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At least 1,135 records · Page 63Linked to original sources
PubMed1999

Vertebrate isoforms of actin capping protein beta have distinct functions In vivo.

Actin capping protein (CP) binds barbed ends of actin filaments to regulate actin assembly. CP is an alpha/beta heterodimer. Vertebrates have conserved isoforms of each subunit. Muscle cells contain two beta isoforms. beta1 is at the Z-line; beta2 is at the intercalated disc and cell periphery in general. To investigate the functions of the isoforms, we replaced one isoform with another using expression in hearts of transgenic mice. Mice expressing beta2 had a severe phenotype with juvenile lethality. Myofibril architecture was severely disrupted. The beta2 did not localize to the Z-line. Therefore, beta1 has a distinct function that includes interactions at the Z-line. Mice expressing beta1 showed altered morphology of the intercalated disc, without the lethality or myofibril disruption of the beta2-expressing mice. The in vivo function of CP is presumed to involve binding barbed ends of actin filaments. To test this hypothesis, we expressed a beta1 mutant that poorly binds actin. These mice showed both myofibril disruption and intercalated disc remodeling, as predicted. Therefore, CPbeta1 and CPbeta2 each have a distinct function that cannot be provided by the other isoform. CPbeta1 attaches actin filaments to the Z-line, and CPbeta2 organizes the actin at the intercalated discs.

Actin Capping Proteins↗
PubMed1956

The sarcoplasmic reticulum in muscle cells of Amblystoma larvae.

Electron microscopy of thin sections of muscle fibers in myotomes of Amblystoma larvae has revealed the presence of a complex, membrane-limited system of canaliculi and vesicles which form a lace-like reticulum around and among the myofibrils. This seems to correspond to the sarcoplasmic reticulum of the earlier light microscopists and the endoplasmic reticulum of other cell types. The elements constituting the reticulum are disposed in a pattern which bears a constant relation to the bands of the adjacent myofibrils and is therefore repeated in each sarcomere. At the H band the system is transversely continuous but not so at other levels. Longitudinally continuity is interrupted at the Z bands where large vesicles belonging to adjacent sarcomere segments of the system face off on opposite sides of the band. The opposing faces of these vesicles are flat and separated by a space of more or less constant width, in which are located small, finger-shaped vesicles. In view of these and other close structural relationships with the myofibrils it seems appropriate to assign to the system a role in the conduction of the excitatory impulse.

Ambystoma↗
PubMed1957

Studies on the endoplasmic reticulum. III. Its form and distribution in striated muscle cells.

Several types of striated muscle have been examined by the technics of electron microscopy and the findings in myotome fibers of Amblystoma larvae, the sartorius, and cardiac muscle of the rat are reported on in some detail. Particular attention has been given to structural components of the interfibrillar sarcoplasm and most especially to a finely divided, vacuolar system known as the sarcoplasmic reticulum. This consists of membrane-limited vesicles, tubules, and cisternae associated in a continuous reticular structure which forms lace-like sleeves around the myofibrils. It shows a definable organization which repeats with each sarcomere of the fiber so that the entire system is segmented in phase with the striations of the associated myofibrils. Details of these repetitive patterns are presented diagrammatically in Text-figs. 1, 2, and 3 on pages 279, 283, and 288 respectively. The system is continuous across the fiber at the H band level and largely discontinuous longitudinally because of interruptions in the structure at the I and Z band levels. The structure of the system relates it to the endoplasmic reticulum of other cell types. The precise morphological relation of the reticulum to the myofibrils, with specializations opposite the different bands, prompts the supposition that the system is functionally important in muscle contraction. In this regard it is proposed that the membrane limiting the system is polarized like the sarcolemma and that the corresponding potential difference is utilized in the intracellular distribution of the excitatory impulse.

Animals↗
PubMed1967

The sarcoplasmic reticulum and its association with the T system in an insect.

The fine structure of the sarcoplasmic reticulum and the transverse tubular system of the femoral muscle of the cockroach, Leucophaea maderae, was studied after prefixation in glutaraldehyde, postfixation in osmium tetroxide, and embedding in Epon. The sarcoplasmic reticulum in this muscle reveals features not previously reported. The sarcoplasmic reticulum is abundant, consisting mainly of a fenestrated envelope which surrounds each myofibril at all levels in the sarcomere. This sarcoplasmic reticulum envelope is continuous transversally as well as longitudinally along the myofibrils. Dyadic junctions are formed by a single T system element which contacts the unfenestrated sarcoplasmic reticulum of adjacent myofibrils in an alternating manner at the ends of the A band. At the dyads, regularly spaced thickenings of the sarcoplasmic reticulum membranes bordering the dyadic spaces are noted. These thickenings, however, do not contact the T tubule membrane. Typical dyadic contacts also are seen between the cell surface membrane and sarcoplasmic reticulum. Z line-like material is seen in contact with the membranes of the cell surface and longitudinal branches of the T systems.

Cell Membrane↗
PubMed1969

Sarcoplasmic reticulum of an unusually fast-acting crustacean muscle.

The fast-acting, synchronous "remotor" muscle of the lobster second antenna was examined by light and electron microscopy and was found to have a more profuse sarcoplasmic reticulum (SR) than any other muscle known. Myofibrils are widely separated from one another and occupy only about one-fourth of the volume of the muscle; most of the remaining volume is taken up by the SR, which resembles the smooth-surfaced reticulum of steroid-secreting cells. Dense granules (0.03-0.1 micro in diameter) are scattered through the reticulum. T-tubules penetrate into the fibers and form dyads along the A bands of myofibrils; however, ferritin-labeling experiments show that the volume of the T-system is very small compared with that of the SR. Myofibrils are approximately 0.5 micro x 1.0 micro in cross section and consist of thick filaments, which appear tubular except at the M region, and thin filaments, which are situated midway between neighboring thick filaments. The ratio of thin to thick filaments is 3:1. The extreme development of the SR in this muscle is discussed in relation to the exceedingly short duration of the contraction-relaxation cycle.

Animals↗
PubMed1976

A Ca2+-activated protease possibly involved in myofibrillar protein turnover. Subcellular localization of the protease in porcine skeletal muscle.

A study was done to determine whether the Ca2+-activated muscle protease (CAF) that removes Z disks from myofibrils in the presence of Ca2+ is located in a sedimentable subcellular organelle. Porcine skeletal muscle cells were diced finely with a scalpel and were suspended in 0.25 M sucrose, 4 mM EDTA with a VIRTIS homogenizer. Filtration of the suspended muscle through four layers of cheesecloth removed most of the myofibrils and stromal protein. Nuclear (1,000 gavg for 15 min), mitochondrial-microsomal (50,000 gavg for 60 min), and supernatant fractions were assayed for succinic dehydrogenase, acid ribonuclease, cathepsin D, and CAF activities. Approximately 96% of total succinic dehydrogenase activity, 81% of cathepsin D activity, and 45% of acid ribonuclease activity, but only 14% of total CAF activity, were found in the nuclear and mitochondrial-microsomal fractions. Cathepsin D activity in the nuclear and mitochondrial-microsomal fractions was decreased if assays were done without prior treatment to rupture membranous structures; hence, our cell rupture and homogenization procedures preserved some intact lysosomal organelles. The results indicate that the small amount of CAF activity in the nuclear and mitochondrial-microsomal fractions was due to contamination by supernate and that CAF is not located in a membrane-bounded subcellular particle. Because CAF is active at the intracellular pH and temperature of living skeletal muscle cells and is in direct contact with the cytoplasm of muscle cells, its activity must be regulated by intracellular cellular Ca2+ concentration to prevent continuous and indiscriminate degradation of myofibrils.

Animals↗
PubMed1979

Immunofluorescent and histochemical localization of AMP deaminase in skeletal muscle.

Fluorescent antibody staining experiments with both isolated myofibrils and muscle fibers grown in culture show that AMP deaminase is bound to the myofibril in the A band. The strongest staining occurs at each end of the A band. The approximate width of the fluorescent stripes and their relation to the A band remains constant as a function of sarcomere length. Removal of enzyme from the myofibrils leads to loss of staining, and readdition of purified enzyme restores the original staining pattern. A histoenzymatic method for the detection of AMP deaminase activity in cultured fibers gives comparable localization. The results are consistent with the previous observation (Ashby, B. and C. Frieden. 1977.J. Biol. Chem. 252:1869--1872) that AMP deaminase forms a tight complex in solution with subfragment-2 (S-2) of myosin or with heavy meromyosin (HMM).

AMP Deaminase↗
PubMed1983

Pattern and time course of rhodamine-actin incorporation in cardiac myocytes.

Microinjection of skeletal actin labeled with rhodamine into cultured cardiac myocytes was followed by rapid incorporation of fluorescence into myofibrils of the cells. Myocytes examined as shortly as 5 min postinjection displayed fluorescent bands corresponding to the sarcomeres. By 10 min, distinct alternating wide and narrow bands of fluorescence were observed. The wide bands appeared to correspond to the full breadth of the I-bands, whereas the narrow bands of fluorescence corresponded to the M-lines. This pattern of fluorescence remained essentially unchanged for at least 15 h postinjection. The myofibrils of cardiac myocytes were functional after rhodamine-actin incorporation as judged by their ability to contract. The results of this study suggest that cardiac myofibrils are morphologically stable structures which, nonetheless, exhibit extensive exchange of actin subunits.

Actins↗
PubMed1954

Electron microscope studies on ultrathin sections of muscle.

Thin sections of striated muscle from frog (sartorius), rabbit (psoas), rat (heart), and fly (flight muscle), and of smooth muscle from clam (adductor) have been obtained using a new microtome. Electron micrographs of them are presented. The sections are sufficiently thin to achieve a resolution of 30 to 40 A. In fly flight muscle, the myofibrils are distinct and well separated morphological units. In frog and rabbit muscle the myofibrils appear to be so closely packed under normal conditions that their identity as separate units is almost lost. In all types of striated muscle examined, it was found that the filaments are arranged within the myofibrils in a continuous and highly regular hexagonal array. The diameter of the filaments in embedded, sectioned muscle appears to be significantly less than that observed in dried shadowed material. The 400 A axial period, observed in frog and rabbit muscle, and in rat heart muscle, was found to extend across the interstitial material between the filaments. The significance of these findings is discussed.

Animals↗
PubMed1967

Evidence from insect fibrillar muscle about the elementary contractile process.

Bundles of myofibrils prepared from the dorsal longitudinal flight muscles of giant water bugs show oscillatory contractile activity in solutions of low ionic strength containing ATP and 10(-8)-10(-7)M Ca(2+). This is due to delay between changes of length and changes of tension under activating conditions. The peculiarities of insect fibrillar muscle which give rise to this behavior are (1) the high elasticity of relaxed myofibrils, (2) a smaller degree of Ca(2+) activation of ATPase activity in unstretched myofibrils and extracted actomyosin, and (3) a direct effect of stretch on ATPase activity. It is shown that the cross-bridges of striated muscle are probably formed from the heads of three myosin molecules and that in insect fibrillar muscle the cycles of mechanochemical energy conversion in the cross-bridges can be synchronized by imposed changes of length. This material is more suitable than vertebrate striated muscle for a study of the nature of the elementary contractile process.

Adenosine Triphosphatases↗
PubMed1968

The site of calcium binding in relation to the activation of myofibrillar contraction.

Skeletal muscle myofibrils, in the presence of 2 mM MgCl(2) at pH 7.0, were found to have two classes of calcium-binding sites with apparent affinity constants of 2.1 x 10(6)M(-1) (class 1) and approximately 3 x 10(4)M(-1) (class 2), respectively. At free calcium concentrations essential for the activation of myofibrillar contraction ( approximately 10(-6)M) there would be significant calcium binding only to the class 1 sites. These sites could bind about 1.3 micromoles of calcium per g protein. Extraction of myosin from the myofibrils did not alter their calcium-binding parameters. Myosin A, under identical experimental conditions, had little affinity for calcium. The class 1 sites are, therefore, presumed to be located in the I filaments. The class 1 sites could only be detected in F actin and myosin B preparations which were contaminated with the tropomyosin-troponin complex. Tropomyosin bound very little calcium. Troponin, which in conjunction with tropomyosin confers calcium sensitivity on actomyosin systems, could bind 22 micromoles of calcium per g protein with an apparent affinity constant of 2.4 x 10(6)M(-1). In view of the identical affinity constants of the myofibrils and troponin and the much greater number of calcium-binding sites on troponin it is suggested that calcium activates myofibrillar contraction by binding to the troponin molecule.

Animals↗
PubMed1973

Graded activation in frog muscle fibers.

The membrane potential of frog single muscle fibers in solutions containing tetrodotoxin was controlled with a two-electrode voltage clamp. Local contractions elicited by 100-ms square steps of depolarization were observed microscopically and recorded on cinefilm. The absence of myofibrillar folding with shortening to striation spacings below 1.95 microm served as a criterion for activation of the entire fiber cross section. With depolarizing steps of increasing magnitude, shortening occurred first in the most superficial myofibrils and spread inward to involve axial myofibrils as the depolarization was increased. In contractions in which the entire fiber cross section shortened actively, both the extent of shortening and the velocity of shortening at a given striation spacing could be graded by varying the magnitude of the depolarization step. The results provide evidence that the degree of activation of individual myofibrils can be graded with membrane depolarization.

Animals↗
PubMed2004

Multiple sources of passive stress relaxation in muscle fibres.

The forces developed during stretch of nonactivated muscle consist of velocity-sensitive (viscous/viscoelastic) and velocity-insensitive (elastic) components. At the myofibrillar level, the elastic-force component has been described in terms of the entropic-spring properties of the giant protein titin, but entropic elasticity cannot account for viscoelastic properties, such as stress relaxation. Here we examine the contribution of titin to passive stress relaxation of isolated rat-cardiac myofibrils depleted of actin by gelsolin treatment. Monte Carlo simulations show that, up to approximately 5 s after a stretch, the time course of stress relaxation can be described assuming unfolding of 1-2 immunoglobulin domains per titin molecule. For extended periods of stress relaxation, the simulations failed to correctly describe the myofibril data, suggesting that in situ, titin-Ig domains may be more stable than predicted in earlier single-molecule atomic-force-microscopy studies. The reasons behind this finding remain unknown; simply assuming a reduced unfolding probability of domains--an effect found here by AFM force spectroscopy on titin-Ig domains in the presence of a chaperone, alpha-B-crystallin--did not help correctly simulate the time course of stress relaxation. We conclude that myofibrillar stress relaxation likely has multiple sources. Evidence is provided that in intact myofibrils, an initial, rapid phase of stress relaxation results from viscous resistance due to the presence of actin filaments.

Actins↗
PubMed1998

Mapping of a myosin-binding domain and a regulatory phosphorylation site in M-protein, a structural protein of the sarcomeric M band.

The myofibrils of cross-striated muscle fibers contain in their M bands cytoskeletal proteins whose main function seems to be the stabilization of the three-dimensional arrangement of thick filaments. We identified two immunoglobin domains (Mp2-Mp3) of M-protein as a site binding to the central region of light meromyosin. This binding is regulated in vitro by phosphorylation of a single serine residue (Ser76) in the immediately adjacent amino-terminal domain Mp1. M-protein phosphorylation by cAMP-dependent kinase A inhibits binding to myosin LMM. Transient transfection studies of cultured cells revealed that the myosin-binding site seems involved in the targeting of M-protein to its location in the myofibril. Using the same method, a second myofibril-binding site was uncovered in domains Mp9-Mp13. These results support the view that specific phosphorylation events could be also important for the control of sarcomeric M band formation and remodeling.

Amino Acid Sequence↗
PubMed1981

Production of cardiac muscle abnormalities in offspring of rats receiving triiodothyroacetic acid (triac) and the effect of beta adrenergic blockade.

As a part of a continuing study on the effects of thyroid hormones on heart muscle, triiodothyroacetic acid (triac), either alone or concurrently with propranolol, has been administered to rats during pregnancy. Control groups received either buffer or propranolol. Offspring, which were given no further treatment, were killed at intervals after birth and their hearts examined histologically, histochemically, and electron microscopically. At 2, 6, and 14 days, offspring of triac-treated rats showed cardiac hypertrophy and, at ultrastructural level, marked disarray of the myofibrils was present. By 28 days, arrangement of the myofibrils had become regular but hypertrophy persisted and was still found in rats examined at 56 days of age, after which time the myocardium was normal. Offspring of rats which had received propranolol at the same time as triac showed a similar pattern of hypertrophy but myofibrillar disarray was not found. Propranolol alone produced no abnormalities. These findings provide further evidence that thyroid hormone analogues can adversely affect heart muscle. When considered in conjunction with previous experiments which showed that thyroxine or triac cause severe hypertrophy but not disarray when given directly to growing rats, they suggest that thyroid hormones can produce a spectrum of abnormalities, thought to depend on the stage of myocardial development at which the stimulus is administered. In the present experiment, the triac-induced myofibrillar disarray but not the hypertrophy was prevented by propranolol, indicating that beta-adrenergic blockade or some other action of propranolol protects the developing myofibrils. Possible mechanisms for the adverse effects of thyroid hormones and the protective action of propranolol are discussed.

Animals↗
PubMed1979

Quantitative morphological analysis of age-related changes in flight muscle of Musca domestica L.

Electron micrographs of dorsal indirect flight muscle of 37 (adult) and 68 (old) day old female houseflies were analyzed by morphometric techniques. In the muscle of the old female housefly there is no significant atrophy and the fine structure is well preserved. There is a small loss in both the myofibril and mitochondrial volumes, an increase in average myofibril size, and a 28% decrease in myofibril number. The thin/thick filament ratio and sarcomere structure are not altered. There is little decrease of particulate glycogen. The fractional volume occupied by SR at the dyads is increased. The most marked change is in the density of mitochondrial cristae which declines by 55% in old muscle and could account for the functional loss. Cristal anomalies and SR changes indicate that the membranes may be the specific target of deleterious aging effects.

Aging↗
PubMed2003

Localization of calpain 3 in human skeletal muscle and its alteration in limb-girdle muscular dystrophy 2A muscle.

Calpain 3/p94, the skeletal muscle-specific isoform of the calpain large subunit family, is a protein product of the gene responsible for limb-girdle muscular dystrophy type 2A (LGMD2A). Through yeast two-hybrid experiments, calpain 3 has been shown to bind to titin in myofibrils [Sorimachi et al. (1995) J. Biol. Chem. 270, 31158-31162]. However, because of extensive autolysis activity, calpain 3 localization in skeletal muscle has been undefined. In this study, we generated a polyclonal antibody against an N-terminal 98-amino-acid calpain 3 fragment, which is not homologous to the corresponding regions of other conventional calpains. This antibody stained myofibrils with a unique repeated doublet-pattern. Confocal microscopic observation with marker antibodies confirmed that calpain 3 is localized in the N2 region of myofibrils. Furthermore, using this antibody, we examined the localization of calpain 3 in LGMD2A muscles.

Calpain↗
PubMed1983

Ultrastructure of the human atrioventricular conduction tissues.

The ultrastructure of the human atrioventricular conduction tissue has been studied by obtaining material from recipient hearts at transplant operations. The hearts were dissected immediately after surgical removal in order to expose the conduction system, and tissue samples were taken directly from the atrioventricular node, the penetrating bundle, the branching bundle, and both bundle branches. Examination with the electron microscope showed that the entire atrioventricular system throughout its length was composed of a spectrum of cells which ranged widely in size and in myofibril content from slim cells resembling cardiac muscle and packed with myofibrils to wide 'empty' cells containing relatively few myofibrils. The cells were polymorphic, and many branched with the branches varying greatly in width. Transverse junctions between cells or between their branches were made by intercalated discs. Lateral connections between cells were extremely rare; they were made by desmosomes only. Nerves were present throughout the axis. The striking features of the atrioventricular conduction system as a whole were firstly that the constituent cells were so widely heterogeneous as to defy any classification into cell types, and secondly that totally dissimilar cells established direct continuity by means of intercalated discs.

Adult↗