ABCA1 mediates cholesterol efflux to the nascent ApoE particle in the brain. After sufficient amount of cholesterol and phospholipids bind to ABCA1, ABCA1 undergoes conformational changes, leading to dimerization and transfer of lipids to ApoE. In addition to its role in lipid transport, ABCA1 also indirectly facilitates amyloid β clearance through ApoE lipidation in brain . Previous animal studies demonstrated that improvement of ApoE lipidation inhibits amyloid deposition in APP mouse models. Deletion of ABCA1 gene increases amyloid β accumulation, whereas its overexpression prevents amyloid β aggregation in amyloid mouse models . In support of the previous cell-based and animal studies, cerebrospinal fluid (CSF) from patients with mild cognitive impairment and Alzheimer's disease showed 30% less efficiency in the ABCA1-mediated cholesterol efflux, compared with cognitively normal participants [7▪]. Although earlier human genetic analyses provided inconsistent results , a recent large-scale study involving more than 92 000 individuals clearly indicated that a well established loss-of-function mutation Asp1800His (N1800H) in ABCA1, is strongly associated with a higher risk of Alzheimer's disease [8▪▪]. Substitution of an uncharged amino acid, asparagine, with a positively charged histidine is responsible for the loss of function. The N1800H variant was observed at 0.2% frequency and significantly increased the hazard ratio to 4.13. In the same population, the hazard ratio for APOE genotype ε4/4 versus ε3/3 was 7.70. Although an association between lower plasma ApoE level and increased risk of Alzheimer's disease was observed in this study, it is unclear whether there is any mechanistic link between them. Because peripheral ApoE cannot penetrate the blood-brain barrier (BBB) , there is no or very low correlation between plasma and CSF ApoE levels [10,11]. In another study, higher CSF ApoE levels were associated with higher CSF tau levels and cognitive decline in human . If CSF ApoE level and amyloid positron emission tomography scan data are available in the future study, they will greatly help in determining how the ABCA1 loss-of-function variant affects the risk of Alzheimer's disease.
Several therapeutic approaches were tested to increase ABCA1 level. Most studies focused on the transcriptional regulation of ABCA1 by targeting transcription factors. Nuclear receptors, liver X receptors (LXRs), retinoid X receptor (RXR), and peroxisome proliferator-activated receptor (PPAR), have been studied extensively in this regard. Agonists of these nuclear receptors increased the levels of ABCA1 and decreased amyloid β accumulation in APP mouse models . Although an initial study suggested that ApoE may be directly responsible for the beneficial effects of agonist, it is important to consider other indirect mechanisms as well. Because LXR and RXR regulate both levels and lipidation of ApoE, it is difficult to distinguish the effect of ApoE levels and lipidation on amyloid β. Subsequent animal studies proposed that ABCA1 and an ApoE receptor may be necessary for the beneficial effects of agonists. However, there are several confounding factors for the proper interpretation. For example, RXR alters the transcription and epigenetic status of numerous genes in mouse brains [13▪▪]. Therefore, it is hard to pinpoint one particular gene as the major mechanism. Furthermore, a recent study also provided an unexpected mechanism. An RXR agonist directly inhibited the initial step of amyloid β aggregation by directly binding to amyloid β [14▪▪]. An increase in the production of CSF may be yet another mechanism that mediates the beneficial effects of nuclear receptors . Along with the positive transcriptional regulation, the negative post-transcriptional regulation by microRNAs has been studied recently. ABCA1 expression is suppressed by multiple microRNAs, such as miR-106b, miR-758, and miR-33 . In one study, pharmacological and genetic inhibition of miR-33 significantly increased ABCA1 level and decreased amyloid β levels in the brain . Given the supportive data from animal and human studies, targeting ABCA1 is a promising therapeutic avenue for Alzheimer's disease.
Structural difference among ApoE isoforms also accounts for the differential ApoE degradation. In neuron, ApoE4 undergoes preferential proteolytic cleavage, generating neurotoxic fragments . Initially, it was demonstrated that the proteolysis of ApoE could be inhibited only by a cysteine protease inhibitor, suggesting that cysteine proteases may play a major role in ApoE degradation. However, subsequent studies demonstrated that aspartic and chymotrypsin-like serine proteases may also mediate the degradation of ApoE. The apparent discrepancy between reports might be attributed to the differences in cell types and the concentrations of protease inhibitors used in each study. Unlike neurons, in monocytes and a microglial cell line, elastase-like proteases mediate the degradation of ApoE4 . A recent study implicated high-temperature requirement serine peptidase A1 (HtrA1) as one of ApoE protease [22▪]. Interestingly, HtrA1 degrades ApoE4 more efficiently than ApoE3. Preliminary data suggest that ApoE4 may inhibit HtrA1-mediated tau degradation in vitro. Although this is an interesting new hypothesis that may explain the effect of ApoE isoform on Alzheimer's disease pathogenesis, its implication to in vivo condition is still unclear. Most previous studies used recombinant ApoE with no or little lipidation under in vitro condition. Because the exact identity of the protease that degrades ApoE in vivo is still elusive, further investigations under physiological conditions are warranted.
ApoE affects cerebral vasculature by affecting cerebral blood flow, neuronal-vascular coupling, and BBB integrity . Its influence on these outcomes stems from the effects of ApoE on cerebral vasculature from both peripheral side and the brain side. In the periphery, ApoE has important implications in atherosclerosis and hyperlipoproteinemia [3,23], which also affect cerebral vasculature. Within the central nervous system, ApoE affects the onset and amount of cerebral amyloid angiopathy (CAA). CAA is amyloid β deposition on the blood vessels in the brain and it is often associated with hemorrhagic lesions, ischemic lesions, encephalopathy, and dementia. Although the peripheral and central nervous system mechanisms both affect cerebral vascular dysfunction, their contributions could be different under different circumstances. For example, although both ApoE2 and ApoE4 are risk factors for microhemorrhage, ApoE2 likely contributes to microhemorrhage via hyperlipidemia-associated blood vessel changes. It is unclear whether ApoE2 exacerbates CAA, given the conflicting data [24–26]. In contrast, ApoE4 causes microhemorrhage largely by increasing CAA. Whether ApoE4 disrupts BBB integrity independent of amyloid β is still under debate. Although some studies have shown that ApoE4 disrupts BBB integrity in non-APP mice, it was recently reported that non-APP ApoE4 knock-in mice do not have any widespread BBB disruption .
Recent studies indicate that ApoE binds a microglial receptor, triggering receptor expressed on myeloid cells 2 (TREM2). TREM2 is a member of the Ig superfamily of receptors. Rare missense variants in TREM2 are associated with an approximately two-fold to four-fold increase in risk of Alzheimer's disease. In several studies using APP mice, TREM2 deficiency consistently decreased the number of plaque-associated microglia and inflammatory cytokines. However, the effects of TREM2 deficiency on amyloid plaque load are not consistent across these studies . Interestingly, when the morphology of individual plaques was analyzed, it was found that TREM2 deficiency results in the decreased ratio of compact region versus diffused region of plaques [38,39]. Similar observation was also made in human Alzheimer's disease brain with TREM2 R47H mutation . TREM2 recognizes a variety of ligands, including ApoJ and ApoE [40▪▪–42▪▪]. In a series of in-vitro experiments, ApoE has been shown to facilitate the TREM2-mediated microglial phagocytosis [40▪▪,41▪▪]. The TREM2 disease variants reduced the affinity between ApoE and TREM2, which led to an impairment of phagocytosis. It was proposed that the lower affinity between ApoE and TREM2 disease variants may decrease the uptake of amyloid β-ApoE complex [40▪▪–42▪▪]. Interestingly, there is no difference in binding affinity between TREM2 and different ApoE isoforms [40▪▪–42▪▪]. Therefore, it appears that the physical interaction between TREM2 and ApoE may not explain the differential effects of ApoE isoforms on Alzheimer's disease pathogenesis. Further studies are warranted to determine whether TREM2 could indirectly affect downstream phenotypes in an ApoE isoform-dependent manner. ApoE is also found to affect TREM2 expression in primary microglia cultured from apoE knock-in mice in response to various microglia activation reagents. ApoE4 led to a lower TREM2 expression as compared to ApoE3 , which might impair the TREM2-mediated amyloid β clearance. In summary, the new connection between ApoE and TREM2 provides new insight into Alzheimer's disease.
Previous studies demonstrated that ApoE isoforms differentially affect synaptic function . Two recent studies provide critical new insights into in-vivo mechanisms. Compared with ApoE3 and ApoE4, ApoE2 strongly enhanced the binding between synaptosomes and astrocytes in vitro and enhanced the phagocytic capacity of astrocytes in ApoE knock-in mice [51▪▪]. ApoE4-mediated synaptic impairment was also attributed to the disruption of slow gamma oscillations in the hippocampus of the apoE knock-in mice [52▪▪]. Taken together, these data suggest that ApoE isoforms affect astrocyte-mediated synaptic pruning and sharp-wave ripples, leading to synapses network dysfunction.
Recently, it was shown that ApoE can translocate to nucleus, bind DNA, and function as a transcription factor in human glioblastoma cells . Genome-wide mapping of ApoE binding site indicated that ApoE4 binds the promoter regions of ∼1700 genes, including genes associated with synaptic function, neuroinflammation, and insulin resistance. In another study, ApoE4 increased the nuclear translocation of histone deacetylases (HDACs) in human neuroblastomas, thereby reducing brain derived neurotrophic factor expression . In contrast, ApoE3 retains HDACs in the cytosol via elevating the expression of protein kinase C ε. In the brains of Alzheimer's disease patients, nuclear translocation of HDA6 was increased as compared with controls. These studies suggested that ApoE may regulate the transcription of certain genes. It will be important to determine the relative contribution of the transcriptional effects of ApoE in Alzheimer's disease pathogenesis, as compared with other well-established effects of ApoE.
Papers of particular interest, published within the annual period of review, have been highlighted as:
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This study demonstrates that ABCA1-mediated cholesterol efflux is impaired in patients with mild cognitive impairment and Alzheimer's disease, providing additional support for ABCA1 induction approach.
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Based on one of the largest sample size so far, this study provided the strong evidence supporting the critical role of ABCA1 in the pathogenesis of Alzheimer's disease. A loss-of-function mutation in ABCA1 was strongly associated with high risk of Alzheimer's disease and cerebrovascular disease.
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Based on ChIP-seq and RNA-seq data, this study identified the cistrome and transcriptome modulated by RXR activation. It provides invaluable resource to better understand the RXR-regulated networks in brain and the effect of amyloid β on such networks.
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This study identified a new class of ApoE protease, HtrA1. Interestingly, HtrA1 is known to affect the degradation of APP and tau as well.
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This study identified a set of lipoprotein particles and apolipoproteins, including apoE, as TREM2 ligands using an unbiased protein microarray screen.
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One of the first two reports identifying apoE as a novel ligand for TREM2.
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One of the first two reports identifying apoE as a novel ligand for TREM2.
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This study analyzed the sizes of CSF apoE lipoprotein particles in human with different apoE genotype. The size of lipoprotein particles was differentially affected by ApoE isoform. Isoform-specific difference indicates that there is a difference in ApoE lipidation.
45. Lee J, Choi J, Wong GW, Wolfgang MJ. Neurometabolic roles of ApoE and Ldl-R in mouse brain. J Bioenerg Biomembr 2016; 48:13–21.
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51▪▪. Chung WS, Verghese PB, Chakraborty C, et al. Novel allele-dependent role for APOE in controlling the rate of synapse pruning by astrocytes. Proc Natl Acad Sci U S A 2016; 113:10186–10191.
This study demonstartes that ApoE isoforms differentially regulate phagocytic capacity of astrocytes and synapse pruning. This novel function of ApoE may explain the benficial effect of ApoE2 isoform on Alzheimer's disease.
52▪▪. Gillespie AK, Jones EA, Lin YH, et al. Apolipoprotein E4 causes age-dependent disruption of slow gamma oscillations during hippocampal sharp-wave ripples. Neuron 2016; 90:740–751.
This study identified a novel molecular mechanism by which GABAergic interneurons affect age-dependent learning and memory impariment.
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