Sestrin2 is a leucine sensor for the mTORC1 pathway

Rags to riches: Amino acid sensing by the Rag GTPases in health and disease

The Rags represent a unique family of evolutionarily conserved, heterodimeric, lysosome-localized small GTPases that play an indispensible role in regulating cellular metabolism in response to various amino acid signaling mechanisms. Rapid progress in the field has begun to unveil a picture in which Rags act as central players in translating information regarding cellular amino acid levels by modulating their nucleotide binding status through an ensemble of support proteins localized in and around the lysosomes. By cooperating with other signaling pathways that converge on the lysosomes, Rags promote anabolic processes through positively affecting mTORC1 signaling in the presence of abundant amino acids. Conversely, Rag inactivation plays an indispensible role in switching cellular metabolism into a catabolic paradigm by promoting the activity of the master lysosomal/autophagic transcription factors TFEB and TFE3. Precise control of Rag signaling is necessary for cells to adapt to constantly changing cellular demands and emerging evidence has highlighted their importance in a wide variety of developmental and pathological conditions.

From Krebs to clinic: glutamine metabolism to cancer therapy

The resurgence of research into cancer metabolism has recently broadened interests beyond glucose and the Warburg effect to other nutrients, including glutamine. Because oncogenic alterations of metabolism render cancer cells addicted to nutrients, pathways involved in glycolysis or glutaminolysis could be exploited for therapeutic purposes. In this Review, we provide an updated overview of glutamine metabolism and its involvement in tumorigenesis in vitro and in vivo, and explore the recent potential applications of basic science discoveries in the clinical setting.

BCKA down-regulates mTORC2-Akt signal and enhances apoptosis susceptibility in cardiomyocytes

Diabetic mellitus (DM) portends poor prognosis concerning pressure overloaded heart disease. Branched-chain amino acids (BCAAs), elements of essential amino acids, have been found altered in its catabolism in diabetes decades ago. However, the relationship between BCAAs and DM induced deterioration of pressure overloaded heart disease remains controversial. This study is aimed to investigate the particular effect of BCKA, a metabolite of BCAA, on myocardial injury induced by pressure overloaded. Primary cardiomyocytes were incubated with or without BCKA and followed by treatment with isoproterenol (ISO); then cell viability was detected by CCK8 and apoptosis was examined by TUNNEL stain and caspase-3 activity analysis. Compared to non-BCKA incubated group, BCKA incubation decreased cell survival and increased apoptosis concentration dependently. Furthermore, Western blot assay showed that mTORC2-Akt pathway was significantly inactivated by BCKA incubation. Moreover, overexpression of rictor, a vital component of mTORC2, significantly abolished the adverse effects of BCKA on apoptosis susceptibility of cardiomyocytes. These results indicate that BCKA contribute to vulnerability of cardiomyocytes in stimulated stress via inactivation of mTORC2-Akt pathway.

Recent Advances in Understanding Amino Acid Sensing Mechanisms that Regulate mTORC1

The mammalian target of rapamycin (mTOR) is the central regulator of mammalian cell growth, and is essential for the formation of two structurally and functionally distinct complexes: mTORC1 and mTORC2. mTORC1 can sense multiple cues such as nutrients, energy status, growth factors and hormones to control cell growth and proliferation, angiogenesis, autophagy, and metabolism. As one of the key environmental stimuli, amino acids (AAs), especially leucine, glutamine and arginine, play a crucial role in mTORC1 activation, but where and how AAs are sensed and signal to mTORC1 are not fully understood. Classically, AAs activate mTORC1 by Rag GTPases which recruit mTORC1 to lysosomes, where AA signaling initiates. Plasma membrane transceptor L amino acid transporter 1 (LAT1)-4F2hc has dual transporter-receptor function that can sense extracellular AA availability upstream of mTORC1. The lysosomal AA sensors (PAT1 and SLC38A9) and cytoplasmic AA sensors (LRS, Sestrin2 and CASTOR1) also participate in regulating mTORC1 activation. Importantly, AAs can be sensed by plasma membrane receptors, like G protein-coupled receptor (GPCR) T1R1/T1R3, and regulate mTORC1 without being transported into the cells. Furthermore, AA-dependent mTORC1 activation also initiates within Golgi, which is regulated by Golgi-localized AA transporter PAT4. This review provides an overview of the research progress of the AA sensing mechanisms that regulate mTORC1 activity.

The impact of protein quality on the promotion of resistance exercise-induced changes in muscle mass

Protein supplementation during resistance exercise training augments hypertrophic gains. Protein ingestion and the resultant hyperaminoacidemia provides the building blocks (indispensable amino acids - IAA) for, and also triggers an increase in, muscle protein synthesis (MPS), suppression of muscle protein breakdown (MPB), and net positive protein balance (i.e., MPS > MPB). The key amino acid triggering the rise in MPS is leucine, which stimulates the mechanistic target of rapamycin complex-1, a key signalling protein, and triggers a rise in MPS. As such, ingested proteins with a high leucine content would be advantageous in triggering a rise in MPS. Thus, protein quality (reflected in IAA content and protein digestibility) has an impact on changes in MPS and could ultimately affect skeletal muscle mass. Protein quality has been measured by the protein digestibility-corrected amino acid score (PDCAAS); however, the digestible indispensable amino acid score (DIAAS) has been recommended as a better method for protein quality scoring. Under DIAAS there is the recognition that amino acids are individual nutrients and that protein quality is contingent on IAA content and ileal (as opposed to fecal) digestibility. Differences in protein quality may have important ramifications for exercise-induced changes in muscle mass gains made with resistance exercise as well as muscle remodelling. Thus, the purpose of this review is a critical appraisal of studies examining the effects of protein quality in supplementation on changes in muscle mass and strength as well as body composition during resistance training.

The effects of bolus supplementation of branched-chain amino acids on skeletal muscle mass, strength, and function in patients with rheumatic disorders during glucocorticoid treatment

OBJECTIVES

To test the effects of bolus supplementation of branched-chain amino acids (BCAA) on skeletal muscle mass, strength, and function in patients with rheumatic disorders taking glucocorticoid (GC).

METHODS

Patients with rheumatic disorders treated with prednisolone (≥10 mg/day) were randomized to ingest additional daily 12 g of BCAA (n = 9) or not (n = 9) for 12 weeks. At baseline, and 4, 8, and 12 weeks, they underwent bioelectrical impedance analysis, muscle strength and functional tests, and computed tomography analysis for cross-sectional area of mid-thigh muscle.

RESULTS

Disease activities of the patients were well controlled and daily GC dose was similarly reduced in both groups. Limb muscle mass was recovered in both groups. Whole-body muscle mass and muscle strength and functional mobility were increased only in BCAA (+) group. The effects of BCAA supplementation on recovering skeletal muscle mass were prominent in particular muscles including biceps femoris muscle.

CONCLUSIONS

This trial is the first-in-man clinical trial to demonstrate that BCAA supplementation might be safe and, at least in part, improve skeletal muscle mass, strength, and function in patients with rheumatic disorders treated with GC.

The apo-structure of the leucine sensor Sestrin2 is still elusive

Sestrin2 is a GATOR2-interacting protein that directly binds leucine and is required for the inhibition of mTORC1 under leucine deprivation, indicating that it is a leucine sensor for the mTORC1 pathway. We recently reported the structure of Sestrin2 in complex with leucine [Protein Data Bank (PDB) ID, 5DJ4] and demonstrated that mutations in the leucine-binding pocket that alter the affinity of Sestrin2 for leucine result in a corresponding change in the leucine sensitivity of mTORC1 in cells. A lower resolution structure of human Sestrin2 (PDB ID, 5CUF), which was crystallized in the absence of exogenous leucine, showed Sestrin2 to be in a nearly identical conformation as the leucine-bound structure. On the basis of this observation, it has been argued that leucine binding does not affect the conformation of Sestrin2 and that Sestrin2 may not be a sensor for leucine. We show that simple analysis of the reported "apo"-Sestrin2 structure reveals the clear presence of prominent, unmodeled electron density in the leucine-binding pocket that exactly accommodates the leucine observed in the higher resolution structure. Refining the reported apo-structure with leucine eliminated the large Fobs-Fcalc difference density at this position and improved the working and free R factors of the model. Consistent with this result, our own structure of Sestrin2 crystallized in the absence of exogenous leucine also contained electron density that is best explained by leucine. Thus, the structure of apo-Sestrin2 remains elusive.

Cardiac Metabolism in Perspective

The heart is a biological pump that converts chemical to mechanical energy. This process of energy conversion is highly regulated to the extent that energy substrate metabolism matches energy use for contraction on a beat-to-beat basis. The biochemistry of cardiac metabolism includes the biochemistry of energy transfer, metabolic regulation, and transcriptional, translational as well as posttranslational control of enzymatic activities. Pathways of energy substrate metabolism in the heart are complex and dynamic, but all of them conform to the First Law of Thermodynamics. The perspectives expand on the overall idea that cardiac metabolism is inextricably linked to both physiology and molecular biology of the heart. The article ends with an outlook on emerging concepts of cardiac metabolism based on new molecular models and new analytical tools. © 2016 American Physiological Society. Compr Physiol 6:1675-1699, 2016.

Structural insight into the arginine-binding specificity of CASTOR1 in amino acid-dependent mTORC1 signaling

The mechanistic Target Of Rapamycin Complex 1 (mTORC1) is central to the cellular response to changes in nutrient signals such as amino acids. CASTOR1 is shown to be an arginine sensor, which plays an important role in the activation of the mTORC1 pathway. In the deficiency of arginine, CASTOR1 interacts with GATOR2, which together with GATOR1 and Rag GTPases controls the relocalization of mTORC1 to lysosomes. The binding of arginine to CASTOR1 disrupts its association with GATOR2 and hence activates the mTORC1 signaling. Here, we report the crystal structure of CASTOR1 in complex with arginine at 2.5 Å resolution. CASTOR1 comprises of four tandem ACT domains with an architecture resembling the C-terminal allosteric domains of aspartate kinases. ACT1 and ACT3 adopt the typical βαββαβ topology and function in dimerization via the conserved residues from helices α1 of ACT1 and α5 of ACT3; whereas ACT 2 and ACT4, both comprising of two non-sequential regions, assume the unusual ββαββα topology and contribute an arginine-binding pocket at the interface. The bound arginine makes a number of hydrogen-bonding interactions and extensive hydrophobic contacts with the surrounding residues of the binding pocket. The functional roles of the key residues are validated by mutagenesis and biochemical assays. Our structural and functional data together reveal the molecular basis for the arginine-binding specificity of CASTOR1 in the arginine-dependent activation of the mTORC1 signaling.

The lysosome as a command-and-control center for cellular metabolism

Lysosomes are membrane-bound organelles found in every eukaryotic cell. They are widely known as terminal catabolic stations that rid cells of waste products and scavenge metabolic building blocks that sustain essential biosynthetic reactions during starvation. In recent years, this classical view has been dramatically expanded by the discovery of new roles of the lysosome in nutrient sensing, transcriptional regulation, and metabolic homeostasis. These discoveries have elevated the lysosome to a decision-making center involved in the control of cellular growth and survival. Here we review these recently discovered properties of the lysosome, with a focus on how lysosomal signaling pathways respond to external and internal cues and how they ultimately enable metabolic homeostasis and cellular adaptation.

Amino Acids Regulate mTORC1 by an Obligate Two-step Mechanism

The mechanistic target of rapamycin complex 1 (mTORC1) coordinates cell growth with its nutritional, hormonal, energy, and stress status. Amino acids are critical regulators of mTORC1 that permit other inputs to mTORC1 activity. However, the roles of individual amino acids and their interactions in mTORC1 activation are not well understood. Here we demonstrate that activation of mTORC1 by amino acids includes two discrete and separable steps: priming and activation. Sensitizing mTORC1 activation by priming amino acids is a prerequisite for subsequent stimulation of mTORC1 by activating amino acids. Priming is achieved by a group of amino acids that includes l-asparagine, l-glutamine, l-threonine, l-arginine, l-glycine, l-proline, l-serine, l-alanine, and l-glutamic acid. The group of activating amino acids is dominated by l-leucine but also includes l-methionine, l-isoleucine, and l-valine. l-Cysteine predominantly inhibits priming but not the activating step. Priming and activating steps differ in their requirements for amino acid concentration and duration of treatment. Priming and activating amino acids use mechanisms that are distinct both from each other and from growth factor signaling. Neither step requires intact tuberous sclerosis complex of proteins to activate mTORC1. Concerted action of priming and activating amino acids is required to localize mTORC1 to lysosomes and achieve its activation.

Perspective: The Potential Role of Essential Amino Acids and the Mechanistic Target of Rapamycin Complex 1 (mTORC1) Pathway in the Pathogenesis of Child Stunting

Stunting is the best summary measure of chronic malnutrition in children. Approximately one-quarter of children under age 5 worldwide are stunted. Lipid-based or micronutrient supplementation has little to no impact in reducing stunting, which suggests that other critical dietary nutrients are missing. A dietary pattern of poor-quality protein is associated with stunting. Stunted children have significantly lower circulating essential amino acids than do nonstunted children. Inadequate dietary intakes of essential amino acids could adversely affect growth, because amino acids are required for synthesis of proteins. The master growth regulation pathway, the mechanistic target of rapamycin complex 1 (mTORC1) pathway, is exquisitely sensitive to amino acid availability. mTORC1 integrates cues such as nutrients, growth factors, oxygen, and energy to regulate growth of bone, skeletal muscle, nervous system, gastrointestinal tract, hematopoietic cells, immune effector cells, organ size, and whole-body energy balance. mTORC1 represses protein and lipid synthesis and cell and organismal growth when amino acids are deficient. Over the past 4 decades, the main paradigm for child nutrition in developing countries has been micronutrient malnutrition, with relatively less attention paid to protein. In this Perspective, we present the view that essential amino acids and the mTORC1 pathway play a key role in child growth. The current assumption that total dietary protein intake is adequate for growth among most children in developing countries needs re-evaluation.

C9orf72 binds SMCR8, localizes to lysosomes, and regulates mTORC1 signaling

C9orf72 interacts strongly with SMCR8 and depends on this interaction for its stability. Lysosomes are major sites of C9orf72 subcellular localization, and abnormal lysosome morphology is seen in its absence. Defects are found in the regulation of the lysosome-localized mTORC1 signaling pathway in C9orf72 KO cells.

Hexanucleotide expansion in an intron of the C9orf72 gene causes amyotrophic lateral sclerosis and frontotemporal dementia. However, beyond bioinformatics predictions that suggested structural similarity to folliculin, the Birt-Hogg-Dubé syndrome tumor suppressor, little is known about the normal functions of the C9orf72 protein. To address this problem, we used genome-editing strategies to investigate C9orf72 interactions, subcellular localization, and knockout (KO) phenotypes. We found that C9orf72 robustly interacts with SMCR8 (a protein of previously unknown function). We also observed that C9orf72 localizes to lysosomes and that such localization is negatively regulated by amino acid availability. Analysis of C9orf72 KO, SMCR8 KO, and double-KO cell lines revealed phenotypes that are consistent with a function for C9orf72 at lysosomes. These include abnormally swollen lysosomes in the absence of C9orf72 and impaired responses of mTORC1 signaling to changes in amino acid availability (a lysosome-dependent process) after depletion of either C9orf72 or SMCR8. Collectively these results identify strong physical and functional interactions between C9orf72 and SMCR8 and support a lysosomal site of action for this protein complex.

CASTORing New Light on Amino Acid Sensing

The activation state of mTORC1, a master regulator of cell growth, is particularly sensitive to changes in the intracellular levels of the amino acid arginine, but the sensing mechanisms are poorly understood. In this issue of Cell, Chantranupong et al. identify CASTOR1 as a direct arginine sensor that acts through the GATOR2 complex to regulate mTORC1.

Long non-coding RNAs in cancer metabolism

Altered cellular metabolism is an emerging hallmark of cancer. Accumulating recent evidence links long non-coding RNAs (lncRNAs), a still poorly understood class of non-coding RNAs, to cancer metabolism. Here we review the emerging findings on the functions of lncRNAs in cancer metabolism, with particular emphasis on how lncRNAs regulate glucose and glutamine metabolism in cancer cells, discuss how lncRNAs regulate various aspects of cancer metabolism through their cross-talk with other macromolecules, explore the mechanistic conceptual framework of lncRNAs in reprogramming metabolism in cancers, and highlight the challenges in this field. A more in-depth understanding of lncRNAs in cancer metabolism may enable the development of novel and effective therapeutic strategies targeting cancer metabolism.

T-cell metabolism governing activation, proliferation and differentiation; a modular view

T lymphocytes are a critical component of the adaptive immune system mediating protection against infection and malignancy, but also implicated in many immune pathologies. Upon recognition of specific antigens T cells clonally expand, traffic to inflamed sites and acquire effector functions, such as the capacity to kill infected and malignantly transformed cells and secrete cytokines to coordinate the immune response. These processes have significant bioenergetic and biosynthetic demands, which are met by dynamic changes in T-cell metabolism, specifically increases in glucose uptake and metabolism; mitochondrial function; amino acid uptake, and cholesterol and lipid synthesis. These metabolic changes are coordinate by key cellular kinases and transcription factors. Dysregulated T-cell metabolism is associated with impaired immunity in chronic infection and cancer and conversely with excessive T-cell activity in autoimmune and inflammatory pathologies. Here we review the key aspects of T-cell metabolism relevant to their immune function, and discuss evidence for the potential to therapeutically modulate T-cell metabolism in disease.

The collagen derived dipeptide hydroxyprolyl-glycine promotes C2C12 myoblast differentiation and myotube hypertrophy

The majority of studies on possible roles for collagen hydrolysates in human health have focused on their effects on bone and skin. Hydroxyprolyl-glycine (Hyp-Gly) was recently identified as a novel collagen hydrolysate-derived dipeptide in human blood. However, any possible health benefits of Hyp-Gly remain unclear. Here, we report the effects of Hyp-Gly on differentiation and hypertrophy of murine skeletal muscle C2C12 cells. Hyp-Gly increased the fusion index, the myotube size, and the expression of the myotube-specific myosin heavy chain (MyHC) and tropomyosin structural proteins. Hyp-Gly increased the phosphorylation of Akt, mTOR, and p70S6K in myoblasts, whereas the phosphatidylinositol 3-kinase (PI3K) inhibitor LY294002 inhibited their phosphorylation by Hyp-Gly. LY294002 and the mammalian target of rapamycin (mTOR) inhibitor rapamycin repressed the enhancing effects of Hyp-Gly on MyHC and tropomyosin expression. The peptide/histidine transporter 1 (PHT1) was highly expressed in both myoblasts and myotubes, and co-administration of histidine inhibited Hyp-Gly-induced phosphorylation of p70S6K in myoblasts and myotubes. These results indicate that Hyp-Gly can induce myogenic differentiation and myotube hypertrophy and suggest that Hyp-Gly promotes myogenic differentiation by activating the PI3K/Akt/mTOR signaling pathway, perhaps depending on PHT1 for entry into cells.

Mechanism of arginine sensing by CASTOR1 upstream of mTORC1

The mechanistic Target of Rapamycin Complex 1 (mTORC1) is a major regulator of eukaryotic growth that coordinates anabolic and catabolic cellular processes with inputs such as growth factors and nutrients, including amino acids. In mammals arginine is particularly important, promoting diverse physiological effects such as immune cell activation, insulin secretion, and muscle growth, largely mediated through activation of mTORC1 (refs 4, 5, 6, 7). Arginine activates mTORC1 upstream of the Rag family of GTPases, through either the lysosomal amino acid transporter SLC38A9 or the GATOR2-interacting Cellular Arginine Sensor for mTORC1 (CASTOR1). However, the mechanism by which the mTORC1 pathway detects and transmits this arginine signal has been elusive. Here, we present the 1.8 Å crystal structure of arginine-bound CASTOR1. Homodimeric CASTOR1 binds arginine at the interface of two Aspartate kinase, Chorismate mutase, TyrA (ACT) domains, enabling allosteric control of the adjacent GATOR2-binding site to trigger dissociation from GATOR2 and downstream activation of mTORC1. Our data reveal that CASTOR1 shares substantial structural homology with the lysine-binding regulatory domain of prokaryotic aspartate kinases, suggesting that the mTORC1 pathway exploited an ancient, amino-acid-dependent allosteric mechanism to acquire arginine sensitivity. Together, these results establish a structural basis for arginine sensing by the mTORC1 pathway and provide insights into the evolution of a mammalian nutrient sensor.

Molecular basis and current strategies of therapeutic arginine depletion for cancer

Renewed interest in the use of therapeutic enzymes combined with an improved knowledge of cancer cell metabolism, has led to the translation of several arginine depletion strategies into early phase clinical trials. Arginine auxotrophic tumors are reliant on extracellular arginine, due to the downregulation of arginosuccinate synthetase or ornithine transcarbamylase-key enzymes for intracellular arginine recycling. Engineered arginine catabolic enzymes such as recombinant human arginase (rh-Arg1-PEG) and arginine deiminase (ADI-PEG) have demonstrated cytotoxicity against arginine auxotrophic tumors. In this review, we discuss the molecular events triggered by extracellular arginine depletion that contribute to tumor cell death.

Leucyl-tRNA Synthetase Activates Vps34 in Amino Acid-Sensing mTORC1 Signaling

Amino acid availability activates signaling by the mammalian target of rapamycin (mTOR) complex 1, mTORC1, a master regulator of cell growth. The class III PI-3-kinase Vps34 mediates amino acid signaling to mTORC1 by regulating lysosomal translocation and activation of the phospholipase PLD1. Here, we identify leucyl-tRNA synthetase (LRS) as a leucine sensor for the activation of Vps34-PLD1 upstream of mTORC1. LRS is necessary for amino acid-induced Vps34 activation, cellular PI(3)P level increase, PLD1 activation, and PLD1 lysosomal translocation. Leucine binding, but not tRNA charging activity of LRS, is required for this regulation. Moreover, LRS physically interacts with Vps34 in amino acid-stimulatable non-autophagic complexes. Finally, purified LRS protein activates Vps34 kinase in vitro in a leucine-dependent manner. Collectively, our findings provide compelling evidence for a direct role of LRS in amino acid activation of Vps34 via a non-canonical mechanism and fill a gap in the amino acid-sensing mTORC1 signaling network.

p53 coordinates decidual sestrin 2/AMPK/mTORC1 signaling to govern parturition timing

Inflammation and oxidative stress are known risk factors for preterm birth (PTB); however, the mechanisms and pathways that influence this condition are not fully described. Previously, we showed that mTORC1 signaling is increased in mice harboring a uterine-specific deletion of transformation-related protein 53 (p53d/d mice), which exhibit premature decidual senescence that triggers spontaneous and inflammation-induced PTB. Treatment with the mTORC1 inhibitor rapamycin reduced the incidence of PTB in the p53d/d mice. Decidual senescence with heightened mTORC1 signaling is also a signature of human PTB. Here, we have identified an underlying mechanism for PTB and a potential therapeutic strategy for treating the condition. Treatment of pregnant p53d/d mice with either the antidiabetic drug metformin or the antioxidant resveratrol activated AMPK signaling and inhibited mTORC1 signaling in decidual cells. Both metformin and resveratrol protected against spontaneous and inflammation-induced PTB in p53d/d females. Using multiple approaches, we determined that p53 interacts with sestrins to coordinate an inverse relationship between AMPK and mTORC1 signaling that determines parturition timing. This signature was also observed in human decidual cells. Together, these results reveal that p53-dependent coordination of AMPK and mTORC1 signaling controls parturition timing and suggest that metformin and resveratrol have therapeutic potential to prevent PTB.

The Emerging Role of Branched-Chain Amino Acids in Insulin Resistance and Metabolism

Insulin is required for maintenance of glucose homeostasis. Despite the importance of insulin sensitivity to metabolic health, the mechanisms that induce insulin resistance remain unclear. Branched-chain amino acids (BCAAs) belong to the essential amino acids, which are both direct and indirect nutrient signals. Even though BCAAs have been reported to improve metabolic health, an increased BCAA plasma level is associated with a high risk of metabolic disorder and future insulin resistance, or type 2 diabetes mellitus (T2DM). The activation of mammalian target of rapamycin complex 1 (mTORC1) by BCAAs has been suggested to cause insulin resistance. In addition, defective BCAA oxidative metabolism might occur in obesity, leading to a further accumulation of BCAAs and toxic intermediates. This review provides the current understanding of the mechanism of BCAA-induced mTORC1 activation, as well as the effect of mTOR activation on metabolic health in terms of insulin sensitivity. Furthermore, the effects of impaired BCAA metabolism will be discussed in detail.

Sensing the Environment Through Sestrins: Implications for Cellular Metabolism

Sestrins are a family of stress-responsive genes that have evolved to attenuate damage induced by stress caused to the cell. By virtue of their antioxidant activity, protein products of Sestrin genes prevent the accumulation of reactive oxygen species within the cell, thereby attenuating the detrimental effects of oxidative stress. In parallel, Sestrins participate in several signaling pathways that control the activity of the target of rapamycin protein kinase (TOR). TOR is a crucial sensor of intracellular and extracellular conditions that promotes cell growth and anabolism when nutrients and growth factors are abundant. In addition to reacting to stress-inducing insults, Sestrins also monitor the changes in the availability of nutrients, which allows them to serve as a key checkpoint for the TOR-regulated signaling pathways. In this review, we will discuss how Sestrins integrate signals from numerous stress- and nutrient-responsive signaling pathways to orchestrate cellular metabolism and support cell viability.

The Mechanistic Target of Rapamycin: The Grand ConducTOR of Metabolism and Aging

Since the discovery that rapamycin, a small molecule inhibitor of the protein kinase mTOR (mechanistic target of rapamycin), can extend the lifespan of model organisms including mice, interest in understanding the physiological role and molecular targets of this pathway has surged. While mTOR was already well known as a regulator of growth and protein translation, it is now clear that mTOR functions as a central coordinator of organismal metabolism in response to both environmental and hormonal signals. This review discusses recent developments in our understanding of how mTOR signaling is regulated by nutrients and the role of the mTOR signaling pathway in key metabolic tissues. Finally, we discuss the molecular basis for the negative metabolic side effects associated with rapamycin treatment, which may serve as barriers to the adoption of rapamycin or similar compounds for the treatment of diseases of aging and metabolism.

Genetic Disruption of the Multifunctional CD98/LAT1 Complex Demonstrates the Key Role of Essential Amino Acid Transport in the Control of mTORC1 and Tumor Growth

The CD98/LAT1 complex is overexpressed in aggressive human cancers and is thereby described as a potential therapeutic target. This complex promotes tumorigenesis with CD98 (4F2hc) engaging β-integrin signaling while LAT1 (SLC7A5) imports essential amino acids (EAA) and promotes mTORC1 activity. However, it is unclear as to which member of the heterodimer carries the most prevalent protumoral action. To answer this question, we explored the tumoral potential of each member by gene disruption of CD98, LAT1, or both and by inhibition of LAT1 with the selective inhibitor (JPH203) in six human cancer cell lines from colon, lung, and kidney. Each knockout respectively ablated 90% (CD98 KO: ) and 100% (LAT1 KO: ) of Na(+)-independent leucine transport activity. LAT1 KO: or JPH203-treated cells presented an amino acid stress response with ATF4, GCN2 activation, mTORC1 inhibition, and severe in vitro and in vivo tumor growth arrest. We show that this severe growth phenotype is independent of the level of expression of CD98 in the six tumor cell lines. Surprisingly, CD98 KO: cells with only 10% EAA transport activity displayed a normal growth phenotype, with mTORC1 activity and tumor growth rate undistinguishable from wild-type cells. However, CD98 KO: cells became extremely sensitive to inhibition or genetic disruption of LAT1 (CD98 KO: /LAT1 KO: ). This finding demonstrates that the tumoral potential of CD98 KO: cells is due to residual LAT1 transport activity. Therefore, these findings clearly establish that LAT1 transport activity is the key growth-limiting step of the heterodimer and advocate the pharmacology development of LAT1 transporter inhibitors as a very promising anticancer target. Cancer Res; 76(15); 4481-92. ©2016 AACR.

GCN2 contributes to mTORC1 inhibition by leucine deprivation through an ATF4 independent mechanism

It is well known that the GCN2 and mTORC1 signaling pathways are regulated by amino acids and share common functions, in particular the control of translation. The regulation of GCN2 activity by amino acid availability relies on the capacity of GCN2 to sense the increased levels of uncharged tRNAs upon amino acid scarcity. In contrast, despite recent progress in the understanding of the regulation of mTORC1 by amino acids, key aspects of this process remain unsolved. In particular, while leucine is well known to be a potent regulator of mTORC1, the mechanisms by which this amino acid is sensed and control mTORC1 activity are not well defined. Our data establish that GCN2 is involved in the inhibition of mTORC1 upon leucine or arginine deprivation. However, the activation of GCN2 alone is not sufficient to inhibit mTORC1 activity, indicating that leucine and arginine exert regulation via additional mechanisms. While the mechanism by which GCN2 contributes to the initial step of mTORC1 inhibition involves the phosphorylation of eIF2α, we show that it is independent of the downstream transcription factor ATF4. These data point to a novel role for GCN2 and phosphorylation of eIF2α in the control of mTORC1 by certain amino acids.

Sestrin regulation of TORC1: Is Sestrin a leucine sensor?

Sestrins are highly conserved, stress-inducible proteins that inhibit target of rapamycin complex 1 (TORC1) signaling. After their transcriptional induction, both vertebrate and invertebrate Sestrins turn on the adenosine monophosphate (AMP)-activated protein kinase (AMPK), which activates the tuberous sclerosis complex (TSC), a key inhibitor of TORC1 activation. However, Sestrin overexpression, on occasion, can result in TORC1 inhibition even in AMPK-deficient cells. This effect has been attributed to Sestrin's ability to bind the TORC1-regulating GATOR2 protein complex, which was postulated to control trafficking of TORC1 to lysosomes. How the binding of Sestrins to GATOR2 is regulated and how it contributes to TORC1 inhibition are unknown. New findings suggest that the amino acid leucine specifically disrupts the association of Sestrin2 with GATOR2, thus explaining how leucine and related amino acids stimulate TORC1 activity. We discuss whether and how these findings fit what has already been learned about Sestrin-mediated TORC1 inhibition from genetic studies conducted in fruit flies and mammals.

Enteral β-hydroxy-β-methylbutyrate supplementation increases protein synthesis in skeletal muscle of neonatal pigs

Many low-birth weight infants are at risk for poor growth due to an inability to achieve adequate protein intake. Administration of the amino acid leucine stimulates protein synthesis in skeletal muscle of neonates. To determine the effects of enteral supplementation of the leucine metabolite β-hydroxy-β-methylbutyrate (HMB) on protein synthesis and the regulation of translation initiation and degradation pathways, overnight-fasted neonatal pigs were studied immediately (F) or fed one of five diets for 24 h: low-protein (LP), high-protein (HP), or LP diet supplemented with 4 (HMB4), 40 (HMB40), or 80 (HMB80) μmol HMB·kg body wt(-1)·day(-1) Cell replication was assessed from nuclear incorporation of BrdU in the longissimus dorsi (LD) muscle and jejunum crypt cells. Protein synthesis rates in LD, gastrocnemius, rhomboideus, and diaphragm muscles, lung, and brain were greater in HMB80 and HP and in brain were greater in HMB40 compared with LP and F groups. Formation of the eIF4E·eIF4G complex and S6K1 and 4E-BP1 phosphorylation in LD, gastrocnemius, and rhomboideus muscles were greater in HMB80 and HP than in LP and F groups. Phosphorylation of eIF2α and eEF2 and expression of SNAT2, LAT1, MuRF1, atrogin-1, and LC3-II were unchanged. Numbers of BrdU-positive myonuclei in the LD were greater in HMB80 and HP than in the LP and F groups; there were no differences in jejunum. The results suggest that enteral supplementation with HMB increases skeletal muscle protein anabolism in neonates by stimulation of protein synthesis and satellite cell proliferation.

Role of GCN2-Independent Signaling Through a Noncanonical PERK/NRF2 Pathway in the Physiological Responses to Dietary Methionine Restriction

Restricting availability of essential amino acids (EAAs) limits aminoacylation of tRNAs by their cognate EAAs and activates the nutrient-sensing kinase, general control nonderepressible 2 (GCN2). Activated GCN2 phosphorylates eukaryotic initiation factor 2 (eIF2), altering gene-specific translation and initiating a transcriptional program collectively described as the integrated stress response (ISR). Central GCN2 activation by EAA deprivation is also linked to an acute aversive feeding response. Dietary methionine restriction (MR) produces a well-documented series of physiological responses (increased energy intake and expenditure, decreased adiposity, and increased insulin sensitivity), but the role of GCN2 in mediating them is unknown. Using Gcn2(-/-) mice, we found that the absence of GCN2 had no effect on the ability of MR to reduce body weight or adiposity, increase energy intake and expenditure, increase hepatic transcription and release of fibroblast growth factor 21, or improve insulin sensitivity. Interestingly, hepatic eIF2 phosphorylation by MR was uncompromised in Gcn2(-/-) mice. Instead, protein kinase R-like endoplasmic reticulum (ER) kinase (PERK) was activated in both intact and Gcn2(-/-) mice. PERK activation corresponded with induction of the ISR and the nuclear respiratory factor 2 antioxidant program but not ER stress. These data uncover a novel glutathione-sensing mechanism that functions independently of GCN2 to link dietary MR to its metabolic phenotype.

Rapamycin: An InhibiTOR of Aging Emerges From the Soil of Easter Island

Rapamycin (sirolimus) is a macrolide immunosuppressant that inhibits the mechanistic target of rapamycin (mTOR) protein kinase and extends lifespan in model organisms including mice. Although rapamycin is an FDA-approved drug for select indications, a diverse set of negative side effects may preclude its wide-scale deployment as an antiaging therapy. mTOR forms two different protein complexes, mTORC1 and mTORC2; the former is acutely sensitive to rapamycin whereas the latter is only chronically sensitive to rapamycin in vivo. Over the past decade, it has become clear that although genetic and pharmacological inhibition of mTORC1 extends lifespan and delays aging, inhibition of mTORC2 has negative effects on mammalian health and longevity and is responsible for many of the negative side effects of rapamycin. In this review, we discuss recent advances in understanding the molecular and physiological effects of rapamycin treatment, and we discuss how the use of alternative rapamycin treatment regimens or rapamycin analogs has the potential to mitigate the deleterious side effects of rapamycin treatment by more specifically targeting mTORC1. Although the side effects of rapamycin are still of significant concern, rapid progress is being made in realizing the revolutionary potential of rapamycin-based therapies for the treatment of diseases of aging.

Advances in Autophagy Regulatory Mechanisms

Autophagy plays a critical role in cell metabolism by degrading and recycling internal components when challenged with limited nutrients. This fundamental and conserved mechanism is based on a membrane trafficking pathway in which nascent autophagosomes engulf cytoplasmic cargo to form vesicles that transport their content to the lysosome for degradation. Based on this simple scheme, autophagy modulates cellular metabolism and cytoplasmic quality control to influence an unexpectedly wide range of normal mammalian physiology and pathophysiology. In this review, we summarise recent advancements in three broad areas of autophagy regulation. We discuss current models on how autophagosomes are initiated from endogenous membranes. We detail how the uncoordinated 51-like kinase (ULK) complex becomes activated downstream of mechanistic target of rapamycin complex 1 (MTORC1). Finally, we summarise the upstream signalling mechanisms that can sense amino acid availability leading to activation of MTORC1.

The GATOR2 Component Wdr24 Regulates TORC1 Activity and Lysosome Function

TORC1 is a master regulator of metabolism in eukaryotes that responds to multiple upstream signaling pathways. The GATOR complex is a newly defined upstream regulator of TORC1 that contains two sub-complexes, GATOR1, which inhibits TORC1 activity in response to amino acid starvation and GATOR2, which opposes the activity of GATOR1. While the GATOR1 complex has been implicated in a wide array of human pathologies including cancer and hereditary forms of epilepsy, the in vivo relevance of the GATOR2 complex remains poorly understood in metazoans. Here we define the in vivo role of the GATOR2 component Wdr24 in Drosophila. Using a combination of genetic, biochemical, and cell biological techniques we demonstrate that Wdr24 has both TORC1 dependent and independent functions in the regulation of cellular metabolism. Through the characterization of a null allele, we show that Wdr24 is a critical effector of the GATOR2 complex that promotes the robust activation of TORC1 and cellular growth in a broad array of Drosophila tissues. Additionally, epistasis analysis between wdr24 and genes that encode components of the GATOR1 complex revealed that Wdr24 has a second critical function, the TORC1 independent regulation of lysosome dynamics and autophagic flux. Notably, we find that two additional members of the GATOR2 complex, Mio and Seh1, also have a TORC1 independent role in the regulation of lysosome function. These findings represent a surprising and previously unrecognized function of GATOR2 complex components in the regulation of lysosomes. Consistent with our findings in Drosophila, through the characterization of a wdr24^-/- knockout HeLa cell line we determined that Wdr24 promotes lysosome acidification and autophagic flux in mammalian cells. Taken together our data support the model that Wdr24 is a key effector of the GATOR2 complex, required for both TORC1 activation and the TORC1 independent regulation of lysosomes.

TORC1 is a conserved multi-protein complex that regulates metabolism and cell growth in response to many upstream inputs including nutrient availability. When amino acids are limiting, the GATOR1 complex inhibits TORC1 activation. The inhibition of TORC1 slows cellular metabolism and promotes cell survival during times of protein scarcity. A second critical response to amino acid limitation is the activation of autophagy. During autophagy cells degrade intracellular components in specialized membrane-bound organelles called autolysosomes that are formed when lysosomes fuse with autophagosomes. In times of nutrient stress, the process of autophagy allows proteins and other building blocks of the cell to be broken down and repurposed for vital cellular functions. Here we demonstrate that Wdr24, a component of the multi-protein GATOR2 complex, has a dual role in the regulation of cellular metabolism in Drosophila. First, Wdr24 is required to oppose the activity of the GATOR1 complex, thus activating TORC1 in a broad array of Drosophila tissues. Second, Wdr24 promotes the acidification of lysosomes and thus facilitates autophagic flux. Our data support the model that Wdr24 uses both TORC1 dependent and independent pathways to regulate cellular metabolism.

Biochemical Basis of Sestrin Physiological Activities

Excessive accumulation of reactive oxygen species (ROS) and chronic activation of mechanistic target of rapamycin (mTOR) complex 1 (mTORC1) are well-characterized promoters of aging and age-associated degenerative pathologies. Sestrins, a family of highly conserved stress-inducible proteins, are important negative regulators of both ROS and mTORC1 signaling pathways; however, the mechanistic basis of how Sestrins suppress these pathways remains elusive. In the past couple of years, breakthrough discoveries about Sestrin signaling and its molecular nature have markedly increased our biochemical understanding of Sestrin function. These discoveries have also uncovered new potential therapeutic strategies that may eventually enable us to attenuate aging and age-associated diseases.

Dietary Protein, Metabolism, and Aging

Dietary restriction (DR), a moderate reduction in food intake, improves health during aging and extends life span across multiple species. Specific nutrients, rather than overall calories, mediate the effects of DR, with protein and specific amino acids (AAs) playing a key role. Modulations of single dietary AAs affect traits including growth, reproduction, physiology, health, and longevity in animals. Epidemiological data in humans also link the quality and quantity of dietary proteins to long-term health. Intricate nutrient-sensing pathways fine tune the metabolic responses to dietary AAs in a highly conserved manner. In turn, these metabolic responses can affect the onset of insulin resistance, obesity, neurodegenerative disease, and other age-related diseases. In this review we discuss how AA requirements are shaped and how ingested AAs regulate a spectrum of homeostatic processes. Finally, we highlight the resulting opportunity to develop nutritional strategies to improve human health during aging.

Amino Acid Sensing by mTORC1: Intracellular Transporters Mark the Spot

Cell metabolism and growth are matched to nutrient availability via the amino-acid-regulated mechanistic target of rapamycin complex 1 (mTORC1). Transporters have emerged as important amino acid sensors controlling mTOR recruitment and activation at the surface of multiple intracellular compartments. Classically, this has involved late endosomes and lysosomes, but now, in a recent twist, also the Golgi apparatus. Here we propose a model in which specific amino acids in assorted compartments activate different mTORC1 complexes, which may have distinct drug sensitivities and functions. We will discuss the implications of this for mTORC1 function in health and disease.

Raptor/mTORC1 loss in adipocytes causes progressive lipodystrophy and fatty liver disease

Objective

Normal adipose tissue growth and function is critical to maintaining metabolic homeostasis and its excess (e.g. obesity) or absence (e.g. lipodystrophy) is associated with severe metabolic disease. The goal of this study was to understand the mechanisms maintaining healthy adipose tissue growth and function.

Methods

Adipose tissue senses and responds to systemic changes in growth factor and nutrient availability; in cells mTORC1 regulates metabolism in response to growth factors and nutrients. Thus, mTORC1 is poised to be a critical intracellular regulator of adipocyte metabolism. Here, we investigate the role of mTORC1 in mature adipocytes by generating and characterizing mice in which the Adiponectin-Cre driver is used to delete floxed alleles of Raptor, which encodes an essential regulatory subunit of mTORC1.

Results

Raptor^Adipoq-cre mice have normal white adipose tissue (WAT) mass for the first few weeks of life, but soon thereafter develop lipodystrophy associated with hepatomegaly, hepatic steatosis, and insulin intolerance. Raptor^Adipoq-cre mice are also resistant to becoming obese when consuming a high fat diet (HFD). Resistance to obesity does not appear to be due to increased energy expenditure, but rather from failed adipose tissue expansion resulting in severe hepatomegaly associated with hyperphagia and defective dietary lipid absorption. Deleting Raptor in WAT also decreases C/EBPα expression and the expression of its downstream target adiponectin, providing one possible mechanism of mTORC1 function in WAT.

Conclusions

mTORC1 activity in mature adipocytes is essential for maintaining normal adipose tissue growth and its selective loss in mature adipocytes leads to a progressive lipodystrophy disorder and systemic metabolic disease that shares many of the hallmarks of human congenital generalized lipodystrophy.

•

A new model of Raptor (mTORC1) loss in adipose tissue is described.

•

Young fat Raptor KO mice have normal WAT mass.

•

Fat Raptor KO mice progressively develop lipodystrophy and metabolic disease.

•

Fat Raptor KO mice resist obesity but develop severe hepatomegaly.

Leucine induced dephosphorylation of Sestrin2 promotes mTORC1 activation

The studies described herein were designed to explore the role of Sestrin2 in mediating the selective action of leucine to activate mTORC1. The results demonstrate that Sestrin2 is a phosphoprotein and that its phosphorylation state is responsive to the availability of leucine, but not other essential amino acids. Moreover, leucine availability-induced alterations in Sestrin2 phosphorylation correlated temporally and dose dependently with the activation state of mTORC1, there being a reciprocal relationship between the degree of phosphorylation of Sestrin2 and the extent of repression of mTORC1. With leucine deprivation, Sestrin2 became more highly phosphorylated and interacted more strongly with proteins of the GATOR2 complex. Notably, in cells lacking the protein kinase ULK1, the activation state of mTORC1 was elevated in leucine-deficient medium, such that the effect of re-addition of the amino acid was blunted. In contrast, overexpression of ULK1 led to hyperphosphorylation of Sestrin2 and enhanced its interaction with GATOR2. Neither rapamycin nor Torin2 had any effect on Sestrin2 phosphorylation, suggesting that leucine deprivation-induced repression of mTORC1 was not responsible for the action of ULK1 on Sestrin2. Mass spectrometry analysis of Sestrin2 revealed three phosphorylation sites that are conserved across mammalian species. Mutation of the three sites to phospho-mimetic amino acids in exogenously expressed Sestrin2 promoted its interaction with GATOR2 and dramatically repressed mTORC1 even in the presence of leucine. Overall, the results support a model in which leucine selectively promotes dephosphorylation of Sestrin2, causing it to dissociate from and thereby activate GATOR2, leading to activation of mTORC1.

VCP and ATL1 regulate endoplasmic reticulum and protein synthesis for dendritic spine formation

Imbalanced protein homeostasis, such as excessive protein synthesis and protein aggregation, is a pathogenic hallmark of a range of neurological disorders. Here, using expression of mutant proteins, a knockdown approach and disease mutation knockin mice, we show that VCP (valosin-containing protein), together with its cofactor P47 and the endoplasmic reticulum (ER) morphology regulator ATL1 (Atlastin-1), regulates tubular ER formation and influences the efficiency of protein synthesis to control dendritic spine formation in neurons. Strengthening the significance of protein synthesis in dendritic spinogenesis, the translation blocker cyclohexamide and the mTOR inhibitor rapamycin reduce dendritic spine density, while a leucine supplement that increases protein synthesis ameliorates the dendritic spine defects caused by Vcp and Atl1 deficiencies. Because VCP and ATL1 are the causative genes of several neurodegenerative and neurodevelopmental disorders, we suggest that impaired ER formation and inefficient protein synthesis are significant in the pathogenesis of multiple neurological disorders.

Protein homeostasis is crucial for maintaining a variety of cellular functions. Here the authors show that valosin-containing protein and its cofactors regulate tubular ER formation and protein synthesis efficiency, thereby control dendritic spine formation in neurons.

The CASTOR Proteins Are Arginine Sensors for the mTORC1 Pathway

Amino acids signal to the mTOR complex I (mTORC1) growth pathway through the Rag GTPases. Multiple distinct complexes regulate the Rags, including GATOR1, a GTPase activating protein (GAP), and GATOR2, a positive regulator of unknown molecular function. Arginine stimulation of cells activates mTORC1, but how it is sensed is not well understood. Recently, SLC38A9 was identified as a putative lysosomal arginine sensor required for arginine to activate mTORC1 but how arginine deprivation represses mTORC1 is unknown. Here, we show that CASTOR1, a previously uncharacterized protein, interacts with GATOR2 and is required for arginine deprivation to inhibit mTORC1. CASTOR1 homodimerizes and can also heterodimerize with the related protein, CASTOR2. Arginine disrupts the CASTOR1-GATOR2 complex by binding to CASTOR1 with a dissociation constant of ~30 μM, and its arginine-binding capacity is required for arginine to activate mTORC1 in cells. Collectively, these results establish CASTOR1 as an arginine sensor for the mTORC1 pathway.

Toward a Molecular Definition of Leucine-Dependent mTORC1 Activation

The mechanistic target of rapamycin complex 1 (mTORC1) coordinates nutrient availability with cell growth. Recent reports by Sabatini and coworkers (Saxton et al., 2016; Wolfson et al., 2016) characterize a cytoplasmic amino acid receptor that couples the binding of leucine to the activation of mTORC1.

Regulation of mTORC1 by growth factors, energy status, amino acids and mechanical stimuli at a glance

The mechanistic/mammalian target of rapamycin complex 1 (mTORC1) plays a pivotal role in the regulation of skeletal muscle protein synthesis. Activation of the complex leads to phosphorylation of two important sets of substrates, namely eIF4E binding proteins and ribosomal S6 kinases. Phosphorylation of these substrates then leads to an increase in protein synthesis, mainly by enhancing translation initiation. mTORC1 activity is regulated by several inputs, such as growth factors, energy status, amino acids and mechanical stimuli. Research in this field is rapidly evolving and unraveling how these inputs regulate the complex. Therefore this review attempts to provide a brief and up-to-date narrative on the regulation of this marvelous protein complex. Additionally, some sports supplements which have been shown to regulate mTORC1 activity are discussed.

Sensing of nutrients and microbes in the gut

PURPOSE OF REVIEW

Sensing of nutrients and microbes in the gut are fundamental processes necessary for life. This review aims to provide an overview of the basic background and new data on cellular nutrient, energy, and microbe sensors.

RECENT FINDINGS

The nutrient sensors 5' adenosine monophosphate-activated protein kinase, activating transcription factor 4 and mechanistic target of rapamycin (mTOR) are critical in control of the cell cycle. Recent data demonstrate their role in metabolic syndrome, in cell growth, and as therapeutic targets. Regulation of mTOR by the amino acids is the subject of intense investigation. Recent studies have further elucidated the exact mechanism of amino acid-dependent mTOR regulation. Pathogen recognition receptors (PRRs) are receptors that recognize conserved microbial molecules. New data demonstrate how lymphocyte-specific PRRs are necessary to maintain homeostasis. Moreover, new studies explore the role of PRRs in controlling the gut bacterial and fungal microbiome.

SUMMARY

Nutrient sensing molecules are central to cell growth and metabolism and are implicated in cancer and the metabolic syndrome. Regulation of nutrient sensors is complex, and may be amenable to therapeutic targeting. Microbial sensors play critical roles in homeostasis and maintenance of the gut fungal and bacterial microbiome.

Tumor suppressive role of sestrin2 during colitis and colon carcinogenesis

The mTOR complex 1 (mTORC1) and endoplasmic reticulum (ER) stress pathways are critical regulators of intestinal inflammation and colon cancer growth. Sestrins are stress-inducible proteins, which suppress both mTORC1 and ER stress; however, the role of Sestrins in colon physiology and tumorigenesis has been elusive due to the lack of studies in human tissues or in appropriate animal models. In this study, we show that human SESN2 expression is elevated in the colon of ulcerative colitis patients but is lost upon p53 inactivation during colon carcinogenesis. In mouse colon, Sestrin2 was critical for limiting ER stress and promoting the recovery of epithelial cells after inflammatory injury. During colitis-promoted tumorigenesis, Sestrin2 was shown to be an important mediator of p53's control over mTORC1 signaling and tumor cell growth. These results highlight Sestrin2 as a novel tumor suppressor, whose downregulation can accelerate both colitis and colon carcinogenesis.

Multiple amino acid sensing inputs to mTORC1

The evolutionarily conserved target of rapamycin complex 1 (TORC1) is a master regulator of cell growth and metabolism. In mammals, growth factors and cellular energy stimulate mTORC1 activity through inhibition of the TSC complex (TSC1-TSC2-TBC1D7), a negative regulator of mTORC1. Amino acids signal to mTORC1 independently of the TSC complex. Here, we review recently identified regulators that link amino acid sufficiency to mTORC1 activity and how mutations affecting these regulators cause human disease.

Structural basis for leucine sensing by the Sestrin2-mTORC1 pathway

Eukaryotic cells coordinate growth with the availability of nutrients through the mechanistic target of rapamycin complex 1 (mTORC1), a master growth regulator. Leucine is of particular importance and activates mTORC1 via the Rag guanosine triphosphatases and their regulators GATOR1 and GATOR2. Sestrin2 interacts with GATOR2 and is a leucine sensor. Here we present the 2.7 angstrom crystal structure of Sestrin2 in complex with leucine. Leucine binds through a single pocket that coordinates its charged functional groups and confers specificity for the hydrophobic side chain. A loop encloses leucine and forms a lid-latch mechanism required for binding. A structure-guided mutation in Sestrin2 that decreases its affinity for leucine leads to a concomitant increase in the leucine concentration required for mTORC1 activation in cells. These results provide a structural mechanism of amino acid sensing by the mTORC1 pathway.