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Keywords:

  • aging;
  • Alzheimer’s disease;
  • memory;
  • rapamycin;
  • TOR

Abstract

  1. Top of page
  2. Abstract
  3. mTOR and rapamycin
  4. mTOR and rapamycin: anti-aging effects
  5. Rapamycin and mTOR: effects on memory and implications for Alzheimer’s disease
  6. Conclusion
  7. Acknowledgements
  8. References

J. Neurochem. (2011) 117, 927–936.

Abstract

Rapamycin is a macrolide immunosuppressant drug, originally used as an anti-fungal agent, which is widely used in transplantation medicine to prevent organ rejection. Target of rapamycin (TOR) is an evolutionarily conserved serine/threonine kinase with pleiotropic cellular functions, regulating processes such as growth and metabolism, cell survival, transcription and autophagy. TOR intervenes in two distinct enzymatic complexes with different functions, a rapamycin-sensitive complex TORC1 and a rapamycin-insensitive complex TORC2. Rapamycin has an inhibitory effect on TORC1 activity and it has been suggested to increase life span, an effect correlated with decreased protein biosynthesis and autophagy activation. In the CNS, rapamycin shows beneficial effects in neuronal survival and plasticity, thus contributing to memory improvement. In this review, evidence implying rapamycin and TOR in aging/life span extension and memory improvement will be discussed. Recent findings about the effects of rapamycin on Alzheimer’s disease-associated neuropathology will be also discussed.

Abbreviations used
4E-BP1

4E-binding protein 1

AD

Alzheimer’s disease

amyloid β

AβPP

amyloid β precursor protein

CR

calorie restriction

FKBP12

12 kDa FK506-binding protein

mTORC

mammalian TOR complex

PI3K

phosphatidylinositol 3-kinase

ROS

reactive oxygen species

S6K

S6 protein kinase

SIRT1

sirtuin 1

TOR

target of rapamycin

TSC

tuberous sclerosis

Rapamycin is naturally produced by a strain of the bacterium Streptomyces hygroscopicus. This discovery was made in 1970 from a soil sample collected in Rapa Nui, commonly known as Easter Island, from where the name rapamycin derives (Sehgal et al. 1975; Vezina et al. 1975). This compound was initially found to have potent anti-fungal properties, but soon a wicked immunosuppressant effect made it be neglected. Nevertheless, because of its immunosuppressant property, rapamycin was approved by the FDA for use in transplantation medicine, particularly in kidney transplantation (Tsang et al. 2007). Because of rapamycin’s anti-proliferative capacity, namely on vascular smooth muscle cells, FDA approved its use in coronary-artery eluting stents (Tsang et al. 2007). Chemically, rapamycin is a lipophilic macrocyclic lactone, virtually insoluble in water, with a melting range of 183–185°C (Sehgal 2003).

The target of rapamycin (TOR) is a 289 kDa protein, which has a C-terminal containing 600 amino acid residues with homology to the p110 subunit of mammalian phosphatidylinositol 3-kinase (PI3K), yeast PI3K VPS34 (Kunz et al. 1993) and the mammalian PI4K (Sabatini et al. 1995). Functionally, TOR is a serine/threonine kinase member of the PI3K-related kinase family (Sarbassov et al. 2005). TOR was first identified in yeast and further recognized as an evolutionarily conserved serine/threonine kinase in eukaryotes, including mammalians (mTOR). mTOR forms two different enzymatic complexes, the mammalian TOR complexes 1 and 2 (mTORC1 and mTORC2, respectively), with distinct functions (Martin and Hall 2005). mTORC1 is a key player in intracellular nutrient sensing controlling protein synthesis, cell growth and proliferation, metabolism and autophagy (Rui 2007; Dunlop and Tee 2009). The best-characterized substrates of mTORC1 are the ribosomal p70 S6 protein kinase (S6K) and the eukaryotic initiation factor 4E-binding protein 1 (4E-BP1) via which mTORC1 controls translation (Sonenberg and Hinnebusch 2009). On the other hand, mTORC2 is the Ser473 kinase for Akt contributing to its activation. mTORC2 also controls the activity of serum- and glucocorticoid-induced kinase and is involved in the organization of actin (Jacinto et al. 2004; Tsang et al. 2007; Kapahi et al. 2010). Both mTORC1 and mTORC2 are activated by growth factors (e.g. insulin and insulin growth factor-1). Growth factors activate mTORC1 via PI3K, phosphoinositide-dependent kinase-1, Akt, the tuberous sclerosis (TSC) 1 – TSC2 complex, and Rheb, a small GTP-binding protein that binds and activates mTORC1 directly (Manning and Cantley 2003; Avruch et al. 2009). In contrast, the way how which growth factors activate mTORC2 has been elusive. Growth factors activate mTORC2 via PI3K (Frias et al. 2006; Yang et al. 2006; García-Martínez and Alessi 2008), but signaling steps beyond PI3K are distinct from those upstream of mTORC1 and unknown (Cybulski and Hall 2009; Sparks and Guertin 2010). Recently, Zinzalla et al. (2011) reported that mTORC2-ribosome interaction is a likely conserved mechanism of mTORC2 activation that is physiologically relevant in both normal and cancer cells. As ribosome content determines growth capacity of a cell, this mechanism of mTORC2 regulation ensures that mTORC2 is active only in growing cells. Noticeably, it is generally considered that mTORC1 activity is rapamycin-sensitive whereas mTORC2 activity is rapamycin-insensitive, although long-term rapamycin treatment can indirectly inhibit mTORC2 in some cell types (Jacinto et al. 2004; Sarbassov et al. 2004, 2006; Cybulski and Hall 2009; Sparks and Guertin 2010).

During the last decade, the mTORC1 pathway has received growing attention since it was found to regulate lifespan in eukaryotes with different levels of complexity. Indeed, mTORC1-dependent regulation of lifespan has been demonstrated in yeast (Kaeberlein et al. 2005; Powers et al. 2006; Bonawitz et al. 2007), worms (Vellai et al. 2003), flies (Kapahi et al. 2004) and rodents (Harrison et al. 2009). The stimulation of autophagy-mediated clearance of damaged cellular constituents assumes most likely a central stage in lifespan extension through rapamycin-induced inhibition of mTORC1 activity (Madeo et al. 2010). It is also known that mTORC1 signaling is involved in the regulation of synaptic plasticity and, consequently, memory formation (Hoeffer and Klann 2010). The fact that mTORC1 is a key regulator of lifespan and integrates signals that guide memory formation in model organisms suggest that its pharmacological modulation by rapamycin has potential practical application to delay the aging process and age-related neurodegenerative diseases, such as Alzheimer’s disease (AD).

mTOR and rapamycin

  1. Top of page
  2. Abstract
  3. mTOR and rapamycin
  4. mTOR and rapamycin: anti-aging effects
  5. Rapamycin and mTOR: effects on memory and implications for Alzheimer’s disease
  6. Conclusion
  7. Acknowledgements
  8. References

Mammalian TOR complex 1 is composed of the serine/threonine kinase mTOR, mammalian lethal with sec-13 protein 8/G-protein β-subunit like protein, regulatory associated protein of TOR (Raptor), proline-rich protein kinase B (PKB/Akt) substrate 40 kDa and DEP domain containing mTOR-interacting protein (Deptor) (Fig. 1), whereas mTORC2 consists of mTOR, mammalian lethal with sec-13 protein 8/G-protein β-subunit like protein, protein observed with Rictor/proline rich protein 5-like (Proctor/PRR5L), rapamycin-insensitive companion of mTOR/mammalian Avo3 (Rictor/mAvo3) and stress-activated protein kinase interacting protein 1/mammalian Avo1 (Sin1/mAvo1) (Guertin and Sabatini 2005, 2009; Wullschleger et al. 2006). The intracellular rapamycin receptor in all eukaryotes is the 12 kDa protein FK506-binding protein (FKBP12) (Fig. 1) (Raught et al. 2001). Rapamycin has a FKBP12-binding moiety and a mTOR-binding moiety. When connected to FKBP12, rapamycin can be anchored by a conserved mTOR domain termed FKBP12-rapamycin binding, enabling the formation of a ternary complex, shutting down downstream signals (Chen et al. 1995; Raught et al. 2001; Banaszynski et al. 2005). FKBP12-rapamycin binding lies the N-terminal mTOR kinase domain and spans residues 2025–2114, being further identified a critical Ser2035 residue essential for FKBP12-rapamycin binding (Chen et al. 1995). The mechanisms by which rapamycin inhibit mTORC1 activity remain somewhat elusive; nevertheless in vitro and in vivo studies demonstrated that rapamycin-FKBP12 promotes the dissociation between Raptor and mTOR, decreasing the phosphorylation of raptor-dependent mTOR substrates (Oshiro et al. 2004). Recently, a model for rapamycin-mediated inhibition of mTORC1 has been proposed: the FKBP12-rapamycin binding to mTOR slightly alters its conformation weakening the mTOR-raptor interaction, although not promoting the dissociation of the dimmer, which consequently obstructs larger-sized substrates to access the active site. This model of rapamycin-mediated inhibition of mTORC1 hypothesizes however that, eventually, the enzymatic complex dissociates (Yip et al. 2010). Concomitantly, rapamycin has also been proved to decrease mTORC1 intrinsic kinase activity as demonstrated by decreased autophosphorylation of Ser2481, reducing the catalytic activity of the enzymatic complex (Soliman et al. 2010). Interestingly, mTORC-specific phosphorylation sites are predominantly different when mTOR associates with mTORC1 or mTORC2. mTORC1 contains mTOR phosphorylated primarily on Ser2448 whereas mTORC2 contains mTOR phosphorylated primarily on Ser2481. Despite mTORC1 predominantly contains P-Ser2448 on mTOR, this phosphorylation site does not always correlate with its catalytic activity (Das et al. 2008; Caccamo et al. 2010). As aforementioned, it is generally considered that only mTORC1 activity is rapamycin-sensitive. In fact, mTORC1 and mTORC2 rather differ in their sensitivity to the drug, mTORC2 assembly being blocked only by exposure to chronic high-dose rapamycin (Sarbassov et al. 2006; Guertin and Sabatini 2009).

image

Figure 1.  Mammalian target of rapamycin complex 1 (mTORC1) stimuli and functions. mTORC1 is an enzymatic complex composed of several proteins in which the main catalytic core is the serine/threonine kinase mammalian target of rapamycin (mTOR). Mammalian lethal with sec-13 protein 8/G-protein β-subunit like protein (mLST8/GβL), regulatory associated protein of TOR (Raptor), proline-rich AKT substrate 40 kDa (PRAS 40) and DEP domain containing mTOR-interacting protein (Deptor) are also constituents of mTORC1, mainly assuming regulatory functions. Evolutionarily, mTORC1 is a sensor of nutrient/energy abundance/scarcity and also mediates other input signals such as growth factors stimuli. Caloric restriction effects on aging are partly mediated by mTORC1 inhibition. Rapamycin is a pharmacological inhibitor of mTORC1 through its binding to the cellular receptor 12 kDa protein FK506-binding protein (FKBP12). mTORC1 activity stimulates protein synthesis through the inhibition of eukaryotic initiation factor 4E-binding protein 1 (4E-BP1) and stimulation of p70 S6 protein kinase (S6K). mTORC1-mediated phosphorylation of ATG13 inhibits the assembly of the complex ATG13/ULK1/FIP200 blunting autophagy. mTORC1 is also capable of decreasing mitochondrial metabolism, mediating a shifting in metabolic energy dependency.

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Physiologically, mTOR integrates a number of extra- and intracellular signals, controlling cell growth, proliferation and metabolism through the regulation of the biosynthetic potential (e.g. ribosome biogenesis), which depends on the nutrient and energy availability (Fig. 1) (Foster and Fingar 2010). On the opposite, when the levels of available nutrients and growth factors are low mTOR signaling is blunted to ensure cell survival under suboptimal conditions, shifting from a predominantly anabolic towards a catabolic metabolism through the activation of macroautophagy (usually referred as autophagy) (Foster and Fingar 2010). The S6K is an established substrate of mTOR known to modulate protein synthesis by a number of mechanisms and to have opposing effects in lifespan. Another well-known substrate of mTOR is the 4E-BP1 that is directly involved in the regulation of mRNA translation (Fig. 1) (Wang and Proud 2010). Furthermore, the catabolic effects of mTOR signaling blunting, as a consequence of autophagy activation, are because of the decay in the phosphorylation of ATG13, which enables its association with ATG1 (ULK1 in mammals) and FIP200 thus conducing to the formation of the autophagosomal membrane (Fig. 1) (Ganley et al. 2009; Hosokawa et al. 2009; Jung et al. 2009; Santos et al. 2010a). The decline of autophagy is also known to be involved in the aging process, especially during the post-reproductive period (Vellai 2009). Since mTOR regulates autophagy as well as other pathways involved in lifespan extension it is plausible that mTOR activity modulation, particularly with rapamycin, holds promise for the improvement of age-related disorders.

mTOR and rapamycin: anti-aging effects

  1. Top of page
  2. Abstract
  3. mTOR and rapamycin
  4. mTOR and rapamycin: anti-aging effects
  5. Rapamycin and mTOR: effects on memory and implications for Alzheimer’s disease
  6. Conclusion
  7. Acknowledgements
  8. References

Aging is a multifactorial process that can be generally defined as the progressive accumulation of deleterious changes occurring in cells, tissues and organisms during life, increasing the odds of disease and death (Harman 2003). Virtually all physiological functions lose efficiency making cells, tissues and organisms more vulnerable to injuries (Vina et al. 2007). The multifactorial aspect of aging implicates the alteration of several physiological processes such as the immunological (Franceschi et al. 2000) and inflammatory (Chung et al. 2001) responses, oxidative status (Harman 2003) and mitochondrial function (Cadenas and Davies 2000). Therefore, understanding the aging process in all its wide complexity has been the major challenge in the biomedicine field, especially because aging constitutes the major risk factor in the development of a number of pathological situations such as sporadic AD. In this regard, it is known that the most robust aging-delaying intervention is calorie restriction (CR). CR consists of experimentally reducing food availability intake by 30–40%, without compromising the supply of essential nutrients, potentially extending lifespan by 15–40%, and also delaying the onset of age-related diseases (Masoro 2003). Indeed, CR has been shown to induce lifespan extension in yeast (Lin et al. 2002; Kaeberlein et al. 2005), worms (Lee et al. 2006; Hansen et al. 2008; Steinkraus et al. 2008; Morselli et al. 2010), flies (Mair et al. 2003; Rogina and Helfand 2004; Bauer et al. 2009), rodents (Weindruch et al. 1986), non-human primates (Colman et al. 2009) and even humans (Roth et al. 1999). Interestingly, TOR is a key molecular interplayer in nutrient sensing, therefore being not surprising its enrollment in lifespan extension. In fact, Powers et al. (2006) demonstrated that TOR signaling down-regulation extends chronological lifespan in yeast.

Also the activation of sirtuins has been implicated in lifespan extension of S. cerevisiae (Howitz et al. 2003) and D. melanogaster (Rogina and Helfand 2004). Accordingly, TOR has been implicated in the interposition of CR-induced silent information regulator 2 (Sir2)-mediated lifespan extension in S. cerevisiae (Medvedik et al. 2007). CR is known to inhibit TOR activity and it has been proposed that TOR inhibition induces relocalization of two transcription factors, Msn2p and Msn4p, from the cytoplasm to the nucleus, where they increase the expression of the nicotinamidase gene PNC1, a regulator of Sir2 activity, promoting its activity and therefore stabilization of yeast rDNA (Medvedik et al. 2007). It was also observed in mammalian cell lines that Sirtuin 1 (SIRT1) (the ortholog of yeast Sir2) is capable of negatively regulate mTOR signaling (Ghosh et al. 2010). These studies suggest that the interplay between mTOR and SIRT1 undergoes a positive feedback loop that amplifies SIRT1 activity.

It is described that Sir2 inhibits rDNA transcription and ribosomal biogenesis (Kim et al. 1999; Imai et al. 2000; Li et al. 2006). The inhibition of S6K, a downstream target of TOR, has been shown to extend life span in D. melanogaster (Kapahi et al. 2004) and C. elegans (Hansen et al. 2007; Pan et al. 2007). In S. cerevisiae, mutations in Sch9 (the ortholog of S6K) (Urban et al. 2007) significantly extend life span (Fabrizio et al. 2001; Kaeberlein et al. 2005). Recently, female but not male mice lacking S6K1 (the mouse genome encodes two paralogs, S6K1 and S6K2) have been shown to display increased life span and slowed progression of age-related pathologies (Selman et al. 2009). Interestingly, it has been suggested that the target of rapamycin, but not Sir2, is associated to the longevity response induced by CR in C. elegans, indicating that neither TOR inhibition nor CR extends lifespan simply by reducing protein synthesis (Hansen et al. 2007). A genetic screening performed in C. elegans implied Rictor (a component of TORC2) in nutrient sensing, activating Akt and serum- and glucocorticoid-induced kinase to modulate feeding behavior, fat metabolism, growth, reproduction and life span (Soukas et al. 2009).

Reduced TOR signaling promotes autophagy, a degradative process that enhances cell survival in face of decreased nutrient availability via the breakdown of cell constituents into amino acids and other small molecules (Chang et al. 2009). Interestingly, in C. elegans and human cells an additional mechanism to SIRT1-mediated increase in longevity was found to be related to SIRT1-dependent induction of autophagy in a TOR-independent pathway (Morselli et al. 2010). It was also shown that autophagy is required for the lifespan-prolonging effects of CR and pharmacological SIRT1 activators (Morselli et al. 2010). Thus, autophagy represents the common link between SIRT1 and TOR molecular pathways and lifespan extension.

Schieke et al. (2006) showed compelling evidence that mTOR/Raptor complex formation assumes a critical role in the overall mitochondrial activity independently of other known mTOR targets such as S6K, and a physical interaction was suggested between mTOR/Raptor complex and mitochondria. Indeed, it was recently demonstrated that mTOR forms a complex with the outer-membrane mitochondrial proteins Bcl-xl and voltage-dependent anion channel 1 (Ramanathan and Schreiber 2009). As shown in mammalian cells inhibition of mTOR activity lowers mitochondrial membrane potential, oxygen consumption and ATP synthase activity, which is correlated with a metabolic shift from aerobic respiration towards enhanced aerobic glycolysis (Fig. 1) (Schieke et al. 2006; Ramanathan and Schreiber 2009). However, contradicting results obtained in S. cerevisiae show that a reduction in TOR signaling extends lifespan through increased mitochondrial respiration (Bonawitz et al. 2007), this apparent contradiction being possibly explained by the different primary sources of energy in yeast and mammals. In both situations, there is a lower propensity to reactive oxygen species (ROS) formation since in the mammalian cells there is a decreased dependency on mitochondrial energy and a shift to aerobic glycolysis, naturally reducing the production of mitochondrial ROS. In S. cerevisiae, because of an increased mitochondrial respiration, there is a reduction in intramitochondrial molecular oxygen, a situation that is unfavorable to ROS production (Vinogradov and Grivennikova 2005). Lower levels of ROS and/or increased antioxidant defenses are known to be ideal conditions to delay the aging process and increase lifespan (Droge and Schipper 2007).

There is plenty of data showing that mTOR is an important cellular participant in a number of pathways, namely in the biosynthetic capacity through the regulation of translation and ribosome biogenesis, elimination of damaged cellular constituents through autophagy to provide new building-blocks to cellular constituents, and regulation of metabolism (Fig. 1). Hence, mTOR proves to be central in the control of several pathways that somehow modulate the aging process. Therefore, mTOR constitutes a good candidate to pharmacological intervention in order to delay the aging process and the onset of age-related disorders such as cardiovascular disease, cancer and AD. Indeed, rapamycin has proved to delay cellular senescence (Demidenko et al. 2009), reduce restenosis of the coronary artery when applied in coated stents (Serruys et al. 2006), prevent age-related weight gain and suppress carcinogenesis in transgenic cancer-prone mice (Anisimov et al. 2010). Furthermore, rapamycin has been demonstrated to extend lifespan in yeast, C. elegans and D. melanogaster under conditions where autophagy is induced (Tavernarakis et al. 2008; Alvers et al. 2009; Bjedov et al. 2010). Curiously, in C. elegans and human cells rapamycin induces autophagy in a SIRT1-independent manner (Morselli et al. 2010). Compelling evidence also demonstrated rapamycin-induced increase in lifespan in mammalians, being demonstrated that both male and female mice undergo an extension in their lifespan (Harrison et al. 2009). Nevertheless, the main mechanism by which rapamycin induces increase in lifespan in mammals remain to be elucidated.

The current evidences about mTOR, rapamycin and their effects on aging, suggest that the inhibition of translation, increase in autophagy and shift of cellular metabolism are mechanisms that adjust cells to physiological states that favor maintenance and repair.

Rapamycin and mTOR: effects on memory and implications for Alzheimer’s disease

  1. Top of page
  2. Abstract
  3. mTOR and rapamycin
  4. mTOR and rapamycin: anti-aging effects
  5. Rapamycin and mTOR: effects on memory and implications for Alzheimer’s disease
  6. Conclusion
  7. Acknowledgements
  8. References

The sporadic form of AD is the most common form of dementia among elderly, being characterized by progressive neuronal impairment and cognitive decline. Clinically, AD is characterized by a decline in several domains such as memory, speech, personality and judgment, vision, association sensory-motor function, and culminates in the death of the individual typically within 3–9 years after diagnosis (Castellani et al. 2010; Santos et al. 2010b). The pathological hallmarks of AD are the formation of extracellular senile plaques, mainly composed of amyloid β (Aβ) peptide, and intracellular neurofibrillary tangles containing hyperphosphorylated tau protein (Moreira et al. 2006, 2007a; Querfurth and LaFerla 2010). Aβ peptides are generated by successive proteolysis of amyloid β precursor protein (AβPP), a large transmembrane glycoprotein that is initially cleaved by the β-site AβPP-cleaving enzyme 1 and subsequently by γ-secretase in the transmembrane domain (Greenfield et al. 2000; Findeis 2007). Aβ peptides span from 36 to 43 aminoacids in length, Aβ40 being the predominant monomeric form and Aβ42 the most toxic form of the peptide. Aβ forms soluble oligomeric species (two to six peptides) and insoluble fibrils (β-pleated sheets), the most toxic species being currently believed to be the soluble/oligomeric Aβ (Glabe and Kayed 2006; Slow et al. 2006; Haass and Selkoe 2007; Querfurth and LaFerla 2010). Hyperphosphorylated tau is insoluble and aggregates in paired helical filamentous structures causing an impaired axonal transport (Castellani et al. 2010; Querfurth and LaFerla 2010). Nevertheless, oligomeric tau is also considered to be the most toxic form, being correlated with memory impairment and neuronal loss (Santacruz et al. 2005; Oddo et al. 2006; Yoshiyama et al. 2007). There are several other alterations that present a good correlation with AD severity such as loss of pyramidal neurons (Mann et al. 1988) and synapses (Masliah et al. 1989) and neuritic alterations (Masliah et al. 1990). The literature also shows that mitochondrial dysfunction and oxidative stress play an important role in the early pathology of AD (Hirai et al. 2001; Moreira et al. 2010). Indeed, there are strong indications that oxidative stress occurs prior to the onset of symptoms in AD and oxidative damage is found not only in the vulnerable regions of the brain affected in disease (Nunomura et al. 2001; Honda et al. 2005) but also peripherally (Migliore et al. 2005, Moreira et al. 2007b). Moreover, it has been shown that oxidative damage occurs before Aβ plaque formation (Nunomura et al. 2001) supporting a causative role of mitochondrial dysfunction and oxidative stress in AD. Studies have also shown increased autophagic degradation of mitochondria in AD brain (Moreira et al. 2007c).

Aging is the main risk factor for the development of sporadic AD and, interestingly, the normal aging brain undergoes events that resemble those occurring in AD brain, namely synapse loss (Geinisman et al. 1986). Mattson et al. (2003) documented that meal size and frequency affect neuronal plasticity that in light of the current knowledge can be easily correlated with mTOR signaling. Indeed, mTOR has been described to be involved in the basic molecular mechanisms of the aging brain, neuronal development and plasticity (Jaworski and Sheng 2006; Bishop et al. 2010). Recently, a genetic screening performed in mice hypothalamus upon CR demonstrated that the expression of up to 490 genes is altered in an age-dependent manner. Interestingly, some of those genes were associated with mTOR, namely the down-regulation of S6K.

It was recently shown that rapamycin prevented paraquat-induced neuronal apoptosis (Wu et al. 2009). Moreover, in an animal model of tuberous sclerosis, expressing heterozygous mutations in the TSC1 or TSC2 gene, inhibitors of mTOR, it was observed an up-regulation of mTOR associated to a deficient long-term potentiation and, consequently, to deficits in memory formation (Ehninger et al. 2008). Rapamycin treatment rescued not only synaptic plasticity but also behavioral deficits (Ehninger et al. 2008). It was found that delta-9-tetrahydrocannabinol-induced memory deficits are rescued upon the administration of low concentrations of rapamycin (Puighermanal et al. 2009). Contradictory findings support the idea that mTOR pathway activation and consequent induction of protein synthesis are essential for the regeneration of axotomized retinal ganglion cells (Park et al. 2008). It could be hypothesized that the beneficial inhibitory effects of rapamycin on mTOR pathway are dependent on the experimental model, age/developmental stage of the organism or cell and, most likely, on the concentration/dosage of rapamycin. As already discussed, rapamycin at higher concentrations can also affect the activity of mTORC2, which interferes with cytoskeleton proteins organization, namely actin. Actin organization and availability within dendritic spines modulates spine size and enlargement, organization of the post-synaptic density, receptor trafficking, and localization of the translational machinery, thus affecting synaptic plasticity and functionality (Bramham 2008).

A previous study demonstrated an impairment of the mTOR signaling pathway in a murine neuroblastoma cell line exposed to Aβ, in the cortex of double AβPP/presenilin-1 transgenic female mice compared with control mice and in lymphocytes of AD patients (Lafay-Chebassier et al. 2005). The phosphorylation of mTOR substrate S6K is decreased in all experimental models and in AD lymphocytes the ratio phospho-S6K/S6K is positively correlated with Mini Mental Status Examination scores (Lafay-Chebassier et al. 2005). Furthermore, Aβ-induced deregulation of mTOR pathway mediates the impairment of synaptic plasticity observed in the Tg2576 transgenic mouse model for AD, this effect being rescued by mTOR up-regulation (Ma et al. 2010). Interestingly, in Tg2576 hippocampal slices the levels of postsynaptic density protein 95 but not synapsin were significantly decreased, an effect similar to that observed in wild type slices treated with rapamycin. However, when Tg2576 slices were treated with rapamycin, there was no further decrease in the levels of postsynaptic density protein 95 compared to untreated Tg2576 slices, suggesting no additional harmful effects induced by rapamycin (Ma et al. 2010). On the opposite, others have demonstrated that rapamycin exacerbates Aβ production (Yu et al. 2005) and Aβ-induced cell death (Lafay-Chebassier et al. 2006). Rapamycin was also shown to promote Aβ production via inhibition of an enzyme belonging to the non-amyloidogenic processing pathway of AβPP, disintegrin and metalloproteinase domain-containing protein 10, thus favoring its amyloidogenic processing (Zhang et al. 2010). Also, Ma et al. (2010) observed that Aβ-induced impairment of mTOR pathway was rescued in hippocampal slices of FKBP12 conditional knockout, facilitating long-term potentiation, which is suggestive of an Aβ-FKBP12 interaction. Indeed, it was previously demonstrated that AβPP is capable of interacting with FKBP12 (Liu et al. 2006).

Whereas some studies point out deleterious effects of rapamycin in models of AD, other studies support its beneficial role in the course of disease. Cell cycle re-entry of neurons has been described as another cause of neurodegeneration in AD (Bonda et al. 2010). Studies have demonstrated that treating neurons with Aβ oligomers, but not fibrils or monomers, elicits an elevation in the levels of activated Akt and mTOR, which was correlated with the retraction of neuronal processes, and induction of cell cycle events in a concentration-dependent manner. The use of PI3K, Akt or mTOR inhibitors blocked Aβ oligomer-induced neuronal cell cycle events (Bhaskar et al. 2009). Recently, two compelling works demonstrated in PDAPP transgenic mice (Spilman et al. 2010) and in the 3xTgAD mice (Caccamo et al. 2010) that long-term feeding of chow containing either microencapsulated rapamycin at 2.24 mg/kg improved their behavioral deficits and showed an autophagy-dependent decrease in Aβ and hyperphosphorylated tau. Indeed, a previous work have demonstrated that rapamycin reduce the aggregation, the levels and the toxicity of wild type and mutant tau protein in an autophagy-dependent manner (Berger et al. 2006). The potential beneficial effects of rapamycin on tau protein levels are corroborated by data showing that in AD brains the up-regulation of mTOR pathway, as suggested by the increase in the phosphorylation of Ser2481 residue, and its downstream targets 4E-BP1, eukaryotic elongation factor 2, and eukaryotic elongation factor 2 kinase is, at least in part, responsible for the increased translation of tau protein (Li et al. 2005). In addition, mTOR has also been implicated as a tight regulator of tau hyperphosphorylation since it coordinates the activity of the enzymes glycogen synthase kinase-3β and protein phosphatase 2A to ensure balanced tau phosphorylation (Meske et al. 2008).

Recent evidence indicates the NAD+-dependent deacetylase SIRT1 is a crucial regulator of α-secretase and, consequently, of the amyloidogenic processing (Donmez et al. 2010; Bonda et al. 2011). It was recently shown that over-expression of SIRT1 improves cognitive performance and reduces AD pathology in a mouse model of AD (Donmez et al. 2010). Mechanistically, SIRT1 deacetylates and activates the retinoic acid receptor β that, in turn, stimulates AβPP, disintegrin and metalloproteinase domain-containing protein 10 gene transcription and a-secretase production (Donmez et al. 2010). It was also shown that a reduction in the levels of SIRT1 correlates with the accumulation of Aβ and tau in the cerebral cortex of AD patients (Julien et al. 2009). The interplay between SIRT1 and mTOR, as previously discussed, suggest that a synergistic modulation of these proteins would most likely yield promising therapeutic results, since SIRT1 modulates Aβ production and mTOR regulates autophagy, a recognized cell clearance pathway.

The literature concerning the effects of rapamycin in AD is controversial since some studies show that it alleviates the neuropathological burden and the phenotypical manifestation of the disease while others show an exacerbation of AD deleterious events. More studies must be done to clarify the role of mTOR as well as the potential therapeutic effects of rapamycin in AD.

Conclusion

  1. Top of page
  2. Abstract
  3. mTOR and rapamycin
  4. mTOR and rapamycin: anti-aging effects
  5. Rapamycin and mTOR: effects on memory and implications for Alzheimer’s disease
  6. Conclusion
  7. Acknowledgements
  8. References

Mammalian TOR complex is an essential evolutionarily conserved cellular sensor of nutrient availability that commands a number of pathways involved in protein biosynthesis, degradation of cellular constituents and metabolism, among others. The control of these pathways involves the assembly of the enzymatic complexes mTORC1 and mTORC2, where mTOR mainly plays a catalytic role. mTORC1 is the most studied complex and the one with higher sensitivity to catalytic inhibition by rapamycin. mTORC1 inhibition, and consequent down-regulation of downstream substrates associated with protein synthesis and up-regulation of autophagy, has been demonstrated to have important aging delaying effects in several model organisms including mammalians, which indicate that mTORC1 is a promising target in cases of age-related disorders. However, rapamycin-induced mTORC1 inhibition provided inconsistent observations in AD models, some pointing to an exacerbation of the neuropathological features and the others to their reduction. Indeed, in animal models of AD a behavioral improvement was achieved upon feeding them with rapamycin. A deeper knowledge of the causes of AD and the role of mTORC1 in the disease progression would help to sharpen strategies involving the modulation of mTORC1 activity to prevent disease development.

Acknowledgements

  1. Top of page
  2. Abstract
  3. mTOR and rapamycin
  4. mTOR and rapamycin: anti-aging effects
  5. Rapamycin and mTOR: effects on memory and implications for Alzheimer’s disease
  6. Conclusion
  7. Acknowledgements
  8. References

Renato X. Santos has a PhD fellowship from the Fundação para a Ciência e a Tecnologia (SFRH/BD/43972/2008). Work in the authors’ laboratories is supported by Fundação para a Ciência e a Tecnologia (PTDC/SAU-NEU/103325/2008 and PTDC/SAU-NMC/110990/2009).

References

  1. Top of page
  2. Abstract
  3. mTOR and rapamycin
  4. mTOR and rapamycin: anti-aging effects
  5. Rapamycin and mTOR: effects on memory and implications for Alzheimer’s disease
  6. Conclusion
  7. Acknowledgements
  8. References