Diet-Dependent Effects of the Drosophila Mnk1/Mnk2 Homolog Lk6 on Growth via eIF4E

eIF4E as a molecular wildcard in metazoans RNA metabolism

The evolutionary origin of eukaryotes spurred the transition from prokaryotic-like translation to a more sophisticated, eukaryotic translation. During this process, successive gene duplication of a single, primordial eIF4E gene encoding the mRNA cap-binding protein eukaryotic translation initiation factor 4E (eIF4E) gave rise to a plethora of paralog genes across eukaryotes that underwent further functional diversification in RNA metabolism. The ability to take different roles is due to eIF4E promiscuity in binding many partner proteins, rendering eIF4E a highly versatile and multifunctional player that functions as a molecular wildcard. Thus, in metazoans, eIF4E paralogs are involved in various processes, including messenger RNA (mRNA) processing, export, translation, storage, and decay. Moreover, some paralogs display differential expression in tissues and developmental stages and show variable biochemical properties. In this review, we discuss recent advances shedding light on the functional diversification of eIF4E in metazoans. We emphasise humans and two phylogenetically distant species which have become paradigms for studies on development, namely the fruit fly Drosophila melanogaster and the roundworm Caenorhabditis elegans.

p38α deficiency restrains liver regeneration after partial hepatectomy triggering oxidative stress and liver injury

p38α MAPK negatively regulates the G₁/S and G₂/M cell cycle transitions. However, liver-specific p38α deficiency impairs cytokinesis and reduces hepatocyte proliferation during cirrhosis and aging in mice. In this work, we have studied how p38α down-regulation affects hepatocyte proliferation after partial hepatectomy, focusing on mitotic progression, cytokinesis and oxidative stress. We found that p38α deficiency triggered up-regulation of cyclins A1, B1, B2, and D1 under basal conditions and after hepatectomy. Moreover, p38α-deficient hepatocytes showed enhanced binucleation and increased levels of phospho-histone H3 but impaired phosphorylation of MNK1 after hepatectomy. The recovery of liver mass was transiently delayed in mice with p38α-deficient hepatocytes vs wild type mice. We also found that p38α deficiency caused glutathione oxidation in the liver, increased plasma aminotransferases and lactate dehydrogenase activities, and decreased plasma protein levels after hepatectomy. Interestingly, p38α silencing in isolated hepatocytes markedly decreased phospho-MNK1 levels, and silencing of either p38α or Mnk1 enhanced binucleation of hepatocytes in culture. In conclusion, p38α deficiency impairs mitotic progression in hepatocytes and restrains the recovery of liver mass after partial hepatectomy. Our results also indicate that p38α regulates cytokinesis by activating MNK1 and redox modulation.

Ras/ERK-signalling promotes tRNA synthesis and growth via the RNA polymerase III repressor Maf1 in Drosophila

The small G-protein Ras is a conserved regulator of cell and tissue growth. These effects of Ras are mediated largely through activation of a canonical RAF-MEK-ERK kinase cascade. An important challenge is to identify how this Ras/ERK pathway alters cellular metabolism to drive growth. Here we report on stimulation of RNA polymerase III (Pol III)-mediated tRNA synthesis as a growth effector of Ras/ERK signalling in Drosophila. We find that activation of Ras/ERK signalling promotes tRNA synthesis both in vivo and in cultured Drosophila S2 cells. We also show that Pol III function is required for Ras/ERK signalling to drive proliferation in both epithelial and stem cells in Drosophila tissues. We find that the transcription factor Myc is required but not sufficient for Ras-mediated stimulation of tRNA synthesis. Instead we show that Ras signalling promotes Pol III function and tRNA synthesis by phosphorylating, and inhibiting the nuclear localization and function of the Pol III repressor Maf1. We propose that inhibition of Maf1 and stimulation of tRNA synthesis is one way by which Ras signalling enhances protein synthesis to promote cell and tissue growth.

The Extracellular-Regulated Kinase Effector Lk6 is Required for Glutamate Receptor Localization at the Drosophila Neuromuscular Junction

The proper localization and synthesis of postsynaptic glutamate receptors are essential for synaptic plasticity. Synaptic translation initiation is thought to occur via the target of rapamycin (TOR) and mitogen-activated protein kinase signal-integrating kinase (Mnk) signaling pathways, which is downstream of extracellular-regulated kinase (ERK). We used the model glutamatergic synapse, the Drosophila neuromuscular junction, to better understand the roles of the Mnk and TOR signaling pathways in synapse development. These synapses contain non-NMDA receptors that are most similar to AMPA receptors. Our data show that Lk6, the Drosophila homolog of Mnk1 and Mnk2, is required in either presynaptic neurons or postsynaptic muscle for the proper localization of the GluRIIA glutamate receptor subunit. Lk6 may signal through eukaryotic initiation factor (eIF) 4E to regulate the synaptic levels of GluRIIA as either interfering with eIF4E binding to eIF4G or expression of a nonphosphorylatable isoform of eIF4E resulted in a significant reduction in GluRIIA at the synapse. We also find that Lk6 and TOR may independently regulate synaptic levels of GluRIIA.

An investigation of nutrient-dependent mRNA translation in Drosophila larvae

The larval period of the Drosophila life cycle is characterized by immense growth. In nutrient rich conditions, larvae increase in mass approximately two hundred-fold in five days. However, upon nutrient deprivation, growth is arrested. The prevailing view is that dietary amino acids drive this larval growth by activating the conserved insulin/PI3 kinase and Target of rapamycin (TOR) pathways and promoting anabolic metabolism. One key anabolic process is protein synthesis. However, few studies have attempted to measure mRNA translation during larval development or examine the signaling requirements for nutrient-dependent regulation. Our work addresses this issue. Using polysome analyses, we observed that starvation rapidly (within thirty minutes) decreased larval mRNA translation, with a maximal decrease at 6–18 hours. By analyzing individual genes, we observed that nutrient-deprivation led to a general reduction in mRNA translation, regardless of any starvation-mediated changes (increase or decrease) in total transcript levels. Although sugars and amino acids are key regulators of translation in animal cells and are the major macronutrients in the larval diet, we found that they alone were not sufficient to maintain mRNA translation in larvae. The insulin/PI3 kinase and TOR pathways are widely proposed as the main link between nutrients and mRNA translation in animal cells. However, we found that genetic activation of PI3K and TOR signaling, or regulation of two effectors – 4EBP and S6K – could not prevent the starvation-mediated translation inhibition. Similarly, we showed that the nutrient stress-activated eIF2α kinases, GCN2 and PERK, were not required for starvation-induced inhibition of translation in larvae. These findings indicate that nutrient control of mRNA translation in larvae is more complex than simply amino acid activation of insulin and TOR signaling.

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.

A Drosophila model of high sugar diet-induced cardiomyopathy

Diets high in carbohydrates have long been linked to progressive heart dysfunction, yet the mechanisms by which chronic high sugar leads to heart failure remain poorly understood. Here we combine diet, genetics, and physiology to establish an adult Drosophila melanogaster model of chronic high sugar-induced heart disease. We demonstrate deterioration of heart function accompanied by fibrosis-like collagen accumulation, insulin signaling defects, and fat accumulation. The result was a shorter life span that was more severe in the presence of reduced insulin and P38 signaling. We provide evidence of a role for hexosamine flux, a metabolic pathway accessed by glucose. Increased hexosamine flux led to heart function defects and structural damage; conversely, cardiac-specific reduction of pathway activity prevented sugar-induced heart dysfunction. Our data establish Drosophila as a useful system for exploring specific aspects of diet-induced heart dysfunction and emphasize enzymes within the hexosamine biosynthetic pathway as candidate therapeutic targets.

On the Diversification of the Translation Apparatus across Eukaryotes

Diversity is one of the most remarkable features of living organisms. Current assessments of eukaryote biodiversity reaches 1.5 million species, but the true figure could be several times that number. Diversity is ingrained in all stages and echelons of life, namely, the occupancy of ecological niches, behavioral patterns, body plans and organismal complexity, as well as metabolic needs and genetics. In this review, we will discuss that diversity also exists in a key biochemical process, translation, across eukaryotes. Translation is a fundamental process for all forms of life, and the basic components and mechanisms of translation in eukaryotes have been largely established upon the study of traditional, so-called model organisms. By using modern genome-wide, high-throughput technologies, recent studies of many nonmodel eukaryotes have unveiled a surprising diversity in the configuration of the translation apparatus across eukaryotes, showing that this apparatus is far from being evolutionarily static. For some of the components of this machinery, functional differences between different species have also been found. The recent research reviewed in this article highlights the molecular and functional diversification the translational machinery has undergone during eukaryotic evolution. A better understanding of all aspects of organismal diversity is key to a more profound knowledge of life.

MNK1 kinase activity is required for abscission

MNK1 is a serine/threonine kinase identified as a target for MAP kinase pathways. Using chemical drug, kinase-dead expression or knockdown by RNA interference, we show that inhibition of MNK1 induces the formation of multinucleated cells, which can be rescued by expressing a form of MNK1 that is resistant to RNA interference. We found that the active human form of MNK1 localises to centrosomes, spindle microtubules and the midbody. Time-lapse recording of MNK1-depleted cells displays cytokinesis defects, as daughter cells fuse back together. When MNK1 activity was inhibited, no microtubule defect at the midbody was detected, however, anchorage of the membrane vesicle at the midbody was impaired as lumenal GFP-positive vesicles did not accumulate at the midbody. At the molecular level, we found that centriolin localisation was impaired at the midbody in MNK1-depleted cells. As a consequence, endobrevin - a v-SNARE protein implicated in the abscission step - was not properly localised to the midbody. Altogether, our data show that MNK1 activity is required for abscission.

Neuroprotective, immunosuppressant and antineoplastic properties of mTOR inhibitors: current and emerging therapeutic options

The acronym mTOR defines a family of serine-threonine protein kinase called mammalian target of rapamycin. The major role of these kinases in the cell is to merge extracellular instructions with information about cellular metabolic resources and to control the rate of anabolic and catabolic processes accordingly. In mammalian cells mTOR is present in two distinct heteromeric protein complexes commonly referred to as mTOR complex 1 (mTORC1) and mTOR complex 2 (mTORC2), involved in the control of a wide variety of cellular processes. It has been recently reported that compounds acting modulating mTOR activity, beside mediating the well recognized processes exploited in the anticancer and immunosuppressant effects, are provided with neuroprotective properties. In fact, mTOR is involved in the mechanism of PI3K/Akt-induced upregulation of glutamate transporter 1, GLT1, that is linked to several neuronal disorders such as stroke, Alzheimer's disease, and amyotrophic lateral sclerosis. Furthermore, in adult brain mTOR is crucial for numerous physiological processes such as synaptic plasticity, learning, memory, and brain control of food uptake. Moreover, the activation of mTOR pathway is involved in neuronal development, dendrite development and spine morphogenesis.

TOR signaling never gets old: aging, longevity and TORC1 activity

The target of rapamycin (TOR) signal transduction network monitors intra- and extracellular conditions that favor cell growth. Research during the last decade has revealed a modular structure of the TOR signaling network. Each signaling module senses a particular set of signals from the cellular milieu and exerts regulatory control towards TOR activity. The TOR pathway responds to growth factor signals, nutrient availability, and cellular stresses like hypoxia and energy stress. The signaling modules and their molecular components constituting the TOR network are remarkably conserved in both sequence and function across species. In yeast, roundworms, flies, and mice, the TOR pathway has been shown to regulate lifespan. Correspondingly, genetic, dietary or pharmacological manipulation of individual signaling modules as well as TOR activity itself extends lifespan in these model organisms. We discuss the potential impact of manipulating TOR activity for human health and lifespan.

Genetic approaches to alcohol addiction: gene expression studies and recent candidates from Drosophila

Ethanol intake causes gene expression changes resulting in cellular and molecular adaptations that could be associated with a predisposition to alcohol dependence. Recently, several research groups have used high-throughput gene expression profiling to search for alcohol-responsive genes in Drosophila melanogaster. Comparison of data from these studies highlights the functional similarities in their results despite differences in their experimental approach and selection cases. Notably, alcohol-responsive gene sets associated with stress response, olfaction, metabolism, proteases, transcriptional regulation, regulation of signal transduction, nucleic acid binding and cytoskeletal organisation were markedly common to these studies. These data support the view that changes in gene expression in alcoholics are associated with widespread cellular functions.

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

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

Madm (Mlf1 adapter molecule) cooperates with Bunched A to promote growth in Drosophila

BACKGROUND

The TSC-22 domain family (TSC22DF) consists of putative transcription factors harboring a DNA-binding TSC-box and an adjacent leucine zipper at their carboxyl termini. Both short and long TSC22DF isoforms are conserved from flies to humans. Whereas the short isoforms include the tumor suppressor TSC-22 (Transforming growth factor-beta1 stimulated clone-22), the long isoforms are largely uncharacterized. In Drosophila, the long isoform Bunched A (BunA) acts as a growth promoter, but how BunA controls growth has remained obscure.

RESULTS

In order to test for functional conservation among TSC22DF members, we expressed the human TSC22DF proteins in the fly and found that all long isoforms can replace BunA function. Furthermore, we combined a proteomics-based approach with a genetic screen to identify proteins that interact with BunA. Madm (Mlf1 adapter molecule) physically associates with BunA via a conserved motif that is only contained in long TSC22DF proteins. Moreover, Drosophila Madm acts as a growth-promoting gene that displays growth phenotypes strikingly similar to bunA phenotypes. When overexpressed, Madm and BunA synergize to increase organ growth.

CONCLUSIONS

The growth-promoting potential of long TSC22DF proteins is evolutionarily conserved. Furthermore, we provide biochemical and genetic evidence for a growth-regulating complex involving the long TSC22DF protein BunA and the adapter molecule Madm.

Expression profiling of prospero in the Drosophila larval chemosensory organ: Between growth and outgrowth

Background

The antenno-maxilary complex (AMC) forms the chemosensory system of the Drosophila larva and is involved in gustatory and olfactory perception. We have previously shown that a mutant allele of the homeodomain transcription factor Prospero (prosVoila1, V1), presents several developmental defects including abnormal growth and altered taste responses. In addition, many neural tracts connecting the AMC to the central nervous system (CNS) were affected. Our earlier reports on larval AMC did not argue in favour of a role of pros in cell fate decision, but strongly suggested that pros could be involved in the control of other aspect of neuronal development. In order to identify these functions, we used microarray analysis of larval AMC and CNS tissue isolated from the wild type, and three other previously characterised prospero alleles, including the V1 mutant, considered as a null allele for the AMC.

Results

A total of 17 samples were first analysed with hierarchical clustering. To determine those genes affected by loss of pros function, we calculated a discriminating score reflecting the differential expression between V1 mutant and other pros alleles. We identified a total of 64 genes in the AMC. Additional manual annotation using all the computed information on the attributed role of these genes in the Drosophila larvae nervous system, enabled us to identify one functional category of potential Prospero target genes known to be involved in neurite outgrowth, synaptic transmission and more specifically in neuronal connectivity remodelling. The second category of genes found to be differentially expressed between the null mutant AMC and the other alleles concerned the development of the sensory organs and more particularly the larval olfactory system. Surprisingly, a third category emerged from our analyses and suggests an association of pros with the genes that regulate autophagy, growth and insulin pathways. Interestingly, EGFR and Notch pathways were represented in all of these three functional categories. We now propose that Pros could perform all of these different functions through the modulation of these two antagonistic and synergic pathways.

Conclusions

The current data contribute to the clarification of the prospero function in the larval AMC and show that pros regulates different function in larvae as compared to those controlled by this gene in embryos. In the future, the possible mechanism by which Pros could achieve its function in the AMC will be explored in detail.

Differential regulation of CHOP translation by phosphorylated eIF4E under stress conditions

Cells respond to environmental stress by inducing translation of a subset of mRNAs important for survival or apoptosis. CHOP, a downstream transcriptional target of stress-induced ATF4, is also regulated translationally in a uORF-dependent manner under stress. Low concentration of anisomycin induces CHOP expression at both transcriptional and translational levels. To study specifically the translational aspect of CHOP expression, and further clarify the regulatory mechanisms underlying stress-induced translation initiation, we developed a CMV promoter-regulated, uORF^chop-driven reporter platform. Here we show that anisomycin-induced CHOP expression depends on phosphorylated eIF4E/S209 and eIF2α/S51. Contrary to phospho-eIF2α/S51, phospho-eIF4E/S209 is not involved in thapsigargin-induced CHOP expression. Studies using various kinase inhibitors and mutants uncovered that both the p38MAPK-Mnk and mTOR signaling pathways contribute to stress-responsive reporter and CHOP expression. We also demonstrated that anisomycin-induced translation is tightly regulated by partner binding preference of eIF4E. Furthermore, mutating the uORF sequence abolished the anisomycin-induced association of chop mRNA with phospho-eIF4E and polysomes, thus demonstrating the significance of this cis-regulatory element in conferring on the transcript a stress-responsive translational inducibility. Strikingly, although insulin treatment activated ERK-Mnk and mTOR pathways, and consequently eIF4E/S209 phosphorylation, it failed to induce phospho-eIF2α/S51 and reporter translation, thus pinpointing a crucial determinant in stress-responsive translation.

mTOR's role in ageing: protein synthesis or autophagy?

The molecular and cellular mechanisms that regulate ageing are currently under scrutiny because ageing is linked to many human diseases. The nutrient sensing TOR pathway is emerging as a key regulator of ageing. TOR signaling is complex affecting several crucial cellular functions and two such functions, which show clear effects on ageing, are protein synthesis and autophagy. In this article we discuss the relative importance of both these processes in ageing, identify how TOR regulates translation and autophagy and speculate on links between the TOR signaling network and ageing pathways.

Activation of p38 mitogen-activated protein kinase is required for in vivo brain-derived neurotrophic factor production in the rat hippocampus

Several lines of evidence strongly suggest that brain-derived neurotrophic factor (BDNF) is associated with the formation, storage and recall of memory in the hippocampus and that it is important to maintain a considerable level of hippocampal BDNF in order to keep normal functions. BDNF can be synthesized in an activity-dependent manner. In fact, kainic acid or AMPA enhances BDNF levels in hippocampal granule neurons. However, the mechanisms of BDNF production are largely unclear. Recently, we have found that riluzole, which blocks voltage-gated sodium channels and thereby reduces glutamate release, actually strengthens immunoreactivity of BDNF in hippocampal granule neurons of rats. Therefore, we examined the riluzole-activated signaling pathways for BDNF production. Riluzole increased levels of phospho-p38 mitogen-activated protein kinase (p38 MAPK), as well as BDNF levels. Inhibition of p38 MAPK by SB203580 reduced riluzole effects, while activation of p38 MAPK by anisomycin increased levels of BDNF, suggesting that p38 MAPK can mediate BDNF production. Riluzole-induced elevation of phospho-activating transcription factor-2, a transcription factor downstream of p38 MAPK, was also observed. A blocker of N-type voltage-gated calcium channels reduced the effects of riluzole on BDNF production and p38 MAPK activation. We also examined a possible involvement of the adenosine A1 receptor in BDNF production because riluzole can influence ecto-nucleotide levels. An A1 receptor agonist inhibited riluzole-induced elevation of BDNF levels, whereas an antagonist not only increased levels of BDNF and active p38 MAPK but also augmented riluzole effects. These results indicate that, in the rat hippocampus, there is an in vivo signaling pathway for BDNF synthesis mediated by p38 MAPK, and that N-type voltage-gated calcium channels and/or adenosine A1 receptors contribute to p38 MAPK activation.

Translational control of long-lasting synaptic plasticity and memory

Long-lasting forms of synaptic plasticity and memory are dependent on new protein synthesis. Recent advances obtained from genetic, physiological, pharmacological, and biochemical studies provide strong evidence that translational control plays a key role in regulating long-term changes in neural circuits and thus long-term modifications in behavior. Translational control is important for regulating both general protein synthesis and synthesis of specific proteins in response to neuronal activity. In this review, we summarize and discuss recent progress in the field and highlight the prospects for better understanding of long-lasting changes in synaptic strength, learning, and memory and implications for neurological diseases.

Translational responses to growth factors and stress

Cellular stresses can induce a wide range of biological responses, depending on the type of stress, the type of cell and the cellular environment. Stress-mediated changes in translational output cover a broad spectrum of potential responses, including an overall decrease in translation or an increase in the translation of specific mRNAs. Many of these changes involve post-translational modifications of components of the translational machinery. The mTOR (mammalian target of rapamycin) pathway is a critical regulator of growth and translation in response to a wide variety of signals, including growth factors, amino acids and energy availability. Through its kinase activity, mTOR activation results in the phosphorylation of translational components and an increase in translation. As stress-mediated changes in translational output are context-dependent, the interplay between stress and mTOR in the control of translation is also likely to depend on factors such as the strength and type of incident stress. In the present paper, we review mTOR-dependent and -independent translational responses, and discuss their regulation by stress.

Ethanol sensitivity and tolerance in long-term memory mutants of Drosophila melanogaster

BACKGROUND

It has become increasingly clear that molecular and neural mechanisms underlying learning and memory and drug addiction are largely shared. To confirm and extend these findings, we analyzed ethanol-responsive behaviors of a collection of Drosophila long-term memory mutants.

METHODS

For each mutant, sensitivity to the acute uncoordinating effects of ethanol was quantified using the inebriometer. Additionally, 2 distinct forms of ethanol tolerance were measured: rapid tolerance, which develops in response to a single brief exposure to a high concentration of ethanol vapor; and chronic tolerance, which develops following a sustained low-level exposure.

RESULTS

Several mutants were identified with altered sensitivity, rapid or chronic tolerance, while a number of mutants exhibited multiple defects.

CONCLUSIONS

The corresponding genes in these mutants represent areas of potential overlap between learning and memory and behavioral responses to alcohol. These genes also define components shared between different ethanol behavioral responses.

Regulation of arsenic trioxide-induced cellular responses by Mnk1 and Mnk2

Arsenic trioxide (As(2)O(3)) is a potent inducer of apoptosis of malignant cells in vitro and in vivo, but the precise mechanisms by which it mediates such effects are not well defined. We provide evidence that As(2)O(3) induces phosphorylation/activation of the MAPK signal-integrating kinases (Mnks) 1 and 2 in leukemia cell lines. Such activation is defective in cells with targeted disruption of the p38alpha MAPK gene, indicating that it requires upstream engagement of the p38 MAPK pathway. Studies using Mnk1(-/-) or Mnk2(-/-), or double Mnk1(-/-)Mnk2(-/-) knock-out cells, establish that activation of Mnk1 and Mnk2 by arsenic trioxide regulates downstream phosphorylation of the eukaryotic initiation factor 4E at Ser-209. Importantly, arsenic-induced apoptosis is enhanced in cells with targeted disruption of the Mnk1 and/or Mnk2 genes, suggesting that these kinases are activated in a negative-feedback regulatory manner, to control generation of arsenic trioxide responses. Consistent with this, pharmacological inhibition of Mnk activity enhances the suppressive effects of arsenic trioxide on primary leukemic progenitors from patients with acute leukemias. Taken together, these findings indicate an important role for Mnk kinases, acting as negative regulators for signals that control generation of arsenic trioxide-dependent apoptosis and antileukemic responses.

Bunched, the Drosophila homolog of the mammalian tumor suppressor TSC-22, promotes cellular growth

BACKGROUND

Transforming Growth Factor-beta1 stimulated clone-22 (TSC-22) is assumed to act as a negative growth regulator and tumor suppressor. TSC-22 belongs to a family of putative transcription factors encoded by four distinct loci in mammals. Possible redundancy among the members of the TSC-22/Dip/Bun protein family complicates a genetic analysis. In Drosophila, all proteins homologous to the TSC-22/Dip/Bun family members are derived from a single locus called bunched (bun).

RESULTS

We have identified bun in an unbiased genetic screen for growth regulators in Drosophila. Rather unexpectedly, bun mutations result in a growth deficit. Under standard conditions, only the long protein isoform BunA - but not the short isoforms BunB and BunC - is essential and affects growth. Whereas reducing bunA function diminishes cell number and cell size, overexpression of the short isoforms BunB and BunC antagonizes bunA function.

CONCLUSION

Our findings establish a growth-promoting function of Drosophila BunA. Since the published studies on mammalian systems have largely neglected the long TSC-22 protein version, we hypothesize that the long TSC-22 protein is a functional homolog of BunA in growth regulation, and that it is antagonized by the short TSC-22 protein.

Nutritional control of protein biosynthetic capacity by insulin via Myc in Drosophila

Animals use the insulin/TOR signaling pathway to mediate their response to fluctuations in nutrient availability. Energy and amino acids are monitored at the single-cell level via the TOR branch of the pathway and systemically via insulin signaling to regulate cellular growth and metabolism. Using a combination of genetics, expression profiling, and chromatin immunoprecipitation, we examine nutritional control of gene expression and identify the transcription factor Myc as an important mediator of TOR-dependent regulation of ribosome biogenesis. We also identify myc as a direct target of FOXO and provide genetic evidence that Myc has a key role in mediating the effects of TOR and FOXO on growth and metabolism. FOXO and TOR also converge to regulate protein synthesis, acting via 4E-BP and Lk6, regulators of the translation factor eIF4E. This study uncovers a network of convergent regulation of protein biosynthesis by the FOXO and TOR branches of the nutrient-sensing pathway.

Diabetic larvae and obese flies-emerging studies of metabolism in Drosophila

The past few years have seen a shift in the use of Drosophila, from studies of growth and development toward genetic characterization of carbohydrate, sterol, and lipid metabolism. This research, reviewed below, establishes a new foundation for using this simple genetic model system to define the basic regulatory mechanisms that underlie metabolic homeostasis and holds the promise of providing new insights into the causes and treatments of critical human disorders such as diabetes and obesity.

Characterization of the activity of human MAP kinase-interacting kinase Mnk1b

Human mitogen-activated protein (MAP) kinase interacting kinase 1b (Mnk1b) is a splice variant of human Mnk1a, which has been identified in our laboratory [A. O'Loghlen, V.M. Gonzalez, D. Pineiro, M.I. Perez-Morgado, M. Salinas, M.E. Martin, Identification and molecular characterization of Mnk1b, a splice variant of human MAP kinase-interacting kinase Mnk1, Exp. Cell Res. 299 (2004) 343-355]. Mnk1b has much higher basal eIF4E kinase activity than Mnk1a. Because Mnk1b presents different features in its C-terminus with respect to Mnk1a, we have studied in this paper the potential role of these structural differences in determining the higher basal eIF4E kinase activity as well as the subcellular localization of Mnk1b. In this paper, we demonstrate that phosphorylation of the Thr209 and Thr214 in the activation loop of Mnk1b is necessary for its activation. However, the different kinase activity between Mnk1a and Mnk1b is independent of the phosphorylation status of the activation loop residues. By deletion of the C-terminal tail in Mnk1a, we confirmed that the absence of this sequence is not responsible for the higher eIF4E kinase activity present in Mnk1b. Moreover, our findings support a crucial role of the 12 amino acids, particularly the Ala344, in the C-terminal specific region of Mnk1b (Mnk1bSR), on the kinase activity of the protein.

eIF4E function in somatic cells modulates ageing in Caenorhabditis elegans

Regulation of protein synthesis is critical for cell growth and maintenance. Ageing in many organisms, including humans, is accompanied by marked alterations in both general and specific protein synthesis. Whether these alterations are simply a corollary of the ageing process or have a causative role in senescent decline remains unclear. An array of protein factors facilitates the tight control of messenger RNA translation initiation. The eukaryotic initiation factor 4E (eIF4E), which binds the 7-monomethyl guanosine cap at the 5' end of all nuclear mRNAs, is a principal regulator of protein synthesis. Here we show that loss of a specific eIF4E isoform (IFE-2) that functions in somatic tissues reduces global protein synthesis, protects from oxidative stress and extends lifespan in Caenorhabditis elegans. Lifespan extension is independent of the forkhead transcription factor DAF-16, which mediates the effects of the insulin-like signalling pathway on ageing. Furthermore, IFE-2 deficiency further extends the lifespan of long-lived age and daf nematode mutants. Similarly, lack of IFE-2 enhances the long-lived phenotype of clk and dietary-restricted eat mutant animals. Knockdown of target of rapamycin (TOR), a phosphatidylinositol kinase-related kinase that controls protein synthesis in response to nutrient cues, further increases the longevity of ife-2 mutants. Thus, signalling via eIF4E in the soma is a newly discovered pathway influencing ageing in C. elegans.

In vivo functions of mitogen-activated protein kinases: conclusions from knock-in and knock-out mice

Multicellular organisms achieve intercellular communication by means of signalling molecules whose effect on the target cell is mediated by signal transduction pathways. Such pathways relay, amplify and integrate signals to elicit appropriate biological responses. Protein kinases form crucial intermediate components of numerous signalling pathways. One group of protein kinases, the mitogen-activated protein kinases (MAP kinases) are kinases involved in signalling pathways that respond primarily to mitogens and stress stimuli. In vitro studies revealed that the MAP kinases are implicated in several cellular processes, including cell division, differentiation, cell survival/apoptosis, gene expression, motility and metabolism. As such, dysfunction of specific MAP kinases is associated with diseases such as cancer and immunological disorders. However, the genuine in vivo functions of many MAP kinases remain elusive. Genetically modified mouse models deficient in a specific MAP kinase or expressing a constitutive active or a dominant negative variant of a particular MAP kinase offer valuable tools for elucidating the biological role of these protein kinases. In this review, we focus on the current status of MAP kinase knock-in and knock-out mouse models and their phenotypes. Moreover, examples of the application of MAP kinase transgenic mice for validating therapeutic properties of specific MAP kinase inhibitors, and for investigating the role of MAP kinase in pathogen-host interactions will be discussed.

Signaling pathways regulating protein synthesis during ageing

Ageing in many organisms, including humans, is accompanied by marked alterations in both general and specific protein synthesis. Protein synthesis is normally under tight control by a broad array of regulatory factors, which facilitate appropriate rates of mRNA translation. Are the wide changes in protein synthesis simply a corollary of the ageing process or do they have a causative role in senescent decline? The jury is still out on this important question. Nevertheless, recent studies reveal an intimate interface between mechanisms that govern the translation of mRNA and molecular pathways implicated in ageing. In our manuscript we consider these links, which potentially underlie age-associated changes in protein synthesis.

Mitogen-activated protein kinases interacting kinases are autoinhibited by a reprogrammed activation segment

Autoinhibition is a recurring mode of protein kinase regulation and can be based on diverse molecular mechanisms. Here, we show by crystal structure analysis, nuclear magnetic resonance (NMR)-based nucleotide affinity studies and rational mutagenesis that nonphosphorylated mitogen-activated protein (MAP) kinases interacting kinase (Mnk) 1 is autoinhibited by conversion of the activation segment into an autoinhibitory module. In a Mnk1 crystal structure, the activation segment is repositioned via a Mnk-specific sequence insertion at the N-terminal lobe with the following consequences: (i) the peptide substrate binding site is deconstructed, (ii) the interlobal cleft is narrowed, (iii) an essential Lys-Glu pair is disrupted and (iv) the magnesium-binding loop is locked into an ATP-competitive conformation. Consistently, deletion of the Mnk-specific insertion or removal of a conserved phenylalanine side chain, which induces a blockade of the ATP pocket, increase the ATP affinity of Mnk1. Structural rearrangements required for the activation of Mnks are apparent from the cocrystal structure of a Mnk2 D228G -staurosporine complex and can be modeled on the basis of crystal packing interactions. Our data suggest a novel regulatory mechanism specific for the Mnk subfamily.

Drosophila lacking microRNA miR-278 are defective in energy homeostasis

The mechanisms that control energy homeostasis and tissue growth during development are closely linked through the insulin signal transduction pathway. Changes in the level of insulin and other hormones reflect the nutritional status of the animal to control circulating sugar levels and fat metabolism. Systemic defects in insulin responsiveness can lead to elevated circulating glucose levels and fat accumulation. Here we present evidence that the microRNA miR-278 plays a role in the control of energy homeostasis in Drosophila. miR-278 mutants have elevated insulin production and are correspondingly lean. Despite the elevated insulin levels, miR-278 mutants have elevated circulating sugar, mobilized from adipose-tissue glycogen stores. We provide evidence that miR-278 mutants are insulin resistant and that miR-278 acts through regulation of the expanded transcript.

Stopped-flow kinetic analysis of eIF4E and phosphorylated eIF4E binding to cap analogs and capped oligoribonucleotides: evidence for a one-step binding mechanism

Recruitment of eukaryotic mRNA to the 48 S initiation complex is rate-limiting for protein synthesis under normal conditions. Binding of the 5' -terminal cap structure of mRNA to eIF4E is a critical event during this process. Mammalian eIF4E is phosphorylated at Ser-209 by Mnk1 and Mnk2 kinases. We investigated the interaction of both eIF4E and phosphorylated eIF4E (eIF4E(P)) with cap analogs and capped oligoribonucleotides by stopped-flow kinetics. For m(7)GpppG, the rate constant of association, k(on), was dependent on ionic strength, decreasing progressively up to 350 mm KCl, but the rate constant of dissociation, k(off), was independent of ionic strength. Phosphorylation of eIF4E decreased k(on) by 2.1-2.3-fold at 50-100 mm KCl but had progressively less effect at higher ionic strengths, being negligible at 350 mm. Contrary to published evidence, eIF4E phosphorylation had no effect on k(off). Several observations supported a simple one-step binding mechanism, in contrast to published reports of a two-step mechanism. The kinetic function that best fit the data changed from single- to double-exponential as the eIF4E concentration was increased. However, measuring k(off) for dissociation of a pre-formed eIF4E.m(7)GpppG complex suggested that the double-exponential kinetics were caused by dissociation of eIF4E dimers, not a two-step mechanism. Addition of a 12-nucleotide chain to the cap structure increased affinity at high ionic strength for both eIF4E (24-fold) and eIF4E(P) (7-fold), primarily due to a decrease in k(off). This suggests that additional stabilizing interactions between capped oligoribonucleotides and eIF4E, which do not occur with cap analogs alone, act to slow dissociation.

Mnk is a negative regulator of cap-dependent translation in Aplysia neurons

To investigate the mechanisms underlying regulation of eukaryotic initiation factor 4E (eIF4E) phosphorylation in Aplysia neurons, we have cloned the Aplysia homolog of the vertebrate eIF4E kinases, Mnk1 and -2. Aplysia Mnk shares many conserved regions with vertebrate Mnk, including putative eukaryotic initiation factor 4G binding regions, activation loop phosphorylation sites, and a carboxy-terminal anchoring site for MAP kinases. As expected, purified Aplysia Mnk phosphorylated Aplysia eIF4E at a conserved carboxy-terminal serine and over-expression of Aplysia Mnk in sensory neurons led to increased phosphorylation of endogenous eIF4E. Over-expression of Aplysia Mnk led to strong decreases in cap-dependent translation, while generally sparing internal ribosomal entry site (IRES)-dependent translation. However, decreases in cap-dependent translation seen after expression of Aplysia Mnk could only be partly explained by increases in eIF4E phosphorylation. In Aplysia sensory neurons, phosphorylation of eIF4E is reduced during intermediate memory formation. However, we found that this physiological regulation of eIF4E phosphorylation was independent of changes in Aplysia Mnk phosphorylation. We propose that changes in eIF4E phosphorylation in Aplysia neurons are a consequence of changes in cap-dependent translation that are independent of regulation of Aplysia Mnk.

Regulation of sprouty stability by Mnk1-dependent phosphorylation

Sprouty (Spry) proteins are negative feedback modulators of receptor tyrosine kinase pathways in Drosophila melanogaster and mammals. Mammalian Spry proteins have been shown to undergo tyrosine and serine phosphorylation in response to growth factor stimulation. While several studies have addressed the function of tyrosine phosphorylation of Spry, little is known about the significance of Spry serine phosphorylation. Here we identify mitogen-activated protein kinase-interacting kinase 1 (Mnk1) as the kinase that phosphorylates human Spry2 (hSpry2) on serines 112 and 121. Mutation of these serine residues to alanine or inhibition of Mnk1 activity increases the rate of ligand-induced degradation of hSpry2. Conversely, enhancement of serine phosphorylation achieved through either the inhibition of cellular phosphatases or the expression of active Mnk1 results in the stabilization of hSpry2. Previous studies have shown that growth factor stimulation induces the proteolytic degradation of hSpry2 by stimulating tyrosine phosphorylation on hSpry2, which in turn promotes c-Cbl binding and polyubiquitination. A mutant of hSpry2 that is deficient in serine phosphorylation displays enhanced tyrosine phosphorylation and c-Cbl binding, indicating that serine phosphorylation stabilizes hSpry2 by exerting an antagonistic effect on tyrosine phosphorylation. Moreover, loss of serine phosphorylation and the resulting enhanced degradation of hSpry2 impair its capacity to antagonize fibroblast growth factor-induced extracellular signal-regulated kinase activation. Our results imply that Mnk1-mediated serine phosphorylation of hSpry2 constitutes a regulatory mechanism to extend the temporal range of Spry activity.

Crystal structures of the Mnk2 kinase domain reveal an inhibitory conformation and a zinc binding site

Human mitogen-activated protein kinases (MAPK)-interacting kinases 1 and 2 (Mnk1 and Mnk2) target the translational machinery by phosphorylation of the eukaryotic initiation factor 4E (eIF4E). Here, we present the 2.1 A crystal structure of a nonphosphorylated Mnk2 fragment that encompasses the kinase domain. The results show Mnk-specific features such as a zinc binding motif and an atypical open conformation of the activation segment. In addition, the ATP binding pocket contains an Asp-Phe-Asp (DFD) in place of the canonical magnesium binding Asp-Phe-Gly (DFG) motif. The phenylalanine of this motif sticks into the ATP binding pocket and blocks ATP binding as observed with inhibitor bound and, thus, inactive p38 kinase. Replacement of the DFD by the canonical DFG motif affects the conformation of Mnk2, but not ATP binding and kinase activity. The results suggest that the ATP binding pocket and the activation segment of Mnk2 require conformational switches to provide kinase activity.

Controlling the size of organs and organisms

A key difference between yeast and metazoans is the need of the latter to regulate cell proliferation and growth to create organs (and organisms) of reproducible size and shape. Great progress has been made in understanding how growth, cell size and the cell cycle are controlled in metazoans. Recent work has shown that disruption of conserved components of the insulin and Tor kinase pathways can alter organ size, indicating that the normal functioning of these pathways is essential for organ size control. However, disruption of genes that regulate patterning and of genes that control cell adhesion and cell polarity has a much more dramatic effect on final organ size than does manipulation of the cell cycle or of basal growth control mechanisms. These data point to an 'organ-size checkpoint' that regulates cell division, cell growth and apoptosis. Recent data suggests that cell competition may play an important role in implementing the organ-size checkpoint.

The Role of the Prothoracic Gland in Determining Critical Weight for Metamorphosis in Drosophila melanogaster

BACKGROUND

The timely onset of metamorphosis in holometabolous insects depends on their reaching the appropriate size known as critical weight. Once critical weight is reached, juvenile hormone (JH) titers decline, resulting in the release of prothoracicotropic hormone (PTTH) at the next photoperiod gate and thereby inducing metamorphosis. How individuals determine when they have reached critical weight is unknown. We present evidence that in Drosophila, a component of the ring gland, the prothoracic gland (PG), assesses growth to determine when critical weight has been achieved.

RESULTS

We used the GAL4/UAS system to suppress or enhance growth by overexpressing PTEN or Dp110, respectively, in various components of the ring gland. Suppression of the growth of the PG and CA, but not of the CA alone, produced larger-than-normal larvae and adults. Suppression of only PG growth resulted in nonviable larvae, but larvae with enlarged PGs produced significantly smaller larvae and adults. Rearing larvae with enlarged PGs under constant light enhanced these effects, suggesting a role for photoperiod-gated PTTH secretion. These larvae are smaller, in part as a result of their repressed growth rates, a phenotype that could be rescued through nutritional supplementation (yeast paste). Most importantly, larvae with enlarged PGs overestimated size so that they initiated metamorphosis before surpassing the minimal viable weight necessary to survive pupation.

CONCLUSIONS

The PG acts as a size-assessing tissue by using insulin-dependent PG cell growth to determine when critical weight has been reached.

4E-BP functions as a metabolic brake used under stress conditions but not during normal growth

4E-BP is an important regulator of overall translation levels in cells. By binding eIF4E, 4E-BP impairs recruitment of the 40S ribosomal subunit to the cap structure present at the 5'-end of all eukaryotic cellular mRNAs. 4E-BP activity is controlled by TOR (Target of Rapamycin). 4E-BP has been studied extensively in cell culture; however, the biological role of 4E-BP in developing organisms is unclear to date. Since TOR has been shown to control tissue growth during animal development, 4E-BP has also been assumed to serve as a growth regulator. Here, we study the relevance of 4E-BP function for organismal development, and present evidence for an alternate view. We show that 4E-BP strongly affects fat metabolism in Drosophila. We suggest that 4E-BP works as a metabolic brake that is activated under conditions of environmental stress to control fat metabolism. 4E-BP mutants lack this regulation, reducing their ability to survive under unfavorable conditions.

Drosophila Melted Modulates FOXO and TOR Activity

The insulin/PI3K signaling pathway controls both tissue growth and metabolism. Here, we identify Melted as a new modulator of this pathway in Drosophila. Melted interacts with both Tsc1 and FOXO and can recruit these proteins to the cell membrane. We provide evidence that in the melted mutant, TOR activity is reduced and FOXO is activated. The melted mutant condition mimics the effects of nutrient deprivation in a normal animal, producing an animal with 40% less fat than normal.

Brummer lipase is an evolutionary conserved fat storage regulator in Drosophila

Energy homeostasis, a fundamental property of all organisms, depends on the ability to control the storage and mobilization of fat, mainly triacylglycerols (TAG), in special organs such as mammalian adipose tissue or the fat body of flies. Malregulation of energy homeostasis underlies the pathogenesis of obesity in mammals including human. We performed a screen to identify nutritionally regulated genes that control energy storage in the model organism Drosophila. The brummer (bmm) gene encodes the lipid storage droplet-associated TAG lipase Brummer, a homolog of human adipocyte triglyceride lipase (ATGL). Food deprivation or chronic bmm overexpression depletes organismal fat stores in vivo, whereas loss of bmm activity causes obesity in flies. Our study identifies a key factor of insect energy homeostasis control. Their evolutionary conservation suggests Brummer/ATGL family members to be implicated in human obesity and establishes a basis for modeling mechanistic and therapeutic aspects of this disease in the fly.