Ca2+ signalling pathways activated by acetylcholine in mouse C2C12 myotubes

Bioluminescent Low-Affinity Ca2+ Indicator for ER with Multicolor Calcium Imaging in Single Living Cells

The sarco/endoplasmic reticulum (SR/ER) is the foremost intercellular Ca²⁺ store (at submillimolar concentrations), playing a crucial role in controlling intracellular Ca²⁺ levels. For the investigation of SR/ER Ca²⁺ dynamics in cells, fluorescent protein-based genetically encoded calcium indicators (GECIs) with low Ca²⁺ affinity have been used. Recently, bioluminescent protein-based GECIs with high brightness have been reported to counter the constraints of fluorescence imaging, such as phototoxicity. However, their Ca²⁺ affinity is high and limited for imaging in the cytosol, nucleus, or mitochondria. In this study, we developed a novel cyan color, low-affinity ( K_d = 110 μM) intensiometric bioluminescent GECI, which enables monitoring of the Ca²⁺ dynamics in the ER of HeLa cells and the SR of C2C12-derived myotubes. To facilitate the broad concentration range of Ca²⁺ in cellular organelles, we additionally developed an intermediate affinity ( K_d = 18 μM), orange color, and bioluminescent GECI, which enables monitoring of Ca²⁺ dynamics in the mitochondria of HeLa cells. With these indicators, in conjunction with an existing high-affinity, green, bioluminescent GECI, we succeeded in multicolor bioluminescent Ca²⁺ imaging in three distinct organelles (nuclei, mitochondria, and ER) simultaneously. The multicolor, live, bioluminescent Ca²⁺ imaging demonstrated here can be used to stably reveal the ER Ca²⁺ homeostasis and cooperative Ca²⁺ regulation among organelles. This will lead to the further understanding of Ca²⁺-related physiological functions and pathophysiological mechanisms.

MyoScreen, a High-Throughput Phenotypic Screening Platform Enabling Muscle Drug Discovery

Despite the need for more effective drug treatments to address muscle atrophy and disease, physiologically accurate in vitro screening models and higher information content preclinical assays that aid in the discovery and development of novel therapies are lacking. To this end, MyoScreen was developed: a robust and versatile high-throughput high-content screening (HT/HCS) platform that integrates a physiologically and pharmacologically relevant micropatterned human primary skeletal muscle model with a panel of pertinent phenotypic and functional assays. MyoScreen myotubes form aligned, striated myofibers, and they show nerve-independent accumulation of acetylcholine receptors (AChRs), excitation-contraction coupling (ECC) properties characteristic of adult skeletal muscle and contraction in response to chemical stimulation. Reproducibility and sensitivity of the fully automated MyoScreen platform are highlighted in assays that quantitatively measure myogenesis, hypertrophy and atrophy, AChR clusterization, and intracellular calcium release dynamics, as well as integrating contractility data. A primary screen of 2560 compounds to identify stimulators of myofiber regeneration and repair, followed by further biological characterization of two hits, validates MyoScreen for the discovery and testing of novel therapeutics. MyoScreen is an improvement of current in vitro muscle models, enabling a more predictive screening strategy for preclinical selection of the most efficacious new chemical entities earlier in the discovery pipeline process.

The potassium channels TASK2 and TREK1 regulate functional differentiation of murine skeletal muscle cells

Two-pore domain potassium (K_2P) channels influence basic cellular parameters such as resting membrane potential, cellular excitability, or intracellular Ca²⁺-concentration [Ca²⁺]_i While the physiological importance of K_2P channels in different organ systems (e.g., heart, central nervous system, or immune system) has become increasingly clear over the last decade, their expression profile and functional role in skeletal muscle cells (SkMC) remain largely unknown. The mouse SkMC cell line C2C12, wild-type mouse muscle tissue, and primary mouse muscle cells (PMMs) were analyzed using quantitative PCR, Western blotting, and immunohistochemical stainings as well as functional analysis including patch-clamp measurements and Ca²⁺ imaging. Mouse SkMC express TWIK-related acid-sensitive K⁺ channel (TASK) 2, TWIK-related K⁺ channel (TREK) 1, TREK2, and TWIK-related arachidonic acid stimulated K⁺ channel (TRAAK). Except TASK2 all mentioned channels were upregulated in vitro during differentiation from myoblasts to myotubes. TASK2 and TREK1 were also functionally expressed and upregulated in PMMs isolated from mouse muscle tissue. Inhibition of TASK2 and TREK1 during differentiation revealed a morphological impairment of myoblast fusion accompanied by a downregulation of maturation markers. TASK2 and TREK1 blockade led to a decreased K⁺ outward current and a decrease of ACh-dependent Ca²⁺ influx in C2C12 cells as potential underlying mechanisms. K_2P-channel expression was also detected in human muscle tissue by immunohistochemistry pointing towards possible relevance for human muscle cell maturation and function. In conclusion, our findings for the first time demonstrate the functional expression of TASK2 and TREK1 in muscle cells with implications for differentiation processes warranting further investigations in physiologic and pathophysiologic scenarios.

Endocytosis function of a ligand-gated ion channel homolog in Caenorhabditis elegans

Ligand-gated ion channels are transmembrane proteins that respond to a variety of transmitters, including acetylcholine, gamma-aminobutyric acid (GABA), glycine, and glutamate [1 and 2]. These proteins play key roles in neurotransmission and are typically found in the nervous system and at neuromuscular junctions [3]. Recently, acetylcholine receptor family members also have been found in nonneuronal cells, including macrophages [4], keratinocytes [5], bronchial epithelial cells [5], and endothelial cells of arteries [6]. The function of these channels in nonneuronal cells in mammals remains to be elucidated, though it has been shown that the acetylcholine receptor alpha7 subunit is required for acetylcholine-mediated inhibition of tumor necrosis factor release by activated macrophages [4]. We show that cup-4, a gene required for efficient endocytosis of fluids by C. elegans coelomocytes, encodes a protein that is homologous to ligand-gated ion channels, with the highest degree of similarity to nicotinic acetylcholine receptors. Worms lacking CUP-4 have reduced phosphatidylinositol 4,5-bisphosphate levels at the plasma membrane, suggesting that CUP-4 regulates endocytosis through modulation of phospholipase C activity.

Involvement of gap junctional communication in myogenesis

Cell-to-cell communication plays important roles in development and in tissue morphogenesis. Gap junctional intercellular communication (GJIC) has been implicated in embryonic development of various tissues and provides a pathway to exchange ions, secondary messengers, and metabolites through the intercellular gap junction channels. Although GJIC is absent in adult skeletal muscles, the formation of skeletal muscles involves a sequence of complex events including cell-cell interaction processes where myogenic cells closely adhere to each other. Much experimental evidence has shown that myogenic precursors and developing muscle fibers can directly communicate through junctional channels. This review summarizes current knowledge on the GJIC and developmental events involved in the formation of skeletal muscle fibers and describes recent progress in the investigation of the role of GJIC in myogenesis: evidence of gap junctions in somitic and myotomal tissue as well as in developing muscle fibers in situ, GJIC between perfusion myoblasts in culture, and involvement of GJIC in cytodifferentiation of skeletal muscle cells and in myoblast fusion. A model of intercellular signaling is proposed where GJIC participates to coordinate a multicellular population of interacting myogenic precursors to allow commitment to the skeletal muscle fate.

Differentiation markers of mouse C2C12 and rat L6 myogenic cell lines and the effect of the differentiation medium

The differentiation grade of cells in culture is dependent on the composition of the culture medium. Two commonly used myogenic cell lines, mouse C2C12 and rat L6, usually differentiate at a low concentration of horse serum. In this study we compared the effect of horse serum with a medium containing a low percentage of Ultroser G and rat brain extract. The maturation grade was evaluated on the basis of various biochemical, (immuno)histochemical and cell-physiological parameters. Substitution of horse serum by Ultroser G and rat brain extract during the differentiation phase resulted in a higher maturation grade of the myotubes of both cell lines, on the basis of creatine kinase activity and the diameter of the myotubes. In addition, the C2C12 myotubes display cross-striation, contain a higher percentage of creatine kinase muscle-specific isoenzyme MM, show a ninefold increase in acetylcholine receptor (AChR) clusters, form a continuous basement membrane, and have a lower resting cytosolic Ca2+ concentration. L6 myotubes show a fivefold increase in AChR clusters and a twofold increase in the expression of the mRNA of the epsilon-subunit of AChR.C2C12 cells show spontaneous contraction and response of cytosolic Ca2+ to various stimulants in contrast to L6 cells which do not. These studies established that the Ultroser G/brain extract medium leads to a higher differentiation grade of both cell lines, but parameters appropriate for use as differentiation markers appear to differ between both cell lines.

Role for calcium from the sarcoplasmic reticulum in coupling muscle activity to nicotinic acetylcholine receptor gene expression in rat

Neurally evoked muscle electrical activity suppresses nicotinic acetylcholine receptor (nAChR) gene expression in extrajunctional domains of adult muscle fibers. It has been proposed that this regulation is mediated by calcium influx through voltage-dependent L-type calcium channels but bypasses the sarcoplasmic reticulum in chick and mouse C2C12 cells. Here we report that in rat muscle calcium influx through L-type calcium channels preferentially reduced nAChR epsilon-subunit RNA via a post-transcriptional mechanism. In contrast, calcium release from the sarcoplasmic reticulum (SR) suppressed nAChR subunit RNA levels as a result of decreasing nAChR subunit promoter activity. Finally, we show that this decreased promoter activity is mediated through the same DNA sequences that control activity-dependent gene expression. Therefore, we propose that in rat muscle, calcium release from the SR participates in coupling muscle depolarization to nAChR gene expression.

Cell-type specificity of preconditioning in an in vitro model

We investigated whether preconditioning could protect several cultured cell lines, to determine whether the protection is specific for cells derived from different myogenic and non-myogenic sources. Ischemia was simulated by centrifugation of cells into a pellet, and cell viability was determined by hypotonic trypan blue solution. Preconditioning was produced by brief exposures to either glucose-free solution or metabolic inhibition. Freshly isolated rabbit ventricular myocytes were studied to confirm that preconditioning occurs in this model. We then compared these results to those in several cultured cell lines, including HEK 293 cells derived from human embryonic kidney, HIT-T15 cells from Syrian hamster pancreatic islets, and C2C12 cells from mouse skeletal muscle. In the latter cell line, we also determined whether differentiation alters preconditioning. Preconditioning protected rabbit ventricular myocytes: the percentage of dead cells was decreased from 36.8 +/- 4.7% in the control group to 23.0 +/- 5.2% in the preconditioned group after 60 min and from 50.7 +/- 2.1% in the control group to 25.5 +/- 4.5% in the preconditioned group after 120 min ischemia (p < 0.02). In contrast, there was no protection from preconditioning in HEK 293 cells or HIT-T15 cells. Preconditioning did not protect C2C12 myoblasts either. Interestingly, after C2C12 myoblasts had differentiated into myotubes (induced by exposing the cells to low-serum medium), they could then be protected by preconditioning (46.3 +/- 3.6% in the control group vs 26.0 +/- 2.7% in the preconditioned group after 60 min and 67.4 +/- 3.6% in the control group vs 46.0 +/- 4.6% in the preconditioned group after 120 min ischemia; p < 0.05). In conclusion, protection from preconditioning is cell-type specific. The presence of endogenous KATP channels (which are plentiful in HIT-T15 cells) is insufficient to enable preconditioning of the cell. Among the various cell types studied, only differentiated muscle cells (rabbit ventricular myocytes and C2C12 myotubes) exhibited preconditioning.

Myoblast fusion requires cytosolic calcium elevation but not activation of voltage-dependent calcium channels

Many studies of in vitro skeletal myogenesis have demonstrated that fusion of myoblasts into multinucleated myotubes is regulated by calcium-dependent processes. Calcium ions appear to be necessary at the outer face of the membrane, and an additional internal calcium increase seems required to promote fusion of aligned myoblasts. It has been proposed that a calcium influx could take place prior to fusion and that this may be mediated by voltage-dependent calcium channels. Previously, we showed that two types of voltage-dependent calcium currents were expressed in multinucleated myotubes but not in rat myoblasts growing in primary culture before the withdrawal of the growth medium. We also showed that the previous formation of multinucleated synticia was not a prerequisite of developmental appearance of calcium currents, suggesting that the two events were time-correlated but not sequentially dependent. These features led us to investigate changes in internal calcium activity and the possible appearance of voltage-dependent calcium influx pathways just after the promotion of fusion by the change of culture medium. The results confirm that a rise in cytosolic calcium activity occurs slightly before fusion in confluent myoblasts and remained in newly formed myotubes. Reducing this elevation by internal calcium buffering lowered myoblast fusion and, reciprocally, blocking cell fusion prevented calcium increase. Treatment with the organic calcium channel blockers nifedipine (5 microM) and PN 200-110 (1 microM) did not alter cytosolic calcium changes nor cell fusion, and voltage-dependent calcium currents were never observed by the perforated patch-clamp technique in aligned fusion-competent myoblasts. Other voltage-operated mechanisms of calcium rise were not detected since depolarization with hyperpotassium solutions failed to elicit increases in intracellular calcium. On the contrary, acetylcholine was able to promote extracellular calcium-dependent calcium transients. Our results confirm the requirement of an increase in resting calcium during fusion, but do not support the hypothesis of an influx through voltage-dependent channels or other voltage-operated pathways. The elevation of internal calcium activity may result from other mechanisms, such as a cholinergic action for example.

The desensitization of the embryonic mouse muscle acetylcholine receptor depends on the cellular environment

The rate of desensitization of nicotinic acetylcholine (ACh) receptor (nAChR), an important characteristic of nAChR function, was studied in myotubes of the mouse C2C12 cell line at different times after fusion, by measuring the decay of ACh-evoked currents (IACh) under various patch-clamp configurations. We observed a progressive slowing of IACh decay rate (half-decay time rose from about 0.5 s to over 5 s) in myotubes of increasing size (i.e. age) under all experimental conditions, except in outside-out patches, when IACh decayed as fast as in the smallest myotubes. Single-channel conductance (about 35 pS) and open time (about 3.5 ms), measured in outside-out and cell-attached patches, were independent of myotube size. In Xenopus oocytes injected with poly(A+)RNA extracted from C2C12 myoblasts or mature myotubes, IACh decay was about 50 times slower than in myotubes. Neither cAMP-dependent nor diacylglycerol-dependent protein kinases, actin nor microtubule polymerization state influenced IACh decay. Our data indicate that the cellular environment, but not readily dialysable cytosolic factors, markedly influences the functional behaviour of nAChR.

Acetylcholine-activated inward current induces cytosolic Ca2+ mobilization in mouse C2C12 myotubes

We examined the spatiotemporal pattern of intracellular Ca2+ liberation in mouse myotubes by means of fluorescence imaging of cytosolic free Ca2+ together with the simultaneous recording of membrane whole-cell currents. Acetylcholine (ACh) applications to C2C12 myotubes equilibrated in Ca(2+)-free medium and voltage clamped at -50 mV evoked localized fluorescence transients of variable amplitude with less than 0.5 s delay. Under the same experimental conditions, fluorescence transients were elicited by ACh also in mouse primary myotubes. Ca2+ transients were inhibited in myotubes clamped at depolarized potentials (-10 mV to +50 mV), or equilibrated in a Na+,Ca(2+)-free medium as well as in cells loaded with heparin, or with inositol (1,4,5) trisphosphate (InsP3). To investigate whether InsP3 could induce Ca2+ mobilization, [Ca2+]i determinations were carried out in myotubes loaded with InsP3 through the whole-cell patch-clamp recording pipette or by extracellular application in permeabilized cells. InsP3 diffusion into the myoplasm caused Ca2+ spikes with 5 +/- 1 s (mean +/- SEM) delay from the rupture of the membrane patch. Spikes were followed by sustained increases in fluorescence or by damped oscillations. In permeabilized myotubes, InsP3 induced the release of sequestered 45Ca2+ with a half-maximally effective concentration (EC50) of 0.28 +/- 0.05 microM, and Hill coefficient of 0.79 +/- 0.09. It is concluded that the ACh-activated inward current in mouse myotubes is coupled to cytosolic Ca2+ mobilization from internal InsP3-sensitive pools.