MALDI–MSI Maps Carboxyls and Aldehydes in Brain Tissue of Nonhuman Parkinson’s Model

Research based out of Uppsala University in Uppsala, Sweden examined an on-tissue chemical derivatization matrix-assisted laser desorption ionization mass spectrometry imaging (MALDI-MSI) approach to comprehensively visualize carboxyls and aldehydes in nonhuman brain tissue sections, attempting to improve upon a process that typically offers low detection sensitivity and high background interference (1).

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MALDI-MSI is used to analyze the spatial distribution of molecules within a sample. It combines mass spectrometry and imaging to provide detailed molecular information. First, a matrix compound is applied to the sample, which helps in the ionization of molecules upon laser irradiation. The laser beam then causes desorption and ionization of molecules from the sample, generating ions that are subsequently analyzed by mass spectrometry. The resulting mass spectra are then used to create two-dimensional images that represent the distribution of different molecules across the sample, allowing researchers to visualize molecular patterns and identify biomarkers or other chemical compounds.

Published in the Journal of the American Society for Mass Spectrometry, this 10-author study describes the experiment carried out with rodent brain tissue sections, plus those of a nonhuman primate Parkinson’s disease (PD) model. Numerous carboxyl- and aldehyde-containing metabolites were detected, including the tricarboxylic acid (TCA) cycle, fatty acid synthesis, glycolysis, lipid peroxidation, and neurotransmitter and amino acid metabolism, and the results also showed an enhancement of previous knowledge of the mechanisms that underlie 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced PD pathophysiology.

While MALDI-MSI offers high-throughput spatial determination of the likes of metabolites, lipids, peptides, proteins, and exogenous pharmaceutical compounds, the detection of carboxyls and aldehydes is more difficult because of low overall abundances and low ionization efficiency, respectively. To combat that, this study employed on-tissue chemical derivatization, which increases the selectivity and sensitivity of MSI toward low-abundance, low-polarity, or poor-ionizing nonpolar compounds.

Furthermore, a dual-purpose reagent was developed for the purpose of on-tissue derivatization in this application, namely (1-(4-(aminomethyl)phenyl)pyridin-1-ium chloride, which the researchers said had previously shown promise in processes involving MALDI-MS as well as liquid chromatography coupled to mass spectrometry (LC–MS). Here, the AMPP successfully produced covalently charge-tagged molecules containing carboxylic acids and aldehydes in the same experiment via a peptide coupling reaction and Schiff base reaction, respectively.

The peptide coupling reagents were O-(7-azabenzotriazol-1-yl)-1,1,3,3-tetramethyluronium hexafluorophosphate (HATU) and O-(benzotriazol-1-yl)-1,1,3,3-tetramethyluronium hexafluorophosphate (HBTU). In tandem, AMPP/HATU derivatization was applied to the probing of regional changes of metabolites within primate brain tissue sections, both control and MPTP-administered, in which marked changes were recorded in distribution of molecules within pathologically relevant regions like the hypothalamus, substantia nigra reticulata, putamen, and globus pallidus.

Through this research, metabolites such as acetic acid, aconitic acid, acrolin, α-ketoglutaric acid, γ-hydroxybutyric acid (GHB), glutamate semialdehyde, N-acetylaspartylglutamate (NAAG), oxaloacetic acid, pantothenic acid, and succinic semialdehyde were all identified in brain regions. The sensitive ultrahigh mass resolution MALDI-MS detection and imaging processes described in this study showed not only a capability to identify previously known quantities within the brain, but also quantified the distributions of these compounds that had not been previously reported in such tissue.

Reference

(1) Kaya, I.; Schembri, L.S.; Nilsson, A.; et al. On-Tissue Chemical Derivatization for Comprehensive Mapping of Brain Carboxyl and Aldehyde Metabolites by MALDI–MS Imaging. J. Am. Soc. Mass Spectrom. 2023, 34 (5), 836–846. DOI: 10.1021/jasms.2c00336