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PubMed2004

Sequential myofibrillar breakdown accompanies mitotic division of mammalian cardiomyocytes.

The contractile tissue of the heart is composed of individual cardiomyocytes. During mammalian embryonic development, heart growth is achieved by cell division while at the same time the heart is already exerting its essential pumping activity. There is still some debate whether the proliferative activity is carried out by a less differentiated, stem cell-like type of cardiomyocytes or whether embryonic cardiomyocytes are able to perform both of these completely different dynamic tasks, contraction and cell division. Our analysis of triple-stained specimen of cultured embryonic cardiomyocytes and of whole mount preparations of embryonic mouse hearts by confocal microscopy revealed that differentiated cardiomyocytes are indeed able to proliferate. However, to go through cell division, a disassembly of the contractile elements, the myofibrils, has to take place. This disassembly occurs in two steps with Z-disk and thin (actin)-filament-associated proteins getting disassembled before disassembly of the M-bands and the thick (myosin) filaments happens. After cytokinesis reassembly of the myofibrillar proteins to their mature cross-striated pattern can be seen. Another interesting observation was that the cell-cell contacts remain seemingly intact during division, probably reflecting the requirement of intact integration sites of the individual cells in the contractile tissue. Our results suggest that embryonic cardiomyocytes have developed an interesting strategy to deal with their major cytoskeletal elements, the myofibrils, during mitosis. The complex disassembly-reassembly process might also provide a mechanistic explanation, why cardiomyocytes cede to divide postnatally.

Actinin↗
PubMed2005

Coxsackievirus-adenovirus receptor (CAR) is essential for early embryonic cardiac development.

The coxsackievirus-adenovirus receptor (CAR) is a cell contact protein on various cell types with unknown physiological function. It belongs to a subfamily of the immunoglobulin-superfamily of which some members are junctional adhesion molecules on epithelial and/or endothelial cells. CAR is dominantly expressed in the hearts and brains of mice until the newborne phase after which it becomes mainly restricted to various epithelial cells. To understand more about the physiological function of CAR, we have generated CAR-deficient mice by gene targeting. We found that these mice die between E11.5 and E13.5 of embryonal development. Ultrastructural analysis of cardiomyocytes revealed that the density of myofibrils was reduced and that their orientation and bundling was disorganized. In addition, mitochondria were enlarged and glycogen storage strongly enriched. In line with these defects, we observed pericardial edema formation as a clear sign of insufficient heart function. Developmental abnormalities likely to be secondary effects of gene ablation were the persistent singular cardial atrio-ventricular canal and dilatations of larger blood vessels such as the cardinal veins. The secondary nature of these defects was supported by the fact that CAR was not expressed on vascular cells or on cells of the vascular wall. No obvious signs for alterations of the histological organization of the placenta were observed. We conclude that CAR is required for embryonal heart development, most likely due to its function during the organization of myofibrils in cardiomyocytes.

Animals↗
PubMed1994

Expression and localization of the phosphoglucomutase-related cytoskeletal protein, aciculin, in skeletal muscle.

Recently, a 60/63 kDa cytoskeletal protein, highly homologous to the glycolytic enzyme phosphoglucomutase (PGM 1), was isolated from smooth muscle tissue and shown to localize in various adherens-type junctions of muscle and some nonmuscle cells. Since this protein, tentatively named 'aciculin', was enriched in muscle tissues and cells, we have attempted to study its expression and localization during myodifferentiation. C2C12 mouse myoblasts did not express any aciculin before cell fusion in culture. Immediately after cell fusion aciculin became detectable and its content continued to rise during myotube maturation. In early myotubes aciculin appeared first at cell tips and was predominantly localized to focal adhesions of immature myotubes. As myotubes matured in culture, aciculin became associated with growing myofibrils, and finally was found redistributed in striations, corresponding to sarcomere Z-discs. Immunoblotting showed that aciculin content in chicken breast skeletal muscle remained very low until day 11 of embryogenesis, but significantly increased in late prenatal and early postnatal development. By immunofluorescence, aciculin was not revealed in thigh skeletal muscle of day 11 chicken embryos, but was prominently localized at myotendinous junctions in thigh muscle of day 16 embryos. Myotendinous junctions appeared to be major sites of aciculin accumulation in developing and mature skeletal muscle fibers in vivo, suggesting some role for this protein in thin filament-membrane interactions and, potentially, in force transmission at these cell-matrix contacts. In adult skeletal muscle faint aciculin staining appeared at the sarcolemma and as striations in register with Z-discs. Since the protein was not identified in glycerinated myofibrils but was localized to striations in C2C12 myotubes and within the limited areas on skeletal muscle tissue sections, we conclude that aciculin is a component of skeletal muscle costameres. In cultured C2C12 myotubes we found some codistribution of aciculin with clusters of acetylcholine receptors, suggesting its presence at neuromuscular junctions. However, we did not detect any significant concentration of aciculin at neuromuscular junctions in both embryonic and adult skeletal muscle. Taken together, our data show that aciculin expression in skeletal muscle is differentiation-dependent and upregulated during muscle development, and that this novel cytoskeletal protein is a component of various cell-matrix adherens junctions in muscle cells.

Animals↗
PubMed1990

The early expression of myofibrillar proteins in round postmitotic myoblasts of embryonic skeletal muscle.

Previous reports on skeletal muscle myogenesis have shown that postmitotic spindle-shaped myoblasts express muscle-specific proteins, some of which are organized into nascent myofibrils. However, we show that, in skeletal muscle cultures derived from 12-day chick embryos, by 6 h after plating the predominant mononucleated cell type that expresses muscle-specific proteins is a round cell. These round myoblasts appear to precede spindle-shaped myoblasts in development, since the latter are more abundant in later cultures and contain larger amounts of muscle proteins and more highly organized myofibrils. By double immunofluorescence microscopy using antibodies specific for the muscle proteins titin, myosin heavy chain (MHC) and zeugmatin we find that 18 h after plating approximately 20% of the round myoblasts that are titin-positive are negative for myofibrillar MHC and zeugmatin. On the other hand, all spindle-shaped myocytes that are positive for titin are also positive for myofibrillar MHC and zeugmatin. These results suggest that titin expression precedes that of myofibrillar MHC and zeugmatin in the non-synchronized round myoblasts, and is consistent with earlier suggestions that titin may function as an initial organizer of myofibrillar proteins during myogenesis. Immunofluorescence data indicate that the earliest localization of the myofibrillar proteins titin, MHC, zeugmatin and alpha-actinin in the round myoblasts is surrounding the nucleus with no immunofluorescent labeling of the cytoplasm or near the plasma membrane. Furthermore, pairwise double immunofluorescence experiments show that these four myofibrillar proteins are all co-localized, at the light-microscopic level of resolution, in irregular patterns that may appear in either a punctate or a basket-like distribution. These labeling patterns around the nucleus are resistant to extraction with Triton X-100, suggesting that the proteins are associated in a stable array. These Triton X-100-resistant assemblies in round myoblasts appear to be composed solely of structural myofibrillar proteins, since the non-structural myofibrillar protein creatine kinase (CK) does not colocalize with the other myofibrillar proteins. These results indicate that in early myoblasts myofibrillar proteins form stable pre-myofibrillar assemblies surrounding the nucleus, and raise the possibility that these initial assemblies may play an organizing role during subsequent early stages of myofibrillogenesis.

Animals↗
PubMed2005

Circulatory variables and the flight performance of birds.

When considering the 'burst' flight performance of birds, such as during take-off, one of the most important structural variables is the ratio of the mass of the flight muscle myofibrils with respect to body mass. However, when considering 'prolonged' flight performance the variable of interest should be the body mass ratio of the mass of the flight muscle myofibrils that can be perfused sustainably with metabolites via the blood supply. The latter variable should be related to blood flow (ml min(-1)), which in turn has been shown to be a function of heart muscle mass, the value of which is more easily obtainable for different species than that for the mass of perfused muscle. The limited empirical evidence available suggests that for birds and mammals the rate of maximum oxygen consumption scales with heart mass (Mh) as Mh 0.88 and that for birds Mh scales with body mass (Mb) as Mb 0.92, leading to the conclusion that the rate of maximum oxygen consumption in birds scales with an exponent of around Mb 0.82. A similar exponent would be expected for the rate of maximum oxygen consumption with respect to the flight muscle mass of birds. This suggests that the sustainable power output from the flight muscles may ultimately be limiting the flight performance of very large flying animals, but as a result of circulatory constraints rather than biomechanical considerations of the flight muscles per se. Under the particular circumstances of sustainable flight performance, calculations of rates of metabolic energy consumed by the flight muscles can be compared directly with the estimates of biomechanical power output required, as calculated using various aerodynamic models. The difference between these calculated values for rates of energy input and output from the muscles is equivalent to the 'apparent' mechanochemical conversion efficiency. The results of one such analysis, of the maximum sustainable flight performance of migratory birds, leads to the conclusion that the efficiency of the flight muscles appears to scale as Mb 0.14. However, much of this apparent scaling may be an artefact of the application and assumptions of the models. The resolution of this issue is only likely to come from studying bird species at either extreme of the size range.

Animals↗
PubMed1984

Differences in temperature dependence of muscle contractile properties and myofibrillar ATPase activity in a cold-temperature fish.

Single muscle fibres were isolated from the fast myotomal muscle of the teleost Myoxocephalus scorpius L. and chemically skinned with 1% Brij. Maximum Ca2+-activated force (P0) increased from 14.5 +/- 1.1 N cm-2 at 2 degrees C to 19.1 +/- 1.8 N cm-2 at 15 degrees C (mean +/- S.E.). Maximum contraction velocity was determined by Hill's slack-test method (V0) and by extrapolation from force-velocity (P-V) relationships (Vmax). There was a linear relation between log10 V0 and temperature below 15 degrees C (Q10 = 1.9, P less than 0.01). The force-velocity characteristics of the fibres were determined at 2 degrees C and 20 degrees C. Points below 0.6 P0 on the P-V curve could be fitted by a linear form of Hill's equation. Extrapolated Vmax values were 0.55 muscle lengths s-1 (L0 s-1) at 2 degrees C and 1.54 L0 s-1 at 20 degrees C. Curvature of the P-V relationship was independent of temperature. The Mg2+, Ca2+-ATPase activity of Triton-X 100 extracted myofibrils was determined under similar ionic conditions to those used in skinned fibre experiments. (Ionic strength 0.16 mmol l-1, pMgATP 2.5). A linear relationship between log10 ATPase and temperature was only obtained below 15 degrees C (P less than 0.001). Above 15 degrees C, the Q10 for ATPase decreased significantly. The Q10(0-15 degrees C) for ATPase activity (3.9) was significantly higher than for unloaded contraction velocity. Supercontraction of isolated myofibrils to very short sarcomere lengths and differences in the mechanical constraints for crossbridge cycling between the preparations probably account for the lack of proportionality between these two parameters.

Adenosine Triphosphatases↗
PubMed2002

Evidence for increased myofibrillar mobility in desmin-null mouse skeletal muscle.

Quantitative electron microscopy was used to characterize the longitudinal mobility of myofibrils during muscle extension to investigate the functional roles of skeletal muscle intermediate filaments. Extensor digitorum longus fifth toe muscles from wild-type (+/+) and desmin-null (des -/-) animals were passively stretched to varying lengths, and the horizontal displacement of adjacent Z-disks in neighboring myofibrils (Deltax(myo)) and average sarcomere length (SL) were calculated. At short SL (<2.20 microm), wild-type and desmin-null Deltax(myo) were not significantly different, although there was a trend towards greater Z-disk misalignment in muscles from knockout animals (Deltax(myo) 0.34+/-0.04 microm versus 0.22+/-0.09 microm; P>0.2; means +/- S.E.M.). However, at higher SL (>2.90 microm), muscles from knockout animals displayed a dramatically increased Deltax(myo) relative to wild-type muscles (0.49+/-0.10 microm versus 0.25+/-0.07 microm; P<0.05). The results, which establish a maximum extension of the desmin network surrounding the Z-disk, provide what we believe to be the first quantitative estimation of the functional limits of the desmin intermediate filament system in the presence of an intact myofibrillar lattice. The existence of a limit on the extension of desmin suggests a mechanism for the recruitment of desmin into a network of force transmission, whether as a longitudinal load bearer or as a component in a radial force-transmission system.

Animals↗
PubMed1997

Myofibrillogenesis in precardiac mesoderm explant culture.

To investigate the cardiac differentiation and the initial stages of myofibrillogenesis, chick precardiac mesoderm explants were cultured and spatiotemporal expressions of various sarcomeric cytoskeletal proteins were examined by immunofluorescence microscope. The precardiac cells differentiated and formed well organized myofibrils in culture. The myofibrillogenesis was similar to the reorganizing process of myofibrils in cultured cardiomyocytes from older embryos.

Animals↗
PubMed1975

Congenital left atrial enlargement. A case report with special reference to myocardial fine structure.

A case of intrapericardial congenital left atrial enlargement was reported. The patient is an asymptomatic 3 year-old girl with abnormal cardiac silhouette seen on a chest roentgenogram. Angiography showed marked left atrial enlargement, and the diagnosis of intrapericardial congenital left atrial enlargement was confirmed on operation. Specimens of the atria were electron microscopically examined. Myocardiocytes of the left atrium showed marked increase of the mitochondria and decrease of myofibrils. Abnormal Z bands and disarrangement of myofibrils were often observed. Atrial specific granules in the left atrium were increased in size and number. The right atrium also showed same changes. The pathogenesis of this condition was discussed.

Cardiomegaly↗
PubMed1995

Ultrastructure of cardiac myocytes in the greater bandicoot rat (Bandicota indica).

Cardiac myocytes in the left ventricle and atrium of the greater bandicoot rat (Bandicota indica) were examined by transmission electron microscopy. The fine structure showed typical features of mammalian cardiac myocytes. In atrial myocytes, however, the areas occupied by mitochondria were much smaller than that filled with myofibrils. The decrease in mitochondria and the abundance of myofibrils are thought to be an ultrastructural adaptation to the large body size of this species. Many conducting myocytes were observed in both atrium and ventricle. The atrial conducting myocytes were ultrastructurally different from the Purkinje fibers of the ventricle. We suggest that the abundance and the ultrastructural variation of conducting myocytes are related to the physiological aspects, such as body size, of this animal.

Animals↗
PubMed2004

Studies of chi(2)/chi(3) tensors in submicron-scaled bio-tissues by polarization harmonics optical microscopy.

Optical second- and third-harmonic generations have attracted a lot of attention in the biomedical imaging research field recently due to their intrinsic sectioning ability and noninvasiveness. Combined with near-infrared excitation sources, their deep-penetration ability makes these imaging modalities suitable for tissue characterization. In this article, we demonstrate a polarization harmonics optical microscopy, or P-HOM, to study the nonlinear optical anisotropy of the nanometer-scaled myosin and actin filaments inside myofibrils. By using tight focusing we can avoid the phase-matching condition due to micron-scaled, high-order structures in skeletal muscle fibers, and obtain the submicron-scaled polarization dependencies of second/third-harmonic generation intensities on the inclination angle between the long axes of the filaments and the polarization direction of the linear polarized fundamental excitation laser light. From these dependencies, detailed information on the tensor elements of the second/third-order nonlinear susceptibilities contributed from the myosin/actin filaments inside myofibrils can thus be analyzed and obtained, reflecting the detailed arrangements and structures of the constructing biomolecules. By acquiring a whole, nonlinearly sectioned image with a submicron spatial resolution, we can also compare the polarization dependency and calculate the nonlinear susceptibilities over a large area of the tissue at the same time-which not only provides statistical information but will be especially useful with complex specimen geometry.

Actins↗
PubMed2005

[Skeletal muscle histomorphometric study of rats in anestrus].

UNLABELLED: Castration, as well as the menopause, represents endocrine suppression that prevails the hypoestrogenism and their larger consequence on the skeletal muscle is to provoke sarcopenia. OBJECTIVE: To study the morphometric alteration of striated muscle of castrated female rats. METHODS: Twenty six female rats Wistar , distributed in two sub-groups, A and B, submitted initially to weigh-in place, vaginal cytology, ovariectomy, and biopsy of the muscle rectus femoris on the back paws, A group on right paw, and B on the left. Elapsed 20 days it was collected vaginal cytology to prove the anestrus status. After 70 days the animals went through weigh-in place, and new muscle biopsy, A group in left paw and B in right paw. The morphometric study was accomplished with the aids of a graduated lens, with reticules of 100 mm2, it was counted the myofibrils with six readings in the vertical and five in the horizontal in each sheet, being obtained a multiple number that applied on a specific formula to calculate the coefficient of muscular density. RESULTS: In A the muscular density varied from 60.0 to 52.33, (p<0.05%), with variation of 14.12%, and in B from 73.5 to 54.0, (p<0.05%), with variation of 26.53%. CONCLUSION: The castration provoked sarcopenia in the striated muscle and reduction of myofibrils number.

Anestrus↗
PubMed2006

Effects of BTS (N-benzyl-p-toluene sulphonamide), an inhibitor for myosin-actin interaction, on myofibrillogenesis in skeletal muscle cells in culture.

Actin filaments align around myosin filaments in the correct polarity and in a hexagonal arrangement to form cross-striated structures. It has been postulated that this myosin-actin interaction is important in the initial phase of myofibrillogenesis. It was previously demonstrated that an inhibitor of actin-myosin interaction, BDM (2,3-butanedione monoxime), suppresses myofibril formation in muscle cells in culture. However, further study showed that BDM also exerts several additional effects on living cells. In this study, we further examined the role of actin-myosin interaction in myofibril assembly in primary cultures of chick embryonic skeletal muscle by applying a more specific inhibitor, BTS (N-benzyl-p-toluene sulphonamide), of myosin ATPase and actin-myosin interaction. The assembly of sarcomeric structures from myofibrillar proteins was examined by immunocytochemical methods with the application of BTS to myotubes just after fusion. Addition of BTS (10-50 microM) significantly suppressed the organization of actin and myosin into cross-striated structures. BTS also interfered in the organization of alpha-actinin, C-protein (or MyBP-C), and connectin (or titin) into ordered striated structures, though the sensitivity was less. Moreover, when myotubes cultured in the presence of BTS were transferred to a control medium, sarcomeric structures were formed in 2-3 days, indicating that the inhibitory effect of BTS on myotubes is reversible. These results show that actin-myosin interaction plays a critical role in the process of myofibrillogenesis.

Actins↗
PubMed1977

Effect of exercise on the skeletal myosin of growing rat.

In order to study the effect of running on ATPase activity of myofibril and myosin in the hindlimb muscles, male wister rats of the same venter weighing approximately 50 g were housed in individual cages and allowed to run unimpeded on a treadwheel for 25-30 days. The myofibril and myosin prepared from the exercised rats had lower activation energy of ATP hydrolysis reaction and higher specific ATPase activity than those from the control rats. The myosin prepared from the running-group contained a much higher amount of the light chain having a molecular weight of 16,000, characteristic of white skeletal myosin, than that from the control group. These observations suggest that physical training changed the ratio of populations in myosin molecules, for instance, white and red skeletal myosin.

Adenosine Triphosphatases↗
PubMed1980

Distribution of sialic acid in frog skeletal muscle and effect of neuraminidase on Ca uptake and ATPase activity of sarcoplasmic reticulum.

The contents of sialic acid in the sarcolemma, sarcoplasmic reticulum (SR) and myofibrils obtained from frog skeletal muscle homogenate were determined. The total sialic acid contents of the sarcolemma and fragmented SR were 2.95 and 3.34 nmols per mg of protein, respectively, while that of myofibrils was 1.47 nmols per mg of protein. Treatment of the fragmented SR with neuraminidase (EC 3.2.1.18; NAase) resulted in the release of sialic acid. Ca uptake and ATPase activity were measured in the NAase-treated fragmented SR. When the fragmented SR stood for 1 hr after treatment and washing, the Ca uptake was decreased slightly and neither basic nor extra ATPase activities were affected. In contrast, when the fragmented SR was allowed to stand for 24 hr after similar treatment, Ca uptake and extra ATPase activity were markedly inhibited, while the duration of extra ATP splitting was markedly prolonged and final ATP hydrolysis was increased without noticeable change in basic ATPase activity. The results obtained suggest that in frog skeletal muscle, sialic acid locates mainly at the surface and SR membranes and that sialic acid is not directly involved in active Ca transport of the SR membrane.

Adenosine Triphosphatases↗
PubMed2000

Histological study of masseter muscle in a mouse muscular dystrophy model (mdx mouse).

Histological changes in the masseter muscle were observed over time in mdx mice, a muscular dystrophy model. It was found that marked necrosis occurs about the time of weaning at around 4 weeks of age; then the tissue actively regenerates at 8 weeks and stabilizes as regenerated muscle with centronuclei at 15 weeks old. This study examined the centronucleus in regenerated muscle. The process from necrosis to regeneration in muscle fibers occurs a little later in the masseter muscle than in other limbic muscles. Regenerated muscles observed around 15 weeks after birth showed a moth-eaten appearance. Transmission Electron Microscope (TEM) observation of transverse sections of muscle fibers revealed that myofibrils surrounded lost regions in the area showing a moth-eaten appearance. Thus, some defensive mechanism may affect the ability of muscle fibers to maintain a function close to normal in mdx mice even though the muscle fibers develop muscular dystrophy. The function of the masseter muscle drastically changes from sucking to mastication behavior at around 4 weeks, and this was considered to influence the morphological changes in the muscle tissue. The moth-eaten appearance seen at 15 weeks may represent an appropriate myofibril reconstruction preventing invasion of the lost regions.

Aging↗
PubMed2001

Calcium sensitivity of force production and myofibrillar ATPase activity in muscles from Thoroughbreds with recurrent exertional rhabdomyolysis.

OBJECTIVE: To determine whether the basis for recurrent exertional rhabdomyolysis (RER) in Thoroughbreds lies in an alteration in the activation and regulation of the myofibrillar contractile apparatus by ionized calcium. ANIMALS: 4 Thoroughbred mares with RER and 4 clinically normal (control) Thoroughbreds. PROCEDURES: Single chemically-skinned type-I (slow-twitch) and type-II (fast-twitch) muscle fibers were obtained from punch biopsy specimens, mounted to a force transducer, and the tensions that developed in response to a series of calcium concentrations were measured. In addition, myofibril preparations were isolated from muscle biopsy specimens and the maximal myofibrillar ATPase activity, as well as its sensitivity to ionized calcium, were measured. RESULTS: Equine type-I muscle fibers were more readily activated by calcium than were type-II muscle fibers. However, there was no difference between the type-II fibers of RER-affected and control horses in terms of calcium sensitivity of force production. There was also no difference between muscle myofibril preparations from RER-affected and control horses in calcium sensitivity of myofibrillar ATPase activity. CONCLUSIONS AND CLINICAL RELEVANCE: An alteration in myofibrillar calcium sensitivity is not a basis for pathologic contracture development in muscles from RER-affected horses. Recurrent exertional rhabdomyolysis in Thoroughbreds may represent a novel heritable defect in the regulation of muscle excitation-contraction coupling or myoplasmic calcium concentration.

Adenosine Triphosphatases↗
PubMed1990

Alterations in postmortem degradation of myofibrillar proteins in muscle of lambs fed a beta-adrenergic agonist.

Dietary administration of 4 ppm of the beta-agonist L-644,969 (Merck Sharpe and Dohme Research Laboratories) to finishing lambs induced a decrease (10 to 14%, P less than .05) in extractable calpain I activity in the longissimus muscle (LD) at death (d 0). At 4 d postmortem (d 4), extractable calpain I levels in the LD of both control and treated lambs were reduced (P less than .001) from those present at d 0, but the extractable activity in the LD was reduced to a greater extent in control than in treated lambs. Calpain II activity was increased 42% (P less than .005) in LD of treated lambs; however, no significant differences were observed between d 0 and d 4 calpain II activity within treated or control LD samples (P greater than .1). Calpastatin activity was higher in the LD of treated lambs (74% on d 0, P less than .001 and 430% on d 4, P less than .001) than in the LD of control lambs. Measurable cathepsin B activity was decreased (29% on d 0, P less than .05) and measurable cathepsin H activity was increased (10% on d 0, P less than .05 and 10% on d 4, P less than .05) in the LD of treated lambs compared with controls. On d 2, 4 and 6 postmortem, degradation in myofibrils isolated from the LD was lower for treated than for control lambs. Warner-Bratzler shear values for loin chops from treated lambs were higher on both d 3 (111%) and 6 (108%) postmortem than for chops from control lambs (P less than .001). L-644,969-induced decreases in muscle proteolytic capacity may limit postmortem myofibril degradation and contribute to the reduced tenderness observed. This decreased proteolytic capacity may contribute to increased muscularity of L-644,969-treated lambs.

Adrenergic beta-Agonists↗