ReviewIn situ metabolomic mass spectrometry imaging: Recent advances and difficulties☆
Graphical abstract
Highlights
► We describe the latest MSI techniques for in situ imaging of endogenous metabolites. ► The representative ionization platforms of such MSI are overviewed. ► Identification strategy of detected mass peaks and its difficulty are discussed. ► These points are important for effective application of MSI in biomedical research.
Introduction
Understanding the complex biochemical processes that occur within living organisms requires not only the elucidation of the molecular entities involved in these processes, but also their spatial distribution within the organism. Analytical technologies for elucidating multiple molecular dynamics in the micro-region that retain the spatial information of the target tissue are thought to be important for understanding biological complexity of disease progress. Chemical stains, immunohistochemical tags and radiolabels are common methods for visualizing and identifying molecular targets. However, there are limits to the sensitivity and specificity of these methods and to the number of target compounds that can be monitored simultaneously. Thus, the simultaneous multiple molecular imaging with high sensitivity will be a technical breakthrough for pathophysiological research.
Metabolites are the result of the interactions of a system's genome with its environment, and are the end products of gene expression. The metabolome is defined as the total quantitative collection of small-molecular-weight metabolites present in a cell, tissue, or organism, that participate in the metabolic reactions required for growth, maintenance, and normal function [1], [2], [3]. Unlike the transcriptome and proteome that represent the processing of information during the expression of genomic information, the metabolome more closely represents the phenotype of an organism under a given set of conditions and can be defined as the “compound-level phenotype” of the genomic information. Metabolomics, the measurement of the global endogenous metabolite profile from a biological sample under different conditions, can lead us to an enhanced understanding of disease mechanisms, the discovery of diagnostic biomarkers, the elucidation of mechanisms for drug action, and an increased ability to predict individual variation in drug response phenotypes [4], [5]. Thus, this rapidly developing discipline has important potential implications in the field of biomedical research.
To date, MS coupled with pre-separation techniques such as LC-MS or GC-MS has been known to be a conventionally used strategy for metabolomics [6], [7], [8]. However, these methods have a drawback in the analysis of tissue samples because of the requirement of metabolite extraction, which causes the loss of information on the spatial localization of the metabolites. In contrast, imaging techniques capable of determining the spatial localization of molecules have revolutionized our approach to diseases by allowing us to directly examine the pathological process, thereby giving us a better understanding of the pathophysiology. In most cases, however, there is a tradeoff among sensitivity, molecular coverage, spatial resolution, and temporal resolution. For example, magnetic resonance imaging (MRI), positron emission tomography (PET), and fluorescence microscopy can visualize the spatial localization of targeted molecules with high sensitivity, but these techniques have low molecular coverage (only a few molecules at a time) [9]. The simultaneous and spatially resolved detection with high sensitivity of a broad range of molecules is still a challenging issue.
MS imaging (MSI) is an emerging technology that makes it possible to determine the distribution of biological molecules present in tissue sections by direct ionization and detection. MSI has received considerable attention as a potential imaging technique for a molecular ex vivo review of tissue sections from an animal or plant based on label-free tracking of endogenous molecules with spatial resolution and molecular specificity [10], [11], [12]. The MSI technique was initially developed as a tool for intact protein imaging from the tissue surface using MALDI-MS [12], [13], [14], [15], [16]. In current research, proteins or peptides are still the main targets. However, the analysis of a wide variety of low-molecular weight compounds, including endogenous metabolites, using MSI combined with several soft ionization methods is emerging as a research target. In this review, we describe recent advances and difficulties in developing an analytical platform for MSI of endogenous small metabolites.
Section snippets
MALDI
MALDI is one of the laser desorption ionization (LDI) methods that can softly ionize several biological molecules. This ionization technique is usually used combined with TOF MS. A conventional MALDI source is equipped with a UV laser such as a nitrogen laser (337 nm) or Nd-YAG (355 nm). Spatial resolution is dependent mostly on the diameter of the laser; the diameter is usually more than 5 μm [17]. However, because MALDI-MSI requires a matrix application step, potential limitations in spatial
Strategy for metabolite identification
When MSI experiments are performed using any of the ionization methods described, tens or hundreds of peaks are simultaneously detected in a single run. Understanding their unique distributions in the tissue section should provide important information. However, the crude analytical data without peak identification is difficult to interpret, and discovery of biomarkers or elucidation of biological and disease mechanisms is hindered. In the metabolomics research field, the comprehensive
Conclusion and future perspectives
In this review, we discussed the recent advances and difficulties of endogenous metabolite MSI. Each MSI platform, MALDI, NIMS, and DESI, has advantages and disadvantages, and improvement of the methods and development of instruments are still in progress in the attempt to overcoming diverse technical disadvantages, especially molecular coverage and metabolite identification.
In the future, a combination of an in situ endogenous metabolite MSI technique with other analytical platforms such as
Acknowledgment
Preparation of this manuscript was supported in part by the Science and Technology Incubation Program in Advanced Region from the funding program “Creation of Innovation Centers for Advanced Interdisciplinary Research Areas” from Japan Science and Technology Agency, commissioned by the Ministry of Education, Culture, Sports, Science, and Technology.
References (61)
- et al.
Metabolomics or metabolite profiles?
Trends Biotechnol
(2005) - et al.
Differential metabolomics reveals ophthalmic acid as an oxidative stress biomarker indicating hepatic glutathione consumption
J Biol Chem
(2006) - et al.
Visualization of the cell-selective distribution of PUFA-containing phosphatidylcholines in mouse brain by imaging mass spectrometry
J Lipid Res
(2009) - et al.
Molecular imaging of amyloid beta peptides in mouse brain sections using mass spectrometry
Anal Biochem
(2002) - et al.
Metabolic phenotyping for monitoring surgical patients
Lancet
(2011) Metabolomics–the link between genotypes and phenotypes
Plant Mol Biol
(2002)- et al.
'Metabonomics': understanding the metabolic responses of living systems to pathophysiological stimuli via multivariate statistical analysis of biological NMR spectroscopic data
Xenobiotica
(1999) - et al.
Metabolomic profiles delineate potential role for sarcosine in prostate cancer progression
Nature
(2009) - et al.
Mass spectrometry-based metabolomics: accelerating the characterization of discriminating signals by combining statistical correlations and ultrahigh resolution
Anal Chem
(2008) - et al.
A metabonomic analysis of plasma from Zucker rat strains using gas chromatography/mass spectrometry and pattern recognition
Rapid Commun Mass Spectrom
(2006)
A multivariate screening strategy for investigating metabolic effects of strenuous physical exercise in human serum
J Proteome Res
Simultaneous PET-MRI: a new approach for functional and morphological imaging
Nat Med
MALDI-FTICR imaging mass spectrometry of drugs and metabolites in tissue
Anal Chem
Imaging mass spectrometry: a new technology for the analysis of protein expression in mammalian tissues
Nat Med
Molecular imaging of biological samples: localization of peptides and proteins using MALDI-TOF MS
Anal Chem
Direct profiling of proteins in biological tissue sections by MALDI mass spectrometry
Anal Chem
New developments in profiling and imaging of proteins from tissue sections by MALDI mass spectrometry
J Proteome Res
Visualization of volatile substances in different organelles with an atmospheric-pressure mass microscope
Anal Chem
Protein identification in DNA databases by peptide mass fingerprinting
Protein Sci
Matrix-assisted laser desorption/ionization time-of-flight mass spectrometric analysis of cellular glycerophospholipids enabled by multiplexed solvent dependent analyte-matrix interactions
Anal Chem
Regulatory functions of phospholipase A2
Crit Rev Immunol
A neuroscientist's guide to lipidomics
Nat Rev Neurosci
Mass microscopy to reveal distinct localization of heme B (m/z 616) in colon cancer liver metastasis
J Mass Spectrom Soc Jpn
Visualization of spatial distribution of gamma-aminobutyric acid in eggplant (Solanum melongena) by matrix-assisted laser desorption/ionization imaging mass spectrometry
Anal Sci
Application of imaging mass spectrometry for the analysis of Oryza sativa rice
Rapid Commun Mass Spectrom
Analysis of low molecular weight acids by negative mode matrix-assisted laser desorption/ionization time-of-flight mass spectrometry
Rapid Commun Mass Spectrom
Mass spectrometric method for analyzing metabolites in yeast with single cell sensitivity
Angew Chem Int Ed Engl
Highly sensitive matrix-assisted laser desorption ionization-mass spectrometry for high-throughput metabolic profiling
Anal Chem
MALDI-MS-Based High-Throughput Metabolite Analysis for Intracellular Metabolic Dynamics
Anal Chem
Shotgun metabolomics approach for the analysis of negatively charged water-soluble cellular metabolites from mouse heart tissue
Anal Chem
Cited by (0)
- ☆
This article is part of a Special Issue entitled: Imaging Mass Spectrometry: A User’s Guide to a New Technique for Biological and Biomedical Research.