Cytoskeletal tension inhibits Hippo signaling through an Ajuba-Warts complex

PI4P-mediated solid-like Merlin condensates orchestrate Hippo pathway regulation

Despite recent studies implicating liquid-like biomolecular condensates in diverse cellular processes, many biomolecular condensates exist in a solid-like state, and their function and regulation are less understood. We show that the tumor suppressor Merlin, an upstream regulator of the Hippo pathway, localizes to both cell junctions and medial apical cortex in Drosophila epithelia, with the latter forming solid-like condensates that activate Hippo signaling. Merlin condensation required phosphatidylinositol-4-phosphate (PI4P)-mediated plasma membrane targeting and was antagonistically controlled by Pez and cytoskeletal tension through plasma membrane PI4P regulation. The solid-like material properties of Merlin condensates are essential for physiological function and protect the condensates against external perturbations. Collectively, these findings uncover an essential role for solid-like condensates in normal physiology and reveal regulatory mechanisms for their formation and disassembly.

Biologically Relevant Laminin-511 Moderates the Derivation and Proliferation of Human Lens Epithelial Stem/Progenitor-Like Cells

Purpose

The role of specific extracellular matrix (ECM) molecules in lens cell development and regeneration is poorly understood, as appropriate cellular models are lacking. Here, a laminin-based lens cell in vitro induction system was developed to study the role of laminin in human lens epithelial stem/progenitor cell (LES/PC) development.

Methods

The human embryonic stem cell-based lens induction system followed a three-stage protocol. The expression profile of laminins during lens induction was screened, and laminin-511 (LN511) was tested as a candidate substitute. LN511 induction system cellular and molecular features, including induction efficiency, transcription factor expression related to different lens development stages, ECM alterations, and Hippo/YAP signaling, were evaluated.

Results

LAMA5, LAMB1, and LAMC1 were highly expressed around the time of LES/PC derivation. We chose LN511 (product of LAMA5, LAMB1, and LAMC1) and found that it considerably enhanced lens cell induction efficiency, compared to that in Matrigel-coated culture, as more and larger lentoid bodies were detected. Notably, LES/PC induction efficiency improved by promoting lens specification-related transcription factor expression and cell proliferation. Transcriptome analysis revealed that compared to those with Matrigel, ECM accumulation and cell adhesion were downregulated in the LN511 system. Hippo/YAP signaling was hypoactive during LES/P-like cell generation, and small molecule inhibitors of YAP/TAZ activity upregulated LES/PC marker expression and promoted the efficiency of LES/P-like cell derivation.

Conclusions

The laminin isoform LN511 is a reliable substitute for the LES/P-like cell induction system, and LN511-YAP acted as efficient modulators of LES/PC derivation; this contributes to knowledge of the role of the ECM in human lens development.

Assembling a Hippo: the evolutionary emergence of an animal developmental signaling pathway

Decades of work in developmental genetics has given us a deep mechanistic understanding of the fundamental signaling pathways underlying animal development. However, little is known about how these pathways emerged and changed over evolutionary time. Here, we review our current understanding of the evolutionary emergence of the Hippo pathway, a conserved signaling pathway that regulates tissue size in animals. This pathway has deep evolutionary roots, emerging piece by piece in the unicellular ancestors of animals, with a complete core pathway predating the origin of animals. Recent functional studies in close unicellular relatives of animals and early-branching animals suggest an ancestral function of the Hippo pathway in cytoskeletal regulation, which was subsequently co-opted to regulate proliferation and animal tissue size.

Scaling between cell cycle duration and wing growth is regulated by Fat-Dachsous signaling in Drosophila

The atypical cadherins Fat and Dachsous (Ds) signal through the Hippo pathway to regulate growth of numerous organs, including the Drosophila wing. Here, we find that Ds-Fat signaling tunes a unique feature of cell proliferation found to control the rate of wing growth during the third instar larval phase. The duration of the cell cycle increases in direct proportion to the size of the wing, leading to linear-like growth during the third instar. Ds-Fat signaling enhances the rate at which the cell cycle lengthens with wing size, thus diminishing the rate of wing growth. We show that this results in a complex but stereotyped relative scaling of wing growth with body growth in Drosophila. Finally, we examine the dynamics of Fat and Ds protein distribution in the wing, observing graded distributions that change during growth. However, the significance of these dynamics is unclear since perturbations in expression have negligible impact on wing growth.

LATS2 condensates organize signalosomes for Hippo pathway signal transduction

Biomolecular condensates have been proposed to mediate cellular signaling transduction. However, the mechanism and functional consequences of signal condensates are not well understood. Here we report that LATS2, the core kinase of the Hippo pathway, responds to F-actin cytoskeleton reduction and forms condensates. The proline-rich motif (PRM) of LATS2 mediates its condensation. LATS2 partitions with the main components of the Hippo pathway to assemble a signalosome for LATS2 activation and for its stability by physically compartmentalizing from E3 ligase FBXL16 complex-dependent degradation, which in turn mediates yes-associated protein (YAP)-transcriptional coactivator with PDZ-binding motif (TAZ) recruitment and inactivation. This oncogenic FBXL16 complex blocks LATS2 condensation by binding to the PRM region to promote its degradation. Disruption of LATS2 condensation leads to tumor progression. Thus, our study uncovers that the signalosomes assembled by LATS2 condensation provide a compartmentalized and reversible platform for Hippo signaling transduction and protein stability, which have potential implications in cancer diagnosis and therapeutics.

Mitf, with Yki and STRIPAK-PP2A, is a key determinant of form and fate in the progenitor epithelium of the Drosophila eye

The Microphthalmia-associated Transcription Factor (MITF) governs numerous cellular and developmental processes. In mice, it promotes specification and differentiation of the retinal pigmented epithelium (RPE), and in humans, some mutations in MITF induce congenital eye malformations. Herein, we explore the function and regulation of Mitf in Drosophila eye development and uncover two roles. We find that knockdown of Mitf results in retinal displacement (RDis), a phenotype associated with abnormal eye formation. Mitf functions in the peripodial epithelium (PE), a retinal support tissue akin to the RPE, to suppress RDis, via the Hippo pathway effector Yorkie (Yki). Yki physically interacts with Mitf and can modify its transcriptional activity in vitro. Severe loss of Mitf, instead, results in the de-repression of retinogenesis in the PE, precluding its development. This activity of Mitf requires the protein phosphatase 2 A holoenzyme STRIPAK-PP2A, but not Yki; Mitf transcriptional activity is potentiated by STRIPAK-PP2A in vitro and in vivo. Knockdown of STRIPAK-PP2A results in cytoplasmic retention of Mitf in vivo and in its decreased stability in vitro, highlighting two potential mechanisms for the control of Mitf function by STRIPAK-PP2A. Thus, Mitf functions in a context-dependent manner as a key determinant of form and fate in the Drosophila eye progenitor epithelium.

A mechanical transition from tension to buckling underlies the jigsaw puzzle shape morphogenesis of histoblasts in the Drosophila epidermis

The polygonal shape of cells in proliferating epithelia is a result of the tensile forces of the cytoskeletal cortex and packing geometry set by the cell cycle. In the larval Drosophila epidermis, two cell populations, histoblasts and larval epithelial cells, compete for space as they grow on a limited body surface. They do so in the absence of cell divisions. We report a striking morphological transition of histoblasts during larval development, where they change from a tensed network configuration with straight cell outlines at the level of adherens junctions to a highly folded morphology. The apical surface of histoblasts shrinks while their growing adherens junctions fold, forming deep lobules. Volume increase of growing histoblasts is accommodated basally, compensating for the shrinking apical area. The folded geometry of apical junctions resembles elastic buckling, and we show that the imbalance between the shrinkage of the apical domain of histoblasts and the continuous growth of junctions triggers buckling. Our model is supported by laser dissections and optical tweezer experiments together with computer simulations. Our analysis pinpoints the ability of histoblasts to store mechanical energy to a much greater extent than most other epithelial cell types investigated so far, while retaining the ability to dissipate stress on the hours time scale. Finally, we propose a possible mechanism for size regulation of histoblast apical size through the lateral pressure of the epidermis, driven by the growth of cells on a limited surface. Buckling effectively compacts histoblasts at their apical plane and may serve to avoid physical harm to these adult epidermis precursors during larval life. Our work indicates that in growing nondividing cells, compressive forces, instead of tension, may drive cell morphology.

Interplay of RAP2 GTPase and the cytoskeleton in Hippo pathway regulation

The Hippo signaling is instrumental in regulating organ size, regeneration, and carcinogenesis. The cytoskeleton emerges as a primary Hippo signaling modulator. Its structural alterations in response to environmental and intrinsic stimuli control Hippo signaling pathway activity. However, the precise mechanisms underlying the cytoskeleton regulation of Hippo signaling are not fully understood. RAP2 GTPase is known to mediate the mechanoresponses of Hippo signaling via activating the core Hippo kinases LATS1/2 through MAP4Ks and MST1/2. Here we show the pivotal role of the reciprocal regulation between RAP2 GTPase and the cytoskeleton in Hippo signaling. RAP2 deletion undermines the responses of the Hippo pathway to external cues tied to RhoA GTPase inhibition and actin cytoskeleton remodeling, such as energy stress and serum deprivation. Notably, RhoA inhibitors and actin disruptors fail to activate LATS1/2 effectively in RAP2-deficient cells. RNA sequencing highlighted differential regulation of both actin and microtubule networks by RAP2 gene deletion. Consistently, Taxol, a microtubule-stabilizing agent, was less effective in activating LATS1/2 and inhibiting cell growth in RAP2 and MAP4K4/6/7 knockout cells. In summary, our findings position RAP2 as a central integrator of cytoskeletal signals for Hippo signaling, which offers new avenues for understanding Hippo regulation and therapeutic interventions in Hippo-impaired cancers.

Scaling between cell cycle duration and wing growth is regulated by Fat-Dachsous signaling in Drosophila

The atypical cadherins Fat and Dachsous (Ds) signal through the Hippo pathway to regulate growth of numerous organs, including the Drosophila wing. Here, we find that Ds-Fat signaling tunes a unique feature of cell proliferation found to control the rate of wing growth during the third instar larval phase. The duration of the cell cycle increases in direct proportion to the size of the wing, leading to linear-like growth during the third instar. Ds-Fat signaling enhances the rate at which the cell cycle lengthens with wing size, thus diminishing the rate of wing growth. We show that this results in a complex but stereotyped relative scaling of wing growth with body growth in Drosophila. Finally, we examine the dynamics of Fat and Ds protein distribution in the wing, observing graded distributions that change during growth. However, the significance of these dynamics is unclear since perturbations in expression have negligible impact on wing growth.

Adherens junctions as molecular regulators of emergent tissue mechanics

Tissue and organ development during embryogenesis relies on the collective and coordinated action of many cells. Recent studies have revealed that tissue material properties, including transitions between fluid and solid tissue states, are controlled in space and time to shape embryonic structures and regulate cell behaviours. Although the collective cellular flows that sculpt tissues are guided by tissue-level physical changes, these ultimately emerge from cellular-level and subcellular-level molecular mechanisms. Adherens junctions are key subcellular structures, built from clusters of classical cadherin receptors. They mediate physical interactions between cells and connect biochemical signalling to the physical characteristics of cell contacts, hence playing a fundamental role in tissue morphogenesis. In this Review, we take advantage of the results of recent, quantitative measurements of tissue mechanics to relate the molecular and cellular characteristics of adherens junctions, including adhesion strength, tension and dynamics, to the emergent physical state of embryonic tissues. We focus on systems in which cell-cell interactions are the primary contributor to morphogenesis, without significant contribution from cell-matrix interactions. We suggest that emergent tissue mechanics is an important direction for future research, bridging cell biology, developmental biology and mechanobiology to provide a holistic understanding of morphogenesis in health and disease.

Identification of a core transcriptional program driving the human renal mesenchymal-to-epithelial transition

During kidney development, nephron epithelia arise de novo from fate-committed mesenchymal progenitors through a mesenchymal-to-epithelial transition (MET). Downstream of fate specification, transcriptional mechanisms that drive establishment of epithelial morphology are poorly understood. We used human iPSC-derived renal organoids, which recapitulate nephrogenesis, to investigate mechanisms controlling renal MET. Multi-ome profiling via snRNA-seq and ATAC-seq of organoids identified dynamic changes in gene expression and chromatin accessibility driven by activators and repressors throughout MET. CRISPR interference identified that paired box 8 (PAX8) is essential for initiation of MET in human renal organoids, contrary to in vivo mouse studies, likely by activating a cell-adhesion program. While Wnt/β-catenin signaling specifies nephron fate, we find that it must be attenuated to allow hepatocyte nuclear factor 1-beta (HNF1B) and TEA-domain (TEAD) transcription factors to drive completion of MET. These results identify the interplay between fate commitment and morphogenesis in the developing human kidney, with implications for understanding both developmental kidney diseases and aberrant epithelial plasticity following adult renal tubular injury.

Hippo-YAP/TAZ signaling in breast cancer: Reciprocal regulation of microRNAs and implications in precision medicine

Breast cancer is a molecularly heterogeneous disease and the most common female malignancy. In recent years, therapy approaches have evolved to accommodate molecular diversity, with a focus on more biologically based therapies to minimize negative consequences. To regulate cell fate in human breast cells, the Hippo signaling pathway has been associated with the alpha subtype of estrogen receptors. This pathway regulates tissue size, regeneration, and healing, as well as the survival of tissue-specific stem cells, proliferation, and apoptosis in a variety of organs, allowing for cell differentiation. Hippo signaling is mediated by the kinases MST1, MST2, LATS1, and LATS2, as well as the adaptor proteins SAV1 and MOB. These kinases phosphorylate the downstream effectors of the Hippo pathway, yes-associated protein (YAP), and transcriptional coactivator with PDZ-binding motif (TAZ), suppressing the expression of their downstream target genes. The Hippo signaling pathway kinase cascade plays a significant role in all cancers. Understanding the principles of this kinase cascade would prevent the occurrence of breast cancer. In recent years, small noncoding RNAs, or microRNAs, have been implicated in the development of several malignancies, including breast cancer. The interconnections between miRNAs and Hippo signaling pathway core proteins in the breast, on the other hand, remain poorly understood. In this review, we focused on highlighting the Hippo signaling system, its key parts, its importance in breast cancer, and its regulation by miRNAs and other related pathways.

The ability of the LIMD1 and TRIP6 LIM domains to bind strained f-actin is critical for their tension dependent localization to adherens junctions and association with the Hippo pathway kinase LATS1

A key step in regulation of Hippo pathway signaling in response to mechanical tension is recruitment of the LIM domain proteins TRIP6 and LIMD1 to adherens junctions. Mechanical tension also triggers TRIP6 and LIMD1 to bind and inhibit the Hippo pathway kinase LATS1. How TRIP6 and LIMD1 are recruited to adherens junctions in response to tension is not clear, but previous studies suggested that they could be regulated by the known mechanosensory proteins α-catenin and vinculin at adherens junctions. We found that the three LIM domains of TRIP6 and LIMD1 are necessary and sufficient for tension-dependent localization to adherens junctions. The LIM domains of TRIP6, LIMD1, and certain other LIM domain proteins have been shown to bind to actin networks under strain/tension. Consistent with this, we show that TRIP6 and LIMD1 colocalize with the ends of actin fibers at adherens junctions. Point mutations in a key conserved residue in each LIM domain that are predicted to impair binding to f-actin under strain inhibits TRIP6 and LIMD1 localization to adherens junctions and their ability to bind to and recruit LATS1 to adherens junctions. Together these results show that the ability of TRIP6 and LIMD1 to bind to strained actin underlies their ability to localize to adherens junctions and regulate LATS1 in response to mechanical tension.

ESCRT-III-dependent adhesive and mechanical changes are triggered by a mechanism detecting alteration of septate junction integrity in Drosophila epithelial cells

Barrier functions of proliferative epithelia are constantly challenged by mechanical and chemical constraints. How epithelia respond to and cope with disturbances of barrier functions to allow tissue integrity maintenance is poorly characterised. Cellular junctions play an important role in this process and intracellular traffic contribute to their homeostasis. Here, we reveal that, in Drosophila pupal notum, alteration of the bi- or tricellular septate junctions (SJs) triggers a mechanism with two prominent outcomes. On one hand, there is an increase in the levels of E-cadherin, F-actin, and non-muscle myosin II in the plane of adherens junctions. On the other hand, β-integrin/Vinculin-positive cell contacts are reinforced along the lateral and basal membranes. We found that the weakening of SJ integrity, caused by the depletion of bi- or tricellular SJ components, alters ESCRT-III/Vps32/Shrub distribution, reduces degradation and instead favours recycling of SJ components, an effect that extends to other recycled transmembrane protein cargoes including Crumbs, its effector β-Heavy Spectrin Karst, and β-integrin. We propose a mechanism by which epithelial cells, upon sensing alterations of the SJ, reroute the function of Shrub to adjust the balance of degradation/recycling of junctional cargoes and thereby compensate for barrier junction defects to maintain epithelial integrity.

To squeeze or not: Regulation of cell size by mechanical forces in development and human diseases

Physical constraints, such as compression, shear stress, stretching and tension play major roles during development and tissue homeostasis. Mechanics directly impact physiology, and their alteration is also recognized as having an active role in driving human diseases. Recently, growing evidence has accumulated on how mechanical forces are translated into a wide panel of biological responses, including metabolism and changes in cell morphology. The aim of this review is to summarize and discuss our knowledge on the impact of mechanical forces on cell size regulation. Other biological consequences of mechanical forces will not be covered by this review. Moreover, wherever possible, we also discuss mechanosensors and molecular and cellular signaling pathways upstream of cell size regulation. We finally highlight the relevance of mechanical forces acting on cell size in physiology and human diseases.

Basal spot junctions of Drosophila epithelial tissues respond to morphogenetic forces and regulate Hippo signaling

Organ size is controlled by numerous factors including mechanical forces, which are mediated in part by the Hippo pathway. In growing Drosophila epithelial tissues, cytoskeletal tension influences Hippo signaling by modulating the localization of key pathway proteins to different apical domains. Here, we discovered a Hippo signaling hub at basal spot junctions, which form at the basal-most point of the lateral membranes and resemble adherens junctions in protein composition. Basal spot junctions recruit the central kinase Warts via Ajuba and E-cadherin, which prevent Warts activation by segregating it from upstream Hippo pathway proteins. Basal spot junctions are prominent when tissues undergo morphogenesis and are highly sensitive to fluctuations in cytoskeletal tension. They are distinct from focal adhesions, but the latter profoundly influences basal spot junction abundance by modulating the basal-medial actomyosin network and tension experienced by spot junctions. Thus, basal spot junctions couple morphogenetic forces to Hippo pathway activity and organ growth.

Extracellular Matrix Cues Regulate Mechanosensing and Mechanotransduction of Cancer Cells

Extracellular biophysical properties have particular implications for a wide spectrum of cellular behaviors and functions, including growth, motility, differentiation, apoptosis, gene expression, cell-matrix and cell-cell adhesion, and signal transduction including mechanotransduction. Cells not only react to unambiguously mechanical cues from the extracellular matrix (ECM), but can occasionally manipulate the mechanical features of the matrix in parallel with biological characteristics, thus interfering with downstream matrix-based cues in both physiological and pathological processes. Bidirectional interactions between cells and (bio)materials in vitro can alter cell phenotype and mechanotransduction, as well as ECM structure, intentionally or unintentionally. Interactions between cell and matrix mechanics in vivo are of particular importance in a variety of diseases, including primarily cancer. Stiffness values between normal and cancerous tissue can range between 500 Pa (soft) and 48 kPa (stiff), respectively. Even the shear flow can increase from 0.1-1 dyn/cm2 (normal tissue) to 1-10 dyn/cm2 (cancerous tissue). There are currently many new areas of activity in tumor research on various biological length scales, which are highlighted in this review. Moreover, the complexity of interactions between ECM and cancer cells is reduced to common features of different tumors and the characteristics are highlighted to identify the main pathways of interaction. This all contributes to the standardization of mechanotransduction models and approaches, which, ultimately, increases the understanding of the complex interaction. Finally, both the in vitro and in vivo effects of this mechanics-biology pairing have key insights and implications for clinical practice in tumor treatment and, consequently, clinical translation.

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.

Mitochondrial phosphagen kinases support the volatile power demands of motor nerve terminals

Motor neurons are the longest neurons in the body, with axon terminals separated from the soma by as much as a meter. These terminals are largely autonomous with regard to their bioenergetic metabolism and must burn energy at a high rate to sustain muscle contraction. Here, through computer simulation and drawing on previously published empirical data, we determined that motor neuron terminals in Drosophila larvae experience highly volatile power demands. It might not be surprising then, that we discovered the mitochondria in the motor neuron terminals of both Drosophila and mice to be heavily decorated with phosphagen kinases - a key element in an energy storage and buffering system well-characterized in fast-twitch muscle fibres. Knockdown of arginine kinase 1 (ArgK1) in Drosophila larval motor neurons led to several bioenergetic deficits, including mitochondrial matrix acidification and a faster decline in the cytosol ATP to ADP ratio during axon burst firing. KEY POINTS: Neurons commonly fire in bursts imposing highly volatile demands on the bioenergetic machinery that generates ATP. Using a computational approach, we built profiles of presynaptic power demand at the level of single action potentials, as well as the transition from rest to sustained activity. Phosphagen systems are known to buffer ATP levels in muscles and we demonstrate that phosphagen kinases, which support such phosphagen systems, also localize to mitochondria in motor nerve terminals of fruit flies and mice. By knocking down phosphagen kinases in fruit fly motor nerve terminals, and using fluorescent reporters of the ATP:ADP ratio, lactate, pH and Ca2+ , we demonstrate a role for phosphagen kinases in stabilizing presynaptic ATP levels. These data indicate that the maintenance of phosphagen systems in motor neurons, and not just muscle, could be a beneficial initiative in sustaining musculoskeletal health and performance.

Epithelial apoptotic pattern emerges from global and local regulation by cell apical area

Geometry is a fundamental attribute of biological systems, and it underlies cell and tissue dynamics. Cell geometry controls cell-cycle progression and mitosis and thus modulates tissue development and homeostasis. In sharp contrast and despite the extensive characterization of the genetic mechanisms of caspase activation, we know little about whether and how cell geometry controls apoptosis commitment in developing tissues. Here, we combined multiscale time-lapse microscopy of developing Drosophila epithelium, quantitative characterization of cell behaviors, and genetic and mechanical perturbations to determine how apoptosis is controlled during epithelial tissue development. We found that early in cell lives and well before extrusion, apoptosis commitment is linked to two distinct geometric features: a small apical area compared with other cells within the tissue and a small relative apical area with respect to the immediate neighboring cells. We showed that these global and local geometric characteristics are sufficient to recapitulate the tissue-scale apoptotic pattern. Furthermore, we established that the coupling between these two geometric features and apoptotic cells is dependent on the Hippo/YAP and Notch pathways. Overall, by exploring the links between cell geometry and apoptosis commitment, our work provides important insights into the spatial regulation of cell death in tissues and improves our understanding of the mechanisms that control cell number and tissue size.

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.

Interplay of RAP2 GTPase and the cytoskeleton in Hippo pathway regulation

The Hippo signaling is instrumental in regulating organ size, regeneration, and carcinogenesis. The cytoskeleton emerges as a primary Hippo signaling modulator. Its structural alterations in response to environmental and intrinsic stimuli control Hippo kinase cascade activity. However, the precise mechanisms underlying the cytoskeleton regulation of Hippo signaling are not fully understood. RAP2 GTPase is known to mediate the mechanoresponses of Hippo signaling via activating the core Hippo kinases LATS1/2 through MAP4Ks and MST1/2. Here we show the pivotal role of the reciprocal regulation between RAP2 GTPase and the cytoskeleton in Hippo signaling. RAP2 deletion undermines the responses of the Hippo pathway to external cues tied to RhoA GTPase inhibition and actin cytoskeleton remodeling, such as energy stress and serum deprivation. Notably, RhoA inhibitors and actin disruptors fail to activate LATS1/2 effectively in RAP2-deficient cells. RNA sequencing highlighted differential regulation of both actin and microtubule networks by RAP2 gene deletion. Consistently, Taxol, a microtubule-stabilizing agent, was less effective in activating LATS1/2 and inhibiting cell growth in RAP2 and MAP4K4/6/7 knockout cells. In summary, our findings position RAP2 as a central integrator of cytoskeletal signals for Hippo signaling, which offers new avenues for understanding Hippo regulation and therapeutic interventions in Hippo-impaired cancers.

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.

The Interaction of Mechanics and the Hippo Pathway in Drosophila melanogaster

Drosophila melanogaster has emerged as an ideal system for studying the networks that control tissue development and homeostasis and, given the similarity of the pathways involved, controlled and uncontrolled growth in mammalian systems. The signaling pathways used in patterning the Drosophila wing disc are well known and result in the emergence of interaction of these pathways with the Hippo signaling pathway, which plays a central role in controlling cell proliferation and apoptosis. Mechanical effects are another major factor in the control of growth, but far less is known about how they exert their control. Herein, we develop a mathematical model that integrates the mechanical interactions between cells, which occur via adherens and tight junctions, with the intracellular actin network and the Hippo pathway so as to better understand cell-autonomous and non-autonomous control of growth in response to mechanical forces.

Proteomic analysis reveals oxidative stress-induced activation of Hippo signaling in thiamethoxam-exposed Drosophila

Thiamethoxam (THIA) is a widely used neonicotinoid insecticide. However, the toxicity and defense mechanisms activated in THIA-exposed insects are unclear. Here, we used isobaric tags for relative and absolute quantitation (iTRAQ) proteomics technology to identify changes in protein expression in THIA-exposed Drosophila. We found that the antioxidant proteins Cyp6a23 and Dys were upregulated, whereas vir-1 was downregulated, which may have been detoxification in response to THIA exposure. Prx5 downregulation promoted the generation of reactive oxygen species. Furthermore, the accumulation of reactive oxygen species led to the induction of antioxidant defenses in THIA-exposed Drosophila, thereby enhancing the levels of oxidative stress markers (e.g., superoxide dismutase, glutathione S-transferase, and glutathione) and reducing catalase expression. Furthermore, the Hippo signaling transcription coactivator Yki was inactivated by THIA. Our results suggesting that Hippo signaling may be necessary to promote insect survival in response to neonicotinoid insecticide toxicity.

Crosstalk between the mTOR and Hippo pathways

Cell behavior changes in response to multiple stimuli, such as growth factors, nutrients, and cell density. The mechanistic target of the rapamycin (mTOR) pathway is activated by growth factors and nutrient stimuli to regulate cell growth and autophagy, whereas the Hippo pathway has negative effects on cell proliferation and tissue growth in response to cell density, DNA damage, and hormonal signals. These two signaling pathways must be precisely regulated and integrated for proper cell behavior. This integrative mechanism is not completely understood; nevertheless, recent studies have suggested that components of the mTOR and Hippo pathways interact with each other. Herein, as per contemporary knowledge, we review the molecular mechanisms of the interaction between the mTOR and Hippo pathways in mammals and Drosophila. Moreover, we discuss the advantage of this interaction in terms of tissue growth and nutrient consumption.

Competition between myosin II and βH-spectrin regulates cytoskeletal tension

Spectrins are membrane cytoskeletal proteins generally thought to function as heterotetramers comprising two α-spectrins and two β-spectrins. They influence cell shape and Hippo signaling, but the mechanism by which they influence Hippo signaling has remained unclear. We have investigated the role and regulation of the Drosophila β-heavy spectrin (β_H-spectrin, encoded by the karst gene) in wing imaginal discs. Our results establish that β_H-spectrin regulates Hippo signaling through the Jub biomechanical pathway due to its influence on cytoskeletal tension. While we find that α-spectrin also regulates Hippo signaling through Jub, unexpectedly, we find that β_H-spectrin localizes and functions independently of α-spectrin. Instead, β_H-spectrin co-localizes with and reciprocally regulates and is regulated by myosin. In vivo and in vitro experiments support a model in which β_H-spectrin and myosin directly compete for binding to apical F-actin. This competition can explain the influence of β_H-spectrin on cytoskeletal tension and myosin accumulation. It also provides new insight into how β_H-spectrin participates in ratcheting mechanisms associated with cell shape change.

Activation of Hippo Pathway Damages Slit Diaphragm by Deprivation of Ajuba Proteins

SIGNIFICANCE STATEMENT

Nuclear exclusion of the cotranscription factor YAP, which is a consequence of activation of the Hippo signaling pathway, leads to FSGS and podocyte apoptosis. Ajuba proteins play an important role in the glomerular filtration barrier by keeping the Hippo pathway inactive. In nephrocytes from Drosophila melanogaster , a well-established model system for podocyte research, Ajuba proteins ensure slit diaphragm (SD) formation and function. Hippo pathway activation leads to mislocalization of Ajuba proteins, decreased SD formation, rearrangement of the actin cytoskeleton, and increased SD permeability. Targeting the kinases of the Hippo pathway with specific inhibitors in the glomerulus could, therefore, be a promising strategy for therapy of FSGS.

BACKGROUND

The highly conserved Hippo pathway, which regulates organ growth and cell proliferation by inhibiting transcriptional cofactors YAP/TAZ, plays a special role in podocytes, where activation of the pathway leads to apoptosis. The Ajuba family proteins (Ajuba, LIM domain-containing protein 1 (LIMD1) and Wilms tumor protein 1-interacting protein [WTIP]) can bind and inactivate large tumor suppressor kinases 1 and 2, (LATS1/2) two of the Hippo pathway key kinases. WTIP, furthermore, connects the slit diaphragm (SD), the specialized cell-cell junction between podocytes, with the actin cytoskeleton.

METHODS

We used garland cell nephrocytes of Drosophila melanogaster to monitor the role of Ajuba proteins in Hippo pathway regulation and structural integrity of the SD. Microscopy and functional assays analyzed the interplay between Ajuba proteins and LATS2 regarding expression, localization, interaction, and effects on the functionality of the SD.

RESULTS

In nephrocytes, the Ajuba homolog Djub recruited Warts (LATS2 homolog) to the SD. Knockdown of Djub activated the Hippo pathway. Reciprocally, Hippo activation reduced the Djub level. Both Djub knockdown and Hippo activation led to morphological changes in the SD, rearrangement of the cortical actin cytoskeleton, and increased SD permeability. Knockdown of Warts or overexpression of constitutively active Yki prevented these effects. In podocytes, Hippo pathway activation or knockdown of YAP also decreased the level of Ajuba proteins.

CONCLUSIONS

Ajuba proteins regulate the structure and function of the SD in nephrocytes, connecting the SD protein complex to the actin cytoskeleton and maintaining the Hippo pathway in an inactive state. Hippo pathway activation directly influencing Djub expression suggests a self-amplifying feedback mechanism.

Transcriptomic analysis provides insight into the mechanism of IKKβ-mediated suppression of HPV18E6-induced cellular abnormalities

High-risk human papillomaviruses (HPVs) 16 and 18 are responsible for more than 70% of cervical cancers and majority of other HPV-associated cancers world-wide. Current treatments for these cancers have limited efficacy, which in turn has resulted in disease recurrence and poor survival rates in advanced disease stages. Hence, there is a significant need for development of novel molecularly-targeted therapeutics. This can only be achieved through improved understanding of disease mechanism. Recently, we developed a Drosophila model of HPV18E6 plus human E3 ubiquitin ligase (hUBE3A) and demonstrated that the E6-induced cellular abnormalities are conserved between humans and flies. Subsequently, we demonstrated that reduced level and activity of IKKβ, a regulator of NF-κB, suppresses the cellular abnormalities induced by E6 oncoprotein and that the interaction of IKKβ and E6 is conserved in human cells. In this study, we performed transcriptomic analysis to identify differentially expressed genes that play a role in IKKβ-mediated suppression of E6-induced defects. Transcriptome analysis identified 215 genes whose expression was altered due to reduced levels of IKKβ. Of these 215 genes, 151 genes showed annotations. These analyses were followed by functional genetic interaction screen using RNAi, overexpression, and mutant fly strains for identified genes. The screen identified several genes including genes involved in Hippo and Toll pathways as well as junctional complexes whose downregulation or upregulation resulted in alterations of E6-induced defects. Subsequently, RT-PCR analysis was performed for validation of altered gene expression level for a few representative genes. Our results indicate an involvement for Hippo and Toll pathways in IKKβ-mediated suppression of E6 + hUBE3A-induced cellular abnormalities. Therefore, this study enhances our understanding of the mechanisms underlying HPV-induced cancer and can potentially lead to identification of novel drug targets for cancers associated with HPV.

RNAi screen in the Drosophila wing of genes encoding proteins related to cytoskeleton organization and cell division

Cell division and cytoskeleton organization are fundamental processes participating in the development of Drosophila imaginal discs. In this manuscript we describe the phenotypes in the adult fly wing generated by knockdowns of 85% of Drosophila genes encoding proteins likely related to the regulation of cell division and cytoskeleton organization. We also compile a molecular classification of these proteins into classes that describe their expected or known main biochemical characteristics, as well as mRNA expression in the wing disc and likely protein subcellular localization for a subset of these genes. Finally, we analyze in more detail one protein family of cytoskeleton genes (Arp2/3 complex), and define the consequences of interfering with cell division for wing growth and patterning.

Studying the Geroprotective Properties of YAP/TAZ Signaling Inhibitors on Drosophila melanogaster Model

The transcriptional coactivators Yes-associated protein (YAP) and transcriptional coactivator with PDZ-binding motif (TAZ) are the main downstream effectors of the evolutionarily conserved Hippo signaling pathway. YAP/TAZ are implicated in the transcriptional regulation of target genes that are involved in a wide range of key biological processes affecting tissue homeostasis and play dual roles in the aging process, depending on the cellular and tissue context. The aim of the present study was to investigate whether pharmacological inhibitors of Yap/Taz increase the lifespan of Drosophila melanogaster. Real-time qRT-PCR was performed to measure the changes in the expression of Yki (Yorkie, the Drosophila homolog of YAP/TAZ) target genes. We have revealed a lifespan-increasing effect of YAP/TAZ inhibitors that was mostly associated with decreased expression levels of the wg and E2f1 genes. However, further analysis is required to understand the link between the YAP/TAZ pathway and aging.

Coordinated growth of linked epithelia is mediated by the Hippo pathway

An epithelium in a living organism seldom develops in isolation. Rather, most epithelia are tethered to other epithelial or non-epithelial tissues, necessitating growth coordination between layers. We investigated how two tethered epithelial layers of the Drosophila larval wing imaginal disc, the disc proper (DP) and the peripodial epithelium (PE), coordinate their growth. DP growth is driven by the morphogens Hedgehog (Hh) and Dpp, but regulation of PE growth is poorly understood. We find that the PE adapts to changes in growth rates of the DP, but not vice versa, suggesting a "leader and follower" mechanism. Moreover, PE growth can occur by cell shape changes, even when proliferation is inhibited. While Hh and Dpp pattern gene expression in both layers, growth of the DP is exquisitely sensitive to Dpp levels, while growth of the PE is not; the PE can achieve an appropriate size even when Dpp signaling is inhibited. Instead, both the growth of the PE and its accompanying cell shape changes require the activity of two components of the mechanosensitive Hippo pathway, the DNA-binding protein Scalloped (Sd) and its co-activator (Yki), which could allow the PE to sense and respond to forces generated by DP growth. Thus, an increased reliance on mechanically-dependent growth mediated by the Hippo pathway, at the expense of morphogen-dependent growth, enables the PE to evade layer-intrinsic growth control mechanisms and coordinate its growth with the DP. This provides a potential paradigm for growth coordination between different components of a developing organ.

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.

Peripodial adherens junctions regulate Ajuba-Yorkie signaling to preserve fly eye morphology

The Drosophila eye develops from the larval eye disc, a flattened vesicle comprised of continuous retinal and peripodial epithelia (PE). The PE is an epithelium that plays a supporting role in retinal neurogenesis, but gives rise to cuticle in the adult. We report here that the PE is also necessary to preserve the morphology of the retinal epithelium. Depletion of the adherens junction (AJ) components β-Catenin (β-Cat), DE-Cadherin or α-Catenin from the PE leads to altered disc morphology, characterized by retinal displacement (RDis); so too does loss of the Ajuba protein Jub, an AJ-associated regulator of the transcriptional coactivator Yorkie (Yki). Restoring AJs or overexpressing Yki in β-Cat deficient PE results in suppression of RDis. Additional suppressors of AJ-dependent RDis include knockdown of Rho kinase (Rok) and Dystrophin (Dys). Furthermore, knockdown of βPS integrin (Mys) from the PE results in RDis, while overexpression of Mys can suppress RDis induced by the loss of β-Cat. We thus propose that AJ-Jub-Yki signaling in PE cells regulates PE cell contractile properties and/or attachment to the extracellular matrix to promote normal eye disc morphology.

TPM2 attenuates progression of prostate cancer by blocking PDLIM7-mediated nuclear translocation of YAP1

BACKGROUND

Prostate cancer (PCa) is a common malignant tumor of the genitourinary system. Clinical intervention in advanced PCa remains challenging. Tropomyosins 2 (TPM2) are actin-binding proteins and have been found as a biomarker candidate for certain cancers. However, no studies have explored the role of TPM2 in PCa and its regulatory mechanism.

METHODS

TPM2 expression was assessed in Gene Expression Omnibus (GEO) and the Cancer Genome Atlas (TCGA) PCa patient dataset. The effect of TPM2 on PCa progression was assessed in vitro and in vivo by quantifying proliferation, migration, invasion and tumor growth assays, and the mechanism of TPM2 in PCa progression was gradually revealed by Western blotting, immunoprecipitation, and immunofluorescence staining arrays.

RESULTS

TPM2 was found to be severely downregulated in tumor tissues of PCa patients compared with tumor-adjacent normal tissues. In vitro experiments revealed that TPM2 overexpression inhibited PCa cell proliferation, invasion and androgen-independent proliferation. Moreover, TPM2 overexpression inhibited the growth of subcutaneous xenograft tumors in vivo. Mechanistically, this effect was noted to be dependent on PDZ-binding motif of TPM2. TPM2 competed with YAP1 for binding to PDLIM7 through the PDZ-binding motif. The binding of TPM2 to PDLIM7 subsequently inhibited the nuclear transport function of PDLIM7 for YAP1. YAP1 sequestered in the cytoplasm phosphorylated at S127, resulting in its inactivation or degradation which in turn inhibited the expression of YAP1 downstream target genes.

CONCLUSIONS

This study investigated the role of TPM2, PDLIM7, and YAP1 in PCa progression and castration resistance. TPM2 attenuates progression of PCa by blocking PDLIM7-mediated nuclear translocation of YAP1. Accordingly, targeting the expression or functional modulation of TPM2, PDLIM7, or YAP1 has the potential to be an effective therapeutic approach to reduce PCa proliferation and prevent the progression of castration-resistant prostate cancer (CRPC).

The α-Catenin mechanosensing M region is required for cell adhesion during tissue morphogenesis

α-Catenin couples the cadherin-catenin complex to the actin cytoskeleton. The mechanosensitive α-Catenin M region undergoes conformational changes upon application of force to recruit interaction partners. Here, we took advantage of the tension landscape in the Drosophila embryo to define three different states of α-Catenin mechanosensing in support of cell adhesion. Low-, medium-, and high-tension contacts showed a corresponding recruitment of Vinculin and Ajuba, which was dependent on the α-Catenin M region. In contrast, the Afadin homolog Canoe acts in parallel to α-Catenin at bicellular low- and medium-tension junctions but requires an interaction with α-Catenin for its tension-sensitive enrichment at high-tension tricellular junctions. Individual M region domains make complex contributions to cell adhesion through their impact on interaction partner recruitment, and redundancies with the function of Canoe. Our data argue that α-Catenin and its interaction partners are part of a cooperative and partially redundant mechanoresponsive network that supports AJs remodeling during morphogenesis.

Digitally predicting protein localization and manipulating protein activity in fluorescence images using 4D reslicing GAN

MOTIVATION

While multi-channel fluorescence microscopy is a vital imaging method in biological studies, the number of channels that can be imaged simultaneously is limited by technical and hardware limitations such as emission spectra cross-talk. One solution is using deep neural networks to model the localization relationship between two proteins so that the localization of one protein can be digitally predicted. Furthermore, the input and predicted localization implicitly reflect the modeled relationship. Accordingly, observing the response of the prediction via manipulating input localization could provide an informative way to analyze the modeled relationships between the input and the predicted proteins.

RESULTS

We propose a protein localization prediction (PLP) method using a cGAN named 4D Reslicing Generative Adversarial Network (4DR-GAN) to digitally generate additional channels. 4DR-GAN models the joint probability distribution of input and output proteins by simultaneously incorporating the protein localization signals in four dimensions including space and time. Because protein localization often correlates with protein activation state, based on accurate PLP, we further propose two novel tools: digital activation (DA) and digital inactivation (DI) to digitally activate and inactivate a protein, in order to observing the response of the predicted protein localization. Compared with genetic approaches, these tools allow precise spatial and temporal control. A comprehensive experiment on six pairs of proteins shows that 4DR-GAN achieves higher-quality PLP than Pix2Pix, and the DA and DI responses are consistent with the known protein functions. The proposed PLP method helps simultaneously visualize additional proteins, and the developed DA and DI tools provide guidance to study localization-based protein functions.

AVAILABILITY AND IMPLEMENTATION

The open-source code is available at https://github.com/YangJiaoUSA/4DR-GAN.

SUPPLEMENTARY INFORMATION

Supplementary data are available at Bioinformatics online.

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.

Myotubularin functions through actomyosin to interact with the Hippo pathway

The Hippo pathway is an evolutionarily conserved developmental pathway that controls organ size by integrating diverse regulatory inputs, including actomyosin-mediated cytoskeletal tension. Despite established connections between the actomyosin cytoskeleton and the Hippo pathway, the upstream regulation of actomyosin in the Hippo pathway is less defined. Here, we identify the phosphoinositide-3-phosphatase Myotubularin (Mtm) as a novel upstream regulator of actomyosin that functions synergistically with the Hippo pathway during growth control. Mechanistically, Mtm regulates membrane phospholipid PI(3)P dynamics, which, in turn, modulates actomyosin activity through Rab11-mediated vesicular trafficking. We reveal PI(3)P dynamics as a novel mode of upstream regulation of actomyosin and establish Rab11-mediated vesicular trafficking as a functional link between membrane lipid dynamics and actomyosin activation in the context of growth control. Our study also shows that MTMR2, the human counterpart of Drosophila Mtm, has conserved functions in regulating actomyosin activity and tissue growth, providing new insights into the molecular basis of MTMR2-related peripheral nerve myelination and human disorders.

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.

AJUBA and WTIP can compete with LIMD1 for junctional localization and LATS regulation

Each of the three mammalian Ajuba family proteins, AJUBA, LIMD1 and WTIP, exhibit tension-dependent localization to adherens junctions, and can associate with Lats kinases. However, only LIMD1 has been directly demonstrated to directly regulate Lats activity in vivo. To assess the relationship of LIMD1 to AJUBA and WTIP, and the potential contributions of AJUBA and WTIP to Lats regulation, we examined the consequences of over-expressing AJUBA and WTIP in MCF10A cells. Over-expression of either AJUBA or WTIP reduced junctional localization of LIMD1, implying that these proteins can compete for binding to adherens junctions. This over-expression also reduced junctional localization of LATS1, implying that AJUBA or WTIP are unable to efficiently recruit Lats kinases to adherens junctions. This over-expression was also associated with increased YAP1 phosphorylation and decreased YAP1 nuclear localization, consistent with increased Lats kinase activity. These observations indicate that AJUBA and WTIP compete with LIMD1 for association with adherens junctions but have activities distinct from LIMD1 in Hippo pathway regulation. They further suggest that the ability of Ajuba family proteins to associate with Lats kinases in solution is not sufficient to enable regulation in vivo, and that tumor suppressor activities of AJUBA and WTIP could stem in part from competition with LIMD1 for regulation of Lats kinases at cell junctions.

The Hippo signalling pathway and its implications in human health and diseases

As an evolutionarily conserved signalling network, the Hippo pathway plays a crucial role in the regulation of numerous biological processes. Thus, substantial efforts have been made to understand the upstream signals that influence the activity of the Hippo pathway, as well as its physiological functions, such as cell proliferation and differentiation, organ growth, embryogenesis, and tissue regeneration/wound healing. However, dysregulation of the Hippo pathway can cause a variety of diseases, including cancer, eye diseases, cardiac diseases, pulmonary diseases, renal diseases, hepatic diseases, and immune dysfunction. Therefore, therapeutic strategies that target dysregulated Hippo components might be promising approaches for the treatment of a wide spectrum of diseases. Here, we review the key components and upstream signals of the Hippo pathway, as well as the critical physiological functions controlled by the Hippo pathway. Additionally, diseases associated with alterations in the Hippo pathway and potential therapies targeting Hippo components will be discussed.

Integrated analysis of cervical squamous cell carcinoma cohorts from three continents reveals conserved subtypes of prognostic significance

Human papillomavirus (HPV)-associated cervical cancer is a leading cause of cancer deaths in women. Here we present an integrated multi-omic analysis of 643 cervical squamous cell carcinomas (CSCC, the most common histological variant of cervical cancer), representing patient populations from the USA, Europe and Sub-Saharan Africa and identify two CSCC subtypes (C1 and C2) with differing prognosis. C1 and C2 tumours can be driven by either of the two most common HPV types in cervical cancer (16 and 18) and while HPV16 and HPV18 are overrepresented among C1 and C2 tumours respectively, the prognostic difference between groups is not due to HPV type. C2 tumours, which comprise approximately 20% of CSCCs across these cohorts, display distinct genomic alterations, including loss or mutation of the STK11 tumour suppressor gene, increased expression of several immune checkpoint genes and differences in the tumour immune microenvironment that may explain the shorter survival associated with this group. In conclusion, we identify two therapy-relevant CSCC subtypes that share the same defining characteristics across three geographically diverse cohorts.

Human papillomavirus (HPV) is a known cause of cervical cancer. Here, the authors perform a multi-omic analysis using published cervical squamous cell carcinoma cohorts from the USA, Europe, and SubSaharan Africa and identify two cervical squamous cell carcinoma subtypes that display prognostic differences.

Intermittent compressive force regulates human periodontal ligament cell behavior via yes-associated protein

Intermittent compressive force influences human periodontal ligament (PDL) cell behavior that facilitates periodontal tissue regeneration. In response to mechanical stimuli, Yes-associated protein (YAP) has been recognized as a mechanosensitive transcriptional activator that regulates cell proliferation and cell fate decisions. This study aimed to investigate whether compressive forces influence cell proliferation and cell fate decisions of human PDL cells via YAP signaling. YAP expression was silenced by shRNA. The effect of YAP on cell proliferation, adipogenesis and osteogenesis of PDL cells under ICF loading were determined. Adipogenic differentiation bias upon ICF loading was confirmed by fourier-transform infrared spectroscopy (FTIR). The results revealed that ICF-induced YAP promotes osteogenesis, but it inhibits adipogenesis in PDL cells. Depletion of YAP results in PDL cells that are irresponsive to ICF and, therefore, the failure of the PDL cells to undergo osteogenic differentiation. This was shown by a significant reduction in calcium deposited in the CF-derived osteoblasts of the YAP-knockdown (YAP-KD) PDL cells. As to control treatment, reduction of YAP promoted adipogenesis, whereas ICF-induced YAP inhibited this mechanism. However, the adipocyte differentiation in YAP-KD cells was not affected upon ICF treatment as the YAP-KD cells still exhibited a better adipogenic differentiation that was unrelated to the ICF. This study demonstrated that, in response to ICF treatment, YAP could be a crucial mechanosensitive transcriptional activator for the regulation of PDL cell behavior through a mechanobiological process. Our results may provide the possibility of facilitating PDL tissue regeneration by manipulation of the Hippo-YAP signaling pathway.

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YAP plays role as a mechanosensitive transcriptional activator of human PDL cells in response to ICF.

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ICF activates YAP and its target genes to promote cell proliferation and osteogenic differentiation of human PDL cells.

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Loss of YAP enhances adipogenic differentiation of human periodontal ligament cells.

Scribble and α-Catenin cooperatively regulate epithelial homeostasis and growth

Epithelial homeostasis is an emergent property of both physical and biochemical signals emanating from neighboring cells and across tissue. A recent study reveals that Scribble, an apico-basal polarity determinant, cooperates with α-Catenin, an adherens junction component, to regulate tissue homeostasis in the Drosophila wing imaginal disc. However, it remains to be addressed whether similar mechanisms are utilized in vertebrates. In this study, we first address how α-Catenin cooperates with Scribble to regulate epithelial homeostasis and growth in mammalian cells. Our data show that α-Catenin and Scribble interact physically in mammalian cells. We then found that both α-Catenin and Scribble are required for regulating nuclear translocation of YAP, an effector of the Hippo signaling pathway. Furthermore, ectopic Scribble suffices to suppress YAP in an α-Catenin-dependent manner. Then, to test our hypothesis that Scribble amounts impact epithelial growth, we use the Drosophila wing imaginal disc. We show that Scribble expression is complementary to Yorkie signal, the Drosophila ortholog of YAP. Ectopic expression of full-length Scribble or Scribble Leucine Rich Region (LRR):α-Catenin chimera sufficiently down-regulates Yorkie signal, leading to smaller wing size. Moreover, Scribble LRR:α-Catenin chimera rescues scribble mutant clones in the wing imaginal disc to maintain tissue homeostasis. Taken together, our studies suggest that the association of cell polarity component Scribble with α-Catenin plays a conserved role in epithelial homeostasis and growth.

YAP signaling is involved in WDR1-regulated proliferation and migration of non-small-cell lung cancer cells

As a major co-factor of F-actin depolymerization, WD-repeat domain 1 (WDR1) affects the cellular microenvironment by cytoskeleton remodeling, thereby influencing cell molecular behavior. Our previous study showed that WDR1 activates YAP (Yes-associated protein) signaling in non-small-cell lung cancer (NSCLC) cells, but the mechanism remains unclear. We discovered that knockdown WDR1 in NSCLC cells decreased the expression of YAP and the nucleus-to-cytoplasm ratio. Disruption of cortical stress by drugs significantly inhibited YAP nuclear trafficking and enhanced YAP phosphorylation. In WDR1-knockdown NSCLC cells, inhibition of Hippo pathway reduced the nuclear exclusion of YAP and phosphorylated YAP. Our data suggest that WDR1-mediated cortical stress might be involved in regulating YAP signaling, thereby affecting the proliferation and migration of NSCLC cells.

Analysis of the Drosophila Ajuba LIM protein defines functions for distinct LIM domains

The Ajuba LIM protein Jub mediates regulation of Hippo signaling by cytoskeletal tension through interaction with the kinase Warts and participates in feedback regulation of junctional tension through regulation of the cytohesin Steppke. To investigate how Jub interacts with and regulates its distinct partners, we investigated the ability of Jub proteins missing different combinations of its three LIM domains to rescue jub phenotypes and to interact with α-catenin, Warts and Steppke. Multiple regions of Jub contribute to its ability to bind α-catenin and to localize to adherens junctions in Drosophila wing imaginal discs. Co-immunoprecipitation experiments in cultured cells identified a specific requirement for LIM2 for binding to Warts. However, in vivo, both LIM1 and LIM2, but not LIM3, were required for regulation of wing growth, Yorkie activity, and Warts localization. Conversely, LIM2 and LIM3, but not LIM1, were required for regulation of cell shape and Steppke localization in vivo, and for maximal Steppke binding in co-immunoprecipitation experiments. These observations identify distinct functions for the different LIM domains of Jub.

The LIM Protein AJUBA is a Potential Oncogenic Target and Prognostic Marker in Human Cancer via Pan-Cancer Analysis

Purpose: The LIM (Lin-11, Isl1, MEC-3) domain protein AJUBA is involved in multiple biological functions, and its aberrant expression is related to the occurrence and progression of various cancers. However, there are no analytical studies on AJUBA in pan-cancer.

Methods: We performed a comprehensive pan-cancer analysis and explored the potential oncogenic roles of AJUBA, including gene expression, genetic mutation, protein phosphorylation, clinical diagnostic biomarker, prognosis, and AJUBA-related immune infiltration based on The Cancer Genome Atlas and Genotype-Tissue Expression databases.

Results: The results revealed that the expression of AJUBA highly correlated with poor clinical outcomes in patients with different types of cancer. Meanwhile, AJUBA expression was positively correlated with cancer-associated fibroblasts in many human cancers, such as breast invasive carcinoma, colon adenocarcinoma, brain lower-grade glioma, lung adenocarcinoma (LUAD), and ovarian serous cystadenocarcinoma (OV). Gene ontology and Kyoto Encyclopedia of Genes and Genomes enrichment analyses showed that AJUBA is mainly involved in protein serine/threonine kinase activity, cell–cell junction, covalent chromatin modification, and Hippo signaling pathway.

Conclusion: The pan-cancer study reveals the oncogenic roles of AJUBA and provides a comprehensive understanding of the molecular biological genetic information of AJUBA in various tumors.