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PubMed1980

The incorporation of radioactive lysine or tyrosine into cardiac and skeletal myofibrillar and non-myofibrillar contractile proteins.

The labelled amino acids incorporation into cardiac and skeletal contractile proteins has been compared after a single injection of 3H lysine, repeated injections of 3H lysine during 1 or 6 hours or after a continuous infusion of both 3H-lysine and 14C-tyrosine. The myofibrillar incorporation was higher in the heart than in the skeletal muscle. Myosin heavy chains and actin have been prepared using gel filtration. The incorporation was again higher in the heart for both these proteins but the labelling of actin in both the muscles reaches rapidly a plateau in contrast with myosin, suggesting that these two proteins possess a different precursor pool. Myosin heavy chains prepared from the supernatant obtained after a relaxing treatment were more labelled than those extracted from myofibrils. These heavy chains from the supernatant were presumably newly synthetized and not yet incorporated into myofibrils. They also were more labelled in the heart than in the skeletal muscle which means that the myosin synthesis itself was different and not the process of assembly of the myofibrils.

Actins↗

PubMed1980

[Ultrastructural stereologic study of cardiac muscle cells during myocardial atrophy].

Wistar rats were exposed to complete starvation over 2, 5 and 10 days. In the course of the experiment the animals developed general atrophy associated with myocardial atrophy. They showed also diminution of the absolute weight of the heart and the diameter of cardiomyocytes. The volumetric and surface density of cell structure were detected stereologically by electron microscopy to evaluate the volumetric and surface-volumetric ratio of the main organellas. It was demonstrated that the volumetric density of mitochondria diminishes in myocardial atrophy while the relative amount of myofibrils increases. The surface density of mitochondria is maintained at the same level as that seen in the control and declines by the end of the experiment. The myocardial atrophy was stereologically ascertained by the volumetric ratio of mitochondria to myofibrils and by that of the T system to myofibrils. This ratio becomes the lesser the higher the degree of atrophy.

Animals↗

PubMed1980

Disproportionate accumulation of myosin and tropomyosin in cultured muscle cells.

Accumulation of two major myofibrillar proteins, myosin and tropomyosin, was monitored in differentiating skeletal muscle cultures. The tropomyosin subunit to myosin heavy chain accumulation rate ratio was more than twice the stoichiometric ratio of tropomyosin subunit to myosin heavy chain in mature skeletal muscle myofibrils and crude myofibrils from cultured muscle cells. Electron microscopy revealed normal patterns of myofibril assembly in these muscle cultures. Therefore, the observed disproportionate accumulation of tropomyosin and myosin heavy chain may be reflecting normal cellular conditions for de novo myofbril assembly.

Animals↗

PubMed1993

RNA from normal anterior endoderm/mesoderm-conditioned medium stimulates myofibrillogenesis in developing mutant axolotl hearts.

In the axolotl, Ambystoma mexicanum, a recessive cardiac lethal mutation causes an incomplete differentiation of the myocardium. Mutant hearts do not contain sarcomeric myofibrils nor do they beat. We have previously shown that normal anterior endoderm, medium conditioned by endoderm, or total RNA extracted from endoderm stimulates differentiation of mutant hearts in culture as indicated by the presence of organized myofibrils and rhythmic contractions of the "rescued" mutant heart tube. In this study, to get a more highly purified sample of the "active" molecule, RNA extracted from endoderm-conditioned medium and was assayed for its ability to promote myofibrillogenesis in mutant hearts. Mutant heart mesoderm responded to conditioned-medium RNA in a dose-dependent manner. Proteinase K treatment of the RNA did not affect inductive activity, while digestion with RNase A completely abolished the ability to rescue mutant hearts. Confocal laser scanning microscopy of immunostained, organ-cultured hearts revealed that mutant hearts contain reduced amounts of the sarcomeric protein tropomyosin in an amorphous distribution, whereas normal and corrected mutant hearts contain tropomyosin primarily in organized myofibrils.

Ambystoma↗

PubMed1993

Preparation of three troponin components from puffer skeletal muscle.

The Ca(2+)-regulatory proteins, i. e., troponin C, I and T, were prepared from puffer skeletal muscle. The SDS-gel electrophoretic study indicated that the molecular weights of troponin C, I and T were 18, 20 and 30k daltons, respectively. The Ca(2+)-sensitivity of the ATPase activity of the puffer myofibrils was abolished by the removal of troponin C by the CDTA-treatment and then restored by the reconstitution with puffer skeletal troponin C. The treatment with excess troponin T of either puffer or rabbit skeletal muscle at slightly acidic condition resulted in the removal of troponins C and I from the puffer myofibrils. The reconstituted puffer troponin C-I-T gave the Ca(2+)-sensitivity to the ATPase activity of puffer desensitized myofibrils in the presence tropomyosin.

Adenosine Triphosphatases↗

PubMed1995

Ultrastructure of voluntary muscle in childhood malnutrition.

This paper is part of a study on the electron microscopy of protein-energy malnutrition, using a rapid autopsy protocol. Samples of voluntary muscle, obtained from eight children dying of severe oedematous malnutrition, were fixed in glutaraldehyde within 75 minutes of death. Atrophy of myofibres, increased prominence of satellite cells, and segmental necrobiosis were seen by light microscopy. Electron microscopy showed variable depletion of myofibrils. In the most severe case, there was focal absence of myofibrils, also disorganized Z lines, and absent M bands. Residual atrophic myofibrils measured less than 0.1 micron in width. Other specimens showed sarcomere disorganization, mitochondrial swelling, glycogen depletion, sarcoplasmic oedema, and focal contractions of sarcomeres. Though non-specific, rigor may be accelerated by free radical damage, calcium release into the cytosol, and low supplies of high-energy phosphates. These conditions may exist in severe malnutrition, complicated by terminal infection and metabolic disturbances.

Female↗

PubMed1988

Physiological factors influencing the growth of skeletal muscle.

The growth of muscle can be regulated by developmental changes or by alterations in hormone levels or in the rate or amount of work demanded. The mechanisms and structures involved in growth processes can be studied by controlling these factors. The models used are chicken anterior latissimus dorsi (ALD) muscle under the influence of overloading and rabbit tibialis anterior (TA) muscle under the influence of chronic nerve stimulation. Both models involve changes in the isoform of myosin that is expressed. Methods of study include quantitative ultrastructural analysis, immunofluorescence and in situ mRNA hybridization. In overloaded chick ALD fibres polysomes are nonuniformly distributed between the myofibrils and in a peripheral annulus even though subcellular concentrations of the new isoform are not found. In normal rabbit muscle the highest concentration of myosin mRNA detected by in situ hybridization is found in the subsarcolemmal zone. In stimulated TA polysomes are found between myofibrils. It appears that the myosin mRNA accumulates at specific cell locations before translation; then diffusion of isomyosin and rapid exchange into myofibrils follows. Therefore, regulation of growth may be possible at the transcriptional, translational and assembly stages.

Animals↗

PubMed1977

Electron microscopic studies on the myotomes of larval lamprey, Lampetra japonica.

Myotomes of the caudal one-third of the body of 26-day-old larval lampreys, Lampetra japonica, were studied by electron microscopy. Each myotome consists of horizontally stacked muscle lamellae. The myotomes are covered laterally by a single layer of flattened cells called here "lateral cells", and the other aspect is covered by an external lamina. The myotomes are midsegmentally innervated. Each muscle lamella usually contains two single cortical layers of myofibrils along the dorsal and ventral sarcolemma with a nucleus and mitochondria interposed between two layers. Numerous peripheral couplings are observed with relatively less developed triads. There are no membrane specializations to connect adjacent muscle lamellae within a myotome. Intermyotomal junctions are, however, noted between tips of cytoplasmic processes of muscle lamellae of adjoining myotomes. They resemble tight or gap junctions. No myofibrils are present in these cytoplasmic processes. Myotendinous junctions, with "terminal couplings" (Nakao, '75), are seen under development at the myoseptal ends of muscle lamellae. Lateral cells contain only ordinary organelles and no special structures such as myofibrils are found in the cytoplasm. They are connected to each other and to muscle lamellae by primitive desmosomes. They generally have no external lamina investment.

Animals↗

PubMed1982

An electron microscopic study on the extraocular muscles of a lamprey, Lampetra japonica.

This paper reports an EM study on the observations of the extraocular muscles of the lamprey, Lampetra japonica. Individual muscles consisted of numerous subunits, each being surrounded by a common external lamina and containing two, S- and F-types, of muscle fibers. A subunit contained an average of 8.25 muscle fibers consisting of 5.58 S-type fibers and 2.67 F-type fibers. In a rectus anterior muscle, the total number of the S-type fiber was 852, the average cross-sectional area being 144 micrometer 2. The total number of the F-type fiber was 397, the average cross-sectional area being 300 micrometer 2. The S-type fiber was characterized by the arrangement of myofibrils as a single layer in the cortical sarcoplasm as well as by the development of peripheral couplings. The F-type fiber was characterized by the presence of central myofibrils in the juxtanuclear sarcoplasm in addition to peripheral myofibrils and by the development of the T-system and triads at the level of the Z line. M lines appeared distinct in the F-type fibers and less distinct in the S-type fibers. Muscle fibers within individual subunits were separated from each other by a space about 20 nm, but no membrane specializations such as gap junction or desmosome have been determined to exist between adjacent muscle fibers.

Animals↗

PubMed1989

Immunofluorescent localization of desmin and vimentin in developing cardiac muscle of Syrian hamster.

The distributions of desmin and vimentin were examined in frozen sections of cardiac muscle from embryonic, newborn, and adult Syrian hamster by using immunofluorescent methods. Frozen sections of newborn and adult skeletal muscle were used for comparison. Cardiac myocytes from day 9 in utero embryos already show a clear association of desmin with the sarcomeric myofibrils. In newborn hearts, desmin is localized in the myofibrillar Z-line areas as well as in the peripheral cytoplasm of the cell. Three days after birth, desmin is associated with the intercalated discs. Thus, in adult cardiac muscle, desmin is present in both Z-bands and intercalated discs. Skeletal muscle of newborn and adult hamster also contains desmin associated with the Z-lines of myofibrils. Vimentin is associated with the myofibrils of day 9 in utero cardiac muscle cells. The protein remains associated with the myofibrillar Z-lines in the newborns and adults. No detectable staining for vimentin was observed in newborn or adult hamster skeletal muscle. The existence of vimentin as well as desmin in differentiated cardiac muscle may be a consequence of the somewhat more epithelial-like nature of cardiac cells as compared to skeletal muscle syncitia.

Animals↗

PubMed2003

ALP and MLP distribution during myofibrillogenesis in cultured cardiomyocytes.

The Z-line is a multifunctional macromolecular complex that anchors sarcomeric actin filaments, mediates interactions with intermediate filaments and costameres, and recruits signaling molecules. Antiparallel alpha-actinin homodimers, present at Z-lines, cross-link overlapping actin filaments and also bind other cytoskeletal and signaling elements. Two LIM domain containing proteins, alpha-actinin associated LIM protein (ALP) and muscle LIM protein (MLP), interact with alpha-actinin, distribute in vivo to Z-lines or costameres, respectively, and, when absent, are associated with heart disease. Here we describe the behavior of ALP and MLP during myofibrillogenesis in cultured embryonic chick cardiomyocytes. As myofibrils develop, ALP and MLP are observed in distinct distribution patterns in the cell. ALP is coincident with alpha-actinin from the first stage of myofibrillogenesis and co-distributes with alpha-actinin to Z-lines and intercalated discs in mature myofibrils. Interestingly, we also demonstrate using ALP-GFP transfection experiments and an in vitro binding assay that the ALP-alpha-actinin binding interaction is not required to target ALP to the Z-line. In contrast, MLP localization is not co-incident with that of alpha-actinin until late stages of myofibrillogenesis; however, it is present in premyofibrils and nascent myofibrils prior to the incorporation of other costameric components such as vinculin, vimentin, or desmin. Our observations support the view that ALP function is required specifically at actin anchorage sites. The subcellular distribution pattern of MLP during myofibrillogenesis suggests that it functions during differentiation prior to the establishment of costameres.

Actinin↗

PubMed2003

Genomic organization, alternative splicing, and expression of human and mouse N-RAP, a nebulin-related LIM protein of striated muscle.

Linkage analysis identifies 10q24-26 as a disease locus for dilated cardiomyopathy (DCM), a region including the N-RAP gene. N-RAP is a nebulin-like LIM protein that may mediate force transmission and myofibril assembly in cardiomyocytes. We describe the sequence, genomic structure, and expression of human N-RAP, as well as an initial screen to determine whether N-RAP mutations cause cardiomyopathy. Human expressed sequence tag databases were searched with the published 3,528-bp mouse N-RAP open reading frame (ORF). Putative cDNA sequences were interrogated by direct sequencing from cardiac and skeletal muscle RNA. We identified two human N-RAP isoforms with ORFs of 5,085 bp (isoform C) and 5,190 bp (isoform S), encoding products of 193-197 kDa. Genomic database searches localize N-RAP to human chromosome 10q25.3 and match isoforms C and S to 41 and 42 exons. Only isoform C is detected in human cardiac RNA; in skeletal muscle, approximately 10% is isoform C and approximately 90% is isoform S. We investigated apparent differences between human N-RAP cDNA and mouse sequences. Two mouse N-RAP isoforms with ORFs of 5,079 and 5,184 bp were identified with approximately 85% similarity to human isoforms; published mouse sequences include cloning artifacts truncating the ORF. Murine and human isoforms have similar gene structure, tissue specificity, and size. N-RAP is especially conserved within its nebulin-like and LIM domains. We expressed both N-RAP isoforms and the previously described truncated N-RAP in embryonic chick cardiomyocytes. All constructs targeted to myofibril precursors and the cell periphery, and inhibited myofibril assembly. Several human N-RAP polymorphisms were detected, but none were unique to cardiomyopathy patients. N-RAP is highly conserved and exclusively expressed in cardiac and skeletal muscle. Genetic abnormalities remain excellent candidate causes for cardiac and skeletal myopathies.

Alternative Splicing↗

PubMed1986

Rearrangement of tubulin, actin, and myosin in cultured ventricular cardiomyocytes of the adult rat.

Antitubulin, phalloidin, and antimyosin were used to study the distribution of microtubules, microfilaments, and myofibrils in cultured adult cardiomyocytes. These cells undergo a stereotypic sequence of morphological change in which myotypic features are lost and then reconstructed during a period of polymorphic growth. Microtubules, though rearranged during these events in culture, are always present in an organized network. Myosin and actin structures, on the other hand, initially degenerate. This initial degeneration is reversed when a cell attaches to the culture substratum. Upon attachment, new microtubules are laid down as a cortical network adjacent to the sarcolemma and, subsequently, as a network in the basal part of the cell. Actin and then myosin filament bundles appear next, in a pattern corresponding to the pattern of the microtubules. Finally, striated myofibrils are formed, first in the central part of the cell, and subsequently in the outgrowing processes of the cell. A mechanism is suggested by which the eventual polymorphic shape of a cell is related to the shape of its initial area of contact with the culture substratum. Finally, a model of myofibrillogenesis is proposed in which microtubules participate in the insertion of myosin among previously formed actin filament bundles to produce myofibrils.

Actins↗

PubMed1987

Binding and distribution of fluorescently labeled filamin in permeabilized and living cells.

This study reports the first development of a fluorescently labeled filamin. Smooth muscle filamin was labeled with fluorescent dyes in order to study its interaction with stress fibers and myofibrils, both in living cells and in permeabilized cells. The labeled filamin bound to the Z bands of isolated cross-striated myofibrils and to the Z bands and intercalated discs in both permeabilized embryonic cardiac myocytes and in frozen sections of adult rat ventricle. In permeabilized embryonic chick myotubes, filamin bound to early myotubes but was absent at later stages. In living embryonic chick myotubes, the fluorescently labeled filamin was incorporated into the Z bands of myofibrils during early and late stages of development but was absent during an intermediate stage. In living cardiac myocytes, filamin-IAR was incorporated into nascent as well as fully formed sarcomeres throughout development. In permeabilized nonmuscle cells, labeled filamin bound to attachment plaques and foci of polygonal networks and to the dense bodies in stress fibers. The periodic bands of filamin in stress fibers had a longer spacing in fibroblasts than in epithelial cells. When injected into living cells, filamin was readily incorporated into stress fibers in a striated pattern. The fluorescent filamin bands were broader in injected cells, however, than they were in permeabilized cells. We have interpreted these results from living and permeabilized cells to mean that native filamin is distributed along the full length of the actin filaments in the stress fibers, with a higher concentration present in the dense bodies. A sarcomeric model is presented indicating the position of filamin with respect to other proteins in the stress fiber.

Actinin↗

PubMed1983

Isozymes of creatine kinase in mammalian cell cultures.

Previous studies on the energy metabolism of rat myocardial cells in culture supported the hypothesis that the creatine-phosphocreatine-creatine kinase system plays an important role in the intracellular transport of energy from the mitochondria to the myofibrils and in the regulation of energy production coupled to energy utilization in this model system. Effective functional compartmentation of ATP could result from the binding of creatine kinase to cellular organelles (e.g., myofibrils and mitochondria) such that high energy charge at the myofibrils is maintained by the reverse creatine kinase reaction, while phosphocreatine is synthesized mainly at the mitochondria in the forward creatine kinase reaction. It was, therefore, essential to demonstrate the presence of mitochondrial creatine kinase in the cultured myocardial cells to support this hypothesis, particularly since the mitochondrial creatine kinase was reportedly absent in fetal hearts. Using electrophoresis on cellulose acetate strips, the mitochondrial creatine kinase isozyme, as well as MM, MB, and BB isozymes, have now been demonstrated in myocardial cultures derived from neonatal rats. The mitochondrial creatine kinase increased with age in culture and with age of animal from which the culture is derived. Furthermore, the addition of creatine to culture media stimulates its synthesis. The mitochondrial creatine kinase isozyme was not detected in nonmuscle cells in culture derived from the neonatal rat hearts, nor in L6 muscle cell line. Phosphocreatine was present in all cells, but the regulation of energy metabolism and energy shuttle by creatine-phosphocreatine-creatine kinase could be operative only in the cells where the mitochondrial creatine kinase is present. This regulatory mechanism provides for an efficient system concomitant with the continuous energy demand of the myocardium; it is not ubiquitous and its development in myocardial cells seems to be triggered postnatally.

Adenosine Triphosphate↗

PubMed1990

Distribution of vinculin in the Z-disk of striated muscle: analysis by laser scanning confocal microscopy.

Vinculin is a major cytoskeletal component in striated muscle, where it has been reported to form a rib-like structure between the cell membrane and the Z-disk termed a costamere. This arrangement of vinculin has been purported to be involved in the alignment of the myofibrils. However, the three-dimensional arrangement of vinculin in relation to the Z-disk of the myofibril was not known. In the present study, we examined the distribution of vinculin in striated muscle with monospecific antibodies using immunofluorescence and laser scanning confocal microscopy. Isolated cardiac and skeletal muscle cells from a variety of species, tissue sections, and neonatal myocytes with developing myofibrils were examined. Optical sectioning in the X-Y and X-Z planes demonstrated that vinculin immunoreactivity was heaviest at the periphery of the cell; however, the immunoreactivity was also distributed within the Z-disk although at a relatively reduced level. This distribution is potentially significant in understanding the physiological significance of vinculin in striated muscle function and in myofibrillogenesis.

Actinin↗

PubMed1993

Fine structure of transverse tubules and the sarcoplasmic reticulum at the myotendinous junction of stretched muscle fibers of the rat.

The transverse (T) tubules and the sarcoplasmic reticulum (SR) at the myotendinous junction of stretched rat skeletal muscle were examined by conventional and intermediate voltage electron microscopy. Stretching induced a large cytoplasmic space devoid of myofibrils at the ends of lengthening fibers. In this space, irregularly running tubular elements were seen. They were connected both with subsarcolemmal caveolae and with T tubules traversing to the A-I junctional level of the preexisting myofibrils. The SR was arranged at regular intervals which were narrower than those of the adult sarcomere. This orderly spacing of the SR seems to indicate that they may play some role(s) in myofibril assembly and/or T tubule arrangement.

Animals↗

PubMed1975

Structural and functional heterogeneity in an insect muscle.

Singing muscles of the katydid, Neoconocephalus robustus (Insecta, Tettigoniidae) are neurogenic, yet perform at contraction-relaxation frequencies as high as 212 Hz (Josephson and Halverson, '71). The mechanical and electrical responses of different bands of one of these muscles (the dorsal longitudinal muscle, DLM) has been examined with respect to ultrastructural features of each part which may be related to muscle performance. The DLM is composed of three bands and is innervated by four motoneurones. The cell bodies of three of these motoneurones occur ipsilaterally in the prothroracic ganglion; the cell body of the other motoneurone is contralateral in the mesothoracic ganglion. Three of the motoneurones (as yet unidentified fast axons) initiate extraordinarily fast twitches (rise time equal 7.3 msec, half duration equals 14.3 msec, 25 C), the fourth (an unidentified slower axon) evokes twitches which are considerably slower (rise time equals 18.9 msec, half duration equals 5.10 msec). Whereas the ventral and medial bands of the muscle are innervated only by fast axons (some fibers of the medial band are doubly innervated), the dorsal band is innervated by both a fast axon and the slower axon. A few fibers of the dorsal band are doubly innervated. The structure of fibers from the ventral and medial bands is very similar, with short sarcomeres (4.0 and 4.3 mum, respectively) and thin strap-like myofibrils delineated by well-developed sarcoplasmic reticulum (SR). Twenty-four percent of the volume of ventral band fibers is SR and the diffusion distance from SR to the center of the adjacent myofibril averages 0.083 mum. Twenty percent of the medial band fiber volume is SR, with a diffusion distance of 0.118 mum. Ventral and medial band fibers contain about 40% mitochondria, and 33% myofibrils. The dorsal band fibers have longer sarcomeres (9.5 mum), and only 10% of the fiber volume is SR. The muscle fibrils of the dorsal band are larger and consequently the diffusion distance is greater (0.227 mum) than in the ventral and medial bands. Mitochondria comprise 23% of the volume of dorsal band fibers. Most dorsal band mitochondria are aggregated into distinct clumps. Although some dorsal band fibers are innervated by a fast axon and some by the slower axon, the dorsal band fibers are structurally homogeneous, suggesting that neurotrophic effects are not important in maintaining the structure of dorsal band fibers. The mechanical-electrical performance and ultrastructure of the ventral and medial bands suggest their roll as fast, metabolically active but weak muscles, used in singing; the dorsal band as a slower but stronger muscle, perhaps involved in postural movements of the wing during singing.

Animals↗