Wenzhen Duan

Evidence that brain-derived neurotrophic factor is required for basal neurogenesis and mediates, in part, the enhancement of neurogenesis by dietary restriction in the hippocampus of adult mice.

To determine the role of brain‐derived neurotrophic factor (BDNF) in the enhancement of hippocampal neurogenesis resulting from dietary restriction (DR), heterozygous BDNF knockout (BDNF +/–) mice and wild‐type mice were maintained for 3 months on DR or ad libitum (AL) diets. Mice were then injected with bromodeoxyuridine (BrdU) and killed either 1 day or 4 weeks later. Levels of BDNF protein in neurons throughout the hippocampus were decreased in BDNF +/– mice, but were increased by DR in wild‐type mice and to a lesser amount in BDNF +/– mice. One day after BrdU injection the number of BrdU‐labeled cells in the dentate gyrus of the hippocampus was significantly decreased in BDNF +/– mice maintained on the AL diet, suggesting that BDNF signaling is important for proliferation of neural stem cells. DR had no effect on the proliferation of neural stem cells in wild‐type or BDNF +/– mice. Four weeks after BrdU injection, numbers of surviving labeled cells were decreased in BDNF +/– mice maintained on either AL or DR diets. DR significantly improved survival of newly generated cells in wild‐type mice, and also improved their survival in BDNF +/– mice, albeit to a lesser extent. The majority of BrdU‐labeled cells in the dentate gyrus exhibited a neuronal phenotype at the 4‐week time point. The reduced neurogenesis in BDNF +/– mice was associated with a significant reduction in the volume of the dentate gyrus. These findings suggest that BDNF plays an important role in the regulation of the basal level of neurogenesis in dentate gyrus of adult mice, and that by promoting the survival of newly generated neurons BDNF contributes to the enhancement of neurogenesis induced by DR.

Dietary restriction normalizes glucose metabolism and BDNF levels, slows disease progression, and increases survival in huntingtin mutant mice

In addition to neurological deficits, Huntingtons disease (HD) patients and transgenic mice expressing mutant human huntingtin exhibit reduced levels of brain-derived neurotrophic factor, hyperglycemia, and tissue wasting. We show that the progression of neuropathological (formation of huntingtin inclusions and apoptotic protease activation), behavioral (motor dysfunction), and metabolic (glucose intolerance and tissue wasting) abnormalities in huntingtin mutant mice, an animal model of HD, are retarded when the mice are maintained on a dietary restriction (DR) feeding regimen resulting in an extension of their life span. DR increases levels of brain-derived neurotrophic factor and the protein chaperone heat-shock protein-70 in the striatum and cortex, which are depleted in HD mice fed a normal diet. The suppression of the pathogenic processes by DR in HD mice suggests that mutant huntingtin promotes neuronal degeneration by impairing cellular stress resistance, and that the body wasting in HD is driven by the neurodegenerative process. Our findings suggest a dietary intervention that may suppress the disease process and increase the life span of humans that carry the mutant huntingtin gene.

Dietary restriction increases the number of newly generated neural cells, and induces BDNF expression, in the dentate gyrus of rats

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 agerelated 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, as determined by stereologic analysis of cells labeled with the DNA precursor analog bromodeoxyuridine. 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 folate deficiency and elevated homocysteine levels endanger dopaminergic neurons in models of Parkinson's disease

Although the cause of Parkinsons disease (PD) is unknown, data suggest roles for environmental factors that may sensitize dopaminergic neurons to age‐related dysfunction and death. Based upon epidemiological data suggesting roles for dietary factors in PD and other age‐related neurodegenerative disorders, we tested the hypothesis that dietary folate can modify vulnerability of dopaminergic neurons to dysfunction and death in a mouse model of PD. We report that dietary folate deficiency sensitizes mice to MPTP‐induced PD‐like pathology and motor dysfunction. Mice on a folate‐deficient diet exhibit elevated levels of plasma homocysteine. When infused directly into either the substantia nigra or striatum, homocysteine exacerbates MPTP‐induced dopamine depletion, neuronal degeneration and motor dysfunction. Homocysteine exacerbates oxidative stress, mitochondrial dysfunction and apoptosis in human dopaminergic cells exposed to the pesticide rotenone or the pro‐oxidant Fe2+. The adverse effects of homocysteine on dopaminergic cells is ameliorated by administration of the antioxidant uric acid and by an inhibitor of poly (ADP‐ribose) polymerase. The ability of folate deficiency and elevated homocysteine levels to sensitize dopaminergic neurons to environmental toxins suggests a mechanism whereby dietary folate may influence risk for PD.

Cellular and Molecular Mechanisms Underlying Perturbed Energy Metabolism and Neuronal Degeneration in Alzheimer's and Parkinson's Diseases

ABSTRACT: Synaptic degeneration and death of nerve cells are defining features of Alzheimers disease (AD) and Parkinsons disease (PD), the two most prevalent age‐related neurodegenerative disorders. In AD, neurons in the hippocampus and basal forebrain (brain regions that subserve learning and memory functions) are selectively vulnerable. In PD dopamine‐producing neurons in the substantia nigra‐striatum (brain regions that control body movements) selectively degenerate. Studies of postmortem brain tissue from AD and PD patients have provided evidence for increased levels of oxidative stress, mitochondrial dysfunction and impaired glucose uptake in vulnerable neuronal populations. Studies of animal and cell culture models of AD and PD suggest that increased levels of oxidative stress (membrane lipid peroxidation, in particular) may disrupt neuronal energy metabolism and ion homeostasis, by impairing the function of membrane ion‐motive ATPases and glucose and glutamate transporters. Such oxidative and metabolic compromise may thereby render neurons vulnerable to excitotoxicity and apoptosis. Studies of the pathogenic mechanisms of AD‐linked mutations in amyloid precursor protein (APP) and presenilins strongly support central roles for perturbed cellular calcium homeostasis and aberrant proteolytic processing of APP as pivotal events that lead to metabolic compromise in neurons. Specific molecular “players” in the neurodegenerative processes in AD and PD are being identified and include Par‐4 and caspases (bad guys) and neurotrophic factors and stress proteins (good guys). Interestingly, while studies continue to elucidate cellular and molecular events occurring in the brain in AD and PD, recent data suggest that both AD and PD can manifest systemic alterations in energy metabolism (e.g., increased insulin resistance and dysregulation of glucose metabolism). Emerging evidence that dietary restriction can forestall the development of AD and PD is consistent with a major “metabolic” component to these disorders, and provides optimism that these devastating brain disorders of aging may be largely preventable.

Neurodegenerative disorders and ischemic brain diseases

Degeneration and death of neurons is the fundamental process responsible for the clinical manifestations of many different neurological disorders of aging, incuding Alzheimers disease, Parkinsons disease and stroke. The death of neurons in such disorders involves apoptotic biochemical cascades involving upstream effectors (Par-4, p53 and pro-apoptotic Bcl-2 family members), mitochondrial alterations and caspase activation. Both genetic and environmental factors, and the aging process itself, contribute to intiation of such neuronal apoptosis. For example, mutations in the amyloid precursor protein and presenilin genes can cause Alzheimers disease, while head injury is a risk factor for both Alzheimers and Parkinsons diseases. At the cellular level, neuronal apoptosis in neurodegenerative disorders may be triggered by oxidative stress, metabolic compromise and disruption of calcium homeostasis. Neuroprotective (anti-apoptotic) signaling pathways involving neurotrophic factors, cytokines and “conditioning responses” can counteract the effects of aging and genetic predisposition in experimental models of neurodegenerative disorders. A better understanding of the molecular underpinnings of neuronal death is leading directly to novel preventative and therapeutic approaches to neurodegenerative disorders.

Meal size and frequency affect neuronal plasticity and vulnerability to disease: cellular and molecular mechanisms

Although all cells in the body require energy to survive and function properly, excessive calorie intake over long time periods can compromise cell function and promote disorders such as cardiovascular disease, type‐2 diabetes and cancers. Accordingly, dietary restriction (DR; either caloric restriction or intermittent fasting, with maintained vitamin and mineral intake) can extend lifespan and can increase disease resistance. Recent studies have shown that DR can have profound effects on brain function and vulnerability to injury and disease. DR can protect neurons against degeneration in animal models of Alzheimers, Parkinsons and Huntingtons diseases and stroke. Moreover, DR can stimulate the production of new neurons from stem cells (neurogenesis) and can enhance synaptic plasticity, which may increase the ability of the brain to resist aging and restore function following injury. Interestingly, increasing the time interval between meals can have beneficial effects on the brain and overall health of mice that are independent of cummulative calorie intake. The beneficial effects of DR, particularly those of intermittent fasting, appear to be the result of a cellular stress response that stimulates the production of proteins that enhance neuronal plasticity and resistance to oxidative and metabolic insults; they include neurotrophic factors such as brain‐derived neurotrophic factor (BDNF), protein chaperones such as heat‐shock proteins, and mitochondrial uncoupling proteins. Some beneficial effects of DR can be achieved by administering hormones that suppress appetite (leptin and ciliary neurotrophic factor) or by supplementing the diet with 2‐deoxy‐d‐glucose, which may act as a calorie restriction mimetic. The profound influences of the quantity and timing of food intake on neuronal function and vulnerability to disease have revealed novel molecular and cellular mechanisms whereby diet affects the nervous system, and are leading to novel preventative and therapeutic approaches for neurodegenerative disorders.

Neuroprotective role of Sirt1 in mammalian models of Huntington's disease through activation of multiple Sirt1 targets

Huntingtons disease is a fatal neurodegenerative disorder caused by an expanded polyglutamine repeat in huntingtin (HTT) protein. We previously showed that calorie restriction ameliorated Huntingtons disease pathogenesis and slowed disease progression in mice that model Huntingtons disease (Huntingtons disease mice). We now report that overexpression of sirtuin 1 (Sirt1), a mediator of the beneficial metabolic effects of calorie restriction, protects neurons against mutant HTT toxicity, whereas reduction of Sirt1 exacerbates mutant HTT toxicity. Overexpression of Sirt1 improves motor function, reduces brain atrophy and attenuates mutant-HTT–mediated metabolic abnormalities in Huntingtons disease mice. Further mechanistic studies suggested that Sirt1 prevents the mutant-HTT–induced decline in brain-derived neurotrophic factor (BDNF) concentrations and the signaling of its receptor, TrkB, and restores dopamine- and cAMP-regulated phosphoprotein, 32 kDa (DARPP32) concentrations in the striatum. Sirt1 deacetylase activity is required for Sirt1-mediated neuroprotection in Huntingtons disease cell models. Notably, we show that mutant HTT interacts with Sirt1 and inhibits Sirt1 deacetylase activity, which results in hyperacetylation of Sirt1 substrates such as forkhead box O3A (Foxo3a), thereby inhibiting its pro-survival function. Overexpression of Sirt1 counteracts the mutant-HTT–induced deacetylase deficit, enhances the deacetylation of Foxo3a and facilitates cell survival. These findings show a neuroprotective role for Sirt1 in mammalian Huntingtons disease models and open new avenues for the development of neuroprotective strategies in Huntingtons disease.

p53 Inhibitors preserve dopamine neurons and motor function in experimental parkinsonism

Drugs currently used for patients with Parkinsons disease provide temporary relief of symptoms but do not halt or slow the underlying neurodegenerative disease process. Increasing evidence suggests that neurons die in Parkinsons disease by a process called apoptosis, which may be triggered by mitochondrial impairment and oxidative stress. We report that two novel synthetic inhibitors of the tumor suppressor protein p53, pifithrin‐α (PFT‐α) and Z‐1‐117, are highly effective in protecting midbrain dopaminergic neurons and improving behavioral outcome in a mouse model of Parkinsons disease. Mice given intraperitoneal injections of PFT‐α or Z‐1‐117 exhibited improved motor function, reduced damage to nigrostriatal dopaminergic neurons and reduced depletion of dopamine and its metabolites after exposure to the toxin 1‐methyl‐4‐phenyl‐1,2,3,6‐tetrahydropyridine (MPTP). MPTP caused an increase in the level of the proapoptotic protein Bax, which was prevented by giving mice PFT‐α and Z‐1‐117. PFT‐α and Z‐1‐117 also suppressed Bax production and apoptosis in cultured dopaminergic cells exposed to MPP+. Our findings demonstrate a pivotal role for p53 in experimental parkinsonism and identify a novel class of synthetic p53 inhibitors with clinical potential.

Brain-derived neurotrophic factor mediates an excitoprotective effect of dietary restriction in mice

Dietary restriction (DR; reduced calorie intake) increases the lifespan of rodents and increases their resistance to cancer, diabetes and other age‐related diseases. DR also exerts beneficial effects on the brain including enhanced learning and memory and increased resistance of neurons to excitotoxic, oxidative and metabolic insults. The mechanisms underlying the effects of DR on neuronal plasticity and survival are unknown. In the present study we show 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.