Abstract
BackgroundAs 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.ResultsTo 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.ConclusionThese 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.
Highlights
As in other vertebrates, avian hindbrain neural crest migrates in streams to specific branchial arches
Co-localization of Eph/ephrin RNA and neural crest Eph receptors and ephrins are known to be expressed in the hindbrain and branchial arches, it is critical to developing a hypothesis for their functions to know which Eph receptors and ephrins are expressed by neural crest and which by surrounding head mesenchyme and ectoderm
Analysis of the cranial neural crest (CNC) subpopulation associated with the first branchial arch has been excluded due to technical problems with resolving neural crest (NC) staining from staining in the hindbrain
Summary
Avian hindbrain neural crest migrates in streams to specific branchial arches. Cranial neural crest express a combination of Eph receptors and ephrins that appear to exclude cells from adjacent tissues by forward and reverse signalling. Hindbrain cranial neural crest (CNC) cells migrate ventrally to the branchial arches, where they contribute to the development of the face, jaw, neck, and heart [2]. It is concluded that in CNC migration, ephrin-B2 functions primarily as a ligand to activate Eph-induced forward signaling that guides migration [4,5]. As a mutation in the PDZ binding domain of ephrin-B1 produces the same defects, it was concluded that in cranial neural crest ephrin B1 acts as a receptor that activates a PDZ mediated signaling cascade [6]
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