Strong additive and synergistic effects of polyoxyethylene nonionic surfactant-assisted protein MALDI imaging mass...

Protein MALDI imaging mass spectrometry (MALDI-IMS) holds a great promise to acquire spatial distribution information of proteins on biological tissue, but it suffers from the small number of proteins detected by direct MALDI-IMS detection. Ionic surfactants have been extensively used for protein extraction to improve the number of proteins detected in tissue samples by LC-MS analysis, but seldom by direct MALDI-IMS detection. Nonionic surfactants are milder than ionic surfactants and protein native structures are remained after extraction, which favors the spatial resolution of direct MALDI-IMS. However, nonionic surfactants are less effective than ionic surfactants. In this report, we utilized polyoxyethylene nonionic surfactants (PNS) to pre-incubate the tissue section, followed by the on-tissue trypsin digestion and then direct MALDI detection of in-situ formed peptides. For the first time, we observed that the additive effect of PNS and the synergistic effect of the mixed PNS in improving the number of peptides detected. Specifically, the peptides detected were 73.0-90.7% distinct when the different PNS (Tween 80 or Triton X-100 alone or their mixture) was used. Taking advantage of this additive effect, the 96 proteins including 12 transmembrane proteins were detected, corresponding to a ~10-fold improvement compared to MALDI-IMS without surfactant. When the mixed surfactants were used to replace Tween 80 and Triton X-100 alone, the optimized surfactant concentration decreased 20-100-fold and the number of peptides detected with m/z > 2500 Da was improved 15-fold. The additive and synergistic effects of PNS suggested that the interaction mode between each PNS and proteins is highly variable. Benefiting from the strong additive effect and diversity of PNS, further improvement of the number of proteins detected by MALDI-IMS is clearly feasible.