The adult fast isozyme of myosin is present in a nerve-muscle tissue culture system

Functional evaluation of nerve-skeletal muscle constructs engineered in vitro

Previously, we have engineered three-dimensional (3-D) skeletal muscle constructs that generate force and display a myosin heavy-chain (MHC) composition of fetal muscle. The purpose of this study was to evaluate the functional characteristics of 3-D skeletal muscle constructs cocultured with fetal nerve explants. We hypothesized that coculture of muscle constructs with neural cells would produce constructs with increased force and adult MHC isoforms. Following introduction of embryonic spinal cord explants to a layer of confluent muscle cells, the neural tissue integrated with the cultured muscle cells to form 3-D muscle constructs with extensions. Immunohistochemical labeling indicated that the extensions were neural tissue and that the junctions between the nerve extensions and the muscle constructs contained clusters of acetylcholine receptors. Compared to muscles cultured without nerve explants, constructs formed from nerve- muscle coculture showed spontaneous contractions with an increase in frequency and force. Upon field stimulation, both twitch (2-fold) and tetanus (1.7-fold) were greater in the nerve-muscle coculture system. Contractions could be elicited by electrically stimulating the neural extensions, although smaller forces are produced than with field stimulation. Severing the extension eliminated the response to electrical stimulation, excluding field stimulation as a contributing factor. Nerve- muscle constructs showed a tendency to have higher contents of adult and lower contents of fetal MHC isoforms, but the differences were not significant. In conclusion, we have successfully engineered a 3-D nerve-muscle construct that displays functional neuromuscular junctions and can be electrically stimulated to contract via the neural extensions projecting from the construct.

C2C12 Co-culture on a fibroblast substratum enables sustained survival of contractile, highly differentiated myotubes with peripheral nuclei and adult fast myosin expression

We describe a simple culture method for obtaining highly differentiated clonal C2C12 myotubes using a feeder layer of confluent fibroblasts, and document the expression of contractile protein expression and aspects of myofibre morphology using this system. Traditional culture methods using collagen- or laminin-coated tissue-culture plastic typically results in a cyclic pattern of detachment and reformation of myotubes, rarely producing myotubes of a mature adult phenotype. C2C12 co-culture on a fibroblast substratum facilitates the sustained culture of contractile myotubes, resulting in a mature sarcomeric register with evidence for peripherally migrating nuclei. Immunoblot analysis demonstrates that desmin, tropomyosin, sarcomeric actin, alpha-actinin-2 and slow myosin are detected throughout myogenic differentiation, whereas adult fast myosin heavy chain isoforms, members of the dystrophin-associated complex, and alpha-actinin-3 are not expressed at significant levels until >6 days of differentiation, coincident with the onset of contractile activity. Electrical stimulation of mature myotubes reveals typical and reproducible calcium transients, demonstrating functional maturation with respect to calcium handling proteins. Immunocytochemical staining demonstrates a well-defined sarcomeric register throughout the majority of myotubes (70-80%) and a striated staining pattern is observed for desmin, indicating alignment of the intermediate filament network with the sarcomeric register. We report that culture volume affects the fusion index and rate of sarcomeric development in developing myotubes and propose that a fibroblast feeder layer provides an elastic substratum to support contractile activity and likely secretes growth factors and extracellular matrix proteins that assist myotube development.

Immunohistochemical characterization of human masseter muscle spindles

An enzyme- and immunohistochemical study has been performed on human masseter muscle spindles. Antibodies selective for different myosin heavy chain (MHC) isoforms and M-band proteins (M-protein, myomesin, and MM-CK) were used. The expression of these proteins was determined in the different intrafusal fiber types. Nuclear bag1 and nuclear bag2 fibers expressed predominantly slow-twitch and slow-tonic MHCs. The bag2 fibers in addition contained fetal MHC. Nuclear chain fibers coexpressed embryonic, fetal, and fast-twitch MHCs. The bag2 and chain fibers contained all three M-band proteins, whereas the bag1 fibers contained only myomesin. In general the MHC expression in the human masseter intrafusal fiber types was similar to that previously reported for limb muscles in man as well as for limb and masseter muscles in other species. However, the number of intrafusal fibers per spindle was unusually high (up to 36). This reinforces the idea that masseter muscle spindles have a strong proprioceptive impact during the control of jaw movements.

Regeneration of cat posterior temporalis muscle in culture

Cat posterior temporalis muscle has a rapid speed of contraction associated with a unique superfast myosin isoform. Superfast myosin expression appears to be an intrinsic property of the muscle fibres and satellite cells, though in culture they failed to express superfast myosin. We have, therefore, cultured this muscle in a system which had previously been shown to encourage the expression of an adult phenotype. The presence of nerve cells resulted in effective regeneration of cat posterior temporalis muscle and even the formation of functional neuromuscular junctions. However, superfast myosin was not found even in mature, contracting, innervated cultures. Thyroid hormone, a known regulator of myosin isoform expression, also failed to elicit superfast myosin expression. Different culture conditions may allow a different outcome, but under circumstances in which mouse muscle expresses an adult phenotype, cat posterior temporalis muscle fails to do so.

Localization of dystrophin in cultures of human muscle

Dystrophin is a protein present in normal human muscle but absent in patients with Duchenne muscular dystrophy (DMD). Using a specific antibody, we have investigated the expression of dystrophin in human muscle which had regenerated in culture in the presence of nerve cells. Dystrophin was present and was correctly localized in the cultures of normal muscle but was absent from cultures of muscle from patients with DMD. Its control and function can now be studied.

Ventral and dorsal horn acetylcholinesterase neurons are maintained in organotypic cultures of postnatal rat spinal cord explants

Transverse sections of postnatal rat spinal cord have been cultured using the organotypic roller tube method. These explant cultures retain identifiable anatomical landmarks, allow identification of individual neurons, can be maintained for up to 8 weeks, and undergo maturational changes in vitro. Putative ventral horn motoneurons were identified in these cultures by localization to ventral horn regions analogous to those of motoneurons in vivo and by staining for choline acetyltransferase (ChAT) immunoreactivity and acetylcholinesterase (AChE) activity. Morphometric studies of the photomicrographic areas of cell bodies of these ventral horn neurons in intact cultures show a range of sizes up to 1635 microns 2 with the average size being 245 +/- 7 microns 2 (n = 724) (average +/- S.E.M.). The size ranges are roughly comparable to cross-sectional areas determined previously for ventral horn motoneurons in vivo. Dorsal horn regions of these cultures also developed prominent AChE activity that was absent at explantation. Biochemical analysis of ChAT and AChE activity in pooled samples of whole cultures showed ChAT activity to be 0.48 +/- 0.08 (n = 7) mumol/min/g protein and AChE activity to be 12.2 +/- 2.0 (n = 7) mumol/min/g protein at 37 degrees C (averages +/- S.E.M.). These values are comparable to previously reported values for neonatal rat spinal cord in situ. Organotypic roller tube cultures of postnatal rat spinal cord provide an attractive system for studies of survival, morphology, growth and differentiation of mammalian ventral horn neurons in vitro.

Myosin heavy chain expression in human muscle cocultured with mouse spinal cord

Monoclonal antibodies which specifically recognise the neonatal or adult fast (type 2A and 2B) or slow myosin heavy chain (MHC) were used to investigate the myosin composition of human muscle which had regenerated in culture in the presence or absence of embryonic mouse spinal cord tissue. Adult fast MHC was found in an average of 75% of the cultured fibres, irrespective of the time in culture, the source of the muscle, the presence of neurones or modifications to the growth medium. It was not found alone but was always in association with the neonatal and/or slow myosin isoforms. The expression of fast MHC in human muscle therefore differs from that in mouse muscle in this culture system (Ecob-Prince et al. 1986), but is still strong evidence for the development of at least one aspect of an adult phenotype in culture.

Parvalbumin in mouse muscle in vivo and in vitro

Parvalbumin is a cytosolic calcium-binding protein found in adult fast-twitch mammalian muscle. Using an antibody to paravalbumin, we have shown that its distribution in adult mouse muscles is associated with certain fibre types. It is absent from slow-twitch type 1 fibres, is absent or at low levels in fast-twitch type 2A fibres, but is present at moderate or high levels in fast-twitch type 2B fibres. When adult mouse muscle is cultured with embryonic mouse spinal cord, the regenerated fibres become innervated, express the adult fast isoform of myosin heavy chain and appear histochemically as fast-twitch fibres. We therefore investigated whether these apparently mature fibres also contained parvalbumin. Parvalbumin was not found in any fibres of twenty mature cultures, suggesting that neurotrophic activity in the absence of specific adult nerve activity patterns was insufficient to cause the expression of parvalbumin in the cultures.

Maintenance of highly contractile tissue-cultured avian skeletal myotubes in collagen gel

Highly contractile skeletal myotubes differentiated in tissue culture are normally difficult to maintain on collagen-coated tissue culture dishes for extended periods because of their propensity to detach as a sheet of cells from their substratum. This detachment results in the release of mechanical tension in the growing cell "sheet" and, consequently, loss of cellular protein. We developed a simple method of culturing high density contractile primary avian myotubes embedded in a collagen gel matrix (collagel) attached to either a stainless steel mesh or nylon support structure. With this system the cells are maintained in a highly contractile state for extended periods in vitro under tension. Structural integrity of the myotubes can be maintained for up to 10 d in basal medium without serum or embryo extract. Total cellular protein and myosin heavy chain accumulation in the cells can be maintained for weeks at levels which are two to three times those found in time-matched controls that are under little tension. Morphologically, the myotubes are well differentiated with structural characteristics of neonatal myofibers. This new collagel culture system should prove useful in the analysis of in vitro gene expression during myotube to myofiber differentiation and its regulation by various environmental factors such as medium growth factors, innervation, and mechanical activity.

Morphological differentiation of human muscle cocultured with mouse spinal cord

Human muscle fibres have been cocultured with sections of embryonic mouse spinal cord for periods of up to 2 months. The muscle fibres regenerated to form a bundle of myotubes, a proportion of which developed cross-striations and contractions. This proportion was variable between biopsies, and morphological differentiation was not as successful as when mouse muscle and mouse nerve were cultured together. Regeneration and morphological differentiation were unaffected by storing samples in liquid nitrogen, and were not improved by the presence of original synaptic areas in the explanted bundle or by alterations in the growth media. These involved changing the levels of serum and embryo extract, using different sources of serum, and the incorporation of additives in the medium. A comparison of the growth characteristics of samples of muscle from 30 patients (including some control samples) indicated that although muscle from younger patients (less than 14 years) regenerated more quickly, the myotubes did not have better differentiation. It also indicated that the growth characteristics of regenerated myotubes from diseased and normal muscle were indistinguishable within the 4-8 weeks observation period. Muscle from patients with Duchenne muscular dystrophy regenerated and differentiated less well than would be expected from age-matched controls, but this was not thought to reflect an intrinsic abnormality in the regenerative capacity of the muscle.

Neuromuscular junctions in adult and developing fast and slow muscles

Functional changes that occur just before hatching in future fast muscles of the chicken are thought to be influenced by the pattern of innervation. We have compared the neuromuscular junctions of two fast muscles, the posterior latissimus dorsi (PLD) and the pectoralis, which differ in their myosin composition at 18 days in ovo. We have also presented new information on the neuromuscular junctions of the adult fast muscles and an adult slow muscle, the anterior latissimus dorsi (ALD). Both categories of adult muscles were heterogeneous, and there was little difference between endplates of the two fast muscles or between the fast and slow muscles. In contrast, there were significant structural differences between the two fast muscles during embryonic development. In early embryonic muscle fibers, which synthesize embryonic forms of myosin, individual motor endplates were contacted by multiple axon terminals. At 18 days in ovo, the majority of the neuromuscular junctions in the pectoralis continued to be multiterminal, whereas all but one of the terminals had been withdrawn from each endplate in the PLD. This single terminal had a unique form that distinguished it from the embryonic pectoralis and also from the two adult muscles. By 7 days after hatching, the neuromuscular junctions of both muscles had single terminals. They were different from the embryonic terminals, though not necessarily equivalent to adult terminals. The results show that multiple terminals persist at 18 days in ovo in the muscle that continues to express an embryonic myosin, but they have been withdrawn from the muscle that has lost this myosin. It is concluded, from combined data on the two muscles, that maturation of the neuromuscular junction during embryonic and late posthatch development is correlated with transitions in the myosin pattern and in contractile properties.

Developmental pattern of mouse skeletal myosin heavy chain gene transcripts in vivo and in vitro

We have studied the transcripts of the embryonic, perinatal, and adult fast myosin heavy chain (MHC) genes in mouse skeletal muscle in vivo before and after birth, and in vitro in myogenic cell lines. In vivo, in 15-day fetal muscle, embryonic and perinatal MHC mRNAs are both present, and the former is the major transcript. By 18 days the perinatal is predominant and the adult MHC mRNA appears. In beta-bungarotoxin-treated fetuses, a similar developmental pattern is detected, suggesting that it is nerve-independent and that primary myotubes alone undergo the same developmental changes. In vitro, in the absence of the nerve, embryonic, perinatal, and adult IIB MHC mRNAs accumulate. The level of the latter two isomRNAs is influenced by culture conditions.

Expression of myosin heavy chain isoforms in regenerating myotubes of innervated and denervated chicken pectoral muscle

Monoclonal antibodies were prepared to stage-specific chicken pectoral muscle myosin heavy chain isoforms. From comparison of serial sections reacted with these antibodies, the myosin heavy chain isoform composition of individual myofibers was determined in denervated pectoral muscle and in regenerating myotubes that developed following cold injury of normal and denervated muscle. It was found that the neonatal myosin heavy chain reappeared in most myofibers following denervation of the pectoral muscle. Regenerating myotubes in both innervated and denervated muscle expressed all of the myosin heavy chain isoforms which have thus far been characterized in developing pectoral muscle. However, the neonatal and adult myosin heavy chains appeared more rapidly in regenerating myotubes compared to myofibers in developing muscle. While the initial expression of these isoforms in the regenerating areas was similar in innervated and denervated muscles, the neonatal myosin heavy chain did not disappear from noninnervated regenerating fibers. These results indicate that innervation is not required for the appearance of fast myosin heavy chain isoforms, but that the nerve plays some role in the repression of the neonatal myosin heavy chain.

Contractile activity is required for the expression of neonatal myosin heavy chain in embryonic chick pectoral muscle cultures

The expression of neonatal myosin heavy chain (MHC) was examined in developing embryonic chicken muscle cultures using a monoclonal antibody (2E9) that has been shown to be specific for that isoform (Bandman, E., 1985, Science (Wash. DC), 227: 780-782). After 1 wk in vitro some myotubes could be stained with the antibody, and the number of cells that reacted with 2E9 increased with time in culture. All myotubes always stained with a second monoclonal antibody that reacted with all MHC isoforms (AG19) or with a third monoclonal antibody that reacted with the embryonic but not the neonatal MHC (EB165). Quantitation by ELISA of an extract from 2-wk cultures demonstrated that the neonatal MHC represented between 10 and 15% of the total myosin. The appearance of the neonatal isoform was inhibited by switching young cultures to medium with a higher [K+] which has been shown to block spontaneous contractions of myotubes in culture. Furthermore, if mature cultures that reacted with the neonatal antibody were placed into high [K+] medium, neonatal MHC disappeared from virtually all myotubes within 3 d. The effect of high [K+] medium was reversible. When cultures maintained in high [K+] medium for 2 wk were placed in standard medium, which permitted the resumption of contractile activity, within 24 h cells began to react with the neonatal specific antibody, and by 72 h many myotubes were strongly positive. Since similar results were also obtained by inhibiting spontaneous contractions with tetrodotoxin, we suggest that the development of contractile activity is not only associated with the maturation of myotubes in culture, but may also be the signal that induces the expression of the neonatal MHC.

Duchenne dystrophic muscle develops lesions in long-term coculture with mouse spinal cord

When strips of human skeletal muscle from biopsies of normal children and donors with Duchenne muscular dystrophy (DMD) are explanted in organotypic coculture with fetal mouse spinal cord, many regenerating muscle fibers develop, become innervated, and maintain a remarkable degree of mature structure and function for more than 3-6 months in vitro. Sequential light microscopy in correlation with electron-microscopic and electrophysiologic analyses showed that despite cross-species innervation, these human muscle fibers develop stable cross-striations, peripherally positioned myonuclei, and mature, functional motor endplates. Of special interest is the onset of significant progressive abnormalities, e.g., unusual focal myofibrillar lesions, in substantial numbers of innervated mature DMD muscle fibers after 2-4 months in culture. The focal myofibrillar lesions were not detected in normal muscle fibers maintained as long as 6 months in coculture, nor are they comparable to the generalized loss of cross-striations observed in muscle atrophy following in vitro denervation of mature DMD fibers.

Developmental progression of myosin gene expression in cultured muscle cells

Myosin heavy chains are encoded by distinct members of a multigene family at different stages of muscle development. Study of the underlying regulatory mechanisms has been hindered because transitions in myosin expression have not been readily attained in tissue culture. Here we show a transition from early (fetal) to late (perinatal/adult) myosins defined by two monoclonal antibodies, F1.652 and N3.36, in the myotubes of mouse C2C12 cells. On day 1 of differentiation, essentially all myosin was early myosin. By day 8, early myosin dropped to 25% of its day 1 value and was replaced by late myosin. The transition occurred without neural contact, connective tissue components, or complex substrates, suggesting that its regulation may be intrinsic to the muscle cell. Our results demonstrate that a developmental progression in myosin gene expression, which occurs rapidly, with high frequency, and under relatively simple conditions, is now amenable to molecular analysis in cultured muscle cells.

Neonatal and adult myosin heavy chain isoforms in a nerve-muscle culture system

When adult mouse muscle fibers are co-cultured with embryonic mouse spinal cord, the muscle regenerates to form myotubes that develop cross-striations and contractions. We have investigated the myosin heavy chain (MHC) isoforms present in these cultures using polyclonal antibodies to the neonatal, adult fast, and slow MHC isoforms of rat (all of which were shown to react specifically with the analogous mouse isoforms) in an immunocytochemical assay. The adult fast MHC was absent in newly formed myotubes but was found at later times, although it was absent when the myotubes myotubes were cultured without spinal cord tissue. When nerve-induced muscle contractions were blocked by the continuous presence of alpha-bungarotoxin, there was no decrease in the proportion of fibers that contained adult fast MHC. Neonatal and slow MHC were found at all times in culture, even in the absence of the spinal cord, and so their expression was not thought to be nerve-dependent. Thus, in this culture system, the expression of adult fast MHC required the presence of the spinal cord, but was probably not dependent upon nerve-induced contractile activity in the muscle fibers.

Altered synthesis of myosin light chains is associated with contractility in cultures of differentiating chick embryo breast muscle

Cultured chick embryo skeletal muscle cells normally synthesize only the embryonic isoform of mysoin. We have found that aneural muscle cultures that become or are provoked into an extremely contractile state will begin to synthesize a pattern of myosin light chains typical of maturing muscle. Immunoblots with neonatal and adult specific monoclonal antibodies did not reveal a corresponding isozyme transition in myosin heavy chain. These results demonstrate a correlation between contractility and the regulation of myosin light chain maturation, and also suggest that the transitions of heavy and light chain synthesis during development do not appear to be under close coordinate regulation.

Pheochromocytoma PC12 cells influence the differentiation of co-cultured skeletal myotubes

Rat skeletal myotubes were cultured in the presence or absence of pheochromocytoma PC12 cells under experimental conditions permitting the maintenance of the muscle cells for up to 5 weeks. The number of acetylcholine receptors (AChRs), estimated using radioiodinated alpha-bungarotoxin (alpha-BTX), was comparable in both types of culture for the first 2 weeks. After prolonged incubation (4 weeks), however, alpha-BTX binding sites were two times more abundant in the PC12-myotube cultures than in the myotube cultures. This is likely to reflect a higher number of AChRs at the surface of the myotubes, for control experiments showed that the contribution of PC12 cells was negligible. Presence of PC12 cells also increased the number of AChR aggregates. Use of fluorescent alpha-BTX demonstrated that mixed cultures contained two times more receptor patches than did myotubes alone during the first 2 weeks of culture, and up to five times more after 4 weeks. Essentially all the tight contacts established between PC12 cells and myotubes were characterized by a receptor cluster and, in some cases, by an aggregate of an acetylcholine esterase (AChE), as visualized by immunohistochemistry. Furthermore an antiserum specific for the adult (fast) isoform of the myosin heavy chain stained some myotubes in long-term, mixed cultures. The antigen was never observed in sister cultures of myotubes alone. These data show that PC12 cells, which are thought to derive from the medullary part of the adrenal gland, can influence the differentiation of skeletal muscle cells in several ways.

Muscle regeneration induced by cells autografted in adult rats

Right triceps surae of 3-week-old Wistar rats were minced and devitalized with liquid nitrogen, a treatment which completely inhibits their ability to regenerate when they are orthotopically autografted. In a first series of experiments, cells were isolated from the left triceps surae, mixed with the devitalized right mince and autografted; in a second series, cells were moreover allowed to proliferate in vitro for a few weeks before being grafted. The regenerates were examined 60 days after surgery. In the first series, all the regenerates were contractile and developed a maximal isometric tetanic force of 18 +/- 6 mN (n = 5); they contained 152 +/- 80 muscle fibres located proximally, the number of which decreased along the proximo-distal axis, being 24 +/- 24 in the median part of the regenerate. The muscle fibres appeared histologically normal except for their shortness (less than 10 mm) and narrowness (mean luminal diameter: 30 microns). In the second series, 2 out of 5 regenerates were comparable with those of the first series except that their fibres were shorter; the 3 other regenerates were unexcitable. These experiments demonstrate that cells isolated from an adult striated muscle are able to regenerate striated muscle fibres in an adult animal and that these cells can retain this property if they are grown in culture.