Probing the expression and function of the P2X7 purinoceptor with antibodies raised by genetic immunization

Animal Models for the Investigation of P2X7 Receptors

The P2X7 receptor is a trimeric ligand-gated cation channel activated by extracellular adenosine 5'-triphosphate. The study of animals has greatly advanced the investigation of P2X7 and helped to establish the numerous physiological and pathophysiological roles of this receptor in human health and disease. Following a short overview of the P2X7 distribution, roles and functional properties, this article discusses how animal models have contributed to the generation of P2X7-specific antibodies and nanobodies (including biologics), recombinant receptors and radioligands to study P2X7 as well as to the pharmacokinetic testing of P2X7 antagonists. This article then outlines how mouse and rat models have been used to study P2X7. These sections include discussions on preclinical disease models, polymorphic P2X7 variants, P2X7 knockout mice (including bone marrow chimeras and conditional knockouts), P2X7 reporter mice, humanized P2X7 mice and P2X7 knockout rats. Finally, this article reviews the limited number of studies involving guinea pigs, rabbits, monkeys (rhesus macaques), dogs, cats, zebrafish, and other fish species (seabream, ayu sweetfish, rainbow trout and Japanese flounder) to study P2X7.

Development of Antibody and Nanobody Tools for P2X7

Antibodies that recognize the ATP-gated P2X7 ion channel are etablished research tools. Nanobodies correspond to the antigen-binding variable immunoglobulin domain (VHH) of heavy chain antibodies that naturally occur in camelids. Nanobodies display better solubility than the variable domains (VH) of conventional antibodies. Therefore, it is much easier to construct bivalent and multivalent fusion proteins with nanobodies than with VH domains or with paired VH-VL domains. Moreover, nanobodies can bind functional crevices that are poorly accessbile to conventional VH-VL domains. This makes nanobodies particulary well suited as functional modulators. Here we provide protocols to raise antibodies and nanobodies against mouse and human P2X7 using cDNA-immunization. This approach evokes antibodies and nanobodies that recognize the P2X7 ion channel in native confirmation, some of which inhibit or potentiate gating of P2X7 by extracellular ATP. Furthermore, we developed protocols for producing P2X7-specific nanobodies and antibodies in vivo using rAAV vectors (AAVnano). This approach can be used either to durably inhibit or potentiate P2X7 function in vivo, or to deplete P2X7-expressing cells.

A P2rx7 Passenger Mutation Affects the Vitality and Function of T cells in Congenic Mice

Among laboratory mouse strains many genes are differentially expressed in the same cell population. As consequence, gene targeting in 129-derived embryonic stem cells (ESCs) and backcrossing the modified mice onto the C57BL/6 background can introduce passenger mutations in the close proximity of the targeted gene. Here, we demonstrate that several transgenic mice carry a P2rx7 passenger mutation that affects the function of T cells. By the example of P2rx4^tm1Rass we demonstrate that P2X4ko T cells express higher levels of P2X7 and are more sensitive toward the P2X7 activators ATP and NAD⁺, rendering these cells more vulnerable toward NAD-induced cell death (NICD) compared with wild type (WT). The enhanced NICD sensitivity confounded functional assays e.g. cytokine production and cell migration. Our results need to be considered when working with P2rx4^tm1Rass mice or other 129-based transgenic strains that target P2rx7 neighboring genes.

•

T cells from 129 mice express higher level of P2X7 compared with T cells from B6 mice

•

P2rx4^tm1Rass T cells express high level of P2X7 due to a P2rx7 passenger mutation

•

P2rx4^tm1Rass T cells are highly susceptible to NAD-induced cell death (NICD)

•

NICD susceptibility of P2rx4^tm1Rass T cells confounds the outcome of functional assays

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.

A Heterologous Model of Thrombospondin Type 1 Domain-Containing 7A-Associated Membranous Nephropathy

Thrombospondin type 1 domain-containing 7A (THSD7A) is a target for autoimmunity in patients with membranous nephropathy (MN). Circulating autoantibodies from patients with THSD7A-associated MN have been demonstrated to cause MN in mice. However, THSD7A-associated MN is a rare disease, preventing the use of patient antibodies for larger experimental procedures. Therefore, we generated antibodies against the human and mouse orthologs of THSD7A in rabbits by coimmunization with the respective cDNAs. Injection of these anti-THSD7A antibodies into mice induced a severe nephrotic syndrome with proteinuria, weight gain, and hyperlipidemia. Immunofluorescence analyses revealed granular antigen-antibody complexes in a subepithelial location along the glomerular filtration barrier 14 days after antibody injection, and immunohistochemistry for rabbit IgG and THSD7A as well as ultrastructural analyses showed the typical characteristics of human MN. Mice injected with purified IgG from rabbit serum that was taken before immunization failed to develop any of these changes. Notably, MN developed in the absence of detectable complement activation, and disease was strain dependent. In vitro, anti-THSD7A antibodies caused cytoskeletal rearrangement and activation of focal adhesion signaling. Knockdown of the THSD7A ortholog, thsd7aa, in zebrafish larvae resulted in altered podocyte differentiation and impaired glomerular filtration barrier function, with development of pericardial edema, suggesting an important role of THSD7A in glomerular filtration barrier integrity. In summary, our study introduces a heterologous mouse model that allows further investigation of the molecular events that underlie MN.

The P2X7 Receptor

The P2X7 receptor is a trimeric ion channel gated by extracellular adenosine 5'-triphosphate. The receptor is present on an increasing number of different cells types including stem, blood, glial, neural, ocular, bone, dental, exocrine, endothelial, muscle, renal and skin cells. The P2X7 receptor induces various downstream events in a cell-specific manner, including inflammatory molecule release, cell proliferation and death, metabolic events, and phagocytosis. As such this receptor plays important roles in heath and disease. Increasing knowledge about the P2X7 receptor has been gained from studies of, but not limited to, protein chemistry including cloning, site-directed mutagenesis, crystal structures and atomic modeling, as well as from studies of primary tissues and transgenic mice. This chapter focuses on the P2X7 receptor itself. This includes the P2RX7 gene and its products including splice and polymorphic variants. This chapter also reviews modulators of P2X7 receptor activation and inhibition, as well as the transcriptional regulation of the P2RX7 gene via its promoter and enhancer regions, and by microRNA and long-coding RNA. Furthermore, this chapter discusses the post-translational modification of the P2X7 receptor by N-linked glycosylation, adenosine 5'-diphosphate ribosylation and palmitoylation. Finally, this chapter reviews interaction partners of the P2X7 receptor, and its cellular localisation and trafficking within cells.

DNA immunization as a technology platform for monoclonal antibody induction

To combat the threat of many emerging infectious diseases, DNA immunization offers a unique and powerful approach to the production of high-quality monoclonal antibodies (mAbs) against various pathogens. Compared with traditional protein-based immunization approaches, DNA immunization is efficient for testing novel immunogen designs, does not require the production or purification of proteins from a pathogen or the use of recombinant protein technology and is effective at generating mAbs against conformation-sensitive targets. Although significant progress in the use of DNA immunization to generate mAbs has been made over the last two decades, the literature does not contain an updated summary of this experience. The current review provides a comprehensive analysis of the literature, including our own work, describing the use of DNA immunization to produce highly functional mAbs, in particular, those against emerging infectious diseases. Critical factors such as immunogen design, delivery approach, immunization schedule, use of immune modulators and the role of final boost immunization are discussed in detail.

Nucleotide-Induced Membrane-Proximal Proteolysis Controls the Substrate Specificity of T Cell Ecto-ADP-Ribosyltransferase ARTC2.2

ARTC2.2 is a toxin-related, GPI-anchored ADP-ribosyltransferase expressed by murine T cells. In response to NAD(+) released from damaged cells during inflammation, ARTC2.2 ADP-ribosylates and thereby gates the P2X7 ion channel. This induces ectodomain shedding of metalloprotease-sensitive cell surface proteins. In this study, we show that ARTC2.2 itself is a target for P2X7-triggered ectodomain shedding. We identify the metalloprotease cleavage site 3 aa upstream of the predicted GPI anchor attachment site of ARTC2.2. Intravenous injection of NAD(+) increased the level of enzymatically active ARTC2.2 in serum, indicating that this mechanism is operative also under inflammatory conditions in vivo. Radio-ADP-ribosylation assays reveal that shedding refocuses the target specificity of ARTC2.2 from membrane proteins to secretory proteins. Our results uncover nucleotide-induced membrane-proximal proteolysis as a regulatory mechanism to control the substrate specificity of ARTC2.2.

CD8-β ADP-ribosylation affects CD8(+) T-cell function

The CD8αβ coreceptor is crucial for effective peptide: MHC-I recognition by the TCR of CD8(+) T cells. Adenosine diphosphate ribosyl transferase 2.2 (ART2.2) utilizes extracellular NAD(+) to transfer ADP-ribose to arginine residues of extracellular domains of surface proteins. Here, we show that in the presence of extracellular NAD(+) , ART2.2 caused ADP-ribosylation of CD8-β on murine CD8(+) T cells in vitro and in vivo. Treatment with NAD(+) prevented binding of anti-CD8-β mAb YTS156.7.7 but not of mAb H35-17.2, indicating that NAD(+) caused modification of certain epitopes and not a general loss of CD8-β. Loss of antibody binding was strictly dependent on ART2.2, because it was not observed on ART2-deficient T cells or in the presence of inhibitory anti-ART2.2 single-domain antibodies. ADP-ribosylation of CD8-β occurred during cell isolation, particularly when cells were isolated from CD38-deficient mice. Incubation of ART2-expressing, but not of ART2-deficient, OVA-specific CD8(+) T cells with NAD(+) interfered with binding of OVA257-264 :MHC-I tetramers. In line with this result, treatment of WT mice with NAD(+) resulted in reduced CD8(+) T-cell mediated cytotoxicity in vivo. We propose that ADP-ribosylation of CD8-β can regulate the coreceptor function of CD8 in the presence of elevated levels of extracellular NAD(+) .

Alternative splicing of the N-terminal cytosolic and transmembrane domains of P2X7 controls gating of the ion channel by ADP-ribosylation

P2X7 is a homotrimeric ion channel with two transmembrane domains and a large extracellular ATP-binding domain. It plays a key role in the response of immune cells to danger signals released from cells at sites of inflammation. Gating of murine P2X7 can be induced by the soluble ligand ATP, as well as by NAD⁺-dependent ADP-ribosylation of arginine 125, a posttranslational protein modification catalyzed by the toxin-related ecto-enzymes ART2.1 and ART2.2. R125 is located at the edge of the ligand-binding crevice. Recently, an alternative splice variant of P2X7, designated P2X7(k), was discovered that differs from the previously described variant P2X7(a) in the N-terminal 42 amino acid residues composing the first cytosolic domain and most of the Tm1 domain. Here we compare the two splice variants of murine P2X7 with respect to their sensitivities to gating by ADP-ribosylation in transfected HEK cells. Our results show that the P2X7(k) variant is sensitive to activation by ADP-ribosylation whereas the P2X7(a) variant is insensitive, despite higher cell surface expression levels. Interestingly, a single point mutation (R276K) renders the P2X7(a) variant sensitive to activation by ADP-ribosylation. Residue 276 is located at the interface of neighboring subunits approximately halfway between the ADP-ribosylation site and the transmembrane domains. Moreover, we show that naive and regulatory T cells preferentially express the more sensitive P2X7(k) variant, while macrophages preferentially express the P2X7(a) variant. Our results indicate that differential splicing of alternative exons encoding the N-terminal cytosolic and transmembrane domains of P2X7 control the sensitivity of different immune cells to extracellular NAD⁺ and ATP.

Murine epidermal Langerhans cells and keratinocytes express functional P2X7 receptors

Extracellular ATP via the activation of purinergic P2 receptors has an emerging role in cutaneous biology; however, the distribution of these receptors in mouse skin is poorly defined. This study investigated whether murine epidermal cell subpopulations express functional purinergic P2X(7) receptors. P2X(7) expression was examined by immunoblotting and immunofluorescence staining of epidermal cells from C57Bl/6 mice. P2X(7) function was evaluated by nucleotide-induced ethidium(+) uptake measurements in epidermal cells from C57Bl/6 mice, and from P2X(7) deficient mice and wild-type littermate controls. P2X(7) was detected in whole epidermal cell preparations, and specifically on Langerhans cells (LCs) and keratinocytes (KCs). ATP induced ethidium(+) uptake into LCs and KCs, with EC(50) values of 503 and 482 microm, respectively. BzATP, and to a lesser extent ATPgammaS and ADP, also induced ethidium(+) uptake; while UTP, alphabeta-meth-ATP and NAD were ineffective. ATP-induced ethidium(+) uptake was impaired by Na(+) and Mg(2+), and the P2X(7) antagonist, A-438079 and was absent in LCs and KCs from P2X(7) deficient mice. These results demonstrate that murine LCs and KCs express functional P2X(7), and support a role for this receptor in cutaneous biology.

Extracellular NAD+ shapes the Foxp3+ regulatory T cell compartment through the ART2-P2X7 pathway

CD4(+)CD25(+)FoxP3(+) regulatory T cells (T reg cells) play a major role in the control of immune responses but the factors controlling their homeostasis and function remain poorly characterized. Nicotinamide adenine dinucleotide (NAD(+)) released during cell damage or inflammation results in ART2.2-mediated ADP-ribosylation of the cytolytic P2X7 receptor on T cells. We show that T reg cells express the ART2.2 enzyme and high levels of P2X7 and that T reg cells can be depleted by intravenous injection of NAD(+). Moreover, lower T reg cell numbers are found in mice deficient for the NAD-hydrolase CD38 than in wild-type, P2X7-deficient, or ART2-deficient mice, indicating a role for extracellular NAD(+) in T reg cell homeostasis. Even routine cell preparation leads to release of NAD(+) in sufficient quantities to profoundly affect T reg cell viability, phenotype, and function. We demonstrate that T reg cells can be protected from the deleterious effects of NAD(+) by an inhibitory ART2.2-specific single domain antibody. Furthermore, selective depletion of T reg cells by systemic administration of NAD(+) can be used to promote an antitumor response in several mouse tumor models. Collectively, our data demonstrate that NAD(+) influences survival, phenotype, and function of T reg cells and provide proof of principle that acting on the ART2-P2X7 pathway represents a new strategy to manipulate T reg cells in vivo.

P2X7 receptor activation induces cell death and microparticle release in murine erythroleukemia cells

Extracellular ATP induces cation fluxes in and impairs the growth of murine erythroleukemia (MEL) cells in a manner characteristic of the purinergic P2X7 receptor, however the presence of P2X7 in these cells is unknown. This study investigated whether MEL cells express functional P2X7. RT-PCR, immunoblotting and immunofluorescence staining demonstrated the presence of P2X7 in MEL cells. Cytofluorometric measurements demonstrated that ATP induced ethidium+ uptake into MEL cells in a concentration-dependent fashion and with an EC(50) of approximately 154 microM. The most potent P2X7 agonist 2'- and 3'-0(4-benzoylbenzoyl) ATP, but not ADP or UTP, induced ethidium+ uptake. ATP-induced ethidium+ and YO-PRO-1(2+) uptake were impaired by the P2X7 antagonist, A-438079. A colourmetric assay demonstrated that ATP impaired MEL cell growth. A cytofluorometric assay showed that ATP induced MEL cell death and that this process was impaired by A-438079. Finally, cytofluorometric measurements of Annexin-V binding and bio-maleimide staining demonstrated that ATP could induce rapid phosphatidylserine exposure and microparticle release in MEL cells respectively, both of which were impaired by A-438079. These results demonstrate that MEL cells express functional P2X7, and indicate that activation of this receptor may be important in the death and release of microparticles from red blood cells in vivo.

Differential regulation of P2X7 receptor activation by extracellular nicotinamide adenine dinucleotide and ecto-ADP-ribosyltransferases in murine macrophages and T cells

Extracellular NAD induces the ATP-independent activation of the ionotropic P2X(7) purinergic receptor (P2X(7)R) in murine T lymphocytes via a novel covalent pathway involving ADP-ribosylation of arginine residues on the P2X(7)R ectodomain. This modification is catalyzed by ART2.2, a GPI-anchored ADP-ribosyltransferase (ART) that is constitutively expressed in murine T cells. We previously reported that ART2.1, a related ecto-ART, is up-regulated in inflammatory murine macrophages that constitutively express P2X(7)R. Thus, we tested the hypothesis that extracellular NAD acts via ART2.1 to regulate P2X(7)R function in murine macrophages. Coexpression of the cloned murine P2X(7)R with ART2.1 or ART2.2 in HEK293 cells verified that P2X(7)R is an equivalent substrate for ADP-ribosylation by either ART2.1 or ART2.2. However, in contrast with T cells, the stimulation of macrophages or HEK293 cells with NAD alone did not activate the P2X(7)R. Rather, NAD potentiated ATP-dependent P2X(7)R activation as indicated by a left shift in the ATP dose-response relationship. Thus, extracellular NAD regulates the P2X(7)R in both macrophages and T cells but via distinct mechanisms. Although ADP-ribosylation is sufficient to gate a P2X(7)R channel opening in T cells, this P2X(7)R modification in macrophages does not gate the channel but decreases the threshold for gating in response to ATP binding. These findings indicate that extracellular NAD and ATP can act synergistically to regulate P2X(7)R signaling in murine macrophages and also suggest that the cellular context in which P2X(7)R signaling occurs differs between myeloid and lymphoid leukocytes.

NAD+ and ATP released from injured cells induce P2X7-dependent shedding of CD62L and externalization of phosphatidylserine by murine T cells

Extracellular NAD(+) and ATP trigger the shedding of CD62L and the externalization of phosphatidylserine on murine T cells. These events depend on the P2X(7) ion channel. Although ATP acts as a soluble ligand to activate P2X(7), gating of P2X(7) by NAD(+) requires ecto-ADP-ribosyltransferase ART2.2-catalyzed transfer of the ADP-ribose moiety from NAD(+) onto Arg125 of P2X(7). Steady-state concentrations of NAD(+) and ATP in extracellular compartments are highly regulated and usually are well below the threshold required for activating P2X(7). The goal of this study was to identify possible endogenous sources of these nucleotides. We show that lysis of erythrocytes releases sufficient levels of NAD(+) and ATP to induce activation of P2X(7). Dilution of erythrocyte lysates or incubation of lysates at 37 degrees C revealed that signaling by ATP fades more rapidly than that by NAD(+). We further show that the routine preparation of primary lymph node and spleen cells induces the release of NAD(+) in sufficient concentrations for ART2.2 to ADP-ribosylate P2X(7), even at 4 degrees C. Gating of P2X(7) occurs when T cells are returned to 37 degrees C, rapidly inducing CD62L-shedding and PS-externalization by a substantial fraction of the cells. The "spontaneous" activation of P2X(7) during preparation of primary T cells could be prevented by i.v. injection of either the surrogate ART substrate etheno-NAD or ART2.2-inhibitory single domain Abs 10 min before sacrificing mice.

Activation of the P2X7 ion channel by soluble and covalently bound ligands

The homotrimeric P2X7 purinergic receptor has sparked interest because of its capacity to sense adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide (NAD) released from cells and to induce calcium signaling and cell death. Here, we examine the response of arginine mutants of P2X7 to soluble and covalently bound ligands. High concentrations of ecto-ATP gate P2X7 by acting as a soluble ligand and low concentrations of ecto-NAD gate P2X7 following ADP-ribosylation at R125 catalyzed by toxin-related ecto-ADP-ribosyltransferase ART2.2. R125 lies on a prominent cysteine-rich finger at the interface of adjacent receptor subunits, and ADP-ribosylation at this site likely places the common adenine nucleotide moiety into the ligand-binding pocket of P2X7.

Characterisation of the R276A gain-of-function mutation in the ectodomain of murine P2X7

The cytolytic P2X7 purinoceptor is widely expressed on leukocytes and has sparked interest because of its key role in the activation of the inflammasome, the release of the pro-inflammatory cytokine IL-1beta and cell death. We report here the functional characterisation of a R276A gain-of-function mutant analysed for its capacities to induce membrane depolarisation, calcium influx and opening of a large membrane pore permeable to YO-PRO-1. Our results highlight the particular sensitivity of R276A mutant to low micromolar adenosine triphosphate (ATP) concentrations, which possibly reflect an increased affinity for its ligands, and a slower closing kinetics of the receptor channel. Our findings support the notion that evolutionary pressures maintain the low sensitivity of P2X7 to ATP. We also believe that the R276A mutant described here may be useful for the generation of new animal models with exacerbated P2X7 functions that will serve to better characterise its role in inflammation and in immune responses.

High sensitivity of intestinal CD8+ T cells to nucleotides indicates P2X7 as a regulator for intestinal T cell responses

The purinoreceptor P2X7 is expressed on subsets of T cells and mediates responses of these cells to extracellular nucleotides such as ATP or NAD(+). We identified P2X7 as a molecule highly up-regulated on conventional CD8alphabeta(+) and unconventional CD8alphaalpha(+) T cells of the intestinal epithelium of mice. In contrast, CD8(+) T cells derived from spleen, mesenteric lymph nodes, and liver expressed only marginal levels of P2X7. However, P2X7 was highly up-regulated on CD8(+) T cells from spleen and lymph nodes when T cells were activated in the presence of retinoic acid. High P2X7 expression on intestinal CD8(+) T cells as well as on CD8(+) T cells incubated with retinoic acid resulted in enhanced sensitivity of cells to extracellular nucleotides. Both cell populations showed a high level of apoptosis following incubation with NAD(+) and the ATP derivative 2',3'-O-(benzoyl-4-benzoyl)-ATP, and injection of NAD(+) caused selective in vivo depletion of intestinal CD8(+) T cells. Following oral infection with Listeria monocytogenes, P2X7-deficient mice showed similar CD8(+) T cell responses in the spleen, but enhanced responses in the intestinal mucosa, when compared with similarly treated wild-type control mice. Overall, our observations define P2X7 as a new regulatory element in the control of CD8(+) T cell responses in the intestinal mucosa.

Preparation of monoclonal antibodies against pseudorabies virus glycoprotein gC by adenovirus immunization alone or as a boost following DNA priming

The objective of the present study was to demonstrate the usefulness of recombinant adenoviral vector in the generation of monoclonal antibodies (MAb) against natural epitopes of proteins using the glycoprotein gC of pseudorabies virus (PRV) as the target antigen. The recombinant adenovirus expressing the glycoprotein gC (Ad-gC) was constructed according to the AdMax method. Three immunization protocols consisting of various combinations of intramuscular injection of Ad-gC and a plasmid DNA expressing gC (pcDNA-gC) were conducted in BALB/c mice at 2-week intervals. The two groups with the highest antibody levels (Ad-gC/Ad-gC and pcDNA-gC/pcDNA-gC/Ad-gC) were selected for fusion following a final protein boost. Nine MAbs against the glycoprotein gC of PRV were subsequently developed and characterized to be isotypes of IgG1, IgG2a, and IgG2b with ascitic titers ranging from 1:2 x 10(5) to 1:5 x 10(6). Immunofluorescence assay (IFA) and Western blotting analysis confirmed that these MAbs could recognize linear epitopes on the glycoprotein gC of PRV. Our results provide a new strategy for preparation of specific MAb against viral protein.

ADP-ribosylation at R125 gates the P2X7 ion channel by presenting a covalent ligand to its nucleotide binding site

ADP-ribosylation is a post-translational modification regulating protein function in which amino acid-specific ADP-ribosyltransferases (ARTs) transfer ADP-ribose from NAD onto specific target proteins. Attachment of the bulky ADP-ribose usually inactivates the target by sterically blocking its interaction with other proteins. P2X7, an ATP-gated ion channel with important roles in inflammation and cell death, in contrast, is activated by ADP-ribosylation. Here, we report the structural basis for this gating and present the first molecular model for the activation of a target protein by ADP-ribosylation. We demonstrate that the ecto-enzyme ART2.2 ADP-ribosylates P2X7 at arginine 125 in a prominent, cysteine-rich region at the interface of 2 receptor subunits. ADP-ribose shares an adenine-ribonucleotide moiety with ATP. Our results indicate that ADP-ribosylation of R125 positions this common chemical framework to fit into the nucleotide-binding site of P2X7 and thereby gates the channel.

Monitoring the expression of purinoceptors and nucleotide-metabolizing ecto-enzymes with antibodies directed against proteins in native conformation

Following their release from cells, ATP and NAD, the universal currencies of energy metabolism, function as extracellular signalling molecules. Mammalian cells express numerous purinoceptors, i.e., the nucleotide-gated P2X ion channels and the G-protein-coupled P2Y receptors. Signalling through purinoceptors is controlled by nucleotide-metabolizing ecto-enzymes, which regulate the availability of extracellular nucleotides. These enzymes include ecto-nucleoside triphosphate diphosphohydrolases (ENTPD, CD39 family) and ecto-nucleotide pyrophosphatase/phosphodiesterases (ENPP, CD203 family). Investigation of these receptors and enzymes has been hampered by the lack of available antibodies, especially ones that recognize these proteins in their native conformation. This study reports the use of genetic immunization to generate such antibodies against P2X(1), P2X(4), P2X(7), ENTPD1, ENPTD2, ENPTD5, ENPTD6, ENPP2, ENPP3, ENPP4, ENPP5, and ENPP6. Genetic immunization ensures expression of the native protein by the cells of the immunized animal and yields antibodies directed against proteins in native conformation (ADAPINCs). Such antibodies are especially useful for immunofluorescence and immunoprecipitation analyses, whereas antibodies against synthetic peptides usually function well only in Western-blot analyses. Here we illustrate the utility of the new antibodies to monitor the cell surface expression of and to purify some key players of purinergic signalling.

NAD+ released during inflammation participates in T cell homeostasis by inducing ART2-mediated death of naive T cells in vivo

Mono ADP-ribosyltransferase 2 (ART2) is an ectoenzyme expressed on mouse T lymphocytes, which catalyze the transfer of ADP-ribose groups from NAD(+) onto several target proteins. In vitro, ADP-ribosylation by ART2 activates the P2X7 ATP receptor and is responsible for NAD(+)-induced T cell death (NICD). Yet, the origin of extracellular NAD(+) and the role of NICD in vivo remain elusive. In a model of acute inflammation induced by polyacrylamide beads, we demonstrate release of NAD(+) into exudates during the early phase of the inflammatory response. This leads to T cell depletion in the draining lymph nodes from wild-type and, more severely, from mice lacking the CD38 NAD(+) glycohydrolase, whereas no effect is observed in ART2-deficient animals. Intravenous injection of NAD(+) used to exacerbate NICD in vivo results in fast and dramatic ART2- and P2X7-dependent depletion of CD4+ and CD8+ T lymphocytes, which can affect up to 80% of peripheral T cells in CD38(-/-) mice. This affects mainly naive T cells as most cells surviving in vivo NAD+ treatment exhibit the phenotype of recently activated/memory cells. Consistently, treatment with NAD(+) abolishes primary Ab response to a T-dependent Ag in NICD-susceptible CD38(-/-) mice but has no effect on the secondary response when given several days after priming. Unexpectedly NAD+ treatment improves the response in their wild-type BALB/c counterparts. We propose that NAD(+) released during early inflammation facilitates the expansion of primed T cells, through ART2-driven death of resting cells, thus contributing to the dynamic regulation of T cell homeostasis.

P2X7 receptor expression levels determine lethal effects of a purine based danger signal in T lymphocytes

Contact of T lymphocytes with nicotinamide adenine dinucleotide (NAD) or ATP causes cell death that requires expression of purinergic receptor P2X(7) (P2X(7)R). T cell subsets differ in their responses to NAD and ATP, which awaits a mechanistic explanation. Here, we show that sensitivity to ATP correlates with P2X(7)R expression levels in CD4 cells, CD8 cells and CD4(+)CD25(+) cells from both C57BL/6 and BALB/c mice. But P2X(7)R ligands do not only induce cell death but also shedding of CD62L. It is shown here that in CD62L(high) T cells, CD62L shedding correlates with low expression of P2X(7)Rs and lower cell death, whereas in CD62L(low) cells P2X(7)R expression and death are higher. The possibility is therefore investigated that P2X(7)Rs induce T cell activation. Experiments show that spontaneous T cell proliferation is somewhat higher in cells expressing P2X(7)Rs, but this effect we suggest is caused by P2X(7)R expression on accessory cells.