Pluripotent stem cells can give rise to all three germ layers: ectoderm, endoderm, and mesoderm. During embryonic development, naïve pluripotent stem cells transition to a formative state, which occurs upon implantation of the embryo. The formative pluripotent epiblast is the only tissue in development that contains the inherent power to initiate differentiation into one of the three germ layers and the germ line. What triggers this fundamental cell fate transition, however, is largely unknown. First author Laura Santini, a former PhD student in the Leeb lab and first author of the study, explains: “In our study, we focused on how the AKT signaling pathway and its downstream targets regulate this transition.” Activation of the PI3K/AKT pathway is known to play a critical role in cell proliferation and differentiation, while its inactivation by the lipid phosphatase PTEN is associated with growth and proliferation arrest.
Having observed that PTEN levels are elevated at the time of transition to the formative state, the Leeb lab used mouse embryonic stem cells lacking PTEN to test whether AKT activity influenced the cell fate transition. “These cells could not exit the naïve state on schedule; their transition was delayed,” Laura explains. Further investigation revealed that upregulated AKT signaling in PTEN mutants led to the retention of FoxO transcription factors in the cytoplasm.
“We were excited to observe that, upon nuclear translocation, FoxO transcription factors contributed to a switch from the naïve gene regulatory network (GRN) to the formative GRN,” Laura adds. GRNs define cell function and identity by regulating the expression and timing of gene activity through interconnected networks of genes, transcription factors, and other regulatory molecules. By conducting a series of chromatin profiling and genetic experiments, the team confirmed that FoxO transcription factors are necessary and sufficient to trigger differentiation by binding to key enhancers in the pluripotency GRN.
Group leader Martin Leeb says: “Our findings reveal that FoxO1 is a central component of the pluripotency transcription factor circuit. The AKT-FoxO1 signaling pathway facilitates the successful establishment of differentiated embryo structures upon implantation.”
DOI: 10.1038/s41467-024-51794-9