Antibodies binding the head domain of P2X4 inhibit channel function and reverse neuropathic pain

Pharmacological differences between human and mouse P2X4 receptor explored using old and new tools

There is growing interest in the P2X4 receptor as a therapeutic target for several cardiovascular, inflammatory and neurological conditions. Key to exploring the physiological and pathophysiological roles of P2X4 is access to selective compounds to probe function in cells, tissues and animal models. There has been a recent growth in selective antagonists for P2X4, though agonist selectivity is less well studied. As there are some known pharmacological differences between P2X receptors from different species, it is important to understand these differences when designing a pharmacological strategy to probe P2X4 function in human tissue and mouse models. Here, we provide a systematic comparison of agonist and antagonist pharmacology in 1321N1 cells expressing either human or mouse P2X4 orthologues. We identify a rank order of agonist potency of ATP > 2-MeSATP > αβmeATP = BzATP > CTP = γ-[(propargyl)-imido]-ATP for human P2X4 and ATP > 2-MeSATP = CTP > ATPγS = γ-[(propargyl)-imido]-ATP = BzATP for mouse. Human P2X4 is not activated by ATPγS but can be activated by αβmeATP. We identify a rank order of antagonist potency of BAY-1797 = PSB-12062 = BX-430 > 5-BDBD > TNP-ATP = PPADS for human P2X4 and BAY-1797 > PSB-12062 = PPADS > TNP-ATP for mouse. Mouse P2X4 is not antagonised by 5-BDBD or BX-430. The study reveals key pharmacological differences between human and mouse P2X4, highlighting caution when selecting tools for comparative studies between human and mouse and ascribing cellular responses of some commonly used agonists to P2X4.

Discovery of Therapeutic Antibodies Targeting Complex Multi-Spanning Membrane Proteins

Complex integral membrane proteins, which are embedded in the cell surface lipid bilayer by multiple transmembrane spanning polypeptides, encompass families of proteins that are important target classes for drug discovery. These protein families include G protein-coupled receptors, ion channels, transporters, enzymes, and adhesion molecules. The high specificity of monoclonal antibodies and the ability to engineer their properties offers a significant opportunity to selectively bind these target proteins, allowing direct modulation of pharmacology or enabling other mechanisms of action such as cell killing. Isolation of antibodies that bind these types of membrane proteins and exhibit the desired pharmacological function has, however, remained challenging due to technical issues in preparing membrane protein antigens suitable for enabling and driving antibody drug discovery strategies. In this article, we review progress and emerging themes in defining discovery strategies for a generation of antibodies that target these complex membrane protein antigens. We also comment on how this field may develop with the emerging implementation of computational techniques, artificial intelligence, and machine learning.

Methods for studying P2X4 receptor ion channels in immune cells

The P2X4 receptor is a trimeric ligand-gated ion channel activated by adenosine 5'-triphosphate (ATP). P2X4 is present in immune cells with emerging roles in inflammation and immunity, and related disorders. This review aims to provide an overview of the methods commonly used to study P2X4 in immune cells, focusing on those methods used to assess P2RX4 gene expression, the presence of the P2X4 protein, and P2X4 ion channel activity in these cells from humans, dogs, mice and rats. P2RX4 gene expression in immune cells is commonly assessed using semi-quantitative and quantitative reverse-transcriptase-PCR. The presence of P2X4 protein in immune cells is mainly assessed using anti-P2X4 polyclonal antibodies with immunoblotting or immunochemistry, but the use of these antibodies, as well as monoclonal antibodies and nanobodies to detect P2X4 with flow cytometry is increasing. Notably, use of an anti-P2X4 monoclonal antibody and flow cytometry has revealed that P2X4 is present on immune cells with a rank order of expression in eosinophils, then neutrophils and monocytes, then basophils and B cells, and finally T cells. P2X4 ion channel activity has been assessed mainly by Ca2+ flux assays using the cell permeable Ca2+-sensitive dyes Fura-2 and Fluo-4 with fluorescence microscopy, spectrophotometry, or flow cytometry. However, other methods including electrophysiology, and fluorescence assays measuring Na+ flux (using sodium green tetra-acetate) and dye uptake (using YO-PRO-12+) have been applied. Collectively, these methods have demonstrated the presence of functional P2X4 in monocytes and macrophages, microglia, eosinophils, mast cells and CD4+ T cells, with other evidence suggestive of functional P2X4 in dendritic cells, neutrophils, B cells and CD8+ T cells.

Microglia in Neuropathic Pain

Neuropathic pain (NP) is pain resulting from lesions or disease of the somatosensory system. A cardinal feature of NP is tactile allodynia (a painful response to normally innocuous stimulation). In 2003, a breakthrough strategy for inducing NP was proposed in which microglia of the spinal dorsal horn (SDH) are activated after peripheral nerve injury (PNI) to overexpress P2X4 receptor (P2X4R) and play an important role in inducing tactile allodynia. In 2005, it was reported that stimulation of microglial P2X4Rs evokes the release of brain-derived neurotrophic factor (BDNF), which causes a depolarizing shift of the anion reversal potential (Eanion) of secondary sensory neurons. These findings and other facts suggest the mechanism by which innocuous touch stimuli cause severe pain and the important role of microglia in the mechanism.

Neuropathic pain: From actual pharmacological treatments to new therapeutic horizons

Neuropathic pain, caused by a lesion or disease affecting the somatosensory system, affects between 3 and 17% of the general population. The treatment of neuropathic pain is challenging due to its heterogeneous etiologies, lack of objective diagnostic tools and resistance to classical analgesic drugs. First-line treatments recommended by the Special Interest Group on Neuropathic Pain (NeuPSIG) and European Federation of Neurological Societies (EFNS) include gabapentinoids, tricyclic antidepressants (TCAs) and selective serotonin noradrenaline reuptake inhibitors (SNRIs). Nevertheless these treatments have modest efficacy or dose limiting side effects. There is therefore a growing number of preclinical and clinical studies aim at developing new treatment strategies to treat neuropathic pain with better efficacy, selectivity, and less side effects. In this review, after a brief description of the mechanisms of action, efficacy, and limitations of current therapeutic drugs, we reviewed new preclinical and clinical targets currently under investigation, as well as promising non-pharmacological alternatives and their potential co-use with pharmacological treatments.

The role of the ATP-adenosine axis in ischemic stroke

In ischemic stroke, the primary neuronal injury caused by the disruption of energy supply is further exacerbated by secondary sterile inflammation. The inflammatory cascade is largely initiated by the purine adenosine triphosphate (ATP) which is extensively released to the interstitial space during brain ischemia and functions as an extracellular danger signaling molecule. By engaging P2 receptors, extracellular ATP activates microglia leading to cytokine and chemokine production and subsequent immune cell recruitment from the periphery which further amplifies post-stroke inflammation. The ectonucleotidases CD39 and CD73 shape and balance the inflammatory environment by stepwise degrading extracellular ATP to adenosine which itself has neuroprotective and anti-inflammatory signaling properties. The neuroprotective effects of adenosine are mainly mediated through A₁ receptors and inhibition of glutamatergic excitotoxicity, while the anti-inflammatory capacities of adenosine have been primarily attributed to A_2A receptor activation on infiltrating immune cells in the subacute phase after stroke. In this review, we summarize the current state of knowledge on the ATP-adenosine axis in ischemic stroke, discuss contradictory results, and point out potential pitfalls towards translating therapeutic approaches from rodent stroke models to human patients.

The core of maintaining neuropathic pain: Crosstalk between glial cells and neurons (neural cell crosstalk at spinal cord)

BACKGROUND

Neuropathic pain (NP) caused by the injury or dysfunction of the nervous system is a chronic pain state accompanied by hyperalgesia, and the available clinical treatment is relatively scarce. Hyperalgesia mediated by pro-inflammatory factors and chemokines plays an important role in the occurrence and maintenance of NP.

DATA TREATMENT

Therefore, we conducted a systematic literature review of experimental NP (PubMed Medline), in order to find the mechanism of inducing central sensitization and explore the intervention methods of hyperalgesia caused by real or simulated injury.

RESULT

In this review, we sorted out the activation pathways of microglia, astrocytes and neurons, and the process of crosstalk among them. It was found that in NP, the microglia P2X4 receptor is the key target, which can activate the mitogen-activated protein kinase pathway inward and then activate astrocytes and outwardly activate neuronal tropomyosin receptor kinase B receptor to activate neurons. At the same time, activated neurons continue to maintain the activation of astrocytes and microglia through chemokines on CXCL13/CXCR5 and CX3CL1/CX3CR1. This crosstalk process is the key to maintaining NP.

CONCLUSION

We summarize the further research on crosstalk among neurons, microglia, and astrocytes in the central nervous system, elaborate the ways and connections of relevant crosstalk, and find potential crosstalk targets, which provides a reference for drug development and preclinical research.

Non-coding RNA and n6-methyladenosine modification play crucial roles in neuropathic pain

After peripheral nerve injury, pain signals are transmitted from primary sensory neurons in the dorsal root ganglion (DRG) to the central nervous system. Epigenetic modification affects neuropathic pain through alterations in the gene expression in pain-related areas and glial cell activation. Recent studies have shown that non-coding RNA and n6-methyladenosine (m6A) methylation modification play pivotal regulatory roles in the occurrence and maintenance of neuropathic pain. Dysregulation of the RNA m6A level via dynamic changes in methyltransferase and demethylase after central or peripheral nerve injury commonly regulates pain-associated genes, contributing to the induction and maintenance of neuropathic pain. The dynamic process has significant implications for the development and maintenance of neuropathic pain. However, the underlying mechanisms by which non-coding RNA and m6A RNA modification regulate neuropathic pain are not well-characterized. This article elucidates the multiple mechanisms of non-coding RNA and m6A methylation in the context of neuropathic pain, and summarizes its potential functions as well as recent advances.

Development of High affinity Nanobodies Specific for NaV1.4 and NaV1.5 Voltage-Gated Sodium Channel Isoforms

Voltage-gated sodium channels, Na_Vs, are responsible for the rapid rise of action potentials in excitable tissues. Na_V channel mutations have been implicated in several human genetic diseases, such as hypokalemic periodic paralysis, myotonia, and long-QT and Brugada syndromes. Here, we generated high-affinity anti-Na_V nanobodies (Nbs), Nb17 and Nb82, that recognize the Na_V1.4 (skeletal muscle) and Na_V1.5 (cardiac muscle) channel isoforms. These Nbs were raised in llama (Lama glama) and selected from a phage display library for high affinity to the C-terminal (CT) region of Na_V1.4. The Nbs were expressed in Escherichia coli, purified, and biophysically characterized. Development of high-affinity Nbs specifically targeting a given human Na_V isoform has been challenging because they usually show undesired crossreactivity for different Na_V isoforms. Our results show, however, that Nb17 and Nb82 recognize the CTNa_V1.4 or CTNa_V1.5 over other CTNav isoforms. Kinetic experiments by biolayer interferometry determined that Nb17 and Nb82 bind to the CTNa_V1.4 and CTNa_V1.5 with high affinity (K_D ∼ 40–60 nM). In addition, as proof of concept, we show that Nb82 could detect Na_V1.4 and Na_V1.5 channels in mammalian cells and tissues by Western blot. Furthermore, human embryonic kidney cells expressing holo Na_V1.5 channels demonstrated a robust FRET-binding efficiency for Nb17 and Nb82. Our work lays the foundation for developing Nbs as anti-Na_V reagents to capture Na_Vs from cell lysates and as molecular visualization agents for Na_Vs.

Structural Basis of the Negative Allosteric Modulation of 5-BDBD at Human P2X4 Receptors

The P2X4 receptor is a ligand-gated ion channel activated by extracellular ATP. P2X4 activity is associated with neuropathic pain, vasodilation, and pulmonary secretion and is therefore of therapeutic interest. The structure-activity relationship of P2X4 antagonists is poorly understood. Here we elucidate the structure-activity of 5-(3-bromophenyl)-1,3-dihydro-2H-benzofuro[3,2-e]-1,4-diazepin-2-one (5-BDBD) at human P2X4 by combining pharmacology, electrophysiology, molecular modeling, and medicinal chemistry. 5-BDBD antagonized P2X4 in a noncompetitive manner but lacked effect at human P2X2. Molecular modeling and site-directed mutagenesis suggested an allosteric binding site for 5-BDBD located between two subunits in the body region of P2X4, with M109, F178, Y300, and I312 on one subunit and R301 on the neighboring subunit as key residues involved in antagonist binding. The bromine group of 5-BDBD was redundant for the antagonist activity of 5-BDBD, although an interaction between the carbonyl group of 5-BDBD and R301 in P2X4 was associated with 5-BDBD activity. 5-BDBD could inhibit the closed channel but poorly inhibited the channel in the open/desensitizing state. We hypothesize that this is due to constriction of the allosteric site after transition from closed to open channel state. We propose that M109, F178, Y300, R301, and I312 are key residues for 5-BDBD binding; provide a structural explanation of how they contribute to 5-BDBD antagonism; and highlight that the limited action of 5-BDBD on open versus closed channels is due to a conformational change in the allosteric site. SIGNIFICANCE STATEMENT: Activity of P2X4 receptor is associated with neuropathic pain, inflammation, and vasodilatation. Molecular information regarding small-molecule interaction with P2X4 is very limited. Here, this study provides a structural explanation for the action of the small-molecule antagonist 5-BDBD at the human P2X4 receptor.

Single-Dose P2 X4R Single-Chain Fragment Variable Antibody Permanently Reverses Chronic Pain in Male Mice

Non-opioid single-chain variable fragment (scFv) small antibodies were generated as pain-reducing block of P2X4R receptor (P2X4R). A panel of scFvs targeting an extracellular peptide sequence of P2X4R was generated followed by cell-free ribosome display for recombinant antibody selection. After three rounds of bio-panning, a panel of recombinant antibodies was isolated and characterized by ELISA, cross-reactivity analysis, and immunoblotting/immunostaining. Generated scFv antibodies feature binding activity similar to monoclonal antibodies but with stronger affinity and increased tissue penetrability due to their ~30% smaller size. Two anti-P2X4R scFv clones (95, 12) with high specificity and affinity binding were selected for in vivo testing in male and female mice with trigeminal nerve chronic neuropathic pain (FRICT-ION model) persisting for several months in untreated BALBc mice. A single dose of P2X4R scFv (4 mg/kg, i.p.) successfully, completely, and permanently reversed chronic neuropathic pain-like measures in male mice only, providing retention of baseline behaviors indefinitely. Untreated mice retained hypersensitivity, and developed anxiety- and depression-like behaviors within 5 weeks. In vitro P2X4R scFv 95 treatment significantly increased the rheobase of larger-diameter (>25 µm) trigeminal ganglia (TG) neurons from FRICT-ION mice compared to controls. The data support use of engineered scFv antibodies as non-opioid biotherapeutic interventions for chronic pain.

Mechanisms of ATP release in pain: role of pannexin and connexin channels

Pain is a physiological response to bodily damage and serves as a warning of potential threat. Pain can also transform from an acute response to noxious stimuli to a chronic condition with notable emotional and psychological components that requires treatment. Indeed, the management of chronic pain is currently an important unmet societal need. Several reports have implicated the release of the neurotransmitter adenosine triphosphate (ATP) and subsequent activation of purinergic receptors in distinct pain etiologies. Purinergic receptors are broadly expressed in peripheral neurons and the spinal cord; thus, purinergic signaling in sensory neurons or in spinal circuits may be critical for pain processing. Nevertheless, an outstanding question remains: what are the mechanisms of ATP release that initiate nociceptive signaling? Connexin and pannexin channels are established conduits of ATP release and have been suggested to play important roles in a variety of pathologies, including several models of pain. As such, these large-pore channels represent a new and exciting putative pharmacological target for pain treatment. Herein, we will review the current evidence for a role of connexin and pannexin channels in ATP release during nociceptive signaling, such as neuropathic and inflammatory pain. Collectively, these studies provide compelling evidence for an important role of connexins and pannexins in pain processing.

Microencapsulated Neural Stem Cells Inhibit Sciatic Nerve Injury-Induced Pain by Reducing P2 × 4 Receptor Expression

Objectives: The purpose of this study is to investigate the effects of transplantation of microencapsulated neural stem cells (MC-NSCs), which downregulate the P2 × 4 receptor (P2 × 4R) overexpression and relieve neuropathic pain (NPP). Methods: Neural stem cells (NSCs) and MC-NSCs were transplanted to the injured sciatic nerve. Transmission electron microscope and immunofluorescence were used to observe the changes of injured sciatic nerve. Behavioral methods were used to detect mechanical withdrawal thresholds (MWT) and thermal withdrawal latency (TWL) of rats. Expression levels of P2 × 4Rs and p-p65 in the spinal cord segment of rats were measured by using molecular biology methods. The concentrations of IL-1β and TNF-α were detected in serum of rats by ELISA. Results: After sciatic nerve injury, the sciatic nerve fibers had the myelinated lamina separated, and disintegrated fragments could be seen. The fluorescence intensity of myelin MBP was weakened. The MWT and TWL were significantly decreased, the expression of P2 × 4Rs and p-p65 were significantly increased, and the concentrations of IL-1β and TNF-α were increased. After NSC and MC-NSC transplantation, the myelin sheath of the sciatic nerve was relatively intact, some demyelination changes could be seen, and the injured sciatic nerve has been improved. The fluorescence intensity of myelin MBP was increased. The MWT and TWL were increased, expression levels of P2 × 4Rs and p-p65 were decreased, and the concentrations of IL-1β and TNF-α were significantly decreased. Compared with NSC transplantation, transplantation of MC-NSCs could better repair the damaged sciatic nerve, decrease the expression of P2 × 4Rs and p-p65, decrease the level of IL-1β and TNF-α, and relieve pain (all p-values < 0.05). Conclusion: NSCs and MC-NSCs transplantation may alleviate pain by reducing the expression of P2 × 4Rs and inhibiting the activation of NF-KB signaling, while MC-NSCs transplantation has a better effect of suppressing pain. Our experimental results provide new data support for the treatment of NPP.

Taspine is a natural product that suppresses P2X4 receptor activity via phosphoinositide 3-kinase inhibition

BACKGROUND & PURPOSE

P2X4 is a ligand-gated cation channel activated by extracellular ATP, involved in neuropathic pain, inflammation and arterial tone.

EXPERIMENTAL APPROACH

Natural products were screened against human or mouse P2X4 activity using fura-2 loaded 1321N1 cells for measurement of intracellular Ca²⁺ responses; whole-cell currents were measured by patch clamp electrophysiological. Human primary macrophage chemokine release was used to assess effect of taspine on inflammatory cell function. An enzymatic assay was performed to assess the effect of taspine on recombinant PI3-kinase.

KEY RESULTS

A natural product screen identified taspine as an inhibitor of human P2X4 activity. Taspine inhibits human and mouse P2X4-mediated Ca²⁺ influx in 1321N1 cells expressing receptors (IC₅₀ 1.6±0.4 μM and 1.6±0.3 μM, respectively), but lacked activity at human P2X2, P2X3, P2X2/3 and P2X7 receptors. Taspine inhibited the maximal response at human and mouse P2X4 but had no effect on ATP potency. Taspine has a slow onset rate (~15 mins for half-maximal inhibition), irreversible over 30 minutes of washout. Taspine inhibits P2X4-mediated Ca²⁺ signalling in mouse BV-2 microglia cells and human primary macrophage. Taspine inhibited P2X4-mediated CXCL5 secretion in human primary macrophage. Taspine reversed ivermectin-induced potentiation of P2X4 currents in 1321N1 stably expressing cells. The known PI3-kinase inhibitor LY294002 mimicked the properties of taspine on P2X4-mediated Ca²⁺ influx and whole-cell currents. Taspine directly inhibited the enzymatic activity of recombinant PI3-kinase in a competitive manner.

CONCLUSIONS AND IMPLICATIONS

Taspine is a novel natural product P2X4 inhibitor, mediating its effect through PI3-kinase inhibitor rather than receptor antagonism. Taspine can inhibit the pro-inflammatory signalling by P2X4 in human primary macrophage.

Analysis of binding residues in monoclonal antibody with high affinity for the head domain of the rat P2X4 receptor

P2X4 receptor is known to be involved in neuropathic pain. In order to detect the expression of P2X4 receptor on microglia at the time of onset of neuropathic pain, one approach consists on the preparation of the monoclonal antibodies with both selective binding and high affinity. We have recently established a monoclonal antibody (named 12-10H) which had high affinity to rat P2X4 receptor expressed in 1321N1 cells. The dissociation constants of the complex between the monoclonal antibodies obtained so far and the head domain (HD) in the rat P2X4 receptor were in the nanomolar range. To improve the affinity by rational mutations, we need to know the precious location of the binding site in these monoclonal antibodies. Here, we have analysed and identified the binding residues in the monoclonal antibody (12-10H) with high affinity for the HD of the rat P2X4 receptor by site-directed mutagenesis.

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.

Antihypersensitivity effect of betanin (red beetroot extract) via modulation of microglial activation in a mouse model of neuropathic pain

BACKGROUND

Neuropathic pain (NeP) medications have several side effects that affect NeP patients' quality of life. Betanin, the most common betacyanin pigment, has been shown to have potent antioxidant and anti-inflammatory properties in vivo; thus, it has potential as a healthcare treatment. In this study, we focused on betanin (red beetroot extract) as a potential therapy for NeP.

METHODS

Mice model of NeP were made by chronic constriction injury (CCI), and the development of mechanical hypersensitivity was confirmed using the von Frey test. Motor coordination and locomotor activity were assessed using open field tests and rotarod tests, respectively. The expression level of glial markers in the spinal cords was analyzed by immunostaining. The direct effects of betanin on microglial cells were investigated using primary cultured microglial cells.

RESULTS

In CCI model mice, repeated betanin treatment, both intraperitoneally and orally, attenuated developing mechanical hypersensitivity in a dose-dependent manner without impairing motor coordination. Betanin treatment also attenuated mechanical hypersensitivity that had developed and prevented the onset of mechanical hypersensitivity in CCI mice. Microglial activation in the spinal cord is known to play a key role in the development of NeP; betanin treatment reduced CCI-induced microglial activation in the spinal cord of model mice. Moreover, in primary microglia cultured cells, the activation of microglia by lipopolysaccharide application was suppressed by betanin treatment.

CONCLUSION

Betanin treatment appears to ameliorate mechanical hypersensitivity related to CCI-induced NeP in mice by inhibiting microglial activation.

SIGNIFICANCE

This article supports findings of the effect of betanin on NeP and provides a potential therapeutic candidate for NeP. Furthermore, elucidating the underlying mechanism of the effect of betanin on microglial activation could assist the development of new treatments for chronic pain.

Role of microglia and P2X4 receptors in chronic pain

This study summarizes current understanding of the role of microglia and P2X4 receptor in chronic pain including neuropathic pain and of their therapeutic potential.

Pain plays an indispensable role as an alarm system to protect us from dangers or injuries. However, neuropathic pain, a debilitating pain condition caused by damage to the nervous system, persists for a long period even in the absence of dangerous stimuli or after injuries have healed. In this condition, pain becomes a disease itself rather than the alarm system and is often resistant to currently available medications. A growing body of evidence indicates that microglia, a type of macrophages residing in the central nervous system, play a crucial role in the pathogenesis of neuropathic pain. Whenever microglia in the spinal cord detect a damaging signal within the nervous system, they become activated and cause diverse alterations that change neural excitability, leading to the development of neuropathic pain. For over a decade, several lines of molecular and cellular mechanisms that define microglial activation and subsequently altered pain transmission have been proposed. In particular, P2X4 receptors (a subtype of purinergic receptors) expressed by microglia have been investigated as an essential molecule for neuropathic pain. In this review article, we describe our understanding of the mechanisms by which activated microglia cause neuropathic pain through P2X4 receptors, their involvement in several pathological contexts, and recent efforts to develop new drugs targeting microglia and P2X4 receptors.

Treatment of chronic neuropathic pain: purine receptor modulation

Extracellular nucleosides and nucleotides have widespread functions in responding to physiological stress. The "purinome" encompasses 4 G-protein-coupled receptors (GPCRs) for adenosine, 8 GPCRs activated by nucleotides, 7 adenosine 5'-triphosphate-gated P2X ion channels, as well as the associated enzymes and transporters that regulate native agonist levels. Purinergic signaling modulators, such as receptor agonists and antagonists, have potential for treating chronic pain. Adenosine and its analogues potently suppress nociception in preclinical models by activating A1 and/or A3 adenosine receptors (ARs), but safely harnessing this pathway to clinically treat pain has not been achieved. Both A2AAR agonists and antagonists are efficacious in pain models. Highly selective A3AR agonists offer a novel approach to treat chronic pain. We have explored the structure activity relationship of nucleoside derivatives at this subtype using a computational structure-based approach. Novel A3AR agonists for pain control containing a bicyclic ring system (bicyclo [3.1.0] hexane) in place of ribose were designed and screened using an in vivo phenotypic model, which reflected both pharmacokinetic and pharmacodynamic parameters. High specificity (>10,000-fold selective for A3AR) was achieved with the aid of receptor homology models based on related GPCR structures. These A3AR agonists are well tolerated in vivo and highly efficacious in models of chronic neuropathic pain. Furthermore, signaling molecules acting at P2X3, P2X4, P2X7, and P2Y12Rs play critical roles in maladaptive pain neuroplasticity, and their antagonists reduce chronic or inflammatory pain, and, therefore, purine receptor modulation is a promising approach for future pain therapeutics. Structurally novel antagonists for these nucleotide receptors were discovered recently.

Structural and Functional Features of the P2X4 Receptor: An Immunological Perspective

Extracellular nucleotides are important mediators of activation, triggering various responses through plasma membrane P2 and P1 receptors. P2 receptors are further subdivided into ionotropic P2X receptors and G protein-coupled P2Y receptors. P2X4 is an ATP-gated cation channel broadly expressed in most tissues of the body. Within the P2X family, P2X4 has a unique subcellular distribution, being preferentially localized in lysosomes. In these organelles, high ATP concentrations do not trigger P2X4 because of the low pH. However, when the pH increases to 7.4, P2X4 can be stimulated by intra-lysosomal ATP, which is in its active, tetra-anionic form. Elucidation of P2X4, P2X3 and P2X7 structures has shed some light on the functional differences between these purinergic receptors. The potential interaction between P2X4 and P2X7 has been extensively studied. Despite intensive effort, it has not been possible yet to determine whether P2X4 and P2X7 interact as heterotrimers or homotrimers at the plasma membrane. However, several publications have shown that functional interactions between P2X4 and P2X7 do occur. Importantly, these studies indicate that P2X4 potentiates P2X7-dependent activation of inflammasomes, leading to increased release of IL-1β and IL-18. The role of P2X4 in various diseases could be beneficial or deleterious even though the pathophysiological mechanisms involved are still poorly defined. However, in diseases whose physiopathology involves activation of the NLRP3 inflammasome, P2X4 was found to exacerbate severity of disease. The recent production of monoclonal antibodies specific for the human and mouse P2X4, some of which are endowed with agonist or antagonist properties, raises the possibility that they could be used therapeutically. Analysis of single nucleotide polymorphisms of the human P2RX4 gene has uncovered the association of P2RX4 gene variants with susceptibility to several human diseases.

Electroacupuncture effects on the P2X4R pathway in microglia regulating the excitability of neurons in the substantia gelatinosa region of rats with spinal nerve ligation

Electroacupuncture (EA) has been used to treat neuropathic pain induced by peripheral nerve injury (PNI) by applying an electrical current to acupoints with acupuncture needles. However, the mechanisms by which EA treats pain remain indistinct. High P2X4 receptor (P2X4R) expression levels demonstrate a notable increase in hyperactive microglia in the ipsilateral spinal dorsal horn following PNI. In order to demonstrate the possibility that EA analgesia is mediated in part by P2X4R in hyperactive microglia, the present study performed mechanical withdrawal threshold (MWT) and thermal withdrawal latency (TWL) tests in male Sprague-Dawley rats that had undergone spinal nerve ligation (SNL). The expression levels of spinal P2X4R were determined using reverse transcription-quantitative PCR, western blotting analysis and immunofluorescence staining. Furthermore, spontaneous excitatory postsynaptic currents (sEPSCs) were recorded using whole-cell patch clamp to demonstrate the effect of EA on synaptic transmission in rat spinal substantia gelatinosa (SG) neurons. The results of the present study demonstrated that EA increased the MWT and TWL and decreased overexpression of P2X4R in hyperactive microglia in SNL rats. Moreover, EA attenuated the frequency of sEPSCs in SG neurons in SNL rats. The results of the present study indicate that EA may mediate P2X4R in hyperactive spinal microglia to inhibit nociceptive transmission of SG neurons, thus relieving pain in SNL rats.

P2X4R Overexpression Upregulates Interleukin-6 and Exacerbates 6-OHDA-Induced Dopaminergic Degeneration in a Rat Model of PD

The pathogenesis of Parkinson’s disease (PD) remains elusive. Current thinking suggests that the activation of microglia and the subsequent release of inflammatory factors, including interleukin-6 (IL-6), are involved in the pathogenesis of PD. P2X4 receptor (P2X4R) is a member of the P2X superfamily of ion channels activated by ATP. To study the possible effect of the ATP-P2X4R signal axis on IL-6 in PD, lentivirus carrying the P2X4R-overexpression gene or empty vector was injected into the substantia nigra (SN) of rats, followed by treatment of 6-hydroxydopamine (6-OHDA) or saline 1 week later. The research found the relative expression of P2X4R in the 6-OHDA-induced PD rat models was notably higher than that in the normal. And P2X4R overexpression could upregulate the expression of IL-6, reduce the amount of dopaminergic (DA) neurons in the SN of PD rats, suggesting that P2X4R may mediate the production of IL-6 to damage DA neurons in the SN. Our data revealed the important role of P2X4R in modulating IL-6, which leads to neuroinflammation involved in PD pathogenesis.

Dissection of P2X4 and P2X7 Receptor Current Components in BV-2 Microglia

Microglia cells represent the immune system of the central nervous system. They become activated by ATP released from damaged and inflamed tissue via purinergic receptors. Ionotropic purinergic P2X4 and P2X7 receptors have been shown to be involved in neurological inflammation and pain sensation. Whether the two receptors assemble exclusively as homotrimers or also as heterotrimers is still a matter of debate. We investigated the expression of P2X receptors in BV-2 microglia cells applying the whole-cell voltage-clamp technique. We dissected P2X4 and P2X7 receptor-mediated current components by using specific P2X4 and P2X7 receptor blockers and by their characteristic current kinetics. We found that P2X4 and P2X7 receptors are activated independently from each other, indicating that P2X4/P2X7 heteromers are not of functional significance in these cells. The pro-inflammatory mediators lipopolysaccharide and interferon γ, if applied in combination, upregulated P2X4, but not P2X7 receptor-dependent current components also arguing against phenotypically relevant heteromerization of P2X4 and P2X7 receptor subunits.

Nociceptive signaling mediated by P2X3, P2X4 and P2X7 receptors

Chronic pain is a debilitating condition that often occurs following peripheral tissue inflammation and nerve injury. This pain, especially neuropathic pain, is a significant clinical problem because of the ineffectiveness of clinically available drugs. Since Burnstock proposed new roles of nucleotides as neurotransmitters, the roles of extracellular ATP and P2 receptors (P2Rs) in pain signaling have been extensively studied, and ATP-P2R signaling has subsequently received much attention as it can provide clues toward elucidating the mechanisms underlying chronic pain and serve as a potential therapeutic target. This review summarizes the literature regarding the role of ATP signaling via P2X3Rs (as well as P2X2/3Rs) in primary afferent neurons and via P2X4Rs and P2X7Rs in spinal cord microglia in chronic pain, and discusses their respective therapeutic potentials.

Role of the immune system in neuropathic pain

Background Acute pain is a warning mechanism that exists to prevent tissue damage, however pain can outlast its protective purpose and persist beyond injury, becoming chronic. Chronic Pain is maladaptive and needs addressing as available medicines are only partially effective and cause severe side effects. There are profound differences between acute and chronic pain. Dramatic changes occur in both peripheral and central pathways resulting in the pain system being sensitised, thereby leading to exaggerated responses to noxious stimuli (hyperalgesia) and responses to non-noxious stimuli (allodynia). Critical role for immune system cells in chronic pain Preclinical models of neuropathic pain provide evidence for a critical mechanistic role for immune cells in the chronicity of pain. Importantly, human imaging studies are consistent with preclinical findings, with glial activation evident in the brain of patients experiencing chronic pain. Indeed, immune cells are no longer considered to be passive bystanders in the nervous system; a consensus is emerging that, through their communication with neurons, they can both propagate and maintain disease states, including neuropathic pain. The focus of this review is on the plastic changes that occur under neuropathic pain conditions at the site of nerve injury, the dorsal root ganglia (DRG) and the dorsal horn of the spinal cord. At these sites both endothelial damage and increased neuronal activity result in recruitment of monocytes/macrophages (peripherally) and activation of microglia (centrally), which release mediators that lead to sensitisation of neurons thereby enabling positive feedback that sustains chronic pain. Immune system reactions to peripheral nerve injuries At the site of peripheral nerve injury following chemotherapy treatment for cancer for example, the occurrence of endothelial activation results in recruitment of CX3C chemokine receptor 1 (CX3CR1)-expressing monocytes/macrophages, which sensitise nociceptive neurons through the release of reactive oxygen species (ROS) that activate transient receptor potential ankyrin 1 (TRPA1) channels to evoke a pain response. In the DRG, neuro-immune cross talk following peripheral nerve injury is accomplished through the release of extracellular vesicles by neurons, which are engulfed by nearby macrophages. These vesicles deliver several determinants including microRNAs (miRs), with the potential to afford long-term alterations in macrophages that impact pain mechanisms. On one hand the delivery of neuron-derived miR-21 to macrophages for example, polarises these cells towards a pro-inflammatory/pro-nociceptive phenotype; on the other hand, silencing miR-21 expression in sensory neurons prevents both development of neuropathic allodynia and recruitment of macrophages in the DRG. Immune system mechanisms in the central nervous system In the dorsal horn of the spinal cord, growing evidence over the last two decades has delineated signalling pathways that mediate neuron-microglia communication such as P2X4/BDNF/GABAA, P2X7/Cathepsin S/Fractalkine/CX3CR1, and CSF-1/CSF-1R/DAP12 pathway-dependent mechanisms. Conclusions and implications Definition of the modalities by which neuron and immune cells communicate at different locations of the pain pathway under neuropathic pain states constitutes innovative biology that takes the pain field in a different direction and provides opportunities for novel approaches for the treatment of chronic pain.

Resolving the Ionotropic P2X4 Receptor Mystery Points Towards a New Therapeutic Target for Cardiovascular Diseases

Adenosine triphosphate (ATP) is a primordial versatile autacoid that changes its role from an intracellular energy saver to a signaling molecule once released to the extracellular milieu. Extracellular ATP and its adenosine metabolite are the main activators of the P2 and P1 purinoceptor families, respectively. Mounting evidence suggests that the ionotropic P2X4 receptor (P2X4R) plays pivotal roles in the regulation of the cardiovascular system, yet further therapeutic advances have been hampered by the lack of selective P2X4R agonists. In this review, we provide the state of the art of the P2X4R activity in the cardiovascular system. We also discuss the role of P2X4R activation in kidney and lungs vis a vis their interplay to control cardiovascular functions and dysfunctions, including putative adverse effects emerging from P2X4R activation. Gathering this information may prompt further development of selective P2X4R agonists and its translation to the clinical practice.

The role of P2X4 receptors in chronic pain: A potential pharmacological target

Chronic pain is a common symptom of most clinical diseases, which seriously affects the psychosomatic health of patients and brings some pain to patients. Due to its pathological mechanism is very complicated and the treatment of chronic pain has always been a difficult problem in clinical. Normally, drugs are usually used to relieve pain, but the analgesic effect is not good, especially for cancer pain patients, the analgesic effect is poor. Therefore, exploring the pathogenesis and treatment of chronic pain has aroused the interest of many researchers. A large number of studies have shown that the role of ATP and P2X4 receptor (P2X4R) play an important role in the pathogenesis of chronic pain. P2X4R is dependent on ATP ligand-gated ion channel receptor, which can be activated by ATP and plays an important role in the information transmission of nerve system and the formation of pain. Therefore, in this paper, we comprehensively described the structure and biological functions of P2X4R, and outlined behavioral evaluation methods of chronic pain models. Moreover, we also explored the inherent relationship between P2X4R and chronic pain, and described the therapeutic effect of P2X4R antagonist on chronic pain, and provided some valuable help for the treatment of chronic pain.

Generating therapeutic monoclonal antibodies to complex multi-spanning membrane targets: Overcoming the antigen challenge and enabling discovery strategies

Complex integral membrane proteins, which are embedded in the cell surface lipid bilayer by multiple transmembrane spanning helices, encompass families of proteins which are important target classes for drug discovery. These protein families include G protein-coupled receptors, ion channels and transporters. Although these proteins have typically been targeted by small molecule drugs and peptides, the high specificity of monoclonal antibodies offers a significant opportunity to selectively modulate these target proteins. However, it remains the case that isolation of antibodies with desired pharmacological function(s) has proven difficult due to technical challenges in preparing membrane protein antigens suitable to support antibody drug discovery. In this review recent progress in defining strategies for generation of membrane protein antigens is outlined. We also highlight antibody isolation strategies which have generated antibodies which bind the membrane protein and modulate the protein function.

The role of P2X4 receptor in neuropathic pain and its pharmacological properties

Neuropathic pain (NPP) is a common symptom of most diseases in clinic, which seriously affects the mental health of patients and brings certain pain to patients. Due to its pathological mechanism is very complicated, and thus, its treatment has been one of the challenges in the field of medicine. Therefore, exploring the pathogenesis and treatment approach of NPP has aroused the interest of many researchers. ATP is an important energy information substance, which participates in the signal transmission in the body. The P2 × 4 receptor (P2 × 4R) is dependent on ATP ligand-gated cationic channel receptor, which can be activated by ATP and plays an important role in the transmission of information in the nervous system and the formation of pain. In this paper, we provide a comprehensive review of the structure and function of the P2 × 4R gene. We also discuss the pathogenesis of NPP and the intrinsic relationship between P2 × 4R and NPP. Moreover, we explore the pharmacological properties of P2 × 4R antagonists or inhibitors used as targeted therapies for NPP.

Microglia-Mediated Regulation of Neuropathic Pain: Molecular and Cellular Mechanisms

Pain is a defense system that responds rapidly to harmful internal and external stimuli through the somatosensory neuronal pathway. However, damage to the nervous system through cancer, diabetes, infection, autoimmune disease, chemotherapy or trauma often leads to neuropathic pain, a debilitating chronic pain condition. Neuropathic pain is not simply a temporal continuum of acute nociceptive signals from the periphery, but rather due to pathologically altered functions in the nervous system, which shift the net neuronal excitatory balance toward excitation. Although alterations were long thought to be a result of changes in neurons, but an increasing body of evidence over the past decades indicates the necessity and sufficiency of microglia, the tissue-resident macrophages of the spinal cord and brain, for nerve injury-induced malfunction of the nervous system. In this review article, I describe our current understanding of the molecular and cellular mechanisms underlying the role of microglia in the pathogenesis of neuropathic pain and discuss the therapeutic potential of microglia from recent advances in the development of new drugs targeting microglia.

Positively Charged Residues in the Head Domain of P2X4 Receptors Assist the Binding of ATP

P2X receptors are a family of trimeric cationic channels located in the membrane of mammalian cells. They open in response to the binding of ATP. The differences between the closed and open structures have been described in detail for some members of the family. However, the order in which the conformational changes take place as ATP enters the binding cleft, and the residues involved in the intermediate stages, are still unknown. Here, we present the results of umbrella sampling simulations aimed to elucidate the sequence of conformational changes that occur during the reversible binding of ATP to the P2X4 receptor. The simulations also provided information about the interactions that develop in the course of the process. In particular, they revealed the existence of a metastable state which assists the binding. This state is stabilized by positively charged residues located in the head domain of the receptor. Based on these findings, we propose a novel mechanism for the capture of ATP by P2X4 receptors.

Developing Modern Pain Therapies

Chronic pain afflicts as much as 50% of the population at any given time but our methods to address pain remain limited, ineffective and addictive. In order to develop new therapies an understanding of the mechanisms of painful sensitization is essential. We discuss here recent progress in the understanding of mechanisms underlying pain, and how these mechanisms are being targeted to produce modern, specific therapies for pain. Finally, we make recommendations for the next generation of targeted, effective, and safe pain therapies.

Generation and Characterization of Specific Monoclonal Antibodies and Nanobodies Directed Against the ATP-Gated Channel P2X4

The P2X4 channel is involved in different physiological and pathological conditions and functions in the nervous system. Despite the existence of several mouse models for which the expression of the gene was manipulated, there is still little information on the expression of the protein at the cellular level. In particular, supposedly specific available antibodies have often proved to recognize unrelated proteins in P2X4-deficient mice. Here, we used an in vivo DNA vaccine approach to generate a series of monoclonal antibodies and nanobodies specific for human, mouse, and rat P2X4 channels. We further characterized these antibodies and show that they solely recognize the native form of the proteins both in biochemical and cytometric applications. Some of these antibodies prove to specifically recognize P2X4 channels by immunostaining in brain or sensory ganglia slices, as well as at the cellular and subcellular levels. Due to their clonality, these different antibodies should represent versatile tools for further characterizing the cellular functions of P2X4 in the nervous system as well as at the periphery.