The initial hyperproliferation phase during

The initial hyperproliferation phase during crypt regeneration is reminiscent of the effects of Apc loss during tumour initiation and genetic studies show Yap and Taz are also required for adenoma formation in mice [10••, 12•, 13]. Furthermore, Yap activation by Mst1/2 or Sav deletion increases crypt proliferation and tumorigenicity [9, 14], with Yap/Taz potentially regulated by additional Lats-related NDR kinases [15]. Studies in human colorectal cancer cell lines similarly show that reducing Yap/Taz levels leads to impaired proliferation, survival and tumourigenicity [14, 16] and a number of clinical reports link high Yap/Taz activity to colorectal cancer progression and overall poor prognosis [17].
Hippo transcriptional program In the crypt epithelium loss and gain of function approaches show that Yap promotes expression of numerous genes implicated in cancer and regenerative signalling (i.e. Egfr ligands, Ctgf, Cyr61, Msln, Il33, etc.) (Figure 1b) [8, 10••]. Studies in intestinal organoids and tumour initiating GSK 2830371 in vivo, suggest that Yap drives Egfr signalling during crypt regeneration and adenoma formation []. Indeed impaired crypt formation in Yap mutant organoids is rescued by the exogenous Egfr ligand, Epiregulin []. In human colorectal cancer cells, Yap also promotes anti-apoptotic gene expression (e.g. Bcl2l1 and Birc5) through a transcriptional complex with β-catenin and Tbx5 [16]. Yap/Taz signalling in the gut epithelium also leads to induction of both negative (Lats2, Amot) and positive (Tead2 and Taz) Hippo regulatory feedback loops that can fine tune Yap/Taz activity (Figure 1b) [10••, 18].
Integration of Wnt and Hippo signalling The Hippo pathway appears intimately entwined with the Wnt pathway, as Yap and Taz inhibit Wnt target genes, many of which are known ISC markers [8, 10••]. Early studies showed that Yap/Taz can sequester Dvl or β-catenin (Figures 1b and 2a) [8, 19, 20]. More recently Azzolin et al. suggested that Yap/Taz bind to the β-catenin destruction complex via Axin1 (Figure 2b) [] and in the absence of Wnt stimulation promotes degradation of β-catenin. Interestingly, Yap regulation of β-catenin may depend on the activity of the methyltransferase Setd7, which associates with Axin1, Yap and β-catenin to methylate Yap and facilitate β-catenin activation [22]. Surprisingly, Yap methylation also leads to cytoplasmic retention and inactivation of Yap [23]. Further studies are required to decipher how the seemingly contradictory actions of Setd7 on Yap and β-catenin are compatible with its protumorigenic effects. Finally Park et al. proposed an entirely different mechanism whereby Yap induces secreted Wnt antagonists (Figure 2d) [24] with three of these, Dkk1, Wnt5a and Bmp4, being well known to counteract the effects of canonical Wnt signalling in the crypt [25, 26, 27]. Despite this, Yap does not appear to induce these genes in the crypt [8, 10••], suggesting that this pathway may not be operational in the gut. Besides ‘how’ Hippo signalling inhibits Wnt signalling, an equally important question is ‘why’. Lineage tracing using Lgr5GFP-CreERT mice unequivocally showed Yap is important for post-irradiation ISC maintenance [], which is seemingly at odds with Yap inhibition of Wnt signalling [], and that sustained nuclear Yap activity depletes ISCs [8]. However, immediately after damage, Yap regulation is dynamic and transiently suspends the Wnt-driven homeostatic program in Lgr5+ ISCs and prevents excessive Paneth cell differentiation (Figure 1b). Since Wnt maintains the self-renewal capacity of ISCs and promotes Paneth cell differentiation [28, 29], Hippo-Wnt crosstalk may thus buffer Wnt activity. Indeed, in Yap mutant intestinal organoids, lowering Wnt activity normalized Paneth differentiation and partially rescued mutant organoid morphogenesis []. However, inhibition of Wnt/β-catenin by Yap is likely not the sole mechanism leading to suppression of Paneth cells. Indeed in cells with constitutive β-catenin signalling, Yap loss still elevates Paneth cells both ex vivo and in vivo. Crosstalk between Hippo and Notch may provide an alternative explanation [14], since deleting the Yap inhibitors Mst1/2 enhanced Notch signalling and reduced Paneth cell formation [30]. In conclusion, Yap is not strictly a driver of cell proliferation and survival during regeneration and cancer, but also functions as a key cell fate regulator.