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WIREs Syst Biol Med
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Imaging mass spectrometry for metabolites: technical progress, multimodal imaging, and biological interactions

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Imaging mass spectrometry (IMS) allows the study of the spatial distribution of small molecules in biological samples. IMS is able to identify and quantify chemicals in situ from whole tissue sections to single cells. Both vacuum mass spectrometry (MS) and ambient MS systems have advanced considerably over the last decade; however, some limitations are still hard to surmount. Sample pretreatment, matrix or solvent choices, and instrument improvement are the key factors that determine the successful application of IMS to different samples and analytes. IMS with innovative MS analyzers, powerful MS spectrum databases, and analysis tools can efficiently dereplicate, identify, and quantify natural products. Moreover, multimodal imaging systems and multiple MS‐based systems provide additional structural, chemical, and morphological information and are applied as complementary tools to explore new fields. IMS has been applied to reveal interactions between living organisms at molecular level. Recently, IMS has helped solve many previously unidentifiable relations between bacteria, fungi, plants, animals, and insects. Other significant interactions on the chemical level can also be resolved using expanding IMS techniques. WIREs Syst Biol Med 2017, 9:e1387. doi: 10.1002/wsbm.1387 This article is categorized under: Analytical and Computational Methods > Analytical Methods Laboratory Methods and Technologies > Imaging Laboratory Methods and Technologies > Metabolomics
Workflow of imaging mass spectrometry (IMS) coupled with liquid chromatography‐mass spectrometry (LC‐MS)/mass spectrometry (MS) for metabolomics. IMS offers a MS spectrum at every single location and LC‐MS/MS supports IMS with overall metabolite information of targets. MS and MS2 data are compared, dereplicated, and identified by networking tools and both authentic and in silico spectral databases.
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Schematic overview of ambient ionization mass spectrometry. (a) The concept and improvement of desorption electrospray ionization (DESI) and its parameters: sample effects, electrospray parameters, chemical parameters, and geometric parameters. (b) The concept and improvement of laser ablation electrospray ionization (LAESI) and its parameters: sample effects, electrospray parameters, and chemical parameters.
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Schematic overview of matrix deposition approaches for a matrix‐assisted laser desorption ionization (MALDI)‐time‐of‐flight (TOF)‐Imaging mass spectrometry (MS) platform. (a) Sieving matrix, matrix sprayers, and matrix sublimation were evaluated according to device cost, time (matrix deposition time‐consumption), versatility (sample types, and matrix choices), and resolution (increasing signal efficiency and matrix crystallite size). After comparison of these three deposition methods, the top 1 was assigned a score of 5, followed by scores of 4 and 3 for the next positions. We chose commercial matrix sprayers (ImagePrep and HTX TM‐Sprayer) as candidates for evaluating with other methods. (b) Schematic overview of nanomaterial substrates and nanostructural‐coated slides on a surface‐assisted laser desorption/ionization (SALDI) imaging MS system. The cross symbols represent analytes.
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Application of imaging mass spectrometry (IMS) to plant–insect interactions, mouse gut, and human surface microbiota. (a) Glucosinolate sinalbin (m/z 424) and an unknown metabolite (m/z 426) from Sinapis alba L. leaves were ingested by Athalia rosae larvae. IMS revealed the distribution of glucosinolate sinalbin and unknown metabolite inside the larvae after ingestion for 0 min, 5 min, 20 min, and 1 day. The intensity scales are represented as color‐coded relative intensities. (b) IMS of gut microbiota in germ‐free (GF), monoassociated Bacteroides thetaiotaomicron (Bt), and biassociated B. thetaiotaomicron plus Bifidobacterium longum (BtBl) mice. From top to bottom, representative metabolites are: choline (m/z 104), carnitine (m/z 217), sodiated hexuronix acid, and sodiated trihexose (m/z 527). (c) Representation of the microbial and molecular diversity of human (male) skin. Bacterial data reveals the microbial diversity according to 16S rRNA amplicon and molecular profiles were measured and identified by UPLC‐QTOF‐MS. For the color scale, blue represents the minimum value and red represents the maximum value. (Reprinted with permission from Refs . Copyright 2014 Elsevier Ltd. (a), . Copyright 2012 American Chemical Society (b), and . Copyright 2015 National Academy of Sciences (c))
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Application of imaging mass spectrometry (IMS) to plant–microbe interactions. (a) IMS of rice resistant and susceptible lines leaves infected by Xoo. From Left to right: Optical image of the rice leaf with a black circle representing the inoculated region, phosphocholine (Pcho), disaccharide (sucrose), chlorophyll‐a fragment, monogalactosyldiacylglycerol (MGDG), momilactone‐A, momilactone‐B, phytocassane‐A, D, or E, phytocassane‐B, and phytocassane‐C. The intensity scales are represented as color‐gradient relative intensities. (b) IMS of fungal pathogen inoculated pea pisatin and pinoresinol monoglucoside. The epidermal layer in the region of the right of the dotted line has been removed. Pisatin and pinoresinol monoglucoside presented in ratio of histology image layer (tissue image): IMS image data for image layer opacity at 100:100, 25:50, and 0:100 (from top to bottom). (c) IMS of maytansine in Putterlickia verrucosa root (X: xylem; C: cortex; P: periderm). 1. Transverse section of P. verrucosa primary root. 2. Distribution of maytansine in the cortex. 3. Distribution of phosphatidylcholine in the periderm. 4. Longitudinal section of P. verrucosa primary root. 5. Distribution of maytansine in the cortex. Black scale bars for panels 1–3 represent 500 µm and 4–5 represent 2000 µm. The intensity scales are represented as color‐gradient relative intensities. (Reprinted with permission from Refs . Copyright 2015 American Chemical Society. (a), . Copyright 2015 Elsevier Ltd. (b), and . Copyright 2014 The American Chemical Society and American Society of Pharmacognosy (c))
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Application of imaging mass spectrometry (IMS) to microbe‐microbe interactions. IMS of Streptomyces–Streptomyces (or Amycolatopsis) interactions. Desferrioxamines B (m/z 561) and E(m/z 601) and acyl‐desferrioxamines (m/z 701, 715, 729, 743, 757, 771, 785) are revealed by matrix‐assisted laser desorption/ionization (MALDI)‐IMS. M (S. coelicolor M145) represents only culture M145. A (Amycolatopsis sp. AA4), E (Streptomyces sp. E14), S (Streptomyces sp. SPB74), and V (S. viridochromogenes DSM 40736) represent the interaction with M145. Brightness indicates the intensity of the mass spectrometry (MS) signal. (Reprinted with permission from Ref . Copyright 2013 Traxler et al.)
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Selected examples of multimodal imaging mass spectrometry. (a) Confocal Raman microscopy (CRM) and secondary ion mass spectrometry (SIMS) imaging MS of quinolone molecules in Pseudomonas‐derived biofilms. (b) Microscopy and matrix‐assisted laser desorption ionization (MALDI) imaging mass spectrometry (MS) of lipids in mouse brain. (c) Magnetic resonance imaging (MRI) and desorption electrospray ionization (DESI) imaging MS of protonated arecoline in in areca nut.(d) DESI then MALDI‐IMS for lipid and protein distribution in mouse brain tissue. (e) MALDI‐guided SIMS for imaging of metabolites in bacterial biofilms. (f) Fluorescence imaging and MALDI‐IMS in Bacillus subtilis biofilms. The chemical information column presents additional chemical information by multimodal combination. The resolution information column presents additional resolution information by multimodal combination. Other columns present additional morphological, and gene expression information by multimodal combination. (Reprinted with permission from Refs . Copyright 2010 American Chemical Society (a), . Copyright 2015 Nature Publishing Group (b), . Copyright 2016 Elsevier Ltd. (c), . Copyright 2012 Elsevier Inc. (d), . Copyright 2014 American Chemical Society (e), and . Copyright 2016 American Chemical Society (f))
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Illustration of depositing internal standard (IS) methods and an IS‐free method for quantitative imaging mass spectrometry (IMS). (a) Depositing IS on the tissue section requires mounting the section followed by application of the IS. Deposition under the tissue section requires spotting the IS first, and mounting the section on top. Deposition in the tissue section requires homogenized tissue samples and then the internal standard is incorporated in the tissue homogenates. Deposition in solution requires that standard solution is directly spotted on the sample plate. (b) Left, animal tissue sections and their matrix‐assisted laser desorption/ionization (MALDI)‐IMS. Right, tissue extinction calculation (TEC) values as histograms for each targeted organ.
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Laboratory Methods and Technologies > Metabolomics
Laboratory Methods and Technologies > Imaging
Analytical and Computational Methods > Analytical Methods

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