Matrix-assisted Laser Desorption/ionization Imaging Mass Spectrometry Analysis Research Articles

Formation of multiple ion types during MALDI imaging mass spectrometry analysis of Mitragyna speciosa alkaloids in dosed rat brain tissue

Mitragyna speciosa, more commonly known as kratom, has emerged as an alternative to treat chronic pain and addiction. However, the alkaloid components of kratom, which are the major contributors to kratom's pharmaceutical properties, have not yet been fully investigated. In this study, matrix-assisted laser desorption/ionization (MALDI) imaging mass spectrometry was used to map the biodistribution of three alkaloids (corynantheidine, mitragynine, and speciogynine) in rat brain tissues. The alkaloids produced three main ion types during MALDI analysis: [M + H]+, [M − H]+, and [M − 3H]+. Contrary to previous reports suggesting that the [M − H]+ and [M − 3H]+ ion types form during laser ablation, these ion types can also be produced during the MALDI matrix application process. Several strategies are proposed to accurately map the biodistribution of the alkaloids. Due to differences in the relative abundances of the ions in different biological regions of the tissue, differences in ionization efficiencies of the ions, and potential overlap of the [M − H]+ and [M − 3H]+ ion types with endogenous metabolites of the same empirical formula, a matrix that mainly produces the [M + H]+ ion type is optimal for accurate mapping of the alkaloids. Alternatively, the most abundant ion type can be mapped or the intensities of all ion types can be summed together to generate a composite image. The accuracy of each of these approaches is explored and validated.

In Situ Detection and Imaging of PFOS in Mouse Kidney by Matrix-Assisted Laser Desorption/Ionization Imaging Mass Spectrometry.

Perfluorooctanesulfonic acid (PFOS) is an emerging environmental organic pollutant that has been widely used in daily life products in the last century. Numerous studies showed that the accumulation of PFOS in human through food chain would lead to various disease. However, there is currently no report about its in situ localization in the tissue. In present study, we aimed to develop a reproductive and less-cost method to quantitatively detect and determine the spatial distribution of PFOS in mouse kidney by matrix-assisted laser desorption/ionization imaging mass spectrometry (MALDI-IMS) with a commercially available matrix. α-Cyano-4-hydroxycinnamic acid (CHCA) matrix was optimized for PFOS detection in MALDI-IMS analysis. Compared to other organic matrices, CHCA used in negative ion mode showed less background interference and enhanced MS signal intensity and high spatial resolution (80 μm) for PFOS analysis. The use of a CHCA matrix with an autospray system led to successful identification of the PFOS ion signals on the perfusion kidney tissue. The detection limit was at the μg/mL level, with direct visualization from a MS image. The developed method with the optimized parameters was successfully employed to obtain the PFOS spatial distribution in the kidney collected from mice after the PFOS exposure for 14 days. PFOS was mainly distributed in the kidney cortex region, which was consistent with the histological analysis results. Taken together, a rapid, economic, and efficient method was developed for PFOS detection by MALDI-IMS using a CHCA matrix. Mapping the distribution of PFOS by MALDI-IMS with a CHCA matrix provides an innovative approach for the analysis of environmental pollutants in animal or human tissues.

MALDI Imaging Mass Spectrometry Profiling of N-Glycans in Formalin-Fixed Paraffin Embedded Clinical Tissue Blocks and Tissue Microarrays

A recently developed matrix-assisted laser desorption/ionization imaging mass spectrometry (MALDI-IMS) method to spatially profile the location and distribution of multiple N-linked glycan species in frozen tissues has been extended and improved for the direct analysis of glycans in clinically derived formalin-fixed paraffin-embedded (FFPE) tissues. Formalin-fixed tissues from normal mouse kidney, human pancreatic and prostate cancers, and a human hepatocellular carcinoma tissue microarray were processed by antigen retrieval followed by on-tissue digestion with peptide N-glycosidase F. The released N-glycans were detected by MALDI-IMS analysis, and the structural composition of a subset of glycans could be verified directly by on-tissue collision-induced fragmentation. Other structural assignments were confirmed by off-tissue permethylation analysis combined with multiple database comparisons. Imaging of mouse kidney tissue sections demonstrates specific tissue distributions of major cellular N-linked glycoforms in the cortex and medulla. Differential tissue distribution of N-linked glycoforms was also observed in the other tissue types. The efficacy of using MALDI-IMS glycan profiling to distinguish tumor from non-tumor tissues in a tumor microarray format is also demonstrated. This MALDI-IMS workflow has the potential to be applied to any FFPE tissue block or tissue microarray to enable higher throughput analysis of the global changes in N-glycosylation associated with cancers.

Enhanced specificity for phosphatidylcholine analysis by positive ion mode matrix‐assisted laser desorption/ionization imaging mass spectrometry

Visualization of the spatial distribution of phosphatidylcholine (PC) in tissues by matrix-assisted laser desorption/ionization imaging mass spectrometry (MALDI-IMS) provides insights into key physiological and pathophysiological processes. In MALDI-IMS analysis, the heterogeneity of adduct ions formed from PC lowers the specificity of detection of PC molecular species and poses a challenge in the identification of these species. To solve this problem, modified matrix solution and desalting with ammonium acetate (NH4 Ac) buffer have been employed. However, the utility of these methods is limited to the analysis of brain sections. The MALDI signal intensities of [PC+H], [PC+Na] and [PC+K] were compared after three different pretreatments (modified matrix solution, desalting with 150 mM ammonium acetate, treatment with 150 mM potassium acetate). Pretreatment of tissue sections with 150 mM potassium acetate resulted in an increase in the signal intensity of [PC+K] ions produced from cryosections of the pancreas, brain, and liver tissues. Pretreatment with potassium acetate can be a simple, improved, and highly useful method for the reliable analysis of PC in tissues.