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

Production of systemically circulating Hedgehog by the intestine couples nutrition to growth and development

Rodenfels et al. show that the Drosophila intestine responds to nutrient availability by regulating production of a circulating lipoprotein-associated form of Hedgehog (Hh). Levels of circulating Hh tune the rates of growth and developmental timing in a coordinated fashion. Circulating Hh is especially important during starvation, when it is also required for mobilization of fat body triacylglycerol stores.

In Drosophila larvae, growth and developmental timing are regulated by nutrition in a tightly coordinated fashion. The networks that couple these processes are far from understood. Here, we show that the intestine responds to nutrient availability by regulating production of a circulating lipoprotein-associated form of the signaling protein Hedgehog (Hh). Levels of circulating Hh tune the rates of growth and developmental timing in a coordinated fashion. Circulating Hh signals to the fat body to control larval growth. It regulates developmental timing by controlling ecdysteroid production in the prothoracic gland. Circulating Hh is especially important during starvation, when it is also required for mobilization of fat body triacylglycerol (TAG) stores. Thus, we demonstrate that Hh, previously known only for its local morphogenetic functions, also acts as a lipoprotein-associated endocrine hormone, coordinating the response of multiple tissues to nutrient availability.

Insulin- and warts-dependent regulation of tracheal plasticity modulates systemic larval growth during hypoxia in Drosophila melanogaster

Adaptation to dynamic environmental cues during organismal development requires coordination of tissue growth with available resources. More specifically, the effects of oxygen availability on body size have been well-documented, but the mechanisms through which hypoxia restricts systemic growth have not been fully elucidated. Here, we characterize the larval growth and metabolic defects in Drosophila that result from hypoxia. Hypoxic conditions reduced fat body opacity and increased lipid droplet accumulation in this tissue, without eliciting lipid aggregation in hepatocyte-like cells called oenocytes. Additionally, hypoxia increased the retention of Dilp2 in the insulin-producing cells of the larval brain, associated with a reduction of insulin signaling in peripheral tissues. Overexpression of the wildtype form of the insulin receptor ubiquitously and in the larval trachea rendered larvae resistant to hypoxia-induced growth restriction. Furthermore, Warts downregulation in the trachea was similar to increased insulin receptor signaling during oxygen deprivation, which both rescued hypoxia-induced growth restriction, inhibition of tracheal molting, and developmental delay. Insulin signaling and loss of Warts function increased tracheal growth and augmented tracheal plasticity under hypoxic conditions, enhancing oxygen delivery during periods of oxygen deprivation. Our findings demonstrate a mechanism that coordinates oxygen availability with systemic growth in which hypoxia-induced reduction of insulin receptor signaling decreases plasticity of the larval trachea that is required for the maintenance of systemic growth during times of limiting oxygen availability.

Overexpression of malic enzyme in the larval stage extends Drosophila lifespan

Metabolic modifications during the developmental period can extend longevity. We found that malic enzyme (Men) overexpression during the larval period lengthened the lifespan of Drosophila. Men overexpression by S106-GeneSwitch-Gal4 driver increased pyruvate content and NADPH/NADP(+) ratio but reduced triglyceride, glycogen, and ATP levels in the larvae. ROS levels increased unexpectedly in Men-overexpressing larvae. Interestingly, adults exposed to larval Men-overexpression maintained ROS tolerance with enhanced expression levels of glutathione-S-transferase D2 and thioredoxin-2. Our results suggest that metabolic changes mediated by Men during development might be related to the control of ROS tolerance and the longevity of Drosophila.

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.

CYP18A1 regulates tissue-specific steroid hormone inactivation in Bombyx mori

Insect development and metamorphosis are regulated by two major hormones, juvenile hormone and ecdysteroids. Despite being the key regulator of insect developmental transitions, the metabolic pathway of the primary steroid hormone, 20-hydroxyecdysone (20E), especially its inactivation pathway, is still not completely elucidated. A cytochrome P450 enzyme, CYP18A1, has been shown to play key roles in insect steroid hormone inactivation through 26-hydroxylation. Here, we identified two CYP18 (BmCYP18A1 and BmCYP18B1) orthologs in the lepidopteran model insect, Bombyx mori. Interestingly, BmCYP18A1 gene is predominantly expressed in the middle silk gland (MSG) while BmCYP18B1 expresses ubiquitously in B. mori. BmCYP18A1 is induced by 20E in vitro, suggesting its role in 20E metabolism. Using the binary Gal4/UAS transgenic system, we ectopically overexpressed BmCYP18A1 in a MSG-specific manner with a Sericin1-Gal4 (Ser-Gal4) driver or in a ubiquitous manner with an Actin3-Gal4 (A3-Gal4) driver. Ectopic overexpression of BmCYP18A1 in MSG or in all tissues resulted in developmental arrestment of transgenic animals during the final instar larval stage. The 20E titers in the transgenic animals expressing BmCYP18A1 were lower compared to the levels in the control animals. Although the biological significance of MSG-specific expression of BmCYP18A1 is unclear, our results provide the first evidence that BmCYP18A1, which is conserved in most arthropods, is involved in a tissue-specific steroid hormone inactivation in B. mori.

Adipocyte amino acid sensing controls adult germline stem cell number via the amino acid response pathway and independently of Target of Rapamycin signaling in Drosophila

How adipocytes contribute to the physiological control of stem cells is a critical question towards understanding the link between obesity and multiple diseases, including cancers. Previous studies have revealed that adult stem cells are influenced by whole-body physiology through multiple diet-dependent factors. For example, nutrient-dependent pathways acting within the Drosophila ovary control the number and proliferation of germline stem cells (GSCs). The potential role of nutrient sensing by adipocytes in modulating stem cells in other organs, however, remains largely unexplored. Here, we report that amino acid sensing by adult adipocytes specifically modulates the maintenance of GSCs through a Target of Rapamycin-independent mechanism. Instead, reduced amino acid levels and the consequent increase in uncoupled tRNAs trigger activation of the GCN2-dependent amino acid response pathway within adipocytes, causing increased rates of GSC loss. These studies reveal a new step in adipocyte-stem cell crosstalk.

TIF-IA-dependent regulation of ribosome synthesis in drosophila muscle is required to maintain systemic insulin signaling and larval growth

The conserved TOR kinase signaling network links nutrient availability to cell, tissue and body growth in animals. One important growth-regulatory target of TOR signaling is ribosome biogenesis. Studies in yeast and mammalian cell culture have described how TOR controls rRNA synthesis—a limiting step in ribosome biogenesis—via the RNA Polymerase I transcription factor TIF-IA. However, the contribution of TOR-dependent ribosome synthesis to tissue and body growth in animals is less clear. Here we show in Drosophila larvae that ribosome synthesis in muscle is required non-autonomously to maintain normal body growth and development. We find that amino acid starvation and TOR inhibition lead to reduced levels of TIF-IA, and decreased rRNA synthesis in larval muscle. When we mimic this decrease in muscle ribosome synthesis using RNAi-mediated knockdown of TIF-IA, we observe delayed larval development and reduced body growth. This reduction in growth is caused by lowered systemic insulin signaling via two endocrine responses: reduced expression of Drosophila insulin-like peptides (dILPs) from the brain and increased expression of Imp-L2—a secreted factor that binds and inhibits dILP activity—from muscle. We also observed that maintaining TIF-IA levels in muscle could partially reverse the starvation-mediated suppression of systemic insulin signaling. Finally, we show that activation of TOR specifically in muscle can increase overall body size and this effect requires TIF-IA function. These data suggest that muscle ribosome synthesis functions as a nutrient-dependent checkpoint for overall body growth: in nutrient rich conditions, TOR is required to maintain levels of TIF-IA and ribosome synthesis to promote high levels of systemic insulin, but under conditions of starvation stress, reduced muscle ribosome synthesis triggers an endocrine response that limits systemic insulin signaling to restrict growth and maintain homeostasis.

All animals need adequate nutrition to grow and develop. Studies in tissue culture and model organisms have identified the TOR kinase signaling pathway as a key nutrient-dependent regulator of growth. Under nutrient rich conditions, TOR kinase is active and stimulates metabolic processes that drive growth. Under nutrient poor conditions, TOR is inhibited and animals alter their metabolism to maintain homeostasis and survival. Here we use Drosophila larvae to identify a role for ribosome synthesis—a key metabolic process—in mediating nutrient and TOR effects on body growth. In particular, we show that ribosome synthesis specifically in larval muscle is necessary to maintain organismal growth. We find that inhibition of muscle ribosome synthesis leads to reduced systemic insulin-like growth factor signaling via two endocrine responses—decreased expression of brain derived Drosophila insulin-like peptides (dILPs) and increased expression of Imp-L2, an inhibitor of insulin signaling. As a result of these effects, body growth is reduced and larval development is delayed. These findings suggest that control of ribosome synthesis, and hence protein synthesis, in specific tissues can exert control on overall body growth.

Drosophila gains traction as a repurposed tool to investigate metabolism

The use of fruit flies has recently emerged as a powerful experimental paradigm to study the core aspects of energy metabolism. The fundamental need for lipid and carbohydrate processing and storage across species dictates that the central regulators that control metabolism are highly conserved through evolution. Accordingly, the Drosophila system is being used to identify human disease genes and has the potential to model successfully human disorders that center on excessive caloric intake and metabolic dysfunction, including diet-induced lipotoxicity and type 2 diabetes. We review here recent progress on this front and contend that increasing such efforts will yield unexpectedly high rates of experimental return, thereby leading to novel approaches in the treatment of obesity and its comorbidities.

An insulin-like androgenic gland hormone gene in the mud crab, Scylla paramamosain, extensively expressed and involved in the processes of growth and female reproduction

Insulin-like androgenic gland hormone (IAG) produced by androgenic gland (AG) in male crustaceans is regarded as a key regulator of sex differentiation. As a member of the insulin/insulin-like growth factor family, IAG is also likely involved in regulating somatic growth. In this study, a full-length cDNA of IAG (termed Sp-IAG) was isolated from the mud crab, Scylla paramamosain. Genomic DNA of Sp-IAG was also cloned, analysis of which reveals that Sp-IAG gene is organized in a 4 exon/3 intron manner. RNA in situ hybridization analysis detected positive signals in both type I and type II AG cells. Reverse transcription-polymerase chain reaction (RT-PCR) analysis showed that Sp-IAG was expressed not only in AG, but also in many other tissues. Sp-IAG expression levels in ovaries were examined at different stages of ovarian development (stages I to V); it was found that the expression was maintained at low levels during undeveloped stage (stage I) to late vitellogenic stage (stage IV) and then increased significantly at mature stage (stage V), suggesting that Sp-IAG may participate in inhibiting oocyte growth and vitellogenesis. The expression pattern of Sp-IAG during the molting cycle of the first stage crabs (C1) was also determined. Sp-IAG expression level continuously decreased from 0 h C1 (postmolt) crabs to 96 h C1 (premolt) crabs, and then increased significantly in the newly molted second stage crabs (C2, postmolt). The combined results suggested for the first time that IAG is involved in regulating ovarian development and somatic growth in crustaceans.

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.

Juvenile hormone regulates body size and perturbs insulin signaling in Drosophila

The role of juvenile hormone (JH) in regulating the timing and nature of insect molts is well-established. Increasing evidence suggests that JH is also involved in regulating final insect size. Here we elucidate the developmental mechanism through which JH regulates body size in developing Drosophila larvae by genetically ablating the JH-producing organ, the corpora allata (CA). We found that larvae that lack CA pupariated at smaller sizes than control larvae due to a reduced larval growth rate. Neither the timing of the metamorphic molt nor the duration of larval growth was affected by the loss of JH. Further, we show that the effects of JH on growth rate are dependent on the forkhead box O transcription factor (FOXO), which is negatively regulated by the insulin-signaling pathway. Larvae that lacked the CA had elevated levels of FOXO activity, whereas a loss-of-function mutation of FOXO rescued the effects of CA ablation on final body size. Finally, the effect of JH on growth appears to be mediated, at least in part, via ecdysone synthesis in the prothoracic gland. These results indicate a role of JH in regulating growth rate via the ecdysone- and insulin-signaling pathways.

Ecdysone differentially regulates metamorphic timing relative to 20-hydroxyecdysone by antagonizing juvenile hormone in Drosophila melanogaster

In insects, a steroid hormone, 20-hydroxyecdysone (20E), plays important roles in the regulation of developmental transitions by initiating signaling cascades via the ecdysone receptor (EcR). Although 20E has been well characterized as the molting hormone, its precursor ecdysone (E) has been considered to be a relatively inactive compound because it has little or no effect on classic EcR mediated responses. I found that feeding E to wild-type third instar larvae of Drosophila melanogaster accelerates the metamorphic timing, which results in elevation of lethality during metamorphosis and reduced body size, while 20E has only a minor effect. The addition of a juvenile hormone analog (JHA) to E impeded their precocious pupariation and thereby rescued the reduced body size. The ability of JHA impeding the effect of E was not observed in the Methoprene-tolerant (Met) and germ-cell expressed (gce) double mutant animals lacking JH signaling, indicating that antagonistic action of JH against E is transduced via a primary JH receptor, Met, or a product of its homolog, Gce. I also found that L3 larvae are susceptible to E around the time when they reach their minimum viable weight. These results indicate that E, and not just 20E, is also essential for proper regulation of developmental timing and body size. Furthermore, the precocious pupariation triggered by E is impeded by the action of JH to ensure that animals attain body size to survive metamorphosis.

Sudestada1, a Drosophila ribosomal prolyl-hydroxylase required for mRNA translation, cell homeostasis, and organ growth

Genome sequences predict the presence of many 2-oxoglutarate (2OG)-dependent oxygenases of unknown biochemical and biological functions in Drosophila. Ribosomal protein hydroxylation is emerging as an important 2OG oxygenase catalyzed pathway, but its biological functions are unclear. We report investigations on the function of Sudestada1 (Sud1), a Drosophila ribosomal oxygenase. As with its human and yeast homologs, OGFOD1 and Tpa1p, respectively, we identified Sud1 to catalyze prolyl-hydroxylation of the small ribosomal subunit protein RPS23. Like OGFOD1, Sud1 catalyzes a single prolyl-hydroxylation of RPS23 in contrast to yeast Tpa1p, where Pro-64 dihydroxylation is observed. RNAi-mediated Sud1 knockdown hinders normal growth in different Drosophila tissues. Growth impairment originates from both reduction of cell size and diminution of the number of cells and correlates with impaired translation efficiency and activation of the unfolded protein response in the endoplasmic reticulum. This is accompanied by phosphorylation of eIF2α and concomitant formation of stress granules, as well as promotion of autophagy and apoptosis. These observations, together with those on enzyme homologs described in the companion articles, reveal conserved biochemical and biological roles for a widely distributed ribosomal oxygenase.

E93 predominantly transduces 20-hydroxyecdysone signaling to induce autophagy and caspase activity in Drosophila fat body

During the larval-prepupal transition in Drosophila, a balancing crosstalk occurs between autophagy and caspase activity in the remodeling fat body: the inhibition of autophagy induces caspase activity and the inhibition of caspases induces autophagy. Both autophagy and caspase activity are induced by a pulse of molting hormone (20-hydroxyecdysone, 20E) via the 20E nuclear receptor complex, EcR-USP. We here demonstrate that E93, a 20E primary-response gene encoding an HTH transcription factor, predominantly transduces 20E signaling to induce autophagy and caspase activity in the remodeling fat body. RNAi knockdown or mutation of E93 blocks autophagy and caspase activity, E93 overexpression induces them both, while E93 overexpression has a better rescuing effect on the inhibition of autophagy than caspase activity caused by EcR(DN) overexpression. At the transcriptional level, E93 not only greatly impacts the 20E-triggered transcriptional cascade, but also upregulates essential autophagy and apoptosis genes. Meanwhile, at the phosphorylational level, E93 blocks the PI3K-TORC1 signaling to initiate autophagy. Taken together, we conclude that autophagy and caspase activity are induced by 20E and predominantly transduced by E93 in the remodeling fat body of Drosophila.

The developmental control of size in insects

The mechanisms that control the sizes of a body and its many parts remain among the great puzzles in developmental biology. Why do animals grow to a species-specific body size, and how is the relative growth of their body parts controlled to so they grow to the right size, and in the correct proportion with body size, giving an animal its species-characteristic shape? Control of size must involve mechanisms that somehow assess some aspect of size and are upstream of mechanisms that regulate growth. These mechanisms are now beginning to be understood in the insects, in particular in Manduca sexta and Drosophila melanogaster. The control of size requires control of the rate of growth and control of the cessation of growth. Growth is controlled by genetic and environmental factors. Insulin and ecdysone, their receptors, and intracellular signaling pathways are the principal genetic regulators of growth. The secretion of these growth hormones, in turn, is controlled by complex interactions of other endocrine and molecular mechanisms, by environmental factors such as nutrition, and by the physiological mechanisms that sense body size. Although the general mechanisms of growth regulation appear to be widely shared, the mechanisms that regulate final size can be quite diverse.

Developmental Biology: miRs and Steroids and Growth Control

The Drosophila miRNA-encoding gene bantam controls cell and tissue growth. A new study now reveals that a large part of its effects on growth could come from its inhibition of the production of the molting hormone ecdysone.

Balancing crosstalk between 20-hydroxyecdysone-induced autophagy and caspase activity in the fat body during Drosophila larval-prepupal transition

In the fruitfly, Drosophila melanogaster, autophagy and caspase activity function in parallel in the salivary gland during metamorphosis and in a common regulatory hierarchy during oogenesis. Both autophagy and caspase activity progressively increase in the remodeling fat body, and they are induced by a pulse of the molting hormone (20-hydroxyecdysone, 20E) during the larval-prepupal transition. Inhibition of autophagy and/or caspase activity in the remodeling fat body results in 25-40% pupal lethality, depending on the genotypes. Interestingly, a balancing crosstalk occurs between autophagy and caspase activity in this tissue: the inhibition of autophagy induces caspase activity and the inhibition of caspases induces autophagy. The Drosophila remodeling fat body provides an in vivo model for understanding the molecular mechanism of the balancing crosstalk between autophagy and caspase activity, which oppose with each other and are induced by the common stimulus 20E, and blockage of either path reinforces the other path.

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.

Mechanisms regulating nutrition-dependent developmental plasticity through organ-specific effects in insects

Nutrition, via the insulin/insulin-like growth factor (IIS)/Target of Rapamycin (TOR) signaling pathway, can provide a strong molding force for determining animal size and shape. For instance, nutrition induces a disproportionate increase in the size of male horns in dung and rhinoceros beetles, or mandibles in staghorn or horned flour beetles, relative to body size. In these species, well-fed male larvae produce adults with greatly enlarged horns or mandibles, whereas males that are starved or poorly fed as larvae bear much more modest appendages. Changes in IIS/TOR signaling plays a key role in appendage development by regulating growth in the horn and mandible primordia. In contrast, changes in the IIS/TOR pathway produce minimal effects on the size of other adult structures, such as the male genitalia in fruit flies and dung beetles. The horn, mandible and genitalia illustrate that although all tissues are exposed to the same hormonal environment within the larval body, the extent to which insulin can induce growth is organ specific. In addition, the IIS/TOR pathway affects body size and shape by controlling production of metamorphic hormones important for regulating developmental timing, like the steroid molting hormone ecdysone and sesquiterpenoid hormone juvenile hormone. In this review, we discuss recent results from Drosophila and other insects that highlight mechanisms allowing tissues to differ in their sensitivity to IIS/TOR and the potential consequences of these differences on body size and shape.

The Ecdysone receptor constrains wingless expression to pattern cell cycle across the Drosophila wing margin in a Cyclin B-dependent manner

Background

Ecdysone triggers transcriptional changes via the ecdysone receptor (EcR) to coordinate developmental programs of apoptosis, cell cycle and differentiation. Data suggests EcR affects cell cycle gene expression indirectly and here we identify Wingless as an intermediary factor linking EcR to cell cycle.

Results

We demonstrate EcR patterns cell cycle across the presumptive Drosophila wing margin by constraining wg transcription to modulate CycB expression, but not the previously identified Wg-targets dMyc or Stg. Furthermore co-knockdown of Wg restores CycB patterning in EcR knockdown clones. Wg is not a direct target of EcR, rather we demonstrate that repression of Wg by EcR is likely mediated by direct interaction between the EcR-responsive zinc finger transcription factor Crol and the wg promoter.

Conclusions

Thus we elucidate a critical mechanism potentially connecting ecdysone with patterning signals to ensure correct timing of cell cycle exit and differentiation during margin wing development.

Transient exposure to low levels of insecticide affects metabolic networks of honeybee larvae

The survival of a species depends on its capacity to adjust to changing environmental conditions, and new stressors. Such new, anthropogenic stressors include the neonicotinoid class of crop-protecting agents, which have been implicated in the population declines of pollinating insects, including honeybees (Apis mellifera). The low-dose effects of these compounds on larval development and physiological responses have remained largely unknown. Over a period of 15 days, we provided syrup tainted with low levels (2 µg/L(-1)) of the neonicotinoid insecticide imidacloprid to beehives located in the field. We measured transcript levels by RNA sequencing and established lipid profiles using liquid chromatography coupled with mass spectrometry from worker-bee larvae of imidacloprid-exposed (IE) and unexposed, control (C) hives. Within a catalogue of 300 differentially expressed transcripts in larvae from IE hives, we detect significant enrichment of genes functioning in lipid-carbohydrate-mitochondrial metabolic networks. Myc-involved transcriptional response to exposure of this neonicotinoid is indicated by overrepresentation of E-box elements in the promoter regions of genes with altered expression. RNA levels for a cluster of genes encoding detoxifying P450 enzymes are elevated, with coordinated downregulation of genes in glycolytic and sugar-metabolising pathways. Expression of the environmentally responsive Hsp90 gene is also reduced, suggesting diminished buffering and stability of the developmental program. The multifaceted, physiological response described here may be of importance to our general understanding of pollinator health. Muscles, for instance, work at high glycolytic rates and flight performance could be impacted should low levels of this evolutionarily novel stressor likewise induce downregulation of energy metabolising genes in adult pollinators.

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.

20-Hydroxyecdysone upregulates Atg genes to induce autophagy in the Bombyx fat body

Autophagy is finely regulated at multiple levels and plays crucial roles in development and disease. In the fat body of the silkworm, Bombyx mori, autophagy occurs and Atg gene expression peaks during the nonfeeding molting and pupation stages when the steroid hormone (20-hydroxyecdysone; 20E) is high. Injection of 20E into the feeding larvae upregulated Atg genes and reduced TORC1 activity resulting in autophagy induction in the fat body. Conversely, RNAi knockdown of the 20E receptor partner (USP) or targeted overexpression of a dominant negative mutant of the 20E receptor (EcR (DN) ) in the larval fat body reduced autophagy and downregulated the Atg genes, confirming the importance of 20E-induction of Atg gene expression during pupation. Moreover, in vitro treatments of the larval fat body with 20E upregulated the Atg genes. Five Atg genes were potentially 20E primary-responsive, and a 20E response element was identified in the Atg1 (ortholog of human ULK1) promoter region. Furthermore, RNAi knockdown of 4 key genes (namely Br-C, E74, HR3 and βftz-F1) in the 20E-triggered transcriptional cascade reduced autophagy and downregulated Atg genes to different levels. Taken together, we conclude that in addition to blocking TORC1 activity for autophagosome initiation, 20E upregulates Atg genes to induce autophagy in the Bombyx fat body.

dMyc expression in the fat body affects DILP2 release and increases the expression of the fat desaturase Desat1 resulting in organismal growth

Drosophila dMyc (dMyc) is known for its role in cell-autonomous regulation of growth. Here we address its role in the fat body (FB), a metabolic tissue that functions as a sensor of circulating nutrients to control the release of Drosophila Insulin-like peptides (Dilps) from the brain influencing growth and development. Our results show that expression of dMyc in the FB affects development and animal size. Expression of dMyc, but not of CycD/cdk4 or Rheb, in the FB diminishes the ability to retain Drosophila Insulin-like peptide-2 (DILP2) in the brain during starvation, suggesting that expression of dMyc mimics the signal that remotely controls the release of Dilps into the hemolymph. dMyc also affects glucose metabolism and increases the transcription of Glucose-transporter-1 mRNA, and of Hexokinase and Pyruvate-Kinase mRNAs, key regulators of glycolysis. These animals are able to counteract the increased levels of circulating trehalose induced by a high sugar diet leading to the conclusion that dMyc activity in the FB promotes glucose disposal. dMyc expression induces cell autonomous accumulation of triglycerides, which correlates with increased levels of Fatty Acid Synthase and Acetyl CoA Carboxylase mRNAs, enzymes responsible for lipid synthesis. We also found the expression of Stearoyl-CoA desaturase, Desat1 mRNA significantly higher in FB overexpressing dMyc. Desat1 is an enzyme that is necessary for monosaturation and production of fatty acids, and its reduction affects dMyc's ability to induce fat storage and resistance to animal survival. In conclusion, here we present novel evidences for dMyc function in the Drosophila FB in controlling systemic growth. We discovered that dMyc expression triggers cell autonomous mechanisms that control glucose and lipid metabolism to favor the storage of nutrients (lipids and sugars). In addition, the regulation of Desat1 controls the synthesis of triglycerides in FB and this may affect the humoral signal that controls DILP2 release in the brain.

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.

Isolation and characterization of the ecdysone receptor and its heterodimeric partner ultraspiracle through development in Sciara coprophila

Regulation of DNA replication is critical, and loss of control can lead to DNA amplification. Naturally occurring, developmentally regulated DNA amplification occurs in the DNA puffs of the late larval salivary gland giant polytene chromosomes in the fungus fly, Sciara coprophila. The steroid hormone ecdysone induces DNA amplification in Sciara, and the amplification origin of DNA puff II/9A contains a putative binding site for the ecdysone receptor (EcR). We report here the isolation, cloning, and characterizing of two ecdysone receptor isoforms in Sciara (ScEcR-A and ScEcR-B) and the heterodimeric partner, ultraspiracle (ScUSP). ScEcR-A is the predominant isoform in larval tissues and ScEcR-B in adult tissues, contrary to the pattern in Drosophila. Moreover, ScEcR-A is produced at amplification but is absent just prior. We discuss these results in relation to the model of ecdysone regulation of DNA amplification.

Developmental checkpoints and feedback circuits time insect maturation

The transition from juvenile to adult is a fundamental process that allows animals to allocate resource toward reproduction after completing a certain amount of growth. In insects, growth to a species-specific target size induces pulses of the steroid hormone ecdysone that triggers metamorphosis and reproductive maturation. The past few years have seen significant progress in understanding the interplay of mechanisms that coordinate timing of ecdysone production and release. These studies show that the neuroendocrine system monitors complex size-related and nutritional signals, as well as external cues, to time production and release of ecdysone. Based on results discussed here, we suggest that developmental progression to adulthood is controlled by checkpoints that regulate the genetic timing program enabling it to adapt to different environmental conditions. These checkpoints utilize a number of signaling pathways to modulate ecdysone production in the prothoracic gland. Release of ecdysone activates an autonomous cascade of both feedforward and feedback signals that determine the duration of the ecdysone pulse at each developmental transitions. Conservation of the genetic mechanisms that coordinate the juvenile-adult transition suggests that insights from the fruit fly Drosophila will provide a framework for future investigation of developmental timing in metazoans.

Conserved microRNA miR-8 controls body size in response to steroid signaling in Drosophila

Body size determination is a process that is tightly linked with developmental maturation. Ecdysone, an insect maturation hormone, contributes to this process by antagonizing insulin signaling and thereby suppressing juvenile growth. Here, we report that the microRNA miR-8 and its target, u-shaped (USH), a conserved microRNA/target axis that regulates insulin signaling, are critical for ecdysone-induced body size determination in Drosophila. We found that the miR-8 level is reduced in response to ecdysone, while the USH level is up-regulated reciprocally, and that miR-8 is transcriptionally repressed by ecdysone's early response genes. Furthermore, modulating the miR-8 level correlatively changes the fly body size; either overexpression or deletion of miR-8 abrogates ecdysone-induced growth control. Consistently, perturbation of USH impedes ecdysone's effect on body growth. Thus, miR-8 acts as a molecular rheostat that tunes organismal growth in response to a developmental maturation signal.

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

Nutrient availability is a key determinant of animal growth. The conserved insulin/PI3 kinase and TOR kinase signaling pathways are two of the best characterized regulators of cell and tissue growth in response to nutritional conditions. Studies in Drosophila larvae show that one mechanism by which these pathways drive growth is by regulating the expression of metabolic genes, especially those genes required for protein synthesis. Here we examine a role for the transcription factor DREF in mediating some of these transcriptional and growth responses. We find that loss of DREF leads to a decrease in organismal growth. These effects are in part due to a requirement for DREF function in cell-autonomous growth. We also uncover a non-autonomous role for DREF activity in the larval fat body. Previous studies show that activation of TOR in the fat body couples nutrition to insulin release from the brain; we find that inhibition of DREF in the fat body can phenocopy effects of nutrient deprivation and fat-specific TOR inhibition, leading to a reduction in systemic insulin signaling, delayed larval growth and smaller final size. Using genetic epistasis, we find that DREF is required for growth downstream of TOR, but not insulin/PI3K signaling. Moreover, we show that TOR can control DREF mRNA levels, in part via the transcription factor dMyc. Finally we show that DREF is required for normal expression of many ribosome biogenesis genes, suggesting that one mechanism by which DREF is required for growth is through the control of protein synthetic capacity. Together these findings suggest DREF is an essential transcription factor in the nutritional control of cell and tissue growth during Drosophila development. Given that DREF is conserved, this role may also be important in the control of growth in other animals.

Controlling animal growth and body size - does fruit fly physiology point the way?

The question of how growth and size are controlled has fascinated generations of biologists. However, the underlying mechanisms still remain unclear. The last year or so has seen a flurry of reports on the control of growth and body size in Drosophila, and a central theme to these papers is the idea of signaling between organs as a control mechanism for overall body growth and development. While this concept is obviously not new, these fly studies now open up the possibility of using a genetically tractable system to dissect in detail how organ-to-organ communication dictates body size.

Ecdysone receptor (EcR) suppresses lipid accumulation in the Drosophila fat body via transcription control

Lipid metabolism drastically changes in response to the environmental factors in metazoans. Lipid is accumulated at the food rich condition, while mobilized in adipocyte tissue in starvation. Such lipid mobilization is also evident during the pupation of the insects. Pupation is induced by metamorphosis hormone, ecdysone via ecdysone receptor (EcR) with lipid mobilization, however, the molecular link of the EcR-mediated signal to the lipid mobilization remains elusive. To address this issue, EcR was genetically knocked-down selectively in 3rd instar larva fat body of Drosophila, corresponding to the adipocyte tissues in mammalians, that contains adipocyte-like cells. In this mutant, lipid accumulation was increased in the fat body. Lipid accumulation was also increased when knocked-down of taiman, which served as the EcR co-activator. Two lipid metabolism regulatory factor, E75B and adipose (adp) as well as cell growth factor, dMyc, were found as EcR target genes in the adipocyte-like cells, and consistently knock-down of these EcR target genes brought phenotypes in lipid accumulation supporting EcR function. These findings suggest that EcR-mediated ecdysone signal is significant in lipid metabolism in insects.

Nutrient/TOR-dependent regulation of RNA polymerase III controls tissue and organismal growth in Drosophila

The nutrient/target-of-rapamycin (TOR) pathway has emerged as a key regulator of tissue and organismal growth in metazoans. The signalling components of the nutrient/TOR pathway are well defined; however, the downstream effectors are less understood. Here, we show that the control of RNA polymerase (Pol) III-dependent transcription is an essential target of TOR in Drosophila. We find that TOR activity controls Pol III in growing larvae via inhibition of the repressor Maf1 and, in part, via the transcription factor Drosophila Myc (dMyc). Moreover, we show that loss of the Pol III factor, Brf, leads to reduced tissue and organismal growth and prevents TOR-induced cellular growth. TOR activity in the larval fat body, a tissue equivalent to vertebrate fat or liver, couples nutrition to insulin release from the brain. Accordingly, we find that fat-specific loss of Brf phenocopies nutrient limitation and TOR inhibition, leading to decreased systemic insulin signalling and reduced organismal growth. Thus, stimulation of Pol III is a key downstream effector of TOR in the control of cellular and systemic growth.

Coordinating growth and maturation - insights from Drosophila

Adult body size in higher animals is dependent on the amount of growth that occurs during the juvenile stage. The duration of juvenile development, therefore, must be flexible and responsive to environmental conditions. When immature animals experience environmental stresses such as malnutrition or disease, maturation can be delayed until conditions improve and normal growth can resume. In contrast, when animals are raised under ideal conditions that promote rapid growth, internal checkpoints ensure that maturation does not occur until juvenile development is complete. Although the mechanisms that regulate growth and gate the onset of maturation have been investigated for decades, the emerging links between childhood obesity, early onset puberty, and adult metabolic disease have placed a new emphasis on this field. Remarkably, genetic studies in the fruit fly Drosophila melanogaster have shown that the central regulatory pathways that control growth and the timing of sexual maturation are conserved through evolution, and suggest that this aspect of animal life history is regulated by a common genetic architecture. This review focuses on these conserved mechanisms and highlights recent studies that explore how Drosophila coordinates developmental growth with environmental conditions.

Drosophila RNA polymerase III repressor Maf1 controls body size and developmental timing by modulating tRNAiMet synthesis and systemic insulin signaling

The target-of-rapamycin pathway couples nutrient availability with tissue and organismal growth in metazoans. The key effectors underlying this growth are, however, unclear. Here we show that Maf1, a repressor of RNA polymerase III-dependent tRNA transcription, is an important mediator of nutrient-dependent growth in Drosophila. We find nutrients promote tRNA synthesis during larval development by inhibiting Maf1. Genetic inhibition of Maf1 accelerates development and increases body size. These phenotypes are due to a non-cell-autonomous effect of Maf1 inhibition in the fat body, the main larval endocrine organ. Inhibiting Maf1 in the fat body increases growth by promoting the expression of brain-derived insulin-like peptides and consequently enhanced systemic insulin signaling. Remarkably, the effects of Maf1 inhibition are reproduced in flies carrying one extra copy of the initiator methionine tRNA, tRNA(i)(Met). These findings suggest the stimulation of tRNA(i)(Met) synthesis via inhibition of dMaf1 is limiting for nutrition-dependent growth during development.

Anaplastic lymphoma kinase spares organ growth during nutrient restriction in Drosophila

Developing animals survive periods of starvation by protecting the growth of critical organs at the expense of other tissues. Here, we use Drosophila to explore the as yet unknown mechanisms regulating this privileged tissue growth. As in mammals, we observe in Drosophila that the CNS is more highly spared than other tissues during nutrient restriction (NR). We demonstrate that anaplastic lymphoma kinase (Alk) efficiently protects neural progenitor (neuroblast) growth against reductions in amino acids and insulin-like peptides during NR via two mechanisms. First, Alk suppresses the growth requirement for amino acid sensing via Slimfast/Rheb/TOR complex 1. And second, Alk, rather than insulin-like receptor, primarily activates PI3-kinase. Alk maintains PI3-kinase signaling during NR as its ligand, Jelly belly (Jeb), is constitutively expressed from a glial cell niche surrounding neuroblasts. Together, these findings identify a brain-sparing mechanism that shares some regulatory features with the starvation-resistant growth programs of mammalian tumors.

Drosophila as a model for interorgan communication: lessons from studies on energy homeostasis

Current studies of physiological communication between Drosophila organs are beginning to address the fundamental problem of how nutrients regulate organismal growth, stem cell behavior, immunity, and aging. Advances in the Drosophila genetic tool kit will allow the design of genetic screens to systematically identify factors involved in organ communication.

The coordination of growth among Drosophila organs in response to localized growth-perturbation

The developmental mechanisms by which growth is coordinated among developing organs are largely unknown and yet are essential to generate a correctly proportioned adult. In particular, such coordinating mechanisms must be able to accommodate perturbations in the growth of individual organs caused by environmental or developmental stress. By autonomously slowing the growth of the developing wing discs within Drosophila larvae, we show that growing organs are able to signal localized growth perturbation to the other organs in the body and slow their growth also. Growth rate is so tightly coordinated among organs that they all show approximately the same reduction in growth rate as the developing wings, thereby maintaining their correct size relationship relative to one another throughout development. Further, we show that the systemic growth effects of localized growth-perturbation are mediated by ecdysone. Application of ecdysone to larvae with growth-perturbed wing discs rescues the growth rate of other organs in the body, indicating that ecdysone is limiting for their growth, and disrupts the coordination of their growth with growth of the wing discs. Collectively our data demonstrate the existence of a novel growth-coordinating mechanism in Drosophila that synchronizes growth among organs in response to localized growth perturbation.

MicroRNAs in Drosophila development

Micro-ribonucleic acids (miRNAs) are small (21-24 nucleotide), endogenously expressed, noncoding RNAs that have emerged as important posttranscriptional regulators of gene expression. MiRNAs have been identified and cloned from diverse eukaryotic organisms where they have been shown to control important physiological and developmental processes such as apoptosis, cell division, and differentiation. A high level of conservation of some miRNAs across phyla further emphasizes their importance as posttranscriptional regulators. Research in a variety of model systems has been instrumental in dissecting the biological functions of miRNAs. In this chapter, we discuss the current literature on the role of miRNAs as developmental regulators in Drosophila.

Steroids make you bigger? Fat chance says Myc

In flies, ecdysone integrates growth with developmental transitions by antagonizing insulin signaling, which links growth with nutritional status. Work in Developmental Cell (Delanoue et. al, 2010) finds that ecdysone represses the transcription factor Myc in the larval fat body to inhibit systemic growth, revealing a mechanism for such coordination.

Myc Function in Drosophila

Myc proteins control several cellular processes, including proliferation and growth, and they play an important role in human tumorigenesis. Several years ago, single homologs of Myc, its interaction partner Max, and its antagonist Mnt were identified in Drosophila melanogaster. Here, we review the function of this so-called Max network in fruit flies, with a particular emphasis on its most obvious biological activity: the control of cellular and organismal growth. We describe the molecular basis for this growth function, as well as the interaction of Myc with other pathways known to control growth, the insulin, TOR, and hippo pathways. In addition, Drosophila Myc also controls DNA replication and influences apoptosis, both cell-autonomously and non-autonomously, in a process known as cell competition. In the future, we expect that further functions of Myc will be uncovered and that genetic approaches will increasingly be used to characterize the evolutionarily conserved molecular mechanism of Myc action, thus also benefitting our understanding of Myc biology in vertebrates.