Reconciling the different roles of Gsk3β in "naïve" and "primed" pluripotent stem cells.

Signaling pathways orchestrated by PI3K/Akt, Raf/Mek/Erk and Wnt/β-catenin are known to play key roles in the self-renewal and differentiation of pluripotent stem cells. The serine/threonine protein kinase Gsk3β has roles in all three pathways, making its exact function difficult to decipher. Consequently, conflicting reports have implicated Gsk3β in promoting self-renewal, while others suggest that it performs roles in the activation of differentiation pathways. Different thresholds of Gsk3β activity also have different biological effects on pluripotent cells, making this situation even more complex. Here, we describe a further level of complexity that is most apparent when comparing "naïve" murine and "primed" human pluripotent stem cells. In naïve cells, Gsk3β activity is restrained by PI3K/Akt, but when released from inhibitory signals it antagonizes self-renewal pathways by targeting pluripotency factors such as Myc and Nanog. This situation also applies in primed cells, but, in addition, a separate pool of Gsk3β is required to suppress canonical Wnt signaling. These observations suggest that different Gsk3β-protein complexes shift the balance between naïve and primed pluripotent cells and identify fundamental differences in their cell signaling. Altogether, these findings have important implications for the mechanisms underpinning the establishment of different pluripotent cell states and for the control of self-renewal and differentiation.