Fluorescence generalized polarization of cell membranes: a two-photon scanning microscopy approach

We use the lipophilic fluorescence probe Laurdan to study cell membranes. The generalized polarization (GP) of Laurdan-labeled cells contains useful information about membrane fluidity and polarity. A high GP is usually associated with low fluidity, low polarity, or high cholesterol content of the membranes, and a low GP is the opposite. We have combined the GP method and two-photon fluorescence microscopy to provide an alternative approach to study cell membranes. Using two-photon excitation in a conventional microscope offers great advantages for studying biological samples. These advantages include efficient background rejection, low photodamage, and improved depth discrimination. We performed GP measurements on mouse fibroblast cells and observed that both intensity and GP images are not spatially uniform. We tested for possible GP artifacts arising from cellular autofluorescence and lifetime quenching, using a procedure for background fluorescence subtraction and by direct lifetime measurements in the microscope. GP measured in a single cell displays a broad distribution, and the GP of 40 different cells grown on the same cover glass is also statistically distributed. The correlations between intensity and GP images were analyzed, and no monotonic dependence between the two was found. By digitally separating high and low GP values, we found that high GP values often associate with the regions of the plasma membrane and low GP values link with the nuclear membranes. Our results also show local GP variations within the plasma and nuclear membranes.