Platelet activation by dimeric beta2-glycoprotein I requires signaling via both glycoprotein Ibalpha and apolipoprotein E receptor 2'

BACKGROUND

Dimerization of beta(2)-glycoprotein I (beta(2)-GPI) by autoantibodies is thought to trigger the clinical manifestations observed in the antiphospholipid syndrome. Arterial thrombosis, a frequently occurring clinical manifestation of the antiphospholipid syndrome, is a process in which platelets play a crucial role. Previous work has shown that binding of dimeric beta(2)-GPI to the platelet receptors apolipoprotein E receptor 2' (ApoER2') and glycoprotein Ibalpha (GPIbalpha) mediates increased platelet activation in an in vitro thrombosis model.

OBJECTIVE

The individual roles of ApoER2' and GPIbalpha in mediating platelet activation by dimeric beta(2)-GPI has hitherto been unclear. In this study, we have determined the roles of either receptor in platelet activation by dimeric beta(2)-GPI.

METHODS

Platelet activation by dimeric beta(2)-GPI was studied under conditions of flow. Intracellular signaling induced by dimeric beta(2)-GPI was subsequently analyzed by means of sodium dodecylsulfate polyacrylamide gel electrophoresis (SDS-PAGE) and western blot analysis.

RESULTS

The increase in platelet deposition onto a fibronectin surface under conditions of flow by dimeric beta(2)-GPI was completely abolished by inhibition of the interaction of dimeric beta(2)-GPI with either GPIbalpha or ApoER2'. Upon platelet stimulation with dimeric beta(2)-GPI, GPIbalpha translocated to the cytoskeleton via the scaffold protein 14-3-3zeta. Concomitantly, ApoER2' dissociated from the adapter protein Disabled1, presumably through phosphorylation of the cytoplasmic tail. Inhibition of one process could not inhibit the other.

CONCLUSION

We show that dimeric beta(2)-GPI signals via two distinct pathways in platelets, both of which are required for platelet activation. Abrogation of either signal results in loss of activation.

Platelets express three different splice variants of ApoER2 that are all involved in signaling

BACKGROUND

beta2-Glycoprotein I is the most relevant antigen in antiphospholipid syndrome. We have shown that binding of dimerized beta2-GPI to platelets via ApoER2' sensitizes platelets for second activating stimuli.

OBJECTIVE

Determine the region of ApoER2 involved in the binding of dimeric beta2-GPI.

METHODS

Cultured human megakaryocytes (MK) and three different human megakaryocytic cell lines were used for mRNA isolation to clone and express recombinant soluble platelet ApoER2. Domain deletion mutants of ApoER2 were constructed to identify the binding site for dimeric beta2-GPI. The presence of ApoER2 splice variants in platelets was demonstrated by immuno-blotting.

RESULTS

Three different mRNA splice variants were isolated from all four types of megakaryocytic cells used. Sequence analysis identified the splice variants: (i) shApoER2Delta5 lacking low-density lipoprotein (LDL) binding domains 4, 5 and 6; (ii) shApoER2Delta4-5 lacking LDL binding domains 3, 4, 5, 6 and (iii) shApoER2Delta3-4-5 lacking LDL binding domains 3, 4, 5, 6 and 7. The presence of three splice variants of ApoER2 on platelets was confirmed by immuno-blotting, with ApoER2Delta4-5 being the most abundantly expressed splice variant. Upon stimulation with dimeric beta2-GPI, all three splice variants were translocated to the cytosol; however, ApoER2Delta4-5 translocation was most prominent. Dimeric beta2-GPI binds platelet ApoER2 variants via LDL-binding domain 1.

CONCLUSIONS

Three different ApoER2 mRNA splice variants were isolated from MK and platelets express all three splice variants. All splice variants were shown to be functional by translocation upon stimulation with dimeric beta2-GPI. All three splice variants express LDL-binding domain 1.

Platelet adhesion to dimeric beta-glycoprotein I under conditions of flow is mediated by at least two receptors: glycoprotein Ibalpha and apolipoprotein E receptor 2'

BACKGROUND

The major antigen implicated in the antiphospholipid syndrome is beta2-glycoprotein I (beta2GPI). Dimerized beta2GPI binds to apolipoprotein E receptor 2' (apoER2') on platelets and increases platelet adhesion to collagen under conditions of flow.

AIM

To investigate whether the interaction between dimerized beta2GPI and platelets is sufficiently strong to resist shear stresses.

METHODS

We studied the interaction of platelets with immobilized dimerized beta2GPI under conditions of flow, and further analyzed the interaction using surface plasmon resonance and solid phase immunoassays.

RESULTS

We found that dimerized beta2GPI supports platelet adhesion and aggregate formation under venous flow conditions. Adhesion of platelets to dimerized beta2GPI was completely inhibited by the addition of soluble forms of both apoER2' and GPIbalpha, and the addition of receptor-associated protein and the removal of GPIbalpha from the platelet surface. GPIbalpha co-precipitated with apoER2', suggesting the presence of complexes between GPIbalpha and apoER2' on platelet membranes. The interaction between GPIbalpha and dimeric beta2GPI was of intermediate affinity (Kd = 180 nM) and Zn2+, but not Ca2+-dependent. Deletion of domain V from dimeric beta2GPI strongly reduced its binding to both GPIbalpha and apoER2'. Antibodies that inhibit the binding of thrombin to GPIbalpha inhibited platelet adhesion to dimeric beta2GPI completely, while antibodies blocking the binding of von Willebrand factor to GPIbalpha had no effect. Dimeric beta2GPI showed reduced binding to low-sulfated GPIbalpha compared to the fully sulfated form.

CONCLUSION

We show that platelets adhere to dimeric beta2GPI under both arterial and venous shear stresses. Platelets adhere via two receptors: GPIbalpha and apoER2'. These receptors are present in a complex on the platelet surface.

Interaction of beta2-glycoprotein I with members of the low density lipoprotein receptor family

The antiphospholipid syndrome (APS) is a non-inflammatory autoimmune disease characterized by arterial and/or venous thrombosis and/or pregnancy morbidity in the presence of autoantibodies that recognize beta2-glycoprotein I (beta2GPI) bound to phospholipids. We have previously demonstrated that dimerization of beta2GPI by autoantibodies induces platelet activation, involving the platelet receptor apolipoprotein E receptor 2' (apoER2') a receptor belonging to the low-density lipoprotein receptor (LDL-R) family. Here, we show that dimeric beta2GPI, but not monomeric beta2GPI, interacts with four other LDL-R family members: the LDL-R related protein (LRP), megalin, the LDL-R and the very-low density lipoprotein receptor (VLDL-R). Interaction between dimeric beta2GPI and LDL-R, apoER2' and VLDL-R was best described with a one-site binding model (half-maximal binding; approximately 20 nm for apoER2' and VLDL-R and approximately 300 nm for LDL-R), whereas the interaction between dimeric beta2GPI and LRP or megalin was best described with a two-site binding model, representing a high- (approximately 3 nm) and a low-affinity site (approximately 0.2 microm). Binding to all receptors tested was unaffected by a tryptophane to serine (W316S) substitution in domain V of beta2GPI, which is known to disrupt the phospholipid binding site of beta2GPI. Also deletion of domain I or II left the interaction with the receptors unaffected. Deletion of domain V, however, significantly decreased the affinity for the receptors. In conclusion, our data show that dimeric beta2GPI can interact with different LDL-R family members. This interaction is dependent on a binding site within domain V of beta2GPI, which does not overlap with the phospholipid-binding site within domain V.

Pathogenic anti-beta2-glycoprotein I antibodies recognize domain I of beta2-glycoprotein I only after a conformational change

Recently, we published the existence of 2 populations of anti-beta2-glycoprotein I (beta2-GPI) IgG antibodies. Type A antibodies recognize epitope G40-R43 in domain I of beta2-GPI and are strongly associated with thrombosis. Type B antibodies recognize other parts of beta2-GPI and are not associated with thrombosis. In this study we demonstrate that type A antibodies only recognize plasma-purified beta2-GPI when coated onto a negatively charged surface and not when coated onto a neutrally charged surface. The affinity of type B antibodies toward plasma-purified beta2-GPI was independent of the charge of the surface to which beta2-GPI was coated. Type A antibodies did not recognize plasma-purified beta2-GPI in solution, whereas they did recognize recombinant beta2-GPI both in solution and coated onto a neutrally charged plate. When the carbohydrate chains were removed from plasma-purified beta2-GPI, we found that type A antibodies did recognize the protein in solution. This supports the hypothesis that the difference in recognition of plasma-purified and recombinant beta2-GPI is caused by the difference in glycosylation and that epitope G40-R43 of plasma-purified beta2-GPI is covered by a carbohydrate chain. Type A anti-beta2-GPI antibodies can only recognize this epitope when this carbohydrate chain is displaced as a result of a conformational change. This finding has major implications both for the detection of pathogenic anti-beta2-GPI antibodies and the comprehension of the pathophysiology of the antiphospholipid syndrome.

The binding site in {beta}2-glycoprotein I for ApoER2' on platelets is located in domain V

The antiphospholipid syndrome is caused by autoantibodies directed against beta(2)-glycoprotein I (beta(2)GPI). Dimerization of beta(2)GPI results in an increased platelet deposition to collagen. We found that apolipoprotein E receptor 2' (apoER2'), a member of the low density lipoprotein receptor family, is involved in activation of platelets by dimeric beta(2)GPI. To identify which domain of dimeric beta(2)GPI interacts with apoER2', we have constructed domain deletion mutants of dimeric beta(2)GPI, lacking domain I (DeltaI), II (DeltaII), or V (DeltaV), and a mutant with a W316S substitution in the phospholipid (PL)-insertion loop of domain V. DeltaI and DeltaII prolonged the clotting time, as did full-length dimeric beta(2)GPI; DeltaV had no effect on the clotting time. Second, DeltaI and DeltaII bound to anionic PL, comparable with full-length dimeric beta(2)GPI. DeltaV and the W316S mutant bound with decreased affinity to anionic PL. Platelet adhesion to collagen increased significantly when full-length dimeric beta(2)GPI, DeltaI, or DeltaII (mean increase 150%) were added to whole blood. No increase was found with plasma beta(2)GPI, DeltaV, or the W316S mutant. Immunoprecipitation indicated that full-length dimeric beta(2)GPI, DeltaI, DeltaII, and the W316S mutant can interact with apoER2' on platelets. DeltaV did not associate with apoER2'. We conclude that domain V is involved in both binding beta(2)GPI to anionic PL and in interaction with apoER2' and subsequent activation of platelets. The binding site in beta(2)GPI for interaction with apoER2' does not overlap with the hydrophobic insertion loop in domain V.

The lipopolysaccharide biosynthesis locus of Campylobacter jejuni 81116

Most Campylobacter jejuni strains express lipo-oligosaccharides. Some strains also express lipopolysaccharides (LPS), with O-antigen-like carbohydrate repeats. C. jejuni 81116 expresses an LPS containing both lipo-oligosaccharides and O-antigen-like repeats, but nothing is known about the structure or sugar composition of these LPS species. A cosmid library of the genome of C. jejuni 81116 was constructed and probed with Campylobacter hyoilei genes involved in LPS synthesis. Five cosmids hybridized with the probe and two of these expressed C. jejuni 81116 LPS in Escherichia coli. By subcloning, a 16 kb DNA region was identified which contains the genetic information required to express C. jejuni LPS. DNA sequence analysis revealed 11 ORFs homologous to genes involved in LPS synthesis of other bacteria. They consisted of three homologues of sugar biosynthesis genes, two homologues of transport genes and six homologues of sugar transferases.