Structural and Functional Basis for Understanding the Biological Significance of P2X7 Receptor

Abstract

The P2X7 receptor (P2X7R) possesses a unique structure associated to an as yet not fully understood mechanism of action that facilitates cell permeability to large ionic molecules through the receptor itself and/or nearby membrane proteins. High extracellular adenosine triphosphate (ATP) levels-inexistent in physiological conditions-are required for the receptor to be triggered and contribute to its role in cell damage signaling. The inconsistent data on its activation pathways and the few studies performed in natively expressed human P2X7R have led us to review the structure, activation pathways, and specific cellular location of P2X7R in order to analyze its biological relevance. The ATP-gated P2X7R is a homo-trimeric receptor channel that is occasionally hetero-trimeric and highly polymorphic, with at least nine human splice variants. It is localized predominantly in the cellular membrane and has a characteristic plasticity due to an extended C-termini, which confers it the capacity of interacting with membrane structural compounds and/or intracellular signaling messengers to mediate flexible transduction pathways. Diverse drugs and a few endogenous molecules have been highlighted as extracellular allosteric modulators of P2X7R. Therefore, studies in human cells that constitutively express P2X7R need to investigate the precise endogenous mediator located nearby the activation/modulation domains of the receptor. Such research could help us understand the possible physiological ATP-mediated P2X7R homeostasis signaling.

Keywords: ATP; P2X7 receptor; P2X7 receptor physiological role; allosteric modulations; channel membrane proteins; human P2X7 receptor isoforms.

Figure 1

Topology of the P2X7 receptor (P2X7R). (A) Secondary structure of the P2X7R monomer with five domains; extracellular, transmembrane 1 (TM1), transmembrane 2 (TM2), intracellular N-termini, and intracellular C-termini. The colors into the domains represent possible specific molecular recognition sites. (B) Co-assembly of the three P2X7R monomers (purple, blue, and orange) position into the trimeric receptor showing possible surrounding molecules that could interact with the different domains, such as intracellular messengers at the N- and/or C- termini (green), membrane residues at the transmembrane domains (blue), and the endogenous agonist adenosine triphosphate (ATP, yellow) and/or other allosteric modulators (red) in the extracellular domain. (C) The P2X7 monomer represented as secondary structure (left) and molecular surface (right) styles resembling a dolphin. (D) Top view of the P2X7R structure embracing the three extracellular ATP-binding pockets (orange dashed lines) and the three additional drug-binding pockets (black dashed lines).

Figure 2

Schematic illustration of the main membrane proteins involved in the regulation of the extracellular adenosine triphosphate (ATP) levels required for P2X7 receptor (P2X7R) function. The P2X7R activation requires large ATP release through nearby membrane structures such as the pannexin channel, but the precise balance of the extracellular ATP concentrations is also dependent on the activity and membrane localization of the enzymes CD39 (NTPDase-1), which dephosphorylates ATP to adenosine diphosphate (ADP) and adenosine monophosphate (AMP), and CD73 (ecto-5′ecto nucleotidase), which turns AMP into adenosine.

Figure 3

Schematic illustrations of the hypothetical states of the P2X7 receptor (P2X7R) activation. (A) Resting state of the P2X7R where the agonist (adenosine triphosphate, ATP) is not present or exists in insufficient concentrations, so that the conformation of the receptor does not open its own ion channel (closed channel). (B) Initial activation of the receptor within milliseconds by the three molecules of ATP, inducing a selective channel opening for small cations (open channels). (C) This initial state is followed by a prolonged phase within seconds to a minute of enlargement of the receptor itself (pore enlargement), allowing the flux of non-selective large ions such as Yo-Pro-1. (D) This prolonged phase of activation can involve other membrane proteins.

Figure 4

Schematic pathway of the role of the P2X7 receptor (P2X7R) in cell damage signaling. In response to a severe damage or an insult signal, the cell releases a huge amount of adenosine triphosphate (ATP) by different mechanisms (from ATP dense granules, ATP channels, …) and the extracellular concentration of ATP reached could be enough to activate the P2X7R, facilitating a flux of large ions and the activation of the inflammasome. The P2X7R/inflammasome complex induces the synthesis of IL-1β and IL-18, also regulated by NF-κB. This hypothesis agrees with the role of the P2X7R in processes such as inflammation, apoptosis, and necrosis.

Figure 5

Schematic pathway of the hypothetical role of the P2X7 receptor (P2X7R) in homeostasis balance. In physiological situations, the specific localization of the P2X7R in the membrane, near finetuned adenosine triphosphate (ATP)-released structures and distant from ATP-degrading enzymes, could provide a small extracellular concentration of ATP which are not enough for the activation of the receptor. However, this activation can take place in the presence of a positive allosteric modulator (M) binding to different domains of the receptor. This will facilitate the selective flux of ions through the P2X7R channel, thus inducing a transduction pathway which could lead to a cellular homeostasis response without inducing necessarily cellular damage.