Warts and Yorkie mediate intestinal regeneration by influencing stem cell proliferation

Drosophila AHR limits tumor growth and stem cell proliferation in the intestine

Background

The aryl hydrocarbon receptor (AHR) plays important roles in intestinal homeostasis, limiting tumour growth and promoting differentiation in the intestinal epithelium. Spineless, the Drosophila homolog of AHR, has only been studied in the context of development but not in the adult intestine.

Methods

The role of Spineless in the Drosophila midgut was studied by overexpression or inactivation of Spineless in infection and tumour models and RNA sequencing of sorted midgut progenitor cells.

Results

We show that spineless is upregulated in the adult intestinal epithelium after infection with Pseudomonas entomophila ( P. e.). Spineless inactivation increased stem cell proliferation following infection-induced injury. Spineless overexpression limited intestinal stem cell proliferation and reduced survival after infection. In two tumour models, using either Notch RNAi or constitutively active Yorkie, Spineless suppressed tumour growth and doubled the lifespan of tumour-bearing flies. At the transcriptional level it reversed the gene expression changes induced in Yorkie tumours, counteracting cell proliferation and altered metabolism.

Conclusions

These findings demonstrate a new role for Spineless in the adult Drosophila midgut and highlight the evolutionarily conserved functions of AHR/Spineless in the control of proliferation and differentiation of the intestinal epithelium.

Drosophila model systems reveal intestinal stem cells as key players in aging

The intestines play important roles in responding immediately and dynamically to food intake, environmental stress, and metabolic dysfunction, and they are involved in various human diseases and aging. A key part of their function is governed by intestinal stem cells (ISCs); therefore, understanding ISCs is vital. Dysregulation of ISC activity, which is influenced by various cell signaling pathways and environmental signals, can lead to inflammatory responses, tissue damage, and increased cancer susceptibility. Aging exacerbates these dynamics and affects ISC function and tissue elasticity. Additionally, proliferation and differentiation profoundly affect ISC behavior and gut health, highlighting the complex interplay between environmental factors and gut homeostasis. Drosophila models help us understand the complex regulatory networks in the gut, providing valuable insights into disease mechanisms and therapeutic strategies targeting human intestinal diseases.

Cell-death induced immune response and coagulopathy promote cachexia in Drosophila

Tumors can exert a far-reaching influence on the body, triggering systemic responses that contribute to debilitating conditions like cancer cachexia. To characterize the mechanisms underlying tumor-host interactions, we utilized a BioID-based proximity labeling method to identify proteins secreted by Ykiact adult Drosophila gut tumors into the bloodstream/hemolymph. Among the major proteins identified are coagulation and immune-responsive factors that contribute to the systemic wasting phenotypes associated with Ykiact tumors. The effect of innate immunity factors is mediated by NFκB transcription factors Relish, dorsal, and Dif, which in turn upregulate the expression of the cachectic factors Pvf1, Impl2, and Upd3. In addition, Ykiact tumors secrete Eiger, a TNF-alpha homolog, which activates the JNK signaling pathway in neighboring non-tumor cells, leading to cell death. The release of damage-associated molecular patterns (DAMPs) from these dying cells presumably amplifies the inflammatory response, exacerbating systemic wasting. Targeting the inflammatory response, the JNK pathway, or the production of cachectic factors could potentially alleviate the debilitating effects of cancer cachexia.

What a tangled web we weave: crosstalk between JAK-STAT and other signalling pathways during development in Drosophila

The Janus kinase-signal transducer and activator of transcription (JAK-STAT) signalling pathway is a key player in animal development and physiology. Although it functions in a variety of processes, the net output of JAK-STAT signalling depends on its spatiotemporal activation, as well as extensive crosstalk with other signalling pathways. Drosophila, with its relatively simple signal transduction pathways and plethora of genetic analysis tools, is an ideal system for dissecting JAK-STAT signalling interactions. In this review, we explore studies in Drosophila revealing that JAK-STAT signalling lies at the nexus of a complex network of interlinked pathways, including epidermal growth factor receptor (EGFR), c-Jun N-terminal kinase (JNK), Notch, Insulin, Hippo, bone morphogenetic protein (BMP), Hedgehog (Hh) and Wingless (Wg). These pathways can synergise with or antagonise one another to produce a variety of outcomes. Given the conserved nature of signal transduction pathways, we conclude with our perspective on the implication of JAK-STAT signalling dysregulation in human diseases, and how studies in Drosophila have the potential to inform and influence clinical research.

A feedback loop between Paxillin and Yorkie sustains Drosophila intestinal homeostasis and regeneration

Balanced self-renewal and differentiation of stem cells are crucial for maintaining tissue homeostasis, but the underlying mechanisms of this process remain poorly understood. Here, from an RNA interference (RNAi) screen in adult Drosophila intestinal stem cells (ISCs), we identify a factor, Pax, which is orthologous to mammalian PXN, coordinates the proliferation and differentiation of ISCs during both normal homeostasis and injury-induced midgut regeneration in Drosophila. Loss of Pax promotes ISC proliferation while suppressing its differentiation into absorptive enterocytes (ECs). Mechanistically, our findings demonstrate that Pax is a conserved target gene of the Hippo signaling pathway in both Drosophila and mammals. Subsequent investigations have revealed Pax interacts with Yki and enhances its cytoplasmic localization, thereby establishing a feedback regulatory mechanism that attenuates Yki activity and ultimately inhibits ISCs proliferation. Additionally, Pax induces the differentiation of ISCs into ECs by activating Notch expression, thus facilitating the differentiation process. Overall, our study highlights Pax as a pivotal component of the Hippo and Notch pathways in regulating midgut homeostasis, shedding light on this growth-related pathway in tissue maintenance and intestinal function.

Sex-dimorphic tumor growth is regulated by tumor microenvironmental and systemic signals

Tumor growth and progression involve coordinated regulation by internal, microenvironmental, and systemic signals and often display conspicuous sexual dimorphism. The mechanisms governing the integration and coordination of these signals, along with their sex-based differences, remain largely unknown. Using a Drosophila tumor model originating from nonreproductive tissue, we show that female-biased tumor growth involves multifaceted communications among tumor cells, hemocytes, and neuroendocrine insulin-producing cells (IPCs). Notch-active tumor cells recruit hemocytes carrying the tumor necrosis factor-α (TNF-α) homolog Eiger to the tumor microenvironment (TME), activating the c-Jun N-terminal kinase (JNK) pathway in tumor cells, instigating the sexually dimorphic up-regulation of cytokine Unpaired 2 (Upd2). Upd2, in turn, exerts a distal influence by modulating the release of a Drosophila insulin-like peptide (Dilp2) from IPCs. Dilp2 then activates the insulin signaling in the tumor, thereby fostering sexual-dimorphic tumor growth. Together, these findings reveal a relay mechanism involving the TME and systemic signals that collectively control the sexual dimorphism of tumor growth.

Regulation of the Intestinal Stem Cell Pool and Proliferation in Drosophila

Understanding the regulation of somatic stem cells, both during homeostasis and in response to environmental challenges like injury, infection, chemical exposure, and nutritional changes, is critical because their dysregulation can result in tissue degeneration or tumorigenesis. The use of models such as the Drosophila and mammalian adult intestines offers valuable insights into tissue homeostasis and regeneration, advancing our knowledge of stem cell biology and cancer development. This review highlights significant findings from recent studies, unveiling the molecular mechanisms that govern self-renewal, proliferation, differentiation, and regeneration of intestinal stem cells (ISCs). These insights not only enhance our understanding of normal tissue maintenance but also provide critical perspectives on how ISC dysfunction can lead to pathological conditions such as colorectal cancer (CRC).

Leptin- and cytokine-like unpaired signaling in Drosophila

Animals have evolved a multitude of signaling pathways that enable them to orchestrate diverse physiological processes to tightly regulate systemic homeostasis. This signaling is mediated by various families of peptide hormones and cytokines that are conserved across the animal kingdom. In this review, we primarily focus on the unpaired (Upd) family of proteins in Drosophila which are evolutionarily related to mammalian leptin and the cytokine interleukin 6. We summarize expression patterns of Upd in Drosophila and discuss the parallels in structure, signaling pathway, and functions between Upd and their mammalian counterparts. In particular, we focus on the roles of Upd in governing metabolic homeostasis, growth and development, and immune responses. We aim to stimulate future studies on leptin-like signaling in other phyla which can help bridge the evolutionary gap between insect Upd and vertebrate leptin and cytokines like interleukin 6.

Epithelial Ca2+ waves triggered by enteric neurons heal the gut

A fundamental and unresolved question in regenerative biology is how tissues return to homeostasis after injury. Answering this question is essential for understanding the etiology of chronic disorders such as inflammatory bowel diseases and cancer. We used the Drosophila midgut to investigate this question and discovered that during regeneration a subpopulation of cholinergic enteric neurons triggers Ca2+ currents among enterocytes to promote return of the epithelium to homeostasis. Specifically, we found that down-regulation of the cholinergic enzyme Acetylcholinesterase in the epithelium enables acetylcholine from defined enteric neurons, referred as ARCENs, to activate nicotinic receptors in enterocytes found near ARCEN-innervations. This activation triggers high Ca2+ influx that spreads in the epithelium through Inx2/Inx7 gap junctions promoting enterocyte maturation followed by reduction of proliferation and inflammation. Disrupting this process causes chronic injury consisting of ion imbalance, Yki activation and increase of inflammatory cytokines together with hyperplasia, reminiscent of inflammatory bowel diseases. Altogether, we found that during gut regeneration the conserved cholinergic pathway facilitates epithelial Ca2+ waves that heal the intestinal epithelium. Our findings demonstrate nerve- and bioelectric-dependent intestinal regeneration which advance the current understanding of how a tissue returns to its homeostatic state after injury and could ultimately help existing therapeutics.

Damage-induced regeneration of the intestinal stem cell pool through enteroblast mitosis in the Drosophila midgut

Many adult tissues and organs including the intestine rely on resident stem cells to maintain homeostasis and regeneration. In mammals, the progenies of intestinal stem cells (ISCs) can dedifferentiate to generate ISCs upon ablation of resident stem cells. However, whether and how mature tissue cells generate ISCs under physiological conditions remains unknown. Here, we show that infection of the Drosophila melanogaster intestine with pathogenic bacteria induces entry of enteroblasts (EBs), which are ISC progenies, into the mitotic cycle through upregulation of epidermal growth factor receptor (EGFR)-Ras signaling. We also show that ectopic activation of EGFR-Ras signaling in EBs is sufficient to drive enteroblast mitosis cell autonomously. Furthermore, we find that the dividing enteroblasts do not gain ISC identity as a prerequisite to divide, and the regenerative ISCs are produced through EB mitosis. Taken together, our work uncovers a new role for EGFR-Ras signaling in driving EB mitosis and replenishing the ISC pool during fly intestinal regeneration, which may have important implications for tissue homeostasis and tumorigenesis in vertebrates.

Tiny Drosophila intestinal stem cells, big power

The signaling pathways are highly conserved between Drosophila and mammals concerning intestinal development, regeneration, and disease. The powerful genetic tools of Drosophila make it a valuable and convenient alternative to answer basic biological questions that can not be addressed using mammalian models. In this review, we discuss recent advances in how we use fly midgut to answer the following key questions: (1) How intestine stem cell niches are established; (2) which factors control asymmetric division of stem cells; (3) how intestinal cells interact with environmental factors, such as tissue damage, microbiota, and diet; (4) how to screen aging/cancer-related factors or drugs by fly intestine stem cells.

Metabolic control of progenitor cell propagation during Drosophila tracheal remodeling

Adult progenitor cells in the trachea of Drosophila larvae are activated and migrate out of niches when metamorphosis induces tracheal remodeling. Here we show that in response to metabolic deficiency in decaying tracheal branches, signaling by the insulin pathway controls the progenitor cells by regulating Yorkie (Yki)-dependent proliferation and migration. Yki, a transcription coactivator that is regulated by Hippo signaling, promotes transcriptional activation of cell cycle regulators and components of the extracellular matrix in tracheal progenitor cells. These findings reveal that regulation of Yki signaling by the insulin pathway governs proliferation and migration of tracheal progenitor cells, thereby identifying the regulatory mechanism by which metabolic depression drives progenitor cell activation and cell division that underlies tracheal remodeling.

Tracheal remodeling is a key step during Drosophila metamorphosis. Here they report that tracheal progenitor cells are activated through Yorkie-dependent proliferation and migration, which is induced by metabolic deficit and insulin signaling.

Wear and Tear of the Intestinal Visceral Musculature by Intrinsic and Extrinsic Factors

BACKGROUND

The gut visceral musculature plays essential roles in not only moving substances through the lumen but also maintaining the function and physiology of the gut. Although the development of the visceral musculature has been studied in multiple model organisms, how it degenerates is poorly understood.

RESULTS

Here, we employ the Drosophila midgut as a model to demonstrate that the visceral musculature is disrupted by intrinsic and extrinsic factors, such as aging, feeding, chemical-induced tissue damage, and oncogenic transformation in the epithelium. Notably, we define four prominent visceral musculature disruption phenotypes, which we refer as 'sprout', 'discontinuity', 'furcation', and 'crossover' of the longitudinal muscle. Given that the occurrence of these phenotypes is increased during aging and under various stresses, we propose that these phenotypes can be used as quantitative readouts of deterioration of the visceral musculature. Intriguingly, administration of a tissue-damaging chemical dextran sulfate sodium (DSS) induced similar visceral musculature disruption phenotypes in zebrafish larvae, indicating that ingestion of a tissue-damaging chemical can disrupt the visceral musculature in a vertebrate as well.

CONCLUSIONS

Our study provides insights into the deterioration of the gut visceral musculature and lays a groundwork for investigating the underlying mechanisms in Drosophila as well as other animals. This article is protected by copyright. All rights reserved.

Investigating local and systemic intestinal signalling in health and disease with Drosophila

Whole-body health relies on complex inter-organ signalling networks that enable organisms to adapt to environmental perturbations and to changes in tissue homeostasis. The intestine plays a major role as a signalling centre by producing local and systemic signals that are relayed to the body and that maintain intestinal and organismal homeostasis. Consequently, disruption of intestinal homeostasis and signalling are associated with systemic diseases and multi-organ dysfunction. In recent years, the fruit fly Drosophila melanogaster has emerged as a prime model organism to study tissue-intrinsic and systemic signalling networks of the adult intestine due to its genetic tractability and functional conservation with mammals. In this Review, we highlight Drosophila research that has contributed to our understanding of how the adult intestine interacts with its microenvironment and with distant organs. We discuss the implications of these findings for understanding intestinal and whole-body pathophysiology, and how future Drosophila studies might advance our knowledge of the complex interplay between the intestine and the rest of the body in health and disease.

Summary: We outline work in the fruit fly Drosophila melanogaster that has contributed knowledge on local and whole-body signalling coordinated by the adult intestine, and discuss its implications in intestinal pathophysiology and associated systemic dysfunction.

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.

Rab21 in enterocytes participates in intestinal epithelium maintenance

Membrane trafficking is defined as the vesicular transport of proteins into, out of, and throughout the cell. In intestinal enterocytes, defects in endocytic/recycling pathways result in impaired function and are linked to diseases. However, how these trafficking pathways regulate intestinal tissue homeostasis is poorly understood. Using the Drosophila intestine as an in vivo system, we investigated enterocyte-specific functions for the early endosomal machinery. We focused on Rab21, which regulates specific steps in early endosomal trafficking. Depletion of Rab21 in enterocytes led to abnormalities in intestinal morphology, with deregulated cellular equilibrium associated with a gain in mitotic cells and increased cell death. Increases in apoptosis and Yorkie signaling were responsible for compensatory proliferation and tissue inflammation. Using an RNAi screen, we identified regulators of autophagy and membrane trafficking that phenocopied Rab21 knockdown. We further showed that Rab21 knockdown-induced hyperplasia was rescued by inhibition of epidermal growth factor receptor signaling. Moreover, quantitative proteomics identified proteins affected by Rab21 depletion. Of these, we validated changes in apolipoprotein ApoLpp and the trehalose transporter Tret1-1, indicating roles for enterocyte Rab21 in lipid and carbohydrate homeostasis, respectively. Our data shed light on an important role for early endosomal trafficking, and Rab21, in enterocyte-mediated intestinal epithelium maintenance. [Media: see text] [Media: see text].

Insect Gut Regeneration

In adult insects, as in vertebrates, the gut epithelium is a highly regenerative tissue that can renew itself rapidly in response to changing inputs from nutrition, the gut microbiota, ingested toxins, and signals from other organs. Because of its cellular and genetic similarities to the mammalian intestine, and its relevance as a target for the control of insect pests and disease vectors, many researchers have used insect intestines to address fundamental questions about stem cell functions during tissue maintenance and regeneration. In Drosophila, where most of the experimental work has been performed, not only are intestinal cell types and behaviors well characterized, but numerous cell signaling interactions have been detailed that mediate gut epithelial regeneration. A prevailing model for regenerative responses in the insect gut invokes stress sensing by damaged enterocytes (ECs) as a principal source for signaling that activates the division of intestinal stem cells (ISCs) and the growth and differentiation of their progeny. However, extant data also reveal alternative mechanisms for regeneration that involve ISC-intrinsic functions, active culling of healthy epithelial cells, enhanced EC growth, and even cytoplasmic shedding by infected ECs. This article reviews current knowledge of the molecular mechanisms involved in gut regeneration in several insect models (Drosophila and Aedes of the order Diptera, and several Lepidoptera).

Tumor-induced disruption of the blood-brain barrier promotes host death

Cancer patients often die from symptoms that manifest at a distance from any tumor. Mechanisms underlying these systemic physiological perturbations, called paraneoplastic syndromes, may benefit from investigation in non-mammalian systems. Using a non-metastatic Drosophila adult model, we find that malignant-tumor-produced cytokines drive widespread host activation of JAK-STAT signaling and cause premature lethality. STAT activity is particularly high in cells of the blood-brain barrier (BBB), where it induces aberrant BBB permeability. Remarkably, inhibiting STAT in the BBB not only rescues barrier function but also extends the lifespan of tumor-bearing hosts. We identify BBB damage in other pathological conditions that cause elevated inflammatory signaling, including obesity and infection, where BBB permeability also regulates host survival. IL-6-dependent BBB dysfunction is further seen in a mouse tumor model, and it again promotes host morbidity. Therefore, BBB alterations constitute a conserved lethal tumor-host interaction that also underlies other physiological morbidities.

Thermal stress induces tissue damage and a broad shift in regenerative signaling pathways in the honey bee digestive tract

Honey bee colonies in the USA have suffered from increased die-off in the last few years with a complex set of interacting stresses playing a key role. With changing climate, an increase in the frequency of severe weather events, such as heat waves, is anticipated. Understanding how these changes may contribute to stress in honey bees is crucial. Individual honey bees appear to have a high capacity to endure thermal stress. One reason for this high-level endurance is likely their robust heat shock response (HSR), which contributes to thermotolerance at the cellular level. However, less is known about other mechanisms of thermotolerance, especially those operating at the tissue level. To elucidate other determinants of resilience in this species, we used thermal stress coupled with RNAseq and identified broad transcriptional remodeling of a number of key signaling pathways in the honey bee, including those pathways known to be involved in digestive tract regeneration in the fruit fly such as the Hippo and JAK/STAT pathways. We also observed cell death and shedding of epithelial cells, which likely leads to induction of this regenerative transcriptional program. We found that thermal stress affects many of these pathways in other tissues, suggesting a shared program of damage response. This study provides important foundational characterization of the tissue damage response program in this key pollinating species. In addition, our data suggest that a robust regeneration program may also be a critical contributor to thermotolerance at the tissue level, a possibility which warrants further exploration in this and other species.

Coordination of tumor growth and host wasting by tumor-derived Upd3

yki-induced gut tumors in Drosophila are associated with host wasting, including muscle dysfunction, lipid loss, and hyperglycemia, a condition reminiscent of human cancer cachexia. We previously used this model to identify tumor-derived ligands that contribute to host wasting. To identify additional molecular networks involved in host-tumor interactions, we develop PathON, a web-based tool analyzing the major signaling pathways in Drosophila, and uncover the Upd3/Jak/Stat axis as an important modulator. We find that yki-gut tumors secrete Upd3 to promote self-overproliferation and enhance Jak/Stat signaling in host organs to cause wasting, including muscle dysfunction, lipid loss, and hyperglycemia. We further reveal that Upd3/Jak/Stat signaling in the host organs directly triggers the expression of ImpL2, an antagonistic binding protein for insulin-like peptides, to impair insulin signaling and energy balance. Altogether, our results demonstrate that yki-gut tumors produce a Jak/Stat pathway ligand, Upd3, that regulates both self-growth and host wasting.

Ding et al. show that yki^3SA-gut tumors produce Upd3 as a cachectic ligand to simultaneously promote self-growth and host organ wasting via systemic activation of Jak/Stat signaling in Drosophila. The Upd3/Jak/Stat axis induces host ImpL2 production and perturbs insulin response, leading to muscle mitochondrial dysfunction, lipid loss, and carbohydrate elevation.

Context-dependent interplay between Hippo and JNK pathway in Drosophila

Both Hippo and JNK signaling have well-established roles in regulating many physiological processes, including cell proliferation, growth, survival, and migration. An increasing body of evidence shows that dysregulation of either Hippo or JNK pathway would lead to tumorigenesis. Recently, studies in Drosophila has coupled Hippo with JNK pathway in numerous ways ranging from tissue regeneration to growth control. In this review, I provide an overview of the current understanding of crosstalk between Hippo and JNK pathway in Drosophila, and discuss their context-dependent interactions in gut homeostasis, regeneration, cell competition and migration.

Microenvironmental regulation of intestinal stem cells in the inflamed intestine

The rapid renewal of intestinal epithelium during homeostasis requires balanced proliferation and differentiation of intestinal stem cells (ISCs) at the base of crypt. Upon intestinal inflammation, the vigorous expansion of surviving ISCs is responsible for epithelial repair. However, it is not well depicted how ISCs adapt to the inflammatory conditions within intestinal tissue and support epithelial repair. In the intestinal inflammation, niche cells around ISCs along with their secreted niche factors can facilitate the regeneration of ISCs via niche signals. Additionally, the growth of ISCs can respond to inflammatory cells, inflammatory cytokines, and inflammatory signals. Understanding the adaptive mechanism of ISCs in supporting intestinal epithelial regeneration during inflammation is a focus on the treatment for patients with intestinal inflammation. Here, we aim to present an overview of how ISCs adapt to the acute inflammation to support intestinal repair, with a focus on the roles and interaction of niche signals.

Hippo Signaling Pathway in Gliomas

The Hippo signaling pathway is a highly conserved pathway involved in tissue development and regeneration that controls organ size through the regulation of cell proliferation and apoptosis. The core Hippo pathway is composed of a block of kinases, MST1/2 (Mammalian STE20-like protein kinase 1/2) and LATS1/2 (Large tumor suppressor 1/2), which inhibits nuclear translocation of YAP/TAZ (Yes-Associated Protein 1/Transcriptional co-activator with PDZ-binding motif) and its downstream association with the TEAD (TEA domain) family of transcription factors. This pathway was recently shown to be involved in tumorigenesis and metastasis in several cancers such as lung, breast, or colorectal cancers but is still poorly investigated in brain tumors. Gliomas are the most common and the most lethal primary brain tumors representing about 80% of malignant central nervous system neoplasms. Despite intensive clinical protocol, the prognosis for patients remains very poor due to systematic relapse and treatment failure. Growing evidence demonstrating the role of Hippo signaling in cancer biology and the lack of efficient treatments for malignant gliomas support the idea that this pathway could represent a potential target paving the way for alternative therapeutics. Based on recent advances in the Hippo pathway deciphering, the main goal of this review is to highlight the role of this pathway in gliomas by a state-of-the-art synthesis.

Inhibition of yes-associated protein dephosphorylation prevents aggravated periodontitis with occlusal trauma

BACKGROUND

Occlusal trauma can aggravate periodontitis, but the mechanism remains unclear. Yes-associated protein (YAP), a mechanical stressor protein, may play an important role in this process.

METHODS

Western blot and quantitative real-time polymerase chain reaction (qRT-PCR) were applied to detect the expression of YAP and inflammatory factors in patients with periodontitis accompanied with or without occlusal trauma. Through local administration of Porphyromonas gingivalis and composite resin bonding on maxillary molars in mice, we established periodontitis and occlusal trauma models. Treatment with or without XAV939, to inhibit YAP activation, was performed in these models. Micro-computed tomography, immunofluorescence (IF), and qRT-PCR were used to explore the YAP pathway in periodontitis with occlusal trauma. Cyclic stress and lipopolysaccharide (LPS) stimuli were applied to the L929 mouse fibroblast cell line with or without XAV939. Western blot, IF, and qRT-PCR were used to verify the in vivo results.

RESULTS

Activated dephosphorylated YAP and increased expression of inflammatory factors were observed in patients with periodontitis accompanied with occlusal trauma. In the mouse model of periodontitis with occlusal trauma, YAP transferred into the nucleus, resulting in Jun N-terminal kinases (JNK) related pro-inflammatory pathway up-regulation. L929 cell cyclic stress and LPS stimulation results confirmed the in vivo results. Application of XAV939 inhibited YAP protein dephosphorylation and reduced JNK pro-inflammatory pathway factor expression in vivo and in vitro.

CONCLUSIONS

Occlusal trauma can activate YAP nuclear transfer, resulting in the up-regulation of the JNK pro-inflammatory pathway. This can be inhibited by the XAV939 YAP inhibitor.

Transiently "Undead" Enterocytes Mediate Homeostatic Tissue Turnover in the Adult Drosophila Midgut

We reveal surprising similarities between homeostatic cell turnover in adult Drosophila midguts and "undead" apoptosis-induced compensatory proliferation (AiP) in imaginal discs. During undead AiP, immortalized cells signal for AiP, allowing its analysis. Critical for undead AiP is the Myo1D-dependent localization of the initiator caspase Dronc to the plasma membrane. Here, we show that Myo1D functions in mature enterocytes (ECs) to control mitotic activity of intestinal stem cells (ISCs). In Myo1D mutant midguts, many signaling events involved in AiP (ROS generation, hemocyte recruitment, and JNK signaling) are affected. Importantly, similar to AiP, Myo1D is required for membrane localization of Dronc in ECs. We propose that ECs destined to die transiently enter an undead-like state through Myo1D-dependent membrane localization of Dronc, which enables them to generate signals for ISC activity and their replacement. The concept of transiently "undead" cells may be relevant for other stem cell models in flies and mammals.

YAP in epithelium senses gut barrier loss to deploy defenses against pathogens

Pathogens commonly disrupt the intestinal epithelial barrier; however, how the epithelial immune system senses the loss of intestinal barrier as a danger signal to activate self-defense is unclear. Through an unbiased approach in the model nematode Caenorhabditis elegans, we found that the EGL-44/TEAD transcription factor and its transcriptional activator YAP-1/YAP (Yes-associated protein) were activated when the intestinal barrier was disrupted by infections with the pathogenic bacterium Pseudomonas aeruginosa PA14. Gene Ontology enrichment analysis of the genes containing the TEAD-binding sites revealed that “innate immune response” and “defense response to Gram-negative bacterium” were two top significantly overrepresented terms. Genetic inactivation of yap-1 and egl-44 significantly reduced the survival rate and promoted bacterial accumulation in worms after bacterial infections. Furthermore, we found that disturbance of the E-cadherin-based adherens junction triggered the nuclear translocation and activation of YAP-1/YAP in the gut of worms. Although YAP is a major downstream effector of the Hippo signaling, our study revealed that the activation of YAP-1/YAP was independent of the Hippo pathway during disruption of intestinal barrier. After screening 10 serine/threonine phosphatases, we identified that PP2A phosphatase was involved in the activation of YAP-1/YAP after intestinal barrier loss induced by bacterial infections. Additionally, our study demonstrated that the function of YAP was evolutionarily conserved in mice. Our study highlights how the intestinal epithelium recognizes the loss of the epithelial barrier as a danger signal to deploy defenses against pathogens, uncovering an immune surveillance program in the intestinal epithelium.

The intestinal epithelial barrier is an important line of defense against pathogenic bacteria infecting the intestine. Persistent bacterial infections can cause disruption of the intestinal barrier; however, how the epithelia immune system recognizes the loss of intestinal barrier as a danger signal to activate self-defense against pathogens is unclear. Using the nematode Caenorhabditis elegans as a model animal, we show that the EGL-44/TEAD transcription factor and its transcriptional activator YAP-1/YAP (Yes-associated protein) are activated when the intestinal barrier is disrupted by bacterial infections. Gene Ontology enrichment reveals that EGL-44/TEAD orchestrates a complex host response composed of innate immune response and defense response to Gram-negative bacteria. Furthermore, our data demonstrate that YAP-1/YAP and EGL-44/TEAD are required for resistance to infections with pathogenic bacteria when the intestinal barrier is disrupted in worms and mice. Our study reveals a novel strategy for the intestinal epithelium to sense danger through its internal architecture and initiate innate immunity.

Targeting the Hippo pathway in cancer, fibrosis, wound healing and regenerative medicine

The Hippo pathway is an evolutionarily conserved signalling pathway with key roles in organ development, epithelial homeostasis, tissue regeneration, wound healing and immune modulation. Many of these roles are mediated by the transcriptional effectors YAP and TAZ, which direct gene expression via control of the TEAD family of transcription factors. Dysregulated Hippo pathway and YAP/TAZ-TEAD activity is associated with various diseases, most notably cancer, making this pathway an attractive target for therapeutic intervention. This Review highlights the key findings from studies of Hippo pathway signalling across biological processes and diseases, and discusses new strategies and therapeutic implications of targeting this pathway.

The Snakeskin-Mesh Complex of Smooth Septate Junction Restricts Yorkie to Regulate Intestinal Homeostasis in Drosophila

Tight junctions in mammals and septate junctions in insects are essential for epithelial integrity. We show here that, in the Drosophila intestine, smooth septate junction proteins provide barrier and signaling functions. During an RNAi screen for genes that regulate adult midgut tissue growth, we found that loss of two smooth septate junction components, Snakeskin and Mesh, caused a hyperproliferation phenotype. By examining epitope-tagged endogenous Snakeskin and Mesh, we demonstrate that the two proteins are present in the cytoplasm of differentiating enteroblasts and in cytoplasm and septate junctions of mature enterocytes. In both enteroblasts and enterocytes, loss of Snakeskin and Mesh causes Yorkie-dependent expression of the JAK-STAT pathway ligand Upd3, which in turn promotes proliferation of intestinal stem cells. Snakeskin and Mesh form a complex with each other, with other septate junction proteins and with Yorkie. Therefore, the Snakeskin-Mesh complex has both barrier and signaling function to maintain stem cell-mediated tissue homeostasis.

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Snakeskin and Mesh are septate junction proteins essential for intestinal homeostasis

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Snakeskin and Mesh act in enteroblasts and enterocytes to regulate stem cell division

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Snakeskin and Mesh form a complex with and restrict the activity of Yorkie

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Loss of Snakeskin and Mesh allows Yorkie to promote Upd3 expression and growth

Model systems for regeneration: Drosophila

Drosophila melanogaster has historically been a workhorse model organism for studying developmental biology. In addition, Drosophila is an excellent model for studying how damaged tissues and organs can regenerate. Recently, new precision approaches that enable both highly targeted injury and genetic manipulation have accelerated progress in this field. Here, we highlight these techniques and review examples of recently discovered mechanisms that regulate regeneration in Drosophila larval and adult tissues. We also discuss how, by applying these powerful approaches, studies of Drosophila can continue to guide the future of regeneration research.

Gilgamesh (Gish)/CK1γ regulates tissue homeostasis and aging in adult Drosophila midgut

Adult tissues and organs rely on resident stem cells to generate new cells that replenish damaged cells. To maintain homeostasis, stem cell activity needs to be tightly controlled throughout the adult life. Here, we show that the membrane-associated kinase Gilgamesh (Gish)/CK1γ maintains Drosophila adult midgut homeostasis by restricting JNK pathway activity and that Gish is essential for intestinal stem cell (ISC) maintenance under stress conditions. Inactivation of Gish resulted in aberrant JNK pathway activation and excessive production of multiple cytokines and growth factors that drive ISC overproliferation. Mechanistically, Gish restricts JNK activation by phosphorylating and destabilizing a small GTPase, Rho1. Interestingly, we find that Gish expression is down-regulated in aging guts and that increasing Gish activity in aging guts can restore tissue homeostasis. Hence, our study identifies Gish/CK1γ as a novel regulator of Rho1 and gatekeeper of tissue homeostasis whose activity is compromised in aging guts.

Colorectal cancer (CRC) as a multifactorial disease and its causal correlations with multiple signaling pathways

Colorectal cancer (CRC) is the third most common malignancy and one of the leading causes of cancer-related death among men worldwide. CRC is a multifactor digestive pathology, which is a huge problem faced not only by clinicians but also by researchers. Importantly, a unique feature of CRC is the dysregulation of molecular signaling pathways. To date, a series of reviews have indicated that different signaling pathways are disordered and have potential as therapeutic targets in CRC. Nevertheless, an overview of the function and interaction of multiple signaling pathways in CRC is needed. Therefore, we summarized the pathways, biological functions and important interactions involved in CRC. First, we investigated the involvement of signaling pathways, including Wnt, PI3K/Akt, Hedgehog, ErbB, RHOA, Notch, BMP, Hippo, AMPK, NF-κB, MAPK and JNK. Subsequently, we discussed the biological function of these pathways in pathophysiological aspects of CRC, such as proliferation, apoptosis and metastasis. Finally, we summarized important interactions among these pathways in CRC. We believe that the interaction of these pathways could provide new strategies for the treatment of CRC.

Pro-apoptotic and pro-proliferation functions of the JNK pathway of Drosophila: roles in cell competition, tumorigenesis and regeneration

The Jun N-terminal kinase (JNK) is a member of the mitogen-activated protein kinase family. It appears to be conserved in all animal species where it regulates important physiological functions involved in apoptosis, cell migration, cell proliferation and regeneration. In this review, we focus on the functions of JNK in Drosophila imaginal discs, where it has been reported that it can induce both cell death and cell proliferation. We discuss this apparent paradox in the light of recent findings and propose that the pro-apoptotic and the pro-proliferative functions are intrinsic properties of JNK activity. Whether one function or another is predominant depends on the cellular context.

Lola regulates Drosophila adult midgut homeostasis via non-canonical hippo signaling

Tissue homeostasis and regeneration in the Drosophila midgut is regulated by a diverse array of signaling pathways including the Hippo pathway. Hippo signaling restricts intestinal stem cell (ISC) proliferation by sequestering the transcription co-factor Yorkie (Yki) in the cytoplasm, a factor required for rapid ISC proliferation under injury-induced regeneration. Nonetheless, the mechanism of Hippo-mediated midgut homeostasis and whether canonical Hippo signaling is involved in ISC basal proliferation are less characterized. Here we identify Lola as a transcription factor acting downstream of Hippo signaling to restrict ISC proliferation in a Yki-independent manner. Not only that Lola interacts with and is stabilized by the Hippo signaling core kinase Warts (Wts), Lola rescues the enhanced ISC proliferation upon Wts depletion via suppressing Dref and SkpA expressions. Our findings reveal that Lola is a non-canonical Hippo signaling component in regulating midgut homeostasis, providing insights on the mechanism of tissue maintenance and intestinal function.

Stem Cell Proliferation Is Kept in Check by the Chromatin Regulators Kismet/CHD7/CHD8 and Trr/MLL3/4

Chromatin remodeling accompanies differentiation, however, its role in self-renewal is less well understood. We report that in Drosophila, the chromatin remodeler Kismet/CHD7/CHD8 limits intestinal stem cell (ISC) number and proliferation without affecting differentiation. Stem-cell-specific whole-genome profiling of Kismet revealed its enrichment at transcriptionally active regions bound by RNA polymerase II and Brahma, its recruitment to the transcription start site of activated genes and developmental enhancers and its depletion from regions bound by Polycomb, Histone H1, and heterochromatin Protein 1. We demonstrate that the Trithorax-related/MLL3/4 chromatin modifier regulates ISC proliferation, colocalizes extensively with Kismet throughout the ISC genome, and co-regulates genes in ISCs, including Cbl, a negative regulator of Epidermal Growth Factor Receptor (EGFR). Loss of kismet or trr leads to elevated levels of EGFR protein and signaling, thereby promoting ISC self-renewal. We propose that Kismet with Trr establishes a chromatin state that limits EGFR proliferative signaling, preventing tumor-like stem cell overgrowths.

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Chromatin modifiers Kismet and Trr limit intestinal stem cell proliferation

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Kismet and Trr colocalize at transcriptionally active regions and co-regulate genes

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EGFR negative regulator Cbl is a target gene of Kismet and Trr

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Kismet and Trr limit EGFR signaling in ISCs, preventing tumor-like ISC accumulation

Heparan sulfate maintains adult midgut homeostasis in Drosophila

Tissue homeostasis is controlled by the differentiated progeny of residential progenitors (stem cells). Adult stem cells constantly adjust their proliferation/differentiation rates to respond to tissue damage and stresses. However, how differentiated cells maintain tissue homeostasis remains unclear. Here, we find that heparan sulfate (HS), a class of glycosaminoglycan (GAG) chains, protects differentiated cells from loss to maintain intestinal homeostasis. HS depletion in enterocytes (ECs) leads to intestinal homeostasis disruption, with accumulation of intestinal stem cell (ISC)-like cells and mis-differentiated progeny. HS-deficient ECs are prone to cell death/stress and induced cytokine and epidermal growth factor (EGF) expression, which, in turn, promote ISC proliferation and differentiation. Interestingly, HS depletion in ECs results in the inactivation of decapentaplegic (Dpp) signaling. Moreover, ectopic Dpp signaling completely rescued the defects caused by HS depletion. Together, our data demonstrate that HS is required for Dpp signal activation in ECs, thereby protecting ECs from ablation to maintain midgut homeostasis. Our data shed light into the regulatory mechanisms of how differentiated cells contribute to tissue homeostasis maintenance.

The role of translationally controlled tumor protein in proliferation of Drosophila intestinal stem cells

Translationally controlled tumor protein (TCTP) is a highly conserved protein functioning in multiple cellular processes, ranging from growth to immune responses. To explore the role of TCTP in tissue maintenance and regeneration, we employed the adult Drosophila midgut, where multiple signaling pathways interact to precisely regulate stem cell division for tissue homeostasis. Tctp levels were significantly increased in stem cells and enteroblasts upon tissue damage or activation of the Hippo pathway that promotes regeneration of intestinal epithelium. Stem cells with reduced Tctp levels failed to proliferate during normal tissue homeostasis and regeneration. Mechanistically, Tctp forms a complex with multiple proteins involved in translation and genetically interacts with ribosomal subunits. In addition, Tctp increases both Akt1 protein abundance and phosphorylation in vivo. Altogether, Tctp regulates stem cell proliferation by interacting with key growth regulatory signaling pathways and the translation process in vivo.

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.

Heparan sulfate negatively regulates intestinal stem cell proliferation in Drosophila adult midgut

Tissue homeostasis is maintained by differentiated progeny of residential stem cells. Both extrinsic signals and intrinsic factors play critical roles in the proliferation and differentiation of adult intestinal stem cells (ISCs). However, how extrinsic signals are transduced into ISCs still remains unclear. Here, we find that heparan sulfate (HS), a class of glycosaminoglycan (GAG) chains, negatively regulates progenitor proliferation and differentiation to maintain midgut homeostasis under physiological conditions. Interestingly, HS depletion in progenitors results in inactivation of Decapentaplegic (Dpp) signaling. Dpp signal inactivation in progenitors resembles HS-deficient intestines. Ectopic Dpp signaling completely rescued the defects caused by HS depletion. Taken together, these data demonstrate that HS is required for Dpp signaling to maintain midgut homeostasis. Our results provide insight into the regulatory mechanisms of how extrinsic signals are transduced into stem cells to regulate their proliferation and differentiation.

Histone demethylase KDM3A is required for enhancer activation of hippo target genes in colorectal cancer

Hippo pathway is involved in tumorigenesis, and its regulation in cytosol has been extensively studied, but its regulatory mechanisms in the nuclear are not clear. In the current study, using a FBS-inducing model following serum starvation, we identified KDM3A, a demethylase of histone H3K9me1/2, as a positive regulator for hippo target genes. KDM3A promotes gene expression through two mechanisms, one is to upregulate YAP1 expression, and the other is to facilitate H3K27ac on the enhancers of hippo target genes. H3K27ac upregulation is more relevant with gene activation, but not H3K4me3; and KDM3A depletion caused H3K9me2 upregulation mainly on TEAD1-binding enhancers rather than gene bodies, further resulting in H3K27ac decrease, less TEAD1 binding on enhancers and impaired transcription. Moreover, KDM3A is associated with p300 and required for p300 recruitment to enhancers. KDM3A deficiency delayed cancer cell growth and migration, which was rescued by YAP1 expression. KDM3A expression is correlated with YAP1 and hippo target genes in colorectal cancer patient tissues, and may serve as a potential prognosis mark. Taken together, our study reveals novel mechanisms for hippo signaling and enhancer activation, which is critical for tumorigenesis of colorectal cancer.

Damage sensing by a Nox-Ask1-MKK3-p38 signaling pathway mediates regeneration in the adult Drosophila midgut

Epithelia are exposed to diverse types of stress and damage from pathogens and the environment, and respond by regenerating. Yet, the proximal mechanisms that sense epithelial damage remain poorly understood. Here we report that p38 signaling is activated in adult Drosophila midgut enterocytes in response to diverse stresses including pathogenic bacterial infection and chemical and mechanical insult. Two upstream kinases, Ask1 and Licorne (MKK3), are required for p38 activation following infection, oxidative stress, detergent exposure and wounding. Ask1-p38 signaling in enterocytes is required upon infection to promote full intestinal stem cell (ISC) activation and regeneration, partly through Upd3/Jak-Stat signaling. Furthermore, reactive oxygen species (ROS) produced by the NADPH oxidase Nox in enterocytes, are required for p38 activation in enterocytes following infection or wounding, and for ISC activation upon infection or detergent exposure. We propose that Nox-ROS-Ask1-MKK3-p38 signaling in enterocytes integrates multiple different stresses to induce regeneration.

AWD regulates timed activation of BMP signaling in intestinal stem cells to maintain tissue homeostasis

Precise control of stem cell (SC) proliferation ensures tissue homeostasis. In the Drosophila intestine, injury-induced regeneration involves initial activation of intestinal SC (ISC) proliferation and subsequent return to quiescence. These two phases of the regenerative response are controlled by differential availability of the BMP type I receptor Thickveins (Tkv), yet how its expression is dynamically regulated remains unclear. Here we show that during homeostasis, the E3 ubiquitin ligase Highwire and the ubiquitin-proteasome system maintain low Tkv protein expression. After ISC activation, Tkv is stabilized by proteasome inhibition and undergoes endocytosis due to the induction of the nucleoside diphosphate kinase Abnormal Wing Disc (AWD). Tkv internalization is required for the activation of the Smad protein Mad, and for the return to quiescence after a regenerative episode. Our data provide insight into the mechanisms ensuring tissue homeostasis by dynamic control of somatic stem cell activity.

Regeneration after injury in the Drosophila intestine involves early activation of intestinal stem cells (ISCs) and subsequent return to quiescence. Here the authors show that return to quiescence by ISCs involves BMP Type I receptor Tkv protein stabilization along with AWD mediated internalization into endocytic vesicles.

Recycling Endosomes in Mature Epithelia Restrain Tumorigenic Signaling

The effects of polarized membrane trafficking in mature epithelial tissue on cell growth and cancer progression have not been fully explored in vivo. A majority of colorectal cancers have reduced and mislocalized Rab11, a small GTPase dedicated to trafficking of recycling endosomes. Patients with low Rab11 protein expression have poor survival rates. Using genetic models across species, we show that intact recycling endosome function restrains aberrant epithelial growth elicited by APC or RAS mutations. Loss of Rab11 protein led to epithelial dysplasia in early animal development and synergized with oncogenic pathways to accelerate tumor progression initiated by carcinogen, genetic mutation, or aging. Transcriptomic analysis uncovered an immediate expansion of the intestinal stem cell pool along with cell-autonomous Yki/Yap activation following disruption of Rab11a-mediated recycling endosomes. Intestinal tumors lacking Rab11a traffic exhibited marked elevation of nuclear Yap, upd3/IL6-Stat3, and amphiregulin-MAPK signaling, whereas suppression of Yki/Yap or upd3/IL6 reduced gut epithelial dysplasia and hyperplasia. Examination of Rab11a function in enteroids or cultured cell lines suggested that this endosome unit is required for suppression of the Yap pathway by Hippo kinases. Thus, recycling endosomes in mature epithelia constitute key tumor suppressors, loss of which accelerates carcinogenesis. SIGNIFICANCE: Recycling endosome traffic in mature epithelia constitutes a novel tumor suppressing mechanism.

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.

Cultured cells and wing disc size of silkworm can be controlled by the Hippo pathway

Hippo signalling represents a cell proliferation and organ-size control pathway. Yorki (Yki), a component of the Hippo pathway, induces the transcription of a number of targets that promote cell proliferation and survival. The functions of Yki have been characterized in Drosophila and mammals, while there are few reports on silkworm, Bombyx mori. In the present study, we found that BmYki3 facilitates cell migration and cell division, and enlarges the cultured cell and wing disc size. Co-immunoprecipitation results indicated that BmYki3 may interact with thymosin, E3 ubiquitin-protein ligase, protein kinase ASK1, dedicator of cytokinesis protein 1, calcium-independent phospholipase A2 and beta-spectrin. RNA-seq results indicated that 4444 genes were upregulated and 10 291 genes were downregulated after BmYki3 was overexpressed in the cultured cells. GO annotation indicated that the up/downregulated genes were enriched in 268/382 GO terms (p < 0.01); KEGG analysis showed that the up/downregulated genes were enriched in 49/101 pathways. These findings provided novel information to understand the functions of BmYki3 in a cell proliferation and organ-size control pathway.

JAK/STAT signaling in stem cells and regeneration: from Drosophila to vertebrates

The JAK/STAT pathway is a conserved metazoan signaling system that transduces cues from extracellular cytokines into transcriptional changes in the nucleus. JAK/STAT signaling is best known for its roles in immunity. However, recent work has demonstrated that it also regulates critical homeostatic processes in germline and somatic stem cells, as well as regenerative processes in several tissues, including the gonad, intestine and appendages. Here, we provide an overview of JAK/STAT signaling in stem cells and regeneration, focusing on Drosophila and highlighting JAK/STAT pathway functions in proliferation, survival and cell competition that are conserved between Drosophila and vertebrates.

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

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

The 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.

Quantitative Real-Time PCR to Measure YAP/TAZ Activity in Human Cells

Transcription coactivators Yes-associated protein (YAP) and transcriptional coactivator with PDZ-binding motif (TAZ, also known as WWTR1) are homologs of the Drosophila Yorkie (Yki) protein and are major downstream effectors of the evolutionarily conserved Hippo pathway. YAP/TAZ play critical roles in regulation of cell proliferation, apoptosis, and stemness, thus mediate functions of the Hippo pathway in organ size control and tumorigenesis. The Hippo pathway inhibits YAP/TAZ through phosphorylation, which leads to YAP/TAZ cytoplasmic retention and degradation. Dephosphorylated and nuclear-localized YAP/TAZ bind to transcription factors, especially the TEAD family proteins, thus transactivate the expression of specific genes. Therefore, measuring the expression level of YAP/TAZ target genes is a critical approach to assess Hippo pathway activity. Through gene expression profiling in different tissues and cells using techniques such as microarray and RNA-seq, many target genes of YAP/TAZ have been identified. Some of these genes were confirmed to be direct YAP/TAZ targets by chromatin immunoprecipitation (ChIP)-PCR or ChIP-seq. These works made it possible to quickly determine YAP/TAZ activity by measuring the mRNA levels of several YAP/TAZ target genes, such as CTGF, CYR61, and miR-130a by quantitative real-time PCR (qPCR). In this chapter, we demonstrate the use of qPCR to measure YAP/TAZ activity in MCF10A cells.