Eph Signaling Research Articles

Conservation of gene function between mammals and flies

Conservation of gene function between mammals and Drosophila is not always easy to demonstrate, given the vast differences in anatomy and embryology between species. Therefore, it comes as a pleasant surprise that Dearborn et al. now demonstrate potentially conserved functions in the eph gene family between higher vertebrates and flies.Eph genes encode Eph tyrosine kinase receptors, which, along with their ligands the ephrins, control vertebrate central nervous system patterning, neural crest migration and axon guidance. There are 14 eph genes in mammals, but only one in the fly, originally designated Dek by Scully and co-workers. There also appears to be only one Ephrin-like protein in Drosophila. Dearborn et al. [1xEph receptor tyrosine kinase-mediated formation of a topographic map in the Drosophila visual system. Dearborn, R et al. J. Neurosci. 2002; 22: 1338–1349PubMedSee all References][1] have investigated the role of eph in the developing fly visual system, thus allowing a comparison with the vertebrate retinotectal system in which Eph/ephrins are clearly implicated in topographic mapping of axons. Vertebrate Ephs and ephrins exhibit topographic countergradients in the retina and optic tectum. In the fly, EPH expression is first seen in early differentiated photoreceptors and their axons, with no dorsoventral gradient of expression. There is an anteroposterior gradient, but this can also be interpreted as being due to developmental timing. In the photoreceptor target fields, the medulla cortex and neuropil also express high amounts of EPH, where there is a distinct, dorsoventral gradient of expression: EPH is highest in the midline, fading away completely in the dorsal and ventral quadrants.Using a series of elegant, genetic tools, the authors suppressed endogenous EPH expression and also established ectopic expression of dominant–negative and wild-type EPH in photoreceptors and the medulla cortex. The conclusion of these studies was that high EPH expression is required in midline neurons of the medulla cortex in order to establish topographic projections to the medulla neuropil and the lobula. Further evidence also suggests that midline photoreceptors require high levels of EPH to establish topographic terminations in the midline of the medulla neuropil.This is the first evidence that Drosophila eph and its vertebrate homologues have maintained a similar function in the topographic mapping of axons, over vast evolutionary time. We can now look forward to the power of fly genetics being employed to dissect further the regulation of EPH signalling.

EphA4 Activity Causes Cell Shape Change and a Loss of Cell Polarity in Xenopus laevis embryos

The Eph family of receptor tyrosine kinases and their ephrin ligands are believed to limit cell-cell interactions during embryonic development via a repulsive mechanism. Little is known, however, about the intracellular effects of Eph signaling that lead to cellular repulsion. We have used scanning and transmission electron microscopy to examine the effects of EphA4 catalytic activity on cells in early embryos of Xenopus laevis. We show that ectopic EphA4 catalytic activity in superficial blastula cells leads to a more rounded cellular morphology, a loss of apical microvilli, and a loss of the apical/basolateral boundary, in addition to the previously reported loss of cell adhesion. These effects indicate that these epithelial cells have lost their apical/basolateral polarity. We also show that EphA4 catalytic activity causes a preferential loss of adherens junctions, compared to tight junctions. Furthermore, EphA4 catalytic activity was found to result in a change in filamentous actin levels in blastomeres. These results taken together suggest that the actin cytoskeleton might be a target of EphA4 signaling.

Eph signalling functions downstream of Val to regulate cell sorting and boundary formation in the caudal hindbrain

Rhombomeres are segmental units of the developing vertebrate hindbrain that underlie the reiterated organisation of cranial neural crest migration and neuronal differentiation. valentino (val), a zebrafish homologue of the mouse bzip transcription factor-encoding gene, kreisler, is required for segment boundary formation caudal to rhombomere 4 (r4). val is normally expressed in r5/6 and is required for cells to contribute to this region. In val(-) mutants, rX, a region one rhombomere in length and of mixed identity, lies between r4 and r7. While a number of genes involved in establishing rhombomeric identity are known, it is still largely unclear how segmental integrity is established and boundaries are formed. Members of the Eph family of receptor tyrosine kinases and their ligands, the ephrins, are candidates for functioning in rhombomere boundary formation. Indeed, expression of the receptor ephB4a coincides with val in r5/6, whilst ephrin-B2a, which encodes a ligand for EphB4a, is expressed in r4 and r7, complementary to the domain of val expression. Here we show that in val(-) embryos, ephB4a expression is downregulated and ephrin-B2a expression is upregulated between r4 and r7, indicating that Val is normally required to establish the mutually exclusive expression domains of these two genes. We show that juxtaposition of ephB4a-expressing cells and ephrin-B2a-expressing cells in the hindbrain leads to boundary formation. Loss of the normal spatial regulation of eph/ephrin expression in val mutants correlates not only with absence of boundaries but also with the inability of mutant cells to contribute to wild-type r5/6. Using a genetic mosaic approach, we show that spatially inappropriate Eph signalling underlies the repulsion of val(-) cells from r5/6. We propose that Val controls eph expression and that interactions between EphB4a and Ephrin-B2a mediate cell sorting and boundary formation in the segmenting caudal hindbrain.

Eph signaling is required for segmentation and differentiation of the somites.

Somitogenesis involves the segmentation of the paraxial mesoderm into units along the anteroposterior axis. Here we show a role for Eph and ephrin signaling in the patterning of presomitic mesoderm and formation of the somites. Ephrin-A-L1 and ephrin-B2 are expressed in an iterative manner in the developing somites and presomitic mesoderm, as is the Eph receptor EphA4. We have examined the role of these proteins by injection of RNA, encoding dominant negative forms of Eph receptors and ephrins. Interruption of Eph signaling leads to abnormal somite boundary formation and reduced or disturbed myoD expression in the myotome. Disruption of Eph family signaling delays the normal down-regulation of her1 and Delta D expression in the anterior presomitic mesoderm and disrupts myogenic differentiation. We suggest that Eph signaling has a key role in the translation of the patterning of presomitic mesoderm into somites.

Embryonic expression of eph signalling factors in Xenopus

Cellular communication in the developing embryo is mediated by receptor-ligand interactions at the cell surface. Receptor protein tyrosine kinases (RTKs) have been shown to play a critical role in the development of the vertebrate embryo. The eph receptors are a large subclass of RTKs for which a corresponding ligand family has only recently been described. The restricted expression patterns of several eph receptors imply roles for these molecules in early vertebrate development. We have isolated both a ligand of the eph ligand family ( ELF), that we have named XELF-a, and an eph-related receptor, XE10, the likely homolog of the murine eck Sek-2 receptor. At least two forms of the XELF-a transcript are present in the developing embryo. A truncated form of the XELF-a ligand, XELF-a′, is the first ELF ligand isolated that lacks both the membrane-spanning and membrane-anchoring motifs conserved among this family, suggesting that ELF ligands can function as fully soluble molecules in vivo. XELF-a and XE10 are expressed maternally and throughout early embryogenesis, while XELF-a′ is only expressed zygotically. The dynamic expression patterns of these signalling molecules, in both mesoderm and neurectoderm, suggest that they may play a role in the patterning of the early vertebrate embryo.