Post-transcriptional regulation of Profilin-2 by microRNAs and RNA-binding proteins forms a critical regulatory node for early embryonic cell fate decisions.

Post-transcriptional control by RNA binding proteins (RBPs) and microRNAs play central roles in mRNA stability and translation, yet how RBPs and microRNAs coordinate in developmental time to regulate cell fate remains poorly understood. Here, we demonstrate that post-transcriptional regulation of the Profilin 2 (Pfn2) transcript is essential for differentiation of embryonic stem cells (ESCs) into the primary germ layer lineages. The Pfn2 3'untranslated region has both an Iron Regulatory Protein binding site (IRE) and a nearby binding site for ESC enriched microRNAs. Deletion of this microRNA site leads to increased PFN2 and reduced FGF signaling during pluripotency transition prior to germ layer formation. In contrast, deletion of the IRE leads to decreased PFN2, a Wnt signaling defect, reduced nuclear beta-catenin, and a subsequent block in mesendodermal lineages during early germ layer formation. We further find that loss of the IRE site results in a cell autonomous defect in Wnt signaling and mesendodermal differentiation. The IRE site acts to stabilize beta-catenin, as disruption of the site leads to reduced nuclear beta-catenin levels. Together, these findings reveal the Pfn2 microRNA-IRE regulatory axis as a critical post-transcriptional regulatory node governing the switch from pluripotency to somatic differentiation.