New methods for multimodal MS imaging of histological tissue sections

Abstract

The insights derived from spatial localization of molecules in tissue sections are of great value for understanding and treating cancer and other diseases. These insights can relate to molecules linked to a disease as well as to drug molecules distributed across organs of interest. Mass spectrometry (MS) imaging can not only provide the molecular and spatial information of single molecules, but can do so for a broad range of substances, from elements to proteins. In one single measurement, the impact of these molecules in multiple physiological and pathological pathways can be analyzed. The value of MS imaging for cancer research is demonstrated by the insights in tumor malignancy generated with MS imaging in this research. Two breast cancer xenograft models, the nonmetastatic MCF-7 and highly metastatic MDA-MB-231 tumors, have been studied with MS imaging in combination with magnetic resonance spectroscopic imaging (MRSI). This study focused on the distribution of phosphocholine (PC), total choline (tCho), free choline (Cho), sodium and potassium in the two different tumors. In vivo 3-dimensional MRSI of MDA-MB-231 tumors demonstrated that high tCho levels (Cho, glycerophosphocholine (GPC) and PC), displayed a heterogeneous spatial distribution in the tumor. MS imaging data provided information about each individual molecule, with increased PC and Cho in viable tumor regions compared to necrotic regions in the highly metastatic MDA-MB-231 tumor type. The same molecules were homogeneously distributed in the nonmetastatic MCF-7 tumor type. Also, significant differences in the spatial distribution of Na+ and K+ in the these two tumor types suggested differential Na+/K+ pump functions and K+ channel expressions. The combined results provide a link to the impact of multiple pathways in tumor malignancy in one single study. In drug discovery research, it is important to know not only in which organs drug and drug metabolites can be found after drug administration, but also how much of drug and metabolites are present in each organ. However, quantification by MS imaging is challenging, with several factors affecting the results of quantitation of drugs in tissue sections. Therefore an experimental setting was specially designed to address the impact of several factors on drug quantitation, such as ion suppression, sample preparation and organ dependence on the mass spectrometric response of the drug using organ homogenates and drugs. Because sample handling is of such importance to the success of MS imaging experiments, a new method was developed for fragile tissue sections and localized washing, which consists of a wetted paper with the solvent of choice applied directly onto the biological tissue surface. The washing solution is spatially contained by the paper structure, thus the formation of a liquid film on the tissue surface is avoided. An additional advantage is that the paper used in the washing step can also be analyzed by MS imaging, providing additional information of components removed from the tissue surface while retaining their spatial organization.