Matrix assisted laser desorption ionization MASS SPECTROMETRY IMAGING for peptide and protein analyses : critical review on on-tissue digestion

Matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI) has established itself among the plethora of mass spectrometry applications. In the biomedical field, MALDI-MSI is being more frequently recognized as a new method for the discovery of biomarkers, targets of treatment, classification of diseased and healthy tissues or to predict the outcome of a pathology. The technology has been used to study the localization of proteolytic peptides directly on tissue sections. A direct correlation between the detected peptides and the distribution and identity of the original precursor protein is the ultimate goal of any MALDI-MSI experiment. Enzymatic digestion protocols are commonly used to reveal the protein signature of these complex tissues. Considerations that pertain to methods of sample preparation, on-tissue digestion, data analysis and visualization will be addressed. This review will also discuss selected applications of on-tissue digestion combined with the MALDI-MSI technology in biomedicine.