Plasma membrane fluidity gradients of human peripheral blood leukocytes

Changes in biophysical parameters of plasma membranes influence cisplatin resistance of sensitive and resistant epidermal carcinoma cells

The mechanism of resistance of cancer cells to the anticancer drug cisplatin is not fully understood. Using cisplatin-sensitive KB-3-1 and -resistant KCP-20 cells, we found that the resistant cells have higher membrane potential, as determined by membrane potential sensing oxonol dye. Electron spin resonance and fluorescence polarization studies revealed that the resistant cells have more "fluid" plasma membranes than the sensitive cells. Because of this observed difference in membrane "fluidity," we attempted modification of the plasma membrane fluidity by the incorporation of heptadecanoic acid into KB-3-1 and KCP-20 cell membranes. We found that such treatment resulted in increased heptadecanoic acid content and increased fluidity in the plasma membranes of both cell types, and also resulted in increased cisplatin resistance in the KCP-20 cells. This finding is in accord with our results, which showed that the cisplatin-resistant KCP-20 cells have more fluid membranes than the cisplatin-sensitive KB-3-1 cells. It remains to be determined whether the observed differences in biophysical status and/or fatty acid composition alone, or the secondary effect of these differences on the structure or function of some transmembrane protein(s), is the reason for increased cisplatin resistance.

Polarization of scatter and fluorescence signals in flow cytometry

BACKGROUND

The pulses of light scatter and fluorescence measured in flow cytometers exhibit varying degrees of polarization. Flow cytometers are heterogeneously sensitive to this polarization, depending on the light source(s), the optical layout, and the types of mirrors and filters used. Therefore, fluorescence polarization can affect apparent intensity ratios between particles and interfere with schemes for interlaboratory standardization.

METHODS

We investigate the degree to which polarization affects common flow cytometry measurements. Our technique for determining polarization differs from previous methods because complete distributions of intensity versus polarization angle are measured, rather than intensities at just two orthogonal polarization angles. Theoretical models for scatter and fluorescence are presented and verified by making polarization measurements of calibration beads.

RESULTS

Measurements of cells stained with a variety of dyes illustrate that fluorescence polarization occurs frequently in flow cytometry.

CONCLUSIONS

Consequences for quantitative cytometry are discussed, and the use of the "magic angle" to make a flow cytometer insensitive to fluorescence polarization is proposed.

Analysis of the fluorescence anisotropy of labelled membranes submitted to a shear stress

Human erythrocyte membranes are elastic and undergo a deformation under shear stress. The phenomenon has been analysed by recording the fluorescence anisotropy of labelled isolated membranes. A model has been developed which assumes an orientation correlation function of a molecular probe incorporated in an elongated membrane. This model has been successfully used to analyse quantitatively data obtained with (1-trimethylamino)-(1,6-diphenyl)-1,3,5-hexatriene (TMA-DPH) and 6-(9-anthroyloxy)-stearic acid (6-AS). In agreement with the model, the effect of the membrane deformation is opposite for these two probes, which corroborates the concept that the alteration of the fluorescence anisotropy reflects mainly the deformation of the membrane and not the rotational freedom of the molecular probe.

Alzheimer's disease and Down syndrome: leukocyte membrane fluidity alterations

Down Syndrome (DS) patients over the age of 40 have brain lesions identical to those of patients with Alzheimer's Disease (AD). We have earlier shown that with some membrane probes, the plasma membranes of circulating leukocytes had increased fluidity in AD compared to the normally more rigid membranes in similarly aged subjects. We next questioned whether the occurrence of AD-like pathological lesions in older DS subjects would be associated with a similar increase in membrane fluidity. Fluidity was assessed by measurements of steady-state fluorescence anisotropy using TMA-DPH, which anchors at the plasma membrane surface, and a series of 9-anthroyloxy fatty acids substituted with the fluorescent moiety at different positions on the fatty acid, which permit measurement of fluidity at different depths of the plasma membrane. This was done simultaneously in neutrophils, lymphocytes, and monocytes utilizing flow cytometry. In older DS subjects (average age 52.6), plasma membrane fluidity was indeed increased, a finding similar to that with AD leukocytes. Membrane fluidity of leukocytes of young DS subjects (average age 23.6 years) was less than that seen in older subjects. Membrane changes may result from lipophilic substances released from the central nervous system, or may reflect intrinsic differences in membrane structure unique in DS.

Membrane structural dynamics of plasma membranes of living human prostatic carcinoma cells differing in metastatic potential

Rigidity of the outer hemileaflet of the plasma membrane of two prostatic carcinoma cell lines with different metastatic potential, 1-LN and 1-LN-EMS-10, was assessed by steady-state anisotropy, using a battery of fluorescent probes. The "bulk" membrane rigidity sensed by diphenylhexatriene, trimethylammonio-DPH, 1-palmitoyl-2-[DPH-ethylcarbonyl]-phosphatidylcholine, and 10-pyrenedecanoic acid indicated slightly higher rigidity in the membrane of the highly metastatic line (1-LN). This was accompanied by 26% greater mole fraction of cholesterol and 9% lower phospholipid, resulting in 40% greater cholesterol/phospholipid ratio. Phosphatidylethanolamine was increased 12%, but corresponding decreases in phosphatidylserine and phosphatidylinositol resulted in no significant change in molar ratio of choline/noncholine phospholipids. Whereas unsaturation index was slightly higher in 1-LN, fatty acids of 1-LN plasma membranes contained 15% more 18:1, 43% more 20:4, 26% more 22:4, and 38% less 18:2. Anisotropy gradients were determined for the two cell lines using a series of n-(9-anthroyloxy) fatty acid probes with n = 2, 3, 6, 7, 9, 12, and 16. Gradients differed only in position of anisotropy maxima, which occurred with n = 6, in 1-LN, and n = 7, in 1-LN-EMS-10. Possible relationships between observed anisotropy gradients and differences in membrane cholesterol and fatty acid composition are discussed.

Comparisons of steady-state anisotropy of the plasma membrane of living cells with different probes

We have used an extended Perrin equation which was in agreement with literature data for steady-state anisotropy (rSS) for a wide variety of artificial and isolated biological membranes labeled with various probes (Van der Meer et al. (1986) Biochim. Biophys. Acta 854, 38-44 to obtain the static component (r infinity) for the intact plasma membranes of living cells. We show that lipid structural order parameters can be obtained for DPH and TMA-DPH in the plasma membranes of intact cells. We have examined the relationship between 'fractional limiting hindered anisotropy', r infinity/r0, which is related to the lipid structural order parameter, of DPH, TMA-DPH, DPHpPC, and a series of depth-dependent probes (n-(9-anthroyloxy) fatty acids, with n = 2-16), using data from 19 cell types. There was a linear relationship between r infinity/r0 values of DPH and TMA-DPH, but the relationship between either of these probes was non-linear with respect to DPHpPC or the series of fatty acid probes. The relationship between r infinity/r0 values of DPHpPC and the series of fatty acid probes was linear, suggesting that they not only undergo similar motions in the membrane, but also experience similar types of restriction to motion, a type which is different from that experienced by DPH and TMA-DPH. We show that for the plasma membranes of living cells, 'second degree' order parameters can be estimated for DPH and TMA-DPH, and propose that the parameter r infinity/r0, or the 'fractional limiting hindered anisotropy', analogous to a 'first degree' order parameter, can be estimated for DPHpPC and the depth-dependent fatty acid probes to evaluate the density of membrane packing.

Altered membrane anisotropy gradients of plasma membranes of living peripheral blood leukocytes in aging and Alzheimer's disease

Several reports have suggested that membrane rigidity, a term that refers to the relative motion of membrane constituents, is decreased in Alzheimer's Disease. Accordingly, a series of fluorescent membrane probes was used to evaluate the rigidity from the surface to the center of the outer hemi-leaflet of the plasma membrane of living neutrophils, monocytes and lymphocytes. Anisotropy, a parameter which increases with increasing membrane rigidity, was calculated from flow cytometric measurements of vertically and horizontally polarized components of the fluorescence emission of the probes. These preliminary experiments suggest that whereas membrane rigidity in certain regions of the plasma membrane of peripheral blood leukocytes is increased as expected in elderly controls, it is decreased in Alzheimer's disease.

Fluorescence quenching of a series of membrane probes measured in living cells by flow cytometry

The transverse location normal to the bilayer surface of a series of n-(9-anthroyloxy) fatty acid probes, where n = 2, 3, 6, 7, 9, 12, and 16, was determined by fluorescence quenching measurements with a flow cytometer. We show that the anthroyloxy moieties of the probes locate at a graded series of depths in the outer leaflet of the plasma membrane of living HeLa cells, in a manner similar to that previously observed for model membrane systems, and mitochondria. For different n, the efficiency of quenching with an aqueous phase quencher, Cu+2, was 2 greater than or equal to 3 greater than 6 greater than or equal to 7 greater than 9 greater than 12 greater than 16. Therefore, flow cytometry permits use of these probes for measurements of dynamic parameters related to membrane fluidity at different depths in the plasma membranes of living cells.