MALDI‐Mass Spectrometric Imaging of Endogenous Metabolites in Biological Systems

Abstract

Matrix‐assisted laser desorption/ionisation (MALDI)‐mass spectrometric imaging (MSI) is a powerful, relatively new technology that allows for the detection of molecules directly from biological systems and also provides the spatial distribution for these analytes within the tissue sample. MSI has gained considerable popularity for the study of endogenous metabolites as metabolites are the end product of various biochemical processes, and represent a biological snapshot of an organism's biochemistry at a given time. Although the underlying concept is simple, factors such as choice of ionisation method, sample preparation, instrumentation and data analysis must be taken into account and tailored for successful applications of MSI, especially for the study of small molecules. MALDI‐MSI has been successfully applied to the study of endogenous metabolites in animal, plant, bacteria and fungal tissues and has provided new insights into biological functions and pathways, interspecies interactions and novel natural products.

Key Concepts:

  • A metabolic profile provides a snapshot of an organism's metabolism and underlying biochemistry at a given time.

  • MALDI‐MSI provides spatial distribution of endogenous or exogenous molecules in a sample.

  • Advantages of MALDI‐MSI include the ability to detect a broad mass range of compounds, the ability to detect multiple analytes in a single experiment without the need for labels or prior knowledge of the analytes and the preservation of biologically relevant spatial information.

  • The study of small molecules with MALDI‐MSI presents unique challenges due to matrix interference and potential analyte delocalisation.

  • MALDI‐MSI has been extensively used to study metabolites and other compounds in animal tissue and has recently been applied to the study of small molecules in plant, bacteria and fungi.

Keywords: MALDI; mass spectrometry; imaging; metabolites; metabolomics; biological tissues; plants; bacteria; agar

Figure 1.

MALDI‐MSI workflow. Samples are prepared by sectioning tissue samples and mounting them on a glass slide or MALDI target plate followed by application of matrix. MSI uses a computer‐controlled X‐Y stage, which holds the sample, and allows for the laser to raster across the sample and mass spectra to be collected at every predefined raster point. After completing the 2D raster, an ion image can be created using software to select a single mass from the spectrum and the software will display the relative abundance of that ion as a heat map of signal intensity at each raster point.

Figure 2.

The result of MSI analysis of testis in negative‐ion mode. Hematoxylin and eosin staining results and ion images of 14 representative peaks. Scale bar is 100 μm. Reproduced with permission from Goto‐Inoue et al. ().

Figure 3.

Comparison of the metabolite distribution in nitrogen‐fixing and non‐fixing Medicago truncatula nodules. (a) Nodules on a wild‐type (WT) Medicago plant inoculated with the WT strain of rhizobia (WT/WT). (b) Nodules on a WT plant inoculated with the fixJ mutant of rhizobia (WT/fixJ). (c) Nodules on a Medicago dnf1–1 mutant inoculated with the WT strain of rhizobia (dnf1/WT). (d) Nodules on a Medicago dnf1–1 mutant inoculated with the fixJ mutant of rhizobia (dnf1/fixJ). (e) Nitrogen fixation activity. Each bar corresponds to the mean value of acetylene reduction for eight plants assayed 3 weeks after inoculation. Error bars indicate standard error (SE). (f–i)Distribution of the haem moiety at m/z 616.18 in the four different nodule types: WT/WT (f), WT/fixJ (g), dnf1/WT (h) and dnf1/fixJ (i). The haem moiety was detected only in fixing nodules in (f). (j–m) Distribution of the metabolite formononetin MalGlc at m/z 517.13 on WT/WT (j), WT/fixJ (k), dnf1/WT (l) and dnf1/fixJ (m), showing great similarity in contrast to (f)–(i). Scale bars=1 mm. Reproduced with permission from Ye et al. ().

Figure 4.

MSI of multiple complex samples. (A) Los Angeles garden soil. (B) La Jolla Shores beach marine sediment. (a) Respective photographs of microbial communities. (b) Overlay of multiple ions observed within the microbial community. Reproduced with permission from Gonzalez et al. (). © Elsevier.

Figure 5.

MALDI‐MSI of the interaction between GA40 (bacteria) and Aspergillu fumigatus fungi (left) compared to interaction of GA40 versus Aspergillu niger fungi (right) including the controls (a) GA40 on the bottom and A. fumigatus on the top (b) A. fumigatus control (c) GA40 control (d) GA40 on the bottom and A. niger on the top (e) A. niger control (half of a colony was imaged) (f) GA40 control. Reproduced with permission from Moree et al. (). © Springer.

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Further Reading

Gemperline E, Chen B and Li L (2014) Challenges and recent advances in mass spectrometric imaging of neurotransmitters. Bioanalysis 6(4): 525–540.

Gemperline E and Li L (2014) MALDI‐mass spectrometric imaging for the investigation of metabolites in Medicago truncatula root nodules. Journal of Visualized Experiments (85): e51434.

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Gemperline, Erin, and Li, Lingjun(Aug 2014) MALDI‐Mass Spectrometric Imaging of Endogenous Metabolites in Biological Systems. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0023207]