ATP regulates the differentiation of mammalian skeletal muscle by activation of a P2X5 receptor on satellite cells

A key role for P2RX5 in brown adipocyte differentiation and energy homeostasis

Brown adipocytes are defined based on a distinct morphology and genetic signature that includes, amongst others, the expression of the Purinergic 2 Receptor X5 (P2RX5). However, the role of P2RX5 in brown adipocyte and brown adipose tissue function is poorly characterized. In the present study, we conducted a metabolic characterization of P2RX5 knockout male mice; next, we characterized this purinergic pathway in a cell-autonomous context in brown adipocytes. We then tested the role of the P2RX5 receptor agonism in metabolic responses in vivo in conditions of minimal adaptive thermogenesis requirements. Our data show that loss of P2RX5 causes reduced brown adipocyte differentiation in vitro, and browning in vivo. Lastly, we unravel a previously unappreciated role for P2RX5 agonism to exert an anti-obesity effect in the presence of enhanced brown adipose tissue recruitment in male mice housed at thermoneutrality. Altogether, our data support a role for P2RX5 in mediating brown adipocyte differentiation and function that could be further targeted for benefits in the context of adipose tissue pathology and metabolic diseases.

Purine Biosynthesis Pathways Are Required for Myogenesis in Xenopus laevis

Purines are required for fundamental biological processes and alterations in their metabolism lead to severe genetic diseases associated with developmental defects whose etiology remains unclear. Here, we studied the developmental requirements for purine metabolism using the amphibian Xenopus laevis as a vertebrate model. We provide the first functional characterization of purine pathway genes and show that these genes are mainly expressed in nervous and muscular embryonic tissues. Morphants were generated to decipher the functions of these genes, with a focus on the adenylosuccinate lyase (ADSL), which is an enzyme required for both salvage and de novo purine pathways. adsl.L knockdown led to a severe reduction in the expression of the myogenic regulatory factors (MRFs: Myod1, Myf5 and Myogenin), thus resulting in defects in somite formation and, at later stages, the development and/or migration of both craniofacial and hypaxial muscle progenitors. The reduced expressions of hprt1.L and ppat, which are two genes specific to the salvage and de novo pathways, respectively, resulted in similar alterations. In conclusion, our data show for the first time that de novo and recycling purine pathways are essential for myogenesis and highlight new mechanisms in the regulation of MRF gene expression.

Dipyridamole activates adenosine A2B receptor and AMPK/cAMP signaling and promotes myogenic differentiation of myoblastic C2C12 cells

Introduction: Sarcopenia is defined as a loss of muscle mass and strength. ATP homeostasis is crucial during myogenesis. We determined how the purinergic system modulates myogenesis using dipyridamole (blocks adenosine taken up by the cells) and tenofovir (inhibits ATP release) in a myoblast cell line. Methods: C2C12 cells were differentiated in the presence/absence of tenofovir/dipyridamole, with/without the A2B selective inhibitor PSB-603. Extra-/intracellular nucleotides were examined via HPLC. The expression of muscle differentiation proteins (Pax7, Mif5, MyoD, MyoG, and MHC), PKA/CREB, adenosine receptors (A1, A2A, A2B, and A3), ATP-channel pannexin-1 and the P2X7 receptor was analyzed via WB and RT-PCR. cAMP and AMPK activation was measured. Results: Tenofovir increased intracellular ATP and reduced extracellular adenosine, decreasing Pax7 expression and increasing MHC expression prematurely. Dipyridamole increased intracellular AMP and extracellular adenosine, counteracting the premature myogenesis promoted by tenofovir. All adenosine receptors were expressed during differentiation with dipyridamole, increasing A2B expression. Tenofovir maintained inactive AMPK and decreased cAMP levels, as well as PKAα and pCREB expression, which were recovered with dipyridamole. Discussion: Adenosine and ATP act as mediators in muscle myogenesis. The blockade of ATP release by tenofovir promotes premature myogenesis, with dipyridamole counteracting the premature differentiation promoted by tenofovir via the adenosine A2B receptor and cAMP/AMPK pathways. Therefore, dipyridamole might be of interest as a therapeutic approach in sarcopenia.

P2Y1R and P2Y2R: potential molecular triggers in muscle regeneration

Muscle regeneration is indispensable for skeletal muscle health and daily life when injury, muscular disease, and aging occur. Among the muscle regeneration, muscle stem cells' (MuSCs) activation, proliferation, and differentiation play a key role in muscle regeneration. Purines bind to its specific receptors during muscle development, which transmit environmental stimuli and play a crucial role of modulator of muscle regeneration. Evidences proved P2R expression during development and regeneration of skeletal muscle, both in human and mouse. In contrast to P2XR, which have been extensively investigated in skeletal muscles, the knowledge of P2YR in this tissue is less comprehensive. This review summarized muscle regeneration via P2Y1R and P2Y2R and speculated that P2Y1R and P2Y2R might be potential molecular triggers for MuSCs' activation and proliferation via the p-ERK1/2 and PLC pathways, explored their cascade effects on skeletal muscle, and proposed P2Y1/2 receptors as potential pharmacological targets in muscle regeneration, to advance the purinergic signaling within muscle and provide promising strategies for alleviating muscular disease.

Dihydropyridines Potentiate ATP-Induced Currents Mediated by the Full-Length Human P2X5 Receptor

The P2X5 receptor, an ATP-gated cation channel, is believed to be involved in tumor development, inflammatory bone loss and inflammasome activation after bacterial infection. Therefore, it is a worthwhile pharmacological target to treat the corresponding diseases, especially in minority populations that have a gene variant coding for functional homotrimeric P2X5 channels. Here, we investigated the effects of dihydropyridines on the human full-length P2X5 receptor (hP2X5^FL) heterologously expressed in Xenopus oocytes using the two-microelectrode voltage clamp method. Agonist dependency, kinetics and permeation behavior, including Cl⁻ permeability, were similar to hP2X5^FL expressed in HEK293 or 1321N1 cells. Additionally, 1,4-dihydropyridines have been shown to interact with various other purinergic receptors, and we have examined them as potential hP2X5 modulators. Of seven commercially available and four newly synthesized dihydropyridines tested at hP2X5^FL, only amlodipine exerted an inhibitory effect, but only at a high concentration of 300 µM. Isradipine and—even more—nimodipine stimulated ATP-induced currents in the low micromolar range. We conclude that common dihydropyridines or four new derivatives of amlodipine are not suitable as hP2X5 antagonists, but amlodipine might serve as a lead for future synthesis to increase its affinity. Furthermore, a side effect of nimodipine therapy could be a stimulatory effect on inflammatory processes.

MicroRNA-128 inhibits mitochondrial biogenesis and function via targeting PGC1α and NDUFS4

The size and morphology of mitochondria are very heterogeneous and correlates well with their healthy functioning. In many pathological conditions, mitochondrial morphology is altered due to impaired mitochondrial dynamics (a collective term for mitochondrial fusion and fission) and dysfunction. The current study aimed at identifying the role of microRNA-128 (miR-128) in regulating mitochondrial biogenesis. Previously, peroxisome proliferator activator receptor γ coactivator 1α (PGC1α) has been shown to co-activate key intermediates of mitochondrial biogenesis, function, and dynamics; however, the upstream regulatory network remains largely unknown. We, herein using in silico analysis followed by in vitro experiments in C2C12 myoblasts, showed that miR-128 reduces mitochondrial biogenesis by directly targeting PGC1α. The expression of downstream genes, nuclear respiratory factors 1 and 2 (NRF1 and NRF2, respectively), and mitochondrial transcription factor A (TFAM) were decreased in C2C12 myoblasts upon overexpression of miR-128. Also, miR-128 is shown to promote mitochondrial dysfunction by directly targeting NADH Dehydrogenase (Ubiquinone) Fe-S Protein 4 (NDUFS4). The mitochondrial dynamics and morphology were impaired post miR-128 overexpression, as revealed by downregulation of fusion proteins (mitofusin1 and 2, i.e., MFN1 and MFN2, respectively) and upregulation of fission protein (dynamin-related protein 1, i.e., DRP1). Conversely, inhibition of miR-128 expression improved mitochondrial biogenesis, function, and dynamics, as evidenced by increased mitochondrial mass and ATP production after antimiR-128 treatment. Our findings reveal that inhibition of miR-128 can be a new potential target for reversing the effects of metabolic disorders of skeletal muscle as observed during many pathophysiological conditions such as obesity and type II diabetes.

Mdfi Promotes C2C12 Cell Differentiation and Positively Modulates Fast-to-Slow-Twitch Muscle Fiber Transformation

Muscle development requires myoblast differentiation and muscle fiber formation. Myod family inhibitor (Mdfi) inhibits myogenic regulatory factors in NIH3T3 cells, but how Mdfi regulates myoblast myogenic development is still unclear. In the present study, we constructed an Mdfi-overexpression (Mdfi-OE) C2C12 cell line by the CRISPR/Cas9 system and performed RNA-seq on Mdfi-OE and wild-type (WT) C2C12 cells. The RNA-seq results showed that the calcium signaling pathway was the most significant. We also established the regulatory networks of Mdfi-OE on C2C12 cell differentiation and muscle fiber type transformation and identified hub genes. Further, both RNA-seq and experimental verification demonstrated that Mdfi promoted C2C12 cell differentiation by upregulating the expression of Myod, Myog, and Myosin. We also found that the positive regulation of Mdfi on fast-to-slow-twitch muscle fiber transformation is mediated by Myod, Camk2b, and its downstream genes, such as Pgc1a, Pdk4, Cs, Cox4, Acadm, Acox1, Cycs, and Atp5a1. In conclusion, our results demonstrated that Mdfi promotes C2C12 cell differentiation and positively modulates fast-to-slow-twitch muscle fiber transformation. These findings further our understanding of the regulatory mechanisms of Mdfi in myogenic development and muscle fiber type transformation. Our results suggest potential therapeutic targets for muscle- and metabolic-related diseases.

Cross-Talk in Nucleotide Signaling in Glioma C6 Cells

The chapter is focused on the mechanism of action of metabotropic P2Y nucleotide receptors: P2Y₁, P2Y₂, P2Y₁₂, P2Y₁₄ and the ionotropic P2X₇ receptor in glioma C6 cells. P2Y₁ and P2Y₁₂ both respond to ADP, but while P2Y₁ links to PLC and elevates cytosolic Ca²⁺ concentration, P2Y₁₂ negatively couples to adenylate cyclase, maintaining cAMP at low level. In glioma C6, these two P2Y receptors modulate activities of ERK1/2 and PI3K/Akt signaling and the effects depend on physiological conditions of the cells. During prolonged serum deprivation, cell growth is arrested, the expression of the P2Y₁ receptor strongly decreases and P2Y₁₂ becomes a major player responsible for ADP-evoked signal transduction. The P2Y₁₂ receptor activates ERK1/2 kinase phosphorylation (a known cell proliferation regulator) and stimulates Akt activity, contributing to glioma invasiveness. In contrast, P2Y₁ has an inhibitory effect on Akt pathway signaling. Furthermore, the P2X₇ receptor, often responsible for apoptotic fate, is not involved in Ca²⁺elevation in C6 cells. The shift in nucleotide receptor expression from P2Y₁ to P2Y₁₂ during serum withdrawal, the cross talk between both receptors and the lack of P2X₇ activity shows the precise self-regulating mechanism, enhancing survival and preserving the neoplastic features of C6 cells.

Altered allostery of the left flipper domain underlies the weak ATP response of rat P2X5 receptors

Although the extracellular ATP-gated cation channel purinergic receptor P2X5 is widely expressed in heart, skeletal muscle, and immune and nervous systems in mammals, little is known about its functions and channel-gating activities. This lack of knowledge is due to P2X5's weak ATP responses in several mammalian species, such as humans, rats, and mice. WT human P2X5 (hP2X5Δ328-349) does not respond to ATP, whereas a full-length variant, hP2X5 (hP2X5-FL), containing exon 10 encoding the second hP2X5 transmembrane domain (TM2), does. However, although rat P2X5 (rP2X5) has a full-length TM2, ATP induces only weak currents in rP2X5, which prompted us to investigate the mechanism underlying this small ATP response. Here, we show that single replacements of specific rP2X5 residues with the corresponding residues in hP2X5 (S191F or F195H) significantly enhance the current amplitude of rP2X5. Using a combination of engineered disulfide cross-linking, single-channel recording, and molecular modeling, we interrogated the effects of S191F and F195H substitutions on the allostery of the left flipper (LF) domain. On the basis of our findings, we propose that the bound ATP-induced distinct allostery of the LF domain with that of other functional subtypes has caused the weak ATP response of rP2X5 receptors. The findings of our study provide the prerequisite for future transgenic studies on the physiological and pathological functions of P2X5 receptors.

Comparative Embryonic Spatio-Temporal Expression Profile Map of the Xenopus P2X Receptor Family

P2X receptors are ATP-gated cations channels formed by the homo or hetero-trimeric association from the seven cloned subunits (P2X1-7). P2X receptors are widely distributed in different organs and cell types throughout the body including the nervous system and are involved in a large variety of physiological but also pathological processes in adult mammals. However, their expression and function during embryogenesis remain poorly understood. Here, we report the cloning and the comparative expression map establishment of the entire P2X subunit family in the clawed frog Xenopus. Orthologous sequences for 6 mammalian P2X subunits were identified in both X. laevis and X. tropicalis, but not for P2X3 subunit, suggesting a potential loss of this subunit in the Pipidae family. Three of these genes (p2rx1, p2rx2, and p2rx5) exist as homeologs in the pseudoallotetraploid X. laevis, making a total of 9 subunits in this species. Phylogenetic analyses demonstrate the high level of conservation of these receptors between amphibian and other vertebrate species. RT-PCR revealed that all subunits are expressed during the development although zygotic p2rx6 and p2rx7 transcripts are mainly detected at late organogenesis stages. Whole mount in situ hybridization shows that each subunit displays a specific spatio-temporal expression profile and that these subunits can therefore be grouped into two groups, based on their expression or not in the developing nervous system. Overlapping expression in the central and peripheral nervous system and in the sensory organs suggests potential heteromerization and/or redundant functions of P2X subunits in Xenopus embryos. The developmental expression of the p2rx subunit family during early phases of embryogenesis indicates that these subunits may have distinct roles during vertebrate development, especially embryonic neurogenesis.

A "noisy" electrical stimulation protocol favors muscle regeneration in vitro through release of endogenous ATP

An in vitro system of electrical stimulation was used to explore whether an innovative "noisy" stimulation protocol derived from human electromyographic recordings (EMGstim) could promote muscle regeneration. EMGstim was delivered to cultured mouse myofibers isolated from Flexor Digitorum Brevis, preserving their satellite cells. In response to EMGstim, immunostaining for the myogenic regulatory factor myogenin, revealed an increased percentage of elongated myogenin-positive cells surrounding the myofibers. Conditioned medium collected from EMGstim-treated cell cultures, promoted satellite cells differentiation in unstimulated myofiber cell cultures, suggesting that extracellular soluble factors could mediate the process. Interestingly, the myogenic effect of EMGstim was mimicked by exogenously applied ATP (0.1 μM), reduced by the ATP diphosphohydrolase apyrase and prevented by blocking endogenous ATP release with carbenoxolone. In conclusion, our results show that "noisy" electrical stimulations favor muscle progenitor cell differentiation most likely via the release of endogenous ATP from contracting myofibres. Our data also suggest that "noisy" stimulation protocols could be potentially more efficient than regular stimulations to promote in vivo muscle regeneration after traumatic injury or in neuropathological diseases.

Nucleoprotein supplementation enhances the recovery of rat soleus mass with reloading after hindlimb unloading-induced atrophy via myonuclei accretion and increased protein synthesis

Hindlimb unloading results in muscle atrophy and a period of reloading has been shown to partially recover the lost muscle mass. Two of the mechanisms involved in this recovery of muscle mass are the activation of protein synthesis pathways and an increase in myonuclei number. The additional myonuclei are provided by satellite cells that are activated by the mechanical stress associated with the reloading of the muscles and eventually incorporated into the muscle fibers. Amino acid supplementation with exercise also can increase skeletal muscle mass through enhancement of protein synthesis and nucleotide supplements can promote cell cycle activity. Therefore, we hypothesized that nucleoprotein supplementation, a combination of amino acids and nucleotides, would enhance the recovery of muscle mass to a greater extent than reloading alone after a period of unloading. Adult rats were assigned to 4 groups: control, hindlimb unloaded (HU; 14 days), reloaded (5 days) after hindlimb unloading (HUR), and reloaded after hindlimb unloading with nucleoprotein supplementation (HUR + NP). Compared with the HUR group, the HUR + NP group had larger soleus muscles and fiber cross-sectional areas, higher levels of phosphorylated rpS6, and higher numbers of myonuclei and myogenin-positive cells. These results suggest that nucleoprotein supplementation has a synergistic effect with reloading in recovering skeletal muscle properties after a period of unloading via rpS6 activation and satellite cell differentiation and incorporation into the muscle fibers. Therefore, this supplement may be an effective therapeutic regimen to include in rehabilitative strategies for a variety of muscle wasting conditions such as aging, cancer cachexia, muscular dystrophy, bed rest, and cast immobilization.

Role of Extracellular Adenosine Triphosphate in Human Skin

Background:

The nucleotide adenosine triphosphate (ATP) has long been known to drive and participate in countless intracellular processes. Extracellular ATP and its metabolite adenosine have also been shown to exert a variety of effects on nearly every cell type in human skin. Knowledge of the sources and effects of extracellular ATP in human skin may help shape new therapies for skin injury, inflammation, and numerous other cutaneous disorders.

Objective:

The objective of this review is to introduce the reader to current knowledge regarding the sources and effects of extracellular ATP in human skin and to outline areas in which further research is necessary to clarify the nature and mechanism of these effects.

Conclusion:

Extracellular ATP seems to play a direct role in triggering skin inflammatory, regenerative, and fibrotic responses to mechanical injury, an indirect role in melanocyte proliferation and apoptosis, and a complex role in Langerhans cell-directed adaptive immunity.

Molecular Structure and Regulation of P2X Receptors With a Special Emphasis on the Role of P2X2 in the Auditory System

The P2X purinergic receptors are cation-selective channels gated by extracellular adenosine 5'-triphosphate (ATP). These purinergic receptors are found in virtually all mammalian cell types and facilitate a number of important physiological processes. Within the past few years, the characterization of crystal structures of the zebrafish P2X4 receptor in its closed and open states has provided critical insights into the mechanisms of ligand binding and channel activation. Understanding of this gating mechanism has facilitated to design and interpret new modeling and structure-function experiments to better elucidate how different agonists and antagonists can affect the receptor with differing levels of potency. This review summarizes the current knowledge on the structure, activation, allosteric modulators, function, and location of the different P2X receptors. Moreover, an emphasis on the P2X2 receptors has been placed in respect to its role in the auditory system. In particular, the discovery of three missense mutations in P2X2 receptors could become important areas of study in the field of gene therapy to treat progressive and noise-induced hearing loss. J. Cell. Physiol. 231: 1656-1670, 2016. © 2015 Wiley Periodicals, Inc.

A novel mechanism of autophagic cell death in dystrophic muscle regulated by P2RX7 receptor large-pore formation and HSP90

P2RX7 is an ATP-gated ion channel, which can also exhibit an open state with a considerably wider permeation. However, the functional significance of the movement of molecules through the large pore (LP) and the intracellular signaling events involved are not known. Here, analyzing the consequences of P2RX7 activation in primary myoblasts and myotubes from the Dmd^mdx mouse model of Duchenne muscular dystrophy, we found ATP-induced P2RX7-dependent autophagic flux, leading to CASP3-CASP7-independent cell death. P2RX7-evoked autophagy was triggered by LP formation but not Ca²⁺ influx or MAPK1-MAPK3 phosphorylation, 2 canonical P2RX7-evoked signals. Phosphoproteomics, protein expression inference and signaling pathway prediction analysis of P2RX7 signaling mediators pointed to HSPA2 and HSP90 proteins. Indeed, specific HSP90 inhibitors prevented LP formation, LC3-II accumulation, and cell death in myoblasts and myotubes but not in macrophages. Pharmacological blockade or genetic ablation of p2rx7 also proved protective against ATP-induced death of muscle cells, as did inhibition of autophagy with 3-MA. The functional significance of the P2RX7 LP is one of the great unknowns of purinergic signaling. Our data demonstrate a novel outcome—autophagy—and show that molecules entering through the LP can be targeted to phagophores. Moreover, we show that in muscles but not in macrophages, autophagy is needed for the formation of this LP. Given that P2RX7-dependent LP and HSP90 are critically interacting in the ATP-evoked autophagic death of dystrophic muscles, treatments targeting this axis could be of therapeutic benefit in this debilitating and incurable form of muscular dystrophy.

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.

Regulation of pannexin and connexin channels and their functional role in skeletal muscles

Myogenic precursor cells express connexins (Cx) and pannexins (Panx), proteins that form different membrane channels involved in cell-cell communication. Cx channels connect either the cytoplasm of adjacent cells, called gap junction channels (GJC), or link the cytoplasm with the extracellular space, termed hemichannels (HC), while Panx channels only support the latter. In myoblasts, Panx1 HCs play a critical role in myogenic differentiation, and Cx GJCs and possibly Cx HCs coordinate metabolic responses during later steps of myogenesis. After innervation, myofibers do not express Cxs, but still express Panx1. In myotubes and innervated myofibers, Panx1 HCs allow release of adenosine triphosphate and thus they might be involved in skeletal muscle plasticity. In addition, Panx1 HCs present in adult myofibers mediate adenosine triphosphate release and glucose uptake required for potentiation of muscle contraction. Under pathological conditions, such as upon denervation and spinal cord injury, levels of Panx1 are upregulated. However, Panx1(-/-) mice show similar degree of atrophy as denervated wild-type muscles. Skeletal muscles also express Cx HCs in the sarcolemma after denervation or spinal cord injury, plus other non-selective membrane channels, including purinergic P2X7 receptors and transient receptor potential type V2 channels. The absence of Cx43 and Cx45 is sufficient to drastically reduce denervation atrophy. Moreover, inflammatory cytokines also induce the expression of Cxs in myofibers, suggesting the expression of these Cxs as a common factor for myofiber degeneration under diverse pathological conditions. Inhibitors of skeletal muscle Cx HCs could be promising tools to prevent muscle wasting induced by conditions associated with synaptic dysfunction and inflammation.

MicroRNAs Regulate Cellular ATP Levels by Targeting Mitochondrial Energy Metabolism Genes during C2C12 Myoblast Differentiation

In our previous study, we identified an miRNA regulatory network involved in energy metabolism in porcine muscle. To better understand the involvement of miRNAs in cellular ATP production and energy metabolism, here we used C2C12 myoblasts, in which ATP levels increase during differentiation, to identify miRNAs modulating these processes. ATP level, miRNA and mRNA microarray expression profiles during C2C12 differentiation into myotubes were assessed. The results suggest 14 miRNAs (miR-423-3p, miR-17, miR-130b, miR-301a/b, miR-345, miR-15a, miR-16a, miR-128, miR-615, miR-1968, miR-1a/b, and miR-194) as cellular ATP regulators targeting genes involved in mitochondrial energy metabolism (Cox4i2, Cox6a2, Ndufb7, Ndufs4, Ndufs5, and Ndufv1) during C2C12 differentiation. Among these, miR-423-3p showed a high inverse correlation with increasing ATP levels. Besides having implications in promoting cell growth and cell cycle progression, its function in cellular ATP regulation is yet unknown. Therefore, miR-423-3p was selected and validated for the function together with its potential target, Cox6a2. Overexpression of miR-423-3p in C2C12 myogenic differentiation lead to decreased cellular ATP level and decreased expression of Cox6a2 compared to the negative control. These results suggest miR-423-3p as a novel regulator of ATP/energy metabolism by targeting Cox6a2.

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.

Pannexin channels mediate the acquisition of myogenic commitment in C2C12 reserve cells promoted by P2 receptor activation

The acquisition of myoblast commitment to the myogenic linage requires rises in intracellular free Ca²⁺ concentration ([Ca²⁺]_i). Putative cell membrane pathways involved in these [Ca²⁺]_i increments are P2 receptors (P2Rs) as well as connexin (Cx) and/or pannexin (Panx) hemichannels and channels (Cx HChs and Panx Chs), respectively, which are known to permeate Ca²⁺. Reserve cells (RCs) are uncommitted myoblasts obtained from differentiated C₂C₁₂ cell cultures, which acquire commitment upon replating. Regarding these cells, we found that extracellular ATP increases the [Ca²⁺]_i via P2Rs. Moreover, ATP increases the plasma membrane permeability to small molecules and a non-selective membrane current, both of which were inhibited by Cx HCh/Panx1Ch blockers. However, RCs exposed to divalent cation-free saline solution, which is known to activate Cx HChs (but not Panx Chs), did not enhance membrane permeability, thus ruling out the possible involvement of Cx HChs. Moreover, ATP-induced membrane permeability was inhibited with blockers of P2Rs that activate Panx Chs. In addition, exogenous ATP induced the expression of myogenic commitment and increased MyoD levels, which was prevented by the inhibition of P2Rs or knockdown of Panx1 Chs. Similarly, increases in MyoD levels induced by ATP released by RCs were inhibited by Panx Ch/Cx HCh blockers. Myogenic commitment acquisition thus requires a feed-forward mechanism mediated by extracellular ATP, P2Rs, and Panx Chs.

Store-operated calcium entry contributes to abnormal Ca²⁺ signalling in dystrophic mdx mouse myoblasts

Sarcolemma damage and activation of various calcium channels are implicated in altered Ca(2+) homeostasis in muscle fibres of both Duchenne muscular dystrophy (DMD) sufferers and in the mdx mouse model of DMD. Previously we have demonstrated that also in mdx myoblasts extracellular nucleotides trigger elevated cytoplasmic Ca(2+) concentrations due to alterations of both ionotropic and metabotropic purinergic receptors. Here we extend these findings to show that the mdx mutation is associated with enhanced store-operated calcium entry (SOCE). Substantially increased rate of SOCE in mdx myoblasts in comparison to that in control cells correlated with significantly elevated STIM1 protein levels. These results reveal that mutation in the dystrophin-encoding Dmd gene may significantly impact cellular calcium response to metabotropic stimulation involving depletion of the intracellular calcium stores followed by activation of the store-operated calcium entry, as early as in undifferentiated myoblasts. These data are in agreement with the increasing number of reports showing that the dystrophic pathology resulting from dystrophin mutations may be developmentally regulated. Moreover, our results showing that aberrant responses to extracellular stimuli may contribute to DMD pathogenesis suggest that treatments inhibiting such responses might alter progression of this lethal disease.

Pannexin‑1 silencing inhibits the proliferation of U87‑MG cells

Pannexin‑1 (Panx‑1) is abundantly expressed in vertebrates and has been shown to assemble into high‑conductance single‑membrane channels, which are permeable to large molecules and regulate cellular function. However, the association between Panx‑1 and astrocyte proliferation is poorly understood. This study provides evidence for a difference in cell proliferation between wild‑type and Panx‑1‑knockdown cells. Proliferation of the U87‑MG malignant glioma cell line was reduced following transfection with Panx‑1‑short interfering RNA. In addition, treatment with the Panx‑1 activator, adenosine triphosphate, significantly reduced cell proliferation at 48 h in Panx‑1‑knockdown cells compared with wild type cells. In conclusion, on the basis of the present findings, Panx‑1 is likely to be important in the regulation of U87‑MG cell proliferation. This provides further support for the hypothesis that there is a correlation between Panx‑1 expression and U87‑MG cell proliferation.

Pannexin 1 and pannexin 3 channels regulate skeletal muscle myoblast proliferation and differentiation

Pannexins constitute a family of three glycoproteins (Panx1, -2, and -3) forming single membrane channels. Recent work demonstrated that Panx1 is expressed in skeletal muscle and involved in the potentiation of contraction. However, Panxs functions in skeletal muscle cell differentiation, and proliferation had yet to be assessed. We show here that Panx1 and Panx3, but not Panx2, are present in human and rodent skeletal muscle, and their various species are differentially expressed in fetal versus adult human skeletal muscle tissue. Panx1 levels were very low in undifferentiated human primary skeletal muscle cells and myoblasts (HSMM) but increased drastically during differentiation and became the main Panx expressed in differentiated cells. Using HSMM, we found that Panx1 expression promotes this process, whereas it was impaired in the presence of probenecid or carbenoxolone. As for Panx3, its lower molecular weight species were prominent in adult skeletal muscle but very low in the fetal tissue and in undifferentiated skeletal muscle cells and myoblasts. Its overexpression (∼43-kDa species) induced HSMM differentiation and also inhibited their proliferation. On the other hand, a ∼70-kDa immunoreactive species of Panx3, likely glycosylated, sialylated, and phosphorylated, was highly expressed in proliferative myoblasts but strikingly down-regulated during their differentiation. Reduction of its endogenous expression using two Panx3 shRNAs significantly inhibited HSMM proliferation without triggering their differentiation. In summary, our results demonstrate that Panx1 and Panx3 are co-expressed in human skeletal muscle myoblasts and play a pivotal role in dictating the proliferation and differentiation status of these cells.

New insights into the relationship between mIGF-1-induced hypertrophy and Ca2+ handling in differentiated satellite cells

Muscle regeneration involves the activation of satellite cells, is regulated at the genetic and epigenetic levels, and is strongly influenced by gene activation and environmental conditions. The aim of this study was to determine whether the overexpression of mIGF-1 can modify functional features of satellite cells during the differentiation process, particularly in relation to modifications of intracellular Ca²⁺ handling.

Satellite cells were isolated from wild-type and MLC/mIGF-1 transgenic mice. The cells were differentiated in vitro, and morphological analyses, intracellular Ca²⁺ measurements, and ionic current recordings were performed.

mIGF-1 overexpression accelerates satellite cell differentiation and promotes myotube hypertrophy. In addition, mIGF-1 overexpression-induced potentiation of myogenesis triggers both quantitative and qualitative changes to the control of intracellular Ca²⁺ handling. In particular, the differentiated MLC/mIGF-1 transgenic myotubes have reduced velocity and amplitude of intracellular Ca²⁺ increases after stimulation with caffeine, KCl and acetylcholine. This appears to be due, at least in part, to changes in the physico-chemical state of the sarcolemma (increased membrane lipid oxidation, increased output currents) and to increased expression of dihydropyridine voltage-operated Ca²⁺ channels. Interestingly, extracellular ATP and GTP evoke intracellular Ca²⁺ mobilization to greater extents in the MLC/mIGF-1 transgenic satellite cells, compared to the wild-type cells.

These data suggest that these MLC/mIGF-1 transgenic satellite cells are more sensitive to trophic stimuli, which can potentiate the effects of mIGF-1 on the myogenic programme.

Purinergic signalling and cancer

Receptors for extracellular nucleotides are widely expressed by mammalian cells. They mediate a large array of responses ranging from growth stimulation to apoptosis, from chemotaxis to cell differentiation and from nociception to cytokine release, as well as neurotransmission. Pharma industry is involved in the development and clinical testing of drugs selectively targeting the different P1 nucleoside and P2 nucleotide receptor subtypes. As described in detail in the present review, P2 receptors are expressed by all tumours, in some cases to a very high level. Activation or inhibition of selected P2 receptor subtypes brings about cancer cell death or growth inhibition. The field has been largely neglected by current research in oncology, yet the evidence presented in this review, most of which is based on in vitro studies, although with a limited amount from in vivo experiments and human studies, warrants further efforts to explore the therapeutic potential of purinoceptor targeting in cancer.

Conductance simulation of the purinergic P2X2, P2X4, and P2X7 ionic channels using a combined Brownian dynamics and molecular dynamics approach

This paper investigates the application of an original combined approach of molecular and Brownian dynamic methods with quantum chemistry calculations for modeling the process of conductance of ion channels using purinergic P2X family receptors P2X2, P2X4, and P2X7 as a case study. A simplified model of the ionic channel in the lipid bilayer has been developed. A high level of conductance (30 pS) of P2X2 ionic channel together with the key role of Asp349 in forming the selectivity filter of P2X2 has been shown by using this approach. Calculated P2X2 permeability to monovalent cations Li(+), Na(+), and K(+) conforms to the free diffusion coefficient of these ions, which shows the low selectivity of P2X2 ionic channel.

Pannexin 3 inhibits proliferation of osteoprogenitor cells by regulating Wnt and p21 signaling

Canonical Wnt signaling and BMP promote the proliferation and differentiation of osteoprogenitors, respectively. However, the regulatory mechanism involved in the transition from proliferation to differentiation is unclear. Here, we show that Panx3 (pannexin 3) plays a key role in this transition by inhibiting the proliferation and promoting the cell cycle exit. Using primary calvarial cells and explants, C3H10T1/2 cells, and C2C12 cells, we found that Panx3 expression inhibited cell growth, whereas the inhibition of endogenous Panx3 expression increased it. We also found that the Panx3 hemichannel inhibited cell growth by promoting β-catenin degradation through GSK3β activation. Additionally, the Panx3 hemichannel inhibited cyclin D1 transcription and Rb phosphorylation through reduced cAMP/PKA/CREB signaling. Furthermore, the Panx3 endoplasmic reticulum Ca(2+) channel induced the transcription and phosphorylation of p21, through the calmodulin/Smad pathway, and resulted in the cell cycle exit. Our results reveal that Panx3 is a new regulator that promotes the switch from proliferation to differentiation of osteoprogenitors via multiple Panx3 signaling pathways.

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.

Electrical stimuli are anti-apoptotic in skeletal muscle via extracellular ATP. Alteration of this signal in Mdx mice is a likely cause of dystrophy

ATP signaling has been shown to regulate gene expression in skeletal muscle and to be altered in models of muscular dystrophy. We have previously shown that in normal muscle fibers, ATP released through Pannexin1 (Panx1) channels after electrical stimulation plays a role in activating some signaling pathways related to gene expression. We searched for a possible role of ATP signaling in the dystrophy phenotype. We used muscle fibers from flexor digitorum brevis isolated from normal and mdx mice. We demonstrated that low frequency electrical stimulation has an anti-apoptotic effect in normal muscle fibers repressing the expression of Bax, Bim and PUMA. Addition of exogenous ATP to the medium has a similar effect. In dystrophic fibers, the basal levels of extracellular ATP were higher compared to normal fibers, but unlike control fibers, they do not present any ATP release after low frequency electrical stimulation, suggesting an uncoupling between electrical stimulation and ATP release in this condition. Elevated levels of Panx1 and decreased levels of Cav1.1 (dihydropyridine receptors) were found in triads fractions prepared from mdx muscles. Moreover, decreased immunoprecipitation of Cav1.1 and Panx1, suggest uncoupling of the signaling machinery. Importantly, in dystrophic fibers, exogenous ATP was pro-apoptotic, inducing the transcription of Bax, Bim and PUMA and increasing the levels of activated Bax and cytosolic cytochrome c. These evidence points to an involvement of the ATP pathway in the activation of mechanisms related with cell death in muscular dystrophy, opening new perspectives towards possible targets for pharmacological therapies.

Microarray-Based Comparisons of Ion Channel Expression Patterns: Human Keratinocytes to Reprogrammed hiPSCs to Differentiated Neuronal and Cardiac Progeny

Ion channels are involved in a large variety of cellular processes including stem cell differentiation. Numerous families of ion channels are present in the organism which can be distinguished by means of, for example, ion selectivity, gating mechanism, composition, or cell biological function. To characterize the distinct expression of this group of ion channels we have compared the mRNA expression levels of ion channel genes between human keratinocyte-derived induced pluripotent stem cells (hiPSCs) and their somatic cell source, keratinocytes from plucked human hair. This comparison revealed that 26% of the analyzed probes showed an upregulation of ion channels in hiPSCs while just 6% were downregulated. Additionally, iPSCs express a much higher number of ion channels compared to keratinocytes. Further, to narrow down specificity of ion channel expression in iPS cells we compared their expression patterns with differentiated progeny, namely, neurons and cardiomyocytes derived from iPS cells. To conclude, hiPSCs exhibit a very considerable and diverse ion channel expression pattern. Their detailed analysis could give an insight into their contribution to many cellular processes and even disease mechanisms.

Altered immune pathway activity under exercise challenge in Gulf War Illness: an exploratory analysis

Though potentially linked to the basic physiology of stress response we still have no clear understanding of Gulf War Illness (GWI), a debilitating illness presenting with a complex constellation of immune, endocrine and neurological symptoms. Here we compared male GWI (n=20) with healthy veterans (n=22) and subjects with chronic fatigue syndrome/myalgic encephalomyelitis (CFS/ME) (n=7). Blood was drawn during a Graded eXercise Test (GXT) prior to exercise, at peak effort (VO2 max) and 4-h post exercise. Affymetrix HG U133 plus 2.0 microarray gene expression profiling in peripheral blood mononuclear cells (PBMCs) was used to estimate activation of over 500 documented pathways. This was cast against ELISA-based measurement of 16 cytokines in plasma and flow cytometric assessment of lymphocyte populations and cytotoxicity. A 2-way ANOVA corrected for multiple comparisons (q statistic <0.05) indicated significant increases in neuroendocrine-immune signaling and inflammatory activity in GWI, with decreased apoptotic signaling. Conversely, cell cycle progression and immune signaling were broadly subdued in CFS. Partial correlation networks linking pathways with symptom severity via changes in immune cell abundance, function and signaling were constructed. Central to these were changes in IL-10 and CD2+ cell abundance and their link to two pathway clusters. The first consisted of pathways supporting neuronal development and migration whereas the second was related to androgen-mediated activation of NF-κB. These exploratory results suggest an over-expression of known exercise response mechanisms as well as illness-specific changes that may involve an overlapping stress-potentiated neuro-inflammatory response.

Purinergic signalling in the lower urinary tract

The aim of this review is to describe the conceptual steps contributing to our current knowledge of purinergic signalling and to consider its involvement in the physiology and pathophysiology of the lower urinary tract. The voiding reflex involves ATP released as a cotransmitter with acetylcholine from parasympathetic nerves supplying the bladder and ATP released from urothelial cells during bladder distension to initiate the voiding reflex via P2X3 receptors on suburothelial low threshold sensory nerve fibres. This mechanosensory transduction pathway also participates, via high threshold sensory nerve fibres, in the initiation of pain in bladder and ureter. Treatment of prostate and bladder cancer with ATP is effective against the primary tumours in animal models and human cell lines, via P2X5 and P2X7 receptors, and also improves the systemic symptoms associated with advanced malignancy. Acupuncture is widely used for the treatment of urinary disorders, and a purinergic hypothesis is discussed for the underlying mechanism.

Cross-talk in nucleotide signaling in glioma C6 cells

The chapter is focused on the mechanism of action of metabotropic P2Y nucleotide receptors: P2Y(1), P2Y(2), P2Y(12), P2Y(14) and the ionotropic P2X(7) receptor in glioma C6 cells. P2Y(1) and P2Y(12) both respond to ADP, but while P2Y(1) links to PLC and elevates cytosolic Ca(2+) concentration, P2Y(12) negatively couples to adenylate cyclase, maintaining cAMP at low level. In glioma C6, these two P2Y receptors modulate activities of ERK1/2 and PI3K/Akt signaling and the effects depend on physiological conditions of the cells. During prolonged serum deprivation, cell growth is arrested, the expression of the P2Y(1) receptor strongly decreases and P2Y(12) becomes a major player responsible for ADP-evoked signal transduction. The P2Y(12) receptor activates ERK1/2 kinase phosphorylation (a known cell proliferation regulator) and stimulates Akt activity, contributing to glioma invasiveness. In contrast, P2Y(1) has an inhibitory effect on Akt pathway signaling. Furthermore, the P2X(7) receptor, often responsible for apoptotic fate, is not involved in Ca(2+)elevation in C6 cells. The shift in nucleotide receptor expression from P2Y(1) to P2Y(12) during serum withdrawal, the cross talk between both receptors and the lack of P2X(7) activity shows the precise self-regulating mechanism, enhancing survival and preserving the neoplastic features of C6 cells.

Developmental expression of P2X5 receptors in the mouse prenatal central and peripheral nervous systems

The functions of P2X purinoceptors (P2X1-7) in the nervous system of adults have been widely studied. However, little is known about their roles during embryonic development. Our previous work has reported an extensive expression of P2X5 receptors in the adult mouse central nervous system. In the present study, we have examined the expression pattern of P2X5 receptor mRNA and protein during prenatal development of the mouse nervous system (from embryonic day E8 to E17). P2X5 receptors appeared in the neural tube as early as E8 and were gradually confined to new-born neurons in the cortical plate and ventral horn of the spinal cord. Heavy signals for P2X5 receptors were also found in dorsal root ganglia (DRG), retina, olfactory epithelium, and nerve fibers in skeletal muscles. In conclusion, P2X5 receptors were strongly represented in the developing mouse nervous system. The transient high expression pattern of P2X5 receptors in epithelium-like structures suggests a role during early neurogenesis.

Extracellular purines promote the differentiation of human bone marrow-derived mesenchymal stem cells to the osteogenic and adipogenic lineages

Extracellular nucleotides are potent signaling molecules mediating cell-specific biological functions, mostly within the processes of tissue damage and repair and flogosis. We previously demonstrated that adenosine 5'-triphosphate (ATP) inhibits the proliferation of human bone marrow-derived mesenchymal stem cells (BM-hMSCs), while stimulating, in vitro and in vivo, their migration. Here, we investigated the effects of ATP on BM-hMSC differentiation capacity. Molecular analysis showed that ATP treatment modulated the expression of several genes governing adipogenic and osteoblastic (i.e., WNT-pathway-related genes) differentiation of MSCs. Functional studies demonstrated that ATP, under specific culture conditions, stimulated adipogenesis by significantly increasing the lipid accumulation and the expression levels of the adipogenic master gene PPARγ (peroxisome proliferator-activated receptor-gamma). In addition, ATP stimulated osteogenic differentiation by promoting mineralization and expression of the osteoblast-related gene RUNX2 (runt-related transcription factor 2). Furthermore, we demonstrated that ATP stimulated adipogenesis via its triphosphate form, while osteogenic differentiation was induced by the nucleoside adenosine, resulting from ATP degradation induced by CD39 and CD73 ectonucleotidases expressed on the MSC membrane. The pharmacological profile of P2 purinergic receptors (P2Rs) suggests that adipogenic differentiation is mainly mediated by the engagement of P2Y1 and P2Y4 receptors, while stimulation of the P1R adenosine-specific subtype A2B is involved in adenosine-induced osteogenic differentiation. Thus, we provide new insights into molecular regulation of MSC differentiation.

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

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

P2X7 purinoceptor alterations in dystrophic mdx mouse muscles: relationship to pathology and potential target for treatment

Duchenne muscular dystrophy (DMD) is a lethal inherited muscle disorder. Pathological characteristics of DMD skeletal muscles include, among others, abnormal Ca(2+) homeostasis and cell signalling. Here, in the mdx mouse model of DMD, we demonstrate significant P2X7 receptor abnormalities in isolated primary muscle cells and cell lines and in dystrophic muscles in vivo. P2X7 mRNA expression in dystrophic muscles was significantly up-regulated but without alterations of specific splice variant patterns. P2X7 protein was also up-regulated and this was associated with altered function of P2X7 receptors producing increased responsiveness of cytoplasmic Ca(2+) and extracellular signal-regulated kinase (ERK) phosphorylation to purinergic stimulation and altered sensitivity to NAD. Ca(2+) influx and ERK signalling were stimulated by ATP and BzATP, inhibited by specific P2X7 antagonists and insensitive to ivermectin, confirming P2X7 receptor involvement. Despite the presence of pannexin-1, prolonged P2X7 activation did not trigger cell permeabilization to propidium iodide or Lucifer yellow. In dystrophic mice, in vivo treatment with the P2X7 antagonist Coomassie Brilliant Blue reduced the number of degeneration-regeneration cycles in mdx skeletal muscles. Altered P2X7 expression and function is thus an important feature in dystrophic mdx muscle and treatments aiming to inhibit P2X7 receptor might slow the progression of this disease.

Connexin- and pannexin-based channels in normal skeletal muscles and their possible role in muscle atrophy

Precursor cells of skeletal muscles express connexins 39, 43 and 45 and pannexin1. In these cells, most connexins form two types of membrane channels, gap junction channels and hemichannels, whereas pannexin1 forms only hemichannels. All these channels are low-resistance pathways permeable to ions and small molecules that coordinate developmental events. During late stages of skeletal muscle differentiation, myofibers become innervated and stop expressing connexins but still express pannexin1 hemichannels that are potential pathways for the ATP release required for potentiation of the contraction response. Adult injured muscles undergo regeneration, and connexins are reexpressed and form membrane channels. In vivo, connexin reexpression occurs in undifferentiated cells that form new myofibers, favoring the healing process of injured muscle. However, differentiated myofibers maintained in culture for 48 h or treated with proinflammatory cytokines for less than 3 h also reexpress connexins and only form functional hemichannels at the cell surface. We propose that opening of these hemichannels contributes to drastic changes in electrochemical gradients, including reduction of membrane potential, increases in intracellular free Ca(2+) concentration and release of diverse metabolites (e.g., NAD(+) and ATP) to the extracellular milieu, contributing to multiple metabolic and physiologic alterations that characterize muscles undergoing atrophy in several acquired and genetic human diseases. Consequently, inhibition of connexin hemichannels expressed by injured or denervated skeletal muscles might reduce or prevent deleterious changes triggered by conditions that promote muscle atrophy.

Molecular and functional properties of P2X receptors--recent progress and persisting challenges

ATP-gated P2X receptors are trimeric ion channels that assemble as homo- or heteromers from seven cloned subunits. Transcripts and/or proteins of P2X subunits have been found in most, if not all, mammalian tissues and are being discovered in an increasing number of non-vertebrates. Both the first crystal structure of a P2X receptor and the generation of knockout (KO) mice for five of the seven cloned subtypes greatly advanced our understanding of their molecular and physiological function and their validation as drug targets. This review summarizes the current understanding of the structure and function of P2X receptors and gives an update on recent developments in the search for P2X subtype-selective ligands. It also provides an overview about the current knowledge of the regulation and modulation of P2X receptors on the cellular level and finally on their physiological roles as inferred from studies on KO mice.

Remodeling of purinergic receptor-mediated Ca2+ signaling as a consequence of EGF-induced epithelial-mesenchymal transition in breast cancer cells

Background

The microenvironment plays a pivotal role in tumor cell proliferation, survival and migration. Invasive cancer cells face a new set of environmental challenges as they breach the basement membrane and colonize distant organs during the process of metastasis. Phenotypic switching, such as that which occurs during epithelial-mesenchymal transition (EMT), may be associated with a remodeling of cell surface receptors and thus altered responses to signals from the tumor microenvironment.

Methodology/Principal Findings

We assessed changes in intracellular Ca²⁺ in cells loaded with Fluo-4 AM using a fluorometric imaging plate reader (FLIPR^TETRA) and observed significant changes in the potency of ATP (EC₅₀ 0.175 µM (−EGF) versus 1.731 µM (+EGF), P<0.05), and the nature of the ATP-induced Ca²⁺ transient, corresponding with a 10-fold increase in the mesenchymal marker vimentin (P<0.05). We observed no change in the sensitivity to PAR2-mediated Ca²⁺ signaling, indicating that these alterations are not simply a consequence of changes in global Ca²⁺ homeostasis. To determine whether changes in ATP-mediated Ca²⁺ signaling are preceded by alterations in the transcriptional profile of purinergic receptors, we analyzed the expression of a panel of P2X ionotropic and P2Y metabotropic purinergic receptors using real-time RT-PCR and found significant and specific alterations in the suite of ATP-activated purinergic receptors during EGF-induced EMT in breast cancer cells. Our studies are the first to show that P2X₅ ionotropic receptors are enriched in the mesenchymal phenotype and that silencing of P2X₅ leads to a significant reduction (25%, P<0.05) in EGF-induced vimentin protein expression.

Conclusions

The acquisition of a new suite of cell surface purinergic receptors is a feature of EGF-mediated EMT in MDA-MB-468 breast cancer cells. Such changes may impart advantageous phenotypic traits and represent a novel mechanism for the targeting of cancer metastasis.

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.

Lipid raft proteome reveals that oxidative phosphorylation system is associated with the plasma membrane

Although accumulating proteomic analyses have supported the fact that mitochondrial oxidative phosphorylation (OXPHOS) complexes are localized in lipid rafts, which mediate cell signaling, immune response and host-pathogen interactions, there has been no in-depth study of the physiological functions of lipid-raft OXPHOS complexes. Here, we show that many subunits of OXPHOS complexes were identified from the lipid rafts of human adipocytes, C2C12 myotubes, Jurkat cells and surface biotin-labeled Jurkat cells via shotgun proteomic analysis. We discuss the findings of OXPHOS complexes in lipid rafts, the role of the surface ATP synthase complex as a receptor for various ligands and extracellular superoxide generation by plasma membrane oxidative phosphorylation complexes.

Expression of P2Y(6) receptors in the developing mouse skeletal muscle and after injury and repair

In this study, single and double-labeling immunofluorescence histochemistry, Western blot and real-time polymerase chain reaction were used to study the expression of P2Y(6) receptors in developing mouse skeletal muscle and during injury and repair. The results show that P2Y(6) receptor immunoreactive (ir) cells were first detected in the dermamyotome at embryonic (E) day 9. The number and immunostaining intensity of the P2Y(6) receptor-ir cells increased from E9 to E13, but decreased from E15 to postnatal day 60 in the developing skeletal muscle system. The expression levels of P2Y(6) receptor protein and mRNA increased rapidly from 1 to 5 days after skeletal muscle injury and then decreased almost to the control level from 7 to 10 days, at the beginning of regeneration. P2Y(6) receptor-immunoreactivity was mainly localized to the ends of single myoblasts and myotube processes in the developing and injury-repair skeletal muscle tissues. These data suggest that the P2Y(6) receptor may be involved in the development and regeneration of skeletal muscle, especially in the migration and extension of the myoblast and myotube in developing and regenerating skeletal muscle.

Therapeutic potential of purinergic signalling for diseases of the urinary tract

This review begins with background information about the discovery and conceptual steps contributing to our current knowledge of purinergic signalling. It then deals with several topics concerned with the physiology and pathophysiology of the lower urinary tract, including: the involvement in the voiding reflex of ATP released as a co-transmitter with acetylcholine from parasympathetic nerves supplying the bladder and ATP released from urothelial cells during bladder distension to initiate the voiding reflex via P2X₃ receptors on suburothelial low-threshold sensory nerve fibres; this latter mechanosensory transduction pathway is also involved via high-threshold fibres in the initiation of pain. Treatment of prostate and bladder cancer with ATP not only appears to be effective against the primary tumours, but also improves the systemic symptoms associated with advanced malignancy. There is dual control of the tone of blood vessels: constriction by ATP released as a co-transmitter from sympathetic nerves and vasodilatation via ATP released from endothelial cells during shear stress acting on endothelial P2 receptors to release nitric oxide. A purinergic hypothesis is discussed for the mechanism underlying acupuncture, widely used for the treatment of urinary disorders.

Extracellular ATP signaling during differentiation of C2C12 skeletal muscle cells: role in proliferation

Evidence shows that extracellular ATP signals influence myogenesis, regeneration and physiology of skeletal muscle. Present work was aimed at characterizing the extracellular ATP signaling system of skeletal muscle C2C12 cells during differentiation. We show that mechanical and electrical stimulation produces substantial release of ATP from differentiated myotubes, but not from proliferating myoblasts. Extracellular ATP-hydrolyzing activity is low in myoblasts and high in myotubes, consistent with the increased expression of extracellular enzymes during differentiation. Stimulation of cells with extracellular nucleotides produces substantial Ca(2+) transients, whose amplitude and shape changed during differentiation. Consistently, C2C12 cells express several P2X and P2Y receptors, whose level changes along with maturation stages. Supplementation with either ATP or UTP stimulates proliferation of C2C12 myoblasts, whereas excessive doses were cytotoxic. The data indicate that skeletal muscle development is accompanied by major functional changes in extracellular ATP signaling.

Sensing muscle ischemia: coincident detection of acid and ATP via interplay of two ion channels

Ischemic pain--examples include the chest pain of a heart attack and the leg pain of a 30 s sprint--occurs when muscle gets too little oxygen for its metabolic need. Lactic acid cannot act alone to trigger ischemic pain because the pH change is so small. Here, we show that another compound released from ischemic muscle, adenosine tri-phosphate (ATP), works together with acid by increasing the pH sensitivity of acid-sensing ion channel number 3 (ASIC3), the molecule used by sensory neurons to detect lactic acidosis. Our data argue that ATP acts by binding to P2X receptors that form a molecular complex with ASICs; the receptor on sensory neurons appears to be P2X5, an electrically quiet ion channel. Coincident detection of acid and ATP should confer sensory selectivity for ischemia over other conditions of acidosis.

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.