The protein phosphatase PP2A-B' subunit Widerborst is a negative regulator of cytoplasmic activated Akt and lipid metabolism in Drosophila

Decoding the connection between lncRNA and obesity: Perspective from humans and Drosophila

Background

Obesity is a burgeoning global health problem with an escalating prevalence and severe implications for public health. New evidence indicates that long non-coding RNAs (lncRNAs) may play a pivotal role in regulating adipose tissue function and energy homeostasis across various species. However, the molecular mechanisms underlying obesity remain elusive.

Scope of review

This review discusses obesity and fat metabolism in general, highlighting the emerging importance of lncRNAs in modulating adipogenesis. It describes the regulatory networks, latest tools, techniques, and approaches to enhance our understanding of obesity and its lncRNA-mediated epigenetic regulation in humans and Drosophila.

Major conclusions

This review analyses large datasets of human and Drosophila lncRNAs from published databases and literature with experimental evidence supporting lncRNAs role in fat metabolism. It concludes that lncRNAs play a crucial role in obesity-related metabolism. Cross-species comparisons highlight the relevance of Drosophila findings to human obesity, emphasizing their potential role in adipose tissue biology. Furthermore, it discusses how recent technological advancements and multi-omics data integration enhance our capacity to characterize lncRNAs and their function. Additionally, this review briefly touches upon innovative methodologies like experimental evolution and advanced sequencing technologies for identifying novel genes and lncRNA regulators in Drosophila, which can potentially contribute to obesity research.

Regulation and role of the PP2A-B56 holoenzyme family in cancer

Protein phosphatase 2A (PP2A) inactivation is common in cancer, leading to sustained activation of pro-survival and growth-promoting pathways. PP2A consists of a scaffolding A-subunit, a catalytic C-subunit, and a regulatory B-subunit. The functional complexity of PP2A holoenzymes arises mainly through the vast repertoire of regulatory B-subunits, which determine both their substrate specificity and their subcellular localization. Therefore, a major challenge for developing more effective therapeutic strategies for cancer is to identify the specific PP2A complexes to be targeted. Of note, the development of small molecules specifically directed at PP2A-B56α has opened new therapeutic avenues in both solid and hematological tumors. Here, we focus on the B56/PR61 family of PP2A regulatory subunits, which have a central role in directing PP2A tumor suppressor activity. We provide an overview of the mechanisms controlling the formation and regulation of these complexes, the pathways they control, and the mechanisms underlying their deregulation in cancer.

Visualizing Lipid Droplets in Drosophila Oogenesis

Lipid droplets (LDs) are fat storage organelles highly abundant in oocytes and eggs of many vertebrates and invertebrates. They have roles both during oogenesis and in provisioning the developing embryo. In Drosophila, large numbers of LDs are generated in nurse cells during mid-oogenesis and then transferred to oocytes. Their number and spatial distribution changes developmentally and in response to various experimental manipulations. This chapter demonstrates how to visualize LDs in Drosophila follicles, both in fixed tissues and living samples. For fixed samples, the protocol explains how to prepare female flies, dissect ovaries, isolate follicles, fix, apply stains, mount the tissue, and perform imaging. For live samples, the protocol shows how to dissect ovaries, apply a fluorescent LD dye, and culture follicles such that they remain alive and healthy during imaging. Finally, a method is provided that employs in vivo centrifugation to assess colocalization of markers with LDs.

Growth arrest of PPP2R5C and PPP2R5D double knockout mice indicates a genetic interaction and conserved function for these PP2A B subunits

Protein phosphatase 2A (PP2A) is a heterotrimeric phosphatase that controls a wide range of cellular functions. The catalytic activity and intracellular location of PP2A are modulated by its association with regulatory B subunits, including B56 proteins, which are encoded by five separate genes in humans and mice. The specific effects of each B56 protein on PP2A activity and function are largely unknown. As part of an effort to identify specific PP2A-B56 functions, we created knockout strains of B56β, B56δ, and B56ε using CRISPR/Cas9n. We found that none of the individual B56 genes are essential for mouse survival. However, mice that have both B56δ and B56γ inactivated (B56δγ-), arrest fetal development around Day E12. The hearts of B56δγ- mice have a single outflow vessel rather than having both an aorta and a pulmonary artery. Thus, there appears to be strong genetic interaction between B56δ and B56γ, and together they are necessary for heart development. Of note, both these proteins have been shown to localize to the nucleus and have the most related peptide sequences of the B56 family members. Our results suggest there are B56 subfamilies, which work in conjunction to regulate specific PP2A functions.

An overview of the insulin signaling pathway in model organisms Drosophila melanogaster and Caenorhabditis elegans

The insulin/insulin-like growth factor signaling pathway is an evolutionary conserved pathway across metazoans and is required for development, metabolism and behavior. This pathway is associated with various human metabolic disorders and cancers. Thus, model organisms including Drosophila melanogaster and Caenorhabditis elegans provide excellent opportunities to examine the structure and function of this pathway and its influence on cellular metabolism and proliferation. In this review, we will provide an overview of human insulin and the human insulin signaling pathway and explore the recent discoveries in model organisms Drosophila melanogaster and Caenorhabditis elegans. Our review will provide information regarding the various insulin-like peptides in model organisms as well as the conserved functions of insulin signaling pathways. Further investigation of the insulin signaling pathway in model organisms could provide a promising opportunity to develop novel therapies for various metabolic disorders and insulin-mediated cancers.

STRIPAK Members Orchestrate Hippo and Insulin Receptor Signaling to Promote Neural Stem Cell Reactivation

Adult stem cells reactivate from quiescence to maintain tissue homeostasis and in response to injury. How the underlying regulatory signals are integrated is largely unknown. Drosophila neural stem cells (NSCs) also leave quiescence to generate adult neurons and glia, a process that is dependent on Hippo signaling inhibition and activation of the insulin-like receptor (InR)/PI3K/Akt cascade. We performed a transcriptome analysis of individual quiescent and reactivating NSCs harvested directly from Drosophila brains and identified the conserved STRIPAK complex members mob4, cka, and PP2A (microtubule star, mts). We show that PP2A/Mts phosphatase, with its regulatory subunit Widerborst, maintains NSC quiescence, preventing premature activation of InR/PI3K/Akt signaling. Conversely, an increase in Mob4 and Cka levels promotes NSC reactivation. Mob4 and Cka are essential to recruit PP2A/Mts into a complex with Hippo kinase, resulting in Hippo pathway inhibition. We propose that Mob4/Cka/Mts functions as an intrinsic molecular switch coordinating Hippo and InR/PI3K/Akt pathways and enabling NSC reactivation.

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Transcriptional profiling of reactivating versus quiescent NSCs identifies STRIPAK members

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PP2A/Mts phosphatase inhibits Akt activation, maintaining NSC quiescence

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Mob4 and Cka target Mts to Hippo to inhibit its activity and promote NSC reactivation

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Mob4/Cka/Mts coordinate Hippo and InR/PI3K/Akt signaling in NSCs

Identification of genes underlying phenotypic plasticity of wing size via insulin signaling pathway by network-based analysis in Sogatella furcifera

Background

Phenotypic plasticity is a common and highly adaptive phenomenon where the same genotype produces different phenotypes in response to environmental cues. Sogatella furcifera, a migratory pest of rice exhibits wing dimorphism, is a model insect for studying phenotypic plasticity of wing size. The Insullin-PI3K-Akt-FOXO signaling pathway plays a crucial role in the manipulation of wing size in the migratory insects. However, the regulatory mechanism via the pathway involved in wing dimorphism are still unexplored.

Results

Accompanied by special alternative splicing, genes involved in muscle contraction and energy metabolism were highly expressed in the wing hinges of macropters, demonstrating their adaptation for energy-demanding long-distance flights. Based on FOXO ChIP-Seq analysis, a total of 1259 putative target genes were observed in the wing hinges, including wing morph development, flight muscle and energy metabolism genes. An integrated gene interaction network was built by combining four heterogeneous datasets, and the IIS-PI3K-Akt-FOXO pathway was clustered in a divided functional module. In total, 45 genes in the module directly interacting with the IIS-PI3K-Akt-FOXO pathway showed differential expression levels between the two wing hinges, thus are regarded as potential Insulin pathway mediated wing dimorphism related genes (IWDRGs). Of the 45 IWDRGs, 5 were selected for verification by gene knockdown experiments, and played significant roles in the insect wing size regulation.

Conclusions

We provided valuable insights on the genetic basis of wing dimorphism, and also demonstrated that network analysis is a powerful approach to identify new genes regulating wing dimorphic development via insulin signaling pathway in the migratory insect.

Electronic supplementary material

The online version of this article (10.1186/s12864-019-5793-z) contains supplementary material, which is available to authorized users.

A network of human functional gene interactions from knockout fitness screens in cancer cells

The function of human genes can be strongly inferred from their knockout fitness profiles across hundreds of CRISPR screens, illuminating the modular organization of the cell.

Genetic interactions mediate the emergence of phenotype from genotype. The systematic survey of genetic interactions in yeast showed that genes operating in the same biological process have highly correlated genetic interaction profiles, and this observation has been exploited to infer gene function in model organisms. Such assays of digenic perturbations in human cells are also highly informative, but are not scalable, even with CRISPR-mediated methods. As an alternative, we developed an indirect method of deriving functional interactions. We show that genes having correlated knockout fitness profiles across diverse, non-isogenic cell lines are analogous to genes having correlated genetic interaction profiles across isogenic query strains and similarly imply shared biological function. We constructed a network of genes with correlated fitness profiles across 276 high-quality CRISPR knockout screens in cancer cell lines into a “coessentiality network,” with up to 500-fold enrichment for co-functional gene pairs, enabling strong inference of gene function and highlighting the modular organization of the cell.

RABL6A inhibits tumor-suppressive PP2A/AKT signaling to drive pancreatic neuroendocrine tumor growth

Hyperactivated AKT/mTOR signaling is a hallmark of pancreatic neuroendocrine tumors (PNETs). Drugs targeting this pathway are used clinically, but tumor resistance invariably develops. A better understanding of factors regulating AKT/mTOR signaling and PNET pathogenesis is needed to improve current therapies. We discovered that RABL6A, a new oncogenic driver of PNET proliferation, is required for AKT activity. Silencing RABL6A caused PNET cell-cycle arrest that coincided with selective loss of AKT-S473 (not T308) phosphorylation and AKT/mTOR inactivation. Restoration of AKT phosphorylation rescued the G1 phase block triggered by RABL6A silencing. Mechanistically, loss of AKT-S473 phosphorylation in RABL6A-depleted cells was the result of increased protein phosphatase 2A (PP2A) activity. Inhibition of PP2A restored phosphorylation of AKT-S473 in RABL6A-depleted cells, whereas PP2A reactivation using a specific small-molecule activator of PP2A (SMAP) abolished that phosphorylation. Moreover, SMAP treatment effectively killed PNET cells in a RABL6A-dependent manner and suppressed PNET growth in vivo. The present work identifies RABL6A as a new inhibitor of the PP2A tumor suppressor and an essential activator of AKT in PNET cells. Our findings offer what we believe is a novel strategy of PP2A reactivation for treatment of PNETs as well as other human cancers driven by RABL6A overexpression and PP2A inactivation.

Regulation of the phosphoprotein phosphatase 2A system and its modulation during oxidative stress: A potential therapeutic target?

Phosphoprotein phosphatases are of growing interest in the pathophysiology of many diseases and are often the neglected partner of protein kinases. One family member, PP2A, accounts for dephosphorylation of ~55-70% of all serine/threonine phosphosites. Interestingly, dysregulation of kinase signalling is a hallmark of many diseases in which an increase in oxidative stress is also noted. With this in mind, we assess the evidence to support oxidative stress-mediated regulation of the PP2A system In this article, we first present an overview of the PP2A system before providing an analysis of the regulation of PP2A by endogenous inhibitors, post translational modification, and miRNA. Next, a detailed critique of data implicating reactive oxygen species, ischaemia, ischaemia-reperfusion, and hypoxia in regulating the PP2A holoenzyme and associated regulators is presented. Finally, the pharmacological targeting of PP2A, its endogenous inhibitors, and enzymes responsible for its post-translational modification are covered. There is extensive evidence that oxidative stress modulates multiple components of the PP2A system, however, most of the data pertains to the catalytic subunit of PP2A. Irrespective of the underlying aetiology, free radical-mediated attenuation of PP2A activity is an emerging theme. However, in many instances, a dichotomy exists, which requires clarification and mechanistic insight. Nevertheless, this raises the possibility that pharmacological activation of PP2A, either through small molecule activators of PP2A or CIP2A/SET antagonists may be beneficial in modulating the cellular response to oxidative stress. A better understanding of which, will have wide ranging implications for cancer, heart disease and inflammatory conditions.

Regulation of FOXO Factors in Mammalian Cells

Forkhead box O (FOXO) transcription factors are central regulators of cellular homeostasis. FOXOs respond to a wide range of external stimuli, including growth factor signaling, oxidative stress, genotoxic stress, and nutrient deprivation. These signaling inputs regulate FOXOs through a number of posttranslational modifications, including phosphorylation, acetylation, ubiquitination, and methylation. Covalent modifications can affect localization, DNA binding, and interactions with other cofactors in the cell. FOXOs integrate the various modifications to regulate cell type-specific gene expression programs that are essential for metabolic homeostasis, redox balance, and the stress response. Together, these functions are critical for coordinating a response to environmental fluctuations in order to maintain cellular homeostasis during development and to support healthy aging.

A Kinome RNAi Screen in Drosophila Identifies Novel Genes Interacting with Lgl, aPKC, and Crb Cell Polarity Genes in Epithelial Tissues

In both Drosophila melanogaster and mammalian systems, epithelial structure and underlying cell polarity are essential for proper tissue morphogenesis and organ growth. Cell polarity interfaces with multiple cellular processes that are regulated by the phosphorylation status of large protein networks. To gain insight into the molecular mechanisms that coordinate cell polarity with tissue growth, we screened a boutique collection of RNAi stocks targeting the kinome for their capacity to modify Drosophila “cell polarity” eye and wing phenotypes. Initially, we identified kinase or phosphatase genes whose depletion modified adult eye phenotypes associated with the manipulation of cell polarity complexes (via overexpression of Crb or aPKC). We next conducted a secondary screen to test whether these cell polarity modifiers altered tissue overgrowth associated with depletion of Lgl in the wing. These screens identified Hippo, Jun kinase (JNK), and Notch signaling pathways, previously linked to cell polarity regulation of tissue growth. Furthermore, novel pathways not previously connected to cell polarity regulation of tissue growth were identified, including Wingless (Wg/Wnt), Ras, and lipid/Phospho-inositol-3-kinase (PI3K) signaling pathways. Additionally, we demonstrated that the “nutrient sensing” kinases Salt Inducible Kinase 2 and 3 (SIK2 and 3) are potent modifiers of cell polarity phenotypes and regulators of tissue growth. Overall, our screen has revealed novel cell polarity-interacting kinases and phosphatases that affect tissue growth, providing a platform for investigating molecular mechanisms coordinating cell polarity and tissue growth during development.

TGF-β signaling regulates p-Akt levels via PP2A during diapause entry in the cotton bollworm, Helicoverpa armigera

Akt, which is a key kinase in the insulin signaling pathway, plays important roles in glucose metabolism, cell proliferation, transcription and cell migration. Our previous studies indicated that low insulin levels and high p-Akt levels are present in diapause-destined individuals. Here, we show that PI3K, which is upstream of Akt, is low in diapause-destined pupal brains but high in p-Akt levels, implying that p-Akt is modified by factors other than the insulin signaling pathway. Protein phosphatase 2A (PP2A), which is a key regulator in the TGF-β signaling pathway, can directly bind to and dephosphorylate Akt. Low PP2A expression and activity in diapause-destined individuals suggest that a weak Akt dephosphorylation contributes to p-Akt accumulation. In addition, transforming growth factor-β receptor I (TβRI), which is upstream of PP2A, increases the activity of PP2A and decreases the p-Akt levels. These results show that TGF-β signaling decreases p-Akt levels by increasing the activity of PP2A. This is the first report showing that TGF-β signaling negatively regulates the insulin pathway in insect development or diapause.

mTORC1 signalling mediates PI3K-dependent large lipid droplet accumulation in Drosophila ovarian nurse cells

Insulin and insulin-like growth factor signalling (IIS), which is primarily mediated by the PI3-kinase (PI3K)/PTEN/Akt kinase signalling cassette, is a highly evolutionarily conserved pathway involved in co-ordinating growth, development, ageing and nutrient homeostasis with dietary intake. It controls transcriptional regulators, in addition to promoting signalling by mechanistic target of rapamycin (mTOR) complex 1 (mTORC1), which stimulates biosynthesis of proteins and other macromolecules, and drives organismal growth. Previous studies in nutrient-storing germline nurse cells of the Drosophila ovary showed that a cytoplasmic pool of activated phosphorylated Akt (pAkt) controlled by Pten, an antagonist of IIS, cell-autonomously regulates accumulation of large lipid droplets in these cells at late stages of oogenesis. Here, we show that the large lipid droplet phenotype induced by Pten mutation is strongly suppressed when mTor function is removed. Furthermore, nurse cells lacking either Tsc1 or Tsc2, which negatively regulate mTORC1 activity, also accumulate large lipid droplets via a mechanism involving Rheb, the downstream G-protein target of TSC2, which positively regulates mTORC1. We conclude that elevated IIS/mTORC1 signalling is both necessary and sufficient to induce large lipid droplet formation in late-stage nurse cells, suggesting roles for this pathway in aspects of lipid droplet biogenesis, in addition to control of lipid metabolism.

Protein phosphatase 2A regulatory subunit B56β modulates erythroid differentiation

Anemia due to attenuated erythroid terminal differentiation is one of the most common hematological disorders occurring at all stages of life. We previously demonstrated that catalytic subunit α of protein phosphatase 2A (PP2Acα) modulates fetal liver erythropoiesis. However the corresponding PP2A regulatory subunit in this process remains unknown. In this study, we report that chemical inhibition of PP2A activity with okadaic acid impairs hemin-induced erythroid differentiation. Interestingly, B56 family member B56β is the only regulatory subunit whose expression is induced by both erythropoietin in fetal liver cells and hemin in erythroleukemia K562 cells. Finally, knockdown of B56β attenuates hemin-induced K562 erythroid differentiation. Collectively, our data identify B56β as the potential functional regulatory subunit of PP2A in erythroid differentiation, shedding light on new target for precise modulation of PP2A activity for treatment of anemia and related diseases.

A triangular connection between Cyclin G, PP2A and Akt1 in the regulation of growth and metabolism in Drosophila

Size and weight control is a tightly regulated process, involving the highly conserved Insulin receptor/target of rapamycin (InR/TOR) signaling cascade. We recently identified Cyclin G (CycG) as an important modulator of InR/TOR signaling activity in Drosophila. cycG mutant flies are underweight and show a disturbed fat metabolism resembling TOR mutants. In fact, InR/TOR signaling activity is disturbed in cycG mutants at the level of Akt1, the central kinase linking InR and TORC1. Akt1 is negatively regulated by protein phosphatase PP2A. Notably the binding of the PP2A B′-regulatory subunit Widerborst (Wdb) to Akt1 is differentially regulated in cycG mutants, presumably by a direct interaction of CycG and Wdb. Since the metabolic defects of cycG mutant animals are abrogated by a concomitant loss of Wdb, CycG presumably influences Akt1 activity at the PP2A nexus. Here we show that Well rounded (Wrd), another B' subunit of PP2A in Drosophila, binds CycG similar to Wdb, and that its loss ameliorates some, but not all, of the metabolic defects of cycG mutants. We propose a model, whereby the binding of CycG to a particular B′-regulatory subunit influences the tissue specific activity of PP2A, required for the fine tuning of the InR/TOR signaling cascade in Drosophila.

Drosophila Cyclin G Is a Regulator of the Notch Signalling Pathway during Wing Development

Notch signalling regulates a multitude of differentiation processes during Drosophila development. For example, Notch activity is required for proper wing vein differentiation which is hampered in mutants of either the receptor Notch, the ligand Delta or the antagonist Hairless. Moreover, the Notch pathway is involved in several aspects of Drosophila oogenesis as well. We have identified Drosophila Cyclin G (CycG) as a molecular interaction partner of Hairless, the major antagonist in the Notch signalling pathway, in vitro and in vivo. Loss of CycG was shown before to cause female sterility and to disturb the architecture of the egg shell. Nevertheless, Notch dependent processes during oogenesis appeared largely unaffected in cycG mutant egg chambers. Loss of CycG modified the dominant wing phenotypes of Notch, Delta and Hairless mutants. Whereas the Notch loss of function phenotype was ameliorated by a loss of CycG, the phenotypes of either Notch gain of function or of Delta or Hairless loss of function were enhanced. In contrast, loss of CycG had only a minor effect on the wing vein phenotype of mutants affecting the EGFR signalling pathway emphasizing the specificity of the interaction of CycG and Notch pathway members.

Suggested Involvement of PP1/PP2A Activity and De Novo Gene Expression in Anhydrobiotic Survival in a Tardigrade, Hypsibius dujardini, by Chemical Genetic Approach

Upon desiccation, some tardigrades enter an ametabolic dehydrated state called anhydrobiosis and can survive a desiccated environment in this state. For successful transition to anhydrobiosis, some anhydrobiotic tardigrades require pre-incubation under high humidity conditions, a process called preconditioning, prior to exposure to severe desiccation. Although tardigrades are thought to prepare for transition to anhydrobiosis during preconditioning, the molecular mechanisms governing such processes remain unknown. In this study, we used chemical genetic approaches to elucidate the regulatory mechanisms of anhydrobiosis in the anhydrobiotic tardigrade, Hypsibius dujardini. We first demonstrated that inhibition of transcription or translation drastically impaired anhydrobiotic survival, suggesting that de novo gene expression is required for successful transition to anhydrobiosis in this tardigrade. We then screened 81 chemicals and identified 5 chemicals that significantly impaired anhydrobiotic survival after severe desiccation, in contrast to little or no effect on survival after high humidity exposure only. In particular, cantharidic acid, a selective inhibitor of protein phosphatase (PP) 1 and PP2A, exhibited the most profound inhibitory effects. Another PP1/PP2A inhibitor, okadaic acid, also significantly and specifically impaired anhydrobiotic survival, suggesting that PP1/PP2A activity plays an important role for anhydrobiosis in this species. This is, to our knowledge, the first report of the required activities of signaling molecules for desiccation tolerance in tardigrades. The identified inhibitory chemicals could provide novel clues to elucidate the regulatory mechanisms underlying anhydrobiosis in tardigrades.

Fine-Tuning of PI3K/AKT Signalling by the Tumour Suppressor PTEN Is Required for Maintenance of Flight Muscle Function and Mitochondrial Integrity in Ageing Adult Drosophila melanogaster

Insulin/insulin-like growth factor signalling (IIS), acting primarily through the PI3-kinase (PI3K)/AKT kinase signalling cassette, plays key evolutionarily conserved regulatory roles in nutrient homeostasis, growth, ageing and longevity. The dysfunction of this pathway has been linked to several age-related human diseases including cancer, Type 2 diabetes and neurodegenerative disorders. However, it remains unclear whether minor defects in IIS can independently induce the age-dependent functional decline in cells that accompany some of these diseases or whether IIS alters the sensitivity to other aberrant signalling. We identified a novel hypomorphic allele of PI3K’s direct antagonist, Phosphatase and tensin homologue on chromosome 10 (Pten), in the fruit fly, Drosophila melanogaster. Adults carrying combinations of this allele, Pten⁵, combined with strong loss-of-function Pten mutations exhibit subtle or no increase in mass, but are highly susceptible to a wide range of stresses. They also exhibit dramatic upregulation of the oxidative stress response gene, GstD1, and a progressive loss of motor function that ultimately leads to defects in climbing and flight ability. The latter phenotype is associated with mitochondrial disruption in indirect flight muscles, although overall muscle structure appears to be maintained. We show that the phenotype is partially rescued by muscle-specific expression of the Bcl-2 homologue Buffy, which in flies, maintains mitochondrial integrity, modulates energy homeostasis and suppresses cell death. The flightless phenotype is also suppressed by mutations in downstream IIS signalling components, including those in the mechanistic Target of Rapamycin Complex 1 (mTORC1) pathway, suggesting that elevated IIS is responsible for functional decline in flight muscle. Our data demonstrate that IIS levels must be precisely regulated by Pten in adults to maintain the function of the highly metabolically active indirect flight muscles, offering a new system to study the in vivo roles of IIS in the maintenance of mitochondrial integrity and adult ageing.

Cyclin G Functions as a Positive Regulator of Growth and Metabolism in Drosophila

In multicellular organisms, growth and proliferation is adjusted to nutritional conditions by a complex signaling network. The Insulin receptor/target of rapamycin (InR/TOR) signaling cascade plays a pivotal role in nutrient dependent growth regulation in Drosophila and mammals alike. Here we identify Cyclin G (CycG) as a regulator of growth and metabolism in Drosophila. CycG mutants have a reduced body size and weight and show signs of starvation accompanied by a disturbed fat metabolism. InR/TOR signaling activity is impaired in cycG mutants, combined with a reduced phosphorylation status of the kinase Akt1 and the downstream factors S6-kinase and eukaryotic translation initiation factor 4E binding protein (4E-BP). Moreover, the expression and accumulation of Drosophila insulin like peptides (dILPs) is disturbed in cycG mutant brains. Using a reporter assay, we show that the activity of one of the first effectors of InR signaling, Phosphoinositide 3-kinase (PI3K92E), is unaffected in cycG mutants. However, the metabolic defects and weight loss in cycG mutants were rescued by overexpression of Akt1 specifically in the fat body and by mutants in widerborst (wdb), the B'-subunit of the phosphatase PP2A, known to downregulate Akt1 by dephosphorylation. Together, our data suggest that CycG acts at the level of Akt1 to regulate growth and metabolism via PP2A in Drosophila.

Size and growth of an organism are adjusted to nutritional conditions by a complex regulatory network involving the Insulin receptor and TOR signaling cascades. Drosophila melanogaster has been used in the past as a genetically tractable model to unravel the complex circuitry by genetic means. We have identified CycG as an important player in the regulation of TOR signaling. CycG mutants are underweight in the midst of food and show typical signs of TOR defects. We provide evidence that CycG acts at the level of Akt1 kinase that links the Insulin receptor and TOR signaling cascades. Molecular and genetic data point to an interplay of CycG and phosphatase PP2A, a well established negative regulator of Akt1 activity. Moreover, CycG may influence PP2A-Akt1 binding. We propose that CycG, by impeding PP2A-Akt1 interaction, acts as a positive regulator of growth in Drosophila.

Ciliary transport regulates PDGF-AA/αα signaling via elevated mammalian target of rapamycin signaling and diminished PP2A activity

Primary cilia are built and maintained by intraflagellar transport (IFT), whereby the two IFT complexes, IFTA and IFTB, carry cargo via kinesin and dynein motors for anterograde and retrograde transport, respectively. Many signaling pathways, including platelet- derived growth factor (PDGF)-AA/αα, are linked to primary cilia. Active PDGF-AA/αα signaling results in phosphorylation of Akt at two residues: P-Akt(T308) and P-Akt(S473), and previous work showed decreased P-Akt(S473) in response to PDGF-AA upon anterograde transport disruption. In this study, we investigated PDGF-AA/αα signaling via P-Akt(T308) and P-Akt(S473) in distinct ciliary transport mutants. We found increased Akt phosphorylation in the absence of PDGF-AA stimulation, which we show is due to impaired dephosphorylation resulting from diminished PP2A activity toward P-Akt(T308). Anterograde transport mutants display low platelet-derived growth factor receptor (PDGFR)α levels, whereas retrograde mutants exhibit normal PDGFRα levels. Despite this, neither shows an increase in P-Akt(S473) or P-Akt(T308) upon PDGF-AA stimulation. Because mammalian target of rapamycin complex 1 (mTORC1) signaling is increased in ciliary transport mutant cells and mTOR signaling inhibits PDGFRα levels, we demonstrate that inhibition of mTORC1 rescues PDGFRα levels as well as PDGF-AA-dependent phosphorylation of Akt(S473) and Akt(T308) in ciliary transport mutant MEFs. Taken together, our data indicate that the regulation of mTORC1 signaling and PP2A activity by ciliary transport plays key roles in PDGF-AA/αα signaling.

The protein phosphatase 2A B56γ regulatory subunit is required for heart development

BACKGROUND

Protein Phosphatase 2A (PP2A) function is controlled by regulatory subunits that modulate the activity of the catalytic subunit and direct the PP2A complex to specific intracellular locations. To study PP2A's role in signal transduction pathways that control growth and differentiation in vivo, a transgenic mouse lacking the B56γ regulatory subunit of PP2A was made.

RESULTS

Lack of PP2A activity specific to the PP2A-B56γ holoenzyme, resulted in the formation of an incomplete ventricular septum and a decrease in the number of ventricular cardiomyocytes. During cardiac development, B56γ is expressed in the nucleus of α-actinin-positive cardiomyocytes that contain Z-bands. The pattern of B56γ expression correlated with the cardiomyocyte apoptosis we observed in B56γ-deficient mice during mid to late gestation. In addition to the cardiac phenotypes, mice lacking B56γ have a decrease in locomotive coordination and gripping strength, indicating that B56γ has a role in controlling PP2A activity required for efficient neuromuscular function.

CONCLUSIONS

PP2A-B56γ activity is required for efficient cardiomyocyte maturation and survival. The PP2A B56γ regulatory subunit controls PP2A substrate specificity in vivo in a manner that cannot be fully compensated for by other B56 subunits.

Insulin signaling and the regulation of insect diapause

A rich chapter in the history of insect endocrinology has focused on hormonal control of diapause, especially the major roles played by juvenile hormones (JHs), ecdysteroids, and the neuropeptides that govern JH and ecdysteroid synthesis. More recently, experiments with adult diapause in Drosophila melanogaster and the mosquito Culex pipiens, and pupal diapause in the flesh fly Sarcophaga crassipalpis provide strong evidence that insulin signaling is also an important component of the regulatory pathway leading to the diapause phenotype. Insects produce many different insulin-like peptides (ILPs), and not all are involved in the diapause response; ILP-1 appears to be the one most closely linked to diapause in C. pipiens. Many steps in the pathway leading from perception of daylength (the primary environmental cue used to program diapause) to generation of the diapause phenotype remain unknown, but the role for insulin signaling in mosquito diapause appears to be upstream of JH, as evidenced by the fact that application of exogenous JH can rescue the effects of knocking down expression of ILP-1 or the Insulin Receptor. Fat accumulation, enhancement of stress tolerance, and other features of the diapause phenotype are likely linked to the insulin pathway through the action of a key transcription factor, FOXO. This review highlights many parallels for the role of insulin signaling as a regulator in insect diapause and dauer formation in the nematode Caenorhabditis elegans.

A targeted genetic modifier screen links the SWI2/SNF2 protein domino to growth and autophagy genes in Drosophila melanogaster

Targeted genetic studies can facilitate phenotypic analyses and provide important insights into development and other complex processes. The SWI2/SNF2 DNA-dependent ATPase Domino (Dom) of Drosophila melanogaster, a component of the Tip60 acetyltransferase complex, has been associated with a wide spectrum of cellular processes at multiple developmental stages. These include hematopoiesis, cell proliferation, homeotic gene regulation, histone exchange during DNA repair, and Notch signaling. To explore the wider gene network associated with Dom action, we used RNAi directed against domino (dom) to mediate loss-of-function at the wing margin, a tissue that is readily scored for phenotypic changes. Dom RNAi driven through GAL4-UAS elicited dominant wing nicking that responded phenotypically to the dose of dom and other loci known to function with dom. We screened for phenotypic modifiers of this wing phenotype among 2500 transpositions of the EP P element and found both enhancers and suppressors. Several classes of modifier were obtained, including those encoding transcription factors, RNA regulatory proteins, and factors that regulate cell growth, proliferation and autophagy, a lysosomal degradation pathway that affects cell growth under conditions of starvation and stress. Our analysis is consistent with prior studies, suggesting that Dom acts pleiotropically as a positive effector of Notch signaling and a repressor of proliferation. This genetic system should facilitate screens for additional loci associated with Dom function, and complement biochemical approaches to their regulatory activity.

Proton-assisted amino acid transporter PAT1 complexes with Rag GTPases and activates TORC1 on late endosomal and lysosomal membranes

Mammalian Target of Rapamycin Complex 1 (mTORC1) is activated by growth factor-regulated phosphoinositide 3-kinase (PI3K)/Akt/Rheb signalling and extracellular amino acids (AAs) to promote growth and proliferation. These AAs induce translocation of mTOR to late endosomes and lysosomes (LELs), subsequent activation via mechanisms involving the presence of intralumenal AAs, and interaction between mTORC1 and a multiprotein assembly containing Rag GTPases and the heterotrimeric Ragulator complex. However, the mechanisms by which AAs control these different aspects of mTORC1 activation are not well understood. We have recently shown that intracellular Proton-assisted Amino acid Transporter 1 (PAT1)/SLC36A1 is an essential mediator of AA-dependent mTORC1 activation. Here we demonstrate in Human Embryonic Kidney (HEK-293) cells that PAT1 is primarily located on LELs, physically interacts with the Rag GTPases and is required for normal AA-dependent mTOR relocalisation. We also use the powerful in vivo genetic methodologies available in Drosophila to investigate the regulation of the PAT1/Rag/Ragulator complex. We show that GFP-tagged PATs reside at both the cell surface and LELs in vivo, mirroring PAT1 distribution in several normal mammalian cell types. Elevated PI3K/Akt/Rheb signalling increases intracellular levels of PATs and synergistically enhances PAT-induced growth via a mechanism requiring endocytosis. In light of the recent identification of the vacuolar H⁺-ATPase as another Rag-interacting component, we propose a model in which PATs function as part of an AA-sensing engine that drives mTORC1 activation from LEL compartments.

PP2A regulates autophagy in two alternative ways in Drosophila

Protein phosphatase 2A (PP2A) holoenzyme is a heterotrimeric complex, consisting of A, B and C subunits. The catalytic subunit PP2A-C (microtubule star/mts) binds to the C-terminal part of the scaffold protein PP2A-A (PP2A-29B). In Drosophila, there are three different forms of B subunits (widerborst/wdb, twins/tws and PP2A-B'), which determine the subcellular localization and substrate specificity of the holoenzyme. Previous studies demonstrated that PP2A is involved in the control of TOR-dependent autophagy both in yeast and mammals. Furthermore, in Drosophila, wdb genetically interacts with the PtdIns3K/PTEN/Akt signaling cascade, which is a main upstream regulatory system of dTOR. Here we demonstrate that in Drosophila, two different PP2A complexes (containing B' or wdb subunit) play essential roles in the regulation of starvation-induced autophagy. The PP2A-A/wdb/C complex acts upstream of dTOR, whereas the PP2A-A/B'/C complex functions as a target of dTOR and may regulate the elongation of autophagosomes and their subsequent fusion with lysosomes. We also identified three Drosophila Atg orthologs (Atg14, Atg17 and Atg101), which represent potential targets of the PP2A-A/B'/C complex during autophagy.

Phosphatases: the new brakes for cancer development?

The phosphatidylinositol 3-kinase (PI3K) pathway plays a pivotal role in the maintenance of processes such as cell growth, proliferation, survival, and metabolism in all cells and tissues. Dysregulation of the PI3K/Akt signaling pathway occurs in patients with many cancers and other disorders. This aberrant activation of PI3K/Akt pathway is primarily caused by loss of function of all negative controllers known as inositol polyphosphate phosphatases and phosphoprotein phosphatases. Recent studies provided evidence of distinct functions of the four main phosphatases—phosphatase and tensin homologue deleted on chromosome 10 (PTEN), Src homology 2-containing inositol 5′-phosphatase (SHIP), inositol polyphosphate 4-phosphatase type II (INPP4B), and protein phosphatase 2A (PP2A)—in different tissues with respect to regulation of cancer development. We will review the structures and functions of PTEN, SHIP, INPP4B, and PP2A phosphatases in suppressing cancer progression and their deregulation in cancer and highlight recent advances in our understanding of the PI3K/Akt signaling axis.

Resistance of Akt kinases to dephosphorylation through ATP-dependent conformational plasticity

Phosphorylation of a threonine residue (T308 in Akt1) in the activation loop of Akt kinases is a prerequisite for deregulated Akt activity frequently observed in neoplasia. Akt phosphorylation in vivo is balanced by the opposite activities of kinases and phosphatases. Here we describe that targeting Akt kinase to the cell membrane markedly reduced sensitivity of phosphorylated Akt to dephosphorylation by protein phosphatase 2A. This effect was amplified by occupancy of the ATP binding pocket by either ATP or ATP-competitive inhibitors. Mutational analysis revealed that R273 in Akt1 and the corresponding R274 in Akt2 are essential for shielding T308 in the activation loop against dephosphorylation. Thus, occupancy of the nucleotide binding pocket of Akt kinases enables intramolecular interactions that restrict phosphatase access and sustain Akt phosphorylation. This mechanism provides an explanation for the "paradoxical" Akt hyperphosphorylation induced by ATP-competitive inhibitor, A-443654. The lack of phosphatase resistance further contributes insight into the mechanism by which the human Akt2 R274H missense mutation may cause autosomal-dominant diabetes mellitus.

Functional genomic screen and network analysis reveal novel modifiers of tauopathy dissociated from tau phosphorylation

A functional genetic screen using loss-of-function and gain-of-function alleles was performed to identify modifiers of tau-induced neurotoxicity using the 2N/4R (full-length) isoform of wild-type human tau expressed in the fly retina. We previously reported eye pigment mutations, which create dysfunctional lysosomes, as potent modifiers; here, we report 37 additional genes identified from ∼1900 genes screened, including the kinases shaggy/GSK-3beta, par-1/MARK, CamKI and Mekk1. Tau acts synergistically with Mekk1 and p38 to down-regulate extracellular regulated kinase activity, with a corresponding decrease in AT8 immunoreactivity (pS202/T205), suggesting that tau can participate in signaling pathways to regulate its own kinases. Modifiers showed poor correlation with tau phosphorylation (using the AT8, 12E8 and AT270 epitopes); moreover, tested suppressors of wild-type tau were equally effective in suppressing toxicity of a phosphorylation-resistant S11A tau construct, demonstrating that changes in tau phosphorylation state are not required to suppress or enhance its toxicity. Genes related to autophagy, the cell cycle, RNA-associated proteins and chromatin-binding proteins constitute a large percentage of identified modifiers. Other functional categories identified include mitochondrial proteins, lipid trafficking, Golgi proteins, kinesins and dynein and the Hsp70/Hsp90-organizing protein (Hop). Network analysis uncovered several other genes highly associated with the functional modifiers, including genes related to the PI3K, Notch, BMP/TGF-β and Hedgehog pathways, and nuclear trafficking. Activity of GSK-3β is strongly upregulated due to TDP-43 expression, and reduced GSK-3β dosage is also a common suppressor of Aβ42 and TDP-43 toxicity. These findings suggest therapeutic targets other than mitigation of tau phosphorylation.

PP2A targeting by viral proteins: a widespread biological strategy from DNA/RNA tumor viruses to HIV-1

Protein phosphatase 2A (PP2A) is a large family of holoenzymes that comprises 1% of total cellular proteins and accounts for the majority of Ser/Thr phosphatase activity in eukaryotic cells. Although initially viewed as constitutive housekeeping enzymes, it is now well established that PP2A proteins represent a family of highly and sophistically regulated phosphatases. The past decade, multiple complementary studies have improved our knowledge about structural and functional regulation of PP2A holoenzymes. In this regard, after summarizing major cellular regulation, this review will mainly focus on discussing a particulate biological strategy, used by various viruses, which is based on the targeting of PP2A enzymes by viral proteins in order to specifically deregulate, for their own benefit, cellular pathways of their hosts. The impact of such PP2A targeting for research in human diseases, and in further therapeutic developments, is also discussed.

Clk2 and B56β mediate insulin-regulated assembly of the PP2A phosphatase holoenzyme complex on Akt

Akt mediates important cellular decisions involved in growth, survival, and metabolism. The mechanisms by which Akt is phosphorylated and activated in response to growth factors or insulin have been extensively studied, but the molecular regulatory components and dynamics of Akt attenuation are poorly understood. Here we show that a downstream target of insulin-induced Akt activation, Clk2, triggers Akt dephosphorylation through the PP2A phosphatase complex. Clk2 phosphorylates the PP2A regulatory subunit B56β (PPP2R5B, B'β), which is a critical regulatory step in the assembly of the PP2A holoenzyme complex on Akt leading to dephosphorylation of both S473 and T308 Akt sites. Since Akt plays a pivotal role in cellular signaling, these results have important implications for our understanding of Akt regulation in many biological processes.

A gain-of-function screen identifies wdb and lkb1 as lifespan-extending genes in Drosophila

The insulin/insulin-like growth factor (IGF) and the target of rapamycin (TOR) signaling pathways are known to regulate lifespan in diverse organisms. However, only a limited number of genes involved in these pathways have been examined regarding their effects on lifespan. Through a gain-of-function screen in Drosophila, we found that overexpression of the wdb gene encoding a regulatory subunit of PP2A, and overexpression of the lkb1 gene encoding a serine/threonine kinase, reduced organ size and extended lifespan. Overexpression of wdb also reduced the level of phosphorylated AKT, while overexpression of lkb1 increased the level of phosphorylated AMPK and decreased the level of phosphorylated S6K. Taken together, our results suggest that wdb- and lkb1-dependent lifespan extension is mediated by downregulation of S6K, a downstream component of the insulin/IGF and TOR signaling pathways.

Quantitative analysis of lipid droplet fusion: inefficient steady state fusion but rapid stimulation by chemical fusogens

Lipid droplets (LDs) are dynamic cytoplasmic organelles containing neutral lipids and bounded by a phospholipid monolayer. Previous studies have suggested that LDs can undergo constitutive homotypic fusion, a process linked to the inhibitory effects of fatty acids on glucose transporter trafficking. Using strict quantitative criteria for LD fusion together with refined light microscopic methods and real-time analysis, we now show that LDs in diverse cell types show low constitutive fusogenic activity under normal growth conditions. To investigate the possible modulation of LD fusion, we screened for agents that can trigger fusion. A number of pharmacological agents caused homotypic fusion of lipid droplets in a variety of cell types. This provided a novel cell system to study rapid regulated fusion between homotypic phospholipid monolayers. LD fusion involved an initial step in which the two adjacent membranes became continuous (<10 s), followed by the slower merging (100 s) of the neutral lipid cores to produce a single spherical LD. These fusion events were accompanied by changes to the LD surface organization. Measurements of LDs undergoing homotypic fusion showed that fused LDs maintained their initial volume, with a corresponding decrease in surface area suggesting rapid removal of membrane from the fused LD. This study provides estimates for the level of constitutive LD fusion in cells and questions the role of LD fusion in vivo. In addition, it highlights the extent of LD restructuring which occurs when homotypic LD fusion is triggered in a variety of cell types.

Functions of B56-containing PP2As in major developmental and cancer signaling pathways

Members of the B'/B56/PR61 family regulatory subunits of PP2A determine the subcellular localization, substrate specificity, and catalytic activity of PP2A in a wide range of biological processes. Here, we summarize the structure and intracellular localization of B56-containing PP2As and review functions of B56-containing PP2As in several major developmental/cancer signaling pathways.

PP2A regulatory subunit PP2A-B' counteracts S6K phosphorylation

The insulin/TOR signaling pathway plays a crucial role in animal homeostasis, sensing nutrient status to regulate organismal growth and metabolism. We identify here the Drosophila B' regulatory subunit of PP2A (PP2A-B') as a novel, conserved component of the insulin pathway that specifically targets the PP2A holoenzyme to dephosphorylate S6K. PP2A-B' knockout flies have elevated S6K phosphorylation and exhibit phenotypes typical of elevated insulin signaling such as reduced total body triglycerides and reduced longevity. We show that PP2A-B' interacts with S6K both physically and genetically. The human homolog of PP2A-B', PPP2R5C, also counteracts S6K1 phosphorylation, indicating a conserved mechanism in mammals. Since S6K affects development of cancer and metabolic disease, our data identify PPP2R5C as a novel factor of potential medical relevance.

Traffic control at the nuclear pore

The proper communication between organelles is essential for many aspects of eukaryotic life. The coordination of nuclear and cytoplasmic activities in particular is of pivotal importance and depends on transport in and out of the nucleus. The material which translocates through nuclear pores is diverse; it includes numerous proteins, RNAs and large ribonucleoprotein complexes like ribosomal subunits. To ensure the correct nucleocytoplasmic distribution of these components, appropriate mechanisms have to be in place which control traffic across the nuclear envelope. A growing number of studies support the notion that transport through nuclear pore complexes is intimately linked to cell physiology. As such, it has become evident that changes in the cellular environment, either by externally applied stress, aging or disease, alter nuclear traffic. Due to the progress made in the past few years, we are now beginning to understand these processes at the molecular level. Thus, the concept emerges that stress or disease conditions correlate with signaling events which aim at the nuclear transport apparatus. Here, we summarize results from recent publications that provide evidence for the hypothesis that changes in cell physiology modulate nuclear traffic by targeting multiple transport factors. We propose that this traffic control is at least in part mediated by specific signaling events.

InAKTivation of insulin/IGF-1 signaling by dephosphorylation

Signal transduction pathways are tightly regulated by phosphorylation-dephosphorylation cycles and yet the mammalian genome contains far more genes that encode for protein kinases than protein phosphatases. Therefore, to target specific substrates, many phosphatases associate with distinct regulatory subunits and thereby modulate multiple cellular processes. One such example is the C. elegans PP2A regulatory subunit PPTR-1 that negatively regulates the insulin/insulin-like growth factor signaling pathway to modulate longevity, dauer diapause, fat metabolism and stress resistance. PPTR-1, as well as its mammalian homolog B56beta, specifically target the PP2A enzyme to AKT and mediate the dephosphorylation of this important kinase at a conserved threonine residue. In C. elegans, the major consequence of this modulation is activation of the FOXO transcription factor homolog DAF-16, which in turn regulates transcription of its many target genes involved in longevity and stress resistance. Understanding the function of B56 subunits may have important consequences in diseases such as Type 2 diabetes and cancer where the balance of Akt phosphorylation is deregulated.

Molecular mechanisms of metabolic regulation by insulin in Drosophila

The insulin signalling pathway is highly conserved from mammals to Drosophila. Insulin signalling in the fly, as in mammals, regulates a number of physiological functions, including carbohydrate and lipid metabolism, tissue growth and longevity. In the present review, I discuss the molecular mechanisms by which insulin signalling regulates metabolism in Drosophila, comparing and contrasting with the mammalian system. I discuss both the intracellular signalling network, as well as the communication between organs in the fly.

Interplay between MEK and PI3 kinase signaling regulates the subcellular localization of protein kinases ERK1/2 and Akt upon oxidative stress

ERK and Akt kinases are key components that participate in numerous regulatory processes, including the response to stress. Using novel tools for quantitative immunofluorescence, we show that oxidant exposure controls the intracellular activation and localization of ERK1/2 and Akt. Oxidative stress alters the nuclear/cytoplasmic levels of the kinases, drastically changing phospho-ERK1/2 and phospho-Akt(Ser473) levels in the nucleus. Moreover, pharmacological inhibition of PI3 kinase modulates the intracellular distribution of phospho-ERK1/2, whereas MEK inhibition affects phospho-Akt(Thr308) and phospho-Akt(Ser473). Our studies identify a new signaling link in the nucleus of stressed cells, where changes in phospho-ERK1/2 levels correlate directly with changes in phospho-Akt(Ser473).

PPTR-1 counterAkts insulin signaling

The serine/threonine kinase Akt is a focal point in signaling pathways that control cell tumorigenesis and insulin resistance. In this issue, Padmanabhan et al. (2009) identify a phosphatase regulatory subunit PPTR-1 that regulates the insulin/insulin-like growth factor 1 pathway by counteracting Akt activity in worms and mammalian cells.