Xenopus neurula left-right asymmetry is respeficied by microinjecting TGF-beta5 protein

A variety of TGF-beta-related ligands regulate the left-right asymmetry of vertebrates but the involvement of TGF-betas in left-right specification has not been reported. We assessed whether TGF-beta signaling is involved in the left-right specification of Xenopus post-gastrula embryos by microinjecting Xenopus TGF-beta5 protein into the left or right flank of neurula-tailbud embryos. Injection on the right side of neurulae caused left-right reversal of the internal organs in 93% of the embryos, while injection on the left side caused less than 5% left-right reversal. Expression of Xenopus nodal related-1 (Xnr-1 ), Xenopus antivin and Xenopus Pitx2, which are normally expressed on the left, was unaltered by the left-side injection. In contrast, right-side injection into neurulae induced the expression of these genes predominantly on the right side. Right-side injection into tailbud embryos caused bilateral expression of these handed genes. Time course analysis of asymmetric gene expression revealed that Xnr-1 could be induced by TGF-beta5 at late neurula stage, while antivin and Pitx2 could be induced by TGF-beta5 at the latertail bud stage. Injection of the antisense morpholino oligonucleotide against Xenopus TGF-beta5 into the left dorsal blastomere inhibited the normal left-handed expression of Xnr-1 and Pitx2, and caused the organ reversal in the injected embryos. These results suggest that normal left-right balance of endogenous TGF-beta5 signaling in the neurula embryo may be needed to determine the laterality of the asymmetric genes and to generate the correct left-right axis.