Changing spatial patterns of DNA replication in the developing wing of Drosophila

Active shape programming drives Drosophila wing disc eversion

How complex 3D tissue shape emerges during animal development remains an important open question in biology and biophysics. Here, we discover a mechanism for 3D epithelial shape change based on active, in-plane cellular events that is analogous to inanimate "shape programmable" materials, which undergo blueprinted 3D shape transformations from in-plane gradients of spontaneous strains. We study eversion of the Drosophila wing disc pouch, when the epithelium transforms from a dome into a curved fold, quantifying 3D tissue shape changes and mapping spatial patterns of cellular behaviors on the evolving geometry using cellular topology. Using a physical model inspired by shape programming, we find that active cell rearrangements are the major contributor to pouch eversion and validate this conclusion using a knockdown of MyoVI, which reduces rearrangements and disrupts morphogenesis. This work shows that shape programming is a mechanism for animal tissue morphogenesis and suggests that patterns in nature could present design strategies for shape-programmable materials.

Transcriptional repression and enhancer decommissioning silence cell cycle genes in postmitotic tissues

The mechanisms that maintain a non-cycling status in postmitotic tissues are not well understood. Many cell cycle genes have promoters and enhancers that remain accessible even when cells are terminally differentiated and in a non-cycling state, suggesting their repression must be maintained long term. In contrast, enhancer decommissioning has been observed for rate-limiting cell cycle genes in the Drosophila wing, a tissue where the cells die soon after eclosion, but it has been unclear if this also occurs in other contexts of terminal differentiation. In this study, we show that enhancer decommissioning also occurs at specific, rate-limiting cell cycle genes in the long-lived tissues of the Drosophila eye and brain, and we propose this loss of chromatin accessibility may help maintain a robust postmitotic state. We examined the decommissioned enhancers at specific rate-limiting cell cycle genes and show that they encode dynamic temporal and spatial expression patterns that include shared, as well as tissue-specific elements, resulting in broad gene expression with developmentally controlled temporal regulation. We extend our analysis to cell cycle gene expression and chromatin accessibility in the mammalian retina using a published dataset, and find that the principles of cell cycle gene regulation identified in terminally differentiating Drosophila tissues are conserved in the differentiating mammalian retina. We propose a robust, non-cycling status is maintained in long-lived postmitotic tissues through a combination of stable repression at most cell cycle gens, alongside enhancer decommissioning at specific rate-limiting cell cycle genes.

Temporal dynamics of apoptosis-induced proliferation in pupal wing development: implications for regenerative ability

BACKGROUND

The ability of animals to regenerate damaged tissue is a complex process that involves various cellular mechanisms. As animals age, they lose their regenerative abilities, making it essential to understand the underlying mechanisms that limit regenerative ability during aging. Drosophila melanogaster wing imaginal discs are epithelial structures that can regenerate after tissue injury. While significant research has focused on investigating regenerative responses during larval stages our comprehension of the regenerative potential of pupal wings and the underlying mechanisms contributing to the decline of regenerative responses remains limited.

RESULTS

Here, we explore the temporal dynamics during pupal development of the proliferative response triggered by the induction of cell death, a typical regenerative response. Our results indicate that the apoptosis-induced proliferative response can continue until 34 h after puparium formation (APF), beyond this point cell death alone is not sufficient to induce a regenerative response. Under normal circumstances, cell proliferation ceases around 24 h APF. Interestingly, the failure of reinitiating the cell cycle beyond this time point is not attributed to an incapacity to activate the JNK pathway. Instead, our results suggest that the function of the ecdysone-responsive transcription factor E93 is involved in limiting the apoptosis-induced proliferative response during pupal development.

CONCLUSIONS

Our study shows that apoptosis can prolong the proliferative period of cells in the wing during pupal development as late as 34 h APF, at least 10 h longer than during normal development. After this time point, the regenerative response is diminished, a process mediated in part by the ecdysone-responsive transcription factor E93.

Epithelial apoptotic pattern emerges from global and local regulation by cell apical area

Geometry is a fundamental attribute of biological systems, and it underlies cell and tissue dynamics. Cell geometry controls cell-cycle progression and mitosis and thus modulates tissue development and homeostasis. In sharp contrast and despite the extensive characterization of the genetic mechanisms of caspase activation, we know little about whether and how cell geometry controls apoptosis commitment in developing tissues. Here, we combined multiscale time-lapse microscopy of developing Drosophila epithelium, quantitative characterization of cell behaviors, and genetic and mechanical perturbations to determine how apoptosis is controlled during epithelial tissue development. We found that early in cell lives and well before extrusion, apoptosis commitment is linked to two distinct geometric features: a small apical area compared with other cells within the tissue and a small relative apical area with respect to the immediate neighboring cells. We showed that these global and local geometric characteristics are sufficient to recapitulate the tissue-scale apoptotic pattern. Furthermore, we established that the coupling between these two geometric features and apoptotic cells is dependent on the Hippo/YAP and Notch pathways. Overall, by exploring the links between cell geometry and apoptosis commitment, our work provides important insights into the spatial regulation of cell death in tissues and improves our understanding of the mechanisms that control cell number and tissue size.

Opportunistic binding of EcR to open chromatin drives tissue-specific developmental responses

Steroid hormones perform diverse biological functions in developing and adult animals. However, the mechanistic basis for their tissue specificity remains unclear. In Drosophila, the ecdysone steroid hormone is essential for coordinating developmental timing across physically separated tissues. Ecdysone directly impacts genome function through its nuclear receptor, a heterodimer of the EcR and ultraspiracle proteins. Ligand binding to EcR triggers a transcriptional cascade, including activation of a set of primary response transcription factors. The hierarchical organization of this pathway has left the direct role of EcR in mediating ecdysone responses obscured. Here, we investigate the role of EcR in controlling tissue-specific ecdysone responses, focusing on two tissues that diverge in their response to rising ecdysone titers: the larval salivary gland, which undergoes programmed destruction, and the wing imaginal disc, which initiates morphogenesis. We find that EcR functions bimodally, with both gene repressive and activating functions, even at the same developmental stage. EcR DNA binding profiles are highly tissue-specific, and transgenic reporter analyses demonstrate that EcR plays a direct role in controlling enhancer activity. Finally, despite a strong correlation between tissue-specific EcR binding and tissue-specific open chromatin, we find that EcR does not control chromatin accessibility at genomic targets. We conclude that EcR contributes extensively to tissue-specific ecdysone responses. However, control over access to its binding sites is subordinated to other transcription factors.

A unified mechanism for the control of Drosophila wing growth by the morphogens Decapentaplegic and Wingless

Development of the Drosophila wing-a paradigm of organ development-is governed by 2 morphogens, Decapentaplegic (Dpp, a BMP) and Wingless (Wg, a Wnt). Both proteins are produced by defined subpopulations of cells and spread outwards, forming gradients that control gene expression and cell pattern as a function of concentration. They also control growth, but how is unknown. Most studies have focused on Dpp and yielded disparate models in which cells throughout the wing grow at similar rates in response to the grade or temporal change in Dpp concentration or to the different amounts of Dpp "equalized" by molecular or mechanical feedbacks. In contrast, a model for Wg posits that growth is governed by a progressive expansion in morphogen range, via a mechanism in which a minimum threshold of Wg sustains the growth of cells within the wing and recruits surrounding "pre-wing" cells to grow and enter the wing. This mechanism depends on the capacity of Wg to fuel the autoregulation of vestigial (vg)-the selector gene that specifies the wing state-both to sustain vg expression in wing cells and by a feed-forward (FF) circuit of Fat (Ft)/Dachsous (Ds) protocadherin signaling to induce vg expression in neighboring pre-wing cells. Here, we have subjected Dpp to the same experimental tests used to elucidate the Wg model and find that it behaves indistinguishably. Hence, we posit that both morphogens act together, via a common mechanism, to control wing growth as a function of morphogen range.

The Krüppel-like factor Cabut has cell cycle regulatory properties similar to E2F1

Using a gain-of-function screen in Drosophila, we identified the Krüppel-like factor Cabut (Cbt) as a positive regulator of cell cycle gene expression and cell proliferation. Enforced cbt expression is sufficient to induce an extra cell division in the differentiating fly wing or eye, and also promotes intestinal stem cell divisions in the adult gut. Although inappropriate cell proliferation also results from forced expression of the E2f1 transcription factor or its target, Cyclin E, Cbt does not increase E2F1 or Cyclin E activity. Instead, Cbt regulates a large set of E2F1 target genes independently of E2F1, and our data suggest that Cbt acts via distinct binding sites in target gene promoters. Although Cbt was not required for cell proliferation during wing or eye development, Cbt is required for normal intestinal stem cell divisions in the midgut, which expresses E2F1 at relatively low levels. The E2F1-like functions of Cbt identify a distinct mechanism for cell cycle regulation that may be important in certain normal cell cycles, or in cells that cycle inappropriately, such as cancer cells.

Control of Drosophila wing size by morphogen range and hormonal gating

The stereotyped dimensions of animal bodies and their component parts result from tight constraints on growth. Yet, the mechanisms that stop growth when organs reach the right size are unknown. Growth of the Drosophila wing-a classic paradigm-is governed by two morphogens, Decapentaplegic (Dpp, a BMP) and Wingless (Wg, a Wnt). Wing growth during larval life ceases when the primordium attains full size, concomitant with the larval-to-pupal molt orchestrated by the steroid hormone ecdysone. Here, we block the molt by genetically dampening ecdysone production, creating an experimental paradigm in which the wing stops growing at the correct size while the larva continues to feed and gain body mass. Under these conditions, we show that wing growth is limited by the ranges of Dpp and Wg, and by ecdysone, which regulates the cellular response to their signaling activities. Further, we present evidence that growth terminates because of the loss of two distinct modes of morphogen action: 1) maintenance of growth within the wing proper and 2) induced growth of surrounding "pre-wing" cells and their recruitment into the wing. Our results provide a precedent for the control of organ size by morphogen range and the hormonal gating of morphogen action.

Regulation of Body Size and Growth Control

The control of body and organ growth is essential for the development of adults with proper size and proportions, which is important for survival and reproduction. In animals, adult body size is determined by the rate and duration of juvenile growth, which are influenced by the environment. In nutrient-scarce environments in which more time is needed for growth, the juvenile growth period can be extended by delaying maturation, whereas juvenile development is rapidly completed in nutrient-rich conditions. This flexibility requires the integration of environmental cues with developmental signals that govern internal checkpoints to ensure that maturation does not begin until sufficient tissue growth has occurred to reach a proper adult size. The Target of Rapamycin (TOR) pathway is the primary cell-autonomous nutrient sensor, while circulating hormones such as steroids and insulin-like growth factors are the main systemic regulators of growth and maturation in animals. We discuss recent findings in Drosophila melanogaster showing that cell-autonomous environment and growth-sensing mechanisms, involving TOR and other growth-regulatory pathways, that converge on insulin and steroid relay centers are responsible for adjusting systemic growth, and development, in response to external and internal conditions. In addition to this, proper organ growth is also monitored and coordinated with whole-body growth and the timing of maturation through modulation of steroid signaling. This coordination involves interorgan communication mediated by Drosophila insulin-like peptide 8 in response to tissue growth status. Together, these multiple nutritional and developmental cues feed into neuroendocrine hubs controlling insulin and steroid signaling, serving as checkpoints at which developmental progression toward maturation can be delayed. This review focuses on these mechanisms by which external and internal conditions can modulate developmental growth and ensure proper adult body size, and highlights the conserved architecture of this system, which has made Drosophila a prime model for understanding the coordination of growth and maturation in animals.

Model systems for regeneration: Drosophila

Drosophila melanogaster has historically been a workhorse model organism for studying developmental biology. In addition, Drosophila is an excellent model for studying how damaged tissues and organs can regenerate. Recently, new precision approaches that enable both highly targeted injury and genetic manipulation have accelerated progress in this field. Here, we highlight these techniques and review examples of recently discovered mechanisms that regulate regeneration in Drosophila larval and adult tissues. We also discuss how, by applying these powerful approaches, studies of Drosophila can continue to guide the future of regeneration research.

Changes in chromatin accessibility ensure robust cell cycle exit in terminally differentiated cells

During terminal differentiation, most cells exit the cell cycle and enter into a prolonged or permanent G0 in which they are refractory to mitogenic signals. Entry into G0 is usually initiated through the repression of cell cycle gene expression by formation of a transcriptional repressor complex called dimerization partner (DP), retinoblastoma (RB)-like, E2F and MuvB (DREAM). However, when DREAM repressive function is compromised during terminal differentiation, additional unknown mechanisms act to stably repress cycling and ensure robust cell cycle exit. Here, we provide evidence that developmentally programmed, temporal changes in chromatin accessibility at a small subset of critical cell cycle genes act to enforce cell cycle exit during terminal differentiation in the Drosophila melanogaster wing. We show that during terminal differentiation, chromatin closes at a set of pupal wing enhancers for the key rate-limiting cell cycle regulators Cyclin E (cycE), E2F transcription factor 1 (e2f1), and string (stg). This closing coincides with wing cells entering a robust postmitotic state that is strongly refractory to cell cycle reactivation, and the regions that close contain known binding sites for effectors of mitogenic signaling pathways such as Yorkie and Notch. When cell cycle exit is genetically disrupted, chromatin accessibility at cell cycle genes remains unaffected, and the closing of distal enhancers at cycE, e2f1, and stg proceeds independent of the cell cycling status. Instead, disruption of cell cycle exit leads to changes in accessibility and expression of a subset of hormone-induced transcription factors involved in the progression of terminal differentiation. Our results uncover a mechanism that acts as a cell cycle–independent timer to limit the response to mitogenic signaling and aberrant cycling in terminally differentiating tissues. In addition, we provide a new molecular description of the cross talk between cell cycle exit and terminal differentiation during metamorphosis.

The longer a cell remains in G0, the more refractory it becomes to re-entering the cell cycle. This study shows that in terminally differentiated cells in vivo, regulatory elements at genes encoding just three key cell cycle regulators (cycE, e2f1 and stg) become inaccessible, limiting their aberrant activation and maintaining a prolonged, robust G0.

Coupling between dynamic 3D tissue architecture and BMP morphogen signaling during Drosophila wing morphogenesis

Tissue morphogenesis is a dynamic process often accompanied by cell patterning and differentiation. Although how conserved growth factor signaling affects cell and tissue shapes has been actively studied, much less is known about how signaling and dynamic morphogenesis are mutually coordinated. Our study shows that BMP signaling and 3D morphogenesis of the Drosophila pupal wing are tightly coupled. These findings are highlighted by the fact that the directionality of BMP signal is changed from lateral planar during the inflation stage to interplanar after re-apposition of the dorsal and ventral wing epithelia. We suspect that the dynamic interplay between planar and interplanar signaling linked to tissue shape changes is likely to be used across species in many developing organs.

At the level of organ formation, tissue morphogenesis drives developmental processes in animals, often involving the rearrangement of two-dimensional (2D) structures into more complex three-dimensional (3D) tissues. These processes can be directed by growth factor signaling pathways. However, little is known about how such morphological changes affect the spatiotemporal distribution of growth factor signaling. Here, using the Drosophila pupal wing, we address how decapentaplegic (Dpp)/bone morphogenetic protein (BMP) signaling and 3D wing morphogenesis are coordinated. Dpp, expressed in the longitudinal veins (LVs) of the pupal wing, initially diffuses laterally within both dorsal and ventral wing epithelia during the inflation stage to regulate cell proliferation. Dpp localization is then refined to the LVs within each epithelial plane, but with active interplanar signaling for vein patterning/differentiation, as the two epithelia appose. Our data further suggest that the 3D architecture of the wing epithelia and the spatial distribution of BMP signaling are tightly coupled, revealing that 3D morphogenesis is an emergent property of the interactions between extracellular signaling and tissue shape changes.

Chromatin organization changes during the establishment and maintenance of the postmitotic state

Background

Genome organization changes during development as cells differentiate. Chromatin motion becomes increasingly constrained and heterochromatin clusters as cells become restricted in their developmental potential. These changes coincide with slowing of the cell cycle, which can also influence chromatin organization and dynamics. Terminal differentiation is often coupled with permanent exit from the cell cycle, and existing data suggest a close relationship between a repressive chromatin structure and silencing of the cell cycle in postmitotic cells. Heterochromatin clustering could also contribute to stable gene repression to maintain terminal differentiation or cell cycle exit, but whether clustering is initiated by differentiation, cell cycle changes, or both is unclear. Here we examine the relationship between chromatin organization, terminal differentiation and cell cycle exit.

Results

We focused our studies on the Drosophila wing, where epithelial cells transition from active proliferation to a postmitotic state in a temporally controlled manner. We find there are two stages of G₀ in this tissue, a flexible G₀ period where cells can be induced to reenter the cell cycle under specific genetic manipulations and a state we call “robust,” where cells become strongly refractory to cell cycle reentry. Compromising the flexible G₀ by driving ectopic expression of cell cycle activators causes a global disruption of the clustering of heterochromatin-associated histone modifications such as H3K27 trimethylation and H3K9 trimethylation, as well as their associated repressors, Polycomb and heterochromatin protein 1 (HP1). However, this disruption is reversible. When cells enter a robust G₀ state, even in the presence of ectopic cell cycle activity, clustering of heterochromatin-associated modifications is restored. If cell cycle exit is bypassed, cells in the wing continue to terminally differentiate, but heterochromatin clustering is severely disrupted. Heterochromatin-dependent gene silencing does not appear to be required for cell cycle exit, as compromising the H3K27 methyltransferase Enhancer of zeste, and/or HP1 cannot prevent the robust cell cycle exit, even in the face of normally oncogenic cell cycle activities.

Conclusions

Heterochromatin clustering during terminal differentiation is a consequence of cell cycle exit, rather than differentiation. Compromising heterochromatin-dependent gene silencing does not disrupt cell cycle exit.

Electronic supplementary material

The online version of this article (10.1186/s13072-017-0159-8) contains supplementary material, which is available to authorized users.

Ecdysone signaling induces two phases of cell cycle exit in Drosophila cells

During development, cell proliferation and differentiation must be tightly coordinated to ensure proper tissue morphogenesis. Because steroid hormones are central regulators of developmental timing, understanding the links between steroid hormone signaling and cell proliferation is crucial to understanding the molecular basis of morphogenesis. Here we examined the mechanism by which the steroid hormone ecdysone regulates the cell cycle in Drosophila. We find that a cell cycle arrest induced by ecdysone in Drosophila cell culture is analogous to a G2 cell cycle arrest observed in the early pupa wing. We show that in the wing, ecdysone signaling at the larva-to-puparium transition induces Broad which in turn represses the cdc25c phosphatase String. The repression of String generates a temporary G2 arrest that synchronizes the cell cycle in the wing epithelium during early pupa wing elongation and flattening. As ecdysone levels decline after the larva-to-puparium pulse during early metamorphosis, Broad expression plummets, allowing String to become re-activated, which promotes rapid G2/M progression and a subsequent synchronized final cell cycle in the wing. In this manner, pulses of ecdysone can both synchronize the final cell cycle and promote the coordinated acquisition of terminal differentiation characteristics in the wing.

Summary: Pulsed ecdysone signaling remodels cell cycle dynamics, causing distinct primary and secondary cell cycle arrests in Drosophila cells, analogous to those observed in the wing during metamorphosis.

TissueMiner: A multiscale analysis toolkit to quantify how cellular processes create tissue dynamics

Segmentation and tracking of cells in long-term time-lapse experiments has emerged as a powerful method to understand how tissue shape changes emerge from the complex choreography of constituent cells. However, methods to store and interrogate the large datasets produced by these experiments are not widely available. Furthermore, recently developed methods for relating tissue shape changes to cell dynamics have not yet been widely applied by biologists because of their technical complexity. We therefore developed a database format that stores cellular connectivity and geometry information of deforming epithelial tissues, and computational tools to interrogate it and perform multi-scale analysis of morphogenesis. We provide tutorials for this computational framework, called TissueMiner, and demonstrate its capabilities by comparing cell and tissue dynamics in vein and inter-vein subregions of the Drosophila pupal wing. These analyses reveal an unexpected role for convergent extension in shaping wing veins.

DOI:http://dx.doi.org/10.7554/eLife.14334.001

Cells interact, divide, rearrange and change shape to build an organ during development. Modern microscopy and computer technology can follow each individual cell of an entire organ in a living organism. However, to understand how the complex choreography of cells leads to well-shaped organs, researchers need tools to help the store and analyze the large amounts of data generated. Tools are also needed to visualize and quantify the complex cell behaviors in an easy and flexible manner.

During its development, a fruit fly’s wings become divided into distinct regions separated by tubular supports called veins. Early on in development, the vein cells are indistinguishable from their neighbors, but at late stages, vein cells become a different shape. Veins also become narrower, which is assumed to be due to the number of vein cells falling. However, the way in which cells behave to bring about these changes has not been studied in detail.

Etournay, Merkel, Popović, Brandl et al. have now developed a toolkit called TissueMiner that enables users to store large amounts of data about cells and analyze how cells collectively shape an organ. TissueMiner was then used to identify vein cells at late stages of wing development and follow them backward in time to reveal their position at early stages. This showed that veins become narrower and more elongated because the cells that make up the veins shrink more than cells in other regions.

TissueMiner was then used to show that vein cells specifically rearrange and elongate to produce thinner regions, while the number of cells increases slightly because the cells divide. These results suggest that the cell behaviors responsible for making veins elongate and narrow are likely to be different from what had previously been assumed.

TissueMiner can be used in future studies to help understand the molecule signals that influence how cells behave in veins during wing development. The toolkit could also now be used to explore the changes involved in the development of other organs in other organisms.

DOI:http://dx.doi.org/10.7554/eLife.14334.002

Roles for the Histone Modifying and Exchange Complex NuA4 in Cell Cycle Progression in Drosophila melanogaster

Robust and synchronous repression of E2F-dependent gene expression is critical to the proper timing of cell cycle exit when cells transition to a postmitotic state. Previously NuA4 was suggested to act as a barrier to proliferation in Drosophila by repressing E2F-dependent gene expression. Here we show that NuA4 activity is required for proper cell cycle exit and the repression of cell cycle genes during the transition to a postmitotic state in vivo However, the delay of cell cycle exit caused by compromising NuA4 is not due to additional proliferation or effects on E2F activity. Instead NuA4 inhibition results in slowed cell cycle progression through late S and G2 phases due to aberrant activation of an intrinsic p53-independent DNA damage response. A reduction in NuA4 function ultimately produces a paradoxical cell cycle gene expression program, where certain cell cycle genes become derepressed in cells that are delayed during the G2 phase of the final cell cycle. Bypassing the G2 delay when NuA4 is inhibited leads to abnormal mitoses and results in severe tissue defects. NuA4 physically and genetically interacts with components of the E2F complex termed D: rosophila, R: bf, E: 2F A: nd M: yb/ M: ulti-vulva class B: (DREAM/MMB), and modulates a DREAM/MMB-dependent ectopic neuron phenotype in the posterior wing margin. However, this effect is also likely due to the cell cycle delay, as simply reducing Cdk1 is sufficient to generate a similar phenotype. Our work reveals that the major requirement for NuA4 in the cell cycle in vivo is to suppress an endogenous DNA damage response, which is required to coordinate proper S and G2 cell cycle progression with differentiation and cell cycle gene expression.

Protein phosphatase 2A promotes the transition to G0 during terminal differentiation in Drosophila

Protein phosphatase type 2A complex (PP2A) has been known as a tumor suppressor for over two decades, but it remains unclear exactly how it suppresses tumor growth. Here, we provide data indicating a novel role for PP2A in promoting the transition to quiescence upon terminal differentiation in vivo. Using Drosophila eyes and wings as a model, we find that compromising PP2A activity during the final cell cycle prior to a developmentally controlled cell cycle exit leads to extra cell divisions and delays entry into quiescence. By systematically testing the regulatory subunits of Drosophila PP2A, we find that the B56 family member widerborst (wdb) is required for the role of PP2A in promoting the transition to quiescence. Cells in differentiating tissues with compromised PP2A retain high Cdk2 activity when they should be quiescent, and genetic epistasis tests demonstrate that ectopic Cyclin E/Cdk2 activity is responsible for the extra cell cycles caused by PP2A inhibition. The loss of wdb/PP2A function cooperates with aberrantly high Cyclin E protein levels, allowing cells to bypass a robust G0 late in development. This provides an example of how loss of PP2A can cooperate with oncogenic mutations in cancer. We propose that the PP2A complex plays a novel role in differentiating tissues to promote developmentally controlled quiescence through the regulation of Cyclin E/Cdk2 activity.

The wing and the eye: a parsimonious theory for scaling and growth control?

How a developing organ grows and patterns to its final shape is an important question in developmental biology. Studies of growth and patterning in the Drosophila wing imaginal disc have identified a key player, the morphogen Decapentaplegic (Dpp). These studies provided insights into our understanding of growth control and scaling: expansion of the Dpp gradient correlated with the growth of the tissue. A recent report on growth of a Drosophila organ other than the wing, the eye imaginal disc, prompts a reconsideration of our models of growth control. Despite striking differences between the two, the Dpp gradient scales with the target tissues of both organs and the growth of both the wing and the eye is controlled by Dpp. The goal of this review is to discuss whether a parsimonious model of scaling and growth control can explain the relationship between the Dpp gradient and growth in these two different developmental systems.

Xrn1/Pacman affects apoptosis and regulates expression of hid and reaper

Programmed cell death, or apoptosis, is a highly conserved cellular process that is crucial for tissue homeostasis under normal development as well as environmental stress. Misregulation of apoptosis is linked to many developmental defects and diseases such as tumour formation, autoimmune diseases and neurological disorders. In this paper, we show a novel role for the exoribonuclease Pacman/Xrn1 in regulating apoptosis. Using Drosophila wing imaginal discs as a model system, we demonstrate that a null mutation in pacman results in small imaginal discs as well as lethality during pupation. Mutant wing discs show an increase in the number of cells undergoing apoptosis, especially in the wing pouch area. Compensatory proliferation also occurs in these mutant discs, but this is insufficient to compensate for the concurrent increase in apoptosis. The phenotypic effects of the pacman null mutation are rescued by a deletion that removes one copy of each of the pro-apoptotic genes reaper, hid and grim, demonstrating that pacman acts through this pathway. The null pacman mutation also results in a significant increase in the expression of the pro-apoptotic mRNAs, hid and reaper, with this increase mostly occurring at the post-transcriptional level, suggesting that Pacman normally targets these mRNAs for degradation. Our results uncover a novel function for the conserved exoribonuclease Pacman and suggest that this exoribonuclease is important in the regulation of apoptosis in other organisms.

Towards long term cultivation of Drosophila wing imaginal discs in vitro

The wing imaginal disc of Drosophila melanogaster is a prominent experimental system for research on control of cell growth, proliferation and death, as well as on pattern formation and morphogenesis during organogenesis. The precise genetic methodology applicable in this system has facilitated conceptual advances of fundamental importance for developmental biology. Experimental accessibility and versatility would gain further if long term development of wing imaginal discs could be studied also in vitro. For example, culture systems would allow live imaging with maximal temporal and spatial resolution. However, as clearly demonstrated here, standard culture methods result in a rapid cell proliferation arrest within hours of cultivation of dissected wing imaginal discs. Analysis with established markers for cells in S- and M phase, as well as with RGB cell cycle tracker, a novel reporter transgene, revealed that in vitro cultivation interferes with cell cycle progression throughout interphase and not just exclusively during G1. Moreover, quantification of EGFP expression from an inducible transgene revealed rapid adverse effects of disc culture on basic cellular functions beyond cell cycle progression. Disc transplantation experiments confirmed that these detrimental consequences do not reflect fatal damage of imaginal discs during isolation, arguing clearly for a medium insufficiency. Alternative culture media were evaluated, including hemolymph, which surrounds imaginal discs during growth in situ. But isolated larval hemolymph was found to be even less adequate than current culture media, presumably as a result of conversion processes during hemolymph isolation or disc culture. The significance of prominent growth-regulating pathways during disc culture was analyzed, as well as effects of insulin and disc co-culture with larval tissues as potential sources of endocrine factors. Based on our analyses, we developed a culture protocol that prolongs cell proliferation in cultured discs.

Identification of novel elements of the Drosophila blisterome sheds light on potential pathological mechanisms of several human diseases

Main developmental programs are highly conserved among species of the animal kingdom. Improper execution of these programs often leads to progression of various diseases and disorders. Here we focused on Drosophila wing tissue morphogenesis, a fairly complex developmental program, one of the steps of which – apposition of the dorsal and ventral wing sheets during metamorphosis – is mediated by integrins. Disruption of this apposition leads to wing blistering which serves as an easily screenable phenotype for components regulating this process. By means of RNAi-silencing technique and the blister phenotype as readout, we identify numerous novel proteins potentially involved in wing sheet adhesion. Remarkably, our results reveal not only participants of the integrin-mediated machinery, but also components of other cellular processes, e.g. cell cycle, RNA splicing, and vesicular trafficking. With the use of bioinformatics tools, these data are assembled into a large blisterome network. Analysis of human orthologues of the Drosophila blisterome components shows that many disease-related genes may contribute to cell adhesion implementation, providing hints on possible mechanisms of these human pathologies.

Temporal regulation of Dpp signaling output in the Drosophila wing

BACKGROUND

The Decapentaplegic (Dpp) signaling pathway is used in many developmental and homeostatic contexts, each time resulting in cellular responses particular to that biological niche. The flexibility of Dpp signaling is clearly evident in epithelial cells of the Drosophila wing imaginal disc. During larval stages of development, Dpp functions as a morphogen, patterning the wing developmental field and stimulating tissue growth. A short time later, however, as wing-epithelial cells exit the cell cycle and begin to differentiate, Dpp is a critical determinant of vein-cell fate. It is likely that the Dpp signaling pathway regulates different sets of target genes at these two developmental time points.

RESULTS

To identify mechanisms that temporally control the transcriptional output of Dpp signaling in this system, we have taken a gene expression profiling approach. We identified genes affected by Dpp signaling at late larval or early pupal developmental time points, thereby identifying patterning- and differentiation-specific downstream targets, respectively.

CONCLUSIONS

Analysis of target genes and transcription factor binding sites associated with these groups of genes revealed potential mechanisms by which target-gene specificity of the Dpp signaling pathway is temporally regulated. In addition, this approach revealed novel mechanisms by which Dpp affects the cellular differentiation of wing-veins.

Hunting complex differential gene interaction patterns across molecular contexts

Heterogeneity in genetic networks across different signaling molecular contexts can suggest molecular regulatory mechanisms. Here we describe a comparative chi-square analysis (CPχ²) method, considerably more flexible and effective than other alternatives, to screen large gene expression data sets for conserved and differential interactions. CPχ² decomposes interactions across conditions to assess homogeneity and heterogeneity. Theoretically, we prove an asymptotic chi-square null distribution for the interaction heterogeneity statistic. Empirically, on synthetic yeast cell cycle data, CPχ² achieved much higher statistical power in detecting differential networks than alternative approaches. We applied CPχ² to Drosophila melanogaster wing gene expression arrays collected under normal conditions, and conditions with overexpressed E2F and Cabut, two transcription factor complexes that promote ectopic cell cycling. The resulting differential networks suggest a mechanism by which E2F and Cabut regulate distinct gene interactions, while still sharing a small core network. Thus, CPχ² is sensitive in detecting network rewiring, useful in comparing related biological systems.

Live cell cycle analysis of Drosophila tissues using the Attune Acoustic Focusing Cytometer and Vybrant DyeCycle violet DNA stain

Flow cytometry has been widely used to obtain information about DNA content in a population of cells, to infer relative percentages in different cell cycle phases. This technique has been successfully extended to the mitotic tissues of the model organism Drosophila melanogaster for genetic studies of cell cycle regulation in vivo. When coupled with cell-type specific fluorescent protein expression and genetic manipulations, one can obtain detailed information about effects on cell number, cell size and cell cycle phasing in vivo. However this live-cell method has relied on the use of the cell permeable Hoechst 33342 DNA-intercalating dye, limiting users to flow cytometers equipped with a UV laser. We have modified this protocol to use a newer live-cell DNA dye, Vybrant DyeCycle Violet, compatible with the more common violet 405nm laser. The protocol presented here allows for efficient cell cycle analysis coupled with cell type, relative cell size and cell number information, in a variety of Drosophila tissues. This protocol extends the useful cell cycle analysis technique for live Drosophila tissues to a small benchtop analyzer, the Attune Acoustic Focusing Cytometer, which can be run and maintained on a single-lab scale.

Combinatorial control of temporal gene expression in the Drosophila wing by enhancers and core promoters

Background

The transformation of a developing epithelium into an adult structure is a complex process, which often involves coordinated changes in cell proliferation, metabolism, adhesion, and shape. To identify genetic mechanisms that control epithelial differentiation, we analyzed the temporal patterns of gene expression during metamorphosis of the Drosophila wing.

Results

We found that a striking number of genes, approximately 50% of the Drosophila transcriptome, exhibited changes in expression during a time course of wing development. While cis-acting enhancer sequences clearly correlated with these changes, a stronger correlation was discovered between core-promoter types and the dynamic patterns of gene expression within this differentiating tissue. In support of the hypothesis that core-promoter type influences the dynamics of expression, expression levels of several TATA-box binding protein associated factors (TAFs) and other core promoter-associated components changed during this developmental time course, and a testes-specific TAF (tTAF) played a critical role in timing cellular differentiation within the wing.

Conclusions

Our results suggest that the combinatorial control of gene expression via cis-acting enhancer sequences and core-promoter types, determine the complex changes in gene expression that drive morphogenesis and terminal differentiation of the Drosophila wing epithelium.

Discontinuities in Rap1 activity determine epithelial cell morphology within the developing wing of Drosophila

Mechanisms that govern cell-fate specification within developing epithelia have been intensely investigated, with many of the critical intercellular signaling pathways identified, and well characterized. Much less is known, however, about downstream events that drive the morphological differentiation of these cells, once their fate has been determined. In the Drosophila wing-blade epithelium, two cell types predominate: vein and intervein. After cell proliferation is complete and adhesive cell-cell contacts have been refined, the vast majority of intervein cells adopt a hexagonal morphology. Within vein territories, however, cell-shape refinement results in trapezoids. Signaling events that differentiate between vein and intervein cell fates are well understood, but the genetic pathways underlying vein/intervein cyto-architectural differences remain largely undescribed. We show here that the Rap1 GTPase plays a critical role in determining cell-type-specific morphologies within the developing wing epithelium. Rap1, together with its effector Canoe, promotes symmetric distribution of the adhesion molecule DE-cadherin about the apicolateral circumference of epithelial cells. We provide evidence that in presumptive vein tissue Rap1/Canoe activity is down-regulated, resulting in adhesive asymmetries and non-hexagonal cell morphologies. In particular Canoe levels are reduced in vein cells as they morphologically differentiate. We also demonstrate that over-expression of Rap1 disrupts vein formation both in the developing epithelium and the adult wing blade. Therefore, vein/intervein morphological differences result, at least in part, from the patterned regulation of Rap1 activity.

Physical mechanisms shaping the Drosophila dorsoventral compartment boundary

BACKGROUND

Separating cells with distinct identities and fates by straight and sharp compartment boundaries is important for growth and pattern formation during animal development. The physical mechanisms shaping compartment boundaries, however, are not fully understood.

RESULTS

We combine theory and quantitative experiments to investigate the roles of different mechanisms to shape compartment boundaries. Our theoretical work shows that cell elongation created by anisotropic stress, cell proliferation rate, orientation of cell division, and cell bond tension all have distinct effects on the morphology of compartment boundaries during tissue growth. Our experiments using the developing Drosophila wing reveal that the roughness of the dorsoventral compartment boundary is dynamic and that it decreases during development. By measuring tissue relaxation in response to laser ablation of cell bonds at different developmental times, we demonstrate that decreased boundary roughness correlates with increased cell bond tension along the compartment boundary. Finally, by using experimentally determined values for cell bond tension, cell elongation and bias in orientation of cell division in simulations of tissue growth, we can reproduce the main features of the time evolution of the dorsoventral compartment boundary shape.

CONCLUSIONS

Local increase of cell bond tension along the boundary as well as global anisotropies in the tissue contribute to shaping boundaries in cell networks. We propose a simple scenario that combines time-dependent cell bond tension at the boundary, oriented cell division, and cell elongation in the tissue that can account for the main features of the dynamics of the shape of the dorsoventral compartment boundary.

Quiescence: early evolutionary origins and universality do not imply uniformity

Cell cycle investigations have focused on relentless exponential proliferation of cells, an unsustainable situation in nature. Proliferation of cells, whether microbial or metazoan, is interrupted by periods of quiescence. The vast majority of cells in an adult metazoan lie quiescent. As disruptions in this quiescence are at the foundation of cancer, it will be important for the field to turn its attention to the mechanisms regulating quiescence. While often presented as a single topic, there are multiple forms of quiescence each with complex inputs, some of which are tied to conceptually challenging aspects of metazoan regulation such as size control. In an effort to expose the enormity of the challenge, I describe the differing biological purposes of quiescence, and the coupling of quiescence in metazoans to growth and to the structuring of tissues during development. I emphasize studies in the organism rather than in tissue culture, because these expose the diversity of regulation. While quiescence is likely to be a primitive biological process, it appears that in adapting quiescence to its many distinct biological settings, evolution has diversified it. Consideration of quiescence in different models gives us an overview of this diversity.

Maintenance of imaginal disc plasticity and regenerative potential in Drosophila by p53

Following irradiation (IR), the DNA damage response (DDR) activates p53, which triggers death of cells in which repair cannot be completed. Lost tissue is then replaced and re-patterned through regeneration. We have examined the role of p53 in co-regulation of the DDR and tissue regeneration following IR damage in Drosophila. We find that after IR, p53 is required for imaginal disc cells to repair DNA, and in its absence the damage marker, γ-H2AX is persistently expressed. p53 is also required for the compensatory proliferation and re-patterning of the damaged discs, and our results indicate that cell death is not required to trigger these processes. We identify an IR-induced delay in developmental patterning in wing discs that accompanies an animal-wide delay of the juvenile-adult transition, and demonstrate that both of these delays require p53. In p53 mutants, the lack of developmental delays and of damage resolution leads to anueploidy and tissue defects, and ultimately to morphological abnormalities and adult inviability. We propose that p53 maintains plasticity of imaginal discs by co-regulating the maintenance of genome integrity and disc regeneration, and coordinating these processes with the physiology of the animal. These findings place p53 in a role as master coordinator of DNA and tissue repair following IR.

Developmental stability: a major role for cyclin G in drosophila melanogaster

Morphological consistency in metazoans is remarkable given the pervasive occurrence of genetic variation, environmental effects, and developmental noise. Developmental stability, the ability to reduce developmental noise, is a fundamental property of multicellular organisms, yet its genetic bases remains elusive. Imperfect bilateral symmetry, or fluctuating asymmetry, is commonly used to estimate developmental stability. We observed that Drosophila melanogaster overexpressing Cyclin G (CycG) exhibit wing asymmetry clearly detectable by sight. Quantification of wing size and shape using geometric morphometrics reveals that this asymmetry is a genuine-but extreme-fluctuating asymmetry. Overexpression of CycG indeed leads to a 40-fold increase of wing fluctuating asymmetry, which is an unprecedented effect, for any organ and in any animal model, either in wild populations or mutants. This asymmetry effect is not restricted to wings, since femur length is affected as well. Inactivating CycG by RNAi also induces fluctuating asymmetry but to a lesser extent. Investigating the cellular bases of the phenotypic effects of CycG deregulation, we found that misregulation of cell size is predominant in asymmetric flies. In particular, the tight negative correlation between cell size and cell number observed in wild-type flies is impaired when CycG is upregulated. Our results highlight the role of CycG in the control of developmental stability in D. melanogaster. Furthermore, they show that wing developmental stability is normally ensured via compensatory processes between cell growth and cell proliferation. We discuss the possible role of CycG as a hub in a genetic network that controls developmental stability.

Morphogens, nutrients, and the basis of organ scaling

The regulation of organ size is a long-standing problem in animal development. Studies in this area have shown that organ-intrinsic patterning morphogens influence organ size, guiding growth in accordance with positional information. However, organ-extrinsic humoral factors such as insulin also affect organ size, synchronizing growth with nutrient levels. Proliferating cells must integrate instructions from morphogens with those from nutrition so that growth proceeds as a function of both inputs. Coordinating cell proliferation with morphogens and nutrients ensures organs scale appropriately with body size, but the basis of this coordination is unclear. Here, the problem is illustrated using the Drosophila wing--a paradigm for organ growth and size control--and a potential solution suggested.

Cell death and selective adhesion reorganize the dorsoventral boundary for zigzag patterning of Drosophila wing margin hairs

Animal tissues and organs are comprised of several types of cells, which are often arranged in a well-ordered pattern. The posterior part of the Drosophila wing margin is covered with a double row of long hairs, which are equally and alternately derived from the dorsal and ventral sides of the wing, exhibiting a zigzag pattern in the lateral view. How this geometrically regular pattern is formed has not been fully understood. In this study, we show that this zigzag pattern is created by rearrangement of wing margin cells along the dorsoventral boundary flanked by the double row of hair cells during metamorphosis. This cell rearrangement is induced by selective apoptosis of wing margin cells that are spatially separated from hair cells. As a result of apoptosis, the remaining wing margin cells are rearranged in a well-ordered manner, which shapes corrugated lateral sides of both dorsal and ventral edges to interlock them for zigzag patterning. We further show that the corrugated topology of the wing edges is achieved by cell-type specific expression and localization of four kinds of NEPH1/nephrin family proteins through heterophilic adhesion between wing margin cells and hair cells. Homophilic E-cadherin adhesion is also required for attachment of the corrugated dorsoventral edges. Taken together, our results demonstrate that sequential coordination of apoptosis and epithelial architecture with selective adhesion creates the zigzag hair alignment. This may be a common mechanism for geometrically ordered repetitive packing of several types of cells in similarly patterned developmental fields such as the mammalian organ of Corti.

Planar cell polarity in Drosophila

In all multicellular organisms, epithelial cells are not only polarized along the apical-basal axis, but also within the epithelial plane, giving cells a sense of direction. Planar cell polarity (PCP) signaling regulates establishment of polarity within the plane of an epithelium. The outcomes of PCP signaling are diverse and include the determination of cell fates, the generation of asymmetric but highly aligned structures, such as the stereocilia in the human inner ear or the hairs on a fly wing, or the directional migration of cells during convergence and extension during vertebrate gastrulation. In humans, aberrant PCP signaling can result in severe developmental defects, such as open neural tubes (spina bifida), and can cause cystic kidneys. In this review, we discuss the basic mechanism and more recent findings of PCP signaling focusing on Drosophila melanogaster, the model organism in which most key PCP components were initially identified.

A novel interaction between hedgehog and Notch promotes proliferation at the anterior-posterior organizer of the Drosophila wing

Notch has multiple roles in the development of the Drosophila melanogaster wing imaginal disc. It helps specify the dorsal-ventral compartment border, and it is needed for the wing margin, veins, and sensory organs. Here we present evidence for a new role: stimulating growth in response to Hedgehog. We show that Notch signaling is activated in the cells of the anterior-posterior organizer that produce the region between wing veins 3 and 4, and we describe strong genetic interactions between the gene that encodes the Hedgehog pathway activator Smoothened and the Notch pathway genes Notch, presenilin, and Suppressor of Hairless and the Enhancer of split complex. This work thus reveals a novel collaboration by the Hedgehog and Notch pathways that regulates proliferation in the 3-4 intervein region independently of Decapentaplegic.

Control of wing size and proportions by Drosophila myc

Generation of an organ of appropriate size and shape requires mechanisms that coordinate growth and patterning, but how this is achieved is not understood. Here we examine the role of the growth regulator dMyc in this process during Drosophila wing imaginal disc development. We find that dMyc is expressed in a dynamic pattern that correlates with fate specification of different regions of the wing disc, leading us to hypothesize that dMyc expression in each region directs its growth. Consistent with this view, clonal analysis of growth in each region demonstrated distinct temporal requirements for dMyc that match its expression. Surprisingly, however, experiments in which dMyc expression is manipulated reveal that the endogenous pattern has only a minor influence on wing shape. Indeed, when dMyc function is completely lacking in the wing disc over most of its development, the discs grow slowly and are small in size but appear morphologically normal. Our experiments indicate, therefore, that rather than directly influence differential growth in the wing disc, the pattern of dMyc expression augments growth directed by other regulators. Overall, however, an appropriate level of dMyc expression in the wing disc is necessary for each region to achieve a proportionately correct size.

Temporal regulation of metamorphic processes in Drosophila by the let-7 and miR-125 heterochronic microRNAs

BACKGROUND

The let-7 and lin-4 microRNAs belong to a class of temporally expressed, noncoding regulatory RNAs that function as heterochronic switch genes in the nematode C. elegans. Heterochronic genes control the relative timing of events during development and are considered a major force in the rapid evolution of new morphologies. let-7 is highly conserved and in Drosophila is temporally coregulated with the lin-4 homolog, miR-125. Little is known, however, about their requirement outside the nematode or whether they universally control the timing of developmental processes.

RESULTS

We report the generation of a Drosophila mutant that lacks let-7 and miR-125 activities and that leads to a pleiotropic phenotype arising during metamorphosis. We focus on two defects and demonstrate that loss of let-7 and miR-125 results in temporal delays in two distinct metamorphic processes: the terminal cell-cycle exit in the wing and maturation of neuromuscular junctions (NMJs) at adult abdominal muscles. We identify the abrupt (ab) gene, encoding a nuclear protein, as a bona fide let-7 target and provide evidence that let-7 governs the maturation rate of abdominal NMJs during metamorphosis by regulating ab expression.

CONCLUSIONS

Drosophila Iet-7 and miR-125 mutants exhibit temporal misregulation of specific metamorphic processes. As in C. elegans, Drosophila let-7 is both necessary and sufficient for the appropriate timing of a specific cell-cycle exit, indicating that its function as a heterochronic microRNA is conserved. The ab gene is a target of let-7, and its repression in muscle is essential for the timing of NMJ maturation during metamorphosis. Our results suggest that let-7 and miR-125 serve as conserved regulators of events necessary for the transition from juvenile to adult life stages.

Normal regulation of Rbf1/E2f1 target genes in Drosophila type 1 protein phosphatase mutants

G1 Cyclin/Cdk complexes phosphorylate and inactivate the pRb tumor suppressor by preventing its ability to bind and repress E2F transcription factors. Current molecular and biochemical evidence suggests that type 1 protein phosphatases (PP1) dephosphorylate and thereby activate pRb, but the functional significance of this has not been addressed in the context of animal development. Here, we use genetic analyses to determine the role of PP1 in the regulation of Rbf1 activity during Drosophila development. While Rbf1 is required for E2f1 inhibition and G1 arrest in the embryonic epidermis and for the periodic expression of E2f1 target genes during endocycle S phase in the embryonic midgut and larval salivary gland, PP1 is not. PP1 regulates periodic cyclin E protein accumulation in ovarian nurse cells independently of Rbf1, which is dispensable for endocycle regulation in this tissue. We conclude that PP1 is not a major regulator of the Rbf1/E2F1 pathway in Drosophila.

A double-assurance mechanism controls cell cycle exit upon terminal differentiation in Drosophila

Terminal differentiation is often coupled with permanent exit from the cell cycle, yet it is unclear how cell proliferation is blocked in differentiated tissues. We examined the process of cell cycle exit in Drosophila wings and eyes and discovered that cell cycle exit can be prevented or even reversed in terminally differentiating cells by the simultaneous activation of E2F1 and either Cyclin E/Cdk2 or Cyclin D/Cdk4. Enforcing both E2F and Cyclin/Cdk activities is required to bypass exit because feedback between E2F and Cyclin E/Cdk2 is inhibited after cells differentiate, ensuring that cell cycle exit is robust. In some differentiating cell types (e.g., neurons), known inhibitors including the retinoblastoma homolog Rbf and the p27 homolog Dacapo contribute to parallel repression of E2F and Cyclin E/Cdk2. In other cell types, however (e.g., wing epithelial cells), unknown mechanisms inhibit E2F and Cyclin/Cdk activity in parallel to enforce permanent cell cycle exit upon terminal differentiation.

Rbf1-independent termination of E2f1-target gene expression during early Drosophila embryogenesis

The initiation and maintenance of G1 cell cycle arrest is a key feature of animal development. In the Drosophila ectoderm, G1 arrest first appears during the seventeenth embryonic cell cycle. The initiation of G1(17) arrest requires the developmentally-induced expression of Dacapo, a p27-like Cyclin E-Cdk2 inhibitor. The maintenance of G1(17) arrest requires Rbf1-dependent repression of E2f1-regulated replication factor genes, which are expressed continuously during cycles 1-16 when S phase immediately follows mitosis. The mechanisms that trigger Rbf1 repressor function and mediate G1(17) maintenance are unknown. Here we show that the initial downregulation of expression of the E2f1-target gene RnrS, which occurs during cycles 15 and 16 prior to entry into G1(17), does not require Rbf1 or p27(Dap). This suggests a mechanism for Rbf1-independent control of E2f1 during early development. We show that E2f1 protein is destroyed in a cell cycle-dependent manner during S phase of cycles 15 and 16. E2f1 is destroyed during early S phase, and requires ongoing DNA replication. E2f1 protein reaccumulates in epidermal cells arrested in G1(17), and in these cells the induction of p27(Dap) activates Rbf1 to repress E2f1-target genes to maintain a stable G1 arrest.

Apoptotic Cells Can Induce Compensatory Cell Proliferation through the JNK and the Wingless Signaling Pathways

In many metazoans, damaged and potentially dangerous cells are rapidly eliminated by apoptosis. In Drosophila, this is often compensated for by extraproliferation of neighboring cells, which allows the organism to tolerate considerable cell death without compromising development and body size. Despite its importance, the mechanistic basis of such compensatory proliferation remains poorly understood. Here, we show that apoptotic cells express the secretory factors wingless (wg) and decapentaplegic (dpp). When cells undergoing apoptosis were kept alive with the caspase inhibitor p35, excessive nonautonomous cell proliferation was observed. Significantly, wg signaling is necessary and, at least in some cells, also sufficient for mitogenesis under these conditions. Finally, we provide evidence that the DIAP1 antagonists reaper and hid can activate the JNK pathway and that this pathway is required for inducing wg and cell proliferation. These findings support a model where apoptotic cells activate signaling cascades for compensatory proliferation.

Cellular immune response to parasitization in Drosophila requires the EBF orthologue collier

Drosophila immune response involves three types of hemocytes ('blood cells'). One cell type, the lamellocyte, is induced to differentiate only under particular conditions, such as parasitization by wasps. Here, we have investigated the mechanisms underlying the specification of lamellocytes. We first show that collier (col), the Drosophila orthologue of the vertebrate gene encoding early B-cell factor (EBF), is expressed very early during ontogeny of the lymph gland, the larval hematopoietic organ. In this organ, Col expression prefigures a specific posterior region recently proposed to act as a signalling centre, the posterior signalling centre (PSC). The complete lack of lamellocytes in parasitized col mutant larvae revealed the critical requirement for Col activity in specification of this cell type. In wild-type larvae, Col expression remains restricted to the PSC following parasitization, despite the massive production of lamellocytes. We therefore propose that Col endows PSC cells with the capacity to relay an instructive signal that orients hematopoietic precursors towards the lamellocyte fate in response to parasitization. Considered together with the role of EBF in lymphopoiesis, these findings suggest new parallels in cellular immunity between Drosophila and vertebrates. Further investigations on Col/EBF expression and function in other phyla should provide fresh insight into the evolutionary origin of lymphoid cells.

Expression of the telomeric retrotransposon HeT-A in Drosophila melanogaster is correlated with cell proliferation

Drosophila melanogaster extends its telomeres by transposition of two non-LTR retrotransposons, HeT-A and TART, to chromosome ends. We have determined the tissue-specific expression of these two elements by whole-mount in situ hybridization with digoxigenin-labeled RNA sense and antisense probes in the germ line and in a variety of larval tissues during normal development in the wild type and in tissues of mutants that cause overproliferation. Our results indicate that transcript levels, which are a key component in the process of telomere elongation in D. melanogaster, are correlated with cell proliferation in normal tissues and that RNA levels are elevated in growth-stimulated tissues.

Connecting Hh, Dpp and EGF signalling in patterning of theDrosophilawing; the pivotal role ofcollier/knotin the AP organiser

Hedgehog (Hh) signalling from posterior (P) to anterior (A) cells is the primary determinant of AP polarity in the limb field in insects and vertebrates. Hh acts in part by inducing expression of Decapentaplegic (Dpp), but how Hh and Dpp together pattern the central region of the Drosophila wing remains largely unknown. We have re-examined the role played by Collier (Col), a dose-dependent Hh target activated in cells along the AP boundary, the AP organiser in the imaginal wing disc. We found that col mutant wings are smaller than wild type and lack L4 vein, in addition to missing the L3-L4 intervein and mis-positioning of the anterior L3 vein. We link these phenotypes to col requirement for the local upregulation of both emc and N, two genes involved in the control of cell proliferation, the EGFR ligand Vein and the intervein determination gene blistered. We further show that attenuation of Dpp signalling in the AP organiser is also col dependent and, in conjunction with Vein upregulation, required for formation of L4 vein. A model recapitulating the molecular interplay between the Hh, Dpp and EGF signalling pathways in the wing AP organiser is presented.

A characterization of the effects of Dpp signaling on cell growth and proliferation in theDrosophilawing

Cell proliferation and patterning must be coordinated for the development of properly proportioned organs. If the same molecules were to control both processes, such coordination would be ensured. Here we address this possibility in the Drosophila wing using the Dpp signaling pathway. Previous studies have shown that Dpp forms a gradient along the AP axis that patterns the wing, that Dpp receptors are autonomously required for wing cell proliferation, and that ectopic expression of either Dpp or an activated Dpp receptor, TkvQ253D, causes overgrowth. We extend these findings with a detailed analysis of the effects of Dpp signaling on wing cell growth and proliferation. Increasing Dpp signaling by expressing TkvQ253D accelerated wing cell growth and cell cycle progression in a coordinate and cell-autonomous manner. Conversely, autonomously inhibiting Dpp signaling using a pathway specific inhibitor, Dad, or a mutation in tkv, slowed wing cell growth and division, also in a coordinate fashion. Stimulation of cell cycle progression by TkvQ253D was blocked by the cell cycle inhibitor RBF, and required normal activity of the growth effector, PI3K. Among the known Dpp targets, vestigial was the only one tested that was required for TkvQ253D-induced growth. The growth response to altering Dpp signaling varied regionally and temporally in the wing disc, indicating that other patterned factors modify the response.

Temperature modulates epidermal cell size in Drosophila melanogaster

Most ectotherms show increased body size at maturity when reared under colder temperatures. In principle, temperature could produce this outcome by influencing growth, proliferation and/or death of epidermal cells. Here we investigated the effects of rearing temperature on the cell size and cell number in the wing blade, the basitarsus of the leg and the cornea of the eye of Drosophila melanogaster from two populations at opposite ends of a South American latitudinal cline. We found that, in both strains of D. melanogaster and in both sexes, a decrease in rearing temperature increases the size of the wings, legs and eyes through an effect on epidermal cell size, with no significant change in cell number. Our results indicate that temperature has a consistent effect on cell size in the Drosophila epidermis and this may also apply to other cell types. In contrast, the evolutionary effects of temperature on the different organs are not consistent. We discuss our findings in the context of growth control in Drosophila.

enok encodes a Drosophila putative histone acetyltransferase required for mushroom body neuroblast proliferation

Mushroom bodies in the Drosophila brain are centers for olfactory learning and memory. We have previously shown that the mushroom bodies comprise three types of neurons with distinct axonal projections. These three types of neurons are generated sequentially from common neuroblasts. We report here the identification of a gene that we have named enoki mushroom (enok), which when it is mutated gives rise to mushroom bodies with reduced axonal structures. enok encodes a putative histone acetyltransferase (HAT) of the MYST family, members of which have been implicated as important modulators of transcriptional activity. A single amino acid change in the zinc finger motif of the putative catalytic HAT domain gives the same phenotype as a null allele, and this finding indicates the importance of HAT activity to Enok's function. Further phenotypic analysis demonstrates that the mushroom body defect is due to an arrest in neuroblast proliferation rather than a failure of either cell fate switching or axon branching. Clonal analyses in the wing discs and the ovaries suggest that enok is essential for normal cell proliferation in some, but not all, tissues. Our results provide in vivo evidence for essential functions of a histone acetyltransferase in the construction of the Drosophila brain.

Larval multidendrite neurons survive metamorphosis and participate in the formation of imaginal sensory axonal pathways in the notum of Drosophila

In each hemimesothorax of Drosophila, a cluster of five larval multidendrite neurons that survives metamorphosis is described. The cell bodies of these neurons, initially grouped together, spread out over the medial heminotum during early pupal stages and extend new dendrites. Growing axons from sensory bristle neurons first appear in a defined orientation specific for each macrochaete. They subsequently contact processes from the larval multidendrite neurons and then appear to follow the preestablished axon trajectories of the latter. Ablation of the multidendrite neurons during the larval stage causes bristle axons to adopt abnormal trajectories. We suggest that the persistent larval neurons participate in guiding axons of the bristles on the medial half of the notum to the posterior dorsal mesothoracic nerve leading to the central nervous system.

Drosophila S6 kinase: a regulator of cell size

Cell proliferation requires cell growth; that is, cells only divide after they reach a critical size. However, the mechanisms by which cells grow and maintain their appropriate size have remained elusive. Drosophila deficient in the S6 kinase gene (dS6K) exhibited an extreme delay in development and a severe reduction in body size. These flies had smaller cells rather than fewer cells. The effect was cell-autonomous, displayed throughout larval development, and distinct from that of ribosomal protein mutants (Minutes). Thus, the dS6K gene product regulates cell size in a cell-autonomous manner without impinging on cell number.

Disruption of selenoprotein biosynthesis affects cell proliferation in the imaginal discs and brain of Drosophila melanogaster

The patufet gene encodes the Drosophila melanogaster homologue of selenophosphate synthetase, an enzyme required for selenoprotein synthesis, and appears to have a role in cell proliferation. In this paper we analyse the expression pattern of patufet during the development of imaginal discs and brain as well as the function of this gene in relation to cell proliferation. Wild-type organisms showed a highly dynamic pattern of ptuf mRNA expression during larval and pupal development. Co-localization analysis of ptuf mRNA expression and BrdU incorporation showed high levels of ptuf mRNA in dividing cells and low or undetectable levels in non-dividing cells. In addition, [(75)Se] incorporation revealed a major selenoprotein band of 42 kDa. Mutant organisms showed no selenoprotein synthesis, lower levels of cell proliferation, a higher proportion of cells arrested in G(2) as seen by cyclin B labeling and increased levels of reactive oxygen species (ROS). Because most selenoproteins identified so far are antioxidants, the role of ptuf in cell proliferation through the control of the cellular redox balance is discussed.