Inhibition of Choline Acetyltransferase as a Mechanism for Cholinergic Dysfunction Induced by Amyloid-β Peptide Oligomers

Dysregulated cholinergic signaling is an early hallmark of Alzheimer disease (AD), usually ascribed to degeneration of cholinergic neurons induced by the amyloid-beta peptide (A beta). It is now generally accepted that neuronal dysfunction and memory deficits in the early stages of AD are caused by the neuronal impact of soluble A beta oligomers (A beta Os). A beta Os build up in AD brain and specifically attach to excitatory synapses, leading to synapse dysfunction. Here, we have investigated the possibility that A beta Os could impact cholinergic signaling. The activity of choline acetyltransferase (ChAT, the enzyme that carries out ACh production) was inhibited by similar to 50% in cultured cholinergic neurons exposed to low nanomolar concentrations of A beta Os. 3-(4,5-Dimethylthiazol-2-yl)-2,5-di-phenyltetrazoliumbromide (MTT) reduction, lactatedehydrogenase release, and [H-3] choline uptake assays showed no evidence of neuronal damage or loss of viability that could account for reduced ChAT activity under these conditions. Glutamate receptor antagonists fully blocked ChAT inhibition and oxidative stress induced by A beta Os. Antioxidant polyunsaturated fatty acids had similar effects, indicating that oxidative damage may be involved in ChAT inhibition. Treatment with insulin, previously shown to down-regulate neuronal A beta O binding sites, fully prevented A beta O-induced inhibition of ChAT. Interestingly, we found that A beta Os selectively bind to similar to 50% of cultured cholinergic neurons, suggesting that ChAT is fully inhibited in A beta O-targeted neurons. Reduction in ChAT activity instigated by A beta Os may thus be a relevant event in early stage AD pathology, preceding the loss of cholinergic neurons commonly observed in AD brains.