In collaboration with the laboratory of Prof. Nathalie Agar at HMS and DFCI/BWH, we have developed new "Slice-MSI" methodology to expand our understanding of cellular brain metabolism beyond the handful of metabolites for which there are good fluorescent biosensors.
Mass spectrometry imaging (MSI) using matrix-assisted laser desorption-ionization (MALDI) is a remarkable technique that can scan a slide-mounted tissue section, pixel by pixel, and produce a full mass spectrum for each pixel. This provides rich chemical detail about the compounds present in each pixel. Each compound produces a very specific peak in the spectrum, according to its mass-to-charge ratio (m/z), with a height related to the quantity of that compound. The separation by mass also allows detection of isotopically labeled variants, derived (for instance) from 13C-labeled fuel molecules.
To study metabolism in living, acutely prepared brain slices from mouse brain, we first place the brain slices in a typical physiology chamber, where we can supply them with oxygenated salines containing selected fuel molecules (sometimes labeled with 13C) and pharmacological agents, and we can stimulate them with electrodes or with chemical stimuli. These slices can't be imaged directly by MALDI-MSI; we first need to preserve their metabolic state and thin-section them.
We designed a fast thermal preservation device that preserves the metabolic state in seconds, by flash heating to denature the metabolic enzymes that interconvert metabolites, followed by flash freezing. We then thin-section the tissue directly from the underlying thermoelectric device, and mount the tissue onto slides for MALDI-MSI imaging.