Keeping the eIF2 alpha kinase Gcn2 in check. Biochim Biophys Acta. 2014;1843:1948-1968. [PMID: 24732012 DOI: 10.1016/j.bbamcr.2014.04.006]

Chaperone Mediated Autophagy in the Crosstalk of Neurodegenerative Diseases and Metabolic Disorders

Chaperone Mediated Autophagy (CMA) is a lysosomal-dependent protein degradation pathway. At least 30% of cytosolic proteins can be degraded by this process. The two major protein players of CMA are LAMP-2A and HSC70. While LAMP-2A works as a receptor for protein substrates at the lysosomal membrane, HSC70 specifically binds protein targets and takes them for CMA degradation. Because of the broad spectrum of proteins able to be degraded by CMA, this pathway has been involved in physiological and pathological processes such as lipid and carbohydrate metabolism, and neurodegenerative diseases, respectively. Both, CMA, and the mentioned processes, are affected by aging and by inadequate nutritional habits such as a high fat diet or a high carbohydrate diet. Little is known regarding about CMA, which is considered a common regulation factor that links metabolism with neurodegenerative disorders. This review summarizes what is known about CMA, focusing on its molecular mechanism, its role in protein, lipid and carbohydrate metabolism. In addition, the review will discuss how CMA could be linked to protein, lipids and carbohydrate metabolism within neurodegenerative diseases. Furthermore, it will be discussed how aging and inadequate nutritional habits can have an impact on both CMA activity and neurodegenerative disorders.

A Rapid Extraction Method for mammalian cell cultures, suitable for quantitative immunoblotting analysis of proteins, including phosphorylated GCN2 and eIF2α

Many studies require the detection and relative quantitation of proteins from cell culture samples using immunoblotting. Limiting factors are the cost of protease inhibitors, the time required to break cells and generate samples, as well as the high risk of protein loss during cell breakage procedures. In addition, a common problem is the viscosity of lysed samples due to the released genomic DNA. As a consequence, the DNA needs to be broken down prior to denaturing polyacrylamide protein gel electrophoresis (SDS-PAGE), e.g. by passing the sample through a syringe gauge needle, sonication, or DNase treatment. In a quest to find a more cost-effective, fast, and yet robust procedure, we found that cell lysis, protein denaturation, and DNA fragmentation can be done in only two steps: harvesting followed by a simple non-laborious 2nd step. Similarly to many pre-existing cell breakage procedures, in our Rapid Protein Extraction (RPE) method, proteins liberated from cells are immediately exposed to a denaturing environment. However, advantages of our method are: •No breaking buffer is needed, instead proteins are liberated directly into the denaturing protein loading buffer used for SDS-PAGE. Consequently, our RPE method does not require any expensive inhibitors.•The RPE method does not involve post-lysis centrifugation steps; instead all cell material is dissolved during the 2nd step, the mixing-heat-treatment step which is new to this method. This prevents potential protein loss that may occur during centrifugation. In addition, this 2nd step simultaneously shears the genomic DNA, making an additional step for DNA fragmentation unnecessary.•The generated samples are suitable for high-quality quantitative immunoblotting. With our RPE method we successfully quantified the phosphorylated forms of protein kinase GCN2 and its substrate eIF2α. In fact, the western signals were stronger and with less background, as compared to samples generated with a pre-existing method.

Hypoxic Stress Upregulates the Expression of Slc38a1 in Brown Adipocytes via Hypoxia-Inducible Factor-1α

The availability of amino acid in the brown adipose tissue (BAT) has been shown to be altered under various conditions; however, little is known about the possible expression and pivotal role of amino acid transporters in BAT under physiological and pathological conditions. The present study comprehensively investigated whether amino acid transporters are regulated by obesogenic conditions in BAT in vivo. Moreover, we investigated the mechanism underlying the regulation of the expression of amino acid transporters by various stressors in brown adipocytes in vitro. The expression of solute carrier family 38 member 1 (Slc38a1; gene encoding sodium-coupled neutral amino acid transporter 1) was preferentially upregulated in the BAT of both genetic and acquired obesity mice in vivo. Moreover, the expression of Slc38a1 was induced by hypoxic stress through hypoxia-inducible factor-1α, which is a master transcription factor of the adaptive response to hypoxic stress, in brown adipocytes in vitro. These results indicate that Slc38a1 is an obesity-associated gene in BAT and a hypoxia-responsive gene in brown adipocytes.

Pulmonary vascular remodeling patterns and expression of general control nonderepressible 2 (GCN2) in pulmonary veno-occlusive disease

BACKGROUND

Heritable pulmonary veno-occlusive disease (PVOD) is linked to mutations in the eukaryotic initiation factor 2 alpha kinase 4 (EIF2AK4) gene, leading to a loss of general control nonderepressible 2 (GCN2). The role of GCN2 expression in pulmonary vascular remodeling remains obscure. We sought to identify specific histologic and biologic features in heritable PVOD.

METHODS

Clinical data and lung histology of 24 PVOD patients (12 EIF2AK4 mutation carriers, 12 non-carriers) were submitted to systematic histologic analysis and semiautomated morphometry. GCN2 expression was quantified by Western blotting in 24 PVOD patients, 44 patients with pulmonary arterial hypertension (PAH; 23 bone morphogenetic protein receptor type II [BMPR2] mutation carriers, 21 non-carriers), and 3 experimental pulmonary hypertension models.

RESULTS

PVOD patients showed a significant decrease of pulmonary arterial patency (p < 0.0001) compared with healthy controls. Histology of EIF2AK4 mutation carriers was distinctive from non-carriers regarding (1) arterial remodeling, with significantly more severe intimal fibrosis (p = 0.001), less severe medial hypertrophy (p = 0.001), and (2) stronger muscular hyperplasia of interlobular septal veins (p = 0.002). GCN2 expression was abolished in heritable PVOD (p < 0.0001), but also importantly decreased in sporadic PVOD (p = 0.03) as well as in heritable (p = 0.002) and idiopathic PAH (p = 0.003); moreover, GCN2 was abolished in 2 experimental pulmonary hypertension models and importantly decreased in 1 model (p < 0.0001 for all models).

CONCLUSIONS

Pulmonary arterial remodeling in PVOD is present to an important extent. A significant decrease of GCN2 expression is a common denominator of all tested groups of PVOD and PAH, including their respective experimental models. Our results underline specific morphologic and biologic similarities between PAH and PVOD and let us consider both conditions rather in one large spectrum of disease than as two distinct and clear-cut entities.

Amino Acid Sensor Kinase Gcn2 Is Required for Conidiation, Secondary Metabolism, and Cell Wall Integrity in the Taxol-Producer Pestalotiopsis microspora

The canonical Gcn2/Cpc1 kinase in fungi coordinates the expression of target genes in response to amino acid starvation. To investigate its possible role in secondary metabolism, we characterized a gcn2 homolog in the taxol-producing fungus Pestalotiopsis microspora. Deletion of the gene led to severe physiological defects under amino acid starvation, suggesting a conserved function of gcn2 in amino acid sensing. The mutant strain Δgcn2 displayed retardation in vegetative growth. It generated dramatically fewer conidia, suggesting a connection between amino acid metabolism and conidiation in this fungus. Importantly, disruption of the gene altered the production of secondary metabolites by HPLC profiling. For instance, under amino acid starvation, the deletion strain Δgcn2 barely produced secondary metabolites including the known natural product pestalotiollide B. Even more, we showed that gcn2 played critical roles in the tolerance to several stress conditions. Δgcn2 exhibited a hypersensitivity to Calcofluor white and Congo red, implying a role of Gcn2 in maintaining the integrity of the cell wall. This study suggests that Gcn2 kinase is an important global regulator in the growth and development of filamentous fungi and will provide knowledge for the manipulation of secondary metabolism in P. microspora.

Regulation of the unfolded protein response by noncoding RNA

Cells are exposed to various intrinsic and extrinsic stresses in both physiological and pathological conditions. To adapt to those conditions, cells have evolved various mechanisms to cope with the disturbances in protein demand, largely through the unfolded protein response (UPR) in the endoplasmic reticulum (ER), but also through the integrated stress response (ISR). Both responses initiate downstream signaling to transcription factors that, in turn, trigger adaptive programs and/or in the case of prolonged stress, cell death mechanisms. Recently, noncoding RNAs, including microRNA and long noncoding RNA, have emerged as key players in the stress responses. These noncoding RNAs act as both regulators and effectors of the UPR and fine-tune the output of the stress signaling pathways. Although much is known about the UPR and the cross talk that exists between pathways, the contribution of small noncoding RNA has not been fully assessed. Herein we bring together and review the current known functions of noncoding RNA in regulating adaptive pathways in both physiological and pathophysiological conditions, illustrating how they operate within the known UPR functions and contribute to diverse cellular outcomes.

The secret life of kinases: insights into non-catalytic signalling functions from pseudokinases

Over the past decade, our understanding of the mechanisms by which pseudokinases, which comprise ∼10% of the human and mouse kinomes, mediate signal transduction has advanced rapidly with increasing structural, biochemical, cellular and genetic studies. Pseudokinases are the catalytically defective counterparts of conventional, active protein kinases and have been attributed functions as protein interaction domains acting variously as allosteric modulators of conventional protein kinases and other enzymes, as regulators of protein trafficking or localisation, as hubs to nucleate assembly of signalling complexes, and as transmembrane effectors of such functions. Here, by categorising mammalian pseudokinases based on their known functions, we illustrate the mechanistic diversity among these proteins, which can be viewed as a window into understanding the non-catalytic functions that can be exerted by conventional protein kinases.

Activation of Gcn2 in response to different stresses

All organisms have evolved pathways to respond to different forms of cellular stress. The Gcn2 kinase is best known as a regulator of translation initiation in response to starvation for amino acids. Work in budding yeast has showed that the molecular mechanism of GCN2 activation involves the binding of uncharged tRNAs, which results in a conformational change and GCN2 activation. This pathway requires GCN1, which ensures delivery of the uncharged tRNA onto GCN2. However, Gcn2 is activated by a number of other stresses which do not obviously involve accumulation of uncharged tRNAs, raising the question how Gcn2 is activated under these conditions. Here we investigate the requirement for ongoing translation and tRNA binding for Gcn2 activation after different stresses in fission yeast. We find that mutating the tRNA-binding site on Gcn2 or deleting Gcn1 abolishes Gcn2 activation under all the investigated conditions. These results suggest that tRNA binding to Gcn2 is required for Gcn2 activation not only in response to starvation but also after UV irradiation and oxidative stress.

Chemical chaperone, TUDCA unlike PBA, mitigates protein aggregation efficiently and resists ER and non-ER stress induced HepG2 cell death

Stress induced BSA (bovine serum albumin) protein aggregation is effectively mitigated in vitro by TUDCA (tauroursodeoxycholic acid) than by PBA (4- phenylbutyric acid), chemical chaperones approved by FDA for the treatment of biliary cirrhosis and urea cycle disorders respectively. TUDCA, unlike PBA, enhances trypsin mediated digestion of BSA. TUDCA activates PERK, an ER-resident kinase that phosphorylates the alpha-subunit of eukaryotic initiation factor2 (eIF2α) and promotes the expression of activated transcription factor 4 (ATF4) in HepG2 cells. In contrast, PBA induced eIF2α phosphorylation is not mediated by PERK activation and results in low ATF4 expression. Neither chaperones promote expression of BiP, an ER chaperone, and CHOP (C/EBP homologous protein), downstream target of eIF2α-ATF4 pathway. Both chaperones mitigate tunicamycin induced PERK-eIF2α-ATF4-CHOP arm of UPR and expression of BiP. TUDCA, unlike PBA does not decrease cell viability and it also mitigates tunicamycin, UV-irradiation and PBA induced PARP (poly ADP-ribose polymerase) cleavage and cell death. These findings therefore suggest that TUDCA’s antiapoptotic activity to protect HepG2 cells and PBA’s activity that limits tumor cell progression may be important while considering their therapeutic potential.

Nutrient Sensing and the Oxidative Stress Response

The simplicity and effectiveness of calorie restriction (CR) in lifespan and healthspan extension have fascinated generations searching for the Fountain of Youth. CR reduces levels of oxidative stress and damage, which have been postulated in the free radical theory of aging as a major cause of aging and diseases of aging. This reduction has long been viewed as a result of passive slowing of metabolism. Recent advances in nutrient sensing have provided molecular insights into the oxidative stress response and suggest that CR triggers an active defense program involving a cascade of molecular regulators to reduce oxidative stress. Physiological studies have provided strong support for oxidative stress in the development of aging-associated conditions and diseases but have also revealed the surprising requirement for oxidative stress to support normal physiological functions and, in some contexts, even slow aging and prevent the progression of cancer. Deciphering the molecular mechanisms and physiological implications of the oxidative stress response during CR will increase our understanding of the basic biology of aging and pave the way for the design of CR mimetics to improve healthspan.

Recognition of a structural domain (RWDBD) in Gcn1 proteins that interacts with the RWD domain containing proteins

Abstract

The protein Gcn1 (General control non-derepressible 1) is found in virtually all eukaryotes, and is a key component of the general amino acid control signal transduction pathway. This pathway is best known for its importance for cells to sense and overcome amino acid starvation. Gcn1 directly binds to the RWD (RING finger-containing proteins, WD-repeat-containing proteins, and yeast DEAD (DEXD)-like helicases) domain of the protein kinase Gcn2, and this is essential for delivering the starvation signal to Gcn2. Gcn2, and thus the GAAC (General Amino Acid Control) pathway, then becomes activated enabling the cell to cope and overcome the starvation condition. Using sensitive homology detection and fold recognition methods a conserved structural domain in Gcn1, RWD Binding Domain (RWDBD), has been recognized that encompasses the region experimentally shown previously to be involved in Gcn2 binding. Further, the structural fold for this domain has been recognized as the ARM (Armadillo) domain, and residues likely to be involved in the binding of Gcn2 RWD domain have been identified within this structural domain. Thus, the current analysis provides a structural basis of Gcn1-Gcn2 association.

Reviewers

This article was reviewed by Dr Oliviero Carugo and Dr Michael Gromiha.

Electronic supplementary material

The online version of this article (doi:10.1186/s13062-017-0184-3) contains supplementary material, which is available to authorized users.

Human Keratinocyte Differentiation Requires Translational Control by the eIF2α Kinase GCN2

Appropriate and sequential differentiation of keratinocytes is essential for all functions of the human epidermis. Although transcriptional regulation has proven to be important for keratinocyte differentiation, little is known about the role of translational control. A key mechanism for modulating translation is through phosphorylation of the α subunit of eukaryotic initiation factor 2 (eIF2). A family of different eIF2α kinases function in the integrative stress response to inhibit general protein synthesis coincident with preferential translation of select mRNAs that participate in stress alleviation. Here we demonstrate that translational control through eIF2α phosphorylation is required for normal keratinocyte differentiation. Analyses of polysome profiles revealed that key differentiation genes, including involucrin, are bound to heavy polysomes during differentiation, despite decreased general protein synthesis. Induced eIF2α phosphorylation by the general control nonderepressible 2 (GCN2) protein kinase facilitated translational control and differentiation-specific protein expression during keratinocyte differentiation. Furthermore, loss of GCN2 thwarted translational control, normal epidermal differentiation, and differentiation gene expression in organotypic skin culture. These findings underscore a previously unknown function for GCN2 phosphorylation of eIF2α and translational control in the formation of an intact human epidermis.

HIV Exploits Antiviral Host Innate GCN2-ATF4 Signaling for Establishing Viral Replication Early in Infection

Antiviral innate host defenses against acute viral infections include suppression of host protein synthesis to restrict viral protein production. Less is known about mechanisms by which viral pathogens subvert host antiviral innate responses for establishing their replication and dissemination. We investigated early innate defense against human immunodeficiency virus (HIV) infection and viral evasion by utilizing human CD4⁺ T cell cultures in vitro and a simian immunodeficiency virus (SIV) model of AIDS in vivo. Our data showed that early host innate defense against the viral infection involves GCN2-ATF4 signaling-mediated suppression of global protein synthesis, which is exploited by the virus for supporting its own replication during early viral infection and dissemination in the gut mucosa. Suppression of protein synthesis and induction of protein kinase GCN2-ATF4 signaling were detected in the gut during acute SIV infection. These changes diminished during chronic viral infection. HIV replication induced by serum deprivation in CD4⁺ T cells was linked to the induction of ATF4 that was recruited to the HIV long terminal repeat (LTR) to promote viral transcription. Experimental inhibition of GCN2-ATF4 signaling either by a specific inhibitor or by amino acid supplementation suppressed the induction of HIV expression. Enhancing ATF4 expression through selenium administration resulted in reactivation of latent HIV in vitro as well as ex vivo in the primary CD4⁺ T cells isolated from patients receiving suppressive antiretroviral therapy (ART). In summary, HIV/SIV exploits the early host antiviral response through GCN2-ATF4 signaling by utilizing ATF4 for activating the viral LTR transcription to establish initial viral replication and is a potential target for HIV prevention and therapy.

Understanding how HIV overcomes host antiviral innate defense response in order to establish infection and dissemination is critical for developing prevention and treatment strategies. Most investigations focused on the viral pathogenic mechanisms leading to immune dysfunction following robust viral infection and dissemination. Less is known about mechanisms that enable HIV to establish its presence despite rapid onset of host antiviral innate response. Our novel findings provide insights into the viral strategy that hijacks the host innate response of the suppression of protein biosynthesis to restrict the virus production. The virus leverages transcription factor ATF4 expression during the GCN2-ATF4 signaling response and utilizes it to activate viral transcription through the LTR to support viral transcription and production in both HIV and SIV infections. This unique viral strategy is exploiting the innate response and is distinct from the mechanisms of immune dysfunction after the critical mass of viral loads is generated.

The CCAAT/enhancer-binding protein-ATF response elements-luciferase mouse model, an innovative tool to monitor the integrated stress response pathway in vivo

PURPOSE OF REVIEW

The article highlights the recent development of an ATF4 (activating transcription factor) inducible luciferase (LUC) mouse model to monitor the integrated stress response pathway (ISR) in vivo.

RECENT FINDING

The ISR pathway plays a key role in cellular adaptation to stress and is dysregulated in numerous diseases. The core event in this pathway is the phosphorylation of eukaryotic translation initiation factor 2 α, which leads to the recruitment of the transcription factor ATF4 to specific CCAAT/enhancer-binding protein-ATF response elements (CAREs) located in the promoters of target genes. To monitor the modulation of this pathway in the whole animal and at tissue and cellular levels, we generated a CARE-driven LUC mouse model. We validated the relevance of this model to study stress-related pathologies and recently observed the correlation between the ISR pathway induction in muscle and the occurrence of stress-induced skeletal muscle atrophy.

SUMMARY

The CARE-LUC mouse model represents an innovative tool for investigating the role of the ISR pathway in physiology and disease and opens new avenues for the development of drugs that could modify this important pathway in stress-related human diseases.

Tryptophan depletion under conditions that imitate insulin resistance enhances fatty acid oxidation and induces endothelial dysfunction through reactive oxygen species-dependent and independent pathways

In atherosclerosis-associated pathologic entities characterized by malnutrition and inflammation, L-tryptophan (TRP) levels are low. Insulin resistance is an independent cardiovascular risk factor and induces endothelial dysfunction by increasing fatty acid oxidation. It is also associated with inflammation and low TRP levels. Low TRP levels have been related to worse cardiovascular outcome. This study evaluated the effect of TRP depletion on endothelial dysfunction under conditions that imitate insulin resistance. Fatty acid oxidation, harmful pathways due to increased fatty acid oxidation, and endothelial dysfunction were assessed in primary human aortic endothelial cells cultured under normal glucose, low insulin conditions in the presence or absence of TRP. TRP depletion activated general control non-derepressible 2 kinase and inhibited aryl hydrocarbon receptor. It increased fatty acid oxidation by increasing expression and activity of carnitine palmitoyltransferase 1. Elevated fatty acid oxidation increased the formation of reactive oxygen species (ROS) triggering the polyol and hexosamine pathways, and enhancing protein kinase C activity and methylglyoxal production. TRP absence inhibited nitric oxide synthase activity in a ROS-dependent way, whereas it increased the expression of ICAM-1 and VCAM-1 in a ROS independent and possibly p53-dependent manner. Thus, TRP depletion, an amino acid whose low levels have been related to worse cardiovascular outcome and to inflammatory atherosclerosis-associated pathologic entities, under conditions that imitate insulin resistance enhances fatty acid oxidation and induces endothelial dysfunction through ROS-dependent and independent pathways. These findings may offer new insights at the molecular mechanisms involved in accelerated atherosclerosis that frequently accompanies malnutrition and inflammation.

Novel therapeutic approaches targeting L-type amino acid transporters for cancer treatment

L-type amino acid transporters (LATs) mainly assist the uptake of neutral amino acids into cells. Four LATs (LAT1, LAT2, LAT3 and LAT4) have so far been identified. LAT1 (SLC7A5) has been attracting much attention in the field of cancer research since it is commonly up-regulated in various cancers. Basic research has made it increasingly clear that LAT1 plays a predominant role in malignancy. The functional significance of LAT1 in cancer and the potential therapeutic application of the features of LAT1 to cancer management are described in this review.

Targeting 14-3-3 adaptor protein-protein interactions to stimulate central nervous system repair

The goal of developing treatments for central nervous system (CNS) injuries is becoming more attainable with the recent identification of various drugs that can repair damaged axons. These discoveries have stemmed from screening efforts, large expression datasets and an improved understanding of the cellular and molecular biology underlying axon growth. It will be important to continue searching for new compounds that can induce axon repair. Here we describe how a family of adaptor proteins called 14-3-3s can be targeted using small molecule drugs to enhance axon outgrowth and regeneration. 14-3-3s bind to many functionally diverse client proteins to regulate their functions. We highlight the recent discovery of the axon-growth promoting activity of fusicoccin-A, a fungus-derived small molecule that stabilizes 14-3-3 interactions with their client proteins. Here we discuss how fusicoccin-A could serve as a starting point for the development of drugs to induce CNS repair.

Regulation of Translation by TOR, eIF4E and eIF2α in Plants: Current Knowledge, Challenges and Future Perspectives

An important step in eukaryotic gene expression is the synthesis of proteins from mRNA, a process classically divided into three stages, initiation, elongation, and termination. Translation is a precisely regulated and conserved process in eukaryotes. The presence of plant-specific translation initiation factors and the lack of well-known translational regulatory pathways in this kingdom nonetheless indicate how a globally conserved process can diversify among organisms. The control of protein translation is a central aspect of plant development and adaptation to environmental stress, but the mechanisms are still poorly understood. Here we discuss current knowledge of the principal mechanisms that regulate translation initiation in plants, with special attention to the singularities of this eukaryotic kingdom. In addition, we highlight the major recent breakthroughs in the field and the main challenges to address in the coming years.

General Amino Acid Control and 14-3-3 Proteins Bmh1/2 Are Required for Nitrogen Catabolite Repression-Sensitive Regulation of Gln3 and Gat1 Localization

Nitrogen catabolite repression (NCR), the ability of Saccharomyces cerevisiae to use good nitrogen sources in preference to poor ones, derives from nitrogen-responsive regulation of the GATA family transcription activators Gln3 and Gat1 In nitrogen-replete conditions, the GATA factors are cytoplasmic and NCR-sensitive transcription minimal. When only poor nitrogen sources are available, Gln3 is nuclear, dramatically increasing GATA factor-mediated transcription. This regulation was originally attributed to mechanistic Tor protein kinase complex 1 (mTorC1)-mediated control of Gln3 However, we recently showed that two regulatory systems act cumulatively to maintain cytoplasmic Gln3 sequestration, only one of which is mTorC1. Present experiments demonstrate that the other previously elusive component is uncharged transfer RNA-activated, Gcn2 protein kinase-mediated general amino acid control (GAAC). Gcn2 and Gcn4 are required for NCR-sensitive nuclear Gln3-Myc¹³ localization, and from epistasis experiments Gcn2 appears to function upstream of Ure2 Bmh1/2 are also required for nuclear Gln3-Myc¹³ localization and appear to function downstream of Ure2 Overall, Gln3 phosphorylation levels decrease upon loss of Gcn2, Gcn4, or Bmh1/2 Our results add a new dimension to nitrogen-responsive GATA-factor regulation and demonstrate the cumulative participation of the mTorC1 and GAAC pathways, which respond oppositely to nitrogen availability, in the nitrogen-responsive control of catabolic gene expression in yeast.

Glucose Starvation Alters Heat Shock Response, Leading to Death of Wild Type Cells and Survival of MAP Kinase Signaling Mutant

A moderate heat shock induces Neurospora crassa to synthesize large quantities of heat shock proteins that are protective against higher, otherwise lethal temperatures. However, wild type cells do not survive when carbohydrate deprivation is added to heat shock. In contrast, a mutant strain defective in a stress-activated protein kinase does survive the combined stresses. In order to understand the basis for this difference in survival, we have determined the relative levels of detected proteins in the mutant and wild type strain during dual stress, and we have identified gene transcripts in both strains whose quantities change in response to heat shock or dual stress. These data and supportive experimental evidence point to reasons for survival of the mutant strain. By using alternative respiratory mechanisms, these cells experience less of the oxidative stress that proves damaging to wild type cells. Of central importance, mutant cells recycle limited resources during dual stress by undergoing autophagy, a process that we find utilized by both wild type and mutant cells during heat shock. Evidence points to inappropriate activation of TORC1, the central metabolic regulator, in wild type cells during dual stress, based upon behavior of an additional signaling mutant and inhibitor studies.

Inappropriate translation inhibition and P-body formation cause cold-sensitivity in tryptophan-auxotroph yeast mutants

In response to different adverse conditions, most eukaryotic organisms, including Saccharomyces cerevisiae, downregulate protein synthesis through the phosphorylation of eIF2α (eukaryotic initiation factor 2α) by Gcn2, a highly conserved protein kinase. Gcn2 also controls the translation of Gcn4, a transcription factor involved in the induction of amino acid biosynthesis enzymes. Here, we have studied the functional role of Gcn2 and Gcn2-regulating proteins, in controlling translation during temperature downshifts of TRP1 and trp1 yeast cells. Our results suggest that neither cold-instigated amino acid limitation nor Gcn2 are involved in the translation suppression at low temperature. However, loss of TRP1 causes increased eIF2α phosphorylation, Gcn2-dependent polysome disassembly and overactivity of Gcn4, which result in cold-sensitivity. Indeed, knock-out of GCN2 improves cold growth of trp1 cells. Likewise, mutation of several Gcn2-regulators and effectors results in cold-growth effects. Remarkably, we found that Hog1, the osmoresponsive MAPK, plays a role in the regulatory mechanism of Gcn2-eIF2α. Finally, we demonstrated that P-body formation responds to a downshift in temperature in a TRP1-dependent manner and is required for cold tolerance.

The genetics of isoflurane-induced developmental neurotoxicity

INTRODUCTION

Neurotoxicity induced by early developmental exposure to volatile anesthetics is a characteristic of organisms across a wide range of species, extending from the nematode C. elegans to mammals. Prevention of anesthetic-induced neurotoxicity (AIN) will rely upon an understanding of its underlying mechanisms. However, no forward genetic screens have been undertaken to identify the critical pathways affected in AIN. By characterizing such pathways, we may identify mechanisms to eliminate isoflurane induced AIN in mammals.

METHODS

Chemotaxis in adult C. elegans after larval exposure to isoflurane was used to measure AIN. We initially compared changes in chemotaxis indices between classical mutants known to affect nervous system development adding mutants in response to data. Activation of specific genes was visualized using fluorescent markers. Animals were then treated with rapamycin or preconditioned with isoflurane to test effects on AIN.

RESULTS

Forty-four mutations, as well as pharmacologic manipulations, identified two pathways, highly conserved from invertebrates to humans, that regulate AIN in C. elegans. Activation of one stress-protective pathway (DAF-2 dependent) eliminates AIN, while activation of a second stress-responsive pathway (endoplasmic reticulum (ER) associated stress) causes AIN. Pharmacologic inhibition of the mechanistic Target of Rapamycin (mTOR) blocks ER-stress and AIN. Preconditioning with isoflurane prior to larval exposure also inhibited AIN.

DISCUSSION

Our data are best explained by a model in which isoflurane acutely inhibits mitochondrial function causing activation of responses that ultimately lead to ER-stress. The neurotoxic effect of isoflurane can be completely prevented by manipulations at multiple points in the pathways that control this response. Endogenous signaling pathways can be recruited to protect organisms from the neurotoxic effects of isoflurane.

The integrated stress response

In response to diverse stress stimuli, eukaryotic cells activate a common adaptive pathway, termed the integrated stress response (ISR), to restore cellular homeostasis. The core event in this pathway is the phosphorylation of eukaryotic translation initiation factor 2 alpha (eIF2α) by one of four members of the eIF2α kinase family, which leads to a decrease in global protein synthesis and the induction of selected genes, including the transcription factor ATF4, that together promote cellular recovery. The gene expression program activated by the ISR optimizes the cellular response to stress and is dependent on the cellular context, as well as on the nature and intensity of the stress stimuli. Although the ISR is primarily a pro-survival, homeostatic program, exposure to severe stress can drive signaling toward cell death. Here, we review current understanding of the ISR signaling and how it regulates cell fate under diverse types of stress.

The NEDD8-activating enzyme inhibitor pevonedistat activates the eIF2α and mTOR pathways inducing UPR-mediated cell death in acute lymphoblastic leukemia

Acute lymphoblastic leukemia (ALL) is the leading cause of cancer-related death in children, and cure rates for adults remain dismal. Further, effective treatment strategies for relapsed/refractory ALL remain elusive. We previously uncovered that ALL cells are prone to apoptosis via endoplasmic reticulum (ER) stress/unfolded protein response (UPR)-mediated mechanisms. We investigated the antineoplastic activity of pevonedistat^®, a novel NEDD8-activating enzyme inhibitor that targets E3 cullin-RING ligases (CRLs) dependent proteasomal protein degradation, in ALL. Herein, we report that pevonedistat induces apoptosis in ALL cells by dysregulating the translational machinery leading to induction of proteotoxic/ER stress and UPR-mediated cell death. Mechanistically, pevonedistat led to P-eIF2a dephosphorylation causing atypical proteotoxic/ER stress from failure to halt protein translation via the UPR and upregulation of mTOR/p70S6K. Additional studies revealed that pevonedistat re-balanced the homeostasis of pro- and anti-apoptotic proteins to favor cell death through altered expression and/or activity of Mcl-1, NOXA, and BIM, suggesting that pevonedistat has a "priming" effect on ALL by altering the apoptotic threshold through modulation of Mcl-1 activity. Further, we demonstrated that pevonedistat synergizes with selected anti-leukemic agents in vitro, and prolongs survival of NSG mice engrafted with ALL cells, lending support for the use of pevonedistat as part of a multi-agent approach.

Regulation of fetal liver growth in a model of diet restriction in the pregnant rat

Limited nutrient availability is a cause of intrauterine growth restriction (IUGR), a condition that has important implications for the well being of the offspring. Using the established IUGR model of maternal fasting in the rat, we investigated mechanisms that control gene expression and mRNA translation in late-gestation fetal liver. Maternal fasting for 48 h during the last one-third of gestation was associated with a 10-15% reduction in fetal body weight and a disproportionate one-third reduction in total fetal liver protein. The fetal liver transcriptome showed only subtle changes consistent with reduced cell proliferation and enhanced differentiation in IUGR. Effects on the transcriptome could not be attributed to specific transcription factors. We purified translating polysomes to profile the population of mRNAs undergoing active translation. Microarray analysis of the fetal liver translatome indicated a global reduction of translation. The only targeted effect was enhanced translation of mitochondrial ribosomal proteins in IUGR, consistent with enhanced mitochondrial biogenesis. There was no evidence for attenuated signaling through the mammalian target of rapamycin (mTOR). Western blot analysis showed no changes in fetal liver mTOR signaling. However, eukaryotic initiation factor 2α (eIF2α) phosphorylation was increased in livers from IUGR fetuses, consistent with a role in global translation control. Our data indicate that IUGR-associated changes in hepatic gene expression and mRNA translation likely involve a network of complex regulatory mechanisms, some of which are novel and distinct from those that mediate the response of the liver to nutrient restriction in the adult rat.

Nutrient shortage triggers the hexosamine biosynthetic pathway via the GCN2-ATF4 signalling pathway

The hexosamine biosynthetic pathway (HBP) is a nutrient-sensing metabolic pathway that produces the activated amino sugar UDP-N-acetylglucosamine, a critical substrate for protein glycosylation. Despite its biological significance, little is known about the regulation of HBP flux during nutrient limitation. Here, we report that amino acid or glucose shortage increase GFAT1 production, the first and rate-limiting enzyme of the HBP. GFAT1 is a transcriptional target of the activating transcription factor 4 (ATF4) induced by the GCN2-eIF2α signalling pathway. The increased production of GFAT1 stimulates HBP flux and results in an increase in O-linked β-N-acetylglucosamine protein modifications. Taken together, these findings demonstrate that ATF4 provides a link between nutritional stress and the HBP for the regulation of the O-GlcNAcylation-dependent cellular signalling.

Activation of GCN2 kinase by ribosome stalling links translation elongation with translation initiation

Ribosome stalling during translation has recently been shown to cause neurodegeneration, yet the signaling pathways triggered by stalled elongation complexes are unknown. To investigate these pathways we analyzed the brain of C57BL/6J-Gtpbp2^nmf205-/- mice in which neuronal elongation complexes are stalled at AGA codons due to deficiencies in a tRNA^Arg_UCU tRNA and GTPBP2, a mammalian ribosome rescue factor. Increased levels of phosphorylation of eIF2α (Ser51) were detected prior to neurodegeneration in these mice and transcriptome analysis demonstrated activation of ATF4, a key transcription factor in the integrated stress response (ISR) pathway. Genetic experiments showed that this pathway was activated by the eIF2α kinase, GCN2, in an apparent deacylated tRNA-independent fashion. Further we found that the ISR attenuates neurodegeneration in C57BL/6J-Gtpbp2^nmf205-/- mice, underscoring the importance of cellular and stress context on the outcome of activation of this pathway. These results demonstrate the critical interplay between translation elongation and initiation in regulating neuron survival during cellular stress.

DOI:http://dx.doi.org/10.7554/eLife.14295.001

Information stored in DNA is used to make proteins in a two-step process. First, the DNA is copied to make molecules of messenger ribonucleic acid (or messenger RNA for short). Next, machines called ribosomes use the messenger RNAs as templates to assemble chains of amino acids – the building blocks of proteins – in a process called translation. Another type of RNA molecule called transfer RNA carries each amino acid to the ribosomes. If a specific transfer RNA is not available for translation at the right time, the ribosome might stall as it moves along the messenger RNA. At this point, the ribosome needs to be restarted or it will fall off the mRNA without finishing the protein.

In 2014, a group of researchers reported that certain types of brain cells are very sensitive to ribosome stalling, and tend to die if translation does not continue. A protein called GTPBP2 was shown to play an important role in restarting stalled ribosomes in these cells. Here, Ishimura, Nagy et al. – including some of the researchers from the earlier work – investigated the molecular pathways that ribosome stalling triggers in brain cells using mutant mice that lacked the GTPBP2 protein. The experiments show that ribosome stalling activates an enzyme known as GCN2, which was already known to sense other types of malfunctions in cellular processes.

Ishimura, Nagy et al. also show that GCN2 triggers stress responses in the cells by activating a communication system called the ATF4 pathway. This pathway protects the cells from damage, and its absence results in more rapid cell deterioration and death. The next challenges are to understand the exact mechanism by which GCN2 senses stalled ribosomes, and to find out how ribosome stalling causes the death of brain cells.

DOI:http://dx.doi.org/10.7554/eLife.14295.002

Differential effects of the two amino acid sensing systems, the GCN2 kinase and the mTOR complex 1, on primary human alloreactive CD4⁺ T-cells

Amino acid deprivation activates general control nonderepressible 2 (GCN2) kinase and inhibits mammalian target of rapamycin (mTOR), affecting the immune response. In this study, the effects of GCN2 kinase activation or mTOR inhibition on human alloreactive CD4+ T-cells were evaluated. The mixed lymphocyte reaction, as a model of alloreactivity, the GCN2 kinase activator, tryptophanol (TRP), and the mTOR complex 1 inhibitor, rapamycin (RAP), were used. Both TRP and RAP suppressed cell proliferation and induced cell apoptosis. These events were p53-independent in the case of RAP, but were accompanied by an increase in p53 levels in the case of TRP. TRP decreased the levels of the Th2 signature transcription factor, GATA-3, as RAP did, yet the latter also decreased the levels of the Th1 and Th17 signature transcription factors, T-bet and RORγt, whereas it increased the levels of the Treg signature transcription factor, FoxP3. Accordingly, TRP decreased the production of interleukin (IL)-4, as RAP did, but RAP also decreased the levels of interferon-γ (IFN-γ) and IL-17. Both TRP and RAP increased the levels of IL-10. As regards hypoxia-inducible factor-1α (HIF-1α), which upregulates the Th17/Treg ratio, its levels were decreased by RAP. TRP increased the HIF-1α levels, which however, remained inactive. In conclusion, our findings indicate that, in primary human alloreactive CD4+ T-cells, the two systems that sense amino acid deprivation affect cell proliferation, apoptosis and differentiation in different ways or through different mechanisms. Both mTOR inhibition and GCN2 kinase activation exert immunosuppressive effects, since they inhibit cell proliferation and induce apoptosis. As regards CD4+ T-cell differentiation, mTOR inhibition exerted a more profound effect, since it suppressed differentiation into the Th1, Th2 and Th17 lineages, while it induced Treg differentiation. On the contrary, the activation of GCN2 kinase suppressed only Th2 differentiation.

Pulmonary veno-occlusive disease

Pulmonary veno-occlusive disease (PVOD) is a rare form of pulmonary hypertension (PH) characterised by preferential remodelling of the pulmonary venules. In the current PH classification, PVOD and pulmonary capillary haemangiomatosis (PCH) are considered to be a common entity and represent varied expressions of the same disease. The recent discovery of biallelic mutations in the EIF2AK4 gene as the cause of heritable PVOD/PCH represents a major milestone in our understanding of the molecular pathogenesis of PVOD. Although PVOD and pulmonary arterial hypertension (PAH) share a similar clinical presentation, with features of severe precapillary PH, it is important to differentiate these two conditions as PVOD carries a worse prognosis and life-threatening pulmonary oedema may occur following the initiation of PAH therapy. An accurate diagnosis of PVOD based on noninvasive investigations is possible utilising oxygen parameters, low diffusing capacity for carbon monoxide and characteristic signs on high-resolution computed tomography of the chest. No evidence-based medical therapy exists for PVOD at present and lung transplantation remains the preferred definitive therapy for eligible patients.

Interactome analysis of herpes simplex virus 1 envelope glycoprotein H

Herpes simplex virus 1 (HSV-1) envelope glycoprotein H (gH) is important for viral entry into cells and nuclear egress of nucleocapsids. To clarify additional novel roles of gH during HSV-1 replication, host cell proteins that interact with gH were screened for by tandem affinity purification coupled with mass spectrometry-based proteomics in 293T cells transiently expressing gH. This screen identified 123 host cell proteins as potential gH interactors. Of these proteins, general control nonderepressive-1 (GCN1), a trans-acting positive effector of GCN2 kinase that regulates phosphorylation of the α subunit of translation initiation factor 2 (eIF2α), was subsequently confirmed to interact with gH in HSV-1-infected cells. eIF2α phosphorylation is known to downregulate protein synthesis, and various viruses have evolved mechanisms to prevent the accumulation of phosphorylated eIF2α in infected cells. Here, it was shown that GCN1 knockdown reduces phosphorylation of eIF2α in HSV-1-infected cells and that the gH-null mutation increases eIF2α in HSV-1-infected cells, whereas gH overexpression in the absence of other HSV-1 proteins reduces eIF2α phosphorylation. These findings suggest that GCN1 can regulate eIF2α phosphorylation in HSV-1-infected cells and that the GCN1-binding viral partner gH is necessary and sufficient to prevent the accumulation of phosphorylated eIF2α. Our database of 123 host cell proteins potentially interacting with gH will be useful for future studies aimed at unveiling further novel functions of gH and the roles of cellular proteins in HSV-1-infected cells.

HOXB9 acts as a negative regulator of activated human T cells in response to amino acid deficiency

T-cell activation is an energy expenditure process and should be properly controlled in accordance with the availability of nutrients such as amino acids to eliminate wasteful energy consumption. However, the details of response to amino acids insufficiency in activated T cells remain largely unknown. Here we show that homeobox B9 (HOXB9), a member of the homeobox gene family that is known as a morphogenesis regulator, acts as a suppressor of activated human T cells to address amino acid starvation. The expression of HOXB9 was triggered by amino acid deprivation as well as functional inhibition of L-type amino acid transporter 1 (also known as SLC7A5) via activating transcription factor 4 in activated T cells. HOXB9 interfered the activities of NF-κB, nuclear factor of activated T-cells (NFAT) and AP-1 but not retinoic acid receptor-related orphan receptor, resulting in attenuation of the production of selective cytokines in activated T cells. Thus, the morphogenetic gene plays an unexpected role in the regulation of cellular metabolism with changes in the nutrition status in human T cells.

Perturbations in actin dynamics reconfigure protein complexes that modulate GCN2 activity and promote an eIF2 response

Genetic and pharmacological interventions in yeast and mammalian cells have suggested a cross-talk between the actin cytoskeleton and protein synthesis. Regulation of the activity of the translation initiation factor 2 (eIF2) is a paramount mechanism for cells to rapidly adjust the rate of protein synthesis and to trigger reprogramming of gene expression to adapt in response to internal and external cues. Here we show that disruption of F-actin in mammalian cells inhibits translation in a GCN2-dependent manner, correlating with increased levels of uncharged tRNA. GCN2 activation increased phosphorylation of its substrate eIF2α and the induction of the integrated stress response master regulator, ATF4. GCN2 activation by latrunculin is dependent on GCN1 and inhibited by IMPACT. Our data suggest that GCN2 occurs in two different complexes, GCN2-eEF1A and GCN2-GCN1. Depolymerization of F-actin shifts GCN2 to favor the complex with GCN1, concomitant with GCN1 being released from its binding to IMPACT, which is sequestered by G-actin. These events may further contribute to GCN2 activation. Our findings indicate that GCN2 is an important sensor of the state of the actin cytoskeleton.

Endoplasmic Reticulum Stress Signaling in Plant Immunity--At the Crossroad of Life and Death

Rapid and complex immune responses are induced in plants upon pathogen recognition. One form of plant defense response is a programmed burst in transcription and translation of pathogenesis-related proteins, of which many rely on ER processing. Interestingly, several ER stress marker genes are up-regulated during early stages of immune responses, suggesting that enhanced ER capacity is needed for immunity. Eukaryotic cells respond to ER stress through conserved signaling networks initiated by specific ER stress sensors tethered to the ER membrane. Depending on the nature of ER stress the cell prioritizes either survival or initiates programmed cell death (PCD). At present two plant ER stress sensors, bZIP28 and IRE1, have been described. Both sensor proteins are involved in ER stress-induced signaling, but only IRE1 has been additionally linked to immunity. A second branch of immune responses relies on PCD. In mammals, ER stress sensors are involved in activation of PCD, but it is unclear if plant ER stress sensors play a role in PCD. Nevertheless, some ER resident proteins have been linked to pathogen-induced cell death in plants. In this review, we will discuss the current understanding of plant ER stress signaling and its cross-talk with immune signaling.

Stress-induced inhibition of translation independently of eIF2α phosphorylation

Exposure of fission yeast cells to ultraviolet (UV) light leads to inhibition of translation and phosphorylation of the eukaryotic initiation factor-2α (eIF2α). This phosphorylation is a common response to stress in all eukaryotes. It leads to inhibition of translation at the initiation stage and is thought to be the main reason why stressed cells dramatically reduce protein synthesis. Phosphorylation of eIF2α has been taken as a readout for downregulation of translation, but the role of eIF2α phosphorylation in the downregulation of general translation has not been much investigated. We show here that UV-induced global inhibition of translation in fission yeast cells is independent of eIF2α phosphorylation and the eIF2α kinase general control nonderepressible-2 protein (Gcn2). Also, in budding yeast and mammalian cells, the UV-induced translational depression is largely independent of GCN2 and eIF2α phosphorylation. Furthermore, exposure of fission yeast cells to oxidative stress generated by hydrogen peroxide induced an inhibition of translation that is also independent of Gcn2 and of eIF2α phosphorylation. Our findings show that stress-induced translational inhibition occurs through an unknown mechanism that is likely to be conserved through evolution.

Summary: In contrast to textbook knowledge, the phosphorylation of translation initiation factor eIF2α is not required for UV-induced inhibition of protein synthesis, which we show in three different cell types.

T cell metabolism drives immunity

Buck et al. discuss the role of lymphocyte metabolism on immune cell development and function.

Lymphocytes must adapt to a wide array of environmental stressors as part of their normal development, during which they undergo a dramatic metabolic remodeling process. Research in this area has yielded surprising findings on the roles of diverse metabolic pathways and metabolites, which have been found to regulate lymphocyte signaling and influence differentiation, function and fate. In this review, we integrate the latest findings in the field to provide an up-to-date resource on lymphocyte metabolism.

Pulmonary capillary hemangiomatosis: a focus on the EIF2AK4 mutation in onset and pathogenesis

Pulmonary capillary hemangiomatosis (PCH) is a pulmonary vascular disease that mainly affects small capillaries in the lung, and is often misdiagnosed as pulmonary arterial hypertension or pulmonary veno-occlusive disease due to similarities in their clinical presentations, prognosis, and management. In patients who are symptomatic, there is a high mortality rate with median survival of 3 years after diagnosis. Both idiopathic and familial PCH cases are being reported, indicating there is genetic component in disease etiology. Mutations in the eukaryotic translation initiation factor 2α kinase 4 (EIF2AK4) gene were identified in familial and idiopathic PCH cases, suggesting EIF2AK4 is a genetic risk factor for PCH. EIF2AK4 mutations were identified in 100% (6/6) of autosomal recessively inherited familial PCH and 20% (2/10) of sporadic PCH cases. EIF2AK4 is a member of serine/threonine kinases. It downregulates protein synthesis in response to a variety of cellular stress such as hypoxia, viral infection, and amino acid deprivation. Bone morphogenetic protein receptor 2 (BMPR2) is a major genetic risk factor in pulmonary arterial hypertension and EIF2AK4 potentially connects with BMPR2 to cause PCH. L-Arginine is substrate of nitric oxide synthase, and L-arginine is depleted during the production of nitric oxide, which may activate EIF2AK4 to inhibit protein synthesis and negatively regulate vasculogenesis. Mammalian target of rapamycin and EIF2α kinase are two major pathways for translational regulation. Mutant EIF2AK4 could promote proliferation of small pulmonary arteries by crosstalk with mammalian targets of the rapamycin signaling pathway. EIF2AK4 may regulate angiogenesis by modulating the immune system in PCH pathogenesis. The mechanisms of abnormal capillary angiogenesis are suggested to be similar to that of tumor vascularization. Specific therapies were developed according to pathogenesis and are proved to be effective in reported cases. Targeting the EIF2AK4 pathway may provide a novel therapy for PCH.

The Gcn2 Regulator Yih1 Interacts with the Cyclin Dependent Kinase Cdc28 and Promotes Cell Cycle Progression through G2/M in Budding Yeast

The Saccharomyces cerevisiae protein Yih1, when overexpressed, inhibits the eIF2 alpha kinase Gcn2 by competing for Gcn1 binding. However, deletion of YIH1 has no detectable effect on Gcn2 activity, suggesting that Yih1 is not a general inhibitor of Gcn2, and has no phenotypic defect identified so far. Thus, its physiological role is largely unknown. Here, we show that Yih1 is involved in the cell cycle. Yeast lacking Yih1 displays morphological patterns and DNA content indicative of a delay in the G2/M phases of the cell cycle, and this phenotype is independent of Gcn1 and Gcn2. Accordingly, the levels of phosphorylated eIF2α, which show a cell cycle-dependent fluctuation, are not altered in cells devoid of Yih1. We present several lines of evidence indicating that Yih1 is in a complex with Cdc28. Yih1 pulls down endogenous Cdc28 in vivo and this interaction is enhanced when Cdc28 is active, suggesting that Yih1 modulates the function of Cdc28 in specific stages of the cell cycle. We also demonstrate, by Bimolecular Fluorescence Complementation, that endogenous Yih1 and Cdc28 interact with each other, confirming Yih1 as a bona fide Cdc28 binding partner. Amino acid substitutions within helix H2 of the RWD domain of Yih1 enhance Yih1-Cdc28 association. Overexpression of this mutant, but not of wild type Yih1, leads to a phenotype similar to that of YIH1 deletion, supporting the view that Yih1 is involved through Cdc28 in the regulation of the cell cycle. We further show that IMPACT, the mammalian homologue of Yih1, interacts with CDK1, the mammalian counterpart of Cdc28, indicating that the involvement with the cell cycle is conserved. Together, these data provide insights into the cellular function of Yih1/IMPACT, and provide the basis for future studies on the role of this protein in the cell cycle.

Gcn1 contacts the small ribosomal protein Rps10, which is required for full activation of the protein kinase Gcn2

General control non-derepressible 2 (Gcn2) is involved in many biological functions. We found that Gcn1 directly contacts small ribosomal protein (Rps)10 on the ribosome and this is required for maintaining basal Gcn2-activity and efficiently activating Gcn2 under amino acid starvation.

GCN2 is required to increase fibroblast growth factor 21 and maintain hepatic triglyceride homeostasis during asparaginase treatment

The antileukemic agent asparaginase triggers the amino acid response (AAR) in the liver by activating the eukaryotic initiation factor 2 (eIF2) kinase general control nonderepressible 2 (GCN2). To explore the mechanism by which AAR induction is necessary to mitigate hepatic lipid accumulation and prevent liver dysfunction during continued asparaginase treatment, wild-type and Gcn2 null mice were injected once daily with asparaginase or phosphate buffered saline for up to 14 days. Asparaginase induced mRNA expression of multiple AAR genes and greatly increased circulating concentrations of the metabolic hormone fibroblast growth factor 21 (FGF21) independent of food intake. Loss of Gcn2 precluded mRNA expression and circulating levels of FGF21 and blocked mRNA expression of multiple genes regulating lipid synthesis and metabolism including Fas, Ppara, Pparg, Acadm, and Scd1 in both liver and white adipose tissue. Furthermore, rates of triglyceride export and protein expression of apolipoproteinB-100 were significantly reduced in the livers of Gcn2 null mice treated with asparaginase, providing a mechanistic basis for the increase in hepatic lipid content. Loss of AAR-regulated antioxidant defenses in Gcn2 null livers was signified by reduced Gpx1 gene expression alongside increased lipid peroxidation. Substantial reductions in antithrombin III hepatic expression and activity in the blood of asparaginase-treated Gcn2 null mice indicated liver dysfunction. These results suggest that the ability of the liver to adapt to prolonged asparaginase treatment is influenced by GCN2-directed regulation of FGF21 and oxidative defenses, which, when lost, corresponds with maladaptive effects on lipid metabolism and hemostasis.

A membrane-bound eIF2 alpha kinase located in endosomes is regulated by heme and controls differentiation and ROS levels in Trypanosoma cruzi

Translation initiation has been described as a key step for the control of growth and differentiation of several protozoan parasites in response to environmental changes. This occurs by the activation of protein kinases that phosphorylate the alpha subunit of the translation initiation factor 2 (eIF2α), which decreases translation, and in higher eukaryotes favors the expression of stress remedial response genes. However, very little is known about the signals that activate eIF2α kinases in protozoan parasites. Here, we characterized an eIF2α kinase of Trypanosoma cruzi (TcK2), the agent of Chagas’ disease, as a transmembrane protein located in organelles that accumulate nutrients in proliferating parasite forms. We found that heme binds specifically to the catalytic domain of the kinase, inhibiting its activity. In the absence of heme, TcK2 is activated, arresting cell growth and inducing differentiation of proliferative into infective and non-proliferative forms. Parasites lacking TcK2 lose this differentiation capacity and heme is not stored in reserve organelles, remaining in the cytosol. TcK2 null cells display growth deficiencies, accumulating hydrogen peroxide that drives the generation of reactive oxygen species. The augmented level of hydrogen peroxide occurs as a consequence of increased superoxide dismutase activity and decreased peroxide activity. These phenotypes could be reverted by the re-expression of the wild type but not of a TcK2 dead mutant. These findings indicate that heme is a key factor for the growth control and differentiation through regulation of an unusual type of eIF2α kinase in T. cruzi.

Trypanosoma cruzi proliferates as epimastigotes in the midgut of the insect vector filled with blood meal. There, it accumulates nutrients in specific endosomal organelles. The parasite moves towards the hindgut and when the blood is completely digested, these organelles are consumed. At this moment, the insect is ready for a new feeding cycle that promotes the release of infective metacyclic-trypomastigote forms. We have previously found that such differentiation involves protein synthesis arrest through the phosphorylation of the eukaryotic translation initiation factor 2α (eIF2α). Now, we show that one of the kinases (TCK2) that phosphorylate eIF2α is localized in these endosomes. TcK2 binds and is specifically inhibited by heme derived from blood hemoglobin. We also found that heme inhibits differentiation, suggesting that it is an important signal for differentiation. By generating knockouts of TcK2, we observed an increased accumulation of heme in the cytosol, which induced cellular damage by affecting the reactive oxygen metabolism in the parasite. We conclude that this eIF2α kinase senses cytosolic heme obtained from the blood meal, promoting its storage in the cytosolic organelles. When heme levels are decreased in the cytosol, TcK2 activation can then arrest protein synthesis that is followed by the induction of the differentiation of proliferative epimastigote forms to infective metacyclic-trypomastigotes.

The varieties of immunological experience: of pathogens, stress, and dendritic cells

In the 40 years since their discovery, dendritic cells (DCs) have been recognized as central players in immune regulation. DCs sense microbial stimuli through pathogen-recognition receptors (PRRs) and decode, integrate, and present information derived from such stimuli to T cells, thus stimulating immune responses. DCs can also regulate the quality of immune responses. Several functionally specialized subsets of DCs exist, but DCs also display functional plasticity in response to diverse stimuli. In addition to sensing pathogens via PRRs, emerging evidence suggests that DCs can also sense stress signals, such as amino acid starvation, through ancient stress and nutrient sensing pathways, to stimulate adaptive immunity. Here, I discuss these exciting advances in the context of a historic perspective on the discovery of DCs and their role in immune regulation. I conclude with a discussion of emerging areas in DC biology in the systems immunology era and suggest that the impact of DCs on immunity can be usefully contextualized in a hierarchy-of-organization model in which DCs, their receptors and signaling networks, cell-cell interactions, tissue microenvironment, and the host macroenvironment represent different levels of the hierarchy. Immunity or tolerance can then be represented as a complex function of each of these hierarchies.

Inhibition of mitochondrial genome expression triggers the activation of CHOP-10 by a cell signaling dependent on the integrated stress response but not the mitochondrial unfolded protein response

Mitochondria-to-nucleus communication, known as retrograde signaling, is important to adjust the nuclear gene expression in response to organelle dysfunction. Among the transcription factors described to respond to mitochondrial stress, CHOP-10 is activated by respiratory chain inhibition, mitochondrial accumulation of unfolded proteins and mtDNA mutations. In this study, we show that altered/impaired expression of mtDNA induces CHOP-10 expression in a signaling pathway that depends on the eIF2α/ATF4 axis of the integrated stress response rather than on the mitochondrial unfolded protein response.

[Adaptation to the availability of essential amino-acids: role of GCN2/eIF2α/ATF4 pathway]

In mammals, metabolic adaptations are required to overcome nutritional deprivation in amino-acids/proteins as well as episodes of malnutrition. GCN2 protein kinase, which phosphorylates the α subunit of the translation initiation factor eIF2, is a sensor of amino-acid(s) deficiencies. On one hand, this review briefly describes the main features of amino-acid metabolism. On the other hand, it describes the role of GCN2 in regulating numerous physiological functions.

Roles of the translation initiation factor eIF2α serine 51 phosphorylation in cancer formation and treatment

Cells respond to various forms of stress by activating anti-proliferative pathways, which allow them to correct the damage caused by stress before re-entering proliferation. If the damage, however, is beyond repair, stressed cells are eliminated from the host by undergoing death. The balance between cell survival and death is essential for cancer formation and is determined by several key pathways that impact on different stages of gene expression. In recent years, it has become evident that phosphorylation of the alpha (α) subunit of the translation initiation factor eIF2 at serine 51 (eIF2αS51P) is an important determinant of cell fate in response to stress. Induction of eIF2αS51P is mediated by a family of four kinases namely, HRI, PKR, PERK and GCN2, each of which responds to distinct forms of stress. Increased eIF2αS51P results in a global inhibition of protein synthesis but at the same time enhances the translation of select mRNAs encoding for proteins that control cell adaptation to stress. Short-term induction of eIF2αS51P has been associated with cell survival whereas long-term induction with cell death. Studies in mouse and human models of cancer have provided compelling evidence that eIF2αS51P plays an essential role in stress-induced tumorigenesis. Increased eIF2αS51P exhibits cell autonomous as well as immune regulatory effects, which can influence tumor growth and the efficacy of anti-tumor therapies. The findings suggest that eIF2αS51P may be of prognostic value and a suitable target for the design and implementation of effective anti-tumor therapies. This article is part of a Special Issue entitled: Translation and Cancer.

A stress assembly that confers cell viability by preserving ERES components during amino-acid starvation

Nutritional restriction leads to protein translation attenuation that results in the storage and degradation of free mRNAs in cytoplasmic assemblies. In this study, we show in Drosophila S2 cells that amino-acid starvation also leads to the inhibition of another major anabolic pathway, the protein transport through the secretory pathway, and to the formation of a novel reversible non-membrane bound stress assembly, the Sec body that incorporates components of the ER exit sites. Sec body formation does not depend on membrane traffic in the early secretory pathway, yet requires both Sec23 and Sec24AB. Sec bodies have liquid droplet-like properties, and they act as a protective reservoir for ERES components to rebuild a functional secretory pathway after re-addition of amino-acids acting as a part of a survival mechanism. Taken together, we propose that the formation of these structures is a novel stress response mechanism to provide cell viability during and after nutrient stress.

DOI:http://dx.doi.org/10.7554/eLife.04132.001

Proteins are needed by living cells to perform vital tasks and are made from building blocks called amino-acids. However, if a cell is starved of amino-acids, protein assembly comes to a halt, and if cells are deprived of amino acids for a long time, the cell may die.

To survive short periods of amino-acid starvation, the cell has developed many protective mechanisms. For example, it can start to break down existing proteins, allowing the cell to scavenge and reuse the amino-acids to make other proteins that are more important for short-term survival. The cell may also temporarily halt certain processes: for example, newly constructed proteins may no longer be transported from the cell structure where they are made—called the endoplasmic reticulum—to their final destinations in the cell. However, the protein transport apparatus is also made of proteins and needs to be protected from being broken down so that once starvation ends, the cell can more quickly return to normal working order.

Zacharogianni et al. identify a strategy cells use to store and protect part of their protein transport apparatus during times of stress. Starving fruit fly cells of amino-acids causes the cells to form protective stress assemblies incorporating the proteins associated with the ‘exit sites’ that release proteins from the endoplasmic reticulum. These assemblies are called Sec bodies, and when amino-acid starvation ends, these bodies release the exit site components unharmed. This allows the cell to quickly resume protein transport and so speeds the cell's recovery. If the Sec bodies do not form, the cells are more likely to die during amino-acid starvation.

The Sec bodies are distinct from previously identified stress assemblies that form in the cell during stress, but they share features with them, such as being liquid droplets. Some of these assemblies have been linked to degenerative diseases like amyotrophic lateral sclerosis (ALS). Further research will be necessary to determine if there are any similar harmful side effects associated with the formation of Sec bodies.

DOI:http://dx.doi.org/10.7554/eLife.04132.002