Proteomic Profiling of Plasma and Brain Tissue from Alzheimer's Disease Patients Reveals Candidate Network of Plasma Biomarkers

Abstract

Background: Alzheimer's disease (AD) is the most prevalent form of dementia with two pathological hallmarks of tau-containing neurofibrillary tangles and amyloid-β protein (Aβ)-containing neuritic plaques. Although Aβ and tau have been explored as potential biomarkers, levels of these pathological proteins in blood fail to distinguish AD from healthy control subjects.

Objective: We aim to discover potential plasma proteins associated with AD pathology by performing tandem mass tag (TMT)-based quantitative proteomic analysis of proteins from peripheral and central nervous system compartments.

Methods: We performed comparative proteomic analyses of plasma collected from AD patients and cognitively normal subjects. In addition, proteomic profiles from the inferior frontal cortex, superior frontal cortex, and cerebellum of postmortem brain tissue from five AD patients and five non-AD controls were compared with plasma proteomic profiles to search for common biomarkers. Liquid chromatography-mass spectrometry was used to analyze plasma and brain tissue labeled with isobaric TMT for relative protein quantification.

Results: Our results showed that the proteins in complement coagulation cascade and interleukin-6 signaling were significantly altered in both plasma and brains of AD patients.

Conclusion: Our results demonstrate the relevance in immune responses between the peripheral and central nervous systems. Those differentially regulated plasma proteins are explored as candidate biomarker profiles that illustrate chronic neuroinflammation in brains of AD patients.

Keywords: Alzheimer’s disease; biomarker; mass spectrometry; plasma; quantitative proteomics; tandem mass tag (TMT).

Fig. 1.

Functional enrichment in the networks of up/downregulated human plasma proteins from 18 AD patients using STRING. Results were obtained from two independent experiments. Biological Process (GO) of acute-phase response and acute inflammatory response (red), KEGG pathways of complement and coagulation cascades (blue), cholesterol metabolism (green), and Reactome pathways of platelet degranulation (yellow) were indicated on the plot.

Fig. 2.

Gene ontology enrichment analysis of up- and downregulated plasma proteins in three categories: A) biological processes, B) molecular function, and C) cellular components. The percentages indicate the enriched proteins among all up- or downregulated proteins. The enrichment analysis was performed using DAVID.

Fig. 3.

Metacore analysis reveals the top scored network of IL-6 pathway. Thick cyan lines indicate the fragments of canonical pathways. The acute-phase response containing IL-6 pathway was enriched.

Fig. 4.

Metacore analysis reveals the top scored network of acute-phase response pathways. Thick cyan lines indicate the fragments of canonical pathways. The acute-phase response containing LRG, lysozyme, and G-CSF pathways was enriched.

Fig. 5.

Comparison of GO functional enrichment analysis between up/downregulated plasma proteins from 18 AD patients and those from three brain regions (SF, IF, and CRLM) of 5 AD patients (p < 0.05). A) GO process. B) GO molecular function (SF, superior frontal cortex; IF, inferior frontal cortex; CRLM, cerebellum).

Fig. 6.

Comparison of the enriched process networks between plasma and brain proteins. Up/downregulated plasma proteins from 18 AD patients were compared to those from two AD brain regions (SF and IF) of five AD patients (p < 0.05). (SF, superior frontal cortex; IF, inferior frontal cortex). Plasma proteins enriched in the inflammation/complement system include CRP, C4a, C8γ, MBL2, Ficolin/H-Ficolin, C9, and FHR-3; IL-6 signal components include ORM1, SAA2, A2M, CRP, APCS, HP, ACT, SAA4, and SAA1; immune response/phagocytosis system include SAA2, CRP, HDL proteins, APOC3, MBL2; blood coagulation components include SAA2, A2M, Coagulation factor XII, Protein C inhibitor, SAA1, and Coagulation factor XIII A.

Fig. 7.

ELISA-based quantification of IL-6 and SAA in plasma from AD and control subjects. The standard error of means (short lines) and means (long middle line) are illustrated for each group. The differences between AD and control subjects are statistically significant (p < 0.05).