AXIN1 and AXIN2 regulate the WNT-signaling landscape to promote distinct mesoderm programs

How distinct lineage identities are specified from pluripotent epiblast cells during gastrulation is a longstanding open question. By investigating AXIN, a negative regulator of the WNT/β-catenin pathway, we have uncovered previously unrecognized roles for WNT signaling in the specification of distinct mesoderm identities. Using complementary approaches, including a detailed analysis of Axin1;Axin2 mutant mouse embryos involving single-cell and single-embryo transcriptomics and in vitro pluripotent stem cell differentiation assays, our data revealed two critical layers of regulation. First, WNT initiates differentiation of primitive streak cells into mesoderm progenitors. Next, WNT amplifies and cooperates with bone morphogenetic protein (BMP)/pSMAD1/5/9 or NODAL/pSMAD2/3 to propel differentiating mesoderm progenitors into either posterior or anterior streak identities, respectively. We propose that Axin1 and Axin2 function to prevent precocious differentiation of pluripotent epiblast cells into mesoderm through the spatiotemporal regulation of WNT-signaling levels.