Potential sources of interference on Abeta immunoassays in biological samples

Development and Implementation of an Internal Quality Control Sample to Standardize Oligomer-Based Diagnostics of Alzheimer's Disease

Protein misfolding and aggregation are pathological hallmarks of various neurodegenerative diseases. In Alzheimer's disease (AD), soluble and toxic amyloid-β (Aβ) oligomers are biomarker candidates for diagnostics and drug development. However, accurate quantification of Aβ oligomers in bodily fluids is challenging because extreme sensitivity and specificity are required. We previously introduced surface-based fluorescence intensity distribution analysis (sFIDA) with single-particle sensitivity. In this report, a preparation protocol for a synthetic Aβ oligomer sample was developed. This sample was used for internal quality control (IQC) to improve standardization, quality assurance, and routine application of oligomer-based diagnostic methods. We established an aggregation protocol for Aβ1-42, characterized the oligomers by atomic force microscopy (AFM), and assessed their application in sFIDA. Globular-shaped oligomers with a median size of 2.67 nm were detected by AFM, and sFIDA analysis of the Aβ1-42 oligomers yielded a femtomolar detection limit with high assay selectivity and dilution linearity over 5 log units. Lastly, we implemented a Shewhart chart for monitoring IQC performance over time, which is another important step toward quality assurance of oligomer-based diagnostic methods.

The Role of Amyloid-β, Tau, and α-Synuclein Proteins as Putative Blood Biomarkers in Patients with Cerebral Amyloid Angiopathy

BACKGROUND

Cerebral amyloid angiopathy (CAA) is a cerebrovascular disorder characterized by the deposition of amyloid-β protein (Aβ) within brain blood vessels that develops in elderly people and Alzheimer's disease (AD) patients. Therefore, the investigation of biomarkers able to differentiate CAA patients from AD patients and healthy controls (HC) is of great interest, in particular in peripheral fluids.

OBJECTIVE

The current study aimed to detect the neurodegenerative disease (ND)-related protein (i.e., Aβ 1 - 40, Aβ 1 - 42, tau, and α-synuclein) levels in both red blood cells (RBCs) and plasma of CAA patients and HC, evaluating their role as putative peripheral biomarkers for CAA.

METHODS

For this purpose, the proteins' concentration was quantified in RBCs and plasma by homemade immunoenzymatic assays in an exploratory cohort of 20 CAA patients and 20 HC.

RESULTS

The results highlighted a significant increase of Aβ 1 - 40 and α-synuclein concentrations in both RBCs and plasma of CAA patients, while higher Aβ 1 - 42 and t-tau levels were detected only in RBCs of CAA individuals compared to HC. Moreover, Aβ 1 - 42/Aβ 1 - 40 ratio increased in RBCs and decreased in plasma of CAA patients. The role of these proteins as candidate peripheral biomarkers easily measurable with a blood sample in CAA needs to be confirmed in larger studies.

CONCLUSION

In conclusion, we provide evidence concerning the possible use of blood biomarkers for contributing to CAA diagnosis and differentiation from other NDs.

Quantitative detection of α-Synuclein and Tau oligomers and other aggregates by digital single particle counting

The pathological hallmark of neurodegenerative diseases is the formation of toxic oligomers by proteins such as alpha-synuclein (aSyn) or microtubule-associated protein tau (Tau). Consequently, such oligomers are promising biomarker candidates for diagnostics as well as drug development. However, measuring oligomers and other aggregates in human biofluids is still challenging as extreme sensitivity and specificity are required. We previously developed surface-based fluorescence intensity distribution analysis (sFIDA) featuring single-particle sensitivity and absolute specificity for aggregates. In this work, we measured aSyn and Tau aggregate concentrations of 237 cerebrospinal fluid (CSF) samples from five cohorts: Parkinson's disease (PD), dementia with Lewy bodies (DLB), Alzheimer's disease (AD), progressive supranuclear palsy (PSP), and a neurologically-normal control group. aSyn aggregate concentration discriminates PD and DLB patients from normal controls (sensitivity 73%, specificity 65%, area under the receiver operating curve (AUC) 0.68). Tau aggregates were significantly elevated in PSP patients compared to all other groups (sensitivity 87%, specificity 70%, AUC 0.76). Further, we found a tight correlation between aSyn and Tau aggregate titers among all patient cohorts (Pearson coefficient of correlation r = 0.81). Our results demonstrate that aSyn and Tau aggregate concentrations measured by sFIDA differentiate neurodegenerative disease diagnostic groups. Moreover, sFIDA-based Tau aggregate measurements might be particularly useful in distinguishing PSP from other parkinsonisms. Finally, our findings suggest that sFIDA can improve pre-clinical and clinical studies by identifying those individuals that will most likely respond to compounds designed to eliminate specific oligomers or to prevent their formation.

Establishing pre-analytical requirements and maximizing peptide recovery in the analytical phase for mass spectrometric quantification of amyloid-β peptides 1-42 and 1-40 in CSF

OBJECTIVES

Amyloid-β (Aβ) peptides in cerebrospinal fluid (CSF), including Aβ42 (residues 1-42) and Aβ40 (residues 1-40), are utilized as biomarkers in the diagnostic workup of Alzheimer's disease. Careful consideration has been given to the pre-analytical and analytical factors associated with measurement of these peptides via immunoassays; however, far less information is available for mass spectrometric methods. As such, we performed a comprehensive evaluation of pre-analytical and analytical factors specific to Aβ quantification using mass spectrometry.

METHODS

Using our quantitative mass spectrometry assay for Aβ42 and Aβ40 in CSF, we investigated the potential for interference from hemolysate, bilirubin, lipids, and anti-Aβ-antibodies. We also optimized the composition of the calibrator surrogate matrix and Aβ recovery during and after solid phase extraction (SPE).

RESULTS

There was no interreference observed with total protein up to 12 g/L, hemolysate up to 10% (v/v), bilirubin up to 0.5% (v/v), intralipid up to 1% (v/v), or anti-Aβ-antibodies at expected therapeutic concentrations. For hemolysate, bilirubin and lipids, visual CSF contamination thresholds were established. In the analytical phase, Aβ recovery was increased by ∼50% via SPE solvent modifications and by over 150% via modification of the SPE collection plate, which also extended analyte stability in the autosampler.

CONCLUSIONS

Attention to mass spectrometric-specific pre-analytical and analytical considerations improved analytical sensitivity and reproducibility, as well as, established CSF specimen acceptance and rejection criteria for use by the clinical laboratory.

Detection of amyloid beta peptides in body fluids for the diagnosis of alzheimer's disease: Where do we stand?

Alzheimer's disease (AD) is an incurable neurodegenerative disease characterized by progressive decline of cognitive abilities. Amyloid beta peptides (Aβ), Tau proteins and the phosphorylated form of the Tau protein, p-Tau, are the core pathological biomarkers of the disease, and their detection for the diagnosis of patients is progressively being implemented. However, to date, their quantification is mostly performed on cerebrospinal fluid (CSF), the collection of which requires an invasive lumbar puncture. Early diagnosis has been shown to be important for disease-modifying treatment, which is currently in development, to limit the progression of the disease. Nevertheless, the diagnosis is often delayed to the point where the disease has already progressed, and the tools currently available do not allow for a systematic follow-up of patients. Thus, the search for a molecular signature of AD in a body fluid such as blood or saliva that can be collected in a minimally invasive way offers hope. A number of methods have been developed for the quantification of core biomarkers, especially in easily accessible fluids such as the blood, that improve their accuracy, specificity and sensitivity. This review summarizes and compares these approaches, focusing in particular on their use for Aβ detection, the earliest biomarker to be modified in the course of AD. The review also discusses biomarker quantification in CSF, blood and saliva and their clinical applications.

The Effect of Propofol Versus Isoflurane Anesthesia on Human Cerebrospinal Fluid Markers of Alzheimer's Disease: Results of a Randomized Trial

BACKGROUND

Preclinical studies have found differential effects of isoflurane and propofol on the Alzheimer's disease (AD)-associated markers tau, phosphorylated tau (p-tau) and amyloid-β (Aβ).

OBJECTIVE

We asked whether isoflurane and propofol have differential effects on the tau/Aβ ratio (the primary outcome), and individual AD biomarkers. We also examined whether genetic/intraoperative factors influenced perioperative changes in AD biomarkers.

METHODS

Patients undergoing neurosurgical/otolaryngology procedures requiring lumbar cerebrospinal fluid (CSF) drain placement were prospectively randomized to receive isoflurane (n = 21) or propofol (n = 18) for anesthetic maintenance. We measured perioperative CSF sample AD markers, performed genotyping assays, and examined intraoperative data from the electronic anesthesia record. A repeated measures ANOVA was used to examine changes in AD markers by anesthetic type over time.

RESULTS

The CSF tau/Aβ ratio did not differ between isoflurane- versus propofol-treated patients (p = 1.000). CSF tau/Aβ ratio and tau levels increased 10 and 24 h after drain placement (p = 2.002×10-6 and p = 1.985×10-6, respectively), mean CSF p-tau levels decreased (p = 0.005), and Aβ levels did not change (p = 0.152). There was no interaction between anesthetic treatment and time for any of these biomarkers. None of the examined genetic polymorphisms, including ApoE4, were associated with tau increase (n = 9 polymorphisms, p > 0.05 for all associations).

CONCLUSION

Neurosurgery/otolaryngology procedures are associated with an increase in the CSF tau/Aβ ratio, and this increase was not influenced by anesthetic type. The increased CSF tau/Aβ ratio was largely driven by increases in tau levels. Future work should determine the functional/prognostic significance of these perioperative CSF tau elevations.

Amyloid Beta and Tau as Alzheimer's Disease Blood Biomarkers: Promise From New Technologies

The utility of the levels of amyloid beta (Aβ) peptide and tau in blood for diagnosis, drug development, and assessment of clinical trials for Alzheimer's disease (AD) has not been established. The lack of availability of ultra-sensitive assays is one critical issue that has impeded progress. The levels of Aβ species and tau in plasma and serum are much lower than levels in cerebrospinal fluid. Furthermore, plasma or serum contain high levels of assay-interfering factors, resulting in difficulties in the commonly used singulex or multiplex ELISA platforms. In this review, we focus on two modern immune-complex-based technologies that show promise to advance this field. These innovative technologies are immunomagnetic reduction technology and single molecule array technology. We describe the technologies and discuss the published studies using these technologies. Currently, the potential of utilizing these technologies to advance Aβ and tau as blood-based biomarkers for AD requires further validation using already collected large sets of samples, as well as new cohorts and population-based longitudinal studies.

Semi-continuous, real-time monitoring of protein biomarker using a recyclable surface plasmon resonance sensor

Although label-free immunosensors based on, for example, surface plasmon resonance (SPR) provide advantages of real-time monitoring of the analyte concentration, its application to routine clinical analysis in a semi-continuous manner is problematic because of the high cost of the sensor chip. The sensor chip is in most cases regenerated by employing an acidic pH. However, this causes gradual deterioration of the activity of the capture antibody immobilized on the sensor surface. To use sensor chips repeatedly, we investigated a novel surface modification method that enables regeneration of the sensor surface under mild conditions. We introduced a monoclonal antibody (anti-CBP Ab) that detects the conformational change in calcium binding protein (CBP) upon Ca²⁺ binding (>1mM). To construct a regenerable SPR-based immunosensor, anti-CBP Ab was first immobilized on the sensor surface, and CBP conjugated to the capture antibody (specific for creatine kinase-MB isoform (CK-MB); CBP-CAb) then bound in the presence of Ca²⁺. A serum sample was mixed with the detection antibody to CK-MB, which generated an SPR signal proportional to the analyte concentration. After each analysis, the sensor surface was regenerated using medium (pH 7) without Ca²⁺, and then adding fresh CBP-CAb in the presence of Ca²⁺ for the subsequent analysis. Analysis of multiple samples using the same sensor was reproducible at a rate >98.7%. The dose-response curve was linear for 1.75-500.75ng/mL CK-MB, with an acceptable coefficient of variation of <8.8%. The performance of the immunosensor showed a strong correlation with that of the Pathfast reference system (R²>96%), and exhibited analytical stability for 1 month. To our knowledge, this is the first report of a renewal of a sensor surface with fresh antibody after each analysis, providing high consistency in the assay during a long-term use (e.g., a month at least).

Elevation of the Plasma Aβ40/Aβ42 Ratio as a Diagnostic Marker of Sporadic Early-Onset Alzheimer's Disease

BACKGROUND

Although plasma amyloid-β (Aβ) levels have been evaluated as a possible diagnostic marker of Alzheimer's disease (AD), the findings are inconsistent.

OBJECTIVE

The present study aimed to validate plasma levels of Aβ40, Aβ42, and the Aβ40/Aβ42 ratio as biomarkers of AD in subjects with early-onset AD (EOAD) without familial AD genetic mutations.

METHODS

Patients with sporadic EOAD (sEOAD) were prospectively recruited by nine neurology clinics. Plasma levels of Aβ40 and Aβ42 were measured using a sandwich enzyme-linked immunosorbent assay (ELISA) in 100 sEOAD (50-69 year-old) and 46 age-matched normal control subjects (50-72 year-old). Cerebrospinal fluid (CSF) was obtained from 32 sEOAD subjects and 25 controls. The integrity of the blood-brain barrier was assessed using the CSF/plasma albumin ratio.

RESULTS

The plasma levels of Aβ42 were significantly lower, while the Aβ40/Aβ42 ratio was significantly higher in sEOAD patients than in controls. The levels of Aβ40, Aβ42, and the Aβ40/Aβ42 ratio did not differ in relation to the APOEɛ4 allele. The CSF/plasma albumin ratio was comparable between the two groups, and the plasma parameters of Aβ proteins were not significantly associated. A multivariate analysis revealed that an increased Aβ40/Aβ42 ratio is valuable for the discrimination of sEOAD from controls (β=0.344, p=0.000). The area under the ROC curve for the Aβ40/Aβ42 ratio was 0.76, and a cut-off ratio of 5.87 was suggested to have 70% sensitivity and 68% specificity.

CONCLUSION

The plasma Aβ40/Aβ42 ratio had moderate validity for the discrimination of sEOAD patients from age-matched controls.

Preclinical Alzheimer's disease: Definition, natural history, and diagnostic criteria

During the past decade, a conceptual shift occurred in the field of Alzheimer's disease (AD) considering the disease as a continuum. Thanks to evolving biomarker research and substantial discoveries, it is now possible to identify the disease even at the preclinical stage before the occurrence of the first clinical symptoms. This preclinical stage of AD has become a major research focus as the field postulates that early intervention may offer the best chance of therapeutic success. To date, very little evidence is established on this "silent" stage of the disease. A clarification is needed about the definitions and lexicon, the limits, the natural history, the markers of progression, and the ethical consequence of detecting the disease at this asymptomatic stage. This article is aimed at addressing all the different issues by providing for each of them an updated review of the literature and evidence, with practical recommendations.