# Protein-O-fucosylation of coreceptors may be required for Nodal signaling in Xenopus

## TL;DR

This study shows that protein-O-fucosylation is important for Nodal signaling in Xenopus embryos, especially for certain Nodal ligands.

## Contribution

The study reveals that XPofut1 is required for a subset of Nodal signaling through EGF-CFC factors in Xenopus.

## Key findings

- XPofut1 knockdown reduces Smad2 phosphorylation and causes axial truncation in Xenopus embryos.
- XPofut1 activity is crucial in the marginal zone but not the vegetal pole of blastula.
- XPofut1 is necessary for Xnr1/Xnr2-induced Smad2 phosphorylation but not for Xnr5/Xnr6.

## Abstract

Nodal-related ligands of TGF-β family play pivotal roles for mesoderm induction and body axis formation during vertebrate early embryogenesis. Nodal ligands are distinct from most other TGF-β ligands family as they require EGF-CFC factors as coreceptors for signaling, in addition to their cognate type I and type II TGF-β receptors. In amphibian Xenopus laevis embryos, 5 Nodal-related genes (Xnr1/2/4/5/6) and 2 EGF-CFC genes (XCR1, XCR3) play roles in mesoderm induction and the accumulation of phosphorylated Smad2, while in mammalian embryos, 1 Nodal gene and 1 EGF-CFC gene (Cripto) play roles during mesoderm induction. Mammalian EGF-CFC factors are reported to be O-fucosylated at a conserved threonine residue of the EGF-like motif by protein-O-fucosyltransferase 1 (Pofut1), but this O-fucose modification is shown to be dispensable for Nodal signaling in mammalian embryos. In this study, we investigated the developmental roles of Xenopus laevis Pofut1 (XPofut1) and its potential function in Nodal signaling. We found that morpholino antisense-mediated knockdown of XPofut1 causes reduction of Smad2 phosphorylation in late blastula and axial truncation in neurula. We also found that the O-fucosyltransferase activity of XPofut1 is important in the marginal zone, but not in the vegetal pole region, of blastula. Interestingly, XPofut1 is necessary for Smad2 phosphorylation induced by Xnr1 or Xnr2, but not by Xnr5 or Xnr6. Among the Nodal signaling components, only EGF-CFC factors are known to be modified by Pofut1. Therefore, based on our current observation, we propose that XPofut1 regulates signaling of a subset of nodal ligands in pregastrulation embryos possibly through modulating the function of EGF-CFC factors.