Exploring the role of TGFß-regulated transcription factor brain acid soluble protein 1 (Basp1) in epithelial plasticity and intestinal repair

The homeostatic renewal of the intestinal epithelium is driven by a stable population of adult stem cells, intestinal stem cells (ISCs), which undergo continuous self-renewal, and from which all intestinal epithelial lineages are derived. Their activity is tightly controlled in the homeostatic crypt niche, regulated primarily by Wnt, Notch, EGF, and BMP signalling. Transforming growth factor ß (TGFß) signalling is likewise thought to be involved in regulating proliferation, differentiation, cell cycle arrest, and apoptosis of epithelial cells in the intestinal villi. Beyond its role in homeostasis, dysregulation of TGFß signalling has been implicated in tissue injury in inflammatory bowel disease (IBD) and is associated with colorectal cancers (CRC) with aggressive features, and which tend to undergo early dissemination.However, ISCs are sensitive to damage, necessitating alternative mechanisms for epithelial regeneration. Our work has shown that following epithelial injury or infection, repair is dependent on the dedifferentiation of lineage-committed epithelial cells into a fetal-like progenitor state, forming a distinct stem cell population, revival stem cells (revSC). These revSCs transiently adopt the embryonic gut profile, in a process known as fetal reprogramming, which is mediated through Hippo (Yap/Taz) signalling and is necessary to reconstitute homeostatic ISCs and promote efficient repair of the intestinal epithelium. TGFß signals have also been shown to induce fetal reprogramming in models of irradiation and helminthic infection. While these studies suggest that Hippo and TGFß signals coordinate regeneration, their downstream transcriptional responses in the gut remain poorly understood. We aim to explore the mechanisms by which TGFß exerts its activity in the intestinal epithelium to promote epithelial plasticity and repair following injury or infection.Prior transcriptional profiling work enabled us to identify brain acid soluble protein 1 (Basp1) as a TGFß-responsive candidate regulator of fetal reprogramming. To assess its function, we generated murine intestinal organoids with inducible Basp1 overexpression and used them to observe how Hippo and TGFß signalling affect organoid morphology and transcriptomic profiles. In parallel, in vivo we used wildtype, Tgfbr2-/-, Yap-/-;Taz-/-, and Yap-/-;Taz-/-;Tgfbr2-/- models to assess Basp1’s expression in distinct epithelial populations after irradiation and parasitic helminth infection.In vivo, Basp1 expression was upregulated in response to irradiation and enriched in populations of revSCs in wildtype mice. Contrarily, Tgfbr2-/-, Yap-/-;Taz-/-, and Yap-/-;Taz-/-;Tgfbr2-/- mice showed either reduced or nonexistent Clu+ revSCs, accompanied by decreased expression of Basp1 in the epithelium. Following parasitic helminth infection, Basp1 expression was high in tuft cells in the wildtype regenerating intestine. In Tgfbr2-/- and Yap-/-;Taz-/-;Tgfbr2-/- mice tuft cell expression was significantly reduced, although Basp1 expression itself was not dependent on Tgfbr2 activity. In vitro, we observed increased revSC and tuft cell differentiation following stimulation with TGFß and cytokine IL-13, respectively. We used both Tgfbr2 knockout organoids and our overexpression model to observe the involvement of TGFß and Basp1 in tuft cell differentiation and specification of tuft cell subtypes. Defining such regulators of epithelial cell plasticity enhances our understanding of intestinal diseases and may inform future therapeutic avenues