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PubMed1975

Donnan potential of rabbit skeletal muscle myofibrils I: electrofluorochromometric detection of potential.

The fluorescence of the dye CC-6 [(3-hexyl-2-(3-hexyl-2-benzoxazolinylidene)-1-propenyl)-benzoxazolium iodide] has been shown to indicate Donnan potentials in rabbit skeletal muscle myofibrils. These results are in agreement with previously published work in which the potentials were measured with microelectrodes on glycerol-extraced muscle fibers. The magnitude of the Donnan potential of the myofibrils has been shown to be dependent on the state (rigor or relaxed) of the system.

Adenosine Triphosphate↗
PubMed1992

A sarcomeric alpha-actinin truncated at the carboxyl end induces the breakdown of stress fibers in PtK2 cells and the formation of nemaline-like bodies and breakdown of myofibrils in myotubes.

In many nonmuscle cells, nonsarcomeric alpha-actinin is distributed in the dense bodies of stress fibers, adhesion plaques, and adherens junctions. In striated muscle, a sarcomeric isoform of alpha-actinin (s-alpha-actinin) is found in the Z-bands of myofibrils and subsarcolemmal adhesion plaques. To understand the role(s) of the alpha-actinin isoforms in the assembly and maintenance of such cytoskeletal structures, full-length or truncated s-alpha-actinin cDNAs were expressed in PtK2 cells and in primary skeletal myogenic cells. We found the following. (i) In transfected PtK2 cells the truncated s-alpha-actinin was rapidly incorporated into preexisting dense bodies, adhesion plaques, and adherens junctions. With time these structures collapsed, and the affected cells detached from the substrate. (ii) In myotubes the truncated s-alpha-actinin was incorporated into nascent Z-bands. Many of these progressively hypertrophied, forming nemaline-like bodies. With time the affected myofibrils fragmented, and the myotubes detached from the substrate. (iii) In both cell types the truncated s-alpha-actinin was significantly more disruptive of the cytoskeletal structures than the full-length molecule. (iv) Pools of "over-expressed" full-length or truncated protein did not self-aggregate into homogeneous, amorphous complexes; rather the exogenous proteins selectively colocalized with the same cohort of cytoskeletal proteins with which the endogenous alpha-actinin normally associates. The similarity among the hypertrophied Z-bands in transfected myotubes, the nemaline bodies in patients with nemaline myopathies, and the streaming Z-bands seen in various muscle pathologies raises the possibility that the genetically determined nemaline bodies and the pathologically induced Z-band alterations may reflect primary and/or post-translational modifications of s-alpha-actinin.

Actinin↗
PubMed1999

Localization of the human 64kD autoantigen D1 to myofibrils in a subset of extraocular muscle fibers.

PURPOSE: To evaluate the tissue-specific expression pattern of the 64kD human autoantigen D1, a tropomodulin-related protein that may be involved in thyroid-associated ophthalmopathy. METHODS: Recombinant 64kD human autoantigen D1 was generated in a bacterial expression system and used to immunize rabbits. Specific antibodies were affinity-purified and used for Western blots on normal and hyperthyroid rat and rabbit tissue, and immunofluorescence localization on cryosections of rat tissue. RESULTS: Anti-64kD human autoantigen D1 antibodies recognize specifically a approximately 70kD polypeptide in western blots of extraocular muscle, sternothyroid muscle, and smooth muscle. Immunofluorescence staining demonstrates that the 64kD human autoantigen D1 localizes to myofibrils in slow fibers from rat extraocular and sternothyroid muscle. The level of this protein is not altered in extraocular muscles from hyperthyroid rabbits. CONCLUSIONS: The 64kD human autoantigen D1 is expressed in slow fibers of extraocular and sternothyroid muscles as a component of myofibrils, and is not upregulated in conditions of hyperthyroidism.

Actins↗
PubMed1997

Myofibrils of skeletal muscle: the activity coefficient of orthophosphate.

In the myofibrils of skeletal muscle, at 22 degrees C, pH 7.1 and at the physiological protein osmotic pressure of 1.8 x 10(5) dynes/cm2, orthophosphate behaves quite ideally, the activity coefficient being 0.85. Under the same conditions and at saturation, 2.67 mumoles of orthophosphate are bound per gram of dry myofibrils, with a dissociation constant of 7 x 10(-5) molal. Work is in progress to determine the activity coefficients of adenine nucleotide analogues. This work is needed to assess the actual value of the free energy of hydrolysis of ATP in muscle.

Adenosine Diphosphate↗
PubMed1987

Sequential disassembly of myofibrils induced by myristate acetate in cultured myotubes.

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.

Actins↗
PubMed1961

The distribution of muscle antigens in contracted myofibrils determined by fluorescein-labeled antibodies.

Chick myofibrils in different states of contraction were treated with fluorescein-labeled antibodies. The rabbit antibodies were prepared against chick myosin, light and heavy meromyosins, and actin. For any one state of contraction, a single myofibril was photographed through the phase contrast microscope, stained with one of the antisera, and photographed through the fluorescence microscope. The cytological changes in the sarcomeres accompanying contraction as observed under phase were correlated with changes in the distribution of the precipitated antibodies as observed under the fluorescence microscope. The changing patterns observed through the fluorescence microscope were compared with those predicted by the sliding filament model of contraction.

Actins↗
PubMed1990

Regulation of binding of subfragment 1 in isolated rigor myofibrils.

A steric-hindrance model has been used to explain the regulation of muscle contraction by tropomyosin-troponin complex. The regulation of binding was studied by microscopic observation of mixtures of fluorescent subfragment 1 (S1) with rigor myofibrils at different actin-to-S1 ratios and in the presence and absence of calcium. Procedures were adapted to protect the critical thiols of S1 before conjugation to thiol-specific fluorochromes, this giving fluorescent S1 with unaltered enzyme activity. S1 binding was greatest in the I band (except at the Z-lines) in the presence of calcium regardless of the [S1]. The patterns in the absence of calcium depended on the actin-to-S1 ratios: low [S1], binding in the myosin-actin overlap region; intermediate [S1], highest binding at the A-I junction; high [S1], greatest binding in the I-band. The two distinct binding patterns observed at low [S1] were demonstrated by dual-channel fluorescence microscopy when myofibrils were sequentially incubated with fluorescent S1 without calcium followed by a different fluorescent S1 with calcium. These observations support the concept of rigor activation of actin sites. The change in the pattern upon increasing [S1] without calcium demonstrate cooperative interactions along the thin filament. However, these interactions (under the conditions used without calcium) do not appear to extend over greater than 2-3 tropomyosin-troponin-7 actin functional units.

Abdominal Muscles↗
PubMed1963

Electron microscopic studies on the indirect flight muscles of Drosophila melanogaster. I. Structure of the myofibrils.

The myofibrils in Drosophila have thick and thin types of myofilaments arranged in the hexagonal pattern described for Calliphora by Huxley and Hanson (15). The thick filaments, along most of their length in the A band, seem to be binary in structure, consisting of a dense cortex and a lighter medulla. In the H zone, however, they show more uniform density; lateral projections (bridges) also appear to be absent in this region. The M band has a varying number of granules (probably of glycogen) distributed between the myofilaments. The myofilaments on reaching the Z region appear to change their hexagonal arrangement and become connected to one another by Z filaments. The regular arrangement of the filaments found in most regions of the fibrils is not seen in the terminal sarcomeres of some flight muscles; the two types of filaments appear to be intermingled in an irregular pattern in these parts of the fibrils. The attachment of myofibrils to the cuticle through the epidermal cells is described.

Actin Cytoskeleton↗
PubMed1963

Electron microscopic studies on the indirect flight muscles of Drosophila melanogaster. II. Differentiation of myofibrils.

The differentiation of the indirect flight muscles was studied in the various pupal stages of Drosophila. Fibrillar material originates in the young basophilic myoblasts in the form of short myofilamants distributed irregularly near the cell membranes. The filaments later become grouped into bundles (fibrils). Certain "Z bodies" appear to be important during this process. The "Z bodies" may possibly be centriolar derivatives and are the precursors of the Z bands. The first formed fibrils (having about 30 thick myofilaments) are already divided into sarcomeres by Z bands. These sarcomeres, however, seem to be shorter than those of the adult fibrils.The H band differentiates in fibrils having about 40 thick myofilaments; the fibrils constrict in the middle of each sarcomere during this process. The individual myofibrils increase from about 0.3 micro to 1.5 micro in diameter during development, apparently by addition of new filaments on the periphery of the fibrils. The ribosomes seem to be the only cytoplasmic inclusions which are closely associated with these growing myofibrils. Disintegration of the plasma membranes limiting individual myoblasts was commonly seen during development of flight muscles, supporting the view that the multinuclear condition of the fibers of these muscles is due to fusion of myoblasts.

Actin Cytoskeleton↗
PubMed1967

An electron microscope study of myofibril formation in embryonic chick skeletal muscle.

The formation of myofibrils in the developing leg muscle of the 12-day chick embryo was studied by electron microscopy. Myofilaments of two varieties, thick (160-170 A in diameter) and thin (60-70 A in diameter), which have been designated myosin and actin filaments, respectively, on the basis of their similarity to natural and synthetic myosin and actin filaments, appear in the cytoplasm of developing muscle cells. There is a greater than 7:1 ratio of thin to thick filaments in these young myofibers. The free myofilaments become aligned in the long axis of the cells, predominantly in subsarcolemmal locations, and aggregate into hexagonally packed arrays of filaments. The presence of Z band material or M band cross-bridges do not appear to be essential for the formation or spacing of these aggregates of filaments. Formation of the Z band lattices occurs coincidentally with the back-to-back apposition of thin filaments. An hypothesis concerning myofibril growth, based on the self-assembly characteristics of the filaments, is presented.

Animals↗
PubMed1967

Morphology of rigor--shortened bovine muscle and the effect of trypsin on pre- and postrigor myofibrils.

Bovine semitendinosus muscles were sampled immediately after death, after 24 hr postmortem with storage at 2 degrees , 16 degrees , or 37 degrees C, and after 312 hr postmortem with storage at 2 degrees and 16 degrees C. A biopsy technique was used to prevent shortening during glutaraldehyde fixation. Postfixation in osmium tetroxide was followed by embedding in an Epon-Araldite mixture. Bovine muscle was supercontracted after 24 hr storage at 27deg; but was only slightly contracted after storage at 16 degrees for 24 hr. Muscle held at 37 degrees for 24 hr was slightly less supercontracted than the 2 degrees muscle. Striking similarities existed between muscles stored at 16 degrees and at 2 degrees C for 312 hr. Both were slightly shortened with narrowed I bands and an area of increased density, probably due to overlap of thin filaments in the middle of the A band. Postmortem shortening was accompanied by banding-pattern changes similar to those predicted for contracting muscle by Huxley and Hanson's sliding filament model. Treatment of myofibrils with 0.05% trypsin resulted in a rapid loss of Z lines and, in supercontracted myofibrils, caused a return of the banding pattern of resting muscle.

Animals↗
PubMed1971

Intrinsic birefringence of glycerinated myofibrils.

Patterns of intrinsic birefringence were revealed in formalin-fixed, glycerinated myofibrils from rabbit striated muscle, by perfusing them with solvents of refractive index near to that of protein, about 1.570. The patterns differ substantially from those obtained in physiological salt solutions, due to the elimination of edge- and form birefringence. Analysis of myofibrils at various stages of shortening has produced results fully consistent with the sliding filament theory of contraction. On a weight basis, the intrinsic birefringence of thick-filament protein is about 2.4 times that of thin-filament protein. Nonadditivity of thick- and thin-filament birefringence in the overlap regions of A bands may indicate an alteration of macromolecular structure due to interaction between the two types of filaments.

Actins↗
PubMed1981

Hybrids of Physarum myosin light chains and desensitized scallop myofibrils.

The two light chains of Physarum myosin have been purified in a 1:1 ratio with a yield of 0.5-1 mg/100 g of plasmodium and a purity of 40-70%; the major contaminant is a 42,000-dalton protein. The 17,700 Mr Physarum myosin light chain (PhLC1) binds to scallop myofibrils, providing the regulatory light chains (ScRLC) have been removed. The 16,500 Mr light (PhLC2) does not bind to scallop myofibrils. The calcium control of scallop myosin ATPase is lost by the removal of one of the two ScRLC's and restored equally well by the binding of either PhLC1 or rabbit skeletal myosin light chains. When both ScRLC's are removed, replacement by two plasmodial light chains does not restore calcium control as platelet or scallop light chains do. Purified plasmodial actomyosin does not bind calcium in 10(-6) M free calcium, 1 mM MgCl2. No tropomyosin was isolated from Physarum by standard methods. Because the Physarum myosin light chains can substitute only partially for light chains from myosin linked systems, because calcium does not bind to the actomyosin, and because tropomyosin is apparently absent, the regulation of plasmodial actomyosin by micromolar Ca++ may involve other mechanisms, possibly phosphorylation.

Actomyosin↗
PubMed1983

Vinculin is a component of an extensive network of myofibril-sarcolemma attachment regions in cardiac muscle fibers.

Immunofluorescent staining of bovine and avian cardiac tissue with affinity-purified antibody to chicken gizzard vinculin reveals two new sites of vinculin reactivity. First, vinculin is organized at the sarcolemma in a striking array of rib-like bands, or costameres. The costameres encircle the cardiac muscle cell perpendicular to the long axis of the fiber and overlie the I bands of the immediately subjacent sarcomeres. The second new site of vinculin reactivity is found in bovine cardiocytes at tubular invaginations of the plasma membrane. The frequency and location of these invaginations correspond to the known frequency and distribution of the transverse tubular system in bovine atrial, ventricular, and Purkinje fibers. We do not detect tubular invaginations that stain with antivinculin in avian cardiocytes and, in fact, a transverse tubular system has not been found in avian cardiac fibers. Apparent lateral Z-line attachments to the sarcolemma and its invaginations have been observed in cardiac muscle by electron microscopy in the same regions where we find vinculin. On the basis of these previous ultrastructural findings and our published evidence for a physical connection between costameres and the underlying myofibrils in skeletal muscle, we interpret the immunofluorescence data of this study to mean that, in cardiac muscle, vinculin is a component of an extensive system of lateral attachment of myofibrils to the plasma membrane and its invaginations.

Animals↗
PubMed1955

Phase and electron microscope studies of the interrelationship of cytochondria and myofibrils in pigeon breast muscle.

MITOCHONDRIA IN PIGEON BREAST MUSCLE ARE COMPOSED OF TWO PROTEIN GELS: a fibrous gel, in the form of a folded ribbon, enclosed within a non-fibrous matrix. An external limiting "membrane" is not demonstrated, and there is poor demarcation between the mitochondria and adjacent structures or sarcoplasm. No internal structure has been determined for sarcosomes. These structures, however, are symplasmic with mitochondria and usually are located within mitochondrial depressions. They apparently have a high lipid content. The myofibrils also have no external limiting "membranes" and the mitochondria and sarcosomes are symplasmic with them. The mitochondria normally lie in the intermyofibrillar sarcoplasm oriented with their longitudinal axes parallel to the myofibrils. Cross-sections show that the myofilaments are oriented into well defined sheets.

Animals↗
PubMed1951

Observations by electron microscopy on contraction of skeletal myofibrils induced with adenosinetriphosphate.

Skeletal myofibrils isolated either by tryptic digestion at 0 degrees C. or by a colloid mill and suspended in buffer solution (pH 7.0, micro; 0.154) containing 20 per cent glycerin and 0.0025 M adenosinetriphosphate at -5 degrees C. contracted slowly and progressively when the temperature was raised above 0 degrees C. Formalin fixation halted this contraction. With the aid of these procedures myofibrils in progressive stages of contraction were then studied with the electron microscope. Electron micrographs showed that uncontracted fibrils isolated by the colloid mill were structurally similar to those described by other workers. Treatment of fibrils with trypsin removed the Z bands and disorganized the I bands. This enzymatic modification of structure did not impair the contractile response. The principal structural changes during contraction consisted of a migration of dense material from the A band into the A-I junction or the Z band, a gradual increase in width of the fibril, a gradual decrease in length of sarcomeres, an apparent increase in the mean diameter of filaments, and a disorientation of these latter from their parallel arrangement.

Adenosine Triphosphate↗
PubMed1965

The location of muscle calcium with respect to the myofibrils.

Autoradiographs have been prepared from frog toe muscles soaked in Ca(45) and fixed with an osmium-oxalate solution. A majority of the grains over the A bands were over the myofibrils. The grain density over the I bands was greatest over the space between the myofibrils. The significance of this distribution is discussed in the light of previous information about the longitudinal distribution of Ca(45) in skeletal muscle.

Animals↗
PubMed1991

Heat shock gene activation by mutant actin is independent of myofibril degeneration in Drosophila muscle.

Artificially mutagenized Drosophila Act88F actin genes with triple and double mutations were expressed in the indirect flight muscles of transgenic flies. The triple mutant actin, GD245T (Gly-36----Glu, Glu-83----Asp, and Gly-245----Asp), induced heat shock protein (hsp) synthesis without affecting flight ability. On the other hand, the double mutation, GD245D (Gly-36----Glu and Glu-83----Asp), disrupted myofibrils but induced little hsp synthesis. These results demonstrate that myofibril degeneration is not the primary cause of the anomalous heat shock gene activation by mutant actins.

Actins↗