Apolipoprotein E ?4 triggers neurotoxicity via cholesterol accumulation, acetylcholine dyshomeostasis, and PKC? mislocalization in cholinergic neuronal cells

The Apolipoprotein E (ApoE) has been known to regulate cholesterol and ?-amyloid (A?) production, redistribution, and elimination, in the central nervous system (CNS). The ApoE ?4 polymorphic variant leads to impaired brain cholesterol homeostasis and amyloidogenic pathway, thus representing the major risk factor for Alzheimer's Disease (AD). Currently, less is known about the molecular mechanisms connecting ApoE ?4-related cholesterol metabolism and cholinergic system degeneration, one of the main AD pathological features. Herein, in vitro cholinergic neuron models were developed in order to study ApoE neuronal expression and investigate the possible interplay between cholesterol metabolism and cholinergic pathway impairment prompted by ?4 isoform. Particularly, alterations specifically occurring in ApoE ?4-carrying neurons (i.e. increased intracellular ApoE, amyloid precursor protein (APP) and A? levels, elevated apoptosis, and reduced cell survival) were recapitulated. ApoE ?4 expression was found to increase intracellular cholesterol accumulation, by regulating the related gene expression, while reducing cholesterol precursor acetyl-CoA, which in turn fuels the acetylcholine (ACh) synthesis route. In parallel, although the ACh intracellular signalling was activated, as demonstrated by the boosted extracellular ACh as well as increased IP3 and Ca2+, the PKC? activation via membrane translocation was surprisingly suppressed, probably explained by the cholesterol overload in ApoE ?4 neuron-like cells. Consequently, the PKC-dependent anti-apoptotic and neuroprotective roles results impaired, reliably adding to other causes of cell death prompted by ApoE ?4. Overall, the obtained data open the way to further critical considerations of ApoE ?4-dependent cholesterol metabolism dysregulation in the alteration of cholinergic pathway, neurotoxicity, and neuronal death.