Mammalian aging can be delayed with genetic, dietary and pharmacologic approaches. Given that the elderly population is dramatically increasing and that aging is the greatest risk factor for a majority of chronic diseases driving both morbidity and mortality, it is critical to expand Geroscience research directed at extending human healthspan.

Rapamycin, an inhibitor of mechanistic target of rapamycin complex 1 (mTORC1), extends the lifespans of yeast, flies, and mice. Calorie restriction, which increases lifespan and insulin sensitivity, is proposed to function by inhibition of mTORC1, yet paradoxically, chronic administration of rapamycin substantially impairs glucose tolerance and insulin action. We demonstrate that rapamycin disrupted a second mTOR complex, mTORC2, in vivo and that mTORC2 was required for the insulin-mediated suppression of hepatic gluconeogenesis. Further, decreased mTORC1 signaling was sufficient to extend lifespan independently from changes in glucose homeostasis, as female mice heterozygous for both mTOR and mLST8 exhibited decreased mTORC1 activity and extended lifespan, but had normal glucose tolerance and insulin sensitivity. Thus, mTORC2 disruption is an important mediator of the effects of rapamycin in vivo.

BackgroundReduced TOR signaling has been shown to significantly increase lifespan in a variety of organisms [1], [2], [3], [4]. It was recently demonstrated that long-term treatment with rapamycin, an inhibitor of the mTOR pathway[5], or ablation of the mTOR target p70S6K[6] extends lifespan in mice, possibly by delaying aging. Whether inhibition of the mTOR pathway would delay or prevent age-associated disease such as AD remained to be determined.Methodology/Principal FindingsWe used rapamycin administration and behavioral tools in a mouse model of AD as well as standard biochemical and immunohistochemical measures in brain tissue to provide answers for this question. Here we show that long-term inhibition of mTOR by rapamycin prevented AD-like cognitive deficits and lowered levels of Aβ42, a major toxic species in AD[7], in the PDAPP transgenic mouse model. These data indicate that inhibition of the mTOR pathway can reduce Aβ42 levels in vivo and block or delay AD in mice. As expected from the inhibition of mTOR, autophagy was increased in neurons of rapamycin-treated transgenic, but not in non-transgenic, PDAPP mice, suggesting that the reduction in Aβ and the improvement in cognitive function are due in part to increased autophagy, possibly as a response to high levels of Aβ.Conclusions/SignificanceOur data suggest that inhibition of mTOR by rapamycin, an intervention that extends lifespan in mice, can slow or block AD progression in a transgenic mouse model of the disease. Rapamycin, already used in clinical settings, may be a potentially effective therapeutic agent for the treatment of AD.

Accumulation of amyloid-␤ (A␤) and Tau is an invariant feature of Alzheimer disease (AD). The upstream role of A␤ accumulation in the disease pathogenesis is widely accepted, and there is strong evidence showing that A␤ accumulation causes cognitive impairments. However, the molecular mechanisms linking A␤ to cognitive decline remain to be elucidated. Here we show that the buildup of A␤ increases the mammalian target of rapamycin (mTOR) signaling, whereas decreasing mTOR signaling reduces A␤ levels, thereby highlighting an interrelation between mTOR signaling and A␤. The mTOR pathway plays a central role in controlling protein homeostasis and hence, neuronal functions; indeed mTOR signaling regulates different forms of learning and memory. Using an animal model of AD, we show that pharmacologically restoring mTOR signaling with rapamycin rescues cognitive deficits and ameliorates A␤ and Tau pathology by increasing autophagy. Indeed, we further show that autophagy induction is necessary for the rapamycinmediated reduction in A␤ levels. The results presented here provide a molecular basis for the A␤-induced cognitive deficits and, moreover, show that rapamycin, an FDA approved drug, improves learning and memory and reduces A␤ and Tau pathology. Neurofibrillary tangles (NFTs)2 and amyloid plaques represent the two major hallmark neuropathological lesions of AD (1). NFTs are intraneuronal inclusions that are mainly formed of the hyperphosphorylated microtubule-binding protein Tau (2-5). In contrast, amyloid plaques accumulate extracellularly and are mainly composed of a peptide called amyloid-␤ (A␤) (6, 7). Although the key role of A␤ accumulation in the pathogenesis of AD is widely accepted, the molecular pathways by which A␤ accumulation leads to cognitive decline and Tau pathology remain to be elucidated.The mammalian target of rapamycin (mTOR) is a conserved Ser/Thr kinase that forms two multiprotein complexes known as mTOR complex (mTORC) 1 and 2 (8). mTORC1 controls cellular homeostasis, and its activity is inhibited by rapamycin; in contrast mTORC2 is insensitive to rapamycin and controls cellular shape by modulating actin function (8, 9). By regulating both protein synthesis and degradation, mTOR plays a key role in controlling protein homeostasis and hence brain function; indeed, mTOR activity has been directly linked to learning and memory (10 -13). Additionally, genetic and pharmacological reduction of mTOR activity has been shown to increase the lifespan in different organisms including yeast, Drosophila, and mice (14 -19).mTOR is an inhibitor of macroautophagy, which is a conserved intracellular system designed for the degradation of long-lived proteins and organelles in lysosomes (20 -22). Cumulative evidence suggests that an age-dependent decrease in the autophagy/lysosome system may account for the accumulation of abnormal proteins during aging (23). Macroautophagy (herein referred to as autophagy) is induced when an isolation membrane is generated surrounding cytosolic components, forming an autopha...

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Mice heterozygous for the Sod2 gene (Sod2+/- mice) have been used to study the phenotype of life-long reduced Mn-superoxide dismutase (MnSOD) activity. The Sod2+/- mice have reduced MnSOD activity (50%) in all tissues throughout life. The Sod2+/- mice have increased oxidative damage as demonstrated by significantly elevated levels of 8-oxo-2-deoxyguanosine (8oxodG) in nuclear DNA in all tissues of Sod2+/- mice studied. The levels of 8oxodG in nuclear DNA increased with age in all tissues of Sod2+/- and wild-type (WT) mice, and at 26 mo of age, the levels of 8oxodG in nuclear DNA were significantly higher (from 15% in heart to over 60% in liver) in the Sod2+/- mice compared with WT mice. The level of 8oxodG was also higher in mitochondrial DNA isolated from liver and brain in Sod2+/- mice compared with WT mice. The increased oxidative damage to DNA in the Sod2+/- mice is associated with a 100% increase in tumor incidence (the number of mice with tumors) in old Sod2+/- mice compared with the old WT mice. However, the life spans (mean and maximum survival) of the Sod2+/- and WT mice were identical. In addition, biomarkers of aging, such as cataract formation, immune response, and formation of glycoxidation products carboxymethyl lysine and pentosidine in skin collagen changed with age to the same extent in both WT and Sod2+/- mice. Thus life-long reduction of MnSOD activity leads to increased levels of oxidative damage to DNA and increased cancer incidence but does not appear to affect aging.

The levels of 8-oxo-2-deoxyguanosine (oxo8dG) in DNA isolated from tissues of rodents (male F344 rats, male B6D2F1 mice, male C57BL͞6 mice, and female C57BL͞6 mice) of various ages were measured using sodium iodide to prevent oxidative damage to DNA during DNA isolation. Oxo8dG was measured in nuclear DNA (nDNA) isolated from liver, heart, brain, kidney, skeletal muscle, and spleen and in mitochondrial DNA (mtDNA) isolated from liver. We observed a significant increase in oxo8dG levels in nDNA with age in all tissues and strains of rodents studied. The age-related increase in oxo8dG in nDNA from old mice was shown not to the result of the tissue's reduced ability to remove the oxo8dG lesion. Rather, the increase in oxo8dG levels appears to arise from an age-related increase in the sensitivity of these tissues to oxidative stress. We also observed an age-related increase in oxo8dG in mtDNA isolated from the livers of the rats and mice. Dietary restriction, which is known to retard aging and increase the lifespan of rodents, was shown to significantly reduce the agerelated accumulation of oxo8dG levels in nDNA in all tissues of male B6D23F1 mice and in most tissues of male F344 rats. Our study also showed that dietary restriction prevented the age-related increase in oxo8dG levels in mtDNA isolated from the livers of both rats and mice. T he oxidative stress hypothesis of aging (or the free radical hypothesis as it was first proposed) is currently one of the most popular explanations for how aging occurs at the biochemical level. The basic tenet of the oxidative stress hypothesis is that the age-related loss of physiological function and aging are because of the progressive and irreversible accumulation of oxidative damage (1). Over the past decade, the oxidative stress hypothesis of aging has gained wide acceptance because numerous studies have shown a strong correlation between increasing age and the accumulation of oxidative damage to cellular macromolecules (2, 3) and because the increased survival observed with dietary restriction has been correlated to reduced oxidative damage (3,4). It also appears that certain types of pathological lesions that arise with age are associated with increased levels of oxidative damage to cellular macromolecules (5, 6).In 1990, Ames' laboratory reported the first data on the effect of aging on DNA oxidation (7). They observed a significant (Ϸ2-fold) increase in 8-oxo-2-deoxyguanosine (oxo8dG) levels in nuclear DNA (nDNA) isolated from liver, kidney, and intestine of male rats between 2 and 24 months of age. Later, Ames et al. (5) reported that the levels of oxo8dG in mitochondrial DNA (mtDNA) isolated from male rat liver increased 2-to 3-fold with age. Since 1990, a number of research groups have observed an age-related increase in the level of oxo8dG in both nDNA and mtDNA in a variety of tissues of rats and mice (8).On the other hand, many investigators have been unable to detect a significant increase in DNA oxidation in rodent tissues with increasing age (9). The most lik...

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