Regulation of expression and cellular release of acetylcholinesterase

Acetylcholinesterase (AChE) is a hydrolytic enzyme which has been linked to the pathological progression of the neurodegenerative disease, Alzheimer’s disease (AD). AD is thought to be driven by the toxic amyloid-β (Aβ) peptide, which derives from proteolytic cleavage of amyloid precursor protein (APP). Here, release of AChE from two neuronal cell lines, SN56 and SH-SY5Y, was investigated and found to be driven by at least two distinct pathways, shedding and exocytosis. The former was found to be mediated by an unknown metalloprotease, sensitive to the inhibitor batimastat. Shedding was also found to be dependent on the action of protein disulphide isomerase. The cellular release of AChE was potentiated by agonism of muscarinic acetylcholine receptors (mAChRs) by carbachol. This process was found to derive, in part, from transcriptional upregulation of AChE by mAChRs, likely involving the Egr-1 transcription factor. Subsequent work established, for the first time, a mechanistic link between APP and regulation of AChE expression. Over-expression of APP in neuronal cell lines led to reductions of AChE mRNA, protein and catalytic activity. Assessment of other cholinergic genes revealed repression, by APP, of the membrane anchor of AChE, PRiMA, but no changes in mRNA levels of butyrylcholinesterase or the high affinity choline transporter, CHT. This regulatory relationship between APP and AChE was confirmed when knockdown of APP in wild type SN56 cells resulted in a significant increase in AChE mRNA. The ability of APP to repress AChE transcription was shown to be independent of proteolytic processing of the former, as inhibition of each of the secretase enzymes responsible for APP proteolysis had no effect on AChE activity. However, APP-mediated repression of AChE was dependent on the N-terminal, extracellular E1 domain and specifically the copper-binding domain within. Deletion of these domains completely ablated the ability of APP to effect transcriptional repression of AChE. These studies have implications for greater understanding of the role of the cholinergic system and AChE in the pathological progression of AD. This work further elucidates a physiological role for APP, the perturbation of which may contribute to neurodegeneration.