Properties of primary mouse myoblasts expanded in culture

Expression of ALS-linked SOD1 Mutation in Motoneurons or Myotubes Induces Differential Effects on Neuromuscular Function In vitro

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease that selectively affects upper and lower motoneurons. Dismantlement of the neuromuscular junction (NMJ) is an early pathological hallmark of the disease whose cellular origin remains still debated. We developed an in vitro NMJ model to investigate the differential contribution of motoneurons and muscle cells expressing ALS-causing mutation in the superoxide dismutase 1 (SOD1) to neuromuscular dysfunction. The primary co-culture system allows the formation of functional NMJs and fosters the expression of the ALS-sensitive fast fatigable type II-b myosin heavy chain (MHC) isoform. Expression of SOD1^G93A in myotubes does not prevent the formation of a functional NMJ but leads to decreased contraction frequency and lowers the slow type I MHC isoform transcript levels. Expression of SOD1^G93A in both motoneurons and myotubes or in motoneurons alone however alters the formation of a functional NMJ. Our results strongly suggest that motoneurons are a major factor involved in the process of NMJ dismantlement in an experimental model of ALS.

Interplay Between Cholinergic and Adenosinergic Systems in Skeletal Muscle

Since the pioneering works of Ricardo Miledi, the neuromuscular junction represents the best example of a synapse where ACh is the neurotransmitter acting on nicotinic ACh receptors. ATP, co-released with ACh, is promptly degraded to Ado, which acts as a modulator of the cholinergic synaptic activity. Consequently, both ACh and adenosine play a crucial role in controlling the nerve-muscle communication. Apart from their role in the context of synaptic transmission, ACh and adenosine are autocrinally released by skeletal muscle cells, suggesting also a non nerve-driven function of these molecules. Indeed, the existence of cholinergic and adenosinergic systems has been widely described in many other non neuronal cell types. In this review, we will describe the two systems and their interplay in non-innervated differentiating skeletal muscle cells, and in innervated adult skeletal muscle fibers. We believe that the better comprehension of the interactions between the activity of nAChRs and adenosine could help the knowledge of skeletal muscle physiology. This article is part of a Special Issue entitled: Honoring Ricardo Miledi - outstanding neuroscientist of XX-XXI centuries.

Long-Term High-Density Extracellular Recordings Enable Studies of Muscle Cell Physiology

Skeletal (voluntary) muscle is the most abundant tissue in the body, thus making it an important biomedical research subject. Studies of neuromuscular transmission, including disorders of ion channels or receptors in autoimmune or genetic neuromuscular disorders, require high-spatial-resolution measurement techniques and an ability to acquire repeated recordings over time in order to track pharmacological interventions. Preclinical techniques for studying diseases of neuromuscular transmission can be enhanced by physiologic ex vivo models of tissue-tissue and cell-cell interactions. Here, we present a method, which allows tracking the development of primary skeletal muscle cells from myoblasts into mature contracting myotubes over more than 2 months. In contrast to most previous studies, the myotubes did not detach from the surface but instead formed functional networks between the myotubes, whose electrical signals were observed over the entire culturing period. Primary cultures of mouse myoblasts differentiated into contracting myotubes on a chip that contained an array of 26,400 platinum electrodes at a density of 3,265 electrodes per mm². Our ability to track extracellular action potentials at subcellular resolution enabled study of skeletal muscle development and kinetics, modes of spiking and spatio-temporal relationships between muscles. The developed system in turn enables creation of a novel electrophysiological platform for establishing ex vivo disease models.

Enhanced Development of Skeletal Myotubes from Porcine Induced Pluripotent Stem Cells

The pig is recognized as a valuable model in biomedical research in addition to its agricultural importance. Here we describe a means for generating skeletal muscle efficiently from porcine induced pluripotent stem cells (piPSC) in vitro thereby providing a versatile platform for applications ranging from regenerative biology to the ex vivo cultivation of meat. The GSK3B inhibitor, CHIR99021 was employed to suppress apoptosis, elicit WNT signaling events and drive naïve-type piPSC along the mesoderm lineage, and, in combination with the DNA methylation inhibitor 5-aza-cytidine, to activate an early skeletal muscle transcription program. Terminal differentiation was then induced by activation of an ectopically expressed MYOD1. Myotubes, characterized by myofibril development and both spontaneous and stimuli-elicited excitation-contraction coupling cycles appeared within 11 days. Efficient lineage-specific differentiation was confirmed by uniform NCAM1 and myosin heavy chain expression. These results provide an approach for generating skeletal muscle that is potentially applicable to other pluripotent cell lines and to generating other forms of muscle.

Combination of agrin and laminin increase acetylcholine receptor clustering and enhance functional neuromuscular junction formation In vitro

Clustering of acetylcholine receptors (AChR) at the postsynaptic membrane is a crucial step in the development of neuromuscular junctions (NMJ). During development and after denervation, aneural AChR clusters form on the sarcolemma. Recent studies suggest that these receptors are critical for guiding and initiating synaptogenesis. The aim of this study is to investigate the effect of agrin and laminin-1; agents with known AChR clustering activity; on NMJ formation and muscle maturation. Primary myoblasts were differentiated in vitro on collagen, laminin or collagen and laminin-coated surfaces in the presence or absence of agrin and laminin. The pretreated cells were then subject to innervation by PC12 cells. The number of neuromuscular junctions was assessed by immunocytochemical co-localization of AChR clusters and the presynaptic marker synaptophysin. Functional neuromuscular junctions were quantitated by analysis of the level of spontaneous as well as neuromuscular blocker responsive contractile activity and muscle maturation was assessed by the degree of myotube striation. Agrin alone did not prime muscle for innervation while a combination of agrin and laminin pretreatment increased the number of neuromuscular junctions formed and enhanced acetylcholine based neurotransmission and myotube striation. This study has direct clinical relevance for treatment of denervation injuries and creating functional neuromuscular constructs for muscle tissue repair.

Adenosine enhances acetylcholine receptor channel openings and intracellular calcium 'spiking' in mouse skeletal myotubes

AIMS

The autocrine activity of the embryonic isoform of the nicotinic acetylcholine receptor is crucial for the correct differentiation and trophism of skeletal muscle cells before innervation. The functional activity of extracellular adenosine and adenosine receptor subtypes expressed in differentiating myotubes is still unknown. In this study, we performed a detailed analysis of the role of adenosine receptor-mediated effects on the autocrine-mediated nicotinic acetylcholine receptor channel openings and the associated spontaneous intracellular calcium 'spikes' generated in differentiating mouse myotubes in vitro.

METHODS

Cell-attached patch-clamp recordings and intracellular calcium imaging experiments were performed in contracting myotubes derived from mouse satellite cells.

RESULTS

The endogenous extracellular adenosine and the adenosine receptor-mediated activity modulated the properties of the embryonic isoform of the nicotinic acetylcholine receptor in myotubes in vitro, by increasing the mean open time and the open probability of the ion channel, and sustaining nicotinic acetylcholine receptor-driven intracellular [Ca(2+) ]i 'spikes'. The pharmacological characterization of the adenosine receptor-mediated effects suggested a prevalent involvement of the A2B adenosine receptor subtype.

CONCLUSION

We propose that the interplay between endogenous adenosine and nicotinic acetylcholine receptors represents a potential novel strategy to improve differentiation/regeneration of skeletal muscle.

In vivo and in vitro effects of 42-hydroxy-palytoxin on mouse skeletal muscle: structural and functional impairment

Palytoxins (PLTXs) are known seafood contaminants and their entrance into the food chain raises concern about possible effects on human health. The increasing number of analogs being identified in edible marine organisms complicates the estimation of the real hazard associated with the presence of PLTX-like compounds. So far, 42-OH-PLTX is one of the few congeners available, and the study of its toxicity represents an important step toward a better comprehension of the mechanism of action of this family of compounds. From this perspective, the aim of this work was to investigate the in vivo and in vitro effect of 42-OH-PLTX on skeletal muscle, one of the most sensitive targets for PLTXs. Our results demonstrate that 42-OH-PLTX causes damage at the skeletal muscle level with a cytotoxic potency similar to that of PLTX. 42-OH-PLTX induces cytotoxicity and cell swelling in a Na(+)-dependent manner similar to the parent compound. However, the limited Ca(2+)-dependence of the toxic insult induced by 42-OH-PLTX suggests a specific mechanism of action for this analog. Our results also suggest an impaired response to the physiological agonist acetylcholine and altered cell elasticity.

Reactive oxygen species contribute to the promotion of the ATP-mediated proliferation of mouse skeletal myoblasts

Reactive oxygen species (ROS) and extracellular adenosine 5'-triphosphate (ATP) participate in autocrine and paracrine regulation in skeletal muscle. However, the link between these two signaling systems is not well established. Here, we studied cell proliferation as a possible consequence of the trophic effect of ATP in cultured skeletal mouse myoblasts and we tested the possibility that low concentrations of ROS represent the intermediate signaling molecule mediating this effect. Exposure to 10 μM ATP increased proliferation of mouse myoblasts by ~20%. ATP also induced intracellular Ca(2+) oscillations, which were independent of extracellular Ca(2+). Both effects of ATP were prevented by suramin, a broad-spectrum purinergic P2 receptor antagonist. In contrast, the adenosine receptor blocker CGS-15943 did not modify the ATP-mediated effects. Consistent with this, adenosine per se did not change myoblast growth, indicating the direct action of ATP via P2 receptor activation. The proliferative effect of ATP was prevented after depletion of hydrogen peroxide (H(2)O(2)) by the peroxidase enzyme catalase. Low-micromolar concentrations of exogenous H(2)O(2) mimicked the stimulatory effect of ATP on myoblast growth. DCF imaging revealed ATP-induced catalase and DPI-sensitive ROS production in myoblasts. In conclusion, our results indicate that extracellular ATP controls mouse myoblast proliferation via induction of ROS generation.

Efficacy of engineered FVIII-producing skeletal muscle enhanced by growth factor-releasing co-axial electrospun fibers

Co-axial electrospun fibers can offer both topographical and biochemical cues for tissue engineering applications. In this study, we demonstrate the sustained treatment of hemophilia through a non-viral, tissue engineering approach facilitated by growth factor-releasing co-axial electrospun fibers. FVIII-producing skeletal myotubes were first engineered on aligned electrospun fibers in vitro, followed by implantation in hemophilic mice with or without a layer of core-shell electrospun fibers designed to provide sustained delivery of angiogenic or lymphangiogenic growth factors, which serves to stimulate the lymphatic or vascular systems to enhance the FVIII transport from the implant site into systemic circulation. Upon subcutaneous implantation into hemophilic mice, the construct seamlessly integrated with the host tissue within one month, and specifically induced either vascular or lymphatic network infiltration in accordance with the growth factors released from the electrospun fibers. Engineered constructs that induced angiogenesis resulted in sustained elevation of plasma FVIII and significantly reduced blood coagulation time for at least 2-months. Biomaterials-assisted functional tissue engineering was shown in this study to offer protein replacement therapy for a genetic disorder such as hemophilia.

Electrically induced contraction of C2C12 myotubes cultured on a porous membrane-based substrate with muscle tissue-like stiffness

A porous membrane-based cell culture device was developed to electrically stimulate a confluent monolayer of C2C12 myotubes. The device's cell culture substrate is a microporous alumina membrane-modified by attaching an atelocollagen membrane on the upperside and a hole-spotted poly(dimethylsiloxane) (PDMS) film on the underside. When electric current is generated between the device's Pt ring electrodes--one of which is placed above the cells and the other below the PDMS layer--the focused current at the PDMS hole can electrically stimulate the cells. C2C12 myoblasts were cultured on the substrate and differentiated into myotubes. When the electrical pulses were applied, myotubes started to contract slightly in and near the hole, and that the continuous stimulation increased both the number of stimuli-responding myotubes and the magnitude of the contraction considerably owing to the underlying atelocollagen membrane with muscle tissue-like stiffness. Also, the generation of contractile myotubes on a wider region of the membrane substrate was possible by applying the electrical pulses through the array of holes in the PDMS film. Using the present system, the glucose uptake by contractile myotubes was examined with fluorescence-labeled glucose, 2-NBDG, which displayed a positive correlation between the contractile activity of myotubes and the uptake of 2-NBDG.

Stereostructure and biological activity of 42-hydroxy-palytoxin: a new palytoxin analogue from Hawaiian Palythoa subspecies

This paper reports on the analysis of the toxin content from Palythoa tuberculosa and Palythoa toxica samples collected off of the Hawaiian coast. Our work, based on in-depth high-resolution liquid chromatography-mass spectrometry analysis along with extensive NMR study, led us to structurally characterize 42-hydroxy-palytoxin, a new palytoxin congener. This toxin and palytoxin itself appeared to be the major components of toxic extract from a P. tuberculosa sample, while 42-hydroxy-palytoxin was proven by far to be the main palytoxin derivative in P. toxica. Functional studies on this new palytoxin-like compound suggest that the new palytoxin analogue and palytoxin itself present similar biological activities.

Localized electrical stimulation to C2C12 myotubes cultured on a porous membrane-based substrate

We report a porous membrane-based cell culture device that can conduct localized electrical stimulation of a cell monolayer. The device's cell culture substrate is a microporous alumina membrane with an underlying thin poly(dimethylsiloxane) (PDMS) film spotted with holes. When electric current is generated between the device's Pt ring electrodes--one of which is placed above the cells and the other below the PDMS layer--the current density condenses at the holes in the PDMS film, and cells located above the holes can be electrically stimulated. C2C12 cells were confluently cultured on the substrate and were differentiated to myotubes. To control the stimulated area in the substrate, we attempted to seal and reopen the holes of the PDMS film by using an air bubble. Since the current pulse could be effectively blocked at the sealed holes, fluorescent Ca2+ transients, indicative of cellular excitation, were observed from the myotubes located above holes in the open state.

Neural agrin controls maturation of the excitation-contraction coupling mechanism in human myotubes developing in vitro

The aim of this study was to elucidate the mechanisms responsible for the effects of innervation on the maturation of excitation-contraction coupling apparatus in human skeletal muscle. For this purpose, we compared the establishment of the excitation-contraction coupling mechanism in myotubes differentiated in four different experimental paradigms: 1) aneurally cultured, 2) cocultured with fetal rat spinal cord explants, 3) aneurally cultured in medium conditioned by cocultures, and 4) aneurally cultured in medium supplemented with purified recombinant chick neural agrin. Ca2+ imaging indicated that coculturing human muscle cells with rat spinal cord explants increased the fraction of cells showing a functional excitation-contraction coupling mechanism. The effect of spinal cord explants was mimicked by treatment with medium conditioned by cocultures or by addition of 1 nM of recombinant neural agrin to the medium. The treatment with neural agrin increased the number of human muscle cells in which functional ryanodine receptors (RyRs) and dihydropyridine-sensitive L-type Ca2+ channels were detectable. Our data are consistent with the hypothesis that agrin, released from neurons, controls the maturation of the excitation-contraction coupling mechanism and that this effect is due to modulation of both RyRs and L-type Ca2+ channels. Thus, a novel role for neural agrin in skeletal muscle maturation is proposed.

Potassium currents in human myogenic cells from donors of different ages

Ageing in humans is accompanied by a reduction in the capacity of satellite cells to proliferate and the forming myoblasts to fuse. The processes of myoblast differentiation and fusion are associated with specific changes in the cells electrical properties. We wanted to elucidate the possible effects of ageing on these parameters and performed whole-cell patch-clamp recordings on human myoblasts obtained from biopsies of skeletal muscles from 2-, 48- and 76-year-old donors. First, we found that resting membrane potential on the 4th day of differentiation in vitro is less negative in the older than in the younger cells. Moreover, the oldest cells showed a smaller density of outward and inward potassium currents. More cells from the old and middle-age donors have a low (less than -40 mV) potential of activation for the outward potassium current. We conclude that in human myoblasts biophysical properties of potassium currents change with donor age.

Intrinsic ionic conductances mediate the spontaneous electrical activity of cultured mouse myotubes

Mouse skeletal myotubes differentiated in vitro exhibited spontaneous contractions associated with electrical activity. The ionic conductances responsible for the origin and modulation of the spontaneous activity were examined using the whole-cell patch-clamp technique and measuring [Ca(2+)](i) transients with the Ca(2+) indicator, fura 2-AM. Regular spontaneous activity was characterized by single TTX-sensitive action potentials, followed by transient increases in [Ca(2+)](i). Since the bath-application of Cd(2+) (300 microM) or Ni(2+) (50 muM) abolished the cell firing, T-type (I(Ca,T)) and L-type (I(Ca,L)) Ca(2+) currents were investigated in spontaneously contracting myotubes. The low activation threshold (around -60 mV) and the high density of I(Ca,T) observed in contracting myotubes suggested that I(Ca,T) initiated action potential firing, by bringing cells to the firing threshold. The results also suggested that the activity of I(Ca,L) could sustain the [Ca(2+)](i) transients associated with the action potential, leading to the activation of apamin-sensitive SK-type Ca(2+)-activated K(+) channels and the afterhyperpolarization (AHP) following single spikes. In conclusion, an interplay between voltage-dependent inward (Na(+) and Ca(2+)) and outward (SK) conductances is proposed to mediate the spontaneous pacemaker activity in cultured muscle myotubes during the process of myogenesis.

Autocrine activation of nicotinic acetylcholine receptors contributes to Ca2+ spikes in mouse myotubes during myogenesis

It is widely accepted that nicotinic acetylcholine receptor (nAChR) channel activity controls myoblast fusion into myotubes during myogenesis. In this study we explored the possible role of nAChR channels after cell fusion in a murine cell model. Using videoimaging techniques we showed that embryonic muscle nAChR channel openings contribute to the spontaneous transients of intracellular concentration of Ca2+ ([Ca2+]i) and to twitches characteristic of developing myotubes before innervation. Moreover, we observed a choline acetyltransferase immunoreactivity in the myotubes and we detected an acetylcholine-like compound in the extracellular solution. Therefore, we suggest that the autocrine activation of nAChR channels gives rise to [Ca2+]i spikes and contractions. Spontaneous openings of the nAChR channels may be an alternative, although less efficient, mechanism. We report also that blocking the nAChRs causes a significant reduction in cell survival, detectable as a decreased number of myotubes in culture. This led us to hypothesize a possible functional role for the autocrine activation of the nAChRs. By triggering mechanical activity, such activation could represent a strategy to ensure the trophism of myotubes in the absence of nerves.

3,4-Methylenedioxymethamphetamine (Ecstasy) Activates Skeletal Muscle Nicotinic Acetylcholine Receptors

Adverse 3,4-methylenedioxymethamphetamine (MDMA; ecstasy) effects are usually ascribed to neurotransmitter release in the central nervous system. Since clinical features such as fasciculations, muscle cramps, rapidly progressing hyperthermia, hyperkalemia, and rhabdomyolysis point to the skeletal muscle as additional target, we studied the effects of MDMA on native and cultured skeletal muscle. We addressed the question whether malignant hyperthermia (MH)-susceptible (MHS) muscle is predisposed to adverse MDMA reactions. Force measurements on muscle strips showed that 100 microM MDMA, a concentration close to that determined in some MDMA users, regularly enhanced the sensitivity of skeletal muscle to caffeine-induced contractures but did not cause contractures on its own. The left-shift of the dose-response curve induced by MDMA was greater in normal than in MHS muscle. Furthermore, MDMA did not release Ca(2+) from isolated sarcoplasmic reticulum vesicles. These findings do not support the view of an MH-triggering effect on muscle. However, MDMA induced Ca(2+) transients in myotubes and increased their acidification rate. Surprisingly, alpha-bungarotoxin, a specific antagonist of the nicotinic acetylcholine receptor (nAChR), abolished these MDMA effects. The nAChR agonistic action of MDMA was confirmed by patch-clamp measurements of ion currents on human embryonic kidney cells expressing nAChR. We conclude that the neuromuscular junction is a target of MDMA and that an activation of nAChR contributes to the muscle-related symptoms of MDMA users. The drug may be of particular risk in individuals with abundant extrajunctional nAChR such as in generalized denervation or muscle regeneration processes and may act on central nAChR.

Restoration of functional motor units in a rat model of sphincter injury by muscle precursor cell autografts1

BACKGROUND

Urinary incontinence is a debilitating condition that affects primarily elderly individuals. One major mechanism results from chronic denervation of the striated urethral sphincter with associated fibrosis. The authors investigated the fate of muscle precursor cells (MPC) injected into a model of striated urethral sphincter injury that reproduces the histopathologic changes of sphincter insufficiency.

METHODS

The striated urethral sphincter of older male rats was damaged by electrocoagulation. MPC were isolated from limb myofiber explants, infected with an adenovirus carrying the transgene encoding beta-galactosidase, and injected into the sphincter of the same animal 37 days after injury. Animals were killed 5 and 30 days after injection for assessment of sphincter function and the formation of motor units.

RESULTS

Electrocoagulation resulted in an irreversible destruction of both sphincteric myofibers and nerve endings, with a functional incapacity of the damaged sphincter to sustain an increase in bladder pressure; atrophy and fibrosis developed after 1 month. Injection of MPC resulted in the formation of beta-galactosidase-expressing myotubes in the sphincter that persisted beyond 30 days. The regenerated myotubes carried acetylcholine receptors associated with a nerve ending and were thus considered to form anatomic motor units. Urodynamic studies confirmed the restoration of 41% of sphincter function 1 month after MPC injection.

CONCLUSIONS

The authors showed that MPC isolated from limb muscles of an older animal can recapitulate a myogenic program when injected into an irreversibly injured sphincter. The maturation of MPC activates nerve regeneration and restores functional motor units.