Competition between human cells by entosis

Human carcinomas are comprised of complex mixtures of tumor cells that are known to compete indirectly for nutrients and growth factors. Whether tumor cells could also compete directly, for example by elimination of rivals, is not known. Here we show that human cells can directly compete by a mechanism of engulfment called entosis. By entosis, cells are engulfed, or cannibalized while alive, and subsequently undergo cell death. We find that the identity of engulfing (“winner”) and engulfed (“loser”) cells is dictated by mechanical deformability controlled by RhoA and actomyosin, where tumor cells with high deformability preferentially engulf and outcompete neighboring cells with low deformability in heterogeneous populations. We further find that activated Kras and Rac signaling impart winner status to cells by downregulating contractile myosin, allowing for the internalization of neighboring cells that eventually undergo cell death. Finally, we compute the energy landscape of cell-in-cell formation, demonstrating that a mechanical differential between winner and loser cells is required for entosis to proceed. These data define a mechanism of competition in mammalian cells that occurs in human tumors.     

The Active Role of Corpse Engulfment Pathways During Cell Competition

Cell competition was first described in imaginal discs of genetically-mosaic Drosophila. In extreme cases, cell competition can replace entire compartments with the descendants of a single cell. We recently identified five genes that are required by wild type epithelial cells to kill neighboring Minute cells during cell competition. These draper, wasp, phosphatidyl-serine receptor, MBC/DOCK180 and Rac1 genes, were each previously implicated in the engulfment of apoptotic corpses. The results draw attention to the active, killing role of engulfing cells during cell competition. Here we discuss the contributions of these engulfment genes to Minute competition in more detail, and compare Minute competition with competition between cells expressing different levels of Myc, or of Warts pathway genes. We also speculate about how cell interactions at clone boundaries may initiate cell competition.     

The active role of corpse engulfment pathways during cell competition

Cell competition was first described in imaginal discs of genetically-mosaic Drosophila. In extreme cases, cell competition can replace entire compartments with the descendents of a single cell. We recently identified five genes that are required by wild-type epithelial cells to kill neighboring Minute cells during cell competition. These draper, wasp, phosphatidyl-serine receptor, MBC/DOCK180 and Rac1 genes, were each previously implicated in the engulfment of apoptotic corpses. The results draw attention to the active, killing role of engulfing cells during cell competition. Here we discuss the contributions of these engulfment genes to Minute competition in more detail, and compare Minute competition with competition between cells expressing different levels of Myc, or of Warts pathway genes. We also speculate about how cell interactions at clone boundaries may initiate cell competition.     

Engulfment and clearance of apoptotic cells based on a GlcNAc-binding lectin-like property of surface vimentin

The clearance of apoptotic cells is important to maintain tissue homeostasis. The engulfment of apoptotic cells is performed by professional phagocytes, such as macrophages, and also by non-professional phagocytes, such as mesenchymal cells. Here, we show that vimentin, a cytoskeletal protein, functions as an engulfment receptor on neighboring phagocytes, which recognize O-linked β-N-acetylglucosamine (O-GlcNAc)-modified proteins from apoptotic cells as "eat me" ligands. Previously, we reported that vimentin possesses a GlcNAc-binding lectin-like property on cell surface. However, the physiological relevance of the surface localization and GlcNAc-binding property of vimentin remained unclear. In the present study, we observed that O-GlcNAc proteins from apoptotic cells interacted with the surface vimentin of neighboring phagocytes and that this interaction induced serine 71-phosphorylation and recruitment of vimentin to the cell surface of the neighboring phagocytes. Moreover, tetrameric vimentin that was disassembled by serine 71-phosphorylation possessed a GlcNAc-binding activity and was localized to the cell surface. We demonstrated our findings in vimentin-expressing common cell lines such as HeLa cells. Furthermore, during normal developmental processes, the phagocytic engulfment and clearance of apoptotic footplate cells in mouse embryos was mediated by the interaction of surface vimentin with O-GlcNAc proteins. Our results suggest a common mechanism for the clearance of apoptotic cells, through the interaction of surface vimentin with O-GlcNAc-modified proteins.     

Cell competition and its implications for development and cancer

Cell competition is a struggle for existence between cells in heterogeneous tissues of multicellular organisms.Loser cells,which die during cell competition,are normally viable when grown only with other loser cells,but when mixed with winner cells,they are at a growth disadvantage and undergo apoptosis.Intriguingly,several recent studies have revealed that cells bearing mutant tumor-suppressor genes,which show overgrowth and tumorigenesis in a homotypic situation,are frequently eliminated,through cell competition,from tissues in which they are surrounded by wild-type cells.Here,we focus on the regulation of cellular competitiveness and the mechanism of cell competition as inferred from two different categories of mutant cells:(1) slower-growing cells and (2) structurally defective cells.We also discuss the possible role of cell competition as an intrinsic homeostasis system through which normal cells sense and remove aberrant cells,such as precancerous cells,to maintain the integrity and normal development of tissues and organs.     

Cell competition and its implications for development and cancer

Cell competition is a struggle for existence between cells in heterogeneous tissues of multicellular organisms.Loser cells,which die during cell competition,are normally viable when grown only with other loser cells,but when mixed with winner cells,they are at a growth disadvantage and undergo apoptosis.Intriguingly,several recent studies have revealed that cells bearing mutant tumor-suppressor genes,which show overgrowth and tumorigenesis in a homotypic situation,are frequently eliminated,through cell competition,from tissues in which they are surrounded by wild-type cells.Here,we focus on the regulation of cellular competitiveness and the mechanism of cell competition as inferred from two different categories of mutant cells:(1) slower-growing cells and (2) structurally defective cells.We also discuss the possible role of cell competition as an intrinsic homeostasis system through which normal cells sense and remove aberrant cells,such as precancerous cells,to maintain the integrity and normal development of tissues and organs.     

Two-step engulfment of apoptotic cells

Apoptotic cells expose phosphatidylserine on their surface as an "eat me" signal, and macrophages respond by engulfing them. Although several molecules that specifically bind phosphatidylserine have been identified, the molecular mechanism that triggers engulfment remains elusive. Here, using a mouse pro-B cell line, Ba/F3, that grows in suspension, we reconstituted the engulfment of apoptotic cells. The parental Ba/F3 cells did not engulf apoptotic cells. Ba/F3 transformants expressing T cell immunoglobulin- and mucin-domain-containing molecule 4 (Tim4), a type I membrane protein that specifically binds phosphatidylserine, efficiently bound apoptotic cells in a phosphatidylserine-dependent manner but did not engulf them. However, Ba/F3 transformants expressing both Tim4 and the integrin α(v)β(3) complex bound to and engulfed apoptotic cells in the presence of milk fat globule epidermal growth factor factor VIII (MFG-E8), a secreted protein that can bind phosphatidylserine and integrin α(v)β(3). These results indicate that the engulfment of apoptotic cells proceeds in two steps: Tim4 tethers apoptotic cells, and the integrin α(v)β(3) complex mediates engulfment in coordination with MFG-E8. A similar two-step engulfment of apoptotic cells was observed with mouse resident peritoneal macrophages. Furthermore, the Tim4/integrin-mediated engulfment by the Ba/F3 cells was enhanced in cells expressing Rac1 and Rab5, suggesting that this system well reproduces the engulfment of apoptotic cells by macrophages.     

Apoptotic cells are cleared by directional migration and elmo1- dependent macrophage engulfment

Apoptotic cell death is essential for development and tissue homeostasis. Failure to clear apoptotic cells can ultimately cause inflammation and autoimmunity. Apoptosis has primarily been studied by staining of fixed tissue sections, and a clear understanding of the behavior of apoptotic cells in living tissue has been elusive. Here, we use a newly developed technique to track apoptotic cells in real time as they emerge and are cleared from the zebrafish brain. We find that apoptotic cells are remarkably motile, frequently migrating several cell diameters to the periphery of living tissues. F-actin remodeling occurs in surrounding cells, but also within the apoptotic cells themselves, suggesting a cell-autonomous component of motility. During the first 2 days of development, engulfment is rare, and most apoptotic cells lyse at the brain periphery. By 3 days postfertilization, most cell corpses are rapidly engulfed by macrophages. This engulfment requires the guanine nucleotide exchange factor elmo1. In elmo1-deficient macrophages, engulfment is rare and may occur through macropinocytosis rather than directed engulfment. These findings suggest that clearance of apoptotic cells in living vertebrates is accomplished by the combined actions of apoptotic cell migration and elmo1-dependent macrophage engulfment.     

Loss of the RhoGAP SRGP-1 promotes the clearance of dead and injured cells in Caenorhabditis elegans

Multicellular animals rapidly clear dying cells from their bodies. Many of the pathways that mediate this cell removal are conserved through evolution. Here, we identify srgp-1 as a negative regulator of cell clearance in both Caenorhabditis elegans and mammalian cells. Loss of srgp-1 function results in improved engulfment of apoptotic cells, whereas srgp-1 overexpression inhibits apoptotic cell corpse removal. We show that SRGP-1 functions in engulfing cells and functions as a GTPase activating protein (GAP) for CED-10 (Rac1). Interestingly, loss of srgp-1 function promotes not only the clearance of already dead cells, but also the removal of cells that have been brought to the verge of death through sublethal apoptotic, necrotic or cytotoxic insults. In contrast, impaired engulfment allows damaged cells to escape clearance, which results in increased long-term survival. We propose that C. elegans uses the engulfment machinery as part of a primitive, but evolutionarily conserved, survey mechanism that identifies and removes unfit cells within a tissue.     

Evidence for a conserved role for CRKII and Rac in engulfment of apoptotic cells

Apoptosis or programmed cell death occurs in multicellular organisms throughout life. The removal of apoptotic cells by phagocytes prevents secondary necrosis and inflammation and also plays a key role in tissue remodeling and regulating immune responses. The molecular mechanisms that regulate the engulfment of apoptotic cells are just beginning to be elucidated. Recent genetic studies in the nematode Caenorhabditis elegans have implicated at least six genes in the removal of apoptotic cell corpses. The gene products of ced-2, ced-5, and ced-10 are thought to be part of a pathway that regulates the reorganization of the cytoskeleton during engulfment. The adapter proteins CrkII and Dock180 and the small GTPase Rac represent the mammalian orthologues of the ced-2, ced-5 and ced-10 gene products, respectively. It is not known whether CrkII, Dock180, or Rac proteins have any role during engulfment in mammalian cells. Here we show, using stable cell lines and transient transfections, that overexpression of wild-type CrkII or an activated form of Rac1 enhances engulfment. Mutants of CrkII failed to mediate this increased engulfment. The higher CrkII-mediated uptake was inhibited by coexpression of a dominant negative form of Rac1 but not by a dominant a negative Rho protein; this suggested that Rac functions downstream of CrkII in this process, which is consistent with genetic studies in the worm that place ced-10 (rac) downstream of ced-2 (crk) in cell corpse removal. Taken together, these data suggest that CED-2/CrkII and CED-10/Rac are part of an evolutionarily conserved pathway in engulfment of apoptotic cells.     

Clearance of apoptotic cells in Caenorhabditis elegans

Programmed cell death, or apoptosis, is a genetically controlled process of cell suicide that is a common fate during an animal's life. In metazoans, apoptotic cells are rapidly removed from the body through the process of phagocytosis. Genetic analyses probing the mechanisms controlling the engulfment of apoptotic cells were pioneered in the nematode Caenorhabditis elegans. So far, at least seven genes have been identified that are required for the recognition and engulfment of apoptotic cells and have been shown to function in two partially redundant signaling pathways. Molecular characterization of their gene products has lead to the finding that similar genes act to control the same processes in other organisms, including mammals. In this paper, we review these exciting findings in C. elegans and discuss their implications in understanding the clearance of apoptotic cells in mammals.     

Forespore engulfment mediated by a ratchet-like mechanism

A key step in bacterial endospore formation is engulfment, during which one bacterial cell engulfs another in a phagocytosis-like process that normally requires SpoIID, SpoIIM, and SpoIIP (DMP). We here describe a second mechanism involving the zipper-like interaction between the forespore protein SpoIIQ and its mother cell ligand SpoIIIAH, which are essential for engulfment when DMP activity is reduced or SpoIIB is absent. They are also required for the rapid engulfment observed during the enzymatic removal of peptidoglycan, a process that does not require DMP. These results suggest the existence of two separate engulfment machineries that compensate for one another in intact cells, thereby rendering engulfment robust. Photobleaching analysis demonstrates that SpoIIQ assembles a stationary structure, suggesting that SpoIIQ and SpoIIIAH function as a ratchet that renders forward membrane movement irreversible. We suggest that ratchet-mediated engulfment minimizes the utilization of chemical energy during this dramatic cellular reorganization, which occurs during starvation.     

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.     

Clearance of apoptotic corpses

Apoptotic corpses can be engulfed and cleared by many other cell types in addition to ‘professional’ phagocytes such as macrophage. Studies of several organisms have contributed to the understanding of apoptotic corpse engulfment. Two partially redundant engulfment pathways have been characterized that act even in non-professional phagocytes to promote corpse engulfment. This review summarizes some recent progress in signaling by these pathways, including the exposure of eat-me-signals on apoptotic cells, and insights from Drosophila on the roles of the bridging receptor Six Microns Under, the non-receptor tyrosine kinase Shark, and store-operated calcium release in the Draper/Ced-1 pathway of corpse recognition and internalization. The mechanism of apoptotic phagosome maturation is outlined, and possible connections between corpse engulfment and proliferation, cell competition, and immunity are discussed.     

Autophagy gene-dependent clearance of apoptotic cells during embryonic development

Autophagy is commonly observed in metazoan organisms during programmed cell death (PCD), but its function in dying cells has been unclear. We studied the role of autophagy in embryonic cavitation, the earliest PCD process in mammalian development. Embryoid bodies (EBs) derived from cells lacking the autophagy genes, atg5 or beclin 1, fail to cavitate. This defect is due to persistence of cell corpses, rather than impairment of PCD. Dying cells in autophagy gene null EBs fail to express the "eat-me" signal, phosphatidylserine exposure, and secrete lower levels of the "come-get-me" signal, lysophosphatidylcholine. These defects are associated with low levels of cellular ATP and are reversed by treatment with the metabolic substrate, methylpyruvate. Moreover, mice lacking atg5 display a defect in apoptotic corpse engulfment during embryonic development. We conclude that autophagy contributes to dead-cell clearance during PCD by a mechanism that likely involves the generation of energy-dependent engulfment signals.     

Involvement of Beclin 1 in engulfment of apoptotic cells

Efficient apoptotic cell engulfment is important for both tissue homeostasis and immune response in mammals. In the present study, we report that Beclin 1 (a regulator of autophagy) is required for apoptotic cell engulfment. The engulfment process was largely abolished in Beclin 1 knock-out cells, and Beclin 1 knockdown significantly decreased apoptotic cell internalization in macrophage and fibroblast cell lines. Beclin 1 was recruited to the early phagocytic cup along with the generation of phosphatidylinositol 3-phosphate and Rac1, which regulates actin dynamics in lamellipodia. No lamellipodia were formed in Beclin 1 knock-out cells, and Beclin 1 knockdown completely inhibited the promotion of engulfment by ectopic expression of Rac1. Beclin 1 was co-immunoprecipitated with Rac1. These data indicate that Beclin 1 coordinates actin dynamics and membrane phospholipid synthesis to promote efficient apoptotic cell engulfment.     

Phagocytosis of apoptotic cells is regulated by a UNC-73/TRIO-MIG-2/RhoG signaling module and armadillo repeats of CED-12/ELMO

BACKGROUND: Phagocytosis of cells undergoing apoptosis is essential during development, cellular turnover, and wound healing. Failure to promptly clear apoptotic cells has been linked to autoimmune disorders. C. elegans CED-12 and mammalian ELMO are evolutionarily conserved scaffolding proteins that play a critical role in engulfment from worm to human. ELMO functions together with Dock180 (a guanine nucleotide exchange factor for Rac) to mediate Rac-dependent cytoskeletal reorganization during engulfment and cell migration. However, the components upstream of ELMO and Dock180 during engulfment remain elusive. RESULTS: Here, we define a conserved signaling module involving the small GTPase RhoG and its exchange factor TRIO, which functions upstream of ELMO/Dock180/Rac during engulfment. Complementary studies in C. elegans show that MIG-2 (which we identify as the homolog of mammalian RhoG) and UNC-73 (the TRIO homolog) also regulate corpse clearance in vivo, upstream of CED-12. At the molecular level, we identify a novel set of evolutionarily conserved Armadillo (ARM) repeats within CED-12/ELMO that mediate an interaction with activated MIG-2/RhoG; this, in turn, promotes Dock180-mediated Rac activation and cytoskeletal reorganization. CONCLUSIONS: The combination of in vitro and in vivo studies presented here identify two evolutionarily conserved players in engulfment, TRIO/UNC73 and RhoG/MIG-2, and the TRIO --> RhoG signaling module is linked by ELMO/CED-12 to Dock180-dependent Rac activation during engulfment. This work also identifies ARM repeats within CED-12/ELMO and their role in linking RhoG and Rac, two GTPases that function in tandem during engulfment.     

Opposite effects of rho family GTPases on engulfment of apoptotic cells by macrophages

The efficient engulfment of apoptotic cells by professional or nonprofessional phagocytes is critical to maintain mammalian homeostasis. To identify molecules involved in the engulfment of apoptotic cells, we established a retrovirus-based expression cloning system coupled with the engulfment assay. By screening a cDNA library of a mouse macrophage cell line, we identified two small GTPase family members (RhoG and Rab5) that enhanced the engulfment of apoptotic cells. By examining other small GTPase family members, we found that Rac1 enhanced the engulfment of apoptotic cells, whereas RhoA inhibited the process. Accordingly, the expression of a dominant-negative form of RhoG or Rac1 in primary macrophage cultures severely reduced the ability of the macrophages to engulf apoptotic cells, and a dominant-negative form of RhoA enhanced the process. These results indicated that the efficient engulfment of apoptotic cells requires the concerted action of small GTPase family members. We demonstrated previously that NIH3T3 cells expressing the alphav beta3 integrin efficiently engulf apoptotic cells in the presence of milk fat globule epidermal growth factor 8 via a phosphatidylserine-dependent mechanism. The dominant-negative form of RhoG or Rac1 inhibited this process, which suggested RhoG and Rac1 are also involved in the integrin-mediated engulfment.     

On a possible relationship between bacterial endospore formation and the origin of eukaryotic cells

The acquisition of intracellular organelles, including mitochondria and plastids and a membrane-bounded nucleus, have been postulated to be key events in the development of the eukaryotic from the prokaryotic ancestral cell. The two major hypotheses to account for such acquisitions are: (1) primitive cells originally obtained organelles by engulfing free-living prokaryotes which then entered into symbiotic association (“endosymbiosis”) with them; (2) organelles arose through the engulfment by the primitive cell of part of its own cytoplasm. To some extent, the former hypothesis has received most support, because endosymbiosis is known to occur in extant organisms, whilst the latter hypothesis has received less support, because cytoplasmic engulfment by prokaryotes is not now thought to occur. However, during the process of endospore formation by extant bacteria, the protoplast within the single cell is observed to divide in a unique manner such that the cell in effect engulfs a portion of its own cytoplasm. The process is strikingly similar to the engulfment suggested by the second hypothesis to have initiated the evolution of eukaryotes. The engulfed cytoplasm is bounded by a double membrane within the “mother cell” and contains enzymes, ribosomes and a complete genome. In many respects this parallels the supposed primitive eukaryotic state and, it is argued, confers potential advantages on the cell, particularly through the control that the “mother cell” can exert on the enclosed compartment. It is hypothesized that bacterial endospore formation is therefore one product of evolution from an early engulfment event that led also to the development of complex eukaryotic cells.     

Integrin α PAT-2/CDC-42 signaling is required for muscle-mediated clearance of apoptotic cells in Caenorhabditis elegans

Clearance of apoptotic cells by engulfment plays an important role in the homeostasis and development of multicellular organisms. Despite the fact that the recognition of apoptotic cells by engulfment receptors is critical in inducing the engulfment process, the molecular mechanisms are still poorly understood. Here, we characterize a novel cell corpse engulfment pathway mediated by the integrin α subunit PAT-2 in Caenorhabditis elegans and show that it specifically functions in muscle-mediated engulfment during embryogenesis. Inactivation of pat-2 results in a defect in apoptotic cell internalization. The PAT-2 extracellular region binds to the surface of apoptotic cells in vivo, and the intracellular region may mediate signaling for engulfment. We identify essential roles of small GTPase CDC-42 and its activator UIG-1, a guanine-nucleotide exchange factor, in PAT-2-mediated cell corpse removal. PAT-2 and CDC-42 both function in muscle cells for apoptotic cell removal and are co-localized in growing muscle pseudopods around apoptotic cells. Our data suggest that PAT-2 functions through UIG-1 for CDC-42 activation, which in turn leads to cytoskeletal rearrangement and apoptotic cell internalization by muscle cells. Moreover, in contrast to PAT-2, the other integrin α subunit INA-1 and the engulfment receptor CED-1, which signal through the conserved signaling molecules CED-5 (DOCK180)/CED-12 (ELMO) or CED-6 (GULP) respectively, preferentially act in epithelial cells to mediate cell corpse removal during mid-embryogenesis. Our results show that different engulfing cells utilize distinct repertoires of receptors for engulfment at the whole organism level.