A Promising Perspective for Pathologies Diagnosis by MALDI In-Source Decay Imaging with a FTMS System.

Introduction
MALDI imaging mass spectrometry has proven to be effective for the discovery and the monitoring of disease-related proteins. With this technique a molecular diagnosis could be done directly on tissue sections in the environment of the diseased area. The use of in-source decay (ISD), that does allow fast and reliable sequences assignments of proteins termini, is a crucial tool for the identification of known biomarkers during MALDI imaging experiments. Combined with ultra-high mass resolution and high mass measurement accuracy of Fourier transform ion-cyclotron (FTICR) mass spectrometry, it is possible to unambiguously assign sequences of proteins present in tissue slices.
In this study, we have shown that FTICR mass spectrometry could be a powerful tool to diagnose pathologies by MALDI-ISD imaging.

Methods
All measurements were carried out on a SolariX FTMS (9.4 tesla) equipped with a Dual Source including smartbeamTMII laser which includes a robust solid state 1 kHz laser with advanced optics for molecular imaging (Bruker Daltonics). Lysozyme (14.3-kDa) or Human Serum Albumin (66.3-kDa) solution (1 mg/ml in 0.1 % TFA) was mixed with 1,5-diaminonaphthalene (DAN) and analyzed by MALDI-ISD and MALDI-ISD imaging. Mouse brain and rabbit eye tissue slices were washed (fixed) to obtain optimal sensitivity and high-quality ion. Before DAN application with an ImagePrep (Bruker Daltonics) and MALDI-ISD imaging analyzes, spots of myelin and crystalline were deposited near mouse brain or rabbit eye tissues, respectively. Results were interpreted using BioToolsTM 3.2 in combination with MascotTM (Matrix Science) for ISD spectra and FlexImagingTM 2.1 for MALDI-ISD imaging experiments.
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Preliminary data
The studies were carried out by MALDI-ISD and MALDI-ISD imaging analyses to evidence the interest on FTICR mass spectrometer for proteins identification in the field of biomarkers characterization.
It is demonstrated that protein ISD leads to the same pattern of fragmentation observed during MALDI-TOF analyzes. Fragmentation generates cn- and zn-series ions of lysozyme and HSA in presence of DAN. Supplementary an-, bn-, xn- and yn-series ions can also be observed. The internal calibration of all the data provides a mass accuracy neighboring 2.5 ppm over the m/z range of interest (300-2500 Da) and a mass resolution of 70000 at m/z 400 Da. It allows the assignment of ISD fragments of proteins, in the low mass range (m/z between 300 and 900), whether from pure solutions or included in tissue slices. Moreover, spots of pure proteins solution (myelin or crystalline) near tissue slices allows to unambiguously validate the proteins identification during MALDI ISD imaging experiments.

Novel aspect
This study evidences the main input of FTICR mass spectrometer for pathologies diagnosis based on biomarkers localization and identification by MALDI-ISD imaging.