3,4-Dimethoxycinnamic Acid as a Novel Matrix for Enhanced In Situ Detection and Imaging of Low-Molecular-Weight Compounds in Biological Tissues by MALDI-MSI

Proteomics mining of cancer hallmarks on a single-cell resolution

Dysregulated proteome is an essential contributor in carcinogenesis. Protein fluctuations fuel the progression of malignant transformation, such as uncontrolled proliferation, metastasis, and chemo/radiotherapy resistance, which severely impair therapeutic effectiveness and cause disease recurrence and eventually mortality among cancer patients. Cellular heterogeneity is widely observed in cancer and numerous cell subtypes have been characterized that greatly influence cancer progression. Population-averaged research may not fully reveal the heterogeneity, leading to inaccurate conclusions. Thus, deep mining of the multiplex proteome at the single-cell resolution will provide new insights into cancer biology, to develop prognostic biomarkers and treatments. Considering the recent advances in single-cell proteomics, herein we review several novel technologies with particular focus on single-cell mass spectrometry analysis, and summarize their advantages and practical applications in the diagnosis and treatment for cancer. Technological development in single-cell proteomics will bring a paradigm shift in cancer detection, intervention, and therapy.

Enhancement of in situ detection and imaging of phytohormones in plant tissues by MALDI-MSI using 2,4-dihydroxy-5-nitrobenzoic acid as a novel matrix

Phytohormones possess unique chemical structures, and their physiological effects are regulated through intricate interactions or crosstalk among multiple phytohormones. MALDI-MSI enables the simultaneous detection and imaging of multiple hormones. However, its application for tracing phytohormones is currently restricted by low abundance of hormone in plant and suboptimal matrix selection. 2,4-Dihydroxy-5-nitrobenzoic acid (DHNBA) was reported as a new MALDI matrix for the enhanced detection and imaging of multiple phytohormones in plant tissues. DHNBA demonstrates remarkable sensitivity improvement when compared to the commonly used matrix, 2,5-dihydroxybenzoic acid (DHB), in the detection of isoprenoid cytokinins (trans-zeatin (tZ), dihy-drozeatin (DHZ), meta-topolin (mT), and N6-(Δ2-isopentenyl) adenine (iP)), jasmonic acid (JA), abscisic acid (ABA), and 1-aminocyclo-propane-1-carboxylic acid (ACC) standards. The distinctive properties of DHNBA (i.e. robust UV absorption, uniform matrix deposition, negligible background interference, and high ionization efficiency of phytohormones) make it as an ideal matrix for enhanced detection and imaging of phytohormones, including tZ, DHZ, ABA, indole-3-acetic acid (IAA), and ACC, by MALDI-MSI in various plant tissues, for example germinating seeds, primary/lateral roots, and nodules. Employing DHNBA significantly enhances our capability to concurrently track complex phytohormone biosynthesis pathways while providing precise differentiation of the specific roles played by individual phytohormones within the same category. This will propel forward the comprehensive exploration of phytohormonal functions in plant science.

Electromagnetic Field-Assisted Frozen Tissue Planarization Enhances MALDI-MSI in Plant Spatial Omics

Plant samples with irregular morphology are challenging for longitudinal tissue sectioning. This has restricted the ability to gain insight into some plants using matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI). Herein, we develop a novel technique termed electromagnetic field-assisted frozen tissue planarization (EMFAFTP). This technique involves using a pair of adjustable electromagnets on both sides of a plant tissue. Under an optimized electromagnetic field strength, nondestructive planarization and regularization of the frozen tissue is induced, allowing the longitudinal tissue sectioning that favors subsequent molecular profiling by MALDI-MSI. As a proof of concept, flowers, leaves and roots with irregular morphology from six plant species are chosen to evaluate the performance of EMFAFTP for MALDI-MSI of secondary metabolites, amino acids, lipids, and proteins among others in the plant samples. The significantly enhanced MALDI-MSI capabilities of these endogenous molecules demonstrate the robustness of EMFAFTP and suggest it has the potential to become a standard technique for advancing MALDI-MSI into a new era of plant spatial omics.

Development of benzimidazole derivatives as efficient matrices for the analysis of acidic small-molecule compounds using matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry in negative ion mode

RATIONALE

With the development of matrix-assisted laser desorption/ionisation (MALDI) mass spectrometry (MS) in spatial localisation omics research on small molecules, the detection sensitivity of the matrix must increase. However, the types of matrices suitable for detecting acidic small molecules in (-) MALDI-MS mode are very limited and are either not sensitive enough or difficult to obtain.

METHODS

More than 10 commercially available benzimidazole and benzothiazole derivatives were selected as MALDI matrices in negative ion mode. MALDI-MS analysis was performed on 38 acidic small molecules and mouse serum, and the matrix effects were compared with those of the common commercial matrices 9-aminoacridine (9AA), 1,5-naphthalenediamine (DAN) and 3-aminoquinoline (3AQ). Moreover, the proton affinity (PA) of the selected potential matrix was calculated, and the relationships among the compound structure, PA value and matrix effect were discussed.

RESULTS

In (-) MALDI-MS mode, a higher PA value generally indicates a better matrix effect. Amino-substituted 2-phenyl-1H-benzo[d]imidazole derivatives had well-defined matrix effects on all analytes and were generally superior to the commonly used matrices 9AA, DAN and 3AQ. Among them, 2-(4-(dimethylamino-phenyl)-1H-benzo[d]imidazole-5-amine (E-4) has the best sensitivity and versatility for detecting different analytes and has the best ability to detect fatty acids in mouse serum; moreover, the limit of detection (LOD) of some analytes can reach as low as ng/L.

CONCLUSIONS

Compared to 9AA, DAN and 3AQ, matrix E-4 is more effective at detecting low-molecular-weight acidic compounds in (-) MALDI-MS mode, with higher sensitivity and better versatility. In addition, there is a clear correlation between compound structure, PA and matrix effects, which provides a basis for designing more efficient matrices.

Absorption and translocation of selected pharmaceuticals in Pistia stratiotes: Spatial distribution analysis using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

Phytoremediation can eliminate pharmaceuticals from aquatic environments through absorption; however, understanding of absorption and transport processes in plants remains limited. In this study, a matrix-assisted laser desorption/ionization time-of-flight mass spectrometry imaging (MALDI-MSI) method was developed to explore the absorption and translocation mechanisms of seven common pharmaceuticals in Pistia stratiotes. Results showed that 2,3-dicyanohydroquinone, an infrequently used matrix, exhibited outstanding performance in MALDI-MSI analysis, producing the highest signal intensity for four of the seven pharmaceuticals. Region of Interest (ROI) analysis revealed that charge speciation of pharmaceuticals significantly influenced their ability to enter vascular bundle. Neutral and positively charged pharmaceuticals easily entered vascular bundle, while negatively charged pharmaceuticals faced difficulty. ROI results for neutral and negatively charged pharmaceuticals exhibited positive correlation with their transfer factor values, indicating that their translocation ability from root to shoot was related to their capacity to enter vascular bundle. However, no correlation was observed for positively charged pharmaceuticals, suggesting that these compounds, upon entering vascular bundle, encountered difficulties in upward translocation through the xylem. This study introduces an innovative approach and offers novel insights into the retention and migration of pharmaceuticals in plant tissues, aiming to enhance the understanding of pharmaceutical accumulation in plants. ENVIRONMENTAL IMPLICATION: Pharmaceuticals in aquatic environment can inflict detrimental effects on both human health and ecosystem. Phytoremediation can remove pharmaceuticals from aquatic environments through absorption. However, our understanding of absorption and transportation of pharmaceuticals in plants remains limited. This study developed a matrix-assisted laser desorption/ionization time-of-flight mass spectrometry imaging (MALDI-MSI) method for pharmaceuticals in plant roots, and to explore the absorption and translocation mechanisms of pharmaceuticals. The study offers direct evidence of differences in accumulation behavior of pharmaceuticals in plants, providing valuable insights for targeted and effective strategies in using plants for remediating the aquatic ecosystem from pharmaceuticals.

2-Hydroxy-5-nitro-3-(trifluoromethyl)pyridine as a Novel Matrix for Enhanced MALDI Imaging of Tissue Metabolites

Matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI), which is a label-free imaging technique, determines the spatial distribution and relative abundance of versatile endogenous metabolites in tissues. Meanwhile, matrix selection is generally regarded as a pivotal step in MALDI tissue imaging. This study presents the first report of a novel MALDI matrix, 2-hydroxy-5-nitro-3-(trifluoromethyl)pyridine (HNTP), for the in situ detection and imaging of endogenous metabolites in rat liver and brain tissues by MALDI-MS in positive-ion mode. The HNTP matrix exhibits excellent characteristics, including strong ultraviolet absorption, μm-scale matrix crystals, high chemical stability, low background ion interference, and high metabolite ionization efficiency. Notably, the HNTP matrix also shows superior detection capabilities, successfully showing 185 detectable metabolites in rat liver tissue sections. This outperforms the commonly used matrices of 2,5-dihydroxybenzoic acid and 2-mercaptobenzothiazole, which detect 145 and 120 metabolites from the rat liver, respectively. Furthermore, a total of 152 metabolites are effectively detected and imaged in rat brain tissue using the HNTP matrix, and the spatial distribution of these compounds clearly shows the heterogeneity of the rat brain. The results demonstrate that HNTP is a new and powerful positive-ion mode matrix to enhance the analysis of metabolites in biological tissues by MALDI-MSI.

2,5-Dihydroxyterephthalic Acid: A Matrix for Improved Detection and Imaging of Amino Acids

Amino acids (AAs), which are low-molecular-weight (low-MW) metabolites, serve as essential building blocks not only for protein synthesis but also for maintaining the nitrogen balance in living systems. In situ detection and imaging of AAs are crucial for understanding more complex biological processes. Matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI) is a label-free mass spectrometric imaging technique that enables the simultaneous detection and imaging of the spatial distribution and relative abundance of different endogenous/exogenous compounds in biological samples. The excellent efficiency of MALDI-MSI is attributed to the choice of the MALDI matrix. However, to the best of our knowledge, no matrix has been specifically developed for AAs. Herein, we report a MALDI matrix, 2,5-dihydroxyterephthalic acid (DHT), which can improve the detection and imaging of AAs in biological samples by MALDI-MS. Our results indicated that DHT exhibited strong ultraviolet-visible (UV-vis) absorption, uniform matrix deposition, and high vacuum stability. Moreover, the matrix-related ion signals produced from DHT were reduced by 50 and 71.8% at m/z < 500 compared to the commonly used matrices of 2,5-dihydroxybenzoic acid (DHB) and α-cyano-4-hydroxycinnamic acid (CHCA), respectively, in their respective organic solvents. In terms of quantitative performance, arginine, glutamic acid, glutamine, and proline can be detected with limits of detection of 6, 4, 6, and 4 ng/mL, respectively, using the DHT as the matrix. Using DHT as the matrix, all 20 protein AAs were successfully detected in human serum by MALDI-MS, whereas only 7 and 10 AAs were detected when DHB and CHCA matrices were used, respectively. Furthermore, 20 protein AAs and taurine were successfully detected and imaged in a section of edible Crassostrea gigas (oyster) tissue for the first time. Our study demonstrates that using DHT as a matrix can improve the detection and imaging of AAs in biological samples by MALDI-MS.

High-throughput MALDI-MSI metabolite analysis of plant tissue microarrays

A novel metabolomics analysis technique, termed matrix-assisted laser desorption/ionization mass spectrometry imaging-based plant tissue microarray (MALDI-MSI-PTMA), was successfully developed for high-throughput metabolite detection and imaging from plant tissues. This technique completely overcomes the disadvantage that metabolites cannot be accessible on an intact plant tissue due to the limitations of the special structures of plant cells (e.g. epicuticular wax, cuticle and cell wall) through homogenization of plant tissues, preparation of PTMA moulds and matrix spraying of PTMA sections. Our study shows several properties of MALDI-MSI-PTMA, including no need of sample separation and enrichment, high-throughput metabolite detection and imaging (>1000 samples per day), high-stability mass spectrometry data acquisition and imaging reconstruction and high reproducibility of data. This novel technique was successfully used to quickly evaluate the effects of two plant growth regulator treatments (i.e. 6-benzylaminopurine and N-phenyl-N'-1,2,3-thiadiazol-5-ylurea) on endogenous metabolite expression in plant tissue culture specimens of Dracocephalum rupestre Hance (D. rupestre). Intra-day and inter-day evaluations indicated that the metabolite data detected on PTMA sections had good reproducibility and stability. A total of 312 metabolite ion signals in leaves tissues of D. rupestre were detected, of which 228 metabolite ion signals were identified, they were composed of 122 primary metabolites, 90 secondary metabolites and 16 identified metabolites of unknown classification. The results demonstrated the advantages of MALDI-MSI-PTMA technique for enhancing the overall detection ability of metabolites in plant tissues, indicating that MALDI-MSI-PTMA has the potential to become a powerful routine practice for high-throughput metabolite study in plant science.

Visualizing the Distribution of Jujube Metabolites at Different Maturity Stages Using Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry Imaging

Chinese jujube (also called Chinese date, Ziziphus jujuba Mill.) is an economically important tree in China and provides a rich source of sugars, vitamins, and bioactive components, all of which are indispensable and essential for the composition and participation in life processes of the human body. However, the location of these metabolites in jujube fruits has not been determined. This study applied matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI) to investigate the spatial distribution of sugars, organic acids, and other key components in jujube fruits at different developmental periods. Soluble sugars such as hexoses and sucrose/maltose significantly increase with fruit ripening, while organic acids show an overall trend of initially increasing and then decreasing. Procyanidins and rutin exhibit specific distributions in the fruit periphery and peel. These findings suggest that MALDI-MSI can be used to study the spatial distribution of nutritional components in jujube fruits, providing insights into the changes and spatial distribution of substances during jujube fruit development. This technique offers a scientific basis for jujube breeding, utilization, and production.

Desorption electrospray ionization and matrix-assisted laser desorption/ionization as imaging approaches for biological samples analysis

The imaging of biological tissues can offer valuable information about the sample composition, which improves the understanding of analyte distribution in such complex samples. Different approaches using mass spectrometry imaging (MSI), also known as imaging mass spectrometry (IMS), enabled the visualization of the distribution of numerous metabolites, drugs, lipids, and glycans in biological samples. The high sensitivity and multiple analyte evaluation/visualization in a single sample provided by MSI methods lead to various advantages and overcome drawbacks of classical microscopy techniques. In this context, the application of MSI methods, such as desorption electrospray ionization-MSI (DESI-MSI) and matrix-assisted laser desorption/ionization-MSI (MALDI-MSI), has significantly contributed to this field. This review discusses the evaluation of exogenous and endogenous molecules in biological samples using DESI and MALDI imaging. It offers rare technical insights not commonly found in the literature (scanning speed and geometric parameters), making it a comprehensive guide for applying these techniques step-by-step. Furthermore, we provide an in-depth discussion of recent research findings on using these methods to study biological tissues.

Spatial Lipidomics of EPSPS and PAT Transgenic and Non-Transgenic Soybean Seeds Using Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry Imaging

Herbicide-resistant soybeans are among the most widely planted transgenic crops. The in situ evaluation of spatial lipidomics in transgenic and non-transgenic soybeans is important for directly assessing the unintended effects of exogenous gene introduction. In this study, matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI)-based non-targeted analytical strategies were used for the first time for in situ detection and imaging of endogenous lipid distributions in transgenic (EPSPS and PAT genes) herbicide-resistant soybean (Glycine max Merrill) (S4003.14) and non-transgenic soybean (JACK) seeds. Statistical analysis revealed significant differences in lipids between S4003.14 and JACK seeds. The variable importance of projection analysis further revealed that 18 identified lipids, including six phosphatidylcholines (PCs), four phosphatidylethanolamines (PEs), five triacylglycerols (TAGs), and three cytidine diphosphate-diacylglycerols (CDP-DAGs), had the strongest differential expression between S4003.14 and JACK seeds. Among those, the upregulated expressions of PC(P-36:1), PC(36:2), PC(P-36:0), PC(37:5), PE(40:2), TAG(52:1), TAG(55:5), and CDP-DAG(37:2) and the downregulated expressions of PC (36:1), TAG(43:0), and three PEs (i.e., PE(P-38:1), PE(P-38:0), and PE(P-40:3)) were successfully found in the S4003.14 seeds, compared to these lipids detected in the JACK seeds. Meanwhile, the lipids of PC (44:8), CDP-DAG(38:0), and CDP-DAG(42:0) were uniquely detected in the S4003.14 soybean seeds, and TAG(45:2) and TAG(57:10) were detected as the unique lipids in the JACK seeds. The heterogeneous distribution of these lipids in the soybean seeds was also clearly visualized using MALDI-MSI. MSI results showed that lipid expression was significantly up/downregulated in S4003.14 seeds, compared to that in JACK seeds. This study improves our understanding of the unintended effects of herbicide-resistant EPSPS and PAT gene transfers on spatial lipidomes in soybean seeds and enables the continued progression of MALDI-MSI as an emerging, reliable, and rapid molecular imaging tool for evaluating unintended effects in transgenic plants.

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.

An efficient Pt@MXene platform for the analysis of small-molecule natural products

Small-molecule (m/z<500) natural products have rich biological activity and significant application value thus need to be effectively detected. Surface-assisted laser desorption/ionization mass spectrometry (SALDI MS) has become a powerful detection tool for small-molecule analysis. However, more efficient substrates need to be developed to improve the efficiency of SALDI MS. Thus, platinum nanoparticle-decorated Ti₃C₂ MXene (Pt@MXene) was synthesized in this study as an ideal substrate for SALDI MS in positive ion mode and exhibited excellent performance for the high-throughput detection of small molecules. Compared with using MXene, GO, and CHCA matrix, a stronger signal peak intensity and wider molecular coverage was obtained using Pt@MXene in the detection of small-molecule natural products, with a lower background, excellent salt and protein tolerance, good repeatability, and high detection sensitivity. The Pt@MXene substrate was also successfully used to quantify target molecules in medicinal plants. The proposed method has potentially wide application.

In Situ Proteomic Analysis of Herbicide-Resistant Soybean and Hybrid Seeds via Matrix-Assisted Laser Desorption/Ionization-Mass Spectrometry Imaging

Transgenic soybean is the commercial crop with the largest cultivation area worldwide. During transgenic soybean cultivation, exogenous genes may be transferred to wild relatives through gene flow, posing unpredictable ecological risks. Accordingly, an environmental risk assessment should focus on fitness changes and underlying mechanisms in hybrids between transgenic and wild soybeans (Glycine soja). Matrix-assisted laser desorption/ionization-mass spectrometry imaging (MALDI-MSI) was used for in situ detection and imaging of protein changes in the seeds of transgenic herbicide-resistant soybean harboring epsps and pat genes, non-transgenic soybean, wild soybean, and their F₂ hybrid. Protein data clearly distinguished wild soybeans, while the F₂ seeds had protein characteristics of both parents and were distinguished from wild soybean seeds. Using UPLC-Q-TOF-MS, 22 differentially expressed proteins (DEPs) were identified, including 13 specific to wild soybean. Sucrose synthase and stress response-related DEPs were differentially expressed in parental and offspring. Differences in these may underpin the greater adaptability of the latter. MSI revealed DEP distribution in transgenic, wild, and F₂ seeds. Identifying DEPs related to fitness may elucidate mechanisms underlying fitness differences among the studied varieties. Our study shows that MALDI-MSI has the potential to become a visual method for transgenic soybean analysis.

Polyacrylamide gel as a new embedding medium for the enhancement of metabolite MALDI imaging

In this study, polyacrylamide gel (PAAG) was successfully used as a new embedding medium to provide the more effective maintenance of biological tissues during the sectioning process, enhancing the tissue imaging of metabolites via matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI). Herein, PAAG, agarose, gelatin, optimal cutting temperature compound (OCT), and ice media were used to embed rat liver and Atlantic salmon (Salmo salar) eyeball samples. These embedded tissues were then sectioned into thin slices and thaw-mounted on conductive microscope glass slides for MALDI-MSI detection to evaluate the embedding effects. The results showed that PAAG embedding has characteristics superior to those of commonly-used embedding media (e.g., agarose, gelatin, OCT, and ice) with the advantages of one-step operation without heating, a better performance of morphology maintenance, the absence of PAAG polymer-ion-related interference below m/z 2000, and the more efficient in situ ionization of metabolites, providing a significant enhancement of both the numbers and intensities of the metabolite ion signals. Our study demonstrates the potential of PAAG embedding as a standard practice for metabolite MALDI tissue imaging, which will lead to an expanded application scope of MALDI-MSI.

Screening of potential chemical marker with interspecific differences in Pterocarpus wood and a spatially-resolved approach to visualize the distribution of the characteristic markers

Profiling the spatial distributions and tissue changes of characteristic compounds with interspecific differences is critical to elucidate the complex species identification during tree species traceability, wood anti-counterfeiting verification and timber trade control. In this research, in order to visualize the spatial position of characteristic compounds in two species with similar morphology (Pterocarpus santalinus and Pterocarpus tinctorius), a high coverage MALDI-TOF-MS imaging method was used to found the mass spectra fingerprints of different wood species. 2-Mercaptobenzothiazole matrix was used to spray wood tissue section to enhance the detection effect of metabolic molecules, and the mass spectrometry imaging data were obtained. Based on this technology, the spatial location of fifteen potential chemical markers with remarkable interspecific differences in 2 Pterocarpus timber species were successfully obtained. Distinct chemical signatures obtained from this method can promote rapid identification at the wood species level. Thus, matrix-assisted laser desorption/time-of-flight/ionization mass spectrometry imaging (MALDI-TOF-MSI) provides a spatial-resolved way for traditional wood morphological classification and breaking through the limitations of traditional wood identification technology.

Spatial Metallomics Reveals Preferable Accumulation of Methylated Selenium in a Single Seed of the Hyperaccumulator Cardamine violifolia†

Cardamine violifolia is a Se hyperaccumulator found in Enshi, China. In this study, spatial metallomics was applied to visualize the distribution and speciation of Se in a single seed of C. violifolia. It was found that Se reached 1729.89 ± 28.14 mg/kg and the main Se species were SeCys and SeMet in bulk seeds. Further in situ study on a single seed found that the methylated Se species located mostly in the episperm. This is the first visualized evidence of the in situ distribution of methylated Se species in the seeds of C. violifolia. In all, spatial metallomics finds a preferable accumulation of methylated Se species in the seed coat, which deepens the understanding of the tolerance of Se by C. violifolia. The protocol applied in this study may also be used for the understanding of the tolerance of heavy metals/metalloids in other hyperaccumulators.

Metabolic profiling, in-situ spatial distribution, and biosynthetic pathway of functional metabolites in Dendrobium nobile stem revealed by combining UPLC-QTOF-MS with MALDI-TOF-MSI

The stem of Dendrobium nobile Lindl. (Orchidaceae), called "Shihu" in traditional Chinese medicine, is a well-known medicinal and edible plant material in China. It is used as an antipyretic, analgesic, and tonic to nourish the stomach and Yin (i.e., to improve the production of body fluids). These therapeutic properties are attributed to its alkaloids, sesquiterpenoids, bibenzyls, fluorenones, and phenanthrenes. However, a comprehensive understanding of these metabolites and their spatial distribution in stems is lacking. In this study, ultra-performance liquid chromatography/quadrupole time-of-flight mass spectrometry (UPLC-QTOF-MS) was performed to obtain detailed metabolites information about D. nobile stems. Then, the spatial distributions of diverse metabolites, including alkaloids and sesquiterpenoids, were characterized and visualized by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry imaging (MALDI-TOF-MSI). Based on the spatial and metabolic profiling data, sesquiterpene alkaloid dendrobine was chosen for the exhaustive study of a biosynthetic pathway in D. nobile. This is the first report on mass spectrometry imaging for Dendrobium species. As a result, critical bioactive metabolites such as 11 alkaloids, 10 sesquiterpenes, and 13 other metabolites were putatively identified and relatively quantified. The identified alkaloids were distributed in the parenchyma or vascular bundle, and sesquiterpenes were present in all regions of the stem with higher abundance in the vascular bundle and cuticle, or in the cuticle and epidermis. The biosynthetic pathway and accumulation pattern of dendrobine in D. nobile stem were also proposed. Our findings not only provided a critical methodology for the thorough understanding of physiological changes in metabolites and precise utilization of D. nobile stem, but also displayed an effective strategy for insight into the biosynthesis of bioactive metabolites in plants.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: An update for 2019-2020

This review is the tenth update of the original article published in 1999 on the application of matrix-assisted laser desorption/ionization (MALDI) mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2020. Also included are papers that describe methods appropriate to analysis by MALDI, such as sample preparation techniques, even though the ionization method is not MALDI. The review is basically divided into three sections: (1) general aspects such as theory of the MALDI process, matrices, derivatization, MALDI imaging, fragmentation, quantification and the use of arrays. (2) Applications to various structural types such as oligo- and polysaccharides, glycoproteins, glycolipids, glycosides and biopharmaceuticals, and (3) other areas such as medicine, industrial processes and glycan synthesis where MALDI is extensively used. Much of the material relating to applications is presented in tabular form. The reported work shows increasing use of incorporation of new techniques such as ion mobility and the enormous impact that MALDI imaging is having. MALDI, although invented nearly 40 years ago is still an ideal technique for carbohydrate analysis and advancements in the technique and range of applications show little sign of diminishing.

Advances in mass spectrometry imaging for toxicological analysis and safety evaluation of pharmaceuticals

Safety issues caused by pharmaceuticals have frequently occurred worldwide, posing a tremendous threat to human health. As an essential part of drug development, the toxicological analysis and safety evaluation is of great significance. In addition, the risk of pharmaceuticals accumulation in the environment and the monitoring of the toxicity from natural medicines have also received ongoing concerns. Due to a lack of spatial distribution information provided by common analytical methods, analyses that provide spatial dimensions could serve as complementary safety evaluation methods for better prediction and evaluation of drug toxicity. With advances in technical solutions and software algorithms, mass spectrometry imaging (MSI) has received increasing attention as a popular analytical tool that enables the simultaneous implementation of qualitative, quantitative, and localization without complex sample pretreatment and labeling steps. In recent years, MSI has become more attractive, powerful, and sensitive and has been applied in several scientific fields that can meet the safety assessment requirements. This review aims to cover a detailed summary of the various MSI technologies utilized in the biomedical and pharmaceutical area, including technical principles, advantages, current status, and future trends. Representative applications and developments in the safety-related issues of different pharmaceuticals and natural medicines are also described to provide a reference for pharmaceutical research, improve rational clinical medicine use, and ensure public safety.

Integrated Experimental Approach, Phytochemistry, and Network Pharmacology to Explore the Potential Mechanisms of Cinnamomi Ramulus for Rheumatoid Arthritis

Cinnamomi Ramulus (CR) has been extensively used as a remedy for inflammatory diseases in China. This study adopted an integrative approach of experimental research, phytochemistry, and network pharmacology to investigate its alleviative effects on rheumatoid arthritis (RA) and the underlying potential mechanisms. CR extract (50, 100, and 200 mg/kg) and methotrexate (MTX) significantly ameliorated RA symptoms in the collagen-induced arthritis (CIA) rat model. They also reduced paw volume, arthritis index, proinflammatory cytokines (TNF-α, IL-17A, IL-6, and IL-1β), and oxidative damage. Sixty-three compounds were systematically identified as the basic components of CR. Fifty-five common genes obtained from compounds and GEO databases were employed to construct the protein-protein interaction (PPI) network. Among them, 20 hub genes were identified via the cytoHubba. Enrichment analysis of the common genes indicated that the TNF signaling pathway and IL-17 signaling pathway might be the potential key pathways. Moreover, molecular docking methods confirmed the high affinity between the top 10 bioactive components of CR and the top 10 targets. In addition, in vitro results showed that CR extract (0.2, 0.4, and 0.8 mg/mL) inhibited inflammation and oxidative damage in MH7A cells stimulated by lipopolysaccharide (LPS). In summary, this study adopted multiple approaches to elucidate the protective effect and potential mechanisms of CR on RA, indicating that CR might be a promising herbal candidate for further investigation of RA treatment.

What are we imaging? Software tools and experimental strategies for annotation and identification of small molecules in mass spectrometry imaging

Mass spectrometry imaging (MSI) has become a widespread analytical technique to perform nonlabeled spatial molecular identification. The Achilles' heel of MSI is the annotation and identification of molecular species due to intrinsic limitations of the technique (lack of chromatographic separation and the difficulty to apply tandem MS). Successful strategies to perform annotation and identification combine extra analytical steps, like using orthogonal analytical techniques to identify compounds; with algorithms that integrate the spectral and spatial information. In this review, we discuss different experimental strategies and bioinformatics tools to annotate and identify compounds in MSI experiments. We target strategies and tools for small molecule applications, such as lipidomics and metabolomics. First, we explain how sample preparation and the acquisition process influences annotation and identification, from sample preservation to the use of orthogonal techniques. Then, we review twelve software tools for annotation and identification in MSI. Finally, we offer perspectives on two current needs of the MSI community: the adaptation of guidelines for communicating confidence levels in identifications; and the creation of a standard format to store and exchange annotations and identifications in MSI.

Nanocomposites of boronic acid-functionalized magnetic multi-walled carbon nanotubes with flexible branched polymers as a novel desorption/ionization matrix for the capture and direct detection of cis-diol-flavonoid compounds coupled with MALDI-TOF-MS

Novel boronic acid-functionalized magnetic multi-walled carbon nanotubes with flexible branched polymer (Fe₃O₄@MWCNTs@ε-PL@BA) nanocomposites were fabricated and applied as the desorption/ionization matrix for the MALDI-TOF-MS determination of low molecular weight flavonoids. The prepared nanocomposite was systematically characterized by various techniques. Compared to the traditional organic matrix, the proposed Fe₃O₄@MWCNTs@ε-PL@BA matrix has excellent ionization efficiency and low-background noise interference due to the MWCNTs unique electron-phonon interaction and the high introduction density of boronic acid functional groups. Good sensitivity and ultra-high salt tolerance of the Fe₃O₄@MWCNTs@ε-PL@BA-assisted MALDI-TOF-MS were permitted for the determination and quantification of flavonoids in actual samples. Noticeably, the limits of detection (LODs) for the target flavonoids were in the range 17-33 nM. The relative standard deviations (RSDs) of spot-to-spot and sample-to-sample (n = 10) were ≤ 9.8% and ≤ 10.1%, respectively. Furthermore, the wide linear ranges (0.1 - 500 µg/mL) and satisfactory calibration plot coefficients (R² > 0.99) of flavonoids were achieved by MALDI-TOF-MS with the Fe₃O₄@MWCNTs@ε-PL@BA matrix. Good recoveries (92-105.5%) were achieved for the target flavonoids in practical food samples. Hence, the prepared Fe₃O₄@MWCNTs@ε-PL@BA nanocomposites have applications in the selective and efficient capture of target flavonoids active biomolecules coupled with MALDI-TOF-MS determination in actual samples.

Synthesis and Investigation of Novel CHCA-Derived Matrices for Matrix-Assisted Laser Desorption/Ionization Mass Spectrometric Analysis of Lipids

A significant area of study and upgrading for increasing sensitivity and general performances of matrix-assisted laser-desorption ionization (MALDI) mass spectrometry (MS) is related to matrix design. Several efforts have been made to address the challenge of low-mass-region interference-free for metabolomics analysis and specifically for lipidomics. To this aim, rationally designed matrices as 4-chloro-α-cyanocinnamic acid (ClCCA) were introduced and reported to provide enhanced analytical performances. We have taken this rational design one step further by developing and optimizing new MALDI matrices with a range of modifications on the CHCA core, involving different functionalities and substituents. Of particular interest was the understanding of the electron-withdrawing (e.g., nitro-) or donating (e.g., methoxy-) effects along with the extent of conjugation on the ionization efficiency. In the present work, ten matrices were designed on a reasonable basis, synthesized, and characterized by NMR and UV spectroscopies and laser desorption ionization. With the assistance of these putative MALDI matrices, samples containing phospholipids (PL), and neutral di-/tri-acylglycerols (DAG, TAG) were investigated using milk, fish, blood, and human plasma extracts. In comparison with CHCA and ClCCA, four of them, viz. [(2E,4E)-2-cyano-5-(4-methoxyphenyl)penta-2,4-dienoic acid] (1), [(2E,4E)-2-cyano-5-(4-nitrophenyl)penta-2,4-dienoic acid] (2), [(E)-2-cyano-3-(6-methoxynaphthalen-2-yl)acrylic acid] (6) and [(E)-2-cyano-3-(naphthalen-2-yl)acrylic acid] (7) displayed good to even excellent performances as MALDI matrices in terms of ionization capability, interference-free spectra, S/N ratio, and reproducibility. Especially compound 7 (cyano naphthyl acrylic acid, CNAA) was the election matrix for PL analysis and matrix 2 (cyano nitrophenyl dienoic acid, CNDA) for neutral lipids such as DAG and TAG in positive ion mode.

Cytology, transcriptomics, and mass spectrometry imaging reveal changes in late-maturation elm (Ulmus pumila) seeds

During seed maturation, the seed deposits storage compounds (starches, oils, and proteins), synthesizes defense compounds, produces a seed coat, initiates embryo dormancy, and becomes desiccated. During the late-maturation stage, seed storage compound contents and compositions change dramatically. Although maturation has been extensively studied in model species and crops, it remains less well characterized in woody perennial plants. In this study, we conducted morphological and cytological observations, transcriptome profiling, and chemical constituent analysis of elm (Ulmus pumila L.) seeds during the late-maturation stage. Light and electron microscopy revealed that closely packed yet discrete lipid bodies frequently surrounded the densely stained protein bodies, and the protein bodies became irregular or even partially disintegrated at the end of seed development. RNA-seq detected substantial transcriptome changes during the late-maturation stage, and pathway enrichment analysis showed that the differentially expressed genes were associated with phenylpropanoid biosynthesis, starch and sucrose metabolism, plant-pathogen interactions, and hormone signal transduction. Furthermore, we used mass spectrometry imaging to detect the relative intensity and spatial distribution of fatty acids, phospholipids, and waxes in elm seeds. Our findings provide a framework for understanding the changes in cytological features and chemical composition during the final stage of elm seed development, and a detailed reference for seed development in woody plants.

Advanced applications of mass spectrometry imaging technology in quality control and safety assessments of traditional Chinese medicines

ETHNOPHARMACOLOGICAL RELEVANCE

Traditional Chinese medicines (TCMs) have made great contributions to the prevention and treatment of human diseases in China, and especially in cases of COVID-19. However, due to quality problems, the lack of standards, and the diversity of dosage forms, adverse reactions to TCMs often occur. Moreover, the composition of TCMs makes them extremely challenging to extract and isolate, complicating studies of toxicity mechanisms.

AIM OF THE REVIEW

The aim of this paper is therefore to summarize the advanced applications of mass spectrometry imaging (MSI) technology in the quality control, safety evaluations, and determination of toxicity mechanisms of TCMs.

MATERIALS AND METHODS

Relevant studies from the literature have been collected from scientific databases, such as "PubMed", "Scifinder", "Elsevier", "Google Scholar" using the keywords "MSI", "traditional Chinese medicines", "quality control", "metabolomics", and "mechanism".

RESULTS

MSI is a new analytical imaging technology that can detect and image the metabolic changes of multiple components of TCMs in plants and animals in a high throughput manner. Compared to other chemical analysis methods, such as liquid chromatography-mass spectrometry (LC-MS), this method does not require the complex extraction and separation of TCMs, and is fast, has high sensitivity, is label-free, and can be performed in high-throughput. Combined with chemometrics methods, MSI can be quickly and easily used for quality screening of TCMs. In addition, this technology can be used to further focus on potential biomarkers and explore the therapeutic/toxic mechanisms of TCMs.

CONCLUSIONS

As a new type of analysis method, MSI has unique advantages to metabolic analysis, quality control, and mechanisms of action explorations of TCMs, and contributes to the establishment of quality standards to explore the safety and toxicology of TCMs.

Michler's ethylketone as a novel negative-ion matrix for the enhancement of lipid MALDI tissue imaging

Michler's ethylketone (MEK, 4,4'-bis(diethylamino)benzophenone), commonly-known as an intermediate in the synthesis of dyes and pigments, was successfully screened and optimized as a novel matrix for the enhancement of lipid in situ detection and imaging in tissues by MALDI-MSI. The results show several properties of MEK as a powerful MALDI matrix, including strong UV absorption, µm-sized crystals and uniform matrix-coating, super high vacuum chemical stability, low matrix-related ion interference, super soft ionization, and high lipid ionization efficiency.

Applications of spatially resolved omics in the field of endocrine tumors

Endocrine tumors derive from endocrine cells with high heterogeneity in function, structure and embryology, and are characteristic of a marked diversity and tissue heterogeneity. There are still challenges in analyzing the molecular alternations within the heterogeneous microenvironment for endocrine tumors. Recently, several proteomic, lipidomic and metabolomic platforms have been applied to the analysis of endocrine tumors to explore the cellular and molecular mechanisms of tumor genesis, progression and metastasis. In this review, we provide a comprehensive overview of spatially resolved proteomics, lipidomics and metabolomics guided by mass spectrometry imaging and spatially resolved microproteomics directed by microextraction and tandem mass spectrometry. In this regard, we will discuss different mass spectrometry imaging techniques, including secondary ion mass spectrometry, matrix-assisted laser desorption/ionization and desorption electrospray ionization. Additionally, we will highlight microextraction approaches such as laser capture microdissection and liquid microjunction extraction. With these methods, proteins can be extracted precisely from specific regions of the endocrine tumor. Finally, we compare applications of proteomic, lipidomic and metabolomic platforms in the field of endocrine tumors and outline their potentials in elucidating cellular and molecular processes involved in endocrine tumors.

Novel Approach for Simultaneous Analysis of Peptide Metabolites from Orally Administered Glycinin in Rat Bloodstream by Coumarin-Tagged MALDI-MS

The lack of an appropriate analytical approach characterizing metabolites from dietary proteins may prevent further studies that could clarify their health benefits. In this study, we attempted to establish a novel analytical assay of peptide metabolites from glycinin using matrix-assisted laser desorption/ionization-mass spectrometry (MALDI-MS), in combination with the amine derivatization technique with coumarin (Cou). Cou (30 mmol/L) derivatization of peptides under rapid (30 min) and mild (25 °C, pH 8.5) conditions caused higher MS detection of the peptides as compared to nonderivatized peptides. In addition, an MS shift of the target by Cou derivatization (+202.0 m/z) can help to easily discriminate peptide metabolites in glycinin-administered blood, by comparing the MALDI-MS spectra of Cou-derivatized plasma with those of preadministered blood. After the oral administration of glycinin (100 mg/kg) to Sprague-Dawley rats, 15 di- to tetrapeptides were successfully characterized as glycinin-derived metabolites, demonstrating that the proposed Cou-tagged MALDI-MS is an appropriate characterization technique for peptide metabolites.

Copper adhesive tape attached to the reverse side of a non-conductive glass slide to achieve protein MALDI-imaging in FFPE-tissue sections

Herein, copper adhesive tape attached to the reverse side of a glass slide was developed as a new method to achieve protein in situ detection and imaging in a formalin fixed paraffin-embedded (FFPE) tissue section on a non-conductive glass slide by MALDI-MSI. The use of this new method led to 223 protein ions being imaged from a rat brain FFPE-tissue section on a non-conductive glass slide by MALDI-MS, compared to only 145 and 163 protein ions detected on an ITO glass slide and an AnchorChip target plate, respectively. This new method has great potential to become standard practice for protein MALDI-imaging in FFPE-tissue sections on non-conductive glass slides.

A Caffeic Acid Matrix Improves In Situ Detection and Imaging of Proteins with High Molecular Weight Close to 200,000 Da in Tissues by Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry Imaging

To our knowledge, this was the first study in which caffeic acid (CA) was successfully evaluated as a matrix to enhance the in situ detection and imaging of endogenous proteins in three biological tissue sections (i.e., a rat brain and Capparis masaikai and germinating soybean seeds) by matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI). Our results show several properties of CA, including strong ultraviolet absorption, a super-wide MS detection mass range close to 200,000 Da, micrometer-sized matrix crystals, uniform matrix deposition, and high ionization efficiency. More high-molecular-weight (HMW) protein ion signals (m/z > 30,000) could be clearly detected in biological tissues with the use of CA, compared to two commonly used MALDI matrices, i.e., sinapinic acid (SA) and ferulic acid (FA). Notably, CA shows excellent performance for HMW protein in situ detection from biological tissues in the mass range m/z > 80,000, compared to the use of SA and FA. Furthermore, the use of a CA matrix also significantly enhanced the imaging of proteins on the surface of selected biological tissue sections. Three HMW protein ion signals (m/z 50,419, m/z 65,874, and m/z 191,872) from a rat brain, two sweet proteins (mabinlin-2 and mabinlin-4) from a Capparis masaikai seed, and three HMW protein ion signals (m/z 94,838, m/z 134,204, and m/z 198,738) from a germinating soybean seed were successfully imaged for the first time. Our study proves that CA has the potential to become a standard organic acid matrix for enhanced tissue imaging of HMW proteins by MALDI-MSI in both animal and plant tissues.

Mass spectrometry imaging for direct visualization of components in plants tissues

Mass spectrometry is considered the most informative technique for components identification and has been widely adopted in plant sciences. However, the spatial distribution of compounds in the plant, which is vital for the exploration of plant physiological mechanisms, is missed in MS analysis. In recent years, mass spectrometry imaging has brought a great breakthrough in plant analysis because it can determine both the molecular compositions and spatial distributions, which is conducive to understand functions and regulation pathways of specific components in plants. Mass spectrometry imaging analysis of plant tissue is toward high sensitivity, high spatial resolution, and even single-cell analysis. Despite many challenges and technical barriers, such as difficulties of sample pretreatment caused by morphological diversity of plant tissues, obstacles for high spatial resolution imaging, and so on, lots of researches have contributed to remarkable progress, including improvement in tissue preparation, matrix innovation, and ionization mode development. This review focuses on the advances of mass spectrometry imaging analysis of plants in the last 5 years, including commonly used ionization techniques, technical advances, and recent applications of mass spectrometry imaging in plants.

Absolute quantification of 2-hydroxyglutarate on tissue by matrix-assisted laser desorption/ionization mass spectrometry imaging for rapid and precise identification of isocitrate dehydrogenase mutations in human glioma

Isocitrate dehydrogenase (IDH) gene mutations are important predictive molecular markers to guide surgical strategy in brain cancer therapy. Herein, we presented a method using matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI MSI) for absolute quantification of 2-hydroxyglutarate (2-HG) on tissues to identify IDH mutations and evaluate tumor residue. This analytical method was tested among 34 glioma patients and validated with gold standard clinical technologies. The cut-off value of 2-HG was set as 0.81 pmol/μg to identify IDH mutant (IDHmt) gliomas with 100% specificity and sensitivity. In addition, 2-HG levels and tumor cell density (TCD) showed positive correlation in IDHmt gliomas by this spatial method. This MALDI MSI-based absolute quantification method has great potentiality for incorporating into surgical workflow in the future.

A general purpose MALDI matrix for the analyses of small organic, peptide and protein molecules

Matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS) has been widely applied for the analysis of large biomolecules. The emergence of inorganic material substrates and new organic matrices extends the use of MALDI MS for small molecule analyses. However, there are usually preferred matrices for different types of analytes. Here, an organic compound, 4-hydroxy-3-nitrobenzonitrile, was found to be a general purpose matrix for the analyses of small organic, peptide and protein molecules. In particular, 4-hydroxy-3-nitrobenzonitrile has a strong UV absorption property, and it provides a clean background in the low mass range. Its analytical performances as a UV-laser matrix were demonstrated for different types of analytes, including organic drugs, peptides, proteins, mouse brain tissue and bacteria. Compared with commercial matrices, this new matrix has better performances when analyzing small molecules, such as drugs, peptides and lipids, while it has similar performances when analyzing proteins.

Imaging of Neurotransmitters and Small Molecules in Brain Tissues Using Laser Desorption/Ionization Mass Spectrometry Assisted with Zinc Oxide Nanoparticles

Inorganic nanostructured materials such as silicon, carbon, metals, and metal oxides have been explored as matrices of low-background signals to assist the laser desorption/ionization (LDI) mass spectrometric (MS) analysis of small molecules, but their applications for imaging of small molecules in biological tissues remain limited in the literature. Titanium dioxide is one of the known nanoparticles (NP) that can effectively assist LDI MS imaging of low molecular weight molecules (LMWM). TiO₂ NP is commercially available as dispersions, which can be applied using a chemical solution sprayer. However, aggregation of NP can occur in the dispersions, and the aggregated NP can slowly clog the sprayer nozzle. In this work, the use of zinc oxide (ZnO) NP for LDI MS imaging is investigated as a superior alternative due to its dissolution in acidic pH. ZnO NP was found to deliver similar or better results in the imaging of LMWM in comparison to TiO₂ NP. The regular acid washes were effective in minimizing clogging and maintaining high reproducibility. High-quality images of mouse sagittal and rat coronal tissue sections were obtained. Ions were detected predominately as Na⁺ or K⁺ adducts in the positive ion mode. The number of LMWM detected with ZnO NP was similar to that obtained with TiO₂ NP, and only a small degree of specificity was observed.

Hybrid MoS2/g-C3N4-assisted LDI mass spectrometry for rapid detection of small molecules and polyethylene glycols and direct determination of uric acid in complicated biological samples

A novel matrix-assisted laser desorption/ionization time-of-flight mass spectrometric method (MALDI-TOF MS) for determination of highly sensitive small molecular compounds was developed based on molybdenum disulfide nanosheets hybridized with ultrathin graphitic carbon nitride (MoS₂/g-C₃N₄) as the matrix. With this approach, the synergistic effects of MoS₂ and g-C₃N₄ enhance the UV absorption of MoS₂/g-C₃N₄, increase both desorption and ionization efficiency in LDI MS, and induce higher signal-to-noise ratio of analytes when compared with the bare MoS₂ and g-C₃N₄ matrix in the determination of amino acids, antibiotics, neutral oligosaccharides, uric acid, and polyethylene glycols (PEGs). The detection limits of these small molecular compounds are in the ranges 0.1 to 10 μg mL^-1, 1*10^-3 to 1.0 μg mL^-1, 1.0 to 10 μg mL^-1, and 2*10^-4 μg mL^-1, respectively, and the polydispersity index of these PEGs is less than 1.02. Moreover, high salt tolera`nce and homogeneous deposition on the spot results in good reproducibility. The relative standard deviations (RSDs) of shot-to-shot and spot-to-spot (n = 15) of these compounds are less than 10.1% and 12.5%, respectively. With MoS₂/g-C₃N₄, the uric acid in complicated biological samples can be directly determined in combination with LDI-TOF MS. We synthesized MoS₂/g-C₃N₄ nanohybrid as an efficient matrix for MALDI-TOF MS analysis of small molecules as well as quantitative detection of uric acid in human urine.

A Novel Cryptococcal Meningitis Therapy: The Combination of Amphotericin B and Posaconazole Promotes the Distribution of Amphotericin B in the Brain Tissue

The deficient brain tissue distribution of amphotericin B (AMPB) seriously restricts its treatment for the clinical efficacy of cryptococcus neoformans meningitis (CNM). We strive to develop a tactic to increase its concentration in brain tissue. We aimed to investigate whether the combination of AMPB and posaconazole (POS) could be more effective in the treatment of CNM and to elucidate its potential mechanisms. HPLC analysis was used to analyze the concentration of AMPB in mouse serum, brain tissue, and BCECs cells. Schrodinger molecular docking, in vitro plasma balance dialysis, and ultrafiltration analysis were performed to evaluate the combinative effect of AMPB and POS with serum albumin and POS on AMPB plasma protein binding. H&E staining and colonization culture experiment of CN were employed to assess the effect of POS on the efficacy of AMPB. POS + AMPB significantly reduced the concentration of plasma total AMPB and increased its concentration in the brain tissue. However, the P-gp inhibitor zosuquidar, BCRP inhibitor Ko143, and a common inhibitor of both, elacridar, had no significant effect on its concentration. Molecular docking, balance dialysis, and ultrafiltration analysis showed that AMPB and POS had potential binding properties to serum albumin. Meanwhile, 4 and 8 μg/mL POS could significantly increase the concentration of free AMPB in plasma. POS and three inhibitors all had no significant effect on the uptake of AMPB by BCECs, but serum albumin had. The therapeutic effect of CNM in mice was confirmed that AMPB and AMPB+POS could restrain the infiltration of neutrophils and lymphocytes in cortical neurons and improve the bleeding and markedly inhibit the proliferation of CN. Collectively, we propose that POS competitively binds to the plasma protein sites of AMPB, thereby increasing its level in the brain tissue. Meanwhile, POS could enhance the efficacy of AMPB in the treatment of CNM, which may be independent of P-gp and BCRP proteins.

Metabolite changes associated with earthworms (Eisenia fetida) graphene exposure revealed by matrix-assisted laser desorption/ionization mass spectrometry imaging

The increased production and environmental release of graphene nanoparticles has raised concerns about its environmental impact, but the effects of graphene on living organisms at the metabolic level remain unknown. In this study, we used matrix assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI)-based untargeted metabolomics to investigate the metabolic response of juvenile earthworms (Eisenia fetida) to graphene exposure in soil tests for the first time. Our results reveal that graphene-exposure significantly disturbs earthworm metabolome, and graphene toxicity on earthworm shows non-concentration-dependent effect. Alanine, phenylalanine, proline, glutamate, arginine, histidine, maltose, glucose, malate, succinate, myo-inositol, and spermidine were successfully screened as significantly change compounds in earthworms for the exposure of graphene. The heterogeneous distributions of these metabolites in earthworm were also clearly imaged by MALDI-MSI. Our MSI results fully showed that the metabolite expression levels in juvenile earthworms significantly changed (up-/down-regulation) after exposure to graphene nanoparticles. This work improves our understanding of graphene nanoparticle toxicity to juvenile earthworms and also enables the continued progression of MALDI-MSI-based metabolomics as an emerging, reliable, and rapid ecotoxicological tool for assessing contaminant toxicity.

Recent developments of novel matrices and on-tissue chemical derivatization reagents for MALDI-MSI

Matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI) is a fast-growing technique for visualization of the spatial distribution of the small molecular and macromolecular biomolecules in tissue sections. Challenges in MALDI-MSI, such as poor sensitivity for some classes of molecules or limited specificity, for instance resulting from the presence of isobaric molecules or limited resolving power of the instrument, have encouraged the MSI scientific community to improve MALDI-MSI sample preparation workflows with innovations in chemistry. Recent developments of novel small organic MALDI matrices play a part in the improvement of image quality and the expansion of the application areas of MALDI-MSI. This includes rationally designed/synthesized as well as commercially available small organic molecules whose superior matrix properties in comparison with common matrices have only recently been discovered. Furthermore, on-tissue chemical derivatization (OTCD) processes get more focused attention, because of their advantages for localization of poorly ionizable metabolites and their‚ in several cases‚ more specific imaging of metabolites in tissue sections. This review will provide an overview about the latest developments of novel small organic matrices and on-tissue chemical derivatization reagents for MALDI-MSI.

Polydopamine-Modified TS-1 Zeolite Framework Nanoparticles as a Matrix for the Analysis of Small Molecules by MALDI-TOF MS

Matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) using conventional organic matrices for detection of small molecules has some limitations, such as heterogeneous analyte/matrix co-crystals, as well as interference of matrices in the low-molecular-weight range. In this work, a zeolite framework nanomaterial, TS-1, was applied as a MALDI matrix for the analysis of small molecules by MALDI-MS for the first time. To improve the signal intensity and reproducibility, TS-1 was modified with polydopamine (TS-1@PDA). Using TS-1@PDA as a matrix, organic substances in the low-molecular-weight region such as amino acids, nucleosides, peptides, oligosaccharides, and fatty acids can be detected by MALDI-MS in positive ion mode. Compared with traditional organic matrices like 2,5-dihydroxybenzoic acid (2,5-DHB) and α-cyano-4-hydroxycinnamic acid (CHCA), TS-1@PDA has the advantages including the formation of uniform sample spots, small background interference at low molecular weight, and better salt tolerance. Furthermore, this matrix was employed for the analysis of endogenous glucose in urine samples, and the level of glucose was quantified with a linear range of 0-10 mM (R ² > 0.98). The results demonstrated that TS-1@PDA has the potential to be used as an effective MALDI matrix for the analysis of small molecules in biological samples with excellent reproducibility and moderate sensitivity.

Differential distribution of characteristic constituents in root, stem and leaf tissues of Salvia miltiorrhiza using MALDI mass spectrometry imaging

Segmentation-quantification is the most commonly used method for studying the tissue distribution of bioactive constituents in plant, but this method would bring uncontrollable pollution, compound migration and denaturation. Mass spectrometry imaging (MSI), as a new method developed in the past 20 years, has high sensitivity, high spatial resolution, high degree of visualization, and low risk of contamination and degeneration when studying tissue distribution of compounds. For the first time we applied matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI) to tissue distribution of characteristic constituents of the medicinal plant Salvia miltiorrhiza. From the collected data, we found the regional differences in root, stem, and leaf tissues, and the ion information with differential distribution characteristics. We also identified 18 bioactive constituents in S. miltiorrhiza with their spatial distribution information. In addition, the plant was divided into five parts, and the identified compounds were analyzed for differences between tissues using LC-MS, which results verified those found from the MSI. It is figured out that MALDI-MSI can be reliably applied to the differential distribution of salvianolic acids and tanshinones.

Imaging of Multiple Plant Hormones in Roots of Rice (Oryza sativa) Using Nanoparticle-Assisted Laser Desorption/Ionization Mass Spectrometry

Plant hormones can act in synergistic and antagonistic ways in response to biotic and abiotic stresses and in plant growth and development. Thus, a technique is needed to simultaneously determine the distributions and concentrations of several plant hormones. Previously, we reported that localizations of two plant hormones [cytokinin (CK) and abscisic acid (ABA)] can be simultaneously visualized in a plant tissue using matrix-assisted laser desorption/ionization (MALDI) mass spectrometry (MS). In MALDI-MS, however, self-ionization of an organic matrix occasionally interferes with ionizations of small molecules (<500 m/z) including most plant hormones. Another technique, nanoparticle-assisted laser desorption/ionization (Nano-PALDI), can avoid matrix self-ionization using nanoparticles to assist the ionization of analytes. Here, we compared the ionization efficiencies of common plant hormones by MALDI-MS and Nano-PALDI-MS. For the comparison, we prepared a standard mix of seven plant hormones [ABA, auxin (IAA), brassinosteroid (Br), two CKs (trans-zeatin, tZ, and 6-(γ,γ-dimethylallylamino) purine, iP), jasmonic acid, and salicylic acid (SA)], an ethylene precursor (1-aminocyclopropane-1-carboxylic acid, ACC), and a heavy hydrogen-labeled ABA (D₆-ABA). Basic MALDI-MS detected all compounds except IAA, Br, and D₆-ABA, while Nano-PALDI-MS detected all nine compounds. By Nano-PALDI-MS imaging (MSI), each of the abovementioned hormones and ACC were also detected in root cross sections of rice which were incubated in the hormone mix for 2 h. In the elongation zone of untreated roots, Nano-PALDI-MSI revealed high levels of ABA and CKs in the outer part of roots and much lower levels in the stele, but Br, SA, and ACC were broadly distributed in the cross section. IAA seemed to be distributed in the epidermis, cortex, and stele. Multiple-hormone imaging using Nano-PALDI-MS has great potential for investigating the roles of hormone signaling in crop development and stress responses.