1.A.7 ATP-gated P2X Receptor Cation Channel (P2X Receptor) Family

Members of the P2X Receptor family respond to ATP, a functional neurotransmitter released by exocytosis from many types of neurons. They have been placed into seven groups (P2X1 - P2X7) based on their pharmacological properties. These channels, which function at neuron-neuron and neuron-smooth muscle junctions, may play roles in the control of blood pressure and pain sensation. They may also function in lymphocyte and platelet physiology. They are found only in animals. Their ATP binding sites are extracellular and involve residues near ile-67. ATP binding causes the channel to go from the closed to the open state (Kracun et al., 2010). The intracellular amino terminus plays a dominant role in desensitization of P2X receptor ion channels (Allsopp and Evans, 2011). Activation and regulation of purinergic P2X receptor channels has been reviewed by Caddou et al. (2011). The gating mechanism has been proposed (Du et al., 2012). The ion access pathway to the transmembrane pore in P2X receptor channels has been estimated (Kawate et al., 2011). P2X receptor channels show threefold symmetry in ionic charge selectivity and unitary conductance (Browne et al., 2011). The binding of ATP to trimeric P2X receptors (P2XR) causes an enlargement of the receptor extracellular vestibule, leading to opening of the cation-selective transmembrane pore, and specific roles of vestibule amino acid residues in receptor activation have been evaluated (Rokic et al. 2013).

The seven different P2X receptors differ in their sensitivities to ATP and various ATP analogues as well as in their inactivation kinetics. ATP binding initially causes opening of the non-selective cation channel, allowing Ca²⁺ entry. Prolonged exposure of slowly inactivating forms to ATP leads to dilation of the pore, making it permeable to larger molecules (up to 1000 Da). Then it functions as a cytolytic pore that is permeable to organic cations such as ethidium and N-methyl-D-glucamine. Formation of this cytolytic pore is regulated by the C-terminal hydrophilic domain in at least one of these receptors (P2X7; Smart et al., 2003). The ion-conducting pathway is formed by three transmembrane domain 2 (TM2) alpha-helices, each being provided by the three subunits of the trimer. P2X receptors are trimeric ATP-activated ion channels permeable to Na⁺, K⁺ and Ca⁺². The seven P2X receptor subtypes are implicated in physiological processes that include modulation of synaptic transmission, contraction of smooth muscle, secretion of chemical transmitters and regulation of immune responses.

The zebrafish chalice-shaped, trimeric P2X(4) receptor (TC#1.A.7.1.4) is knit together by subunit-subunit contacts implicated in ion channel gating and receptor assembly. Extracellular domains, rich in beta-strands, have large acidic patches that may attract cations, through fenestrations, to vestibules near the ion channel. In the transmembrane pore, the 'gate' is defined by an approximately 8 A slab of protein. There are three non-canonical, intersubunit ATP-binding sites. ATP binding may promote subunit rearrangement and ion channel opening (Kawate et al., 2009).

The P2X1 receptor is the dominant P2X type in smooth muscle neurons. P2X2/P2X3 heterooligomers mediate sensory signals in many other sensory neurons. P2X4 and P2X6 receptors are highly expressed in the central nervous system and probably form heterooligomers. P2X7 is expressed in cells of the immune system and of hematopoetic origin. These channels have a homo- or heterotrimeric architecture (Aschrafi et al., 2004). The carboxy termini influence the regulation of P2X1 receptors (Wen and Evans 2011).

The proteins of the P2X Receptor family are quite similar in sequence (>35% identity), but they possess 380-1000 amino acyl residues per subunit with variability in length localized primarily to the C-terminal domains. They possess two transmembrane spanners, one about 30-50 residues from their N-termini, the other near residues 320-340. The extracellular receptor domains between these two spanners (of about 270 residues) are well conserved with several conserved glycyl residues and 10 conserved cysteyl residues. The hydrophilic C-termini vary in length from 25 to 240 residues. Topologically, they resemble the epithelial Na⁺ channel (ENaC) proteins (TC #1.A.6) in possessing (a) N- and C-termini localized intracellularly, (b) two putative transmembrane spanners, (c) a large extracellular loop domain, and (d) many conserved extracellular cysteyl residues. P2X Receptor family members are, however, not demonstrably homologous with them. P2X Receptor channels are probably hetero- or homomultimers of several subunits and transport small monovalent cations (Me⁺). Some also transport Ca²⁺, and after prolonged exposure to ATP, various metabolites as noted above.

The three-dimensional structure of a P2X receptor is known (Burnstock and Kennedy, 2011). When ATP binds the pore opens within milliseconds, allowing the cations to flow. P2X receptors are expressed in both central and peripheral neurons where they are involved in neuromuscular and synaptic neurotransmission and neuromodulation. They are also expressed in most types of nonneuronal cells and mediate a wide range of actions, such as contraction of smooth muscle, secretion and immunomodulation. Changes in the expression of P2X receptors have been characterized in many pathological conditions of the cardiovascular, gastrointestinal, respiratory, and urinogenital systems and in the brain and special senses. The therapeutic potential of P2X receptor agonists and antagonists is currently being investigated in a range of disorders, including chronic neuropathic and inflammatory pain, depression, cystic fibrosis, dry eye, irritable bowel syndrome, interstitial cystitis, dysfunctional urinary bladder, and cancer.

Hattori and Gouaux (2012) reported the crystal structure of the zebrafish P2X(4) receptor in complex with ATP and a new structure of the apo receptor. The agonist-bound structure reveals an ATP-binding motif and an open ion channel. ATP binding induces cleft closure of the nucleotide-binding pocket, flexing of the lower body β-sheet and a radial expansion of the extracellular vestibule. The structural widening of the extracellular vestibule is directly coupled to the opening of the ion channel pore by way of an iris-like expansion of the transmembrane helices.

Phosphoinositides modulate the functions of most P2X receptor channels in neurons and glia. A dual polybasic motif has been shown to determine phosphoinositide binding and regulation in members of the P2X channel family (Bernier et al., 2012). 

Cys loop, glutamate, and P2X receptors are ligand-gated ion channels (LGICs) with 5, 4, and 3 protomers, respectively. Agonists and competitive antagonists apparently induce opposite motions of the binding pocket (Du et al., 2012b). Agonists, usually small, induce closure of the binding pocket, leading to opening of the channel pore, whereas antagonists, usually large, induce opening of the binding pocket, thereby stabilizing the closed pore.

The strong expression of ATP-gated P2X3 receptors by a subpopulation of sensory neurons indicates the important role of these membrane proteins in nociceptive signaling in health and disease, especially when the latter is accompanied by chronic pain syndromes. These receptors exist mainly as trimeric homomers, and, in part, as heteromers (assembly of two P2X3 subunits with one P2X2). Recent investigations have suggested distinct molecular determinants responsible for agonist binding and channel opening for transmembrane flux of sodium, calcium and potassium ions. Trimeric P2X3 receptors are rapidly activated by ATP and can be strongly desensitized in the continuous presence of the agonist. Endogenous substances, widely thought to be involved in triggering pain, especially in pathological conditions, can potently modulate the expression and function of P2X3 receptors, with differential changes in response amplitude, desensitization and recovery. Strong facilitation of P2X3 receptor function is induced by enodogenous substances like the neuropeptide calcitonin gene-related peptide and the neurotrophins nerve growth factor and brain-derived neurotrophic factor. These substances possess distinct mechanisms of action on P2X3 receptors, generally attributable to discrete phosphorylation of N- or C-terminal P2X3 domains (Fabbretti and Nistri, 2012).

Two structural classes of pore-opening mechanisms have been established: bending of pore-lining helices in the case of tetrameric cation channels, and tilting of such helices in mechanosensitive channels. Expansion of the gate region in the external pore in P2X receptors is accompanied by a narrowing of the inner pore, indicating that pore-forming helices straighten on ATP binding to open the channel (Li et al., 2010). This pore-opening mechanism has implications for the role of subunit interfaces in the gating mechanism and points to a role of the internal pore in ion permeation.  Amino terminal residues are involved in lipid, cholesterol and lipid raft regulation of P2X1 (Allsopp et al. 2010).

The generalized transport reaction is:

Me⁺ (out) Me⁺ (in).