P2X7受体与疾病的相关性

P2X受体是一类离子型配体门控通道,分为7个不同的亚型(P2X1～7)。嘌呤能离子通道型受体7(purinergic ligand-gated ion channel 7 receptor, P2X7R)是ATP门控的,非选择性的阳离子通道,属于嘌呤受体P2X家族。P2X7受体广泛表达于神经系统、肌肉组织和免疫系统。在胞外ATP作用下,P2X7受体偶联多种胞内信号通路,参与细胞增殖、凋亡及炎症因子的释放等多种生理功能。研究发现,P2X7受体与诸多疾病有着密切联系,包括自身免疫性疾病(如关节炎和炎症性肠病)、神经退行性疾病、慢性疼痛、情绪障碍和癌症等。P2X7受体异常表达会导致这些疾病的发生,增加疾病的易感性与病变程度。现就P2X7受体的生物学特征、P2X7受体与疾病的关系及其特异性阻断剂和激动剂进行综述。     

中枢神经系统离子通道受体P2X7的研究进展

在ATP门控离子通道P2X受体家族中,P2X7受体由于在结构和功能上与其他(P2X1-P2X6)受体的显著差别而备受关注.P2X7受体是由3个同源亚基组成的多聚体,其C端为P2X受体家族中最长的,与其他已知蛋白没有同源性.P2X7受体具有独特的双功能性,被ATP激活后形成非选择性阳离子通道,允许钾、钠、钙等阳离子跨膜流动,而对二价阳离子表现出相对较强的选择性,在低浓度二价阳离子环境及ATP的持续刺激下,激活的P2X7受体能形成大的孔通道.P2X7受体广泛分布在血液系统、免疫系统和骨组织等多种组织器官中,通过信号转导参与细胞增殖、蛋白合成和细胞凋亡等事件.近年来在中枢神经系统中的研究发现,P2X7受体参与神经突触传递等生理过程,并在神经性退变等病理过程中发挥重要的调节作用.其中,多种假说支持它与少突胶质细胞的损伤有密切关系.     

P2X7受体与炎症相关性疾病

P2X7是一种在多种免疫细胞中广泛表达的以ATP为配体的阳离子通道受体,它的激活能引起和加重炎症反应。当细胞处于损伤、缺氧或炎症状态时, P2X7受体可被释放到胞外的大量ATP激活,进而通过活化NLRP3炎症小体、调节基因转录等方式,影响炎症介质(IL-1β、IL-18等)的释放从而参与多种炎症性疾病,如糖尿病肾病、系统性红斑狼疮(systemic lupus erythematosus,SLE)等。近年来,细胞外ATP-P2X7受体信号通路已成为炎症性疾病研究较多的通路之一。大量研究表明, P2X7受体是治疗炎症性疾病的潜在靶点。该文将对P2X7受体及其参与的炎症相关性疾病的关系作一综述。     

P2Y_6受体生物学效应的研究进展

嘌呤受体分为P1和P2受体两大类,其中,P2受体又分为配体门控离子通道型受体(P2X受体)和G蛋白偶联型受体(P2Y受体)。P2Y6受体是P2Y家族中的一员,P2Y6受体参与心血管疾病、内分泌疾病及神经病变等疾病的发生。随着氯吡格雷(P2Y12受体阻断剂)等嘌呤受体阻断剂被FDA批准应用于临床,且表现出良好的疗效,P2Y6受体的生物学效应的研究,亦成为人们开展针对P2Y受体的新型靶向性药物研究的热点之一。基于分子生物学技术的发展,P2Y6受体生物学效应的研究取得了显著进展。     

P2X4受体的结构功能关系研究进展

嘌呤能受体P2X4是三磷酸腺苷(ATP)-门控的阳离子通道,对生物体内多种重要生命活动起一定的调节作用。二次跨膜的三聚体通道P2X4受体的三维空间组成是由胞外结构域、跨膜域及胞内N-、C-端组成。ATP的三磷酸基团能被位于亚基界面的ATP结合口袋的带正电氨基酸特异性识别,嘌呤环则被疏水氨基酸和部分氨基酸的主链氧所识别。P2X4受体激活后,胞外阳离子更多是通过侧窗路径进入细胞内。就P2X4受体的空间结构、配体的识别、离子通透途径及门控机制作一综述。     

人白血病细胞系P2X7受体的表达及其介导的细胞内钙浓度变化

采用半定量RT-PCR和流式细胞术,在基因和蛋白水平研究了白血病细胞系U937、HL60和Ramos细胞P2X7受体的表达。荧光染料Fura-2／AM负载后,用荧光分光光度计测定P2X7受体激动剂三磷酸腺苷(adenosine 5′-triphosphate,ATP)和苯甲酰苯甲酸ATP(2′,3′-O-(4-benzoyl)benzoyl-ATP,BzATP)刺激前后细胞内钙离子浓度的变化,以确认其功能。结果表明：U937和HL60细胞系表达P2X7受体的mRNA和蛋白,Ramos不表达;在激动剂的刺激下,可引发U937和HL60细胞胞内钙浓度的显著升高,但对Ramos没有作用。当去除胞外钙离子时,ATP和BzATP刺激均不能引起U937和HL60细胞胞内钙离子浓度的升高。提示U937和HL60细胞表达P2X7受体的基因和功能蛋白,Ramos细胞则不表达该受体。     

克隆的P2受体亚型的药理学研究进展

细胞外嘌呤（腺苷,ADP,ATP）及嘧啶（UDP,UTP）为重要的信使分子,通过细胞表面P2受体介导产生不同的生物效应,P2嘌吟受体的概念于1978年被提出,随后根据药理学特征又被分为P2X及P2X嘌呤受体,90年代,采用分子生物学手段,一系列配体门控的P2X受体及G蛋白耦联的P2Y受体被克隆及功能表达,迄今为止,已有七型P2X受体亚型（P2X1－7）及六型P2Y受体亚型被克隆（P2Y1,2,4,6,11,12）,各型具有不同的分子结构,药理学特征及组织分布,本文还讨论了目前可用于区分各亚型激动剂及拮抗剂。     

包含P2X3亚基的受体在介导痛觉中的作用

包含P2X3亚基的受体为三磷酸腺苷 (ATP)门控的阳离子通道 ,包括P2X3亚基的同源多聚体(P2X3受体 )和异源多聚体 (P2X2 /3受体 )。大量研究表明包含P2X3亚基的受体在介导多种类型痛觉中有重要作用     

P2X受体调节剂的作用机制与最新研究进展

ATP激活的配体门控阳离子通道P2X受体参与多种生理病理功能,是一类重要的新药靶标。目前发现的多种靶向P2X受体的小分子药物多数因结果未达到预期或存在明显的副作用而止步于临床研究。明确的调节机制可为靶向P2X受体的理性药物设计提供新的方向,从而提高药物发现概率和有效性。本文就P2X受体调节剂的作用机制进行综述,并对靶向P2X受体的小分子药物最新研究进展进行总结。     

不同诱导因子对人外周血单个核细胞P2X7受体表达的作用

ATP激活P2X7受体可产生一系列的白细胞功能反应,因此P2X7受体的表达调控引起我们的兴趣。然而P2X7受体在正常人外周血单个核细胞(peripheral blood mononuclear cells,PBMC)、单核细胞中的表达调控机制尚未阐明。本文用半定量RT-PCR方法检测多种细胞因子、细菌抗原、丝裂原对P2X7受体表达的诱导作用,探索P2X7受体的诱导表达模式。结果表明,单个核细胞和单核细胞可检出P2X7受体的表达;白细胞介素2、4、6(interleukin-2、-4、-6,IL-2、IL-4、IL-6)、肿瘤坏死因子仪(tumour necrosis factor-α,TNF-α)等细胞因子和金黄色葡萄球菌CowanⅠ株(Staphylococcus aureus Cowan strainⅠ,SAC)、脂多糖(lipopolysaccharide,LPS)能上调PBMC的P2X7受体表达,而γ干扰素(interferon-γ,IFN-γ)、粒-巨噬细胞集落刺激因子(granulocyte-macrophage colony-stimulating factor,GM-CSF)、巨噬细胞集落刺激因子(macmphage colony-stimulating factor,M-CSF)和植物血凝素(phytohemagglutinin-M,PHA-M)等则没有作用;LPS和M-CSF可以提高单核细胞的P2X7受体表达,IFN-γ、TNF-α、GM-CSF作用较弱,但是这些因子的预处理并不能增强LPS对P2X7受体表达的诱导。炎症因子促进P2X7受体的表达,提示P2X7受体可能在对抗细菌感染的免疫反应中起一定作用,这有待于进一步研究。     

The P2X7 receptor as a route for non‐exocytotic glutamate release: dependence on the carboxyl tail

P2X7 receptors trigger Ca²⁺‐dependent exocytotic glutamate release, but also function as a route for non‐exocytotic glutamate release from neurons or astrocytes. To gain an insight into the mechanisms involving the P2X7 receptor as a direct pathway for glutamate release, we compared the behavior of a full‐length rat P2X7 receptor, a truncated rat P2X7 receptor in which the carboxyl tail had been deleted, a rat P2X7 receptor with the 18‐amino acid cysteine‐rich motif of the carboxyl tail deleted, and a rat P2X2 receptor, all of which are expressed in HEK293 cells. We found that the P2X7 receptor function as a route for glutamate release was antagonized in a non‐competitive way by extracellular Mg²⁺, did not require the recruitment of pore‐forming molecules, and was dependent on the carboxyl tail. Indeed, the truncated P2X7 receptor and the P2X7 receptor with the deleted cysteine‐rich motif both lost their function as a pathway for glutamate release, while still evoking intracellular Ca²⁺ elevation. No glutamate efflux was observed through the P2X2 receptor. Notably, HEK293 cells (lacking the machinery for Ca²⁺‐dependent exocytosis), when transfected with P2X7 receptors, appear to be a suitable model for investigating the P2X7 receptor as a route for non‐exocytotic glutamate efflux.     

Differential sensitivity of human platelet P2X1 and P2Y1 receptors to disruption of lipid rafts

ATP-stimulated P2X1 and ADP-stimulated P2Y1 receptors play important roles in platelet activation. An increase in intracellular Ca2+ represents a key signalling event coupled to both of these receptors, mediated via direct gating of Ca2+-permeable channels in the case of P2X1 and phospholipase-C-dependent Ca2+ mobilisation for P2Y1. We show that disruption of cholesterol-rich membrane lipid rafts reduces P2X1 receptor-mediated calcium increases by approximately 80%, while P2Y1 receptor-dependent Ca2+ release is unaffected. In contrast to artery, vas deferens, bladder smooth muscle, and recombinant expression in cell lines, where P2X1 receptors show almost exclusive association with lipid rafts, only approximately 20% of platelet P2X1 receptors are co-expressed with the lipid raft marker flotillin-2. We conclude that lipid rafts play a significant role in the regulation of P2X1 but not P2Y1 receptors in human platelets and that a reserve of non-functional P2X1 receptors may exist.     

Lack of evidence for direct phosphorylation of recombinantly expressed P2X2 and P2X3 receptors by protein kinase C

P2X₃ and P2X₂₊₃ receptors are present on sensory neurons, where they contribute not only to transient nociceptive responses, but also to hypersensitivity underlying pathological pain states elicited by nerve injuries. Increased signalling through P2X₃ and P2X₂₊₃ receptors may arise from an increased routing to the plasma membrane and/or gain of function of pre-existing receptors. An obvious effector mechanism for functional modulation is protein kinase C (PKC)-mediated phosphorylation, since all P2X family members share a conserved consensus sequence for PKC, TXR/K, within the intracellularly located N-terminal domain. Contradictory reports have been published regarding the exact role of this motif. In the present study, we confirm that site-directed elimination of the potential phosphor-acceptor threonine or the basic residue in the P+2 position of the TXR/K sequence accelerates desensitization of P2X₂ receptors and abolishes P2X₃ receptor function. Moreover, the PKC activator phorbol 12-myristate 13-acetate increased P2X₃ (but not P2X₂) receptor-mediated currents. Biochemically, however, we were unable to demonstrate by various experimental approaches a direct phosphorylation of wild-type P2X₂ and P2X₃ receptors expressed in both Xenopus laevis oocytes and HEK293 cells. In conclusion, our data support the view that the TXR/K motif plays an important role in P2X function and that phorbol 12-myristate 13-acetate is capable of modulating some P2X receptor subtypes. The underlying mechanism, however, is unlikely to involve direct PKC-mediated P2X receptor phosphorylation.     

ATP activates P2X receptors to mediate gap junctional coupling in the cochlea

ATP is an important extracellular signaling molecule and can activate both ionotropic (P2X) and metabotropic purinergic (P2Y) receptors to influence cellular function in many aspects. Gap junction is an intercellular channel and plays a critical role in hearing. Here, we report that stimulation of ATP reduced gap junctional coupling between cochlear supporting cells. This uncoupling effect could be evoked by nanomolar physiological levels of ATP. A P2X receptor agonist benzoylbenzoyl-ATP (BzATP) but not a P2Y receptor agonist UTP stimulated this uncoupling effect. Application of P2X receptor antagonists pyridoxalphosphate-6-azophenyl-2′,4′-disulfonic acid (PPADS, 50 μM) or oxidized ATP (oATP, 0.1 mM) eliminated this uncoupling effect. We further found that ATP activated P2X receptors in the cochlear supporting cells allowing Ca²⁺ influxing, thereby increasing intracellular Ca²⁺ concentration to mediate gap junctions. These data suggest that ATP can mediate cochlear gap junctions at the physiological level by the activation of P2X receptors rather than P2Y receptors. This P2X receptor-mediated purinergic control on the cochlear gap junctions may play an important role in the regulation of K⁺-recycling for ionic homeostasis in the cochlea and the reduction of hearing sensitivity under noise stress for protection.     

Regulation of P2X(7) nucleotide receptor function in human monocytes by extracellular ions and receptor density

P2X receptors function as ATP-gated cation channels. The P2X(7) receptor subtype is distinguished from other P2X family members by a very low affinity for extracellular ATP (millimolar EC50) and its ability to trigger induction of nonselective pores on repeated or prolonged stimulation. Previous studies have indicated that certain P2X(7) receptor-positive cell types, such as human blood monocytes and murine thymocytes, lack this pore-forming response. In the present study we compared pore formation in response to P2X(7) receptor activation in human blood monocytes with that in macrophages derived from these monocytes by in vitro tissue culture. ATP induced nonselective pores in macrophages but not in freshly isolated monocytes when both cell types were identically stimulated in standard NaCl-based salines. However, ion substitution studies revealed that replacement of extracellular Na+ and Cl- with K+ and nonhalide anions strongly facilitated ATP-dependent pore formation in monocytes. These ionic conditions also resulted in increased agonist affinity, such that 30-100 microM ATP was sufficient for activation of nonselective pores by P2X(7) receptors. Comparison of P2X(7) receptor expression in blood monocytes with that in macrophages indicated no differences in steady-state receptor mRNA levels but significant increases (up to 10-fold) in the amount of immunoreactive P2X(7) receptor protein at the cell surface of macrophages. Thus ability of ATP to activate nonselective pores in cells that natively express P2X(7) receptors can be modulated by receptor subunit density at the cell surface and ambient levels of extracellular Na+ and Cl-. These mechanisms may prevent adventitious P2X(7) receptor activation in monocytes until these proinflammatory leukocytes migrate to extravascular sites of tissue damage.     

Biochemical and functional evidence for heteromeric assembly of P2X1 and P2X4 subunits

P2X receptors are ligand-gated ion channels activated by extracellular ATP. In expression systems, P2X subunits form homo- and heterotrimeric receptors. Heteromerization is also likely to occur in vivo as (i) most P2X subunits show overlapping distribution in different tissues and (ii) the functional properties of many native P2X receptors differ from those of heterologously expressed homomeric receptors. Here, we used the Xenopus laevis oocyte expression system to test for heteromerization of P2X1 and P2X4 subunits. Upon co-injection, P2X4 subunits were co-purified with hexahistidyl-tagged P2X1 subunits indicating heteromerization. Blue native polyacrylamide gel electrophoresis (BN-PAGE) analysis of these P2X complexes excluded artificial aggregation and confirmed that both subunits were present in trimeric complexes of the same size. Two-electrode voltage-clamp experiments revealed functional P2X receptors with kinetic properties resembling homomeric P2X4 receptors and a pharmacological profile similar to homomeric P2X1 receptors. Thus, application of alpha,beta-methylene ATP evoked a slowly desensitizing current sensitive to the antagonists suramin and 2',3'-O-(2,4,6-trinitrophenyl)-ATP. This study provides for the first time biochemical and functional evidence for the formation of heteromeric P2X(1+4) receptors. These receptors may account for native P2X mediated responses that until now could not be correlated with previously described recombinant P2X receptors.     

Dependence of purinergic P2X receptor activity on ectodomain structure

Purinergic receptors (P2XRs) activate and desensitize in response to the binding of extracellular nucleotides in a receptor- and ligand-specific manner, but the structural bases of their ligand preferences and channel kinetics have been incompletely characterized. Here we tested the hypothesis that affinity of agonists for binding domain accounts for a ligand-specific desensitization pattern. We generated chimeras using receptors with variable sensitivity to ATP in order: P2X(4)R > P2X(2a)R = P2X(2b)R P2X(7)R. Chimeras having the ectodomain Ile(66)-Tyr(310) sequence of P2X(2)R and Val(61)-Phe(313) sequence of P2X(7)R in the backbone of P2X(4)R were expressed but were non-functioning channels. P2X(2a) + X(4)R and P2X(2b) + X(4)R chimeras having the Val(66)-Tyr(315) ectodomain sequence of P2X(4)R in the backbones of P2X(2a)R and P2X(2b)R were functional and exhibited increased sensitivity to ligands as compared with both parental receptors. These chimeras also desensitized faster than parental receptors and in a ligand-nonspecific manner. However, like parental P2X(2b)R and P2X(2a)R, chimeric P2X(2b) + X(4)R desensitized more rapidly than P2X(2a) + X(4)R, and the rate of desensitization of P2X(2a)+X(4)R increased by substituting its Arg(371)-Pro(376) intracellular C-terminal sequence with the Glu(376)-Gly(381) sequence of P2X(4)R. These results indicate the relevance of interaction between the ectodomain and flanking regions around the transmembrane domains on ligand potency and receptor activation. Furthermore, the ligand potency positively correlates with the rate of receptor desensitization but does not affect the C-terminal-specific pattern of desensitization.     

Acidic amino acids impart enhanced Ca2+ permeability and flux in two members of the ATP-gated P2X receptor family

P2X receptors are ATP-gated cation channels expressed in nerve, muscle, bone, glands, and the immune system. The seven family members display variable Ca2+ permeabilities that are amongst the highest of all ligand-gated channels (Egan and Khakh, 2004). We previously reported that polar residues regulate the Ca2+ permeability of the P2X2 receptor (Migita et al., 2001). Here, we test the hypothesis that the formal charge of acidic amino acids underlies the higher fractional Ca2+ currents (Pf%) of the rat and human P2X1 and P2X4 subtypes. We used patch-clamp photometry to measure the Pf% of HEK-293 cells transiently expressing a range of wild-type and genetically altered receptors. Lowering the pH of the extracellular solution reduced the higher Pf% of the P2X1 receptor but had no effect on the lower Pf% of the P2X2 receptor, suggesting that ionized side chains regulate the Ca2+ flux of some family members. Removing the fixed negative charges found at the extracellular ends of the transmembrane domains also reduced the higher Pf% of P2X1 and P2X4 receptors, and introducing these charges at homologous positions increased the lower Pf% of the P2X2 receptor. Taken together, the data suggest that COO- side chains provide an electrostatic force that interacts with Ca2+ in the mouth of the pore. Surprisingly, the glutamate residue that is partly responsible for the higher Pf% of the P2X1 and P2X4 receptors is conserved in the P2X3 receptor that has the lowest Pf% of all family members. We found that neutralizing an upstream His45 increased Pf% of the P2X3 channel, suggesting that this positive charge masks the facilitation of Ca2+ flux by the neighboring Glu46. The data support the hypothesis that formal charges near the extracellular ends of transmembrane domains contribute to the high Ca2+ permeability and flux of some P2X receptors.     

A Start Point for Extracellular Nucleotide Signaling

A START POINT FOR EXTRACELLULAR NUCLEOTIDE SIGNALING
The recent discovery of a plant receptor for extracellu- lar nucleotides, reported by Choi et al. （2014）, is a major breakthrough that had been anticipated for over a dec- ade. Plants release ATP into their extracellular matrix （ECM） during growth and when they are induced by vari- ous biotic and abiotic stimuli （Clark and Roux, 2011）. That these extracellular nucleotides would activate receptors in plants was predicted by two sets of discoveries： that low- and sub-micromolar ATP could induce increases in [Ca2＋]cyt, NO, and superoxide signaling intermediates that led to downstream growth, stomatal, and defense responses, and that these changes could be blocked by antagonists that blocked extracellular nucleotide receptors in animals （Demidchik et al., 2003; Song et al., 2006; Clark et al., 2011; Demidchik et al., 2009, 2011）. Although mammalian biolo- gists had discovered two classes of receptors for extracel- lular nucleotides （P2X and P2Y） decades ago （Burnstock, 2007）, there were no plant proteins obviously similar to these in any sequence data available. Clearly, if there were plant purinoceptors, they would be different from the mammalian receptors, and they could not be discovered by motif searches.