Cholesterol can be detrimental or vital, and must be present in the right place at the right time and in the right amount. This is well known in the heart and the vascular system. However, in the CNS cholesterol is still an enigma, although several of its fundamental functions in the brain have been identified. Brain cholesterol has attracted additional attention owing to its close connection to ApoE, a key polymorphic transporter of extracellular cholesterol in humans. Indeed, both cholesterol and ApoE are so critical to fundamental activities of the brain, that the brain regulates their synthesis autonomously. Yet, similar control mechanisms of ApoE and cholesterol homeostasis may exist on either sides of the blood–brain barrier. One indication is that the APOE ε4 allele is associated with hypercholesterolemia and a proatherogenic profile on the vascular side and with increased risk of Alzheimer’s disease on the CNS side. In this review, we draw attention to the association between cholesterol and ApoE in the aging and diseased brain, and to the behavior of the ApoE4 protein at the molecular level. The attempt to correlate in vivo and in vitro observations is challenging but crucial for developing future strategies to address ApoE-related aberrations in cholesterol metabolism selectively in the brain.

Extracellular accumulation of β-amyloid peptide (Aβ) has been linked to the development of Alzheimer disease. The importance of intraneuronal Aβ has been recognized more recently. Although considerable evidence indicates that extracellular Aβ contributes to the intracellular pool of Aβ, the mechanisms involved in Aβ uptake by neurons are poorly understood. We examined the molecular mechanisms involved in Aβ-(1–42) internalization by primary neurons in the absence of apolipoprotein E. We demonstrated that Aβ-(1–42) is more efficiently internalized by axons than by cell bodies of sympathetic neurons, suggesting that Aβ-(1–42) uptake might be mediated by proteins enriched in the axons. Although the acetylcholine receptor α7nAChR, previously suggested to be involved in Aβ internalization, is enriched in axons, our results indicate that it does not mediate Aβ-(1–42) internalization. Moreover, receptors of the low density lipoprotein receptor family are not essential for Aβ-(1–42) uptake in the absence of apolipoprotein E because receptor-associated protein had no effect on Aβ uptake. By expressing the inactive dynamin mutant dynK44A and the clathrin hub we found that Aβ-(1–42) internalization is independent of clathrin but dependent on dynamin, which suggests an endocytic pathway involving caveolae/lipid rafts. Confocal microscopy studies showing that Aβ did not co-localize with the early endosome marker EEA1 further support a clathrin-independent mechanism. The lack of co-localization of Aβ with caveolin in intracellular vesicles and the normal uptake of Aβ by neurons that do not express caveolin indicate that Aβ does not require caveolin either. Instead partial co-localization of Aβ-(1–42) with cholera toxin subunit B and sensitivity to reduction of cellular cholesterol and sphingolipid levels suggest a caveolae-independent, raft-mediated mechanism. Understanding the molecular events involved in neuronal Aβ internalization might identify potential therapeutic targets for Alzheimer disease.

In the brain, the amyloid β peptide (Aβ) exists extracellularly and inside neurons. The intracellular accumulation of Aβ in Alzheimer′s disease brain has been questioned for a long time. However, there is now sufficient strong evidence indicating that accumulation of Aβ inside neurons plays an important role in the pathogenesis of Alzheimer′s disease. Intraneuronal Aβ originates from intracellular cleavage of APP and from Aβ internalization from the extracellular milieu. We discuss here the different molecular mechanisms that are responsible for Aβ internalization in neurons and the links between Aβ internalization and neuronal dysfunction and death. A brief description of Aβ uptake by glia is also presented.

Accumulation of β-amyloid (Aβ) inside brain neurons is an early and crucial event in Alzheimer's disease (AD). Studies in brains of AD patients and mice models of AD suggested that cholesterol homeostasis is altered in neurons that accumulate Aβ. Here we directly investigated the role of intracellular oligomeric Aβ₄₂(oAβ₄₂) in neuronal cholesterol homeostasis. We report that oAβ₄₂induces cholesterol sequestration without increasing cellular cholesterol mass. Several features of AD, such as endosomal abnormalities, brain accumulation of Aβ and neurofibrillary tangles, and influence of apolipoprotein E genotype, are also present in Niemann-Pick type C, a disease characterized by impairment of intracellular cholesterol trafficking. These common features and data presented here suggest that a pathological mechanism involving abnormal cholesterol trafficking could take place in AD. Cholesterol sequestration in Aβ-treated neurons results from impairment of intracellular cholesterol trafficking secondary to inhibition of protein prenylation. oAβ₄₂reduces sterol regulatory element-binding protein-2 (SREBP-2) cleavage, causing decrease of protein prenylation. Inhibition of protein prenylation represents a mechanism of oAβ₄₂-induced neuronal death. Supply of the isoprenoid geranylgeranyl pyrophosphate to oAβ₄₂-treated neurons recovers normal protein prenylation, reduces cholesterol sequestration, and prevents Aβ-induced neurotoxicity. Significant to AD, reduced levels of protein prenylation are present in the cerebral cortex of the TgCRND8 mouse model. In conclusion, we demonstrate a significant inhibitory effect of Aβ on protein prenylation and identify SREBP-2 as a target of oAβ₄₂, directly linking Aβ to cholesterol homeostasis impairment.

Amany Mohamed, Anissa Viveiros, Kathleen Williams, Elena Posse de Chaves