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* National Institute of Arthritis, Musculoskeletal and Skin Diseases, National Institutes of Health, Department of Health and Human Services, Bethesda, Maryland; and Department of Bioengineering, University of Washington, Seattle, Washington
Correspondence: Address reprint requests to Leepo C. Yu, E-mail: yule{at}mail.nih.gov.
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ABSTRACT |
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSREFERENCES |
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). Biochemically, the cardiac actomyosin ATP hydrolysis cycle follows the same pathway with kinetics similar to those of the skeletal ATPase cycle (2).
One striking mechanical feature of cardiac muscle that differs from skeletal muscle is the steep force-length relation on the "ascending limb" (sarcomere length between 1.7 µm and 2.3 µm) and the pronounced length dependence of the Ca2+ sensitivity—i.e., the Frank-Starling law of the heart. Several experimental results are consistent with the idea that variations in the interfilament separation (lattice spacing) as a result of changes in sarcomere length modulate the actin-myosin interaction (3), although lattice spacing may not be the only factor (4).
There exists a vast literature characterizing structural, mechanical, and biochemical properties of the skeletal actomyosin interactions, whereas comparatively little is known about cardiac muscle. One approach to gain insight into cardiac muscle contraction is to compare various characteristics of the cross-bridges in the two types of muscle fibers. The purpose of this study is to use low angle x-ray diffraction to compare the structures of the thick filaments in relaxed cardiac and skeletal muscles under closely matched conditions known to alter cardiac contractility. In a relaxed muscle, interactions between actin and myosin are mostly limited to the low affinity states—the weak binding states—in the ATP hydrolysis cycle. The weak binding states are obligatory and required for isomerization or transition to force-generating, strongly bound cross-bridges (5). Characterizing the weak binding states would therefore have implications for understanding the mechanism of force generation.
Equatorial x-ray diffraction and mechanical stiffness measurements of relaxed cardiac muscle fibers showed that when myofilament lattice spacing decreased, the binding between actin and myosin increased (6). However, equatorial measurements did not provide information about the structural state of the cross-bridges beyond its radial position in the lattice. Two-dimensional x-ray diffraction from mammalian skeletal muscle reveals not only the physical distribution but also the biochemical state of myosins in the thick filaments. It was shown that the distribution of the cross-bridges on the mammalian skeletal thick filament surface was highly temperature dependent (7,8) and was correlated with myosin conformations and biochemical states (9,8). The thick filaments are helically ordered only at higher temperatures (>20°C) and become progressively more disordered as temperature is lowered. In MgATP-containing solutions, a well-ordered thick filament signifies that most of the myosins contain MgADP.Pi at their active site (10). In contrast, the myosin·ATP state is disordered, as is the analogous state myosin·AMPPNP (11).
Matsubara and colleagues (12) pioneered two-dimensional x-ray diffraction studies from intact heart muscle. With the technology available at the time, information extracted from the studies was limited. In this study, two-dimensional x-ray diffraction patterns from relaxed permeabilized rabbit cardiac and psoas muscle fibers are compared at several temperatures and ionic strengths (µ). Similar temperature dependence found in the myosin layer line (MLL) intensities suggests that under physiological conditions, the cardiac cross-bridges are largely distributed in the M.ADP.Pi state, but M.ATP is increasingly favored as the temperature is lowered. In the cardiac muscle, MLL are considerably weaker, the I1,1/I1,0 ratio is generally higher, and the lattice spacing larger, compared to the skeletal muscle. The results are consistent with the idea that a greater fraction of the cardiac cross-bridges is weakly bound to actin, probably in the A·M·ATP state.
Preliminary results were presented at the 2006 Biophysical Society meeting (13).
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METHODS |
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20 min, the heart was opened and placed in the skinning solution with 1% triton X-100 for 30 min at 5°C. Subsequently, the trabeculae (0.5 mm in diameter, 6 mm in length) were dissected from the right ventricle, fixed at the two ends at resting length. The specimens were kept in the skinning solution at 5°C with 1% triton X-100 for 12 h and then were stored in the skinning solution without the detergent, ready for use. The specimens were used for experiments within 3 days. Sarcomere length was adjusted to 2.1–2.2 µm under laser light diffraction. For comparison, permeabilized single bundles of fibers from rabbit psoas major were used (for details, see Xu et al. (14)).
Solutions
The following solutions were used for the experiments. 1), Skinning solution contained (in mM) 5 KH2PO4, 5 MgAc, 5 EGTA, 3 Na2ATP, 50 CrP, 5 NaN3, 2 dithiothreitol (DTT), protease inhibitor cocktail from Sigma (St. Louis, MO; 100 µM [4-(2-aminoethyl)benzenesulfonyl fluoride hydrochloride] (AEBSF), 4 µM bestatin, 1.4 µM E-64, 2.2 µM leupeptin, 1.5 µM pepstatin A, 80 µM aprotinin), pH = 7.0. 2), Relaxing solution contained (in mM) 2 MgATP, 2 MgCl2, 2 EGTA, 5 DTT, 10 imidazole, pH 7.0, µ = 27 mM. µ was adjusted by adding 10 mM creatine phosphate and varied potassium propionate for µ = 50 mM (low µ) or 200 mM (high µ). An ATP-backup system, 109 unit/ml creatine kinase was added in the relaxing solution. 3), Rigor solution contained (in mM) 2.5 EGTA, 2.5 EDTA, 10 imidazole, 5 DTT, 150 potassium propionate, pH 7.0, µ = 170 mM. Before applying the rigor solution, the sample was always rinsed several times with a "quick rinse" solution containing (in mM) 5 EGTA, 15 EDTA, 20 imidazole, pH 7.0, µ = 70 mM (14).
During the entire course of the experiments, the solution in the chamber was continuously mixed by a push-pull syringe pump at the rate of 0.5 ml/s to minimize any concentration gradient along the length of the muscle. To reduce radiation damage, the specimen chamber was moved up and down continuously for a range of 4 mm at a constant rate of 4 mm/s by a stepping motor (Aerotech, Pittsburgh, PA).
X-ray source, camera, and detector system
The experiments were performed at beamline X27C (Advanced Polymer PRT) at the National Synchrotron Light Source, Brookhaven National Laboratory, Upton, NY. The optics of X27C used a double-multilayer (silicon/tungsten) monochromator. A three-pinhole system was used for collimating the monochromatized beam. The beam size at the specimen was 0.4 mm in diameter and specimen-to-detector distance was 1500 mm. A MAR Research CCD detector (Hamburg, Germany) with 0.08 x 0.08 mm pixel size was used for collecting the x-ray data.
The sample was held vertical. The exposure time for each pattern was 2 min in general. The maximum accumulated exposure time for each muscle sample was 
12 min. In some cases, the solution background patterns were taken at the end of a series of x-ray exposures on the sample.
To directly compare the intensities while minimizing random errors (e.g., muscle size and beam quality), a stringent experimental protocol was followed. Only experiments where diffraction patterns were obtained for a complete set of conditions were selected for this work. In general, six exposures were taken for each experiment in the following sequence: i), in relaxing solution µ = 200 mM (or 50 mM) at 5°C; ii), 25°C; iii), µ = 50 mM (or 200 mM) at 25°C; iv), 5°C; v), in rigor condition µ = 170 mM at 5°C; and vi), 25°C. The spacings of all reflections were calibrated at the beginning of this series of experiments by the 1/144.3 Å–1 meridional reflection from skinned rabbit psoas muscle in rigor at µ = 170 mM and T = 25°C (see Xu et al. (14) for details).
In summary, the experimental protocol included stirring the chamber to avoid solution gradient, scanning the fibers through the beam to minimize local radiation damage, and comparing multiple experimental conditions from the same sample to ensure the reproducibility of the recorded data.
Data reduction and analysis
The data were displayed and analyzed on a Silicon Graphics Indigo workstation (Mountain View, CA). The data in the four quadrants were first rotated, folded, and averaged. The program made slices parallel to the meridian and slices parallel to the equator of the diffraction patterns. Intensities within each slice were integrated to generate one-dimensional intensity profiles for further analysis. All integrated intensities (I) in Table 1 were normalized by the integrated intensity of the first MLL (IMLL1) obtained from the muscles in the relaxing solution at µ = 50 mM and 25°C.
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). Intensities above the background were then integrated.
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RESULTS |
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Effects of temperature on layer lines
In general, the diffraction patterns from the relaxed cardiac muscle are affected by factors such as temperature and µ, similar to those from relaxed skeletal muscle. Temperature, in particular, has a strong effect on the intensity of the MLL. Fig. 1 shows typical diffraction patterns from a single trabeculae at 25°C and 5°C when µ was either 200 mM (Fig. 1, A and B) or 50 mM (Fig. 1, D and E). A total of three complete sets of data similar to Fig. 1 from three separate preparations was obtained. For technical reasons (different camera lengths), the diffraction patterns could not be combined into one for display.
The pattern (Fig. 1 A) at 25°C and µ = 200 mM shows visible MLL, indicating a certain degree of helical order in the arrangement of the cross-bridges in the thick filament. In contrast at 5°C the layer lines are practically undetectable (Fig. 1 B), indicating that the helical array of the cross-bridges becomes highly disordered. Myosin-related reflections, particularly the first MLL and the third meridional reflections, are clearly stronger at the higher temperature.
When µ is lowered to 50 mM, the layer lines are stronger. A similar effect of temperature on the MLL and the meridional reflections is found, as shown in Fig. 1, D and E. The pattern shown in Fig. 1 F is the difference between Fig. 1, D and E. It emphasizes the increase in layer line intensities with higher temperature.
Integrated off-meridional intensity profiles: comparison to skeletal muscle
Fig. 2 displays the intensity profiles of vertical slices taken between 0.002377 and 0.006997 Å–1 (between the dotted lines shown in Fig. 1, A and C) from Fig. 1, A–E. The width between the dotted lines covers most of the first MLL and the first actin layer line of the diffraction patterns. At µ = 200 mM, decreasing the temperature from 25°C to 5°C causes nearly complete loss of the first MLL (Fig. 2 A). At low µ, the first MLL is evident at both temperatures (Fig. 2 A). For comparison, the much stronger intensity profiles from skinned rabbit psoas fibers obtained under identical conditions are shown in Fig. 2 B. For both muscles, the MLL are indexed on 430 Å and there is little density located at the first actin layer line (365 Å). In rigor, the MLL from both preparations become undetectable, whereas the actin layer line at 365 Å becomes predominant (Fig. 2). Changes in the layer line intensities are similar in that with increasing temperature, the intensity of the first MLL increases. Effects of ionic strength are also similar (see below).
Changes in the first MLL intensity as a function of temperature and ionic strength are summarized in Table 1 (three fiber bundles used). Intensity of the first MLL (IMLL1) at 200 mM and 5°C, with few or no detectable layer lines, was taken as the baseline. It was subtracted from IMLL1 obtained under other conditions. Although the range of the variance is large, the combined data reinforce the observation in Fig. 1 that layer line intensities in both skeletal and cardiac muscle are strengthened by increasing temperature and lowered ionic strength.
Effects of ionic strength on layer line intensities
Ionic strength also affects the helical order of the myosin filament. By comparing the diffraction patterns at the same temperature (either at 25°C or 5°C) (Fig. 1, A and D, or Fig. 1, B and E, respectively) it is evident that raising µ decreases the layer line intensities. Similar effects have been found in rabbit psoas muscle (14). The changes cannot be attributed to myosin interaction with thin filaments, since in skeletal muscle, similar order-disorder transformation occurs when there is no actin-myosin overlap (10). The mechanism of the order-disorder effect will be addressed in Discussion.
Equatorial intensities
The equatorial intensities generally agree with our previous results from rat cardiac trabeculae and skinned rabbit psoas muscle: lowering temperature increases the I1,1/I1,0 ratio, indicating enhanced weak cross-bridge binding in both muscles (6,14). In Fig. 3 equatorial patterns obtained from cardiac trabeculae based on Fig. 1 (Fig. 3, A, C, and E) are compared to patterns from skinned skeletal fibers (Fig. 3, B, D, and F). The ratio of intensities of the [1,1] and [1,0] reflections (I1,1/I1,0) and the lattice spacing (D1,0) from three separate bundles are summarized in Table 2.
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1.5-fold, respectively (Table 2). Myofilament lattice spacing (D1,0) is influenced similarly by temperature and µ in both preparations. Lowering µ decreases D1,0, whereas at each µ lowering temperature increases D1,0 (Table 2). The changes in equatorial intensities with temperature and µ can be compared to the maximal changes observed in rigor for the cardiac (Fig. 3 E) and the skeletal (Fig. 3 F) bundles. As expected, the I1,1/I1,0 ratio increases significantly in rigor.
Meridional reflections
The intensities and spacings of the third myosin based meridional reflections (M3) from a trabecula are shown in Fig. 4 A and from psoas fibers in Fig. 4 B. In the relaxed state, M3 intensity increased with increasing temperature at both ionic strengths in skeletal fibers (Fig. 4 B). In cardiac muscle, M3 intensity increased at elevated temperature when µ = 50 mm but was unchanged when µ = 200 mM. The M3 spacing was 143.4 Å at 25°C for µ = 200 mM and 50 mM in both preparations. At 5°C, the spacings of M3 diverged depending on µ and muscle. For the trabeculae, at µ = 50 mM, it remained at 143.4 Å, whereas at µ = 200 mM it shifted to 145.1 Å. For the psoas at µ = 200 mM, one peak appeared at 147.6 Å and a small peak remained at 143 Å (Fig. 4 B). The rigor spacing in panels A and B was 144.3 Å and was used as calibration for all the measured spacings. The results indicate that, in general, changes in temperature and µ have similar effects on the M3 reflections from cardiac and skeletal muscles.
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DISCUSSION |
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disordered arrays of the myosin heads is strongly correlated with the equilibrium between the [M.ADP.Pi] and [M.ATP] states (15,10,8). An ordered array signifies an equilibrium in favor of the [M.ADP.Pi] population whereas a disordered array, the [M.ATP] population.
Although there is no direct correlation between filament structure and solution biochemistry for the cardiac muscle, the results obtained for skeletal myosin are applicable to cardiac myosin. The key characteristics of the diffraction patterns (e.g., the layer line intensities (Table 1), equatorial intensity ratio (Table 2), and the meridional reflections (Fig. 4)) show similar dependence on temperature. Furthermore, ATP hydrolysis by (porcine) cardiac S1 follows the same pathways as skeletal S1 with similar kinetics (2). Therefore, the order/disorder equilibrium in a relaxed cardiac muscle should also closely reflect the [M.ADP.Pi]/[M.ATP] equilibrium. It follows that at physiological temperatures large fractions of the cross-bridges in the cardiac muscle are expected to be in the [M.ADP.Pi] state. As temperature is lowered, [M.ATP] and [A.M.ATP] states are increasingly favored. To derive quantitative equilibrium constants for order/disorder and [M.ADP.Pi]/[M.ATP] at a given temperature, however, requires further experiments.
Ionic strength effects are distinct from the temperature effects
Evidence suggests that disorder by µ has an origin different from temperature. In solution, higher µ promotes hydrolysis of ATP by myosin S1 (16). One might expect improved helical order in the myosin filament as µ is raised, but the opposite has been observed (Figs. 1–3
). Increasing µ significantly decreases, rather than increases, the intensities of the MLL (Fig. 1; (14)). At very high µ the myosin filament is dissolved. These results lead to the conclusion that increasing ionic strength has a predominant effect on disrupting the stability of the entire myosin filament, and its effect cannot be correlated with the biochemical states of the myosin heads.
The results in Table 2 suggest some differences in the equatorial diffraction patterns from the two muscles: the I1,1/I1,0 ratio is generally higher and the lattice spacing D10 is larger in the cardiac muscle. Since diffraction patterns were obtained under identical conditions, it is reasonable to make direct quantitative comparisons between the cardiac and skeletal results. Every mean value of I1,1/I1,0 listed in Table 2 from cardiac muscle appears to be higher, with some as high as doubling that of the skeletal data (at µ = 50 mM and 5°C), although some of the differences in I11/I10 are marginally significant and the variance of the cardiac data is greater than from skeletal fibers. Nevertheless, the current data point to a trend for a higher I1,1/I1,0 in cardiac muscle.
The higher I1,1/I1,0 for cardiac muscle suggests higher affinity between actin and myosin. Increases in I1,1/I1,0 in relaxed cardiac trabeculae were correlated with increased weak cross-bridge binding, as suggested by measurements of chord stiffness (6). Higher affinity has also been observed in solution. Kapp for activation of actomyosin ATP hydrolysis is significantly lower for cardiac than for skeletal thin filament (H. White, Eastern Virginia Medical School, 2006, personal communication). In electron microscopic studies, it has been observed that interactions between the isolated thick and thin filaments under relaxing conditions appear to be more frequent for cardiac than for skeletal filaments (17,1).
The more abundant weakly bound cross-bridges could be populated in the A.M.ATP state, since the difference in I1,1/I1,0 is more prominent (P = 0.05) at low temperature and µ = 200 mM (Table 2), where [M.ATP] and [A.M.ATP] are favored. The larger D10 spacing is also consistent with a larger fraction of cross-bridges attached in the [A.M.ATP] state (14). Furthermore, the M.ATP state is disordered on the thick filament, and the A.M.ATP attached state only gives rise to faint myosin and actin layer lines (11,14). This could be one of the causes for the weak MLL in the cardiac muscle, although fiber branching could also be an explanation (17,1).
One of the explanations for greater weak cross-bridge binding in cardiac muscle could be that weak binding is favored by a greater proportion of thin filament cross-bridge-binding sites in the "closed" state, as compared to skeletal thin filaments without Ca2+ (for reviews, see Gordon et al. (18) and Lehrer and Geeves (19)). Biochemical studies have indicated that the equilibrium distribution of skeletal and cardiac thin filaments between the "blocked" and "closed" states in the absence of Ca2+ may differ (20). Measurement of myosin S1 subfragment binding to thin filaments that were either regulated by skeletal or cardiac TnC complexed with skeletal TnI and TnT indicated that the fraction of thin filament sites in the "closed" state (permissive to weak cross-bridge binding) was greater with cardiac TnC than for the skeletal isoform in the absence of Ca2 (20). If this comparison is valid for thin filaments reconstituted with skeletal Tn versus cardiac Tn, the increased equatorial intensity ratio for cardiac muscle (Table 1) could, at least in part, result from the increased availability of cardiac thin filament sites in the "closed" state for weak cross-bridge-thin filament interaction. Determination of the M.ATP M.ADP.Pi equilibrium for cardiac myosin and determination of the equilibrium distribution of cardiac thin filaments constituted with all cardiac regulatory protein isoforms will be needed to clarify the observed differences.
The order-disorder transition in the thick filaments may reflect changes in the flexibility of the myosin heads: physiological relevance of helical order and disorder
In the crystal structure of myosin S1 containing Mg.ADP.AlF4 at its active site, the switch-II loop is "closed", whereas those containing Mg.ATP, the switch-II is "open" (for review, see Holmes and Geeves (21)). S1.ADP.AlF4 is a close analog of the S1.ADP.Pi. S1 in the switch-II closed conformation is a stable and tightly packed structure, whereas in the switch-II open conformation the various domains are "internally uncoupled" with increased interdomain flexibility (22,23). A stable and less flexible S1 structure should result in decreased spatial fluctuations on the surface of the thick filament, revealing a well-ordered helical structure of the filament which gives rise to strong MLL. Conversely, an S1 with uncoupled internal domains would lead to a disordered filament and decreased layer line intensity.
A well-ordered filament consisting of rather inflexible myosin heads in the M.ADP.Pi state might appear somewhat paradoxical in view of the fact that maximal Ca2+-activated force in skinned cardiac trabeculae increases with increasing temperature (24,25). In skinned rabbit psoas muscle at 25°C the myosin heads are arranged close to the surface of the thick filament (the center of mass of cross-bridges is at 135 Å from the center of the myosin filament) (7). Although precise measurement for the cardiac thick filament is not yet available, the center of mass of cardiac cross-bridges appears to be similarly located. Increased temperature favors cross-bridge proximity to the thick filament surface with apparently decreased cross-bridge mobility. In contracting muscle, where the attachment of the weak-binding M.ADP.Pi intermediate to the thin filaments is a critical step in the cross-bridge cycle (26), sequestering of the head away from the thin filament would be expected to reduce the rate of this reaction.
Several factors may resolve these apparent inconsistencies. Even in the ordered state, the heads are by no means completely rigid but show a degree of thermal/spatial disorder. In the model of Malinchik et al. (7) for skeletal muscle, for the "ordered" state the root mean square isotropic displacement was 20 Å, making the thin filament within reach. Second, at physiological temperature the myosin heads are mostly in the prepower stroke, M.ADP.Pi state, poised to generate force. With increased rates of the ATP hydrolysis cycle at higher temperatures, the well-ordered filament structure should present no hindrance for generating higher force levels.
The disordered state of the thick filament may increase the proximity of the myosin heads to the thin filament and enhance the probability of cross-bridge-thin filament interaction. Phosphorylation of the regulatory light chain in skeletal muscle disorders myosin heads in isolated thick filaments (27), increases force calcium sensitivity (24,28), and increases the tension redevelopment rate at submaximal Ca2+ activation (29,30). Sweeney et al. (31) suggested that the enhancement of submaximal force and tension redevelopment kinetics could be due to closer proximity of the disordered myosin heads to thin filaments. In skinned cardiac trabeculae, regulatory light chain phosphorylation increased both maximal force and Ca2+ sensitivity and increased force redevelopment kinetics at submaximal [Ca2+] (32). It could be that some flexibility is introduced into the rather inflexible M.ADP.Pi state by phosphorylation, and the number of cross-bridges bound to actin thus increases. Further x-ray diffraction experiments plus mechanical measurements are needed to test these ideas.
Implications for understanding cardiac function—the Frank-Starling relation
There is significant evidence that the steep enhancement of force with increasing sarcomere length in cardiac muscle results from increased cross-bridge proximity to thin filament binding sites (33,34), although there is evidence that lattice spacing may not be the only factor (4). It should be noted that the steep length dependence of force is maintained only at higher temperatures, whereas lowering temperature eliminates the effects of sarcomere length on force at all activating [Ca2+] (24). The recent suggestions that cross-bridge affinity for actin in the weak-binding states (M.ATP and M.ADP.Pi) is modulated by lattice spacing offers a potential explanation for the strong temperature dependence of length-dependent activation (6,33,34). At low temperature, where the M.ATP state is favored, a significant fraction of the cross-bridges, with their flexible structures, may have ready access to binding sites on the thin filament. Under these conditions the influence of the lattice spacing on cross-bridge-thin filament interaction may be rendered less crucial to force generation. Increasing temperature shifts the cross-bridge population in favor of the M.ADP.Pi state in the putative prepowerstroke conformation. Because of decreased flexibility of myosin heads, narrowing lattice spacing as sarcomere length increases could facilitate attachment and hence become a key modulator of the active force level. Further studies on lattice spacing effects are in progress.
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ACKNOWLEDGEMENTS |
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This research was in part supported by the Intramural Research Program of the National Institute of Arthritis and Musculoskeletal and Skin Diseases of the National Institutes of Health, and by an NIH grant to D.M. (HL 67071).
Submitted on May 25, 2006; accepted for publication August 8, 2006.
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REFERENCES |
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IMLL1) as a function of temperature and ionic strength
