Accelerated death of megakaryocytes from Wiskott-Aldrich syndrome patients.

Wiskott-Aldrich syndrome (WAS) is an X-linked recessive disorder caused by WAS gene mutations resulting in haematopoietic/immune cell defects. Recent studies report accelerated death of WAS platelets and lymphocytes. Data on megakaryocyte (MK) maturation, viability and their possible role in thrombocytopenia development in WAS are limited. In this study we evaluate the MK viability and morphology in untreated, romiplostim-treated WAS patients compared with normal controls. The study included 32 WAS patients and 17 healthy donors. MKs were captured from bone marrow aspirates by surface-immobilized anti-GPIIb-IIIa antibody. Viability (by phosphatidylserine [PS] externalization), distribution by maturation stages and size of MK were determined by light microscopy. MK distribution by maturation stages in patients differed from controls. 40 ± 22% of WAS MKs versus 23 ± 11% of normal MKs were at maturation stage 3 (p = 0.02), whereas 24 ± 20% in WAS and 39 ± 14% in controls had megakaryoblast morphology (p = 0.05). Romiplostim treatment changed the MK maturation stages distribution close to normal. PS-positive (PS+) MK in WAS was significantly higher (21 ± 21%) than in healthy controls (2 ± 4%, p < 0.01). WAS patients with more damaging truncating mutations and higher disease score had higher PS+ MK fraction (Spearman r = 0.6, p < 0.003). We conclude that WAS MKs have increased cell death tendency and changes in maturation pattern. Both could contribute to thrombocytopenia in WAS patients.

Effects of bacterial lipopolysaccharides on platelet function: inhibition of weak platelet activation.

Platelets are anucleate blood cells with reported roles in hemostasis and immune responses, which possess a functional receptor for bacterial lipopolysaccharides (LPSs), the well-known inducers of inflammation. However, LPSs effects on platelets are contradictory. Here we aim to investigate mechanisms of platelet functioning in the presence of LPS and to find the cause of the discrepancy in the previously published data. Cell activity was analyzed by flow cytometry, western blotting, and aggregometry. Thrombus growth was assessed by fluorescent microscopy. LPS' activity was checked by their capability to induce PMN activation. However, LPSs did not substantially affect either thrombus growth in flow chambers, irreversible platelet aggregation, or platelet responses to strong activation. Platelet aggregation in response to 1 µM of ADP was significantly inhibited by LPSs. Flow cytometry analysis revealed that platelet activation responses to weak stimulation were also diminished by LPSs, while VASP phosphorylation was weakly increased. Additionally, LPSs were capable of inhibition of ADP-induced P2-receptor desensitization. Incubation of platelets with a pan-PDE inhibitor IBMX significantly enhanced the LPSs-induced platelet inhibition, implying cAMP/cGMP dependent mechanism. The discrepancy in the previously published data could be explained by LPS-induced weak inhibition of platelet activation and the prevention of platelet desensitization.

Mechanisms of increased mitochondria-dependent necrosis in Wiskott-Aldrich syndrome platelets.

Wiskott-Aldrich syndrome (WAS) is associated with thrombocytopenia of unclear origin. We investigated real-time cytosolic calcium dynamics, mitochondrial membrane potential and phoszphatidylserine (PS) exposure in single fibrinogen-bound platelets using confocal microscopy. The WAS platelets had higher resting calcium levels, more frequent spikes, and their mitochondria more frequently lost membrane potential followed by PS exposure (in 22.9% of platelets vs 3.9% in controls; P<0.001) after the collapse of the last mitochondria. This phenomenon was inhibited by the mitochondrial permeability transition pore inhibitor cyclosporine A, as well by xestospongin C and lack of extracellular calcium. Thapsigargin by itself caused accelerated cell death in the WAS platelets. The number of mitochondria was predictive of PS exposure: 33% of platelets from WAS patients with fewer than five mitochondria exposed PS, while only 12% did among those that had five or more mitochondria. Interestingly, healthy donor platelets with fewer mitochondria also more readily became procoagulant upon PAR1/PAR4 stimulation. Collapse of single mitochondria led to greater cytosolic calcium increase in WAS platelets if they had one to three mitochondria compared with platelets containing higher numbers. A computer systems biology model of platelet calcium homeostasis showed that smaller platelets with fewer mitochondria could have impaired calcium homeostasis because of higher surface-to-volume ratio and greater metabolic load, respectively. There was a correlation (C=0.81, P<0.02) between the mean platelet size and platelet count in the WAS patients. We conclude that WAS platelets readily expose PS via a mitochondria-dependent necrotic mechanism caused by their smaller size, which could contribute to the development of thrombocytopenia.

Calpain-controlled detachment of major glycoproteins from the cytoskeleton regulates adhesive properties of activated phosphatidylserine-positive platelets.

In resting platelets, adhesive membrane glycoproteins are attached to the cytoskeleton. On strong activation, phosphatidylserine(PS)-positive and -negative platelet subpopulations are formed. Platelet activation is accompanied by cytoskeletal rearrangement, although the glycoprotein attachment status in these two subpopulations is not clear. We developed a new, flow cytometry-based, single-cell approach to investigate attachment of membrane glycoproteins to the cytoskeleton in cell subpopulations. In PS-negative platelets, adhesive glycoproteins integrin αIIbß3, glycoprotein Ib and, as shown for the first time, P-selectin were associated with the cytoskeleton. In contrast, this attachment was disrupted in PS-positive platelets; it was retained to some extent only in the small convex regions or 'caps'. It correlated with the degradation of talin and filamin observed only in PS-positive platelets. Calpain inhibitors essentially prevented the disruption of membrane glycoprotein attachment in PS-positive platelets, as well as talin and filamin degradation. With the suggestion that detachment of glycoproteins from the cytoskeleton may affect platelet adhesive properties, we investigated the ability of PS-positive platelets to resist shear-induced breakaway from the immobilized fibrinogen. Shear rates of 500/s caused PS-positive platelet breakaway, but their adhesion stability increased more than 10-fold after pretreatment of the platelets with calpain inhibitor. In contrast, the ability of PS-positive platelets to adhere to immobilized von Willebrand's factor at 100/s was low, but this was not affected by the preincubation of platelets with a calpain inhibitor. Our data suggest that calpain-controlled detachment of membrane glycoproteins is a new mechanism that is responsible for the loss of ability of the procoagulant platelets to resist detachment from thrombi by high shear stress.

Platelet surface-associated activation and secretion-mediated inhibition of coagulation factor XII.

Coagulation factor XII (fXII) is important for arterial thrombosis, but its physiological activation mechanisms are unclear. In this study, we elucidated the role of platelets and platelet-derived material in fXII activation. FXII activation was only observed upon potent platelet stimulation (with thrombin, collagen-related peptide, or calcium ionophore, but not ADP) accompanied by phosphatidylserine exposure and was localised to the platelet surface. Platelets from three patients with grey platelet syndrome did not activate fXII, which suggests that platelet-associated fXII-activating material might be released from α-granules. FXII was preferentially bound by phosphotidylserine-positive platelets and annexin V abrogated platelet-dependent fXII activation; however, artificial phosphotidylserine/phosphatidylcholine microvesicles did not support fXII activation under the conditions herein. Confocal microscopy using DAPI as a poly-phosphate marker did not reveal poly-phosphates associated with an activated platelet surface. Experimental data for fXII activation indicates an auto-inhibition mechanism (ki/ka = 180 molecules/platelet). Unlike surface-associated fXII activation, platelet secretion inhibited activated fXII (fXIIa), particularly due to a released C1-inhibitor. Platelet surface-associated fXIIa formation triggered contact pathway-dependent clotting in recalcified plasma. Computer modelling suggests that fXIIa inactivation was greatly decreased in thrombi under high blood flow due to inhibitor washout. Combined, the surface-associated fXII activation and its inhibition in solution herein may be regarded as a flow-sensitive regulator that can shift the balance between surface-associated clotting and plasma-dependent inhibition, which may explain the role of fXII at high shear and why fXII is important for thrombosis but negligible in haemostasis.

Antiplatelet agents can promote two-peaked thrombin generation in platelet rich plasma: mechanism and possible applications.

BACKGROUND: Thrombin generation assay is a convenient and widely used method for analysis of the blood coagulation system status. Thrombin generation curve (TGC) is usually bell-shaped with a single peak, but there are exceptions. In particular, TGC in platelet-rich plasma (PRP) can sometimes have two peaks. OBJECTIVE: We sought to understand the mechanism underlying the occurrence of two peaks in the PRP thrombin generation curve. METHODS: Tissue factor-induced thrombin generation in PRP and platelet-poor plasma (PPP) was monitored using continuous measurement of the hydrolysis rate of the thrombin-specific fluorogenic substrate Z-Gly-Gly-Arg-AMC. Expression of phosphatidylserine (PS) and CD62P on the surface of activated platelets was measured by flow cytometry using corresponding fluorescently labeled markers. RESULTS: The addition of the P(2)Y(12) receptor antagonist MeS-AMP (160 µM), 83 nM prostaglandin E(1) (PGE(1)), or 1.6% DMSO to PRP caused the appearance of two peaks in the TGC. The PS exposure after thrombin activation on washed platelets in a suspension supplemented with DMSO, PGE(1) or MeS-AMP was delayed, which could indicate mechanism of the second peak formation. Supplementation of PRP with 1.6% DMSO plus 830 nM PGE(1) mediated the disappearance of the second peak and decreased the amplitude of the first peak. Increasing the platelet concentration in the PRP promoted the consolidation of the two peaks into one. CONCLUSIONS: Procoagulant tenase and prothrombinase complexes in PRP assemble on phospholipid surfaces containing PS of two types--plasma lipoproteins and the surface of activated platelets. Thrombin generation in the PRP can be two-peaked. The second peak appears in the presence of platelet antagonists as a result of delayed PS expression on platelets, which leads to delayed assembly of the membrane-dependent procoagulant complexes and a second wave of thrombin generation.

Two types of procoagulant platelets are formed upon physiological activation and are controlled by integrin α(IIb)ß(3).

OBJECTIVE: Phosphatidylserine (PS) externalization by platelets upon activation is a key event in hemostasis and thrombosis. It is currently believed that strong stimulation of platelets forms 2 subpopulations, only 1 of which expresses PS. METHODS AND RESULTS: Here, we demonstrate that physiological stimulation leads to the formation of not 1 but 2 types of PS-expressing activated platelets, with dramatically different properties. One subpopulation sustained increased calcium level after activation, whereas another returned to the basal low-calcium state. High-calcium PS-positive platelets had smaller size, high surface density of fibrin(ogen), no active integrin α(IIb)ß(3), depolarized mitochondrial membranes, gradually lost cytoplasmic membrane integrity, and were poorly aggregated. In contrast, the low-calcium PS-positive platelets had normal size, retained mitochondrial membrane potential and cytoplasmic membrane integrity, and combined retention of fibrin(ogen) with active α(IIb)ß(3) and high proaggregatory function. Formation of low-calcium PS-positive platelets was promoted by platelet concentration increase or shaking and was decreased by integrin α(IIb)ß(3) antagonists, platelet dilution, or in platelets from kindlin-3-deficient and Glanzmann thrombasthenia patients. CONCLUSIONS: Identification of a novel PS-expressing platelet subpopulation with low calcium regulated by integrin α(IIb)ß(3) can be important for understanding the mechanisms of PS exposure and thrombus formation.

Spatial structure and pH-dependent conformational diversity of dimeric transmembrane domain of the receptor tyrosine kinase EphA1.

Eph receptors are found in a wide variety of cells in developing and mature tissues and represent the largest family of receptor tyrosine kinases, regulating cell shape, movements, and attachment. The receptor tyrosine kinases conduct biochemical signals across plasma membrane via lateral dimerization in which their transmembrane domains play an important role. Structural-dynamic properties of the homodimeric transmembrane domain of the EphA1 receptor were investigated with the aid of solution NMR in lipid bicelles and molecular dynamics in explicit lipid bilayer. EphA1 transmembrane segments associate in a right-handed parallel alpha-helical bundle, region (544-569)(2), through the N-terminal glycine zipper motif A(550)X(3)G(554)X(3)G(558). Under acidic conditions, the N terminus of the transmembrane helix is stabilized by an N-capping box formed by the uncharged carboxyl group of Glu(547), whereas its deprotonation results in a rearrangement of hydrogen bonds, fractional unfolding of the helix, and a realignment of the helix-helix packing with appearance of additional minor dimer conformation utilizing seemingly the C-terminal GG4-like dimerization motif A(560)X(3)G(564). This can be interpreted as the ability of the EphA1 receptor to adjust its response to ligand binding according to extracellular pH. The dependence of the pK(a) value of Glu(547) and the dimer conformational equilibrium on the lipid head charge suggests that both local environment and membrane surface potential can modulate dimerization and activation of the receptor. This makes the EphA1 receptor unique among the Eph family, implying its possible physiological role as an "extracellular pH sensor," and can have relevant physiological implications.

Transmembrane domain of EphA1 receptor forms dimers in membrane-like environment.

Eph receptor tyrosine kinases (RTKs) are activated by a ligand-mediated dimerization in the plasma membrane and subjected to clusterization at a high local density of receptors and their membrane-anchored ligands. Interactions between transmembrane domains (TMDs) were recognized to assist to the ligand-binding extracellular domains in the dimerization of some RTKs, whereas a functional role of Eph-receptor TMDs remains unknown. We have studied a propensity of EphA1-receptor TMDs (TMA1) to self-association in membrane-mimetic environment. Dimerization of TMA1 in SDS environment was revealed by SDS-PAGE and confirmed by FRET analysis of the fluorescently labeled peptide (Kd=7.2+/-0.4 microM at 1.5 mM SDS). TMA1 dimerization was also found in 1,2-dimyristoyl-sn-glycero-3-phosphocholine liposomes (DeltaG=-15.4+/-0.5 kJ/mol). Stability of TMA1 dimers is comparable to the reported earlier stability of TMD dimers of fibroblast growth factor receptor 3 and tenfold weaker than the stability of TMD dimers of glycophorin A possessing high propensity to dimerization. Our results suggest that EphA1-receptor TMD contribute to the dimerization-mediated receptor activation. An assumed role of the TMD interactions is the efficient signal transduction due to TMD-driving mutual orientation of kinase domains in dimers, while a relatively low force of the TMD interactions does not prevent a ligand-controlled regulation of the receptor dimerization.