Fats, Aβ, Oxidative Stress: Feeding Forward and Backward, Killing Neurons?

Add a new strand to the growing web of relationships among production of the Aβ peptide, regulation of cholesterol and other lipids, oxidative stress, and the death of neurons in Alzheimer's. Mark Mattson and colleagues at the National Institute on Aging in Baltimore, Maryland, believe that Aβ drives an increase in the conversion of sphingomyelin back into its precursor ceramide; this, in turn, increases cholesterol production, oxidative stress (resulting in more Aβ), and culminates in cell death. The researchers reported a series of experiments supporting this cycle of events last week at the annual meeting of the American Association for the Advancement of Science in Seattle, and published them February 17 in the early online edition of PNAS.

Ceramides, which can be produced de novo from serine and palmitoyl CoA, are short-lived intermediates in the production of sphingolipids such as the sphingomyelin component of membranes. Degradation of sphingomyelin is the other main source of ceramides. Though they are transient and not particularly abundant, these lipids are known to regulate various cellular processes ranging from developmental events to apoptosis, and they have been implicated in cell death in stroke and Parkinson's disease. Ceramides also help regulate cellular levels of cholesterol. Alzheimer's researchers have noted that ceramide is significantly increased in patients with mild cognitive impairment, leading to suggestions that this could be a biomarker for incipient AD (see consensus report from the Antecedent Biomarkers Group).

A putative role in Aβ generation has also been assigned to ceramides. Luigi Puglielli, working with Dora Kovacs at Massachusetts General Hospital in Charlestown, has found that increased ceramide levels stabilize and increase levels of BACE in vitro, with resultant increases in Aβ (see ARF related news story).

The new study by Mattson’s team explores changes in ceramide/sphingomyelin and cholesterol metabolism in animal and human tissues with an eye toward correlating these processes with both oxidative stress and Aβ-induced cell death. In an initial set of experiments, first author Roy Cutler and colleagues showed that in aging mice, ceramide and cholesterol levels increase, while sphingomyelin levels decrease, in an AD-vulnerable area (the middle frontal gyrus) but not in cerebellum. These changes correlated with increases in oxidative stress (as measured by increases in 4-hydoxynonenal adducts of lysine and histidines) in the middle frontal gyrus, but not in cerebellum.

In autopsy tissue from seven AD patients, the researchers detected similar changes. Ceramides and cholesterol, as well as oxidative stress, were increased in middle frontal gyrus but not cerebellum in AD patients versus controls. Sphingomyelin was decreased in middle temporal gyrus but not cerebellum. The researchers then examined brain tissue from four separate groups—AD patients with mild, moderate, or severe disease, and controls. This time looking only at membrane fractions from superior frontal cortex tissue, they found that increases in ceramides, and cholesterol, as well as oxidative stress correlated with the severity of disease.

The researchers then tested in cultured hippocampal neurons their hypothesis that Aβ deposition and the resulting oxidative stress could be responsible for these irregularities in lipid metabolism. They found that exposure to Aβ42 led to increases in ceramide, cholesterol, and cholesterol ester levels, along with decreases in sphingomyelin. Similarly, oxidative stress was increased in these neurons, which died within 24 hours of exposure to Aβ. Pretreating neurons with the antioxidant vitamin E reduced ceramide and cholesterol levels, and protected the neurons from Aβ-induced death. Similarly, pretreating the neurons to lower ceramide levels protected them from Aβ.

Mattson and colleagues have previously published evidence linking misregulation of sphingolipids to the death of cells in amyotrophic lateral sclerosis (Cutler et al., 2002), and just this month reported similar processes in HIV dementia (see Haughey et al., 2004). Based on their current findings, they propose a model whereby aging conspires with AD-specific genetic or environmental factors to increase Aβ production. Perturbed cholesterol and sphingolipid regulation, and oxidative stress, can both be causes and results of increased Aβ levels by this model. If this holds up in other studies, pharmacologic agents that affect sphingolipid metabolism could become therapeutic candidates, the authors suggest.—Hakon Heimer

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Comments

  1. This article provides an intriguing link among aging, oxidation, ceramides, and cholesterol. Mark Mattson’s group analyzed the brain content of sphingomyelin and ceramides in young and old animals. They observed a striking >100-fold increase in the levels of C24:0 galactosyl-ceramide in old C57B6 mice compared to young (three-month-old) mice. In contrast, the amount of C24:0 sphingomyelin was less in six-month-old mice than in 24-month-old mice. Other small changes were observed, including a modest increase in C24:0 ceramide and even small increases in free cholesterol. The group went on to show similar changes in Alzheimer's brain and in hippocampal neurons following treatment with Aβ; they also show that treating the neurons with antioxidants or ISP-1 (an inhibitor of sphingomyelin synthesis) prevents the accumulation of these species.

    Ceramides are known to stimulate apoptosis, and inhibition of ceramide production would be expected to be protective. In this context, the moderate increase in C24:0 ceramide could be deleterious for the neuron. Similarly, the modest increase in cholesterol might also be deleterious because increased cholesterol is associated with increased Aβ production. However, a critical question is the site of cholesterol production. Increased cholesterol associated with myelin would be less significant than increases in neuronal cholesterol. Prior studies provide conflicting reports on cholesterol levels in Alzheimer’s disease. Most studies do not find a general disease-related increase in cholesterol, although several studies have noted increases in neuronal cholesterol in AD and increases in plaque-associated cholesterol (1-3).

    The most striking aspect of this manuscript is >100-fold increase in galactosyl-ceramide levels. A quick perusal of the literature suggests that galactosyl-ceramide increases when production of glucosyl-ceramide is inhibited, and that galactosyl-ceramide is protective (4). Thus, the increased production of galactosyl-ceramide might be a protective response to oxidative damage in aging.

    References:

    Heverin M, Bogdanovic N, Lütjohann D, Bayer T, Pikuleva I, Bretillon L, Diczfalusy U, Winblad B, Björkhem I. Changes in the levels of cerebral and extracerebral sterols in the brain of patients with Alzheimer's disease. J Lipid Res. 2004 Jan;45(1):186-93. PubMed.

    Burns MP, Noble WJ, Olm V, Gaynor K, Casey E, LaFrancois J, Wang L, Duff K. Co-localization of cholesterol, apolipoprotein E and fibrillar Abeta in amyloid plaques. Brain Res Mol Brain Res. 2003 Jan 31;110(1):119-25. PubMed.

    Distl R, Meske V, Ohm TG. Tangle-bearing neurons contain more free cholesterol than adjacent tangle-free neurons. Acta Neuropathol. 2001 Jun;101(6):547-54. PubMed.

    Grazide S, Terrisse AD, Lerouge S, Laurent G, Jaffrézou JP. Cytoprotective effect of glucosylceramide synthase inhibition against daunorubicin-induced apoptosis in human leukemic cell lines. J Biol Chem. 2004 Apr 30;279(18):18256-61. PubMed.

    View all comments by Benjamin Wolozin

  2. Three recent articles (refs. 1, 2, and 3) have suggested that sphingolipid metabolism may be involved in the pathogenesis of Alzheimer’s disease (AD). The first one, from Xianlin Han and colleagues (1), has identified a ~threefold increase in ceramide levels in the brain of AD patients, when compared to age-matched controls. In that study, the increase in ceramide levels was accompanied by a concomitant decrease in sulfatide content in the brain, suggesting that ceramide was mainly produced by degradation of these highly abundant glycosylated forms of sphingolipids. The second article (2), from our group, has shown that cell-surface hydrolysis of sphingomyelin (SM) can regulate the rate of amyloid-β peptide (Aβ) generation via the second messenger ceramide. Finally, Sawamura and colleagues (3) have shown that sphingosine biosynthesis in the endoplasmic reticulum can also affect APP processing.

    Ceramide is the backbone of all sphingolipids, including SM and glycosphingolipids (among which are the sulfatides). Glycosphingolipids are important components of myelin membranes and are highly enriched in white matter. SM serves both as structural component for the organization of cell membranes and as substrate for the generation of ceramide. Ceramide, in turn, when produced at the cell surface, acts as second messenger and activates several signaling events.

    A new article published in the February 17 issue of PNAS by Roy Cutler and colleagues in Mark Mattson’s group (4) now extends our information by showing that aging is accompanied by specific (and not generalized) changes in ceramide/sphingolipid metabolism in the brain. The authors first analyzed sphingolipid levels in three- and 25-month-old mice, and found a specific age-related increase in two species of ceramide—the C18:0 and C24:0 isoforms. They also found increased levels of C24:0 galactosyl-ceramide. Galactosyl-ceramide serves as substrate for the biosynthesis of sulfatides and is also the first bio-product of sulfatide hydrolysis. Further hydrolysis of C24:0 galactosyl-ceramide would then liberate C24:0 ceramide. This sequence of events most likely occurs outside the cell in the white matter, is associated with loss of myelin membranes, and can explain the rise of only one isoform of ceramide. However, it remains unclear where the C18:0 isoform comes from.

    After these initial results, Mark Mattson’s group analyzed different areas of AD brains and compared them to age-matched controls. Once again, they found a twofold increase in both ceramide C24:0 and galactosyl-ceramide C24:0. However, this time they also found a specific decrease of C24:0 SM. Therefore, C24:0 ceramide can originate from hydrolysis of both C24:0 sulfatides and C24:0 SM. These changes were only observed in the middle frontal gyrus (with extensive Aβ deposition and neurofibrillary tangles), but not in the cerebellum (with little or no Aβ deposits or tangles). This correlation seems to suggest that the changes are a consequence, rather than a cause, of neurodegeneration. Finally, Cutler et al. found increased lipid oxidation—once again, only in the “AD-susceptible” areas. However, when they analyzed cell membranes, they could not find any changes in SM, sulfatide, or galactosyl-ceramide levels. They only found an increase in ceramide levels, mostly C18:0 and C24:0, which correlated with the severity of the disease. These results provide the first evidence for specific changes in cell-surface ceramide in AD brains. Since the “signaling-active” ceramide is generated in the cell membrane, the above results further strengthen the possibility that this second messenger might be involved in the pathogenesis of AD. Obviously, production of ceramide from sulfatide degradation in myelin sheets may further aggravate the situation.

    The second messenger, ceramide, is mostly produced by receptor-mediated activation of neutral sphingomyelinase (nSMase). However, it has long been known that oxidative stress can also activate nSMase and induce hydrolysis of cell-surface SM. This has been shown only in cell culture; direct evidence in vivo is lacking. Mark Mattson’s group tried to address this issue by analyzing lipid oxidation in AD brains and by treating primary neurons with Aβ42 (as oxidative stressor). Their results indicate that Aβ can alter ceramide/SM metabolism and induce oxidative damage in cell culture, and suggest that such events may play a role in the progression of disease. However, the definitive experiments to test/confirm such a hypothesis remain to be performed.

    Throughout their paper, Cutler et al. mostly focused on ceramide and sphingolipids. However, they also analyzed cholesterol, which is strongly implicated in AD. The authors detected a constant increase in cholesterol levels in brain membranes from AD patients, which correlated with the severity of the disease. They also showed (and this is very puzzling) that Aβ itself can affect cholesterol levels in neuronal cultures, probably in association with oxidative stress, as suggested by increased levels of lipid peroxidation. However, they do not provide evidence as to whether this is due to increased biosynthesis or uptake of cholesterol. Obviously, it will be very interesting to test these possibilities in the near future. In addition to membrane cholesterol, the authors also detected changes in the levels of cholesterol esters, indicating that oxidative damage can induce profound alterations of cholesterol homeostasis and distribution in neurons. Once again, the question is: How? The answer will probably come only once we identify the intracellular determinants (lipids or proteins) that regulate cholesterol metabolism in neurons.

    The results found by Cutler et al. (4) in AD brains are very similar to those the same group already published from patients with amyotrophic lateral sclerosis (5), suggesting that we are only looking at events produced by the degenerative disorder. These may be common events/alterations that occur during neurodegeneration and are induced by loss of neurons and myelin. Obviously, it is still very important to understand whether they affect the progression of the disease.

    References:

    Han X, M Holtzman D, McKeel DW, Kelley J, Morris JC. Substantial sulfatide deficiency and ceramide elevation in very early Alzheimer's disease: potential role in disease pathogenesis. J Neurochem. 2002 Aug;82(4):809-18. PubMed.

    Puglielli L, Ellis BC, Saunders AJ, Kovacs DM. Ceramide stabilizes beta-site amyloid precursor protein-cleaving enzyme 1 and promotes amyloid beta-peptide biogenesis. J Biol Chem. 2003 May 30;278(22):19777-83. PubMed. RETRACTED

    Sawamura N, Ko M, Yu W, Zou K, Hanada K, Suzuki T, Gong JS, Yanagisawa K, Michikawa M. Modulation of amyloid precursor protein cleavage by cellular sphingolipids. J Biol Chem. 2004 Mar 19;279(12):11984-91. PubMed.

    Cutler RG, Kelly J, Storie K, Pedersen WA, Tammara A, Hatanpaa K, Troncoso JC, Mattson MP. Involvement of oxidative stress-induced abnormalities in ceramide and cholesterol metabolism in brain aging and Alzheimer's disease. Proc Natl Acad Sci U S A. 2004 Feb 17;101(7):2070-5. PubMed.

    Cutler RG, Pedersen WA, Camandola S, Rothstein JD, Mattson MP. Evidence that accumulation of ceramides and cholesterol esters mediates oxidative stress-induced death of motor neurons in amyotrophic lateral sclerosis. Ann Neurol. 2002 Oct;52(4):448-57. PubMed.

    View all comments by Luigi Puglielli

  3. Ceramide, Cholesterol, and Oxidative Stress
    In a very exciting study, Cutler and colleagues provide insight to link membrane-associated oxidative stress and neuronal degeneration in aging and Alzheimer disease (AD). Many lines of evidence suggest that oxidative stress plays a pivotal role in the pathogenesis of AD, and various views have been proposed (Mattson, 2002; Smith et al., 2002; Butterfield, 2003). Also, as suggested by the involvement of apolipoprotein E in AD and epidemiological reports showing that treatment to lower blood cholesterol level reduces the risk of AD (Wolozin et al., 2000), cholesterol metabolism appears to play a role in the pathophysiology of AD. Although oxidative modification of lipid has been extensively studied, a mechanism to explain the link between oxidative stress and cholesterol metabolism remains largely unknown. In this regard, Cutler and colleagues demonstrate that aging itself results in increases in the amount of ceramide and related compounds. Furthermore, ceramide and related lipids accumulate in brain regions vulnerable to AD. This observation may provide an important clue to answer the question of why age-associated neuronal loss is inevitable in the senescent brain. Since the profile of various lipids extracted from AD brains is similar to that from control, neurotoxicity of amyloid-β (Aβ) might be related to the same pathways as neuronal aging.

    If this is the case, how can we cope with this problem? Since ceramide would play an important role for neuronal function, manipulation of oxidative stress would seem to be a safer option. With this in mind, the authors demonstrate that α-tocopherol is effective in suppressing the increase of ceramide and HNE induced by Aβ without changing the level of sphingomyelin. While the effect of α-tocopherol on reducing the amount of ceramide is smaller than that of an inhibitor of sphingomyelin synthesis, α-tocopherol is superior in suppressing the accumulation of HNE. This study, coupled with recent reports showing the protective role of antioxidant vitamins against AD (Zandi et al., 2004), bolsters the notion that neurotoxicity induced by Aβ and oxidative stress is a fundamental determinant in senescence and AD pathophysiology.

    References:

    Butterfield DA. Amyloid beta-peptide [1-42]-associated free radical-induced oxidative stress and neurodegeneration in Alzheimer's disease brain: mechanisms and consequences. Curr Med Chem. 2003 Dec;10(24):2651-9. PubMed.

    Cutler RG, Kelly J, Storie K, Pedersen WA, Tammara A, Hatanpaa K, Troncoso JC, Mattson MP. Involvement of oxidative stress-induced abnormalities in ceramide and cholesterol metabolism in brain aging and Alzheimer's disease. Proc Natl Acad Sci U S A. 2004 Feb 17;101(7):2070-5. PubMed.

    Mattson MP. Oxidative stress, perturbed calcium homeostasis, and immune dysfunction in Alzheimer's disease. J Neurovirol. 2002 Dec;8(6):539-50. PubMed.

    Smith MA, Casadesus G, Joseph JA, Perry G. Amyloid-beta and tau serve antioxidant functions in the aging and Alzheimer brain. Free Radic Biol Med. 2002 Nov 1;33(9):1194-9. PubMed.

    Wolozin B, Kellman W, Ruosseau P, Celesia GG, Siegel G. Decreased prevalence of Alzheimer disease associated with 3-hydroxy-3-methyglutaryl coenzyme A reductase inhibitors. Arch Neurol. 2000 Oct;57(10):1439-43. PubMed.

    Zandi PP, Anthony JC, Khachaturian AS, Stone SV, Gustafson D, Tschanz JT, Norton MC, Welsh-Bohmer KA, Breitner JC, . Reduced risk of Alzheimer disease in users of antioxidant vitamin supplements: the Cache County Study. Arch Neurol. 2004 Jan;61(1):82-8. PubMed.

    View all comments by Kazuhiro Honda

  4. Lipids May Play an Important Role in AD Pathogenesis
    It has long been recognized that lipid alterations play an essential role in many major diseases such as diabetes, obesity, atherosclerosis, and myocardial dysfunction (see Stanley, 2001; Unger & Orci, 2001; Unger, 2002 for recent reviews). Very recently, we have found that lipid alterations are a very early event in the pathogenesis of Alzheimer’s disease (Han et al., 2001; 2002; 2003; Cheng et al., 2003), which can affect APP processing (Puglielli et al., 2003; Sawamura et al., 2004). The newly published article by Cutler and colleagues in Mark Mattson’s group (Cutler et al., 2004) further supports the notion that lipids may play an important role in AD pathogenesis.

    By using a lipidomics approach (Han and Gross, 2003), we have demonstrated changes in three types of specialized lipids in both postmortem brain tissues and lumbar cerebrospinal fluid (CSF) of AD patients at the earliest clinical stage of AD. These lipids include plasmalogen (Han et al., 2001), sulfatide (Han et al., 2002; 2003; Cheng et al., 2003), and ceramide (Han et al., 2002). Plasmalogen is a class of lipids that is enriched in both gray and white matter of the brain and protects cells from the effects of oxidation (Zoeller et al., 1988; Murphy, 2001). Substantial loss of plasmalogen content in brains of AD subjects and in brain tissues of AD animal models suggests that increased oxidative damage is present in brains of AD subjects and the accumulation of amyloid-β peptide (Aβ) may cause oxidation stress, resulting in the plasmalogen deficiency (Han et al., 2001; Cheng et al., 2003). Thus, alterations in plasmalogen content can serve as an indicator of oxidative stress. Therefore, it would be of interest if the study conducted by Cutler and colleagues could examine the alterations in plasmalogen content in cultured neuronal cells and animal brain tissues to support their hypothesis regarding the membrane-associated oxidative stress.

    Previous studies have demonstrated that substantial sulfatide depletion was present in both gray matter and white matter of brains from AD subjects in comparison to cognitively normal controls (Svennerholm and Gottfries, 1994; Han et al., 2002; Cheng et al., 2003). Moreover, we demonstrated that ceramide content in gray and white matter of AD brains was quite different, and the mass content of ceramides was highest at the very mild stage of AD in comparison to other AD stages. In contrast to these previous studies, Cutler and colleagues found that the mass content of sulfatides in isolated membrane rafts was not significantly different at different AD stages, whereas the ceramide content in the rafts increased with AD severity. Furthermore, it seems that the ceramide content in their raft preparation is unusually high relative to the mass contents of sulfatides and cerebrosides. Since markedly different sulfatide levels are present in white and gray matter, it is important to recognize that sampling different brain regions leads to observed differences in the overall mass of sulfatides, the percentage change with disease, and the intrinsic statistical evaluations of significance. Thus, selection and reproducibility of sampled regions is essential to observing differences in neuronal diseases in which regionally specific alterations in lipid metabolism likely occur. We found that the presence of cross contamination between gray and white matter could be determined by profiling ethanolamine glycerophospholipid molecular species (Han et al., 2001).

    It is well-known that ceramides act as second messengers and activate several signaling events. Accumulation of excessive ceramide has diverse biological consequences, including upregulation of cytokines, generation of reactive oxygen species, interruption of the mitochondrial respiratory chain, and apoptosis. Both Cutler et al. (2004) and Lee et al. (2004) have recently demonstrated that treatment of cultured cells (either primary neurons or oligodendrocytes, respectively) with Aβ-activated neutral sphingomyelinase (nSMase) and increased sphingomyelin hydrolysis, which led to the accumulation of ceramides. However, ceramide production could affect the production rate of Aβ (Puglielli et al., 2003) as well as APP processing (Sawamra et al., 2004). It appears that the generation of both Aβ and ceramides should be balanced under normal physiological conditions. Disruption of this balance will result in multiple downstream reactions, ultimately leading to cell death. As discussed by Cutler and colleagues, altered lipid metabolism likely represents one of the key elements leading to neuronal dysfunction and cell death in AD.

    See also:

    Stanley, J. (2001) Lipoproteins. 3. Why do lipoproteins promote atherosclerosis? Lipid Technol. 13, 89-90.

    References:

    Cheng H, Xu J, McKeel DW, Han X. Specificity and potential mechanism of sulfatide deficiency in Alzheimer's disease: an electrospray ionization mass spectrometric study. Cell Mol Biol (Noisy-le-grand). 2003 Jul;49(5):809-18. PubMed.

    Cutler RG, Kelly J, Storie K, Pedersen WA, Tammara A, Hatanpaa K, Troncoso JC, Mattson MP. Involvement of oxidative stress-induced abnormalities in ceramide and cholesterol metabolism in brain aging and Alzheimer's disease. Proc Natl Acad Sci U S A. 2004 Feb 17;101(7):2070-5. PubMed.

    Han X, Fagan AM, Cheng H, Morris JC, Xiong C, Holtzman DM. Cerebrospinal fluid sulfatide is decreased in subjects with incipient dementia. Ann Neurol. 2003 Jul;54(1):115-9. PubMed.

    Han X, Gross RW. Global analyses of cellular lipidomes directly from crude extracts of biological samples by ESI mass spectrometry: a bridge to lipidomics. J Lipid Res. 2003 Jun;44(6):1071-9. PubMed.

    Han X, Holtzman DM, McKeel DW. Plasmalogen deficiency in early Alzheimer's disease subjects and in animal models: molecular characterization using electrospray ionization mass spectrometry. J Neurochem. 2001 May;77(4):1168-80. PubMed.

    Han X, M Holtzman D, McKeel DW, Kelley J, Morris JC. Substantial sulfatide deficiency and ceramide elevation in very early Alzheimer's disease: potential role in disease pathogenesis. J Neurochem. 2002 Aug;82(4):809-18. PubMed.

    Lee JT, Xu J, Lee JM, Ku G, Han X, Yang DI, Chen S, Hsu CY. Amyloid-beta peptide induces oligodendrocyte death by activating the neutral sphingomyelinase-ceramide pathway. J Cell Biol. 2004 Jan 5;164(1):123-31. PubMed.

    Murphy RC. Free-radical-induced oxidation of arachidonoyl plasmalogen phospholipids: antioxidant mechanism and precursor pathway for bioactive eicosanoids. Chem Res Toxicol. 2001 May;14(5):463-72. PubMed.

    Puglielli L, Ellis BC, Saunders AJ, Kovacs DM. Ceramide stabilizes beta-site amyloid precursor protein-cleaving enzyme 1 and promotes amyloid beta-peptide biogenesis. J Biol Chem. 2003 May 30;278(22):19777-83. PubMed. RETRACTED

    Sawamura N, Ko M, Yu W, Zou K, Hanada K, Suzuki T, Gong JS, Yanagisawa K, Michikawa M. Modulation of amyloid precursor protein cleavage by cellular sphingolipids. J Biol Chem. 2004 Mar 19;279(12):11984-91. PubMed.

    Svennerholm L, Gottfries CG. Membrane lipids, selectively diminished in Alzheimer brains, suggest synapse loss as a primary event in early-onset form (type I) and demyelination in late-onset form (type II). J Neurochem. 1994 Mar;62(3):1039-47. PubMed.

    Unger RH. Lipotoxic diseases. Annu Rev Med. 2002;53:319-36. PubMed.

    Unger RH, Orci L. Diseases of liporegulation: new perspective on obesity and related disorders. FASEB J. 2001 Feb;15(2):312-21. PubMed.

    Zoeller RA, Morand OH, Raetz CR. A possible role for plasmalogens in protecting animal cells against photosensitized killing. J Biol Chem. 1988 Aug 15;263(23):11590-6. PubMed.

    View all comments by Xianlin Han

  5. Changes in brain lipid composition have been determined in 24 month-old Fischer rats compared with 6 months-old ones. The cerebral levels of sphingomyelin and cholesterol were found to be significantly increased in aged rats, whereas the amount of phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, phosphatidylinositol, and phosphatidic acid appear to be unaffected by aging. Long-term feeding with acetyl-L-carnitine was able to reduce the age-dependent increase of both sphingomyelin and cholesterol cerebral levels with no effect on the other measured phospholipids. These findings shown that changes in membrane lipid metabolism and/or composition represent one of the alterations occurring in rat brain with aging, and that long-term feeding with acetyl-L-carnitine can be useful in normalizing these age-dependent disturbances.

    References:

    Aureli T, Di Cocco ME, Capuani G, Ricciolini R, Manetti C, Miccheli A, Conti F. Effect of long-term feeding with acetyl-L-carnitine on the age-related changes in rat brain lipid composition: a study by 31P NMR spectroscopy. Neurochem Res. 2000 Mar;25(3):395-9. PubMed.

    View all comments by Timothy Tiedemann
    • User Profile Image Yusuf Hannun
      Medical University of South Carolina
    • Posted:

    The study by Cutler et al. reports on increases in the levels of the lipids ceramide and cholesterol in the aging brain. These changes are further exacerbated in Alzheimer’s disease (AD) brains, which also show decreased levels of sphingomyelin (a precursor/product to ceramide). Notably, these changes are accompanied by increased lipid oxidation products, and they were manifest in the middle frontal gyrus (an area highly affected in AD) but not in cerebellum (which demonstrated modest disease changes). Moreover, the increases in ceramide correlated with the severity of the disease.

    Given the known roles of ceramide in stress responses and in mediating cytotoxic responses, the authors then exposed hippocampal neurons (in primary culture) to amyloid-β peptide (Aβ), and found that Aβ induced oxidation of membrane lipids and accumulation of ceramide and cholesterol. The authors then implicate the accumulated ceramide in mediating the toxicity of Aβ by demonstrating that an inhibitor of ceramide synthesis (ISP-1/myriocin) inhibited cell death. Using inhibitors of oxidation (a-tocopherol) and inducers of oxidative stress (HNE), they order the cytotoxic pathway such that Aβ incites an oxidative stress followed by ceramide accumulation, which then mediates cytotoxicity.

    Another recent study on the role of ceramide in mediating oligodendrocyte death in response to Aβ reached similar conclusions and provided further evidence for activation of neutral sphingomyelinase as the key step responsive to the action of Aβ and the attendant oxidative stress (Lee et al., 2004). A recent in-vivo study demonstrated that intracerebral injection of Aβ induced lipid peroxidation, which was responsible for activation of neutral sphingomyelinase and a significant and prolonged increase in the levels of ceramide (Alessenko et al., 2004).

    Together, these results provide significant evidence for the operation of an oxidation/ceramide-based mechanism of stress response in aging brain and AD. For investigators studying AD, this mechanism begins to bring together previously known correlates and players invoked in the pathogenesis of this disorder, including the cytotoxic actions of Aβ, the increased oxidative stress and load of oxidative injury in aging and AD brain, the previously noted increases in ceramide levels in AD brain (e.g., Han et al., 2002), the established roles of sphingolipids in mediating and regulating stress responses, and the noted roles of ceramide in neuronal apoptosis (e.g., Puranam et al., 1999).

    What is most remarkable and gratifying to those of us who have been studying bioactive lipids are the current rewards in translating basic research in this field to establishing the roles of these molecules in human disease. Indeed, this study illustrates how the development of basic research now allows significant and likely predictions. In the case of ceramide, multiple studies had established its roles and participation in various apoptotic responses and the ability of inhibitors of ceramide formation to ameliorate cytotoxic responses. Mechanistic studies had disclosed that oxidative stimuli indirectly activate neutral sphingomyelinase leading to the generation of ceramide; this prompted the proposal that ceramide provides a molecular link in transducing cell responses to oxidative stress.

    Obviously, more mechanistic insight is required. For example, how does oxidation activate sphingomyelinase? How does ceramide transduce cytotoxic effects of stimuli such as Aβ? At the same time, it is now clear that pursuing the oxidation/sphingolipid connection promises novel insight as well as the possibilities of specific therapeutics for AD, for example, inhibitors of neutral sphingomyelinase.

    References:

    Lee JT, Xu J, Lee JM, Ku G, Han X, Yang DI, Chen S, Hsu CY. Amyloid-beta peptide induces oligodendrocyte death by activating the neutral sphingomyelinase-ceramide pathway. J Cell Biol. 2004 Jan 5;164(1):123-31. PubMed.

    Alessenko AV, Bugrova AE, Dudnik LB. Connection of lipid peroxide oxidation with the sphingomyelin pathway in the development of Alzheimer's disease. Biochem Soc Trans. 2004 Feb;32(Pt 1):144-6. PubMed.

    Han X, M Holtzman D, McKeel DW, Kelley J, Morris JC. Substantial sulfatide deficiency and ceramide elevation in very early Alzheimer's disease: potential role in disease pathogenesis. J Neurochem. 2002 Aug;82(4):809-18. PubMed.

    Puranam KL, Guo WX, Qian WH, Nikbakht K, Boustany RM. CLN3 defines a novel antiapoptotic pathway operative in neurodegeneration and mediated by ceramide. Mol Genet Metab. 1999 Apr;66(4):294-308. PubMed.

    View all comments by Yusuf Hannun

  7. Fats, Amyloid-β, Oxidative Stress: Restoring Neuronal Function?

    We read with interest this article, news summary, and the responses by knowledgeable commentators. Preliminary and related findings were presented by authors at the Society for Neuroscience Annual Meeting 2002 (Abstract
    #884.10    ) and 2003 (Abstract
    #406.7    ).

    We are very glad that authors experimentally added to our findings that there are complex biochemical relations among Aβ, cholesterol, phospholipids, oxidative stress, and neuronal function. A few years ago, we were the first to show that Aβ modulates neural lipid synthesis (in PC12 cells, in primary cerebral cell cultures, in utero in rat fetuses, and ex vivo in hippocampal and cortical slices), and that oxidative stress reverses the stimulatory effect of the peptide. We explain such interrelation in functional,
    not pathological terms. We also are intrigued by the PNAS article proposal of "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."

    Alzheimer's changes in neurochemistry of Aβ and oxidative stress represent physiological transitory mechanisms aiming to compensate impaired dynamics of cholesterol and other lipid components of neural membranes that we believe are primary causes of neurotransmission and synaptic plasticity failure in Alzheimer's patients.

    Therefore, in contrast to the reasoning of Cutler et al., we earlier proposed that Aβ and oxidative stress modulation are integrated compensatory factors that help cure the neural plasticity impaired by lipid metabolism break. This is because the oxidative stress cascade affects membrane fluidity and itself is critical for synaptic function and plasticity (as shown by others) and because
    Aβ may improve synaptic plasticity by modulating neural cholesterol dynamics.

    Moreover, a further link could be drawn based on the observation of antioxidant properties of Aβ under physiological conditions. Thus, the slow-onset component of the long-term potentiation (LTP) can be pharmacologically induced by Aα-tocopherol (vitamin E) and Aβ; it is impaired in the transgenic mice overexpressing enzyme Cu/Zn superoxide dismutase (SOD-1), and may be attributed to the lipid antioxidant modulation by vitamin E or Aβ, and
    dependency of slow LTP component on a unique molecular mechanism.

    In summary, we appreciate this fascinating report. We would like to encourage others to join the research linking lipids, oxidative stress, and neurodegeneration.

    References:
    Koudinov AR, Koudinova NV. Amyloid beta protein restores hippocampal long term potentiation: A central role for cholesterol? Neurobiol Lipids, 15 Sept 2003. 1, 8. Full text.

    View all comments by Alexei Koudinov

  8. Acetyl-L-Carnitine has shown some efficacy in treating early onset AD in some small, short term trials on several parameters of memory and in one as an adjuvant therapy in combination with acetylcholinesterase inhibitors.

    References:

    Bianchetti A, Rozzini R, Trabucchi M. Effects of acetyl-L-carnitine in Alzheimer's disease patients unresponsive to acetylcholinesterase inhibitors. Curr Med Res Opin. 2003;19(4):350-3. PubMed.

    Montgomery SA, Thal LJ, Amrein R. Meta-analysis of double blind randomized controlled clinical trials of acetyl-L-carnitine versus placebo in the treatment of mild cognitive impairment and mild Alzheimer's disease. Int Clin Psychopharmacol. 2003 Mar;18(2):61-71. PubMed.

    Brooks JO, Yesavage JA, Carta A, Bravi D. Acetyl L-carnitine slows decline in younger patients with Alzheimer's disease: a reanalysis of a double-blind, placebo-controlled study using the trilinear approach. Int Psychogeriatr. 1998 Jun;10(2):193-203. PubMed.

    Bigini P, Larini S, Pasquali C, Muzio V, Mennini T. Acetyl-L-carnitine shows neuroprotective and neurotrophic activity in primary culture of rat embryo motoneurons. Neurosci Lett. 2002 Sep 6;329(3):334-8. PubMed.

    View all comments by Timothy Tiedemann
    • User Profile Image Jacob Mack
      bio-chemistry-psychology/neuroscience graduate.
    • Posted:

    Superb paper! I think linking this research with receptor-dependent and -independent pathways (CB1 receptors) (arachidonic acid, anadamide) will present an insightful link between lipids, cholesterol, various esters, and signal transduction via neuronal cell membranes. It might also elucidate mechanisms of AD pathogenesis and new treatment options, for example, cannabinoid manipulation of lipid levels or modulation of intracellular cholesterol production through nanogene therapy. Great insights in this paper.

    View all comments by Jacob Mack

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References

News Citations

  1. Ceramide Leads to Higher BACE Levels

Paper Citations

  1. Cutler RG, Pedersen WA, Camandola S, Rothstein JD, Mattson MP. Evidence that accumulation of ceramides and cholesterol esters mediates oxidative stress-induced death of motor neurons in amyotrophic lateral sclerosis. Ann Neurol. 2002 Oct;52(4):448-57. PubMed.
  2. Haughey NJ, Cutler RG, Tamara A, McArthur JC, Vargas DL, Pardo CA, Turchan J, Nath A, Mattson MP. Perturbation of sphingolipid metabolism and ceramide production in HIV-dementia. Ann Neurol. 2004 Feb;55(2):257-67. PubMed.

Other Citations

  1. Antecedent Biomarkers Group

External Citations

  1. Lipid Library

Further Reading

Primary Papers

  1. Cutler RG, Kelly J, Storie K, Pedersen WA, Tammara A, Hatanpaa K, Troncoso JC, Mattson MP. Involvement of oxidative stress-induced abnormalities in ceramide and cholesterol metabolism in brain aging and Alzheimer's disease. Proc Natl Acad Sci U S A. 2004 Feb 17;101(7):2070-5. PubMed.

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