Cell Competition Time Line: Winners Kill Losers, which Are Extruded and Engulfed by Hemocytes

Progression of m6A in the tumor microenvironment: hypoxia, immune and metabolic reprogramming

Recently, N6-methyladenosine (m6A) has aroused widespread discussion in the scientific community as a mode of RNA modification. m6A comprises writers, erasers, and readers, which regulates RNA production, nuclear export, and translation and is very important for human health. A large number of studies have found that the regulation of m6A is closely related to the occurrence and invasion of tumors, while the homeostasis and function of the tumor microenvironment (TME) determine the occurrence and development of tumors to some extent. TME is composed of a variety of immune cells (T cells, B cells, etc.) and nonimmune cells (tumor-associated mesenchymal stem cells (TA-MSCs), cancer-associated fibroblasts (CAFs), etc.). Current studies suggest that m6A is involved in regulating the function of various cells in the TME, thereby affecting tumor progression. In this manuscript, we present the composition of m6A and TME, the relationship between m6A methylation and characteristic changes in TME, the role of m6A methylation in TME, and potential therapeutic strategies to provide new perspectives for better treatment of tumors in clinical work.

In Drosophila Hemolymph, Serine Proteases Are the Major Gelatinases and Caseinases

After separation on gel zymography, Drosophila melanogaster hemolymph displays gelatinase and caseinase bands of varying sizes, ranging from over 140 to 25 kDa. Qualitative and quantitative variations in these bands were observed during larval development and between different D. melanogaster strains and Drosophila species. The activities of these Drosophila hemolymph gelatinase and caseinase were strongly inhibited by serine protease inhibitors, but not by EDTA. Mass spectrometry identified over 60 serine proteases (SPs) in gel bands corresponding to the major D. melanogaster gelatinases and caseinases, but no matrix metalloproteinases (MMPs) were found. The most abundant proteases were tequila and members of the Jonah and trypsin families. However, the gelatinase bands did not show any change in the tequila null mutant. Additionally, no clear changes could be observed in D. melanogaster gel bands 24 h after injection of bacterial lipopolysaccharides (LPS) or after oviposition by Leptopilina boulardi endoparasitoid wasps. It can be concluded that the primary gelatinases and caseinases in Drosophila larval hemolymph are serine proteases (SPs) rather than matrix metalloproteinases (MMPs). Furthermore, the gelatinase pattern remains relatively stable even after short-term exposure to pathogenic challenges.

RF, Mohan H, Annaert W, Chaudhary V. 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.

Planarians require ced-12/elmo-1 to clear dead cells by excretion through the gut

Cell corpse removal is a critical component of both development and homeostasis throughout the animal kingdom. Extensive research has revealed many of the mechanisms involved in corpse removal, typically involving engulfment and digestion by another cell; however, the dynamics of cell corpse clearance in adult tissues remain unclear. Here, we track cell death in the adult planarian Schmidtea mediterranea and find that, following light-induced cell death, pigment cell corpses transit to the gut and are excreted from the animal. Gut phagocytes, previously only known to phagocytose food, are required for pigment cells to enter the gut lumen. Finally, we show that the planarian ortholog of ced-12/engulfment and cell motility (ELMO) is required for corpse phagocytosis and removal through the gut. In total, we present a mechanism of cell clearance in an adult organism involving transit of dead cells to the gut, transport into the gut by phagocytes, and physical excretion of debris.

Hijacking homeostasis: Regulation of the tumor microenvironment by apoptosis

Cancers are genetically driven, rogue tissues which generate dysfunctional, obdurate organs by hijacking normal, homeostatic programs. Apoptosis is an evolutionarily conserved regulated cell death program and a profoundly important homeostatic mechanism that is common (alongside tumor cell proliferation) in actively growing cancers, as well as in tumors responding to cytotoxic anti-cancer therapies. Although well known for its cell-autonomous tumor-suppressive qualities, apoptosis harbors pro-oncogenic properties which are deployed through non-cell-autonomous mechanisms and which generally remain poorly defined. Here, the roles of apoptosis in tumor biology are reviewed, with particular focus on the secreted and fragmentation products of apoptotic tumor cells and their effects on tumor-associated macrophages, key supportive cells in the aberrant homeostasis of the tumor microenvironment. Historical aspects of cell loss in tumor growth kinetics are considered and the impact (and potential impact) on tumor growth of apoptotic-cell clearance (efferocytosis) as well as released soluble and extracellular vesicle-associated factors are discussed from the perspectives of inflammation, tissue repair, and regeneration programs. An "apoptosis-centric" view is proposed in which dying tumor cells provide an important platform for intricate intercellular communication networks in growing cancers. The perspective has implications for future research and for improving cancer diagnosis and therapy.

B7-H3 Associates with IMPDH2 and Regulates Cancer Cell Survival

Lung cancer is one of the most common cancers worldwide, and despite improvements in treatment regimens, patient prognosis remains poor. Lung adenocarcinomas develop from the lung epithelia and understanding how specific genetic and environmental factors lead to oncogenic transformation in these cells is of great importance to define the pathways that contribute to tumorigenesis. The recent rise in the use of immunotherapy to treat different cancers has prompted the exploration of immune modulators in tumour cells that may provide new targets to manipulate this process. Of these, the B7 family of cell surface receptors, which includes PD-1, is of particular interest due to its role in modulating immune cell responses within the tumour microenvironment. B7-H3 (CD276) is one family member that is upregulated in many cancer types and suggested to contribute to tumour-immune interactions. However, the function and ligand(s) for this receptor in normal lung epithelia and the mechanisms through which the overexpression of B7-H3 regulate cancer progression in the absence of immune cell interactions remain unclear. Here, we present evidence that B7-H3 is associated with one of the key rate-limiting metabolic enzymes IMPDH2, and the localisation of this complex is altered in human lung cancer cells that express high levels of B7-H3. Mechanistically, the IMPDH2:B7-H3 complex provides a protective role in cancer cells to escape oxidative stress triggered by chemotherapy, thus leading to cell survival. We further demonstrate that the loss of B7-H3 in cancer cells has no effect on growth or migration in 2D but promotes the expansion of 3D spheroids in an IMPDH2-dependent manner. These findings provide new insights into the B7-H3 function in the metabolic homeostasis of normal and transformed lung cancer cells, and whilst this molecule remains an interesting target for immunotherapy, these findings caution against the use of anti-B7-H3 therapies in certain clinical settings.

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.

The PECAn image and statistical analysis pipeline identifies Minute cell competition genes and features

Investigating organ biology often requires methodologies to induce genetically distinct clones within a living tissue. However, the 3D nature of clones makes sample image analysis challenging and slow, limiting the amount of information that can be extracted manually. Here we develop PECAn, a pipeline for image processing and statistical data analysis of complex multi-genotype 3D images. PECAn includes data handling, machine-learning-enabled segmentation, multivariant statistical analysis, and graph generation. This enables researchers to perform rigorous analyses rapidly and at scale, without requiring programming skills. We demonstrate the power of this pipeline by applying it to the study of Minute cell competition. We find an unappreciated sexual dimorphism in Minute cell growth in competing wing discs and identify, by statistical regression analysis, tissue parameters that model and correlate with competitive death. Furthermore, using PECAn, we identify several genes with a role in cell competition by conducting an RNAi-based screen.

To not love thy neighbor: mechanisms of cell competition in stem cells and beyond

Cell competition describes the process in which cells of greater fitness are capable of sensing and instructing elimination of lesser fit mutant cells. Since its discovery in Drosophila, cell competition has been established as a critical regulator of organismal development, homeostasis, and disease progression. It is therefore unsurprising that stem cells (SCs), which are central to these processes, harness cell competition to remove aberrant cells and preserve tissue integrity. Here, we describe pioneering studies of cell competition across a variety of cellular contexts and organisms, with the ultimate goal of better understanding competition in mammalian SCs. Furthermore, we explore the modes through which SC competition takes place and how this facilitates normal cellular function or contributes to pathological states. Finally, we discuss how understanding of this critical phenomenon will enable targeting of SC-driven processes, including regeneration and tumor progression.

Therapeutic Cell Repopulation of the Liver: From Fetal Rat Cells to Synthetic Human Tissues

Progenitor cells isolated from the fetal liver can provide a unique cell source to generate new healthy tissue mass. Almost 20 years ago, it was demonstrated that rat fetal liver cells repopulate the normal host liver environment via a mechanism akin to cell competition. Activin A, which is produced by hepatocytes, was identified as an important player during cell competition. Because of reduced activin receptor expression, highly proliferative fetal liver stem/progenitor cells are resistant to activin A and therefore exhibit a growth advantage compared to hepatocytes. As a result, transplanted fetal liver cells are capable of repopulating normal livers. Important for cell-based therapies, hepatic stem/progenitor cells containing repopulation potential can be separated from fetal hematopoietic cells using the cell surface marker δ-like 1 (Dlk-1). In livers with advanced fibrosis, fetal epithelial stem/progenitor cells differentiate into functional hepatic cells and out-compete injured endogenous hepatocytes, which cause anti-fibrotic effects. Although fetal liver cells efficiently repopulate the liver, they will likely not be used for human cell transplantation. Thus, utilizing the underlying mechanism of repopulation and developed methods to produce similar growth-advantaged cells in vitro, e.g., human induced pluripotent stem cells (iPSCs), this approach has great potential for developing novel cell-based therapies in patients with liver disease. The present review gives a brief overview of the classic cell transplantation models and various cell sources studied as donor cell candidates. The advantages of fetal liver-derived stem/progenitor cells are discussed, as well as the mechanism of liver repopulation. Moreover, this article reviews the potential of in vitro developed synthetic human fetal livers from iPSCs and their therapeutic benefits.

Cell polarity and extrusion: How to polarize extrusion and extrude misspolarized cells?

The barrier function of epithelia is one of the cornerstones of the body plan organization of metazoans. It relies on the polarity of epithelial cells which organizes along the apico-basal axis the mechanical properties, signaling as well as transport. This barrier function is however constantly challenged by the fast turnover of epithelia occurring during morphogenesis or adult tissue homeostasis. Yet, the sealing property of the tissue can be maintained thanks to cell extrusion: a series of remodeling steps involving the dying cell and its neighbors leading to seamless cell expulsion. Alternatively, the tissue architecture can also be challenged by local damages or the emergence of mutant cells that may alter its organization. This includes mutants of the polarity complexes which can generate neoplastic overgrowths or be eliminated by cell competition when surrounded by wild type cells. In this review, we will provide an overview of the regulation of cell extrusion in various tissues focusing on the relationship between cell polarity, cell organization and the direction of cell expulsion. We will then describe how local perturbations of polarity can also trigger cell elimination either by apoptosis or by cell exclusion, focusing specifically on how polarity defects can be directly causal to cell elimination. Overall, we propose a general framework connecting the influence of polarity on cell extrusion and its contribution to aberrant cell elimination.

Fat body-derived Spz5 remotely facilitates tumor-suppressive cell competition through Toll-6-α-Spectrin axis-mediated Hippo activation

Tumor-suppressive cell competition is an evolutionarily conserved process that selectively removes precancerous cells to maintain tissue homeostasis. Using the polarity-deficiency-induced cell competition model in Drosophila, we identify Toll-6, a Toll-like receptor family member, as a driver of tension-mediated cell competition through α-Spectrin (α-Spec)-Yorkie (Yki) cascade. Toll-6 aggregates along the boundary between wild-type and polarity-deficient clones, where Toll-6 physically interacts with the cytoskeleton network protein α-Spec to increase mechanical tension, resulting in actomyosin-dependent Hippo pathway activation and the elimination of scrib mutant cells. Furthermore, we show that Spz5 secreted from fat body, the key innate organ in fly, facilitates the elimination of scrib clones by binding to Toll-6. These findings uncover mechanisms by which fat bodies remotely regulate tumor-suppressive cell competition of polarity-deficient tumors through inter-organ crosstalk and identified the Toll-6-α-Spec axis as an essential guardian that prevents tumorigenesis via tension-mediated cell elimination.

The Reservoir of Persistent Human Papillomavirus Infection; Strategies for Elimination Using Anti-Viral Therapies

Human Papillomaviruses have co-evolved with their human host, with each of the over 200 known HPV types infecting distinct epithelial niches to cause diverse disease pathologies. Despite the success of prophylactic vaccines in preventing high-risk HPV infection, the development of HPV anti-viral therapies has been hampered by the lack of enzymatic viral functions, and by difficulties in translating the results of in vitro experiments into clinically useful treatment regimes. In this review, we discuss recent advances in anti-HPV drug development, and highlight the importance of understanding persistent HPV infections for future anti-viral design. In the infected epithelial basal layer, HPV genomes are maintained at a very low copy number, with only limited viral gene expression; factors which allow them to hide from the host immune system. However, HPV gene expression confers an elevated proliferative potential, a delayed commitment to differentiation, and preferential persistence of the infected cell in the epithelial basal layer, when compared to their uninfected neighbours. To a large extent, this is driven by the viral E6 protein, which functions in the HPV life cycle as a modulator of epithelial homeostasis. By targeting HPV gene products involved in the maintenance of the viral reservoir, there appears to be new opportunities for the control or elimination of chronic HPV infections.

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.

A novel injury paradigm in the central nervous system of adult Drosophila: molecular, cellular and functional aspects

The mammalian central nervous system (CNS) exhibits limited regenerative capacity and the mechanisms that mediate its regeneration are not fully understood. Here, we present a novel experimental design to damage the CNS by using a contusion injury paradigm. The design of this protocol allows the study of long-term and short-term cellular responses, including those of the CNS and the immune system, and of any implications regarding functional recovery. We demonstrate for the first time that adult Drosophilamelanogaster glial cells undergo spontaneous functional recovery following crush injury. This crush injury leads to an intermediate level of functional recovery after damage, which is ideal to screen for genes that facilitate or prevent the regeneration process. Here, we validate this model and analyse the immune responses of glial cells as a central regulator of functional regeneration. Additionally, we demonstrate that glial cells and macrophages contribute to functional regeneration through mechanisms involving the Jun N-terminal kinase (JNK) pathway and the Drosophila protein Draper (Drpr), characteristic of other neural injury paradigms. We show that macrophages are recruited to the injury site and are required for functional recovery. Further, we show that the proteins Grindelwald and Drpr in Drosophila glial cells mediate activation of JNK, and that expression of drpr is dependent on JNK activation. Finally, we link neuron-glial communication and the requirement of neuronal vesicular transport to regulation of the JNK pathway and functional recovery.

This article has an associated First Person interview with the first author of the paper.

Summary: Central nervous system crush injury paradigm in adult Drosophilamelanogaster is a suitable model to study the cellular events, and genetic pathways behind injury responses and functional regeneration. We describe the immune responses of glial cells, neurons and macrophages following injury, and the functional relevance of each response.

Role of cell competition in ageing

Recent advances in rapid medical detection and diagnostic technology have extended both human health and life expectancy. However, ageing remains one of the critical risk factors in contributing to major incapacitating and fatal conditions, including cancer and neurodegeneration. Therefore, it is vital to study how ageing attributes to (or participates in) endangering human health via infliction of age-related diseases and what must be done to tackle this intractable process. This review encompasses the most recent literature elaborating the role of cell competition (CC) during ageing. CC is a process that occurs between two heterogeneous populations, where the cells with higher fitness levels have a competitive advantage over the neighbouring cells that have comparatively lower fitness levels. This interaction results in the selection of the fit cells, within a population, and elimination of the viable yet suboptimal cells. Therefore, it is tempting to speculate that, if this quality control mechanism works efficiently throughout life, can it ultimately lead to a healthier ageing and extended lifespan. Furthermore, the review aims to collate all the important state of the art publications that provides evidence of the relevance of CC in dietary restriction, stem cell dynamics, and cell senescence, thus, prompting us to advocate its contribution and in exploring new avenues and opportunities in fighting age-related conditions.

Culling Less Fit Neurons Protects against Amyloid-β-Induced Brain Damage and Cognitive and Motor Decline

Alzheimer’s disease (AD) is the most common form of dementia, impairing cognitive and motor functions. One of the pathological hallmarks of AD is neuronal loss, which is not reflected in mouse models of AD. Therefore, the role of neuronal death is still uncertain. Here, we used a Drosophila AD model expressing a secreted form of human amyloid-β42 peptide and showed that it recapitulates key aspects of AD pathology, including neuronal death and impaired long-term memory. We found that neuronal apoptosis is mediated by cell fitness-driven neuronal culling, which selectively eliminates impaired neurons from brain circuits. We demonstrated that removal of less fit neurons delays β-amyloid-induced brain damage and protects against cognitive and motor decline, suggesting that contrary to common knowledge, neuronal death may have a beneficial effect in AD.

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Peptides linked to neurodegenerative diseases reduce neuronal fitness in Drosophila

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β-amyloid-induced neuronal death is mediated by fitness regulators flower and azot

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Suppression of fitness-based neuronal culling aggravates cognitive and motor decline

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Neuronal death related to fitness-based selection has a beneficial net effect

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.

Exploring MYC relevance to cancer biology from the perspective of cell competition

Cancer has long been regarded and treated as a foreign body appearing by mistake inside a living organism. However, now we know that cancer cells communicate with neighbours, thereby creating modified environments able to support their unusual need for nutrients and space. Understanding the molecular basis of these bi-directional interactions is thus mandatory to approach the complex nature of cancer. Since their discovery, MYC proteins have been showing to regulate a steadily increasing number of processes impacting cell fitness, and are consistently found upregulated in almost all human tumours. Of interest, MYC takes part in cell competition, an evolutionarily conserved fitness comparison strategy aimed at detecting weakened cells, which are then committed to death, removed from the tissue and replaced by fitter neighbours. During physiological development, MYC-mediated cell competition is engaged to eliminate cells with suboptimal MYC levels, so as to guarantee selective growth of the fittest and proper homeostasis, while transformed cells expressing high levels of MYC coopt cell competition to subvert tissue constraints, ultimately disrupting homeostasis. Therefore, the interplay between cells with different MYC levels may result in opposite functional outcomes, depending on the nature of the players. In the present review, we describe the most recent findings on the role of MYC-mediated cell competition in different contexts, with a special emphasis on its impact on cancer initiation and progression. We also discuss the relevance of competition-associated cell death to cancer disease.

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.

Systemic signalling and local effectors in developmental stability, body symmetry, and size

Symmetric growth and the origins of fluctuating asymmetry are unresolved phenomena of biology. Small, and sometimes noticeable, deviations from perfect bilateral symmetry reflect the vulnerability of development to perturbations. The degree of asymmetry is related to the magnitude of the perturbations and the ability of an individual to cope with them. As the left and right sides of an individual were presumed to be genetically identical, deviations of symmetry were traditionally attributed to non-genetic effects such as environmental and developmental noise. In this review, we draw attention to other possible sources of variability, especially to somatic mutations and transposons. Mutations are a major source of phenotypic variability and recent genomic data have highlighted somatic mutations as ubiquitous, even in phenotypically normal individuals. We discuss the importance of factors that are responsible for buffering and stabilizing the genome and for maintaining size robustness and quality through elimination of less-fit or damaged cells. However, the important question that arises from these studies is whether this self-correcting capacity and intrinsic organ size controls are sufficient to explain how symmetric structures can reach an identical size and shape. Indeed, recent discoveries in the fruit fly have uncovered a conserved hormone of the insulin/IGF/relaxin family, Dilp8, that is responsible for stabilizing body size and symmetry in the face of growth perturbations. Dilp8 alarm signals periphery growth status to the brain, where it acts on its receptor Lgr3. Loss of Dilp8-Lgr3 signaling renders flies incapable of detecting growth perturbations and thus maintaining a stable size and symmetry. These findings help to understand how size and symmetry of somatic tissues remain undeterred in noisy environments, after injury or illnesses, and in the presence of accumulated somatic mutations.

Learning on the Fly: The Interplay between Caspases and Cancer

The ease of genetic manipulation, as well as the evolutionary conservation of gene function, has placed Drosophila melanogaster as one of the leading model organisms used to understand the implication of many proteins with disease development, including caspases and their relation to cancer. The family of proteases referred to as caspases have been studied over the years as the major regulators of apoptosis: the most common cellular mechanism involved in eliminating unwanted or defective cells, such as cancerous cells. Indeed, the evasion of the apoptotic programme resulting from caspase downregulation is considered one of the hallmarks of cancer. Recent investigations have also shown an instrumental role for caspases in non-lethal biological processes, such as cell proliferation, cell differentiation, intercellular communication, and cell migration. Importantly, malfunction of these essential biological tasks can deeply impact the initiation and progression of cancer. Here, we provide an extensive review of the literature surrounding caspase biology and its interplay with many aspects of cancer, emphasising some of the key findings obtained from Drosophila studies. We also briefly describe the therapeutic potential of caspase modulation in relation to cancer, highlighting shortcomings and hopeful promises.

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.

Cell Extrusion: A Stress-Responsive Force for Good or Evil in Epithelial Homeostasis

Epithelial tissues robustly respond to internal and external stressors via dynamic cellular rearrangements. Cell extrusion acts as a key regulator of epithelial homeostasis by removing apoptotic cells, orchestrating morphogenesis, and mediating competitive cellular battles during tumorigenesis. Here, we delineate the diverse functions of cell extrusion during development and disease. We emphasize the expanding role for apoptotic cell extrusion in exerting morphogenetic forces, as well as the strong intersection of cell extrusion with cell competition, a homeostatic mechanism that eliminates aberrant or unfit cells. While cell competition and extrusion can exert potent, tumor-suppressive effects, dysregulation of either critical homeostatic program can fuel cancer progression.

Troponin-I enhances and is required for oncogenic overgrowth

Human tumors of various tissue origins show an intriguing over-expression of genes not considered oncogenes, such as that encoding Troponin-I (TnI), a well-known muscle protein. Out of the three TnI genes known in humans, the slow form, TNNI1, is affected the most. Drosophila has only one TnI gene, wupA. Here, we studied excess- and loss-of function of wupA in Drosophila, and assayed TNNI1 down regulation in human tumors growing in mice. Drosophila TnI excess-of-function increases proliferation and potentiates oncogenic mutations in Ras, Notch and Lgl genes. By contrast, TnI loss-of-function reduces proliferation and antagonizes the overgrowth due to these oncogenic mutations. Troponin-I defective cells undergo Flower- and Sparc-dependent cell competition. TnI can localize to the nucleus and its excess elicits transcriptional up-regulation of InR, Rap1 and Dilp8, which is consistent with the increased cell proliferation. Human tumor cell lines treated with a human Troponin-I peptide arrest in G₀/G_1. In addition, proliferation of non-small-cell lung carcinoma xenografts in mice is restrained by TNNI1 down-regulation. Thus, Troponin-I reveals a novel function in cell proliferation that may be of therapeutic interest in certain types of cancer.

Elimination of classically-activated macrophages in tumor-conditioned medium by alternatively-activated macrophages

Cellular interactions are critical during development, tissue fitness and epithelial tumor development. The expression levels of specific genes confer to tumoral cells a survival advantage versus the normal neighboring cells. As a consequence, cells surrounding tumors are eliminated and engulfed by macrophages. We propose a novel scenario in which circulating cells facing a tumor can reproduce these cellular interactions. In vitro cultured macrophages from murine bone marrow were used to investigate this hypothesis. M1 macrophages in tumoral medium upregulated markers of a suboptimal condition, such as Sparc and TyrRS, and undergo apoptosis. However, M2 macrophages display higher Myc expression levels and proliferate at the expense of M1. Resulting M1 apoptotic debris is engulfed by M2 in a Sparc- and TyrRS-dependent manner. These findings suggest that tumor-dependent macrophage elimination could deplete immune response against tumors. This possibility could be relevant for macrophage based anti-tumoral strategies.

Human Cancer Cells Signal Their Competitive Fitness Through MYC Activity

MYC-mediated cell competition is a cell-cell interaction mechanism known to play an evolutionary role during development from Drosophila to mammals. Cells expressing low levels of MYC, called losers, are committed to die by nearby cells with high MYC activity, called winners, that overproliferate to compensate for cell loss, so that the fittest cells be selected for organ formation. Given MYC's consolidated role in oncogenesis, cell competition is supposed to be relevant to cancer, but its significance in human malignant contexts is largely uncharacterised. Here we show stereotypical patterns of MYC-mediated cell competition in human cancers: MYC-upregulating cells and apoptotic cells were indeed repeatedly found at the tumour-stroma interface and within the tumour parenchyma. Cell death amount in the stromal compartment and MYC protein level in the tumour were highly correlated regardless of tumour type and stage. Moreover, we show that MYC modulation in heterotypic co-cultures of human cancer cells is sufficient as to subvert their competitive state, regardless of genetic heterogeneity. Altogether, our findings suggest that the innate role of MYC-mediated cell competition in development is conserved in human cancer, with malignant cells using MYC activity to colonise the organ at the expense of less performant neighbours.

Slit-Robo Repulsive Signaling Extrudes Tumorigenic Cells from Epithelia

Cells dynamically interact throughout animal development to coordinate growth and deter disease. For example, cell-cell competition weeds out aberrant cells to enforce homeostasis. In Drosophila, tumorigenic cells mutant for the cell polarity gene scribble (scrib) are actively eliminated from epithelia when surrounded by wild-type cells. While scrib cell elimination depends critically on JNK signaling, JNK-dependent cell death cannot sufficiently explain scrib cell extirpation. Thus, how JNK executed cell elimination remained elusive. Here, we show that repulsive Slit-Robo2-Ena signaling exerts an extrusive force downstream of JNK to eliminate scrib cells from epithelia by disrupting E-cadherin. While loss of Slit-Robo2-Ena in scrib cells potentiates scrib tumor formation within the epithelium, Robo2-Ena hyperactivation surprisingly triggers luminal scrib tumor growth following excess extrusion. This extrusive signaling is amplified by a positive feedback loop between Slit-Robo2-Ena and JNK. Our observations provide a potential causal mechanism for Slit-Robo dysregulation in numerous human cancers.

MYC, Cell Competition, and Cell Death in Cancer: The Inseparable Triad

Deregulation of MYC family proteins in cancer is associated with a global reprogramming of gene expression, ultimately promoting glycolytic pathways, cell growth, and proliferation. It is well known that MYC upregulation triggers cell-autonomous apoptosis in normal tissues, while frankly malignant cells develop resistance to apoptotic stimuli, partly resulting from MYC addiction. As well as inducing cell-autonomous apoptosis, MYC upregulation is able to trigger non cell-autonomous apoptotic death through an evolutionarily conserved mechanism known as “cell competition”. With regard to this intimate and dual relationship between MYC and cell death, recent evidence obtained in Drosophila models of cancer has revealed that, in early tumourigenesis, MYC upregulation guides the clonal expansion of mutant cells, while the surrounding tissue undergoes non-cell autonomous death. Apoptosis inhibition in this context was shown to restrain tumour growth and to restore a wild-type phenotype. This suggests that cell-autonomous and non cell-autonomous apoptosis dependent on MYC upregulation may shape tumour growth in different ways, soliciting the need to reconsider the role of cell death in cancer in the light of this new level of complexity. Here we review recent literature about MYC and cell competition obtained in Drosophila, with a particular emphasis on the relevance of cell death to cell competition and, more generally, to cancer. Possible implications of these findings for the understanding of mammalian cancers are also discussed.

The SWI/SNF Complex Protein Snr1 Is a Tumor Suppressor in Drosophila Imaginal Tissues

Components of the SWI/SNF chromatin-remodeling complex are among the most frequently mutated genes in various human cancers, yet only SMARCB1/hSNF5, a core member of the SWI/SNF complex, is mutated in malignant rhabdoid tumors (MRT). How SMARCB1/hSNF5 functions differently from other members of the SWI/SNF complex remains unclear. Here, we use Drosophila imaginal epithelial tissues to demonstrate that Snr1, the conserved homolog of human SMARCB1/hSNF5, prevents tumorigenesis by maintaining normal endosomal trafficking-mediated signaling cascades. Removal of Snr1 resulted in neoplastic tumorigenic overgrowth in imaginal epithelial tissues, whereas depletion of any other members of the SWI/SNF complex did not induce similar phenotypes. Unlike other components of the SWI/SNF complex that were detected only in the nucleus, Snr1 was observed in both the nucleus and the cytoplasm. Aberrant regulation of multiple signaling pathways, including Notch, JNK, and JAK/STAT, was responsible for tumor progression upon snr1-depletion. Our results suggest that the cytoplasmic Snr1 may play a tumor suppressive role in Drosophila imaginal tissues, offering a foundation for understanding the pivotal role of SMARCB1/hSNF5 in suppressing MRT during early childhood. Cancer Res; 77(4); 862-73. ©2017 AACR.

Mechanisms and consequences of entosis

Multiple mechanisms have emerged where the engulfment of whole live cells, leading to the formation of what are called 'cell-in-cell' structures, induces cell death. Entosis is one such mechanism that drives cell-in-cell formation during carcinogenesis and development. Curiously, entotic cells participate actively in their own engulfment, by invading into their hosts, and are then killed non-cell-autonomously. Here we review the mechanisms of entosis and entotic cell death and the consequences of entosis on cell populations.

Both the apoptotic suicide pathway and phagocytosis are required for a programmed cell death in Caenorhabditis elegans

Background

Programmed cell deaths in the nematode Caenorhabditis elegans are generally considered suicides. Dying cells are engulfed by neighboring cells in a process of phagocytosis. To better understand the interaction between the engulfment and death processes, we analyzed B.al/rapaav cell death, which has been previously described as engulfment-dependent and hence as a possible murder.

Results

We found that B.al/rapaav is resistant to caspase-pathway activation: the caspase-mediated suicide pathway initiates the cell-death process but is insufficient to cause B.al/rapaav death without the subsequent assistance of engulfment. When the engulfing cell P12.pa is absent, other typically non-phagocytic cells can display cryptic engulfment potential and facilitate this death.

Conclusions

We term this death an “assisted suicide” and propose that assisted suicides likely occur in other organisms. The study of assisted suicides might provide insight into non-cell autonomous influences on cell death. Understanding the mechanism that causes B.al/rapaav to be resistant to activation of the caspase pathway might reveal the basis of differences in the sensitivity to apoptotic stimuli of tumor and normal cells, a key issue in the field of cancer therapeutics.

Electronic supplementary material

The online version of this article (doi:10.1186/s12915-016-0262-5) contains supplementary material, which is available to authorized users.

Drosophila and the hallmarks of cancer

: Cancer was the disease of the twentieth century. Today it is still a leading cause of death worldwide despite being intensively investigated. Abundant knowledge exists regarding the pathological and molecular mechanisms that drive healthy cells to become malignant and form metastatic tumors. The relation of oncogenes and tumor suppressors to the genetic trigger of carcinogenesis is unquestionable. However, the development of the disease requires many characteristics that due to their proven role in cancer are collectively described as the "hallmarks of cancer." We highlight here the historic discoveries made using the model organism Drosophila melanogaster and its contributions to biomedical and cancer research. Flies are utilized as a model organism for the investigation of each and every aspect of cancer hallmarks. Due to the significant conservation between flies and mammals at the signaling and tissue physiology level it is possible to explore the genes and mechanisms responsible for cancer pathogenesis in flies. Recent Drosophila studies suggest novel aspects of therapeutic intervention and are expected to guide cancer research in the twenty-first century.

Cell Competition Drives the Formation of Metastatic Tumors in a Drosophila Model of Epithelial Tumor Formation

Cell competition is a homeostatic process in which proliferating cells compete for survival. Elimination of otherwise normal healthy cells through competition is important during development and has recently been shown to contribute to maintaining tissue health during organismal aging. The mechanisms that allow for ongoing cell competition during adult life could, in principle, contribute to tumorigenesis. However, direct evidence supporting this hypothesis has been lacking. Here, we provide evidence that cell competition drives tumor formation in a Drosophila model of epithelial cancer. Cells expressing EGFR together with the conserved microRNA miR-8 acquire the properties of supercompetitors. Neoplastic transformation and metastasis depend on the ability of these cells to induce apoptosis and engulf nearby cells. miR-8 expression causes genome instability by downregulating expression of the Septin family protein Peanut. Cytokinesis failure due to downregulation of Peanut is required for tumorigenesis. This study provides evidence that the cellular mechanisms that drive cell competition during normal tissue growth can be co-opted to drive tumor formation and metastasis. Analogous mechanisms for cytokinesis failure may lead to polyploid intermediates in tumorigenesis in mammalian cancer models.

Spontaneous Cell Competition in Immortalized Mammalian Cell Lines

Cell competition is a form of cell-cell interaction by which cells compare relative levels of fitness, resulting in the active elimination of less-fit cells, “losers,” by more-fit cells, “winners.” Here, we show that in three routinely-used mammalian cell lines – U2OS, 3T3, and MDCK cells – sub-clones arise stochastically that exhibit context-dependent competitive behavior. Specifically, cell death is elicited when winner and loser sub-clones are cultured together but not alone. Cell competition and elimination in these cell lines is caspase-dependent and requires cell-cell contact but does not require de novo RNA synthesis. Moreover, we show that the phenomenon involves differences in cellular metabolism. Hence, our study demonstrates that cell competition is a common feature of immortalized mammalian cells in vitro and implicates cellular metabolism as a mechanism by which cells sense relative levels of “fitness.”

Epithelial homeostasis

Epithelia form intelligent, dynamic barriers between the external environment and an organism's interior. Intercellular cadherin-based adhesions adapt and respond to mechanical forces and cell density, while tight junctions flexibly control diffusion both within the plasma membrane and between adjacent cells. Epithelial integrity and homeostasis are of central importance to survival, and mechanisms have evolved to ensure these processes are maintained during growth and in response to damage. For instance, cell competition surveys the fitness of cells within epithelia and removes the less fit; extrusion or delamination can remove apoptotic or defective cells from the epithelial sheet and can restore homeostasis when an epithelial layer becomes too crowded; spindle orientation ensures two-dimensional growth in simple epithelia and controls stratification in complex epithelia; and transition to a mesenchymal phenotype enables active escape from an epithelial layer. This review will discuss these various mechanisms and consider how they are subverted in disease.

Elimination of unfit cells maintains tissue health and prolongs lifespan

Viable yet damaged cells can accumulate during development and aging. Although eliminating those cells may benefit organ function, identification of this less fit cell population remains challenging. Previously, we identified a molecular mechanism, based on “fitness fingerprints” displayed on cell membranes, which allows direct fitness comparison among cells in Drosophila. Here, we study the physiological consequences of efficient cell selection for the whole organism. We find that fitness-based cell culling is naturally used to maintain tissue health, delay aging, and extend lifespan in Drosophila. We identify a gene, azot, which ensures the elimination of less fit cells. Lack of azot increases morphological malformations and susceptibility to random mutations and accelerates tissue degeneration. On the contrary, improving the efficiency of cell selection is beneficial for tissue health and extends lifespan.

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Fitness-based cell culling maintains tissue health

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Azot ensures the elimination of less fit cells

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Lack of azot accelerates tissue degeneration

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Improving the efficiency of cell selection extends lifespan

Of flies, mice, and men: evolutionarily conserved tissue damage responses and aging

Studies in flies, mice, and human models have provided a conceptual framework for how paracrine interactions between damaged cells and the surrounding tissue control tissue repair. These studies have amassed evidence for an evolutionarily conserved secretory program that regulates tissue homeostasis. This program coordinates cell survival and proliferation during tissue regeneration and repair in young animals. By virtue of chronic engagement, however, it also contributes to the age-related decline of tissue homeostasis leading to degeneration, metabolic dysfunction, and cancer. Here, we review recent studies that shed light on the nature and regulation of this evolutionarily conserved secretory program.

Darwin's multicellularity: from neurotrophic theories and cell competition to fitness fingerprints

Metazoans have evolved ways to engage only the most appropriate cells for long-term tissue development and homeostasis. In many cases, competitive interactions have been shown to guide such cell selection events. In Drosophila, a process termed cell competition eliminates slow proliferating cells from growing epithelia. Recent studies show that cell competition is conserved in mammals with crucial functions like the elimination of suboptimal stem cells from the early embryo and the replacement of old T-cell progenitors in the thymus to prevent tumor formation. Moreover, new data in Drosophila has revealed that fitness indicator proteins, required for cell competition, are also involved in the culling of retinal neurons suggesting that 'fitness fingerprints' may play a general role in cell selection.

Signaling by the engulfment receptor draper: a screen in Drosophila melanogaster implicates cytoskeletal regulators, Jun N-terminal Kinase, and Yorkie

Draper, the Drosophila melanogaster homolog of the Ced-1 protein of Caenorhabditis elegans, is a cell-surface receptor required for the recognition and engulfment of apoptotic cells, glial clearance of axon fragments and dendritic pruning, and salivary gland autophagy. To further elucidate mechanisms of Draper signaling, we screened chromosomal deficiencies to identify loci that dominantly modify the phenotype of overexpression of Draper isoform II (suppressed differentiation of the posterior crossvein in the wing). We found evidence for 43 genetic modifiers of Draper II. Twenty-four of the 37 suppressor loci and 3 of the 6 enhancer loci were identified. An additional 5 suppressors and 2 enhancers were identified among mutations in functionally related genes. These studies reveal positive contributions to Drpr signaling for the Jun N-terminal Kinase pathway, supported by genetic interactions with hemipterous, basket, jun, and puckered, and for cytoskeleton regulation as indicated by genetic interactions with rac1, rac2, RhoA, myoblast city, Wiskcott-Aldrich syndrome protein, and the formin CG32138, and for yorkie and expanded. These findings indicate that Jun N-terminal Kinase activation and cytoskeletal remodeling collaborate in Draper signaling. Relationships between Draper signaling and Decapentaplegic signaling, insulin signaling, Salvador/Warts/Hippo signaling, apical-basal cell polarity, and cellular responses to mechanical forces are also discussed.