GLI1-expressing mesenchymal cells form the essential Wnt-secreting niche for colon stem cells.

Wnt-ß-catenin signalling plays a pivotal role in the homeostasis of the intestinal epithelium by promoting stem cell renewal1,2. In the small intestine, epithelial Paneth cells secrete Wnt ligands and thus adopt the function of the stem cell niche to maintain epithelial homeostasis3,4. It is unclear which cells comprise the stem cell niche in the colon. Here we show that subepithelial mesenchymal GLI1-expressing cells form this essential niche. Blocking Wnt secretion from GLI1-expressing cells prevents colonic stem cell renewal in mice: the stem cells are lost and, as a consequence, the integrity of the colonic epithelium is corrupted, leading to death. GLI1-expressing cells also play an important role in the maintenance of the small intestine, where they serve as a reserve Wnt source that becomes critical when Wnt secretion from epithelial cells is prevented. Our data suggest a mechanism by which the stem cell niche is adjusted to meet the needs of the intestine via adaptive changes in the number of mesenchymal GLI1-expressing cells.

Bone marrow mesenchymal stromal cells support regeneration of intestinal damage in a colitis mouse model, independent of their CXCR4 expression.

Inflammatory bowel disease (IBD) is characterized by a chronically dysregulated immune response in the gastrointestinal tract. Bone marrow multipotent mesenchymal stromal cells have an important immunomodulatory function and support regeneration of inflamed tissue by secretion of soluble factors as well as through direct local differentiation. CXCR4 is the receptor for CXCL12 (SDF-1, stromal-derived factor-1) and has been shown to be the main chemokine receptor, required for homing of MSCs. Increased expression of CXCL12 by inflamed intestinal tissue causes constitutive inflammation by attracting lymphocytes but can also be used to direct MSCs to sites of injury/inflammation. Trypsin is typically used to dissociate MSCs into single-cell suspensions but has also been shown to digest surface CXCR4. Here, we assessed the regenerative effects of CXCR4high and CXCR4low MSCs in an immune-deficient mouse model of DSS-induced colitis. We found that transplantation of MSCs resulted in clinical improvement and histological recovery of intestinal epithelium. In contrary to our expectations, the levels of CXCR4 on transplanted MSCs did not affect their regenerative supporting potential, indicating that paracrine effects of MSCs may be largely responsible for their regenerative/protective effects.

Epithelial Wnt secretion drives the progression of inflammation-induced colon carcinoma in murine model.

Colon cancer is initiated by stem cells that escape the strict control. This process is often driven through aberrant activation of Wnt signaling by mutations in components acting downstream of the receptor complex that unfetter tumor cells from the need for Wnts. Here we describe a class of colon cancer that does not depend on mutated core components of the Wnt pathway. Genetically blocking Wnt secretion from epithelial cells of such tumors results in apoptosis, reduced expression of colon cancer markers, followed by enhanced tumor differentiation. In contrast to the normal colonic epithelium, such tumor cells autosecrete Wnts to maintain their uncontrolled proliferative behavior. In humans, we determined certain cases of colon cancers in which the Wnt pathway is hyperactive, but not through mutations in its core components. Our findings illuminate the path in therapy to find further subtypes of Wnt-dependent colon cancer that might be responsive to Wnt secretion inhibitors.

Wnt Ligands as a Part of the Stem Cell Niche in the Intestine and the Liver.

The term "Wnt signaling" does not refer to one uniform signal transduction cascade. Instead, it describes the multiple discrete signals elicited by Wnt ligands following their interaction with distinct receptor complexes. The interaction of stem cells with niche cells is coordinated by the involvement of different signaling pathways, including Wnt signaling. The stem cell populations are highly sensitive to modulation of Wnt pathway activity. Wnt signaling is of paramount importance for stem cell self-renewal, survival, proliferation, differentiation, movement, and cell polarity. Aberrant activation of Wnt/ß-catenin signaling is associated with the pathology of many types of cancer, such as colorectal cancer and hepatocellular carcinoma. Importantly, although often initiated by mutation(s) downstream of the Wnt-receptor complex, the progression of colorectal cancer still seems to be augmented by Wnt ligand-mediated signaling. This chapter focuses on the role of Wnt ligands in the intestine and the liver during homeostasis and cancer.

A small molecule identified through an in silico screen inhibits Aurora B-INCENP interaction.

Aurora B is a serine/threonine kinase that has a central role in the regulation of mitosis. The observation of Aurora B overexpression in cancer makes it a promising target to develop antitumoral inhibitors. We describe a new potential inhibitor that exclusively targets the interaction site of Aurora B and its activator INCENP. We performed a structure-based virtual screening and determined five potential candidates of 200 000 compounds, which selectively bind to the Aurora B::INCENP interaction site, but not to the ATP-binding site (kinase pocket) of Aurora B or other related kinases. Further characterization in vivo validated the inhibitory role of one of these five compounds in Aurora B::INCENP complex formation and exhibited hallmarks of Aurora inhibition such as chromosome congression and segregation defects that interfere with the progression into cytokinesis and result in multinuclear cells. Our results provide an alternative approach on the way of exploring specific kinase inhibitors.

Wnt Ligands Secreted by Subepithelial Mesenchymal Cells Are Essential for the Survival of Intestinal Stem Cells and Gut Homeostasis.

Targeting of Wnt signaling represents a promising anti-cancer therapy. However, the consequences of systemically attenuating the Wnt pathway in an adult organism are unknown. Here, we globally prevent Wnt secretion by genetically ablating Wntless. We find that preventing Wnt signaling in the entire body causes mortality due to impaired intestinal homeostasis. This is caused by the loss of intestinal stem cells. Reconstitution of Wnt/ß-catenin signaling via delivery of external Wnt ligands prolongs the survival of intestinal stem cells and reveals the essential role of extra-epithelial Wnt ligands for the renewal of the intestinal epithelium. Wnt2b is a key extra-epithelial Wnt ligand capable of promoting Wnt/ß-catenin signaling and intestinal homeostasis. Wnt2b is secreted by subepithelial mesenchymal cells that co-express either Gli1 or Acta2. Subepithelial mesenchymal cells expressing high levels of Wnt2b are predominantly Gli1 positive.