Genetic polymorphisms of the human P2X7 receptor and relationship to function

Dysregulation of the inflammatory response to the parasite, Toxoplasma gondii, in P2X7 receptor-deficient mice

The P2X(7) receptor (P2X(7)R) is a two transmembrane receptor that is highly expressed on the surface of immune cells. Loss of function polymorphisms in this receptor have been linked to increased susceptibility to intracellular pathogens. P2X(7)R gene knockout (P2X(7)R(-/-); on a C57Bl/6J background), C57Bl/6J and BALB/c mice were infected with the avirulent ME49 strain of the intracellular parasite, Toxoplasma gondii, and susceptibility determined by monitoring weight loss. P2X(7)R(-/-) mice lost significantly more weight than C57Bl/6J mice from day 8p.i. C57Bl/6J, in turn, lost significantly more weight than BALB/c mice. Thus, by day 10p.i., P2X(7)R(-/-) mice had lost 5.7 ± 0.7% of their weight versus 2.4 ± 0.6% for C57Bl/6J mice, whereas BALB/c mice had gained 1.9 ± 0.5%; by day 12p.i., P2X(7)R(-/-) mice had lost 15.1±0.6%, C57Bl/6J had lost 10.1±0.8% and BALB/c had lost 4.8 ± 0.8% of their weight. Neither parasite burden nor liver pathology was greater in the P2X(7)R(-/-) mice than in C57Bl/6J mice but BALB/c mice had significantly smaller numbers of parasites and less pathology in their livers than these strains. Absence of the P2X(7) receptor did not affect IFN-γ, IL-12, IL-1β, monocyte chemoattractant protein-1 (MCP-1) or TNF production. However, both P2X(7)R(-/-) and C57Bl/6J mice produced more IL-1β and TNF than BALB/c mice. There was one important point of differentiation between the P2X(7)R(-/-) and C57Bl/6J mice, namely the significantly enhanced and prolonged production of nitric oxide, accompanied by delayed production of IL-10 in the P2X(7)R-deficient mice.

A Thr357 to Ser polymorphism in homozygous and compound heterozygous subjects causes absent or reduced P2X7 function and impairs ATP-induced mycobacterial killing by macrophages

The P2X(7) receptor is a ligand-gated cation channel that is highly expressed on mononuclear leukocytes and that mediates ATP-induced apoptosis and killing of intracellular pathogens. There is a wide variation in P2X(7) receptor function between subjects, explained in part by four loss-of-function polymorphisms (R307Q, E496A, I568N, and a 5'-intronic splice site polymorphism), as well as rare mutations. In this study, we report the allele frequencies of 11 non-synonymous P2X(7) polymorphisms and describe a fifth loss-of-function polymorphism in the gene (1096C --> G), which changes Thr(357) to Ser (T357S) with an allele frequency of 0.08 in the Caucasian population. P2X(7) function was measured by ATP-induced ethidium(+) influx into peripheral blood lymphocytes and monocytes and, when compared with wild-type subjects, was reduced to 10-65% in heterozygotes, 1-18% in homozygotes, and 0-10% in compound heterozygotes carrying T357S and a second loss-of-function polymorphism. Overexpression of the T357S mutant P2X(7) in either HEK-293 cells or Xenopus oocytes gave P2X(7) function of approximately 50% that of wild-type constructs. Differentiation of monocytes to macrophages, which also up-regulates P2X(7), restored P2X(7) function to near normal in cells heterozygous for T357S and to a value 50-65% of wild-type in cells homozygous for T357S or compound heterozygous for T357S/E496A. However, macrophages from subjects that are compound heterozygous for either T357S/R307Q or T357S/stop codon had near-to-absent P2X(7) function. These functional deficits induced by T357S were paralleled by impaired ATP-induced apoptosis and mycobacteria killing in macrophages from these subjects. Lymphocytes, monocytes, and macrophages from subjects homozygous for T357S or compound heterozygous for T357S and a second loss-of-function allele have reduced or absent P2X(7) receptor function.

Chelerythrine and other benzophenanthridine alkaloids block the human P2X7 receptor

1 Extracellular ATP can activate a cation-selective channel/pore on human B-lymphocytes, known as the P2X7 receptor. Activation of this receptor is linked to PLD stimulation. We have used ATP-induced 86Rb+ (K+) efflux to examine the effect of benzophenanthridine alkaloids on P2X7 channel/pore function in human B-lymphocytes. 2 Both ATP and the nucleotide analogue 2'-3'-O-(4-benzoylbenzoyl)-ATP (BzATP) induced an 86Rb+ efflux, which was completely inhibited by the isoquinoline derivative 1-(N,O-bis[5-isoquinolinesulphonyl]-N-methyl-l-tyrosyl)-4-phenylpiperazine (KN-62), a potent P2X7 receptor antagonist. 3 The benzophenanthridine alkaloid chelerythrine, a potent PKC inhibitor, inhibited the ATP-induced 86Rb+ efflux by 73.4+/-3.5% and with an IC50 of 5.6+/-2.3 microm. Similarly, other members of this family of compounds, sanguinarine and berberine, blocked the ATP-induced 86Rb+ efflux by 58.8+/-4.8 and 61.1+/-8.0%, respectively. 4 Concentration-effect curves to ATP estimated an EC50 value of 78 microm and in the presence of 5 and 10 microm chelerythrine this increased slightly to 110 and 150 microm, respectively, which fits a noncompetitive inhibitor profile for chelerythrine. 5 Chelerythrine at 10 microm was effective at inhibiting the ATP-induced PLD stimulation in B-lymphocytes by 94.2+/-21.9% and the phorbol 12-myristate 13-acetate-induced PLD stimulation by 68.2+/-7.4%. 6 This study demonstrates that chelerythrine in addition to PKC inhibition has a noncompetitive inhibitory action on the P2X7 receptor itself.

Early prostate cancer detected using expression of non-functional cytolytic P2X7 receptors

AIMS

To detect early prostate cancer reliably by monitoring the expression of non-functional P2X(7) cytolytic purinergic receptors.

METHODS AND RESULTS

P2X(7) receptors were absent from normal prostate epithelium obtained from post mortem tissue and tissue from cases of transurethral resection collected from young men (n = 23) who were confirmed to be free of cancer at later procedures 5-10 years after collection of the original samples. However, P2X(7) was present in every case of 116 confirmed prostate cancers regardless of Gleason grade or patient age. P2X(7) was present in apparently normal epithelial cells in acini well outside the tumour margins, but appeared in a distinct stage-specific manner commencing with the nucleus, progressing to the cytoplasm and collecting finally on the apical membrane of the epithelial cells in morphologically distinct cancer. The pattern of P2X(7) receptor localization in the epithelial cells was recorded in earlier biopsies obtained from the same patient cohort. One hundred and fourteen of 116 prostates stained positively for P2X(7) at the earliest biopsy, though generally with a less advanced pattern of distribution.

CONCLUSIONS

The appearance of P2X(7) receptors in normal prostate tissue adjacent to prostate tumours makes direct tumour biopsy less critical for positive cancer diagnosis and enables cancer progression to be monitored.

Point mutations confer loss of ATP-induced human P2X(7) receptor function

Residues considered essential for ATP binding to the human P2X(7) receptor (hP2X(7)R) were investigated. HEK293 cells or Xenopus oocytes were transfected with wild-type or site-directed mutants of hP2X(7)R constructs and channel/pore activity measured in the presence of ATP or 2',3'-O-(4-benzoylbenzoyl)-ATP (BzATP). Barium uptake and ethidium influx into HEK293 cells were abolished in cells expressing K193A and K311A mutants, and were partially reduced in cells expressing mutant P210A. K193A and K311A mutations also completely abolished responses to ATP and BzATP in Xenopus oocytes as measured by electrophysiology. These results indicate that K193 and K311 are essential residues in ATP binding in the hP2X(7)R.