Divergent roles for Eph and Ephrin in Avian Cranial Neural Crest

<p>Abstract</p> <p>Background</p> <p>As in other vertebrates, avian hindbrain neural crest migrates in streams to specific branchial arches. Signalling from Eph receptors and ephrins has been proposed to provide a molecular mechanism that guides the cells restricting them to streams. In mice and frogs, cranial neural crest express a combination of Eph receptors and ephrins that appear to exclude cells from adjacent tissues by forward and reverse signalling. The objective of this study was to provide comparative data on the distribution and function of Eph receptors and ephrins in avian embryos.</p> <p>Results</p> <p>To distinguish neural crest from bordering ectoderm and head mesenchyme, we have co-labelled embryos for Eph or ephrin RNA and a neural crest marker protein. Throughout their migration avian cranial neural crest cells express EphA3, EphA4, EphA7, EphB1, and EphB3 and move along pathways bordered by non-neural crest cells expressing ephrin-B1. In addition, avian cranial neural crest cells express ephrin-B2 and migrate along pathways bordered by non-neural crest cells expressing EphB2. Thus, the distribution of avian Eph receptors and ephrins differs from those reported in other vertebrates. In stripe assays when explanted cranial neural crest were given the choice between FN or FN plus clustered ephrin-B1 or EphB2 fusion protein, the cells strongly localize to lanes containing only FN. This preference is mitigated in the presence of soluble ephrin-B1 or EphB2 fusion protein.</p> <p>Conclusion</p> <p>These findings show that avian cranial neural crest use Eph and ephrin receptors as other vertebrates in guiding migration. However, the Eph receptors are expressed in different combinations by neural crest destined for each branchial arch and ephrin-B1 and ephrin-B2 appear to have opposite roles to those reported to guide cranial neural crest migration in mice. Unlike many of the signalling, specification, and effector pathways of neural crest, the roles of Eph receptors and ephrins have not been rigorously conserved. This suggests diversification of receptor and ligand expression is less constrained, possibly by promiscuous binding and use of common downstream pathways.</p

The molecular phylogeny of eph receptors and ephrin ligands

<p>Abstract</p> <p>Background</p> <p>The tissue distributions and functions of Eph receptors and their ephrin ligands have been well studied, however less is known about their evolutionary history. We have undertaken a phylogenetic analysis of Eph receptors and ephrins from a number of invertebrate and vertebrate species.</p> <p>Results</p> <p>Our findings indicate that Eph receptors form three major clades: one comprised of non-chordate and cephalochordate Eph receptors, a second comprised of urochordate Eph receptors, and a third comprised of vertebrate Eph receptors. Ephrins, on the other hand, fall into either a clade made up of the non-chordate and cephalochordate ephrins plus the urochordate and vertebrate ephrin-Bs or a clade made up of the urochordate and vertebrate ephrin-As.</p> <p>Conclusion</p> <p>We have concluded that Eph receptors and ephrins diverged into A and B-types at different points in their evolutionary history, such that primitive chordates likely possessed an ancestral ephrin-A and an ancestral ephrin-B, but only a single Eph receptor. Furthermore, ephrin-As appear to have arisen in the common ancestor of urochordates and vertebrates, whereas ephrin-Bs have a more ancient bilaterian origin. Ancestral ephrin-B-like ligands had transmembrane domains; as GPI anchors appear to have arisen or been lost at least 3 times.</p

EphA4 probe hybridizes to r3, r5, and a streak of cells caudal to the OV (arrow) that coincide with the stream of HNK-1-positive cells associated with BAIII (a, b)

EphB1 probe binding cells have an identical distribution (c, d). Insets show the same embryos at lower magnification. Ec, ectoderm; My, mesenchyme; s, somite; BA, branchial arch; OV, otic vesicle.<p><b>Copyright information:</b></p><p>Taken from "Divergent roles for Eph and Ephrin in Avian Cranial Neural Crest"</p><p>http://www.biomedcentral.com/1471-213X/8/56</p><p>BMC Developmental Biology 2008;8():56-56.</p><p>Published online 21 May 2008</p><p>PMCID:PMC2405773.</p><p></p

Distribution of ephrin-B2 mRNA and HNK-1 protein in the hindbrain region of the stage 12–14 embryos

At stage 14, ephrin-B2 probe labels streams of cells rostral and caudal to the otic vesicle (arrows), as does HNK-1 (a, b). The probe has a similar distribution in stage 13 (c) and stage 12 (d) embryos. Insets show the same embryos at lower magnification. In cross section, ephrin-B2 probe binding cells again co-localize with cells stained by HNK-1 (arrowheads) (e, f). Inset shows the same section at lower magnification. BA, branchial arch; Ec, ectoderm; My, mesenchyme; NT, neural tube; OV, otic vesicle. Distribution of ephrin-B2 mRNA and HNK-1 protein in the embryonic avian hindbrain. Wholemount (a, b) and cross-section (c, d) images of a stage 14 chicken embryo double labeled for ephrin-B2 and HNK-1. (a, b) Ephrin-B2 probe labels cells rostral and caudal to the OV (arrows), as does HNK-1. Inset shows embryo at lower magnification. (c, d) In cross-section, ephrin-B2 probe binding cells again colocalize with cells stained by HNK-1 (arrowheads). Inset shows section at lower magnification. Ec, ectoderm; My, mesenchyme; s, somite.<p><b>Copyright information:</b></p><p>Taken from "Divergent roles for Eph and Ephrin in Avian Cranial Neural Crest"</p><p>http://www.biomedcentral.com/1471-213X/8/56</p><p>BMC Developmental Biology 2008;8():56-56.</p><p>Published online 21 May 2008</p><p>PMCID:PMC2405773.</p><p></p

Wholemount (a-c) and cross-section (d-f) images of a stage 14 chicken embryo double labeled for EphB2 and HNK-1

(a-c) EphB2 probe staining is in patches (arrows) bordering cells labeled by HNK-1. Inset shows embryo at lower magnification. The margins of the regions expressing EphB2 have been traced with a thin white line in panel a. The tracing from panel a has been superimposed on the image in panel b to show the margins relative to the HNK-1 expressing cells. (d-fi) In cross-section (taken at black line in c), probe (arrow) and HNK-1 stained cells have a non-overlapping distribution. Inset shows section at lower magnification. Ec, ectoderm; My, mesenchyme; s, somite.<p><b>Copyright information:</b></p><p>Taken from "Divergent roles for Eph and Ephrin in Avian Cranial Neural Crest"</p><p>http://www.biomedcentral.com/1471-213X/8/56</p><p>BMC Developmental Biology 2008;8():56-56.</p><p>Published online 21 May 2008</p><p>PMCID:PMC2405773.</p><p></p

There are significantly more cells found growing on lanes of substrate bound ephrin-B1/Fc or EphB2/Fc protein in the presence of soluble competitor than in the presence of soluble Fc

<p><b>Copyright information:</b></p><p>Taken from "Divergent roles for Eph and Ephrin in Avian Cranial Neural Crest"</p><p>http://www.biomedcentral.com/1471-213X/8/56</p><p>BMC Developmental Biology 2008;8():56-56.</p><p>Published online 21 May 2008</p><p>PMCID:PMC2405773.</p><p></p

Cells labeled by EphA3 probe and HNK-1 antibody (arrowheads) form a narrow, sub-ectodermal band that reaches from the neural tube to the branchial arches (a, b)

This is also the case with EphA7 (c, d) and EphB3 (e, f). Insets show the same sections at lower magnification. BA, branchial arch; Ec, ectoderm; My, mesenchyme; s, somite.<p><b>Copyright information:</b></p><p>Taken from "Divergent roles for Eph and Ephrin in Avian Cranial Neural Crest"</p><p>http://www.biomedcentral.com/1471-213X/8/56</p><p>BMC Developmental Biology 2008;8():56-56.</p><p>Published online 21 May 2008</p><p>PMCID:PMC2405773.</p><p></p

EphA3 probe labels streams of cells (arrows) rostral and caudal to the OV, as does the HNK-1 antibody (a, b)

EphA7 (c, d) and EphB3 (e, f) probe binding cells have a similar distribution. Insets show the same embryos at lower magnification. BA, branchial arch; Ec, ectoderm; My, mesenchyme; s, somite.<p><b>Copyright information:</b></p><p>Taken from "Divergent roles for Eph and Ephrin in Avian Cranial Neural Crest"</p><p>http://www.biomedcentral.com/1471-213X/8/56</p><p>BMC Developmental Biology 2008;8():56-56.</p><p>Published online 21 May 2008</p><p>PMCID:PMC2405773.</p><p></p