Stem cells can differentiate to replace dead and damaged cells. But how do stem cells decide what type of cell to become in a given situation? Using intestinal organoids, Bon-Kyoung Koo’s group at IMBA and the Institute of Basic Science identified a new gene, Daam1, that plays an essential role, activating the growth of secretory cells in the intestine. This finding, published Nov. 24 in Science Advances, opens up new perspectives in cancer research.
Our bodies are, in a way, like cars — to keep them running, they need to be checked and repaired regularly. In the case of our body, any cells that are damaged or dead must be replaced to keep the organs functioning. This replacement occurs thanks to adult stem cells that reside in the tissue. Unlike embryonic stem cells, which can form any type of cell in the body, adult stem cells will only form the types of cells found in the tissue they belong to. But how do tissue-specific stem cells know which type of cell to make? Gabriele Colozza, a postdoctoral researcher in Bon-Kyoung Koo’s lab at IMBA — now director at the Center for Genome Engineering, Institute of Basic Science in South Korea — decided to investigate this question using intestinal stem cells.
Guts — a constant construction site
“In our intestines, cells are exposed to extreme conditions,” explains Colozza. Mechanical wear and tear, but also digestive enzymes and varying pH values affect all intestinal cells. In turn, stem cells in the gut lining differentiate to form new intestinal cells. “Damaged cells must be replaced, but it is a delicate balance between stem cell renewal and differentiation into other cell types: uncontrolled proliferation of stem cells can lead to tumor formation; on the other hand, if too many stem cells differentiate, the tissue will to be depleted of stem cells and ultimately unable to self-renew.”
This balance is finely tuned by signaling pathways and feedback loops, which allow cells to communicate with each other. An important pathway is called Wnt. The Wnt pathway is known for its role in embryonic development, and if left unchecked, an overactive Wnt pathway can lead to excessive cell division and tumor formation.
Molecular partner identified
A well-known antagonist of Wnt signaling — keeping Wnt in check — is Rnf43, first identified by Bon-Kyoung Koo. Prior to this study, Rnf43 was known to target the Wnt receptor Frizzled and mark it for degradation. “We wanted to know how Rnf43 works and also what – in turn – controls Rnf43 and helps it regulate Wnt signaling.” From previous research, the scientists knew that Rnf43 alone was not sufficient to cleave the Wnt receptor Frizzled, which is located in the plasma membrane. “In our work, we used biochemical analyzes to determine which proteins interact with Rnf43.” A key partner of Rnf43 was shown to be the protein Daam1.
To understand how Daam1 regulates Rnf43 and affects the tissues it acts on, Colozza turned to intestinal organelles. “We found that Daam1 is required for Rnf43 to be active, so that Rnf43 regulates Wnt signaling at all. Further work in cells showed that Rnf43 needs Daam1 to move the Wnt receptor Frizzled into vesicles called endosomes. endosomes, Frizzled is transported to lysosomes where it is degraded, reducing Wnt signaling,” adds Colozza.
Intestinal organoids are 3D cell cultures grown from adult intestinal stem cells, allowing researchers to mimic the intestinal mucosa. For Colozza, the organoids were an opportunity to understand how Rnf43 and Daam1 affect the delicate balance of stem cell renewal and differentiation in the gut. “We found that when we knock out Rnf43 or Daam1, the organelles grow into tumor-like structures. These tumor-like organelles continue to grow, even if we withdraw the growth factors they normally depend on, such as R-spondin .”
Activation of Paneth cell formation
When Colozza followed up on this effect in mouse tissue, the researchers were surprised. “When Rnf43 was missing, the intestines grew tumors — as expected. But when Daam1 was missing, no tumors developed. We were puzzled by this striking difference: how can the loss of factors in the same pathway, which behave similarly in organelles, lead to to such different results?’
Looking closely at the intestines, Colozza saw that the intestines that lacked Rnf43 were filled with a specific type of secretory cell, Paneth cells. Intestines lacking Daam1, on the other hand, contained no additional Paneth cells. Paneth cells secrete growth factors, such as Wnt, that stimulate cell division. “Daam1 is required for efficient formation of Paneth cells. When Daam1 is active, stem cells differentiate to form Paneth cells. When Daam1 is not active, stem cells differentiate into another cell type.”
Tumors change their location to grow
This link between molecular outcomes and Paneth cells explains the puzzling difference between intestines and organoids. “In organoid culture, we scientists provide growth factors, so knocking out both Rnf43 and Daam1 leads to tumor-like organoids. But in the gut, there is no little scientist providing growth factors. Instead, Paneth cells provide growth factors, such as Wnt, and create the right conditions for stem cells to survive and divide. When Paneth cells are missing — such as when Daam1 is not active to drive the cells to become Paneth cells — the stem cells will not divide much. But when there are too many Paneth cells — as in intestines that lack Rnf43 — the excess growth factors can contribute to tumor formation.
The study by Colozza and colleagues is the first genetic evidence that Daam1, a member of the noncanonical Wnt pathway, is important for Paneth cell determination and is directly involved in the development of this critical secretory cell. The results also shed light on the importance of stem cell location. “We show that cancer cells modify their microenvironment and influence their supportive environment so that they can grow better.”