Assembly and trafficking of P2X purinergic receptors (Review)

Figures & data

Figure 1.  Trafficking of P2X receptors to and from the plasma membrane. The P2X receptor subtypes are broadly classified into four groups based upon their trafficking behaviour. Motifs that have been shown to be determinants of receptor trafficking are indicated. Plasma membrane (1–2), P2X_1-7 receptors stabilized by YXXXK motif. Deletions within LPS-binding motif of P2X₇, inhibit plasma membrane expression. Endosomes (4–6), P2X₄ constitutively internalized from the plasma membrane via dynamin and AP-2 dependent process involving IL and YXXGL motifs. Receptors can be recycled but are predominantly targeted to lysosomes. P2X_1,5 and ₇ are internalized in response to chronic agonist application. They recycle back to the plasma membrane. Lysosomes (3, 7–9), P2X₄ is targeted to lysosomes, protected from degradation and released via exocytosis. P2X_4/6 and P2X_4/5 assemblies are also targeted here. Delivery to lysosomes may be via a direct pathway from the TGN as well as an indirect pathway via the plasma membrane. ER, P2X_1,2,3,5 exported as homo- and heterotrimers. P2X₄ rapidly exported as homo- and hetero-trimers. P2X₆ retained as a monomer or exported as a heterotrimer. P2X₇ exported as a trimer or retained incorrectly assembled.

Supplementary Table I. Physiological and pathological role of P2X receptors confirmed by knocking out or knocking down their expression.

	Distribution
P2X1	Smooth muscle	In P2X1-deficient mice there is a reduction of contraction of the vas deferens in response to sympathetic nerve stimulation, which results in reduced sperm in the ejaculate and loss of fertility [1].
		P2X1 receptors contribute to sympathetic-induced vasoconstriction in the smooth muscle of the mesenteric artery [2] and bladder [3].
P2X2	Central and peripheral nervous systems including sensory neurones	Mice deficient in P2X2 receptors show attenuated ventilatory response to hypoxia because of a reduction in the response of the carotid sinus nerve [4].
P2X3	Restricted to sensory neurones	Knockout of P2X3 receptors results in relatively small defects in inflammatory pain but obvious deficits in responsiveness to non-noxious stimuli [5]. The use of antisense oligonucleotides, however, showed a more significant role for P2X3 in chronic neuropathic and inflammatory pain [6]. Knockout of P2X3 and P2X2 also caused urinary bladder hyporeflexia [7], reduced afferent nerve activity in response to bladder distension [8] impaired peristalsis [9] and elimination of taste responses in gustatory nerve [10].
P2X4	Widespread throughout peripheral tissues, including immune, epithelial, endothelial and cardiac tissue and the CNS.	Using antisense oligonucleotides, receptors were shown to mediate a calcium influx into vascular endothelial cells in response to shear stress, which triggers secretion of nitric oxide [11], [12]. P2X4 knockout mice show impaired flow induced release of NO and defects in both the control of blood pressure and vascular remodelling [13].
		In human airway epithelial cells, knockdown of P2X4 and P2X6 by siRNA inhibited zinc-induced calcium entry [14]. This calcium influx was shown to restore chloride secretion across cystic fibrosis airway epithelia [15], suggesting that these receptors are possible therapeutic targets for treatment of cystic fibrosis [16].
		Receptors in spinal cord microglia/macrophages play an important role in peripheral nerve injury-induced pain hypersensitivity (tactile allodynia) [17]. P2X4 receptor expression is increased in spinal cord microglia following nerve injury and antisense oligonucleotides against the P2X4 receptor reduced the induction of tactile allodynia [17].
		In the CNS, long term potentiation at hippocampal synapses was reduced in P2X4 knock out mice suggesting a role for calcium entry through these receptors contributing to synaptic plasticity [18].
P2X7	Immune cells, epithelial cells.	Knockdown of the P2X7 receptor abolished the ATP-stimulated processing and release of Il-1β in mouse macrophages [19]. These mice also show reduced cartilage destruction in an antibody-induced arthritis state [20] and reduced bone formation [21].
		Disruption of the P2X7 gene reduced inflammatory and neuropathic pain [22].
		Loss-of-function polymorphisms in P2X7 receptors are linked to epithelial cell cancers and lymphomas [23], [24].

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