Expression of the alpha 7 subunit of the nicotinic acetylcholine receptor in normal and myasthenic human thymuses.

The nicotinic acetylcholine receptor (AChR) is a transmembrane glycoprotein composed of five homologous subunits. Different isoforms of the AChR alpha subunit exist (alpha 1 to alpha 9). Of them, alpha 1 is expressed in muscle, alpha 2 to alpha 9 in neuronal cells. Muscle AChR is the target autoantigen in the autoimmune disease myasthenia gravis (MG). The thymus is implicated in MG pathogenesis, and the anti-AChR autoimmune response may start in this tissue, that expresses the muscle-type alpha 1 subunit as well as other muscle AChR subunits. The thymus also expresses the "neuronal" alpha 3 and alpha 5 subunits. By using polymerase chain reaction and other molecular techniques, we demonstrate here expression of the AChR alpha 7 subunit transcript in thymuses from both myasthenic patients and normal subjects. The alpha 7 subunit can form homo-oligomeric functional AChR complexes that, like muscle AChR, bind alpha-bungarotoxin. The demonstration of expression of the alpha 7 subunit in the thymus suggests that alpha-bungarotoxin binding, functional AChRs of the neuronal type are normally present in the thymus.