mTOR drives cerebrovascular, synaptic, and cognitive dysfunction in normative aging.

Cerebrovascular dysfunction and cognitive decline are highly prevalent in aging, but the mechanisms underlying these impairments are unclear. Cerebral blood flow decreases with aging and is one of the earliest events in the pathogenesis of Alzheimer's disease (AD). We have previously shown that the mechanistic/mammalian target of rapamycin (mTOR) drives disease progression in mouse models of AD and in models of cognitive impairment associated with atherosclerosis, closely recapitulating vascular cognitive impairment. In the present studies, we sought to determine whether mTOR plays a role in cerebrovascular dysfunction and cognitive decline during normative aging in rats. Using behavioral tools and MRI-based functional imaging, together with biochemical and immunohistochemical approaches, we demonstrate that chronic mTOR attenuation with rapamycin ameliorates deficits in learning and memory, prevents neurovascular uncoupling, and restores cerebral perfusion in aged rats. Additionally, morphometric and biochemical analyses of hippocampus and cortex revealed that mTOR drives age-related declines in synaptic and vascular density during aging. These data indicate that in addition to mediating AD-like cognitive and cerebrovascular deficits in models of AD and atherosclerosis, mTOR drives cerebrovascular, neuronal, and cognitive deficits associated with normative aging. Thus, inhibitors of mTOR may have potential to treat age-related cerebrovascular dysfunction and cognitive decline. Since treatment of age-related cerebrovascular dysfunction in older adults is expected to prevent further deterioration of cerebral perfusion, recently identified as a biomarker for the very early (preclinical) stages of AD, mTOR attenuation may potentially block the initiation and progression of AD.

Lifelong reduction in complex IV induces tissue-specific metabolic effects but does not reduce lifespan or healthspan in mice.

Loss of SURF1, a Complex IV assembly protein, was reported to increase lifespan in mice despite dramatically lower cytochrome oxidase (COX) activity. Consistent with this, our previous studies found advantageous changes in metabolism (reduced adiposity, increased insulin sensitivity, and mitochondrial biogenesis) in Surf1-/- mice. The lack of deleterious phenotypes in Surf1-/- mice is contrary to the hypothesis that mitochondrial dysfunction contributes to aging. We found only a modest (nonsignificant) extension of lifespan (7% median, 16% maximum) and no change in healthspan indices in Surf1-/- vs. Surf1+/+ mice despite substantial decreases in COX activity (22%-87% across tissues). Dietary restriction (DR) increased median lifespan in both Surf1+/+ and Surf1-/- mice (36% and 19%, respectively). We measured gene expression, metabolites, and targeted expression of key metabolic proteins in adipose tissue, liver, and brain in Surf1+/+ and Surf1-/- mice. Gene expression was differentially regulated in a tissue-specific manner. Many proteins and metabolites are downregulated in Surf1-/- adipose tissue and reversed by DR, while in brain, most metabolites that changed were elevated in Surf1-/- mice. Finally, mitochondrial unfolded protein response (UPRmt )-associated proteins were not uniformly altered by age or genotype, suggesting the UPRmt is not a key player in aging or in response to reduced COX activity. While the changes in gene expression and metabolism may represent compensatory responses to mitochondrial stress, the important outcome of this study is that lifespan and healthspan are not compromised in Surf1-/- mice, suggesting that not all mitochondrial deficiencies are a critical determinant of lifespan.

Sco2 deficient mice develop increased adiposity and insulin resistance.

Cytochrome c oxidase (COX) is an essential transmembrane protein complex (Complex IV) in the mitochondrial respiratory electron chain. Mutations in genes responsible for the assembly of COX are associated with Leigh syndrome, cardiomyopathy, spinal muscular atrophy and other fatal metabolic disorders in humans. Previous studies have shown that mice lacking the COX assembly protein Surf1 (Surf1-/- mice) paradoxically show a number of beneficial metabolic phenotypes including increased insulin sensitivity, upregulation of mitochondrial biogenesis, induction of stress response pathways and increased lifespan. To determine whether these effects are specific to the Surf1 mutation or a more general effect of reduced COX activity, we asked whether a different mutation causing reduced COX activity would have similar molecular and physiologic changes. Sco2 knock-in/knock-out (KI/KO) mice in which one allele of the Sco2 gene that encodes a copper chaperone required for COX activity is deleted and the second allele is mutated, have previously been shown to be viable despite a 30-60% reduction in COX activity. In contrast to the Surf1-/- mice, we show that Sco2 KI/KO mice have increased fat mass, associated with reduced ß-oxidation and increased adipogenesis markers, reduced insulin receptor beta (IR-ß levels in adipose tissue, reduced muscle glucose transporter 4 (Glut4) levels and a impaired response to the insulin tolerance test consistent with insulin resistance. COX activity and protein are reduced approximately 50% in adipose tissue from the Sco2 KI/KO mice. Consistent with the increase in adipose tissue mass, the Sco2 KI/KO mice also show increased hepatosteatosis, elevated serum and liver triglyceride and increased serum cholesterol levels compared to wild-type controls. In contrast to the Surf1-/- mice, which show increased mitochondrial number, upregulation of the mitochondrial unfolded protein response (UPRMT) pathway and no significant change in mitochondrial respiration in several tissues, Sco2 KI/KO mice do not upregulate the UPRMT, and tissue oxygen consumption and levels of several proteins involved in mitochondrial function are reduced in adipose tissue compared to wild type mice. Thus, the metabolic effects of the Sco2 and Surf1-/- mutations are opposite, despite comparable changes in COX activity, illuminating the complex impact of mitochondrial dysfunction on physiology and pointing to an important role for complex IV in regulating metabolism.

A cross-sectional study of male and female C57BL/6Nia mice suggests lifespan and healthspan are not necessarily correlated.

Lifespan provides a discrete metric that is intuitively appealing and the assumption has been that healthspan is extended concomitant with lifespan. Medicine has been more successful at extending life than preserving health during aging. Interventions that extend lifespan in model organisms do not always result in a corresponding increase in healthspan, suggesting that lifespan and healthspan may be uncoupled. To understand how interventions that extend life affect healthspan, we need measures that distinguish between young and old animals. Here we measured age-related changes in healthspan in male and female C57BL/6JNia mice assessed at 4 distinct ages (4 months, 20 months, 28 months and 32 months). Correlations between health parameters and age varied. Some parameters show consistent patterns with age across studies and in both sexes, others changed in one sex only and others showed no significant differences in mice of different ages. Few correlations existed among health assays, suggesting that physiological function in domains we assessed change independently in aging mice. With one exception, health parameters were not significantly associated with an increased probability of premature death. Our results show the need for more robust measures of murine health and suggest a potential disconnect between health and lifespan in mice.

Health Effects of Long-Term Rapamycin Treatment: The Impact on Mouse Health of Enteric Rapamycin Treatment from Four Months of Age throughout Life.

Rapamycin, an mTOR inhibitor, has been shown to extend lifespan in a range of model organisms. It has been reported to extend lifespan in multiple strains of mice, administered chronically or acutely early or late in life. The ability of rapamycin to extend health (healthspan) as opposed to life is less well documented. To assess the effects chronic rapamycin treatment on healthspan, enteric rapamycin was given to male and female C57BL/6J mice starting at 4 months of age and continued throughout life. Repeated, longitudinal assessments of health in individual animals were made starting at 16 months of age (=12 months of treatment) until death. A number of health parameters were improved (female grip strength, female body mass and reduced sleep fragmentation in both sexes), others showed no significant difference, while at least one (male rotarod performance) was negatively affected. Rapamycin treatment affected many measures of health in a highly sex-specific manner. While sex-specific phenotypic effects of rapamycin treatment have been widely reported, in this study we document sex differences in the direction of phenotypic change. Rapamycin-fed males and females were both significantly different from controls; however the differences were in the opposite direction in measures of body mass, percent fat and resting metabolic rate, a pattern not previously reported.

Obesity-induced oxidative stress, accelerated functional decline with age and increased mortality in mice.

Obesity is a serious chronic disease that increases the risk of numerous co-morbidities including metabolic syndrome, cardiovascular disease and cancer as well as increases risk of mortality, leading some to suggest this condition represents accelerated aging. Obesity is associated with significant increases in oxidative stress in vivo and, despite the well-explored relationship between oxidative stress and aging, the role this plays in the increased mortality of obese subjects remains an unanswered question. Here, we addressed this by undertaking a comprehensive, longitudinal study of a group of high fat-fed obese mice and assessed both their changes in oxidative stress and in their performance in physiological assays known to decline with aging. In female C57BL/6J mice fed a high-fat diet starting in adulthood, mortality was significantly increased as was oxidative damage in vivo. High fat-feeding significantly accelerated the decline in performance in several assays, including activity, gait, and rotarod. However, we also found that obesity had little effect on other markers of function and actually improved performance in grip strength, a marker of muscular function. Together, this first comprehensive assessment of longitudinal, functional changes in high fat-fed mice suggests that obesity may induce segmental acceleration of some of the aging process.

Rapamycin extends life and health in C57BL/6 mice.

Target of rapamycin inhibition by rapamycin feeding has previously been shown to extend life in genetically heterogeneous mice. To examine whether it similarly affected mouse health, we fed encapsulated rapamycin or a control diet to C57BL/6Nia mice of both sexes starting at 19 months of age. We performed a range of health assessments 6 and 12 months later. Rapamycin feeding significantly reduced mTOR activity in most but not all tissues. It also reduced total and resting metabolic rate during the light (inactive) phase of the light:dark cycle in females only but had no effect on spontaneous activity or metabolism during the dark (active) phase of either sex. Males only had less fragmented sleep when fed rapamycin, whereas stride length and rotarod performance were improved in both sexes. Survival was also improved by this late-life rapamycin feeding, and some pathological lesions were delayed. We found no adverse health consequences associated with rapamycin treatment.