Acidic Methanol Treatment Facilitates Matrix-Assisted Laser Desorption Ionization-Mass Spectrometry Imaging of Energy Metabolism

Detection of small molecule metabolites (SMM), particularly
those
involved in energy metabolism using MALDI-mass spectrometry imaging
(MSI), is challenging due to factors including ion suppression from
other analytes present (e.g., proteins and lipids). One potential
solution to enhance SMM detection is to remove analytes that cause
ion suppression from tissue sections before matrix deposition through
solvent washes. Here, we systematically investigated solvent treatment
conditions to improve SMM signal and preserve metabolite localization.
Washing with acidic methanol significantly enhances the detection
of phosphate-containing metabolites involved in energy metabolism.
The improved detection is due to removing lipids and highly polar
metabolites that cause ion suppression and denaturing proteins that
release bound phosphate-containing metabolites. Stable isotope infusions
of [13C6]nicotinamide coupled to MALDI-MSI (“Iso-imaging”)
in the kidney reveal patterns that indicate blood vessels, medulla,
outer stripe, and cortex. We also observed different ATP:ADP raw signals
across mouse kidney regions, consistent with regional differences
in glucose metabolism favoring either gluconeogenesis or glycolysis.
In mouse muscle, Iso-imaging using [13C6]glucose
shows high glycolytic flux from infused circulating glucose in type
1 and 2a fibers (soleus) and relatively lower glycolytic flux in type
2b fiber type (gastrocnemius). Thus, improved detection of phosphate-containing
metabolites due to acidic methanol treatment combined with isotope
tracing provides an improved way to probe energy metabolism with spatial
resolution in vivo.