Recent advances in ambient mass spectrometry imaging

Rapid determination of glyphosate and glufosinate in human blood by probe electrospray ionization tandem mass spectrometry

In forensic science, glyphosate (GLYP) and glufosinate (GLUF), a class of non-selective broad-spectrum herbicides, have been frequently encountered in many fatal poisoning and suicide cases due to their widespread availability. Therefore, it is essential to develop an effective method for detecting these compounds. Some conventional methods, such as gas chromatography-mass spectrometry (GC-MS) or liquid chromatography-mass spectrometry (LC-MS), have been reported to detect these compounds. However, these methods are not ideal for their time-consuming and non-sensitive feature. Herein, probe electrospray ionization (PESI) tandem mass spectrometry (MS/MS), a fast and sensitive technique, was applied for the determination of GLYP and GLUF in human blood, which can obtain analytical results within 0.5 min without derivatization and chromatographic separation. After protein precipitation of blood samples, the supernatant was mixed with isopropanol and ultra-pure water (1:1 v/v). Then, 8 μL of the mixture was introduced into the plastic sample plate for PESI-MS/MS analysis. The limits of detection (LODs) of the method were 0.50 μg/mL and 0.25 μg/mL for two analytes, and the limits of quantitation (LOQs) were both 1.00 μg/mL, which are higher than the concentration of reported poisoning and fatal cases. In the linear range of 1-500 μg/mL, the regression coefficients (r2) for GLYP and GLUF were over 0.99. The matrix effects ranged from 94.8 % to 119.5 %, and the biases were below 4.3 %. The recoveries ranged between 84.8 % and 107.4 %, and the biases were below 7.6 %. Meanwhile, the method was effectively utilized to detect and quantify the blood, urine, and other samples. Consequently, the results suggest that PESI-MS/MS is a straightforward, fast, and sensitive method for detecting GLUF and GLYP in forensics. In the future, PESI-MS/MS will become an indispensable technique for polar substances in grassroots units of public security where rapid detection is essential.

Imaging plant metabolism in situ

Mass spectrometry imaging (MSI) has emerged as an invaluable analytical technique for investigating the spatial distribution of molecules within biological systems. In the realm of plant science, MSI is increasingly employed to explore metabolic processes across a wide array of plant tissues, including those in leaves, fruits, stems, roots, and seeds, spanning various plant systems such as model species, staple and energy crops, and medicinal plants. By generating spatial maps of metabolites, MSI has elucidated the distribution patterns of diverse metabolites and phytochemicals, encompassing lipids, carbohydrates, amino acids, organic acids, phenolics, terpenes, alkaloids, vitamins, pigments, and others, thereby providing insights into their metabolic pathways and functional roles. In this review, we present recent MSI studies that demonstrate the advances made in visualizing the plant spatial metabolome. Moreover, we emphasize the technical progress that enhances the identification and interpretation of spatial metabolite maps. Within a mere decade since the inception of plant MSI studies, this robust technology is poised to continue as a vital tool for tackling complex challenges in plant metabolism.

Lewis Acidic Metal-Organic Framework Assisted Ambient Liquid Extraction Mass Spectrometry Imaging for Enhancing the Coverage of Poorly Ionizable Lipids in Brain Tissue

The spatial distribution of lipidomes in tissues is of great importance in studies of living processes, diseases, and therapies. Mass spectrometry imaging (MSI) has become a critical technique for spatial lipidomics. However, MSI of low-abundance or poorly ionizable lipids is still challenging because of the ion suppression from high-abundance lipids. Here, a metal-organic framework (MOF) Zr6O4(OH)4(1,3,5-Tris(4-carboxyphenyl) benzene)2(triflate)6(Zr6OTf-BTB) was prepared and used for selective on-tissue adsorption of phospholipids to reduce ion suppression from them to poorly ionizable lipids. The results show that Zr6OTf-BTB with strong Lewis acidic sites and a large specific surface area (647.9 m2·g-1) could selectively adsorb phospholipids under 1% FA-MeOH. Adsorption efficiencies of phospholipids are 88.4-144.9 times higher than those of other neutral lipids. Moreover, the adsorption capacity and the adsorption kinetic rate constant of the new material to phospholipids are higher than those of Zr6-BTB (242.72 vs 73.96 mg·g-1, 0.0442 vs 0.0220 g·mg-1·min-1). A Zr6OTf-BTB sheet was prepared by a lamination technique for on-tissue phospholipid adsorption from brain tissue. Then, the tissue section on the Zr6OTf-BTB sheet was directly imaged via ambient liquid extraction-MSI with 1% FA-MeOH as the sampling solvent. The results showed that phospholipids could be 100% removed directly on tissue, and the detection coverage of the Zr6OTf-BTB-enhanced MSI method to ceramides (Cers) and hexosylceramides (HexCers) was increased by 5-26 times compared with direct tissue MSI (26 vs 1 and 17 vs 3). The new method provides an efficient and convenient way to eliminate the ion suppression from phospholipids in MSI, largely improving the detection coverage of low-abundance and poorly ionizable lipids.

Optical sensors (optodes) for multiparameter chemical imaging: classification, challenges, and prospects

Chemical gradients and uneven distribution of analytes are common in natural and artificial systems. As a result, the ability to visualize chemical distributions in two or more dimensions has gained significant importance in recent years. This has led to the integration of chemical imaging techniques into all domains of analytical chemistry. In this review, we focus on the use of optical sensors, so-called optodes, to obtain real-time and multidimensional images of two or more parameters simultaneously. It is important to emphasize that multiparameter imaging in this context is not confined solely to multiple chemical parameters (analytes) but also encompasses physical (e.g., temperature or flow) or biological (e.g., metabolic activity) parameters. First, we discuss the technological milestones that have paved the way for chemical imaging using optodes. Later, we delve into various strategies that can be taken to enable multiparameter imaging. The latter spans from developing novel receptors that enable the recognition of multiple parameters to chemometrics and machine learning-based techniques for data analysis. We also explore ongoing trends, challenges, and prospects for future developments in this field. Optode-based multiparameter imaging is a rapidly expanding field that is being fueled by cutting-edge technologies. Chemical imaging possesses the potential to provide novel insights into complex samples, bridging not only across various scientific disciplines but also between research and society.

A monolithic microfluidic probe for ambient mass spectrometry imaging of biological tissues

Ambient mass spectrometry imaging (MSI) is a powerful technique that allows for the simultaneous mapping of hundreds of molecules in biological samples under atmospheric conditions, requiring minimal sample preparation. We have developed nanospray desorption electrospray ionization (nano-DESI), a liquid extraction-based ambient ionization technique, which has proven to be sensitive and capable of achieving high spatial resolution. We have previously described an integrated microfluidic probe, which simplifies the nano-DESI setup, but is quite difficult to fabricate. Herein, we introduce a facile and scalable strategy for fabricating microfluidic devices for nano-DESI MSI applications. Our approach involves the use of selective laser-assisted etching (SLE) of fused silica to create a monolithic microfluidic probe (SLE-MFP). Unlike the traditional photolithography-based fabrication, SLE eliminates the need for the wafer bonding process and allows for automated, scalable fabrication of the probe. The chamfered design of the sampling port and ESI emitter significantly reduces the amount of polishing required to fine-tune the probe thereby streamlining and simplifying the fabrication process. We have also examined the performance of a V-shaped probe, in which only the sampling port is fabricated using SLE technology. The V-shaped design of the probe is easy to fabricate and provides an opportunity to independently optimize the size and shape of the electrospray emitter. We have evaluated the performance of SLE-MFP by imaging mouse tissue sections. Our results demonstrate that SLE technology enables the fabrication of robust monolithic microfluidic probes for MSI experiments. This development expands the capabilities of nano-DESI MSI and makes the technique more accessible to the broader scientific community.

Guided analysis of ambient ionization mass spectrometry data with the MQ_Assistant

RATIONALE

Ambient ionization mass spectrometry (AIMS) delivers realistic data from samples in their native state. In addition, AIMS methods reduce time and costs for sample preparation and have less environmental impact. However, AIMS data are often complex and require substantial processing before interpretation.

METHODS

We developed an interactive R script for guided mass spectrometry (MS) data processing. The "MQ_Assistant" is based on MALDIquant, a popular R package for MS data processing. In each step, the user can try and preview the effect of chosen parameters before deciding on the values with the best result and proceeding to the next stage. The outcome of the MQ_Assistant is a feature matrix that can be further analyzed in R and statistics tools such as MetaboAnalyst.

RESULTS

Using 360 AIMS example spectra, we demonstrate the step-by-step processing for creating a feature matrix. In addition, we show how to visualize the results of three biological replicates of a plant-microbe interaction between Arabidopsis and Trichoderma as a heatmap using R and upload them to MetaboAnalyst. The final parameter set can be saved for reuse in MALDIquant workflows of similar data.

CONCLUSIONS

The MQ_Assistant helps novices and experienced users to develop workflows for (AI)MS data processing. The interactive procedure supports the quick finding of appropriate settings. These parameters can be exported and reused in future projects. The stepwise operation with visual feedback also suggests the use of the MQ_Assistant in education.

Imidazolium-based mass tags for protein biomarker detection using laser desorption ionization mass spectrometry

Novel imidazolium-based mass tags (IMTs) were designed, synthesized and applied to simultaneous in situ analysis of multiple biomarkers on less than 10 cells. The high sensitivity, flexible extensibility and excellent distinguishability of IMTs open new avenues for designing common mass tag templates suitable for mass spectrometric immunoassay and provide an ideal option for multiplex-sensitive detection at the cellular scale.

High-Throughput Mass Spectrometry Imaging of Biological Systems: Current Approaches and Future Directions

In the past two decades, the power of mass spectrometry imaging (MSI) for the label free spatial mapping of molecules in biological systems has been substantially enhanced by the development of approaches for imaging with high spatial resolution. With the increase in the spatial resolution, the experimental throughput has become a limiting factor for imaging of large samples with high spatial resolution and 3D imaging of tissues. Several experimental and computational approaches have been recently developed to enhance the throughput of MSI. In this critical review, we provide a succinct summary of the current approaches used to improve the throughput of MSI experiments. These approaches are focused on speeding up sampling, reducing the mass spectrometer acquisition time, and reducing the number of sampling locations. We discuss the rate-determining steps for different MSI methods and future directions in the development of high-throughput MSI techniques.

Enhancement of Ambient Mass Spectrometry Imaging Data by Image Restoration

Mass spectrometry imaging (MSI) has been a key driver of groundbreaking discoveries in a number of fields since its inception more than 50 years ago. Recently, MSI development trends have shifted towards ambient MSI (AMSI) as the removal of sample-preparation steps and the possibility of analysing biological specimens in their natural state have drawn the attention of multiple groups across the world. Nevertheless, the lack of spatial resolution has been cited as one of the main limitations of AMSI. While significant research effort has presented hardware solutions for improving the resolution, software solutions are often overlooked, although they can usually be applied in a cost-effective manner after image acquisition. In this vein, we present two computational methods that we have developed to directly enhance the image resolution post-acquisition. Robust and quantitative resolution improvement is demonstrated for 12 cases of openly accessible datasets across laboratories around the globe. Using the same universally applicable Fourier imaging model, we discuss the possibility of true super-resolution by software for future studies.

Pyrylium-Based Derivatization for Rapid Labeling and Enhanced Detection of Cholesterol in Mass Spectrometry Imaging

Cholesterol in the central nervous system has been increasingly found to be closely related to neurodegenerative diseases. Defects in cholesterol metabolism can cause structural and functional disorders of the central nervous system. The detection of abnormal cholesterol is of great significance for the cognition of physiological and pathological states of organisms, and the spatial distribution of cholesterol can also provide more clues for our understanding of the complex mechanism of disease. Here, we developed a novel pyrylium-based derivatization reagent combined with matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI) to visualize cholesterol in biological tissues. A new class of charged hydroxyl derivatization reagents was designed and synthesized, and finally 1-(carboxymethyl)-2,4,6-trimethylpyridinium (CTMP) was screened for tissue derivatization of cholesterol. Different from the shortcomings of traditional hydroxyl labeling methods such as harsh reaction conditions and long reaction time, in our study, we combined the advantages of CTMP itself and the EDCl/HOBt reaction system to achieve instant labeling of cholesterol on tissues through two-step activation. In addition, we also reported changes in cholesterol content in different stages and different brain regions during disease development in SOD1 mutant mouse model. The cholesterol derivatization method we developed provides an efficient way to explore the distribution and spatial metabolic network of cholesterol in biological tissues.

The Exploration of Microbial Natural Products and Metabolic Interaction Guided by Mass Spectrometry Imaging

As an impressive mass spectrometry technology, mass spectrometric imaging (MSI) can provide mass spectra data and spatial distribution of analytes simultaneously. MSI has been widely used in diverse fields such as clinical diagnosis, the pharmaceutical industry and environmental study due to its accuracy, high resolution and developing reproducibility. Natural products (NPs) have been a critical source of leading drugs; almost half of marketed drugs are derived from NPs or their derivatives. The continuous search for bioactive NPs from microorganisms or microbiomes has always been attractive. MSI allows us to analyze and characterize NPs directly in monocultured microorganisms or a microbial community. In this review, we briefly introduce current mainstream ionization technologies for microbial samples and the key issue of sample preparation, and then summarize some applications of MSI in the exploration of microbial NPs and metabolic interaction, especially NPs from marine microbes. Additionally, remaining challenges and future prospects are discussed.

Increasing DESI-MS Ion Signal by Plasma Treatment

Many studies are focused on using plasma in mass spectrometry as an ionization source or postionization method. In this study, the effect of plasma treatment in the sample preparation step of desorption electrospray ionization (DESI) has been investigated. The plasma treatment of polar samples, including morphine, codeine, captopril, theophylline, fructose, and amphiphilic compounds such as phosphatidylethanolamine (PE) in E. coli bacteria, as well as nonpolar compounds, including thebaine, papaverine, and noscapine, has been followed for ionization efficiency in DESI technique. An atmospheric-pressure glow discharge plasma (GDP) along with the electrospray ionization technique is examined. Plasma treatment before ambient ionization has a dramatic effect on polar and nonpolar sample signals in DESI-TOF mass spectrometry. The intensity of the mass spectrum shows an increase of 1.9-3.4 times for polar compounds, 2.1-2.5 times for nonpolar compounds, and 3.0 times for PE in E. coli bacteria (N = 4). Plasma is a source of reactive atoms, molecules, ions, radicals, and ultraviolet radiation. Plasma surface treatment before DESI analysis by energetic species through momentum/energy transfer yields higher energy surface molecules, leading to more/easier desorption. Under optimal treatment conditions, an improved ion signal intensity is observed without any fragmentation, decomposition, or chemical changes. Ion signals are increased possibly by both increased ionization through protonation of molecules and enhanced subsequent desorption during DESI analysis.

Routine analysis of pesticides in foodstuffs: Emerging ambient ionization mass spectrometry as an alternative strategy to be on your radar

Pesticides residues in foodstuffs are longstanding of great concern to consumers and governments, thus reliable evaluation techniques for these residues are necessary to ensure food safety. Emerging ambient ionization mass spectrometry (AIMS), a transformative technology in the field of analytical chemistry, is becoming a promising and solid evaluation technology due to its advantages of direct, real-time and in-situ ionization on samples without complex pretreatments. To provide useful guidance on the evaluation techniques in the field of food safety, we offered a comprehensive review on the AIMS technology and introduced their novel applications for the analysis of residual pesticides in foodstuffs under different testing scenarios (i.e., quantitative, screening, imaging, high-throughput detection and rapid on-site analysis). Meanwhile, the creative combination of AIMS with high-resolution mass analyzer (e.g., orbitrap and time-of-flight) was fundamentally mentioned based on recent studies about the detection and evaluation of multi-residual pesticides between 2015 and 2021. Finally, the technical challenges and prospects associated with AIMS operation in food industry were discussed.

Molecular differentiation of Panax notoginseng grown under different conditions by internal extractive electrospray ionization mass spectrometry and multivariate analysis

Panax notoginseng is a highly valuable and widely used herb in traditional Chinese medicine. The quality and efficacy of Panax notoginseng grown under different conditions can greatly vary due to the differences in chemical composition. The analysis of chemical composition in Panax notoginseng typically involves various experimental steps including extraction, chromatographic separation and characterization, which can be time- and labor-consuming. Therefore, the efficient quality assessment and control of Panax notoginseng requires the development of more rapid methods for the chemical characterization and classification of Panax notoginseng. In this study, a method based on internal extractive electrospray ionization mass spectrometry (iEESI-MS) was developed to characterize chemical components of Panax notoginseng samples under different growth conditions (e.g., place of origin, soil quality, growth season) at the speed of 0.5 min per sample, without sample pretreatment and chromatographic separation. A total of 35 chemical components, including sugars, saponins, organic acids, etc., were identified in Panax notoginseng samples. Clear separation was observed in the multivariate analysis of the iEESI-MS data from Panax notoginseng samples grown under different conditions. The difference in the content of sucrose, fructose, Rg1, Rf, Rb1, Noto-R1, malonyl-Rb1, malonyl-Rg1, malonyl-Rf, Rd, Re, linoleic acid, palmitic acid and malic acid can be used as key characteristic indicators to discriminate origin, commercial specifications, and cultivation conditions of Panax notoginseng samples. The results of our study indicate the high power of iEESI-MS for the rapid molecular characterization and classification of Panax notoginseng under different growth conditions, which can be used for the quality assessment of traditional herbal medicines as well as in pharmaceutical and clinical analysis.

Unraveling metabolic alterations in transgenic mouse model of Alzheimer's disease using MALDI MS imaging with 4-aminocinnoline-3-carboxamide matrix

Revealing the metabolic abnormalities of central and peripheral systems in Alzheimer's disease (AD) mouse model is of paramount importance for understanding AD disease. Matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI MSI) is a powerful label-free technique that has been extensively utilized for the interrogation of spatial changes of various metabolites in neurodegenerative disease. However, technical limitations still exist in MALDI MS, and there is a need to improve the performance of traditional MALDI for a deeper investigation of metabolic alterations in the AD mouse model. In this work, 4-aminocinnoline-3-carboxamide (4-AC) was developed into a novel dual-polarity MALDI matrix. Compared with traditionally used MALDI matrices such as 2,5-dihydroxybenzoic acid (DHB) and 9-aminoacridine (9-AA), 4-AC exhibited superior performance in UV absorption at 355 nm, ion yields, background interference, and vacuum stability, making it an ideal MALDI matrix for comprehensive evaluation of metabolic alteration in the brain and serum of APP/PS1 transgenic mouse model of AD. In total, 93 metabolites exhibited different levels of regional changes in the brain of AD mice as compared to the age-matched controls. Moreover, in the serum of AD mice, 81 altered metabolites distinguishing the AD group from the control were observed by using multivariate statistical analysis. It is expected that the application of the MALDI MSI method developed in this work to visualize the spatio-chemical change of various metabolites may improve our understanding of the etiopathogenesis of AD.

Nanomaterial-assisted thread-based isotachophoresis with on-thread solute trapping

This research describes a nanomaterial-assisted TB-ITP setup for the clean-up, preconcentration, and trapping of alkaloids in biological fluids, followed by their on-thread DESI-MS determination.

Nanoscale Three-Dimensional Imaging of Drug Distributions in Single Cells via Laser Desorption Post-Ionization Mass Spectrometry

Exploring the three-dimensional (3D) drug distribution within a single cell at nanoscale resolution with mass spectrometry imaging (MSI) techniques is crucial in cellular biology, yet it remains a great challenge due to limited lateral resolution, detection sensitivities, and reconstruction problems. Herein, a microlensed fiber laser desorption post-ionization time-of-flight mass spectrometer (MLF-LDPI-TOFMS) was developed for the 3D imaging of two anticancer drugs within single cells at a 500 × 500 × 500 nm³ voxel resolution. Nanoscale desorption was obtained with a microlensed fiber (MLF), and a 157 nm post-ionization laser was introduced to enhance the ionization yield. Furthermore, a new type of alignment method for 3D reconstruction was developed on the basis of our embedded uniform circular polystyrene microspheres (PMs). Our findings demonstrate that this 3D imaging technique has the potential to provide information about the 3D distributions of specific molecules at the nanoscale level.

Recent Advances of Ambient Mass Spectrometry Imaging and Its Applications in Lipid and Metabolite Analysis

Ambient mass spectrometry imaging (AMSI) has attracted much attention in recent years. As a kind of unlabeled molecular imaging technique, AMSI can enable in situ visualization of a large number of compounds in biological tissue sections in ambient conditions. In this review, the developments of various AMSI techniques are discussed according to one-step and two-step ionization strategies. In addition, recent applications of AMSI for lipid and metabolite analysis (from 2016 to 2021) in disease diagnosis, animal model research, plant science, drug metabolism and toxicology research, etc., are summarized. Finally, further perspectives of AMSI in spatial resolution, sensitivity, quantitative ability, convenience and software development are proposed.

Direct Liquid Extraction and Ionization Techniques for Understanding Multimolecular Environments in Biological Systems (Secondary Publication)

A combination of direct liquid extraction using a small volume of solvent and electrospray ionization allows the rapid measurement of complex chemical components in biological samples and visualization of their distribution in tissue sections. This review describes the development of such techniques and their application to biological research since the first reports in the early 2000s. An overview of electrospray ionization, ion suppression in samples, and the acceleration of specific chemical reactions in charged droplets is also presented. Potential future applications for visualizing multimolecular environments in biological systems are discussed.

Spatial proteomics for understanding the tissue microenvironment

The human body comprises rich populations of cells, which are arranged into tissues and organs with diverse functionalities. These cells exhibit a broad spectrum of phenotypes and are often organized as a heterogeneous but sophisticatedly regulated ecosystem - tissue microenvironment, inside which every cell interacts with and is reciprocally influenced by its surroundings through its life span. Therefore, it is critical to comprehensively explore the cellular machinery and biological processes in the tissue microenvironment, which is best exemplified by the tumor microenvironment (TME). The past decade has seen increasing advances in the field of spatial proteomics, the main purpose of which is to characterize the abundance and spatial distribution of proteins and their post-translational modifications in the microenvironment of diseased tissues. Herein, we outline the achievements and remaining challenges of mass spectrometry-based tissue spatial proteomics. Exciting technology developments along with important biomedical applications of spatial proteomics are highlighted. In detail, we focus on high-quality resources built by scalpel macrodissection-based region-resolved proteomics, method development of sensitive sample preparation for laser microdissection-based spatial proteomics, and antibody recognition-based multiplexed tissue imaging. In the end, critical issues and potential future directions for spatial proteomics are also discussed.

Mapping the distribution of perfluoroalkyl substances in zebrafishes by liquid extraction surface analysis mass spectrometry

Investigation on the distribution of persistent organic pollutants (POPs) in aquatic organisms is of great importance for exploring the biological toxicity and health risks of environmental pollutants. In this study, a liquid extraction surface analysis mass spectrometry (LESA-MS) method was developed for rapid and in situ analysis of the spatial distribution of perfluoroalkyl substances (PFASs) in zebrafish. By combining the high-precision automated moving platform of LESA device and the high-resolution MS, quantitative analysis of perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS) in zebrafish tissue section were easily achieved. A tissue-specific ionization efficiency factor (TSF) strategy was also proposed to correct the matrix effect in different parts of zebrafish tissue. By using the developed method, high sensitive and efficient imaging of PFOA and PFOS in zebrafish tissue was achieved, and the distributions of PFOA and PFOS in descending order were gills, organs, roes, pelvic fin, muscle, and brain. The experimental results demonstrated that the coupling of LESA-MS method with TFS strategy is an efficient and reliable approach for monitoring the content distribution of environmental pollutants in biological tissues.

Development of a new atmospheric pressure plasmaspray ionization for ambient mass spectrometry

A new atmospheric pressure ionization method, plasmaspray ionization, termed as PSI, was developed to be an alternative ambient ion source for mass spectrometry. It comprises a plasma jet device and a sample spray part. While the nonthermal plasma jet strikes the surface of stainless steel tube out of the spray capillary, the sprayed sample will be ionized with the assistant of auxiliary gas. Although PSI is a little bit more complex than electrospray ionization (ESI) in instrument, it shows both better linearity and higher sensitivity for organic compounds. For protein samples, it presents wider distributions of multiply charged ions and higher mass resolution without sacrificing any sensitivity. For the mechanism of PSI, the charge build-up process on the tip of capillary should play a key role for the ion formation, and the stimulated pulsed voltage on the flow tube will promote the ion aggregation speed until the charge density is high enough. PSI source contains the features of plasma ionization and ESI and can be considered as a novel combo bridging these techniques. These results reflect that this method of PSI can be applied and further developed as a versatile new ion source for a wild range of organic and biological samples.

CpG preconditioning reduces accumulation of lysophosphatidylcholine in ischemic brain tissue after middle cerebral artery occlusion

Ischemic stroke is one of the major causes of death and permanent disability in the world. However, the molecular mechanisms surrounding tissue damage are complex and further studies are needed to gain insights necessary for development of treatment. Prophylactic treatment by administration of cytosine-guanine (CpG) oligodeoxynucleotides has been shown to provide neuroprotection against anticipated ischemic injury. CpG binds to Toll-like receptor 9 (TLR9) causing initialization of an inflammatory response that limits visible ischemic damages upon subsequent stroke. Here, we use nanospray desorption electrospray ionization (nano-DESI) mass spectrometry imaging (MSI) to characterize molecular effects of CpG preconditioning prior to middle cerebral artery occlusion (MCAO) and reperfusion. By doping the nano-DESI solvent with appropriate internal standards, we can study and compare distributions of phosphatidylcholine (PC) and lysophosphatidylcholine (LPC) in the ischemic hemisphere of the brain despite the large changes in alkali metal abundances. Our results show that CpG preconditioning not only reduces the infarct size but it also decreases the degradation of PC and accumulation of LPC species, which indicates reduced cell membrane breakdown and overall ischemic damage. Our findings show that molecular mechanisms of PC degradation are intact despite CpG preconditioning but that these are limited due to the initialized inflammatory response.

High-Spatial-Resolution Multimodal Imaging by Tapping-Mode Scanning Probe Electrospray Ionization with Feedback Control

Direct extraction and ionization techniques using minute amounts of solvent can be employed for the rapid analysis of chemical components in a sample without any sample preparation steps. This type of approach is important for mass spectrometry imaging of samples with multiple chemical components that have different spatial distributions (i.e., biological tissues). To improve the spatial resolution of such imaging, it is necessary to reduce the solvent volume for extraction and deliver it to the sample surface. This report describes a feedback control system applied to tapping-mode scanning probe electrospray ionization. By combining the measurement technique of capillary probe vibration with the dynamic distance control system between the probe and the sample, the vibration amplitude of the probe is maintained while the probe scans over uneven samples. This method allows simultaneous high-resolution imaging of molecular distribution, surface topography, and amplitude/phase changes in the probe vibration. Such multimodal imaging is demonstrated on rhodamine B thin films in microwells and on a mouse brain tissue section. This technique can generally be applied to examine the multidimensional molecular distribution and the surface profiles of various objects.

Addressing Cellular Heterogeneity in Cancer through Precision Proteomics

Cells exhibit a broad spectrum of functions driven by differences in molecular phenotype. Understanding the heterogeneity between and within cell types has led to advances in our ability to diagnose and manipulate biological systems. Heterogeneity within and between tumors still poses a challenge to the development and efficacy of therapeutics. In this Perspective we review the toolkit of protein-level experimental approaches for investigating cellular heterogeneity. We describe how innovative approaches and technical developments have supported the advent of bottom-up single-cell proteomic analysis and present opportunities and challenges within cancer research. Finally, we introduce the concept of "precision proteomics" and discuss how the advantages and limitations of various experimental approaches render them suitable for different biological systems and questions.

The choice of tissue fixative is a key determinant for mass spectrometry imaging based tumor metabolic reprogramming characterization

The application of mass spectrometry imaging (MSI) for the study of spatiotemporal alterations of the metabolites in tumors has brought a number of significant biological results. At present, metabolite profiling based on MSI is typically performed on frozen tissue sections; however, the majority of clinical specimens need to be fixed in tissue fixative to avoid autolysis and to preserve antigenicity. In this study, we present the global impacts of different fixatives on the MS imaging of gastric cancer tissue metabolites. The MSI performances of 17 kinds of metabolites, such as amino acids, polyamines, cholines, organic acids, polypeptides, nucleotides, nucleosides, nitrogen bases, cholesterols, fatty acids, and phospholipids, in untreated, 10% formalin-, 4% paraformaldehyde-, acetone-, and 95% ethanol-fixed gastric cancer tissues were thoroughly explored for the first time. Furthermore, we also investigated the spatial expressions of 6 metabolic enzymes, namely, GLS, FASN, CHKA, PLD2, cPLA2, and EGFR, closely related to tumor-associated metabolites. Immunohistochemical staining carried out on the same tissue sections' which have undergone MSI analysis' suggests that enzymatic characterization is feasible after metabolite imaging. Combining the spatial signatures of metabolites and pathway-related metabolic enzymes in heterogeneous tumor tissues offers an insight to understand the complex tumor metabolism. Graphical abstract.