Ephrin-A5 (EFNA5) belongs to the ephrin family,contains 1 ephrin RBD (ephrin receptor-binding) domain. Ephrin-A5 is a cell surface GPI-bound ligand for Eph receptors, a family of receptor tyrosine kinases which are crucial for migration, repulsion and adhesion during neuronal, vascular and epithelial development. It binds promiscuously Eph receptors residing on adjacent cells, leading to contact-dependent bidirectional signaling into neighboring cells. The interaction of EFNA5 with EPHA5 also mediates communication between pancreatic islet cells to regulate glucose-stimulated insulin secretion. Cognate/functional ligand for EPHA7, their interaction regulates brain development modulating cell-cell adhesion and repulsion.
Ephrin-B1 also known as EFNB1, is a member of the ephrin family. The transmembrane- associated ephrin ligands and their Eph family of receptor tyrosine kinases are expressed by cells of the SVZ. Eph/ephrin interactions are implicated in axon guidance, neural crest cell migration, establishment of segmental boundaries, and formation of angiogenic capillary plexi. Eph receptors and ephrins are divided into two subclasses, A and B, based on binding specificities. Ephrin subclasses are further distinguished by their mode of attachment to the plasma membrane: ephrin-A ligands bind EphA receptors and are anchored to the plasma membrane via a glycosylphosphatidylinositol (GPI) linkage, whereas ephrin-B ligands bind EphB receptors and are anchored via a transmembrane domain. An exception is the EphA4 receptor, which binds both subclasses of ephrins. EphrinB1 and B class Eph receptors provide positional cues required for the normal morphogenesis of skeletal elements. Another malformation, preaxial polydactyly, was exclusively seen in heterozygous females in which expression of the X-linked ephrinB1 gene was mosaic, so that ectopic EphB-ephrinB1 interactions led to restricted cell movements and the bifurcation of digital rays.
Ephrin-B1, also named EFL-3, ELK ligand, EPH-related receptor tyrosine kinase ligand 2, is a single-pass type I membrane protein. It contains 1 ephrin RBD (ephrin receptor-binding) domain and belongs to the ephrin family. Ephrins are divided into the ephrin-A (EFNA) class, which are anchored to the membrane by a glycosylphosphatidylinositol linkage, and the ephrin-B (EFNB) class, which are transmembrane proteins. All ephrins share a conserved extracellular sequence, which most likely corresponds to the receptor-binding domain. Ephrin-B1 has been shown to bind EphA3, EphB1, EphB2, EphB3, and EphB4. The extracellular domains of human and mouse ephrin-B1 share 94% amino acid identity.
Ephrin-B2 is a type I transmembrane protein and belongs the Ephrin family. It binds to the receptor tyrosine kinases, such as EPHA4, EPHB4 and EPHA3. Ephrin-B2 has been implicated in mediating developmental events, especially in the nervous system, erythropoiesis and tumour metastasis. Ligation of Ephrin-B2 with complementary EphB receptors on adjacent cells results in a combination of forward (EphB receptors) and reverse (Ephrin-B2) signalling, which is central to tissue development and remodelling functions. In addition, Ephrin-B2 may play a role in constraining the orientation of longitudinally projecting axons.
Ephrin Type-B Receptor 1 (EPHB1) is a single-pass type I membrane protein that belongs to the Ephrin-B family of receptor tyrosine kinases that is involved in embryonic nervous and vascular system development. EPHB1/EPHT2 contains two fibronectin type-III domains, one protein kinase domain and one SAM (sterile α motif) domain. EPHB1 could stimulate fibroblast motility on extracellular matrix in a kinase-dependent manner, which also correlated with its association with Grb7, an adaptor molecule implicated in the regulation of cell migration. It binds to ephrin-B1, ephrin-B2 and ephrin-B3. EPHB1 plays an important roles in diverse biological processes including nervous system development, angiogenesis, and neural synapsis formation and maturation and may be involved in cell-cell interactions in the nervous system.
Erythropoietin (EPO) is a 34 kDa glycoprotein hormone in the type I cytokine family and is related to thrombopoietin. Its three N-glycosylation sites, four alpha helices, and N-to C-terminal disulfide bond are conserved across species. Glycosylation of the EPO protein is required for biological activities in vivo. The mature human EPO protein shares 75%-84% amino acid sequence identity with bovine, canine, equine, feline, mouse, ovine, porcine, and rat EPO. EPO is primarily produced in the kidney by a population of fibroblast-like cortical interstitial cells adjacent to the proximal tubules. It is also produced in much lower, but functionally significant amounts by fetal hepatocytes and in adult liver and brain. EPO promotes erythrocyte formation by preventing the apoptosis of early erythroid precursors which express the erythropoietin receptor (EPO R). EPO R has also been described in brain, retina, heart, skeletal muscle, kidney, endothelial cells, and a variety of tumor cells. Ligand induced dimerization of EPO R triggers JAK2-mediated signaling pathways followed by receptor/ligand endocytosis and degradation. Rapid regulation of circulating EPO allows tight control of erythrocyte production and hemoglobin concentrations. Anemia or other causes of low tissue oxygen tension induce erythropoietin production by stabilizing the hypoxia-induceable transcription factors HIF-1 alpha and HIF-2 alpha. EPO additionally plays a tissue-protective role in ischemia by blocking apoptosis and inducing angiogenesis.
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