A shared mechanism for TNP-ATP recognition by members of the P2X receptor family

P2X receptors (P2X1-7) are non-selective cation channels involved in many physiological activities such as synaptic transmission, immunological modulation, and cardiovascular function. These receptors share a conserved mechanism to sense extracellular ATP. TNP-ATP is an ATP derivative acting as a nonselective competitive P2X antagonist. Understanding how it occupies the orthosteric site in the absence of agonism may help reveal the key allostery during P2X gating. However, TNP-ATP/P2X complexes (TNP-ATP/human P2X3 (hP2X3) and TNP-ATP/chicken P2X7 (ckP2X7)) with distinct conformations and different mechanisms of action have been proposed. Whether these represent species and subtype variations or experimental differences remains unclear. Here, we show that a common mechanism of TNP-ATP recognition exists for the P2X family members by combining enhanced conformation sampling, engineered disulfide bond analysis, and covalent occupancy. In this model, the polar triphosphate moiety of TNP-ATP interacts with the orthosteric site, while its TNP-moiety is deeply embedded in the head and dorsal fin (DF) interface, creating a restrictive allostery in these two domains that results in a partly enlarged yet ion-impermeable pore. Similar results were obtained from multiple P2X subtypes of different species, including ckP2X7, hP2X3, rat P2X2 (rP2X2), and human P2X1 (hP2X1). Thus, TNP-ATP uses a common mechanism for P2X recognition and modulation by restricting the movements of the head and DF domains which are essential for P2X activation. This knowledge is applicable to the development of new P2X inhibitors.

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.

Stress/cell death pathways, neuroinflammation, and neuropathic pain

Neuropathic pain is a common and debilitating modality of chronic pain induced by a lesion or disease of the somatosensory nervous system. Albeit the elucidation of numerous pathophysiological mechanisms and the development of potential treatment compounds, safe and reliable therapies of neuropathic pain remain poor. Multiple stress/cell death pathways have been shown to be implicated in neuroinflammation during neuropathic pain. Here, we summarize the current knowledge of stress/cell death pathways and present an overview of the roles and molecular mechanisms of stress/cell death pathways in neuroinflammation during neuropathic pain, covering intrinsic and extrinsic apoptosis, autophagy, mitophagy, ferroptosis, pyroptosis, necroptosis, and phagoptosis. Small molecule compounds that modulate stress/cell death pathways in alleviating neuropathic pain are discussed mainly based on preclinical neuropathic pain models. These findings will contribute to in-depth understanding of the pathological processes during neuropathic pain as well as bridge the gap between basic and translational research to uncover new neuroprotective interventions.

Microglial P2X4 receptors promote ApoE degradation and contribute to memory deficits in Alzheimer's disease

Numerous evidences support that microglia contributes to the progression of Alzheimer's disease. P2X4 receptors are ATP-gated channels with high calcium permeability, which are de novo expressed in a subset of reactive microglia associated with various pathological contexts, contributing to microglial functions. P2X4 receptors are mainly localized in lysosomes and trafficking to the plasma membrane is tightly regulated. Here, we investigated the role of P2X4 in the context of Alzheimer's disease (AD). Using proteomics, we identified Apolipoprotein E (ApoE) as a specific P2X4 interacting protein. We found that P2X4 regulates lysosomal cathepsin B (CatB) activity promoting ApoE degradation; P2rX4 deletion results in higher amounts of intracellular and secreted ApoE in both bone-marrow-derived macrophage (BMDM) and microglia from APPswe/PSEN1dE9 brain. In both human AD brain and APP/PS1 mice, P2X4 and ApoE are almost exclusively expressed in plaque-associated microglia. In 12-month-old APP/PS1 mice, genetic deletion of P2rX4 reverses topographical and spatial memory impairment and reduces amount of soluble small aggregates of Aß1-42 peptide, while no obvious alteration of plaque-associated microglia characteristics is observed. Our results support that microglial P2X4 promotes lysosomal ApoE degradation, indirectly altering Aß peptide clearance, which in turn might promotes synaptic dysfunctions and cognitive deficits. Our findings uncover a specific interplay between purinergic signaling, microglial ApoE, soluble Aß (sAß) species and cognitive deficits associated with AD.

Neuronal P2X4 receptor may contribute to peripheral inflammatory pain in rat spinal dorsal horn

Objective

Intense inflammation may result in pain, which manifests as spinal central sensitization. There is growing evidence that purinergic signaling plays a pivotal role in the orchestration of pain processing. Over the last decade the ionotropic P2X purino receptor 4 (P2X4) got into spotlight in neuropathic disorders, however its precise spinal expression was scantily characterized during inflammatory pain. Thus, we intended to analyze the receptor distribution within spinal dorsal horn and lumbar dorsal root ganglia (DRG) of rats suffering in inflammatory pain induced by complete Freund adjuvant (CFA).

Methods

CFA-induced peripheral inflammation was validated by mechanical and thermal behavioral tests. In order to ensure about the putative alteration of spinal P2X4 receptor gene expression qPCR reactions were designed, followed by immunoperoxidase and Western blot experiments to assess changes at a protein level. Colocalization of P2X4 with neuronal and glial markers was investigated by double immunofluorescent labelings, which were subsequently analyzed with IMARIS software. Transmission electronmicroscopy was applied to study the ultrastructural localization of the receptor. Concurrently, in lumbar DRG cells similar methodology has been carried out to complete our observations.

Results

The figures of mechanical and thermal behavioral tests proved the establishment of CFA-induced inflammatory pain. We observed significant enhancement of P2X4 transcript level within the spinal dorsal horn 3 days upon CFA administration. Elevation of P2X4 immunoreactivity within Rexed lamina I-II of the spinal gray matter was synchronous with mRNA expression, and confirmed by protein blotting. According to IMARIS analysis the robust protein increase was mainly detected on primary afferent axonterminals and GFAP-labelled astrocyte membrane compartments, but not on postsynaptic dendrites was also validated ultrastructurally within the spinal dorsal horn. Furthermore, lumbar DRG analysis demonstrated that peptidergic and non-peptidergic nociceptive subsets of ganglia cells were also abundantly positive for P2X4 receptor in CFA model.

Conclusion

Here we provide novel evidence about involvement of neuronal and glial P2X4 receptor in the establishment of inflammatory pain.

Lysosomal Potassium Channels

Lysosomes are acidic membrane-bound organelles that use hydrolytic enzymes to break down material through pathways such as endocytosis, phagocytosis, mitophagy, and autophagy. To function properly, intralysosomal environments are strictly controlled by a set of integral membrane proteins such as ion channels and transporters. Potassium ion (K⁺) channels are a large and diverse family of membrane proteins that control K⁺ flux across both the plasma membrane and intracellular membranes. In the plasma membrane, they are essential in both excitable and non-excitable cells for the control of membrane potential and cell signaling. However, our understanding of intracellular K⁺ channels is very limited. In this review, we summarize the recent development in studies of K⁺ channels in the lysosome. We focus on their characterization, potential roles in maintaining lysosomal membrane potential and lysosomal function, and pathological implications.

The role of microglial/macrophagic salt-inducible kinase 3 on normal and excessive phagocytosis after transient focal cerebral ischemia

Previous studies suggested that anti-inflammatory microglia/macrophages (Mi/MΦ) play a role in “normal phagocytosis,” which promoted the rapid clearance of necrotic substances and apoptotic cells. More recently, a few studies have found that Mi/MΦ also play a role in “pathological phagocytosis” in the form of excessive or reduced phagocytosis, thereby worsening damage induced by CNS diseases. However, the underlying mechanisms and the Mi/MΦ subtypes related to this pathological phagocytosis are still unknown. Salt-inducible kinase 3 (SIK3), a member of the 5’ adenosine monophosphate-activated protein kinase (AMPK) family, has been shown to regulate inflammation in several peripheral diseases. Whether SIK3 also regulates the inflammatory response in CNS diseases is currently unknown. Therefore, in this study, we created a transgenic tamoxifen-induced Mi/MΦ-specific SIK3 conditional knockout (SIK3-cKO) mouse to examine SIK3’s role in phagocytotic function induced by transient focal cerebral ischemia (tFCI). By single-cell RNA-seq, we found the pro-inflammatory Mi/MΦ phenotype performed an excessive phagocytotic function, but the anti-inflammatory Mi/MΦ phenotype performed a normal phagocytotic function. We found that SIK3-cKO caused Mi/MΦ heterogenization from the transitional phenotype to the anti-inflammatory phenotype after tFCI. This phenotypic shift corresponded with enhanced phagocytosis of both apoptotic and live neurons. Interestingly, SIK3-cKO enhanced normal phagocytosis of myelin debris but attenuating excessive phagocytosis of non-damaged myelin sheath, thereby protecting white matter integrity after tFCI. CD16, a pro-inflammation marker, was decreased significantly by SIK3-cKO and correlated with “excessive phagocytosis.” SIK3-cKO promoted long-term recovery of white matter function and neurological function as assessed with electrophysiological compound action potential (CAPs) and behavioral analysis. This study is the first to show a role of SIK3 in Mi/MΦ phagocytosis in CNS diseases, and reveals that promoting Mi/MΦ anti-inflammatory heterogenization inhibits “excessive phagocytosis” of live cells and facilitates “normal phagocytosis” of apoptotic cells. Therefore, inhibition of SIK3 in Mi/MΦ may be a potential therapeutic target in stroke and other CNS diseases with accompanying white matter destruction.

Increased surface P2X4 receptors by mutant SOD1 proteins contribute to ALS pathogenesis in SOD1-G93A mice

Amyotrophic lateral sclerosis (ALS) is a fatal motoneuron (MN) disease characterized by protein misfolding and aggregation leading to cellular degeneration. So far neither biomarker, nor effective treatment has been found. ATP signaling and P2X4 receptors (P2X4) are upregulated in various neurodegenerative diseases. Here we show that several ALS-related misfolded proteins including mutants of SOD1 or TDP-43 lead to a significant increase in surface P2X4 receptor density and function in vitro. In addition, we demonstrate in the spinal the cord of SOD1-G93A (SOD1) mice that misfolded SOD1-G93A proteins directly interact with endocytic adaptor protein-2 (AP2); thus, acting as negative competitors for the interaction between AP2 and P2X4, impairing constitutive P2X4 endocytosis. The higher P2X4 surface density was particularly observed in peripheral macrophages of SOD1 mice before the onset and during the progression of ALS symptoms positioning P2X4 as a potential early biomarker for ALS. P2X4 expression was also upregulated in spinal microglia of SOD1 mice during ALS and affect microglial inflammatory responses. Importantly, we report using double transgenic SOD1 mice expressing internalization-defective P2X4mCherryIN knock-in gene or invalidated for the P2X4 gene that P2X4 is instrumental for motor symptoms, ALS progression and survival. This study highlights the role of P2X4 in the pathophysiology of ALS and thus its potential for the development of biomarkers and treatments. We also decipher the molecular mechanism by which misfolded proteins related to ALS impact P2X4 trafficking at early pathological stage in cells expressing-P2X4.

Blocking P2X purinoceptor 4 signalling alleviates cigarette smoke induced pulmonary inflammation

Background

Chronic obstructive pulmonary disease (COPD) is associated with elevated ATP levels in the extracellular space. Once released, ATP serves as danger signal modulating immune responses by activating purinergic receptors. Accordingly, purinergic signalling has been implicated in respiratory inflammation associated with cigarette smoke exposure. However, the role of P2X4-signalling has not been fully elucidated yet.

Methods

Here, we analysed the P2X4 mRNA expression in COPD patients as well as cigarette smoke-exposed mice. Furthermore, P2X4-signalling was blocked by either using a specific antagonist or genetic depletion of P2rx4 in mice applied to an acute and prolonged model of cigarette smoke exposure. Finally, we inhibited P2X4-signalling in macrophages derived from THP-1 before stimulation with cigarette smoke extract.

Results

COPD patients exhibited an increased P2X4 mRNA expression in cells isolated from the bronchoalveolar lavage fluid and peripheral mononuclear cells. Similarly, P2rx4 expression was elevated in lung tissue of mice exposed to cigarette smoke. Blocking P2X4-signalling in mice alleviated cigarette smoke induced airway inflammation as well as lung parenchyma destruction. Additionally, human macrophages derived from THP-1 cells released reduced concentrations of proinflammatory cytokines in response to cigarette smoke extract stimulation when P2X4 was inhibited.

Conclusion

Taken together, we provide evidence that P2X4-signalling promotes innate immunity in the immunopathologic responses induced by cigarette smoke exposure.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12931-022-02072-z.

The P2X4 Receptor: Cellular and Molecular Characteristics of a Promising Neuroinflammatory Target

The adenosine 5′-triphosphate-gated P2X4 receptor channel is a promising target in neuroinflammatory disorders, but the ability to effectively target these receptors in models of neuroinflammation has presented a constant challenge. As such, the exact role of P2X4 receptors and their cell signalling mechanisms in human physiology and pathophysiology still requires further elucidation. To this end, research into the molecular mechanisms of P2X4 receptor activation, modulation, and inhibition has continued to gain momentum in an attempt to further describe the role of P2X4 receptors in neuroinflammation and other disease settings. Here we provide an overview of the current understanding of the P2X4 receptor, including its expression and function in cells involved in neuroinflammatory signalling. We discuss the pharmacology of P2X4 receptors and provide an overview of P2X4-targeting molecules, including agonists, positive allosteric modulators, and antagonists. Finally, we discuss the use of P2X4 receptor modulators and antagonists in models of neuroinflammatory cell signalling and disease.

Augmentation of Extracellular ATP Synergizes With Chemotherapy in Triple Negative Breast Cancer

Introduction

Breast cancer affects two million patients worldwide every year and is the most common cause of cancer-related death among women. The triple-negative breast cancer (TNBC) sub-type is associated with an especially poor prognosis because currently available therapies fail to induce long-lasting responses. Therefore, there is an urgent need to develop novel therapies that result in durable responses. One universal characteristic of the tumor microenvironment is a markedly elevated concentration of extracellular adenosine triphosphate (eATP). Chemotherapy exposure results in further increases in eATP through its release into the extracellular space of cancer cells via P2RX channels. eATP is degraded by eATPases. Given that eATP is toxic to cancer cells, we hypothesized that augmenting the release of eATP through P2RX channels and inhibiting extracellular ATPases would sensitize TNBC cells to chemotherapy.

Methods

TNBC cell lines MDA-MB 231, Hs 578t and MDA-MB 468 and non-tumorigenic immortal mammary epithelial MCF-10A cells were treated with increasing concentrations the chemotherapeutic agent paclitaxel in the presence of eATPases or specific antagonists of P2RXs with cell viability and eATP content being measured. Additionally, the mRNA, protein and cell surface expressions of the purinergic receptors P2RX4 and P2RX7 were evaluated in all examined cell lines via qRT-PCR, western blot, and flow cytometry analyses, respectively.

Results

In the present study, we observed dose-dependent declines of cell viability and increases in eATP of paclitaxel-treated TNBC cell lines in the presence of inhibitors of eATPases, but not of the MCF-10A cell line. These effects were reversed by specific antagonists of P2RXs. Similar results, as those observed with eATPase inhibitors, were seen with P2RX activators. All examined cell lines expressed both P2RX4 and P2RX7 at the mRNA, protein and cell surface levels.

Conclusion

These results reveal that eATP modulates the chemotherapeutic response in TNBC cell lines, which could be exploited to enhance the efficacy of chemotherapy regimens for TNBC.

The Role of Microglial Purinergic Receptors in Pain Signaling

Pain is an essential modality of sensation in the body. Purinergic signaling plays an important role in nociceptive pain transmission, under both physiological and pathophysiological conditions, and is important for communication between both neuronal and non-neuronal cells. Microglia and astrocytes express a variety of purinergic effectors, and a variety of receptors play critical roles in the pathogenesis of neuropathic pain. In this review, we discuss our current knowledge of purinergic signaling and of the compounds that modulate purinergic transmission, with the aim of highlighting the importance of purinergic pathways as targets for the treatment of persistent pain.

Lysosomal potassium channels

The lysosome is an important membrane-bound acidic organelle that is regarded as the degradative center as well as multifunctional signaling hub. It digests unwanted macromolecules, damaged organelles, microbes, and other materials derived from endocytosis, autophagy, and phagocytosis. To function properly, the ionic homeostasis and membrane potential of the lysosome are strictly regulated by transporters and ion channels. As the most abundant cation inside the cell, potassium ions (K⁺) are vital for lysosomal membrane potential and lysosomal calcium (Ca²⁺) signaling. However, our understanding about how lysosomal K⁺homeostasis is regulated and what are the functions of K⁺in the lysosome is very limited. Currently, two lysosomal K⁺channels have been identified: large-conductance Ca²⁺-activated K⁺channel (BK) and transmembrane Protein 175 (TMEM175). In this review, we summarize recent development in our understanding of K⁺ homeostasis and K⁺channels in the lysosome. We hope to guide the readers into a more in-depth discussion of lysosomal K⁺ channels in lysosomal physiology and human diseases.

Structural biology of cation channels important for lysosomal calcium release

Calcium is one of the most important second messengers in cells. The uptake and release of calcium ions are conducted by channels and transporters. Inside a eukaryotic cell, calcium is stored in intracellular organelles including the endoplasmic reticulum (ER), mitochondrion, and lysosome. Lysosomes are acid membrane-bounded organelles serving as the crucial degradation and recycling center of the cell. Lysosomes involve in multiple important signaling events, including nutrient sensing, lipid metabolism, and trafficking. Hitherto, two lysosomal cation channel families have been suggested to function as calcium release channels, namely the Two-pore Channel (TPC) family, and the Transient Receptor Potential Channel Mucolipin (TRPML) family. Additionally, a few plasma membrane calcium channels have also been found in the lysosomal membrane under certain circumstances. In this review, we will discuss the structural mechanism of the cation channels that may be important for lysosomal calcium release, primarily focusing on the TPCs and TRPMLs.

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.

Acidic Ca2+ stores and immune-cell function

Acidic organelles act as intracellular Ca²⁺ stores; they actively sequester Ca²⁺ in their lumina and release it to the cytosol upon activation of endo-lysosomal Ca²⁺ channels. Recent data suggest important roles of endo-lysosomal Ca²⁺ channels, the Two-Pore Channels (TPCs) and the TRPML channels (mucolipins), in different aspects of immune-cell function, particularly impacting membrane trafficking, vesicle fusion/fission and secretion. Remarkably, different channels on the same acidic vesicles can couple to different downstream physiology. Endo-lysosomal Ca²⁺ stores can act under different modalities, be they acting alone (via local Ca²⁺ nanodomains around TPCs/TRPMLs) or in conjunction with the ER Ca²⁺ store (to either promote or suppress global ER Ca²⁺ release). These different modalities impinge upon functions as broad as phagocytosis, cell-killing, anaphylaxis, immune memory, thrombostasis, and chemotaxis.

P2X4 receptor participates in autophagy regulation in Parkinson's disease

Dysfunctional autophagy often occurs during the development of neurodegenerative diseases, such as Parkinson's disease, Huntington's disease, and Alzheimer's disease. The purinergic P2X4 receptor is an ATP-gated ion channel that is widely expressed in the microglia, astrocytes, and neurons of the central and peripheral nervous systems. P2X4R is involved in the regulation of cellular excitability, synaptic transmission, and neuroinflammation. However, the role played by P2X4R in Parkinson's disease remains poorly understood. Rat models of Parkinson's disease were established by injecting 6-hydroxydopamine into the substantia nigra pars compacta. P2X4R-targeted small interfering RNA (siRNA) was injected into the same area 1 week before injury induction to inhibit the expression of the P2X4 receptor. The results showed that the inhibition of P2X4 receptor expression in Parkinson's disease model rats reduced the rotation behavior induced by apomorphine treatment, increased the latency on the rotarod test, and upregulated the expression of tyrosine hydroxylase, brain-derived neurotrophic factor, LC3-II/LC3-I, Beclin-1, and phosphorylated tropomyosin receptor kinase B (TrkB) in brain tissue, while simultaneously reducing p62 levels. These findings suggest that P2X4 receptor activation might inhibit neuronal autophagy through the regulation of the brain-derived neurotrophic factor/TrkB signaling pathway, leading to dopaminergic neuron damage in the substantia nigra and the further inhibition of P2X4 receptor-mediated autophagy. These results indicate that P2X4 receptor might serve as a potential novel target for the treatment of Parkinson's disease. This study was approved by the Animal Ethics Committee of Affiliated Hospital of Qingdao University (approval No. QYFYWZLL26119) on April 12, 2016.

P2X4 Receptors Mediate Ca2+ Release from Lysosomes in Response to Stimulation of P2X7 and H1 Histamine Receptors

The P2X4 purinergic receptor is targeted to endolysosomes, where it mediates an inward current dependent on luminal ATP and pH. Activation of P2X4 receptors was previously shown to trigger lysosome fusion, but the regulation of P2X4 receptors and their role in lysosomal Ca²⁺ signaling are poorly understood. We show that lysosomal P2X4 receptors are activated downstream of plasma membrane P2X7 and H₁ histamine receptor stimulation. When P2X4 receptors are expressed, the increase in near-lysosome cytosolic [Ca²⁺] is exaggerated, as detected with a low-affinity targeted Ca²⁺ sensor. P2X4-dependent changes in lysosome properties were triggered downstream of P2X7 receptor activation, including an enlargement of lysosomes indicative of homotypic fusion and a redistribution of lysosomes towards the periphery of the cell. Lysosomal P2X4 receptors, therefore, have a role in regulating lysosomal Ca²⁺ release and the regulation of lysosomal membrane trafficking.

The glyco-redox interplay: Principles and consequences on the role of reactive oxygen species during protein glycosylation

Reactive oxygen species (ROS) carry out prime physiological roles as intracellular signaling agents, yet pathologically high concentrations of ROS cause irreversible damage to biomolecules, alter cellular programs and contribute to various diseases. While decades of intensive research have identified redox-related patterns and signaling pathways, very few addressed how the glycosylation machinery senses and responds to oxidative stress. A common trait among ROS and glycans residing on glycoconjugates is that they are both highly dynamic, as they are quickly fine-tuned in response to stressors such as inflammation, cancer and infectious diseases. On this account, the delicate balance of the redox potential, which is tightly regulated by dozens of enzymes including NOXs, and the mitochondrial electron transport chain as well as the fluidity of glycan biosynthesis resulting from the cooperation of glycosyltransferases, glycosidases, and nucleotide sugar transporters, is paramount to cell survival. Here, we review the broad spectrum of the interplay between redox changes and glycosylation with respect to their principle consequences on human physiology.

Should I stay or should I go? Golgi membrane spatial organization for protein sorting and retention

The Golgi complex is the membrane-bound organelle that lies at the center of the secretory pathway. Its main functions are to maintain cellular lipid homeostasis, to orchestrate protein processing and maturation, and to mediate protein sorting and export. These functions are not independent of one another, and they all require that the membranes of the Golgi complex have a well-defined biochemical composition. Importantly, a finely-regulated spatiotemporal organization of the Golgi membrane components is essential for the correct performance of the organelle. In here, we review our current mechanistic and molecular understanding of how Golgi membranes are spatially organized in the lateral and axial directions to fulfill their functions. In particular, we highlight the current evidence and proposed models of intra-Golgi transport, as well as the known mechanisms for the retention of Golgi residents and for the sorting and export of transmembrane cargo proteins. Despite the controversies, conflicting evidence, clashes between models, and technical limitations, the field has moved forward and we have gained extensive knowledge in this fascinating topic. However, there are still many important questions that remain to be completely answered. We hope that this review will help boost future investigations on these issues.

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.

Increased surface P2X4 receptor regulates anxiety and memory in P2X4 internalization-defective knock-in mice

ATP signaling and surface P2X4 receptors are upregulated selectively in neurons and/or glia in various CNS disorders including anxiety, chronic pain, epilepsy, ischemia, and neurodegenerative diseases. However, the cell-specific functions of P2X4 in pathological contexts remain elusive. To elucidate P2X4 functions, we created a conditional transgenic knock-in P2X4 mouse line (Floxed P2X4mCherryIN) allowing the Cre activity-dependent genetic swapping of the internalization motif of P2X4 by the fluorescent mCherry protein to prevent constitutive endocytosis of P2X4. By combining molecular, cellular, electrophysiological, and behavioral approaches, we characterized two distinct knock-in mouse lines expressing noninternalized P2X4mCherryIN either exclusively in excitatory forebrain neurons or in all cells natively expressing P2X4. The genetic substitution of wild-type P2X4 by noninternalized P2X4mCherryIN in both knock-in mouse models did not alter the sparse distribution and subcellular localization of P2X4 but increased the number of P2X4 receptors at the surface of the targeted cells mimicking the pathological increased surface P2X4 state. Increased surface P2X4 density in the hippocampus of knock-in mice altered LTP and LTD plasticity phenomena at CA1 synapses without affecting basal excitatory transmission. Moreover, these cellular events translated into anxiolytic effects and deficits in spatial memory. Our results show that increased surface density of neuronal P2X4 contributes to synaptic deficits and alterations in anxiety and memory functions consistent with the implication of P2X4 in neuropsychiatric and neurodegenerative disorders. Furthermore, these conditional P2X4mCherryIN knock-in mice will allow exploring the cell-specific roles of P2X4 in various physiological and pathological contexts.

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.

Deviant reporter expression and P2X4 passenger gene overexpression in the soluble EGFP BAC transgenic P2X7 reporter mouse model

The ATP-gated P2X7 receptor is highly expressed in microglia and has been involved in diverse brain diseases. P2X7 effects were also described in neurons and astrocytes but its localisation and function in these cell types has been challenging to demonstrate in situ. BAC transgenic mouse lines have greatly advanced neuroscience research and two BAC-transgenic P2X7 reporter mouse models exist in which either a soluble EGFP (sEGFP) or an EGFP-tagged P2X7 receptor (P2X7-EGFP) is expressed under the control of a BAC-derived P2rx7 promoter. Here we evaluate both mouse models and find striking differences in both P2X expression levels and EGFP reporter expression patterns. Most remarkably, the sEGFP model overexpresses a P2X4 passenger gene and sEGFP shows clear neuronal localisation but appears to be absent in microglia. Preliminary functional analysis in a status epilepticus model suggests functional consequences of the observed P2X receptor overexpression. In summary, an aberrant EGFP reporter pattern and possible effects of P2X4 and/or P2X7 protein overexpression need to be considered when working with this model. We further discuss reasons for the observed differences and possible caveats in BAC transgenic approaches.

P2Y2 and P2X4 Receptors Mediate Ca2+ Mobilization in DH82 Canine Macrophage Cells

Purinergic receptors of the P2 subclass are commonly found in human and rodent macrophages where they can be activated by adenosine 5'-triphosphate (ATP) or uridine 5'-triphosphate (UTP) to mediate Ca²⁺ mobilization, resulting in downstream signalling to promote inflammation and pain. However, little is understood regarding these receptors in canine macrophages. To establish a macrophage model of canine P2 receptor signalling, the expression of these receptors in the DH82 canine macrophage cell line was determined by reverse transcription polymerase chain reaction (RT-PCR) and immunocytochemistry. P2 receptor function in DH82 cells was pharmacologically characterised using nucleotide-induced measurements of Fura-2 AM-bound intracellular Ca²⁺. RT-PCR revealed predominant expression of P2X4 receptors, while immunocytochemistry confirmed predominant expression of P2Y₂ receptors, with low levels of P2X4 receptor expression. ATP and UTP induced robust Ca²⁺ responses in the absence or presence of extracellular Ca²⁺. ATP-induced responses were only partially inhibited by the P2X4 receptor antagonists, 2',3'-O-(2,4,6-trinitrophenyl)-ATP (TNP-ATP), paroxetine and 5-BDBD, but were strongly potentiated by ivermectin. UTP-induced responses were near completely inhibited by the P2Y₂ receptor antagonists, suramin and AR-C118925. P2Y₂ receptor-mediated Ca²⁺ mobilization was inhibited by U-73122 and 2-aminoethoxydiphenyl borate (2-APB), indicating P2Y₂ receptor coupling to the phospholipase C and inositol triphosphate signal transduction pathway. Together this data demonstrates, for the first time, the expression of functional P2 receptors in DH82 canine macrophage cells and identifies a potential cell model for studying macrophage-mediated purinergic signalling in inflammation and pain in dogs.

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.

Implication of Neuronal Versus Microglial P2X4 Receptors in Central Nervous System Disorders

The P2X4 receptor (P2X4) is an ATP-gated cation channel that is highly permeable to Ca²⁺ and widely expressed in neuronal and glial cell types throughout the central nervous system (CNS). A growing body of evidence indicates that P2X4 plays key roles in numerous central disorders. P2X4 trafficking is highly regulated and consequently in normal situations, P2X4 is present on the plasma membrane at low density and found mostly within intracellular endosomal/lysosomal compartments. An increase in the de novo expression and/or surface density of P2X4 has been observed in microglia and/or neurons during pathological states. This review aims to summarize knowledge on P2X4 functions in CNS disorders and provide some insights into the relative contributions of neuronal and glial P2X4 in pathological contexts. However, determination of the cell-specific functions of P2X4 along with its intracellular and cell surface roles remain to be elucidated before its potential as a therapeutic target in multiple disorders can be defined.

Contribution of P2X4 Receptors to CNS Function and Pathophysiology

The release and extracellular action of ATP are a widespread mechanism for cell-to-cell communication in living organisms through activation of P2X and P2Y receptors expressed at the cell surface of most tissues, including the nervous system. Among ionototropic receptors, P2X4 receptors have emerged in the last decade as a potential target for CNS disorders such as epilepsy, ischemia, chronic pain, anxiety, multiple sclerosis and neurodegenerative diseases. However, the role of P2X4 receptor in each pathology ranges from beneficial to detrimental, although the mechanisms are still mostly unknown. P2X4 is expressed at low levels in CNS cells including neurons and glial cells. In normal conditions, P2X4 activation contributes to synaptic transmission and synaptic plasticity. Importantly, one of the genes present in the transcriptional program of myeloid cell activation is P2X4. Microglial P2X4 upregulation, the P2X4⁺ state of microglia, seems to be common in most acute and chronic neurodegenerative diseases associated with inflammation. In this review, we summarize knowledge about the role of P2X4 receptors in the CNS physiology and discuss potential pitfalls and open questions about the therapeutic potential of blocking or potentiation of P2X4 for different pathologies.

Lysosomal Ca2+ Homeostasis and Signaling in Health and Disease

Calcium (Ca²⁺) signaling is an essential process in all cells that is maintained by a plethora of channels, pumps, transporters, receptors, and intracellular Ca²⁺ sequestering stores. Changes in cytosolic Ca²⁺ concentration govern processes as far reaching as fertilization, cell growth, and motility through to cell death. In recent years, lysosomes have emerged as a major intracellular Ca²⁺ storage organelle with an increasing involvement in triggering or regulating cellular functions such as endocytosis, autophagy, and Ca²⁺ release from the endoplasmic reticulum. This review will summarize recent work in the area of lysosomal Ca²⁺ signaling and homeostasis, including newly identified functions, and the involvement of lysosome-derived Ca²⁺ signals in human disease. In addition, we explore recent controversies in the techniques used for measurement of lysosomal Ca²⁺ content.

Lysosomal Biology and Function: Modern View of Cellular Debris Bin

Lysosomes are the main proteolytic compartments of mammalian cells comprising of a battery of hydrolases. Lysosomes dispose and recycle extracellular or intracellular macromolecules by fusing with endosomes or autophagosomes through specific waste clearance processes such as chaperone-mediated autophagy or microautophagy. The proteolytic end product is transported out of lysosomes via transporters or vesicular membrane trafficking. Recent studies have demonstrated lysosomes as a signaling node which sense, adapt and respond to changes in substrate metabolism to maintain cellular function. Lysosomal dysfunction not only influence pathways mediating membrane trafficking that culminate in the lysosome but also govern metabolic and signaling processes regulating protein sorting and targeting. In this review, we describe the current knowledge of lysosome in influencing sorting and nutrient signaling. We further present a mechanistic overview of intra-lysosomal processes, along with extra-lysosomal processes, governing lysosomal fusion and fission, exocytosis, positioning and membrane contact site formation. This review compiles existing knowledge in the field of lysosomal biology by describing various lysosomal events necessary to maintain cellular homeostasis facilitating development of therapies maintaining lysosomal function.

Regulation of Microglial Functions by Purinergic Mechanisms in the Healthy and Diseased CNS

Microglial cells, the resident macrophages of the central nervous system (CNS), exist in a process-bearing, ramified/surveying phenotype under resting conditions. Upon activation by cell-damaging factors, they get transformed into an amoeboid phenotype releasing various cell products including pro-inflammatory cytokines, chemokines, proteases, reactive oxygen/nitrogen species, and the excytotoxic ATP and glutamate. In addition, they engulf pathogenic bacteria or cell debris and phagocytose them. However, already resting/surveying microglia have a number of important physiological functions in the CNS; for example, they shield small disruptions of the blood–brain barrier by their processes, dynamically interact with synaptic structures, and clear surplus synapses during development. In neurodegenerative illnesses, they aggravate the original disease by a microglia-based compulsory neuroinflammatory reaction. Therefore, the blockade of this reaction improves the outcome of Alzheimer’s Disease, Parkinson’s Disease, multiple sclerosis, amyotrophic lateral sclerosis, etc. The function of microglia is regulated by a whole array of purinergic receptors classified as P2Y12, P2Y6, P2Y4, P2X4, P2X7, A2A, and A3, as targets of endogenous ATP, ADP, or adenosine. ATP is sequentially degraded by the ecto-nucleotidases and 5′-nucleotidase enzymes to the almost inactive inosine as an end product. The appropriate selective agonists/antagonists for purinergic receptors as well as the respective enzyme inhibitors may profoundly interfere with microglial functions and reconstitute the homeostasis of the CNS disturbed by neuroinflammation.

Pharmacological and genetic characterisation of the canine P2X4 receptor

BACKGROUND AND PURPOSE

P2X4 receptors are emerging therapeutic targets for treating chronic pain and cardiovascular disease. Dogs are well-recognised natural models of human disease, but information regarding P2X4 receptors in dogs is lacking. To aid the development and validation of P2X4 receptor ligands, we have characterised and compared canine and human P2X4 receptors.

EXPERIMENTAL APPROACH

Genomic DNA was extracted from whole blood samples from 101 randomly selected dogs and sequenced across the P2RX4 gene to identify potential missense variants. Recombinant canine and human P2X4 receptors tagged with Emerald GFP were expressed in 1321N1 and HEK293 cells and analysed by immunoblotting and confocal microscopy. In these cells, receptor pharmacology was characterised using nucleotide-induced Fura-2 AM measurements of intracellular Ca²⁺ and known P2X4 receptor antagonists. P2X4 receptor-mediated inward currents in HEK293 cells were assessed by automated patch clamp.

KEY RESULTS

No P2RX4 missense variants were identified in any canine samples. Canine and human P2X4 receptors were localised primarily to lysosomal compartments. ATP was the primary agonist of canine P2X4 receptors with near identical efficacy and potency at human receptors. 2'(3')-O-(4-benzoylbenzoyl)-ATP, but not ADP, was a partial agonist with reduced potency for canine P2X4 receptors compared to the human orthologues. Five antagonists inhibited canine P2X4 receptors, with 1-(2,6-dibromo-4-isopropyl-phenyl)-3-(3-pyridyl)urea displaying reduced sensitivity and potency at canine P2X4 receptors.

CONCLUSION AND IMPLICATIONS

P2X4 receptors are highly conserved across dog pedigrees and display expression patterns and pharmacological profiles similar to human receptors, supporting validation and use of therapeutic agents for P2X4 receptor-related disease onset and management in dogs and humans.

Nanoparticle-Mediated Therapeutic Application for Modulation of Lysosomal Ion Channels and Functions

Applications of nanoparticles in various fields have been addressed. Nanomaterials serve as carriers for transporting conventional drugs or proteins through lysosomes to various cellular targets. The basic function of lysosomes is to trigger degradation of proteins and lipids. Understanding of lysosomal functions is essential for enhancing the efficacy of nanoparticles-mediated therapy and reducing the malfunctions of cellular metabolism. The lysosomal function is modulated by the movement of ions through various ion channels. Thus, in this review, we have focused on the recruited ion channels for lysosomal function, to understand the lysosomal modulation through the nanoparticles and its applications. In the future, lysosomal channels-based targets will expand the therapeutic application of nanoparticles-associated drugs.

Endo-Lysosomal Cation Channels and Infectious Diseases

Among the infectious diseases caused by pathogenic microorganisms such as bacteria, viruses, parasites, or fungi, the most prevalent ones today are malaria, tuberculosis, influenza, HIV/AIDS, Ebola, dengue fever, and methicillin-resistant Staphylococcus aureus (MRSA) infection, and most recently Covid-19 (SARS-CoV2). Others with a rather devastating history and high fatality rates such as plague, cholera, or typhus seem less threatening today but have not been eradicated, and with a declining efficacy of current antibiotics they ought to be watched carefully. Another emerging issue in this context is health-care associated infection. About 100,000 hospitalized patients in the USA ( www.cdc.gov ) and 33,000 in Europe ( https://www.ecdc.europa.eu ) die each year as a direct consequence of an infection caused by bacteria resistant to antibiotics. Among viral infections, influenza is responsible for about 3-5 million cases of severe illness, and about 250,000 to 500,000 deaths annually ( www.who.int ). About 37 million people are currently living with HIV infection and about one million die from it each year. Coronaviruses such as MERS-CoV, SARS-CoV, but in particular the recent outbreak of Covid-19 (caused by SARS-CoV2) have resulted in large numbers of infections worldwide with an estimated several hundred thousand deaths (anticipated fatality rate: <5%). With a comparatively low mortality rate dengue virus causes between 50 and 100 million infections every year, leading to 50,000 deaths. In contrast, Ebola virus is the causative agent for one of the deadliest viral diseases. The Ebola outbreak in West Africa in 2014 is considered the largest outbreak in history with more than 11,000 deaths. Many of the deadliest pathogens such as Ebola virus, influenza virus, mycobacterium tuberculosis, dengue virus, and cholera exploit the endo-lysosomal trafficking system of host cells for penetration into the cytosol and replication. Defects in endo-lysosomal maturation, trafficking, fusion, or pH homeostasis can efficiently reduce the cytotoxicity caused by these pathogens. Most of these functions critically depend on endo-lysosomal membrane proteins such as transporters and ion channels. In particular, cation channels such as the mucolipins (TRPMLs) or the two-pore channels (TPCs) are involved in all of these aspects of endo-lysosomal integrity. In this review we will discuss the correlations between pathogen toxicity and endo-lysosomal cation channel function, and their potential as drug targets for infectious disease therapy.

Molecular Mechanisms of Calcium Signaling During Phagocytosis

Calcium (Ca²⁺) is a ubiquitous second messenger involved in the regulation of numerous cellular functions including vesicular trafficking, cytoskeletal rearrangements and gene transcription. Both global as well as localized Ca²⁺ signals occur during phagocytosis, although their functional impact on the phagocytic process has been debated. After nearly 40 years of research, a consensus may now be reached that although not strictly required, Ca²⁺ signals render phagocytic ingestion and phagosome maturation more efficient, and their manipulation make an attractive avenue for therapeutic interventions. In the last decade many efforts have been made to identify the channels and regulators involved in generating and shaping phagocytic Ca²⁺ signals. While molecules involved in store-operated calcium entry (SOCE) of the STIM and ORAI family have taken center stage, members of the canonical, melastatin, mucolipin and vanilloid transient receptor potential (TRP), as well as purinergic P2X receptor families are now recognized to play significant roles. In this chapter, we review the recent literature on research that has linked specific Ca²⁺-permeable channels and regulators to phagocytic function. We highlight the fact that lipid mediators are emerging as important regulators of channel gating and that phagosomal ionic homeostasis and Ca²⁺ release also play essential parts. We predict that improved methodologies for measuring these factors will be critical for future advances in dissecting the intricate biology of this fascinating immune process.

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.

NAADP Receptors

Of the established Ca²⁺-mobilizing messengers, NAADP is arguably the most tantalizing. It is the most potent, often efficacious at low nanomolar concentrations, and its receptors undergo dramatic desensitization. Recent studies have identified a new class of calcium-release channel, the two-pore channels (TPCs), as the likely targets for NAADP regulation, even though the effect may be indirect. These channels localized at endolysosomes, where they mediate local Ca²⁺ release, and have highlighted a new role of acidic organelles as targets for messenger-evoked Ca²⁺ mobilization. Three distinct roles of TPCs have been identified. The first is to effect local Ca²⁺ release that may play a role in endolysosomal function including vesicular fusion and trafficking. The second is to trigger global calcium release by recruiting Ca²⁺-induced Ca²⁺-release (CICR) channels at lysosomal-endoplasmic reticulum (ER) junctions. The third is to regulate plasma membrane excitability by the targeting of Ca²⁺ release from appropriately positioned subplasma membrane stores to regulate plasma membrane Ca²⁺-activated channels. In this review, I discuss the role of nicotinic acid adenine nucleotide diphosphate (NAADP)-mediated Ca²⁺ release from endolysosomal stores as a widespread trigger for intracellular calcium signaling mechanisms, and how studies of TPCs are beginning to enhance our understanding of the central role of lysosomes in Ca²⁺ signaling.

Human Peripheral Blood Eosinophils Express High Levels of the Purinergic Receptor P2X4

Extracellular nucleotides are important mediators of cell activation and trigger multiple responses via membrane receptors known as purinergic receptors (P2). P2X receptors are ligand-gated ion channels, activated by extracellular ATP. P2X4 is one of the most sensitive purinergic receptors, that is typically expressed by neurons, microglia, and some epithelial and endothelial cells. P2X4 mediates neuropathic pain via brain-derived neurotrophic factor and is also involved in inflammation in response to high ATP release. It is therefore involved in multiple inflammatory pathologies as well as neurodegenerative diseases. We have produced monoclonal antibodies (mAb) directed against this important human P2X4 receptor. Focusing on two mAbs, we showed that they also recognize mouse and rat P2X4. We demonstrated that these mAbs can be used in flow cytometry, immunoprecipitation, and immunohistochemistry, but not in Western blot assays, indicating that they target conformational epitopes. We also characterized the expression of P2X4 receptor on mouse and human peripheral blood lymphocytes (PBL). We showed that P2X4 is expressed at the surface of several leukocyte cell types, with the highest expression level on eosinophils, making them potentially sensitive to adenosine triphosphate (ATP). P2X4 is expressed by leucocytes, in human and mouse, with a significant gender difference, males having higher surface expression levels than females. Our findings reveal that PBL express significant levels of P2X4 receptor, and suggest an important role of this receptor in leukocyte activation by ATP, particularly in P2X4^high expressing eosinophils.

Cell Death Induced by Cationic Amphiphilic Drugs Depends on Lysosomal Ca2+ Release and Cyclic AMP

Repurposing cationic amphiphilic drugs (CAD) for cancer treatment is emerging as an attractive means to enhance the efficacy of chemotherapy. Many commonly used CADs, including several cation amphiphilic antihistamines and antidepressants, induce cancer-specific, lysosome-dependent cell death and sensitize cancer cells to chemotherapy. CAD-induced inhibition of lysosomal acid sphingomyelinase is necessary, but not sufficient, for the subsequent lysosomal membrane permeabilization and cell death, while other pathways regulating this cell death pathway are largely unknown. Prompted by significant changes in the expression of genes involved in Ca2+ and cyclic AMP (cAMP) signaling pathways in CAD-resistant MCF7 breast cancer cells, we identified here an early lysosomal Ca2+ release through P2X purinergic receptor 4 (P2RX4) and subsequent Ca2+- and adenylyl cyclase 1 (ADCY1)-dependent synthesis of cAMP as a signaling route mediating CAD-induced lysosomal membrane permeabilization and cell death. Importantly, pharmacologic and genetic means to increase cellular cAMP levels either by activating cAMP-inducing G-protein-coupled receptors (GPR3 or β2 adrenergic receptor) or ADCY1, or by inhibiting cAMP-reducing guanine nucleotide-binding protein G(i) subunit α2, C-X-C motif chemokine receptor type 4, or cAMP phosphodiesterases, sensitized cancer cells to CADs. These data reveal a previously unrecognized lysosomal P2RX4- and ADCY1-dependent signaling cascade as a pathway essential for CAD-induced lysosome-dependent cell death and encourage further investigations to find the most potent combinations of CADs and cAMP-inducing drugs for cancer therapy.

P2X7 Interactions and Signaling - Making Head or Tail of It

Extracellular adenine nucleotides play important roles in cell-cell communication and tissue homeostasis. High concentrations of extracellular ATP released by dying cells are sensed as a danger signal by the P2X7 receptor, a non-specific cation channel. Studies in P2X7 knockout mice and numerous disease models have demonstrated an important role of this receptor in inflammatory processes. P2X7 activation has been shown to induce a variety of cellular responses that are not usually associated with ion channel function, for example changes in the plasma membrane composition and morphology, ectodomain shedding, activation of lipases, kinases, and transcription factors, as well as cytokine release and apoptosis. In contrast to all other P2X family members, the P2X7 receptor contains a long intracellular C-terminus that constitutes 40% of the whole protein and is considered essential for most of these effects. So far, over 50 different proteins have been identified to physically interact with the P2X7 receptor. However, few of these interactions have been confirmed in independent studies and for the majority of these proteins, the interaction domains and the physiological consequences of the interactions are only poorly described. Also, while the structure of the P2X7 extracellular domain has recently been resolved, information about the organization and structure of its C-terminal tail remains elusive. After shortly describing the structure and assembly of the P2X7 receptor, this review gives an update of the identified or proposed interaction domains within the P2X7 C-terminus, describes signaling pathways in which this receptor has been involved, and provides an overlook of the identified interaction partners.

‬The expression of purinergic P2X4 and P2X7 receptors in selected mesolimbic structures during morphine withdrawal in rats

Morphine is one of the most potent analgesics used in medicine and it's long-term use is associated with the risk of the state of dependence. The cessation of chronic morphine administration leads to withdrawal signs which are associated with neurotransmitter dysregulations within mesolimbic system. Adenosine 5'-triphosphate (ATP) and purinergic system play an important role in the activity of central nervous system (CNS). Purinergic receptors are widely distributed in neurons and glial cells throughout the CNS taking part in integration of functional activity between neurons, glial and vascular cells. In the present study the mRNA and protein expression of purinergic P2X4 and P2X7 receptors in selected mesolimbic structures (striatum, hippocampus and prefrontal cortex) during morphine withdrawal in rats was investigated by RT-PCR and Western Blot analysis. Two experimental models of morphine withdrawal were studied: single and repeated morphine withdrawal. We demonstrated that expression of P2X4 and P2X7 receptors was altered during morphine withdrawal period in rats. These alterations were varied in particular mesolimbic areas depending on the scheme of morphine administration. Our results extend the current knowledge on morphine withdrawal and for the first time high-light interactions between purinergic system and morphine withdrawal. It seems, the purinergic system may be a new, valuable tool in searching for a new strategy of management of opioid dependence.

P2X4 and lysosome fusion

Similar to other members of the P2X receptor family, the P2X4 receptor at the plasma membrane forms a highly Ca²⁺ permeable, non-selective cation channel that is activated by extracellular ATP. Yet, P2X4 differs from the other subtypes, as it is predominantly localized on late endosomal, lysosomal and/or lysosome-related organelles. It is targeted there by virtue of tyrosine-based and di-leucine like trafficking motifs contained within its C-terminal and N-terminal regions respectively. The physiological role of the stable intracellular expression of P2X4 in acidic compartments has been a long-standing puzzle. Recent evidence, however, points to a dual role in the regulation of ion fluxes across lysosomal membranes to control lysosome membrane fusion and in the re-sensitization of receptors exposed to extracellular ATP.

Lysosomal Ion Channels as Decoders of Cellular Signals

Lysosomes, the degradation center of the cell, are filled with acidic hydrolases. Lysosomes generate nutrient-sensitive signals to regulate the import of H+, hydrolases, and endocytic and autophagic cargos, as well as the export of their degradation products (catabolites). In response to environmental and cellular signals, lysosomes change their positioning, number, morphology, size, composition, and activity within minutes to hours to meet the changing cellular needs. Ion channels in the lysosome are essential transducers that mediate signal-initiated Ca2+/Fe2+/Zn2+ release and H+/Na+/K+-dependent changes of membrane potential across the perimeter membrane. Dysregulation of lysosomal ion flux impairs lysosome movement, membrane trafficking, nutrient sensing, membrane repair, organelle membrane contact, and lysosome biogenesis and adaptation. Hence, activation and inhibition of lysosomal channels by synthetic modulators may tune lysosome function to maintain cellular health and promote cellular clearance in lysosome storage disorders.

Endolysosomal Ca2+ Signalling and Cancer Hallmarks: Two-Pore Channels on the Move, TRPML1 Lags Behind!

The acidic vesicles of the endolysosomal (EL) system are emerging as an intracellular Ca²⁺ store implicated in the regulation of multiple cellular functions. The EL Ca²⁺ store releases Ca²⁺ through a variety of Ca²⁺-permeable channels, including Transient Receptor Potential (TRP) Mucolipin 1-3 (TRPML1-3) and two-pore channels 1-2 (TPC1-2), whereas EL Ca²⁺ refilling is sustained by the proton gradient across the EL membrane and/or by the endoplasmic reticulum (ER). EL Ca²⁺ signals may be either spatially restricted to control vesicle trafficking, autophagy and membrane repair or may be amplified into a global Ca²⁺ signal through the Ca²⁺-dependent recruitment of ER-embedded channels. Emerging evidence suggested that nicotinic acid adenine dinucleotide phosphate (NAADP)-gated TPCs sustain multiple cancer hallmarks, such as migration, invasiveness and angiogenesis. Herein, we first survey the EL Ca²⁺ refilling and release mechanisms and then focus on the oncogenic role of EL Ca²⁺ signaling. While the evidence in favor of TRPML1 involvement in neoplastic transformation is yet to be clearly provided, TPCs are emerging as an alternative target for anticancer therapies.

Ginsenosides Act As Positive Modulators of P2X4 Receptors

We investigated the selectivity of protopanaxadiol ginsenosides from Panax ginseng acting as positive allosteric modulators on P2X receptors. ATP-induced responses were measured in stable cell lines overexpressing human P2X4 using a YOPRO-1 dye uptake assay, intracellular calcium measurements, and whole-cell patch-clamp recordings. Ginsenosides CK and Rd were demonstrated to enhance ATP responses at P2X4 by ∼twofold, similar to potentiation by the known positive modulator ivermectin. Investigations into the role of P2X4 in mediating a cytotoxic effect showed that only P2X7 expression in HEK-293 cells induces cell death in response to high concentrations of ATP, and that ginsenosides can enhance this process. Generation of a P2X7-deficient clone of BV-2 microglial cells using CRISPR/Cas9 gene editing enabled an investigation of endogenous P2X4 in a microglial cell line. Compared with parental BV-2 cells, P2X7-deficient BV-2 cells showed minor potentiation of ATP responses by ginsenosides, and insensitivity to ATP⁻ or ATP⁺ ginsenoside-induced cell death, indicating a primary role for P2X7 receptors in both of these effects. Computational docking to a homology model of human P2X4, based on the open state of zfP2X4, yielded evidence of a putative ginsenoside binding site in P2X4 in the central vestibule region of the large ectodomain.

A state-of-the-art perspective on microgliopathic pain

Acute nociceptive pain is an undesirable feeling but has a physiological significance as a warning system for living organisms. Conversely, chronic pain is lacking physiological significance, but rather represents a confusion of nerve functions. The neuropathic pain that occurs after peripheral nerve injury (PNI) is perhaps the most important type of chronic pain because it is refractory to available medications and thus remains a heavy clinical burden. In recent decades, studies have shown that spinal microglia play a principal role in the alterations in synaptic functions evoking this pain. It is also clear that the P2X4 receptor (P2X4R), a subtype of ionotropic ATP receptors, is upregulated exclusively in spinal microglia after PNI and plays a key role in evoking neuropathic pain. Neuropathic pain is caused by several conditions associated with activated microglia without nerve damage. ‘Microgliopathic pain’ is a new concept indicating such abnormal pain related to activated microglia.