Rare Missense Variants of the Human β4 Subunit Alter Nicotinic α3β4 Receptor Plasma Membrane Localisation.

α3β4 nicotinic acetylcholine receptors (nARs) are pentameric ligand-gated cation channels that function in peripheral tissue and in the peripheral and central nervous systems, where they are critical mediators of ganglionic synaptic transmission and modulators of reward-related behaviours. In the pentamer, two α3β4 subunit couples provide ligand-binding sites, and the fifth single (accessory) subunit (α3 or β4) regulates receptor trafficking from the endoplasmic reticulum to the cell surface. A number of rare missense variants of the human β4 subunit have recently been linked to nicotine dependence and/or sporadic amyotrophic lateral sclerosis, and altered responses to nicotine have been reported for these variants; however, it is unknown whether the effects of mutations depend on the subunit within the ligand-binding couples and/or on the fifth subunit. Here, by expressing single populations of pentameric receptors with fixed stoichiometry in cultured cells, we investigated the effect of β4 variants in the fifth position on the assembly and surface exposure of α3β4 nAChRs. The results demonstrate that the missense mutations in the accessory subunit alone, despite not affecting the assembly of α3β4 receptors, alter their trafficking and surface localisation. Thus, altered trafficking of an otherwise functional nAChR may underlie the pathogenic effects of these mutations.