MALDI imaging and profiling MS of higher mass proteins from tissue

Lipidomics for clinical diagnosis: Dye-Assisted Laser Desorption/Ionization (DALDI) method for lipids detection in MALDI mass spectrometry imaging

Lipid-based biomarkers for research and diagnosis are rapidly emerging to unpack the basis of person-to-person and population variations in disease susceptibility, drug and nutritional responses, to name but a few. Hence, with the advent of MALDI Mass Spectrometry Imaging, lipids have begun to be investigated intensively. However, lipids are highly mobile during tissue preparation, and are soluble in the solvent used for matrix preparation or in the fixing fluid such as formalin, resulting in substantial delocalization. In the present article, we investigated as another alternative, the possibility of using specific dyes that can absorb UV wavelengths, in order to desorb the lipids specifically from tissue sections, and are known to immobilize them in tissues. Indeed, after lipid insolubilization with chromate solution or chemical fixation with osmium tetroxide, heterocyclic-based dyes can be directly used without matrix. Taking into account the fact that some dyes have this matrix-free capability, we identified particular dyes dedicated to histological staining of lipids that could be used with MALDI mass spectrometry imaging. We stained tissue sections with either Sudan Black B, Nile Blue A, or Oil Red O. An important advantage of this assay relies on its compatibility with usual practices of histopathological investigation of lipids. As a new method, DALDI stands for Dye-Assisted Laser Desorption Ionization and allows for future clinical and histopathological applications using routine histological protocols. Additionally, this novel methodology was validated in human ovarian cancer biopsies to demonstrate its use as a suitable procedure, for histological diagnosis in lipidomics field.

Advanced mass spectrometry technologies for the study of microbial pathogenesis

Matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS) has been successfully applied to the field of microbial pathogenesis with promising results, principally in diagnostic microbiology to rapidly identify bacteria based on the molecular profiles of small cell populations. Direct profiling of molecules from serum and tissue samples by MALDI MS provides a means to study the pathogen-host interaction and to discover potential markers of infection. Systematic molecular profiling across tissue sections represents a new imaging modality, enabling regiospecific molecular measurements to be made in situ, in both two-dimensional and three-dimensional analyses. Herein, we briefly summarize work that employs MALDI MS to study the pathogenesis of microbial infection.

HFIP extraction followed by 2D CTAB/SDS-PAGE separation: a new methodology for protein identification from tissue sections after MALDI mass spectrometry profiling for personalized medicine research

Matrix-assisted laser desorption ionization mass spectrometry imaging (MALDI-MSI) and profiling technology have become the easiest methods for quickly accessing the protein composition of a tissue area. Unfortunately, the demand for the identification of these proteins remains unmet. To overcome this bottleneck, we combined several strategies to identify the proteins detected via MALDI profiling including on-tissue protein extraction using hexafluoroIsopropanol (1,1,1,3,3,3-hexafluoro-2-propanol, HFIP) coupled with two-dimensional cetyl trimethylammonium bromide/sodium dodecyl sulfate-polyacrylamide gel electrophoresis (2D CTAB/SDS-PAGE) for separation followed by trypsin digestion and MALDI-MS analyses for identification. This strategy was compared with an on-tissue bottom-up strategy that we previously developed. The data reflect the complementarity of the approaches. An increase in the number of specific proteins identified has been established. This approach demonstrates the potential of adapted extraction procedures and the combination of parallel identification approaches for personalized medicine applications. The anatomical context provides important insight into identifying biomarkers and may be considered a first step for tissue-based biomarker research, as well as the extemporaneous examination of biopsies during surgery.

Matrix pre-coated targets for high throughput MALDI imaging of proteins

We have developed matrix pre-coated targets for imaging proteins in thin tissue sections by matrix-assisted laser desorption/ionization mass spectrometry. Gold covered microscope slides were coated with sinapinic acid (SA) in batches in advance and were shown to be stable for over 6 months when kept in the dark. The sample preparation protocol using these SA pre-coated targets involves treatment with diisopropylethylamine (DIEA)-H2 O vapor, transforming the matrix layer to a viscous ionic liquid. This SA-DIEA ionic liquid layer extracts proteins and other analytes from tissue sections that are thaw mounted to this target. DIEA is removed by the immersion of the target into diluted acetic acid, allowing SA to co-crystallize with extracted analytes directly on the target. Ion images (3-70 kDa) of sections of mouse brain and rat kidney at spatial resolution down to 10 µm were obtained. Use of pre-coated slides greatly reduces sample preparation time for matrix-assisted laser desorption/ionization imaging while providing high throughput, low cost and high spatial resolution images.

Imaging lipids with secondary ion mass spectrometry

This review discusses the application of time-of-flight secondary ion mass spectrometry (TOF-SIMS) and magnetic sector SIMS with high lateral resolution performed on a Cameca NanoSIMS 50(L) to imaging lipids. The similarities between the two SIMS approaches and the differences that impart them with complementary strengths are described, and various strategies for sample preparation and to optimize the quality of the SIMS data are presented. Recent reports that demonstrate the new insight into lipid biochemistry that can be acquired with SIMS are also highlighted. This article is part of a Special Issue entitled Tools to study lipid functions.

Detection of high molecular weight proteins by MALDI imaging mass spectrometry

MALDI imaging mass spectrometry (IMS) is a unique technology to explore the spatial distribution of biomolecules directly on tissues. It allows the in situ investigation of a large number of small proteins and peptides. Detection of high molecular weight proteins through MALDI IMS still represents an important challenge, as it would allow the direct investigation of the distribution of more proteins involved in biological processes, such as cytokines, enzymes, neuropeptide precursors and receptors. In this work we compare the traditional method performed with sinapinic acid with a comparable protocol using ferulic acid as the matrix. Data show a remarkable increase of signal acquisition in the mass range of 20k to 150k Th. Moreover, we report molecular images of biomolecules above 70k Th, demonstrating the possibility of expanding the application of this technology both in clinical investigations and basic science.

MALDI MS imaging as a tool for biomarker discovery: methodological challenges in a clinical setting

MALDI MS imaging (MSI) is an analytical tool capable of providing spatial distribution and relative abundance of biomolecules directly in tissue. After 15 years of intense efforts to improve the acquisition and quality of molecular images, MSI has matured into an asset of the proteomic toolbox. The power of MSI lies in the ability to differentiate tissue regions that are not histologically distinct but are characterized by different MS profiles. Recently, MSI has been gaining momentum in biomedical research and has found applications in disease diagnosis and prognosis, biomarker discovery, and drug therapy. Although the technology holds great promise, MSI is still faced with a set of methodological challenges presented by the clinical setting. There is a growing awareness regarding this topic and efforts are being taken to develop clear and practical standards to overcome these challenges. This review presents an overview of MALDI MSI as a biomarker discovery tool and recent methodological progress in the field.

Mass spectrometry-based imaging of metabolites and proteins

Imaging techniques based on mass spectrometry (MS) have become powerful approaches to decipher the spatial distribution of metabolites and proteins. MS imaging (MSI) mostly relies on matrix-assisted laser desorption/ionization coupled to MS detection, but desorption electrospray ionization is also frequently used. Here we describe our current protocols for MALDI-MSI of seed sections and for root tissue. Detailed procedures for cryo-sectioning, matrix application, image capture, mass spectrometry measurement and data analysis are given.

Enhanced detection of high-mass proteins by using an active pixel detector

Flying high: Application of an active pixel detector with high charge sensitivity to a linear time-of-flight mass spectrometer results in enhanced detection of high-mass proteins (such as Immunoglobulin G; IgG) using a conventional microchannel plate detection system. This technique thus provides a means to extend the mass range of such detectors as well as allowing direct visualization of mass-dependent ion-focusing phenomena.

MALDI mass spectrometry in prostate cancer biomarker discovery

Matrix-assisted laser desorption/ionisation (MALDI) mass spectrometry (MS) is a highly versatile and sensitive analytical technique, which is known for its soft ionisation of biomolecules such as peptides and proteins. Generally, MALDI MS analysis requires little sample preparation, and in some cases like MS profiling it can be automated through the use of robotic liquid-handling systems. For more than a decade now, MALDI MS has been extensively utilised in the search for biomarkers that could aid clinicians in diagnosis, prognosis, and treatment decision making. This review examines the various MALDI-based MS techniques like MS imaging, MS profiling and proteomics in-depth analysis where MALDI MS follows fractionation and separation methods such as gel electrophoresis, and how these have contributed to prostate cancer biomarker research. This article is part of a Special Issue entitled: Biomarkers: A Proteomic Challenge.

Imaging mass spectrometry: from tissue sections to cell cultures

Imaging mass spectrometry (IMS) has been a useful tool for investigating protein, peptide, drug and metabolite distributions in human and animal tissue samples for almost 15years. The major advantages of this method include a broad mass range, the ability to detect multiple analytes in a single experiment without the use of labels and the preservation of biologically relevant spatial information. Currently the majority of IMS experiments are based on imaging animal tissue sections or small tumor biopsies. An alternative method currently being developed is the application of IMS to three-dimensional cell and tissue culture systems. With new advances in tissue culture and engineering, these model systems are able to provide increasingly accurate, high-throughput and cost-effective models that recapitulate important characteristics of cell and tissue growth in vivo. This review will describe the most recent advances in IMS technology and the bright future of applying IMS to the field of three-dimensional cell and tissue culture.

Matrix-assisted laser desorption ionization imaging mass spectrometry: in situ molecular mapping

Matrix-assisted laser desorption ionization imaging mass spectrometry (IMS) is a relatively new imaging modality that allows mapping of a wide range of biomolecules within a thin tissue section. The technology uses a laser beam to directly desorb and ionize molecules from discrete locations on the tissue that are subsequently recorded in a mass spectrometer. IMS is distinguished by the ability to directly measure molecules in situ ranging from small metabolites to proteins, reporting hundreds to thousands of expression patterns from a single imaging experiment. This article reviews recent advances in IMS technology, applications, and experimental strategies that allow it to significantly aid in the discovery and understanding of molecular processes in biological and clinical samples.

Mass spectrometry imaging with high resolution in mass and space

Mass spectrometry (MS) imaging links molecular information and the spatial distribution of analytes within a sample. In contrast to most histochemical techniques, mass spectrometry imaging can differentiate molecular modifications and does not require labeling of targeted compounds. We have recently introduced the first mass spectrometry imaging method that provides highly specific molecular information (high resolution and accuracy in mass) at cellular dimensions (high resolution in space). This method is based on a matrix-assisted laser desorption/ionization (MALDI) imaging source working at atmospheric pressure which is coupled to an orbital trapping mass spectrometer. Here, we present a number of application examples and demonstrate the benefit of ‘mass spectrometry imaging with high resolution in mass and space.’ Phospholipids, peptides and drug compounds were imaged in a number of tissue samples at a spatial resolution of 5–10 μm. Proteins were analyzed after on-tissue tryptic digestion at 50-μm resolution. Additional applications include the analysis of single cells and of human lung carcinoma tissue as well as the first MALDI imaging measurement of tissue at 3 μm pixel size. MS image analysis for all these experiments showed excellent correlation with histological staining evaluation. The high mass resolution (R = 30,000) and mass accuracy (typically 1 ppm) proved to be essential for specific image generation and reliable identification of analytes in tissue samples. The ability to combine the required high-quality mass analysis with spatial resolution in the range of single cells is a unique feature of our method. With that, it has the potential to supplement classical histochemical protocols and to provide new insights about molecular processes on the cellular level.

Detection systems for mass spectrometry imaging: a perspective on novel developments with a focus on active pixel detectors

Instrumental developments for imaging and individual particle detection for biomolecular mass spectrometry (imaging) and fundamental atomic and molecular physics studies are reviewed. Ion-counting detectors, array detection systems and high mass detectors for mass spectrometry (imaging) are treated. State-of-the-art detection systems for multi-dimensional ion, electron and photon detection are highlighted. Their application and performance in three different imaging modes--integrated, selected and spectral image detection--are described. Electro-optical and microchannel-plate-based systems are contrasted. The analytical capabilities of solid-state pixel detectors--both charge coupled device (CCD) and complementary metal oxide semiconductor (CMOS) chips--are introduced. The Medipix/Timepix detector family is described as an example of a CMOS hybrid active pixel sensor. Alternative imaging methods for particle detection and their potential for future applications are investigated.

Laserspray ionization imaging of multiply charged ions using a commercial vacuum MALDI ion source

This is the first report of imaging mass spectrometry (MS) from multiply charged ions at vacuum. Laserspray ionization (LSI) was recently extended to applications at vacuum producing electrospray ionization-like multiply charged ions directly from surfaces using a commercial intermediate pressure matrix-assisted laser desorption/ionization ion mobility spectrometry (IMS) MS instrument. Here, we developed a strategy to image multiply charged peptide ions. This is achieved by the use of 2-nitrophloroglucinol as matrix for spray deposition onto the tissue section and implementation of "soft" acquisition conditions including lower laser power and ion accelerating voltages similar to electrospray ionization-like conditions. Sufficient ion abundance is generated by the vacuum LSI method to employ IMS separation in imaging multiply charged ions obtained on a commercial mass spectrometer ion source without physical instrument modifications using the laser in the commercially available reflection geometry alignment. IMS gas-phase separation reduces the complexity of the ion signal from the tissue, especially for multiply charged relative to abundant singly charged ions from tissue lipids. We show examples of LSI tissue imaging from charge state +2 of three endogenous peptides consisting of between 1 and 16 amino acid residues from the acetylated N-terminal end of myelin basic protein: mass-to-charge (m/z) 795.81 (+2) molecular weight (MW) 1589.6, m/z 831.35 (+2) MW 1660.7, and m/z 917.40 (+2) MW 1832.8.

MALDI-imaging segmentation is a powerful tool for spatial functional proteomic analysis of human larynx carcinoma

PURPOSE

For several decades, conventional histological staining and immunohistochemistry (IHC) have been the main tools to visualize and understand tissue morphology and structure. IHC visualizes the spatial distribution of individual protein species directly in tissue. However, a specific antibody is required for each protein, and multiplexing capabilities are extremely limited, rarely visualizing more than two proteins simultaneously. With the recent emergence of matrix-assisted laser desorption/ionization imaging mass spectrometry (MALDI-imaging), it is becoming possible to study more complex proteomic patterns directly in tissue. However, the analysis and interpretation of large and complex MALDI-imaging data requires advanced computational methods. In this paper, we show how the recently introduced method of spatial segmentation can be applied to analysis and interpretation of a larynx carcinoma section and compare the spatial segmentation with the histological annotation of the same tissue section.

METHODS

Matrix-assisted laser desorption/ionization imaging is a label-free spatially resolved analytical technique, which allows detection and visualization of hundreds of proteins at once. Spatial segmentation of the MALDI-imaging data by clustering of spectra by their similarity was performed, automatically generating a spatial segmentation map of the tissue section, where regions of similar proteomic patterns were highlighted. The tissue was stained with the hematoxylin and eosin (H&E), histopathologically analyzed and annotated. The segmentation map was interpreted after its overlay with the H&E microscopy image.

RESULTS

The automatically generated segmentation map exhibits high correspondence to the detailed histological annotation of the larynx carcinoma tissue section. By superimposing, the segmentation map based on the proteomic profiles with H&E-stained microscopic images, we demonstrate precise localization of complex and histopathologically relevant tissue features in an automated way.

CONCLUSIONS

The combination of MALDI-imaging and automatic spatial segmentation is a useful approach in analyzing carcinoma tissue and provides a deeper insight into the functional proteomic organization of the respective tissue.

The proteomic future: where mass spectrometry should be taking us

A newcomer to the -omics era, proteomics, is a broad instrument-intensive research area that has advanced rapidly since its inception less than 20 years ago. Although the 'wet-bench' aspects of proteomics have undergone a renaissance with the improvement in protein and peptide separation techniques, including various improvements in two-dimensional gel electrophoresis and gel-free or off-gel protein focusing, it has been the seminal advances in MS that have led to the ascension of this field. Recent improvements in sensitivity, mass accuracy and fragmentation have led to achievements previously only dreamed of, including whole-proteome identification, and quantification and extensive mapping of specific PTMs (post-translational modifications). With such capabilities at present, one might conclude that proteomics has already reached its zenith; however, 'capability' indicates that the envisioned goals have not yet been achieved. In the present review we focus on what we perceive as the areas requiring more attention to achieve the improvements in workflow and instrumentation that will bridge the gap between capability and achievement for at least most proteomes and PTMs. Additionally, it is essential that we extend our ability to understand protein structures, interactions and localizations. Towards these ends, we briefly focus on selected methods and research areas where we anticipate the next wave of proteomic advances.

Data processing for 3D mass spectrometry imaging

Data processing for three dimensional mass spectrometry (3D-MS) imaging was investigated, starting with a consideration of the challenges in its practical implementation using a series of sections of a tissue volume. The technical issues related to data reduction, 2D imaging data alignment, 3D visualization, and statistical data analysis were identified. Software solutions for these tasks were developed using functions in MATLAB. Peak detection and peak alignment were applied to reduce the data size, while retaining the mass accuracy. The main morphologic features of tissue sections were extracted using a classification method for data alignment. Data insertion was performed to construct a 3D data set with spectral information that can be used for generating 3D views and for data analysis. The imaging data previously obtained for a mouse brain using desorption electrospray ionization mass spectrometry (DESI-MS) imaging have been used to test and demonstrate the new methodology.

Controlled-pH tissue cleanup protocol for signal enhancement of small molecule drugs analyzed by MALDI-MS imaging

The limit of detection of low-molecular weight compounds in tissue sections, analyzed by matrix assisted laser desorption ionization mass spectrometry imaging (MALDI-MSI), was significantly improved by employing sample washing using a pH-controlled buffer solution. The pH of the washing solutions were set at values whereby the target analytes would have low solubility. Washing the tissue sections in the buffered solution resulted in removal of endogenous soluble ionization-suppressing compounds and salts, while the target compound remained in situ with minor or no delocalization during the buffered washing procedure. Two pharmaceutical compounds (cimetidine and imipramine) and one new protease inhibitor compound were successfully used to evaluate the feasibility of the pH-controlled tissue washing protocol for MALDI-MSI. Enhancement in signal-to-noise ratio was achieved by a factor of up to 10.

Activity-based probes linked with laser-cleavable mass tags for signal amplification in imaging mass spectrometry: analysis of serine hydrolase enzymes in mammalian tissue

A novel functional imaging mass spectrometry technology is described that utilizes activity-based probes for imaging enzyme active sites in tissue sections. We demonstrate this technology using an activity-based probe (fluorophosphate) that is specific for serine hydrolases. A dendrimer containing multiple mass tags that is attached to the activity-based probe is used to analyze the binding sites of the probe through release and measurement of the mass tags on laser irradiation. A generation 8 poly(amido amine) dendrimer with 1024 amino groups was labeled with an azide group, and then, more than 900 mass tags were attached in order to achieve signal amplification of nearly 3 orders of magnitude. The experimental protocol first involves binding of the activity-based probe containing an alkyne group to serine hydrolases in the tissue section followed by attachment of the dendrimer labeled with mass tags to the bound probe by Click chemistry. On irradiation of the labeled tissue by the laser beam in a raster pattern, the mass tags are liberated and recorded by the mass analyzer; consequently, the ion image of the mass tag reveals the distribution of serine hydrolases in the tissue. This process was shown using rat brain and mouse embryo sections. Targeted imaging has the advantage of providing high spatial resolution and high sensitivity through the use of signal amplification chemistry with high target specificity through the use of an enzyme activity probe.

Localization and imaging of gangliosides in mouse brain tissue sections by laserspray ionization inlet

A new ionization method for the analysis of fragile gangliosides without undesired fragmentation or salt adduction is presented. In laserspray ionization inlet (LSII), the matrix/analyte sample is ablated at atmospheric pressure, and ionization takes place in the ion transfer capillary of the mass spectrometer inlet by a process that is independent of a laser wavelength or voltage. The softness of LSII allows the identification of gangliosides up to GQ1 with negligible sialic acid loss. This is of importance to the field of MS imaging, as undesired fragmentation has made it difficult to accurately map the spatial distribution of fragile ganglioside lipids in tissue. Proof-of-principle structural characterization of endogenous gangliosides using MS(n) fragmentation of multiply charged negative ions on a LTQ Velos and subsequent imaging of the GD1 ganglioside is demonstrated. This is the first report of multiply charged negative ions using inlet ionization. We find that GD1 is detected at higher levels in the mouse cortex and hippocampus compared with the thalamus. In LSII with the laser aligned in transmission geometry relative to the inlet, images were obtained in approximately 60 min using an inexpensive nitrogen laser.

Ionic liquids as matrices in microfluidic sample deposition for high-mass matrix- assisted laser desorption/ionization mass spectrometry

Sample preparation for matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) via a microfluidic deposition device using ionic liquid matrices addresses several problems of standard protocols with crystalline matrices, such as the heterogeneity of sample spots due to the co-crystallization of sample and matrix and the limited capability for high-throughput analysis. Since ionic liquid matrices do not solidify during the measurement, the resulting sample spots are homogeneous. The use of these matrices is also beneficial for automated sample preparation, since crystallization of the matrix is avoided and, thus, no clogging of the spotting device can occur. The applicability of ionic liquids to the analysis of biomolecules with high molecular weights, up to ≈ 1 MDa is shown, as well as a good sensitivity (5 fmol) for recombinant human fibronectin, a protein with a molecular weight of 226 kDa. Microfluidic sample deposition of proteins with high molecular weights will, in the future, allow parallel sample preparation for MALDI-MS and for electron microscopy.

Tissue analysis by imaging MS

Imaging MS (IMS) is generating tremendous interest in scientific communities because of its unparalleled capabilities to provide chemical analysis of intact tissue. Advances in analytical chemistry and MS are providing new insights into chemical and biological processes. This review will discuss various IMS platforms and their applications in biomedical and pharmaceutical research.

MALDI-ToF mass spectrometry for studying noncovalent complexes of biomolecules

Matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) has been demonstrated to be a valuable tool to investigate noncovalent interactions of biomolecules. The direct detection of noncovalent assemblies is often more troublesome than with electrospray ionization. Using dedicated sample preparation techniques and carefully optimized instrumental parameters, a number of biomolecule assemblies were successfully analyzed. For complexes dissociating under MALDI conditions, covalent stabilization with chemical cross-linking is a suitable alternative. Indirect methods allow the detection of noncovalent assemblies by monitoring the fading of binding partners or altered H/D exchange patterns.

MALDI imaging mass spectrometry for direct tissue analysis: technological advancements and recent applications

Matrix assisted laser desorption/ionization (MALDI) imaging mass spectrometry (IMS) is a method that allows the investigation of the molecular content of tissues within its morphological context. Since it is able to measure the distribution of hundreds of analytes at once, while being label free, this method has great potential which has been increasingly recognized in the field of tissue-based research. In the last few years, MALDI-IMS has been successfully used for the molecular assessment of tissue samples mainly in biomedical research and also in other scientific fields. The present article will give an update on the application of MALDI-IMS in clinical and preclinical research. It will also give an overview of the multitude of technical advancements of this method in recent years. This includes developments in instrumentation, sample preparation, computational data analysis and protein identification. It will also highlight a number of emerging fields for application of MALDI-IMS like drug imaging where MALDI-IMS is used for studying the spatial distribution of drugs in tissues.

MALDI tissue imaging: from biomarker discovery to clinical applications

Matrix-assisted laser desorption ionization (MALDI) imaging mass spectrometry (IMS) is a powerful tool for the generation of multidimensional spatial expression maps of biomolecules directly from a tissue section. From a clinical proteomics perspective, this method correlates molecular detail to histopathological changes found in patient-derived tissues, enhancing the ability to identify candidates for disease biomarkers. The unbiased analysis and spatial mapping of a variety of molecules directly from clinical tissue sections can be achieved through this method. Conversely, targeted IMS, by the incorporation of laser-reactive molecular tags onto antibodies, aptamers, and other affinity molecules, enables analysis of specific molecules or a class of molecules. In addition to exploring tissue during biomarker discovery, the integration of MALDI-IMS methods into existing clinical pathology laboratory practices could prove beneficial to diagnostics. Querying tissue for the expression of specific biomarkers in a biopsy is a critical component in clinical decision-making and such markers are a major goal of translational research. An important challenge in cancer diagnostics will be to assay multiple parameters in a single slide when tissue quantities are limited. The development of multiplexed assays that maximize the yield of information from a small biopsy will help meet a critical challenge to current biomarker research. This review focuses on the use of MALDI-IMS in biomarker discovery and its potential as a clinical diagnostic tool with specific reference to our application of this technology to prostate cancer.

MALDI imaging techniques dedicated to drug-distribution studies

Previously, MS was often used to analyze the composition and structure of biological molecules present in solutions. Today, technology developments enable the application of MS for the analysis of localized biomolecules on biological tissue surfaces. This technique is called MS imaging. MALDI imaging MS is a technique whereby thousands of compounds present in a tissue section are detected simultaneously without labeling. Although initially used for the detection of biomolecules such as peptides and proteins, this technology is also used today for drug detection. These characteristics make MS imaging an ideal technology that is perfectly adapted for ADME (absorption, distribution, metabolism and excretion) studies. In fact, this technology facilitates the tracking of one or several administered drugs, as well as the metabolites that result from their assimilations. In this article, we will present the various possibilities that MALDI imaging MS approaches have to offer for the study of drugs and their metabolites using MS, MS/MS, FAST-SRM and MRM modes. In this context, we investigate two studies: the distribution of olanzapine in the kidney and the overall distribution of BDM31343 in mouse whole-body section.

MALDI imaging mass spectrometry of lipids by adding lithium salts to the matrix solution

Mass spectrometry imaging of lipids using MALDI-TOF/TOF mass spectrometers is of growing interest for chemical mapping of organic compounds at the surface of tissue sections. Many efforts have been devoted to the best matrix choice and deposition technique. Nevertheless, the identification of lipid species desorbed from tissue sections remains problematic. It is now well-known that protonated, sodium- and potassium-cationized lipids are detected from biological samples, thus complicating the data analysis. A new sample preparation method is proposed, involving the use of lithium salts in the matrix solution in order to simplify the mass spectra with only lithium-cationized molecules instead of a mixture of various cationized species. Five different lithium salts were tested. Among them, lithium trifluoroacetate and lithium iodide merged the different lipid adducts into one single lithium-cationized species. An optimized sample preparation protocol demonstrated that the lithium trifluoroacetate salt slightly increased desorption of phosphatidylcholines. Mass spectrometry images acquired on rat brain tissue sections by adding lithium trifluoroacetate showed the best results in terms of image contrast. Moreover, more structurally relevant fragments were generated by tandem mass spectrometry when analyzing lithium-cationized species.

The evolving field of imaging mass spectrometry and its impact on future biological research

Within the past decade, imaging mass spectrometry (IMS) has been increasingly recognized as an indispensable technique for studying biological systems. Its rapid evolution has resulted in an impressive array of instrument variations and sample applications, yet the tools and data are largely confined to specialists. It is therefore important that at this junction the IMS community begin to establish IMS as a permanent fixture in life science research thereby making the technology and/or the data approachable by non-mass spectrometrists, leading to further integration into biological and clinical research. In this perspective article, we provide insight into the evolution and current state of IMS and propose some of the directions that IMS could develop in order to stay on course to become one of the most promising new tools in life science research.