Membrane dynamic alterations associated with activation of human platelets by thrombin

Evidence for a physiological role for membrane rafts in human platelets

We have investigated raft formation in human platelets in response to cell activation. Lipid phase separation and domain formation were detected using the fluorescent dye 1,1'-dioctadecyl-3,3,3',3'-tetramethyl-indocarbocyanine perchlorate (diI-C(18)) that preferentially partitions into gel-like lipid domains. We showed that when human platelets are activated by cold and physiological agonists, rafts coalesce into visible aggregates. These events were disrupted by depletion of membrane cholesterol. Using Fourier transform infrared spectroscopy (FTIR), we measured a thermal phase transition at around 30 degrees C in intact platelets, which we have assigned as the liquid-ordered to the liquid-disordered phase transition of rafts. Phase separation of the phospholipid and the sphingomyelin-enriched rafts could be observed as two phase transitions at around 15 and 30 degrees C, respectively. The higher transition, assigned to the rafts, was greatly enhanced with removal of membrane cholesterol. Detergent-resistant membranes (DRMs) were enriched in cholesterol (50%) and sphingomyelin (20%). The multi-functional platelet receptor CD36 selectively partitioned into DRMs, whereas the GPI-linked protein CD55 and the major platelet integrin alpha(IIb)beta(3a) did not, which suggests that the clustering of proteins within rafts is a regulated process dependent on specific lipid protein interactions. We suggest that raft aggregation is a dynamic, reversible physiological event triggered by cell activation.

Platelet membrane fluidity and intraplatelet Ca2+ mobilization are affected in uraemia

In present investigations, platelet membrane fluidity and intraplatelet Ca2+ mobilization were analysed in uraemic platelets by fluorescence techniques. Thirteen non-dialyzed uraemic patients and 16 control subjects were examined. Anisotropy of DPH-probe, measured at 37 degrees C, was significantly higher in control (0.2236 +/- 0.0050) than in uraemic platelets (0.1969 +/- 0.0082; p < 0.01). There was no difference between control (109.8 +/- 6.0 nM) and uraemic platelets (100.0 +/- 7.3 nM) when the basal [Ca2+]i in resting platelets was determined. Activation of platelets by ADP (12.5 microM) or by thrombin (0.1 U/ml) resulted in an increase in [Ca2+]i. It was significantly higher (p* < 0.003 for ADP and p* < 0.009 for thrombin, respectively) in control platelets (383.6 +/- 56.3 nM and 2031.0 +/- 298.8 nM, respectively) than in uraemic ones (191.0 +/- 21.3 nM and 838.7 +/- 144.1 nM, respectively). The amount of released Ca2+ was higher in control platelets activated by both ADP and thrombin (157.6 +/- 21.4 nM and 409.3 +/- 71.0 nM, respectively) than in uraemic platelets (76.7 +/- 15.7 nM and 203.0 +/- 29.3 nM, respectively) and the differences were significant (p < 0.01 and p* < 0.01, respectively). These results indicate an abnormal intracellular Ca2+ mobilization in uraemic platelets. Both increased membrane fluidity and decreased Ca2+ mobilization should be considered as a possible reason of reduced fibrinogen receptor exposure on uraemic platelets.

Molecular order and fluidity of the plasma membrane of human platelets from time-resolved fluorescence depolarization

The ability of seven fluorescence polarization probes (1,6-diphenyl-1,3,5-hexatriene, 1-[(4-trimethyl- amino)phenyl]-6-phenyl-1,3,5-hexatriene, (2-carboxy- ethyl)-1,6-diphenyl-1,3,5-hexatriene, 16(9-anthroyloxy)-palmitic acid, CIS-parinaric acid, trans-parinaric acid and perylene) to report changes induced by temperature and Ca2+ in the plasma membrane of human platelets has been examined. The steady-state fluorescence anisotropy of the probes was compared after being incorporated into whole resting platelets, fragments of platelet plasma membrane and multilayers of lipids extracted from these membranes. In addition, we have investigated the molecular order and dynamics of the three preparations by time-resolved fluorescence depolarization of DPH and CE-DPH as a function of temperature and Ca2+ concentration. The high values of the order parameters found in intact platelets (SDPH, 36 degrees C = 0.70) were almost identical to those in membrane fragments and lipid vesicles, suggesting that lipid-lipid interactions and, therefore, the lipid composition are the main factors influencing the probe order parameter. Other lipid interactions such as those with membrane proteins and intracellular components have little effect on the SDPH in platelets. These measurements also showed that the stationary fluorescence anisotropy of DPH and CE-DPH in platelets is largely determined (80%) by the structural order of the lipid bilayer. Therefore, the previous "microviscosity" values based on stationary anisotropy data reflect the alignment and packing rather than the mobility of the bilayer components. The dynamic component of the anisotropy decay of these probes was analyzed in terms of the wobbling-in-cone model, allowing an estimation of the apparent viscosity of platelet plasma membrane (eta DPH, 36 degrees C = 0.5 P) that is similar to that of the erythrocyte membrane. This value decreased substantially in multilayers of native lipids, indicating a large effect of the lipid-protein interactions on the probe dynamics within the bilayer. When the temperature was raised from 25 degrees to 36 degrees C a pronounced decrease was observed in the order parameter and apparent viscosity, followed by a tendency to level-off in the 36 degrees-40 degrees C interval. This may be related to the end-point of the lipid phase separation reported by Gordon et al. (1983). Finally, the rigidifying (lipid ordering) effect of Ca2+ on the platelet plasma membrane could also be observed by the fluorescence anisotropy measurements, in the form of an increase (approximately 2%) of the order parameter of CE-DPH for Ca2+ concentrations in the millimolar range.

Parallel investigation of exocytosis kinetics and membrane fluidity changes in human platelets with the fluorescent probe, trimethylammonio-diphenylhexatriene

A simple, flexible and sensitive fluorescence method is described, which, from the same experiment, provides coupled quantitative informations on membrane fluidity changes and exocytosis, and reliable kinetic analyses of these effects, in intact cell suspensions. The method is based on the features peculiar to trimethylammonio-diphenylhexatriene (TMA-DPH), a fluorescent hydrophobic probe, which, in intact cells, is incorporated specifically into the plasma membranes, according to an instantaneous partition equilibrium. The method was tested on human platelets upon stimulation with various agents, such as human alpha-thrombin, adenosine diphosphate (ADP), adrenaline and ionomycin, which act through different types of mechanism. The experimental conditions were chosen to allow platelet shape change and exocytosis, but no aggregation. The kinetics and the dose-dependence of the changes in TMA-DPH fluorescence intensity and anisotropy were compared to the simultaneous physiological responses of platelets to the same stimuli, under the same conditions. Quantitative correlations were established between serotonin secretion and the increase in fluorescence intensity, whereas fluorescence anisotropy, which monitors membrane fluidity changes was associated with platelet shape change. The specificity of the effects was confirmed with appropriate antagonistic or modulating agents.

Functionally thrombasthenic state in normal platelets following the administration of ticlopidine

To elucidate the bleeding tendency that follows the administration of ticlopidine, we investigated the skin bleeding time and some ex vivo functions of platelets obtained from eight healthy volunteers before and 1 wk after daily administration of 500 mg of ticlopidine. We found the following: ticlopidine significantly (P less than 0.001) prolonged the skin bleeding time and impaired the binding of radiolabeled fibrinogen and von Willebrand Factor, the clot retraction and the aggregation of platelets in response to ADP, epinephrine, thrombin, ionophore A23187, collagen, or arachidonic acid. In contrast, the administration of this drug did not affect intraplatelet levels of cAMP, agglutination and binding of von Willebrand Factor in response to ristocetin, shape change in response to ADP, collagen, thrombin, or arachidonic acid, or binding of prostaglandin E1 to resting platelets. Secretion of ATP in response to ADP or epinephrine was completely inhibited, whereas secretion as well as thromboxane synthesis in response to high concentrations of collagen, arachidonic acid, calcium ionophore A23187, or thrombin was unaffected. Studies with monoclonal antibodies showed that the glycoprotein IIb-IIIa complex (the putative receptor for fibrinogen and von Willebrand Factor on the surface of platelets exposed to naturally occurring aggregating agents) was quantitatively unaffected by ticlopidine. This observation was further confirmed by densitometric scannings of Periodic Acid-Schiff-stained gels of platelet suspensions. The onset, as well as the cessation of the inhibitory effect of ticlopidine on platelets was very slow, and reached a maximum after a 3-5-d administration. In addition, ticlopidine appeared to be a much more potent inhibitor when administered to subjects than when added in vitro to platelets. Finally, abnormalities comparable to those found in volunteers taking ticlopidine were observed when platelets from untreated subjects were incubated in the plasma of ticlopidine-treated subjects. We conclude that ticlopidine induces a thrombasthenic state in normal platelets without affecting the glycoprotein IIb-IIIa complex quantitatively. Furthermore, our data suggest that one or more active metabolites rather than the native drug mediate the abnormalities of platelet function observed in ticlopidine-treated subjects.

Change in the physical state of platelet plasma membranes upon ionophore A23187 activation. A fluorescence polarization study

Human platelets were isolated and fluorescence-labelled by 1,6-diphenylhexatriene. Diphenylhexatriene was essentially localized in the plasma membrane, as indicated by trinitrobenzenesulfonate-quenching experiments. A decrease of the fluorescence polarization of diphenylhexatriene was observed upon ionophore A23187 addition in the absence of aggregation. 0.3 microM ionophore allowed to reach the maximum rate of the decrease of fluorescence polarization; it also maximally stimulated the light transmission change, the serotonin release and the thromboxane B2 synthesis. The amplitude of the fluorescence polarization decrease was maximum at platelet concentrations between 4 X 10(7) and 7 X 10(7)/ml. The presence of Ca2+ in the medium increased the rate constant of the polarization change. Chlorpromazine (60 microM) completely inhibited this transition, but at 30 microM its inhibitory effect was reversed by Ca2+. The membrane events implied in platelet activation very likely lead to fluidization of the plasma membrane, perhaps by its fusion with the membranes of internal granules which are relatively depleted of cholesterol. Ca2+ plays a central role in the triggering of the observed effects at the membrane level.

Changes in the distribution of platelet membrane proteins revealed by energy transfer

The redistribution of platelet membrane proteins in response to platelet activation was studied. To investigate this process we prepared a variety of platelet ligands, including di- and tetrameric concanavalin A, IgG, thrombin, wheat-germ agglutinin and other lectins. These ligands were conjugated either with acceptor (rhodamine isothiocyanate) or donor (fluoresceine isothiocyanate) fluorophore. Platelets exposed to various combinations of ligand species were stimulated with different aggregating agents, and changes in sensitized fluorescence emission or donor quenching were recorded. Energy transfer was observed with thrombin, dimeric concanavalin A after addition of thrombin and various combinations of dimeric concanavalin A with other membrane ligands. The preincubation of platelets with colchicine prevented energy transfer between appropriate ligand pairs and platelet activator. Our studies show that nonradiative energy transfer can be used to analyze redistribution of membrane receptor sites in platelets.

Cell membrane changes after in vivo acute Na+ load in normotensive and spontaneously hypertensive rats

Our previous observation of a greater increase in erythrocyte Na+ in SHR than in WKY after an acute Na+ load may result either from a genetic membrane property or from a specific plasma influence. In order to elucidate this question, membrane characteristics were compared with or without an acute Na+ load. Na+ transport was measured in Ringer and in plasma on Na+ enriched and K+ depleted red cells. Platelet microviscosity was measured as an index of membrane structural changes. After acute Na+ load a similar reduction of net Na+ extrusion and of K+ influx was observed in both strains. This indicates an inhibition of the Na+,K+-pump. Platelet microviscosity was similarly increased in SHR and WKY. Thus an acute Na+ load induced alterations of membrane properties in both SHR and WKY. The higher erythrocyte Na+ content in SHR stems rather from their intrinsic membrane properties than from a plasma factor.

Fluorescence studies of the blood platelet membranes associated with fibrinogen

To follow microviscosity changes in membranes associated with fibrinogen binding to human platelets, specific fluorescent probes were used and their fluorescence anisotropy was analysed. The degree of fluorescence anisotropy of diphenylhexatriene, anilinonaphthalene sulfonate (ANS) and fluorescamine increased significantly when fibrinogen reacted with its membrane receptors. Fluorescence polarization analyses showed that fibrinogen binding to platelet membranes is accompanied by an increase in the membrane lipid rigidity. On the other hand, changes in the fluorescence anisotropy of membrane tryptophans and N-(3-pyrene)maleimide suggest augmented mobility of the membrane proteins. The binding of fibrinogen to the membrane receptors is not accompanied by any change in the fluorescence intensity of ANS attached to the membranes. This may suggest that covering of platelets with fibrinogen molecules does not influence the surface membrane charge.

Microenvironment changes of human blood platelet membranes associated with fibrinogen binding

Alterations in the membrane organization caused by fibrinogen binding to human blood platelets and their isolated membranes were analyzed by fluorescence and electron spin resonance measurements. The degree of fluorescent anisotropy of DPH, ANS and fluorescamine increased significantly when fibrinogen reacted with its membrane receptors. Both fluorescence and ESR analyses showed that fibrinogen binding to platelet membranes is accompanied by an increase of the membrane lipid rigidity. This effect seems to be indirect in nature and is mediated by altered membrane protein interactions. As it has been shown that an increased membrane lipid rigidity leads to a greater exposure of membrane proteins, including fibrinogen receptors, this might facilitate a formation of molecular linkages between neighboring platelets. On the other hand, changes of fluorescence anisotropy of membrane tryptophans and N-(3-pyrene) maleimide suggest the augmented mobility of the membrane proteins. Evidence is presented which indicated that the binding of fibrinogen to the membrane receptors is not accompanied by any changes in the fluorescence intensity of ANS attached to the membranes. It may suggest that the covering of platelets with fibrinogen does not influence the surface membrane charge. In contrast to fibrinogen, calcium ions caused an increase of the fluorescence intensity resulting from the more efficient binding of ANS to the platelet membranes.

Abnormal platelet aggregation response in Huntington's disease

Platelet aggregation response to epinephrine, dopamine, serotonin, adenosine diphosphate, arachidonic acid, and collagen was examined in seven patients with Huntington's disease and nine of their relatives. All patients, except for two cases that were in terminal states, showed enhanced response to all the stimulants, especially to dopamine and epinephrine. The platelet aggregation response in many relatives also deviated from the normal limit. The relationship between platelet aggregation abnormality in Huntington's disease and the pathophysiology of the disease was discussed from the view of a generalized membrane defect hypothesis in Huntington's disease, and of disturbed cathecholamine metabolism, both in the CNS and periphery. A possibility that platelet aggregation response examination will be a useful screening test of offspring at risk was proposed.

Fluorescent substrate analogue for adenosine deaminase: 3'-O-[5-(dimethylamino)naphthalene-1-sulfonyl]adenosine

The synthesis of the fluorescent derivative of adenosine, by reaction with 5-(dimethylamino)naphthalene-1-sulfonyl chloride in dry pyridine at low temperature, yielding 3'-O-[5-(dimethylamino)naphthalene-1-sulfonyl]adenosine (3'-O-dansyladenosine), is here described. 3'-O-Dansyladenosine is partially soluble in water (approximately 10(-4) M) and upon excitation at 325 nm exhibits maximum fluorescence emission at 516 +/- 22 nm (corrected) in buffered aqueous solution at pH 7.6 with a quantum yield of 0.21 and a lifetime of 11.8 +/- 0.2 ns. The fluorescence of 3'-O-dansyladenosine is sensitive to the polarity of its solvent: in pyridine, a quantum yield of 0.61 at the emission maximum of 435 nm was observed. 3'-O-Dansyladenosine is a reversible competitive inhibitor of adenosine deaminase with a moderate inhibitive dissociation constant K1 = (1.54 +/- 0.13) X 10(-5) M. The enzyme-substrate analogue association constant was determined by equilibrium dialysis to be K = (0.69 +/- 0.05) X 10(5) M-1, very close to KI-1. The hydrophobic nature of its binding site in adenosine deaminase is evident from the strong blue shift of the fluorescence emission maximum to 440 nm, the 4-fold increase in fluorescence quantum yield, and the longer lifetime of 15.8 +/- 0.2 ns; the tight, rigid nature of the complex is evident from its high fluorescence polarization value, 0.23. An 85% decrease in the fluorescence emission intensity of the adenosine deaminase-3'-O-dansyladenosine complex in the presence of adenosine indicates the selective binding to the enzyme active site. Correlation between the conformation of the probe, either when free in various solvents or when bound to the enzyme, and its fluorescence quantum yield is noted. 3'-O-Dansyladenosine is suitable for fluorescent labeling of adenosine deaminase in cell systems.