Mini Review: Highlight of Recent Advances and Applications of MALDI Mass Spectrometry Imaging in 2024

Abstract

Matrix-assisted laser desorption/ionisation mass spectrometry imaging (MALDI-MSI) is an emerging imaging tool that allows visualisation of hundreds of analytes unbiasedly in a single experiment. This paper highlights the adaptations of MALDI-MSI in different context in 2024, such as clinical diagnostic, pharmacology, forensics applications, plant metabolism and biology. Challenges and advancements were also discussed regarding sample preparation, instrumentations, data analysis, and integration of machine learning in the trend of single cell resolution and multi-omics. There are still rooms for improvements in sensitivity, spatial resolution, acquisition algorithm and data integration across multi-omics data to enable MALDI-MSI at subcellular level.

FIGURE 1

Publication trends of MSI studies. (A) Line graph showing the number of PubMed‐listed publications with search query of ‘mass spectrometry imaging’ from year 2014 to 2024. (B) Proportional representation of four ionisation sources mentioned in MSI studies published in 2024. Search queries used were ‘mass spectrometry imaging’ AND ‘MALDI’ OR ‘SIMS’ OR ‘DESI’ OR ‘laser ablation’. Data was obtained as of 31 December 2024.

FIGURE 2

A typical MALDI‐MSI workflow. Key steps include: (1) Tissue preservation via formalin‐fixing or snap‐freezing and sample embedding. (2) Embedded tissues are sectioned and mounted on ITO slides. Sample processing steps such as de‐waxing, antigen retrieval and washing may follow based on analyte(s) of interest. (3) Samples may undergo enzymatic digestion. For example, trypsin and PNGase F are applied for the analysis of peptides and N‐glycans respectively. (4) Matrix is deposited onto the tissue by spraying or sublimation. (5) Data is acquired in a MS coupled with MALDI source. (6) Generation of ion distribution images by mapping ion intensities obtained from the MS to specific locations in the tissue, creating spatial maps of analyte distribution. Figure is created using the BioRender software.

FIGURE 3

Diverse range of MALDI‐MSI applications. (A) Peptide MALDI‐MSI of focal cortical dysplasia section. (i, ii) Spatial segmentation of grey matter (GM), white matter (WM), perilesional (PL) and lesional (L) regions. (iii–v) Ion distribution images of peptides from myelin basic protein (MBP, m/z 726.41), vimentin (Vim, m/z 1428.72) and neuronal nuclei (NeuN, m/z 2407.26). Adapted and modified from Cagnoli et al. [20] (fig. 3) with permission from John Wiley and Sons. (B) (i) Ion distribution image of zolpidem ions (m/z 308.17578, indicated by red signals) and (ii) overlaid optical image of a hair sample from an individual who ingested the drug. (iii) Ion distribution image of zolpidem, and (iv) overlaid optical image of soaked hair. Adapted and modified from Ji et al. [22] (figs. S1 and S2) with permission from Elsevier. (C) Ion distribution images of a phosphatidylcholine (PC (32:0), m/z 734.56), sphingolipid (SM(d17:1/19:0), m/z 731.60) and arginine (m/z 175.11) in control and nickel‐treated skin samples. Adapted and modified from Rezaei et al. [26] (fig. 4) with permission from John Wiley and Sons. (D) Ion distribution images of 4‐formylsalicylic acid (m/z 166.026) in cultivated (G. max) and wild soybeans (G. soja). Adapted and modified from Yin et al. [39] (fig. 5) with permission from Elsevier. (E) Ion distribution images of tobramycin (m/z 468.26) in P. aeruginosa biofilm colonies at multiple post‐treatment stages with a three‐dimensional perspective. Adapted and modified from Shen et al. [41] (fig. 6) with permission from Elsevier.

FIGURE 4

MALDI‐MSI coupled with trapped ion mobility separation (TIMS) enabled differentiation of isobaric molecules with distinct distribution patterns. (A) Identification of isobaric species (m/z 758.56) with different collision cross section (CCS) values in mouse lung tissue. Ion distribution image revealed distinct localisation patterns of isobaric species. Adapted from Ngai et al. [12] (fig. 3) with permission from John Wiley and Sons. (B) Similar observation was made in oat grain tissue for m/z 756.55, with one species distributed throughout the grain (red), while the other localised in the white endosperm region (green). Adapted and modified from Lau et al. [81] (fig. 6) with permission from Elsevier.