Imaging of whole zebra fish (Danio rerio) by desorption electrospray ionization mass spectrometry

Development of mass spectrometry imaging techniques and its latest applications

Mass spectrometry imaging (MSI) is a novel molecular imaging technology that collects molecular information from the surface of samples in situ. The spatial distribution and relative content of various compounds can be visualized simultaneously with high spatial resolution. The prominent advantages of MSI promote the active development of ionization technology and its broader applications in diverse fields. This article first gives a brief introduction to the vital parts of the processes during MSI. On this basis, provides a comprehensive overview of the most relevant MS-based imaging techniques from their mechanisms, pros and cons, and applications. In addition, a critical issue in MSI, matrix effects is also discussed. Then, the representative applications of MSI in biological, forensic, and environmental fields in the past 5 years have been summarized, with a focus on various types of analytes (e.g., proteins, lipids, polymers, etc.) Finally, the challenges and further perspectives of MSI are proposed and concluded.

Desorption electrospray ionization mass spectrometry imaging in discovery and development of novel therapies

Desorption electrospray ionization mass spectrometry imaging (DESI-MSI) is one of the least specimen destructive ambient ionization mass spectrometry tissue imaging methods. It enables rapid simultaneous mapping, measurement, and identification of hundreds of molecules from an unmodified tissue sample. Over the years, since its first introduction as an imaging technique in 2005, DESI-MSI has been extensively developed as a tool for separating tissue regions of various histopathologic classes for diagnostic applications. Recently, DESI-MSI has also emerged as a versatile technique that enables drug discovery and can guide the efficient development of drug delivery systems. For example, it has been increasingly employed for uncovering unique patterns of in vivo drug distribution, the discovery of potentially treatable biochemical pathways, revealing novel druggable targets, predicting therapeutic sensitivity of diseased tissues, and identifying early tissue response to pharmacological treatment. These and other recent advances in implementing DESI-MSI as the tool for the development of novel therapies are highlighted in this review.

Spatially Mapping the Baseline and Bisphenol-A Exposed Daphnia magna Lipidome Using Desorption Electrospray Ionization-Mass Spectrometry

Untargeted lipidomics has previously been applied to the study of daphnids and the discovery of biomarkers that are indicative of toxicity. Typically, liquid chromatography—mass spectrometry is used to measure the changes in lipid abundance in whole-body homogenates of daphnids, each only ca. 3 mm in length which limits any biochemical interpretation of site-specific toxicity. Here, we applied mass spectrometry imaging of Daphnia magna to combine untargeted lipidomics with spatial resolution to map the molecular perturbations to defined anatomical regions. A desorption electrospray ionization—mass spectrometry (DESI-MS) method was optimized and applied to tissue sections of daphnids exposed to bisphenol-A (BPA) compared to unexposed controls, generating an untargeted mass spectrum at each pixel (35 µm²/pixel) within each section. First, unique lipid profiles from distinct tissue types were identified in whole-body daphnids using principal component analysis, specifically distinguishing appendages, eggs, eye, and gut. Second, changes in the lipidome were mapped over four stages of normal egg development and then the effect of BPA exposure on the egg lipidome was characterized. The primary perturbations to the lipidome were annotated as triacylglycerides and phosphatidylcholine, and the distributions of the individual lipid species within these classes were visualized in whole-body D. magna sections as ion images. Using an optimized DESI-MS workflow, the first ion images of D. magna tissue sections were generated, mapping both their baseline and BPA-perturbed lipidomes.

Mass spectrometry imaging as a potential technique for diagnostic of Huanglongbing disease using fast and simple sample preparation

Huanglongbing (HLB) is a disease of worldwide incidence that affects orange trees, among other commercial varieties, implicating in great losses to the citrus industry. The disease is transmitted through Diaphorina citri vector, which inoculates Candidatus Liberibacter spp. in the plant sap. HLB disease lead to blotchy mottle and fruit deformation, among other characteristic symptoms, which induce fruit drop and affect negatively the juice quality. Nowadays, the disease is controlled by eradication of sick, symptomatic plants, coupled with psyllid control. Polymerase chain reaction (PCR) is the technique most used to diagnose the disease; however, this methodology involves high cost and extensive sample preparation. Mass spectrometry imaging (MSI) technique is a fast and easily handled sample analysis that, in the case of Huanglongbing allows the detection of increased concentration of metabolites associated to the disease, including quinic acid, phenylalanine, nobiletin and sucrose. The metabolites abieta-8,11,13-trien-18-oic acid, suggested by global natural product social molecular networking (GNPS) analysis, and 4-acetyl-1-methylcyclohexene showed a higher distribution in symptomatic leaves and have been directly associated to HLB disease. Desorption electrospray ionization coupled to mass spectrometry imaging (DESI-MSI) allows the rapid and efficient detection of biomarkers in sweet oranges infected with Candidatus Liberibacter asiaticus and can be developed into a real-time, fast-diagnostic technique.

Methods for Cryosectioning and Mass Spectrometry Imaging of Whole-Body Zebrafish

The zebrafish (Danio rerio) is an ideal model for whole animal studies of lipid metabolism and lipid-related disease. In this work, infrared matrix-assisted laser desorption electrospray ionization (IR-MALDESI) mass spectrometry imaging (MSI) was applied for direct visualization of lipid and metabolite distributions across various organs in whole-body zebrafish tissue sections. Detailed methods for overcoming the challenges of cryosectioning adult male zebrafish for MSI and complementary histological imaging are described. Representative two-dimensional ion maps demonstrated organ specific localization of lipid analytes allowing for visualization of areas of interest including the brain, liver, intestines, and skeletal muscle. A high resolving power mass spectrometer was utilized for accurate mass measurements, which permitted the use of open-source, web-based tools for MS1 annotations including METASPACE and METLIN. Whole-body MSI with IR-MALDESI allowed for broad lipid coverage with high spatial resolution, illustrating the potential of this technique for studying lipid-related diseases using zebrafish as a model organism.

Review and perspectives on the applications of mass spectrometry imaging under ambient conditions

Ambient mass spectrometry (AMS)-based techniques are performed under ambient conditions in which the ionization and desorption occur in the open environment allowing the direct analysis of molecules with minimal or no sample preparation. A selected group of AMS techniques demonstrate imaging capabilities that can provide information about the localization of molecules on complex sample surfaces such as biological tissues. 2D, 3D, and multimodal imaging have unlocked an array of applications to systematically address complex problems in many areas of research such as drug monitoring, natural products, forensics, and cancer diagnostics. In the present review, we summarize recent advances in the field with respect to the implementation of new ambient ionization techniques and current applications in the last 5 years. In more detail, we mainly focus on imaging applications in topics related to animal whole bodies and tissues, single cells, cancer diagnostics and biomarkers, microbial cultures and co-cultures, plant and natural product metabolomics, and forensic applications. Finally, we discuss new areas of research, future perspectives, and the overall direction that the field may take in the years to come.

Software solutions for evaluation and visualization of laser ablation inductively coupled plasma mass spectrometry imaging (LA-ICP-MSI) data: a short overview

Mass spectrometry imaging (MSI) using laser ablation (LA) inductively coupled plasma (ICP) is an innovative and exciting methodology to perform highly sensitive elemental analyses. LA-ICP-MSI of metals, trace elements or isotopes in tissues has been applied to a range of biological samples. Several LA-ICP-MSI studies have shown that metals have a highly compartmentalized distribution in some organs, which might be altered in consequence of genetic diseases, intoxication, or malnutrition. Although metal imaging by LA-ICP-MSI is an established methodology, potential pitfalls in the determination of metal concentrations might result from erroneous calibration, standardization, and normalization. In addition, for simple display of final imaging results, most LA-ICP-MSI users prefer to process their measurements by commercial processing software. Such programs typically visualize the regional metal differences in colorful and vivid imaging maps, but might not represent the actual signal densities correctly. There is a great abundance of such MSI data processing programs available differing in quality, usability, integrated features, workflow, reliability, system requirements, speed of data processing, and price. Some software packages contain a multitude of features which are superfluous for most users. In contrast, often only few data formats are used, in case of commercial programs even only the instrument provider’s own raw data format. Therefore, first time and average users are often confused and helpless in choosing the correct software for processing their data. Here we have briefly summarized software packages, data routines, macros, programming tools, scripts, algorithms, or self-written patches and updates for existing programs presently in use for mining LA-ICP-MSI data.

Lipid dynamics in zebrafish embryonic development observed by DESI-MS imaging and nanoelectrospray-MS

The zebrafish Danio rerio is a model vertebrate organism for understanding biological mechanisms. Recent studies have explored using zebrafish as a model for lipid-related diseases, for in vivo fish bioassays, and for embryonic toxicity experiments. Mass spectrometry (MS) and MS imaging are established tools for lipid profiling and spatial mapping of biomolecules and offer rapid, sensitive, and simple analytical protocols for zebrafish analysis. When ambient ionization techniques are used, ions are generated in native environmental conditions, requiring neither sample preparation nor separation of molecules prior to MS. We used two direct MS techniques to describe the dynamics of the lipid profile during zebrafish embryonic development from 0 to 96 hours post-fertilization and to explore these analytical approaches as molecular diagnostic assays. Desorption electrospray ionization (DESI) MS imaging followed by nanoelectrospray (nESI) MS and tandem MS (MS/MS) were used in positive and negative ion modes, allowing the detection of a large variety of phosphatidylglycerols, phosphatidylcholines, phosphatidylinositols, free fatty acids, triacylglycerols, ubiquinone, squalene, and other lipids, and revealed information on the spatial distributions of lipids within the embryo and on lipid molecular structure. Differences were observed in the relative ion abundances of free fatty acids, triacylglycerols, and ubiquinone - essentially localized to the yolk - across developmental stages, whereas no relevant differences were found in the distribution of complex membrane glycerophospholipids, indicating conserved lipid constitution. Embryos exposed to trichloroethylene for 72 hours exhibited an altered lipid profile, indicating the potential utility of this technique for testing the effects of environmental contaminants.

Monitoring Toxic Ionic Liquids in Zebrafish (Danio rerio) with Desorption Electrospray Ionization Mass Spectrometry Imaging (DESI-MSI)

Ambient mass spectrometry imaging has become an increasingly powerful technique for the direct analysis of biological tissues in the open environment with minimal sample preparation and fast analysis times. In this study, we introduce desorption electrospray ionization mass spectrometry imaging (DESI-MSI) as a novel, rapid, and sensitive approach to localize the accumulation of a mildly toxic ionic liquid (IL), AMMOENG 130 in zebrafish (Danio rerio). The work demonstrates that DESI-MSI has the potential to rapidly monitor the accumulation of IL pollutants in aquatic organisms. AMMOENG 130 is a quaternary ammonium-based IL reported to be broadly used as a surfactant in commercialized detergents. It is known to exhibit acute toxicity to zebrafish causing extensive damage to gill secondary lamellae and increasing membrane permeability. Zebrafish were exposed to the IL in a static 96-h exposure study in concentrations near the LC₅₀ of 1.25, 2.5, and 5.0 mg/L. DESI-MS analysis of zebrafish gills demonstrated the appearance of a dealkylated AMMOENG 130 metabolite in the lowest concentration of exposure identified by a high resolution hybrid LTQ-Orbitrap mass spectrometer as the trimethylstearylammonium ion, [C₂₁H₄₆N]⁺. With DESI-MSI, the accumulation of AMMOENG 130 and its dealkylated metabolite in zebrafish tissue was found in the nervous and respiratory systems. AMMOENG 130 and the metabolite were capable of penetrating the blood brain barrier of the fish with significant accumulation in the brain. Hence, we report for the first time the simultaneous characterization, distribution, and metabolism of a toxic IL in whole body zebrafish analyzed by DESI-MSI. This ambient mass spectrometry imaging technique shows great promise for the direct analysis of biological tissues to qualitatively monitor foreign, toxic, and persistent compounds in aquatic organisms from the environment. Graphical Abstract ᅟ.

MALDI imaging mass spectrometry of Pacific White Shrimp L. vannamei and identification of abdominal muscle proteins

MALDI imaging mass spectrometry (IMS) has been applied to whole animal tissue sections of Pacific White Shrimp, Litopenaeus vannamei, in an effort to identify and spatially localize proteins in specific organ systems. Frozen shrimp were sectioned along the ventral-dorsal axis and methods were optimized for matrix application. In addition, tissue microextraction and homogenization was conducted followed by top-down LC-MS/MS analysis of intact proteins and searches of shrimp EST databases to identify imaged proteins. IMS images revealed organ system specific protein signals that highlighted the hepatopancreas, heart, nervous system, musculature, and cuticle. Top-down proteomics identification of abdominal muscle proteins revealed the sequence of the most abundant muscle protein that has no sequence homology to known proteins. Additional identifications of abdominal muscle proteins included titin, troponin-I, ubiquitin, as well as intact and multiple truncated forms of flightin; a protein known to function in high frequency contraction of insect wing muscles. The combined use of imaging mass spectrometry and top-down proteomics allowed for identification of novel proteins from the sparsely populated shrimp protein databases.

Matrix-assisted laser desorption/ionization mass spectrometry imaging of cell cultures for the lipidomic analysis of potential lipid markers in human breast cancer invasion

RATIONALE

Breast cancer is the leading cause of cancer death among women worldwide. Identification of lipid targets that play a role in breast cancer invasion may advance our understanding of the rapid progression of cancer and may lead to the development of new biomarkers for the disease.

METHODS

Matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI MSI) was applied for the lipidomic profiling of two poorly invasive and two highly invasive breast cancer cell lines to identify the differentially accumulated lipids related to the invasive phenotype. The four cell lines were individually grown on indium tin oxide (ITO)-coated glass slides, analyzed as cell cultures. The raster width and matrix for detection were optimized to improve detection sensitivity.

RESULTS

Optimized MSI measurements were performed directly on the cell culture with 9-aminoacridine as matrix, resulting in 215 endogenous compounds detected in positive ion mode and 267 endogenous compounds in negative ion mode in all the four cell lines, representing the largest group of analytes that have been analyzed from cells by a single MSI study. In highly invasive cell lines, 31 lipids including phosphatidylglycerol (PG) and phosphatidic acids were found upregulated and eight lipids including sphingomyelin (SM) downregulated in negative ion mode. The products of de novo fatty acid synthesis incorporated into membrane phospholipids, like oleic-acid-containing PG, may be involved in mitochondrial dysfunction and thus affect the invasion of breast cancer cells. The deficiency of SM may be related to the disruption of apoptosis in highly invasive cancer cells.

CONCLUSIONS

This work uncovered more analytes in cells by MSI than previous reports, providing a better visualization and novel insights to advance our understanding of the relationship between rapid progression of breast cancer and lipid metabolism. The most altered lipids may aid the discovery of diagnostic markers and therapeutic targets of breast cancer.

Reactive DESI-MS imaging of biological tissues with dicationic ion-pairing compounds

This work illustrates reactive desorption electrospray ionization mass spectrometry (DESI-MS) with a stable dication on biological tissues. Rat brain and zebra fish tissues were investigated with reactive DESI-MS in which the dictation forms a stable bond with biological tissue fatty acids and lipids. Tandem mass spectrometry (MS/MS) was used to characterize the dication (DC9) and to identify linked lipid-dication compounds formed. The fragment m/z 85 common to both DC9 fragmentation and DC9-lipid fragmentation was used to confirm that DC9 is indeed bonded with the lipids. Lipid signals in the range of m/z 250-350 and phosphoethanolamines (PE) m/z 700-800 observed in negative ion mode were also detected in positive ion mode with reactive DESI-MS with enhanced signal intensity. Reactive DESI-MS imaging in positive ion mode of rat brain and zebra fish tissues allowed enhanced detection of compounds commonly observed in the negative ion mode.

DESI imaging of small molecules in biological tissues

Desorption electrospray ionization (DESI) allows the direct analysis of ordinary objects or preprocessed samples under ambient conditions. Among other applications, DESI is used to identify and to record spatial distributions of small molecules in situ, sliced or imprinted biological tissue. Manipulation of the chemistry accompanying ambient analysis ionization can be used to optimize chemical analysis, including molecular imprinting. Images are obtained by continuously moving the sample relative to the DESI sprayer and the inlet of the mass spectrometer. The acquisition time depends on the size of the surface to be analyzed and on the desired resolution.