Non-invasive imaging of intracellular lipid metabolism in macrophages by Raman microscopy in combination with stable isotopic labeling

Monocyte-derived macrophages play a key role in atherogenesis because their transformation into foam cells is responsible for deposition of lipids in plaques within arterial walls. The appearance of cytosolic lipid droplets is a hallmark of macrophage foam cell formation and the molecular basics involved in this process are not well-understood. Of particular interest is the intracellular fate of different individual lipid species, such as fatty acids or cholesterol. Here, we utilize Raman microscopy to image the metabolism of such lipids and to trace their subsequent storage patterns. The combination of microscopic information with Raman spectroscopy provides a powerful molecular imaging method, which allows visualization at the diffraction limit of the employed laser light and biochemical characterization through associated spectral information. In order to distinguish the molecules of interest from other naturally occurring lipids spectroscopically, deuterium labels were introduced. Intracellular distribution and metabolic changes were observed for serum albumin-complexed palmitic and oleic acid, as well as cholesterol and quantitatively evaluated by monitoring the increase in C-D scattering intensities at time points 0.5, 1, 3, 6, 24, 30 and 36 hours. This approach may also allow for investigating the cellular trafficking of other molecules, such as nutrients, metabolites and drugs.