A dual role for apolipoprotein e in neuroinflammation: anti- and pro-inflammatory activity

Quantitative proteomics of acutely-isolated mouse microglia identifies novel immune Alzheimer's disease-related proteins

BACKGROUND

Microglia are innate immune cells of the brain that perform phagocytic and inflammatory functions in disease conditions. Transcriptomic studies of acutely-isolated microglia have provided novel insights into their molecular and functional diversity in homeostatic and neurodegenerative disease states. State-of-the-art mass spectrometry methods can comprehensively characterize proteomic alterations in microglia in neurodegenerative disorders, potentially providing novel functionally relevant molecular insights that are not provided by transcriptomics. However, comprehensive proteomic profiling of adult primary microglia in neurodegenerative disease conditions has not been performed.

METHODS

We performed quantitative mass spectrometry based proteomic analyses of purified CD11b⁺ acutely-isolated microglia from adult (6 mo) mice in normal, acute neuroinflammatory (LPS-treatment) and chronic neurodegenerative states (5xFAD model of Alzheimer's disease [AD]). Differential expression analyses were performed to characterize specific microglial proteomic changes in 5xFAD mice and identify overlap with LPS-induced pro-inflammatory changes. Our results were also contrasted with existing proteomic data from wild-type mouse microglia and from existing microglial transcriptomic data from wild-type and 5xFAD mice. Neuropathological validation studies of select proteins were performed in human AD and 5xFAD brains.

RESULTS

Of 4133 proteins identified, 187 microglial proteins were differentially expressed in the 5xFAD mouse model of AD pathology, including proteins with previously known (Apoe, Clu and Htra1) as well as previously unreported relevance to AD biology (Cotl1 and Hexb). Proteins upregulated in 5xFAD microglia shared significant overlap with pro-inflammatory changes observed in LPS-treated mice. Several proteins increased in human AD brain were also upregulated by 5xFAD microglia (Aβ peptide, Apoe, Htra1, Cotl1 and Clu). Cotl1 was identified as a novel microglia-specific marker with increased expression and strong association with AD neuropathology. Apoe protein was also detected within plaque-associated microglia in which Apoe and Aβ were highly co-localized, suggesting a role for Apoe in phagocytic clearance of Aβ.

CONCLUSIONS

We report a comprehensive proteomic study of adult mouse microglia derived from acute neuroinflammation and AD models, representing a valuable resource to the neuroscience research community. We highlight shared and unique microglial proteomic changes in acute neuroinflammation aging and AD mouse models and identify novel roles for microglial proteins in human neurodegeneration.

Microglia-targeted stem cell therapies for Alzheimer disease: A preclinical data review

Alzheimer disease (AD) is a severe, life-threatening illness characterized by gradual memory loss. The classic histological features of AD include extracellular formation of β-amyloid plaques (Aβ), intracellular neurofibrillary tangles (NFT), and synaptic loss. Recently, accumulated evidence has confirmed the critical role of microglia in the development and exacerbation of AD. When Aβ forms deposits, microglia quickly respond to restore brain physiology by activating a series of repair mechanisms. However, prolonged microglial activation is considered detrimental and may aggravate AD progression. To date, there are no curative therapies for AD. The advent of stem cell transplantation offers novel strategies to treat AD in animal models. Furthermore, studies have reported that transplanted stem cells might ameliorate AD symptoms by regulating microglial functions, from detrimental to protective. This review focuses on the crucial functions of microglia in AD and examines the reactions of microglia to transplanted stem cells.

16S rRNA Next Generation Sequencing Analysis Shows Bacteria in Alzheimer's Post-Mortem Brain

The neurological deterioration associated with Alzheimer's disease (AD), involving accumulation of amyloid-beta peptides and neurofibrillary tangles, is associated with evident neuroinflammation. This is now seen to be a significant contributor to pathology. Recently the tenet of the privileged status of the brain, regarding microbial compromise, has been questioned, particularly in terms of neurodegenerative diseases. It is now being considered that microbiological incursion into the central nervous system could be either an initiator or significant contributor to these. This is a novel study using 16S ribosomal gene-specific Next generation sequencing (NGS) of extracted brain tissue. A comparison was made of the bacterial species content of both frozen and formaldehyde fixed sections of a small cohort of Alzheimer-affected cases with those of cognitively unimpaired (normal). Our findings suggest an increase in bacterial populations in Alzheimer brain tissue compared with normal.

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.

Pathological vascular and inflammatory biomarkers of acute- and chronic-phase traumatic brain injury

Given the demand for developing objective methods for characterizing traumatic brain injury (TBI), research dedicated to evaluating putative biomarkers has burgeoned over the past decade. Since it is critical to elucidate the underlying pathological processes that underlie the higher diverse outcomes that follow neurotrauma, considerable efforts have been aimed at identifying biomarkers of both the acute- and chronic-phase TBI. Such information is not only critical for helping to elucidate the pathological changes that lead to poor long-term outcomes following TBI but it may also assist in the identification of possible prevention and interventions for individuals who sustain head trauma. In the current review, we discuss the potential role of vascular dysfunction and chronic inflammation in both acute- and chronic-phase TBI, and we also highlight existing studies that have investigated inflammation biomarkers associated with poorer injury outcome.

The Genetic Determinants of Renal Impairment Following Cardiac Surgery

Cardiac surgery is frequently performed, and acute renal dysfunction is a common adverse event following this procedure. Cardiac surgery-related renal injury independently predicts longer hospital stays and greater rates of morbidity and mortality. Although much work has been completed toward better understanding of this phenomenon, the state of knowledge concerning surgery-related renal injury remains limited. Currently, there is no effective paradigm to identify patients who are at risk for this condition; the specific mechanisms of renal injury during surgery are incompletely understood; and few therapies exist to prevent or treat this phenomenon. To better understand this common clinical problem, recent research has focused on the importance of genetic variability within the physiological and patho-physiological systems that underlie renal dysfunction following cardiac surgery. Emphasizing the importance of using genetics to elucidate molecular mechanisms of this disease, this article reviews the current literature on genetic polymorphisms and post cardiac surgery-related renal dysfunction.

APOE Genotype Alters Immunoglobulin Subtypes in Knock-In Mice

Apolipoprotein E (APOE) alleles are strongly related to the risk of Alzheimer's disease (AD). APOE genotype also affects inflammatory processes in response to damage. We tested whether APOE genotype affected the levels of specific immunoglobulins in healthy, uninfected APOE knock-in mice. We measured specific immunoglobulins in brain, spleen, and plasma. Levels of total IgG in brain and spleen were highest in APOE-ɛ3 mice, significantly higher than in APOE-ɛ2 and APOE-ɛ4 mice; no differences were observed for levels of total IgG in plasma. We also measured specific subtypes of IgG. IgG1 was only detectable in plasma and did not differ by APOE genotype. IgG3 was detectable in plasma and spleen, and also did not differ by APOE genotype. IgG2b showed the same pattern as levels of total IgG by APOE genotype, with the highest levels of IgG2b in brain, spleen, and plasma of APOE-ɛ3 mice. IgG2a showed an entirely different pattern, with significantly higher levels in spleen and plasma of APOE-ɛ4 mice compared to APOE-ɛ2 and APOE-ɛ3 mice. We also measured IgM and IgA in spleens and plasma of these mice. In spleen, APOE-ɛ4 mice had the lowest IgA levels and the highest levels of IgM; both being significantly different from APOE-ɛ2 mice. In total, murine IgG2a and IgM were highest in APOE-ɛ4 mice, while total IgG and Ig2b were highest in APOE-ɛ3 mice. These dramatically different distributions of immunoglobulins could allow for human AD risk biomarkers based on specific immunoglobulin subtypes.

Arabidopsis thaliana extracts optimized for polyphenols production as potential therapeutics for the APOE-modulated neuroinflammation characteristic of Alzheimer's disease in vitro

Although the cause of Alzheimer's disease (AD) is unknown, glial-induced neuroinflammation is an early symptom. Familial AD is caused by increases in amyloid-beta (Aβ) peptide, particularly soluble oligomeric (oAβ), considered a proximal neurotoxin and neuroinflammatory stimuli. APOE4, a naturally occurring genotype of APOE, is the greatest genetic risk factor for AD; increasing risk up to 12-fold compared to APOE3 and APOE2. oAβ-induced neuroinflammation is greater with APOE4 compared to APOE3 and APOE2. As sinapates and flavonoids have anti-inflammatory properties, a protocol was developed for optimizing polyphenol production in seedlings of Arabidopsis thaliana (A. thaliana). Three mutants (cop1, prn1, xpf3) were identified, and the extracts treated with liver microsomes to mimic physiological metabolism, with HPLC and MS performed on the resulting metabolites for peak identification. These extracts were used to treat primary glial cells isolated from human APOE-targeted-replacement (APOE-TR) and APOE-knock-out (KO) mice, with neuroinflammation induced by lipopolysaccharide (LPS) or oAβ. The dose-response data for TNFα secretion demonstrate the followed the order: APOE-KO > APOE4 > APOE3 > APOE2, with xpf3 the most effective anti-neuroinflammatory across APOE genotypes. Thus, the plant-based approach described herein may be particularly valuable in treating the APOE4-induced neuroinflammatory component of AD risk.

Interactive effects of C-reactive protein levels on the association between APOE variants and triglyceride levels in a Taiwanese population

Background

Apolipoprotein E (APOE) plays a major role in lipid metabolism and inflammation. However, the association between APOE gene polymorphisms and serum triglyceride levels remains controversial. We tested the effects of APOE variants on triglyceride levels and their interactions with the inflammatory marker C-reactive protein (CRP) in a Taiwanese population.

Methods

Two APOE single nucleotide polymorphisms (SNPs) rs429358 and rs7412 were genotyped by TaqMan Assay using real time PCR in 595 healthy subjects attending the clinic for routine visits.

Results

After adjustment for clinical covariates, subjects carrying the rs429358-TT genotype and non-ε4 alleles were found to have higher CRP levels, whereas those with rs7412-CC genotype and non-ε2 alleles had significantly higher total and low-density lipoprotein cholesterol levels (all P < 0.01). Using subgroup and interaction analyses, we observed significantly lower triglyceride levels in subjects carrying the rs429358-TT genotype and non-ε4 alleles in the low CRP group (P = 2.71× 10⁻⁴ and P = 4.32 × 10⁻⁴, respectively), but not in those in the high CRP group (interaction P = 0.013 and 0.045, respectively). In addition, multivariate stepwise linear regression analysis showed that subjects carrying the rs429358-TT genotype and non-ε4 alleles with low CRP levels had significantly lower triglyceride levels (P < 0.001 and P < 0.001, respectively). In addition, when combined with the risk alleles of GCKR, APOA5 and LPL gene variants, we observed that triglyceride levels increased significantly with the number of risk alleles (P = 2.9 × 10⁻¹²).

Conclusions

The combination of SNPs and ε alleles at the APOE locus is involved in managing lipid and CRP levels in the Taiwanese population. APOE polymorphisms interact with CRP to regulate triglyceride levels, thus triglyceride concentration is influenced by both the genetic background of the APOE locus and the inflammatory status of a subject.

Electronic supplementary material

The online version of this article (doi:10.1186/s12944-016-0262-z) contains supplementary material, which is available to authorized users.

Identification and modification of amyloid-independent phenotypes of APOE4 mice

BACKGROUND

Over 70 million Americans inherit the strongest genetic risk factor for Alzheimer's disease (AD), apolipoprotein E4 (APOE4), but have no course for reducing their risk. The association of non-steroidal anti-inflammatory drug (NSAID) use with reduced risk of AD for APOE4-carriers suggests that NSAIDs may be useful in AD prevention.

METHODS

We identified phenotypes associated with APOE4 in APOE knock-in mice in order to define modifiable measures that correlate with risk of AD.

RESULTS

APOE4 mouse brains showed altered post-translational modifications and biochemical distribution of APOE compared to APOE3 mice; these differences were also observed in brains of human APOE4 carriers. Two-month treatment with ibuprofen significantly altered the expression pattern of APOE in APOE4 mice to that of APOE3 mice; PPAR-γ agonist pioglitazone also had a significant effect. APOE4 mice also show deficits in dendritic spine density, and ibuprofen and pioglitazone significantly increased dendritic spine density.

CONCLUSIONS

We report new phenotypes associated with APOE4 in human and APOE knock-in mice and their mitigation with NSAID treatment, through COX-2 inhibition and PPAR-γ activation.

Epigenetic considerations of the APOE gene. [corrected]

The apolipoprotein E (APOE) gene is robustly linked with numerous physiological conditions, including healthy aging, altered cardiovascular fitness, and cognitive function. These connections have been established primarily by phenotype-genotype association studies using APOE's three common genetic variants (ε2, ε3, and ε4). These variants encode for the three apoE protein isoforms (E2, E3, and E4), which have slightly different structures and, consequently, distinct functions in lipid metabolism. However, the differential lipid binding and transferring properties of these isoforms cannot fully explain the association of APOE with such a wide range of physiological phenotypes. One potential explanation for APOE's pleiotropic roles may lie in its unique epigenetic properties. In this article, we present a brief review of the APOE gene and protein, its disease associations, and epigenetic components, with a focus on DNA methylation. We close with a discussion of the prospective epigenetic implications of APOE in disease.

Genetic and Transcriptomic Profiles of Inflammation in Neurodegenerative Diseases: Alzheimer, Parkinson, Creutzfeldt-Jakob and Tauopathies

Polymorphisms in certain inflammatory-related genes have been identified as putative differential risk factors of neurodegenerative diseases with abnormal protein aggregates, such as sporadic Alzheimer’s disease (AD) and sporadic Parkinson’s disease (sPD). Gene expression studies of cytokines and mediators of the immune response have been made in post-mortem human brain samples in AD, sPD, sporadic Creutzfeldt-Jakob disease (sCJD) subtypes MM1 and VV2, Pick’s disease (PiD), progressive supranuclear palsy (PSP) and frontotemporal lobar degeneration linked to mutation P301L in MAPT Frontotemporal lobar degeneration-tau (FTLD-tau). The studies have disclosed variable gene regulation which is: (1) disease-dependent in the frontal cortex area 8 in AD, sPD, sCJD MM1 and VV2, PiD, PSP and FTLD-tau; (2) region-dependent as seen when comparing the entorhinal cortex, orbitofrontal cortex, and frontal cortex area 8 (FC) in AD; the substantia nigra, putamen, FC, and angular gyrus in PD, as well as the FC and cerebellum in sCJD; (3) genotype-dependent as seen considering sCJD MM1 and VV2; and (4) stage-dependent as seen in AD at different stages of disease progression. These observations show that regulation of inflammation is much more complicated and diverse than currently understood, and that new therapeutic approaches must be designed in order to selectively act on specific targets in particular diseases and at different time points of disease progression.

Apolipoprotein E Isoforms and AMD

The cholesterol transporting protein apolipoprotein E (ApoE) occurs in three allelic variants in humans unlike in other species. The resulting protein isoforms E2, E3 and E4 exhibit differences in lipid binding, integrating into lipoprotein particles and affinity for lipoprotein receptors. ApoE isoforms confer genetic risk for several diseases of aging including atherosclerosis, Alzheimer's disease, and age-related macular degeneration (AMD). A single E4 allele increases the risk of developing Alzheimer's disease, whereas the E2 allele is protective. Intriguingly, the E4 allele is protective in AMD. Current thinking about different functions of ApoE isoforms comes largely from studies on Alzheimer's disease. These data cannot be directly extrapolated to AMD since the primary cells affected in these diseases (neurons vs. retinal pigment epithelium) are so different. Here, we propose that ApoE serves a fundamentally different purpose in regulating cholesterol homeostasis in the retinal pigment epithelium and this could explain why allelic risk factors are flipped for AMD compared to Alzheimer's disease.

Obesity and sex interact in the regulation of Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative disorder, for which a number of genetic, environmental, and lifestyle risk factors have been identified. A significant modifiable risk factor is obesity in mid-life. Interestingly, both obesity and AD exhibit sex differences and are regulated by sex steroid hormones. Accumulating evidence suggests interactions between obesity and sex in regulation of AD risk, although the pathways underlying this relationship are unclear. Inflammation and the E4 allele of apolipoprotein E have been identified as independent risk factors for AD and both interact with obesity and sex steroid hormones. We review the individual and cooperative effects of obesity and sex on development of AD and examine the potential contributions of apolipoprotein E, inflammation, and their interactions to this relationship.

ApoE4 upregulates the activity of mitochondria-associated ER membranes

In addition to the appearance of senile plaques and neurofibrillary tangles, Alzheimer's disease (AD) is characterized by aberrant lipid metabolism and early mitochondrial dysfunction. We recently showed that there was increased functionality of mitochondria-associated endoplasmic reticulum (ER) membranes (MAM), a subdomain of the ER involved in lipid and cholesterol homeostasis, in presenilin-deficient cells and in fibroblasts from familial and sporadic AD patients. Individuals carrying the ε4 allele of apolipoprotein E (ApoE4) are at increased risk for developing AD compared to those carrying ApoE3. While the reason for this increased risk is unknown, we hypothesized that it might be associated with elevated MAM function. Using an astrocyte-conditioned media (ACM) model, we now show that ER-mitochondrial communication and MAM function-as measured by the synthesis of phospholipids and of cholesteryl esters, respectively-are increased significantly in cells treated with ApoE4-containing ACM as compared to those treated with ApoE3-containing ACM. Notably, this effect was seen with lipoprotein-enriched preparations, but not with lipid-free ApoE protein. These data are consistent with a role of upregulated MAM function in the pathogenesis of AD and may help explain, in part, the contribution of ApoE4 as a risk factor in the disease.

Microglial genes regulating neuroinflammation in the progression of Alzheimer's disease

Neuroinflammation is a pathological hallmark of Alzheimer's disease (AD), and microglia, the brain's resident phagocyte, are pivotal for the immune response observed in AD. Microglia act as sentinel and protective cells, but may become inappropriately reactive in AD to drive neuropathology. Recent Genome Wide Association Studies (GWAS) have identified more than 20 gene variants associated with an increased risk of late-onset AD (LOAD), the most prevalent form of AD [1]. The findings strongly implicate genes related to the immune response (CR1, CD33, MS4A, CLU, ABCA7, EPHA1 and HLA-DRB5-HLA-DRB1), endocytosis (BIN1, PICALM, CD2AP, EPHA1 and SORL1) and lipid biology (CLU, ABCA7 and SORL1) [2-8], and many encode proteins which are highly expressed in microglia [1]. Furthermore, recent identification of a low frequency mutation in the gene encoding the triggering receptor expressed in myeloid cells 2 protein (TREM2) confers increased risk of AD in LOAD cohorts with an effect size similar to that for APOE, until recently the only identified genetic risk factor associated with LOAD [9,10(••)] (Figure 1). The present review summarises our current understanding of the probable roles of microglial genes in the regulation of neuroinflammatory processes in AD and their relation to other processes affecting the disease's progression.

Neuronal Regulation of Neuroprotective Microglial Apolipoprotein E Secretion in Rat In Vitro Models of Brain Pathophysiology

Apolipoprotein E (ApoE) is mainly secreted by glial cells and is involved in many brain functions, including neuronal plasticity, β-amyloid clearance, and neuroprotection. Microglia--the main immune cells of the brain--are one source of ApoE, but little is known about the physiologic regulation of microglial ApoE secretion by neurons and whether this release changes under inflammatory or neurodegenerative conditions. Using rat primary neural cell cultures, we show that microglia release ApoE through a Golgi-mediated secretion pathway and that ApoE progressively accumulates in neuroprotective microglia-conditioned medium. This constitutive ApoE release is negatively affected by microglial activation both with lipopolysaccharide and with ATP. Microglial ApoE release is stimulated by neuron-conditioned media and under coculture conditions. Neuron-stimulated microglial ApoE release is mediated by serine and glutamate through N-methyl-D-aspartate receptors and is differently regulated by activation states (i.e. lipopolysaccharide vs ATP) and by 6-hydroxydopamine. Microglial ApoE silencing abrogated protection of cerebellar granule neurons against 6-hydroxydopamine toxicity in cocultures, indicating that microglial ApoE release is neuroprotective. Our findings shed light on the reciprocal cross-talk between neurons and microglia that is crucial for normal brain functions. They also open the way for the identification of possible pharmacologic targets that can modulate neuroprotective microglial ApoE release under pathologic conditions.

Apolipoprotein E promotes subretinal mononuclear phagocyte survival and chronic inflammation in age-related macular degeneration

Physiologically, the retinal pigment epithelium (RPE) expresses immunosuppressive signals such as FAS ligand (FASL), which prevents the accumulation of leukocytes in the subretinal space. Age-related macular degeneration (AMD) is associated with a breakdown of the subretinal immunosuppressive environment and chronic accumulation of mononuclear phagocytes (MPs). We show that subretinal MPs in AMD patients accumulate on the RPE and express high levels of APOE. MPs of Cx3cr1(-/-) mice that develop MP accumulation on the RPE, photoreceptor degeneration, and increased choroidal neovascularization similarly express high levels of APOE. ApoE deletion in Cx3cr1(-/-) mice prevents pathogenic age- and stress-induced subretinal MP accumulation. We demonstrate that increased APOE levels induce IL-6 in MPs via the activation of the TLR2-CD14-dependent innate immunity receptor cluster. IL-6 in turn represses RPE FasL expression and prolongs subretinal MP survival. This mechanism may account, in part, for the MP accumulation observed in Cx3cr1(-/-) mice. Our results underline the inflammatory role of APOE in sterile inflammation in the immunosuppressive subretinal space. They provide rationale for the implication of IL-6 in AMD and open avenues toward therapies inhibiting pathogenic chronic inflammation in late AMD.

Association of the Apolipoprotein E polymorphism with migraine: a meta-analysis

Background

Apolipoprotein E (ApoE) gene has been reported to be associated with migraine and tension-type headache (TTH), but the results are conflicting. This study aimed to evaluate the association of ApoE with migraine by a meta-analysis.

Methods

MEDLINE, ISI Web of Knowledge, The Cochrane Central Register of Controlled Trials, and EMBASE databases were searched to identify eligible studies published in English from 2000 to 2014. Data were extracted using standardized forms. The association was assessed by relative risk (RR) with 95 % confidence intervals (CIs) using a fixed or random effects model.

Results

Four studies, comprising 649 migraineurs, 229 TTH subjects and 975 controls, met all the criteria and were included in the meta-analysis. No significant difference was found comparing genotypic and allelic frequencies in the case of migraineurs versus controls and TTH subjects versus controls. Only when migraineurs and TTH subjects were considered as a whole group, ApoE4 was found to increase the relative risk of headache by 1.48 (95 % CI 1.16, 1.90; P = 0.002), compared to controls.

Conclusions

ApoE ε4 allele is not associated with migraine susceptibility, but is positively related to headache (including migraine and TTH).

Inflammation in Alzheimer's Disease and Molecular Genetics: Recent Update

Alzheimer's disease (AD) is a complex age-related neurodegenerative disorder of the central nervous system. Since the first description of AD in 1907, many hypotheses have been established to explain its causes. The inflammation theory is one of them. Pathological and biochemical studies of brains from AD individuals have provided solid evidence of the activation of inflammatory pathways. Furthermore, people with long-term medication of anti-inflammatory drugs have shown a reduced risk to develop the disease. After three decades of genetic study in AD, dozens of loci harboring genetic variants influencing inflammatory pathways in AD patients has been identified through genome-wide association studies (GWAS). The most well-known GWAS risk factor that is responsible for immune response and inflammation in AD development should be APOE ε4 allele. However, a growing number of other GWAS risk AD candidate genes in inflammation have recently been discovered. In the present study, we try to review the inflammation in AD and immunity-associated GWAS risk genes like HLA-DRB5/DRB1, INPP5D, MEF2C, CR1, CLU and TREM2.

APOE-modulated Aβ-induced neuroinflammation in Alzheimer's disease: current landscape, novel data, and future perspective

Chronic glial activation and neuroinflammation induced by the amyloid-β peptide (Aβ) contribute to Alzheimer's disease (AD) pathology. APOE4 is the greatest AD-genetic risk factor; increasing risk up to 12-fold compared to APOE3, with APOE4-specific neuroinflammation an important component of this risk. This editorial review discusses the role of APOE in inflammation and AD, via a literature review, presentation of novel data on Aβ-induced neuroinflammation, and discussion of future research directions. The complexity of chronic neuroinflammation, including multiple detrimental and beneficial effects occurring in a temporal and cell-specific manner, has resulted in conflicting functional data for virtually every inflammatory mediator. Defining a neuroinflammatory phenotype (NIP) is one way to address this issue, focusing on profiling the changes in inflammatory mediator expression during disease progression. Although many studies have shown that APOE4 induces a detrimental NIP in peripheral inflammation and Aβ-independent neuroinflammation, data for APOE-modulated Aβ-induced neuroinflammation are surprisingly limited. We present data supporting the hypothesis that impaired apoE4 function modulates Aβ-induced effects on inflammatory receptor signaling, including amplification of detrimental (toll-like receptor 4-p38α) and suppression of beneficial (IL-4R-nuclear receptor) pathways. To ultimately develop APOE genotype-specific therapeutics, it is critical that future studies define the dynamic NIP profile and pathways that underlie APOE-modulated chronic neuroinflammation. In this editorial review, we present data supporting the hypothesis that impaired apoE4 function modulates Aβ-induced effects on inflammatory receptor signaling, including amplification of detrimental (TLR4-p38α) and suppression of beneficial (IL-4R-nuclear receptor) pathways, resulting in an adverse NIP that causes neuronal dysfunction. NIP, Neuroinflammatory phenotype; P.I., pro-inflammatory; A.I., anti-inflammatory.

Child apolipoprotein E gene variants and risk of cerebral palsy: estimation from case-parent triads

OBJECTIVE

To use case-parent triad data to investigate if cerebral palsy (CP) is associated with variants of the APOE gene, the rs59007384 SNP of the TOMM40 gene or combined haplotypes of the two genes.

STUDY DESIGN

DNA was analyzed in buccal swabs from 235 children with CP, their parents and a sibling. The relative risks (RR) with 95% confidence intervals (CI) that the children would have a distribution of APOE genotypes, rs59007384 variants or combined haplotypes deviating from Mendelian inheritance were estimated.

RESULTS

Children with CP were more likely than expected to carry the APOEε3 allele (RR 7.5; CI: 0.99-53.7 for heterozygotes and 10.3; CI: 1.4-79.6 for homozygotes), and to have the haplotype of APOEε3 and rs59007384 G (RR 2.4; CI: 1-5.7 for heterozygotes, RR 3.7; CI: 1.4-9.5 for homozygotes) whereas the distribution was as expected for rs59007384 alone. In the subgroup analyses the findings were confined to children born preterm. Among siblings the distribution of these genes was as expected according to Mendelian inheritance.

CONCLUSION

We speculate that children with APOEε2/APOEε4 alleles are more likely to die following cerebral injury in utero, resulting in a higher than expected proportion of children with CP carrying the APOEε3 allele.

Two Alzheimer's disease risk genes increase entorhinal cortex volume in young adults

Alzheimer's disease (AD) risk genes alter brain structure and function decades before disease onset. Apolipoprotein E (APOE) is the strongest known genetic risk factor for AD, and a related gene, apolipoprotein J (APOJ), also affects disease risk. However, the extent to which these genes affect brain structure in young adults remains unclear. Here, we report that AD risk alleles of these two genes, APOE-ε4 and APOJ-C, cumulatively alter brain volume in young adults. Using voxel-based morphometry (VBM) in 57 individuals, we examined the entorhinal cortex, one of the earliest brain regions affected in AD pathogenesis. Apolipoprotein E-ε4 carriers exhibited higher right entorhinal cortex volume compared to non-carriers. Interestingly, APOJ-C risk genotype was associated with higher bilateral entorhinal cortex volume in non-APOE-ε4 carriers. To determine the combined disease risk of APOE and APOJ status per subject, we used cumulative odds ratios as regressors for volumetric measurements. Higher disease risk corresponded to greater right entorhinal cortex volume. These results suggest that, years before disease onset, two key AD genetic risk factors may exert influence on the structure of a brain region where AD pathogenesis takes root.

The role of the immune system in neurodegenerative disorders: Adaptive or maladaptive?

Neurodegenerative diseases share common features, including catastrophic neuronal loss that leads to cognitive or motor dysfunction. Neuronal injury occurs in an inflammatory milieu that is populated by resident and sometimes, infiltrating, immune cells - all of which participate in a complex interplay between secreted inflammatory modulators and activated immune cell surface receptors. The importance of these immunomodulators is highlighted by the number of immune factors that have been associated with increased risk of neurodegeneration in recent genome-wide association studies. One of the more difficult tasks for designing therapeutic strategies for immune modulation against neurodegenerative diseases is teasing apart beneficial from harmful signals. In this regard, learning more about the immune components of these diseases has yielded common themes. These unifying concepts should eventually enable immune-based therapeutics for treatment of Alzheimer׳s and Parkinson׳s diseases and amyotrophic lateral sclerosis. Targeted immune modulation should be possible to temper maladaptive factors, enabling beneficial immune responses in the context of neurodegenerative diseases. This article is part of a Special Issue entitled SI: Neuroimmunology in Health And Disease.

Gene sequences regulating the production of apoE and cerebral palsy of variable severity

BACKGROUND

The apoE protein is the most important lipid transporter in the brain and has also been shown to have several regulatory functions in the central nervous system. The production of apoE is regulated by a number of genes and increases under certain conditions such as cerebral injury in adults.

AIMS

Our aim was to study whether variations in genes regulating the expression of the APOE gene were associated with severity of cerebral palsy (CP).

METHODS

Children enrolled in the Cerebral Palsy Register of Norway (CPRN) were invited to participate in this cross-sectional study; 281 of the invited 703 children (40%) returned swabs with buccal cells collected by parents. Six genetic variations thought to affect the production of apoE were genotyped and correlated with clinical data recorded in the CPRN.

RESULTS

Compared with children carrying the GG allele, children with genotype GT or TT in a specific genetic variation (rs59007384 located in the nearby TOMM40 gene) had excess risk for worse fine motor function (Odds ratio (OR): 1.82; 95% Confidence interval (CI): 1.10-2.99; p = 0.019) and epilepsy (OR: 2.32; CI: 1.17-4.61; p = 0.016). There was no association between severity of CP and any of the other five genetic variations analyzed.

CONCLUSION

Our findings suggest that genetic variations in one of the sequences regulating the expression of APOE, may be associated with worse clinical outcome in children with cerebral palsy.

Alzheimer's disease: relevant molecular and physiopathological events affecting amyloid-β brain balance and the putative role of PPARs

Alzheimer’s disease (AD) is the most common form of age-related dementia. With the expected aging of the human population, the estimated morbidity of AD suggests a critical upcoming health problem. Several lines of research are focused on understanding AD pathophysiology, and although the etiology of the disease remains a matter of intense debate, increased brain levels of amyloid-β (Aβ) appear to be a critical event in triggering a wide range of molecular alterations leading to AD. It has become evident in recent years that an altered balance between production and clearance is responsible for the accumulation of brain Aβ. Moreover, Aβ clearance is a complex event that involves more than neurons and microglia. The status of the blood-brain barrier (BBB) and choroid plexus, along with hepatic functionality, should be considered when Aβ balance is addressed. Furthermore, it has been proposed that exposure to sub-toxic concentrations of metals, such as copper, could both directly affect these secondary structures and act as a seeding or nucleation core that facilitates Aβ aggregation. Recently, we have addressed peroxisomal proliferator-activated receptors (PPARs)-related mechanisms, including the direct modulation of mitochondrial dynamics through the PPARγ-coactivator-1α (PGC-1α) axis and the crosstalk with critical aging- and neurodegenerative-related cellular pathways. In the present review, we revise the current knowledge regarding the molecular aspects of Aβ production and clearance and provide a physiological context that gives a more complete view of this issue. Additionally, we consider the different structures involved in AD-altered Aβ brain balance, which could be directly or indirectly affected by a nuclear receptor (NR)/PPAR-related mechanism.

The potential applications of Apolipoprotein E in personalized medicine

Personalized medicine uses various individual characteristics to guide medical decisions. Apolipoprotein (ApoE), the most studied polymorphism in humans, has been associated with several diseases. The purpose of this review is to elucidate the potential role of ApoE polymorphisms in personalized medicine, with a specific focus on neurodegenerative diseases, by giving an overview of its influence on disease risk assessment, diagnosis, prognosis, and therapy. This review is not a systematic inventory of the literature, but rather a summary and discussion of novel, influential and promising works in the field of ApoE research that could be valuable for personalized medicine. Empirical evidence suggests that ApoE genotype informs pre-symptomatic risk for a wide variety of diseases, is valuable for the diagnosis of type III dysbetalipoproteinemia, increases risk of dementia in neurodegenerative diseases, and is associated with a poor prognosis following acute brain damage. ApoE status appears to influence the efficacy of certain drugs, outcome of clinical trials, and might also give insight into disease prevention. Assessing ApoE genotype might therefore help to guide medical decisions in clinical practice.

Apolipoprotein E, amyloid-beta, and neuroinflammation in Alzheimer's disease

Alzheimer's disease (AD) is characterized by the accumulation and deposition of amyloid-beta (Aβ) peptides in the brain. Neuroinflammation occurs in the AD brain and plays a critical role in the neurodegenerative pathology. Particularly, Aβ evokes an inflammatory response that leads to synaptic dysfunction, neuronal death, and neurodegeneration. Apolipoprotein E (ApoE) proteins are involved in cholesterol transport, Aβ binding and clearance, and synaptic functions in the brain. The ApoE4 isoform is a key risk factor for AD, while the ApoE2 isoform has a neuroprotective effect. However, studies have reached different conclusions about the roles of the isoforms; some show that both ApoE3 and ApoE4 have anti-inflammatory effects, while others show that ApoE4 causes a predisposition to inflammation or promotes an inflammatory response following lipopolysaccharide treatment. These discrepancies may result from the differences in models, cell types, experimental conditions, and inflammatory stimuli used. Further, little was known about the role of ApoE isoforms in the Aβ-induced inflammatory response and in the neuroinflammation of AD. Our recent work showed that ApoE isoforms differentially regulate and modify the Aβ-induced inflammatory response in neural cells, with ApoE2 suppressing and ApoE4 promoting the response. In this article, we review the roles, mechanisms, and interrelations among Aβ, ApoE, and neuroinflammation in AD.

Islet adaptation to obesity and insulin resistance in WNIN/GR-Ob rats

WNIN/GR-Ob mutant rat is a novel animal model to study metabolic syndrome (obesity, insulin resistance, hyperinsulinemia, impaired glucose tolerance and cardiovascular diseases). We have investigated the islet characteristics of obese mutants at different age groups (1, 6 and 12 months) to assess the islet changes in response to early and chronic metabolic stress. Our data demonstrates altered islet cell morphology and function (hypertrophy, fibrotic lesions, vacuolation, decreased stimulation index, increased TNFα, ROS and TBARS levels) in mutants as compared to controls. Furthermore, network analysis (gene-gene interaction) studied in pancreas demonstrated increased inflammation as a key factor underlying obesity/metabolic syndrome in mutants. These observations pave way to explore this model to understand islet adaptation in response to metabolic syndrome.

Effects of apoE deficiency and occlusal disharmony on amyloid-beta production and spatial memory in rats

Amyloid-β (Aβ) plays a causative role in Alzheimer’s disease. Apolipoprotein E (apoE) is involved in Aβ accumulation, whereas occlusal disharmony increases Aβ production in the rat hippocampus. The purpose of the present study was to investigate the effects of apoE deficiency and occlusal disharmony on Aβ production and spatial memory. Wild-type (WT) (n = 12) and apoE-deficient [ApoE(−/−)] (n = 12) rats (Sprague-Dawley; 8 weeks old) were used. These rats were randomly divided into four groups of six rats each: two control (C) groups: WT (C-WT) and ApoE [C-ApoE(−/−)], and two occlusal disharmony (D) groups: WT (D-WT) and ApoE [D-ApoE(−/−)]. The C group received no treatment for 8 weeks. In the D group, the maxillary molar cusps were cut off for 8 weeks. The spatial memory of rats was assessed according to their behavioral performance in a radial arm maze. In both genotypes of rats, significant differences in the reference memory, Aβ42 production, β-secretase expression and plasma corticosterone levels were observed between the C and D groups (P < 0.0125). The levels of Aβ42 and glucocorticoid receptor in the C-ApoE(−/−) group were also significantly higher than those in the C-WT group (P < 0.0125). However, no significant differences in these parameters were found between the two genotypes with occlusal disharmony. In conclusion, occlusal disharmony induces cognitive dysfunction and Aβ accumulation in the rat hippocampus, and the effects of occlusal disharmony on Aβ accumulation and cognitive dysfunction were larger than those of apoE deficiency.

Apolipoprotein E as a β-amyloid-independent factor in Alzheimer's disease

APOE, which encodes apolipoprotein E, is the most prevalent and best established genetic risk factor for late-onset Alzheimer’s disease. Current understanding of Alzheimer’s disease pathophysiology posits an important role for apolipoprotein E in the disease cascade via its interplay with β-amyloid. However, evidence is also emerging for roles of apolipoprotein E in the disease process that are independent of β-amyloid. Particular areas of interest are lipid metabolism, tau pathology, neuroenergetics, neurodevelopment, synaptic plasticity, the neurovasculature, and neuroinflammation. The intent of this article is to review the literature in each of these areas.

Epigenetic signature and enhancer activity of the human APOE gene

The human apolipoprotein E (APOE) gene plays an important role in lipid metabolism. It has three common genetic variants, alleles ε2/ε3/ε4, which translate into three protein isoforms of apoE2, E3 and E4. These isoforms can differentially influence total serum cholesterol levels; therefore, APOE has been linked with cardiovascular disease. Additionally, its ε4 allele is strongly associated with the risk of Alzheimer's disease (AD), whereas the ε2 allele appears to have a modest protective effect for AD. Despite decades of research having illuminated multiple functional differences among the three apoE isoforms, the precise mechanisms through which different APOE alleles modify diseases risk remain incompletely understood. In this study, we examined the genomic structure of APOE in search for properties that may contribute novel biological consequences to the risk of disease. We identify one such element in the ε2/ε3/ε4 allele-carrying 3'-exon of APOE. We show that this exon is imbedded in a well-defined CpG island (CGI) that is highly methylated in the human postmortem brain. We demonstrate that this APOE CGI exhibits transcriptional enhancer/silencer activity. We provide evidence that this APOE CGI differentially modulates expression of genes at the APOE locus in a cell type-, DNA methylation- and ε2/ε3/ε4 allele-specific manner. These findings implicate a novel functional role for a 3'-exon CGI and support a modified mechanism of action for APOE in disease risk, involving not only the protein isoforms but also an epigenetically regulated transcriptional program at the APOE locus driven by the APOE CGI.

CNF1 increases brain energy level, counteracts neuroinflammatory markers and rescues cognitive deficits in a murine model of Alzheimer's disease

Overexpression of pro-inflammatory cytokines and cellular energy failure are associated with neuroinflammatory disorders, such as Alzheimer's disease. Transgenic mice homozygous for human ApoE4 gene, a well known AD and atherosclerosis animal model, show decreased levels of ATP, increased inflammatory cytokines level and accumulation of beta amyloid in the brain. All these findings are considered responsible for triggering cognitive decline. We have demonstrated that a single administration of the bacterial E. coli protein toxin CNF1 to aged apoE4 mice, beside inducing a strong amelioration of both spatial and emotional memory deficits, favored the cell energy restore through an increment of ATP content. This was accompanied by a modulation of cerebral Rho and Rac1 activity. Furthermore, CNF1 decreased the levels of beta amyloid accumulation and interleukin-1β expression in the hippocampus. Altogether, these data suggest that the pharmacological modulation of Rho GTPases by CNF1 can improve memory performances in an animal model of Alzheimer's disease via a control of neuroinflammation and a rescue of systemic energy homeostasis.

The influence of Apolipoprotein E genotype on regional pathology in Alzheimer's disease

Background

Carriers of the ApoE ϵ4 allele are at a greater risk for developing Alzheimer’s disease (AD) and those who do develop AD tend to have a much greater neuropathological disease burden. Although several studies have shown significant differences in AD pathology among ϵ4 carriers and non-carriers, few have characterized these differences in terms of brain region and neuropathological score frequency.

Methods

566 pathologically-confirmed AD cases who were followed prospectively with antemortem dementia diagnoses (312 ApoE ϵ4 carriers and 254 ApoE ϵ4 non-carriers) were compared on the frequencies of neuropathological frequency scores (none, sparse, moderate, frequent) among several different brain regions (frontal, temporal, parietal, hippocampal, and entorhinal) using the CERAD scoring system. Pathology score frequencies were analyzed by carrier status (ϵ4 carrier vs. ϵ4 non-carrier) and by genotype (2/3, 3/3, 2/4, 3/4, 4/4). Both analyses investigated pathology score frequencies among different brain regions (frontal, temporal, parietal, hippocampal, and entorhinal).

Results

ϵ4 carriers had a significantly lower age at death (p <0.001) and significantly higher Braak scores (p <0.001) than ϵ4 non-carriers. Genotype comparison revealed that plaque and tangle pathologies increased in the following pattern, 2/3<3/3<2/4<3/4<4/4, for several brain regions. When stratified by age and ApoE ϵ4 carrier status, ϵ4 carriers tended to have significantly more frequent scores across most cortical areas. However, non-carriers age 90 and older tended to have greater plaque pathology than carriers. For tangle pathology, ϵ4 carriers tended to have significantly more “frequent” scores than non-carriers, except for the hippocampal and entorhinal areas in individuals age 90 and older.

Conclusions

ApoE ϵ4 carriers had a significantly higher percentage of “frequent” scores for plaques and tangles when compared to ApoE ϵ4 non-carriers for several brain regions. However, ϵ4 non-carriers age 90 and older tended to have less plaque and tangle pathology in certain brain regions. These results demonstrate that AD pathology may manifest itself differently based on ApoE genotype and suggest that ApoE carriers and non-carriers may have different patterns of AD neuropathology location and density.

Apolipoprotein E polymorphisms and severity of cerebral palsy: a cross-sectional study in 255 children in Norway

AIM

The aim of this study was to examine whether the presence of the apolipoprotein E (ApoE) allele APOEε4 is associated with less severe manifestations of cerebral palsy (CP), consistent with the suggested beneficial effect of this allele on neurodevelopment in children.

METHOD

ApoE genotyping was performed on buccal epithelial cells from 255 children (141 males 114 females; mean age 12y, SD 2y 3mo, range 9-17y) recorded in the Cerebral Palsy Register of Norway. The main outcome measure of CP severity was the Gross Motor Function Classification System (GMFCS). Secondary outcome measures were fine motor function, epilepsy, and the need for gastrostomy tube feeding (GTF).

RESULTS

There was no association between the APOEε4 genotype and GMFCS levels (odds ratio [OR] 1.15; 95% confidence interval [CI] 0.66-1.99). However, the APOEε4 genotype was more often present among children with epilepsy (OR 2.2; 95% CI 1.1-4.2) and/or receiving GTF (OR 2.7; 95% CI 1.1-6.6). Among children with unilateral CP, the presence of APOEε4 was associated with more severe fine motor impairment (OR 2.6; 95% CI 1.3-6.9).

INTERPRETATION

Our main hypothesis that APOEε4 would have a protective effect on neurodevelopment was not supported. Instead, subgroup analyses suggested an adverse effect of the APOEε4 genotype on the developing brain after injury.

Apolipoprotein e sets the stage: response to injury triggers neuropathology

Apolipoprotein (apo) E4 is the major genetic risk factor for Alzheimer's disease and is associated with poor clinical outcome following traumatic brain injury and other neuropathological disorders. Protein instability and an isoform-specific apoE property called domain interaction are responsible for these neuropathological effects. ApoE4 is the most neurotoxic isoform and can induce neuropathology through various cellular pathways. Neuronal damage or stress induces apoE synthesis as part of the repair response; however, when apoE4 is expressed in neurons, its unique conformation makes it susceptible to proteolysis, resulting in the generation of neurotoxic fragments. These fragments cause pathological mitochondrial dysfunction and cytoskeletal alterations. Here, we review data supporting the hypothesis that apoE4 (> apoE3 > apoE2) has direct neurotoxic effects and highlight studies showing that blocking domain interaction reverses these detrimental effects.

A multifaceted role for apoE in the clearance of beta-amyloid across the blood-brain barrier

BACKGROUND

While apolipoprotein E4 (apoE4) is highly correlated with the development of Alzheimer's disease (AD), its role in AD pathology and, in particular, beta-amyloid (Aβ) removal from the brain, is not clearly defined.

OBJECTIVE

To elucidate the influence of apoE on the clearance of Aβ across the blood-brain barrier (BBB).

METHODS

Aβ(1-42) was intracerebrally administered to transgenic mice expressing human apoE isoforms and examined in the periphery.

RESULTS

apoE3 and apoE4 mice had 5 times and 2 times, respectively, more Aβ(1-42) appearing in the plasma than wild-type or apoE knockout mice, indicating an enhanced clearance of Aβ from the brain to the periphery. In vitro, unbound basolateral apoE3 (i.e., not bound to Aβ), and to a lesser extent unbound apoE4, at concentrations ≤10 nM facilitated basolateral-to-apical fluorescein-Aβ(1-42) transcytosis across a BBB model, while apoE isoforms bound to Aβ significantly disrupted Aβ transcytosis. Additionally, following apical exposure to the BBB model, we found that apoE4 bound to Aβ is able to penetrate the BBB more readily than apoE3 bound to Aβ and does so via the RAGE (receptor for advanced glycation end products) transporter.

CONCLUSION

These studies indicate a multifaceted, isoform-dependent role for apoE in the exchange of Aβ across the BBB and may partially explain the association of apoE4 and Aβ brain accumulation in AD.

ApoE genotype: from geographic distribution to function and responsiveness to dietary factors

ApoE is a key protein in lipid metabolism with three major isoforms.ApoEallele frequencies show non-random global distribution especially in Europe with highapoEε3frequency in the Mediterranean area, whereas theapoEε4genotype is enriched in Northern Europe. TheapoEε4genotype is one of the most important genetic risk factors for age-dependent chronic diseases, including CVD and Alzheimer's disease (AD). The apoE polymorphism has been shown to impact on blood lipids, biomarkers of oxidative stress and chronic inflammation, which all may contribute to the isoform-dependent disease risk. Studies in mice and human subjects indicate that theapoEε3but not theapoEε4genotype may significantly benefit from dietary flavonoids (e.g. quercetin) andn-3 fatty acids. Metabolism of lipid soluble vitamins E and D is likewise differentially affected by theapoEgenotype. Epidemiological and experimental evidence suggest a better vitamin D status inapoEε4than ε3subjects indicating a certain advantage of ε4over ε3. The present review aims at evaluation of current data available on interactions between apoE polymorphism and dietary responsiveness to flavonoids, fat soluble vitamins andn-3 fatty acids. Likewise, distinct geographic distribution and chronic disease risk of the different apoE isoforms are addressed.

An apolipoprotein E4 fragment affects matrix metalloproteinase 9, tissue inhibitor of metalloproteinase 1 and cytokine levels in brain cell lines

Apolipoprotein (apo) E4 isoform, a major risk factor for Alzheimer disease (AD), is more susceptible to proteolysis than apoE2 and apoE3 isoforms. ApoE4 fragments have been found in AD patients' brain. In the present study, we examined the effect of full-length apoE4 and apoE4 fragments apoE4[Δ(186-299)] and apoE4[Δ(166-299)] on inflammation in human neuroblastoma SK-N-SH and human astrocytoma SW-1783 cells. Western blot and zymography analysis showed that treatment of SK-N-SH cells with apoE4[Δ(186-299)], but not full-length apoE4 or the shorter apoE4[Δ(166-299)] fragment, leads to increased extracellular levels of matrix metalloproteinase 9 (MMP9) and tissue inhibitor of metalloproteinase 1 (TIMP1). Real-time PCR showed that interleukin (IL)-1β gene expression is also increased in SK-N-SH cells treated with apoE4[Δ(186-299)]. Treatment of SK-N-SH cells with IL-1β leads to increased MMP9 and TIMP1 extracellular levels, suggesting that the induction of IL-1β may be the mechanism by which apoE4[Δ(186-299)] regulates MMP9 and TIMP1 levels in these cells. In contrast to SK-N-SH cells, treatment of SW-1783 cells with apoE4[Δ(186-299)], and to a lesser extent with apoE4, leads to increased TIMP1 extracellular levels without affecting MMP9 levels. Additionally, apoE4[Δ(186-299)] leads to decreased IL-10 gene expression in SK-N-SH cells, whereas both apoE4 and apoE4[Δ(186-299)] lead to decreased TNFα gene expression without affecting IL-1β and IL-10 gene expression in SW-1783 cells. Overall, our findings indicate that a specific apoE4 fragment (apoE4[Δ(186-299)]), with molecular mass similar that of apoE4 fragments detected in AD patients' brain, can influence the level of inflammatory molecules in brain cell lines. It is possible that these phenomena contribute to AD pathogenesis.

Butyrylcholinesterase genotype and gender influence Alzheimer's disease phenotype

Retrospective data are presented to support a spectrum of early Alzheimer's disease (AD) along a continuum defined by gender and genotype. The putative neurodegenerative mechanisms driving distinct phenotypes at each end of the spectrum are glial hypoactivity associated with early failure of synaptic cholinergic neurotransmission and glial overactivation associated with loss of neural network connectivity due to accelerated age-related breakdown of myelin. In early AD, male butyrylcholinesterase K-variant carriers with one or two apolipoprotein ɛ4 alleles have prominent medial temporal atrophy, synaptic failure, cognitive decline, and accumulation of aggregated beta-amyloid peptide. Increasing synaptic acetylcholine in damaged but still functional cholinergic synapses improves cognitive symptoms, whereas increasing the ability of glia to support synapses and to clear beta-amyloid peptide might be disease-modifying. Conversely, chronic glial overactivation can also drive degenerative processes and in butyrylcholinesterase K-variant negative females generalized glial overactivation may be the main driver from mild cognitive impairment to AD. Females are more likely than males to have accelerated age-related myelin breakdown, more widespread white matter loss, loss of neural network connectivity, whole brain atrophy, and functional decline. Increasing extracellular acetylcholine levels blocks glial activation, reduces myelin loss and damage to neural network connectivity, and is disease-modifying. Between extremes characterized by gender, genotype, and age, pathophysiology may be mixed and this spectrum may explain much of the heterogeneity of amnestic mild cognitive impairment. Preservation of the functional integrity of the neural network may be an important component of strengthening cognitive reserve and significantly delaying the onset and progression of dementia, particularly in females. Prospective confirmation of these hypotheses is required. Implications for future research and therapeutic opportunities are discussed.

Neuroinflammation: modulation by flavonoids and mechanisms of action

Neuroinflammatory processes are known to contribute to the cascade of events culminating in the neuronal damage that underpins neurodegenerative disorders such as Parkinson's and Alzheimer's disease. Recently, there has been much interest in the potential neuroprotective effects of flavonoids, a group of plant secondary metabolites known to have diverse biological activity in vivo. With respect to the brain, flavonoids, such as those found in cocoa, tea, berries and citrus, have been shown to be highly effective in preventing age-related cognitive decline and neurodegeneration in both animals and humans. Evidence suggests that flavonoids may express such ability through a multitude of physiological functions, including an ability to modulate the brains immune system. This review will highlight the evidence for their potential to inhibit neuroinflammation through an attenuation of microglial activation and associated cytokine release, iNOS expression, nitric oxide production and NADPH oxidase activity. We will also detail the current evidence indicting that their regulation of these immune events appear to be mediated by their actions on intracellular signaling pathways, including the nuclear factor-κB (NF-κB) cascade and mitogen-activated protein kinase (MAPK) pathway. As such, flavonoids represent important precursor molecules in the quest to develop of a new generation of drugs capable of counteracting neuroinflammation and neurodegenerative disease.

Apolipoprotein E expression is elevated by interleukin 1 and other interleukin 1-induced factors

Background

We have previously outlined functional interactions, including feedback cycles, between several of the gene products implicated in the pathogenesis of Alzheimer's disease. A number of Alzheimer-related stressors induce neuronal expression of apolipoprotein E (ApoE), β-amyloid precursor protein (βAPP), and fragments of the latter such as amyloid β-peptide (Aβ) and secreted APP (sAPP). These stressors include interleukin-1 (IL-1)-mediated neuroinflammation and glutamate-mediated excitotoxicity. Such circumstances are especially powerful when they transpire in the context of an APOE ε4 allele.

Methods

Semi-quantitative immunofluorescence imaging was used to analyze rat brains implanted with IL-1β slow-release pellets, sham pellets, or no pellets. Primary neuronal or NT2 cell cultures were treated with IL-1β, glutamate, Aβ, or sAPP; relative levels of ApoE mRNA and protein were measured by RT-PCR, qRT-PCR, and western immunoblot analysis. Cultures were also treated with inhibitors of multi-lineage kinases--in particular MAPK-p38 (SB203580), ERK (U0126), or JNK (SP600125)--prior to exposure of cultures to IL-1β, Aβ, sAPP, or glutamate.

Results

Immunofluorescence of tissue sections from pellet-implanted rats showed that IL-1β induces expression of βAPP, IL-1α, and ApoE; the latter was confirmed by western blot analysis. These protein changes were mirrored by increases in their mRNAs, as well as in those encoding IL-1β, IL-1β-converting enzyme (ICE), and tumor necrosis factor (TNF). IL-1β also increased ApoE expression in neuronal cultures. It stimulated release of sAPP and glutamate in these cultures too, and both of these agents--as well as Aβ--stimulated ApoE expression themselves, suggesting that they may contribute to the effect of IL-1β on ApoE levels. Inhibitors of MAPK-p38, ERK, and JNK inhibited ApoE induction by all these agents except glutamate, which was sensitive only to inhibitors of ERK and JNK.

Conclusion

Conditions of glial activation and hyperexcitation can elevate proinflammatory cytokines, ApoE, glutamate, βAPP, and its secreted fragments. Because each of these factors promotes glial activation and neuronal hyperexcitation, these relationships have the potential to sustain self-propagating neurodegenerative cycles that could culminate in a progressive neurodegenerative disorder such as Alzheimer's disease.

Characterisation of amyloid-induced inflammatory responses in the rat retina

Amyloid-induced inflammation is thought to play a critical and early role in the pathophysiology of Alzheimer's disease. As such, robust models with relevant and accessible compartments that provide a means of assessing anti-inflammatory agents are essential for the development of therapeutic agents. In the present work, we have characterised the induction of inflammation in the rat retina following intravitreal administration of amyloid-beta protein (Aβ). Histology and mRNA endpoints in the retina demonstrate Aβ1-42-, but not Aβ42-1-, induced inflammatory responses characterised by increases in markers for microglia and astrocytes (ionised calcium-binding adaptor molecule 1 (iba-1), GFAP and nestin) and increases in mRNA for inflammatory cytokines and chemokines such as IL1-β, MIP1α and TNFα. Likewise, analysis of vitreal cytokines also revealed increases in inflammatory cytokines and chemokines, including IL1-β, MIP1α and MCP1, induced by Aβ1-42 but not Aβ42-1. This profile of pro-inflammatory gene and protein expression is consistent with that observed in the Alzheimer's disease brain and suggest that this preclinical model may provide a useful relevant tool in the development of anti-inflammatory approaches directed towards Alzheimer's disease therapy.

Nitric oxide counteracts the hyperoxia-induced proliferation and proinflammatory responses of mouse astrocytes

Preclinical studies in the premature baboon evaluating the efficacy and potential toxicity of inhaled nitric oxide indicated a significant effect on astrocyte area density, suggesting phenotypic and functional changes in astrocytes upon exposure to nitric oxide. However, the effects of nitric oxide and oxygen, the two major therapeutic gases utilized in neonatal intensive care, on astrocyte morphology and function remain vastly unknown. Herein, we report that exposure of mouse neonatal cortical astrocytes to hyperoxia results in a proinflammatory phenotype and increase in proliferation without significant changes in cellular morphology or levels of intermediate filament proteins. The proinflammatory phenotype was evident by a significant increase in cellular levels of cyclooxygenase-2 and a concomitant increase in prostaglandin E(2) secretion, a decline in the intracellular and secreted levels of apolipoprotein E, and a significant increase in the intracellular levels of clusterin. This proinflammatory phenotype was not evident upon simultaneous exposure to hyperoxia and nitric oxide. These results suggest that exposure to nitric oxide in the setting of hyperoxia confers unrecognized beneficial effects by suppressing astrocytic inflammation.

Cross-talk between apolipoprotein E and cytokines

Apolipoprotein E (apoE) is a multifunctional glycosylated protein characterized by its wide tissue distribution. Despite its importance in lipid transport and atherosclerosis pathogenesis, apoE is associated with neurodegenerative disorders such as Alzheimer's disease (AD) and Parkinson disease, and autoimmune disorders such as multiple sclerosis and psoriasis. Among others, the role of apoE in modulating inflammation and oxidation is crucial in elucidating the risk factors of the above diseases since the function of apoE is closely linked with both proinflammatory and antiinflammatory cytokines. Moreover, apoE modulates inflammatory and immune responses in an isoform-dependent manner. Correspondingly, inflammatory cytokines can either upregulate or downregulate the production of apoE in various tissue types. However, studies on the interactions between apoE and cytokines occasionally yield conflicting results, highlighting the complex roles of apoE and cytokines in various disorders. The present paper summarizes the current knowledge about the cross-talk between apoE and cytokines, with emphasis on the effects of apoE on the Th1/Th2 balance.

Apolipoprotein E in Alzheimer's disease and other neurological disorders

Apolipoprotein E (APOE) is a 299-aminoacid protein encoded by the APOE gene. Three common polymorphisms in the APOE gene, ɛ2, ɛ3, and ɛ4, result in a single aminoacid change in the APOE protein. APOE ɛ2, ɛ3, and ɛ4 alleles strongly alter, in a dose-dependent manner, the likelihood of developing Alzheimer's disease and cerebral amyloid angiopathy. In particular, APOE ɛ4 is associated with increased risk for Alzheimer's disease whereas APOE ɛ2 is associated with decreased risk. The effects of APOE genotype on risk of these diseases are likely to be mediated by differential effects of APOE on amyloid-β accumulation in the brain and its vasculature. Response to treatment for Alzheimer's disease might differ according to APOE genotype. Because convincing evidence ties the APOE genotype to risk of Alzheimer's disease and cerebral amyloid angiopathy, APOE has been studied in other neurological diseases. APOE ɛ4 is associated with poor outcome after traumatic brain injury and brain haemorrhage, although the mechanisms underlying these associations are unclear. The possibility that APOE has a role in these and other neurological diseases has been of great interest, but convincing associations have not yet emerged.

Function and comorbidities of apolipoprotein e in Alzheimer's disease

Alzheimer's disease (AD)—the most common type of dementia among the elderly—represents one of the most challenging and urgent medical mysteries affecting our aging population. Although dominant inherited mutation in genes involved in the amyloid metabolism can elicit familial AD, the overwhelming majority of AD cases, dubbed sporadic AD, do not display this Mendelian inheritance pattern. Apolipoprotein E (APOE), the main lipid carrier protein in the central nervous system, is the only gene that has been robustly and consistently associated with AD risk. The purpose of the current paper is thus to highlight the pleiotropic roles and the structure-function relationship of APOE to stimulate both the functional characterization and the identification of novel lipid homeostasis-related molecular targets involved in AD.