The use of synchrotron radiation in time-resolved X-ray diffraction studies of myosin layer-line reflections during muscle contraction

In vivo imaging of skeletal muscle form and function: 50 years of insight

Skeletal muscle form and function has fascinated scientists for centuries. Our understanding of muscle function has long been driven by advancements in imaging techniques. For example, the sliding filament theory of muscle, which is now widely leveraged in biomechanics research, stemmed from observations made possible by scanning electron microscopy. Over the last 50 years, advancing in medical imaging, combined with ingenuity and creativity of biomechanists, have provide a wealth of new and important insights into in vivo human muscle function. Incorporation of in vivo imaging has also advanced computational modeling and allowed our research to have an impact in many clinical populations. While this review does not provide a comprehensive or meta-analysis of the all the in vivo muscle imaging work over the last five decades, it provides a narrative about the past, present, and future of in vivo muscle imaging.

The Sliding Filament Theory Since Andrew Huxley: Multiscale and Multidisciplinary Muscle Research

Two groundbreaking papers published in 1954 laid out the theory of the mechanism of muscle contraction based on force-generating interactions between myofilaments in the sarcomere that cause filaments to slide past one another during muscle contraction. The succeeding decades of research in muscle physiology have revealed a unifying interest: to understand the multiscale processes-from atom to organ-that govern muscle function. Such an understanding would have profound consequences for a vast array of applications, from developing new biomimetic technologies to treating heart disease. However, connecting structural and functional properties that are relevant at one spatiotemporal scale to those that are relevant at other scales remains a great challenge. Through a lens of multiscale dynamics, we review in this article current and historical research in muscle physiology sparked by the sliding filament theory.

Evolution of the analytical scattering model of live Escherichia coli

A previously reported multi-scale model for (ultra-)small-angle X-ray (USAXS/SAXS) and (very) small-angle neutron scattering (VSANS/SANS) of live Escherichia coli was revised on the basis of compositional/metabolomic and ultrastructural constraints. The cellular body is modeled, as previously described, by an ellipsoid with multiple shells. However, scattering originating from flagella was replaced by a term accounting for the oligosaccharide cores of the lipopolysaccharide leaflet of the outer membrane including its cross-term with the cellular body. This was mainly motivated by (U)SAXS experiments showing indistinguishable scattering for bacteria in the presence and absence of flagella or fimbrae. The revised model succeeded in fitting USAXS/SAXS and differently contrasted VSANS/SANS data of E. coli ATCC 25922 over four orders of magnitude in length scale. Specifically, this approach provides detailed insight into structural features of the cellular envelope, including the distance of the inner and outer membranes, as well as the scattering length densities of all bacterial compartments. The model was also successfully applied to E. coli K12, used for the authors' original modeling, as well as for two other E. coli strains. Significant differences were detected between the different strains in terms of bacterial size, intermembrane distance and its positional fluctuations. These findings corroborate the general applicability of the approach outlined here to quantitatively study the effect of bactericidal compounds on ultrastructural features of Gram-negative bacteria without the need to resort to any invasive staining or labeling agents.

Functional Requirements of a Mathematical Model of Muscle Contraction

Muscle modeling has a long history. Models typically belong to the class of lumped models built around Hill's contractile element, or to crossbridge models at the ultrastructure level. Lumped models built on the contractile element are not sufficiently dynamic, since the muscle's contractile properties are set a priori by the force-velocity relation. We have shown that the force-velocity relation is not a unique descriptor of muscle's contractile state [1]. Alternatively, description of muscle as a time and length dependent force generator may be used to model muscle dynamics. This paper defines the functional requirements of a mathematical model of muscle contraction and evaluates how closely a lumped model based on ultrastructural dynamics resembles muscle. This single model was found able to describe isometric and isotonic contractions, the force-length relation, the force-velocity relation, and changes in contractile state and contraction rate. This approach may help link macro contractile properties, such as the force-velocity relation, to micro dynamics of crossbridge bonds. It may also be useful as a component of larger physiological models and simulations.

Synchrotron radiation X-ray diffraction studies on muscle: past, present, and future

X-ray diffraction is a technique to study the structure of materials at spatial resolutions up to an atomic scale. In the field of life science, the X-ray diffraction technique is especially suited to study materials having periodical structures, such as protein crystals, nucleic acids, and muscle. Among others, muscle is a dynamic structure and the molecular events occurring during muscle contraction have been the main interest among muscle researchers. In early days, the laboratory X-ray generators were unable to deliver X-ray flux strong enough to resolve the dynamic molecular events in muscle. This situation has dramatically been changed by the advent of intense synchrotron radiation X-rays and advanced detectors, and today X-ray diffraction patterns can be recorded from muscle at sub-millisecond time resolutions. In this review, we shed light mainly on the technical aspects of the history and the current status of the X-ray diffraction studies on muscle and discuss what will be made possible for muscle studies by the advance of new techniques.

A compact tunable polarized X-ray source based on laser-plasma helical undulators

Laser wakefield accelerators have great potential as the basis for next generation compact radiation sources because of their extremely high accelerating gradients. However, X-ray radiation from such devices still lacks tunability, especially of the intensity and polarization distributions. Here we propose a tunable polarized radiation source based on a helical plasma undulator in a plasma channel guided wakefield accelerator. When a laser pulse is initially incident with a skew angle relative to the channel axis, the laser and accelerated electrons experience collective spiral motions, which leads to elliptically polarized synchrotron-like radiation with flexible tunability on radiation intensity, spectra and polarization. We demonstrate that a radiation source with millimeter size and peak brilliance of 2 × 10(19) photons/s/mm(2)/mrad(2)/0.1% bandwidth can be made with moderate laser and electron beam parameters. This brilliance is comparable with third generation synchrotron radiation facilities running at similar photon energies, suggesting that laser plasma based radiation sources are promising for advanced applications.

Application of advanced X-ray methods in life sciences

BACKGROUND

Synchrotron radiation (SR) sources provide diverse X-ray methods for the investigation of structure-function relationships in biological macromolecules.

SCOPE OF REVIEW

Recent developments in SR sources and in the X-ray tools they offer for life sciences are reviewed. Specifically, advances in macromolecular crystallography, small angle X-ray solution scattering, X-ray absorption and fluorescence spectroscopy, and imaging are discussed with examples.

MAJOR CONCLUSIONS

SR sources offer a range of X-ray techniques that can be used in a complementary fashion in studies of biological systems at a wide range of resolutions from atomic to cellular scale. Emerging applications of X-ray techniques include the characterization of disordered proteins, noncrystalline and nonequilibrium systems, elemental imaging of tissues, cells and organs, and detection of time-resolved changes in molecular structures.

GENERAL SIGNIFICANCE

X-ray techniques are in the center of hybrid approaches that are used to gain insight into complex problems relating to biomolecular mechanisms, disease and possible therapeutic solutions. This article is part of a Special Issue entitled "Science for Life". Guest Editors: Dr. Austen Angell, Dr. Salvatore Magazù and Dr. Federica Migliardo.

Tunable synchrotron-like radiation from centimeter scale plasma channels

Synchrotron radiation (SR) sources are immensely useful tools for scientific researches and many practical applications. Currently, the state-of-the-art synchrotrons rely on conventional accelerators, where electrons are accelerated in a straight line and radiate in bending magnets or other insertion devices. However, these facilities are usually large and costly. Here, we study a compact all optical synchrotron-like radiation source based on laser-plasma acceleration either in a straight or a curved plasma channel. With the laser pulse off-axially injected, its centroid oscillates transversely in the plasma channel. This results in a wiggler motion of the whole accelerating structure and the self-trapped electrons behind the laser pulse, leading to strong synchrotron-like radiations with tunable spectra. It is further shown that a palmtop ring-shaped synchrotron is possible with current high power laser technologies. With its potential of high flexibility and tunability, such light sources once realized would find applications in wide areas and make up the shortage of large SR facilities.

Hugh E. Huxley: the compleat biophysicist

The sliding filament model of muscle contraction, put forward by Hugh Huxley and Jean Hanson in 1954, is 60 years old in 2014. Formulation of the model and subsequent proof was driven by the pioneering work of Hugh Huxley (1924-2013). We celebrate Huxley's integrative approach to the study of muscle contraction; how he persevered throughout his career, to the end of his life at 89 years, to understand at the molecular level how muscle contracts and develops force. Here we show how his life and work, with its focus on a single scientific problem, had impact far beyond the field of muscle contraction to the benefit of multiple fields of cellular and structural biology. Huxley introduced the use of x-ray diffraction to study the contraction in living striated muscle, taking advantage of the paracrystalline lattice that would ultimately allow understanding contraction in terms of single molecules. Progress required design of instrumentation with ever-increasing spatial and temporal resolution, providing the impetus for the development of synchrotron facilities used for most protein crystallography and muscle studies today. From the time of his early work, Huxley combined electron microscopy and biochemistry to understand and interpret the changes in x-ray patterns. He developed improved electron-microscopy techniques, thin sections and negative staining, that enabled answering major questions relating to the structure and organization of thick and thin filaments in muscle and the interaction of myosin with actin and its regulation. Huxley established that the ATPase domain of myosin forms the crossbridges of thick filaments that bind actin, and introduced the idea that myosin makes discrete steps on actin. These concepts form the underpinning of cellular motility, in particular the study of how myosin, kinesin, and dynein motors move on their actin and tubulin tracks, making Huxley a founder of the field of cellular motility.

The value of mechanistic biophysical information for systems-level understanding of complex biological processes such as cytokinesis

This review illustrates the value of quantitative information including concentrations, kinetic constants and equilibrium constants in modeling and simulating complex biological processes. Although much has been learned about some biological systems without these parameter values, they greatly strengthen mechanistic accounts of dynamical systems. The analysis of muscle contraction is a classic example of the value of combining an inventory of the molecules, atomic structures of the molecules, kinetic constants for the reactions, reconstitutions with purified proteins and theoretical modeling to account for the contraction of whole muscles. A similar strategy is now being used to understand the mechanism of cytokinesis using fission yeast as a favorable model system.

Remembrance of Hugh E. Huxley, a founder of our field

Hugh E. Huxley (1924-2013) carried out structural studies by X-ray fiber diffraction and electron microscopy that established how muscle contracts. Huxley's sliding filament mechanism with an ATPase motor protein taking steps along an actin filament, established the paradigm not only for muscle contraction but also for other motile systems using actin and unconventional myosins, microtubules and dynein and microtubules and kinesin.

Modeling muscle's nonlinear viscoelastic dynamics

Muscle's contractile properties are complicated by its viscoelastic properties. Failure of early viscoelastic muscle models led to Hill's force-velocity relation embodied as the contractile element. Adopting a particular force-velocity relation to describe muscle is neither easy, nor unique [1]. Time-varying elastance based models of the left ventricle have been popular since the idea was presented in 1969 [2]. This paper investigates adoption of the time-varying elastance concept to describe the viscoelastic properties of muscle. It will be shown that a time-varying elastance must be extended to a time, length, and velocity dependent elastance. Results show how a generalized force generator description of muscle [3] may be used to realistically model muscle's viscoelasticity.

Structure of the rigor actin-tropomyosin-myosin complex

Regulation of myosin and filamentous actin interaction by tropomyosin is a central feature of contractile events in muscle and nonmuscle cells. However, little is known about molecular interactions within the complex and the trajectory of tropomyosin movement between its "open" and "closed" positions on the actin filament. Here, we report the 8 Å resolution structure of the rigor (nucleotide-free) actin-tropomyosin-myosin complex determined by cryo-electron microscopy. The pseudoatomic model of the complex, obtained from fitting crystal structures into the map, defines the large interface involving two adjacent actin monomers and one tropomyosin pseudorepeat per myosin contact. Severe forms of hereditary myopathies are linked to mutations that critically perturb this interface. Myosin binding results in a 23 Å shift of tropomyosin along actin. Complex domain motions occur in myosin, but not in actin. Based on our results, we propose a structural model for the tropomyosin-dependent modulation of myosin binding to actin.

50 years of fiber diffraction

In 1955 Ken Holmes started working on tobacco mosaic virus (TMV) as a research student with Rosalind Franklin at Birkbeck College, London. Afterward he spent 18months as a post doc with Don Caspar and Carolyn Cohen at the Children's Hospital, Boston where he continued the work on TMV and also showed that the core of the thick filament of byssus retractor muscle from mussels is made of two-stranded alpha-helical coiled-coils. Returning to England he joined Aaron Klug's group at the newly founded Laboratory of Molecular Biology in Cambridge. Besides continuing the TMV studies, which were aimed at calculating the three-dimensional density map of the virus, he collaborated with Pringle's group in Oxford to show that two conformation of the myosin cross-bridge could be identified in insect flight muscle. In 1968 he opened the biophysics department at the Max Planck Institute for Medical Research in Heidelberg, Germany. With Gerd Rosenbaum he initiated the use of synchrotron radiation as a source for X-ray diffraction. In his lab the TMV structure was pushed to 4A resolution and showed how the RNA binds to the protein. With his co-workers he solved the structure of g-actin as a crystalline complex and then solved the structure of the f-actin filament by orientating the g-actin structure so as to give the f-actin fiber diffraction pattern. He was also able to solve the structure of the complex of actin with tropomyosin from fiber diffraction.

SoftWAXS: a computational tool for modeling wide-angle X-ray solution scattering from biomolecules

This paper describes a computational approach to estimating wide-angle X-ray solution scattering (WAXS) from proteins, which has been implemented in a computer program called SoftWAXS. The accuracy and efficiency of SoftWAXS are analyzed for analytically solvable model problems as well as for proteins. Key features of the approach include a numerical procedure for performing the required spherical averaging and explicit representation of the solute-solvent boundary and the surface of the hydration layer. These features allow the Fourier transform of the excluded volume and hydration layer to be computed directly and with high accuracy. This approach will allow future investigation of different treatments of the electron density in the hydration shell. Numerical results illustrate the differences between this approach to modeling the excluded volume and a widely used model that treats the excluded-volume function as a sum of Gaussians representing the individual atomic excluded volumes. Comparison of the results obtained here with those from explicit-solvent molecular dynamics clarifies shortcomings inherent to the representation of solvent as a time-averaged electron-density profile. In addition, an assessment is made of how the calculated scattering patterns depend on input parameters such as the solute-atom radii, the width of the hydration shell and the hydration-layer contrast. These results suggest that obtaining predictive calculations of high-resolution WAXS patterns may require sophisticated treatments of solvent.

Defining muscle elastance as a parameter

Functional descriptions of striated muscle are often based on the measured variables force and initial velocity of shortening, embodied as Hill's contractile element. The fundamental difficulty of describing the mechanical properties of muscle with a force-velocity relation that is set a priori, and the practical problem of the act of measurement changing muscle's force-velocity relation or elastance curve, are described. As an alternative, a new model of muscle contraction is presented, which characterizes muscle's contractile state with parameters, rather than variables. Muscle is treated as a force generator that is time, length, and velocity dependent. Muscle dynamics develop from a single equation based on the formation and relaxation of crossbridge bonds. This analytical function permits the calculation of muscle elastance via E(m)=[abstract: see text]. This new muscle model is defined independently from load properties, and muscle elastance is dynamic and reflects changing numbers of crossbridge bonds. This parameter is more representative of the mechanical properties of muscle than are variables such as muscle force and shortening velocity.

Structural changes of actin-bound myosin heads after a quick length change in frog skeletal muscle

Changes in the x-ray diffraction pattern from a frog skeletal muscle were recorded after a quick release or stretch, which was completed within one millisecond, at a time resolution of 0.53 ms using the high-flux beamline at the SPring-8 third-generation synchrotron radiation facility. Reversibility of the effects of the length changes was checked by quickly restoring the muscle length. Intensities of seven reflections were measured. A large, instantaneous intensity drop of a layer line at an axial spacing of 1/10.3 nm(-1) after a quick release and stretch, and its partial recovery by reversal of the length change, indicate a conformational change of myosin heads that are attached to actin. Intensity changes on the 14.5-nm myosin layer line suggest that the attached heads alter their radial mass distribution upon filament sliding. Intensity changes of the myosin reflections at 1/21.5 and 1/7.2 nm(-1) are not readily explained by a simple axial swing of cross-bridges. Intensity changes of the actin-based layer lines at 1/36 and 1/5.9 nm(-1) are not explained by it either, suggesting a structural change in actin molecules.

Myosin head movements during isometric contraction studied by X-ray diffraction of single frog muscle fibres

Time resolved X-ray diffraction experiments in single muscle fibres of the frog at 2.15 microns sarcomere length and 4 degrees C were performed at ID2 (SAXS), ESRF, Grenoble (France) to investigate the structural aspects of cross-bridge action during the development of the isometric tetanic tension (T0). Changes in the low angle myosin-based reflections were measured with 5 ms time resolution by signal averaging data collected with a 10 m camera length and a 2D gas-filled detector. Upon activation the intensity of the first order myosin layer line reflection, I(M1), and the intensity of the second order meridional reflection, I(M2), reduced practically to zero with a half-time which leads the tension rise by 15-20 ms. The complex changes of the intensity of the third order myosin meridional reflection, I(M3), and the increase of its axial spacing from 14.34 nm (at rest) to 14.57 nm (at T0) could be analysed by assuming that they were the result of the combination of the time dependent modulation in intensity of two closely spaced periodicities, one at 14.34 nm, characteristics of the myosin molecule at rest and the other at 14.57 nm, assumed by the myosin as a consequence of the activation and force production. I(14.34) drops monotonically in advance to isometric tension development with a half-time similar to that of I(M1) and I(M2), while I(14.57) rises from zero to a maximum in parallel with tension.

Myosin head configuration in relaxed fish muscle: resting state myosin heads must swing axially by up to 150 A or turn upside down to reach rigor

The arrangement and shape of myosin heads in relaxed muscle have been determined by analysis of low-angle X-ray diffraction data from a very highly ordered vertebrate muscle in bony fish. This reveals the arrangement and interactions between the two heads of the same myosin molecule, the shape of the resting myosin head (M.ADP.Pi) assuming a putative hinge between the myosin catalytic domain and the light chain binding-domain, and the way that the actin-binding sites on myosin are arrayed around the actin filaments in the bony fish muscle A-band cell unit. The results are discussed in terms of possible force-generating mechanisms. Changes in myosin head shape or tilt have been implicated in the mechanism of force generation. The myosin head arrangement, including perturbations from perfect helical symmetry, has all heads oriented roughly the same way up (there is only a small range of rotations around the head long axis). X-ray data do not define the absolute polarity of the myosin head array. The resting head rotation is either similar to (65 degrees difference) or opposite to (115 degrees difference) the rotation in the rigor state. If the rotations are similar, probably the more likely possibility, then the average relative axial displacement of the inner and outer ends of the heads from the resting state to rigor is about 140 to 150 A. If (less likely) the resting head rotation is opposite to rigor, then the heads would need to turn over (i.e. rotate about 115 degrees around their own long axes) and the mean relative axial displacement from relaxed to rigor would only be 20 to 30 A.

Structure and periodicities of cross-bridges in relaxation, in rigor, and during contractions initiated by photolysis of caged Ca2+

Ultra-rapid freezing and electron microscopy were used to directly observe structural details of frog muscle fibers in rigor, in relaxation, and during force development initiated by laser photolysis of DM-nitrophen (a caged Ca2+). Longitudinal sections from relaxed fibers show helical tracks of the myosin heads on the surface of the thick filaments. Fibers frozen at approximately 13, approximately 34, and approximately 220 ms after activation from the relaxed state by photorelease of Ca2+ all show surprisingly similar cross-bridge dispositions. In sections along the 1,1 lattice plane of activated fibers, individual cross-bridge densities have a wide range of shapes and angles, perpendicular to the fiber axis or pointing toward or away from the Z line. This highly variable distribution is established very early during development of contraction. Cross-bridge density across the interfilament space is more uniform than in rigor, wherein the cross-bridges are more dense near the thin filaments. Optical diffraction (OD) patterns and computed power density spectra of the electron micrographs were used to analyze periodicities of structures within the overlap regions of the sarcomeres. Most aspects of these patterns are consistent with time resolved x-ray diffraction data from the corresponding states of intact muscle, but some features are different, presumably reflecting different origins of contrast between the two methods and possible alterations in the structure of the electron microscopy samples during processing. In relaxed fibers, OD patterns show strong meridional spots and layer lines up to the sixth order of the 43-nm myosin repeat, indicating preservation and resolution of periodic structures smaller than 10 nm. In rigor, layer lines at 18, 24, and 36 nm indicate cross-bridge attachment along the thin filament helix. After activation by photorelease of Ca2+, the 14.3-nm meridional spot is present, but the second-order meridional spot (22 nm) disappears. The myosin 43-nm layer line becomes less intense, and higher orders of 43-nm layer lines disappear. A 36-nm layer line is apparent by 13 ms and becomes progressively stronger while moving laterally away from the meridian of the pattern at later times, indicating cross-bridges labeling the actin helix at decreasing radius.

Simulation of the rapid regeneration of the actin-myosin working stroke with a tight coupling model of muscle contraction

A. F. Huxley's suggestion in Nature (1992) that a structural modification in the myosin head driven by phosphate release can explain the rapid regeneration of the working stroke, which follows the quick recovery elicited by a step release of moderate size (3-6 nm per half-sarcomere), has been tested with a theoretical model. It is assumed that, in the shortening muscle, cross-bridges can undergo their work producing interaction in two ways distinct for the biochemical state and for the amount of filament sliding allowed. During shortening at low speed, as well as after a shortening step of moderate size, phosphate release from the cross-bridge in the AM-ADP-P state promotes a 100 s-1 structural change which resets the myosin head in a configuration that allows for a new complete working stroke in the AM-ADP state. In this case the total sliding distance for interaction is about 15 nm. With the increase in shortening velocity a progressively larger fraction of interacting cross-bridges remains in the AM-ADP-P state throughout the working stroke and the sliding distance for interaction is about 11 nm. Reattachment of detached cross-bridges occurs at moderate rate whichever is the pathway from which they originate. The model predicts satisfactorily the time course of the rapid regeneration of the working stroke in double step experiments, but fails to simulate the transition to the steady state response in staircase experiments, the maximum power output during steady shortening and the decrease in rate of energy liberation at high shortening velocities. These results strengthen the conclusion of our previous modelling work where we demonstrated that the condition necessary to fit the mechanical and energetic properties of shortening muscle is to assume two pathways for cross-bridge cycling distinct for the kinetics of detachment and reattachment.

A cross-bridge model that is able to explain mechanical and energetic properties of shortening muscle

The responses of muscle to steady and stepwise shortening are simulated with a model in which actin-myosin cross-bridges cycle through two pathways distinct for the attachment-detachment kinetics and for the proportion of energy converted into work. Small step releases and steady shortening at low velocity (high load) favor the cycle implying approximately 5 nm sliding per cross-bridge interaction and approximately 100/s detachment-reattachment process; large step releases and steady shortening at high velocity (low load) favor the cycle implying approximately 10 nm sliding per cross-bridge interaction and approximately 20/s detachment-reattachment process. The model satisfactorily predicts specific mechanical properties of frog skeletal muscle, such as the rate of regeneration of the working stroke as measured by double-step release experiments and the transition to steady state during multiple step releases (staircase shortening). The rate of energy liberation under different mechanical conditions is correctly reproduced by the model. During steady shortening, the relation of energy liberation rate versus shortening speed attains a maximum (approximately 6 times the isometric rate) for shortening velocities lower than half the maximum velocity of shortening and declines for higher velocities. In addition, the model provides a clue for explaining how, in different muscle types, the higher the isometric maintenance heat, the higher the power output during steady shortening.

The chicken muscle thick filament: temperature and the relaxed cross-bridge arrangement

Although chicken myosin S1 has recently been crystallized and its structure analysed, the relaxed periodic arrangement of myosin heads on the chicken thick filament has not been determined. We report here that the cross-bridge array of chicken filaments is temperature sensitive, and the myosin heads become disordered at temperatures near 4 degrees C. At 25 degrees C, however, thick filaments from chicken pectoralis muscle can be isolated with a well ordered, near-helical, arrangement of cross-bridges as seen in negatively stained preparations. This periodicity is confirmed by optical diffraction and computed transforms of images of the filaments. These show a strong series of layer lines near the orders of a 43 nm near-helical periodicity as expected from X-ray diffraction. Both analysis of phases on the first layer line, and computer filtered images of the filaments, are consistent with a three-stranded arrangement of the myosin heads on the filament.

Ultrastructure of skeletal muscle fibers studied by a plunge quick freezing method: myofilament lengths

We have set up a system to rapidly freeze muscle fibers during contraction to investigate by electron microscopy the ultrastructure of active muscles. Glycerinated fiber bundles of rabbit psoas muscles were frozen in conditions of rigor, relaxation, isometric contraction, and active shortening. Freezing was carried out by plunging the bundles into liquid ethane. The frozen bundles were then freeze-substituted, plastic-embedded, and sectioned for electron microscopic observation. X-ray diffraction patterns of the embedded bundles and optical diffraction patterns of the micrographs resemble the x-ray diffraction patterns of unfixed muscles, showing the ability of the method to preserve the muscle ultrastructure. In the optical diffraction patterns layer lines up to 1/5.9 nm-1 were observed. Using this method we have investigated the myofilament lengths and concluded that there are no major changes in length in either the actin or the myosin filaments under any of the conditions explored.

Molecular movements in contracting muscle: towards "muscle--the movie"

The recent publication of the crystal structures of G-actin and of myosin subfragment-1, together with analysis of a time-resolved series of well sampled low-angle 2D X-ray diffraction patterns from bony fish muscle permits the study of the molecular movements in muscle that are associated with generation and regulation of contractile force. Here it is shown that even though low-angle (i.e. low resolution) X-ray diffraction patterns are being used, these patterns are sensitive, for example, to sub-domain movements of as little as 3 A or 4 degrees within the actin monomers of actin filaments. Actin filament diffraction patterns from whole muscle are being used to define actin domain and tropomyosin movements involved in regulation. Myosin and actin filament diffraction patterns are being used together to start to show how the complete "quasi-crystalline" unit cell in the bony fish muscle A-band can be modelled as a series of time-slices through a typical tetanic contraction of the muscle. In this way, the time sequence of images can be used to create "muscle--the movie".

The relaxed crossbridge pattern in isolated rabbit psoas muscle thick filaments

Rabbit muscle is a major source of material for biochemical experiments and spin labelling studies of contraction, and so it is important to establish how closely this material resembles the frog and fish muscles usually used for structural studies. Previous studies have shown that relaxed rabbit muscle thick filaments lose the characteristic order of their crossbridges when they are cooled below about 15–19 degrees C, whereas the order of fish and frog muscles is retained above 0 degrees C. The lack of order has frustrated attempts to examine rabbit thick filament structure and has raised questions about how closely they might resemble other thick filaments. We have therefore developed a procedure for preserving the crossbridge order in isolated filaments. Electron microscopy of these thick filaments after either negative staining or metal shadowing has shown that the crossbridge pattern has a 43 nm axial repeat and is based on three near-helical strands. Computed transforms of either type of image show a series of layer lines confirming that the native relaxed pattern has been preserved, and computer reconstructions show the individual crossbridges lying on a slightly perturbed 3-stranded lattice. These data indicate an unexpectedly high degree of similarity between the rabbit and frog patterns and indicate that, in fully preserved material, there is little structural difference between the two thick filaments at the temperature at which each normally functions.(ABSTRACT TRUNCATED AT 250 WORDS)

Time-resolved studies of crossbridge movement: why use X-rays? Why use fish muscle?

The advantages of using time-resolved X-ray diffraction as a means of probing myosin cross-bridge behaviour in active muscle are outlined, together with the reasons that bony fish muscle has advantages in such studies. We show that the observed X-ray diffraction patterns from fish muscle can be analysed in a way that is rigorous enough to allow reliable information about crossbridge activity to be defined. Among the advantages of this muscle are that diffraction patterns from resting, active and rigor muscles are all well-sampled at least out to the 30 row-line, that the resting myosin layer-line pattern can be 'solved' crystallographically to define the starting position of the crossbridges in resting muscle, and that the equatorial intensity distribution, which in all patterns from vertebrate skeletal muscles comprises overlapping peaks from the A-band and the Z-band, can be analysed sufficiently rigorously to allow separation of the two patterns, both of which change when the muscle is active. Finally, we present results both on a new set of myosin-based layer-lines in patterns from active muscle (consistent with the presence of low-force bridges as also indicated by the time-courses of the intensity changes on the equator and the changing mass distribution in the A-band unit cell) and also on changes of the actin-based layer-lines (consistent with stereospecific labelling of the actin filaments by force-producing crossbridges). Our results to date, which demonstrate the enormous power of time-resolved X-ray diffraction studies, strongly support the swinging of myosin heads on actin as part of the contractile cycle.

Evidence for structurally different attached states of myosin cross-bridges on actin during contraction of fish muscle

Using data from fast time-resolved x-ray diffraction experiments on the synchrotrons at Daresbury and (Deutsches Elektronen Synchrotron [DESY]), it is shown that during contraction of fish muscle there are at least two distinct configurations of myosin cross-bridges on actin, that they appear to have different tension producing properties and that they probably differ in the axial tilt of the cross-bridges on actin. Evidence is presented for newly observed myosin-based layer lines in patterns from active fish muscle, together with intensity changes of the actin layer lines. On the equator, the 110 reflection changes much faster (time for 50% change t1/2 = 21 +/- 4 ms after activation) than the 100 reflection (t1/2 = 35 +/- 8 ms) and tension (t1/2 = 41 +/- 3 ms) during the rising phase of tetanic contractions. These and higher order reflections have been used to show the time course of mass attachment at actin during this rising phase. Mass arrival (t1/2 = 25 ms) precedes tension by approximately 15 ms. Analysis has been carried out to evaluate the effects of changes in sarcomere length during the tetanus. It is shown that any such effects are very small. Difference "equatorial" electron density maps between active muscle at a time when mass arrival at actin is just complete, but the tension is still rising, and at a later time well into the tension plateau, show that the structural difference between the lower and higher force states corresponds to mass movement consistent with axial swinging of heads from a nonstereospecific actin attached state (low force) to a more stereospecific (high force) state.

Rapid regeneration of the actin-myosin power stroke in contracting muscle

At the molecular level, muscle contraction is the result of cyclic interaction between myosin crossbridges, which extend from the thick filament, and the thin filament, which consists mainly of actin. The energy for work done by a single crossbridge during a cycle of attachment, generation of force, shortening and detachment is believed to be coupled to the hydrolysis of one molecule of ATP. The distance the actin filament slides relative to the myosin filament in one crossbridge cycle has been estimated as 12 nm by step-length perturbation studies on single fibres from frog muscle. The 'mechanical' power stroke of the attached crossbridge can therefore be defined as 12-nm shortening with a force profile like that shown by the quick recovery of force following a length perturbation. According to this definition, power strokes cannot be repeated faster than the overall ATPase rate. Here, however, we show that the power stroke can be regenerated much faster than expected from the ATPase rate. This contradiction can be resolved if, in the shortening muscle, the free energy of ATP hydrolysis is used in several actin-myosin interactions consisting of elementary power strokes each of 5-10 nm.

Actin mediated regulation of muscle contraction

Striated and smooth muscles have different mechanisms of regulation of contraction which can be the basis for selective pharmacological alteration of the contractility of these muscle types. The progression in our understanding of the tropomyosin-troponin regulatory system of striated muscle from the early 1970s through the early 1990s is described along with key concepts required for understanding this complex system. This review also examines the recent history of the putative contractile regulatory proteins of smooth muscle, caldesmon and calponin. A contrast is made between the actin linked regulatory systems of striated and smooth muscle.

Twitch relaxation of the cat soleus muscle at different lengths and temperatures

We recorded isometric and isotonic twitches, in situ, from the cat soleus at various muscle lengths and temperatures. At a given temperature the duration of isometric twitches increased approximately 60% for each 10% increase in muscle length, which was primarily owing to decreases in the rate of relaxation. For the relaxation of isometric twitches recorded at different muscle lengths, the equivalent activation energies determined were the same (13.2 +/- 0.3 kcal/M). The duration of isotonic twitch contractions increased only 20% for each 10% increase in muscle length. Even a small amount of shortening (3%) diminished the dependence of twitch duration on muscle length. In this case, twitch duration increased approximately 30% for every 10% increase in muscle length. Hence, even small changes in internal and/or external compliance (eg, changes in the tendon-fiber continuity) can greatly influence twitch duration. Our findings are consistent with the hypotheses that in the cat soleus, Ca2+ sequestration is primarily governed by a single energy dependent process and that the Ca2+ sensitivity of the contractile apparatus increases with increasing sarcomere length.

Disorder induced in nonoverlap myosin cross-bridges by loss of adenosine triphosphate

Adenosine triphosphate-dependent changes in myosin filament structure have been directly observed in whole muscle by electron microscopy of thin sections of rapidly frozen, demembranated frog sartorius specimens. In the presence of ATP the thick filaments show an ordered, helical array of cross-bridges except in the bare zone. In the absence of ATP they show two distinct appearances: in the region of overlap with actin, there is an ordered, rigorlike array of cross-bridges between the thick and thin filaments, whereas in the nonoverlap region (H-zone) the myosin heads move away from the thick filament backbone and lose their helical order. This result suggests that the presence of ATP is necessary for maintenance of the helical array of cross-bridges characteristic of the relaxed state. The primary effect of ATP removal on the myosin heads appears to be weaken their binding to the thick filament backbone; released heads that are close to an actin filament subsequently form a new actin-based, ordered array.

Microsecond rotational motion of spin-labeled myosin heads during isometric muscle contraction. Saturation transfer electron paramagnetic resonance

We have used saturation transfer electron paramagnetic resonance (ST-EPR) to detect the microsecond rotational motions of spin-labeled myosin heads in bundles of skinned muscle fibers, under conditions of rigor, relaxation, and isometric contraction. Experiments were performed on fiber bundles perfused continuously with an ATP-regenerating system. Conditions were identical to those we have used in previous studies of myosin head orientation, except that the fibers were perpendicular to the magnetic field, making the spectra primarily sensitive to rotational motion rather than to the orientational distribution. In rigor, the high intensity of the ST-EPR signal indicates the absence of microsecond rotational motion, showing that heads are all rigidly bound to actin. However, in both relaxation and contraction, considerable microsecond rotational motion is observed, implying that the previously reported orientational disorder under these conditions is dynamic, not static, on the microsecond time scale. The behavior in relaxation is essentially the same as that observed when myosin heads are detached from actin in the absence of ATP (Barnett and Thomas, 1984), corresponding to an effective rotational correlation time of approximately 10 microseconds. Slightly less mobility is observed during contraction. One possible interpretation is that in contraction all heads have the same mobility, corresponding to a correlation time of approximately 25 microseconds. Alternatively, more than one motional population may be present. For example, assuming that the spectrum in contraction is a linear combination of those in relaxation (mobile) and rigor (immobile), we obtained a good fit with a mole fraction of 78-88% of the heads in the mobile state. These results are consistent with previous STEPR studies on contracting myofibrils(Thomas et al., 1980). Thus most myosin heads undergo microsecond rotational motions most of the time during isometric contraction, at least in the probed region of the myosin head.These motions could arise primarily from the free rotations of heads detached from actin. However, if most of these heads are attached to actin during contraction, as suggested by stiffness measurements, this result provides support for the hypothesis that sub-millisecond rotational motions of actin-attached myosin heads play an important role in force generation.

Structural changes in the thin filament during activation studied by X-ray diffraction of highly stretched skeletal muscle

The actin layer-lines were recorded from a frog semitendinosus muscle stretched to a sarcomere length greater than 4.4 microM. On activation of the muscle, the equator, the second layer-line at 1/18 nm-1 and the 5.9 nm layer-line increased in integrated intensity. On the other hand, the integrated intensity of the first layer-line at 1/36 nm-1 decreased markedly on activation. This decrease was not fully attributable to shifts of tropomyosin strands and therefore suggested a structural change in the actin subunit. The decrease may account for the apparent lack of an intensity increase of this layer-line on activation at normal muscle lengths where attachment of myosin heads to actin increases the intensities of other layer-lines.

The effects of temperature on relaxation in frog skeletal muscle: the role of parvalbumin

Isolated muscle fibers from Rana temporaria tibialis anterior muscles were microinjected with aequorin. The force responses and the Ca2+ transients associated with twitch and tetanic contractions were studied at several temperatures. The declines of the Ca2+ transients were well described by single exponential equations and the effects of temperature were complex (multi-exponential). To determine if these temperature effects on the Ca2+ transients were influenced by the Ca2+ indicator itself, samples of the injected aequorin were studied in vitro using a Gibson stopped-flow apparatus. The quenching of aequorin luminescence with either EGTA or de-calcified Rana temporaria parvalbumin were mono-exponential. These overall quenching reactions had single exponential temperature dependencies. The effects of temperature on the declines of the single fiber Ca2+ transients did not appear to be influenced by the kinetics of the aequorin reaction. The disparity in the effects of temperature on the single fiber Ca2+ transients versus the in vitro quenching of aequorin luminescence with parvalbumin, were interpreted to indicate that in twitch and tetanic contractions of these fibers, it was unlikely that soluble Ca2+ binding proteins played a major role in the regulation of myoplasmic Ca2+.

Ionic effects on the rotational dynamics of cross-bridges in myosin filaments, measured by triplet absorption anisotropy

We have measured the rotational motion of myosin heads in synthetic thick filaments at 4 degrees C in the time range from 10(-7) to 10(-4) seconds, by measuring transient absorption anisotropy of an eosin probe attached to a reactive sulfhydryl on the myosin head. Under conditions that result in monomeric myosin (500 mM ionic strength), the anisotropy decay is independent of pH in the range from 7.0 to 8.2 and [Mg2+] in the range from 0.1 to 10 mM; the anisotropy decays bi-exponentially with correlation times of 0.4 and 2 microseconds to a constant value of 0.016. Under more physiological conditions (115 mM ionic strength), resulting in filament formation, the anisotropy decay is sensitive to both pH and [Mg2+]. The anisotropy at pH 8.2 and 0.1 mM-Mg2+ decays with correlation times of 0.5 and 3.8 microseconds to a constant limiting anisotropy of 0.038. When the [Mg2+] is increased to 10 mM, the correlation times are 0.6 and 5.7 microseconds and the limiting anisotropy value is 0.055. Identical changes in the anisotropy decay are caused by an increase in [H+] to pH 7.0, in the presence of 0.1 mM-Mg2+. Increasing the total ionic strength to 187 mM decreases the amplitude of the cation effects. These results provide direct evidence that the rotational dynamics of myosin heads in thick filaments are influenced by physiological concentrations of cations. The results are qualitatively consistent with the proposal that these and other ionic conditions regulate transitions between "spread" and "compact" cross-bridge conformations, but the quantitative results indicate that cross-bridges undergo large-amplitude microsecond rotations even under conditions where the compact state should predominate.

Time-resolved x-ray diffraction of biological materials

Instrumental and specimen considerations pertinent to performing time-resolved x-ray diffraction on biological materials are discussed. Existing synchrotron x-ray sources, in conjunction with integrating x-ray detectors, have made millisecond diffraction experiments feasible; exposure times several orders of magnitude shorter than this will be possible with synchrotron sources now on the drawing boards. Experience gained from time-resolved studies together with order-of-magnitude estimates of specimen requirements can be used to determine the instrumental capabilities needed for various time-resolved experiments. Existing instrumental capabilities and methods of dealing with time-resolved specimens are reviewed.

Laser-stimulated luminescence used to measure x-ray diffraction of a contracting striated muscle

An integrating x-ray area detector that operates on the basis of laser-stimulated luminescence was used in a diffraction study of muscle contraction. The area detector has a dynamic range of 1 to 10(5), a sensitivity about 60 times greater with approximately 1/300 as much fog background as x-ray film. It is erasable and reusable but, like film, can integrate at a practically unlimited counting rate. The high sensitivity and wide dynamic range of the detector resulted in a sufficient reduction in the exposure time to make possible the recording of a clear x-ray diffraction pattern, with up to 2.0-nanometer axial spacing, from a contracting frog skeletal muscle in as little as 10 seconds with synchrotron radiation. During the isometric contraction of the muscle, most of the actin diffraction lines increased in intensity without noticeable changes in their peak positions. Changes also occurred in diffraction intensities from the myosin heads. The results indicate that during contraction the structure of the actin filaments differs from that in the rigor state, suggesting a possible structural change in the actin subunits themselves; the myosin heads during contraction retain the axial periodicity of the myosin filament and become aligned in a more perpendicular manner to the actin filaments.

Arrangement of myosin heads in relaxed thick filaments from frog skeletal muscle

The distribution of myosin heads on the surface of frog skeletal muscle thick filaments has been determined by computer processing of electron micrographs of isolated filaments stained with tannic acid and uranyl acetate. The heads are arranged in three strands but not in a strictly helical manner and so the structure has cylindrical symmetry. This accounts for the "forbidden" meridional reflections seen in diffraction patterns. Each layer-line therefore represents the sum of terms of Bessel orders 0, +/- 3, +/- 6, +/- 9 and so on. These terms interact so that, unlike a helical object without terms from overlapping Bessel orders, as the azimuth is changed, the amplitude on a layer-line at a particular radius varies substantially and its phase does not alter linearly. Consequently, a three-dimensional reconstruction cannot be produced from a single view. We have therefore used tilt series of three individual filaments to decompose the data on layer-lines 0 to 6 into terms of Bessel orders up to +/- 9 using a least-squares procedure. These data had a least-squares residual of 0.32 and enabled a three-dimensional reconstruction to be obtained at a nominal resolution of 6 nm. This showed, at a radius of about 10 nm, three strands of projecting morphological units with three units spaced along each strand every 42.9 nm axially. We have identified these units with pairs of myosin heads. Successive units along a strand are perturbed axially, azimuthally and radially from the positions expected if the structure was perfectly helical. This may simply be a consequence of steric restrictions in packing the heads on the thick filament surface, but could also reflect an underlying non-helical arrangement of myosin tails, which would be consistent with the thick filament shaft being constructed from three subfilaments in which the tails were arranged regularly. There was also material at a radius of about 6 nm spaced 42.9 nm axially, which we tentatively identified with accessory proteins. The filament shaft had a pronounced pattern of axial staining.

Time-resolved X-ray diffraction studies of frog skeletal muscle isometrically twitched by two successive stimuli using synchrotron radiation

In order to clarify the delay between muscular structural changes and mechanical responses, the intensity changes of the equatorial and myosin layer-line reflections were studied by a time-resolved X-ray diffraction technique using synchrotron radiation. The muscle was stimulated at 12-13 degrees C by two successive stimuli at an interval (80-100 ms) during which the second twitch started while tension was still being exerted by the muscle. At the first twitch, the intensity changes of the 1.0 and 1.1 equatorial reflections reached 65 and 200% of the resting values, and further changes to 55 and 220% were seen at the second twitch, respectively. Although the second twitch decreased not only the time to peak tension but also that to the maximum intensity changes of the equatorial reflections (in both cases, about 15 ms), the delay (about 20 ms) between the intensity changes and the development of tension at the first twitch were still observed at the second twitch. On the other hand, the intensities of the 42.9 nm off-meridional and the 21.5 nm meridional myosin reflections decreased at the first twitch to the levels found when a muscle was isometrically tetanized, and no further decrease in their intensities was observed at the second twitch. These results indicate that a certain period of time is necessary for myosin heads to contribute to tension development after their arrival in the vicinity of the thin filaments during contraction.

Intensity increases of actin layer-lines on activation of the Limulus muscle

Small angle x-ray diffraction patterns were recorded from isometrically contracting Limulus (horseshoe crab) telson levator muscle using a multiwire proportional-area detector on the storage ring DORIS. In the pattern a substantial increase in intensity is observed on the thin-filament-associated layer-line at 1/38 nm-1 (the first actin layer-line) with a maximum increase at a radial spacing of R = 0.07 nm-1 but there is a much smaller change in the intensity of the 5.9-nm layer-line, which also arises from the thin filament structure. The results suggest that during contraction the myosin heads, presumably being attached to the thin filaments, are arranged along the long-stranded helical tracks of the thin filaments but that the spatial relationship between the heads and the actin monomers varies. Intensity increases have also been observed (Maéda et al., manuscript in preparation) in the part of the patterns from frog muscle and barnacle muscle, which are attributable to the first actin layer-line. It is thus likely that the intensity increase of the first actin layer-line on the Limulus pattern is associated not with structural features which are special to Limulus muscle, but with the tension generating processes that are shared by muscles in general.

The role of tropomyosin-troponin in the regulation of skeletal muscle contraction

Steric blocking of actin-myosin interaction by tropomyosin has been a working hypothesis in the study of the regulation of skeletal muscle contraction, yet the simple movement of actin-associated tropomyosin from a myosin-blocking position (relaxation) to a nonblocking position (contraction) cannot adequately account for all of the biophysical and biochemical observations which have been made to date. Ambiguous assignment of tropomyosin positions on actin during contraction, due in part to the limited resolution of reconstruction techniques, may also hint at a real lack of clearcut 'on' and 'off' positioning of tropomyosin and tropomyosin-troponin complex. Recent biochemical evidence suggests processes relatively independent of tropomyosin-troponin may have a governing effect on contraction, involving kinetic constraints on actin-myosin interaction influenced by the binding of ATP and the intermediates of ATP hydrolysis. Based on our current understanding put forth in this review, it is clear that regulatory interactions in muscle contraction do not consist solely of steric effects but involve kinetic factors as well. Where the latter are being defined in systems reconstituted from purified proteins and their fragments, the steric components of regulation are most clearly observed in studies of structurally more intact physiologic systems (e.g. intact or skinned whole muscle fibres). The fine detail of the processes and their interplay remains an intriguing question. Likewise, the precise physical relationship of myosin with actin in the crossbridge cycle continues to elude definition. Refinement of several methodologies (X-ray crystallography, three-dimensional reconstruction, time-resolved X-ray diffraction) will increase the potential for detailing the molecular basis of the regulation of muscle contraction.

Structural changes during activation of frog muscle studied by time-resolved X-ray diffraction

The pattern given by contracting frog muscle can be followed with high time resolution using synchrotron radiation as a high-intensity X-ray source. We have studied the behaviour of the second actin layer-line (axial spacing of approximately 179 A) at an off-meridional spacing of approximately 0.023 A-1, a region of the diagram that is sensitive to the position of tropomyosin in the thin filaments. In confirmation of earlier work, we find that there is a substantial increase in the intensity of this part of the pattern during contraction. We find that the reflection reaches half its final intensity about 17 milliseconds after the stimulus at 6 degrees C. The changes in the equatorial reflections, which arise from movement of crossbridges towards the thin filaments, occur with a delay of about 12 to 17 milliseconds relative to this change in the actin pattern. In over-stretched muscle, where thick and thin filaments no longer overlap, the changes in the actin second layer-line still take place upon stimulation with a time course and intensity similar to that observed at full overlap. This indicates that tropomyosin movement, in response to calcium binding to troponin, is the first structural step in muscular contraction, and is the prerequisite for myosin binding. A change in intensity similar to that found in contracting muscle is seen in rigor, where tropomyosin is probably locked in the active position. During relaxation the earlier stages in the decrease in intensity of the second actin layer-line take place significantly sooner after the last stimulus than tension decay. In over-stretched muscles the intensity decay is appreciably faster than in the same muscles at rest length, where attached crossbridges may interfere with the return of tropomyosin to its resting position.