End-plate acetylcholine receptor: structure, mechanism, pharmacology, and disease

Abstract

The synapse is a localized neurohumoral contact between a neuron and an effector cell and may be considered the quantum of fast intercellular communication. Analogously, the postsynaptic neurotransmitter receptor may be considered the quantum of fast chemical to electrical transduction. Our understanding of postsynaptic receptors began to develop about a hundred years ago with the demonstration that electrical stimulation of the vagus nerve released acetylcholine and slowed the heart beat. During the past 50 years, advances in understanding postsynaptic receptors increased at a rapid pace, owing largely to studies of the acetylcholine receptor (AChR) at the motor endplate. The endplate AChR belongs to a large superfamily of neurotransmitter receptors, called Cys-loop receptors, and has served as an exemplar receptor for probing fundamental structures and mechanisms that underlie fast synaptic transmission in the central and peripheral nervous systems. Recent studies provide an increasingly detailed picture of the structure of the AChR and the symphony of molecular motions that underpin its remarkably fast and efficient chemoelectrical transduction.

FIGURE 1

A: electron micrograph of rat neuromuscular junction treated with peroxidase-conjugated α-bungarotoxin to label postjunctional AChRs. (From Engel et al. Neurology 27: 307–325, 1977, with permission.) B: time course of endplate potential and current. Horizontal bar, 2 ms; vertical bar, 40 nA (EPC) or 1 mV (EPP). (From Barrett EF and Magleby KL. Biology of Cholinergic Function, edited by A. M. Goldberg and I. Hanin. New York: Raven, 1976, p. 29–100, with permission.)

FIGURE 2

Patch-clamp recording of a single acetylcholine receptor (AChR) channel activation episode elicited by a low concentration of ACh applied to adult human endplate AChRs expressed in 293 HEK cells (S. M. Sine and N. Mukhtasimova, unpublished data). Channel openings are upward deflections. Membrane potential, −120 mV; Gaussian filter, 20 kHz.

FIGURE 3

Single-channel currents through adult human AChRs elicited by the indicated concentrations of ACh. The extended del Castillo and Katz mechanism was fitted simultaneously to the global set of closed and open dwell times for the indicated concentrations of ACh. [From Mukhtasimova et al. (184).]

FIGURE 4

Progression of AChR atomic structure determination. A: Torpedo AChR at 9 Å resolution obtained by cryo-EM (270), with permission from N. Unwin. B: crystal structure of AChBP at 2.7 Å resolution (32; PDB code 1I9B). C: Torpedo AChR at 4 Å resolution (271; PDB code 2BG9). D: GLIC at 3.3 Å resolution (PDB code 3EHZ).

FIGURE 5

Interface dividing ligand binding and pore domains of the α-subunit from a homology model of the human AChR generated using the Torpdeo AChR as a template (276). Consecutive residues are highlighted in a single color and labeled. In the left panel, the pore runs vertically along the left side, and in the right panel, the pore is just beneath the β1-β2 loop coming out of the plane of the page.

FIGURE 6

Ligand binding domain of a homology model of the adult human AChR [276] highlighting the subunit interface and seven discontinuous loops A–C at the principal face and D–G at the complementary face. Each loop is highlighted in a single color.

FIGURE 7

Secondary structure of the ligand binding domain of the human AChR α-subunit. The two views differ by 90° rotation about an axis passing vertically through the domain.

FIGURE 8

Aromatic residues (space-filling) that form the ligand binding pocket of AChBP (PDB code 1I9B) with acetylcholine (ball and stick) docked to the site (93).

FIGURE 9

Rings of charged or polar residues along the ion permeation pathway through a homology model of the adult human AChR (276). The α-subunit is blue. Two subunits facing the viewer are removed for clarity.

FIGURE 10

Interface dividing ligand binding and pore domains in the α-subunit from the Torpedo AChR (PDB code 2BG9) showing energetically coupled residues from the principal (155) (top) and secondary (153) (bottom) coupling pathways.

FIGURE 11

Key residues from the ligand binding domain that juxtapose the pore domain: comparison of the AChR α1 crystal structure (PDB code 2QC1; Ref. 73), GLIC (PDB code 3EHZ; Ref. 118), and ELIC (PDB code 2VI0; Ref. 119).

FIGURE 12

Comparison of AChBP ligand binding site without (yellow ribbons) and with (blue ribbons) bound ACh following prolonged all atom MD simulation (93). ACh is shown in space-filling representation.

FIGURE 13

Steered MD simulation of human AChR homology model beginning with the apo conformation (aromatic residues in red), using PDB code 2BG9 as a template, and ending with the agonist bound conformation (aromatic residues in yellow), using PDB code 1UV6 as a template. Arrows indicate direction of force application. Large inset shows TMD2s before (blue) and after (yellow) simulation. [From Wang et al. (280).]

FIGURE 14

Structures of AChR agonists and antagonists.

FIGURE 15

Orientation of epibatidine bound to Aplysia AChBP (PDB code 2BYQ; Ref. 110). Left: principal and complementary faces are shown as ribbons, while epibatidine is shown as space-filling. Right: residues of closest approach in AChBP are shown as space-filling (Trp143, Tyr185, Tyr192, Trp53, Met114) and epibatidine as ball and stick.

FIGURE 16

Orientation of d-tubocurarine bound to AChBP (92). Left: principal subunit is yellow and the complementary subunit blue, while d-tubocurarine is shown as space-filling. Right: key contact residues are shown as space-filling (Y192, Y89, W143, M114, L112), while d-tubocurarine is shown as ball and stick.

FIGURE 17

Orientation of α-conotoxin ImI bound to Aplysia AChBP (PDB code 2BYP; Ref. 110). Left: principal and complementary faces are shown as ribbons, while α-conotoxin ImI is shown as space-filling. Right: key AChBP determinants of affinity, based on mutagenesis analyses of α7 AChRs (213, 215), are shown as space-filling (Tyr91, Tyr186,Tyr193, Asp195, Met114, Arg77), while key determinants of α-conotoxin ImI affinity (216) are shown as ball and stick (Asp5, Pro6, Arg7, Trp10).

FIGURE 18

Orientation of cobra α-neurotoxin bound to Lymnaea AChBP (PDB code 1YI5; Ref. 29). Left: principal (yellow) and complementary (blue) faces of AChBP and cobra α-neurotoxin (green) are shown as ribbons. Right: close-up view of the complex, with residues in finger II of α-neurotoxin shown as ball and stick (Trp25, Asp27, Phe29, Arg33, Arg36) and residues of the aromatic binding pocket of AChBP (Trp143, Tyr185, Tyr192, Trp53) shown as space-filling.

FIGURE 19

Overview of congenital myasthenic syndromes due to mutations in AChR transmembrane domains. Top panel compares time courses of miniature endplate currents from control (black), fast channel (green), and slow channel (red) endplates [From Engel et al. (84)]. Bottom panel shows a homology model of the AChR α-subunit (based on PDB code 2BG9) as space-filling, with TMD2 highlighted in blue, TMD3 in magenta, and TMD1 in orange. Identified CMS mutations are indicated and shown with side chains white.

FIGURE 20

First identified slow channel CMS. Left: Subunit TMDs are shown with the location of the mutant residue εT264 indicated by arrow. Right: compares single-channel currents for wild-type and mutant AChRs (channel openings are upward deflections). [From Ohno et al. (199).]

FIGURE 21

Fast channel CMS due to mutation at the entrance to the α-ε binding site (246). Left: extracellular domains of the α- and ε-subunits are shown with the mutant residue εAsp175 in standard atom colors. Right: compares single-channel currents for wild-type and mutant AChRs (channel openings are upward deflections). [From Sine et al. (246).]

FIGURE 22

Fast channel CMS due to a mode-switching mutation in the intracellular domain of the ε-subunit (279). Left: a homology model of the AChR (based on PDB code 2BG9) in ribbon representation with regions in which CMS localize highlighted in white space-filling representation. Right: compares individual episodes of single-channel currents for wild-type and ε411P AChRs. [From Wang et al. (279).]