Hippo activation through homodimerization and membrane association for growth inhibition and organ size control

MST2 methylation by PRMT5 inhibits Hippo signaling and promotes pancreatic cancer progression

The Hippo signaling axis is a tumor suppressor pathway that is activated by various extra-pathway factors to regulate cell differentiation and organ development. Recent studies have reported that autophosphorylation of the core kinase cassette stimulates activation of the Hippo signaling cascade. Here, we demonstrate that protein arginine methyltransferase 5 (PRMT5) contributes to inactivation of the Hippo signaling pathway in pancreatic cancer. We show that the Hippo pathway initiator serine/threonine kinase 3 (STK3, also known as MST2) of Hippo signaling pathway can be symmetrically di-methylated by PRMT5 at arginine-461 (R461) and arginine-467 (R467) in its SARAH domain. Methylation suppresses MST2 autophosphorylation and kinase activity by blocking its homodimerization, thereby inactivating Hippo signaling pathway in pancreatic cancer. Moreover, we also show that the specific PRMT5 inhibitor GSK3326595 re-activates the dysregulated Hippo signaling pathway and inhibits the growth of human pancreatic cancer xenografts in immunodeficient mice, thus suggesting potential clinical application of PRMT5 inhibitors in pancreatic cancer.

Dimerization and autophosphorylation of the MST family of kinases are controlled by the same set of residues

The Hippo pathway controls tissue growth and regulates stem cell fate through the activities of core kinase cassette that begins with the Sterile 20-like kinase MST1/2. Activation of MST1/2 relies on trans-autophosphorylation but the details of the mechanisms regulating that reaction are not fully elucidated. Proposals include dimerization as a first step and include multiple models for potential kinase-domain dimers. Efforts to verify and link these dimers to trans-autophosphorylation were unsuccessful. We explored the link between dimerization and trans-autophosphorylation for MST2 and the entire family of MST kinases. We analyzed crystal lattice contacts of structures of MST kinases and identified an ensemble of kinase-domain dimers compatible with trans-autophosphorylation. These dimers share a common dimerization interface comprised of the activation loop and αG-helix while the arrangements of the kinase-domains within the dimer varied depending on their activation state. We then verified the dimerization interface and determined its function using MST2. Variants bearing alanine substitutions of the αG-helix prevented dimerization of the MST2 kinase domain both in solution and in cells. These substitutions also blocked autophosphorylation of full-length MST2 and its Drosophila homolog Hippo in cells. These variants retain the same secondary structure as wild-type and capacity to phosphorylate a protein substrate, indicating the loss of MST2 activation can be directly attributed to a loss of dimerization rather than loss of either fold or catalytic function. Together this data functionally links dimerization and autophosphorylation for MST2 and suggests this activation mechanism is conserved across both species and the entire MST family.

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.

Dimerization and autophosphorylation of the MST family of kinases are controlled by the same set of residues

The Hippo pathway controls tissue growth and regulates stem cell fate through the activities of core kinase cassette that begins with the Sterile 20-like kinase MST1/2. Activation of MST1/2 relies on trans -autophosphorylation but the details of the mechanisms regulating that reaction are not fully elucidated. Proposals include dimerization as a first step and include multiple models for potential kinase-domain dimers. Efforts to verify and link these dimers to trans -autophosphorylation were unsuccessful. We explored the link between dimerization and trans -autophosphorylation for MST2 and the entire family of MST kinases. We analyzed crystal lattice contacts of structures of MST kinases and identified an ensemble of kinase-domain dimers compatible with trans -autophosphorylation. These dimers share a common dimerization interface comprised of the activation loop and αG-helix while the arrangements of the kinase-domains within the dimer varied depending on their activation state. We then verified the dimerization interface and determined its function using MST2. Variants bearing alanine substitutions of the αG-helix prevented dimerization of the MST2 kinase domain both in solution and in cells. These substitutions also blocked autophosphorylation of full-length MST2 and its Drosophila homolog Hippo in cells. These variants retain the same secondary structure as wild-type and capacity to phosphorylate a protein substrate, indicating the loss of MST2 activation can be directly attributed to a loss of dimerization rather than loss of either fold or catalytic function. Together this data functionally links dimerization and autophosphorylation for MST2 and suggests this activation mechanism is conserved across both species and the entire MST family.

Kinetic Regulation of the Mammalian Sterile 20-like Kinase 2 (MST2)

Canonically, MST1/2 functions as a core kinase of the Hippo pathway and noncanonically during both apoptotic signaling and with RASSFs in T-cells. Faithful signal transduction by MST1/2 relies on both appropriate activation and regulated substrate phosphorylation by the activated kinase. Considerable progress has been made in understanding the molecular mechanisms regulating the activation of MST1/2 and identifying downstream signaling events. Here, we investigated the ability of MST2 to phosphorylate a peptide substrate and how that activity is regulated. Using a steady-state kinetic system, we parse the contribution of different factors to substrate phosphorylation, including the domains of MST2, phosphorylation, caspase cleavage, and complex formation. We found that in the unphosphorylated state, the SARAH domain stabilizes interactions with a peptide substrate and promotes turnover. Phosphorylation drives the activity of MST2, and once activated, MST2 is not further regulated by complex formation with other Hippo pathway components (SAV1, MOB1A, and RASSF5). We also show that the phosphorylated, caspase-cleaved MST2 is as active as the full-length one, suggesting that caspase-stimulated activity arises through noncatalytic mechanisms. The kinetic analysis presented here establishes a framework for interpreting how signaling events and post-translational modifications contribute to the signaling of MST2 in vivo.

Increasing kinase domain proximity promotes MST2 autophosphorylation during Hippo signaling

The Hippo pathway plays an important role in developmental biology, mediating organ size by controlling cell proliferation through the activity of a core kinase cassette. Multiple upstream events activate the pathway, but how each controls this core kinase cassette is not fully understood. Activation of the core kinase cassette begins with phosphorylation of the kinase MST1/2 (also known as STK3/4). Here, using a combination of in vitro biochemistry and cell-based assays, including chemically induced dimerization and single-molecule pulldown, we revealed that increasing the proximity of adjacent kinase domains, rather than formation of a specific protein assembly, is sufficient to trigger autophosphorylation. We validate this mechanism in cells and demonstrate that multiple events associated with the active pathway, including SARAH domain-mediated homodimerization, membrane recruitment, and complex formation with the effector protein SAV1, each increase the kinase domain proximity and autophosphorylation of MST2. Together, our results reveal that multiple and distinct upstream signals each utilize the same common molecular mechanism to stimulate MST2 autophosphorylation. This mechanism is likely conserved among MST2 homologs. Our work also highlights potential differences in Hippo signal propagation between each activating event owing to differences in the dynamics and regulation of each protein ensemble that triggers MST2 autophosphorylation and possible redundancy in activation.

Biophysical characterization of SARAH domain-mediated multimerization of Hippo pathway complexes in Drosophila

Hippo pathway signaling limits cell growth and proliferation and maintains the stem-cell niche. These cellular events result from the coordinated activity of a core kinase cassette that is regulated, in part, by interactions involving Hippo, Salvador, and dRassF. These interactions are mediated by a conserved coiled-coil domain, termed SARAH, in each of these proteins. SARAH domain-mediated homodimerization of Hippo kinase leads to autophosphorylation and activation. Paradoxically, SARAH domain-mediated heterodimerization between Hippo and Salvador enhances Hippo kinase activity in cells, whereas complex formation with dRassF inhibits it. To better understand the mechanism by which each complex distinctly modulates Hippo kinase and pathway activity, here we biophysically characterized the entire suite of SARAH domain-mediated complexes. We purified the three SARAH domains from Drosophila melanogaster and performed an unbiased pulldown assay to identify all possible interactions, revealing that isolated SARAH domains are sufficient to recapitulate the cellular assemblies and that Hippo is a universal binding partner. Additionally, we found that the Salvador SARAH domain homodimerizes and demonstrate that this interaction is conserved in Salvador's mammalian homolog. Using native MS, we show that each of these complexes is dimeric in solution. We also measured the stability of each SARAH domain complex, finding that despite similarities at both the sequence and structural levels, SARAH domain complexes differ in stability. The identity, stoichiometry, and stability of these interactions characterized here comprehensively reveal the nature of SARAH domain-mediated complex formation and provide mechanistic insights into how SARAH domain-mediated interactions influence Hippo pathway activity.

Systematic analysis of the Hippo pathway organization and oncogenic alteration in evolution

The Hippo pathway is a central regulator of organ size and a key tumor suppressor via coordinating cell proliferation and death. Initially discovered in Drosophila, the Hippo pathway has been implicated as an evolutionarily conserved pathway in mammals; however, how this pathway was evolved to be functional from its origin is still largely unknown. In this study, we traced the Hippo pathway in premetazoan species, characterized the intrinsic functions of its ancestor components, and unveiled the evolutionary history of this key signaling pathway from its unicellular origin. In addition, we elucidated the paralogous gene history for the mammalian Hippo pathway components and characterized their cancer-derived somatic mutations from an evolutionary perspective. Taken together, our findings not only traced the conserved function of the Hippo pathway to its unicellular ancestor components, but also provided novel evolutionary insights into the Hippo pathway organization and oncogenic alteration.

Bimolecular Fluorescence Complementation (BiFC) in Tissue Culture and in Developing Tissues of Drosophila to Study Protein-Protein Interactions

Protein-protein interactions provide a common mechanism for regulating protein functions and also serve as the fundamental step of many biochemical reactions. To accurately determine the involvement and function of protein-protein interactions, it is crucial to detect the interactions with the minimum number of artifacts. In this chapter, we report the method of bimolecular fluorescence complementation (BiFC) in tissue culture and developing tissues of Drosophila, which allows the visualization of subcellular localization of protein-protein interactions in living cells.

The Cross-Talk Between the TNF-α and RASSF-Hippo Signalling Pathways

The regulation of cell death through apoptosis is essential to a number of physiological processes. Defective apoptosis regulation is associated with many abnormalities including anomalies in organ development, altered immune response and the development of cancer. Several signalling pathways are known to regulate apoptosis including the Tumour Necrosis Factor-α (TNF-α) and Hippo signalling pathways. In this paper we review the cross-talk between the TNF-α pathway and the Hippo signalling pathway. Several molecules that tightly regulate the Hippo pathway, such as members of the Ras-association domain family member (RASSF) family proteins, interact and modulate some key proteins within the TNF-α pathway. Meanwhile, TNF-α stimulation also affects the expression and activation of core components of the Hippo pathway. This implies the crucial role of signal integration between these two major pathways in regulating apoptosis.

The Septate Junction Protein Tsp2A Restricts Intestinal Stem Cell Activity via Endocytic Regulation of aPKC and Hippo Signaling

Hippo signaling and the activity of its transcriptional coactivator, Yorkie (Yki), are conserved and crucial regulators of tissue homeostasis. In the Drosophila midgut, after tissue damage, Yki activity increases to stimulate stem cell proliferation, but how Yki activity is turned off once the tissue is repaired is unknown. From an RNAi screen, we identified the septate junction (SJ) protein tetraspanin 2A (Tsp2A) as a tumor suppressor. Tsp2A undergoes internalization to facilitate the endocytic degradation of atypical protein kinase C (aPKC), a negative regulator of Hippo signaling. In the Drosophila midgut epithelium, adherens junctions (AJs) and SJs are prominent in intestinal stem cells or enteroblasts (ISCs or EBs) and enterocytes (ECs), respectively. We show that when ISCs differentiate toward ECs, Tsp2A is produced, participates in SJ assembly, and turns off aPKC and Yki-JAK-Stat activity. Altogether, our study uncovers a mechanism allowing the midgut to restore Hippo signaling and restrict proliferation once tissue repair is accomplished.

The Hippo Signaling Network and Its Biological Functions

Hippo signaling is an evolutionarily conserved network that has a central role in regulating cell proliferation and cell fate to control organ growth and regeneration. It promotes activation of the LATS kinases, which control gene expression by inhibiting the activity of the transcriptional coactivator proteins YAP and TAZ in mammals and Yorkie in Drosophila. Diverse upstream inputs, including both biochemical cues and biomechanical cues, regulate Hippo signaling and enable it to have a key role as a sensor of cells' physical environment and an integrator of growth control signals. Several components of this pathway localize to cell-cell junctions and contribute to regulation of Hippo signaling by cell polarity, cell contacts, and the cytoskeleton. Downregulation of Hippo signaling promotes uncontrolled cell proliferation, impairs differentiation, and is associated with cancer. We review the current understanding of Hippo signaling and highlight progress in the elucidation of its regulatory mechanisms and biological functions.

Homeostatic Control of Hpo/MST Kinase Activity through Autophosphorylation-Dependent Recruitment of the STRIPAK PP2A Phosphatase Complex

The Hippo pathway controls organ size and tissue homeostasis through a kinase cascade leading from the Ste20-like kinase Hpo (MST1/2 in mammals) to the transcriptional coactivator Yki (YAP/TAZ in mammals). Whereas previous studies have uncovered positive and negative regulators of Hpo/MST, how they are integrated to maintain signaling homeostasis remains poorly understood. Here, we identify a self-restricting mechanism whereby autophosphorylation of an unstructured linker in Hpo/MST creates docking sites for the STRIPAK PP2A phosphatase complex to inactivate Hpo/MST. Mutation of the phospho-dependent docking sites in Hpo/MST or deletion of Slmap, the STRIPAK subunit recognizing these docking sites, results in constitutive activation of Hpo/MST in both Drosophila and mammalian cells. In contrast, autophosphorylation of the Hpo/MST linker at distinct sites is known to recruit Mats/MOB1 to facilitate Hippo signaling. Thus, multisite autophosphorylation of Hpo/MST linker provides an evolutionarily conserved built-in molecular platform to maintain signaling homeostasis by coupling antagonistic signaling activities.

Mechanical strain regulates the Hippo pathway in Drosophila

Animal cells are thought to sense mechanical forces via the transcriptional co-activators YAP (or YAP1) and TAZ (or WWTR1), the sole Drosophila homolog of which is named Yorkie (Yki). In mammalian cells in culture, artificial mechanical forces induce nuclear translocation of YAP and TAZ. Here, we show that physiological mechanical strain can also drive nuclear localisation of Yki and activation of Yki target genes in the Drosophila follicular epithelium. Mechanical strain activates Yki by stretching the apical domain, reducing the concentration of apical Crumbs, Expanded, Kibra and Merlin, and reducing apical Hippo kinase dimerisation. Overexpressing Hippo kinase to induce ectopic activation in the cytoplasm is sufficient to prevent Yki nuclear localisation even in flattened follicle cells. Conversely, blocking Hippo signalling in warts clones causes Yki nuclear localisation even in columnar follicle cells. We find no evidence for involvement of other pathways, such as Src42A kinase, in regulation of Yki. Finally, our results in follicle cells appear generally applicable to other tissues, as nuclear translocation of Yki is also readily detectable in other flattened epithelial cells such as the peripodial epithelium of the wing imaginal disc, where it promotes cell flattening.

Drosophila Schip1 Links Expanded and Tao-1 to Regulate Hippo Signaling

Regulation of organ size is essential in animal development, and Hippo (Hpo) signaling is a major conserved mechanism for controlling organ growth. In Drosophila, Hpo and Warts kinases are core components of this pathway and function as tumor suppressors by inhibiting Yorkie (Yki). Expanded (Ex) is a regulator of the Hpo activity, but how they are linked is unknown. Here, we show that Schip1, a Drosophila homolog of the mammalian Schwannomin interacting protein 1 (SCHIP1), provides a link between Ex and Hpo. Ex is required for apical localization of Schip1 in imaginal discs. Schip1 is necessary for promoting membrane localization and phosphorylation of Hpo by recruiting the Hpo kinase Tao-1. Taking these findings together, we conclude that Schip1 directly links Ex to Hpo signaling by recruiting Tao-1. This study provides insights into the mechanism of Tao-1 regulation and a potential growth control function for SCHIP1 in mammals.

Cellular Organization and Cytoskeletal Regulation of the Hippo Signaling Network

The Hippo signaling network integrates diverse upstream signals to control cell fate decisions and regulate organ growth. Recent studies have provided new insights into the cellular organization of Hippo signaling, its relationship to cell-cell junctions, and how the cytoskeleton modulates Hippo signaling. Cell-cell junctions serve as platforms for Hippo signaling by localizing scaffolding proteins that interact with core components of the pathway. Interactions of Hippo pathway components with cell-cell junctions and the cytoskeleton also suggest potential mechanisms for the regulation of the pathway by cell contact and cell polarity. As our understanding of the complexity of Hippo signaling increases, a future challenge will be to understand how the diverse inputs into the pathway are integrated and to define their respective contributions in vivo.

Localization of Hippo signalling complexes and Warts activation in vivo

Hippo signalling controls organ growth and cell fate by regulating the activity of the kinase Warts. Multiple Hippo pathway components localize to apical junctions in epithelial cells, but the spatial and functional relationships among components have not been clarified, nor is it known where Warts activation occurs. We report here that Hippo pathway components in Drosophila wing imaginal discs are organized into distinct junctional complexes, including separate distributions for Salvador, Expanded, Warts and Hippo. These complexes are reorganized on Hippo pathway activation, when Warts shifts from associating with its inhibitor Jub to its activator Expanded, and Hippo concentrates at Salvador sites. We identify mechanisms promoting Warts relocalization, and using a phospho-specific antisera and genetic manipulations, identify where Warts activation occurs: at apical junctions where Expanded, Salvador, Hippo and Warts overlap. Our observations define spatial relationships among Hippo signalling components and establish the functional importance of their localization to Warts activation.

Components of the Hippo signalling pathway localize to apical junctions in epithelial cells, where they regulate growth in response to mechanical and biochemical cues. Sun et al. show that these proteins are organized into distinct junctional complexes, which reorganize up on Hippo pathway activation.

Hippo Stabilises Its Adaptor Salvador by Antagonising the HECT Ubiquitin Ligase Herc4

Signalling through the Hippo (Hpo) pathway involves a kinase cascade, which leads to the phosphorylation and inactivation of the pro-growth transcriptional co-activator Yorkie (Yki). Despite the identification of a large number of pathway members and modulators, our understanding of the molecular events that lead to activation of Hpo and the downstream kinase Warts (Wts) remain incomplete. Recently, targeted degradation of several Hpo pathway components has been demonstrated as a means of regulating pathway activity. In particular, the stability of scaffold protein Salvador (Sav), which is believed to promote Hpo/Wts association, is crucially dependent on its binding partner Hpo. In a cell-based RNAi screen for ubiquitin regulators involved in Sav stability, we identify the HECT domain protein Herc4 (HECT and RLD domain containing E3 ligase) as a Sav E3 ligase. Herc4 expression promotes Sav ubiquitylation and degradation, while Herc4 depletion stabilises Sav. Interestingly, Hpo reduces Sav/Herc4 interaction in a kinase-dependent manner. This suggests the existence of a positive feedback loop, where Hpo stabilises its own positive regulator by antagonising Herc4-mediated degradation of Sav.

The GTPase regulatory proteins Pix and Git control tissue growth via the Hippo pathway

The Salvador-Warts-Hippo (Hippo) pathway is a conserved regulator of organ size and is deregulated in human cancers. In epithelial tissues, the Hippo pathway is regulated by fundamental cell biological properties, such as polarity and adhesion, and coordinates these with tissue growth. Despite its importance in disease, development, and regeneration, the complete set of proteins that regulate Hippo signaling remain undefined. To address this, we used proteomics to identify proteins that bind to the Hippo (Hpo) kinase. Prominent among these were PAK-interacting exchange factor (known as Pix or RtGEF) and G-protein-coupled receptor kinase-interacting protein (Git). Pix is a conserved Rho-type guanine nucleotide exchange factor (Rho-GEF) homologous to Beta-PIX and Alpha-PIX in mammals. Git is the single Drosophila melanogaster homolog of the mammalian GIT1 and GIT2 proteins, which were originally identified in the search for molecules that interact with G-protein-coupled receptor kinases. Pix and Git form an oligomeric scaffold to facilitate sterile 20-like kinase activation and have also been linked to GTPase regulation. We show that Pix and Git regulate Hippo-pathway-dependent tissue growth in D. melanogaster and that they do this in parallel to the known upstream regulator Fat cadherin. Pix and Git influence activity of the Hpo kinase by acting as a scaffold complex, rather than enzymes, and promote Hpo dimerization and autophosphorylation of Hpo's activation loop. Therefore, we provide important new insights into an ancient signaling network that controls the growth of metazoan tissues.

Mutual regulation between Hippo signaling and actin cytoskeleton

Hippo signaling plays a crucial role in growth control and tumor suppression by regulating cell proliferation, apoptosis, and differentiation. How Hippo signaling is regulated has been under extensive investigation. Over the past three years, an increasing amount of data have supported a model of actin cytoskeleton blocking Hippo signaling activity to allow nuclear accumulation of a downstream effector, Yki/Yap/Taz. On the other hand, Hippo signaling negatively regulates actin cytoskeleton organization. This review provides insight on the mutual regulatory mechanisms between Hippo signaling and actin cytoskeleton for a tight control of cell behaviors during animal development, and points out outstanding questions for further investigations.

The Hippo pathway polarizes the actin cytoskeleton during collective migration of Drosophila border cells

Collective migration of Drosophila border cells depends on a dynamic actin cytoskeleton that is highly polarized such that it concentrates around the outer rim of the migrating cluster of cells. How the actin cytoskeleton becomes polarized in these cells to enable collective movement remains unknown. Here we show that the Hippo signaling pathway links determinants of cell polarity to polarization of the actin cytoskeleton in border cells. Upstream Hippo pathway components localize to contacts between border cells inside the cluster and signal through the Hippo and Warts kinases to polarize actin and promote border cell migration. Phosphorylation of the transcriptional coactivator Yorkie (Yki)/YAP by Warts does not mediate the function of this pathway in promoting border cell migration, but rather provides negative feedback to limit the speed of migration. Instead, Warts phosphorylates and inhibits the actin regulator Ena to activate F-actin Capping protein activity on inner membranes and thereby restricts F-actin polymerization mainly to the outer rim of the migrating cluster.