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1/7Copyright 2015 Wolters Kluwer Health Inc. All rights reserved.

CURRENTOPINION Amino acid sensing and activation of mechanistictarget of rapamycin complex 1: implications forskeletal muscle

Daniel J. Ham, Gordon S. Lynch, and Rene Koopman

Purpose of review

This article evaluates recent studies on the mechanisms involved in sensing changes in amino acidavailability and activation of the mechanistic target of rapamycin complex 1 (mTORC1).

Recent findings

mTORC1 is sensitive to changes in amino acid availability and a well known regulator of protein turnover.The mechanisms of amino acid sensing and mTORC1 signaling are emerging with multiple potential

sensors (e.g., solute carrier family 38, member 9, lysosomal protein transmembrane 4 beta/solute carrierfamily 7, member 5-solute carrier family 3, member 2) and signal transducers (e.g., Sestrins, ADP-ribosylation factor 1, and microspherule protein 1) identified. Studies in various cell lines have unveiled theimportance of the lysosome in amino acid sensing and signal transmission.

Summary

Recent discoveries in amino acid sensing highlight a complex scenario, whereby mTORC1 is not merelysensitive to some amino acids and not others, but where specific amino acids are sensed by specificpathways under specific conditions. The physiological purpose of such an arrangement remains to beunraveled, but it would allow mTORC1 to precisely regulate growth during different metabolic conditions.Understanding the mechanisms responsible for sensing amino acid availability and regulating mTORC1activity is an important prerequisite for the development of nutritional strategies to combat skeletal musclewasting disorders.

Keywords

amino acid metabolism, lysosome, protein synthesis

INTRODUCTION

As early as 1975, it was clear that not all amino acidshad a similar capacity to regulate protein metab-olism. In a landmark study, Buse and Reid [1] dis-covered that unlike other amino acids, the essentialbranched-chain amino acid leucine rapidly stimu-lated muscle protein synthesis and reduced proteinbreakdown in skeletal muscle in vitro. Since then a

multitude of studies in cells, animals, and humanshave demonstrated a potent stimulation of proteinsynthesis by essential amino acids, particularly leu-cine (for reviews see [2,3]). Although the signalingproteins that regulate protein metabolism are exten-sive, the mechanistic target of rapamycin complex 1(mTORC1) has emerged as a central regulator thatintegrates signals from nutrients (e.g., amino acids),growth factors (e.g., insulin), energy status (ATP),and stress (e.g., oxidative stress) [4,5]. Growth fac-tors lead to mTORC1 activation by a well definedmechanism involving the phosphorylation and

inhibition of two key suppressors of mTORC1 acti-vation, tuberous sclerosis 2 (TSC2) and proline-richAkt substrate of 40 kDa[4]. However, growth factorscannot effectively activate mTORC1 without thepresence of amino acids, which are both necessaryand sufficient to activate mTORC1[5].

Modulating mTORC1 and protein metabolismthrough amino acid supplementation has received

considerable attention as a potential treatment formuscle wasting. However, despite an abundance ofamino acid supplementation studies, understand-ing the underlying mechanisms by which amino

Basic and Clinical Myology Laboratory, Department of Physiology, The

University of Melbourne, Victoria, Australia

Correspondence to ReneKoopman, PhD, Department of Physiology, The

University of Melbourne, VIC 3010, Australia. Tel: +61 3 8344 0243;

fax: +61 3 8344 5818; e-mail: [email protected]

Curr Opin Clin Nutr Metab Care 2016, 19:6773

DOI:10.1097/MCO.0000000000000240

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REVIEW
mailto:[email protected]:[email protected]

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acid availability is sensed by skeletal muscle cells toregulate the mTORC1 network remains elusive. Incontrast, over the last 5 years, elegant mechanisticstudies in human embryonic kidney (HEK) cells,mouse embryonic fibroblasts (MEF), and variouscancer cell lines have advanced our understandingof how cellular amino acid sensing regulatesmTORC1 and controls cell growth. These mecha-

nistic studies paint a complex picture regarding thecontrol of protein metabolism and cell growth byamino acids. A better understanding of how aminoacid availability is sensed by cells will enhance ourinterpretation of studies on amino acid supplement-ation relevant to nutrition and physiology.

The ever-expanding network of amino acid-sensitive events upstream of mTORC1 continuesto expand and the mechanisms of amino acidsensing appear specific for different amino acids.Perhaps the most important advance in recent timeswas the identification of the lysosomal amino acid

transporter SLC38A9 asthe most promising aminoacid-sensor candidate [6

&&

,7&&

]. In this review, wespecifically highlight how our understanding ofamino acid sensing has progressed over the last yearand discuss gaps in our current knowledge andimplications for future research in skeletal muscle.

Lysosomal localization is important formechanistic target of rapamycin complex 1activationStudies in many different cell lines (HEK, MEF, andvarious cancer types) have revealed the importance

of the lysosome in amino acid sensing and signaltransmission. Upon amino acid stimulation,mTORC1 colocalizes with the lysosome, where itinteracts with key components of the amino acidsensing machinery [e.g., vacuolar adenosine tri-phosphatase (v-ATPase), Rags, and the Ragulator]along with the growth factor sensitive protein,

Rheb. The lysosome is the endpoint for many catab-olic processes, such as autophagy, where it isresponsible for breaking down proteins and organ-elles into constituent amino acids. The lysosome isan ideal storage site for amino acid sensing, provid-ing access to an enriched source of amino acidseven during conditions of low extracellular aminoacid availability, through the catabolism of proteinsand organelles. As such, the lysosomal localizationof mTORC1 signaling is ideal for mTORC1 to assessthe metabolic state of the cell and orchestrate appro-priate growth signals[8].

Amino acid sensitive signals converge uponthe Rag guanosine triphosphatase (GTPases),which are anchored to the lysosomal surface bythe Ragulator complex [8]. Rags function as het-erodimers with Ras-related GTP-binding protein(Rag) A or RagB binding to RagC or RagD. Aminoacid stimulation activates the Rag complex bymodulating the guanine nucleotide-loading statusof the Rags. The active complex consists of RagA/Bin the GTP-bound state and RagC/D in the gua-nosine diphosphate (GDP)-bound state. The gua-nine nucleotide-loading status of Rags can beregulated by multiple factors including: GTPase-

activating proteins (GAPs) that stimulate GTPhydrolysis [e.g., GTPase-activating proteins towardRags 1 (GATOR1) complex [9]]; guanine nucleo-tide exchange factors that facilitate GDP dis-sociation (e.g., the Ragulator complex); andnucleotide dissociation inhibitors [e.g., GDP-dis-sociation inhibitor (GDIs)] that prevent GDP dis-sociation, which until the recent discovery of theactions of the Sestrin family of proteins, had notbeen described for Rags. Upon activation, the Ragcomplex promotes mTORC1 translocation to thelysosome where it can interact with Rheb, result-

ing in activation of mTORC1 and its downstreamtargets ribosomal protein S6 kinase 1, 70 kDa(S6K1) and eukaryotic translation initiation factor4E binding protein 1 (4EBP1) [2].

The lysosome is not only where mTORC1 inter-acts with Rheb to drive protein synthesis and cellgrowth, but rather a key point of control formTORC1 activation and repression. The regulationof Rags and their guanine nucleotide-loadingstatus has emerged as an important point ofcontrol for sensing amino acid sufficiency andinsufficiency and promoting or repressing the

KEY POINTS

mTORC1 is sensitive to changes in amino acidavailability and a well known regulator ofprotein turnover.

The mechanisms of amino acid sensing and mTORC1signaling are emerging with multiple potential sensors

[e.g., solute carrier family 38, member 9 (SLC38A9),lysosomal protein transmembrane 4 beta (LAPTM4b)/solute carrier family 7, member 5-solute carrier family3, member 2 (LAT1-4F2hc)] and signal transducers[e.g., Sestrins, ADP-ribosylation factor 1 (ARF-1), andMCRS1] identified.

Studies in various (cancer) cell lines have unveiled theimportance of the lysosome in amino acid sensing andsignal transmission.

Understanding the mechanisms responsible for sensingamino acid availability and regulating mTORC1 activityis an important prerequisite for the development ofnutritional strategies to combat skeletal musclewasting disorders.

Protein, amino acid metabolism, and therapy

68 www.co-clinicalnutrition.com Volume 19 Number 1 January 2016

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activity of mTORC1. It is important to notethat it is equally important that mTORC1 isrepressed during conditions of reduced aminoacid availability and activated during amino acidrepletion. For example, in RagAGTP/GTP knockinmice where RagA is perpetually GTP-bound(active), the mice are incapable of disabling

mTORC1 signaling and arresting growth processesduring amino acid deprivation. As a result, RagAGTP/GTP knockin mice have increased neonatal lethalityduring the period of fasting between birth andsuckling[10].

Sestrins suppress mechanistic target ofrapamycin complex 1 activity during aminoacid insufficiency

Recently, the Sestrin family of proteins haveemerged as important amino acid-sensitive regula-tors of mTORC1, potently suppressing its activityduring amino acid insufficiency [11

&&

,12&

]. Thecritical importance of Sestrins in signaling aminoacid availability to mTORC1 was highlighted byknockout studies in mice, where knockout of allthree Sestrins (13) was associated with ineffectiveinactivation of mTORC1 signaling in skeletalmuscle, liver, and heart tissue during neonatalfasting and dramatically reduced postnatalsurvival [11

&&

]. Although it is well known thatthe Sestrins regulate mTORC1 activity, the mech-anisms responsible have remained elusive. Earlystudies concluded that Sestrins act through TSC2

and AMP-dependent protein kinase to inhibitmTORC1, but recent work [11

&&

] demonstratedthat neither protein was essential for the regula-tion of mTORC1 by Sestrin. Peng etal. [11

&&

] showedthat Sestrins can act as GDIs for Rags, lockingRagA/B in either the GTP or GDP-bound form,thereby suppressing both activation and deactiva-tion of mTORC1 in response to amino acid avail-ability. In addition, an amino acid-sensitiveinteraction has been reported between Sestrin2and the GTPase-activating proteins toward Rags 2(GATOR2) complex, a poorly understood positive

regulator of mTORC1, which may act upstreamor in parallel to the RagA/B GAP GATOR1 [12

&

].However, despite the requirement for both GATOR1and Rags for the inhibition of mTORC1 signalingby Sestrins, the interaction between GATOR2 andGATOR1 was not disrupted nor was the GAP activityof GATOR1 to RagA/B affected by Sestrins [12

&

].Further work is required to fully understand themechanisms responsible for the regulation ofmTORC1 by Sestrins, and the upstream mechanismsresponsible for its activation.

Microspherule protein 1 facilitateslysosomal localization and couplingof Rheb to mechanistic target ofrapamycin complex 1Once amino acid sufficiency stimulates the trans-location of mTORC1 to the lysosome, it can interactwith Rheb, which integrates signals from various

growth factors (e.g., insulin and insulin-like growthfactor 1), although the mechanisms responsible forthis interaction are unclear. Growth factors activateRheb, but cannot activate mTORC1 in the absence ofamino acids. A recent breakthrough identified micro-spherule protein 1 (MCRS1) as an amino acid sensi-tive molecular link between activated Rheb andmTORC1 [13

&&

]. MCRS1 interacts with both mTORC1and Rheb independently. Interaction of MCRS1 withmTORC1 is essential for mTORC1 activity, but isinsensitive to amino acids. On the other hand, inter-action of MCRS1 with GTP-bound (activated) Rheb isamino acid sensitive, with the MCRS1/Rheb inter-action being reduced by amino acid depletion andstrongly enhanced by amino acid stimulation. Theinteraction between MCRS1 and Rheb is essential forthe lysosomal localization of Rheb, butnot mTORC1,since MCRS1 suppression abolished the lysosomallocalization of Rheb but not mTORC1 [13

&&

]. Further-more, in confirmation of therole of MCRS1 in linkingRheb and mTORC1 at the lysosome, MCRS1depletion impaired the Rheb/mTORC1 interactionin multiple cell lines, whereas MCRS1 knockdownexperiments in mice resulted in severe mTORC1activity inhibition [13

&&

].

Mechanistic target of rapamycin complex 1represses extracellular protein utilizationduring amino acid insufficiency

The Rags themselves can also play a role in theinhibition of mTORC1. In RagC/D knockout cellsthe repression of mTORC1 by amino acid depri-vation is attenuated [14

&

]. In fact, the Rags appearto also play a role in recruiting TSC2 to the lysoso-mal membrane where it inhibits mTORC1 [15].Further complicating the role of mTORC1 in amino

acid sensing and growth, Palmet al. [16&&

]observedan inhibitory effect of mTORC1 on cancer cellproliferation when cells were forced to rely onextracellular proteins as an amino acid source.Although the lysosomal degradation of extracellularproteins provided sufficient amino acids to promotemTORC1 activation and sustain cell survival,significant cell accumulation did not occur in theabsence of free amino acids. In fact, in the absenceof free amino acids, mTORC1 inhibition (usingTorin1 or rapamycin) increased the catabolism

Amino acid sensing Ham et al.

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of extracellular proteins and enhanced cellproliferation [16

&&

]. As such, by controlling the lyso-somal degradation of extracellular proteins,mTORC1 precisely matches cell growth with theavailability of free amino acids. Intuitively, it makessense that mTORC1 would inhibit cell growth whenthe cell relies on the breakdown of extracellular

proteins to survive. Although the physiological roleof these processes in skeletal muscle is currentlyunknown, the control of autophagy by mTORC1is well documented in this tissue [17,18]. Autophagyis tightly regulated in skeletal muscle, with bothexcessive activation and inhibition resulting in atro-phy [17]. Interestingly, it was demonstrated recentlyin skeletal muscle cells that mTORC1 not onlyregulates autophagy, but autophagy also regulatesmTORC1 and the amino acid signaling network[19

&

].

Recently discovered intracellular amino acidsensors

Despite the identification of many proteinsinvolved in signaling amino acid sufficiency tomTORC1, the actual amino acid sensors havelargely remained elusive. Given the importance ofthe lysosome in amino acid signaling and therequirement for amino acids in the lysosomallumen, it would be logical for an amino acid sensorto have a lysosomal transmembrane domain andinteract with known components of the aminoacid sensing pathway. This premise led to the dis-

covery of SLC38A9, a member of the solute carrier38 family which, unlike other solute carrierproteins, colocalizes with both lysosomal markers(e.g., lysosomal-associated membrane protein 1and CD63) and key components of the amino acidsensing pathway, including Ragulator, RagAand C, and the v-ATPase [6

&&

,7&&

]. SLC38A9 overex-pression renders mTORC1 resistant to amino acidstarvation, whereas silencing blunts amino acid-induced mTORC1 activation [6

&&

,7&&

]. The N-termi-nal interacts with the RagulatorRag GTPase com-plex, which is the key step required for mTORC1

activation, but requires the transmembrane domainto confer amino acid responsiveness [7

&&

]. AlthoughSLC38A9 is capable of bidirectional amino acid trans-port, this does not appear to be its primary purpose,since the measured rates of glutamine influx andefflux are moderate comparedwith other lysosomalamino acid transporters [6

&&

,7&&

]. Thus, SLC38A9could be considered a transporter that also acts as areceptor, or a transceptor, with properties similar tothose of the amino acid sensors GAP1 and yeastextracellular amino acid sensor described in yeastandDrosophila[6

&&

].

Intriguingly, SLC38A9 has a particular penchantfor arginine. Recent experiments revealed knockoutof SLC38A9 strongly inhibited arginine-inducedmTORC1 activation but cells remained sensitiveto leucine [7

&&

]. Furthermore, SLC38A9 is capableof bidirectional transport of [3H]arginine and[3H]asparagine, but not [3H]leucine or [3H]histidine.

As such, SLC38A9 appears to play a central role insensing and relaying arginine but not leucine levelsto mTORC1. Of course, SLC38A9 is one of manyamino acid sensors, since amino acid-sensitiveevents have been reported upstream of folliculin[20,21]and GATOR1 [12

&

]and in response to otheramino acids such as glutamine and leucine.

Another important question regarding aminoacid sensing at the lysosome, is how lysosomesbecome enriched with amino acids to activatemTORC1 during periods of high extracellular aminoacid availability. Recent findings suggest a mechan-ism involving the leucine transporter LAT1-4F2hc.LAT1/4F2hc (or SLC7A5/SLC3A2) is a bidirectionalamino acid transporter that controls the simul-taneous cellular efflux of glutamine/nonessentialamino acids and cellular influx of L-leucine/essen-tial amino acids[22]. A new role has been identifiedfor this transporter in mediating the transport ofleucine into lysosomes for inside-out activation ofmTORC1 through the v-ATPase (Fig. 1). Uponamino acid stimulation, LAT1-4F2hc is recruitedto the lysosome from the cell membrane and/orthe Golgi by LAPTM4b, where it commences bidirec-tional transport of leucine into the lysosome and

glutamine out of thelysosome, and is necessary formTORC1 activation [23

&&

]. Therefore, LAT1-4F2hcplays an important role in transporting extracellularleucine into the cytosol and then transporting cyto-solic leucine into the lysosome. As such, for acti-vation of mTORC1 by lysosomal leucine, substantialextracellular and cytosolic leucine concentrationsalong with sufficient cytosolic and lysosomal con-centrations of neutral nonessential amino acids(e.g., glutamine) would be required. The require-ment for cytosolic and lysosomal nonessentialamino acids also implicates other transporters

capable of promoting nonessential amino aciduptake, such as SNAT3 (SLC38A3) and SNAT5(SLC38A5). For a review on the role of amino acidtransporters in amino acid sensing [24]. Althoughthe precise mechanisms responsible for leucine sens-ing remain to be determined, the discovery ofLAPTM4b-induced translocation of a functional leu-cine transporter to the lysosome is a major stepforward in our understanding of the inside-outmechanism of mTORC1 activation by lysosomalamino acids, originally described by Zoncu et al.[25].


http://-/?-http://-/?-

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Glutamine modulates mechanistic target ofrapamycin complex 1 independent of theRagulator complexThe particular sensitivity of mTORC1 to someamino acids, the most notable being leucine,arginine, and glutamine, has been known for sometime but all were thought to act through changes inRag GTPase loading at the lysosomal surface. How-ever, a recent study casted significant doubt on thisassertion. Interestingly, in Rag A/B knockout cells,

amino acid sensitivity was only partially impededcompared with control cells [26

&&

]. By stimulatingRag A/B knockout cells with each of the standard20 amino acids, Jewell et al. [26

&&

]determined thatmTORC1 activation by both leucine and argininewas abolished, but glutamine-induced activationof mTORC1 was unperturbed. Although glutaminestill required a functional v-ATPase, the Ragulatorcomplex was not essential for mTORC1 activation.Instead, the ARF1 GTPase mediated the trans-location of mTORC1 to the lysosome in gluta-mine-stimulated Rag A/B knockout cells [26

&&

].

In an effort to transition some of the excellentprogress made toward understanding amino acidsensing in more physiological contexts, Averouset al. [14

&

] investigated the effect of amino acid orleucine deprivation and stimulation on lysosomallocalization and mTORC1 activity in MEF cells withor without serum. Evidence of amino acid specificcontrol of mTORC1 was also observed, but in con-trast to Jewellet al.[26

&&

]this group observed modu-lation of mTORC1 activity but not lysosomal

localization by leucine. Leucine deprivation andthen leucine stimulation first blunted and thenrestored mTORC1 activity based on the phosphoryl-ation status of S6K1 and 4EBP1, without affectingthe lysosomal localization of mTORC1. Further-more, in RagC/D knockdown cells, mTORC1activity remained sensitive to leucine availabilitywithout localization to the lysosome [14

&

].Together, these results highlight that control ofmTORC1 activity is highly context dependent andamino acid specific, and mTORC1 maintains aminoacid sensitivity in the absence of Rag GTPases

FIGURE 1. Key mechanisms in amino acid sensing and mTORC1 activation. Newly discovered players (in bold) include:SLC38A9, described as a transceptor, which senses lysosomal arginine availability [6&&,7&&]; the sestrins, which repressmTORC1 activity during amino acid insufficiency [15,16&&]; microspherule protein 1, which couples Rheb to mTORC1 in anamino acid-sensitive fashion [13&&]; LAPTM4b, which facilitates the translocation of LAT1-4F2hc and influx of leucine into thelysosome[27]; and ARF1, which mediates glutamine induced mTORC1 activation independently of the Rags [26&&]. mTORC1,

mechanistic target of rapamycin complex 1.



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[14&

,26&&

]and, at least in MEF cells [14&

], independ-ent of lysosomal localization.

Translating amino acid sensing mechanismsto skeletal muscle

It is important to note that amino acid sensing

may vary widely between different tissues and celllines [27]. Although some studies have demon-strated skeletal muscle effects of amino acidsensitive proteins (e.g., the Sestrins [11

&&

]), themajority of our understanding of amino acidsensing comes from mechanistic studies in HEK,MEF, and cancer cell lines and may not reflectsensing mechanisms in skeletal muscle. Someprogress has been made in applying some of thesefindings to skeletal muscle. For example, increasesin RagB mRNA and protein in response toincreased amino acid availability have beenobserved in human skeletal muscle [28

&

]. Sensi-tivity of mTORC1 to arginine was also recentlydemonstrated in a mouse skeletal muscle cell line[29]. Although it has been known for many yearsthat leucine is a potent acute modulator of proteinmetabolism in human skeletal muscle, chronicleucine supplementation studies failed to demon-strate changes in skeletal muscle mass or strengthduring muscle wasting conditions [2]. Further-more, under many catabolic conditions, skeletalmuscle becomes insensitive to changes in aminoacid availability, an observation termed anabolicresistance[30]. Further work is needed to increase

our understanding of the mechanisms involvedin amino acid sensing and their regulation ofmuscle mass during health and disease. Thisknowledge is essential for the development ofefficacious nutritional interventions for skeletalmuscle wasting disorders.

CONCLUSION

Recent discoveries on the mechanisms of aminoacid signaling highlights the complexity of aminoacid sensing, whereby mTORC1 is not merely

sensitive to some amino acids and not others,but where specific amino acids are sensed inspecific ways. The physiological purpose of suchan arrangement remains to be unraveled, but itwould allow mTORC1 to precisely regulate growthduring different metabolic conditions. A greaterunderstanding of the mechanisms responsible foramino acid sensing, and their dysregulationduring skeletal muscle diseases and disorders, isan important prerequisite for the development ofoptimal nutritional strategies to combat skeletalmuscle wasting.

Acknowledgements

None.

Financial support and sponsorship

D.H. is supported by a Research Fellowship from theEuropean Society for Clinical Nutrition and Metabolism(ESPEN).

Conflicts of interest

There are no conflicts of interest.

REFERENCES AND RECOMMENDED

READINGPapers of particular interest, published within the annual period of review, havebeen highlighted as:& of special interest&& of outstanding interest

1. Buse MG, Reid SS. Leucine. A possible regulator of protein turnover inmuscle. J Clin Invest 1975; 56:12501261.

2. Ham DJ, Caldow MK, Lynch GS, Koopman R. Leucine as a treatment formuscle wasting: a critical review. Clin Nutr 2014; 33:937945.

3. Dickinson JM, Rasmussen BB. Essential amino acid sensing, signaling, and

transport in the regulation of human muscle protein metabolism. Curr OpinClin Nutr Metab Care 2011; 14:8388.

4. Efeyan A, ZoncuR, Sabatini DM. Amino acids and mTORC1: from lysosomesto disease. Trends Mol Med 2012; 18:524533.

5. Jewell JL,Russell RC,Guan KL.Amino acid signalling upstream of mTOR. NatRev Mol Cell Biol 2013; 14:133139.

6.

&&

Rebsamen M, Pochini L, Stasyk T, et al. SLC38A9 is a component of thelysosomal amino acid sensing machinery that controls mTORC1. Nature2015; 519:477481.

Codiscovery of the promising amino acid sensor candidate, which activatesmTORC1 in response to lysosomal arginine availability.7.

&&

Wang S, Tsun ZY, Wolfson RL, et al. Metabolism. Lysosomal amino acidtransporter SLC38A9 signals arginine sufficiency to mTORC1. Science2015; 347:188194.

Codiscovery of the promising amino acid sensor candidate, which activatesmTORC1 in response to lysosomal arginine availability.8. Bar-Peled L, Sabatini DM. Regulation of mTORC1 by amino acids. Trends

Cell Biol 2014; 24:400406.

9. Bar-Peled L, Chantranupong L, Cherniack AD, et al. A tumor suppressorcomplex with GAP activity for the Rag GTPases that signal amino acidsufficiency to mTORC1. Science 2013; 340:11001106.

10. Efeyan A, Zoncu R, Chang S, et al. Regulation of mTORC1 by the RagGTPases is necessary for neonatal autophagy and survival. Nature 2013;493:679683.

11.

&&

Peng M, Yin N, Li MO. Sestrins function as guanine nucleotide dissociationinhibitors for Rag GTPases to control mTORC1 signaling. Cell 2014;159:122133.

Report the crucial role of sestrins in supressing mTORC1 activity during aminoacid insufficiency, and provide the first evidence of a GDI for Rags.12.

&

Chantranupong L, Wolfson RL, Orozco JM, et al. The sestrins interact withGATOR2 to negatively regulate the amino-acid-sensing pathway upstream ofmTORC1. Cell Rep 2014; 9:18.

Demonstrate the requirement of GATOR1 and the Rags for sestrin-inducedmTORC1 inhibition.13.

&&

Fawal MA, Brandt M, Djouder N. MCRS1 binds and couples Rheb to aminoacid-dependent mTORC1 activation. Dev Cell 2015; 33:6781.

Report amino acid sensitive regulation of Rheb by MCRS1, which serves as amolecular link between Rheb and mTORC1.14.

&

Averous J, Lambert-Langlais S, Carraro V, et al. Requirement for lysosomallocalizationof mTOR forits activation differs between leucine andother aminoacids. Cell Signal 2014; 26:19181927.

Undermore physiological contexts, this groupreport aminoacid specific mTORC1regulatory mechanisms.15. Demetriades C, Doumpas N, Teleman AA. Regulation of TORC1 in response

to amino acid starvation via lysosomal recruitment of TSC2. Cell 2014;156:786799.

16.

&&

Palm W, Park Y, Wright K, et al. The utilization of extracellular proteins asnutrients is suppressed by mTORC1. Cell 2015; 162:259270.

Demonstrated that mTORC1 couples cell growth with free amino acid availability,supressing growth when amino acids are derived from the breakdown of extra-cellular proteins.17. Castets P, Lin S, Rion N, et al. Sustained activation of mTORC1 in skeletal

muscle inhibits constitutive and starvation-induced autophagy and causes asevere, late-onset myopathy. Cell Metab 2013; 17:731744.



7/26/2019 00075197-201601000-00013


18. Castets P, Ruegg MA. MTORC1 determines autophagy through ULK1regulation in skeletal muscle. Autophagy 2013; 9:14351437.

19.

&

Yu X, Long YC. Autophagy modulates amino acid signaling network inmyotubes: differential effects on mTORC1 pathway and the integrated stressresponse. Faseb J 2015; 29:394407.

First to show a reciprocal regulatory relationship between mTORC1 and autop-hagy.20. Tsun ZY, Bar-Peled L, Chantranupong L, et al. The folliculin tumorsuppressor

is a GAP for the RagC/D GTPases that signal amino acid levels to mTORC1.Mol Cell 2013; 52:495505.

21. Petit CS, Roczniak-Ferguson A, Ferguson SM. Recruitment of folliculin to

lysosomes supports the amino acid-dependent activation of Rag GTPases.J Cell Biol 2013; 202:11071122.22. Nicklin P, Bergman P, Zhang B, et al. Bidirectional transport of amino acids

regulates mTOR and autophagy. Cell 2009; 136:521534.23.

&&

Milkereit R, Persaud A, Vanoaica L, et al.LAPTM4b recruits the LAT1-4F2hcLeu transporter to lysosomes and promotes mTORC1 activation. Nat Com-mun 2015; 6:7250.

Describes amino acid sensitivetranslocation of a functional leucine receptor to thelysosome, which is essential for mTORC1 activation.

24. Taylor PM. Role of amino acid transporters in amino acid sensing. Am J ClinNutr 2014; 99:223S230S.

25. Zoncu R, Bar-Peled L, Efeyan A, et al. mTORC1 senses lysosomal aminoacids through an inside-out mechanism that requires the vacuolar H()-ATPase. Science 2011; 334:678683.

26.

&&

Jewell JL, Kim YC, Russell RC, et al. Metabolism. Differential regulation ofmTORC1 by leucine and glutamine. Science 2015; 347:194198.

Demonstrated aminoacid specific regulators mechanismsfor mTORC1 activation.27. Chantranupong L, Wolfson RL, Sabatini DM. Nutrient-sensing mechanisms

across evolution. Cell 2015; 161:6783.28.

&

Carlin MB, Tanner RE, Agergaard J, et al. Skeletal muscle Ras-related GTP

binding B mRNA and protein expression is increased after essential aminoacid ingestion in healthy humans. J Nutr 2014; 144:14091414.First group to show changes in Rag mRNA and protein in human skeletal muscle.29. Ham DJ, Caldow MK, Lynch GS, Koopman R. Arginine protects muscle cells

fromwasting in vitroin an mTORC1-dependent and NO-independent manner.Amino Acids 2014; 46:26432652.

30. Phillips SM, Glover EI, Rennie MJ. Alterations of protein turnover underlyingdisuse atrophy in human skeletal muscle. J Appl Physiol 2009; 107:645654.



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