Molecular and cell biological characterisation of neuronal nicotinic acetylcholine receptors
Molecular and cell biological characterisation of neuronal nicotinic acetylcholine receptors (nAChRs) provides an insight into their functional roles and potential as therapeutic targets for neurological disorders. Nicotinic receptors are oligomeric ligand-gated ion channels, comprising five subunits. Twelve vertebrate neuronal nAChR subunits (2-10 and 2-4) have been cloned to date, with considerable diversity observed in nAChR subunit composition. Heterologous expression of cloned subunits is a powerful method for investigating ion channel receptor pharmacology and subunit composition, but achieving efficient expression of some nAChRs in cultured cell lines has proved difficult. In this study, chimeras containing the N-terminal domain of the nAChR subunits, fused to the C-terminal region of the 5-hydroxtryptamine type 3 receptor subunit, 5HT3A, were constructed to overcome some of the challenges of recombinant nAChR expression. When combinations of wild-type and chimeric subunits were expressed in human embryonic kidney tsA201 cells, inclusion of nAChR/5HT3A chimeras enhanced the expression of nAChRs containing each of the 2, 3, 4, 6, 7, 9, 10, 2 or 4 nAChR subunits, determined by detection of radioligand binding sites. This was particularly significant for 6- or 9/10-containing nAChRs, as radioligand binding to wild-type nAChRs containing these subunits was not detected in tsA201 cells. A detailed pharmacological characterisation of receptors containing 9/5HT3A + 10/5HT3A chimeras in tsA201 cells via competition binding suggested that the chimeras provide suitable models for characterisation of wild-type nAChRs. Radioligand binding to intact cells and enzyme-linked assays to detect epitope-tagged subunits expressed in transfected cells, suggested that 9/5HT3A- containing receptors were expressed at the cell surface in high levels. Comparison of radioligand binding to nAChR subtypes containing combinations of wild-type and chimeric 2-6 and 2-4 subunits implicated the N- and C-terminal domains of both the alpha and beta type nAChR subunits in subunit oligomerisation events and provided an insight into nAChR assembly.