Toward Mass Spectrometry Imaging in the Metabolomics Scale: Increasing Metabolic Coverage Through Multiple On-Tissue Chemical Modifications

Abstract

Exploring the metabolic differences directly on tissues is essential for the comprehensive understanding of how multicellular organisms function. Mass spectrometry imaging (MSI) is an attractive technique toward this goal; however, MSI in metabolomics scale has been hindered by multiple limitations. This is most notable for single cell level high-spatial resolution imaging because of the limited number of molecules in small sampling size and the low ionization yields of many metabolites. Several on-tissue chemical derivatization approaches have been reported to increase MSI signals of targeted compounds, especially in matrix-assisted laser desorption/ionization (MALDI)-MSI. Herein, we adopt a combination of chemical derivatization reactions, to selectively enhance metabolite signals of a specific functional group for each consecutive tissue section. Three well-known on-tissue derivatization methods were used as a proof of concept experiment: coniferyl aldehyde for primary amines, Girard’s reagent T for carbonyl groups, and 2-picolylamine for carboxylic acids. This strategy was applied to the cross-sections of leaves and roots from two different maize genotypes (B73 and Mo17), and enabled the detection of over six hundred new unique metabolite features compared to without modification. Statistical analysis indicated quantitative variation between metabolites in the tissue sections, while MS images revealed differences in localization of these metabolites. Combined, this untargeted approach facilitated the visualization of various classes of compounds, demonstrating the potential for untargeted MSI in the metabolomics scale.

Recent developments of novel matrices and on-tissue chemical derivatization reagents for MALDI-MSI

“…3 Recent developments of novel matrices and on-tissue chemical derivatization reagents for MALDI-MSI CA was also used by Esteve and coworkers [61] for the comparison with THAS and DPP-TFB. Dueñas and coworkers [62] also applied CA for a proof of concept experiment, in which various classes of compounds containing a primary amine group were visualized in tissue sections of maize leaves and roots. Guo and coworkers [63] combined a laser-assisted tissue transfer (LATT) technique with the CA derivatization for imaging of up to 67 small molecule metabolites including amino acids, neurotransmitters and dipeptides, and others in brain tissue sections of rats.…”

Plant Single-Cell Metabolomics—Challenges and Perspectives

“…That said, matrix optimization is an active field in technological developments for MALDI imaging applications [43]. Several works have described matrix optimization for specific compound classes [44], as well as exciting approaches to expand the coverage based on derivatizations [45] and post-ionization strategies [46]. Another recent trend involves using nanoparticles instead of organic matrixes, and it shows promising results for ionizing the smaller range of metabolites and providing increased spatial resolution [47,48].…”

Metabolomics meets lipidomics: Assessing the small molecule component of metabolism

“…Major improvements in TA B L E 1 Technological platforms used to resolve polar and lipid metabolite diversity. The most common and successful applications are described [31][32][33][34][35][36] Flow [37] Complex lipids: Glycerolipids, glycerophospholipids, sphingolipids [38][39][40] Surface [41][42][43] Fatty acids Complex lipids: Glycerolipids, glycerophospholipids, sphingolipids [44][45][46][47][48] Gas [49][50][51] Fatty acids, oxysterols, oxylipins, steroids, aldehydes [24,[52][53][54][55][56] Liquid [57][58][59][60][61] Fatty acids, oxysterols, oxylipins, steroids, acyl carnitines [56,[62][63][64] Complex lipids: Glycerolipids, glycerophospholipids, sphingolipids [65,66] Capillary electrophoresis mass spectrometry (CE-MS) [67][68][69][70]…”