Electrospray mass spectrometry of phospholipids

Ammonium bicarbonate buffers combined with hybrid surface technology columns improve the peak shape of strongly tailing lipids

BACKGROUND

Lipids such as phosphatidic acids (PAs) and cardiolipins (CLs) present strongly tailing peaks in reversed phase liquid chromatography, which entails low detectability. They are usually analyzed by hydrophilic interaction liquid chromatography (HILIC), which hampers high-throughput lipidomics. Thus, there is a great need for improved analytical methods in order to obtain a broader coverage of the lipidome in a single chromatographic method. We investigated the effect of ammonium bicarbonate (ABC) on peak asymmetry and detectability, in comparison with ammonium formate (AFO) on both a conventional BEH C18 column and an HST-CSH C18 column.

RESULTS

The combination of 2.5 mM ABC buffer pH 8 with an HST-CSH C18 column produced significantly improved results, reducing the asymmetry factor at 10 % peak height of PA 16:0/18:1 from 8.4 to 1.6. Furthermore, on average, there was up to a 54-fold enhancement in the peak height of its [M - H]- ion compared to AFO and the BEH C18 column. We confirmed this beneficial effect on other strongly tailing lipids, with accessible phosphate moieties e.g., cardiolipins, phosphatidylinositol phosphate, phosphatidylinositol bisphosphate, phosphorylated ceramide and phosphorylated sphingosine. Furthermore, we found an increased detectability of phospho- and sphingolipids up to 28 times in negative mode when using an HST-CSH C18 column. The method was successfully applied to mouse liver samples, where previously undetected endogenous phospholipids could be analyzed with improved chromatographic separation.

SIGNIFICANCE

In conclusion, the use of 2.5 mM ABC substantially improved the peak shape of PAs and enhanced the detectability of the lipidome in negative mode on an RPLC-ESI-Q-TOF-MS system on both BEH C18 and HST-CSH C18 columns. This method provides a wider coverage of the lipidome with one single injection for future lipidomic applications in negative mode.

Lipidomics of homeoviscous adaptation to low temperatures in Staphylococcus aureus utilizing exogenous straight-chain unsaturated fatty acids

It is well established that Staphylococcus aureus can incorporate exogenous straight-chain unsaturated fatty acids (SCUFAs) into membrane phospho- and glyco-lipids from various sources in supplemented culture media and when growing in vivo during infection. Given the enhancement of membrane fluidity when oleic acid (C18:1Δ9) is incorporated into lipids, we were prompted to examine the effect of medium supplementation with C18:1Δ9 on growth at low temperatures. C18:1Δ9 supported the growth of a cold-sensitive, branched-chain fatty acid (BCFA)-deficient mutant at 12°C. Interestingly, we found similar results in the BCFA-sufficient parental strain, supported by the fact that the incorporation of C18:1Δ9 into the membrane increased membrane fluidity in both strains. We show that the incorporation of C18:1Δ9 and its elongation product C20:1Δ11 into membrane lipids was required for growth stimulation and relied on a functional FakAB incorporation system. Lipidomics analysis of the phosphatidylglycerol and diglycosyldiacylglycerol lipid classes revealed major impacts of C18:1Δ9 and temperature on lipid species. Growth at 12°C in the presence of C18:1Δ9 also led to increased production of the carotenoid pigment staphyloxanthin. The enhancement of growth by C18:1Δ9 is an example of homeoviscous adaptation to low temperatures utilizing an exogenous fatty acid. This may be significant in the growth of S. aureus at low temperatures in foods that commonly contain C18:1Δ9 and other SCUFAs in various forms.

IMPORTANCE

We show that Staphylococcus aureus can use its known ability to incorporate exogenous fatty acids to enhance its growth at low temperatures. Individual species of phosphatidylglycerols and diglycosyldiacylglycerols bearing one or two degrees of unsaturation derived from the incorporation of C18:1Δ9 at 12°C are described for the first time. In addition, enhanced production of the carotenoid staphyloxanthin occurs at low temperatures. The studies describe a biochemical reality underlying membrane biophysics. This is an example of homeoviscous adaptation to low temperatures utilizing exogenous fatty acids over the regulation of the biosynthesis of endogenous fatty acids. The studies have likely relevance to food safety in that unsaturated fatty acids may enhance the growth of S. aureus in the food environment.

Structural characterization of fatty acid anions via gas-phase charge inversion using Mg(tri-butyl-terpyridine)2 2+ reagent ions

RATIONALE

Free fatty acids and lipid classes containing fatty acid esters are major components of lipidome. In the absence of a chemical derivatization step, FA anions do not yield all of the structural information that may be of interest under commonly used collision-induced dissociation (CID) conditions. A line of work that avoids condensed-phase derivatization takes advantage of gas-phase ion/ion chemistry to charge invert FA anions to an ion type that provides the structural information of interest using conventional CID. This work was motivated by the potential for significant improvement in overall efficiency for obtaining FA chain structural information.

METHODS

A hybrid triple quadrupole/linear ion-trap tandem mass spectrometer that has been modified to enable the execution of ion/ion reaction experiments was used to evaluate the use of 4,4',4″-tri-tert-butyl-2,2':6',2″-terpyridine (ttb-Terpy) as the ligand in divalent magnesium complexes for charge inversion of FA anions.

RESULTS

Mg(ttb-Terpy)2 2+ complexes provide significantly improved efficiency in producing structurally informative products from FA ions relative to Mg(Terpy)2 2+ complexes, as demonstrated for straight-chain FAs, branched-chain FAs, unsaturated FAs, and cyclopropane-containing FAs. It was discovered that most of the structurally informative fragmentation from [FA-H + Mg(ttb-Terpy)]+ results from the loss of a methyl radical from the ligand followed by radical-directed dissociation (RDD), which stands in contrast to the charge-remote fragmentation (CRF) believed to be operative with the [FA-H + Mg(Terpy)]+ ions.

CONCLUSIONS

This work demonstrates that a large fraction of product ions from the CID of ions of the form [FA-H + Mg(ttb-Terpy)]+ are derived from RDD of the FA backbone, with a very minor fraction arising from structurally uninformative dissociation channels. This ligand provides an alternative to previously used ligands for the structural characterization of FAs via CRF.

Is Lipid Metabolism of Value in Cancer Research and Treatment? Part I- Lipid Metabolism in Cancer

For either healthy or diseased organisms, lipids are key components for cellular membranes; they play important roles in numerous cellular processes including cell growth, proliferation, differentiation, energy storage and signaling. Exercise and disease development are examples of cellular environment alterations which produce changes in these networks. There are indications that alterations in lipid metabolism contribute to the development and progression of a variety of cancers. Measuring such alterations and understanding the pathways involved is critical to fully understand cellular metabolism. The demands for this information have led to the emergence of lipidomics, which enables the large-scale study of lipids using mass spectrometry (MS) techniques. Mass spectrometry has been widely used in lipidomics and allows us to analyze detailed lipid profiles of cancers. In this article, we discuss emerging strategies for lipidomics by mass spectrometry; targeted, as opposed to global, lipid analysis provides an exciting new alternative method. Additionally, we provide an introduction to lipidomics, lipid categories and their major biological functions, along with lipidomics studies by mass spectrometry in cancer samples. Further, we summarize the importance of lipid metabolism in oncology and tumor microenvironment, some of the challenges for lipodomics, and the potential for targeted approaches for screening pharmaceutical candidates to improve the therapeutic efficacy of treatment in cancer patients.

A combined lipidomic and proteomic profiling of Arabidopsis thaliana plasma membrane

The plant plasma membrane (PM) plays a key role in perception of environmental signals, and set-up of adaptive responses. An exhaustive and quantitative description of the whole set of lipids and proteins constituting the PM is necessary to understand how these components allow to fulfill such essential physiological functions. Here we provide by state-of-the-art approaches the first combined reference of the plant PM lipidome and proteome from Arabidopsis thaliana suspension cell culture. We identified and quantified a reproducible core set of 2165 proteins, which is by far the largest set of available data concerning this plant PM proteome. Using the same samples, combined lipidomic approaches, allowing the identification and quantification of an unprecedented repertoire of 414 molecular species of lipids showed that sterols, phospholipids, and sphingolipids are present in similar proportions in the plant PM. Within each lipid class, the precise amount of each lipid family and the relative proportion of each molecular species were further determined, allowing to establish the complete lipidome of Arabidopsis PM, and highlighting specific characteristics of the different molecular species of lipids. Results obtained point to a finely tuned adjustment of the molecular characteristics of lipids and proteins. More than a hundred proteins related to lipid metabolism, transport, or signaling have been identified and put in perspective of the lipids with which they are associated. This set of data represents an innovative resource to guide further research relative to the organization and functions of the plant PM.

Comprehensive analysis of phospholipid in milk and their biological roles as nutrients and biomarkers

Phospholipids (PL) have garnered significant attention due to their physiological activities. Milk and other dairy products are important dietary sources for humans and have been extensively used to analyze the presence of PL by various analytical techniques. In this paper, the analysis techniques of PL were reviewed with the eight trigrams of phospholipidomics and a comprehensive fingerprint of 1295 PLs covering 8 subclasses in milk and other dairy products, especially. Technology is the primary productive force. Based on phospholipidomics technology, we further review the relationship between the composition of PL and factors that may be involved in processing and experimental operation, and emphasized the significance of the biological role played by PL in dietary supplements and biomarkers (production, processing and clinical research), and providing the future research directions.

Shotgun lipidomics of human subretinal fluids under rod-dominant retina reveals cone-dominated lipids

Subretinal fluid (SRF) accumulates between photoreceptor outer segments and retinal pigment epithelium during rhegmatogenous retinal detachment. Biomolecular components such as lipids originate from cells surrounding the SRF. Knowledge of the composition of these molecules in SRF potentially provides mechanistic insight into the physiologic transfer of lipids between retinal tissue compartments. Using mass spectrometry and tandem mass spectrometry analysis on an electrospray ionization quadrupole-time-of-flight mass spectrometer, we identified a total of 115 lipid molecular species of 11 subclasses and 9 classes in two samples from two patients with rhegmatogenous retinal detachment. These included 47 glycerophosphocholines, 6 glycerophosphoethanolamines, 1 glycerophosphoinositol, 18 sphingomyelins, 9 cholesteryl esters, free cholesterol, 3 ceramides, 22 triacylglycerols and 8 free fatty acids. Glycerophosphocholines were of the highest intensity. By minimizing the formation of different adduct forms or clustering ions of different adducts, we determined the relative intensity of lipid molecular species within the same subclasses. The profiles were compared with those of retinal cells available in the published literature. The glycerophosphocholine profile of SRF was similar to that of cone outer segments, suggesting that outer segment degradation products are constitutively released into the interphotoreceptor matrix, appearing in SRF during detachment. This hypothesis was supported by the retinal distributions of corresponding lipid synthases' mRNA expression obtained from an online resource based on publicly available single-cell sequencing data. In contrast, based on lipid profiles and relevant gene expression in this study, the sources of free cholesterol and cholesteryl esters in SRF appeared more ambiguous, possibly reflecting that outer retina takes up plasma lipoproteins. Further studies to identify and quantify lipids in SRF will help better understand etiology of diseases relevant to outer retina.

Gas-Phase Ion/Ion Reactions to Enable Radical-Directed Dissociation of Fatty Acid Ions: Application to Localization of Methyl Branching

Methyl branching on the carbon chains of fatty acids and fatty esters is among the structural variations encountered with fatty acids and fatty esters. Branching in fatty acid/ester chains is particularly prominent in bacterial species and, for example, in vernix caseosa and sebum. The distinction of branched chains from isomeric straight-chain species and the localization of branching can be challenging to determine by mass spectrometry (MS). Condensed-phase derivatization strategies, often used in conjunction with separations, are most commonly used to address the identification and characterization of branched fatty acids. In this work, a gas-phase ion/ion strategy is presented that obviates condensed-phase derivatization and introduces a radical site into fatty acid ions to facilitate radical-directed dissociation (RDD). The gas-phase approach is also directly amenable to fatty acid anions generated via collision-induced dissociation from lipid classes that contain fatty esters. Specifically, divalent magnesium complexes bound to two terpyridine ligands that each incorporate a ((2,2,6,6-tetramethyl-1-piperidine-1-yl)oxy) (TEMPO) moiety are used to charge-invert fatty acid anions. Following the facile loss of one of the ligands and the TEMPO group of the remaining ligand, a radical site is introduced into the complex. Subsequent collision-induced dissociation (CID) of the complex exhibits preferred cleavages that localize the site(s) of branching. The approach is illustrated with iso-, anteiso-, and isoprenoid branched-chain fatty acids and an intact glycerophospholipid and is applied to a mixture of branched- and straight-chain fatty acids derived from Bacillus subtilis.

Mobilize a Proton to Transform the Collision-Induced Dissociation Spectral Pattern of a Cyclic Peptide

The collision-induced dissociation (CID) behaviors of protonated molecules of anabaenopeptins, a group of cyanobacterial cyclic peptides, were investigated in detail using liquid chromatography-tandem mass spectrometry. Although anabaenopeptin A and B share a macrocyclic peptide structure, they give strikingly different fragmentation patterns; the former gives a variety of product ions including cleavages in the cyclic peptide structure, which is useful for structural analysis; whereas the latter gives far fewer product ions and no fragmentation in the cyclic moiety. Energy-resolved CID experiments clarified the mechanism behind the striking difference attributable to the difference in exocyclic amino acid residues, Tyr or Arg. The guanidino group in Arg-containing analogue, anabaenopeptin B, should be by far the most preferred protonation site; the proton would be sequestered at the guanidino group in the protonated molecule, with the lack of proton mobility prohibiting opening of the charge-directed fragmentation channels in the cyclic moiety. Enzymatic hydrolysis of the guanidino group to give citrullinated-anabaenopeptin B restored proton mobility. The fragmentation pattern of the citrullinated peptide became almost identical to that of anabaenopeptin A. The observed fragmentation behaviors of these cyclic peptides were consistent with those of linear peptides, which have been well understood based on the mobile proton model.

Evaluation of Surface-Induced Dissociation Ion Mobility-Mass Spectrometry for Lipid Structural Characterization

The complexity of the lipidome has necessitated the development of novel analytical approaches for the identification and structural analysis of morphologically diverse classes of lipids. At this time, a variety of dissociation techniques have been utilized to probe lipid decomposition pathways in search of structurally diagnostic fragment ions. Here, we investigate the application of surface-induced dissociation (SID), a fragmentation technique that imparts energy to the target molecule via collision with a coated surface, for the fragmentation of seven lipids across four major lipid subclasses. We have developed a tuning methodology for guiding the efficient operation of a previously developed custom SID device for molecules as small as ca. 300 Da with ion mobility analysis of the fragmentation products. SID fragmentation of the various lipids analyzed was found to generate fragment ions similar to those observed in CID spectra, but fragment ion lab frame onset energies were lower in SID due to the higher energy deposition via a more massive target. For the largest lipid evaluated (cardiolipin 18:1), SID produced chain fragment ions, which yielded analytically useful information regarding the composition of the acyl tails. Ion mobility provided an orthogonal dimension of separation and aided in assigning product ions to their precursors. Overall, the combination of SID and IM-MS is another potential methodology in the analytical toolkit for lipid structural analysis.

Variations of intact phospholipid compositions in the digestive system of Antarctic krill, Euphausia superba, between summer and autumn

The biochemical composition of Antarctic krill, Euphausia superba, is largely determined by their feeding behaviour. As they supply energy for animals of a higher trophic level and are also commercialized for human consumption, the interest in research on the species is high. Lipids, especially phospholipids, make up a high proportion of dry weight in krill. Seasonal changes are well documented in the fingerprint of free fatty acids analysed after hydrolysis of phospholipids, but the underlying intact polar lipids are rarely considered. In this study, we evaluated the compositions of intact phospholipids (IPLs) in the stomach, digestive gland and hind gut of Antarctic krill caught in summer and autumn at the Antarctic Peninsula region. Using high-resolution mass spectrometry, the fatty acid composition of 179 intact phospholipids could be resolved. Most IPLs were phosphatidylcholines, followed by phosphatidylethanolamines. Several very long chain polyunsaturated fatty acids up to 38:8, which have not been reported in krill before, were identified. The composition shifted to higher molecular weight IPLs with a higher degree of unsaturation for summer samples, especially for samples of the digestive gland. The data supplied in this paper provides new insights into lipid dynamics between summer and autumn usually described by free fatty acid biomarkers.

Interfacial Electromigration for Analysis of Biofluid Lipids in Small Volumes

Lipids are important biomarkers within the field of disease diagnostics and can serve as indicators of disease progression and predictors of treatment effectiveness. Although lipids can provide important insight into how diseases initiate and progress, mass spectrometric methods for lipid characterization and profiling are limited due to lipid structural diversity, particularly the presence of various lipid isomers. Moreover, the difficulty of handling small-volume samples exacerbates the intricacies of biological analyses. In this work, we have developed a strategy that electromigrates a thin film of a small-volume biological sample directly to the air-liquid interface formed at the tip of a theta capillary. Importantly, we seamlessly integrated in situ biological lipid extraction with accelerated chemical derivatization, enabled by the air-liquid interface, and conducted isomeric structural characterization within a unified platform utilizing theta capillary nanoelectrospray ionization mass spectrometry, all tailored for small-volume sample analysis. We applied this unified platform to the analysis of lipids from small-volume human plasma and Alzheimer's disease mouse serum samples. Accelerated electro-epoxidation of unsaturated lipids at the interface allowed us to characterize lipid double-bond positional isomers. The unique application of electromigration of a thin film to the air-liquid interface in combination with accelerated interfacial reactions holds great potential in small-volume sample analysis for disease diagnosis and prevention.

On-tissue dataset-dependent MALDI-TIMS-MS2 bioimaging

Trapped ion mobility spectrometry (TIMS) adds an additional separation dimension to mass spectrometry (MS) imaging, however, the lack of fragmentation spectra (MS2) impedes confident compound annotation in spatial metabolomics. Here, we describe spatial ion mobility-scheduled exhaustive fragmentation (SIMSEF), a dataset-dependent acquisition strategy that augments TIMS-MS imaging datasets with MS2 spectra. The fragmentation experiments are systematically distributed across the sample and scheduled for multiple collision energies per precursor ion. Extendable data processing and evaluation workflows are implemented into the open source software MZmine. The workflow and annotation capabilities are demonstrated on rat brain tissue thin sections, measured by matrix-assisted laser desorption/ionisation (MALDI)-TIMS-MS, where SIMSEF enables on-tissue compound annotation through spectral library matching and rule-based lipid annotation within MZmine and maps the (un)known chemical space by molecular networking. The SIMSEF algorithm and data analysis pipelines are open source and modular to provide a community resource.

Mass Spectrometry Based on Chemical Derivatization Has Brought Novel Discoveries to Lipidomics: A Comprehensive Review

Lipids, as one of the most important organic compounds in organisms, are important components of cells and participate in energy storage and signal transduction of living organisms. As a rapidly rising field, lipidomics research involves the identification and quantification of multiple classes of lipid molecules, as well as the structure, function, dynamics, and interactions of lipids in living organisms. Due to its inherent high selectivity and high sensitivity, mass spectrometry (MS) is the "gold standard" analysis technique for small molecules in biological samples. The combination chemical derivatization with MS detection is a unique strategy that could improve MS ionization efficiency, facilitate structure identification and quantitative analysis. Herein, this review discusses derivatization-based MS strategies for lipidomic analysis over the past decade and focuses on all the reported lipid categories, including fatty acids and modified fatty acids, glycerolipids, glycerophospholipids, sterols and saccharolipids. The functional groups of lipids mainly involved in chemical derivatization include the C=C group, carboxyl group, hydroxyl group, amino group, carbonyl group. Furthermore, representative applications of these derivatization-based lipid profiling methods were summarized. Finally, challenges and countermeasures of lipid derivatization are mentioned and highlighted to guide future studies of derivatization-based MS strategy in lipidomics.

Separation of Isobaric Lipids in Imaging Mass Spectrometry Using Gas-Phase Charge Inversion Ion/Ion Reactions

The diverse array of chemical compounds present in tissue samples results in many isobaric (i.e., same nominal mass) compounds in imaging mass spectrometry experiments. Adequate separation and differentiation of these compounds is necessary to ensure accurate analyte identification and avoid composite images comprising multiple compounds. High-resolution accurate mass (HRAM) measurements are able to resolve these compounds in some instances, but HRAM measurements are not always feasible depending on the instrument platform and the desired experimental time scale. Alternatively, tandem mass spectrometry (MS/MS) can be used to perform gas-phase transformations that improve molecular specificity. While conventional MS/MS methods employ collision induced dissociation (CID) to fragment compounds of interest and then analyze the product masses, gas-phase ion/ion reactions can be used to instead selectively react with desired classes of analytes. Herein, we have used gas-phase charge inversion ion/ion reactions to selectively resolve phosphatidylcholines (PCs) in isobaric lipid mixtures. A 1,4-phenylenedipropionic acid (PDPA) reagent dianion readily reacts with [M + H]+, [M + Na]+, and [M + K]+ ion types to produce demethylated product anions for each PC, [PC - CH3]-. These product anions are no longer isobaric and now differ in mass by 22 Da (protonated versus sodiated) and 16 Da (sodiated versus potassiated), respectively. This reaction has been used to differentiate isobaric lipids in the imaging mass spectrometry analysis of rat brain tissue.

Discriminative Metabolomics Analysis and Cytotoxic Evaluation of Flowers, Leaves, and Roots Extracts of Matthiola longipetala subsp. livida

Matthiola longipetala subsp. livida is an annual herb in Brassicaceae that has received little attention despite the family's high reputation for health benefits, particularly cancer prevention. In this study, UPLC-HRMS-MS analysis was used for mapping the chemical constituents of different plant parts (i.e., flowers, leaves, and roots). Also, spectral similarity networks via the Global Natural Products Social Molecular Networking (GNPS) were employed to visualize their chemical differences and similarities. Additionally, the cytotoxic activity on HCT-116, HeLa, and HepG2 cell lines was evaluated. Throughout the current analysis, 154 compounds were annotated, with the prevalence of phenolic acids, glucosinolates, flavonol glucosides, lipids, peptides, and others. Predictably, secondary metabolites (phenolic acids, flavonoids, and glucosinolates) were predominant in flowers and leaves, while the roots were characterized by primary metabolites (peptides and fatty acids). Four diacetyl derivatives tentatively assigned as O-acetyl O-malonyl glucoside of quercetin (103), kaempferol (108 and 112), and isorhamnetin (114) were detected for the first time in nature. The flowers and leaves extracts showed significant inhibition of HeLa cell line propagation with LC50 values of 18.1 ± 0.42 and 29.6 ± 0.35 µg/mL, respectively, whereas the flowers extract inhibited HCT-116 with LC50 24.8 ± 0.45 µg/mL, compared to those of Doxorubicin (26.1 ± 0.27 and 37.6 ± 0.21 µg/mL), respectively. In conclusion, the flowers of M. longipetala are responsible for the abundance of bioactive compounds with cytotoxic properties.

Effects of Silane Monolayers on Lysophosphatidylcholine (LysoPC) Detection by Desorption Ionization on Silicon Mass Spectrometry (DIOS-MS) in Solution and Plasma

Desorption ionization on silicon mass spectrometry (DIOS-MS) enables high throughput analysis of low-molecular-weight biomolecules. However, detection of metabolite biomarkers in complex fluids such as plasma requires sample pretreatment, limiting clinical application. Here, we show that porous silicon, chemically modified using monolayers of n-propyldimethylmethoxysilane molecules, is a good candidate for fingerprinting lysophosphatidylcholine (lysoPC) in plasma, without sample pretreatment, for DIOS-MS-based diagnosis (e.g., sepsis). Results were correlated to lysoPC molecule location inside/outside the pores, determined by time-of-flight secondary ion mass spectrometry profiling, and to physicochemical properties.

Expanding the Molecular Disturbances of Lipoproteins in Cardiometabolic Diseases: Lessons from Lipidomics

The increasing global burden of cardiometabolic diseases highlights the urgent clinical need for better personalized prediction and intervention strategies. Early diagnosis and prevention could greatly reduce the enormous socio-economic burden posed by these states. Plasma lipids including total cholesterol, triglycerides, HDL-C, and LDL-C have been at the center stage of the prediction and prevention strategies for cardiovascular disease; however, the bulk of cardiovascular disease events cannot be explained sufficiently by these lipid parameters. The shift from traditional serum lipid measurements that are poorly descriptive of the total serum lipidomic profile to comprehensive lipid profiling is an urgent need, since a wealth of metabolic information is currently underutilized in the clinical setting. The tremendous advances in the field of lipidomics in the last two decades has facilitated the research efforts to unravel the lipid dysregulation in cardiometabolic diseases, enabling the understanding of the underlying pathophysiological mechanisms and identification of predictive biomarkers beyond traditional lipids. This review presents an overview of the application of lipidomics in the study of serum lipoproteins in cardiometabolic diseases. Integrating the emerging multiomics with lipidomics holds great potential in moving toward this goal.

Untargeted Metabolomics by Liquid Chromatography-Mass Spectrometry in Biomedical Research

Metabolomics, alone or in combination with other omics sciences, has shown great relevance in a large number of investigations in different branches of biomedicine, often providing novel discoveries and helping to expand the knowledge. Metabolomics analyses are carried out using different techniques, but in this chapter, we focus on liquid chromatography coupled to high-resolution mass spectrometry. The designated methodology consists of an untargeted approach for the analysis of plasma samples. The use of this method, with a reverse-phase column and electrospray ionization in positive mode, covers the detection of a broad range of metabolites, mainly of nonpolar and of intermediate polarity. This chapter also reviews the mass fragmentation spectra for the identification of bile acids, acylcarnitines, and glycerophospholipids.

Chemical derivatization strategy for mass spectrometry-based lipidomics

Lipids, serving as the structural components of cellular membranes, energy storage, and signaling molecules, play the essential and multiple roles in biological functions of mammals. Mass spectrometry (MS) is widely accepted as the first choice for lipid analysis, offering good performance in sensitivity, accuracy, and structural characterization. However, the untargeted qualitative profiling and absolute quantitation of lipids are still challenged by great structural diversity and high structural similarity. In recent decade, chemical derivatization mainly targeting carboxyl group and carbon-carbon double bond of lipids have been developed for lipidomic analysis with diverse advantages: (i) offering more characteristic structural information; (ii) improving the analytical performance, including chromatographic separation and MS sensitivity; (iii) providing one-to-one chemical isotope labeling internal standards based on the isotope derivatization regent in quantitative analysis. Moreover, the chemical derivatization strategy has shown great potential in combination with ion mobility mass spectrometry and ambient mass spectrometry. Herein, we summarized the current states and advances in chemical derivatization-assisted MS techniques for lipidomic analysis, and their strengths and challenges are also given. In summary, the chemical derivatization-based lipidomic approach has become a promising and reliable technique for the analysis of lipidome in complex biological samples.

Identification of Monomethyl Branched-Chain Lipids by a Combination of Liquid Chromatography Tandem Mass Spectrometry and Charge-Switching Chemistries

While various mass spectrometric approaches have been applied to lipid analysis, unraveling the extensive structural diversity of lipids remains a significant challenge. Notably, these approaches often fail to differentiate between isomeric lipids─a challenge that is particularly acute for branched-chain fatty acids (FAs) that often share similar (or identical) mass spectra to their straight-chain isomers. Here, we utilize charge-switching strategies that combine ligated magnesium dications with deprotonated fatty acid anions. Subsequent activation of these charge inverted anions yields mass spectra that differentiate anteiso-branched- from straight-chain and iso-branched-chain FA isomers with the predictable fragmentation enabling de novo assignment of anteiso branch points. The application of these charge-inversion chemistries in both gas- and solution-phase modalities is demonstrated to assign the position of anteiso-methyl branch-points in FAs and, with the aid of liquid chromatography, can be extended to de novo assignment of additional branching sites via predictable fragmentation patterns as methyl branching site(s) move closer to the carboxyl carbon. The gas-phase approach is shown to be compatible with top-down structure elucidation of complex lipids such as phosphatidylcholines, while the integration of solution-phase charge-inversion with reversed phase liquid chromatography enables separation and unambiguous identification of FA structures within isomeric mixtures. Taken together, the presented charge-switching MS-based technique, in combination with liquid chromatography, enables the structural identification of branched-chain FA without the requirement of authentic methyl-branched FA reference standards.

Quantitative determination of sn-positional phospholipid isomers in MSn using silver cationization

Glycerophospholipids are one of the fundamental building blocks for life. The acyl chain connectivity to the glycerol backbone constitutes different sn-positional isomers, which have great diversity and importance for biological function. However, to fully realize their impact on function, analytical techniques that can identify and quantify sn-positional isomers in chemically complex biological samples are needed. Here, we utilize silver ion cationization in combination with tandem mass spectrometry (MSn) to identify sn-positional isomers of phosphatidylcholine (PC) species. In particular, a labile carbocation is generated through a neutral loss (NL) of AgH, the dissociation of which provides diagnostic product ions that correspond to acyl chains at the sn-1 or sn-2 position. The method is comparable to currently available methods, has a sensitivity in the nM-µM range, and is compatible with quantitative imaging using mass spectrometry in MS4. The results reveal a large difference in isomer concentrations and the ion images show that the sn-positional isomers PC 18:1_18:0 are homogeneously distributed, whereas PC 18:1_16:0 and PC 20:1_16:0 show distinct localizations to sub-hippocampal structures.

Scalable Microfabrication of Multi-Emitter Arrays in Silicon for a Compact Microfluidic Electrospray Propulsion System

The recent proliferation of SmallSats and their use in increasingly demanding applications require the development of onboard electric propulsion compatible with the power, mass, and volume constraints of these spacecraft. Electrospray propulsion is a promising technology for SmallSats due to its unique high efficiency and scalability across the wide power range of these platforms, for example, from a few watts available in a CubeSat to a few hundred watts in a MiniSat. The implementation of electrospray propulsion requires the use of microfabrication techniques to create compact arrays of thousands of electrospray emitters. This article demonstrates the microfabrication of multi-emitter electrospray sources of a scalable size for electrospray propulsion. In particular, a microfabrication and assembly process is developed and demonstrated by fabricating sources with arrays of 1, 64, and 256 emitters. The electrospray sources are tested in a relevant environment for space propulsion (inside a vacuum chamber), exhibiting excellent propulsive performance (e.g., absence of beam impingement in the extractor electrode, absence of hysteresis in the beam current versus propellant flow rate characteristic, proper operation in the cone-jet electrospraying mode, etc.) and nearly coincident output per emitter. Several design elements contribute to this performance: the even distribution of the propellant among all emitters made possible by the implementation of a network of microfluidic channels in the backside of the emitter array; the small dead volume of the network of microfluidic channels; the accurate alignment between the emitters and extractor orifices; and the use of a pipe-flow configuration to drive the propellant through closed conduits, which protects the propellant.

Orthogonality in Principal Component Analysis Allows the Discovery of Lipids in the Jejunum That Are Independent of Ad Libitum Feeding

Ad libitum feeding of experimental animals is preferred because of medical relevance together with technical and practical considerations. In addition, ethical committees may require ad libitum feeding. However, feeding affects the metabolism so ad libitum feeding may mask the effects of drugs on tissues directly involved in the digestion process (e.g., jejunum and liver). Despite this effect, principal component analysis has the potential of identifying metabolic traits that are statistically independent (orthogonal) to ad libitum feeding. Consequently, we used principal component analysis to discover the metabolic effects of doxorubicin independent of ad libitum feeding. First, we analyzed the lipidome of the jejunum and the liver of rats treated with vehicle or doxorubicin. Subsequently, we performed principal component analysis. We could identify a principal component associated to the hydrolysis of lipids during digestion and a group of lipids that were orthogonal. These lipids in the jejunum increased with the treatment time and presented a polyunsaturated fatty acid as common structural trait. This characteristic suggests that doxorubicin increases polyunsaturated fatty acids. This behavior agrees with our previous in vitro results and suggests that doxorubicin sensitized the jejunum to ferroptosis, which may partially explain the toxicity of doxorubicin in the intestines.

Cryogenic infrared spectroscopy provides mechanistic insight into the fragmentation of phospholipid silver adducts

Tandem mass spectrometry is arguably the most important analytical tool for structure elucidation of lipids and other metabolites. By fragmenting intact lipid ions, valuable structural information such as the lipid class and fatty acyl composition are readily obtainable. The information content of a fragment spectrum can often be increased by the addition of metal cations. In particular, the use of silver ions is deeply rooted in the history of lipidomics due to their propensity to coordinate both electron-rich heteroatoms and C = C bonds in aliphatic chains. Not surprisingly, coordination of silver ions was found to enable the distinction of sn-isomers in glycerolipids by inducing reproducible intensity differences in the fragment spectra, which could, however, not be rationalized. Here, we investigate the fragmentation behaviors of silver-adducted sn- and double bond glycerophospholipid isomers by probing fragment structures using cryogenic gas-phase infrared (IR) spectroscopy. Our results confirm that neutral headgroup loss from silver-adducted glycerophospholipids leads to dioxolane-type fragments generated by intramolecular cyclization. By combining high-resolution IR spectroscopy and computational modelling of silver-adducted fragments, we offer qualitative explanations for different fragmentation behaviors of glycerophospholipid isomers. Overall, the results demonstrate that gas-phase IR spectroscopy of fragment ions can significantly contribute to our understanding of lipid dissociation mechanisms and the influence of coordinating cations.

Newborn Screen for X-Linked Adrenoleukodystrophy Using Flow Injection Tandem Mass Spectrometry in Negative Ion Mode

X-linked adrenoleukodystrophy (X-ALD) is a genetic disorder caused by pathogenic variants in the ATP-binding cassette subfamily D member 1 gene (ABCD1) that encodes the adrenoleukodystrophy protein (ALDP). Defects in ALDP result in elevated cerotic acid, and lead to C26:0-lysophosphatidylcholine (C26:0-LPC) accumulation, which is the primary biomarker used in newborn screening (NBS) for X-ALD. C26:0-LPC levels were measured in dried blood spot (DBS) NBS specimens using a flow injection analysis (FIA) coupled with electrospray ionization (ESI) tandem mass spectrometry (MS/MS) performed in negative ion mode. The method was validated by assessing and confirming linearity, accuracy, and precision. We have also established C26:0-LPC cutoff values that identify newborns at risk for X-ALD. The mean concentration of C26:0-LPC in 5881 de-identified residual routine NBS specimens was 0.07 ± 0.02 µM (mean + 1 standard deviation (SD)). All tested true X-ALD positive and negative samples were correctly identified based on C26:0-LPC cutoff concentrations for borderline between 0.15 µM and 0.22 µM (mean + 4 SD) and presumptive screening positive at ≥0.23 µM (mean + 8 SD). The presented FIA method shortens analysis run-time to 1.7 min, while maintaining the previously established advantage of utilizing negative mode MS to eliminate isobaric interferences that could lead to screening false positives.

Comprehensive Lipidomic Analysis of Three Edible Brown Seaweeds Based on Reversed-Phase Liquid Chromatography Coupled with Quadrupole Time-of-Flight Mass Spectrometry

A comprehensive lipidomic analysis was performed onto three edible brown seaweeds, namely Laminaria japonica, Undaria pinnatifida, and Scagassum natans, using reversed-phase liquid chromatography coupled with quadrupole time-of-flight mass spectrometry (RPLC-Q-TOF-MS/MS). In total, 675 lipid molecules, including glycolipids (GLs), phospholipids, sphingolipids (SLs), betaine lipids, and glycerolipids, were identified and semiquantified. With the exception of the high content of diacylglycerols found in L. japonica (54.6% of total lipids), GLs were the dominant component in the three brown seaweeds (27.7-56.7% of total lipids), containing a high proportion of eicosapentaenoic acid. Interestingly, SLs represented by ceramide and hexosylceramide containing phytosphingosine and α-hydroxy fatty acid structures were detected in the three brown seaweeds. A large number of acylated GLs were identified and reported for the first time in these seaweeds, including acylated monogalactosyldiacylglycerol and acylated digalactosyldiacylglycerol containing nonoxidized fatty acids. The bioactive lipids identified herein could be considered potential biomarkers for identifying these seaweeds, evaluating their nutritional value and further promoting their utilization.

A new update of MALDI-TOF mass spectrometry in lipid research

Matrix-assisted laser desorption and ionization (MALDI) mass spectrometry (MS) is an indispensable tool in modern lipid research since it is fast, sensitive, tolerates sample impurities and provides spectra without major analyte fragmentation. We will discuss some methodological aspects, the related ion-forming processes and the MALDI MS characteristics of the different lipid classes (with the focus on glycerophospholipids) and the progress, which was achieved during the last ten years. Particular attention will be given to quantitative aspects of MALDI MS since this is widely considered as the most serious drawback of the method. Although the detailed role of the matrix is not yet completely understood, it will be explicitly shown that the careful choice of the matrix is crucial (besides the careful evaluation of the positive and negative ion mass spectra) in order to be able to detect all lipid classes of interest. Two developments will be highlighted: spatially resolved Imaging MS is nowadays well established and the distribution of lipids in tissues merits increasing interest because lipids are readily detectable and represent ubiquitous compounds. It will also be shown that a combination of MALDI MS with thin-layer chromatography (TLC) enables a fast spatially resolved screening of an entire TLC plate which makes the method competitive with LC/MS.

Rapid identification of plasmalogen molecular species using targeted multiplexed selected reaction monitoring mass spectrometry

Plasmalogens (Pls) levels are reported to be altered in several neurological and metabolic diseases. Identification of sn-1 fatty alcohols and sn-2 fatty acids of different Pls species is necessary to determine the roles and mechanisms of action of Pls in different diseases. Previously, full-scan tandem mass spectrometry (MS/MS) was used for this purpose but is not effective for low-abundance Pls species. Recently, multiplexed selected reaction monitoring MS (SRM/MS) was found to be more selective and sensitive than conventional full-scan MS/MS for the identification of low-abundance compounds. In the present study, we developed a liquid chromatography (LC)-targeted multiplexed SRM/MS system for the identification and quantification of different Pls choline (Pls-PC) and Pls ethanolamine (Pls-PE) species. We determined five precursor-product ion transitions to identify sn-1 and sn-2 fragments of each Pls species. Consequently, sn-1 and sn-2 fatty acyl chains of 22 Pls-PC and 55 Pls-PE species were identified in mouse brain samples. Among them, some species had C20:0 and C20:1 fatty alcohols at the sn-1 position. For quantification of Pls species in mouse brain samples, a single SRM transition was employed. Thus, our results suggest that the LC-targeted multiplexed SRM/MS system is very sensitive for the identification and quantification of low-abundance lipids such as Pls, and is thus expected to make a significant contribution to basic and clinical research in this field in the future.

Protective Effect of l-Theanine against DSS-Induced Colitis by Regulating the Lipid Metabolism and Reducing Inflammation via the NF-κB Signaling Pathway

The present study revealed the phylactic effects of l-theanine on a DSS-induced colitis mice model. The results showed that 3% DSS treatment significantly induced intestinal damage as reflected by DAI, histopathological feature, and colon length, while l-theanine pretreatment markedly prevented these trends to exert protective effects. Meanwhile, l-theanine pretreatment decreased the levels of TNF-α, IL-1β, IL-6, iNOS, and COX2 on DSS-induced colitis. Notably, DSS inhibited the proliferation and promoted the apoptosis of intestinal epithelial cells, thereby damaging the integrity of the intestinal epithelial barrier, whereas l-theanine also played a protective role by attenuating these deteriorated effects. It was also observed that l-theanine treatment downregulated the levels of p-p65, p65, p-p53, p53, and p-AKT protein expression in acute DSS-induced colitis, which showed the protective function of l-theanine, mainly via the NF-κB signaling pathway. Furthermore, the results of lipid analysis and transcriptome analysis show that l-theanine reversed transcriptional profiles and lipid profiles of colitis models, mainly via the inflammatory reactivity-related pathway. Interestingly, the correlation analysis between transcriptional profiles and lipid profiles showed that inflammatory response-related genes were almost significantly correlated with differential lipid metabolites. In summary, l-theanine plays a protective role in DSS-induced colitis via downregulating the NF-κB signaling pathway and regulating lipid metabolism disorders.

Grape Lipidomics: An Extensive Profiling thorough UHPLC-MS/MS Method

Lipids play many essential roles in living organisms, which accounts for the great diversity of these amphiphilic molecules within the individual lipid classes, while their composition depends on intrinsic and extrinsic factors. Recent developments in mass spectrometric methods have significantly contributed to the widespread application of the liquid chromatography-mass spectrometry (LC-MS) approach to the analysis of plant lipids. However, only a few investigators have studied the extensive composition of grape lipids. The present work describes the development of an ultrahigh performance liquid chromatography tandem mass spectrometry (UHPLC-MS/MS) method that includes 8098 MRM; the method has been validated using a reference sample of grapes at maturity with a successful analysis and semi-quantification of 412 compounds. The aforementioned method was subsequently applied also to the analysis of the lipid profile variation during the Ribolla Gialla cv. grape maturation process. The partial least squares (PLS) regression model fitted to our experimental data showed that a higher proportion of certain glycerophospholipids (i.e., glycerophosphoethanolamines, PE and glycerophosphoglycerols, PG) and of some hydrolysates from those groups (i.e., lyso-glycerophosphocholines, LPC and lyso-glycerophosphoethanolamines, LPE) can be positively associated with the increasing °Brix rate, while a negative association was found for ceramides (CER) and galactolipids digalactosyldiacylglycerols (DGDG). The validated method has proven to be robust and informative for profiling grape lipids, with the possibility of application to other studies and matrices.

Analytical Methodologies for Lipidomics in Hemp Plant

The chemical composition of Cannabis sativa L. has been extensively studied for tens of years, but little is known about its lipidome. This chapter describes an analytical workflow for polar lipid determination in hemp. After extraction, lipids are enriched and isolated by graphitized carbon black sorbent, and the isolated lipid is analyzed by liquid chromatography (LC) coupled with high resolution mass spectrometry, leading to identification of many lipid species. We have developed a semi-automated platform using commercially available Lipostar software for lipid identification. Our approach affords the identification of 189 polar lipids in hemp extract, including sulfolipids and phospholipids. The number of the identified lipid species is by far the highest ever reported for Cannabis sativa.

Lipidomics of the Edible Brown Alga Wakame (Undaria pinnatifida) by Liquid Chromatography Coupled to Electrospray Ionization and Tandem Mass Spectrometry

The lipidome of a brown seaweed commonly known as wakame (Undaria pinnatifida), which is grown and consumed around the world, including Western countries, as a healthy nutraceutical food or supplement, was here extensively examined. The study was focused on the characterization of phospholipids (PL) and glycolipids (GL) by liquid chromatography (LC), either hydrophilic interaction LC (HILIC) or reversed-phase LC (RPLC), coupled to electrospray ionization (ESI) and mass spectrometry (MS), operated both in high and in low-resolution mode. Through the acquisition of single (MS) and tandem (MS/MS) mass spectra more than 200 PL and GL of U. pinnatifida extracts were characterized in terms of lipid class, fatty acyl (FA) chain composition (length and number of unsaturations), and regiochemistry, namely 16 SQDG, 6 SQMG, 12 DGDG, 5 DGMG, 29 PG, 8 LPG, 19 PI, 14 PA, 19 PE, 8 PE, 38 PC, and 27 LPC. The FA (C16:0) was the most abundant saturated acyl chain, whereas the monounsaturated C18:1 and the polyunsaturated C18:2 and C20:4 chains were the prevailing ones. Odd-numbered acyl chains, iJ., C15:0, C17:0, C19:0, and C19:1, were also recognized. While SQDG exhibited the longest and most unsaturated acyl chains, C18:1, C18:2, and C18:3, in the sn-1 position of glycerol, they were preferentially located in the sn-2 position in the case of PL. The developed analytical approach might pave the way to extend lipidomic investigations also for other edible marine algae, thus emphasizing their potential role as a source of bioactive lipids.

Ultrasonic Sputter Desorption Mass Spectrometry Technique for Minimally Invasive Tissue Analysis

Minimally invasive techniques for in vivo tissue analysis are desired by life science and medical research. Herein, a new ionization interface coupled with ultrasonic sputter desorption (USD) was developed for in vitro and in vivo tissue analysis. Sample molecules were effectively sputtered out when the high-frequency ultrasonic probe touched the tissue. Then, the sputtered molecules were collected and ionized by a custom-made heated quartz tube and finally analyzed by mass spectrometry (MS) online. The sample pretreatment of the USD-MS technique was quite simple and required no other steps except for wetting the tissue surface with ethanol to assist molecular extraction. Experimental results demonstrated that the proposed method was suitable for the analysis of different morphologies of tissues (such as liver, brain, kidney, and lung) and performed well in the analysis of liver tumors and paracancerous tissues. Moreover, as the proposed method caused little damage to the tissues during analysis, rats and mice with orthotopic tumors still survived after the experiments. Overall, the newly developed USD-MS technique was an effective tool for minimally invasive tissue analysis and could be used as a new candidate method for in situ and real-time analysis of biological tissues in vitro and in vivo.

Direct Mapping of Phospholipid Ferroptotic Death Signals in Cells and Tissues by Gas Cluster Ion Beam Secondary Ion Mass Spectrometry (GCIB-SIMS)

Peroxidized phosphatidylethanolamine (PEox) species have been identified by liquid chromatography mass spectrometry (LC-MS) as predictive biomarkers of ferroptosis, a new program of regulated cell death. However, the presence and subcellular distribution of PEox in specific cell types and tissues have not been directly detected by imaging protocols. By applying gas cluster ion beam secondary ion mass spectrometry (GCIB-SIMS) imaging with a 70 keV (H2 O)n+ (n>28 000) cluster ion beam, we were able to map PEox with 1.2 μm spatial resolution at the single cell/subcellular level in ferroptotic H9c2 cardiomyocytes and cortical/hippocampal neurons after traumatic brain injury. Application of this protocol affords visualization of physiologically relevant levels of very low abundance (20 pmol μmol-1 lipid) peroxidized lipids in subcellular compartments and their accumulation in disease conditions.

Lipid Secretion by Parasitic Cells of Coccidioides Contributes to Disseminated Disease

Coccidioides is a soil-borne fungal pathogen and causative agent of a human respiratory disease (coccidioidomycosis) endemic to semi-desert regions of southwestern United States, Mexico, Central and South America. Aerosolized arthroconidia inhaled by the mammalian host first undergo conversion to large parasitic cells (spherules, 80-100 μm diameter) followed by endosporulation, a process by which the contents of spherules give rise to multiple endospores. The latter are released upon rupture of the maternal spherules and establish new foci of lung infection. A novel feature of spherule maturation prior to endosporulation is the secretion of a lipid-rich, membranous cell surface layer shed in vivo during growth of the parasitic cells and secretion into liquid culture medium during in vitro growth. Chemical analysis of the culture derived spherule outer wall (SOW) fraction showed that it is composed largely of phospholipids and is enriched with saturated fatty acids, including myristic, palmitic, elaidic, oleic, and stearic acid. NMR revealed the presence of monosaccharide- and disaccharide-linked acylglycerols and sphingolipids. The major sphingolipid components are sphingosine and ceramide. Primary neutrophils derived from healthy C57BL/6 and DBA/2 mice incubated with SOW lipids revealed a significant reduction in fungicidal activity against viable Coccidioides arthroconidia compared to incubation of neutrophils with arthroconidia alone. Host cell exposure to SOW lipids had no effect on neutrophil viability. Furthermore, C57BL/6 mice that were challenged subcutaneously with Coccidioides arthroconidia in the presence of the isolated SOW fraction developed disseminated disease, while control mice challenged with arthroconidia alone by the same route showed no dissemination of infection. We hypothesize that SOW lipids contribute to suppression of inflammatory response to Coccidioides infection. Studies are underway to characterize the immunosuppressive mechanism(s) of SOW lipids.

Anthracyclins Increase PUFAs: Potential Implications in ER Stress and Cell Death

Metabolic and personalized interventions in cancer treatment require a better understanding of the relationship between the induction of cell death and metabolism. Consequently, we treated three primary liver cancer cell lines with two anthracyclins (doxorubicin and idarubin) and studied the changes in the lipidome. We found that both anthracyclins in the three cell lines increased the levels of polyunsaturated fatty acids (PUFAs) and alkylacylglycerophosphoethanolamines (etherPEs) with PUFAs. As PUFAs and alkylacylglycerophospholipids with PUFAs are fundamental in lipid peroxidation during ferroptotic cell death, our results suggest supplementation with PUFAs and/or etherPEs with PUFAs as a potential general adjuvant of anthracyclins. In contrast, neither the markers of de novo lipogenesis nor cholesterol lipids presented the same trend in all cell lines and treatments. In agreement with previous research, this suggests that modulation of the metabolism of cholesterol could be considered a specific adjuvant of anthracyclins depending on the type of tumor and the individual. Finally, in agreement with previous research, we found a relationship across the different cell types between: (i) the change in endoplasmic reticulum (ER) stress, and (ii) the imbalance between PUFAs and cholesterol and saturated lipids. In the light of previous research, this imbalance partially explains the sensitivity to anthracyclins of the different cells. In conclusion, our results suggest that the modulation of different lipid metabolic pathways may be considered for generalized and personalized metabochemotherapies.

Perspective on Emerging Mass Spectrometry Technologies for Comprehensive Lipid Structural Elucidation

Lipids and metabolites are of interest in many clinical and research settings because it is the metabolome that is increasingly recognized as a more dynamic and sensitive molecular measure of phenotype. The enormous diversity of lipid structures and the importance of biological structure-function relationships in a wide variety of applications makes accurate identification a challenging yet crucial area of research in the lipid community. Indeed, subtle differences in the chemical structures of lipids can have important implications in cellular metabolism and many disease pathologies. The speed, sensitivity, and molecular specificity afforded by modern mass spectrometry has led to its widespread adoption in the field of lipidomics on many different instrument platforms and experimental workflows. However, unambiguous and complete structural identification of lipids by mass spectrometry remains challenging. Increasingly sophisticated tandem mass spectrometry (MS/MS) approaches are now being developed and seamlessly integrated into lipidomics workflows to meet this challenge. These approaches generally either (i) alter the type of ion that is interrogated or (ii) alter the dissociation method in order to improve the structural information obtained from the MS/MS experiment. In this Perspective, we highlight recent advances in both ion type alteration and ion dissociation methods for lipid identification by mass spectrometry. This discussion is aimed to engage investigators involved in fundamental ion chemistry and technology developments as well as practitioners of lipidomics and its many applications. The rapid rate of technology development in recent years has accelerated and strengthened the ties between these two research communities. We identify the common characteristics and practical figures of merit of these emerging approaches and discuss ways these may catalyze future directions of lipid structural elucidation research.

Lipidomics as a Diagnostic Tool for Prostate Cancer

The main goal of this study was to explore the phospholipid alterations associated with the development of prostate cancer (PCa) using two imaging methods: matrix-assisted laser desorption ionization with time-of-flight mass spectrometer (MALDI-TOF/MS), and electrospray ionization with triple quadrupole mass spectrometer (ESI-QqQ/MS). For this purpose, samples of PCa tissue (n = 40) were evaluated in comparison to the controls (n = 40). As a result, few classes of compounds, namely phosphatidylcholines (PCs), lysophosphatidylcholines (LPCs), sphingomyelins (SMs), and phosphatidylethanolamines (PEs), were determined. The obtained results were evaluated by univariate (Mann-Whitney U-test) and multivariate statistical analysis (principal component analysis, correlation analysis, volcano plot, artificial neural network, and random forest algorithm), in order to select the most discriminative features and to search for the relationships between the responses of these groups of substances, also in terms of the used analytical technique. Based on previous literature and our results, it can be assumed that PCa is linked with both the synthesis of fatty acids and lipid oxidation. Among the compounds, phospholipids, namely PC 16:0/16:1, PC 16:0/18:2, PC 18:0/22:5, PC 18:1/18:2, PC 18:1/20:0, PC 18:1/20:4, and SM d18:1/24:0, were assigned as metabolites with the best discriminative power for the tested groups. Based on the results, lipidomics can be found as alternative diagnostic tool for CaP diagnosis.

Qualitative distribution of endogenous sphingolipids in plasma of human and rodent species by UPLC-Q-Exactive-MS

Sphingolipids (SLs) are endogenously bioactive molecules with diverse structures, and its metabolic disorders are involved in the progression of many diseases. In this study, an ultra-performance liquid chromatography quadrupole exactive mass spectrometry (UPLC-Q-Exactive-MS) method was established to comprehensively profile SLs in plasma. First, the fragment patterns of SL standards of each subclass were investigated. Then, the SL species in plasma were characterized based on the fragmentation rules. Finally, a total of 144 endogenous SL species consisting of 216 regioisomers were identified in plasma of human, golden hamster and C57BL/6 mice, which was the most comprehensive identification for SLs in plasma. In addition to the known species, 19 SL species that have never been reported were also identified. The profile of SLs in plasma of human and two rodent species was compared subsequently. It was worth noting that a total of 9 SL molecular species consisting of 11 regioisomers with low abundance were successfully identified in human plasma through comparison among species. Those findings contribute to a deeper understanding of SLs in human plasma and provide scientific basis for the selection of animal model. The established profile of SLs in plasma could be used for screening of lipid biomarkers of various diseases.

Brain Lipid Dynamics in Amyloid Precursor Protein/Presenilin 1 Mouse Model of Early Alzheimer's Disease by Desorption Electrospray Ionization and Matrix Assisted Laser Desorption Ionization-Mass Spectrometry Imaging Techniques

Alzheimer's disease (AD) is closely associated with lipid metabolism dysfunction. However, space distribution and metabolism of aberrant lipids in the brain of early-stage AD mouse remain unclear. In our current work, a novel lipidomics method based on mass spectrometry imaging was developed to visually disclose molecular perturbation and characterize space distribution in the brain of double transgenic amyloid precursor protein/presenilin 1 mouse (2 and 3 months old). Significant changes were detected, including phosphatidylethanolamines, phosphatidylcholines, fatty acids, lysophospholipids, and glycerides in AD mouse brain. The results in this study suggest that these significantly altered lipid metabolic pathways (glycerophospholipid metabolism) may be implicated in early-stage AD. Our work deepens the understanding of the physio-pathologic mechanism of early-stage AD.

Localization of Carbon-Carbon Double Bond and Cyclopropane Sites in Cardiolipins via Gas-Phase Charge Inversion Reactions

Cardiolipins (CLs) are comprised of two phosphatic acid moieties bound to a central glycerol backbone and are substituted with four acyl chains. Consequently, a vast number of distinct CL structures are possible in different biological contexts, representing a significant analytical challenge. Electrospray ionization tandem mass spectrometry (ESI-MS/MS) has become a widely used approach for the detection, characterization, and quantitation of complex lipids, including CLs. Central to this approach is fragmentation of the [CLs - H]- or [CL - 2H]2- anions by collision-induced dissociation (CID). Product ions in the resulting tandem mass spectra confirm the CL subclass assignment and reveal the numbers of carbons and degrees of unsaturation in each of the acyl chains. Conventional CID, however, affords limited structural elucidation of the fatty acyl chains, failing to discriminate isomers arising from different site(s) of unsaturation or cyclopropanation and potentially obscuring their metabolic origins. Here, we report the application of charge inversion ion/ion chemistry in the gas phase to enhance the structural elucidation of CLs. Briefly, CID of [CL - H]2- anions generated via negative ion ESI allowed for the assignment of individual fatty acyl substituents and phosphatidic acid moieties. Next, gas-phase derivatization of the resulting CL product ions, including fatty acyl carboxylate anions, was effected with gas-phase ion/ion charge inversion reactions with tris-phenanthroline magnesium reagent dications. Subsequent isolation and activation of the charge-inverted fatty acyl complex cations permitted the localization of both carbon-carbon double bond and cyclopropane motifs within each of the four acyl chains of CLs. This approach was applied to the de novo elucidation of unknown CLs in a biological extract revealing distinct isomeric populations and regiochemical relationships between double bonds and carbocyles.

Matrix-assisted ionization Fourier transform mass spectrometry for the analysis of lipids

RATIONALE

Assessing the utility of vacuum matrix-assisted ionization (MAI) for the direct and rapid analysis of lipids in complex samples with emphasis on bacterial taxonomy.

METHODS

Matrix-assisted ionization Fourier transform mass spectrometry (MAI-FTMS) was used to characterize polar and non-polar lipids in mixtures.

RESULTS

For non-polar lipid triacylglycerols (TAGs), MAI-FTMS produced lipid-specific ions for eight different edible oils and allowed these oils to be identified based on their MAI-FTMS profiles. For polar lipids from bacteria, MAI-FTMS of crude lipid extracts allowed taxonomic identification of eight blind-coded samples based on taxonomy-specific phospholipid profiles. MAI produced results comparable and complementary to benchmark MALDI and ESI methods currently used for characterization of polar and non-polar lipids in the same mixtures.

CONCLUSIONS

The newly developed MAI technique is a rapid, simple and complementary method for the characterization of polar and non-polar lipids in complex mixtures.

Animal- free skin permeation analysis using mass spectrometry imaging

Here we demonstrate an animal-free skin permeation analytical approach suitable for testing pharmaceuticals, cosmetics, occupational skin hazards and skin allergens. The method aims to replace or significantly reduce existing in-vivo models and improve on already established in-vitro models. This by offering a more sensitive and flexible analytical approach that can replace and/or complement existing methods in the OECD guidelines for skin adsorption (no 427 and no 428) and measure multiple compounds simultaneously in the skin while being able to also trace endogenous effects in cells. We demonstrate this here by studying how active ingredients in sunscreen permeate through left-over human skin, from routine surgery, in a in a Franz-cell permeation model. Two common sunscreens were therefore applied to the human skin and Time of flight secondary ion mass spectrometry (ToF-SIMS) was used to trace the molecules through the skin. We show that that ToF-SIMS imaging can be applied in visualizing the distribution of Avobenzone, Bemotrizinol, Biscotrizole and Ethyl hexyl triazine at subcellular resolution in the skin. The UV-blockers could be visualized at the same time in one single experiment without any probes or antibodies used. The UV-blockers mostly remained in the stratum corneum. However, in certain features of the skin, such as sebaceous glands, the penetration of the UV-blockers was more prominent, and the compounds reached deeper into the epidermis.

Identification of unusual phospholipids from bovine heart mitochondria by HPLC-MS/MS

Phospholipids, including ether phospholipids, are composed of numerous isomeric and isobaric species that have the same backbone and acyl chains. This structural resemblance results in similar fragmentation patterns by collision-induced dissociation of phospholipids regardless of class, yielding complicated MS/MS spectra when isobaric species are analyzed together. Furthermore, the presence of isobaric species can lead to misassignment of species when made solely based on their molecular weights. In this study, we used normal-phase HPLC for ESI-MS/MS analysis of phospholipids from bovine heart mitochondria. Class separation by HPLC eliminates chances for misidentification of isobaric species from different classes of phospholipids. Chromatography yields simple MS/MS spectra without interference from isobaric species, allowing clear identification of peaks corresponding to fragmented ions containing monoacylglycerol backbone derived from losing one acyl chain. Using these fragmented ions, we characterized individual and isomeric species in each class of mitochondrial phospholipids, including unusual species, such as PS, containing an ether linkage and species containing odd-numbered acyl chains in cardiolipin, PS, PI, and PG. We also characterized monolysocardiolipin and dilysocardiolipin, the least abundant but nevertheless important mitochondrial phospholipids. The results clearly show the power of HPLC-MS/MS for identification and characterization of phospholipids, including minor species.

Evaluating a targeted multiple reaction monitoring approach to global untargeted lipidomic analyses of human plasma

RATIONALE

The Lipidyzer platform was recently updated on a SCIEX QTRAP 6500+ mass spectrometer and offers a targeted lipidomics assay including 1150 different lipids. We evaluated this targeted approach using human plasma samples and compared the results against a global untargeted lipidomics method using a high-resolution Q Exactive HF Orbitrap mass spectrometer.

METHODS

Lipids from human plasma samples (N = 5) were extracted using a modified Bligh-Dyer approach. A global untargeted analysis was performed using a Thermo Orbitrap Q Exactive HF mass spectrometer, followed by data analysis using Progenesis QI software. Multiple reaction monitoring (MRM)-based targeted analysis was performed using a QTRAP 6500+ mass spectrometer, followed by data analysis using SCIEX OS software. The samples were injected on three separate days to assess reproducibility for both approaches.

RESULTS

Overall, 465 lipids were identified from 11 lipid classes in both approaches, of which 159 were similar between the methods, 168 lipids were unique to the MRM approach, and 138 lipids were unique to the untargeted approach. Phosphatidylcholine and phosphatidylethanolamine species were the most commonly identified using the untargeted approach, while triacylglycerol species were the most commonly identified using the targeted MRM approach. The targeted MRM approach had more consistent relative abundances across the three days than the untargeted approach. Overall, the coefficient of variation for inter-day comparisons across all lipid classes was ∼ 23% for the untargeted approach and ∼ 9% for the targeted MRM approach.

CONCLUSIONS

The targeted MRM approach identified similar numbers of lipids to a conventional untargeted approach, but had better representation of 11 lipid classes commonly identified by both approaches. Based on the separation methods employed, the conventional untargeted approach could better detect phosphatidylcholine and sphingomyelin lipid classes. The targeted MRM approach had lower inter-day variability than the untargeted approach when tested using a small group of plasma samples. These studies highlight the advantages in using targeted MRM approaches for human plasma lipidomics analysis.