Alternative fatty acid desaturation pathways revealed by deep profiling of total fatty acids in RAW 264.7 cell line

Skylite: Skyline-Based Lipid Isomer Retention Time Evaluation for Lipidomics in Metabolic Dysfunction-Associated Steatohepatitis

Metabolic dysfunction-associated steatotic liver disease (MASLD) is the most prevalent liver disorder worldwide and can progress to steatohepatitis. Elevated de novo lipogenesis (DNL) is a key contributor to hepatic steatosis. Fatty acid (FA) desaturation produces several unsaturated lipid isomers that are structurally very similar but have diverse biological functions. However, due to their structural similarity, many conventional mass spectrometry approaches cannot detect such metabolic alterations. Thus, we introduce the Skylite (Skyline-based lipid isomer retention time evaluation) workflow using conventional liquid chromatography-mass spectrometry (LC-MS) to identify important isomer features. Retention times of isomeric phosphatidylcholines are compared with the well-characterized human plasma reference standard NIST 1950. Retention time trends correlate well with fixed-charge derivatized FA in liquid chromatography and ozone-induced dissociation mass spectrometry data. The interpretation is supported by double bond diagnostic fragments in LC-MS/MS experiments of epoxidized hydrolyzed fatty acids. We investigate hepatic lipid profiles, focusing on esterified fatty acids in two mouse models of metabolic dysfunction-associated steatohepatitis (MASH). Out of 37 phosphatidylcholine sum compositions, the workflow identifies 123 lipid features. Importantly, CCl4-induced and melanocortin-4 receptor knockout mice on a western diet (WD) have significantly higher levels of mead acid, branched-chain fatty acid, and n-7 PUFA incorporated into phosphatidylcholines. While the MASH mouse liver tissues contain notable amounts of n-7 PUFA, no n-10 PUFA were detected, potentially indicating a unique desaturation pattern. The screening for altered lipid isomer profiles bridges the gap between high-throughput analyses and specialized structure-resolved techniques.

Mass Spectrometry Imaging for the Characterization of C═C Localization in Unsaturated Lipid Isomers at the Single-Cell Level

Unsaturated lipids with carbon-carbon double bonds (C═C) have been implicated in the pathogenesis of various diseases. While mass spectrometry imaging (MSI) has been employed to map the distribution of lipid isomers in tissue sections, the identification of lipid C═C positional isomers at the single-cell level using MSI poses a significant challenge. In this study, we developed a novel approach utilizing ToF-SIMS in conjunction with the Paternò-Büchi (P-B) photochemical reaction to characterize the C═C localization in unsaturated lipid isomers at the single-cell level. The P-B reaction was employed to produce adduct products, which were subsequently subjected to collision-induced dissociation by the primary ion beam of ToF-SIMS to generate characteristic ion pairs indicative of the presence of C═C bonds. Utilizing this approach, lipid isomers in brain and skeletal tissues from mice, as well as different cell lines, were visualized at single-cell resolution. Furthermore, distinct variations in the composition of FA 18:1 isomers across different microregions and cell types were revealed. Our P-B ToF-SIMS approach enables the accurate identification and characterization of complex lipid structures with remarkable spatial resolution and can be helpful in understanding the physiological role of these C═C positional isomers.

Sample preparation for fatty acid analysis in biological samples with mass spectrometry-based strategies

Fatty acids (FAs) have attracted many interests for their pivotal roles in many biological processes. Imbalance of FAs is related to a variety of diseases, which makes the measurement of them important in biological samples. Over the past two decades, mass spectrometry (MS) has become an indispensable technique for the analysis of FAs owing to its high sensitivity and precision. Due to complex matrix effect of biological samples and inherent poor ionization efficiency of FAs in MS, sample preparation including extraction and chemical derivatization prior to analysis are often employed. Here, we describe an updated overview of FA extraction techniques, as well as representative derivatization methods utilized in different MS platforms including gas chromatography-MS, liquid chromatography-MS, and mass spectrometry imaging based on different chain lengths of FAs. Derivatization strategies for the identification of double bond location in unsaturated FAs are also summarized and highlighted. The advantages, disadvantages, and prospects of these methods are compared and discussed. This review provides the development and valuable information for sample pretreatment approaches and qualitative and quantitative analysis of interested FAs using different MS-based platforms in complex biological matrices. Finally, the challenges of FA analysis are summarized and the future perspectives are prospected.

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.