Selected protein monitoring in histological sections by targeted MALDI-FTICR in-source decay imaging

Matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI MSI) is a rapidly growing method in biomedical research allowing molecular mapping of proteins on histological sections. The images can be analyzed in terms of spectral pattern to define regions of interest. However, the identification and the differential quantitative analysis of proteins require off line or in situ proteomic methods using enzymatic digestion. The rapid identification of biomarkers holds great promise for diagnostic research, but the major obstacle is the absence of a rapid and direct method to detect and identify with a sufficient dynamic range a set of specific biomarkers. In the current work, we present a proof of concept for a method allowing one to identify simultaneously a set of selected biomarkers on histological slices with minimal sample treatment using in-source decay (ISD) MSI and MALDI-Fourier transform ion cyclotron resonance (FTICR). In the proposed method, known biomarkers are spotted next to the tissue of interest, the whole MALDI plate being coated with 1,5-diaminonaphthalene (1,5-DAN) matrix. The latter enhances MALDI radical-induced ISD, providing large tags of the amino acid sequences. Comparative analysis of ISD fragments between the reference spots and the specimen in imaging mode allows for unambiguous identification of the selected biomarker while preserving full spatial resolution. Moreover, the high resolution/high mass accuracy provided by FTICR mass spectrometry allows the identification of proteins. Well-resolved peaks and precise measurements of masses and mass differences allow the construction of reliable sequence tags for protein identification. The method will allow the use of MALDI-FTICR MSI as a method for rapid targeted biomarker detection in complement to classical histology.