Alternatively Spliced Isoforms of the P2X7 Receptor: Structure, Function and Disease Associations

Abstract

The P2X7 receptor (P2X7R) is an ATP-gated membrane ion channel that is expressed by multiple cell types. Following activation by extracellular ATP, the P2X7R mediates a broad range of cellular responses including cytokine and chemokine release, cell survival and differentiation, the activation of transcription factors, and apoptosis. The P2X7R is made up of three P2X7 subunits that contain specific domains essential for the receptor's varied functions. Alternative splicing produces P2X7 isoforms that exclude one or more of these domains and assemble in combinations that alter P2X7R function. The modification of the structure and function of the P2X7R may adversely affect cellular responses to carcinogens and pathogens, and alternatively spliced (AS) P2X7 isoforms have been associated with several cancers. This review summarizes recent advances in understanding the structure and function of AS P2X7 isoforms and their associations with cancer and potential role in modulating the inflammatory response.

Keywords: ATP; P2X7R; alternative splicing; cancer; inflammation.

Figure 3

P2RX7 mRNA isoforms and predicted proteins. Diagrammatic representation of the exons (open numbered boxes) comprising each transcript (mRNA variant names are listed on the left of each transcript). The thin line joining exon 10 to exon 11 represents the retained intron 10. N3 and N4 are alternative exons; p7 indicates that part of the sequence for exon 7 is missing. Thick black lines represent the proteins that could be translated beginning at the primary start codon in exon 1 or at an alternative site.

Figure 1

Model of the full-length human P2X7R. (A) Structure of the trimeric P2X7R within an epithelial membrane model. The three P2X7 subunits are shown in magenta, cyan and green ribbon representations. The epithelial membrane is shown in yellow. (B) A zoomed in view of a P2X7R ATP-binding site located between two P2X7 subunits in the extracellular domain of the receptor. ATP is shown in orange interacting with N292, R294, and K311 on one subunit and K64 and 66 on the adjacent P2X7 subunit. Structural model produced using PyMOL software version 2.5.2, Schrödinger, Inc., New York, NY, USA [23].

Figure 2

Overview of the regions constituting the P2X7A subunit. (A) Ribbon representation of the P2X7A subunit structure. The intracellular amino and carboxy termini are shown in purple, the transmembrane domains (TM1 and TM2) are shown in orange, and the extracellular domain is shown in green. (B) The exonic structure of the full-length P2RX7 messenger ribonucleic acid (mRNA) (exons 1 to 13) that encodes the 595 amino acid P2X7A subunit. The amino acid exon boundaries are numbered above the exonic structure (i.e., exon 1 encodes amino acids 1 to 42 of the P2X7R protein, exon 2 encodes amino acids 43 to 99, and so on). The corresponding P2X7A subunit regions that are translated from the mRNA are illustrated below the exonic structure with the colours aligning to the regions shown in (A). (C) Domains of interest located within the carboxy terminal of the P2X7A subunit including the C rich domain, actin filament binding domain, SH3 binding domain, TNFR1 death domain, trafficking domain, and LPS domain. Structural model produced using PyMOL software version 2.5.2 [23].

Figure 2

Overview of the regions constituting the P2X7A subunit. (A) Ribbon representation of the P2X7A subunit structure. The intracellular amino and carboxy termini are shown in purple, the transmembrane domains (TM1 and TM2) are shown in orange, and the extracellular domain is shown in green. (B) The exonic structure of the full-length P2RX7 messenger ribonucleic acid (mRNA) (exons 1 to 13) that encodes the 595 amino acid P2X7A subunit. The amino acid exon boundaries are numbered above the exonic structure (i.e., exon 1 encodes amino acids 1 to 42 of the P2X7R protein, exon 2 encodes amino acids 43 to 99, and so on). The corresponding P2X7A subunit regions that are translated from the mRNA are illustrated below the exonic structure with the colours aligning to the regions shown in (A). (C) Domains of interest located within the carboxy terminal of the P2X7A subunit including the C rich domain, actin filament binding domain, SH3 binding domain, TNFR1 death domain, trafficking domain, and LPS domain. Structural model produced using PyMOL software version 2.5.2 [23].

Figure 4

Ribbon representation of the heterotrimeric (P2X7A)₂/P2X7B receptor. The P2X7B subunit is shown in green, lacking the carboxy terminal as highlighted by the green circle, while the P2X7A subunits are shown in pink and blue. Molecular surface representations of the P2X7A carboxy termini are shown in pink and blue. The structural model has been produced using PyMOL software version 2.5.2 [23].