Lack of hepatic apoE does not influence early Aβ deposition: observations from a new APOE knock-in model

Abstract

Background The apolipoprotein E (APOE) gene is the strongest genetic risk factor for late-onset Alzheimer disease (AD). ApoE is produced by both astrocytes and microglia in the brain, whereas hepatocytes produce the majority of apoE found in the periphery. Studies using APOE knock-in and transgenic mice have demonstrated a strong isoform-dependent effect of apoE on the accumulation of amyloid-β (Aβ) deposition in the brain in the form of both Aβ-containing amyloid plaques and cerebral amyloid angiopathy. However, the specific contributions of different apoE pools to AD pathogenesis remain unknown. Methods We have begun to address these questions by generating new lines of APOE knock-in (APOE-KI) mice (ε2/ε2, ε3/ε3, and ε4/ε4) where the exons in the coding region of APOE are flanked by loxP sites, allowing for cell type-specific manipulation of gene expression. We assessed these mice both alone and after crossing them with mice with amyloid deposition in the brain. Using biochemical and histological methods. We also investigated how removal of APOE expression from hepatocytes affected cerebral amyloid deposition. Results As in other APOE knock-in mice, apoE protein was present predominantly in astrocytes in the brain under basal conditions and was also detected in reactive microglia surrounding amyloid plaques. Primary cultured astrocytes and microglia from the APOE-KI mice secreted apoE in lipoprotein particles of distinct size distribution upon native gel analysis with microglial particles being substantially smaller than the HDL-like particles secreted by astrocytes. Crossing of APP/PS1 transgenic mice to the different APOE-KI mice recapitulated the previously described isoform-specific effect (ε4 > ε3) on amyloid plaque and Aβ accumulation. Deletion of APOE in hepatocytes did not alter brain apoE levels but did lead to a marked decrease in plasma apoE levels and changes in plasma lipid profile. Despite these changes in peripheral apoE and on plasma lipids, cerebral accumulation of amyloid plaques in APP/PS1 mice was not affected. Conclusions Altogether, these new knock-in strains offer a novel and dynamic tool to study the role of APOE in AD pathogenesis in a spatially and temporally controlled manner.

Axl receptor tyrosine kinase is a regulator of apolipoprotein E

“…Importantly, we also observed that AZ7235 has little to no effect on apoE and ABCA1 in murine glial cells, even when these cells express human apoE protein. The human APOE targeted-replacement mice we used to cultivate primary glial cells were generated using a knock-in strategy that retains human introns between exons 2-3 and 3-4 but retains the murine 5'UTR and 3'UTR sequences [37]. Murine apoE is known to be much less responsive to known apoE regulators such as GW3965 [32] and Bexarotene [63], thus we expected a reduced signal in primary murine glia.…”

“…Primary human astrocytes (cat# 1800) and brain vascular pericytes (cat# 1200) were acquired from ScienCell. Human APOE3 targeted-replacement mice [37] were obtained from the Cure Alzheimer Fund and primary murine mixed glia were cultured from postnatal day 0-2 pups as described [38]. The LXR agonist T0901317 was acquired from Sigma-Aldrich (cat# 575310).…”

The Role of HDL and HDL Mimetic Peptides as Potential Therapeutics for Alzheimer’s Disease

“…The majority of these findings have been obtained by studying human APOE isoform-specific targeted replacement (TR) mice crossed with transgenic AD mouse models. Recently, to address the role of peripheral and central APOE, respectively, in AD, novel lines of human APOE-knock in (KI) mice have been generated [79]. It was shown that deletion of hepatic APOE, which led to a marked decrease in peripheral APOE levels, did not affect cerebral Aβ deposition in the APP/PS1 transgenic mouse model of AD.…”

“…More recently, it has been shown that microglia drive tau pathology and neurodegeneration depending on APOE [123]. It also appears that the inflammatory response in the brain is regulated specifically by microglia-derived APOE [79]. In addition, TREM2 has been shown to regulate cholesterol metabolism in microglia, particularly upon chronic phagocytic challenge, and TREM2-deficient microglia fail to upregulate lipid metabolism genes including APOE, leading to intracellular accumulation of cholesteryl esters, which is rescued by treatment with pharmacological agents to inhibit CE synthesis or increase expression of cholesterol efflux genes [124].…”

Therapeutic approaches targeting Apolipoprotein E function in Alzheimer’s disease

“…These mice have impaired synaptic plasticity but their spatial memory skills are intact [24], suggesting that peripheral and CNS APOE may have distinct effects on CNS function. On the other hand, absence of hepatic APOE does not affect the APOE4dependent induction of Aβ pathologies in young APP/ PS1 female mice, suggesting that plasma APOE4 may have little influence on initiation of Aβ pathologies in the brain [27]. With this knowledge, it is reasonable to explore treatment options that would preferentially modify the CNS pool of APOE without affecting the peripheral sources, thus also avoiding systemic metabolic syndromes.…”