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PI3K Signaling Inhibition - A New Way to Culture Trophoblast Stem Cells

Review of “Inhibition of Phosphoinositide-3-kinase signaling promotes the stem cell state of trophoblast” from STEM CELLS by Stuart P. Atkinson

Trophoblast stem cells (TSCs), a self‐renewing population of extra‐embryonic cells that can differentiate into all trophoblastic cell types of the placenta, are derived from mouse blastocysts and early postimplantation stage embryos [1, 2]. Unfortunately, we know little regarding the precise signaling pathways required to support the stem cell state of TSCs, a fact exemplified by the generally heterogeneous nature of in vitro TSC cultures.

In the presence of inhibitors of specific pathways, embryonic stem cells (ESCs) form homogeneous colonies of cells that maintain epigenomic and transcriptional characteristics of a naïve ESC state [3, 4]. Researchers led by Myriam Hemberger (University of Calgary, Alberta, Canada) and Cheryl Q.E. Lee (Institute of Medical Biology, Singapore) speculated that similar conditions might promote the culture of murine TSCs in an elevated state of potency. In a new STEM CELLS study, the authors now report phosphoinositide 3-kinases (PI3K) signaling as a vitally important pathway that fine‐tunes stemness in the trophoblast compartment [5].

The authors of this new study employed a small molecule kinase inhibitor screen to identify those signaling pathways critically involved in the maintenance and differentiation of TSCs. While the vast majority of inhibitors promoted TSC differentiation affected signaling pathways and cell cycle proteins involved in TSC self‐renewal and proliferation, they discovered that inhibition of PI3K signaling actually increased TSC marker expression (e.g., Cdx2, Egr1, Eomes, and Esrrb), thereby promoting a tighter epithelial colony morphology, as observed in ESCs when cultured in naïve conditions, with fewer differentiated or apoptotic cells. This can be viewed as a surprising finding, as PI3K inhibition generally leads to differentiation and apoptosis in other stem cells [6]. 

TSC growth conditions generally employ fibroblast growth factor (FGF) and transforming growth factor β (TGFB) as crucial growth factors; however, PI3K inhibition could not substitute for FGF or TGFB and, furthermore, failed to affect the phosphorylation status of Extracellular signal-regulated kinases (ERKs), suggesting that PI3K inhibition functions independently of these pathways in TSCs. Finally, the authors studied the consequences of PI3K inhibition removal, finding that TSCs reverted to their previous phenotype, thereby suggesting that murine TSCs reversibly switch between two states, as observed for ESCs.

Overall, this new study highlights the ability of PI3K inhibition to reversibly reduce TSC heterogeneity and promote a heightened stem cell-like state; for this reason, the authors hypothesize that the PI3K signaling pathway “fine-tunes” the stemness of TSCs.

For more on trophoblast stem cells and new ways to enhance their culture conditions, and hence, their study and therapeutic application, stay tuned to the Stem Cells Portal.

References

  1. Tanaka S, Kunath T, Hadjantonakis A-K, et al., Promotion of Trophoblast Stem Cell Proliferation by FGF4. Science 1998;282:2072.
  2. Uy GD, Downs KM, and Gardner RL, Inhibition of trophoblast stem cell potential in chorionic ectoderm coincides with occlusion of the ectoplacental cavity in the mouse. Development 2002;129:3913.
  3. Martello G and Smith A, The Nature of Embryonic Stem Cells. Annual Review of Cell and Developmental Biology 2014;30:647-675.
  4. Marks H, Kalkan T, Menafra R, et al., The Transcriptional and Epigenomic Foundations of Ground State Pluripotency. Cell 2012;149:590-604.
  5. Lee CQE, Bailey A, Lopez-Tello J, et al., Inhibition of Phosphoinositide-3-Kinase Signaling Promotes the Stem Cell State of Trophoblast. STEM CELLS 2019;37:1307-1318.
  6. Yu JSL and Cui W, Proliferation, survival and metabolism: the role of PI3K/AKT/mTOR signalling in pluripotency and cell fate determination. Development 2016;143:3050.