Differentiating stem cells can replace damaged and dying cells. However, how can stem cells choose which kind of cell to form in a particular circumstance? The team of Bon-Kyoung Koo at IMBA and the Institute for Basic Science discovered a new gene, Daam1, using intestinal organoids. Daam1 is crucial because it turns on the formation of secretory cells in the intestine. This discovery, which was published in Science Advances on November 24, offers fresh insights into the study of cancer.
Our bodies are similar to cars in that they require routine maintenance in order to continue working properly. To maintain the functionality of our organs, any damaged or dead cells in our bodies must be replaced. Adult stem cells that reside in the tissue are responsible for this replenishment. Adult stem cells can only form the cell types present in the tissue to which they belong, in contrast to embryonic stem cells, which can form every type of cell in the body. However, how can stem cells specific to a given tissue decide which kind of cell to produce? Former postdoctoral researcher in Bon-Kyoung Koo's lab at IMBA, Gabriele Colozza is currently the director of the Centre for Genome Engineering at the Institute for Basic Science in South Korea. choose to use intestinal stem cells to research this query.
The intestines are a never-ending construction site
According to Colozza, "our intestines expose cells to extreme conditions." Intestinal cells are impacted by mechanical wear and tear, digesting enzymes, and changes in pH. The mucosa of the intestine then differentiates stem cells to produce new intestinal cells. "Damaged cells must be replaced, but the renewal of stem cells and their differentiation into other cell types must be carefully balanced. Too many stem cell differentiations can eventually deplete the tissue of its stem cells, making it unable to self-renew. On the other hand, unchecked stem cell proliferation can result in tumor formation."
Cells can communicate with one another through signaling pathways and feedback loops, which are responsible for maintaining this delicate balance. The Wnt pathway is one significant one. The overactive Wnt pathway can result in excessive cell division and the formation of tumors if left unchecked. The Wnt pathway is well-known for its role in embryonic development.
Identified molecular partner
The well-known Wnt signaling antagonist Rnf43, first discovered by Bon-Kyoung Koo, functions to restrain Wnt. Rnf43 was previously known to target and mark the Wnt receptor Frizzled for degradation. "We were interested in learning how Rnf43 functions as well as what, in turn, controls Rnf43 and aids in its regulation of Wnt signaling." The scientists knew from past studies that Rnf43 alone was insufficient to degrade the plasma membrane-bound Wnt receptor Frizzled. "In our project, we determined which proteins interact with Rnf43 by using biochemical assays." The protein Daam1 was found to be an important Rnf43 partner.
Colozza used intestinal organoids to learn how Daam1 controls Rnf43 and impacts the tissues it interacts with. We discovered that Daam1 is necessary for Rnf43 to function and, consequently, for Rnf43 to control any Wnt signaling. Additional research in cells revealed that Rnf43 requires Daam1 in order to transfer the Wnt receptor Frizzled into endosome-like vesicles. Frizzled is transported from the endosomes to the lysosomes, where it undergoes degradation and inhibits Wnt signaling, according to Colozza.
Researchers can replicate the intestinal mucosa using intestinal organoids, which are three-dimensional cell cultures derived from adult intestinal stem cells. Organoids provided Colozza with a chance to study the effects of Rnf43 and Daam1 on the delicate balance between intestinal stem cell renewal and differentiation. We discovered that the organoids develop into structures resembling tumors when Rnf43 or Daam1 are knocked out. These tumor-like organoids proliferate even in the absence of the growth factors, like R-spondin, that they typically require.
Activating Paneth cell production
Colozza's follow-up on this result in mouse tissue surprised the researchers. As might be expected, tumors developed in the intestines when Rnf43 was absent. However, no tumors developed when Daam1 was absent. This stark difference perplexed us: how could the loss of similar-behaving factors in the same pathway produce such disparate results in organoids?
By closely examining the intestines, Colozza discovered that the absence of Rnf43 in the intestines resulted in an abundance of Paneth cells, a particular kind of secretory cell. Conversely, intestinal devoid of Daam1 did not have any additional Paneth cells. Growth factors that promote cell division, such as Wnt, are secreted by path cells. "The effective formation of Paneth cells requires Daam1." Stem cells differentiate into Paneth cells when Daam1 is active. The stem cells differentiate into a different type of cell when Daam1 is not active.
Tumors alter their niche to expand
The mysterious distinction between intestines and organoids is explained by the connection between the molecular findings and Paneth cells. Because growth factors are supplied by scientists in organoid culture, Rnf43 and Daam1 knockouts result in organoids that resemble tumors. However, there isn't a tiny scientist supplying growth factors in the intestine. Rather, Wnt and other growth factors are supplied by Paneth cells, which also establish the ideal environment for stem cell survival and division. Stem cells will not proliferate much in the absence of Paneth cells, such as when Daam1 is not active to stimulate cells to become Paneth cells. However, an excess of Paneth cells—like in intestines devoid of Rnf43—can result in an overabundance of growth factors that aid in the development of tumors.
The work by Colozza and associates provides the first genetic evidence that Daam1, a component of the non-canonical Wnt pathway, is essential for Paneth cell specification and directly contributes to the development of this vital secretory cell. The significance of the stem cell niche is further illuminated by the results. It is demonstrated to us that tumor cells alter their microenvironment and impact their surrounding environment to enhance their growth.