Fat cadherin modulates organ size in Drosophila via the Salvador/Warts/Hippo signaling pathway

YAP in development and disease: Navigating the regulatory landscape from retina to brain

The distinctive role of Yes-associated protein (YAP) in the nervous system has attracted widespread attention. This comprehensive review strategically uses the retina as a vantage point, embarking on an extensive exploration of YAP's multifaceted impact from the retina to the brain in development and pathology. Initially, we explore the crucial roles of YAP in embryonic and cerebral development. Our focus then shifts to retinal development, examining in detail YAP's regulatory influence on the development of retinal pigment epithelium (RPE) and retinal progenitor cells (RPCs), and its significant effects on the hierarchical structure and functionality of the retina. We also investigate the essential contributions of YAP in maintaining retinal homeostasis, highlighting its precise regulation of retinal cell proliferation and survival. In terms of retinal-related diseases, we explore the epigenetic connections and pathophysiological regulation of YAP in diabetic retinopathy (DR), glaucoma, and proliferative vitreoretinopathy (PVR). Lastly, we broaden our exploration from the retina to the brain, emphasizing the research paradigm of "retina: a window to the brain." Special focus is given to the emerging studies on YAP in brain disorders such as Alzheimer's disease (AD) and Parkinson's disease (PD), underlining its potential therapeutic value in neurodegenerative disorders and neuroinflammation.

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 Hippo pathway of cytoskeletal regulation, which was subsequently co-opted to regulate proliferation and animal tissue size.

Natural compounds targeting YAP/TAZ axis in cancer: Current state of art and challenges

Cancer has become a burgeoning global healthcare concern marked by its exponential growth and significant economic ramifications. Though advancements in the treatment modalities have increased the overall survival and quality of life, there are no definite treatments for the advanced stages of this malady. Hence, understanding the diseases etiologies and the underlying molecular complexities, will usher in the development of innovative therapeutics. Recently, YAP/TAZ transcriptional regulation has been of immense interest due to their role in development, tissue homeostasis and oncogenic transformations. YAP/TAZ axis functions as coactivators within the Hippo signaling cascade, exerting pivotal influence on processes such as proliferation, regeneration, development, and tissue renewal. In cancer, YAP is overexpressed in multiple tumor types and is associated with cancer stem cell attributes, chemoresistance, and metastasis. Activation of YAP/TAZ mirrors the cellular "social" behavior, encompassing factors such as cell adhesion and the mechanical signals transmitted to the cell from tissue structure and the surrounding extracellular matrix. Therefore, it presents a significant vulnerability in the clogs of tumors that could provide a wide window of therapeutic effectiveness. Natural compounds have been utilized extensively as successful interventions in the management of diverse chronic illnesses, including cancer. Owing to their capacity to influence multiple genes and pathways, natural compounds exhibit significant potential either as adjuvant therapy or in combination with conventional treatment options. In this review, we delineate the signaling nexus of YAP/TAZ axis, and present natural compounds as an alternate strategy to target cancer.

Contributions of the Dachsous intracellular domain to Dachsous-Fat signaling

The protocadherins Fat and Dachsous regulate organ growth, shape, patterning, and planar cell polarity. Although Dachsous and Fat have been described as ligand and receptor, respectively, in a signal transduction pathway, there is also evidence for bidirectional signaling. Here we assess signaling downstream of Dachsous through analysis of its intracellular domain. Genomic deletions of conserved sequences within dachsous identified regions of the intracellular domain required for normal development. Deletion of the A motif increased Dachsous protein levels and decreased wing size. Deletion of the D motif decreased Dachsous levels at cell membranes, increased wing size, and disrupted wing, leg and hindgut patterning and planar cell polarity. Co-immunoprecipitation experiments established that the D motif is necessary and sufficient for association of Dachsous with four key partners: Lowfat, Dachs, Spiny-legs, and MyoID. Subdivision of the D motif identified distinct regions that are preferentially responsible for association with Lft versus Dachs. Our results identify motifs that are essential for Dachsous function and are consistent with the hypothesis that the key function of Dachsous is regulation of Fat.

Exploring the molecular makeup of support cells in insect camera eyes

Animals typically have either compound eyes, or camera-type eyes, both of which have evolved repeatedly in the animal kingdom. Both eye types include two important kinds of cells: photoreceptor cells, which can be excited by light, and non-neuronal support cells (SupCs), which provide essential support to photoreceptors. At the molecular level deeply conserved genes that relate to the differentiation of photoreceptor cells have fueled a discussion on whether or not a shared evolutionary origin might be considered for this cell type. In contrast, only a handful of studies, primarily on the compound eyes of Drosophila melanogaster, have demonstrated molecular similarities in SupCs. D. melanogaster SupCs (Semper cells and primary pigment cells) are specialized eye glia that share several molecular similarities with certain vertebrate eye glia, including Müller glia. This led us to question if there could be conserved molecular signatures of SupCs, even in functionally different eyes such as the image-forming larval camera eyes of the sunburst diving beetle Thermonectus marmoratus. To investigate this possibility, we used an in-depth comparative whole-tissue transcriptomics approach. Specifically, we dissected the larval principal camera eyes into SupC- and retina-containing regions and generated the respective transcriptomes. Our analysis revealed several common features of SupCs including enrichment of genes that are important for glial function (e.g. gap junction proteins such as innexin 3), glycogen production (glycogenin), and energy metabolism (glutamine synthetase 1 and 2). To evaluate similarities, we compared our transcriptomes with those of fly (Semper cells) and vertebrate (Müller glia) eye glia as well as respective retinas. T. marmoratus SupCs were found to have distinct genetic overlap with both fly and vertebrate eye glia. These results suggest that T. marmoratus SupCs are a form of glia, and like photoreceptors, may be deeply conserved.

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.

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.

E2 enzyme Bruce negatively regulates Hippo signaling through POSH-mediated expanded degradation

The Hippo pathway is a master regulator of organ growth, stem cell renewal, and tumorigenesis, its activation is tightly controlled by various post-translational modifications, including ubiquitination. While several E3 ubiquitin ligases have been identified as regulators of Hippo pathway, the corresponding E2 ubiquitin-conjugating enzymes (E2s) remain unknown. Here, we performed a screen in Drosophila to identify E2s involved in regulating wing overgrowth caused by the overexpression of Crumbs (Crb) intracellular domain and identified Bruce as a critical regulator. Loss of Bruce downregulates Hippo target gene expression and suppresses Hippo signaling inactivation induced tissue growth. Unexpectedly, our genetic data indicate that Bruce acts upstream of Expanded (Ex) but in parallel with the canonical Hippo (Hpo) -Warts (Wts) cascade to regulate Yorkie (Yki), the downstream effector of Hippo pathway. Mechanistically, Bruce synergizes with E3 ligase POSH to regulate growth and ubiquitination-mediated Ex degradation. Moreover, we demonstrate that Bruce is required for Hippo-mediated malignant tumor progression. Altogether, our findings unveil Bruce as a crucial E2 enzyme that bridges the signal from the cell surface to regulate Hippo pathway activation in Drosophila.

The cell adhesion molecule Echinoid promotes tissue survival and separately restricts tissue overgrowth in Drosophila imaginal discs

The interactions that cells in Drosophila imaginal discs have with their neighbors are known to regulate their ability to survive. In a screen of genes encoding cell surface proteins for gene knockdowns that affect the size or shape of mutant clones, we found that clones of cells with reduced levels of echinoid (ed) are fewer, smaller, and can be eliminated during development. In contrast, discs composed mostly of ed mutant tissue are overgrown. We find that ed mutant tissue has lower levels of the anti-apoptotic protein Diap1 and has increased levels of apoptosis which is consistent with the observed underrepresentation of ed mutant clones and the slow growth of ed mutant tissue. The eventual overgrowth of ed mutant tissue results not from accelerated growth, but from prolonged growth resulting from a failure to arrest growth at the appropriate final size. Ed has previously been shown to physically interact with multiple Hippo-pathway components and it has been proposed to promote Hippo pathway signaling, to exclude Yorkie (Yki) from the nucleus, and restrain the expression of Yki-target genes. We did not observe changes in Yki localization in ed mutant tissue and found decreased levels of expression of several Yorkie-target genes, findings inconsistent with the proposed effect of Ed on Yki. We did, however, observe increased expression of several Yki-target genes in wild-type cells neighboring ed mutant cells, which may contribute to elimination of ed mutant clones. Thus, ed has two distinct functions: an anti-apoptotic function by maintaining Diap1 levels, and a function to arrest growth at the appropriate final size. Both of these are unlikely to be explained by a simple effect on the Hippo pathway.

Exploring the molecular makeup of support cells in insect camera eyes

Animals generally have either compound eyes, which have evolved repeatedly in different invertebrates, or camera eyes, which have evolved many times across the animal kingdom. Both eye types include two important kinds of cells: photoreceptor cells, which can be excited by light, and non-neuronal support cells (SupCs), which provide essential support to photoreceptors. Despite many examples of convergence in eye evolution, similarities in the gross developmental plan and molecular signatures have been discovered, even between phylogenetically distant and functionally different eye types. For this reason, a shared evolutionary origin has been considered for photoreceptors. In contrast, only a handful of studies, primarily on the compound eyes of Drosophila melanogaster , have demonstrated molecular similarities in SupCs. D. melanogaster SupCs (Semper cells and primary pigment cells) are specialized eye glia that share several molecular similarities with certain vertebrate eye glia, including Müller glia. This led us to speculate whether there are conserved molecular signatures of SupCs, even in functionally different eyes such as the image-forming larval camera eyes of the sunburst diving beetle Thermonectus marmoratus . To investigate this possibility, we used an in-depth comparative whole-tissue transcriptomics approach. Specifically, we dissected the larval principal camera eyes into SupC- and retina-containing regions and generated the respective transcriptomes. Our analysis revealed several conserved features of SupCs including enrichment of genes that are important for glial function (e.g. gap junction proteins such as innexin 3), glycogen production (glycogenin), and energy metabolism (glutamine synthetase 1 and 2). To evaluate the extent of conservation, we compared our transcriptomes with those of fly (Semper cells) and vertebrate (Müller glia) eye glia as well as respective retinas. T. marmoratus SupCs were found to have distinct genetic overlap with both fly and vertebrate eye glia. These results provide molecular evidence for the deep conservation of SupCs in addition to photoreceptor cells, raising essential questions about the evolutionary origin of eye-specific glia in animals.

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

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

The role of the Hippo pathway in autophagy in the heart

The Hippo pathway, an evolutionarily conserved signalling mechanism, controls organ size and tumourigenesis. Increasing lines of evidence suggest that autophagy, an important mechanism of lysosome-mediated cellular degradation, is regulated by the Hippo pathway, which thereby profoundly affects cell growth and death responses in various cell types. In the heart, Mst1, an upstream component of the Hippo pathway, not only induces apoptosis but also inhibits autophagy through phosphorylation of Beclin 1. YAP/TAZ, transcription factor co-factors and the terminal effectors of the Hippo pathway, affect autophagy through transcriptional activation of TFEB, a master regulator of autophagy and lysosomal biogenesis. The cellular abundance of YAP is negatively regulated by autophagy and suppression of autophagy induces accumulation of YAP, which, in turn, acts as a feedback mechanism to induce autophagosome formation. Thus, the Hippo pathway and autophagy regulate each other, thereby profoundly affecting cardiomyocyte survival and death. This review discusses the interaction between the Hippo pathway and autophagy and its functional significance during stress conditions in the heart and the cardiomyocytes therein.

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.

Trimethyltin induces apoptosis and necroptosis of mouse liver by oxidative stress through YAP phosphorylation

Trimethyltin (TMT) is widely used as a major component of plastic stabilizers in agriculture and industry, and can accumulate in large quantities in the liver. To investigate the relationship between liver tissue damage induced by TMT exposure and YAP phosphorylation in mice, we gave the mice drinking water containing 0.01 mg/mL TMT for 14 days to establish an in vivo experimental model, and continuously treated AML12 cells with 20 μM TMT for 24 h to establish an in vitro experimental model. Transcriptomics revealed that TMT exposure altered 62,466 apparently diversely expressed genes, including 1197 upregulated and 899 downregulated genes, and that enrichment of the Hippo pathway occurred. Moreover, western blotting (WB) and quantitative real-time PCR (qRTPCR) results showed that TMT exposure triggered an increase in the expression of P-YAP, apoptosis and necroptosis-interrelated genes, and a decrease in Bcl-2 expression in mouse livers tissues and AML12 cells. The expression of P-YAP was significantly suppressed in the TRULI-treated TMT-exposed AML12 cells, while oxidative stress levels and damage were also significantly attenuated. In conclusion, TMT triggers YAP phosphorylation to induce oxidative stress inducing apoptosis and necroptosis in mouse livers tissues. Our results confirm the liver toxic effect and specific mechanism of TMT.

Shaggy regulates tissue growth through Hippo pathway in Drosophila

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

Somatic mutations in FAT cadherin family members constitute an underrecognized subtype of colorectal adenocarcinoma with unique clinicopathologic features

BACKGROUND

The FAT cadherin family members (FAT1, FAT2, FAT3 and FAT4) are conserved tumor suppressors that are recurrently mutated in several types of human cancers, including colorectal carcinoma (CRC).

AIM

To characterize the clinicopathologic features of CRC patients with somatic mutations in FAT cadherin family members.

METHODS

We analyzed 526 CRC cases from The Cancer Genome Atlas PanCancer Atlas dataset. CRC samples were subclassified into 2 groups based on the presence or absence of somatic mutations in FAT1, FAT2, FAT3 and FAT4. Individual clinicopathological data were collected after digital slide review. Statistical analysis was performed using t tests and chi-square tests.

RESULTS

This CRC study cohort had frequent mutations in the FAT1 (10.5%), FAT2 (11.2%), FAT3 (15.4%) and FAT4 (23.4%) genes. Two hundred CRC patients (38.0%) harbored somatic mutations in one or more of the FAT family genes and were grouped into the FAT mutated CRC subtype. The FAT-mutated CRC subtype was more commonly located on the right side of the colon (51.0%) than in the rest of the cohort (30.1%, P < 0.001). It showed favorable clinicopathologic features, including a lower rate of positive lymph nodes (pN1-2: 33.5% vs 46.4%, P = 0.005), a lower rate of metastasis to another site or organ (pM1: 7.5% vs 16.3%, P = 0.006), and a trend toward an early tumor stage (pT1-2: 25.0% vs 18.7%, P = 0.093). FAT somatic mutations were significantly enriched in microsatellite instability CRC (28.0% vs 2.1%, P < 0.001). However, FAT somatic mutations in microsatellite stable CRC demonstrated similar clinicopathologic behaviors, as well as a trend of a better disease-free survival rate (hazard ratio = 0.539; 95% confidence interval: 0.301-0.967; log-rank P = 0.073).

CONCLUSION

FAT cadherin family genes are frequently mutated in CRC, and their mutation profile defines a subtype of CRC with favorable clinicopathologic characteristics.

The unfolding of the Hippo signaling pathway

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

Comparative Transcriptome Analysis Provides Insight into Spatio-Temporal Expression Characteristics and Genetic Regulatory Network in Postnatal Developing Subcutaneous and Visceral Fat of Bama Pig

The depot differences between Subcutaneous Fat (SAF) and Visceral Fat (VAF) are critical for human well-being and disease processes in regard to energy metabolism and endocrine function. Miniature pigs (Sus scrofa) are ideal biomedical models for human energy metabolism and obesity due to the similarity of their lipid metabolism with that of humans. However, the regulation of differences in fat deposition and development remains unclear. In this study, the development of SAF and VAF was characterized and compared in Bama pig during postnatal development (infancy, puberty and adulthood), using RNA sequencing techniques (RNA-Seq). The transcriptome of SAF and VAF was profiled and isolated from 1-, 3- and 6 months-old pigs and identified 23,636 expressed genes, of which 1,165 genes were differentially expressed between the depots and/or developmental stages. Upregulated genes in SAF showed significant function and pathway enrichment in the central nervous system development, lipid metabolism, oxidation-reduction process and cell adhesion, whereas genes involved in the immune system, actin cytoskeleton organization, male gonad development and the hippo signaling pathway were preferentially expressed in VAF. Miner analysis of short time-series expression demonstrated that differentiation in gene expression patterns between the two depots corresponded to their distinct responses in sexual development, hormone signaling pathways, lipid metabolism and the hippo signaling pathway. Transcriptome analysis of SAF and VAF suggested that the depot differences in adipose tissue are not only related to lipid metabolism and endocrine function, but are closely associated with sexual development and organ size regulation.

Emerging Mechanisms of Growth and Patterning Regulation by Dachsous and Fat Protocadherins

Dachsous (Ds) and Fat are evolutionarily conserved cell adhesion molecules that play a critical role in development of multiple organ systems, where they coordinate tissue growth and morphogenesis. Much of our understanding of Ds-Fat signaling pathway comes from studies in Drosophila, where they initiate a signaling pathway that regulate growth by influencing Hippo signaling and morphogenesis by regulating Planar Cell Polarity (PCP). In this review, we discuss recent advances in our understanding of the mechanisms by which Ds-Fat signaling pathway regulates these critical developmental processes. Further, we discuss the progress in our understanding about how they function in mammals.

The wing imaginal disc

The Drosophila wing imaginal disc is a tissue of undifferentiated cells that are precursors of the wing and most of the notum of the adult fly. The wing disc first forms during embryogenesis from a cluster of ∼30 cells located in the second thoracic segment, which invaginate to form a sac-like structure. They undergo extensive proliferation during larval stages to form a mature larval wing disc of ∼35,000 cells. During this time, distinct cell fates are assigned to different regions, and the wing disc develops a complex morphology. Finally, during pupal stages the wing disc undergoes morphogenetic processes and then differentiates to form the adult wing and notum. While the bulk of the wing disc comprises epithelial cells, it also includes neurons and glia, and is associated with tracheal cells and muscle precursor cells. The relative simplicity and accessibility of the wing disc, combined with the wealth of genetic tools available in Drosophila, have combined to make it a premier system for identifying genes and deciphering systems that play crucial roles in animal development. Studies in wing imaginal discs have made key contributions to many areas of biology, including tissue patterning, signal transduction, growth control, regeneration, planar cell polarity, morphogenesis, and tissue mechanics.

The ZO-1 protein Polychaetoid as an upstream regulator of the Hippo pathway in Drosophila

The generation of a diversity of photoreceptor (PR) subtypes with different spectral sensitivities is essential for color vision in animals. In the Drosophila eye, the Hippo pathway has been implicated in blue- and green-sensitive PR subtype fate specification. Specifically, Hippo pathway activation promotes green-sensitive PR fate at the expense of blue-sensitive PRs. Here, using a sensitized triple heterozygote-based genetic screening approach, we report the identification of the single Drosophila zonula occludens-1 (ZO-1) protein Polychaetoid (Pyd) as a new regulator of the Hippo pathway during the blue- and green-sensitive PR subtype binary fate choice. We demonstrate that Pyd acts upstream of the core components and the upstream regulator Pez in the Hippo pathway. Furthermore, We found that Pyd represses the activity of Su(dx), a E3 ligase that negatively regulates Pez and can physically interact with Pyd, during PR subtype fate specification. Together, our results identify a new mechanism underlying the Hippo signaling pathway in post-mitotic neuronal fate specification.

The Hippo signaling pathway was originally discovered for its critical role in tissue growth and organ size control. Its evolutionarily conserved roles in various biological processes, including cell differentiation, stem cell regeneration and homeostasis, innate immune biology, as well as tumorigenesis, have been subsequently found in other species. During the development of the Drosophila eye, the Hippo pathway promotes green- and represses blue-sensitive photoreceptor (PR) subtype fate specification. Taking advantage of this binary PR fate choice, we screened Drosophila chromosomal deficiency lines to seek new regulators of the Hippo signaling pathway. We identified the Drosophila membrane-associated ZO-1 protein Pyd as an upstream regulator of the Hippo pathway to specify PR subtypes. Our results have demonstrated that Pyd represses Su(dx)’s activity in the Hippo pathway to specify PR subtypes. Our results demonstrate a new mechanism underlying the Hippo signaling pathway in post-mitotic neuronal fate specification.

Comparative transcriptome analysis revealed omnivorous adaptation of the small intestine of Melinae

As the main digestive organ, the small intestine plays a vital role in the digestion of animals. At present, most of the research on animal feeding habits focuses on carnivores and herbivores. However, the mechanism of feeding and digestion in omnivores remains unclear. This study aims to reveal the molecular basis of the omnivorous adaptive evolution of Melinae by comparing the transcriptome of the small intestines of Asian Badgers (Meles leucurus) and Northern Hog Badgers (Arctonyx albogularis). We obtained high-quality small intestinal transcriptome data from these two species. Key genes and signalling pathways were analysed through Gene Ontology (GO), Kyoto Encyclopedia of Genes and Genomes (KEGG) and other databases. Research has mainly found that orthologous genes related to six enzymes have undergone adaptive evolution. In addition, the study also found three digestion-related pathways (cGMP-PKG, cAMP, and Hippo). They are related to the digestion and absorption of nutrients, the secretion of intestinal fluids, and the transport of food through the small intestine, which may help omnivorous animals adapt to an omnivorous diet. Our study provides insight into the adaptation of Melinae to omnivores and affords a valuable transcriptome resource for future research.

A tale of two cell-fates: role of the Hippo signaling pathway and transcription factors in early lineage formation in mouse preimplantation embryos

In mammals, the first cell-fate decision occurs during preimplantation embryo development when the inner cell mass (ICM) and trophectoderm (TE) lineages are established. The ICM develops into the embryo proper, while the TE lineage forms the placenta. The underlying molecular mechanisms that govern lineage formation involve cell-to-cell interactions, cell polarization, cell signaling and transcriptional regulation. In this review, we will discuss the current understanding regarding the cellular and molecular events that regulate lineage formation in mouse preimplantation embryos with an emphasis on cell polarity and the Hippo signaling pathway. Moreover, we will provide an overview on some of the molecular tools that are used to manipulate the Hippo pathway and study cell-fate decisions in early embryos. Lastly, we will provide exciting future perspectives on transcriptional regulatory mechanisms that modulate the activity of the Hippo pathway in preimplantation embryos to ensure robust lineage segregation.

Emerging Principles in the Transcriptional Control by YAP and TAZ

Simple Summary

YAP and TAZ are transcriptional cofactors that integrate several upstream signals to generate context-dependent transcriptional responses. This requires extensive integration with epigenetic regulators and other transcription factors. The molecular and genomic characterization of YAP and TAZ nuclear function has broad implications both in physiological and pathological settings.

Abstract

Yes-associated protein (YAP) and TAZ are transcriptional cofactors that sit at the crossroad of several signaling pathways involved in cell growth and differentiation. As such, they play essential functions during embryonic development, regeneration, and, once deregulated, in cancer progression. In this review, we will revise the current literature and provide an overview of how YAP/TAZ control transcription. We will focus on data concerning the modulation of the basal transcriptional machinery, their ability to epigenetically remodel the enhancer–promoter landscape, and the mechanisms used to integrate transcriptional cues from multiple pathways. This reveals how YAP/TAZ activation in cancer cells leads to extensive transcriptional control that spans several hallmarks of cancer. The definition of the molecular mechanism of transcriptional control and the identification of the pathways regulated by YAP/TAZ may provide therapeutic opportunities for the effective treatment of YAP/TAZ-driven tumors.

The two sides of Hippo pathway in cancer

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

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

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

MicroRNAs Regulating Hippo-YAP Signaling in Liver Cancer

Liver cancer is one of the most common cancers worldwide, and its prevalence and mortality rate are increasing due to the lack of biomarkers and effective treatments. The Hippo signaling pathway has long been known to control liver size, and genetic depletion of Hippo kinases leads to liver cancer in mice through activation of the downstream effectors yes-associated protein (YAP) and transcriptional coactivator with PDZ-binding motif (TAZ). Both YAP and TAZ not only reprogram tumor cells but also alter the tumor microenvironment to exert carcinogenic effects. Therefore, understanding the mechanisms of YAP/TAZ-mediated liver tumorigenesis will help overcome liver cancer. For decades, small noncoding RNAs, microRNAs (miRNAs), have been reported to play critical roles in the pathogenesis of many cancers, including liver cancer. However, the interactions between miRNAs and Hippo-YAP/TAZ signaling in the liver are still largely unknown. Here, we review miRNAs that influence the proliferation, migration and apoptosis of tumor cells by modulating Hippo-YAP/TAZ signaling during hepatic tumorigenesis. Previous findings suggest that these miRNAs are potential biomarkers and therapeutic targets for the diagnosis, prognosis, and treatment of liver cancer.

Role of FAT1 in health and disease

FAT atypical cadherin 1 (FAT1), which encodes a protocadherin, is one of the most frequently mutated genes in human cancer. Over the past 20 years, the role of FAT1 in tissue growth and in the development of diseases has been extensively studied. There is definitive evidence that FAT1 serves a substantial role in the maintenance of organs and development, and its expression appears to be tissue-specific. FAT1 activates a variety of signaling pathways through protein-protein interactions, including the Wnt/β-catenin, Hippo and MAPK/ERK signaling pathways, which affect cell proliferation, migration and invasion. Abnormal FAT1 expression may lead to the development of tumors and may affect prognosis. Therefore, FAT1 may have potential in tumor therapy. The structural and functional changes mediated by FAT1, its tissue distribution and changes in FAT1 expression in human diseases are described in the present review, which provides further insight for understanding the role of FAT1 in development and disease.

Negative feedback couples Hippo pathway activation with Kibra degradation independent of Yorkie-mediated transcription

The Hippo (Hpo) pathway regulates tissue growth in many animals. Multiple upstream components promote Hpo pathway activity, but the organization of these different inputs, the degree of crosstalk between them, and whether they are regulated in a distinct manner is not well understood. Kibra (Kib) activates the Hpo pathway by recruiting the core Hpo kinase cassette to the apical cortex. Here, we show that the Hpo pathway downregulates Drosophila Kib levels independently of Yorkie-mediated transcription. We find that Hpo signaling complex formation promotes Kib degradation via SCF^Slimb-mediated ubiquitination, that this effect requires Merlin, Salvador, Hpo, and Warts, and that this mechanism functions independently of other upstream Hpo pathway activators. Moreover, Kib degradation appears patterned by differences in mechanical tension across the wing. We propose that Kib degradation mediated by Hpo pathway components and regulated by cytoskeletal tension serves to control Kib-driven Hpo pathway activation and ensure optimally scaled and patterned tissue growth.

Nuclear exclusion of YAP exacerbates podocyte apoptosis and disease progression in Adriamycin-induced focal segmental glomerulosclerosis

Focal segmental glomerulosclerosis (FSGS) is a chronic glomerular disease with poor clinical outcomes. Podocyte loss via apoptosis is one important mechanism underlying the pathogenesis of FSGS. Recently, Yes-associated-protein (YAP), a key downstream protein in the Hippo pathway, was identified as an activator for multiple gene transcriptional factors in the nucleus to control cell proliferation and apoptosis. To investigate the potential role of YAP in the progression of FSGS, we examined kidney samples from patients with minimal change disease or FSGS and found that increases in podocyte apoptosis is positively correlated with the cytoplasmic distribution of YAP in human FSGS. Utilizing an established mT/mG transgenic mouse model and primary cultured podocytes, we found that YAP was distributed uniformly in nucleus and cytoplasm in the podocytes of control animals. Adriamycin treatment induced gradual nuclear exclusion of YAP with enhanced phospho-YAP/YAP ratio, accompanied by the induction of podocyte apoptosis both in vivo and in vitro. Moreover, we used verteporfin to treat an Adriamycin-induced FSGS mouse model, and found YAP inhibition by verteporfin induced nuclear exclusion of YAP, thus increasing podocyte apoptosis and accelerating disease progression. Therefore, our findings suggest that YAP nuclear distribution and activation in podocytes is an important endogenous anti-apoptotic mechanism during the progression of FSGS.

Deficiency of large tumor suppressor kinase 1 causes congenital hearing loss associated with cochlear abnormalities in mice

Mammalian auditory hair cells are not spontaneously replaced. Their number and coordinated polarization are fairly well-maintained and both these factors might be essential for the cochlear amplifier. Cell cycle regulation has critical roles in regulating appropriate cell size and cell number. However, little is known about the physiological roles of the Hippo pathway, which is one of the most important signaling cascades that regulates cell growth, differentiation, and regenerative capacity in the cochlear sensory epithelium. Herein, we investigated the in vivo role of the large tumor suppressor 1 (LATS1), an essential kinase in the Hippo/yes-associated protein pathway, in the cochlea using the LATS1 knockout mice. LATS1 was expressed in hair cells and supporting cells. It was strongly expressed on the surface of the cuticular plate of the organ of Corti. We found that LATS1 knockout caused congenital hearing loss due to the irregular orientation and slightly reduced number of hair cells, whereas the number of supporting cells remained unchanged. On the surface of the hair cells, the kinocilium and stereocilia were dispersed during and after morphogenesis. However, the expression of the receptor-independent polarity regulators, such as Par3 or Gαi, was not affected. We concluded that LATS1 has an indispensable role in the maturation of mammalian auditory hair cells, but not in the development of the supporting cells, and thus, has a role in the hearing acquisition.

Identification and Analysis of MicroRNAs Associated with Wing Polyphenism in the Brown Planthopper, Nilaparvata lugens

Many insects are capable of developing two types of wings (i.e., wing polyphenism) to adapt to various environments. Though the roles of microRNAs (miRNAs) in regulating animal growth and development have been well studied, their potential roles in modulating wing polyphenism remain largely elusive. To identify wing polyphenism-related miRNAs, we isolated small RNAs from 1st to 5th instar nymphs of long-wing (LW) and short-wing (SW) strains of the brown planthopper (BPH), Nilaparvata lugens. Small RNA libraries were then constructed and sequenced, yielding 158 conserved and 96 novel miRNAs. Among these, 122 miRNAs were differentially expressed between the two BPH strains. Specifically, 47, 2, 27 and 41 miRNAs were more highly expressed in the 1st, 3rd, 4th and 5th instars, respectively, of the LW strain compared with the SW strain. In contrast, 47, 3, 29 and 25 miRNAs were more highly expressed in the 1st, 3rd, 4th and 5th instars, respectively, of the SW strain compared with the LW strain. Next, we predicted the targets of these miRNAs and carried out Gene Ontology and Kyoto Encyclopedia of Genes and Genomes pathway analysis. We found that a number of pathways might be involved in wing form determination, such as the insulin, MAPK, mTOR, FoxO and thyroid hormone signaling pathways and the thyroid hormone synthesis pathway. Thirty and 45 differentially expressed miRNAs targeted genes in the insulin signaling and insect hormone biosynthesis pathways, respectively, which are related to wing dimorphism. Among these miRNAs, Nlu-miR-14-3p, Nlu-miR-9a-5p and Nlu-miR-315-5p, were confirmed to interact with insulin receptors (NlInRs) in dual luciferase reporter assays. These discoveries are helpful for understanding the miRNA-mediated regulatory mechanism of wing polyphenism in BPHs and shed new light on how insects respond to environmental cues through developmental plasticity.

Chromosomal and molecular pathway alterations in the neuroendocrine carcinoma and adenocarcinoma components of gastric mixed neuroendocrine-nonneuroendocrine neoplasm

Gastric mixed adenoneuroendocrine carcinoma (MANEC) is a clinically aggressive subtype of mixed neuroendocrine-nonneuroendocrine neoplasm (MiNEN) with unclear clonal origin. In this study, we analyzed high-resolution copy number (CN) profiling data using the OncoScan CNV Assay in the neuroendocrine carcinoma (NEC) and adenocarcinoma components of eight MANECs. Some common CNVs, including the gain of CCNE1 (19q12) and the loss of FAT1 (4q35.2), were frequently detected in both components; these CNVs were verified by FISH, qPCR and immunohistochemistry staining assays in samples with sufficient material. The identification of common CNVs in both components supports the likelihood of single clonal origin of morphologically heterogeneous tumor cells and suggests several novel genetic events potentially involved in the development of gastric MANEC. We also detected and validated some CNVs and alterations specific for the NEC component, such as MAPK1 loss and MAPK signaling pathway alterations, which could contribute to the neuroendocrine differentiation of gastric MANEC. In addition, we found that the NEC component presented more CNVs and greater CN loss than the adenocarcinoma component (P = 0.007 and P = 0.004, respectively); the NEC components from different cases were not clustered in the hierarchical clustering analysis, indicating the marked genetic heterogenicity of the NEC component in gastric MANEC. In summary, this study describes the cytogenetic characteristics of each component of gastric MANEC, providing some clues for further studies on the development and progression of gastric MANEC as well as providing some potential therapeutic targets.

STRIPAK-PP2A regulates Hippo-Yorkie signaling to suppress retinal fate in the Drosophila eye disc peripodial epithelium

The specification of organs, tissues and cell types results from cell fate restrictions enacted by nuclear transcription factors under the control of conserved signaling pathways. The progenitor epithelium of the Drosophila compound eye, the eye imaginal disc, is a premier model for the study of such processes. Early in development, apposing cells of the eye disc are established as either retinal progenitors or support cells of the peripodial epithelium (PE), in a process whose genetic and mechanistic determinants are poorly understood. We have identified protein phosphatase 2A (PP2A), and specifically a STRIPAK-PP2A complex that includes the scaffolding and substrate-specificity components Cka, Strip and SLMAP, as a critical player in the retina-PE fate choice. We show that these factors suppress ectopic retina formation in the presumptive PE and do so via the Hippo signaling axis. STRIPAK-PP2A negatively regulates Hippo kinase, and consequently its substrate Warts, to release the transcriptional co-activator Yorkie into the nucleus. Thus, a modular higher-order PP2A complex refines the activity of this general phosphatase to act in a precise specification of cell fate.

The Hippo Pathway as a Driver of Select Human Cancers

The Hippo pathway regulates myriad biological processes in diverse species and is a key cancer signaling network in humans. Although Hippo has been linked to multiple aspects of cancer, its role in this disease is incompletely understood. Large-scale pan-cancer analyses of core Hippo pathway genes reveal that the pathway is mutated at a high frequency only in select human cancers, including malignant mesothelioma and meningioma. Hippo pathway deregulation is also enriched in squamous epithelial cancers. We discuss cancer-related functions of the Hippo pathway and potential explanations for the cancer-restricted mutation profile of core Hippo pathway genes. Greater understanding of Hippo pathway deregulation in cancers will be essential to guide the imminent use of Hippo-targeted therapies.

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.

Reduced SERCA Function Preferentially Affects Wnt Signaling by Retaining E-Cadherin in the Endoplasmic Reticulum

Calcium homeostasis in the lumen of the endoplasmic reticulum is required for correct processing and trafficking of transmembrane proteins, and defects in protein trafficking can impinge on cell signaling pathways. We show here that mutations in the endoplasmic reticulum calcium pump SERCA disrupt Wingless signaling by sequestering Armadillo/β-catenin away from the signaling pool. Armadillo remains bound to E-cadherin, which is retained in the endoplasmic reticulum when calcium levels there are reduced. Using hypomorphic and null SERCA alleles in combination with the loss of the plasma membrane calcium channel Orai allowed us to define three distinct thresholds of endoplasmic reticulum calcium. Wingless signaling is sensitive to even a small reduction, while Notch and Hippo signaling are disrupted at intermediate levels, and elimination of SERCA function results in apoptosis. These differential and opposing effects on three oncogenic signaling pathways may complicate the use of SERCA inhibitors as cancer therapeutics.

Suisse and Treisman describe genetic conditions that reduce calcium in the endoplasmic reticulum to three distinct extents. They find that Wnt signaling is more sensitive to changes in calcium levels than the Notch and Hippo pathways, potentially complicating the use of calcium pump inhibitors as cancer therapeutics.

A palmitoyltransferase Approximated gene Bm-app regulates wing development in Bombyx mori

The silkworm Bombyx mori is an important lepidopteran model insect in which many kinds of natural mutants have been identified. However, molecular mechanisms of most of these mutants remain to be explored. Here we report the identification of a gene Bm-app is responsible for the silkworm minute wing (mw) mutation which exhibits exceedingly small wings during pupal and adult stages. Compared with the wild type silkworm, relative messenger RNA expression of Bm-app is significantly decreased in the u11 mutant strain which shows mw phenotype. A 10 bp insertion in the putative promoter region of the Bm-app gene in mw mutant strain was identified and the dual luciferase assay revealed that this insertion decreased Bm-app promoter activity. Furthermore, clustered regularly interspaced short palindromic repeats/RNA-guided Cas9 nucleases-mediated depletion of the Bm-app induced similar wing defects which appeared in the mw mutant, demonstrating that Bm-app controls wing development in B. mori. Bm-app encodes a palmitoyltransferase and is responsible for the palmitoylation of selected cytoplasmic proteins, indicating that it is required for cell mitosis and growth during wing development. We also discuss the possibility that Bm-app regulates wing development through the Hippo signaling pathway in B. mori.

Hippo-Yap/Taz signaling: Complex network interactions and impact in epithelial cell behavior

The Hippo pathway has emerged as a crucial integrator of signals in biological events from development to adulthood and in diseases. Although extensively studied in Drosophila and in cell cultures, major gaps of knowledge still remain on how this pathway functions in mammalian systems. The pathway consists of a growing number of components, including core kinases and adaptor proteins, which control the subcellular localization of the transcriptional co-activators Yap and Taz through phosphorylation of serines at key sites. When localized to the nucleus, Yap/Taz interact with TEAD transcription factors to induce transcriptional programs of proliferation, stemness, and growth. In the cytoplasm, Yap/Taz interact with multiple pathways to regulate a variety of cellular functions or are targeted for degradation. The Hippo pathway receives cues from diverse intracellular and extracellular inputs, including growth factor and integrin signaling, polarity complexes, and cell-cell junctions. This review highlights the mechanisms of regulation of Yap/Taz nucleocytoplasmic shuttling and their implications for epithelial cell behavior using the lung as an intriguing example of this paradigm. This article is categorized under: Gene Expression and Transcriptional Hierarchies > Regulatory Mechanisms Signaling Pathways > Cell Fate Signaling Establishment of Spatial and Temporal Patterns > Cytoplasmic Localization.

Casein kinase 1 family proteins promote Slimb-dependent Expanded degradation

Hippo signalling integrates diverse stimuli related to epithelial architecture to regulate tissue growth and cell fate decisions. The Hippo kinase cascade represses the growth-promoting transcription co-activator Yorkie. The FERM protein Expanded is one of the main upstream Hippo signalling regulators in Drosophila as it promotes Hippo kinase signalling and directly inhibits Yorkie. To fulfil its function, Expanded is recruited to the plasma membrane by the polarity protein Crumbs. However, Crumbs-mediated recruitment also promotes Expanded turnover via a phosphodegron-mediated interaction with a Slimb/β-TrCP SCF E3 ligase complex. Here, we show that the Casein Kinase 1 (CKI) family is required for Expanded phosphorylation. CKI expression promotes Expanded phosphorylation and interaction with Slimb/β-TrCP. Conversely, CKI depletion in S2 cells impairs Expanded degradation downstream of Crumbs. In wing imaginal discs, CKI loss leads to elevated Expanded and Crumbs levels. Thus, phospho-dependent Expanded turnover ensures a tight coupling of Hippo pathway activity to epithelial architecture.

Hippo Signaling in Cancer: Lessons From Drosophila Models

Hippo pathway was initially identified through genetic screens for genes regulating organ size in fruitflies. Recent studies have highlighted the role of Hippo signaling as a key regulator of homeostasis, and in tumorigenesis. Hippo pathway is comprised of genes that act as tumor suppressor genes like hippo (hpo) and warts (wts), and oncogenes like yorkie (yki). YAP and TAZ are two related mammalian homologs of Drosophila Yki that act as effectors of the Hippo pathway. Hippo signaling deficiency can cause YAP- or TAZ-dependent oncogene addiction for cancer cells. YAP and TAZ are often activated in human malignant cancers. These transcriptional regulators may initiate tumorigenic changes in solid tumors by inducing cancer stem cells and proliferation, culminating in metastasis and chemo-resistance. Given the complex mechanisms (e.g., of the cancer microenvironment, and the extrinsic and intrinsic cues) that overpower YAP/TAZ inhibition, the molecular roles of the Hippo pathway in tumor growth and progression remain poorly defined. Here we review recent findings from studies in whole animal model organism like Drosophila on the role of Hippo signaling regarding its connection to inflammation, tumor microenvironment, and other oncogenic signaling in cancer growth and progression.

Transcriptional versus metabolic control of cell fitness during cell competition

The maintenance of tissue homeostasis and health relies on the efficient removal of damaged or otherwise suboptimal cells. One way this is achieved is through cell competition, a fitness quality control mechanism that eliminates cells that are less fit than their neighbours. Through this process, cell competition has been shown to play diverse roles in development and in the adult, including in homeostasis and tumour suppression. However, over the last few years it has also become apparent that certain oncogenic mutations can provide cells with a competitive advantage that promotes their expansion via the elimination of surrounding wild-type cells. Thus, understanding how this process is initiated and regulated will provide important insights with relevance to a number of different research areas. A key question in cell competition is what determines the competitive fitness of a cell. Here, we will review what is known about this question by focussing on two non-mutually exclusive possibilities; first, that the activity of a subset of transcription factors determines competitive fitness, and second, that the outcome of cell competition is determined by the relative cellular metabolic status.

Septate junction components control Drosophila hematopoiesis through the Hippo pathway

Hematopoiesis requires coordinated cell signals to control the proliferation and differentiation of progenitor cells. In Drosophila, blood progenitors, called prohemocytes, which are located in a hematopoietic organ called the lymph gland, are regulated by the Salvador-Warts-Hippo pathway. In epithelial cells, the Hippo pathway integrates diverse biological inputs, such as cell polarity and cell-cell contacts, but Drosophila blood cells lack the conspicuous polarity of epithelial cells. Here, we show that the septate-junction components Cora and NrxIV promote Hippo signaling in the lymph gland. Depletion of septate-junction components in hemocytes produces similar phenotypes to those observed in Hippo pathway mutants, including increased differentiation of immune cells. Our analysis places septate-junction components as upstream regulators of the Hippo pathway where they recruit Merlin to the membrane. Finally, we show that interactions of septate-junction components with the Hippo pathway are a key functional component of the cellular immune response following infection.

Early girl is a novel component of the Fat signaling pathway

The Drosophila protocadherins Dachsous and Fat regulate growth and tissue polarity by modulating the levels, membrane localization and polarity of the atypical myosin Dachs. Localization to the apical junctional membrane is critical for Dachs function, and the adapter protein Vamana/Dlish and palmitoyl transferase Approximated are required for Dachs membrane localization. However, how Dachs levels are regulated is poorly understood. Here we identify the early girl gene as playing an essential role in Fat signaling by limiting the levels of Dachs protein. early girl mutants display overgrowth of the wings and reduced cross vein spacing, hallmark features of mutations affecting Fat signaling. Genetic experiments reveal that it functions in parallel with Fat to regulate Dachs. early girl encodes an E3 ubiquitin ligase, physically interacts with Dachs, and regulates its protein stability. Concomitant loss of early girl and approximated results in accumulation of Dachs and Vamana in cytoplasmic punctae, suggesting that it also regulates their trafficking to the apical membrane. Our findings establish a crucial role for early girl in Fat signaling, involving regulation of Dachs and Vamana, two key downstream effectors of this pathway.

During development, organs grow to achieve a consistent final size. The evolutionarily conserved Hippo signaling network plays a central role in organ size control, and when dysregulated can be associated with cancer and other diseases. Fat signaling is one of several upstream pathways that impinge on Hippo signaling to regulate organ growth. We describe here identification of the Drosophila early girl gene as a new component of the Fat signaling pathway. We show that Early girl controls Fat signaling by regulating the levels of the Dachs protein. However Early girl differs from other Fat signaling regulators in that it doesn’t influence planar cell polarity or control the polarity of Dachs localization. early girl encodes a conserved protein that is predicted to influence protein stability, and it can physically associate with Dachs. We also discovered that Early girl acts together with another protein, called Approximated, to regulate the sub-cellular localization of Dachs and a Dachs-interacting protein called Vamana. Altogether, our observations establish Early girl as an essential component of Fat signaling that acts to regulate the levels and localization of Dachs and Vamana.

Fat-regulated adaptor protein Dlish binds the growth suppressor Expanded and controls its stability and ubiquitination

To regulate the growth and size of organs, cells can use information from their neighbors to modify intracellular mediators of cell proliferation. The intracellular Hippo pathway is a widely utilized nexus for growth control in animals, but its regulation by extracellular signals is not fully understood. We here identify a pathway that regulates organ size in Drosophila, triggered by the transmembrane receptor, the giant protocadherin Fat. We show that the Fat-regulated SH3 domain adaptor protein Dlish binds to and reduces the stability of the growth suppressor Expanded, a known regulator of the Hippo pathway. The destabilization of Expanded by Dlish works in parallel to a previously established pathway in which Dlish increases levels of the growth-stimulating protein Dachs.

The Drosophila protocadherin Fat controls organ size through the Hippo pathway, but the biochemical links to the Hippo pathway components are still poorly defined. We previously identified Dlish, an SH3 domain protein that physically interacts with Fat and the type XX myosin Dachs, and showed that Fat’s regulation of Dlish levels and activity helps limit Dachs-mediated inhibition of Hippo pathway activity. We here characterize a parallel growth control pathway downstream of Fat and Dlish. Using immunoprecipitation and mass spectrometry to search for Dlish partners, we find that Dlish binds the FERM domain growth repressor Expanded (Ex); Dlish SH3 domains directly bind sites in the Ex C terminus. We further show that, in vivo, Dlish reduces the subapical accumulation of Ex, and that loss of Dlish blocks the destabilization of Ex caused by loss of Fat. Moreover, Dlish can bind the F-box E3 ubiquitin ligase Slimb and promote Slimb-mediated ubiquitination of Expanded in vitro. Both the in vitro and in vivo effects of Dlish on Ex require Slimb, strongly suggesting that Dlish destabilizes Ex by helping recruit Slimb-containing E3 ubiquitin ligase complexes to Ex.

Drosophila Genetics: The Power of Genetic Mosaic Approaches

Drosophila melanogaster has been a central player in the discovery of the Hippo pathway and in understanding its in vivo functions. From a technique standpoint, the Flp-FRT system for the generation of genetic mosaics has been a principle tool. It has broadly been used in the discovery of Hippo pathway members in mutagenesis screens, in the analysis of target gene expression, and in genetic epistasis. Here we briefly introduce this tool, summarize its use in the Hippo pathway field, and provide a protocol for the generation of Flp-FRT clones in imaginal discs with dissection and staining for reporter gene expression to characterize candidate Hippo pathway genes.

Evolution and Regulation of Limb Regeneration in Arthropods

Regeneration has fascinated both scientists and non-scientists for centuries. Many organisms can regenerate, and arthropod limbs are no exception although their ability to regenerate is a product shaped by natural and sexual selection. Recent studies have begun to uncover cellular and molecular processes underlying limb regeneration in several arthropod species. Here we argue that an evo-devo approach to the study of arthropod limb regeneration is needed to understand aspects of limb regeneration that are conserved and divergent. In particular, we argue that limbs of different species are comprised of cells at distinct stages of differentiation at the time of limb loss and therefore provide insights into regeneration involving both stem cell-like cells/precursor cells and differentiated cells. In addition, we review recent studies that demonstrate how limb regeneration impacts the development of the whole organism and argue that studies on the link between local tissue damage and the rest of the body should provide insights into the integrative nature of development. Molecular studies on limb regeneration are only beginning to take off, but comparative studies on the mechanisms of limb regeneration across various taxa should not only yield interesting insights into development but also answer how this remarkable ability evolved across arthropods and beyond.

The Power of Drosophila Genetics: The Discovery of the Hippo Pathway

The Hippo Pathway comprises a vast network of components that integrate diverse signals including mechanical cues and cell surface or cell-surface-associated molecules to define cellular outputs of growth, proliferation, cell fate, and cell survival on both the cellular and tissue level. Because of the importance of the regulators, core components, and targets of this pathway in human health and disease, individual components were often identified by efforts in mammalian models or for a role in a specific process such as stress response or cell death. However, multiple components were originally discovered in the Drosophila system, and the breakthrough of conceiving that these components worked together in a signaling pathway came from a series of Drosophila genetic screens and fundamental genetic and phenotypic characterization efforts. In this chapter, we will review the original discoveries leading to the conceptual framework of these components as a tumor suppressor network. We will review chronologically the early efforts that established our initial understanding of the core machinery that then launched the growing and vibrant field to be discussed throughout later chapters of this book.

Loss of the FAT1 Tumor Suppressor Promotes Resistance to CDK4/6 Inhibitors via the Hippo Pathway

Cyclin dependent kinase 4/6 (CDK4/6) inhibitors (CDK4/6i) are effective in breast cancer; however, drug resistance is frequently encountered and poorly understood. We conducted a genomic analysis of 348 estrogen receptor-positive (ER⁺) breast cancers treated with CDK4/6i and identified loss-of-function mutations affecting FAT1 and RB1 linked to drug resistance. FAT1 loss led to marked elevations in CDK6, the suppression of which restored sensitivity to CDK4/6i. The induction of CDK6 was mediated by the Hippo pathway with accumulation of YAP and TAZ transcription factors on the CDK6 promoter. Genomic alterations in other Hippo pathway components were also found to promote CDK4/6i resistance. These findings uncover a tumor suppressor function of Hippo signaling in ER⁺ breast cancer and establish FAT1 loss as a mechanism of resistance to CDK4/6i.