix metalloproteinase-2 has a biological action within the cell.

Structural analysis of F18 fimbriae expressed by porcine toxigenic Escherichia coli

The F18 fimbriae expressed by porcine toxigenic Escherichia coli strains are 1- to 2-mm-long filaments that mediate the adhesion of the bacteria to enterocytes. The backbone of these fimbriae is built from a major structural 15.1-kDa protein, FedA. The structure of isolated negatively stained F18 fimbriae imaged by dark-field scanning transmission electron microscopy (STEM) was resolved to approximately 2 nm. Analyzing their helical symmetry showed the axially repeating units to alternate in a "zigzag" manner around the helical axis with an axial rise of 2.2 nm. Two repeating units give rise to the observed 4.3-nm helical repeat, which is practically identical to the pitch of the one-start helix formed. Additionally, an axially repeating pattern with a 27-nm spacing was found on rotary-shadowed fimbriae. Mass-per-length determination of unstained F18 fimbriae by STEM revealed the axially repeating unit to have a molecular mass of 25.4 kDa, indicating that it is a FedA monomer, with the difference in mass arising from the minor subunits, FedE and FedF. The presence of the latter two proteins might cause the observed 27-nm axial pattern.