Stacy A. Hussong

Chronic inhibition of mammalian target of rapamycin by rapamycin modulates cognitive and non-cognitive components of behavior throughout lifespan in mice

Aging is, by far, the greatest risk factor for most neurodegenerative diseases. In non-diseased conditions, normal aging can also be associated with declines in cognitive function that significantly affect quality of life in the elderly. It was recently shown that inhibition of Mammalian TOR (mTOR) activity in mice by chronic rapamycin treatment extends lifespan, possibly by delaying aging {Harrison, 2009 #4}{Miller, 2011 #168}. To explore the effect of chronic rapamycin treatment on normal brain aging we determined cognitive and non-cognitive components of behavior throughout lifespan in male and female C57BL/6 mice that were fed control- or rapamycin-supplemented chow. Our studies show that rapamycin enhances cognitive function in young adult mice and blocks age-associated cognitive decline in older animals. In addition, mice fed with rapamycin-supplemented chow showed decreased anxiety and depressive-like behavior at all ages tested. Levels of three major monoamines (norepinephrine, dopamine and 5-hydroxytryptamine) and their metabolites (3,4-dihydroxyphenylacetic acid, homovanillic acid, and 5-hydroxyindolacetic acid) were significantly augmented in midbrain of rapamycin-treated mice compared to controls. Our results suggest that chronic, partial inhibition of mTOR by oral rapamycin enhances learning and memory in young adults, maintains memory in old C57BL/6J mice, and has concomitant anxiolytic and antidepressant-like effects, possibly by stimulating major monoamine pathways in brain.

Chronic Rapamycin Restores Brain Vascular Integrity and Function Through NO Synthase Activation and Improves Memory in Symptomatic Mice Modeling Alzheimer’s Disease

Vascular pathology is a major feature of Alzheimer’s disease (AD) and other dementias. We recently showed that chronic administration of the target-of-rapamycin (TOR) inhibitor rapamycin, which extends lifespan and delays aging, halts the progression of AD-like disease in transgenic human (h)APP mice modeling AD when administered before disease onset. Here we demonstrate that chronic reduction of TOR activity by rapamycin treatment started after disease onset restored cerebral blood flow (CBF) and brain vascular density, reduced cerebral amyloid angiopathy and microhemorrhages, decreased amyloid burden, and improved cognitive function in symptomatic hAPP (AD) mice. Like acetylcholine (ACh), a potent vasodilator, acute rapamycin treatment induced the phosphorylation of endothelial nitric oxide (NO) synthase (eNOS) and NO release in brain endothelium. Administration of the NOS inhibitor L-NG-Nitroarginine methyl ester reversed vasodilation as well as the protective effects of rapamycin on CBF and vasculature integrity, indicating that rapamycin preserves vascular density and CBF in AD mouse brains through NOS activation. Taken together, our data suggest that chronic reduction of TOR activity by rapamycin blocked the progression of AD-like cognitive and histopathological deficits by preserving brain vascular integrity and function. Drugs that inhibit the TOR pathway may have promise as a therapy for AD and possibly for vascular dementias.

Over‐expression of heat shock factor 1 phenocopies the effect of chronic inhibition of TOR by rapamycin and is sufficient to ameliorate Alzheimer's‐like deficits in mice modeling the disease

Rapamycin, an inhibitor of target‐of‐rapamycin, extends lifespan in mice, possibly by delaying aging. We recently showed that rapamycin halts the progression of Alzheimers (AD)‐like deficits, reduces amyloid‐beta (Aβ) and induces autophagy in the human amyloid precursor protein (PDAPP) mouse model. To delineate the mechanisms by which chronic rapamycin delays AD we determined proteomic signatures in brains of control‐ and rapamycin‐treated PDAPP mice. Proteins with reported chaperone‐like activity were overrepresented among proteins up‐regulated in rapamycin‐fed PDAPP mice and the master regulator of the heat‐shock response, heat‐shock factor 1, was activated. This was accompanied by the up‐regulation of classical chaperones/heat shock proteins (HSPs) in brains of rapamycin‐fed PDAPP mice. The abundance of most HSP mRNAs except for alpha B‐crystallin, however, was unchanged, and the cap‐dependent translation inhibitor 4E‐BP was active, suggesting that increased expression of HSPs and proteins with chaperone activity may result from preferential translation of pre‐existing mRNAs as a consequence of inhibition of cap‐dependent translation. The effects of rapamycin on the reduction of Aβ, up‐regulation of chaperones, and amelioration of AD‐like cognitive deficits were recapitulated by transgenic over‐expression of heat‐shock factor 1 in PDAPP mice. These results suggest that, in addition to inducing autophagy, rapamycin preserves proteostasis by increasing chaperones. We propose that the failure of proteostasis associated with aging may be a key event enabling AD, and that chronic inhibition of target‐of‐rapamycin may delay AD by maintaining proteostasis in brain.

Decreased in vitro mitochondrial function is associated with enhanced brain metabolism, blood flow, and memory in Surf1-deficient mice

Recent studies have challenged the prevailing view that reduced mitochondrial function and increased oxidative stress are correlated with reduced longevity. Mice carrying a homozygous knockout (KO) of the Surf1 gene showed a significant decrease in mitochondrial electron transport chain Complex IV activity, yet displayed increased lifespan and reduced brain damage after excitotoxic insults. In the present study, we examined brain metabolism, brain hemodynamics, and memory of Surf1 KO mice using in vitro measures of mitochondrial function, in vivo neuroimaging, and behavioral testing. We show that decreased respiration and increased generation of hydrogen peroxide in isolated Surf1 KO brain mitochondria are associated with increased brain glucose metabolism, cerebral blood flow, and lactate levels, and with enhanced memory in Surf1 KO mice. These metabolic and functional changes in Surf1 KO brains were accompanied by higher levels of hypoxia-inducible factor 1 alpha, and by increases in the activated form of cyclic AMP response element-binding factor, which is integral to memory formation. These findings suggest that Surf1 deficiency-induced metabolic alterations may have positive effects on brain function. Exploring the relationship between mitochondrial activity, oxidative stress, and brain function will enhance our understanding of cognitive aging and of age-related neurologic disorders.

mTOR drives cerebral blood flow and memory deficits in LDLR−/− mice modeling atherosclerosis and vascular cognitive impairment:

We recently showed that mTOR attenuation blocks progression and abrogates established cognitive deficits in Alzheimer’s disease (AD) mouse models. These outcomes were associated with the restoration of cerebral blood flow (CBF) and brain vascular density (BVD) resulting from relief of mTOR inhibition of NO release. Recent reports suggested a role of mTOR in atherosclerosis. Because mTOR drives aging and vascular dysfunction is a universal feature of aging, we hypothesized that mTOR may contribute to brain vascular and cognitive dysfunction associated with atherosclerosis. We measured CBF, BVD, cognitive function, markers of inflammation, and parameters of cardiovascular disease in LDLR−/− mice fed maintenance or high-fat diet ± rapamycin. Cardiovascular pathologies were proportional to severity of brain vascular dysfunction. Aortic atheromas were reduced, CBF and BVD were restored, and cognitive dysfunction was attenuated potentially through reduction in systemic and brain inflammation following chronic mTOR attenuation. Our studies suggest that mTOR regulates vascular integrity and function and that mTOR attenuation may restore neurovascular function and cardiovascular health. Together with our previous studies in AD models, our data suggest mTOR-driven vascular damage may be a mechanism shared by age-associated neurological diseases. Therefore, mTOR attenuation may have promise for treatment of cognitive impairment in atherosclerosis.

Inhibition of mTOR protects the blood-brain barrier in models of Alzheimer’s disease and vascular cognitive impairment

An intact blood-brain barrier (BBB) limits entry of proinflammatory and neurotoxic blood-derived factors into the brain parenchyma. The BBB is damaged in Alzheimers disease (AD), which contributes significantly to the progression of AD pathologies and cognitive decline. However, the mechanisms underlying BBB breakdown in AD remain elusive, and no interventions are available for treatment or prevention. We and others recently established that inhibition of the mammalian/mechanistic target of rapamycin (mTOR) pathway with rapamycin yields significant neuroprotective effects, improving cerebrovascular and cognitive function in mouse models of AD. To test whether mTOR inhibition protects the BBB in neurological diseases of aging, we treated hAPP(J20) mice modeling AD and low-density lipoprotein receptor-null (LDLR-/-) mice modeling vascular cognitive impairment with rapamycin. We found that inhibition of mTOR abrogates BBB breakdown in hAPP(J20) and LDLR-/- mice. Experiments using an in vitro BBB model indicated that mTOR attenuation preserves BBB integrity through upregulation of specific tight junction proteins and downregulation of matrix metalloproteinase-9 activity. Together, our data establish mTOR activity as a critical mediator of BBB breakdown in AD and, potentially, vascular cognitive impairment and suggest that rapamycin and/or rapalogs could be used for the restoration of BBB integrity. NEW & NOTEWORTHY This report establishes mammalian/mechanistic target of rapamycin as a critical mediator of blood-brain barrier breakdown in models of Alzheimers disease and vascular cognitive impairment and suggests that drugs targeting the target of rapamycin pathway could be used for the restoration of blood-brain barrier integrity in disease states.