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. 2013 May 7:4:55.
doi: 10.3389/fphar.2013.00055. eCollection 2013.

Insights into the Molecular Mechanisms Underlying Mammalian P2X7 Receptor Functions and Contributions in Diseases, Revealed by Structural Modeling and Single Nucleotide Polymorphisms

Lin-Hua Jiang  1 , Jocelyn M Baldwin, Sebastien Roger, Stephen A Baldwin
Affiliations

Insights into the Molecular Mechanisms Underlying Mammalian P2X7 Receptor Functions and Contributions in Diseases, Revealed by Structural Modeling and Single Nucleotide Polymorphisms

Lin-Hua Jiang et al. Front Pharmacol. .

Abstract

The mammalian P2X7 receptors (P2X7Rs), a member of the ionotropic P2X receptor family with distinctive functional properties, play an important part in mediating extracellular ATP signaling in health and disease. A clear delineation of the molecular mechanisms underlying the key receptor properties, such as ATP-binding, ion permeation, and large pore formation of the mammalian P2X7Rs, is still lacking, but such knowledge is crucial for a better understanding of their physiological functions and contributions in diseases and for development of therapeutics. The recent breakthroughs in determining the atomic structures of the zebrafish P2X4.1R in the closed and ATP-bound open states have provided the long-awaited structural information. The human P2RX7 gene is abundant with non-synonymous single nucleotide polymorphisms (NS-SNPs), which generate a repertoire of human P2X7Rs with point mutations. Characterizations of the NS-SNPs identified in patients of various disease conditions and the resulting mutations have informed previously unknown molecular mechanisms determining the mammalian P2X7R functions and diseases. In this review, we will discuss the new insights into such mechanisms provided by structural modeling and recent functional and genetic linkage studies of NS-SNPs.

Keywords: ATP-binding; NS-SNPs; P2X7R; extracellular ATP; ion channel; large pore; structural modeling.

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Figures

Figure 1
Figure 1
Sequence comparison of the zebrafish P2X4.1R and mammalian P2X7Rs. The amino acid sequences of the mammalian P2X7Rs are aligned with the sequences of the truncated zebrafish P2X4.1Rs with determined atomic structures (Hattori and Gouaux, 2012). The ΔP2X4.1-B2 (B2) consisting of Ser28-Phe381 with changes at three positions (C51F, N78K, and N187R) and the ΔP2X4.1-C (C) Ser28-Lys365 with changes at two positions (N78K and N187R) were used to determine the structures in the closed (PDB accession number: 4DW0) and ATP-bound open states (4DW1), respectively. The species abbreviations are: H, human; MM, macaque monkey; D, dog; R, rat; M, mouse; GP, guinea pig. The sequences in italics are absent in the atomic structures or structural models. The same color scheme is used to indicate the different domains in the human P2X7R sequence here and the dolphin-shaped single subunit structural models shown in the following figures. The nine residues in the mammalian P2X7Rs (10 in the zebrafish P2X4.1R, including Leu217) involved in forming the inter-subunit ATP-binding site (shown in Figure 3B) are highlighted with six residues from one subunit in pink and another three residues from the complementary subunit in cyan (shown in Figure 4). The residues in green are present in the structural regions surrounding the ATP-binding site (also shown in Figure 4), changes of which alter the agonist sensitivity or other functional properties of the P2X7Rs (see text for details). The residues contributing to the transmembrane ion-conducting pathway (shown in Figure 5B) and residues in the extracellular lateral fenestrations are indicated in blue. The underlined residues in the human and mouse P2X7Rs (shown in Figure 6) are mutated by NS-SNPs to the residues shown underneath.
Figure 2
Figure 2
The dolphin-like architectures of the zebrafish P2X4.1R and human P2X7R subunits. (A,B) The structures of the zebrafish P2X4.1R subunit in the closed (A) (PDB accession number: 4DW0) and ATP-bound open states (B) (4DW1), respectively. (C,D) The structural models of the human P2X7R subunit in the closed (C) and open states (D) generated based on the structures of the zebrafish P2X4.1R (4DW0 and 4DW1, respectively). The overall architecture of single P2XR subunit is analogous to the shape of a leaping dolphin, with the extracellular and TM domains akin to the body and the tail, respectively. The different domains are shown in colors; tail in green, lower body in cyan, upper body in blue, head in purple, dorsal fin in orange, right flipper in red, and left flipper in yellow (the same color scheme as used in Figure 1). The five conserved disulfide bonds are shown in white. The dotted circles (C) highlight the major differences in the human P2X7R subunit compared with the zebrafish P2X4.1R subunit, and the solid circles the major differences between the closed and open states of the same receptors (B,D).
Figure 3
Figure 3
The chalice-like structural models of the human P2X7R. The structural models of the trimeric human P2X7R in the closed (A) and ATP-bound open state (B), based on the structures of the zebrafish P2X4.1R (4DW0 and 4DW1, respectively), are viewed parallel to the plasma membrane (top) or from the extracellular side of the membrane (bottom). Each subunit is shown in a different color. Three ATP molecules shown in space filling representation bind to the three inter-subunit interfaces. The circle [top in (B)] shows one of the three lateral fenestrations for ions to enter or exit from the transmembrane ion-conducting pathway in the open state.
Figure 4
Figure 4
The inter-subunit ATP-binding site in the human P2X7R. Binding of ATP to the inter-subunit site in the human P2X7R in the open state involves nine residues from two adjacent subunits. The subunit-B contributes four hydrophilic residues (Lys64, Lys66, Thr187, and Lys197) and a further two hydrophobic residues (Leu191 and Ile228) shown in magenta, and the complementary subunit-A provides another three hydrophilic residues (Gln292, Arg294, and Lys311) in cyan. The circle shows the approximate boundary of the ATP-binding site. Changes of residues shown in green alter the agonist sensitivity or ion channel gating of the mammalian P2X7Rs with the exception of Arg125, which is the ADP-ribosylation site.
Figure 5
Figure 5
The transmembrane ion-conducting pathway in the human P2X7R. The arrangements of the transmembrane domains (TM1s and TM2s) in the structural models of the human P2X7R in the closed (A) and open states (B), viewed from the intracellular side of the membrane. The three TM1 helices are located at the periphery and the three TM2 helices in the center form the ion-conducting pathway. Val335 and Ser342 represent the extracellular and intracellular ends of the gate that restricts the ion flow in the closed state (A). Val335, Ser339, Ser342, Leu346, Ala348, Phe350, and Asp352 form the ion-conducting pathway in the open state (B). The circles in brown illustrate the change in size of the ion-conducting pathway from the closed to the open state.
Figure 6
Figure 6
Location of residues mutated by NS-SNPs in the extracellular and TM domains. The residues in the extracellular and TM2 domains that are mutated by NS-SNPs are shown in the structural models of the human P2X7R subunit in the closed (A) and open states (B). The same color scheme is used to indicate the different domains as in other figures. The residue at position 283 is mutated by NS-SNP in the mouse P2X7R.
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