Development\nof a Desorption Electrospray Ionization–Multiple-Reaction-Monitoring\nMass Spectrometry (DESI-MRM) Workflow for Spatially Mapping Oxylipins\nin Pulmonary Tissue

Oxylipins are a class\nof low-abundance lipids formed\nvia oxygenation\nof fatty acids. These compounds include potent signaling molecules\n(e.g., octadecanoids, eicosanoids) that can exert essential functions\nin the pathophysiology of inflammatory diseases including asthma.\nWhile some oxylipin signaling cascades have been unraveled using LC-MS/MS-based\nmethods, measurements in homogenate samples do not represent the spatial\nheterogeneity of lipid metabolism. Mass spectrometry imaging (MSI)\ndirectly detects analytes from a surface, which enables spatial mapping\nof oxylipin biosynthesis and migration within the tissue. MSI has\nlacked the sensitivity to routinely detect low-abundance oxylipins;\nhowever, new multiple-reaction-monitoring (MRM)-based MSI technologies\nshow increased sensitivity. In this study, we developed a workflow\nto apply desorption electrospray ionization coupled to a triple quadrupole\nmass spectrometer (DESI-MRM) to spatially map oxylipins in pulmonary\ntissue. The targeted MSI workflow screened guinea pig lung extracts\nusing LC-MS/MS to filter oxylipin targets based on their detectability\nby DESI-MRM. A panel of 5 oxylipins was then selected for DESI-MRM\nimaging derived from arachidonic acid (TXB<sub>2</sub>, 11-HETE, 12-HETE),\nlinoleic acid (12,13-DiHOME), and α-linolenic acid (16-HOTrE).\nTo parse this new data type, a custom-built R package (quantMSImageR)\nwas developed with functionality to label regions of interest as well\nas quantify and analyze lipid distributions. The spatial distributions\nquantified by DESI-MRM were supported by LC-MS/MS analysis, with both\nindicating that 16-HOTrE and 12-HETE were associated with airways,\nwhile 12,13-DiHOME and arachidonic acid were mapped to parenchyma.\nThis study realizes the potential of targeted MSI to routinely map\nlow-abundance oxylipins with high specificity at scale.