Antagonism of Nodal signaling by BMP/Smad5 prevents ectopic primitive streak formation in the mouse amnion

Abstract

SUMMARYThe strength and spatiotemporal activity of Nodal signaling is tightly controlled in early implantation mouse embryos, including by autoregulation and feedback loops, and involves secreted and intracellular antagonists. These control mechanisms, which are established at the extra-embryonic/embryonic interfaces, are essential for anterior-posterior patterning of the epiblast and correct positioning of the primitive streak. Formation of an ectopic primitive streak, or streak expansion, has previously been reported in mutants lacking antagonists that target Nodal signaling. Here, we demonstrate that loss-of-function of a major bone morphogenetic protein (BMP) effector, Smad5, results in formation of an ectopic primitive streak-like structure in mutant amnion accompanied by ectopic Nodal expression. This suggests that BMP/Smad5 signaling contributes to negative regulation of Nodal. In cultured cells, we find that BMP-activated Smad5 antagonizes Nodal signaling by interfering with the Nodal-Smad2/4-Foxh1 autoregulatory pathway through the formation of an unusual BMP4-induced Smad complex containing Smad2 and Smad5. Quantitative expression analysis supports that ectopic Nodal expression in the Smad5 mutant amnion is induced by the Nodal autoregulatory loop and a slow positive-feedback loop. The latter involves BMP4 signaling and also induction of ectopic Wnt3. Ectopic activation of these Nodal feedback loops in the Smad5 mutant amnion results in the eventual formation of an ectopic primitive streak-like structure. We conclude that antagonism of Nodal signaling by BMP/Smad5 signaling prevents primitive streak formation in the amnion of normal mouse embryos.

Signaling Pathways and Gene Regulatory Networks in Cardiomyocyte Differentiation

“…BMPs represses Nodal by sequestration of the shared Smad-4 54 or by promoting the formation of Smad-5/-2 complexes disrupting the Nodal autoregulatory pathway with Smad-2, -4, and Foxh1. 55 The fibroblast growth factor (FGF), Erk, p38, and JNK/mitogen-activated protein kinase (MAPK) pathways phosphorylate Smad-1 contributing to its degradation. There are also interactions with the Wnt/b-catenin pathway since Wnt3A can stabilize and prolong Smad-1 signal by preventing GSK-3-and MAPKmediated Smad-1 degradation.…”

Smad2 Is Essential for Maintenance of the Human and Mouse Primed Pluripotent Stem Cell State

“…In mice, the site of distal visceral endoderm formation is defined by antagonism between Smad2 and Smad1 (60). Similarly, antagonism of Smad5 with nodal/Smad2 signaling in mouse amnion prevents ectopic primitive streak formation (61). Antagonism between Smad2 and Smad1/5 was also shown to be critical in Xenopus dorsal-ventral mesoderm polarity (62) and in Drosophila wing development (63).…”

Distinct SoxB1 networks are required for naïve and primed pluripotency

“…Depicted below are diagrams of pre- (E3.5) and postimplantation (E5.5 and E7.5) mouse embryos indicating the published expression patterns of SoxB1 mRNAs (Wood & Episkopou, 1999; Uchikawa et al, 2011; Avilion et al, 2003; Cajal et al, 2012), and the areas of BMP/Nodal signalling and inhibition during gastrulation (Constam & Robertson, 2000; Bachiller et al, 2000; Kinder et al, 2001; Levine et al, 2006; Pereira et al, 2012; Norris et al, 2002; Lawson et al, 1999; Perea-Gomez et al, 2002). …”