MS Imaging – Another Modality for Digital Pathology?

Abstract

A single modality is very often not good enough to understand all functional, structural, temporal and chemical relations underlying certain biological conditions. Imaging a specimen with two or more complementary modalities creates an informative, composite view of a sample that spans all relevant resolution ranges. Here we shortly introduce the concept of MALDI Mass Spectrometry based imaging (MALDI MSI) and its potential to localize molecules in a tissue without a priori knowledge. Advantages, limitations, and potentials for applications are briefly highlighted.

In this talk our investigations on aging skin are presented to show the potential of MALDI MSI. Our skin is constantly exposed to solar radiation, high oxygen levels, and environmental pollutants. These are accelerant stress factors for premature skin aging, tissue inflammation, and photocarcinogenesis. In skin aging, the evident and well-known clinical effect is the alteration of the skin appearance. To counteract oxidative stress, the cells activate several epidermal and dermal lipoxygenases. Indeed, oxidized lipids can act as danger-associated molecular patterns (DAMPs) messengers and are involved in the senescence-associated secretory phenotype (SASP). In previous work from our collaborator F. Gruber1, several oxidized phosphatidylcholines (OxPCs) were characterized in a semitargeted lipidomic approach. The method involved the exposure of human dermal fibroblasts at defined UVA fluences, followed by liquid-liquid extraction, and lastly, mass spectrometric analysis by means of reversed-phase HPLC coupled to electrospray ionization MS/MS. We now went one step further and studied the localization and relative quantity of certain OxPCs in a 3D skin model. Our study aims to evaluate the epilipidomic effects via an untargeted multimodal approach focusing on OxPCs species. We employed high-resolution matrix-assisted laser desorption/ionization Fourier-transform ion cyclotron resonance mass spectrometry (MALDI-FT-ICR MS) to generate OxPC distribution images. The high mass accuracy of FT-ICR MS instrumentation, combined with the analytes’ spatial localization and relative abundance, allow us to locate and putatively identify several OxPCs with great confidence. Lastly, the integration with high-definition images from light-microscopy and immunohistology yield a detailed and comprehensive result of the effects of UV-induced alteration.