We have identified several dietary factors that may increase the risk of age-related neurodegenerative disorders including Alzheimer's and Parkinson's diseases, and stroke. We found that dietary restriction (reduced calorie intake) can protect neurons against dysfunction and degeneration in mouse and rat models of Alzheimer's disease, Parkinson's disease, stroke and Huntington's disease. Dietary restriction induces the expression of neurotrophic factors and """"""""stress"""""""" proteins that may increase resistance of neurons to oxidative and metabolic stress. We are developing novel dietary supplements that mimic the beneficial effects of caloric restriction. In another series of studies, we have obtained evidence that folic acid can protect neurons in models relevant to Alzheimer's and Parkinson's diseases. --Molecular and Cellular Mechanisms of Action of Dietary Restriction in the Brain: We have discovered that levels of brain-derived neurotrophic factor (BDNF) are significantly increased in the hippocampus, cerebral cortex and striatum of mice maintained on an alternate day feeding DR regimen compared to animals fed ad libitum. Damage to hippocampal neurons induced by the excitotoxin kainic acid was significantly reduced in mice maintained on DR, and this neuroprotective effect was attenuated by intraventricular administration of a BDNF-blocking antibody Our findings show that simply reducing food intake results in increased levels of BDNF in brain cells, and suggest that the resulting activation of BDNF signaling pathways plays a key role in the neuroprotective effect of DR. These results bolster accumulating evidence that DR may be an effective approach for increasing the resistance of the brain to damage and enhancing brain neuronal plasticity. The adult brain contains neural stem cells that are capable of proliferating, differentiating into neurons or glia, and then either surviving or dying. This process of neural-cell production (neurogenesis) in the dentate gyrus of the hippocampus is responsive to brain injury, and both mental and physical activity. We now report that neurogenesis in the dentate gyrus can also be modified by diet. Previous studies have shown that dietary restriction (DR) can suppress age-related deficits in learning and memory, and can increase resistance of neurons to degeneration in experimental models of neurodegenerative disorders. We found that maintenance of adult rats on a DR regimen results in a significant increase in the numbers of newly produced neural cells in the dentate gyrus of the hippocampus. The increase in neurogenesis in rats maintained on DR appears to result from decreased death of newly produced cells, rather than from increased cell proliferation. We further show that the expression of brain-derived neurotrophic factor, a trophic factor recently associated with neurogenesis, is increased in hippocampal cells of rats maintained on DR. Our data are the first evidence that diet can affect the process of neurogenesis, as well as the first evidence that diet can affect neurotrophic factor production. These findings provide insight into the mechanisms whereby diet impacts on brain plasticity, aging and neurodegenerative disorders. --Dietary Restriction and Caloric Restriction Dietary Supplements in Monkeys: Rhesus monkeys were divided into three groups (6-8 monkeys per group): normal diet, 30% caloric restriction, and a third group whose diet was supplemented with 0.4% 2DG. Monkeys were maintained on the diets for 8 months, during which time various physiological parameters were measured, and MRI and PET brain imaging analyses and tests of motor function were performed. The monkeys were then administered the dopaminergic toxin MPTP via unilateral intracarotid infusion; this toxin selectively damages substantia nigra dopaminergic neurons and renders the monkeys hemi-Parkinsonian (motor dysfuntion on one side of the body). During a 3 month post-MPTP period, motor function was assessed and brain imaging was performed again. The monkeys were then euthanatized and various histological and molecular analyses of their brains are in progress. The prediction is that caloric restriction and 2-deoxy-D-glucose supplementation will reduce damage to dopaminergic neurons and improve functional outcome. The underlying mechanisms will be explored in studies of their brain tissues and will involve biochemical, molecular and histological analyses similar to those used in our previous studies in mice, plus additional analyses including gene expression analysis using the NIA 26K array. We expect these analyses to confirm our findings of increased neurotrophic factor, protein chaperone and UCP expression in the brains of mice and rats maintained on DR. We also expect novel genes affected by DR and 2DG supplementation to emerge from this study. --Folic Acid Deficiency and Homocysteine in Alzheimer's and Parkinson's Diseases: We began our investigations of the possible roles of homocysteine and folate in brain aging and neurodegenerative disorders with a study in which we showed that homocysteine can induce apoptosis of cultured hippocampal neurons and can increase their vulnerability to excitotoxicity. We further showed that homocysteine kills neurons by inducing DNA damage which triggers activation of PARP and p53. Folate deficiency can also induce neuronal apoptosis, and can increase the vulnerability of neurons to oxidative and excitotoxic insults. We have performed studies aimed at determining the impact of homocysteine and dietary folate on neuronal vulnerability in mouse models of AD and PD. When wild-type and APP mutant mice were maintained for 3 months on the usual diet or a folate-deficient diet; levels of plasma homocysteine were increased to similar amounts in both wild-type and APP mutant mice maintained on the folate-deficient diet. Counts of hippocampal CA3 and CA1 neurons revealed a significant decrease in the number of CA3 neurons in the APP mutant mice on the folate-deficient diet compared to APP mutant mice on the normal diet, and to wild-type mice on either diet. In cell culture studies we further showed that folate deficiency and homocysteine render neurons vulnerable to being killed by Abeta. These results suggest that, as a result of increased homocysteine levels, folate deficiency can render neurons vulnerable to Abeta toxicity. These findings provide experimental evidence supporting epidemiological and clinical data that suggest that individuals with elevated homocysteine levels are at increased risk of AD. We found that dietary folate deficiency sensitizes mice to MPTP-induced PD-like pathology and motor dysfunction. --We have recently provided evidence that dietary lipids may modulate risk of AD and ALS. Levels of cholesterol and long-chain ceramides are increased in membranes of cells in the brains of AD patients and spinal cords of ALS patients. Additional data in studies of cell culture and animal models of AD and ALS suggest that ceramides may play an important role in the cell death process in these disorders. Because levels of cholesterol, sphingolipids and ceramides can be modulated by changes in diet, our data suggest that dietary lipids may modify the vulnerability of neurons to age-related diseases.
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