Although the transcriptome, proteome, and interactome of several eukaryotic model organisms have been described in detail, lipidomes remain relatively uncharacterized. Using Saccharomyces cerevisiae as an example, we demonstrate that automated shotgun lipidomics analysis enabled lipidome-wide absolute quantification of individual molecular lipid species by streamlined processing of a single sample of only 2 million yeast cells. By comparative lipidomics, we achieved the absolute quantification of 250 molecular lipid species covering 21 major lipid classes. This analysis provided Ϸ95% coverage of the yeast lipidome achieved with 125-fold improvement in sensitivity compared with previous approaches. Comparative lipidomics demonstrated that growth temperature and defects in lipid biosynthesis induce ripple effects throughout the molecular composition of the yeast lipidome. This work serves as a resource for molecular characterization of eukaryotic lipidomes, and establishes shotgun lipidomics as a powerful platform for complementing biochemical studies and other systems-level approaches.fatty acid elongation ͉ S. cerevisiae ͉ shotgun lipidomics T he lipidome of eukaryotic cells consists of hundreds to thousands of individual lipid species that constitute membranes, store metabolic energy and function as bioactive molecules (1-3). Despite the extensive characterization of proteins, their association into complexes and activities (4-6), it is still difficult to assess how perturbations within the lipid metabolic network affect the full lipidome of cells. This work shows that lipidome-wide quantification of individual molecular lipid species (molecules with defined chemical structure) by absolute quantification (expressed in mol or mol%) provides a new approach to relate lipidomics and functional genomics studies.The yeast Saccharomyces cerevisiae serves as a prime model organism for studying the molecular organization and regulatory circuitry of eukaryotic lipidomes (7-9). It uses a relatively simple and conserved network of lipid metabolic pathways (Fig. 1) that synthesize a few hundred molecular lipid species constituting its full lipidome (3). The lipidome diversity is primarily determined by the fatty acid synthase (10), the ⌬-9 desaturase (11) and the fatty acid elongation complex (12) that produce only saturated or mono-unsaturated fatty acids having 10 to 26 carbon atoms for the biosynthesis of glycerolipids, glycerophospholipids, and sphingolipids. Importantly, several metabolic conversions interlink sphingolipid, glycerophospholipid, and glycerolipid metabolism such that any perturbation within the metabolic network is prone to induce lipidome-wide ripple effects. Remarkably, numerous genes involved in lipid metabolism and trafficking can be mutated or deleted without apparent physiological consequences (Fig. 1).Despite remarkable methodological advances, lipidomics seldom complements functional genomics efforts owing to three major factors. First, analysis of glycerophospholipids and sphingolipids requires ...

We report a method for the identification and quantification of glycerophospholipid molecular species that is based on the simultaneous automated acquisition and processing of 41 precursor ion spectra, specific for acyl anions of common fatty acids moieties and several lipid class-specific fragment ions. Absolute quantification of identified species was linear within a concentration range of 10 nM-100 microM and was achieved by spiking into total lipid extracts a set of synthetic lipid standards with diheptadecanoyl (17:0/17:0) fatty acid moieties, representing six common classes of glycerophospholipids. The automated analysis of total lipid extracts was powered by a robotic nanoflow ion source and produced currently the most detailed description of the glycerophospholipidome.

Comparisons between tears and meibum indicate that meibum is likely to supply the majority of lipids in the tear film lipid layer. However, the observed higher mole ratio of phospholipid in tears shows that analysis of meibum alone does not provide a complete understanding of the tear film lipid composition.

The analysis of lipids by mass spectrometry (MS) can provide in-depth characterization for many forms of biological samples. However, such workflows can also be hampered by challenges like low chromatographic resolution for lipid separations and the convolution of mass spectra from isomeric and isobaric species. To address these issues, we describe the use of differential mobility spectrometry (DMS) as a rapid and predictable separation technique within a shotgun lipidomics workflow, with a special focus on phospholipids (PLs). These analytes, ionized by electrospray ionization (ESI), are filtered using DMS prior to MS analysis. The observed separation (measured in terms of DMS compensation voltage) is affected by several factors, including the m/z of the lipid ion, the structure of an individual ion, and the presence of chemical modifiers in the DMS cell. Such DMS separations can simplify the analysis of complex extracts in a robust and reproducible manner, independent of utilized MS instrumentation. The predictable separation achieved with DMS can facilitate correct lipid assignments among many isobaric and isomeric species independent of the resolution settings of the MS analysis. This leads to highly comprehensive and quantitative lipidomic outputs through rapid profiling analyses, such as Q1 and MRM scans. The ultimate benefit of the DMS separation in this unique shotgun lipidomics workflow is its ability to separate many isobaric and isomeric lipids that by standard shotgun lipidomics workflows are difficult to assess precisely, for example, ether and diacyl species and phosphatidylcholine (PC) and sphingomyelin (SM) lipids.

This article is available online at http://www.jlr.org glycerophospholipids (GPs). Numerous accounts have examined the role of GPs in cellular biochemistries including membrane permeability, protein aggregation, and receptor activation ( 1-6 ). MS is a powerful tool for GP structure elucidation and is commonly used in contemporary lipidomics studies in complex biological extracts. MS/MS that uses collision-induced dissociation (CID) is central to most protocols in modern lipidomics and can identify headgroup class, acyl chain length, and degree of acyl chain unsaturation ( 7-11 ). However, there are numerous important structural features of GPs that are not easily discerned by CID, including identifi cation of carbon-carbon double bond position(s), the stereochemistry of carboncarbon double bonds, and the position of substitution of each acyl chain on the glycerol backbone (i.e., sn -position) ( 9, 12, 13 ). The inability to discriminate between snpositional isomers, or even to unequivocally exclude the presence of both isomers, is an impediment to our understanding of the roles of these distinct molecular structures in biological systems.Recent reports point to specifi c arrangements of acyl chains in GPs being responsible for structural interactions that induce specifi c activity. This has been noted particularly in the interactions of GPs toward nuclear receptor proteins. For example, Liu et al. ( 14 )

Triacylglycerols are neutral lipids present in all mammalian cells as energy reserves and diacylglycerols as intermediates in phospholipid biosynthesis and as signaling molecules. The molecular species of triacylglycerols and diacylglycerols present in mammalian cells are quite complex and previous investigations revealed multiple isobaric species having molecular weights at virtually every even mass between 600-900 daltons, making it difficult to assess changes of individual molecular species after cell activation. A method has been developed using tandem mass spectrometry and neutral loss scanning to quantitatively analyze changes in those glyceryl ester molecular species containing identical fatty acyl groups. This was carried out by neutral loss scanning of 18 common fatty acyl groups where the neutral loss corresponded to the free carboxylic acid plus NH 3 . Deuterium labeled internal standards were used to normalize the signal for each nominal [M +NH 4 ] + ion undergoing this neutral loss reaction. This method was applied in studies of triacylglycerols in RAW 264.7 cells treated with the toll-like receptor 4 ligand Kdo 2 -lipid A. A 50:1-TAG containing 18:1 was found to increase significantly over a 24 hr time course after Kdo 2 -lipid A exposure whereas an isobaric 50:1-TAG containing 16:1 did not change relative to controls.

Shotgun lipidomics has evolved into a myriad of multi-dimensional strategies for molecular lipid characterization, including bioinformatics tools for mass spectrum interpretation and quantitative measurements to study systems-lipidomics in complex biological extracts. Taking advantage of spectral mass accuracy, scan speed and sensitivity of improved quadrupole linked time-of-flight mass analyzers, we developed a bias-free global lipid profiling acquisition technique of sequential precursor ion fragmentation called MS/MSALL. This generic information-independent tandem mass spectrometry (MS) technique consists of a Q1 stepped mass isolation window through a set mass range in small increments, fragmenting and recording all product ions and neutral losses. Through the accurate MS and MS/MS information, the molecular lipid species are resolved, including distinction of isobaric and isomeric species, and composed into more precise lipidomic outputs. The method demonstrates good reproducibility and at least 3 orders of dynamic quantification range for isomeric ceramides in human plasma. More than 400 molecular lipids in human plasma were uncovered and quantified in less than 12 min, including acquisitions in both positive and negative polarity modes. We anticipate that the performance of sequential precursor ion fragmentation both in quality and throughput will lead to the uncovering of new avenues throughout the biomedical research community, enhance biomarker discovery and provide novel information target discovery programs as it will prospectively shed new insight into affected metabolic and signaling pathways.

The yeast Saccharomyces cerevisiae synthesizes three classes of sphingolipids: inositolphosphoceramides (IPCs), mannosyl-inositolphosphoceramides (MIPCs), and mannosyl-diinositolphosphoceramides (M(IP)2C). Tandem mass spectrometry of their molecular anions on a hybrid quadrupole time-of-flight (QqTOF) instrument produced fragments of inositol-containing head groups, which were specific for each lipid class. MS(n) analysis performed on a hybrid linear ion trap-orbitrap (LTQ Orbitrap) mass spectrometer with better than 3 ppm mass accuracy identified fragment ions specific for the amide-linked fatty acid and the long chain base moieties in individual molecular species. By selecting m/z of class-specific fragment ions for multiple precursor ion scanning, we profiled yeast sphingolipids in total lipid extracts on a QqTOF mass spectrometer. Thus, a combination of QqTOF and LTQ Orbitrap mass spectrometry lends itself to rapid, comprehensive and structure-specific profiling of the molecular composition of sphingolipids and glycerophospholipids in important model organisms, such as fungi and plants.