M1 muscarinic acetylcholine receptor interacts with BACE1 and regulates its proteosomal degradation

Abstract

A prime culprit in the pathogenesis of Alzheimer's disease (AD) is overproduction/aggregation of β-amyloid (Aβ), which is derived from β-Amyloid Precursor Protein through sequential cleavages by β-site APP cleaving protein 1 (BACE1) and γ-secretase. The level/activity of BACE1 is elevated in sporadic AD and identification of proteins that affect BACE1 is important in AD research. Here we found that M1 Muscarinic acetylcholine receptor (M1 mAChR), an important G protein-coupled receptor involved in cholinergic neuronal activity, can interact with BACE1 and mediate its proteosomal degradation. Moreover, overexpression and downregulation of M1 mAChR can decrease and increase the levels of BACE1, as well as the generation of Aβ, respectively. These findings point to a novel coupling of BACE1 and M1 mAChR in AD and possibly schizophrenia.

Introduction

One important pathological hallmark of Alzheimer's disease (AD) is the formation of senile plaques in the brain, whose major components are aggregated β-amyloid (Aβ) peptides that are derived from the β-amyloid precursor protein (APP) through sequential cleavages by β- and γ-secretases [24]. Multiple lines of evidence suggest that overproduction/aggregation of Aβ is a primary cause of AD, since Aβ is neurotoxic and can trigger a cascade of neurodegenerative steps ending in the formation of senile plaques and intra-neuronal fibrillary tangles that provoke subsequent neuronal loss [9]. In addition to the amyloidogenic processing by β- and γ-secretases, APP can be alternatively cleaved by α-secretase within the Aβ domain to release the soluble ectodomain of APP (sAPPα) that is neuroprotective [8], [16].

The putative β-secretase is β-site APP-cleaving enzyme 1 (BACE1), also known as Asp2 or memapsin 2 [20], [22], [23]. BACE1 is a membrane-bound aspartyl protease with a characteristic type I transmembrane domain near the C-terminus [20], [22]. Its precursor, pro-BACE1, can be cleaved by a furin-like endoprotease and modified by glycosylation to produce mature BACE1 [1]. Optimal BACE1 activity requires an acidic environment and the major cellular compartments in various pre-mitotic cell lines overexpressing exogenous BACE1 include the early Golgi, late Golgi/early endosomes, and endosomes. In addition, BACE1 is found at the cell surface [13], [14]. During its degradation, BACE1 is ubiquitinated and subjected to the proteasome pathway [25]. Since BACE1 activity is thought to be the rate-limiting factor in Aβ generation, the identification of factors that modulate the level, trafficking, and activity of BACE1 is important in AD research.

Muscarinic acetylcholine receptors (mAChRs) are members of the G protein-coupled receptor superfamily. There are five identified mAChR subtypes, M1–M5. One of them, M1 mAChR, has been found involved in cognitive processing relevant to AD and memory [3], [17]. Cholinergic stimulation of M1 mAChR can activate α-secretase, increasing sAPPα levels and preventing the formation of Aβ via MAPK- and PKC-dependent pathways [2], [10]. Furthermore, stimulation of M1 mAChR decreases tau protein phosphorylation [7], [19]. Since M1 mAChR is found to be associated with important hallmarks of AD, it has been presumed to be a pivotal target in AD treatment [3], [6]. In addition to α-secretase, BACE1 has been found to be regulated by M1 mAChR but the results are controversial: one study showed that application of an M1 mAChR agonist in vivo resulted in a decreased BACE1 level [4], whereas another study reported that an M1 mAChR agonist treatment in cell culture increased BACE1 expression, possibly through the MEK/ERK pathway [26]. Herein, we carried out additional studies to determine the correlation between M1 mAChR and BACE1.