Mechanistic target of rapamycin

The mechanistic target of rapamycin, also known as mammalian target of rapamycin (mTOR) or FK506-binding protein 12-rapamycin-associated protein 1 (FRAP1), is a is_associated_with::protein that in humans is encoded by the MTOR is_associated_with::gene. MTOR is a is_associated_with::serine/threonine protein kinase that regulates cell growth, is_associated_with::cell proliferation, cell is_associated_with::motility, cell survival, is_associated_with::protein synthesis, is_associated_with::autophagy, and transcription. MTOR belongs to the is_associated_with::phosphatidylinositol 3-kinase-related kinase protein family.

Discovery
MTOR was first named as the mammalian target of rapamycin. is_associated_with::Rapamycin was discovered in a soil sample from is_associated_with::Easter Island, known locally as Rapa Nui, in the 1970s. The bacterium is_associated_with::Streptomyces hygroscopicus, isolated from that sample, produces an antifungal that researchers named rapamycin after the island.

Rapamycin arrests fungal activity at the is_associated_with::G1 phase of the cell cycle. In mammals, it suppresses the immune system by blocking the G1 to S phase transition in is_associated_with::T-lymphocytes. Thus, it is used as an is_associated_with::immunosuppressant following organ transplantation.

Molecular genetic studies in yeast (published in 1991) first identified FKBP12, TOR1, and TOR2 as the targets of rapamycin; these studies were conducted at the Biozentrum in Basel, Switzerland and is_associated_with::Sandoz Pharmaceuticals (now Novartis) by Joseph Heitman, Rao Movva, and Mike Hall. They isolated rapamycin-resistant mutants of is_associated_with::Saccharomyces cerevisiae and discovered that mutations in any of three genes can confer rapamycin resistance. Two of the genes were named TOR1 and TOR2 for targets of rapamycin (TOR) and in honor of the Spalentor, a gate to the city of Basel where TOR was first discovered. The third gene is FPR1, which encodes the yeast ortholog of FKBP12 binding protein in the TOR complexes. Loss of function mutations in FPR1 confer resistance to rapamycin, and also to FK506, providing genetic evidence the FKBP12-drug complexes are the active intracellular agents. Mutations in TOR1 or TOR2 that confer FKBP12-rapamycin resistance are dominant gain of function mutations that alter single amino acid residues within the FRB domain and thereby block FKBP12-rapamycin binding. Several years later, in 1994 the mammalian target of rapamycin (mTOR) was identified and found to be the ortholog of the yeast Tor1/2 proteins and defined as the rapamycin target in mammals by is_associated_with::David M. Sabatini and is_associated_with::Solomon H. Snyder (Johns Hopkins University) and also by Robert Abraham (who first named it mTOR) and is_associated_with::Stuart L. Schreiber (Harvard University). mTOR stands for mammalian Target Of Rapamycin and was named based on the precedent that TOR was first discovered via genetic and molecular studies of rapamycin-resistant mutants of Saccharomyces cerevisiae that identified Tor1 and Tor2 as the targets of rapamycin.

Function
MTOR integrates the input from upstream pathways, including is_associated_with::insulin, is_associated_with::growth factors (such as is_associated_with::IGF-1 and is_associated_with::IGF-2), and is_associated_with::amino acids. mTOR also senses cellular nutrient, oxygen, and energy levels. The mTOR pathway is dysregulated in human diseases, such as is_associated_with::diabetes, is_associated_with::obesity, depression, and certain is_associated_with::cancers. is_associated_with::Rapamycin inhibits mTOR by associating with its intracellular receptor FKBP12. The FKBP12-is_associated_with::rapamycin complex binds directly to the FKBP12-Rapamycin Binding (FRB) domain of mTOR, inhibiting its activity.

Complexes
MTOR is the is_associated_with::catalytic subunit of two structurally distinct complexes: MTORC1 and MTORC2. Both complexes localize to different subcellular compartments, thus affecting their activation and function.

mTORC1
mTOR Complex 1 (mTORC1) is composed of mTOR, regulatory-associated protein of mTOR (Raptor), mammalian lethal with SEC13 protein 8 (is_associated_with::MLST8) and the non-core components PRAS40 and is_associated_with::DEPTOR. This complex functions as a nutrient/energy/redox sensor and controls protein synthesis. The activity of mTORC1 is stimulated by insulin, growth factors, serum, is_associated_with::phosphatidic acid, is_associated_with::amino acids (particularly is_associated_with::leucine), and is_associated_with::oxidative stress.

mTORC2
mTOR Complex 2 (mTORC2) is composed of mTOR, rapamycin-insensitive companion of mTOR (is_associated_with::RICTOR), is_associated_with::MLST8, and mammalian stress-activated protein kinase interacting protein 1 (mSIN1). mTORC2 has been shown to function as an important regulator of the is_associated_with::cytoskeleton through its stimulation of F-is_associated_with::actin stress fibers, is_associated_with::paxillin, RhoA, Rac1, Cdc42, and is_associated_with::protein kinase C α (PKCα). mTORC2 also phosphorylates the serine/threonine protein kinase Akt/PKB at the serine residue S473, thus affecting metabolism and survival. Phosphorylation of the serine stimulates Akt phosphorylation at a threonine T308 residue by PDK1 and leads to full Akt activation;

Inhibition by rapamycin
Rapamycin inhibits mTORC1, and this appears to provide most of the beneficial effects of the drug (including life-span extension in animal studies). Rapamycin has a more complex effect on mTORC2, inhibiting it only in certain cell types under prolonged exposure. Disruption of mTORC2 produces the diabetic-like symptoms of decreased glucose tolerance and insensitivity to insulin.

Gene deletion experiments
The mTORC2 signaling pathway is less defined than the mTORC1 signaling pathway. The functions of the components of the mTORC complexes have been studied using knockdowns and knockouts and were found to produce the following phenotypes:
 * is_associated_with::NIP7: Knockdown reduced mTORC2 activity that is indicated by decreased phosphorylation of mTORC2 substrates.
 * is_associated_with::RICTOR: Overexpression leads to metastasis and knockdown inhibits growth factor-induced PKC-phosphorylation.
 * mTOR: Inhibition of mTORC1 and mTORC2 by PP242 [2-(4-Amino-1-isopropyl-1H-pyrazolo[3,4-d]pyrimidin-3-yl)-1H-indol-5-ol] leads to is_associated_with::autophagy or is_associated_with::apoptosis; inhibition of mTORC2 alone by PP242 prevents phosphorylation of Ser-473 site on AKT and arrests the cells in is_associated_with::G1 phase of the is_associated_with::cell cycle.
 * PDK1: Knockout is lethal; hypomorphic allele results in smaller organ volume and organism size but normal AKT activation.
 * is_associated_with::AKT: Knockout mice experience spontaneous apoptosis (is_associated_with::AKT1), severe diabetes (is_associated_with::AKT2), small brains (is_associated_with::AKT3), and growth deficiency (AKT1/AKT2)
 * TOR1, the S. cerevisiae orthologue of mTORC1, is a regulator of both carbon and nitrogen metabolism; TOR1 KO strains regulate response to nitrogen as well as carbon availability, indicating that it is a key nutritional transducer in yeast.

Autism
MTOR is implicated in the failure of a 'pruning' mechanism of the excitatory synapses in is_associated_with::autism spectrum disorders.

Aging
Decreased TOR activity has been found to increase life span in S. cerevisiae, C. elegans, and D. melanogaster. The mTOR inhibitor is_associated_with::rapamycin has been confirmed to increase lifespan in mice by independent groups at the is_associated_with::Jackson Laboratory, is_associated_with::University of Texas Health Science Center, and the is_associated_with::University of Michigan.

It is hypothesized that some dietary regimes, like is_associated_with::caloric restriction and is_associated_with::methionine restriction, cause lifespan extension by decreasing mTOR activity. It is believed that this is achieved by limiting the essential amino acid leucine, a potent activator of mTOR. The administration of is_associated_with::leucine into the rat brain has been shown to decrease food intake and body weight via activation of the mTOR pathway.

According to the is_associated_with::free radical theory of aging, is_associated_with::reactive oxygen species cause damage of is_associated_with::mitochondrial proteins and decrease of ATP production. Subsequently, via ATP sensitive AMPK, the mTOR pathway is inhibited and ATP consuming protein synthesis is downregulated, since mTORC1 initiates a phosphorylation cascade activating the is_associated_with::ribosome. Hence, the proportion of damaged proteins is enhanced. Moreover, disruption of mTORC1 directly inhibits mitochondrial respiration. These positive feedbacks on the aging process are counteracted by protective mechanisms: Decreased mTOR activity (among other factors) upregulates is_associated_with::glycolysis and removal of dysfunctional cellular components via is_associated_with::autophagy.

Alzheimer’s disease
mTOR signaling intersects with Alzheimer’s disease (AD) pathology in several aspects, suggesting its potential role as a contributor to disease progression. In general, findings demonstrate mTOR signaling hyperactivity in AD brains. For example, postmortem studies of human AD brain reveal dysregulation in PTEN, Akt, S6K, and mTOR. mTOR signaling appears to be closely related to the presence of soluble amyloid beta (Aβ) and tau proteins, which aggregate and form two hallmarks of the disease, Aβ plaques and neurofibrillary tangles, respectively. In vitro studies have shown Aβ to be an activator of the PI3K/AKT pathway, which in turn activates mTOR. In addition, applying Aβ to N2K cells increases the expression of p70S6K, a downstream target of mTOR known to have higher expression in neurons that eventually develop neurofibrillary tangles. Chinese hamster ovary cells transfected with the 7PA2 familial AD mutation also exhibit increased mTOR activity compared to controls, and the hyperactivity is blocked using a gamma-secretase inhibitor. These in vitro studies suggest that increasing Aβ concentrations increases mTOR signaling; however, significantly large, cytotoxic Aβ concentrations are thought to decrease mTOR signaling.

Consistent with data observed in vitro, mTOR activity and activated p70S6K have been shown to be significantly increased in the cortex and hippocampus of animal models of AD compared to controls. Pharmacologic or genetic removal of the Aβ in animal models of AD eliminates the disruption in normal mTOR activity, pointing to the direct involvement of Aβ in mTOR signaling. In addition, by injecting Aβ oligomers into the hippocampi of normal mice, mTOR hyperactivity is observed. Cognitive impairments characteristic of AD appear to be mediated by the phosphorylation of PRAS-40, which detaches from and allows for the mTOR hyperactivity when it is phosphorylated; inhibiting PRAS-40 phosphorylation prevents Aβ-induced mTOR hyperactivity. Given these findings, the mTOR signaling pathway appears to be one mechanism of Aβ-induced toxicity in AD.

The hyperphosphorylation of tau proteins into neurofibrillary tangles is one hallmark of AD. p70S6K activation has been shown to promote tangle formation as well as mTOR hyperactivity through increased phosphorylation and reduced dephosphorylation. It has also been proposed that mTOR contributes to tau pathology by increasing the translation of tau and other proteins. Synaptic plasticity is a key contributor to learning and memory, two processes that are severely impaired in AD patients. Translational control, or the maintenance of protein homeostasis, has been shown to be essential for neural plasticity and is regulated by mTOR. Both protein over- and under-production via mTOR activity seem to contribute to impaired learning and memory. Furthermore, given that deficits resulting from mTOR overactivity can be alleviated through treatment with rapamycin, it is possible that mTOR plays an important role in affecting cognitive functioning through synaptic plasticity. Further evidence for mTOR activity in neurodegeneration comes from recent findings demonstrating that eIF2α-P, an upstream target of the mTOR pathway, mediates cell death in prion diseases through sustained translational inhibition. Some evidence points to mTOR’s role in reduced Aβ clearance as well. mTOR is a negative regulator of autophagy; therefore, hyperactivity in mTOR signaling should reduce Aβ clearance in the AD brain. Several groups have proposed that disruptions in autophagy may be a potential source of pathogenesis in protein misfolding diseases, including AD. Studies using mouse models of Huntington’s disease demonstrate that treatment with rapamycin facilitates the clearance of huntingtin aggregates. Perhaps the same treatment may be useful in clearing Aβ deposits as well.

Cancer
Over-activation of mTOR signaling significantly contributes to the initiation and development of tumors and mTOR activity was found to be deregulated in many types of cancer including breast, prostate, lung, melanoma, bladder, brain, and renal carcinomas. Reasons for constitutive activation are several. Among the most common are mutations in tumor suppressor PTEN gene. is_associated_with::PTEN phosphatase negatively affects mTOR signalling through interfering with the effect of is_associated_with::PI3K, an upstream effector of mTOR. Additionally, mTOR activity is deregulated in many cancers as a result of increased activity of is_associated_with::PI3K or is_associated_with::Akt. Similarly, overexpression of downstream mTOR effectors is_associated_with::4E-BP1, is_associated_with::S6K and is_associated_with::eIF4E leads to poor cancer prognosis. Also, mutations in TSC protein that inhibits the activity of mTOR may lead to a condition named tuberous sclerosis complex, which exhibits as benign lesions and increases the risk of is_associated_with::renal cell carcinoma.

Increasing mTOR activity was shown to drive cell cycle progression and increase cell proliferation mainly thanks to its effect on protein synthesis. Moreover, active mTOR supports tumor growth also indirectly by inhibiting is_associated_with::autophagy. Constitutively activated mTOR functions in supplying carcinoma cells with oxygen and nutrients by increasing the translation of is_associated_with::HIF1A and supporting is_associated_with::angiogenesis. mTOR also aids in another metabolic adaptation of cancerous cells to support their increased growth rate - activation of glycolytic metabolism. is_associated_with::Akt2, a substrate of mTOR, specifically of is_associated_with::mTORC2, upregulates expression of the glycolytic enzyme is_associated_with::PKM2 thus contributing to the is_associated_with::Warburg effect.

mTOR inhibitors as therapies
mTOR inhibitors, e.g. is_associated_with::rapamycin, are already used to prevent is_associated_with::transplant rejection. Rapamycin is also related to the therapy of glycogen storage disease (GSD). Some articles reported that rapamycin can inhibit mTORC1 so that the phosphorylation of GS(glycogen synthase) can be increased in skeletal muscle. This discovery represents a potential novel therapeutic approach for glycogen storage diseases that involve glycogen accumulation in muscle. Various natural compounds, including is_associated_with::epigallocatechin gallate (EGCG), is_associated_with::caffeine, is_associated_with::curcumin, and is_associated_with::resveratrol, have also been reported to inhibit mTOR when applied to isolated cells in culture; however, there is as yet no evidence that these substances inhibit mTOR when taken as is_associated_with::dietary supplements.

Some mTOR inhibitors (e.g. is_associated_with::temsirolimus, is_associated_with::everolimus) are beginning to be used in the treatment of cancer. mTOR inhibitors may also be useful for treating several age-associated diseases including neurodegenerative diseases such as is_associated_with::Alzheimer's disease and is_associated_with::Parkinson's disease. is_associated_with::Ridaforolimus is another mTOR inhibitor, currently in clinical development.

Interactions
Mammalian target of rapamycin has been shown to interact with:


 * is_associated_with::ABL1,
 * is_associated_with::AKT1,
 * is_associated_with::CLIP1,
 * is_associated_with::EIF3F
 * is_associated_with::EIF4EBP1,
 * is_associated_with::FKBP1A,
 * is_associated_with::GPHN,
 * is_associated_with::KIAA1303,
 * is_associated_with::PRKCD,
 * is_associated_with::RHEB,
 * is_associated_with::RICTOR,
 * RPS6KB1,
 * is_associated_with::STAT1,
 * is_associated_with::STAT3, and
 * is_associated_with::UBQLN1.