N-Terminally Truncated and Pyroglutamate-Modified Aβ Forms Are Measurable in Human Cerebrospinal Fluid and Are Potential Markers of Disease Progression in Alzheimer's Disease

Alzheimer's disease (AD) is a pathology characterized by the accumulation in the brain of intracellular and extracellular amyloid-beta (A beta) aggregates, especially of A beta 1-40 and A beta 1-42 peptides. It is known that N-terminally truncated or modified A beta forms also exist in AD brains and cerebrospinal fluid (CSF), and they play a key role in the pathogenesis of the disease. Herein, we developed an antibody-free method based on Solid-Phase Extraction and Electrospray Ionization Liquid Chromatography Mass Spectrometry for the identification and quantitation in human CSF of A beta isoforms. In human CSF, we could detect and quantify a panel of 19 A beta isoforms, including N-terminally truncated and pyroglutamate-modified forms, never quantified before in CSF. Among these, we identified novel N-terminally truncated A beta species: four bound to copper and two phosphorylated forms, which were found to be the most common proteoforms in human CSF along with A beta 1-40, A beta 3-40, and A beta pE11-42. We tested the newly developed and validated method in a pilot study on CSF from elderly individuals with subjective memory complaints (SMCs, n = 9), mild cognitive impairment (MCI, n = 18), and AD (n = 15); along with A beta 1-42, five N-terminally truncated forms (A beta 11-40, A beta 3-42, A beta pE11-42, A beta pE3-40, and A beta 4-40 Cu2+) are altered in AD/MCI. Thus, we demonstrated that N-terminally truncated and pyroglutamate-modified A beta can be quantified in human CSF, and five of them, along with A beta 1-42, are potential markers of AD progression. The described method could represent a useful tool for patients' stratification and monitoring. Moreover, the newly identified A beta CSF species might represent new potential therapeutic targets.

Automated summary

A newly developed antibody-free method was used to identify and quantify Aβ isoforms in human CSF, revealing the presence of novel Aβ species. Among these, five Aβ isoforms found in CSF of elderly individuals may serve as potential markers for Alzheimer's disease progression.