Insights into the channel gating of P2X receptors from structures, dynamics and small molecules

Abstract

P2X receptors, as ATP-gated non-selective trimeric ion channels, are permeable to Na(+), K(+) and Ca(2+). Comparing with other ligand-gated ion channel families, P2X receptors are distinct in their unique gating properties and pathophysiological roles, and have attracted attention as promising drug targets for a variety of diseases, such as neuropathic pain, multiple sclerosis, rheumatoid arthritis and thrombus. Several small molecule inhibitors for distinct P2X subtypes have entered into clinical trials. However, many questions regarding the gating mechanism of P2X remain unsolved. The structural determinations of P2X receptors at the resting and ATP-bound open states revealed that P2X receptor gating is a cooperative allosteric process involving multiple domains, which marks the beginning of the post-structure era of P2X research at atomic level. Here, we review the current knowledge on the structure-function relationship of P2X receptors, depict the whole picture of allosteric changes during the channel gating, and summarize the active sites that may contribute to new strategies for developing novel allosteric drugs targeting P2X receptors.

Figure 1

The three-dimensional architectures of the zfP2X4 receptor viewed from the extracellular side (upper), parallel to the membrane (middle) and the intracellular side (lower) at resting (left, PDB ID code: 3H9V) and ATP-bound open (right, PDB ID code: 4DW1) states. The red dashed line indicates the distance between the N9 atom of the purine ring of ATP and the C_α atom of A347. The black dashed arrow indicates the ion influx pathway. Subunits A, B and C are colored by green, magenta and cyan cartoon, respectively. ATP molecules are shown as spheres. Red dashed circles indicate the boundary of the upper vestibule or the gate. All figures were made with PyMol (http://www.pymol.org).

Figure 2

ATP-induced conformational changes of zfP2X4 receptors. (A) The P2X4 subunit has a dolphin-like shape. Distinctive body parts are shown in different colors. (B) Superposition of a single P2X4 subunit at resting (green) and open (red) states. (C–I) Superposition of head (C), dorsal fin (D), left flipper (E), right flipper (F), upper body (G), lower body (H) and fluke (I) domains at resting (green) and open (red) states. The grey arrows indicate the conformational changes after ATP binding.

Figure 3

Amino residues involved in small molecule recognition of P2X receptors.

Figure 4

Bound ATP-evoked allosteric changes associated with channel opening in the P2X4 receptor. (A–D) Bound-ATP evoked structural rearrangements at the ATP binding site (A), body domain (B), TM region (C) and overall structure of zfP2X4 receptors (D). The dark-blue arrows indicate the conformational changes after ATP binding. Structures at the resting and open states of zfP2X4 receptors are displayed in green and red, respectively.