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David W. Russell - One of the best experts on this subject based on the ideXlab platform.
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Cholesterol 24-Hydroxylase: An Enzyme of Cholesterol Turnover in the Brain
Annual Review of Biochemistry, 2009Co-Authors: David W. Russell, Rebekkah W. Halford, Denise M.o. Ramirez, Rahul Shah, Tiina KottiAbstract:Cholesterol 24-Hydroxylase is a highly conserved cytochrome P450 that is responsible for the majority of Cholesterol turnover in the vertebrate central nervous system. The enzyme is expressed in neurons, including hippocampal and cortical neurons that are important for learning and memory formation. Disruption of the Cholesterol 24-Hydroxylase gene in the mouse reduces both Cholesterol turnover and synthesis in the brain but does not alter steady-state levels of Cholesterol in the tissue. The decline in synthesis reduces the flow of metabolites through the Cholesterol biosynthetic pathway, of which one, geranylgeraniol diphosphate, is required for learning in the whole animal and for synaptic plasticity in vitro. This review focuses on how the link between Cholesterol metabolism and higher-order brain function was experimentally established.
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Reduction of Cholesterol synthesis in the mouse brain does not affect amyloid formation in Alzheimer's disease, but does extend lifespan
Proceedings of the National Academy of Sciences of the United States of America, 2009Co-Authors: Rebekkah W. Halford, David W. RussellAbstract:Alterations in cellular Cholesterol synthesis or content in cultured neurons affect the cleavage of amyloid precursor protein to amyloidogenic Aβ40 and Aβ42 peptides characteristic of Alzheimer's disease. To determine whether a decrease in Cholesterol synthesis affects amyloid precursor protein processing in vivo, we crossed Cholesterol 24-Hydroxylase knockout mice, which exhibit a 50% reduction in brain sterol synthesis, with transgenic mice [B6.Cg-Tg(APPswe, PSEN1E9)85Dbo/J] that develop Alzheimer's disease-like pathology. Amyloid precursor protein expression and amyloid plaque deposition in the cortex and hippocampus of male and female Alzheimer's disease mice between the ages of 3 to 15 months were similar in the presence and absence of Cholesterol 24-Hydroxylase. A modest but statistically significant decline in insoluble Aβ42 peptide levels was detected in the hippocampus of 12-month-old knockout/Alzheimer's disease males. The levels of insoluble Aβ40 and Aβ42 peptides in 15-month-old knockout/Alzheimer's disease females were also reduced slightly. Although amyloid plaque accumulation did not affect brain sterol or fatty acid synthesis rates in 24-Hydroxylase WT or knockout mice, loss of one or both Cholesterol 24-Hydroxylase alleles increased longevity in Alzheimer's disease mice. These studies suggest that reducing de novo Cholesterol synthesis in the brain will not substantially alter the course of Alzheimer's disease, but may confer a survival advantage.
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Biphasic requirement for geranylgeraniol in hippocampal long-term potentiation
Proceedings of the National Academy of Sciences of the United States of America, 2008Co-Authors: Tiina Kotti, Daphne D. Head, Charles E. Mckenna, David W. RussellAbstract:Mice deficient in Cholesterol 24-Hydroxylase exhibit reduced rates of Cholesterol synthesis and other non-sterol isoprenoids that arise from the mevalonate pathway. These metabolic abnormalities, in turn, impair learning in the whole animal and hippocampal long-term potentiation (LTP) in vitro. Here, we report pharmacogenetic experiments in hippocampal slices from wild-type and mutant mice that characterize the dependence of LTP on the non-sterol isoprenoid, geranylgeraniol. Addition of geranylgeraniol to slices from 24-Hydroxylase knockout mice restores LTP to wild-type levels; however, farnesol, a chemically related compound, does not substitute for geranylgeraniol nor does another animal model of impaired LTP (apolipoprotein E deficiency) respond to this isoprenoid. The requirement for geranylgeraniol is independent of acute protein isoprenylation as judged in experiments employing cell-permeable inhibitors of protein farnesyl transferase and geranylgeranyl transferase enzymes and in mutant mice hypomorphic for geranylgeranyltransferase II. Time course studies show that geranylgeraniol acts within 5 min and at 2 different times during the establishment of LTP: just before electrical stimulation and approximately 15 min thereafter. Localized delivery of geranylgeraniol to the dendritic trees of CA1 hippocampal neurons via the recording electrode is sufficient to restore LTP in slices from 24-Hydroxylase knockout mice. We conclude that geranylgeraniol acts specifically and quickly to affect LTP in the Schaffer collaterals of the hippocampus.
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neuronal expression and subcellular localization of Cholesterol 24 Hydroxylase in the mouse brain
The Journal of Comparative Neurology, 2008Co-Authors: Denise M.o. Ramirez, Stefan Andersson, David W. RussellAbstract:Cholesterol 24-Hydroxylase is a cytochrome P450 (CYP46A1) that is selectively expressed in the brain and is responsible for the majority of Cholesterol turnover in the central nervous system. Mice deficient in 24-Hydroxylase exhibit impaired learning and defective hippocampal long-term potentiation, suggesting that the metabolism of Cholesterol by this enzyme is required for learning and memory formation. To determine where in the neuron Cholesterol turnover was taking place, monoclonal antibodies directed against 24-Hydroxylase were generated by immunization of mice with recombinant protein and used to detect the enzyme in brain homogenates, cultured neurons, and histological sections. 24-Hydroxylase was localized to the endoplasmic reticulum and was distributed throughout the cell bodies and dendrites of multiple types of neurons; the enzyme was not detected in axon terminals or in the cells of 24-Hydroxylase knockout mice. 24-Hydroxylase was highly expressed in pyramidal neurons of the hippocampus and cortex, in Purkinje cells of the cerebellum, and in hippocampal and cerebellar interneurons. Within the retina, 24-Hydroxylase was detected in ganglion cells and some but not all cells of the inner nuclear layer. These findings reveal the microsomal localization of 24-Hydroxylase and provide subcellular insight into Cholesterol turnover in the brain. J. Comp. Neurol. 507:1676‐1693, 2008. © 2008 Wiley-Liss, Inc.
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Brain Cholesterol turnover required for geranylgeraniol production and learning in mice
Proceedings of the National Academy of Sciences of the United States of America, 2006Co-Authors: Tiina Kotti, Denise M.o. Ramirez, Brad E. Pfeiffer, Kimberly M. Huber, David W. RussellAbstract:The mevalonate pathway produces Cholesterol and nonsterol isoprenoids, such as geranylgeraniol. In the brain, a fraction of Cholesterol is metabolized in neurons by the enzyme Cholesterol 24-Hydroxylase, and this depletion activates the mevalonate pathway. Brains from mice lacking 24-Hydroxylase excrete Cholesterol more slowly, and the tissue compensates by suppressing the mevalonate pathway. Here we report that this suppression causes a defect in learning. 24-Hydroxylase knockout mice exhibit severe deficiencies in spatial, associative, and motor learning, and in hippocampal long-term potentiation (LTP). Acute treatment of wild-type hippocampal slices with an inhibitor of the mevalonate pathway (a statin) also impairs LTP. The effects of statin treatment and genetic elimination of 24-Hydroxylase on LTP are reversed by a 20-min treatment with geranylgeraniol but not by Cholesterol. We conclude that Cholesterol turnover in brain activates the mevalonate pathway and that a constant production of geranylgeraniol in a small subset of neurons is required for LTP and learning.
Steve Meaney - One of the best experts on this subject based on the ideXlab platform.
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Enhanced production of 24S-hydroxyCholesterol is not sufficient to drive liver X receptor target genes in vivo
Journal of Internal Medicine, 2011Co-Authors: Marjan Shafaati, Steve Meaney, Maria Olin, Ann Båvner, Hanna Pettersson, Björn Rozell, Paolo Parini, Ingemar BjorkhemAbstract:Abstract. Shafaati M, Olin M, Bavner A, Pettersson H, Rozell B, Meaney S, Parini P, Bjorkhem I (Karolinska University Hospital Huddinge, Huddinge, Sweden; Dublin Institute of Technology, Dublin, Ireland). Enhanced production of 24S-hydroxyCholesterol is not sufficient to drive liver X receptor target genes in vivo. J Intern Med 2011; 270: 377–387. Background. Oxysterols such as 24S-hydroxyCholesterol (OHC) and 27-OHC are intermediates of Cholesterol excretion pathways. In addition, they are putative endogenous agonists of the liver X receptor (LXR) class of nuclear hormone receptors and are thought to be important mediators of Cholesterol-dependent gene regulation. 24S-OHC is one of the most efficient endogenous LXR agonists known and is present in the brain and in the circulation at relatively high levels. Objectives. To explore the regulatory importance of 24S-OHC in vivo. Design. We developed a transgenic mouse model in which human Cholesterol 24-Hydroxylase, the enzyme responsible for the formation of 24S-OHC, was expressed under the control of a promoter derived from the β-actin gene. Results. Both male and female transgenic mice had elevated levels of cerebral, plasma, biliary and faecal 24S-OHC. According to the faecal excretion results, production of 24S-OHC was increased four- to sevenfold. Gene expression profiling revealed that the elevated production of 24S-OHC did not result in the anticipated activation of LXR target genes in the brain or liver. Conclusion. In spite of the fact that 24S-OHC is a highly effective agonist of LXRs in vitro, it is not a critical activator of target genes to this nuclear receptor in vivo, either in the brain or in the liver.
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transcriptional regulation of Cholesterol 24 Hydroxylase by histone deacetylase inhibitors
Biochemical and Biophysical Research Communications, 2009Co-Authors: Marjan Shafaati, Riona Odriscoll, Ingemar Bjorkhem, Steve MeaneyAbstract:Abstract The mechanistic basis for the tissue specific expression of Cholesterol elimination pathways is poorly understood. To gain additional insight into this phenomenon we considered it of interest to investigate if epigenetic mechanisms are involved in the regulation of the brain-specific enzyme Cholesterol 24-Hydroxylase (CYP46A1), a key regulator of brain Cholesterol elimination. We demonstrated a marked time-dependent derepression of the expression of CYP46A1, in response to treatment with the potent histone deacetylase (HDAC) inhibitor Trichostatin A. The pattern of expression of the genes in the genomic region surrounding CYP46A1 was found to be diametrically opposite in brain and liver. Intraperitoneal injection of HDAC inhibitors in mice led to a significant derepression of hepatic Cyp46a1 mRNA expression and tissue specific changes in Hmgcr and Cyp39a1 mRNA expression. These results are discussed in the context of the phenomenology of tissue specific Cholesterol balance.
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Regulation of α- and β-secretase activity by oxysterols: Cerebrosterol stimulates processing of APP via the α-secretase pathway
Biochemical and Biophysical Research Communications, 2007Co-Authors: D. Famer, Ingemar Bjorkhem, Steve Meaney, M. Mousavi, Agneta Nordberg, Milita CrisbyAbstract:Abstract The Cholesterol 24-Hydroxylase encoded by the gene CYP46 is expressed almost exclusively in central nervous system (CNS) neurons and catalyzes the formation of 24S-hydroxyCholesterol (24S-OHC) from Cholesterol. This conversion corresponds to a major pathway for excretion of excess Cholesterol from the brain. There is a significant flux of another oxysterol, 27-hydroxyCholesterol (27-OHC) from the circulation into the brain. Polymorphisms within the CYP46A1 gene have been associated with Alzheimer’s disease (AD) incidence. In this study, we examined the effects of 24S-OHC and 27-OHC on the α- and β-secretase activity in the human neuroblastoma cell line SH-SY5Y. Furthermore, we examined the effects of the two oxysterols on the levels of extra- and intracellular proteins of secreted APPα (sAPPα). Our findings suggest that 24S-OHC may exert a unique modulatory effect on APP processing and that this oxysterol increases the α-secretase activity as well as the α/β-secretase activity ratio. The possibility is discussed that the ratio between 24S-OHC and 27-OHC is of importance for the generation of amyloid in the brain.
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studies on the transcriptional regulation of Cholesterol 24 Hydroxylase cyp46a1 marked insensitivity toward different regulatory axes
Journal of Biological Chemistry, 2006Co-Authors: Yoshihiko Ohyama, Steve Meaney, Maura Heverin, Lena Ekstrom, Anat Brafman, Millicent Shafir, Ulla Andersson, Maria Olin, Gosta Eggertsen, Ulf DiczfalusyAbstract:Abstract 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.
Marieanne Burlot - One of the best experts on this subject based on the ideXlab platform.
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Cholesterol 24 Hydroxylase defect is implicated in memory impairments associated with alzheimer like tau pathology
Human Molecular Genetics, 2015Co-Authors: Marieanne Burlot, Jerome Braudeau, Kristin Michaelsenpreusse, Brigitte Potier, Sophie Ayciriex, Jennifer Varin, Benoit GautierAbstract:: Alzheimer's disease (AD) is characterized by both amyloid and Tau pathologies. The amyloid component and altered Cholesterol metabolism are closely linked, but the relationship between Tau pathology and Cholesterol is currently unclear. Brain Cholesterol is synthesized in situ and cannot cross the blood-brain barrier: to be exported from the central nervous system into the blood circuit, excess Cholesterol must be converted to 24S-hydroxyCholesterol by the Cholesterol 24-Hydroxylase encoded by the CYP46A1 gene. In AD patients, the concentration of 24S-hydroxyCholesterol in the plasma and the cerebrospinal fluid are lower than in healthy controls. The THY-Tau22 mouse is a model of AD-like Tau pathology without amyloid pathology. We used this model to investigate the potential association between Tau pathology and CYP46A1 modulation. The amounts of CYP46A1 and 24S-hydroxyCholesterol in the hippocampus were lower in THY-Tau22 than control mice. We used an adeno-associated virus (AAV) gene transfer strategy to increase CYP46A1 expression in order to investigate the consequences on THY-Tau22 mouse phenotype. Injection of the AAV-CYP46A1 vector into the hippocampus of THY-Tau22 mice led to CYP46A1 and 24S-hydroxyCholesterol content normalization. The cognitive deficits, impaired long-term depression and spine defects that characterize the THY-Tau22 model were completely rescued, whereas Tau hyperphosphorylation and associated gliosis were unaffected. These results argue for a causal link between CYP46A1 protein content and memory impairments that result from Tau pathology. Therefore, CYP46A1 may be a relevant therapeutic target for Tauopathies and especially for AD.
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modulation du metabolisme du Cholesterol dans un modele murin de tauopathie evaluation de la Cholesterol 24 Hydroxylase comme cible therapeutique dans la maladie d alzheimer
2014Co-Authors: Marieanne BurlotAbstract:La maladie d’Alzheimer (MA) se caracterise par une perte mnesique progressive et au plan neuropathologique par le depot extracellulaire de plaques amyloides, resultant de l’agregation de peptides amyloides (Aβ), et par l’apparition d’une degenerescence neurofibrillaire (DNF) constituee d’agregats intraneuronaux de proteines Tau hyper et anormalement phosphorylees. L’evolution des deficits cognitifs des patients est particulierement correlee a la progression spatio-temporelle de la DNF. A l’heure actuelle, il n’existe aucun traitement curatif de la maladie. Le Cholesterol joue un role central dans la physiopathologie de la MA. En particulier, l’allele e4 du gene de l’apolipoproteine E, transporteur cerebral essentiel du Cholesterol, est le principal facteur de risque genetique des formes sporadiques de la MA. De nombreuses etudes in vitro montrent qu’une surcharge en Cholesterol induit la production d’Aβ pathogenes et qu’inversement, une depletion en Cholesterol entraine une diminution de la voie amyloide. Le Cholesterol ne peut pas passer librement la barriere hemato-encephalique (BHE). Le Cholesterol cerebral est exclusivement synthetise in situ. Le Cholesterol cerebral en exces doit etre exporte dans la circulation sanguine pour etre metabolise. Pour franchir la BHE, sa conversion en 24(S)-hydroxyCholesterol est necessaire, etape controlee par la Cholesterol 24-Hydroxylase (CYP46A1). Deux precedents travaux de these effectues dans le laboratoire ont permis de mettre en evidence des connexions etroites entre le metabolisme du Cholesterol et la MA in vivo. La surexpression intracerebrale de CYP46A1 dans un modele murin amyloide a l’aide d’un vecteur viral adeno-associe (AAV) a conduit a la diminution de la production d’Aβ, des plaques amyloides et a l’amelioration des performances mnesiques des animaux. A l’inverse, l’inhibition de l’expression de CYP46A1 dans l’hippocampe de souris sauvages induit la production d’Aβ, la phosphorylation de Tau et des defauts mnesiques chez la souris. L’objectif de mon travail de doctorat a ete de determiner s’il existait un lien direct entre CYP46A1 et la pathologie Tau et si la modulation du metabolisme du Cholesterol pourrait avoir un effet benefique sur la pathologie Tau associee a la MA. Pour repondre a ces questions, le modele murin THY-Tau22, qui developpe une pathologie Tau de type Alzheimer, a ete utilise. Cette pathologie, essentiellement hippocampique, est evolutive et associee a des deficits mnesiques. Dans l’hippocampe des souris THY-Tau22, le Cholesterol libre total n’est pas modifie, alors que l’expression proteique de CYP46A1 est diminuee, et en consequence le contenu en 24(S)-hydroxyCholesterol. L’expression proteique de CYP46A1 dans l’hippocampe est egalement reduite dans un autre modele murin de pathologie Tau, le modele THY-Tau30. Ainsi, la pathologie Tau semble etre a l’origine de la diminution de l’expression proteique de CYP46A1. Afin de determiner si la surexpression de CYP46A1 chez la souris THY-Tau22 pouvait ameliorer son phenotype biochimique, neuropathologique et clinique, un vecteur AAV codant pour CYP46A1 a ete injecte dans l’hippocampe de souris THY-Tau22 âgees de trois mois et demi. Deux mois et demi apres injection, la surexpression de CYP46A1 chez les souris THY-Tau22 induit une restauration de la concentration hippocampique en 24(S)-hydroxyCholesterol et une augmentation de l’expression des genes impliques dans la synthese du Cholesterol, et plus particulierement dans la voie du mevalonate. Deux mois et demi et cinq mois et demi post-injection, la surexpression de CYP46A1 entraine une restauration complete des performances mnesiques des animaux qui s’accompagne d’un retablissement de la depression a long terme, de la longueur des dendrites secondaires, de la densite synaptique et de l’expression des genes d’activite precoce dans l’hippocampe. (...)
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evaluation de la Cholesterol 24 Hydroxylase cyp46a1 comme cible therapeutique dans le modele de souris thy tau22 modulation du metabolisme du Cholesterol et maladie d alzheimer
2011Co-Authors: Marieanne BurlotAbstract:La maladie d’Alzheimer (MA) se caracterise par des plaques amyloides extracellulaires et des corps neurofibrillaires intracellulaires (NFT) formes de proteines tau hyperphosphorylee. Le Cholesterol joue un role central dans la physiopathologie de la maladie. Synthetise in situ, le Cholesterol cerebral en exces doit etre exporte dans la circulation pour etre metabolise. Pour franchir la barriere hemato-meningee, il est converti par la Cholesterol-24-hydrolaxe (CYP). Dans le modele murin THY-Tau22, j’ai etudie les consequences de la surexpression de CYP dans le cerveau (via l’injection stereotaxique de vecteurs AAV codant le gene CYP46A1). Le modele utilise reproduit le developpement des NFT dans le cerveau et presente des deficits mnesiques a partir de 3 mois et une pathologie Tau severe a 12. Mes resultats montrent que la surexposition de CYP dans le cerveau des souris a 3,5 mois reduit l’atteinte evaluee a 9 mois et semble affecter la phosphorylation pathologique de Tau.
Erik G. Lund - One of the best experts on this subject based on the ideXlab platform.
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knockout of the Cholesterol 24 Hydroxylase gene in mice reveals a brain specific mechanism of Cholesterol turnover
Journal of Biological Chemistry, 2003Co-Authors: Erik G. Lund, Stephen D. Turley, John M. Dietschy, Tiina Kotti, David W. RussellAbstract:Abstract Most Cholesterol turnover takes place in the liver and involves the conversion of Cholesterol into soluble and readily excreted bile acids. The synthesis of bile acids is limited to the liver, but several enzymes in the bile acid biosynthetic pathway are expressed in extra-hepatic tissues and there also may contribute to Cholesterol turnover. An example of the latter type of enzyme is Cholesterol 24-Hydroxylase, a cytochrome P450 (CYP46A1) that is expressed at 100-fold higher levels in the brain than in the liver. Cholesterol 24-Hydroxylase catalyzes the synthesis of the oxysterol 24(S)-hydroxyCholesterol. To assess the relative contribution of the 24-hydroxylation pathway to Cholesterol turnover, we performed balance studies in mice lacking the Cholesterol 24-Hydroxylase gene (Cyp46a1–/– mice). Parameters of hepatic Cholesterol and bile acid metabolism in the mutant mice remained unchanged relative to wild type controls. In contrast to the liver, the synthesis of new Cholesterol was reduced by ∼40% in the brain, despite steady-state levels of Cholesterol being similar in the knockout mice. These data suggest that the synthesis of new Cholesterol and the secretion of 24(S)-hydroxyCholesterol are closely coupled and that at least 40% of Cholesterol turnover in the brain is dependent on the action of Cholesterol 24-Hydroxylase. We conclude that Cholesterol 24-Hydroxylase constitutes a major tissue-specific pathway for Cholesterol turnover in the brain.
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Quantitation of two pathways for Cholesterol excretion from the brain in normal mice and mice with neurodegeneration.
Journal of Lipid Research, 2003Co-Authors: Erik G. Lund, Stephen D. Turley, David W. Russell, John M. DietschyAbstract:Although the pool of Cholesterol in the adult cen- tral nervous system (CNS) is large and of constant size, lit- tle is known of the process(es) involved in regulation of ste- rol turnover in this pool. In 7-week-old mice, net excretion of Cholesterol from the brain equaled 1.4 mg/day/kg body weight, and from the whole animal was 179 mg/day/kg. De- letion of Cholesterol 24-Hydroxylase, an enzyme highly ex- pressed in the CNS, did not alter brain growth or myelina- tion, but reduced sterol excretion from the CNS 64% to 0.5 mg/day/kg. In mice with a mutation in the Niemann-Pick C gene that had ongoing neurodegeneration, sterol excretion from the CNS was increased to 2.3 mg/day/kg. Deletion of Cholesterol 24-Hydroxylase activity in these animals reduced net excretion only 22% to 1.8 mg/day/kg. Thus, at least two different pathways promote net sterol excretion from the CNS. One uses Cholesterol 24-Hydroxylase and may re- flect sterol turnover in large neurons in the brain. The other probably involves the movement of Cholesterol or one of its metabolites across the blood-brain barrier and may more closely mirror sterol turnover in pools such as glial cell membranes and myelin. —Xie, C., E. G. Lund, S. D. Turley, D. W. Russell, and J. M. Dietschy. Quantitation of two path- ways for Cholesterol excretion from the brain in normal mice and mice with neurodegeneration. J. Lipid Res. 2003. 44: 1780-1789.
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cdna cloning of Cholesterol 24 Hydroxylase a mediator of Cholesterol homeostasis in the brain
Proceedings of the National Academy of Sciences of the United States of America, 1999Co-Authors: Erik G. Lund, Joseph M Guileyardo, David W. RussellAbstract:The turnover of Cholesterol in the brain is thought to occur via conversion of excess Cholesterol into 24S-hydroxyCholesterol, an oxysterol that is readily secreted from the central nervous system into the plasma. To gain molecular insight into this pathway of Cholesterol metabolism, we used expression cloning to isolate cDNAs that encode murine and human Cholesterol 24-Hydroxylases. DNA sequence analysis indicates that both proteins are localized to the endoplasmic reticulum, share 95% identity, and represent a new cytochrome P450 subfamily (CYP46). When transfected into cultured cells, the cDNAs produce an enzymatic activity that converts Cholesterol into 24S-hydroxyCholesterol, and to a lesser extent, 25-hydroxyCholesterol. The Cholesterol 24-Hydroxylase gene contains 15 exons and is located on human chromosome 14q32.1. Cholesterol 24-Hydroxylase is expressed predominantly in the brain as judged by RNA and protein blotting. In situ mRNA hybridization and immunohistochemistry localize the expression of this P450 to neurons in multiple subregions of the brain. The concentrations of 24S-hydroxyCholesterol in serum are low in newborn mice, reach a peak between postnatal days 12 and 15, and thereafter decline to baseline levels. In contrast, Cholesterol 24-Hydroxylase protein is first detected in the brain of mice at birth and continues to accumulate with age. We conclude that the cloned cDNAs encode Cholesterol 24-Hydroxylases that synthesize oxysterols in neurons of the brain and that secretion of 24S-hydroxyCholesterol from this tissue in the mouse is developmentally regulated.
Ulf Diczfalusy - One of the best experts on this subject based on the ideXlab platform.
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studies on the transcriptional regulation of Cholesterol 24 Hydroxylase cyp46a1 marked insensitivity toward different regulatory axes
Journal of Biological Chemistry, 2006Co-Authors: Yoshihiko Ohyama, Steve Meaney, Maura Heverin, Lena Ekstrom, Anat Brafman, Millicent Shafir, Ulla Andersson, Maria Olin, Gosta Eggertsen, Ulf DiczfalusyAbstract:Abstract 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.