Cholinergic denervation exacerbates amyloid pathology and induces hippocampal atrophy in Tg2576 mice

The main pathological hallmarks of Alzheimer's disease (AD) consist of amyloid plaques and neurofibrillary tangles. Hippocampal cell loss, atrophy and cholinergic dysfunction are also features of AD. The present work is aimed at studying the interactions between cholinergic denervation, APP processing and hippocampal integrity. The cholinergic immunotoxin mu p-75-saporin was injected into the 3rd ventricle of 6- to 8-month-old Tg2576 mice to induce a cholinergic denervation. Four weeks after cholinergic immunolesion, a significant 14-fold increase of soluble Aβ1-42 was observed. Cholinergically lesioned Tg2576 mice showed hippocampal atrophy together with degenerating FluoroJade-B-stained neurons and reduction of synaptophysin expression in CA1-3 pyramidal layers. We also found that cholinergic denervation led to reduced levels of ADAM17 in hippocampus of Tg2576 mice. Inhibition of ADAM17 with TAPI-2 (5 μM) decreased viability of hippocampal primary neurons from Tg2576 brains and decreased phosphorylation of downstream effectors of trophic signalling (ERK and Akt). The cholinergic agonist carbachol (100 μM) rescued these effects, suggesting that cholinergic deficits might render hippocampus more vulnerable to neurotoxicity upon certain toxic environments. The present work proposes a novel model of AD that worsens the patent amyloid pathology of Tg2576 mice together with hippocampal synaptic pathology and neurodegeneration. Drugs aimed at favoring cholinergic transmission should still be considered as potential treatments of AD.