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The competitive nature of cells
Experimental Cell Research 306 (2005) 317 – 322 www.elsevier.com/locate/yexcr
Review
The competitive nature of cells Begon˜a Dı´az, Eduardo Moreno* Centro Nacional de Investigaciones Oncolo´gicas (C.N.I.O), Melchor Ferna´ndez Almagro, 3. E-28029 Madrid, Spain Received 2 February 2005, revised version received 2 February 2005 Available online 18 April 2005
Abstract The possibility that cells of multicellular organisms may compete with one another has been postulated several times. It was experimentally confirmed in Drosophila, probably for the first time, when cells with different metabolic rates were mixed: cells that would have been viable on their own disappeared due to the presence of metabolically more active cells. After almost 30 years of neglect, genetic analysis in Drosophila has started to reveal a gene network that regulates the competitive behavior of cells. If the genes regulating cellular competitiveness in Drosophila have a conserved function in mammals, the study of cell competition could have an impact in several biomedical fields, including functional degeneration, cancer, or stem cell therapies. D 2005 Elsevier Inc. All rights reserved. Keywords: Drosophila; Cell competition; Stem cell
Contents A genetic network for cell competitiveness Revealing cellular fitness. . . . . . . . . . Stem cells and cell competition . . . . . . Acknowledgments . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . .
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A genetic network for cell competitiveness The possibility that cells of multicellular organisms may compete with one another has been postulated several times over the past two centuries. In 1888, Wilhelm Roux proposed the idea of a cellular struggle for survival during development [1]. Wilhelm Roux transferred Charles Darwin’s theory of the struggle for existence to the fight among cells and ‘‘parts’’ of the organism in the process of ontogenesis. As evidence for the conflict between cell types, he referred to pathological processes in which cells of one tissue start to invade another [1]. However, his idea faced a lot of opposition, since the harmonic behavior of * Corresponding author. Fax: +34 912 246 980. E-mail address: [email protected] (E. Moreno). 0014-4827/$ - see front matter D 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.yexcr.2005.03.017
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cells within multicellular organisms argued against a fierce competition among their parts and in favor of a cooperation of the different cell types. Twenty years later, Ilya Mechnikov considered the likelihood of cells continuously fighting to survive in light of his studies on phagocytosis, but did not developed the idea much further. During his studies on the structure of the nervous system, Santiago Ramon y Cajal was even more explicit, suggesting that during neurogenesis there might be a competitive struggle among neurons for space and nutrition. With the discovery of growth factors, the idea of neuronal competition at the level of axon pathfinding was recovered as a marginal part of the neurotrophic theory by Rita Levy-Montalcini and refined by Dale Purves and Martin Raff [2,3]. In this view, all cells require signals from their neighbors to survive [3]. This social nature of cells is now widely accepted: the
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dependence of most cell types on survival signals provided by their neighbors also faces no opposition by those that view the cellular societies of multicellular organisms as a gathering of genetically programmed ‘‘harmonic cooperators’’. Cells need each other to survive, so that must provide a solid ground for cooperation [3]. But what happens if the resources required for cell survival are limiting? Then some cells must die. Do they die randomly, without noticing, in a passive non-competitive way, or do they actively compete among them for the limiting resources and struggle to survive? This simple concept of cells competing among them remained difficult to prove, and hence remained controversial. The main reason why it was difficult to prove experimentally is because it is not easy to distinguish between a competitive interaction that results in the death of one of the competitors and a passive non-competitive death, or even a ‘‘self-sacrifice’’. Examples of such ‘‘self-sacrifice’’ do actually exist in multicellular animals. One famous example is the death of cells in the nematode Caenorhabditis elegans, where cells kill themselves for the good development of the organism, accepting their fatal fate without trying to fight back or compete actively with the neighbors [4]. But do competitive interactions among cells also exist? For a competitive interaction between cells to be demonstrated unequivocally, an experimental situation must be created where the interaction between two defined groups of cells can be monitored, so that the effects on one population can be proved to be due to the presence of the other cell group. As opposed to a random mechanism, a truly competitive situation will then cause a decrease of cell numbers in the weaker of the two populations and a corresponding increase in the other. Given enough time, the group with the competitive advantage will take over, and eventually eliminate, the weaker population. Cellular competition was experimentally confirmed in Drosophila, probably for the first time, almost a century after Roux’s initial formulation, when cells with different metabolic rates were mixed: cells that would have been viable on their own disappeared due to the presence of metabolically more active cells [5]. The particular mutants used are the so-called Minutes, which are mutants in ribosomal proteins. The first Minute genes were discovered by Thomas Hunt Morgan and Calvin Bridges at Columbia University in New York and later at Caltech in Pasadena. Nowadays, there are at least 50 different Minute genes, since many mutations in several ribosomal proteins have been obtained over the years. Minute homozygote flies are cell lethal, due to a lack of functional ribosomes and no protein synthesis, but Minute heterozygote flies are perfectly viable and normally sized, although it takes them a few days longer than wild-type flies to complete embryonic development, probably due to the lack of a fully active ribosomal machinery. The crucial observation regarding competitive interactions came when mosaics containing both population types, Minute heterozygous cells together with wild-type
cells, were created [5]. The experiment was performed by two graduate students, Gine´s Morata and Pedro Ripoll at the laboratory of Antonio Garcı´a-Bellido in the CSIC in Madrid. Before the experiment was performed, Antonio Garcı´a-Bellido predicted that the growth delay will not be cell autonomous and that the cells will not proliferate at different rates when mixed. That was not the case, and cells did indeed proliferate at different rates. This caused Garcı´aBellido not to co-author that paper. But even more strikingly, Minute heterozygous cells not only proliferated more slowly. Unlike what was expected, the Minute heterozygote cells were not even viable in the Drosophila wing anymore. Presumably, the presence of wild-type surrounding cells must be killing them, despite they would have been viable on their own, a possibility that was confirmed later [6]. Morata and Ripoll submitted the paper to several journals and was rejected. The reviewers did not believe it. The idea of cells competing among them during development was ‘‘havoc’’ and ‘‘nonsense’’ and the referees felt there should be something wrong with the experiment, since cells need to cooperate rather than compete. The paper was finally published in Developmental Biology in 1975, and since then, it has become a highly cited paper. OK, now we believe it, so what? It appears clear that cells can fight in many epithelial tissues during fly development if they have different metabolic rates, but should we care? And if we do, can we find genes regulating cell competitiveness? In principle, competition among cells provides an efficient mechanism for selecting cell quality and thereby ensuring that the requisite cellular tasks will be done by the most efficient ones. It seems unlikely that such an effective mechanism to select for cell fitness is confined to flies. Most importantly, after almost 30 years of neglect [5,6], genetic analysis in Drosophila has started to reveal a gene network that controls cell competition [6]. Just as there are pathways that regulate the cell cycle [7– 9] or programmed cell death [10], at least in the fly there seems to be genes that regulate cell competitiveness [6,11 – 14]. In support of a global relevance, we will speculate that genetically programmed cell competition is used throughout the animal kingdom during development and homeostasis. To unify the following examples, in addition to the classical term of cell competition, and without the aim of substituting it, but rather to more rigorously define a group of common phenomena, we propose the term cytagon (from the ancient Greek ‘‘cell war’’), as synonym of ‘‘programmed cell competition’’. Classical experiments in Drosophila pioneered the idea that cells compete with each other to fill a limited space that appears to be mapped out in advance [5,11]. But what can the success of cells in the battlefield be attributed to? And how are the dimensions of such a field determined? The simplest possibility would provide answers to both questions by a single molecular principle, i.e., that the same factor(s) that selects for cell quality are also governing field
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size. This appears to be the case in Drosophila, where the main factor controlling cell competition [6], Decapentaplegic (Dpp, a homologue of bone morphogenetic proteins (BMPs)), is also responsible for the spatial dimensions of the field within which cells compete [15,16]. Is this just a coincidence, or could it be advantageous to regulate quality and size via the same molecule? Could this even be a general principle? It is possible to spot other cases in development and homeostasis where cell competition may occur within a limited space predefined by a mighty morphogen. As a starting example, the random patterns of proliferation and apoptosis that are normally observed during embryogenesis in mammals may be the outputs of a continuous developmental cytagon. Recent preliminary evidence suggests that cell competition may be at play in mammals during embryonic development [17], in a similar way as it had been described in Drosophila. If the process is finally confirmed to be present during development [17], it is likely that a similar situation may occur later, during homeostasis of selfrenewing cell pools like stem cells [18] or lymphocytes [19]. In the hematopoietic system, new cells are constantly produced but the size of the stem cell niche remains constant. There is a continuous migration of cells leaving from and returning to the niche. It is possible that for blood cells to re-engraft in the bone marrow niche, cytagon may be at play, as a mechanism to ensure that the size of the pool is maintained and the most competitive cells are selected to remain. BMPs have been implicated in regulating survival and maintenance of the bone marrow stem cell population [20] and thus may define its size and quality. Also, controlling size and quality of the lymphocyte pool is of crucial importance in mammals: the number of lymphocytes must be large enough to detect a diverse range of pathogens, but there is limited physical space in the body to host all lymphocytes [19]. Due to this size constraint, the immune system has to sacrifice some of the pre-existing cells to make room for new lymphocytes that are continuously produced. In several of these cases, cell competition could be epigenetically regulated to favor the expansion of one cell population within a limited space at the expense of another population or cell type.
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While endocytosis has traditionally been considered a slow desensitization mechanism for signal transduction pathways triggered by extracellular ligands, more and more evidence indicates that endocytosis plays an important positive role in the activation and propagation of certain signaling pathways [21], including Dpp signaling [6,22,23]. Two theoretical frameworks, depending on whether a ligand is provided in limiting supply or in excess, can account for cell selection based on ligand internalization. If the extracellular ligand concentration is limiting, a ‘‘winnertakes-it-all’’ situation could apply (Fig. 1). Weak cells which are metabolically less active do not obtain enough survival factor and die. Several mechanisms can limit ligand concentration in the intercellular space, such as the rate of synthesis in producing cells, sequestration of free ligand in the extracellular matrix, changes in cell number, and the fate of the ligand in the endocytic pathway (degradation versus recycling). From the perspective of cell competition, the endocytic pathway is of special interest as it could mechanistically couple the availability of free ligand with competition for ligand internalization [6]. A shift of the balance towards ligand degradation decreases the amounts of available growth factors and thereby reduces organ size [22]. Tipping the balance towards recycling [24] would lower cytagon and, simultaneously, increase organ size. The connection between organ size and cell competition would only correlate because the same factor is used for inducing growth and for revealing cell fitness [6,16].
Revealing cellular fitness The surest foundation is quality. Andrew Carnegie.
To achieve epigenetic modulation, cell competition must be enabled genetically. But by what means is cell competitiveness assessed and compared from cell to cell? Some of the genes involved in this process are starting to be identified, and it appears that endocytic internalization of extracellular ligands could be one of the principles by which cell competition is implemented on a population basis [6].
Fig. 1. Programmed cell competition or cytagon could act as a cell selection mechanism at the population level. (A) Cell selection inside a population based upon growth factor accessibility. Growth factor (red spots) is available in limiting quantities, and only those cells in which growth factor signaling is above a threshold can survive. In this scenario, the mechanism sets the ‘‘sub-optimal’’ threshold for survival. (B) Growth factor (red spots) is not limiting, so all the cells in the population have enough growth factordependent survival signaling. However, not all of them are equally competitive. Cytagon allows the most competitive to eliminate the others after ‘‘cell – cell-comparison’’. By this mechanism, cells that could survive by their own are eliminated by the presence of more fit cells. In this scenario, this mechanism would set the ‘‘optimal’’ threshold for survival.
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But what if the ligand is not limiting, as it may be the case for a morphogen? Then a cell –cell communication mechanism could permit cells to compare their respective signaling levels [25,26] and cause a ‘‘you lose’’ signal to be sent to the less efficient cell(s), which as a result would undergo apoptosis (Fig. 1). It is possible that members of the TNF superfamily might represent a molecular implementation of such a ‘‘you lose’’ signal [27], but the discovery of the genes implicated in the cell –cell communication mechanism that may help cells compare relative signaling levels among them remains a challenge. Such hypothetical mechanism may help one cell to spy the signaling levels of the neighboring cells in a first step, and trigger apoptosis through a second step using the ‘‘you lose signal’’. Is the existence of such ‘‘cell –cell spying system’’ wild speculation? We do not think so. Several lines of evidence coming from studies of the Drosophila imaginal discs suggest that such a spying genetic network must exist in order to allow cells to monitor the signaling levels of their neighbors [6,25,26], but the molecular nature of such spy molecules remains unknown. We would like to note that such a ‘‘you lose’’ scenario has the advantage that it does not depend on conditions of limiting survival factor(s), since cells with different signaling levels, due for example to different rates of endocytosis, will be able to still spy and recognize each other, even if the extra-cellular concentrations of ligand are not limiting. Until here, cell competition makes perfect sense as a physiological mechanism to select cells of optimal quality and to maximize tissue fitness [6]. However, a surprising discovery has been made with the identification of genes that are able to induce cell competition above wild-type levels, what has been termed super-competition [13,14]. In particular, it has been recently proposed that oncogenes of the myc family can transform cells into super-competitors [13,14], able to expand at the expense of normal surrounding tissue by killing it by apoptosis, in a way that total cell numbers are unchanged [14]. This phenomenon has been hypothesized to be involved in early stages of cancer formation (Fig. 2) and in the explanation of poorly understood clinical observations like the one termed ‘‘field cancerization’’ [14]. A gene duplication event, a translocation, or some other means of causing heritable overexpression of a human myc family member may help an originally transformed cell to success-
fully establish its descendants within an epithelial cell group. If such super-competitor cells behave like those in Drosophila, their expansion would occur at the expense of surrounding cells and hence not be detectable by morphological examination [14]. Competition for growth and survival factors could help expand the initial pre-cancerous population within a local trophic compartment. Further oncogenic mutations are then more likely to occur in an expanded population of cells that already exhibit this competitive advantage (Fig. 2). The discovery comes as a surprise, because there is no clear physiological function for it. Could cell super-competition be modulated epigenetically, for example, to contribute to tissue repair and regeneration, by promoting the proliferation of new cells at the expense of the old damaged, and/or displaced, tissue? In any case, from a historical point of view, super-competition can be seen as a possible validation of Roux controversial idea of a cellular struggle for survival and its connection to pathological processes, during which cells of one region start to invade the territory occupied by another population [1]. Study of model organisms such as yeast, C. elegans, sea urchins, or Drosophila has pioneered crucial contributions to processes with important implications in neoplasia like the nature and role of environmental mutagens [28], cell proliferation [7– 9], or apoptosis [4,10]. May the phenomenon of super-competition described in Drosophila [13,14] also apply to humans and help us understand tumor progression [29,30]? In this regard, the competitive expansion of super-competitor cells in Drosophila reveals a scenario that is strikingly reminiscent of, and may explain, the clinical finding termed ‘‘field cancerization’’, in which a field of cells of monoclonal origin is associated with a proliferative advantage and expands at the expense of normal tissue. Initially, neither invasive growth nor aberrant histology are present; but as the field becomes larger, additional genetic hits give rise to various subclones that eventually evolve into primary and ‘‘second field tumors’’ that share a common clonal origin [14,29,30]. Dmyc, like other myc family proteins, is a transcription factor known to regulate genes involved in cellular metabolism, like ribosomal proteins. One of the hypothesis as to the way by which increased dMyc levels confer a proliferation advantage is that cells with an enhanced
Fig. 2. Super-competition could contribute to early cancer progression. A mutation of a myc proto-oncogen could transform a normal epithelial cell into a ‘‘super-competitor’’ (A), able to undergo clonal expansion at the expense of the normal surrounding tissue without any morphological alterations (B and C). Secondary mutations in a super-competitor background could initiate tumor formation (D).
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translational capacity might compete more successfully than surrounding cells for the active uptake of extracellular survival and growth factors, and in doing so may expand within the community by killing or attenuating normal cells [14]. Likewise, tumors with high metastatic potential may be explained by success in cell competition during tissue invasion. The ability of advanced tumors to metastasize may thus be linked to the same early event conferring a Darwinian advantage to primary tumors. Due to the increasing evidence favoring a role for stem cells in tumor formation [31,32], it would be interesting to know if cell competition induced by oncogenes like myc can occur in stem cell niches [44]. Moreover, in adult tissues that undergo continuous cell turnover, stem cells are responsible for tissue renewal throughout adult life. Therefore, maintenance of sufficient number of optimal quality stem cells may be essential to ensure efficient tissue renewal and repair. But, is there a quality control that helps selecting optimal stem cells at a population level? Stem cells and cell competition Should I stay or should I go now? If I go there will be trouble. . . And if I stay it will be double! ‘‘Combat Rock’’. The Clash, 1982.
Adult stem cells are often found at specific locations called niches that provide the special tissue microenvironment required for the stem cell to maintain a stable undifferentiated state [33 –35]. Very much like a young girl in the dilemma to leave the family house, the daughters of proliferating stem cells must face the fate decision whether to remain in the niche, youthful and undifferentiated, preserving the pool of stem cells; or to leave the special microenvironment and differentiate into novel cell types [33 –35]. The mechanisms by which stem cells decide to remain in the niche or to leave it are a major player in regulating the balance between stem cell self-renewal and differentiation. If all of them go, ‘‘there will be trouble’’, since the niche will be emptied and the tissue will degenerate sooner or later, but if they all stay, the number of stem cells ‘‘will be double’’. How is this dilemma resolved? Should I stay or should I go? Two main hypothesis have been proposed to explain the mechanisms governing stem cell self-renewal [33 – 36]. The first suggests that the process cells use to remain or exit the stem cell niche is deterministic and based in asymmetric cell divisions. Asymmetric cell divisions could be implemented by orienting the plane of cell division so that one of the daughter cells remains in the niche while the other moves away as division proceeds. The second postulates a probabilistic mechanism, where cells do not divide in any oriented direction, but since the niche is a fixed space, some cells may end out of the niche. Although the original concept of niche favored the probabilistic
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mechanism [36], recent evidence is accumulating that asymmetric cell divisions may be a more common [37 – 39], if not universal, mechanism to regulate the balance between differentiation and self-renewal. The first cellular network to be defined at the functional level as a niche was identified in the Drosophila ovary by Ting Xie and Allan Spradling [40]. Although the niche was described as a functional unit in the year 2000, the number of stem cells that should reside in that niche had been previously estimated, before even seeing them, in a beautiful genetic study done by Eric Wieschaus and Janosz Szabad in 1979 [41]. The number of germline stem cells of the ovary was then estimated to be around 2.8 per niche, inferred by the proportions of eggs produced by mosaic flies [41]. Since cells are indivisible units, this decimal number predicted that most niches will contain either two or three stem cells each, assuming there is not much variation from one ovary germ stem cell niche to another. Each one of these 2 or 3 stem cells is attached tightly to the niche [42] and is maintained undifferentiated by the extracellular stem cell factor Decapentaplegic (Dpp), a BMP2/4 homologue [43]. A niche geometer might characterize the niche as a box of somatic cells harboring the germline stem cells. Dpp is thought to be produced by those somatic cells surrounding the stem cells and secreted to the territory occupied by the 2 or 3 stem cells that need it to divide and self-renew [43]. The germ stem cell divides continuously, approximately once per day, and after each division, one daughter cell stays in the niche as a stem cell, while the other moves out of the niche and begins a program of differentiation that ultimately will produce one oocyte and 15 nurse cells. It has been recently shown [44] that cell competition can contribute to select stem cells of optimal quality within the Drosophila ovary germ line niche, in a similar way as it does in the Drosophila wing [6]. Results are consistent with a model where dpp controls niche size [40,43] and d-Myc activates the competitive behavior of cells, so that cells within the niche are induced to compete for Dpp [44]. dMyc expression in the niche seems to create a cell-based competitive environment oriented to maximize cellular fitness among stem cells [44]. The existence of a mechanism governing stem cell renewal oriented to select for cell quality and maximize the cellular fitness of the cells that remain within the niche has the caveat that the accumulation of mutations transforming cells into super-competitors may promote the generation of cancer stem cells. Is it possible that animals have developed mechanisms to promote cell competition while reducing super-competition in order to maintain the cell fitness selection mechanisms while reducing the occurrence of tumor promoting processes? Finally, one could ask whether cytagon could serve as a target for therapeutic interventions to combat processes such as functional degeneration and aging [6] or cancer [13,14], or to improve (or supplant) stem cell therapies [44], i.e., medicine in search of excellence. In any case, those are all challenges for the future study of this field of research. After
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all, cells are an organism’s most important asset and quality may be its surest foundation. [21]
Acknowledgments We thank O. Fernandez-Capetillo and C. Rhiner for reading the manuscript and suggestions. Work is supported by a Caja Madrid-CNIO junior group leader grant to EM.
[22]
[23]
[24]
References [25] [1] W. Roux, Der Kampf der Theile im Organismus, W. Engelmann, Leipzig, 1881. [2] D. Purves, Neuronal competition, Nature 287 (5783) (1980 Oct. 16) 585 – 586. [3] M.C. Raff, Social controls on cell survival and cell death, Nature 356 (6368) (1992 Apr. 2) 397 – 400. [4] J.Y. Yuan, H.R. Horvitz, The Caenorhabditis elegans genes ced-3 and ced-4 act cell-autonomously to cause programmed cell death. Dev. Biol. 138 (1990) 33 – 41. [5] G. Morata, P. Ripoll, Minutes: mutants of Drosophila autonomously affecting cell division rate, Dev. Biol. 42 (2) (1975 Feb.) 211 – 221. [6] E. Moreno, K. Basler, G. Morata, Cells compete for decapentaplegic survival factor to prevent apoptosis in Drosophila wing development, Nature 416 (6882) (2002 Apr. 18) 755 – 759. [7] L.H. Hartwell, M.B. Kastan, Cell cycle control and cancer, Science 266 (5192) (1994) 1821 – 1832. [8] P. Nurse, Genetic control of cell size at cell division in yeast, Nature 256 (5518) (1975 Aug. 14) 547 – 551. [9] T. Evans, E.T. Rosenthal, J. Youngblom, D. Distel, T. Hunt, Cyclin: a protein specified by maternal mRNA in sea urchin eggs that is destroyed at each cleavage division, Cell 33 (2) (1983 Jun.) 389 – 396. [10] H.M. Ellis, H.R. Horvitz, Genetic control of programmed cell death in the nematode C. elegans, Cell 44 (6) (1986 Mar. 28) 817 – 829. [11] P. Simpson, G. Morata, Differential mitotic rates and patterns of growth in compartments in the Drosophila wing, Dev. Biol. 85 (2) (1981 Jul. 30) 299 – 308. [12] L.A. Johnston, D.A. Prober, B.A. Edgar, R.N. Eisenman, P. Gallant, Drosophila Myc regulates cellular growth during development, Cell 98 (6) (1999 Sep. 17) 779 – 790. [13] C. de la Cova, M. Abril, P. Bellosta, P. Gallant, L.A. Johnston, Drosophila myc regulates organ size by inducing cell competition, Cell 117 (1) (2004 Apr. 2) 107 – 116. [14] E. Moreno, K. Basler, dMyc transforms cells into super-competitors, Cell 117 (1) (2004 Apr. 2) 117 – 129. [15] R. Burke, K. Basler, Dpp receptors are autonomously required for cell proliferation in the entire developing Drosophila wing, Development 122 (7) (1996 Jul.) 2261 – 2269. [16] F.A. Martin, A. Perez-Garijo, E. Moreno, G. Morata, The brinker gradient controls wing growth in Drosophila, Development 131 (20) (2004 Oct.) 4921 – 4930. [17] E.R. Oliver, T.L. Saunders, S.A. Tarle, T. Glaser, Ribosomal protein L24 defect in belly spot and tail (Bst), a mouse Minute, Development 131 (16) (2004 Aug.) 3907 – 3920. [18] J. Domen, The role of apoptosis in regulating hematopoietic stem cell numbers, Apoptosis 6 (4) (2001 Aug.) 239 – 252. [19] E. Gaudin, M. Rosado, F. Agenes, A. McLean, A.A. Freitas, B-cell homeostasis, competition, resources, and positive selection by selfantigens, Immunol. Rev. 197 (2004 Feb.) 102 – 115. [20] G. Bhardwaj, B. Murdoch, D. Wu, D.P. Baker, K.P. Williams, K.
[26]
[27]
[28] [29] [30] [31]
[32] [33] [34] [35] [36]
[37] [38] [39]
[40] [41]
[42]
[43]
[44]
Chadwick, L.E. Ling, F.N. Karanu, M. Bhatia, Sonic hedgehog induces the proliferation of primitive human hematopoietic cells via BMP regulation, Nat. Immunol. 2 (2) (2001 Feb.) 172 – 180. P.S. McPherson, B.K. Kay, N.K. Hussain, Signaling on the endocytic pathway, Traffic 2 (6) (2001 Jun.) 375 – 384. E.V. Entchev, A. Schwabedissen, M. Gonzalez-Gaitan, Gradient formation of the TGF-beta homolog Dpp, Cell 103 (6) (2000 Dec. 8) 981 – 991. T.Y. Belenkaya, C. Han, D. Yan, R.J. Opoka, M. Khodoun, H. Liu, X. Lin, Drosophila Dpp morphogen movement is independent of dynamin-mediated endocytosis but regulated by the glypican members of heparan sulfate proteoglycans, Cell 119 (2) (2004 Oct. 15) 231 – 244. S. Pfeiffer, S. Ricardo, J.B. Manneville, C. Alexandre, J.P. Vincent, Producing cells retain and recycle wingless in Drosophila embryos, Curr. Biol. 12 (11) (2002 Jun 4) 957 – 962. M. Milan, L. Perez, S.M. Cohen, Short-range cell interactions and cell survival in the Drosophila wing, Dev. Cell. 2 (6) (2002 Jun.) 797 – 805. T. Adachi-Yamada, M.B. O’Connor, Morphogenetic apoptosis: a mechanism for correcting discontinuities in morphogen gradients, Dev. Biol. 251 (1) (2002 Nov. 1) 74 – 90. E. Moreno, M. Yan, K. Basler, Evolution of TNF signaling mechanisms. JNK-dependent apoptosis triggered by Eiger, the Drosophila homolog of the TNF superfamily, Curr. Biol. 12 (14) (2002 Jul. 23) 1263. H.J. Muller, Artificial transmutation of the gene, Science 46 (84) (1927). J. Secombe, S.B. Pierce, R.N. Eisenman, Myc: a weapon of mass destruction, Cell 117 (2) (2004 Apr. 16) 153 – 156. T.D. Donaldson, R.J. Duronio, Cancer cell biology: Myc wins the competition, Curr. Biol. 14 (11) (2004 Jun. 8) R425 – R427. R. Pardal, M.F. Clarke, S.J. Morrison, Applying the principles of stem-cell biology to cancer, Nat. Rev., Cancer 3 (12) (2003 Dec.) 895 – 902. G.Q. Daley, Chronic myeloid leukemia: providing ground for cancer stem cells, Cell 119 (3) (2004 Oct. 29) 314 – 316. A. Spradling, D. Drummond-Barbosa, T. Kai, Stem cells find their niche, Nature 414 (6859) (2001 Nov. 1) 98 – 104. E. Fuchs, T. Tumbar, G. Guasch, Socializing with the neighbors: stem cells and their niche, Cell 116 (6) (2004 Mar. 19) 769 – 778. F.M. Watt, B.L. Hogan, Out of Eden: stem cells and their niches, Science 287 (5457) (2000 Feb. 25) 1427 – 1430. R. Schofield, The relationship between the spleen colony-forming cell and the haemopoietic stem cell, Blood Cells 4 (1 – 2) (1978) 7 – 25. M.R. Wallenfang, E. Matunis, Orienting stem cells, Science 301 (5639) (2003 Sep. 12) 1490 – 1491. H. Lin, Stem cells: to be and not to be, Nature 425 (6956) (2003 Sep. 25) 53 – 55. Y.M. Yamashita, D.L. Jones, M.T. Fuller, Orientation of asymmetric stem cell division by the APC tumor suppressor and centrosome, Science 301 (5639) (2003 Sep. 12) 1547 – 1550. T. Xie, A.C. Spradling, A niche maintaining germ line stem cells in the Drosophila ovary, Science 290 (5490) (2000 Oct. 13) 328 – 330. E. Wieschaus, J. Szabad, The development and function of the female germ line in Drosophila melanogaster: a cell lineage study, Dev. Biol. 68 (1) (1979 Jan.) 29 – 46. X. Song, C.H. Zhu, C. Doan, T. Xie, Germline stem cells anchored by adherens junctions in the Drosophila ovary niches, Science 296 (5574) (2002 Jun. 7) 1855 – 1857. T. Xie, A.C. Spradling, Decapentaplegic is essential for the maintenance and division of germline stem cells in the Drosophila ovary, Cell 94 (2) (1998 Jul. 24) 251 – 260. E. Moreno, Cell-competition regulates ‘‘stemness’’ (submitted for publication).
Review
The competitive nature of cells Begon˜a Dı´az, Eduardo Moreno* Centro Nacional de Investigaciones Oncolo´gicas (C.N.I.O), Melchor Ferna´ndez Almagro, 3. E-28029 Madrid, Spain Received 2 February 2005, revised version received 2 February 2005 Available online 18 April 2005
Abstract The possibility that cells of multicellular organisms may compete with one another has been postulated several times. It was experimentally confirmed in Drosophila, probably for the first time, when cells with different metabolic rates were mixed: cells that would have been viable on their own disappeared due to the presence of metabolically more active cells. After almost 30 years of neglect, genetic analysis in Drosophila has started to reveal a gene network that regulates the competitive behavior of cells. If the genes regulating cellular competitiveness in Drosophila have a conserved function in mammals, the study of cell competition could have an impact in several biomedical fields, including functional degeneration, cancer, or stem cell therapies. D 2005 Elsevier Inc. All rights reserved. Keywords: Drosophila; Cell competition; Stem cell
Contents A genetic network for cell competitiveness Revealing cellular fitness. . . . . . . . . . Stem cells and cell competition . . . . . . Acknowledgments . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . .
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A genetic network for cell competitiveness The possibility that cells of multicellular organisms may compete with one another has been postulated several times over the past two centuries. In 1888, Wilhelm Roux proposed the idea of a cellular struggle for survival during development [1]. Wilhelm Roux transferred Charles Darwin’s theory of the struggle for existence to the fight among cells and ‘‘parts’’ of the organism in the process of ontogenesis. As evidence for the conflict between cell types, he referred to pathological processes in which cells of one tissue start to invade another [1]. However, his idea faced a lot of opposition, since the harmonic behavior of * Corresponding author. Fax: +34 912 246 980. E-mail address: [email protected] (E. Moreno). 0014-4827/$ - see front matter D 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.yexcr.2005.03.017
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cells within multicellular organisms argued against a fierce competition among their parts and in favor of a cooperation of the different cell types. Twenty years later, Ilya Mechnikov considered the likelihood of cells continuously fighting to survive in light of his studies on phagocytosis, but did not developed the idea much further. During his studies on the structure of the nervous system, Santiago Ramon y Cajal was even more explicit, suggesting that during neurogenesis there might be a competitive struggle among neurons for space and nutrition. With the discovery of growth factors, the idea of neuronal competition at the level of axon pathfinding was recovered as a marginal part of the neurotrophic theory by Rita Levy-Montalcini and refined by Dale Purves and Martin Raff [2,3]. In this view, all cells require signals from their neighbors to survive [3]. This social nature of cells is now widely accepted: the
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dependence of most cell types on survival signals provided by their neighbors also faces no opposition by those that view the cellular societies of multicellular organisms as a gathering of genetically programmed ‘‘harmonic cooperators’’. Cells need each other to survive, so that must provide a solid ground for cooperation [3]. But what happens if the resources required for cell survival are limiting? Then some cells must die. Do they die randomly, without noticing, in a passive non-competitive way, or do they actively compete among them for the limiting resources and struggle to survive? This simple concept of cells competing among them remained difficult to prove, and hence remained controversial. The main reason why it was difficult to prove experimentally is because it is not easy to distinguish between a competitive interaction that results in the death of one of the competitors and a passive non-competitive death, or even a ‘‘self-sacrifice’’. Examples of such ‘‘self-sacrifice’’ do actually exist in multicellular animals. One famous example is the death of cells in the nematode Caenorhabditis elegans, where cells kill themselves for the good development of the organism, accepting their fatal fate without trying to fight back or compete actively with the neighbors [4]. But do competitive interactions among cells also exist? For a competitive interaction between cells to be demonstrated unequivocally, an experimental situation must be created where the interaction between two defined groups of cells can be monitored, so that the effects on one population can be proved to be due to the presence of the other cell group. As opposed to a random mechanism, a truly competitive situation will then cause a decrease of cell numbers in the weaker of the two populations and a corresponding increase in the other. Given enough time, the group with the competitive advantage will take over, and eventually eliminate, the weaker population. Cellular competition was experimentally confirmed in Drosophila, probably for the first time, almost a century after Roux’s initial formulation, when cells with different metabolic rates were mixed: cells that would have been viable on their own disappeared due to the presence of metabolically more active cells [5]. The particular mutants used are the so-called Minutes, which are mutants in ribosomal proteins. The first Minute genes were discovered by Thomas Hunt Morgan and Calvin Bridges at Columbia University in New York and later at Caltech in Pasadena. Nowadays, there are at least 50 different Minute genes, since many mutations in several ribosomal proteins have been obtained over the years. Minute homozygote flies are cell lethal, due to a lack of functional ribosomes and no protein synthesis, but Minute heterozygote flies are perfectly viable and normally sized, although it takes them a few days longer than wild-type flies to complete embryonic development, probably due to the lack of a fully active ribosomal machinery. The crucial observation regarding competitive interactions came when mosaics containing both population types, Minute heterozygous cells together with wild-type
cells, were created [5]. The experiment was performed by two graduate students, Gine´s Morata and Pedro Ripoll at the laboratory of Antonio Garcı´a-Bellido in the CSIC in Madrid. Before the experiment was performed, Antonio Garcı´a-Bellido predicted that the growth delay will not be cell autonomous and that the cells will not proliferate at different rates when mixed. That was not the case, and cells did indeed proliferate at different rates. This caused Garcı´aBellido not to co-author that paper. But even more strikingly, Minute heterozygous cells not only proliferated more slowly. Unlike what was expected, the Minute heterozygote cells were not even viable in the Drosophila wing anymore. Presumably, the presence of wild-type surrounding cells must be killing them, despite they would have been viable on their own, a possibility that was confirmed later [6]. Morata and Ripoll submitted the paper to several journals and was rejected. The reviewers did not believe it. The idea of cells competing among them during development was ‘‘havoc’’ and ‘‘nonsense’’ and the referees felt there should be something wrong with the experiment, since cells need to cooperate rather than compete. The paper was finally published in Developmental Biology in 1975, and since then, it has become a highly cited paper. OK, now we believe it, so what? It appears clear that cells can fight in many epithelial tissues during fly development if they have different metabolic rates, but should we care? And if we do, can we find genes regulating cell competitiveness? In principle, competition among cells provides an efficient mechanism for selecting cell quality and thereby ensuring that the requisite cellular tasks will be done by the most efficient ones. It seems unlikely that such an effective mechanism to select for cell fitness is confined to flies. Most importantly, after almost 30 years of neglect [5,6], genetic analysis in Drosophila has started to reveal a gene network that controls cell competition [6]. Just as there are pathways that regulate the cell cycle [7– 9] or programmed cell death [10], at least in the fly there seems to be genes that regulate cell competitiveness [6,11 – 14]. In support of a global relevance, we will speculate that genetically programmed cell competition is used throughout the animal kingdom during development and homeostasis. To unify the following examples, in addition to the classical term of cell competition, and without the aim of substituting it, but rather to more rigorously define a group of common phenomena, we propose the term cytagon (from the ancient Greek ‘‘cell war’’), as synonym of ‘‘programmed cell competition’’. Classical experiments in Drosophila pioneered the idea that cells compete with each other to fill a limited space that appears to be mapped out in advance [5,11]. But what can the success of cells in the battlefield be attributed to? And how are the dimensions of such a field determined? The simplest possibility would provide answers to both questions by a single molecular principle, i.e., that the same factor(s) that selects for cell quality are also governing field
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size. This appears to be the case in Drosophila, where the main factor controlling cell competition [6], Decapentaplegic (Dpp, a homologue of bone morphogenetic proteins (BMPs)), is also responsible for the spatial dimensions of the field within which cells compete [15,16]. Is this just a coincidence, or could it be advantageous to regulate quality and size via the same molecule? Could this even be a general principle? It is possible to spot other cases in development and homeostasis where cell competition may occur within a limited space predefined by a mighty morphogen. As a starting example, the random patterns of proliferation and apoptosis that are normally observed during embryogenesis in mammals may be the outputs of a continuous developmental cytagon. Recent preliminary evidence suggests that cell competition may be at play in mammals during embryonic development [17], in a similar way as it had been described in Drosophila. If the process is finally confirmed to be present during development [17], it is likely that a similar situation may occur later, during homeostasis of selfrenewing cell pools like stem cells [18] or lymphocytes [19]. In the hematopoietic system, new cells are constantly produced but the size of the stem cell niche remains constant. There is a continuous migration of cells leaving from and returning to the niche. It is possible that for blood cells to re-engraft in the bone marrow niche, cytagon may be at play, as a mechanism to ensure that the size of the pool is maintained and the most competitive cells are selected to remain. BMPs have been implicated in regulating survival and maintenance of the bone marrow stem cell population [20] and thus may define its size and quality. Also, controlling size and quality of the lymphocyte pool is of crucial importance in mammals: the number of lymphocytes must be large enough to detect a diverse range of pathogens, but there is limited physical space in the body to host all lymphocytes [19]. Due to this size constraint, the immune system has to sacrifice some of the pre-existing cells to make room for new lymphocytes that are continuously produced. In several of these cases, cell competition could be epigenetically regulated to favor the expansion of one cell population within a limited space at the expense of another population or cell type.
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While endocytosis has traditionally been considered a slow desensitization mechanism for signal transduction pathways triggered by extracellular ligands, more and more evidence indicates that endocytosis plays an important positive role in the activation and propagation of certain signaling pathways [21], including Dpp signaling [6,22,23]. Two theoretical frameworks, depending on whether a ligand is provided in limiting supply or in excess, can account for cell selection based on ligand internalization. If the extracellular ligand concentration is limiting, a ‘‘winnertakes-it-all’’ situation could apply (Fig. 1). Weak cells which are metabolically less active do not obtain enough survival factor and die. Several mechanisms can limit ligand concentration in the intercellular space, such as the rate of synthesis in producing cells, sequestration of free ligand in the extracellular matrix, changes in cell number, and the fate of the ligand in the endocytic pathway (degradation versus recycling). From the perspective of cell competition, the endocytic pathway is of special interest as it could mechanistically couple the availability of free ligand with competition for ligand internalization [6]. A shift of the balance towards ligand degradation decreases the amounts of available growth factors and thereby reduces organ size [22]. Tipping the balance towards recycling [24] would lower cytagon and, simultaneously, increase organ size. The connection between organ size and cell competition would only correlate because the same factor is used for inducing growth and for revealing cell fitness [6,16].
Revealing cellular fitness The surest foundation is quality. Andrew Carnegie.
To achieve epigenetic modulation, cell competition must be enabled genetically. But by what means is cell competitiveness assessed and compared from cell to cell? Some of the genes involved in this process are starting to be identified, and it appears that endocytic internalization of extracellular ligands could be one of the principles by which cell competition is implemented on a population basis [6].
Fig. 1. Programmed cell competition or cytagon could act as a cell selection mechanism at the population level. (A) Cell selection inside a population based upon growth factor accessibility. Growth factor (red spots) is available in limiting quantities, and only those cells in which growth factor signaling is above a threshold can survive. In this scenario, the mechanism sets the ‘‘sub-optimal’’ threshold for survival. (B) Growth factor (red spots) is not limiting, so all the cells in the population have enough growth factordependent survival signaling. However, not all of them are equally competitive. Cytagon allows the most competitive to eliminate the others after ‘‘cell – cell-comparison’’. By this mechanism, cells that could survive by their own are eliminated by the presence of more fit cells. In this scenario, this mechanism would set the ‘‘optimal’’ threshold for survival.
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But what if the ligand is not limiting, as it may be the case for a morphogen? Then a cell –cell communication mechanism could permit cells to compare their respective signaling levels [25,26] and cause a ‘‘you lose’’ signal to be sent to the less efficient cell(s), which as a result would undergo apoptosis (Fig. 1). It is possible that members of the TNF superfamily might represent a molecular implementation of such a ‘‘you lose’’ signal [27], but the discovery of the genes implicated in the cell –cell communication mechanism that may help cells compare relative signaling levels among them remains a challenge. Such hypothetical mechanism may help one cell to spy the signaling levels of the neighboring cells in a first step, and trigger apoptosis through a second step using the ‘‘you lose signal’’. Is the existence of such ‘‘cell –cell spying system’’ wild speculation? We do not think so. Several lines of evidence coming from studies of the Drosophila imaginal discs suggest that such a spying genetic network must exist in order to allow cells to monitor the signaling levels of their neighbors [6,25,26], but the molecular nature of such spy molecules remains unknown. We would like to note that such a ‘‘you lose’’ scenario has the advantage that it does not depend on conditions of limiting survival factor(s), since cells with different signaling levels, due for example to different rates of endocytosis, will be able to still spy and recognize each other, even if the extra-cellular concentrations of ligand are not limiting. Until here, cell competition makes perfect sense as a physiological mechanism to select cells of optimal quality and to maximize tissue fitness [6]. However, a surprising discovery has been made with the identification of genes that are able to induce cell competition above wild-type levels, what has been termed super-competition [13,14]. In particular, it has been recently proposed that oncogenes of the myc family can transform cells into super-competitors [13,14], able to expand at the expense of normal surrounding tissue by killing it by apoptosis, in a way that total cell numbers are unchanged [14]. This phenomenon has been hypothesized to be involved in early stages of cancer formation (Fig. 2) and in the explanation of poorly understood clinical observations like the one termed ‘‘field cancerization’’ [14]. A gene duplication event, a translocation, or some other means of causing heritable overexpression of a human myc family member may help an originally transformed cell to success-
fully establish its descendants within an epithelial cell group. If such super-competitor cells behave like those in Drosophila, their expansion would occur at the expense of surrounding cells and hence not be detectable by morphological examination [14]. Competition for growth and survival factors could help expand the initial pre-cancerous population within a local trophic compartment. Further oncogenic mutations are then more likely to occur in an expanded population of cells that already exhibit this competitive advantage (Fig. 2). The discovery comes as a surprise, because there is no clear physiological function for it. Could cell super-competition be modulated epigenetically, for example, to contribute to tissue repair and regeneration, by promoting the proliferation of new cells at the expense of the old damaged, and/or displaced, tissue? In any case, from a historical point of view, super-competition can be seen as a possible validation of Roux controversial idea of a cellular struggle for survival and its connection to pathological processes, during which cells of one region start to invade the territory occupied by another population [1]. Study of model organisms such as yeast, C. elegans, sea urchins, or Drosophila has pioneered crucial contributions to processes with important implications in neoplasia like the nature and role of environmental mutagens [28], cell proliferation [7– 9], or apoptosis [4,10]. May the phenomenon of super-competition described in Drosophila [13,14] also apply to humans and help us understand tumor progression [29,30]? In this regard, the competitive expansion of super-competitor cells in Drosophila reveals a scenario that is strikingly reminiscent of, and may explain, the clinical finding termed ‘‘field cancerization’’, in which a field of cells of monoclonal origin is associated with a proliferative advantage and expands at the expense of normal tissue. Initially, neither invasive growth nor aberrant histology are present; but as the field becomes larger, additional genetic hits give rise to various subclones that eventually evolve into primary and ‘‘second field tumors’’ that share a common clonal origin [14,29,30]. Dmyc, like other myc family proteins, is a transcription factor known to regulate genes involved in cellular metabolism, like ribosomal proteins. One of the hypothesis as to the way by which increased dMyc levels confer a proliferation advantage is that cells with an enhanced
Fig. 2. Super-competition could contribute to early cancer progression. A mutation of a myc proto-oncogen could transform a normal epithelial cell into a ‘‘super-competitor’’ (A), able to undergo clonal expansion at the expense of the normal surrounding tissue without any morphological alterations (B and C). Secondary mutations in a super-competitor background could initiate tumor formation (D).
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translational capacity might compete more successfully than surrounding cells for the active uptake of extracellular survival and growth factors, and in doing so may expand within the community by killing or attenuating normal cells [14]. Likewise, tumors with high metastatic potential may be explained by success in cell competition during tissue invasion. The ability of advanced tumors to metastasize may thus be linked to the same early event conferring a Darwinian advantage to primary tumors. Due to the increasing evidence favoring a role for stem cells in tumor formation [31,32], it would be interesting to know if cell competition induced by oncogenes like myc can occur in stem cell niches [44]. Moreover, in adult tissues that undergo continuous cell turnover, stem cells are responsible for tissue renewal throughout adult life. Therefore, maintenance of sufficient number of optimal quality stem cells may be essential to ensure efficient tissue renewal and repair. But, is there a quality control that helps selecting optimal stem cells at a population level? Stem cells and cell competition Should I stay or should I go now? If I go there will be trouble. . . And if I stay it will be double! ‘‘Combat Rock’’. The Clash, 1982.
Adult stem cells are often found at specific locations called niches that provide the special tissue microenvironment required for the stem cell to maintain a stable undifferentiated state [33 –35]. Very much like a young girl in the dilemma to leave the family house, the daughters of proliferating stem cells must face the fate decision whether to remain in the niche, youthful and undifferentiated, preserving the pool of stem cells; or to leave the special microenvironment and differentiate into novel cell types [33 –35]. The mechanisms by which stem cells decide to remain in the niche or to leave it are a major player in regulating the balance between stem cell self-renewal and differentiation. If all of them go, ‘‘there will be trouble’’, since the niche will be emptied and the tissue will degenerate sooner or later, but if they all stay, the number of stem cells ‘‘will be double’’. How is this dilemma resolved? Should I stay or should I go? Two main hypothesis have been proposed to explain the mechanisms governing stem cell self-renewal [33 – 36]. The first suggests that the process cells use to remain or exit the stem cell niche is deterministic and based in asymmetric cell divisions. Asymmetric cell divisions could be implemented by orienting the plane of cell division so that one of the daughter cells remains in the niche while the other moves away as division proceeds. The second postulates a probabilistic mechanism, where cells do not divide in any oriented direction, but since the niche is a fixed space, some cells may end out of the niche. Although the original concept of niche favored the probabilistic
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mechanism [36], recent evidence is accumulating that asymmetric cell divisions may be a more common [37 – 39], if not universal, mechanism to regulate the balance between differentiation and self-renewal. The first cellular network to be defined at the functional level as a niche was identified in the Drosophila ovary by Ting Xie and Allan Spradling [40]. Although the niche was described as a functional unit in the year 2000, the number of stem cells that should reside in that niche had been previously estimated, before even seeing them, in a beautiful genetic study done by Eric Wieschaus and Janosz Szabad in 1979 [41]. The number of germline stem cells of the ovary was then estimated to be around 2.8 per niche, inferred by the proportions of eggs produced by mosaic flies [41]. Since cells are indivisible units, this decimal number predicted that most niches will contain either two or three stem cells each, assuming there is not much variation from one ovary germ stem cell niche to another. Each one of these 2 or 3 stem cells is attached tightly to the niche [42] and is maintained undifferentiated by the extracellular stem cell factor Decapentaplegic (Dpp), a BMP2/4 homologue [43]. A niche geometer might characterize the niche as a box of somatic cells harboring the germline stem cells. Dpp is thought to be produced by those somatic cells surrounding the stem cells and secreted to the territory occupied by the 2 or 3 stem cells that need it to divide and self-renew [43]. The germ stem cell divides continuously, approximately once per day, and after each division, one daughter cell stays in the niche as a stem cell, while the other moves out of the niche and begins a program of differentiation that ultimately will produce one oocyte and 15 nurse cells. It has been recently shown [44] that cell competition can contribute to select stem cells of optimal quality within the Drosophila ovary germ line niche, in a similar way as it does in the Drosophila wing [6]. Results are consistent with a model where dpp controls niche size [40,43] and d-Myc activates the competitive behavior of cells, so that cells within the niche are induced to compete for Dpp [44]. dMyc expression in the niche seems to create a cell-based competitive environment oriented to maximize cellular fitness among stem cells [44]. The existence of a mechanism governing stem cell renewal oriented to select for cell quality and maximize the cellular fitness of the cells that remain within the niche has the caveat that the accumulation of mutations transforming cells into super-competitors may promote the generation of cancer stem cells. Is it possible that animals have developed mechanisms to promote cell competition while reducing super-competition in order to maintain the cell fitness selection mechanisms while reducing the occurrence of tumor promoting processes? Finally, one could ask whether cytagon could serve as a target for therapeutic interventions to combat processes such as functional degeneration and aging [6] or cancer [13,14], or to improve (or supplant) stem cell therapies [44], i.e., medicine in search of excellence. In any case, those are all challenges for the future study of this field of research. After
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all, cells are an organism’s most important asset and quality may be its surest foundation. [21]
Acknowledgments We thank O. Fernandez-Capetillo and C. Rhiner for reading the manuscript and suggestions. Work is supported by a Caja Madrid-CNIO junior group leader grant to EM.
[22]
[23]
[24]
References [25] [1] W. Roux, Der Kampf der Theile im Organismus, W. Engelmann, Leipzig, 1881. [2] D. Purves, Neuronal competition, Nature 287 (5783) (1980 Oct. 16) 585 – 586. [3] M.C. Raff, Social controls on cell survival and cell death, Nature 356 (6368) (1992 Apr. 2) 397 – 400. [4] J.Y. Yuan, H.R. Horvitz, The Caenorhabditis elegans genes ced-3 and ced-4 act cell-autonomously to cause programmed cell death. Dev. Biol. 138 (1990) 33 – 41. [5] G. Morata, P. Ripoll, Minutes: mutants of Drosophila autonomously affecting cell division rate, Dev. Biol. 42 (2) (1975 Feb.) 211 – 221. [6] E. Moreno, K. Basler, G. Morata, Cells compete for decapentaplegic survival factor to prevent apoptosis in Drosophila wing development, Nature 416 (6882) (2002 Apr. 18) 755 – 759. [7] L.H. Hartwell, M.B. Kastan, Cell cycle control and cancer, Science 266 (5192) (1994) 1821 – 1832. [8] P. Nurse, Genetic control of cell size at cell division in yeast, Nature 256 (5518) (1975 Aug. 14) 547 – 551. [9] T. Evans, E.T. Rosenthal, J. Youngblom, D. Distel, T. Hunt, Cyclin: a protein specified by maternal mRNA in sea urchin eggs that is destroyed at each cleavage division, Cell 33 (2) (1983 Jun.) 389 – 396. [10] H.M. Ellis, H.R. Horvitz, Genetic control of programmed cell death in the nematode C. elegans, Cell 44 (6) (1986 Mar. 28) 817 – 829. [11] P. Simpson, G. Morata, Differential mitotic rates and patterns of growth in compartments in the Drosophila wing, Dev. Biol. 85 (2) (1981 Jul. 30) 299 – 308. [12] L.A. Johnston, D.A. Prober, B.A. Edgar, R.N. Eisenman, P. Gallant, Drosophila Myc regulates cellular growth during development, Cell 98 (6) (1999 Sep. 17) 779 – 790. [13] C. de la Cova, M. Abril, P. Bellosta, P. Gallant, L.A. Johnston, Drosophila myc regulates organ size by inducing cell competition, Cell 117 (1) (2004 Apr. 2) 107 – 116. [14] E. Moreno, K. Basler, dMyc transforms cells into super-competitors, Cell 117 (1) (2004 Apr. 2) 117 – 129. [15] R. Burke, K. Basler, Dpp receptors are autonomously required for cell proliferation in the entire developing Drosophila wing, Development 122 (7) (1996 Jul.) 2261 – 2269. [16] F.A. Martin, A. Perez-Garijo, E. Moreno, G. Morata, The brinker gradient controls wing growth in Drosophila, Development 131 (20) (2004 Oct.) 4921 – 4930. [17] E.R. Oliver, T.L. Saunders, S.A. Tarle, T. Glaser, Ribosomal protein L24 defect in belly spot and tail (Bst), a mouse Minute, Development 131 (16) (2004 Aug.) 3907 – 3920. [18] J. Domen, The role of apoptosis in regulating hematopoietic stem cell numbers, Apoptosis 6 (4) (2001 Aug.) 239 – 252. [19] E. Gaudin, M. Rosado, F. Agenes, A. McLean, A.A. Freitas, B-cell homeostasis, competition, resources, and positive selection by selfantigens, Immunol. Rev. 197 (2004 Feb.) 102 – 115. [20] G. Bhardwaj, B. Murdoch, D. Wu, D.P. Baker, K.P. Williams, K.
[26]
[27]
[28] [29] [30] [31]
[32] [33] [34] [35] [36]
[37] [38] [39]
[40] [41]
[42]
[43]
[44]
Chadwick, L.E. Ling, F.N. Karanu, M. Bhatia, Sonic hedgehog induces the proliferation of primitive human hematopoietic cells via BMP regulation, Nat. Immunol. 2 (2) (2001 Feb.) 172 – 180. P.S. McPherson, B.K. Kay, N.K. Hussain, Signaling on the endocytic pathway, Traffic 2 (6) (2001 Jun.) 375 – 384. E.V. Entchev, A. Schwabedissen, M. Gonzalez-Gaitan, Gradient formation of the TGF-beta homolog Dpp, Cell 103 (6) (2000 Dec. 8) 981 – 991. T.Y. Belenkaya, C. Han, D. Yan, R.J. Opoka, M. Khodoun, H. Liu, X. Lin, Drosophila Dpp morphogen movement is independent of dynamin-mediated endocytosis but regulated by the glypican members of heparan sulfate proteoglycans, Cell 119 (2) (2004 Oct. 15) 231 – 244. S. Pfeiffer, S. Ricardo, J.B. Manneville, C. Alexandre, J.P. Vincent, Producing cells retain and recycle wingless in Drosophila embryos, Curr. Biol. 12 (11) (2002 Jun 4) 957 – 962. M. Milan, L. Perez, S.M. Cohen, Short-range cell interactions and cell survival in the Drosophila wing, Dev. Cell. 2 (6) (2002 Jun.) 797 – 805. T. Adachi-Yamada, M.B. O’Connor, Morphogenetic apoptosis: a mechanism for correcting discontinuities in morphogen gradients, Dev. Biol. 251 (1) (2002 Nov. 1) 74 – 90. E. Moreno, M. Yan, K. Basler, Evolution of TNF signaling mechanisms. JNK-dependent apoptosis triggered by Eiger, the Drosophila homolog of the TNF superfamily, Curr. Biol. 12 (14) (2002 Jul. 23) 1263. H.J. Muller, Artificial transmutation of the gene, Science 46 (84) (1927). J. Secombe, S.B. Pierce, R.N. Eisenman, Myc: a weapon of mass destruction, Cell 117 (2) (2004 Apr. 16) 153 – 156. T.D. Donaldson, R.J. Duronio, Cancer cell biology: Myc wins the competition, Curr. Biol. 14 (11) (2004 Jun. 8) R425 – R427. R. Pardal, M.F. Clarke, S.J. Morrison, Applying the principles of stem-cell biology to cancer, Nat. Rev., Cancer 3 (12) (2003 Dec.) 895 – 902. G.Q. Daley, Chronic myeloid leukemia: providing ground for cancer stem cells, Cell 119 (3) (2004 Oct. 29) 314 – 316. A. Spradling, D. Drummond-Barbosa, T. Kai, Stem cells find their niche, Nature 414 (6859) (2001 Nov. 1) 98 – 104. E. Fuchs, T. Tumbar, G. Guasch, Socializing with the neighbors: stem cells and their niche, Cell 116 (6) (2004 Mar. 19) 769 – 778. F.M. Watt, B.L. Hogan, Out of Eden: stem cells and their niches, Science 287 (5457) (2000 Feb. 25) 1427 – 1430. R. Schofield, The relationship between the spleen colony-forming cell and the haemopoietic stem cell, Blood Cells 4 (1 – 2) (1978) 7 – 25. M.R. Wallenfang, E. Matunis, Orienting stem cells, Science 301 (5639) (2003 Sep. 12) 1490 – 1491. H. Lin, Stem cells: to be and not to be, Nature 425 (6956) (2003 Sep. 25) 53 – 55. Y.M. Yamashita, D.L. Jones, M.T. Fuller, Orientation of asymmetric stem cell division by the APC tumor suppressor and centrosome, Science 301 (5639) (2003 Sep. 12) 1547 – 1550. T. Xie, A.C. Spradling, A niche maintaining germ line stem cells in the Drosophila ovary, Science 290 (5490) (2000 Oct. 13) 328 – 330. E. Wieschaus, J. Szabad, The development and function of the female germ line in Drosophila melanogaster: a cell lineage study, Dev. Biol. 68 (1) (1979 Jan.) 29 – 46. X. Song, C.H. Zhu, C. Doan, T. Xie, Germline stem cells anchored by adherens junctions in the Drosophila ovary niches, Science 296 (5574) (2002 Jun. 7) 1855 – 1857. T. Xie, A.C. Spradling, Decapentaplegic is essential for the maintenance and division of germline stem cells in the Drosophila ovary, Cell 94 (2) (1998 Jul. 24) 251 – 260. E. Moreno, Cell-competition regulates ‘‘stemness’’ (submitted for publication).