Fertilization of the Xenopus egg leads to the asymmetrical distribution of components of the Wnt/β-catenin pathway to what will become the dorsal side of the embryo. This leads to asymmetric signaling by Nodal, a transforming growth factor–β ligand critical to embryonic axis specification and to mesoderm induction. Martello et al. found that microRNAs (miRNAs) from gastrula-stage Xenopus embryos inhibited embryonic Nodal signaling. Computational analysis revealed binding sites for two related miRNAs, miR-15 and miR-16, in the 3′ untranslated region (3′ UTR) of mRNA encoding the type II Nodal receptor Acvr2a. Activity of a reporter containing a wild-type miR-15–binding site was reduced in Xenopus embryos and human HepG2 cells compared with that of a reporter containing a mutated binding site. In addition, activity of the former was further decreased by overexpression of miR-15 or its primary precursor (pri-miR-15/16). Injection of miR-15 inhibited endogenous Nodal signaling and attenuated downstream sequelae such as mesoderm induction and development of Spemann’s organizer, phenotypic defects that were rescued by coinjection of Acvr2a mRNA lacking the 3′ UTR. In contrast, inhibitory oligonucleotides directed against miR-15 and miR-16 increased Acvr2a abundance and promoted expansion of Spemann’s organizer. The abundance and activity of miR-15 and miR-16 were greatest ventrally; Acvr2a abundance and Smad2 phosphorylation (p-Smad2, a marker for Nodal signaling) were greatest dorsally. Injection of pri-miR-15/16 led to a decrease in dorsal p-Smad2, whereas inhibition of miR-15 and miR-16 led to an increase in ventral p-Smad2. Overexpression of Xwnt-8 or β-catenin mRNA decreased the abundance and activity of miR-15 and miR-16, whereas β-catenin knockdown increased them. Thus, miR-15 and miR-16 appear to constitute a link between asymmetry in Wnt/β-catenin and asymmetry in Nodal signaling and, thereby, to play a crucial role in the regulation of embryonic patterning.
G. Martello, L. Zacchigna, M. Inui, M. Montagner, M. Adorno, A. Mamidi, L. Morsut, S. Soligo, U. Tran, S. Dupont, M. Cordenonsi, O. Wessely, S. Piccolo, MicroRNA control of Nodal signalling. Nature 449, 183-188 (2007). [PubMed]