Serine residues in the α4 nicotinic acetylcholine receptor subunit regulate surface α4β2* receptor expression and clustering.

Chronic nicotine exposure upregulates α4β2* nicotinic acetylcholine receptors (nAChRs) in the brain. The goal of this study was to examine the role of three serine residues in the large cytoplasmic loop of the α4 subunit on α4β2* upregulation in neurons. Serine residues S336, S470 and S530 in mouse α4 were mutated to alanine and then re-expressed in primary neurons from cortex, hippocampus and subcortex of α4 KO mice. Mutant and wild type α4 expressing neurons were treated with nicotine (0.1, 1 and 10 μM) and assessed for α4β2* upregulation. α4β2* nAChRs expressing S336A or S470A mutants were deficient at cell surface upregulation in both subcortex and hippocampal neurons. S530A α4β2* mutants exhibited aberrant surface upregulation in subcortical neurons. None of the mutants affected surface upregulation in cortical neurons or upregulation of total α4β2* binding sites in any region. Further, dense domains or clusters of α4β2* nAChRs were observed in the neuronal surface. The impact of nicotine exposure on the intensity, area, and density of these clusters was dependent upon individual mutations. Effects of α4 nAChR mutants on surface upregulation varied among brain regions, suggesting that the cellular mechanism of α4β2* upregulation is complex and involves cellular identity. We also report for the first time that α4β2* nAChRs form clusters on the neuronal surface and that nicotine treatment alters the characteristics of the clusters in an α4 mutant-dependent manner. This finding adds a previously unknown layer of complexity to the effects of nicotine on α4β2* expression and function.