SCP3 Regulates Germ Layer Formation In Xenopus Embryos By Removing The Inhibitory Phosphorylation Of R-smads

Germ layer specification is one of the key events shortly after fertilization that initiates body formation of vertebrate embryos. In Xenopus, the maternally deposited transcriptional factor VegT promotes the expression of zygotic Nodal/Activin ligands that further form a morphogen gradient along the vegetal-animal axis and trigger the induction of the three germ layers. However, in the complex organism with redundant regulation, is there a mechanism that cleavaging blastomeres of Xenopus embryos modulate their ability to response to the morphogen signals?To explore the mechanism of germ layer specification, we identified 43 asymmetrically localized maternal mRNAs along the animal-vegetal axis in cleavage Xenopus embryos by using RNA-seq technology. Among them, 12 candidates genes lacking previous known maternal function were further verified and their asymmetrical expression pattern was confirmed by the quantitative real-time PCR. After microinjection of mRNA and MO(morpholino), we further identified the candidates that may contribute to germ layer formation.Here we found SCP3(small C-terminal domain phosphatase 3) is maternally expressed and vegetally enriched in Xenopus embryos and is essential for the timely induction of germ layers. SCP3 is required for the full activation of Nodal/Activin and BMP signals and functions via dephosphorylation of linker regions of R-Smads. Consistently, the linker regions of R-Smads are heavily phosphorylated in fertilized eggs and this phosphorylation is gradually removed when embryos approach the midblastula transition(MBT). Knocking-down of maternal SCP3 attenuates these dephosphorylation events and the activation of Nodal/Activin and BMP signals after MBT.This study thus suggested that the maternal SCP3 serves as a vegetally enriched, intrinsic factor to ensure a prepared status of Smads for their activation by the upcoming ligands during germ layer induction of Xenopus embryos.