Xenopuscadherin-11 restrains cranial neural crest migration and influences neural crest specification

Cranial neural crest (CNC) cells migrate extensively, typically in a pattern of cell streams. In Xenopus, these cells express the adhesion molecule Xcadherin-11 (Xcad-11) as they begin to emigrate from the neural fold. In order to study the function of this molecule, we have overexpressed wild-type Xcad-11 as well as Xcad-11 mutants with cytoplasmic(ΔcXcad-11) or extracellular (ΔeXcad-11) deletions. Green fluorescent protein (GFP) was used to mark injected cells. We then transplanted parts of the fluorescent CNC at the premigratory stage into non-injected host embryos. This altered not only migration, but also the expression of neural crest markers. Migration of transplanted cranial neural crest cells was blocked when full-length Xcad-11 or its mutant lacking the β-catenin-binding site(ΔcXcad-11) was overexpressed. In addition, the expression of neural crest markers (AP-2, Snail and twist) diminished within the first four hours after grafting, and disappeared completely after 18 hours. Instead, these grafts expressed neural markers (2G9, nrp-1 andN-Tubulin). β-catenin co-expression, heterotopic transplantation of CNC cells into the pharyngeal pouch area or both in combination failed to prevent neural differentiation of the grafts. By contrast, ΔeXcad-11 overexpression resulted in premature emigration of cells from the transplants. The AP-2 and Snailpatterns remained unaffected in these migrating grafts, while twistexpression was strongly reduced. Co-expression of ΔeXcad-11 andβ-catenin was able to rescue the loss of twist expression,indicating that Wnt/β-catenin signalling is required to maintaintwist expression during migration. These results show that migration is a prerequisite for neural crest differentiation. Endogenous Xcad-11 delays CNC migration. Xcad-11 expression must, however, be balanced, as overexpression prevents migration and leads to neural marker expression. Although Wnt/β-catenin signalling is required to sustain twist expression during migration, it is not sufficient to block neural differentiation in non-migrating grafts.