Coordination of germ layer lineage choice by TET1 during primed pluripotency.

Gastrulation in the early postimplantation stage mammalian embryo begins when epiblast cells ingress to form the primitive streak or develop as the embryonic ectoderm. The DNA dioxygenase Tet1 is highly expressed in the epiblast and yet continues to regulate lineage specification during gastrulation when its expression is diminished. Here, we show how Tet1 plays a pivotal role upstream of germ layer lineage bifurcation. During the transition from naive pluripotency to lineage priming, a global reconfiguration redistributes Tet1 from Oct4-cobound promoters to distal regulatory elements at lineage differentiation genes, which are distinct from high-affinity sites engaged by Oct4. An altered chromatin landscape in Tet1-deficient primed epiblast-like cells is associated with enhanced Oct4 expression and binding to Nodal and Wnt target genes, resulting in collaborative signals that enhance mesendodermal and inhibit neuroectodermal gene expression during lineage segregation. A permissive role for Tet1 in neural fate induction involves Zic2-dependent engagement at neural target genes at lineage priming, is dependent on the signaling environment during gastrulation, and impacts neural tube closure after gastrulation. Our findings provide mechanistic information for epigenetic integration of pluripotency and signal-induced differentiation cues.

Nodal signaling has dual roles in fate specification and directed migration during germ layer segregation in zebrafish.

During gastrulation, endodermal cells actively migrate to the interior of the embryo, but the signals that initiate and coordinate this migration are poorly understood. By transplanting ectopically induced endodermal cells far from the normal location of endoderm specification, we identified the inputs that drive internalization without the confounding influences of fate specification and global morphogenic movements. We find that Nodal signaling triggers an autocrine circuit for initiating endodermal internalization. Activation of the Nodal receptor directs endodermal specification through sox32 and also induces expression of more Nodal ligands. These ligands act in an autocrine fashion to initiate endodermal cell sorting. Our work defines an 'AND' gate consisting of sox32-dependent endodermal specification and Nodal ligand reception controlling endodermal cell sorting to the inner layer of the embryo at the onset of gastrulation.