ATP potentiates the formation of AChR aggregate in the co-culture of NG108-15 cells with C2C12 myotubes

ATP-Induced Increase in Intracellular Calcium Levels and Subsequent Activation of mTOR as Regulators of Skeletal Muscle Hypertrophy

Intracellular signaling pathways, including the mammalian target of rapamycin (mTOR) and the mitogen-activated protein kinase (MAPK) pathway, are activated by exercise, and promote skeletal muscle hypertrophy. However, the mechanisms by which these pathways are activated by physiological stimulation are not fully understood. Here we show that extracellular ATP activates these pathways by increasing intracellular Ca²⁺ levels ([Ca²⁺]_i), and promotes muscle hypertrophy. [Ca²⁺]_i in skeletal muscle was transiently increased after exercise. Treatment with ATP induced the increase in [Ca²⁺]_i through the P2Y₂ receptor/inositol 1,4,5-trisphosphate receptor pathway, and subsequent activation of mTOR in vitro. In addition, the ATP-induced increase in [Ca²⁺]_i coordinately activated Erk1/2, p38 MAPK and mTOR that upregulated translation of JunB and interleukin-6. ATP also induced an increase in [Ca²⁺]_i in isolated soleus muscle fibers, but not in extensor digitorum longus muscle fibers. Furthermore, administration of ATP led to muscle hypertrophy in an mTOR- and Ca²⁺-dependent manner in soleus, but not in plantaris muscle, suggesting that ATP specifically regulated [Ca²⁺]_i in slow muscles. These findings suggest that ATP and [Ca²⁺]_i are important mediators that convert mechanical stimulation into the activation of intracellular signaling pathways, and point to the P2Y receptor as a therapeutic target for treating muscle atrophy.

Purinergic signalling during development and ageing

Extracellular purines and pyrimidines play major roles during embryogenesis, organogenesis, postnatal development and ageing in vertebrates, including humans. Pluripotent stem cells can differentiate into three primary germ layers of the embryo but may also be involved in plasticity and repair of the adult brain. These cells express the molecular components necessary for purinergic signalling, and their developmental fates can be manipulated via this signalling pathway. Functional P1, P2Y and P2X receptor subtypes and ectonucleotidases are involved in the development of different organ systems, including heart, blood vessels, skeletal muscle, urinary bladder, central and peripheral neurons, retina, inner ear, gut, lung and vas deferens. The importance of purinergic signalling in the ageing process is suggested by changes in expression of A1 and A2 receptors in old rat brains and reduction of P2X receptor expression in ageing mouse brain. By contrast, in the periphery, increases in expression of P2X3 and P2X4 receptors are seen in bladder and pancreas.

Effects of acetylcholine and electrical stimulation on glial cell line-derived neurotrophic factor production in skeletal muscle cells

Glial cell line-derived neurotrophic factor (GDNF) is a neurotrophic factor required for survival of neurons in the central and peripheral nervous system. Specifically, GDNF has been characterized as a survival factor for spinal motor neurons. GDNF is synthesized and secreted by neuronal target tissues, including skeletal muscle in the peripheral nervous system; however, the mechanisms by which GDNF is synthesized and released by skeletal muscle are not fully understood. Previous results suggested that cholinergic neurons regulate secretion of GDNF by skeletal muscle. In the current study, GDNF production by skeletal muscle myotubes following treatment with acetylcholine was examined. Acetylcholine receptors on myotubes were identified with labeled alpha-bungarotoxin and were blocked using unlabeled alpha-bungarotoxin. The question of whether electrical stimulation has a similar effect to that of acetylcholine was also investigated. Cells were stimulated with voltage pulses; at 1 and 5 Hz frequencies for times ranging from 30 min to 48 h. GDNF content in myotubes and GDNF in conditioned culture medium were quantified by enzyme-linked immunosorbant assay. Results suggest that acetylcholine and short-term electrical stimulation reduce GDNF secretion, while treatment with carbachol or long-term electrical stimulation enhances GDNF production by skeletal muscle.

Purinergic signaling in embryonic and stem cell development

Nucleotides are of crucial importance as carriers of energy in all organisms. However, the concept that in addition to their intracellular roles, nucleotides act as extracellular ligands specifically on receptors of the plasma membrane took longer to be accepted. Purinergic signaling exerted by purines and pyrimidines, principally ATP and adenosine, occurs throughout embryologic development in a wide variety of organisms, including amphibians, birds, and mammals. Cellular signaling, mediated by ATP, is present in development at very early stages, e.g., gastrulation of Xenopus and germ layer definition of chick embryo cells. Purinergic receptor expression and functions have been studied in the development of many organs, including the heart, eye, skeletal muscle and the nervous system. In vitro studies with stem cells revealed that purinergic receptors are involved in the processes of proliferation, differentiation, and phenotype determination of differentiated cells. Thus, nucleotides are able to induce various intracellular signaling pathways via crosstalk with other bioactive molecules acting on growth factor and neurotransmitter receptors. Since normal development is disturbed by dysfunction of purinergic signaling in animal models, further studies are needed to elucidate the functions of purinoceptor subtypes in developmental processes.

Cholinergic neurons regulate secretion of glial cell line-derived neurotrophic factor by skeletal muscle cells in culture

Glial cell line-derived neurotrophic factor (GDNF) has been identified as a potent survival factor for both central and peripheral neurons. GDNF has been shown to be a potent survival factor for motor neurons during programmed cell death and continuous treatment with GDNF maintains hyperinnervation of skeletal muscle in adulthood. However, little is known about factors regulating normal production of endogenous GDNF in skeletal muscle. This study aimed to examine the role that motor neurons play in regulating GDNF secretion by skeletal muscle. A co-culture of skeletal muscle cells (C2C12) and cholinergic neurons, glioma×neuroblastoma hybrid cells (NG108-15) were used to create nerve-muscle interactions in vitro. Acetylcholine receptors (AChRs) on nerve-myotube co-cultures were blocked with alpha-bungarotoxin (α-BTX). GDNF protein content in cells and in culture medium was analyzed by enzyme-linked immunosorbant assay (ELISA) and western blotting. GDNF localization was examined by immunocytochemistry. The nerve-muscle co-culture study indicated that the addition of motor neurons to skeletal muscle cells reduced the secretion of GDNF by skeletal muscle. The results also showed that blocking AChRs with α-BTX reversed the action of neural cells on GDNF secretion by skeletal muscle. Although ELISA results showed no GDNF in differentiated NG108-15 cells grown alone, immunocytochemical analysis showed that GDNF was localized in NG108-15 cells co-cultured with C2C12 myotubes. These results suggest that motor neurons may be regulating their own supply of GDNF secreted by skeletal muscle and that activation of AChRs may be involved in this process.

ATP induces synaptic gene expressions in cortical neurons: transduction and transcription control via P2Y1 receptors

Studies in vertebrate neuromuscular synapses have revealed previously that ATP, via P2Y receptors, plays a critical role in regulating postsynaptic gene expressions. An equivalent regulatory role of ATP and its P2Y receptors would not necessarily be expected for the very different situation of the brain synapses, but we provide evidence here for a brain version of that role. In cultured cortical neurons, the expression of P2Y(1) receptors increased sharply during neuronal differentiation. Those receptors were found mainly colocalized with the postsynaptic scaffold postsynaptic density protein 95 (PSD-95). This arises through a direct interaction of a PDZ domain of PSD-95 with the C-terminal PDZ-binding motif, D-T-S-L of the P2Y(1) receptor, confirmed by the full suppression of the colocalization upon mutation of two amino acids therein. This interaction is effective in recruiting PSD-95 to the membrane. Specific activation of P2Y(1) (G-protein-coupled) receptors induced the elevation of intracellular Ca(2+) and activation of a mitogen-activated protein kinase/Raf-1 signaling cascade. This led to distinct up-regulation of the genes encoding acetylcholinesterase (AChE(T) variant), choline acetyltransferase, and the N-methyl-d-aspartate receptor subunit NR2A. This was confirmed, in the example of AChE, to arise from P2Y(1)-dependent stimulation of a human ACHE gene promoter. That involved activation of the transcription factor Elk-1; mutagenesis of the ACHE promoter revealed that Elk-1 binding at its specific responsive elements in that promoter was induced by P2Y(1) receptor activation. The combined findings reveal that ATP, via its P2Y(1) receptor, can act trophically in brain neurons to regulate the gene expression of direct effectors of synaptic transmission.

Purinergic (ATP) signaling stimulates JNK1 but not JNK2 MAPK in osteoblast-like cells: contribution of intracellular Ca2+ release, stress activated and L-voltage-dependent calcium influx, PKC and Src kinases

This work shows that ATP activates JNK1, but not JNK2, in rat osteoblasts and ROS-A 17/2.8 osteoblast-like cells. In ROS-A 17/2.8 cells ATP induced JNK1 phosphorylation in a dose- and time-dependent manner. JNK1 phosphorylation also increased after osteoblast stimulation with ATPgammaS and UTP, but not with ADPbetaS. RT-PCR studies supported the expression of P2Y(2) receptor subtype. ATP-induced JNK1 activation was reduced by PI-PLC, IP(3) receptor, PKC and Src inhibitors and by gadolinium, nifedipine and verapamil or a Ca(2+)-free medium. ERK 1/2 or p38 MAPK inhibitors diminished JNK1 activation by ATP, suggesting a cross-talk between these pathways. ATP stimulated osteoblast-like cell proliferation consistent with the participation of P2Y(2) receptors. These results show that P2Y(2) receptor stimulation by ATP induces JNK1 phosphorylation in ROS-A 17/2.8 cells in a way dependent on PI-PLC/IP(3)/intracellular Ca(2+) release and Ca(2+) influx through stress activated and L-type voltage-dependent calcium channels and involves PKC and Src kinases.

Photolithographic patterning of C2C12 myotubes using vitronectin as growth substrate in serum-free medium

The C2C12 cell line is frequently used as a model of skeletal muscle differentiation. In our serum-free defined culture system, differentiation of C2C12 cells into myotubes required surface-bound signals such as substrate-adsorbed vitronectin or laminin. On the basis of this substrate requirement of myotube formation, we developed a photolithography-based method to pattern C2C12 myotubes, where myotubes formed exclusively on vitronectin surface patterns. We have determined that the optimal line width to form single myotubes is approximately 30 mum. To illustrate a possible application of this method, we patterned myotubes on the top of commercial substrate-embedded microelectrodes. In contrast to previous experiments where cell patterning was achieved by selective attachment of the cells to patterned surfaces in a medium that contained all of the factors necessary for differentiation, this study illustrates that surface patterning of a signaling molecule, which is essential for skeletal muscle differentiation in a defined system, can result in the formation of aligned myotubes on the patterns. This technique is being developed for applications in cell biology, tissue engineering, and robotics.

Characterization of aconitine-induced block of delayed rectifier K+ current in differentiated NG108-15 neuronal cells

The effects of aconitine (ACO), a highly toxic alkaloid, on ion currents in differentiated NG108-15 neuronal cells were investigated in this study. ACO (0.3-30 microM) suppressed the amplitude of delayed rectifier K+ current (I K(DR)) in a concentration-dependent manner with an IC50 value of 3.1 microM. The presence of ACO enhanced the rate and extent of I K(DR) inactivation, although it had no effect on the initial activation phase of I K(DR). It could shift the inactivation curve of I K(DR) to a hyperpolarized potential with no change in the slope factor. Cumulative inactivation for I K(DR) was also enhanced by ACO. Orphenadrine (30 microM) or methyllycaconitine (30 microM) slightly suppressed I K(DR) without modifying current decay. ACO (10 microM) had an inhibitory effect on voltage-dependent Na+ current (I Na). Under current-clamp recordings, ACO increased the firing and widening of action potentials in these cells. With the aid of the minimal binding scheme, the ACO actions on I K(DR) was quantitatively provided with a dissociation constant of 0.6 microM. A modeled cell was designed to duplicate its inhibitory effect on spontaneous pacemaking. ACO also blocked I K(DR) in neuroblastoma SH-SY5Y cells. Taken together, the experimental data and simulations show that ACO can block delayed rectifier K+ channels of neurons in a concentration- and state-dependent manner. Changes in action potentials induced by ACO in neurons in vivo can be explained mainly by its blocking actions on I K(DR) and I Na.

Skeletal muscle expresses the extracellular cyclic AMP-adenosine pathway

BACKGROUND AND PURPOSE

cAMP is a key intracellular signalling molecule that regulates multiple processes of the vertebrate skeletal muscle. We have shown that cAMP can be actively pumped out from the skeletal muscle cell. Since in other tissues, cAMP efflux had been associated with extracellular generation of adenosine, in the present study we have assessed the fate of interstitial cAMP and the existence of an extracellular cAMP-adenosine signalling pathway in skeletal muscle.

EXPERIMENTAL APPROACH

cAMP efflux and/or its extracellular degradation were analysed by incubating rat cultured skeletal muscle with exogenous cAMP, forskolin or isoprenaline. cAMP and its metabolites were quantified by radioassay or HPLC, respectively.

KEY RESULTS

Incubation of cells with exogenous cAMP was followed by interstitial accumulation of 5'-AMP and adenosine, a phenomenon inhibited by selective inhibitors of ecto-phosphodiesterase (DPSPX) and ecto-nucleotidase (AMPCP). Activation of adenylyl cyclase (AC) in cultured cells with forskolin or isoprenaline increased cAMP efflux and extracellular generation of 5'-AMP and adenosine. Extracellular cAMP-adenosine pathway was also observed after direct and receptor-dependent stimulation of AC in rat extensor muscle ex vivo. These events were attenuated by probenecid, an inhibitor of ATP binding cassette family transporters.

CONCLUSIONS AND IMPLICATIONS

Our results show the existence of an extracellular biochemical cascade that converts cAMP into adenosine. The functional relevance of this extracellular signalling system may involve a feedback modulation of cellular response initiated by several G protein-coupled receptor ligands, amplifying cAMP influence to a paracrine mode, through its metabolite, adenosine.

Physiology and pathophysiology of purinergic neurotransmission

This review is focused on purinergic neurotransmission, i.e., ATP released from nerves as a transmitter or cotransmitter to act as an extracellular signaling molecule on both pre- and postjunctional membranes at neuroeffector junctions and synapses, as well as acting as a trophic factor during development and regeneration. Emphasis is placed on the physiology and pathophysiology of ATP, but extracellular roles of its breakdown product, adenosine, are also considered because of their intimate interactions. The early history of the involvement of ATP in autonomic and skeletal neuromuscular transmission and in activities in the central nervous system and ganglia is reviewed. Brief background information is given about the identification of receptor subtypes for purines and pyrimidines and about ATP storage, release, and ectoenzymatic breakdown. Evidence that ATP is a cotransmitter in most, if not all, peripheral and central neurons is presented, as well as full accounts of neurotransmission and neuromodulation in autonomic and sensory ganglia and in the brain and spinal cord. There is coverage of neuron-glia interactions and of purinergic neuroeffector transmission to nonmuscular cells. To establish the primitive and widespread nature of purinergic neurotransmission, both the ontogeny and phylogeny of purinergic signaling are considered. Finally, the pathophysiology of purinergic neurotransmission in both peripheral and central nervous systems is reviewed, and speculations are made about future developments.