There is very strong evidence from many laboratories that have consistently shown that hyperactive mTOR contributes to tau pathology.
In postmortem human AD brains, hyperactive mTOR signaling was found in neurons that were predicted to develop tau pathology. "Up-regulation of Phosphorylated/Activated p70 S6 Kinase and its Relationship to Neurofibrillary Pathology in Alzheimer's Disease"; JJ Pei, 2003, The American Journal of Pathology.(36)
"Phosphorylated/Activated p70 S6 Kinase" is the immediate downstream product of mTOR and "Neurofibrillary" is hyperphosphorylated Tau.
They looked at AD brains that span the pre-clinical, symptomatic and late stages corresponding to Braak's stages of AD pathology. They examined various areas of the brain, especially the hippocampus and temporal cortex areas of brain most involved in AD. The brains were studied with confocal microscopy and special stains. The findings were summarized:
"We found that activated p70 S6 kinase (activated mTOR) is co-distributed with the neurofibrillary pathology (Tau) in a predictable sequence from the entorhinal cortex, to the hippocampal CA1 and layers III and V of the temporal cortex. Activated (mTOR) was obviously increased in neurons before developing NFTs. Only levels of (activated mTOR) showed a dependent correlation with total tau and hyperphosphorylated tau."
They found that a significant amount (@ 60%) of normal tau remained in supernatant of AD brains. They hypothesized that to keep the cell functioning, neurons enduring hyperphosphorylated tau which fails to function would compensate by making more tau. The problem being as made more Tau that would be converted to hyperphosphorylated tau; so that attempt to compensate just made pathology worse.
In an animal model of tau pathology, "Hyperactive TOR in Drosophilia (fly) facilitates the development of tau pathology and the associated neurodegeneration." [Oddo, 2015, mTOR at Crossroads].(24)
"mTOR Regulates Tau Phosphorylation and Degradation: Implications for Alzheimer's disease and other Tauopathies", Caccamo...Oddo, 2013. (7)
Oddo, who was the lead researcher in 2013 study and also authored 2015 review article, "mTOR at crossroads" (23) states:
"Mice with hyperactive mTOR also have increased brain levels of total and phosphorylated tau. Conversely, reducing mTOR has beneficial effects on tau pathology. To this end, reducing mTOR with rapamycin in a transgenic mouse expressing mutant human tau decreased tau pathology and improved the associated motor deficits."
The study consisted of two parts. The first part used a genetically modified mouse heterozygous for TSC gene. TSC inhibits mTOR so missing one gene would reduce inhibition of mTOR and result in increased expression of mTOR. These mice were called TSC+/-.
First: Measure activity of mTOR in hippocampus of the TSC+/- mice. mTOR activity was elevated showing hyperactive mTOR.
Second: Measured Tau in hippocampus of mouse brain. Tau was elevated 1.5 fold.
Third: Measured the degree of phosphorylation of Tau. Tau was significantly hyperphosphorylated.
Fourth: Determined that GSK3B was elevated. This is kinase enzyme determined to be responsible for increased phosphorylation of Tau.
Fifth: mRNA was not elevated showing there was not increased production of Tau.
Sixth: Two major forms of protein degradation are protesome and autophagy. Protesome activity was normal. Autophagy related enzymes were decreased. This showed autophagy was decreased.
The results showed that hyperactive mTOR increased level of Tau by increased pathologic phosphorylation combined with decreased autophagy.
Second part of study used a pharmacologic approach to determine the relationship between Tau and mTOR. They used a transgenic mouse that harbors a mutant human gene which causes mice to develop tau pathology. The mice developed age-dependent accumulation of tau inclusions starting at 4-5 months. As the mice age the tau pathology becomes worse with neuronal loss at 12 months. Mice with the abnormal gene were called TG mice and controls called non-TG mice.
At two months of age TG mice were divided into group fed rapamycin and group fed regular chow. The rapamycin preparation was same low dose preparation Harrison used to extend lifespan in mice in 2009 study.
TG mice were treated with rapamycin chow or regular chow for 6 months and then tested at 8 months of age.
TG mice fed regular chow had significant motor impairment. TG mice fed rapamycin had no motor impairment and performed at same level as control mice.
TG mice showed elevated activity of mTOR in brain.
TG mice had robust tau accumulation and phosphorylation in brain. Rapamycin treated mice had same brain findings as normal controls.
TG mice showed elevated activity of GSK3B kinase, responsible for hyperphosphorylation.
TG mice had reduced autophagy and rapamycin treated mice showed induction of autophagy.
They concluded reduction of autophagy was partly responsible for elevated Tau.
They state in discussion: " Here we offer first evidence in mammals of a direct link between mTOR signaling and tau accumulation. Notably, not only did we show that genetically increasing mTOR signaling increases tau levels and phosphorylation; but we also showed that reducing mTOR signaling with rapamycin ameliorated tau pathology and rescues motor deficits in a mouse model of tauopathies.
Summary: "Accumulation of tau is a critical event in several neurodegenerative disorders, collectively known as tauopathies, which include Alzheimer's disease and frontotemporal dementia. Pathological tau is hyperphosphorylated and aggregates to form neurofibrillary tangles...Here, we used multiple animal models and complimentary genetic and pharmacologic approaches to show mTOR regulates tau phosphorylation and degradation. Specifically, we show that genetically increasing mTOR activity elevates endogenous mouse tau levels and phosphorylation. Complementary to it, we further demonstrate that pharmacologically reducing mTOR signaling with rapamyin ameliorates tau pathology and the associated behavioral deficits in a mouse model overexpressing mutant human tau.
"In summary, we show that increasing mTOR facilitates tau pathology, while reducing mTOR signaling ameliorates tau pathology. Given the overwhelming evidence showing that reducing mTOR signaling increases lifespan and health span, the data presented here may have profound clinical implications for aging and taupathies and provide the molecular basis for how aging may contribute to tau pathology. Additionally, these results provide pre-clinical data indicating that reducing mTOR signaling may be a VALID THERAPEUTIC APPROACH FOR TAUOPATHIES."
Oddo in 2015 paper, "at the crossroads" continued: " ((24) Similar to these observations, chronic treatment with a rapamycin ...(rapalog) in mutant tau mice, decreased mTOR signaling, stimulated autophagy, reduced tau levels and neurofibrillary tangle density, which led to attenuation of motor deficits."
Oddo continued: "The mechanism underlying these observations is likely multifactorial. For example hyperactive mTOR signaling decreased autophagy turnover, which is known degradation pathway for tau. mTOR can also regulate tau levels by increasing translation of it's mRNA. Indeed, direct evidence from primary hippocampal neurons showed that inhibition of mTOR suppress tau translation, while constitutively active mTOR signaling increased tau translation
An excellent analysis of the abnormal metabolism of Tau in AD is presented by JJ Pei, a leading expert on Tau in paper, "Mammalian target of rapamycin (mTOR) mediates tau protein dyshomeostasis: implications for Alzheimer disease", 2013. (34)
Five molecular events can be described in regard to abnormal Tau metabolism in AD. All five of these steps are mediated and controlled by mTOR.
1. Degradation: autophagy described above.
2. Translation (synthesis)
3. Phosphorylation, Kinases, adding
4. Phosphorylation, Phosphatase, removal
5. Aggregation (fibrillation)
Translation, Phosphorylation and Aggregation are controlled by both mTORC1 and mTORC2 and taken together constitute "Dyshomeostasis. By this is meant an imbalance of tau homeostasis, a condition required for neurons to maintain physiologic function. The result is both a loss of function and toxic function.
Translation: There is increase in tau synthesis. This is caused by up-regulated mTORC1 activity.
This is adding phosphate groups. Tau becomes hyperphosphorylated, adding too many phosphate groups. This is mediated by both mTORC1 and mTORC2 increased activity. A critical factor is location of where phosphate groups are added. mTORC2, acting through phosphatase GSK-3B adds phosphate groups to flanking region. This inhibits microtubule binding capacity and results in loss of function and converts tau into toxic molecule.
Phosphorylation: phosphatase. This is removing phosphate groups. Main phosphatase is protein phosphatase 2A (PP2A). This activity is impaired by mTORC1 and mTORC2. Activity is restored by rapamycin by lowering activity of mTOR.
[Activity is also restored by metformin. (see paper "Biguanide metformin acts on tau phosphorylation via mTOR/protein phosphatase 2A (PP2A) signaling"; Kickstein, 2010].(38)
The balance between Kinase action (adding phosphate groups) and phosphatase action (pruning phosphatase groups) is critical to maintain proper level of phosphorylation. Increased phosphorylation is caused by increased activity mTOR.
Aggregation (fibrillation). This is process by which soluble tau becomes insoluble fibrillates. The process is mediated through mTORC2 and includes apoptosis.
The pathogenesis of TAU abnormal translation, hyperphosphorylation and aggregation causes neurodegenerative tauopathies.
Oddo in 2015 paper concludes: (24)
"One startling implication of these observations is that long-term exposure to hyperactive mTOR might increase tau translation and decrease its degradation/turnover, while concomitantly increasing tau phosphorylation."
"Collectively, these studies highlight multiple pathways by which mTOR signaling contributes to tau pathology."