Amyloid beta-protein reduces acetylcholine synthesis in a cell line derived from cholinergic neurons of the basal forebrain

The characteristic features of a brain with Alzheimer disease (AD) include the presence of neuritic plaques composed of amyloid beta-protein (A beta) and reductions in the levels of cholinergic markers. Neurotoxic responses to A beta have been reported in vise and in vitro, suggesting that the cholinergic deficit in AD brain may be secondary to the degeneration of cholinergic neurons caused by A beta. However, it remains to be determined if A beta contributes to the cholinergic deficit in AD brain by nontoxic effects, We examined the effects of synthetic A beta peptides on the cholinergic properties of a mouse cell line, SN56, derived from basal forebrain cholinergic neurons. A beta 1-42 and A beta 1-28 reduced the acetylcholine (AcCho) content of the cells in a concentration-dependent fashion, whereas A beta 1-16 was inactive. Maximal reductions of 43% and 33% were observed after a 48-h treatment with 100 nM of A beta 1-42 and 50 pM of A beta 1-28, respectively. Neither A beta 1-28 nor A beta 1-42 at a concentration of 100 nM and a treatment period of 2 weeks was toxic to the cells. Treatment of the cells with A beta 25-28 (48 h; 100 nM) significantly decreased AcCho levels, suggesting that the sequence GSNK (aa 25-28) is responsible for the AcCho-reducing effect of A beta. The reductions in AcCho levels caused by A beta 1-42 and A beta 1-28 were accompanied by proportional decreases in choline acetyltransferase activity. In contrast, acetylcholinesterase activity was unaltered, indicating that A beta specifically reduces the synthesis of AcCho in SN56 cells. The reductions in AcCho content caused by A beta 1-42 could be prevented by a cotreatment with all-trans-retinoic acid (10 nM), a compound previously shown to increase choline acetyltransferase mRNA expression in SN56 cells, These results demonstrate a nontoxic, suppressive effect of A beta on AcCho synthesis, an action that may contribute to the cholinergic deficit in AD brain.