Optimization of Multiple Glycosidase and Chemical Stabilization Strategies for N-Glycan Isomer Detection by Mass Spectrometry Imaging in Formalin-Fixed, Paraffin-Embedded Tissues

The analysis of N-glycan distributions in formalin-fixed, paraffin-embedded (FFPE) tissues by matrix-assisted laser desorption/ionization (MALDI) imaging mass spectrometry (IMS) is an effective approach for characterization of many disease states. As the workflow has matured and new technology emerged, approaches are needed to more efficiently characterize the isomeric structures of these N-glycans to expand on the specificity of their localization within tissue. Sialic acid chemical derivatization can be used to determine the isomeric linkage (α2,3 or α2,6) of sialic acids attached to N-glycans, while endoglycosidase F3 (Endo F3) can be enzymatically applied to preferentially release α1,6-linked core fucosylated glycans, further describing the linkage of fucose on N-glycans. Here we describe workflows where N-glycans are chemically derivatized to reveal sialic acid isomeric linkages, combined with a dual-enzymatic approach of endoglycosidase F3 and PNGase F to further elucidate fucosylation isomers on the same tissue section.

MALDI Imaging Mass Spectrometry of N-glycans and Tryptic Peptides from the Same Formalin-Fixed, Paraffin-Embedded Tissue Section

Matrix-assisted laser desorption/ionization (MALDI) imaging mass spectrometry (IMS) is a unique and well developed tool for probing the protein content of formalin-fixed, paraffin-embedded tissue (FFPE). Integral to this approach is the application of trypsin, and more recently peptide N-glycosidase F, to release tryptic peptides or N-glycans from tissue and report localization of distinct species. This is typically done on serial or adjacent tissue sections, and there is an emerging need to understand the colocalized protein population linked to the exact same regions of N-glycans. Here we describe an approach where N-glycans are first imaged from a tissue section followed by reprocessing of the same tissue section for tryptic peptide MALDI IMS. Strategies for colocalizing peptides to target N-glycans or N-glycan regions are described.