Plasma concentrations of free amyloid β cannot predict the development of Alzheimer's disease

The free plasma amyloid Aβ1-42/Aβ1-40 ratio predicts conversion to dementia for subjects with mild cognitive impairment with performance equivalent to that of the total plasma Aβ1-42/Aβ1-40 ratio. The BALTAZAR study

BACKGROUND AND PURPOSE

Blood-based biomarkers are a non-invasive solution to predict the risk of conversion of mild cognitive impairment (MCI) to dementia. The utility of free plasma amyloid peptides (not bound to plasma proteins and/or cells) as an early indicator of conversion to dementia is still debated, as the results of studies have been contradictory. In this context, we investigated whether plasma levels of the free amyloid peptides Aβ1-42 and Aβ1-40 and the free plasma Aβ1-42/Aβ1-40 ratio are associated with the conversion of MCI to dementia, in particular AD, over three years of follow-up in a subgroup of the BALTAZAR cohort. We also compared their predictive value to that of total plasma Aβ1-42 and Aβ1-40 levels and the total plasma Aβ1-42/Aβ1-40 ratio.

METHODS

The plasma Aβ1-42 and Aβ1-40 peptide assay was performed using the INNO-BIA kit (Fujirebio Europe). Free amyloid levels (defined by the amyloid fraction directly accessible to antibodies of the assay) were obtained with the undiluted plasma, whereas total amyloid levels were obtained after the dilution of plasma (1/3) with a denaturing buffer. Free and total Aβ1-42 and Aβ1-40 levels were measured at inclusion for a subgroup of participants (N = 106) with mild cognitive impairment (MCI) from the BALTAZAR study (a large-scale longitudinal multicenter cohort with a three-year follow-up). Associations between conversion and the free/total plasma Aβ1-42 and Aβ1-40 levels and Aβ1-42/Aβ1-40 ratio were analyzed using logistic and Cox Proportional Hazards models. Demographic, clinical, cognitive (MMSE, ADL and IADL), APOE, and MRI characteristics (relative hippocampal volume) were compared using non-parametric (Mann-Whitney) or parametric (Student) tests for quantitative variables and Chi-square or Fisher exact tests for qualitative variables.

RESULTS

The risk of conversion to dementia was lower for patients in the highest quartile of free plasma Aβ1-42/Aβ1-40 (≥ 25.8%) than those in the three lower quartiles: hazard ratio = 0.36 (95% confidence interval [0.15-0.87]), after adjustment for age, sex, education, and APOE ε4 (p-value = 0.022). This was comparable to the risk of conversion in the highest quartile of total plasma Aβ1-42/Aβ1-40: hazard ratio = 0.37 (95% confidence interval [0.16-0.89], p-value = 0.027). However, while patients in the highest quartile of total plasma Aβ1-42/Aβ1-40 showed higher MMSE scores and a higher hippocampal volume than patients in the three lowest quartiles of total plasma Aβ1-42/Aβ1-40, as well as normal CSF biomarker levels, the patients in the highest quartile of free plasma Aβ1-42/Aβ1-40 did not show any significant differences in MMSE scores, hippocampal volume, or CSF biomarker levels relative to the three lowest quartiles of free plasma Aβ1-42/Aβ1-40.

CONCLUSION

The free plasma Aβ1-42/Aβ1-40 ratio is associated with a risk of conversion from MCI to dementia within three years, with performance comparable to that of the total plasma Aβ1-42/Aβ1-40 ratio. Threshold levels of the free and total plasma Aβ1-42/Aβ1-40 ratio could be determined, with a 60% lower risk of conversion for patients above the threshold than those below.

Based on molecular structures: Amyloid-β generation, clearance, toxicity and therapeutic strategies

Amyloid-β (Aβ) has long been considered as one of the most important pathogenic factors in Alzheimer’s disease (AD), but the specific pathogenic mechanism of Aβ is still not completely understood. In recent years, the development of structural biology technology has led to new understandings about Aβ molecular structures, Aβ generation and clearance from the brain and peripheral tissues, and its pathological toxicity. The purpose of the review is to discuss Aβ metabolism and toxicity, and the therapeutic strategy of AD based on the latest progress in molecular structures of Aβ. The Aβ structure at the atomic level has been analyzed, which provides a new and refined perspective to comprehend the role of Aβ in AD and to formulate therapeutic strategies of AD.

Biomarkers and Tools for Predicting Alzheimer's Disease in the Preclinical Stage

Alzheimer's disease (AD) is the only leading cause of death for which no disease-modifying therapy is currently available. Over the past decade, a string of disappointing clinical trial results has forced us to shift our focus to the preclinical stage of AD, which represents the most promising therapeutic window. However, the accurate diagnosis of preclinical AD requires the presence of brain β- amyloid deposition determined by cerebrospinal fluid or amyloid-positron emission tomography, significantly limiting routine screening and diagnosis in non-tertiary hospital settings. Thus, an easily accessible marker or tool with high sensitivity and specificity is highly needed. Recently, it has been discovered that individuals in the late stage of preclinical AD may not be truly "asymptomatic" in that they may have already developed subtle or subjective cognitive decline. In addition, advances in bloodderived biomarker studies have also allowed the detection of pathologic changes in preclinical AD. Exosomes, as cell-to-cell communication messengers, can reflect the functional changes of their source cell. Methodological advances have made it possible to extract brain-derived exosomes from peripheral blood, making exosomes an emerging biomarker carrier and liquid biopsy tool for preclinical AD. The eye and its associated structures have rich sensory-motor innervation. In this regard, studies have indicated that they may also provide reliable markers. Here, our report covers the current state of knowledge of neuropsychological and eye tests as screening tools for preclinical AD and assesses the value of blood and brain-derived exosomes as carriers of biomarkers in conjunction with the current diagnostic paradigm.

Validation of Plasma Amyloid-β 42/40 for Detecting Alzheimer Disease Amyloid Plaques

BACKGROUND AND OBJECTIVES

To determine the diagnostic accuracy of a plasma Aβ42/Aβ40 assay in classifying amyloid PET status across global research studies using samples collected by multiple centers that utilize different blood collection and processing protocols.

METHODS

Plasma samples (n = 465) were obtained from 3 large Alzheimer disease (AD) research cohorts in the United States (n = 182), Australia (n = 183), and Sweden (n = 100). Plasma Aβ42/Aβ40 was measured by a high precision immunoprecipitation mass spectrometry (IPMS) assay and compared to the reference standards of amyloid PET and CSF Aβ42/Aβ40.

RESULTS

In the combined cohort of 465 participants, plasma Aβ42/Aβ40 had good concordance with amyloid PET status (receiver operating characteristic area under the curve [AUC] 0.84, 95% confidence interval [CI] 0.80-0.87); concordance improved with the inclusion of APOE ε4 carrier status (AUC 0.88, 95% CI 0.85-0.91). The AUC of plasma Aβ42/Aβ40 with CSF amyloid status was 0.85 (95% CI 0.78-0.91) and improved to 0.93 (95% CI 0.89-0.97) with APOE ε4 status. These findings were consistent across the 3 cohorts, despite differences in protocols. The assay performed similarly in both cognitively unimpaired and impaired individuals.

DISCUSSION

Plasma Aβ42/Aβ40 is a robust measure for detecting amyloid plaques and can be utilized to aid in the diagnosis of AD, identify those at risk for future dementia due to AD, and improve the diversity of populations enrolled in AD research and clinical trials.

CLASSIFICATION OF EVIDENCE

This study provides Class II evidence that plasma Aβ42/Aβ40, as measured by a high precision IPMS assay, accurately diagnoses brain amyloidosis in both cognitively unimpaired and impaired research participants.

Combination of gut microbiota and plasma amyloid-β as a potential index for identifying preclinical Alzheimer's disease: a cross-sectional analysis from the SILCODE study

Background

Plasma amyloid-β (Aβ) may facilitate identification of individuals with brain amyloidosis. Gut microbial dysbiosis in Alzheimer’s disease (AD) is increasingly being recognized. However, knowledge about alterations of gut microbiota in preclinical AD, as well as whether the combination of plasma Aβ and gut microbiota could identify preclinical AD, remains largely unknown.

Methods

This study recruited 34 Aβ-negative cognitively normal (CN−) participants, 32 Aβ-positive cognitively normal (CN+) participants, and 22 patients with cognitive impairment (CI), including 11 patients with mild cognitive impairment (MCI) and 11 AD dementia patients. All participants underwent neuropsychological assessments and fecal microbiota analysis through 16S ribosomal RNA (rRNA) Illumina Miseq sequencing technique. Meso Scale Discovery (MSD) kits were used to quantify the plasma Aβ₄₀, Aβ₄₂, and Aβ₄₂/Aβ₄₀ in CN− and CN+ participants. Using Spearman’s correlation analysis, the associations of global standard uptake value rate (SUVR) with altered gut microbiota and plasma Aβ markers were separately evaluated. Furthermore, the discriminative power of the combination of gut microbiota and plasma Aβ markers for identifying CN+ individuals was investigated.

Results

Compared with the CN− group, the CN+ group showed significantly reduced plasma Aβ₄₂ (p = 0.011) and Aβ₄₂/Aβ₄₀ (p = 0.003). The relative abundance of phylum Bacteroidetes was significantly enriched, whereas phylum Firmicutes and class Deltaproteobacteria were significantly decreased in CN+ individuals in comparison with that in CN− individuals. Particularly, the relative abundance of phylum Firmicutes and its corresponding SCFA-producing bacteria exhibited a progressive decline tendency from CN− to CN+ and CI. Besides, the global brain Aβ burden was negatively associated with the plasma Aβ₄₂/Aβ₄₀ (r = −0.298, p = 0.015), family Desulfovibrionaceae (r = −0.331, p = 0.007), genus Bilophila (r = −0.247, p = 0.046), and genus Faecalibacterium (r = −0.291, p = 0.018) for all CN participants. Finally, the combination of plasma Aβ markers, altered gut microbiota, and cognitive performance reached a relatively good discriminative power in identifying individuals with CN+ from CN− (AUC = 0.869, 95% CI 0.782 ~ 0.955).

Conclusions

This study provided the evidence that the gut microbial composition was altered in preclinical AD. The combination of plasma Aβ and gut microbiota may serve as a non-invasive, cost-effective diagnostic tool for early AD screening. Targeting the gut microbiota may be a novel therapeutic strategy for AD.

Trial registration

This study has been registered in ClinicalTrials.gov (NCT03370744, https://www.clinicaltrials.gov) in November 15, 2017.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13195-022-00977-x.

A transcriptome-wide association study identifies novel blood-based gene biomarker candidates for Alzheimer's disease risk

Alzheimer's disease (ad) adversely affects the health, quality of life and independence of patients. There is a critical need to identify novel blood gene biomarkers for ad risk assessment. We performed a transcriptome-wide association study to identify biomarker candidates for ad risk. We leveraged two sets of gene expression prediction models of blood developed using different reference panels and modeling strategies. By applying the prediction models to a meta-GWAS including 71 880 (proxy) cases and 383 378 (proxy) controls, we identified significant associations of genetically determined expression of 108 genes in blood with ad risk. Of these, 15 genes were differentially expressed between ad patients and controls with concordant directions in measured expression data. With evidence from the analyses based on both genetic instruments and directly measured expression levels, this study identifies 15 genes with strong support as biomarkers in blood for ad risk, which may enhance ad risk assessment and mechanism-focused studies.

CCR5 antagonist reduces HIV-induced amyloidogenesis, tau pathology, neurodegeneration, and blood-brain barrier alterations in HIV-infected hu-PBL-NSG mice

BACKGROUND

Neurocognitive impairment is present in 50% of HIV-infected individuals and is often associated with Alzheimer's Disease (AD)-like brain pathologies, including increased amyloid-beta (Aβ) and Tau hyperphosphorylation. Here, we aimed to determine whether HIV-1 infection causes AD-like pathologies in an HIV/AIDS humanized mouse model, and whether the CCR5 antagonist maraviroc alters HIV-induced pathologies.

METHODS

NOD/scid-IL-2Rγcnull mice engrafted with human blood leukocytes were infected with HIV-1, left untreated or treated with maraviroc (120 mg/kg twice/day). Human cells in animal's blood were quantified weekly by flow cytometry. Animals were sacrificed at week-3 post-infection; blood and tissues viral loads were quantified using p24 antigen ELISA, RNAscope, and qPCR. Human (HLA-DR+) cells, Aβ-42, phospho-Tau, neuronal markers (MAP 2, NeuN, neurofilament-L), gamma-secretase activating protein (GSAP), and blood-brain barrier (BBB) tight junction (TJ) proteins expression and transcription were quantified in brain tissues by immunohistochemistry, immunofluorescence, immunoblotting, and qPCR. Plasma Aβ-42, Aβ-42 cellular uptake, release and transendothelial transport were quantified by ELISA.

RESULTS

HIV-1 significantly decreased human (h)CD4+ T-cells and hCD4/hCD8 ratios; decreased the expression of BBB TJ proteins claudin-5, ZO-1, ZO-2; and increased HLA-DR+ cells in brain tissues. Significantly, HIV-infected animals showed increased plasma and brain Aβ-42 and phospho-Tau (threonine181, threonine231, serine396, serine199), associated with transcriptional upregulation of GSAP, an enzyme that catalyzes Aβ formation, and loss of MAP 2, NeuN, and neurofilament-L. Maraviroc treatment significantly reduced blood and brain viral loads, prevented HIV-induced loss of neuronal markers and TJ proteins; decreased HLA-DR+ cells infiltration in brain tissues, significantly reduced HIV-induced increase in Aβ-42, GSAP, and phospho-Tau. Maraviroc also reduced Aβ retention and increased Aβ release in human macrophages; decreased the receptor for advanced glycation end products (RAGE) and increased low-density lipoprotein receptor-related protein-1 (LRP1) expression in human brain endothelial cells. Maraviroc induced Aβ transendothelial transport, which was blocked by LRP1 antagonist but not RAGE antagonist.

CONCLUSIONS

Maraviroc significantly reduced HIV-induced amyloidogenesis, GSAP, phospho-Tau, neurodegeneration, BBB alterations, and leukocytes infiltration into the CNS. Maraviroc increased cellular Aβ efflux and transendothelial Aβ transport via LRP1 pathways. Thus, therapeutically targeting CCR5 could reduce viremia, preserve the BBB and neurons, increased brain Aβ efflux, and reduce AD-like neuropathologies.

Performance of Plasma Amyloid β, Total Tau, and Neurofilament Light Chain in the Identification of Probable Alzheimer's Disease in South China

Background: Alzheimer's disease (AD) is the most common type of dementia and has no effective treatment to date. It is essential to develop a minimally invasive blood-based biomarker as a tool for screening the general population, but the efficacy remains controversial. This cross-sectional study aimed to evaluate the ability of plasma biomarkers, including amyloid β (Aβ), total tau (t-tau), and neurofilament light chain (NfL), to detect probable AD in the South Chinese population.

Methods: A total of 277 patients with a clinical diagnosis of probable AD and 153 healthy controls with normal cognitive function (CN) were enrolled in this study. The levels of plasma Aβ42, Aβ40, t-tau, and NfL were detected using ultra-sensitive immune-based assays (SIMOA). Lumbar puncture was conducted in 89 patients with AD to detect Aβ42, Aβ40, t-tau, and phosphorylated (p)-tau levels in the cerebrospinal fluid (CSF) and to evaluate the consistency between plasma and CSF biomarkers through correlation analysis. Finally, the diagnostic value of plasma biomarkers was further assessed by constructing a receiver operating characteristic (ROC) curve.

Results: After adjusting for age, sex, and the apolipoprotein E (APOE) alleles, compared to the CN group, the plasma t-tau, and NfL were significantly increased in the AD group (p < 0.01, Bonferroni correction). Correlation analysis showed that only the plasma t-tau level was positively correlated with the CSF t-tau levels (r = 0.319, p = 0.003). The diagnostic model combining plasma t-tau and NfL levels, and age, sex, and APOE alleles, showed the best performance for the identification of probable AD [area under the curve (AUC) = 0.89, sensitivity = 82.31%, specificity = 83.66%].

Conclusion: Blood biomarkers can effectively distinguish patients with probable AD from controls and may be a non-invasive and efficient method for AD pre-screening.

Abnormal amyloid beta metabolism in systemic abnormalities and Alzheimer's pathology: Insights and therapeutic approaches from periphery

Alzheimer's disease (AD) is an age-associated, multifactorial neurodegenerative disorder that is incurable. Despite recent success in treatments that partially improve symptomatic relief, they have failed in most clinical trials. Re-holding AD for accurate diagnosis and treatment is widely known as a challenging task. Lack of knowledge of basic molecular pathogenesis might be a possible reason for ineffective AD treatment. Historically, a majority of therapy-based studies have investigated the role of amyloid-β (Aβ peptide) in the central nervous system (CNS), whereas less is known about Aβ peptide in the periphery in AD. In this review, we provide a comprehensive summary of the current understanding of Aβ peptide metabolism (anabolism and catabolism) in the brain and periphery. We show that the abnormal metabolism of Aβ peptide is significantly linked with central-brain and peripheral abnormalities; the interaction between peripheral Aβ peptide metabolism and peripheral abnormalities affects central-brain Aβ peptide metabolism, suggesting the existence of significant communication between these two pathways of Aβ peptide metabolism. This close interaction between the central brain and periphery in abnormal Aβ peptide metabolism plays a key role in the development and progression of AD. In conclusion, we need to obtain a full understanding of the dynamic roles of Aβ peptide at the molecular level in both the brain and periphery in relation to the pathology of AD. This will not only provide new information regarding the complex disease pathology, but also offer potential new clues to improve therapeutic strategies and diagnostic biomarkers for the successful treatment of AD.

Transitioning from cerebrospinal fluid to blood tests to facilitate diagnosis and disease monitoring in Alzheimer's disease

Alzheimer's disease (AD) is increasingly prevalent worldwide, and disease-modifying treatments may soon be at hand; hence, now, more than ever, there is a need to develop techniques that allow earlier and more secure diagnosis. Current biomarker-based guidelines for AD diagnosis, which have replaced the historical symptom-based guidelines, rely heavily on neuroimaging and cerebrospinal fluid (CSF) sampling. While these have greatly improved the diagnostic accuracy of AD pathophysiology, they are less practical for application in primary care, population-based and epidemiological settings, or where resources are limited. In contrast, blood is a more accessible and cost-effective source of biomarkers in AD. In this review paper, using the recently proposed amyloid, tau and neurodegeneration [AT(N)] criteria as a framework towards a biological definition of AD, we discuss recent advances in biofluid-based biomarkers, with a particular emphasis on those with potential to be translated into blood-based biomarkers. We provide an overview of the research conducted both in CSF and in blood to draw conclusions on biomarkers that show promise. Given the evidence collated in this review, plasma neurofilament light chain (N) and phosphorylated tau (p-tau; T) show particular potential for translation into clinical practice. However, p-tau requires more comparisons to be conducted between its various epitopes before conclusions can be made as to which one most robustly differentiates AD from non-AD dementias. Plasma amyloid beta (A) would prove invaluable as an early screening modality, but it requires very precise tests and robust pre-analytical protocols.

Association of Midlife Plasma Amyloid-β Levels With Cognitive Impairment in Late Life: The ARIC Neurocognitive Study

OBJECTIVE

To evaluate the association between midlife plasma amyloid-beta (Aβ_1-42, Aβ_1-40, Aβ₄₂:Aβ₄₀) and risk of MCI and dementia.

METHODS

Plasma Aβ₄₂ and Aβ₄₀ were retrospectively measured using a fluorimetric bead-based immunoassay in a subsample of the Atherosclerosis Risk in Communities cohort study. We investigated the relationship of plasma Aβ₄₂, Aβ₄₀, and Aβ₄₂:Aβ₄₀ ratio measured in midlife, late-life, and the change from midlife to late-life, to risk of MCI, dementia, and combined MCI/dementia outcomes in late-life (from 2011-19). We used multinomial logistic regressions estimating relative risk ratios (RRR) of these cognitive outcomes vs cognitively normal adjusted for age, sex, education, site-race, APOE, hypertension, diabetes, and body mass index.

RESULTS

A total of 2284 participants were included (midlife mean age=59.2±5.2, 57% female, 22% Black). Each doubling of midlife Aβ₄₂:Aβ₄₀ was associated with 37% lower risk of MCI/dementia (RRR=0.63, 95% CI: 0.46, 0.87), but only up to approximately the median (spline model threshold 0.20). Every standard deviation increase in plasma Aβ₄₂ (10 pg/mL) was associated with 13% lower risk of MCI/dementia (RRR=0.87, 95% CI: 0.77, 0.98), whereas every standard deviation increase in plasma Aβ₄₀ (67 pg/mL) was associated with 15% higher risk of MCI/dementia (RRR=1.15, 95% CI: 1.01, 1.29). Associations were comparable, but slightly weaker statistically, when repeating models using late-life plasma Aβ predictors. Aβ₄₂ and Aβ₄₀ increased from midlife to late-life, but changes were not associated with cognitive outcomes.

CONCLUSION

Midlife measurement of plasma Aβ may have utility as a blood-based biomarker indicative of risk for future cognitive impairment.

The Emerging Role of Circular RNAs in Alzheimer's Disease and Parkinson's Disease

Alzheimer’s disease (AD) and Parkinson’s disease (PD) are two neurodegenerative diseases (NDDs) commonly found in elderly patients that are difficult to diagnose and lack effective treatment. Currently, the available diagnostic methods for these two NDDs do not meet clinical diagnostic expectations. Circular RNAs (circRNAs) are a diverse group of endogenous non-coding RNAs (ncRNAs) found in eukaryotic cells. Emerging studies suggest that altered expression of circRNAs is involved in the pathological processes of NDDs. CircRNAs could also prove to be promising biomarkers for the early diagnosis of NDDs such as AD and PD. Growing evidence has improved our knowledge of the roles of circRNAs in NDDs, which may lead to new therapeutic approaches that target transcription for preventing neurodegeneration. In this review, we describe the formation mechanisms and functions of circRNAs as well as methods of validation. We also discuss the emerging role of circRNAs in the pathophysiology of AD and PD and their potential value as biomarkers and therapeutic targets for AD and PD in the future.

Lipid Peroxidation Assessment in Preclinical Alzheimer Disease Diagnosis

BACKGROUND

Alzheimer disease (AD) is an increasingly common neurodegenerative disease, especially in countries with aging populations. Its diagnosis is complex and is usually carried out in advanced stages of the disease. In addition, lipids and oxidative stress have been related to AD since the earliest stages. A diagnosis in the initial or preclinical stages of the disease could help in a more effective action of the treatments.

METHODS

Isoprostanoid biomarkers were determined in plasma samples from preclinical AD participants (n = 12) and healthy controls (n = 31) by chromatography and mass spectrometry (UPLC-MS/MS). Participants were accurately classified according to cerebrospinal fluid (CSF) biomarkers and neuropsychological examination.

RESULTS

Isoprostanoid levels did not show differences between groups. However, some of them correlated with CSF biomarkers (t-tau, p-tau) and with cognitive decline. In addition, a panel including 10 biomarkers showed an area under curve (AUC) of 0.96 (0.903-1) and a validation AUC of 0.90 in preclinical AD prediction.

CONCLUSIONS

Plasma isoprostanoids could be useful biomarkers in preclinical diagnosis for AD. However, these results would require a further validation with an external cohort.

Diagnostic and Prognostic Laboratory Testing for Alzheimer Disease

This article focuses on current clinical laboratory testing to diagnose Alzheimer disease and monitor its progression throughout its disease course. Several clinically available tests focus on analysis of amyloid and tau levels in cerebrospinal fluid as well as autosomal dominant and risk factor genes. Although the current armament of clinical laboratory testing is limited by invasiveness of cerebrospinal fluid collection, rarity of autosomal dominant genetic mutations, and uncertainties of risk inherent in nonpenetrant genes, the field is poised to advance the clinical repertoire of laboratory diagnostic testing.

Alzheimer's Disease Animal Models: Elucidation of Biomarkers and Therapeutic Approaches for Cognitive Impairment

Alzheimer’s disease (AD) is an age-related and progressive neurodegenerative disorder. It is widely accepted that AD is mainly caused by the accumulation of extracellular amyloid β (Aβ) and intracellular neurofibrillary tau tangles. Aβ begins to accumulate years before the onset of cognitive impairment, suggesting that the benefit of currently available interventions would be greater if they were initiated in the early phases of AD. To understand the mechanisms of AD pathogenesis, various transgenic mouse models with an accelerated accumulation of Aβ and tau tangles have been developed. However, none of these models exhibit all pathologies present in human AD. To overcome these undesirable phenotypes, APP knock-in mice, which were presented with touchscreen-based tasks, were developed to better evaluate the efficacy of candidate therapeutics in mouse models of early-stage AD. This review assesses several AD mouse models from the aspect of biomarkers and cognitive impairment and discusses their potential as tools to provide novel AD therapeutic approaches.

Plasma Amyloid Concentration in Alzheimer's Disease: Performance of a High-Throughput Amyloid Assay in Distinguishing Alzheimer's Disease Cases from Controls

Background:

Collection of cerebrospinal fluid (CSF) for measurement of amyloid-β (Aβ) species is a gold standard in Alzheimer’s disease (AD) diagnosis, but has risks. Thus, establishing a low-risk blood Aβ test with high AD sensitivity and specificity is of outmost interest.

Objective:

We evaluated the ability of a commercially available plasma Aβ assay to distinguish AD patients from biomarker-healthy controls.

Method:

In a case-control design, we examined plasma samples from 44 AD patients (A + N+) and 49 controls (A–N–) from a memory clinic. AD was diagnosed using a combination of neuropsychological examination, CSF biomarker analysis and brain imaging. Total Aβ₄₀ and total Aβ₄₂ in plasma were measured through enzyme-linked immunosorbent assay (ELISA) technology using ABtest40 and ABtest42 test kits (Araclon Biotech Ltd.). Receiver operating characteristic (ROC) analyses with outcome AD were performed, and sensitivity and specificity were calculated.

Results:

Plasma Aβ_42/40 was weakly positively correlated with CSF Aβ_42/40 (Spearman’s rho 0.22; p = 0.037). Plasma Aβ_42/40 alone was not able to statistically significantly distinguish between AD patients and controls (AUC 0.58; 95% CI 0.46, 0.70). At a cut-point of 0.076 maximizing sensitivity and specificity, plasma Aβ_42/40 had a sensitivity of 61.2% and a specificity of 63.6%.

Conclusion:

In this sample, the high-throughput blood Aβ assay was not able to distinguish well between AD patients and controls. Whether or not the assay may be useful in large-scale epidemiological settings remains to be seen.

Associations Between Physical Activity, Blood-Based Biomarkers of Neurodegeneration, and Cognition in Healthy Older Adults: The MAPT Study

Physical activity (PA) demonstrated benefits on brain health, but its relationship with blood biomarkers of neurodegeneration remains poorly investigated. We explored the cross-sectional associations of PA with blood concentrations of neurofilament light chain (NFL) and beta amyloid (Aβ)42/40. We further examined whether the interaction between PA and these biomarkers was longitudinally related to cognition. Four-hundred and sixty-five nondemented older adults engaged in an interventional study and who had a concomitant assessment of PA levels and blood measurements of NFL (pg/mL) and Aβ 42/40 were analyzed. A composite Z-score combining 4 cognitive tests was used for cognitive assessment up to a 4-year follow-up. Multiple linear regressions demonstrated that people achieving 500-999 and 2000+ MET-min/week of PA had lower (ln)NFL concentrations than their inactive peers. Logistic regressions revealed that achieving at least 90 MET-min/week of PA was associated with a lower probability of having high NFL concentrations (ie, ≥91.961 pg/mL [third quartile]). PA was not associated with (Aβ)42/40. Mixed-model linear regressions demonstrated that the reverse relationship between PA and cognitive decline tended to be more pronounced as Aβ 42/40 increased, while it was dampened with increasing levels of (ln)NFL concentrations. This study demonstrates that PA is associated with blood NFL but not with Aβ 42/40. Furthermore, it suggests that PA may attenuate the negative association between amyloid load and cognition, while having high NFL levels mitigates the favorable relationship between PA and cognition. More investigations on non demented older adults are required for further validation of the present findings.

Plasma Amyloid-β in Relation to Antibodies Against Herpes Simplex Virus, Cytomegalovirus, and Chlamydophila pneumoniae

Background:

Amyloid-β (Aβ), the key constituent of Alzheimer’s disease (AD) plaques, has antimicrobial properties.

Objective:

To investigate the association between plasma Aβ and antibodies against the AD-related pathogens herpes simplex virus (HSV), cytomegalovirus (CMV), and C. pneumoniae.

Methods:

Plasma from 339 AD cases, obtained on average 9.4 years (±4.00) before diagnosis, and their matched controls were analyzed for Aβ₄₀ and Aβ₄₂ concentrations with Luminex xMAP technology and INNOBIA plasma Aβ-form assays. Enzyme-linked immunosorbent assays were utilized for analyses of anti-HSV immunoglobulin (Ig) G, anti-HSV1 IgG, anti-HSV2 IgG, anti-CMV IgG, and anti-C. pneumoniae IgG. Follow-up samples were available for 150 of the cases.

Results:

Presence and levels of anti-HSV1 IgG, anti-HSV2 IgG, anti-CMV IgG, and anti-C. pneumoniae IgG did not correlate with concentrations of Aβ₄₂ or Aβ₄₀ in cases or controls.

Conclusion:

Levels of plasma Aβ were not associated with antibodies against different AD-related pathogens.

Biomarkers in Alzheimer's disease

Background

Alzheimer's disease (AD) is a progressive neurodegenerative disease. AD is the main cause of dementia worldwide and aging is the main risk factor for developing the illness. AD classical diagnostic criteria rely on clinical data. However, the development of a biological definition of AD using biomarkers that reflect the underling neuropathology is needed.

Content

The aim of this review is to describe the main outcomes when measuring classical and novel biomarkers in biological fluids or neuroimaging.

Summary

Nowadays, there are three classical biomarkers for the diagnosis of AD: Aβ42, t-Tau and p-Tau. The diagnostic use of cerebrospinal fluid biomarkers is limited due to invasive collection by lumbar puncture with potential side effects. Plasma/serum measurements are the gold standard in clinics, because they are minimally invasive and, in consequence, easily collected and processed. The two main proteins implicated in the pathological process, Aβ and Tau, can be visualized using neuroimaging techniques, such as positron emission tomography.

Outlook

As it is currently accepted that AD starts decades before clinical symptoms could be diagnosed, the opportunity to detect biological alterations prior to clinical symptoms would allow early diagnosis or even perhaps change treatment possibilities.

Plasma β-Amyloid Levels Associated With Structural Integrity Based on Diffusion Tensor Imaging in Subjective Cognitive Decline: The SILCODE Study

Background: Accumulating evidence has demonstrated that plasma β-amyloid (Aβ) levels are useful biomarkers to reflect brain amyloidosis and gray matter structure, but little is known about their correlation with subclinical white matter (WM) integrity in individuals at risk of Alzheimer's disease (AD). Here, we investigated the microstructural changes in WM between subjects with low and high plasma Aβ levels among individuals with subjective cognitive decline (SCD).

Methods: This study included 142 cognitively normal individuals with SCD who underwent a battery of neuropsychological tests, plasma Aβ measurements, and diffusion tensor imaging (DTI) based on the Sino Longitudinal Study on Cognitive Decline (SILCODE). Using tract-based spatial statistics (TBSS), we compared fractional anisotropy (FA), and mean diffusivity (MD) in WM between subjects with low (N = 71) and high (N = 71) plasma Aβ levels (cut-off: 761.45 pg/ml for Aβ40 and 10.74 pg/ml for Aβ42).

Results: We observed significantly decreased FA and increased MD in the high Aβ40 group compared to the low Aβ40 group in various regions, including the body, the genu, and the splenium of the corpus callosum; the superior longitudinal fasciculus; the corona radiata; the thalamic radiation; the external and internal capsules; the inferior fronto-occipital fasciculus; and the sagittal stratum [p < 0.05, familywise error (FWE) corrected]. Average FA values were associated with poor performance on executive and memory assessments. No significant differences were found in either MD or FA between the low and high Aβ42 groups.

Conclusion: Our results suggest that a correlation exists between WM integrity and plasma Aβ40 levels in individuals with SCD.

Amyloid-β misfolding as a plasma biomarker indicates risk for future clinical Alzheimer's disease in individuals with subjective cognitive decline

Background

We evaluated Aβ misfolding in combination with Aβ_42/40 ratio as a prognostic tool for future clinical progression to mild cognitive impairment (MCI) or dementia due to Alzheimer’s disease (AD) in individuals with subjective cognitive decline (SCD).

Methods

Baseline plasma samples (n = 203) from SCD subjects in the SCIENCe project and Amsterdam Dementia Cohort (age 61 ± 9 years; 57% male, mean follow-up time 2.7 years) were analyzed using immuno-infrared-sensor technology. Within 6 years of follow-up, 22 (11%) individuals progressed to MCI or dementia due to AD. Sensor readout values > 1646 cm^− 1 reflected normal Aβ folding; readouts at ≤ 1646 cm^− 1 reflected low and at < 1644 cm^− 1 high misfolding. We used Cox proportional hazard models to quantify Aβ misfolding as a prognostic biomarker for progression to MCI and dementia due to AD. The accuracy of the predicted development of MCI/AD was determined by time-dependent receiver operating characteristic (t-ROC) curve analyses that take individual follow-up and conversion times into account. Statistical models were adjusted for age, sex, and APOEε4 status. Additionally, plasma Aβ_42/40 data measured by SIMOA were statistically analyzed and compared.

Results

All 22 patients who converted to MCI or AD-dementia within 6 years exhibited Aβ misfolding at baseline. Cox analyses revealed a hazard ratio (HR) of 19 (95% confidence interval [CI] 2.2–157.8) for future conversion of SCD subjects with high misfolding and of 11 (95% CI 1.0–110.1) for those with low misfolding. T-ROC curve analyses yielded an area under the curve (AUC) of 0.94 (95% CI 0.86–1.00; 6-year follow-up) for Aβ misfolding in an age, sex, and APOEε4 model. A similar model with plasma Aβ_42/40 ratio yielded an AUC of 0.92 (95% CI, 0.82–1.00). The AUC increased to 0.99 (95% CI, 0.99–1.00) after inclusion of both Aβ misfolding and the Aβ_42/40 ratio.

Conclusions

A panel of structure- and concentration-based plasma amyloid biomarkers may predict conversion to clinical MCI and dementia due to AD in cognitively unimpaired subjects. These plasma biomarkers provide a noninvasive and cost-effective alternative for screening early AD pathological changes. Follow-up studies and external validation in larger cohorts are in progress for further validation of our findings.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13195-020-00738-8.

Plasma Aβ42 as a Biomarker of Prodromal Alzheimer's Disease Progression in Patients with Amnestic Mild Cognitive Impairment: Evidence from the PharmaCog/E-ADNI Study

It is an open issue whether blood biomarkers serve to diagnose Alzheimer's disease (AD) or monitor its progression over time from prodromal stages. Here, we addressed this question starting from data of the European FP7 IMI-PharmaCog/E-ADNI longitudinal study in amnesic mild cognitive impairment (aMCI) patients including biological, clinical, neuropsychological (e.g., ADAS-Cog13), neuroimaging, and electroencephalographic measures. PharmaCog/E-ADNI patients were classified as "positive" (i.e., "prodromal AD" n = 76) or "negative" (n = 52) based on a diagnostic cut-off of Aβ42/P-tau in cerebrospinal fluid as well as APOE ε 4 genotype. Blood was sampled at baseline and at two follow-ups (12 and 18 months), when plasma amyloid peptide 42 and 40 (Aβ42, Aβ40) and apolipoprotein J (clusterin, CLU) were assessed. Linear Mixed Models found no significant differences in plasma molecules between the "positive" (i.e., prodromal AD) and "negative" groups at baseline. In contrast, plasma Aβ42 showed a greater reduction over time in the prodromal AD than the "negative" aMCI group (p = 0.048), while CLU and Aβ40 increased, but similarly in the two groups. Furthermore, plasma Aβ42 correlated with the ADAS-Cog13 score both in aMCI patients as a whole and the prodromal AD group alone. Finally, CLU correlated with the ADAS-Cog13 only in the whole aMCI group, and no association with ADAS-Cog13 was found for Aβ40. In conclusion, plasma Aβ42 showed disease progression-related features in aMCI patients with prodromal AD.

Serum Tau Proteins as Potential Biomarkers for the Assessment of Alzheimer's Disease Progression

Total tau (t-tau) and phosphorylated tau (p-tau) protein elevations in cerebrospinal fluid (CFS) are well-established hallmarks of Alzheimer’s disease (AD), while the associations of serum t-tau and p-tau levels with AD have been inconsistent across studies. To identify more accessible non-invasive AD biomarkers, we measured serum tau proteins and associations with cognitive function in age-matched controls (AMC, n = 26), mild cognitive impairment group (MCI, n = 30), and mild-AD group (n = 20) according to the Mini-mental State Examination (MMSE), Clinical Dementia Rating (CDR), and Global Deterioration Scale (GDS) scores. Serum t-tau, but not p-tau, was significantly higher in the mild-AD group than AMC subjects (p < 0.05), and there were significant correlations of serum t-tau with MMSE and GDS scores. Receiver operating characteristic (ROC) analysis distinguished mild-AD from AMC subjects with moderate sensitivity and specificity (AUC = 0.675). We speculated that tau proteins in neuronal cell-derived exosomes (NEX) isolated from serum would be more strongly associated with brain tau levels and disease characteristics, as these exosomes can penetrate the blood-brain barrier. Indeed, ELISA and Western blotting indicated that both NEX t-tau and p-tau (S202) were significantly higher in the mild-AD group compared to AMC (p < 0.05) and MCI groups (p < 0.01). In contrast, serum amyloid β (Aβ_1–42) was lower in the mild-AD group compared to MCI groups (p < 0.001). During the 4-year follow-up, NEX t-tau and p-tau (S202) levels were correlated with the changes in GDS and MMSE scores. In JNPL3 transgenic (Tg) mice expressing a human tau mutation, t-tau and p-tau expression levels in NEX increased with neuropathological progression, and NEX tau was correlated with tau in brain tissue exosomes (tEX), suggesting that tau proteins reach the circulation via exosomes. Taken together, our data suggest that serum tau proteins, especially NEX tau proteins, are useful biomarkers for monitoring AD progression.

Correlation between cognition and plasma noradrenaline level in Alzheimer's disease: a potential new blood marker of disease evolution

Recent evidence showing degeneration of the noradrenergic system in the locus coeruleus (LC) in Alzheimer's disease (AD) has motivated great interest in noradrenaline (NA) as a potential brain hallmark of the disease. Despite the current exploration of blood markers for AD, the deregulation of the plasma NA concentration ([NA]_plasma) in AD is currently not well understood. This retrospective study includes a cohort of 71 patients (32 AD patients, 22 with other dementia and 17 without dementia) who were given consultations for memory complaints in the Cognitive Neurology Center of Lariboisière (Paris) between 2009 and 2014. As previously described in brain tissue, we show for the first time a linear correlation between [NA]_plasma and Mini Mental State Examination (MMSE) score in AD patients. We observed that high [NA]_plasma in AD patients was associated with higher [Aβ_1-42]_CSF than in other AD patients with [NA]_plasma similar to NC patients. In parallel, we observed a lower (p-Tau/Tau)_CSF in AD patients with low [NA]_plasma than in non-AD patients with [NA]_plasma similar to [NA]_plasma in NC patients. Our data suggest that [NA]_plasma could be a potential biomarker of disease evolution in the context of AD and could possibly improve early diagnosis.

Amyloid-beta plasma and cerebrospinal fluid biomarkers in aged dogs with cognitive dysfunction syndrome

Background

Cognitive dysfunction syndrome (CDS) is a common progressive neurodegenerative disease that is poorly defined. Specific multitargeted protocols do not exist for setting the diagnosis and the prognosis of the syndrome.

Hypothesis/Objectives

To quantify Aβ42 and Aβ40 peptides in blood and cerebrospinal fluid (CSF) and to investigate their contribution to CCDS.

Animals

A total of 61 dogs from a hospital population.

Methods

Case‐control study. Six young (YG: 0‐4 years old), 8 middle‐aged (4‐8 years old), 17 cognitively unimpaired and aged (CU: 8‐20 years old), and 30 cognitively impaired and aged (CI: 8‐17 years). From the CI group, 10 dogs exhibited mild impairment (CI‐MCI) and 20 exhibited severe impairment (CI‐SCI). Cognitive status was assessed using a validated owner‐based questionnaire. Direct and indirect Aβ markers were determined in plasma fractions (total‐TP, free‐FP, bound to plasma components‐CP) and CSF using commercial ELISA assays (AΒtest, Araclon Biotech).

Results

TPAβ42/40 facilitated discrimination between CI‐MCI and CU aged dogs with area under curve ≥ 0.79. CSFAβ42 levels were higher (P = .09) in CU (1.25 ± 0.28 ng/mL) than in MCI (1.04 ± 0.32 ng/mL) dogs. CSF Aβ42 levels were correlated with the CP fragment (CPAβ40: P = .02, CPAβ42: P = .02). CPAβ42 was higher in the CI‐MCI (23.03 ± 11.79 pg/μL) group compared to the other aged dogs (CU: 10.42 ± 7.18 pg/μL, P = .02, SCI: 11.40 ± 12.98 pg/μL, P = .26).

Conclusion and Clinical Importance

The Aβ should be determined in all of the 3 plasma fractions (TP, FP, CP). In the clinical approach, TPAβ42/40 could be used as an efficient preselection tool for the aged canine population targeting dogs with mild cognitive impairment.

Identifying Protein Biomarkers in Blood for Alzheimer's Disease

Background: At present, the main diagnostic methods for Alzheimer's disease (AD) are positron emission tomography (PET) scanning of the brain and analysis of cerebrospinal fluid (CSF) sample, but these methods are expensive and harmful to patients. Recently, more researchers focus on diagnosing AD by detecting biomarkers in blood, which is a cheaper and harmless way. Therefore, identifying AD-related proteins in blood can help treatment and diagnosis. Methods: We proposed a hypothesis that similar diseases share similar proteins. Diseases with similar symptoms are caused by abnormalities of similar proteins. Assuming that the similarities between AD and other diseases obey the normal distribution, we developed an iterative method based on disease similarity (IBDS). We combined Elastic Network (EN) with Minimum angle regression (MAR) to find the optimal solution. Finally, we used case studies and Summary data Mendelian Random (SMR) to verify our method. Results: We selected 39 diseases which are highly related to AD. They correspond 1,481 kinds of proteins. One hundred and eighty-four proteins are reported to be related to AD in Uniprot and the number would be 284 with our method. The AUC of our method by cross-validation is 0.9251 which is much higher than previous methods. Conclusion: In this paper, we presented a novel method for prioritizing AD-related proteins. Seven proteins have tissue specificity in blood among these 284 proteins, which could be used to diagnose AD in future. Case studies and SMR have been used to prove the relationship between these 7 proteins and AD. Availability and Implementation: https://github.com/zty2009/Identifying-Protein-Biomarkers-in-Blood-for-Alzheimer-s-Disease.

Paper-Based Detection Device for Alzheimer's Disease-Detecting β-amyloid Peptides (1-42) in Human Plasma

The diagnosis of Alzheimer's disease (AD) is frequently missed or delayed in clinical practice. To remedy this situation, we developed a screening, paper-based (P-ELISA) platform to detect β-amyloid peptide 1-42 (Aβ42) and provide rapid results using a small volume, easily accessible plasma sample instead of cerebrospinal fluid. The protocol outlined herein only requires 3 μL of sample per well and a short operating time (i.e., only 90 min). The detection limit of Aβ42 is 63.04 pg/mL in a buffer system. This P-ELISA-based approach can be used for early, preclinical stage AD screening, including screening for amnestic mild cognitive impairment (MCI) due to AD. It may also be used for treatment and stage monitoring purposes. The implementation of this approach may provide tremendous impact for an afflicted population and may well prompt additional and expanded efforts in both academic and commercial communities.

Total Aβ42/Aβ40 ratio in plasma predicts amyloid-PET status, independent of clinical AD diagnosis

OBJECTIVE

To explore whether the plasma total β-amyloid (Aβ) Aβ₄₂/Aβ₄₀ ratio is a reliable predictor of the amyloid-PET status by exploring the association between these 2 variables in a subset of the Australian Imaging, Biomarkers and Lifestyle (AIBL) study of aging cohort.

METHODS

Taking plasma samples at 3 separate time points, month 18 (n = 176), month 36 (n = 169), and month 54 (n = 135), we assessed the total Aβ₄₂/Aβ₄₀ ratio in plasma (TP42/40) with regard to neocortical Aβ burden via PET standardized uptake value ratio (SUVR) and investigated both association with Aβ-PET status and correlation (and agreement) with SUVR.

RESULTS

The TP42/40 plasma ratio was significantly reduced in amyloid-PET-positive participants at all time points (p < 0.0001). Adjusting for covariates age, gender, APOE ε4 allele status, and clinical classification clearly affects the significance, with p values reduced and only comparisons at 54 months retaining significance (p = 0.006). Correlations with SUVR were similar across each time point, with Spearman ρ reaching -0.64 (p < 0.0001). Area under the curve values were highly reproducible over time points, with values ranging from 0.880 at 36 months to 0.913 at 54 months. In assessments of the healthy control group only, the same relationships were found.

CONCLUSIONS

The current study demonstrates reproducibility of the plasma assay to discriminate between amyloid-PET positive and negative over 3 time points, which can help to substantially reducing the screening rate of failure for clinical trials targeting preclinical or prodromal disease.

CLASSIFICATION OF EVIDENCE

This study provides Class II evidence that plasma total Aβ₄₂/Aβ₄₀ ratio is associated with neocortical amyloid burden as measured by PET SUVR.

Diagnosis of Mild Cognitive Impairment and Alzheimer's Disease by the Plasma and Serum Amyloid-beta 42 Assay through Highly Sensitive Peptoid Nanosheet Sensor

Alzheimer's disease (AD) is the most common neurodegenerative disorder with a continuous pathophysiological process starting from the preclinical and mild cognitive impairment (MCI) phases to the dementia phase. Early diagnosis is prerequisite for the early intervention of AD but meanwhile challenging. Amyloid-beta 1-42 (Aβ₄₂) plays a crucial part in AD pathology. Positron-emission tomography (PET) imaging of Aβ₄₂ in the brain and the measurement of Aβ₄₂ in the cerebrospinal fluid (CSF) have been adopted for the auxiliary diagnosis of AD, but their widespread clinical application has been limited due to the radiation and the high-cost of PET and the invasive lumbar puncture for collecting CSF. Noninvasive and cost-effective blood-based assay is desirable for the early diagnosis of AD. Here, a label-free assay for the quantification of blood Aβ₄₂ was developed using the high-throughput surface plasmon resonance imaging method with the aid of an antibody-mimetic peptoid nanosheet equipping Aβ₄₂-recognizing loops. We demonstrated that this nanosheet-based sensor system could distinguish the plasma and sera from normal individuals and patients suffering AD and amnestic MCI with high sensitivity and specificity, preceding the diagnostic performance of the Aβ₄₂-recognizing molecule and the antibody specific to Aβ₄₂. This work provides a label-free, cost-effective, highly sensitive, and high-throughput blood-based assay for early detection of AD.

Role of Fluid Biomarkers and PET Imaging in Early Diagnosis and its Clinical Implication in the Management of Alzheimer's Disease

Clinical diagnosis of Alzheimer's disease (AD) is based on symptoms; however, the challenge is to diagnose AD at the preclinical stage with the application of biomarkers and initiate early treatment (still not widely available). Currently, cerebrospinal fluid (CSF) amyloid-β 42 (Aβ42) and tau are used in the clinical diagnosis of AD; nevertheless, blood biomarkers (Aβ42 and tau) are less predictive. Amyloid-positron emission tomography (PET) imaging is an advancement in technology that uses approved radioactive diagnostic agents (florbetapir, flutemetamol, or florbetaben) to estimate Aβ neuritic plaque density in adults with cognitive impairment evaluated for AD and other causes of cognitive decline. There is no cure for AD to date-the disease progression cannot be stopped or reversed; approved pharmacological agents (donepezil, galantamine, and rivastigmine; memantine) provide symptomatic treatment. However, the disease-modifying therapies are promising; aducanumab and CAD106 are in phase III trials for the early stages of AD. In conclusion, core CSF biomarkers reflect pathophysiology of AD in the early and late stages; the application of approved radiotracers have potential in amyloid-PET brain imaging to detect early AD.

Alterations of aqueous humor Aβ levels in Aβ-infused and transgenic mouse models of Alzheimer disease

Alzheimer's disease (AD) is an ageing-related neurodegenerative disease characterized and diagnosed by deposition of insoluble amyloid-β (Aβ) plaques in the brain. The plaque accumulation in the brain directly affects reduced levels of Aβ in cerebrospinal fluid (CSF) and blood, as Aβ can freely transport the blood-brain barrier, and clinical investigations have suggested these two biofluids as promising samples for in vitro diagnosis. Given that the human eye structurally resembles the brain and Aβ accumulation often observed in the ocular region of AD patients, in this study, we examined aqueous humor Aβ as another possible surrogate biomarker. First, using the acute Aβ-infused AD mouse model by injecting Aβ to the CSF in intracerebroventricular region of normal ICR mice, we investigated whether Aβ concentration in the aqueous humor in AD models is positively correlated with the concentration in the CSF. Then, we examined the correlation of aqueous humor Aβ levels with increased plaque deposition in the brain and reduced Aβ levels in both CSF and blood in adult and aged 5XFAD Alzheimer transgenic mice. Collectively, the synthetic Aβ injected into CSF immediately migrate to the aqueous humor, however, the age-dependently reducing pattern of Aβ levels in CSF and blood was not observed in the aqueous humor.

Latest advances in cerebrospinal fluid and blood biomarkers of Alzheimer's disease

Alzheimer's disease (AD) is the most common neurodegenerative disease and its diagnosis has classically been based on clinical symptoms. Recently, a biological rather than a syndromic definition of the disease has been proposed that is based on biomarkers that reflect neuropathological changes. In AD, there are two main biomarker categories, namely neuroimaging and fluid biomarkers [cerebrospinal fluid (CSF) and blood]. As a complex and multifactorial disease, AD biomarkers are important for an accurate diagnosis and to stage the disease, assess the prognosis, test target engagement, and measure the response to treatment. In addition, biomarkers provide us with information that, even if it does not have a current clinical use, helps us to understand the mechanisms of the disease. In addition to the pathological hallmarks of AD, which include amyloid-β and tau deposition, there are multiple concomitant pathological events that play a key role in the disease. These include, but are not limited to, neurodegeneration, inflammation, vascular dysregulation or synaptic dysfunction. In addition, AD patients often have an accumulation of other proteins including α-synuclein and TDP-43, which may have a pathogenic effect on AD. In combination, there is a need to have biomarkers that reflect different aspects of AD pathogenesis and this will be important in the future to establish what are the most suitable applications for each of these AD-related biomarkers. It is unclear whether sex, gender, or both have an effect on the causes of AD. There may be differences in fluid biomarkers due to sex but this issue has often been neglected and warrants further research. In this review, we summarize the current state of the principal AD fluid biomarkers and discuss the effect of sex on these biomarkers.

Plasma Aβ42/40 ratio alone or combined with FDG-PET can accurately predict amyloid-PET positivity: a cross-sectional analysis from the AB255 Study

BACKGROUND

To facilitate population screening and clinical trials of disease-modifying therapies for Alzheimer's disease, supportive biomarker information is necessary. This study was aimed to investigate the association of plasma amyloid-beta (Aβ) levels with the presence of pathological accumulation of Aβ in the brain measured by amyloid-PET. Both plasma Aβ42/40 ratio alone or combined with an FDG-PET-based biomarker of neurodegeneration were assessed as potential AD biomarkers.

METHODS

We included 39 cognitively normal subjects and 20 patients with mild cognitive impairment from the AB255 Study who had undergone PiB-PET scans. Total Aβ40 and Aβ42 levels in plasma (TP42/40) were quantified using ABtest kits. Subjects were dichotomized as Aβ-PET positive or negative, and the ability of TP42/40 to detect Aβ-PET positivity was assessed by logistic regression and receiver operating characteristic analyses. Combination of plasma Aβ biomarkers and FDG-PET was further assessed as an improvement for brain amyloidosis detection and diagnosis classification.

RESULTS

Eighteen (30.5%) subjects were Aβ-PET positive. TP42/40 ratio alone identified Aβ-PET status with an area under the curve (AUC) of 0.881 (95% confidence interval [CI] = 0.779-0.982). Discriminating performance of TP42/40 to detect Aβ-PET-positive subjects yielded sensitivity and specificity values at Youden's cutoff of 77.8% and 87.5%, respectively, with a positive predictive value of 0.732 and negative predictive value of 0.900. All these parameters improved after adjusting the model for significant covariates. Applying TP42/40 as the first screening tool in a sequential diagnostic work-up would reduce the number of Aβ-PET scans by 64%. Combination of both FDG-PET scores and plasma Aβ biomarkers was found to be the most accurate Aβ-PET predictor, with an AUC of 0.965 (95% CI = 0.913-0.100).

CONCLUSIONS

Plasma TP42/40 ratio showed a relevant and significant potential as a screening tool to identify brain Aβ positivity in preclinical and prodromal stages of Alzheimer's disease.

Plasma amyloid and tau as dementia biomarkers in Down syndrome: Systematic review and meta-analyses

Individuals with Down syndrome (DS) are at high risk of developing Alzheimer's disease (AD). Discovering reliable biomarkers which could facilitate early AD diagnosis and be used to predict/monitor disease course would be extremely valuable. To examine if analytes in blood related to amyloid plaques may constitute such biomarkers, we conducted meta‐analyses of studies comparing plasma amyloid beta (Aβ) levels between DS individuals and controls, and between DS individuals with and without dementia. PubMed, Embase, and Google Scholar were searched for studies investigating the relationship between Aβ plasma concentrations and dementia in DS and 10 studies collectively comprising >1,600 adults, including >1,400 individuals with DS, were included. RevMan 5.3 was used to perform meta‐analyses. Meta‐analyses showed higher plasma Aβ₄₀ (SMD = 1.79, 95% CI [1.14, 2.44], Z = 5.40, p < .00001) and plasma Aβ₄₂ levels (SMD = 1.41, 95% CI [1.15, 1.68], Z = 10.46, p < .00001) in DS individuals than controls, and revealed that DS individuals with dementia had higher plasma Aβ₄₀ levels (SMD = 0.23, 95% CI [0.05, 0.41], Z = 2.54, p = .01) and lower Aβ₄₂/Aβ₄₀ ratios (SMD = −0.33, 95% CI [−0.63, −0.03], Z = 2.15, p = .03) than DS individuals without dementia. Our results indicate that plasma Aβ₄₀ levels may constitute a promising biomarker for predicting dementia status in individuals with DS. Further investigations using new ultra‐sensitive assays are required to obtain more reliable results and to investigate to what extent these results may be generalizable beyond the DS population.

Age-Dependent Relationship Between Plasma Aβ40 and Aβ42 and Total Tau Levels in Cognitively Normal Subjects

Both amyloid plaques and neurofibrillary tangles are pathological hallmarks in the brains of patients with Alzheimer’s disease (AD). However, the constituents of these hallmarks, amyloid beta (Aβ) 40, Aβ42, and total Tau (t-Tau), have been detected in the blood of cognitively normal subjects by using an immunomagnetic reduction (IMR) assay. Whether these levels are age-dependent is not known, and their interrelation remains undefined. We determined the levels of these biomarkers in cognitively normal subjects of different age groups. A total of 391 cognitively normal subjects aged 23–91 were enrolled from hospitals in Asia, Europe, and North America. Healthy cognition was evaluated by NIA-AA guidelines to exclude subjects with mild cognitive impairment (MCI) and AD and by cognitive assessment using the Mini Mental State Examination and Clinical Dementia Rating (CDR). We examined the effect of age on plasma levels of Aβ40, Aβ42, and t-Tau and the relationship between these biomarkers during aging. Additionally, we explored age-related reference intervals for each biomarker. Plasma t-Tau and Aβ42 levels had modest but significant correlations with chronological age (r = 0.127, p = 0.0120 for t-Tau; r = −0.126, p = 0.0128 for Aβ42), ranging from ages 23 to 91. Significant positive correlations were detected between Aβ42 and t-Tau in the groups aged 50 years and older, with Rho values ranging from 0.249 to 0.474. Significant negative correlations were detected between Aβ40 and t-Tau from age 40 to 91 (r ranged from −0.293 to −0.582) and between Aβ40 and Aβ42 in the age groups of 30–39 (r = −0.562, p = 0.0235), 50–59 (r = −0.261, p = 0.0142), 60–69 (r = −0.303, p = 0.0004), and 80–91 (r = 0.459, p = 0.0083). We also provided age-related reference intervals for each biomarker. In this multicenter study, age had weak but significant effects on the levels of Aβ42 and t-Tau in plasma. However, the age group defined by decade revealed the emergence of a relationship between Aβ40, Aβ42, and t-Tau in the 6th and 7th decades. Validation of our findings in a large-scale and longitudinal study is warranted.

A systemic view of Alzheimer disease - insights from amyloid-β metabolism beyond the brain

Alzheimer disease (AD) is the most common type of dementia, and is currently incurable; existing treatments for AD produce only a modest amelioration of symptoms. Research into this disease has conventionally focused on the CNS. However, several peripheral and systemic abnormalities are now understood to be linked to AD, and our understanding of how these alterations contribute to AD is becoming more clearly defined. This Review focuses on amyloid-β (Aβ), a major hallmark of AD. We review emerging findings of associations between systemic abnormalities and Aβ metabolism, and describe how these associations might interact with or reflect on the central pathways of Aβ production and clearance. On the basis of these findings, we propose that these abnormal systemic changes might not only develop secondary to brain dysfunction but might also affect AD progression, suggesting that the interactions between the brain and the periphery have a crucial role in the development and progression of AD. Such a systemic view of the molecular pathogenesis of AD could provide a novel perspective for understanding this disease and present new opportunities for its early diagnosis and treatment.

Plasma amyloid beta levels are associated with cerebral amyloid and tau deposition

Introduction

We investigated the relationship of plasma amyloid beta (Aβ) with cerebral deposition of Aβ and tau on positron emission tomography (PET).

Methods

Forty-four participants (18 cognitively normal older adults [CN], 10 mild cognitive impairment, 16 Alzheimer's disease [AD]) underwent amyloid PET and a blood draw. Free and total plasma Aβ40 and Aβ42 were assessed using a validated assay. Thirty-seven participants (17 CN, 8 mild cognitive impairment, 12 AD) also underwent a [¹⁸F]flortaucipir scan. Scans were preprocessed by standard techniques, and mean global and regional amyloid and tau values were extracted. Free Aβ42/Aβ40 (Aβ F42:F40) and total Aβ42/Aβ40 (Aβ T42:T40) were evaluated for differences by diagnosis and relation to PET Aβ positivity. Relationships between these measures and cerebral Aβ and tau on both regional and voxel-wise basis were also evaluated.

Results

Lower Aβ T42:T40 was associated with diagnosis and PET Aβ positivity. Lower plasma Aβ T42:T40 ratios predicted cerebral Aβ positivity, both across the full sample and in CN only. Finally, lower plasma Aβ T42:T40 ratios were associated with increased cortical Aβ and tau in AD-related regions on both regional and voxel-wise analyses.

Discussion

Plasma Aβ measures may be useful biomarkers for predicting cerebral Aβ and tau. Additional studies in larger samples are warranted.

Luminex-based quantification of Alzheimer's disease neuropathologic change in formalin-fixed post-mortem human brain tissue

The vast majority of archived research and clinical pathological specimens are stored in the form of formalin fixed, paraffin-embedded (FFPE) tissues, but, unlike fresh frozen tissue samples, highly quantitative measures in FFPE tissues are limited to immunohistochemical and immunofluorescence thresholding image analysis studies, cell counting, and ordinal ranking systems. This poses a significant obstacle for clinical investigations that aim to correlate diagnostic markers of neurodegenerative diseases like Alzheimer's disease (AD) with parameters like age, gender, drug exposures, genotype, disease stage, co-morbidities, or environmental factors. To overcome this limitation, we have developed Luminex-based techniques and protocols for the quantification of amyloid β and hyperphosphorylated Tau in FFPE brain sections. We validated the Luminex assay in FFPE sections from prefrontal cortex, hippocampus, and neostriatum from 30 cases that underwent prior neuropathological diagnostic assessment of AD following the current NIA-AA recommendations for AD: 10 cases diagnosed as not or low, 10 cases as intermediate, and 10 cases as high AD neuropathologic change. Consistent with the neuropathologic assessment, Luminex assay detected high amounts of amyloid beta in the frontal cortex and striatum, and high amounts of hyperphosphorylated Tau in the frontal cortex and hippocampus, of cases with high AD neuropathologic change. This assay can be expanded to detect diverse antigenic targets of interest, as we show here with IBA1 and GFAP. This novel approach supports multiplexed highly quantitative, molecularly specific neuropathology measures to further explore mechanisms of neurodegeneration in AD.

Revealing heterogeneity of brain imaging phenotypes in Alzheimer's disease based on unsupervised clustering of blood marker profiles

Alzheimer's disease (AD) affects millions of people and is a major rising problem in health care worldwide. Recent research suggests that AD could have different subtypes, presenting differences in how the disease develops. Characterizing those subtypes could be key to deepen the understanding of this complex disease. In this paper, we used a multivariate, non-supervised clustering method over blood-based markers to find subgroups of patients defined by distinctive blood marker profiles. Our analysis on ADNI database identified 4 possible subgroups, each with a different blood profile. More importantly, we show that subgroups with different profiles have a different relationship between brain phenotypes detected in magnetic resonance imaging and disease condition.

Mass spectrometry: A platform for biomarker discovery and validation for Alzheimer's and Parkinson's diseases

Accurate, reliable, and objective biomarkers for Alzheimer's disease (AD), Parkinson's disease (PD), and related age-associated neurodegenerative disorders are urgently needed to assist in both diagnosis, particularly at early stages, and monitoring of disease progression. Technological advancements in protein detection platforms over the last few decades have resulted in a plethora of reported molecular biomarker candidates for both AD and PD; however, very few of these candidates are developed beyond the discovery phase of the biomarker development pipeline, a reflection of the current bottleneck within the field. In this review, the expanded use of selected reaction monitoring (SRM) targeted mass spectrometry will be discussed in detail as a platform for systematic verification of large panels of protein biomarker candidates prior to costly validation testing. We also advocate for the coupling of discovery-based proteomics with modern targeted MS-based approaches (e.g., SRM) within a single study in future workflows to expedite biomarker development and validation for AD and PD. It is our hope that improving the efficiency within the biomarker development process by use of an SRM pipeline may ultimately hasten the development of biomarkers that both decrease misdiagnosis of AD and PD and ultimately lead to detection at early stages of disease and objective assessment of disease progression. This article is part of the special issue "Proteomics".

A two-step immunoassay for the simultaneous assessment of Aβ38, Aβ40 and Aβ42 in human blood plasma supports the Aβ42/Aβ40 ratio as a promising biomarker candidate of Alzheimer's disease

Background

The quantification of amyloid-beta (Aβ) peptides in blood plasma as potential biomarkers of Alzheimer’s disease (AD) is hampered by very low Aβ concentrations and the presence of matrix components that may interfere with the measurements.

Methods

We developed a two-step immunoassay for the simultaneous measurement of the relative levels of Aβ38, Aβ40 and Aβ42 in human EDTA plasma. The assay was employed for the study of 23 patients with dementia of the Alzheimer’s type (AD-D) and 17 patients with dementia due to other reasons (OD). We examined relationships with the clinical diagnosis, cerebral Aβ load as quantified by amyloid-positron emission tomography, apolipoprotein E genotype, Aβ levels and Tau protein in cerebrospinal fluid.

Results

Preconcentration of plasma Aβ peptides by immunoprecipitation substantially facilitated their immunological measurements. The Aβ42/Aβ40 and Aβ42/Aβ38 ratios were statistically significantly lower in the AD-D patients than in the OD group. The areas under the receiver operating characteristic curves reached 0.87 for the Aβ42/Aβ40 ratio and 0.80 for the Aβ42/Aβ38 ratio.

Conclusions

The measurement of plasma Aβ peptides with an immunological assay can be improved by preconcentration via immunoprecipitation with an antibody against the Aβ amino-terminus and elution of the captured peptides by heating in a mild detergent-containing buffer. Our findings support the Aβ42/Aβ40 ratio in blood plasma as a promising AD biomarker candidate which correlates significantly with the validated core biomarkers of AD. Further studies will be needed for technical advancement of the assay and validation of the biomarker findings.

Electronic supplementary material

The online version of this article (10.1186/s13195-018-0448-x) contains supplementary material, which is available to authorized users.

Current state of Alzheimer's fluid biomarkers

Alzheimer’s disease (AD) is a progressive neurodegenerative disease with a complex and heterogeneous pathophysiology. The number of people living with AD is predicted to increase; however, there are no disease-modifying therapies currently available and none have been successful in late-stage clinical trials. Fluid biomarkers measured in cerebrospinal fluid (CSF) or blood hold promise for enabling more effective drug development and establishing a more personalized medicine approach for AD diagnosis and treatment. Biomarkers used in drug development programmes should be qualified for a specific context of use (COU). These COUs include, but are not limited to, subject/patient selection, assessment of disease state and/or prognosis, assessment of mechanism of action, dose optimization, drug response monitoring, efficacy maximization, and toxicity/adverse reactions identification and minimization. The core AD CSF biomarkers Aβ42, t-tau, and p-tau are recognized by research guidelines for their diagnostic utility and are being considered for qualification for subject selection in clinical trials. However, there is a need to better understand their potential for other COUs, as well as identify additional fluid biomarkers reflecting other aspects of AD pathophysiology. Several novel fluid biomarkers have been proposed, but their role in AD pathology and their use as AD biomarkers have yet to be validated. In this review, we summarize some of the pathological mechanisms implicated in the sporadic AD and highlight the data for several established and novel fluid biomarkers (including BACE1, TREM2, YKL-40, IP-10, neurogranin, SNAP-25, synaptotagmin, α-synuclein, TDP-43, ferritin, VILIP-1, and NF-L) associated with each mechanism. We discuss the potential COUs for each biomarker.

Correlations between plasma and PET beta-amyloid levels in individuals with subjective cognitive decline: the Fundació ACE Healthy Brain Initiative (FACEHBI)

BACKGROUND

Peripheral biomarkers that identify individuals at risk of developing Alzheimer's disease (AD) or predicting high amyloid beta (Aβ) brain burden would be highly valuable. To facilitate clinical trials of disease-modifying therapies, plasma concentrations of Aβ species are good candidates for peripheral AD biomarkers, but studies to date have generated conflicting results.

METHODS

The Fundació ACE Healthy Brain Initiative (FACEHBI) study uses a convenience sample of 200 individuals diagnosed with subjective cognitive decline (SCD) at the Fundació ACE (Barcelona, Spain) who underwent amyloid florbetaben(18F) (FBB) positron emission tomography (PET) brain imaging. Baseline plasma samples from FACEHBI subjects (aged 65.9 ± 7.2 years) were analyzed using the ABtest (Araclon Biotech). This test directly determines the free plasma (FP) and total plasma (TP) levels of Aβ40 and Aβ42 peptides. The association between Aβ40 and Aβ42 plasma levels and FBB-PET global standardized uptake value ratio (SUVR) was determined using correlations and linear regression-based methods. The effect of the APOE genotype on plasma Aβ levels and FBB-PET was also assessed. Finally, various models including different combinations of demographics, genetics, and Aβ plasma levels were constructed using logistic regression and area under the receiver operating characteristic curve (AUROC) analyses to evaluate their ability for discriminating which subjects presented brain amyloidosis.

RESULTS

FBB-PET global SUVR correlated weakly but significantly with Aβ42/40 plasma ratios. For TP42/40, this observation persisted after controlling for age and APOE ε4 allele carrier status (R2 = 0.193, p = 1.01E-09). The ROC curve demonstrated that plasma Aβ measurements are not superior to APOE and age in combination in predicting brain amyloidosis. It is noteworthy that using a simple preselection tool (the TP42/40 ratio with an empirical cut-off value of 0.08) optimizes the sensitivity and reduces the number of individuals subjected to Aβ FBB-PET scanners to 52.8%. No significant dependency was observed between APOE genotype and plasma Aβ measurements (p value for interaction = 0.105).

CONCLUSION

Brain and plasma Aβ levels are partially correlated in individuals diagnosed with SCD. Aβ plasma measurements, particularly the TP42/40 ratio, could generate a new recruitment strategy independent of the APOE genotype that would improve identification of SCD subjects with brain amyloidosis and reduce the rate of screening failures in preclinical AD studies. Independent replication of these findings is warranted.

Diagnosis and Management of Neuropsychiatric Symptoms in Alzheimer's Disease

PURPOSE OF REVIEW

To explore the most recent developments in the effective diagnosis and treatment of neuropsychiatric symptoms (NPS) in Alzheimer's disease (AD).

RECENT FINDINGS

The clinical diagnosis of NPS in AD is facilitated by the use of the Neuropsychiatric Inventory (NPI). CT and MRI scans can be useful for detecting structural changes indicating AD. Other promising diagnostic methodologies that are less frequently used in the clinical setting include positron emission tomography (PET) scans for detecting amyloid and blood tests for detecting serum biomarkers. Numerous pharmaceutical agents have been studied for their use in managing NPS, with antipsychotics being popular for managing agitation but also having significant side effects. Non-pharmacological interventions, such as reminiscence therapy and the Describe, Investigate, Create, Evaluate (DICE) approach may be able to provide treatment without such adverse effects. Diagnosing AD and the comorbid NPS remains primarily a clinical endeavor with CT and MRI scans sometimes used, but evidence is amassing for the use of other imaging modalities and different lab tests for convenient and empiric diagnosis of AD to distinguish it from other psychiatric illnesses. The number of pharmacologic treatments for NPS that are safe as well as efficacious remains limited, yet non-pharmacologic interventions have clear clinical utility. In addition to searching for more successful pharmacological treatments, further research should focus on novel diagnostic tests and non-pharmacologic therapies.

Aging and APOE-ε4 are determinative factors of plasma Aβ42 levels

Objective

The aim of this study was to confirm determinative factors for plasma Aβ and its association with cognitive function.

Methods

Fasting plasma Aβ40 and Aβ42 levels were measured by ELISA in 1019 participants in the Iwaki Health Promotion Project. The relationships between plasma Aβ and health-related items, including physical characteristics, cognitive function tests, blood chemistry, and APOE-ε4 genotype were analyzed.

Results

The plasma levels of Aβ40 and Aβ42, and Aβ40/42 ratio were found to significantly increase with aging. The age-dependent increase in Aβ42 level was significantly suppressed by APOE-ε4. Renal function was an associated factor for the plasma Aβ40 level. The plasma Aβ42 level and Aβ40/42 ratio correlated with cognitive function.

Interpretation

Age and APOE-ε4 are major determinative factors of plasma levels of Aβ42 and the Aβ40/42 ratio. These factors are critical adjustment factors for the usage of plasma Aβ as a biomarker of central nervous system amyloidosis.