Mass spectrometry imaging (MSI) is more advantageous than traditional imaging methods because a range of molecular distributions can be studied simultaneously with MSI. However, MSI is still a novel technique that needs to be developed further before it “can fulfill its potential,” Mats Borén, PhD, head of development at Denator, a company focused on the stabilization of biologic samples upstream, told BioPharma-Reporter.com.
He said the major limitations of MSI include ionization “of only a very small fraction of available molecules,” that the results “are only intensity distributions of masses which for the most part remain unidentified,” especially for larger proteins and peptides.
“The images are more like fingerprints to identify disease changes then actual identifications,” Borén said.
Heat stabilization, however, can address the problem of sample changes during work up procedures, Borén said.
“During sample work up to MALDI [Matrix-assisted laser desorption/ionization] imaging the sample is frozen and a thin section, ~12µm, is cut and thawed onto a glass slide. The section is then generally refrozen and later dried at room temperature, either in a desiccator or under a stream of nitrogen,” he explained. And although brief periods of thawing may constitute a “substantial risk,” heat stabilization during these thawing steps can minimize risk.
Borén said his company’s Stabilizor system works “by raising the temperature of the sample above the protein denaturation temperature,” or over 90 degrees Celsius in the case of the Stabilizor T1.
This denaturation, or the loss of the 3-D structure though unfolding, “renders the enzymes inactive,” he said. “This prevents further change to the sample” so that when it’s “further processed for downstream analysis the results will be closer to the in vivo situation and less disturbed by post sampling changes which might have occurred during the process due to residual enzymatic activity in the sample,” Borén added.
In regards to the problem of changes during sample processing “there are no other solutions,” Borén said. “Inhibitors are not used as they are difficult to apply to the frozen section before thawing and the organic solvent wash that are used for imaging larger proteins are only used after the sections have been dried and not [on] all of metabolites and lipids as they will be washed away from the sections during the wash.”
The more general problems with IMS are also being addressed by various ways that “will enable more and partly different ionization, giving more coverage of what’s really in the sample. By using different matrices” to differentiate sections of the same sample, “it is possible to focus on mainly analyzing one class of molecules.”
And the problem of identifying large proteins is “generally addressed by applying trypsin to the section and digesting the large proteins into peptides.”