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???displayArticle.abstract??? Nodal proteins are secreted signaling factors of the transforming growth factor beta (TGFbeta) family with essential roles in embryonic development in vertebrates. Mutations affecting the Nodal factors have severe consequences in mammals and fish. Furthermore, increased Nodal levels have been associated with melanoma tumor progression. Like other TGFbeta-related proteins, Nodal factors consist of a pro-domain and a mature domain. The pro-domain of mouse Nodal protein stabilizes its precursor. However, the mechanisms by which the pro-domains exert their activities are unknown. Here, we characterize the zebrafish Nodal-related factor Cyclops (Cyc) and find unexpected functions for the pro-domain in regulating Cyc activity. We identified a lysosome-targeting region in the Cyc pro-domain that destabilizes the precursor and restricts Cyc activity, revealing the molecular basis for the short-range signaling activities of Cyc. We show that both the pro- and mature-domains of Cyc regulate its stability. We also characterize a mutation in the pro-domain of human NODAL (hNODAL) that underlies congenital heterotaxia. Heterologous expression of mutant hNODAL increases expression of Nodal-response genes. Our studies reveal unexpected roles for the pro-domain of the Nodal factors and provide a possible mechanism for familial heterotaxia.
Fig. 5. Mutation of a conserved arginine residue in hNODAL renders the protein more active and can induce left-right asymmetry defects. (A,B) CycR314Q and hNODALR183Q are more active than Cyc and hNODAL. Zebrafish embryos at the one-cell stage were injected with 5 or 25 pg of RNA encoding either Cyc, CycR314Q, hNODAL or hNODALR183Q. Expression of gsc (A) or ntl (B) was examined at 50% epiboly. The Arg-Gln mutations render CycR314Q and hNODALR183Q more active than the wild-type proteins. Animal pole views shown. Scale bar: 100 μm. (C) Overexpression of CycR314Q and hNODALR183Q in Xenopus embryos causes perturbations in left-sided expression of Xpitx2. Xenopus embryos at the four-cell stage were injected with 25 pg of DNA expression constructs encoding either Cyc, CycR314Q, hNODAL or hNODALR183Q in the dorsal-left or dorsal-right blastomeres and cultured until stage 28 and processed for in situ hybridization to detect Xpitx2 expression. Embryos were scored as left, right, bilateral or no Xpitx2 expression. Ventral views of stage 28 embryos with Xpitx2 expression in the cardiac primordia are shown. Injections of hNODALR183Q or CycR314Q can induce right-sided, bilateral or no Xpitx2 expression at a higher frequency than observed after injections of hNODAL or Cyc. (D) Gut looping is also perturbed by injection of hNODALR183Q. Xenopus embryos at the four-cell stage were injected with 25 pg of DNA encoding either hNODAL or hNODALR183Q in the dorsal-left or dorsal-right blastomeres and cultured until stage 45 and scored for the direction of intestinal coiling. Injected embryos manifested either normal, reversed or abnormal looping. In some embryos, heterotaxia of intestinal coiling and cardiac laterality was observed. Right-sided injections of hNODALR183Q increase the frequency of reversed intestinal coiling in comparison with hNODAL.
