Tissue differentiation is an important process that involves major cellular membrane remodeling. We used Madin-Darby canine kidney cells as a model for epithelium formation and investigated the remodeling of the total cell membrane lipidome during the transition from a nonpolarized morphology to an epithelial morphology and vice versa. To achieve this, we developed a shotgun-based lipidomics workflow that enabled the absolute quantification of mammalian membrane lipidomes with minimal sample processing from low sample amounts. Epithelial morphogenesis was accompanied by a major shift from sphingomyelin to glycosphingolipid, together with an increase in plasmalogen, phosphatidylethanolamine, and cholesterol content, whereas the opposite changes took place during an epithelial-to-mesenchymal transition. Moreover, during polarization, the sphingolipids became longer, more saturated, and more hydroxylated as required to generate an apical membrane domain that serves as a protective barrier for the epithelial sheet.epithelial polarity | shotgun lipidomics | high resolution mass spectrometry | Forssman glycosphingolipid | epithelial-mesenchymal transition G ain and loss of epithelial polarity are fundamental processes in tissue differentiation (1, 2). These processes have been extensively studied not only in the context of embryogenesis but in tumor progression and metastasis (1). Although cell polarization necessarily implies major changes in the structure and properties of the cell membranes, the exact changes in the molecular composition of their membrane lipidome remain unknown.Madin-Darby canine kidney (MDCK) cells are a cell culture model for epithelial polarization (3,4). When seeded at low density, MDCK cells undergo a morphological change from a fibroblast-like morphology to a well-polarized epithelium (5) and have recently also been adapted to undergo an epithelial-mesenchymal transition (EMT) (6). Epithelial cells have evolved characteristic apical and basolateral membrane domains, separated by tight junctions, with specific protein and lipid composition (7,8). The unusual robustness of the apical membrane is largely attributable to its special lipid composition (9-11).In this work, we applied shotgun MS for comprehensive and quantitative characterization of mammalian cellular membranes (12). By combining high mass resolution MS, an improved extraction procedure that increases the recovery of polar lipids (13), and the use of internal standard mixtures for absolute quantification, we expanded the lipid repertoire covered by shotgun lipidomics to glycosphingolipids (GSPs). Importantly, the analysis of the GSPs did not require the hydrolysis of glycerophospholipids (GPs) (14), and this simplification allowed us to determine the membrane lipidome with unprecedented comprehensiveness (more than 300 lipid species from 14 different lipid classes) from a single sample of 10 5 cells.We further elucidated how the lipidome is remodeled during the course of MDCK cell epithelial polarization. We found that when these c...

Acute metabolic changes of plasma membrane (PM) lipids, such as those mediating signaling reactions, are rapidly compensated by homeostatic responses whose molecular basis is poorly understood. Here we show that the Extended-Synaptotagmins (E-Syts), ER proteins which function as PI(4,5)P2 and Ca2+-regulated tethers to the PM, participate in these responses. E-Syts transfer glycerolipids between bilayers in vitro and such transfer requires Ca2+ and their SMP domain, a lipid-harboring module. Genome edited cells lacking E-Syts do not exhibit abnormalities in the major glycerolipids at rest, but display enhanced and sustained accumulation of PM diacylglycerol (DAG) upon PI(4,5)P2 hydrolysis by PLC activation, which can be rescued by expression of E-Syt1, but not by mutant E-Syt1 lacking the SMP domain. The formation of E-Syts-dependent ER-PM tethers in response to stimuli that cleave PI(4,5)P2 and elevate Ca2+ may help reverse accumulation of DAG in the PM by transferring it to the ER for metabolic recycling.

As the lipidomics field continues to advance, self-evaluation within the community is critical. Here, we performed an interlaboratory comparison exercise for lipidomics using Standard Reference Material (SRM) 1950-Metabolites in Frozen Human Plasma, a commercially available reference material. The interlaboratory study comprised 31 diverse laboratories, with each laboratory using a different lipidomics workflow. A total of 1,527 unique lipids were measured across all laboratories and consensus location estimates and associated uncertainties were determined for 339 of these lipids measured at the sum composition level by five or more participating laboratories. These evaluated lipids detected in SRM 1950 serve as community-wide benchmarks for intra- and interlaboratory quality control and method validation. These analyses were performed using nonstandardized laboratory-independent workflows. The consensus locations were also compared with a previous examination of SRM 1950 by the LIPID MAPS consortium. While the central theme of the interlaboratory study was to provide values to help harmonize lipids, lipid mediators, and precursor measurements across the community, it was also initiated to stimulate a discussion regarding areas in need of improvement.

Mass spectrometry-based shotgun lipidomics has enabled the quantitative and comprehensive assessment of cellular lipid compositions. The yeast Saccharomyces cerevisiae has proven to be a particularly valuable experimental system for studying lipid-related cellular processes. Here, by applying our shotgun lipidomics platform, we investigated the influence of a variety of commonly used growth conditions on the yeast lipidome, including glycerophospholipids, triglycerides, ergosterol as well as complex sphingolipids. This extensive dataset allowed for a quantitative description of the intrinsic flexibility of a eukaryotic lipidome, thereby providing new insights into the adjustments of lipid biosynthetic pathways. In addition, we established a baseline for future lipidomic experiments in yeast. Finally, flexibility of lipidomic features is proposed as a new parameter for the description of the physiological state of an organism.