Progressive loss of synaptic integrity in human apolipoprotein E4 targeted replacement mice and attenuation by apolipoprotein E2

APOE2 gene therapy reduces amyloid deposition and improves markers of neuroinflammation and neurodegeneration in a mouse model of Alzheimer disease

Epidemiological studies show that individuals who carry the relatively uncommon APOE ε2 allele rarely develop Alzheimer disease, and if they do, they have a later age of onset, milder clinical course, and less severe neuropathological findings than people without this allele. The contrast is especially stark when compared with the major genetic risk factor for Alzheimer disease, APOE ε4, which has an age of onset several decades earlier, a more aggressive clinical course and more severe neuropathological findings, especially in terms of the amount of amyloid deposition. Here, we demonstrate that brain exposure to APOE ε2 via a gene therapy approach, which bathes the entire cortical mantle in the gene product after transduction of the ependyma, reduces Aβ plaque deposition, neurodegenerative synaptic loss, and, remarkably, reduces microglial activation in an APP/PS1 mouse model despite continued expression of human APOE ε4. This result suggests a promising protective effect of exogenous APOE ε2 and reveals a cell nonautonomous effect of the protein on microglial activation, which we show is similar to plaque-associated microglia in the brain of Alzheimer disease patients who inherit APOE ε2. These data increase the potential that an APOE ε2 therapeutic could be effective in Alzheimer disease, even in individuals born with the risky ε4 allele.

APOE2 Heterozygosity Reduces Hippocampal Soluble Amyloid-β42 Levels in Non-Hyperlipidemic Mice

APOE2 lowers Alzheimer's disease (AD) risk; unfortunately, the mechanism remains poorly understood and the use of mice models is problematic as APOE2 homozygosity is associated with hyperlipidemia. In this study, we developed mice that are heterozygous for APOE2 and APOE3 or APOE4 and overexpress amyloid-β peptide (Aβ) (EFAD) to evaluate the effect of APOE2 dosage on Aβ pathology. We found that heterozygous mice do not exhibit hyperlipidemia. Hippocampal but not cortical levels of soluble Aβ42 followed the order E2/2FAD > E2/3FAD≤E3/3FAD and E2/2FAD > E2/4FAD < E4/4FAD without an effect on insoluble Aβ42. These findings offer initial insights on the impact of APOE2 on Aβ pathology.

APOE4 is a Risk Factor and Potential Therapeutic Target for Alzheimer's Disease

Alzheimer's disease (AD) is a progressive neurodegenerative disease, the main pathological hallmark of which is the loss of neurons, resulting in cognitive and memory impairments. Sporadic late-onset AD is a prevalent form of the disease and the apolipoprotein E4 (APOE4) genotype is the strongest predictor of the disease development. The structural variations of APOE isoforms affect their roles in synaptic maintenance, lipid trafficking, energy metabolism, inflammatory response, and BBB integrity. In the context of AD, APOE isoforms variously control the key pathological elements of the disease, including Aβ plaque formation, tau aggregation, and neuroinflammation. Taking into consideration the limited number of therapy choices that can alleviate symptoms and have little impact on the AD etiology and progression to date, the precise research strategies guided by apolipoprotein E (APOE) polymorphisms are required to assess the potential risk of age-related cognitive decline in people carrying APOE4 genotype. In this review, we summarize the evidence implicating the significance of APOE isoforms on brain functions in health and pathology with the aim to identify the possible targets that should be addressed to prevent AD manifestation in individuals with the APOE4 genotype and to explore proper treatment strategies.

Astrocytic APOE4 genotype-mediated negative impacts on synaptic architecture in human pluripotent stem cell model

The APOE4 genotype is the strongest risk factor for the pathogenesis of sporadic Alzheimer's disease (AD), but the detailed molecular mechanism of APOE4-mediated synaptic impairment remains to be determined. In this study, we generated a human astrocyte model carrying the APOE3 or APOE4 genotype using human induced pluripotent stem cells (iPSCs) in which isogenic APOE4 iPSCs were genome edited from healthy control APOE3 iPSCs. Next, we demonstrated that the astrocytic APOE4 genotype negatively affects dendritic spine dynamics in a co-culture system with primary neurons. Transcriptome analysis revealed an increase of EDIL3, an extracellular matrix glycoprotein, in human APOE4 astrocytes, which could underlie dendritic spine reduction in neuronal cultures. Accordingly, postmortem AD brains carrying the APOE4 allele have elevated levels of EDIL3 protein deposits within amyloid plaques. Together, these results demonstrate the novel deleterious effect of human APOE4 astrocytes on synaptic architecture and may help to elucidate the mechanism of APOE4-linked AD pathogenesis.

APOE2 gene therapy reduces amyloid deposition, and improves markers of neuroinflammation and neurodegeneration in a mouse model of Alzheimer disease

Epidemiological studies show that individuals who carry the relatively uncommon APOE ε2 allele rarely develop Alzheimer disease, and if they do they have a later age of onset, milder clinical course, and less severe neuropathological findings than others with Alzheimer disease. The contrast is especially stark in comparison to the phenotype associated with the major genetic risk factor for Alzheimer disease, APOE ε4, which has an age of onset several decades earlier, as well as a more aggressive clinical course and notably more severe neuropathological findings, especially in terms of the amount of amyloid deposition. Even one APOE ε2 allele improves phenotype, but it is uncertain if that is due to the replacement of a more toxic allele by APOE ε2, or if APOE ε2 has a protective, neuro-modulatory effect. Here, we demonstrate that brain exposure to APOE2 via a gene therapy approach which bathes the entire cortical mantle in the gene product after transduction of the ependyma, rapidly ameliorates established Aβ plaque deposition, neurodegenerative synaptic loss, and, remarkably, reduces microglial activation in an APP/PS1 mouse model despite continued expression of human APOE4. This result suggests a promising protective effect of exogenous APOE2, revealing a cell non-autonomous effect of the protein on microglial activation. We also show that plaque associated microglia in the brain of patients who inherit APOE2 similarly have less microglial reactivity to plaques. These data raise the potential that an APOE2 therapeutic could be effective in Alzheimer disease even in individuals born with the risk ε4 allele.

One Sentence Summary

Introduction of ApoE2 using an AAV that transduces the ependymal cells of the ventricle causes a reduction in amyloid load and plaque associated synapse loss, and reduces neuroinflammation by modulating microglial responsiveness to plaques.

Ketogenic interventions in mild cognitive impairment, Alzheimer's disease, and Parkinson's disease: A systematic review and critical appraisal

Background

There is increasing interest in therapeutic ketosis as a potential therapy for neurodegenerative disorders-in particular, mild cognitive impairment (MCI), Alzheimer's disease (AD), and Parkinson's disease (PD)-following a proof-of-concept study in Parkinson's disease published in 2005.

Methods

To provide an objective assessment of emerging clinical evidence and targeted recommendations for future research, we reviewed clinical trials involving ketogenic interventions in mild cognitive impairment, Alzheimer's disease, and Parkinson's disease reported since 2005. Levels of clinical evidence were systematically reviewed using the American Academy of Neurology criteria for rating therapeutic trials.

Results

10 AD, 3 MCI, and 5 PD therapeutic ketogenic trials were identified. Respective grades of clinical evidence were objectively assessed using the American Academy of Neurology criteria for rating therapeutic trials. We found class "B" evidence (probably effective) for cognitive improvement in subjects with mild cognitive impairment and subjects with mild-to-moderate Alzheimer's disease negative for the apolipoprotein ε4 allele (APOε4-). We found class "U" evidence (unproven) for cognitive stabilization in individuals with mild-to-moderate Alzheimer's disease positive for the apolipoprotein ε4 allele (APOε4+). We found class "C" evidence (possibly effective) for improvement of non-motor features and class "U" evidence (unproven) for motor features in individuals with Parkinson's disease. The number of trials in Parkinson's disease is very small with best evidence that acute supplementation holds promise for improving exercise endurance.

Conclusions

Limitations of the literature to date include the range of ketogenic interventions currently assessed in the literature (i.e., primarily diet or medium-chain triglyceride interventions), with fewer studies using more potent formulations (e.g., exogenous ketone esters). Collectively, the strongest evidence to date exists for cognitive improvement in individuals with mild cognitive impairment and in individuals with mild-to-moderate Alzheimer's disease negative for the apolipoprotein ε4 allele. Larger-scale, pivotal trials are justified in these populations. Further research is required to optimize the utilization of ketogenic interventions in differing clinical contexts and to better characterize the response to therapeutic ketosis in patients who are positive for the apolipoprotein ε4 allele, as modified interventions may be necessary.

Linking Diabetes to Alzheimer's Disease: Potential Roles of Glucose Metabolism and Alpha-Glucosidase

Alzheimer's disease (AD) and type 2 diabetes mellitus (DM) are more prevalent with ageing and cause a substantial global socio-economic burden. The biology of these two conditions is well elaborated, but whether AD and type 2 DM arise from coincidental roots in ageing or are linked by pathophysiological mechanisms remains unclear. Research findings involving animal models have identified mechanisms shared by both AD and type 2 DM. Deposition of β-amyloid peptides and formation of intracellular neurofibrillary tangles are pathological hallmarks of AD. Type 2 DM, on the other hand, is a metabolic disorder characterised by hyperglycaemia and insulin resistance. Several studies show that improving type 2 DM can delay or prevent the development of AD, and hence, prevention and control of type 2 DM may reduce the risk of AD later in life. Alpha-glucosidase is an enzyme that is commonly associated with hyperglycaemia in type 2 DM. However, it is uncertain if this enzyme may play a role in the progression of AD. This review explores the experimental evidence that depicts the relationship between dysregulation of glucose metabolism and AD. We also delineate the links between alpha-glucosidase and AD and the potential role of alpha-glucosidase inhibitors in treating AD.

Current understanding of the interactions between metal ions and Apolipoprotein E in Alzheimer's disease

Alzheimer's disease (AD), the most common type of dementia in the elderly, is a chronic and progressive neurodegenerative disorder with no effective disease-modifying treatments to date. Studies have shown that an imbalance in brain metal ions, such as zinc, copper, and iron, is closely related to the onset and progression of AD. Many efforts have been made to understand metal-related mechanisms and therapeutic strategies for AD. Emerging evidence suggests that interactions of brain metal ions and apolipoprotein E (ApoE), which is the strongest genetic risk factor for late-onset AD, may be one of the mechanisms for neurodegeneration. Here, we summarize the key points regarding how metal ions and ApoE contribute to the pathogenesis of AD. We further describe the interactions between metal ions and ApoE in the brain and propose that their interactions play an important role in neuropathological alterations and cognitive decline in AD.

Neuronal hyperexcitability in Alzheimer's disease: what are the drivers behind this aberrant phenotype?

Alzheimer's disease (AD) is a progressive neurodegenerative disorder leading to loss of cognitive abilities and ultimately, death. With no cure available, limited treatments mostly focus on symptom management. Identifying early changes in the disease course may provide new therapeutic targets to halt or reverse disease progression. Clinical studies have shown that cortical and hippocampal hyperactivity are a feature shared by patients in the early stages of disease, progressing to hypoactivity during later stages of neurodegeneration. The exact mechanisms causing neuronal excitability changes are not fully characterized; however, animal and cell models have provided insights into some of the factors involved in this phenotype. In this review, we summarize the evidence for neuronal excitability changes over the course of AD onset and progression and the molecular mechanisms underpinning these differences. Specifically, we discuss contributors to aberrant neuronal excitability, including abnormal levels of intracellular Ca²⁺ and glutamate, pathological amyloid β (Aβ) and tau, genetic risk factors, including APOE, and impaired inhibitory interneuron and glial function. In light of recent research indicating hyperexcitability could be a predictive marker of cognitive dysfunction, we further argue that the hyperexcitability phenotype could be leveraged to improve the diagnosis and treatment of AD, and present potential targets for future AD treatment development.

Targeted Suppression of Lipoprotein Receptor LSR in Astrocytes Leads to Olfactory and Memory Deficits in Mice

Perturbations of cholesterol metabolism have been linked to neurodegenerative diseases. Glia–neuron crosstalk is essential to achieve a tight regulation of brain cholesterol trafficking. Adequate cholesterol supply from glia via apolipoprotein E-containing lipoproteins ensures neuronal development and function. The lipolysis-stimulated lipoprotein receptor (LSR), plays an important role in brain cholesterol homeostasis. Aged heterozygote Lsr+/− mice show altered brain cholesterol distribution and increased susceptibility to amyloid stress. Since LSR expression is higher in astroglia as compared to neurons, we sought to determine if astroglial LSR deficiency could lead to cognitive defects similar to those of Alzheimer’s disease (AD). Cre recombinase was activated in adult Glast-CreERT/lsrfl/fl mice by tamoxifen to induce astroglial Lsr deletion. Behavioral phenotyping of young and old astroglial Lsr KO animals revealed hyperactivity during the nocturnal period, deficits in olfactory function affecting social memory and causing possible apathy, as well as visual memory and short-term working memory problems, and deficits similar to those reported in neurodegenerative diseases, such as AD. Furthermore, GFAP staining revealed astroglial activation in the olfactory bulb. Therefore, astroglial LSR is important for working, spatial, and social memory related to sensory input, and represents a novel pathway for the study of brain aging and neurodegeneration.

Effects of sex and genotype in human APOE-targeted replacement mice on alcohol self-administration measured with the automated IntelliCage system before and after repeated mild traumatic brain injury

BACKGROUND

Few studies have examined the association between APOE genotype and alcohol use. Although some of these studies have reported outcomes associated with a history of drinking, none have examined alcohol-seeking behavior. In addition, no preclinical studies have examined alcohol use as a function of APOE genotype with or without traumatic brain injury.

METHODS

Male and female human APOE3- and APOE4-targeted replacement (TR) mice were used to assess voluntary alcohol seeking longitudinally using a 2-bottle choice paradigm conducted within the automated IntelliCage system prior to and following repeated mild TBI (rmTBI). Following an acquisition phase in which the concentration of ethanol (EtOH) was increased to 12%, a variety of drinking paradigms that included extended alcohol access (EAA1 and EAA2), alcohol deprivation effect (ADE), limited access drinking in the dark (DID), and progressive ratio (PR) were used to assess alcohol-seeking behavior. Additional behavioral tasks were performed to measure cognitive function and anxiety-like behavior.

RESULTS

All groups readily consumed increasing concentrations of EtOH (4-12%) during the acquisition phase. During the EAA1 period (12% EtOH), there was a significant genotype effect in both males and females for EtOH preference. Following a 3-week abstinence period, mice received sham or rmTBI resulting in a genotype- and sex-independent main effect of rmTBI on the recovery of righting reflex and a main effect of rmTBI on spontaneous home-cage activity in females only. Reintroduction of 12% EtOH (EAA2) resulted in a significant effect genotype for alcohol preference in males with APOE4 mice displaying increased preference and motivation for alcohol compared with APOE3 mice independent of TBI while in females, there was a significant genotype × TBI interaction under the ADE and DID paradigms. Finally, there was a main effect of rmTBI on increased risk-seeking behavior in both sexes, but no effect on spatial learning or cognitive flexibility.

CONCLUSION

These results suggest that sex and APOE genotype play a significant role in alcohol consumption and may subsequently influence long-term recovery following traumatic brain insults.

A meta-analysis of Alzheimer's disease's relationship with human ApoE gene variants

PURPOSE

To explore the association between Alzheimer's disease and apolipoprotein E (ApoE). Studies on this relationship are plentiful, but they mostly suffer from the disadvantage of inadequate sample size, so we conducted this meta-analysis to assess the association between ApoE polymorphisms and AD in humans.

METHOD

The research literature centered on the association between Alzheimer's disease and ApoE polymorphisms was searched using databases including EMBASE, CQVIP, Medline, Web of knowledge, PubMed, Cochrane Library, CNKI, and Wanfang Data up to July 2020. The quality of the included literature was assessed using the NOS scale. We used RevMan 5.3 statistical software for the data extraction and meta-analysis.

RESULTS

A total of 569 studies were retrieved according to the search strategy and the inclusion criteria. After removing the duplicate studies and studies that did not match the topic, 155 studies were obtained. 39 publications were finally included according to the inclusion and exclusion criteria. Five of them were selected for the meta-analysis after a careful evaluation.

CONCLUSION

Patients with Alzheimer's disease have a high positive rate of the ε4 allele (OR = 2.19, 95% CI: 1.38-3.48) and a low positive rate of the ε3 allele, but there is no significant association between the ApoE ε2 allele and AD (OR = 0.71, 95% CI: 0.19-2.58). The positivity rates of the ε4/ε4 and ε3/ε4 genotypes were higher in the case group (OR = 3.82, 95% CI: 1.86-7.84; OR = 2.07, 95% CI: 1.40-3.06), but the positivity rates of the ε2/ε3 and ε3/ε3 genotypes were significantly lower in the case group than in the control group (OR = 0.62, 95% CI: 0.18-2.11; OR = 0.52, 95% CI: 0.36-0.75).

Cognitively normal APOE ε4 carriers have specific elevation of CSF SNAP-25

Cerebrospinal fluid (CSF) synaptosomal-associated protein 25 (SNAP-25) and neurogranin (Ng) are recently described biomarkers for pre- and postsynaptic integrity known to be elevated in symptomatic Alzheimer disease (AD). Their relationship with Apolipoprotein E (APOE) ε4 carrier status, the major genetic risk factor for AD, remains unclear. In this study, CSF SNAP-25 and Ng were compared in cognitively normal APOE ε4 carriers and noncarriers (n = 274, mean age 65 ± 9.0 years, 39% APOE ε4 carriers, 58% female). CSF SNAP-25, not CSF Ng, was specifically elevated in APOE ε4 carriers versus noncarriers (5.95 ± 1.72 pg/mL, 4.44 ± 1.40 pg/mL, p < 0.0001), even after adjusting for age, sex, years of education, and amyloid status (p < 0.0001). CSF total tau (t-tau), phosphorylated-tau-181 (ptau181), and neurofilament light chain (NfL) also did not vary by APOE ε4 status. Our findings suggest APOE ε4 carriers have amyloid-related and amyloid-independent presynaptic disruption as reflected by elevated CSF SNAP-25 levels. In contrast, postsynaptic disruption as reflected by elevations in CSF neurogranin is related to amyloid status.

Novel Influences of Sex and APOE Genotype on Spinal Plasticity and Recovery of Function after Spinal Cord Injury

Spinal cord injuries can abolish both motor and sensory function throughout the body. Spontaneous recovery after injury is limited and can vary substantially between individuals. Despite an abundance of therapeutic approaches that have shown promise in preclinical models, there is currently a lack of effective treatment strategies that have been translated to restore function after spinal cord injury (SCI) in the human population. We hypothesized that sex and genetic background of injured individuals could impact how they respond to treatment strategies, presenting a barrier to translating therapies that are not tailored to the individual. One gene of particular interest is APOE, which has been extensively studied in the brain because of its allele-specific influences on synaptic plasticity, metabolism, inflammation, and neurodegeneration. Despite its prominence as a therapeutic target in brain injury and disease, little is known about how it influences neural plasticity and repair processes in the spinal cord. Using humanized mice, we examined how the ε3 and ε4 alleles of APOE influence the efficacy of therapeutic intermittent hypoxia (IH) in inducing spinally-mediated plasticity after cervical SCI (cSCI). IH is sufficient to enhance plasticity and restore motor function after experimental SCI in genetically similar rodent populations, but its effect in human subjects is more variable (Golder and Mitchell, 2005; Hayes et al., 2014). Our results demonstrate that both sex and APOE genotype determine the extent of respiratory motor plasticity that is elicited by IH, highlighting the importance of considering these clinically relevant variables when translating therapeutic approaches for the SCI community.

Effect of Combined Electroacupuncture and Selegiline Treatment in Alzheimer's Disease: An Animal Model

The complexity of pathological mechanisms in Alzheimer's disease (AD) poses significant challenges to the development of corresponding drugs. Symptom-specific pharmacological interventions and alternative treatments provide promising treatment possibilities. Therefore, we considered a combination of selegiline (SEL) and electroacupuncture (EA). We used an animal model with AD to investigate the effect of a combination of these treatments on cognitive function. 5XFAD mice received a week of SEL treatment and 2 weeks of EA. Novel object recognition and Y-maze tests were subsequently performed to assess their cognitive functions. To determine the molecular action of the combination treatment, Western blots, Aβ_1-42 enzyme-linked immunosorbent assays (ELISA), and micro-positron-emission tomography were also performed to assess pathological markers and processes. The results were assessed based on the difference between untreated transgenic, SEL-treated, and SEL- and EA-treated groups of mice. Mice in the combined treatment group demonstrated significantly better cognitive functions, and lesser neuroinflammation than the comparative groups. In addition, mice treated with a combination of SEL and EA did not demonstrate a direct modulation of insoluble Aβ but demonstrated greater glucose metabolism. Our findings demonstrated that SEL combined with EA treatment was associated with better cognitive functioning due to inhibition of neuroinflammation and increased glucose metabolism relative to the comparative groups in a mouse model with AD.

APP-Induced Patterned Neurodegeneration Is Exacerbated by APOE4 in Caenorhabditis elegans

Genetic and epidemiological studies have found that variations in the amyloid precursor protein (APP) and the apoliopoprotein E (APOE) genes represent major modifiers of the progressive neurodegeneration in Alzheimer's disease (AD). An extra copy of or gain-of-function mutations in APP correlate with early onset AD. Compared to the other variants (APOE2 and APOE3), the ε4 allele of APOE (APOE4) hastens and exacerbates early and late onset forms of AD. Convenient in vivo models to study how APP and APOE4 interact at the cellular and molecular level to influence neurodegeneration are lacking. Here, we show that the nematode C. elegans can model important aspects of AD including age-related, patterned neurodegeneration that is exacerbated by APOE4 Specifically, we found that APOE4, but not APOE3, acts with APP to hasten and expand the pattern of cholinergic neurodegeneration caused by APP Molecular mechanisms underlying how APP and APOE4 synergize to kill some neurons while leaving others unaffected may be uncovered using this convenient worm model of neurodegeneration.

Synaptic Elimination in Neurological Disorders

Synapses are well known as the main structures responsible for transmitting information through the release and recognition of neurotransmitters by pre- and post-synaptic neurons. These structures are widely formed and eliminated throughout the whole lifespan via processes termed synaptogenesis and synaptic pruning, respectively. Whilst the first pro-cess is needed for ensuring proper connectivity between brain regions and also with the periphery, the second phenomenon is important for their refinement by eliminating weaker and unnecessary synapses and, at the same time, maintaining and fa-voring the stronger ones, thus ensuring proper synaptic transmission. It is well-known that synaptic elimination is modulated by neuronal activity. However, only recently the role of the classical complement cascade in promoting this phenomenon has been demonstrated. Specifically, microglial cells recognize activated complement component 3 (C3) bound to synapses tar-geted for elimination, triggering their engulfment. As this is a highly relevant process for adequate neuronal functioning, dis-ruptions or exacerbations in synaptic pruning could lead to severe circuitry alterations that could underlie neuropathological alterations typical of neurological and neuropsychiatric disorders. In this review, we focus on discussing the possible in-volvement of excessive synaptic elimination in Alzheimer’s disease, as it has already been reported dendritic spine loss in post-synaptic neurons, increased association of complement proteins with its synapses and, hence, augmented microglia-mediated pruning in animal models of this disorder. In addition, we briefly discuss how this phenomenon could be related to other neurological disorders, including multiple sclerosis and schizophrenia.

Apolipoprotein E and Alzheimer disease: pathobiology and targeting strategies

Polymorphism in the apolipoprotein E (APOE) gene is a major genetic risk determinant of late-onset Alzheimer disease (AD), with the APOE*ε4 allele conferring an increased risk and the APOE*ε2 allele conferring a decreased risk relative to the common APOE*ε3 allele. Strong evidence from clinical and basic research suggests that a major pathway by which APOE4 increases the risk of AD is by driving earlier and more abundant amyloid pathology in the brains of APOE*ε4 carriers. The number of amyloid-β (Aβ)-dependent and Aβ-independent pathways that are known to be differentially modulated by APOE isoforms is increasing. For example, evidence is accumulating that APOE influences tau pathology, tau-mediated neurodegeneration and microglial responses to AD-related pathologies. In addition, APOE4 is either pathogenic or shows reduced efficiency in multiple brain homeostatic pathways, including lipid transport, synaptic integrity and plasticity, glucose metabolism and cerebrovascular function. Here, we review the recent progress in clinical and basic research into the role of APOE in AD pathogenesis. We also discuss how APOE can be targeted for AD therapy using a precision medicine approach.

Developmental roles of microglia: A window into mechanisms of disease

Microglia are engineers of the central nervous system (CNS) both in health and disease. In addition to the canonical immunological roles of clearing damaging entities and limiting the spread of toxicity and death, microglia remodel the CNS throughout life. While they have been extensively studied in disease and injury, due to their highly variable functions, their precise role in these contexts still remains uncertain. Over the past decade, we have greatly expanded our understanding of microglial function, including their essential homeostatic roles during development. Here, we review these developmental roles, identify parallels in disease, and speculate whether developmental mechanisms re-emerge in disease and injury. Developmental Dynamics 248:98-117, 2019. © 2018 Wiley Periodicals, Inc.

The Synergistic Effects of APOE Genotype and Obesity on Alzheimer's Disease Risk

The APOE gene has three common alleles—E2, E3, and E4, with APOE4 being the strongest genetic risk factor for developing Alzheimer’s Disease (AD). Obesity is a global epidemic and contributes to multiple metabolic problems. Obesity is also a risk factor for cognitive decline. Here, we review the effects of APOE4 and obesity on cognition and AD development, independently and together. We describe studies that have associated APOE4 with cognitive deficits and AD, as well as studies that have associated obesity to cognitive deficits and AD. We then describe studies that have examined the effects of obesity and APOE genotypes together, with a focus on APOE4 and high fat diets. Both human studies and rodent models have contributed to understanding the effects of obesity on the different APOE genotypes, and we outline possible underlying mechanisms associated with these effects. Data across approaches support a model in which APOE4 and obesity combine for greater detrimental effects on metabolism and cognition, in ways that are influenced by both age and sex.

Human ApoE Isoforms Differentially Modulate Brain Glucose and Ketone Body Metabolism: Implications for Alzheimer's Disease Risk Reduction and Early Intervention

Humans possess three genetic isoforms of apolipoprotein E (ApoE)-ApoE2, ApoE3, and ApoE4-that confer differential risk for Alzheimer's disease (AD); however, the underlying mechanisms are poorly understood. This study sought to investigate the impact of human ApoE isoforms on brain energy metabolism, an area significantly perturbed in preclinical AD. A TaqMan custom array was performed to examine the expression of a total of 43 genes involved in glucose and ketone body transport and metabolism, focusing on pathways leading to the generation of acetyl-CoA, in human ApoE gene-targeted replacement female mice. Consistent with our previous findings, brains expressing ApoE2 exhibited the most robust profile, whereas brains expressing ApoE4 displayed the most deficient profile on the uptake and metabolism of glucose, the primary fuel for the brain. Specifically, the three ApoE brains differed significantly in facilitated glucose transporters, which mediate the entry of glucose into neurons, and hexokinases, which act as the "gateway enzyme" in glucose metabolism. Interestingly, on the uptake and metabolism of ketone bodies, the secondary energy source for the brain, ApoE2 and ApoE4 brains showed a similar level of robustness, whereas ApoE3 brains presented a relatively deficient profile. Further, ingenuity pathway analysis indicated that the PPAR-γ/PGC-1α signaling pathway could be activated in the ApoE2 brain and inhibited in the ApoE4 brain. Notably, PGC-1α overexpression ameliorated ApoE4-induced deficits in glycolysis and mitochondrial respiration. Overall, our data provide additional evidence that human ApoE isoforms differentially modulate brain bioenergetic metabolism, which could serve as a potential mechanism contributing to their discrete risk impact in AD.SIGNIFICANCE STATEMENT We uncovered hexokinase as a key cytosolic point in the glucose metabolism that is differentially modulated by the three ApoE genotypes. The differences in hexokinase expression and activity exhibited in the three ApoE brains may underlie their distinct impact on brain glucose utilization and further susceptibility to AD. Therefore, a therapeutic approach that could circumvent the deficiencies in the cytosolic metabolism of glucose by providing glucose metabolizing intermediates, e.g., pyruvate, may hold benefits for ApoE4 carriers, who are at high risk for AD. The bioenergetic robustness may translate into enhanced synaptic activity and, ultimately, reduces the risk of developing AD and/or delays the onset of clinical manifestation.

Neuronal pentraxin 1: A synaptic-derived plasma biomarker in Alzheimer's disease

Synaptic neurodegeneration is thought to be an early event initiated by soluble β-amyloid (Aβ) aggregates that closely correlates with cognitive decline in Alzheimer disease (AD). Apolipoprotein ε4 (APOE4) is the most common genetic risk factor for both familial AD (FAD) and sporadic AD; it accelerates Aβ aggregation and selectively impairs glutamate receptor function and synaptic plasticity. However, its molecular mechanisms remain elusive and these synaptic deficits are difficult to monitor. AD- and APOE4-dependent plasma biomarkers have been proposed, but synapse-related plasma biomarkers are lacking. We evaluated neuronal pentraxin 1 (NP1), a potential CNS-derived plasma biomarker of excitatory synaptic pathology. NP1 is preferentially expressed in brain and involved in glutamate receptor internalization. NP1 is secreted presynaptically induced by Aβ oligomers, and implicated in excitatory synaptic and mitochondrial deficits. Levels of NP1 and its fragments were increased in a correlated fashion in both brain and plasma of 7-8 month-old E4FAD mice relative to E3FAD mice. NP1 was also found in exosome preparations and reduced by dietary DHA supplementation. Plasma NP1 was higher in E4FAD+ (APOE4^+/+/FAD^+/-) relative to E4FAD- (non-carrier; APOE4^+/+/FAD^-/-) mice, suggesting NP1 is modulated by Aβ expression. Finally, relative to normal elderly, plasma NP1 was also elevated in patients with mild cognitive impairment (MCI) and elevated further in the subset who progressed to early-stage AD. In those patients, there was a trend towards increased NP1 levels in APOE4 carriers relative to non-carriers. These findings indicate that NP1 may represent a potential synapse-derived plasma biomarker relevant to early alterations in excitatory synapses in MCI and early-stage AD.

Apolipoprotein E, Receptors, and Modulation of Alzheimer's Disease

Apolipoprotein E (apoE) is a lipid carrier in both the peripheral and the central nervous systems. Lipid-loaded apoE lipoprotein particles bind to several cell surface receptors to support membrane homeostasis and injury repair in the brain. Considering prevalence and relative risk magnitude, the ε4 allele of the APOE gene is the strongest genetic risk factor for late-onset Alzheimer's disease (AD). ApoE4 contributes to AD pathogenesis by modulating multiple pathways, including but not limited to the metabolism, aggregation, and toxicity of amyloid-β peptide, tauopathy, synaptic plasticity, lipid transport, glucose metabolism, mitochondrial function, vascular integrity, and neuroinflammation. Emerging knowledge on apoE-related pathways in the pathophysiology of AD presents new opportunities for AD therapy. We describe the biochemical and biological features of apoE and apoE receptors in the central nervous system. We also discuss the evidence and mechanisms addressing differential effects of apoE isoforms and the role of apoE receptors in AD pathogenesis, with a particular emphasis on the clinical and preclinical studies related to amyloid-β pathology. Finally, we summarize the current strategies of AD therapy targeting apoE, and postulate that effective strategies require an apoE isoform-specific approach.

Human ApoE ɛ2 Promotes Regulatory Mechanisms of Bioenergetic and Synaptic Function in Female Brain: A Focus on V-type H+-ATPase

Humans possess three major isoforms of the apolipoprotein E (ApoE) gene encoded by three alleles: ApoE ɛ2 (ApoE2), ApoE ɛ3 (ApoE3), and ApoE ɛ4 (ApoE4). It is established that the three ApoE isoforms confer differential susceptibility to Alzheimer's disease (AD); however, an in-depth molecular understanding of the underlying mechanisms is currently unavailable. In this study, we examined the cortical proteome differences among the three ApoE isoforms using 6-month-old female, human ApoE2, ApoE3, and ApoE4 gene-targeted replacement mice and two-dimensional proteomic analyses. The results reveal that the three ApoE brains differ primarily in two areas: cellular bioenergetics and synaptic transmission. Of particular significance, we show for the first time that the three ApoE brains differentially express a key component of the catalytic domain of the V-type H+-ATPase (Atp6v), a proton pump that mediates the concentration of neurotransmitters into synaptic vesicles and thus is crucial in synaptic transmission. Specifically, our data demonstrate that ApoE2 brain exhibits significantly higher levels of the B subunit of Atp6v (Atp6v1B2) when compared to both ApoE3 and ApoE4 brains, with ApoE4 brain exhibiting the lowest expression. Our additional analyses show that Atp6v1B2 is significantly impacted by aging and AD pathology and the data suggest that Atp6v1B2 deficiency could be involved in the progressive loss of synaptic integrity during early development of AD. Collectively, our findings indicate that human ApoE isoforms differentially modulate regulatory mechanisms of bioenergetic and synaptic function in female brain. A more efficient and robust status in both areas-in which Atp6v may play a role-could serve as a potential mechanism contributing to the neuroprotective and cognition-favoring properties associated with the ApoE2 genotype.

Differential Effect of APOE Alleles on Brain Glucose Metabolism in Targeted Replacement Mice: An [18F]FDG-μPET Study

Background:

The Apolipoprotein E (ApoE) alleles ɛ2, ɛ3, and ɛ4 are known to differentially modulate cerebral glucose metabolism and the risk for Alzheimer’s disease (AD) via both amyloid-β (Aβ)-dependent and independent mechanisms.

Objective:

We investigated the influence of ApoE on cerebral glucose metabolism in humanized APOE Targeted Replacement (TR) mice at ages that precede the comparison of Aβ parenchymal deposits in APOE4-TR mice.

Methods:

Fludeoxyglucose ([¹⁸F]FDG) positron emission tomography (PET) measures were performed longitudinally in homozygous APOE-TR mice (APOE2, APOE3, APOE4; n = 10 for each group) at 3, 5, 11, and 15 months. Results were quantified using standard uptake values and analyzed statistically using a linear mixed effects model. Levels of the Aβ₄₀ and Aβ₄₂ peptides were quantified ex vivo using enzyme-linked immunosorbent assay (ELISA) at 15 months in the same animals.

Results:

APOE2 mice (versus APOE3) showed a significant increase in glucose metabolism starting at 6 months, peaking at 9 months. No evidence of hypometabolism was apparent in any region or time point for APOE4 mice, which instead displayed a hypermetabolism at 15 months. Whole brain soluble Aβ₄₀ and Aβ₄₂ levels were not significantly different between genotypes at 15 months.

Conclusions:

Introduction of human APOE alleles ɛ2 and ɛ4 is sufficient to produce alterations in brain glucose metabolism in comparison to the control allele ɛ3, without a concomitant alteration in Aβ₄₀ and Aβ₄₂ levels. These results suggest novel Aβ-independent metabolic phenotypes conferred by ɛ2 and ɛ4 alleles and have important implications for preclinical studies using TR-mice.

The effects of apolipoprotein E genotype, α-synuclein deficiency, and sex on brain synaptic and Alzheimer's disease-related pathology

Introduction

Alzheimer's disease (AD) and synucleinopathies share common pathological mechanisms. Apolipoprotein E4 (apoE4), the most prevalent genetic risk factor for AD, also increases the risk for dementia in pure synucleinopathies. We presently examined the effects of α-synuclein deficiency (α-syn−/−) and sex on apoE4-driven pathologies.

Methods

AD-related, synaptic, and vascular markers were analyzed in female and male α-syn−/− and α-syn+/+ apoE4, apoE3, and apoE3/E4 mice.

Results

ApoE4 was hypolipidated, and this effect was unchanged by α-syn−/− and sex. The levels of synaptic markers were lower, and the levels of AD-related parameters were higher in female α-syn−/− apoE4 mice compared with the corresponding apoE3 mice. By comparison, apoE4 had small effects on the AD parameters of male and female α-syn+/+ apoE4 mice.

Discussion

Although α-syn−/− does not affect the upstream lipidation impairment of apoE4, it acts as a “second hit” enhancer of the subsequent apoE4-driven pathologies.

Effect of APOE Genotype on Synaptic Proteins in Earlier Adult Life

Background:

Possession of APOEɛ4 is a strong risk factor for late-onset Alzheimer’s disease and is associated with loss of synaptic proteins in the elderly even in the absence of Alzheimer’s disease.

Objective:

We hypothesized that ɛ4 allele possession in non-demented adults aged under-75 would also be associated with alterations in the levels of synaptic proteins.

Methods:

We measured synaptophysin, PSD95, drebrin, SNAP-25, and septin 7 by ELISA in hippocampus and superior temporal gyrus from 103 adults aged <75 without dementia. Corresponding gene expression was measured by RT-PCR.

Results:

There was no evidence that ɛ4 affected levels of the proteins measured. Instead we found an increase in post-synaptic proteins in the hippocampi of those with an ɛ32 genotype. The evidence was strongest for drebrin (p = 0.011). There was some evidence of increased synaptic protein gene expression in ɛ4 carriers.

Conclusions:

People with an APOEɛ32 genotype have a reduced risk of Alzheimer’s disease. It may be relevant that they have a higher level of post-synaptic proteins in the hippocampus even in earlier adulthood.

ApoE isoforms differentially regulates cleavage and secretion of BDNF

Apolipoprotein E4 (ApoE4) is a major genetic risk factor for sporadic or late onset Alzheimer’s disease (AD). Brain-derived neurotrophic factor (BDNF) is decreased by 3 to 4-fold in the brains of AD patients at autopsy. ApoE4 mice also have reduced BDNF levels. However, there have been no reports relating the different ApoE isoforms or AD to differential regulation of BDNF. Here we report that in the hippocampal regions of AD patients both prepro-BDNF and pro-BDNF expression showed a 40 and 60% decrease respectively compared to that expression in the hippocampi of age-matched control patients. We further report that ApoE isoforms differentially regulate maturation and secretion of BDNF from primary human astrocytes. After 24 h, ApoE3 treated astrocytes secreted 1.75- fold higher pro-BDNF than ApoE2-treated astrocytes, and ApoE2-treated astrocytes secreted 3-fold more mature-BDNF (m-BDNF) than ApoE3-treated astrocytes. In contrast, ApoE4-treated cells secreted negligible amounts of m-BDNF or pro-BDNF. ApoE2 increased the level of intracellular pre-pro BDNF by 19.04 ± 6.68%, while ApoE4 reduced the pre-pro BDNF by 21.61 ± 5.9% compared to untreated cells. Similar results were also seen in ApoE2, ApoE3 or ApoE4 treated cells at 4 h. Together, these results indicate that an ApoE2 or ApoE3 mediated positive regulation of BDNF may be protective while ApoE4 related defects in BDNF processing could lead to AD pathophysiology. These interactions of the ApoE isoforms with BDNF may help explain the increased risk of AD associated with the ApoE4 isoform.

The role of APOE on lipid homeostasis and inflammation in normal brains

The role of APOE in the risk of Alzheimer's disease (AD) has largely focused on its effects on AD pathological processes. However, there are increasing data that APOE genotype affects processes in normal brains. Studies of young cognitively normal humans show effects of APOE genotype on brain structure and activity. Studies of normal APOE knock-in mice show effects of APOE genotype on brain structure, neuronal markers, and behavior. APOE interactions with molecules important for lipid efflux and lipid endocytosis underlie effects of APOE genotype on neuroinflammation and lipoprotein composition. These effects provide important targets for new therapies for reduction of the risk of AD before any signs of pathogenesis.

Attentional performance, impulsivity, and related neurotransmitter systems in apoE2, apoE3, and apoE4 female transgenic mice

RATIONALE

The apolipoprotein E (apoE) genotype influences cognitive performance in humans depending on age and sex. While the detrimental role of the apoE4 isoform on spatial learning and memory has been well-established in humans and rodents, less is known on its impact on the executive functions.

OBJECTIVES

We aimed to evaluate the effect of apoE isoforms (apoE2, apoE3, apoE4) on visuospatial attention and inhibitory control performance in female transgenic mice, and to determine the neurochemical and neuropharmacological basis of this potential relationship.

METHODS

Female mice carrying apoE2, apoE3, and apoE4 were trained in the five-choice serial reaction time task (5-CSRTT). Upon a stable performance, we manipulated the inter-trial interval and the stimulus duration to elicit impulsive responding and engage attention respectively. We further performed a pharmacological challenge by administering cholinergic and GABAergic agents. Finally, we analyzed the levels of brain amino acids and monoamines by using reversed phase high-performance liquid chromatography (HPLC).

RESULTS

ApoE4 mice showed a deficient inhibitory control as revealed by increased perseveration and premature responding. When attention was challenged, apoE4 mice also showed a higher drop in accuracy. The adverse effect of scopolamine on the task was attenuated in apoE4 mice compared to apoE2 and apoE3. Furthermore, apoE4 mice showed less dopamine in the frontal cortex than apoE2 mice.

CONCLUSIONS

We confirmed that the apoE genotype influences attention and inhibitory control in female transgenic mice. The influence of apoE isoforms in the brain neuromodulatory system may explain the cognitive and behavioral differences attributable to the genotype.

The basolateral amygdala γ-aminobutyric acidergic system in health and disease

The brain comprises an excitatory/inhibitory neuronal network that maintains a finely tuned balance of activity critical for normal functioning. Excitatory activity in the basolateral amygdala (BLA), a brain region that plays a central role in emotion and motivational processing, is tightly regulated by a relatively small population of γ-aminobutyric acid (GABA) inhibitory neurons. Disruption in GABAergic inhibition in the BLA can occur when there is a loss of local GABAergic interneurons, an alteration in GABAA receptor activation, or a dysregulation of mechanisms that modulate BLA GABAergic inhibition. Disruptions in GABAergic control of the BLA emerge during development, in aging populations, or after trauma, ultimately resulting in hyperexcitability. BLA hyperexcitability manifests behaviorally as an increase in anxiety, emotional dysregulation, or development of seizure activity. This Review discusses the anatomy, development, and physiology of the GABAergic system in the BLA and circuits that modulate GABAergic inhibition, including the dopaminergic, serotonergic, noradrenergic, and cholinergic systems. We highlight how alterations in various neurotransmitter receptors, including the acid-sensing ion channel 1a, cannabinoid receptor 1, and glutamate receptor subtypes, expressed on BLA interneurons, modulate GABAergic transmission and how defects of these systems affect inhibitory tonus within the BLA. Finally, we discuss alterations in the BLA GABAergic system in neurodevelopmental (autism/fragile X syndrome) and neurodegenerative (Alzheimer's disease) diseases and after the development of epilepsy, anxiety, and traumatic brain injury. A more complete understanding of the intrinsic excitatory/inhibitory circuit balance of the amygdala and how imbalances in inhibitory control contribute to excessive BLA excitability will guide the development of novel therapeutic approaches in neuropsychiatric diseases.

ApoE4 and Aβ Oligomers Reduce BDNF Expression via HDAC Nuclear Translocation

Apolipoprotein E4 (ApoE4) is a major genetic risk factor for several neurodegenerative disorders, including Alzheimer's disease (AD). Epigenetic dysregulation, including aberrations in histone acetylation, is also associated with AD. We show here for the first time that ApoE4 increases nuclear translocation of histone deacetylases (HDACs) in human neurons, thereby reducing BDNF expression, whereas ApoE3 increases histone 3 acetylation and upregulates BDNF expression. Amyloid-β (Aβ) oligomers, which have been implicated in AD, caused effects similar to ApoE4. Blocking low-density lipoprotein receptor-related protein 1 (LRP-1) receptor with receptor-associated protein (RAP) or LRP-1 siRNA abolished the ApoE effects. ApoE3 also induced expression of protein kinase C ε (PKCε) and PKCε retained HDACs in the cytosol. PKCε activation and ApoE3 supplementation prevented ApoE4-mediated BDNF downregulation. PKCε activation also reversed Aβ oligomer- and ApoE4-induced nuclear import of HDACs, preventing the loss in BDNF. ApoE4 induced HDAC6-BDNF promoter IV binding, which reduced BDNF exon IV expression. Nuclear HDAC4 and HDAC6 were more abundant in the hippocampus of ApoE4 transgenic mice than in ApoE3 transgenic mice or wild-type controls. Nuclear translocation of HDA6 was also elevated in the hippocampus of AD patients compared with age-matched controls. These results provide new insight into the cause of synaptic loss that is the most important pathologic correlate of cognitive deficits in AD.

Fatty acid metabolism in carriers of apolipoprotein E epsilon 4 allele: is it contributing to higher risk of cognitive decline and coronary heart disease?

Apolipoprotein E (ApoE) is a protein playing a pivotal role in lipid homeostasis since it regulates cholesterol, triglyceride and phospholipid metabolism in the blood and the brain. APOE gene regulates the expression of this protein and has three different alleles: ε2, ε3 and ε4. Carrying an APOE4 allele is recognised as a genetic risk factor of late-onset Alzheimer’s disease (LOAD) and coronary heart disease (CHD). Consuming fatty fish, rich in long chain omega-3 fatty acids (LC omega-3), seems to be associated with risk reduction of developing LOAD and CHD but this link seems not to hold in APOE4 carriers, at least in LOAD. In CHD trials, APOE4 carriers supplemented with LC omega-3 were categorized as differential responders to the treatment with regards to CHD risk markers. This is potentially because fatty acid metabolism is disturbed in APOE4 carriers compared to the non-carriers. More specifically, homeostasis of LC omega-3 is disrupted in carriers of APOE4 allele and this is potentially because they β-oxidize more LC omega-3 than the non-carriers. Therefore, there is a potential shift in fatty acid selection for β-oxidation towards LC omega-3 which are usually highly preserved for incorporation into cell membranes.

Apolipoprotein E in Alzheimer's disease: an update

The vast majority of Alzheimer's disease (AD) cases are late onset (LOAD), which is genetically complex with heritability estimates up to 80%. Apolipoprotein E (APOE) has been irrefutably recognized as the major genetic risk factor, with semidominant inheritance, for LOAD. Although the mechanisms that underlie the pathogenic nature of APOE in AD are still not completely understood, emerging data suggest that APOE contributes to AD pathogenesis through both amyloid-β (Aβ)-dependent and Aβ-independent pathways. Given the central role for APOE in the modulation of AD pathogenesis, many therapeutic strategies have emerged, including converting APOE conformation, regulating APOE expression, mimicking APOE peptides, blocking the APOE/Aβ interaction, modulating APOE lipidation state, and gene therapy. Accumulating evidence also suggests the utility of APOE genotyping in AD diagnosis, risk assessment, prevention, and treatment response.

Regional-specific effects of ovarian hormone loss on synaptic plasticity in adult human APOE targeted replacement mice

The human apolipoprotein ε4 allele (APOE4) has been implicated as one of the strongest genetic risk factors associated with Alzheimer’s disease (AD) and in influencing normal cognitive functioning. Previous studies have demonstrated that mice expressing human apoE4 display deficits in behavioral and neurophysiological outcomes compared to those with apoE3. Ovarian hormones have also been shown to be important in modulating synaptic processes underlying cognitive function, yet little is known about how their effects are influenced by apoE. In the current study, female adult human APOE targeted replacement (TR) mice were utilized to examine the effects of human APOE genotype and long-term ovarian hormone loss on synaptic plasticity in limbic regions by measuring dendritic spine density and electrophysiological function. No significant genotype differences were observed on any outcomes within intact mice. However, there was a significant main effect of genotype on total spine density in apical dendrites in the hippocampus, with post-hoc t-tests revealing a significant reduction in spine density in apoE3 ovariectomized (OVX) mice compared to sham operated mice. There was also a significant main effect of OVX on the magnitude of LTP, with post-hoc t-tests revealing a decrease in apoE3 OVX mice relative to sham. In contrast, apoE4 OVX mice showed increased synaptic activity relative to sham. In the lateral amygdala, there was a significant increase in total spine density in apoE4 OVX mice relative to sham. This increase in spine density was consistent with a significant increase in spontaneous excitatory activity in apoE4 OVX mice. These findings suggest that ovarian hormones differentially modulate synaptic integrity in an apoE-dependent manner within brain regions that are susceptible to neurophysiological dysfunction associated with AD.

The dendritic hypothesis for Alzheimer's disease pathophysiology

Converging evidence indicates that processes occurring in and around neuronal dendrites are central to the pathogenesis of Alzheimer's disease. These data support the concept of a "dendritic hypothesis" of AD, closely related to the existing synaptic hypothesis. Here we detail dendritic neuropathology in the disease and examine how Aβ, tau, and AD genetic risk factors affect dendritic structure and function. Finally, we consider potential mechanisms by which these key drivers could affect dendritic integrity and disease progression. These dendritic mechanisms serve as a framework for therapeutic target identification and for efforts to develop disease-modifying therapeutics for Alzheimer's disease.

Altered neurotransmission in the lateral amygdala in aged human apoE4 targeted replacement mice

The human APOE4 allele is associated with an early age of onset and increased risk of Alzheimer's disease (AD). Apolipoprotein E is secreted as part of a high-density lipoprotein-like particle by glial cells in the brain for the primary purpose of transport of lipophilic compounds involved in the maintenance of synapses. Previous studies examining synaptic integrity in the amygdala of human apoE targeted replacement (TR) mice showed a decrease in spontaneous excitatory synaptic activity, dendritic arbor, and spine density associated with apoE4 compared with apoE3 and apoE2 in adult male mice. In the present study, we assessed how APOE genotype affects synaptic integrity of amygdala neurons by comparing electrophysiological and morphometric properties in human apoE3, E4, and E2/4 TR mice at the age of 18-20 months. In contrast to adult mice, we found that aged apoE4 TR mice exhibited the highest level of excitatory synaptic activity compared with other cohorts. Additionally, apoE4 mice had significantly greater spontaneous inhibitory activity than all other cohorts. Taken together, there was a significant interaction between genotypes when comparing inhibition relative to excitation; there was a simple main effect of frequency type with an imbalance toward inhibition in apoE4 mice but not in apoE3 or apoE2/4 mice. These results suggest that apoE isoforms differentially influence synaptic transmission throughout the life span, where aging coupled with apoE4 expression, results in an imbalance in maintaining integrity of synaptic transmission.

The dark sides of amyloid in Alzheimer's disease pathogenesis

Although widely explored, the pathogenesis of Alzheimer's disease (AD) has yet to be cleared. Over the past twenty years the so call amyloid cascade hypothesis represented the main research paradigm in AD pathogenesis. In spite of its large consensus, the proposed role of β-amyloid (Aβ) remain to be elucidated. Many evidences are starting to cast doubt on Aβ as the primary causative factor in AD. For instance, Aβ is deposited in the brain following many different kinds of injury. Also, concentration of Aβ needed to induce toxicity in vitro are never reached in vivo. In this review we propose an amyloid-independent interpretation of several AD pathogenic features, such as synaptic plasticity, endo-lysosomal trafficking, cell cycle regulation and neuronal survival.

Reduction in DHA transport to the brain of mice expressing human APOE4 compared to APOE2

Benefits on cognition from docosahexaenoic acid (DHA, 22 : 6 n-3) intake are absent in humans carrying apolipoprotein E ε4 allele (APOE4), the most important genetic risk factor for Alzheimer's disease (AD). To test the hypothesis that carrying APOE4 impairs DHA distribution, we evaluated plasma and brain fatty acid profiles and uptake of [(14) C]-DHA using in situ cerebral perfusion through the blood-brain barrier in 4- and 13-month-old male and female APOE-targeted replacement mice (APOE2, APOE3, and APOE4), fed with a DHA-depleted diet. Cortical and plasma DHA were 9% lower and 34% higher in APOE4 compared to APOE2 mice, respectively. Brain uptake of [(14) C]-DHA was 24% lower in APOE4 versus APOE2 mice. A significant relationship was established between DHA and apoE concentrations in the cortex of mice (r(2) = 0.21) and AD patients (r(2) = 0.32). Altogether, our results suggest that lower brain uptake of DHA in APOE4 than in APOE2 mice may limit the accumulation of DHA in cerebral tissues. These data provide a mechanistic explanation for the lack of benefit of DHA in APOE4 carriers on cognitive function and the risk of AD. Using human APOE2, 3, and 4 isoform-specific transgenic mice, we found a lower brain uptake of docosahexaenoic acid (DHA) in APOE4 than in APOE2 mice that may limit the biodistribution of DHA in cerebral tissues. These data provide a mechanistic explanation for the lack of benefit of DHA in APOE4 carriers on cognitive function and the risk of Alzheimer's disease (AD).

Western-type diet modulates inflammatory responses and impairs functional outcome following permanent middle cerebral artery occlusion in aged mice expressing the human apolipoprotein E4 allele

Background

Numerous clinical trials in stroke have failed, most probably partially due to preclinical studies using young, healthy male rodents with little relevance to the heterogenic conditions of human stroke. Co-morbid conditions such as atherosclerosis and infections coupled with advanced age are known to contribute to increased risk of cerebrovascular diseases. Clinical and preclinical studies have shown that the E4 allele of human apolipoprotein (ApoE4) is linked to poorer outcome in various conditions of brain injury and neurodegeneration, including cerebral ischemia. Since ApoE is a known regulator of lipid homeostasis, we studied the impact of a high-cholesterol diet in aged mice in the context of relevant human ApoE isoforms on the outcome of focal brain ischemia.

Methods

Aged mice expressing human E3 and E4 isoforms of ApoE in C57BL/6J background and C57BL/6J mice fed on either a high-fat diet or a normal diet underwent permanent middle cerebral artery occlusion. The impact of a high-cholesterol diet was assessed by measuring the serum cholesterol level and the infarction volume was determined by magnetic resonance imaging. Sensorimotor deficits were assessed using an adhesive removal test and the findings were correlated with inflammatory markers.

Results

We show that expression of human ApoE4 renders aged mice fed with a western-type diet more susceptible to sensorimotor deficits upon stroke. These deficits are not associated with atherosclerosis but are accompanied with altered astroglial activation, neurogenesis, cyclooxygenase-2 immunoreactivity and increased plasma IL-6.

Conclusions

Our results support the hypothesis that ApoE alleles modify the inflammatory responses in the brain and the periphery, thus contributing to altered functional outcome following stroke.

Cerebrospinal fluid biomarkers of Alzheimer's disease in healthy elderly

Numerous studies have shown that Alzheimer's Disease (AD) pathology begins before the onset of clinical symptoms. Because therapies are likely to be more effective if they are implemented early in the disease progression, it is necessary to identify reliable biomarkers to detect AD pathology in the early stages of the disease, ideally in presymptomatic individuals. Recent research has identified three candidate cerebrospinal fluid (CSF) biomarkers that reflect AD pathology: amyloid beta, total tau protein (t-tau), and tau protein phosphorylated at AD-specific epitopes (p-tau). They are useful in supporting the AD diagnosis and have predictive value for AD when patients are in the stage of mild cognitive impairment (MCI). However, their predictive utility in cognitively healthy subjects is still being evaluated. We conducted a review of studies published between 1993 and 2011 and summarized their findings on the role of CSF biomarkers for AD in healthy elderly.

ApoE4 induces synaptic and ERG impairments in the retina of young targeted replacement apoE4 mice

The vertebrate retina, which is part of the central nervous system, is a window into the brain. The present study investigated the extent to which the retina can be used as a model for studying the pathological effects of apolipoprotein E4 (apoE4), the most prevalent genetic risk factor for Alzheimer's disease (AD). Immunohistochemical studies of retinas from young (4 months old) apoE4-targeted replacement mice and from corresponding mice which express the AD benign apoE3 allele, revealed that the density of the perikarya of the different classes of retinal neurons was not affected by apoE4. In contrast, the synaptic density of the retinal synaptic layers, which was assessed immunohistochemically and by immunoblot experiments, was significantly lower in the apoE4 than in the apoE3 mice. This was associated with reduced levels of the presynaptic vesicular glutamatergic transporter, VGluT1, but not of either the GABAergic vesicular transporter, VGaT, or the cholinergic vesicular transporter, VAChT, suggesting that the glutamatergic nerve terminals are preferentially affected by apoE4. In contrast, the post synaptic scaffold proteins PSD-95 and Gephyrin, which reside in excitatory and inhibitory synapses, respectively, were both elevated, and their ratio was not affected by apoE4. Electroretinogram (ERG) recordings revealed significant attenuation of mixed rod-cone responses in dark-adapted eyes of apoE4 mice. These findings suggest that the reduced ERG response in the apoE4 mice may be related to the observed decrease in the retinal nerve terminals and that the retina could be used as a novel model for non-invasive monitoring of the effects of apoE4 on the CNS.

ApoE4 induces Aβ42, tau, and neuronal pathology in the hippocampus of young targeted replacement apoE4 mice

Background

Recent findings suggest that the pathological effects of apoE4, the most prevalent genetic risk factor for Alzheimer’s disease (AD), start many years before the onset of the disease and are already detectable at a young age. In the present study we investigated the extent to which such pathological and cognitive impairments also occur in young apoE4 mice.

Results

This study revealed that the levels of the presynaptic glutamatergic vesicular transporter, VGlut, in the CA3, CA1, and DG hippocampal subfields were lower in hippocampal neurons of young (4-month-old) apoE4-targeted replacement mice than in those of the apoE3 mice. In contrast, the corresponding inhibitory GABAergic nerve terminals and perikarya were not affected by apoE4.

This synaptic effect was associated with hyperphosphorylation of tau in these neurons. In addition, apoE4 increased the accumulation of neuronal Aβ42 and induced mitochondrial changes, both of which were specifically pronounced in CA3 neurons. Spatial navigation behavioral studies revealed that these hippocampal pathological effects of apoE4 are associated with corresponding behavioral impairments. Time-course studies revealed that the effects of apoE4 on tau hyperphosphorylation and the mitochondria were already apparent at the age of 1 month and that the apoE4-driven accumulation of neuronal Aβ and reduced VGlut levels evolve later and are apparent at the age of 2–4 months. Furthermore, the levels of tau phosphorylation decrease in apoE3 mice and increase in apoE4 mice between 1 and 4 months, whereas the levels of Aβ42 decrease in apoE3 mice and are not affected in apoE4 mice over the same time period.

Conclusions

These findings show that apoE4 stimulates the accumulation of Aβ42 and hyperphosphorylated tau and reduces the levels of VGlut in hippocampal neurons of young apoE4-targeted replacement mice and that these neurochemical effects are associated with cognitive impairments. This model is not associated with hypothesis-driven mechanistic manipulations and is thus most suitable for unbiased studies of the mechanisms underlying the pathological effects of apoE4.

Apolipoprotein E and Alzheimer disease: risk, mechanisms and therapy

Apolipoprotein E (Apo-E) is a major cholesterol carrier that supports lipid transport and injury repair in the brain. APOE polymorphic alleles are the main genetic determinants of Alzheimer disease (AD) risk: individuals carrying the ε4 allele are at increased risk of AD compared with those carrying the more common ε3 allele, whereas the ε2 allele decreases risk. Presence of the APOE ε4 allele is also associated with increased risk of cerebral amyloid angiopathy and age-related cognitive decline during normal ageing. Apo-E-lipoproteins bind to several cell-surface receptors to deliver lipids, and also to hydrophobic amyloid-β (Aβ) peptide, which is thought to initiate toxic events that lead to synaptic dysfunction and neurodegeneration in AD. Apo-E isoforms differentially regulate Aβ aggregation and clearance in the brain, and have distinct functions in regulating brain lipid transport, glucose metabolism, neuronal signalling, neuroinflammation, and mitochondrial function. In this Review, we describe current knowledge on Apo-E in the CNS, with a particular emphasis on the clinical and pathological features associated with carriers of different Apo-E isoforms. We also discuss Aβ-dependent and Aβ-independent mechanisms that link Apo-E4 status with AD risk, and consider how to design effective strategies for AD therapy by targeting Apo-E.