Alzheimer's disease (AD) is an age-related disorder characterized by deposition of amyloid ␤-peptide (A␤) and degeneration of neurons in brain regions such as the hippocampus, resulting in progressive cognitive dysfunction. The pathogenesis of AD is tightly linked to A␤ 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 A␤ induces membrane oxidative stress and the accumulation of ceramide species and cholesterol. Treatment of neurons with ␣-tocopherol or an inhibitor of sphingomyelin synthesis prevents accumulation of ceramides and cholesterol and protects them against death induced by A␤. Our findings suggest a sequence of events in the pathogenesis of AD in which A␤ induces membrane-associated oxidative stress, resulting in perturbed ceramide and cholesterol metabolism which, in turn, triggers a neurodegenerative cascade that leads to clinical disease.amyloid ͉ apoptosis ͉ hippocampus ͉ lipid peroxidation ͉ sphingomyelin C hanges that occur in the brain during aging increase the risk of Alzheimer's disease (AD), a disorder involving the progressive deposition of amyloid ␤-peptide (A␤) and associated degeneration of neurons in brain regions involved in learning and memory (1). Two factors that are believed to contribute to neuronal dysfunction and degeneration in AD are increased oxidative stress and increased production of neurotoxic forms of A␤ (2). Alterations in lipid metabolism may also play roles in AD because the risk of AD is affected by inheritance of different isoforms of apolipoprotein E (3), changes in cholesterol metabolism can affect A␤ production in cell culture and in vivo (4-6), and drugs that lower cholesterol levels may reduce the risk of AD (7,8). However, a direct link between alterations in the metabolism of cholesterol and other membrane lipids in AD has not been established, and it is not known whether and how such lipid alterations might lead to neuronal dysfunction and death.Membrane microdomains that are rich in cholesterol and sphingolipids play important roles in various cellular signaling pathways (9, 10). Sphingomyelin is a major source of ceramides, lipid mediators that are generated when sphingomyelin is cleaved by sphingomyelinases, enzymes activated by inflammatory cytokines (11) and oxidative stress (12). Ceramides play important roles in regulating an array of physiological processes, including cell proliferation and differentiation, and a form of programmed cell death called apoptosis (13). Ceramides have been implicated in the pathological death of neurons that occurs in ischemic stroke (14) and Parkinson's disease (15). In the present study, we document significant increases in levels of m...

Recent epidemiological and clinical data suggest that persons with low folic acid levels and elevated homocysteine levels are at increased risk of Alzheimer's 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 beta-peptide (Abeta) 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 Abeta. Methyl donor deficiency caused uracil misincorporation and DNA damage and greatly potentiated Abeta toxicity as the result of reduced repair of Abeta-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 Abeta 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 Abeta.

Amyotrophic lateral sclerosis (ALS) is characterized by degeneration of motor neurons in the spinal cord resulting in progressive paralysis and death. The pathogenic mechanism of ALS is unknown but may involve increased oxidative stress, overactivation of glutamate receptors, and apoptosis. We report abnormalities in sphingolipid and cholesterol metabolism in the spinal cords of ALS patients and in a transgenic mouse model (Cu/ZnSOD mutant mice), which manifest increased levels of sphingomyelin, ceramides, and cholesterol esters; in the Cu/ZnSOD mutant mice, these abnormalities precede the clinical phenotype. In ALS patients and Cu/Zn-SOD mutant mice, increased oxidative stress occurs in association with the lipid alterations, and exposure of cultured motor neurons to oxidative stress increases the accumulation of sphingomyelin, ceramides, and cholesterol esters. Pharmacological inhibition of sphingolipid synthesis prevents accumulation of ceramides, sphingomyelin, and cholesterol esters and protects motor neurons against death induced by oxidative and excitotoxic insults. These findings suggest a pivotal role for altered sphingolipid metabolism in the pathogenesis of ALS.

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We report increased modification of proteins by 4-hydroxynonenal (HNE), a product of membrane lipid peroxidation, in the lumbar spinal cord of sporadic amyotrophic lateral sclerosis (ALS) patients versus that of neurologically normal controls. By immunohistochemistry, HNE-protein modification was detected in ventral horn motor neurons, and immunoprecipitation analysis revealed that one of the proteins modified by HNE was the astrocytic glutamate transporter EAAT2. Given that the function of proteins modified by HNE can be severely compromised as previously demonstrated for glutamate transporters in cortical synaptosome preparations, our findings suggest a scenario in which oxidative stress leads to the production of HNE, impairment of glutamate transport, and excitotoxic motor neuron degeneration in ALS.

Synaptic degeneration and death of nerve cells are defining features of Alzheimer's disease (AD) and Parkinson's 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 there-by 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.

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Many cases of autosomal dominant inherited forms of early‐onset Alzheimer's disease are caused by mutations in the genes encoding presenilin‐1 (PS‐1; chromosome 14) and presenilin‐2 (PS‐2; chromosome 1). PSs are expressed in neurons throughout the brain wherein they appear to be localized primarily to the endoplasmic reticulum (ER) of cell bodies and dendrities. PS‐1 and PS‐2 show high homology and are predicted to have eight transmembrane domains with the C terminus, N terminus, and a loop domain all on the cytosolic side of the membrane; an enzymatic cleavage of PSs occurs at a site near the loop domain. The normal function of PSs is unknown, but data suggest roles in membrane trafficking, amyloid precursor protein processing, and regulation of ER calcium homeostasis. Homology of PSs to the C. elegans gene sel‐12, which is involved in Notch signaling, and phenotypic similarities of PS‐1 and Notch knockout mice suggest a developmental role for PSs in the nervous system. When expressed in cultured cells and transgenic mice, mutant PSs promote increased production of a long form of amyloid β‐peptide (Aβ1‐42) that may possess enhanced amyloidogenic and neurotoxic properties. PS mutations sensitize cultured neural cells to apoptosis induced by trophic factor withdrawal, metabolic insults, and amyloid β‐peptide. The mechanism responsible for the proapoptotic action of mutant PSs may involve perturbed calcium release from ER stores and increased levels of oxidative stress. Recent studies of apoptosis in many different cell types suggest that ER calcium signaling can modulate apoptosis. The evolving picture of PS roles in neuronal plasticity and Alzheimer's disease is bringing to the forefront the ER, an organelle increasingly recognized as a key regulator of neuronal plasticity and survival.