Force-velocity relations of rat cardiac myosin isozymes sliding on algal cell actin cables in vitro

Investigations of Molecular Mechanisms of Actin-Myosin Interactions in Cardiac Muscle

The functional characteristics of cardiac muscle depend on the composition of protein isoforms in the cardiomyocyte contractile machinery. In the ventricular myocardium of mammals, several isoforms of contractile and regulatory proteins are expressed - two isoforms of myosin (V1 and V3) and three isoforms of tropomyosin chains (α, β, and κ). Expression of protein isoforms depends on the animal species, its age and hormonal status, and this can change with pathologies of the myocardium. Mutations in these proteins can lead to cardiomyopathies. The functional significance of the protein isoform composition has been studied mainly on intact hearts or on isolated preparations of myocardium, which could not provide a clear comprehension of the role of each particular isoform. Present-day experimental techniques such as an optical trap and in vitro motility assay make it possible to investigate the phenomena of interactions of contractile and regulatory proteins on the molecular level, thus avoiding effects associated with properties of a whole muscle or muscle tissue. These methods enable free combining of the isoforms to test the molecular mechanisms of their participation in the actin-myosin interaction. Using the optical trap and the in vitro motility assay, we have studied functional characteristics of the cardiac myosin isoforms, molecular mechanisms of the calcium-dependent regulation of actin-myosin interaction, and the role of myosin and tropomyosin isoforms in the cooperativity mechanisms in myocardium. The knowledge of molecular mechanisms underlying myocardial contractility and its regulation is necessary for comprehension of cardiac muscle functioning, its disorders in pathologies, and for development of approaches for their correction.

Effects of cardiac myosin isoform variation on myofilament function and crossbridge kinetics in transgenic rabbits

BACKGROUND

The left ventricles of both rabbits and humans express predominantly beta-myosin heavy chain (MHC). Transgenic (TG) rabbits expressing 40% alpha-MHC are protected against tachycardia-induced cardiomyopathy, but the normal amount of alpha-MHC expressed in humans is only 5% to 7% and its functional importance is questionable. This study was undertaken to identify a myofilament-based mechanism underlying tachycardia-induced cardiomyopathy protection and to extrapolate the impact of MHC isoform variation on myofilament function in human hearts.

METHODS AND RESULTS

Papillary muscle strips from TG rabbits expressing 40% (TG40) and 15% alpha-MHC (TG15) and from nontransgenic (NTG) controls expressing approximately 100% beta-MHC (NTG40 and NTG15) were demembranated and calcium activated. Myofilament tension and calcium sensitivity were similar in TGs and respective NTGs. Force-clamp measurements revealed approximately 50% higher power production in TG40 versus NTG40 (P<0.001) and approximately 20% higher power in TG15 versus NTG15 (P<0.05). A characteristic of acto-myosin crossbridge kinetics, the "dip" frequency, was significantly higher in TG40 versus NTG40 (0.70+/-0.04 versus 0.39+/-0.09 Hz, P<0.01) but not in TG15 versus NTG15. The calculated crossbridge time-on was also significantly shorter in TG40 (102.3+/-14.2 ms) versus NTG40 (175.7+/-19.7 ms) but not in TG15 versus NTG15.

CONCLUSIONS

The incorporation of 40% alpha-MHC leads to greater myofilament power production and more rapid crossbridge cycling, which facilitate ejection and relengthening during short cycle intervals, and thus protect against tachycardia-induced cardiomyopathy. Our results suggest, however, that, even when compared with the virtual absence of alpha-MHC in the failing heart, the 5% to 7% alpha-MHC content of the normal human heart has little if any functional significance.

Thick Filament Proteins and Performance in Human Heart Failure

Modifications in thick filament protein content and performance are thought to underlie contraction-relaxation dysfunction in human heart failure. It has been found that myofibrillar Mg.ATPase is reduced in failing myocardium, which may be due in part to the reduction in alpha-myosin heavy chain (MHC) isoform content from approximately 5-10% in normal myocardium to <2% in failing myocardium. The physiological importance of this seemingly small amount of alpha-MHC appears substantiated by the development of cardiopathologies in humans with mutated alpha-MHC at normal abundance. Therefore, the replacement of alpha-MHC by beta-MHC (possessing slower actomyosin enzymatic kinetics) may underlie to a significant degree the reduced myocardial shortening velocity and reduced relaxation function in human heart failure. The atrial isoform of myosin essential light chain (ELC) may replace up to 25% of the ventricular isoform in failing ventricles and in so doing promotes myocardial shortening velocity. An elevated accumulation of the higher performing atrial-ELC, unlike the reduced content of the higher performing alpha-MHC, is therefore considered a compensatory response in heart failure. Phosphorylation of the myofilament proteins myosin regulatory light chain and troponin-I are both reduced in heart failure and collectively result in an elevated myofilament sensitivity to calcium activation, which inhibits relaxation function. These and other modifications in thick filament proteins, as discussed in this review, directly affect mechanical power output and relaxation function of the myocardium and thereby may be considered to cause or in some cases to compensate for the otherwise ineffective myocardial performance in heart failure.

Molecular mechanics of mouse cardiac myosin isoforms

Two myosin isoforms are expressed in myocardium, alphaalpha-homodimers (V(1)) and betabeta-homodimers (V(3)). V(1) exhibits higher velocities and myofibrillar ATPase activities compared with V(3). We also observed this for cardiac myosin from normal (V(1)) and propylthiouracil-treated (V(3)) mice. Actin velocity in a motility assay (V(actin)) over V(1) myosin was twice that of V(3) as was the myofibrillar ATPase. Myosin's average force (F(avg)) was similar for V(1) and V(3). Comparing V(actin) and F(avg) across species for both V(1) and V(3), our laboratory showed previously (VanBuren P, Harris DE, Alpert NR, and Warshaw DM. Circ Res 77: 439-444, 1995) that mouse V(1) has greater V(actin) and F(avg) compared with rabbit V(1). Mouse V(3) V(actin) was twice that of rabbit V(actin). To understand myosin's molecular structure and function, we compared alpha- and beta-cardiac myosin sequences from rodents and rabbits. The rabbit alpha- and beta-cardiac myosin differed by eight and four amino acids, respectively, compared with rodents. These residues are localized to both the motor domain and the rod. These differences in sequence and mechanical performance may be an evolutionary attempt to match a myosin's mechanical behavior to the heart's power requirements.

Kinetic differences at the single molecule level account for the functional diversity of rabbit cardiac myosin isoforms

1. Cardiac V3 myosin generates slower actin filament velocities and higher average isometric forces (in an in vitro motility assay) when compared with the V1 isoform. 2. To account for differences in V1 and V3 force and motion generation at the molecular level, we characterized the mechanics and kinetics of single V1 and V3 myosin molecules using a dual laser trap setup. 3. No differences in either unitary displacement (approximately 7 nm) or force (approximately 0.8 pN) were observed between isoforms; however, the duration of unitary displacement events was significantly longer for the V3 isoform at MgATP concentrations > 10 microM. 4. Our results were interpreted on the basis of a cross-bridge model in which displacement event durations were determined by the rates of MgADP release from, and MgATP binding to, myosin. 5. We propose that the release rate of MgADP from V3 myosin is half that of V1 myosin without any difference in their rates of MgATP binding; thus, kinetic differences between the two cardiac myosin isoforms are sufficient to account for their functional diversity.

Distinct kinetic properties of cardiac myosin isoforms revealed by in vitro studies

To clarify the physiological significance of myosin isoform redistribution in cardiac adaptation process, we compared the kinetic property of the two cardiac myosin isoforms using in vitro motility assay techniques. Cardiac myosin isoforms V1 and V3 were obtained from ventricular muscle of young rats and hypothyroid rats respectively. On each of these myosin isoforms fixed on a glass coverslip, fluorescently labeled actin filaments were made to slide in the presence of ATP. To measure the force generated by actomyosin interaction, a small latex bead was attached to the barbed end of an actin filament and the bead was captured by the laser optical trap installed in a microscope. The force was determined from the distance between the bead and the trap positions under either auxotonic or isometric conditions. The time-averaged force generated by multiple cross-bridges did not differ significantly between the two isoforms. On the other hand, the unitary force measurement revealed the same level of amplitude but a longer duration for V3 isoform. The same level of time-averaged force is in agreement with not only our previous finding but the results of maximum force measurement in muscle preparations. The difference in kinetic characteristics of the two isoforms could account for the difference in economy of force development and the basis for cardiac adaptation mechanism.

Comparison of unitary displacements and forces between 2 cardiac myosin isoforms by the optical trap technique: molecular basis for cardiac adaptation

To provide information on the mechanism of cardiac adaptation at the molecular level, we compared the unitary displacements and forces between the 2 rat cardiac myosin isoforms, V1 and V3. A fluorescently labeled actin filament, with a polystyrene bead attached, was caught by an optical trap and brought close to a glass surface sparsely coated with either of the 2 isoforms, so that the actin-myosin interaction took place in the presence of a low concentration of ATP (0.5 micromol/L). Discrete displacement events were recorded with a low trap stiffness (0.03 to 0.06 pN/nm). Frequency distribution of the amplitude of the displacements consisted of 2 gaussian curves with peaks at 9 to 10 and 18 to 20 nm for both V1 and V3, suggesting that 9 to 10 nm is the unitary displacement for both isoforms. The duration of the displacement events was longer for V3 than for V1. On the other hand, discrete force transients were recorded with a high trap stiffness (2.1 pN/nm), and their amplitude showed a broad distribution with mean values between 1 and 2 pN for V1 and V3. The durations of the force transients were also longer for V3 than for V1. These results indicate that both the unitary displacements and forces are similar in amplitude but different in duration between the 2 cardiac myosin isoforms, being consistent with the reports that the tension cost is higher in muscles consisting mainly of V1 than those consisting mainly of V3.

Force-velocity relation of sliding of skeletal muscle myosin, arranged on a paramyosin filament, on actin cables

To investigate in vitro ATP-dependent sliding of regularly arranged myosin molecules on actin filaments, we prepared thick hybrid filaments in which myosin molecules isolated from rabbit skeletal muscle were arranged around the paramyosin core (length, 10-20 micron; diameter, </=0.2 micron) obtained from a molluscan smooth muscle. A single to a few thick hybrid filaments were attached to a polystyrene bead (diameter, 4.5 micron; specific gravity, 1.5) and made to slide on actin filament arrays (actin cables) in the internodal cell of an alga, mounted on the rotor of a centrifuge microscope. The bead was subjected to centrifugal forces serving as external loads to the ATP-dependent actin-myosin sliding. The maximum unloaded sliding velocity of the thick filament attached-bead (mean, 3.4 micron/s; 20-23 degrees C) was significantly higher than that of the bead coated with randomly oriented myosin molecules reported previously. The steady-state force-velocity (P-V) relations obtained were qualitatively similar to those in intact skeletal muscle fibers. These results indicate that this in vitro motility assay system retains the basic characteristics of contracting skeletal muscle fibers, and that it may be effectively used to study mechanisms underlying the steady-state P-V characteristics of ATP-dependent actin-myosin sliding using various recombinant myosins produced in nonmuscle cells.

Molecular motor mechanics in the contracting heart. V1 versus V3 myosin heavy chain

The amount of iron in the low molecular weight pool (LMW) increases during no-flow ischemia and is thought to be essential to oxygen radical-derived damage upon reperfusion. Applying three short ischemic periods (5 min) preconditioning before 15 min ischemia results in an improved contractility compared to a direct 15 min ischemic insult. This raises the question whether preconditioning leads to a decrease in hte LMW iron pool. We therefore investigated the change in in hte LMW iron pool during ischemic insult after applying preconditioning. It is assumed that an increase in LMW iron is dependent on the accumulation of reduction equivalents derived from the anaerobic glycolysis. Therefore the glycogen content was also reduced by administration by anoxia and glucagon administration to study the effect on the LMW iron pool.

Different cardiac myosin isoforms exhibit equal force-generating ability in vitro

We measured forces generated by myosin molecules and a single actin filament using an optical trap system. The force per unit length of actin filament did not differ significantly between cardiac myosin isoforms. V1 and V3. This indicates that the ability to generate force is equal between V1 and V3, despite their difference in the unloaded sliding velocity past actin.