Size compensation in Drosophila after generalised cell death.

Regeneration is a response mechanism aimed to restore tissues that have been damaged. We are studying in the wing disc of Drosophila the regenerative response to a dose of Ionizing Radiation that kills over 35% of the cells distributed all over the disc. After such treatment the discs are able to restore normal size, indicating there is a mechanism that repairs generalised damage. We have tested the role of the JNK, JAK/STAT and Wg pathways, known to be required for regeneration after localised damage in the disc. We find that after irradiation there is size compensation in the absence of function of these pathways, indicating that they are not necessary for the compensation. Furthermore, we also find that generalised damage does not cause an increase in the proliferation rate of surviving cells. We propose that irradiated discs suffer a developmental delay and resume growth at normal rate until they reach the final stereotyped size. The delay appears to be associated with a developmental reversion, because discs undergo rejuvenation towards an earlier developmental stage. We argue that the response to generalized damage is fundamentally different from that to localized damage, which requires activity of JNK and Wg.

Lack of apoptosis leads to cellular senescence and tumorigenesis in Drosophila epithelial cells.

Programmed cell death (apoptosis) is a homeostasis program of animal tissues designed to remove cells that are unwanted or are damaged by physiological insults. To assess the functional role of apoptosis, we have studied the consequences of subjecting Drosophila epithelial cells defective in apoptosis to stress or genetic perturbations that normally cause massive cell death. We find that many of those cells acquire persistent activity of the JNK pathway, which drives them into senescent status, characterized by arrest of cell division, cell hypertrophy, Senescent Associated ß-gal activity (SA-ß-gal), reactive oxygen species (ROS) production, Senescent Associated Secretory Phenotype (SASP) and migratory behaviour. We have identified two classes of senescent cells in the wing disc: 1) those that localize to the appendage part of the disc, express the upd, wg and dpp signalling genes and generate tumour overgrowths, and 2) those located in the thoracic region do not express wg and dpp nor they induce tumour overgrowths. Whether to become tumorigenic or non-tumorigenic depends on the original identity of the cell prior to the transformation. We also find that the p53 gene contributes to senescence by enhancing the activity of JNK.

An exciting period of Drosophila developmental biology: Of imaginal discs, clones, compartments, parasegments and homeotic genes.

In this review we recall a number of important discoveries that took place in Drosophila during the seventies and eighties of the last century. The development of cell lineage methods and of powerful modifications of same, such as the Minute technique, led to the discovery of compartments and provided a clearer picture of the body organization: that came to be seen as a chain of metameric lineage units along the A/P body axis. Further, genetic screens allowed the identification of the genes involved in the establishment of the metameric scaffold - the segmentation genes- and also of Hox genes that are responsible for the specific development of individual body parts. As cloning methods became available, many of the most relevant of these developmental genes were cloned and a molecular analysis of development initiated. The discovery of the homeobox, a molecular mark of Hox and other relevant developmental genes, allowed the finding of Hox genes in animal species, like humans, in which they could not be identified by genetic methods. Analysis of the structure and function of Hox genes provided a general image of the genetic design of the metazoan body.

Chromatin remodelling and retrotransposons activities during regeneration in Drosophila.

Regeneration is a response mechanism aiming to reconstruct lost or damaged structures. To achieve this, the cells repopulating the lost tissue often have to change their original identity, a process that involves chromatin remodelling.We have analysed the issue of chromatin remodelling during regeneration in the wing disc of Drosophila . In this disc the ablation of the central region (the pouch) induces the regenerative response of the cells from the lateral region (the hinge), which reconstitute the wing pouch. We have examined euchromatin and heterochromatin histone marks during the process and find that heterochromatin marks disappear but are recovered when regeneration is complete. Euchromatin marks are not modified. We also describe the transcription of two retrotransposons, Roo and F-element in the regenerating cells. We have established a temporal correlation between the alterations of heterochromatin marks and the levels of transcription of two retrotransposons, Roo and F-element, both during embryonic development and in the regeneration process.

Tumorigenesis and cell competition in Drosophila in the absence of polyhomeotic function.

Cell competition is a homeostatic process that eliminates by apoptosis unfit or undesirable cells from animal tissues, including tumor cells that appear during the life of the organism. In Drosophila there is evidence that many types of oncogenic cells are eliminated by cell competition. One exception is cells mutant for polyhomeotic (ph), a member of the Polycomb family of genes; most of the isolated mutant ph clones survive and develop tumorous overgrowths in imaginal discs. To characterize the tumorigenic effect of the lack of ph, we first studied the growth of different regions of the wing disc deficient in ph activity and found that the effect is restricted to the proximal appendage. Moreover, we found that ph-deficient tissue is partially refractory to apoptosis. Second, we analyzed the behavior of clones lacking ph function and found that many suffer cell competition but are not completely eliminated. Unexpectedly, we found that nonmutant cells also undergo cell competition when surrounded by ph-deficient cells, indicating that within the same tissue cell competition may operate in opposite directions. We suggest two reasons for the incompleteness of cell competition in ph mutant cells: 1) These cells are partially refractory to apoptosis, and 2) the loss of ph function alters the identity of imaginal cells and subsequently their cell affinities. It compromises the winner/loser interaction, a prerequisite for cell competition.

Cell competition: A historical perspective.

Cell competition is a homeostatic process designed to remove from animal tissues viable cells that are unfit, abnormal or malignant and that may compromise the general fitness or the viability of the organism. Originally discovered in Drosophila in the mid-seventies of last century, there is strong evidence that it also occurs in other metazoans, where cell competition appears to play a similar surveillance role. In this review I summarize the field of cell competition, with special emphasis in the history of the phenomenon within the general frame of Developmental Biology in the second half of the XX century, pointing out the key observations and the evolution of ideas that have led to the current understanding.

Cell competition and tumorigenesis in the imaginal discs of Drosophila.

Cancer is a major health issue and the object of investigations in thousands of laboratories all over the world. Most of cancer research is being carried out in in vitro systems or in animal models, generally mice or rats. However, the discovery of the high degree of genetic identity among metazoans has prompted investigation in organisms like Drosophila, on the idea that the genetic basis of cancer in flies and humans may have many aspects in common. Moreover, the sophisticated genetic methodology of Drosophila offers operational advantages and allows experimental approaches inaccessible in other species. Cell competition is a cell-quality control process that aims to identifying and subsequently removing cells within animal tissues that are unfit, abnormal or aberrant, and that may compromise the fitness or the viability of the organism. It was originally described in Drosophila imaginal discs but later work has shown it occurs in mammalian tissues where it fulfils similar roles. One aspect of the surveillance role of cell competition is to eliminate oncogenic cells that may appear during development or the life of an organism. In this review we have focussed on the work on Drosophila imaginal discs relating cell competition and tumorigenic processes. We briefly discuss related work in mammalian tissues.

JNK-mediated Slit-Robo signaling facilitates epithelial wound repair by extruding dying cells.

Multicellular organisms repair injured epithelium by evolutionarily conserved biological processes including activation of c-Jun N-terminal kinase (JNK) signaling. Here, we show in Drosophila imaginal epithelium that physical injury leads to the emergence of dying cells, which are extruded from the wounded tissue by JNK-induced Slit-Roundabout2 (Robo2) repulsive signaling. Reducing Slit-Robo2 signaling in the wounded tissue suppresses extrusion of dying cells and generates aberrant cells with highly upregulated growth factors Wingless (Wg) and Decapentaplegic (Dpp). The inappropriately elevated Wg and Dpp impairs wound repair, as halving one of these growth factor genes cancelled wound healing defects caused by Slit-Robo2 downregulation. Our data suggest that JNK-mediated Slit-Robo2 signaling contributes to epithelial wound repair by promoting extrusion of dying cells from the wounded tissue, which facilitates transient and appropriate induction of growth factors for proper wound healing.

A refutation to 'A new A-P compartment boundary and organizer in holometabolous insect wings'.

We respond to a recent report by Abbasi and Marcus who present two main findings: first they argue that there is an organiser and a compartment boundary within the posterior compartment of the butterfly wing. Second, they present evidence for a previously undiscovered lineage boundary near wing vein 5 in Drosophila, a boundary that delineates a "far posterior" compartment. Clones of cells were marked with the yellow mutation and they reported that these clones always fail to cross a line close to vein 5 on the Drosophila wing. In our hands yellow proved an unusable marker for clones in the wing blade and therefore we reexamined the matter. We marked clones of cells with multiple wing hairs or forked and found a substantial proportion of these clones cross the proposed lineage boundary near vein 5, in conflict with their findings and conclusion. As internal controls we showed that these same clones respect the other two well established compartment boundaries: the anteroposterior compartment boundary is always respected. The dorsoventral boundary is mostly respected, and is crossed only by clones that are induced early in development, consistent with many reports. We question the validity of Abbasi and Marcus' conclusions regarding the butterfly wing but present no new data.Arising from: R. Abbasi and J. M. Marcus Sci. Rep. 7, 16337 (2017); https://doi.org/10.1038/s41598-017-16553-5 .

Pro-apoptotic and pro-proliferation functions of the JNK pathway of Drosophila: roles in cell competition, tumorigenesis and regeneration.

The Jun N-terminal kinase (JNK) is a member of the mitogen-activated protein kinase family. It appears to be conserved in all animal species where it regulates important physiological functions involved in apoptosis, cell migration, cell proliferation and regeneration. In this review, we focus on the functions of JNK in Drosophila imaginal discs, where it has been reported that it can induce both cell death and cell proliferation. We discuss this apparent paradox in the light of recent findings and propose that the pro-apoptotic and the pro-proliferative functions are intrinsic properties of JNK activity. Whether one function or another is predominant depends on the cellular context.

Regenerative response of different regions of Drosophila imaginal discs.

Thanks to the introduction of new methods to induce massive damage under controlled conditions, much information about regeneration in Drosophila imaginal discs has accumulated in recent years. In this review, we discuss results concerning primarily the wing disc, putting emphasis on the different regenerative responses of the wing appendage, which exhibits a robust regenerative potential, and the trunk region, the notum, which regenerates very poorly. The wing disc may be a paradigm of a tissue in which a common original lineage generates cells with distinct regenerative potential. We argue that a key factor in those differences is the activity of the Jun N-terminal Kinase (JNK) pathway, which functions differently in the appendage and the body trunk.

Short-term activation of the Jun N-terminal kinase pathway in apoptosis-deficient cells of Drosophila induces tumorigenesis.

In Drosophila, the JNK pathway eliminates by apoptosis aberrant cells that appear in development. It also performs other functions associated with cell proliferation, but analysis of the latter is hindered by the pro-apoptotic activity. We report the response of apoptosis-deficient cells to transient activation of JNK and show that it causes persistent JNK function during the rest of the development. As a consequence, there is continuous activity of the downstream pathways JAK/STAT, Wg and Dpp, which results in tumour overgrowths. We also show that the oncogenic potential of the Ras-MAPK pathway resides largely on its ability to suppress apoptosis. It has been proposed that a hallmark of tumour cells is that they can evade apoptosis. In reverse, we propose that, in Drosophila, apoptosis-deficient cells become tumorigenic due to their property of acquiring persistent JNK activity after stress events that are inconsequential in tissues in which cells are open to apoptosis.

Distinct regenerative potential of trunk and appendages of Drosophila mediated by JNK signalling.

The Drosophila body comprises a central part, the trunk, and outgrowths of the trunk, the appendages. Much is known about appendage regeneration, but little about the trunk. As the wing imaginal disc contains a trunk component, the notum, and a wing appendage, we have investigated the response to ablation of these two components. We find that, in contrast with the strong regenerative response of the wing, the notum does not regenerate. Nevertheless, the elimination of the wing primordium elicits a proliferative response of notum cells, but they do not regenerate wing; they form a notum duplicate. Conversely, the wing cells cannot regenerate an ablated notum; they overproliferate and generate a hinge overgrowth. These results suggest that trunk and appendages cannot be reprogrammed to generate each other. Our experiments demonstrate that the proliferative response is mediated by JNK signalling from dying cells, but JNK functions differently in the trunk and the appendages, which may explain their distinct regenerative potential.

Homeostatic response to blocking cell division in Drosophila imaginal discs: Role of the Fat/Dachsous (Ft/Ds) pathway.

One major problem in developmental biology is the identification of the mechanisms that control the final size of tissues and organs. We are addressing this issue in the imaginal discs of Drosophila by analysing the response to blocking cell division in large domains in the wing and leg discs. The affected domains may be zones of restricted lineage like compartments, or zones of open lineage that may integrate cells from the surrounding territory. Our results reveal the existence of a powerful homeostatic mechanism that can compensate for gross differences in growth rates and builds structures of normal size. This mechanism functions at the level of whole discs, inducing additional cell proliferation to generate the cells that populate the cell division-arrested territory and generating an active recruitment process to integrate those cells. The activation of this response mechanism is mediated by alterations in the normal activity of PCP genes of the Fat/Ds system: in discs mutant for dachs, ds or four jointed the response mechanism is not activated.

Cell reprogramming during regeneration in Drosophila: transgression of compartment boundaries.

We discuss recent work about cellular reprogramming during regeneration of the imaginal discs of Drosophila. These contain various lineage blocks, compartments, which express distinct genetic programmes. It has been found that after massive damage to a compartment cells from a neighbour compartment can transgress the compartment border and contribute to its regeneration. The transgressing cells are genetically reprogrammed and acquire a new identity, a process facilitated by up regulation of the JNK pathway and transient loss of epigenetic control by the Pc-G and trx-G genes. The final acquisition of the new identity appears to be mediated by induction by neighbour cells, a phenomenon akin the Community Effect described for the specification of amphibian muscle cells.

Tumorigenic Properties of Drosophila Epithelial Cells Mutant for lethal giant larvae.

BACKGROUND: Mutations in Drosophila tumor suppressor genes (TSGs) lead to the formation of invasive tumors in the brain and imaginal discs. RESULTS: Here we studied the tumorigenic properties of imaginal discs mutant for the TSG gene lethal giant larvae (lgl). lgl mutant cells display the characteristic features of mammalian tumor cells: they can proliferate indefinitely, induce additional tracheogenesis (an insect counterpart of vasculogenesis) and invade neighboring tissues. Lgl mutant tissues exhibit high apoptotic levels, which lead to the activation of the Jun-N-Terminal Kinase (JNK) pathway. We propose that JNK is a key factor in the acquisition of these tumorigenic properties; it promotes cell proliferation and induces high levels of Mmp1 and confers tumor cells capacity to invade wild-type tissue. Noteworthy, lgl RNAi-mediated down-regulation does not produce similar transformations in the central nervous system (CNS), thereby indicating a fundamental difference between the cells of developing imaginal discs and those of differentiated organs. We discuss these results in the light of the "single big-hit origin" of some human pediatric or developmental cancers. CONCLUSIONS: Down-regulation of lgl in imaginal discs is sufficient to enhance tracheogenesis and to promote invasion and colonization of other larval structures including the CNS. Developmental Dynamics 245:834-843, 2016. © 2016 Wiley Periodicals, Inc.

Cell competition, apoptosis and tumour development.

The phenomenon of cell competition is an interactive process originally discovered in the imaginal discs of Drosophila; it is a developmental mechanism that identifies and eliminates cells that are weaker than their neighbours or have features that make them different or not well adapted to their surroundings. It appears to be an important homeostatic mechanism to contribute to the general fitness of developing tissues. Here we discuss some of the basic features of cell competition and then focus on results indicating that cell competition is responsible for the removal of malignant or aberrant cells that may appear during development, although in certain circumstances it can revert its role to promote tumour growth. We also consider several recent studies that indicate that cell competition also occurs in vertebrates where it performs similar functions.

Transgressions of compartment boundaries and cell reprogramming during regeneration in Drosophila.

Animals have developed mechanisms to reconstruct lost or damaged tissues. To regenerate those tissues the cells implicated have to undergo developmental reprogramming. The imaginal discs of Drosophila are subdivided into distinct compartments, which derive from different genetic programs. This feature makes them a convenient system to study reprogramming during regeneration. We find that massive damage inflicted to the posterior or the dorsal compartment of the wing disc causes a transient breakdown of compartment boundaries, which are quickly reconstructed. The cells involved in the reconstruction often modify their original identity, visualized by changes in the expression of developmental genes like engrailed or cubitus interruptus. This reprogramming is mediated by up regulation of the JNK pathway and transient debilitation of the epigenetic control mechanism. Our results also show that the local developmental context plays a role in the acquisition of new cell identities: cells expressing engrailed induce engrailed expression in neighbor cells. DOI: http://dx.doi.org/10.7554/eLife.01831.001.