Ex vivo 18O-labeling mass spectrometry identifies a peripheral amyloid ß clearance pathway.

ABSTRACT

BACKGROUND: Proteolytic degradation of amyloid ß (Aß) peptides has been intensely studied due to the central role of Aß in Alzheimer's disease (AD) pathogenesis. While several enzymes have been shown to degrade Aß peptides, the main pathway of Aß degradation in vivo is unknown. Cerebrospinal fluid (CSF) Aß42 is reduced in AD, reflecting aggregation and deposition in the brain, but low CSF Aß42 is, for unknown reasons, also found in some inflammatory brain disorders such as bacterial meningitis. METHOD: Using 18O-labeling mass spectrometry and immune-affinity purification, we examined endogenous proteolytic processing of Aß in human CSF. RESULTS: The Aß peptide profile was stable in CSF samples from healthy controls but in CSF samples from patients with bacterial meningitis, showing increased leukocyte cell count, 18O-labeling mass spectrometry identified proteolytic activities degrading Aß into several short fragments, including abundant Aß1-19 and 1-20. After antibiotic treatment, no degradation of Aß was detected. In vitro experiments located the source of the proteolytic activity to blood components, including leukocytes and erythrocytes, with insulin-degrading enzyme as the likely protease. A recombinant version of the mid-domain anti-Aß antibody solanezumab was found to inhibit insulin-degrading enzyme-mediated Aß degradation. CONCLUSION: 18O labeling-mass spectrometry can be used to detect endogenous proteolytic activity in human CSF. Using this technique, we found an enzymatic activity that was identified as insulin-degrading enzyme that cleaves Aß in the mid-domain of the peptide, and could be inhibited by a recombinant version of the mid-domain anti-Aß antibody solanezumab.