Decreased expression and increased oxidation of plasma haptoglobin in Alzheimer disease: Insights from redox proteomics

Alzheimer disease (AD) is one of the most disabling disorders of the elderly and the number of people worldwide facing dementia is expected to dramatically increase in the near future. Thus, one of the major concerns of modern society is to identify putative biomarkers that serve as a valuable early diagnostic tool to identify a subset of patients with increased risk to develop AD. An ideal biomarker should be present in blood before dementia is clinically confirmed, have high sensitivity and specificity, and be reproducible. Proteomics platforms offer a powerful strategy to reach these goals and recently have been demonstrated to be promising approaches. However, the high variability of technologies and studied populations has led to contrasting results. To increase specificity, we analyzed both protein expression profiles and oxidative modifications (carbonylation) of plasma proteins in mild cognitive impairment (MCI) and AD subjects compared with age-matched controls. Most of the proteins found to have differential levels in MCI and AD confirmed results already obtained in other cohort studies. Interestingly, we applied for the first time in MCI a redox proteomics approach to specifically identify oxidized proteins. Among them, haptoglobin, one of the most abundantly secreted glycoproteins with chaperone function, was found to be either increasingly downregulated or increasingly oxidized in AD and MCI compared with controls. We also demonstrated that in vitro oxidation of haptoglobin affects the formation of amyloid-β fibrils, thus suggesting that oxidized haptoglobin is not able to act as an extracellular chaperone to prevent or slow formation of amyloid-β aggregates. Another chaperone protein, α2-macroglobulin, was found to be selectively oxidized in AD patients compared with controls. Our findings suggest that alterations in proteins acting as extracellular chaperones may contribute to exacerbating amyloid-β toxicity in the peripheral system and may be considered a putative marker of disease progression.