An emerging role for TOR signaling in mammalian tissue and stem cell physiology

Abstract

The mammalian target of rapamycin (mTOR) is a kinase that responds to a myriad of signals, ranging from nutrient availability and energy status, to cellular stressors, oxygen sensors and growth factors. The finely tuned response of mTOR to these stimuli results in alterations to cell metabolism and cell growth. Recent studies of conditional knockouts of mTOR pathway components in mice have affirmed the role of mTOR signaling in energy balance, both at the cell and whole organism levels. Such studies have also highlighted a role for mTOR in stem cell homeostasis and lifespan determination. Here, we discuss the molecular mechanisms of TOR signaling and review recent in vitro and in vivo studies of mTOR tissue-specific activities in mammals.

Fig. 1.

The domain structure of mTOR. Mammalian target of rapamycin (mTOR) is an atypical serine-threonine protein kinase that belongs to the phosphatidylinositol 3-kinase (PI3K)-related kinase protein (PIKK) family. Along with other members of the PIKK family, mTOR contains a defining C-terminal kinase domain (orange) that bears similarity to the kinase domain of PI3K. In addition, mTOR possesses numerous N-terminal HEAT (huntingtin, elongation factor 3, protein phosphatase 2A, TOR1) repeats (green) that are thought to mediate the bulk of protein-protein interactions between mTOR and other signaling proteins. Other domains include a FAT (FRAP, ATM and TTRAP) domain (pink), which is also found in other PIKK family members, a C-terminal FAT (FATC) domain (blue) of unknown function and a FKBP12/rapamycin-binding (FRB) domain (brown), which is adjacent to the kinase domain in TOR.

Fig. 2.

Composition of TORC1 and TORC2. (A) Target of rapamycin complex 1 (TORC1) (shown as a monomer) consists of mammalian TOR (mTOR), regulatory associated protein of mTOR (raptor), proline-rich AKT substrate 40 KDa (PRAS40), mammalian lethal with Sec-13 protein 8 (mLST8) and DEP domain TOR-binding protein (Deptor). Raptor interacts with some substrates and promotes dimerization of TORC1 complexes by direct interaction with TOR subunits from each monomer. PRAS40 binding to TORC1 is inhibitory and may be mediated by direct interaction with either mTOR or raptor. mLST8, via its multiple WD40 repeats, binds to the kinase domain of mTOR. Deptor binds the FAT (FRAP, ATM and TTRAP) domain of TOR and is capable of inhibiting both TORC1 and TORC2. Rag A/B and Rag C/D bind TORC1 through direct interaction with raptor. (B) TORC2 complex members include mTOR, rapamycin-insensitive companion of mTOR (Rictor), stress-activated protein kinase-interacting protein 1 (Sin1), mLST8, Deptor and protein-binding Rictor (protor). Rictor contains conserved domains that are hypothesized to be important for TORC2 complex formation and substrate recruitment. Sin1 has been described to promote Rictor-mTOR binding and regulate substrate specificity. Protor binds Rictor, although its function is currently unclear. mLST8 also binds mTOR and may be required for TORC2 function in vivo based on knockout data.

Fig. 3.

Upstream regulation of mTORC signaling. Mammalian target of rapamycin complex (mTORC) activity is influenced by a number of positive (shown in blue) and negative (shown in pink) regulators. mTORC1 is activated by growth factors and insulin signaling. This activation is mediated by phosphatidylinositol 3-kinase (PI3K) and AKT (a serine/threonine protein kinase), which inhibit the tuberous sclerosis complex (TSC) 1/2 complex, thereby relieving the TSC1/2-mediated repression of Ras homolog enriched in brain (Rheb) and allowing activation of TORC1. By contrast, low cellular energy levels (conveyed by AMP) and hypoxia activate AMP kinase, which represses mTORC1 both through direct phosphorylation of TSC2 and through regulatory associated protein of mTOR (raptor) inhibition. The TSC1/2 complex thus acts to integrate intracellular cues and extracellular demands, and under unfavorable cellular conditions represses the activity of mTORC1. Intracellularly, mTORC1 can also be activated by amino acids at the lysosomal membrane, which act via the Rag proteins and the Ragulator complex (which consists of p14, p18 and MP1). mTORC2 can also be activated by growth factors through PI3K and through undetermined effectors (indicated by the broken arrow). In turn, mTORC2 can regulate AKT, thereby placing it upstream of TORC1. mTORC2 can also be activated through its association with ribosomes. Abbreviations: GSK3β, glycogen synthase 3β; IRS, insulin receptor substrate; LKB1, liver kinase B1; MP1, MEK partner 1; PRAS40, proline-rich AKT substrate 40 KDa; PTEN, phosphatase and tensin homolog protein; S6K, ribosomal S6 kinase.

Fig. 4.

Downstream effectors of mTORC signaling. (A) Mammalian target of rapamycin complex 1 (mTORC1) phosphorylation of eukaryotic initiation factor 4E (eIF4E)-binding protein (4E-BP) disrupts the interaction of 4E-BP with eIF4E, which leaves eIF4A free to promote the binding of ribosomes to the transcriptional start site. mTORC1 also activates ribosomal S6 kinase (S6K), which activates the translational initiation factor eIF4B and the S6 ribosomal protein by direct phosphorylation. mTOR can also associate with general transcription factor III C (TFIIIC) and relieve its inhibitor Maf1, leading to increased tRNA production. TORC1 activity also promotes association between transcription initiation factor 1A (TIF-1A) and polymerase I (PolI), thereby promoting rRNA synthesis. Finally, mTORC1 signaling can inhibit autophagy through phosphorylation of Unc-51-like kinase 1 (ULK1) and autophagy related 13 homolog (ATG13). (B) mTORC2 phosphorylates and activates AKT (a serine/threonine protein kinase) and serum/glucocorticoid regulated kinase (SGK), and has been implicated in signaling via protein kinase C (PKC) and Rac/Rho. Collectively, these mTORC2-mediated activities promote growth factor signaling and cytoskeletal reorganization. In response to insulin signaling, mTORC2 can also interact with ribosomes. Abbreviation: ATG101, autophagy-related protein 101; FIP200, FAK family interacting protein of 200 kDa.