ATP acts via P2Y1 receptors to stimulate acetylcholinesterase and acetylcholine receptor expression: transduction and transcription control

Single-nuclei sequencing of skeletal muscle reveals subsynaptic-specific transcripts involved in neuromuscular junction maintenance

The neuromuscular junction (NMJ) is the synapse formed between motor neurons and skeletal muscle fibers. Its stability relies on the continued expression of genes in a subset of myonuclei, called NMJ myonuclei. Here, we use single-nuclei RNA-sequencing (snRNA-seq) to identify numerous NMJ-specific transcripts. To elucidate how the NMJ transcriptome is regulated, we also performed snRNA-seq on sciatic nerve transected, botulinum toxin injected, and Musk knockout muscles. The data show that NMJ gene expression is not only driven by agrin-Lrp4/MuSK signaling but is also affected by electrical activity and trophic factors other than agrin. By selecting the three NMJ genes Etv4, Lrtm1, and Pdzrn4, we further characterize novel contributors to NMJ stability and function. AAV-mediated overexpression shows that Etv4 is sufficient to upregulate the expression of -50% of the NMJ genes in non-synaptic myonuclei, while AAV-CRISPR/Cas9-mediated muscle-specific knockout of Pdzrn4 induces NMJ fragmentation. Further investigation of Pdzrn4 revealed that it localizes to the Golgi apparatus and interacts with MuSK protein. Collectively, our data provide a rich resource of NMJ transcripts, highlight the importance of ETS transcription factors at the NMJ, and suggest a novel pathway for NMJ post-translational modifications.

Effect of creatine administration on locomotor activity and stress response in olive flounder (Paralichthys olivaceus)

The creatine kinase system is crucial for maintaining cellular energy homeostasis and plays a role in regulating locomotor behavior in organisms, but its significance in the regulating the motionless behavior in olive flounder is limited. In the first experiment of this study, elevated levels of creatine kinase (CK) activity in the spinal cord were detected in the juvenile group (JG) flounder compared to the adult group (AG) flounder. In the second experiment, to further confirm the involvement of CK in the locomotor behavior, the adult flounder was given an intraperitoneal injection of creatine (150 mg/kg), while the flounder in the control group received a saline solution. After one week post-injection, the behavioral analysis revealed that the flounder in the creatine-treated group displayed higher levels of locomotor activity and a greater number of escape attempts in response to external stimuli when compared to the control group. However, the acute stress response, induced by intraperitoneal injection and characterized by tail beating, was significantly alleviated in the flounder in the creatine-treated group. Additionally, there was an upregulation of the UII and AchR genes in the spinal cord, as well as increased levels of UII and AchR in the muscle tissues of the creatine-treated flounder. However, a reduction in UI mRNA levels was observed in the brain of the flounder. Collectively, our data provide the evidence that the elevated enzyme activity and gene expression of creatine kinase play important roles in off-bottom swimming behavior in the JG flounder. Furthermore, administration of creatine improved the locomotor activity and alleviated the stress response in flounder, which is associated with regulation of the locomotor- and stress-related gene in the brain, spinal cord, and muscle.

Aflatoxin B1-induced redox imbalance in the hippocampus and cerebral cortex of male Wistar rats is accompanied by altered cholinergic, indoleaminergic, and purinergic pathways: Abatement by dietary rutin

The neurotoxic impact of dietary exposure to aflatoxin B1 (AFB1) is well documented in experimental studies. Rutin is a phytochemical with prominent anti-inflammatory and antioxidant activities. There is an information gap on the influence of rutin on AFB1-induced neurotoxicity. This study investigated the influence of rutin on neurobehavioral and biochemical abnormalities in male Wistar rats (six weeks old) orally treated with AFB1 (0.75, and 1.5 mg/kg body weight) or co-administered with rutin (50 mg/kg) for 30 uninterrupted days. Results indicate that AFB1-induced depression-like behavior by Tail Suspension Test (TST) and cognitive impairment by Y-maze was abated following rutin co-administration. Abatement of AFB1-induced decreases in acetylcholinesterase (AChE) activity, and increased antioxidant status, by rutin was accompanied by a marked reduction in oxidative stress markers and increased hydrolysis of the purinergic molecules in the cerebral cortex and hippocampus of rats. Additionally, rutin co-treatment abrogated AFB1-mediated elevation of interleukin-6 (IL-6), nitric oxide (NO) levels, and activity of myeloperoxidase (MPO). Correspondingly, rutin co-treatment lowered the activity and immunocontent of immunosuppressive indoleamine 2, 3-dioxygenase (IDO). Further, rutin co-treatment prevented histological injuries in the cerebral cortex and hippocampus. In conclusion, abatement of AFB1-induced neurobehavioral abnormalities by rutin involves the mechanisms of anti-inflammatory, antioxidant, and regulation of cholinergic, purinergic, and indoleaminergic pathways in rats.

Mechanical Disturbance of Osteoclasts Induces ATP Release That Leads to Protein Synthesis in Skeletal Muscle through an Akt-mTOR Signaling Pathway

Muscle and bone are tightly integrated through mechanical and biochemical signals. Osteoclasts are cells mostly related to pathological bone loss; however, they also start physiological bone remodeling. Therefore, osteoclast signals released during bone remodeling could improve both bone and skeletal muscle mass. Extracellular ATP is an autocrine/paracrine signaling molecule released by bone and muscle cells. Then, in the present work, it was hypothesized that ATP is a paracrine mediator released by osteoclasts and leads to skeletal muscle protein synthesis. RAW264.7-derived osteoclasts were co-cultured in Transwell^® chambers with flexor digitorum brevis (FDB) muscle isolated from adult BalbC mice. The osteoclasts at the upper chamber were mechanically stimulated by controlled culture medium perturbation, resulting in a two-fold increase in protein synthesis in FDB muscle at the lower chamber. Osteoclasts released ATP to the extracellular medium in response to mechanical stimulation, proportional to the magnitude of the stimulus and partly dependent on the P2X₇ receptor. On the other hand, exogenous ATP promoted Akt phosphorylation (S473) in isolated FDB muscle in a time- and concentration-dependent manner. ATP also induced phosphorylation of proteins downstream Akt: mTOR (S2448), p70S6K (T389) and 4E-BP1 (T37/46). Exogenous ATP increased the protein synthesis rate in FDB muscle 2.2-fold; this effect was blocked by Suramin (general P2X/P2Y antagonist), LY294002 (phosphatidylinositol 3 kinase inhibitor) and Rapamycin (mTOR inhibitor). These blockers, as well as apyrase (ATP metabolizing enzyme), also abolished the induction of FDB protein synthesis evoked by mechanical stimulation of osteoclasts in the co-culture model. Therefore, the present findings suggest that mechanically stimulated osteoclasts release ATP, leading to protein synthesis in isolated FDB muscle, by activating the P2-PI3K-Akt-mTOR pathway. These results open a new area for research and clinical interest in bone-to-muscle crosstalk in adaptive processes related to muscle use/disuse or in musculoskeletal pathologies.

Induction of aggression and anxiety-like responses by perfluorooctanoic acid is accompanied by modulation of cholinergic- and purinergic signaling-related parameters in adult zebrafish

Perfluorooctanoic acid (PFOA) is a contaminant of global concern owing to its prevalent occurrence in aquatic and terrestrial environments with potential hazardous impact on living organisms. Here, we investigated the influence of realistic environmental concentrations of PFOA (0, 0.25, 0.5, or 1.0 mg/L) on relevant behaviors of adult zebrafish (Danio rerio) (e.g., exploration to novelty, social preference, and aggression) and the possible role of PFOA in modulating cholinergic and purinergic signaling in the brain after exposure for 7 consecutive days. PFOA significantly increased geotaxis as well as reduced vertical exploration (a behavioral endpoint for anxiety), and increased the frequency and duration of aggressive episodes without affecting their social preference. Exposure to PFOA did not affect ADP hydrolysis, whereas ATP and AMP hydrolysis were significantly increased at the highest concentration tested. However, AChE activity was markedly decreased in all PFOA-exposed groups when compared with control. In conclusion, PFOA induces aggression and anxiety-like behavior in adult zebrafish and modulates both cholinergic and purinergic signaling biomarkers. These novel data can provide valuable insights into possible health threats related to human activities, demonstrating the utility of adult zebrafish to elucidate how PFOA affects neurobehavioral responses in aquatic organisms.

Purinergic signaling in hepatic disease

Extracellular purines (ATP and adenosine) are ubiquitous intercellular messengers. During tissular damage, they function as damage-associated molecular patterns (DAMPs). In this context, purines announce tissue alterations to initiate a reparative response that involve the formation of the inflammasome complex and the recruitment of specialized cells of the immune system. The present review focuses on the role of the purinergic system in liver damage, mainly during the onset and development of fibrosis. After hepatocellular injury, extracellular ATP promotes a signaling cascade that ameliorates tissue alterations to restore the hepatic function. However, if cellular damage becomes chronic, ATP orchestrates an aberrant reparative process that results in severe liver diseases such as fibrosis and cirrhosis. ATP and adenosine, their receptors, and extracellular ectonucleotidases are mediators of unique processes that will be reviewed in detail.

Blocking skeletal muscle DHPRs/Ryr1 prevents neuromuscular synapse loss in mutant mice deficient in type III Neuregulin 1 (CRD-Nrg1)

Schwann cells are integral components of vertebrate neuromuscular synapses; in their absence, pre-synaptic nerve terminals withdraw from post-synaptic muscles, leading to muscle denervation and synapse loss at the developing neuromuscular junction (NMJ). Here, we report a rescue of muscle denervation and neuromuscular synapses loss in type III Neuregulin 1 mutant mice (CRD-Nrg1-/-), which lack Schwann cells. We found that muscle denervation and neuromuscular synapse loss were prevented in CRD-Nrg1-/-mice when presynaptic activity was blocked by ablating a specific gene, such as Snap25 (synaptosomal-associated 25 kDa protein) or Chat (choline acetyltransferase). Further, these effects were mediated by a pathway that requires postsynaptic acetylcholine receptors (AChRs), because ablating Chrna1 (acetylcholine receptor α1 subunit), which encodes muscle-specific AChRs in CRD-Nrg1-/-mice also rescued muscle denervation. Moreover, genetically ablating muscle dihydropyridine receptor (DHPR) β1 subunit (Cacnb1) or ryanodine receptor 1 (Ryr1) also rescued muscle denervation and neuromuscular synapse loss in CRD-Nrg1-/-mice. Thus, these genetic manipulations follow a pathway-from presynaptic to postsynaptic, and, ultimately to muscle activity mediated by DHPRs and Ryr1. Importantly, electrophysiological analyses reveal robust synaptic activity in the rescued, Schwann-cell deficient NMJs in CRD-Nrg1-/-Cacnb1-/-or CRD-Nrg1-/-Ryr1-/-mutant mice. Thus, a blockade of synaptic activity, although sufficient, is not necessary to preserve NMJs that lack Schwann cells. Instead, a blockade of muscle activity mediated by DHRPs and Ryr1 is both necessary and sufficient for preserving NMJs that lack Schwann cells. These findings suggest that muscle activity mediated by DHPRs/Ryr1 may destabilize developing NMJs and that Schwann cells play crucial roles in counteracting such a destabilizing activity to preserve neuromuscular synapses during development.

Erythropoietin regulates the expression of dimeric form of acetylcholinesterase during differentiation of erythroblast

Acetylcholinesterase (AChE; EC 3.1.1.7) is known to hydrolyze acetylcholine at cholinergic synapses. In mammalian erythrocyte, AChE exists as a dimer (G₂ ) and is proposed to play role in erythropoiesis. To reveal the regulation of AChE during differentiation of erythroblast, erythroblast-like cells (TF-1) were induced to differentiate by application of erythropoietin (EPO). The expression of AChE was increased in parallel to the stages of differentiation. Application of EPO in cultured TF-1 cells induced transcriptional activity of ACHE gene, as well as its protein product. This EPO-induced event was in parallel with erythrocytic proteins, for example, α- and β-globins. The EPO-induced AChE expression was mediated by phosphorylations of Akt and GATA-1; because the application of Akt kinase inhibitor blocked the gene activation. Erythroid transcription factor also known as GATA-1, a downstream transcription factor of EPO signaling, was proposed here to account for regulation of AChE in TF-1 cell. A binding sequence of GATA-1 was identified in ACHE gene promoter, which was further confirmed by chromatin immunoprecipitation (ChIP) assay. Over-expression of GATA-1 in TF-1 cultures induced AChE expression, as well as activity of ACHE promoter tagged with luciferase gene (pAChE-Luc). The deletion of GATA-1 sequence on the ACHE promoter, pAChE_ΔGATA-1 -Luc, reduced the promoter activity during erythroblastic differentiation. On the contrary, the knock-down of AChE in TF-1 cultures could lead to a reduction in EPO-induced expression of erythrocytic proteins. These findings indicated specific regulation of AChE during maturation of erythroblast, which provided an insight into elucidating possible mechanisms in regulating erythropoiesis.

Wnt3a induces the expression of acetylcholinesterase during osteoblast differentiation via the Runx2 transcription factor

Acetylcholinesterase (AChE) hydrolyzes acetylcholine to terminate cholinergic transmission in neurons. Apart from this AChE activity, emerging evidence suggests that AChE could also function in other, non-neuronal cells. For instance, in bone, AChE exists as a proline-rich membrane anchor (PRiMA)-linked globular form in osteoblasts, in which it is proposed to play a noncholinergic role in differentiation. However, this hypothesis is untested. Here, we found that in cultured rat osteoblasts, AChE expression was increased in parallel with osteoblastic differentiation. Because several lines of evidence indicate that AChE activity in osteoblast could be triggered by Wnt/β-catenin signaling, we added recombinant human Wnt3a to cultured osteoblasts and found that this addition induced expression of the ACHE gene and protein product. This Wnt3a-induced AChE expression was blocked by the Wnt-signaling inhibitor Dickkopf protein-1 (DKK-1). We hypothesized that the Runt-related transcription factor 2 (Runx2), a downstream transcription factor in Wnt/β-catenin signaling, is involved in AChE regulation in osteoblasts, confirmed by the identification of a Runx2-binding site in the ACHE gene promoter, further corroborated by ChIP. Of note, Runx2 overexpression in osteoblasts induced AChE expression and activity of the ACHE promoter tagged with the luciferase gene. Moreover, deletion of the Runx2-binding site in the ACHE promoter reduced its activity during osteoblastic differentiation, and addition of 5-azacytidine and trichostatin A to differentiating osteoblasts affected AChE expression, suggesting epigenetic regulation of the ACHE gene. We conclude that AChE plays a role in osteoblastic differentiation and is regulated by both Wnt3a and Runx2.

Expression of globular form acetylcholinesterase is not altered in P2Y1R knock-out mouse brain

Adenosine 5'-triphosphate (ATP), a neurotransmitter and a neuromodulator, has been shown to be co-stored and co-released with acetylcholine (ACh) at the pre-synaptic vesicles in vertebrate neuromuscular junction (nmj). Several lines of studies demonstrated that binding of ATP to its corresponding P2Y₁ receptors (P2Y₁R) in muscle and neuron regulated the post-synaptic gene expressions. Indeed, the expression of acetylcholinesterase (AChE) in muscle was markedly decreased in P2Y1R^-/- (P2Y₁R knock-out) mice. In order to search for possible role of P2Y₁R in cholinergic function of the brain, the expression of globular form AChE was determined in the brain of P2Y1R^-/- mice. In contrast to that in muscle, the amounts of AChE activity, AChE catalytic subunit, structure subunit PRiMA and the amount of ACh, in the brain were not, significantly, altered, suggesting the role of P2Y₁R in neuron could have different function as that in muscle. However, the expressions of a series of neuronal development markers, i.e. neurofilaments, were reduced in P2Y1R^-/- mouse brain, indicating P2Y₁R may be involved in neuronal development process.

Flavonoids induce the expression of acetylcholinesterase in cultured osteoblasts

Flavonoids, a group of natural compounds mainly derived from plants, are known to possess osteogenic effects in bone cells. Here, we aimed to test if flavonoid could induce a cholinergic enzyme, acetylcholinesterase (AChE), as well as bone differentiation. In cultured rat osteoblasts, twenty flavonoids, deriving from Chinese herbs and having known induction of alkaline phosphatase (ALP¹) expression, were tested for its induction activity on AChE expression. Eleven flavonoids showed the induction, and five of them had robust activation of AChE expression, including baicalin, calycosin, genistin, hyperin and pratensein: the induction of AChE included the levels of mRNA, protein and enzymatic activity. Moreover, the flavonoid-induced AChE expression in cultured osteoblast was in proline-rich membrane anchor (PRiMA)-linked tetrameric globular form (G₄) only. In parallel, the expression of PRiMA was also induced by the application of flavonoids. The flavonoid-induced AChE in the cultures was not affected by estrogen receptor blocker, ICI 182,780. Taken together, the induction of PRiMA-linked AChE in osteoblast should be independent to classical estrogen signaling pathway.

Transcriptional activity of acetylcholinesterase gene is regulated by DNA methylation during C2C12 myogenesis

The expression of acetylcholinesterase (AChE), an enzyme hydrolyzes neurotransmitter acetylcholine at vertebrate neuromuscular junction, is regulated during myogenesis, indicating the significance of muscle intrinsic factors in controlling the enzyme expression. DNA methylation is essential for temporal control of myogenic gene expression during myogenesis; however, its role in AChE regulation is not known. The promoter of vertebrate ACHE gene carries highly conserved CG-rich regions, implying its likeliness to be methylated for epigenetic regulation. A DNA methyltransferase inhibitor, 5-azacytidine (5-Aza), was applied onto C2C12 cells throughout the myotube formation. When DNA methylation was inhibited, the promoter activity, transcript expression and enzymatic activity of AChE were markedly increased after day 3 of differentiation, which indicated the putative role of DNA methylation. By bisulfite pyrosequencing, the overall methylation rate was found to peak at day 3 during C2C12 cell differentiation; a SP1 site located at -1826bp upstream of mouse ACHE gene was revealed to be heavily methylated. The involvement of transcriptional factor SP1 in epigenetic regulation of AChE was illustrated here: (i) the SP1-driven transcriptional activity was increased in 5-Aza-treated C2C12 culture; (ii) the binding of SP1 onto the SP1 site of ACHE gene was fully blocked by the DNA methylation; and (iii) the sequence flanking SP1 sites of ACHE gene was precipitated by chromatin immuno-precipitation assay. The findings suggested the role of DNA methylation on AChE transcriptional regulation and provided insight in elucidating the DNA methylation-mediated regulatory mechanism on AChE expression during muscle differentiation.

Reduced Expression of P2Y2 Receptor and Acetylcholinesterase at Neuromuscular Junction of P2Y1 Receptor Knock-out Mice

ATP is co-stored and co-released with acetylcholine (ACh) at the pre-synaptic vesicles in vertebrate neuromuscular junction (nmj). Several lines of studies demonstrated that binding of ATP to its corresponding P2Y1 and P2Y2 receptors in the muscle regulated post-synaptic gene expressions. To further support the notion that P2Y receptors are playing indispensable role in formation of post-synaptic specifications at the nmj, the knock-out mice of P2Y1 receptor (P2Y1R (-/-)) were employed here for analyses. In P2Y1R (-/-) mice, the expression of P2Y2 receptor in muscle was reduced by over 50 %, as compared to P2Y1R (+/+) mice. In parallel, the expression of acetylcholinesterase (AChE) in muscle was markedly decreased. In the analysis of the expression of anchoring subunits of AChE in P2Y1R (-/-) mice, the proline-rich membrane anchor (PRiMA) subunit was reduced by 60 %; while the collagen tail (ColQ) subunit was reduced by 50 %. AChE molecular forms in the muscle were not changed, except the amount of enzyme was reduced. Immuno-staining of P2Y1R (-/-) mice nmj, both AChE and AChR were still co-localized at the nmj, and the staining was diminished. Taken together our data demonstrated that P2Y1 receptor regulated the nmj gene expression.

The role of purinergic signalling in the musculoskeletal system

Accumulating evidence now suggests that purinergic signalling exerts significant regulatory effects in the musculoskeletal system. In particular, it has emerged that extracellular nucleotides are key regulators of bone cell differentiation, survival and function. This review discusses our current understanding of the direct effects of purinergic signalling in bone, cartilage and muscle.

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.

Electrical stimulation induces IL-6 in skeletal muscle through extracellular ATP by activating Ca(2+) signals and an IL-6 autocrine loop

Interleukin-6 (IL-6) is an important myokine that is highly expressed in skeletal muscle cells upon exercise. We assessed IL-6 expression in response to electrical stimulation (ES) or extracellular ATP as a known mediator of the excitation-transcription mechanism in skeletal muscle. We examined whether the canonical signaling cascade downstream of IL-6 (IL-6/JAK2/STAT3) also responds to muscle cell excitation, concluding that IL-6 influences its own expression through a positive loop. Either ES or exogenous ATP (100 μM) increased both IL-6 expression and p-STAT3 levels in rat myotubes, a process inhibited by 100 μM suramin and 2 U/ml apyrase. ATP also evoked IL-6 expression in both isolated skeletal fibers and extracts derived from whole FDB muscles. ATP increased IL-6 release up to 10-fold. STAT3 activation evoked by ATP was abolished by the JAK2 inhibitor HBC. Blockade of secreted IL-6 with a neutralizing antibody or preincubation with the STAT3 inhibitor VIII reduced STAT3 activation evoked by extracellular ATP by 70%. Inhibitor VIII also reduced by 70% IL-6 expression evoked by ATP, suggesting a positive IL-6 loop. In addition, ATP increased up to 60% the protein levels of SOCS3, a negative regulator of the IL-6 signaling pathway. On the other hand, intracellular calcium chelation or blockade of IP3-dependent calcium signals abolished STAT3 phosphorylation evoked by either extracellular ATP or ES. These results suggest that expression of IL-6 in stimulated skeletal muscle cells is mediated by extracellular ATP and nucleotide receptors, involving IP3-dependent calcium signals as an early step that triggers a positive IL-6 autocrine loop.

Abundant expression and functional participation of TRPV1 at Zusanli acupoint (ST36) in mice: mechanosensitive TRPV1 as an "acupuncture-responding channel"

Background

Acupuncture is a therapy that involves applying mechanical stimulation to acupoints using needles. Although acupuncture is believed to trigger neural regulation by opioids or adenosine, still little is known about how physical stimulation is turned into neurological signaling. The transient receptor potential vanilloid receptors 1 and 4 (TRPV1 and TRPV4) and the acid-sensing ion channel 3 (ASIC3) are regarded as mechanosensitive channels. This study aimed to clarify their role at the Zusanli acupoint (ST36) and propose possible sensing pathways linking channel activation to neurological signaling.

Methods

First, tissues from different anatomical layers of ST36 and the sham point were sampled, and channel expressions between the two points were compared using western blotting. Second, immunofluorescence was performed at ST36 to reveal distribution pattern of the channels. Third, agonist of the channels were injected into ST36 and tested in a mouse inflammatory pain model to seek if agonist injection could replicate acupuncture-like analgesic effect. Last, the components of proposed downstream sensing pathway were tested with western blotting to determine if they were expressed in tissues with positive mechanosensitive channel expression.

Results

The results from western blotting demonstrated an abundance of TRPV1, TRPV4, and ASIC3 in anatomical layers of ST36. Furthermore, immunofluorescence showed these channels were expressed in both neural and non-neural cells at ST36. However, only capsaicin, a TRPV1 agonist, replicated the analgesic effect of acupuncture when injected into ST36. Components of calcium wave propagation (CWP, the proposed downstream sensing pathway) were also expressed in tissues with abundant TRPV1 expression, the muscle and epimysium layers.

Conclusions

The results demonstrated mechanosensitive channel TRPV1 is highly expressed at ST36 and possibly participated in acupuncture related analgesia. Since CWP was reported by other to occur during acupuncture and its components were shown here to express in tissues with positive TRPV1 expression. These findings suggest TRPV1 might act as acupuncture-responding channel by sensing physical stimulation from acupuncture and conducting the signaling via CWP to nerve terminals. This study provided a better understanding between physical stimulation from acupuncture to neurological signaling.

Purinergic signalling in the musculoskeletal system

It is now widely recognised that extracellular nucleotides, signalling via purinergic receptors, participate in numerous biological processes in most tissues. It has become evident that extracellular nucleotides have significant regulatory effects in the musculoskeletal system. In early development, ATP released from motor nerves along with acetylcholine acts as a cotransmitter in neuromuscular transmission; in mature animals, ATP functions as a neuromodulator. Purinergic receptors expressed by skeletal muscle and satellite cells play important pathophysiological roles in their development or repair. In many cell types, expression of purinergic receptors is often dependent on differentiation. For example, sequential expression of P2X5, P2Y1 and P2X2 receptors occurs during muscle regeneration in the mdx model of muscular dystrophy. In bone and cartilage cells, the functional effects of purinergic signalling appear to be largely negative. ATP stimulates the formation and activation of osteoclasts, the bone-destroying cells. Another role appears to be as a potent local inhibitor of mineralisation. In osteoblasts, the bone-forming cells, ATP acts via P2 receptors to limit bone mineralisation by inhibiting alkaline phosphatase expression and activity. Extracellular ATP additionally exerts significant effects on mineralisation via its hydrolysis product, pyrophosphate. Evidence now suggests that purinergic signalling is potentially important in several bone and joint disorders including osteoporosis, rheumatoid arthritis and cancers. Strategies for future musculoskeletal therapies might involve modulation of purinergic receptor function or of the ecto-nucleotidases responsible for ATP breakdown or ATP transport inhibitors.

Activation of UTP-sensitive P2Y2 receptor induces the expression of cholinergic genes in cultured cortical neurons: a signaling cascade triggered by Ca2+ mobilization and extracellular regulated kinase phosphorylation

ATP functions as an extracellular signaling molecule that is costored and coreleased with neurotransmitters at central and peripheral neuronal synapses. Stimulation by ATP upregulates the expression of synaptic genes in muscle-including the genes for nicotine acetylcholine receptor (α-, δ-, and ε-subunits) and acetylcholinesterase (AChE)-via the P2Y receptor (P2YR), but the trophic response of neurons to the activation of P2YRs is less well understood. We reported that cultured cortical neurons and the developing rat brain expressed different types of P2YRs, and among these the UTP-sensitive P2Y2R was the most abundant. P2Y2R was found to exist in membrane rafts and it colocalized with the postsynaptic protein PSD-95 in cortical neurons. Notably, agonist-dependent stimulation of P2Y2R elevated the neuronal expression of cholinergic genes encoding AChE, PRiMA (an anchor for the globular form AChE), and choline acetyltransferase, and this induction was mediated by a signaling cascade that involved Ca(2+) mobilization and extracellular regulated kinases 1/2 activation. The importance of P2Y2R action was further shown by the receptor's synergistic effect with P2Y1R in enhancing cholinergic gene expression via the robust stimulation of Ca(2+) influx. Taken together our results revealed a developmental function of P2Y2R in promoting synaptic gene expression and demonstrated the influence of costimulation of P2Y1R and P2Y2R in neurons.

Proteomic and metabolomic profiling reveals time-dependent changes in hippocampal metabolism upon paroxetine treatment and biomarker candidates

Most of the commonly used antidepressants block monoamine reuptake transporters to enhance serotonergic or noradrenergic neurotransmission. Effects besides or downstream of monoamine reuptake inhibition are poorly understood and yet presumably important for the drugs' mode of action. In the present study we aimed at identifying hippocampal cellular pathway alterations in DBA/2 mice using paroxetine as a representative Selective Serotonin Reuptake Inhibitor (SSRI). Furthermore we identified biomarker candidates for the assessment of antidepressant treatment effects in plasma. Hippocampal protein levels were compared between chronic paroxetine- and vehicle-treated animals using in vivo(15)N metabolic labeling combined with mass spectrometry. We also studied the time course of metabolite level changes in hippocampus and plasma using a targeted polar metabolomics profiling platform. In silico pathway analyses revealed profound alterations related to hippocampal energy metabolism. Glycolytic metabolite levels acutely increased while Krebs cycle metabolite levels decreased upon chronic treatment. Changes in energy metabolism were influenced by altered glycogen metabolism rather than by altered glycolytic or Krebs cycle enzyme levels. Increased energy levels were reflected by an increased ATP/ADP ratio and by increased ratios of high-to-low energy purines and pyrimidines. In the course of our analyses we also identified myo-inositol as a biomarker candidate for the assessment of antidepressant treatment effects in the periphery. This study defines the cellular response to paroxetine treatment at the proteome and metabolome levels in the hippocampus of DBA/2 mice and suggests novel SSRI modes of action that warrant consideration in antidepressant development efforts.

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.

Counter-regulation of cholinesterases: differential activation of PKC and ERK signaling in retinal cells through BChE knockdown

The ubiquitous cholinesterase (ChE) enzymes, functioning in the termination of acetylcholine mediated neural transmission, are also reported to have additional functions. Through application of siRNAs against butyrylcholinesterase (BChE) in R28 cells, a retinal cell line with pluripotent properties, a counter-regulation between ChEs was revealed. BChE knock down resulted in an up-regulation of not only acetylcholinesterase (AChE), but also altered the signaling status of PKC and ERK. Knockdown of BChE modified ERK signaling most notably through ERK1/2 proteins, together with the transcription activator P90RSK1 and c-fos. Stimulation of the R28 cell line by forskolin revealed that ChEs are involved in an intricate cross talk between different signaling pathways. Forskolin-stimulated R28 cells displayed a robust cholinergic response, as detected by both electrophysiology and ChE expression, and changed the activation status of PKC/ERK signaling pathways. The findings in R28 cells show that ChE expressions are inversely co-regulated and act through the transcription factors c-fos and P90RSK1. Since R28 cells have the capacity to differentiate into different cell types through stimulation of signaling pathways, ChEs are likely to be associated with cell fate determination, rather than just terminating cholinergic responses.

Effect of purinergic receptor activation on Na+-K+ pump activity, excitability, and function in depolarized skeletal muscle

Activity-induced elevation of extracellular purines and pyrimidines has been associated with autocrine and paracrine signaling in many tissues. Here we investigate the effect of purinergic signaling for the excitability and contractility of depolarized skeletal muscle. Muscle excitability was experimentally depressed by elevating the extracellular K(+) from 4 to 10 mM, which reduced the tetanic force to 24 +/- 2% of the force at 4 mM K(+). Upon addition of 1 mM ATP, however, the force recovered to 65 +/- 8% of the control force (P < 0.001, n = 5). A similar recovery was seen with ADP, but not with UTP or adenosine. The ATP-induced force recovery could be inhibited by P2Y(1) receptor antagonists (3 muM SCH-202676 or 1 muM MRS-2500). A fourfold increase in M-wave area demonstrated that the ATP-induced force recovery was associated with restoration of muscle excitability (P < 0.05, n = 4). Experiments using (86)Rb(+) as a tracer for K(+) showed that ATP also induced a twofold increase in the activity of muscle Na(+)-K(+) pumps. The force recovery and the stimulation of the Na(+)-K(+) pump activity by ATP were inhibited by 50 muM of the phospholipase C inhibitor U-73122. It is concluded that purinergic signaling can increase the Na(+)-K(+) pump activity and improve force and excitability of depolarized skeletal muscles. This novel purinergic regulation may be important for the maintenance of muscle excitability during intense exercise, where the extracellular K(+) can increase substantially.

ATP released by electrical stimuli elicits calcium transients and gene expression in skeletal muscle

ATP released from cells is known to activate plasma membrane P2X (ionotropic) or P2Y (metabotropic) receptors. In skeletal muscle cells, depolarizing stimuli induce both a fast calcium signal associated with contraction and a slow signal that regulates gene expression. Here we show that nucleotides released to the extracellular medium by electrical stimulation are partly involved in the fast component and are largely responsible for the slow signals. In rat skeletal myotubes, a tetanic stimulus (45 Hz, 400 1-ms pulses) rapidly increased extracellular levels of ATP, ADP, and AMP after 15 s to 3 min. Exogenous ATP induced an increase in intracellular free Ca(2+) concentration, with an EC(50) value of 7.8 +/- 3.1 microm. Exogenous ADP, UTP, and UDP also promoted calcium transients. Both fast and slow calcium signals evoked by tetanic stimulation were inhibited by either 100 mum suramin or 2 units/ml apyrase. Apyrase also reduced fast and slow calcium signals evoked by tetanus (45 Hz, 400 0.3-ms pulses) in isolated mouse adult skeletal fibers. A likely candidate for the ATP release pathway is the pannexin-1 hemichannel; its blockers inhibited both calcium transients and ATP release. The dihydropyridine receptor co-precipitated with both the P2Y(2) receptor and pannexin-1. As reported previously for electrical stimulation, 500 mum ATP significantly increased mRNA expression for both c-fos and interleukin 6. Our results suggest that nucleotides released during skeletal muscle activity through pannexin-1 hemichannels act through P2X and P2Y receptors to modulate both Ca(2+) homeostasis and muscle physiology.

Extracellular ATP inhibits chloride channels in mature mammalian skeletal muscle by activating P2Y1 receptors

ATP is released from skeletal muscle during exercise, a discovery dating back to 1969. Surprisingly, few studies have examined the effects of extracellular ATP on mature mammalian skeletal muscle. This electrophysiological study examined the effects of extracellular ATP on fully innervated rat levator auris longus using two intracellular microelectrodes. The effects of ATP were determined by measuring the relative changes of miniature endplate potentials (mEPPs) and voltage responses to step current pulses in individual muscle fibres. Exposure to ATP (20 microm) prolonged the mEPP falling phase by 31 +/- 7.5% (values +/- s.d., n = 3 fibres). Concurrently, the input resistance increased by 31 +/- 2.0% and the time course of the voltage responses increased by 59 +/- 3.0%. Analogous effects were observed using 2 and 5 microm ATP, and on regions distal from the neuromuscular junction, indicating that physiologically relevant levels of ATP enhanced electrical signalling over the entire muscle fibre. The effects of extracellular ATP were blocked by 200 microm anthracene-9-carboxylic acid, a chloride channel inhibitor, and reduced concentrations of extracellular chloride, indicating that ATP inhibited chloride channels. A high affinity agonist for P2Y receptors, 2-methylthioadenosine-5-O-diphosphate (2MeSADP), induced similar effects to ATP with an EC(50) of 160 +/- 30 nm. The effects of 250 nm2MeSADP were blocked by 500 nmMRS2179, a specific P2Y(1) receptor inhibitor, suggesting that ATP acts on P2Y(1) receptors to inhibit chloride channels. The inhibition of chloride channels by extracellular ATP has implications for muscle excitability and fatigue, and the pathophysiology of myotonias.

Calcium signaling-induced Smad3 nuclear accumulation induces acetylcholinesterase transcription in apoptotic HeLa cells

Recently, acetylcholinesterase (AChE) has been studied as an important apoptosis regulator. We previously showed that cellular calcium mobilization upregulated AChE expression by modulating promoter activity and mRNA stability. In this study, we have identified a potential Smad3/4 binding element, TGCCAGACA, located within the -601 to -571 bp fragment of the AChE promoter, as an important calcium response motif. Smad2/3 and Smad4 were shown to bind this element. Overexpression of human Smad3 increased AChE transcription activity in a dose-dependent manner in HeLa cells, whereas dominant-negative Smad3 blocked this activation. Upon A23187 and thapsigargin treatment, nuclear Smad3 accumulation was observed, an effect that was blocked by the intracellular Ca(2+) chelator BAPTA-AM. Calcium-induced AChE transcriptional activation was significantly blocked when the nuclear localization signal of Smad3 was destroyed. Taken together, our data suggest Smad3 can regulate AChE transcriptional activation following calcium-induced nuclear accumulation.

Evolutionary origins of the purinergic signalling system

Purines appear to be the most primitive and widespread chemical messengers in the animal and plant kingdoms. The evidence for purinergic signalling in plants, invertebrates and lower vertebrates is reviewed. Much is based on pharmacological studies, but important recent studies have utilized the techniques of molecular biology and receptors have been cloned and characterized in primitive invertebrates, including the social amoeba Dictyostelium and the platyhelminth Schistosoma, as well as the green algae Ostreococcus, which resemble P2X receptors identified in mammals. This suggests that contrary to earlier speculations, P2X ion channel receptors appeared early in evolution, while G protein-coupled P1 and P2Y receptors were introduced either at the same time or perhaps even later. The absence of gene coding for P2X receptors in some animal groups [e.g. in some insects, roundworms (Caenorhabditis elegans) and the plant Arabidopsis] in contrast to the potent pharmacological actions of nucleotides in the same species, suggests that novel receptors are still to be discovered.

Transcriptional regulation of proline-rich membrane anchor (PRiMA) of globular form acetylcholinesterase in neuron: an inductive effect of neuron differentiation

The transcriptional regulation of proline-rich membrane anchor (PRiMA), an anchoring protein of tetrameric globular form of acetylcholinesterase (G(4) AChE), was revealed in cultured cortical neurons during differentiation. The level of AChE(T) protein, total enzymatic activity and the amount of G(4) AChE were dramatically increased during the neuron differentiation. RT-PCR analyses revealed that the transcript encoding PRiMA was significantly up-regulated in the differentiated neurons. To investigate the transcriptional mechanism on PRiMA regulation, a reporter construct of human PRiMA promoter-tagged luciferase was employed in this study. Upon the neuronal differentiation in cortical neurons, a mitogen-activated protein (MAP) kinase-dependent pathway was stimulated: this signaling cascade was shown to regulate the transcriptional activity of PRiMA. In addition, both PRiMA and AChE(T) transcripts were induced by the over expression of an active mutant of Raf in the cultured neurons. The treatment of a MAP kinase inhibitor (U0126) significantly blocked the expression of PRiMA transcript and promoter-driven luciferase activity as induced by the differentiation of cortical neurons. These results suggested that a MAP kinase signaling pathway served as one of the transcriptional regulators in controlling PRiMA gene expression during the neuronal differentiation process.

Purinergic activation of anion conductance and osmolyte efflux in cultured rat hippocampal neurons

The majority of mammalian cells demonstrate regulatory volume decrease (RVD) following swelling caused by hyposmotic exposure. A critical signal initiating RVD is activation of nucleotide receptors by ATP. Elevated extracellular ATP in response to cytotoxic cell swelling during pathological conditions also may initiate loss of taurine and other intracellular osmolytes via anion channels. This study characterizes neuronal ATP-activated anion current and explores its role in net loss of amino acid osmolytes. To isolate anion currents, we used CsCl as the major electrolyte in patch electrode and bath solutions and blocked residual cation currents with NiCl(2) and tetraethylammonium. Anion currents were activated by extracellular ATP with a K(m) of 70 microM and increased over fourfold during several minutes of ATP exposure, reaching a maximum after 9.0 min (SD 4.2). The currents were blocked by inhibitors of nucleotide receptors and volume-regulated anion channels (VRAC). Currents showed outward rectification and inactivation at highly depolarizing membrane potentials, characteristics of swelling-activated anion currents. P2X agonists failed to activate the anion current, and an inhibitor of P2X receptors did not block the effect of ATP. Furthermore, current activation was observed with extracellular ADP and 2-(methylthio)adenosine 5'-diphosphate, a P2Y(1) receptor-specific agonist. Much less current activation was observed with extracellular UTP, suggesting the response is mediated predominantly by P2Y(1) receptors. ATP caused a dose-dependent loss of taurine and alanine that could be blocked by inhibitors of VRAC. ATP did not inhibit the taurine uptake transporter. Thus extracellular ATP triggers a loss of intracellular organic osmolytes via activation of anion channels. This mechanism may facilitate neuronal volume homeostasis during cytotoxic edema.

Constitutive and agonist-induced dimerizations of the P2Y1 receptor: relationship to internalization and scaffolding

In living cells, P2Y(1) receptor dimerization was quantitated by an improved version of fluorescence resonance energy transfer donor photobleaching analysis. 44% of the P2Y(1) receptors expressed in HEK293 cell membranes exist as dimers in the resting state, inducible by agonist exposure to give 85-100% dimerization. Monomer and constitutive dimers are fully active. Agonist-induced dimerization follows desensitization and is fully reversible upon withdrawal of agonist. Receptor dimers are required for internalization at 37 degrees C but are not sufficient; at 20 degrees C dimerization also occurs, but endocytosis is abolished. Removal of the C-terminal 19 amino acids abolished both dimerization and internalization, whereas full activation by agonists was retained up to a loss of 39 amino acids, confirming active monomers. This receptor is known to bind through its last four amino acids (DTSL) to a scaffolding protein, Na/H exchanger regulatory factor-2, which was endogenous here, and DTSL removal blocked constitutive dimerization specifically. Distinction should therefore be made between the following: 1) constitutive dimers tethered to a scaffolding protein, together with effector proteins, within a signaling micro-domain, and 2) free dimers in the cell membrane, which here are inducible by agonist exposure. For the class A G-protein-coupled receptors, we suggest that the percentages of free monomers, and in many cases of induced free dimers, commonly become artifactually increased; this would arise from an excess there of the receptor over its specific scaffold and from a lack of the native targeting of the receptor to that site.

Calcitonin gene-related peptide induces the expression of acetylcholinesterase-associated collagen ColQ in muscle: a distinction in driving two different promoters between fast- and slow-twitch muscle fibers

The presence of a collagenous protein (ColQ) characterizes the collagen-tailed forms of acetylcholinesterase at vertebrate neuromuscular junctions (nmjs). Two ColQ transcripts as ColQ-1 and ColQ-1a, driven by two promoters: pColQ-1 and pColQ-1a, were found in mammalian slow- and fast-twitch muscles, respectively, which have distinct expression pattern in different muscle fibers. In this study, we show the differential expression of CoQ in different muscles is triggered by calcitonin gene-related peptide (CGRP), a known motor neuron-derived factor. Application of CGRP, or dibutyryl-cAMP (Bt(2)-cAMP), in cultured myotubes induced the expression of ColQ-1a transcript and promoter activity; however, the expression of ColQ-1 transcript did not respond to CGRP or Bt(2)-cAMP. The CGRP-induced gene activation was blocked by an adenylyl cyclase inhibitor or a dominant negative mutant of cAMP-responsive element (CRE) binding protein (CREB). Two CRE sites were mapped within the ColQ-1a promoter, and mutations of the CRE sites abolished the response of CGRP or Bt(2)-cAMP. In parallel, CGRP receptor complex was dominantly expressed at the nmjs of fast muscle but not of slow muscle. These results suggested that the expression of ColQ-1a at the nmjs of fast-twitch muscle was governed by a CGRP-mediated cAMP signaling mechanism.

Stimulation of the P2Y1 receptor up-regulates nucleoside-triphosphate diphosphohydrolase-1 in human retinal pigment epithelial cells

Stimulation of receptors for either ATP or adenosine leads to physiologic changes in retinal pigment epithelial (RPE) cells that may influence their relationship with the adjacent photoreceptors. The ectoenzyme nucleoside-triphosphate diphosphohydrolase-1 (NTPDase1) catalyzes the dual dephosphorylation of ATP and ADP to AMP. Although NTPDase1 can consequently control the balance between ATP and adenosine, it is unclear how its expression and activity are regulated. Classic negative feedback theory predicts an increase in enzyme activity in response to enhanced exposure to substrate. This study asked whether exposure to ATP increases NTPDase1 activity in RPE cells. Although levels of NTPDase1 mRNA and protein in cultured human ARPE-19 cells were generally low under control conditions, exposure to slowly hydrolyzable ATPgammaS led to a time-dependent increase in NTPDase1 mRNA that was accompanied by a rise in levels of the functional 78-kDa protein. Neither NTPDase2 nor NTPDase3 mRNA message was elevated by ATPgammaS. The ATPase activity of cells increased in parallel, indicating the up-regulation of NTPDase1 was functionally relevant. The up-regulation of NTPDase1 protein was partially blocked by P2Y1 receptor inhibitors MRS2179 (N6-methyl-2'-deoxyadenosine-3',5'-bisphosphate) and MRS2500 [2-iodo-N6-methyl-(N)-methanocarba-2'-deoxyadenosine 3',5'-bisphosphate] and increased by P2Y1 receptor agonist MRS2365 [(N)-methanocarba-2MeSADP]. In conclusion, prolonged exposure to extracellular ATPgammaS increased NTPDase1 message and protein levels and increased ecto-ATPase activity. This up-regulation reflects a feedback circuit, mediated at least in part by the P2Y1 receptor, to regulate levels of extracellular purines in subretinal space. NTPDase1 levels may thus serve as an index for increased extracellular ATP levels under certain pathologic conditions, although other mechanisms could also contribute.

Cortical spreading depression releases ATP into the extracellular space and purinergic receptor activation contributes to the induction of ischemic tolerance

Cortical Spreading Depression (CSD) is a well-studied model of preconditioning that provides a high degree of tolerance to a subsequent ischemic event in the brain. The present study was undertaken in order to determine whether the release of ATP during CSD could contribute to the induction of ischemic tolerance. Direct measurement of ATP levels during CSD indicates that with each CSD wave ATP is released into the extracellular space at levels exceeding 100 microM. Cultures of rat primary cortical neurons exposed to low levels of extracellular ATP developed tolerance to subsequent oxygen-glucose deprivation (OGD) or metabolic hypoxia. The preconditioning effect requires new protein synthesis and develops with time, suggesting that a complex genomic response is required for the induction of tolerance. Multiple purinergic receptors are involved in mediating tolerance, with P2Y receptor activation having the greatest effect. Although extracellular adenosine or glutamate may make a small contribution, most of the tolerance was found to be induced independently of adenosine or glutamate receptor activation. Multiple signal transduction pathways mediate the response to extracellular ATP with the protein kinase A pathway and activation of phospholipase C contributing the most. The results are consistent with the proposal that CSD releases ATP into the extracellular space and the subsequent activation of P2Y receptors makes a major contribution to the induction of ischemic tolerance in the brain.

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.

Regulation of acetylcholinesterase expression by calcium signaling during calcium ionophore A23187- and thapsigargin-induced apoptosis

We have recently reported that acetylcholinesterase expression was induced during apoptosis in various cell types. In the current study we provide evidence to suggest that the induction of acetylcholinesterase expression during apoptosis is regulated by the mobilization of intracellular Ca(2+). During apoptosis, treatment of HeLa and MDA-MB-435s cells with the calcium ionophore A23187 resulted in a significant increase in acetylcholinesterase mRNA and protein levels. Chelation of intracellular Ca(2+) by BAPTA-AM (1,2-bis-(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-acetoxymethyl ester), an intracellular Ca(2+) chelator, inhibited acetylcholinesterase expression. A23187 also enhanced the stability of acetylcholinesterase mRNA and increased the activity of acetylcholinesterase promoter, effects that were blocked by BAPTA-AM. Perturbations of cellular Ca(2+) homeostasis by thapsigargin resulted in the increase of acetylcholinesterase expression as well as acetylcholinesterase promoter activity during thapsigargin induced apoptosis in HeLa and MDA-MB-435s cells, effects that were also inhibited by BAPTA-AM. We further demonstrated that the transactivation of the human acetylcholinesterase promoter by A23187 and thapsigargin was partially mediated by a CCAAT motif within the -1270 to -1248 fragment of the human acetylcholinesterase promoter. This motif was able to bind to CCAAT binding factor (CBF/NF-Y). These results strongly suggest that cytosolic Ca(2+) plays a key role in acetylcholinesterase regulation during apoptosis induced by A23187 and thapsigargin.

Different effects of ATP on the contractile activity of mice diaphragmatic and skeletal muscles

Apart from acetyl-choline (Ach), adenosine-5'-trisphosphate (ATP) is thought to play a role in neuromuscular function, however little information is available on its cellular physiology. As such, effects of ATP and adenosine on contractility of mice diaphragmatic and skeletal muscles (m. extensor digitorum longa-MEDL) have been investigated in in vitro experiments. Application of carbacholine (CCh) in vitro in different concentrations led to pronounced muscle contractions, varying from 9.15+/-4.76 to 513.13+/-15.4 mg and from 44.65+/-5.01 to 101.46+/-9.11 mg for diaphragm and MEDL, respectively. Two hundred micromolars of CCh in both muscles caused the contraction with the 65% (diaphragm) to 75% (MEDL) of maximal contraction force-this concentration was thus used in further experiments. It was found that application of ATP (100 microM) increased the force of diaphragmatic contraction caused by CCh (200 microM) from 335.2+/-51.4 mg (n=21) in controls to 426.5+/-47.8 mg (n=10; P<0.05), but decreased the contractions of MEDL of CCh from 76.6+/-6.5mg (n=26) in control to 40.2+/-9.0mg (n=8; P<0.05). Application of adenosine (100 microM) had no effect on CCh-induced contractions of these muscles. Resting membrane potential (MP) measurements using sharp electrodes were done at 10, 20 and 30 min after the application of ATP and adenosine. Diaphragm showed depolarization from 75+/-0.6 down to 63.2+/-1.05, 57.2+/-0.96 and 53.6+/-1.1 mV after 10, 20 and 30 min of exposition, respectively (20 fibers from 4 muscles each, P<0.05 in all three cases). Adenosine showed no effect on diaphragmatic MP. Both agents were ineffective in case of MEDL. The effects of ATP in both tissues were abolished by suramin (100 microM), a P2-receptor antagonist, and chelerythrin (50 microM), a specific protein-kinase C (PKC) inhibitor, but were not affected by 1H-[1,2,4]-oxadiazolo-[4,3-alpha]-quinoxalin-1-one (ODQ, 1 microM), a guanylyl-cyclase inhibitor, or by adenosine-3,5-monophosphothioate (Rp-cAMP, 1 microM), a protein-kinase A (PKA) inhibitor. Besides the action on contractile activity, ATP (100 microM) led to a significant (P<0.001) depolarization of diaphragm muscle fibers from 74.5+/-2.3 down to 64+/-2.1, 58.2+/-2.2 and 54.3+/-2.4 mV after 10, 20 and 30 min of incubation, respectively. Incubation of MEDL with the same ATP concentration showed no significant change of MP. Denervation of the muscles for 28 days led to a decrease of CCh-induced contractions of diaphragm down to 171.1+/-34.5mg (n=11, P<0.05), but increased the contractile force of MEDL up to 723.9+/-82.3mg (n=9, P<0.01). Application of ATP elevated the contractility of denervated diaphragm caused by CCh up to normal values (311.1+/-79.7 mg, n=6, P>0.05 versus control), but did not significantly affect of contractility of MEDL, which became 848.1+/-62.7 mg (n=6). These results show that the effects of ATP on both diaphragmatic and skeletal muscles are mediated through P2Y receptors coupled to chelerytrin-sensitive protein-kinase C.

Purinergic signalling in neuron-glia interactions

Activity-dependent release of ATP from synapses, axons and glia activates purinergic membrane receptors that modulate intracellular calcium and cyclic AMP. This enables glia to detect neural activity and communicate among other glial cells by releasing ATP through membrane channels and vesicles. Through purinergic signalling, impulse activity regulates glial proliferation, motility, survival, differentiation and myelination, and facilitates interactions between neurons, and vascular and immune system cells. Interactions among purinergic, growth factor and cytokine signalling regulate synaptic strength, development and responses to injury. We review the involvement of ATP and adenosine receptors in neuron-glia signalling, including the release and hydrolysis of ATP, how the receptors signal, the pharmacological tools used to study them, and their functional significance.

Increased susceptibility to ATP via alteration of P2X receptor function in dystrophic mdx mouse muscle cells

Pathological cellular hallmarks of Duchenne muscular dystrophy (DMD) include, among others, abnormal calcium homeostasis. Changes in the expression of specific receptors for extracellular ATP in dystrophic muscle have been recently documented: here, we demonstrate that at the earliest, myoblast stage of developing dystrophic muscle a purinergic dystrophic phenotype arises. In myoblasts of a dystrophin-negative muscle cell line established from the mdx mouse model of DMD but not in normal myoblasts, exposure to extracellular ATP triggered a strong increase in cytoplasmic Ca2+ concentrations. Influx of extracellular Ca2+ was stimulated by ATP and BzATP and inhibited by zinc, Coomassie Brilliant Blue-G, and KN-62, demonstrating activation of P2X7 receptors. Significant expression of P2X4 and P2X7 proteins was immunodetected in dystrophic myoblasts. Therefore, full-length dystrophin appears, surprisingly, to play an important role in myoblasts in controlling responses to ATP. Our results suggest that altered function of P2X receptors may be an important contributor to pathogenic Ca2+ entry in dystrophic mouse muscle and may have implications for the pathogenesis of muscular dystrophies. Treatments aiming at inhibition of specific ATP receptors could be of a potential therapeutic benefit.

Regulation of expression of early growth response transcription factors in rat primary cortical neurons by extracellular ATP

The zinc finger transcription factor early growth response-1 (Egr-1, NGFI-A, zif268, Krox 24, TIS8, ZENK) is upregulated immediately in the brain by cortical spreading depression (CSD) and other preconditioning stimuli and thus might participate in regulation of the overall genomic response to preconditioning. In the present study, the induction of expression of Egr-1 and other early growth response family members was characterized in rat primary cortical neuronal cultures. In neuronal cultures in vitro, depolarization or exposure to extracellular glutamate caused a 4-fold increase in egr-1 mRNA while exposure to extracellular ATP caused a 10-fold increase. The presence of mRNA encoding for multiple types of purinergic receptors was confirmed by RT-PCR. A number of nucleotide agonists proved effective in eliciting an increase in egr-1 mRNA. Over a limited range of concentration, the most effective agonists were ATP > ADP > alpha, beta-methylene ATP > UTP > cAMP > UDP > AMP > adenosine. Pertussis toxin, suramin, reactive blue 2, PPADS, DPCPX and inhibitors of Protein Kinase C, Protein Kinase A and PI3 kinase significantly reduced the upregulation of egr-1 by exposure to extracellular ATP. These findings suggest that neuronal metabotropic purinergic receptor activation contributes to the induction of early growth response transcription factors and may provide a target that can be manipulated to increase ischemic tolerance of the brain in vivo.

Developing skeletal muscle cells express functional muscarinic acetylcholine receptors coupled to different intracellular signaling systems

This study analyzed the expression of muscarinic acetylcholine receptors (mAChRs) in the rat cultured skeletal muscle cells and their coupling to G protein, phospholipase C and adenylyl cyclase (AC). Our results showed the presence of a homogeneous population of [(3)H]methyl-quinuclidinyl benzilate-binding sites in the membrane fraction from the rat cultured muscle (K(D) = 0.4 nM, B(max) = 8.9 fmol mg protein(-1)). Specific muscarinic binding sites were also detected in denervated diaphragm muscles from adult rats and in myoblasts isolated from newborn rats. Activation of mAChRs with carbachol induced specific [(35)S]GTPgammaS binding to cultured muscle membranes and potentiated the forskolin-dependent stimulation of AC. These effects were totally inhibited by 0.1-1 microM atropine. In addition, mAChRs were able to stimulate generation of diacylglycerol (DAG) in response to acetylcholine, carbachol or selective mAChR agonist oxotremorine-M. The carbachol-dependent increase in DAG was inhibited in a concentration-dependent manner by mAChR antagonists atropine, pirenzepine and 4-DAMP mustard. Finally, activation of these receptors was correlated with increased synthesis of acetylcholinesterase, via a PKC-dependent pathway. Taken together, these results indicate that expression of mAChRs, coupled to G protein and distinct intracellular signaling systems, is a characteristic of noninnervated skeletal muscle cells and may be responsible for trophic influences of acetylcholine during formation of the neuromuscular synapse.

Purinergic modulation of synaptic signalling at the neuromuscular junction

Purines have physiologically important functions throughout the nervous system. In both the central (CNS) and peripheral nervous systems (PNS), purines in the form of adenosine triphosphate and adenosine can play a number of roles in neuronal activation and inhibition. In addition, purines are known to be important for glial cell signaling in both the CNS and PNS. In the PNS, the neuromuscular junction (NMJ) is an excellent model for studying simple synaptic interactions. It is well suited to investigations of neuron-glia interactions because synaptic properties are well defined and perisynaptic Schwann cells (PSCs), glial cells at the NMJ, dynamically interact with the pre- and postsynaptic elements. At the NMJ, purines are critical for presynaptic modulation but also for neuron-glia interactions. Purines signal to PSCs through metabotropic and ionotropic receptors and activation of these receptors can have both modulatory and activating functions. This review will discuss recent developments in our understanding of purinergic modulation of the NMJ with an emphasis on the involvement of purines in neuron-glia interactions at this synapse.

Extracellular ATP activates ERK1/ERK2 via a metabotropic P2Y1 receptor in a Ca2+ independent manner in differentiated human skeletal muscle cells

ATP is released at the neuromuscular junction to regulate development and proliferation. The sequential expression of P2X and P2Y receptors has been correlated to these effects in many species and cell lines. We have therefore investigated ATP mediated signalling in differentiated primary human skeletal muscle cells. ATP was capable to trigger Ca2+ transients in these cells via P2Y receptors which were not attributable to Ca2+ influx via P2X receptors. Instead, ATP propagated the formation of inositol phosphate (IP) with an EC50 of 21.3 microM. The Ca2+ transient provoked by ATP was abrogated roughly 75% by the phospholipase C (PLC) inhibitor, U73122. Interestingly, the ryanodine sensitive Ca2+ pool was not involved in ATP triggered Ca2+ release. On mRNA level and by a pharmacological approach we confirmed the presence of the P2Y1, P2Y2, P2Y4 and P2Y6 receptors. Substantially, ATP activated IP formation via a P2Y1 receptor. In addition, ATP elicited extracellular signal regulated kinase (ERK)1/2 phosphorylation in a time and concentration dependent manner, again mainly via P2Y1 receptors. The ATP mediated ERK1/2 phosphorylation was strictly dependent on phospholipase C and PI3 kinase activity. Importantly, ATP mediated ERK1/2 phosphorylation was Ca2+ independent. This observation was corroborated by the finding that conventional protein kinase C inhibitors did not suppress ATP triggered ERK1/2 phosphorylation. Taken together, these observations highlight the importance of ATP as a co-neurotransmitter at the neuromuscular junction via dual signalling, i.e. IP3 receptor mediated Ca2+ transients and Ca2+ insensitive phosphorylation of ERK1/2.

ATP induces the post-synaptic gene expression in neuron-neuron synapses: Transcriptional regulation of AChE catalytic subunit

Adenosine 5'-triphosphate (ATP) is an important trophic factor, which is co-stored and co-released at central and peripheral cholinergic synapses. The synaptic ATP induces post-synaptic gene transcription during the formation and maintenance of vertebrate neuromuscular junction (nmj) via a mitogen-activaton protein (MAP) kinase signaling pathway and subsequently activates acetylcholinesterase (AChE) and acetylcholine receptor (AChR) genes. However, the role of ATP in the central nervous system is still not clear. Primary culture of rat cortical neurons was used as a model system to study the biological functions of ATP in neuron-neuron synapses. During the differentiation of cultured cortical neurons, the protein levels of AChE and one of the ATP receptor subtypes, P2Y1 receptor, were increased. By using a human AChE promoter tagged with a luciferase-reporter gene, the transcriptional regulation of AChE gene by ATP could be monitored. The activation of P2Y1 receptors could regulate the AChE promoter activity in cultured cortical neurons. These results suggested the activation of P2Y receptors may play role(s) in synaptic gene expression of neuron-neuron synapses in the brain.

Transcriptional regulation of acetylcholinesterase-associated collagen ColQ in fast- and slow-twitch muscle fibers

The presence of a collagenous protein (ColQ) characterizes the collagen-tailed forms of acetylcholinesterase (AChE) and butyrylcholinesterase at vertebrate neuromuscular junctions, which is tethered in the synaptic basal lamina. ColQ subunits, differing mostly by their signal sequences, are encoded by transcripts ColQ-1 and ColQ-1a, which are differentially expressed in slow- and fast-twitch muscles in mammals, respectively. Both ColQ transcripts are derived from a single COLQ gene. Transcripts encoding ColQ increased during myogenic differentiation of C2C12 cells; the increase was in parallel with AChE catalytic subunit. Quantitative PCR analysis indicated that the increase during the myotube formation was due to the up regulation of ColQ-1 transcript instead of ColQ-1a. In order to reveal the regulatory mechanism of ColQ transcripts, two distinct promoters, pColQ-1 and pColQ-1a, were isolated from human COLQ gene. The ColQ promoters showed a muscle fiber type-specific expression pattern, and which was in line with the expression of endogenous transcript. After in vivo DNA transfection, pColQ-1 showed strong activity in slow-twitch muscle (e.g. soleus), while pColQ-1a was preferably expressed in fast-twitch muscle (e.g. tibialis). Mutation analysis of the ColQ promoters suggested that the muscle fiber type-specific expression pattern of ColQ transcripts was regulated by a slow upsteam regulatory element (SURE) and a fast intronic regulatory element (FIRE). These results explain the specific expression patterns of collagen-tailed AChE in slow and fast muscle fibers.

Calcineurin-NFAT signaling, together with GABP and peroxisome PGC-1α, drives utrophin gene expression at the neuromuscular junction

We examined whether calcineurin-NFAT (nuclear factors of activated T cells) signaling plays a role in specifically directing the expression of utrophin in the synaptic compartment of muscle fibers. Immunofluorescence experiments revealed the accumulation of components of the calcineurin-NFAT signaling cascade within the postsynaptic membrane domain of the neuromuscular junction. RT-PCR analysis using synaptic vs. extrasynaptic regions of muscle fibers confirmed these findings by showing an accumulation of calcineurin transcripts within the synaptic compartment. We also examined the effect of calcineurin on utrophin gene expression. Pharmacological inhibition of calcineurin in mice with either cyclosporin A or FK506 resulted in a marked decrease in utrophin A expression at synaptic sites, whereas constitutive activation of calcineurin had the opposite effect. Mutation of the previously identified NFAT binding site in the utrophin A promoter region, followed by direct gene transfer studies in mouse muscle, led to an inhibition in the synaptic expression of a lacZ reporter gene construct. Transfection assays performed with cultured myogenic cells indicated that calcineurin acted additively with GA binding protein (GABP) to transactivate utrophin A gene expression. Because both GABP- and calcineurin-mediated pathways are targeted by peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α), we examined whether this coactivator contributes to utrophin gene expression. In vitro and in vivo transfection experiments showed that PGC-1α alone induces transcription from the utrophin A promoter. Interestingly, this induction is largely potentiated by coexpression of PGC-1α with GABP. Together, these studies indicate that the synaptic expression of utrophin is also driven by calcineurin-NFAT signaling and occurs in conjunction with signaling events that involve GABP and PGC-1α.