A review of complementary separation methods and matrix assisted laser desorption ionization-mass spectrometry imaging: Lowering sample complexity

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.

Implications of Peak Selection in the Interpretation of Unsupervised Mass Spectrometry Imaging Data Analyses

Mass spectrometry imaging can produce large amounts of complex spectral and spatial data. Such data sets are often analyzed with unsupervised machine learning approaches, which aim at reducing their complexity and facilitating their interpretation. However, choices made during data processing can impact the overall interpretation of these analyses. This work investigates the impact of the choices made at the peak selection step, which often occurs early in the data processing pipeline. The discussion is done in terms of visualization and interpretation of the results of two commonly used unsupervised approaches: t-distributed stochastic neighbor embedding and k-means clustering, which differ in nature and complexity. Criteria considered for peak selection include those based on hypotheses (exemplified herein in the analysis of metabolic alterations in genetically engineered mouse models of human colorectal cancer), particular molecular classes, and ion intensity. The results suggest that the choices made at the peak selection step have a significant impact in the visual interpretation of the results of either dimensionality reduction or clustering techniques and consequently in any downstream analysis that relies on these. Of particular significance, the results of this work show that while using the most abundant ions can result in interesting structure-related segmentation patterns that correlate well with histological features, using a smaller number of ions specifically selected based on prior knowledge about the biochemistry of the tissues under investigation can result in an easier-to-interpret, potentially more valuable, hypothesis-confirming result. Findings presented will help researchers understand and better utilize unsupervised machine learning approaches to mine high-dimensionality data.

Gold Nanoparticle-Assisted Black Silicon Substrates for Mass Spectrometry Imaging Applications

Mass spectrometry imaging (MSI) based on matrix-assisted laser desorption ionization (MALDI) is widely used in proteomics. However, matrix-free technologies are gaining popularity for detecting low molecular mass compounds. Small molecules were analyzed with nanostructured materials as ionization promoters, which produce low-to-no background signal, and facilitate enhanced specificity and sensitivity through functionalization. We investigated the fabrication and the use of black silicon and gold-coated black silicon substrates for surface-assisted laser desorption/ionization mass spectrometry imaging (SALDI-MSI) of animal tissues and human fingerprints. Black silicon was created using dry etching, while gold nanoparticles were deposited by sputtering. Both methods are safe for the user. Physicochemical characterization and MSI measurements revealed the optimal properties of the substrates for SALDI applications. The gold-coated black silicon worked considerably better than black silicon as the LDI-MSI substrate. The substrate was also compatible with imprinting, as a sample-simplification method that allows efficient transference of metabolites from the tissues to the substrate surface, without compound delocalization. Moreover, by modifying the surface with hydrophilic and hydrophobic groups, specific interactions were stimulated between surface and sample, leading to a selective analysis of molecules. Thus, our substrate facilitates targeted and/or untargeted in situ metabolomics studies for various fields such as clinical, environmental, forensics, and pharmaceutical research.

Imaging mass spectrometry allows for neuroanatomic-specific detection of gangliosides in the healthy and diseased brain

Gangliosides have a wide variety of biological functions due to their location on the outer leaflet of plasma membranes. They form a critical component of membrane rafts, or ganglioside-enriched microdomains, where they influence the physical properties of the membrane as well as its function. Gangliosides can change their structure to meet their external and internal environmental demands. This ability to change structure makes gangliosides both fascinating and technologically challenging targets to identify and understand. A full understanding on how gangliosides are regulated within the central nervous system (CNS) is critical, as ganglioside dysregulation is observed in the aging brain as well as in several neurodegenerative injuries and diseases such as stroke, Alzheimer's disease, Parkinson's disease, Huntington's disease and several lysosomal storage disorders diseases, including Tay Sach's disease. Mass spectrometry (MS) has become a useful means to better understand ganglioside composition and function. Imaging mass spectrometry (IMS) provides the added benefit of placing analytical information within an anatomical context. This review article will discuss recent advances in MS-based detection methods, with a focus on IMS-based approaches to help understand the spatial-specific role gangliosides in the healthy brain as in CNS injuries and disease.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: An update for 2013-2014

This review is the eighth update of the original article published in 1999 on the application of Matrix-assisted laser desorption/ionization mass spectrometry (MALDI) mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2014. Topics covered in the first part of the review include general aspects such as theory of the MALDI process, matrices, derivatization, MALDI imaging, fragmentation, and arrays. The second part of the review is devoted to applications to various structural types such as oligo- and poly- saccharides, glycoproteins, glycolipids, glycosides, and biopharmaceuticals. Much of this material is presented in tabular form. The third part of the review covers medical and industrial applications of the technique, studies of enzyme reactions, and applications to chemical synthesis. © 2018 Wiley Periodicals, Inc. Mass Spec Rev 37:353-491, 2018.

Imaging mass spectrometry (IMS) of cortical lipids from preclinical to severe stages of Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative disease affecting millions of patients worldwide. Previous studies have demonstrated alterations in the lipid composition of lipid extracts from plasma and brain samples of AD patients. However, there is no consensus regarding the qualitative and quantitative changes of lipids in brains from AD patients. In addition, the recent developments in imaging mass spectrometry methods are leading to a new stage in the in situ analysis of lipid species in brain tissue slices from human postmortem samples. The present study uses the matrix-assisted laser desorption/ionization imaging mass spectrometry (MALDI-IMS), permitting the direct anatomical analysis of lipids in postmortem brain sections from AD patients, which are compared with the intensity of the lipid signal in samples from matched subjects with no neurological diseases. The frontal cortex samples from AD patients were classified in three groups based on Braak's histochemical criteria, ranging from non-cognitively impaired patients to those severely affected. The main results indicate a depletion of different sulfatide lipid species from the earliest stages of the disease in both white and gray matter areas of the frontal cortex. Therefore, the decrease in sulfatides in cortical areas could be considered as a marker of the disease, but may also indicate neurochemical modifications related to the pathogenesis of the disease. This article is part of a Special Issue entitled: Membrane Lipid Therapy: Drugs Targeting Biomembranes edited by Pablo V. Escribá.

Enhanced capabilities for imaging gangliosides in murine brain with matrix-assisted laser desorption/ionization and desorption electrospray ionization mass spectrometry coupled to ion mobility separation

The increased interest in lipidomics calls for improved yet simplified methods of lipid analysis. Over the past two decades, mass spectrometry imaging (MSI) has been established as a powerful technique for the analysis of molecular distribution of a variety of compounds across tissue surfaces. Matrix-assisted laser desorption/ionization (MALDI) MSI is widely used to study the spatial distribution of common lipids. However, a thorough sample preparation and necessity of vacuum for efficient ionization might hamper its use for high-throughput lipid analysis. Desorption electrospray ionization (DESI) is a relatively young MS technique. In DESI, ionization of molecules occurs under ambient conditions, which alleviates sample preparation. Moreover, DESI does not require the application of an external matrix, making the detection of low mass species more feasible due to the lack of chemical matrix background. However, irrespective of the ionization method, the final information obtained during an MSI experiment is very complex and its analysis becomes challenging. It was shown that coupling MSI to ion mobility separation (IMS) simplifies imaging data interpretation. Here we employed DESI and MALDI MSI for a lipidomic analysis of the murine brain using the same IMS-enabled instrument. We report for the first time on the DESI IMS-MSI of multiply sialylated ganglioside species, as well as their acetylated versions, which we detected directly from the murine brain tissue. We show that poly-sialylated gangliosides can be imaged as multiply charged ions using DESI, while they are clearly separated from the rest of the lipid classes based on their charge state using ion mobility. This represents a major improvement in MSI of intact fragile lipid species. We additionally show that complementary lipid information is reached under particular conditions when DESI is compared to MALDI MSI.

Laser Ablation with Vacuum Capture for MALDI Mass Spectrometry of Tissue

We have developed a laser ablation sampling technique for matrix-assisted laser desorption ionization (MALDI) mass spectrometry and tandem mass spectrometry (MS/MS) analyses of in-situ digested tissue proteins. Infrared laser ablation was used to remove biomolecules from tissue sections for collection by vacuum capture and analysis by MALDI. Ablation and transfer of compounds from tissue removes biomolecules from the tissue and allows further analysis of the collected material to facilitate their identification. Laser ablated material was captured in a vacuum aspirated pipette-tip packed with C18 stationary phase and the captured material was dissolved, eluted, and analyzed by MALDI. Rat brain and lung tissue sections 10 μm thick were processed by in-situ trypsin digestion after lipid and salt removal. The tryptic peptides were ablated with a focused mid-infrared laser, vacuum captured, and eluted with an acetonitrile/water mixture. Eluted components were deposited on a MALDI target and mixed with matrix for mass spectrometry analysis. Initial experiments were conducted with peptide and protein standards for evaluation of transfer efficiency: a transfer efficiency of 16% was obtained using seven different standards. Laser ablation vacuum capture was applied to freshly digested tissue sections and compared with sections processed with conventional MALDI imaging. A greater signal intensity and lower background was observed in comparison with the conventional MALDI analysis. Tandem time-of-flight MALDI mass spectrometry was used for compound identification in the tissue.

Decrease in Sphingomyelin (d18:1/16:0) in Stem Villi and Phosphatidylcholine (16:0/20:4) in Terminal Villi of Human Term Placentas with Pathohistological Maternal Malperfusion

Placental villi play pivotal roles in feto-maternal transportation and phospholipids constitute a major part of the villous membrane. We have been developing and optimizing an imaging system based on a matrix-assisted laser desorption/ionization (MALDI)-based mass spectrometer, which provides clear two-dimensional molecular distribution patterns using highly sensitive mass spectrometry from mixtures of ions generated on tissue surfaces. We recently applied this technology to normal human uncomplicated term placentas and detected the specific distribution of sphingomyelin (SM) (d18:1/16:0) in stem villi and phosphatidylcholine (PC) (16:0/20:4) in terminal villi. In the present study, we applied this technology to nine placentas with maternal or fetal complications, and determined whether a relationship existed between these specific distribution patterns of phospholipid molecules and the six representative pathological findings of placentas, i.e., villitis of unknown etiology (VUE), thrombus, atherosis, chorioamnionitis (CAM), immature terminal villi, and multiple branched terminal villi. In two placentas with the first and second largest total number of positive pathological findings, i.e., five and three positive findings, the specific distribution of SM (d18:1/16:0) in stem villi and PC (16:0/20:4) in terminal villi disappeared. The common pathological findings in these two placentas were atherosis, immature terminal villi, and multiple branched terminal villi, suggesting the possible involvement of the underperfusion of maternal blood into the intervillous space. On the other hand, the number of pathological findings were two or less in the seven other placentas, in which no specific relationships were observed between the differential expression patterns of these two phospholipids in stem and terminal villi and the pathological findings of the placentas; however, the specific distribution pattern of SM (d18:1/16:0) in stem villi disappeared in four placentas, while that of PC (16:0/20:4) in terminal villi was preserved. These results suggested that the absence of the specific distribution of PC (16:0/20:4) in terminal villi, possibly in combination with the absence of SM (d18:1/16:0) in stem villi, was linked to placental morphological changes in response to maternal underperfusion of the placenta.

In situ drug and metabolite analysis [corrected] in biological and clinical research by MALDI MS imaging

In recent years the analysis in mass spectrometry (MS) [corrected] imaging has been expanded to detect a wide variety of low molecular weight compounds (LMWC), including exogenous and endogenous compounds. The high sensitivity and selectivity of MS imaging combined with visualization of molecular spatial distribution in tissues, makes it a valuable [corrected] platform in targeted drug and untargeted metabolomic analysis [corrected] in biological and clinical research. Here, we review the current and potential applications of MALDI MS imaging in these areas. The aim of advancing MALDI MS imaging in the field of LMWC is to support clinical applications by understanding drug and drug-metabolite distribution, investigating toxicity and discovering [corrected] new biomarkers.

Solvent separating secondary metabolites directly from biosynthetic tissue for surface-assisted laser desorption ionisation mass spectrometry

Marine bioactive metabolites are often heterogeneously expressed in tissues both spatially and over time. Therefore, traditional solvent extraction methods benefit from an understanding of the in situ sites of biosynthesis and storage to deal with heterogeneity and maximize yield. Recently, surface-assisted mass spectrometry (MS) methods namely nanostructure-assisted laser desorption ionisation (NALDI) and desorption ionisation on porous silicon (DIOS) surfaces have been developed to enable the direct detection of low molecular weight metabolites. Since direct tissue NALDI-MS or DIOS-MS produce complex spectra due to the wide variety of other metabolites and fragments present in the low mass range, we report here the use of "on surface" solvent separation directly from mollusc tissue onto nanostructured surfaces for MS analysis, as a mechanism for simplifying data annotation and detecting possible artefacts from compound delocalization during the preparative steps. Water, ethanol, chloroform and hexane selectively extracted a range of choline esters, brominated indoles and lipids from Dicathais orbita hypobranchial tissue imprints. These compounds could be quantified on the nanostructured surfaces by comparison to standard curves generated from the pure compounds. Surface-assisted MS could have broad utility for detecting a broad range of secondary metabolites in complex marine tissue samples.