Semi‐quantitative assay for neurogranin peptides using IP‐MALDI‐MS

Abstract

AbstractBackgroundNeurogranin (Ng) is a cerebrospinal fluid (CSF) biomarker for synaptic dysfunction at early stage of Alzheimer’s disease (AD). It has been reported that a variety of truncated peptides from Ng is increased in AD brain tissue. However, the clinical significance of Ng peptides in plasma has remained unclear. We have developed a simultaneous detection method of a plurality of amyloid βpeptides in plasma by using immunoprecipitation combined with matrix‐assisted laser desorption ionization time‐of‐flight mass spectrometry (IP‐MALDI‐MS) (Kaneko et al., Proc Jpn Acad Ser B Phys Biol Sci. 2014). In this study, we applied the IP‐MALDI‐MS technique to Ng measurement.MethodNg peptides and proteins were immunoprecipitated from human plasma or standard sample spiked by stable isotope‐labeled (SIL) Ng50‐78 using magnetic beads coupled with anti‐Ng monoclonal antibody. After the IP was performed two times consecutively, Ng proteins and peptides eluted from antibody beads were applied on the MALDI plate. SIL‐Ng50‐78 was used as an internal standard for normalizing an intensity of each Ng peptide.ResultWe optimized the condition such as a kind of detergent and an amount of antibody beads in IP for enhancing Ng signal intensity in MS. In the examination of detergents, UDM showed the highest sensitivity among six tested detergents. Using IP‐MALDI‐MS, we tried to detect Ng peptides and proteins in human plasma. As a result, some peaks corresponding to a variety of Ng peptides (m/z m/z > 5000.0) were detected in a plasma sample. To evaluate our semi‐quantitative assay using the internal standard, we selected some Ng peptides with low and high mass (m/z 2331.5–4084.7) to examine a linearity of the normalized intensity of Ng peptides. The accuracy in linear regression model showed within 80‐120% over a range of 5–160 pM in Ng48‐75 and 10‐160 pM in Ng50–78 and Ng33–75. Intra assay precision of Ng peptides in plasma was less than 20%CV.ConclusionIP‐MALDI‐MS enable the highly sensitive and simultaneous semi‐quantitative measurement of multiple Ng peptides levels. It could be helpful in the analysis of plasma Ng peptides for AD biomarker.