Stem cells possess the remarkable ability to transform into specific cell types, replenishing dead or damaged cells within our bodies. Unraveling the mystery of how these versatile cells determine their fate in various situations, a team led by Bon-Kyoung Koo at IMBA and the Institute for Basic Science identified a pivotal gene, Daam1, through their work with intestinal organoids.
In the intricate machinery of our bodies, akin to cars needing regular checks and repairs, damaged or deceased cells must be replaced to sustain organ function. Tissue-resident adult stem cells are the heroes in this regeneration process.
While embryonic stem cells can develop into any cell type, adult stem cells exhibit specificity, generating only the cell types present in their respective tissues. The question of how these tissue-specific stem cells precisely determine the cell type they give rise to intrigued Gabriele Colozza, a postdoctoral researcher in Bon-Kyoung Koo's lab at IMBA, who is now the director at the Center for Genome Engineering, Institute for Basic Science in South Korea.
Choosing intestinal stem cells as the focus of their investigation, the research team pinpointed Daam1 as a crucial gene orchestrating the development of secretory cells in the intestine. This breakthrough not only deepens our understanding of the intricate processes governing stem cell behavior but also opens up new vistas in the realm of cancer research.
Intestines—a constant construction site
Within our intestines, cells face formidable conditions, as Gabriele Colozza explains—the challenges range from mechanical wear and tear to the impact of digestive enzymes and fluctuating pH values. In response to these environmental factors, stem cells in the intestinal mucosa undergo differentiation to generate new intestinal cells.
The imperative to replace damaged cells introduces a nuanced dance between stem cell renewal and differentiation into distinct cell types. Striking this balance is critical, as unchecked stem cell proliferation may pave the way for tumor formation. Conversely, an excess of stem cell differentiation can deplete the tissue of its stem cells, rendering it unable to self-renew.
This intricate equilibrium is finely regulated by signaling pathways and feedback loops that facilitate cellular communication. One such pivotal pathway is the Wnt pathway, recognized for its role in embryonic development. However, an unbridled Wnt pathway can trigger excessive cell division, culminating in the formation of tumors. The delicacy of these regulatory mechanisms underscores the complexity of maintaining tissue integrity and highlights the potential implications for diseases such as cancer.
Molecular partner identified
In the intricate dance of Wnt signaling regulation, Rnf43 emerges as a well-known antagonist, serving as a check on Wnt activity—an insight initially uncovered by Bon-Kyoung Koo. Rnf43's role in targeting the Wnt receptor Frizzled for degradation was established prior to this study.
The quest to unravel the workings of Rnf43 and its regulatory influences led the researchers to explore its interactions with other proteins. Through biochemical assays, they identified a crucial partner: the protein Daam1.
Gabriele Colozza, delving into the role of Daam1, employed intestinal organoids as a tool to decipher its impact on Rnf43 and the tissues it influences. The findings unveiled that Daam1 is indispensable for Rnf43 to be active in regulating Wnt signaling. Detailed cellular investigations demonstrated that Rnf43 relies on Daam1 to transport the Wnt receptor Frizzled into endosomes, which then shuttle it to lysosomes for degradation, effectively dampening Wnt signaling.
Utilizing three-dimensional cell cultures derived from adult intestinal stem cells, known as intestinal organoids, Colozza examined the repercussions of manipulating Rnf43 and Daam1. Strikingly, when Rnf43 or Daam1 was knocked out, the organoids exhibited tumor-like growth. Even without the typical growth factors, such as R-spondin, these tumor-like structures persisted, highlighting the pivotal role of Rnf43 and Daam1 in maintaining the delicate balance between stem cell renewal and differentiation in the intestine.
Switching on Paneth cell formation
The researchers encountered an unexpected twist when examining the impact of Rnf43 and Daam1 in mouse tissue. The anticipated tumor growth occurred when Rnf43 was absent, aligning with previous findings. However, the absence of Daam1 yielded a surprising result—no tumor formation. This stark difference prompted a deeper exploration into the intricacies of these seemingly similar factors within the same pathway.
Upon scrutinizing the intestines, Gabriele Colozza observed a distinctive pattern: intestines lacking Rnf43 were abundant in a specific type of secretory cells known as Paneth cells. In contrast, intestines lacking Daam1 displayed no excess of Paneth cells. These cells play a crucial role by secreting growth factors like Wnt, which stimulate cell division.
The pivotal revelation was that Daam1 is essential for the efficient formation of Paneth cells. When Daam1 is active, stem cells undergo differentiation to become Paneth cells. However, when Daam1 is inactive, stem cells take on a different cellular identity, emphasizing the intricate role Daam1 plays in orchestrating the differentiation of stem cells within the intestinal environment.
Tumors modify their niche to grow
The connection between molecular outcomes and the presence of Paneth cells unravels the enigma behind the divergent results observed in intestines and organoids, as explained by Colozza.
“In organoid culture, where scientists supply growth factors, knocking out both Rnf43 and Daam1 results in tumor-like organoids. Conversely, in the intestine, there's no external supply of growth factors; instead, Paneth cells take on this role, providing growth factors like Wnt. They create the conducive environment for stem cells to thrive and proliferate.”
Colozza emphasizes that when Paneth cells are scarce, as seen when Daam1 is inactive and unable to drive cell differentiation into Paneth cells, stem cell division is limited. On the flip side, an excess of Paneth cells, observed in intestines lacking Rnf43, contributes to tumor formation due to an abundance of growth factors.
This groundbreaking study by Colozza and colleagues provides the first genetic evidence highlighting the significance of Daam1, a component of the non-canonical Wnt pathway, in specifying Paneth cells and directly influencing the development of these crucial secretory cells. Additionally, the findings underscore the critical role of the stem cell niche, revealing that tumor cells can reshape their microenvironment to enhance their growth and influence the supportive conditions around them.