β-catenin safeguards cell survival via a transcription-independent mechanism during the induction of primitive streak from hESCs.

The emergence of the primitive streak, representing an organizing center for gastrulation, marks the mesendodermal lineage specification from epiblast, in which the epiblast cells undergo highly organized collective behaviors to form mesendodermal cells properly. Cell death is observed at the peri-gastrulation stage, especially in the primitive streak region. However, the dynamic and regulatory mechanism of cell death in the primitive streak formation is unclear. Here, we observed that a quick inhibition of the fast elevated cell death is coinciding with an accumulation of β-catenin during the early stage of primitive streak induction from human embryonic stem cells (hESCs). Deficiency of β-catenin in hESCs does not affect their self-renewal but cause robust cell death after primitive streak induction, while neuroectodermal differentiation remains unchanged. Overexpression of full-length β-catenin in β-catenin-deficient hESCs restores the cell death restriction during induction of primitive streak. Mechanistically, the β-catenin-restricted cell death during primitive streak is transcription-independent. The accumulated β-catenin traps casein kinase-1 in β-catenin destruction complex following WNT activation via its ARM repeat domain, resulting in the inhibition of mTORC1 by stabilizing DEPTOR, subsequently attenuates mitochondrial translocation of p53 and enhances mitophagy to promote cell survival. Consistently, mTORC1 inhibition by rapamycin or RAD001 attenuates the cell death in β-catenin-deficient cells during induction of primitive streak. In addition, only the β-catenin retains activations of cell death restriction and transcriptional activity can promote hESCs to successfully differentiate into primitive streak and cardiomyocytes, suggesting that β-catenin-restricted cell death safeguards the fate transition during the primitive streak induction via offering a crucial window for the accumulation of β-catenin to induce lineage-specific genes. These findings provide new insights into the function and mechanisms by which β-catenin coordinates the cell death and early lineage commitment.