Fluorescent probes for microscopy visualization of cholesterol topography and dynamics in membranes

Abstract: In attempting to interrogate the structural and functional properties of cell membranes with minimal invasiveness, fluorescence microscopy remains the gold-standard technique. Fluorescence is a highly versatile spectroscopic property covering a wide range of the spectrum, with inherent advantages in both the spatial and time domains. Fluorescent probes have been key elements in the constellation of tools used to push the limits of spatial resolution down to single-molecule localization, optimize visualization of delicate biological structures in the living cell, and enable the tracking of molecular motions with high temporal precision. Cholesterol is an essential component of eukaryotic membranes; it interacts with other lipids and proteins and has a modulatory function on many membrane-resident hormone and neurotransmitter receptors. Fluorescent sensors have played an important role in revealing different biological and physicochemical properties of cholesterol. In cell biology applications, fluorescent cholesterol sensors helped determine uptake mechanisms and topography in the plasma membrane and follow the intracellular trafficking of cholesterol. Biophysical investigations have resorted to fluorescent probes to establish orientation, rotational, and translational motions in the membrane, order/disorder, and physical changes brought about by the presence of the sterol in the phospholipid bilayer. The ample spectral range and varying specificities of direct and indirect probes designed to visualize cholesterol enable the experimentalist to study a wide array of cholesterol properties and/or learn about the influence of the sterol on other lipids, membrane proteins, and transient supramolecular assemblies like membrane-resident lipid domains.