Assembly and trafficking of P2X purinergic receptors (Review)

High-affinity agonism at the P2X7 receptor is mediated by three residues outside the orthosteric pocket

P2X receptors are trimeric ATP-gated ion channels that activate diverse signaling cascades. Due to its role in apoptotic pathways, selective activation of P2X7 is a potential experimental tool and therapeutic approach in cancer biology. However, mechanisms of high-affinity P2X7 activation have not been defined. We report high-resolution cryo-EM structures of wild-type rat P2X7 bound to the high-affinity agonist BzATP as well as significantly improved apo receptor structures in the presence and absence of sodium. Apo structures define molecular details of pore architecture and reveal how a partially hydrated Na+ ion interacts with the conductance pathway in the closed state. Structural, electrophysiological, and direct binding data of BzATP reveal that three residues just outside the orthosteric ATP-binding site are responsible for its high-affinity agonism. This work provides insights into high-affinity agonism for any P2X receptor and lays the groundwork for development of subtype-specific agonists applicable to cancer therapeutics.

Human P2X4 receptor gating is modulated by a stable cytoplasmic cap and a unique allosteric pocket

P2X receptors (P2XRs) are a family of ATP-gated ion channels comprising homomeric and heteromeric trimers of seven subunits (P2X 1 - P2X 7 ) that confer different rates of desensitization. The helical recoil model of P2XR desensitization proposes the stability of the cytoplasmic cap sets the rate of desensitization, but timing of its formation is unclear for slow-desensitizing P2XRs. We report cryo-EM structures of full-length, wild-type human P2X 4 receptor in apo, antagonist-bound, and desensitized states. Because the apo and antagonist-bound structures of this slow-desensitizing P2XR include an intact cytoplasmic cap while the desensitized state structure does not, the cytoplasmic cap forms before agonist binding. Furthermore, structural and functional data suggests the cytoplasmic cap is stabilized by lipids to slow desensitization and that P2X 4 is further modified by glycosylation and palmitoylation. Finally, our antagonist-bound inhibited state structure reveals features specific to the allosteric ligand-binding pocket in human receptors that empower the development of small-molecule modulators.

P2X7R and P2X4R expression of mice submandibular gland in high-fat diet/streptozotocin-induced type 2 diabetes

Type 2 diabetes mellitus (T2DM) is a chronic inflammatory disease that can compromise the functioning of various organs, including the salivary glands (SG). The purinergic system is one of the most important inflammatory pathways in T2DM condition, and P2X7R and P2X4R are the primary purinergic receptors in SG that regulate inflammatory homeostasis. This study aimed to evaluate P2X7R and P2X4R expression, and morphological changes in the submandibular gland (SMG) in T2DM. Twenty-four 5-week-old mice were randomly assigned to control (CON) and diabetes mellitus (DM) groups (n = 12 each). Body weight, diet, and blood glucose levels were monitored weekly. The histomorphology of the SMG and the expression of the P2X7R, and P2X7R was evaluated by immunohistochemistry (IHC) staining and reverse transcription-quantitative polymerase chain reaction (RT-qPCR) at 11 and 13 weeks of age. Our findings indicate a significant increase in food consumption, body weight, and blood glucose levels in the DM group. Although a significant increase in P2X7R and P2X4R expression was observed in the DM groups, the receptor location remained unchanged. We also observed a significant increase in the acinar area in the DM13w group, and a significant decrease in the ductal area in the DM11w and DM13w groups. Targeting purinergic receptors may offer novel therapeutic methods for diabetic complications.

Oxidative stress-mediated proapoptosis signaling: A novel theory on the mechanism underlying the pathogenesis of burning mouth syndrome

BACKGROUND

Burning mouth syndrome (BMS) is a chronic oral pain disorder characterized by a generalized burning sensation in the oral mucosa without apparent medical or dental causes. Despite various hypotheses proposed to explain BMS pathogenesis, a clear understanding of the cellular-level events and associated histologic and molecular findings is lacking. Advancing our understanding of BMS pathogenesis could facilitate the development of more targeted therapeutic interventions.

TYPES OF STUDIES REVIEWED

The authors conducted an extensive literature search and review of cellular mechanisms, focusing on evidence-based data that support a comprehensive hypothesis for BMS pathogenesis. The authors explored novel and detailed mechanisms that may account for the characteristic features of BMS.

RESULTS

The authors proposed that BMS symptoms arise from the uncontrolled activation of proapoptotic transmembrane calcium permeable channels expressed in intraoral mucosal nerve fibers. Elevated levels of reactive oxygen species or dysfunctional antiapoptosis pathways may lead to uncontrolled oxidative stress-mediated apoptosis signaling, resulting in upregulation of transmembrane transient receptor potential vanilloid type 1 and P2X 3 calcium channels in nociceptive fibers. Activation of these channels can cause nerve terminal depolarization, leading to generation of action potentials that are centrally interpreted as pain.

CONCLUSIONS AND PRACTICAL IMPLICATIONS

The authors present a novel hypothesis for BMS pathogenesis, highlighting the role of proapoptotic transmembrane calcium permeable channels and oxidative stress-mediated apoptosis signaling in the development of BMS symptoms. Understanding these underlying mechanisms could provide new insights into the development of targeted therapeutic interventions for BMS. Additional research is warranted to validate this hypothesis and explore potential avenues for effective management of BMS.

Modulation of Microglial Function by ATP-Gated P2X7 Receptors: Studies in Rat, Mice and Human

P2X receptors are a family of seven ATP-gated ion channels that trigger physiological and pathophysiological responses in a variety of cells. Five of the family members are sensitive to low concentrations of extracellular ATP, while the P2X6 receptor has an unknown affinity. The last subtype, the P2X7 receptor, is unique in requiring millimolar concentrations to fully activate in humans. This low sensitivity imparts the agonist with the ability to act as a damage-associated molecular pattern that triggers the innate immune response in response to the elevated levels of extracellular ATP that accompany inflammation and tissue damage. In this review, we focus on microglia because they are the primary immune cells of the central nervous system, and they activate in response to ATP or its synthetic analog, BzATP. We start by introducing purinergic receptors and then briefly consider the roles that microglia play in neurodevelopment and disease by referencing both original works and relevant reviews. Next, we move to the role of extracellular ATP and P2X receptors in initiating and/or modulating innate immunity in the central nervous system. While most of the data that we review involve work on mice and rats, we highlight human studies of P2X7R whenever possible.

Role of purinergic receptors in cardiac sympathetic nerve injury in diabetes mellitus

Diabetic cardiac autonomic neuropathy is a common and serious chronic complication of diabetes, which can lead to sympathetic and parasympathetic nerve imbalance and a relative excitation of the sympathetic nerve. Purinergic receptors play a crucial role in this process. Diabetic cardiac sympathetic nerve injury affects the expression of purinergic receptors, and activated purinergic receptors affect the phosphorylation of different signaling pathways and the regulation of inflammatory processes. This paper introduces the abnormal changes of sympathetic nerve in diabetes mellitus and summarizes the recently published studies on the role of several purinergic receptor subtypes in diabetic cardiac sympathetic nerve injury. These studies suggest that purinergic receptors as novel drug targets are of great significance for the treatment of diabetic autonomic neuropathy. This article is part of the Special Issue on "Purinergic Signaling: 50 years".

Molecular Pharmacology of P2X Receptors: Exploring Druggable Domains Revealed by Structural Biology

Extracellular ATP is a critical signaling molecule that is found in a wide range of concentrations across cellular environments. The family of nonselective cation channels that sense extracellular ATP, termed P2X receptors (P2XRs), is composed of seven subtypes (P2X₁-P2X₇) that assemble as functional homotrimeric and heterotrimeric ion channels. Each P2XR is activated by a distinct concentration of extracellular ATP, spanning from high nanomolar to low millimolar. P2XRs are implicated in a variety of physiological and pathophysiological processes in the cardiovascular, immune, and central nervous systems, corresponding to the spatiotemporal expression, regulation, and activation of each subtype. The therapeutic potential of P2XRs is an emerging area of research in which structural biology has seemingly exceeded medicinal chemistry, as there are several published P2XR structures but currently no FDA-approved drugs targeting these ion channels. Cryogenic electron microscopy is ideally suited to facilitate structure-based drug design for P2XRs by revealing and characterizing novel ligand-binding sites. This review covers structural elements in P2XRs including the extracellular orthosteric ATP-binding site, extracellular allosteric modulator sites, channel pore, and cytoplasmic substructures, with an emphasis on potential therapeutic ligand development.

Contribution of P2X4 receptor in pain associated with rheumatoid arthritis: a review

Pain is the most common symptom reported by patients with rheumatoid arthritis (RA) even after the resolution of chronic joint inflammation. It is believed that RA-associated pain is not solely due to inflammation, but could also be attributed to aberrant modifications to the central nervous system. The P2X4 receptor (P2X4R) is an ATP-activated purinergic receptor that plays a significant role in the transmission of information in the nervous system and pain. The involvement of P2X4R during the pathogenesis of chronic inflammatory pain and neuropathic pain is well-established. The attenuation of this receptor alleviates disease pathogenesis and related symptoms, including hyperalgesia and allodynia. Although some studies have revealed the contribution of P2X4R in promoting joint inflammation in RA, how it implicates pain associated with RA at peripheral and central nervous systems is still lacking. In this review, the possible contributions of P2X4R in the nervous system and how it implicates pain transmission and responses were examined.

Effect of PKC/NF-κB on the Regulation of P2X3 Receptor in Dorsal Root Ganglion in Rats with Sciatic Nerve Injury

Background

Protein kinase C (PKC), nuclear factor-kappa B p65 (NF-κB p65), and P2X₃ receptor (P2X₃R) play significant roles in the sensitization and transduction of nociceptive signals, which are considered as potential targets for the treatment of neuropathic pain. However, the mechanisms and relationships among them have not been clearly clarified.

Methods

80 rats were randomized and divided into 10 groups (n = 8). Sciatic chronic constriction injury (CCI) rats were intrathecally administered with bisindolylmaleimide I (GF109203X), a PKC-selective antagonist once a day, or pyrrolidine dithiocarbamate (PDTC), an NF-κB inhibitor twice a day. Sham-operated rats were intrathecally administered with saline. Thermal withdrawal latency (TWL) and mechanical withdrawal threshold (MWT) were evaluated in all the groups before CCI operation (baseline) and on the 1st, 3rd, 7th, 10th, and 14th day after CCI operation. Protein levels of p-PKCα, p-NF-κB p65, and P2X₃R were analyzed in the CCI ipsilateral L_4-6 dorsal root ganglions (DRGs).

Results

Intrathecal injection of GF109203X or PDTC alleviated the TWL and MWT in the following 2 weeks after CCI surgery. The protein levels of p-PKCα, p-NF-κB p65, and P2X₃R in the ipsilateral DRGs significantly increased after CCI operation, which could be partly reversed by intrathecal administration of GF109203X or PDTC.

Conclusion

The upregulation of p-PKCα, p-NF-κB p65, and P2X₃R expression in the DRGs of CCI rats was involved in the occurrence and development of neuropathic pain. Phosphorylated PKCα and phosphorylated NF-κB p65 regulated with each other. Phosphorylated NF-κB p65 and PKCα have a mutual regulation relationship with P2X₃R, respectively, while the specific regulatory mechanism needs further research.

P2X4 receptor as a modulator in the function of P2X receptor in CD4+ T cells from peripheral blood and adipose tissue

Obesity is characterized by immune cell infiltration and inflammation. Purinergic receptors such as P2X1, 4 and 7 are expressed on immune cells and their activation contributes with an inflammatory response. However, the simultaneous expression of P2X1, 4 and 7 during overweight or obesity have not been described. Therefore, the aim of this study was to determine single and simultaneously expression and function of the P2X1, 4 and 7 receptors in lymphocytes and CD4 + T cells from peripheral blood (PB) and adipose tissue (AT). Our results showed a higher expression of the P2X4 receptor on CD4 + T cells from PB regarding P2X7 and P2X1 receptor expression. In addition, P2X4 receptor expression on CD4 + T cells from PB and AT was increased in individuals with BMI ≥ 25 Kg/m². Moreover, a higher simultaneous expression of the P2X4 and P2X7 receptors on CD4 + T cells from AT compared to CD4 + T cells expressing P2X1 and P2X7 receptors simultaneously. Besides, CD4 + T cells expressing P2X4 and P2X7 receptors from PB and AT were augmented in individuals with BMI ≥ 25 Kg/m². In addition, the percentage of lymphocytes and also CD4 + T cells expressing P2X4 receptor were elevated both in PB and AT compared to cells expressing P2X7 or P2X1. However, CD4 + T cells expressing P2X4 and P2X7 were augmented in AT compared to PB. The function of the receptors showed a lower shedding of CD62 L in adipose tissue mononuclear cells (ATMC) compared with peripheral blood mononuclear cells (PBMC) and a greater participation of P2X4 in the mobilization of intracellular calcium. We concluded that it was possible to determine for the first time the simultaneous expression of purinergic receptors in ATMC, where the P2X4 receptor has a greater participation in the activation of CD4 + T cells possibly modulating the function of the other two receptors.

Alternative splicing of P2RX7 pre-messenger RNA in health and diseases: Myth or reality?

Alternative splicing (AS) tremendously increases the use of genetic information by generating protein isoforms that differ in protein-protein interactions, catalytic activity and/or subcellular localization. This review is not dedicated to AS in general, but rather we focus our attention on AS of P2RX7 pre-mRNA. Whereas P2RX7 mRNA is expressed by virtually all eukaryotic mammalian cells, the expression of this channel receptor is restrained to certain cells. When expressed at the cell membrane, P2RX7 controls downstream events including release of inflammatory molecules, phagocytosis, cell proliferation and death and metabolic events. Therefore, P2RX7 is an important actor of health and diseases. In this review, we summarize the general mechanisms leading to AS. Further, we recapitulate our current knowledge concerning the functional regions in P2RX7, identified at the genetic or exonic levels, and how AS may affect the expression of these regions. Finally, the potential of P2RX7 splice variants to control the fate of cancer cells is discussed.

The role of the 12(S)-HETE/GPR31/12-HETER axis in cancer and ischemia-reperfusion injury

The G protein-coupled receptors (GPCRs) constitute a large superfamily of seven transmembrane-spanning receptors that are activated by several classes of ligands, including bioactive lipids. GPCRs are attractive therapeutic targets for the treatment of human diseases, as they finely regulate a wide array of cellular functions. In this minireview, we summarized what is currently known about the G protein-coupled receptor GPR31/12-HETER. We highlighted, in particular, its structural similarity with human homologs, the biological functions of its recognized ligand 12(S)-hydroxyeicosatetraenoic acid (HETE), an arachidonic acid metabolite, and the role that GPR31/12-HETER-mediated signals play in cancer cell growth, invasion and metastasis, and in liver ischemia-reperfusion (IR) injury. Recent studies shed light and interest on the 12(S)-HETE/GPR31/12-HETER-activated signaling pathways and functions. The full spectrum of GPR31/12-HETER-mediated biological functions has yet to be characterized. Further studies are needed to identify other potential ligands, i.e. other than 12(S)-HETE. Another important remaining question is whether the multiple 12(S)-HETE-induced biological activities, including its role in diabetes, neurodegeneration, neuroprotection, and platelet function, occur via GPR31/12-HETER and/or involve the activation of other receptor molecules and pathways.

Inflammation-induced upregulation of P2X4 expression augments mucin secretion in airway epithelia

Mucus clearance provides an essential innate defense mechanism to keep the airways and lungs free of particles and pathogens. Baseline and stimulated mucin secretion from secretory airway epithelial cells need to be tightly regulated to prevent mucus hypersecretion and mucus plugging of the airways. It is well established that extracellular ATP is a potent stimulus for regulated mucus secretion. Previous studies revealed that ATP acts via metabotropic P2Y₂ purinoreceptors on goblet cells. Extracellular ATP, however, is also a potent agonist for ionotropic P2X purinoreceptors. Expression of several P2X isoforms has been reported in airways, but cell type-specific expression and the function thereof remained elusive. With this study, we now provide evidence that P2X₄ is the predominant P2X isoform expressed in secretory airway epithelial cells. After IL-13 treatment of either human primary tracheal epithelial cells or mice, P2X₄ expression is upregulated in vitro and in vivo under conditions of chronic inflammation, mucous metaplasia, and hyperplasia. Upregulation of P2X₄ is strongest in MUC5AC-positive goblet cells. Moreover, activation of P2X₄ by extracellular ATP augments intracellular Ca²⁺ signals and mucin secretion, whereas Ca²⁺ signals and mucin secretion are dampened by inhibition of P2X₄ receptors. These data provide new insights into the purinergic regulation of mucin secretion and add to the emerging picture that P2X receptors modulate exocytosis of large secretory organelles and secretion of macromolecular vesicle cargo.

Gardenoside combined with ozone inhibits the expression of P2X3 and P2X7 purine receptors in rats with sciatic nerve injury

Neuropathic pain is a severe health problem for which there is a lack of effective therapy. Ozone and Gardenia fruits have been used separately in pain relief for many years; however, their underlying mechanisms remain unclear. To investigate the pain‑relieving effects of combined ozone and Gardenia, a chronic constriction sciatic nerve injury (CCI) rat model was constructed and treated with ozone and gardenoside (Ozo&Gar), which is a compound found in Gardenia fruits. A total of 70 rats were randomly divided into five groups: Control (Ctrl), Ctrl + Ozo&Gar, Sham, CCI, and CCI + Ozo&Gar. The rats in the Ctrl + Ozo&Gar and CCI + Ozo&Gar groups were administered an intravenous injection of 30 µg/ml ozone and 300 µmol/l gardenoside. The rats in the Ctrl, Sham and CCI groups were administered the same volume of saline. Pain behavior, mechanical hyperalgesia, thermal hyperalgesia, and the protein expression levels of P2X3 and P2X7 purine receptors in L4‑L5 dorsal root ganglion (DRG) were determined 15 days post‑surgery. The results demonstrated that treatment with a combination of ozone and gardenoside increased mechanical withdrawal threshold and thermal withdrawal latency, thus confirming their pain‑relieving effects. In addition, a significant increase in the mRNA and protein expression levels of P2X3 and P2X7 was detected in the DRG of rats in the CCI group compared with in the control groups; however, following treatment with a combination of ozone and gardenoside, the mRNA and protein expression levels of P2X3 and P2X7 receptors were significantly reduced compared with in the CCI group. These results indicated that the mechanism underlying the pain‑relieving effects of ozone and gardenoside may be mediated by inhibition of P2X3 and P2X7 purine receptors in the DRG. This finding suggested that ozone and gardenoside may be considered potential drug candidates that target P2X3 and P2X7 purine receptors.

P2X receptor overexpression induced by soluble oligomers of amyloid beta peptide potentiates synaptic failure and neuronal dyshomeostasis in cellular models of Alzheimer's disease

The most common cause of dementia is Alzheimer's disease. The etiology of the disease is unknown, although considerable evidence suggests a critical role for the soluble oligomers of amyloid beta peptide (Aβ). Because Aβ increases the expression of purinergic receptors (P2XRs) in vitro and in vivo, we studied the functional correlation between long-term exposure to Aβ and the ability of P2XRs to modulate network synaptic tone. We used electrophysiological recordings and Ca²⁺ microfluorimetry to assess the effects of chronic exposure (24 h) to Aβ oligomers (0.5 μM) together with known inhibitors of P2XRs, such as PPADS and apyrase on synaptic function. Changes in the expression of P2XR were quantified using RT-qPCR. We observed changes in the expression of P2X1R, P2X7R and an increase in P2X2R; and also in protein levels in PC12 cells (143%) and hippocampal neurons (120%) with Aβ. In parallel, the reduction on the frequency and amplitude of mEPSCs (72% and 35%, respectively) were prevented by P2XR inhibition using a low PPADS concentration. Additionally, the current amplitude and intracellular Ca²⁺ signals evoked by extracellular ATP were increased (70% and 75%, respectively), suggesting an over activation of purinergic neurotransmission in cells pre-treated with Aβ. Taken together, our findings suggest that Aβ disrupts the main components of synaptic transmission at both pre- and post-synaptic sites, and induces changes in the expression of key P2XRs, especially P2X2R; changing the neuromodulator function of the purinergic tone that could involve the P2X2R as a key factor for cytotoxic mechanisms. These results identify novel targets for the treatment of dementia and other diseases characterized by increased purinergic transmission.

Evidence that activation of P2X7R does not exacerbate neuronal death after optic nerve transection and focal cerebral ischemia in mice

Conflicting data in the literature about the function of P2X7R in survival following ischemia necessitates the conductance of in-depth studies. To investigate the impacts of activation vs inhibition of the receptor on neuronal survival as well as the downstream signaling cascades, in addition to optic nerve transection (ONT), 30min and 90min of middle cerebral artery occlusion (MCAo) models were performed in mice. Intracellular calcium levels were assessed in primary cortical neuron cultures. Here, we show that P2X7R antagonist Brilliant Blue G (BBG) decreased DNA fragmentation, infarct volume, brain swelling, neurological deficit scores and activation of microglial cells after focal cerebral ischemia. BBG also significantly increased the number of surviving retinal ganglion cells (RGCs) after ONT and the number of surviving neurons following MCAo. Importantly, receptor agonist BzATP resulted in increased activation of microglial cells and induced phosphorylation of ERK, AKT and JNK. These results indicated that inhibition of P2X7R with BBG promoted neuronal survival, not through the activation of survival kinase pathways, but possibly by improved intracellular Ca²⁺ overload and decreased the levels of Caspase 1, IL-1β and Bax proteins. On the other hand, BzATP-mediated increased number of activated microglia and increased survival kinase levels in addition to increased caspase-1 and IL-1β levels indicate the complex nature of the P2X7 receptor-mediated signaling in neuronal injury.

Exploring the Ca2+-dependent synaptic dynamics in vibro-dissociated cells

Dynamic alteration of the synaptic strength is one of the most important processes occurring in the nervous system. Combination of electrophysiology, confocal imaging and molecular biology led to significant advances in this research field. Yet, a progress in this area, in particular in studies of changes in the quantal behavior of central synapses and impact of glial cells on individual synapses, is hampered by technical difficulties of resolving small quantal synaptic currents. In this paper we will show how the technique of non-enzymatic vibro-dissociation, which enables to isolate living neurons avoiding artifacts of cell culture and preserving functional synapse, can be used to obtain a valuable information on fine details and mechanisms of synaptic plasticity. In particular, we will describe our recent results on Ca²⁺-dependent modulation of the postsynaptic AMPA and NMDA receptors in the individual synaptic boutons.

P2X4 Receptor Reporter Mice: Sparse Brain Expression and Feeding-Related Presynaptic Facilitation in the Arcuate Nucleus

P2X4 receptors are ATP-gated cation channels that are widely expressed in the nervous system. To identify P2X4 receptor-expressing cells, we generated BAC transgenic mice expressing tdTomato under the control of the P2X4 receptor gene (P2rx4). We found sparse populations of tdTomato-positive neurons in most brain areas with patterns that matched P2X4 mRNA distribution. tdTomato expression within microglia was low but was increased by an experimental manipulation that triggered microglial activation. We found surprisingly high tdTomato expression in the hypothalamic arcuate nucleus (Arc) (i.e., within parts of the neural circuitry controlling feeding). Immunohistochemistry and genetic crosses of P2rx4 tdTomato mice with cell-specific GFP reporter lines showed that the tdTomato-expressing cells were mainly AgRP-NPY neurons and tanycytes. There was no electrophysiological evidence for functional expression of P2X4 receptors on AgRP-NPY neuron somata, but instead, we found clear evidence for functional presynaptic P2X4 receptor-mediated responses in terminals of AgRP-NPY neurons onto two of their postsynaptic targets (Arc POMC and paraventricular nucleus neurons), where ATP dramatically facilitated GABA release. The presynaptic responses onto POMC neurons, and the expression of tdTomato in AgRP-NPY neurons and tanycytes, were significantly decreased by food deprivation in male mice in a manner that was partially reversed by the satiety-related peptide leptin. Overall, we provide well-characterized tdTomato reporter mice to study P2X4-expressing cells in the brain, new insights on feeding-related regulation of presynaptic P2X4 receptor responses, and the rationale to explore extracellular ATP signaling in the control of feeding behaviors.

SIGNIFICANCE STATEMENT

Cells expressing ATP-gated P2X4 receptors have proven problematic to identify and study in brain slice preparations because P2X4 expression is sparse. To address this limitation, we generated and characterized BAC transgenic P2rx4 tdTomato reporter mice. We report the distribution of tdTomato-expressing cells throughout the brain and particularly strong expression in the hypothalamic arcuate nucleus. Together, our studies provide a new, well-characterized tool with which to study P2X4 receptor-expressing cells. The electrophysiological studies enabled by this mouse suggest previously unanticipated roles for ATP and P2X4 receptors in the neural circuitry controlling feeding.

Differential expression of ATP-gated P2X receptors in DRG between chronic neuropathic pain and visceralgia rat models

There are divergences between neuropathic pain and visceralgia in terms of the duration, location, and character of hyperalgesia. It is generally recognized that nociceptive receptors, including P2X receptors, may play different roles in nociceptive mechanisms. The different roles of P2X1-7 receptors have not been fully understood both in neuropathic pain and visceral hyperalgesia. In order to explore the different expressions of P2X1-7 receptors in these two hyperalgesia models, the lumbosacral dorsal root ganglion (DRG) neurons from rat sciatic nerve chronic constriction injury (CCI) model and neonatal colorectal distention (NCRD) model were studied (both the primary nociceptive neuron afferents of those two models projected to the same segment of spinal cord). Both immunohistochemistry (IHC) technique and real-time fluorescence quantitative polymerase chain reaction (RT-PCR) technology were applied to analyze the protein expression levels and nucleic acid of P2X1-7 receptors. We found that except P2X2 and P2X3, the expression levels of P2X1 and P2X5 receptors increased in neuropathic pain while those expression levels of P2X4, P2X6, and P2X7 receptors increased in visceral pain. Our results also suggested that in addition to P2X2/3 heteromeric, other P2X subunits may also involved in generation heteromeric such as P2X1/5 and/or P2X2/5 in neuropathic pain and P2X4/6 and/or P2X4/7 in visceral pain.

CaMKIIα and caveolin-1 cooperate to drive ATP-induced membrane delivery of the P2X3 receptor

The P2X3 receptor plays a vital role in sensory processing and transmission. The assembly and trafficking of the P2X3 receptor are important for its function in primary sensory neurons. As an important inflammation mediator, ATP is released from different cell types around primary sensory neurons, especially under pathological pain conditions. Here, we show that α, β-MeATP dramatically promoted membrane delivery of the P2X3 receptor both in HEK293T cells expressing recombinant P2X3 receptor and in rat primary sensory neurons. α, β-MeATP induced P2X3 receptor-mediated Ca²⁺ influx, which further activated Ca²⁺/calmodulin-dependent protein kinase IIα (CaMKIIα). The N terminus of the P2X3 receptor was responsible for CaMKIIα binding, whereas Thr³⁸⁸ in the C terminus was phosphorylated by CaMKIIα. Thr³⁸⁸ phosphorylation increased P2X3 receptor binding to caveolin-1. Caveolin-1 knockdown abrogated the α, β-MeATP-induced membrane insertion of the P2X3 receptor. Moreover, α, β-MeATP drove the CaMKIIα-mediated membrane coinsertion of the P2X2 receptor with the P2X3 receptor. The increased P2X3 receptors on the cell membrane that are due to Thr³⁸⁸ phosphorylation facilitated P2X3 receptor-mediated signal transduction. Together, our data indicate that CaMKIIα and caveolin-1 cooperate to drive ligand-induced membrane delivery of the P2X3 receptor and may provide a mechanism of P2X3 receptor sensitization in pain development.

Imaging P2X4 receptor subcellular distribution, trafficking, and regulation using P2X4-pHluorin

A P2X4 receptor labeled with the pH-sensitive GFP superecliptic pHluorin represents a useful probe to investigate P2X4 receptor distribution, trafficking, and up-regulation.

P2X4 receptors are adenosine triphosphate (ATP)-gated cation channels present on the plasma membrane (PM) and also within intracellular compartments such as vesicles, vacuoles, lamellar bodies (LBs), and lysosomes. P2X4 receptors in microglia are up-regulated in epilepsy and in neuropathic pain; that is to say, their total and/or PM expression levels increase. However, the mechanisms underlying up-regulation of microglial P2X4 receptors remain unclear, in part because it has not been possible to image P2X4 receptor distribution within, or trafficking between, cellular compartments. Here, we report the generation of pH-sensitive fluorescently tagged P2X4 receptors that permit evaluations of cell surface and total receptor pools. Capitalizing on information gained from zebrafish P2X4.1 crystal structures, we designed a series of mouse P2X4 constructs in which a pH-sensitive green fluorescent protein, superecliptic pHluorin (pHluorin), was inserted into nonconserved regions located within flexible loops of the P2X4 receptor extracellular domain. One of these constructs, in which pHluorin was inserted after lysine 122 (P2X4-pHluorin123), functioned like wild-type P2X4 in terms of its peak ATP-evoked responses, macroscopic kinetics, calcium flux, current–voltage relationship, and sensitivity to ATP. P2X4-pHluorin123 also showed pH-dependent fluorescence changes, and was robustly expressed on the membrane and within intracellular compartments. P2X4-pHluorin123 identified cell surface and intracellular fractions of receptors in HEK-293 cells, hippocampal neurons, C8-B4 microglia, and alveolar type II (ATII) cells. Furthermore, it showed that the subcellular fractions of P2X4-pHluorin123 receptors were cell and compartment specific, for example, being larger in hippocampal neuron somata than in C8-B4 cell somata, and larger in C8-B4 microglial processes than in their somata. In ATII cells, P2X4-pHluorin123 showed that P2X4 receptors were secreted onto the PM when LBs undergo exocytosis. Finally, the use of P2X4-pHluorin123 showed that the modulator ivermectin did not increase the PM fraction of P2X4 receptors and acted allosterically to potentiate P2X4 receptor responses. Collectively, our data suggest that P2X4-pHluorin123 represents a useful optical probe to quantitatively explore P2X4 receptor distribution, trafficking, and up-regulation.

Comparative analysis of P2X1, P2X2, P2X3, and P2X4 receptor subunits in rat nodose ganglion neurons

Nodose ganglion (NG) neurons are visceral primary sensory neurons. The transmission and regulation of visceral sensation is mediated mainly by the P2X purinoceptor (P2X receptor). Although the characteristics of different P2X receptor subunits in the NG have been studied previously, comprehensive analyses have not been performed. In this study, we used immunohistochemistry, immunocytochemistry, and whole cell patch clamp techniques to compare the expression and function of P2X1, P2X2, P2X3, and P2X4 receptor subunits in adult rat NG neurons. Polyclonal antibodies against the four P2X subunits labeled different subpopulations of NG neurons. P2X1 and P2X3 were expressed mainly in small-to-medium sized NG neurons, whereas P2X2 and P2X4 were located mostly in medium- and larger-sized NG neurons. Over 36% of NG neurons were P2X3 positive, which was higher than the other three P2X subunits. In addition, different types of currents were recorded from neurons expressing different P2X subunits. The fast type of ATP current was recorded from neurons containing P2X1–4 subunits, the intermediate type of current was recorded from neurons containing the P2X1, P2X3, and P2X4 subunits, the slow type was recorded from neurons expressing P2X1–3, and/or P2X4 subunits, whereas the very slow type was recorded from neurons containing the P2X2 and P2X3 subunits. These comparative results provide an anatomical verification of the different subunits in NG neurons, and offer direct support for the idea that various functional NG populations have distinct responses to ATP, which might be in part due to the different expression profiles of diverse P2X subunits.

The relationship between P2X4 and P2X7: a physiologically important interaction?

Purinergic signaling within the kidney is becoming an important focus in the study of renal health and disease. The effectors of ATP signaling, the P2Y and P2X receptors, are expressed to varying extents in and along the nephron. There are many studies demonstrating the importance of the P2Y₂ receptor on kidney function, and other P2 receptors are now emerging as participants in renal regulation. The P2X4 receptor has been linked to epithelial sodium transport in the nephron and expression levels of the P2X7 receptor are up-regulated in certain pathophysiological states. P2X7 antagonism has been shown to ameliorate rodent models of DOCA salt-induced hypertension and P2X4 null mice are hypertensive. Interestingly, polymorphisms in the genetic loci of P2X4 and P2X7 have been linked to blood pressure variation in human studies. In addition to the increasing evidence linking these two P2X receptors to renal function and health, a number of studies link the two receptors in terms of physical associations between their subunits, demonstrated both in vitro and in vivo. This review will analyze the current literature regarding interactions between P2X4 and P2X7 and assess the potential impact of these with respect to renal function.

Hypoxia Attenuates Purinergic P2X Receptor-Induced Inflammatory Gene Expression in Brainstem Microglia

Hypoxia and increased extracellular nucleotides are frequently coincident in the brainstem. Extracellular nucleotides are potent modulators of microglial inflammatory gene expression via P2X purinergic receptor activation. Although hypoxia is also known to modulate inflammatory gene expression, little is known about how hypoxia or P2X receptor activation alone affects inflammatory molecule production in brainstem microglia, nor how hypoxia and P2X receptor signaling interact when they occur together. In the study reported here, we investigated the ability of a brief episode of hypoxia (2 hours) in the presence and absence of the nonselective P2X receptor agonist 2′(3′)-O-(4-benzoylbenzoyl)adenosine-5′-triphosphate (BzATP) to promote inflammatory gene expression in brainstem microglia in adult rats. We evaluated inducible nitric oxide synthase (iNOS), tumor necrosis factor alpha (TNFα), and interleukin (IL)-6 messenger RNA levels in immunomagnetically isolated brainstem microglia. While iNOS and IL-6 gene expression increased with hypoxia and BzATP alone, TNFα expression was unaffected. Surprisingly, BzATP-induced inflammatory effects were lost after hypoxia, suggesting that hypoxia impairs proinflammatory P2X-receptor signaling. We also evaluated the expression of key P2X receptors activated by BzATP, namely P2X1, P2X4, and P2X7. While hypoxia did not alter their expression, BzATP upregulated P2X4 and P2X7 mRNAs; these effects were ablated in hypoxia. Although both P2X4 and P2X7 receptor expression correlated with increased microglial iNOS and IL-6 levels in microglia from normoxic rats, in hypoxia, P2X7 only correlated with IL-6, and P2X4 correlated only with iNOS. In addition, correlations between P2X7 and P2X4 were lost following hypoxia, suggesting that P2X4 and P2X7 receptor signaling differs in normoxia and hypoxia. Together, these data suggest that hypoxia suppresses P2X receptor-induced inflammatory gene expression, indicating a potentially immunosuppressive role of extracellular nucleotides in brainstem microglia following exposure to hypoxia.

Positional editing of transmembrane domains during ion channel assembly

The integration of transmembrane (TM)-spanning regions of many channels and ion transporters is potentially compromised by the presence of polar and charged residues required for biological function. Although the two TMs of the ATP-gated ion channel subunit P2X2 each contain charged/polar amino acids, we found that each TM is efficiently membrane inserted when it is analysed in isolation, and uncovered no evidence for cooperativity between these two TMs during P2X2 integration. However, using minimal N-glycosylation distance mapping, we find that the positioning of TM2 in newly synthesized P2X2 monomers is distinct from that seen in subunits of the high-resolution structures of assembled homologous trimers. We conclude that P2X2 monomers are initially synthesised at the endoplasmic reticulum in a distinct conformation, where the extent of the TM-spanning regions is primarily defined by the thermodynamic cost of their membrane integration at the Sec61 translocon. In this model, TM2 of P2X2 subsequently undergoes a process of positional editing within the membrane that correlates with trimerisation of the monomer, a process requiring specific polar/charged residues in both TM1 and TM2. We postulate that the assembly process offsets any energetic cost of relocating TM2, and find evidence that positional editing of TM2 in the acid-sensing ion channel (ASIC1a) is even more pronounced than that observed for P2X2. Taken together, these data further underline the potential complexities involved in accurately predicting TM domains. We propose that the orchestrated repositioning of TM segments during subunit oligomerisation plays an important role in generating the functional architecture of active ion channels, and suggest that the regulation of this underappreciated biosynthetic step may provide an elegant mechanism for maintaining ER homeostasis.

Expression, assembly and function of novel C-terminal truncated variants of the mouse P2X7 receptor: re-evaluation of P2X7 knockouts

BACKGROUND AND PURPOSE

Splice variants of P2X7 receptor transcripts contribute to the diversity of receptor-mediated responses. Here, we investigated expression and function of C-terminal truncated (ΔC) variants of the mP2X7 receptor, which are predicted to escape inactivation in one strain of P2X7(-/-) mice (Pfizer KO).

EXPERIMENTAL APPROACH

Expression in wild-type (WT) and Pfizer KO tissue was investigated by reverse transcription (RT)-PCR and Western blot analysis. ΔC variants were also cloned and expressed in HEK293 cells to investigate their assembly, trafficking and function.

KEY RESULTS

RT-PCR indicates expression of a ΔC splice variant in brain, salivary gland (SG) and spleen from WT and Pfizer KO mice. An additional ΔC hybrid transcript, containing sequences of P2X7 upstream of exon 12, part of exon 13 followed in-frame by the sequence of the vector used to disrupt the P2X7 gene, was also identified in the KO mice. By blue native (BN) PAGE analysis and the use of cross linking reagents followed by SDS-PAGE, P2X7 trimers, dimers and monomers were detected in the spleen and SG of Pfizer KO mice. The molecular mass was reduced compared with P2X7 in WT mice tissue, consistent with a ΔC variant. When expressed in HEK293 cells the ΔC variants were inefficiently trafficked to the cell surface and agonist-evoked whole cell currents were small. Co-expressed with P2X7A, the ΔC splice variant acted in a dominant negative fashion to inhibit function.

CONCLUSIONS AND IMPLICATIONS

Pfizer KO mice are not null for P2X7 receptor expression but express ΔC variants with reduced function.

Imaging P2X4 receptor lateral mobility in microglia: regulation by calcium and p38 MAPK

ATP-gated ionotropic P2X4 receptors are up-regulated in activated microglia and are critical for the development of neuropathic pain, a microglia-associated disorder. However, the nature of how plasma membrane P2X4 receptors are regulated in microglia is not fully understood. We used single-molecule imaging to track quantum dot-labeled P2X4 receptors to explore P2X4 receptor mobility in the processes of resting and activated microglia. We find that plasma membrane P2X4 receptor lateral mobility in resting microglial processes is largely random, consisting of mobile and slowly mobile receptors. Moreover, lateral mobility is P2X subunit- and cell-specific, increased in an ATP activation and calcium-dependent manner, and enhanced in activated microglia by the p38 MAPK pathway that selectively regulates slowly mobile receptors. Thus, our data indicate that P2X4 receptors are dynamically regulated mobile ATP sensors, sampling more of the plasma membrane in response to ATP and during the activated state of microglia that is associated with nervous system dysfunction.

Neuronal P2X2 receptors are mobile ATP sensors that explore the plasma membrane when activated

ATP-gated ionotropic P2X2 receptors are widely expressed in neurons. Although the electrophysiological properties of P2X2 receptors have been extensively studied, little is known about the plasma membrane lateral mobility of P2X2 receptors or whether receptor mobility is regulated by ATP. Here we used single-molecule imaging with simultaneous whole-cell voltage-clamp recordings to track quantum dot-labeled P2X2 receptors in the dendrites of rat hippocampal neurons to explore P2X2 receptor mobility and its regulation. We find that plasma membrane P2X2 receptor lateral mobility in dendrites is heterogeneous but mostly Brownian in nature, consisting of mobile and slowly mobile receptor pools. Moreover, lateral mobility is P2X2 subunit and cell specific, is increased in an activation-dependent manner, and is regulated by cytosolic VILIP1, a calcium binding protein. Our data provide the first direct measures of P2X receptor mobility and show that P2X2 receptors are mobile ATP sensors, sampling more of the dendritic plasma membrane in response to ATP.

Role of purinergic P2X receptors in the control of liver homeostasis

It is now accepted that extracellular ATP and other nucleotides are potent signaling molecules, akin to neurotransmitters, hormones and lipid mediators. In the liver, several clues support a significant role for extracellular ATP-induced signaling pathways in the control of tissue homeostasis. First, ATP and other nucleotides are physiologically detected in extracellular fluids within the liver, including sinusoidal blood and intraductular bile, in various mammalian species including human and rodents. Moreover, finely tuned mechanisms of ATP release by different liver cell types have been described, under physiological cellular changes. In addition, most hepatic cells constitutively express, at the membrane level, several ATP-metabolizing ectoenzymes and ATP-sensitive receptors that modulate and transduce these mediator signals respectively. Finally, hepatic cells also express numerous membrane transporters that actively contribute to purinergic salvage pathways. Once released in the extracellular medium, unmetabolised ATP molecules can bind to purinergic P2X and P2Y receptors, and subsequently trigger various intracellular signal transduction pathways collectively referred to as purinergic signaling. In the liver, purinergic signaling has been shown to regulate key basic cellular functions, such as glucose/lipid metabolism, protein synthesis and ionic secretion, and homeostatic processes, such as cell cycle, inflammatory response and immunity. Whilst the functional relevance of P2Y receptors in liver physiology has been well documented, limited information is available regarding the potential role of hepatic P2X receptors in the modulation of liver homeostasis.

CCL2 promotes P2X4 receptor trafficking to the cell surface of microglia

P2X4 receptors (P2X4Rs), a subtype of the purinergic P2X family, play important roles in regulating neuronal and glial functions in the nervous system. We have previously shown that the expression of P2X4Rs is upregulated in activated microglia after peripheral nerve injury and that activation of the receptors by extracellular ATP is crucial for maintaining nerve injury-induced pain hypersensitivity. However, the regulation of P2X4R expression on the cell surface of microglia is poorly understood. Here, we identify the CC chemokine receptor CCR2 as a regulator of P2X4R trafficking to the cell surface of microglia. In a quantitative cell surface biotinylation assay, we found that applying CCL2 or CCL12, endogenous ligands for CCR2, to primary cultured microglial cells, increased the levels of P2X4R protein on the cell surface without changing total cellular expression. This effect of CCL2 was prevented by an antagonist of CCR2. Time-lapse imaging of green fluorescent protein (GFP)-tagged P2X4R in living microglial cells showed that CCL2 stimulation increased the movement of P2X4R-GFP particles. The subcellular localization of P2X4R immunofluorescence was restricted to lysosomes around the perinuclear region. Notably, CCL2 changed the distribution of lysosomes with P2X4R immunofluorescence within microglial cells and induced release of the lysosomal enzyme β-hexosaminidase, indicating lysosomal exocytosis. Moreover, CCL2-stimulated microglia enhanced Akt phosphorylation by ATP applied extracellularly, a P2X4R-mediated response. These results indicate that CCL2 promotes expression of P2X4R protein on the cell surface of microglia through exocytosis of P2X4R-containing lysosomes, which may be a possible mechanism for pain hypersensitivity after nerve injury.

P2X4 receptors interact with both P2X2 and P2X7 receptors in the form of homotrimers

BACKGROUND AND PURPOSE

The P2X receptor family consists of seven subunit types - P2X1-P2X7. All but P2X6 are able to assemble as homotrimers. In addition, various subunit permutations have been reported to form heterotrimers. Evidence for heterotrimer formation includes co-localization, co-immunoprecipitation and the generation of receptors with novel functional properties; however, direct structural evidence for heteromer formation, such as chemical cross-linking and single-molecule imaging, is available in only a few cases. Here we examined the nature of the interaction between two pairs of subunits - P2X2 and P2X4, and P2X4 and P2X7.

EXPERIMENTAL APPROACH

We used several experimental approaches, including in situ proximity ligation, co-immunoprecipitation, co-isolation on affinity beads, chemical cross-linking and atomic force microscopy (AFM) imaging.

KEY RESULTS

Both pairs of subunits co-localize upon co-transfection, interact intimately within cells, and can be co-immunoprecipitated and co-isolated from cell extracts. Despite this, chemical cross-linking failed to show evidence for heteromer formation. AFM imaging of isolated receptors showed that all three subunits had the propensity to form receptor dimers. This self-association is likely to account for the observed close interaction between the subunit pairs, in the absence of true heteromer formation.

CONCLUSIONS AND IMPLICATIONS

We conclude that both pairs of receptors interact in the form of distinct homomers. We urge caution in the interpretation of biochemical evidence indicating heteromer formation in other cases.

Ionotropic ATP receptors in neuronal-glial communication

In the central nervous system ATP is released from both neurones and astroglial cells acting as a homo- and heterocellular neurotransmitter. Glial cells express numerous purinoceptors of both ionotropic (P2X) and metabotropic (P2Y) varieties. Astroglial P2X receptors can be activated by ongoing synaptic transmission and can mediate fast local signalling through elevation in cytoplasmic Ca(2+) and Na(+) concentrations. These ionic signals can be translated into various physiological messages by numerous pathways, including release of gliotransmitters, metabolic support of neurones and regulation of activity of postsynaptic glutamate and GABA receptors. Ionotropic purinoceptors represent a novel pathway of glia-driven modulation of synaptic signalling that involves the release of ATP from neurones and astrocytes followed by activation of P2X receptors which can regulate synaptic activity by variety of mechanisms expressed in both neuronal and glial compartments.

Amino acid residues constituting the agonist binding site of the human P2X3 receptor

Homomeric P2X3 receptors are present in sensory ganglia and participate in pain perception. Amino acid (AA) residues were replaced in the four supposed nucleotide binding segments (NBSs) of the human (h) P2X3 receptor by alanine, and these mutants were expressed in HEK293 cells and Xenopus laevis oocytes. Patch clamp and two-electrode voltage clamp measurements as well as the Ca(2+) imaging technique were used to compare the concentration-response curves of the selective P2X1,3 agonist α,β-methylene ATP obtained at the wild-type P2X3 receptor and its NBS mutants. Within these NBSs, certain Gly (Gly-66), Lys (Lys-63, Lys-176, Lys-284, Lys-299), Asn (Asn-177, Asn-279), Arg (Arg-281, Arg-295), and Thr (Thr-172) residues were of great importance for a full agonist response. However, the replacement of further AAs in the NBSs by Ala also appeared to modify the amplitude of the current and/or [Ca(2+)](i) responses, although sometimes to a minor degree. The agonist potency decrease was additive after the simultaneous replacement of two adjacent AAs by Ala (K65A/G66A, F171A/T172A, N279A/F280A, F280A/R281A) but was not altered after Ala substitution of two non-adjacent AAs within the same NBS (F171A/N177A). SDS-PAGE in the Cy5 cell surface-labeled form demonstrated that the mutants appeared at the cell surface in oocytes. Thus, groups of AAs organized in NBSs rather than individual amino acids appear to be responsible for agonist binding at the hP2X3 receptor. These NBSs are located at the interface of the three subunits forming a functional receptor.

Modulation of heteromeric P2X1/5 receptors by phosphoinositides in astrocytes depends on the P2X1 subunit

Purinergic signaling is critical for neuron-glia communication. Glial cells participate in synaptic transmission and express metabotropic P2Y as well as ionotropic P2X ATP receptors. In astrocytes, endogenous ATP-evoked currents with kinetics and pharmacology characteristic of the heteromeric P2X1/5 receptor channel have recently been reported. We investigated the interaction of major phosphoinositides with heteromeric P2X1/5 channels. Using patch-clamp electrophysiology on enhanced green fluorescent protein-expressing astrocytes acutely isolated from cortical slices of transgenic mice, we report a strong modulation of P2X1/5-like currents by phosphoinositides. Wortmannin-induced depletion of phosphoinositides decreases the amplitude of both the fast and sustained component of the P2X1/5-like currents although recovery and kinetics remain intact. In transfected human embryonic kidney cells, we provide evidence that depleting phosphatidylinositol 4,5-bisphosphate [PI(4,5)P(2)] levels significantly decreases P2X1/5 currents while intracellular application of PI(4,5)P(2) completely rescued P2X1/5 currents, ruling out the involvement of phosphatidylinositol 3,4,5-trisphosphate. In contrast to P2X1, homomeric P2X5 current responses were found insensitive to phosphoinositides, and the C-terminus of P2X5 subunit lacked binding to phospholipids in an overlay assay. Our results suggest that the contribution of calcium-permeable heteromeric P2X1/5 receptor channels to the excitability of astrocytes is modulated by PI(4,5)P(2) through the P2X1 lipid-binding domain.

Ca(2+) channels on the move

The versatility of Ca(2+) as an intracellular messenger derives largely from the spatial organization of cytosolic Ca(2+) signals, most of which are generated by regulated openings of Ca(2+)-permeable channels. Most Ca(2+) channels are expressed in the plasma membrane (PM). Others, including the almost ubiquitous inositol 1,4,5-trisphosphate receptors (IP(3)R) and their relatives, the ryanodine receptors (RyR), are predominantly expressed in membranes of the sarcoplasmic or endoplasmic reticulum (ER). Targeting of these channels to appropriate destinations underpins their ability to generate spatially organized Ca(2+) signals. All Ca(2+) channels begin life in the cytosol, and the vast majority are then functionally assembled in the ER, where they may either remain or be dispatched to other membranes. Here, by means of selective examples, we review two issues related to this trafficking of Ca(2+) channels via the ER. How do cells avoid wayward activity of Ca(2+) channels in transit as they pass from the ER via other membranes to their final destination? How and why do some cells express small numbers of the archetypal intracellular Ca(2+) channels, IP(3)R and RyR, in the PM?

An evolutionary history of P2X receptors

Adenosine triphosphate (ATP) is an ancient and fundamentally important biological molecule involved in both intracellular and extracellular activities. P2X ionotropic and P2Y metabotropic receptors have been cloned and characterised in mammals. ATP plays a central physiological role as a transmitter molecule in processes including the sensation of pain, taste, breathing and inflammation via the activation of P2X receptors. P2X receptors are structurally distinct from glutamate and Cys-loop/nicotinic receptors and form the third major class of ligand-gated ion channel. Yet, despite the importance of P2X receptors, both as physiological mediators and therapeutic targets, the evolutionary origins and phylogenicity of ATP signalling via P2X receptors remain unclear.

Dissecting the physiological role of selective transmembrane-segment retention at the ER translocon

The membrane integration of polytopic proteins is coordinated at the endoplasmic reticulum (ER) by the conserved Sec61 translocon, which facilitates the lateral release of transmembrane (TM) segments into the lipid phase during polypeptide translocation. Here we use a site-specific crosslinking strategy to study the membrane integration of a new model protein and show that the TM segments of the P2X2 receptor are retained at the Sec61 complex for the entire duration of the biosynthetic process. This extremely prolonged association implicates the Sec61 complex in the regulation of the membrane integration process, and we use both in vitro and in vivo analyses to study this effect further. TM-segment retention depends on the association of the ribosome with the Sec61 complex, and complete lateral exit of the P2X2 TM segments was only induced by the artificial termination of translation. In the event of the premature release of P2X2 TM1 from the ER translocon, the truncated polypeptide fragment was to found aggregate in the ER membrane, suggesting a distinct physiological requirement for the delayed release of TM segments from the ER translocon site.

Functional interactions between P2X4 and P2X7 receptors from mouse salivary epithelia

Mouse parotid acinar cells express P2X4 and P2X7 receptors (mP2X4R and mP2X7R) whose physiological function remains undetermined. Here we show that mP2X4R expressed in HEK-293 cells do not allow the passage of tetraethylammonium (TEA+) and promote little, if any, ethidium bromide (EtBr) uptake when stimulated with ATP or BzATP. In contrast, mP2X7R generates slowly decaying TEA+ current, sustained Na+ current and promotes robust EtBr uptake. However, ATP-activated TEA+ current from acinar cells was unlike that generated by mP2X7R or mP2X4R. Functional interactions between mP2X4R and mP2X7R were investigated in HEK cells co-transfected with different mP2X4 : mP2X7 cDNA ratios and using solutions containing either TEA+ or Na+ ions. Co-expressed channels generated a TEA+ current that displayed faster decay during ATP stimulation than mP2X7R alone. Moreover, cells transfected with a 2 : 1 cDNA ratio displayed decaying kinetics similar to those observed in acinar cells. Concentration-response curves in Na+-containing solutions were constructed for heterologously expressed mP2X4R, mP2X7R and mP2X4R:mP2X7R co-expressions as well as acinar cells. The EC50 values determined were 11, 220, 434 and 442 microM, respectively. Na+ currents generated by expressing mP2X4R or mP2X7R alone were potentiated by ivermectin (IVM). In contrast, IVM potentiation in acinar cells and HEK cells co-expressing P2X4 and P2X7 (1 : 1 or 2 : 1 cDNA ratios) was seen only when the ATP concentration was lowered from 5 to 0.03 mM. Taken together our observations indicate a functional interaction between murine P2X7 and P2X4 receptors. Such interaction might occur in acinar cells to shape the response to extracellular ATP in salivary epithelia.

Analysis of assembly and trafficking of native P2X4 and P2X7 receptor complexes in rodent immune cells

P2X4 and P2X7 are the predominant P2X receptor subtypes expressed in immune cells. Having previously shown a structural and functional interaction between the two recombinant receptors, our aims here were to identify the preferred assembly pathway of the endogenous receptors in macrophage-like cells and to investigate the trafficking of these receptors between the plasma membrane and intracellular sites. We exploited the difference in size between the two subunits, and we used a combination of cross-linkers and blue native-PAGE analysis to investigate the subunit composition of complexes present in primary cultures of rat microglia and macrophages from wild type and P2X7(-/-) mice. Our results indicate that the preferred assembly pathway for both receptors is the formation of homotrimers. Homotrimers of P2X7 were able to co-immunoprecipitate with P2X4, suggesting that an interaction occurs between rather than within receptor complexes. In both macrophages and microglia, P2X7 receptors were predominantly at the cell surface, whereas P2X4 receptors were predominantly intracellular. There were clear cell type-dependent differences in the extent to which P2X4 receptors trafficked to and from the surface; trafficking was much more dynamic in microglia than in the macrophages, and further activation of cultured microglia with relatively short (3-h) incubations with lipopolysaccharide caused an approximately 4-fold increase in the fraction of receptors at the surface with only a 1.2-fold increase in total expression. The redistribution of intracellular receptors is thus an efficient means of enhancing the functional expression of P2X4 at the plasma membrane of microglia.

Differential membrane redistribution of P2X receptor isoforms in response to osmotic and hyperglycemic stress in the rat lens

P2X(1, 2, 3, 4, 6 and 7) are all expressed in a differentiation-dependent manner in the rat lens. However, in the lens outer cortex the subcellular distribution of all P2X isoforms is predominantly associated with a pool of receptors located in cytoplasmic vesicles. Here we investigate whether osmotic and hyperglycemic stress can alter the subcellular distribution of this cytoplasmic pool of P2X receptors. We show that in a discrete zone of the deeper outer cortex an isoform and stimulus-specific shift in the subcellular distribution of P2X receptors occurs from the cytoplasm to defined membrane domains. In response to hypertonic stress P2X(1) and P2X(4) isoforms became more closely associated with the broad sides of fiber cells, while under hypotonic conditions P2X(4) and P2X(6) isoforms associate with the narrow side membranes. No such changes in subcellular distribution were observed for P2X(2,3 and 7) isoforms. Lens cultured in 50 mM glucose exhibited cell swelling in this zone but only P2X(4) associated with narrow side membranes. Our results indicate P2X receptors can be differentially recruited to specific membrane domains of lens fiber cells by osmotic and hyperglycemic stress. Furthermore they suggest the involvement of specific P2X isoforms in the regulation of fiber cell volume and the initiation of diabetic cataract.