Fibroblast growth factor, epidermal growth factor and insulin exert a neuronal-like influence on acetylcholine receptors in aneurally cultured human muscle

Insulin resistance induced by long-term hyperinsulinemia abolishes the effects of acute insulin exposure on cell-surface nicotinic acetylcholine receptor levels and actin cytoskeleton morphology

Using CHO-K1/A5 cells, a clonal cell line that robustly expresses adult muscle-type nicotinic acetylcholine receptor (nAChR), we explored whether insulin resistance in these mammalian cells affects cell-surface expression of the nAChR, its endocytic internalization, and actin cytoskeleton integrity. Acute nanomolar insulin stimulation resulted in a slow increase in nAChR cell-surface levels, reaching maximum levels at ∼1 h. Long periods of insulin incubation caused CHO-K1/A5 cells to become insulin resistant, as previously observed with several other cell types. Furthermore, long-term insulin treatment abolished the effects of short-term insulin exposure on cell-surface nAChR levels, suggestive of a desensitization phenomenon. It also affected the kinetics of ligand-induced nAChR internalization. Since the integrity of the cortical actin cytoskeleton affects nAChR endocytosis, we also studied the effects of long-term insulin treatment on this meshwork. We found that it significantly affected the cortical actin morphology of CHO-K1/A5 cells and the response of the actin cytoskeleton to a subsequent short-term insulin stimulus. Overall, the present results show for the first time the effects of insulin signaling on cell-surface nAChR expression and actin cytoskeleton-associated internalization.

Evidence that acidic fibroblast growth factor promotes maturation of rat satellite-cell-derived myotubes in vitro

Satellite cells isolated from fast tibialis anterior (TA) and slow soleus (SOL) rat muscles were cultivated on matrigel, and treated with acidic fibroblast growth factor (aFGF). The following observations were made: 1) aFGF-treated cultures exhibited enhanced proliferation as mirrored by a twofold increase in DNA content. 2) Compared to the untreated cultures, myotubes in the aFGF cultures were larger; 3) Using reverse transcriptase polymerase chain reaction (RT-PCR) and northern blot analyses, we observed enhanced expression of all adult myosin heavy chain (MHC) isoforms, as well as of myogenin. These findings indicate that, under the culture conditions used, aFGF has a stimulatory effect on proliferation but also on maturation and differentiation of satellite cells. Furthermore, transcript levels of FGF receptor 1 (FGFR1) and 4 (FGFR4) isoforms, as well as of aFGF and bFGF were assessed by RT-PCR. aFGF-treated myotubes displayed increased expression of aFGF and bFGF, suggesting a paracrine effect of exogenous aFGF. In this regard, SOL-derived cultures responded more strongly than TA-derived cultures. The effects of aFGF treatment on the two receptors consisted of a decrease in FGFR1 and an increase in FGFR4 mRNA levels in 5-day-old cultures. In 8-day-old TA cultures, effects of FGF were similar to those in 5-day-old cultures. 8-day FGF-treated SOL cultures treated with FGF for 8 days exhibited higher FGFR1 and FGFR4 mRNA levels than the respective untreated cultures. Compared to 5 day-treated cultures, FGFR1 increased and FGFR4 decreased. This led to a shift in the ratio of FGFR1 to FGFR4 in the FGF-treated cultures which may explain the ability of satellite cells to differentiate under the influence of aFGF.

Hydrocortisone influences voltage-dependent L-type calcium channels in cultured human skeletal muscle

The glucocorticoid hydrocortisone (HC), applied for up to 2 weeks to either aneurally or innervated cultured human muscle, produced 2-fold increase of the number of dihydropyridine ([3H]PN200-110) binding sites. The K(+)-induced, nifedipine-inhibited Ca2+ uptake was increased 40%. The effect of HC was concentration- and time-dependent. [3H]PN200-110 affinity for its receptor was not affected by HC treatment. HC did not exert significant influence on the total amount of protein, CK activity, and the number of myotubes. These results indicate that voltage-dependent L-type Ca2+ channel expression in human muscle is regulated by glucocorticoid.

Induction of acetylcholine receptor cluster formation by local application of growth factors in cultured Xenopus muscle cells

A survey of several growth factors was carried out to determine their effect on the formation of acetylcholine receptor (AChR) clusters in cultured Xenopus muscle cells. Factors were locally applied via polystyrene beads and AChR clustering at bead-muscle contacts was assessed. Among the factors tested, basic fibroblast growth factor, insulin and insulin-like growth factor-I, as well as several polycations, were found to be effective in inducing AChR clusters. The action of these molecules appears to be mediated by tyrosine kinase receptors, since it can be blocked by a tyrosine kinase inhibitor. These results suggest that local ligand-mediated tyrosine phosphorylation is involved in the formation of AChR clusters.

Improved medium with EGF and BSA for differentiated human skeletal muscle cells

Epidermal growth factor (EGF) and bovine serum albumin (BSA) are both required for serum-free clonal growth of human muscle satellite cells (HMSC). However, neither inhibits differentiation of HMSC, and when both are added to a minimal serum-free differentiation medium, they enhance survival and maintenance of human myotubes. A combination of 10 ng/mL EGF and 0.5 mg/mL BSA, added to MCDB 120 plus 10 micrograms/mL insulin, increases both total protein per dish and total creatine kinase activity, and keeps the myotubes in good condition for a longer period of time. The myotubes become cross-striated and exhibit frequent spontaneous twitching. Substantial amounts of neonatal myosin heavy chain and the MM isozyme of creatine kinase are expressed, together with detectable amounts of adult fast myosin heavy chain. With regular feeding, these cultures can be maintained for at least 3 weeks with no overgrowth by mononucleate cells, and with far less degeneration than with insulin as the only supplement.

Ultroser G and brain extract induce a continuous basement membrane with specific synaptic elements in aneurally cultured human skeletal muscle cells

Basement membrane (BM) components were studied on human muscle and skeletal muscle cells cultured on different media by immunofluorescence and electron microscopy. Their topographical relation with acetylcholine receptors was investigated. Myotubes cultured on a combination of the serum substitute Ultroser G and brain extract show a continuous layer of heparan sulfate proteoglycans (HSPGs), laminin, and type IV collagen. In contrast, myotubes cultured on serum-containing media are associated with granular depositions of HSPG and laminin and only with wisps of type IV collagen. Omission of brain extract or substitution by chicken embryo extract results in an intermediate staining pattern. For all types of cultures, fibronectin is localized in and around mononuclear cells, but hardly associated with myotubes. A codistribution between clusters of acetylcholine receptors and HSPG and laminin and Vicia villosa B4 lectin-positive material exists only in Ultroser G/brain extract-based myotubes like in muscle in vivo. No clustering is observed in serum-based myotubes. Electron microscopy reveals that the former myotubes are surrounded by a continuous BM consisting of a lamina lucida, lamina densa, and lamina fibroreticularis. Proteoglycans are present on the external site of the lamina densa and associated in a regular fashion with collagen fibrils. In conclusion, BMs associated with myotubes cultured on Ultroser G/brain extract resemble in many ways the in vivo situation, including synaptic specializations. Cultured myotubes may serve as a model system for studies on the structure and function of human muscular (synaptic) BM under normal and pathological conditions.

Different degradation rates of junctional and extrajunctional acetylcholine receptors of human muscle cultured in monolayer and innervated by fetal rat spinal cord neurons

It is well demonstrated that in intact animals the degradation rate of the junctional acetylcholine receptor (AChR) is significantly slower than that of the extrajunctional receptor. Such data, however, are not available for human AChRs because the required experimentation cannot be performed in humans. We have now studied the degradation rate of the junctional and extrajunctional AChRs, utilizing our tissue culture model, in which well-differentiated neuromuscular junctions (NMJs) form on human muscle cultured in monolayer and innervated long-term by fetal rat spinal cord neurons. Half-life of AChRs was studied by a method utilizing the autoradiography of 125I-alpha bungarotoxin and computerized video image analysis. Extrajunctional AChRs degraded with a half-life of 1.3 days whereas junctional AChRs degraded with a half-life of 3.5 days. Our studies demonstrate for the first time that in innervated cultured human muscle: (a) the life span of human junctional AChR, is approximately 3 times longer than that of the extrajunctional AChR and (b) the stability of human AChR is neuronally regulated. This system can now be applied to evaluate the influence of pharmacologic agents on the stability of human junctional AChR, which is of potential importance in the treatment of myasthenia gravis and other diseases of the NMJ.

Induction of synaptic development in cultured muscle cells by basic fibroblast growth factor

The role of basic fibroblast growth factor (bFGF) in signaling the development of the neuromuscular junction was examined. Beads coated with bFGF induced the formation of acetylcholine receptor (AChR) clusters in cultured Xenopus myotomal muscle cells. Tyrphostin, a tyrosine kinase inhibitor, abolished AChR clustering induced by bFGF beads, suggesting a role of tyrosine kinase activation in AChR clustering. Using specific antibodies, we demonstrated the presence of both bFGF and its receptor in the myotomal muscle in vivo during the period of neuromuscular connection. However, similar tissue from older animals with mature neuromuscular junctions showed an apparently truncated form of the bFGF receptor. These data suggest that bFGF may play a role in signaling synaptogenesis in skeletal muscle.

Expression of muscle-specific isozymes of phosphorylase and creatine kinase in human muscle fibers cultured aneurally in serum-free, hormonally/chemically enriched medium

Primary cultures of muscle cells derived from a biopsied adult human skeletal muscle were grown up to 6 weeks in a hormonally/chemically enriched serum-free medium. The expression of muscle-specific isozymes of creatine kinase, glycogen phosphorylase, and phosphoglycerate mutase, indicative of muscle cell maturation, was studied after 1, 4 and 6 weeks of growth. The maturation of muscle fibers cultured in serum-free medium was comparable to that achieved by muscle fibers cultured in medium containing 10% serum and supplemented with growth factors (insulin, epidermal growth factor, and fibroblastic growth factor) and was greater than that achieved in medium containing 10% serum only. Our study demonstrates that adult human muscle can be cultured aneurally for a long period of time in a serum-free medium, and that it can achieve a high degree of maturation. This study provides an important basis for investigations related to: (1) assessment of the influence of individual components of the medium on human muscle maturation in culture; (2) studies of regulation of abnormal gene expression in diseased human muscle cultured in serum-fre medium.

Improved media for normal human muscle satellite cells: serum-free clonal growth and enhanced growth with low serum

We have developed a serum-free medium for clonal growth of normal human muscle satellite cells (HMSC). It consists of an optimized nutrient medium, MCDB 120, plus a serum-free supplement, designated SF, that contains epidermal growth factor (EGF), insulin, dexamethasone, bovine serum albumin, and fetuin. Fibroblast growth factor was needed with dialyzed fetal bovine serum (dFBS) as the only other supplement, but in media containing SF, it was only slightly beneficial, and was omitted from the final medium without significant loss. Clonal growth of HMSC in MCDB 120 plus SF is as good as with 15% serum and 0.5% chicken embryo or bovine pituitary extract. However, growth is further improved by use of a doubly-supplemented (DS) medium containing both SF and 5% dFBS. Clonal growth of HMSC in the DS medium far exceeds that in previous media with any amount of serum, and monolayer growth is at least equal to that in conventional media with higher levels of serum. Cells grown in these media exhibit little differentiation, even when grown to high densities. However, they retain the capacity for extensive fusion and synthesis of increased creatine kinase when transferred to a serum-free differentiation-promoting medium, such as Dulbecco's modified Eagle's medium plus insulin. All experiments were done with clonal cultures of HMSC to insure that observed growth responses were always those of muscle cells.

Molecular events in synaptogenesis: nerve-muscle adhesion and postsynaptic differentiation

The clustering of acetylcholine receptors (AChR) in the postsynaptic membrane of newly innervated muscle fibers is one of the earliest events in the development of the vertebrate neuromuscular junction. Here, we describe two hypotheses that can account for AChR clustering in response to innervation. The "trophic factor" hypothesis proposes that the neuron releases a soluble factor that interacts with the muscle cell in a specific manner and that this interaction results in the local accumulation of AChR. The "contact and adhesion" hypothesis proposes that the binding of the nerve to the muscle cell surface is itself sufficient to induce AChR clustering, without the participation of soluble factors. We present a model for the molecular assembly of AChR clusters based on the contact and adhesion hypothesis. The model involves the sequential assembly of three distinct membrane domains. The first domain to form serves to attach microfilaments to the cytoplasmic surface of the muscle cell membrane at sites of muscle-nerve adhesion. The second domain to form is clathrin-coated membrane; it serves as a site of insertion of additional membrane elements, including AChR. Upon insertion of AChR into the cell surface, a membrane skeleton assembles by anchoring itself to the AChR. The skeleton, composed in part of actin and spectrin, binds and immobilizes significant numbers of AChR, thereby forming the third membrane domain of the AChR cluster. We make several predictions that should distinguish this model of AChR clustering from one that invokes soluble, trophic factors.

Protein phosphorylation in response to the tumor promoter TPA is dependent on the state of differentiation of muscle cells

We have shown previously (A. Sobel and A. H. Tashjian, Jr. (1983). J. Biol. Chem. 258, 10,312-10,324;A. Sobel and M.C. Boutterin (1985). Neurochem. Int. 7, 995-1006) that, in the pituitary-derived GH4C1 cells, thyrotropin-releasing hormone or the tumor promoter TPA (12-O-tetradecanoylphorbol-13-acetate) stimulates the phosphorylation of two sets of cytoplasmic proteins related to the regulation of prolactin synthesis and release, respectively. Interestingly, phosphoproteins with identical electrophoretic migration properties on two-dimensional gels were detected in cultured neonate or adult mouse muscle cells and in the L6 and C2 myogenic cell lines. In addition TPA, which is known to have many actions on muscle cell functions, proliferation, and differentiation, stimulated the phosphorylation of these same proteins in myoblasts in culture. After fusion of the proliferating myoblasts into differentiated myotubes, this TPA-induced stimulation was strongly reduced in normal muscle cell cultures where some mononucleate muscle and non-muscle cells remained present. It was totally abolished in the homogeneous L6 and C2 cell lines. These observations suggest that the same phosphoproteins may be related to the intracellular mechanisms involved in the transduction of extracellular regulatory signals in such distinct differentiated environments as those of pituitary and muscle cells. In muscle cells themselves, the regulation of the phosphorylation of these proteins is function of the cell's state of differentiation.

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.

Effects of electrical stimulation and tetrodotoxin paralysis on expression of muscle-specific isozymes of four enzymes in aneurally cultured embryonic rat muscle

We studied the effect of electrical stimulation and a sodium channel blocker (tetrodotoxin) on the expression of muscle-specific isozymes of creatine kinase, glycogen phosphorylase, phosphoglycerate mutase, and lactate dehydrogenase in aneurally cultured embryonic rat muscle. Muscle contractile activity slightly accelerated the accumulation of muscle-specific isozyme of creatine kinase in early cultures (4 days of experiment), but no increase in the expression of muscle-specific isozymes of any enzyme was present in older cultures (11 days of experiment). We conclude that muscle contractile activity is not a main regulator of isozyme maturation in this system.

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.