Acetylcholine sensitivity in replicating satellite cells

Calcium influx through muscle nAChR-channels: One route, multiple roles

Although Ca²⁺ influx through muscle nAChR-channels has been described over the past 40 years, its functions remain still poorly understood. In this review we suggest possible roles of Ca²⁺ entry at all stages of muscle development, summarizing the evidence present in literature. nAChRs are expressed in myoblasts prior to fusion, and can be activated in the absence of an ACh-releasing nerve terminal, with Ca²⁺ influx likely contributing to regulate cell fusion. Upon establishment of nerve-muscle contact, Ca²⁺ influx contributes to orchestrate the signaling required for the correct formation of the neuromuscular junction. Finally, in the mature synapse, Ca²⁺ entry through postsynaptic nAChR-channels - highly Ca²⁺ permeable, in particular in humans - acts on K⁺ and Na⁺ channels to shape endplate excitability. However, when genetic defects cause excessive channel activation, Ca²⁺ influx becomes toxic and causes endplate myopathy. Throughout the review, we highlight how Ricardo Miledi has contributed to construct this whole body of knowledge, from the initial description of Ca²⁺ permeability of endplate nAChR channels, to the rationale for the treatment of endplate excitotoxic damage under pathological conditions. This article is part of a Special Issue entitled: SI: Honoring Ricardo Miledi - outstanding neuroscientist of XX-XXI centuries.

Green fluorescent protein incorporation by mouse myoblasts may yield false evidence of myogenic differentiation of human haematopoietic stem cells

AIMS

Haematopoietic CD34+ stem cells are able to differentiate into skeletal muscle, a potentially invaluable tool for treating degenerative diseases such as muscular dystrophy. However, some studies argue that the differentiative potential of these cells might have been overestimated. In vitro studies provide a controlled environment in which to investigate this point.

METHODS

CD34+ stem cells from human peripheral blood, labelled with green fluorescent protein (GFP), were co-cultured with mouse myogenic C2C12 cells. The functional properties of mononucleated GFP+ cells were determined using electrophysiological techniques and were related to protein profiling determined by immunofluorescence staining and single-cell RT-PCR. Mouse mesoangioblasts co-cultured with human myotubes provided methodological controls.

RESULTS

After 2-4 days, mononucleated adherent GFP+ cells showed acetylcholine-evoked current responses, typical of myogenic cells, as if stem cells had integrated into the host environment. In contrast to this hypothesis, human nuclei could not be detected in adherent GFP+ cells by immunofluorescence. Moreover, single-cell RT-PCR showed that adherent GFP+ cells responsive to acetylcholine expressed mouse markers while loose unresponsive GFP+ cells were of human origin. The transcripts of the human alpha1 subunit of the acetylcholine muscle receptor were not amplified in co-cultures.

CONCLUSION

Single-cell analysis of functional properties combined with other markers revealed that, under the co-culture conditions used, GFP was transferred from human CD34+ stem cells to C2C12 myoblasts by mechanisms unrelated to myogenic stem cell differentiation. Our results emphasize the need for careful controls using several markers when investigating the myogenic differentiation of circulating stem cells.

Fusion-independent expression of functional ACh receptors in mouse mesoangioblast stem cells contacting muscle cells

Mesoangioblasts are vessel-associated fetal stem cells that can be induced to differentiate into skeletal muscle, both in vitro and in vivo. Whether this is due to fusion or to transdifferentiation into bona fide satellite cells is still an open question, for mesoangioblasts as well as for other types of stem cells. The early steps of satellite cell myogenic differentiation involve MyoD activation, membrane hyperpolarization and the appearance of ACh sensitivity and gap junctional communication. If mesoangioblasts differentiate into satellite cells, these characteristics should be observed in stem cells prior to fusion into multinucleated myotubes. We have investigated the functional properties acquired by mononucleated green fluorescent protein (GFP)-positive mesoangioblasts co-cultured with differentiating C2C12 myogenic cells, using the patch-clamp technique. Mesoangioblasts whose membrane contacted myogenic cells developed a hyperpolarized membrane resting potential and ACh-evoked current responses. Dye and electrical coupling was observed among mesoangioblasts but not between mesoangioblasts and myotubes. Mouse MyoD was detected by RT-PCR both in single, mononucleated mesoangioblasts co-cultured with C2C12 myotubes and in the total mRNA from mouse mesoangioblasts co-cultured with human myotubes, but not in human myotubes or stem cells cultured in isolation. In conclusion, when co-cultured with muscle cells, mesoangioblasts acquire many of the functional characteristics of differentiating satellite cells in the absence of cell fusion, strongly indicating that these stem cells undergo transdifferentiation into satellite cells, when exposed to a myogenic environment.

Activation of nicotinic acetylcholine receptors increases the rate of fusion of cultured human myoblasts

1. Fusion of myogenic cells is important for muscle growth and repair. The aim of this study was to examine the possible involvement of nicotinic acetylcholine receptors (nAChR) in the fusion process of myoblasts derived from postnatal human satellite cells. 2. Acetylcholine-activated currents (ACh currents) were characterized in pure preparations of freshly isolated satellite cells, proliferating myoblasts, myoblasts triggered to fuse and myotubes, using whole-cell and single-channel voltage clamp recordings. Also, the effect of cholinergic agonists on myoblast fusion was tested. 3. No nAChR were observed in freshly isolated satellite cells. nAChR were first observed in proliferating myoblasts, but ACh current densities increased markedly only just before fusion. At that time most mononucleated myoblasts had ACh current densities similar to those of myotubes. ACh channels had similar properties at all stages of myoblast maturation. 4. The fraction of myoblasts that did not fuse under fusion-promoting conditions had no ACh current and thus resembled freshly isolated satellite cells. 5. The rate of myoblast fusion was increased by carbachol, an effect antagonized by alpha-bungarotoxin, curare and decamethonium, but not by atropine, indicating that nAChR were involved. Even though a prolonged exposure to carbachol led to desensitization, a residual ACh current persisted after several days of exposure to the nicotinic agonist. 6. Our observations suggest that nAChR play a role in myoblast fusion and that part of this role is mediated by the flow of ions through open ACh channels.

Synthesis and release of an acetylcholine-like compound by human myoblasts and myotubes

1. Exogenously applied acetylcholine (ACh) is a modulator of human myoblast fusion. Using a chemiluminescent method, we examined whether an endogenous ACh-like compound (ACh-lc) was present in, and released by, pure human myogenic cells. 2. Single, freshly isolated satellite cells and proliferating myoblasts contained 15 and 0.5 fmol ACh-lc, respectively. Cultured myotubes contained ACh-lc as well. Also, ACh-like immunoreactivity was detected in all myogenic cells. 3. Part of the ACh-lc was synthesized by choline acetyltransferase (ChAT), as indicated by the reduction of ACh-lc content when bromoACh was present in the culture medium, and by direct measurements of ChAT activity. Also, ChAT-like immunoreactivity was observed in all myogenic cells. 4. Myoblasts and myotubes released ACh-lc spontaneously by a partially Ca(2+)-dependent mechanism. 5. The application by microperfusion of medium conditioned beforehand by myoblasts (thus presumably containing ACh-lc) onto a voltage-clamped myotube induced inward currents resembling ACh-induced currents in their kinetics, reversal potential, and sensitivity to nicotinic antagonists. 6. In vitro, the spontaneously released ACh-lc promoted myoblast fusion but only in the presence of an anticholinesterase. 7. Our observations indicate that human myogenic cells synthesize and release an ACh-lc and thereby promote the fusion process that occurs in muscle during growth or regeneration.

Skeletal muscle satellite cells

Evidence now suggests that satellite cells constitute a class of myogenic cells that differ distinctly from other embryonic myoblasts. Satellite cells arise from somites and first appear as a distinct myoblast type well before birth. Satellite cells from different muscles cannot be functionally distinguished from one another and are able to provide nuclei to all fibers without regard to phenotype. Thus, it is difficult to ascribe any significant function to establishing or stabilizing fiber type, even during regeneration. Within a muscle, satellite cells exhibit marked heterogeneity with respect to their proliferative behavior. The satellite cell population on a fiber can be partitioned into those that function as stem cells and those which are readily available for fusion. Recent studies have shown that the cells are not simply spindle shaped, but are very diverse in their morphology and have multiple branches emanating from the poles of the cells. This finding is consistent with other studies indicating that the cells have the capacity for extensive migration within, and perhaps between, muscles. Complexity of cell shape usually reflects increased cytoplasmic volume and organelles including a well developed Golgi, and is usually associated with growing postnatal muscle or muscles undergoing some form of induced adaptive change or repair. The appearance of activated satellite cells suggests some function of the cells in the adaptive process through elaboration and secretion of a product. Significant advances have been made in determining the potential secretion products that satellite cells make. The manner in which satellite cell proliferative and fusion behavior is controlled has also been studied. There seems to be little doubt that cellcell coupling is not how satellite cells and myofibers communicate. Rather satellite cell regulation is through a number of potential growth factors that arise from a number of sources. Critical to the understanding of this form of control is to determine which of the many growth factors that can alter satellite cell behavior in vitro are at work in vivo. Little work has been done to determine what controls are at work after a regeneration response has been initiated. It seems likely that, after injury, growth factors are liberated through proteolytic activity and initiate an activation process whereby cells enter into a proliferative phase. After myofibers are formed, it also seems likely that satellite cell behavior is regulated through diffusible factors arising from the fibers rather than continuous control by circulating factors.(ABSTRACT TRUNCATED AT 400 WORDS)

Blockage of nicotinic acetylcholine receptors by 5-hydroxytryptamine

The action of 5-hydroxytryptamine (5HT) on nicotinic acetylcholine receptor (nAChR) channels was investigated in mouse myotubes, human cloned TE671/RD cells, and Xenopus laevis oocytes. The decay of the ACh-activated whole-cell currents was reversibly accelerated in the presence of 5HT (10(-5) to 10(-3) M), in a dose-dependent manner. 5HT also reduced the size and accelerated the decay of currents elicited by ACh in Xenopus oocytes injected with mRNA extracted from C2 myotubes or Torpedo electroplaques, or oocytes injected with cloned mouse muscle AChR subunit mRNAs. The effect of 5HT was promptly reversed after washout, or by depolarizing the oocyte beyond -10 mV. In patch-clamp recordings from myotubes, bath-application of 5HT did not exert an indirect influence on the ACh-activated channels within the patch membrane. In contrast, when the patch membrane was exposed to 5HT (10(-6) M), ACh unit responses appeared as bursts of short pulses. It is concluded that the regulation of ACh responses by 5HT results from a fast noncompetitive blocking action of nAChR-channels. These results show that ligand-gated channels, activated by their specific neurotransmitter, may be regulated by a different neurotransmitter through a direct action on the receptor molecule.

Hemin enhances differentiation and maturation of cultured regenerated skeletal myotubes

Satellite cells, isolated from marcaine-damaged rat skeletal muscle, differentiate in culture to form contracting, cross-striated myotubes. Addition of 20 microM hemin (ferriprotoporphyrin IX chloride) to the culture medium resulted in increases in the number, size, and alignment of myotubes; in the number of myotubes that exhibited cross-striations; and in the strength and frequency of myotube contractions. Hemin increased satellite cell fusion by 27%, but decreased cell proliferative rate by 30%. Hemin increased the specific activity of creatine kinase (CK), a sensitive indicator of muscle differentiation, by 157%. Separation of CK isoenzymes by agarose gel electrophoresis showed that hemin increased only the muscle-specific CK isoenzymes (MM-CK and MB-CK). Thus, hemin seems to duplicate some of the effects of innervation on cultured myotubes by increasing contraction frequency and strength, appearance of cross-striations, and muscle-specific isoenzymes. In contrast, 3-amino-1,2,4-triazole, an inhibitor of heme biosynthesis, decreased the number of cross-striated myotubes, the strength and frequency of myotube contractions, and CK activity. These inhibitory effects were reversed by hemin. Collectively, these results demonstrate a physiologically significant role for heme in myotube maturation.

Hemin increases aerobic capacity of cultured regenerating skeletal myotubes

Regeneration of damaged, mature muscle occurs by differentiation of satellite cells. In culture, satellite cell myoblasts proliferate, align, and fuse to form cross-striated, contracting myotubes. The biochemical changes and the factors that regulate differentiation in satellite cells have not been investigated previously. We report here that no significant differences in glucose uptake rate or glucose oxidation rate were observed between regenerating myoblasts and myotubes, whereas the aerobic oxidation of palmitic acid increased 7.3-fold between these differentiation states. Specific activities of enzymes of critical importance in aerobic metabolism or in production of ATP were increased 2- to 3.5-fold during fusion. Addition of 20 microM hemin to regenerating muscle cultures potentiated the aerobic capacity as evidenced by a 23.6% increase in palmitate oxidation rate. Hemin also increased the specific activities of all nonheme enzymes investigated with the exception of phosphofructokinase. This augmentation of aerobic metabolism together with the time frame of active muscle differentiation suggests a complex role for hemin in myogenesis.

Acetylcholine receptor channels are present in undifferentiated satellite cells but not in embryonic myoblasts in culture

The expression and the physiological properties of acetylcholine receptors (AChRs) of mononucleated myogenic cells, isolated from either embryonic or adult muscle of the mouse, have been investigated using the gigaohm seal patch-clamp technique in combination with immunocytochemistry (with an anti-myosin antibody) and alpha-bungarotoxin binding techniques. Undifferentiated (myosin-negative) embryonic myoblasts, grown either in mass culture or under clonal conditions, were found to be unresponsive to ACh and did not bind alpha-bungarotoxin. On the contrary, undifferentiated satellite cells (from adult muscle) exhibited channels activated by ACh and alpha-bungarotoxin binding sites similar to those observed in differentiated (myosin-positive) embryonic myoblasts and myotubes. Two classes of ACh-activated channels with different opening frequencies were identified. The major class of channels had a conductance of about 42 pS and mean open time of 3.1-8.2 msec. The minor class of channels had smaller conductance (about 17 pS) and similar open time. During differentiation, the conductance of the two channels did not change significantly, while channel lifetime became shorter in myotubes derived from satellite cells but not in myotubes derived from embryonic myoblasts. The relative proportion of small over large channels was significantly larger in embryonic than in adult myogenic cells.

Single acetylcholine-activated channels in cultured rhabdomyoblasts

Acetylcholine receptor (AChR) was found to be present on the cell surface of the human rhabdomyoblast (RD) cell line. Two classes of ACh-activated channels have been observed, one with a large conductance and long duration and the other with smaller conductance and short duration, similar to those of human myotubes. RD membrane exhibited a specific binding to the alpha-bungarotoxin indicating the presence of nicotinic AChRs. These results support the hypothesis that rhabdomyosarcomas derive from myogenic precursors.

Agents that activate protein kinase C reduce acetylcholine sensitivity in cultured myotubes

We have examined acetylcholine (ACh)-elicited potentials or currents in current- or voltage-clamped cultured myotubes exposed to 12-O-tetradecanoyl-phorbol-13-acetate (TPA), a potent tumor promoter that activates protein kinase C. Although this agent had little action on either membrane resting potential or electrical resistance, a reversible decrease in ACh sensitivity was induced on 3-4-d-old chick myotubes. Depression of transmitter action by TPA was extended to 7-8-d mouse myotubes only when they were treated with phosphatidylserine. Glyceryl dioleate had effects on myotubes similar to those of TPA but with a reduced efficacy. We conclude that the activation of protein kinase C might be involved with the capacity of ACh receptors to respond to transmitter stimulation.