Coordination of growth and cell division in the Drosophila wing

Activated STAT regulates growth and induces competitive interactions independently of Myc, Yorkie, Wingless and ribosome biogenesis

Cell competition is a conserved mechanism that regulates organ size and shares properties with the early stages of cancer. In Drosophila, wing cells with increased Myc or with optimum ribosome function become supercompetitors that kill their wild-type neighbors (called losers) up to several cell diameters away. Here, we report that modulating STAT activity levels regulates competitor status. Cells lacking STAT become losers that are killed by neighboring wild-type cells. By contrast, cells with hyper-activated STAT become supercompetitors that kill losers located at a distance in a manner that is dependent on hid but independent of Myc, Yorkie, Wingless signaling, and of ribosome biogenesis. These results indicate that STAT, Wingless and Myc are major parallel regulators of cell competition, which may converge on signals that non-autonomously kill losers. As hyper-activated STATs are causal to tumorigenesis and stem cell niche occupancy, our results have therapeutic implications for cancer and regenerative medicine.

S phase-coupled E2f1 destruction ensures homeostasis in proliferating tissues

Precise control of cell cycle regulators is critical for normal development and tissue homeostasis. E2F transcription factors are activated during G1 to drive the G1-S transition and are then inhibited during S phase by a variety of mechanisms. Here, we genetically manipulate the single Drosophila activator E2F (E2f1) to explore the developmental requirement for S phase–coupled E2F down-regulation. Expression of an E2f1 mutant that is not destroyed during S phase drives cell cycle progression and causes apoptosis. Interestingly, this apoptosis is not exclusively the result of inappropriate cell cycle progression, because a stable E2f1 mutant that cannot function as a transcription factor or drive cell cycle progression also triggers apoptosis. This observation suggests that the inappropriate presence of E2f1 protein during S phase can trigger apoptosis by mechanisms that are independent of E2F acting directly at target genes. The ability of S phase-stabilized E2f1 to trigger apoptosis requires an interaction between E2f1 and the Drosophila pRb homolog, Rbf1, and involves induction of the pro-apoptotic gene, hid. Simultaneously blocking E2f1 destruction during S phase and inhibiting the induction of apoptosis results in tissue overgrowth and lethality. We propose that inappropriate accumulation of E2f1 protein during S phase triggers the elimination of potentially hyperplastic cells via apoptosis in order to ensure normal development of rapidly proliferating tissues.

Rapidly growing tissues provide an excellent opportunity to study the careful balance between cell proliferation and apoptosis needed for normal organ structure and function in developing organisms. We present evidence that a transcription factor critical for regulating progression of the Drosophila melanogaster cell cycle, E2f1, serves also as an indicator of normal tissue development. E2f1 activation during G1 phase of the cell cycle triggers entry into S phase. E2f1 activity is then rapidly inhibited during S phase by a mechanism that couples E2f1 proteolysis directly to DNA synthesis. Expression during larval development of an S phase-stabilized form of E2f1 results in apoptosis in rapidly proliferating adult wing precursor cells, even when this stabilized E2f1 protein is mutated such that it cannot induce transcription or cell cycle progression. Preventing the ability of S phase-stabilized E2f1 to induce apoptosis results in massive tissue overgrowth. We propose that aberrant E2f1 accumulation during S phase triggers apoptosis in order to remove potentially hyper-proliferative cells and to maintain homeostasis during tissue growth.

FACS purification and transcriptome analysis of drosophila neural stem cells reveals a role for Klumpfuss in self-renewal

Drosophila neuroblasts (NBs) have emerged as a model for stem cell biology that is ideal for genetic analysis but is limited by the lack of cell-type-specific gene expression data. Here, we describe a method for isolating large numbers of pure NBs and differentiating neurons that retain both cell-cycle and lineage characteristics. We determine transcriptional profiles by mRNA sequencing and identify 28 predicted NB-specific transcription factors that can be arranged in a network containing hubs for Notch signaling, growth control, and chromatin regulation. Overexpression and RNA interference for these factors identify Klumpfuss as a regulator of self-renewal. We show that loss of Klumpfuss function causes premature differentiation and that overexpression results in the formation of transplantable brain tumors. Our data represent a valuable resource for investigating Drosophila developmental neurobiology, and the described method can be applied to other invertebrate stem cell lineages as well.

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.

Akt is negatively regulated by Hippo signaling for growth inhibition in Drosophila

Tissue growth is achieved through coordinated cellular growth, cell division and apoptosis. Hippo signaling is critical for monitoring tissue growth during animal development. Loss of Hippo signaling leads to tissue overgrowth due to continuous cell proliferation and block of apoptosis. As cells lacking Hippo signaling are similar in size compared to normal cells, cellular growth must be properly maintained in Hippo signaling-deficient cells. However, it is not clear how Hippo signaling might regulate cellular growth. Here we show that loss of Hippo signaling increased Akt (also called Protein Kinase B, PKB) expression and activity, whereas activation of Hippo signaling reduced Akt expression in developing tissues in Drosophila. While yorkie (yki) is sufficient to increase Akt expression, Akt up-regulation caused by the loss of Hippo signaling is strongly dependent on yki, indicating that Hippo signaling negatively regulates Akt expression through Yki inhibition. Consistently, genetic analysis revealed that Akt plays a critical role in facilitating growth of Hippo signaling-defective tissues. Thus, Hippo signaling not only blocks cell division and promotes apoptosis, but also regulates cellular growth by inhibiting the Akt pathway activity.

Hypertrophy and unconventional cell division of hepatocytes underlie liver regeneration

BACKGROUND

The size of organs and tissues is basically determined by the number and size of their cells. However, little attention has been paid to this fundamental concept. The liver has a remarkable ability to regenerate after surgical resection (partial hepatectomy [PHx]), and hepatocytes account for about 80% of liver weight, so we investigate how the number and size of hepatocytes contribute to liver regeneration in mice. It has been generally accepted that hepatocytes undergo one or two rounds of cell division after 70% PHx. However, ploidy of hepatocytes is known to increase during regeneration, suggesting an unconventional cell cycle. We therefore examine cell cycle of hepatocytes in detail.

RESULTS

By developing a method for genetic fate mapping and a high-throughput imaging system of individual hepatocytes, we show that cellular hypertrophy makes the first contribution to liver regeneration; i.e., regeneration after 30% PHx is achieved solely by hypertrophy without cell division, and hypertrophy precedes proliferation after 70% PHx. Proliferation and hypertrophy almost equally contribute to regeneration after 70% PHx. Furthermore, although most hepatocytes enter cell cycle after 70% PHx, not all hepatocytes undergo cell division. In addition, binuclear hepatocytes undergo reductive divisions to generate two mononuclear daughter hepatocytes in some cases.

CONCLUSIONS

Our findings demonstrate the importance of hypertrophy and the unconventional cell division cycle of hepatocytes in regeneration, prompting a significant revision of the generally accepted model of liver regeneration.

Control of organ size in plants

The size of plant organs, such as leaves and flowers, is determined by an interaction of genotype and environmental influences. Organ growth occurs through the two successive processes of cell proliferation followed by cell expansion. A number of genes influencing either or both of these processes and thus contributing to the control of final organ size have been identified in the last decade. Although the overall picture of the genetic regulation of organ size remains fragmentary, two transcription factor/microRNA-based genetic pathways are emerging in the control of cell proliferation. However, despite this progress, fundamental questions remain unanswered, such as the problem of how the size of a growing organ could be monitored to determine the appropriate time for terminating growth. While genetic analysis will undoubtedly continue to advance our knowledge about size control in plants, a deeper understanding of this and other basic questions will require including advanced live-imaging and mathematical modeling, as impressively demonstrated by some recent examples. This should ultimately allow the comparison of the mechanisms underlying size control in plants and in animals to extract common principles and lineage-specific solutions.

Organ size control by Hippo and TOR pathways

The determination of final organ size is a highly coordinated and complex process that relies on the precise regulation of cell number and/or cell size. Perturbation of organ size control contributes to many human diseases, including hypertrophy, degenerative diseases, and cancer. Hippo and TOR are among the key signaling pathways involved in the regulation of organ size through their respective functions in the regulation of cell number and cell size. Here, we review the general mechanisms that regulate organ growth, describe how Hippo and TOR control key aspects of growth, and discuss recent findings that highlight a possible coordination between Hippo and TOR in organ size regulation.

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.

A network of broadly expressed HLH genes regulates tissue-specific cell fates

Spatial and temporal expression of specific basic-helix-loop-helix (bHLH) transcription factors defines many types of cellular differentiation. We find that a distinct mechanism regulates the much broader expression of the heterodimer partners of these specific factors and impinges on differentiation. In Drosophila, a cross-interacting regulatory network links expression of the E protein Daughterless (Da), which heterodimerizes with bHLH proteins to activate them, with expression of the Id protein Extramacrochaetae (Emc), which antagonizes bHLH proteins. Coupled transcriptional feedback loops maintain the widespread Emc expression that restrains Da expression, opposing bHLH-dependent differentiation while enhancing growth and cell survival. Where extracellular signals repress emc, Da expression can increase. This defines regions of proneural ectoderm independently from the proneural bHLH genes. Similar regulation is found in multiple Drosophila tissues and in mammalian cells and therefore is likely to be a conserved general feature of developmental regulation by HLH proteins.

CDNA cloning and characterization of S6 kinase and its effect on yolk protein gene expression in the oriental fruit fly Bactrocera dorsalis (Hendel)

p70 S6 kinase (S6K), a serine/threonine protein kinase, is a downstream target of target of rapamycin (TOR) gene and an important regulator of protein synthesis responsible for cell growth and reproduction. In this study, a S6K gene, named BdS6K (GenBank Accession No. GQ203802), was isolated from the oriental fruit fly Bactrocera dorsalis (Hendel). Quantitative RT-PCR showed that BdS6K mRNA is expressed at a higher level in egg than in other developmental stages, as well as in ovary than in fat body. Downregulation of BdS6K activity by rapamycin treatment in larval stage resulted in the developmental defects of larvae, pupae, and adults, with a reduced yolk protein (YP) expression in the fat body throughout the first reproductive cycle with a substantial reduction in ovary size, and also repressed the egg development in female fruit fly. Knockdown of BdS6K gene by RNA interference in the adult significantly decreased the YP expression. These observations support the involvement of BdS6K signaling in the regulation of the YP synthesis and egg development in B. dorsalis.

Control of Drosophila endocycles by E2F and CRL4(CDT2)

Endocycles are variant cell cycles comprised of DNA Synthesis (S)- and Gap (G)- phases but lacking mitosis^1,2. Such cycles facilitate post-mitotic growth in many invertebrate and plant cells, and are so ubiquitous that they may account for up to half the world’s biomass^3,4. DNA replication in endocycling Drosophila cells is triggered by Cyclin E/Cyclin Dependent Kinase 2 (CycE/Cdk2), but this kinase must be inactivated during each G-phase to allow the assembly of pre-Replication Complexes (preRCs) for the next S-phase^5,6. How CycE/Cdk2 is periodically silenced to allow re-replication has not been established. Here, using genetic tests in parallel with computational modeling, we show that Drosophila’s endocycles are driven by a molecular oscillator in which the E2F1 transcription factor promotes CycE expression and S-phase initiation, S-phase then activates the CRL4^Cdt2 ubiquitin ligase, and this in turn mediates the destruction of E2F1⁷. We propose that it is the transient loss of E2F1 during S-phases that creates the window of low Cdk activity required for preRC formation. In support of this model over-expressed E2F1 accelerated endocycling, whereas a stabilized variant of E2F1 blocked endocycling by de-regulating target genes including CycE, as well as Cdk1 and mitotic Cyclins. Moreover, we find that altering cell growth by changing nutrition or TOR signaling impacts E2F1 translation, thereby making endocycle progression growth-dependent. Many of the regulatory interactions essential to this novel cell cycle oscillator are conserved in animals and plants^1,2,8, suggesting that elements of this mechanism act in most growth-dependent cell cycles.

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.

The missing link: implementation of morphogenetic growth control on the cellular and molecular level

In the wing imaginal disc of Drosophila melanogaster, the morphogen Dpp controls growth, probably in an instructive manner. Many models for growth control by Dpp have been proposed and have been extensively discussed elsewhere. In this review, we speculate on how instructive growth control could provide a link between Dpp signaling and cell growth and/or cell cycle progression and so implement morphogenetic growth control on the cellular and molecular levels.

Myc localizes to histone locus bodies during replication in Drosophila

Myc is an important protein at the center of multiple pathways required for growth and proliferation in animals. The absence of Myc is lethal in flies and mice, and its over-production is a potent inducer of over-proliferation and cancer. Myc protein is localized to the nucleus where it executes its many functions, however the specific sub-nuclear localization of Myc has rarely been reported. The work we describe here began with an observation of unexpected, punctate spots of Myc protein in certain regions of Drosophila embryos. We investigated the identity of these puncta and demonstrate that Myc is co-localized with coilin, a marker for sub-nuclear organelles known as Cajal Bodies (CBs), in embryos, larvae and ovaries. Using antibodies specific for U7 snRNP component Lsm11, we show that the majority of Myc and coilin co-localization occurs in Histone Locus Bodies (HLBs), the sites of histone mRNA transcription and processing. Furthermore, Myc localizes to HLBs only during replication in mitotic and endocycling cells, suggesting that its role there relates to replication-dependent canonical histone gene transcription. These results provide evidence that sub-nuclear localization of Myc is cell-cycle dependent and potentially important for histone mRNA production and processing.

Understanding morphogenetic growth control -- lessons from flies

Morphogens are secreted signalling molecules that control the patterning and growth of developing organs. How morphogens regulate patterning is fairly well understood; however, how they control growth is less clear. Four principal models have been proposed to explain how the morphogenetic protein Decapentaplegic (DPP) controls the growth of the wing imaginal disc in the fly. Recent studies in this model system have provided a wealth of experimental data on growth and DPP gradient properties, as well as on the interactions of DPP with other signalling pathways. These findings have allowed a more precise formulation and evaluation of morphogenetic growth models. The insights into growth control by the DPP gradient will also be useful for understanding other morphogenetic growth systems.

Arabidopsis ORGAN SIZE RELATED1 regulates organ growth and final organ size in orchestration with ARGOS and ARL

• The growth of a plant organ to its characteristic size is regulated by an elaborate developmental program involving both internal and external signals. Here, we identify a novel Arabidopsis gene, ORGAN SIZE RELATED1 (OSR1), that is involved in regulation of organ growth and overall organ size. • A combination of genetic, cytological and molecular approaches was used to characterize the expression profile, subcellular localization and roles of OSR1 during organ growth. • Ectopic expression of OSR1 in Arabidopsis resulted in enlarged organs, as a consequence of increases in both cell number and cell size. OSR1 shares a conserved OSR domain with ARGOS and ARGOS-LIKE (ARL), which is sufficient for their functions in promoting organ growth. OSR1 is a plant hormone-responsive gene and appears to act redundantly with ARGOS and ARL during organ growth. The OSR proteins are localized to the endoplasmic reticulum. • Our results suggest that three co-evolved members of the OSR family may act coordinately to orchestrate growth signals and cell proliferation and expansion, thereby affecting organ growth and final organ size.

Genetics and molecular biology of tuberous sclerosis complex

Tuberous Sclerosis Complex is a multisystem disorder exhibiting a wide range of manifestations characterized by tumour-like lesions called hamartomas in the brain, skin, eyes, heart, lungs and kidneys. Tuberous Sclerosis Complex is genetically determined with an autosomal dominant inheritance and is caused by inactivating mutations in either the TSC1 or TSC2 genes. TSC1/2 genes play a fundamental role in the regulation of phosphoinositide 3-kinase (PI3K) signalling pathway, inhibiting the mammalian target of rapamycin (mTOR) through activation of the GTPase activity of Rheb. Mutations in TSC1/2 genes impair the inhibitory function of the hamartin/tuberin complex, leading to phosphorylation of the downstream effectors of mTOR, p70 S6 kinase (S6K), ribosomal protein S6 and the elongation factor binding protein 4E-BP1, resulting in uncontrolled cell growth and tumourigenesis.

Despite recent promising genetic, diagnostic, and therapeutic advances in Tuberous Sclerosis Complex, continuing research in all aspects of this complex disease will be pivotal to decrease its associated morbidity and mortality. In this review we will discuss and analyse all the important findings in the molecular pathogenesis of Tuberous Sclerosis Complex, focusing on genetics and the molecular mechanisms that define this multisystemic disorder.

Mechanisms limiting body growth in mammals

Recent studies have begun to provide insight into a long-standing mystery in biology-why body growth in animals is rapid in early life but then progressively slows, thus imposing a limit on adult body size. This growth deceleration in mammals is caused by potent suppression of cell proliferation in multiple tissues and is driven primarily by local, rather than systemic, mechanisms. Recent evidence suggests that this progressive decline in proliferation results from a genetic program that occurs in multiple organs and involves the down-regulation of a large set of growth-promoting genes. This program does not appear to be driven simply by time, but rather depends on growth itself, suggesting that the limit on adult body size is imposed by a negative feedback loop. Different organs appear to use different types of information to precisely target their adult size. For example, skeletal and cardiac muscle growth are negatively regulated by myostatin, the concentration of which depends on muscle mass itself. Liver growth appears to be modulated by bile acid flux, a parameter that reflects organ function. In pancreas, organ size appears to be limited by the initial number of progenitor cells, suggesting a mechanism based on cell-cycle counting. Further elucidation of the fundamental mechanisms suppressing juvenile growth is likely to yield important insights into the pathophysiology of childhood growth disorders and of the unrestrained growth of cancer. In addition, improved understanding of these growth-suppressing mechanisms may someday allow their therapeutic suspension in adult tissues to facilitate tissue regeneration.

Molecular mechanism of size control in development and human diseases

How multicellular organisms control their size is a fundamental question that fascinated generations of biologists. In the past 10 years, tremendous progress has been made toward our understanding of the molecular mechanism underlying size control. Original studies from Drosophila showed that in addition to extrinsic nutritional and hormonal cues, intrinsic mechanisms also play important roles in the control of organ size during development. Several novel signaling pathways such as insulin and Hippo-LATS signaling pathways have been identified that control organ size by regulating cell size and/or cell number through modulation of cell growth, cell division, and cell death. Later studies using mammalian cell and mouse models also demonstrated that the signaling pathways identified in flies are also conserved in mammals. Significantly, recent studies showed that dysregulation of size control plays important roles in the development of many human diseases such as cancer, diabetes, and hypertrophy.

Targeted interactomics reveals a complex core cell cycle machinery in Arabidopsis thaliana

A protein interactome focused towards cell proliferation was mapped comprising 857 interactions among 393 proteins, leading to many new insights in plant cell cycle regulation.

A comprehensive view on heterodimeric cyclin-dependent kinase (CDK)/cyclin complexes in plants is obtained, in relation with their regulators.

Over 100 new candidate cell cycle proteins were predicted.

The basic underlying mechanisms that govern the cell cycle are conserved among all eukaryotes. Peculiar for plants, however, is that their genome contains a collection of cell cycle regulatory genes that is intriguingly large (Vandepoele et al, 2002; Menges et al, 2005) compared to other eukaryotes. Arabidopsis thaliana (Arabidopsis) encodes 71 genes in five regulatory classes versus only 15 in yeast and 23 in human.

Despite the discovery of numerous cell cycle genes, little is known about the protein complex machinery that steers plant cell division. Therefore, we applied tandem affinity purification (TAP) approach coupled with mass spectrometry (MS) on Arabidopsis cell suspension cultures to isolate and analyze protein complexes involved in the cell cycle. This approach allowed us to successfully map a first draft of the basic cell cycle complex machinery of Arabidopsis, providing many new insights into plant cell division.

To map the interactome, we relied on a streamlined platform comprising generic Gateway-based vectors with high cloning flexibility, the fast generation of transgenic suspension cultures, TAP adapted for plant cells, and matrix-assisted laser desorption ionization (MALDI) tandem-MS for the identification of purified proteins (Van Leene et al, 2007, 2008Van Leene et al, 2007, 2008). Complexes for 102 cell cycle proteins were analyzed using this approach, leading to a non-redundant data set of 857 interactions among 393 proteins (Figure 1A). Two subspaces were identified in this data set, domain I1, containing interactions confirmed in at least two independent experimental repeats or in the reciprocal purification experiment, and domain I2 consisting of uniquely observed interactions.

Several observations underlined the quality of both domains. All tested reverse purifications found the original interaction, and 150 known or predicted interactions were confirmed, meaning that also a huge stack of new interactions was revealed. An in-depth computational analysis revealed enrichment for many cell cycle-related features among the proteins of the network (Figure 1B), and many protein pairs were coregulated at the transcriptional level (Figure 1C). Through integration of known cell cycle-related features, more than 100 new candidate cell cycle proteins were predicted (Figure 1D). Besides common qualities of both interactome domains, their real significance appeared through mutual differences exposing two subspaces in the cell cycle interactome: a central regulatory network of stable complexes that are repeatedly isolated and represent core regulatory units, and a peripheral network comprising transient interactions identified less frequently, which are involved in other aspects of the process, such as crosstalk between core complexes or connections with other pathways. To evaluate the biological relevance of the cell cycle interactome in plants, we validated interactions from both domains by a transient split-luciferase assay in Arabidopsis plants (Marion et al, 2008), further sustaining the hypothesis-generating power of the data set to understand plant growth.

With respect to insights into the cell cycle physiology, the interactome was subdivided according to the functional classes of the baits and core protein complexes were extracted, covering cyclin-dependent kinase (CDK)/cyclin core complexes together with their positive and negative regulation networks, DNA replication complexes, the anaphase-promoting complex, and spindle checkpoint complexes. The data imply that mitotic A- and B-type cyclins exclusively form heterodimeric complexes with the plant-specific B-type CDKs and not with CDKA;1, whereas D-type cyclins seem to associate with CDKA;1. Besides the extraction of complexes previously shown in other organisms, our data also suggested many new functional links; for example, the link coupling cell division with the regulation of transcript splicing. The association of negative regulators of CDK/cyclin complexes with transcription factors suggests that their role in reallocation is not solely targeted to CDK/cyclin complexes. New members of the Siamese-related inhibitory proteins were identified, and for the first time potential inhibitors of plant-specific mitotic B-type CDKs have been found in plants. New evidence that the E2F–DP–RBR network is not only active at G1-to-S, but also at the G2-to-M transition is provided and many complexes involved in DNA replication or repair were isolated. For the first time, a plant APC has been isolated biochemically, identifying three potential new plant-specific APC interactors, and finally, complexes involved in the spindle checkpoint were isolated mapping many new but specific interactions.

Finally, to get a general view on the complex machinery, modules of interacting cyclins and core cell cycle regulators were ranked along the cell cycle phases according to the transcript expression peak of the cyclins, showing an assorted set of CDK–cyclin complexes with high regulatory differentiation (Figure 4). Even within the same subfamily (e.g. cyclin A3, B1, B2, D3, and D4), cyclins differ not only in their functional time frame but also in the type and number of CDKs, inhibitors, and scaffolding proteins they bind, further indicating their functional diversification. According to our interaction data, at least 92 different variants of CDK–cyclin complexes are found in Arabidopsis.

In conclusion, these results reflect how several rounds of gene duplication (Sterck et al, 2007) led to the evolution of a large set of cyclin paralogs and a myriad of regulators, resulting in a significant jump in the complexity of the cell cycle machinery that could accommodate unique plant-specific features such as an indeterminate mode of postembryonic development. Through their extensive regulation and connection with a myriad of up- and downstream pathways, the core cell cycle complexes might offer the plant a flexible toolkit to fine-tune cell proliferation in response to an ever-changing environment.

Cell proliferation is the main driving force for plant growth. Although genome sequence analysis revealed a high number of cell cycle genes in plants, little is known about the molecular complexes steering cell division. In a targeted proteomics approach, we mapped the core complex machinery at the heart of the Arabidopsis thaliana cell cycle control. Besides a central regulatory network of core complexes, we distinguished a peripheral network that links the core machinery to up- and downstream pathways. Over 100 new candidate cell cycle proteins were predicted and an in-depth biological interpretation demonstrated the hypothesis-generating power of the interaction data. The data set provided a comprehensive view on heterodimeric cyclin-dependent kinase (CDK)–cyclin complexes in plants. For the first time, inhibitory proteins of plant-specific B-type CDKs were discovered and the anaphase-promoting complex was characterized and extended. Important conclusions were that mitotic A- and B-type cyclins form complexes with the plant-specific B-type CDKs and not with CDKA;1, and that D-type cyclins and S-phase-specific A-type cyclins seem to be associated exclusively with CDKA;1. Furthermore, we could show that plants have evolved a combinatorial toolkit consisting of at least 92 different CDK–cyclin complex variants, which strongly underscores the functional diversification among the large family of cyclins and reflects the pivotal role of cell cycle regulation in the developmental plasticity of plants.

Spatiotemporally modulated Vestigial gradient by Wingless signaling adaptively regulates cell division for precise wing size control

In animal development, the growth of a tissue or organ is timely arrested when it reaches the stereotyped correct size. How this is robustly controlled remains poorly understood. The prevalent viewpoint, which is that morphogen gradients, due to their organizing roles in development, are directly responsible for growth arrest, cannot explain a number of observations. Recent findings from studies of the Drosophila wing have revealed that the interpretation of the Wingless gradient requires signaling-induced self-inhibition and that cell proliferation is controlled by graded vestigial expression. These findings highlight a growth control mechanism that involves Wingless regulated vestigial expression, but a question is whether they can quantitatively explain the observed precision and robustness of wing size control. Quantitative and systematic investigation into Wingless signaling using a mathematical model has elucidated two points. First, negative regulation of the Vestigial gradient by Wingless signaling makes vestigial expression precise and robust. Second, weak Wingless signaling in a primarily small wing pouch causes a short and steep Vestigial gradient, which stimulates more cell divisions and leads to a significant expansion of the wing pouch; however, strong Wingless signaling in a primarily large wing pouch causes a long and smooth Vestigial gradient, which stimulates fewer cell divisions and results in a slight expansion of the wing pouch. These results substantially decipher an inherent mechanism of tissue and organ size control. Our model explains, and is supported by, a number of experimental observations.

A Drosophila model for fetal alcohol syndrome disorders: role for the insulin pathway

Prenatal exposure to ethanol in humans results in a wide range of developmental abnormalities, including growth deficiency, developmental delay, reduced brain size, permanent neurobehavioral abnormalities and fetal death. Here we describe the use of Drosophila melanogaster as a model for exploring the effects of ethanol exposure on development and behavior. We show that developmental ethanol exposure causes reduced viability, developmental delay and reduced adult body size. We find that flies reared on ethanol-containing food have smaller brains and imaginal discs, which is due to reduced cell division rather than increased apoptosis. Additionally, we show that, as in mammals, flies reared on ethanol have altered responses to ethanol vapor exposure as adults, including increased locomotor activation, resistance to the sedating effects of the drug and reduced tolerance development upon repeated ethanol exposure. We have found that the developmental and behavioral defects are largely due to the effects of ethanol on insulin signaling; specifically, a reduction in Drosophila insulin-like peptide (Dilp) and insulin receptor expression. Transgenic expression of Dilp proteins in the larval brain suppressed both the developmental and behavioral abnormalities displayed by ethanol-reared adult flies. Our results thus establish Drosophila as a useful model system to uncover the complex etiology of fetal alcohol syndrome.

Rab11 regulates JNK and Raf/MAPK-ERK signalling pathways during Drosophila wing development

Developmental signalling pathways are regulated by intracellular vesicle trafficking in multicellular organisms. In our earlier communication, we have shown that mutation in Rab11 (a subfamily of the Ypt/Rab gene family) results in the activation of JNK signalling pathways in Drosophila eye. Here, we report that Rab11 regulates JNK and Raf/MAPK-ERK signalling pathways during Drosophila wing development. Using immunofluorescence and immunohistochemical analyses, we show that overexpression of Rab11 in mutant wing imaginal disc cells triggers the induction of apoptosis and activation of JNK and ERK. Further, using a genetic approach it has been shown that Rab11 interacts with the components of these pathways during Drosophila wing development. In addition to this, in Rab11 mutant wing imaginal discs JNK activity was monitored using puc(E)⁶⁹, a P-lacZ enhancer-trap line inserted in puckered (puc). A strong induction of puc in Rab11 mutant wing imaginal disc cells provided a strong support that Rab11 regulates the JNK signalling pathway during Drosophila wing development.

E2F1 regulates cellular growth by mTORC1 signaling

During cell proliferation, growth must occur to maintain homeostatic cell size. Here we show that E2F1 is capable of inducing growth by regulating mTORC1 activity. The activation of cell growth and mTORC1 by E2F1 is dependent on both E2F1's ability to bind DNA and to regulate gene transcription, demonstrating that a gene induction expression program is required in this process. Unlike E2F1, E2F3 is unable to activate mTORC1, suggesting that growth activity could be restricted to individual E2F members. The effect of E2F1 on the activation of mTORC1 does not depend on Akt. Furthermore, over-expression of TSC2 does not interfere with the effect of E2F1, indicating that the E2F1-induced signal pathway can compensate for the inhibitory effect of TSC2 on Rheb. Immunolocalization studies demonstrate that E2F1 induces the translocation of mTORC1 to the late endosome vesicles, in a mechanism dependent of leucine. E2F1 and leucine, or insulin, together affect the activation of S6K stronger than alone suggesting that they are complementary in activating the signal pathway. From these studies, E2F1 emerges as a key protein that integrates cell division and growth, both of which are essential for cell proliferation.

The Drosophila FoxA ortholog Fork head regulates growth and gene expression downstream of Target of rapamycin

Forkhead transcription factors of the FoxO subfamily regulate gene expression programs downstream of the insulin signaling network. It is less clear which proteins mediate transcriptional control exerted by Target of rapamycin (TOR) signaling, but recent studies in nematodes suggest a role for FoxA transcription factors downstream of TOR. In this study we present evidence that outlines a similar connection in Drosophila, in which the FoxA protein Fork head (FKH) regulates cellular and organismal size downstream of TOR. We find that ectopic expression and targeted knockdown of FKH in larval tissues elicits different size phenotypes depending on nutrient state and TOR signaling levels. FKH overexpression has a negative effect on growth under fed conditions, and this phenotype is not further exacerbated by inhibition of TOR via rapamycin feeding. Under conditions of starvation or low TOR signaling levels, knockdown of FKH attenuates the size reduction associated with these conditions. Subcellular localization of endogenous FKH protein is shifted from predominantly cytoplasmic on a high-protein diet to a pronounced nuclear accumulation in animals with reduced levels of TOR or fed with rapamycin. Two putative FKH target genes, CG6770 and cabut, are transcriptionally induced by rapamycin or FKH expression, and silenced by FKH knockdown. Induction of both target genes in heterozygous TOR mutant animals is suppressed by mutations in fkh. Furthermore, TOR signaling levels and FKH impact on transcription of the dFOXO target gene d4E-BP, implying a point of crosstalk with the insulin pathway. In summary, our observations show that an alteration of FKH levels has an effect on cellular and organismal size, and that FKH function is required for the growth inhibition and target gene induction caused by low TOR signaling levels.

A dp53-dependent mechanism involved in coordinating tissue growth in Drosophila

A study in the Drosophila wing suggests a crucial role of p53 in the coordination of growth between adjacent cell populations to maintain organ proportions and shape.

Coordination of growth between and within organs contributes to the generation of well-proportioned organs and functionally integrated adults. The mechanisms that help to coordinate the growth between different organs start to be unravelled. However, whether an organ is able to respond in a coordinated manner to local variations in growth caused by developmental or environmental stress and the nature of the underlying molecular mechanisms that contribute to generating well-proportioned adult organs under these circumstances remain largely unknown. By reducing the growth rates of defined territories in the developing wing primordium of Drosophila, we present evidence that the tissue responds as a whole and the adjacent cell populations decrease their growth and proliferation rates. This non-autonomous response occurs independently of where growth is affected, and it is functional all throughout development and contributes to generate well-proportioned adult structures. Strikingly, we underscore a central role of Drosophila p53 (dp53) and the apoptotic machinery in these processes. While activation of dp53 in the growth-depleted territory mediates the non-autonomous regulation of growth and proliferation rates, effector caspases have a unique role, downstream of dp53, in reducing proliferation rates in adjacent cell populations. These new findings indicate the existence of a stress response mechanism involved in the coordination of tissue growth between adjacent cell populations and that tissue size and cell cycle proliferation can be uncoupled and are independently and non-autonomously regulated by dp53.

The coordination of growth within and between organs contributes to the generation of functionally integrated structures and well-proportioned animals and plants. Though these issues have fascinated biologists for centuries, the responsible molecular mechanisms remain largely uncharacterized. In this work, we have used the Drosophila wing primordium to show that adjacent cell populations grow and proliferate in a coordinated manner. By reducing growth rates in specific territories within the developing wing, we showed that the tissue responds as a whole and that in adjacent cell populations the growth and cell cycle rates are concomitantly reduced, thus maintaining tissue proportions and normal wing shape. Interestingly, we show that the Drosophila tumour suppressor protein dp53 and apoptotic machinery play a key role in coordinating this tissue-wide response. Both growth and proliferation rates are regulated in a coordinated and non-autonomous manner by the activity of dp53, whilst the apoptotic pathway has a specific and non-autonomous role in regulating cell proliferation rates. Our studies describe a novel mechanism for regulating tissue growth in developing organs that may ultimately be relevant for other processes involving coordination of growth, such as tissue renewal, regeneration, and cancer.

Induction of cellular growth by the axon guidance regulators netrin A and semaphorin-1a

Although neurite outgrowth has been linked to axon guidance regulators, the effects of guidance molecules on cellular growth are not well understood. Use of the Drosophila wing imaginal disc, an epithelial tissue and a well-characterized system for analysis of cellular growth regulation, permits analysis of the impacts of guidance molecules on cellular growth in a setting in which axon guidance is not a confounding factor. In this investigation, the impacts of Netrin A (NetA) and Semaphorin-1a (Sema1a) signaling on cellular growth are examined during wing development. Levels of these genes were modulated in somatic clones in the developing wing disc, and clone areas, as well as individual sizes of clonal cells were assessed. NetA and Sema1a signaling were found to induce cellular growth in these assays. Furthermore, immunohistochemical analyses indicated that NetA and Sema1a signaling induce expression of several growth regulators, including myc, cycD, cdk4, PCNA, and MapK in the wing disc. These data illustrate that NetA and Sema1a can specifically promote growth through induction of key cellular growth regulators. The abilities of NetA and Sema1a to regulate cellular growth are likely critical to their functions in both nervous system development and oncogenesis.

A buoyancy-based screen of Drosophila larvae for fat-storage mutants reveals a role for Sir2 in coupling fat storage to nutrient availability

Obesity has a strong genetic component, but few of the genes that predispose to obesity are known. Genetic screens in invertebrates have the potential to identify genes and pathways that regulate the levels of stored fat, many of which are likely to be conserved in humans. To facilitate such screens, we have developed a simple buoyancy-based screening method for identifying mutant Drosophila larvae with increased levels of stored fat. Using this approach, we have identified 66 genes that when mutated increase organismal fat levels. Among these was a sirtuin family member, Sir2. Sirtuins regulate the storage and metabolism of carbohydrates and lipids by deacetylating key regulatory proteins. However, since mammalian sirtuins function in many tissues in different ways, it has been difficult to define their role in energy homeostasis accurately under normal feeding conditions. We show that knockdown of Sir2 in the larval fat body results in increased fat levels. Moreover, using genetic mosaics, we demonstrate that Sir2 restricts fat accumulation in individual cells of the fat body in a cell-autonomous manner. Consistent with this function, changes in the expression of metabolic enzymes in Sir2 mutants point to a shift away from catabolism. Surprisingly, although Sir2 is typically upregulated under conditions of starvation, Sir2 mutant larvae survive better than wild type under conditions of amino-acid starvation as long as sugars are provided. Our findings point to a Sir2-mediated pathway that activates a catabolic response to amino-acid starvation irrespective of the sugar content of the diet.

Obesity is a major problem in affluent societies. In addition to dietary intake, there are clearly genetic factors that make some people more likely to become obese. At present, we have a poor understanding of what the genetic differences are that predispose some individuals to obesity. In order to discover genes that regulate the amount of stored fat, we have conducted a study using larvae of the fruit fly Drosophila and shown that 66 different genes, when mutated, cause these larvae to store more fat. For the majority of these genes, very similar genes exist in humans. We have also shown that the Sir2 gene has a role in protecting these larvae from storing excessive amounts of fat and that it does so by regulating the synthesis and breakdown of fat in individual cells of a tissue where fat is stored. Finally, we demonstrate a role for Sir2 in changing metabolism when certain types of nutrients (amino acids) are lacking in the diet.

EndoGI modulates Notch signaling and axon guidance in Drosophila

Signaling through the Notch receptor has dramatically different effects depending on cell type and developmental timing. While a myriad of biological systems affected by Notch have been described, the molecular mechanisms by which a generic Notch signal is translated into a cell-type-specific output are less clear. Canonically, the Notch intracellular domain (NICD) translocates into the nucleus upon ligand binding to transcriptionally regulate target genes. In order to generate specificity, therefore, additional factors must exist that modulate NICD activity. Here we describe a novel regulator of the Notch pathway, Endonuclease GI (EndoGI). EndoGI localizes to the nucleus of most cells and activates Notch signaling when overexpressed. In the absence of endoGI, mutant animals are viable, but uncoordinated as motor neurons fail to innervate their appropriate muscle targets. Our data is therefore consistent with EndoGI functioning as a positive regulator of the Notch signaling pathway, playing a critical role during axon guidance of motor neurons.

Dampened activity of E2F1-DP and Myb-MuvB transcription factors in Drosophila endocycling cells

The endocycle is a variant cell cycle comprised of alternating gap (G) and DNA synthesis (S) phases (endoreplication) without mitosis (M), which results in DNA polyploidy and large cell size. Endocycles occur widely in nature, but much remains to be learned about the regulation of this modified cell cycle. Here, we compared gene expression profiles of mitotic cycling larval brain and disc cells with the endocycling cells of fat body and salivary gland of the Drosophila larva. The results indicated that many genes that are positively regulated by the heterodimeric E2F1-DP or Myb-MuvB complex transcription factors are expressed at lower levels in endocycling cells. Many of these target genes have functions in M phase, suggesting that dampened E2F1 and Myb activity promote endocycles. Many other E2F1 target genes that are required for DNA replication were also repressed in endocycling cells, an unexpected result given that these cells must duplicate up to thousands of genome copies during each S phase. For some EF2-regulated genes, the lower level of mRNA in endocycling cells resulted in lower protein concentration, whereas for other genes it did not, suggesting a contribution of post-transcriptional regulation. Both knockdown and overexpression of E2F1-DP and Myb-MuvB impaired endocycles, indicating that transcriptional activation and repression must be balanced. Our data suggest that dampened transcriptional activation by E2F1-DP and Myb-MuvB is important to repress mitosis and coordinate the endocycle transcriptional and protein stability oscillators.

Regulation of organ growth by morphogen gradients

Morphogen gradients play a fundamental role in organ patterning and organ growth. Unlike their role in patterning, their function in regulating the growth and the size of organs is poorly understood. How and why do morphogen gradients exert their mitogenic effects to generate uniform proliferation in developing organs, and by what means can morphogens impinge on the final size of organs? The decapentaplegic (Dpp) gradient in the Drosophila wing imaginal disc has emerged as a suitable and established system to study organ growth. Here, we review models and recent findings that attempt to address how the Dpp morphogen contributes to uniform proliferation of cells, and how it may regulate the final size of wing discs.

Pathogenic LRRK2 negatively regulates microRNA-mediated translational repression

Gain-of-function mutations in leucine-rich repeat kinase 2 (LRRK2) cause familial as well as sporadic Parkinson's disease characterized by age-dependent degeneration of dopaminergic neurons. The molecular mechanism of LRRK2 action is not known. Here we show that LRRK2 interacts with the microRNA (miRNA) pathway to regulate protein synthesis. Drosophila e2f1 and dp messenger RNAs are translationally repressed by let-7 and miR-184*, respectively. Pathogenic LRRK2 antagonizes these miRNAs, leading to the overproduction of E2F1/DP, previously implicated in cell cycle and survival control and shown here to be critical for LRRK2 pathogenesis. Genetic deletion of let-7, antagomir-mediated blockage of let-7 and miR-184* action, transgenic expression of dp target protector, or replacement of endogenous dp with a dp transgene non-responsive to let-7 each had toxic effects similar to those of pathogenic LRRK2. Conversely, increasing the level of let-7 or miR-184* attenuated pathogenic LRRK2 effects. LRRK2 associated with Drosophila Argonaute-1 (dAgo1) or human Argonaute-2 (hAgo2) of the RNA-induced silencing complex (RISC). In aged fly brain, dAgo1 protein level was negatively regulated by LRRK2. Further, pathogenic LRRK2 promoted the association of phospho-4E-BP1 with hAgo2. Our results implicate deregulated synthesis of E2F1/DP caused by the miRNA pathway impairment as a key event in LRRK2 pathogenesis and suggest novel miRNA-based therapeutic strategies.

The steroid hormone ecdysone controls systemic growth by repressing dMyc function in Drosophila fat cells

How steroid hormones shape animal growth remains poorly understood. In Drosophila, the main steroid hormone, ecdysone, limits systemic growth during juvenile development. Here we show that ecdysone controls animal growth rate by specifically acting on the fat body, an organ that retains endocrine and storage functions of the vertebrate liver and fat. We demonstrate that fat body-targeted loss of function of the Ecdysone receptor (EcR) increases dMyc expression and its cellular functions such as ribosome biogenesis. Moreover, changing dMyc levels in this tissue is sufficient to affect animal growth rate. Finally, the growth increase induced by silencing EcR in the fat body is suppressed by cosilencing dMyc. In conclusion, the present work reveals an unexpected function of dMyc in the systemic control of growth in response to steroid hormone signaling.

Tuberous sclerosis complex 1 regulates dE2F1 expression during development and cooperates with RBF1 to control proliferation and survival

Previous studies in Drosophila melanogaster have demonstrated that many tumor suppressor pathways impinge on Rb/E2F to regulate proliferation and survival. Here, we report that Tuberous Sclerosis Complex 1 (TSC1), a well-established tumor suppressor that regulates cell size, is an important regulator of dE2F1 during development. In eye imaginal discs, the loss of tsc1 cooperates with rbf1 mutations to promote ectopic S-phase and cell death. This cooperative effect between tsc1 and rbf1 mutations can be explained, at least in part, by the observation that TSC1 post-transcriptionally regulates dE2F1 expression. Clonal analysis revealed that the protein level of dE2F1 is increased in tsc1 or tsc2 mutant cells and conversely decreased in rheb or dTor mutant cells. Interestingly, while s6k mutations have no effect on dE2F1 expression in the wild-type background, S6k is absolutely required for the increase of dE2F1 expression in tsc2 mutant cells. The canonical TSC/Rheb/Tor/S6k pathway is also an important determinant of dE2F1-dependent cell death, since rheb or s6k mutations suppress the developmentally regulated cell death observed in rbf1 mutant eye discs. Our results provide evidence to suggest that dE2F1 is an important cell cycle regulator that translates the growth-promoting signal downstream of the TSC/Rheb/Tor/S6k pathway.

Tuberous Sclerosis Complex genes 1 (TSC1) is a downstream component of the Insulin Receptor signaling pathway that is often deregulated in many tumors. In this study, we discovered that the fruit fly homolog of TSC1 regulates E2F transcription factor by controlling protein expression. E2F family proteins are key regulators of cellular division, and other tumor promoting events are previously shown to regulate E2F activity. Our findings demonstrate the importance of altering the E2F activity during tumorigenesis and provide new insights into the crosstalk between tumor promoting events.

Mitotic raptor promotes mTORC1 activity, G(2)/M cell cycle progression, and internal ribosome entry site-mediated mRNA translation

The mTOR signaling complex integrates signals from growth factors and nutrient availability to control cell growth and proliferation, in part through effects on the protein-synthetic machinery. Protein synthesis rates fluctuate throughout the cell cycle but diminish significantly during the G(2)/M transition. The fate of the mTOR complex and its role in coordinating cell growth and proliferation signals with protein synthesis during mitosis remain unknown. Here we demonstrate that the mTOR complex 1 (mTORC1) pathway, which stimulates protein synthesis, is actually hyperactive during mitosis despite decreased protein synthesis and reduced activity of mTORC1 upstream activators. We describe previously unknown G(2)/M-specific phosphorylation of a component of mTORC1, the protein raptor, and demonstrate that mitotic raptor phosphorylation alters mTORC1 function during mitosis. Phosphopeptide mapping and mutational analysis demonstrate that mitotic phosphorylation of raptor facilitates cell cycle transit through G(2)/M. Phosphorylation-deficient mutants of raptor cause cells to delay in G(2)/M, whereas depletion of raptor causes cells to accumulate in G(1). We identify cyclin-dependent kinase 1 (cdk1 [cdc2]) and glycogen synthase kinase 3 (GSK3) pathways as two probable mitosis-regulated protein kinase pathways involved in mitosis-specific raptor phosphorylation and altered mTORC1 activity. In addition, mitotic raptor promotes translation by internal ribosome entry sites (IRES) on mRNA during mitosis and is demonstrated to be associated with rapamycin resistance. These data suggest that this pathway may play a role in increased IRES-dependent mRNA translation during mitosis and in rapamycin insensitivity.

Non-redundant selector and growth-promoting functions of two sister genes, buttonhead and Sp1, in Drosophila leg development

The radically distinct morphologies of arthropod and tetrapod legs argue that these appendages do not share a common evolutionary origin. Yet, despite dramatic differences in morphology, it has been known for some time that transcription factors encoded by the Distalless (Dll)/Dlx gene family play a critical role in the development of both structures. Here we show that a second transcription factor family encoded by the Sp8 gene family, previously implicated in vertebrate limb development, also plays an early and fundamental role in arthropod leg development. By simultaneously removing the function of two Sp8 orthologs, buttonhead (btd) and Sp1, during Drosophila embryogenesis, we find that adult leg development is completely abolished. Remarkably, in the absence of these factors, transformations from ventral to dorsal appendage identities are observed, suggesting that adult dorsal fates become derepressed when ventral fates are eliminated. Further, we show that Sp1 plays a much more important role in ventral appendage specification than btd and that Sp1 lies genetically upstream of Dll. In addition to these selector-like gene functions, Sp1 and btd are also required during larval stages for the growth of the leg. Vertebrate Sp8 can rescue many of the functions of the Drosophila genes, arguing that these activities have been conserved, despite more than 500 million years of independent evolution. These observations suggest that an ancient Sp8/Dlx gene cassette was used in an early metazoan for primitive limb-like outgrowths and that this cassette was co-opted multiple times for appendage formation in multiple animal phyla.

A robust cell cycle control mechanism limits E2F-induced proliferation of terminally differentiated cells in vivo

Terminally differentiated cells in Drosophila melanogaster wings and eyes are largely resistant to proliferation upon deregulation of either E2F or cyclin E (CycE), but exogenous expression of both factors together can bypass cell cycle exit. In this study, we show this is the result of cooperation of cell cycle control mechanisms that limit E2F-CycE positive feedback and prevent cycling after terminal differentiation. Aberrant CycE activity after differentiation leads to the degradation of E2F activator complexes, which increases the proportion of CycE-resistant E2F repressor complexes, resulting in stable E2F target gene repression. If E2F-dependent repression is lost after differentiation, high anaphase-promoting complex/cyclosome (APC/C) activity degrades key E2F targets to limit cell cycle reentry. Providing both CycE and E2F activities bypasses exit by simultaneously inhibiting the APC/C and inducing a group of E2F target genes essential for cell cycle reentry after differentiation. These mechanisms are essential for proper development, as evading them leads to tissue outgrowths composed of dividing but terminally differentiated cells.

MAP kinase phosphorylation is dispensable for cell division, but required for cell growth in Drosophila

Proper activation of the Ras/MAPK pathway is broadly required during development, and in many cases, signal transduction downstream of the receptor is linear. Thus, different mechanisms exist to properly regulate the large number of specific developmental outputs that are required by the activation of this pathway. Previously, we have reported a regulated cytoplasmic sequestration of phosphorylated MAPK (pMAPK) in developing Drosophila compound eyes and wings "called MAPK Cytoplasmic Hold". In the developing wing, we have shown that cytoplasmic hold promotes the differentiation of wing vein tissue, while pMAPK nuclear translocation regulates growth and division. We had also suggested that the Ras pathway signals for inducing cell growth and cell division split upstream of the nuclear translocation of MAPK itself. Here, we further refine the role of MAPK in Drosophila. We report evidence that suggests, for the first time, that the phosphorylation of MAPK is itself another step in the regulation of cell growth and division in both Drosophila wing and eye cells. We show that inhibition of MAPK phosphorylation, or pMAPK nuclear translocation, is sufficient to block cell growth, but not cell division. These data suggest that non-phosphorylated MAPK is sufficient to induce cell division, but not cell growth, once inside the nucleus of the cell.

Imaginal discs: Renaissance of a model for regenerative biology

Many animals display a capacity to regenerate tissues or even a complete body. One of the main goals of regenerative biology is to identify the genes and genetic networks necessary for this process. Drosophila offers an ideal model system for such studies. The wide range of genetic and genomic approaches available for use in flies has helped in initiating the deciphering of the mechanisms underlying regeneration, and the results may be applicable to other organisms, including mammals. Moreover, most models of regeneration require experimental manipulation, whereas in Drosophila discrete domains can be ablated by genetically induced methods. Here, we present a summary of current research into imaginal disc regeneration and discuss the power of this tissue as a tool for understanding the genetics of regeneration.

The nuclear receptor DHR3 modulates dS6 kinase-dependent growth in Drosophila

S6 kinases (S6Ks) act to integrate nutrient and insulin signaling pathways and, as such, function as positive effectors in cell growth and organismal development. However, they also have been shown to play a key role in limiting insulin signaling and in mediating the autophagic response. To identify novel regulators of S6K signaling, we have used a Drosophila-based, sensitized, gain-of-function genetic screen. Unexpectedly, one of the strongest enhancers to emerge from this screen was the nuclear receptor (NR), Drosophila hormone receptor 3 (DHR3), a critical constituent in the coordination of Drosophila metamorphosis. Here we demonstrate that DHR3, through dS6K, also acts to regulate cell-autonomous growth. Moreover, we show that the ligand-binding domain (LBD) of DHR3 is essential for mediating this response. Consistent with these findings, we have identified an endogenous DHR3 isoform that lacks the DBD. These results provide the first molecular link between the dS6K pathway, critical in controlling nutrient-dependent growth, and that of DHR3, a major mediator of ecdysone signaling, which, acting together, coordinate metamorphosis.

The miRNA machinery targets Mei-P26 and regulates Myc protein levels in the Drosophila wing

MicroRNAs (miRNAs) have been implicated in cell-cycle regulation and in some cases shown to have a role in tissue growth control. Depletion of miRNAs was found to have an effect on tissue growth rates in the wing primordium of Drosophila, a highly proliferative epithelium. Dicer-1 (Dcr-1) is a double-stranded RNAseIII essential for miRNA biogenesis. Adult cells lacking dcr-1, or with reduced dcr-1 activity, were smaller than normal cells and gave rise to smaller wings. dcr-1 mutant cells showed evidence of being susceptible to competition by faster growing cells in vivo and the miRNA machinery was shown to promote G(1)-S transition. We present evidence that Dcr-1 acts by regulating the TRIM-NHL protein Mei-P26, which in turn regulates dMyc protein levels. Mei-P26 is a direct target of miRNAs, including the growth-promoting bantam miRNA. Thus, regulation of tissue growth by the miRNA pathway involves a double repression mechanism to control dMyc protein levels in a highly proliferative and growing epithelium.

Exploring the effects of mechanical feedback on epithelial topology

Apical cell surfaces in metazoan epithelia, such as the wing disc of Drosophila, resemble polygons with different numbers of neighboring cells. The distribution of these polygon numbers has been shown to be conserved. Revealing the mechanisms that lead to this topology might yield insights into how the structural integrity of epithelial tissues is maintained. It has previously been proposed that cell division alone, or cell division in combination with cell rearrangements, is sufficient to explain the observed epithelial topology. Here, we extend this work by including an analysis of the clustering and the polygon distribution of mitotic cells. In addition, we study possible effects of cellular growth regulation by mechanical forces, as such regulation has been proposed to be involved in wing disc size regulation. We formulated several theoretical scenarios that differ with respect to whether cell rearrangements are allowed and whether cellular growth rates are dependent on mechanical stress. We then compared these scenarios with experimental data on the polygon distribution of the entire cell population, that of mitotic cells, as well as with data on mitotic clustering. Surprisingly, we observed considerably less clustering in our experiments than has been reported previously. Only scenarios that include mechanical-stress-dependent growth rates are in agreement with the experimental data. Interestingly, simulations of these scenarios showed a large decrease in rearrangements and elimination of cells. Thus, a possible growth regulation by mechanical force could have a function in releasing the mechanical stress that evolves when all cells have similar growth rates.

Notch signaling and developmental cell-cycle arrest in Drosophila polar follicle cells

Temporal and spatial regulation of cell division is critical for proper development of multicellular organisms. An important aspect of this regulation is cell-cycle arrest, which in many cell types is coupled with differentiated status. Here we report that the polar cells--a group of follicle cells differentiated early during Drosophila oogenesis--are arrested at G2 phase and can serve as a model cell type for investigation of developmental regulation of cell-cycle arrest. On examining the effects of String, a mitosis-promoting phosphatase Cdc25 homolog, and Notch signaling in polar cells, we found that misexpression of String can trigger mitosis in existing polar cells to induce extra polar cells. Normally, differentiation of the polar cells requires Notch signaling. We found that the Notch-induced extra polar cells arise through recruitment of the neighboring cells rather than promotion of proliferation, and they are also arrested at G2 phase. Notch signaling is probably involved in down-regulating String in polar cells, thus inducing the G2 cell-cycle arrest.

Mechanisms of growth and homeostasis in the Drosophila wing

Animal shape and size is controlled with amazing precision during development. External factors such as nutrient availability and crowding can alter overall animal size, but individual body parts scale reproducibly to match the body even with challenges from a changing environment. How is such precision achieved? Here, we review selected research from the last few years in Drosophila--arguably the premier genetic model for the study of animal growth--that sheds light on how body and tissue size are regulated by forces intrinsic to individual organs. We focus on two topics currently under intense study: the influence of pattern regulators on organ and tissue growth and the role of local competitive interactions between cells in tissue homeostasis and final size.