Hetero-oligomeric Assembly of P2X Receptor Subunits

Abstract

P2X receptors are a distinct family of ligand-gated ion channels activated by extracellular ATP. Each of the seven identified subunit proteins (P2X 1 through P2X 7 ) has been reported to form functional homo-oligomeric channels when expressed in heterologous systems. Functional studies of native receptors, together with patterns of subunit gene expression, suggest that hetero-oligomeric assembly among members of this family may also occur. This prediction is supported by reports describing hetero-oligomeric assembly for three different recombinant subunit combinations. In this report, we systematically examined the ability of all members of the P2X receptor family to interact using a co-immunoprecipitation assay. The seven P2X receptor subunits were differentially epitope-tagged and expressed in various combinations in human embryonic kidney 293 cells. It was found that six of the seven subunits formed homooligomeric complexes, the exception being P2X 6 . When co-assembly between pairs of subunits was examined, all were able to form hetero-oligomeric assemblies with the exception of P2X 7 . Whereas P2X 1 , P2X 2 , P2X 5 , and P2X 6 were able to assemble with most subunits, P2X 3 and P2X 4 presented a more restricted pattern of coassociation. These results suggest that hetero-oligomeric assembly might underlie functional discrepancies observed between P2X responses seen in the native and recombinant settings, while providing for an increased diversity of signaling by ATP.Investigation of the native receptors mediating extracellular ATP signaling in tissues has been a difficult task due to the lack of useful pharmacological tools. For this reason, the cloning of ATP receptors (the P2 receptor family) and their recombinant expression has proven extremely useful in elucidating the basic properties of these proteins and for providing a template for further study into native P2 receptors. Two families of proteins mediating the actions of ATP have been identified: the metabotropic G protein-coupled P2Y receptors and the ionotropic P2X receptors (1). The P2X receptors are nonselective ion channels thought to be oligomeric in nature, and they are expressed in many excitable and nonexcitable cells, where they mediate a variety of physiological actions, including smooth muscle contractility, neuroendocrine secretion, and modulation of synaptic transmission (2, 3). In addition, recent reports suggest that they may also play an important role in the transmission of pain perception (4, 5). With the molecular identification of seven P2X receptor subunits, our understanding of the biophysical and pharmacological properties of these channels has been considerably increased. However, relatively little is known about the multimeric organization of this new class of channel receptors.Almost all known ionotropic receptors exist as hetero-oligomers (6). Importantly, their functional and pharmacological properties are directly determined by their subunit composition, with different subunit combinations yielding different phenotypes (e...

Distribution of purinergic P2X receptors in the equine digit, cervical spinal cord and dorsal root ganglia

“…P2X receptor subunits were expressed in most tissues, specifically P2X [1][2][3][4][5]7 mRNA was found in all tissues, whereas P2X 6 was well expressed in the dorsal root ganglia and spinal cord but only faintly in other tissues (data not shown). The identification of the PCR amplified products as P2X [1][2][3][4][5][6][7] amplicons was based on their size and on subsequent DNA sequencing and genome-wide similarity searches (Blast, NCBI). A comparison of the sequenced nucleotide sequences to the predicted ones gave a perfect match for all, but the P2X 5 transcript which was found to combine parts of sequences matching the NCBI predicted transcript (XM_001918102.1), the Ensembl predicted transcripts (ENSECAT00000026624, ENSECAT00000026645) or neither the NCBI/Ensembl predicted transcripts (Tables S2 and S3).…”

“…Protein samples (25 μg) were mixed with lane marker non-reducing sample buffer (ThermoScientific) and DTT (0.1 M) and denatured at 70°C for 10 min. Samples used in Western blots for P2X 1,[3][4][5][6][7] proteins were denatured, whereas samples for P2X 2 protein did not undergo denaturation since P2X 2 antibody showed better results with non-denatured samples. Samples and molecular weight markers were loaded and separated on NuPage 4-12 % Bis-Tris gel (Invitrogen) and then transferred onto nitrocellulose membrane (Whatman).…”

“…Tissue samples for immunohistochemical analysis of P2X [1][2][3]7 were fixed overnight in 4 % buffered PFA, paraffin embedded then cut in 6-μm sections. Sections were deparaffinised and rehydrated before being subjected to antigen retrieval …”

“…Upon binding ATP (and related nucleotides), conformational changes result in channel opening and flux of monovalent and divalent cations including Na + , K + and Ca 2+ [3]. Currently seven P2X receptor subunits are recognised in vertebrates (P2X 1-7 ), each being encoded by its own gene [4].…”

“…mRNA signal for most of the tested P2X receptor subunits (P2X 1-5, 7 ) was detected in all sampled equine tissues, whereas P2X 6 receptor subunit was predominantly expressed in the dorsal root ganglia and spinal cord. Western blot analysis validated the specificity of P2X [1][2][3]7 antibodies, and these were used in immunohistochemistry studies. P2X 1-3, 7 receptor subunits were found in smooth muscle cells in the palmar digital artery and vein with the exception of the P2X 3 subunit that was present only in the vein.…”