Muscarinic acetylcholine receptor inhibition in transgenic Alzheimer‐like Tg2576 mice by scopolamine favours the amyloidogenic route of processing of amyloid precursor protein

Abstract

The molecular mechanisms of the interrelationship between cholinergic neurotransmission, processing of amyloid precursor protein (APP) and beta-amyloid (Abeta) production in vivo are still less understood. To reveal any effect of cholinergic dysfunction on APP processing in vivo, 11-month-old transgenic Tg2576 mice with Abeta plaque pathology received intraperitoneal injections of scopolamine at a daily dosage of 2mg/kg body weight for 14 days in order to suppress cortical cholinergic transmission by chronic inhibition of muscarinic acetylcholine receptors. Scopolamine treatment of transgenic Tg2576 mice resulted in increased levels of fibrillar Abeta(1-40) and Abeta(1-42), while the soluble, SDS-extractable Abeta level remained unchanged as compared to vehicle-injected Tg2576 mice. alpha-Secretase activity determined in cortical tissue from scopolamine-treated Tg2576 mice was lower by about 30% as compared to that assayed in control mice, while beta-secretase activity and BACE1 protein expression appeared unaffected by scopolamine treatment. The amount of sAPPalpha, the product secreted by alpha-secretase-mediated APP cleavage, and the unprocessed APP were assayed in the soluble and membrane fraction, respectively, of cortical tissue preparations from treated and control mice by Western blotting. Using the anti antibody 6E10 which specifically labels human sAPPalpha and full length APP in transgenic Tg2576, an enhanced APP level was detected in the membrane fraction from treated mice as compared to controls, while in the soluble fraction scopolamine treatment did not affect the protein level of sAPPalpha. These data indicate an accumulation of APP in cortical membrane fraction in scopolamine-treated Tg2576 mice presumably due to the decreased level of alpha-secretase-mediated APP cleavage, and further suggest that chronic suppression of cortical muscarinic cholinergic transmission may alter the balance between alpha- and beta-secretory APP processing by favouring the amyloidogenic route.

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“…An increased AD-type pathology (plaques and tangles) in Parkinson's disease is associated with chronic treatment with relatively selective M1 antagonists [14] . This is in line, inter alia, with (1) the effects of the selective M1 mAChR antagonist dicyclomine in triple transgenic AD mice that showed a marked increase in A ␤ deposition and tau pathology [4] ; (2) the effects of scopolamine in Tg2576 mice that elevates A ␤ deposition [15] , and (3) cell cultures from M1 knockout mice that exhibit a shift towards amyloidogenic APP processing [6] . Thus, the M1 mAChR is robustly linked with A ␤ processing and tau hyperphosphorylation.…”

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“…The main findings supporting this premise are the fact that non-selective muscarinic antagonists such as scopolamine-induced cognitive impairment, favor the production of beta-amyloid peptide and decrease the activity of α-secretase [5] . Some triterpenoid saponins have shown to reduce scopolamine-induced amnesia [6][7][8] and non-selective and selective muscarinic agonists have shown to improve learning and memory.…”

“…This postulate dictates that acetylcholine and its receptors, especially (α7) 5 are considered as neuroprotective by modulating glutamatemediated neuronal excitability [21,22] . In AD abnormalities in glutamatergic, neurotransmission is initially observed at the entorhinal cortex (EC), which is followed by further neurotransmission defects in the hippocampus, amygdala, frontal cortex, and parietal cortex [23] .…”

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