Rapid alterations in lateral mobility of lipids in the plasma membrane of activated human megakaryocytes

A new look at the lipid composition of the plasma membrane of human blood platelets relative to the GPIIb/IIIa (integrin cxIIβ3) content

The total lipids of the human platelet plasma membrane (HPPM) from 50 ml of blood of healthy subjects were extracted, quantified and related to the mass content of the major intrinsic membrane protein, the glycoprotein IIb/IIIa (GPIIb/IIIa). The HPPM total lipid/GPIIb/IIIa weight ratio determined was 5.40 ± 0.20, independently of the membrane washing procedure used, with the cholesterol/GPIIb/IIIa and phospholipid/GPIIb/IIIa molar ratios of 800 ± 50 and 1200 ± 40, respectively. If the distribution of lipids around each intrinsic protein were proportional to its mass, the lipids around a molecule of GPIIb/IIIa will occupy about 120 nm(2) of the membrane plane, which is about one and a half times the cross-sectional area of the extracellular head of GPIIb/IIIa, as estimated by electron microscopy. The lipid extracts were further subjected to thin-layer chromatography to separate and quantify the different phospholipid fractions, the free fatty acids and the neutral lipid fraction and the distribution of fatty acids in each fraction was determined by gas chromatography after methanolysis. The phospholipid molar distribution was SPM(22.3 ± 0.9%), PC(36.2 ± 1.0%), PE(24.9 ± 0.9%), PS(12.1 ± 0.6%) and PI(4.5 ± 0.4%) and the free fatty acid fraction represented 2.9 ± 0.4% of the total fatty acids in HPPM. The fatty acid chain length ranged from 14 to 24 carbons, comprising unsaturated fatty acids (47.3% molar per cent of the total) of which 40.7 ± 2.0% were monosaturated and 40.7 ± 0.9% tetraunsaturated. Palmitic, stearic, oleic and arachidonic acids represent 66% of the total fatty acids of HPPM, being: 68.9 ± 5.3% of palmitic acid and 63.3 ± 6.9% of oleic acid in PC; 50.9 ± 3.8% of arachidonic acid in PE; and 30.5 ± 2.4% of stearic acid in PS. We discuss the methodological modifications and the new data in relation with the major differences in HPPM lipid composition found in the literature. The data obtained provides a comprehensive and accurate description of the lipid composition of HPPM on which to rely as a reference for basic and medical studies.

Cholesterol modulation of nicotinic acetylcholine receptor surface mobility

Nicotinic acetylcholine receptor (AChR) function and distribution are quite sensitive to cholesterol (Chol) levels in the plasma membrane (reviewed by Barrantes in J Neurochem 103 (suppl 1):72-80, 2007). Here we combined confocal fluorescence recovery after photobleaching (FRAP) and confocal fluorescence correlation spectroscopy (FCS) to examine the mobility of the AChR and its dependence on Chol content at the cell surface of a mammalian cell line. Plasma membrane AChR exhibited limited mobility and only ~55% of the fluorescence was recovered within 10 min after photobleaching. Depletion of membrane Chol by methyl-beta-cyclodextrin strongly affected the mobility of the AChR at the plasma membrane; the fraction of mobile AChR fell from 55 to 20% in Chol-depleted cells, whereas Chol enrichment by methyl-beta-cyclodextrin-Chol treatment did not reduce receptor mobility at the cell surface. Actin depolymerization caused by latrunculin A partially restored receptor mobility in Chol-depleted cells. In agreement with the FRAP data, scanning FCS experiments showed that the diffusion coefficient of the AChR was about 30% lower upon Chol depletion. Taken together, these results suggest that membrane Chol modulates AChR mobility at the plasma membrane through a Chol-dependent mechanism sensitive to cortical actin.

Relation between membrane fluidity and signal transduction in the human megakaryoblastic cell line MEG-01

The fluidity of the plasma membrane is thought to affect the responsiveness of blood platelets. We measured membrane fluidity in a single cell by Fluorescence Recovery after Photobleaching (FRAP) of the lipophilic probe DiIC14. Since platelets are too small for this technique, we used the human megakaryoblastic cell-line MEG-01, which shares many properties with platelets. MEG-01 cells were cultured for 44 h with simvastatin or mevalonate to change the cholesterol content, enabling analysis of signal processing at cholesterol/phospholipid ratios (C/P) between 0.20 and 0.31. The diffusion of DiIC14 correlated inversely with the C/P ratio with lateral diffusion coefficients (D) of 3.28 x 10(-9) cm2/s at a low C/P decreasing to 2.55 x 10(-9) cm2/s at a high C/P ratio. The mobile fraction was 65% and constant at the different C/P ratios. The relation between lipid diffusion and signal processing was measured following stimulation with 10 U/ml thrombin at 22 degrees C. There were only little differences in phosphatidylinositol metabolism, Ca2+ influx or mobilization and prostaglandin I2-induced formation of cyclic AMP. At 37 degrees C, cells with a high C/P ratio showed increased phosphatidylinositol metabolism, but these differences had no major effect on the Ca2+ responses. These data demonstrate that in megakaryoblasts the lateral diffusion of lipids is inversely correlated with the C/P ratio, but within the range of 0.20-0.31 the influence on signal processing is minor.

Cytosolic calcium ions regulate lipid mobility in the plasma membrane of the human megakaryoblastic cell line MEG-01

The fluidity of the plasma membrane is thought to play a role in the activation of blood platelets. We investigated the lateral diffusion of the lipophilic probe 1,1'-ditetradecyl-3,3,3',3'-tetramethyl-indocarbocyanine perchlorate (DiIC14) and derivatives in the plasma membrane of the megakaryoblast MEG-01 by fluorescence recovery after photobleaching. The lateral diffusion coefficient (D) of DiIC14 in an unstimulated cell was (3.53 +/- 0.06) x 10(-9) cm2/s with a mobile fraction of 75%. Similar data were found with DiIC12 and DiIC18, but lipophilic probes specific for the outer leaflet showed a slower diffusion with a D value of (2.99 +/- 0.31) x 10(-9) cm2/s and a mobile fraction of 58%. Stimulation with platelet-activating agents decreased the diffusion of DiIC14 within 2 min, but left the mobile fraction unchanged. Signal processing was required for the decrease in D as D-Phenylalanyl-L-prolyl-L-arginyl-chloromethane-treated thrombin, which binds normally to the thrombin receptor but fails to activate the cell, had no effect. The decrease in D was accompanied by an increase in cytosolic Ca2+ content, [Ca2+]i, and studies using different concentrations of thrombin, the Ca2+ chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid acetoxymethylester and the Ca2+ ionophore ionomycin revealed that lipid mobilty in the plasma membrane is regulated by Ca2+. In contrast, treatments thought to interfere with the mobility of membrane proteins had little effect. We conclude that the rigidification of the plasma membrane during cell activation is caused by an increase in [Ca2+]i and is therefore a late event and might only contribute to signal transduction at steps downstream of the mobilization/influx of Ca2+.