Roy G. Cutler

Involvement of oxidative stress-induced abnormalities in ceramide and cholesterol metabolism in brain aging and Alzheimer's disease

Alzheimers disease (AD) is an age-related disorder characterized by deposition of amyloid β-peptide (Aβ) and degeneration of neurons in brain regions such as the hippocampus, resulting in progressive cognitive dysfunction. The pathogenesis of AD is tightly linked to Aβ deposition and oxidative stress, but it remains unclear as to how these factors result in neuronal dysfunction and death. We report alterations in sphingolipid and cholesterol metabolism during normal brain aging and in the brains of AD patients that result in accumulation of long-chain ceramides and cholesterol. Membrane-associated oxidative stress occurs in association with the lipid alterations, and exposure of hippocampal neurons to Aβ induces membrane oxidative stress and the accumulation of ceramide species and cholesterol. Treatment of neurons with α-tocopherol or an inhibitor of sphingomyelin synthesis prevents accumulation of ceramides and cholesterol and protects them against death induced by Aβ. Our findings suggest a sequence of events in the pathogenesis of AD in which Aβ induces membrane-associated oxidative stress, resulting in perturbed ceramide and cholesterol metabolism which, in turn, triggers a neurodegenerative cascade that leads to clinical disease.

Folic Acid Deficiency and Homocysteine Impair DNA Repair in Hippocampal Neurons and Sensitize Them to Amyloid Toxicity in Experimental Models of Alzheimer's Disease

Recent epidemiological and clinical data suggest that persons with low folic acid levels and elevated homocysteine levels are at increased risk of Alzheimers disease (AD), but the underlying mechanism is unknown. We tested the hypothesis that impaired one-carbon metabolism resulting from folic acid deficiency and high homocysteine levels promotes accumulation of DNA damage and sensitizes neurons to amyloid β-peptide (Aβ) toxicity. Incubation of hippocampal cultures in folic acid-deficient medium or in the presence of methotrexate (an inhibitor of folic acid metabolism) or homocysteine induced cell death and rendered neurons vulnerable to death induced by Aβ. Methyl donor deficiency caused uracil misincorporation and DNA damage and greatly potentiated Aβ toxicity as the result of reduced repair of Aβ-induced oxidative modification of DNA bases. When maintained on a folic acid-deficient diet, amyloid precursor protein (APP) mutant transgenic mice, but not wild-type mice, exhibited increased cellular DNA damage and hippocampal neurodegeneration. Levels of Aβ were unchanged in the brains of folate-deficient APP mutant mice. Our data suggest that folic acid deficiency and homocysteine impair DNA repair in neurons, which sensitizes them to oxidative damage induced by Aβ.

Diet-induced insulin resistance impairs hippocampal synaptic plasticity and cognition in middle-aged rats

Overall dietary energy intake, particularly the consumption of simple sugars such as fructose, has been increasing steadily in Western societies, but the effects of such diets on the brain are poorly understood. Here, we used functional and structural assays to characterize the effects of excessive caloric intake on the hippocampus, a brain region important for learning and memory. Rats fed with a high‐fat, high‐glucose diet supplemented with high‐fructose corn syrup showed alterations in energy and lipid metabolism similar to clinical diabetes, with elevated fasting glucose and increased cholesterol and triglycerides. Rats maintained on this diet for 8 months exhibited impaired spatial learning ability, reduced hippocampal dendritic spine density, and reduced long‐term potentiation at Schaffer collateral—CA1 synapses. These changes occurred concurrently with reductions in levels of brain‐derived neurotrophic factor in the hippocampus. We conclude that a high‐calorie diet reduces hippocampal synaptic plasticity and impairs cognitive function, possibly through BDNF‐mediated effects on dendritic spines. Published 2008 Wiley‐Liss, Inc.

Diabetes impairs hippocampal function through glucocorticoid-mediated effects on new and mature neurons

Many organ systems are adversely affected by diabetes, including the brain, which undergoes changes that may increase the risk of cognitive decline. Although diabetes influences the hypothalamic-pituitary-adrenal axis, the role of this neuroendocrine system in diabetes-induced cognitive dysfunction remains unexplored. Here we demonstrate that, in both insulin-deficient rats and insulin-resistant mice, diabetes impairs hippocampus-dependent memory, perforant path synaptic plasticity and adult neurogenesis, and the adrenal steroid corticosterone contributes to these adverse effects. Rats treated with streptozocin have reduced insulin and show hyperglycemia, increased corticosterone, and impairments in hippocampal neurogenesis, synaptic plasticity and learning. Similar deficits are observed in db/db mice, which are characterized by insulin resistance, elevated corticosterone and obesity. Changes in hippocampal plasticity and function in both models are reversed when normal physiological levels of corticosterone are maintained, suggesting that cognitive impairment in diabetes may result from glucocorticoid-mediated deficits in neurogenesis and synaptic plasticity.

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.

Intermittent fasting and caloric restriction ameliorate age-related behavioral deficits in the triple-transgenic mouse model of Alzheimer's disease.

Alzheimers disease (AD) is a neurodegenerative disorder characterized by progressive decline in cognitive function associated with the neuropathological hallmarks amyloid beta-peptide (Abeta) plaques and neurofibrillary tangles. Because aging is the major risk factor for AD, and dietary energy restriction can retard aging processes in the brain, we tested the hypothesis that two different energy restriction regimens, 40% calorie restriction (CR) and intermittent fasting (IF) can protect against cognitive decline in the triple-transgenic mouse model of AD (3xTgAD mice). Groups of 3xTgAD mice were maintained on an ad libitum control diet, or CR or IF diets, beginning at 3 months of age. Half of the mice in each diet group were subjected to behavioral testing (Morris swim task and open field apparatus) at 10 months of age and the other half at 17 months of age. At 10 months 3xTgAD mice on the control diet exhibited reduced exploratory activity compared to non-transgenic mice and to 3xTgAD mice on CR and IF diets. Overall, there were no major differences in performance in the water maze among genotypes or diets in 10-month-old mice. In 17-month-old 3xTgAD mice the CR and IF groups exhibited higher levels of exploratory behavior, and performed better in both the goal latency and probe trials of the swim task, compared to 3xTgAD mice on the control diet. 3xTgAD mice in the CR group showed lower levels of Abeta1-40, Abeta1-42 and phospho-tau in the hippocampus compared to the control diet group, whereas Abeta and phospho-tau levels were not decreased in 3xTgAD mice in the IF group. IF may therefore protect neurons against adverse effects of Abeta and tau pathologies on synaptic function. We conclude that CR and IF dietary regimens can ameliorate age-related deficits in cognitive function by mechanisms that may or may not be related to Abeta and tau pathologies.

The effects of intermittent or continuous energy restriction on weight loss and metabolic disease risk markers: a randomized trial in young overweight women

Background:The problems of adherence to energy restriction in humans are well known.Objective:To compare the feasibility and effectiveness of intermittent continuous energy (IER) with continuous energy restriction (CER) for weight loss, insulin sensitivity and other metabolic disease risk markers.Design:Randomized comparison of a 25% energy restriction as IER (∼2710 kJ/day for 2 days/week) or CER (∼6276 kJ/day for 7 days/week) in 107 overweight or obese (mean (±s.d.) body mass index 30.6 (±5.1) kg m−2) premenopausal women observed over a period of 6 months. Weight, anthropometry, biomarkers for breast cancer, diabetes, cardiovascular disease and dementia risk; insulin resistance (HOMA), oxidative stress markers, leptin, adiponectin, insulin-like growth factor (IGF)-1 and IGF binding proteins 1 and 2, androgens, prolactin, inflammatory markers (high sensitivity C-reactive protein and sialic acid), lipids, blood pressure and brain-derived neurotrophic factor were assessed at baseline and after 1, 3 and 6 months.Results:Last observation carried forward analysis showed that IER and CER are equally effective for weight loss: mean (95% confidence interval ) weight change for IER was −6.4 (−7.9 to −4.8) kg vs −5.6 (−6.9 to −4.4) kg for CER (P-value for difference between groups=0.4). Both groups experienced comparable reductions in leptin, free androgen index, high-sensitivity C-reactive protein, total and LDL cholesterol, triglycerides, blood pressure and increases in sex hormone binding globulin, IGF binding proteins 1 and 2. Reductions in fasting insulin and insulin resistance were modest in both groups, but greater with IER than with CER; difference between groups for fasting insulin was −1.2 (−1.4 to −1.0) μU ml−1 and for insulin resistance was −1.2 (−1.5 to −1.0) μU mmol−1 l−1 (both P=0.04).Conclusion:IER is as effective as CER with regard to weight loss, insulin sensitivity and other health biomarkers, and may be offered as an alternative equivalent to CER for weight loss and reducing disease risk.

Voluntary exercise and caloric restriction enhance hippocampal dendritic spine density and BDNF levels in diabetic mice

Diabetes may adversely affect cognitive function, but the underlying mechanisms are unknown. To investigate whether manipulations that enhance neurotrophin levels will also restore neuronal structure and function in diabetes, we examined the effects of wheel running and dietary energy restriction on hippocampal neuron morphology and brain‐derived neurotrophic factor (BDNF) levels in db/db mice, a model of insulin resistant diabetes. Running wheel activity, caloric restriction, or the combination of the two treatments increased levels of BDNF in the hippocampus of db/db mice. Enhancement of hippocampal BDNF was accompanied by increases in dendritic spine density on the secondary and tertiary dendrites of dentate granule neurons. These studies suggest that diabetes exerts detrimental effects on hippocampal structure, and that this state can be attenuated by increasing energy expenditure and decreasing energy intake. Published 2009 Wiley‐Liss, Inc.

Perturbation of sphingolipid metabolism and ceramide production in HIV-dementia

Infection by the human immunodeficiency virus type 1 (HIV‐1) often results in neurological dysfunction including HIV dementia (HIVD). Alterations in cytokine and redox balance are thought to play important roles in the pathogenesis of HIVD, but the specific mechanisms underlying neuronal dysfunction and death are unknown. Activation of cytokine receptors and oxidative stress can induce the production of ceramide from membrane sphingomyelin, and recent findings suggest that ceramide is an important mediator of a form of programmed cell death called apoptosis. We now report that levels of ceramide, sphingomyelin, and hydroxynonenal (HNE) are significantly increased in brain tissues and cerebrospinal fluid of HIVD patients. Exposure of cultured neurons to the neurotoxic HIV proteins gp120 and Tat resulted in increased cellular levels of sphingomyelin, ceramide, and HNE. The ceramide precursor palmitoyl‐CoA sensitized neurons to Tat and gp120 toxicity, whereas an inhibitor of ceramide production reduced Tat and gp120‐induced increases of ceramide and HNE and protected the neurons from Tat and gp120‐induced death. These results suggest that HIV‐1 infection may promote a lipid imbalance in neural cells, resulting in an overproduction of ceramide and consequent cellular dysfunction and death.

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.