Deletion of M1 Muscarinic Acetylcholine Receptors Increases Amyloid Pathology In Vitro and In Vivo

Alzheimer's disease (AD) is a progressive neurological disorder that causes dementia and poses a major public health crisis as the population ages. Aberrant processing of the amyloid precursor protein (APP) is strongly implicated as a proximal event in AD pathophysiology, but the neurochemical signals that regulate APP processing in the brain are not completely understood. Activation of muscarinic acetylcholine receptors (mAChRs) has been shown to affect APP processing and AD pathology, but less is known about the roles of specific mAChR subtypes. In this study, we used M-1 mAChR knock-out mice (M1KO) to isolate the effects of the M-1 mAChR on APP processing in primary neurons and on the development of amyloid pathology in a transgenic mouse model of AD. We demonstrate that the loss of M-1 mAChRs increases amyloidogenic APP processing in neurons, as evidenced by decreased agonist-regulated shedding of the neuroprotective APP ectodomain APPs alpha and increased production of toxic A beta peptides. Expression of M-1 mAChRs on the M1KO background rescued this phenotype, indicating that M-1 mAChRs are sufficient to modulate nonamyloidogenic APP processing. In APP(Swe/Ind) transgenic mice, the loss of M-1 mAChRs resulted in increased levels of brain A beta and greater accumulation of amyloid plaque pathology. Analysis of APP metabolites in APP(Swe/Ind) brain tissue indicates that the loss of M-1 mAChRs increases amyloidogenic APP processing. These results indicate that the M-1 mAChR is an important regulator of amyloidogenesis in the brain and provide strong support for targeting the M-1 mAChR as a therapeutic candidate in AD.