Investigating mechanisms underlying rapamycin's effects on age-related motor deficits

Aging is characterized by changes in metabolic pathways that can affect whole body homeostasis. These physiological changes with age in brain may be responsible for phenotypic alterations in sensory, motor and cognitive function. The TOR (Target of rapamycin) pathway regulates growth and metabolism by sensing nutrient and growth factors in wide range of organisms. It is well established that genetic mutations that prevent TOR signaling increases lifespan in a wide variety of species, from yeast to mice. Recently, it has been shown that rapamycin, a natural macrolide that inhibits mechanistic target of rapamycin complex 1 (mTORC1), and is used as immunosuppressant and antitumor agent, can delay aging. We hypothesized that if rapamycin treatment slows aging, it should also prevent or delay age-sensitive traits.;The decline in speed and accuracy of locomotor function is one of the most universal age-sensitive traits. Therefore, we tested the hypothesis that rapamycin can slow the age-related decline in locomotor function. To test our hypothesis, we used young and old genetically heterogeneous UM-HET3 mice of both sexes. Mouse chow supplemented with microencapsulated rapamycin at 14 parts per million (2.25 mg/kg body weight/day) was fed to mice starting at 12 months of age. The goal of this study was to determine whether the pharmacological inhibition of mTOR by rapamycin attenuates the age-related decline in motor function. We found that spontaneous ambulatory activity during a 16 minute period after introducing the mice to a novel cage declined with age, but only in female mice. This effect of aging in female mice was prevented by chronic rapamycin treatment. On the other hand, spontaneous rearing (vertical) activity during that same 16 minute period declined in both sexes. Rapamycin prevented the age-related decline in this measure. We also measured performance on a rotarod as a measure of motor coordination. We found that rotarod performance declined with age in both sexes. Rapamycin treatment attenuated the decline in rotarod performance in both males and females.;Because it had been previously reported that age-related deficits in motor function are related to increased protein carbonyls, we investigated the effect of rapamycin on protein carbonyls in brain regions associated with motor function. Using a fluorescence-based carbonyl assay, we found that detergent-soluble protein carbonyls were increased with age in striatum and hippocampus of both male and female mice. Additionally, females showed elevated carbonyl levels in the cytosolic protein fraction in hippocampus and cerebellum. Rapamycin treatment prevented the increase in protein carbonyl levels in both cytosolic and detergent soluble protein fractions in brain regions involved in motor function. Taken together, the data show that rapamycin prevented oxidative damage to proteins in the brain.;It has been previously reported that increased oxidative damage to proteins is accompanied by an increase in reactive astrogliosis, an inflammatory process that is activated in astrocytes following injury and which is commonly observed during aging and in age-associated neurodegenerative diseases including- Alzheimer's and Parkinson's disease. We hypothesized that, because rapamycin decreased protein carbonyls, then it would also reduce reactive astrogliosis, which can be measured by increased expression levels of glial fibrillary acidic protein (GFAP). Consistent with previous reports, GFAP expression was elevated in the aged striatum of both male and female mice. Surprisingly, rapamycin treatments further increased, rather than reduced, the age-related increase in expression of GFAP in the striatum of both sexes. We note that reactive astrogliosis is generally considered beneficial after brain injuries in helping to promote healing and/or minimize damage. However, prolonged gliosis can lead to scarring and is generally considered to be detrimental to brain health.;We followed this potential link between rapamycin and CHOP expression in cell culture with an examination of the effects of rapamycin treatment on apoptosis in-vivo. Interestingly, we found that CHOP expression increased with age, as expected, and that rapamycin treatment prevented this increase in CHOP levels in female mice, although these changes were not significant in the brains of male mice.;In summary, our findings suggest that chronic rapamycin treatment improves age related behavioral deficits in a sex-specific manner. With age, females showed greater motor deficits than males. Rapamycin prevented this decline in motor performance in both sexes. In addition, sex-specific differences in oxidative protein damage in different brain regions associated with motor function were also prevented by rapamycin. Furthermore, an age-related increase in CHOP in females was significantly reduced by rapamycin treatment. Overall, our data show for the first time the beneficial effect of rapamycin on age-related motor deficits and that these changes are correlated with a reduction in protein carbonyls in soluble and insoluble protein fraction of brain regions associated with motor function in both sexes. (Abstract shortened by ProQuest.).