Galantamine-induced Amyloid-β Clearance Mediated via Stimulation of Microglial Nicotinic Acetylcholine Receptors

Reduction of brain amyloid-beta (A beta) has been proposed as a therapeutic target for Alzheimer disease (AD), and microglial A beta phagocytosis is noted as an A beta clearance system in brains. Galantamine is an acetylcholinesterase inhibitor approved for symptomatic treatment of AD. Galantamine also acts as an allosterically potentiating ligand (APL) for nicotinic acetylcholine receptors (nAChRs). APL-binding site is located close to but distinct from that for acetylcholine on nAChRs, and FK1 antibody specifically binds to the APL-binding site without interfering with the acetylcholine-binding site. We found that in human AD brain, microglia accumulated on A beta deposits and expressed alpha 7 nAChRs including the APL-binding site recognized with FK1 antibody. Treatment of rat microglia with galantamine significantly enhanced microglial A beta phagocytosis, and acetylcholine competitive antagonists as well as FK1 antibody inhibited the enhancement. Thus, the galantamine-enhanced microglial A beta phagocytosis required the combined actions of an acetylcholine competitive agonist and the APL for nAChRs. Indeed, depletion of choline, an acetylcholine-competitive alpha 7 nAChR agonist, from the culture medium impeded the enhancement. Similarly, Ca2+ depletion or inhibition of the calmodulin-dependent pathways for the actin reorganization abolished the enhancement. These results suggest that galantamine sensitizes microglial alpha 7 nAChRs to choline and induces Ca2+ influx into microglia. The Ca2+-induced intracellular signaling cascades may then stimulate A beta phagocytosis through the actin reorganization. We further demonstrated that galantamine treatment facilitated A beta clearance in brains of rodent AD models. In conclusion, we propose a further advantage of galantamine in clinical AD treatment and microglial nAChRs as a new therapeutic target.