Mass Spectrometric Imaging

The ability to observe the distribution of molecules in neuronal tissues can be of great value towards developing an understanding of cell to cell signaling in the nervous system—particularly at single cell and subcellular spatial resolutions. Mass spectrometric imaging (MSI) utilizes mass spectrometry to produce distribution maps of selected compounds by rastering the ionization beam across the sample. A marked advantage of MSI results from the notion that analytes need not be labeled or characterized prior to data acquisition. In addition, numerous compounds may be studied within a single experiment as a result of the multiplexed detection scheme afforded by the mass spectrometer.

We use both matrix assisted laser desorption/ionization (MALDI) and secondary ion mass spectrometry (SIMS) to study the distribution of a wide range of neurologically relevant compounds ranging from lipids and small metabolite molecules to peptides and proteins. With MALDI imaging, a focused laser beam is used to ablate a matrix-coated sample, while in SIMS, a focused ion beam is used to raster across a pristine sample surface. MALDI and SIMS are complementary to each other in that SIMS provides submicron resolution suitable for single cell imaging, while MALDI provides a wide mass range for the detection of peptides and proteins.

In addition, several novel preparative protocols are being developed in order to allow high resolution images to be produced at cellular and even subcellular spatial resolutions, especially for MALDI imaging. We have observed the subcellular localization of vitamin E at the soma-neurite junction with SIMS imaging of single, isolated neurons, which suggests that vitamin E may have an important role in neuronal function [Monroe, J. Am. Chem. Soc. 2005]. We are continuing to use SIMS for the imaging of single cells as well as tissues.  Additionally, a multifaceted approach that uses both SIMS and MALDI imaging has been applied to the analysis of rat spinal cord tissue to provide a comprehensive chemical map of compounds [Monroe, Proteomics 2008].

Examples of SIMS and MALDI imaging experiments

MSI images
A. SIMS imaging of a single isolated neuron total ion image and m/z 430 identified as Vitamin E (scale bar is 10 µm). B. Subcellular SIMS imaging of cholesterol (m/z 385) within a single cultured mammalian neuron (scale bar is 100 µm).

MSI images

C. MALDI imaging of a spinal cord section and m/z 1347.7 identified as Substance P (scale bar is 1 µm). D. SIMS imaging of a spinal cord section m/z 184 identified as phosphatidylcholine and m/z 385 identified as cholesterol (scale bar is 100 µm).

Relevant Publications:
E.B. Monroe, J.C. Jurchen, J. Lee, S.S. Rubakhin, J.V. Sweedler, Vitamin E imaging and localization in the neuronal membrane, J. Am. Chem. Soc. 127, 2005, 12152-12153.

E.B. Monroe, S.P. Annangudi, N.G. Hatcher, H.B. Gutstein, S.S. Rubakhin, J.V. Sweedler, SIMS and MALDI MS imaging of the spinal cord, Proteomics 8, 2008, 3746-3754.

Mass Spectrometry