Steep differences in wingless signaling trigger Myc-independent competitive cell interactions

The Drosophila EcR-Hippo component Taiman promotes epithelial cell fitness by control of the Dally-like glypican and Wg gradient

Rapidly dividing cells can eliminate slow growing neighbors through the apoptotic process of cell competition. This process ensures that only high fitness cells populate embryonic tissues and is proposed to underlie the ability of oncogene-transformed cells to progressively replace normal cells within a tissue. Patches of cells in the Drosophila wing disc overexpressing the oncogenic Taiman (Tai) transcriptional coactivator kill normal neighbors by secreting Spätzle ligands that trigger pro-apoptotic Toll signaling in receiving cells. However, extracellular signaling mechanisms responsible for elimination of slow growing cells by normal neighbors remain poorly defined. Here we show that slow growing cells with reduced Tai (Tailow) are killed by normal neighbors through a mechanism involving competition for the Wingless (Wg/Wnt) ligand. Elevated Wg signaling significantly rescues elimination of Tailow cells in multiple organs, suggesting that Tai may normally promote Wg activity. Examining distribution of Wg components reveals that Tai promotes extracellular spread of the Wg ligand from source cells across the wing disc, thus ensuring patterned expression of multiple Wg-regulated target genes. Tai controls Wg spread indirectly through the extracellular glypican Dally-like protein (Dlp), which binds Wg and promotes its extracellular diffusion and capture by receptors. Data indicate that Tai likely controls Dlp at two levels: transcription of dlp mRNA and Dlp intracellular trafficking. Overall, these data indicate that the Tai acts through Dlp to enable Wg transport and signaling, and that cell competition in the Tailow model arises due to inequity in the ability of epithelial cells to sequester limiting amounts of the Wg growth factor.

Cell competition and the regulation of protein homeostasis

The process of embryonic development involves remarkable cellular plasticity, which governs the coordination between cells necessary to build an organism. One role of this plasticity is to ensure that when aberrant cells are eliminated, growth adjustment occurs so that the size of the tissue is maintained. An important regulator of cellular plasticity that ensures cellular cooperation is a fitness-sensing mechanism termed cell competition. During cell competition, cells with defects that lower fitness but do not affect viability, such as those that cause impaired signal transduction, slower cellular growth, mitochondrial dysregulation or impaired protein homeostasis, are killed when surrounded by fitter cells. This is accompanied by the compensatory proliferation of the surviving cells. The underlying factors and mechanisms that demarcate certain cells as less fit than their neighbouring cells and losers of cell competition are still relatively unknown. Recent evidence has pointed to mitochondrial defects and proteotoxic stress as important hallmarks of these loser cells. Here, we review recent advances in this area, focussing on the role of mitochondrial activity and protein homeostasis as major mechanisms determining competitive cell fitness during development and the importance of cell proteostasis in determining cell fitness.

Toward a predictive understanding of epithelial cell death

Epithelial cell death is highly prevalent during development and tissue homeostasis. While we have a rather good understanding of the molecular regulators of programmed cell death, especially for apoptosis, we still fail to predict when, where, how many and which specific cells will die in a tissue. This likely relies on the much more complex picture of apoptosis regulation in a tissular and epithelial context, which entails cell autonomous but also non-cell autonomous factors, diverse feedback and multiple layers of regulation of the commitment to apoptosis. In this review, we illustrate this complexity of epithelial apoptosis regulation by describing these different layers of control, all demonstrating that local cell death probability is a complex emerging feature. We first focus on non-cell autonomous factors that can locally modulate the rate of cell death, including cell competition, mechanical input and geometry as well as systemic effects. We then describe the multiple feedback mechanisms generated by cell death itself. We also outline the multiple layers of regulation of epithelial cell death, including the coordination of extrusion and regulation occurring downstream of effector caspases. Eventually, we propose a roadmap to reach a more predictive understanding of cell death regulation in an epithelial context.

Maintenance of proteostasis by Drosophila Rer1 is essential for competitive cell survival and Myc-driven overgrowth

Defects in protein homeostasis can induce proteotoxic stress, affecting cellular fitness and, consequently, overall tissue health. In various growing tissues, cell competition based mechanisms facilitate detection and elimination of these compromised, often referred to as 'loser', cells by the healthier neighbors. The precise connection between proteotoxic stress and competitive cell survival remains largely elusive. Here, we reveal the function of an endoplasmic reticulum (ER) and Golgi localized protein Rer1 in the regulation of protein homeostasis in the developing Drosophila wing epithelium. Our results show that loss of Rer1 leads to proteotoxic stress and PERK-mediated phosphorylation of eukaryotic initiation factor 2α. Clonal analysis showed that rer1 mutant cells are identified as losers and eliminated through cell competition. Interestingly, we find that Rer1 levels are upregulated upon Myc-overexpression that causes overgrowth, albeit under high proteotoxic stress. Our results suggest that increased levels of Rer1 provide cytoprotection to Myc-overexpressing cells by alleviating the proteotoxic stress and thereby supporting Myc-driven overgrowth. In summary, these observations demonstrate that Rer1 acts as a novel regulator of proteostasis in Drosophila and reveal its role in competitive cell survival.

Cell competition: emerging signaling and unsolved questions

Multicellular communities have an intrinsic mechanism that optimizes their structure and function via cell-cell communication. One of the driving forces for such self-organization of the multicellular system is cell competition, the elimination of viable unfit or deleterious cells via cell-cell interaction. Studies in Drosophila and mammals have identified multiple mechanisms of cell competition caused by different types of mutations or cellular changes. Intriguingly, recent studies have found that different types of "losers" of cell competition commonly show reduced protein synthesis. In Drosophila, the reduction in protein synthesis levels in loser cells is caused by phosphorylation of the translation initiation factor eIF2α via a bZip transcription factor Xrp1. Given that a variety of cellular stresses converge on eIF2α phosphorylation and thus global inhibition of protein synthesis, cell competition may be a machinery that optimizes multicellular fitness by removing stressed cells. In this review, we summarize and discuss emerging signaling mechanisms and critical unsolved questions, as well as the role of protein synthesis in cell competition.

The Wnt pathway regulates wing morph determination in Acyrthosiphon pisum

Wing dimorphism occurs in insects as a survival strategy to adapt to environmental changes. In response to environmental cues, mother aphids transmit signals to their offspring, and the offspring either emerge as winged adults or develop as wingless adults with degeneration of the wing primordia in the early instar stage. However, how the wing morph is determined in the early instar stage is still unclear. Here, we established a surgical sampling method to obtain precise wing primordium tissues for transcriptome analysis. We identified Wnt as a regulator of wing determination in the early second instar stage in the pea aphid. Inhibiting Wnt signaling via knockdown of Wnt2, Wnt11b, the Wnt receptor-encoding gene fz2 or the downstream targets vg and omb resulted in a decreased proportion of winged aphids. Activation of Wnt signaling via knockdown of miR-8, an inhibitor of the Wnt/Wg pathway, led to an increased proportion of winged aphids. Furthermore, the wing primordia of wingless nymphs underwent apoptosis in the early second instar, and cell death was activated by knockdown of fz2 under the wing-inducing condition. These results indicate that the developmental plasticity of aphid wings is modulated by the intrinsic Wnt pathway in response to environmental challenges.

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.

Ribosomal protein mutations and cell competition: autonomous and nonautonomous effects on a stress response

Ribosomal proteins (Rps) are essential for viability. Genetic mutations affecting Rp genes were first discovered in Drosophila, where they represent a major class of haploinsufficient mutations. One mutant copy gives rise to the dominant "Minute" phenotype, characterized by slow growth and small, thin bristles. Wild-type (WT) and Minute cells compete in mosaics, that is, Rp+/- are preferentially lost when their neighbors are of the wild-type genotype. Many features of Rp gene haploinsufficiency (i.e. Rp+/- phenotypes) are mediated by a transcriptional program. In Drosophila, reduced translation and slow growth are under the control of Xrp1, a bZip-domain transcription factor induced in Rp mutant cells that leads ultimately to the phosphorylation of eIF2α and consequently inhibition of most translation. Rp mutant phenotypes are also mediated transcriptionally in yeast and in mammals. In mammals, the Impaired Ribosome Biogenesis Checkpoint activates p53. Recent findings link Rp mutant phenotypes to other cellular stresses, including the DNA damage response and endoplasmic reticulum stress. We suggest that cell competition results from nonautonomous inputs to stress responses, bringing decisions between adaptive and apoptotic outcomes under the influence of nearby cells. In Drosophila, cell competition eliminates aneuploid cells in which loss of chromosome leads to Rp gene haploinsufficiency. The effects of Rp gene mutations on the whole organism, in Minute flies or in humans with Diamond-Blackfan Anemia, may be inevitable consequences of pathways that are useful in eliminating individual cells from mosaics. Alternatively, apparently deleterious whole organism phenotypes might be adaptive, preventing even more detrimental outcomes. In mammals, for example, p53 activation appears to suppress oncogenic effects of Rp gene haploinsufficiency.

Stem cell competition driven by the Axin2-p53 axis controls brain size during murine development

Cell competition acts as a quality-control mechanism that eliminates cells less fit than their neighbors to optimize organ development. Whether and how competitive interactions occur between neural progenitor cells (NPCs) in the developing brain remains unknown. Here, we show that endogenous cell competition occurs and intrinsically correlates with the Axin2 expression level during normal brain development. Induction of genetic mosaicism predisposes Axin2-deficient NPCs to behave as "losers" in mice and undergo apoptotic elimination, but homogeneous ablation of Axin2 does not promote cell death. Mechanistically, Axin2 suppresses the p53 signaling pathway at the post-transcriptional level to maintain cell fitness, and Axin2-deficient cell elimination requires p53-dependent signaling. Furthermore, mosaic Trp53 deletion confers a "winner" status to p53-deficient cells that outcompete their neighbors. Conditional loss of both Axin2 and Trp53 increases cortical area and thickness, suggesting that the Axin2-p53 axis may coordinate to survey cell fitness, regulate natural cell competition, and optimize brain size during neurodevelopment.

Stemming Tumoral Growth: A Matter of Grotesque Organogenesis

The earliest metazoans probably evolved from single-celled organisms which found the colonial system to be a beneficial organization. Over the course of their evolution, these primary colonial organisms increased in size, and division of labour among the cells became a remarkable feature, leading to a higher level of organization: the biological organs. Primitive metazoans were the first organisms in evolution to show organ-type structures, which set the grounds for complex organs to evolve. Throughout evolution, and concomitant with organogenesis, is the appearance of tissue-specific stem cells. Tissue-specific stem cells gave rise to multicellular living systems with distinct organs which perform specific physiological functions. This setting is a constructive role of evolution; however, rebel cells can take over the molecular mechanisms for other purposes: nowadays we know that cancer stem cells, which generate aberrant organ-like structures, are at the top of a hierarchy. Furthermore, cancer stem cells are the root of metastasis, therapy resistance, and relapse. At present, most therapeutic drugs are unable to target cancer stem cells and therefore, treatment becomes a challenging issue. We expect that future research will uncover the mechanistic “forces” driving organ growth, paving the way to the implementation of new strategies to impair human tumorigenesis.

Cell competition in development, homeostasis and cancer

Organ development and homeostasis involve dynamic interactions between individual cells that collectively regulate tissue architecture and function. To ensure the highest tissue fidelity, equally fit cell populations are continuously renewed by stochastic replacement events, while cells perceived as less fit are actively removed by their fitter counterparts. This renewal is mediated by surveillance mechanisms that are collectively known as cell competition. Recent studies have revealed that cell competition has roles in most, if not all, developing and adult tissues. They have also established that cell competition functions both as a tumour-suppressive mechanism and as a tumour-promoting mechanism, thereby critically influencing cancer initiation and development. This Review discusses the latest insights into the mechanisms of cell competition and its different roles during embryonic development, homeostasis and cancer.

Clone wars: From molecules to cell competition in intestinal stem cell homeostasis and disease

The small intestine is among the fastest self-renewing tissues in adult mammals. This rapid turnover is fueled by the intestinal stem cells residing in the intestinal crypt. Wnt signaling plays a pivotal role in regulating intestinal stem cell renewal and differentiation, and the dysregulation of this pathway leads to cancer formation. Several studies demonstrate that intestinal stem cells follow neutral drift dynamics, as they divide symmetrically to generate other equipotent stem cells. Competition for niche space and extrinsic signals in the intestinal crypt is the governing mechanism that regulates stemness versus cell differentiation, but the underlying molecular mechanisms are still poorly understood, and it is not yet clear how this process changes during disease. In this review, we highlight the mechanisms that regulate stem cell homeostasis in the small intestine, focusing on Wnt signaling and its regulation by RNF43 and ZNRF3, key inhibitors of the Wnt pathway. Furthermore, we summarize the evidence supporting the current model of intestinal stem cell regulation, highlighting the principles of neutral drift at the basis of intestinal stem cell homeostasis. Finally, we discuss recent studies showing how cancer cells bypass this mechanism to gain a competitive advantage against neighboring normal cells.

Stem cells in the gut rapidly renew themselves through processes that cancer cells co-opt to trigger tumor development. Gabriele Colozza from the Institute of Molecular Biotechnology in Vienna, Austria, and colleagues review how a network of critical molecular signals and competition for limited space help to regulate the dynamics of stem cells in the intestines. The correct balance between self-renewal and differentiation is tightly controlled by the so-called Wnt signaling pathway and its inhibitors. Competition between dividing cells in the intestinal crypts, the locations between finger-like protrusions in the gut where stem cells are found, provides another protective mechanism against runaway stem cell growth. However, intestinal cancer cells, thanks to their activating mutations, bypass these safeguards to gain a survival advantage. Drugs that target these ‘super-competitive’ behaviors could therefore help combat tumor proliferation.

Game of clones: Battles in the field of carcinogenesis

Recent advances in bulk sequencing approaches as well as genomic decoding at the single-cell level have revealed surprisingly high somatic mutational burdens in normal tissues, as well as increased our understanding of the landscape of "field cancerization", that is, molecular and immune alterations in mutagen-exposed normal-appearing tissues that recapitulated those present in tumors. Charting the somatic mutational landscapes in normal tissues can have strong implications on our understanding of how tumors arise from mutagenized epithelium. Making sense of those mutations to understand the progression along the pathologic continuum of normal epithelia, preneoplasias, up to malignant tissues will help pave way for identification of ideal targets that can guide new strategies for preventing or eliminating cancers at their earliest stages of development. In this review, we will provide a brief history of field cancerization and its implications on understanding early stages of cancer pathogenesis and deviation from the pathologically "normal" state. The review will provide an overview of how mutations accumulating in normal tissues can lead to a patchwork of mutated cell clones that compete while maintaining an overall state of functional homeostasis. The review also explores the role of clonal competition in directing the fate of normal tissues and summarizes multiple mechanisms elicited in this phenomenon and which have been linked to cancer development. Finally, we highlight the importance of understanding mutations in normal tissues, as well as clonal competition dynamics (in both the epithelium and the microenvironment) and their significance in exploring new approaches to combatting cancer.

Cell autonomous and non-autonomous consequences of deviations in translation machinery on organism growth and the connecting signalling pathways

Translation machinery is responsible for the production of cellular proteins; thus, cells devote the majority of their resources to ribosome biogenesis and protein synthesis. Single-copy loss of function in the translation machinery components results in rare ribosomopathy disorders, such as Diamond-Blackfan anaemia in humans and similar developmental defects in various model organisms. Somatic copy number alterations of translation machinery components are also observed in specific tumours. The organism-wide response to haploinsufficient loss-of-function mutations in ribosomal proteins or translation machinery components is complex: variations in translation machinery lead to reduced ribosome biogenesis, protein translation and altered protein homeostasis and cellular signalling pathways. Cells are affected both autonomously and non-autonomously by changes in translation machinery or ribosome biogenesis through cell-cell interactions and secreted hormones. We first briefly introduce the model organisms where mutants or knockdowns of protein synthesis and ribosome biogenesis are characterized. Next, we specifically describe observations in Caenorhabditis elegans and Drosophila melanogaster, where insufficient protein synthesis in a subset of cells triggers cell non-autonomous growth or apoptosis responses that affect nearby cells and tissues. We then cover the characterized signalling pathways that interact with ribosome biogenesis/protein synthesis machinery with an emphasis on their respective functions during organism development.

A role for Flower and cell death in controlling morphogen gradient scaling

During development, morphogen gradients encode positional information to pattern morphological structures during organogenesis¹. Some gradients, like that of Dpp in the fly wing, remain proportional to the size of growing organs-that is, they scale. Gradient scaling keeps morphological patterns proportioned in organs of different sizes^2,3. Here we show a mechanism of scaling that ensures that, when the gradient is smaller than the organ, cell death trims the developing tissue to match the size of the gradient. Scaling is controlled by molecular associations between Dally and Pentagone, known factors involved in scaling, and a key factor that mediates cell death, Flower^4-6. We show that Flower activity in gradient expansion is not dominated by cell death, but by the activity of Dally/Pentagone on scaling. Here we show a potential connection between scaling and cell death that may uncover a molecular toolbox hijacked by tumours.

Inter-Species Rescue of Mutant Phenotype—The Standard for Genetic Analysis of Human Genetic Disorders in Drosophila melanogaster Model

Drosophila melanogaster (the fruit fly) is arguably a superstar of genetics, an astonishing versatile experimental model which fueled no less than six Nobel prizes in medicine. Nowadays, an evolving research endeavor is to simulate and investigate human genetic diseases in the powerful D. melanogaster platform. Such a translational experimental strategy is expected to allow scientists not only to understand the molecular mechanisms of the respective disorders but also to alleviate or even cure them. In this regard, functional gene orthology should be initially confirmed in vivo by transferring human or vertebrate orthologous transgenes in specific mutant backgrounds of D. melanogaster. If such a transgene rescues, at least partially, the mutant phenotype, then it qualifies as a strong candidate for modeling the respective genetic disorder in the fruit fly. Herein, we review various examples of inter-species rescue of relevant mutant phenotypes of the fruit fly and discuss how these results recommend several human genes as candidates to study and validate genetic variants associated with human diseases. We also consider that a wider implementation of this evolutionist exploratory approach as a standard for the medicine of genetic disorders would allow this particular field of human health to advance at a faster pace.

Differential gene expression analysis identified determinants of cell fate plasticity during radiation-induced regeneration in Drosophila

Ionizing radiation (IR) is used to treat half of all cancer patients because of its ability to kill cells. IR, however, can induce stem cell-like properties in non-stem cancer cells, potentiating tumor regrowth and reduced therapeutic success. We identified previously a subpopulation of cells in Drosophila larval wing discs that exhibit IR-induced stem cell-like properties. These cells reside in the future wing hinge, are resistant to IR-induced apoptosis, and are capable of translocating, changing fate, and participating in regenerating the pouch that suffers more IR-induced apoptosis. We used here a combination of lineage tracing, FACS-sorting of cells that change fate, genome-wide RNAseq, and functional testing of 42 genes, to identify two key changes that are required cell-autonomously for IR-induced hinge-to-pouch fate change: (1) repression of hinge determinants Wg (Drosophila Wnt1) and conserved zinc-finger transcription factor Zfh2 and (2) upregulation of three ribosome biogenesis factors. Additional data indicate a role for Myc, a transcriptional activator of ribosome biogenesis genes, in the process. These results provide a molecular understanding of IR-induced cell fate plasticity that may be leveraged to improve radiation therapy.

Ionizing radiation (IR) is used to treat half of all cancer patients because of its ability to kill cells but treatment failures are common because tumors grow back (regenerate). Here, we asked which changes in the properties of cells facilitate regeneration in Drosophila (fruit flies) after exposure to radiation. We identified six genes whose products increase or decrease the regenerative potential of cells. These results help us understand how tissues regenerate after IR damage and will aid in designing better therapies that involve radiation.

Xrp1 and Irbp18 trigger a feed-forward loop of proteotoxic stress to induce the loser status

Cell competition induces the elimination of less-fit "loser" cells by fitter "winner" cells. In Drosophila, cells heterozygous mutant in ribosome genes, Rp/+, known as Minutes, are outcompeted by wild-type cells. Rp/+ cells display proteotoxic stress and the oxidative stress response, which drive the loser status. Minute cell competition also requires the transcription factors Irbp18 and Xrp1, but how these contribute to the loser status is partially understood. Here we provide evidence that initial proteotoxic stress in RpS3/+ cells is Xrp1-independent. However, Xrp1 is sufficient to induce proteotoxic stress in otherwise wild-type cells and is necessary for the high levels of proteotoxic stress found in RpS3/+ cells. Surprisingly, Xrp1 is also induced downstream of proteotoxic stress, and is required for the competitive elimination of cells suffering from proteotoxic stress or overexpressing Nrf2. Our data suggests that a feed-forward loop between Xrp1, proteotoxic stress, and Nrf2 drives Minute cells to become losers.

Cell competition is driven by Xrp1-mediated phosphorylation of eukaryotic initiation factor 2α

Cell competition is a context-dependent cell elimination via cell-cell interaction whereby unfit cells ('losers') are eliminated from the tissue when confronted with fitter cells ('winners'). Despite extensive studies, the mechanism that drives loser's death and its physiological triggers remained elusive. Here, through a genetic screen in Drosophila, we find that endoplasmic reticulum (ER) stress causes cell competition. Mechanistically, ER stress upregulates the bZIP transcription factor Xrp1, which promotes phosphorylation of the eukaryotic translation initiation factor eIF2α via the kinase PERK, leading to cell elimination. Surprisingly, our genetic data show that different cell competition triggers such as ribosomal protein mutations or RNA helicase Hel25E mutations converge on upregulation of Xrp1, which leads to phosphorylation of eIF2α and thus causes reduction in global protein synthesis and apoptosis when confronted with wild-type cells. These findings not only uncover a core pathway of cell competition but also open the way to understanding the physiological triggers of cell competition.

Cell competition in intratumoral and tumor microenvironment interactions

Tumors are complex cellular and acellular environments within which cancer clones are under continuous selection pressures. Cancer cells are in a permanent mode of interaction and competition with each other as well as with the immediate microenvironment. In the course of these competitive interactions, cells share information regarding their general state of fitness, with less-fit cells being typically eliminated via apoptosis at the hands of those cells with greater cellular fitness. Competitive interactions involving exchange of cell fitness information have implications for tumor growth, metastasis, and therapy outcomes. Recent research has highlighted sophisticated pathways such as Flower, Hippo, Myc, and p53 signaling, which are employed by cancer cells and the surrounding microenvironment cells to achieve their evolutionary goals by means of cell competition mechanisms. In this review, we discuss these recent findings and explain their importance and role in evolution, growth, and treatment of cancer. We further consider potential physiological conditions, such as hypoxia and chemotherapy, that can function as selective pressures under which cell competition mechanisms may evolve differently or synergistically to confer oncogenic advantages to cancer.

Cell competition from development to neurodegeneration

Cell competition is a process by which suboptimal cells are eliminated to the benefit of cells with higher fitness. It is a surveillance mechanism that senses differences in the fitness status by several modes, such as expression of fitness fingerprints, survival factor uptake rate and resistance to mechanical stress. Fitness fingerprints-mediated cell competition recognizes isoforms of the transmembrane protein Flower, and translates the relative fitness of cells into distinct fates through the Flower code. Impairments in cell competition potentiate the development of diseases like cancer and ageing-related pathologies. In cancer, malignant cells acquire a supercompetitor behaviour, killing the neighbouring cells and overtaking the tissue, thus avoiding elimination. Neurodegenerative disorders affect millions of people and are characterized by cognitive decline and locomotor deficits. Alzheimer's disease is the most common form of dementia, and one of the largely studied diseases. However, the cellular processes taking place remain unclear. Drosophila melanogaster is an emerging neurodegeneration model due to its versatility as a tool for genetic studies. Research in a Drosophila Alzheimer's disease model detected fitness markers in the suboptimal and hyperactive neurons, thus establishing a link between cell competition and Alzheimer's disease. In this Review, we overview cell competition and the new insights related to neurodegenerative disorders, and discuss how research in the field might contribute to the development of new therapeutic targets for these diseases.

NOTUM from Apc-mutant cells biases clonal competition to initiate cancer

The tumour suppressor APC is the most commonly mutated gene in colorectal cancer. Loss of Apc in intestinal stem cells drives the formation of adenomas in mice via increased WNT signalling1, but reduced secretion of WNT ligands increases the ability of Apc-mutant intestinal stem cells to colonize a crypt (known as fixation)2. Here we investigated how Apc-mutant cells gain a clonal advantage over wild-type counterparts to achieve fixation. We found that Apc-mutant cells are enriched for transcripts that encode several secreted WNT antagonists, with Notum being the most highly expressed. Conditioned medium from Apc-mutant cells suppressed the growth of wild-type organoids in a NOTUM-dependent manner. Furthermore, NOTUM-secreting Apc-mutant clones actively inhibited the proliferation of surrounding wild-type crypt cells and drove their differentiation, thereby outcompeting crypt cells from the niche. Genetic or pharmacological inhibition of NOTUM abrogated the ability of Apc-mutant cells to expand and form intestinal adenomas. We identify NOTUM as a key mediator during the early stages of mutation fixation that can be targeted to restore wild-type cell competitiveness and provide preventative strategies for people at a high risk of developing colorectal cancer.

Apc-mutant cells act as supercompetitors in intestinal tumour initiation

A delicate equilibrium of WNT agonists and antagonists in the intestinal stem cell (ISC) niche is critical to maintaining the ISC compartment, as it accommodates the rapid renewal of the gut lining. Disruption of this balance by mutations in the tumour suppressor gene APC, which are found in approximately 80% of all human colon cancers, leads to unrestrained activation of the WNT pathway^1,2. It has previously been established that Apc-mutant cells have a competitive advantage over wild-type ISCs³. Consequently, Apc-mutant ISCs frequently outcompete all wild-type stem cells within a crypt, thereby reaching clonal fixation in the tissue and initiating cancer formation. However, whether the increased relative fitness of Apc-mutant ISCs involves only cell-intrinsic features or whether Apc mutants are actively involved in the elimination of their wild-type neighbours remains unresolved. Here we show that Apc-mutant ISCs function as bona fide supercompetitors by secreting WNT antagonists, thereby inducing differentiation of neighbouring wild-type ISCs. Lithium chloride prevented the expansion of Apc-mutant clones and the formation of adenomas by rendering wild-type ISCs insensitive to WNT antagonists through downstream activation of WNT by inhibition of GSK3β. Our work suggests that boosting the fitness of healthy cells to limit the expansion of pre-malignant clones may be a powerful strategy to limit the formation of cancers in high-risk individuals.

Tiny Drosophila makes giant strides in cancer research

Cancer burden has been increasing worldwide, making cancer the second leading cause of death in the world. Over the past decades, various experimental models have provided important insights into the nature of cancer. Among them, the fruit fly Drosophila as a whole-animal toolkit has made a decisive contribution to our understanding of fundamental mechanisms of cancer development including loss of cell polarity. In recent years, scalable Drosophila platforms have proven useful also in developing anti-cancer regimens that are effective not only in mammalian models but also in patients. Here, we review studies using Drosophila as a tool to advance cancer study by complementing other traditional research systems.

Epithelial cell-turnover ensures robust coordination of tissue growth in Drosophila ribosomal protein mutants

Highly reproducible tissue development is achieved by robust, time-dependent coordination of cell proliferation and cell death. To study the mechanisms underlying robust tissue growth, we analyzed the developmental process of wing imaginal discs in Drosophila Minute mutants, a series of heterozygous mutants for a ribosomal protein gene. Minute animals show significant developmental delay during the larval period but develop into essentially normal flies, suggesting there exists a mechanism ensuring robust tissue growth during abnormally prolonged developmental time. Surprisingly, we found that both cell death and compensatory cell proliferation were dramatically increased in developing wing pouches of Minute animals. Blocking the cell-turnover by inhibiting cell death resulted in morphological defects, indicating the essential role of cell-turnover in Minute wing morphogenesis. Our analyses showed that Minute wing discs elevate Wg expression and JNK-mediated Dilp8 expression that causes developmental delay, both of which are necessary for the induction of cell-turnover. Furthermore, forced increase in Wg expression together with developmental delay caused by ecdysone depletion induced cell-turnover in the wing pouches of non-Minute animals. Our findings suggest a novel paradigm for robust coordination of tissue growth by cell-turnover, which is induced when developmental time axis is distorted.

Animal development can be disturbed by various stimuli such as genetic mutations, environmental fluctuations, and physical injuries. However, animals often accomplish normal tissue growth and morphogenesis even in the presence of developmental perturbations. Drosophila Minute mutants, a series of fly mutants for a ribosomal protein gene, show significantly prolonged larval period but develop into essentially normal flies. We found an unexpected massive cell death and subsequent compensatory cell proliferation in developing wing discs of Minute animals. This ‘cell-turnover’ was essential for normal wing morphogenesis in Minute flies. We found that the cell-turnover was induced by elevated Wg expression in the wing pouch and JNK-mediated Dilp8 expression that causes developmental delay. Indeed, cell-turnover was reproduced in non-Minute animals’ wing discs by overexpressing Wg using the wg promoter together with developmental delay caused by ecdysone depletion. Our findings propose a novel paradigm for morphogenetic robustness by cell-turnover, which ensures normal wing growth during the abnormally prolonged larval period, possibly by creating a flexible cell death and proliferation platform to adjust cell numbers in the prospective wing blade.

Oncological Ligand-Target Binding Systems and Developmental Approaches for Cancer Theranostics

Targeted treatment of cancer hinges on the identification of specific intracellular molecular receptors on cancer cells to stimulate apoptosis for eventually inhibiting growth; the development of novel ligands to target biomarkers expressed by the cancer cells; and the creation of novel multifunctional carrier systems for targeted delivery of anticancer drugs to specific malignant sites. There are numerous receptors, antigens, and biomarkers that have been discovered as oncological targets (oncotargets) for cancer diagnosis and treatment applications. Oncotargets are critically important to navigate active anticancer drug ingredients to specific disease sites with no/minimal effect on surrounding normal cells. In silico techniques relating to genomics, proteomics, and bioinformatics have catalyzed the discovery of oncotargets for various cancer types. Effective oncotargeting requires high-affinity probes engineered for specific binding of receptors associated with the malignancy. Computational methods such as structural modeling and molecular dynamic (MD) simulations offer opportunities to structurally design novel ligands and optimize binding affinity for specific oncotargets. This article proposes a streamlined approach for the development of ligand-oncotarget bioaffinity systems via integrated structural modeling and MD simulations, making use of proteomics, genomic, and X-ray crystallographic resources, to support targeted diagnosis and treatment of cancers and tumors.

Neuronal Selection Based on Relative Fitness Comparison Detects and Eliminates Amyloid-β-Induced Hyperactive Neurons in Drosophila

During adult life, damaged but viable neurons can accumulate in the organism, creating increasingly heterogeneous and dysfunctional neural circuits. One intriguing example is the aberrant increased activity of cerebral networks detected in vulnerable brain regions during preclinical stages of Alzheimer's disease. The pathophysiological contribution of these early functional alterations to the progression of Alzheimer's disease is uncertain. We found that a unique cell selection mechanism based on relative fitness comparison between neurons is able to target and remove aberrantly active neurons generated by heterologous human amyloid-β in Drosophila. Sustained neuronal activity is sufficient to compromise neuronal fitness and upregulate the expression of the low fitness indicators FlowerLoseB and Azot in the fly. Conversely, forced silencing of neurons restores brain fitness and reduces amyloid-β-induced cell death. The manipulation of this cell selection process, which was already proved to be conserved in humans, might be a promising new avenue to treat Alzheimer's.

Slow-Cycling Cancer Stem Cells Regulate Progression and Chemoresistance in Colon Cancer

Cancer chemoresistance is often attributed to the presence of cancer stem cell (CSC)-like cells, but whether they are homogeneously chemoresistant remains unclear. We previously showed that in colon tumors, a subpopulation of LGR5⁺ CSC-like cells driven by TCF1 (TCF7), a Wnt-responsive transcription factor, were responsible for tumorigenicity. Here we demonstrate that the tumorigenic subpopulation of mouse LGR5⁺ cells exists in a slow-cycling state and identify a unique 22-gene signature that characterizes these slow-cycling CSC. Seven of the signature genes are specifically expressed in slow-cycling LGR5⁺ cells from xenografted human colon tumors and are upregulated in colon cancer clinical specimens. Among these seven, four genes (APCDD1, NOTUM, PROX1, and SP5) are known to be direct Wnt target genes, and PROX1 was expressed in the invasive fronts of colon tumors. PROX1 was activated by TCF1 to induce CDKN1C and maintain a slow-cycling state in colon cancer organoids. Strikingly, PROX1 was required for recurrent growth after chemotherapeutic treatment, suggesting that inhibition of slow-cycling CSC by targeting the TCF1-PROX1-CDKN1C pathway is an effective strategy to combat refractory colon cancer in combination with conventional chemotherapy. SIGNIFICANCE: These findings illustrate the importance of a slow-cycling CSC subpopulation in colon cancer development and chemoresistance, with potential implications for the identified slow-cycling CSC signatures and the TCF1-PROX1-CDKN1C pathway as therapeutic targets.

Emerging mechanisms of cell competition

The growth and survival of cells within tissues can be affected by 'cell competition' between different cell clones. This phenomenon was initially recognized between wild-type cells and cells with mutations in ribosomal protein (Rp) genes in Drosophila melanogaster. However, competition also affects D. melanogaster cells with mutations in epithelial polarity genes, and wild-type cells exposed to 'super-competitor' cells with mutation in the Salvador-Warts-Hippo tumour suppressor pathway or expressing elevated levels of Myc. More recently, cell competition and super-competition were recognized in mammalian development, organ homeostasis and cancer. Genetic and cell biological studies have revealed that mechanisms underlying cell competition include the molecular recognition of 'different' cells, signalling imbalances between distinct cell populations and the mechanical consequences of differential growth rates; these mechanisms may also involve innate immune proteins, p53 and changes in translation.

Cell competition controls differentiation in mouse embryos and stem cells

Cell competition is a short-range intercellular communication, in which cells compare their fitness with that of their neighbors and eliminate the cells with relatively lower fitness. It is considered important for the formation and maintenance of healthy tissues; however, its exact role during development, especially in mammals, has been obscure. Recent studies in mouse embryonic epiblast and skin tissues revealed that cell differentiation in early embryos and stem cell proliferation tends to produce suboptimal cells, especially during early developmental stages. Cell competition occurs at multiple stages and via multiple mechanisms during development to ensure elimination of such low-quality cells. Thus, quality control via cell competition supports correct development by overcoming the heterogeneity produced during cell differentiation and stem cell proliferation.

Outcompeting cancer

The tumour microenvironment plays a critical role in determining tumour fate. Within that environment, and indeed throughout epithelial tissues, cells experience competition with their neighbours, with those less fit being eliminated by fitter adjacent cells. Herein we discuss evidence suggesting that mutations in cancer cells may be selected for their ability to exploit cell competition to kill neighbouring host cells, thereby facilitating tumour expansion. In some instances, cell competition may help host tissues to defend against cancer, by removing neoplastic and aneuploid cells. Cancer risk factors, such as high-sugar or high-fat diet and inflammation, impact cell competition-based host defences, suggesting that their effect on tumour risk may in part be accounted for by their influence on cell competition. We propose that interventions aimed at modifying the strength and direction of cell competition could induce cancer cell killing and form the basis for novel anticancer therapies.

Regulation of Anisotropic Tissue Growth by Two Orthogonal Signaling Centers

The Drosophila wing has served as a paradigm to mechanistically characterize the role of morphogens in patterning and growth. Wingless (Wg) and Decapentaplegic (Dpp) are expressed in two orthogonal signaling centers, and their gradients organize patterning by regulating the expression of well-defined target genes. By contrast, graded activity of these morphogens is not an absolute requirement for wing growth. Despite their permissive role in regulating growth, here we show that Wg and Dpp are utilized in a non-interchangeable manner by the two existing orthogonal signaling centers to promote preferential growth along the two different axes of the developing wing. Our data indicate that these morphogens promote anisotropic growth by making use of distinct and non-interchangeable molecular mechanisms. Whereas Dpp drives growth along the anterior-posterior axis by maintaining Brinker levels below a growth-repressing threshold, Wg exerts its action along the proximal-distal axis through a double repression mechanism involving T cell factor (TCF).

Cell elimination strategies upon identity switch via modulation of apterous in Drosophila wing disc

The ability to establish spatial organization is an essential feature of any developing tissue and is achieved through well-defined rules of cell-cell communication. Maintenance of this organization requires elimination of cells with inappropriate positional identity, a poorly understood phenomenon. Here we studied mechanisms regulating cell elimination in the context of a growing tissue, the Drosophila wing disc and its dorsal determinant Apterous. Systematic analysis of apterous mutant clones along with their twin spots shows that they are eliminated from the dorsal compartment via three different mechanisms: relocation to the ventral compartment, basal extrusion, and death, depending on the position of the clone in the wing disc. We find that basal extrusion is the main elimination mechanism in the hinge, whereas apoptosis dominates in the pouch and in the notum. In the absence of apoptosis, extrusion takes over to ensure clearance in all regions. Notably, clones in the hinge grow larger than those in the pouch, emphasizing spatial differences. Mechanistically, we find that limiting cell division within the clones does not prevent their extrusion. Indeed, even clones of one or two cells can be extruded basally, demonstrating that the clone size is not the main determinant of the elimination mechanism to be used. Overall, we revealed three elimination mechanisms and their spatial biases for preserving pattern in a growing organ.

As development proceeds, cells become more specialized and the compartmentalization ensures spatial separation of the specialized cells. This process of pattern formation is rather well understood. How the pattern is maintained afterwards though is largely unknown. Using the Drosophila wing disc as a model organ, we examined what happens to dorsal cells if they lose their dorsal identity. Formerly, it was shown that these cells are eliminated from the dorsal compartment via apoptosis or through relocation to the ventral compartment. Here we show that a third mode of elimination, basal extrusion, also contributes to their clearing. We quantified, for the first time, contributions of each mechanism and discovered a regional bias in their operation. Importantly, if apoptosis is blocked, basal extrusion takes over to ensure clearance from all regions. Recent modeling approaches suggested that there is a lower limit to the clone size for extrusion. Therefore, we tested the hypothesis that the choice of elimination mechanism may be dictated by the clone size. We prevented cell divisions within the clones to be eliminated and found that even 1–2 cell clones readily underwent basal extrusion, demonstrating that there is no lower limit to the clone size for extrusion.

Cell competition corrects noisy Wnt morphogen gradients to achieve robust patterning in the zebrafish embryo

Morphogen signalling forms an activity gradient and instructs cell identities in a signalling strength-dependent manner to pattern developing tissues. However, developing tissues also undergo dynamic morphogenesis, which may produce cells with unfit morphogen signalling and consequent noisy morphogen gradients. Here we show that a cell competition-related system corrects such noisy morphogen gradients. Zebrafish imaging analyses of the Wnt/β-catenin signalling gradient, which acts as a morphogen to establish embryonic anterior-posterior patterning, identify that unfit cells with abnormal Wnt/β-catenin activity spontaneously appear and produce noise in the gradient. Communication between unfit and neighbouring fit cells via cadherin proteins stimulates apoptosis of the unfit cells by activating Smad signalling and reactive oxygen species production. This unfit cell elimination is required for proper Wnt/β-catenin gradient formation and consequent anterior-posterior patterning. Because this gradient controls patterning not only in the embryo but also in adult tissues, this system may support tissue robustness and disease prevention.

Gradients of morphogens such as Wnt provide instructive cues for cell identities during development. Here, the authors report that in the developing zebrafish embryo, cell competition and elimination of unfit cells are required for proper Wnt gradient formation.

Cell Competition Is Driven by Autophagy

Cell competition is a quality control process that selectively eliminates unfit cells from the growing tissue via cell-cell interaction. Despite extensive mechanistic studies, the mechanism by which cell elimination is triggered has been elusive. Here, through a genetic screen in Drosophila, we discover that V-ATPase, an essential factor for autophagy, is required for triggering cell competition. Strikingly, autophagy is specifically elevated in prospective "loser" cells nearby wild-type "winner" cells, and blocking autophagy in loser cells abolishes their elimination. Mechanistically, elevated autophagy upregulates a proapoptotic gene hid through NFκB, and the elevated hid cooperates with JNK signaling to effectively induce loser's death. Crucially, this mechanism generally applies to cell competition caused by differences in protein synthesis between cells. Our findings establish a common mechanism of cell competition whereby cells with higher protein synthesis induce autophagy in their neighboring cells, leading to elimination of unfit cells.

The Initial Stage of Tumorigenesis in Drosophila Epithelial Tissues

Cancer development originates in a single mutant cell transformed from a normal cell, including further evolution of pro-tumor cells through additional mutations into malignant cancer tissues. Data from recent studies, however, suggest that most pro-tumor cells do not develop into tumors but remain dormant within or are prophylactically eliminated from tissues unless bestowed with additional driver mutations. Drosophila melanogaster has provided very efficient model systems, such as imaginal discs and ovarian follicular epithelia, to study the initial stage of tumorigenesis. This review will focus on the behaviors of emerging pro-tumor cells surrounded by normal cells and situations where they initiate tumor development.

Next-Generation Sequencing Reveals Increased Anti-oxidant Response and Ecdysone Signaling in STAT Supercompetitors in Drosophila

Cell competition is the elimination of one viable population of cells (the losers) by a neighboring fitter population (the winners) and was discovered by studies in the Drosophila melanogaster wing imaginal disc. Supercompetition is a process in which cells with elevated JAK/STAT signaling or increased Myc become winners and outcompete wild-type neighbors. To identify the genes that are differentially regulated in STAT supercompetitors, we purified these cells from Drosophila wing imaginal discs and performed next-generation sequencing. Their transcriptome was compared to those of control wing disc cells and Myc supercompetitors. Bioinformatics revealed that STAT and Myc supercompetitors have distinct transcriptomes with only 41 common differentially regulated genes. Furthermore, STAT supercompetitors have elevated reactive oxygen species, an anti-oxidant response and increased ecdysone signaling. Using a combination of methods, we validated 13 differentially expressed genes. These data sets will be useful resources to the community.

Cell competition: the winners and losers of fitness selection

The process of cell competition results in the 'elimination of cells that are viable but less fit than surrounding cells'. Given the highly heterogeneous nature of our tissues, it seems increasingly likely that cells are engaged in a 'survival of the fittest' battle throughout life. The process has a myriad of positive roles in the organism: it selects against mutant cells in developing tissues, prevents the propagation of oncogenic cells and eliminates damaged cells during ageing. However, 'super-fit' cancer cells can exploit cell competition mechanisms to expand and spread. Here, we review the regulation, roles and risks of cell competition in organism development, ageing and disease.

Emerging links between cell competition and Alzheimer's disease

Alzheimer's disease (AD) causes a progressive loss of memory and other cognitive functions, which inexorably debilitates patients. There is still no cure for AD and effective treatments to delay or revert AD are urgently needed. On a molecular level, the excessive accumulation of amyloid-β (Aβ) peptides triggers a complex cascade of pathological events underlying neuronal death, whose details are not yet completely understood. Our laboratory recently discovered that cell competition may play a protective role against AD by eliminating less fit neurons from the brain of Aβ-transgenic flies. Loss of Aβ-damaged neurons through fitness comparison with healthy counterparts is beneficial for the organism, delaying cognitive decline and motor disability. In this Review, we introduce the molecular mechanisms of cell competition, including seminal works on the field and latest advances regarding genetic triggers and effectors of cell elimination. We then describe the biological relevance of competition in the nervous system and discuss how competitive interactions between neurons may arise and be exacerbated in the context of AD. Selection of neurons through fitness comparison is a promising, but still emerging, research field that may open new avenues for the treatment of neurological disorders.

Harnessing epithelial homeostatic mechanisms to fight cancer

Cancer treatments have, in general, targeted the cancer cell itself. This approach has often been unsuccessful in the long term, especially for solid tumors. Even targeted therapies based on sequencing cancer genomes can be thwarted by genetic heterogeneity within tumors. Furthermore, genomic instability in cancer cells accelerates the generation of variants that are resistant to the treatment. Immunotherapies and anti-angiogenic treatments, which target the tumor-interacting and tumor-adjacent cells, have overcome some of these challenges, suggesting that other methods that target wild-type cells could be valuable in arresting tumor progression. Studies in Drosophila have uncovered mechanisms by which cells within an epithelium can react to neighboring cells that have genetic differences, resulting in the elimination of one population at the expense of another. Some of these mechanisms are now known to be conserved in mammals. The possibility of harnessing such mechanisms to empower normal epithelial cells to eliminate their precancerous neighbors before they develop into fully fledged cancers is an area of research that merits more attention.

dTcf/Pangolin suppresses growth and tumor formation in Drosophila

Wnt/Wingless (Wg) signaling controls many aspects of animal development and is deregulated in different human cancers. The transcription factor dTcf/Pangolin (Pan) is the final effector of the Wg pathway in Drosophila and has a dual role in regulating the expression of Wg target genes. In the presence of Wg, dTcf/Pan interacts with β-catenin/Armadillo (Arm) and induces the transcription of Wg targets. In absence of Wg, dTcf/Pan partners with the transcriptional corepressor TLE/Groucho (Gro) and inhibits gene expression. Here, we use the wing imaginal disk of Drosophila as a model to examine the functions that dTcf/Pan plays in a proliferating epithelium. We report a function of dTcf/Pan in growth control and tumorigenesis. Our results show that dTcf/Pan can limit tissue growth in normal development and suppresses tumorigenesis in the context of oncogene up-regulation. We identify the conserved transcription factors Sox box protein 15 (Sox15) and Ftz transcription factor 1 (Ftz-f1) as genes controlled by dTcf/Pan involved in tumor development. In conclusion, this study reports a role for dTcf/Pan as a repressor of normal and oncogenic growth and identifies the genes inducing tumorigenesis downstream of dTcf/Pan.

Mechanisms of oncogenic cell competition-Paths of victory

Cancer is a multistep process. In the early phases of this disease, mutations in oncogenes and tumor suppressors are thought to promote clonal expansion. These mutations can increase cell competitiveness, allowing tumor cells to grow within the tissue by eliminating wild type host cells. Recent studies have shown that cell competition can also function in later phases of cancer. Here, we examine the existing evidence linking cell competition and tumorigenesis. We focus on the mechanisms underlying cell competition and their contribution to disease pathogenesis.

Solid stress, competition for space and cancer: The opposing roles of mechanical cell competition in tumour initiation and growth

The regulation of cell growth, cell proliferation and cell death is at the basis of the homeostasis of tissues. While they can be regulated by intrinsic and genetic factors, their response to external signals emanating from the local environment is also essential for tissue homeostasis. Tumour initiation and progression is based on the misregulation of growth, proliferation and death mostly through the accumulation of genetic mutations. Yet, there is an increasing body of evidences showing that tumour microenvironment also has a strong impact on cancer initiation and progression. This includes the mechanical constrains and the compressive forces generated by the resistance of the surrounding tissue/matrix to tumour expansion. Recently, mechanical stress has been proposed to promote competitive interactions between cells through a process called mechanical cell competition. Cell population with a high proliferative rate can compact and eliminate the neighbouring cells which are more sensitive to compaction. While this emerging concept has been recently validated in vivo, the relevance of this process during tumour progression has never been discussed extensively. In this review, I will first describe the phenomenology of mechanical cell competition focusing on the main parameters and the pathways regulating cell elimination. I will then discuss the relevance of mechanical cell competition in tumour initiation and expansion while emphasizing its potential opposing contributions to tumourogenesis.

Drosophila Models of Cell Polarity and Cell Competition in Tumourigenesis

Cell competition is an important surveillance mechanism that measures relative fitness between cells in a tissue during development, homeostasis, and disease. Specifically, cells that are "less fit" (losers) are actively eliminated by relatively "more fit" (winners) neighbours, despite the less fit cells otherwise being able to survive in a genetically uniform tissue. Originally described in the epithelial tissues of Drosophila larval imaginal discs, cell competition has since been shown to occur in other epithelial and non-epithelial Drosophila tissues, as well as in mammalian model systems. Many genes and signalling pathways have been identified as playing conserved roles in the mechanisms of cell competition. Among them are genes required for the establishment and maintenance of apico-basal cell polarity: the Crumbs/Stardust/Patj (Crb/Sdt/Patj), Bazooka/Par-6/atypical Protein Kinase C (Baz/Par-6/aPKC), and Scribbled/Discs large 1/Lethal (2) giant larvae (Scrib/Dlg1/L(2)gl) modules. In this chapter, we describe the concepts and mechanisms of cell competition, with emphasis on the relationship between cell polarity proteins and cell competition, particularly the Scrib/Dlg1/L(2)gl module, since this is the best described module in this emerging field.

Xrp1 is a transcription factor required for cell competition-driven elimination of loser cells

The elimination of unfit cells from a tissue is a process known in Drosophila and mammals as cell competition. In a well-studied paradigm “loser” cells that are heterozygous mutant for a haploinsufficient ribosomal protein gene are eliminated from developing tissues via apoptosis when surrounded by fitter wild-type cells, referred to as “winner” cells. However, the mechanisms underlying the induction of this phenomenon are not fully understood. Here we report that a CCAAT-Enhancer-Binding Protein (C/EBP), Xrp1, which is known to help maintaining genomic stability after genotoxic stress, is necessary for the elimination of loser clones in cell competition. In loser cells, Xrp1 is transcriptionally upregulated by an autoregulatory loop and is able to trigger apoptosis - driving cell elimination. We further show that Xrp1 acts in the nucleus to regulate the transcription of several genes that have been previously involved in cell competition. We therefore speculate that Xrp1 might play a fundamental role as a molecular caretaker of the genomic integrity of tissues.

Super-Competitors Game the Fitness Sensing System

Competitive interactions between neighboring cells require fitness comparison and local killing, but the signals regulating these processes are unknown. In this issue, Alpar et al. (2018) demonstrate that fitter cells secrete serine proteases to create a local burst of active Spätzle, triggering Toll signaling and apoptosis in less fit neighbors.

Modelling Cooperative Tumorigenesis in Drosophila

The development of human metastatic cancer is a multistep process, involving the acquisition of several genetic mutations, tumour heterogeneity, and interactions with the surrounding microenvironment. Due to the complexity of cancer development in mammals, simpler model organisms, such as the vinegar fly, Drosophila melanogaster, are being utilized to provide novel insights into the molecular mechanisms involved. In this review, we highlight recent advances in modelling tumorigenesis using the Drosophila model, focusing on the cooperation of oncogenes or tumour suppressors, and the interaction of mutant cells with the surrounding tissue in epithelial tumour initiation and progression.