Laurdan fluorescence lifetime discriminates cholesterol content from changes in fluidity in living cell membranes

Abstract

Detection of the fluorescent properties of Laurdan has been proven to be an efficient tool to investigate membrane packing and ordered lipid phases in model membranes and living cells. Traditionally the spectral shift of Laurdan's emission from blue in the ordered lipid phase of the membrane (more rigid) toward green in the disordered lipid phase (more fluid) is quantified by the generalized polarization function. Here, we investigate the fluorescence lifetime of Laurdan at two different emission wavelengths and find that when the dipolar relaxation of Laurdan's emission is spectrally isolated, analysis of the fluorescence decay can distinguish changes in membrane fluidity from changes in cholesterol content. Using the phasor representation to analyze changes in Laurdan's fluorescence lifetime we obtain two different phasor trajectories for changes in polarity versus changes in cholesterol content. This gives us the ability to resolve in vivo membranes with different properties such as water content and cholesterol content and thus perform a more comprehensive analysis of cell membrane heterogeneity. We demonstrate this analysis in NIH3T3 cells using Laurdan as a biosensor to monitor changes in the membrane water content during cell migration.

Figure 1

Laurdan GP analysis. (A) Fluorescence-intensity images of NIH3T3 cells at different z planes 1 μm apart (from the ventral region to the dorsal region) observed in the blue channel (460/80 nm). (B) GP images. The GP scale used to pseudocolor the intensity image is shown at right. (C) Binary GP image: the GP histogram from the corresponding image in B is decomposed into two Gaussian components. (D) Pixels belonging to the low-GP component (green Gaussian) are green; pixels belonging to the high-GP component (red Gaussian) are red.

Figure 2

Laurdan decay transformed into a phasor plot for solution and POPC vesicle experiments detected at two wavelength ranges, 460/80 nm (A and B) and 540/50 nm (C, D, and F). In the blue channel (A and B), the phasor position is determined by polarity effects, whereas in the green channel (C, D, and F), dipolar relaxations determine the phasor position, which is outside the universal circle. Laurdan phasor is measured in pure 1-butanol (A) and glycerol (C) with increasing amounts of nanopure water. Water causes a shortening of Laurdan lifetime. In B and D, the effect of adding cholesterol to a solution of 30% water in glycerol shows that at 460 nm the phasor distribution moves toward a longer lifetime, whereas at 540 nm, the addition of cholesterol increases dipolar relaxations, as shown by the shift of the phasor distribution toward the outside of the universal circle. (E) In POPC vesicles, addition of cholesterol also moves the phasor outside of the universal circle but also toward longer lifetimes.

Figure 3

Influence of cholesterol on decay of Laurdan in the blue wavelength range. Phasor distribution of Laurdan in NIH3T3 cells detected through a 460/80-nm filter (blue filter) before (A) and after (B) treatment with 1 and 5 mM M-β-CD. (C) NIH3T3 cells pseudocolored according to the palette defined in A, where green pixels correspond to the average phasor position (green cursor), and yellow and red pixels correspond to movement of the phasor distribution toward the left or right, respectively, upon M-β-CD treatment. (D) The NIH3T3 cells of C after treatment with M-β-CD pseudocolored according to the same palette. Pixels within the plasma membrane move toward the lower-polarity region, whereas pixels corresponding to the internal membranes move in the direction of increased polarity.

Figure 4

Influence of cholesterol on decay of Laurdan in the green wavelength range. Phasor distribution of Laurdan in NIH3T3 cells detected through a 540/40-nm filter (green filter) before (A) and after (B) treatment with 1 and 5 mM M-β-CD. Before treatment (A), the phasor distribution is completely outside of the universal circle, indicating that the emission decay is dominated by dipolar relaxations. After treatment with M-β-CD (B), the phasor distribution is moved inside the universal circle. (C) NIH3T3 cells pseudocolored according to the palette defined in A. (D) The NIH3T3 cells of C after treatment with M-β-CD pseudocolored according to the palette defined in B, where the two cursors in A are simply rotated to reflect the change in dipolar relaxation. After cholesterol removal, the two cursors select the plasma membrane (red cursor) and the internal membranes (green cursor).

Figure 5

Gradual depletion of cholesterol from a single cell monitored by decay of Laurdan in the green wavelength range. (A) Time series of the fluorescence-intensity images of a group of NIH3T3 cells observed in the green channel (540/40 nm), upon treatment with 5 mM M-β-CD. (B and C) Lifetime images of the time series depicted in A, pseudocolored according to the two different palettes defined in D, which highlight the high-cholesterol environment (B) and the low-cholesterol environment (C). The difference between the two palettes is a small rotation in the placement of the red and green cursors, to reflect the change in dipolar relaxation. As can be seen with increasing time after addition of 5 mM M-β-CD, more pixels are highlighted in the low-cholesterol environment. (E) Quantification of this effect, where we plot the fractions of pixels in the high- and low-cholesterol environments as a function of time during M-β-CD treatment.

Figure 6

Monitoring fluidity changes in NIH3T3 cells after EGF stimulation by using Laurdan phasor distribution in the two detection wavelength ranges. (A) Intensity images collected in the blue channel before and after EGF stimulation; arrows point to the retracting tail and the leading edge of the cell, respectively. The fluorescence signal due to free Laurdan outside the cell (i.e., indicated by the white circle) is easily distinguishable and it can be ignored in our analysis. (B) FLIM images collected in the blue channel (460/80 nm): we highlight the fluidity/polarity changes using a color scale from red (rigid) to green (fluid). To obtain this color scale, in the phasor plot (C), the points lying on the line connecting the red and the green circle are used to color the corresponding pixels in FLIM images. (D) Intensity images collected in the green channel before and after EGF stimulation; arrows point to the retracting tail and the leading edge of the cell, respectively. (E–G) FLIM images collected in the green channel (540/50 nm). Pixels are first selected according to their position along the low- (E) or high-cholesterol content (F) and then colored according to a fluidity scale shown in the phasor plots (G). As the cell responds to stimulation, the retracting tail, which is characterized by low cholesterol content, becomes more rigid. The leading edge, which is characterized by higher cholesterol content, becomes more rigid.