The ε4 allele of the apolipoprotein E gene (APOE4) is a strong genetic risk factor for Alzheimer’s disease (AD) and several other neurodegenerative conditions, including Lewy body dementia (LBD). The ...three APOE alleles encode protein isoforms that differ from one another only at amino acid positions 112 and 158: apoE2 (C112, C158), apoE3 (C112, R158), and apoE4 (R112, R158). Despite progress, it remains unclear how these small amino acid differences in apoE sequence among the three isoforms lead to profound effects on aging and disease-related pathways. Here, we propose a novel “ApoE Cascade Hypothesis” in AD and age-related cognitive decline, which states that the biochemical and biophysical properties of apoE impact a cascade of events at the cellular and systems levels, ultimately impacting aging-related pathogenic conditions including AD. As such, apoE-targeted therapeutic interventions are predicted to be more effective by addressing the biochemical phase of the cascade.
In this review, Martens et al. propose a novel “ApoE Cascade Hypothesis,” which states that the biochemical and biophysical properties of apoE impact a cascade of events at the cellular and systems levels, ultimately leading to Alzheimer’s disease and age-related cognitive decline.
Diabetes and impaired brain insulin signaling are linked to the pathogenesis of Alzheimer’s disease (AD). The association between diabetes and AD-associated amyloid pathology is stronger among ...carriers of the apolipoprotein E (APOE) ε4 gene allele, the strongest genetic risk factor for late-onset AD. Here we report that apoE4 impairs neuronal insulin signaling in human apoE-targeted replacement (TR) mice in an age-dependent manner. High-fat diet (HFD) accelerates these effects in apoE4-TR mice at middle age. In primary neurons, apoE4 interacts with insulin receptor and impairs its trafficking by trapping it in the endosomes, leading to impaired insulin signaling and insulin-stimulated mitochondrial respiration and glycolysis. In aging brains, the increased apoE4 aggregation and compromised endosomal function further exacerbate the inhibitory effects of apoE4 on insulin signaling and related functions. Together, our study provides novel mechanistic insights into the pathogenic mechanisms of apoE4 and insulin resistance in AD.
•ApoE4 impairs cerebral insulin signaling in an age-dependent manner•Peripheral insulin resistance and apoE4 synergistically impair insulin signaling•ApoE4 reduces insulin-IR interaction and impairs IR trafficking•ApoE4 aggregation and endosomal dysfunction impair insulin signaling with aging
Zhao et al. demonstrates that aging and peripheral insulin resistance induces impairment of cerebral insulin signaling in human apoE4-targeted replacement mice. ApoE4 interferes with insulin receptor signaling by blocking its interaction with insulin and trapping the receptor in the endosomes.
APOE4 is the strongest genetic risk factor associated with late-onset Alzheimer's disease (AD). To address the underlying mechanism, we develop cerebral organoid models using induced pluripotent stem ...cells (iPSCs) with APOE ε3/ε3 or ε4/ε4 genotype from individuals with either normal cognition or AD dementia. Cerebral organoids from AD patients carrying APOE ε4/ε4 show greater apoptosis and decreased synaptic integrity. While AD patient-derived cerebral organoids have increased levels of Aβ and phosphorylated tau compared to healthy subject-derived cerebral organoids, APOE4 exacerbates tau pathology in both healthy subject-derived and AD patient-derived organoids. Transcriptomics analysis by RNA-sequencing reveals that cerebral organoids from AD patients are associated with an enhancement of stress granules and disrupted RNA metabolism. Importantly, isogenic conversion of APOE4 to APOE3 attenuates the APOE4-related phenotypes in cerebral organoids from AD patients. Together, our study using human iPSC-organoids recapitulates APOE4-related phenotypes and suggests APOE4-related degenerative pathways contributing to AD pathogenesis.
Evidence suggests interplay among the three major risk factors for Alzheimer’s disease (AD): age, APOE genotype, and sex. Here, we present comprehensive datasets and analyses of brain transcriptomes ...and blood metabolomes from human apoE2-, apoE3-, and apoE4-targeted replacement mice across young, middle, and old ages with both sexes. We found that age had the greatest impact on brain transcriptomes highlighted by an immune module led by Trem2 and Tyrobp, whereas APOE4 was associated with upregulation of multiple Serpina3 genes. Importantly, these networks and gene expression changes were mostly conserved in human brains. Finally, we observed a significant interaction between age, APOE genotype, and sex on unfolded protein response pathway. In the periphery, APOE2 drove distinct blood metabolome profile highlighted by the upregulation of lipid metabolites. Our work identifies unique and interactive molecular pathways underlying AD risk factors providing valuable resources for discovery and validation research in model systems and humans.
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•Aging leads to the most profound changes in brain gene expression networks•Immune module led by Alzheimer’s risk genes Trem2/Tyrobp is upregulated with aging•Alzheimer’s risk allele APOE4 increases the expression of Serpina3 family genes•Alzheimer’s protective allele APOE2 drives unique serum metabolome profiles
Zhao et al. present comprehensive datasets and analyses of brain transcriptomes and blood metabolomes from human apoE2-, apoE3-, and apoE4-targeted replacement mice across young, middle, and old ages with both sexes. The study provides critical insight on the molecular pathways underlying three major Alzheimer’s risk factors age, APOE, and sex.
The brain, as one of the most lipid-rich organs, heavily relies on lipid transport and distribution to maintain homeostasis and neuronal function. Lipid transport mediated by lipoprotein particles, ...which are complex structures composed of apolipoproteins and lipids, has been thoroughly characterized in the periphery. Although lipoproteins in the central nervous system (CNS) were reported over half a century ago, the identification of
APOE4
as the strongest genetic risk factor for Alzheimer's disease has accelerated investigation of the biology and pathobiology of lipoproteins in the CNS. This review provides an overview of the different components of lipoprotein particles, in particular apolipoproteins, and their involvements in both physiological functions and pathological mechanisms in the CNS.
TREM2 is an innate immune receptor specifically expressed in microglia. Coding variations in TREM2 have been reported to increase the risk for Alzheimer's disease (AD) and other neurodegenerative ...diseases. While multiple studies support a role for TREM2 in microglial recruitment to amyloid plaques, the chemoattractant factor modulating TREM2-dependent microglial responses has not been defined.
Potential binding of oligomeric amyloid-β 1-42 (oAβ
) to TREM2 was tested by complementary approaches including solid phase binding, surface plasmon resonance and immunoprecipitation assays. The ability of oAβ
to activate TREM2 signaling pathways was examined by analyzing the phosphorylation of Syk and Akt in primary microglia as well as TREM2-mediated signaling in a reporter cell system. Lastly, the functional outcome of oAβ
-TREM2 interaction was tested by examining impacts on microglial migration in vitro and clustering around oAβ
-bearing brain areas in vivo.
We found that oAβ
bound to TREM2 with high affinity and activated TREM2-dependent signaling pathway. Neither monomeric nor scrambled Aβ bound to TREM2 supporting a specific interaction between oAβ and TREM2. The disease-associated mutations of TREM2 reduced its binding affinity to oAβ
. Furthermore, we identified several positively charged amino acids within residues 31-91 of TREM2 that were crucial for its interaction with oAβ
. Importantly, oAβ
promoted microglial migration in vitro and clustering in vivo in a TREM2-dependent manner.
Our data establish a critical link between oAβ
, a major pathological component of AD, and TREM2, a strong genetic risk factor for AD expressed in microglia, and suggest that such interaction contributes to the pathogenic events in AD by modulating microglial responses.
The ε4 allele of the APOE gene encoding apolipoprotein E (apoE) is a strong genetic risk factor for aging-related cognitive decline as well as late-onset Alzheimer's disease (AD) compared to the ...common ε3 allele. In the central nervous system, apoE is produced primarily by astrocytes and functions in transporting lipids including cholesterol to support neuronal homeostasis and synaptic integrity. Although mouse models and corresponding primary cells have provided valuable tools for studying apoE isoform-dependent functions, recent studies have shown that human astrocytes have a distinct gene expression profile compare with rodent astrocytes. Human induced pluripotent stem cells (iPSCs) derived from individuals carrying specific gene variants or mutations provide an alternative cellular model more relevant to humans upon differentiation into specific cell types. Thus, we reprogramed human skin fibroblasts from cognitively normal individuals carrying APOE ε3/ε3 or ε4/ε4 genotype to iPSC clones and further differentiated them into neural progenitor cells and then astrocytes. We found that human iPSC-derived astrocytes secreted abundant apoE with apoE4 lipoprotein particles less lipidated compared to apoE3 particles. More importantly, human iPSC-derived astrocytes were capable of promoting neuronal survival and synaptogenesis when co-cultured with iPSC-derived neurons with APOE ε4/ε4 astrocytes less effective in supporting these neurotrophic functions than those with APOE ε3/ε3 genotype. Taken together, our findings demonstrate APOE genotype-dependent effects using human iPSC-derived astrocytes and provide novel evidence that the human iPSC-based model system is a strong tool to explore how apoE isoforms contribute to neurodegenerative diseases.
Carrying premature termination codons in 1 allele of the ABCA7 gene is associated with an increased risk for Alzheimer’s disease (AD). While the primary function of ABCA7 is to regulate the transport ...of phospholipids and cholesterol, ABCA7 is also involved in maintaining homeostasis of the immune system. Since inflammatory pathways causatively or consequently participate in AD pathogenesis, we studied the effects of Abca7 haplodeficiency in mice on brain immune responses under acute and chronic conditions. When acute inflammation was induced through peripheral lipopolysaccharide injection in control or heterozygous Abca7 knockout mice, partial ABCA7 deficiency diminished proinflammatory responses by impairing CD14 expression in the brain. On breeding to AppNL-G-F
knockin mice, we observed increased amyloid-β (Aβ) accumulation and abnormal endosomal morphology in microglia. Taken together, our results demonstrate that ABCA7 loss of function may contribute to AD pathogenesis by altering proper microglial responses to acute inflammatory challenges and during the development of amyloid pathology, providing insight into disease mechanisms and possible treatment strategies.
Apolipoprotein E (APOE) ε4 allele is the strongest genetic risk factor for late-onset Alzheimer's disease mainly by modulating amyloid-β pathology. APOE ε4 is also shown to exacerbate ...neurodegeneration and neuroinflammation in a tau transgenic mouse model. To further evaluate the association of APOE genotype with the presence and severity of tau pathology, we express human tau via an adeno-associated virus gene delivery approach in human APOE targeted replacement mice. We find increased hyperphosphorylated tau species, tau aggregates, and behavioral abnormalities in mice expressing APOE ε2/ε2. We also show that in humans, the APOE ε2 allele is associated with increased tau pathology in the brains of progressive supranuclear palsy (PSP) cases. Finally, we identify an association between the APOE ε2/ε2 genotype and risk of tauopathies using two series of pathologically-confirmed cases of PSP and corticobasal degeneration. Our data together suggest APOE ε2 status may influence the risk and progression of tauopathy.
Carrying the ε4 allele of the APOE gene encoding apolipoprotein E (APOE4) markedly increases the risk for late-onset Alzheimer's disease (AD), in which APOE4 exacerbates the brain accumulation and ...subsequent deposition of amyloid-β (Aβ) peptides. While the LDL receptor-related protein 1 (LRP1) is a major apoE receptor in the brain, we found that its levels are associated with those of insoluble Aβ depending on APOE genotype status in postmortem AD brains. Thus, to determine the functional interaction of apoE4 and LRP1 in brain Aβ metabolism, we crossed neuronal LRP1-knockout mice with amyloid model APP/PS1 mice and APOE3-targeted replacement (APO3-TR) or APOE4-TR mice. Consistent with previous findings, mice expressing apoE4 had increased Aβ deposition and insoluble amounts of Aβ40 and Aβ42 in the hippocampus of APP/PS1 mice compared with those expressing apoE3. Intriguingly, such effects were reversed in the absence of neuronal LRP1. Neuronal LRP1 deficiency also increased detergent-soluble apoE4 levels, which may contribute to the inhibition of Aβ deposition. Together, our results suggest that apoE4 exacerbates Aβ pathology through a mechanism that depends on neuronal LRP1. A better understanding of apoE isoform-specific interaction with their metabolic receptor LRP1 on Aβ metabolism is crucial for defining APOE4-related risk for AD.
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