Functional regulation of P2X6 receptors by N-linked glycosylation: identification of a novel alpha beta-methylene ATP-sensitive phenotype

Biochemical and functional evidence for heteromeric assembly of P2X1 and P2X4 subunits

P2X receptors are ligand-gated ion channels activated by extracellular ATP. In expression systems, P2X subunits form homo- and heterotrimeric receptors. Heteromerization is also likely to occur in vivo as (i) most P2X subunits show overlapping distribution in different tissues and (ii) the functional properties of many native P2X receptors differ from those of heterologously expressed homomeric receptors. Here, we used the Xenopus laevis oocyte expression system to test for heteromerization of P2X1 and P2X4 subunits. Upon co-injection, P2X4 subunits were co-purified with hexahistidyl-tagged P2X1 subunits indicating heteromerization. Blue native polyacrylamide gel electrophoresis (BN-PAGE) analysis of these P2X complexes excluded artificial aggregation and confirmed that both subunits were present in trimeric complexes of the same size. Two-electrode voltage-clamp experiments revealed functional P2X receptors with kinetic properties resembling homomeric P2X4 receptors and a pharmacological profile similar to homomeric P2X1 receptors. Thus, application of alpha,beta-methylene ATP evoked a slowly desensitizing current sensitive to the antagonists suramin and 2',3'-O-(2,4,6-trinitrophenyl)-ATP. This study provides for the first time biochemical and functional evidence for the formation of heteromeric P2X(1+4) receptors. These receptors may account for native P2X mediated responses that until now could not be correlated with previously described recombinant P2X receptors.

Atomic Force Microscopy Imaging Demonstrates that P2X2 Receptors Are Trimers but That P2X6 Receptor Subunits Do Not Oligomerize

P2X receptors are cation-selective channels activated by extracellular ATP. The architecture of these receptors is still not completely clear. Here we have addressed this issue by both chemical cross-linking and direct imaging of individual receptors by atomic force microscopy (AFM). Cross-linking of the P2X(2) receptor produced higher order adducts, consistent with the presence of trimers. The mean molecular volume of the receptor determined by AFM (409 nm(3)) also points to a trimeric structure. P2X(2) receptors bearing His(6) epitope tags were incubated with anti-His(6) antibodies, and the resultant complexes were imaged by AFM. For receptors with two bound antibodies, the mean angle between the antibodies was 123 degrees , again indicating that the receptor is a trimer. In contrast, cross-linking of the P2X(6) receptor did not produce higher order adducts, and the mean molecular volume of the receptor was 145 nm(3). We conclude that P2X(2) receptors are trimers, whereas the P2X(6) receptor subunits do not form stable oligomers.

Multiple P2X and P2Y receptor subtypes in mouse J774, spleen and peritoneal macrophages

We investigated P2 receptor expression and function in macrophages from mouse, and in the J774 cell line, and revealed a larger spectrum of P2 receptor subtypes than previously recognised. The nucleotides adenosine triphosphate (ATP), adenosine diphosphate, uridine triphosphate and uridine diphosphate evoked an increase in intracellular calcium and the activation of a potassium current. The sensitivity of these responses to the antagonists suramin, PPADS, MRS 2179 and Cibacron blue suggest the presence of at least three functional P2Y receptor subtypes, most probably P2Y(2), P2Y(4) and P2Y(6). ATP also activated P2X receptors, giving rise to a rapidly activating cation conductance. This response was insensitive to the antagonists suramin and Cibacron blue, was potentiated by Zn(2+) and inhibited by acidification suggesting involvement of P2X(4) receptors. In low divalent cation solution, responses to ATP became larger, and dibenzoyl-ATP became more potent than ATP, indicating the presence of P2X(7) receptors. Immunofluorescence, flow cytometry, Western blots and RT-PCR show that P2X(4) and P2X(7) receptors are the most prominent in both macrophage types, while the expression of the other P2X subunits is variable and sometimes weak or undetectable. These techniques also demonstrated the presence of mRNA for P2Y(1), P2Y(2), P2Y(4) and P2Y(6) receptors along with protein expression for the three subtypes we investigated, namely, P2Y(1), P2Y(2) and P2Y(4).

Epidermal growth factor facilitates epinephrine inhibition of P2X7-receptor-mediated pore formation and apoptosis: a novel signaling network

Epidermal growth factor (EGF), epinephrine, and the P2X7 receptor system regulate growth of human uterine cervical epithelial cells, but little is known about how these systems intercommunicate in exerting their actions. The objective of this study was to understand the mechanisms of EGF and epinephrine regulation of growth of cervical cells. Treatment of cultured CaSki cells with 0.2 nM EGF increased cell number via a PD98059-sensitive pathway. Treatment with 2 nM epinephrine increased cell number, and the effect was facilitated by cotreatment with EGF. Whereas the effect of EGF alone involved up-regulation of [3H]-thymidine incorporation and an increase in cell proliferation, the effect of epinephrine was mediated by inhibition of apoptosis. Epinephrine inhibited apoptosis induced by the P2X7 receptor ligand 2',3'-0-(4-benzoylbenzoyl)-ATP, by attenuation of P2X7 receptor plasma membrane pore formation. Cotreatment with EGF facilitated epinephrine effect via a phosphoinositide 3-kinase-dependent mechanism. CaSki cells express the beta2-adrenoceptor, and the epinephrine antiapoptotic effect could be mimicked by beta2-adrenoceptor agonists and by activators of adenylyl cyclase. Likewise, the effect could be blocked by beta2-adrenoceptor blockers and by the inhibitor of protein kinase-A H-89. Western immunoblot analysis revealed that epinephrine decreased the levels of the glycosylated 85-kDa form of the P2X7 receptor and increased receptor degradation, and that EGF potentiated these effects of epinephrine. EGF did not affect cellular levels of the beta2-adrenoceptor. In contrast, EGF, acting via the EGF receptor, augmented beta2-adrenoceptor recycling, and it inhibited beta2-adrenoceptor internalization via a phosphoinositide 3-kinase-dependent mechanism. We conclude that, in cervical epithelial cells, EGF has a dual role: as mitogen, acting via the MAPK/MAPK kinase pathway, and as an antiapoptotic factor by facilitating epinephrine effect and resulting in greater expression of beta2-adrenoceptors in the plasma membrane. These findings underscore a novel signaling network of communication between the receptor tyrosine kinases, the G protein-coupled receptors, and the purinergic P2X7 receptor.

Trimeric Architecture of Homomeric P2X2 and Heteromeric P2X1+2 Receptor Subtypes

Of the three major classes of ligand-gated ion channels, nicotinic receptors and ionotropic glutamate receptors are known to be organized as pentamers and tetramers, respectively. The architecture of the third class, P2X receptors, is under debate, although evidence for a trimeric assembly is accumulating. Here we provide biochemical evidence that in addition to the rapidly desensitising P2X1 and P2X3 receptors, the slowly desensitising subtypes P2X2, P2X4, and P2X5 are trimers of identical subunits. Similar (heteromeric) P2X subunits also formed trimers, as shown for co-expressed P2X1 and P2X2 subunits, which assembled efficiently to a P2X1+2 receptor that was exported to the plasma membrane. In contrast, P2X6 subunits, which are incapable of forming functional homomeric channels in Xenopus oocytes, were retained in the ER as apparent tetramers and high molecular mass aggregates. Altogether, we conclude from these data that a trimeric architecture is the structural hallmark of functional homomeric and heteromeric P2X receptors.

Functional characterization of a P2X receptor from Schistosoma mansoni

The cloning and characterization of a P2X receptor (schP2X) from the parasitic blood fluke Schistosoma mansoni provides the first example of a non-vertebrate ATP-gated ion channel. A number of functionally important amino acid residues conserved throughout vertebrate P2X receptors, including 10 extracellular cysteines, aromatic and positively charged residues involved in ATP recognition, and a consensus protein kinase C site in the amino-terminal tail, are also present in schP2X. Overall, the amino acid sequence identity of schP2X with human P2X(1-7) receptors ranges from 25.8 to 36.6%. ATP evoked concentration-dependent currents at schP2X channels expressed in Xenopus oocytes with an EC(50) of 22.1 microM. 2',3'-O-(4-Benzoylbenzoyl)adenosine 5'-triphosphate (Bz-ATP) was a partial agonist (maximum response 75.4 +/- 4.4% that of ATP) with a higher potency (EC(50) of 3.6 microM) than ATP. Suramin and pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid blocked schP2X responses to 100 microm ATP with IC(50) values of 9.6 and 0.5 microM, respectively. Ivermectin (10 microM) potentiated currents to both ATP and Bz-ATP by approximately 60% with a minimal effect on potency (EC(50) of 18.2 and 1.6 microM, respectively). The relative permeability of schP2X expressed in HEK293 cells to various cations was determined under bi-ionic conditions. schP2X has a relatively high calcium permeability (P(Ca)/P(Na) = 3.80 +/- 0.29) and an estimated minimum pore diameter similar to that of vertebrate P2X receptors. SchP2X provides a useful comparative model for the better understanding of human P2X receptor function and may also provide an alternative drug target for treatment of schistosomiasis.

P2X receptor subtype-specific modulation of excitatory and inhibitory synaptic inputs in the rat brainstem

The role of P2 receptors in synaptic transmission to the rat medial nucleus of the trapezoid body (MNTB) was studied in an in vitro brain slice preparation. Whole-cell patch recordings were made and spontaneous synaptic responses studied under voltage clamp during application of P2X receptor agonists. ATPgammaS (100 microm) had no effect on holding current, but facilitated spontaneous excitatory postsynaptic current (sEPSC) frequency in 41% of recordings and facilitated spontaneous inhibitory postsynaptic currents (sIPSCs) in 20% of recordings. These were blocked by the P2 receptor antagonist suramin (100 microm). alpha,beta-meATP also facilitated sEPSC and sIPSC frequency, while l-beta,gamma-meATP facilitated only sIPSCs. The sEPSC facilitation by ATPgammaS was blocked by TTX (but did not block facilitation of sIPSCs). sEPSC facilitation was blocked by PPADS (30 microm) and the selective P2X(3) receptor antagonist A-317491 (3 microm), suggesting that modulation of sEPSCs involves P2X(3) receptor subunits. alpha,beta-meATP-facilitated sIPSCs were also recorded in wild-type mouse MNTB neurones, but were absent in the MNTB from P2X(1) receptor-deficient mice demonstrating a functional role for P2X(1) receptors in the CNS.

Identification of a trafficking motif involved in the stabilization and polarization of P2X receptors

Extracellular ATP-gated channels (P2X receptors) define the third major family of ionotropic receptors, and they are expressed widely in nerve cells, muscles, and endocrine and exocrine glands. P2X subunits have two membrane-spanning domains, and a receptor is thought to be formed by oligomerization of three subunits. We have identified a conserved motif in the cytoplasmic C termini of P2X subunits that is necessary for their surface expression; mutations in this motif result in a marked reduction of the receptors at the plasma membrane because of a rapid internalization. Transfer of the motif to a reporter protein (CD(4)) enhances the surface expression of the chimera, indicating that this motif is likely involved in the stabilization of P2X receptor at the cell surface. In neurons, mutated P2X(2) subunits showed reduced membrane expression and an altered axodendritic distribution. This motif is also present in intracellular regions of other membrane proteins, such as in the third intracellular loop of some G protein-coupled receptors, suggesting that it might be involve in their cellular stabilization and polarization.