Regulation of imaginal disc cell size, cell number and organ size by Drosophila class I(A) phosphoinositide 3-kinase and its adaptor

Interplay between growth factor and nutrient signaling: lessons from Drosophila TOR

During normal development, cellular and organismal growth is coordinately regulated. Each cell and each individual organ integrates information about nutrient availability, hormonal signals, and intrinsic growth programs. Describing the signaling pathways involved in these processes and how they are integrated is important to understand how growth is controlled during development and may also permit the development of means to curb uncontrolled growth in disease. In recent years, the biochemical analysis of cellular growth in cultured cells and the genetic dissection of growth control in model organisms has identified two conserved signaling pathways dedicated to cellular growth. The target of rapamycin (TOR) pathway regulates growth in response to nutrients, and the insulin/IGF pathways are involved in coordinating cellular growth in response to endocrine signals. This review discusses recent advances in the understanding of the interaction between these pathways, with a special focus on the contribution of the genetic analysis of these pathways in Drosophila.

Genetic analysis of TOR signaling in Drosophila

Over a 4-day period of development, Drosophila larvae undergo a roughly 1,000-fold increase in mass. This impressive growth requires a continuous source of dietary protein; in the absence of amino acids, growth is arrested and various larval tissues display characteristic cell-cycle, metabolic, and structural changes. Mutations in the Drosophila target of rapamycin (dTOR) gene result in strikingly similar phenotypes, suggesting that dTOR acts in a signaling pathway responsive to nutrient availability. Genetic epistasis experiments indicate that dTOR is also required for cell growth in response to insulin and PI3K signaling, and that S6K activation can partially rescue dTOR loss of function. Thus dTOR has roles in both nutrient- and growth factor-mediated signaling, and may act to coordinate the activities of these pathways during development. Here we describe the use of mutations in dTOR to dissect its role in various signaling events, to gain insight into TOR protein structure, and to identify novel factors involved in TOR signaling.

HIDden targets of microRNAs for growth control

How is the size of an animal determined? Why is it that humans grow larger than mice? Certainly, one of the most astonishing features of animal development is that every animal of a given species, and its organs and appendages, grow to approximately the same size. Surprisingly little is known about the biology of tissue growth and size control. Recent advances in Drosophila research have implicated a microRNA as an important regulator of animal size. These studies reveal an unexpected layer of size regulation in higher animals.

Akt promotes increased mammalian cell size by stimulating protein synthesis and inhibiting protein degradation

Expression of constitutively active Akt3 was found to increase the size of MCF-7 cells approximately twofold both in vitro and in vivo. A regulatable version of Akt1 (MER-Akt) was also found capable of inducing a twofold increase in the size of H4IIE rat hepatoma cells. Rapamycin, a specific inhibitor of mTOR function, was found to inhibit the Akt-induced increase in cell size by 70%, presumably via inhibition of the Akt-induced increase in protein synthesis. To determine whether Akt could be inhibiting protein degradation, thereby contributing to its ability to induce an increase in cell size, we conducted protein degradation experiments in the H4IIE cell line. Activation of MER-Akt was found to inhibit protein degradation to a degree comparable to insulin treatment. The effects of these two agents on protein degradation were not additive, thereby suggesting that they were acting on a similar pathway. An inhibitor of the phosphatidylinositol 3-kinase pathway, LY-294002, blocked both insulin- and Akt-induced inhibition of protein degradation, again consistent with the hypothesis that both agents were acting on the same pathway. In contrast, rapamycin did not block the ability of either agent to inhibit protein degradation. These results indicate that Akt increases cell size through both mTOR-dependent and -independent pathways and that the latter involves inhibition of protein degradation. These studies are also consistent with the hypothesis that insulin's ability to regulate protein degradation is to a large extent mediated via Akt.

Development and the Genetics of Evolutionary Change Within Insect Species

▪ Abstract  Changes in genes and in developmental processes generate the phenotypic variation that is sorted by natural selection in adaptive evolution. We review several case studies in which artificial selection experiments in insects have led to divergent morphologies, and where further work has revealed information about the underlying changes at both the genetic and developmental levels. In addition, we examine several studies of phenotypic plasticity where multidisciplinary approaches are also beginning to reveal more about how developmental processes are modulated. Such integrated research will lead to a richer understanding of the changes in development that occur during evolutionary responses to natural selection, and it will also more rigorously examine how developmental processes can influence the tempo and direction of evolutionary change.

The functions of insulin signaling: size isn't everything, even in Drosophila

Mammalian insulin and insulin-like growth factors (IGFs) signal through several receptors with different ligand specificities to regulate metabolism and growth. This regulation is defective in diabetes and in a wide variety of human tumors. Recent analysis in Drosophila melanogaster has revealed that insulin-like molecules (known as DILPs in flies) also control growth and metabolism, but probably do so by signaling through a single insulin receptor (InR). The intracellular signaling molecules regulated by this receptor are highly evolutionarily conserved. Work in flies has helped to dissect the network of InR-regulated intracellular signaling pathways and identify some of the critical players in these pathways and in interacting signaling cascades. Surprisingly, these studies have shown that DILPs control tissue and body growth primarily by regulating cell growth and cell size. Changes in cell growth produced by these molecules may subsequently modulate the rate of cell proliferation in a cell type-specific fashion. At least part of this growth effect is mediated by two small groups of neurons in the Drosophila brain, which secrete DILPs into the circulatory system at levels that are modulated by nutrition. This signaling center is also involved in DILP-dependent control of the fly's rate of development, fertility, and life span. These surprisingly diverse functions of InR signaling, which appear to be conserved in all higher animals, reflect a central role for this pathway in coordinating development, physiology, and properly proportioned growth of the organism in response to its nutritional state. Studies in flies are providing important new insights into the biology of this system, and the identification of novel components in the InR-regulated signaling cascade is already beginning to inform the development of new therapeutic strategies for insulin-linked diseases in the clinic.

The control of body size in insects

Control mechanisms that regulate body size and tissue size have been sought at both the cellular and organismal level. Cell-level studies have revealed much about the control of cell growth and cell division, and how these processes are regulated by nutrition. Insulin signaling is the key mediator between nutrition and the growth of internal organs, such as imaginal disks, and is required for the normal proportional growth of the body and its various parts. The insulin-related peptides of insects do not appear to control growth by themselves, but act in conjunction with other hormones and signaling molecules, such as ecdysone and IDGFs. Size regulation cannot be understood solely on the basis of the mechanisms that control cell size and cell number. Size regulation requires mechanisms that gather information on a scale appropriate to the tissue or organ being regulated. A new model mechanism, using autocrine signaling, is outlined by which tissue and organ size regulation can be achieved. Body size regulation likewise requires a mechanism that integrates information at an appropriate scale. In insects, this mechanism operates by controlling the secretion of ecdysone, which is the signal that terminates the growth phase of development. The mechanisms for size assessment and the pathways by which they trigger ecdysone secretion are diverse and can be complex. The ways in which these higher-level regulatory mechanisms interact with cell- and molecular- level mechanisms are beginning to be elucidated.

Tumour suppressors hamartin and tuberin: intracellular signalling

Tumour suppressors hamartin and tuberin, encoded by tuberous sclerosis complex 1(TSC1) and TSC2 genes, respectively, are critical regulators of cell growth and proliferation. Mutations in TSC1 and TSC2 genes are the cause of an autosomal dominant disorder known as tuberous sclerosis complex (TSC). Another genetic disorder, lymphangioleiomyomatosis (LAM), is also associated with mutations in the TSC2 gene. Hamartin and tuberin control cell growth by negatively regulating S6 kinase 1 (S6K1) and eukaryotic initiation factor 4E binding protein 1 (4E-BP1), potentially through their upstream modulator mammalian target of rapamycin (mTOR). Growth factors and insulin promote Akt/PKB-dependent phosphorylation of tuberin, which in turn, releases S6K1 from negative regulation by tuberin and results in the activation of S6K1. Although much has been written regarding the molecular genetics of TSC and LAM, which is associated with either the loss of or mutation in the TSC1 and TSC2 genes, few reviews have addressed the intracellular signalling pathways regulated by hamartin and tuberin. The current review will fill the gap in our understanding of their role in cellular signalling networks, and by improving this understanding, an integrated picture regarding the normal function of tuberin and hamartin is beginning to emerge.

The Drosophila bZIP transcription factor Vrille is involved in hair and cell growth

Vri is closely related to bZIP transcription factors involved in growth or cell death. vri clonal and overexpression analyses revealed defects at the cellular level. vri clones in the adult cuticle contain smaller cells with atrophic bristles. The phenotypes are strictly cell autonomous. Clones induced in the eye precursor cells lead to individuals with smaller eyes and reduced number of ommatidia with an abnormal morphology and shorter photoreceptor cell stalks. Overexpression of vri is anti-proliferative in embryonic dorsal epidermis and in imaginal discs, and induces apoptosis. On the wing surface, larger cells with multiple trichomes are observed, suggesting cytoskeletal defects. In salivary glands, vri overexpression leads to smaller cells and organs. We also show that vri is involved in locomotion and flight and interacts genetically with genes encoding actin-binding proteins. The phenotypes observed are consistent with the hypothesis that vri is required for normal cell growth and proliferation via the regulation of the actin cytoskeleton.

Phosphoinositide 3-kinase activation regulates cell division time by coordinated control of cell mass and cell cycle progression rate

Cells must increase their mass in coordination with cell cycle progression to ensure that their size and macromolecular composition remain constant for any given proliferation rate. To this end, growth factors activate early signaling cascades that simultaneously promote cell mass increase and induce cell cycle entry. Nonetheless, the mechanism that controls the concerted regulation of cell growth and cell cycle entry in mammals remains unknown. The phosphatidylinositol 3-kinase (PI3K)/protein kinase B pathway regulates cell cycle entry by inactivating forkhead transcription factors and promoting cyclin D synthesis. PI3K/protein kinase B-derived signals also affect activation of p70 S6 kinase and the mammalian target of rapamycin, enzymes involved in cell growth control. We previously showed that enhancement of PI3K activation accelerates cell cycle entry, whereas reduction of PI3K activation retarded this process. Here we examined whether expression of different PI3K mutants affects cell growth during cell division. We show that diminishing or enhancing the magnitude of PI3K activation in a transient manner reduces or increases, respectively, the protein synthesis rate. Alteration of cell growth and cell cycle entry by PI3K forms appears to be concerted, because it results in lengthening or shortening of cell division time without altering cell size. In support of a central role for PI3K in growth control, expression of a deregulated, constitutive active PI3K mutant affects p70 S6 kinase and mammalian target of rapamycin activities and increases cell size. Together, the results show that transient PI3K activation regulates cell growth and cell cycle in a coordinated manner, which in turn controls cell division time.

Expression of Drosophila FOXO regulates growth and can phenocopy starvation

BACKGROUND

Components of the insulin signaling pathway are important regulators of growth. The FOXO (forkhead box, sub-group "O") transcription factors regulate cellular processes under conditions of low levels of insulin signaling. Studies in mammalian cell culture show that activation of FOXO transcription factors causes cell death or cell cycle arrest. The Caenorhabditis elegans homologue of FOXO, Daf-16, is required for the formation of dauer larvae in response to nutritional stress. In addition, FOXO factors have been implicated in stress resistance and longevity.

RESULTS

We have identified the Drosophila melanogaster homologue of FOXO (dFOXO), which is conserved in amino acid sequence compared with the mammalian FOXO homologues and Daf-16. Expression of dFOXO during early larval development causes inhibition of larval growth and alterations in feeding behavior. Inhibition of larval growth is reversible upon discontinuation of dFOXO expression. Expression of dFOXO during the third larval instar or at low levels during development leads to the generation of adults that are reduced in size. Analysis of the wings and eyes of these small flies indicates that the reduction in size is due to decreases in cell size and cell number. Overexpression of dFOXO in the developing eye leads to a characteristic phenotype with reductions in cell size and cell number. This phenotype can be rescued by co-expression of upstream insulin signaling components, dPI3K and dAkt, however, this rescue is not seen when FOXO is mutated to a constitutively active form.

CONCLUSIONS

dFOXO is conserved in both sequence and regulatory mechanisms when compared with other FOXO homologues. The establishment of Drosophila as a model for the study of FOXO transcription factors should prove beneficial to determining the biological role of these signaling molecules. The alterations in larval development seen upon overexpression of dFOXO closely mimic the phenotypic effects of starvation, suggesting a role for dFOXO in the response to nutritional adversity. This work has implications in the understanding of cancer and insulin related disorders, such as diabetes and obesity.

A new genetic locus controlling growth and proliferation in Drosophila melanogaster

Multicellular organisms grow through both proliferation and growth of their individual cells. We have conducted a P-element-based misexpression screen for genes whose upregulation alters wing disc growth during development. One particular group of four P elements, all inserted at cytological location 61C7-8, exhibited specific overgrowth upon misexpression in proliferating imaginal tissues. Clonal analysis revealed that upon misexpression, cell number was increased but cell size was not affected, indicating that cell growth and proliferation were induced in a coordinate manner. Loss of function at the locus produced small flies with reduced cell number, consistent with the presence of a gene encoding a positive growth regulator. We characterized a new transcription unit initiating in a region adjacent to the P insertions, which generated a complex series of polyadenylated transcripts. Although these RNAs were induced in response to misexpression, none was sufficient by itself to recapitulate overgrowth when overexpressed. This suggested either that a particular combination of these transcripts was necessary or that other sequences are involved.

Modulo is a target of Myc selectively required for growth of proliferative cells in Drosophila

In Drosophila, the homologue of the proto-oncogene Myc is a key regulator of both cell size and cell growth. The identities and roles of dMyc target genes in these processes, however, remain largely unexplored. Here, we investigate the function of the modulo (mod) gene, which encodes a nucleolus localized protein. In gain of function or loss of function experiments, we demonstrate that mod is directly controlled by dMyc. Strikingly, in proliferative imaginal cells, mod loss-of-function impairs both cell growth and cell size, whereas larval endoreplicative tissues grow normally. In contrast to dMyc, over-expressing Mod in wing imaginal discs is not sufficient to induce cell growth. Taken together, our results indicate that mod does not possess the full spectrum of dMyc activities, but is required selectively in proliferative cells to sustain their growth and to maintain their specific size.

Coordinate regulation of translation by the PI 3-kinase and mTOR pathways

Control of translation initiation is an important means by which cells tightly regulate the critical processes of growth and proliferation. Multiple effector proteins contribute to translation initiation of specially modified mRNAs that modulate these processes. Coordinated regulation of these translational effectors by multiple signaling pathways allows the integration of information regarding mitogenic signals, energy levels, and nutrient sufficiency. The mTOR protein, in particular, serves as a sensor of all of these signals and is thought to thus serve as a crucial checkpoint control protein. Signals from the mTOR pathway converge with mitogenic inputs from the phosphoinositide (PI) 3-kinase pathway on translational effector proteins to coordinately control cellular growth, size, and cell proliferation. The translational effectors regulated by the PI 3-kinase and mTOR pathways and their roles in regulation of cellular growth will be the primary focus of this review.

Cell cycle withdrawal, progression, and cell survival regulation by EGFR and its effectors in the differentiating Drosophila eye

Receptor tyrosine kinases such as the EGF receptor transduce extracellular signals into multiple cellular responses. In the developing Drosophila eye, EGFR activity triggers cell differentiation. Here we focus on three additional cell autonomous aspects of EGFR function and their coordination with differentiation, namely, withdrawal from the cell cycle, mitosis, and cell survival. We find that, whereas differentiation requires intense signaling, dependent on multiple reinforcing ligands, lesser EGFR activity maintains cell cycle arrest, promotes mitosis, and protects against cell death. Each response requires the same Ras, Raf, MAPK, and Pnt signal transduction pathway. Mitotic and survival responses also involve Pnt-independent branches, perhaps explaining how survival and mitosis can occur independently. Our results suggest that, rather than triggering all or none responses, EGFR coordinates partially independent processes as the eye differentiates.

Synaptic Drosophila UNC-13 is regulated by antagonistic G-protein pathways via a proteasome-dependent degradation mechanism

UNC-13 is a highly conserved plasma membrane-associated synaptic protein implicated in the regulation of neurotransmitter release through the direct modulation of the SNARE exocytosis complex. Previously, we characterized the Drosophila homologue (DUNC-13) and showed it to be essential for neurotransmitter release immediately upstream of vesicular fusion ("priming") at the neuromuscular junction (NMJ). Here, we show that the abundance of DUNC-13 in NMJ synaptic boutons is regulated downstream of GalphaS and Galphaq pathways, which have inhibitory and facilitatory roles, respectively. Both cAMP modulation and PKA function are required for DUNC-13 synaptic up-regulation, suggesting that the cAMP pathway enhances synaptic efficacy via DUNC-13. Similarly, PLC function and DAG modulation also regulate the synaptic levels of DUNC-13, through a mechanism that appears independent of PKC. Our results suggest that proteasome-mediated protein degradation is the primary mechanism regulating DUNC-13 levels at the synapse. Both PLC- and PKA-mediated pathways appear to regulate synaptic levels of DUNC-13 through controlling the rate of proteasome-dependent DUNC-13 degradation. We conclude that the functional abundance of DUNC-13 at the synapse, a key determinant of synaptic vesicle priming and neurotransmitter release probability, is primarily regulated by the rate of protein degradation, rather than translocation or transport, convergently controlled via both cAMP and DAG signal transduction pathways.

A Ral guanine exchange factor-Ral pathway is conserved in Drosophila melanogaster and sheds new light on the connectivity of the Ral, Ras, and Rap pathways

Ras GTPases are central to many physiological and pathological signaling pathways and act via a combination of effectors. In mammals, at least three Ral exchange factors (RalGEFs) contain a Ras association domain and constitute a discrete subgroup of Ras effectors. Despite their ability to bind activated Rap as well as activated Ras, they seem to act downstream of Ras but not downstream of Rap. We have revisited the Ras/Rap-Ral connections in Drosophila melanogaster by using iterative two-hybrid screens with these three GTPases as primary baits and a subsequent genetic approach. We show that (i) the Ral-centered protein network appears to be extremely conserved in human and flies, (ii) in this network, RGL is a functional Drosophila orthologue of RalGEFs, and (iii) the RGL-Ral pathway functionally interacts with both the Ras and Rap pathways. Our data do not support the paradigmatic model where Ral is in the effector pathway of Ras. They reveal a signaling circuitry where Ral is functionally downstream of the Rap GTPase, at odds with the pathways described for mammalian cell lines. Thus, in vivo data show variations in the connectivity of pathways described for cell lines which might display only a subset of the biological possibilities.

Insulin/IGF and target of rapamycin signaling: a TOR de force in growth control

'They come in all sizes.' Apart from its origin and use in the clothing industry, this saying reflects the fact that the size of organisms spans an enormous range. Whether destined to be large or small, species grow in an organized fashion to reach their final specified size. For growth to proceed, food must be metabolized to liberate energy in the form of adenosine triphosphate (ATP) and protein building blocks in the form of amino acids. One major orchestrator of this complex growth process in diverse metazoan species is the insulin/insulin-like growth factor (IGF) system. This review summarizes current studies primarily from Drosophila regarding the function of the insulin/IGF system in the control of growth.

The Drosophila forkhead transcription factor FOXO mediates the reduction in cell number associated with reduced insulin signaling

BACKGROUND

Forkhead transcription factors belonging to the FOXO subfamily are negatively regulated by protein kinase B (PKB) in response to signaling by insulin and insulin-like growth factor in Caenorhabditis elegans and mammals. In Drosophila, the insulin-signaling pathway regulates the size of cells, organs, and the entire body in response to nutrient availability, by controlling both cell size and cell number. In this study, we present a genetic characterization of dFOXO, the only Drosophila FOXO ortholog.

RESULTS

Ectopic expression of dFOXO and human FOXO3a induced organ-size reduction and cell death in a manner dependent on phosphoinositide (PI) 3-kinase and nutrient levels. Surprisingly, flies homozygous for dFOXO null alleles are viable and of normal size. They are, however, more sensitive to oxidative stress. Furthermore, dFOXO function is required for growth inhibition associated with reduced insulin signaling. Loss of dFOXO suppresses the reduction in cell number but not the cell-size reduction elicited by mutations in the insulin-signaling pathway. By microarray analysis and subsequent genetic validation, we have identified d4E-BP, which encodes a translation inhibitor, as a relevant dFOXO target gene.

CONCLUSION

Our results show that dFOXO is a crucial mediator of insulin signaling in Drosophila, mediating the reduction in cell number in insulin-signaling mutants. We propose that in response to cellular stresses, such as nutrient deprivation or increased levels of reactive oxygen species, dFOXO is activated and inhibits growth through the action of target genes such as d4E-BP.

Shrinkage control: regulation of insulin-mediated growth by FOXO transcription factors

The insulin signaling pathway regulates organismal growth in response to nutrient conditions by controlling a range of metabolic and biosynthetic processes. Recent studies in Drosophila have shown how transcriptional responses to reduced insulin and nutrient levels can act to inhibit growth.

The Rb-family protein p107 inhibits translation by a PDK1-dependent mechanism

The Rb family of proteins, which consists of Rb, p107 and p130, are critical regulators of cell proliferation. In addition to their inhibitory effects on cell cycle progression, Rb-family proteins repress transcription by RNA polymerases I and III, and may therefore restrain cell growth. However, it is not known if Rb, p107 or p130 have direct effects on protein synthesis. Here we report that ectopic expression of p107 in rat fibroblasts markedly attenuates the stimulation of mRNA translation and global protein synthesis by serum growth factors. This effect is associated with a reduction in the phosphorylation and activation of the serine-threonine kinases Akt1 and p70 S6 kinase (S6K1), two downstream targets of phosphoinositide-dependent kinase 1 (PDK1). We show that overexpression of p107 interferes with the recruitment of PDK1 to the plasma membrane in response to growth factors. Overexpression of PDK1 restores the defect in translation elicited by p107. These results suggest that p107 restricts cell growth by interfering with the phosphoinositide 3-kinase (PI3K) signaling pathway.

PTEN function in mammalian cell size regulation

The PTEN tumor suppressor gene is a lipid phosphatase that negatively regulates cell survival mediated by the phosphatidyl inositol 3' kinase-protein kinase B/Akt signaling pathway. Recent in vivo studies have revealed a novel role for PTEN in the size control of neurons. Dysregulation of cell growth control by PTEN is associated with the neurological disorder Lhermitte-Duclos disease. PTEN may regulate cell size through effects on protein translation.

The Drosophila insulin/IGF receptor controls growth and size by modulating PtdInsP3 levels

Understanding the control of size is of fundamental biological and clinical importance. Insulin/IGF signaling during development controls growth and size, possibly by coordinating the activities of the Ras and PI 3-kinase signaling pathways. We show that in Drosophila mutating the consensus binding site for the Ras pathway adaptor Drk/Grb2 in Chico/IRS does not interfere with growth whereas mutating the binding sites of the PI 3-kinase adaptor p60 completely abrogates Chico function. Furthermore, we present biochemical and genetic evidence that loss of the homolog of the tumor suppressor gene, Pten, results in increased PtdInsP3 levels and that these increased levels are sufficient to compensate for the complete loss of the Insulin/insulin-like growth factor receptor function. This reduction of Pten activity is also sufficient to vastly increase organism size. These results suggest that PtdInsP3 is a second messenger for growth and that levels of PtdInsP3 during development regulate organismal size.

Interactions between Ras1, dMyc, and dPI3K signaling in the developing Drosophila wing

The Ras GTPase links extracellular signals to intracellular mechanisms that control cell growth, the cell cycle, and cell identity. An activated form of Drosophila Ras (Ras(V12)) promotes these processes in the developing wing, but the effector pathways involved are unclear. Here, we present evidence indicating that Ras(V12) promotes cell growth and G(1)/S progression by increasing dMyc protein levels and activating dPI3K signaling, and that it does so via separate effector pathways. We also show that endogenous Ras is required to maintain normal levels of dMyc, but not dPI3K signaling during wing development. Finally, we show that induction of dMyc and regulation of cell identity are separable effects of Raf/MAPK signaling. These results suggest that Ras may only affect PI3K signaling when mutationally activated, such as in Ras(V12)-transformed cells, and provide a basis for understanding the synergy between Ras and other growth-promoting oncogenes in cancer.

Nutrient-dependent expression of insulin-like peptides from neuroendocrine cells in the CNS contributes to growth regulation in Drosophila

BACKGROUND

The insulin/IGF-1 signaling pathway controls cellular and organismal growth in many multicellular organisms. In Drosophila, genetic defects in components of the insulin signaling pathway produce small flies that are delayed in development and possess fewer and smaller cells as well as female sterility, reminiscent of the phenotypes of starved flies.

RESULTS

Here we establish a causal link between nutrient availability and insulin-dependent growth. We show that in addition to the Drosophila insulin-like peptide 2 (dilp2) gene, overexpression of dilp1 and dilp3-7 is sufficient to promote growth. Three of the dilp genes are expressed in seven median neurosecretory cells (m-NSCs) in the brain. These m-NSCs possess axon terminals in the larval endocrine gland and on the aorta, from which DILPs may be released into the circulatory system. Although expressed in the same cells, the expression of the three genes is controlled by unrelated cis-regulatory elements. The expression of two of the three genes is regulated by nutrient availability. Genetic ablation of these neurosecretory cells mimics the phenotype of starved or insulin signaling mutant flies.

CONCLUSIONS

These results point to a conserved role of the neuroendocrine axis in growth control in multicellular organisms.

The bantam gene regulates Drosophila growth

We report here the consequences of mutations of a novel locus, named bantam, whose product is involved in the regulation of growth in Drosophila. bantam mutant animals are smaller than wild type, due to a reduction in cell number but not cell size, and do not have significant disruptions in patterning. Conversely, overexpression of the bantam product using the EP element EP(3)3622 causes overgrowth of wing and eye tissue. Overexpression in clones of cells results in an increased rate of cell proliferation and a matched increase in cellular growth rate, such that the resulting tissue is composed of more cells of a size comparable to wild type. These effects are strikingly similar to those associated with alterations in the activity of the cyclinD-cdk4 complex. However, epistasis and genetic interaction analyses indicate that bantam and cyclinD-cdk4 operate independently. Thus, the bantam locus represents a novel regulator of tissue growth.

Structural insight into substrate specificity and regulatory mechanisms of phosphoinositide 3-kinases

Phosphoinositide 3-kinases (PI3Ks) are implicated in a variety of fundamental cellular processes. These enzymes catalyse phosphorylation of the 3'-OH position of myo-inositol lipids that serve as secondary messengers. The catalytic subunit for one of the family members, PI3K gamma, has been structurally characterized, independently, in complexes with kinase inhibitors and with the p21(Ras) GTPase. These atomic structures provide a basis for the rationalization of some PI3K substrate specificities and regulatory mechanisms, establishing links to functional and cellular data. Ongoing comprehensive structural and functional studies are essential to realize the promise of PI3K isozyme-specific therapeutic agents.

Essential role of PDK1 in regulating cell size and development in mice

PDK1 functions as a master kinase, phosphorylating and activating PKB/Akt, S6K and RSK. To learn more about the roles of PDK1, we generated mice that either lack PDK1 or possess PDK1 hypomorphic alleles, expressing only approximately 10% of the normal level of PDK1. PDK1(-/-) embryos die at embryonic day 9.5, displaying multiple abnormalities including lack of somites, forebrain and neural crest derived tissues; however, development of hind- and midbrain proceed relatively normally. In contrast, hypomorphic PDK1 mice are viable and fertile, and insulin injection induces the normal activation of PKB, S6K and RSK. Nevertheless, these mice are 40-50% smaller than control animals. The organ volumes from the PDK1 hypomorphic mice are reduced proportionately. We also establish that the volume of a number of PDK1-deficient cells is reduced by 35-60%, and show that PDK1 deficiency does not affect cell number, nuclear size or proliferation. We provide genetic evidence that PDK1 is essential for mouse embryonic development, and regulates cell size independently of cell number or proliferation, as well as insulin's ability to activate PKB, S6K and RSK.

Mammalian cell size is controlled by mTOR and its downstream targets S6K1 and 4EBP1/eIF4E

The coordinated action of cell cycle progression and cell growth (an increase in cell size and cell mass) is critical for sustained cellular proliferation, yet the biochemical signals that control cell growth are poorly defined, particularly in mammalian systems. We find that cell growth and cell cycle progression are separable processes in mammalian cells and that growth to appropriate cell size requires mTOR- and PI3K-dependent signals. Expression of a rapamycin-resistant mutant of mTOR rescues the reduced cell size phenotype induced by rapamycin in a kinase-dependent manner, showing the evolutionarily conserved role of mTOR in control of cell growth. Expression of S6K1 mutants that possess partial rapamycin-resistant activity or overexpression of eIF4E individually and additively partially rescues the rapamycin-induced decrease in cell size. In the absence of rapamycin, overexpression of S6K1 or eIF4E increases cell size, and, when coexpressed, they cooperate to increase cell size further. Expression of a phosphorylation site-defective mutant of 4EBP1 that constitutively binds the eIF4E-Cap complex to inhibit translation initiation reduces cell size and blocks eIF4E effects on cell size. These data show that mTOR signals downstream to at least two independent targets, S6K1 and 4EBP1/eIF4E, that function in translational control to regulate mammalian cell size.

The genetic basis of the interspecific differences in wing size in Nasonia (Hymenoptera; Pteromalidae): major quantitative trait loci and epistasis

There is a 2.5-fold difference in male wing size between two haplodiploid insect species, Nasonia vitripennis and N. giraulti. The haploidy of males facilitated a full genomic screen for quantitative trait loci (QTL) affecting wing size and the detection of epistatic interactions. A QTL analysis of the interspecific wing-size difference revealed QTL with major effects and epistatic interactions among loci affecting the trait. We analyzed 178 hybrid males and initially found two major QTL for wing length, one for wing width, three for a normalized wing-size variable, and five for wing seta density. One QTL for wing width explains 38.1% of the phenotypic variance, and the same QTL explains 22% of the phenotypic variance in normalized wing size. This corresponds to a region previously introgressed from N. giraulti into N. vitripennis that accounts for 44% of the normalized wing-size difference between the species. Significant epistatic interactions were also found that affect wing size and density of setae on the wing. Screening for pairwise epistatic interactions between loci on different linkage groups revealed four additional loci for wing length and four loci for normalized wing size that were not detected in the original QTL analysis. We propose that the evolution of smaller wings in N. vitripennis males is primarily the result of major mutations at few genomic regions and involves epistatic interactions among some loci.

Genetic analysis of insulin signaling in Drosophila

Studies in the fruit fly Drosophila melanogaster and the nematode Caenorhabditis elegans have revealed that components of the insulin signaling pathway have been highly conserved during evolution. Genetic analysis in Drosophila suggests that structural conservation also extends to the functional level. Flies carrying mutations that reduce insulin signaling have a growth deficiency phenotype similar to that seen in mice with disruptions of genes encoding insulin-like growth factors (IGFs) or the IGF-I receptor. Recent studies in flies have demonstrated a role for the insulin signaling pathway in the regulation of metabolism, reproduction and lifespan via modulation of central neuroendocrine pathways. Similarly, mice with loss of brain insulin receptors or insulin receptor substrate 2 deficiency exhibit neuroendocrine defects and female infertility. These parallels suggest that the insulin system has multiple conserved roles, acting directly to modulate growth and indirectly, via the neuroendocrine system, to modulate peripheral physiology in response to changes in nutrient availability.

Drosophila extracellular signal-regulated kinase involves the insulin-mediated proliferation of Schneider cells

The Drosophila insulin pathway is involved in the control of the proliferation and size of the cell. The stimulation of Schneider cells with human insulin has been observed to activate Drosophila extracellular signal regulated kinase (DERK). However, the role of DERK in the regulation of proliferation is unknown. In this study, we have identified a role of DERK in the proliferation of Drosophila Schneider cells. The inhibition of DERK activity by the overexpression of DMKP-3, an ERK-specific mitogen-activated protein kinase (MAPK) phosphatase, inhibited G(1) to S phase cell cycle progression as well as bromodeoxyuridine (BrdU) incorporation, which were previously increased by human insulin. However, DMKP-3 overexpression did not significantly reduce cell size that was also enlarged by insulin treatment, which suggests the specificity of the ERK pathway in proliferation but not for cell size. G1 to S phase cell cycle progression and BrdU incorporation were also reduced by catalytically inactive DMKP-3 mutant, and they may be acquired by the trapping of DERK into cytosol. The depletion of DERK or DMKP-3 by inhibitory double-stranded RNA decreased and increased BrdU incorporation, respectively. Thus, we propose that DERK is involved in the proliferation of Schneider cells via the insulin pathway.

Synthesis and function of 3-phosphorylated inositol lipids

The 3-phosphorylated inositol lipids fulfill roles as second messengers by interacting with the lipid binding domains of a variety of cellular proteins. Such interactions can affect the subcellular localization and aggregation of target proteins, and through allosteric effects, their activity. Generation of 3-phosphoinositides has been documented to influence diverse cellular pathways and hence alter a spectrum of fundamental cellular activities. This review is focused on the 3-phosphoinositide lipids, the synthesis of which is acutely triggered by extracellular stimuli, the enzymes responsible for their synthesis and metabolism, and their cell biological roles. Much knowledge has recently been gained through structural insights into the lipid kinases, their interaction with inhibitors, and the way their 3-phosphoinositide products interact with protein targets. This field is now moving toward a genetic dissection of 3-phosphoinositide action in a variety of model organisms. Such approaches will reveal the true role of the 3-phosphoinositides at the organismal level in health and disease.

dS6K-regulated cell growth is dPKB/dPI(3)K-independent, but requires dPDK1

Genetic studies in Drosophila melanogaster underscore the importance of the insulin-signalling pathway in controlling cell, organ and animal size. Effectors of this pathway include Chico (the insulin receptor substrate homologue), dPI(3)K, dPKB, dPTEN, and dS6K. Mutations in any of these components have a striking effect on cell size and number, with the exception of dS6K. Mutants in dS6K affect cell size but not cell number, seemingly consistent with arguments that dS6K is a distal effector in the signalling pathway, directly controlled by dTOR, a downstream effector of dPI(3)K and dPKB. Unexpectedly, recent studies showed that dS6K activity is unimpaired in chico-deficient larvae, suggesting that dS6K activation may be mediated through the dPI(3)K docking sites of the Drosophila insulin receptor. Here, we show genetically, pharmacologically and biochemically that dS6K resides on an insulin signalling pathway distinct from that of dPKB, and surprisingly also from that of dPI(3)K. More striking, despite dPKB-dPI(3)K-independence, dS6K activity is dependent on the Drosophila homologue of the phosphoinositide-dependent protein kinase 1, dPDK1, demonstrating that both dPDK1, as well as dTOR, mediated dS6K activation is phosphatidylinositide-3,4,5-trisphosphate (PIP3)-independent.

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

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

Temperature modulates epidermal cell size in Drosophila melanogaster

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

Drosophila's insulin/PI3-kinase pathway coordinates cellular metabolism with nutritional conditions

Studies in Drosophila have characterized insulin receptor/phosphoinositide 3-kinase (Inr/PI3K) signaling as a potent regulator of cell growth, but its function during development has remained uncertain. Here we show that inhibiting Inr/PI3K signaling phenocopies the cellular and organismal effects of starvation, whereas activating this pathway bypasses the nutritional requirement for cell growth, causing starvation sensitivity at the organismal level. Consistent with these findings, studies using a pleckstrin homology domain-green fluorescent protein (PH-GFP) fusion as an indicator for PI3K activity show that PI3K is regulated by the availability of dietary protein in vivo. Hence we surmise that an essential function of insulin/PI3K signaling in Drosophila is to coordinate cellular metabolism with nutritional conditions.

TheDrosophila melanogastergenebrain tumornegatively regulates cell growth and ribosomal RNA synthesis

The regulation of ribosome synthesis is likely to play an important role in the regulation of cell growth. Previously, we have shown that the ncl-1 gene in Caenorhabditis elegans functions as an inhibitor of cell growth and ribosome synthesis. We now indicate that the Drosophila melanogaster tumor suppressor brain tumor (brat) is an inhibitor of cell growth and is a functional homolog of the C. elegans gene ncl-1. The brat gene is able to rescue the large nucleolus phenotype of ncl-1 mutants. We also show that brat mutant cells are larger, have larger nucleoli, and have more ribosomal RNA than wild-type cells. Furthermore, brat overexpressing cells contain less ribosomal RNA than control cells. These results suggest that the tumorous phenotype of brat mutants may be due to excess cell growth and ribosome synthesis.

Control of growth and organ size in Drosophila

Transplantation experiments have shown that developing metazoan organs carry intrinsic information about their size and shape. Organ and body size are also sensitive to extrinsic cues provided by the environment, such as the availability of nutrients. The genetic and molecular pathways that contribute to animal size and shape are numerous, yet how they cooperate to control growth is mysterious. The recent identification and characterization of several mutations affecting growth in Drosophila melanogaster promises to provide insights. Many of these mutations affect the extrinsic control of animal size; others affect the organ-intrinsic control of pattern and size. In this review, we summarize the characteristics of some of these mutations and their roles in growth and size control. In addition, we speculate about possible connections between the extrinsic and intrinsic pathways controlling growth and pattern.

Regulation of Drosophila tracheal system development by protein kinase B

Protein kinase B (PKB, also termed Akt) is a phosphatidylinositol 3' kinase (PI3'K)-dependent enzyme implicated in survival signaling and human tumorigenesis. To identify potential targets of this protein kinase, we employed a genetic screen in Drosophila. Among several genes that genetically interacted with PKB was trachealess (trh), which encodes a bHLH-PAS domain transcription factor required for development of the trachea and other tubular organs. Trh activates expression of the fibroblast growth factor receptor Breathless, which, in turn, is required for directed migration of all tracheal branches. Using a combination of biochemical and transgenic approaches, we show that direct phosphorylation of Trh by PKB at serine 665 is essential for nuclear localization and functional activation of this regulator of branching morphogenesis.

Regulation of translation via TOR signaling: insights from Drosophila melanogaster

The target of rapamycin (TOR) proteins are large protein kinases evolutionarily conserved from yeast to human. A large body of evidence demonstrates that TOR proteins function in a nutrient-sensing checkpoint whose role is to restrict growth under conditions of low nutrient availability. Under such conditions, TOR blocks the transmission of growth-promoting signals from extracellular stimuli. Recent data obtained by genetic studies in the fruit fly Drosophila melanogaster demonstrate the importance of both insulin-like signaling and TOR signaling in promoting growth. Importantly, these studies identified a major downstream target of TOR and insulin-like signaling as the translational machinery.

Insulin signaling: lessons from the Drosophila tuberous sclerosis complex, a tumor suppressor

The genes that encode the proteins composing the tuberous sclerosis complex (TSC) are tumor suppressors. Experiments in the model organism Drosophila melanogaster have provided insight into the identity of these genes and their functions in regulating cell size and proliferation. Montagne et al. describe the various genetic interactions that show TSC to be a regulator of the insulin signaling pathway and a regulator of progression through the cell cycle, which explains its effects on cell size and tissue and tumor growth.

Akt1/PKBalpha is required for normal growth but dispensable for maintenance of glucose homeostasis in mice

The serine-threonine kinase Akt, also known as protein kinase B (PKB), is an important effector for phosphatidylinositol 3-kinase signaling initiated by numerous growth factors and hormones. Akt2/PKBbeta, one of three known mammalian isoforms of Akt/PKB, has been demonstrated recently to be required for at least some of the metabolic actions of insulin (Cho, H., Mu, J., Kim, J. K., Thorvaldsen, J. L., Chu, Q., Crenshaw, E. B., Kaestner, K. H., Bartolomei, M. S., Shulman, G. I., and Birnbaum, M. J. (2001) Science 292, 1728-1731). Here we show that mice deficient in another closely related isoform of the kinase, Akt1/PKBalpha, display a conspicuous impairment in organismal growth. Akt1(-/-) mice demonstrated defects in both fetal and postnatal growth, and these persisted into adulthood. However, in striking contrast to Akt2/PKBbeta null mice, Akt1/PKBalpha-deficient mice are normal with regard to glucose tolerance and insulin-stimulated disposal of blood glucose. Thus, the characterization of the Akt1 knockout mice and its comparison to the previously reported Akt2 deficiency phenotype reveals the non-redundant functions of Akt1 and Akt2 genes with respect to organismal growth and insulin-regulated glucose metabolism.

Cell survival and proliferation are modified by insulin-like growth factor 2 between days 9 and 10 of mouse gestation

The size of mammalian species involves the interaction of multiple genetic modifiers that control the timing and extent of growth mechanisms. Disruption of the paternal allele of the imprinted embryonic gene coding for insulin-like growth factor 2 (IGF2, Igf2+m/−p), results in viable mice that are 60% the weight of wild-type littermates. Differences in weight are first detected at embryonic day (E) 11, and the growth deficit is maintained throughout life. We report the mechanisms that account for this unusual phenotype. In order to quantify growth, we used novel methods to generate single cell suspensions of post-implantation mouse embryos. We were then able to quantify cell number, cell proliferation and cell death between E8.5 and E11.5 using flow cytometry. Determination of total embryo cell number also allowed us to time litters by a method other than by plugging. Wild-type and Igf2+m/−p embryos accumulated similar total cell numbers up to E9.25, but cell number began to diverge by around E9.5, with significant differences by E11 (75% of wild type). A relative increase in pyknotic nuclei, sub-GI cytometry counts and caspase activity, all indicative of cell death, occurred in Igf2+m/−p embryos at E9.25, reverting to wild-type levels by E9.75. This was followed at E9.75 by a significant reduction in the proportion of cells in S phase, quantified by S-phase cytometry counts and BrdU labelling. No significant differences in cell size were detected. We conclude that the majority of the cell number differences between wild-type and Igf2+m/−p mice can be accounted for by modification of cell survival and proliferation during the period (E9 to E10) of post-implantation development.

Regulation of pancreatic beta-cell growth and survival by the serine/threonine protein kinase Akt1/PKBalpha

The physiological performance of an organ depends on an interplay between changes in cellular function and organ size, determined by cell growth, proliferation and death. Nowhere is this more evident than in the endocrine pancreas, where disturbances in function or mass result in severe disease. Recently, the insulin signal-transduction pathway has been implicated in both the regulation of hormone secretion from beta cells in mammals as well as the determination of cell and organ size in Drosophila melanogaster. A prominent mediator of the actions of insulin and insulin-like growth factor 1 (IGF-1) is the 3'-phosphoinositide-dependent protein kinase Akt, also known as protein kinase B (PKB). Here we report that overexpression of active Akt1 in the mouse beta cell substantially affects compartment size and function. There was a significant increase in both beta-cell size and total islet mass, accompanied by improved glucose tolerance and complete resistance to experimental diabetes.

Extracellular control of cell size

Both cell growth (cell mass increase) and progression through the cell division cycle are required for sustained cell proliferation. Proliferating cells in culture tend to double in mass before each division, but it is not known how growth and division rates are co-ordinated to ensure that cell size is maintained. The prevailing view is that coordination is achieved because cell growth is rate-limiting for cell-cycle progression. Here, we challenge this view. We have investigated the relationship between cell growth and cell-cycle progression in purified rat Schwann cells, using two extracellular signal proteins that are known to influence these cells. We find that glial growth factor (GGF) can stimulate cell-cycle progression without promoting cell growth. We have used this restricted action of GGF to show that, for cultured Schwann cells, cell growth rate alone does not determine the rate of cell-cycle progression and that cell size at division is variable and depends on the concentrations of extracellular signal proteins that stimulate cell-cycle progression, cell growth, or both.