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 28,634 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Synthetic filaments prepared from column-purified rabbit skeletal myosin by slow dialysis exhibit characteristic bipolar organization and 14-nm axial subunit spacing. Backbone substructure can be discerned in high resolution micrographs in the form of striations of 3--4-nm width and slight angular tilt from the direction of the filament axis. Filament backbone diameters vary over the population, although remaining relatively constant for a single filament. Approximately 25% of the filaments appear poorly stained and frayed, which may be due to collapse on the electron microscope grid. Optical diffraction studies reveal a 43-nm axial repeat as well as the 14.3-nm subunit repeat, indicating a structural homology with natural filaments. A model for synthetic filament aggregation is presented that is consistent with observations of backbone diameter variation, absence of bare zones, and the presence of fraying filaments.
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PMID:Structural studies of synthetic filaments prepared from column-purified myosin. 26 30

The various manifestations of platelet activation are derived from a reorganization of components of the contractile and microtubular systems. The controversial initial stages of excitation-contraction coupling in platelets lead to the release of calcium from the dense tubular system, the morphological counterpart of the muscle sarcotubular closed vesicles. Calcium triggers the actin-myosin interaction and the developing force, possibly together with a local increase of the cation concentration, may cause the collapse of the microtubular ring and its reappearance in the forming long pseudopodia. Actin-myosin interaction is modulated by several factors among which tropomyosin-troponin, responsible for the calcium-sensitivity of contractile processes, and phosphorylation of one of the myosin light chains. Platelet actin is anchored to the membrane and its sliding towards the short myosin filaments may form the basis for platelet shape change. Platelet alpha-actinin and actin-binding protein are able to aggregate actin into an impressive gel. Therefore, the contractile proteins seem to have a double role in controlling the consistency of the cytoplasmic gel on the one hand, and the contractile manifestations related to motility on the other hand. One of the most important features of the 'contracted' platelet is the rigidity of the pseudopodia brought about by the 'gelification' of actin filaments and the presence of microtubules. A new model for clot contraction is proposed, based on the rigidity of the long spiky pseudopodia and on the motile properties of platelets. While migrating towards each other, the interlocking pseudopodia from different platelets adhere to the polymerizing fibrin, compressing the fibrin nets in their pathway. Since the anchoring of contractile fibers to membranes is crucial for the platelet contractile manifestations, the integrity of the membrane structure should be considered in the study of pathological aspects of platelet function.
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PMID:The contractile system of blood platelets and its function. 36 20

Global cytoskeleton dynamics is likely to exist in animal cells and some experimental evidence for this has recently been obtained in cells from the human lymphoblastic cell line KE37. We have further investigated the dramatic and reversible microtubule-dependent cell elongation which occurs upon treatment of KE37 cells with cytochalasin D. This phenomenon results in a non-locomotory cell with definite polarity. It involves a sustained equatorial myosin II-dependent contraction of cortical, most of the myosin II being accumulated on segments of the main cellular extension. We report here that such a cell lengthening is energy-dependent and can be inhibited, or suppressed, by surface ligands such as wheat germ agglutinin but not by concanavalin A. Suppression of the cytochalasin D effect by wheat germ agglutinin is rapid and appears to be collapse of the cell extension and relocalization of the contracted actomyosin as a whole. It suggests that the binding of the wheat germ agglutinin to the cell surface results in the transient disassembly of microtubules, a possibility also raised by the potent antagonist effect of taxol on wheat germ agglutinin action. Taken together, the data are consistent with a specific role of microtubules in the control of the activity of the cortical actomyosin system.
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PMID:The cortical actomyosin system of cytochalasin D-treated lymphoblasts. 189 39

Metabolic fatigue is a characteristic muscle response to intense exercise that has outstripped the rate of ATP replacement. The accumulation of metabolic by-products, namely hydrogen ions and diprotonated phosphate, interferes with actin-myosin interaction, effectively preserving muscle ATP levels by preventing further ATP hydrolysis. Muscle force and metabolite concentrations return to normal in about 5 minutes. Less intense exercise causes a more subtle, non-metabolic fatigue due to a still-undefined disturbance of excitation-contraction coupling, which can last for several hours. In this type of fatigue, greater effort is required to generate submaximal forces. Endurance exercise is mainly limited by the size of muscle glycogen stores and how efficiently they are used. Endurance training permits an athlete to work aerobically at high rates, consuming a mixture of lipid and carbohydrate fuels. When muscle glycogen is used up, exercise can only continue at the relatively low rate supportable by lipid metabolism. Anaerobic exercise is also limited by subjective factors such as dyspnoea and muscle pain, which have objective determinants. Extremely prolonged exercise can lead to general collapse because of dehydration, hyperthermia, or hypoglycaemia. None of these factors explains the phenomenon of asthenia, a subjective sense of exhaustion that produces no objective impairment of physical performance. The metabolic myopathies are experiments of nature that promise to shed new light on the biochemical basis of muscle fatigue. This will require quantitative studies of the kind provided by topical magnetic resonance spectroscopy, correlating physiology and metabolism in vivo.
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PMID:Muscle metabolism during fatigue and work. 226 24

Twenty-three cases of an arterial disease that affects competition cyclists are reported. Patients complained of intermittent acute claudication appearing on one lower limb only at the time of a maximal strain while cycling. Doppler hemodynamic investigation on an ergometric bicycle revealed a collapse of the ankle systolic pressure. Arteriography showed a sinuous lengthening and moderate stenosis of the external iliac artery. Pathologic examination of the artery disclosed a stenotic intimal thickening due to moderately cellular loose connective tissue with a variable distribution of collagen and elastic fibers. The cells in the affected zone were readily labeled with anti-actin and anti-myosin antibodies, and electron microscopy revealed features of synthetic smooth muscle cells. The lesion observed differs from intimal fibrodysplasia and from artherosclerosis. Abnormal local hemodynamic conditions may lead to this type of lesion. Thus, stenotic intimal thickening of the external iliac artery appears to be a new arterial disease defined by clinical, arteriographic, and pathologic features.
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PMID:Stenotic intimal thickening of the external iliac artery in competition cyclists. 233 32

The phorbol ester TPA induces the sequential disassembly of myofibrils. First the alpha-actin thin filaments are disrupted and then, hours later, the myosin heavy chain (MHC) thick filaments. TPA does not induce the disassembly of the beta- and gamma-actin thin filaments of stress fibers in presumptive myoblasts or fibroblasts, nor does it block the reemergence of stress fibers in 72-h myosacs that have been depleted of all myofibrillar molecules. There are differences in where, when, and how myofibrillar alpha-actin and MHC are degraded and eliminated from TPA-myosacs. Though the anisodiametric myotubes have begun to retract into isodiametric myosacs after 5 h in TPA, staining with anti-MHC reveals normal tandem A bands. In contrast, staining with mAb to muscle actin fails to reveal tandem I bands. Instead, both mAb to muscle actin and rhophalloidin brilliantly stain numerous disk-like bodies approximately 3.0 micron in diameter. These muscle actin bodies do not fuse with one another, nor do they costain with anti-MHC. All muscle actin bodies and/or molecules disappear in 36-h myosacs. The collapse of A bands is first initiated in 10-h myosacs. Their loss correlates with the appearance of immense, amorphous MHC patches. MHC patches range from a few micrometers to over 60 micron in size. They do not costain with antimuscle actin or rho-phalloidin. While diminishing in number and fluorescence intensity, MHC aggregates are present in 30% of the 72-h myosacs. Myosacs removed from TPA rapidly elongate, and after 48 h display normal newly assembled myofibrils. TPA reversibly blocks incorporation of [35S]methionine into myofibrillar alpha-actin, MHC, myosin light chains 1 and 2, the tropomyosins, and troponin C. It does not block the synthesis of beta- or gamma-actins, the nonmyofibrillar MHC or light chains, tubulin, vimentin, desmin, or most household molecules.
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PMID:Sequential disassembly of myofibrils induced by myristate acetate in cultured myotubes. 365 56

The role of calcium in neurulation in mammalian embryos has been studied by culturing rat embryos at 10.4 days of gestation, when the cephalic neural folds have elevated but not fused, in serum containing cytoskeletal inhibitors or calcium antagonists. The effects of these antagonists on the morphology of the cephalic neural folds have been examined by scanning electron microscopy. The different agents caused the cephalic neural folds to part to varying degrees. The neural folds were classified as intact (normal), open (folds parted up to 90 degrees with each other), flattened (folds parted from 90 degrees to 180 degrees) or collapsed (folds parted more than 180 degrees). The microtubule inhibitors colchicine and nocodazole at 10(-4) M respectively cause the cephalic neural folds of 10.4-day embryos to collapse after 60 min. At 5.2 X 10(-6)M the microfilament inhibitor cytochalasin B causes the folds to open after 60 min. Longer term culture of 9.5-day embryos for 24 h in diazepam, which is reported to inhibit myosin synthesis, causes general developmental retardation including a delay in the closure of the neural tube. Culture of 10.4-day rat embryos for 60 min in papaverine at 2.4 X 10(-4) M or gallopamil (D-600) at 5.0 X 10(-4) M, agents which reduce the entry of calcium into cells, causes opening of the elevated cephalic neural folds. In contrast TMB-8, which is purported to perturb some intracellular calcium-dependent functions, does not cause opening of the elevated cephalic neural folds, even at high concentrations. The results suggest that both microtubules and microfilaments are essential to the maintenance of the elevated cephalic neural folds in rat embryos. The results are also compatible with the idea that calcium ion flux across the membranes of the neuroepithelial cells might be important for the elevation of the neural folds, and thus for successful neurulation.
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PMID:Calcium and neurulation in mammalian embryos. II. Effects of cytoskeletal inhibitors and calcium antagonists on the neural folds of rat embryos. 373 82

A highly branched filament network is the principal structure in the periphery of detergent-extracted cytoskeletons of macrophages that have been spread on a surface and either freeze or critical point dried, and then rotary shadowed with platinum-carbon. This array of filaments completely fills lamellae extended from the cell and bifurcates to form 0.2-0.5 micron thick layers on the top and bottom of the cell body. Reaction of the macrophage cytoskeletons with anti-actin IgG and with anti-IgG bound to colloidal gold produces dense staining of these filaments, and incubation with myosin subfragment 1 uniformly decorates these filaments, identifying them as actin. 45% of the total cellular actin and approximately 70% of actin-binding protein remains in the detergent-insoluble cell residue. The soluble actin is not filamentous as determined by sedimentation analysis, the DNAase I inhibition assay, and electron microscopy, indicating that the cytoskeleton is not fragmented by detergent extraction. The spacing between the ramifications of the actin network is 94 +/- 47 nm and 118 +/- 72 nm in cytoskeletons prepared for electron microscopy by freeze drying and critical point drying, respectively. Free filament ends are rare, except for a few which project upward from the body of the network or which extend down to the substrate. Filaments of the network intersect predominantly at right angles to form either T-shaped and X-shaped overlaps having striking perpendicularity or else Y-shaped intersections composed of filaments intersecting at 120-130 degrees angles. The actin filament concentration in the lamellae is high, with an average value of 12.5 mg/ml. The concentration was much more uniform in freeze-dried preparations than in critical point-dried specimens, indicating that there is less collapse associated with the freezing technique. The orthogonal actin network of the macrophage cortical cytoplasm resembles actin gels made with actin-binding protein. Reaction of cell cytoskeletons and of an actin gel made with actin-binding protein with anti-actin-binding protein IgG and anti-IgG-coated gold beads resulted in the deposition of clusters of gold at points where filaments intersect and at the ends of filaments that may have been in contact with the membrane before its removal with detergent. In the actin gel made with actin-binding protein, 75% of actin-fiber intersections labeled, and the filament spacing between intersections is consistent with that predicted on theoretical grounds if each added actin-binding protein molecule cross-links two filaments to form an intersection in the gel.(ABSTRACT TRUNCATED AT 400 WORDS)
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PMID:The architecture of actin filaments and the ultrastructural location of actin-binding protein in the periphery of lung macrophages. 374 63

A monoclonal antibody specific to bovine cardiac titin has been identified. The antibody recognizes a common antigenic site in striated muscles of several species. In relaxed myofibrils, specific staining at the A-I junction resulted in a doublet of fluorescent bands within a sarcomere. The distance between the doublets in successive sarcomeres varied according to the degree of myofibrillar contraction. Staining on formamide-extracted myofibrils has confirmed that this epitope is located near the outer edges of isolated A bands. Selective extraction of myofibrillar proteins resulted in different staining patterns. Disrupting the structural integrity of the M-line or the A-band centre caused a significant amount of titin to translocate toward the Z-line region. In contrast, shortening of the A-band by removal of myosin from the ends of the thick filaments resulted in anti-titin staining moving closer to the M-line region. Several conclusions can be drawn from this study: (a) two aligned groups of titin molecules are placed symmetrically to the M-line in a sarcomere; (b) titin may attach directly or via intermediary protein(s) to sites near the M-line and Z-line such that the protein is under tension and (c) removal of proteins from either region results in titin staining in the opposite region. However, the edges of the A-band give some hindrance to collapse of the titin toward the M-line.
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PMID:Immunocytochemical studies using a monoclonal antibody to bovine cardiac titin on intact and extracted myofibrils. 390 57

Actin-rich cortical cytoplasm of phagocytic leucocytes forms pseudopodia and controls cell shape and movement by generating directional propulsive and contractile forces. Proteins purified from leucocytes form and deform an actin matrix. Actin-binding protein (ABP) cross-links actin filaments into a three-dimensional lattice with perpendicular branches. This structure, which can be visualized in the electron microscope, is consistent with physical properties of actin-ABP matrices. Gelsolin binds one end of actin filaments with high affinity in the presence of calcium; acumentin, another protein, constitutively binds the other end with low affinity. Together these proteins can control actin filament length and thereby regulate expansion (propulsion) or collapse of the actin network. The assembly state of the network also controls myosin-based contractile forces. A tug-of-war decides the direction of lattice movement, regions of lesser structure tending to move toward regions of greater structure.
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PMID:The structure of cortical cytoplasm. 612 62


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