Amyloid β concentrations and stable isotope labeling kinetics of human plasma specific to central nervous system amyloidosis

Abstract

Introduction: Cerebrospinal fluid analysis and other measurements of amyloidosis, such as amyloid-binding positron emission tomography studies, are limited by cost and availability. There is a need for a more practical amyloid β (Aβ) biomarker for central nervous system amyloid deposition.

Methods: We adapted our previously reported stable isotope labeling kinetics protocol to analyze the turnover kinetics and concentrations of Aβ38, Aβ40, and Aβ42 in human plasma.

Results: Aβ isoforms have a half-life of approximately 3 hours in plasma. Aβ38 demonstrated faster turnover kinetics compared with Aβ40 and Aβ42. Faster fractional turnover of Aβ42 relative to Aβ40 and lower Aβ42 and Aβ42/Aβ40 concentrations in amyloid-positive participants were observed.

Discussion: Blood plasma Aβ42 shows similar amyloid-associated alterations as we have previously reported in cerebrospinal fluid, suggesting a blood-brain transportation mechanism of Aβ. The stability and sensitivity of plasma Aβ measurements suggest this may be a useful screening test for central nervous system amyloidosis.

Keywords: Amyloid β; Aβ42; Human; Kinetics; Plasma; Turnover.

Figure 1. Plasma Aβ SILK for Aβ38, Aβ40, and Aβ42

(A): Average isotopic enrichment time course profiles normalized to plasma leucine for plasma Aβ38 (blue), Aβ40 (green), and Aβ42 (red) (mean +/− 95% CI) by labeling protocol (left: IV bolus; right: oral). Kinetic profiles of all three isoforms appear similar between labeling protocols, with Aβ38 reaching its labeling peak before Aβ40 and Aβ42. (B): Average isotopic enrichment ratios for plasma Aβ38/Aβ40 displaying both amyloid groups on the same plot (blue, amyloid negative; red, amyloid positive) (mean +/− 95% CI) demonstrates similar rates of plasma Aβ38/Aβ40 turnover regardless of amyloid status or labeling protocol (left, IV bolus; right, oral). (C): Average isotopic enrichment ratios for plasma Aβ42/Aβ40 displaying both amyloid groups on the same plot (blue, amyloid negative; red, amyloid positive) highlights the faster Aβ42 turnover kinetics in the amyloid positive group (mean +/− 95% CI) for both the IV-bolus (left) and oral-labeled groups (right).

Figure 2. Absolute concentrations of Aβ by clinical group

(A): Absolute concentrations of Aβ42/Aβ40 over time averaged by clinical group (blue, amyloid negative; red, amyloid positive) (mean +/− 95% CI). Aβ42/Aβ40 concentrations were 10–15% lower in the amyloid positive group compared to the amyloid negative group at all time points measured. (B): Absolute concentrations of Aβ42 over time averaged by clinical group (blue, amyloid negative; red, amyloid positive) (mean +/− 95% CI). Aβ42 concentrations were 10–15% lower in the amyloid positive group compared to the amyloid negative group at all time points measured. (C): Absolute concentrations of Aβ42/Aβ40 over time with individual participant time courses illustrates the consistency of concentration measurements (blue, amyloid negative; red, amyloid positive).

Figure 3. Absolute concentrations of Aβ42/Aβ40 remain relatively stable over time with measurable separation of clinical groups

(A): Average Aβ42/Aβ40 concentrations for each participant at all time points separated by amyloid status (blue, amyloid negative; red, amyloid positive) demonstrate the stability and reproducibility of this measurement over time. (B): Aβ42/Aβ40 concentrations by amyloid status as an average of all time points (0 – 24 hours). On average, the Aβ42/Aβ40 concentration was 0.1297 +/− 0.0033 in the amyloid negative group (blue) and 0.1111 +/− 0.0019 in the amyloid positive group (red). This reflects a 14.3% lower Aβ42/Aβ40 concentration in amyloid positive individuals compared to amyloid negative individuals overall (p value < 0.0001, mean +/− 95% CI shown).

Figure 4. Plasma Aβ42/Aβ40 concentrations correlate with two different measures of amyloidosis

Plasma Aβ42/Aβ40 concentration ratios are lower in amyloid positive individuals (red) compared to amyloid negative individuals (blue) when amyloid status is determined by CSF Aβ42/Aβ40 concentrations. The relationship between decreased Aβ42/Aβ40 in both blood and CSF in the presence of amyloidosis has a correlation coefficient of 0.6999, indicating a strong positive correlation between these measurements (p < 0.0001). Similarly, plasma Aβ42/Aβ40 concentration ratios are lower in amyloid positive individuals (red) compared to amyloid negative individuals (blue) when amyloid status is determined by [¹¹C]PIB-PET imaging with amyloid positive individuals having a mean cortical binding potential (MCBP) > 0.18. While all participants with known amyloidosis by PIB-PET had correspondingly low plasma Aβ42/Aβ40 measurements, several participants classified as amyloid negative by PIB-PET were also found to have similarly low plasma Aβ42/Aβ40 values (below the threshold of 0.1243).

Figure 5. ROC curve

ROC curve analysis using average plasma Aβ42/Aβ40 concentration ratios over 24 hours demonstrates an AUC of 0.8865, indicating this assay has good accuracy as a diagnostic test to detect amyloidosis.