Intestinal Wnt signalling: the key to its control?

Castillo-Azofeifa D, Wald T, Reyes E, et al. A DLG1-ARHGAP31-CDC42 axis is essential for the intestinal stem cell response to fluctuating niche Wnt signalling. Cell Stem Cell 2023; 30 (2): 188–206. doi: 10.1016/j.stem.2022.12.008

Small intestinal epithelium is made up of crypt-villus units with proliferative stem cells at the base, largely driven by Wnt (wingless-related integration site) signalling. After injury Wnt signalling transiently increases, but overactivation of Wnt can lead to tumourigenesis. DLG1 (Discs large 1) is a tumour suppressor protein which interacts with adenomatous polyposis coli and regulates cell polarity. Castillo-Azofeifa et al analysed the link between DLG1 and canonical Wnt signalling.

Dlg1 was conditionally deleted in mouse gut epithelium using Villin-Cre (Cre recombinase). This did not affect intestinal cell polarity. However, when canonical Wnt signalling was activated in these mice using a mitogen-activated protein kinase inhibitor, more cells died at the crypt. When Dlg1 was deleted in intestinal organoids, exposure to exogenous canonical Wnt3A (Wnt family member 3A) did not lead to the expected hyperproliferation with spheroid formation. This was not the case for non-canonical Wnt5A (Wnt family member 5A) ligand exposure. Dlg1, therefore, seems critical for intestinal stem cell (ISC) response when canonical Wnt signalling is activated. To evaluate the molecular mechanism the transcriptional profile of Dlg1-ISCs was studied, by fluorescence-activated cell sorting isolation and RNA sequencing. The only gene differentially expressed was Arhgap31 (Rho GTPase activating protein 31), a GTPase activating protein. Organoids with Arhgap31 deletion showed the same phenotype following WNT3A exposure as Dlg1 organoids. Dlg1 expression was unaffected, suggesting that Dlg1 acts upstream of Arhgap31. Arhgap31 deletion led to activation of CDC42 (Cell division control protein 42 homolog), a Wnt pathway activator. Dlg organoids had higher levels of active CDC42, which in turn led to reduced ISC motility and cell division.

In summary, …