Imaging Mass Spectrometry – Molecular Microscopy for Biological and Clinical Research

Abstract

Imaging mass spectrometry has seen tremendous advances over recent years and has become an important tool for molecular discovery in intact tissues with high molecular fidelity and spatial resolution. This chapter will focus on MALDI Imaging Mass Spectrometry (IMS) because it is the most useful technology for the mapping of metabolites, lipids, peptides and proteins at high sensitivity in intact tissue sections. Desorption of biomolecules is accomplished by direct laser irradiation of an array of spots (i.e., pixels) on the tissue to map the location of specific molecules. Both fresh-frozen and formalin-fixed tissue sections can be imaged without the need for specific targeting reagents such as antibodies. Molecular images of this nature are produced based on specific m/z (mass-to-charge) values, or ranges of values. Thus, each specimen gives rise to many hundreds of specific molecular images from a single raster of the tissue. In a complementary approach, where only discrete areas within the tissue are of interest, such as in anatomic pathology, we have developed a histology-directed approach that integrates mass spectrometry and microscopy. Mass spectra are collected from selected discrete areas of cells within the tissue for laser ablation and analysis, and these are then correlated with microscopic images of the tissue section.

This chapter will illustrate the use of IMS in several biologically and medically relevant research projects. One area of interest is the mapping of molecular changes occurring in diabetes in both a mouse model and in the human disease. Major molecular alterations have been recorded in advanced diabetic nephropathy involving both proteins and lipids. Another application employs IMS to differentiate benign skin lesions from melanomas using in-house developed PIMS (Pathology Interface for Mass Spectrometry) software.

Although not primarily a technology review, this chapter briefly describes recent technological advances both in sample preparation and instrumental performance to achieve images at high spatial resolution (1–10 microns) and at high speeds (a typical sample tissue once prepared can be imaged in minutes). Applications will include the use of tandem mass spectrometry (MS/MS), ultra-high mass resolution, and ion accumulation technology for IMS. Finally, new biocomputational approaches will be described that address the high dimensionality of IMS data as well as our implementation of ‘image fusion’ in predictive integration applications of MS images with microscopy and other imaging modalities.