Defects in yolk sac vasculogenesis, chorioallantoic fusion, and embryonic axis elongation in mice with targeted disruption of Yap65

Structural basis of YAP recognition by TEAD4 in the hippo pathway

The Hippo signaling pathway controls cell growth, proliferation, and apoptosis by regulating the expression of target genes that execute these processes. Acting downstream from this pathway is the YAP transcriptional coactivator, whose biological function is mediated by the conserved TEAD family transcription factors. The interaction of YAP with TEADs is critical to regulate Hippo pathway-responsive genes. Here, we describe the crystal structure of the YAP-interacting C-terminal domain of TEAD4 in complex with the TEAD-interacting N-terminal domain of YAP. The structure reveals that the N-terminal region of YAP is folded into two short helices with an extended loop containing the PXXPhiP motif in between, while the C-terminal domain of TEAD4 has an immunoglobulin-like fold. YAP interacts with TEAD4 mainly through the two short helices. Point mutations of TEAD4 indicate that the residues important for YAP interaction are required for its transforming activity. Mutagenesis reveals that the PXXPhiP motif of YAP, although making few contacts with TEAD4, is important for TEAD4 interaction as well as for the transforming activity.

Structural insights into the YAP and TEAD complex

The Yes-associated protein (YAP) transcriptional coactivator is a key regulator of organ size and a candidate human oncogene inhibited by the Hippo tumor suppressor pathway. The TEAD family of transcription factors binds directly to and mediates YAP-induced gene expression. Here we report the three-dimensional structure of the YAP (residues 50-171)-TEAD1 (residues 194-411) complex, in which YAP wraps around the globular structure of TEAD1 and forms extensive interactions via three highly conserved interfaces. Interface 3, including YAP residues 86-100, is most critical for complex formation. Our study reveals the biochemical nature of the YAP-TEAD interaction, and provides a basis for pharmacological intervention of YAP-TEAD hyperactivation in human diseases.

Yes-associated protein is an independent prognostic marker in hepatocellular carcinoma

BACKGROUND

Yes-associated protein (YAP), a downstream target of the Hippo signaling pathway, was recently linked to hepatocarcinogenesis in a mouse hepatocellular carcinoma (HCC) model. The objective of the current study was to investigate the clinical significance of YAP in HCC and its prognostic values in predicting survival and tumor recurrence.

METHODS

The authors collected 177 pairs of tumor and adjacent nontumor tissue from HCC patients with definitive clinicopathologic and follow-up data. YAP expression was determined by immunohistochemistry, Western blot analysis, and quantitative polymerase chain reaction. Association of YAP with each clinicopathologic feature was analyzed by Pearson chi-square test, and HCC-specific disease-free survival and overall survival by Kaplan-Meier curves and log-rank test. Multivariate Cox regression analyses of YAP in HCC were also performed.

RESULTS

YAP was expressed in the majority of HCC cases (approximately 62%) and mainly accumulated in the tumor nucleus. Overexpression of YAP in HCC was significantly associated with poorer tumor differentiation (Edmonson grade; P = .021) and high serum alpha-fetoprotein (AFP) level (P < .001). Kaplan-Meier and Cox regression data indicated that YAP was an independent predictor for HCC-specific disease-free survival (hazards ratio [HR], 1.653; 95% confidence interval [95% CI], 1.081-2.528 [P = .02]) and overall survival (HR, 2.148; 95% CI, 1.255-3.677 [P = .005]).

CONCLUSIONS

YAP is an independent prognostic marker for overall survival and disease-free survival times of HCC patients and clinicopathologically associated with tumor differentiation and serum AFP level. It is a potential therapeutic target for this aggressive malignancy.

Yes-associated protein is an independent prognostic marker in hepatocellular carcinoma

BACKGROUND

Yes-associated protein (YAP), a downstream target of the Hippo signaling pathway, was recently linked to hepatocarcinogenesis in a mouse hepatocellular carcinoma (HCC) model. The objective of the current study was to investigate the clinical significance of YAP in HCC and its prognostic values in predicting survival and tumor recurrence.

METHODS

The authors collected 177 pairs of tumor and adjacent nontumor tissue from HCC patients with definitive clinicopathologic and follow-up data. YAP expression was determined by immunohistochemistry, Western blot analysis, and quantitative polymerase chain reaction. Association of YAP with each clinicopathologic feature was analyzed by Pearson chi-square test, and HCC-specific disease-free survival and overall survival by Kaplan-Meier curves and log-rank test. Multivariate Cox regression analyses of YAP in HCC were also performed.

RESULTS

YAP was expressed in the majority of HCC cases (approximately 62%) and mainly accumulated in the tumor nucleus. Overexpression of YAP in HCC was significantly associated with poorer tumor differentiation (Edmonson grade; P = .021) and high serum alpha-fetoprotein (AFP) level (P < .001). Kaplan-Meier and Cox regression data indicated that YAP was an independent predictor for HCC-specific disease-free survival (hazards ratio [HR], 1.653; 95% confidence interval [95% CI], 1.081-2.528 [P = .02]) and overall survival (HR, 2.148; 95% CI, 1.255-3.677 [P = .005]).

CONCLUSIONS

YAP is an independent prognostic marker for overall survival and disease-free survival times of HCC patients and clinicopathologically associated with tumor differentiation and serum AFP level. It is a potential therapeutic target for this aggressive malignancy.

Crucial role for Mst1 and Mst2 kinases in early embryonic development of the mouse

Mammalian sterile 20-like kinases 1 and 2 (Mst1 and Mst2, respectively) are potent serine/threonine kinases that are involved in cell proliferation and cell death. To investigate the physiological functions of Mst1 and Mst2, we generated Mst1 and Mst2 mutant mice. Mst1(-/-) and Mst2(-/-) mice were viable and fertile and developed normally, suggesting possible functional overlaps between the two genes. A characterization of heterozygous and homozygous combinations of Mst1 and Mst2 mutant mice showed that mice containing a single copy of either gene underwent normal organ development; however, Mst1(-/-); Mst2(-/-) mice lacking both Mst1 and Mst2 genes started dying in utero at approximately embryonic day 8.5. Mst1(-/-); Mst2(-/-) mice exhibited severe growth retardation, failed placental development, impaired yolk sac/embryo vascular patterning and primitive hematopoiesis, increased apoptosis in placentas and embryos, and disorganized proliferating cells in the embryo proper. These findings indicate that both Mst1 and Mst2 kinases play essential roles in early mouse development, regulating placental development, vascular patterning, primitive hematopoiesis, and cell proliferation and survival.

Mammalian NDR/LATS protein kinases in hippo tumor suppressor signaling

The NDR/LATS family of kinases is a subgroup of the AGC group of protein kinases and is conserved from lower eukaryotes to humans. Like other AGC kinases, NDR/LATS kinases require phosphorylation of conserved Ser/Thr residues for activation. On the one hand, binding of the coactivator MOB to NDR/LATS allows autophosphorylation. On the other hand, MST kinases directly phosphorylate NDR/LATS kinases. In addition to our understanding of the molecular activation mechanisms, recent studies have shown that LATS kinases play a central role in Hippo/SWH (Salvador/Warts/Hippo) tumor suppressor pathways, which coordinate cell proliferation and apoptosis by regulating proto-oncogenes, such as YAP and TAZ. In this review, we summarize current knowledge of Merlin/MST/SAV/MOB/LATS/NDR/YAP/TAZ networks (also termed mammalian Hippo signaling) and their roles in mammalian cellular transformation.

YAP, TAZ, and Yorkie: a conserved family of signal-responsive transcriptional coregulators in animal development and human disease

How extracellular cues are transduced to the nucleus is a fundamental issue in biology. The paralogous WW-domain proteins YAP (Yes-associated protein) and TAZ (transcriptional coactivator with PDZ-binding motif; also known as WWTR1, for WW-domain containing transcription regulator 1) constitute a pair of transducers linking cytoplasmic signaling events to transcriptional regulation in the nucleus. A cascade composed of mammalian Ste20-like (MST) and large tumor suppressor (LATS) kinases directs multisite phosphorylation, promotes 14-3-3 binding, and hinders nuclear import of YAP and TAZ, thereby inhibiting their transcriptional coactivator and growth-promoting activities. A similar cascade regulates the trafficking and function of Yorkie, the fly orthologue of YAP. Mammalian YAP and TAZ are expressed in various tissues and serve as coregulators for transcriptional enhancer factors (TEFs; also referred to as TEADs, for TEA-domain proteins), runt-domain transcription factors (Runxs), peroxisome proliferator-activated receptor gamma (PPARgamma), T-box transcription factor 5 (Tbx5), and several others. YAP and TAZ play distinct roles during mouse development. Both, and their upstream regulators, are intimately linked to tumorigenesis and other pathogenic processes. Here, we review studies on this family of signal-responsive transcriptional coregulators and emphasize how relative sequence conservation predicates their function and regulation, to provide a conceptual framework for organizing available information and seeking new knowledge about these signal transducers.

Cellular and molecular origin of circumpapillary dysgenesis of the pigment epithelium

PURPOSE

We studied clinical phenotyping and TEAD1 expression in mice and humans to gain a better understanding of the primary origin in the pathogenesis of circumpapillary dysgenesis of the pigment epithelium.

DESIGN

Observational case series and experimental study.

PARTICIPANTS

Three female patients from an affected family were included for phenotypic study. Mice and human tissues were used for biochemistry and immunohistochemistry studies.

METHODS

We performed genetic analyses and longitudinal clinical, imaging, and electrophysiologic studies in a 3-generation family. Western blotting and immunohistochemistry were used to detect TEAD1 expression in mice and human retinal tissues.

MAIN OUTCOME MEASURES

Autofluorescence and optical coherence tomography (OCT) imaging were compared and reviewed from 3 patients. TEAD1 expression was compared in different tissues from mice and human samples.

RESULTS

A point mutation at T1261 in TEAD1 was detected in the mother. Autofluorescence and OCT imaging studies revealed choroid is involved earlier than retinal pigment epithelium (RPE). From immunoblot analysis, we discovered that TEAD1 and its cofactors YAP65 and FOXA2 are expressed in the choroid. Immunohistochemical analysis on frozen sections of mouse retina supports immunoblot results.

CONCLUSIONS

The primary cellular origin of circumpapillary dysgenesis of the pigment epithelium is within the choroid instead of the pigment epithelium. The loss of the RPE and photoreceptors in later stages of the disease is a secondary consequence of choroidal degeneration. Studies of the downstream targets of TEAD1 in choroidal cells will provide promising new research opportunities for the development of treatments for choroidal diseases.

FINANCIAL DISCLOSURE(S)

The author(s) have no proprietary or commercial interest in any materials discussed in this article.

TEADs mediate nuclear retention of TAZ to promote oncogenic transformation

The transcriptional coactivators YAP and TAZ are downstream targets inhibited by the Hippo tumor suppressor pathway. The expression level of TAZ is recently shown to be elevated in invasive breast cancer cells and some primary breast cancers. TAZ is important for breast cancer cell migration, invasion, and tumorigenesis, but the underlying mechanism is not defined. In this study, we show that TAZ interacts with TEAD transcriptional factors. Knockdown of TEADs suppresses TAZ-mediated oncogenic transformation of MCF10A cells. Uncoupling TAZ from Hippo regulation by S89A mutation enhances its transforming ability. Several residues located in the N-terminal region of TAZ are identified to be important for interaction with TEADs, and these same residues are equally important for TAZ to transform MCF10A cells. Mechanistically, TAZ mutants defective in interaction with TEADs fail to accumulate in the nucleus. Live cell imaging of enhanced green fluorescent protein-TAZ and its mutant defective in TEAD interaction suggests that TEAD interaction mediates nuclear retention. These results reveal a novel mechanism for TEADs to regulate nuclear retention and thus the transforming ability of TAZ.

The E3 ubiquitin ligase WWP1 selectively targets HER4 and its proteolytically derived signaling isoforms for degradation

In general, epidermal growth factor receptor family members stimulate cell proliferation. In contrast, at least one HER4 isoform, JM-a/Cyt1, inhibits cell growth after undergoing a two-step proteolytic cleavage that first produces a membrane-anchored 80-kDa fragment (m80(HER4)) and subsequently liberates a soluble 80-kDa fragment, s80(HER4). Here we report that s80(HER4) Cyt1 action increased the expression of WWP1 (for WW domain-containing protein 1), an E3 ubiquitin ligase, but not other members of the Nedd4 E3 ligase family. The HER4 Cyt1 isoform contains three proline-rich tyrosine (PY) WW binding motifs, while Cyt2 has only two. WWP1 binds to all three Cyt1 PY motifs; the interaction with PY2 found exclusively in Cyt1 was strongest. WWP1 ubiquitinated and caused the degradation of HER4 but not of EGFR, HER2, or HER3. The HER4-WWP1 interaction also accelerated WWP1 degradation. Membrane HER4 (full length and m80(HER4), the product of the first proteolytic cleavage) were the preferred targets of WWP1, correlating with the membrane localization of WWP1. Conversely s80(HER4), a poorer WWP1 substrate, was found in the cell nucleus, while WWP1 was not. Deletion of the C2 membrane association domain of WWP1 allowed more efficient s80(HER4) degradation, suggesting that WWP1 is normally part of a membrane complex that regulates HER4 membrane species levels, with a predilection for the growth-inhibitory Cyt1 isoform. Finally, WWP1 expression diminished HER4 biologic activity in MCF-7 cells. We previously showed that nuclear s80(HER4) is ubiquitinated and degraded by the anaphase-promoting complex, suggesting that HER4 ubiquitination within specific cellular compartments helps regulate the unique HER4 signaling capabilities.

The Hippo-YAP pathway: new connections between regulation of organ size and cancer

The control of organ size is a basic biological question. In the past several years, the Hippo signaling pathway has been delineated and shown to be crucial in control of organ size in both Drosophila and mammals. Acting downstream of the Hippo pathway is the Yki/YAP/TAZ transcription co-activators. In mammalian cells, the Hippo pathway kinase cascade inhibits YAP and its paralog TAZ by phosphorylation and promotion of their cytoplasmic localization. The TEAD family transcription factors have recently been identified as evolutionarily conserved key mediators of YAP biological functions. yap is a candidate oncogene, and several other components of the Hippo pathway are tumor suppressors. Dysregulation of the Hippo pathway contributes to the loss of contact inhibition observed in cancer cells. Therefore, the Hippo-YAP pathway connects the regulation of organ size and tumorigenesis.

YAP regulates neural progenitor cell number via the TEA domain transcription factor

Tight control of cell proliferation is essential for proper growth during development and for tissue homeostasis in mature animals. The evolutionarily conserved Hippo pathway restrains proliferation through a kinase cascade that culminates in the inhibition of the transcriptional coactivator YAP. Unphosphorylated YAP activates genes involved in cell proliferation and survival by interacting with a DNA-binding factor. Here we show that during vertebrate neural tube development, the TEA domain transcription factor (TEAD) is the cognate DNA-binding partner of YAP. YAP and TEAD gain of function causes marked expansion of the neural progenitor population, partly owing to their ability to promote cell cycle progression by inducing cyclin D1 and to inhibit differentiation by suppressing NeuroM. Their loss of function results in increased apoptosis, whereas repressing their target genes leads to premature neuronal differentiation. Inhibiting the upstream kinases of the Hippo pathway also causes neural progenitor overproliferation. Thus, the Hippo pathway plays critical roles in regulating neural progenitor cell number by affecting proliferation, fate choice, and cell survival.

Mammalian Tead proteins regulate cell proliferation and contact inhibition as transcriptional mediators of Hippo signaling

Regulation of organ size is important for development and tissue homeostasis. In Drosophila, Hippo signaling controls organ size by regulating the activity of a TEAD transcription factor, Scalloped, through modulation of its co-activator protein Yki. Here, we show that mouse Tead proteins regulate cell proliferation by mediating Hippo signaling. In NIH3T3 cells, cell density and Hippo signaling regulated the activity of endogenous Tead proteins by modulating nuclear localization of a Yki homolog, Yap1, and the resulting change in Tead activity altered cell proliferation. Tead2-VP16 mimicked Yap1 overexpression, including increased cell proliferation, reduced cell death, promotion of EMT, lack of cell contact inhibition and promotion of tumor formation. Growth-promoting activities of various Yap1 mutants correlated with their Tead-co-activator activities. Tead2-VP16 and Yap1 regulated largely overlapping sets of genes. However, only a few of the Tead/Yap1-regulated genes in NIH3T3 cells were affected in Tead1(-/-);Tead2(-/-) or Yap1(-/-) embryos. Most of the previously identified Yap1-regulated genes were not affected in NIH3T3 cells or mutant mice. In embryos, levels of nuclear Yap1 and Tead1 varied depending on cell type. Strong nuclear accumulation of Yap1 and Tead1 were seen in myocardium, correlating with requirements of Tead1 for proliferation. However, their distribution did not always correlate with proliferation. Taken together, mammalian Tead proteins regulate cell proliferation and contact inhibition as a transcriptional mediator of Hippo signaling, but the mechanisms by which Tead/Yap1 regulate cell proliferation differ depending on the cell type, and Tead, Yap1 and Hippo signaling may play multiple roles in mouse embryos.

Chato, a KRAB zinc-finger protein, regulates convergent extension in the mouse embryo

In Xenopus and zebrafish embryos, elongation of the anterior-posterior body axis depends on convergent extension, a process that involves polarized cell movements and is regulated by non-canonical Wnt signaling. The mechanisms that control axis elongation of the mouse embryo are much less well understood. Here, we characterize the ENU-induced mouse mutation chato, which causes arrest at midgestation and defects characteristic of convergent extension mutants, including a shortened body axis, mediolaterally extended somites and an open neural tube. The chato mutation disrupts Zfp568, a Krüppel-associated box (KRAB) domain zinc-finger protein. Morphometric analysis revealed that the definitive endoderm of mouse wild-type embryos undergoes cell rearrangements that lead to convergent extension during early somite stages, and that these cell rearrangements fail in chato embryos. Although non-canonical Wnt signaling is important for convergent extension in the mouse notochord and neural plate, the results indicate that chato regulates body axis elongation in all embryonic tissues through a process independent of non-canonical Wnt signaling.

Expression of Yes-associated protein in common solid tumors

The Hippo signaling pathway is a highly conserved potent regulator of cell growth, division, and apoptosis. Yes-associated protein (YAP), the nuclear effector of the Hippo pathway, is a highly conserved component of this pathway in mammalian systems. In humans, amplification of the chromosome region containing the YAP gene (11q22) has been reported in several tumor types. This study was performed to determine if YAP expression was present in 4 common types of malignant tumors that have the highest lifetime risk of causing cancer death among men and women in the United States. The YAP expression intensity and distribution were evaluated in normal tissues and compared to the most frequently occurring malignant tumors in these tissues (colonic adenocarcinoma, lung adenocarcinoma, ovarian serous cystadenocarcinoma, and ductal carcinoma of the breast). For each tissue, the nuclear and cytoplasmic YAP expression intensity was scored as negative, low, or high. We found focal expression of YAP in the progenitor and reparative cellular compartments of normal tissue. In contrast, there was strong and diffuse nuclear and cytoplasmic YAP expression in colonic adenocarcinoma, lung adenocarcinoma, and ovarian serous cystadenocarcinoma. We concluded that the potent Hippo growth regulatory pathway shows markedly different expression patterns in normal tissues of the colon, lung, and ovary compared to the 3 common malignant tumor types we examined in these tissues. Our findings suggest that activation of the Hippo signaling pathway may occur through YAP as part of cell proliferation in normal tissue homeostasis and also might be a frequently activated oncogenic pathway in 3 common malignant tumor types.

A crucial role of WW45 in developing epithelial tissues in the mouse

The role and molecular mechanisms of a new Hippo signalling pathway are not fully understood in mammals. Here, we generated mice that lack WW45 and revealed a crucial role for WW45 in cell-cycle exit and epithelial terminal differentiation. Many organs in the mutant mouse embryos displayed hyperplasia accompanied by defects in terminal differentiation of epithelial progenitor cells owing to impaired proliferation arrest rather than intrinsic acceleration of proliferation during differentiation. Importantly, the MST1 signalling pathway is specifically activated in differentiating epithelial cells. Moreover, WW45 is required for MST1 activation and translocation to the nucleus for subsequent LATS1/2 activation upon differentiation signal. LATS1/2 phosphorylates YAP, which, in turn, translocates from the nucleus into the cytoplasm, resulting in cell-cycle exit and terminal differentiation of epithelial progenitor cells. Collectively, these data provide compelling evidence that WW45 is a key mediator of MST1 signalling in the coordinate coupling of proliferation arrest with terminal differentiation for proper epithelial tissue development in mammals.

Deficiency of zonula occludens-1 causes embryonic lethal phenotype associated with defected yolk sac angiogenesis and apoptosis of embryonic cells

Zonula occludens (ZO)-1/2/3 are the members of the TJ-MAGUK family of membrane-associated guanylate kinases associated with tight junctions. To investigate the role of ZO-1 (encoded by Tjp1) in vivo, ZO-1 knockout (Tjp1(-/-)) mice were generated by gene targeting. Although heterozygous mice showed normal development and fertility, delayed growth and development were evident from E8.5 onward in Tjp1(-/-) embryos, and no viable Tjp1(-/-) embryos were observed beyond E11.5. Tjp1(-/-) embryos exhibited massive apoptosis in the notochord, neural tube area, and allantois at embryonic day (E)9.5. In the yolk sac, the ZO-1 deficiency induced defects in vascular development, with impaired formation of vascular trees, along with defective chorioallantoic fusion. Immunostaining of wild-type embryos at E8.5 for ZO-1/2/3 revealed that ZO-1/2 were expressed in almost all embryonic cells, showing tight junction-localizing patterns, with or without ZO-3, which was confined to the epithelial cells. ZO-1 deficiency depleted ZO-1-expression without influence on ZO-2/3 expression. In Tjp1(+/+) yolk sac extraembryonic mesoderm, ZO-1 was dominant without ZO-2/3 expression. Thus, ZO-1 deficiency resulted in mesoderms with no ZO-1/2/3, associated with mislocalization of endothelial junctional adhesion molecules. As a result, angiogenesis was defected in Tjp1(-/-) yolk sac, although differentiation of endothelial cells seemed to be normal. In conclusion, ZO-1 may be functionally important for cell remodeling and tissue organization in both the embryonic and extraembryonic regions, thus playing an essential role in embryonic development.

Redundant roles of Tead1 and Tead2 in notochord development and the regulation of cell proliferation and survival

Four members of the TEAD/TEF family of transcription factors are expressed widely in mouse embryos and adult tissues. Although in vitro studies have suggested various roles for TEAD proteins, their in vivo functions remain poorly understood. Here we examined the role of Tead genes by generating mouse mutants for Tead1 and Tead2. Tead2(-/-) mice appeared normal, but Tead1(-/-); Tead2(-/-) embryos died at embryonic day 9.5 (E9.5) with severe growth defects and morphological abnormalities. At E8.5, Tead1(-/-); Tead2(-/-) embryos were already small and lacked characteristic structures such as a closed neural tube, a notochord, and somites. Despite these overt abnormalities, differentiation and patterning of the neural plate and endoderm were relatively normal. In contrast, the paraxial mesoderm and lateral plate mesoderm were displaced laterally, and a differentiated notochord was not maintained. These abnormalities and defects in yolk sac vasculature organization resemble those of mutants for Yap, which encodes a coactivator of TEAD proteins. Moreover, we demonstrated genetic interactions between Tead1 and Tead2 and Yap. Finally, Tead1(-/-); Tead2(-/-) embryos showed reduced cell proliferation and increased apoptosis. These results suggest that Tead1 and Tead2 are functionally redundant, use YAP as a major coactivator, and support notochord maintenance as well as cell proliferation and survival in mouse development.

The emerging role of the hippo pathway in cell contact inhibition, organ size control, and cancer development in mammals

The Hippo pathway defined originally in Drosophila melanogaster is conserved in mammals. The fly core components Hippo, Sav, Wts, and Mats are conserved in mammals as Mst1/2, WW45, LATS1/2, and Mob1. The pathway impinges on transcriptional coactivator Yorkie in fly and YAP in mammals to coordinate cell proliferation and apoptosis. Several recent publications establish that the pathway is one major conserved mechanism governing cell contact inhibition, organ size control, and cancer development. This advance opens new vistas in exploring fundamental mechanisms in cell and developmental biology and offers potential targets to interfere with cancer development.

Elucidation of a universal size-control mechanism in Drosophila and mammals

Coordination of cell proliferation and cell death is essential to attain proper organ size during development and for maintaining tissue homeostasis throughout postnatal life. In Drosophila, these two processes are orchestrated by the Hippo kinase cascade, a growth-suppressive pathway that ultimately antagonizes the transcriptional coactivator Yorkie (Yki). Here we demonstrate that a single phosphorylation site in Yki mediates the growth-suppressive output of the Hippo pathway. Hippo-mediated phosphorylation inactivates Yki by excluding it from the nucleus, whereas loss of Hippo signaling leads to nuclear accumulation and therefore increased Yki activity. We further delineate a mammalian Hippo signaling pathway that culminates in the phosphorylation of YAP, the mammalian homolog of Yki. Using a conditional YAP transgenic mouse model, we demonstrate that the mammalian Hippo pathway is a potent regulator of organ size, and that its dysregulation leads to tumorigenesis. These results uncover a universal size-control mechanism in metazoan.

Hippo signaling in organ size control

The control of organ (or organism) size is a fundamental aspect of life that has long captured human imagination. What makes an elephant grow a million times larger than a mouse? How do our two hands develop independently of each other yet reach very similar size? How does a liver precisely regenerate its original mass when two-thirds of it is removed? The recent discovery of a novel signaling network in Drosophila, known as the Hippo (Hpo) pathway, might provide an important entry point to these fascinating questions. The Hpo pathway consists of several negative growth regulators acting in a kinase cascade that ultimately phosphorylates and inactivates Yorkie (Yki), a transcriptional coactivator that positively regulates cell growth, survival, and proliferation. Components of the Hpo pathway are highly conserved throughout evolution, suggesting that this pathway may function as a global regulator of tissue homeostasis in all metazoan animals. Here, I provide a historical review of this potent growth-regulatory pathway and highlight outstanding questions that will likely be the focus of future investigation.