Heterogeneity of P2X Receptors in Sympathetic Neurons: Contribution of Neuronal P2X1 Receptors Revealed Using Knockout Mice

Purinergic Signaling and its Role in the Stem Cell Differentiation

Purinergic signaling is a mechanism in which extracellular purines and pyrimidines interact with specialized cell surface receptors known as purinergic receptors. These receptors are divided into two families of P1 and P2 receptors, each responding to different nucleosides and nucleotides. P1 receptors are activated by adenosine, while P2 receptors are activated by pyrimidine and purines. P2X receptors are ligand-gated ion channels, including seven subunits (P2X1-7). However, P2Y receptors are the G-protein coupled receptors comprising eight subtypes (P2Y1/2/4/6/11/12/13/14). The disorder in purinergic signaling leads to various health-related issues and diseases. In various aspects, it influences the activity of non-neuronal cells and neurons. The molecular mechanism of purinergic signaling provides insight into treating various human diseases. On the contrary, stem cells have been investigated for therapeutic applications. Purinergic signaling has shown promising effect in stem cell engraftment. The immune system promotes the autocrine and paracrine mechanisms and releases the significant factors essential for successful stem cell therapy. Each subtype of purinergic receptor exerts a beneficial effect on the damaged tissue. The most common effect caused by purinergic signaling is the proliferation and differentiation that treat different health-related conditions.

The P2X1 receptor as a therapeutic target

Within the family of purinergic receptors, the P2X1 receptor is a ligand-gated ion channel that plays a role in urogenital, immune and cardiovascular function. Specifically, the P2X1 receptor has been implicated in controlling smooth muscle contractions of the vas deferens and therefore has emerged as an exciting drug target for male contraception. In addition, the P2X1 receptor contributes to smooth muscle contractions of the bladder and is a target to treat bladder dysfunction. Finally, platelets and neutrophils have populations of P2X1 receptors that could be targeted for thrombosis and inflammatory conditions. Drugs that specifically target the P2X1 receptor have been challenging to develop, and only recently have small molecule antagonists of the P2X1 receptor been available. However, these ligands need further biological validation for appropriate selectivity and drug-like properties before they will be suitable for use in preclinical models of disease. Although the atomic structure of the P2X1 receptor has yet to be determined, the recent discovery of several other P2X receptor structures and improvements in the field of structural biology suggests that this is now a distinct possibility. Such efforts may significantly improve drug discovery efforts at the P2X1 receptor.

ATP-sensitive K+ channels and mitochondrial permeability transition pore mediate effects of hydrogen sulfide on cytosolic Ca2+ homeostasis and insulin secretion in β-cells

Hydrogen sulfide (H₂S) is endogenously produced in pancreatic ß cells and its level is elevated in diabetes. Here, we report that H₂S affects insulin secretion via two mechanisms that converge on cytosolic free Ca²⁺ ([Ca²⁺]_i), a key mediator of insulin exocytosis. Cellular calcium imaging, using Fura-2 or Fluo-4, showed that exposure of INS-1E cells to H₂S (30-100 μM) reduced both [Ca²⁺]_i levels (by 21.7 ± 2.3%) and oscillation frequency (p < 0.01, n = 4). Consistent with a role of plasma membrane K_ATP channels (plasma-K_ATP), the effects of H₂S on [Ca²⁺]_i were blocked by gliclazide (a blocker of plasma-K_ATP channels), but were mimicked by diazoxide (an activator of plasma-K_ATP channels). Surprisingly, when Ca²⁺ entry via plasma membrane was inhibited using Ca²⁺-free external solutions, H₂S increased [Ca²⁺]_i by 39.7 ± 3.6% suggesting Ca²⁺ release from intracellular stores. H₂S-induced [Ca²⁺]_i increases were abolished by either FCCP (which depletes Ca²⁺ stored in mitochondria) or cyclosporine A (an inhibitor of mitochondrial permeability transition pore, mPTP) suggesting that H₂S induces Ca²⁺ release from mitochondria. Measurement of mitochondrial membrane potential (MMP) suggested that H₂S causes MMP depolarization, which was blocked by cyclosporine A. Finally, insulin measurements by ELISA indicated that H₂S decreased insulin release from INS-1E cells, but after plasma membrane Ca²⁺ entry was blocked by nifedipine, H₂S-induced mitochondrial Ca²⁺ release is able to increase insulin release. Together, our results indicate that H₂S has dual effects on insulin release suggesting that, with different metabolic conditions, H₂S may differentially modulate the insulin release from pancreatic ß cells and play a role in ß cell dysfunction.

Purine and purinergic receptors

Adenosine 5′-triphosphate acts as an extracellular signalling molecule (purinergic signalling), as well as an intracellular energy source. Adenosine 5′-triphosphate receptors have been cloned and characterised. P1 receptors are selective for adenosine, a breakdown product of adenosine 5′-triphosphate after degradation by ectonucleotidases. Four subtypes are recognised, A₁, A_2A, A_2B and A₃ receptors. P2 receptors are activated by purine and by pyrimidine nucleotides. P2X receptors are ligand-gated ion channel receptors (seven subunits (P2X1-7)), which form trimers as both homomultimers and heteromultimers. P2Y receptors are G protein-coupled receptors (eight subtypes (P2Y_{1/2/4/6/11/12/13/14})). There is both purinergic short-term signalling and long-term (trophic) signalling. The cloning of P2X-like receptors in primitive invertebrates suggests that adenosine 5′-triphosphate is an early evolutionary extracellular signalling molecule. Selective purinoceptor agonists and antagonists with therapeutic potential have been developed for a wide range of diseases, including thrombosis and stroke, dry eye, atherosclerosis, kidney failure, osteoporosis, bladder incontinence, colitis, neurodegenerative diseases and cancer.

Neurotrophic Factors NGF, GDNF and NTN Selectively Modulate HSV1 and HSV2 Lytic Infection and Reactivation in Primary Adult Sensory and Autonomic Neurons

Herpes simplex viruses (HSV1 and HSV2) establish latency in peripheral ganglia after ocular or genital infection, and can reactivate to produce different patterns and frequencies of recurrent disease. Previous studies showed that nerve growth factor (NGF) maintains HSV1 latency in embryonic sympathetic and sensory neurons. However, adult sensory neurons are no longer dependent on NGF for survival, some populations cease expression of NGF receptors postnatally, and the viruses preferentially establish latency in different populations of sensory neurons responsive to other neurotrophic factors (NTFs). Thus, NGF may not maintain latency in adult sensory neurons. To identify NTFs important for maintaining HSV1 and HSV2 latency in adult neurons, we investigated acute and latently-infected primary adult sensory trigeminal (TG) and sympathetic superior cervical ganglia (SCG) after NTF removal. NGF and glial cell line-derived neurotrophic factor (GDNF) deprivation induced HSV1 reactivation in adult sympathetic neurons. In adult sensory neurons, however, neurturin (NTN) and GDNF deprivation induced HSV1 and HSV2 reactivation, respectively, while NGF deprivation had no effects. Furthermore, HSV1 and HSV2 preferentially reactivated from neurons expressing GFRα2 and GFRα1, the high affinity receptors for NTN and GDNF, respectively. Thus, NTN and GDNF play a critical role in selective maintenance of HSV1 and HSV2 latency in primary adult sensory neurons.

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.

E-NTPDase1/CD39 modulates renin release from heart mast cells during ischemia/reperfusion: a novel cardioprotective role

Ischemia/reperfusion (I/R) elicits renin release from cardiac mast cells (MC), thus activating a local renin-angiotensin system (RAS), culminating in ventricular fibrillation. We hypothesized that in I/R, neurogenic ATP could degranulate juxtaposed MC and that ecto-nucleoside triphosphate diphosphohydrolase 1/CD39 (CD39) on MC membrane could modulate ATP-induced renin release. We report that pharmacological inhibition of CD39 in a cultured human mastocytoma cell line (HMC-1) and murine bone marrow-derived MC with ARL67156 (100 µM) increased ATP-induced renin release (≥2-fold), whereas purinergic P2X7 receptors (P2X7R) blockade with A740003 (3 µM) prevented it. Likewise, CD39 RNA silencing in HMC-1 increased ATP-induced renin release (≥2-fold), whereas CD39 overexpression prevented it. Acetaldehyde, an I/R product (300 µM), elicited an 80% increase in ATP release from HMC-1, in turn, causing an autocrine 20% increase in renin release. This effect was inhibited or potentiated when CD39 was overexpressed or silenced, respectively. Moreover, P2X7R silencing prevented ATP- and acetaldehyde-induced renin release. I/R-induced RAS activation in ex vivo murine hearts, characterized by renin and norepinephrine overflow and ventricular fibrillation, was potentiated (∼2-fold) by CD39 inhibition, an effect prevented by P2X7R blockade. Our data indicate that by regulating ATP availability at the MC surface, CD39 modulates local renin release and thus, RAS activation, ultimately exerting a cardioprotective effect.

Heteromeric assembly of P2X subunits

Transcripts and/or proteins of P2X receptor (P2XR) subunits have been found in virtually all mammalian tissues. Generally more than one of the seven known P2X subunits have been identified in a given cell type. Six of the seven cloned P2X subunits can efficiently form functional homotrimeric ion channels in recombinant expression systems. This is in contrast to other ligand-gated ion channel families, such as the Cys-loop or glutamate receptors, where homomeric assemblies seem to represent the exception rather than the rule. P2XR mediated responses recorded from native tissues rarely match exactly the biophysical and pharmacological properties of heterologously expressed homomeric P2XRs. Heterotrimerization of P2X subunits is likely to account for this observed diversity. While the existence of heterotrimeric P2X2/3Rs and their role in physiological processes is well established, the composition of most other P2XR heteromers and/or the interplay between distinct trimeric receptor complexes in native tissues is not clear. After a description of P2XR assembly and the structure of the intersubunit ATP-binding site, this review summarizes the distribution of P2XR subunits in selected mammalian cell types and the biochemically and/or functionally characterized heteromeric P2XRs that have been observed upon heterologous co-expression of P2XR subunits. We further provide examples where the postulated heteromeric P2XRs have been suggested to occur in native tissues and an overview of the currently available pharmacological tools that have been used to discriminate between homo- and heteromeric P2XRs.

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.

P2 receptors for extracellular nucleotides in the central nervous system: role of P2X7 and P2Y₂ receptor interactions in neuroinflammation

Extracellular nucleotides induce cellular responses in the central nervous system (CNS) through the activation of ionotropic P2X and metabotropic P2Y nucleotide receptors. Activation of these receptors regulates a wide range of physiological and pathological processes. In this review, we present an overview of the current literature regarding P2X and P2Y receptors in the CNS with a focus on the contribution of P2X7 and P2Y(2) receptor-mediated responses to neuroinflammatory and neuroprotective mechanisms.

The participation of plasma membrane hemichannels to purinergic signaling

The field of hemichannels is closely related to the purinergic signaling and both areas have been growing in parallel. Hemichannels open in response to a wide range of stressful conditions, such as ischemia, pressure or swelling. Hemichannels represent an important mechanism for the cellular release of adenosine 5'-triphosphate (ATP), which is an agonist of the P2Y and P2X family of purinergic receptors. Therefore, hemichannels are key molecules in the regulation of purinergic receptor activation, during physiological and pathophysiological conditions. Furthermore, purinergic receptor activation can also lead to the opening of hemichannels and the subsequent amplification of purinergic signaling via a positive signaling feedback loop, giving rise to the concept of ATP-induced ATP release. Purinergic receptor signaling is involved in regulating many physiological and pathophysiological processes. P2Y receptors activate inositol trisphosphate and transiently increase intracellular calcium. This signaling opens both connexin and pannexin channels, therefore contributing to the expansion of calcium waves across astrocytes and epithelial cells. In addition, several of the P2X receptor subtypes, including the P2X2, P2X4 and P2X7 receptors, activate select cellular permeation pathways to large molecules, including the pannexin-1 channels, which are involved in the initiation of inflammatory responses and cell death. Consequently, the interplay between purinergic receptors and hemichannels could represent a novel target with substantial therapeutic implications in areas such as chronic pain, inflammation or atherosclerosis. This article is part of a Special Issue entitled: The communicating junctions, roles and dysfunctions.

Activation and regulation of purinergic P2X receptor channels

Mammalian ATP-gated nonselective cation channels (P2XRs) can be composed of seven possible subunits, denoted P2X1 to P2X7. Each subunit contains a large ectodomain, two transmembrane domains, and intracellular N and C termini. Functional P2XRs are organized as homomeric and heteromeric trimers. This review focuses on the binding sites involved in the activation (orthosteric) and regulation (allosteric) of P2XRs. The ectodomains contain three ATP binding sites, presumably located between neighboring subunits and formed by highly conserved residues. The detection and coordination of three ATP phosphate residues by positively charged amino acids are likely to play a dominant role in determining agonist potency, whereas an AsnPheArg motif may contribute to binding by coordinating the adenine ring. Nonconserved ectodomain histidines provide the binding sites for trace metals, divalent cations, and protons. The transmembrane domains account not only for the formation of the channel pore but also for the binding of ivermectin (a specific P2X4R allosteric regulator) and alcohols. The N- and C- domains provide the structures that determine the kinetics of receptor desensitization and/or pore dilation and are critical for the regulation of receptor functions by intracellular messengers, kinases, reactive oxygen species and mercury. The recent publication of the crystal structure of the zebrafish P2X4.1R in a closed state provides a major advance in the understanding of this family of receptor channels. We will discuss data obtained from numerous site-directed mutagenesis experiments accumulated during the last 15 years with reference to the crystal structure, allowing a structural interpretation of the molecular basis of orthosteric and allosteric ligand actions.

P2X1 receptor mobility and trafficking; regulation by receptor insertion and activation

P2X1 receptors for ATP contribute to signalling in a variety of cell types and following stimulation undergo rapid desensitisation (within 1 s), and require ∼5 min to recover. In HEK293 cells P2X1 receptors C-terminally tagged with enhanced green fluorescent protein (P2X1-eGFP) were predominantly expressed at the cell surface. Following > 90% photo-bleaching of P2X1-eGFP within a 6 μm² circle at the cell surface fluorescence recovery after photo-bleaching (FRAP) was fit with a time constant of ∼60 s and recovered to ∼75% of pre-bleach levels. Following activation of the P2X1 receptor with α,β-methylene ATP the associated calcium influx doubled the FRAP recovery rate. The protein synthesis inhibitor cycloheximide had only a small effect on repeated FRAP and indicated a limited contribution of new P2X1 receptors to the FRAP. Inhibition of trafficking with brefeldin A reduced recovery and this effect could be reversed following receptor activation. In contrast, the dynamin inhibitor dynasore had no effect on FRAP under unstimulated conditions but reduced the level of recovery following agonist stimulation. In functional studies both brefeldin A and dynasore increased the recovery time from desensitisation. Taken together these studies demonstrate for the first time an important role of receptor recycling on P2X1 receptor responsiveness.

P2 receptor-mediated modulation of neurotransmitter release-an update

Presynaptic nerve terminals are equipped with a number of presynaptic auto- and heteroreceptors, including ionotropic P2X and metabotropic P2Y receptors. P2 receptors serve as modulation sites of transmitter release by ATP and other nucleotides released by neuronal activity and pathological signals. A wide variety of P2X and P2Y receptors expressed at pre- and postsynaptic sites as well as in glial cells are involved directly or indirectly in the modulation of neurotransmitter release. Nucleotides are released from synaptic and nonsynaptic sites throughout the nervous system and might reach concentrations high enough to activate these receptors. By providing a fine-tuning mechanism these receptors also offer attractive sites for pharmacotherapy in nervous system diseases. Here we review the rapidly emerging data on the modulation of transmitter release by facilitatory and inhibitory P2 receptors and the receptor subtypes involved in these interactions.

Inositol triphosphate-mediated Ca2+ signals direct purinergic P2Y receptor regulation of neuronal ion channels

Purinergic P2Y receptors are one of four types of G(q/11)-coupled receptors in rat superior cervical ganglia (SCG) sympathetic neurons. In cultured SCG neurons, purinergic and bradykinin suppression of I(M) were similar in magnitude and somewhat less than that by muscarinic agonists. The effects of the P2Y receptor agonist UTP on neuronal excitability and discharge properties were studied. Under current clamp, UTP increased action potential (AP) firing in response to depolarizing current steps, depolarized the resting potential, decreased the threshold current required to fire an AP, and decreased spike-frequency adaptation. These effects were very similar to those resulting from bradykinin stimulation and not as profound as from muscarinic stimulation or full M-current blockade. We then examined the P2Y mechanism of action. Like bradykinin, but unlike muscarinic, purinergic stimulation induced rises in intracellular [Ca(2+)](i). Tests using expression of IP(3)"sponge" or IP(3) phosphatase constructs implicated IP(3) accumulation as necessary for purinergic suppression of I(M). Overexpression of wild-type or dominant-negative calmodulin (CaM) implicated Ca(2+)/CaM in the purinergic action. Both sets of results were similar to bradykinin, and opposite to muscarinic, suppression. We also examined modulation of Ca(2+) channels. As for bradykinin, purinergic stimulation did not suppress I(Ca), unless neuronal calcium sensor-1 (NCS-1) activity was blocked by a dominant-negative NCS-1 construct. Our results indicate that P2Y receptors modulate M-type channels in SCG cells via IP(3)-mediated [Ca(2+)](i) signals in concert with CaM and not by depletion of phosphatidylinositol-4, 5-biphosphate. We group purinergic P2Y and bradykinin B(2) receptors together as having a common mode of action.

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.

Metabotropic P2Y receptors inhibit P2X3 receptor-channels via G protein-dependent facilitation of their desensitization

BACKGROUND AND PURPOSE

The aim of the present study was to investigate whether the endogenous metabotropic P2Y receptors modulate ionotropic P2X(3) receptor-channels.

EXPERIMENTAL APPROACH

Whole-cell patch-clamp experiments were carried out on HEK293 cells permanently transfected with human P2X(3) receptors (HEK293-hP2X(3) cells) and rat dorsal root ganglion (DRG) neurons.

KEY RESULTS

In both cell types, the P2Y(1,12,13) receptor agonist, ADP-beta-S, inhibited P2X(3) currents evoked by the selective agonist, alpha,beta-methylene ATP (alpha,beta-meATP). This inhibition could be markedly counteracted by replacing in the pipette solution the usual GTP with GDP-beta-S, a procedure known to block all G protein heterotrimers. P2X(3) currents evoked by ATP, activating both P2Y and P2X receptors, caused a smaller peak amplitude and desensitized faster than those currents evoked by the selective P2X(3) receptor agonist alpha,beta-meATP. In the presence of intracellular GDP-beta-S, ATP- and alpha,beta-meATP-induced currents were identical. Recovery from P2X(3) receptor desensitization induced by repetitive ATP application was slower than the recovery from alpha,beta-meATP-induced desensitization. When G proteins were blocked by intracellular GDP-beta-S, the recovery from the ATP- and alpha,beta-meATP-induced desensitization were of comparable speed.

CONCLUSIONS AND IMPLICATIONS

Our results suggest that the activation of P2Y receptors G protein-dependently facilitates the desensitization of P2X(3) receptors and suppresses the recovery from the desensitized state. Hence, the concomitant stimulation of P2X(3) and P2Y receptors of DRG neurons by ATP may result both in an algesic effect and a partly counterbalancing analgesic activity.

Contribution of P2X1 receptor intracellular basic residues to channel properties

The intracellular amino and carboxy termini of P2X receptors have been shown to contribute to the regulation of ATP evoked currents. In this study we produced, and expressed in Xenopus oocytes, individual alanine point mutants of positively charged amino acids (eight lysine, seven arginine and one histidine) in the intracellular domains of the human P2X1 receptor. The majority of these mutations had no effect on the amplitude, time-course or rectification of ATP evoked currents. In contrast the mutant K367A was expressed at normal levels at the cell surface however ATP evoked currents were reduced by >99% and desensitised more rapidly demonstrating a role of K367 in channel regulation. This is similar to that previously described for T18A mutant channels. Co-expression of T18A and K367A mutant P2X1 receptors produced larger ATP evoked responses than either mutant alone and suggests that these amino and carboxy terminal regions interact to regulate channel function.

Excitatory P2-receptors at sympathetic axon terminals: role in temperature control of cutaneous blood flow

The mechanisms underlying the reduction in cutaneous blood flow in response to cooling are only partially understood. A study published in this issue of the British Journal of Pharmacology now provides evidence for the involvement of excitatory P2-receptors located at sympathetic axon terminals in the cooling-induced vasoconstriction in the skin. Cooling appears to cause the release of adenine nucleotides followed by the activation of excitatory presynaptic P2-receptors at noradrenergic axon terminals. Activation of these excitatory P2-receptors induces the release of noradrenaline, which subsequently causes constriction of blood vessels in the skin by action on smooth muscle alpha(1)- and alpha(2)-adrenoceptors. The commentary discusses the implication of the results and remaining questions.

Pharmacology of P2X channels

Significant progress in understanding the pharmacological characteristics and physiological importance of homomeric and heteromeric P2X channels has been achieved in recent years. P2X channels, gated by ATP and most likely trimerically assembled from seven known P2X subunits, are present in a broad distribution of tissues and are thought to play an important role in a variety of physiological functions, including peripheral and central neuronal transmission, smooth muscle contraction, and inflammation. The known homomeric and heteromeric P2X channels can be distinguished from each other on the basis of pharmacological differences when expressed recombinantly in cell lines, but whether this pharmacological classification holds true in native cells and in vivo is less well-established. Nevertheless, several potent and selective P2X antagonists have been discovered in recent years and shown to be efficacious in various animal models including those for visceral organ function, chronic inflammatory and neuropathic pain, and inflammation. The recent advancement of drug candidates targeting P2X channels into human trials, confirms the medicinal exploitability of this novel target family and provides hope that safe and effective medicines for the treatment of disorders involving P2X channels may be identified in the near future.

Molecular properties of P2X receptors

P2X receptors for adenosine tri-phosphate (ATP) are a distinct family of ligand-gated cation channels with two transmembrane domains, intracellular amino and carboxy termini and a large extracellular ligand binding loop. Seven genes (P2X(1-7)) have been cloned and the channels form as either homo or heterotrimeric channels giving rise to a wide range of phenotypes. This review aims to give an account of recent work on the molecular basis of the properties of P2X receptors. In particular, to consider emerging information on the assembly of P2X receptor subunits, channel regulation and desensitisation, targeting, the molecular basis of drug action and the functional contribution of P2X receptors to physiological processes.

An evaluation of antibody detection of the P2X1 receptor subunit in the CNS of wild type and P2X1-knockout mice

P2X receptors in the nervous system are involved in both neuronal and glial functions. Pharmacological tools to differentiate the seven P2X subunits are limited, hence localisation studies have examined their distribution. Such studies have provided conflicting data on the localisation of the P2X1 receptor subunit in the CNS. We therefore tested for the presence of the P2X1 subunit in the CNS, using tissue from P2X1 subunit knockout (KO) mice to test the antisera. There was no difference between the CNS of wild type (WT) and KO mice using anti-P2X1 in immunohistochemistry and Western blots. In contrast, the bladder reacted positively only in WT tissue. Thus the P2X1 subunit antibodies cross-react with non-P2X1 receptor proteins in the CNS. Antibody specificity tests should therefore be conducted in the specific tissue to be examined and on KO tissue where available.

Developmental changes in heteromeric P2X(2/3) receptor expression in rat sympathetic ganglion neurons

We have used whole cell patch clamp recording and immunohistochemistry to investigate the expression of P2X(2/3) receptors in rat superior cervical ganglion neurons during late embryonic and early post-natal development. Neurons from E18 and P1 animals responded to the nicotinic agonist dimethylphenylpiperazinium (DMPP), and the purinoceptor agonists ATP and alpha,beta-meATP with sustained inward currents. Responsiveness to DMPP was maintained at P 17, while that to ATP declined dramatically, and responses to alpha,beta-meATP were rarely detected. Immunohistochemistry for the P2X(3) subunit revealed widespread staining in superior cervical ganglia from P1 rats, but little immunoreactivity in ganglia from P17 animals. In neurons from P1 animals, the response to alpha,beta-meATP exhibited pharmacological properties of the heteromeric P2X(2/3) receptor. In conclusion, sympathetic neurons of the rat superior cervical ganglion are more responsive to ATP and alpha,beta-meATP at birth and during the early post-natal period, due largely to the expression of the P2X(3) subunit, but these responses are much reduced in mature rats.

Contribution of conserved polar glutamine, asparagine and threonine residues and glycosylation to agonist action at human P2X1 receptors for ATP

The role of conserved polar glutamine, asparagine and threonine residues in the large extracellular loop, and glycosylation, to agonist action at human P2X1 receptors was tested by generating alanine substitution mutants. For the majority of mutants (Q56A, Q95A, T104A, T109A, Q112A, Q114A, T146A, N153A, T158A, N184A, N191A, N242A, N300A) alanine substitution had no effect on ATP potency. The mutants Q95A, Q112A, Q114A and T158A showed changes in efficacy for the partial agonists BzATP and Ap5A, suggesting that these polar residues may contribute to the gating of the channel. The mutants T186A, N204A and N290A had six-, three- and 60-fold decreases in ATP potency, respectively. For T186A and N290A, the partial agonists BzATP and Ap5A were no longer agonists but still bind to the receptor as shown by the ability to modulate the response to co-applied ATP. N153, N184 and N242 are glycosylated in the endoplasmic reticulum and N300 acquires complex glycosylation in the golgi. These results aid in refining a model for ATP binding at the P2X1 receptor where the residues F185T186, and the conserved triplet N290F291R292, are likely to play a role in ATP action at the receptor.

P2X2 knockout mice and P2X2/P2X3 double knockout mice reveal a role for the P2X2 receptor subunit in mediating multiple sensory effects of ATP

Extracellular ATP plays a role in nociceptive signalling and sensory regulation of visceral function through ionotropic receptors variably composed of P2X2 and P2X3 subunits. P2X2 and P2X3 subunits can form homomultimeric P2X2, homomultimeric P2X3, or heteromultimeric P2X2/3 receptors. However, the relative contribution of these receptor subtypes to afferent functions of ATP in vivo is poorly understood. Here we describe null mutant mice lacking the P2X2 receptor subunit (P2X2-/-) and double mutant mice lacking both P2X2 and P2X3 subunits (P2X2/P2X3(Dbl-/-)), and compare these with previously characterized P2X3-/- mice. In patch-clamp studies, nodose, coeliac and superior cervical ganglia (SCG) neurones from wild-type mice responded to ATP with sustained inward currents, while dorsal root ganglia (DRG) neurones gave predominantly transient currents. Sensory neurones from P2X2-/- mice responded to ATP with only transient inward currents, while sympathetic neurones had barely detectable responses. Neurones from P2X2/P2X3(Dbl-/-) mice had minimal to no response to ATP. These data indicate that P2X receptors on sensory and sympathetic ganglion neurones involve almost exclusively P2X2 and P2X3 subunits. P2X2-/- and P2X2/P2X3(Dbl-/-) mice had reduced pain-related behaviours in response to intraplantar injection of formalin. Significantly, P2X3-/-, P2X2-/-, and P2X2/P2X3(Dbl-/-) mice had reduced urinary bladder reflexes and decreased pelvic afferent nerve activity in response to bladder distension. No deficits in a wide variety of CNS behavioural tests were observed in P2X2-/- mice. Taken together, these data extend our findings for P2X3-/- mice, and reveal an important contribution of heteromeric P2X2/3 receptors to nociceptive responses and mechanosensory transduction within the urinary bladder.

Mutagenesis studies of conserved proline residues of human P2X receptors for ATP indicate that proline 272 contributes to channel function

Proline residues can play a major role in the secondary structure of proteins. In the extracellular ATP binding loop of P2X receptors there are four totally conserved proline residues (P2X1 receptor numbering; P93, P166, P228 and P272) and three less conserved residues P196 (six of seven isoforms), P174 and P225 (five of seven isoforms). We have mutated individual conserved proline residues in the human P2X1 receptor and determined their properties. Mutants were expressed in Xenopus oocytes and characterized using a two-electrode voltage clamp. Mutants P166A, P174A, P196A, P225A and P228A had no effect on ATP potency compared with wild-type and P93A had a fourfold decrease in ATP potency. The P272A, P272D and P272K receptor mutants were expressed at the cell surface; however, these mutants were non-functional. In contrast, P272I, P272G and P272F produced functional channels, with either no effect or a 2.5- or 6.5-fold increase in ATP potency, respectively. At P272F receptors the apparent affinity of the ATP analogue antagonist 2',3'-O-(2,4,6-trinitrophenyl)-ATP was increased by 12.5-fold. These results suggest that individual proline residues are not essential for normal P2X receptor function and that the receptor conformation around P272 contributes to ATP binding at the receptor.

Biochemical and functional evidence for heteromeric assembly of P2X1 and P2X4 subunits

P2X receptors are ligand-gated ion channels activated by extracellular ATP. In expression systems, P2X subunits form homo- and heterotrimeric receptors. Heteromerization is also likely to occur in vivo as (i) most P2X subunits show overlapping distribution in different tissues and (ii) the functional properties of many native P2X receptors differ from those of heterologously expressed homomeric receptors. Here, we used the Xenopus laevis oocyte expression system to test for heteromerization of P2X1 and P2X4 subunits. Upon co-injection, P2X4 subunits were co-purified with hexahistidyl-tagged P2X1 subunits indicating heteromerization. Blue native polyacrylamide gel electrophoresis (BN-PAGE) analysis of these P2X complexes excluded artificial aggregation and confirmed that both subunits were present in trimeric complexes of the same size. Two-electrode voltage-clamp experiments revealed functional P2X receptors with kinetic properties resembling homomeric P2X4 receptors and a pharmacological profile similar to homomeric P2X1 receptors. Thus, application of alpha,beta-methylene ATP evoked a slowly desensitizing current sensitive to the antagonists suramin and 2',3'-O-(2,4,6-trinitrophenyl)-ATP. This study provides for the first time biochemical and functional evidence for the formation of heteromeric P2X(1+4) receptors. These receptors may account for native P2X mediated responses that until now could not be correlated with previously described recombinant P2X receptors.

Evidence for P2Y1, P2Y2, P2Y6 and atypical UTP-sensitive receptors coupled to rises in intracellular calcium in mouse cultured superior cervical ganglion neurons and glia

1 P2Y receptors are expressed in the nervous system and are involved in calcium signalling in neurons and glia. In the superior cervical ganglion (SCG), RT-PCR analysis indicated the presence of P2Y(1,2&6) receptors. Rises in intracellular calcium in response to P2Y receptor stimulation were determined from adult mouse cultured SCG neurons and glia. 2 ADP evoked suramin (100 microM)- and pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS, 30 microM)-sensitive rises in intracellular calcium in approximately 80% of SCG neurons (EC50 approximately 20 microM). ADP-evoked responses were abolished in neurons from P2Y1 receptor-deficient mice (responses to UTP were unaffected). 3 The pyrimidines UTP (EC50 approximately 85 microM) and UDP (EC50>90 microM) evoked PPADS- and suramin-sensitive responses in approximately 70 and approximately 20% of SCG neurons, respectively. 4 In SCG glial cells, ADP (EC50 approximately 30 microM) evoked calcium responses in approximately 50% of glia. These were suramin and PPADS sensitive and essentially abolished in SCG glial cells cultured from adult P2Y1 receptor-deficient mice. 5 UTP (EC50 approximately 25 microM) and UDP (EC50>200 microM) evoked suramin- and pyridoxalphosphate-6-azophenyl-2',5'-disulphonate-sensitive rises in calcium in approximately 60 and 20% SCG glial cells, respectively. 6 These results indicate the presence of several P2Y receptors coupled to an increase in intracellular calcium in the SCG: ADP-sensitive P2Y1 receptors and UDP-sensitive P2Y6 receptors in SCG neurons and glial cells, a novel UTP-sensitive P2Y receptor in SCG neurons and UTP- and ATP-sensitive P2Y2 receptors in SCG glia.

P2X receptor subtype-specific modulation of excitatory and inhibitory synaptic inputs in the rat brainstem

The role of P2 receptors in synaptic transmission to the rat medial nucleus of the trapezoid body (MNTB) was studied in an in vitro brain slice preparation. Whole-cell patch recordings were made and spontaneous synaptic responses studied under voltage clamp during application of P2X receptor agonists. ATPgammaS (100 microm) had no effect on holding current, but facilitated spontaneous excitatory postsynaptic current (sEPSC) frequency in 41% of recordings and facilitated spontaneous inhibitory postsynaptic currents (sIPSCs) in 20% of recordings. These were blocked by the P2 receptor antagonist suramin (100 microm). alpha,beta-meATP also facilitated sEPSC and sIPSC frequency, while l-beta,gamma-meATP facilitated only sIPSCs. The sEPSC facilitation by ATPgammaS was blocked by TTX (but did not block facilitation of sIPSCs). sEPSC facilitation was blocked by PPADS (30 microm) and the selective P2X(3) receptor antagonist A-317491 (3 microm), suggesting that modulation of sEPSCs involves P2X(3) receptor subunits. alpha,beta-meATP-facilitated sIPSCs were also recorded in wild-type mouse MNTB neurones, but were absent in the MNTB from P2X(1) receptor-deficient mice demonstrating a functional role for P2X(1) receptors in the CNS.