DREF is required for cell and organismal growth in Drosophila and functions downstream of the nutrition/TOR pathway

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 showed that they encode for 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 genes, alongside enhancer decommissioning at specific rate-limiting cell cycle genes.

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.

TOR signalling is required for host lipid metabolic remodelling and survival following enteric infection in Drosophila

When infected by enteric pathogenic bacteria, animals need to initiate local and whole-body defence strategies. Although most attention has focused on the role of innate immune anti-bacterial responses, less is known about how changes in host metabolism contribute to host defence. Using Drosophila as a model system, we identify induction of intestinal target-of-rapamycin (TOR) kinase signalling as a key adaptive metabolic response to enteric infection. We find that enteric infection induces both local and systemic induction of TOR independently of the Immune deficiency (IMD) innate immune pathway, and we see that TOR functions together with IMD signalling to promote infection survival. These protective effects of TOR signalling are associated with remodelling of host lipid metabolism. Thus, we see that TOR is required to limit excessive infection-mediated wasting of host lipid stores by promoting an increase in the levels of gut- and fat body-expressed lipid synthesis genes. Our data support a model in which induction of TOR represents a host tolerance response to counteract infection-mediated lipid wasting in order to promote survival. This article has an associated First Person interview with the first author of the paper.

A low-sugar diet enhances Drosophila body size in males and females via sex-specific mechanisms

In Drosophila, changes to dietary protein elicit different body size responses between the sexes. Whether these differential body size effects extend to other macronutrients remains unclear. Here, we show that lowering dietary sugar (0S diet) enhanced body size in male and female larvae. Despite an equivalent phenotypic effect between the sexes, we detected sex-specific changes to signalling pathways, transcription and whole-body glycogen and protein. In males, the low-sugar diet augmented insulin/insulin-like growth factor signalling pathway (IIS) activity by increasing insulin sensitivity, where increased IIS was required for male metabolic and body size responses in 0S. In females reared on low sugar, IIS activity and insulin sensitivity were unaffected, and IIS function did not fully account for metabolic and body size responses. Instead, we identified a female-biased requirement for the Target of rapamycin pathway in regulating metabolic and body size responses. Together, our data suggest the mechanisms underlying the low-sugar-induced increase in body size are not fully shared between the sexes, highlighting the importance of including males and females in larval studies even when similar phenotypic outcomes are observed.

The Nutrient-Responsive Molecular Chaperone Hsp90 Supports Growth and Development in Drosophila

Animals can sense internal nutrients, such as amino acids/proteins, and are able to modify their developmental programs in accordance with their nutrient status. In the fruit fly, Drosophila melanogaster, amino acid/protein is sensed by the fat body, an insect adipose tissue, through a nutrient sensor, target of rapamycin (TOR) complex 1 (TORC1). TORC1 promotes the secretion of various peptide hormones from the fat body in an amino acid/protein-dependent manner. Fat-body-derived peptide hormones stimulate the release of insulin-like peptides, which are essential growth-promoting anabolic hormones, from neuroendocrine cells called insulin-producing cells (IPCs). Although the importance of TORC1 and the fat body-IPC axis has been elucidated, the mechanism by which TORC1 regulates the expression of insulinotropic signal peptides remains unclear. Here, we show that an evolutionarily conserved molecular chaperone, heat shock protein 90 (Hsp90), promotes the expression of insulinotropic signal peptides. Fat-body-selective Hsp90 knockdown caused the transcriptional downregulation of insulinotropic signal peptides. IPC activity and systemic growth were also impaired in fat-body-selective Hsp90 knockdown animals. Furthermore, Hsp90 expression depended on protein/amino acid availability and TORC1 signaling. These results strongly suggest that Hsp90 serves as a nutrient-responsive gene that upregulates the fat body-IPC axis and systemic growth. We propose that Hsp90 is induced in a nutrient-dependent manner to support anabolic metabolism during the juvenile growth period.

Mitochondrial genotype alters the impact of rapamycin on the transcriptional response to nutrients in Drosophila

BACKGROUND

In addition to their well characterized role in cellular energy production, new evidence has revealed the involvement of mitochondria in diverse signaling pathways that regulate a broad array of cellular functions. The mitochondrial genome (mtDNA) encodes essential components of the oxidative phosphorylation (OXPHOS) pathway whose expression must be coordinated with the components transcribed from the nuclear genome. Mitochondrial dysfunction is associated with disorders including cancer and neurodegenerative diseases, yet the role of the complex interactions between the mitochondrial and nuclear genomes are poorly understood.

RESULTS

Using a Drosophila model in which alternative mtDNAs are present on a common nuclear background, we studied the effects of this altered mitonuclear communication on the transcriptomic response to altered nutrient status. Adult flies with the 'native' and 'disrupted' genotypes were re-fed following brief starvation, with or without exposure to rapamycin, the cognate inhibitor of the nutrient-sensing target of rapamycin (TOR). RNAseq showed that alternative mtDNA genotypes affect the temporal transcriptional response to nutrients in a rapamycin-dependent manner. Pathways most greatly affected were OXPHOS, protein metabolism and fatty acid metabolism. A distinct set of testis-specific genes was also differentially regulated in the experiment.

CONCLUSIONS

Many of the differentially expressed genes between alternative mitonuclear genotypes have no direct interaction with mtDNA gene products, suggesting that the mtDNA genotype contributes to retrograde signaling from mitochondria to the nucleus. The interaction of mitochondrial genotype (mtDNA) with rapamycin treatment identifies new links between mitochondria and the nutrient-sensing mTORC1 (mechanistic target of rapamycin complex 1) signaling pathway.

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.

The Hippo pathway integrates PI3K-Akt signals with mechanical and polarity cues to control tissue growth

The Hippo signalling pathway restricts cell proliferation in animal tissues by inhibiting Yes-associated protein (YAP or YAP1) and Transcriptional Activator with a PDZ domain (TAZ or WW-domain-containing transcriptional activator [WWTR1]), coactivators of the Scalloped (Sd or TEAD) DNA-binding transcription factor. Drosophila has a single YAP/TAZ homolog named Yorkie (Yki) that is regulated by Hippo pathway signalling in response to epithelial polarity and tissue mechanics during development. Here, we show that Yki translocates to the nucleus to drive Sd-mediated cell proliferation in the ovarian follicle cell epithelium in response to mechanical stretching caused by the growth of the germline. Importantly, mechanically induced Yki nuclear localisation also requires nutritionally induced insulin/insulin-like growth factor 1 (IGF-1) signalling (IIS) via phosphatidyl inositol-3-kinase (PI3K), phosphoinositide-dependent kinase 1 (PDK1 or PDPK1), and protein kinase B (Akt or PKB) in the follicular epithelium. We find similar results in the developing Drosophila wing, where Yki becomes nuclear in the mechanically stretched cells of the wing pouch during larval feeding, which induces IIS, but translocates to the cytoplasm upon cessation of feeding in the third instar stage. Inactivating Akt prevents nuclear Yki localisation in the wing disc, while ectopic activation of the insulin receptor, PI3K, or Akt/PKB is sufficient to maintain nuclear Yki in mechanically stimulated cells of the wing pouch even after feeding ceases. Finally, IIS also promotes YAP nuclear localisation in response to mechanical cues in mammalian skin epithelia. Thus, the Hippo pathway has a physiological function as an integrator of epithelial cell polarity, tissue mechanics, and nutritional cues to control cell proliferation and tissue growth in both Drosophila and mammals.

DREF Genetically Counteracts Mi-2 and Caf1 to Regulate Adult Stem Cell Maintenance

Active adult stem cells maintain a bipotential state with progeny able to either self-renew or initiate differentiation depending on extrinsic signals from the surrounding microenvironment. However, the intrinsic gene regulatory networks and chromatin states that allow adult stem cells to make these cell fate choices are not entirely understood. Here we show that the transcription factor DNA Replication-related Element Factor (DREF) regulates adult stem cell maintenance in the Drosophila male germline. A temperature-sensitive allele of DREF described in this study genetically separated a role for DREF in germline stem cell self-renewal from the general roles of DREF in cell proliferation. The DREF temperature-sensitive allele caused defects in germline stem cell self-renewal but allowed viability and division of germline stem cells as well as cell viability, growth and division of somatic cyst stem cells in the testes and cells in the Drosophila eye. Germline stem cells mutant for the temperature sensitive DREF allele exhibited lower activation of a TGF-beta reporter, and their progeny turned on expression of the differentiation factor Bam prematurely. Results of genetic interaction analyses revealed that Mi-2 and Caf1/p55, components of the Nucleosome Remodeling and Deacetylase (NuRD) complex, genetically antagonize the role of DREF in germline stem cell maintenance. Taken together, these data suggest that DREF contributes to intrinsic components of the germline stem cell regulatory network that maintains competence to self-renew.

Many adult tissues are maintained throughout life by the dual ability of adult stem cells to produce progeny that either self-renew or differentiate to replace specialized cells lost to turnover or damage. Although signals from the surrounding microenvironment have been shown to regulate the choice between self-renewal and onset of differentiation, the intrinsic gene regulatory programs that set up and maintain this bipotential state are not well understood. In this report we describe antagonistic components of an intrinsic stem cell program important for maintaining the balance between self-renewal and differentiation in Drosophila male germline adult stem cell lineage. We identified a temperature-sensitive mutant in the transcription factor DNA Replication-related Element Factor (DREF) gene that disrupts the ability of germline stem cells to self-renew, but not stem cell viability, ability to divide or differentiate under the same conditions. DREF mutant germline stem cells showed defects in the TGF-beta signaling pathway, a pathway that is critical for maintaining the stem cell population. Genetic interaction analyses revealed that Mi-2 and Caf1/p55, components of the Nucleosome Remodeling and Deacetylase complex genetically antagonize the role of DREF in germline stem cell maintenance. We propose that DREF contributes to a transcriptional environment necessary for maintaining a bi-potential stem cell state able to properly respond to extrinsic niche signals.

Transcriptional responses to hyperplastic MRL signalling in Drosophila

Recent work has implicated the actin cytoskeleton in tissue size control and tumourigenesis, but how changes in actin dynamics contribute to hyperplastic growth is still unclear. Overexpression of Pico, the only Drosophila Mig-10/RIAM/Lamellipodin adapter protein family member, has been linked to tissue overgrowth via its effect on the myocardin-related transcription factor (Mrtf), an F-actin sensor capable of activating serum response factor (SRF). Transcriptional changes induced by acute Mrtf/SRF signalling have been largely linked to actin biosynthesis and cytoskeletal regulation. However, by RNA profiling, we find that the common response to chronic mrtf and pico overexpression in wing discs was upregulation of ribosome protein and mitochondrial genes, which are conserved targets for Mrtf/SRF and are known growth drivers. Consistent with their ability to induce a common transcriptional response and activate SRF signalling in vitro, we found that both pico and mrtf stimulate expression of an SRF-responsive reporter gene in wing discs. In a functional genetic screen, we also identified deterin, which encodes Drosophila Survivin, as a putative Mrtf/SRF target that is necessary for pico-mediated tissue overgrowth by suppressing proliferation-associated cell death. Taken together, our findings raise the possibility that distinct targets of Mrtf/SRF may be transcriptionally induced depending on the duration of upstream signalling.

Epigenetic factors Polycomb (Pc) and Suppressor of zeste (Su(z)2) negatively regulate longevity in Drosophila melanogaster

The process of aging is a hallmark of the natural life span of all organisms and individuals within a population show variability in the measures of age related performance. Longevity and the rate of aging are influenced by several factors such as genetics, nutrition, stress, and environment. Many studies have focused on the genes that impact aging and there is increasing evidence that epigenetic factors regulate these genes to control life span. Polycomb (PcG) and trithorax (trxG) protein complexes maintain the expression profiles of developmentally important genes and regulate many cellular processes. Here, we report that mutations of PcG and trxG members affect the process of aging in Drosophila melanogaster, with perturbations mostly associated with retardation in aging. We find that mutations in polycomb repressive complex (PRC1) components Pc and Su(z)2 increase fly survival. Using an inducible UAS-GAL4 system, we show that this effect is tissue-specific; knockdown in fat body, but not in muscle or brain tissues, enhances life span. We hypothesize that these two proteins influence life span via pathways independent of their PRC1 functions, with distinct effects on response to oxidative stress. Our observations highlight the role of global epigenetic regulators in determining life span.

Drosophila muscles regulate the immune response against wasp infection via carbohydrate metabolism

We recently found that JAK/STAT signaling in skeletal muscles is important for the immune response of Drosophila larvae against wasp infection, but it was not clear how muscles could affect the immune response. Here we show that insulin signaling is required in muscles, but not in fat body or hemocytes, during larval development for an efficient encapsulation response and for the formation of lamellocytes. This effect requires TOR signaling. We show that muscle tissue affects the immune response by acting as a master regulator of carbohydrate metabolism in the infected animal, via JAK/STAT and insulin signaling in the muscles, and that there is indirect positive feedback between JAK/STAT and insulin signaling in the muscles. Specifically, stimulation of JAK/STAT signaling in the muscles can rescue the deficient immune response when insulin signaling is suppressed. Our results shed new light on the interaction between metabolism, immunity, and tissue communication.

DREF plays multiple roles during Drosophila development

DREF was originally identified as a transcription factor that coordinately regulates the expression of DNA replication- and proliferation-related genes in Drosophila. Subsequent studies demonstrated that DREF is involved in tumor suppressor pathways including p53 and Hippo signaling. DREF also regulates the expression of genes encoding components of the JNK and EGFR pathways during Drosophila development. DREF itself is under the control of the TOR pathway during cell and tissue growth responding to nutrition. Recent studies revealed that DREF plays a role in chromatin organization including insulator function, chromatin remodeling, and telomere maintenance. DREF is also involved in the regulation of genes related to mitochondrial biogenesis, linking it to cellular proliferation. Thus, DREF is now emerging as not only a transcription factor, but also a multi-functional protein. In this review, we summarize current advances in studies on the novel functions of Drosophila DREF.

Regulatory Enhancer-Core-Promoter Communication via Transcription Factors and Cofactors

Gene expression is regulated by genomic enhancers that recruit transcription factors and cofactors to activate transcription from target core promoters. Over the past years, thousands of enhancers and core promoters in animal genomes have been annotated, and we have learned much about the domain structure in which regulatory genomes are organized in animals. Enhancer-core-promoter targeting occurs at several levels, including regulatory domains, DNA accessibility, and sequence-encoded core-promoter specificities that are likely mediated by different regulatory proteins. We review here current knowledge about enhancer-core-promoter targeting, regulatory communication between enhancers and core promoters, and the protein factors involved. We conclude with an outlook on open questions that we find particularly interesting and that will likely lead to additional insights in the upcoming years.

Stress signaling between organs in metazoa

Many organisms have developed a robust ability to adapt and survive in the face of environmental perturbations that threaten the integrity of their genome, proteome, or metabolome. Studies in multiple model organisms have shown that, in general, when exposed to stress, cells activate a complex prosurvival signaling network that includes immune and DNA damage response genes, chaperones, antioxidant enzymes, structural proteins, metabolic enzymes, and noncoding RNAs. The manner of activation runs the gamut from transcriptional induction of genes to increased stability of transcripts to posttranslational modification of important biosynthetic proteins within the stressed tissue. Superimposed on these largely autonomous effects are nonautonomous responses in which the stressed tissue secretes peptides and other factors that stimulate tissues in different organs to embark on processes that ultimately help the organism as a whole cope with stress. This review focuses on the mechanisms by which tissues in one organ adapt to environmental challenges by regulating stress responses in tissues of different organs.

The Systemic Control of Growth

Growth is a complex process that is intimately linked to the developmental program to form adults with proper size and proportions. Genetics is an important determinant of growth, as exemplified by the role of local diffusible molecules setting up organ proportions. In addition, organisms use adaptive responses allowing modulating the size of individuals according to environmental cues, for example, nutrition. Here, we describe some of the physiological principles participating in the determination of final individual size.

High sucrose consumption promotes obesity whereas its low consumption induces oxidative stress in Drosophila melanogaster

The effects of sucrose in varied concentrations (0.25-20%) with constant amount of yeasts in larval diet on development and metabolic parameters of adult fruit fly Drosophila melanogaster were studied. Larvae consumed more food at low sucrose diet, overeating with yeast. On high sucrose diet, larvae ingested more carbohydrates, despite consuming less food and obtaining less protein derived from yeast. High sucrose diet slowed down pupation and increased pupa mortality, enhanced levels of lipids and glycogen, increased dry body mass, decreased water content, i.e. resulted in obese phenotype. Furthermore, it suppressed reactive oxygen species-induced oxidation of lipids and proteins as well as the activity of superoxide dismutase. The activity of catalase was gender-related. In males, at all sucrose concentrations used catalase activity was higher than at its concentration of 0.25%, whereas in females sucrose concentration virtually did not influence the activity. High sucrose diet increased content of protein thiols and the activity of glucose-6-phosphate dehydrogenase. The increase in sucrose concentration also enhanced uric acid level in females, but caused opposite effects in males. Development on high sucrose diets was accompanied by elevated steady-state insulin-like peptide 3 mRNA level. Finally, carbohydrate starvation at yeast overfeeding on low sucrose diets resulted in oxidative stress reflected by higher levels of oxidized lipids and proteins accompanied by increased superoxide dismutase activity. Potential mechanisms involved in regulation of redox processes by carbohydrates are discussed.

A transcriptional network controlling glial development in the Drosophila visual system

In the nervous system, glial cells need to be specified from a set of progenitor cells. In the developing Drosophila eye, perineurial glia proliferate and differentiate as wrapping glia in response to a neuronal signal conveyed by the FGF receptor pathway. To unravel the underlying transcriptional network we silenced all genes encoding predicted DNA-binding proteins in glial cells using RNAi. Dref and other factors of the TATA box-binding protein-related factor 2 (TRF2) complex were previously predicted to be involved in cellular metabolism and cell growth. Silencing of these genes impaired early glia proliferation and subsequent differentiation. Dref controls proliferation via activation of the Pdm3 transcription factor, whereas glial differentiation is regulated via Dref and the homeodomain protein Cut. Cut expression is controlled independently of Dref by FGF receptor activity. Loss- and gain-of-function studies show that Cut is required for glial differentiation and is sufficient to instruct the formation of membrane protrusions, a hallmark of wrapping glial morphology. Our work discloses a network of transcriptional regulators controlling the progression of a naïve perineurial glia towards the fully differentiated wrapping glia.

The Hippo pathway as a target of the Drosophila DRE/DREF transcriptional regulatory pathway

The DRE/DREF transcriptional regulatory system has been demonstrated to activate a wide variety of genes with various functions. In Drosophila, the Hippo pathway is known to suppress cell proliferation by inducing apoptosis and cell cycle arrest through inactivation of Yorkie, a transcription co-activator. In the present study, we found that half dose reduction of the hippo (hpo) gene induces ectopic DNA synthesis in eye discs that is suppressed by overexpression of DREF. Half reduction of the hpo gene dose reduced apoptosis in DREF-overexpressing flies. Consistent with these observations, overexpression of DREF increased the levels of hpo and phosphorylated Yorkie in eye discs. Interestingly, the diap1-lacZ reporter was seen to be significantly decreased by overexpression of DREF. Luciferase reporter assays in cultured S2 cells revealed that one of two DREs identified in the hpo gene promoter region was responsible for promoter activity in S2 cells. Furthermore, endogenous hpo mRNA was reduced in DREF knockdown S2 cells, and chromatin immnunoprecipitation assays with anti-DREF antibodies proved that DREF binds specifically to the hpo gene promoter region containing DREs in vivo. Together, these results indicate that the DRE/DREF pathway is required for transcriptional activation of the hpo gene to positively control Hippo pathways.

p53- and ERK7-dependent ribosome surveillance response regulates Drosophila insulin-like peptide secretion

Insulin-like signalling is a conserved mechanism that coordinates animal growth and metabolism with nutrient status. In Drosophila, insulin-producing median neurosecretory cells (IPCs) regulate larval growth by secreting insulin-like peptides (dILPs) in a diet-dependent manner. Previous studies have shown that nutrition affects dILP secretion through humoral signals derived from the fat body. Here we uncover a novel mechanism that operates cell autonomously in the IPCs to regulate dILP secretion. We observed that impairment of ribosome biogenesis specifically in the IPCs strongly inhibits dILP secretion, which consequently leads to reduced body size and a delay in larval development. This response is dependent on p53, a known surveillance factor for ribosome biogenesis. A downstream effector of this growth inhibitory response is an atypical MAP kinase ERK7 (ERK8/MAPK15), which is upregulated in the IPCs following impaired ribosome biogenesis as well as starvation. We show that ERK7 is sufficient and essential to inhibit dILP secretion upon impaired ribosome biogenesis, and it acts epistatically to p53. Moreover, we provide evidence that p53 and ERK7 contribute to the inhibition of dILP secretion upon starvation. Thus, we conclude that a cell autonomous ribosome surveillance response, which leads to upregulation of ERK7, inhibits dILP secretion to impede tissue growth under limiting dietary conditions.

Ribosome biogenesis is a major consumer of cellular energy and a rate-limiting process during cell growth. The ribosome biogenesis pathway is tightly connected with signaling pathways that regulate tissue growth. For example, nutrient-regulated signaling cues adjust the rate of ribosome biogenesis. On the other hand, the process of ribosome biogenesis is closely monitored by so-called surveillance mechanisms. The best-known ribosome surveillance factor is the transcription factor and tumor suppressor p53. In proliferating cells, activation of p53 upon disturbed ribosome biogenesis leads to cell cycle arrest and inhibition of proliferation. Here we show that ribosome surveillance not only regulates growth locally in proliferating cells, but is also coupled to hormonal growth control through regulation of insulin like peptide (dILPs) secretion. We observed that inhibition of ribosome biogenesis in the Drosophila insulin-producing cells generates a strong cell autonomous signal to inhibit dILP secretion. We identify two downstream effectors of this ribosome surveillance response by showing that p53 as well as an atypical MAP kinase ERK7 are mediators of the inhibition of dILP secretion. We also provide evidence that this ribosome surveillance mechanism contributes to nutrient-dependent regulation of dILP secretion.

Drosophila neuroblasts as a new model for the study of stem cell self-renewal and tumour formation

Drosophila larval brain stem cells (neuroblasts) have emerged as an important model for the study of stem cell asymmetric division and the mechanisms underlying the transformation of neural stem cells into tumour-forming cancer stem cells. Each Drosophila neuroblast divides asymmetrically to produce a larger daughter cell that retains neuroblast identity, and a smaller daughter cell that is committed to undergo differentiation. Neuroblast self-renewal and differentiation are tightly controlled by a set of intrinsic factors that regulate ACD (asymmetric cell division). Any disruption of these two processes may deleteriously affect the delicate balance between neuroblast self-renewal and progenitor cell fate specification and differentiation, causing neuroblast overgrowth and ultimately lead to tumour formation in the fly. In this review, we discuss the mechanisms underlying Drosophila neural stem cell self-renewal and differentiation. Furthermore, we highlight emerging evidence in support of the notion that defects in ACD in mammalian systems, which may play significant roles in the series of pathogenic events leading to the development of brain cancers.

Drosophila Myc: A master regulator of cellular performance

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

Myc function in Drosophila

Drosophila contains a single MYC gene. Like its vertebrate homologs, it encodes a transcription factor that activates many targets, including prominently genes involved in ribosome biogenesis and translation. This activity makes Myc a central regulator of growth and/or proliferation of many cell types, such as imaginal disc cells, polyploid cells, stem cells, and blood cells. Importantly, not only does Myc act cell autonomously but it also affects the fate of adjacent cells and tissues. This potential of Myc is harnessed by many different signaling pathways, involving, among others, Wg, Dpp, Hpo, ecdysone, insulin, and mTOR.

Control of e2f1 and PCNA by Drosophila transcription factor DREF

DREF (DNA replication-related element-binding factor), a zinc finger type transcription factor required for proper cell cycle progression in both mitotic and endocycling cells, is a positive regulator of E2F1, an important transcription factor which regulates genes related to the S-phase of the cell cycle. DREF and E2F1 regulate similar sets of replication-related genes, including proliferating cell nuclear antigen (PCNA), and play roles in the G1 to S phase transition. However, the relationships between dref and e2f1 or PCNA during development are poorly understood. Here, we provided evidence for novel control of e2f1 and PCNA involving DREF in endocycling cells. Somatic clone analysis demonstrated that dref knockdown stabilized E2F1 expression at posttranscriptional levels in endocycling salivary gland cells. Similarly, PCNA expression was up-regulated in the endocycling salivary gland cells. Genetic interaction analysis indicated that the endoreplication defects are partly caused via possible enhancement of E2F1 activity. From these results and previous reports, we conclude that regulation of e2f1 and PCNA by DREF in vivo is complex and the regulation mechanism may differ with the tissue and/or positions in the tissue.

Drosophila nuclear factor DREF regulates the expression of the mitochondrial DNA helicase and mitochondrial transcription factor B2 but not the mitochondrial translation factor B1

DREF [DRE (DNA replication-related element)-binding factor] controls the transcription of numerous genes in Drosophila, many involved in nuclear DNA (nDNA) replication and cell proliferation, three in mitochondrial DNA (mtDNA) replication and two in mtDNA transcription termination. In this work, we have analysed the involvement of DREF in the expression of the known remaining genes engaged in the minimal mtDNA replication (d-mtDNA helicase) and transcription (the activator d-mtTFB2) machineries and of a gene involved in mitochondrial mRNA translation (d-mtTFB1). We have identified their transcriptional initiation sites and DRE sequences in their promoter regions. Gel-shift and chromatin immunoprecipitation assays demonstrate that DREF interacts in vitro and in vivo with the d-mtDNA helicase and d-mtTFB2, but not with the d-mtTFB1 promoters. Transient transfection assays in Drosophila S2 cells with mutated DRE motifs and truncated promoter regions show that DREF controls the transcription of d-mtDNA helicase and d-mtTFB2, but not that of d-mtTFB1. RNA interference of DREF in S2 cells reinforces these results showing a decrease in the mRNA levels of d-mtDNA helicase and d-mtTFB2 and no changes in those of the d-mtTFB1. These results link the genetic regulation of nuclear DNA replication with the genetic control of mtDNA replication and transcriptional activation in Drosophila.

Body size regulation and insulin-like growth factor signaling

How animals achieve their specific body size is a fundamental, but still largely unresolved, biological question. Over the past decades, studies on the insect model system have provided some important insights into the process of body size determination and highlighted the importance of insulin/insulin-like growth factor signaling. Fat body, the Drosophila counterpart of liver and adipose tissue, senses nutrient availability and controls larval growth rate by modulating peripheral insulin signaling. Similarly, insulin-like growth factor I produced from liver and muscle promotes postnatal body growth in mammals. Organismal growth is tightly coupled with the process of sexual maturation wherein the sex steroid hormone attenuates body growth. This review summarizes some important findings from Drosophila and mammalian studies that shed light on the general mechanism of animal size determination.

Coordination of organ growth: principles and outstanding questions from the world of insects

In animal species undergoing determinate growth, the making of a full-size adult body requires a series of coordinated growth events culminating in the cessation of growth that precedes sexual maturation. The merger between physiology and genetics now coming to pass in the Drosophila model allows us to decipher these growth events with an unsurpassed level of sophistication. Here, we review several coordination mechanisms that represent fundamental aspects of growth control: adaptation of growth to environmental cues, interorgan coordination, and the coordination of growth with developmental transitions. The view is emerging of an integrated process where organ-autonomous growth is coordinated with both developmental and environmental cues to define final body size.

Developmental regulation of nucleolus size during Drosophila eye differentiation

When cell cycle withdrawal accompanies terminal differentiation, biosynthesis and cellular growth are likely to change also. In this study, nucleolus size was monitored during cell fate specification in the Drosophila eye imaginal disc using fibrillarin antibody labeling. Nucleolus size is an indicator of ribosome biogenesis and can correlate with cellular growth rate. Nucleolar size was reduced significantly during cell fate specification and differentiation, predominantly as eye disc cells entered a cell cycle arrest that preceded cell fate specification. This reduction in nucleolus size required Dpp and Hh signaling. A transient enlargement of the nucleolus accompanied cell division in the Second Mitotic Wave. Nucleoli continued to diminish in postmitotic cells following fate specification. These results suggest that cellular growth is regulated early in the transition from proliferating progenitor cells to terminal cell fate specification, contemporary with regulation of the cell cycle, and requiring the same extracellular signals.