Biology:P2RX7

Short description: Protein-coding gene in the species Homo sapiens

Generic protein structure example

P2X purinoceptor 7 is a protein that in humans is encoded by the P2RX7 gene.

The product of this gene belongs to the family of purinoceptors for ATP. Multiple alternatively spliced variants which would encode different isoforms have been identified although some fit nonsense-mediated decay criteria.

The receptor is found in the central and peripheral nervous systems, in microglia, in macrophages, in uterine endometrium, and in the retina. The P2X₇ receptor also serves as a pattern recognition receptor for extracellular ATP-mediated apoptotic cell death, regulation of receptor trafficking, mast cell degranulation, and inflammation. Regarding inflammation, P2X7 receptor induces the NLRP3 inflammasome in myeloid cells and leads to interleukin-1beta release.

Structure and kinetics

The P2X₇ subunits can form homomeric receptors only with a typical P2X receptor structure. The P2X₇ receptor is a ligand-gated cation channel that opens in response to ATP binding and leads to cell depolarization. The P2X₇ receptor requires higher levels of ATP than other P2X receptors; however, the response can be potentiated by reducing the concentration of divalent cations such as calcium or magnesium. Continued binding leads to increased permeability to N-methyl-D-glucamine (NMDG⁺). P2X₇ receptors do not become desensitized readily and continued signaling leads to the aforementioned increased permeability and an increase in current amplitude.

Pharmacology

Agonists

• P2X₇ receptors respond to BzATP more readily than ATP.
• ADP and AMP are weak agonists of P2X₇ receptors, but a brief exposure to ATP can increase their effectiveness.
• Glutathione has been proposed to act as a P2X₇ receptor agonist when present at milimolar levels, inducing calcium transients and GABA release from retinal cells.

Antagonists

• The P2X₇ receptor current can be blocked by zinc, calcium, magnesium, and copper.
• P2X₇ receptors are sensitive to pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) and relatively insensitive to suramin, but the suramin analog, NF279, is much more effective.
• Oxidized ATP (OxATP) and Brilliant Blue G has also been used for blocking P2X₇ in inflammation.
• Other blockers include the large organic cations calmidazolium (a calmodulin antagonist) and KN-62 (a CaM kinase II antagonist).
• JNJ-54175446 and JNJ-55308942 are selective antagonists

Receptor trafficking

In microglia, P2X₇ receptors are found mostly on the cell surface. Conserved cysteine residues located in the carboxyl terminus seem to be important for receptor trafficking to the cell membrane. These receptors are upregulated in response to peripheral nerve injury.

In melanocytic cells P2X₇ gene expression may be regulated by MITF.

Recruitment of pannexin

Activation of the P2X₇ receptor by ATP leads to recruitment of pannexin pores which allow small molecules such as ATP to leak out of cells. This allows further activation of purinergic receptors and physiological responses such a spreading cytoplasmic waves of calcium. Moreover, this could be responsible for ATP-dependent lysis of macrophages through the formation of membrane pores permeable to larger molecules.

Clinical significance

Inflammation

On T cells activation of P2X₇ receptors can activate the T cells or cause T cell differentiation, can affect T cell migration or (at high extracellular levels of ATP and/or NAD+) can induce cell death. The CD38 enzyme on B lymphocytes and macrophages reduces extracellular NAD+, promoting the survival of T cells.

Neuropathic pain

Microglial P2X₇ receptors are thought to be involved in neuropathic pain because blockade or deletion of P2X₇ receptors results in decreased responses to pain, as demonstrated in vivo. Moreover, P2X₇ receptor signaling increases the release of proinflammatory molecules such as IL-1β, IL-6, and TNF-α. In addition, P2X₇ receptors have been linked to increases in proinflammatory cytokines such as CXCL2 and CCL3. P2X₇ receptors are also linked to P2X₄ receptors, which are also associated with neuropathic pain mediated by microglia.

Osteoporosis

Mutations in this gene have been associated to low lumbar spine bone mineral density and accelerated bone loss in post-menopausal women.

Diabetes

The ATP/P2X7R pathway may trigger T-cell attacks on the pancreas, rendering it unable to produce insulin. This autoimmune response may be an early mechanism by which the onset of diabetes is caused.

Research

Possible link to hepatic fibrosis

One study in mice showed that blockade of P2X7 receptors attenuates onset of liver fibrosis.

See also

• Purinergic receptor
• P2X receptor

Further reading

• "P2 receptors in bone--modulation of osteoclast formation and activity via P2X7 activation". Critical Reviews in Eukaryotic Gene Expression 13 (2–4): 237–42. 2003. doi:10.1615/CritRevEukaryotGeneExpr.v13.i24.150. PMID 14696970. 
• "Blockade of the pore-forming P2X7 receptor inhibits formation of multinucleated human osteoclasts in vitro". Calcified Tissue International 73 (4): 361–9. October 2003. doi:10.1007/s00223-002-2098-y. PMID 12874700. 
• "Extracellular nucleotide signaling: a mechanism for integrating local and systemic responses in the activation of bone remodeling". Bone 28 (5): 507–12. May 2001. doi:10.1016/S8756-3282(01)00430-6. PMID 11344050. 
• "Expression of a P2X7 receptor by a subpopulation of human osteoblasts". Journal of Bone and Mineral Research 16 (5): 846–56. May 2001. doi:10.1359/jbmr.2001.16.5.846. PMID 11341329. 
• "Multinucleated osteoclast formation in vivo and in vitro by P2X7 receptor-deficient mice". Critical Reviews in Eukaryotic Gene Expression 13 (2–4): 243–53. 2003. doi:10.1615/CritRevEukaryotGeneExpr.v13.i24.160. PMID 14696971. 
• "Oligo-capping: a simple method to replace the cap structure of eukaryotic mRNAs with oligoribonucleotides". Gene 138 (1–2): 171–4. January 1994. doi:10.1016/0378-1119(94)90802-8. PMID 8125298. 
• "Construction and characterization of a full length-enriched and a 5'-end-enriched cDNA library". Gene 200 (1–2): 149–56. October 1997. doi:10.1016/S0378-1119(97)00411-3. PMID 9373149. 
• "A Glu-496 to Ala polymorphism leads to loss of function of the human P2X7 receptor". The Journal of Biological Chemistry 276 (14): 11135–42. April 2001. doi:10.1074/jbc.M010353200. PMID 11150303. 
• "Proteomic and functional evidence for a P2X7 receptor signalling complex". The EMBO Journal 20 (22): 6347–58. November 2001. doi:10.1093/emboj/20.22.6347. PMID 11707406. 
• "Point mutations confer loss of ATP-induced human P2X(7) receptor function". FEBS Letters 512 (1–3): 43–6. February 2002. doi:10.1016/S0014-5793(01)03311-7. PMID 11852049. 
• "A loss-of-function polymorphic mutation in the cytolytic P2X7 receptor gene and chronic lymphocytic leukaemia: a molecular study". Lancet 359 (9312): 1114–9. March 2002. doi:10.1016/S0140-6736(02)08156-4. PMID 11943260. 
• "Epithelial membrane proteins induce membrane blebbing and interact with the P2X7 receptor C terminus". The Journal of Biological Chemistry 277 (37): 34017–23. September 2002. doi:10.1074/jbc.M205120200. PMID 12107182. 
• "An ATP-gated ion channel at the cell nucleus". Nature 420 (6911): 42. November 2002. doi:10.1038/420042a. PMID 12422208. 
• "Extracellular adenosine 5'-triphosphate induces a loss of CD23 from human dendritic cells via activation of P2X7 receptors". International Immunology 14 (12): 1415–21. December 2002. doi:10.1093/intimm/dxf111. PMID 12456589. 
• "Generation and initial analysis of more than 15,000 full-length human and mouse cDNA sequences". Proceedings of the National Academy of Sciences of the United States of America 99 (26): 16899–903. December 2002. doi:10.1073/pnas.242603899. PMID 12477932. Bibcode: 2002PNAS...9916899M. 
• "An Ile-568 to Asn polymorphism prevents normal trafficking and function of the human P2X7 receptor". The Journal of Biological Chemistry 278 (19): 17108–13. May 2003. doi:10.1074/jbc.M212759200. PMID 12586825. 
• "Specific detection of non-functional human P2X(7) receptors in HEK293 cells and B-lymphocytes". FEBS Letters 538 (1–3): 159–62. March 2003. doi:10.1016/S0014-5793(03)00172-8. PMID 12633871. 
• "P2X7 receptor-dependent blebbing and the activation of Rho-effector kinases, caspases, and IL-1 beta release". Journal of Immunology 170 (11): 5728–38. June 2003. doi:10.4049/jimmunol.170.11.5728. PMID 12759456. 
• "Purinergic receptors are part of a functional signaling system for proliferation and differentiation of human epidermal keratinocytes". The Journal of Investigative Dermatology 120 (6): 1007–15. June 2003. doi:10.1046/j.1523-1747.2003.12261.x. PMID 12787128. 
• "Mutation of a dibasic amino acid motif within the C terminus of the P2X7 nucleotide receptor results in trafficking defects and impaired function". Journal of Immunology 171 (3): 1304–11. August 2003. doi:10.4049/jimmunol.171.3.1304. PMID 12874219. 

External links

• P2RX7+protein,+human at the US National Library of Medicine Medical Subject Headings (MeSH)

This article incorporates text from the United States National Library of Medicine, which is in the public domain.