Mass Spectrometry Imaging Elucidates the Precise Localization and Site-Specific Functions of Skin Lipids

Lipids are essential for the skin, playing a crucial role in forming plasma membranes and maintaining the skin's permeability barrier and hydration. Intercellular lipids fill the spaces between corneocytes and contribute to the barrier function. Lipid abnormalities in the skin have been observed in many skin diseases, including atopic dermatitis and psoriasis. However, the specific localization and roles of skin lipids at particular sites remain incompletely elucidated due to the limited methods available for comprehensive lipid analysis. This study aims to precisely determine the localization of skin lipids, especially intercellular lipids, and investigate their roles and metabolism using mass spectrometry imaging (MSI). We conducted high-resolution (spatial resolution: 5 µm) matrix-assisted laser desorption/ionization (MALDI)-MSI on the lower back and buttocks and created overlay images of skin lipids to clarify their precise localizations. Ceramide was localized in the outermost layer among intercellular lipids. Cholesterol and free fatty acids were present in the stratum corneum but were at trace levels in the outermost layer. Cholesterol sulfate was abundant in the granular layer and gradually decreased in the stratum corneum, promoting desquamation. Phospholipids were confined to the viable epidermis (stratum corneum-/epidermis+), which forms the plasma membrane. A significant increase in mass intensity in the stratum corneum was observed for ceramide, sphingoid base, cholesterol, and free fatty acids, along with a decrease in phospholipids compared with those in the viable epidermis, based on region of interest analysis (Mann-Whitney test, p < 0.0005). We clarified the precise localization of skin lipids, particularly intercellular lipids. Our findings supported the reported functions of skin lipids at specific sites. Skin lipids are metabolized to form intercellular lipids in the stratum corneum, which are essential for the skin barrier. Our current lipid localization data serve as a baseline, or healthy control dataset, for future MSI-based lipid biomarker research in disease groups.