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PubMed1989

[Quantitative ultrastructural characteristics of mitochondria and myofibrils in the myocardium in chronic hemodynamic overload].

Stereological morphometry was used to determine ultrastructural changes of the myocardium in the course of well defined chronic hemodynamic overload. Overload was induced in the hearts of rabbits by perforation of aortic valves. Relative volumes of mitochondria, myofibrils were measured in tissue samples taken from subendocardial and subepicardial region of the left ventricle. In the course of developing hypertrophy relative volume of mitochondria significantly increased as well as volume ratio Mi/Mf, while those of myofibrils decreased. Increased volume of mitochondria prolonged during the period of developed hypertrophy and its normalization occurred in the hypertrophy regression. Failure of the heart was characterized by increased volume of mitochondria and decrease those of myofibrils. Comparison of given morphometric data in subendocardium and subepicardium of the left ventricle did not find regional differences in subcellular composition of cardiomyocytes in the course of chronic hemodynamic overload.

Animals↗
PubMedDate not supplied

[Interrelated effects of ATP and ionic strength on Ca2+-sensitivity of isolated myofibrils].

Ability of isolated myofibrils for relaxation was studied in a medium with KCl concentrations of 6 to 250 mM with ATP content varying from 0.01 to 1.0 mM. It was shown that myofibrils can relax at all the studied values of the ionic strength, but ATP concentration required for the relaxation is to increase with decreasing the ionic strength A stable state of Ca2+-sensitive system of myofibrils is maintained at approximately the same values of ATP and KCl concentrations. A possible mechanism of this phenomenon is discussed.

Adenosine Triphosphate↗
PubMed1979

X-ray microanalytical studies on native myofibrils and mitochondria isolated by microdissection from honey-bee flight muscle.

The distribution of elements was studied by means of energy dispersive X ray microanalysis in the honey-bee flight muscle. Semi-quantitative analysis of bulk muscle specimen, isolated mitochondria and isolated myofibrils either in resting or stretched state was performed. High K peak was observed in the spectra of bulk muscle specimen resembling the spectra of myofibrils and high P peak resembling the spectra of mitochondria. In the myofibrils no substantial difference was found between the spectra of A bands and Z lines, but they contained much higher K peak than the spectra of I bands.

Animals↗
PubMed1984

Myofibrils and transverse tubules in glycerol-treated frog skeletal muscle.

Effects of glycerol (400 mM) on the structure of frog skeletal muscle were reported based on measurements of the diameters of myofibrils and transverse tubules (T-tubules). The diameter of the myofibrils decreased to about 0.58 micron from the normal size of 0.79 micron during glycerol treatment, but the spacing between the myofibrils became wider. The size did not recover to the initial value even after return of the muscle to the Ringer solution. T-tubule diameters increased to 35-55 nm from the normal size of about 30 nm in glycerol-treated muscle, and the size was maintained after return of the muscle to the Ringer solution.

Animals↗
PubMed1995

The inhibitory effect of troponin I on the ATPase activity of myofibrils treated with troponin T and troponin T1.

Troponin in bullfrog skeletal myofibrils was substituted by troponin T and troponin T1 from rabbit skeletal muscle. The inhibitory effect of troponin I was examined on the ATPase activity of these troponin-substituted myofibrillar preparations. The ATPase of myofibrils substituted by troponin T was more inhibited by troponin I than the ATPase of myofibrils substituted by troponin T1.

Adenosine Triphosphatases↗
PubMed1983

Gamma actin, spectrin, and intermediate filament proteins colocalize with vinculin at costameres, myofibril-to-sarcolemma attachment sites.

Localization of vinculin at the sarcolemma of striated muscle fibers defines an orthogonal lattice. The costameres of the lattice are the riblike bands of vinculin that run perpendicular to the long axis of the fiber, repeat in register with I bands of the subjacent myofibrils, and seem to couple the myofibril to the sarcolemma [Pardo et al 1982, 1983a]. The colocalization studies presented in this paper show that gamma actin, spectrin, and intermediate filament antigens are additional components of this lattice of costameres. In addition, the results show that gamma actin and spectrin are also components of the internal network of collars, first visualized with antibody to desmin [Granger and Lazarides, 1978], that connects the myofibrils to each other at the level of the Z line.

Actins↗
PubMed1989

Manifestation of the stripes of minor proteins location in A-bands of rabbit cardiac myofibrils.

Cardiac myofibrils were isolated from rabbit ventricular muscle by a method that preserves well the integrity of the A-band structure. For the first time electron microscopic observations using the negative staining method revealed, in cardiac A-bands, a full complement of pronounced transverse stripes which indicate the locations of minor proteins in skeletal muscles. The manifestation of some transverse stripes in the cardiac A-band was shown to depend on the duration of muscle incubation in a Ca2(+)-depleting and ATP-free solution before its homogenization into myofibrils. The clear visibility of fine structural details in electron micrographs allowed us to resolve morphological features specific for cardiac muscle at both the central and end parts of the A-bands. The myofibrils demonstrated here are expected to be useful for elucidating the fine structure of cardiac thick filaments and in particular the locations of minor proteins.

Animals↗
PubMed2005

Disruption in the tropomodulin1 (Tmod1) gene compromises cardiomyocyte development in murine embryonic stem cells by arresting myofibril maturation.

Tropomodulins (Tmods) comprise a family of capping proteins for actin filament pointed ends. To decipher the significance of Tmod1 functions during de novo myofibrillogenesis, we generated Tmod1 null embryonic stem (ES) cells and studied their differentiation into cardiomyocytes. Strikingly, in vitro cardiomyocyte differentiation of wild type (WT) ES cells faithfully recapitulates in vivo cardiomyocyte differentiation, allowing us to evaluate the phenotypes of Tmod1 knockout (KO) myofibrils irrespective of embryonic lethality of Tmod1 KO mice. Immunofluorescence and electron microscopy studies revealed that Tmod1 null cardiac myocytes were round, morphologically immature, and contained underdeveloped myofibrils that were shorter, narrower, and had fewer thin filaments than those in WT cells. Unexpectedly, clear gaps in the staining pattern for F-actin at the H-zone were detected in most KO cells, indicating the presence of filaments at uniform lengths. This indicates that additional mechanisms other than capping proteins are responsible for thin filament length maintenance in cardiac myocytes. Also unexpectedly, approximately 40% of the KO cardiac myocytes exhibited contractile activity. Our data indicate that differentiating ES cells are a powerful system to investigate the functional properties of contractile proteins and that Tmod1 functions are critical for late stages of myofibrillogenesis, and for the maturation of myofibrils.

Animals↗
PubMed1995

Microscopic analysis of the elastic properties of nebulin in skeletal myofibrils.

The elastic properties of nebulin were studied by measuring the elasticity of single skeletal myofibrils, from which the portion of the thin filament located at the I band had been selectively removed by treatment with plasma gelsolin under rigor conditions. In this myofibril model, a portion of each nebulin molecule at the I band was expected to be free of actin filaments and exposed. The length of the exposed portion of the nebulin molecule was controlled by performing the gelsolin treatment at various sarcomere lengths. The relation between the passive tension and extension of the exposed portion of the nebulin showed a convex curve starting from a slack length, apparently in a fashion similar to that of wool. The slack sarcomere length shifted depending on the length of the exposed portion of the nebulin, however, the relation being represented by a single master curve. The elastic modulus of nebulin was estimated to be two to three orders of magnitude smaller than that of an actin filament. Based on these results, we conclude that nebulin attaches to an actin filament in a side-by-side fashion and that it does not significantly contribute to the elastic modulus of thin filaments. The relation between the passive tension and extension of connectin (titin) was obtained for a myofibril from which thin filaments had been completely removed with gelsolin under contracting conditions; this showed a concave curve, consistent with the previous results obtained in single fibers.

Actins↗
PubMed1999

Exchange of alpha-actinin in isolated rigor myofibrils.

In the current study, the process of alpha-actinin binding to the myofibrillar Z-line was investigated to determine its mechanism. Pretreatment of rigor myofibrils with unlabeled alpha-actinin did not prevent or slow the incorporation of fluorescein skeletal alpha-actinin into myofibrils suggesting that incorporation was not the filling of empty binding sites but rather an exchange reaction. Further support for this was obtained using quantitative measures of labeled alpha-actinin incorporation and measures of total myofibrillar alpha-actinin. These results showed that there was no change in myofibrillar alpha-actinin content when up to 15% of the total alpha-actinin was the labeled protein. Measurement of the time-course of fluorescein alpha-actinin incorporation by quantitative fluorescence microscopy showed that the increase in Z-line fluorescence was well described by a rapid (unresolved) incorporation of fluorescence followed by a much slower phase. The slower phase was independent of fluorescein alpha-actinin concentration (2.5-160 nM) and had an apparent rate of 0.008-0.016 min(-1). Pretreatment of myofibrils with fluorescein alpha-actinin followed by incubation with unlabeled alpha-actinin resulted in a decrease in Z-line fluorescence with an apparent rate of 0.021 min(-1). The slow phase was interpreted as representing the dissociation rate of intrinsic Z-line alpha-actinin. Thus, the dissociation rate for the in situ interaction of alpha-actinin with actin appears to be three orders of magnitude slower than that determined from solution studies.

Actinin↗
PubMed1994

Insulin-like growth factor I stimulates myofibril development and decreases smooth muscle alpha-actin of adult cardiomyocytes.

Adult rat cardiomyocytes in long-term culture express type 1 insulin-like growth factor (IGF) receptors. In contrast to insulin receptors, type 1 IGF receptors are up-regulated during culturing. IGF-I added to the cells at plating increased granular density and pseudopodia number per cell after 7 days. After 16 days, IGF-I-treated cells showed, as compared with controls, a dramatic increase of the number of newly built sarcomeres and were packed with myofibrils. At the same time, IGF-I suppressed the accumulation of smooth muscle alpha-actin (sm-alpha-actin) in a dose-dependent manner. Under the conditions of this in vitro system, growth hormone had no effect on cell morphology or sm-alpha-actin. sm-alpha-Actin, a nonsarcomeric isoform of actin expressed in early fetal cardiac development, reappears both during long-term culture of adult rat cardiomyocytes and during heart hypertrophy. This study shows that type 1 IGF receptors are up-regulated in adult rat cardiomyocytes in long-term culture and that IGF-I enhances myofibril development and concomitantly down-regulates sm-alpha-actin. This protein forms stress-fiber-like structures and may temporarily serve as a scaffold for the formation of new sarcomeres until myofibrils have developed throughout the cell and the scaffold is no longer needed. Our findings thus allow us to propose another hypothesis for the mechanism leading to overload heart hypertrophy.

Actins↗
PubMed1981

Morphological changes of myofibril in the carcinogenetic course of 20-methylcholanthrene-induced rhabdomyosarcoma.

In this study 20-methylcholanthrene (MC) was embedded in skeletal muscle tissue of mice. The resulting morphological changes were observed by light and electron microscope until the occurrence of rhabdomyosarcoma. The injected region of the muscle tissue in which MC was embedded went through three stages: destruction and degeneration, repair and appearance of atypical cells. The morphologic figures of the myofibrils seen in the skeletal muscle cell during destruction and degeneration were similar to nonspecific. The muscle tissue then regenerated in the same manner as seen during experimentally induced muscle tissue regeneration in both humans and animals. Abnormal mitotic figures were found 60 to 70 days after the injection of MC. Immature muscle cells, with tuft-formed myofibrils present, underwent abnormal mitosis in which one of the nuclei divided. The rhabdomyosarcoma cells produced by this abnormal mitosis began to appear in the lesion 70 days after the initial embedding of MC. The rhabdomyosarcoma cells in the induced tumor formed undifferentiated myofibrils in various degrees of maturation, and were therefore considered to play a part in the abnormal myofibrillar formation.

Animals↗
PubMed1996

Myosin heavy chain isoforms regulate muscle function but not myofibril assembly.

Myosin heavy chain (MHC) is the motor protein of muscle thick filaments. Most organisms produce many muscle MHC isoforms with temporally and spatially regulated expression patterns. This suggests that isoforms of MHC have different characteristics necessary for defining specific muscle properties. The single Drosophila muscle Mhc gene yields various isoforms as a result of alternative RNA splicing. To determine whether this multiplicity of MHC isoforms is critical to myofibril assembly and function, we introduced a gene encoding only an embryonic MHC into Drosophila melanogaster. The embryonic transgene acts in a dominant antimorphic manner to disrupt flight muscle function. The transgene was genetically crossed into an MHC null background. Unexpectedly, transformed flies expressing only the embryonic isoform are viable. Adult muscles containing embryonic MHC assemble normally, indicating that the isoform of MHC does not determine the dramatic ultrastructural variation among different muscle types. However, transformed flies are flightless and show reduced jumping and mating ability. Their indirect flight muscle myofibrils progressively deteriorate. Our data show that the proper MHC isoform is critical for specialized muscle function and myofibril stability.

Animals↗
PubMed1998

Effects of thiol protease inhibitors on myoblast fusion and myofibril assembly in vitro.

To investigate the roles of thiol proteases such as cathepsins and calpains in muscle differentiation, we have treated primary cultures of pectoralis muscle with a variety of protease inhibitors and examined the effects these agents have on myoblast fusion and myofibrillogenesis. We have found that a membrane-permeable inhibitor, E64D, has dramatic effects on both events of muscle differentiation. Cells treated with this inhibitor display gross morphological changes, severe delays in myofibril assembly, and reduced ability to fuse in culture. These morphological changes are correlated with a build up of beta1-integrin throughout the cytoplasm. These effects could also be produced using NH4Cl, a lysosomotrophic agent. In addition, we show that two nonpermeable inhibitors (leupeptin and E64) slightly decrease myoblast fusion, but have no effects on the ability of the cells to form mature myofibrils. These results are discussed in terms of their relevance to the inheritable disease of muscular dystrophy and I-cell disease (mucolipodosis II).

Animals↗
PubMed1997

Evidence for myosin-binding phosphatase in heart myofibrils.

Protein phosphatase was partially purified from myofibrils of bovine heart by sequential column chromatographies. The purified protein phosphatase was immunologically identified as a delta isoform of PP1 (PP1delta). The myosin-binding subunit (MBS) of myosin-binding phosphatase (MBP) in smooth muscle was co-purified with PP1delta at each step of the sequential column chromatographies. The immunoprecipitation experiment using the polyclonal antibody to MBS showed that PP1delta associates with MBS in the purified phosphatase. In addition, the myosin-binding assay showed that the purified phosphatase has the characteristics of binding to cardiac myosin. These data strongly suggest that MBP, the holoenzyme composed of PP1delta and MBS, is expressed in heart myofibrils.

Animals↗
PubMed2001

Specific interaction of the potassium channel beta-subunit minK with the sarcomeric protein T-cap suggests a T-tubule-myofibril linking system.

Ion-channel beta-subunits are ancillary proteins that co-assemble with alpha-subunits to modulate gating kinetics and enhance stability of multimeric channel complexes. They provide binding sites for other regulatory proteins and are medically important as the targets of many pharmacological compounds. MinK is the beta-subunit of the slow activating component of the delayed rectifier potassium current (I(Ks)) channel, and associates with the alpha-subunit, KvLQT1. We report here that minK specifically interacts with the sarcomeric Z-line component, T-cap (also called telethonin). In vitro interaction studies indicated that the cytoplasmic domain of minK specifically binds to the sixteen C-terminal residues of T-cap; these residues are sufficient for its interaction with minK. Consistent with our in vitro studies, immunofluorescence staining followed by confocal analysis revealed that both minK and T-cap are localized within the Z-line region in cardiac muscle. Striated staining of minK was observed in non-washed, membrane-intact cardiac myofibrils, but not in well-washed, membrane-removed cardiac myofibrils, suggesting that minK localizes on T-tubular membranes surrounding the Z-line in the inner ventricular myocardium. Together with our previous data on the colocalization and interaction of T-cap with the N-terminus of the giant protein titin in the periphery of the Z-line, these data suggest that T-cap functions as an adapter protein to link together myofibrillar components with the membranous beta-subunit of the I(Ks) channel. We speculate that this interaction may contribute to a stretch-dependent regulation of potassium flux in cardiac muscle, providing a "mechano-electrical feedback" system.

Amino Acid Sequence↗
PubMed2003

Molecular organizations of myofibrils of skeletal muscle studied by atomic force microscopy.

By applying AFM technology, we studied mechanical characteristics of myofibrils of skeletal muscle. The obtained results indicate that (1) the Z-band is the most rigid sarcomere component stabilizing the myofibril structures, (2) various filamentous components are inter-connected in sarcomere with sufficient mechanical strength to support the contractile force, and (3) the molecular structure of the overlap region between actin and myosin filaments is anisotropic. In any case the present studies clearly indicate that the AFM technique is a powerful tool to investigate the mechanical characteristics of sarcomere structure of muscle fiber.

Actins↗
PubMed2001

Mg2+-dependent ATPase activity in cardiac myofibrils from the insulin-resistant JCR:LA-cp rat.

There is a great deal of information presently available documenting a cardiomyopathic condition in insulin-deficient models of diabetes. Less information is available documenting a similar status in non insulin-dependent models of diabetes. We have studied the functional integrity of the myofibrils isolated from hearts of JCR:LA rats. The JCR:LA rat is hyperinsulinemic, hyperlipidemic, glucose intolerant and obese. As such, it carries many of the characteristics found in humans with non insulin-dependent diabetes mellitus. These animals also have many indications of heart disease. However, it is not clear if the hearts suffer from vascular complications or are cardiomyopathic in nature. We examined Mg2+-dependent myofibrillar ATPase in hearts of JCR:LA-cp/cp rats and their corresponding control animals (+/?) and found no significant differences (P> 0.05). This is in striking contrast to the depression in this activity exhibited by cardiac myofibrils isolated from insulin-deficient models of diabetes. Our data demonstrate that myofibrillar functional integrity is normal in JCR:LA-cp rats and suggest that these hearts are not in a cardiomyopathic state. Insulin status may be critical in generating a cardiomyopathic condition in diabetes.

Animals↗