ApoE3 vs. ApoE4 Astrocytes: A Detailed Analysis Provides New Insights into Differences in Cholesterol Homeostasis

Abstract

The strongest genetic risk factor for sporadic Alzheimer’s disease (AD) is the presence of the ε4 allele of the apolipoprotein E (ApoE) gene, the major apolipoprotein involved in brain cholesterol homeostasis. Being astrocytes the main producers of cholesterol and ApoE in the brain, we investigated the impact of the ApoE genotype on astrocyte cholesterol homeostasis. Two mouse astrocytic cell lines expressing the human ApoE3 or ApoE4 isoform were employed. Gas chromatography–mass spectrometry (GC-MS) analysis pointed out that the levels of total cholesterol, cholesterol precursors, and various oxysterols are altered in ApoE4 astrocytes. Moreover, the gene expression analysis of more than 40 lipid-related genes by qRT-PCR showed that certain genes are up-regulated (e.g., CYP27A1) and others down-regulated (e.g., PPARγ, LXRα) in ApoE4, compared to ApoE3 astrocytes. Beyond confirming the significant reduction in the levels of PPARγ, a key transcription factor involved in the maintenance of lipid homeostasis, Western blotting showed that both intracellular and secreted ApoE levels are altered in ApoE4 astrocytes, as well as the levels of receptors and transporters involved in lipid uptake/efflux (ABCA1, LDLR, LRP1, and ApoER2). Data showed that the ApoE genotype clearly affects astrocytic cholesterol homeostasis; however, further investigation is needed to clarify the mechanisms underlying these differences and the consequences on neighboring cells. Indeed, drug development aimed at restoring cholesterol homeostasis could be a potential strategy to counteract AD.

Altered Brain Cholesterol Machinery in a Down Syndrome Mouse Model: A Possible Common Feature with Alzheimer’s Disease

“…Notably, some of the current results resemble the data reported by our group in a previous study focused on cholesterol homeostasis in ApoE4 astrocytes. Indeed, we have demonstrated that Ts2 mice, in comparison to Eu mice, show HMGCR, DHCR24, and cholesterol trends similar to those already observed in ApoE4 astrocytes compared to ApoE3 [29]. However, we must take into account that the two experimental models are different.…”

“…The limited expression of HMGCR and DHCR24 and lower cholesterol content in our Ts2 mouse model and ApoE4 astrocytes [29] imply meaningful issues about the pharmacological treatment of AD-like neurodegeneration with the use of lipid lowering agents, in particular statins which are HMGCR inhibitors. Although the effects of statins are mainly at the systemic level, these drugs, in particular the lipophilic simvastatin which appears the most effective at crossing the BBB, are proposed as neuroprotective in the early stages of AD thanks to their hypocholesterolemic activity, and antioxidant and anti-inflammatory properties [49][50][51].…”

“…Cholesterol, the cholesterol precursors (lanosterol, lathosterol, and desmosterol), and the oxysterols 7α-OHC, 7β-OHC, 24-OHC, 25-OHC, 27-OHC, 7-KC, α-EPOX, and β-EPOX were measured using isotope dilution gas chromatography-mass spectrometry (GC-MS), as described elsewhere [29].…”

Cell type-specific functions of Alzheimer's disease endocytic risk genes

“…Aside from being the main APOE producers in the brain, astrocytes alongside microglia are crucial for the uptake and endocytosis of substrates from the extracellular space including Aβ, tau and α-synuclein [123]. Aβ uptake/clearance in astrocytes is facilitated via lipid-associated endocytic receptors such as LDLR or LRP1 and has been shown to occur via APOE isoform-dependent complex binding, with -POE2/Aβ and APOE3/Aβ complexes resulting in faster clearance than APOE4/Aβ complexes in the mouse brain [124][125][126]. Indeed, reduced LRP1 expression and Aβ uptake in astrocytes was linked to receptor entrapment in the EEs due to late APOE4 receptor dissociation in the EE, an effect that was counteracted through inhibition of nuclear histone deacetylation 4 (figure 2, parts 2-4) [127].…”

Aging and brain free cholesterol concentration on amyloid‐β peptide accumulation in guinea pigs

“…Astrocytes are more efficient than neurons at cholesterol production as they contain copious precursors for cholesterol synthesis (Nieweg et al., 2009). Astrocyte‐derived cholesterol upregulates Aβ production (Wang et al., 2021) and produces apolipoprotein E (APOE) as cargo carriers supplying cholesterol esters to neurons via LRP1 and LDLR (low density lipoprotein receptor) (Staurenghi et al., 2022). Brain cholesterol is continuously being shed from the plasma membrane by transporters ABCG1/ABCG5 and ABCA1, the ATP binding cassette subfamily A member 1 (Koldamova et al., 2014), and is metabolized by CYP46A1, the cholesterol 24(S)‐hydroxylase in neurons to 24(S)‐hydroxycholesterol.…”