Gating of nicotinic ACh receptors: latest insights into ligand binding and function

Abstract

Nicotinic acetylcholine receptors (nAChRs) are in the superfamily of cys-loop receptors, and are widely expressed in the nervous system where they participate in a variety of physiological functions, including regulating excitability and neurotransmitter release, as well as neuromuscular contraction. Members of the cys-loop family of receptors, which also includes the molluscan ACh-binding protein (AChBP), a soluble protein that is analogous to the extracellular ligand-binding domain of the cys-loop receptors, are pentameric assemblies of five subunits, with each subunit arranged around a central pore. The binding of ACh to the extracellular interface between two subunits induces channel opening. With the recent 4 A resolution of the Torpedo nAChR, and the crystal structure of the AChBP, much has been learned about the structure of the ligand-binding domain and the channel pore, as well as major structural rearrangements that may confer channel opening, including a major rearrangement of the C-loop within the ligand binding pocket, and perhaps other regions including the F-loop (the beta8-beta9 linker), the beta1-beta2 linker and the cys-loop. Here I will review the latest findings from my lab aimed at a further understanding of the function of the neuronal nAChR channels (and in particular the role of desensitization), and our search for novel AChBP species that may lead to a further understanding of the function of the cys-loop receptor family.

Jerrel Yakel received his BS from Oregon State University, and his PhD from the University of California, Los Angeles, where he studied ligand-gated ion channels and serotonin receptors in cultured hippocampal neurons and cell lines with Meyer Jackson. During a postdoctoral fellowship with Hersch Gerschenfeld at the Ecole Normale Superieure (Paris, France), he investigated the regulation of voltage-gated calcium channels by G protein-coupled receptors. During a second postdoctoral stage at the Vollum Institute with Alan North and Tom Soderling, he studied the function of regulation of ligand-gated ion channels. He joined NIEHS as an investigator in 1993, and is currently a Senior Investigator in the Laboratory of Neurobiology. His laboratory explores the function and regulation of ligand-gated ion channels, in particular the neuronal nicotinic receptor channels, in the hippocampus.

Figure 1. Molecular model of the rat α7 nAChR

A, a side view of the pentameric α7 receptor model is shown. The model was developed based on sequence alignment with the cryoelectron microscopy structure of the T. marmorata nAChR as indicated in Gay et al. (2007). The α helices are shown in red and the β strands in blue. The extracellular ligand binding domain is shown close up in B. The ligand binding site is composed of a cluster of aromatic residues from both the principal and complementary subunits and is capped by the C-loop. The transition domain consists of several loops including: Cys-loop, β1–β2 linker, β8–β9 linker, β10–M1 linker and the M2–M3 linker. C, a top-down view of the α7 receptor model.

Figure 2

The rate of α7 nAChR desensitization is slowed by the W55A mutation A, representative inward current responses due to the rapid and sustained application of ACh (1 mm; indicated by the horizontal bar) for wildtype (left) and W55A mutant α7 receptor (right) expressed in Xenopus oocytes. B, a close-up of the ligand binding domain of the α7 receptor highlighting the tryptophan 55 (W55) that is within the β2 strand across from the C-loop.