Tissue repair through cell competition and compensatory cellular hypertrophy in postmitotic epithelia

Cell competition promotes phenotypically silent cardiomyocyte replacement in the mammalian heart

Heterogeneous anabolic capacity in cell populations can trigger a phenomenon known as cell competition, through which less active cells are eliminated. Cell competition has been induced experimentally in stem/precursor cell populations in insects and mammals and takes place endogenously in early mouse embryonic cells. Here, we show that cell competition can be efficiently induced in mouse cardiomyocytes by mosaic overexpression of Myc during both gestation and adult life. The expansion of the Myc-overexpressing cardiomyocyte population is driven by the elimination of wild-type cardiomyocytes. Importantly, this cardiomyocyte replacement is phenotypically silent and does not affect heart anatomy or function. These results show that the capacity for cell competition in mammals is not restricted to stem cell populations and suggest that stimulated cell competition has potential as a cardiomyocyte-replacement strategy.

Drosophila Myc: A master regulator of cellular performance

The identification of the Drosophila homolog of the human MYC oncogene has fostered a series of studies aimed to address its functions in development and cancer biology. Due to its essential roles in many fundamental biological processes it is hard to imagine a molecular mechanism in which MYC function is not required. For this reason, the easily manipulated Drosophila system has greatly helped in the dissection of the genetic and molecular pathways that regulate and are regulated by MYC function. In this review, we focus on studies of MYC in the fruitfly with particular emphasis on metabolism and cell competition, highlighting the contributions of this model system in the last decade to our understanding of MYC's complex biological nature. This article is part of a Special Issue entitled: Myc proteins in cell biology and pathology.

Starvation induces FoxO-dependent mitotic-to-endocycle switch pausing during Drosophila oogenesis

When exposed to nutrient challenge, organisms have to adapt their physiology in order to balance reproduction with adult fitness. In mammals, ovarian follicles enter a massive growth phase during which they become highly dependent on gonadotrophic factors and nutrients. Somatic tissues play a crucial role in integrating these signals, controlling ovarian follicle atresia and eventually leading to the selection of a single follicle for ovulation. We used Drosophila follicles as a model to study the effect of starvation on follicle maturation. Upon starvation, Drosophila vitellogenic follicles adopt an 'atresia-like' behavior, in which some slow down their development whereas others enter degeneration. The mitotic-to-endocycle (M/E) transition is a critical step during Drosophila oogenesis, allowing the entry of egg chambers into vitellogenesis. Here, we describe a specific and transient phase during M/E switching that is paused upon starvation. The Insulin pathway induces the pausing of the M/E switch, blocking the entry of egg chambers into vitellogenesis. Pausing of the M/E switch involves a previously unknown crosstalk between FoxO, Cut and Notch that ensures full reversion of the process and rapid resumption of oogenesis upon refeeding. Our work reveals a novel genetic mechanism controlling the extent of the M/E switch upon starvation, thus integrating metabolic cues with development, growth and reproduction.

Interplay of cell proliferation and cell death in Drosophila tissue regeneration

Regeneration is a fascinating process that allows some organisms to reconstruct damaged tissues. In addition to the classical regeneration model of the Drosophila larval imaginal discs, the genetically induced tissue ablation model has promoted the understanding of molecular mechanisms underlying cell death, proliferation, and remodeling for tissue regeneration. Recent studies have also revealed that tissue injury responses occur not only locally but also systemically, even in the uninjured region. Genetic studies in Drosophila have demonstrated the dynamic role of the cell death-induced tissue response in the reconstruction of damaged tissues.

Cell competition: how to eliminate your neighbours

A conventional view of development is that cells cooperate to build an organism. However, based on studies of Drosophila, it has been known for years that viable cells can be eliminated by their neighbours through a process termed cell competition. New studies in mammals have revealed that this process is universal and that many factors and mechanisms are conserved. During cell competition, cells with lower translation rates or those with lower levels of proteins involved in signal transduction, polarity and cellular growth can survive in a homogenous environment but are killed when surrounded by cells of higher fitness. Here, we discuss recent advances in the field as well as the mechanistic steps involved in this phenomenon, which have shed light on how and why cell competition exists in developing and adult organisms.

Endocycles: a recurrent evolutionary innovation for post-mitotic cell growth

In endoreplication cell cycles, known as endocycles, cells successively replicate their genomes without segregating chromosomes during mitosis and thereby become polyploid. Such cycles, for which there are many variants, are widespread in protozoa, plants and animals. Endocycling cells can achieve ploidies of >200,000 C (chromatin-value); this increase in genomic DNA content allows a higher genomic output, which can facilitate the construction of very large cells or enhance macromolecular secretion. These cells execute normal S phases, using a G1-S regulatory apparatus similar to the one used by mitotic cells, but their capability to segregate chromosomes has been suppressed, typically by downregulation of mitotic cyclin-dependent kinase activity. Endocycles probably evolved many times, and the various endocycle mechanisms found in nature highlight the versatility of the cell cycle control machinery.

Socializing with MYC: cell competition in development and as a model for premalignant cancer

Studies in Drosophila and mammals have made it clear that genetic mutations that arise in somatic tissues are rapidly recognized and eliminated, suggesting that cellular fitness is tightly monitored. During development, damaged, mutant, or otherwise unfit cells are prevented from contributing to the tissue and are instructed to die, whereas healthy cells benefit and populate the animal. This cell selection process, known as cell competition, eliminates somatic genetic heterogeneity and promotes tissue fitness during development. Yet cell competition also has a dark side. Super competition can be exploited by incipient cancers to subvert cellular cooperation and promote selfish behavior. Evidence is accumulating that MYC plays a key role in regulation of social behavior within tissues. Given the high number of tumors with deregulated MYC, studies of cell competition promise to yield insight into how the local environment yields to and participates in the early stages of tumor formation.

Compensatory cellular hypertrophy: the other strategy for tissue homeostasis

Metazoan tissues have the ability to maintain tissue size and morphology while eliminating aberrant or damaged cells. In the tissue homeostasis system, cell division is the primary strategy cells use not only to increase tissue size during development but also to compensate for cell loss in tissue repair. Recent studies in Drosophila, however, have shown that cells in postmitotic tissues undergo hypertrophic growth without division, contributing to tissue repair as well as organ development. Indeed, similar compensatory cellular hypertrophy (CCH) can be observed in different contexts such as mammalian hepatocytes or corneal endothelial cells. Here we highlight these findings and discuss the underlying mechanisms of CCH, which is likely an evolutionarily conserved strategy for homeostatic tissue growth in metazoans.

Polyploidization and cell fusion contribute to wound healing in the adult Drosophila epithelium

BACKGROUND

Reestablishing epithelial integrity and biosynthetic capacity is critically important following tissue damage. The adult Drosophila abdominal epithelium provides an attractive new system to address how postmitotic diploid cells contribute to repair.

RESULTS

Puncture wounds to the adult Drosophila epidermis close initially by forming a melanized scab. We found that epithelial cells near the wound site fuse to form a giant syncytium, which sends lamellae under the scab to re-epithelialize the damaged site. Other large cells arise more peripherally by initiating endocycles and becoming polyploid, or by cell fusion. Rac GTPase activity is needed for syncytium formation, while the Hippo signaling effector Yorkie modulates both polyploidization and cell fusion. Large cell formation is functionally important because when both polyploidization and fusion are blocked, wounds do not re-epithelialize.

CONCLUSIONS

Our observations indicate that cell mass lost upon wounding can be replaced by polyploidization instead of mitotic proliferation. We propose that large cells generated by polyploidization or cell fusion are essential because they are better able than diploid cells to mechanically stabilize wounds, especially those containing permanent acellular structures, such as scar tissue.