The effect of calcium activation of skinned fiber bundles on the structure of Limulus thick filaments

Invertebrate muscles: thin and thick filament structure; molecular basis of contraction and its regulation, catch and asynchronous muscle

This is the second in a series of canonical reviews on invertebrate muscle. We cover here thin and thick filament structure, the molecular basis of force generation and its regulation, and two special properties of some invertebrate muscle, catch and asynchronous muscle. Invertebrate thin filaments resemble vertebrate thin filaments, although helix structure and tropomyosin arrangement show small differences. Invertebrate thick filaments, alternatively, are very different from vertebrate striated thick filaments and show great variation within invertebrates. Part of this diversity stems from variation in paramyosin content, which is greatly increased in very large diameter invertebrate thick filaments. Other of it arises from relatively small changes in filament backbone structure, which results in filaments with grossly similar myosin head placements (rotating crowns of heads every 14.5 nm) but large changes in detail (distances between heads in azimuthal registration varying from three to thousands of crowns). The lever arm basis of force generation is common to both vertebrates and invertebrates, and in some invertebrates this process is understood on the near atomic level. Invertebrate actomyosin is both thin (tropomyosin:troponin) and thick (primarily via direct Ca(++) binding to myosin) filament regulated, and most invertebrate muscles are dually regulated. These mechanisms are well understood on the molecular level, but the behavioral utility of dual regulation is less so. The phosphorylation state of the thick filament associated giant protein, twitchin, has been recently shown to be the molecular basis of catch. The molecular basis of the stretch activation underlying asynchronous muscle activity, however, remains unresolved.

Simultaneous stretching and contraction of stress fibers in vivo

To study the dynamics of stress fiber components in cultured fibroblasts, we expressed alpha-actinin and the myosin II regulatory myosin light chain (MLC) as fusion proteins with green fluorescent protein. Myosin activation was stimulated by treatment with calyculin A, a serine/threonine phosphatase inhibitor that elevates MLC phosphorylation, or with LPA, another agent that ultimately stimulates phosphorylation of MLC via a RhoA-mediated pathway. The resulting contraction caused stress fiber shortening and allowed observation of changes in the spacing of stress fiber components. We have observed that stress fibers, unlike muscle myofibrils, do not contract uniformly along their lengths. Although peripheral regions shortened, more central regions stretched. We detected higher levels of MLC and phosphorylated MLC in the peripheral region of stress fibers. Fluorescence recovery after photobleaching revealed more rapid exchange of myosin and alpha-actinin in the middle of stress fibers, compared with the periphery. Surprisingly, the widths of the myosin and alpha-actinin bands in stress fibers also varied in different regions. In the periphery, the banding patterns for both proteins were shorter, whereas in central regions, where stretching occurred, the bands were wider.

Elastic properties of isolated thick filaments measured by nanofabricated cantilevers

Using newly developed nanofabricated cantilever force transducers, we have measured the mechanical properties of isolated thick filaments from the anterior byssus retractor muscle of the blue mussel Mytilus edulis and the telson levator muscle of the horseshoe crab Limulus polyphemus. The single thick filament specimen was suspended between the tip of a flexible cantilever and the tip of a stiff reference beam. Axial stress was placed on the filament, which bent the flexible cantilever. Cantilever tips were microscopically imaged onto a photodiode array to extract tip positions, which could be converted into force by using the cantilever stiffness value. Length changes up to 23% initial length (Mytilus) and 66% initial length (Limulus) were fully reversible and took place within the physiological force range. When stretch exceeded two to three times initial length (Mytilus) or five to six times initial length (Limulus), at forces approximately 18 nN and approximately 7 nN, respectively, the filaments broke. Appreciable and reversible strain within the physiological force range implies that thick-filament length changes could play a significant physiological role, at least in invertebrate muscles.

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.