Comprehensive analysis of platelet glycoprotein Ibα ectodomain glycosylation

BACKGROUND

Platelet glycoprotein (GP) Ibα is the major ligand-binding subunit of the GPIb-IX-V complex that binds von Willebrand factor. GPIbα is heavily glycosylated, and its glycans have been proposed to play key roles in platelet clearance, von Willebrand factor binding, and as target antigens in immune thrombocytopenia syndromes. Despite its importance in platelet biology, the glycosylation profile of GPIbα is not well characterized.

OBJECTIVES

The aim of this study was to comprehensively analyze GPIbα amino acid sites of glycosylation (glycosites) and glycan structures.

METHODS

GPIbα ectodomain that was recombinantly expressed or that was purified from human platelets was analyzed by Western blot, mass spectrometry glycomics, and mass spectrometry glycopeptide analysis to define glycosites and the structures of the attached glycans.

RESULTS

We identified a diverse repertoire of N- and O-glycans, including sialoglycans, Tn antigen, T antigen, and ABO(H) blood group antigens. In the analysis of the recombinant protein, we identified 62 unique O-glycosites. In the analysis of the endogenous protein purified from platelets, we identified 48 unique O-glycosites and 1 N-glycosite. The GPIbα mucin domain is densely O-glycosylated. Glycosites are also located within the macroglycopeptide domain and mechanosensory domain.

CONCLUSIONS

This comprehensive analysis of GPIbα glycosylation lays the foundation for further studies to determine the functional and structural roles of GPIbα glycans.

Fast clearance of platelets in a commonly used mouse model for GPIbα is impeded by an anti-GPIbβ antibody derivative

BACKGROUND

Glycoprotein (GP)Ibα plays a critical role in regulating platelet clearance. Recently, we identified the mechanosensory domain (MSD) in GPIbα and reported evidence to suggest that unfolding of the GPIbα MSD induces exposure of the Trigger sequence therein and subsequent GPIb-IX signaling that accelerates platelet clearance. In a commonly used transgenic mouse model, IL4R-IbαTg, where the Trigger sequence is constitutively exposed, constitutive GPIb-IX-mediated cellular signals are present. Clearance of their platelets is also significantly faster than that of wild-type mice. Previously, an anti-GPIbβ antibody RAM.1 was developed. RAM.1 inhibits GPIbα-dependent platelet signaling and activation. Further, RAM.1 also inhibits anti-GPIbα antibody-mediated filopodia formation.

OBJECTIVE

To investigate whether RAM.1 can ameliorate trigger sequence exposure-mediated platelet clearance.

METHODS

Spontaneous filopodia were measured by confocal microscopy. Other platelet signaling events were measured by flow cytometry. Endogenous platelet life span was tracked by Alexa 488-labeled anti-mouse GPIX antibody.

RESULT

Transfected Chinese hamster ovary cells stably expressing the same chimeric IL4R-Ibα protein complex as in IL4R-IbαTg mice also constitutively exhibit filopodia, and that such filopodia could be abolished by treatment of RAM.1. Further, transfusion of a recombinant RAM.1 derivative that is devoid of its Fc portion significantly extends the endogenous life span of IL4R-IbαTg platelets.

CONCLUSION

These results provide the key evidence supporting the causative link of Trigger sequence exposure to accelerated platelet clearance, and suggest that a RAM.1 derivative may be therapeutically developed to treat GPIb-IX-mediated thrombocytopenia.

Extracellular signal-regulated kinase 5 associates with casein kinase II to regulate GPIb-IX-mediated platelet activation via the PTEN/PI3K/Akt pathway

Essentials The mechanisms of extracellular signal-regulated kinase 5 (ERK5) in GPIb-IX signaling are unclear. Function of ERK5 in GPIb-IX was tested using aggregation, western blotting, and mass spectrometry. The protein interacting with ERK5 in human platelets was identified as casein kinase II (CKII). ERK5 associates with CKII to regulate the activation of the PI3K/Akt pathway in GPIb-IX signaling.

SUMMARY

Background The platelet glycoprotein (GP) Ib-IX complex plays essential roles in thrombosis and hemostasis. The mitogen-activated protein kinases (MAPKs) ERK1/2 and p38 have been shown to be important in the GPIb-IX-mediated signaling leading to integrin activation. However, the roles of the MAPK extracellular signal-regulated kinase 5 (ERK5) in GPIb-IX-mediated platelet activation are unknown. Objective To reveal the function and mechanisms of ERK5 in GPIb-IX-mediated platelet activation. Methods The functions of ERK5 in GPIb-IX-mediated human platelet activation were assessed using botrocetin/VWF, ristocetin/VWF, or platelet adhesion to von Willebrand factor (VWF) under shear stress in the presence of a specific inhibitor of ERK5. ERK5-associated proteins were pulled down from Chinese hamster ovary (CHO) cells transfected with HA-tagged-ERK5, identified by mass spectrometry, and confirmed in human platelets. Roles of ERK5-associated proteins in GPIb-IX-mediated platelet activation were clarified using specific inhibitors. Results The phosphorylation levels of ERK5 were significantly enhanced in human platelets stimulated with botrocetin/VWF or ristocetin/VWF. The ERK5 inhibitor XMD8-92 suppressed the second wave of human platelet aggregation induced by botrocetin/VWF or ristocetin/VWF and inhibited human platelet adhesion on immobilized VWF under shear stress. Casein kinase II (CKII) was identified as an ERK5-associated protein in human platelets. The CKII inhibitor TBB, similar to the ERK5 inhibitor XMD8-92, specifically restrained PTEN phosphorylation, therefore suppressing Akt phosphorylation in human platelets treated with botrocetin/VWF. Conclusion ERK5 associates with CKII to play essential roles in GPIb-IX-mediated platelet activation via the PTEN/PI3K/Akt pathway.

Lentiviral gene rescue of a Bernard-Soulier mouse model to study platelet glycoprotein Ibβ function

Essentials A signaling role of glycoprotein (GP)Ibβ is postulated but not formally demonstrated in platelets. Lentiviral-mediated rescue in knock-out mice can be used to evaluate GPIbβ function in vivo. Transduction of the native subunit corrected the main defects associated with GPIb-IX deficiency Deletion of intracellular 159-170 segment increased thrombosis, 150-160 removal increased bleeding.

SUMMARY

Background The platelet glycoprotein (GP)Ib-V-IX complex is required for normal hemostasis and megakaryopoiesis. A role in GPIb-dependent responses has been ascribed to the less well characterized GPIbβ subunit using a specific antibody and GPIb-IX transfected cells. Objectives Our aim was to evaluate, in vivo, the role of the GPIbβ in hemostasis and thrombosis. Methods GPIbβ(null) Sca-1(+) progenitors transduced with viral particles harboring hGPIbβ were transplanted into lethally irradiated GPIbβ(-/-) recipient mice. Results hGPIbβ transplanted into the bone marrow of GPIbβ(null) mice rescued GPIb-IX expression in 97% of circulating platelets. These platelets efficiently bound von Willebrand factor (VWF) and extended filopodia on a VWF matrix, demonstrating the restoration of GPIb-dependent adhesive and signaling properties. These mice exhibited less severe macrothrombocytopenia and had normal tail bleeding times as compared with GPIbβ(null) mice. This strategy was employed to manipulate and evaluate the role of the GPIbβ intracellular domain. Removal of the membrane proximal segment (Δ(150-160) ) decreased GPIb-IX expression by 43%, confirming its involvement in receptor assembly and biosynthesis, and resulted in increased bleeding times and decreased thrombosis in a mechanical injury model in the aorta. On the other hand, deletion of the C-flanking 159-170 segment allowed normal GPIb-IX expression, VWF-dependent responses and bleeding times, but resulted in enhanced arterial thrombosis. Conclusion This pointed to a repressor role of GPIbβ in thrombus formation in vivo that was not predicted in studies of heterologous cells. These results highlight the utility of this lentiviral strategy for the structure-function evaluation of GPIb-IX in platelets.

Successful treatment with oseltamivir phosphate in a patient with chronic immune thrombocytopenia positive for anti-GPIb/IX autoantibody

The management of chronic immune thrombocytopenia (ITP) remains to be a challenge. Oseltamivir phosphate is a sialidase inhibitor agent used to treat influenza in the conventional sense. At present, we demonstrate for the first time that an adult chronic ITP patient with anti-GP Ib/IX autoantibody, who was resistant to corticosteroids, IVIG, recombinant human thrombopoietin, rituximab, danazol and vindesine, but was successfully treated with oseltamivir phosphate. Through flow cytometric analysis of β-galactose and β-GlcNAc exposure on platelet surfaces, we showed that oseltamivir phosphate could reduce the desialylation level of platelet glycoproteins in ITP patient. The substantial alleviation of thrombocytopenia in this case, though not leading to conclusions, lays a foundation for a novel approach for the treatment of ITP.

Platelet glycoprotein Ib-IX as a regulator of systemic inflammation

OBJECTIVE

The platelet glycoprotein Ib-IX (GP Ib-IX) receptor is a well-characterized adhesion receptor supporting hemostasis and thrombosis via interactions with von Willebrand factor. We examine the GP Ib-IX/von Willebrand factor axis in murine polymicrobial sepsis, as modeled by cecal ligation and puncture (CLP).

APPROACH AND RESULTS

Genetic absence of the GP Ib-IX ligand, von Willebrand factor, prolongs survival after CLP, but absence of the receptor, GP Ib-IX, does not. Because absence of either von Willebrand factor or GP Ib-IX significantly impairs hemostasis and thrombosis, we sought to define additional GP Ib-IX-dependent pathways impacting survival in the CLP model. We document that the absence of GP Ib-IX leads to reduced platelet-neutrophil and platelet-monocyte interactions. Twenty-four hours after CLP, absence of GP Ib-IX coincides with an alteration in cytokine levels, such as tumor necrosis factor-α secreted by monocytes, and increased macrophage-1 antigen expression by neutrophils.

CONCLUSIONS

In contrast to the well-characterized proinflammatory properties of platelets, we describe in the CLP model an anti-inflammatory property associated with platelet GP Ib-IX. Thus, a single platelet receptor displays a dual modulatory role in both the thrombotic and inflammatory pathways associated with polymicrobial sepsis. In sharing leucine-rich motifs with toll-like receptors, platelet GP Ib-IX can be considered a multifunctional participant in hemostasis, thrombosis, and the inflammatory cascade. The results highlight a dynamic role for platelets in systemic inflammation and add to the complex pathophysiologic events that occur during the dysregulated coagulation and inflammation associated with sepsis.

Analysis of inter-subunit contacts reveals the structural malleability of extracellular domains in platelet glycoprotein Ib-IX complex

BACKGROUND

The glycoprotein (GP)Ib-IX complex is critical to hemostasis and thrombosis. Its proper assembly is closely correlated with its surface expression level and requires cooperative interactions among extracellular and transmembrane domains of Ibα, Ibβ and IX subunits. Two interfaces have been previously identified between the extracellular domains of Ibβ and IX.

OBJECTIVE

To understand how extracellular domains interact in GPIb-IX.

METHODS

The Ibβ extracellular domain (IbβE ) or the IX counterpart (IXE ) in GPIb-IX was replaced with a well-folded IbβE /IXE chimera called IbβEabc , and the effect of domain replacement on assembly and expression of the receptor complex in transiently transfected Chinese hamster ovary cells was analyzed.

RESULTS

Replacing IXE with IbβEabc in GPIb-IX retained interface 1 but not interface 2 between the extracellular domains. While this domain replacement preserved complex integrity, the expression levels of Ibβ and Ibα were significantly reduced. Additional domain replacement with IbβEabc or IbβE in GPIb-IX produced the complex at disparate expression levels that cannot be simply explained by two separate interfaces. In particular, when IbβE in GPIb-IX was replaced by IbβEabc , Ibα and IX were expressed at approximately 70% of the wild-type level. Their levels were not reduced when IXE was changed further to IbβE .

CONCLUSIONS

Our results demonstrate the importance of the association between Ibβ and IX extracellular domains for complex assembly and efficient expression, and provide evidence for the structural malleability of these domains that may accommodate and propagate conformational changes therein.