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mTOR is implicated in the failure of a 'pruning' mechanism of the excitatory synapses in autism spectrum disorders. [98]
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. [99] [100] [101] 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. [102] In vitro studies have shown Aβ to be an activator of the PI3K/AKT pathway, which in turn activates mTOR. [103] 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. [104] [105] 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. [106] [107] These in vitro studies suggest that increasing Aβ concentrations increases mTOR signaling; however, significantly large, cytotoxic Aβ concentrations are thought to decrease mTOR signaling. [108]
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. [107] [109] 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. [109] In addition, by injecting Aβ oligomers into the hippocampi of normal mice, mTOR hyperactivity is observed. [109] 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. [109] [110] [111] 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. [104] [112] [113] [114] It has also been proposed that mTOR contributes to tau pathology by increasing the translation of tau and other proteins. [115]
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. [107] [116] [117] [118] [119] 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. [103] [120] 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. [121]
Some evidence points to mTOR's role in reduced Aβ clearance as well. mTOR is a negative regulator of autophagy; [122] therefore, hyperactivity in mTOR signaling should reduce Aβ clearance in the AD brain. Disruptions in autophagy may be a potential source of pathogenesis in protein misfolding diseases, including AD. [123] [124] [125] [126] [127] [128] Studies using mouse models of Huntington's disease demonstrate that treatment with rapamycin facilitates the clearance of huntingtin aggregates. [129] [130] Perhaps the same treatment may be useful in clearing Aβ deposits as well.
Hyperactive mTOR pathways have been identified in certain lymphoproliferative diseases such as autoimmune lymphoproliferative syndrome (ALPS), [131] multicentric Castleman disease, [132] and post-transplant lymphoproliferative disorder (PTLD). [133]
mTORC1 activation is required for myofibrillar muscle protein synthesis and skeletal muscle hypertrophy in humans in response to both physical exercise and ingestion of certain amino acids or amino acid derivatives. [134] [135] Persistent inactivation of mTORC1 signaling in skeletal muscle facilitates the loss of muscle mass and strength during muscle wasting in old age, cancer cachexia, and muscle atrophy from physical inactivity. [134] [135] [136] mTORC2 activation appears to mediate neurite outgrowth in differentiated mouse neuro2a cells. [137] Intermittent mTOR activation in prefrontal neurons by β-hydroxy β-methylbutyrate inhibits age-related cognitive decline associated with dendritic pruning in animals, which is a phenomenon also observed in humans. [138]
Active mTORC1 is positioned on lysosomes. mTOR is inhibited [140] when lysosomal membrane is damaged by various exogenous or endogenous agents, such as invading bacteria, membrane-permeant chemicals yielding osmotically active products (this type of injury can be modeled using membrane-permeant dipeptide precursors that polymerize in lysosomes), amyloid protein aggregates (see above section on Alzheimer's disease) and cytoplasmic organic or inorganic inclusions including urate crystals and crystalline silica. [140] The process of mTOR inactivation following lysosomal/endomembrane is mediated by the protein complex termed GALTOR. [140] At the heart of GALTOR [140] is galectin-8, a member of β-galactoside binding superfamily of cytosolic lectins termed galectins, which recognizes lysosomal membrane damage by binding to the exposed glycans on the lumenal side of the delimiting endomembrane. Following membrane damage, galectin-8, which normally associates with mTOR under homeostatic conditions, no longer interacts with mTOR but now instead binds to SLC38A9, RRAGA/RRAGB, and LAMTOR1, inhibiting Ragulator's (LAMTOR1-5 complex) guanine nucleotide exchange function- [140]
TOR is a negative regulator of autophagy in general, best studied during response to starvation, [141] [142] [143] [144] [145] which is a metabolic response. During lysosomal damage however, mTOR inhibition activates autophagy response in its quality control function, leading to the process termed lysophagy [146] that removes damaged lysosomes. At this stage another galectin, galectin-3, interacts with TRIM16 to guide selective autophagy of damaged lysosomes. [147] [148] TRIM16 gathers ULK1 and principal components (Beclin 1 and ATG16L1) of other complexes (Beclin 1-VPS34-ATG14 and ATG16L1-ATG5-ATG12) initiating autophagy, [148] many of them being under negative control of mTOR directly such as the ULK1-ATG13 complex, [143] [144] [145] or indirectly, such as components of the class III PI3K (Beclin 1, ATG14 and VPS34) since they depend on activating phosphorylations by ULK1 when it is not inhibited by mTOR. These autophagy-driving components physically and functionally link up with each other integrating all processes necessary for autophagosomal formation: (i) the ULK1-ATG13-FIP200/RB1CC1 complex associates with the LC3B/GABARAP conjugation machinery through direct interactions between FIP200/RB1CC1 and ATG16L1, [149] [150] [151] (ii) ULK1-ATG13-FIP200/RB1CC1 complex associates with the Beclin 1-VPS34-ATG14 via direct interactions between ATG13's HORMA domain and ATG14, [152] (iii) ATG16L1 interacts with WIPI2, which binds to PI3P, the enzymatic product of the class III PI3K Beclin 1-VPS34-ATG14. [153] Thus, mTOR inactivation, initiated through GALTOR [140] upon lysosomal damage, plus a simultaneous activation via galectin-9 (which also recognizes lysosomal membrane breach) of AMPK [140] that directly phosphorylates and activates key components (ULK1, [154] Beclin 1 [155] ) of the autophagy systems listed above and further inactivates mTORC1, [156] [157] allows for strong autophagy induction and autophagic removal of damaged lysosomes.
Additionally, several types of ubiquitination events parallel and complement the galectin-driven processes: Ubiquitination of TRIM16-ULK1-Beclin-1 stabilizes these complexes to promote autophagy activation as described above. [148] ATG16L1 has an intrinsic binding affinity for ubiquitin [151] ); whereas ubiquitination by a glycoprotein-specific FBXO27-endowed ubiquitin ligase of several damage-exposed glycosylated lysosomal membrane proteins such as LAMP1, LAMP2, GNS/N-acetylglucosamine-6-sulfatase, TSPAN6/tetraspanin-6, PSAP/prosaposin, and TMEM192/transmembrane protein 192 [158] may contribute to the execution of lysophagy via autophagic receptors such as p62/SQSTM1, which is recruited during lysophagy, [151] or other to be determined functions.
Scleroderma, also known as systemic sclerosis, is a chronic systemic autoimmune disease characterised by hardening (sclero) of the skin (derma) that affects internal organs in its more severe forms. [159] [160] mTOR plays a role in fibrotic diseases and autoimmunity, and blockade of the mTORC pathway is under investigation as a treatment for scleroderma. [9]
A rare gain-of-function mutation causes Smith-Kingsmore syndrome. [161]
mTOR inhibitors, e.g. rapamycin, are already used to prevent transplant rejection.
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 disease that involve glycogen accumulation in muscle.
There are two primary mTOR inhibitors used in the treatment of human cancers, temsirolimus and everolimus. mTOR inhibitors have found use in the treatment of a variety of malignancies, including renal cell carcinoma (temsirolimus) and pancreatic cancer, breast cancer, and renal cell carcinoma (everolimus). [162] The complete mechanism of these agents is not clear, but they are thought to function by impairing tumour angiogenesis and causing impairment of the G1/S transition. [163]
mTOR inhibitors may be useful for treating/preventing several age-associated conditions, [164] including neurodegenerative diseases such as Alzheimer's disease and Parkinson's disease. [165] After a short-term treatment with the mTOR inhibitors dactolisib and everolimus, in elderly (65 and older), treated subjects had a reduced number of infections over the course of a year. [166]
Various natural compounds, including epigallocatechin gallate (EGCG), caffeine, curcumin, berberine, quercetin, resveratrol and pterostilbene, have been reported to inhibit mTOR when applied to isolated cells in culture. [167] [168] [169] As yet no high quality evidence exists that these substances inhibit mTOR signaling or extend lifespan when taken as dietary supplements by humans, despite encouraging results in animals such as fruit flies and mice. Various trials are ongoing. [170] [171]
Mechanistic target of rapamycin has been shown to interact with: [172]
Sirolimus, also known as rapamycin and sold under the brand name Rapamune among others, is a macrolide compound that is used to coat coronary stents, prevent organ transplant rejection, treat a rare lung disease called lymphangioleiomyomatosis, and treat perivascular epithelioid cell tumour (PEComa). It has immunosuppressant functions in humans and is especially useful in preventing the rejection of kidney transplants. It is a mammalian target of rapamycin (mTOR) kinase inhibitor that reduces the sensitivity of T cells and B cells to interleukin-2 (IL-2), inhibiting their activity.
Protein kinase B (PKB), also known as Akt, is the collective name of a set of three serine/threonine-specific protein kinases that play key roles in multiple cellular processes such as glucose metabolism, apoptosis, cell proliferation, transcription, and cell migration.
Tuberous sclerosis complex 2 (TSC2), also known as tuberin, is a protein that in humans is encoded by the TSC2 gene.
RHEB also known as Ras homolog enriched in brain (RHEB) is a GTP-binding protein that is ubiquitously expressed in humans and other mammals. The protein is largely involved in the mTOR pathway and the regulation of the cell cycle.
Ribosomal protein S6 kinase beta-1 (S6K1), also known as p70S6 kinase, is an enzyme that in humans is encoded by the RPS6KB1 gene. It is a serine/threonine kinase that acts downstream of PIP3 and phosphoinositide-dependent kinase-1 in the PI3 kinase pathway. As the name suggests, its target substrate is the S6 ribosomal protein. Phosphorylation of S6 induces protein synthesis at the ribosome.
Regulatory-associated protein of mTOR also known as raptor or KIAA1303 is an adapter protein that is encoded in humans by the RPTOR gene. Two mRNAs from the gene have been identified that encode proteins of 1335 and 1177 amino acids long.
Rapamycin-insensitive companion of mammalian target of rapamycin (RICTOR) is a protein that in humans is encoded by the RICTOR gene.
Serine/threonine-protein kinase ULK1 is an enzyme that in humans is encoded by the ULK1 gene.
AuTophaGy related 1 (Atg1) is a 101.7kDa serine/threonine kinase in S.cerevisiae, encoded by the gene ATG1. It is essential for the initial building of the autophagosome and Cvt vesicles. In a non-kinase role it is - through complex formation with Atg13 and Atg17 - directly controlled by the TOR kinase, a sensor for nutrient availability.
Autophagy-related protein 13 also known as ATG13 is a protein that in humans is encoded by the KIAA0652 gene.
Target of rapamycin complex subunit LST8, also known as mammalian lethal with SEC13 protein 8 (mLST8) or TORC subunit LST8 or G protein beta subunit-like, is a protein that in humans is encoded by the MLST8 gene. It is a subunit of both mTORC1 and mTORC2, complexes that regulate cell growth and survival in response to nutrient, energy, redox, and hormonal signals. It is upregulated in several human colon and prostate cancer cell lines and tissues. Knockdown of mLST8 prevented mTORC formation and inhibited tumor growth and invasiveness.
mTOR inhibitors are a class of drugs used to treat several human diseases, including cancer, autoimmune diseases, and neurodegeneration. They function by inhibiting the mammalian target of rapamycin (mTOR), which is a serine/threonine-specific protein kinase that belongs to the family of phosphatidylinositol-3 kinase (PI3K) related kinases (PIKKs). mTOR regulates cellular metabolism, growth, and proliferation by forming and signaling through two protein complexes, mTORC1 and mTORC2. The most established mTOR inhibitors are so-called rapalogs, which have shown tumor responses in clinical trials against various tumor types.
mTORC1, also known as mammalian target of rapamycin complex 1 or mechanistic target of rapamycin complex 1, is a protein complex that functions as a nutrient/energy/redox sensor and controls protein synthesis.
mTOR Complex 2 (mTORC2) is an acutely rapamycin-insensitive protein complex formed by serine/threonine kinase mTOR that regulates cell proliferation and survival, cell migration and cytoskeletal remodeling. The complex itself is rather large, consisting of seven protein subunits. The catalytic mTOR subunit, DEP domain containing mTOR-interacting protein (DEPTOR), mammalian lethal with sec-13 protein 8, and TTI1/TEL2 complex are shared by both mTORC2 and mTORC1. Rapamycin-insensitive companion of mTOR (RICTOR), mammalian stress-activated protein kinase interacting protein 1 (mSIN1), and protein observed with rictor 1 and 2 (Protor1/2) can only be found in mTORC2. Rictor has been shown to be the scaffold protein for substrate binding to mTORC2.
David M. Sabatini is an American scientist and a former professor of biology at the Massachusetts Institute of Technology. From 2002 to 2021, he was a member of the Whitehead Institute for Biomedical Research. He was also an investigator of the Howard Hughes Medical Institute from 2008 to 2021 and was elected to the National Academy of Sciences in 2016. He is known for his contributions in the areas of cell signaling and cancer metabolism, most notably the co-discovery of mTOR.
Michael Nip Hall is an American-Swiss molecular biologist and professor at the Biozentrum of the University of Basel, Switzerland. He discovered TOR, a protein central for regulating cell growth.
The Ragulator-Rag complex is a regulator of lysosomal signalling and trafficking in eukaryotic cells, which plays an important role in regulating cell metabolism and growth in response to nutrient availability in the cell. The Ragulator-Rag Complex is composed of five LAMTOR subunits, which work to regulate MAPK and mTOR complex 1. The LAMTOR subunits form a complex with Rag GTPase and v-ATPase, which sits on the cell’s lysosomes and detects the availability of amino acids. If the Ragulator complex receives signals for low amino acid count, it will start the process of catabolizing the cell. If there is an abundance of amino acids available to the cell, the Ragulator complex will signal that the cell can continue to grow. Ragulator proteins come in two different forms: Rag A/Rag B and Rag C/Rag D. These interact to form heterodimers with one another.
Immunometabolism is a branch of biology that studies the interplay between metabolism and immunology in all organisms. In particular, immunometabolism is the study of the molecular and biochemical underpinninngs for i) the metabolic regulation of immune function, and ii) the regulation of metabolism by molecules and cells of the immune system. Further categorization includes i) systemic immunometabolism and ii) cellular immunometabolism. Immunometabolism includes metabolic inflammation:a chronic, systemic, low grade inflammation, orchestrated by metabolic deregulation caused by obesity or aging.
HY-124798 (Rheb inhibitor NR1) is the first compound to be developed that acts as a potent and selective inhibitor of Rheb, a GTP-binding protein which acts as an endogenous activator of the mechanistic target of rapamycin (mTOR) subtype mTORC1. Since many of the side effects of rapamycin and its analogues are thought to result from binding to the other subtype mTORC2, it is hoped that selective inhibition of mTORC1 should have a more selective effects profile. As mTORC1 and mTORC2 have binding sites that are very similar in structure, it has been challenging to develop highly subtype selective inhibitors, making indirect inhibition via modulation of other messenger proteins such as Rheb an attractive approach. However, since HY-124798 has a relatively weak IC50 of 2.1μM, and Rheb also has other targets in addition to mTORC1, it remains to be established whether it will deliver the hoped for improvements in pharmacological profile.
Torin_1 is a drug which was one of the first non-rapalog derived inhibitors of the mechanistic target of rapamycin (mTOR) subtypes mTORC1 and mTORC2. In animal studies it has anti-inflammatory, anti-cancer, and anti-aging properties, and shows activity against neuropathic pain.
The mTOR signaling pathway acts as a molecular systems integrator to support organismal and cellular interactions with the environment. The mTOR pathway regulates homeostasis by directly influencing protein synthesis, transcription, autophagy, metabolism, and organelle biogenesis and maintenance. It is not surprising then that mTOR signaling is implicated in the entire hierarchy of brain function including the proliferation of neural stem cells, the assembly and maintenance of circuits, experience-dependent plasticity and regulation of complex behaviors like feeding, sleep and circadian rhythms. ...
mTOR function is mediated through two large biochemical complexes defined by their respective protein composition and have been extensively reviewed elsewhere(Dibble and Manning, 2013; Laplante and Sabatini, 2012)(Figure 1B). In brief, common to both mTOR complex 1 (mTORC1) and mTOR complex 2 (mTORC2) are: mTOR itself, mammalian lethal with sec13 protein 8 (mLST8; also known as GβL), and the inhibitory DEP domain containing mTOR-interacting protein (DEPTOR). Specific to mTORC1 is the regulator-associated protein of the mammalian target of rapamycin (Raptor) and proline-rich Akt substrate of 40 kDa (PRAS40)(Kim et al., 2002; Laplante and Sabatini, 2012). Raptor is essential to mTORC1 activity. The mTORC2 complex includes the rapamycin insensitive companion of mTOR (Rictor), mammalian stress activated MAP kinase-interacting protein 1 (mSIN1), and proteins observed with rictor 1 and 2 (PROTOR 1 and 2)(Jacinto et al., 2006; Jacinto et al., 2004; Pearce et al., 2007; Sarbassov et al., 2004)(Figure 1B). Rictor and mSIN1 are both critical to mTORC2 function.