Apolipoprotein E (ApoE) isoform-dependent lipid release from astrocytes prepared from human ApoE3 and ApoE4 knock-in mice

APOE and Alzheimer's disease: Pathologic clues from transgenic Drosophila melanogaster

Alzheimer's disease (AD) is one of the most common forms of neurodegenerative diseases. Apolipoprotein E4 (ApoE4) is the main genetic risk factor in the development of late-onset AD. However, the exact mechanism underlying ApoE4-mediated neurodegeneration remains unclear. We utilized Drosophila melanogaster to examine the neurotoxic effects of various human APOE isoforms when expressed specifically in glial and neural cells. We assessed impacts on mitochondrial dynamics, ER stress, lipid metabolism, and bio-metal ion concentrations in the central nervous system (CNS) of the transgenic flies. Dachshund antibody staining revealed a reduction in the number of Kenyon cells. Behavioral investigations including ethanol tolerance and learning and memory performance demonstrated neuronal dysfunction in APOE4-expressing larvae and adult flies. Transcription level of marf and drp-1 were found to be elevated in APOE4 flies, while atf4, atf6, and xbp-1 s showed down regulation. Enhanced concentrations of triglyceride and cholesterol in the CNS were observed in APOE4 transgenic flies, with especially pronounced effects upon glial-specific expression of the gene. Spectrophotometry of brain homogenate revealed enhanced Fe++ and Zn++ ion levels in conjunction with diminished Cu++ levels upon APOE4 expression. To explore therapeutic strategies, we subjected the flies to heat-shock treatment, aiming to activate heat-shock proteins (HSPs) and assess their potential to mitigate the neurotoxic effects of APOE isoforms. The results showed potential therapeutic benefits for APOE4-expressing flies, hinting at an ability to attenuate memory deterioration. Overall, our findings suggest that APOE4 can alter lipid metabolism, bio metal ion homeostasis, and disrupt the harmonious fission-fusion balance of neuronal and glial mitochondria, ultimately inducing ER stress. These alterations mirror the main clinical manifestations of AD in patients. Therefore, our work underscores the suitability of Drosophila as a fertile model for probing the pathological roles of APOE and furthering our understanding of diverse isoform-specific functions.

Lysosomal cholesterol accumulation in aged astrocytes impairs cholesterol delivery to neurons and can be rescued by cannabinoids

Cholesterol is crucial for the proper functioning of eukaryotic cells, especially neurons, which rely on cholesterol to maintain their complex structure and facilitate synaptic transmission. However, brain cells are isolated from peripheral cholesterol by the blood-brain barrier and mature neurons primarily uptake the cholesterol synthesized by astrocytes for proper function. This study aimed to investigate the effect of aging on cholesterol trafficking in astrocytes and its delivery to neurons. We found that aged astrocytes accumulated high levels of cholesterol in the lysosomal compartment, and this cholesterol buildup can be attributed to the simultaneous occurrence of two events: decreased levels of the ABCA1 transporter, which impairs ApoE-cholesterol export from astrocytes, and reduced expression of NPC1, which hinders cholesterol release from lysosomes. We show that these two events are accompanied by increased microR-33 in aged astrocytes, which targets ABCA1 and NPC1. In addition, we demonstrate that the microR-33 increase is triggered by oxidative stress, one of the hallmarks of aging. By coculture experiments, we show that cholesterol accumulation in astrocytes impairs the cholesterol delivery from astrocytes to neurons. Remarkably, we found that this altered transport of cholesterol could be alleviated through treatment with endocannabinoids as well as cannabidiol or CBD. Finally, according to data demonstrating that aged astrocytes develop an A1 phenotype, we found that cholesterol buildup is also observed in reactive C3+ astrocytes. Given that reduced neuronal cholesterol affects synaptic plasticity, the ability of cannabinoids to restore cholesterol transport from aged astrocytes to neurons holds significant implications in aging and inflammation.

Relationships of brain cholesterol and cholesterol biosynthetic enzymes to Alzheimer's pathology and dementia in the CFAS population-derived neuropathology cohort

Altered cholesterol metabolism is implicated in brain ageing and Alzheimer's disease. We examined whether key genes regulating cholesterol metabolism and levels of brain cholesterol are altered in dementia and Alzheimer's disease neuropathological change (ADNC). Temporal cortex (n = 99) was obtained from the Cognitive Function and Ageing Study. Expression of the cholesterol biosynthesis rate-limiting enzyme HMG-CoA reductase (HMGCR) and its regulator, SREBP2, were detected using immunohistochemistry. Expression of HMGCR, SREBP2, CYP46A1 and ABCA1 were quantified by qPCR in samples enriched for astrocyte and neuronal RNA following laser-capture microdissection. Total cortical cholesterol was measured using the Amplex Red assay. HMGCR and SREBP2 proteins were predominantly expressed in pyramidal neurones, and in glia. Neuronal HMGCR did not vary with ADNC, oxidative stress, neuroinflammation or dementia status. Expression of HMGCR neuronal mRNA decreased with ADNC (p = 0.022) and increased with neuronal DNA damage (p = 0.049), whilst SREBP2 increased with ADNC (p = 0.005). High or moderate tertiles for cholesterol levels were associated with increased dementia risk (OR 1.44, 1.58). APOE ε4 allele was not associated with cortical cholesterol levels. ADNC is associated with gene expression changes that may impair cholesterol biosynthesis in neurones but not astrocytes, whilst levels of cortical cholesterol show a weak relationship to dementia status.

The Contributions of the Endolysosomal Compartment and Autophagy to APOEɛ4 Allele-Mediated Increase in Alzheimer's Disease Risk

Apolipoprotein E4 (APOE4), although yet-to-be fully understood, increases the risk and lowers the age of onset of Alzheimer's disease (AD), which is the major cause of dementia among elderly individuals. The endosome-lysosome and autophagy pathways, which are necessary for homeostasis in both neurons and glia, are dysregulated even in early AD. Nonetheless, the contributory roles of these pathways to developing AD-related pathologies in APOE4 individuals and models are unclear. Therefore, this review summarizes the dysregulations in the endosome-lysosome and autophagy pathways in APOE4 individuals and non-human models, and how these anomalies contribute to developing AD-relevant pathologies. The available literature suggests that APOE4 causes endosomal enlargement, increases endosomal acidification, impairs endosomal recycling, and downregulates exosome production. APOE4 impairs autophagy initiation and inhibits basal autophagy and autophagy flux. APOE4 promotes lysosome formation and trafficking and causes ApoE to accumulate in lysosomes. APOE4-mediated changes in the endosome, autophagosome and lysosome could promote AD-related features including Aβ accumulation, tau hyperphosphorylation, glial dysfunction, lipid dyshomeostasis, and synaptic defects. ApoE4 protein could mediate APOE4-mediated endosome-lysosome-autophagy changes. ApoE4 impairs vesicle recycling and endosome trafficking, impairs the synthesis of autophagy genes, resists being dissociated from its receptors and degradation, and forms a stable folding intermediate that could disrupt lysosome structure. Drugs such as molecular correctors that target ApoE4 molecular structure and enhance autophagy may ameliorate the endosome-lysosome-autophagy-mediated increase in AD risk in APOE4 individuals.

APOE expression and secretion are modulated by mitochondrial dysfunction

Mitochondria influence cellular function through both cell-autonomous and non-cell autonomous mechanisms, such as production of paracrine and endocrine factors. Here, we demonstrate that mitochondrial regulation of the secretome is more extensive than previously appreciated, as both genetic and pharmacological disruption of the electron transport chain caused upregulation of the Alzheimer's disease risk factor apolipoprotein E (APOE) and other secretome components. Indirect disruption of the electron transport chain by gene editing of SLC25A mitochondrial membrane transporters as well as direct genetic and pharmacological disruption of either complexes I, III, or the copper-containing complex IV of the electron transport chain elicited upregulation of APOE transcript, protein, and secretion, up to 49-fold. These APOE phenotypes were robustly expressed in diverse cell types and iPSC-derived human astrocytes as part of an inflammatory gene expression program. Moreover, age- and genotype-dependent decline in brain levels of respiratory complex I preceded an increase in APOE in the 5xFAD mouse model. We propose that mitochondria act as novel upstream regulators of APOE-dependent cellular processes in health and disease.

A Function of Amyloid-β in Mediating Activity-Dependent Axon/Synapse Competition May Unify Its Roles in Brain Physiology and Pathology

Amyloid-β protein precursor (AβPP) gives rise to amyloid-β (Aβ), a peptide at the center of Alzheimer's disease (AD). AβPP, however, is also an ancient molecule dating back in evolution to some of the earliest forms of metazoans. This suggests a possible ancestral function that may have been obscured by those that evolve later. Based on literature from the functions of Aβ/AβPP in nervous system development, plasticity, and disease, to those of anti-microbial peptides (AMPs) in bacterial competition as well as mechanisms of cell competition uncovered first by Drosophila genetics, I propose that Aβ/AβPP may be part of an ancient mechanism employed in cell competition, which is subsequently co-opted during evolution for the regulation of activity-dependent neural circuit development and plasticity. This hypothesis is supported by foremost the high similarities of Aβ to AMPs, both of which possess unique, opposite (i.e., trophic versus toxic) activities as monomers and oligomers. A large body of data further suggests that the different Aβ oligomeric isoforms may serve as the protective and punishment signals long predicted to mediate activity-dependent axonal/synaptic competition in the developing nervous system and that the imbalance in their opposite regulation of innate immune and glial cells in the brain may ultimately underpin AD pathogenesis. This hypothesis can not only explain the diverse roles observed of Aβ and AβPP family molecules, but also provide a conceptual framework that can unify current hypotheses on AD. Furthermore, it may explain major clinical observations not accounted for and identify approaches for overcoming shortfalls in AD animal modeling.

Apolipoprotein E4 inhibits γ-secretase activity via binding to the γ-secretase complex

The mechanisms of amyloid accumulation in familial Alzheimer's disease (FAD) and sporadic AD (SAD) are controversial. In FAD, mutations in presenilin (PSEN) impair γ-secretase activity and lead to abnormal amyloid β-protein (Aβ) production, thereby increasing the Aβ42/40 ratio. SAD is postulated to be caused by decreased Aβ clearance of apolipoprotein E4 (APOE4), the strongest risk factor for SAD. However, whether intracellular APOE4 affects Aβ production is unclear. Using APOE3 and APOE4 knock-in (KI) mouse brain and primary cultured fibroblasts from these mice, in this study, we demonstrated that APOE3 and APOE4 bind to the γ-secretase complex and isoform-dependently regulate its activity and Aβ production. We found that Aβ40 levels and γ-secretase activity were higher in APOE knockout mouse brain than in wild-type mouse brain. APOE4-KI fibroblasts had significant lower Aβ levels and γ-secretase activity but higher Aβ42/40 ratio compared with APOE3-KI cells, indicating that APOE4-KI reduces Aβ production by inhibiting γ-secretase activity. Interestingly, the levels of γ-secretase complex bound to APOE4 are higher than those bound to APOE3, and the levels of γ-secretase complex in the brain and fibroblasts of APOE4-KI mice were higher than those of APOE3-KI mice. Taken together, our findings demonstrate that intracellular APOE4 inhibits Aβ production, more preferentially inhibits Aβ40 production, and thereby induces an increase in the Aβ42/40 ratio via binding to the γ-secretase complex. These results suggest a novel mechanism in which intracellular APOE4 contributes to the pathogenesis of SAD by inhibiting γ-secretase activity.

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

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.

APOE in the bullseye of neurodegenerative diseases: impact of the APOE genotype in Alzheimer’s disease pathology and brain diseases

ApoE is the major lipid and cholesterol carrier in the CNS. There are three major human polymorphisms, apoE2, apoE3, and apoE4, and the genetic expression of APOE4 is one of the most influential risk factors for the development of late-onset Alzheimer's disease (AD). Neuroinflammation has become the third hallmark of AD, together with Amyloid-β plaques and neurofibrillary tangles of hyperphosphorylated aggregated tau protein. This review aims to broadly and extensively describe the differential aspects concerning apoE. Starting from the evolution of apoE to how APOE's single-nucleotide polymorphisms affect its structure, function, and involvement during health and disease. This review reflects on how APOE's polymorphisms impact critical aspects of AD pathology, such as the neuroinflammatory response, particularly the effect of APOE on astrocytic and microglial function and microglial dynamics, synaptic function, amyloid-β load, tau pathology, autophagy, and cell–cell communication. We discuss influential factors affecting AD pathology combined with the APOE genotype, such as sex, age, diet, physical exercise, current therapies and clinical trials in the AD field. The impact of the APOE genotype in other neurodegenerative diseases characterized by overt inflammation, e.g., alpha- synucleinopathies and Parkinson's disease, traumatic brain injury, stroke, amyotrophic lateral sclerosis, and multiple sclerosis, is also addressed. Therefore, this review gathers the most relevant findings related to the APOE genotype up to date and its implications on AD and CNS pathologies to provide a deeper understanding of the knowledge in the APOE field.

Lipolysis-Stimulated Lipoprotein Receptor Acts as Sensor to Regulate ApoE Release in Astrocytes

Astroglia play an important role, providing de novo synthesized cholesterol to neurons in the form of ApoE-lipidated particles; disruption of this process can increase the risk of Alzheimer’s disease. We recently reported that glia-specific suppression of the lipolysis-stimulated lipoprotein receptor (LSR) gene leads to Alzheimer’s disease-like memory deficits. Since LSR is an Apo-E lipoprotein receptor, our objective of this study was to determine the effect of LSR expression modulation on cholesterol and ApoE output in mouse astrocytes expressing human ApoE3. qPCR analysis showed that siRNA-mediated lsr knockdown significantly increased expression of the genes involved in cholesterol synthesis, secretion, and metabolism. Analysis of media and lipoprotein fractions showed increased cholesterol and lipidated ApoE output in HDL-like particles. Further, lsr expression could be upregulated when astrocytes were incubated 5 days in media containing high levels (two-fold) of lipoprotein, or after 8 h treatment with 1 µM LXR agonist T0901317 in lipoprotein-deficient media. In both conditions of increased lsr expression, the ApoE output was repressed or unchanged despite increased abca1 mRNA levels and cholesterol production. We conclude that LSR acts as a sensor of lipoprotein content in the medium and repressor of ApoE release, while ABCA1 drives cholesterol efflux, thereby potentially affecting cholesterol load, ApoE lipidation, and limiting cholesterol trafficking towards the neuron.

Cholesterol and matrisome pathways dysregulated in astrocytes and microglia

The impact of apolipoprotein E ε4 (APOE4), the strongest genetic risk factor for Alzheimer's disease (AD), on human brain cellular function remains unclear. Here, we investigated the effects of APOE4 on brain cell types derived from population and isogenic human induced pluripotent stem cells, post-mortem brain, and APOE targeted replacement mice. Population and isogenic models demonstrate that APOE4 local haplotype, rather than a single risk allele, contributes to risk. Global transcriptomic analyses reveal human-specific, APOE4-driven lipid metabolic dysregulation in astrocytes and microglia. APOE4 enhances de novo cholesterol synthesis despite elevated intracellular cholesterol due to lysosomal cholesterol sequestration in astrocytes. Further, matrisome dysregulation is associated with upregulated chemotaxis, glial activation, and lipid biosynthesis in astrocytes co-cultured with neurons, which recapitulates altered astrocyte matrisome signaling in human brain. Thus, APOE4 initiates glia-specific cell and non-cell autonomous dysregulation that may contribute to increased AD risk.

Apolipoprotein E4 Effects a Distinct Transcriptomic Profile and Dendritic Arbor Characteristics in Hippocampal Neurons Cultured in vitro

The APOE gene is diversified by three alleles ε2, ε3, and ε4 encoding corresponding apolipoprotein (apo) E isoforms. Possession of the ε4 allele is signified by increased risks of age-related cognitive decline, Alzheimer's disease (AD), and the rate of AD dementia progression. ApoE is secreted by astrocytes as high-density lipoprotein-like particles and these are internalized by neurons upon binding to neuron-expressed apoE receptors. ApoE isoforms differentially engage neuronal plasticity through poorly understood mechanisms. We examined here the effects of native apoE lipoproteins produced by immortalized astrocytes homozygous for ε2, ε3, and ε4 alleles on the maturation and the transcriptomic profile of primary hippocampal neurons. Control neurons were grown in the presence of conditioned media from Apoe -/- astrocytes. ApoE2 and apoE3 significantly increase the dendritic arbor branching, the combined neurite length, and the total arbor surface of the hippocampal neurons, while apoE4 fails to produce similar effects and even significantly reduces the combined neurite length compared to the control. ApoE lipoproteins show no systemic effect on dendritic spine density, yet apoE2 and apoE3 increase the mature spines fraction, while apoE4 increases the immature spine fraction. This is associated with opposing effects of apoE2 or apoE3 and apoE4 on the expression of NR1 NMDA receptor subunit and PSD95. There are 1,062 genes differentially expressed across neurons cultured in the presence of apoE lipoproteins compared to the control. KEGG enrichment and gene ontology analyses show apoE2 and apoE3 commonly activate expression of genes involved in neurite branching, and synaptic signaling. In contrast, apoE4 cultured neurons show upregulation of genes related to the glycolipid metabolism, which are involved in dendritic spine turnover, and those which are usually silent in neurons and are related to cell cycle and DNA repair. In conclusion, our work reveals that lipoprotein particles comprised of various apoE isoforms differentially regulate various neuronal arbor characteristics through interaction with neuronal transcriptome. ApoE4 produces a functionally distinct transcriptomic profile, which is associated with attenuated neuronal development. Differential regulation of neuronal transcriptome by apoE isoforms is a newly identified biological mechanism, which has both implication in the development and aging of the CNS.

Isoform- and cell-state-specific lipidation of ApoE in astrocytes

Apolipoprotein E transports lipids and couples metabolism between astrocytes and neurons. The E4 variant (APOE4) affects these functions and represents a genetic predisposition for Alzheimer's disease, but the molecular mechanisms remain elusive. We show that ApoE produces different types of lipoproteins via distinct lipidation pathways. ApoE forms high-density lipoprotein (HDL)-like, cholesterol-rich particles via the ATP-binding cassette transporter 1 (ABCA1), a mechanism largely unaffected by ApoE polymorphism. Alternatively, ectopic accumulation of fat in astrocytes, a stress-associated condition, redirects ApoE toward the assembly and secretion of triacylglycerol-rich lipoproteins, a process boosted by the APOE4 variant. We demonstrate in vitro that ApoE can detect triacylglycerol in membranes and spontaneously assemble lipoprotein particles (10-20 nm) rich in unsaturated triacylglycerol, and that APOE4 has remarkable properties behaving as a strong triacylglycerol binder. We propose that fatty APOE4 astrocytes have reduced ability to clear toxic fatty acids from the extracellular milieu, because APOE4 reroutes them back to secretion.

Alzheimer's Disease: Current Perspectives and Advances in Physiological Modeling

Though Alzheimer's disease (AD) is the most common cause of dementia, complete disease-modifying treatments are yet to be fully attained. Until recently, transgenic mice constituted most in vitro model systems of AD used for preclinical drug screening; however, these models have so far failed to adequately replicate the disease's pathophysiology. However, the generation of humanized APOE4 mouse models has led to key discoveries. Recent advances in stem cell differentiation techniques and the development of induced pluripotent stem cells (iPSCs) have facilitated the development of novel in vitro devices. These "microphysiological" systems-in vitro human cell culture systems designed to replicate in vivo physiology-employ varying levels of biomimicry and engineering control. Spheroid-based organoids, 3D cell culture systems, and microfluidic devices or a combination of these have the potential to replicate AD pathophysiology and pathogenesis in vitro and thus serve as both tools for testing therapeutics and models for experimental manipulation.

Perihematomal Edema After Intracerebral Hemorrhage: An Update on Pathogenesis, Risk Factors, and Therapeutic Advances

Intracerebral hemorrhage (ICH) has one of the worst prognoses among patients with stroke. Surgical measures have been adopted to relieve the mass effect of the hematoma, and developing targeted therapy against secondary brain injury (SBI) after ICH is equally essential. Numerous preclinical and clinical studies have demonstrated that perihematomal edema (PHE) is a quantifiable marker of SBI after ICH and is associated with a poor prognosis. Thus, PHE has been considered a promising therapeutic target for ICH. However, the findings derived from existing studies on PHE are disparate and unclear. Therefore, it is necessary to classify, compare, and summarize the existing studies on PHE. In this review, we describe the growth characteristics and relevant underlying mechanism of PHE, analyze the contributions of different risk factors to PHE, present the potential impact of PHE on patient outcomes, and discuss the currently available therapeutic strategies.

Cholesterol Regulates Exosome Release in Cultured Astrocytes

Exosomes are vesicles secreted by various kinds of cells, and they are rich in cholesterol, sphingomyelin (SM), phosphatidylcholine, and phosphatidylserine. Although cellular sphingolipid-mediated exosome release has been reported, the involvement of other lipid components of cell membranes in the regulation of exosome release is poorly understood. Here, we show that the level of exosome release into conditioned media is significantly reduced in cultured astrocytes prepared from apolipoprotein E (ApoE) knock-out mice when compared to those prepared from wild-type (WT) mice. The reduced level of exosome release was accompanied by elevated levels of cellular cholesterol. The addition of cholesterol to WT astrocytes significantly increased the cellular cholesterol levels and reduced exosome release. PI3K/Akt phosphorylation was enhanced in ApoE-deficient and cholesterol-treated WT astrocytes. In contrast, the depletion of cholesterol in ApoE-deficient astrocytes due to treatment with β-cyclodextrin recovered the exosome release level to a level similar to that in WT astrocytes. In addition, the reduced levels of exosome release due to the addition of cholesterol recovered to the control levels after treatment with a PI3K inhibitor (LY294002). The cholesterol-dependent regulation of exosome release was also confirmed by in vivo experiments; that is, exosome levels were significantly reduced in the CSF and blood serum of WT mice that were fed a high-fat diet and had increased cholesterol levels when compared to those in WT mice that were fed a normal diet. These results suggest that exosome release is regulated by cellular cholesterol via stimulation of the PI3K/Akt signal pathway.

APOE4 genotype exacerbates the depression-like behavior of mice during aging through ATP decline

Population-based studies reveal that apolipoprotein E (APOE) ε4 gene allele is closely associated with late-life depression (LLD). However, its exact role and underlying mechanism remain obscure. The current study found that aged apoE4-targeted replacement (TR) mice displayed obvious depression-like behavior when compared with age-matched apoE3-TR mice. Furthermore, apoE4 increased stress-induced depression-like behaviors, accompanied by declines in the hippocampal 5-HT (1A) radioligand [18F] MPPF uptake evidenced by positron emission tomography (PET). In [18F]-fluorodeoxyglucose PET ([18F]-FDG PET) analyses, the FDG uptake in the prefrontal cortex, temporal cortex and hippocampus of apoE4-TR mice significantly declined when compared with that of apoE3-TR mice after acute stress. Further biochemical analysis revealed that ATP levels in the prefrontal cortex of apoE4-TR mice decreased during aging or stress process and ATP supplementation effectively rescued the depression-like behaviors of elderly apoE4-TR mice. In primary cultured astrocytes from the cortex of apoE-TR mice, apoE4, when compared with apoE3, obviously decreased the mitochondrial membrane potential, mitochondrial respiration, and glycolysis in a culture time-dependent manner. Our findings highlight that apoE4 is a potential risk factor of depression in elderly population by impairing the glucose metabolism, reducing ATP level, and damaging mitochondrial functions in astrocytes, which indicates that in clinical settings ATP supplementation may be effective for elderly depression patients with apoE4 carrier.

The redox status of cysteine thiol residues of apolipoprotein E impacts on its lipid interactions

Redox-mediated modulation of cysteine (Cys) thiols has roles in various pathophysiological functions. We recently found that formation of disulfide-linked complexes of apolipoprotein (apo) E3 prevented apoE3 from irreversible oxidation. In this report, the influence of modification of Cys thiols in apoE2 and apoE3 on interactions with lipids was investigated. The apoE redox status was examined by a band-shift assay using a maleimide compound, and interactions with lipids were evaluated by a kinetic assay using dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and non-denaturing polyacrylamide gel electrophoresis. A reduction in DMPC clearance activity of apoE2 and apoE3 but not apoE4 was observed. Although hydrogen peroxide-induced oxidation decreased the clearance activity of the isoforms, apoE2 showed the greatest residual activity. Both Cys thiol masking and dimerization decreased the activity of apoE2 and apoE3 but not apoE4. In contrast, apoAII preincubation markedly increased the activity (apoE2 > apoE3 > apoE4), in accordance with the formation of apoE-AII and apoAII-E2-AII complexes. ApoAII preincubation also reduced the particle size of apoE-DMPC liposome complexes, especially for apoE2. Redox-mediated modification of Cys thiols of apoE2 or apoE3, especially disulfide bond formation with apoAII, affects lipid metabolism and consequently may be responsible for the diverse isoform specificity of apoE.

ApoE and ApoE Nascent-Like HDL Particles at Model Cellular Membranes: Effect of Protein Isoform and Membrane Composition

Apolipoprotein E (ApoE), an important mediator of lipid transportation in plasma and the nervous system, plays a large role in diseases such as atherosclerosis and Alzheimer's. The major allele variants ApoE3 and ApoE4 differ only by one amino acid. However, this difference has major consequences for the physiological behaviour of each variant. In this paper, we follow (i) the initial interaction of lipid-free ApoE variants with model membranes as a function of lipid saturation, (ii) the formation of reconstituted High-Density Lipoprotein-like particles (rHDL) and their structural characterisation, and (iii) the rHDL ability to exchange lipids with model membranes made of saturated lipids in the presence and absence of cholesterol [1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) or 1-palmitoyl-2-oleoyl-glycero-3-phosphocholine (POPC) with and without 20 mol% cholesterol]. Our neutron reflection results demonstrate that the protein variants interact differently with the model membranes, adopting different protein conformations. Moreover, the ApoE3 structure at the model membrane is sensitive to the level of lipid unsaturation. Small-angle neutron scattering shows that the ApoE containing lipid particles form elliptical disc-like structures, similar in shape but larger than nascent or discoidal HDL based on Apolipoprotein A1 (ApoA1). Neutron reflection shows that ApoE-rHDL do not remove cholesterol but rather exchange saturated lipids, as occurs in the brain. In contrast, ApoA1-containing particles remove and exchange lipids to a greater extent as occurs elsewhere in the body.

Precision Nutrition for Alzheimer's Prevention in ApoE4 Carriers

The ApoE4 allele is the most well-studied genetic risk factor for Alzheimer’s disease, a condition that is increasing in prevalence and remains without a cure. Precision nutrition targeting metabolic pathways altered by ApoE4 provides a tool for the potential prevention of disease. However, no long-term human studies have been conducted to determine effective nutritional protocols for the prevention of Alzheimer’s disease in ApoE4 carriers. This may be because relatively little is yet known about the precise mechanisms by which the genetic variant confers an increased risk of dementia. Fortunately, recent research is beginning to shine a spotlight on these mechanisms. These new data open up the opportunity for speculation as to how carriers might ameliorate risk through lifestyle and nutrition. Herein, we review recent discoveries about how ApoE4 differentially impacts microglia and inflammatory pathways, astrocytes and lipid metabolism, pericytes and blood–brain barrier integrity, and insulin resistance and glucose metabolism. We use these data as a basis to speculate a precision nutrition approach for ApoE4 carriers, including a low-glycemic index diet with a ketogenic option, specific Mediterranean-style food choices, and a panel of seven nutritional supplements. Where possible, we integrate basic scientific mechanisms with human observational studies to create a more complete and convincing rationale for this precision nutrition approach. Until recent research discoveries can be translated into long-term human studies, a mechanism-informed practical clinical approach may be useful for clinicians and patients with ApoE4 to adopt a lifestyle and nutrition plan geared towards Alzheimer’s risk reduction.

Astrocytic expression of the Alzheimer's disease risk allele, ApoEε4, potentiates neuronal tau pathology in multiple preclinical models

ApoEε4 is a major genetic risk factor for Alzheimer's disease (AD), a disease hallmarked by extracellular amyloid-beta (Aβ) plaques and intracellular neurofibrillary tangles (NFTs). The presence of the ApoEε4 allele is associated with increased Aβ deposition and a role for ApoEε4 in the potentiation of tau pathology has recently emerged. This study focused on comparing the effects of adeno-associated virus (AAV)-mediated overexpression of the three predominant human ApoE isoforms within astrocytes. The isoform-specific effects of human ApoE were evaluated within in vitro models of tau pathology within neuron/astrocyte co-cultures, as well as in a transgenic tau mouse model. Tau aggregation, accumulation, and phosphorylation were measured to determine if the three isoforms of human ApoE had differential effects on tau. Astrocytic overexpression of the human ApoEε4 allele increased phosphorylation and misfolding of overexpressed neuronal tau in multiple models, including the aggregation and accumulation of added tau oligomers, in an isoform-specific manner. The ability of ApoEε4 to increase tau aggregation could be inhibited by an ApoEε4-specific antibody. This study indicates that astrocytic expression of ApoEε4 can potentiate tau aggregation and phosphorylation within neurons and supports a gain of toxic function hypothesis for the effect of hApoEε4 on tau.

Cholesterol Metabolism in Neurodegenerative Diseases: Molecular Mechanisms and Therapeutic Targets

Cholesterol is an indispensable component of the cell membrane and plays vital roles in critical physiological processes. Brain cholesterol accounts for a large portion of total cholesterol in the human body, and its content must be tightly regulated to ensure normal brain function. Disorders of cholesterol metabolism in the brain are linked to neurodegenerative diseases, including Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease (HD), and other atypical cognitive deficits that arise at old age. However, the specific role of cholesterol metabolism disorder in the pathogenesis of neurodegenerative diseases has not been fully elucidated. Statins that are a class of lipid-lowering drugs have been reported to have a positive effect on neurodegenerative diseases. Herein, we reviewed the physiological and pathological conditions of cholesterol metabolism and discussed the possible mechanisms of cholesterol metabolism and statin therapy in neurodegenerative diseases.

APOE-ε4-related differences in left thalamic microstructure in cognitively healthy adults

APOE-ε4 is a main genetic risk factor for developing late onset Alzheimer's disease (LOAD) and is thought to interact adversely with other risk factors on the brain. However, evidence regarding the impact of APOE-ε4 on grey matter structure in asymptomatic individuals remains mixed. Much attention has been devoted to characterising APOE-ε4-related changes in the hippocampus, but LOAD pathology is known to spread through the whole of the Papez circuit including the limbic thalamus. Here, we tested the impact of APOE-ε4 and two other risk factors, a family history of dementia and obesity, on grey matter macro- and microstructure across the whole brain in 165 asymptomatic individuals (38-71 years). Microstructural properties of apparent neurite density and dispersion, free water, myelin and cell metabolism were assessed with Neurite Orientation Density and Dispersion (NODDI) and quantitative magnetization transfer (qMT) imaging. APOE-ε4 carriers relative to non-carriers had a lower macromolecular proton fraction (MPF) in the left thalamus. No risk effects were present for cortical thickness, subcortical volume, or NODDI indices. Reduced thalamic MPF may reflect inflammation-related tissue swelling and/or myelin loss in APOE-ε4. Future prospective studies should investigate the sensitivity and specificity of qMT-based MPF as a non-invasive biomarker for LOAD risk.

Non-hematopoietic erythropoietin-derived peptides for atheroprotection and treatment of cardiovascular diseases

Relevance: Cardiovascular diseases continue to be the leading cause of premature adult death.

Lipid profile and atherogenesis: Dislipidaemia leads to subsequent lipid accumulation and migration of immunocompetent cells into the vessel intima. Macrophages accumulate cholesterol forming foam cells – the morphological substrate of atherosclerosis in its initial stage.

Inflammation and atherogenesis: Pro-inflammatory factors provoke oxidative stress, vascular wall damage and foam cells formation.

Endothelial and mitochondrial dysfunction in the development of atherosclerosis: Endothelial mitochondria are some of the organelles most sensitive to oxidative stress. Damaged mitochondria produce excess superoxide and H2O2, which are the main factors of intracellular damage, further increasing endothelial dysfunction.

Short non-hematopoietic erythropoietin-based peptides as innovative atheroprotectors: Research in recent decades has shown that erythropoietin has a high cytoprotective activity, which is mainly associated with exposure to the mitochondrial link and has been confirmed in various experimental models. There is also a short-chain derivative, the 11-amino acid pyroglutamate helix B surface peptide (PHBSP), which selectively binds to the erythropoietin heterodymic receptor and reproduces its cytoprotective properties. This indicates the promising use of short-chain derivatives of erythropoietin for the treatment and prevention of atherosclerotic vascular injury. In the future, it is planned to study the PHBSP derivatives, the modification of which consists in adding RGD and PGP tripeptides with antiaggregant properties to the original 11-member peptide.

Association of APOE Gene Polymorphism with Stroke Patients from Rural Eastern India

CONTEXT

Studies from the different ethnic regions of the world have reported variable results on association of APOE gene polymorphism in stroke.

AIM

The aim of this study is to find out the possible association of APOE polymorphism in stroke patients in ethnic Bengali population.

SETTINGS AND DESIGN

A prospective case-control study was undertaken in the Department of Neurology, Burdwan Medical College, Burdwan, West Bengal, India, over a period of 3 years.

METHODS

We collected 10 ml venous blood samples from 148 clinically and radiologically diagnosed acute stroke patients (80 of ischemic stroke and 68 of intracerebral hemorrhage) and consecutive 108 ethnic age- and sex-matched controls, in ethylenediaminetetraacetic acid vials after informed written consent. Genomic DNA was prepared at S.N. Pradhan Centre of Neurosciences, University of Calcutta, Kolkata, India. Exotic single-nucleotide polymorphisms (rs429358, rs 7412) were analyzed by polymerase chain reaction-restriction fragment length polymorphism for genotype of APOE.

RESULTS

The frequencies of different APOE allele among 80 ischemic stroke patients were 5.6% (n = 9) for E2, 75.68% (n = 121) for E3, and 18.7% (n = 30) for E4. The E3 allele is significantly over-represented (P = 0.004) in controls compared to the patients (88% in controls vs 75.6% ischemic stroke patients and 80% hemorrhagic patients). A significantly high frequency of APOE4 allele was observed in ischemic (18.7%) and hemorrhagic patients (11%) compared to controls (8%). The E4 allele plays a major risk for developing ischemic stroke [odds ratio (OR) = 2.744; 95% confidence interval (CI): 1.43-5.10] and E3 plays a protective role for hemorrhagic stroke (OR = 0.53; 95% CI: 0.29-0.96), while E4 allele plays a nonsignificant (P = 0.31) increase in trend in hemorrhage stroke (OR = 1.4).

CONCLUSIONS

There is significant association of APOE gene polymorphism in stroke patients of ethnic Bengali population. The E4 allele increases significant risk for development of ischemic strokes, and it also plays nonsignificant increase in trend in hemorrhagic strokes.

Genetic risk of dementia modifies obesity effects on white matter myelin in cognitively healthy adults

APOE-ε4 is a major genetic risk factor for late-onset Alzheimer's disease that interacts with other risk factors, but the nature of such combined effects remains poorly understood. We quantified the impact of APOE-ε4, family history (FH) of dementia, and obesity on white matter (WM) microstructure in 165 asymptomatic adults (38-71 years old) using quantitative magnetization transfer and neurite orientation dispersion and density imaging. Microstructural properties of the fornix, parahippocampal cingulum, and uncinate fasciculus were compared with those in motor and whole-brain WM regions. Widespread interaction effects between APOE, FH, and waist-hip ratio were found in the myelin-sensitive macromolecular proton fraction from quantitative magnetization transfer. Among individuals with the highest genetic risk (FH+ and APOE-ε4), obesity was associated with reduced macromolecular proton fraction in the right parahippocampal cingulum, whereas no effects were present for those without FH. Risk effects on apparent myelin were moderated by hypertension and inflammation-related markers. These findings suggest that genetic risk modifies the impact of obesity on WM myelin consistent with neuroglia models of aging and late-onset Alzheimer's disease.

APOE Alleles and Diet in Brain Aging and Alzheimer's Disease

The APOE gene alleles modify human aging and the response to the diet at many levels with diverse pleotropic effects from gut to brain. To understand the interactions of APOE isoforms and diet, we analyze how cellular trafficking of apoE proteins affects energy metabolism, the immune system, and reproduction. The age-accelerating APOE4 allele alters the endosomal trafficking of cell surface receptors that mediate lipid and glucose metabolism. The APOE4 allele is the ancestral human allele, joined by APOE3 and then APOE2 in the human species. Under conditions of high infection, uncertain food, and shorter life expectancy, APOE4 may be adaptive for reducing mortality. As humans transitioned into modern less-infectious environments and longer life spans, APOE4 increased risks of aging-related diseases, particularly impacting arteries and the brain. The association of APOE4 with glucose dysregulation and body weight promotes many aging-associated diseases. Additionally, the APOE gene locus interacts with adjacent genes on chromosome 19 in haplotypes that modify neurodegeneration and metabolism, for which we anticipate complex gene-environment interactions. We summarize how diet and Alzheimer's disease (AD) risk are altered by APOE genotype in both animal and human studies and identify gaps. Much remains obscure in how APOE alleles modify nutritional factors in human aging. Identifying risk variant haplotypes in the APOE gene complex will clarify homeostatic adaptive responses to environmental conditions.

Axl receptor tyrosine kinase is a regulator of apolipoprotein E

Alzheimer's disease (AD), the leading cause of dementia, is a chronic neurodegenerative disease. Apolipoprotein E (apoE), which carries lipids in the brain in the form of lipoproteins, plays an undisputed role in AD pathophysiology. A high-throughput phenotypic screen was conducted using a CCF-STTG1 human astrocytoma cell line to identify small molecules that could upregulate apoE secretion. AZ7235, a previously discovered Axl kinase inhibitor, was identified to have robust apoE activity in brain microglia, astrocytes and pericytes. AZ7235 also increased expression of ATP-binding cassette protein A1 (ABCA1), which is involved in the lipidation and secretion of apoE. Moreover, AZ7235 did not exhibit Liver-X-Receptor (LXR) activity and stimulated apoE and ABCA1 expression in the absence of LXR. Target validation studies using AXL-/- CCF-STTG1 cells showed that Axl is required to mediate AZ7235 upregulation of apoE and ABCA1. Intriguingly, apoE expression and secretion was significantly attenuated in AXL-deficient CCF-STTG1 cells and reconstitution of Axl or kinase-dead Axl significantly restored apoE baseline levels, demonstrating that Axl also plays a role in maintaining apoE homeostasis in astrocytes independent of its kinase activity. Lastly, these effects may require human apoE regulatory sequences, as AZ7235 exhibited little stimulatory activity toward apoE and ABCA1 in primary murine glia derived from neonatal human APOE3 targeted-replacement mice. Collectively, we identified a small molecule that exhibits robust apoE and ABCA1 activity independent of the LXR pathway in human cells and elucidated a novel relationship between Axl and apoE homeostasis in human astrocytes.

Using human induced pluripotent stem cells (hiPSCs) to investigate the mechanisms by which Apolipoprotein E (APOE) contributes to Alzheimer's disease (AD) risk

Although the biochemical and pathological hallmarks of Alzheimer's disease (AD), such as axonal transport defects, synaptic loss, and selective neuronal death, are well characterized, the underlying mechanisms that cause AD are largely unknown, thereby making it difficult to design effective therapeutic interventions. Genome-wide association studies (GWAS) studies have identified several factors associated with increased AD risk. Of these genetic factors, polymorphisms in the Apolipoprotein E (APOE) gene are the strongest and most prevalent. While it has been established that the ApoE protein modulates the formation of amyloid plaques and neurofibrillary tangles, the precise molecular mechanisms by which various ApoE isoforms enhance or mitigate AD onset and progression in aging adults are yet to be elucidated. Advances in cellular reprogramming to generate disease-in-a-dish models now provide a simplified and accessible system that complements animal and primary cell models to study ApoE in the context of AD. In this review, we will describe the use and manipulation of human induced pluripotent stem cells (hiPSCs) in dissecting the interaction between ApoE and AD. First, we will provide an overview of the proposed roles that ApoE plays in modulating pathophysiology of AD. Next, we will summarize the recent studies that have employed hiPSCs to model familial and sporadic AD. Lastly, we will speculate on how current advances in genome editing technologies and organoid culture systems can be used to improve hiPSC-based tools to investigate ApoE-dependent modulation of AD onset and progression.

Tight junction modulation at the blood-brain barrier: Current and future perspectives

The blood-brain barrier (BBB) is the one of the most robust physical barriers in the body, comprised of tight junction (TJ) proteins in brain microvascular endothelial cells. The need for drugs to treat central nervous systems diseases is ever increasing, however the presence of the BBB significantly hampers the uptake of drugs into the brain. To overcome or circumvent the barrier, many kinds of techniques are being developed. Modulating the paracellular route by disruption of the TJ complex has been proposed as a potential drug delivery system to treat brain diseases, however, it has several limitations and is still in a developmental stage. However, recent significant advance in medical equipment /tools such as targeted ultra-sound technologies may resolve these limitations. In this review, we introduce recent advances in site- or molecular size-selective BBB disruption/modulation technologies and we include details on pharmacological inhibitory molecules against intercellular TJ proteins to modulate the BBB.

Elevated TREM2 Gene Dosage Reprograms Microglia Responsivity and Ameliorates Pathological Phenotypes in Alzheimer's Disease Models

Variants of TREM2 are associated with Alzheimer's disease (AD). To study whether increasing TREM2 gene dosage could modify the disease pathogenesis, we developed BAC transgenic mice expressing human TREM2 (BAC-TREM2) in microglia. We found that elevated TREM2 expression reduced amyloid burden in the 5xFAD mouse model. Transcriptomic profiling demonstrated that increasing TREM2 levels conferred a rescuing effect, which includes dampening the expression of multiple disease-associated microglial genes and augmenting downregulated neuronal genes. Interestingly, 5xFAD/BAC-TREM2 mice showed further upregulation of several reactive microglial genes linked to phagocytosis and negative regulation of immune cell activation. Moreover, these mice showed enhanced process ramification and phagocytic marker expression in plaque-associated microglia and reduced neuritic dystrophy. Finally, elevated TREM2 gene dosage led to improved memory performance in AD models. In summary, our study shows that a genomic transgene-driven increase in TREM2 expression reprograms microglia responsivity and ameliorates neuropathological and behavioral deficits in AD mouse models.

Peripheral versus central nervous system APOE in Alzheimer's disease: Interplay across the blood-brain barrier

The apolipoprotein E (APOE) ε4 allele has been demonstrated as the preeminent genetic risk factor for late onset Alzheimer's disease (AD), which comprises greater than 90% of all AD cases. The discovery of the connection between different APOE genotypes and AD risk in the early 1990s spurred three decades of intense and comprehensive research into the function of APOE in the normal and diseased brain. The importance of APOE in the periphery has been well established, due to its pivotal role in maintaining cholesterol homeostasis and cardiovascular health. The influence of vascular factors on brain function and AD risk has been extensively studied in recent years. As a major apolipoprotein regulating multiple molecular pathways beyond its canonical lipid-related functions in the periphery and the central nervous system, APOE represents a critical link between the two compartments, and may influence AD risk from both sides of the blood-brain barrier. This review discusses recent advances in understanding the different functions of APOE in the periphery and in the brain, and highlights several promising APOE-targeted therapeutic strategies for AD.

Cerebrospinal fluid biomarkers for understanding multiple aspects of Alzheimer's disease pathogenesis

Alzheimer's disease (AD) is a multifactorial age-related brain disease. Numerous pathological events run forth in the brain leading to AD. There is an initial long, dormant phase before the clinical symptoms become evident. There is a need to diagnose the disease at the preclinical stage since therapeutic interventions are most likely to be effective if initiated early. Undoubtedly, the core cerebrospinal fluid (CSF) biomarkers have a good diagnostic accuracy and have been used in clinical trials as end point measures. However, looking into the multifactorial nature of AD and the overlapping pathology with other forms of dementia, it is important to integrate the core CSF biomarkers with a broader panel of other biomarkers reflecting different aspects of pathology. The review is focused upon a panel of biomarkers that relate to different aspects of AD pathology, as well as various studies that have evaluated their diagnostic potential. The panel includes markers of neurodegeneration: neurofilament light chain and visinin-like protein (VILIP-1); markers of amyloidogenesis and brain amyloidosis: apolipoproteins; markers of inflammation: YKL-40 and monocyte chemoattractant protein 1; marker of synaptic dysfunction: neurogranin. These markers can highlight on the state and stage-associated changes that occur in AD brain with disease progression. A combination of these biomarkers would not only aid in preclinical diagnosis, but would also help in identifying early brain changes during the onset of disease. Successful treatment strategies can be devised by understanding the contribution of these markers in different aspects of disease pathogenesis.

Redox equilibrium of serum apolipoprotein E3: a buffering effect of disulfide-linked complexes against oxidative stress on apolipoprotein E3-containing lipoproteins

Reversible redox modification of cysteine thiols is crucial for protecting proteins from irreversible detrimental change. However, the physiological significance of the redox modification of apolipoprotein (apo) E is unclear. Here, we hypothesized that the disulfide-linked complexes of apoE3 corresponding to the representative reversible-modified apoE3 play a protective role against oxidative stress. The effects of disulfide bond formation on oxidative stress on apoE3 were evaluated with a band-shift assay. Maleimide-labeled apoE3 and unlabeled apoE3 were defined as the reduced (r)-apoE3 and non-reduced (nr)-apoE3 forms, respectively. Hydrogen peroxide-induced oxidation decreased for reduced-form apoE (r-apoE3) but increased for nr-apoE3. Induction of apoE3-AII complex formation with excess of apoAII markedly suppressed the oxidative stress-induced increase in nr-apoE3 (P<0.001) and enhanced homodimer formation. The apoE3-AII complex was more dominant in high-density lipoprotein (HDL) than in very low-density lipoprotein. Under oxidative stress, HDL showed a significant decrease, rather than an increase, in nr-apoE3 levels with a concomitant significant increase in apoE3-AII levels (P<0.005). This finding suggests that the majority of nr-apoE3 in HDL exists in a reversible oxidized form. The apoE3-AII complex, formed from the reversible oxidized apoE3, is beneficial for maintaining the redox equilibrium of apoE3 by preventing the modification of apoE3 to its irreversible oxidized form. The apoE3-AII complex may be possibly implicated in the pathophysiology of various apoE-related diseases.

The role of APOE4 in Alzheimer's disease: strategies for future therapeutic interventions

Alzheimer’s disease (AD) is the leading cause of dementia affecting almost 50 million people worldwide. The ε4 allele of Apolipoprotein E (APOE) is the strongest known genetic risk factor for late-onset AD cases, with homozygous APOE4 carriers being approximately 15-times more likely to develop the disease. With 25% of the population being APOE4 carriers, understanding the role of this allele in AD pathogenesis and pathophysiology is crucial. Though the exact mechanism by which ε4 allele increases the risk for AD is unknown, the processes mediated by APOE, including cholesterol transport, synapse formation, modulation of neurite outgrowth, synaptic plasticity, destabilization of microtubules, and β-amyloid clearance, suggest potential therapeutic targets. This review will summarize the impact of APOE on neurons and neuronal signaling, the interactions between APOE and AD pathology, and the association with memory decline. We will then describe current treatments targeting APOE4, complications associated with the current therapies, and suggestions for future areas of research and treatment.

Multivariate MR biomarkers better predict cognitive dysfunction in mouse models of Alzheimer's disease

To understand multifactorial conditions such as Alzheimer's disease (AD) we need brain signatures that predict the impact of multiple pathologies and their interactions. To help uncover the relationships between pathology affected brain circuits and cognitive markers we have used mouse models that represent, at least in part, the complex interactions altered in AD, while being raised in uniform environments and with known genotype alterations. In particular, we aimed to understand the relationship between vulnerable brain circuits and memory deficits measured in the Morris water maze, and we tested several predictive modeling approaches. We used in vivo manganese enhanced MRI traditional voxel based analyses to reveal regional differences in volume (morphometry), signal intensity (activity), and magnetic susceptibility (iron deposition, demyelination). These regions included hippocampus, olfactory areas, entorhinal cortex and cerebellum, as well as the frontal association area. The properties of these regions, extracted from each of the imaging markers, were used to predict spatial memory. We next used eigenanatomy, which reduces dimensionality to produce sets of regions that explain the variance in the data. For each imaging marker, eigenanatomy revealed networks underpinning a range of cognitive functions including memory, motor function, and associative learning, allowing the detection of associations between context, location, and responses. Finally, the integration of multivariate markers in a supervised sparse canonical correlation approach outperformed single predictor models and had significant correlates to spatial memory. Among a priori selected regions, expected to play a role in memory dysfunction, the fornix also provided good predictors, raising the possibility of investigating how disease propagation within brain networks leads to cognitive deterioration. Our cross-sectional results support that modeling approaches integrating multivariate imaging markers provide sensitive predictors of AD-like behaviors. Such strategies for mapping brain circuits responsible for behaviors may help in the future predict disease progression, or response to interventions.

The Role of APOE4 in Disrupting the Homeostatic Functions of Astrocytes and Microglia in Aging and Alzheimer's Disease

APOE4 is the greatest genetic risk factor for late-onset Alzheimer’s disease (AD), increasing the risk of developing the disease by 3-fold in the 14% of the population that are carriers. Despite 25 years of research, the exact mechanisms underlying how APOE4 contributes to AD pathogenesis remain incompletely defined. APOE in the brain is primarily expressed by astrocytes and microglia, cell types that are now widely appreciated to play key roles in the pathogenesis of AD; thus, a picture is emerging wherein APOE4 disrupts normal glial cell biology, intersecting with changes that occur during normal aging to ultimately cause neurodegeneration and cognitive dysfunction. This review article will summarize how APOE4 alters specific pathways in astrocytes and microglia in the context of AD and the aging brain. APOE itself, as a secreted lipoprotein without enzymatic activity, may prove challenging to directly target therapeutically in the classical sense. Therefore, a deeper understanding of the underlying pathways responsible for APOE4 toxicity is needed so that more tractable pathways and drug targets can be identified to reduce APOE4-mediated disease risk.

Neurointegrity and neurophysiology: astrocyte, glutamate, and carbon monoxide interactions

Astrocyte contributions to brain function and prevention of neuropathologies are as extensive as that of neurons. Astroglial regulation of glutamate, a primary neurotransmitter, is through uptake, release through vesicular and non-vesicular pathways, and catabolism to intermediates. Homeostasis by astrocytes is considered to be of primary importance in determining normal central nervous system health and central nervous system physiology - glutamate is central to dynamic physiologic changes and central nervous system stability. Gasotransmitters may affect diverse glutamate interactions positively or negatively. The effect of carbon monoxide, an intrinsic central nervous system gasotransmitter, in the complex astrocyte homeostasis of glutamate may offer insights to normal brain development, protection, and its use as a neuromodulator and neurotherapeutic. In this article, we will review the effects of carbon monoxide on astrocyte homeostasis of glutamate.

Brain APOE expression quantitative trait loci-based association study identified one susceptibility locus for Alzheimer's disease by interacting with APOE ε4

Some studies have demonstrated interactions of AD-risk single nucleotide polymorphisms (SNPs) in non-APOE regions with APOE genotype. Nevertheless, no study reported interactions of expression quantitative trait locus (eQTL) for APOE with APOE genotype. In present study, we included 9286 unrelated AD patients and 8479 normal controls from 12 cohorts of NIA Genetics of Alzheimer’s Disease Data Storage Site (NIAGADS) and Alzheimer’s Disease Neuroimaging Initiative (ADNI). 34 unrelated brain eQTLs for APOE were compiled from BRAINEAC and GTEx. We used multi-covariate logistic regression analysis to identify eQTLs interacted with APOE ε4. Adjusted for age and gender, substantia nigra eQTL rs438811 for APOE showed significantly strong interaction with APOE ε4 status (OR, 1.448; CI, 1.124–1.430; P-value = 7.94 × 10⁻⁶). APOE ε4-based sub-group analyses revealed that carrying one minor allele T of rs438811 can increase the opportunity of developing to AD by 26.75% in APOE ε4 carriers but not in non-carriers. We revealed substantia nigra eQTL rs438811 for APOE can interact with APOE ε4 and confers risk in APOE ε4 carriers only.

Frontiers of Complex Disease Mechanisms: Membrane Surface Tension May Link Genotype to Phenotype in Glaucoma

Although many monogenic diseases are understood based upon structural changes of gene products, less progress has been made concerning polygenic disease mechanisms. This article presents a new interdisciplinary approach to understand complex diseases, especially their genetic polymorphisms. I focus upon primary open angle glaucoma (POAG). Although elevated intraocular pressure (IOP) and oxidative stress are glaucoma hallmarks, the linkages between these factors and cell death are obscure. Reactive oxygen species (ROS) promote the formation of oxidatively truncated phosphoglycerides (OTP), free fatty acids, lysophosphoglycerides, oxysterols, and other chemical species that promote membrane disruption and decrease membrane surface tension. Several POAG-linked gene polymorphisms identify proteins that manage damaged lipids and/or influence membrane surface tension. POAG-related genes expected to participate in these processes include: ELOVL5, ABCA1, APOE4, GST, CYP46A1, MYOC, and CAV. POAG-related gene products are expected to influence membrane surface tension, strength, and repair. I propose that heightened IOP overcomes retinal ganglion cell (RGC) membrane compressive strength, weakened by damaged lipid accumulation, to form pores. The ensuing structural failure promotes apoptosis and blindness. The linkage between glaucoma genotype and phenotype is mediated by physical events. Force balancing between the IOP and compressive strength regulates pore nucleation; force balancing between pore line tension and membrane surface tension regulates pore growth. Similar events may contribute to traumatic brain injury, Alzheimer's disease, and macular degeneration.

APOE gene and neuropsychiatric disorders and endophenotypes: A comprehensive review

The Apolipoprotein E (APOE) gene is one of the main candidates in neuropsychiatric genetics, with hundreds of studies carried out in order to explore the possible role of polymorphisms in the APOE gene in a large number of neurological diseases, psychiatric disorders, and related endophenotypes. In the current article, we provide a comprehensive review of the structural and functional aspects of the APOE gene and its relationship with brain disorders. Evidence from genome-wide association studies and meta-analyses shows that the APOE gene has been significantly associated with several neurodegenerative disorders. Cellular and animal models show growing evidence of the key role of APOE in mechanisms of brain plasticity and behavior. Future analyses of the APOE gene might find a possible role in other neurological diseases and psychiatric disorders and related endophenotypes. © 2016 Wiley Periodicals, Inc.

Astrocytes and the Warning Signs of Intracerebral Hemorrhagic Stroke

Two decades into the two thousands, intracerebral hemorrhagic stroke (ICH) continues to reap lives across the globe. In the US, nearly 12,000 people suffer from ICH every year. Half of them survive, but many are left with permanent physical and cognitive disabilities, the severity of which depends on the location and broadness of the brain region affected by the hemorrhage. The ongoing efforts to identify risk factors for hemorrhagic stroke have been instrumental for the development of new medical practices to prevent, aid the recovery and reduce the risk of recurring ICH. Recent efforts approach the study of ICH from a different angle, providing information on how we can limit brain damage by manipulating astrocyte receptors. These results provide a novel understanding of how astrocytes contribute to brain injury and recovery from small ICH. Here, we discuss current knowledge on the risk factors and molecular pathology of ICH and the functional properties of astrocytes and their role in ICH. Last, we discuss candidate astrocyte receptors that may prove to be valuable therapeutic targets to treat ICH. Together, these findings provide basic and clinical scientists useful information for the future development of strategies to improve the detection of small ICH, limit brain damage, and prevent the onset of more severe episodes of brain hemorrhage.

Amyloid-β Reduces Exosome Release from Astrocytes by Enhancing JNK Phosphorylation

Exosomes are small extracellular vesicles secreted by variety of cell types such as neurons, astrocytes, and oligodendrocytes. It is suggested that exosomes play essential role in the maintenance of the neuronal functions and also in the clearance of amyloid-β (Aβ) from the brain. Aβ is well known to cause neuronal cell death, whereas little is known about its effect on astrocytes. In this study, we examined the effect of Aβ on release of exosomes from astrocytes in culture. We analyzed release of exosomes and apoE, both of which are known to remove/clear Aβ from the brain, in the culture medium of astrocytes. We found that exosome and apoE-HDL were successfully separated by density gradient ultracentrifugation demonstrated by distribution of their specific markers, flotillin and HSP90, and cholesterol, and morphological analysis using electron microscopy. Exosome release was significantly reduced by Aβ1-42 treatment in cultured astrocytes accompanied by an increased JNK phosphorylation. Whereas, apoE-HDL release remained unchanged. A JNK inhibitor restored the decreased levels of exosome release induced by Aβ treatment to levels similar to those of control, suggesting that Aβ1-42 inhibits exosome release via stimulation of JNK signal pathway. Because exosomes are shown to remove Aβ in the brain, our findings suggest that increased Aβ levels in the brain may impair the exosome-mediated Aβ clearance pathway.

ApoE4 Accelerates Early Seeding of Amyloid Pathology

Accumulation and aggregation of amyloid-β (Aβ) in the brain is an initiating step in the pathogenesis of Alzheimer's disease (AD). The ε4 allele of apolipoprotein E (apoE) gene is the strongest genetic risk factor for late-onset AD. Although there is strong evidence showing that apoE4 enhances amyloid pathology, it is not clear what the critical stage(s) is during amyloid development in which apoE4 has the strongest impact. Using apoE inducible mouse models, we show that increased expression of astrocytic apoE4, but not apoE3, during the seeding stage of amyloid development enhanced amyloid deposition and neuritic dystrophy in amyloid model mice. ApoE4, but not apoE3, significantly increased brain Aβ half-life measured by in vivo microdialysis. Furthermore, apoE4 expression increased whereas apoE3 reduced amyloid-related gliosis in the mouse brains. Together, our results demonstrate that apoE4 has the greatest impact on amyloid during the seeding stage, likely by perturbing Aβ clearance and enhancing Aβ aggregation.

Redox status of serum apolipoprotein E and its impact on HDL cholesterol levels

BACKGROUND

Apolipoprotein E (apoE) is closely involved in the pathogenesis of apoE-related diseases, such as Alzheimer's disease and cardiovascular disease. The redox modulation of cysteine-thiols in a protein is involved in various pathophysiological regulations; however, that of apoE has not been studied in detail. Herein, we devised an analytical method to determine the redox status of serum apoE and assessed its relation to serum cholesterol levels and apoE phenotype.

METHODS

The present method was based on a band shift assay, using a photocleavable maleimide-conjugated polyethylene glycol.

RESULTS

The basic characteristics of the present method were found to be satisfactory to determine the redox status of serum apoE quantitatively. Serum apoE was separated into its reduced-form (r-), non-reduced-form (nr-), apoE-AII complex, and homodimer using this method. R-apoE could be detected as a 40-kDa band, whereas nr-apoE remained as monomeric apoE. R-apoE displayed a preference for VLDL; however, the levels showed the correlation with HDL-cholesterol levels (p<0.005). Redox status of serum apoE was significantly different among apoE phenotypes. The quantitative ratios of nr-apoE to total apoE in serum from subjects with apoE4/E3 were higher than in serum from subjects with apoE3/E3 (p<0.0001) and apoE3/E2 (p<0.001).

CONCLUSION

The redox status of serum apoE might be related to the synthesis of HDL. The information concerning the redox status of serum apoE provided by the present method may be a potent indicator to evaluate various apoE-related diseases.

Integrated late onset Alzheimer's disease (LOAD) susceptibility genes: Cholesterol metabolism and trafficking perspectives

Late onset Alzheimer's disease (LOAD) is the most common type of dementia and is characterized by decreased amyloid-β (Aβ) clearance from the brain. Cholesterol regulates the production and clearance of Aβ. Genome-wide association study (GWAS) suggests that at least 20 genes are associated with LOAD. The genes APOE, CLU, SORL1, PICALM, and BIN1 have a relatively high LOAD susceptibility. Additional experimental and bioinformatic approaches to integrate data from genetics, epigenetics, and molecular networks may further increase our understanding of LOAD in relation to cholesterol metabolism and trafficking.