The Hippo pathway and its downstream effectors, the transcriptional co-activators YAP and TAZ, regulate organ growth and cell plasticity during animal development and regeneration. Remarkably, experimental activation of YAP/TAZ in the mouse can promote regeneration in organs with poor or compromised regenerative capacity, such as the adult heart, and the liver and intestine of old or diseased mice. However, therapeutic YAP/TAZ activation may cause serious side effects. Most notably, YAP/TAZ are hyperactivated in human cancers and prolonged activation of YAP/TAZ triggers cancer development in mice. Thus, can the power of YAP/TAZ to promote regeneration be harnessed in a safe way? Here we review the role of Hippo signaling in animal regeneration, examine the promises and risks of YAP/TAZ activation for regenerative medicine, and discuss strategies to activate YAP/TAZ for regenerative therapy while minimizing adverse side effects. D. melanogaster Hpo kinase), the large tumor suppressor kinases 1 and 2 (LATS1 and LATS2, Warts in D. melanogaster), the adaptor proteins Salvador1, MOB1A/B (SAV1 and Mats in D. melanogaster), the homologous transcriptional co-activators YAP and TAZ (Yorkie in D. melanogaster), and the TEAD transcription factors (TEAD 1-4, Scalloped in D. melanogaster) (Fig.1) 9,12,39,40. Mechanistically, YAP/TAZ in complex with a TEAD transcription factor bind to gene enhancers, interact with chromatin remodelling factors, and modulate RNA Polymerase II (Pol II) to drive or repress the expression of target genes, which prominently include cell cycle, cell migration and cell fate regulators (see also below) 40-48. Of note, although TEAD transcription factors are required for YAP/TAZ target gene expression, YAP-TEAD complexes alone may not be sufficient to activate the different genetic programs. Indeed, bioinformatics analyses of the regulatory regions that are bound by YAP/TAZ-TEAD complexes identified cooperation between YAP/TEAD and other transcription factors 45,49-52 (Fig. 1). In addition, although TEAD factors are their main interaction partners, YAP/TAZ can interact with other DNA binding transcription factors such as p73 53 , RUNX 54 , and TBX5 55,56. Therefore, YAP/TAZ cooperate with various transcription factors to regulate target gene expression. Activation of MST1/2 induces the phosphorylation of SAV1 and MOB1A/B 57,58 , which assist MST1/2 in the recruitment, phosphorylation, and activation of LATS1/2 58-60. LATS1/2 can also be phosphorylated and activated by the MAP4K1-7 family kinases 61-63. Subsequently, LATS1/2 phosphorylate YAP and TAZ 32,39,40,46,64-66. YAP/TAZ phosphorylation by LATS1/2 causes their cytoplasmic sequestration by 14-3-3 proteins and triggers further phosphorylation by Casein kinase 1δ/ε as well as ubiquitylation by the SCF E3 ubiquitin ligase complex and proteasomal degradation 67-70. Thus, the core Hippo kinases inhibit YAP/TAZ activity and suppress the transcriptional output of the Hippo pathway.

Vascular endothelial growth factor (VEGF) is a major driver of blood vessel formation. However, the signal transduction pathways culminating in the biological consequences of VEGF signaling are only partially understood. Here, we show that the Hippo pathway effectors YAP and TAZ work as crucial signal transducers to mediate VEGF-VEGFR2 signaling during angiogenesis. We demonstrate that YAP/TAZ are essential for vascular development as endothelium-specific deletion of YAP/TAZ leads to impaired vascularization and embryonic lethality. Mechanistically, we show that VEGF activates YAP/TAZ via its effects on actin cytoskeleton and that activated YAP/TAZ induce a transcriptional program to further control cytoskeleton dynamics and thus establish a feedforward loop that ensures a proper angiogenic response. Lack of YAP/TAZ also results in altered cellular distribution of VEGFR2 due to trafficking defects from the Golgi apparatus to the plasma membrane. Altogether, our study identifies YAP/TAZ as central mediators of VEGF signaling and therefore as important regulators of angiogenesis.

Gradients of vascular endothelial growth factor (VEGF) induce single endothelial cells to become leading tip cells of emerging angiogenic sprouts. Tip cells then suppress tip-cell features in adjacent stalk cells via Dll4/Notch-mediated lateral inhibition. We report here that Smad1/Smad5-mediated BMP signaling synergizes with Notch signaling during selection of tip and stalk cells. Endothelium-specific inactivation of Smad1/Smad5 in mouse embryos results in impaired Dll4/Notch signaling and increased numbers of tip-cell-like cells at the expense of stalk cells. Smad1/5 downregulation in cultured endothelial cells reduced the expression of several target genes of Notch and of other stalk-cell-enriched transcripts (Hes1, Hey1, Jagged1, VEGFR1, and Id1-3). Moreover, Id proteins act as competence factors for stalk cells and form complexes with Hes1, which augment Hes1 levels in the endothelium. Our findings provide in vivo evidence for a regulatory loop between BMP/TGFβ-Smad1/5 and Notch signaling that orchestrates tip- versus stalk-cell selection and vessel plasticity.

The Hippo signaling pathway and its two downstream effectors, the YAP and TAZ transcriptional coactivators, are drivers of tumor growth in experimental models. Studying mouse models, we show that YAP and TAZ can also exert a tumor-suppressive function. We found that normal hepatocytes surrounding liver tumors displayed activation of YAP and TAZ and that deletion of Yap and Taz in these peritumoral hepatocytes accelerated tumor growth. Conversely, experimental hyperactivation of YAP in peritumoral hepatocytes triggered regression of primary liver tumors and melanoma-derived liver metastases. Furthermore, whereas tumor cells growing in wild-type livers required YAP and TAZ for their survival, those surrounded by Yap- and Taz-deficient hepatocytes were not dependent on YAP and TAZ. Tumor cell survival thus depends on the relative activity of YAP and TAZ in tumor cells and their surrounding tissue, suggesting that YAP and TAZ act through a mechanism of cell competition to eliminate tumor cells.