Cholesterol dependence of cell membrane dynamics

Membranes of living cells are characterized by laser-assisted fluorescence microscopy, in particular a combination of microspectrofluorometry, total internal reflection fluorescence microscopy (TIRFM) and fluorescence decay kinetics. The generalized polarization (GP, characterizing a spectral shift which depends on the phase of membrane lipids), the time constant of fluorescence anisotropy (tau(r)) as well as the fluorescence lifetime (tau) of the membrane marker laurdan revealed to be appropriate measures for membrane stiffness and fluidity. GP decreased with increasing temperature and was always higher for the plasma membrane than for intracellular membranes. The latter effect was correlated with the intracellular content of cholesterol, which could be modified using defined protocols of depletion or enrichment. Concomitant with generalized polarization the fluorescence lifetime tau increased with the content of cholesterol. Changes of cholesterol amounts in cell membranes have previously been related to specific diseases and may have some influence on the uptake of pharmaceutical agents.