Abstract :
[en] Since its first implementation in 1997, MALDI Mass Spectrometry Imaging (MALDI MSI) has become an important tool in the proteomic arsenal, especially for biomarker hunting. First dedicated to high molecular weight, MALDI MSI is more and more used to map the distribution of small molecules too (lipids, drugs and metabolites,…). Last developments tend to improve the sample treatments to obtain the best spatial resolution as possible. From this perspective, great efforts have been made on the MALDI matrix deposition methods.
Now, one of the remaining challenges for MALDI-MSI users consists of identification of detected molecules. For high molecular weight, methods inspired by classical proteomics techniques, are regularly used. Bottom-Up (PMF obtained after in situ trypsin digestion) and Top-Down (in situ In-Source Decay) approaches have been used directly from a tissue slice, leading to the identification of some of the most abundant proteins present at the surface of the tissue. When small molecules are analyzed, the identification is more straightforward. Indeed, tandem mass spectrometry can easily be used, leading to the fragmentation of the detected compounds which allows their unambiguous identification. This identification is even more reliable when high resolution exact mass measurements can be performed.
In this talk, I will present how in our lab, we profit of the exceptional features of FT-ICR mass spectrometry for imaging and especially for identification purposes. The first example will deal with the benefit of high mass accuracy and high mass resolution for ISD-based protein identification. The mass accuracy and high mass resolution coupled with the use of a “cleaning” software allow unequivocal assignment of ISD fragments of proteins, in the low mass range (m/z between 300 and 900), whether from pure solutions or from tissue slices. The next examples will deal with the imaging of small molecules. The identification of drugs and their metabolites is facilitated with high mass accuracy. In our lab, we work on the localization of methadone and its first metabolite, EDDP in necrophagous fly larvae. In the mass range of these compounds (278-310 m/z), many matrix ion peaks are detected and the unique features of FT-ICR allows for unambiguous identification thanks to exact mass measurements. We also use MALDI Imaging to map the messenger molecules between plant roots and beneficial bacteria. The comparison of spectra recorded with a TOF/TOF instrument and with a FT-ICR demonstrates that high resolution allows for detecting molecules which could have been missed otherwise. It also allows to distinguish unknown compounds from alkali adducts of known molecules.
