Jurnal Internasional EPHB2 mengatur aktivasi trombosit | Jurnal Darah
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The families of ephrin tyrosine kinase receptor kinases (subclass A and B) and their membrane-related ephrin ligands are highly conserved throughout multicellular organisms, from primitive invertebrates to mammals, consistent with their important role in regulating cell interactions -cell. 2 Ephrin ligands that are many phosphatidylinositol (subclass A) or transmembrane (B) form transelular complexes with specific epinine receptors. Ephrin receptors consist of a conserved extracellular ligand ligand domain, fibronectin-rich cysteine-like domains, a transmembrane domain, and an intracellular tyrosine kinase domain. These ephrin / receptor complexes are expressed in the confines of cells and regulate cell signaling pathways, rearranging actin cytoskeleton, redox regulation through activation of nicotinamide adenine dinucleotide phosphate oxidase (Nox), and activation of proteases, including disintegrin and metalloproteinase-10 ( ADAM10) is involved in downregulating EPH receptor expression. 3 Platelets express Eph kinase, EphA4 and EphB1, and their ligands, ephrinB1, and contribute to the stability of thrombus. 4 More recently, platelets have also been found to express EPHB2 5 Thrombocytes from genetically modified mice in which the entire cytoplasmic tail of EPHB2 is replaced by β-galactosidase which shows not only the role for efra / receptor complexes in the gap between adhesive platelets, especially those which regulate αIIbβ3 outside-signal platinum integrins in thrombus formation and clot retraction, but also the end-contact role for EPHB2 in regulating thrombin and collagen signal pathways and αIIbβ3 in-out signaling . 5 In this issue, Berrou et al have confirmed the role for EPHB2 in regulating this platelet activation pathway independently of receptor ligation (see figure).
Two patients, brothers and sisters of people old relatives, initially studied in their early teens. They showed spontaneous subcutaneous bleeding and severe bleeding after minor injuries. 1 In brothers, there were also anemia in children due to excessive gastrointestinal bleeding. No parent shows a tendency to bleed. The platelet count for all family members is within the normal range or quite reduced. Patient platelet aggregation was compromised in response to adenosine 5′-diphosphate (ADP), arachidonic acid, and collagen, but not to ristocetin, while the amount of solid granules and secretions appeared normal. 1 Whereas in previous decades identifying specific causes or mutations associated with unexplained bleeding was a difficult attempt, 6 a key feature of this study was the application of complete sequential sequencing to identified 6 rare homozygous nonsynonymous variants along with 2 patients, which together with others. the evidence described in detail in the publication allows homozygous missile mutations 2233C> T (Arg745Cys) in genes for tyrosine kinase EPHB2 receptors to be identified as the most likely major cause of defects. 1 Both parents are heterozygous. for this mutation, which is located within the intracellular tyrosine kinase motif which is highly conserved immediately before Asp746, a proton acceptor which is thought to be important for the active site adenosine triphosphate-dependent kinase of EPHB2. Disruption of tyrosine kinase activity by the Arg745Cys mutation is confirmed by decreased tyrosine convulsion phosphorylation in Tyr594 / Tyr604 despite similar levels of EPHB2 expression in patients and control platelets.
Whereas previous studies involving EPHB1 / B2 complex showed a significant role in regulating the αIIbβ3 integrin in signal and function, 4 5 αIIbβ3 external signaling (clot retraction, platelet adhesion, and spread in fibrinogen) is only slightly affected in the patient's platelets. In contrast, patient platelets show a damaged activation of αIIbβ3 (out-in signal) in response to activation by agonists acting in GPCR (ADP, thrombin receptor) or agonists acting at GPVI (collagen, convulxin snake venom) and thrombus formation disorders in collagen-coated surface under ex vivo flow at the equivalent arterial shear rate 1500 / s. Tyrosine phosphorylation of signaling molecules, Lyn kinase, spleen tyrosine kinase (Syk), and Fc receptor γ-chain (FcRγ), all previously involved in GPVI / FcRγ complex signaling, are greatly reduced in the absence of platelets that show a direct or no role directly for EPHB2 in regulating GPVI-dependent platelet activation (see picture panel B). Convulxin which works in GPVI induces phosphorylation of Tyr594 / Tyr604 from wild types but not EPHB2 mutants. This finding is confirmed in studies of wild type or mutant overexpression of basophilic leukemia (RBL-2H3) mice (which express the GPVI / FcRγ complex), where the Arg745Cys mutation inhibits phosphorylation of EPHB2 dependent convulsions, but not ligand-induced ephrin. Grouping EPHB2, which previously supported the relevance of Arg745 / Asp746 in disrupting tyrosine phosphorylation EPHB2
This finding raises interesting questions regarding the potential mechanism for how mutant EPHB2 acts to inhibit dependent signaling pathways GPVI / FcRγ- and / or platelet GPCR, apparently without requirements for ephrin platelet / ligand contact, and although there is no difference evidenced in the expression levels of other receptors. This is most likely a combination of factors that contribute to platelet defects. In particular, electron microscopy revealed significant differences in the shape and tendency of fragmentation in platelets Arg745Cys EPHB2, consistent with the role of EPHB2 in circulating megakaryocytes / platelets in platelet production and cytoskeletal maintenance. The lack of phosphorylation of mutated EPHB2 can also have an impact on the localization or recombinant activity of kinase and / or phosphatase pathways, which affect the signal by GPVI. Interestingly, such as ephrin / ephrin receptor complex, 3 4 GPVI involvement leads to activation of both Nox1 / 2 and ADAM10, 7 ⇓ – 9  involved each in the generation of intracellular reactive oxygen species through the Syk-dependent and Syk-independent pathways, and in ectodomain metalodroteinase-mediated shedding. In this case, the cytoplasmic domain of GPVI human platelets also contains unpaired thiols, which cause GPVI disulfide fast-dependent dimerization after agonist stimulation, 10 and although there is no evidence, it is interesting to speculate. in deviant interactions involving Cys745 in mutant EPHB2, especially if EPHB2 is usually associated with the GPVI / FcR complex.
In short, identification of specific mutations in EPHB2 in 2 family members with GPVI / GPCR heavy and defective bleeding defects – signaling dependent lines new understanding of human platelet production and function. This would be interesting in the future to identify and characterize this and other mutations in the population that help determine the exact mechanism that regulates platelet reactivity.
- © 2018 by The American Society of Hematology