Molecular imaging of adrenal gland by desorption electrospray ionization mass spectrometry

DESI-TQ-MS imaging for ex vivo brain biodistribution assessment: evaluation of LBT-999, a ligand of the dopamine transporter (DAT)

BACKGROUND

Selection of the most promising radiotracer candidates for radiolabeling is a difficult step in the development of radiotracer pharmaceuticals, especially for the brain. Mass spectrometry (MS) is an alternative to study ex vivo the characteristics of candidates, but most MS studies are complicated by the pharmacologic doses injected and the dissection of regions to study candidate biodistribution. In this study, we tested the ability of a triple quadrupole analyzer (TQ LC-MS/MS) to quantify low concentrations of a validated precursor of a radiotracer targeting the DAT (LBT-999) in dissected regions. We also investigated its biodistribution on brain slices using MS imaging with desorption electrospray ionization (DESI) coupled to time-of-flight (TOF) vs. TQ mass analyzers.

RESULTS

TQ LC-MS/MS enabled quantification of LBT-999 injected at sub-tracer doses in dissected striata. DESI-MS imaging (DESI-MSI) with both analyzers provided images of LBT-999 biodistribution on sagittal slices that were consistent with positron emission tomography (PET). However, the TOF analyzer only obtained biodistribution images at a high injected dose of LBT-999, while the TQ analyzer provided biodistribution images at lower injected doses of LBT-999 with a better signal-to-noise ratio. It also allowed simultaneous visualization of endogenous metabolites such as dopamine.

CONCLUSIONS

Our results show that LC-TQ MS/MS in combination with DESI-MSI can provide important information (biodistribution, specific and selective binding) that can facilitate the selection of the most promising candidates for radiolabeling and support the development of radiotracers.

Mass Spectrometry Imaging Techniques: Non-Ambient and Ambient Ionization Approaches

Molecular information can be acquired from sample surfaces in real time using a revolutionary molecular imaging technique called mass spectrometry imaging (MSI). The technique can concurrently provide high spatial resolution information on the spatial distribution and relative proportion of many different compounds. Thus, many scientists have been drawn to the innovative capabilities of the MSI approach, leading to significant focus in various fields during the past few decades. This review describes the sampling protocol, working principle and applications of a few non-ambient and ambient ionization mass spectrometry imaging techniques. The non-ambient techniques include secondary ionization mass spectrometry and matrix-assisted laser desorption ionization, while the ambient techniques include desorption electrospray ionization, laser ablation electrospray ionization, probe electro-spray ionization, desorption atmospheric pressure photo-ionization and femtosecond laser desorption ionization. The review additionally addresses the advantages and disadvantages of ambient and non-ambient MSI techniques in relation to their suitability, particularly for biological samples used in tissue diagnostics. Last but not least, suggestions and conclusions are made regarding the challenges and future prospects of MSI.

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.

On-tissue chemical derivatization in mass spectrometry imaging

Mass spectrometry imaging (MSI) combines molecular and spatial information in a valuable tool for a wide range of applications. Matrix-assisted laser desorption/ionization (MALDI) is at the forefront of MSI ionization due to its wide availability and increasing improvement in spatial resolution and analysis speed. However, ionization suppression, low concentrations, and endogenous and methodological interferences cause visualization problems for certain molecules. Chemical derivatization (CD) has proven a viable solution to these issues when applied in mass spectrometry platforms. Chemical tagging of target analytes with larger, precharged moieties aids ionization efficiency and removes analytes from areas of potential isobaric interferences. Here, we address the application of CD on tissue samples for MSI analysis, termed on-tissue chemical derivatization (OTCD). MALDI MSI will remain the focus platform due to its popularity, however, alternative ionization techniques such as liquid extraction surface analysis and desorption electrospray ionization will also be recognized. OTCD reagent selection, application, and optimization methods will be discussed in detail. MSI with OTCD is a powerful tool to study the spatial distribution of poorly ionizable molecules within tissues. Most importantly, the use of OTCD-MSI facilitates the analysis of previously inaccessible biologically relevant molecules through the adaptation of existing CD methods. Though further experimental optimization steps are necessary, the benefits of this technique are extensive.

Absolute quantification of cholesterol from thin tissue sections by silver-assisted laser desorption ionization mass spectrometry imaging

Cholesterol is essential to all animal life, and its dysregulation is observed in many diseases. For some of these, the precise determination of cholesterol's histological location and absolute abundance at cellular length scales within tissue samples would open the door to a more fundamental understanding of the role of cholesterol in disease onset and progression. We have developed a fast and simple method for absolute quantification of cholesterol within brain samples based on the sensitive detection and mapping of cholesterol by silver-assisted laser desorption ionization mass spectrometry imaging (AgLDI MSI) from thin tissue sections. Reproducible calibration curves were generated by depositing a range of cholesterol-D₇ concentrations on brain homogenate tissue sections combined with the homogeneous spray deposition of a non-animal steroid reference standard detectable by AgLDI MSI to minimize experimental variability. Results obtained from serial brain sections gave consistent cholesterol quantitative values in very good agreement with those obtained with other mass spectrometry-based methods.

SDHB knockout and succinate accumulation are insufficient for tumorigenesis but dual SDHB/NF1 loss yields SDHx-like pheochromocytomas

Inherited pathogenic succinate dehydrogenase (SDHx) gene mutations cause the hereditary pheochromocytoma and paraganglioma tumor syndrome. Syndromic tumors exhibit elevated succinate, an oncometabolite that is proposed to drive tumorigenesis via DNA and histone hypermethylation, mitochondrial expansion, and pseudohypoxia-related gene expression. To interrogate this prevailing model, we disrupt mouse adrenal medulla SDHB expression, which recapitulates several key molecular features of human SDHx tumors, including succinate accumulation but not 5hmC loss, HIF accumulation, or tumorigenesis. By contrast, concomitant SDHB and the neurofibromin 1 tumor suppressor disruption yields SDHx-like pheochromocytomas. Unexpectedly, in vivo depletion of the 2-oxoglutarate (2-OG) dioxygenase cofactor ascorbate reduces SDHB-deficient cell survival, indicating that SDHx loss may be better tolerated by tissues with high antioxidant capacity. Contrary to the prevailing oncometabolite model, succinate accumulation and 2-OG-dependent dioxygenase inhibition are insufficient for mouse pheochromocytoma tumorigenesis, which requires additional growth-regulatory pathway activation.

Online Quaternized Derivatization Mapping and Glycerides Profiling of Cancer Tissues by Laser Ablation Carbon Fiber Ionization Mass Spectrometry

Mass spectrometry imaging has become a hot research field owing to its ability to reflect the distribution of multiple metabolites in tissue. However, not all kinds of metabolites have great ionization efficiency in mass spectrometry imaging. The mass signals of low polar metabolites like monoglycerides and diglycerides may be seriously suppressed. Many strategies have been proposed to fix the problem, such as on-tissue derivatization and online derivatization. Also, some challenges were encountered when implementing these approaches. Herein, a platform coupled online quaternized derivatization and laser ablation carbon fiber ionization mass spectrometry imaging has been developed. The mass signals of monoglycerides and diglycerides were drastically increased in the platform, and high-quality mass images of these metabolites could be acquired readily. In the platform, metabolites were first desorbed by a laser and then reacted online with a derivatization reagent transmitted by carbon fiber ionization, which also undertook the postionization of derivatization products. Pyridine acted as the main derivatization reagent to target metabolites with hydroxyl groups. Remarkably, the derivatization reaction proceeded rapidly without any catalyst owing to the high energy provided by the laser. The mass images of eight monoglycerides and 21 diglycerides were achieved after applying the platform into human ovarian cancer tissues. Notably, a higher mass intensity of these glycerides was captured in cancerous tissues than in para-cancerous tissues, which might infer aberrations in glyceride metabolisms of cancerous tissues.

Organic matrix-free imaging mass spectrometry

Mass spectrometry (MS) is an ideal tool for analyzing multiple types of (bio)molecular information simultaneously in complex biological systems. In addition, MS provides structural information on targets, and can easily discriminate between true analytes and background. Therefore, imaging mass spectrometry (IMS) enables not only visualization of tissues to give positional information on targets but also allows for molecular analysis of targets by affording the molecular weights. Matrix-assisted laser desorption/ionization-time of flight (MALDI-TOF) MS is particularly effective and is generally used for IMS. However, the requirement for an organic matrix raises several limitations that get in the way of accurate and reliable images and hampers imaging of small molecules such as drugs and their metabolites. To overcome these problems, various organic matrix-free LDI IMS systems have been developed, mostly utilizing nanostructured surfaces and inorganic nanoparticles as an alternative to the organic matrix. This minireview highlights and focuses on the progress in organic matrix-free LDI IMS and briefly discusses the use of other IMS techniques such as desorption electrospray ionization, laser ablation electrospray ionization, and secondary ion mass spectrometry.

Metabolites and Lipids Associated with Fetal Swine Anatomy via Desorption Electrospray Ionization - Mass Spectrometry Imaging

Chemical imaging by mass spectrometry (MS) has been largely used to study diseases in animals and humans, especially cancer; however, this technology has been minimally explored to study the complex chemical changes associated with fetal development. In this work, we report the histologically-compatible chemical imaging of small molecules by desorption electrospray ionization (DESI) - MS of a complete swine fetus at 50 days of gestation. Tissue morphology was unperturbed by morphologically-friendly DESI-MS analysis while allowing detection of a wide range of small molecules. We observed organ-dependent localization of lipids, e.g. a large diversity of phosphatidylserine lipids in brain compared to other organs, as well as metabolites such as N-acetyl-aspartic acid in the developing nervous system and N-acetyl-L-glutamine in the heart. Some lipids abundant in the lungs, such as PC(32:0) and PS(40:6), were  similar to surfactant composition reported previously. Sulfatides were highly concentrated in the fetus liver, while hexoses were barely detected at this organ but were abundant in lung and heart. The chemical information on small molecules recorded via DESI-MS imaging coupled with traditional anatomical evaluation is a powerful source of bioanalytical information which reveals the chemical changes associated with embryonic and fetal development that, when disturbed, causes congenital diseases such as spina bifida and cleft palate.

Novel methods in adrenal research: a metabolomics approach

Metabolic alterations have implications in a spectrum of tissue functions and disease. Aided by novel molecular biological and computational tools, our understanding of physiological and pathological processes underpinning endocrine and endocrine-related disease has significantly expanded over the last decade. Herein, we focus on novel metabolomics-related methodologies in adrenal research: in situ metabolomics by mass spectrometry imaging, steroid metabolomics by gas and liquid chromatography-mass spectrometry, energy pathway metabologenomics by liquid chromatography-mass spectrometry-based metabolomics of Krebs cycle intermediates, and cellular reprogramming to generate functional steroidogenic cells and hence to modulate the steroid metabolome. All four techniques to assess and/or modulate the metabolome in biological systems provide tremendous opportunities to manage neoplastic and non-neoplastic disease of the adrenal glands in the era of precision medicine. In this context, we discuss emerging clinical applications and/or promising metabolic-driven research towards diagnostic, prognostic, predictive and therapeutic biomarkers in tumours arising from the adrenal gland and extra-adrenal paraganglia as well as modern approaches to delineate and reprogram adrenal metabolism.

Mass Spectrometry Imaging of Brain Cholesterol and Metabolites with Trifluoroacetic Acid-Enhanced Desorption Electrospray Ionization

Imaging of cholesterol and other metabolites simultaneously by ambient mass spectrometry will greatly benefit biological studies, however, it still remains challenging. Herein, by adding acid into the desorption electrospray ionization (DESI) spray solvent, we achieved simultaneous mass spectrometry imaging of cholesterol and other metabolites directly from mouse brain sections. The introduction of acid increased the signal intensity of cholesterol in mouse brain tissues by approximately 21-fold. Additionally, the present strategy provided increased signal intensities for other metabolites up to 62-fold, as well as identification of seven more metabolites (23 vs 16 for acid-enhanced DESI vs DESI). Moreover, increased corelationships for alanine as well as putrescine and spermidine with cholesterol were discovered under acid-enhanced DESI. The potential of the present strategy in the fields of biological and medical research was demonstrated by investigating the level change for cholesterol, alanine, putrescine, and spermidine in Alzheimer's disease (AD) mouse brain.

Imaging with mass spectrometry, the next frontier in sphingolipid research? A discussion on where we stand and the possibilities ahead

In the last ten years, mass spectrometry (MS) has become the favored analytical technique for sphingolipid (SPL) analysis and measurements. Indeed MS has the unique ability to both acquire sensitive and quantitative measurements and to resolve the molecular complexity characteristic of SPL molecules, both across the different SPL families and within the same SPL family. Currently, two complementary MS-based approaches are used for lipid research: analysis of lipid extracts, mainly by infusion electrospray ionization (ESI), and mass spectrometry imaging (MSI) from a sample surface (i.e. intact tissue sections, cells, model membranes, thin layer chromatography plates) (Fig. 1). The first allows for sensitive and quantitative information about total lipid molecular species from a given specimen from which lipids have been extracted and chromatographically separated prior to the analysis; the second, albeit generally less quantitative and less specific in the identification of molecular species due to the complexity of the sample, allows for spatial information of lipid molecules from biological specimens. In the field of SPL research, MS analysis of lipid extracts from biological samples has been commonly utilized to implicate the role of these lipids in specific biological functions. On the other hand, the utilization of MSI in SPL research represents a more recent development that has started to provide interesting descriptive observations regarding the distribution of specific classes of SPLs within tissues. Thus, it is the aim of this review to discuss how MSI technology has been employed to extend the study of SPL metabolism and the type of information that has been obtained from model membranes, single cells and tissues. We envision this discussion as a complementary compendium to the excellent technical reviews recently published about the specifics of MSI technologies, including their application to SPL analysis (Fuchs et al., 2010; Berry et al., 2011; Ellis et al., 2013; Eberlin et al., 2011; Kraft and Klitzing, 2014).

On-Tissue Chemical Derivatization of Catecholamines Using 4-( N-Methyl)pyridinium Boronic Acid for ToF-SIMS and LDI-ToF Mass Spectrometry Imaging

The analysis of small polar compounds with ToF-SIMS and MALDI-ToF-MS have been generally hindered by low detection sensitivity, poor ionization efficiency, ion suppression, analyte in-source fragmentation, and background spectral interferences from either a MALDI matrix and/or endogenous tissue components. Chemical derivatization has been a well-established strategy for improved mass spectrometric detection of many small molecular weight endogenous compounds in tissues. Here, we present a devised strategy to selectively derivatize and sensitively detect catecholamines with both secondary ion ejection and laser desorption ionization strategies, which are used in many imaging mass spectrometry (IMS) experiments. Chemical derivatization of catecholamines was performed by a reaction with a synthesized permanent pyridinium-cation-containing boronic acid molecule, 4-( N-methyl)pyridinium boronic acid, through boronate ester formation (boronic acid-diol reaction). The derivatization facilitates their sensitive detection with ToF-SIMS and LDI-ToF mass spectrometric techniques. 4-( N-Methyl)pyridinium boronic acid worked as a reactive matrix for catecholamines with LDI and improved the sensitivity of detection for both SIMS and LDI, while the isotopic abundances of the boron atom reflect a unique isotopic pattern for derivatized catecholamines in MS analysis. Finally, the devised strategy was applied, as a proof of concept, for on-tissue chemical derivatization and GCIB-ToF-SIMS (down to 3 μm per pixel spatial resolution) and LDI-ToF mass spectrometry imaging of dopamine, epinephrine, and norepinephrine in porcine adrenal gland tissue sections. MS/MS using collision-induced dissociation (CID)-ToF-ToF-SIMS was subsequently employed on the same tissue sections after SIMS and LDI mass spectrometry imaging experiments, which provided tandem MS information for the validation of the derivatized catecholamines in situ. This methodology can be a powerful approach for the selective and sensitive ionization/detection and spatial localization of diol-containing molecules such as aminols, vic-diols, saccharides, and glycans along with catecholamines in tissue sections with both SIMS and LDI/MALDI-MS techniques.

Tip-Enhanced Photoinduced Electron Transfer and Ionization on Vertical Silicon Nanowires

Nanostructured semiconductors are one of the most potent candidates for matrix-free laser desorption/ionization mass spectrometric (LDI-MS) analysis of low-molecular-weight molecules. Herein, the enhanced photoinduced electron transfer and LDI on the tip of a vertical silicon nanowire (SiNW) array were investigated. Theoretical simulation and LDI detection of indigo and isatin molecules in negative ion mode revealed that the electric field can be enhanced on the tip end of SiNWs, thereby promoting the energy and electron transfer to the analytes adsorbed on the tip of SiNWs. On the basis of this finding, a tip-contact sampling method coupled with LDI-MS detection was established. In this strategy, the tip of SiNWs can be regarded as microextraction heads for the sampling of molecules when they come in contact with analytes. Impression of skin, tissue, and pericarp on the vertical SiNW array can effectively transfer endogenous metabolites or exogenous substances onto the tip. Upon laser irradiation, the adsorbed molecules on the SiNW tip can be efficiently ionized and detected in negative ion mode because of the tip-enhanced electron transfer and LDI effect. We believe this work may significantly expand the application of LDI-MS in various fields.

High-Resolution Tissue Mass Spectrometry Imaging Reveals a Refined Functional Anatomy of the Human Adult Adrenal Gland

In the adrenal gland, neuroendocrine cells that synthesize catecholamines and epithelial cells that produce steroid hormones are united beneath a common organ capsule to function as a single stress-responsive organ. The functional anatomy of the steroid hormone-producing adrenal cortex and the catecholamine-producing medulla is ill defined at the level of small molecules. Here, we report a comprehensive high-resolution mass spectrometry imaging (MSI) map of the normal human adrenal gland. A large variety of biomolecules was accessible by matrix-assisted laser desorption/ionization-Fourier transform-ion cyclotron resonance MSI, including nucleoside phosphates indicative of oxidative phosphorylation, sterol and steroid metabolites, intermediates of glycolysis and the tricarboxylic acid cycle, lipids, and fatty acids. Statistical clustering analyses yielded a molecularly defined adrenal anatomy of 10 distinct molecular zones including a highly structured corticomedullary interface. By incorporating pathway information, activities of carbohydrate, amino acid, and lipid metabolism as well as endocrine bioactivity were revealed to be highly spatially organized, which could be visualized as different molecularly defined zones. Together, these findings provide a molecular definition of human adult adrenal gland structure beyond classical histological anatomy.

Direct Visualization of Neurotransmitters in Rat Brain Slices by Desorption Electrospray Ionization Mass Spectrometry Imaging (DESI - MS)

Mass spectrometry imaging (MSI) of neurotransmitters has so far been mainly performed by matrix-assisted laser desorption/ionization (MALDI) where derivatization reagents, deuterated matrix and/or high resolution, or tandem MS have been applied to circumvent problems with interfering ion peaks from matrix and from isobaric species. We herein describe the application of desorption electrospray ionization mass spectrometry imaging (DESI)-MSI in rat brain coronal and sagittal slices for direct spatial monitoring of neurotransmitters and choline with no need of derivatization reagents and/or deuterated materials. The amino acids γ-aminobutyric (GABA), glutamate, aspartate, serine, as well as acetylcholine, dopamine, and choline were successfully imaged using a commercial DESI source coupled to a hybrid quadrupole-Orbitrap mass spectrometer. The spatial distribution of the analyzed compounds in different brain regions was determined. We conclude that the ambient matrix-free DESI-MSI is suitable for neurotransmitter imaging and could be applied in studies that involve evaluation of imbalances in neurotransmitters levels. Graphical Abstract ᅟ.

Simultaneous imaging of multiple neurotransmitters and neuroactive substances in the brain by desorption electrospray ionization mass spectrometry

With neurological processes involving multiple neurotransmitters and neuromodulators, it is important to have the ability to directly map and quantify multiple signaling molecules simultaneously in a single analysis. By utilizing a molecular-specific approach, namely desorption electrospray ionization mass spectrometry imaging (DESI-MSI), we demonstrated that the technique can be used to image multiple neurotransmitters and their metabolites (dopamine, dihydroxyphenylacetic acid, 3-methoxytyramine, serotonin, glutamate, glutamine, aspartate, γ-aminobutyric acid, adenosine) as well as neuroactive drugs (amphetamine, sibutramine, fluvoxamine) and drug metabolites in situ directly in brain tissue sections. The use of both positive and negative ionization modes increased the number of identified molecular targets. Chemical derivatization by charge-tagging the primary amines of molecules significantly increased the sensitivity, enabling the detection of low abundant neurotransmitters and other neuroactive substances previously undetectable by MSI. The sensitivity of the imaging approach of neurochemicals has a great potential in many diverse applications in fields such as neuroscience, pharmacology, drug discovery, neurochemistry, and medicine.

Mass spectrometry imaging for visualizing organic analytes in food

The demand for rapid chemical imaging of food products steadily increases. Mass spectrometry (MS) is featured by excellent molecular specificity of analysis and is, therefore, a very attractive method for chemical profiling. MS for food imaging has increased significantly over the past decade, aided by the emergence of various ambient ionization techniques that allow direct and rapid analysis in ambient environment. In this article, the current status of food imaging with MSI is reviewed. The described approaches include matrix-assisted laser desorption/ionization (MALDI), but emphasize desorption atmospheric pressure photoionization (DAPPI), electrospray-assisted laser desorption/ionization (ELDI), probe electrospray ionization (PESI), surface desorption atmospheric pressure chemical ionization (SDAPCI), and laser ablation flowing atmospheric pressure afterglow (LA-FAPA). The methods are compared with regard to spatial resolution; analysis speed and time; limit of detection; and technical aspects. The performance of each method is illustrated with the description of a related application. Specific requirements in food imaging are discussed.

In situ biomarker discovery and label-free molecular histopathological diagnosis of lung cancer by ambient mass spectrometry imaging

Sensitive and spatial exploration of the metabolism of tumors at the metabolome level is highly challenging. In this study, we developed an in situ metabolomics method based on ambient mass spectrometry imaging using air flow-assisted desorption electrospray ionization (AFADESI), which can spatially explore the alteration of global metabolites in tissues with high sensitivity. Using this method, we discovered potential histopathological diagnosis biomarkers (including lipids, amino acids, choline, peptides, and carnitine) from 52 postoperative lung cancer tissue samples and then subsequently used these biomarkers to generate images for rapid and label-free histopathological diagnosis. These biomarkers were validated with a sensitivity and a specificity of 93.5% and 100%, respectively. Moreover, a single imaging analysis of a cryosection that visualized all these biomarkers, taking tens of minutes, revealed the type and subtype of the cancer. This method could potentially be used as a molecular pathological tool for rapid clinical lung cancer diagnosis and immediate image-guided surgery.

Ambient ionization MS for bioanalysis: recent developments and challenges

Ambient ionization MS has become very popular in analytical science and has now evolved as an effective analytical tool in metabolomics, biological tissue imaging, protein and small molecule drug analysis, where biological samples are probed in a rapid and direct fashion with minimal sample preparation at ambient conditions. However, certain inherent challenges continue to hinder the vibrant prospects of these methods for in situ analyses or to replace conventional methods in bioanalysis. This review provides an introduction to the field and its application in bioanalysis, with an emphasis on the most recent developments and applications. Furthermore, ongoing challenges or limitations related to quantitation, sensitivity, selectivity, instrumentation and mass range of these ambient methods will also be discussed.

Analysis of metabolic changes in plant pathosystems by imprint imaging DESI-MS

The response of plants to microbial pathogens is based on the production of secondary metabolites. The complexity of plant-pathogen interactions makes their understanding a challenging task for metabolomic studies requiring powerful analytical approaches. In this paper, the ability of ambient mass spectrometry to provide a snapshot of plant metabolic response to pathogen invasion was tested. The fluctuations of glycoalkaloids present in sprouted potatoes infected by the phytopathogen Pythium ultimum were monitored by imprint imaging desorption electrospray ionization mass spectrometry (DESI-MS). After 8 d from the inoculation, a decrease of the relative abundance of potato glycoalkaloids α-solanine (m/z 706) and α-chaconine (m/z 722) was observed, whereas the relative intensity of solanidine (m/z 398), solasodenone (m/z 412), solanaviol (m/z 430), solasodiene (m/z 396), solaspiralidine (m/z 428), γ-solanine/γ-chaconine (m/z 560) , β-solanine (m/z 706), and β-chaconine (m/z 722) increased. The progression of the disease, expressed by the development of brown necrotic lesions on the potato, led to the further decrease of all the glycoalkaloid metabolites. Therefore, the applicability of imprint imaging DESI-MS in studying the plant metabolic changes in a simple pathosystem was demonstrated with minimal sample preparation.

Imaging Mass Spectrometric Analysis of Neurotransmitters: A Review

Imaging mass spectrometry (IMS) is a toolbox of versatile techniques that enable us to investigate analytes in samples at molecular level. In recent years, IMS, and especially matrix-assisted laser desorption/ionisation (MALDI), has been used to visualise a wide range of metabolites in biological samples. Simultaneous visualisation of the spatial distribution of metabolites in a single sample with little tissue disruption can be considered as one important advantage of MALDI over other techniques. However, several technical hurdles including low concentrations and rapid degradation rates of small molecule metabolites, matrix interference of signals and poor ionisation, need to be addressed before MALDI can be considered as a reliable tool for the analysis of metabolites such as neurotransmitters in brain tissues from different sources including humans. In the present review we will briefly describe current MALDI IMS techniques used to study neurotransmitters and discuss their current status, challenges, as well as future prospects.

Mass spectrometry coupled to imaging techniques: the better the view the greater the challenge

These are definitively exciting times for membrane lipid researchers. Once considered just as the cell membrane building blocks, the important role these lipids play is steadily being acknowledged. The improvement occurred in mass spectrometry techniques (MS) allows the establishment of the precise lipid composition of biological extracts. However, to fully understand the biological function of each individual lipid species, we need to know its spatial distribution and dynamics. In the past 10 years, the field has experienced a profound revolution thanks to the development of MS-based techniques allowing lipid imaging (MSI). Images reveal and verify what many lipid researchers had already shown by different means, but none as convincing as an image: each cell type presents a specific lipid composition, which is highly sensitive to its physiological and pathological state. While these techniques will help to place membrane lipids in the position they deserve, they also open the black box containing all the unknown regulatory mechanisms accounting for such tailored lipid composition. Thus, these results urges to different disciplines to redefine their paradigm of study by including the complexity revealed by the MSI techniques.

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.

Data processing and analysis for mass spectrometry imaging

Mass spectrometry imaging produces large numbers of spectra that need to be efficiently stored, processed, and analyzed. In this chapter, we describe the protocol and methods for data processing, visualization, and statistical analysis, with related techniques and tools available presented. Examples are given with data collected for a 3D MS imaging of a mouse brain and 2D MS imaging of human bladder tissues.

Challenges and recent advances in mass spectrometric imaging of neurotransmitters

Mass spectrometric imaging (MSI) is a powerful tool that grants the ability to investigate a broad mass range of molecules, from small molecules to large proteins, by creating detailed distribution maps of selected compounds. To date, MSI has demonstrated its versatility in the study of neurotransmitters and neuropeptides of different classes toward investigation of neurobiological functions and diseases. These studies have provided significant insight in neurobiology over the years and current technical advances are facilitating further improvements in this field. Herein, we briefly review new MSI studies of neurotransmitters, focusing specifically on the challenges and recent advances of MSI of neurotransmitters.

A derivatization and validation strategy for determining the spatial localization of endogenous amine metabolites in tissues using MALDI imaging mass spectrometry

Imaging mass spectrometry (IMS) studies increasingly focus on endogenous small molecular weight metabolites and consequently bring special analytical challenges. Since analytical tissue blanks do not exist for endogenous metabolites, careful consideration must be given to confirm molecular identity. Here, we present approaches for the improvement in detection of endogenous amine metabolites such as amino acids and neurotransmitters in tissues through chemical derivatization and matrix-assisted laser desorption/ionization (MALDI) IMS. Chemical derivatization with 4-hydroxy-3-methoxycinnamaldehyde (CA) was used to improve sensitivity and specificity. CA was applied to the tissue via MALDI sample targets precoated with a mixture of derivatization reagent and ferulic acid as a MALDI matrix. Spatial distributions of chemically derivatized endogenous metabolites in tissue were determined by high-mass resolution and MS(n) IMS. We highlight an analytical strategy for metabolite validation whereby tissue extracts are analyzed by high-performance liquid chromatography (HPLC)-MS/MS to unambiguously identify metabolites and distinguish them from isobaric compounds.

Mass spectrometry imaging, an emerging technology in neuropsychopharmacology

Mass spectrometry imaging is a powerful tool for directly determining the distribution of proteins, peptides, lipids, neurotransmitters, metabolites and drugs in neural tissue sections in situ. Molecule-specific imaging can be achieved using various ionization techniques that are suited to different applications but which all yield data with high mass accuracies and spatial resolutions. The ability to simultaneously obtain images showing the distributions of chemical species ranging from metal ions to macromolecules makes it possible to explore the chemical organization of a sample and to correlate the results obtained with specific anatomical features. The imaging of biomolecules has provided new insights into multiple neurological diseases, including Parkinson's and Alzheimer's disease. Mass spectrometry imaging can also be used in conjunction with other imaging techniques in order to identify correlations between changes in the distribution of important chemical species and other changes in the properties of the tissue. Here we review the applications of mass spectrometry imaging in neuroscience research and discuss its potential. The results presented demonstrate that mass spectrometry imaging is a useful experimental method with diverse applications in neuroscience.

Imaging of biological tissues by desorption electrospray ionization mass spectrometry

Mass spectrometry imaging (MSI) provides untargeted molecular information with the highest specificity and spatial resolution for investigating biological tissues at the hundreds to tens of microns scale. When performed under ambient conditions, sample pre-treatment becomes unnecessary, thus simplifying the protocol while maintaining the high quality of information obtained. Desorption electrospray ionization (DESI) is a spray-based ambient MSI technique that allows for the direct sampling of surfaces in the open air, even in vivo. When used with a software-controlled sample stage, the sample is rastered underneath the DESI ionization probe, and through the time domain, m/z information is correlated with the chemical species' spatial distribution. The fidelity of the DESI-MSI output depends on the source orientation and positioning with respect to the sample surface and mass spectrometer inlet. Herein, we review how to prepare tissue sections for DESI imaging and additional experimental conditions that directly affect image quality. Specifically, we describe the protocol for the imaging of rat brain tissue sections by DESI-MSI.

Blotting assisted by heating and solvent extraction for DESI-MS imaging

Imprints of potato sprout (Solanum tuberosum L.), gingko leaves (Gingko biloba L.) and strawberries (Fragaria x ananassa Duch.) were successfully imaged by desorption electrospray ionization mass spectrometry (DESI-MS) on TLC plates through blotting assisted by heating and/or solvent extraction. Ion images showing the distribution of significant compounds such as glycoalkaloid toxins in potato sprout, ginkgolic acids and flavonoids in ginkgo leaves, and sugars and anthocyanidin in strawberry were obtained. Practical implications of this work include analysis of a wide range of irregular or soft materials by different imprinting conditions without requiring the addition of matrices or use of specific kinds of surfaces.

Mass spectrometry imaging under ambient conditions

Mass spectrometry imaging (MSI) has emerged as an important tool in the last decade and it is beginning to show potential to provide new information in many fields owing to its unique ability to acquire molecularly specific images and to provide multiplexed information, without the need for labeling or staining. In MSI, the chemical identity of molecules present on a surface is investigated as a function of spatial distribution. In addition to now standard methods involving MSI in vacuum, recently developed ambient ionization techniques allow MSI to be performed under atmospheric pressure on untreated samples outside the mass spectrometer. Here we review recent developments and applications of MSI emphasizing the ambient ionization techniques of desorption electrospray ionization (DESI), laser ablation electrospray ionization (LAESI), probe electrospray ionization (PESI), desorption atmospheric pressure photoionization (DAPPI), femtosecond laser desorption ionization (fs-LDI), laser electrospray mass spectrometry (LEMS), infrared laser ablation metastable-induced chemical ionization (IR-LAMICI), liquid microjunction surface sampling probe mass spectrometry (LMJ-SSP MS), nanospray desorption electrospray ionization (nano-DESI), and plasma sources such as the low temperature plasma (LTP) probe and laser ablation coupled to flowing atmospheric-pressure afterglow (LA-FAPA). Included are discussions of some of the features of ambient MSI for example the ability to implement chemical reactions with the goal of providing high abundance ions characteristic of specific compounds of interest and the use of tandem mass spectrometry to either map the distribution of targeted molecules with high specificity or to provide additional MS information on the structural identification of compounds. We also describe the role of bioinformatics in acquiring and interpreting the chemical and spatial information obtained through MSI, especially in biological applications for tissue diagnostic purposes. Finally, we discuss the challenges in ambient MSI and include perspectives on the future of the field.

Surface analysis of lipids by mass spectrometry: more than just imaging

Mass spectrometry is now an indispensable tool for lipid analysis and is arguably the driving force in the renaissance of lipid research. In its various forms, mass spectrometry is uniquely capable of resolving the extensive compositional and structural diversity of lipids in biological systems. Furthermore, it provides the ability to accurately quantify molecular-level changes in lipid populations associated with changes in metabolism and environment; bringing lipid science to the "omics" age. The recent explosion of mass spectrometry-based surface analysis techniques is fuelling further expansion of the lipidomics field. This is evidenced by the numerous papers published on the subject of mass spectrometric imaging of lipids in recent years. While imaging mass spectrometry provides new and exciting possibilities, it is but one of the many opportunities direct surface analysis offers the lipid researcher. In this review we describe the current state-of-the-art in the direct surface analysis of lipids with a focus on tissue sections, intact cells and thin-layer chromatography substrates. The suitability of these different approaches towards analysis of the major lipid classes along with their current and potential applications in the field of lipid analysis are evaluated.

Correlated imaging--a grand challenge in chemical analysis

Correlated chemical imaging is an emerging strategy for acquisition of images by combining information from multiplexed measurement platforms to track, visualize, and interpret in situ changes in the structure, organization, and activities of interesting chemical systems, frequently spanning multiple decades in space and time. Acquiring and correlating information from complementary imaging experiments has the potential to expose complex chemical behavior in ways that are simply not available from single methods applied in isolation, thereby greatly amplifying the information gathering power of imaging experiments. However, in order to correlate image information across platforms, a number of issues must be addressed. First, signals are obtained from disparate experiments with fundamentally different figures of merit, including pixel size, spatial resolution, dynamic range, and acquisition rates. In addition, images are often acquired on different instruments in different locations, so the sample must be registered spatially so that the same area of the sample landscape is addressed. The signals acquired must be correlated in both spatial and temporal domains, and the resulting information has to be presented in a way that is readily understood. These requirements pose special challenges for image cross-correlation that go well beyond those posed in single technique imaging approaches. The special opportunities and challenges that attend correlated imaging are explored by specific reference to correlated mass spectrometric and Raman imaging, a topic of substantial and growing interest.

Generation of picoliter droplets of liquid for electrospray ionization with piezoelectric inkjet

We report the association of inkjet and electrospray ionization MS to detect picoliter droplet, where the liquid volume and its position onto the tip can be precisely controlled to form ultrafine droplets for successive ionization of the analyte. Single rectangle pulse was applied to piezoelectric device on inkjet microchip for the ejection of each picoliter droplet, and it was controlled by a computer. The voltage and width of driving pulse for the inkjet were optimized to make reproducible ejection of the solvent with low viscosity. The volume of each droplet was about 600 pl, and a trigger of 10 droplets was selected as the best inlet mode taking relative standard derivation of the droplets into consideration. The target substrate used with high voltage to form ionization was graphite, after several attempts with some materials. High-speed camera was used to capture the breaking-up process of a droplet. The distance between the inkjet nozzle and the tip was set at 2 cm to avoid short circuit. The influences on the mass intensity of the diameter of the tip, the volume and the concentration of the sample were examined. The tip with a small diameter performed greater intensity, and the limit of detection decreased, whereas the small volume of liquid played high ionization efficiency. Linear regression in the range between 1 and 200 ppm for caffeine was conducted, where internal standard theobromine was used. Some real samples were also detected with the instrument.

Chemical aspects of the extractive methods of ambient ionization mass spectrometry

Ambient ionization techniques allow complex chemical samples to be analyzed in their native state with minimal sample preparation. This brings the obvious advantages of simplicity, speed, and versatility to mass spectrometry: Desorption electrospray ionization (DESI), for example, is used in chemical imaging for tumor margin diagnosis. This review on the extractive methods of ambient ionization focuses on chemical aspects, mechanistic considerations, and the accelerated chemical reactions occurring in charged liquid droplets generated in the spray process. DESI uses high-velocity solvent droplets to extract analytes from surfaces. Nano-DESI employs liquid microjunctions for analyte dissolution, whereas paper-spray ionization uses DC potentials applied to wet porous material such as paper or biological tissue to field emit charged analyte-containing solvent droplets. These methods also operate in a reactive mode in which added reagents allow derivatization during ionization. The accelerated reaction rates seen in charged microdroplets are useful in small-scale rapid chemical synthesis.