Nicotinic receptor agonists and antagonists increase sAPPα secretion and decrease Aβ levels in vitro

We have earlier reported that Aβ were significantly reduced in brains of smoking Alzheimer patients and control subjects compared with non-smokers, as well as in nicotine treated APPsw transgenic mice. To examine the mechanisms by which nicotine modulates APP processing we here measured levels of secreted amyloid precursor protein (sAPPα), total sAPP, Aβ40 and Aβ42 in different cell lines expressing different nicotinic receptor (nAChR) subtypes or no nAChRs. Treatment with nicotine increased release of sAPPα and at the same time lowered Aβ levels in both SH-SY5Y and SH-SY5Y/APPsw cells expressing α3 and α7 nAChR subtypes. These effects could also be evoked by co-treatment with the competitive α7 nAChR antagonists α-bungarotoxin and methyllycaconitine (MLA), and by these antagonists alone, suggesting that binding to the agonist binding site, rather than activation of the receptor, may be sufficient to trigger changes in APP processing. The nicotine-induced increase in sAPPα could only be blocked by co-treatment with the open channel blocker mecamylamine. In addition to nicotine, the agonists epibatidine and cytisine both significantly increased the release of sAPP in M10 cells expressing the α4/β2 nAChR subtype, and this effect was blocked by co-treatment with mecamylamine but not by the α4/β2 competitive antagonist dihydro-β-erythroidine. The lack of effect of nicotine on sAPPα and Aβ levels in HEK 293/APPsw cells, which do not express any nAChRs, demonstrates that the nicotine-induced attenuation of β-amyloidosis is mediated by nAChRs and not by a direct effect of nicotine. Our data show that nicotinic compounds stimulate the non-amyloidogenic pathway and that α4 and α7 nAChRs play a major role in modulating this process. Nicotinic drugs directed towards specific nAChR subtypes might therefore be beneficial for the treatment of AD not only by lowering Aβ production but also by enhance release of neuroprotective sAPPα.