Mechanics and energetics of contraction in striated muscle of the sea scallop, Placopecten magellanicus

The mechanical properties of the mantle muscle of European cuttlefish (Sepia officinalis)

The circular muscles surrounding the mantle cavity of European cuttlefish (Sepia officinalis) generate the mechanical power to compress the cavity, forcing a jet of water out of the funnel, propelling the animal during jet propulsion swimming. During ontogeny, jetting frequency decreases in adults compared with juveniles, and this is expected to be reflected in the contractile properties of the locomotory muscles. To develop greater insight into how the locomotion of these animals is powered during ontogeny, we determined the mechanical properties of bundles of muscle fascicles during isometric, isotonic and cyclic length changes in vitro, at two life stages: juveniles and adults. The twitch kinetics were faster in juveniles than in adults (twitch rise time 257 ms compared with 371 ms; half-twitch relaxation 257 ms compared with 677 ms in juveniles and adults, respectively); however, twitch and tetanic stress, the maximum velocity of shortening and curvature of the force-velocity relationship did not differ. Under cyclic conditions, net power exhibited an inverted U-shaped relationship with cycle frequency in both juveniles and adults; the frequency at which maximum net power was achieved was shifted to lower cycle frequencies with increased maturity, which is consistent with the slower contraction and relaxation kinetics in adults compared with juveniles. The cycle frequency at which peak power was achieved during cyclical contractions in vitro was found to match that seen in vivo in juveniles, suggesting power is being maximised during jet propulsion swimming.

Escape responses by jet propulsion in scallops

The impressive swimming escape response of scallops uses a simple locomotor system that facilitates analysis of the functional relationships between its primary components. One large adductor muscle, two valves, the muscular mantle, and the rubbery hinge ligament are the basic elements allowing swimming by jet propulsion. Although these basic functional elements are shared among scallop species, the exact nature of the escape response varies considerably within and among species. Valve shape and density have opposing influences upon the capacity for swimming and the ease of attack by predators once captured. Patterns of muscle use can partly overcome the constraints imposed by shell characteristics. The depletion of muscle reserves during gametogenesis leads to a trade-off between escape response performance and reproductive investment. However, changes in muscle energetic status influence repeat performance more than initial escape performance. Escape response performance is influenced by habitat temperature and mariculture techniques. During scallop ontogeny, changes in susceptibility to predation and in reproductive investment may influence escape response capacities. These ontogenetic patterns are likely to vary with the longevity and maximal size of each species. Although the basic elements allowing swimming by jet propulsion are common to scallops, their exact use varies considerably among species.

Structural basis of the relaxed state of a Ca2+-regulated myosin filament and its evolutionary implications

Myosin filaments of muscle are regulated either by phosphorylation of their regulatory light chains or Ca(2+) binding to the essential light chains, contributing to on-off switching or modulation of contraction. Phosphorylation-regulated filaments in the relaxed state are characterized by an asymmetric interaction between the two myosin heads, inhibiting their actin binding or ATPase activity. Here, we have tested whether a similar interaction switches off activity in myosin filaments regulated by Ca(2+) binding. Cryo-electron microscopy and single-particle image reconstruction of Ca(2+)-regulated (scallop) filaments reveals a helical array of myosin head-pair motifs above the filament surface. Docking of atomic models of scallop myosin head domains into the motifs reveals that the heads interact in a similar way to those in phosphorylation-regulated filaments. The results imply that the two major evolutionary branches of myosin regulation--involving phosphorylation or Ca(2+) binding--share a common structural mechanism for switching off thick-filament activity in relaxed muscle. We suggest that the Ca(2+)-binding mechanism evolved from the more ancient phosphorylation-based system to enable rapid response of myosin-regulated muscles to activation. Although the motifs are similar in both systems, the scallop structure is more tilted and higher above the filament backbone, leading to different intermolecular interactions. The reconstruction reveals how the myosin tail emerges from the motif, connecting the heads to the filament backbone, and shows that the backbone is built from supramolecular assemblies of myosin tails. The reconstruction provides a native structural context for understanding past biochemical and biophysical studies of this model Ca(2+)-regulated myosin.

The ultrastructure and contractile properties of a fast-acting, obliquely striated, myosin-regulated muscle: the funnel retractor of squids

We investigated the ultrastructure, contractile properties, and in vivo length changes of the fast-acting funnel retractor muscle of the long-finned squid Doryteuthis pealeii. This muscle is composed of obliquely striated, spindle-shaped fibers ~3 mum across that have an abundant sarcoplasmic reticulum, consisting primarily of membranous sacs that form 'dyads' along the surface of each cell. The contractile apparatus consists of 'myofibrils' approximately 0.25-0.5 microm wide in cross section arrayed around the periphery of each cell, surrounding a central core that contains the nucleus and large mitochondria. Thick myofilaments are approximately 25 nm in diameter and approximately 2.8 microm long. 'Dense bodies' are narrow, resembling Z lines, but are discontinuous and are not associated with the cytoskeletal fibrillar elements that are so prominent in slower obliquely striated muscles. The cells approximate each other closely with minimal intervening intercellular connective tissue. Our physiological experiments, conducted at 17 degrees C, showed that the longitudinal muscle fibers of the funnel retractor were activated rapidly (8 ms latent period following stimulation) and generated force rapidly (peak twitch force occurred within 50 ms). The longitudinal fibers had low V(max) (2.15 +/-0.26 L(0) s(-1), where L(0) was the length that generated peak isometric force) but generated relatively high isometric stress (270+/-20 mN mm(-2) physiological cross section). The fibers exhibited a moderate maximum power output (49.9 W kg(-1)), compared with vertebrate and arthropod cross striated fibers, at a V/V(max) of 0.33+/-0.044. During ventilation of the mantle cavity and locomotion, the funnel retractor muscle operated in vivo over a limited range of strains (+0.075 to -0.15 relative to resting length, L(R)) and at low strain rates (from 0.16 to 0.91 L(R) s(-1) ), corresponding to a range of V/V(max) from 0.073 to 0.42. During the exhalant phase of the jet the range of strains was even narrower: maximum range less than +/-0.04, with the muscle operating nearly isometrically during ventilation and slow, arms-first swimming. The limited length operating range of the funnel retractor muscles, especially during ventilation and slow jetting, suggests that they may act as muscular struts.

Millisecond time-resolved changes occurring in Ca2+-regulated myosin filaments upon relaxation

Contraction of many muscles is activated in part by the binding of Ca(2+) to, or phosphorylation of, the myosin heads on the surface of the thick filaments. In relaxed muscle, the myosin heads are helically ordered and undergo minimal interaction with actin. On Ca(2+) binding or phosphorylation, the head array becomes disordered, reflecting breakage of the head-head and other interactions that underlie the ordered structure. Loosening of the heads from the filament surface enables them to interact with actin filaments, bringing about contraction. On relaxation, the heads return to their ordered positions on the filament backbone. In scallop striated adductor muscle, the disordering that takes place on Ca(2+) binding occurs on the millisecond time scale, suggesting that it is a key element of muscle activation. Here we have studied the reverse process. Using time-resolved negative staining electron microscopy, we show that the rate of reordering on removal of Ca(2+) also occurs on the same physiological time scale. Direct observation of images together with analysis of their Fourier transforms shows that activated heads regain their axial ordering within 20 ms and become ordered in their final helical positions within 50 ms. This rapid reordering suggests that reformation of the ordered structure, and the head-head and other interactions that underlie it, is a critical element of the relaxation process.

Jet propulsion in the cold: mechanics of swimming in the Antarctic scallopAdamussium colbecki

Unlike most bivalves, scallops are able to swim, relying on a shell with reduced mass and streamlined proportions, a large fast-twitch adductor muscle and the elastic characteristics of the shell's hinge. Despite these adaptations, swimming in scallops is never far from failure, and it is surprising to find a swimming scallop in Antarctica, where low temperature increases the viscosity of seawater, decreases the power output of the adductor muscle and potentially compromises the energy storage capability of the hinge material (abductin, a protein rubber). How does the Antarctic scallop, Adamussium colbecki, cope with the cold? Its shell mass is substantially reduced relative to that of temperate and tropical scallops, but this potential advantage is more than offset by a drastic reduction in adductor-muscle mass. By contrast, A. colbecki's abductin maintains a higher resilience at low temperatures than does the abductin of a temperate scallop. This resilience may help to compensate for reduced muscle mass,assisting the Antarctic scallop to maintain its marginal swimming ability. However, theory suggests that this assistance should be slight, so the adaptive value of increased resilience remains open to question. The high resilience of A. colbecki abductin at low temperatures may be of interest to materials engineers.

Ca2+ causes release of myosin heads from the thick filament surface on the milliseconds time scale

We have used electron microscopy to study the structural changes induced when myosin filaments are activated by Ca2+. Negative staining reveals that when Ca2+ binds to the heads of relaxed Ca2+ -regulated myosin filaments, the helically ordered myosin heads become disordered and project further from the filament surface. Cryo-electron microscopy of unstained, frozen-hydrated specimens supports this finding, and shows that disordering is reversible on removal of Ca2+. The structural change is thus a result of Ca2+ binding alone and not an artifact of staining. Comparison of the two techniques suggests that negative staining preserves the structure induced by Ca2+ -binding. We therefore used a time-resolved negative staining technique to determine the time scale of the structural change. Full disordering was observed within 30 ms of Ca2+ addition, and had started to occur within 10 ms, showing that the change occurs on the physiological time scale. Comparison with studies of single heavy meromyosin molecules suggests that an increased mobility of myosin heads induced by Ca2+ binding underlies the changes in filament structure that we observe. We conclude that the loosening of the array of myosin heads that occurs on activation is real and physiological; it may function to make activated myosin heads freer to contact actin filaments during muscle contraction.

Capturing time-resolved changes in molecular structure by negative staining

Imaging structural intermediates of biological processes is a key step in understanding biological function. Because intermediates are commonly short-lived, lasting only milliseconds, the main methods used to capture them have been conventional imaging of analog or inhibited states, having extended lifetimes, or rapid (millisecond timescale) freezing of intermediates with subsequent observation by cryo-EM. We have developed a simpler method that fixes structure on the millisecond timescale. The procedure consists of briefly (milliseconds) exposing the macromolecular structure of interest on an EM grid to conditions that initiate the structural change, then immediately fixing with uranyl acetate or tannic acid. Specimens are then observed by negative staining. The key finding that validates this approach is our demonstration that uranyl acetate, and in some cases tannic acid, fixes protein molecular structure on the millisecond timescale. This is demonstrated by our observation that exposure of actin and myosin filaments to these fixatives for as little as 10 ms is sufficient to fully preserve them against changes that normally induce rapid and major alteration in their molecular structure. Fixation appears to stabilize both ionic and hydrophobic bonds. This approach should be of general utility for studying transient molecular changes in many systems.

Calcium-dependent structural changes in scallop heavy meromyosin

The mechanism of calcium regulation of scallop myosin is not understood, although it is known that both myosin heads are required. We have explored possible interactions between the heads of heavy meromyosin (HMM) in the presence and absence of calcium and nucleotides by sedimentation and electron microscope studies. The ATPase activity of the HMM preparation was activated over tenfold by calcium, indicating that the preparation contained mostly regulated molecules. In the presence of ADP or ATP analogs, calcium increased the asymmetry of the HMM molecule as judged by its slower sedimentation velocity compared with that in EGTA. In the absence of nucleotide the asymmetry was high even in EGTA. The shift in sedimentation occurred with a sharp midpoint at a calcium level of about 0.5 microM. Sedimentation of subfragment 1 was not dependent on calcium or on nucleotides. Modeling accounted for the observed sedimentation behavior by assuming that both HMM heads bent toward the tail in the absence of calcium, while in its presence the heads had random positions. The sedimentation pattern showed a single peak at all calcium concentrations, indicating equilibration between the two forms with a t(1/2) less than 70 seconds. Electron micrographs of crosslinked, rotary shadowed specimens indicated that 81 % of HMM molecules in the presence of nucleotide had both heads pointing back towards the tail in the absence of calcium, as compared with 41 % in its presence. This is consistent with the sedimentation data. We conclude that in the "off" state, scallop myosin heads interact with each other, forming a rigid structure with low ATPase activity. When molecules are switched "on" by binding of calcium, communication between the heads is lost, allowing them to flex randomly about the junction with the tail; this could facilitate their interaction with actin in contracting muscle.

Relaxation rate of intact striated muscle fibres after flash photolysis of a caged calcium chelator (diazo-2)

Single twitch fibres from lumbrical muscles of Xenopus have been loaded with the photolysable calcium-chelator diazo-2 by incubation in Ringer solution containing the membrane permeable acetoxymethyl ester (AM) form of diazo-2. Incubation caused a progressive slowing of tetanus rise and relaxation which is ascribed to calcium-buffering by unphotolyzed diazo-2 (Kd = 2.2 microM). After incubation, exposure to a brief UV flash caused a three to four fold increase in the rate of tension fall. A flash given 16-18 ms after the last tetanic stimulus (at 22-24 degrees C) resulted in 10% increase in relaxation rate compared with the control before incubation. A much bigger effect was observed when a flash was given half-way into the slow phase, where an 1.8-1.9-fold increase in relaxation rate, above the preincubation slope, was observed. It is concluded that rapid lowering of [Ca]i, and hence more rapid removal of Ca2+ from troponin, speeds up relaxation, indicating that calcium translocation is the major determinant of the rate of tension fall during the isometric phase of relaxation.

A folded (10 S) conformer of myosin from a striated muscle and its implications for regulation of ATPase activity

Myosin from the striated adductor muscle of the scallop Pecten maximus is shown to fold into a compact 10 S conformer under relaxing conditions, as has been characterized for smooth and non-muscle myosins. The folding transition is accompanied by the trapping of nucleotide at the active site to give a species with a half-life of about an hour at 20 degrees C. Ca2+ binding to the specific, regulatory sites on a myosin head promotes unfolding to the extended 6 S conformer and activates product release by 60-fold. The unfolding transition, however, remains much slower than the contraction-relaxation cycle of scallop striated muscle and could not play a role in the regulation of these events. The dissociation of products from myosin heads in native thick filaments is Ca2(+)-regulated, but under relaxing conditions the nucleotide is released at least an order of magnitude faster than from the 10 S monomeric myosin, at a rate similar to that observed with heavy meromyosin. Thus, there is no evidence for any intermolecular interaction between neighbouring molecules in the filament analogous to the head-neck intramolecular interaction in the 10 S conformer. It is possible that the 10 S myosin state represents an inert form involved in the control of filament assembly during muscle growth and development. Removal of regulatory light chains or labelling the reactive heavy chain thiol of myosin prevents, or at least disfavours, formation of the folded 10 S conformer and allows separation of the modified protein from the native molecules.

Striated scallop muscle relaxation: fast force transients produced by photolysis of Diazo-2

Relaxation of the myosin regulated striated adductor muscles of Pecten maximus was initiated by the photolysis of the caged Ca2+ chelator, Diazo-2. The fibres relaxed to approximately 30% of the maximum tension with a mean half-time of 17.9 +/- 1.6 ms (n = 7, temp 12 degrees C), much faster than the rates observed in intact muscle at the same temperature. This indicates that in the intact adductor muscle the slower relaxation rate is determined by the speed of Ca2+ removal from the sarcoplasm. The faster rate of relaxation of scallop muscle in vitro, compared with frog skeletal muscle may reflect different mechanisms of regulation of the crossbridge cycle.

Transient-kinetic studies of the adenosine triphosphatase activity of scallop heavy meromyosin

Fluorescence stopped-flow experiments were performed to elucidate the elementary steps of the ATPase mechanism of scallop heavy meromyosin in the presence and in the absence of Ca2+. ATP binding and hydrolysis, as monitored by the change in tryptophan fluorescence, appear to be Ca2+-insensitive, whereas both Pi release and ADP release are markedly suppressed in the absence of Ca2+. Rate constants for Pi release are 0.2 s-1 and 0.002 s-1 and for ADP release are 6 s-1 and 0.01 s-1 in the presence and in the absence of Ca2+ respectively. Ca2+ binding to the specific site of the regulatory domain is rapid and its release occurs at 25 s-1, consistent with the time scale of a twitch of the striated adductor muscle. Nucleotide binding is a multi-step process requiring a minimum of three states. In such a model Ca2+ controls the rate of conformational changes at the active site in both the forward and the reverse direction, leading to a large dependence of the rate of nucleotide release, but a lesser effect on the overall equilibrium position. The kinetic trapping of nucleotides and Pi at the active site, in the absence of Ca2+, appears to be a fundamental step in suppressing the interaction of the myosin head with the thin filaments in relaxed molluscan muscle.

A mechanical study of regulation in the striated adductor muscle of the scallop

Chemically skinned fibre bundles were prepared from the striated adductor muscle of the sea scallop, Placopecten magellanicus. The relation between tension and calcium concentration was determined in activating solutions containing 5 mM-MgATP, ionic strength 0.2, pH 7.1 at 20 degrees C. The isometric tension rose from zero to its maximum value between pCa 6.0 and 5.2. The steepness of the relation cannot be accounted for in terms of the binding of calcium to the two known sites on myosin and suggests that there must be an additional, co-operative mechanism. The regulatory light chain content of the fibre bundles was determined by urea gel electrophoresis and was found to be approximately 2 light chains per myosin molecule. The regulatory light chains were removed completely by treatment with EDTA at 25-30 degrees C. Fibre bundles then showed a total loss of control over contraction; a high tension was generated whether or not calcium was present in the bathing solution. Complete removal of the regulatory light chains did not greatly affect the tension generated or the stiffness in the rigor state. Control of contraction could be restored completely by the addition of regulatory light chains from scallop muscle. Treatment with EDTA at 0-12 degrees C resulted in the removal of 0.76-2.0 regulatory light chains per myosin molecule. Fibre bundles for which removal was less than complete were partially sensitive to calcium, i.e. tension was higher in the presence of calcium than in its absence. The results indicate that the normal mechanism of tension generation in scallop muscle is mediated primarily through myosin and not thin filament control. This finding is consistent with previous studies of the ATPase activity of myofibrils from scallop muscle.