Kibra is a regulator of the Salvador/Warts/Hippo signaling network

Advancements of anticancer agents by targeting the Hippo signalling pathway: biological activity, selectivity, docking analysis, and structure-activity relationship

The Hippo signalling pathway is prominent and governs cell proliferation and stem cell activity, acting as a growth regulator and tumour suppressor. Defects in Hippo signalling and hyperactivation of its downstream effector's Yes-associated protein (YAP) and transcriptional co-activator with PDZ-binding motif (TAZ) play roles in cancer development, implying that pharmacological inhibition of YAP and TAZ activity could be an effective cancer treatment strategy. Conversely, YAP and TAZ can also have beneficial effects in promoting tissue repair and regeneration following damage, therefore their activation may be therapeutically effective in certain instances. Recently, a complex network of intracellular and extracellular signalling mechanisms that affect YAP and TAZ activity has been uncovered. The YAP/TAZ-TEAD interaction leads to tumour development and the protein structure of YAP/TAZ-TEAD includes three interfaces and one hydrophobic pocket. There are clinical and preclinical trial drugs available to inhibit the hippo signalling pathway, but these drugs have moderate to severe side effects, so researchers are in search of novel, potent, and selective hippo signalling pathway inhibitors. In this review, we have discussed the hippo pathway in detail, including its structure, activation, and role in cancer. We have also provided the various inhibitors under clinical and preclinical trials, and advancement of small molecules their detailed docking analysis, structure-activity relationship, and biological activity. We anticipate that the current study will be a helpful resource for researchers.

TSG-6-Mediated Extracellular Matrix Modifications Regulate Hypoxic-Ischemic Brain Injury

Proteoglycans containing link domains modify the extracellular matrix (ECM) to regulate cellular homeostasis and can also sensitize tissues/organs to injury and stress. Hypoxic-ischemic (H-I) injury disrupts cellular homeostasis by activating inflammation and attenuating regeneration and repair pathways. In the brain, the main component of the ECM is the glycosaminoglycan hyaluronic acid (HA), but whether HA modifications of the ECM regulate cellular homeostasis and response to H-I injury is not known. In this report, employing both male and female mice, we demonstrate that link-domain-containing proteoglycan, TNFα-stimulated gene-6 (TSG-6), is active in the brain from birth onward and differentially modifies ECM HA during discrete neurodevelopmental windows. ECM HA modification by TSG-6 enables it to serve as a developmental switch to regulate the activity of the Hippo pathway effector protein, yes-associated protein 1 (YAP1), in the maturing brain and in response to H-I injury. Mice that lack TSG-6 expression display dysregulated expression of YAP1 targets, excitatory amino acid transporter 1 (EAAT1; glutamate-aspartate transporter) and 2 (EAAT2; glutamate transporter-1). Dysregulation of YAP1 activation in TSG-6-/- mice coincides with age- and sex-dependent sensitization of the brain to H-I injury such that 1-week-old neonates display an anti-inflammatory response in contrast to an enhanced proinflammatory injury reaction in 3-month-old adult males but not females. Our findings thus support that a key regulator of age- and sex-dependent H-I injury response in the mouse brain is modulation of the Hippo-YAP1 pathway by TSG-6-dependent ECM modifications.

Hnrnpk protects against osteoarthritis through targeting WWC1 mRNA and inhibiting Hippo signaling pathway

Osteoarthritis (OA) is an age-related or post-traumatic degenerative whole joint disease characterized by the rupture of articular cartilage homeostasis, the regulatory mechanisms of which remain elusive. This study identifies the essential role of heterogeneous nuclear ribonucleoprotein K (hnRNPK) in maintaining articular cartilage homeostasis. Hnrnpk expression is markedly downregulated in human and mice OA cartilage. The deletion of Hnrnpk effectively accelerates the development of post-traumatic and age-dependent OA in mice. Mechanistically, the KH1 and KH2 domain of Hnrnpk bind and degrade the mRNA of WWC1. Hnrnpk deletion increases WWC1 expression, which in turn leads to the activation of Hippo signaling and ultimately aggravates OA. In particular, intra-articular injection of LPA and adeno-associated virus serotype 5 expressing WWC1 RNA interference ameliorates cartilage degeneration induced by Hnrnpk deletion, and intra-articular injection of adeno-associated virus serotype 5 expressing Hnrnpk protects against OA. Collectively, this study reveals the critical roles of Hnrnpk in inhibiting OA development through WWC1-dependent downregulation of Hippo signaling in chondrocytes and defines a potential target for the prevention and treatment of OA.

Inhibiting Hippo pathway kinases releases WWC1 to promote AMPAR-dependent synaptic plasticity and long-term memory in mice

The localization, number, and function of postsynaptic AMPA-type glutamate receptors (AMPARs) are crucial for synaptic plasticity, a cellular correlate for learning and memory. The Hippo pathway member WWC1 is an important component of AMPAR-containing protein complexes. However, the availability of WWC1 is constrained by its interaction with the Hippo pathway kinases LATS1 and LATS2 (LATS1/2). Here, we explored the biochemical regulation of this interaction and found that it is pharmacologically targetable in vivo. In primary hippocampal neurons, phosphorylation of LATS1/2 by the upstream kinases MST1 and MST2 (MST1/2) enhanced the interaction between WWC1 and LATS1/2, which sequestered WWC1. Pharmacologically inhibiting MST1/2 in male mice and in human brain-derived organoids promoted the dissociation of WWC1 from LATS1/2, leading to an increase in WWC1 in AMPAR-containing complexes. MST1/2 inhibition enhanced synaptic transmission in mouse hippocampal brain slices and improved cognition in healthy male mice and in male mouse models of Alzheimer's disease and aging. Thus, compounds that disrupt the interaction between WWC1 and LATS1/2 might be explored for development as cognitive enhancers.

The Hippo signaling pathway in development and regeneration

The Hippo signaling pathway is a central growth control mechanism in multicellular organisms. By integrating diverse mechanical, biochemical, and stress cues, the Hippo pathway orchestrates proliferation, survival, differentiation, and mechanics of cells, which in turn regulate organ development, homeostasis, and regeneration. A deep understanding of the regulation and function of the Hippo pathway therefore holds great promise for developing novel therapeutics in regenerative medicine. Here, we provide updates on the molecular organization of the mammalian Hippo signaling network, review the regulatory signals and functional outputs of the pathway, and discuss the roles of Hippo signaling in development and regeneration.

Inhibition of RhoGEF/RhoA alleviates regorafenib resistance and cancer stemness via Hippo signaling pathway in hepatocellular carcinoma

Patients with hepatocellular carcinoma (HCC) are vulnerable to drug resistance. Although drug resistance has been taken much attention to HCC therapy, little is known of regorafenib and regorafenib resistance (RR). This study aimed to determine the drug resistance pattern and the role of RhoA in RR. Two regorafenib-resistant cell lines were constructed based on Huh7 and Hep3B cell lines. In vitro and in vivo assays were conducted to study RhoA expression, the activity of Hippo signaling pathway and cancer stem cell (CSC) traits. The data showed that RhoA was highly expressed, Hippo signaling was hypoactivated and CSC traits were more prominent in RR cells. Inhibiting RhoA could reverse RR, and the alliance of RhoA inhibition and regorafenib synergistically attenuated CSC phenotype. Furthermore, inhibiting LARG/RhoA increased Kibra/NF2 complex formation, prevented YAP from shuttling into the nucleus and repressed CD44 mRNA expression. Clinically, the high expression of RhoA correlated with poor prognosis. LARG, RhoA, YAP1 and CD44 show positive correlation with each other. Thus, inhibition of RhoGEF/RhoA has the potential to reverse RR and repress CSC phenotype in HCC.

Comprehensive split TEV based protein-protein interaction screening reveals TAOK2 as a key modulator of Hippo signalling to limit growth

The conserved Hippo signalling pathway plays a crucial role in tumour formation by limiting tissue growth and proliferation. At the core of this pathway are tumour suppressor kinases STK3/4 and LATS1/2, which limit the activity of the oncogene YAP1, the primary downstream effector. Here, we employed a split TEV-based protein-protein interaction screen to assess the physical interactions among 28 key Hippo pathway components and potential upstream modulators. This screen led us to the discovery of TAOK2 as pivotal modulator of Hippo signalling, as it binds to the pathway's core kinases, STK3/4 and LATS1/2, and leads to their phosphorylation. Specifically, our findings revealed that TAOK2 binds to and phosphorylates LATS1, resulting in the reduction of YAP1 phosphorylation and subsequent transcription of oncogenes. Consequently, this decrease led to a decrease in cell proliferation and migration. Interestingly, a correlation was observed between reduced TAOK2 expression and decreased patient survival time in certain types of human cancers, including lung and kidney cancer as well as glioma. Moreover, in cellular models corresponding to these cancer types the downregulation of TAOK2 by CRISPR inhibition led to reduced phosphorylation of LATS1 and increased proliferation rates, supporting TAOK2's role as tumour suppressor gene. By contrast, overexpression of TAOK2 in these cellular models lead to increased phospho-LATS1 but reduced cell proliferation. As TAOK2 is a druggable kinase, targeting TAOK2 could serve as an attractive pharmacological approach to modulate cell growth and potentially offer strategies for combating cancer.

Role of YAP Signaling in Regulation of Programmed Cell Death and Drug Resistance in Cancer

Although recent advances in cancer treatment significantly improved the prognosis of patients, drug resistance remains a major challenge. Targeting programmed cell death is a major approach of antitumor drug development. Deregulation of programmed cell death (PCD) contributes to resistance to a variety of cancer therapeutics. Yes-associated protein (YAP) and its paralog TAZ, the main downstream effectors of the Hippo pathway, are aberrantly activated in a variety of human malignancies. The Hippo-YAP pathway, which was originally identified in Drosophila, is well conserved in humans and plays a defining role in regulation of cell fate, tissue growth and regeneration. Activation of YAP signaling has emerged as a key mechanism involved in promoting cancer cell proliferation, metastasis, and drug resistance. Understanding the role of YAP/TAZ signaling network in PCD and drug resistance could facilitate the development of effective strategies for cancer therapeutics.

Cortical tension promotes Kibra degradation via Par-1

The Hippo pathway is an evolutionarily conserved regulator of tissue growth. Multiple Hippo signaling components are regulated via proteolytic degradation. However, how these degradation mechanisms are themselves modulated remains unexplored. Kibra is a key upstream pathway activator that promotes its own ubiquitin-mediated degradation upon assembling a Hippo signaling complex. Here, we demonstrate that Hippo complex-dependent Kibra degradation is modulated by cortical tension. Using classical genetic, osmotic, and pharmacological manipulations of myosin activity and cortical tension, we show that increasing cortical tension leads to Kibra degradation, whereas decreasing cortical tension increases Kibra abundance. Our study also implicates Par-1 in regulating Kib abundance downstream of cortical tension. We demonstrate that Par-1 promotes ubiquitin-mediated Kib degradation in a Hippo complex-dependent manner and is required for tension-induced Kib degradation. Collectively, our results reveal a previously unknown molecular mechanism by which cortical tension affects Hippo signaling and provide novel insights into the role of mechanical forces in growth control.

Control of stem cell renewal and fate by YAP and TAZ

Complex physiological processes control whether stem cells self-renew, differentiate or remain quiescent. Two decades of research have placed the Hippo pathway, a highly conserved kinase signalling cascade, and its downstream molecular effectors YAP and TAZ at the nexus of this decision. YAP and TAZ translate complex biological cues acting on stem cells - from mechanical forces to cellular metabolism - into genome-wide effects to mediate stem cell functions. While aberrant YAP/TAZ activity drives stem cell dysfunction in ageing, tumorigenesis and disease, therapeutic targeting of Hippo signalling and YAP/TAZ can boost stem cell activity to enhance regeneration. In this Review, we discuss how YAP/TAZ control the self-renewal, fate and plasticity of stem cells in different contexts, how dysregulation of YAP/TAZ in stem cells leads to disease, and how therapeutic modalities targeting YAP/TAZ may benefit regenerative medicine and cancer therapy.

New Insights into YAP/TAZ-TEAD-Mediated Gene Regulation and Biological Processes in Cancer

The Hippo pathway is conserved across species. Key mammalian Hippo pathway kinases, including MST1/2 and LATS1/2, inhibit cellular growth by inactivating the TEAD coactivators, YAP, and TAZ. Extensive research has illuminated the roles of Hippo signaling in cancer, development, and regeneration. Notably, dysregulation of Hippo pathway components not only contributes to tumor growth and metastasis, but also renders tumors resistant to therapies. This review delves into recent research on YAP/TAZ-TEAD-mediated gene regulation and biological processes in cancer. We focus on several key areas: newly identified molecular patterns of YAP/TAZ activation, emerging mechanisms that contribute to metastasis and cancer therapy resistance, unexpected roles in tumor suppression, and advances in therapeutic strategies targeting this pathway. Moreover, we provide an updated view of YAP/TAZ's biological functions, discuss ongoing controversies, and offer perspectives on specific debated topics in this rapidly evolving field.

SPTAN1/NUMB axis senses cell density to restrain cell growth and oncogenesis through Hippo signaling

The loss of contact inhibition is a key step during carcinogenesis. The Hippo-Yes-associated protein (Hippo/YAP) pathway is an important regulator of cell growth in a cell density-dependent manner. However, how Hippo signaling senses cell density in this context remains elusive. Here, we report that high cell density induced the phosphorylation of spectrin α chain, nonerythrocytic 1 (SPTAN1), a plasma membrane-stabilizing protein, to recruit NUMB endocytic adaptor protein isoforms 1 and 2 (NUMB1/2), which further sequestered microtubule affinity-regulating kinases (MARKs) in the plasma membrane and rendered them inaccessible for phosphorylation and inhibition of the Hippo kinases sterile 20-like kinases MST1 and MST2 (MST1/2). WW45 interaction with MST1/2 was thereby enhanced, resulting in the activation of Hippo signaling to block YAP activity for cell contact inhibition. Importantly, low cell density led to SPTAN1 dephosphorylation and NUMB cytoplasmic location, along with MST1/2 inhibition and, consequently, YAP activation. Moreover, double KO of NUMB and WW45 in the liver led to appreciable organ enlargement and rapid tumorigenesis. Interestingly, NUMB isoforms 3 and 4, which have a truncated phosphotyrosine-binding (PTB) domain and are thus unable to interact with phosphorylated SPTAN1 and activate MST1/2, were selectively upregulated in liver cancer, which correlated with YAP activation. We have thus revealed a SPTAN1/NUMB1/2 axis that acts as a cell density sensor to restrain cell growth and oncogenesis by coupling external cell-cell contact signals to intracellular Hippo signaling.

Apical polarity and actomyosin dynamics control Kibra subcellular localization and function in Drosophila Hippo signaling

The Hippo pathway is an evolutionarily conserved regulator of tissue growth that integrates inputs from both polarity and actomyosin networks. An upstream activator of the Hippo pathway, Kibra, localizes at the junctional and medial regions of the apical cortex in epithelial cells, and medial accumulation promotes Kibra activity. Here, we demonstrate that cortical Kibra distribution is controlled by a tug-of-war between apical polarity and actomyosin dynamics. We show that while the apical polarity network, in part via atypical protein kinase C (aPKC), tethers Kibra at the junctional cortex to silence its activity, medial actomyosin flows promote Kibra-mediated Hippo complex formation at the medial cortex, thereby activating the Hippo pathway. This study provides a mechanistic understanding of the relationship between the Hippo pathway, polarity, and actomyosin cytoskeleton, and it offers novel insights into how fundamental features of epithelial tissue architecture can serve as inputs into signaling cascades that control tissue growth, patterning, and morphogenesis.

WWC1/2 regulate spinogenesis and cognition in mice by stabilizing AMOT

WWC1 regulates episodic learning and memory, and genetic nucleotide polymorphism of WWC1 is associated with neurodegenerative diseases such as Alzheimer's disease. However, the molecular mechanism through which WWC1 regulates neuronal function has not been fully elucidated. Here, we show that WWC1 and its paralogs (WWC2/3) bind directly to angiomotin (AMOT) family proteins (Motins), and recruit USP9X to deubiquitinate and stabilize Motins. Deletion of WWC genes in different cell types leads to reduced protein levels of Motins. In mice, neuron-specific deletion of Wwc1 and Wwc2 results in reduced expression of Motins and lower density of dendritic spines in the cortex and hippocampus, in association with impaired cognitive functions such as memory and learning. Interestingly, ectopic expression of AMOT partially rescues the neuronal phenotypes associated with Wwc1/2 deletion. Thus, WWC proteins modulate spinogenesis and cognition, at least in part, by regulating the protein stability of Motins.

The Hippo Pathway Effectors YAP/TAZ-TEAD Oncoproteins as Emerging Therapeutic Targets in the Tumor Microenvironment

Various cancer cell-associated intrinsic and extrinsic inputs act on YAP/TAZ proteins to mediate the hyperactivation of the TEAD transcription factor-based transcriptome. This YAP/TAZ-TEAD activity can override the growth-limiting Hippo tumor-suppressor pathway that maintains normal tissue homeostasis. Herein, we provide an integrated summary of the contrasting roles of YAP/TAZ during normal tissue homeostasis versus tumor initiation and progression. In addition to upstream factors that regulate YAP/TAZ in the TME, critical insights on the emerging functions of YAP/TAZ in immune suppression and abnormal vasculature development during tumorigenesis are illustrated. Lastly, we discuss the current methods that intervene with the YAP/TAZ-TEAD oncogenic signaling pathway and the emerging applications of combination therapies, gut microbiota, and epigenetic plasticity that could potentiate the efficacy of chemo/immunotherapy as improved cancer therapeutic strategies.

miR-497 Regulates LATS1 through the PPARG Pathway to Participate in Fatty Acid Synthesis in Bovine Mammary Epithelial Cells

Nutrient metabolism is required to maintain energy balance in animal organisms, and fatty acids play an irreplaceable role in fat metabolism. In this study, microRNA sequencing was performed on mammary gland tissues collected from cows during early, peak, and late lactation to determine miRNA expression profiles. Differentially expressed miRNA (miR-497) was selected for functional studies of fatty acid substitution. Simulants of miR-497 impaired fat metabolism [triacylglycerol (TAG) and cholesterol], whereas knockdown of miR-497 promoted fat metabolism in bovine mammary epithelial cells (BMECs) in vitro. In addition, in vitro experiments on BMECs showed that miR-497 could down-regulate C16:1, C17:1, C18:1, and C20:1 as well as long-chain polyunsaturated fats. Thus, these data expand the discovery of a critical role for miR-497 in mediating adipocyte differentiation. Through bioinformatics analysis and further validation, we identified large tumor suppressor kinase 1 (LATS1) as a target of miR-497. siRNA-LATS1 increased concentrations of fatty acids, TAG, and cholesterol in cells, indicating an active role of LATS1 in milk fat metabolism. In summary, miR-497/LATS1 can regulate the biological processes associated with TAG, cholesterol, and unsaturated fatty acid synthesis in cells, providing an experimental basis for further elucidating the mechanistic regulation of lipid metabolism in BMECs.

A tale of two Hippo pathway modules

The Hippo pathway is an evolutionarily conserved pathway with crucial roles in development, organ size control, tissue homeostasis and cancer. Over two decades of research have elucidated the core Hippo pathway kinase cascade, but its precise organization has not been fully understood. In this issue of The EMBO Journal, Qi et al (2023) report a new model of two modules for the Hippo kinase cascade, providing new insights into this long-standing question.

Two Hippo signaling modules orchestrate liver size and tumorigenesis

The Hippo pathway is a central regulator of organ size and tumorigenesis and is commonly depicted as a kinase cascade, with an increasing number of regulatory and adaptor proteins linked to its regulation over recent years. Here, we propose that two Hippo signaling modules, MST1/2-SAV1-WWC1-3 (HPO1) and MAP4K1-7-NF2 (HPO2), together regulate the activity of LATS1/2 kinases and YAP/TAZ transcriptional co-activators. In mouse livers, the genetic inactivation of either HPO1 or HPO2 module results in partial activation of YAP/TAZ, bile duct hyperplasia, and hepatocellular carcinoma (HCC). On the contrary, inactivation of both HPO1 and HPO2 modules results in full activation of YAP/TAZ, rapid development of intrahepatic cholangiocarcinoma (iCCA), and early lethality. Interestingly, HPO1 has a predominant role in regulating organ size. HPO1 inactivation causes a homogenous YAP/TAZ activation and cell proliferation across the whole liver, resulting in a proportional and rapid increase in liver size. Thus, this study has reconstructed the order of the Hippo signaling network and suggests that LATS1/2 and YAP/TAZ activities are finetuned by HPO1 and HPO2 modules to cause different cell fates, organ size changes, and tumorigenesis trajectories.

Expanded directly binds conserved regions of Fat to restrain growth via the Hippo pathway

The Hippo pathway is a conserved and critical regulator of tissue growth. The FERM protein Expanded is a key signaling hub that promotes activation of the Hippo pathway, thereby inhibiting the transcriptional co-activator Yorkie. Previous work identified the polarity determinant Crumbs as a primary regulator of Expanded. Here, we show that the giant cadherin Fat also regulates Expanded directly and independently of Crumbs. We show that direct binding between Expanded and a highly conserved region of the Fat cytoplasmic domain recruits Expanded to the apicolateral junctional zone and stabilizes Expanded. In vivo deletion of Expanded binding regions in Fat causes loss of apical Expanded and promotes tissue overgrowth. Unexpectedly, we find Fat can bind its ligand Dachsous via interactions of their cytoplasmic domains, in addition to the known extracellular interactions. Importantly, Expanded is stabilized by Fat independently of Dachsous binding. These data provide new mechanistic insights into how Fat regulates Expanded, and how Hippo signaling is regulated during organ growth.

KIBRA upregulation increases susceptibility to podocyte injury and glomerular disease progression

Despite recent progress in the identification of mediators of podocyte injury, mechanisms underlying podocyte loss remain poorly understood, and cell-specific therapy is lacking. We previously reported that kidney and brain expressed protein (KIBRA), encoded by WWC1, promotes podocyte injury in vitro through activation of the Hippo signaling pathway. KIBRA expression is increased in the glomeruli of patients with focal segmental glomerulosclerosis, and KIBRA depletion in vivo is protective against acute podocyte injury. Here, we tested the consequences of transgenic podocyte-specific WWC1 expression in immortalized human podocytes and in mice, and we explored the association between glomerular WWC1 expression and glomerular disease progression. We found that KIBRA overexpression in immortalized human podocytes promoted cytoplasmic localization of Yes-associated protein (YAP), induced actin cytoskeletal reorganization, and altered focal adhesion expression and morphology. WWC1-transgenic (KIBRA-overexpressing) mice were more susceptible to acute and chronic glomerular injury, with evidence of YAP inhibition in vivo. Of clinical relevance, glomerular WWC1 expression negatively correlated with renal survival among patients with primary glomerular diseases. These findings highlight the importance of KIBRA/YAP signaling to the regulation of podocyte structural integrity and identify KIBRA-mediated injury as a potential target for podocyte-specific therapy in glomerular disease.

Phase separation of Hippo signalling complexes

The Hippo pathway was originally discovered to control tissue growth in Drosophila and includes the Hippo kinase (Hpo; MST1/2 in mammals), scaffold protein Salvador (Sav; SAV1 in mammals) and the Warts kinase (Wts; LATS1/2 in mammals). The Hpo kinase is activated by binding to Crumbs-Expanded (Crb-Ex) and/or Merlin-Kibra (Mer-Kib) proteins at the apical domain of epithelial cells. Here we show that activation of Hpo also involves the formation of supramolecular complexes with properties of a biomolecular condensate, including concentration dependence and sensitivity to starvation, macromolecular crowding, or 1,6-hexanediol treatment. Overexpressing Ex or Kib induces formation of micron-scale Hpo condensates in the cytoplasm, rather than at the apical membrane. Several Hippo pathway components contain unstructured low-complexity domains and purified Hpo-Sav complexes undergo phase separation in vitro. Formation of Hpo condensates is conserved in human cells. We propose that apical Hpo kinase activation occurs in phase separated "signalosomes" induced by clustering of upstream pathway components.

Fat and Dachsous cadherins in mammalian development

Cell growth and patterning are critical for tissue development. Here we discuss the evolutionarily conserved cadherins, Fat and Dachsous, and the roles they play during mammalian tissue development and disease. In Drosophila, Fat and Dachsous regulate tissue growth via the Hippo pathway and planar cell polarity (PCP). The Drosophila wing has been an ideal tissue to observe how mutations in these cadherins affect tissue development. In mammals, there are multiple Fat and Dachsous cadherins, which are expressed in many tissues, but mutations in these cadherins that affect growth and tissue organization are context dependent. Here we examine how mutations in the Fat and Dachsous mammalian genes affect development in mammals and contribute to human disease.

Hippo-released WWC1 facilitates AMPA receptor regulatory complexes for hippocampal learning

Learning and memory rely on changes in postsynaptic glutamergic α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-type receptor (AMPAR) number, spatial organization, and function. The Hippo pathway component WW and C2 domain-containing protein 1 (WWC1) regulates AMPAR surface expression and impacts on memory performance. However, synaptic binding partners of WWC1 and its hierarchical position in AMPAR complexes are largely unclear. Using cell-surface proteomics in hippocampal tissue of Wwc1-deficient mice and by generating a hippocampus-specific interactome, we show that WWC1 is a major regulatory platform in AMPAR signaling networks. Under basal conditions, the Hippo pathway members WWC1 and large tumor-suppressor kinase (LATS) are associated, which might prevent WWC1 effects on synaptic proteins. Reduction of WWC1/LATS binding through a point mutation at WWC1 elevates the abundance of WWC1 in AMPAR complexes and improves hippocampal-dependent learning and memory. Thus, uncoupling of WWC1 from the Hippo pathway to AMPAR-regulatory complexes provides an innovative strategy to enhance synaptic transmission.

Hippo signaling instructs ectopic but not normal organ growth

The Hippo signaling pathway is widely considered a master regulator of organ growth because of the prominent overgrowth phenotypes caused by experimental manipulation of its activity. Contrary to this model, we show here that removing Hippo transcriptional output did not impair the ability of the mouse liver and Drosophila eyes to grow to their normal size. Moreover, the transcriptional activity of the Hippo pathway effectors Yap/Taz/Yki did not correlate with cell proliferation, and hyperactivation of these effectors induced gene expression programs that did not recapitulate normal development. Concordantly, a functional screen in Drosophila identified several Hippo pathway target genes that were required for ectopic overgrowth but not normal growth. Thus, Hippo signaling does not instruct normal growth, and the Hippo-induced overgrowth phenotypes are caused by the activation of abnormal genetic programs.

Multiphase coalescence mediates Hippo pathway activation

The function of biomolecular condensates is often restricted by condensate dissolution. Whether condensates can be suppressed without condensate dissolution is unclear. Here, we show that upstream regulators of the Hippo signaling pathway form functionally antagonizing condensates, and their coalescence into a common phase provides a mode of counteracting the function of biomolecular condensates without condensate dissolution. Specifically, the negative regulator SLMAP forms Hippo-inactivating condensates to facilitate pathway inhibition by the STRIPAK complex. In response to cell-cell contact or osmotic stress, the positive regulators AMOT and KIBRA form Hippo-activating condensates to facilitate pathway activation. The functionally antagonizing SLMAP and AMOT/KIBRA condensates further coalesce into a common phase to inhibit STRIPAK function. These findings provide a paradigm for restricting the activity of biomolecular condensates without condensate dissolution, shed light on the molecular principles of multiphase organization, and offer a conceptual framework for understanding upstream regulation of the Hippo signaling pathway.

The Regulation of the Hippo Pathway by Intercellular Junction Proteins

The Hippo pathway is an evolutionarily conserved pathway that serves to promote cell death and differentiation while inhibiting cellular proliferation across species. The downstream effectors of this pathway, yes-associated protein (YAP) and transcriptional co-activator with PDZ-binding motif (TAZ), are considered vital in promoting the output of the Hippo pathway, with activation of upstream kinases negatively regulating YAP/TAZ activity. The upstream regulation of the Hippo pathway is not entirely understood on a molecular level. However, several studies have shown that numerous cellular and non-cellular mechanisms such as cell polarity, contact inhibition, soluble factors, mechanical forces, and metabolism can convey external stimuli to the intracellular kinase cascade, promoting the activation of key components of the Hippo pathway and therefore regulating the subcellular localisation and protein activity of YAP/TAZ. This review will summarise what we have learnt about the role of intercellular junction-associated proteins in the activation of this pathway, including adherens junctions and tight junctions, and in particular our latest findings about the desmosomal components, including desmoglein-3 (DSG3), in the regulation of YAP signalling, phosphorylation, and subcellular translocation.

Shaggy regulates tissue growth through Hippo pathway in Drosophila

The evolutionarily conserved Hippo pathway coordinates cell proliferation, differentiation and apoptosis to regulate organ growth and tumorigenesis. Hippo signaling activity is tightly controlled by various upstream signals including growth factors and cell polarity, but the full extent to which the pathway is regulated during development remains to be resolved. Here, we report the identification of Shaggy, the homolog of mammalian Gsk3β, as a novel regulator of the Hippo pathway in Drosophila. Our results show that Shaggy promotes the expression of Hippo target genes in a manner that is dependent on its kinase activity. Loss of Shaggy leads to Yorkie inhibition and downregulation of Hippo pathway target genes. Mechanistically, Shaggy acts upstream of the Hippo pathway and negatively regulates the abundance of the FERM domain containing adaptor protein Expanded. Our results reveal that Shaggy is functionally required for Crumbs/Slmb-mediated downregulation of Expanded in vivo, providing a potential molecular link between cellular architecture and the Hippo signaling pathway.

Transmembrane protein KIRREL1 regulates Hippo signaling via a feedback loop and represents a therapeutic target in YAP/TAZ-active cancers

The Hippo tumor-suppressor pathway is frequently dysregulated in human cancers and represents a therapeutic target. However, strategies targeting the mammalian Hippo pathway are limited because of the lack of a well-established cell-surface regulator. Here, we show that transmembrane protein KIRREL1, by interacting with both SAV1 and LATS1/2, promotes LATS1/2 activation by MST1/2 (Hippo kinases), and LATS1/2 activation, in turn, inhibits activity of YAP/TAZ oncoproteins. Conversely, YAP/TAZ directly induce the expression of KIRREL1 in a TEAD1-4-dependent manner. Indeed, KIRREL1 expression positively correlates with canonical YAP/TAZ target gene expression in clinical tumor specimens and predicts poor prognosis. Moreover, transgenic expression of KIRREL1 effectively blocks tumorigenesis in a mouse intrahepatic cholangiocarcinoma model, indicating a tumor-suppressor role of KIRREL1. Hence, KIRREL1 constitutes a negative feedback mechanism regulating the Hippo pathway and serves as a cell-surface marker and potential drug target in cancers with YAP/TAZ dependency.

Thalamic gray matter volume mediates the association between KIBRA polymorphism and olfactory function among older adults: a population-based study

The kidney and brain expressed protein (KIBRA) rs17070145 polymorphism is associated with both structure and activation of the olfactory cortex. However, no studies have thus far examined whether KIBRA can be linked with olfactory function and whether brain structure plays any role in the association. We addressed these questions in a population-based cross-sectional study among rural-dwelling older adults. This study included 1087 participants derived from the Multidomain Interventions to Delay Dementia and Disability in Rural China, who underwent the brain MRI scans in August 2018 to October 2020; of these, 1016 took the 16-item Sniffin' Sticks identification test and 634 (62.40%) were defined with olfactory impairment (OI). Data were analyzed using the voxel-based morphometry analysis and general linear, logistic, and structural equation models. The KIBRA rs17070145 C-allele (CC or CT vs. TT genotype) was significantly associated with greater gray matter volume (GMV) mainly in the bilateral orbitofrontal cortex and left thalamus (P < 0.05) and with the multi-adjusted odds ratio of 0.73 (95% confidence interval 0.56-0.95) for OI. The left thalamic GMV could mediate 8.08% of the KIBRA-olfaction association (P < 0.05). These data suggest that the KIBRA rs17070145 C-allele is associated with a reduced likelihood of OI among older adults, partly mediated through left thalamic GMV.

YAP induces an oncogenic transcriptional program through TET1-mediated epigenetic remodeling in liver growth and tumorigenesis

Epigenetic remodeling is essential for oncogene-induced cellular transformation and malignancy. In contrast to histone post-translational modifications, how DNA methylation is remodeled by oncogenic signaling remains poorly understood. The oncoprotein YAP, a coactivator of the TEAD transcription factors mediating Hippo signaling, is widely activated in human cancers. Here, we identify the 5-methylcytosine dioxygenase TET1 as a direct YAP target and a master regulator that coordinates the genome-wide epigenetic and transcriptional reprogramming of YAP target genes in the liver. YAP activation induces the expression of TET1, which physically interacts with TEAD to cause regional DNA demethylation, histone H3K27 acetylation and chromatin opening in YAP target genes to facilitate transcriptional activation. Loss of TET1 not only reverses YAP-induced epigenetic and transcriptional changes but also suppresses YAP-induced hepatomegaly and tumorigenesis. These findings exemplify how oncogenic signaling regulates the site specificity of DNA demethylation to promote tumorigenesis and implicate TET1 as a potential target for modulating YAP signaling in physiology and disease.

Roles of Yes-associated protein and transcriptional coactivator with PDZ-binding motif in non-neoplastic liver diseases

The prevalence of liver disease has been increasing worldwide. Moreover, the burden of end-stage liver disease, including cirrhosis and liver cancer, is high because of high mortality and suboptimal treatment. The pathological process of liver disease includes steatosis, hepatocyte death, and fibrosis, which ultimately lead to cirrhosis and liver cancer. Clinical and preclinical evidence indicates that non-neoplastic liver diseases, particularly cirrhosis, are major risk factors for liver cancer, although the mechanism underlying this association remains unclear. Yes-associated protein (YAP) and transcriptional coactivator with PDZ-binding motif (TAZ) are transcriptional activators that regulate organ size and cancer development. YAP and TAZ play important roles in liver development, regeneration, and homeostasis. Abnormal YAP and TAZ levels have also been implicated in non-neoplastic liver diseases (e.g., non-alcoholic fatty liver disease, alcoholic liver disease, liver injury, and liver fibrosis). Here, we review recent findings on the roles of YAP and TAZ in non-neoplastic liver diseases and discuss directions for future research. This review provides a basis for the study of non-neoplastic liver diseases.

Molecular Alterations in Malignant Pleural Mesothelioma: A Hope for Effective Treatment by Targeting YAP

Malignant pleural mesothelioma is a rare and aggressive neoplasm, which has primarily been attributed to the exposure to asbestos fibers (83% of cases); yet, despite a ban of using asbestos in many countries, the incidence of malignant pleural mesothelioma failed to decline worldwide. While little progress has been made in malignant pleural mesothelioma diagnosis, bevacizumab at first, then followed by double immunotherapy (nivolumab plus ipilumumab), were all shown to improve survival in large phase III randomized trials. The morphological analysis of the histological subtyping remains the primary indicator for therapeutic decision making at an advanced disease stage, while a platinum-based chemotherapy regimen combined with pemetrexed, either with or without bevacizumab, is still the main treatment option. Consequently, malignant pleural mesothelioma still represents a significant health concern owing to poor median survival (12-18 months). Given this context, both diagnosis and therapy improvements require better knowledge of the molecular mechanisms underlying malignant pleural mesothelioma's carcinogenesis and progression. Hence, the Hippo pathway in malignant pleural mesothelioma initiation and progression has recently received increasing attention, as the aberrant expression of its core components may be closely related to patient prognosis. The purpose of this review was to provide a critical analysis of our current knowledge on these topics, the main focus being on the available evidence concerning the role of each Hippo pathway's member as a promising biomarker, enabling detection of the disease at earlier stages and thus improving prognosis.

Self-Sustained Regulation or Self-Perpetuating Dysregulation: ROS-dependent HIF-YAP-Notch Signaling as a Double-Edged Sword on Stem Cell Physiology and Tumorigenesis

Organ development, homeostasis, and repair often rely on bidirectional, self-organized cell-niche interactions, through which cells select cell fate, such as stem cell self-renewal and differentiation. The niche contains multiplexed chemical and mechanical factors. How cells interpret niche structural information such as the 3D topology of organs and integrate with multiplexed mechano-chemical signals is an open and active research field. Among all the niche factors, reactive oxygen species (ROS) have recently gained growing interest. Once considered harmful, ROS are now recognized as an important niche factor in the regulation of tissue mechanics and topology through, for example, the HIF-YAP-Notch signaling pathways. These pathways are not only involved in the regulation of stem cell physiology but also associated with inflammation, neurological disorder, aging, tumorigenesis, and the regulation of the immune checkpoint molecule PD-L1. Positive feedback circuits have been identified in the interplay of ROS and HIF-YAP-Notch signaling, leading to the possibility that under aberrant conditions, self-organized, ROS-dependent physiological regulations can be switched to self-perpetuating dysregulation, making ROS a double-edged sword at the interface of stem cell physiology and tumorigenesis. In this review, we discuss the recent findings on how ROS and tissue mechanics affect YAP-HIF-Notch-PD-L1 signaling, hoping that the knowledge can be used to design strategies for stem cell-based and ROS-targeting therapy and tissue engineering.

The Role of Hippo-YAP/TAZ Signaling in Brain Development

In order for our complex nervous system to develop normally, both precise spatial and temporal regulation of a number of different signaling pathways is critical. During both early embryogenesis and in organ development, one pathway that has been repeatedly implicated is the Hippo-YAP/TAZ signaling pathway. The paralogs YAP and TAZ are transcriptional co-activators that play an important role in cell proliferation, cell differentiation, and organ growth. Regulation of these proteins by the Hippo kinase cascade is therefore important for normal development. In this article, we review the growing field of research surrounding the role of Hippo-YAP/TAZ signaling in normal and atypical brain development. Starting from the development of the neural tube to the development and refinement of the cerebral cortex, cerebellum, and ventricular system, we address the typical role of these transcriptional co-activators, the functional consequences that manipulation of YAP/TAZ and their upstream regulators have on brain development, and where further research may be of benefit. This article is protected by copyright. All rights reserved.

The unfolding of the Hippo signaling pathway

The development of a functional organ requires not only patterning mechanisms that confer proper identities to individual cells, but also growth-regulatory mechanisms that specify the final size of the organ. At the turn of the 21st century, comprehensive genetic screens in model organisms had successfully uncovered the major signaling pathways that mediate pattern formation in metazoans. In contrast, signaling pathways dedicated to growth control were less explored. The past two decades has witnessed the emergence of the Hippo signaling pathway as a central mediator of organ size control through coordinated regulation of cell proliferation and apoptosis. Here I reflect on the early discoveries in Drosophila that elucidated the core kinase cascade and transcriptional machinery of the Hippo pathway, highlight its deep evolutionary conservation from humans to unicellular relatives of metazoan, and discuss the complex regulation of Hippo signaling by upstream inputs. This historical perspective underscores the importance of model organisms in uncovering fundamental and universal mechanisms of life processes.

Hippo Signaling in the Endometrium

The uterus is essential for embryo implantation and fetal development. During the estrous cycle, the uterine endometrium undergoes dramatic remodeling to prepare for pregnancy. Angiogenesis is an essential biological process in endometrial remodeling. Steroid hormones regulate the series of events that occur during such remodeling. Researchers have investigated the potential factors, including angiofactors, involved in endometrial remodeling. The Hippo signaling pathway discovered in the 21st century, plays important roles in various cellular functions, including cell proliferation and cell death. However, its role in the endometrium remains unclear. In this review, we describe the female reproductive system and its association with the Hippo signaling pathway, as well as novel Hippo pathway genes and potential target genes.

The Hippo pathway in cancer: YAP/TAZ and TEAD as therapeutic targets in cancer

Tumorigenesis is a highly complex process, involving many interrelated and cross-acting signalling pathways. One such pathway that has garnered much attention in the field of cancer research over the last decade is the Hippo signalling pathway. Consisting of two antagonistic modules, the pathway plays an integral role in both tumour suppressive and oncogenic processes, generally via regulation of a diverse set of genes involved in a range of biological functions. This review discusses the history of the pathway within the context of cancer and explores some of the most recent discoveries as to how this critical transducer of cellular signalling can influence cancer progression. A special focus is on the various recent efforts to therapeutically target the key effectors of the pathway in both preclinical and clinical settings.

Modulation of Hippo signaling by Mnat9 N-acetyltransferase for normal growth and tumorigenesis in Drosophila

Hippo signaling is a conserved mechanism for controlling organ growth. Increasing evidence suggests that Hippo signaling is modulated by various cellular factors for normal development and tumorigenesis. Hence, identification of these factors is pivotal for understanding the mechanism for the regulation of Hippo signaling. Drosophila Mnat9 is a putative N-acetyltransferase that is required for cell survival by affecting JNK signaling. Here we show that Mnat9 is involved in the negative regulation of Hippo signaling. RNAi knockdown of Mnat9 in the eye disc suppresses the rough eye phenotype of overexpressing Crumbs (Crb), an upstream factor of the Hippo pathway. Conversely, Mnat9 RNAi enhances the eye phenotype caused by overexpressing Expanded (Ex) or Warts (Wts) that acts downstream to Crb. Similar genetic interactions between Mnat9 and Hippo pathway genes are found in the wing. The reduced wing phenotype of Mnat9 RNAi is suppressed by overexpression of Yorkie (Yki), while it is suppressed by knockdown of Hippo upstream factors like Ex, Merlin, or Kibra. Mnat9 co-immunoprecipitates with Mer, implying their function in a protein complex. Furthermore, Mnat9 overexpression together with Hpo knockdown causes tumorous overgrowth in the abdomen. Our data suggest that Mnat9 is required for organ growth and can induce tumorous growth by negatively regulating the Hippo signaling pathway.

YAP/TAZ in Bone and Cartilage Biology

YAP and TAZ were initially described as the main regulators of organ growth during development and more recently implicated in bone biology. YAP and TAZ are regulated by mechanical and cytoskeletal cues that lead to the control of cell fate in response to the cellular microenvironment. The mechanical component represents a major signal for bone tissue adaptation and remodelling, so YAP/TAZ contributes significantly in bone and cartilage homeostasis. Recently, mice and cellular models have been developed to investigate the precise roles of YAP/TAZ in bone and cartilage cells, and which appear to be crucial. This review provides an overview of YAP/TAZ regulation and function, notably providing new insights into the role of YAP/TAZ in bone biology.

Ecdysone regulates Drosophila wing disc size via a TORC1 dependent mechanism

Most cells in a developing organ stop proliferating when the organ reaches a correct, final size. The underlying molecular mechanisms are not understood. We find that in Drosophila the hormone ecdysone controls wing disc size. To study how ecdysone affects wing size, we inhibit endogenous ecdysone synthesis and feed larvae exogenous ecdysone in a dose-controlled manner. For any given ecdysone dose, discs stop proliferating at a particular size, with higher doses enabling discs to reach larger sizes. Termination of proliferation coincides with a drop in TORC1, but not Dpp or Yki signaling. Reactivating TORC1 bypasses the termination of proliferation, indicating that TORC1 is a main downstream effector causing proliferation termination at the maximal ecdysone-dependent size. Experimental manipulation of Dpp or Yki signaling can bypass proliferation termination in hinge and notum regions, but not the pouch, suggesting that the mechanisms regulating proliferation termination may be distinct in different disc regions.

Epigenetic Regulation of the Wnt/β-Catenin Signaling Pathway in Cancer

Studies over the past four decades have elucidated the role of Wnt/β-catenin mediated regulation in cell proliferation, differentiation and migration. These processes are fundamental to embryonic development, regeneration potential of tissues, as well as cancer initiation and progression. In this review, we focus on the epigenetic players which influence the Wnt/β-catenin pathway via modulation of its components and coordinated regulation of the Wnt target genes. The role played by crosstalk with other signaling pathways mediating tumorigenesis is also elaborated. The Hippo/YAP pathway is particularly emphasized due to its extensive crosstalk via the Wnt destruction complex. Further, we highlight the recent advances in developing potential therapeutic interventions targeting the epigenetic machinery based on the characterization of these regulatory networks for effective treatment of various cancers and also for regenerative therapies.

The Hippo pathway uses different machinery to control cell fate and organ size

The Hippo pathway is a conserved signaling network that regulates organ growth and cell fate. One such cell fate decision is that of R8 photoreceptor cells in the Drosophila eye, where Hippo specifies whether cells sense blue or green light. We show that only a subset of proteins that control organ growth via the Hippo pathway also regulate R8 cell fate choice, including the STRIPAK complex, Tao, Pez, and 14-3-3 proteins. Furthermore, key Hippo pathway proteins were primarily cytoplasmic in R8 cells rather than localized to specific membrane domains, as in cells of growing epithelial organs. Additionally, Warts was the only Hippo pathway protein to be differentially expressed between R8 subtypes, while central Hippo pathway proteins were expressed at dramatically lower levels in adult and pupal eyes than in growing larval eyes. Therefore, we reveal several important differences in Hippo signaling in the contexts of organ growth and cell fate.

The two sides of Hippo pathway in cancer

The Hippo signaling pathway was originally characterized by genetic studies in Drosophila to regulate tissue growth and organ size, and the core components of this pathway are highly conserved in mammals. Studies over the past two decades have revealed critical physiological and pathological functions of the Hippo tumor-suppressor pathway, which is tightly regulated by a broad range of intracellular and extracellular signals. These properties enable the Hippo pathway to serve as an important controller in organismal development and adult tissue homeostasis. Dysregulation of the Hippo signaling has been observed in many cancer types, suggesting the possibility of cancer treatment by targeting the Hippo pathway. The general consensus is that Hippo has tumor suppressor function. However, growing evidence also suggests that the function of the Hippo pathway in malignancy is cancer context dependent as recent studies indicating tumor promoting function of LATS. This article surveys the Hippo pathway signaling mechanisms and then reviews both the tumor suppressing and promoting function of this pathway. A comprehensive understanding of the dual roles of the Hippo pathway in cancer will benefit future therapeutic targeting of the Hippo pathway for cancer treatment.

Role of the Hippo pathway and mechanisms for controlling cellular localization of YAP/TAZ

The Hippo pathway is a crucial signaling mechanism that inhibits the growth of cells and organs during development and in disease. When the Hippo pathway is activated, YAP/TAZ transcriptional coactivators are phosphorylated by upstream kinases, preventing nuclear localization of YAP/TAZ. However, when the Hippo pathway is inhibited, YAP/TAZ localize mainly in the nucleus and induce the expression of target genes related to cell proliferation. Abnormal proliferation of cells is one of the hallmarks of cancer initiation, and activation of Hippo pathway dampens such cell proliferation. Various types of diseases including cancer can occur due to the dysregulation of the Hippo pathway. Therefore, a better understanding of the Hippo pathway signaling mechanisms, and in particular how YAP/TAZ exist in the nucleus, may lead to the identification of new therapeutic targets for treating cancer and other diseases. In this review, we summarize the overall Hippo pathway and discuss mechanisms related to nuclear localization of YAP/TAZ.

Hippo-YAP signaling in digestive system tumors

The Hippo pathway is an evolutionally conserved pathway and plays an important role in regulating tissue hemostasis and organ size control. Deregulation of the Hippo pathway is implicated in various human digestive system tumors. The past two decades have witnessed the discovery and elucidation of key signaling components and molecular mechanisms of the Hippo pathway. Among these, the signaling transducers YAP/TAZ are in the center of this complex network to sense and respond to extracellular cues such as cell contact, matrix stiffness and growth factors. In this review, we summarize the biological and clinical significance of Hippo-YAP signaling in the digestive system tumors, and explore the novel therapeutic strategies for targeting Hippo-YAP signaling.

Crumbs and the apical spectrin cytoskeleton regulate R8 cell fate in the Drosophila eye

The Hippo pathway is an important regulator of organ growth and cell fate. In the R8 photoreceptor cells of the Drosophila melanogaster eye, the Hippo pathway controls the fate choice between one of two subtypes that express either the blue light-sensitive Rhodopsin 5 (Hippo inactive R8 subtype) or the green light-sensitive Rhodopsin 6 (Hippo active R8 subtype). The degree to which the mechanism of Hippo signal transduction and the proteins that mediate it are conserved in organ growth and R8 cell fate choice is currently unclear. Here, we identify Crumbs and the apical spectrin cytoskeleton as regulators of R8 cell fate. By contrast, other proteins that influence Hippo-dependent organ growth, such as the basolateral spectrin cytoskeleton and Ajuba, are dispensable for the R8 cell fate choice. Surprisingly, Crumbs promotes the Rhodopsin 5 cell fate, which is driven by Yorkie, rather than the Rhodopsin 6 cell fate, which is driven by Warts and the Hippo pathway, which contrasts with its impact on Hippo activity in organ growth. Furthermore, neither the apical spectrin cytoskeleton nor Crumbs appear to regulate the Hippo pathway through mechanisms that have been observed in growing organs. Together, these results show that only a subset of Hippo pathway proteins regulate the R8 binary cell fate decision and that aspects of Hippo signalling differ between growing organs and post-mitotic R8 cells.