Nicotine Acts as a Pharmacological Chaperone to Up-Regulate Human α4β2 Acetylcholine Receptors

Abstract

Human neuronal nicotinic acetylcholine receptor (AChR) alpha4 subunits and an alpha4 mutant (S247Falpha4) found in autosomal-dominant nocturnal frontal lobe epilepsy (ADNFLE) were expressed along with beta2 in permanently transfected tsA201 human embryonic kidney cell lines. Their sensitivity to activation, desensitization, and up-regulation by cholinergic ligands was investigated. Up-regulation after 3 to 24 h resulted primarily from an increase in assembly of AChRs from large pools of unassembled subunits, but up-regulation also resulted from a 5-fold increase in the lifetime of AChRs in the surface membrane. Up-regulation does not require current flow through surface membrane AChRs, because up-regulation occurs in the presence of the channel blocker mecamylamine and with the alpha4 mutant, which prevents nearly all AChR function. Both membrane-permeable ligands like nicotine and much less permeable quaternary amine cholinergic ligands can act as pharmacological chaperones within the endoplasmatic reticulum to promote the assembly of AChRs. Agonists are more potent pharmacological chaperones than antagonists, presumably because activated or desensitized conformations assemble more efficiently. Assembly intermediates are disrupted by solubilization in Triton X-100, but chemical cross-linking stabilizes a putative assembly intermediate approximately the size of an alpha4beta2alpha4beta2 tetramer.

The Transmembrane Domain Mediates Tetramerization of α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid (AMPA) Receptors

“…Biogenesis and assembly of nAChR are enhanced by the binding of nicotine (43,44). Similarly, binding of glutamate, which exists in the endoplasmic reticulum at high concentrations (45), could influence AMPAR assembly as well, albeit negatively.…”

Unraveling the High- and Low-Sensitivity Agonist Responses of Nicotinic Acetylcholine Receptors

“…Expressing ␣4 and ␤2 subunits in equal amounts in oocytes gives rise to a receptor population, resulting in biphasic concentration-response curves to the endogenous ligand acetylcholine (ACh) and other agonists. The different agonist sensitivities are clearly related to the subunit stoichiometry, and, based on studies with different ␣4:␤2 expression ratios and concatenated subunits Zhou et al, 2003;Kuryatov et al, 2005;Moroni et al, 2006;Zwart et al, 2006;Carbone et al, 2009), the high-and low-sensitivity components of the biphasic curves have been ascribed to the (␣4) 2 (␤2) 3 and (␣4) 3 (␤2) 2 receptor subtypes, respectively. Although mixtures of different stoichiometries can explain biphasic concentration-response curves, the lack of a detailed explanation for the different agonist sensitivities has made interpretation of certain experimental data difficult and has hampered rational drug design.…”

“…Both subtypes may be pharmacologically important because they seem to be present in native brain (Marks et al, 2007) and they display significantly different physiological properties Kuryatov et al, 2005;Moroni et al, 2006Moroni et al, , 2008Tapia et al, 2007). Expressing ␣4 and ␤2 subunits in equal amounts in oocytes gives rise to a receptor population, resulting in biphasic concentration-response curves to the endogenous ligand acetylcholine (ACh) and other agonists.…”

Unfolding the therapeutic potential of chemical chaperones for age-related macular degeneration

“…Another example is nicotine, a membrane-permeable ligand of human neuronal nicotinic acetylcholine receptors (AChR), which can act as a pharmacological chaperone to promote AChR assembly in the ER. Less permeable quaternary amine cholinergic ligands were also found to have chaperone-like function [37]. The role of chaperone therapy in the treatment of diseases characterized by ER stress but not a single, mutated and misfolded protein is further supported by in vivo studies demonstrating the effectiveness of chemical chaperones in the treatment of diabetes and hypoxic injury [31,34,35,38].…”