Adult cardiac muscle cells in long-term serum-free culture: myofibrillar organization and expression of myosin heavy chain isoforms

A culture system for adult rat cardiac muscle cells has been established without exposure of cells to serum at any step of the procedure. The methodology has been standardized and optimized to obtain better quality and high yield of cells and culture. Subsequent to enzyme perfusion, the release of myocytes from enzyme-perfused tissues was carried out in enzyme-free Joklik's medium instead of exposing cells to proteolytic enzyme(s) as done previously. Approximately 5 million cylindrical muscle cells per ventricle were obtained. The culture medium contained Eagle's minimum essential medium with Earle's salts, basic fibroblast growth factor, epidermal growth factor, insulin, transferrin, selenium, norepinephrine, triiodothyronine (T3), bovine serum albumin, nonessential amino acids, and ascorbic acid. The plating efficiency of the experimental cultures was comparable to that of the control cultures grown in the presence of serum. The cells in the serum-free medium contained myofibrillar and myosin isoforms characteristics of the adult myocytes. The cells underwent cellular reorganization comparable to that of the controls. The initial phase of reorganization involved the breakdown of myofibrils and extrusion of mitochondria, degraded myofibrils, and other cellular organelles. The latter phase of reorganization included myofibrillogenesis and organellogenesis resulting in the development of myofibrillar apparatus with cellular organelles. Myocytes were contractile throughout the culture period. Cardiac myocytes grown in serum-free medium expressed the predominant myosin isoform V1 similar to their counterparts in vivo. T3 is essential for the expression of isomyosin V1.(ABSTRACT TRUNCATED AT 250 WORDS)

A myofibrillar protein of insect muscle related to vertebrate titin connects Z band and A band: purification and molecular characterization of invertebrate mini-titin

We show that myofibrils of insect flight and leg muscle contain a doublet of polypeptides with apparent molecular weights of 700K (K = 10(3) Mr) (Hmp I) and 600K (Hmp II), respectively. In Locusta migratoria high ionic strength extraction solubilizes only Hmp II, which is readily purified in native form. It probably reflects a proteolytic derivative of the non-extractable Hmp I. On the basis of its viscosity radius and sedimentation coefficient, Hmp II has a molecular weight of 600K and seems to consist of a single polypeptide chain. The highly asymmetric structure of the molecule is confirmed by rotary shadowing. The flexible rods have a uniform diameter of 3–4 nm and an average length of 260 nm. Polyclonal antibodies show cross-reactivity between Hmp II and its putative precursor Hmp I. We discuss the similarities and differences between the larger titin I/titin II of vertebrate sarcomeric muscle and the smaller Hmp I/Hmp II of invertebrate muscle and conclude that the latter may reflect a mini-titin. In line with the smaller length, immunoelectron microscopy locates the insect mini-titin to the I band and a very short portion of the A band only, while vertebrate titin is known to connect the Z band to the M band. Mini-titin has also been purified from several other insects including Drosophila. Immunofluorescence microscopy on frozen sections shows that mini-titin is present in the sarcomeric muscles of various species from different invertebrate phyla. These include Annelida, Nematomorpha, Plathelmintha, Nemertea and Nematoda like Ascaris lumbricoides and Caenorhabditis elegans. This wide-spread occurrence of invertebrate mini-titin is confirmed by immunoblotting experiments.

Z-band proteins in the flight muscle and leg muscle of the honeybee

Monoclonal antibodies (mAb's) have been raised against proteins in preparations of Z-discs isolated from honeybee fibrillar flight muscle. These antibodies have identified four Z-disc antigens on immunoblots of honeybee fibrillar proteins. Antibody alpha binds to the 90-100 kD protein, alpha-actinin; mAb P interacts with the protein, projectin, an extremely large polypeptide (greater than 600kD) found in the connecting filaments which link thick filaments to the Z-band in insect asynchronous flight muscle. Two other mAb's recognize previously uncharacterized insect Z-band proteins. Monoclonal antibody Z(400) binds to a pair of proteins with molecular masses near 400 kD and 600 kD. Antibody Z(175) recognizes two components, 158 kD and 175 kD, that are not only immunologically similar but have nearly identical peptide maps. Indirect immunofluorescence microscopy studies show that the proteins recognized by mAb's alpha, Z(175) and Z(400) are located at the Z-band, while the mAb P antigen is found on either side of it. Three of the four antibodies we have obtained recognize leg muscle proteins. Monoclonal antibodies alpha and P comigrate on SDS gels with analogous components from flight muscle. Only the smaller of the two proteins identified in flight muscle by mAb Z(400) is found in leg muscle, however. Furthermore, no Z(175) antigens have been detected in the non-fibrillar tissue by either monoclonal or polyclonal antibodies. Immunofluorescence microscopy studies localize the alpha and Z(400) antigens at the Z-line in leg muscle fibrils. Surprisingly, however, mAb P binds within the A-bands of synchronous fibres, not between the A- and Z-bands as in asynchronous fibrillar muscle.

Fast and slow isoforms of troponin I and troponin C. Distribution in normal rabbit muscles and effects of chronic stimulation

Polyclonal antibodies were raised against troponin I (TnI) and troponin C (TnC) purified from fast-twitch and slow-twitch rabbit muscles. These antibodies were used to elucidate the distribution of fast and slow isoforms of TnI and TnC in normal and chronically stimulated rabbit hind limb muscles by immunoblots of one-dimensional and two-dimensional electrophoreses. In contrast to the multiplicity of fast and slow troponin T (TnT) isoforms, TnI and TnC were present as unique fast and slow isoforms. Whereas no charge variants were detected for slow TnI, fast TnI was present in at least three charge variants. As judged from the results of alkaline phosphatase digestion, these charge variants represent differently phosphorylated forms. Fast and slow TnC both exist as two charge variants which, however, were unaffected by alkaline phosphatase treatment. Chronic low-frequency stimulation of fast-twitch muscles induced progressive increases in the slow isoforms of TnC and TnI at the expense of their fast isoforms. The extent of the fast-to-slow transition was more pronounced in the case of TnC than in that of TnI. Long-term stimulated muscles with a complete fast-to-slow transition, at the level of the TnT isoforms, still contained fast and slow isoforms of both TnI and TnC. The coexistence of fast and slow isoforms of the three troponin subunits in the transforming muscle was interpreted as indicating the presence of hybrid troponin molecules composed of fast and slow isoforms. Studies at the mRNA level showed changes similar to those at the protein level. However, in long-term stimulated muscles, the fast-to-slow transition of TnI was more pronounced at the mRNA level than at the protein level.

Structural change of troponin C molecule and its domains upon Ca2+ binding in the presence of Mg2+ ions measured by a solution X-ray scattering technique

A small-angle X-ray scattering study on troponin C showed that two domains of the molecule move closer to each other and the molecule shrinks along its long axis upon Ca2+ binding in the absence of Mg2+ ions (Fujisawa, T., Ueki, T., & Iida S. (1988) J. Biochem. 105, 377-383). When Mg2+ ions bind to troponin-C, the radius of gyration changes from 27.8 to 24.3 A and the average radius of gyration of the two domains is estimated to be 15.1 A. These radii indicate that the distance between the centers of the two domains is 38.1 A. Such a change is analogous to the previous result for troponin C with two Ca2+ ions bound at the high-affinity sites. Thus, the structural behavior of troponin C molecule is essentially the same when Ca2+/Mg2+ ions bind to its high-affinity sites. On the other hand, the effect of Ca2+ binding to the low-affinity sites in the presence of Mg2+ ions is quite different from the previous result. The binding of Ca2+ ions causes a dimerization of troponin C molecules with an apparent constant of 511 M-1. Such a characteristic behavior, implying the occurrence of a surface property change, may be related to the physiological role of troponin C molecule in the muscle. The scattering experiments on the tryptic fragments of troponin C also had interesting and important results: the C-domain shrinks, with the radius of gyration changing from 17.0 to 14.9 A while the N-domain swells from 13.9 to 15.0 A upon Ca2+ binding. Such an opposite change is consistent with the results of circular dichroism and spectroscopic studies of the domains.

Tropomyosin-troponin complex stabilizes the pointed ends of actin filaments against polymerization and depolymerization

In striated muscle the pointed ends of polar actin filaments are directed toward the center of the sarcomere. Formed filaments keep a constant length of about 1 micron. As polymerization and depolymerization at free pointed ends are not sufficiently slow to account for the constant length of the filaments, we searched for proteins which occur in sarcomeres and can stabilize the pointed ends of actin filaments. We observed that tropomyosin-troponin complex reduces the rate of association and dissociation of actin molecules at the pointed ends more than 30-fold. On the average, every 600 s one association or dissociation reaction has been found to occur at the pointed ends near the critical actin monomer concentration.

Inhibition reactivation myofibrillar ATPase technique for demonstration of three fiber types in a single cryostat muscle section

An inhibition reactivation technique was used for histochemical staining of human skeletal muscle sections. Myofibrillar ATPase activity was inhibited by sodium hydroxymercuribenzoate (2.5 mM in 0.1 M Tris-HCl buffer, pH 7.2-7.5, 30 min) and successively reactivated by cysteine which was added to incubation solution (10 mM cysteine-HCl, 2.5 mM ATP-disodium salt, 50 mM potassium chloride and 27 mM calcium chloride in barbital buffer, pH 9.4, 35 min at 37 C). This technique allows the distinction of three fiber categories with different staining intensities in single cross-section. Dark, intermediate and light fibers correspond to IIB, I, and IIA types, respectively. Storage of air dried sections in the freezer at -20 C for one month had no influence on staining characteristics.

Structure and function of contractile proteins in human dilated cardiomyopathy

The pathogenesis of reduced systolic left ventricular function in dilated cardiomyopathy is yet unclear. To analyze a possible involvement of contractile protein, function and structure of left ventricular myofibrils were examined in hearts of patients with advanced cardiomyopathy undergoing heart transplantation and in normal control hearts (from renal transplant donors). Myosin and actin content of the left ventricular myocardium was slightly reduced in cardiomyopathic hearts. Myofibrillar polypeptide composition was determined using two-dimensional electrophoresis and immunoblotting. No differences in constituting polypeptides were apparent, including Z-line proteins and proteins of the endosarcomeric lattice. M-line-bound creatine kinase was identical in both groups. Further, basal and maximal myofibrillar adenosine triphosphatase (ATPase) activities were unaltered in dilated cardiomyopathy. The structure of purified myosin was identical in both groups by the following criteria: electrophoretic mobility of native myosin, identical pattern of light chains after isoelectric focusing, identical cleavage peptides of myosin's heavy chain, and identical patterns after immunoblotting of heavy chain cleavage peptides using polyclonal antibodies generated against myosin from normal and cardiomyopathic ventricles. Ca2+-activated, K+-EDTA-activated and actin-activated myosin ATPase activities were identical in control and cardiomyopathic hearts. A structural alteration or functional defect of myofibrils does not seem to be primarily involved in the pathogenesis of reduced myocardial contractility in dilated cardiomyopathy.

Characterization of components of Z-bands in the fibrillar flight muscle of Drosophila melanogaster

Twelve monoclonal antibodies have been raised against proteins in preparations of Z-disks isolated from Drosophila melanogaster flight muscle. The monoclonal antibodies that recognized Z-band components were identified by immunofluorescence microscopy of flight muscle myofibrils. These antibodies have identified three Z-disk antigens on immunoblots of myofibrillar proteins. Monoclonal antibodies alpha:1-4 recognize a 90-100-kD protein which we identify as alpha-actinin on the basis of cross-reactivity with antibodies raised against honeybee and vertebrate alpha-actinins. Monoclonal antibodies P:1-4 bind to the high molecular mass protein, projectin, a component of connecting filaments that link the ends of thick filaments to the Z-band in insect asynchronous flight muscles. The anti-projectin antibodies also stain synchronous muscle, but, surprisingly, the epitopes here are within the A-bands, not between the A- and Z-bands, as in flight muscle. Monoclonal antibodies Z(210):1-4 recognize a 210-kD protein that has not been previously shown to be a Z-band structural component. A fourth antigen, resolved as a doublet (approximately 400/600 kD) on immunoblots of Drosophila fibrillar proteins, is detected by a cross reacting antibody, Z(400):2, raised against a protein in isolated honeybee Z-disks. On Lowicryl sections of asynchronous flight muscle, indirect immunogold staining has localized alpha-actinin and the 210-kD protein throughout the matrix of the Z-band, projectin between the Z- and A-bands, and the 400/600-kD components at the I-band/Z-band junction. Drosophila alpha-actinin, projectin, and the 400/600-kD components share some antigenic determinants with corresponding honeybee proteins, but no honeybee protein interacts with any of the Z(210) antibodies.

Equilibrium distribution of ions in a muscle fiber

We have developed a mathematical description of the equilibrium (Donnan) distribution of mobile ions between two phases containing fixed charges. This differs from the classical Donnan derivation by including mobile polyvalent ions such as those present in intact muscle fibers and in solutions used with skinned muscle fibers. Given the average concentrations of ionic species present in intact frog muscle, we calculate that the myofibrillar fixed charge density (-42 meq/liter cytoplasmic fluid) is in close agreement with estimates from amino acid analysis of myofibrillar proteins. As expected, with negative fixed charges in the myofibril, anions are excluded from the myofibrillar space while cations are concentrated in this space; the ratio between the average intra- and extramyofibrillar concentrations for an ion of valence n is (1.11)n. This model allowed us to design a bathing solution for skinned muscle fibers in which the intramyofibrillar ion concentrations closely approximate those found in intact frog muscle cells. Our model, applied to the A- and I-bands of the sarcomere, suggests that likely differences in fixed charge densities in these regions accounts for only a small fraction of the extreme concentration of phosphocreatine observed in the I-bands of intact frog muscle.

Short-term stability and muscle adaptation after mandibular advancement surgery with and without suprahyoid myotomy in juvenile Macaca mulatta

The purpose of this study was to examine the short-term adaptations that occur within the mandible and anterior digastric muscle complex after mandibular advancement with and without suprahyoid myotomy in 20 juvenile rhesus monkeys. The results showed that the animals that did not undergo myotomy experienced relapse equivalent to 13% of the surgical advancement. Those animals that underwent a myotomy of the digastric muscle complex showed complete stability of the surgical lengthening of the mandible. Both groups of animals grew normally after the fixation period when compared to age-matched control animals. Analysis of adaptations within the digastric muscle complex was performed with the use of radiopaque muscle and tendon markers. The results showed an immediate lengthening of the entire digastric muscle complex with mandibular advancement surgery in the group that underwent advancement without myotomy. Further analysis showed that most lengthening in these animals occurred at the connective tissue interfaces of the complex--at the muscle-bone and muscle-tendon interfaces. No significant changes in sarcomere or fiber length were found in the group that did not undergo myotomy, although there was a significant shortening of muscle fibres resulting from loss of serial sarcomeres in the myotomy group. Comparison of histochemical characteristics of the anterior digastric muscle before and after surgery revealed the following findings: (1) there were no significant differences in percentage of composition between control and experimental muscles; (2) despite fixation of the jaws and myotomy, there was no evidence of atrophy of the anterior digastric muscle at any experimental interval; and (3) the type I fibers of the anterior digastric muscle underwent significant stretch-induced hypertrophy after lengthening. The results of this study support the hypothesis that tension produced by stretching of the connective tissues associated with the digastric muscle complex can contribute to postsurgical relapse of the surgically advanced mandible. However, no adverse effect on future growth of the mandible was observed from stretching the digastric muscle complex by mandibular advancement surgery in juvenile subjects.

Factors affecting polyacrylamide gel electrophoresis and electroblotting of high-molecular-weight myofibrillar proteins

Electrophoresis of the high-molecular-mass proteins (greater than 500 kDa) of muscle myofibrils is difficult using conventional procedures. The mobility of these proteins was influenced by the heating time in sample buffer, the use of 2-mercaptoethanol in the upper reservoir buffer, and the pH of the resolving gel in a stacking sodium dodecyl sulfate gel system. Heating samples for 4 min (versus shorter times), addition of 2-mercaptoethanol to the upper reservoir buffer, and reducing the pH of the resolving gel to 8.6 all enhanced the mobility and resolution of the high-molecular-weight proteins on polyacrylamide gels. The sulfhydryl reducing agents commonly used in protein sample buffers (2-mercaptoethanol and dithiothreitol) were found to migrate at the electrophoretic dye front. Inclusion of 10 mM 2-mercaptoethanol in the upper reservoir buffer or blocking free sulfhydryl groups with N-ethylmaleimide prevented intermolecular disulfide bond formation during electrophoresis. The addition of 10 mM 2-mercaptoethanol to the buffer used for electroblotting also improved efficiency of protein transfer to nitrocellulose.

Differential response of myofibrillar and cytoskeletal proteins in cells treated with phorbol myristate acetate

Muscle-specific and nonmuscle contractile protein isoforms responded in opposite ways to 12-o-tetradecanoyl phorbol-13-acetate (TPA). Loss of Z band density was observed in day-4-5 cultured chick myotubes after 2 h in the phorbol ester, TPA. By 5-10 h, most I-Z-I complexes were selectively deleted from the myofibril, although the A bands remained intact and longitudinally aligned. The deletion of I-Z-I complexes was inversely related to the appearance of numerous cortical, alpha-actinin containing bodies (CABs), transitory structures approximately 3.0 microns in diameter. Each CAB consisted of a filamentous core that costained with antibodies to alpha-actin and sarcomeric alpha-actinin. In turn each CAB was encaged by a discontinuous rim that costained with antibodies to vinculin and talin. Vimentin and desmin intermediate filaments and most cell organelles were excluded from the membrane-free CABs. These curious bodies disappeared over the next 10 h so that in 30-h myosacs all alpha-actin and sarcomeric alpha-actinin structures had been eliminated. On the other hand vinculin and talin adhesion plaques remained prominent even in 72-h myosacs. Disruption of the A bands was first initiated after 15-20 h in TPA (e.g., 15-20-h myosacs). Thick filaments of apparently normal length and structure were progressively released from A segments, and by 40 h all A bands had been broken down into enormous numbers of randomly dispersed, but still intact single thick filaments. This breakdown correlated with the formation of amorphous cytoplasmic aggregates which invariably colocalized antibodies to myosin heavy chain, MLC 1-3, myomesin, and C protein. Complete elimination of all immunoreactive thick filament proteins required 60-72 h of TPA exposure. The elimination of the thick filament-associated proteins did not involve the participation of vinculin or talin. In contrast to its effects on myofibrils, TPA did not induce the disassembly of the contractile proteins in stress fibers and microfilaments either in myosacs or in fibroblastic cells. Similarly, TPA, which rapidly induces the translocation of vinculin and talin to ectopic sites in many types of immortalized cells, had no gross effect on the adhesion plaques of myosacs, primary fibroblastic cells, or presumptive myoblasts. Clearly, the response to TPA of contractile protein and some cytoskeletal isoforms not only varies among phenotypes, but even within the domains of a given myotube the myofibrils respond one way, the stress fibers/microfilaments another.

Detection of a specific isoform of alpha-actinin with antisera directed against dystrophin

We have characterized a protein immunologically related to dystrophin, the protein product of the Duchenne muscular dystrophy gene. We identify this related protein as a fast-twitch glycolytic isoform (mouse extensor digitorum longus-specific) of myofibrillar alpha-actinin. This specific isoform of alpha-actinin exhibits a more restricted pattern of expression in skeletal muscle than fast-twitch-specific isoforms of both myosin and Ca2+-ATPase. Our results provide evidence that dystrophin and myofibrillar alpha-actinin are related proteins, reinforcing the previous data concerning the sequence homologies noted between nonmuscle cytoskeletal alpha-actinin and dystrophin. In addition, we describe the first antisera directed against a specific myofibrillar skeletal muscle isoform of alpha-actinin.

Immunocytochemical studies of spectrin in hamster cardiac tissue

The spectrins are a family of cytoskeletal-membrane proteins that have a wide tissue distribution. In the present study, we employed polyclonal antibodies made against mammalian and avian erythroid spectrins as well as mammalian brain spectrin to assess their presence and distributions in the mammalian heart. Western blot analyses revealed that all three antibodies were specific for a 240,000 molecular weight alpha-spectrin subunit found in hamster erythrocyte ghost homogenates, whole hamster heart, and isolated hamster cardiac myofibril homogenates. Spectrin staining was absent from the Triton X-100-extracted supernatant fraction of myofibril preparations, suggesting that the protein is linked to the myofibril precipitate after exposure to the detergent. Frozen, unfixed, 2-microns-thick; sections of adult. Syrian golden hamster cardiac tissue exhibited strong immunofluorescent staining of intercalated discs and Z-bands using all three antibodies. In addition, the mammalian erythroid spectrin antibodies showed staining of the sarcolemma, and in cross section, revealed a delicate internal network of staining that appears to surround individual myofibrils. This may be T-tubule-associated staining. Myofibrils isolated from cardiac myocytes using Triton X-100 show positive Z-band staining using all three antibodies. Double staining with Texas Red-labeled monoclonal desmin and FITC-labeled polyclonal spectrin antibodies revealed that both stained the myofibrillar Z-line regions. These results demonstrate that spectrin is closely associated with the membranes, myofibrils, and intermediate filaments in the mammalian heart.

Immunocytochemical studies of cardiac myofibrillogenesis in early chick embryos. III. Generation of fasciae adherentes and costameres

To study whether the first myofibrils are separate from or firmly bound to the myocytic cell membranes, whole mount preparations of 6-12-somite-stage chick embryonic hearts were examined by fluorescence microscopy after double labeling with antibodies to vinculin (fluorescein-conjugated) and rhodamine-phalloidin, or with antibodies to titin (rhodamine-conjugated) and nitrobenz-oxadiazole-phallacidin. When a small number of myofibrils appeared for the first time at the nine somite stage, most of them were already bound to the cell membranes through zonulae adherentes, fasciae adherentes, or costameres. In the outer of the two myocardial cell layers, in which the myocytes were closely in contact with each other along polygonal boundaries, fasciae adherentes and costameres developed at the boundaries, apparently by conversion of preexisting zonulae adherentes. On the other hand, in the inner cell layer, in which myocytes were more loosely associated with each other, both costameres and fasciae adherentes appeared to develop de novo, the former in association with the inner surface of the myocardial wall and the latter at the intercellular boundaries. The myofibrillar tracks in the inner layer followed long and smooth courses and were as a whole aligned in the circumferential direction of the tubular heart wall from the earliest stage of myofibril formation. Those in the outer layer were arranged in a pattern of two- or three-dimensional networks in the 9-10 somite stage, although many myofibrils were also circumferentially directed. The fact that the majority of the first myofibrils were already bound to the cell membranes in a directed manner suggests that myocytes at the earliest stage of myofibril formation are endowed with spatial information that directs the organization of nascent myofibrils. It is proposed that the myocyte cell membranes perform an essential role in cardiac myofibrillogenesis.

A silver-reducing component in rat striated muscle. I. Selective localization at the level of the terminal cistern/transverse tubule system. Light and electron microscope studies with a new histochemical procedure

This study reports the presence of a silver-reducing constituent in rat striated muscle fiber located selectively at the level of the terminal cistern/transverse tubule system. It is related to the T tubule network at or near sites that participate in junctions with terminal cisternae, i.e., at both sides of the T tubule in skeletal muscle (triad) and, predominantly, at one side in the ventricle (dyad). Little reactivity is present in the auricle due to the scarcity of those membrane systems. The longitudinal sarcoplasmic reticulum, the sarcolemma, mitochondria and myofibrils are not outlined by the reaction product. Extraction of low molecular weight substances, nucleic acids and lipids did not suppress the chemical reaction. A new argentaffin (Hg--Ag) technique is described. Ethanol or aldehyde fixed muscles were passed to water, postfixed 6-24 h with mercuric acetate (5% w/v in 1% acetic acid), washed with 1% acetic acid and distilled water, stained 12-24 h at 43 degrees C with ammoniacal silver nitrate (60% w/v) and washed in 10% sodium sulfite (three changes) and water. All steps were carried out in darkness. Postfixation with mercuric acetate proved to be essential for immobilizing the argentaffin component without interfering with its strong argentaffinity. The procedure also provides a simple method for tracing the pathway of transversally oriented membrane systems in skeletal and cardiac muscle cells.

Myofibril assembly is linked with vinculin, alpha-actinin, and cell-substrate contacts in embryonic cardiac myocytes in vitro

The relationship of nascent myofibrils with the accumulation of adhesion plaque proteins and the formation of focal cell contacts was studied in embryonic chick cardiac myocytes in vitro. The cultures were double-stained with various combinations of the specific antiactin drug phalloidin and antibodies against vinculin, alpha-actinin, connectin (titin), myosin heavy chain, fibronectin, and desmin and examined under fluorescence and interference reflection microscopy. In the areas of myofibril assembly, vinculin and alpha-actinin plaques were formed at the ventral sarcolemmae. These areas overlapped with the sites of cell-to-substrate focal contacts and extracellular fibronectin. Because the myofibrils always ran in a straight line between these sites, polarized lines appeared to be generated within the cells in response to their physical (e.g., stress) and/or biochemical environment (e.g., adhesion plaque proteins). The possible presence of other factors cannot be ruled out for the proper alignment of myofibrils. As soon as myofibrils came to span between these adhesion sites, they exhibited typically mature cross-striated characteristics. Thus, the formation of these inferred lines has some relation to, or is in fact necessary for, the maturation of myofibrils, in addition to the directional arrangement of sarcomeric proteins. Additionally, synthesis and distribution of myosin and connectin were tightly linked during early developmental (premyofibril and myofibril) stages. The spatial deployment of desmin was not coupled with vinculin. Thus, connectin and desmin do not appear to form the initial scaffold of sarcomeres.

Histochemical characteristics of masseter and temporalis muscles after 5 weeks of maxillomandibular fixation--an investigation in Macaca mulatta

This study describes the histochemical characteristics and cross-sectional areas of the superficial masseter and temporalis muscles in juvenile rhesus monkeys after 5 weeks of maxillomandibular fixation. Four juvenile male Macaca mulatta underwent mandibular surgery and 5 weeks of maxillomandibular fixation as part of a study of temporomandibular joint (TMJ) adaptations after condylar replacement. Immediately before the time the animals were killed (5 weeks postsurgically), biopsies of the masseter and temporalis muscles were obtained and submitted to histochemical analysis and calculation of muscle-fiber areas. The data were compared to histochemistry from 12 juvenile control Macaca mulatta. Significant decreases in mean cross-sectional area were exhibited in both type I (p less than 0.05) and type II (p less than 0.01) fibers in all muscles when compared to controls (n = 12). The ratio of type I to type II fibers, however, remained constant during maxillomandibular fixation in masseter and temporalis muscle samples, indicating no change in relative types of fibers. We conclude from this experimental investigation that (1) significant atrophy occurs in the temporalis and masseter muscles after 5 weeks of maxillomandibular fixation, and (2) this atrophy occurs in both type I and type II fibers, indicating that overall recruitment of the muscle (and not just of one fiber type of motor unit) was affected during fixation.

Studies on cardiac myofibrillogenesis with antibodies to titin, actin, tropomyosin, and myosin

Cardiac myofibrillogenesis was examined in cultured chick cardiac cells by immunofluorescence using antibodies against titin, actin, tropomyosin, and myosin. Primitive cardiomyocytes initially contained stress fiber-like structures (SFLS) that stained positively for alpha actin and/or muscle tropomyosin. In some cases the staining for muscle tropomyosin and alpha actin was disproportionate; this suggests that the synthesis and/or assembly of these two isoforms into the SFLS may not be stoichiometric. The alpha actin containing SFLS in these myocytes could be classified as either central or peripheral; central SFLS showed developing sarcomeric titin while peripheral SFLS had weak titin fluorescence and a more uniform stain distribution. Sarcomeric patterns of titin and myosin were present at multiple sites on these structures. A pair of titin staining bands was clearly associated with each developing A band even at the two or three sarcomere stage, although occasional examples of a titin band being associated with a half sarcomere were noted. The appearance of sarcomeric titin patterns coincided or preceded sarcomere periodicity of either alpha actin or muscle tropomyosin. The early appearance of titin in myofibrillogenesis suggests it may have a role in filament alignment during sarcomere assembly.

Type I protein is a slow isoform of troponin T

Type I protein, a myofibrillar protein thought to be specific to slow-twitch skeletal muscle fibers, was purified. Two-dimensional electrophoresis indicated its identity with the purified slow troponin-T1s isoform. Immunochemical analyses using antibodies raised against type I protein and slow Tn-T1s, further substantiated the identity of the two proteins.

[Effect of short-term space flights on physiological properties and composition of myofibrillar proteins of the skeletal muscles of rats]

Contractile properties of preparations of glycerinated myofibers and subunit composition of myofibrillar proteins of skeletal muscles were studied using rats flown on Kosmos-1514 (pregnant females) and Kosmos-1667 (males). After the 5- and 7-day flights the strength and velocity of contraction of myofibers decreased, although this change was not correlated with functional differentiation of muscles. The myosin population tended to vary in terms of the proportion of fast and slow isoforms. It is concluded that physiological properties of skeletal muscles at an early stage of orbital flights deteriorated primarily due to a decline in the functional activity of the excitation-contraction conjugation system of myofibers.

[Interactions of fats and proteins in meat. 3. Changes in tryptophan content and available forms of lysine, methionine and cystine]

In experimental models the effect of hydroperoxides of the methyl ester of linoleic acid, and hexanal, on the contents of tryptophan, "available" forms of lysine, methionine and cystine in proteins were analyzed. Conditions typical for technological treatment of meat were simulated. A muscle gel containing large quantities of myofibrillar proteins served as meat substrate. It was found that thermal denaturation and the presence of sodium chloride in the medium stimulate the reactivity of methyl linoleate hydroperoxides and hexanal with proteins. Cystine and lysine demonstrated the highest reactivity with oxidated fats. Addition of hexanal and sodium chloride to the muscle gel caused a distinct decrease in the accessible sulfhydryl groups content.

The question of chymase in cultured muscle cells

An alkaline proteinase, identical to mast cell chymase, has been described by a number of laboratories as being associated with myofibrils extracted from adult rat skeletal muscle tissue. A more recent study has indicated that chymase may be an intrinsic protein in the rat myocyte. The present study of rat myogenic cell lines, using more stringent controls and a probe of more highly defined specificity, supports the view that (i) chymase originates from mast cells of the interstitium and (ii) chymase from mast cells becomes adsorbed to myofibrils of adult muscle during homogenization of this complex tissue.

The organization of titin filaments in the half-sarcomere revealed by monoclonal antibodies in immunoelectron microscopy: a map of ten nonrepetitive epitopes starting at the Z line extends close to the M line

mAbs specific for titin or nebulin were characterized by immunoblotting and fluorescence microscopy. Immunoelectron microscopy on relaxed chicken breast muscle revealed unique transverse striping patterns. Each of the 10 distinct titin antibodies provided a pair of delicate decoration lines per sarcomere. The position of these pairs was centrally symmetric to the M line and was antibody dependent. The results provided a linear epitope map, which starts at the Z line (antibody T20), covers five distinct positions along the I band (T21, T12, T4, T1, T11), the A-I junction (T3), and three distinct positions within the A band (T10, T22, T23). The epitope of T23 locates 0.2 micron before the M line. In immunoblots, the two antibodies decorating at or just before the Z line (T20, T21) specifically recognized the insoluble titin TI component but did not recognize TII, a proteolytic derivative. All other titin antibodies recognized TI and TII. Thus titin molecules appear as polar structures lacking over large regions repetitive epitopes. One physical end seems related to Z line anchorage, while the other may bind close to the M line. Titin epitopes influenced by the contractional state of the sarcomere locate between the N1 line and the A-I junction (T4, T1, T11). We discuss the results in relation to titin molecules having half-sarcomere lengths. The three nebulin antibodies so far characterized again give rise to distinct pairs of stripes. These locate close to the N2 line.

Localization of paratropomyosin in skeletal muscle myofibrils and its translocation during postmortem storage of muscles

Using polyclonal antibodies against paratropomyosin, which is believed to modify the actin-myosin interaction in postrigor skeletal muscles, we studied the localization of paratropomyosin in chicken breast muscle myofibrils. Intact myofibrils stained with fluorescent antibodies showed that paratropomyosin was exclusively located at the A-I junction region of sarcomeres. In stretched myofibrils (3.7 micron in sarcomere length), the approximate width of the fluorescent stripes and their relation to the A band remained constant. Removal of the A band from myofibrils led to loss of stainability. During postmortem storage of muscles, on the other hand, paratropomyosin was translocated from its original position at the A-I junction region onto thin filaments. The translocation of paratropomyosin was successfully induced with a calcium ion concentration of 10(-4) M in the presence of protease inhibitors. We therefore conclude that in postrigor muscles, paratropomyosin is released from the A-I junction region following the increase in the sarcoplasmic calcium ion concentration to 10(-4) M, and then binds to thin filaments, which results in weakening of rigor linkages formed between actin and myosin.

Distribution and organization of the elastic system fibres in healthy human gingiva. Ultrastructural and immunohistochemical study

The ultrastructural distribution and organization of the elastic system fibres, i.e. oxytalan, elaunin and elastic fibres, were studied by transmission electron microscopy and by an immunohistochemical method for the detection of elastin in healthy human gingiva. The morphological distribution of these fibres was characterized by the presence of oxytalan, elaunin and elastic fibres, respectively, in the upper, medium, and deep layers of gingival connective tissue. Anti-elastin antibody reacted with microfibrils and amorphous material of the elastic system fibres throughout the gingival connective tissue. These findings were interpreted as indicating that the microfibrils were associated with small amounts of elastin at their surface.

Characteristic structures of actin gels induced with hepatic actinogelin or with chicken gizzard alpha-actinin: implication for their function

We studied the properties of actinogelin, a Ca2+-regulated actin cross-linking protein isolated from Ehrlich tumor cells or rat liver. Chicken gizzard alpha-actinin was used as a Ca2+-insensitive control. Actinogelin, which has very high gelation activity under low Ca2+ conditions, was found using electron microscopic or fluorescence studies to induce formation of a characteristic structure in which actin filaments and bundles radiate to (or converge from) all directions from spot-like core structures. A similar structure was induced with actinogelin, even in the presence of 0.7 saturation of tropomyosin. No such structure was detected with actinogelin under high Ca2+ conditions, and only a few were found with gizzard alpha-actinin. Because reconstituted structures are similar to those observed intracellularly, actinogelin may be important in the formation of similar microfilament organization in the cells. It seems also important that these structures are reconstituted with only two purified protein components, i.e., actinogelin and actin. Immunocompetition studies showed that actinogelin and gizzard alpha-actinin partially shared antigenicity, and their molecular shape and peptide maps were similar. Their amino acid compositions [Kuo et al., 1982], subunit and domain structures, and binding sites on actin [Mimura and Asano, 1987] are also very similar. Therefore, it is concluded that actinogelin belongs to alpha-actinin superfamily proteins. Furthermore, the presence of functionally different subfamilies concerned with Ca2+ sensitivity, gelation-efficiency, and others is discussed. Actinogelin, which induces networks of actin filaments, may be classified as high gelation type.

Paratropomyosin, a new myofibrillar protein, weakens rigor linkages formed between actin and myosin

We have used an enzymatic technique to determine the weakening effect of paratropomyosin, a new myofibrillar protein, on rigor linkages formed between actin and myosin, and to clarify the distinct function of paratropomyosin, as to that of tropomyosin. Paratropomyosin inhibited the Mg-ATPase activity and enhanced the K-ATPase activity of reconstituted actomyosin stoichiometrically, and its maximal binding to actin was estimated to occur at a molar ratio of 1: 12.5. Paratropomyosin also inhibited the myofibrillar Mg-ATPase activity by 49% and enhanced the myofibrillar K-ATPase activity to 126%, while tropomyosin had no effect on these ATPases. These results indicate that paratropomyosin is able to bind to thin filaments of myofibrils, because the binding site for paratropomyosin on F-actin is different from that for tropomyosin, and that, due to its greater affinity for the myosin binding site on actin, paratropomyosin competes for the binding site and helps weaken rigor linkages.

Alterations in fine structures of myofibrils and structural proteins in patients with dilated cardiomyopathy--studies with biopsied heart tissues

Ultrastructural and biochemical alterations in myofibrils (Mf) were studied in biopsied myocardial tissues in 11 patients with dilated cardiomyopathy (DCM) and compared with those in non-hypertrophic control and secondary hypertrophic (SHT) heart muscles. Transverse diameters of biopsied cardiac myocytes increased significantly in both of SHT and DCM, and ultrastructural changes were similar in quality in both of them. Volume densities of Mf were 61.1 and 59.9% on average in control and SHT myocardial cells, respectively, and they were not significantly different. But in DCM volume density was significantly less (49.8% in left ventricular myocytes), and inverse relation between that and diameter of cardiac myocytes was observed (p less than 0.01). Electrophoretic pattern and relative composition of major structural proteins from control and SHT heart muscles were similar and statistically insignificant. In DCM, relative contents of myosin heavy chain and alpha-actinin decreased significantly and distinctively in all of cases suggesting primary degradation of myofibrils.

Cloning and characterization of mammalian myosin regulatory light chain (RLC) cDNA: the RLC gene is expressed in smooth, sarcomeric, and nonmuscle tissues

The 20-kD regulatory light chain (RLC) plays a central role in the regulation of smooth muscle contraction. Little is known about the structure or expression of smooth muscle myosin light chain (MLC) genes. A cDNA library was constructed in the expression vector, lambda gt-11, with mRNA derived from cultured rat aortic smooth muscle cells. Using antibody generated against tracheal smooth muscle myosin, three cDNA clones encoding a RLC were isolated, one of which, SmRLC-2, represents a full-length transcript of the RLC mRNA. The derived amino acid sequence shows 94.2% homology with the chicken gizzard RLC, and 70 and 52% homology with the rat skeletal and cardiac muscle MLC-2 proteins, respectively. Thus, the gene encoding the putative smooth muscle RLC appears to have originated by duplication of the same ancestor that gave rise to the sarcomeric MLC-2 genes. Contrary to the stringent tissue-specific expression of sarcomeric MLC-2 genes, RNA blot hybridization and S1 nuclease mapping demonstrates that the putative smooth muscle RLC gene is expressed in smooth, sarcomeric, and nonmuscle tissues at significant levels. Primer extension analysis suggests that the same promoter region is used in these different tissues. Thus the putative smooth muscle RLC gene appears to be a gene that is constitutively expressed in a large variety of cells and has a differentiated function in smooth muscle.

Developmental and hormonal regulation of sarcomeric myosin heavy chain gene family

Sarcomeric myosin heavy chain (MHC), the main component of the sarcomere, contains the ATPase activity that generates the contractile force of cardiac and skeletal muscles. The different MHC isoforms are encoded by a closely related multigene family. Most members (seven) of this gene family have been isolated and characterized in the rat, including the alpha- and beta-cardiac, skeletal embryonic, neonatal, fast IIA, fast IIB, and extraocular specific MHC. The slow type I skeletal MHC is encoded by the same gene that codes for the cardiac beta-MHC. Each MHC gene studied displays a pattern of expression that is tissue and developmental stage specific, both in cardiac and skeletal muscles. Furthermore, more than one MHC gene is expressed in each muscle while each gene is expressed in more than one tissue. The expression of each MHC gene in cardiac and skeletal muscles is modulated by thyroid hormone. Surprisingly, however, the same MHC gene can be regulated by the hormone in a significantly different manner, even in opposite directions, depending on the muscle in which it is expressed. Moreover, the skeletal embryonic and neonatal MHC genes, so far considered specific to these 2 developmental stages, are normally expressed in certain adult muscles and can be reinduced by hypothyroidism in specific muscles. This complex pattern of expression and regulation of the MHC gene family in cardiac and skeletal muscle sheds new light on the mechanisms involved in determining the biochemical basis of the contractile state. It also indicates that the cardiac contractile system needs to be examined in a broader context, including skeletal muscles, in order to understand fully its developmental and physiologic regulation.

The multiplicity of combinations of myosin light chains and heavy chains in histochemically typed single fibres. Rabbit soleus muscle

1. Six adult rabbit soleus muscles were analysed by isolating histochemically identified fibre pieces from freeze-dried serial cross-sections. 2. By the use of this method, four fibre types (I, IC, IIC and IIA) were identified and analysed micro-electrophoretically. 3. Type I fibres contained the slow myosin heavy chain HCI and the slow myosin light chains LC1s and LC2s. 4. Type IIA fibres contained the fast myosin HCIIa with the fast light chains and, in addition, either LC1s or both LC1s and LC2s. 5. The C fibres (IC and IIC) represented intermediate populations between types I and IIC (IC) and between IC and IIA (IIC). They contained varied ratios of HCI/HCIIa with both sets of fast and slow light chains. With regard to myosin composition and isoforms of other myofibrillar proteins (M- and C-proteins, alpha-tropomyosin, troponin I), IC fibres resembled type I and IIC fibres resembled type IIA. 6. The presence of various myosin light and heavy chains within a specific fibre suggests a multiplicity of isomyosins. Without consideration of LC1sa and LC1sb differences, at least 54 possible isomyosins can be derived: type I fibres contain one isomyosin, types IC and IIC 54 possible isomyosins, and type IIA up to 18.

The multiplicity of combinations of myosin light chains and heavy chains in histochemically typed single fibres. Rabbit tibialis anterior muscle

1. Combined histochemical and biochemical single-fibre analyses [Staron & Pette (1987) Biochem. J. 243, 687-693], were used to investigate the rabbit tibialis-anterior fibre population. 2. This muscle is composed of four histochemically defined fibre types (I, IIC, IIA and IIB). 3. Type I fibres contain slow myosin light chains LC1s and LC2 and the slow myosin heavy chain HCI, and types IIA and IIB contain the fast myosin light chains LC1f, LC2f and LC3f and the fast heavy chains HCIIa and HCIIb respectively. 4. A small fraction of fibres (IIAB), histochemically intermediate between types IIA and IIB, contain the fast light myosin chains but display a coexistence of HCIIa and HCIIb. 5. Similarly to the soleus muscle, C fibres in the tibialis anterior muscle contain both fast and slow myosin light chains and heavy chains. The IIC fibres show a predominance of the fast forms and the IC fibres (histochemically intermediate between types I and IIC) a predominance of the slow forms. 6. A total of 60 theoretical isomyosins can be derived from these findings on the distribution of fast and slow myosin light and heavy chains in the fibres of rabbit tibialis anterior muscle.

Characterization and fibre type distribution of a new myofibrillar protein of molecular weight 32 kDa

A new basic protein of molecular weight 32 kDa has been isolated and purified to homogeneity from skeletal muscles rich in type I fibres. By the use of a specific monoclonal antibody, the protein has been shown to be present in all type I fibres and some type II fibres, the number of which varies with the muscle and the region of the muscle sectioned. A protein of similar properties could not be isolated from rabbit muscles consisting predominantly of type II fibres. By fluorescence microscopy, the protein has been shown to be located in the Z-disc from which the presence of divalent cations, probably calcium, facilitates its extraction at low ionic strength. The protein is unusual in that its distribution does not correlate completely with the known muscle fibre types and in that as yet there is no evidence for the presence of an isoform in those cells that do not stain with the specific antibody for the 32 kDa protein isolated from slow muscles.

[Electron microscope study of protein P55 interaction with myosin]

It was shown by electron microscopic study that in the presence of inhibitor of myosin ATPase protein P55 the long fibrils of myosin became short.

Isolation and localization of filamin in heart muscle

High-molecular-mass protein was isolated from chicken heart muscle. The apparent molecular mass of a single polypeptide chain is similar to that of chicken gizzard filamin: 250-270 kDa. The protein interacts with antibodies against chicken gizzard filamin and induces F-actin gelation in a concentration-dependent manner. Immunofluorescent staining of cardiomyocytes and chicken heart sections with antifilamin antibody demonstrates two types of filamin localization: filamin was located on the sarcomere border in the periphery of the Z-disk; filamin was found in intercalated disks between cardiomyocytes.

Actin-attached and detached crossbridges in myofibrils: segregation into two populations according to their sensitivity to proteolytic digestion of myosin heavy chain

Tryptic digestion of myofibrils was used to assess the interaction of crossbridges with thin filaments in the presence of ATP analogues. The relative amounts of 200 kDa fragment produced by trypsin from myosin heavy chain when the crossbridge is attached to actin, and of 160 kDa fragment produced when the crossbridge is detached from actin, served as a measure of crossbridge-actin interaction. In rigor only the 200 kDa fragment was produced suggesting that a great majority of the crossbridges were strongly attached to actin; in the presence of MgPPi at 0 degrees C only the 160 kDa fragment was finally produced suggesting that eventually all crossbridges detached from actin. In the presence of MgPPi or MgAMPPNP at 25 degrees C both 200 and 160 kDa fragments were present for several minutes after myosin heavy chain had been completely digested, suggesting that two populations of crossbridges (attached and detached) co-existed at the same time within the myofibril. It is concluded that the addition of ATP analogues to muscle does not simply affect the chemical equilibrium of binding of myosin heads to actin but that it causes rapid dissociation of one crossbridge population without significant effect on binding to actin of the remaining crossbridge population.

The ATPase kinetics of insect fibrillar flight muscle myosin subfragment-1

Myosin subfragment-1 (S1) has been prepared from the fibrillar flight muscles of the giant water bug Lethocerus by chymotryptic digestion of myofibrillar suspensions in the absence of magnesium ions. The S1 obtained has a single light chain and a heavy chain with molecular weights of about 18 kDa and 90 kDa respectively. The kinetics of the elementary steps of the magnesium-dependent ATPase of insect S1 and rabbit S1 are similar, both with ATP and with ATP analogues as substrates. However, the presence of variable amounts of inactive protein within our preparation means that several rate constants cannot be obtained with as much precision in the case of insect S1. The most striking differences between the rabbit and insect S1 are values for the Vmax and the Km of actin during actin-activation of the MgATPase activity, which are up to an order of magnitude lower and greater in the insect than in the rabbit, respectively. The mechanical properties of strain activation and of capacity to do extended oscillatory work are unique to insect fibrillar flight muscle and distinguish it from vertebrate striated muscle. It is likely that these properties reflect differences in the organization of actin and myosin within the respective filament lattices rather than intrinsic differences in the ATPase mechanisms of the isolated myosin molecules from the two types of muscle.

Development of myofibrils in the gizzard of chicken embryos. Intracellular distribution of structural proteins and development of contractility

The intracellular distributions of major muscle proteins, myosin, actin, tropomyosin, alpha-actinin, and desmin, in smooth muscle cells of chicken gizzard at various stages of embryogenesis were investigated by immunofluorescence-labeling of enzyme-dispersed cells cultured up to three hours. These muscle proteins, except some part of myosin, were organized into fibrous structures as soon as synthesis and accumulation of proteins started. As for myosin, a considerable amount of it was dispersed in soluble cytoplasm as well. On the other hand, Ca++-dependent contractility was detected with detergent-extracted myoblasts and glycerinated tissue from embryos older than 7 days. Although the nascent myofibrils bear a resemblance to "stress fibers," the former could be distinguished from the latter by their high stability in dispersed, spherical cells. The above findings, therefore, show that the synthesis of contractile proteins is followed by immediate assembly of them into functional myofibrils without undergoing any intermediate structure. Based on these findings, the mechanism of myofibril formation in developing smooth muscle cells is discussed.

Isolation and characterization of sarcomeric actin genes expressed in Xenopus laevis embryos

A Xenopus laevis complementary DNA (cDNA) library prepared from messenger RNAs extracted from embryos has been screened for actin-coding sequences. Two cDNA clones corresponding to an alpha cardiac and an alpha skeletal muscle actin mRNA have been identified and characterized. From a genomic library, we have furthermore isolated the genes that correspond to the characterized cDNAs. In addition we have identified an actin processed gene which seems to be derived from a second type of skeletal muscle actin gene. Southern blot analysis of X. laevis DNA reveals that each of the three genes is present in at least two copies. In Xenopus tropicalis, a similar Southern blot analysis demonstrates that the three alpha actin genes exist as single copy. This result correlates with the genome duplication that has been proposed to have occurred recently in a X. laevis ancestor. A sequence comparison of the X. laevis cardiac and skeletal muscle actin cDNAs shows that the encoded peptides are highly conserved. Nevertheless, the numerous nucleotide changes at silent mutation sites suggest that the genes originated before the amphibia/reptile-bird divergence, more than 350 million years ago. Comparison of the promoters of the cardiac and skeletal actin genes, which are co-expressed in embryos, reveals a few common structural sequence elements.

Caldesmon is an elongated, flexible molecule localized in the actomyosin domains of smooth muscle

A rapid purification procedure has been developed for the isolation of caldesmon from hog stomach smooth muscle utilizing a KI extract of washed myofibrils as source material. On SDS-PAGE this mammalian caldesmon showed a closely-spaced doublet around 155 kd. By low-angle rotary shadowing caldesmon was shown to be an elongated, highly flexible molecule which tends to form end-to-end dimers that are structurally very similar to filamin. When added to F-actin solutions caldesmon increased the high-shear viscosity considerably, but by an extent that depended on sample preparation. The effect was shown to be due to caldesmon and not to a trace contaminant by its full reversibility after addition of a monospecific caldesmon antibody. Recent investigations have shown that in smooth muscle two structurally distinct domains can be distinguished: an actomyosin domain and an actin-intermediate filament domain. Immunocytochemistry of ultrathin sections of smooth muscle at the light and electron microscope level revealed that caldesmon is present in the actomyosin domain. Caldesmon is thus a potential regulator of the actomyosin system in smooth muscle.

Fragmentation of myofibrils, limited proteolysis and water holding capacity of meat

Protein changes in ageing meat result in increased vulnerability of the myofibrils to fragmentation, caused possibly by limited proteolysis. It was investigated which groups of muscle proteases, if any, were involved and what was the relation between fragmentation and hydration of beef meat. In samples ranging in natural pH from 5.4 to 7.0 the least fragmentation after 3 days at 2 degrees C was at pH 6. This could suggest the role of both the cathepsins and neutral proteases. In samples aged in the presence of EDTA fragmentation was significantly lower than in the controls. This could indicate the role of Ca2+ activated neutral proteases, or support the hypothesis on the nonenzymatic mechanism involving Ca2+. The results of PAG electrophoresis could not have been due to the neutral proteases, as the 30,000 g X mol-1 component, resulting from the hydrolysis of troponin T, did not accumulate at pH 7 until the 9th day of ageing, but at pH 5.4 the intensity of this band increased markedly already after 3 days. There was no correlation between the fragmentation and the hydration of the aged meat after cooking. The addition of 0.001% of trypsin or 0.0005% of papain to minced meat did not cause after 9 days any increase in the contents of free amino acids and peptides or significant changes in the PAGE pattern as compared to those in the controls. However, the fragmentation and hydration of the raw meat was larger in the samples containing added enzymes. After cooking the hydration of the samples did not differ.(ABSTRACT TRUNCATED AT 250 WORDS)

Myofibrillar M-band proteins represent constituents of native thick filaments, frayed filaments and bare zone assemblages

A-segments, native thick filaments, frayed filaments, bare zone assemblages, as well as completely disassembled and reassembled thick filaments from chicken pectoralis major were investigated for the presence of M-band proteins by the colloidal gold labelling technique. Specific polyclonal antibodies against the three M-band proteins identified to date, MM-creatine kinase, M-protein (165 kDa) and a 185 kDa protein myomesin, were prepared. Incubation with anti-M-protein and anti-myomesin antibodies resulted in heavy labelling of all thick filament types mentioned above, with the exception of the completely disassembled and reassembled thick filaments. In that case no labelling was detected with either antibody. In contrast, MM-creatine kinase which is an integral component of the intact M-band structure was detectable on isolated native thick filaments with lower frequency and to a variable extent. Also, bare zone assemblages were only rarely labelled by anti-MM-creatine kinase antibodies. This study shows that the 'cuff-like' additional material which had previously been observed in the middle of the bare zone of isolated thick filaments represent remnants of all three M-band proteins, whereas the extra material in intact bare zone assemblages mainly consists of myomesin and M-protein, but not of MM-creatine kinase. Myomesin and M-line protein may be important for the assembly and structural maintenance of thick filaments as well as for anchoring of additional M-band proteins, e.g. MM-creatine kinase which is bound less tightly to thick filaments and, in accordance with earlier results, seems to represent within the M-band some of the prominent bridge-forming structures.