Imaging and Mapping of Tissue Constituents at the Single-Cell Level Using MALDI MSI and Quantitative Laser Scanning Cytometry

Genetically Encoded Fluorescent Proteins Enable High-Throughput Assignment of Cell Cohorts Directly from MALDI-MS Images

Matrix-assisted laser desorption/ionization (MALDI) mass spectrometry imaging (MSI) provides a unique in situ chemical profile that can include drugs, nucleic acids, metabolites, lipids, and proteins. MSI of individual cells (of a known cell type) affords a unique insight into normal and disease-related processes and is a prerequisite for combining the results of MSI and other single-cell modalities (e.g. mass cytometry and next-generation sequencing). Technological barriers have prevented the high-throughput assignment of MSI spectra from solid tissue preparations to their cell type. These barriers include obtaining a suitable cell-identifying image (e.g. immunohistochemistry) and obtaining sufficiently accurate registration of the cell-identifying and MALDI-MS images. This study introduces a technique that overcame these barriers by assigning cell type directly from mass spectra. We hypothesized that, in MSI from mice with a defined fluorescent protein expression pattern, the fluorescent protein's molecular ion could be used to identify cell cohorts. A method was developed for the purification of enhanced yellow fluorescent protein (EYFP) from mice. To determine EYFP's molecular mass for MSI studies, we performed intact mass analysis and characterized the protein's primary structure and post-translational modifications through various techniques. MALDI-MSI methods were developed to enhance the detection of EYFP in situ, and by extraction of EYFP's molecular ion from MALDI-MS images, automated, whole-image assignment of cell cohorts was achieved. This method was validated using a well-characterized mouse line that expresses EYFP in motor and sensory neurons and should be applicable to hundreds of commercially available mice (and other animal) strains comprising a multitude of cell-specific fluorescent labels.

Toxicokinetics, Tissue Distribution, and Excretion of Dufulin Racemate and Its R ( S)-Enantiomers in Rats

Dufulin is a plant antiviral agent with a novel molecular structure and has been used widely to prevent and control tobacco and rice viral diseases. In this study, an UHPLC-MS/MS method was developed for rapid determination of dufulin racemate ( rac-DFL) and its R ( S)-enantiomers in rat plasma, tissues, urine, and feces. A MALDI-MSI method was further used for visual research on tissue distribution after intragastric administration of the three analytes. Toxicokinetic study showed that both ( R)-enantiomer of dufulin (( R)-DFL) and ( S)-enantiomer of dufulin (( S)-DFL) had a faster ability to reach C_max than that of rac-DFL. ( R)-DFL and ( S)-DFL had a similar T_1/2, though both were significantly lower than rac-DFL. C_max of rac-DFL was obviously higher than ( R)-DFL or ( S)-DFL. Meanwhile, C_max of ( S)-DFL was only about 60% of ( R)-DFL. Rac-DFL and its R ( S)-enantiomers had a dose-dependent toxicokinetic profile. Tissue distribution results revealed rac-DFL, ( R)-DFL, and ( S)-DFL mainly distributed in the liver and kidney, but the maximum concentration was only ng/g grade and could significantly degrade within 3 h. This indicates that dufulin does not cause liver and kidney toxicity in animals. In addition, rac-DFL and its R ( S)-enantiomers have not been detected in brain tissue. Cumulative excretion of rac-DFL and its R ( S)-enantiomers within 24 h in urine and feces were less than 22.85% indicating that they mainly excreted as metabolites. These results could provide evidence for the in-depth toxicity evaluation of dufulin pesticide. In addition, its metabolic selectivity information in vivo has also been obtained.