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Title: In situ cell-by-cell imaging and analysis of small cell populations by mass spectrometry

Author
item SHRESTHA, BINDESH - George Washington University
item Patt, Joseph - Joe
item VERTES, AKOS - George Washington University

Submitted to: Analytical Chemist
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/1/2010
Publication Date: 12/31/2010
Citation: Shrestha, B., Patt, J.M., Vertes, A. 2011. In situ cell-by-cell imaging and analysis of small cell populations by mass spectrometry. Analytical Chemist. 83(8):2947-2955.

Interpretive Summary: Imaging of molecules by mass spectrometry (MS) is emerging as a tool to determine the distribution of proteins, lipids and metabolites in cells and tissues. Existing imaging methods rely on artificial grids for sampling cells. These grids ignore the natural cellular organization of the tissue and reduce the precision with which the samples are collected. Here we demonstrate that laser ablation electrospray ionization (LAESI) MS can be utilized for sampling molecules from adjacent cells within plant tissue. The cell-by-cell molecular image of the metabolite cyanidin, the ion responsible for purple pigmentation in onion (Allium cepa) epidermal cells, correlated well with the color of cells in the tissue. Chemical imaging using single-cells as pixels reflects the spatial distribution of biochemical differences within a tissue without the distortion stemming from sampling multiple cells within the laser focal spot. This approach has the added benefit of enabling the analysis of small cell populations for biochemical heterogeneity. To explore cell-to-cell variations within and between tissues, multivariate statistical analysis on LAESI-MS data from A. cepa, sour orange (Citrus aurantium) leaf and human cheek cell populations was performed using the a statistical method called orthogonal projections to latent structures (OPLS), which is a type of prinicipal components analysis. The OPLS analysis of spectra containing over 300 ion peaks each provided guidance in identifying a small number of metabolites most responsible for the variance between the cell populations.

Technical Abstract: Molecular imaging by mass spectrometry (MS) is emerging as a tool to determine the distribution of proteins, lipids and metabolites in tissues. The existing imaging methods, however, rely on predefined typically rectangular grids for sampling that ignore the natural cellular organization of the tissue. Here we demonstrate that laser ablation electrospray ionization (LAESI) MS can be utilized for in situ cell-by-cell imaging of plant tissues. The cell-by-cell molecular image of the metabolite cyanidin, the ion responsible for purple pigmentation in onion (Allium cepa) epidermal cells, correlated well with the color of cells in the tissue. Chemical imaging using single-cells as pixels reflects the spatial distribution of biochemical differences within a tissue without the distortion stemming from sampling multiple cells within the laser focal spot. This approach has the added benefit of enabling the analysis of small cell populations for biochemical heterogeneity. To explore cell-to-cell variations within and between tissues, multivariate statistical analysis on LAESI-MS data from A. cepa, sour orange (Citrus aurantium) leaf and human buccal epithelial cell populations was performed using the method of orthogonal projections to latent structures (OPLS). The OPLS analysis of spectra containing over 300 ion peaks each provided guidance in identifying a small number of metabolites most responsible for the variance between the cell populations.