Eph kinases and ephrins support thrombus growth and stability by regulating integrin outside-in signaling in platelets

EPH/Ephrin Signaling in Normal Hematopoiesis and Hematologic Malignancies: Deciphering Their Intricate Role and Unraveling Possible New Therapeutic Targets

Erythropoietin-producing hepatocellular carcinoma receptors (EPHs) represent the largest family of receptor tyrosine kinases (RTKs). EPH interaction with ephrins, their membrane-bound ligands, holds a pivotal role in embryonic development, while, though less active, it is also implicated in various physiological functions during adult life. In normal hematopoiesis, different patterns of EPH/ephrin expression have been correlated with hematopoietic stem cell (HSC) maintenance and lineage-committed hematopoietic progenitor cell (HPC) differentiation, as well as with the functional properties of their mature offspring. Research in the field of hematologic malignancies has unveiled a rather complex involvement of the EPH/ephrinsignaling pathway in the pathophysiology of these neoplasms. Aberrations in genetic, epigenetic, and protein levels have been identified as possible players implicated both in tumor progression and suppression, while correlations have also been highlighted regarding prognosis and response to treatment. Initial efforts to therapeutically target the EPH/ephrin axis have been undertaken in the setting of hematologic neoplasia but are mainly confined to the preclinical level. To this end, deciphering the complexity of this signaling pathway both in normal and malignant hematopoiesis is necessary.

Platelet formation and activation are influenced by neuronal guidance proteins

Platelets are anucleate blood cells derived from megakaryocytes. They link the fundamental functions of hemostasis, inflammation and host defense. They undergo intracellular calcium flux, negatively charged phospholipid translocation, granule release and shape change to adhere to collagen, fibrin and each other, forming aggregates, which are key to several of their functions. In all these dynamic processes, the cytoskeleton plays a crucial role. Neuronal guidance proteins (NGPs) form attractive and repulsive signals to drive neuronal axon navigation and thus refine neuronal circuits. By binding to their target receptors, NGPs rearrange the cytoskeleton to mediate neuron motility. In recent decades, evidence has indicated that NGPs perform important immunomodulatory functions and influence platelet function. In this review, we highlight the roles of NGPs in platelet formation and activation.

A Phase II Study of sEphB4-HSA in Metastatic Castration-Resistant Prostate Cancer

INTRODUCTION

Ephrin receptors and their membrane-localized ligands induce bidirectional signaling and facilitate tumor-stroma interactions. Blocking the EphB4-EphrinB2 pathway, which can be accomplished by soluble EphB4 conjugated to human serum albumin (sEphB4-HSA), promotes cell death in preclinical models of aggressive prostate cancer. We hypothesized that targeting the EphB4-EphrinB2 pathway may serve as a therapeutic target in the treatment of metastatic castration resistant prostate cancer (mCRPC).

PATIENTS AND METHODS

We conducted a single arm, phase II trial in patients with progressive mCRPC who had received no more than 3 prior therapies for mCRPC. sEphB4-HSA 1000 mg IV was administered every 2 weeks, extending to 3 weeks starting from cycle 7. The primary endpoint was confirmed prostate specific antigen (PSA) response rate. We employed a Simon 2-stage Minimax design with 15 patients in the first stage and 10 additional patients in the second stage.

RESULTS

Fourteen eligible patients enrolled in the study with median age of 73.5 years (range: 52-83) and median baseline PSA of 65.11 ng/mL (range: 7.77-2850 ng/mL). Most patients received 3 prior therapies for mCRPC. The median treatment duration with sEphB4-HSA was 6.5 weeks (range: 2-35 weeks). Three patients experienced a serious adverse event potentially related to therapy, including 1 patient with a grade 5 event (cerebral vascular accident) possibly related to the study drug. No patient had a confirmed PSA response, and the study was stopped for futility. Thirteen patients had PSA progression. The median time to PSA progression was 28 days (90% CI: 28-42 days), and median time to radiologic progression was 55 days (90% CI: 54-72 days). Of 3 patients with measurable disease, 2 had stable disease and one had progressive disease.

CONCLUSION

In patients with mCRPC who progressed on prior second generation AR-targeted therapy, sEphB4-HSA monotherapy had no discernable anti-tumor activity.

Mechanistic and therapeutic implications of EphA-4 receptor tyrosine kinase in the pathogenesis of Alzheimer's disease

Erythropoietin-producing hepatoma (Eph) receptors belong to a family of tyrosine kinase receptors that plays a pivotal role in the development of the brain. Eph can be divided broadly into two groups, namely, EphA and EphB, comprising nine and five members, respectively. In recent years, the role of EphA-4 has become increasingly apparent in the onset of Alzheimer's disease (AD). Emerging evidence suggests that EphA-4 results in synaptic dysfunction, which in turn promotes the progression of AD. Moreover, pharmacological or genetic ablation of EphA-4 in the murine model of AD can alleviate the symptoms. The current review summarizes different pathways by which EphA-4 can influence pathogenesis. Since, majority of the studies had reported the protective effect of EphA-4 inhibition during AD, designing therapeutics based on decreasing its enzymatic activity might be necessary for introducing the novel interventions. Therefore, the review described peptide and nanobodies inhibitors of EphA-4 that exhibit the potential to modulate EphA-4 and could be used as lead molecules for the targeted therapy of AD.

Recent advances in inherited platelet disorders

PURPOSE OF REVIEW

The increasing use of high throughput sequencing and genomic analysis has facilitated the discovery of new causes of inherited platelet disorders. Studies of these disorders and their respective mouse models have been central to understanding their biology, and also in revealing new aspects of platelet function and production. This review covers recent contributions to the identification of genes, proteins and variants associated with inherited platelet defects, and highlights how these studies have provided insights into platelet development and function.

RECENT FINDINGS

Novel genes recently implicated in human platelet dysfunction include the galactose metabolism enzyme UDP-galactose-4-epimerase in macrothrombocytopenia, and erythropoietin-producing hepatoma-amplified sequence receptor transmembrane tyrosine kinase EPHB2 in a severe bleeding disorder with deficiencies in platelet agonist response and granule secretion. Recent studies of disease-associated variants established or clarified roles in platelet function and/or production for the membrane receptor G6b-B, the FYN-binding protein FYB1/ADAP, the RAS guanyl-releasing protein RASGRP2/CalDAG-GEFI and the receptor-like protein tyrosine phosphatase PTPRJ/CD148. Studies of genes associated with platelet disorders advanced understanding of the cellular roles of neurobeachin-like 2, as well as several genes influenced by the transcription regulator RUNT-related transcription factor 1 (RUNX1), including NOTCH4.

SUMMARY

The molecular bases of many hereditary platelet disorders have been elucidated by the application of recent advances in cell imaging and manipulation, genomics and protein function analysis. These techniques have also aided the detection of new disorders, and enabled studies of disease-associated genes and variants to enhance understanding of platelet development and function.

EphB1 interaction with caveolin-1 in endothelial cells modulates caveolae biogenesis

Caveolae, the cave-like structures abundant in endothelial cells (ECs), are important for multiple signaling processes such as production of nitric oxide and caveolae-mediated intracellular trafficking. Using superresolution microscopy, fluorescence resonance energy transfer, and biochemical analysis, we observed that the EphB1 receptor tyrosine kinase constitutively interacts with caveolin-1 (Cav-1), the key structural protein of caveolae. Activation of EphB1 with its ligand Ephrin B1 induced EphB1 phosphorylation and the uncoupling EphB1 from Cav-1 and thereby promoted phosphorylation of Cav-1 by Src. Deletion of Cav-1 scaffold domain binding (CSD) motif in EphB1 prevented EphB1 binding to Cav-1 as well as Src-dependent Cav-1 phosphorylation, indicating the importance of CSD in the interaction. We also observed that Cav-1 protein expression and caveolae numbers were markedly reduced in ECs from EphB1-deficient (EphB1-/-) mice. The loss of EphB1 binding to Cav-1 promoted Cav-1 ubiquitination and degradation, and hence the loss of Cav-1 was responsible for reducing the caveolae numbers. These studies identify the crucial role of EphB1/Cav-1 interaction in the biogenesis of caveolae and in coordinating the signaling function of Cav-1 in ECs.

Platelets as Modulators of Cerebral Ischemia/Reperfusion Injury

Ischemic stroke is among the leading causes of disability and death worldwide. In acute ischemic stroke, the rapid recanalization of occluded cranial vessels is the primary therapeutic aim. However, experimental data (obtained using mostly the transient middle cerebral artery occlusion model) indicates that progressive stroke can still develop despite successful recanalization, a process termed "reperfusion injury." Mounting experimental evidence suggests that platelets and T cells contribute to cerebral ischemia/reperfusion injury, and ischemic stroke is increasingly considered a thrombo-inflammatory disease. The interaction of von Willebrand factor and its receptor on the platelet surface, glycoprotein Ib, as well as many activatory platelet receptors and platelet degranulation contribute to secondary infarct growth in this setting. In contrast, interference with GPIIb/IIIa-dependent platelet aggregation and thrombus formation does not improve the outcome of acute brain ischemia but dramatically increases the susceptibility to intracranial hemorrhage. Here, we summarize the current understanding of the mechanisms and the potential translational impact of platelet contributions to cerebral ischemia/reperfusion injury.

Emerging Roles for Eph Receptors and Ephrin Ligands in Immunity

Eph receptors are the largest family of receptor tyrosine kinases and mediate a myriad of essential processes in humans from embryonic development to adult tissue homeostasis through interactions with membrane-bound ephrin ligands. The ubiquitous expression of Eph receptors and ephrin ligands among the cellular players of the immune system underscores the importance of these molecules in orchestrating an optimal immune response. This review provides an overview of the various roles of Eph receptors and ephrin ligands in immune cell development, activation, and migration. We also discuss the role of Eph receptors in disease pathogenesis as well as the implications of Eph receptors as future immunotherapy targets. Given the diverse and critical roles of Eph receptors and ephrin ligands throughout the immune system during both resting and activated states, this review aims to highlight the critical yet underappreciated roles of this family of signaling molecules in the immune system.

Platelet Shp2 negatively regulates thrombus stability under high shear stress

Essentials Shp2 negatively regulates thrombus stability under pathological shear rate. Shp2 suppresses TXA2 receptor-mediated platelet dense granule secretion. Through αIIbβ3 outside-in signaling, Shp2 targets calmodulin-dependent activation of Akt. Shp2 may serve to prevent the formation of unwanted occlusive thrombi. SUMMARY: Background Perpetuation is the final phase of thrombus formation; however, its mechanisms and regulation are poorly understood. Objective To investigate the mechanism of Shp2 in platelet function and thrombosis. Methods and results We demonstrate that the platelet-expressed Src homology region 2 domain-containing protein tyrosine phosphatase Shp2 is a negative regulator of thrombus stability under high shear stress. In a ferric chloride-induced mesenteric arteriole thrombosis model, megakaryocyte/platelet-specific Shp2-deficient mice showed less thrombi shedding than wild-type mice, although their occlusion times were comparable. In accordance with this in vivo phenotype, a microfluidic whole-blood perfusion assay revealed that the thrombi formed on collagen surfaces by Shp2-deficient platelets were more stable under high shear rates than those produced by wild-type platelets. Whereas Shp2 deficiency did not alter platelet responsiveness towards thrombin, ADP and collagen stimulation, Shp2-deficient platelets showed increased dense granule secretion when stimulated by the thromboxane A2 analog U46619. Shp2 appears to act downstream of integrin αIIb β3 outside-in signaling, inhibiting the phosphorylation of Akt (Ser473 and Thr308) and dense granule secretion. Calmodulin was also shown to bind both Shp2 and Akt, linking Shp2 to Akt activation. Conclusions Platelet Shp2 negatively regulates thrombus perpetuation under high shear stress. This signaling pathway may constitute an important mechanism for the prevention of unwanted occlusive thrombus formation, without dramatically interfering with hemostasis.

Association of Genetic Variants With Warfarin-Associated Bleeding Among Patients of African Descent

IMPORTANCE

Major warfarin-related bleeding occurs more frequently in African Americans than in other populations. Identification of potential genetic factors related to this adverse event may help identify at-risk patients.

OBJECTIVE

To identify genetic factors associated with warfarin-related bleeding in patients of African descent at an international normalized ratio (INR) of less than 4.

DESIGN, SETTING, AND PARTICIPANTS

A case-control genome-wide association study involving patients of African descent taking warfarin was conducted in a discovery cohort (University of Chicago [2009-2011] and the University of Illinois at Chicago [2002-2011]), and associations were confirmed in a replication cohort (University of Chicago [2015-2016]). Potential population stratification was examined in the discovery cohort by principal component analysis. Odds ratios (ORs) and 95% CIs were computed for bleeding risk by logistic regression analysis. Summary statistics from the discovery and the replication cohorts were analyzed with a fixed effects meta-analysis. The potential influence of single-nucleotide polymorphisms (SNPs) on gene expression was studied by luciferase expression assays.

EXPOSURES

Single-nucleotide polymorphisms associated with warfarin-related bleeding.

MAIN OUTCOMES AND MEASURES

Major bleeding-defined as bleeding requiring hospitalization, causing a decrease in hemoglobin level of more than 2 g/dL, requiring blood transfusion, or any combination of the 3-while taking warfarin at an INR of less than 4.

RESULTS

The discovery cohort consisted of 31 cases (mean age, 60.1 years [SD, 14.9 years], 26 women [83.9%]) and 184 warfarin-treated controls (mean age, 57.1 years [SD, 15.7 years]) with no documented bleeding. The replication cohort consisted of 40 cases (mean age, 55.6 years [SD, 17.3 years], 27 women [67.5%]), and 148 warfarin-treated controls (mean age, 55.4 years [SD, 17.1 years]; 98 women [66.2%]) with no documented bleeding. In the discovery cohort, 4 SNPs in linkage disequilibrium on chromosome 6 (rs115112393, rs16871327, rs78132896, and rs114504854) were associated with warfarin-related bleeding but did not reach genome-wide significance. The SNP rs78132896 occurred in 11 cases (35.5%) and 9 controls (4.9%) in the discovery cohort (OR, 8.31; 95% CI, 3.2-21.5; P < 6.21 × 10-8), and the association was confirmed in the replication cohort (the SNP was present in 14 cases [35.0%] and 7 controls [4.8%]; OR, 8.24; 95% CI, 3.1-25.3, P = 5.64 × 10-5). Genome-wide significance of this SNP was achieved when the cohorts were combined via meta-analysis (OR, 8.27; 95% CI, 4.18-16.38; P = 2.05 × 10-11). These SNPs are found only in people of African descent. In vitro luciferase expression assays demonstrated that rs16871327 (enhancer SNP) and rs78132896 (promoter SNP) risk alleles together increased EPHA7 gene (Entrez Gene 2045) transcription by a mean of 14.95 (SD, 1.7) compared with wild-type alleles (mean, 9.56 [SD, 0.84]; difference, 5.39; 95% CI, 4.1-6.6; P < .001).

CONCLUSIONS AND RELEVANCE

In this preliminary study involving patients of African descent taking warfarin, 4 single-nucleotide polymorphisms in linkage disequilibrium on chromosome 6 were associated with an increased risk of major bleeding at INR of less than 4. Validation of these findings in an independent prospective cohort is required.

Loss of EfnB1 in the osteogenic lineage compromises their capacity to support hematopoietic stem/progenitor cell maintenance

The bone marrow stromal microenvironment contributes to the maintenance and function of hematopoietic stem/progenitor cells (HSPCs). The Eph receptor tyrosine kinase family members have been implicated in bone homeostasis and stromal support of HSPCs. The present study examined the influence of EfnB1-expressing osteogenic lineage on HSPC function. Mice with conditional deletion of EfnB1 in the osteogenic lineage (EfnB1_OB^-/-), driven by the Osterix promoter, exhibited a reduced prevalence of osteogenic progenitors and osteoblasts, correlating to lower numbers of HSPCs compared with Osx:Cre mice. Long-term culture-initiating cell (LTC-IC) assays confirmed that the loss of EfnB1 within bone cells hindered HSPC function, with a significant reduction in colony formation in EfnB1_OB^-/- mice compared with Osx:Cre mice. Human studies confirmed that activation of EPHB2 on CD34⁺ HSPCs via EFNB1-Fc stimulation enhanced myeloid/erythroid colony formation, whereas functional blocking of either EPHB1 or EPHB2 inhibited the maintenance of LTC-ICs. Moreover, EFNB1 reverse signaling in human and mouse stromal cells was found to be required for the activation of the HSPC-promoting factor CXCL12. Collectively, the results of this study confirm that EfnB1 contributes to the stromal support of HSPC function and maintenance and may be an important factor in regulating the HSPC niche.

A mutation of the human EPHB2 gene leads to a major platelet functional defect

The ephrin transmembrane receptor family of tyrosine kinases is involved in platelet function. We report the first EPHB2 variant affecting platelets in 2 siblings (P1 and P2) from a consanguineous family with recurrent bleeding and normal platelet counts. Whole-exome sequencing identified a c.2233C>T variant (missense p.R745C) of the EPHB2 gene. P1 and P2 were homozygous for this variant, while their asymptomatic parents were heterozygous. The p.R745C variant within the tyrosine kinase domain was associated with defects in platelet aggregation, αIIbβ3 activation, and granule secretion induced by G-protein-coupled receptor (GPCR) agonists and convulxin, as well as in thrombus formation on collagen under flow. In contrast, clot retraction, flow-dependent platelet adhesion, and spreading on fibrinogen were only mildly affected, indicating limited effects on αIIbβ3 outside-in signaling. Most importantly, Lyn, Syk, and FcRγ phosphorylation, the initial steps in glycoprotein VI (GPVI) platelet signaling were drastically impaired in the absence of platelet-platelet contact, indicating a positive role for EPHB2 in GPVI activation. Likewise platelet activation by PAR4-AP showed defective Src activation, as opposed to normal protein kinase C activity and Ca²⁺ mobilization. Overexpression of wild-type and R745C EPHB2 variant in RBL-2H3 (rat basophilic leukemia) cells stably expressing human GPVI confirmed that EPHB2 R745C mutation impaired EPHB2 autophosphorylation but had no effect on ephrin ligand-induced EPHB2 clustering, suggesting it did not interfere with EPHB2-ephrin-mediated cell-to-cell contact. In conclusion, this novel inherited platelet disorder affecting EPHB2 demonstrates this tyrosine kinase receptor plays an important role in platelet function through crosstalk with GPVI and GPCR signaling.

AMPK-ACC signaling modulates platelet phospholipids and potentiates thrombus formation

AMP-activated protein kinase (AMPK) α1 is activated in platelets on thrombin or collagen stimulation, and as a consequence, phosphorylates and inhibits acetyl-CoA carboxylase (ACC). Because ACC is crucial for the synthesis of fatty acids, which are essential for platelet activation, we hypothesized that this enzyme plays a central regulatory role in platelet function. To investigate this, we used a double knock-in (DKI) mouse model in which the AMPK phosphorylation sites Ser79 on ACC1 and Ser212 on ACC2 were mutated to prevent AMPK signaling to ACC. Suppression of ACC phosphorylation promoted injury-induced arterial thrombosis in vivo and enhanced thrombus growth ex vivo on collagen-coated surfaces under flow. After collagen stimulation, loss of AMPK-ACC signaling was associated with amplified thromboxane generation and dense granule secretion. ACC DKI platelets had increased arachidonic acid-containing phosphatidylethanolamine plasmalogen lipids. In conclusion, AMPK-ACC signaling is coupled to the control of thrombosis by specifically modulating thromboxane and granule release in response to collagen. It appears to achieve this by increasing platelet phospholipid content required for the generation of arachidonic acid, a key mediator of platelet activation.

Connexins and Pannexins in Vascular Function and Disease

Connexins (Cxs) and pannexins (Panxs) are ubiquitous membrane channel forming proteins that are critically involved in many aspects of vascular physiology and pathology. The permeation of ions and small metabolites through Panx channels, Cx hemichannels and gap junction channels confers a crucial role to these proteins in intercellular communication and in maintaining tissue homeostasis. This review provides an overview of current knowledge with respect to the pathophysiological role of these channels in large arteries, the microcirculation, veins, the lymphatic system and platelet function. The essential nature of these membrane proteins in vascular homeostasis is further emphasized by the pathologies that are linked to mutations and polymorphisms in Cx and Panx genes.

EphA receptors and ephrin-A ligands are upregulated by monocytic differentiation/maturation and promote cell adhesion and protrusion formation in HL60 monocytes

Background

Eph signaling is known to induce contrasting cell behaviors such as promoting and inhibiting cell adhesion/spreading by altering F-actin organization and influencing integrin activities. We have previously demonstrated that EphA2 stimulation by ephrin-A1 promotes cell adhesion through interaction with integrins and integrin ligands in two monocyte/macrophage cell lines. Although mature mononuclear leukocytes express several members of the EphA/ephrin-A subclass, their expression has not been examined in monocytes undergoing during differentiation and maturation.

Results

Using RT-PCR, we have shown that EphA2, ephrin-A1, and ephrin-A2 expression was upregulated in murine bone marrow mononuclear cells during monocyte maturation. Moreover, EphA2 and EphA4 expression was induced, and ephrin-A4 expression was upregulated, in a human promyelocytic leukemia cell line, HL60, along with monocyte differentiation toward the classical CD14⁺⁺CD16⁻ monocyte subset. Using RT-PCR and flow cytometry, we have also shown that expression levels of αL, αM, αX, and β2 integrin subunits were upregulated in HL60 cells along with monocyte differentiation while those of α4, α5, α6, and β1 subunits were unchanged. Using a cell attachment stripe assay, we have shown that stimulation by EphA as well as ephrin-A, likely promoted adhesion to an integrin ligand-coated surface in HL60 monocytes. Moreover, EphA and ephrin-A stimulation likely promoted the formation of protrusions in HL60 monocytes.

Conclusions

Notably, this study is the first analysis of EphA/ephrin-A expression during monocytic differentiation/maturation and of ephrin-A stimulation affecting monocyte adhesion to an integrin ligand-coated surface. Thus, we propose that monocyte adhesion via integrin activation and the formation of protrusions is likely promoted by stimulation of EphA as well as of ephrin-A.

Electronic supplementary material

The online version of this article (doi:10.1186/s12860-017-0144-x) contains supplementary material, which is available to authorized users.

Targeted therapies in hematological malignancies using therapeutic monoclonal antibodies against Eph family receptors

The use of monoclonal antibodies (mAbs) and molecules derived from them has achieved considerable attention and success in recent years, establishing this mode of therapy as an important therapeutic strategy in many cancers, in particular hematological tumors. mAbs recognize cell surface antigens expressed on target cells and mediate their function through various mechanisms such as antibody-dependent cellular cytotoxicity, complement-dependent cytotoxicity, or immune system modulation. The efficacy of mAb therapy can be improved when they are conjugated to a highly potent payloads, including cytotoxic drugs and radiolabeled isotopes. The Eph family of proteins has received considerable attention in recent years as therapeutic targets for treatment of both solid and hematological cancers. High expression of Eph receptors on cancer cells compared with low expression levels in normal adult tissues makes them an attractive candidate for cancer immunotherapy. In this review, we detail the modes of action of antibody-based therapies with a focus on the Eph family of proteins as potential targets for therapy in hematological malignancies.

Pannexin- and Connexin-Mediated Intercellular Communication in Platelet Function

The three major blood cell types, i.e., platelets, erythrocytes and leukocytes, are all produced in the bone marrow. While red blood cells are the most numerous and white cells are the largest, platelets are small fragments and account for a minor part of blood volume. However, platelets display a crucial function by preventing bleeding. Upon vessel wall injury, platelets adhere to exposed extracellular matrix, become activated, and form a platelet plug preventing hemorrhagic events. However, when platelet activation is exacerbated, as in rupture of an atherosclerotic plaque, the same mechanism may lead to acute thrombosis causing major ischemic events such as myocardial infarction or stroke. In the past few years, major progress has been made in understanding of platelet function modulation. In this respect, membrane channels formed by connexins and/or pannexins are of particular interest. While it is still not completely understood whether connexins function as hemichannels or gap junction channels to inhibit platelet aggregation, there is clear-cut evidence for a specific implication of pannexin1 channels in collagen-induced aggregation. The focus of this review is to summarize current knowledge of the role of connexins and pannexins in platelet aggregation and to discuss possible pharmacological approaches along with their limitations and future perspectives for new potential therapies.

The role of Eph/ephrin molecules in stromal–hematopoietic interactions

Bone marrow mesenchymal stromal/stem cells(BMSC) are fundamental regulatory elements of the hematopoietic stem cell niche; however, the molecular signals that mediate BMSC support of hematopoiesis are poorly understood. Recent studies indicate that BMSC and hematopoietic stem/progenitors cells differentially express the Eph cell surface tyrosine kinase receptors, and their ephrinligands. Eph/ephrin interactions are thought to mediate cross-talk between BMSC and different hematopoietic cell populations to influence cell development, migration and function. This review summarizes Eph/ephrin interactions in the regulation of BMSC communication with hematopoietic stem/progenitor cells and discusses Eph/ephrintargeted therapeutic strategies that are currently being pursued or various hematotological malignancies.

Loss of ephrinB1 in osteogenic progenitor cells impedes endochondral ossification and compromises bone strength integrity during skeletal development

The EphB receptor tyrosine kinase family and their ephrinB ligands have been implicated as mediators of skeletal development and bone homeostasis in humans, where mutations in ephrinB1 contribute to frontonasal dysplasia and coronal craniosynostosis. In mouse models, ephrinB1 has been shown to be a critical factor mediating osteoblast function. The present study examined the functional importance of ephrinB1 during endochondral ossification using the Cre recombination system with targeted deletion of ephrinB1 (EfnB1^fl/fl) in osteogenic progenitor cells, under the control of the osterix (Osx:Cre) promoter. The Osx:EfnB1^-/- mice displayed aberrant bone growth during embryonic and postnatal skeletal development up to 4weeks of age, when compared to the Osx:Cre controls. Furthermore, compared to the Osx:Cre control mice, the Osx:EfnB1^-/- mice exhibited significantly weaker and less rigid bones, with a reduction in trabecular/ cortical bone formation, reduced trabecular architecture and a reduction in the size of the growth plates at the distal end of the femora from newborn through to 4weeks of age. The aberrant bone formation correlated with increased numbers of tartrate resistant acid phosphatase positive osteoclasts and decreased numbers of bone lining osteoblasts in 4week old Osx:EfnB1^-/- mice, compared to Osx:Cre control mice. Taken together, these observations demonstrate the importance of ephrinB1 signalling between cells of the skeleton required for endochondral ossification.

EphA2 promotes cell adhesion and spreading of monocyte and monocyte/macrophage cell lines on integrin ligand-coated surfaces

Eph signaling, which arises following stimulation by ephrins, is known to induce opposite cell behaviors such as promoting and inhibiting cell adhesion as well as promoting cell-cell adhesion and repulsion by altering the organization of the actin cytoskeleton and influencing the adhesion activities of integrins. However, crosstalk between Eph/ephrin with integrin signaling has not been fully elucidated in leukocytes, including monocytes and their related cells. Using a cell attachment stripe assay, we have shown that, following stimulation with ephrin-A1, kinase-independent EphA2 promoted cell spreading/elongation as well as adhesion to integrin ligand-coated surfaces in cultured U937 (monocyte) and J774.1 (monocyte/macrophage) cells as well as sublines of these cells expressing dominant negative EphA2 that lacks most of the intracellular region. Moreover, a pull-down assay showed that dominant negative EphA2 is recruited to the β2 integrin/ICAM1 and β2 integrin/VCAM1 molecular complexes in the subline cells following stimulation with ephrin-A1-Fc. Notably, this study is the first comprehensive analysis of the effects of EphA2 receptors on integrin-mediated cell adhesion in monocytic cells. Based on these findings we propose that EphA2 promotes cell adhesion by an unknown signaling pathway that largely depends on the extracellular region of EphA2 and the activation of outside-in integrin signaling.

Gap junctions and connexin hemichannels in the regulation of haemostasis and thrombosis

Platelets are involved in the maintenance of haemostasis but their inappropriate activation leads to thrombosis, a principal trigger for heart attack and ischaemic stroke. Although platelets circulate in isolation, upon activation they accumulate or aggregate together to form a thrombus, where they function in a co-ordinated manner to prevent loss of blood and control wound repair. Previous report (1) indicates that the stability and functions of a thrombus are maintained through sustained, contact-dependent signalling between platelets. Given the role of gap junctions in the co-ordination of tissue responses, it was hypothesized that gap junctions may be present within a thrombus and mediate intercellular communication between platelets. Therefore studies were performed to explore the presence and functions of connexins in platelets. In this brief review, the roles of hemichannels and gap junctions in the control of thrombosis and haemostasis and the future directions for this research will be discussed.

Expression of the Receptor Tyrosine Kinase EphB2 on Dendritic Cells Is Modulated by Toll-Like Receptor Ligation but Is Not Required for T Cell Activation

The Eph receptor tyrosine kinases interact with their ephrin ligands on adjacent cells to facilitate contact-dependent cell communication. Ephrin B ligands are expressed on T cells and have been suggested to act as co-stimulatory molecules during T cell activation. There are no detailed reports of the expression and modulation of EphB receptors on dendritic cells, the main antigen presenting cells that interact with T cells. Here we show that mouse splenic dendritic cells (DC) and bone-marrow derived DCs (BMDC) express EphB2, a member of the EphB family. EphB2 expression is modulated by ligation of TLR4 and TLR9 and also by interaction with ephrin B ligands. Co-localization of EphB2 with MHC-II is also consistent with a potential role in T cell activation. However, BMDCs derived from EphB2 deficient mice were able to present antigen in the context of MHC-II and produce T cell activating cytokines to the same extent as intact DCs. Collectively our data suggest that EphB2 may contribute to DC responses, but that EphB2 is not required for T cell activation. This result may have arisen because DCs express other members of the EphB receptor family, EphB3, EphB4 and EphB6, all of which can interact with ephrin B ligands, or because EphB2 may be playing a role in another aspect of DC biology such as migration.

PADPIN: protein-protein interaction networks of angiogenesis, arteriogenesis, and inflammation in peripheral arterial disease

Peripheral arterial disease (PAD) results from an obstruction of blood flow in the arteries other than the heart, most commonly the arteries that supply the legs. The complexity of the known signaling pathways involved in PAD, including various growth factor pathways and their cross talks, suggests that analyses of high-throughput experimental data could lead to a new level of understanding of the disease as well as novel and heretofore unanticipated potential targets. Such bioinformatic analyses have not been systematically performed for PAD. We constructed global protein-protein interaction networks of angiogenesis (Angiome), immune response (Immunome), and arteriogenesis (Arteriome) using our previously developed algorithm GeneHits. The term "PADPIN" refers to the angiome, immunome, and arteriome in PAD. Here we analyze four microarray gene expression datasets from ischemic and nonischemic gastrocnemius muscles at day 3 posthindlimb ischemia (HLI) in two genetically different C57BL/6 and BALB/c mouse strains that display differential susceptibility to HLI to identify potential targets and signaling pathways in angiogenesis, immune, and arteriogenesis networks. We hypothesize that identification of the differentially expressed genes in ischemic and nonischemic muscles between the strains that recovers better (C57BL/6) vs. the strain that recovers more poorly (BALB/c) will help for the prediction of target genes in PAD. Our bioinformatics analysis identified several genes that are differentially expressed between the two mouse strains with known functions in PAD including TLR4, THBS1, and PRKAA2 and several genes with unknown functions in PAD including EphA4, TSPAN7, SLC22A4, and EIF2a.

Evaluation of targeting c-Src by the RGT-containing peptide as a novel antithrombotic strategy

Background

Interaction of integrin β3 with c-Src plays critical roles in cellular signaling which is heavily implicated in platelet adhesion and aggregation, as well as in tumor cell proliferation and metastasis or in osteoclastic bone resorption. Selectively blocking integrin αIIbβ3 outside-in signaling in platelets has been a focus of attention because of its effective antithrombotic potential together with a sufficient hemostatic capacity. The myristoylated RGT peptide has been shown to achieve this blockade by targeting the association of c-Src with the integrin β3 tail, but the lack of key information regarding the mechanisms of action prevents this strategy from being further developed into practical antithrombotics. Therefore, in-depth knowledge of the precise mechanisms for RGT peptide in regulating platelet function is needed to establish the basis for a potential antithrombotic therapy by targeting c-Src.

Methods

The reduction-sensitive peptides were applied to rule out the membrane anchorage after cytoplasmic delivery. The c-Src activity was assayed at living cell or at protein levels to assess the direct effect of RGT targeting on c-Src. Thrombus formation under flow in the presence of cytoplasmic RGT peptide was observed by perfusing whole blood through the collagen-coated micro-chamber.

Results

The RGT peptide did not depend on the membrane anchorage to inhibit outside-in signaling in platelets. The myr-AC ~ CRGT peptide readily blocked agonist-induced c-Src activation by disrupting the Src/β3 association and inhibited the RhoA activation and collagen-induced platelet aggregation in addition to the typical outside-in signaling events. The myr-AC ~ CRGT had no direct effect on the kinase activity of c-Src in living cells as evidenced by its inability to dissociate Csk from c-Src or to alter the phosphorylation level of c-Src Y⁴¹⁶ and Y⁵²⁷, consistent results were also from in vitro kinase assays. Under flow conditions, the myr-AC ~ CRGT peptide caused an inhibition of platelet thrombus formation predominantly at high shear rates.

Conclusions

These findings provide novel insights into the molecular mechanisms by which the RGT peptide regulates integrin signaling and platelet function and reinforce the potential of the RGT peptide-induced disruption of Src/β3 association as a druggable target that would finally enable in vivo and clinical studies using the structure-based small molecular mimetics.

Electronic supplementary material

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Mice lacking the SLAM family member CD84 display unaltered platelet function in hemostasis and thrombosis

Background

Platelets are anuclear cell fragments derived from bone marrow megakaryocytes that safeguard vascular integrity by forming thrombi at sites of vascular injury. Although the early events of thrombus formation—platelet adhesion and aggregation—have been intensively studied, less is known about the mechanisms and receptors that stabilize platelet-platelet interactions once a thrombus has formed. One receptor that has been implicated in this process is the signaling lymphocyte activation molecule (SLAM) family member CD84, which can undergo homophilic interactions and becomes phosphorylated upon platelet aggregation.

Objective

The role of CD84 in platelet physiology and thrombus formation was investigated in CD84-deficient mice.

Methods and Results

We generated CD84-deficient mice and analyzed their platelets in vitro and in vivo. Cd84^−/− platelets exhibited normal activation and aggregation responses to classical platelet agonists. Furthermore, CD84 deficiency did not affect integrin-mediated clot retraction and spreading of activated platelets on fibrinogen. Notably, also the formation of stable three-dimensional thrombi on collagen-coated surfaces under flow ex vivo was unaltered in the blood of Cd84^−/− mice. In vivo, Cd84^−/− mice exhibited unaltered hemostatic function and arterial thrombus formation.

Conclusion

These results show that CD84 is dispensable for thrombus formation and stabilization, indicating that its deficiency may be functionally compensated by other receptors or that it may be important for platelet functions different from platelet-platelet interactions.

EphB2 regulates contact-dependent and contact-independent signaling to control platelet function

The Eph kinases, EphA4 and EphB1, and their ligand, ephrinB1, have been previously reported to be present in platelets where they contribute to thrombus stability. Although thrombus formation allows for Eph-ephrin engagement and bidirectional signaling, the importance specifically of Eph kinase or ephrin signaling in regulating platelet function remained unidentified. In the present study, a genetic approach was used in mice to establish the contribution of signaling orchestrated by the cytoplasmic domain of EphB2 (a newly discovered Eph kinase in platelets) in platelet activation and thrombus formation. We conclude that EphB2 signaling is involved in the regulation of thrombus formation and clot retraction. Furthermore, the cytoplasmic tail of this Eph kinase regulates initial platelet activation in a contact-independent manner in the absence of Eph-ephrin ligation between platelets. Together, these data demonstrate that EphB2 signaling not only modulates platelet function within a thrombus but is also involved in the regulation of the function of isolated platelets in a contact-independent manner.

Eph- and ephrin-dependent mechanisms in tumor and stem cell dynamics

The erythropoietin-producing hepatocellular (Eph) receptors comprise the largest family of receptor tyrosine kinases (RTKs). Initially regarded as axon-guidance and tissue-patterning molecules, Eph receptors have now been attributed with various functions during development, tissue homeostasis, and disease pathogenesis. Their ligands, ephrins, are synthesized as membrane-associated molecules. At least two properties make this signaling system unique: (1) the signal can be simultaneously transduced in the receptor- and the ligand-expressing cell, (2) the signaling outcome through the same molecules can be opposite depending on cellular context. Moreover, shedding of Eph and ephrin ectodomains as well as ligand-dependent and -independent receptor crosstalk with other RTKs, proteases, and adhesion molecules broadens the repertoire of Eph/ephrin functions. These integrated pathways provide plasticity to cell-microenvironment communication in varying tissue contexts. The complex molecular networks and dynamic cellular outcomes connected to the Eph/ephrin signaling in tumor-host communication and stem cell niche are the main focus of this review.

EphA4 receptor is a novel negative regulator of osteoclast activity

Of the ephrin (Eph) receptors, mature osteoclasts express predominantly EphA4. This study sought to determine if EphA4 has a regulatory role in osteoclasts. Treatment of RAW/C4 cells with Epha4 small interfering RNAs (siRNAs) increased average size, Ctsk mRNA expression level, and bone resorption activity of the derived osteoclast-like cells. Activation of the EphA4 signaling in osteoclast precursors with EfnA4-fc chimeric protein reduced cell size and resorption activity of the derived osteoclasts. Homozygous Epha4 null mice had substantially less trabecular bone in femur and vertebra compared to wild-type controls. The bone loss was due to a decrease in trabecular number and an increase in trabecular spacing, but not to an increase in osteoclast-lined bone surface or an increase in the number of osteoclasts on bone surface. Dynamic histomorphometry and serum biomarker analyses indicate that bone formation in Epha4 null mice was reduced slightly but not significantly. Osteoclasts of Epha4 null mice were also larger, expressed higher levels of Mmp3 and Mmp9 mRNAs, and exhibited greater bone resorption activity than wild-type osteoclasts in vitro. Deficient Epha4 expression had no effects on the total number of osteoclast formed in response to receptor activator of NF-κB ligand nor on apoptosis of osteoclasts in vitro. It also did not affect the protein-tyrosine phosphorylation status of its ligands, EfnB2, EfnA2, and EfnA4, in osteoclasts. Deficient Epha4 expression in Epha4 null osteoclasts activated the β3 -integrin signaling through reduced phosphorylation of the tyr-747 residue, which led to increased binding of the stimulatory talin and reduced binding of the inhibitory Dok1 to β3 -integrin. This in turn activated Vav3 and the bone resorption activity of osteoclasts. In conclusion, we demonstrate for the first time that EphA4 is a potent negative regulator of osteoclastic activity, mediated in part through increased Dok1 binding to β3 -integrin via an increase in EphA4-dependent tyr-747 phosphorylation.

Involvement of neutrophils in thrombus formation in living mice

Thrombosis is one of the major causes of human death worldwide. Identification of the cellular and molecular mechanisms leading to thrombus formation is thus crucial for the understanding of the thrombotic process. To examine thrombus formation in a living mouse, new technologies have been developed. Digital intravital microscopy allows to visualize the development of thrombosis and generation of fibrin in real-time within living animal in a physiological context. This specific system allowed the identification of new cellular partners involved in platelet adhesion and activation. Furthermore, it improved, especially, the knowledge of the early phase of thrombus formation and fibrin generation in vivo. Until now, platelets used to be considered the sole central player in thrombus generation. However, recently, it has been demonstrated that leukocytes, particularly neutrophils, play a crucial role in the activation of the blood coagulation cascade leading to thrombosis. In this review, we summarized the mechanisms leading to thrombus formation in the microcirculation according to the method of injury in mice with a special focus on the new identified roles of neutrophils in this process.

Ephrin-As, Eph receptors and integrin α3 interact and colocalise at membrane protrusions of U251MG glioblastoma cells

Glioblastoma is the most common brain cancer. Ephrins and their Eph receptors play important roles in the development of central nervous system and the regulation of cancer cell migration and invasion. In a search for the Eph receptor complexes, we used tandem affinity purification based interaction screening with tagged ephrins A1, A3 and A4 combined with protein identification by mass-spectrometry in U251MG glioblastoma cells. Ephrins bound to Eph receptors, mainly to EphA2 in these cells. Integrin α3 was identified in protein complexes with ephrin-As. Soluble ephrin-A1 colocalised with integrin α3 at the cell surface, and was rapidly endocytosed by the cells. However, integrin α3 did not colocalise with internalised ephrin-A1, whereas EphA2 receptor did. In U251MG cells, integrin α3 colocalised with EphA2 receptor at the cell edges and protrusions. Sites of EphA2-integrin α3 colocalisation were positive for vinculin, focal adhesion kinase and phosphotyrosine, that is, markers for cell adhesion and active signalling. The interaction between ephrin-As, Eph receptors and integrin α3 is plausibly important for the crosstalk between Eph and integrin signalling pathways at the membrane protrusions and in the migration of brain cancer cells.

Neferine exerts its antithrombotic effect by inhibiting platelet aggregation and promoting dissociation of platelet aggregates

INTRODUCTION

Neferine, a kind of isoquinoline alkaloid, extracted from the seed embryo of Nelumbo nucifera Gaertn, has long been recognized in traditional medicine as a medicinal plant with various usages. Neferine has many biological activities, including anti-hypertensive, anti-arrhythmic, negative inotropic effect and relaxation on vascular smooth muscle. Although previous studies have reported its antithrombotic effect, the mechanisms by which it exerts antithrombotic effect have not been thoroughly studied.

METHOD

Washed mice platelets and mice platelet-rich-plasma (PRP) were used to investigate the effects of neferine on platelet aggregation, secretion induced by various agonists and dissociation of agonist-formed platelet aggregates. Bioflux plates coated with collagen were used to investigate the effect of neferine on platelet adhesion and thrombosis in vitro. With collagen-epinephrine-induced acute pulmonary thrombus formation mouse model, the effect of neferine on thrombosis in vivo was also examined.

RESULTS

Neferine, significantly and dose-dependently, inhibited collagen-, thrombin-, U46619-induced platelet aggregation in mice washed platelets, or ADP-induced platelet aggregation in PRP. Neferine treatment decreased platelet dense granule secretion initiated by collagen, thrombin and U46619. Also, Neferine dramatically and dose-dependently promoted the dissociation of platelet aggregates pre-formed by various agonists including collagen, thrombin, U46619 or ADP. Neferine can significantly reduce the area of mice platelets adhesion to the collagen and inhibit thrombosis in vitro. In collagen-epinephrine-induced acute pulmonary thrombus mouse model, neferine, at 6 mg/kg, significantly attenuated thrombus formation.

CONCLUSIONS

Neferine remarkably prevents thrombus formation by inhibiting platelet activation, adhesion and aggregation, as well as promoting disassembly of pre-formed platelet aggregates. The inhibitory effects of neferine on platelet activation might be relevant in cases involving aberrant platelet activation where neferine could be used as an anti-platelet and antithrombotic agent.

Platelet lysates stimulate angiogenesis, neurogenesis and neuroprotection after stroke

Platelets contain chemo-attractants and mitogens that have a major role in tissue repair. Therefore we hypothesised that tissue regeneration secondary to activation of endogenous neural stem cells (eNSC) can be enhanced by delivering platelets to the ischaemic brain. To examine these potential therapeutic effects we injected platelet-poor plasma (PPP), fibroblast growth factor (FGF2) and platelet lysate (PLT) to the lateral ventricles after permanent middle cerebral artery occlusion (PMCAO) in rats. The animals were tested with the neurological severity score, and infarct volumes were measured at 90 days post-PMCAO. Immunohistochemistry was used to determine the fate of newborn cells and to count blood vessels in the ischaemic brain. Platelets significantly increased eNSC proliferation and angiogenesis in the subventricular zone (SVZ) and in the peri-lesion cortex. Functional outcome was significantly improved and injury size was significantly reduced in rats treated with PLT suggesting additional neuroprotective effects. In conclusion, local delivery of PLT to the lateral ventricles induces angiogenesis, neurogenesis and neuroprotection and reduces behavioural deficits after brain ischaemia.

The effects of arterial flow on platelet activation, thrombus growth, and stabilization

Injury of an arterial vessel wall acutely triggers a multifaceted process of thrombus formation, which is dictated by the high-shear flow conditions in the artery. In this overview, we describe how the classical concept of arterial thrombus formation and vascular occlusion, driven by platelet activation and fibrin formation, can be extended and fine-tuned. This has become possible because of recent insight into the mechanisms of: (i) platelet-vessel wall and platelet-platelet communication, (ii) autocrine platelet activation, and (iii) platelet-coagulation interactions, in relation to blood flow dynamics. We list over 40 studies with genetically modified mice showing a role of platelet and plasma proteins in the control of thrombus stability after vascular injury. These include multiple platelet adhesive receptors and other junctional molecules, components of the ADP receptor signalling cascade to integrin activation, proteins controlling platelet shape, and autocrine activation processes, as well as multiple plasma proteins binding to platelets and proteins of the intrinsic coagulation cascade. Regulatory roles herein of the endothelium and other blood cells are recapitulated as well. Patient studies support the contribution of platelet- and coagulation activation in the regulation of thrombus stability. Analysis of the factors determining flow-dependent thrombus stabilization and embolus formation in mice will help to understand the regulation of this process in human arterial disease.

Harnessing the platelet signaling network to produce an optimal hemostatic response

Once released into the circulation by megakaryocytes, circulating platelets can undergo rapid activation at sites of vascular injury and resist unwarranted activation, which can lead to heart attacks and strokes. Historically, the signaling mechanisms underlying the regulation of platelet activation have been approached as a collection of individual pathways unique to agonist. This review takes a different approach, casting platelet activation as the product of a signaling network, in which activating and restraining mechanisms interact in a flexible network that regulates platelet adhesiveness, cohesion between platelets, granule secretion, and the formation of a stable hemostatic thrombus.

The cell motility modulator Slit2 is a potent inhibitor of platelet function

BACKGROUND

Vascular injury and atherothrombosis involve vessel infiltration by inflammatory leukocytes, migration of medial vascular smooth muscle cells to the intimal layer, and ultimately acute thrombosis. A strategy to simultaneously target these pathological processes has yet to be identified. The secreted protein, Slit2, and its transmembrane receptor, Robo-1, repel neuronal migration in the developing central nervous system. More recently, it has been appreciated that Slit2 impairs chemotaxis of leukocytes and vascular smooth muscle cells toward diverse inflammatory attractants. The effects of Slit2 on platelet function and thrombus formation have never been explored.

METHODS AND RESULTS

We detected Robo-1 expression in human and murine platelets and megakaryocytes and confirmed its presence via immunofluorescence microscopy and flow cytometry. In both static and shear microfluidic assays, Slit2 impaired platelet adhesion and spreading on diverse extracellular matrix substrates by suppressing activation of Akt. Slit2 also prevented platelet activation on exposure to ADP. In in vivo studies, Slit2 prolonged bleeding times in murine tail bleeding assays. Using intravital microscopy, we found that after mesenteric arteriolar and carotid artery injury, Slit2 delayed vessel occlusion time and prevented the stable formation of occlusive arteriolar thrombi.

CONCLUSIONS

These data demonstrate that Slit2 is a powerful negative regulator of platelet function and thrombus formation. The ability to simultaneously block multiple events in vascular injury may allow Slit2 to effectively prevent and treat thrombotic disorders such as myocardial infarction and stroke.

Developing T-cell migration: role of semaphorins and ephrins

Cell migration is a crucial event for normal T-cell development, and various ligand/receptor pairs have been implicated. Most of them, including chemokines and extracellular matrix proteins, have attractant properties on thymocytes. We discuss herein two further groups of ligand/receptor pairs, semaphorins/neuropilins and ephs/ephrins, which are constitutively expressed by thymocytes and thymic microenvironmental cells. Evidence shows that the corresponding interactions are relevant for developing T-cell migration, including the entry of bone marrow progenitor cells, migration of CD4/CD8-defined thymocyte subpopulations triggered by chemokines and/or extracellular matrix proteins, and thymocyte export. Conceptually, the data summarized here show that thymocyte migration results from a complex network of molecular interactions, which generate not only attraction, but also repulsion of migrating T-cell precursors.

Gap junctions and connexin hemichannels underpin hemostasis and thrombosis

BACKGROUND

Connexins are a widespread family of membrane proteins that assemble into hexameric hemichannels, also known as connexons. Connexons regulate membrane permeability in individual cells or couple between adjacent cells to form gap junctions and thereby provide a pathway for regulated intercellular communication. We have examined the role of connexins in platelets, blood cells that circulate in isolation but on tissue injury adhere to each other and the vessel wall to prevent blood loss and to facilitate wound repair.

METHODS AND RESULTS

We report the presence of connexins in platelets, notably connexin37, and that the formation of gap junctions within platelet thrombi is required for the control of clot retraction. Inhibition of connexin function modulated a range of platelet functional responses before platelet-platelet contact and reduced laser-induced thrombosis in vivo in mice. Deletion of the Cx37 gene (Gja4) in transgenic mice reduced platelet aggregation, fibrinogen binding, granule secretion, and clot retraction, indicating an important role for connexin37 hemichannels and gap junctions in platelet thrombus function.

CONCLUSIONS

Together, these data demonstrate that platelet gap junctions and hemichannels underpin the control of hemostasis and thrombosis and represent potential therapeutic targets.

Differential gene expression in the EphA4 knockout spinal cord and analysis of the inflammatory response following spinal cord injury

Mice lacking the axon guidance molecule EphA4 have been shown to exhibit extensive axonal regeneration and functional recovery following spinal cord injury. To assess mechanisms by which EphA4 may modify the response to neural injury a microarray was performed on spinal cord tissue from mice with spinal cord injury and sham injured controls. RNA was purified from spinal cords of adult EphA4 knockout and wild-type mice four days following lumbar spinal cord hemisection or laminectomy only and was hybridised to Affymetrix All-Exon Array 1.0 GeneChips™. While subsequent analyses indicated that several pathways were altered in EphA4 knockout mice, of particular interest was the attenuated expression of a number of inflammatory genes, including Arginase 1, expression of which was lower in injured EphA4 knockout compared to wild-type mice. Immunohistological analyses of different cellular components of the immune response were then performed in injured EphA4 knockout and wildtype spinal cords. While numbers of infiltrating CD3+ T cells were low in the hemisection model, a robust CD11b+ macrophage/microglial response was observed post-injury. There was no difference in the overall number or spread of macrophages/activated microglia in injured EphA4 knockout compared to wild-type spinal cords at 2, 4 or 14 days post-injury, however a lower proportion of Arginase-1 immunoreactive macrophages/activated microglia was observed in EphA4 knockout spinal cords at 4 days post-injury. Subtle alterations in the neuroinflammatory response in injured EphA4 knockout spinal cords may contribute to the regeneration and recovery observed in these mice following injury.

A novel mechanism of sustained platelet αIIbβ3 activation via PEAR1

Because single nucleotide polymorphisms (SNPs) in platelet endothelial aggregation receptor 1 (PEAR1) are associated with differential functional platelet responses in healthy subjects, we studied the function of PEAR1 in human platelets. During platelet aggregation by various agonists, the membrane expression of PEAR1 and its tyrosine phosphorylation increased. The recombinant PEAR1 EMI domain (GST-EMI) competitively reduced platelet adhesion to surface-coated PEAR1, diminished platelet aggregation, and eliminated PEAR1 phosphorylation. Polyclonal antibodies against the extracellular PEAR1 domain triggered PEAR1 phosphorylation in a src family kinase (SFK)–dependent manner. Such resulted in downstream signaling, culminating in extensive platelet degranulation and irreversible aggregation reactions interrupted by excess monovalent anti–GST-EMI F(ab) fragments. In resting platelets, the cytoplasmic tail of PEAR1 was found complexed to c-Src and Fyn, but on its phosphorylation, phospho-PEAR1 recruited p85 PI3K, resulting in persistent activation of PI3K and Akt. Thus, αIIbβ3 activation was amplified, hence stabilizing platelet aggregates, a signaling cascade fully interrupted by the SFK inhibitor PP1 and the PI3K inhibitor LY294002. This study is the first demonstration of a functional role for PEAR1 in platelet activation, underpinning the observed association between PEAR1 and platelet function in genome-wide association studies.

Protein dynamics at Eph receptor-ligand interfaces as revealed by crystallography, NMR and MD simulations

Background

The role of dynamics in protein functions including signal transduction is just starting to be deciphered. Eph receptors with 16 members divided into A- and B- subclasses are respectively activated by 9 A- and B-ephrin ligands. EphA4 is the only receptor capable of binding to all 9 ephrins and small molecules with overlapped interfaces.

Results

We first determined the structures of the EphA4 ligand binding domain (LBD) in two crystals of P1 space group. Noticeably, 8 EphA4 molecules were found in one asymmetric unit and consequently from two crystals we obtained 16 structures, which show significant conformational variations over the functionally critical A-C, D-E, G-H and J-K loops. The 16 new structures, together with previous 9 ones, can be categorized into two groups: closed and open forms which resemble the uncomplexed and complexed structures of the EphA4 LBD respectively. To assess whether the conformational diversity over the loops primarily results from the intrinsic dynamics, we initiated 30-ns molecular dynamics (MD) simulations for both closed and open forms. The results indicate that the loops do have much higher intrinsic dynamics, which is further unravelled by NMR H/D exchange experiments. During simulations, the open form has the RMS deviations slightly larger than those of the closed one, suggesting the open form may be less stable in the absence of external contacts. Furthermore, no obvious exchange between two forms is observed within 30 ns, implying that they are dynamically separated.

Conclusions

Our study provides the first experimental and computational result revealing that the intrinsic dynamics are most likely underlying the conformational diversity observed for the EphA4 LBD loops mediating the binding affinity and specificity. Interestingly, the open conformation of the EphA4 LBD is slightly unstable in the absence of it natural ligand ephrins, implying that the conformational transition from the closed to open has to be driven by the high-affinity interaction with ephrins because the weak interaction with small molecule was found to be insufficient to trigger the transition. Our results therefore highlight the key role of protein dynamics in Eph-ephrin signalling and would benefit future design of agonists/antagonists targeting Eph receptors.

The platelet P2Y12 receptor contributes to granule secretion through Ephrin A4 receptor

The main responses of P2Y(1) ligation are platelet shape change and transient aggregation while P2Y(12) ligation amplifies P2Y(1)-induced aggregation and accelerates aggregation, secretion and thromboxane A(2) production induced by other agonist-receptor complexes. We searched for new targets of P2Y signalling using micro-arrays with 144 peptides representing known phosphosites of protein tyrosine kinases. ADP induced phosphorylation of peptides representing surface receptors, second messenger enzymes and cytoskeletal proteins. Strong phosphorylation was found in peptides representing Ephrin-receptor family members. Blockade of P2Y(1/12) inhibited phosphorylation of EphA4- and EphB1-peptides on micro-arrays. The EphA2/4 inhibitor 2,5-dimethylpyrrolyl benzoic acid derivative interfered with P2Y(1/12)-induced EphA4 phosphorylation, left P2Y(1)-induced aggregation unchanged but inhibited with P2Y(12)-induced secretion, second phase aggregation and thrombus formation on collagen at 1600 s(-1). These results show that platelet EphA4 is an important intermediate in P2Y(12)-induced granule secretion.

Platelet signaling

This chapter summarizes current ideas about the intracellular signaling that drives platelet responses to vascular injury. After a brief overview of platelet activation intended to place the signaling pathways into context, the first section considers the early events of platelet activation leading up to integrin activation and platelet aggregation. The focus is on the G protein-mediated events utilized by agonists such as thrombin and ADP, and the tyrosine kinase-based signaling triggered by collagen. The second section considers the events that occur after integrin engagement, some of which are dependent on close physical contact between platelets. A third section addresses the regulatory events that help to avoid unprovoked or excessive platelet activation, after which the final section briefly considers individual variations in platelet reactivity and the role of platelet signaling in the innate immune response and embryonic development.