Shedding new light on lipid biology with coherent anti-Stokes Raman scattering microscopy

Abstract

Despite the ubiquitous roles of lipids in biology, the detection of lipids has relied on invasive techniques and population measurements. The molecular profi les of the lipid-rich structures are evaluated by measurements of total lipid extracts with liquid or gas chromatography coupled to mass spectrometry ( 8 ). This approach is inexact because it measures lipid molecules from areas other than the structures of interest. Furthermore, spatial information, which is critical to understanding the function of lipids ( 9 ), is inaccessible with the bulk measurement techniques. To visualize the lipid-rich structures, fl uorescently tagged lipid molecules, which intercalate with the lipidrich structures, are used ( 10, 11 ). Nevertheless, the use of the fl uorescent lipid molecules to label lipid-rich structures is problematic for several reasons. First, to facilitate the transport of the fl uorescent lipid molecules into cells, cell fi xation procedures are often performed ( 12 ). This procedure prevents monitoring the dynamic responses of lipid-rich structures to stimuli or the disease processes. Second, labeling effi ciency is highly dependent on the type of fl uorescent lipid molecules ( 12 ), thus posing a signifi cant problem to the quantitation of lipid-rich structures. Third, the intercalation of fl uorescent lipid molecules can lead to changes in the properties of lipidrich structures. For instance, incorporation of fl uorescent lipid molecules into the cell membrane can induce membrane phase separation and membrane microdomain formation, which can perturb critical cellular processes including signal transduction and endocytosis ( 13 ).As an alternative to fl uorescence, signals from molecular vibration provide an attractive means for label-free chemical imaging. In particular, Raman scattering has been widely used for spectroscopic study of biomolecules ( 14 ). The Raman-scattered photons exhibit frequency Abstract Despite the ubiquitous roles of lipids in biology, the detection of lipids has relied on invasive techniques, population measurements, or nonspecifi c labeling. Such diffi culties can be circumvented by a label-free imaging technique known as coherent anti-Stokes Raman (CARS) microscopy, which is capable of chemically selective, highly sensitive, and high-speed imaging of lipid-rich structures with submicron three-dimensional spatial resolution. We review the broad applications of CARS microscopy to studies of lipid biology in cell cultures, tissue biopsies, and model organisms. Recent technical advances, limitations of the technique, and perspectives are discussed. Lipids play a critical role in human health and diseases. Phospholipids, glycolipids, and sterol lipids are the major components of the cell membrane ( 1 ). Sphingolipids constitute up to 80% of the myelin sheaths, which are the membranous structures that wrap around the axons for insulation and for proper propagation of electrical impulses ( 2 ). Glycerolipids or triglycerides serve as the cytoplasmic energy depots ( 3 ). Bioactive lip...

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