Contribution of calcium ions to P2X channel responses

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

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

Association of polymorphisms within P2RX4 with type 2 diabetes mellitus: a preliminary case-control study

OBJECTIVE

Type 2 diabetes mellitus (T2DM) is a complex heterogenic metabolic with a wide range of etiology. Purinergic receptors have pivotal roles in different processes and are hypothesized to have roles in the pathogenesis of T2DM.

MATERIALS AND METHODS

Three hundred subjects affected by T2DM and 300 healthy subjects were genotyped by amplification refractory mutation system polymerase chain reaction (ARMS-PCR). SPSS V16.0 was recruited for statistical analysis.

RESULTS

The findings showed that the G allele of rs25644A > G increases the risk of T2DM in our population statistically (OR = 1.51, 95% CI = 1.14-1.99, p = 0.003). This allele in some genotype models, including the dominant model, caused an increase in the risk of T2DM. The interaction of genotypes between studied variants in the P2XR4 gene increased the risk of T2DM. Haplotype analysis showed that Ars1169727/Grs25644 haplotype caused an increase in the risk of T2DM.

CONCLUSIONS

The findings suggest that rs25644A > G plays a role in our population's increased risk of T2DM.

The ion channels of endomembranes

The endomembrane system consists of organellar membranes in the biosynthetic pathway [endoplasmic reticulum (ER), Golgi apparatus, and secretory vesicles] as well as those in the degradative pathway (early endosomes, macropinosomes, phagosomes, autophagosomes, late endosomes, and lysosomes). These endomembrane organelles/vesicles work together to synthesize, modify, package, transport, and degrade proteins, carbohydrates, and lipids, regulating the balance between cellular anabolism and catabolism. Large ion concentration gradients exist across endomembranes: Ca2+ gradients for most endomembrane organelles and H+ gradients for the acidic compartments. Ion (Na+, K+, H+, Ca2+, and Cl-) channels on the organellar membranes control ion flux in response to cellular cues, allowing rapid informational exchange between the cytosol and organelle lumen. Recent advances in organelle proteomics, organellar electrophysiology, and luminal and juxtaorganellar ion imaging have led to molecular identification and functional characterization of about two dozen endomembrane ion channels. For example, whereas IP3R1-3 channels mediate Ca2+ release from the ER in response to neurotransmitter and hormone stimulation, TRPML1-3 and TMEM175 channels mediate lysosomal Ca2+ and H+ release, respectively, in response to nutritional and trafficking cues. This review aims to summarize the current understanding of these endomembrane channels, with a focus on their subcellular localizations, ion permeation properties, gating mechanisms, cell biological functions, and disease relevance.

Extracellular ATP inhibits excitatory synaptic input on parvalbumin positive interneurons and attenuates gamma oscillations via P2X4 receptors

BACKGROUND AND PURPOSE

P2X4 receptors (P2X4R) are ligand gated cation channels that are activated by extracellular ATP released by neurons and glia. The receptors are widely expressed in the brain and have fractional calcium currents comparable with NMDA receptors. Although P2X4Rs have been reported to modulate synaptic transmission and plasticity, their involvement in shaping neuronal network activity remains to be elucidated.

EXPERIMENTAL APPROACH

We investigated the effects of P2X receptors at network and synaptic level using local field potential electrophysiology, whole cell patch clamp recordings and calcium imaging in fast spiking parvalbumin positive interneurons (PVINs) in rat and mouse hippocampal slices. The stable ATP analogue ATPγS, selective antagonists and P2X4R knockout mice were used.

KEY RESULTS

The P2XR agonist ATPγS reversibly decreased the power of gamma oscillations. This inhibition could be antagonized by the selective P2X4R antagonist PSB-12062 and was not observed in P2X4-/- mice. The phasic excitatory inputs of CA3 PVINs were one of the main regulators of the gamma power. Associational fibre compound excitatory postsynaptic currents (cEPSCs) in CA3 PVINs were inhibited by P2X4R activation. This effect was reversible, dependent on intracellular calcium and dynamin-dependent internalization of AMPA receptors.

CONCLUSIONS AND IMPLICATIONS

The results indicate that P2X4Rs are an important source of dendritic calcium in CA3 PVINs, thereby regulating excitatory synaptic inputs onto the cells and presumably the state of gamma oscillations in the hippocampus. P2X4Rs represent an effective target to modulate hippocampal network activity in pathophysiological conditions such as Alzheimer's disease and schizophrenia.

Interaction of Angiotensin II AT1 Receptors with Purinergic P2X Receptors in Regulating Renal Afferent Arterioles in Angiotensin II-Dependent Hypertension

In angiotensin II (Ang II)-dependent hypertension, Ang II activates angiotensin II type 1 receptors (AT1R) on renal vascular smooth muscle cells, leading to renal vasoconstriction with eventual glomerular and tubular injury and interstitial inflammation. While afferent arteriolar vasoconstriction is initiated by the increased intrarenal levels of Ang II activating AT1R, the progressive increases in arterial pressure stimulate the paracrine secretion of adenosine triphosphate (ATP), leading to the purinergic P2X receptor (P2XR)-mediated constriction of afferent arterioles. Thus, the afferent arteriolar tone is maintained by two powerful systems eliciting the co-existing activation of P2XR and AT1R. This raises the conundrum of how the AT1R and P2XR can both be responsible for most of the increased renal afferent vascular resistance existing in angiotensin-dependent hypertension. Its resolution implies that AT1R and P2XR share common receptor or post receptor signaling mechanisms which converge to maintain renal vasoconstriction in Ang II-dependent hypertension. In this review, we briefly discuss (1) the regulation of renal afferent arterioles in Ang II-dependent hypertension, (2) the interaction of AT1R and P2XR activation in regulating renal afferent arterioles in a setting of hypertension, (3) mechanisms regulating ATP release and effect of angiotensin II on ATP release, and (4) the possible intracellular pathways involved in AT1R and P2XR interactions. Emerging evidence supports the hypothesis that P2X1R, P2X7R, and AT1R actions converge at receptor or post-receptor signaling pathways but that P2XR exerts a dominant influence abrogating the actions of AT1R on renal afferent arterioles in Ang II-dependent hypertension. This finding raises clinical implications for the design of therapeutic interventions that will prevent the impairment of kidney function and subsequent tissue injury.

Identification of residues in the first transmembrane domain of the P2X7 that regulates receptor trafficking, sensitization, and dye uptake function

P2X receptors (P2X1-7) are trimeric ion channels activated by extracellular ATP. Each P2X subunit contains two transmembrane helices (TM1 and TM2). We substituted all residues in TM1 of rat P2X7 with alanine or leucine one by one, expressed mutants in HEK293T cells, and examined the pore permeability by recording both membrane currents and fluorescent dye uptake in response to agonist application. Alanine substitution of G27, K30, H34, Y40, F43, L45, M46, and D48 inhibited agonist-stimulated membrane current and dye uptake, and all but one substitution, D48A, prevented surface expression. Mutation V41A partially reduced both membrane current and dye uptake, while W31A and A44L showed reduced dye uptake not accompanied by reduced membrane current. Mutations T28A, I29A, and L33A showed small changes in agonist sensitivity, but they had no or small impact on dye uptake function. Replacing charged residues with residues of the same charge (K30R, H34K, and D48E) rescued receptor function, while replacement with residues of opposite charge inhibited (K30E and H34E) or potentiated (D48K) receptor function. Prolonged stimulation with agonist-induced current facilitation and a leftward shift in the dose-response curve in the P2X7 wild-type and most functional mutants, but sensitization was absent in the W31A, L33A, and A44L. Detailed analysis of the decay of responses revealed two kinetically distinct mechanisms of P2X7 deactivation: fast represents agonist unbinding, and slow might represent resetting of the receptor to the resting closed state. These results indicate that conserved and receptor-specific TM1 residues control surface expression of the P2X7 protein, non-polar residues control receptor sensitization, and D48 regulates intrinsic channel properties.

cGAMP the travelling messenger

2'3'-cGAMP is a key molecule in the cGAS-STING pathway. This cyclic dinucleotide is produced by the cytosolic DNA sensor cGAS in response to the presence of aberrant dsDNA in the cytoplasm which is associated with microbial invasion or cellular damage. 2'3'-cGAMP acts as a second messenger and activates STING, the central hub of DNA sensing, to induce type-I interferons and pro-inflammatory cytokines necessary for responses against infection, cancer or cellular stress. Classically, detection of pathogens or danger by pattern recognition receptors (PRR) was thought to signal and induce the production of interferon and pro-inflammatory cytokines in the cell where sensing occurred. These interferon and cytokines then signal in both an autocrine and paracrine manner to induce responses in neighboring cells. Deviating from this dogma, recent studies have identified multiple mechanisms by which 2'3'-cGAMP can travel to neighboring cells where it activates STING independent of DNA sensing by cGAS. This observation is of great importance, as the cGAS-STING pathway is involved in immune responses against microbial invaders and cancer while its dysregulation drives the pathology of a wide range of inflammatory diseases to which antagonists have been elusive. In this review, we describe the fast-paced discoveries of the mechanisms by which 2'3'-cGAMP can be transported. We further highlight the diseases where they are important and detail how this change in perspective can be applied to vaccine design, cancer immunotherapies and treatment of cGAS-STING associated disease.

Ion permeation pathway within the internal pore of P2X receptor channels

P2X receptor channels are trimeric ATP-activated ion channels expressed in neuronal and non-neuronal cells that are attractive therapeutic targets for human disorders. Seven subtypes of P2X receptor channels have been identified in mammals that can form both homomeric and heteromeric channels. P2X1-4 and P2X7 receptor channels are cation-selective, whereas P2X5 has been reported to have both cation and anion permeability. P2X receptor channel structures reveal that each subunit is comprised of two transmembrane helices, with both N-and C-termini on the intracellular side of the membrane and a large extracellular domain that contains the ATP binding sites at subunit interfaces. Recent structures of ATP-bound P2X receptors with the activation gate open reveal the unanticipated presence of a cytoplasmic cap over the central ion permeation pathway, leaving lateral fenestrations that may be largely buried within the membrane as potential pathways for ions to permeate the intracellular end of the pore. In the present study, we identify a critical residue within the intracellular lateral fenestrations that is readily accessible to thiol-reactive compounds from both sides of the membrane and where substitutions influence the relative permeability of the channel to cations and anions. Taken together, our results demonstrate that ions can enter or exit the internal pore through lateral fenestrations that play a critical role in determining the ion selectivity of P2X receptor channels.

Purinergic signaling: Diverse effects and therapeutic potential in cancer

Regardless of improved biological insights and therapeutic advances, cancer is consuming multiple lives worldwide. Cancer is a complex disease with diverse cellular, metabolic, and physiological parameters as its hallmarks. This instigates a need to uncover the latest therapeutic targets to advance the treatment of cancer patients. Purines are building blocks of nucleic acids but also function as metabolic intermediates and messengers, as part of a signaling pathway known as purinergic signaling. Purinergic signaling comprises primarily adenosine triphosphate (ATP) and adenosine (ADO), their analogous membrane receptors, and a set of ectonucleotidases, and has both short- and long-term (trophic) effects. Cells release ATP and ADO to modulate cellular function in an autocrine or paracrine manner by activating membrane-localized purinergic receptors (purinoceptors, P1 and P2). P1 receptors are selective for ADO and have four recognized subtypes-A1, A2A, A2B, and A3. Purines and pyrimidines activate P2 receptors, and the P2X subtype is ligand-gated ion channel receptors. P2X has seven subtypes (P2X1-7) and forms homo- and heterotrimers. The P2Y subtype is a G protein-coupled receptor with eight subtypes (P2Y1/2/4/6/11/12/13/14). ATP, its derivatives, and purinoceptors are widely distributed in all cell types for cellular communication, and any imbalance compromises the homeostasis of the cell. Neurotransmission, neuromodulation, and secretion employ fast purinergic signaling, while trophic purinergic signaling regulates cell metabolism, proliferation, differentiation, survival, migration, invasion, and immune response during tumor progression. Thus, purinergic signaling is a prospective therapeutic target in cancer and therapy resistance.

Purinoreceptors and ectonucleotidases control ATP-induced calcium waveforms and calcium-dependent responses in microglia: Roles of P2 receptors and CD39 in ATP-stimulated microglia

Adenosine triphosphate (ATP) and its metabolites drive microglia migration and cytokine production by activating P2X- and P2Y- class purinergic receptors. Purinergic receptor activation gives rise to diverse intracellular calcium (Ca2+ signals, or waveforms, that differ in amplitude, duration, and frequency. Whether and how these characteristics of diverse waveforms influence microglia function is not well-established. We developed a computational model trained with data from published primary murine microglia studies. We simulate how purinoreceptors influence Ca2+ signaling and migration, as well as, how purinoreceptor expression modifies these processes. Our simulation confirmed that P2 receptors encode the amplitude and duration of the ATP-induced Ca2+ waveforms. Our simulations also implicate CD39, an ectonucleotidase that rapidly degrades ATP, as a regulator of purinergic receptor-induced Ca2+ responses. Namely, it was necessary to account for CD39 metabolism of ATP to align the model's predicted purinoreceptor responses with published experimental data. In addition, our modeling results indicate that small Ca2+ transients accompany migration, while large and sustained transients are needed for cytokine responses. Lastly, as a proof-of-principal, we predict Ca2+ transients and cell membrane displacements in a BV2 microglia cell line using published P2 receptor mRNA data to illustrate how our computer model may be extrapolated to other microglia subtypes. These findings provide important insights into how differences in purinergic receptor expression influence microglial responses to ATP.

The P2X1 receptor as a therapeutic target

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

Alternative Pharmacological Strategies for the Treatment of Alzheimer's Disease: Focus on Neuromodulator Function

Alzheimer's disease (AD) is a neurodegenerative disorder, comprising 70% of dementia diagnoses worldwide and affecting 1 in 9 people over the age of 65. However, the majority of its treatments, which predominantly target the cholinergic system, remain insufficient at reversing pathology and act simply to slow the inevitable progression of the disease. The most recent neurotransmitter-targeting drug for AD was approved in 2003, strongly suggesting that targeting neurotransmitter systems alone is unlikely to be sufficient, and that research into alternate treatment avenues is urgently required. Neuromodulators are substances released by neurons which influence neurotransmitter release and signal transmission across synapses. Neuromodulators including neuropeptides, hormones, neurotrophins, ATP and metal ions display altered function in AD, which underlies aberrant neuronal activity and pathology. However, research into how the manipulation of neuromodulators may be useful in the treatment of AD is relatively understudied. Combining neuromodulator targeting with more novel methods of drug delivery, such as the use of multi-targeted directed ligands, combinatorial drugs and encapsulated nanoparticle delivery systems, may help to overcome limitations of conventional treatments. These include difficulty crossing the blood-brain-barrier and the exertion of effects on a single target only. This review aims to highlight the ways in which neuromodulator functions are altered in AD and investigate how future therapies targeting such substances, which act upstream to classical neurotransmitter systems, may be of potential therapeutic benefit in the sustained search for more effective treatments.

Effective targeting of microglial P2X7 following intracerebroventricular delivery of nanobodies and nanobody-encoding AAVs

The P2X7 ion channel is a key sensor for extracellular ATP and a key trigger of sterile inflammation. Intravenous injection of nanobodies that block P2X7 has shown to be beneficial in mouse models of systemic inflammation. P2X7 has also emerged as an attractive therapeutic target for inflammatory brain diseases. However, little is known about the ability of nanobodies to cross the BBB. Here we evaluated the ability of P2X7-specific nanobodies to reach and to block P2X7 on microglia following intravenous or intracerebral administration. For this study, we reformatted and sequence-optimized P2X7 nanobodies for higher stability and elevated isoelectric point. Following injection of nanobodies or nanobody-encoding adeno-associated viral vectors (AAV), we monitored the occupancy and blockade of microglial P2X7 in vivo using ex vivo flow cytometry. Our results show that P2X7 on microglia was within minutes completely occupied and blocked by intracerebroventricularly injected nanobodies, even at low doses. In contrast, very high doses were required to achieve similar effects when injected intravenously. The endogenous production of P2X7-antagonistic nanobodies following intracerebral or intramuscular injection of nanobody-encoding AAVs resulted in a long-term occupancy and blockade of P2X7 on microglia. Our results provide new insights into the conditions for the delivery of nanobodies to microglial P2X7 and point to AAV-mediated delivery of P2X7 nanobodies as a promising strategy for the treatment of sterile brain inflammation.

5-HT3 Receptors in Rat Dorsal Root Ganglion Neurons: Ca2+ Entry and Modulation of Neurotransmitter Release

Rat dorsal root ganglion (DRG) neurons express 5-hydroxytryptamine receptors (5-HT3Rs). To elucidate their physiological role in the modulation of sensory signaling, we aimed to quantify their functional expression in newborn and adult rat DRG neurons, as well as their ability to modulate the Ca²⁺-dependent neurotransmitter release, by means of electrophysiological techniques combined with fluorescence-based Ca²⁺ imaging. The selective 5-HT3R agonist mCPBG (10 μM) elicited whole-cell currents in 92.5% of adult DRG neurons with a significantly higher density current than in responding newborn cells (52.2%), suggesting an increasing serotoninergic modulation on primary afferent cells during development. Briefly, 5-HT3Rs expressed by adult DRG neurons are permeable to Ca²⁺ ions, with a measured fractional Ca²⁺ current (i.e., the percentage of total current carried by Ca²⁺ ions, Pf) of 1.0%, similar to the value measured for the human heteromeric 5-HT3_A/B receptor (P_f = 1.1%), but lower than that of the human homomeric 5-HT3_A receptor (P_f = 3.5%). mCPBG applied to co-cultures of newborn DRG and spinal neurons significantly increased the miniature excitatory postsynaptic currents (mEPSCs) frequency in a subset of recorded spinal neurons, even in the presence of Cd²⁺, a voltage-activated Ca²⁺ channel blocker. Considered together, our findings indicate that the Ca²⁺ influx through heteromeric 5-HT3Rs is sufficient to increase the spontaneous neurotransmitter release from DRG to spinal neurons.

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.

Enlightening activation gating in P2X receptors

P2X receptors are trimeric nonselective cation channels gated by ATP. They assemble from seven distinct subunit isoforms as either homo- or heteromeric complexes and contain three extracellularly located binding sites for ATP. P2X receptors are expressed in nearly all tissues and are there involved in physiological processes like synaptic transmission, pain, and inflammation. Thus, they are a challenging pharmacological target. The determination of crystal and cryo-EM structures of several isoforms in the last decade in closed, open, and desensitized states has provided a firm basis for interpreting the huge amount of functional and biochemical data. Electrophysiological characterization in conjugation with optical approaches has generated significant insights into structure–function relationships of P2X receptors. This review focuses on novel optical and related approaches to better understand the conformational changes underlying the activation of these receptors.

How Structural Biology Has Directly Impacted Our Understanding of P2X Receptor Function and Gating

P2X receptors are ATP-gated ion channels expressed in a wide variety of eukaryotic cells. They play key roles in diverse processes such as platelet activation, smooth muscle contraction, synaptic transmission, nociception, cell proliferation, and inflammation making this receptor family an important pharmacological target. Structures of P2X receptors solved by X-ray crystallography have been instrumental in helping to define mechanisms of molecular P2X receptor function. In 2009, the first X-ray structure of the P2X4 receptor subtype confirmed a trimeric stoichiometry and revealed the overall architecture of the functional ion channel. Subsequent X-ray structures have provided the molecular details to define the orthosteric ATP binding pocket, the orthosteric antagonist binding pocket, an allosteric antagonist binding pocket, and the pore architecture in each of the major conformational states of the receptor gating cycle. Moreover, the unique gating mechanism by which P2X receptor subtypes desensitize at differing rates, referred to as the helical recoil model of receptor desensitization, was discovered directly from X-ray structures of the P2X3 receptor. However, structures of P2X receptors solved by X-ray crystallography have only been able to provide limited information on the cytoplasmic domain of this receptor family, as this domain was always truncated to varying degrees in order to facilitate crystallization. Because the P2X7 receptor subtype has a significantly larger cytoplasmic domain that has been shown to be necessary for its ability to initiate apoptosis, an absence of structural information on the P2X7 receptor cytoplasmic domain has limited our understanding of its complex signaling pathways as well as its unusual ability to remain open without undergoing desensitization. This absence of cytoplasmic structural information for P2X7 receptors was recently overcome when the first full-length P2X7 receptor structures were solved by single-particle cryogenic electron microscopy. These structures finally provide insight into the large and unique P2X7 receptor cytoplasmic domain and revealed two novel structural elements and several surprising findings: first, a cytoplasmic structural element called the cytoplasmic ballast was identified that contains a dinuclear zinc ion complex and a high affinity guanosine nucleotide binding site and second, a palmitoylated membrane proximal structural element called the C-cys anchor was identified which prevents P2X7 receptor desensitization. This chapter will highlight the major structural and functional aspects of P2X receptors discovered through structural biology, with a key emphasis on the most recent cryogenic electron microscopy structures of the full-length, wild-type P2X7 receptor.

Using Whole-Cell Electrophysiology and Patch-Clamp Photometry to Characterize P2X7 Receptor Currents

The fundamental property of P2X7 receptor (P2X7R) channels is the transport of cations across the cell surface membrane. Electrophysiology and patch-clamp photometry are readily accessible methods of measuring this flux in a wide range of cell types. They are important tools used to characterize the functional properties of native cells studied in cell culture, in vitro tissue slices, and, in some cases, in situ single cells. Further, they are efficient methods of probing the relation of structure to function of recombinant receptors expressed in heterologous systems. Here, we provide step-by-step procedures for use of two standard recording protocols, broken-patch and perforated-patch voltage clamp. Further, we describe a third technique, called the dye-overload method, that uses simultaneous measurement of membrane current and fura-2 fluorescence to quantify the contribution of Ca²⁺ flux to the ATP-gated current.

Ion Selectivity in P2X Receptors: A Comparison between hP2X3 and zfP2X4

Charge discrimination in P2X receptors occurs in two stages. The first stage takes place in the extracellular vestibule. The second one happens as the ions travel across the pore. The search of the amino acids required to achieve these goals has focused on negatively charged residues conserved among the family members. This strategy, however, has afforded baffling results since residues that strongly influence ion selectivity in a given member are not present in others. This finding suggests that alternative family members could achieve the same goal using different molecular approaches. We have compared the mechanisms of charge discrimination in the extracellular vestibule of zebrafish P2X4 (zfP2X4) and human P2X3 (hP2X3), employing molecular dynamics simulations. In particular, we have analyzed how the mutation of residues D59 and D61 of zfP2X4 and residues E46, D53, and E57 of hP2X3 influence ion behavior. The results indicate that both D59 and D61 are required to confer the extracellular vestibule of zfP2X4 a preference for cations. In contrast, the presence of D53 suffices to provide that capacity to hP2X3. We also computed the potentials of mean force for the passage of Na⁺ and Cl^- through the pore of hP2X3. These profiles were compared against those already available for zfP2X4. Altogether, the results provide a detailed description of the mechanisms employed by these receptors to discriminate between cations and anions.

Evolution of Mechanical Stress (In-vitro) on Properties of Granulocytes in Acute Lymphoblastic Leukemia

Acute lymphoid leukemia (ALL) affects the lymph cells or lymphocytes that make up the lymph tissue and prevents the proper maturation of the bone marrow cells. The processes through which cells convert mechanical stimuli into biochemical signals are called mechanical transitions and result in the sensation of specific cellular responses. In the present study, the functional properties of granulocytes of the patients with ALL were investigated using the in vitro mechanical stress model. The experimental part of the work was executed using blood from patients with ALL (n=30) being treated in the Hematological Department of Belgorod Region Hospital, Belgorod, Russia. The patients were in the age range of 18-45 years. Sample blood was obtained from all the patients who underwent a standard course of chemotherapy. Blood sampling was performed using a venepuncture and collected into the vacuum tubes Vacuette K3E. Blood samples from each experimental group were divided into two groups of control and experiment. The injection model of mechanical stress was used for the experiment group in vitro. Subsequently, the adenosine triphosphate (ATP) concentration increased by 1.8 times in this group, compared with the controls. Young's module, which numerically characterizes the rigidity of the granulocytes' plasmalemma, decreased by 54.4% (P<0.05) under the influence of mechanical stress. The surface potential of plasmalemma was not significantly different between samples in the group of control and experiment in patients with ALL. However, the adhesive force between erythrocyte and granulocyte increased by 30.7% (P<0.05). The osmotic load test showed an increase in the cell's volume during incubation. The use of membrane reserve by granulocytes increased by 47% (P<0.05) at the initial seconds of incubation. The obtained results pointed to the regulatory role of ATP molecules in intercellular signaling and add to the present literature regarding the mechanisms of intercellular interaction in the microvasculature on the development of leukemia. Moreover, the obtained results can be taken into account for the development of new pharmacological immune correctors.

Regulation of P2X1 receptors by modulators of the cAMP effectors PKA and EPAC

P2X1 receptors are adenosine triphosphate (ATP)-gated cation channels that are functionally important for male fertility, bladder contraction, and platelet aggregation. The activity of P2X1 receptors is modulated by lipids and intracellular messengers such as cAMP, which can stimulate protein kinase A (PKA). Exchange protein activated by cAMP (EPAC) is another cAMP effector; however, its effect on P2X1 receptors has not yet been determined. Here, we demonstrate that P2X1 currents, recorded from human embryonic kidney (HEK) cells transiently transfected with P2X1 cDNA, were inhibited by the highly selective EPAC activator 007-AM. In contrast, EPAC activation enhanced P2X2 current amplitude. The PKA activator 6-MB-cAMP did not affect P2X1 currents, but inhibited P2X2 currents. The inhibitory effects of EPAC on P2X1 were prevented by triple mutation of residues 21 to 23 on the amino terminus of P2X1 subunits to the equivalent amino acids on P2X2 receptors. Double mutation of residues 21 and 22 and single mutation of residue 23 also protected P2X1 receptors from inhibition by EPAC activation. Finally, the inhibitory effects of EPAC on P2X1 were also prevented by NSC23766, an inhibitor of Rac1, a member of the Rho family of small GTPases. These data suggest that EPAC is an important regulator of P2X1 and P2X2 receptors.

Specific and behaviorally consequential astrocyte Gq GPCR signaling attenuation in vivo with iβARK

Astrocytes respond to neurotransmitters and neuromodulators using G-protein-coupled receptors (GPCRs) to mediate physiological responses. Despite their importance, there has been no method to genetically, specifically, and effectively attenuate astrocyte Gq GPCR pathways to explore consequences of this prevalent signaling mechanism in vivo. We report a 122-residue inhibitory peptide from β-adrenergic receptor kinase 1 (iβARK; and inactive D110A control) to attenuate astrocyte Gq GPCR signaling. iβARK significantly attenuated Gq GPCR Ca2+ signaling in brain slices and, in vivo, altered behavioral responses, spared other GPCR responses, and did not alter astrocyte spontaneous Ca2+ signals, morphology, electrophysiological properties, or gene expression in the striatum. Furthermore, brain-wide attenuation of astrocyte Gq GPCR signaling with iβARK using PHP.eB adeno-associated viruses (AAVs), when combined with c-Fos mapping, suggested nuclei-specific contributions to behavioral adaptation and spatial memory. iβARK extends the toolkit needed to explore functions of astrocyte Gq GPCR signaling within neural circuits in vivo.

High-affinity P2Y2 and low-affinity P2X7 receptor interaction modulates ATP-mediated calcium signaling in murine osteoblasts

The P2 purinergic receptor family implicated in many physiological processes, including neurotransmission, mechanical adaptation and inflammation, consists of ATP-gated non-specific cation channels P2XRs and G-protein coupled receptors P2YRs. Different cells, including bone forming osteoblasts, express multiple P2 receptors; however, how P2X and P2Y receptors interact in generating cellular responses to various doses of [ATP] remains poorly understood. Using primary bone marrow and compact bone derived osteoblasts and BMP2-expressing C2C12 osteoblastic cells, we demonstrated conserved features in the P2-mediated Ca2+ responses to ATP, including a transition of Ca2+ response signatures from transient at low [ATP] to oscillatory at moderate [ATP], and back to transient at high [ATP], and a non-monotonic changes in the response magnitudes which exhibited two troughs at 10-4 and 10-2 M [ATP]. We identified P2Y2 and P2X7 receptors as predominantly contributing to these responses and constructed a mathematical model of P2Y2R-induced inositol trisphosphate (IP3) mediated Ca2+ release coupled to a Markov model of P2X7R dynamics to study this system. Model predictions were validated using parental and CRISPR/Cas9-generated P2Y2 and P2Y7 knockouts in osteoblastic C2C12-BMP cells. Activation of P2Y2 by progressively increasing [ATP] induced a transition from transient to oscillatory to transient Ca2+ responses due to the biphasic nature of IP3Rs and the interaction of SERCA pumps with IP3Rs. At high [ATP], activation of P2X7R modulated the response magnitudes through an interplay between the biphasic nature of IP3Rs and the desensitization kinetics of P2X7Rs. Moreover, we found that P2Y2 activity may alter the kinetics of P2X7 towards favouring naïve state activation. Finally, we demonstrated the functional consequences of lacking P2Y2 or P2X7 in osteoblast mechanotransduction. This study thus provides important insights into the biophysical mechanisms underlying ATP-dependent Ca2+ response signatures, which are important in mediating bone mechanoadaptation.

Enhanced expression of P2X4 purinoceptors in pyramidal neurons of the rat hippocampal CA1 region may be involved ischemia-reperfusion injury

Ischemic stroke is the most serious disease that harms human beings. In principle, its treatment is to restore blood flow supply as soon as possible. However, after the blood flow is restored, it will lead to secondary brain injury, that is, ischemia-reperfusion injury. The mechanism of ischemia-reperfusion injury is very complicated. This study showed that P2X4 receptors in the pyramidal neurons of rat hippocampus were significantly upregulated in the early stage of ischemia-reperfusion injury. Neurons with high expression of P2X4 receptors are neurons that are undergoing apoptosis. Intraventricular injection of the P2X4 receptor antagonist 5-(3-bromophenyl)-1,3-dihydro-2H-benzofuro[3,2-e]-1,4-diazepin-2-one (5-BDBD) and PSB-12062 can partially block neuronal apoptosis, to promote the survival of neurons, indicating that ATP through P2X4 receptors is involved in the process of cerebral ischemia-reperfusion injury. Therefore, identifying the mechanism of neuronal degeneration induced by extracellular ATP via P2X4 receptors after ischemia-reperfusion will likely find new targets for the treatment of ischemia-reperfusion injury, and will provide a useful theoretical basis for the treatment of ischemia-reperfusion injury.

Farnesyl pyrophosphate is a new danger signal inducing acute cell death

Cell death is a vital event in life. Infections and injuries cause lytic cell death, which gives rise to danger signals that can further induce cell death, inflammation, and tissue damage. The mevalonate (MVA) pathway is an essential, highly conserved and dynamic metabolic pathway. Here, we discover that farnesyl pyrophosphate (FPP), a metabolic intermediate of the MVA pathway, functions as a newly identified danger signal to trigger acute cell death leading to neuron loss in stroke. Harboring both a hydrophobic 15-carbon isoprenyl chain and a heavily charged pyrophosphate head, FPP leads to acute cell death independent of its downstream metabolic pathways. Mechanistically, extracellular calcium influx and the cation channel transient receptor potential melastatin 2 (TRPM2) exhibit essential roles in FPP-induced cell death. FPP activates TRPM2 opening for ion influx. Furthermore, in terms of a mouse model constructing by middle cerebral artery occlusion (MCAO), FPP accumulates in the brain, which indicates the function of the FPP and TRPM2 danger signal axis in ischemic injury. Overall, our data have revealed a novel function of the MVA pathway intermediate metabolite FPP as a danger signal via transient receptor potential cation channels.

Regional Neurodegeneration in vitro: The Protective Role of Neural Activity

Traumatic brain injury is a devastating public health problem, the eighth leading cause of death across the world. To improve our understanding of how injury at the cellular scale affects neural circuit function, we developed a protocol to precisely injure individual neurons within an in vitro neural network. We used high speed calcium imaging to estimate alterations in neural activity and connectivity that occur followed targeted microtrauma. Our studies show that mechanically injured neurons inactivate following microtrauma and eventually re-integrate into the network. Single neuron re-integration is dependent on its activity prior to injury and initial connections in the network: more active and integrated neurons are more resistant to microtrauma and more likely to re-integrate into the network. Micromechanical injury leads to neuronal death 6 h post-injury in a subset of both injured and uninjured neurons. Interestingly, neural activity and network participation after injury were associated with survival in linear discriminate analysis (77.3% correct prediction, Wilks' Lambda = 0.838). Based on this observation, we modulated neuronal activity to rescue neurons after microtrauma. Inhibition of neuronal activity provided much greater survivability than did activation of neurons (ANOVA, p < 0.01 with post-hoc Tukey HSD, p < 0.01). Rescue of neurons by blocking activity in the post-acute period is partially mediated by mitochondrial energetics, as we observed silencing neurons after micromechanical injury led to a significant reduction in mitochondrial calcium accumulation. Overall, the present study provides deeper insight into the propagation of injury within networks, demonstrating that together the initial activity, network structure, and post-injury activity levels contribute to the progressive changes in a neural circuit after mechanical trauma.

Optogenetic activation of spinal microglia triggers chronic pain in mice

Spinal microglia are highly responsive to peripheral nerve injury and are known to be a key player in pain. However, there has not been direct evidence showing that selective microglial activation in vivo is sufficient to induce chronic pain. Here, we used optogenetic approaches in microglia to address this question employing CX3CR1creER/+: R26LSL-ReaChR/+ transgenic mice, in which red-activated channelrhodopsin (ReaChR) is inducibly and specifically expressed in microglia. We found that activation of ReaChR by red light in spinal microglia evoked reliable inward currents and membrane depolarization. In vivo optogenetic activation of microglial ReaChR in the spinal cord triggered chronic pain hypersensitivity in both male and female mice. In addition, activation of microglial ReaChR up-regulated neuronal c-Fos expression and enhanced C-fiber responses. Mechanistically, ReaChR activation led to a reactive microglial phenotype with increased interleukin (IL)-1β production, which is likely mediated by inflammasome activation and calcium elevation. IL-1 receptor antagonist (IL-1ra) was able to reverse the pain hypersensitivity and neuronal hyperactivity induced by microglial ReaChR activation. Therefore, our work demonstrates that optogenetic activation of spinal microglia is sufficient to trigger chronic pain phenotypes by increasing neuronal activity via IL-1 signaling.

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.

Intracellular Ca2+ Signaling in Protozoan Parasites: An Overview with a Focus on Mitochondria

Ca²⁺ signaling has been involved in controling critical cellular functions such as activation of proteases, cell death, and cell cycle control. The endoplasmatic reticulum plays a significant role in Ca²⁺ storage inside the cell, but mitochondria have long been recognized as a fundamental Ca²⁺ pool. Protozoan parasites such as Plasmodium falciparum, Toxoplasma gondii, and Trypanosoma cruzi display a Ca²⁺ signaling toolkit with similarities to higher eukaryotes, including the participation of mitochondria in Ca²⁺-dependent signaling events. This review summarizes the most recent knowledge in mitochondrial Ca²⁺ signaling in protozoan parasites, focusing on the mechanism involved in mitochondrial Ca²⁺ uptake by pathogenic protists.

KCa3.1-dependent uptake of the cytotoxic DNA-binding dye Hoechst 33258 into cancerous but not healthy cervical cells

The poor and nonselective penetration of current chemotherapeutics across the plasma membranes of cancer cells, which is necessary for the targeted disruption of the intracellular machinery, remains a major pharmaceutical challenge. In several cell types, including mast cells and macrophages, exposure to extracellular ATP is known to stimulate passive entry of large and otherwise membrane impermeable cationic dyes, which is usually attributed to conduction through ionotropic P2X receptors. Here, we report that elevations in cytosolic Ca2+ stimulate the rapid uptake and nuclear accumulation of a DNA-binding fluorescent cation, Hoechst 33258 (H33258), in cervical cancer cells. The H33258 uptake was dependent on activation of intermediate conductance Ca2+-activated K+ channels (KCa3.1), and direct stimulation of the channel with the activators SKA 31 and DCEBIO was sufficient to induce cellular uptake of H33258 directly. In contrast to the results from cancerous cervical cells, KCa3.1-dependent H33258 uptake was rarely observed in epithelial cells derived from the ectocervix and transformation zone of healthy cervical tissue. Furthermore, whole-cell patch clamp experiments and assessment of membrane potential using the slow voltage-sensitive dye bis-(1,3-diethylthiobarbituric acid)trimethine oxonol revealed a significant difference in functional KCa3.1 activity between cancerous and healthy cervical epithelial cells, which correlated strongly with the incidence of KCa3.1-dependent H33258 uptake. Finally, we show that activation of KCa3.1 channels caused a modest but significant sensitization of cancer cells to the growth suppressant effects of H33258, lending plausibility to the idea of using KCa3.1 channel activators to enhance cell penetration of small cationic toxins into cancer cells expressing these channels.

Update of P2X receptor properties and their pharmacology: IUPHAR Review 30

The known seven mammalian receptor subunits (P2X1–7) form cationic channels gated by ATP. Three subunits compose a receptor channel. Each subunit is a polypeptide consisting of two transmembrane regions (TM1 and TM2), intracellular N- and C-termini, and a bulky extracellular loop. Crystallization allowed the identification of the 3D structure and gating cycle of P2X receptors. The agonist-binding pocket is located at the intersection of two neighbouring subunits. In addition to the mammalian P2X receptors, their primitive ligand-gated counterparts with little structural similarity have also been cloned. Selective agonists for P2X receptor subtypes are not available, but medicinal chemistry supplied a range of subtype-selective antagonists, as well as positive and negative allosteric modulators. Knockout mice and selective antagonists helped to identify pathological functions due to defective P2X receptors, such as male infertility (P2X1), hearing loss (P2X2), pain/cough (P2X3), neuropathic pain (P2X4), inflammatory bone loss (P2X5), and faulty immune reactions (P2X7).

Role of Conserved Residues and F322 in the Extracellular Vestibule of the Rat P2X7 Receptor in Its Expression, Function and Dye Uptake Ability

Activation of the P2X7 receptor results in the opening of a large pore that plays a role in immune responses, apoptosis, and many other physiological and pathological processes. Here, we investigated the role of conserved and unique residues in the extracellular vestibule connecting the agonist-binding domain with the transmembrane domain of rat P2X7 receptor. We found that all residues that are conserved among the P2X receptor subtypes respond to alanine mutagenesis with an inhibition (Y51, Q52, and G323) or a significant decrease (K49, G326, K327, and F328) of 2',3'-O-(benzoyl-4-benzoyl)-ATP (BzATP)-induced current and permeability to ethidium bromide, while the nonconserved residue (F322), which is also present in P2X4 receptor, responds with a 10-fold higher sensitivity to BzATP, much slower deactivation kinetics, and a higher propensity to form the large dye-permeable pore. We examined the membrane expression of conserved mutants and found that Y51, Q52, G323, and F328 play a role in the trafficking of the receptor to the plasma membrane, while K49 controls receptor responsiveness to agonists. Finally, we studied the importance of the physicochemical properties of these residues and observed that the K49R, F322Y, F322W, and F322L mutants significantly reversed the receptor function, indicating that positively charged and large hydrophobic residues are important at positions 49 and 322, respectively. These results show that clusters of conserved residues above the transmembrane domain 1 (K49-Y51-Q52) and transmembrane domain 2 (G326-K327-F328) are important for receptor structure, membrane expression, and channel gating and that the nonconserved residue (F322) at the top of the extracellular vestibule is involved in hydrophobic inter-subunit interaction which stabilizes the closed state of the P2X7 receptor channel.

The interplay between α7 nicotinic acetylcholine receptors, pannexin-1 channels and P2X7 receptors elicit exocytosis in chromaffin cells

Pannexin-1 (Panx1) forms plasma membrane channels that allow the exchange of small molecules between the intracellular and extracellular compartments, and are involved in diverse physiological and pathological responses in the nervous system. However, the signaling mechanisms that induce their opening still remain elusive. Here, we propose a new mechanism for Panx1 channel activation through a functional crosstalk with the highly Ca²⁺ permeable α7 nicotinic acetylcholine receptor (nAChR). Consistent with this hypothesis, we found that activation of α7 nAChRs induces Panx1-mediated dye uptake and ATP release in the neuroblastoma cell line SH-SY5Y-α7. Using membrane permeant Ca²⁺ chelators, total internal reflection fluorescence microscopy in SH-SY5Y-α7 cells expressing a membrane-tethered GCAMP3, and Src kinase inhibitors, we further demonstrated that Panx1 channel opening depends on Ca²⁺ signals localized in submembrane areas, as well as on Src kinases. In turn, Panx1 channels amplify cytosolic Ca²⁺ signals induced by the activation of α7 nAChRs, by a mechanism that seems to involve ATP release and P2X7 receptor activation, as hydrolysis of extracellular ATP with apyrase or blockage of P2X7 receptors with oxidized ATP significantly reduces the α7 nAChR-Ca²⁺ signal. The physiological relevance of this crosstalk was also demonstrated in neuroendocrine chromaffin cells, wherein Panx1 channels and P2X7 receptors contribute to the exocytotic release of catecholamines triggered by α7 nAChRs, as measured by amperometry. Together these findings point to a functional coupling between α7 nAChRs, Panx1 channels and P2X7 receptors with physiological relevance in neurosecretion.

P2X3-Containing Receptors as Targets for the Treatment of Chronic Pain

Current therapies for the treatment of chronic pain provide inadequate relief for millions of suffering patients, demonstrating the need for better therapies that will treat pain effectively and improve the quality of patient's lives. Better understanding of the mechanisms that mediate chronic pain is critical for developing drugs with improved clinical outcomes. Adenosine triphosphate (ATP) is a key modulator in nociceptive pathways. Release of ATP from injured tissue or sympathetic efferents has sensitizing effects on sensory neurons in the periphery, and presynaptic vesicular release of ATP from the central terminals can increase glutamate release thereby potentiating downstream central sensitization mechanisms, a condition thought to underlie many chronic pain conditions. The purinergic receptors on sensory nerves primarily responsible for ATP signaling are P2X3 and P2X2/3. Selective knockdown experiments, or inhibition with small molecules, demonstrate P2X3-containing receptors are key targets to modulate nociceptive signals. Preclinical studies have identified that P2X3-containing receptors are critical for sensory transduction for bladder function, and clinical studies have shown promise in treatment for bladder pain and pain associated with osteoarthritis. Further clinical characterization of antagonists to P2X3-containing receptors may lead to improved therapies in the treatment of chronic pain.

Drosophila taste neurons as an agonist-screening platform for P2X receptors

The P2X receptor family of ATP-gated cation channels are attractive drug targets for pain and inflammatory disease, but no subtype-selective agonists, and few partially selective agonists have been described to date. As proof-of-concept for the discovery of novel P2X receptor agonists, here we demonstrate the use of Drosophila taste neurons heterologously expressing rat P2X2 receptors as a screening platform. We demonstrate that wild-type rat P2X2 expressed in Drosophila is fully functional (ATP EC₅₀ 8.7 µM), and that screening of small (2 µl) volumes of a library of 80 adenosine nucleotide analogues is rapid and straightforward. We have determined agonist potency and specificity profiles for rat P2X2 receptors; triphosphate-bearing analogues display broad activity, tolerating a number of substitutions, and diphosphate and monophosphate analogues display very little activity. While several ATP analogues gave responses of similar magnitude to ATP, including the previously identified agonists ATPγS and ATPαS, we were also able to identify a novel agonist, the synthetic analogue 2-fluoro-ATP, and to confirm its agonist activity on rat P2X2 receptors expressed in human cells. These data validate our Drosophila platform as a useful tool for the analysis of agonist structure-activity relationships, and for the screening and discovery of novel P2X receptor agonists.

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.

Purinergic P2X1 receptor, purinergic P2X7 receptor, and angiotensin II type 1 receptor interactions in the regulation of renal afferent arterioles in angiotensin II-dependent hypertension

In ANG II-dependent hypertension, ANG II activates ANG II type 1 receptors (AT₁Rs), elevating blood pressure and increasing renal afferent arteriolar resistance (AAR). The increased arterial pressure augments interstitial ATP concentrations activating purinergic P2X receptors (P2XRs) also increasing AAR. Interestingly, P2X₁R and P2X₇R inhibition reduces AAR to the normal range, raising the conundrum regarding the apparent disappearance of AT₁R influence. To evaluate the interactions between P2XRs and AT₁Rs in mediating the increased AAR elicited by chronic ANG II infusions, experiments using the isolated blood perfused juxtamedullary nephron preparation allowed visualization of afferent arteriolar diameters (AAD). Normotensive and ANG II-infused hypertensive rats showed AAD responses to increases in renal perfusion pressure from 100 to 140 mmHg by decreasing AAD by 26 ± 10% and 19 ± 4%. Superfusion with the inhibitor P2X₁Ri (NF4490; 1 μM) increased AAD. In normotensive kidneys, superfusion with ANG II (1 nM) decreased AAD by 16 ± 4% and decreased further by 19 ± 5% with an increase in renal perfusion pressure. Treatment with P2X₁Ri increased AAD by 30 ± 6% to values higher than those at 100 mmHg plus ANG II. In hypertensive kidneys, the inhibitor AT₁Ri (SML1394; 1 μM) increased AAD by 10 ± 7%. In contrast, treatment with P2X₁Ri increased AAD by 21 ± 14%; combination with P2X₁Ri plus P2X₇Ri (A438079; 1 μM) increased AAD further by 25 ± 8%. The results indicate that P2X₁R, P2X₇R, and AT₁R actions converge at receptor or postreceptor signaling pathways, but P2XR exerts a dominant influence abrogating the actions of AT₁Rs on AAR in ANG II-dependent hypertension.

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.

High-resolution detection of ATP release from single cultured mouse dorsal horn spinal cord glial cells and its modulation by noradrenaline

Human embryonic kidney 293 (HEK293) cells stably transfected with the rat P2X2 receptor subunit were preincubated with 200 nM progesterone (HEK293-P2X2-PROG), a potent positive allosteric modulator of homomeric P2X2 receptors, and used to detect low nanomolar concentrations of extracellular ATP. Fura-2-loaded HEK293-P2X2-PROG cells were acutely plated on top of cultured DH glial cells to quantify ATP release from single DH glial cells. Application of the α1 adrenoceptor agonist phenylephrine (PHE, 20 μM) or of a low K⁺ (0.2 mM) solution evoked reversible increases in the intracellular calcium concentration ([Ca²⁺]_i) in the biosensor cells. A reversible increase in [Ca²⁺]_i was also detected in half of the biosensor cells following the interruption of general extracellular perfusion. All increases in [Ca²⁺]_i were blocked in the presence of the P2X2 antagonist PPADS or after preloading the glial cells with the calcium chelator BAPTA, indicating that they were due to calcium-dependent ATP release from the glial cells. ATP release induced by PHE was blocked by -L-phenylalanine 2-naphtylamide (GPN) that permeabilizes secretory lysosomes and bafilomycin A1 (Baf A1), an inhibitor of the H⁺-pump of acidic secretory vesicles. By contrast, ATP release induced by application of a low-K⁺ solution was abolished by Baf A1 but not by GPN. Finally, spontaneous ATP release observed after interrupting general perfusion was insensitive to both GPN and Baf A1 pretreatment. Our results indicate that ATP is released in a calcium-dependent manner from two distinct vesicular pools and one non-vesicular pool coexisting in DH glial cells and that noradrenaline and PHE selectively target the secretory lysosome pool.

Ca2+ flux through splice variants of the ATP-gated ionotropic receptor P2X7 is regulated by its cytoplasmic N terminus

Activation of ionotropic P2X receptors increases free intracellular Ca²⁺ ([Ca²⁺] _i ) by initiating a transmembrane cation flux. We studied the "a" and "k" splice variants of the rat purinergic P2X7 receptor (rP2X7aR and rP2X7kR) to exhibit a significant difference in Ca²⁺ flux through this channel. This difference is surprising because the variants share absolute sequence identity in the area of the pore that defines ionic selectivity. Here, we used patch-clamp fluorometry and chimeric receptors to show that the fraction of the total current carried by Ca²⁺ is a function of the primary sequence of the cytoplasmic N terminus. Using scanning mutagenesis, we identified five sites within the N terminus that respond to mutagenesis with a decrease in fractional calcium current and an increase in permeability to the polyatomic cation, N-methyl-d-glucamine (NMDG⁺), relative to Na⁺ (P _NMDG/P _Na). We tested the hypothesis that these sites line the permeation pathway by measuring the ability of thiol-reactive MTSET⁺ to alter the current of cysteine-substituted variants, but we detected no effect. Finally, we studied the homologous sites of the rat P2X2 receptor (rP2X2R) and observed that substitutions at Glu¹⁷ significantly reduced the fractional calcium current. Taken together, our results suggest that a change in the structure of the N terminus alters the ability of an intra-pore Ca²⁺ selectivity filter to discriminate among permeating cations. These results are noteworthy for two reasons: they identify a previously unknown outcome of mutagenesis of the N-terminal domain, and they suggest caution when assigning structure to function for truncated P2X receptors that lack a part of the N terminus.

P2X4 receptors, immunity, and sepsis

Sepsis is life-threatening systemic organ dysfunction caused by a deregulated host response to an infectious insult. Currently, the treatment of sepsis is limited to the use of antibiotics, fluids, and cardiovascular/respiratory support. Despite these interventions, septic mortality remains high, with reduced life quality in survivors. For this reason, the identification of novel drug targets is a pressing task of modern pharmacology. According to a recent research, it appears that P2 purinergic receptors, which can regulate the host's response to infections, have been identified as potential targets for the treatment of sepsis. Among P2 receptors, the P2X₄ receptor has recently captured the attention of the research community owing to its role in protecting against infections, inflammation, and organ injury. The present review provides an outline of the role played by P2X₄ receptors in the modulation of the host's response to sepsis and the promise that targeting this receptor holds in the treatment of sepsis.

The Role of Connexin and Pannexin Channels in Perinatal Brain Injury and Inflammation

Perinatal brain injury remains a major cause of death and life-long disability. Perinatal brain injury is typically associated with hypoxia-ischemia and/or infection/inflammation. Both hypoxia-ischemia and infection trigger an inflammatory response in the brain. The inflammatory response can contribute to brain cell loss and chronic neuroinflammation leading to neurological impairments. It is now well-established that brain injury evolves over time, and shows a striking spread from injured to previously uninjured regions of the brain. There is increasing evidence that this spread is related to opening of connexin hemichannels and pannexin channels, both of which are large conductance membrane channels found in almost all cell types in the brain. Blocking connexin hemichannels within the first 3 h after hypoxia-ischemia has been shown to improve outcomes in term equivalent fetal sheep but it is important to also understand the downstream pathways linking membrane channel opening with the development of injury in order to identify new therapeutic targets. Open membrane channels release adenosine triphosphate (ATP), and other neuroactive molecules, into the extracellular space. ATP has an important physiological role, but has also been reported to act as a damage-associated molecular pattern (DAMP) signal mediated through specific purinergic receptors and so act as a primary signal 1 in the innate immune system inflammasome pathway. More crucially, extracellular ATP is a key inflammasome signal 2 activator, with purinergic receptor binding triggering the assembly of the multi-protein inflammasome complex. The inflammasome pathway and complex formation contribute to activation of inflammatory caspases, and the release of inflammatory cytokines, including interleukin (IL)-1β, tumor necrosis factor (TNF)-α, IL-18, and vascular endothelial growth factor (VEGF). We propose that the NOD-like receptor protein-3 (NLRP3) inflammasome, which has been linked to inflammatory responses in models of ischemic stroke and various inflammatory diseases, may be one mechanism by which connexin hemichannel opening especially mediates perinatal brain injury.

Coordinated calcium signalling in cochlear sensory and non-sensory cells refines afferent innervation of outer hair cells

Outer hair cells (OHCs) are highly specialized sensory cells conferring the fine‐tuning and high sensitivity of the mammalian cochlea to acoustic stimuli. Here, by genetically manipulating spontaneous Ca²⁺ signalling in mice in vivo, through a period of early postnatal development, we find that the refinement of OHC afferent innervation is regulated by complementary spontaneous Ca²⁺ signals originating in OHCs and non‐sensory cells. OHCs fire spontaneous Ca²⁺ action potentials during a narrow period of neonatal development. Simultaneously, waves of Ca²⁺ activity in the non‐sensory cells of the greater epithelial ridge cause, via ATP‐induced activation of P2X₃ receptors, the increase and synchronization of the Ca²⁺ activity in nearby OHCs. This synchronization is required for the refinement of their immature afferent innervation. In the absence of connexin channels, Ca²⁺ waves are impaired, leading to a reduction in the number of ribbon synapses and afferent fibres on OHCs. We propose that the correct maturation of the afferent connectivity of OHCs requires experience‐independent Ca²⁺ signals from sensory and non‐sensory cells.

Diabetes-induced damage of gastric nitric oxide neurons mediated by P2X7R in diabetic mice

It is generally considered that enteric neuropathy is one of the causative factors in diabetic gastroparesis. Our previous study demonstrated that there is a loss of NOS neurons in diabetic mice. However, the underlying mechanism remains unclear. The present study was designed to clarify the relationship between neuronal P2X7R and NOS neuron damage. The effect of P2X7R on diabetes-induced gastric NOS neurons damage and its mechanism were investigated by using quantitative RT-PCR，immunofluorescence, western blot, isometric force recording, intracellular calcium ([Ca²⁺]i) measurement and whole-cell patch clamp techniques. The immunohistochemistry and western blot results showed that nNOS expression was significantly down-regulated in diabetic mice, meanwhile, electric field stimulation-induced NOS sensitive relaxation was significantly suppressed. Myenteric neurons expressed P2X7R and pannexin1, and the mRNA and protein level of P2X7R and pannexin1 were up-regulated in diabetic mice. BzATP, a P2X7R activator, evoked [Ca²⁺]i increase in Hek293 cells with heterologous expression of P2X7R (Hek293-P2X7R cells) and the same dose of ATP-induced [Ca²⁺]i was more obvious in Hek293-P2X7R cells than in Hek293 cells. Application of BzATP activated an inward current of Hek293-P2X7R in a dose dependent manner. Hek293-P2X7R but not untransfected Hek293 cells could take up of YO-PRO-1. In addition, the uptake of YO-PRO-1 by Hek293-P2X7R was blocked by oxATP, a P2X7 antagonist and CBX, a pannexin1 inhibitor. The results suggest that the P2X7R of enteric neurons may be involved in diabetes-induced NOS neuron damage via combining with pannexin-1 to form transmembrane pores which induce macromolecular substances and calcium into the cells.

Role of P2 receptors in normal brain development and in neurodevelopmental psychiatric disorders

The purinergic signaling system, including P2 receptors, plays an important role in various central nervous system (CNS) disorders. Over the last few decades, a substantial amount of accumulated data suggest that most P2 receptor subtypes (P2X1, 2, 3, 4, 6, and 7, and P2Y₁, ₂, ₆, ₁₂, and ₁₃) regulate neuronal/neuroglial developmental processes, such as proliferation, differentiation, migration of neuronal precursors, and neurite outgrowth. However, only a few of these subtypes (P2X2, P2X3, P2X4, P2X7, P2Y₁, and P2Y₂) have been investigated in the context of neurodevelopmental psychiatric disorders. The activation of these potential target receptors and their underlying mechanisms mainly influence the process of neuroinflammation. In particular, P2 receptor-mediated inflammatory cytokine release has been indicated to contribute to the complex mechanisms of a variety of CNS disorders. The released inflammatory cytokines could be utilized as biomarkers for neurodevelopmental and psychiatric disorders to improve the early diagnosis intervention, and prognosis. The population changes in gut microbiota after birth are closely linked to neurodevelopmental/neuropsychiatric disorders in later life; thus, the dynamic expression and function of P2 receptors on gut epithelial cells during disease processes indicate a novel avenue for the evaluation of disease progression and for the discovery of related therapeutic compounds.

The interaction between P2X7Rs and T-type calcium ion channels in penicillin-induced epileptiform activity

Limited information exists on the link between purinergic class P2X7 receptors (P2X7Rs) and calcium ion channels in epilepsy; no data has been reported regarding the interaction between P2X7Rs and T-type calcium ion channels in epilepsy. Thus, this study is an evaluation of the role that T-type calcium ion channels play in the effect of P2X7Rs on penicillin-induced epileptiform activity. In the first set of experiments, P2X7R agonist BzATP (at 25-, 50-, 100- and 200-μg doses), P2X7R antagonist A-438079 (at 5-, 10-, 20- and 40-μg doses) and T-type calcium ion channel antagonist, NNC-550396 were administered for electrophysiological analyses 30 min after penicillin injection (2.5 μl, 500 IU). In the second set of experiments, the effective doses of these substances were used for biochemical analyses. Malondialdehyde (MDA), advanced oxidation protein product (AOPP), glutathione (GSH), glutathione reductase (GR), glutathione peroxide (GPx), catalase (CAT) and superoxide dismutase (SOD) levels were measured in the cerebrum, cerebellum and brainstem of rats. BzATP (100 μg, icv) increased the mean frequency of epileptiform activity, whereas A-438079 (40 μg, icv) and NNC-550396 (30 μg, ic) reduced it. Both A-438079 and NNC-550396 reversed BzATP's proconvulsant action. BzATP increased lipid peroxidation and protein oxidation; it also altered other antioxidant enzymes measured in this study, which were all then reversed via A-438079 and NNC-550396, at least in the cerebrum. The electrophysiological and biochemical analysis of present study suggest that P2X7Rs and its interaction with T-type calcium ion channels play an important role in the experimental model of epilepsy.

Simulation of P2X-mediated calcium signalling in microglia

KEY POINTS

A computational model of P2X channel activation in microglia was developed that includes downfield Ca²⁺ -dependent signalling pathways. This model provides quantitative insights into how diverse signalling pathways in microglia converge to control microglial function.

ABSTRACT

Microglia function is orchestrated through highly coupled signalling pathways that depend on calcium (Ca²⁺ ). In response to extracellular ATP, transient increases in intracellular Ca²⁺ driven through the activation of purinergic receptors, P2X and P2Y, are sufficient to promote cytokine synthesis. Although the steps comprising the pathways bridging purinergic receptor activation with transcriptional responses have been probed in great detail, a quantitative model for how these steps collectively control cytokine production has not been established. Here we developed a minimal computational model that quantitatively links extracellular stimulation of two prominent ionotropic purinergic receptors, P2X4 and P2X7, with the graded production of a gene product, namely the tumour necrosis factor α (TNFα) cytokine. In addition to Ca²⁺ handling mechanisms common to eukaryotic cells, our model includes microglia-specific processes including ATP-dependent P2X4 and P2X7 activation, activation of nuclear factor of activated T-cells (NFAT) transcription factors, and TNFα production. Parameters for this model were optimized to reproduce published data for these processes, where available. With this model, we determined the propensity for TNFα production in microglia, subject to a wide range of ATP exposure amplitudes, frequencies and durations that the cells could encounter in vivo. Furthermore, we have investigated the extent to which modulation of the signal transduction pathways influence TNFα production. Our results suggest that pulsatile stimulation of P2X4 via micromolar ATP may be sufficient to promote TNFα production, whereas high-amplitude ATP exposure is necessary for production via P2X7. Furthermore, under conditions that increase P2X4 expression, for instance, following activation by pathogen-associated molecular factors, P2X4-associated TNFα production is greatly enhanced. Given that Ca²⁺ homeostasis in microglia is profoundly important to its function, this computational model provides a quantitative framework to explore hypotheses pertaining to microglial physiology.