RecA in Escherichia coli and its homolog, ScRad51 in Saccharomyces cerevisiae, are known to be essential for recombinational repair. The homolog of RecA and ScRad51 in mice, MmRad51, was mutated to determine its function. Mutant embryos arrested early during development. A decrease in cell proliferation, followed by programmed cell death and chromosome loss, was observed. Radiation sensitivity was demonstrated in trophectoderm-derived cells. Interestingly, embryonic development progressed further in a p53 null background; however, fibroblasts derived from double-mutant embryos failed to proliferate in tissue culture.

Angiogenesis is a multistep process that requires coordinated migration, proliferation, and junction formation of vascular endothelial cells (ECs) to form new vessel branches in response to growth stimuli. Major intracellular signaling pathways that regulate angiogenesis have been well elucidated, but key transcriptional regulators that mediate these signaling pathways and control EC behaviors are only beginning to be understood. Here, we show that YAP/TAZ, a transcriptional coactivator that acts as an end effector of Hippo signaling, is critical for sprouting angiogenesis and vascular barrier formation and maturation. In mice, endothelial-specific deletion of Yap/Taz led to blunted-end, aneurysm-like tip ECs with fewer and dysmorphic filopodia at the vascular front, a hyper-pruned vascular network, reduced and disarranged distributions of tight and adherens junction proteins, disrupted barrier integrity, subsequent hemorrhage in growing retina and brain vessels, and reduced pathological choroidal neovascularization. Mechanistically, YAP/TAZ activates actin cytoskeleton remodeling, an important component of filopodia formation and junction assembly. Moreover, YAP/TAZ coordinates EC proliferation and metabolic activity by upregulating MYC signaling. Overall, these results show that YAP/TAZ plays multifaceted roles for EC behaviors, proliferation, junction assembly, and metabolism in sprouting angiogenesis and barrier formation and maturation and could be a potential therapeutic target for treating neovascular diseases.

The Hippo pathway regulates the self-renewal and differentiation of various adult stem cells, but its role in cell fate determination and differentiation during liver development remains unclear. Here we report that the Hippo pathway controls liver cell lineage specification and proliferation separately from Notch signalling, using mice and primary hepatoblasts with liver-specific knockout of Lats1 and Lats2 kinase, the direct upstream regulators of YAP and TAZ. During and after liver development, the activation of YAP/TAZ induced by loss of Lats1/2 forces hepatoblasts or hepatocytes to commit to the biliary epithelial cell (BEC) lineage. It increases BEC and fibroblast proliferation by up-regulating TGFβ signalling, but suppresses hepatoblast to hepatocyte differentiation by repressing Hnf4α expression. Notably, oncogenic YAP/TAZ activation in hepatocytes induces massive p53-dependent cell senescence/death. Together, our results reveal that YAP/TAZ activity levels govern liver cell differentiation and proliferation in a context-dependent manner.

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IntroductionNAFLD is characterized by an excessive accumulation of fat in the liver. The most severe form of NAFLD, NASH, often progresses to liver cancer (1-3). Tracking with the increasing prevalence of general obesity, 30% of the US population is now estimated to have NAFLD, and 25% of these individuals will develop NASH (1, 3). Currently, there are no effective therapies to prevent the incidence and progression of NAFLD or NASH (4). This makes clarification of the detailed mechanisms of disease progression using appropriate animal models that much more urgent.Insulin signaling begins with the binding of insulin to the insulin receptor (IR). This then phosphorylates insulin receptor substrates (IRSs) and subsequently triggers the recruitment of PI3K and the activation of AKT (5-9). Deletion of IRS1 and IRS2 in the mouse liver reduces AKT activity and gives rise to insulin resistance (10). Excessive AKT activation leads to the development of NAFLD by promoting the maturation of the transcription factor SREBP1c (11,12). In its mature form, SREBP1c contributes to the induction of fatty acid synthase (FAS) and acetyl-CoA carboxylase (ACC), which are key enzymes in de novo lipogenesis (13). Phosphatase and tensin homolog (PTEN) is a negative regulator of AKT signaling, and several human cancers are associated with mutations or downregulation of the PTEN gene (14-16). Liver-specific PTEN-knockout mice progressively develop NAFLD, NASH, and HCC (17, 18) as a result of increased AKT signaling (19).The Hippo signaling pathway has been implicated in the suppression of tissue regeneration, the proliferation of stem cells, and the development of cancer by inhibiting the oncogenic activity of the transcriptional coactivators YAP and TAZ (20,21). In mice, liverspecific knockout of the Hippo pathway components MST1/2, SAV1, or NF2 induces the expansion of hepatic progenitor cells via YAP/TAZ activation and leads eventually to the development of liver cancer (HCC, cholangiocarcinoma [CC], or both) (22)(23)(24)(25). Despite its importance in tumorigenesis, the role of Hippo signaling in the metabolic dysregulation that precedes the development of liver cancer remains unclear.Previous studies have suggested that YAP regulates components of the AKT pathway (i.e., PI3K, PTEN, and AKT) and that the Drosophila Hippo ortholog MST1 binds and inhibits AKT directly (26)(27)(28)(29). Increased YAP expression in human liver tumors is associated with high levels of p- AKT (30,31). This suggests that crosstalk between the Hippo and AKT pathways may be important in the maintenance of functional liver homeostasis. The molecular coordination of these 2 pathways in liver tumorigenesis, however, has not been revealed. Using several mouse models, we now show Nonalcoholic fatty liver disease (NAFLD) is a major risk factor for liver cancer; therefore, its prevention is an important clinical goal. Ablation of phosphatase and tensin homolog (PTEN) or the protein kinase Hippo signaling pathway induces liver cancer via activation of AKT or the transc...