Genetic Risk Factor For Alzheimer's Disease Research Articles

Bioinformatics Analysis of Publicly Available Single-Nuclei Transcriptomics Alzheimer’s Disease Datasets Reveals APOE Genotype-Specific Changes Across Cell Types in Two Brain Regions

Alzheimer’s Disease (AD) is a complex neurodegenerative disease that gravely affects patients and imposes an immense burden on caregivers. Apolipoprotein E4 (APOE4) has been identified as the most common genetic risk factor for AD, yet the molecular mechanisms connecting APOE4 to AD are not well understood. Past transcriptomic analyses in AD have revealed APOE genotype-specific transcriptomic differences; however, these differences have not been explored at a single-cell level. To elucidate more complex APOE genotype-specific disease-relevant changes masked by the bulk analysis, we leverage the first two single-nucleus RNA sequencing AD datasets from human brain samples, including nearly 55,000 cells from the prefrontal and entorhinal cortices. In each brain region, we performed a case versus control APOE genotype-stratified differential gene expression analysis and pathway network enrichment in astrocytes, microglia, neurons, oligodendrocytes, and oligodendrocyte progenitor cells. We observed more global transcriptomic changes in APOE4 positive AD cells and identified differences across APOE genotypes primarily in glial cell types. Our findings highlight the differential transcriptomic perturbations of APOE isoforms at a single-cell level in AD pathogenesis and have implications for precision medicine development in the diagnosis and treatment of AD.

APOE4 exacerbates α-synuclein seeding activity and contributes to neurotoxicity in Alzheimer’s disease with Lewy body pathology

Approximately half of Alzheimer’s disease (AD) brains have concomitant Lewy pathology at autopsy, suggesting that α-synuclein (α-SYN) aggregation is a regulated event in the pathogenesis of AD. Genome-wide association studies revealed that the ε4 allele of the apolipoprotein E (APOE4) gene, the strongest genetic risk factor for AD, is also the most replicated genetic risk factor for Lewy body dementia (LBD), signifying an important role of APOE4 in both amyloid-β (Aβ) and α-SYN pathogenesis. How APOE4 modulates α-SYN aggregation in AD is unclear. In this study, we aimed to determine how α-SYN is associated with AD-related pathology and how APOE4 impacts α-SYN seeding and toxicity. We measured α-SYN levels and their association with other established AD-related markers in brain samples from autopsy-confirmed AD patients (N = 469), where 54% had concomitant LB pathology (AD + LB). We found significant correlations between the levels of α-SYN and those of Aβ40, Aβ42, tau and APOE, particularly in insoluble fractions of AD + LB. Using a real-time quaking-induced conversion (RT-QuIC) assay, we measured the seeding activity of soluble α-SYN and found that α-SYN seeding was exacerbated by APOE4 in the AD cohort, as well as a small cohort of autopsy-confirmed LBD brains with minimal Alzheimer type pathology. We further fractionated the soluble AD brain lysates by size exclusion chromatography (SEC) ran on fast protein liquid chromatography (FPLC) and identified the α-SYN species (~ 96 kDa) that showed the strongest seeding activity. Finally, using human induced pluripotent stem cell (iPSC)-derived neurons, we showed that amplified α-SYN aggregates from AD + LB brain of patients with APOE4 were highly toxic to neurons, whereas the same amount of α-SYN monomer was not toxic. Our findings suggest that the presence of LB pathology correlates with AD-related pathologies and that APOE4 exacerbates α-SYN seeding activity and neurotoxicity, providing mechanistic insight into how APOE4 affects α-SYN pathogenesis in AD.

Sex- and age-specific associations between cardiometabolic risk and white matter brain age in the UK Biobank cohort.

Cardiometabolic risk (CMR) factors are associated with accelerated brain aging and increased risk for sex‐dimorphic illnesses such as Alzheimer's disease (AD). Yet, it is unknown how CMRs interact with sex and apolipoprotein E‐ϵ4 (APOE4), a known genetic risk factor for AD, to influence brain age across different life stages. Using age prediction based on multi‐shell diffusion‐weighted imaging data in 21,308 UK Biobank participants, we investigated whether associations between white matter Brain Age Gap (BAG) and body mass index (BMI), waist‐to‐hip ratio (WHR), body fat percentage (BF%), and APOE4 status varied (i) between males and females, (ii) according to age at menopause in females, and (iii) across different age groups in males and females. We report sex differences in associations between BAG and all three CMRs, with stronger positive associations among males compared to females. Independent of APOE4 status, higher BAG (older brain age relative to chronological age) was associated with greater BMI, WHR, and BF% in males, whereas in females, higher BAG was associated with greater WHR, but not BMI and BF%. These divergent associations were most prominent within the oldest group of females (66–81 years), where greater BF% was linked to lower BAG. Earlier menopause transition was associated with higher BAG, but no interactions were found with CMRs. In conclusion, the findings point to sex‐ and age‐specific associations between CMRs and brain age. Incorporating sex as a factor of interest in studies addressing CMR may promote sex‐specific precision medicine, consequently improving health care for both males and females.

Involvement of APOE polymorphism in Alzheimer's disease susceptibility and Donepezil response in Moroccan patients

BackgroundDonepezil (DNP) is an acetylcholinesterase inhibitor used for the treatment of Alzheimer's disease (AD). Apolipoprotein E (ApoE) is reported to be a strong genetic risk factor for AD. The aim of this study was to evaluate the impact of ApoE gene polymorphism on susceptibility to AD and response to Donepezil in Moroccan patients. MethodsA total of 70 Moroccan AD patients and 63 healthy controls participated to this study. Patients were treated with 5 to 10 mg of DNP daily for 6 months and 12 months later. The genotype analysis of ApoE gene was performed by PCR-RFLP method. ResultsA statistically significant difference was observed in allelic and genotypic frequencies of ApoE gene polymorphism among cases and controls. The ɛ2 allele was significantly associated with increased risk of AD (OR [95% CI] = 3.3 [1.8–6.08], p = 0.0001). Furthermore, the ɛ2/ɛ2 and ɛ2/ɛ4 genotypes were more frequent in AD patients than controls (OR [95% CI] = 17.64 [3.68–84.9], p = 0.0003; OR [95% CI] = 3.53 [1.03–12.04], p = 0.044, respectively). However, no association was observed between ApoE ɛ4 status and clinical features, as well as between ApoE gene polymorphism and response to DNP treatment. In addition, a higher rate of the ɛ2/ɛ2 genotype was observed in patients with severe stage of AD. ConclusionsOur study suggests that ɛ2 allele of the ApoE gene may represent a genetic risk factor for AD in Moroccan patients. On the other hand, ApoE genotypes seem to have no effect on response to DNP. However, more larger studies must be conducted in order to confirm the present findings.

A Levee to the Flood: Pre-injury Neuroinflammation and Immune Stress Influence Traumatic Brain Injury Outcome.

Increasing evidence demonstrates that aging influences the brain's response to traumatic brain injury (TBI), setting the stage for neurodegenerative pathology like Alzheimer's disease (AD). This topic is often dominated by discussions of post-injury aging and inflammation, which can diminish the consideration of those same factors before TBI. In fact, pre-TBI aging and inflammation may be just as critical in mediating outcomes. For example, elderly individuals suffer from the highest rates of TBI of all severities. Additionally, pre-injury immune challenges or stressors may alter pathology and outcome independent of age. The inflammatory response to TBI is malleable and influenced by previous, coincident, and subsequent immune insults. Therefore, pre-existing conditions that elicit or include an inflammatory response could substantially influence the brain's ability to respond to traumatic injury and ultimately affect chronic outcome. The purpose of this review is to detail how age-related cellular and molecular changes, as well as genetic risk variants for AD affect the neuroinflammatory response to TBI. First, we will review the sources and pathology of neuroinflammation following TBI. Then, we will highlight the significance of age-related, endogenous sources of inflammation, including changes in cytokine expression, reactive oxygen species processing, and mitochondrial function. Heightened focus is placed on the mitochondria as an integral link between inflammation and various genetic risk factors for AD. Together, this review will compile current clinical and experimental research to highlight how pre-existing inflammatory changes associated with infection and stress, aging, and genetic risk factors can alter response to TBI.

APOE4 confers transcriptomic and functional alterations to primary mouse microglia

Common genetic variants in more than forty loci modulate risk for Alzheimer’s disease (AD). AD risk alleles are enriched within enhancers active in myeloid cells, suggesting that microglia, the brain-resident macrophages, may play a key role in the etiology of AD. A major genetic risk factor for AD is Apolipoprotein E (APOE) genotype, with the ε4/ε4 (E4) genotype increasing risk for AD by approximately 15 fold compared to the most common ε3/ε3 (E3) genotype. However, the impact of APOE genotype on microglial function has not been thoroughly investigated. To address this, we cultured primary microglia from mice in which both alleles of the mouse Apoe gene have been humanized to encode either human APOE ε3 or APOE ε4. Relative to E3 microglia, E4 microglia exhibit altered morphology, increased endolysosomal mass, increased cytokine/chemokine production, and increased lipid and lipid droplet accumulation at baseline. These changes were accompanied by decreased translation and increased phosphorylation of eIF2ɑ and eIF2ɑ-kinases that participate in the integrated stress response, suggesting that E4 genotype leads to elevated levels of cellular stress in microglia relative to E3 genotype. Using live-cell imaging and flow cytometry, we also show that E4 microglia exhibited increased phagocytic uptake of myelin and other substrates compared to E3 microglia. While transcriptomic profiling of myelin-challenged microglia revealed a largely overlapping response profile across genotypes, differential enrichment of genes in interferon signaling, extracellular matrix and translation-related pathways was identified in E4 versus E3 microglia both at baseline and following myelin challenge. Together, our results suggest E4 genotype confers several important functional alterations to microglia even prior to myelin challenge, providing insight into the molecular and cellular mechanisms by which APOE4 may increase risk for AD.

Genetic dissection of glutathione S-transferase omega-1: identification of novel downstream targets and Alzheimer's disease pathways.

Alzheimer's disease (AD) is affected by genetic factors. Polymorphisms in the glutathione S-transferase omega-1 (Gsto1) gene have been shown by genetic correlation analyses performed in different ethnic populations to be genetic risk factors for AD. Gene expression profile data from BXD recombinant inbred mice were used in combination with genetic and bioinformatic analyses to characterize the mechanisms underlying regulation of Gsto1 variation regulation and to identify network members that may contribute to AD risk or progression. Allele-specific assays confirmed that variation in Gsto1 expression is controlled by cis-expression quantitative trait loci. We found that Gsto1 mRNA levels were related to several central nervous system traits, such as glial acidic fibrillary protein levels in the caudate putamen, cortical gray matter volume, and hippocampus mossy fiber pathway volume. We identified 2168 genes whose expression was highly correlated with that of Gsto1. Some genes were enriched for the most common neurodegenerative diseases. Some Gsto1-related genes identified in this study had previously been identified as susceptibility genes for AD, such as APP, Grin2b, Ide, and Psenen. To evaluate the relationships between Gsto1 and candidate network members, we transfected astrocytes with Gsto1 siRNA and assessed the effect on putative downstream effectors. We confirmed that knockdown of Gsto1 had a significant influence on Pa2g4 expression, suggesting that Pa2g4 may be a downstream effector of Gsto1, and that both genes interact with other genes in a network during AD pathogenesis.

Neuronal ROS-induced glial lipid droplet formation is altered by loss of Alzheimer’s disease–associated genes

A growing list of Alzheimer's disease (AD) genetic risk factors is being identified, but the contribution of each variant to disease mechanism remains largely unknown. We have previously shown that elevated levels of reactive oxygen species (ROS) induces lipid synthesis in neurons leading to the sequestration of peroxidated lipids in glial lipid droplets (LD), delaying neurotoxicity. This neuron-to-glia lipid transport is APOD/E-dependent. To identify proteins that modulate these neuroprotective effects, we tested the role of AD risk genes in ROS-induced LD formation and demonstrate that several genes impact neuroprotective LD formation, including homologs of human ABCA1, ABCA7, VLDLR, VPS26, VPS35, AP2A, PICALM, and CD2AP Our data also show that ROS enhances Aβ42 phenotypes in flies and mice. Finally, a peptide agonist of ABCA1 restores glial LD formation in a humanized APOE4 fly model, highlighting a potentially therapeutic avenue to prevent ROS-induced neurotoxicity. This study places many AD genetic risk factors in a ROS-induced neuron-to-glia lipid transfer pathway with a critical role in protecting against neurotoxicity.

APOE4 homozygotes are resistant to the mnemonic benefits of 17β-estradiol in the EFAD mouse model of Alzheimer's disease.

Loss of circulating estrogens at menopause may contribute to increased risk for Alzheimer's disease (AD) in females relative to males. APOE4, the greatest genetic risk factor for AD, acts synergistically with estrogens to increase risk in females. However, despite the relative contributions of APOE genotype and estrogen loss to sex differences in AD risk, these factors are rarely examined in concert in basic research. Hence, our knowledge of how APOE genotype may interact with estrogens to contribute to AD risk, or whether estrogens may be of benefit to particular genotypes, is limited. The present study was designed to test whether APOE genotype interacts with E2 to modulate cognition and dendritic spine density in a mouse model of AD. Effects of 17β-estradiol (E2 ) treatment on memory were examined in ovariectomized (OVXed) transgenic mice expressing 5 familial AD mutations (5XFAD) and two copies of human APOE3 (E3FAD), APOE4 (E4FAD), or one copy each of APOE3/APOE4 (E3/4FAD). OVXed female E3FAD, E3/4FAD, and E4FAD mice received bilateral dorsal hippocampal (DH) infusion of E2 immediately after training in object recognition (OR) and object placement (OP) tasks. Next, the effect of APOE genotype to modulate estrogenic effects on spines was examined. E3FAD, E3/4FAD, and E4FAD mice received DH infusion of E2 and brains were collected for Golgi impregnation and quantification of dendritic spine density 2 h later. DH infusion of E2 facilitated OR and OP memory in E3FAD and E3/4FADs, but not E4FADs. Similarly, DH infusion of E2 increased CA1 apical dendritic spine density in E3FADs and E3/4FADs, but not E4FADs. CA1 basal spine density was significantly reduced by E4FAD genotype. The significant benefit of E2 only in E3FAD and E3/4FAD mice suggests that having two copies of the APOE4 allele results in lower susceptibility to the memory-enhancing effects of E2 in the EFAD model. These findings provide novel evidence that E2 may be of clinical benefit to APOE3 and APOE3/4, but not homozygous APOE4, carriers.

Using a virtual reality path integration task to investigate entorhinal functioning in individuals with and without subjective cognitive decline

AbstractBackgroundThe entorhinal cortex (EC) contains spatially modulated cells, so called grid cells, and plays a central role in spatial navigation. Studies show that tauopathy in the EC is already observed in the early stages of Alzheimer's disease (AD) and is responsible for impairments in spatial memory performance. Moreover, recent studies demonstrate that young, healthy volunteers with the APOE‐ε4‐allele, a genetic risk factor for AD, show impaired grid‐like representations in the EC and reduced path integration performance in environments without spatial cues. People with subjective cognitive decline (SCD) perceive a subjective deterioration in their cognitive memory performance with unimpaired performance on standardized neuropsychological tests. SCD corresponds to the recently defined stage 2 of AD and is also a risk factor for dementia. Here, we tested whether subjects with SCD show path integration impairment, presumably related to malfunctioning in the EC.Method68 participants (31 SCD patients recruited through a memory clinic, age: 68.8 ± 9.2; 37 controls without SCD, age: 69.1 ± 7.9) performed a path integration task on a laptop. In addition, a neuropsychological test (CERAD+) was carried out to rule out an objective cognitive impairment.ResultThe two groups did not differ in age, sex, medical history and number of educational years. We replicated previous findings indicating better path integration performance with spatial cues such as boundaries and landmarks (F(2,128) = 20.83, P < 0.001). In addition, we showed that women performed worse than men (F(1,64) = 6.00, P = 0.017) and that path integration performance declined with increasing age (F(1,64) = 32.14, P < 0.001). There was no significant difference in performance between SCD and controls (F(1,64) = 2.20, P = 0.143).ConclusionGenerally, SCD was not associated with lower path integration performance. We are in the process of characterizing the sample based on plasma amyloid beta 42/40, NFL, pTau and APOE genotype to test, if AD biomarkers and genetic risk factors are associated with path integration performance.

Mitochondrial DNA and Alzheimer's disease: a first case-control study of the Tunisian population.

Alzheimer's disease (AD) is the most common neurodegenerative disorder in humans and presents a major health problem throughout the world. The etiology of AD is complex, and many factors are implicated, including mitochondria. Mitochondrial alteration has been proposed as a possible cause of AD. Therefore, several studies have focused on finding an association between inherited mitochondrial DNA variants and AD onset. In this study, we looked, for the first time, for a potential association between mitochondrial haplogroups or polymorphisms and AD in the Tunisian population. We also evaluated the distribution of the major genetic risk factor for AD, the apolipoprotein E epsilon 4 (APOE ε4), in this population. In total, 159 single-nucleotide polymorphisms (SNPs) of mitochondrial DNA haplogroups were genotyped in 254 individuals (58 patients and 196 controls). An additional genotyping of APOE ε4 was performed. No significant association between mitochondrial haplogroups and AD was found. However, two individual SNPs, A5656G (p = 0.03821, OR = 10.46) and A13759G (p = 0.03719, OR = 10.78), showed a significant association with AD. APOE 4 was confirmed as a risk factor for AD (p = 0.000014). Our findings may confirm the absence of a relation between mitochondrial haplogroups and AD and support the possible involvement of some inherited variants in the pathogenicity of AD.

It takes two: APOE4 and air pollution synergistically affect hippocampal volume in women

AbstractBackgroundThe APOE e4 allele (APOE4) is the strongest genetic risk factor for Alzheimer's Disease (AD), and exposure to air pollution is emerging as a major environmental AD risk factor. These factors, individually, do not explain most of AD cases though. It is possible that more than one “hit” is required for AD to develop, so different risk factors acting together might synergistically promote AD‐related pathology.MethodIn this study, we explored how different combinations of APOE4 carrier status and exposure to traffic‐generated air pollution may influence right hippocampal volume (RHV) in 10,623 females and 9,682 males aged 60‐75, using the UK Biobank Resource. The higher exposure to traffic pollution was approximated by closeness (50 meters or less) of subject’s primary residence to a major road. The APOE4 status was defined as carrying C allele of rs429358. The RHV was evaluated by the magnetic resonance imaging (MRI). We performed the analysis of variance (ANOVA) and Tukey's honestly significant difference (HSD) test.ResultThe RHV was significantly lower in female APOE4 carriers who live 50 meters or less from a major road (N=199) compared to non‐carriers living farther from the road (N=7195) (P‐Value=0.01). Separately, APOE4 or closeness to the road did not significantly influence RHV in females. No significant effects were found in males.ConclusionTwo hits (APOE4 and high exposure to car exhaust fumes) may be needed to cause brain changes associated with AD pathology, such as reduced RHV. A major component of car exhaust fumes is nitrogen oxides that have been linked to disruption of blood‐brain barrier and increased brain permeability to toxins. The APOE4 was previously associated with reduced myelin damage repair. A potential mechanism might, therefore, involve a synergistic effect of increased brain permeability to damaging factors and decreased brain ability to repair relevant damage. Results of this study also suggest that negative effects of the strongest genetic risk factor for AD (APOE4) on the brain may be attenuated by avoiding damaging exposures, such as to air pollution from car exhaust fumes.

Exome sequencing identifies rare damaging variants in the ATB8B4 and ABCA1 genes as novel risk factors for Alzheimer's disease.

Damaging rare variants in the TREM2, SORL1 and ABCA7 genes have been associated with an increased risk of developing Alzheimer's Disease (AD) with odds ratios that were not observed since the identification of the main AD genetic risk factor, the APOE-ε4 allele. Here, we aimed to identify additional AD-associated genes by investigating the burden of rare damaging variants in the exomes of AD cases and controls. On a single server, we analyzed in two stages, the data from 52,270 exome sequences from several independent datasets from Europe and the United States. After comprehensive QC, Stage-1 and Stage-2 datasets comprised in total 16,396 AD cases (5,672 EOAD) and 18,107 controls with European ancestry. All detected non-synonymous and loss-of-function rare variants were prioritized by REVEL and LOFTEE, and analyzed in a per-gene burden analysis. After a Stage-1 discovery analysis, we replicated findings in an independent dataset (Stage-2). We combined the Stage-1 and Stage-2 datasets and determined, for each gene, the features of the variants that drive the burden-associations. We confirmed the AD-association of rare damaging variants SORL1, TREM2, ABCA7, and newly identified a significant AD-association of rare damaging variants in the ATP8B4 and ABCA1 genes. In addition, we find a strong indication for the AD-association of ADAM10 and SRC genes (Stage-2 p<0.05). For most genes, we observed a larger effect size for LOF variants compared to missense variants (Figure-A). High-impact variants in these genes are mostly extremely rare and enriched in AD patients with early ages at onset (Figure-B). We identified, for the first time, the AD-association of rare damaging variants in two genes: (i) microglial ATP8B4 which is involved in phospholipid transport, and (ii) ABCA1 which plays a critical role in lipidation of apoE thereby supporting Aβ processing. Further, we found strong evidence for the AD-association of damaging variants in ADAM10 and SRC genes. ADAM10 is involved in the proteolytic processing of APP, while SRC is a Non-Receptor Tyrosine Kinase which binds PTK2B/Pyk2, a known AD risk factor. Together, our study provides further evidence for the role of Aβ and microglia in AD pathophysiology.

Evidence against altered excitatory/inhibitory balance in the posteromedial cortex of young adult APOE E4 carriers: A resting state 1H-MRS study

A strategy to gain insight into early changes that may predispose people to Alzheimer's disease (AD) is to study the brains of younger cognitively healthy people that are at increased genetic risk of AD. The Apolipoprotein (APOE) E4 allele is the strongest genetic risk factor for AD, and several neuroimaging studies comparing APOE E4 carriers with non-carriers at age ∼20–30 years have detected hyperactivity (or reduced deactivation) in posteromedial cortex (PMC), a key hub of the default network (DN), which has a high susceptibility to early amyloid deposition in AD. Transgenic mouse models suggest such early network activity alterations may result from altered excitatory/inhibitory (E/I) balance, but this is yet to be examined in humans. Here we test the hypothesis that PMC fMRI hyperactivity could be underpinned by altered levels of excitatory (glutamate) and/or inhibitory (GABA) neurotransmitters in this brain region. Forty-seven participants (20 APOE E4 carriers and 27 non-carriers) aged 18–25 years underwent resting-state proton magnetic resonance spectroscopy (1H-MRS), a non-invasive neuroimaging technique to measure glutamate and GABA in vivo. Metabolites were measured in a PMC voxel of interest and in a comparison voxel in the occipital cortex (OCC). There was no difference in either glutamate or GABA between the E4 carriers and non-carriers in either MRS voxel, or in the ratio of glutamate to GABA, a measure of E/I balance. Default Bayesian t-tests revealed evidence in support of this null finding. Our findings suggest that PMC hyperactivity in APOE E4 carriers is unlikely to be associated with, or possibly may precede, alterations in local resting-state PMC neurotransmitters, thus informing our understanding of the spatio-temporal sequence of early network alterations underlying APOE E4 related AD risk.

Diet enriched in omega-3 fatty acids alleviates olfactory system deficits in APOE4 transgenic mice.

Olfactory dysfunction is observed in several neurological disorders including Mild Cognitive Impairment (MCI) and Alzheimer disease (AD). These deficits occur early and correlate with global cognitive performance, depression and degeneration of olfactory regions in the brain. Despite extensive human studies, there has been little characterization of the olfactory system in models of AD. In order to determine if olfactory structural and/or molecular phenotypes are observed in a model expressing a genetic risk factor for AD, we assessed the olfactory bulb (OB) in APOE4 transgenic mice. A significant decrease in OB weight was observed at 12 months of age in APOE4 mice concurrent with inflammation and decreased NeuN expression. In order to determine if a diet rich in omega-3s may alleviate the olfactory system phenotypes observed, we assessed WT and APOE4 mice on a docosahexaenoic acid (DHA) diet. APOE4 mice on a DHA diet did not present with atrophy of the OB, and the alterations in NeuN and IBA-1 expression were alleviated. Furthermore, alterations in caspase mRNA and protein expression in the APOE4 OB were not observed with a DHA diet. Similar to the human AD condition, OB atrophy is an early phenotype in the APOE4 mice and concurrent with inflammation. These data support a link between the structural olfactory brain region atrophy and the olfactory dysfunction observed in AD and suggest that inflammation and cell death pathways may contribute to the olfactory deficits observed. Furthermore, the results suggest that diets enriched in DHA may provide benefit to APOE4 allele carriers.

Targeting Default Mode Network Dysfunction in Persons at Risk of Alzheimer's Disease with Transcranial Magnetic Stimulation (NEST4AD): Rationale and Study Design.

Default mode network (DMN) dysfunction is well established in Alzheimer's disease (AD) and documented in both preclinical stages and at-risk subjects, thus representing a potential disease target. Multi-sessions of repetitive transcranial magnetic stimulation (rTMS) seem capable of modulating DMN dynamics and memory in healthy individuals and AD patients; however, the potential of this approach in at-risk subjects has yet to be tested. This study will test the effect of rTMS on the DMN in healthy older individuals carrying the strongest genetic risk factor for AD, the Apolipoprotein E (APOE) ɛ4 allele. We will recruit 64 older participants without cognitive deficits, 32 APOE ɛ4 allele carriers and 32 non-carriers as a reference group. Participants will undergo four rTMS sessions of active (high frequency) or sham DMN stimulation. Multimodal imaging exam (including structural, resting-state, and task functional MRI, and diffusion tensor imaging), TMS with concurrent electroencephalography (TMS-EEG), and cognitive assessment will be performed at baseline and after the stimulation sessions. We will assess changes in DMN connectivity with resting-state functional MRI and TMS-EEG, as well as changes in memory performance in APOE ɛ4 carriers. We will also investigate the mechanisms underlying DMN modulation through the assessment of correlations with measures of neuronal activity, excitability, and structural connectivity with multimodal imaging. The results of this study will inform on the physiological and cognitive outcomes of DMN stimulation in subjects at risk for AD and on the possible mechanisms. These results may outline the design of future non-pharmacological preventive interventions for AD.

APOE3-Jacksonville (V236E) variant reduces self-aggregation and risk of dementia.

Apolipoprotein E (APOE) genetic variants have been shown to modify Alzheimer’s disease (AD) risk. We previously identified an APOE3 variant (APOE3-V236E), named APOE3-Jacksonville (APOE3-Jac), associated with healthy brain aging and reduced risk for AD and dementia with Lewy bodies (DLB). Herein, we resolved the functional mechanism by which APOE3-Jac reduces APOE aggregation and enhances its lipidation in human brains, as well as in cellular and biochemical assays. Compared to APOE3, expression of APOE3-Jac in astrocytes increases several classes of lipids in the brain including phosphatidylserine, phosphatidylethanolamine, phosphatidic acid, and sulfatide, critical for synaptic functions. Mice expressing APOE3-Jac have reduced amyloid pathology, plaque-associated immune responses, and neuritic dystrophy. The V236E substitution is also sufficient to reduce the aggregation of APOE4, whose gene allele is a major genetic risk factor for AD and DLB. These findings suggest that targeting APOE aggregation might be an effective strategy for treating a subgroup of individuals with AD and DLB.

Reelin Regulates Neuronal Excitability through Striatal-Enriched Protein Tyrosine Phosphatase (STEP61) and Calcium Permeable AMPARs in an NMDAR-Dependent Manner.

Alzheimer's disease (AD) is a progressive neurodegenerative disease marked by the accumulation of amyloid-β (Aβ) plaques and neurofibrillary tangles. Aβ oligomers cause synaptic dysfunction early in AD by enhancing long-term depression (LTD; a paradigm for forgetfulness) via metabotropic glutamate receptor (mGluR)-dependent regulation of striatal-enriched tyrosine phosphatase (STEP61). Reelin is a neuromodulator that signals through ApoE (apolipoprotein E) receptors to protect the synapse against Aβ toxicity (Durakoglugil et al., 2009) Reelin signaling is impaired by ApoE4, the most important genetic risk factor for AD, and Aβ-oligomers activate metabotropic glutamate receptors (Renner et al., 2010). We therefore asked whether Reelin might also affect mGluR-LTD. To this end, we induced chemical mGluR-LTD using DHPG (Dihydroxyphenylglycine), a selective mGluR5 agonist. We found that exogenous Reelin reduces the DHPG-induced increase in STEP61, prevents the dephosphorylation of GluA2, and concomitantly blocks mGluR-mediated LTD. By contrast, Reelin deficiency increased expression of Ca2+-permeable GluA2-lacking AMPA receptors along with higher STEP61 levels, resulting in occlusion of DHPG-induced LTD in hippocampal CA1 neurons. We propose a model in which Reelin modulates local protein synthesis as well as AMPA receptor subunit composition through modulation of mGluR-mediated signaling with implications for memory consolidation or neurodegeneration.SIGNIFICANCE STATEMENT Reelin is an important neuromodulator, which in the adult brain controls synaptic plasticity and protects against neurodegeneration. Amyloid-β has been shown to use mGluRs to induce synaptic depression through endocytosis of NMDA and AMPA receptors, a mechanism referred to as LTD, a paradigm of forgetfulness. Our results show that Reelin regulates the phosphatase STEP, which plays an important role in neurodegeneration, as well as the expression of calcium-permeable AMPA receptors, which play a role in memory formation. These data suggest that Reelin uses mGluR LTD pathways to regulate memory formation as well as neurodegeneration.

APOE Genotype and Alzheimer's Disease: The Influence of Lifestyle and Environmental Factors.

Alzheimer's disease (AD) is the most common neurodegenerative disorder with obscure pathogenesis and no disease-modifying therapy to date. AD is multifactorial disease that develops from the complex interplay of genetic factors and environmental exposures. The E4 allele of the gene encoding apolipoprotein E (APOE) is the most common genetic risk factor for AD, whereas the E2 allele acts in a protective manner. A growing amount of epidemiological evidence suggests that several lifestyle habits and environmental factors may interact with APOE alleles to synergistically affect the risk of AD development. Among them, physical exercise, dietary habits including fat intake and ketogenic diet, higher education, traumatic brain injury, cigarette smoking, coffee consumption, alcohol intake, and exposure to pesticides and sunlight have gained increasing attention. Although the current evidence is inconsistent, it seems that younger APOE4 carriers in preclinical stages may benefit mostly from preventive lifestyle interventions, whereas older APOE4 noncarriers with dementia may show the most pronounced effects. The large discrepancies between the epidemiological studies may be attributed to differences in the sample sizes, the demographic characteristics of the participants, including age and sex, the methodological design, and potential related exposures and comorbidities as possible cofounding factors. In this Review, we aim to discuss available evidence of the prominent APOE genotype-environment interactions in regard to cognitive decline with a focus on AD, providing an overview of the current landscape in this field and suggesting future directions.

Genetic overlap between Alzheimer's disease and blood lipid levels

Late-onset Alzheimer's disease (AD) has a significant genetic component, but the molecular mechanisms through which genetic risk factors contribute to AD pathogenesis are unclear. We screened for genetic sharing between AD and the blood levels of 615 metabolites to elucidate how the polygenic architecture of AD affects metabolomic profiles. We retrieved summary statistics from genome-wide association studies of AD and the metabolite blood levels and assessed for shared genetic etiology, using a polygenic risk score-based approach. For the blood levels of 31 metabolites, all of which were lipids, we identified and replicated genetic sharing with AD. We also found a positive genetic concordance - implying that genetic risk factors for AD are associated with higher blood levels - for 16 of the 31 replicated metabolites. In the brain, lipids and their intermediate metabolites have essential structural and functional roles, such as forming and dynamically regulating synaptic membranes. Our results imply that genetic risk factors for AD affect lipid levels, which may be leveraged to develop novel treatment strategies for AD.