Ammonium Sulfate Improves Detection of Hydrophilic Quaternary Ammonium Compounds through Decreased Ion Suppression in Matrix-Assisted Laser Desorption/Ionization Imaging Mass Spectrometry

Catheter-Directed Ionic Liquid Embolic Agent for Rapid Portal Vein Embolization, Segmentectomy, and Bile Duct Ablation

Embolic materials currently in use for portal vein embolization (PVE) do not treat the tumor, which poses a risk for tumor progression during the interval between PVE and surgical resection. Here, is developed an ionic-liquid-based embolic material (LEAD) for portal vein embolization, liver ablation, and drug delivery. LEAD is optimized and characterized for diffusivity, X-ray visibility, and cytotoxicity. In the porcine renal embolization model, LEAD delivered from the main renal artery reached vasculature down to 10 microns with uniform tissue ablation and delivery of small and large therapeutics. In non-survival and survival porcine experiments, successful PVE is achieved in minutes, leading to the expected chemical segmentectomy, and delivery of a large protein drug (i.e., Nivolumab) with LEAD. In cholangiocarcinoma mouse tumor models and in ex vivo human tumors, LEAD consistently achieved an effective ablation and wide drug distribution. Furthermore, various strains of drug-resistant patient-derived bacteria showed significant susceptibility to LEAD, suggesting that LEAD may also prevent infectious complications resulting from tissue ablation. With its capabilities to embolize, ablate, and deliver therapeutics, ease of use, and a high safety profile demonstrated in animal studies, LEAD offers a potential alternative to tumor ablation with or without PVE for FLR growth.

Evaluation of 6 MALDI-Matrices for 10 μm Lipid Imaging and On-Tissue MSn with AP-MALDI-Orbitrap

Mass spectrometry imaging is a technique uniquely suited to localize and identify lipids in a tissue sample. Using an atmospheric pressure (AP-) matrix-assisted laser desorption ionization (MALDI) source coupled to an Orbitrap Elite, numerous lipid locations and structures can be determined in high mass resolution spectra and at cellular spatial resolution, but careful sample preparation is necessary. We tested 11 protocols on serial brain sections for the commonly used MALDI matrices CHCA, norharmane, DHB, DHAP, THAP, and DAN in combination with tissue washing and matrix additives to determine the lipid coverage, signal intensity, and spatial resolution achievable with AP-MALDI. In positive-ion mode, the most lipids could be detected with CHCA and THAP, while THAP and DAN without additional treatment offered the best signal intensities. In negative-ion mode, DAN showed the best lipid coverage and DHAP performed superiorly for gangliosides. DHB produced intense cholesterol signals in the white matter. One hundred fifty-five lipids were assigned in positive-ion mode (THAP) and 137 in negative-ion mode (DAN), and 76 peaks were identified using on-tissue tandem-MS. The spatial resolution achievable with DAN was 10 μm, confirmed with on tissue line-scans. This enabled the association of lipid species to single neurons in AP-MALDI images. The results show that the performance of AP-MALDI is comparable to vacuum MALDI techniques for lipid imaging.

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.

Quantitative Mass Spectrometry Imaging of Biological Systems

Mass spectrometry imaging (MSI) is a powerful, label-free technique that provides detailed maps of hundreds of molecules in complex samples with high sensitivity and subcellular spatial resolution. Accurate quantification in MSI relies on a detailed understanding of matrix effects associated with the ionization process along with evaluation of the extraction efficiency and mass-dependent ion losses occurring in the analysis step. We present a critical summary of approaches developed for quantitative MSI of metabolites, lipids, and proteins in biological tissues and discuss their current and future applications.

Supermolecule-assisted imaging of low-molecular-weight quaternary-ammonium compounds by MALDI-MS of their non-covalent complexes with cucurbit[7]uril

Cucurbit[7]uril was used to form non-covalent complexes with low-molecular-weight quaternary-ammonium compounds for their indirect analysis by MALDI-MS. By shifting the ion signals to a higher and interference-free mass region, the distributions of neurine, choline, and phosphocholine in rat brain were visualized by MALDI imaging with high selectivity and good sensitivity.

Cucurbit[7]uril was used to form non-covalent complexes with low-molecular-weight quaternary-ammonium compounds for their indirect analysis by MALDI-MS.

Development of sheet-enhanced technique (Set) method for matrix-assisted laser desorption/ionization imaging mass spectrometry

RATIONALE

The key to successful experiments in matrix-assisted laser desorption/ionization imaging mass spectrometry (MALDI-IMS) is to apply the matrix uniformly to the sample. With the development of automated equipment, uniform matrix application has made great progress while the sample preparation required to acquire a better image becomes complicated.

METHODS

The approach is to apply the matrix uniformly to tape and adhere it to the tissue section. We call this the sheet-enhanced technique (Set) method.

RESULTS

The Set method promotes ionization of biomolecules as well as the spray method. This procedure does not require the preparation and application of a matrix solution for each experiment, dramatically reducing the time and effort of matrix deposition.

CONCLUSIONS

In the present study, we have developed the Set method as a new matrix application method. The method promotes ionization of biomolecules as well as the spray method for MALDI-IMS.

Unsupervised machine learning using an imaging mass spectrometry dataset automatically reassembles grey and white matter

Current histological and anatomical analysis techniques, including fluorescence in situ hybridisation, immunohistochemistry, immunofluorescence, immunoelectron microscopy and fluorescent fusion protein, have revealed great distribution diversity of mRNA and proteins in the brain. However, the distributional pattern of small biomolecules, such as lipids, remains unclear. To this end, we have developed and optimised imaging mass spectrometry (IMS), a combined technique incorporating mass spectrometry and microscopy, which is capable of comprehensively visualising biomolecule distribution. We demonstrated the differential distribution of phospholipids throughout the cell body and axon of neuronal cells using IMS analysis. In this study, we used solarix XR, a high mass resolution and highly sensitive MALDI-FT-ICR-MS capable of detecting higher number of molecules than conventional MALDI-TOF-MS instruments, to create a molecular distribution dataset. We examined the diversity of biomolecule distribution in rat brains using IMS and hypothesised that unsupervised machine learning reconstructs brain structures such as the grey and white matters. We have demonstrated that principal component analysis (PCA) can reassemble the grey and white matters without assigning brain anatomical regions. Hierarchical clustering allowed us to classify the 10 groups of observed molecules according to their distributions. Furthermore, the group of molecules specifically localised in the cerebellar cortex was estimated to be composed of phospholipids.

Change in Brain Plasmalogen Composition by Exposure to Prenatal Undernutrition Leads to Behavioral Impairment of Rats

Epidemiological studies suggest that poor nutrition during pregnancy influences offspring predisposition to experience developmental and psychiatric disorders. Animal studies have shown that maternal undernutrition leads to behavioral impairment, which is linked to alterations in monoaminergic systems and inflammation in the brain. In this study, we focused on the ethanolamine plasmalogen of the brain as a possible contributor to behavioral disturbances observed in offspring exposed to maternal undernutrition. Maternal food or protein restriction between gestational day (GD) 5.5 and GD 10.5 resulted in hyperactivity of rat male adult offspring. Genes related to the phospholipid biosynthesis were found to be activated in the PFC, but not in the NAcc or striatum, in the offspring exposed to prenatal undernutrition. Corresponding to these gene activations, increased ethanolamine plasmalogen (18:0p-22:6) was observed in the PFC using mass spectrometry imaging. A high number of crossings and the long time spent in the center area were observed in the offspring exposed to prenatal undernutrition and were mimicked in adult rats via the intravenous injection of ethanolamine plasmalogen (18:0p-22:6) incorporated into the liposome. Additionally, plasmalogen (18:0p-22:6) increased only in the PFC, and not in the NAcc or striatum. These results suggest that brain plasmalogen is one of the key molecules to control behavior, and its injection using liposome is a potential therapeutic approach for cognitive impairment.SIGNIFICANCE STATEMENT Maternal undernutrition correlates to developmental and psychiatric disorders. Here, we found that maternal undernutrition in early pregnancy led to hyperactivity in rat male offspring and induced gene activation of phospholipid-synthesizing enzyme and elevation of ethanolamine plasmalogen (18:0p-22:6) level in the PFC. Intravenous injection of ethanolamine plasmalogen (18:0p-22:6) incorporated into the liposome maintained crossing activity and the activity was circumscribed to the center area for a long time period, as in prenatally undernourished offspring with aberrant behavior. Furthermore, the amount of ethanolamine plasmalogen (18:0p-22:6) increased in the PFC of the rat after injection. Our result suggests that brain plasmalogen is one of the key molecules to control behavior and that its injection using liposome is a potential therapeutic approach for cognitive impairment.

Novel approach to enhance sensitivity in surface-assisted laser desorption/ionization mass spectrometry imaging using deposited organic-inorganic hybrid matrices

Sample pretreatment is key to obtaining good data in matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI). Although sublimation is one of the best methods for obtaining homogenously fine organic matrix crystals, its sensitivity can be low due to the lack of a solvent extraction effect. We investigated the effect of incorporating a thin film of metal formed by zirconium (Zr) sputtering into the sublimation process for MALDI matrix deposition for improving the detection sensitivity in mouse liver tissue sections treated with olanzapine. The matrix-enhanced surface-assisted laser desorption/ionization (ME-SALDI) method, where a matrix was formed by sputtering Zr to form a thin nanoparticle layer before depositing MALDI organic matrix comprising α-cyano-4-hydroxycinnamic acid (CHCA) by sublimation, resulted in a significant improvement in sensitivity, with the ion intensity of olanzapine being about 1800 times that observed using the MALDI method, comprising CHCA sublimation alone. When Zr sputtering was performed after CHCA deposition, however, no such enhancement in sensitivity was observed. The enhanced sensitivity due to Zr sputtering was also observed when the CHCA solution was applied by spraying, being about twice as high as that observed by CHCA spraying alone. In addition, the detection sensitivity of these various pretreatment methods was similar for endogenous glutathione. Given that sample preparation using the ME-SALDI-MSI method, which combines Zr sputtering with the sublimation method for depositing an organic matrix, does not involve a solvent, delocalization problems such as migration of analytes observed after matrix spraying and washing with aqueous solutions as sample pretreatment are not expected. Therefore, ME-Zr-SALDI-MSI is a novel sample pretreatment method that can improve the sensitivity of analytes while maintaining high spatial resolution in MALDI-MSI.

Tandem Mass Spectrometry Imaging Reveals Distinct Accumulation Patterns of Steroid Structural Isomers in Human Adrenal Glands

Visualizing tissue distribution of steroid hormones is a promising application of MALDI mass spectrometry imaging (MSI). On-tissue chemical derivatization using Girard's T reagent has enhanced the ionization efficiency of steroids. However, discriminating between structural isomers with distinct bioactivities remains a challenge. Herein, we used ion trap MS/tandem MS (MS³) to distinguish a mineralcorticoid aldosterone (Aldo) and a glucocorticoid cortisol (F), from their structural isomers. Our method is also useful to detect hybrid steroids (18-hydroxycortisol [18-OHF] and 18-oxocortisol) with sufficient signal-to-noise ratio. The clinical applicability of the tandem MS method was evaluated by analyzing F, Aldo, and 18-OHF distributions in human adrenal glands. In such clinical specimens, small Aldo-producing cell clusters (APCCs) were identified and were first found to produce a high level of Aldo and not to contain F. Moreover, a part of APCCs produced 18-OHF, presumably converted from F by APCC-specific CYP11B2 activity. Catecholamine species were also visualized with another derivatization reagent (TAHS), and those profiling successfully discriminated pheochromocytoma species. These tandem MSI-methods, coupled with on-tissue chemical derivatization has proven to be useful for detecting low-abundance steroids, including Aldo and hybrid steroids and thus identifying steroid hormone-producing lesions.

Derivatization Strategy for Simultaneous Molecular Imaging of Phospholipids and Low-Abundance Free Fatty Acids in Thyroid Cancer Tissue Sections

Matrix-assisted laser desorption/ionization (MALDI) mass spectrometry imaging (MSI) has been applied in many fields for detecting and imaging a variety of metabolites. In cancer research, this fast-growing imaging method also helps to elucidate the connection between the changes of metabolites in the microenvironment and the proliferation and survival of cancer cells. Free fatty acids (FFAs) are a vital building block of phospholipids (PLs) that can serve as a second cellular messenger and provide nutrients in the cancer microenvironment. The metabolism process of FFAs and PLs is highly relevant to the initiation and progression of different cancers. To better understand the metabolism process in cancer tissues, simultaneously detecting and imaging FFAs and PLs is essential. Despite the crucial developments that have been performed in the field of lipids imaging, FFAs and PLs have rarely been detected and imaged simultaneously in positive ion mode with good detection sensitivity. In this work, an on-tissue derivatization method was used to add a permanently quaternary amine onto FFAs; then, the FFAs and PLs were simultaneously imaged in positive ion mode. The derivatized FFAs are suitable for detection in positive ion mode. In comparison with the traditional matrix and the previous derivatization method, our derivatization reagent has a higher sensitivity for imaging FFAs. In addition, for simultaneous imaging analysis of FFAs and PLs, the number of imaged FFAs and PLs is greater than that with the previous on-tissue derivatization method. This high-sensitivity on-tissue derivatization method was applied to detect and image PLs and fatty acids in thyroid cancer tissues. In the MSI experiment, FFA derivatives and PLs were imaged while molecular localization and tissue integrity were maintained. Meanwhile, the correlation between PLs and FFAs was also studied, and the results showed that the correlations between saturated FFAs of C16:0 and C18:0 and PLs are better than the correlations of unsaturated FFAs with PLs.

Characterizing and alleviating ion suppression effects in atmospheric pressure matrix-assisted laser desorption/ionization

RATIONALE

As a powerful ambient ion source, atmospheric pressure (AP) matrix-assisted laser desorption/ionization (MALDI) enables direct analysis at atmospheric pressure/temperature and minimal sample preparation. With the increasing usage of AP-MALDI sources with Orbitrap instruments, systematic characterization of the extent of ion suppression effect (ISE) in AP-MALDI-Orbitrap mass spectrometry imaging (MSI) is desirable. Recently, a new low-pressure MALDI platform has been introduced that reportedly provided better sensitivity. While extensive research efforts have been devoted to improving spatial resolution, fewer studies focused on the characterization and sensitivity improvement of these MALDI platforms that, coupled with high-resolution Orbitraps, provide powerful strategy for MSI.

METHODS

We compared the analytical performance of AP and low-pressure (subatmospheric) MALDI sources to study the effect of pressure control in the ion source. Using a model peptide/protein mixture, we systematically evaluated the factors influencing ISE. Furthermore, the effect of laser spot size was evaluated through tissue imaging analysis of lipids and neuropeptides. The effects of ion suppression and laser spot size have also been examined by comparing the number of identified molecular species during MSI analysis.

RESULTS

Several key operating parameters including source pressure, laser energy, laser repetition rate, and microscopic slide coating materials were optimized to minimize the ISE. Under the optimal conditions, the subatmospheric AP-MALDI-Orbitrap platform with high spatial and mass spectral resolution enabled significantly improved coverage of several lipid and neuropeptide families in the MS analysis of mouse brain tissue sections.

CONCLUSIONS

The new SubAP-MALDI source coupled with an Orbitrap mass spectrometer was established as a viable platform for in situ endogenous biomolecular analysis with increased sensitivity compared with conventional AP-MALDI sources as evidenced by the confident identification of neuropeptides from mouse brain imaging analyses. The alleviated ISE was key to substantial performance improvement due to optimized intermediate pressure conditions and better ion collection by the ion funnel.

Toward Higher Sensitivity in Quantitative MALDI Imaging Mass Spectrometry of CNS Drugs Using a Nonpolar Matrix

Tissue-specific ion suppression is an unavoidable matrix effect in MALDI mass spectrometry imaging (MALDI-MSI), the negative impact of which on precision and accuracy in quantitative MALDI-MSI can be reduced to some extent by applying isotope internal standards for normalization and matrix-matched calibration routines. The detection sensitivity still suffers, however, often resulting in significant loss of signal for the investigated analytes. An MSI application considerably affected by this phenomenon is the quantitative spatial analysis of central nervous system (CNS) drugs. Most of these drugs are low molecular weight, lipophilic compounds, which exhibit inefficient desorption and ionization during MALDI using conventional polar acidic matrices (CHCA, DHB). Here, we present the application of the (2-[(2 E)-3-(4- tert-butylphenyl)-2-methylprop-2-enylidene]malononitrile) matrix for high sensitivity imaging of CNS drugs in mouse brain sections. Since DCTB is usually described as an electron-transfer matrix, we provide a rationale (i.e., computational calculations of gas-phase proton affinity and ionization energy) for an additional proton-transfer ionization mechanism with this matrix. Furthermore, we compare the extent of signal suppression for five different CNS drugs when employing DCTB versus CHCA matrices. The results showed that the signal suppression was not only several times lower with DCTB than with CHCA but also depended on the specific tissue investigated. Finally, we present the application of DCTB and ultrahigh resolution Fourier transform ion cyclotron resonance mass spectrometry to quantitative MALDI imaging of the anesthetic drug xylazine in mouse brain sections based on a linear matrix-matched calibration curve. DCTB afforded up to 100-fold signal intensity improvement over CHCA when comparing representative single MSI pixels and >440-fold improvement for the averaged mass spectrum of the adjacent tissue sections.

Distribution of Antisense Oligonucleotides in Rat Eyeballs Using MALDI Imaging Mass Spectrometry

Oligonucleotide-based therapeutics such as antisense oligonucleotides, small interfering RNAs (siRNAs), decoy and aptamer have been extensively developed. To investigate the pharmacokinetics of oligonucleotide therapeutics, it is common to label a radioisotope in a nucleic acid and visualize it. However, if the labeled terminal nucleotide is decomposed by a nuclease in vivo, only the labeled nucleotide is detected, and it is impossible to observe the nucleic acid exhibiting the drug effect. The distribution of biomolecules, such as phospholipids, proteins, and glycolipids, can be obtained and visualized without labeling using matrix-assisted laser desorption/ionization imaging mass spectrometry (MALDI-IMS). MALDI-IMS is also used in pharmacokinetic analysis to visualize a parent drug and its metabolites simultaneously. In this study, we reported a methodology for oligonucleotides analysis by MALDI-IMS. When phosphorothioate antisense oligonucleotide was administered into the eyeball of rats, it reached the retina after 30 min without undergoing decomposition by nucleases.

Profiling of small molecule metabolites and neurotransmitters in crustacean hemolymph and neuronal tissues using reversed-phase LC-MS/MS

Crustaceans have been long used as model animals for neuromodulation studies because of their well-defined neural circuitry. The identification of small molecule metabolites and signaling molecules in circulating fluids and neuronal tissues presents unique challenges due to their diverse structures, biological functions, and wide range of concentrations. LC combined with high resolution MS/MS is one of the most powerful tools to uncover endogenous small molecules. Here we explored several sample preparation techniques (solid-phase extraction and denaturing) and MS data acquisition strategies (data-dependent acquisition and targeted MS2-based acquisition) that provided complementary coverage and improved overall identification rate in C18 LC-MS/MS experiment. By MS/MS spectral matching with mzCloud database and those generated from standard compounds, a total of 129 small molecule metabolites and neurotransmitters were identified from crustacean hemolymph and neuronal tissues. These confidently identified small molecules covered predominant biosynthetic pathways for major neurotransmitters, validating the effectiveness of the high-throughput RPLC-MS/MS approach in studying the metabolism of neurotransmitters.

Distribution Analysis via Mass Spectrometry Imaging of Ephedrine in the Lungs of Rats Orally Administered the Japanese Kampo Medicine Maoto

Maoto, a traditional Japanese Kampo medicine, has been used to treat various respiratory diseases, including respiratory infections and influenza. Ephedrine (EPD), the main ingredient in maoto, is also clinically used to treat respiratory diseases. However, the pharmacokinetics and distribution of EPD in the lungs after the administration of maoto have not been demonstrated. This study aimed to determine the concentrations, distribution, and pharmacokinetics of EPD and its precursor methylephedrine (MEPD) in the lungs of rats orally administered maoto (1 and 4 g/kg). We used liquid chromatography–electrospray ionization-tandem mass spectrometry to measure the ingredient concentrations. Both ingredients were detected in maoto-treated lung homogenates. Next, we examined the distribution of both ingredients in lung sections by using matrix-assisted laser desorption/ionization-mass spectrometry imaging, a powerful tool for the visualization of the distribution of biological molecules. The mass spectrometry imaging analysis detected only EPD and provided the first visual demonstration that EPD is distributed in the alveoli, bronchi, and bronchioles in the lungs of rats orally administered maoto (4 g/kg, three times at 2-h intervals). These results suggest that the pharmacological efficacy of maoto for the amelioration of respiratory symptoms is related to the distribution of EPD in the lung.