Diagnostic use of cerebral and extracerebral oxysterols

Toward biomarkers in multiple sclerosis: new advances

Multiple sclerosis is an autoimmune disease that commonly affects young adults. If initially characterized by acute relapses, it is later followed by only incomplete remission. Over years, progressive disability and irreversible deficit lead to chronic neurological deficits in the majority of patients. The clinical course is protracted and unpredictable, and no biological marker is useful in predicting the evolution of autoaggression and disability. It is difficult to diagnose and to monitor disease progression after the initial symptoms or even during the major clinical manifestations, and it is difficult to treat. In this review, the authors report recent advances in the field, focusing on the search of new antigens as a marker of the disease, in their relevance to the pathophysiology and diagnosis of the disease.

7α-hydroxy-3-oxo-4-cholestenoic acid in cerebrospinal fluid reflects the integrity of the blood-brain barrier

There is a continuous flux of the oxysterol 27-hydroxycholesterol (27-OHC) from the circulation across the blood-brain barrier (BBB) into the brain. The major metabolite of 27-OHC in the brain is 7α-hydroxy-3-oxo-4-cholestenoic acid (7-HOCA). We confirm a recent report describing the presence of this metabolite in cerebrospinal fluid (CSF) at a relatively high concentration. A simple and accurate method was developed for assay of 7-HOCA in CSF based on isotope dilution-mass spectrometry and use of (2)H4-labeled internal standard. The concentration of this metabolite was found to be markedly increased in CSF from patients with a dysfunctional BBB. There was a high correlation between the levels of 7-HOCA in CSF and the CSF/serum albumin ratio. The concentration of 7-HOCA in CSF was not significantly affected by neurodegeneration. Our findings suggest that 7-HOCA could be used as a diagnostic marker for conditions with a dysfunctional BBB.

Brain cholesterol metabolism, oxysterols, and dementia

Cholesterol metabolism is implicated in the etiology of Alzheimer's disease (AD) and amyloid production in the brain. While brain cholesterol cannot be measured directly in vivo, the oxysterol, 24S-hydroxycholesterol (24-OHC), is the predominant metabolite of brain cholesterol and can be measured in the blood. The aim of this review is to evaluate plasma 24-OHC as a potential biomarker of AD risk and discuss factors related to its levels in the brain and blood. This systematic review examines studies published between 1950 and June 2012 that examined the relationship between plasma 24-OHC, cognition, brain structure, and dementia using the following key words ("24S-hydroxycholesterol" or "24-hydroxycholesterol") and ("Brain" or "Cognitive"). We found a total of 28 studies of plasma 24-OHC and neurodegenerative disease, including a subset of 12 that used dementia as a clinical endpoint. These studies vary in the direction of the observed associations. Results suggest plasma 24-OHC may be higher in the early stages of cognitive impairment and lower in more advanced stages of AD when compared to cognitively normal controls. Measures of 24-OHC in the blood may be an important potential marker for cholesterol metabolism in the brain and risk of AD. Further studies of plasma 24-OHC and dementia must account for the stage of disease, establish the temporal trends in oxysterol concentrations, and employ neuroimaging modalities to assess the structural and metabolic changes occurring in the brain prior to the onset of cognitive impairment.

Oxysterols and cholesterol precursors correlate to magnetic resonance imaging measures of neurodegeneration in multiple sclerosis

BACKGROUND

Cholesterol homeostasis is important for formation and maintenance of myelin and axonal membranes in the central nervous system (CNS). The concentrations of the brain specific cholesterol metabolite 24S-hydroxycholesterol (24OHC) and cholesterol precursors have been shown to be altered in multiple sclerosis (MS). However, how changes in sterol levels relate to the pathological processes in MS is not clear.

METHODS

In this study, we compared serum and cerebrospinal fluid (CSF) sterol levels between 105 MS (51 relapsing-remitting (RR); 39 secondary progressive (SP) and 15 primary progressive (PP)) and 49 control patients. Sterol levels were correlated to magnetic resonance imaging (MRI) markers of disease activity.

RESULTS

We found decreased serum 24OHC and 27-hydroxycholesterol (27OHC) and increased CSF lathosterol in MS patients compared to control patients (p=0.018, p=0.002 and p=0.002, respectively). Subgroup analysis showed that serum 24OHC levels were negatively correlated to normalized brain volume measurements in relapse-onset MS patients (r= -0.326, p=0.004).

CONCLUSIONS

These results confirm that cholesterol homeostasis is disturbed in MS and suggest that changes in cholesterol synthesis are related to neurodegenerative pathological processes as seen on the MRI. The data seem to be in line with the recently reported observation that high dose statins may have a positive effect on clinical disability in secondary progressive MS.

Lipid anti-lipid antibody responses correlate with disease activity in systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is a chronic autoimmune disorder characterized by broad clinical manifestations including cardiovascular and renal complications with periodic disease flares and significant morbidity and mortality. One of the main contributing factors to the pathology of SLE is the accumulation and impaired clearance of immune complexes of which the principle components are host auto-antigens and antibodies. The contribution of host lipids to the formation of these autoimmune complexes remains poorly defined. The aim of the present study was to identify and analyze candidate lipid autoantigens and their corresponding anti-lipid antibody responses in a well-defined SLE patient cohort using a combination of immunological and biophysical techniques. Disease monitoring in the SLE cohort was undertaken with serial British Isles Lupus Assessment Group (BILAG) scoring. Correlations between specific lipid/anti-lipid responses were investigated as disease activity developed from active flares to quiescent during a follow up period. We report a significant negative correlation between anti-lipid antibodies for 24S-hydroxycholesterol, cardiolipin and phosphatidylserine with SLE disease activity. Taken together, these data suggest that lipid autoantigens represent a new family of biomarkers that can be employed to monitor disease activity plus the efficacy of therapeutic intervention in SLE.

Methods for oxysterol analysis: past, present and future

Oxysterols are oxidised forms of cholesterol or its precursors. In this article we will concentrate specifically on those formed in mammalian systems. Oxidation may be catalysed by endogenous enzymes or through reactive oxygen species forming a myriad of potential products. A number of these products are biologically active, and oxysterols may have roles in cholesterol homeostasis, neurogenesis, protein prenylation and in the immune system. Oxysterols are also implicated in aetiology of disease states including atherosclerosis, neurodegenerative and inflammatory diseases. Reports indicating the levels of oxysterols in plasma, cerebrospinal fluid and various tissues are in many cases unrealistic owing to a lack of attention to the possibility of autoxidation, a process by which oxysterols are formed from cholesterol by oxygen in air. This article comprises a critical assessment of the technical difficulties of oxysterol analysis, highlights methodologies utilising best practise and discusses newer procedures.

24S-hydroxycholesterol in plasma: a marker of cholesterol turnover in neurodegenerative diseases

Brain cholesterol is mainly involved in the cell membrane structure, in signal transduction, neurotransmitter release, synaptogenesis and membrane trafficking. Impairment of brain cholesterol metabolism was described in neurodegenerative diseases, such as Multiple Sclerosis, Alzheimer and Huntington Diseases. Since the blood-brain barrier efficiently prevents cholesterol uptake from the circulation into the brain, de novo synthesis is responsible for almost all cholesterol present there. Cholesterol is converted into 24S-hydroxycholesterol (24OHC) by cholesterol 24-hydroxylase (CYP46A1) expressed in neural cells. Plasma concentration of 24OHC depends upon the balance between cerebral production and hepatic elimination and is related to the number of metabolically active neurons in the brain. Factors affecting brain cholesterol turnover and liver elimination of oxysterols, together with the metabolism of plasma lipoproteins, genetic background, nutrition and lifestyle habits were found to significantly affect its plasma levels. Either increased or decreased plasma 24OHC concentrations were described in patients with neurodegenerative diseases. A group of evidence suggests that reduced levels of 24OHC are related to the loss of metabolically active cells and the degree of brain atrophy. Inflammation, dysfunction of BBB, increased cholesterol turnover might counteract this tendency resulting in increased levels or, in some cases, in unsignificant changes. The study of plasma 24OHC is likely to offer an insight about brain cholesterol turnover with a limited diagnostic power.

The link between altered cholesterol metabolism and Alzheimer's disease

Alzheimer's disease (AD), the most common form of dementia, is characterized by the progressive loss of neurons and synapses, and by extracellular deposits of amyloid-β (Aβ) as senile plaques, Aβ deposits in the cerebral blood vessels, and intracellular inclusions of hyperphosphorylated tau in the form of neurofibrillary tangles. Several mechanisms contribute to AD development and progression, and increasing epidemiological and molecular evidence suggests a key role of cholesterol in its initiation and progression. Altered cholesterol metabolism and hypercholesterolemia appear to play fundamental roles in amyloid plaque formation and tau hyperphosphorylation. Over the last decade, growing evidence supports the idea that cholesterol oxidation products, known as oxysterols, may be the missing link between altered brain cholesterol metabolism and AD pathogenesis, as their involvement in neurotoxicity, mainly by interacting with Aβ peptides, is reported.

Markers of cholesterol metabolism in the brain show stronger associations with cerebrovascular disease than Alzheimer's disease

Cholesterol metabolism is believed to play a role in the development of Alzheimer's disease (AD). Oxysterol metabolites of cholesterol, 24S-hydroxycholesterol (24-OHC, a brain-derived oxysterol) and 27-hydroxycholesterol (27-OHC, a peripherally derived oxysterol) cross the blood brain barrier and have been associated with AD. We investigated whether oxysterols were associated with markers of cerebrovascular disease prior to the onset of cognitive impairment. Oxysterols were quantified in 105 participants (average age: 80 ± 4 years) from the Pittsburgh Cardiovascular Health Study Cognition Study who remained cognitively normal at blood draw in 2002, had MRI in 1992 and 1998, and annual cognitive assessment for incident AD and mild cognitive impairment made by consensus conference between 1998 and 2010. Higher plasma levels of 24-OHC were associated with age, gender, the presence of high grade white matter hyperintensities, and brain infarcts on prior MRI. Participants with higher plasma 24-OHC and a greater ratio of 24-OHC/27-OHC were also more likely to develop incident cognitive impairment over 8 years of follow-up. Higher levels of 24-OHC suggest increased cholesterol metabolism occurring in the brains of participants with cerebrovascular disease prior to the onset of cognitive impairment. Measurement of oxysterols may provide information about cholesterol metabolism and brain disease over the cognitive impairment process.

Inflammatory-induced hibernation in the fetus: priming of fetal sheep metabolism correlates with developmental brain injury

Prenatal inflammation is considered an important factor contributing to preterm birth and neonatal mortality and morbidity. The impact of prenatal inflammation on fetal bioenergetic status and the correlation of specific metabolites to inflammatory-induced developmental brain injury are unknown. We used a global metabolomics approach to examine plasma metabolites differentially regulated by intrauterine inflammation. Preterm-equivalent sheep fetuses were randomized to i.v. bolus infusion of either saline-vehicle or LPS. Blood samples were collected at baseline 2 h, 6 h and daily up to 10 days for metabolite quantification. Animals were killed at 10 days after LPS injection, and brain injury was assessed by histopathology. We detected both acute and delayed effects of LPS on fetal metabolism, with a long-term down-regulation of fetal energy metabolism. Within the first 3 days after LPS, 121 metabolites were up-regulated or down-regulated. A transient phase (4–6 days), in which metabolite levels recovered to baseline, was followed by a second phase marked by an opposing down-regulation of energy metabolites, increased pO₂ and increased markers of inflammation and ADMA. The characteristics of the metabolite response to LPS in these two phases, defined as 2 h to 2 days and at 6–9 days, respectively, were strongly correlated with white and grey matter volumes at 10 days recovery. Based on these results we propose a novel concept of inflammatory-induced hibernation of the fetus. Inflammatory priming of fetal metabolism correlated with measures of brain injury, suggesting potential for future biomarker research and the identification of therapeutic targets.

Relationship between cholesterol metabolism, ApoE and brain volumes in Alzheimer’s disease

APOE genotype, aging and midlife hypercholesterolemia are well-established risk factors for late-onset Alzheimer’s disease (AD). ApoE and cholesterol are involved in the pathogenesis of AD since they influence amyloid-β accumulation and Tau pathology. APOE ε4 carriers were found to present lower levels of amyloid-β1–42, higher tau and phosphorylated tau and a higher degree of brain atrophy at any disease stage. Presence of ApoE4 shifts the onset of the disease towards a younger age and makes progression faster. Hypercholesterolemia together with other major cardiovascular risk factors were found to be involved in the pathogenesis of AD, but reduced plasma cholesterol levels were described in demented patients. Significant correlations were found between cholesterol precursors lathosterol, lanosterol and 24S-hydroxycholesterol (a putative marker of brain cholesterol turnover) in plasma and brain atrophy as quantified by MRI. It is likely that neurodegeneration affects both brain and whole-body cholesterol metabolism in AD.

Side chain-oxidized oxysterols regulate the brain renin-angiotensin system through a liver X receptor-dependent mechanism

Disturbances in cholesterol metabolism have been associated with hypertension and neurodegenerative disorders. Because cholesterol metabolism in the brain is efficiently separated from plasma cholesterol by the blood-brain barrier (BBB), it is an unsolved paradox how high blood cholesterol can cause an effect in the brain. Here, we discuss the possibility that cholesterol metabolites permeable to the BBB might account for these effects. We show that 27-hydroxycholesterol (27-OH) and 24S-hydroxycholesterol (24S-OH) up-regulate the renin-angiotensin system (RAS) in the brain. Brains of mice on a cholesterol-enriched diet showed up-regulated angiotensin converting enzyme (ACE), angiotensinogen (AGT), and increased JAK/STAT activity. These effects were confirmed in in vitro studies with primary neurons and astrocytes exposed to 27-OH or 24S-OH, and were partially mediated by liver X receptors. In contrast, brain RAS activity was decreased in Cyp27a1-deficient mice, a model exhibiting reduced 27-OH production from cholesterol. Moreover, in humans, normocholesterolemic patients with elevated 27-OH levels, due to a CYP7B1 mutation, had markers of activated RAS in their cerebrospinal fluid. Our results demonstrate that side chain-oxidized oxysterols are modulators of brain RAS. Considering that levels of cholesterol and 27-OH correlate in the circulation and 27-OH can pass the BBB into the brain, we suggest that this cholesterol metabolite could be a link between high plasma cholesterol levels, hypertension, and neurodegeneration.

α-Tocopherol impairs 7-ketocholesterol-induced caspase-3-dependent apoptosis involving GSK-3 activation and Mcl-1 degradation on 158N murine oligodendrocytes

In important and severe neurodegenerative pathologies, 7-ketocholesterol, mainly resulting from cholesterol autoxidation, may contribute to dys- or demyelination processes. On various cell types, 7-ketocholesterol has often been shown to induce a complex mode of cell death by apoptosis associated with phospholipidosis. On 158N murine oligodendrocytes treated with 7-ketocholesterol (20 μg/mL corresponding to 50 μM, 24-48 h), the induction of a mode of cell death by apoptosis characterised by the occurrence of cells with condensed and/or fragmented nuclei, caspase activation (including caspase-3) and internucleosomal DNA fragmentation was observed. It was associated with a loss of transmembrane mitochondrial potential (ΔΨm) measured with JC-1, with a dephosphorylation of Akt and GSK3 (especially GSK3β), and with degradation of Mcl-1. With α-tocopherol (400 μM), which was capable of counteracting 7-ketocholesterol-induced apoptosis, Akt and GSK3β dephosphorylation were inhibited as well as Mcl-1 degradation. These data underline that the potential protective effects of α-tocopherol against 7-ketocholesterol-induced apoptosis do not depend on the cell line considered, and that the cascade of events (Akt/GSK3β/Mcl-1) constitutes a link between 7-ketocholesterol-induced cytoplasmic membrane dysfunctions and mitochondrial depolarisation leading to apoptosis.

Cerebrospinal fluid and blood biomarkers of neuroaxonal damage in multiple sclerosis

Following emerging evidence that neurodegenerative processes in multiple sclerosis (MS) are present from its early stages, an intensive scientific interest has been directed to biomarkers of neuro-axonal damage in body fluids of MS patients. Recent research has introduced new candidate biomarkers but also elucidated pathogenetic and clinical relevance of the well-known ones. This paper reviews the existing data on blood and cerebrospinal fluid biomarkers of neuroaxonal damage in MS and highlights their relation to clinical parameters, as well as their potential predictive value to estimate future disease course, disability, and treatment response. Strategies for future research in this field are suggested.

Marked accumulation of 27-hydroxycholesterol in the brains of Alzheimer's patients with the Swedish APP 670/671 mutation

There is a significant flux of the neurotoxic oxysterol 27-hydroxycholesterol (27OHC) from the circulation across the blood-brain barrier. Because there is a correlation between 27OHC and cholesterol in the circulation and lipoprotein-bound cholesterol does not pass the blood-brain barrier, we have suggested that 27OHC may mediate the effects of hypercholesterolemia on the brain. We previously demonstrated a modest accumulation of 27OHC in brains of patients with sporadic Alzheimer's disease (AD), consistent with a role of 27OHC as a primary pathogenetic factor. We show here that there is a 4-fold accumulation of 27OHC in different regions of the cortexes of patients carrying the Swedish amyloid precursor protein (APPswe) 670/671 mutation. The brain levels of sitosterol and campesterol were not significantly different in the AD patients compared with the controls, suggesting that the blood-brain barrier was intact in the AD patients. We conclude that accumulation of 27OHC is likely to be secondary to neurodegeneration, possibly a result of reduced activity of CYP7B1, the neuronal enzyme responsible for metabolism of 27OHC. We discuss the possibility of a vicious circle in the brains of the patients with familial AD whereby neurodegenerative changes cause an accumulation of 27OHC that further accelerates neurodegeneration.

Advances in methods for the determination of biologically relevant lipid peroxidation products

Lipid peroxidation is recognized to be an important contributor to many chronic diseases, especially those of an inflammatory pathology. In addition to their value as markers of oxidative damage, lipid peroxidation products have also been shown to have a wide variety of biological and cell signalling effects. In view of this, accurate and sensitive methods for the measurement of lipid peroxidation products are essential. Although some assays have been described for many years, improvements in protocols are continually being reported and, with recent advances in instrumentation and technology, highly specialized and informative techniques are increasingly used. This article gives an overview of the most currently used methods and then addresses the recent advances in some specific approaches. The focus is on analysis of oxysterols, F(2)-isoprostanes and oxidized phospholipids by gas chromatography or liquid chromatography mass spectrometry techniques and immunoassays for the detection of 4-hydroxynonenal.

Links between ApoE, brain cholesterol metabolism, tau and amyloid beta-peptide in patients with cognitive impairment

Brain neurons remove the excess of cholesterol via conversion into the more polar 24OHC [(24S)-hydroxycholesterol]. 24OHC acts as a signalling molecule inducing ApoE (apolipoprotein E)-mediated cholesterol efflux from astrocytes, by a direct effect on ApoE transcription, protein synthesis and secretion. In CSF (cerebrospinal fluid) collected form from patients with cognitive impairment (Alzheimer's disease and patients with mild cognitive impairment) the levels of ApoE, tau, p-tau (hyperphosphorylated tau) were significantly increased, together with 24OHC, compared with controls. We also found that the levels of tau and p-tau were significantly correlated with ApoE and 24OHC in the same samples. Such a correlation was not found in control patients. Increased levels of cholesterol in membranes and impairment in brain cholesterol metabolism were found to be involved both in APP (amyloid precursor protein) processing and amyloid beta-peptide deposition and, recently, in tau pathology. The CSF tau levels are considered to be related to the neurodegenerative process in Alzheimer's disease. During neurodegeneration, the cholesterol accumulated in neurons is converted into 24OHC. The release of 24OHC from neurons induces ApoE secretion by astrocytes, and both are related to the intensity of the neurodegenerative process and neuronal injury. ApoE can also be involved in the scavenging of tau from neurons. The direct correlations between ApoE, 24OHC and tau suggest that cholesterol metabolism may be involved in generation of both tau and amyloid beta-peptide and that the ApoE is released by astrocytes in order to counteract this ongoing process.

Marked accumulation of 27-hydroxycholesterol in SPG5 patients with hereditary spastic paresis

Patients with a recessively inherited "pure" hereditary spastic paresis (SPG5) have mutations in the gene coding for the oxysterol 7 alpha hydroxylase (CYP7B1). One of the expected metabolic consequences of such mutations is accumulation of oxysterol substrates due to decreased enzyme activity. In accordance with this, we demonstrate here that four patients with the SPG5 disease have 6- to 9-fold increased plasma levels of 27-hydroxycholesterol. A much higher increase, 30- to 50-fold, was found in cerebrospinal fluid. The plasma levels of 25-hydroxycholesterol were increased about 100-fold. There were no measurable levels of this oxysterol in cerebrospinal fluid. The pattern of bile acids in serum was normal, suggesting a normal bile acid synthesis. The findings are discussed in relation to two transgenic mouse models with increased levels of 27-hydroxy cholesterol in the circulation but without neurological symptoms: the cyp27a1 transgenic mouse and the cyp7b1 knockout mouse. The absolute plasma levels of 27-hydroxycholesterol in the latter models are, however, only about 20% of those in the SPG5 patients. If the accumulation of 27-hydroxycholesterol is an important pathogenetic factor, a reduction of its levels may reduce or prevent the neurological symptoms. A possible strategy to achieve this is discussed.

Analysis of neurosterols by GC-MS and LC-MS/MS

The term neurosteroid was coined by Baulieu and colleagues in Paris towards the end of the last century to describe steroids which are synthesised in the central or peripheral nervous system [E.E. Baulieu, Psychoneuroendocrinology 23 (1998) 963-87]. This definition was restricted to side-chain "shortened" steroids with 21 carbon atoms or less, and excluded sterols and their carboxylic acids with an intact side-chain. By analogy, we now use the term neurosterol to describe C(27) sterols synthesised in the nervous system. In this review we discuss the biological importance of neurosterols, and how they are extracted, isolated, and analysed by GC-MS and LC-MS/MS, from brain and relevant body fluids. We present applications of methodology employed for analysis of specific sterols and comment on the relative merits of the methods employed. Finally, the importance of future in-depth "sterolomic" investigations of brain is highlighted.

Apolipoprotein E and cholesterol in aging and disease in the brain

Cholesterol can be detrimental or vital, and must be present in the right place at the right time and in the right amount. This is well known in the heart and the vascular system. However, in the CNS cholesterol is still an enigma, although several of its fundamental functions in the brain have been identified. Brain cholesterol has attracted additional attention owing to its close connection to ApoE, a key polymorphic transporter of extracellular cholesterol in humans. Indeed, both cholesterol and ApoE are so critical to fundamental activities of the brain, that the brain regulates their synthesis autonomously. Yet, similar control mechanisms of ApoE and cholesterol homeostasis may exist on either sides of the blood-brain barrier. One indication is that the APOE ε4 allele is associated with hypercholesterolemia and a proatherogenic profile on the vascular side and with increased risk of Alzheimer's disease on the CNS side. In this review, we draw attention to the association between cholesterol and ApoE in the aging and diseased brain, and to the behavior of the ApoE4 protein at the molecular level. The attempt to correlate in vivo and in vitro observations is challenging but crucial for developing future strategies to address ApoE-related aberrations in cholesterol metabolism selectively in the brain.

Levels of ApoE in cerebrospinal fluid are correlated with Tau and 24S-hydroxycholesterol in patients with cognitive disorders

Evidence was recently presented from in vitro studies that 24S-hydroxycholesterol acts as a signalling molecule inducing apoE-mediated cholesterol efflux from astrocytoma cells, and that there is a direct effect of the oxysterol on apoE transcription, protein synthesis and secretion. Consistent with this mechanism, a significant correlation is demonstrated here between levels of apoE and 24S-hydroxycholesterol in cerebrospinal fluid from patients with Alzheimer's disease and patients with mild cognitive impairment. Such a correlation was not found in control patients. There was no correlation between levels of apoE and cholesterol in cerebrospinal fluid from controls. The results are consistent with a close coupling between release of 24S-hydroxycholesterol and apoE secretion under conditions with neuronal degeneration. The levels of apoE in cerebrospinal fluid were also correlated to the levels of Tau and the possibility is discussed that the level of apoE in cerebrospinal fluid may be used as a marker of neurodegeneration.

24S-hydroxycholesterol in relation to disease manifestations of acute experimental autoimmune encephalomyelitis

Levels of the brain-specific cholesterol metabolite 24S-hydroxycholesterol are proposed as possible biomarkers for multiple sclerosis (MS). It is not yet clear for which aspect of the MS disease manifestations 24S-hydroxycholesterol is a reflection. We studied the relation of serum levels of 24S-hydroxycholesterol and other sterols to the disease characteristics of acute experimental autoimmune encephalomyelitis (EAE), an animal model for MS. Serum was analyzed for cholesterol precursors, oxysterols, and plant sterols during the course of disease development. Significantly increased levels of the cholesterol metabolites 24S-hydroxycholesterol and 27-hydroxycholesterol were observed on day 9, before the onset of clinical signs. The serum levels of these oxysterols gradually increased up to 193% and 415%, respectively, at day 17, when clinical symptoms had recovered. Total cholesterol levels were slightly but significantly decreased on day 9 and day 17 in treated animals. Serum levels of cholesterol precursors and plant sterols decreased gradually from day 11 and day 14, respectively. Immunostaining of the 24S-hydroxycholesterol-forming enzyme Cyp46 was shown in macrophage infiltrates. In vitro experiments confirmed the presence of Cyp46 in macrophages and showed a decreased expression after LPS treatment. The data indicate that changes in serum oxysterols occur early in EAE and can be formed by macrophages. These early changes indicate an important role for oxysterols in the development of EAE.

Rediscovery of cerebrosterol

24S-hydroxycholesterol was identified more than half a century ago and was initially given the name "cerebrosterol" due to the fact that it was abundant in the brain. A decade ago, we showed that the most important mechanism by which cholesterol is eliminated from the mammalian brain involves a hydroxylation into cerebrosterol followed by diffusion of this steroid over the blood-brain barrier. Using an (18)O(2) inhalation technique, we showed that about two-thirds of the cholesterol synthesis in rat brain is balanced by conversion into cerebrosterol. The hydroxylase responsible for the reaction was found to be dependent upon NADPH and oxygen, consistent with involvement of a species of cytochrome, P-450. The gene coding for the cytochrome P-450 responsible for the reaction was later cloned by the group of David Russell in Dallas and the enzyme was found to be located to neuronal cells in the brain. Recent studies by us and others on this new pathway for elimination of cholesterol from the brain have given new insights into the mechanisms by which cholesterol homeostasis is maintained in this organ. In addition, these studies have resulted in new diagnostic and prognostic tools in connection with neurological and neurodegenerative diseases. An overview of the studies is presented here and the possibility is discussed that the cholesterol 24S-hydroxylase in the brain may be a new drug target in connection with neurodegenerative diseases.

Crossing the barrier: oxysterols as cholesterol transporters and metabolic modulators in the brain

A normal brain function requires constant levels of cholesterol, and the need for constancy seems to be higher here than in any other organ. Nature has met this need by isolation of brain cholesterol by a highly efficient blood-brain barrier. As a low synthesis of cholesterol is present in the brain, a mechanism for compensatory elimination is required. A decade ago we made the unexpected finding that the favoured mechanism for this involves conversion into 24S-hydroxycholesterol, followed by diffusion over the blood-brain barrier. Recent studies by us and others on this new pathway have given new insights into the mechanisms by which cholesterol homeostasis is maintained in the brain. We recently demonstrated a flux of another oxygenated product of cholesterol, 27-hydroxycholesterol, in the opposite direction. The latter flux may be important for neurodegeneration, and may be the link between hypercholesterolaemia and Alzheimer's disease. An overview of the above studies is presented and the possibility that the cholesterol 24S-hydroxylase in the brain may be important for memory and learning and that it may be a new drug target is discussed.

Oxysterols and Alzheimer's disease

There is a clear link between cholesterol turnover and neurodegenerative diseases and hypercholesterolemia is an established risk factor for Alzheimer's disease (AD). The failure to demonstrate a transfer of cholesterol from the circulation into the brain in humans and experimental animals makes it difficult to explain the link between hypercholesterolemia and AD. In contrast to cholesterol itself, side-chain oxidized cholesterol metabolites such as 24S-hydroxycholesterol and 27-hydroxycholesterol are able to pass the blood-brain barrier (BBB). Formation of 24S-hydroxycholesterol is the quantitatively most important mechanism for elimination of cholesterol from the brain and we recently demonstrated a significant net uptake of 27-hydroxycholesterol by the brain from the circulation. We have also shown that patients with AD have increased brain levels of 27-hydroxycholesterol, which may affect the production of beta-amyloid in the brain. The levels of 27-hydroxycholesterol in the circulation are correlated with the levels of cholesterol and the possibility must be considered that the flux of 27-hydroxycholesterol into the brain is the missing link between hypercholesterolemia and Alzheimer's disease. Current knowledge about the role of the two oxysterols for cholesterol homeostasis in the brain as well as their diagnostic potential are reviewed.

Are the CSF levels of 24S-hydroxycholesterol a sensitive biomarker for mild cognitive impairment?

There is a need for effective biomarkers showing whether or not a patient with mild cognitive impairment (MCI) will progress to Alzheimer's disease (AD) with dementia. At the present three cerebrospinal fluid (CSF) biomarkers are in general use: total tau, phospho-tau and beta-Amyloid. These markers are regarded to have high capacity to differentiate early AD from normal ageing. We have analysed CSF levels of a new marker for neuronal degeneration, 24S-hydroxycholesterol (24OHC) in patients with MCI. For reasons of comparison, we also analysed these levels in patients with AD. There was a significant correlation between CSF levels of 24OHC and total tau (as well as phospho-tau) in both groups of patients. Fifty percent of the patients contemplated for MCI were found to have elevated levels of 24OHC (using a 95th upper percentile set cut-off). All the MCI patients with normal levels of 24OHC had normal levels of the other markers. In patients with AD, the percentages of those with increased levels of 24OHC, tau and phospho tau were similar (55-67%). In this pilot study, we discuss the possibility that 24OHC may be a sensitive test for MCI.

Cholesterol in Alzheimer's disease

Alzheimer's disease (AD) is the most common form of neurodegenerative dementia and affects up to 15 million people worldwide. Although no single cause of AD has been identified, recent research has suggested that several pathogenetic factors influence risk and expression. A growing amount of evidence underscores a mechanistic link between cholesterol metabolism in the brain and the formation of amyloid plaques. Excess brain cholesterol has been associated with increased formation and deposition of amyloid-beta peptide from amyloid precursor protein. Cholesterol-lowering statins have become a focus of research in AD. Genetic polymorphisms associated with pivotal points in cholesterol metabolism in brain tissues may contribute to the risk and pathogenesis of AD. In this review, we summarise current knowledge of the role of cholesterol metabolism in the pathogenesis of AD and examine the potential of statins in the prevention and treatment of AD.

Studies on the transcriptional regulation of cholesterol 24-hydroxylase (CYP46A1): marked insensitivity toward different regulatory axes

Mammalian CNS contains a disproportionally large and remarkably stable pool of cholesterol. Despite an efficient recycling there is some requirement for elimination of brain cholesterol. Conversion of cholesterol into 24S-hydroxycholesterol by the cholesterol 24-hydroxylase (CYP46A1) is the quantitatively most important mechanism. Based on the protein expression and plasma levels of 24S-hydroxycholesterol, CYP46A1 activity appears to be highly stable in adults. Here we have made a structural and functional characterization of the promoter of the human CYP46A1 gene. No canonical TATA or CAAT boxes were found in the promoter region. Moreover this region had a high GC content, a feature often found in genes considered to have a largely housekeeping function. A broad spectrum of regulatory axes using a variety of promoter constructs did not result in a significant transcriptional regulation. Oxidative stress caused a significant increase in transcriptional activity. The possibility of a substrate-dependent transcriptional regulation was explored in vivo in a sterol-deficient mouse model (Dhcr24 null) in which almost all cholesterol had been replaced with desmosterol, which is not a substrate for CYP46A1. Compared with heterozygous littermates there was no statistically significant difference in the mRNA levels of Cyp46a1. During the first 2 weeks of life in the wild-type mouse, however, a significant increase of Cyp46a1 mRNA levels was found, in parallel with an increase in 24S-hydroxycholesterol level and a reduction of cholesterol synthesis. The failure to demonstrate a significant transcriptional regulation under most conditions is discussed in relation to the turnover of brain and neuronal cholesterol.

Polymorphism in neuropeptide Y influences CSF cholesterol levels but is no major risk factor of Alzheimer's disease

Neuropeptide Y (NPY) is a neurotransmitter expressed in the central nervous system and involved in learning and memory. The NPY L7P polymorphism has been associated with altered cholesterol levels in obese patients. Since altered cholesterol metabolism is also involved in Alzheimer's disease (AD), the effects of two NPY polymorphisms (L7P and IVS1-100 T/G) on CSF and plasma cholesterol and 24S-hydroxycholesterol were investigated in AD patients and non-demented controls. Furtheremore, the effect of both NPY polymorphisms on the risk of AD was studied. The NPY IVS1-100 T/G polymorphism influenced CSF levels of cholesterol, whereas CSF and plasma levels of 24S-hydroxycholesterol and plasma cholesterol were not altered by genotype. NPY L7P polymorphism did not influence CSF or plasma cholesterol or 24S-hydroxycholesterol. Both NPY polymorphisms did not influence the risk of AD. Our data support the observation, that NPY polymorphisms might influence cholesterol metabolism, but might not act as major risk factor in AD.

Crossing the barrier: net flux of 27-hydroxycholesterol into the human brain

Side chain oxidized oxysterols have a unique ability to traverse lipophilic membranes. We tested the hypothesis that there is a net flux of 27-hydroxycholesterol from the circulation into the brain using plasma samples collected from the internal jugular vein and an artery of healthy male volunteers. Two independent studies were performed, one in which total levels of 27-hydroxycholesterol were measured and one in which the free fraction of 27-hydroxycholesterol was measured. In the majority of subjects studied, the level of 27-hydroxycholesterol was higher in the artery than in the vein, and uptake from the circulation was calculated to be about 5 mg/24 h. The distribution of 27-hydroxycholesterol in human brain was found to be consistent with an extracerebral origin, with a concentration gradient from the white to the gray matter--a situation opposite that of 24S-hydroxycholesterol, which os exclusively formed in brain. In view of the fact that the blood-brain barrier is impermeable to cholesterol and that 27-hydroxycholesterol is a potent regulator of several cholesterol-sensitive genes, the flux of 27-hydroxycholesterol into the brain may be and important link between intra- and extracerebral cholesterol homeostasis.

Altered levels of plasma 24S- and 27-hydroxycholesterol in demented patients

Alterations in brain cholesterol metabolism and reduced 24S-hydroxycholesterol plasma levels have been described in Alzheimer's disease (AD) and vascular dementia (VD). We hypothesize that changes in peripheral cholesterol metabolism, such as alterations in the plasma levels of 27-hydroxycholesterol, might also be involved. Plasma levels of 24S-hydroxycholesterol and 27-hydroxycholesterol in patients suffering from dementing disorders such as AD, VD, and mild cognitive impairment (MCI) were compared to those in age- and cholesterol matched non-demented and depressed subjects. Cholesterol corrected concentrations of plasma 24S-hydroxycholesterol and 27-hydroxycholesterol were significantly reduced in patients with dementing disorders compared to non-demented subjects and depressed patients. A strong positive correlation between plasma 24S-hydroxycholesterol and 27-hydroxycholesterol levels was observed. The ratios of plasma 24S-hydroxycholesterol to 27-hydroxycholesterol were higher in patients with dementing disorders compared to non-demented subjects. Our results support the observation, that cholesterol metabolism is altered in dementing disorders, indicated by different plasma concentrations of brain specific and peripherally produced oxysterols.

Biological markers in CSF and blood for axonal degeneration in multiple sclerosis

Biomarkers in body fluids could help to predict and monitor neurological decline in people with multiple sclerosis (MS). We discuss markers for axonal damage in body fluids in people with MS. The most promising axonal marker for discriminating patients with MS from those with other neurological diseases is the neurofilament light chain in CSF. Antibodies against the heavy-chain isoform are associated with disease progression. Other studies have shown altered CSF concentrations of tau proteins, actin, tubulin, and 14-3-3 protein. Interestingly, the concentration of 24S-hydroxycholesterol was decreased in serum of patients with MS. No clear changes have been shown for the markers apolipoprotein E and neurospecific enolase. We describe three types of markers for axonal damage: markers that reflect processes in the CNS, those that reflect extraneural processes, and those that reflect whole-body changes. These concepts may be helpful for biomarker research in various neurodegenerative diseases.