Tumour immune escape via P2X7 receptor signalling

While P2X7 receptor expression on tumour cells has been characterized as a promotor of cancer growth and metastasis, its expression by the host immune system is central for orchestration of both innate and adaptive immune responses against cancer. The role of P2X7R in anti-tumour immunity is complex and preclinical studies have described opposing roles of the P2X7R in regulating immune responses against tumours. Therefore, few P2X7R modulators have reached clinical testing in cancer patients. Here, we review the prognostic value of P2X7R in cancer, how P2X7R have been targeted to date in tumour models, and we discuss four aspects of how tumours skew immune responses to promote immune escape via the P2X7R; non-pore functional P2X7Rs, mono-ADP-ribosyltransferases, ectonucleotidases, and immunoregulatory cells. Lastly, we discuss alternative approaches to offset tumour immune escape via P2X7R to enhance immunotherapeutic strategies in cancer patients.

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.

Key sites for P2X receptor function and multimerization: overview of mutagenesis studies on a structural basis

P2X receptors constitute a seven-member family (P2X1-7) of extracellular ATP-gated cation channels of widespread expression. Because P2X receptors have been implicated in neurological, inflammatory and cardiovascular diseases, they constitute promising drug targets. Since the first P2X cDNA sequences became available in 1994, numerous site-directed mutagenesis studies have been conducted to disclose key sites of P2X receptor function and oligomerization. The publication of the 3-Å crystal structures of the zebrafish P2X4 (zfP2X4) receptor in the homotrimeric apo-closed and ATP-bound open states in 2009 and 2012, respectively, has ushered a new era by allowing for the interpretation of the wealth of molecular data in terms of specific three-dimensional models and by paving the way for designing more-decisive experiments. Thanks to these structures, the last five years have provided invaluable insight into our understanding of the structure and function of the P2X receptor class of ligandgated ion channels. In this review, we provide an overview of mutagenesis studies of the pre- and post-crystal structure eras that identified amino acid residues of key importance for ligand binding, channel gating, ion flow, formation of the pore and the channel gate, and desensitization. In addition, the sites that are involved in the trimerization of P2X receptors are reviewed based on mutagenesis studies and interface contacts that were predicted by the zfP2X4 crystal structures.

Flexible subunit stoichiometry of functional human P2X2/3 heteromeric receptors

The aim of the present work was to clarify whether heterotrimeric P2X2/3 receptors have a fixed subunit stoichiometry consisting of one P2X2 and two P2X3 subunits as previously suggested, or a flexible stoichiometry containing also the inverse subunit composition. For this purpose we transfected HEK293 cells with P2X2 and P2X3 encoding cDNA at the ratios of 1:2 and 4:1, and analysed the biophysical and pharmacological properties of the generated receptors by means of the whole-cell patch-clamp technique. The concentration-response curves for the selective agonist α,β-meATP did not differ from each other under the two transfection ratios. However, co-expression of an inactive P2X2 mutant and the wild type P2X3 subunit and vice versa resulted in characteristic distortions of the α,β-meATP concentration-response relationships, depending on which subunit was expressed in excess, suggesting that HEK293 cells express mixtures of (P2X2)1/(P2X3)2 and (P2X2)2/(P2X3)1 receptors. Whereas the allosteric modulators H+ and Zn2+ failed to discriminate between the two possible heterotrimeric receptor variants, the α,β-meATP-induced responses were blocked more potently by the competitive antagonist A317491, when the P2X2 subunit was expressed in deficit of the P2X3 subunit. Furthermore, blue-native PAGE analysis of P2X2 and P2X3 subunits co-expressed in Xenopus laevis oocytes and HEK293 cells revealed that plasma membrane-bound P2X2/3 receptors appeared in two clearly distinct heterotrimeric complexes: a (P2X2-GFP)2/(P2X3)1 complex and a (P2X2-GFP)1/(P2X3)2 complex. These data strongly indicate that the stoichiometry of the heteromeric P2X2/3 receptor is not fixed, but determined in a permutational manner by the relative availability of P2X2 and P2X3 subunits.

Conformational flexibility of the agonist binding jaw of the human P2X3 receptor is a prerequisite for channel opening

BACKGROUND AND PURPOSE

It is assumed that ATP induces closure of the binding jaw of ligand-gated P2X receptors, which eventually results in the opening of the membrane channel and the flux of cations. Immobilization by cysteine mutagenesis of the binding jaw inhibited ATP-induced current responses, but did not allow discrimination between disturbances of binding, gating, subunit assembly or trafficking to the plasma membrane.

EXPERIMENTAL APPROACH

A molecular model of the pain-relevant human (h)P2X3 receptor was used to identify amino acid pairs, which were located at the lips of the binding jaw and did not participate in agonist binding but strongly approached each other even in the absence of ATP.

KEY RESULTS

A series of cysteine double mutant hP2X3 receptors, expressed in HEK293 cells or Xenopus laevis oocytes, exhibited depressed current responses to α,β-methylene ATP (α,β-meATP) due to the formation of spontaneous inter-subunit disulfide bonds. Reducing these bonds with dithiothreitol reversed the blockade of the α,β-meATP transmembrane current. Amino-reactive fluorescence labelling of the His-tagged hP2X3 receptor and its mutants expressed in HEK293 or X. laevis oocytes demonstrated the formation of inter-subunit cross links in cysteine double mutants and, in addition, confirmed their correct trimeric assembly and cell surface expression.

CONCLUSIONS AND IMPLICATIONS

In conclusion, spontaneous tightening of the binding jaw of the hP2X3 receptor by inter-subunit cross-linking of cysteine residues substituted at positions not directly involved in agonist binding inhibited agonist-evoked currents without interfering with binding, subunit assembly or trafficking.

Heteromeric assembly of P2X subunits

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

Desensitization properties of P2X3 receptors shaping pain signaling

ATP-gated P2X3 receptors are mostly expressed by nociceptive sensory neurons and participate in transduction of pain signals. P2X3 receptors show a combination of fast desensitization onset and slow recovery. Moreover, even low nanomolar agonist concentrations unable to evoke a response, can induce desensitization via a phenomenon called “high affinity desensitization.” We have also observed that recovery from desensitization is agonist-specific and can range from seconds to minutes. The recovery process displays unusually high temperature dependence. Likewise, recycling of P2X3 receptors in peri-membrane regions shows unexpectedly large temperature sensitivity. By applying kinetic modeling, we have previously shown that desensitization characteristics of P2X3 receptor are best explained with a cyclic model of receptor operation involving three agonist molecules binding a single receptor and that desensitization is primarily developing from the open receptor state. Mutagenesis experiments suggested that desensitization depends on a certain conformation of the ATP binding pocket and on the structure of the transmembrane domains forming the ion pore. Further molecular determinants of desensitization have been identified by mutating the intracellular N- and C-termini of P2X3 receptor. Unlike other P2X receptors, the P2X3 subtype is facilitated by extracellular calcium that acts via specific sites in the ectodomain neighboring the ATP binding pocket. Thus, substitution of serine275 in this region (called “left flipper”) converts the natural facilitation induced by extracellular calcium to receptor inhibition. Given their strategic location in nociceptive neurons and unique desensitization properties, P2X3 receptors represent an attractive target for development of new analgesic drugs via promotion of desensitization aimed at suppressing chronic pain.

Agonist antagonist interactions at the rapidly desensitizing P2X3 receptor

P2X3 receptors (P2XRs), as members of the purine receptor family, are deeply involved in chronic pain sensation and therefore, specific, competitive antagonists are of great interest for perspective pain management. Heretofore, Schild plot analysis has been commonly used for studying the interaction of competitive antagonists and the corresponding receptor. Unfortunately, the steady-state between antagonist and agonist, as a precondition for this kind of analysis, cannot be reached at fast desensitizing receptors like P2X3R making Schild plot analysis inappropriate. The aim of this study was to establish a new method to analyze the interaction of antagonists with their binding sites at the rapidly desensitizing human P2X3R. The patch-clamp technique was used to investigate the structurally divergent, preferential antagonists A317491, TNP-ATP and PPADS. The P2X1,3-selective α,β-methylene ATP (α,β-meATP) was used as an agonist to induce current responses at the wild-type (wt) P2X3R and several agonist binding site mutants. Afterwards a Markov model combining sequential transitions of the receptor from the closed to the open and desensitized mode in the presence or absence of associated antagonist molecules was developed according to the measured data. The P2X3R-induced currents could be fitted correctly with the help of this Markov model allowing identification of amino acids within the binding site which are important for antagonist binding. In conclusion, Markov models are suitable to simulate agonist antagonist interactions at fast desensitizing receptors such as the P2X3R. Among the antagonists investigated, TNP-ATP and A317491 acted in a competitive manner, while PPADS was identified as a (pseudo)irreversible blocker.

Functional differences between ATP-gated human and rat P2X3 receptors are caused by critical residues of the intracellular C-terminal domain

ATP-activated P2X3 receptors of sensory ganglion neurons contribute to pain transduction and are involved in chronic pain signaling. Although highly homologous (97%) in rat and human species, it is unclear whether P2X3 receptors have identical function. Studying human and rat P2X3 receptors expressed in patch-clamped human embryonic kidney (HEK) cells, we investigated the role of non-conserved tyrosine residues in the C-terminal domain (rat tyrosine-393 and human tyrosine-376) as key determinants of receptor function. In comparison with rat P2X3 receptors, human P2X3 receptors were more expressed and produced larger responses with slower desensitization and faster recovery. In general, desensitization was closely related to peak current amplitude for rat and human receptors. Downsizing human receptor expression to the same level of the rat one still yielded larger responses retaining slower desensitization and faster recovery. Mutating phenylalanine-376 into tyrosine in the rat receptor did not change current amplitude; yet, it retarded desensitization onset, demonstrating how this residue was important to functionally link these two receptor states. Conversely, removing tyrosine from position 376 strongly down-regulated human receptor function. The different topology of tyrosine residues in the C-terminal domain has contrasting functional consequences and is sufficient to account for species-specific properties of this pain-transducing channel.

Effects of nucleotide analogs at the P2X3 receptor and its mutants identify the agonist binding pouch

In this study, we investigated the effects of single alanine substitutions of amino acid residues in the supposed ATP binding site of the human P2X3 receptor on the agonistic effect of nucleotide analogs. The wild-type and mutant receptors were expressed in HEK293 cells, and the nucleotide effects were measured by means of the whole-cell patch-clamp method. Modifications in the receptor binding site changed the concentration-response relationship, the current kinetics, and the recovery from desensitization during fast, pulsed, local agonist applications. On the basis of this fact, we were able to distinguish binding from other effects, such as gating/desensitization, by using a hidden Markov model that describes the complete channel behavior with a matrix of rate constants. The binding energies of the nucleotide analogs were calculated and compared with the binding energies of ATP at both the wild-type receptor and its alanine mutants. Changes in the binding energies caused by alterations in the receptor and/or agonist structures were concluded to be attributable to the preferential binding of certain structural constituents of ATP to certain amino acid moieties of the receptor. The results were also checked for consistency with a P2X3 homology model that we developed from the known zebrafish P2X4 crystal structure in the closed state. The functional data correlated well with the predictions of the agonist dockings to the structural model.

ATP binding site mutagenesis reveals different subunit stoichiometry of functional P2X2/3 and P2X2/6 receptors

The aim of the present experiments was to clarify the subunit stoichiometry of P2X2/3 and P2X2/6 receptors, where the same subunit (P2X2) forms a receptor with two different partners (P2X3 or P2X6). For this purpose, four non-functional Ala mutants of the P2X2, P2X3, and P2X6 subunits were generated by replacing single, homologous amino acids particularly important for agonist binding. Co-expression of these mutants in HEK293 cells to yield the P2X2 WT/P2X3 mutant or P2X2 mutant/P2X3 WT receptors resulted in a selective blockade of agonist responses in the former combination only. In contrast, of the P2X2 WT/P2X6 mutant and P2X2 mutant/P2X6 WT receptors, only the latter combination failed to respond to agonists. The effects of α,β-methylene-ATP and 2-methylthio-ATP were determined by measuring transmembrane currents by the patch clamp technique and intracellular Ca(2+) transients by the Ca(2+)-imaging method. Protein labeling, purification, and PAGE confirmed the assembly and surface trafficking of the investigated WT and WT/mutant combinations in Xenopus laevis oocytes. In conclusion, both electrophysiological and biochemical investigations uniformly indicate that one subunit of P2X2 and two subunits of P2X3 form P2X2/3 heteromeric receptors, whereas two subunits of P2X2 and one subunit of P2X6 constitute P2X2/6 receptors. Further, it was shown that already two binding sites of the three possible ones are sufficient to allow these receptors to react with their agonists.

Highly conserved tyrosine 37 stabilizes desensitized states and restricts calcium permeability of ATP-gated P2X3 receptor

Tyrosine 37 in the first transmembrane (TM1) domain is highly conserved in ATP-gated P2X receptors suggesting its fundamental role. We tested whether Y37 contributes to the desensitization of P2X3 receptors, which is currently not well understood. By combining electrophysiological, imaging and modeling approaches, we studied desensitization of various Y37 P2X3 mutants and potential partners of Y37. Unlike the membrane current of the WT receptor, which desensitized in seconds, Y37A mutant current did not fully desensitize even after minutes-long applications of β,γ-meATP, α,β-meATP, ATP or 2MeS-ATP. The fractional calcium current was enhanced in the Y37A mutant. Y37F did not rescue the native P2X3 phenotype indicating a role for the hydroxyl group of Y37 for the WT receptor. Homology modeling indicated I318 or I319 in TM2 as potential partners for Y37 in the receptor closed state. We tested this hypothesis by creating a permanent interaction between the two residues via disulfide bond. Whereas single Y37C, I318C and I319C mutants were functional, the double mutants Y37C-I318C and Y37C-I319C were non-functional. Using a cyclic model of receptor operation, we suggest that the conserved tyrosine 37 links TM1 to TM2 of adjacent subunit to stabilize desensitized states and restricts calcium permeability through the ion channel.

Lipid rafts control P2X3 receptor distribution and function in trigeminal sensory neurons of a transgenic migraine mouse model

Background

A genetic knock-in mouse model expressing the R192Q mutation of the α1-subunit of the Ca_V2.1 channels frequently found in patients with familial hemiplegic migraine shows functional upregulation of ATP-sensitive P2X3 receptors of trigeminal sensory neurons that transduce nociceptive inputs to the brainstem. In an attempt to understand the basic mechanisms linked to the upregulation of P2X3 receptor activity, we investigated the influence of the lipid domain of these trigeminal sensory neurons on receptor compartmentalization and function.

Results

Knock-in neurons were strongly enriched with lipid rafts containing a larger fraction of P2X3 receptors at membrane level. Pretreatment with the Ca_V2.1 channel blocker ω-agatoxin significantly decreased the lipid raft content of KI membranes. After pharmacologically disrupting the cholesterol component of lipid rafts, P2X3 receptors became confined to non-raft compartments and lost their functional potentiation typically observed in KI neurons with whole-cell patch-clamp recording. Following cholesterol depletion, all P2X3 receptor currents decayed more rapidly and showed delayed recovery indicating that alteration of the lipid raft milieu reduced the effectiveness of P2X3 receptor signalling and changed their desensitization process. Kinetic modeling could reproduce the observed data when slower receptor activation was simulated and entry into desensitization was presumed to be faster.

Conclusions

The more abundant lipid raft compartment of knock-in neurons was enriched in P2X3 receptors that exhibited stronger functional responses. These results suggest that the membrane microenvironment of trigeminal sensory neurons is an important factor in determining sensitization of P2X3 receptors and could contribute to a migraine phenotype by enhancing ATP-mediated responses.

Role of the ectodomain serine 275 in shaping the binding pocket of the ATP-gated P2X3 receptor

ATP-activated P2X3 receptors expressed in nociceptive sensory neurons play an important role in pain signaling. Basic properties of this receptor subtype, including very strong desensitization, depend on the rate of dissociation of the agonist from the binding site. Even though the rough structure of the ATP binding site has been proposed on the basis of the X-ray structure of the zebrafish P2X4 receptor and mutagenesis studies, the fine subunit-specific structural properties predisposing the receptor to tight capture of the agonist inside the binding pocket have not been elucidated. In this work, by exploring in silico the functional role for the left flipper located in the ectodomain region, we identified within this loop a candidate residue S275, which could contribute to the closure of the agonist-binding pocket. Testing of the S275 mutants using the patch-clamp technique revealed a crucial role for S275 in agonist binding and receptor desensitization. The S275A mutant showed a reduced rate of onset of desensitization and accelerated resensitization and was weakly inhibited by nanomolar agonist. Extracellular calcium application produced inhibition instead of facilitation of membrane currents. Moreover, some full agonists became only partial agonists when applied to the S275A receptor. These effects were stronger with the more hydrophobic mutants S275C and S275V. Taken together, our data suggest that S275 contributes to the closure of the agonist-binding pocket and that effective capture of the agonist provided by the left flipper in calcium-dependent manner determines the high rate of desensitization, slow recovery, and sensitivity to nanomolar agonist of the P2X3 receptor.

P2 receptor signaling in neurons and glial cells of the central nervous system

Purine and pyrimidine nucleotides are extracellular signaling molecules in the central nervous system (CNS) leaving the intracellular space of various CNS cell types via nonexocytotic mechanisms. In addition, ATP is a neuro-and gliotransmitter released by exocytosis from neurons and neuroglia. These nucleotides activate P2 receptors of the P2X (ligand-gated cationic channels) and P2Y (G protein-coupled receptors) types. In mammalians, seven P2X and eight P2Y receptor subunits occur; three P2X subtypes form homomeric or heteromeric P2X receptors. P2Y subtypes may also hetero-oligomerize with each other as well as with other G protein-coupled receptors. P2X receptors are able to physically associate with various types of ligand-gated ion channels and thereby to interact with them. The P2 receptor homomers or heteromers exhibit specific sensitivities against pharmacological ligands and have preferential functional roles. They may be situated at both presynaptic (nerve terminals) and postsynaptic (somatodendritic) sites of neurons, where they modulate either transmitter release or the postsynaptic sensitivity to neurotransmitters. P2 receptors exist at neuroglia (e.g., astrocytes, oligodendrocytes) and microglia in the CNS. The neuroglial P2 receptors subserve the neuron-glia cross talk especially via their end-feets projecting to neighboring synapses. In addition, glial networks are able to communicate through coordinated oscillations of their intracellular Ca(2+) over considerable distances. P2 receptors are involved in the physiological regulation of CNS functions as well as in its pathophysiological dysregulation. Normal (motivation, reward, embryonic and postnatal development, neuroregeneration) and abnormal regulatory mechanisms (pain, neuroinflammation, neurodegeneration, epilepsy) are important examples for the significance of P2 receptor-mediated/modulated processes.

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

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