Nerfin-1 represses transcriptional output of Hippo signaling in cell competition

The Drosophila Hippo pathway transcription factor Scalloped and its co-factors alter each other's chromatin binding dynamics and transcription in vivo

The Hippo pathway is an important regulator of organ growth and cell fate. The major mechanism by which Hippo is known to control transcription is by dictating the nucleo-cytoplasmic shuttling rate of Yorkie, a transcription co-activator, which promotes transcription with the DNA binding protein Scalloped. The nuclear biophysical behavior of Yorkie and Scalloped, and whether this is regulated by the Hippo pathway, remains unexplored. Using multiple live-imaging modalities on Drosophila tissues, we found that Scalloped interacts with DNA on a broad range of timescales, and enrichment of Scalloped at sites of active transcription is mediated by longer DNA dwell times. Further, Yorkie increased Scalloped's DNA dwell time, whereas the repressors Nervous fingers 1 (Nerfin-1) and Tondu-domain-containing growth inhibitor (Tgi) decreased it. Therefore, the Hippo pathway influences transcription not only by controlling nuclear abundance of Yorkie but also by modifying the DNA binding kinetics of the transcription factor Scalloped.

Therapeutic targeting of TEAD transcription factors in cancer

The Hippo signaling pathway inhibits the activity of the oncogenic YAP (Yes-associated protein)/TAZ (transcriptional co-activator with PDZ-binding motif)-TEAD (TEA/ATTS domain) transcriptional complex. In cancers, inactivating mutations in upstream Hippo components and/or enhanced activity of YAP/TAZ and TEAD have been observed. The activity of this transcriptional complex can be effectively inhibited by targeting the TEAD family of transcription factors. The development of TEAD inhibitors has been driven by the discovery that TEAD has druggable hydrophobic pockets, and is currently at the clinical development stage. Three small molecule TEAD inhibitors are currently being tested in Phase I clinical trials. In this review, we highlight the role of TEADs in cancer, discuss various avenues through which TEAD activity can be inhibited, and outline the opportunities for the administration of TEAD inhibitors.

Transcriptome-wide analysis of cellular immune response stimulated by nuclear input of different down syndrome cell adhesion molecule intracellular domains

In arthropods, Dscam (Down syndrome cell adhesion molecule) produces multiple pathogen specific receptors via immune responsive alternative splicing, generating molecular complexity analogous to vertebrate antibodies. Fewer isoforms are produced by the exons encoding Dscam's intracellular domain (ICD); therefore, the present study aimed to determine the transcriptional response of Eriocheir sinensis to Dscam ICDs. In the group overexpressing all cytoplasmic tail exons (ICD-FL), 1401 differentially expressed genes (DEGs) were identified; overexpressed of ICD constructs lacking exon-35 (ICD-△35) identified 413 DEGs; and overexpression of ICD constructs lacking exon-35 and exon-36 (ICD-△35 + 36) identified 22 DEGs. The DEGs were enriched in immunity and metabolism-related pathways. The expression of selected genes was confirmed using quantitative real-time reverse transcription PCR. The transcriptomes of Drosophila S2 cells overexpressing different ICDs were then determined. We identified key immune, metabolic, and cell proliferation-regulated genes and gene networks, providing insights into the membrane-to-nuclear signaling pathway of Dscam.

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.

Snail regulates Hippo signalling-mediated cell proliferation and tissue growth in Drosophila

Snail (Sna) plays a pivotal role in epithelia-mesenchymal transition and cancer metastasis, yet its functions in normal tissue development remain elusive. Here, using Drosophila as a model organism, we identified Sna as an essential regulator of Hippo signalling-mediated cell proliferation and tissue growth. First, Sna is necessary and sufficient for impaired Hippo signalling-induced cell proliferation and tissue overgrowth. Second, Sna is necessary and sufficient for the expression of Hippo pathway target genes. Third, genetic epistasis data indicate Sna acts downstream of Yki in the Hippo signalling. Finally, Sna is physiologically required for tissue growth in normal development. Mechanistically, Yki activates the transcription of sna, whose protein product binds to Scalloped (Sd) and promotes Sd-dependent cell proliferation. Thus, this study uncovered a previously unknown physiological function of Sna in normal tissue development and revealed the underlying mechanism by which Sna modulates Hippo signalling-mediated cell proliferation and tissue growth.

Reciprocal YAP1 loss and INSM1 expression in neuroendocrine prostate cancer

Neuroendocrine prostate cancer (NEPC) is a rare but aggressive histologic variant of prostate cancer that responds poorly to androgen deprivation therapy. Hybrid NEPC-adenocarcinoma (AdCa) tumors are common, often eluding accurate pathologic diagnosis and requiring ancillary markers for classification. We recently performed an outlier-based meta-analysis across a number of independent gene expression microarray datasets to identify novel markers that differentiate NEPC from AdCa, including up-regulation of insulinoma-associated protein 1 (INSM1) and loss of Yes-associated protein 1 (YAP1). Here, using diverse cancer gene expression datasets, we show that Hippo pathway-related genes, including YAP1, are among the top down-regulated gene sets with expression of the neuroendocrine transcription factors, including INSM1. In prostate cancer cell lines, transgenic mouse models, and human prostate tumor cohorts, we confirm that YAP1 RNA and YAP1 protein expression are silenced in NEPC and demonstrate that the inverse correlation of INSM1 and YAP1 expression helps to distinguish AdCa from NEPC. Mechanistically, we find that YAP1 loss in NEPC may help to maintain INSM1 expression in prostate cancer cell lines and we further demonstrate that YAP1 silencing likely occurs epigenetically, via CpG hypermethylation near its transcriptional start site. Taken together, these data nominate two additional markers to distinguish NEPC from AdCa and add to data from other tumor types suggesting that Hippo signaling is tightly reciprocally regulated with neuroendocrine transcription factor expression. © 2021 The Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Application of CRISPR screens to investigate mammalian cell competition

Cell competition is defined as the context-dependent elimination of cells that is mediated by intercellular communication, such as paracrine or contact-dependent cell signaling, and/or mechanical stresses. It is considered to be a quality control mechanism that facilitates the removal of suboptimal cells from both adult and embryonic tissues. Cell competition, however, can also be hijacked by transformed cells to acquire a 'super-competitor' status and outcompete the normal epithelium to establish a precancerous field. To date, many genetic drivers of cell competition have been identified predominately through studies in Drosophila. Especially during the last couple of years, ethylmethanesulfonate-based genetic screens have been instrumental to our understanding of the molecular regulators behind some of the most common competition mechanisms in Drosophila, namely competition due to impaired ribosomal function (or anabolism) and mechanical sensitivity. Despite recent findings in Drosophila and in mammalian models of cell competition, the drivers of mammalian cell competition remain largely elusive. Since the discovery of CRISPR/Cas9, its use in functional genomics has been indispensable to uncover novel cancer vulnerabilities. We envision that CRISPR/Cas9 screens will enable systematic, genome-scale probing of mammalian cell competition to discover novel mutations that not only trigger cell competition but also identify novel molecular components that are essential for the recognition and elimination of less fit cells. In this review, we summarize recent contributions that further our understanding of the molecular mechanisms of cell competition by genetic screening in Drosophila, and provide our perspective on how similar and novel screening strategies made possible by whole-genome CRISPR/Cas9 screening can advance our understanding of mammalian cell competition in the future.

Transcriptomic and proteomic signatures of stemness and differentiation in the colon crypt

Intestinal stem cells are non-quiescent, dividing epithelial cells that rapidly differentiate into progenitor cells of the absorptive and secretory cell lineages. The kinetics of this process is rapid such that the epithelium is replaced weekly. To determine how the transcriptome and proteome keep pace with rapid differentiation, we developed a new cell sorting method to purify mouse colon epithelial cells. Here we show that alternative mRNA splicing and polyadenylation dominate changes in the transcriptome as stem cells differentiate into progenitors. In contrast, as progenitors differentiate into mature cell types, changes in mRNA levels dominate the transcriptome. RNA processing targets regulators of cell cycle, RNA, cell adhesion, SUMOylation, and Wnt and Notch signaling. Additionally, global proteome profiling detected >2,800 proteins and revealed RNA:protein patterns of abundance and correlation. Paired together, these data highlight new potentials for autocrine and feedback regulation and provide new insights into cell state transitions in the crypt.

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.

Pits and CtBP Control Tissue Growth in Drosophila melanogaster with the Hippo Pathway Transcription Repressor Tgi

The Hippo pathway is an evolutionarily conserved signaling network that regulates organ size, cell fate, and tumorigenesis. In the context of organ size control, the pathway incorporates a large variety of cellular cues, such as cell polarity and adhesion, into an integrated transcriptional response. The central Hippo signaling effector is the transcriptional coactivator Yorkie, which controls gene expression in partnership with different transcription factors, most notably Scalloped. When it is not activated by Yorkie, Scalloped can act as a repressor of transcription, at least in part due to its interaction with the corepressor protein Tgi. The mechanism by which Tgi represses transcription is incompletely understood, and therefore we sought to identify proteins that potentially operate together with Tgi. Using an affinity purification and mass-spectrometry approach we identified Pits and CtBP as Tgi-interacting proteins, both of which have been linked to transcriptional repression. Both Pits and CtBP were required for Tgi to suppress the growth of the Drosophila melanogaster eye and CtBP loss suppressed the undergrowth of yorkie mutant eye tissue. Furthermore, as reported previously for Tgi, overexpression of Pits repressed transcription of Hippo pathway target genes. These findings suggest that Tgi might operate together with Pits and CtBP to repress transcription of genes that normally promote tissue growth. The human orthologs of Tgi, CtBP, and Pits (VGLL4, CTBP2, and IRF2BP2) have previously been shown to physically and functionally interact to control transcription, implying that the mechanism by which these proteins control transcriptional repression is conserved throughout evolution.

The regulation of Yorkie, YAP and TAZ: new insights into the Hippo pathway

The Hippo pathway is a highly conserved signalling pathway that regulates multiple biological processes, including organ size control and cell fate. Since its discovery, genetic and biochemical studies have elucidated several key signalling steps important for pathway activation and deactivation. In recent years, technical advances in microscopy and genome modification have allowed new insights into Hippo signalling to be revealed. These studies have highlighted that the nuclear-cytoplasmic shuttling behaviour of the Hippo pathway transcriptional co-activators Yorkie, YAP and TAZ is far more dynamic than previously appreciated, and YAP and TAZ are also regulated by liquid-liquid phase separation. Here, we review our current understanding of Yorkie, YAP and TAZ regulation, with a focus on recent microscopy-based studies.

BTG4 is A Novel p53 Target Gene That Inhibits Cell Growth and Induces Apoptosis

BTG4 is the last cloned and poorly studied member of BTG/Tob family. Studies have suggested that BTG4 is critical for the degradation of maternal mRNAs in mice during the process of maternal-to-zygotic transition, and downregulated in cancers, such as gastric cancer. However, the regulatory mechanism of BTG4 and its function in cancers remain elusive. In this study, we have for the first time identified the promoter region of the human BTG4 gene. Serial luciferase reporter assay demonstrated that the core promoter of BTG4 is mainly located within the 388 bp region near its transcription initiation site. Transcription factor binding site analysis revealed that the BTG4 promoter contains binding sites for canonical transcription factors, such as Sp1, whereas its first intron contains two overlapped consensus p53 binding sites. However, overexpression of Sp1 has negligible effects on BTG4 promoter activity, and site-directed mutagenesis assay further suggested that Sp1 is not a critical transcription factor for the transcriptional regulation of BTG4. Of note, luciferase assay revealed that one of the intronic p53 binding sites is highly responsive to p53. Both exogenous p53 overexpression and adriamycin-mediated endogenous p53 activation result in the transcriptional upregulation of BTG4. In addition, BTG4 is downregulated in lung and colorectal cancers, and overexpression of BTG4 inhibits cell growth and induces apoptosis in cancer cells. Taken together, our results strongly suggest that BTG4 is a novel p53-regulated gene and probably functions as a tumor suppressor in lung and colorectal cancers.

Identification of Quinolinols as Activators of TEAD-Dependent Transcription

The transcriptional co-regulators YAP (Yes-associated protein) and TAZ (transcriptional coactivator with PDZ-binding motif) are the vertebrate downstream effectors of the Hippo signaling pathway that controls various physiological and pathological processes. YAP and TAZ pair with the TEAD (TEA domain) family of transcription factors to initiate transcription. We previously identified a tractable pocket in TEADs, which has been physiologically shown to bind palmitate. Herein, a TEAD-palmitate interaction screen was developed to select small molecules occupying the palmitate-binding pocket (PBP) of TEADs. We show that quinolinols were TEAD-binding compounds that augment YAP/TAZ-TEAD activity, which was verified using TEAD reporter assay, RT-qPCR, and RNA-Seq analyses. Structure-activity relationship investigations uncovered the quinolinol substituents that are necessary for TEAD activation. We reveal a novel mechanism where quinolinols stabilize YAP/TAZ protein levels by occupying the PBP. The enhancement of YAP activity by quinolinols accelerates the in vivo wound closure in a mouse wound-healing model. Although small molecules that occupy the PBP have been shown to inhibit YAP/TAZ-TEAD activity, leveraging PBP to activate TEADs is a novel approach.

The Hippo Signaling Pathway in Development and Disease

The Hippo signaling pathway regulates diverse physiological processes, and its dysfunction has been implicated in an increasing number of human diseases, including cancer. Here, we provide an updated review of the Hippo pathway; discuss its roles in development, homeostasis, regeneration, and diseases; and highlight outstanding questions for future investigation and opportunities for Hippo-targeted therapies.

Opposing transcriptional and post-transcriptional roles for Scalloped in binary Hippo-dependent neural fate decisions

The Hippo tumor suppressor pathway plays many fundamental cell biological roles during animal development. Two central players in controlling Hippo-dependent gene expression are the TEAD transcription factor Scalloped (Sd) and its transcriptional co-activator Yorkie (Yki). Hippo signaling phosphorylates Yki, thereby blocking Yki-dependent transcriptional control. In post-mitotic Drosophila photoreceptors, a bistable negative feedback loop forms between the Hippo-dependent kinase Warts/Lats and Yki to lock in green vs blue-sensitive neuronal subtype choices, respectively. Previous experiments indicate that sd and yki mutants phenocopy each other's functions, both being required for promoting the expression of the blue photoreceptor fate determinant melted (melt) and the blue-sensitive opsin Rh5. Here, we demonstrate that Sd ensures the robustness of this neuronal fate decision via multiple antagonistic gene regulatory roles. In Hippo-positive (green) photoreceptors, Sd directly represses both melt and Rh5 gene expression through defined TEAD binding sites, a mechanism that is antagonized by Yki in Hippo-negative (blue) cells. Additionally, in blue photoreceptors, Sd is required to promote the translation of the Rh5 protein through a 3'UTR-dependent and microRNA-mediated process. Together, these studies reveal that Sd can drive context-dependent cell fate decisions through opposing transcriptional and post-transcriptional mechanisms.

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.