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TOPICAL REVIEW |
1 Autonomic Neuroscience Centre, Royal Free and University College Medical School, Rowland Hill Street, London NW3 2PF, UK
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(Received 25 April 2008;
accepted after revision 19 May 2008;
first published online 22 May 2008)
Corresponding author G. Burnstock: Autonomic Neuroscience Centre, Royal Free and University College Medical School, Rowland Hill Street, London NW3 2PF, UK. Email: g.burnstock{at}ucl.ac.uk
The concept of purinergic signalling (i.e. ATP as an extracellular signalling molecule and neurotransmitter) was introduced in a Pharmacological Reviews article in 1972 (Burnstock, 1972). Perhaps because of the emphasis on ATP as an intracellular energy source, it was regarded by many as unlikely that such a ubiquitous molecule would be an extracellular messenger. However, after receptor subtypes for ATP and its breakdown product, adenosine, were cloned and characterized in the early 1990s and purinergic synaptic, as well as neuromuscular, transmission was established, the concept is now well established and indeed the field is expanding rapidly (see Burnstock, 2007a). Nevertheless, we are still on the steep slope of the discovery curve and there are plenty of gaps in our knowledge, a number of controversial areas still to be resolved and exciting new areas to explore, some of which will be addressed in this short review.
Mechanism(s) of ATP transport
For many years it was thought that, apart from nerves, the only source of extracellular ATP acting on purinoceptors was damaged or dying cells. However, it is now recognized that ATP release from healthy cells is a physiological mechanism. ATP is released from many non-neuronal cell types during mechanical deformation in response to shear stress, stretch, or osmotic swelling, as well as hypoxia and stimulation by various agents. There is an active debate about the precise transport mechanism(s) involved in ATP release. There is compelling evidence for exocytotic vesicular release of ATP from nerves, but for ATP release from non-neuronal cells various transport mechanisms have been proposed, including ATP-binding cassette (ABC) transporters, connexin or pannexin hemichannels, and possibly plasmalemmal voltage-dependent anion or P2X7 receptor channels, as well as vesicular release. Perhaps surprisingly, evidence was presented that the release of ATP from vascular endothelial cells (Bodin & Burnstock, 2001) and from urothelial cells in the ureter (Knight et al. 2002) is largely vesicular, since monensin and brefeldin A, which interfere with vesicular formation and trafficking, inhibited distension-evoked ATP release, but not gadolinium, an inhibitor of stretch-activated channels, or glibenclamide, an inhibitor of members of the ABC protein family. Since then, exocytotic vesicular release of ATP from osteoblasts, fibroblasts and astrocytes has also been reported. There is increased release of ATP from endothelial cells during acute inflammation. Local probes for real-time measurement of ATP release in biological tissues have been developed recently (see Corriden et al. 2007).
Another question that has been raised over the years concerns the mechanism of transport of ATP into synaptic vesicles, but this might have been resolved recently with the identification of a vesicular nucleotide transporter (Sawada et al. 2008).
Multiple receptors and single cells
On the basis of cloning, studies of transduction mechanisms and pharmacology, receptor subtypes for purines and pyrimidines have been identified: P1 adenosine receptors (A1, A2A, A2B and A3 subtypes), P2X ionotropic receptors (P2X1–7 subtypes forming both homomultimers and heteromultimers) and P2Y metabotropic receptors (P2Y1, P2Y2, P2Y4, P2Y6, P2Y11, P2Y12, P2Y13 and P2Y14), some of which are also sensitive to pyrimidines (Ralevic & Burnstock, 1998; Burnstock, 2007b). Many non-neuronal cells, as well as neurones, express multiple receptor subtypes (see Burnstock & Knight, 2004) and we face questions about how they interact in controlling cellular activity (see Volonté et al. 2006). Some receptors mediate fast signalling, such as secretion, neurotransmission or neuromodulation, while others mediate longer-term ‘trophic’ signalling, regulating cell proliferation, differentiation, motility and death. Some receptors appear to operate only in pathological conditions. Perhaps the best known example of cells with multiple receptors with different functional roles is microglia. P2Y12 and P2X4 receptors mediate chemotaxis and migration to sites of injury, while P2X1 and/or P2X2 receptors may mediate the change of phenotype from ‘resting’ microglia with elaborate processes to the ‘activated’ amoeboid form at the sites of injury. P2Y6 receptors are strongly expressed in activated microglia, where they mediate phagocytotic activity (Koizumi et al. 2007). P2X4 and P2X7 receptors mediate neuropathic pain and P1 (adenosine) receptors mediate neuroprotection, although the underlying mechanisms are not yet understood.
Neuropathic and inflammatory pain
This is an exciting area of strong current interest. P2X3 receptors were cloned in 1995 (Chen et al. 1995; Lewis et al. 1995) and shown to be localized in the periphery on nociceptive sensory fibres arising from sensory ganglia, and on the central terminals in inner lamina 2 of the dorsal horn of the spinal cord (Bradbury et al. 1998). P2X3 receptors and P2X2/3 heteromultimer receptors were shown to mediate purinergic mechanosensory transduction that initiates pain in visceral organs, including bladder, ureter and gut (Burnstock, 2001a).
P2X3 receptors in the central nervous system (CNS) have also been shown to be involved in neuropathic and inflammatory pain (see Jarvis, 2003). Selective antagonists to P2X3 and P2X2/3 receptors prevent allodynia produced by intrathecal administration of ATP in rats (Nakagawa et al. 2007). P2X3 and P2X2/3 receptor-dependent cytosolic phospholipase A2 activity in primary sensory neurons is claimed to be a key event in neuropathic pain (Tsuda et al. 2007; Wirkner et al. 2007). The cyclic AMP sensor Epac appears to play a critical role in P2X3 sensitization in inflammation. Short interfering RNA (siRNA) for the P2X3 receptor relieves chronic neuropathic pain (Dorn et al. 2004). P2X3 receptors mediate heat hyperalgesia in a rat model of trigeminal neuropathic pain (Shinoda et al. 2007).
Perhaps surprisingly, in a seminal paper, Tsuda et al. (2003) showed that, after nerve injury, up-regulated expression of P2X4 receptors on spinal microglia mediate allodynia and supporting evidence has followed (see Trang et al. 2006; Zhang et al. 2008). Further, a number of papers have shown that antagonists to P2X7 receptors expressed on microglia also significantly reduce neuropathic pain (Donnelly-Roberts & Jarvis, 2007; Fulgenzi et al. 2008), which has also been shown to be abolished in P2X7 knockout mice (Chessell et al. 2005). P2Y12 receptor up-regulation in activated microglia has also been claimed recently as a gateway of p38 signalling in neuropathic pain (Kobayashi et al. 2008). However, unresolved issues concern the mechanism underlying these striking effects and the interactions of the different P2 receptor subtypes involved in neuropathic and inflammatory pain (see Khakh & North, 2006).
P2X7 receptors
One of the most controversial areas concerns the multiple identity and modes of action of P2X7 receptors (North, 2002; Sperlágh et al. 2006; Anderson & Nedergaard, 2006; Khakh & North, 2006). Of the P2X receptor family, it is unusual in that it has an extremely long C-terminal sequence (240 amino acids) (Fig. 1A). In addition to the opening of the usual cation channel with low concentrations of ATP, when occupied by high concentrations of ATP (or 2'-3'-O-(4-benzoylbenzoyl)-ATP), large pores permeable to molecules up to 900 Da are opened (Fig. 1B), which lead to apoptosis and cell death in most situations, although proliferative actions via P2X7 receptors have also been described. Occupation of P2X7 receptors leads to blebbing (the function of which is still not clear) and shedding of microvesicles associated with the production of pro-inflammatory cytokines, including interleukin-1β (IL-1β) and transforming growth factor β mRNA expression. The P2X7 receptor field is further complicated by the identification of a number of polymorphisms or spliced variants (Cheewatrakoolpong et al. 2005; Shemon et al. 2006; Fonfria et al. 2008) (Fig. 1C and D) and by a complex story regarding antagonists, some of which work in rats but not in humans and vice versa (Gever et al. 2006; Gunosewoyo et al. 2007).
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Evolution of purinergic signalling
There are many studies of the effect of nucleotides and nucleosides in most invertebrate phyla as well as in lower vertebrates, suggesting that P1 (adenosine), P2X and P2Y receptors were all present early in evolution (see Burnstock, 1996). More recently receptors have been cloned in both Dictyostelium discoideum (a social amoeba) and Schistosoma mansoni (a parasitic trematode) that resemble P2X receptors in mammals (Agboh et al. 2004; Fountain et al. 2007). Also there have been recent papers suggesting that purinergic signalling is involved in regeneration of plants (Demidchik & Maathuis, 2007; Roux & Steinebrunner, 2007). It is important that more molecular studies are carried out to identify purinoceptors in invertebrates, to establish the primitive nature of the purinergic signalling system and to give insights into the advantages of this system leading to its retention in mammals.
Development and regeneration
The importance of purinergic signalling in embryological and postnatal development is emerging (see Burnstock, 2001b; Zimmermann, 2006), where transient expression of purinoceptor subtypes suggests that they may be involved in proliferation, differentiation and cell death of specific sequential cellular steps in the developmental process. However, hopefully this is only the beginning of studies in this interesting area with the need for further work, particularly those relating purinergic messengers to the influence of growth factors and genetic programming.
There are also an increasing number of studies of the role of purines and pyrimidines in regeneration and wound healing (see Abbracchio & Burnstock, 1998; Burnstock, 2007a) and studies of the involvement of purinergic signalling in stem cell activities are just beginning (e.g. Mishra et al. 2006; Heo & Han, 2006; Coppi et al. 2007), but more are needed.
Behavioural studies
At one physiological level, it is now clear that purinergic synaptic neurotransmission and neuromodulation is widespread in the CNS and that there is complex expression of different purinoceptor subtypes on both neurons and glial cells in different regions of the brain (see Burnstock, 2007a). However, there are relatively few studies of the behavioural roles of purines and pyrimidines. There are some reports of their roles in learning and memory, feeding and locomotive behaviour and some hints about their involvement in neurodegenerative diseases and psychiatric disorders, but much more research is needed in this area. Perhaps the main limiting factor for such studies is the absence at present of selective and stable purinoceptor subtype agonists and antagonists that can cross the blood–brain barrier.
Therapeutic developments
There has been considerable enthusiastic exploration in recent years for the use of purinergic agents for the treatment of a number of diseases, including: stroke, thrombosis, bladder incontinence, pain, cystic fibrosis, dry eye, cancer, diabetes and osteoporosis (see Burnstock, 2006). Clopidogrel, a breakdown product of which blocks P2Y12 receptor-mediated platelet aggregation, is now widely used for the treatment of stroke and thrombosis. However, for other diseases, we still await the synthesis by medicinal chemists of small molecule drugs that are stable in vivo and are orally bioavailable. For example, Roche Bioscience (Palo Alto) have recently produced a P2X3 receptor antagonist (RO3) that seems to satisfy these criteria and is in clinical trial for pain and bladder incontinence (Gever et al. 2006).
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References |
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TopAbstractReferences |
|---|
Agboh KC, Webb TE, Evans RJ & Ennion SJ (2004). Functional characterization of a P2X receptor from Schistosoma mansoni. J Biol Chem 279, 41650–41657.
Anderson CM & Nedergaard M (2006). Emerging challenges of assigning P2X7 receptor function and immunoreactivity in neurons. Trends Neurosci 29, 257–262.[CrossRef][Medline]
Atkinson L, Milligan CJ, Buckley NJ & Deuchars J (2002). An ATP-gated ion channel at the cell nucleus. Nature 420, 42.
Bodin P & Burnstock G (2001). Evidence that release of ATP from endothelial cells during increased shear stress is vesicular. J Cardiovasc Pharmacol 38, 900–908.[CrossRef][Medline]
Bradbury EJ, Burnstock G & McMahon SB (1998). The expression of P2X3 purinoreceptors in sensory neurons: effects of axotomy and glial-derived neurotrophic factor. Mol Cell Neurosci 12, 256–268.[CrossRef][Medline]
Burnstock G (1972). Purinergic nerves. Pharmacol Rev 24, 509–581.
Burnstock G (1996). Purinoceptors: ontogeny and phylogeny. Drug Dev Res 39, 204–242.[CrossRef]
Burnstock G (2001a). Purine-mediated signalling in pain and visceral perception. Trends Pharmacol Sci 22, 182–188.[CrossRef][Medline]
Burnstock G (2001b). Purinergic signalling in development. In Handbook of Experimental Pharmacology, vol. 151/I, Purinergic and Pyrimidinergic Signalling I – Molecular, Nervous and Urinogenitary System Function, ed. Abbracchio MP & Williams M, pp. 89–127. Springer-Verlag, Berlin.
Burnstock G (2006). Pathophysiology and therapeutic potential of purinergic signaling. Pharmacol Rev 58, 58–86.
Burnstock G (2007a). Physiology and pathophysiology of purinergic neurotransmission. Physiol Rev 87, 659–797.
Burnstock G (2007b). Purine and pyrimidine receptors. Cell Mol Life Sci 64, 1471–1483.[CrossRef][Medline]
Burnstock G & Knight GE (2004). Cellular distribution and functions of P2 receptor subtypes in different systems. Int Rev Cytol 240, 31–304.[Medline]
Cheewatrakoolpong B, Gilchrest H, Anthes JC & Greenfeder S (2005). Identification and characterization of splice variants of the human P2X7 ATP channel. Biochem Biophys Res Commun 332, 17–27.[CrossRef][Medline]
Chen CC, Akopian AN, Sivilotti L, Colquhoun D, Burnstock G & Wood JN (1995). A P2X purinoceptor expressed by a subset of sensory neurons. Nature 377, 428–431.
Chen L & Brosnan CF (2006). Regulation of immune response by P2X7 receptor. Crit Rev Immunol 26, 499–513.[Medline]
Chessell IP, Hatcher JP, Bountra C, Michel AD, Hughes JP, Green P, Egerton J, Murfin M, Richardson J, Peck WL, Grahames CB, Casula MA, Yiangou Y, Birch R, Anand P & Buell GN (2005). Disruption of the P2X7 purinoceptor gene abolishes chronic inflammatory and neuropathic pain. Pain 114, 386–396.[CrossRef][Medline]
Coppi E, Pugliese AM, Urbani S, Melani A, Cerbai E, Mazzanti B, Bosi A, Saccardi R & Pedata F (2007). ATP modulates cell proliferation and elicits two different electrophysiological responses in human mesenchymal stem cells. Stem Cells 25, 1840–1849.
Corriden R, Insel PA & Junger WG (2007). A novel method using fluorescence microscopy for real-time assessment of ATP release from individual cells. Am J Physiol Cell Physiol 293, C1420–C1425.
Demidchik V & Maathuis FJ (2007). Physiological roles of nonselective cation channels in plants: from salt stress to signalling and development. New Phytol 175, 387–404.[CrossRef][Medline]
Donnelly-Roberts DL & Jarvis MF (2007). Discovery of P2X7 receptor-selective antagonists offers new insights into P2X7 receptor function and indicates a role in chronic pain states. Br J Pharmacol 151, 571–579.[CrossRef][Medline]
Dorn G, Patel S, Wotherspoon G, Hemmings-Mieszczak M, Barclay J, Natt FJ, Martin P, Bevan S, Fox A, Ganju P, Wishart W & Hall J (2004). siRNA relieves chronic neuropathic pain. Nucleic Acids Res 32, e49.
Fonfria E, Clay WC, Levy DS, Goodwin JA, Roman S, Smith GD, Condreay JP & Michel AD (2008). Cloning and pharmacological characterization of the guinea pig P2X7 receptor orthologue. Br J Pharmacol 153, 544–556.[CrossRef][Medline]
Fountain SJ, Parkinson K, Young MT, Cao L, Thompson CR & North RA (2007). An intracellular P2X receptor required for osmoregulation in Dictyostelium discoideum. Nature 448, 200–203.
Fulgenzi A, Ticozzi P, Gabel CA, Dell AG, Quattrini A, Franzone JS & Ferrero ME (2008). Periodate oxidized ATP. (oATP) reduces hyperalgesia in mice: involvement of P2X7 receptors and implications for therapy. Int J Immunopathol Pharmacol 21, 61–71.[Medline]
Gever J, Cockayne DA, Dillon MP, Burnstock G & Ford APDW (2006). Pharmacology of P2X channels. Pflugers Arch 452, 513–537.[CrossRef][Medline]
Gunosewoyo H, Coster MJ & Kassiou M (2007). Molecular probes for P2X7 receptor studies. Curr Med Chem 14, 1505–1523.[CrossRef][Medline]
Heo JS & Han HJ (2006). ATP stimulates mouse embryonic stem cell proliferation via protein kinase C, phosphatidylinositol 3-kinase/Akt, and mitogen-activated protein kinase signaling pathways. Stem Cells 24, 2637–2648.
Jarvis MF (2003). Contributions of P2X3 homomeric and heteromeric channels to acute and chronic pain. Expert Opin Ther Targets 7, 513–522.[CrossRef][Medline]
Khakh BS & North RA (2006). P2X receptors as cell-surface ATP sensors in health and disease. Nature 442, 527–532.
Knight GE, Bodin P, de Groat WC & Burnstock G (2002). ATP is released from guinea pig ureter epithelium on distension. Am J Physiol Renal Physiol 282, F281–F288.
Kobayashi K, Yamanaka H, Fukuoka T, Dai Y, Obata K & Noguchi K (2008). P2Y12 receptor upregulation in activated microglia is a gateway of p38 signaling and neuropathic pain. J Neurosci 28, 2892–2902.
Koizumi S, Shigemoto-Mogami Y, Nasu-Tada K, Shinozaki Y, Ohsawa K, Tsuda M, Joshi BV, Jacobson KA, Kohsaka S & Inoue K (2007). UDP acting at P2Y6 receptors is a mediator of microglial phagocytosis. Nature 446, 1091–1095.
Lewis C, Neidhart S, Holy C, North RA, Buell G & Surprenant A (1995). Coexpression of P2X2 and P2X3 receptor subunits can account for ATP-gated currents in sensory neurons. Nature 377, 432–435.
Locovei S, Scemes E, Qiu F, Spray DC & Dahl G (2007). Pannexin1 is part of the pore forming unit of the P2X7 receptor death complex. FEBS Lett 581, 483–488.[CrossRef][Medline]
Mishra SK, Braun N, Shukla V, Füllgrabe M, Schomerus C, Korf HW, Gachet C, Ikehara Y, Sévigny J, Robson SC & Zimmermann H (2006). Extracellular nucleotide signaling in adult neural stem cells: synergism with growth factor-mediated cellular proliferation. Development 133, 675–684.
Nakagawa T, Wakamatsu K, Zhang N, Maeda S, Minami M, Satoh M & Kaneko S (2007). Intrathecal administration of ATP produces long-lasting allodynia in rats: differential mechanisms in the phase of the induction and maintenance. Neuroscience 147, 445–455.[CrossRef][Medline]
North RA (2002). Molecular physiology of P2X receptors. Physiol Rev 82, 1013–1067.
Pelegrin P & Surprenant A (2007). Pannexin-1 couples to maitotoxin- and nigericin-induced interleukin-1β release through a dye uptake-independent pathway. J Biol Chem 282, 2386–2394.
Ralevic V & Burnstock G (1998). Receptors for purines and pyrimidines. Pharmacol Rev 50, 413–492.
Roux SJ & Steinebrunner I (2007). Extracellular ATP: an unexpected role as a signaler in plants. Trends Plant Sci 12, 522–527.[CrossRef][Medline]
Sawada K, Echigo N, Juge N, Miyaji T, Otsuka M, Omote H, Yamamoto A & Moriyama Y (2008). Identification of a vesicular nucleotide transporter. Proc Natl Acad Sci U S A 105, 5683–5686.
Shemon AN, Sluyter R, Fernando SL, Clarke AL, Dao-Ung LP, Skarratt KK, Saunders BM, Tan KS, Gu BJ, Fuller SJ, Britton WJ, Petrou S & Wiley JS (2006). A Thr357 to Ser polymorphism in homozygous and compound heterozygous subjects causes absent or reduced P2X7 function and impairs ATP-induced mycobacterial killing by macrophages. J Biol Chem 281, 2079–2086.
Shinoda M, Kawashima K, Ozaki N, Asai H, Nagamine K & Sugiura Y (2007). P2X3 receptor mediates heat hyperalgesia in a rat model of trigeminal neuropathic pain. J Pain 8, 588–597.[CrossRef][Medline]
Sperlágh B, Vizi ES, Wirkner K & Illes P (2006). P2X7 receptors in the nervous system. Prog Neurobiol 78, 327–346.[CrossRef][Medline]
Trang T, Beggs S & Salter MW (2006). Purinoceptors in microglia and neuropathic pain. Pflugers Arch 452, 645–652.[CrossRef][Medline]
Tsuda M, Hasegawa S & Inoue K (2007). P2X receptors-mediated cytosolic phospholipase A2 activation in primary afferent sensory neurons contributes to neuropathic pain. J Neurochem 103, 1408–1416.[CrossRef][Medline]
Tsuda M, Shigemoto-Mogami Y, Koizumi S, Mizokoshi A, Kohsaka S, Salter MW & Inoue K (2003). P2X4 receptors induced in spinal microglia gate tactile allodynia after nerve injury. Nature 424, 778–783.
Volonté C, Amadio S, D'Ambrosi N, Colpi M & Burnstock G (2006). P2 receptor web: complexity and fine-tuning. Pharmacol Ther 112, 264–280.[CrossRef][Medline]
Wirkner K, Sperlagh B & Illes P (2007). P2X3 receptor involvement in pain states. Mol Neurobiol 36, 165–183.[CrossRef][Medline]
Zhang Z, Zhang ZY, Fauser U & Schluesener HJ (2008). Mechanical allodynia and spinal up-regulation of P2X4 receptor in experimental autoimmune neuritis rats. Neuroscience 152, 495–501.[CrossRef][Medline]
Zimmermann H (2006). Nucleotide signaling in nervous system development. Pflugers Arch 452, 573–588.[CrossRef][Medline]
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