Characterizing Stem Cell Antigen-1 Positive Trophoblast Cells in the Trophoblast Stem Cell Niche and Placenta Development in the Mouse

Abstract

Oxygen concentration and transforming growth factor-β (TGF-β) are important for placenta development and the regulation of trophoblast cell proliferation and differentiation. Stem cell antigen-1 (Sca-1) is a cell surface marker commonly used to enrich mouse stem and progenitor cells and is highly expressed in mouse trophoblast stem (TS) cell cultures and increased in a model of placenta insufficiency. Sca-1 positive (POS) cells from different tissues respond to both hypoxia (state of insufficient oxygen availability for homeostasis maintenance) and TGF-β signals, resulting in altered cellular characteristics and function. We hypothesized that Sca-1POS trophoblasts are undifferentiated and can contribute to the labyrinth and junctional zone placenta layers and that Sca-1 identifies a unique subpopulation of TS cells that could increase their proliferation in response to hypoxia and can suppress the TGF-β signalling pathway. Using mouse models to visualize Sca-1POS cells and their progeny, we confirmed that differentiated trophoblasts do not express Sca-1 and that trophoblasts of Sca-1POS lineage contribute to both placenta layers. Mouse TS cell culture studies identified that Sca-1POS cells had increased TS cell gene expression, with both populations able to self-renew in normoxia and hypoxia long-term (10-passages). In response to hypoxia (72 hours), Sca-1POS TS cells had increased proliferation and Sca-1 protein expression when compared with Sca-1 negative (NEG) hypoxia and Sca-1POS normoxia groups. In culture, maintaining homogeneous Sca-1NEG TS cells proved challenging, as Sca-1NEG cells gained Sca-1 cell-surface expression, suggesting that Sca-1 may be required for self-renewal and maintenance of TS cell's undifferentiated state. TGF- treatment did not affect cellular proliferation or Sca-1 mRNA or protein expression. Sca-1 downregulated TGF-β pathway activation, as there was greater activation in Sca-1NEG compared to Sca-1POS TS cells. Future work to determine the function of Sca-1 is required. Currently, the role of undifferentiated trophoblast cells in the placenta and their potential to respond and adapt to physiological stress and insults that compromise placental health is unknown. Together, my results highlight the important role of Sca-1 in the TS cell niche and placenta development and provide insight into a population of undifferentiated, multipotent trophoblast cells that appear to confer a unique stress response.