The Tacrine-Induced Endoplasmic Reticulum Stress in AChE-Expressed Cells Leads to Improper Assembly and Transport of the Oligomeric Enzyme: Reversal by Trehalose

Abstract

Alzheimer's disease (AD) is the most common dementia with progressive loss of cognitive functions. Acetylcholinesterase (AChE) inhibitors have been approved as conventional pharmacotherapies for AD. Tacrine was the first AChE inhibitor introduced into clinics for AD; however, it was withdrawn from use in 2013 because of safety concerns. In cultured neurons, as well as in mice, tacrine was found to induce endoplasmic reticulum (ER) stress and finally lead to cell apoptosis: the event was triggered by binding the inhibitor to the intracellular enzyme serving as a pharmacological chaperone. Trehalose, a known ER stress reducer, was shown here to ameliorate the ER stress induced by tacrine in AChE-overexpressed NG108-15 cells, with the increased level of C/EBP homologous protein (CHOP) and phosphorylated eukaryotic initiation factor 2 alpha (p-eIF2α). In tetrameric G4 AChE overexpressed cells, the tacrine-exposed cultures revealed considerable G1/G2 forms of AChE accumulated in the ER fraction, whereas the treatment of trehalose decreased the accumulation of G1/G2 AChE. Meanwhile, trehalose reduced the ER stress induced by other AChE inhibitors, for example, lycobetaine, bis(3)-cognitin, daurisoline, and dauricine, in the cultured neuronal cells. Besides, this tacrine-induced ER stress was identified in all AChE isoforms, as well as in butyrylcholinesterase (BChE) expressing cells. Thus, we proposed that the AChE inhibitors, particularly tacrine, could act as ‘chemical/pharmacological chaperones’ during AChE biosynthesis in the ER, disrupting the proper folding of AChE in neurons as a result of ER stress. Trehalose possesses the ability to relieve ER stress by promoting the proper assembly of AChE. The results provide guidance for the drug design and discovery of AChE inhibitors for AD treatment.