Characterization of the human ephrin-A4 promoter

Expression of the ephrin-A4 ligand, a family member of ligands binding the Eph receptor tyrosine kinases, is induced after an antigen-receptor stimulation of lymphocytes. To understand the transcription regulation of the ephrin-A4 gene, its promoter was identified and regulating elements were characterized. The ephrin-A4 promoter contains cis elements directing the cell-specific expression. By deletion studies, three specific regions, which were contributing to the transcription activity in lymphoid cells, were localized. In one of these regions, an inverted CCAAT box was identified and shown to bind the transcription activator nuclear factor-Y (NF-Y). The importance of NF-Y binding for the ephrin-A4 promoter activity is shown by a total abrogation of promoter activity after destruction of its binding site. NF-Y binding and activity are also crucially dependent on the integrity of the surrounding sequence. In addition, electrophoretic mobility-shift assay and serial-mutation analysis of the two remaining regulating regions revealed cis regulatory elements contributing to the transcription activity of the ephrin-A4 promoter.

Human osteosarcoma expresses specific ephrin profiles: implications for tumorigenicity and prognosis

BACKGROUND

The molecular mechanisms underlying malignancy of osteosarcoma are unknown. It has been reported that eph receptor protein tyrosine kinases and their ligands, ephrins, are associated with increased tumorigenicity in patients with breast carcinoma and melanoma. The expression and role of eph/ephrins in human osteosarcoma has not yet been characterized.

METHODS

Ephrin-A1, ephrin-A3, ephrin-A4, ephrin-A5, ephrin-B1, ephrin-B2, and ephrin-B3 mRNA expression was examined by reverse transcription polymerase chain reaction analysis in nine specimens of human osteosarcoma tissue and five human osteosarcoma cell lines. Ephrin-B1 protein expression was detected immunohistochemically in human osteosarcoma tissue. Clinicopathologic correlation was made between the osteosarcoma specimens and their ephrin expression profiles.

RESULTS

Normal bone specimens, osteosarcoma tissue specimens, and osteosarcoma cell lines expressed a distinct mRNA profile of ephrin-A1, ephrin-A4, and ephrin-B2. A second mRNA profile that included ephrin-A3, ephrin-A5, and ephrin-B1 was expressed by a subset of tumors. The expression of ephrin-B1 was correlated with a poorer clinical prognosis. Ephrin-B1 protein was expressed by osteosarcoma cells and blood vessels.

CONCLUSIONS

The results of this study suggest that ephrin-B1 expressed by osteosarcoma may be a poor prognostic marker through increased tumorigenicity.

Expression and function of the Eph A receptors and their ligands ephrins A in the rat thymus

Thymus development and function are dependent on the definition of different and graded microenvironments that provide the maturing T cell with the different signals that drive its maturation to a functional T lymphocyte. In these processes, cell-cell interactions, cell migration, and positioning are clues for the correct functioning of the organ. The Eph family of receptor tyrosine kinases and their ligands, the ephrins, has been implicated in all these processes by regulating cytoskeleton and adhesion functioning, but a systemic analysis of their presence and possible functional role in thymus has not yet been conducted. In this regard, the current study combines different experimental approaches for analyzing the expression of four members of the Eph A family and their ligands, ephrins A, in the embryonic and adult rat thymus. The patterns of Eph and ephrin expression in the distinct thymic regions were different but overlapping. In general, the studied Eph A were expressed on thymic epithelial cells, whereas ephrins A seem to be more restricted to thymocytes, although Eph A1 and ephrin A1 are expressed on both cell types. Furthermore, the supply of either Eph A-Fc or ephrin A-Fc fusion proteins to fetal thymus organ cultures interferes with T cell development, suggesting an important role for this family of proteins in the cell mechanisms that drive intrathymic T cell development.

Loss of Eph-receptor expression correlates with loss of cell adhesion and chondrogenic capacity in Hoxa13 mutant limbs

Mesenchymal patterning is an active process whereby genetic commands coordinate cell adhesion, sorting and condensation, and thereby direct the formation of morphological structures. In mice that lack the Hoxa13 gene, the mesenchymal condensations that form the autopod skeletal elements are poorly resolved, resulting in missing digit, carpal and tarsal elements. In addition, mesenchymal and endothelial cell layers of the umbilical arteries (UAs) are disorganized, resulting in their stenosis and in embryonic death. To further investigate the role of Hoxa13 in these phenotypes, we generated a loss-of-function allele in which the GFP gene was targeted into the Hoxa13 locus. This allele allowed FACS isolation of mesenchymal cells from Hoxa13 heterozygous and mutant homozygous limb buds. Hoxa13GFP expressing mesenchymal cells from Hoxa13 mutant homozygous embryos are defective in forming chondrogenic condensations in vitro. Analysis of pro-adhesion molecules in the autopod of Hoxa13 mutants revealed a marked reduction in EphA7 expression in affected digits, as well as in micromass cell cultures prepared from mutant mesenchymal cells. Finally, antibody blocking of the EphA7 extracellular domain severely inhibits the capacity of Hoxa13GFP heterozygous cells to condense and form chondrogenic nodules in vitro, which is consistent with the hypothesis that reduction in EphA7 expression affects the capacity of Hoxa13–/– mesenchymal cells to form chondrogenic condensations in vivo and in vitro. EphA7 and EphA4 expression were also decreased in the mesenchymal and endothelial cells that form the umbilical arteries in Hoxa13 mutant homozygous embryos. These results suggest that an important role for Hoxa13 during limb and UA development is to regulate genes whose products are required for mesenchymal cell adhesion, sorting and boundary formation.

Graded expression patterns of ephrin-As in the superior colliculus after lesion of the adult mouse optic nerve

The idea has been put forward that molecules and mechanisms acting during development are re-used during regeneration in the adult, for example in response to traumatic injury in order to re-establish the functional integrity of neuronal circuits. Members of the Eph family of receptor tyrosine kinases and their 'ligands', the ephrins, play a prominent role during development of the retinocollicular projection in rodents, where EphA receptors and ephrin-As are expressed in gradients in both the retina and the superior colliculi (SC). We were interested in investigating whether EphA family members are also expressed or re-expressed in the adult after optic nerve lesion, since the presence of axon guidance information is an important prerequisite for a topographically appropriate re-connection by retinal ganglion cell (RGC) axons. This analysis was encouraged by results showing that RGC axons do not exert guidance preferences in response to membranes from adult unlesioned SC, but in response to membranes from the adult deafferented SC. We found a graded expression pattern of ephrin-As in the SC both before and after deafferentation, which was remarkably similar to those found during development. EphA receptor levels were reduced in the SC after deafferentation and the expression patterns of the EphB family were not changed. In particular, the presence of a graded ephrin-A expression in the deafferented SC suggests that - if robust regeneration of RGC axons can be achieved - topographic guidance information as a likely requirement for a functionally successful re-establishment of the retinocollicular projection is available.

Kinase-dependent and kinase-independent functions of EphA4 receptors in major axon tract formation in vivo

The EphA4 receptor tyrosine kinase regulates the formation of the corticospinal tract (CST), a pathway controlling voluntary movements, and of the anterior commissure (AC), connecting the neocortical temporal lobes. To study EphA4 kinase signaling in these processes, we generated mice expressing mutant EphA4 receptors either lacking kinase activity or with severely downregulated kinase activity. We demonstrate that EphA4 is required for CST formation as a receptor for which it requires an active kinase domain. In contrast, the formation of the AC is rescued by kinase-dead EphA4, suggesting that in this structure EphA4 acts as a ligand for which its kinase activity is not required. Unexpectedly, the cytoplasmic sterile-alpha motif (SAM) domain is not required for EphA4 functions. Our findings establish both kinase-dependent and kinase-independent functions of EphA4 in the formation of major axon tracts.

Roles of Eph receptors and ephrins in segmental patterning

Eph receptor tyrosine kinases and their membrane-bound ligands, ephrins, have key roles in patterning and morphogenesis. Interactions between these molecules are promiscuous, but largely fall into two groups: EphA receptors bind to glycosylphosphatidyl inositol-anchored ephrin-A ligands, and EphB receptors bind to transmembrane ephrin-B proteins. Ephrin-B proteins transduce signals, such that bidirectional signalling can occur upon interaction with the Eph receptor. In many tissues, there are complementary and overlapping expression domains of interacting Eph receptors and ephrins. An important role of Eph receptors and ephrins is to mediate cell contact-dependent repulsion, and this has been implicated in the pathfinding of axons and neural crest cells, and the restriction of cell intermingling between hindbrain segments. Studies in an in vitro system show that bidirectional activation is required to prevent intermingling between cell populations, whereas unidirectional activation can restrict cell communication via gap junctions. Recent work indicates that Eph receptors can also upregulate cell adhesion, but the biochemical basis of repulsion versus adhesion responses is unclear. Eph receptors and ephrins have thus emerged as key regulators that, in parallel with cell adhesion molecules, underlie the establishment and maintenance of patterns of cellular organization.

A splice variant of human ephrin-A4 encodes a soluble molecule that is secreted by activated human B lymphocytes

Ephrin-A4 is a ligand for the erythropoietin-producing hepatocellular (Eph) receptor family of tyrosine kinases. We have identified a secreted form of ephrin-A4, denoted ephrin-A4 (s), which is encoded by an alternatively spliced mRNA and is produced by in vivo activated B cells in tonsils. Blood B cells secrete ephrin-A4 (s) upon stimulation via the B-cell antigen receptor. A subpopulation of tonsil cells in the crypts with a dendritic cell phenotype was shown to express EphA2, an Eph receptor tyrosine kinase that was found to be capable of binding an ephrin-A4 immunoglobulin chimeric protein. We conclude that ephrin-A4 (s) may play a role in the interaction between activated B lymphocytes and dendritic cells in human tonsils. (Blood. 2000;95:221-230)

Retinoic acid synthesis and hindbrain patterning in the mouse embryo

Targeted disruption of the murine retinaldehyde dehydrogenase 2 (Raldh2) gene precludes embryonic retinoic acid (RA) synthesis, leading to midgestational lethality (Niederreither, K., Subbarayan, V., Dolle, P. and Chambon, P. (1999). Nature Genet. 21, 444–448). We describe here the effects of this RA deficiency on the development of the hindbrain and associated neural crest. Morphological segmentation is impaired throughout the hindbrain of Raldh2−/− embryos, but its caudal portion becomes preferentially reduced in size during development. Specification of the midbrain region and of the rostralmost rhombomeres is apparently normal in the absence of RA synthesis. In contrast, marked alterations are seen throughout the caudal hindbrain of mutant embryos. Instead of being expressed in two alternate rhombomeres (r3 and r5), Krox20 is expressed in a single broad domain, correlating with an abnormal expansion of the r2-r3 marker Meis2. Instead of forming a defined r4, Hoxb1- and Wnt8A-expressing cells are scattered throughout the caudal hindbrain, whereas r5/r8 markers such as kreisler or group 3/4 Hox genes are undetectable or markedly downregulated. Lack of alternate Eph receptor gene expression could explain the failure to establish rhombomere boundaries. Increased apoptosis and altered migratory pathways of the posterior rhombencephalic neural crest cells are associated with impaired branchial arch morphogenesis in mutant embryos. We conclude that RA produced by the embryo is required to generate posterior cell fates in the developing mouse hindbrain, its absence leading to an abnormal r3 (and, to a lesser extent, r4) identity of the caudal hindbrain cells.

Involvement of long- and short-range signalling during early tendon development

Tendons connect muscle to skeletal elements. Although tendons have been shown to originate from the lateral plate mesoderm, very little is known at the molecular level about how they are formed. We have found that two genes, Follistatin and Eph-A4, are expressed in regions associated with tendon formation in developing chick limbs. Follistatin is expressed near the tip of the digits and subsequently around the tendon, whereas Eph A4 transcripts were localized in a slightly more proximal region and later in the body of the tendon. Previous work has demonstrated that application of TGFbeta1 or TGFbeta2 to inter-digital regions or the removal of ectoderm in the foot plate induces ectopic cartilage formation, while removal of ectoderm or application of FGF to tips of developing digits leads to truncation. Here we show that TGFbeta1 or removal of ectoderm is also able to induce the expression of both Eph-A4 and Follistatin and that manipulations that cause truncations affect these genes. Thus cartilage and tendon development appear to be coordinated. Ectopic application of recombinant human Follistatin, an antgaonist of certain TGFbeta super-family proteins including Activin and Bmp-4, results in the loss of tendon, implicating signalling by TGFbeta super-family in the development of tendon during chick embryogenesis. Signalling by TGFbeta family members, antagonised by Noggin is known to regulate skeletal development. Thus we suggest that parallel pathways govern both skeletal and tendon patterning.

Regional differences in the developing cerebral cortex revealed by ephrin-A5 expression

The development of axonal connections between thalamic nuclei and their cortical target areas occurs in a highly specific manner. To explore the mechanisms of thalamocortical axon pathfinding, we investigated the expression of several members of the ephrin and Eph gene families in the forebrain. The Eph ligand ephrin-A5 was expressed in three distinct gradients during the development of the telencephalon. The first gradient occurred in the cortical ventricular zone and established ephrin-A5 as one of the earliest markers distinguishing cells residing in the anterior versus posterior cortical neuroepithelium. The second gradient was apparent in the subplate and occurred in spatial opposition to a distinct gradient for the low-affinity NGF receptor p75. This finding reveals that different regions of the early subplate are molecularly heterogeneous. Third, we confirmed that ephrin-A5 is expressed in a bi-directional gradient in the cortical plate, with highest levels in the somatomotor cortex. Three putative receptors for ephrin-A5 -- EphA3, EphA4 and EphA5 -- showed distinct expression patterns in the developing thalamus. The graded distributions of ephrin-A5 in the developing subplate and cortex and the expression of its receptors in the thalamus are consistent with the notion that the Eph ligands and their receptors may function in the topographic mapping of thalamic axons to specific cortical areas.

Characterization of ephrin-A1 and ephrin-A4 as ligands for the EphA8 receptor protein tyrosine kinase

The Eph receptors are the largest known family of receptor protein tyrosine kinases, which play important roles with their ligands called ephrin in the neural development, angiogenesis, and vascular network assembly. It was previously shown that ephrin-A2, -A3 and -A5 bind to, and activate the EphA8 receptor tyrosine kinase, respectively. In this study, we have examined if there are other additional ephrin ligands interacting with the EphA8 receptor tyrosine kinase expressed in NIH3T3 fibroblasts. For this purpose, we have constructed chimeric ephrin-A1, -A4, -B1, -B2 or -B3 ligands consisting of the Fc portion of human IgG fused to their carboxyl-terminus. Both ephrin-A1 and ephrin-A4 chimeric ligands efficiently bound to the EphA8 receptor expressed in NIH3T3 fibroblasts, whereas the transmembrane ligands including ephrin-B1, -B2 and -B3 did not. Additionally we have demonstrated that both the EphA8-TrkB chimeric receptor and the EphA8 receptor expressed in NIH3T3 fibroblasts are efficiently tyrosine-phosphorylated upon stimulating with epthin-A1 or -A4 but none of transmembrane ephrin-B proteins. These results strongly indicate that the EphA8 receptor functions exclusively as an glycosyl phosphatidylinositol (GPI)-linked ephrin ligand-dependent receptor protein tyrosine kinase.

Ephrin-A binding and EphA receptor expression delineate the matrix compartment of the striatum

The striatum integrates limbic and neocortical inputs to regulate sensorimotor and psychomotor behaviors. This function is dependent on the segregation of striatal projection neurons into anatomical and functional components, such as the striosome and matrix compartments. In the present study the association of ephrin-A cell surface ligands and EphA receptor tyrosine kinases (RTKs) with the organization of these compartments was determined in postnatal rats. Ephrin-A1 and ephrin-A4 selectively bind to EphA receptors on neurons restricted to the matrix compartment. Binding is absent from the striosomes, which were identified by mu-opioid receptor immunostaining. In contrast, ephrin-A2, ephrin-A3, and ephrin-A5 exhibit a different mosaic binding pattern that appears to define a subset of matrix neurons. In situ hybridization for EphA RTKs reveals that the two different ligand binding patterns strictly match the mRNA expression patterns of EphA4 and EphA7. Ligand-receptor binding assays indicate that ephrin-A1 and ephrin-A4 selectively bind EphA4 but not EphA7 in the lysates of striatal tissue. Conversely, ephrin-A2, ephrin-A3, and ephrin-A5 bind EphA7 but not EphA4. These observations implicate selective interactions between ephrin-A molecules and EphA RTKs as potential mechanisms for regulating the compartmental organization of the striatum.

In vivo cell sorting in complementary segmental domains mediated by Eph receptors and ephrins

The restriction of intermingling between specific cell populations is crucial for the maintenance of organized patterns during development. A striking example is the restriction of cell mixing between segments in the insect epidermis and the vertebrate hindbrain that may enable each segment to maintain a distinct identity. In the hindbrain, this is a result of different adhesive properties of odd- and even-numbered segments (rhombomeres), but an adhesion molecule with alternating segmental expression has not been found. However, blocking experiments suggest that Eph-receptor tyrosine kinases may be required for the segmental restriction of cells. Eph receptors and their membrane-bound ligands, ephrins, are expressed in complementary rhombomeres and, by analogy with their roles in axon pathfinding, could mediate cell repulsion at boundaries. Remarkably, transmembrane ephrins can themselves transduce signals, raising the possibility that bi-directional signalling occurs between adjacent ephrin- and Eph-receptor-expressing cells. We report here that mosaic activation of Eph receptors leads to sorting of cells to boundaries in odd-numbered rhombomeres, whereas mosaic activation of ephrins results in sorting to boundaries in even-numbered rhombomeres. These data implicate Eph receptors and ephrins in the segmental restriction of cell intermingling.

Characterization of the genes for mouse LERK-3/Ephrin-A3 (Epl3), mouse LERK-4/Ephrin-A4 (Epl4), and human LERK-6/Ephrin-A2 (EPLG6): conservation of intron/exon structure

We have isolated the genes for the eph receptor family ligands mouse LERK-3/Ephrin-A3 (Epl3), mouse LERK-4/Ephrin-A4 (Epl4), and human LERK-6/Ephrin-A2 (EPLG6). These genes show a high level of conservation in their intron/exon structures encoding the receptor-binding region. In addition, the nucleotide sequences of the genes reveal the predicted cDNA sequence of mouse LERK-3/Ephrin-A3, mouse LERK-4/Ephrin-A4, and human LERK-6/Ephrin-A2.

Ligand for EPH-related kinase (LERK) 7 is the preferred high affinity ligand for the HEK receptor

HEK is a member of the EPH-like receptor tyrosine kinase family, which appear to have roles in development and oncogenesis. Recently, we purified a soluble HEK ligand which is also a ligand (AL1) for the HEK-related receptor EHK1. Promiscuity appears to be a characteristic feature of interactions between the EPH-like receptors and their ligands, termed ligands for EPH-related kinases (LERKs). This prompted us to analyze the interactions between the HEK exodomain and fusion proteins comprising candidate LERKs and the Fc portion of human IgG1 (Fc) or a FLAGTM-peptide tag by surface plasmon resonance, size exclusion high performance liquid chromatography, sedimentation equilibrium, and transphosphorylation. Our results indicate that AL1/LERK7 is the preferred high-affinity ligand for HEK, forming a stable 1:1 complex with a dissociation constant of 12 nM. As expected the apparent affinities of bivalent fusion proteins of LERKs and the Fc portion of human IgG1 had significantly reduced dissociation rates compared with their monovalent, FLAGTM-tagged derivatives. High-avidity binding of monovalent ligands can be achieved by antibody-mediated cross-linking of monovalent ligands and with LERK7 results in specific phosphorylation of the receptor. By extrapolation, our findings indicate that some of the reported LERK-receptor interactions are a consequence of the use of bivalent ligand or receptor constructs and may be functionally irrelevant.

The genes encoding the eph-related receptor tyrosine kinase ligands LERK-1 (EPLG1, Epl1), LERK-3 (EPLG3, Epl3), and LERK-4 (EPLG4, Epl4) are clustered on human chromosome 1 and mouse chromosome 3

Hek and elk are members of the eph-related family of receptor tyrosine kinases. Recently, we isolated five cDNAs encoding membrane-bound ligands to hek and elk. Because of the promiscuous nature of their binding, we have termed these proteins ligands of the eph-related kinases or LERKs. The LERKs can be divided into two subgroups by virtue of their sequence identity, binding properties, and mode of cell membrane attachment. For example, LERK-2 (EPLG2, Epl2) and LERK-5 (EPLG5, Epl5) are type 1 transmembrane proteins, while LERK-1 (EPLG1, Epl1), LERK-3 (EPLG3, Epl3), and LERK-4 (EPLG4, Epl4) are anchored to the membrane by glycosyl-phosphatidylinositol (GPI) linkage. Using Southern hybridization analysis of human x rodent somatic cell hybrid DNAs, we have assigned the genes that encode the GPI-anchored LERKs (EPLG1, EPLG3, and EPLG4) to human chromosome 1. Fluorescence in situ hybridization to metaphase chromosome preparations using genomic clones from each locus refined this localization to chromosome 1, bands q21-q22. In addition, Southern blot analysis of DNA from interspecific backcross mice indicated that the mouse homologues Epl1, Epl3, and Epl4 map to a homologous region on mouse chromosome 3.

Ligands for EPH-related tyrosine kinase receptors are developmentally regulated in the CNS

Elk is a member of the eph family of receptor-like tyrosine kinases. Although its function is unknown, elk is postulated to play a role in nervous system development. Using Northern analysis, we examined the developmental regulation of RNAs encoding elk, and several ligands for the eph family of RTKs, the LERKs. Expression of elk, LERK-1, and LERK-2 RNAs is high in all regions examined in the embryonic and postnatal rat brain and decreases to low levels with age. One exception is the adult olfactory bulb which continues to express a moderate level of LERK-2. In contrast, moderate LERK-4 expression was limited to the developing hippocampus and cerebral cortex. These data indicate that elk and some of the LERKs may play a role in nervous system development, maintenance, and/or regeneration.

Ligands for the receptor tyrosine kinases hek and elk: isolation of cDNAs encoding a family of proteins

Hek is a member of the eph subfamily of receptor tyrosine kinases whose members include elk, hek2, sek, eph and eck among others. Using a soluble form of hek consisting of the extracellular region of the receptor fused to the Fc domain of human IgG1 and an expression cloning strategy, we have isolated two different but related cDNAs from the human T-lymphoma line HSB-2 that encode ligands for hek. The cDNAs encode proteins of 238 and 201 amino acids (44% amino acid identity) that are anchored to the membrane by glycosylphosphatidylinositol (GPI)-linkage. The proteins encoded by these cDNAs are bound by hek with affinity constants of 2 x 10(8) M-1. These proteins also bind the elk tyrosine kinase receptor. These cDNAs are related to other cDNAs that we have recently isolated from a human placental library that encode ligands for both hek and elk and define an emerging family of ligands for eph-related kinases (LERKs).