P2RX7

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. P2X₇ receptors have been implicated in ATP-mediated cell death, regulation of receptor trafficking, and inflammation.

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.

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. Other blockers include the large organic cations calmidazolium (a calmodulin antagonist) and KN-62 (a CaM kinase II antagonist).

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

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. Interestingly, 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.

Possible link to diabetes

Paolo Fiorina, MD, PhD, of the Nephrology Division at Boston Children’s Hospital and his team studied hundreds of pathways in animals with diabetes. They eventually isolated one, known as ATP/P2X7R, which triggers the T-cell attacks on the pancreas, rendering it unable to produce insulin.

“By identifying the ATP/P2X7R pathway as the early mechanism in the body that fires up an autoimmune response, we found the root cause of diabetes” says Fiorina.

Researches

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

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.