Every month, wounds open in women of child-bearing age. To release an egg during ovulation, the ovary ruptures, bursting a single layer of cells known as the ovarian surface epithelium (OSE). The body quickly patches the wound, but how that repair process works is a bit of a mystery. Tissues such as intestinal crypts have distinct stem cell populations that facilitate tissue maintenance. But in the OSE, pinpointing a defined stem cell population has proven elusive.
Barbara Vanderhyden, an ovarian cancer biologist, and her team at the University of Ottawa recently found that OSE cells respond to local environmental conditions, such as tissue damage, and shift to become more epithelial or more mesenchymal and stem-like.1 The team reported in Communications Biology that stemness emerges from a variety of environmental cues, and that the ovarian epithelium may not require a distinct population of stem cells for tissue maintenance.
“It's not just a black or white thing. It's not just one or the other,” said Trevor Shepherd, an ovarian cancer biologist at London Regional Cancer Centre in Ontario, who was not involved in the research. “These cells retain this plasticity to allow them to respond to this really harsh microenvironment.”
Very little is known about the OSE other than that it associates with ovulation, a major risk factor for ovarian cancer. “The more ovulations a woman has, the higher her risk of ovarian cancer,” Vanderhyden said, “and we have no idea why.” She wanted to find out what OSE cells contribute to that risk. So, she and her team went looking for that elusive stem cell population.
They started with sphere formation, a standard assay for stem cells. Compared with cells cultured as a monolayer, OSE spheres ramped up gene expression across a number of pathways, including wound repair programs. However, putative OSE stem cell markers, including Lgr5 and Sca-1 remained unchanged.
The team previously showed that mouse OSE cells acquire stem cell characteristics when exposed to TGFb,2 so they next treated OSE cells cultured as monolayers or as spheroids with TGFb and assessed gene expression changes using RNA-seq.
What came out of the analysis didn’t overlap with the stem cell markers they found in the spheres enough to offer a clear picture. Although TGFb treatment increased Cd44 expression, for example, it did not ramp expression up four-fold as it had in OSE spheres. Also, in contrast to what the researchers had seen in OSE spheres, TGFb treatment increased Sca-1 expression.
The team next tried their luck with overexpressing Snai1, an epithelial-mesenchymal transition transcription factor activated by TGFb signaling. They hoped to see changes in gene expression or signaling pathways that occurred when they cultured the cells as spheroids or overexpressed TGFb. But again, they found little overlap. Snai1 overexpression had no effect on Cd44 or Sca-1 expression, suggesting that these stemness markers are context-specific.
Finally, the team turned to Brca1, which had decreased in expression in spheroid cultured cells. Although reduced Brca1 expression led to huge changes in gene expression, the changes did not line up with results from the other three methods.
Vanderhyden’s team approached the cells from four different angles and came out with four sets of potential stem cell markers and signaling pathways, with little overlap between them.
“We’re identifying signaling pathways at every step of every approach, and there’s just not a common panel that is coming out every single time, which I have to say is frustrating,” Vanderhyden recalled. “You begin to wonder, what’s going on here?”
The conclusion Vanderhyden and her team came to was heterogeneity in how the signals elicit a stem cell-like response.
“What this [study] really adds is the additional layer that that heterogeneity is not static. It is constantly dynamic,” Shepherd said.
“It definitely was not what we expected,” Vanderhyden said. “But, what [the data] is saying is that this is a heterogeneous population, and how it achieves stem cell-like behavior is entirely context-dependent.”
- L.E. Carter et al., "Transcriptional heterogeneity of stemness phenotypes in the ovarian epithelium," Commun Biol, 4:1–11, 2021.
- L.F. Gamwell et al., "The mouse ovarian surface epithelium contains a population of LY6A (SCA-1) expressing progenitor cells that are regulated by ovulation-associated factors," Biol Reprod, 87,80:1–10, 2012.