Tissue culture studies of muscle disorders: Part 1. Techniques, cell growth, morphology, cell surface

Analyses of the differentiation potential of satellite cells from myoD-/-, mdx, and PMP22 C22 mice

Background

Sporadic and sometimes contradictory studies have indicated changes in satellite cell behaviour associated with the progressive nature of human Duchenne muscular dystrophy (DMD). Satellite cell proliferation and number are reportedly altered in DMD and the mdx mouse model. We recently found that satellite cells in MSVski transgenic mice, a muscle hypertrophy model showing progressive muscle degeneration, display a severe ageing-related differentiation defect in vitro. We tested the hypothesis that similar changes contribute to the gradual loss of muscle function with age in mdx and PMP22 mice, a model of human motor and sensory neuropathy type 1A (HMSN1A).

Methods

Single extensor digitorum longus muscle fibres were cultured from mdx and PMP22 mice and age- and genetic background-matched controls. Mice at several ages were compared with regard to the differentiation of satellite cells, assayed as the proportion of desmin-expressing cells that accumulated sarcomeric myosin heavy chain.

Results

Satellite cells of 2 month, 6 month, and 12 month old mdx mice were capable of differentiating to a similar extent to age-matched wild type control animals in an in vitro proliferation/differentiation model. Strikingly, differentiation efficiency in individual 6 month and 12 month old mdx animals varies to a much higher extent than in age-matched controls, younger mdx animals, or PMP22 mice. In contrast, differentiation of myoblasts from all myoD null mice assayed was severely impaired in this assay system. The defect in satellite cell differentiation that occurs in some mdx animals arises from a delay in differentiation that is not overcome by IGF-1 treatment at any phase of cultivation.

Conclusion

Overall, a defect in satellite cell differentiation above that arising through normal ageing does not occur in mdx or PMP22 mouse models of human disease. Nonetheless, the impaired differentiation of satellite cells from some mdx animals suggests that additional factors, environmental or epigenetic, may lead to deteriorating muscle repair through poor differentiation of satellite cells in genetically predisposed individuals.

Aging normal and dystrophic mouse muscle: analysis of myogenicity in cultures of living single fibers

In the early stages of Duchenne muscular dystrophy, chronic muscle degeneration is counterbalanced by regeneration whose progressive failure beyond the fifth year is attributed to an accelerated senescence associated with excessive myogenic cell division. This idea was tested by counting the numbers of myogenic cells accumulating over 90 h around individual living fibers isolated from muscles of dystrophic (mdx) and normal mice, 14-550 days of age. In cultures of normal muscle fibers, the number of myogenic cells per fiber decreased with increasing age of the animal. Cultures from young mdx mice exhibited an age-related increase in myogenic cell number, attaining at 50 days three times the number in control cultures. Thereafter, myogenic cell number progressively declined with animal age to control values. Thus, there is no evidence that extensive myoblast proliferation in young dystrophic mice-and, by association, in Duchenne muscular dystrophy patients-depletes their myogenic responsiveness in later life when they become weak and wasted.

Acetylcholine receptors in innervated muscles of dystrophic mdx mice degrade as after denervation

Acetylcholine receptors (AChRs) are present at the top of the postsynaptic membrane of the neuromuscular junction (NMJ) at very high density, possibly anchored to cytoskeletal elements. The present study investigated whether AChR degradation is affected in animals lacking dystrophin, a protein that is an integral part of the cytoskeletal complex and is missing in Duchenne muscular dystrophy. The animal model for Duchenne muscular dystrophy, the mutant mdx mouse, was used to determine whether disruption of the cytoskeleton, caused by the absence of dystrophin, affects AChR degradation. Of the two populations of junctional AChRs, Rs (expressed in innervated adult muscles) and Rr (expressed in embryonic or denervated muscles), only Rs are affected in mdx animals. In innervated mdx soleus, diaphragm, and sternomastoid muscles, the AChRs have an accelerated degradation rate (t1/2 of approximately 3-5 d), similar to that acquired by Rs in control muscles after denervation. The Rs in mdx NMJs do not accelerate further when the muscles are denervated. The absence of dystrophin does not affect the degradation rate of the Rr AChRs (t1/2 of 1 d), which are expressed after denervation in mdx as in control muscles. These results suggest that dystrophin or an intact cytoskeletal complex may be required for neuronal stabilization of Rs receptors at the adult neuromuscular junctions.

Ion channels in a skeletal muscle cell line from a Duchenne muscular dystrophy patient

A cell line (RCDMD), derived from a muscle biopsy taken from a 7-year-old patient with Duchenne muscular dystrophy (DMD), was established in vitro using conditioned media from the UCHT1 thyroid cell line as described elsewhere (Biochim Biophys Acta 1992; 1134:247-255). Unlike other cell lines established by the same procedure, RCDMD cells were highly refractory to transformation and the resulting cell line grew slowly with a doubling time of approximately 72 h. Further, cells continue to grow after more than 20 doublings and 15 passages. Some of the characteristics of the cell line include lack of reaction with antidystrophin antibodies and the presence of receptors for the dihydropyridine PN200-110 (Kd) = 0.3 +/- 0.05 nmol/L and Bmax = 1.06 +/- 0.03 pmol/mg protein) and for alpha-bungarotoxin (Kd = 1.02 +/- 0.17 nmol/L and Bmax = 4.2 +/- 0.37 pmol/mg protein). Patch clamped cells in the voltage clamp configuration lack ion currents when growing in complete medium with high serum, but they can be induced to differentiate by serum deprivation and addition of hormones and trace elements. After 5 days in differentiating medium, noninactivating, delayed rectifier potassium currents are seen. At day 12, A-type, inactivating potassium currents as well as transient inward currents are seen. In conditions in which sodium and potassium currents are absent, a very fast activating and fast inactivating calcium current was evident. The cell line offers the possibility of studying cellular mechanisms in the pathophysiology of DMD.

Diseases and disorders of muscle

Muscle may suffer from a number of diseases or disorders, some being fatal to humans and animals. Their management or treatment depends on correct diagnosis. Although no single method may be used to identify all diseases, recognition depends on the following diagnostic procedures: (1) history and clinical examination, (2) blood biochemistry, (3) electromyography, (4) muscle biopsy, (5) nuclear magnetic resonance, (6) measurement of muscle cross-sectional area, (7) tests of muscle function, (8) provocation tests, and (9) studies on protein turnover. One or all of these procedures may prove helpful in diagnosis, but even then identification of the disorder may not be possible. Nevertheless, each of these procedures can provide useful information. Among the most common diseases in muscle are the muscular dystrophies, in which the newly identified muscle protein dystrophin is either absent or present at less than normal amounts in both Duchenne and Becker's muscular dystrophy. Although the identification of dystrophin represents a major breakthrough, treatment has not progressed to the experimental stage. Other major diseases of muscle include the inflammatory myopathies and neuropathies. Atrophy and hypertrophy of muscle and the relationship of aging, exercise, and fatigue all add to our understanding of the behavior of normal and abnormal muscle. Some other interesting related diseases and disorders of muscle include myasthenia gravis, muscular dysgenesis, and myclonus. Disorders of energy metabolism include those caused by abnormal glycolysis (Von Gierke's, Pompe's, Cori-Forbes, Andersen's, McArdle's, Hers', and Tauri's diseases) and by the acquired diseases of glycolysis (disorders of mitochondrial oxidation). Still other diseases associated with abnormal energy metabolism include lipid-related disorders (carnitine and carnitine palmitoyl-transferase deficiencies) and myotonic syndromes (myotonia congenita, paramyotonia congenita, hypokalemic and hyperkalemic periodic paralysis, and malignant hyperexia). Diseases of the connective tissues discussed include those of nutritional origin (scurvy, lathyrism, starvation, and protein deficiency), the genetic diseases (dermatosparaxis, Ehlers-Danlos syndrome, osteogenesis imperfecta, Marfan syndrome, homocystinuria, alcaptonuria, epidermolysis bullosa, rheumatoid arthritis in humans, polyarthritis in swine, Aleutian disease of mink, and the several types of systemic lupus erythematosus) and the acquired diseases of connective tissues (abnormal calcification, systemic sclerosis, interstitial lung disease, hepatic fibrosis, and carcinomas of the connective tissues). Several of the diseases of connective tissues may prove to be useful models for determining the relationship of collagen to meat tenderness and its other physical properties. Several other promising models for studying the nutrition-related disorders and the quality-related characteristics of meat are also reviewed.

A human skeletal muscle cell line obtained from an adult donor

A cell line (RCMH) in permanent culture was established from surgically removed adult normal human skeletal muscle by exposure to conditioned media obtained from thyroid cells. Cells proliferated indefinitely but displayed density inhibition of growth while maintaining some differentiated markers. Under certain incubation conditions, cells fused into myotube-like structures, with a concomitant increase in muscle specific proteins, such as human myoglobin, skeletal muscle myosin, desmin and dystrophin, as identified using immunocytochemical procedures. In addition, RCMH cells displayed high affinity receptors for alpha-bungarotoxin (Bmax = 0.7 pmol/mg protein, Kd = 1.5 nM) and dihydropyridines (Bmax = 0.3 pmol/mg protein, Kd = 0.5 nM for [3H]PN200-110); these values are comparable to those reported for muscle cells in primary culture. Patch-clamp studies showed the presence of 42 pS carbachol gated channels and of 5 pS calcium channels (current carried by barium); chloride and potassium channels were also seen. This new cell line appears to be a convenient model system to study skeletal muscle function.

Cellular ageing of Alzheimer's disease and Down syndrome cells in culture

In Alzheimer's disease, the typical clinical symptoms and the pathological findings are restricted to the nervous system. Nevertheless, like in some other neurologic-metabolic disorders, several alterations are found in peripheral tissues. The aim of this study was to examine whether cellular properties which can be studied in vitro on skin fibroblast cultures obtained from Alzheimer's disease patients differ from those of age-matched controls. Down syndrome patients were also included, since the same neuropathological findings are present in nearly 100% of Down syndrome patients. Since Alzheimer's disease is an age-related disorder, we examined the growth characteristics of skin fibroblast cultures. The in vitro senescence of cultured fibroblasts is widely accepted as a model for in vivo ageing. Normal growth properties were found. We can conclude that there is no premature ageing in Alzheimer's disease nor in Down syndrome and that the abnormalities found in peripheral tissues are related to the disease itself. The beta amyloid precursor protein (beta APP) has been shown to have adhesive interactions. We therefore investigated several parameters of adhesion in the skin fibroblast cultures: adhesion to a fibronectin coat, adhesion to extracellular matrix of Alzheimer's disease cultures and semi-quantification of adhesion-related molecules (beta 1-integrin, cell surface proteoglycans, extracellular matrix proteoglycans, extracellular matrix fibronectin). No significant difference was found in the parameters examined.

Effect of various agents on the cytoplasmic calcium concentration in cultured human muscle cells

1. We determined the cytoplasmic Ca2+ concentration ([Ca2+]i) in cultured human muscle cells using the fluorescent indicator Quin-2. 2. The [Ca2+]i was dependent on the external Ca2+ concentration. Acetylcholine in the presence of external Ca2+ caused a transient increase in [Ca2+]i. Inhibition by nifedipine indicated that this response was mediated through activated voltage-operated channels. In nominally Ca2(+)-free buffer acetylcholine did not markedly increase [Ca2+]i. Therefore, the increase in [Ca2+]i as a response to depolarization is mainly due to influx of external Ca2+. 3. Various concentrations of caffeine did not influence the [Ca2+]i. Dantrolene decreased [Ca2+]i, both in the presence and absence of external Ca2+. The reduction probably resulted from an action of dantrolene on the intracellular Ca2+ stores, since dantrolene did not influence 45Ca2+ influx or efflux and caffeine partially counteracted the reduction.

Calcium currents in normal and dystrophic human skeletal muscle cells in culture

Human muscle cells obtained from biopsy specimens were grown in a primary culture system and electrophysiologically studied. Whole cell patch-clamp recordings revealed the presence of two types of calcium currents: (i) a low-threshold (-60 mV) one (ICa, T) with fast activation and inactivation kinetics (time-to-peak: 39 ms at -30 mV); and (ii) a high-threshold (-10 mV) one (ICa,L) with slower kinetics (time-to-peak: 550 ms at 20 mV). These two types of calcium currents could be also distinguished by their pharmacological characteristics since ICa,L was sensitive to the antagonist and agonist dihydropyridine derivatives contrary to ICa,T which was completely resistant to these compounds. These functional calcium channels existed both in normal and Duchenne dystrophic (DMD) human skeletal muscle cells in culture. We discuss a possible role of these two types of calcium channels in the myoplasmic calcium accumulation observed in the Duchenne muscular dystrophy.

Molecular and functional analysis of the muscle-specific promoter region of the Duchenne muscular dystrophy gene

Duchenne muscular dystrophy (DMD) gene transcripts are most abundant in normal skeletal and cardiac muscle and accumulate as normal myoblasts differentiate into multinucleated myotubes. In this report we describe our initial studies aimed at defining the cis-acting sequences and trans-acting factors involved in the myogenic regulation of DMD gene transcription. A cosmid clone containing the first exon of the DMD gene has been isolated, and sequences lying upstream of exon 1 were analyzed for homologies to other muscle-specific gene promoters and for their ability to direct muscle-specific transcription of chimeric chloramphenicol acetyltransferase (CAT) gene constructs. The results indicate that the transcriptional start site for this gene lies 37 base pairs (bp) upstream of the 5' end of the published cDNA sequence and that 850 bp of upstream sequence can direct CAT gene expression in a muscle-specific manner. Sequence analysis indicates that in addition to an ATA and GC box, this region contains domains that have been implicated in the regulation of other muscle-specific genes: a CArG box at -91 bp; myocyte-specific enhancer-binding nuclear factor 1 binding site homologies at -58, -535, and -583 bp; and a muscle-CAAT consensus sequence at -394 bp relative to the cap site. Our observation that only 149 bp of upstream sequence is required for muscle-specific expression of a chimeric CAT gene construct further implicates the CArG and myocyte-specific enhancer-binding nuclear factor 1 binding homologies as important domains in the regulation of this gene. On the other hand, the unique profile of myogenic cell line-specific induction displayed by our DMD promoter-CAT gene constructs suggests that other as yet undefined cis-acting sequences and/or trans-acting factors may also be involved.

Molecular and functional analysis of the muscle-specific promoter region of the Duchenne muscular dystrophy gene

Duchenne muscular dystrophy (DMD) gene transcripts are most abundant in normal skeletal and cardiac muscle and accumulate as normal myoblasts differentiate into multinucleated myotubes. In this report we describe our initial studies aimed at defining the cis-acting sequences and trans-acting factors involved in the myogenic regulation of DMD gene transcription. A cosmid clone containing the first exon of the DMD gene has been isolated, and sequences lying upstream of exon 1 were analyzed for homologies to other muscle-specific gene promoters and for their ability to direct muscle-specific transcription of chimeric chloramphenicol acetyltransferase (CAT) gene constructs. The results indicate that the transcriptional start site for this gene lies 37 base pairs (bp) upstream of the 5' end of the published cDNA sequence and that 850 bp of upstream sequence can direct CAT gene expression in a muscle-specific manner. Sequence analysis indicates that in addition to an ATA and GC box, this region contains domains that have been implicated in the regulation of other muscle-specific genes: a CArG box at -91 bp; myocyte-specific enhancer-binding nuclear factor 1 binding site homologies at -58, -535, and -583 bp; and a muscle-CAAT consensus sequence at -394 bp relative to the cap site. Our observation that only 149 bp of upstream sequence is required for muscle-specific expression of a chimeric CAT gene construct further implicates the CArG and myocyte-specific enhancer-binding nuclear factor 1 binding homologies as important domains in the regulation of this gene. On the other hand, the unique profile of myogenic cell line-specific induction displayed by our DMD promoter-CAT gene constructs suggests that other as yet undefined cis-acting sequences and/or trans-acting factors may also be involved.

Muscle cell cultures in Menkes' disease: copper accumulation in myotubes

We present 64Cu uptake studies in cultured muscle cells from a one-year-old patient with Menkes' disease. The cultured muscle cells from the patient showed a five-fold higher 64Cu uptake than control muscle cells. Copper uptake in muscle cells was of the same magnitude as that found in fibroblasts from the patient and also from other Menkes' patients. The copper content of a muscle biopsy from the patient was twice that of a control biopsy. The enhanced uptake is probably copper specific, since zinc uptake was unaltered in both muscle cells and fibroblasts from the patient. Cytochrome c oxidase in the muscle of the patient was reduced to one-third of the value for controls, which is in agreement with the hypothesis that in Menkes' disease copper accumulates in a biologically non-active form. However, in cultured muscle cells and fibroblasts from the patient the cytochrome c oxidase activity was in the normal range, probably because of the relatively large amount of copper already available in the culture medium.

Dystrophin-related muscular dystrophies

The gene for the locus involved in Duchenne and Becker muscular dystrophies has been cloned and subject to intense analysis. The protein product of the locus is called dystrophin, and it has been shown to be associated with the muscle fiber membrane. The new knowledge of the molecular genetics of these disorders is being applied rapidly in clinical practice. Carrier detection and prenatal diagnosis have been revolutionized by the use of probes for the gene. These probes are also being employed to clarify cases where conventional clinical examination results in equivocal diagnoses. It is suggested that the disorders characterized by dystrophin abnormalities should be called dystrophin-related muscular dystrophies (DRMD). There are mouse and dog models for DRMD and these are being used to explore therapeutic strategies for treating DRMD patients.

Oxidative stress and muscular dystrophy

Oxidative stress may be the fundamental basis of many of the structural, functional and biochemical changes characteristic of the inherited muscular dystrophies in animals and humans. The presence of by-products of oxidative damage, and the compensatory increases in cellular antioxidants, both indicate oxidative stress may be occurring in dystrophic muscle. Changes in the proportions and metabolism of cellular lipids, abnormal functions of cellular membranes, altered activity of membrane-bound enzymes such as the SR Ca2+-ATPase, disturbances in cellular protein turnover and energy production and a variety of other changes all indicate that these inherited muscular dystrophies appear more like the results of oxidative stress to muscle than any other type of underlying muscle disturbance. Particular details of these altered characteristics of dystrophic muscle, in combination with current knowledge on the processes of oxidative damage to cells, may provide some insight into the underlying biochemical defect responsible for the disease, as well as direct research towards the ultimate goal of an effective treatment.

The binding of lipoproteins to human muscle cells: binding and uptake of LDL, HDL, and alpha-tocopherol

The specific binding of low-density lipoprotein (LDL) to cells and its subsequent uptake into these cells is well documented, but little is known of the LDL binding and uptake by skeletal muscle. Lipoproteins are the major transporters of tocopherols, deficiencies of which have been associated with a number of muscle diseases of animals. Their possible implication in human muscle diseases prompted our investigation of LDL and high-density lipoprotein (HDL) binding and uptake into human muscle cells in culture. Cultured human muscle cells were used at both the myoblast and myotube stage. They were incubated with LDL or HDL which were labelled by protein iodination or with (3H) alpha-tocopherol and receptor binding and cell uptake characteristics established. LDL binds to both myoblasts and myotubes, but the binding affinity increases significantly with the more highly differentiated cells. This binding appears to be specific to LDL receptors. The LDL is taken into the muscle cell and protein is degraded, as with other types of cells. HDL also binds to muscle cells, but there is no evidence of internalization. alpha-Tocopherol is transferred to muscle cells from both LDL and HDL, but the transfer is not dependent on lipoprotein internalization. HDL is effective as a means of transport of alpha-tocopherol to muscle cells, but LDL appears to be about one order more effective.

Dual-label autoradiographic analysis of human skin fibroblast and myoblast proteins by two-dimensional polyacrylamide gel electrophoresis using immobilised pH gradients in the first dimension

Horizontal two-dimensional polyacrylamide gel electrophoresis with immobilised pH gradients in the first dimension has been applied to the analysis of human skin fibroblast and muscle myoblast total cell proteins. Excellent two-dimensional separations of skin fibroblast proteins were obtained using pH 4-10 immobilised pH gradient gels with a long interelectrode distance (16 cm), but resolution was degraded, particularly of the more acidic proteins, by the use of shorter (10 cm) gels. Improved resolution of acidic and basic proteins was obtained using separate pH 4-7 and pH 7-10 immobilised pH gradient gels respectively in the first dimension. Two-dimensional protein maps of skin fibroblast proteins were visualised both by silver staining and by autoradiography of samples labelled synthetically with [35S]methionine. Horizontal two-dimensional electrophoresis, using pH 4-7 and pH 7-10 immobilised pH gradient gels in the first dimension, was applied to the analysis of protein samples from skin fibroblasts and muscle myoblasts dual-labelled synthetically with [35S]methionine and [75Se]selenomethionine in an attempt to identify sets of proteins specific to each cell type. In addition, two-dimensional maps or protein samples derived from normal individuals and patients with Duchenne muscular dystrophy were compared to search for protein changes associated with the disease state. Although sets of qualitative protein spot differences were observed by visual inspection of the two-dimensional gels, more rigorous qualitative and quantitative analysis of the patterns using a computerised analysis system will be required to obtain the maximum amount of information from these data.

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.

Transmembrane electrical characteristics of cultured human skeletal muscle cells

Skeletal muscle explants from normal subjects were established from biopsy material on collagen. Cellular outgrowth appeared within 3-4 days, and fusion of myoblasts was observed in 5-10 days. Multinucleated myotubes were impaled under high optical magnification, at 37 degrees C, with conventional glass microelectrodes. The mean resting potential was -44.4 mV +/- 2.4 (n = 399); -33 +/- 2.3 mV at 9 days (n = 10) vs -48 +/- 2.5 mV (n = 15) at 27 days. The average input resistance (Rin) was 9.7 M omega (n = 83). Action potentials could be elicited by electrical stimulation and had a mean amplitude of 55.9 +/- 2.1 mV with a mean maximum rate of rise (Vmax) of 72.1 +/- 7.5 V/s. The mean overshoot was 13.9 +/- 2.3 mV, and the action potential duration determined at 50% of repolarization (APD50) was 8.0 msec (n = 7). The resting membrane potential showed a depolarization of 23 mV/decade for extracellular potassium ion concentration ([K]o) between 4.5-100 mM. Thus, we have established the normal resting potential and maximum rate of rise of the action potential for human myotubes in culture. We have shown that the values for these are less than those previously reported in cultured avian and rodent cells. In addition, we have shown that the response in our system of the resting potential to change in extracellular potassium concentration is blunted compared to studies using isolated muscle, suggesting an increase in ratio of sodium to potassium permeability. Cultured human muscle cells depolarized in the presence of ouabain.

Insulin enhances development of functional voltage-dependent Ca2+ channels in aneurally cultured human muscle

Voltage-dependent Ca2+ channels were studied by the binding of the potent Ca2+ channel antagonist PN200-110 and by the K+-induced 45Ca2+ uptake in human muscle cultured aneurally in the presence of insulin, fibroblast growth factor, and epidermal growth factor, added in combination or individually. Compared to the muscle grown in medium without growth factors, 14-15 days of treatment with insulin (10 micrograms/ml) alone or in combination with two other growth factors caused a 3.4- and 3.8-fold increase per culture dish in the number of PN200-110 binding sites, respectively. There was no change in the affinity of the ligand-receptor complex. Under the same conditions, there was also fourfold increase of the K+-induced 45Ca2+ uptake in cultured human muscle. Neither fibroblast growth factor nor epidermal growth factor alone influenced PN200-110 binding sites. Our study demonstrates that insulin enhances the development of functional voltage-dependent Ca2+ channels in cultured human muscle.

Hormonal control of muscle growth

In muscle of whole animals, pituitary growth hormone, the thyroid hormones, and insulin are major growth-promoting hormones, and the glucocorticoids have significant catabolic actions. At the cellular level the primary anabolic hormones for cultured myoblasts are the somatomedins (insulin-like growth factors) and fibroblast growth factor. In these cells physiological concentrations of growth hormone, thyroid hormones, and insulin have no growth-promoting effect; some of the reported actions of insulin probably result from cross-reaction with the somatomedin receptor. Results with purified proteins do not support the view that mitogens block myoblast differentiation; transforming growth factor-beta and interferon are nonmitogenic proteins that inhibit differentiation, insulin-like growth factors are mitogens that stimulate differentiation, and fibroblast growth factor is the only purified mitogen that inhibits differentiation. At least six serum-free media have now been devised for the growth of various kinds of muscle cells under closely defined conditions.

De novo neuromuscular junction formation on human muscle fibres cultured in monolayer and innervated by foetal rat spinal cord: ultrastructural and ultrastructural--cytochemical studies

Ultrastructural features of neuromuscular junction formation and transverse tubule development were studied utilizing a newly developed model in which human muscle fibres cultured in monolayer are innervated by foetal rat spinal cord with dorsal root ganglia attached. At early innervation (7-10 days), when distinct 'boutons' are contacting muscle fibres, the contacts of nerve terminals with the muscle fibres are, ultrastructurally, superficial and unorganized, and there is no basal lamina-like material between nerve terminals and muscle fibres. A bouton consists, ultrastructurally, of a cluster of small nerve terminals contacting the muscle fibre. At 2-3 weeks of innervation, shallow 'beds' are formed on the muscle fibre just beneath nerve terminals, and occasionally there are irregular and miniscule fragments of basal lamina-like material in the cleft. There is no Schwann cell apposing the nerve terminal at this stage of innervation. After 4-5 weeks of innervation there is more definite basal lamina material in the cleft and suggestive postsynaptic plasmalemmal densities and invaginations. However, there is no Schwann cell apposing the nerve terminal at this stage. At 6-8 weeks of innervation, deep postsynaptic folds are present, a Schwann cell apposes the nerve terminal, and basal lamina surrounds the entire muscle fibre. At all four stages of innervation examined, ultrastructural cytochemistry of alpha-bungarotoxin binding reveals that nicotinic ACh receptors are located exclusively at the neuromuscular junctions. After 1-2 weeks of innervation, very few lanthanum-positive transverse tubules are observed and only in close proximity to the surface membrane. After 3 weeks of innervation, more lanthanum-positive tubules are present, and they are located deeper within the muscle fibre. Five weeks after innervation, somewhat more elaborated tubules (but no lateral sacs) appear, and honeycomb structures are often present. After 6-7 weeks of innervation the tubular system is very elaborate and lateral sacs are present. Hence, this study describes consecutive stages of the formation of neuromuscular junctions and transverse tubules in innervated cultured human muscle, and provides an important basis to which similar studies related to the diseased human muscle can be compared.

Peroxisomes and peroxisomal functions in muscle. Studies with muscle cells from controls and a patient with the cerebro-hepato-renal (Zellweger) syndrome

In the present study we investigated peroxisomal functions in cultured human muscle cells from control subjects and from a patient with the Zellweger syndrome, a genetic disease characterized by the absence of morphologically distinguishable peroxisomes in liver and kidney. In homogenates of cultured muscle cells from control subjects, catalase is contained within subcellular particles, acyl-CoA:dihydroxyacetonephosphate acyltransferase activity is present and palmitoyl-CoA can be oxidized by a peroxisomal beta-oxidative pathway; these findings are indicative of the presence of peroxisomes in the cells. In homogenates of cultured muscle cells from the patient with the Zellweger syndrome, acyl-CoA:dihydroxyacetonephosphate acyltransferase activity was deficient, peroxisomal beta-oxidation of palmitoyl-CoA was impaired and catalase was not particle-bound. These findings indicate that functional peroxisomes are absent in muscle from patients with the Zellweger syndrome. We conclude that cultured human muscle cells can be used as a model system to study peroxisomal functions in muscle and the consequences for this tissue of a generalized dysfunction of peroxisomes.

Tissue culture studies of muscle disorders: Part 2. Biochemical studies, nerve-muscle culture, metabolic myopathies, and animal models

This review continues with studies of protein, lipid, and purine metabolism of Duchenne muscular dystrophy (DMD) cells in vitro and of muscle cells in combined culture with nerve cells. In vitro studies of human metabolic myopathies are tabulated. Results using the hamster, chicken, and mouse (dy25, dy, mdg, and mdx) myopathies are discussed. Interesting findings include suggestions of altered collagen synthesis by DMD cells. Analysis of cell proteins by two-dimensional gel electrophoresis and the use of combined nerve-muscle cultures remain important areas of development. It is disappointing that so few attempts have been made to repeat significant findings in this field, and when a number of laboratories have examined the same phenomenon, the results are often contradictory. It remains to be shown how these various abnormalities found in cells in vitro are related to each other and to those pathologic features of diseased muscle observed in vivo.