Peripheral apoE isoform levels in cognitively normal APOE ε3/ε4 individuals are associated with regional gray matter volume and cerebral glucose metabolism

Background

Carriers of the APOE ε4 allele are at increased risk of developing Alzheimer’s disease (AD), and have been shown to have reduced cerebral metabolic rate of glucose (CMRgl) in the same brain areas frequently affected in AD. These individuals also exhibit reduced plasma levels of apolipoprotein E (apoE) attributed to a specific decrease in the apoE4 isoform as determined by quantification of individual apoE isoforms in APOE ε4 heterozygotes. Whether low plasma apoE levels are associated with structural and functional brain measurements and cognitive performance remains to be investigated.

Methods

Using quantitative mass spectrometry we quantified the plasma levels of total apoE and the individual apoE3 and apoE4 isoforms in 128 cognitively normal APOE ε3/ε4 individuals included in the Arizona APOE cohort. All included individuals had undergone extensive neuropsychological testing and 25 had in addition undergone FDG-PET and MRI to determine CMRgl and regional gray matter volume (GMV).

Results

Our results demonstrated higher apoE4 levels in females versus males and an age-dependent increase in the apoE3 isoform levels in females only. Importantly, a higher relative ratio of apoE4 over apoE3 was associated with GMV loss in the right posterior cingulate and with reduced CMRgl bilaterally in the anterior cingulate and in the right hippocampal area. Additional exploratory analysis revealed several negative associations between total plasma apoE, individual apoE isoform levels, GMV and CMRgl predominantly in the frontal, occipital and temporal areas. Finally, our results indicated only weak associations between apoE plasma levels and cognitive performance which further appear to be affected by sex.

Conclusions

Our study proposes a sex-dependent and age-dependent variation in plasma apoE isoform levels and concludes that peripheral apoE levels are associated with GMV, CMRgl and possibly cognitive performance in cognitively healthy individuals with a genetic predisposition to AD.

Electronic supplementary material

The online version of this article (doi:10.1186/s13195-016-0231-9) contains supplementary material, which is available to authorized users.

Apolipoprotein E lipoprotein particles inhibit amyloid-β uptake through cell surface heparan sulphate proteoglycan

BACKGROUND

The accumulation, aggregation and deposition of amyloid-β (Aβ) peptides in the brain are central to the pathogenesis of Alzheimer's disease (AD). Alzheimer's disease risk increases significantly in individuals carrying one or two copies of APOE ε4 allele compared to individuals with an ε3/ε3 genotype. Growing evidence has demonstrated that apolipoprotein E (apoE) strongly influences AD pathogenesis by controlling Aβ aggregation and metabolism. Heparan sulphate proteoglycans (HSPGs) are abundant cell surface molecules that bind to both apoE and Aβ. HSPGs have been associated with Aβ aggregation and deposition. Although several lines of research have shown that apoE influences Aβ clearance in the brain, it is not clear how apoE influences HSPG-mediated cellular uptake of Aβ.

RESULTS

In this study, we show that apoE lipoprotein particles from conditioned media of immortalized astrocytes isolated from human APOE-targeted replacement (TR) mice significantly suppress cellular Aβ42 and Aβ40 uptake through cell surface HSPG. ApoE3 and apoE4 particles have similar binding affinity to heparin, while apoE4 particles are likely hypolipidated compared to apoE particles. We also found that the apoE particles antagonize Aβ binding to cell surface, and inhibited Aβ uptake in a concentration-dependent manner in Chinese hamster ovary (CHO) cells. While the effect was not apoE isoform-dependent, the suppressive effect of apoE particles on Aβ uptake was not observed in HSPG-deficient CHO cells. We further demonstrated that apoE particles reduced the internalization of Aβ in mouse primary neurons, an effect that is eliminated by the presence of heparin.

CONCLUSIONS

Taken together, our findings indicate that apoE particles irrespective of isoform inhibit HSPG-dependent cellular Aβ uptake. Modulating the ability of apoE particles to affect Aβ cellular uptake may hold promises for developing new strategies for AD therapy.

Total apolipoprotein E levels and specific isoform composition in cerebrospinal fluid and plasma from Alzheimer's disease patients and controls

The apolipoprotein E (ApoE) ε4 allele is the strongest risk factor of sporadic Alzheimer's disease (AD), however, the fluid concentrations of ApoE and its different isoforms (ApoE2, ApoE3 and ApoE4) in AD patients and among APOE genotypes (APOE ε2, ε3, ε4) remain controversial. Using a novel mass spectrometry-based method, we quantified total ApoE and specific ApoE isoform concentrations and potential associations with age, cognitive status, cholesterol levels and established AD biomarkers in cerebrospinal fluid (CSF) from AD patients versus non-AD individuals with different APOE genotypes. We also investigated plasma total ApoE and ApoE isoform composition in a subset of these individuals. In total n = 43 AD and n = 43 non-AD subjects were included. We found that CSF and plasma total ApoE levels did not correlate with age or cognitive status and did not differ between AD and non-AD subjects deeming ApoE as an unfit diagnostic marker for AD. Also, whereas CSF ApoE levels did not vary between APOE genotypes APOE ε4 carriers exhibited significantly decreased plasma ApoE levels attributed to a specific decrease in the ApoE4 isoform concentrations. CSF total ApoE concentrations were positively associated with CSF, total tau, tau phosphorylated at Thr181 and Aβ1-42 of which the latter association was weaker and only present in APOE ε4 carriers indicating a differential involvement of ApoE in tau versus Aβ-linked neuropathological processes. Future studies need to elucidate whether the observed plasma ApoE4 deficiency is a life-long condition in APOE ɛ4 carriers and whether this decrease in plasma ApoE predisposes APOE ɛ4 carriers to AD.

Kynurenic Acid levels in cerebrospinal fluid from patients with Alzheimer's disease or dementia with lewy bodies

Kynurenic acid (KYNA) is implicated in cognitive functions. Altered concentrations of the compound are found in serum and cerebrospinal fluid (CSF) of patients with Alzheimer’s disease (AD). Further studies to determine whether KYNA serves as a biomarker for cognitive decline and dementia progression are required. In this study, we measured CSF KYNA levels in AD patients (n = 19), patients with dementia with Lewy bodies (DLB) (n = 18), and healthy age-matched controls (Ctrls)) (n = 20) to further explore possible correlations between KYNA levels, cognitive decline, and well-established AD and inflammatory markers. Neither DLB patients nor AD patients showed significantly altered CSF KYNA levels compared to Ctrls. However, female AD patients displayed significantly higher KYNA levels compared to male AD patients, a gender difference not seen in the Ctrl or DLB group. Levels of KYNA significantly correlated with the AD-biomarker P-tau and the inflammation marker soluble intercellular adhesion molecule-1 (sICAM-1) in the AD patient group. No associations between KYNA and cognitive functions were found. Our study shows that, although KYNA was not associated with cognitive decline in AD or DLB patients, it may be implicated in AD-related hyperphosphorylation of tau and inflammation. Further studies on larger patient cohorts are required to understand the potential role of KYNA in AD and DLB.

Retinoic acid isomers facilitate apolipoprotein E production and lipidation in astrocytes through the retinoid X receptor/retinoic acid receptor pathway

Apolipoprotein E (apoE) is the major cholesterol transport protein in the brain. Among the three human APOE alleles (APOE2, APOE3, and APOE4), APOE4 is the strongest genetic risk factor for late-onset Alzheimer disease (AD). The accumulation of amyloid-β (Aβ) is a central event in AD pathogenesis. Increasing evidence demonstrates that apoE isoforms differentially regulate AD-related pathways through both Aβ-dependent and -independent mechanisms; therefore, modulating apoE secretion, lipidation, and function might be an attractive approach for AD therapy. We performed a drug screen for compounds that modulate apoE production in immortalized astrocytes derived from apoE3-targeted replacement mice. Here, we report that retinoic acid (RA) isomers, including all-trans-RA, 9-cis-RA, and 13-cis-RA, significantly increase apoE secretion to ~4-fold of control through retinoid X receptor (RXR) and RA receptor. These effects on modulating apoE are comparable with the effects recently reported for the RXR agonist bexarotene. Furthermore, all of these compounds increased the expression of the cholesterol transporter ABCA1 and ABCG1 levels and decreased cellular uptake of Aβ in an apoE-dependent manner. Both bexarotene and 9-cis-RA promote the lipidation status of apoE, in which 9-cis-RA promotes a stronger effect and exhibits less cytotoxicity compared with bexarotene. Importantly, we showed that oral administration of bexarotene and 9-cis-RA significantly increases apoE, ABCA1, and ABCG1 levels in mouse brains. Taken together, our results demonstrate that RXR/RA receptor agonists, including several RA isomers, are effective modulators of apoE secretion and lipidation and may be explored as potential drugs for AD therapy.

Apolipoproteins E and J interfere with amyloid-beta uptake by primary human astrocytes and microglia in vitro

Defective clearance of the amyloid-β peptide (Aβ) from the brain is considered a strong promoter in Alzheimer's disease (AD) pathogenesis. Astrocytes and microglia are important mediators of Aβ clearance and Aβ aggregation state and the presence of amyloid associated proteins (AAPs), such as Apolipoproteins E and J (ApoE and ApoJ), may influence Aβ clearance by these cells. Here we set out to investigate whether astrocytes and microglia differ in uptake efficiency of Aβ oligomers (Aβoligo ) and Aβ fibrils (Aβfib ), and whether the Aβ aggregation state and/or presence of AAPs affect Aβ uptake in these cells in vitro. Adult human primary microglia and astrocytes, isolated from short delay post-mortem brain tissue, were exposed to either Aβoligo or Aβfib alone or combined with a panel of certain AAPs whereafter Aβ-positive cells were quantified using flow cytometry. Upon exposure to Aβ combined with ApoE, ApoJ, α1-antichymotrypsin (ACT) and a combination of serum amyloid P and complement C1q (SAP-C1q), a clear reduction in astrocytic but not microglial Aβoligo uptake, was observed. In contrast, Aβfib uptake was strongly reduced in the presence of AAPs in microglia, but not in astrocytes. These data provide the first evidence of distinct roles of microglia and astrocytes in Aβ clearance. More importantly we show that Aβ clearance by glial cells is negatively affected by AAPs like ApoE and ApoJ. Thus, targeting the association of Aβ with AAPs, such as ApoE and ApoJ, could serve as a therapeutic strategy to increase Aβ clearance by glial cells.

Assessment of peptide chemical modifications on the development of an accurate and precise multiplex selected reaction monitoring assay for apolipoprotein e isoforms

Apolipoprotein E (ApoE) is a polymorphic protein that plays a major role in lipid metabolism in the central nervous system and periphery. It has three common allelic isoforms, ApoE2, ApoE3, and ApoE4, that differ in only one or two amino acids. ApoE isoforms have been associated with the occurrence and progression of several pathological conditions, such as coronary atherosclerosis and Alzheimer's disease. The aim of this study was to develop a mass spectrometry (MS)-based assay for absolute quantification of ApoE isoforms in cerebrospinal fluid and plasma samples using isotope-labeled peptides. The assay included five tryptic peptides: CLAVYQAGAR (ApoE2), LGADMEDVCGR (ApoE2 and 3), LAVYQAGAR (ApoE3 and 4), LGADMEDVR (ApoE4), and LGPLVEQGR (total ApoE). Both cerebrospinal fluid and plasma samples were assayed to validate the method. The digestion yield and the extension of chemical modifications in selected amino acid residues (methionine oxidation, glutamine deamidation, and cyclization of N-terminus carbamidomethylcysteine) were also studied. The ApoE phenotype was successfully assigned to all samples analyzed in a blinded manner. The method showed good linearity (R(2) > 0.99) and reproducibility (within laboratory imprecision <13%). The comparison of the MS-based assay with an ELISA for total ApoE concentration showed a moderate correlation (R(2) = 0.59). This MS-based assay can serve as an important tool in clinical studies aiming to elucidate the association between ApoE genotype, total ApoE, and ApoE isoform concentrations in various disorders related to ApoE polymorphisms.

Cerebrospinal fluid inflammatory markers in Parkinson's disease--associations with depression, fatigue, and cognitive impairment

Neuroinflammation may be involved in the pathophysiology of Parkinson's disease (PD) and specifically in non-motor symptoms such as depression, fatigue and cognitive impairment. The aim of this study was to measure inflammatory markers in cerebrospinal fluid (CSF) samples from PD patients and a reference group, and to investigate correlations between non-motor symptoms and inflammation. We quantified C-reactive protein (CRP), interleukin-6, tumor necrosis factor-alpha, eotaxin, interferon gamma-induced protein-10, monocyte chemotactic protein-1 (MCP-1), and macrophage inflammatory protein 1-β in CSF samples from PD patients (N=87) and the reference group (N=33). Sixteen of the PD patients had a dementia diagnosis (PDD). We assessed symptoms of fatigue, depression, anxiety and cognitive function using the Functional Assessment of Chronic Illness Therapy-Fatigue, the Hospital Anxiety and Depression Scale, and the Mini Mental State Examination, respectively. There were no significant differences in mean levels of inflammatory markers between PD patients and the reference group. After controlling for age, gender and somatic illness, patients with PDD had significantly higher levels of CRP compared to non-demented PD patients (p=0.032) and the reference group (p=0.026). Increased levels of inflammatory markers in CSF were significantly associated with more severe symptoms of depression, anxiety, fatigue, and cognition in the entire PD group. After controlling for PD duration, age, gender, somatic illness and dementia diagnosis, high CRP levels were significantly associated with more severe symptoms of depression (p=0.010) and fatigue (p=0.008), and high MCP-1 levels were significantly associated with more severe symptoms of depression (p=0.032). Our results indicate that non-motor features of PD such as depression, fatigue, and cognitive impairment are associated with higher CSF levels of inflammatory markers.

NG2 cells, a new trail for Alzheimer's disease mechanisms?

Background

Neuron Glial 2 (NG2) cells are glial cells known to serve as oligodendrocyte progenitors as well as modulators of the neuronal network. Altered NG2 cell morphology and up-regulation as well as increased shedding of the proteoglycan NG2 expressed on the cell surface have been described in rodent models of brain injury. Here we describe alterations in the human NG2 cell population in response to pathological changes characteristic of Alzheimer’s disease (AD).

Results

Immunohistological stainings of postmortem brain specimens from clinically diagnosed and postmortem verified AD patients and non-demented controls revealed reduced NG2 immunoreactivity as well as large numbers of NG2 positive astrocytes in individuals with high amyloid beta plaque load. Since fibrillar amyloid beta (Aβ)1-42 is the major component of AD-related senile plaques, we exposed human NG2 cells to oligomer- and fibril enriched preparations of Aβ1-42. We found that both oligomeric and fibrillar Aβ1-42 induced changes in NG2 cell morphology. Further, in vitro exposure to fibrillar Aβ1-42 decreased the NG2 concentrations in both cell lysates and supernatants. Interestingly, we also found significantly decreased levels of soluble NG2 in the cerebrospinal fluid (CSF) from clinically diagnosed AD patients compared to non-demented individuals. Additionally, the CSF NG2 levels were found to significantly correlate with the core AD biomarkers Aß1-42, T-tau and P-tau.

Conclusion

Our results demonstrate major alterations in the NG2 cell population in relation to AD pathology which highlights the NG2 cell population as a new attractive research target in the search for cellular mechanisms associated with AD pathogenesis.

Electronic supplementary material

The online version of this article (doi:10.1186/2051-5960-1-7) contains supplementary material, which is available to authorized users.

Low CSF levels of both α-synuclein and the α-synuclein cleaving enzyme neurosin in patients with synucleinopathy

Neurosin is a protease that in vitro degrades α-synuclein, the main constituent of Lewy bodies found in brains of patients with synucleinopathy including Parkinson's disease (PD) and dementia with Lewy bodies (DLB). Several studies have reported reduced cerebrospinal fluid (CSF) levels of α-synuclein in synucleinopathy patients and recent data also proposes a significant role of α-synuclein in the pathophysiology of Alzheimer's disease (AD). To investigate potential links between neurosin and its substrate α-synuclein in vivo we used a commercially available sandwich ELISA and an in-house developed direct ELISA to quantify CSF levels of α-synuclein and neurosin in patients diagnosed with DLB, PD and PD dementia (PDD) versus AD patients and non-demented controls. We found that patients with synucleinopathy displayed lower CSF levels of neurosin and α-synuclein compared to controls and AD patients. In contrast, AD patients demonstrated significantly increased CSF α-synuclein but similar neurosin levels compared to non-demented controls. Further, CSF neurosin and α-synuclein concentrations were positively associated in controls, PD and PDD patients and both proteins were highly correlated to CSF levels of phosphorylated tau in all investigated groups. We observed no effect of gender or presence of the apolipoprotein Eε4 allele on neither neurosin or α-synuclein CSF levels. In concordance with the current literature our study demonstrates decreased CSF levels of α-synuclein in synucleinopathy patients versus AD patients and controls. Importantly, decreased α-synuclein levels in patients with synucleinopathy appear linked to low levels of the α-synuclein cleaving enzyme neurosin. In contrast, elevated levels of α-synuclein in AD patients were not related to any altered CSF neurosin levels. Thus, altered CSF levels of α-synuclein and neurosin in patients with synucleinopathy versus AD may not only mirror disease-specific neuropathological mechanisms but may also serve as fit candidates for future biomarker studies aiming at identifying specific markers of synucleinopathy.

Gender-dependent levels of hyaluronic acid in cerebrospinal fluid of patients with neurodegenerative dementia

Numerous reports over the years have described neuroinflammatory events and vascular changes in neurodegenerative diseases such as Alzheimers disease (AD) and Dementia with Lewy bodies (DLB). Interestingly, recent reports from other research areas suggest that inflammatory and vascular processes are influenced by gender. These findings are intriguing from the perspective that women show a higher incidence of AD and warrant investigations on how gender influences various processes in neurodegenerative dementia. In the current study we measured the cerebrospinal fluid (CSF) and plasma concentrations of hyaluroinic acid (HA), an adhesionmolecule known to regulate both vascular and inflammatory processes, in AD and DLB patients as well as in healthy elders. Our analysis showed that male AD and DLB patients had almost double the amount of HA compared to female patients whereas no gender differences were observed in the controls. Furthermore, we found that CSF levels of HA in foremost female AD patients correlated with various AD related biomarkers. Correlations between HA levels and markers of inflammation and vascular changes were only detected in female AD patients but in both male and female DLB patients. We conclude that HA may be linked to several pathological events present in AD, as reflected in CSF protein concentrations. The HA profile in CSF, but not in plasma, and associations to other markers appear to be gender-dependent which should be taken into account in clinical examinations and future biomarker studies.

Cell adhesion molecules in Alzheimer's disease

Cell adhesion molecules (CAMs) mediate interactions between cells and their surroundings that are vital to processes controlling for cell survival, activation, migration, and plasticity. However, increasing evidence suggests that CAMs also mediate mechanisms involved in several neurological diseases. This article reviews the current knowledge on the role of CAMs in amyloid-β (Aβ) metabolism, cell plasticity, neuroinflammation, and vascular changes, all of which are considered central to the pathogenesis and progression of Alzheimer's disease (AD). This paper also outlines the possible roles of CAMs in current and novel AD treatment strategies.

Complement in the brain

The brain is considered to be an immune privileged site, because the blood-brain barrier limits entry of blood borne cells and proteins into the central nervous system (CNS). As a result, the detection and clearance of invading microorganisms and senescent cells as well as surplus neurotransmitters, aged and glycated proteins, in order to maintain a healthy environment for neuronal and glial cells, is largely confined to the innate immune system. In recent years it has become clear that many factors of innate immunity are expressed throughout the brain. Neuronal and glial cells express Toll like receptors as well as complement receptors, and virtually all complement components can be locally produced in the brain, often in response to injury or developmental cues. However, as inflammatory reactions could interfere with proper functioning of the brain, tight and fine tuned regulatory mechanisms are warranted. In age related diseases, such as Alzheimer's disease (AD), accumulating amyloid proteins elicit complement activation and a local, chronic inflammatory response that leads to attraction and activation of glial cells that, under such activation conditions, can produce neurotoxic substances, including pro-inflammatory cytokines and oxygen radicals. This process may be exacerbated by a disturbed balance between complement activators and complement regulatory proteins such as occurs in AD, as the local synthesis of these proteins is differentially regulated by pro-inflammatory cytokines. Much knowledge about the role of complement in neurodegenerative diseases has been derived from animal studies with transgenic overexpressing or knockout mice for specific complement factors or receptors. These studies have provided insight into the potential therapeutic use of complement regulators and complement receptor antagonists in chronic neurodegenerative diseases as well as in acute conditions, such as stroke. Interestingly, recent animal studies have also indicated that complement activation products are involved in brain development and synapse formation. Not only are these findings important for the understanding of how brain development and neural network formation is organized, it may also give insights into the role of complement in processes of neurodegeneration and neuroprotection in the injured or aged and diseased adult central nervous system, and thus aid in identifying novel and specific targets for therapeutic intervention.

Optimum contribution selection using traditional best linear unbiased prediction and genomic breeding values in aquaculture breeding schemes

The aim of this study was to compare genetic gain for a traditional aquaculture sib breeding scheme with breeding values based on phenotypic data (TBLUP) with a breeding scheme with genome-wide (GW) breeding values. Both breeding schemes were closed nuclei with discrete generations modeled by stochastic simulation. Optimum contribution selection was applied to restrict pedigree-based inbreeding to either 0.5 or 1% per generation. There were 1,000 selection candidates and a sib test group of either 4,000 or 8,000 fish. The number of selected dams and sires to create full sib families in each generation was determined from the optimum contribution selection method. True breeding values for a trait were simulated by summing the number of each QTL allele and the true effect of each of the 1,000 simulated QTL. Breeding values in TBLUP were predicted from phenotypic and pedigree information, whereas genomic breeding values were computed from genetic markers whose effects were estimated using a genomic BLUP model. In generation 5, genetic gain was 70 and 74% greater for the GW scheme than for the TBLUP scheme for inbreeding rates of 0.5 and 1%. The reduction in genetic variance was, however, greater for the GW scheme than for the TBLUP scheme due to fixation of some QTL. As expected, accuracy of selection increased with increasing heritability (e.g., from 0.77 with a heritability of 0.2 to 0.87 with a heritability of 0.6 for GW, and from 0.53 and 0.58 for TBLUP in generation 5 with sib information only). When the trait was measured on the selection candidate compared with only on sibs and the heritability was 0.4, accuracy increased from 0.55 to 0.69 for TBLUP and from 0.83 to 0.86 for GW. The number of selected sires to get the desired rate of inbreeding was in general less in GW than in TBLUP and was 33 for GW and 83 for TBLUP (rate of inbreeding 1% and heritability 0.4). With truncation selection, genetic gain for the scheme with GW breeding values was nearly twice as large as a scheme with traditional BLUP breeding values. The results indicate that the benefits of applying GW breeding values compared with TBLUP are reduced when contributions are optimized. In conclusion, genetic gain in aquaculture breeding schemes with optimized contributions can increase by as much as 81% by applying genome-wide breeding values compared with traditional BLUP breeding values.

Astrocytic A beta 1-42 uptake is determined by A beta-aggregation state and the presence of amyloid-associated proteins

Intracerebral accumulation of amyloid-beta (A beta) leading to A beta plaque formation, is the main hallmark of Alzheimer's disease and might be caused by defective A beta-clearance. We previously found primary human astrocytes and microglia able to bind and ingest A beta 1-42 in vitro, which appeared to be limited by A beta 1-42 fibril formation. We now confirm that astrocytic A beta-uptake depends on size and/or composition of A beta-aggregates as astrocytes preferably take up oligomeric A beta over fibrillar A beta. Upon exposure to either fluorescence-labelled A beta 1-42 oligomers (A beta(oligo)) or fibrils (A beta(fib)), a larger (3.7 times more) proportion of astrocytes ingested oligomers compared to fibrils, as determined by flow cytometry. A beta-internalization was verified using confocal microscopy and live-cell imaging. Neither uptake of A beta(oligo) nor A beta(fib), triggered proinflammatory activation of the astrocytes, as judged by quantification of interleukin-6 and monocyte-chemoattractant protein-1 release. Amyloid-associated proteins, including alpha1-antichymotrypsin (ACT), serum amyloid P component (SAP), C1q and apolipoproteins E (ApoE) and J (ApoJ) were earlier found to influence A beta-aggregation. Here, astrocytic uptake of A beta(fib) increased when added to the cells in combination with SAP and C1q (SAP/C1q), but was unchanged in the presence of ApoE, ApoJ and ACT. Interestingly, ApoJ and ApoE dramatically reduced the number of A beta(oligo)-positive astrocytes, whereas SAP/C1q slightly reduced A beta(oligo) uptake. Thus, amyloid-associated proteins, especially ApoJ and ApoE, can alter A beta-uptake in vitro and hence may influence A beta clearance and plaque formation in vivo.