Look Beyond Plasma Membrane Biophysics: Revealing Considerable Variability of the Dipole Potential Between Plasma and Organelle Membranes of Living Cells.

Due to the lack of measurement techniques suitable for examining compartments of intact, living cells, membrane biophysics is almost exclusively investigated in the plasma membrane despite the fact that its alterations in intracellular organelles may also contribute to disease pathogenesis. Here, we employ a novel, easy-to-use, confocal microscopy-based approach utilizing F66, an environment-sensitive fluorophore in combination with fluorescent organelle markers and quantitative image analysis to determine the magnitude of the molecular order-related dipole potential in the plasma membrane and intracellular organelles of various tumor and neural cell lines. Our comparative analysis demonstrates considerable intracellular variations of the dipole potential that may be large enough to modulate protein functions, with an inward decreasing gradient on the route of the secretory/endocytic pathway (plasma membrane >> lysosome > Golgi > endoplasmic reticulum), whereas mitochondrial membranes are characterized by a dipole potential slightly larger than that of lysosomes. Our approach is suitable and sensitive enough to quantify membrane biophysical properties selectively in intracellular compartments and their comparative analysis in intact, living cells, and, therefore, to identify the affected organelles and potential therapeutic targets in diseases associated with alterations in membrane lipid composition and thus biophysics such as tumors, metabolic, neurodegenerative, or lysosomal storage disorders.