Suppression of P-cadherin expression as a key regulatory element for embryonic stem cell stemness.

In embryonic stem (ES) cell colonies, a small subpopulation that changes cell shape and loses pluripotency often appears in two-dimensional (2D) cultures, even in the presence of a stemness factor. We have previously shown that membrane translocation of the syntaxin4, t-SNARE protein contributes to this phenomenon. Here, we show that ES cells in three-dimensional (3D) aggregates do not succumb to extruded syntaxin4 owing to suppressed expression of P-cadherin protein. While extracellular expression of syntaxin4 led to the striking upregulation of P-cadherin mRNA in both 2D and 3D-ES cells, morphological changes and appreciable expression of P-cadherin protein were detected only in 2D-ES cells. Importantly, the introduction of an expression cassette for P-cadherin practically reproduced the effects induced by extracellular syntaxin4, where the transgene product was clearly detected in 2D-, but not 3D-ES cells. An expression construct for P-cadherin-Venus harboring an in-frame insertion of the P2A sequence at the joint region gave fluorescent signals only in the cytoplasm of 2D-ES cells, demonstrating translational regulation of P-cadherin. These results provide the mechanistic insight into the uncontrollable differentiation in 2D-ES cells and shed light on the validity of the "embryoid body protocol commonly used for ES cell handling" for directional differentiation.Key words: differentiation, embryoid body, ES cells, P-cadherin, syntaxin4.