Laurdan is a fluorescent probe that detects changes in membrane phase properties through its sensitivity to the polarity of its environment in the bilayer. Variations in membrane water content cause shifts in the laurdan emission spectrum, which are quantified by calculating the generalized polarization (GP). We tested whether laurdan fluorescence could be used to distinguish differences in phospholipid order from changes in membrane fluidity by examining the temperature dependence of laurdan GP and fluorescence anisotropy in dipalmitoylphosphatidylcholine (DPPC) vesicles. The phase transition from the solid ordered phase to the liquid disordered phase was observed as a decrease in laurdan GP values from 0.7 to -0.14 and a reduction in anisotropy from 0.25 to 0.12. Inclusion of various amounts of cholesterol in the membranes to generate a liquid ordered phase caused an increase in the apparent melting temperature detected by laurdan GP. In contrast, cholesterol decreased the apparent melting temperature estimated from anisotropy measurements. Based on these results, it appeared that laurdan anisotropy detected changes in membrane fluidity while laurdan GP sensed changes in phospholipid order. Thus, the same fluorescent probe can be used to distinguish effects of perturbations on membrane order and fluidity by comparing the results of fluorescence emission and anisotropy measurements.

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Normally, cell membranes resist hydrolysis by secretory phospholipase A 2 . However, upon elevation of intracellular calcium, the cells become susceptible. Previous investigations demonstrated a possible relationship between changes in lipid order caused by increased calcium and susceptibility to phospholipase A 2 . To further explore this relationship, we used temperature as an experimental means of manipulating membrane physical properties. We then compared the response of human erythrocytes to calcium ionophore at various temperatures in the range of 20-50 °C using fluorescence spectroscopy and two-photon fluorescence microscopy. The steady state fluorescence emission of the environmentsensitive probe, laurdan, revealed that erythrocyte membrane order decreases systematically with temperature throughout this range, especially between 28 and 45 °C. Furthermore, the ability of calcium ionophore to induce increased membrane order and susceptibility to phospholipase A 2 depended similarly on temperature. Both responses to calcium influx were enhanced as membrane fluidity increased. Analysis of the spatial distribution of laurdan fluorescence at several temperatures indicated that the ordering effect of intracellular calcium on fluid membranes generates an increase in the number of fluid-solid boundaries. Hydrolysis of the membrane appeared to progress outward from these boundaries. We conclude that phospholipase A 2 prefers to hydrolyze lipids in fluid regions of human erythrocyte membranes, but primarily when those regions coexist with domains of ordered lipids.

Exposure of human erythrocytes to elevated intracellular calcium causes fragments of the cell membrane to be shed as microvesicles. This study tested the hypothesis that microvesicle release depends on microscopic membrane physical properties such as lipid order, fluidity, and composition. Membrane properties were manipulated by varying the experimental temperature, membrane cholesterol content, and the activity of the trans-membrane phospholipid transporter, scramblase. Microvesicle release was enhanced by increasing the experimental temperature. Reduction in membrane cholesterol content by treatment with methyl-beta-cyclodextrin also facilitated vesicle shedding. Inhibition of scramblase with R5421 impaired vesicle release. These data were interpreted in the context of membrane characteristics assessed previously by fluorescence spectroscopy with environment-sensitive probes such as laurdan, diphenylhexatriene, and merocyanine 540. The observations supported the following conclusions: 1) calcium-induced microvesicle shedding in erythrocytes relates more to membrane properties detected by diphenylhexatriene than by the other probes; 2) loss of trans-membrane phospholipid asymmetry is required for microvesicle release.PACS Codes: 87.16.dj, 87.16.dt.

Accumulating evidence implicates the voltage-dependent anion channel (VDAC) as functioning in mitochondria-mediated apoptosis involving cytochrome c release, leading to caspases activation and apoptosis. The mechanisms regulating cytochrome c release and the molecular architecture of the cytochrome c conducting channel remain unknown. Previously, we demonstrated that apoptosis induction was accompanied by VDAC oligomerization, as revealed by cross-linking and directly monitored in living cells using Bioluminescence Resonance Energy Transfer technology. Moreover, apoptosis inhibitors inhibited VDAC oligomerization and a correlation between the levels of VDAC oligomerization and apoptosis was observed. Here, we combined sitedirected mutagenesis with chemical cross-linking to reveal the contact sites between VDAC1 molecules in dimers and higher oligomers. Replacing hydrophobic amino acids with charged amino acids in b-strands 1,2 and 19, but not 14, interfered with VDAC1 oligomerization and apoptosis induction. Cysteine cross-linking results, from introducing cysteine at a defined position in cysteineless VDAC1 and applying the cysteine-specific cross-linker, BMOE, supported the close vicinity of b-strands 1,2 and 19 in VDAC1 dimer. Moreover, the results suggest that VDAC1 exists as a dimer that undergoes conformational changes upon apoptosis induction to assemble into a higher oligomeric state. Additionally we demonstrated that the N-terminal region of VDAC1 lies inside the pore, but could also move and interact with the N-terminus from a second molecule to form a dimer. Our results suggest that the glycine rich sequence 21-GYGFG-25 is involved in the N-terminus translocation from the internal pore to the channel face. These results provide structural insight into cellular VDAC1's oligomeric state and its N-terminal region location and translocation.

Normally, human erythrocytes display several responses to elevated intracellular calcium levels. These include a shape transition from discocyte to spherocyte, shedding of microvesicles into the extracellular fluid, and enhanced susceptibility to the hydrolytic action of secretory phospholipase A 2 . These responses to elevated intracellular calcium were all blunted in erythrocytes containing hemoglobin S. The reduction of both the shape transition and the shedding of microvesicles were greater than the impairment of phospholipase susceptibility, and both correlated strongly with the intracellular content of hemoglobin S. In contrast to the response to elevated intracellular calcium, erythrocytes containing hemoglobin S displayed a 2.5-fold increase in basal susceptibility to phospholipase A 2 compared to control erythrocytes in the absence of ionophore. The effect was more prominent among samples from patients heterozygous for hemoglobin S than in samples from homozygous individuals. These results reveal additional abnormalities in the membranes of sickle cell erythrocytes beyond those described previously and demonstrate that red blood cells from both heterozygous and homozygous are affected. Furthermore, they suggest a possible means by which sickle cell disease and trait patients may display enhanced vulnerability to inflammatory stimuli. Am. J. Hematol. 72:162-169, 2003.