Ror2 signaling is required for local upregulation of GDF6 and activation of BMP signaling at the neural plate border

Abstract

The receptor tyrosine kinase Ror2 is a major Wnt receptor that activates β-catenin-independent signaling and plays a conserved role in the regulation of convergent extension movements and planar cell polarity in vertebrates. Mutations in the ROR2 gene cause recessive Robinow syndrome in humans, a short-limbed dwarfism associated with craniofacial malformations. Here, we show that Ror2 is required for local upregulation of gdf6 at the neural plate border in Xenopus embryos. Ror2 morphant embryos fail to upregulate neural plate border genes and show defects in the induction of neural crest cell fate. These embryos lack the spatially restricted activation of BMP signaling at the neural plate border at early neurula stages, which is required for neural crest induction. Ror2-dependent planar cell polarity signaling is required in the dorsolateral marginal zone during gastrulation indirectly to upregulate the BMP ligand Gdf6 at the neural plate border and Gdf6 is sufficient to rescue neural plate border specification in Ror2 morphant embryos. Thereby, Ror2 links Wnt/planar cell polarity signaling to BMP signaling in neural plate border specification and neural crest induction.

Signaling pathways and tissue interactions in neural plate border formation

“…48,49 Antisense-morpholino-induced loss of Ror2 function in Xenopus embryos leads to defective NPB formation and downregulation of most NPB genes such as tfap2a, gbx2, zic1, and msx1 and msx2. 50 Moreover, the BMP ligand gdf6 was downregulated in Ror2 morphant embryos already at late blastula and early gastrula stages, which is consistent with an early positive role of BMP signaling in NPB specification and importantly, functionally links Wnt and BMP signaling.…”

“…However, the requirement of Notch signaling in NC development has been shown only in late gastrula and early neurula stages whereas hairy2 expression in the NPB is dependent on BMP, FGF and Wnt but independent of Notch. 57 Notably, the expression levels of bmpr1a, pcns and gdf6 are higher in the adjacent ectoderm at the onset of gastrulation, which additionally expresses bmpr1b, fgfr1, fgfr2, fgfr4 and weakly fgfr3, 50,58,59 suggesting that Wnt, FGF and BMP ligands secreted by the DLMZ are received in the adjacent ectoderm.…”

“…In Ror2 depleted embryos, both, elevation of the neural folds and thickening of the NPB was impaired or delayed. 50 Moreover, cell polarity in the lateral neural plate was disrupted, indicating that Ror2-dependent PCP signaling controls not only NPB specification but also cell polarity in the NPB and neural plate as well as NPB morphogenesis. Thus, Ror2 appears to coordinate and integrate tissue specification and morphogenesis.…”

“…2) and further expresses hairy2, fgfr1, fgfr4a, bmpr1a, pcns and ror2. 4,14,15,50 The early expression of hairy2, a Notch target, suggested a contribution of Notch signaling already in this early stage of NPB specification. However, the requirement of Notch signaling in NC development has been shown only in late gastrula and early neurula stages whereas hairy2 expression in the NPB is dependent on BMP, FGF and Wnt but independent of Notch.…”

“…24,59 Consistently, the NPB expresses the BMP ligands bmp4 and gdf6 as well as the receptors bmpr1a and bmpr1b. 8,27,50,67 Expression of gdf6 at the NPB depends on Ror2 signaling during gastrulation and seems to be important for the spatially restricted activation of BMP signaling at the NPB, which is required for specification and maintenance of NC cell fate also in zebrafish embryos. 68,69 Physical, genetic and functional interactions between BMP, Wnt and FGF combined with the transcription factors expressed in the NPB and additional signaling pathways form a gene regulatory network (for review see ref.…”