Epithelial stem cells are maintained within niches that promote self-renewal by providing signals that specify the stem cell fate. In the Drosophila ovary, epithelial follicle stem cells (FSCs) reside in niches at the anterior tip of the tissue and support continuous growth of the ovarian follicle epithelium. Here, we demonstrate that a neighboring dynamic population of stromal cells, called escort cells, are FSC niche cells. We show that escort cells produce both Wingless and Hedgehog ligands for the FSC lineage, and that Wingless signaling is specific for the FSC niche whereas Hedgehog signaling is active in both FSCs and daughter cells. In addition, we show that multiple escort cells simultaneously encapsulate germ cell cysts and contact FSCs. Thus, FSCs are maintained in a dynamic niche by a non-dedicated population of niche cells.

Epithelial stem cells divide asymmetrically, such that one daughter replenishes the stem cell pool and the other differentiates. We found that in the epithelial follicle stem cell (FSC) lineage of the Drosophila ovary, EGFR signaling functions specifically in the FSCs to promote the unique partially polarized state of the FSC, establish apical-basal polarity throughout the lineage, and promote FSC maintenance in the niche. In addition, we identified a novel connection between EGFR signaling and the cell polarity regulator, LKB1, which indicates that EGFR signals through both the Ras-Raf-MEK-Erk pathway and through the LKB1-AMPK pathway to suppress apical identity. The development of apical-basal polarity is the earliest visible difference between FSCs and their daughters, and our findings demonstrate that the EGFR-mediated regulation of apical-basal polarity is essential for the segregation of stem cell and daughter cell fates.

Despite current understanding of transcriptional and epigenetic programs regulating transitions of human embryonic stem cells between distinct stages of pluripotency, our knowledge of the cell biology of changes in pluripotency states remains limited. We report a dynamic remodeling of the actin cytoskeleton of human embryonic stem cells (hESCs) as they transition from primed to naïve pluripotency that includes assembly of a ring of contractile actin filaments encapsulating colonies of naïve hESCs. The actin ring requires activity of the Arp2/3 complex and traction force microscopy suggests a role in limiting cell-substrate tensional forces. Arp2/3 complex activity is also necessary for effective transition to the naïve pluripotency state, including translocation of the Hippo pathway effectors YAP and TAZ from the cytosol to the nucleus. In hESCs with inhibited Arp2/3 complex activity, expressing a nuclear-localized YAP-S127A restores assembly of the actin ring and the naïve pluripotency state, including established markers and colony formation. Together these new findings on the cell biology of hESC reveal a signaling network of Arp2/3 complex activity for dynamic remodeling of contractile actin filaments and YAP/TAZ activity for transition to the naïve pluripotency state.

The adult stem cell niche microenvironment is essential for maintenance and self renewal of the cell within that environment, ensuring a consistent population of stem cells available for regeneration of differentiated daughter cells. In the Drosophila ovary, follicle stem cells (FSCs), produce and maintain the epithelium of the eventual oocytes. These FSCs reside at the anterior edge of the tissue where they regularly divide throughout oogenesis. Like many epithelial tissues, the FSCs require the Wnt/Wingless and EGF signaling pathways for self-renewal. Abrogation of either pathway leads to FSC loss whereas constitutive activation of either pathway inhibits daughter cell differentiation. These findings demonstrate that precise activation of Wingless and EGF signaling within the narrow range of the FSC niche is essential to preserve stem cell identity while allowing daughter cells to differentiate. However, it remains unclear how this specificity is achieved, and how these signals cooperate to maintain FSC self renewal. We found that the specificity of Wingless signaling is not due to an inability of FSC daughter cells to activate Wingless signaling or to cross-talk with the Hedgehog or Notch pathways. Instead, our data suggest that the Wingless ligand is locally delivered by nearby niche cells and spatially restricted to FSCs. We also found that FSCs with impaired Wingless signaling lack pERK, an indicator of EGF signaling, suggesting that Wingless signaling functions upstream of EGF signaling in a hierarchy of self-renewal pathways. Finally, we found that overactive EGF signaling can partially rescue the FSC loss caused by knocking out wingless and that wingless promotes transcription of the EGF ligand spitz. Collectively, these findings demonstrate that the specific signal Wingless acts upstream of EGFR to maintain FSCs. These studies will contribute to our understanding of how epithelial stem cells are maintained both specifically and robustly, especially in a dynamic niche environment.