Role of Key Aromatic Residues in the Ligand-binding Domain of α7 Nicotinic Receptors in the Agonist Action of β-Amyloid

Mei Tong,Michael M. White,Komal Arora,Robert A. Nichols

doi:10.1074/jbc.m111.241299

Mei Tong, Michael M. White + Show 2 more

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https://doi.org/10.1074/jbc.m111.241299

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Abstract

Soluble β-amyloid (Aβ) resides in certain regions of the brain at or near picomolar concentration, rising in level during the prodromic stage of Alzheimer disease. Recently, we identified the homomeric α7 nicotinic acetylcholine receptor (α7-nAChR) as one possible functional target for picomolar Aβ. This study was aimed at addressing which residues in α7-nAChRs potentially interact with Aβ to regulate the presynaptic function of this receptor. Site-directed mutagenesis was carried out to study the key aromatic residues in the mouse α7-nAChR agonist-binding pocket. Mutations of tyrosine188 resulted in a decrease in activation of presynaptic α7-nAChRs by ACh and Aβ but with no change in response to nicotine, indicating the critical role of Tyr-188 in presynaptic regulation by Aβ. Coimmunoprecipitation additionally revealed direct binding of Aβ to α7-nAChRs and to the Tyr-188 mutant receptor. In contrast, mutations of Tyr-195 in α7-nAChR led to decreased activation by nicotine without apparent effects on ACh- or Aβ-induced responses. Agonist-induced responses of Tyr-93 mutant α7-nAChRs indicated possible interactions of nicotine and Aβ with its hydroxyl group, but there was no change in presynaptic responses after mutation of Trp-149. All of the mutants were shown to be expressed on the plasma membrane using cell surface labeling. Together, these results directly demonstrate an essential role for the aromatic residue Tyr-188 as a key component in the agonist binding domain for the activation of α7-nAChRs by Aβ.

Highlights

Alzheimer disease (AD)2 is a neurodegenerative disease characterized by distinct pathologies
NG108-15 was used as a model neuronal system for expression of nicotinic acetylcholine receptor (nAChR), as they are normally devoid of functional nAChRs
The axonal varicosities of the NG108-15 cells are presynaptic-like structures capable of synapsing on the appropriate postsynaptic target (26 –28), and expression of nAChRs in the varicosities serves as a reconstituted presynaptic system for examining the agonist-like action of A␤

Summary

Introduction

Alzheimer disease (AD) is a neurodegenerative disease characterized by distinct pathologies. Over the course of the disease, there is loss of synapses and neuronal cells in select regions of the brain, those critical for memory and cognitive processes. ␣7-nAChRs are highly expressed on the presynaptic inputs in the hippocampus and cortex, which are critical regions for synaptic plasticity and memory formation. A␤ may play distinct regulatory roles depending on the location of the nAChRs. In particular, nAChRs at presynaptic sites possibly mediate an agonist-like effect of pM-nM A␤. A␤ applied to NG108-15 varicosities expressing ␣7-nAChRs induced pronounced and sustained increases in Ca2ϩ, reconstituting the agonist-like effect of A␤ in presynaptic-like structures, and the response could be fully blocked by the selective ␣7-nAChR antagonist ␣-bungarotoxin [23]. The structural description of the mouse ␣7-nAChR ligand binding site was derived from that of the Torpedo nAChR (a muscle-type nAChR) and the ACh binding protein [24]

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