Force generation by skeletal muscle is controlled by mechanosensing in myosin filaments

Abstract

Contraction of both skeletal muscle and the heart is thought to be controlled by a calcium-dependent structural change in the actin-containing thin filaments, which permits the binding of myosin motors from the neighbouring thick filaments to drive filament sliding. Here we show by synchrotron small-angle X-ray diffraction of frog (Rana temporaria) single skeletal muscle cells that, although the well-known thin-filament mechanism is sufficient for regulation of muscle shortening against low load, force generation against high load requires a second permissive step linked to a change in the structure of the thick filament. The resting (switched 'OFF') structure of the thick filament is characterized by helical tracks of myosin motors on the filament surface and a short backbone periodicity. This OFF structure is almost completely preserved during low-load shortening, which is driven by a small fraction of constitutively active (switched 'ON') myosin motors outside thick-filament control. At higher load, these motors generate sufficient thick-filament stress to trigger the transition to its long-periodicity ON structure, unlocking the major population of motors required for high-load contraction. This concept of the thick filament as a regulatory mechanosensor provides a novel explanation for the dynamic and energetic properties of skeletal muscle. A similar mechanism probably operates in the heart.

Myosin filament activation in the heart is tuned to the mechanical task

“…This work, by combining sarcomere-level mechanics with nanometermicrometer-scale X-ray diffraction in intact trabeculae from the rat cardiac ventricle, describes the changes in the thick filament and the myosin motors after force development in the cardiac twitch and their relation to SL, with the subnanometer precision achieved by using the X-ray interference between the two halves of the thick filament. We find that, in diastole, all of the myosinbased reflections mark the quasihelical three-stranded symmetry followed by the myosin molecules when they are in their off state on the surface of the thick filament with a short periodicity (2,4,7). At the peak of twitch force, the intensities of all of the meridional reflections and that of the ML1 decrease because of the myosin motors leaving their helical tracks as the thick filament switches on.…”

“…2A and Fig. S2A) shows the first-order layer line reflection (ML1) at an axial spacing of 43 nm and up to the sixth order of the meridional reflections (M1-M6) indexing on the quasihelical three-stranded symmetry with 43-nm periodicity followed by the myosin molecules when they are on the surface of the thick filament in their resting (off) state (2,4,7). The spatial resolution achieved along the meridian (parallel to the trabecula axis) with vertically mounted trabeculae is adequate to record the reflection fine structure (Fig.…”

“…In diastole, the motors are on the surface of the thick filament folded back toward the center of the sarcomere (blue in the cartoon in Fig. 3A), and the backbone has a short periodicity (2,4,7). The corresponding axial mass density is represented by a Gaussian with the center at an axial position (z) displaced by −7.97 nm from the head-rod junction (negative direction is toward the center of the thick filament) and σ = 3.5 nm (Fig.…”

“…However, in these muscles at rest, most of the myosin motors are in the off state and packed into helical tracks with 43-nm periodicity on the surface of the thick filaments (1-4), making them unavailable for binding to the thin filament and ATP hydrolysis (5,6). Recent X-ray diffraction experiments on single fibers from skeletal muscle showed that, in addition to the canonical thin filament activation system, a thick filament mechanosensing mechanism provides a way for selective unlocking of myosin motors during high load contraction (7). This thick filament-based regulation has not yet been shown in cardiac muscle, in which several regulatory systems are significant.…”

Interfibrillar stiffening of echinoderm mutable collagenous tissue demonstrated at the nanoscale

“…Shifts in these peaks, as would be induced by mechanical loading or ionic treatments, are therefore a measure of the nanoscale fibril strain demonstrated for vertebrate tissues (41-43, 46, 47). By combining micromechanics with in situ small-angle X-ray scattering, it has been possible to shed light on the fundamental ultrastructural mechanisms enabling viscoelasticity, toughness, and force generation in vertebrate tissues ranging from tendon (48), bone (44,46), and aorta (49) to muscle (50), as well as more unusual examples of biological optimization such as armored fish scales (43). Using SAXD, it was found that in crosslink-deficient fibrils, increased molecular slippage led to larger fibril strains, compared with normal collagen fibrils (48); that high toughness of antler bone was due to inorganic/organic friction at the intrafibrillar level (41); and that fibrillar reorientation blunted crack propagation in skin (51), among other examples.…”