Brain Cholesterol Content Research Articles

Targeting cytochrome P450 46A1 and brain cholesterol 24-hydroxylation to treat neurodegenerative diseases.

The brain cholesterol content is determined by the balance between the pathways of in situ biosynthesis and cholesterol elimination via 24-hydroxylation catalyzed by CYP46A1 (cytochrome P450 46A1). Both pathways are tightly coupled and determine the rate of brain cholesterol turnover. Evidence is accumulating that modulation of CYP46A1 activity by gene therapy or pharmacologic means could be beneficial in case neurodegenerative and other brain diseases and affect brain processes other than cholesterol biosynthesis and elimination. This minireview summarizes these other processes, most common of which include abnormal protein accumulation, memory and cognition, motor behavior, gene transcription, protein phosphorylation as well as autophagy and lysosomal processing. The unifying mechanisms, by which these processes could be affected by CYP46A targeting are also discussed.

Amphetamine self‐administration alters the cholesterol content and G protein membrane compartmentalization in the rat striatum.

IntroductionChronic amphetamine use causes profound adaptations in the brain including desensitization and sensitization of many GPCRs and altered signaling of G proteins coupled to these receptors. These changes are associated with vulnerability to relapse. However, the mechanisms underlying changes in receptor‐mediated G protein signaling remain elusive.One possible mechanism is an alteration in the compartmentalization of G proteins in the plasma membrane. Lipid rafts are the subcompartment of the plasma membranes characterized by high cholesterol content. They act as microdomains to accommodate cellular signaling and protein trafficking. Changes in the lipid composition of the plasma membrane would alter protein localization and associated signaling. Little is known about the effects of psychostimulant treatments on G protein distribution in lipid raft and nonraft domains of striatal cell membranes.The purpose of this study was to use an animal model of amphetamine self‐administration to investigate the parallel changes in the brain cholesterol content and G protein membrane compartmentalization. This study may shed light on the contribution of membrane cholesterol content to amphetamine‐induced alteration in G protein signaling and receptor responses to stimuli.MethodsMale Sprague Dawley rats were anesthetized, implanted with catheters, and allowed to self‐administer amphetamine (0.189 mg/kg/infusion) on a fixed ratio one schedule. Animals were allowed to self‐administer up to 40 injections in 6 hrs per day over 5 or 14 days. Rats were sacrificed and the striata were dissected. Striatal tissues were fractionated using a sucrose density gradient and centrifugation to isolate lipid raft and non‐lipid raft fractions. Fractions were subjected to immunoblotting to determine G protein localization. Amplex Red Hydrogen Peroxide/Peroxidase assay kit was used to measure the striatal cholesterol levels.ResultsOur data showed both 5 and 14 days of amphetamine self‐administration significantly reduced the total cholesterol levels in the striatum. Interestingly, we found that amphetamine self‐administration differentially regulated the membrane compartmentalization of Gα subtypes. For Gαo and Gαs, there was a notable translocation from nonraft to raft fractions in both 5 and 14 days amphetamine self‐administering rats. For Gαi2 and Gαi3, there was no change in membrane compartmentalization following 5 days of amphetamine self‐administration; however, 14 days of amphetamine exposure caused these G‐proteins to translocate from raft to nonraft fractions. No significant membrane translocation of Gαq was observed. This study provides first time evidence that amphetamine self‐administration selectively alters the membrane compartmentalization of Gα subunits, which may be associated with changes in receptor function and addiction‐like behavior. Future research on the contribution of the membrane cholesterol content to receptor function and addiction behavior is warranted.Support or Funding InformationThis project is supported by NIH DA006634.

Activation of α‐secretase cleavage

Alpha-secretase-mediated cleavage of the amyloid precursor protein (APP) releases the neuroprotective APP fragment sαAPP and prevents amyloid β peptide (Aβ) generation. Moreover, α-secretase-like cleavage of the Aβ transporter 'receptor for advanced glycation end products' counteracts the import of blood Aβ into the brain. Assuming that Aβ is responsible for the development of Alzheimer's disease (AD), activation of α-secretase should be preventive. α-Secretase-mediated APP cleavage can be activated via several G protein-coupled receptors and receptor tyrosine kinases. Protein kinase C, mitogen-activated protein kinases, phosphatidylinositol 3-kinase, cAMP and calcium are activators of receptor-induced α-secretase cleavage. Selective targeting of receptor subtypes expressed in brain regions affected by AD appears reasonable. Therefore, the PACAP receptor PAC1 and possibly the serotonin 5-HT(6) receptor subtype are promising targets. Activation of APP α-secretase cleavage also occurs upon blockade of cholesterol synthesis by statins or zaragozic acid A. Under physiological statin concentrations, the brain cholesterol content is not influenced. Statins likely inhibit Aβ production in the blood by α-secretase activation which is possibly sufficient to inhibit AD development. A disintegrin and metalloproteinase 10 (ADAM10) acts as α-secretase on APP. By targeting the nuclear retinoic acid receptor β, the expression of ADAM10 and non-amyloidogenic APP processing can be enhanced. Excessive activation of ADAM10 should be avoided because ADAM10 and also ADAM17 are not APP-specific. Both ADAM proteins cleave various substrates, and therefore have been associated with tumorigenesis and tumor progression.

Specific Loss of Brain ABCA1 Increases Brain Cholesterol Uptake and Influences Neuronal Structure and Function

The expression of the cholesterol transporter ATP-binding cassette transporter A1 (ABCA1) in the brain and its role in the lipidation of apolipoproteins indicate that ABCA1 may play a critical role in brain cholesterol metabolism. To investigate the role of ABCA1 in brain cholesterol homeostasis and trafficking, we characterized mice that specifically lacked ABCA1 in the CNS, generated using the Cre/loxP recombination system. These mice showed reduced plasma high-density lipoprotein (HDL) cholesterol levels associated with decreased brain cholesterol content and enhanced brain uptake of esterified cholesterol from plasma HDL. Increased levels of HDL receptor SR-BI in brain capillaries and apolipoprotein A-I in brain and CSF of mutant mice were evident. Cholesterol homeostasis changes were mirrored by disturbances in motor activity and sensorimotor function. Changes in synaptic ultrastructure including reduced synapse and synaptic vesicle numbers were observed. These data show that ABCA1 is a key regulator of brain cholesterol metabolism and that disturbances in cholesterol transport in the CNS are associated with structural and functional deficits in neurons. Moreover, our findings also demonstrate that specific changes in brain cholesterol metabolism can lead to alterations in cholesterol uptake from plasma to brain.

Effect of docosahexaenoic acid on brain 3-hydroxy-3-methylglutaryl-coenzyme A reductase activity in male ICR mice

We investigated the influence of docosahexaenoic acid ethyl ester (DHA-EE) on 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase activity in the brains of adult and aged mice. Male mice (Crlj:CD-1) were fed diets containing 3% lard plus 2% linoleic acid ethyl ester (LA-EE), or 2% DHA-EE, for 3 months. The brain HMG-CoA reductase activity of 8-month-old (adult) mice was not significantly influenced by dietary intake of DHA-EE. However, in 18-month-old (aged) mice, its activity was enhanced with dietary intake of DHA-EE. Brain HMG-CoA reductase activity and brain cholesterol content significantly increased with age. Hepatic HMG-CoA reductase activity and the cholesterol content of both adult and aged mice were reduced in DHA-EE diet groups, compared with LA-EE diet groups. The DHA percentages of brain and liver microsomal fractions increased with the intake of DHA-EE in adult and aged mice. These results suggest that DHA may enhance brain HMG-CoA reductase activity in aged mice.

Amyloid excess in Alzheimer’s disease: What is cholesterol to be blamed for?

A link between alterations in cholesterol homeostasis and Alzheimer’s disease (AD) is nowadays widely accepted. However, the molecular mechanism/s underlying such link remain unclear. Numerous experimental evidences support the view that changes in neuronal membrane cholesterol levels and/or subcellular distribution determine the aberrant accumulation of the amyloid peptide in the disease. Still, this view comes from rather contradictory data supporting the existence of either high or low brain cholesterol content. This is of particular concern considering that therapeutical strategies aimed to reduce cholesterol levels are already being tested in humans. Here, we review the molecular mechanisms proposed and discuss the perspectives they open.

Cholesterol inhibits the insertion of the Alzheimer’s peptide Aβ(25–35) in lipid bilayers

The physiological relationship between brain cholesterol content and the action of amyloid beta (Abeta) peptide in Alzheimer's disease (AD) is a highly controversially discussed topic. Evidences for modulations of the Abeta/membrane interaction induced by plasma membrane cholesterol have already been observed. We have recently reported that Abeta(25-35) is capable of inserting in lipid membranes and perturbing their structure. Applying neutron diffraction and selective deuteration, we now demonstrate that cholesterol alters, at the molecular level, the capability of Abeta(25-35) to penetrate into the lipid bilayers; in particular, a molar weight content of 20% of cholesterol hinders the intercalation of monomeric Abeta(25-35) completely. At very low cholesterol content (about 1% molar weight) the location of the C-terminal part of Abeta(25-35) has been unequivocally established in the hydrocarbon region of the membrane, in agreement with our previous results on pure phospholipids membrane. These results link a structural property to a physiological and functional behavior and point to a therapeutical approach to prevent the AD by modulation of membrane properties.

High doses of simvastatin, pravastatin, and cholesterol reduce brain cholesterol synthesis in guinea pigs

Recent epidemiological studies suggest that inhibitors of 3-hydroxy-3-methyl-glutaryl CoA reductase, so-called statins, are effective in lowering the prevalence of Alzheimer’s disease. Whether the effect of statins is due to a local inhibition of cholesterol synthesis in the brain or whether it is mediated by the reduced levels of cholesterol in the circulation is not known. In the present work, we tested the possibility that high doses of lipophilic and hydrophilic statins, simvastatin and pravastatin, respectively, or a diet high in cholesterol could affect cholesterol homeostasis in the brain of guinea pigs. The total brain cholesterol levels were not affected by high-dose simvastatin or pravastatin treatment. Significantly lower levels of the cholesterol precursor lathosterol and its ratio to cholesterol were found in the brains of simvastatin and pravastatin-treated animals. 24S-Hydroxycholesterol, the transportable form of cholesterol across the blood–brain barrier, was significantly lower in the brain of pravastatin-treated animals. Excessive cholesterol feeding resulted in higher serum cholesterol levels but did not affect total brain cholesterol level. However, de novo cholesterol synthesis in the brain seemed to be down-regulated, as indicated by lower absolute levels and cholesterol-related ratios of lathosterol compared with controls. The passage of deuterium-labeled cholesterol across the blood–brain barrier in one animal was found to be approximately 1%. Our results suggest that brain cholesterol synthesis in guinea pigs can be slightly, but significantly, influenced by high doses of lipophilic and hydrophilic statins as well as by high dietary cholesterol intake, while total brain cholesterol content and thus, cholesterol homeostasis is maintained.

Ontogenic profile of brain lipids following perinatal exposure to cadmium.

Uniparous female rats were exposed to 10 ppm cadmium (as Cd(CH3COO)2) in drinking water, ad libitum, from day 0 of pregnancy; neonates exposed to Cd gestationally and lactationally were separated from their mothers at 21 days of age and were given 10 ppm Cd in drinking water up to 45 days of age. The sequential analysis of brain lipids during development showed a significant reduction in the total lipid levels at 15, 21, 30 and 45 days of age compared to the respective controls. No change in the triglyceride levels was observed in the Cd-exposed group, except for a slight but statistically significant increase (12%, p < 0.05) at day 15 of postnatal life. Metallic exposure also decreased the cholesterol content of brain at different time intervals in comparison to its levels in age-matched control rats. The analysis of sialic acid content of the total gangliosides revealed elevated levels of this group of glycolipids at 21 (25%), 30 (17%) and 45 (23%) days of age compared to their counterpart controls. However, a remarkable reduction in the concentration of brain galactosylceramide (30-43%) and 3'-sulphogalactosylceramide (24-37%) at 21, 30 and 45 days of age was observed following pre- and postnatal exposure of cadmium in comparison to the respective controls. The ontogenic profile of different brain phospholipids in the Cd-exposed group showed an increase in the levels of phosphatidylethanolamine at 21 (31%), 30 (25%) and 45 (19%) days of age; the phosphatidylcholine contents increased at day 21 (14%), followed by a significant decrease at 30 (14%) and 45 (19%) days compared to age-matched controls. Brain phosphatidylinositol, phosphatidylserine and sphingomyelin did not show any alteration at early periods of exposure but decreased significantly following continued exposure by 34%, 45% and 21%, respectively, at 45 days of age. Metal analysis in the brain showed a reduction in zinc and copper levels at 15 and 21 days of age in Cd-exposed animals; however, iron levels remained comparable with control values throughout the experiment. It may be concluded that an early exposure of Cd may produce alteration in the development of different lipids, which may produce CNS dysfunctions with a possibility of being manifested even in later life.

Age-related changes in brain and spinal cord lipids of the male garden lizard

The lipid composition (cholesterol, phospholipid triglyceride and fatty acids) of brain and spinal cord of the male garden lizard was traced during maturation and ageing. The total brain cholesterol content of both the tissues showed a significant rise during maturation, remaining almost constant thereafter. The free cholesterol content of the brain increased between middle and old age and that of the spinal cord increased during maturation. The esterified cholesterol content of the brain increased during maturation, the same parameter being not age-dependent in the spinal cord. While in the brain the phospholipid content increased during maturation followed by a decrease in old age, it showed a reverse trend in the spinal cord. The level of triglyceride in the brain declined during maturation followed by an increase in old age with no appreciable age change in the spinal cord. A comparison between young and old age-groups revealed a decrease in fatty acid content of the brain. A similar trend of decline was observed during maturation in the spinal cord. The pattern of age changes in the lipid profile of nervous tissue of the lizard, a non-mammalian vertebrate, almost conform to the pattern observed in a majority of mammals.

Hypolipidemic activity of dipyridamole: Effects on the main regulatory enzymes of cholesterogenesis

The in vivo dipyridamole treatment for 16 days produced a significant decrease in chick plasma cholesterol, mainly due to the esterified form. This effect was especially patent in the VLDL+LDL fraction. Similar results were observed in triglyceride content. To our knowledge, this is the first report on this hypolipidemic effects of dipyridamole. Total and esterified cholesterol increased after the same treatment in chick liver, while brain cholesterol content was not affected. Hepatic 3-hydroxy-3- methylglutaryl-CoA reductase activity was drastically reduced, while other secondary regulatory enzymes such as mevalonate kinase, mevalonate 5-phosphate kinase and mevalonate 5-pyrophosphate decarboxylase did not change significantly. No significant differences were found in cholesterol and lipidic phosphorus from liver microsomes, so that the effect of dipyridamole on reductase activity cannot be due to modifications in cholesterol/lipidic phosphorus molar ratio. Neither of these enzyme activities was affected in vitro by dipyridamole.

Altered thyroidal states modulate the insulin receptor characteristics of the developing rabbit brain.

We investigated the effect of propylthiouracil (PTU)-induced hypothyroidism and T4-induced hyperthyroidism on the fetal and neonatal rabbit brain insulin receptors (number and affinity) using plasma membranes. PTU administration to the pregnant mothers resulted in low serum-free T4 and normal total T3 concentrations, while T4 therapy to the mothers resulted in high serum-free T4 and high total T3 concentrations in the fetus and neonate. PTU-induced hypothyroidism did not affect the fetal brain insulin receptors, cholesterol content (brain homogenate) or protein content. On the other hand, brain insulin receptor number and total brain cholesterol content decreased in the neonate. T4 therapy at 100 micrograms/kg reversed the serum T4 to the control value and normalized the neonatal brain insulin receptor number and cholesterol content while a higher dose of T4 (200 micrograms/kg) increased the neonatal brain insulin receptor number, cholesterol and protein content. We conclude that altered thyroidal states modulate the brain insulin receptor (number and affinity) in neonatal, but not fetal brain plasma membranes.

Cytosolic 3-hydroxy-3-methyl glutaryl coenzyme a synthase in rat brain: properties and developmental change.

The properties and developmental change in the activity of cytosolic 3-hydroxy-3-methyl glutaryl coenzyme A (HMG-CoA) synthase in brain was examined and whether or not HMG-CoA lyase is present in cytosol and mitochondria from brain was determined. Although mitochondrial fractions contained significant HMG-CoA lyase activity, the enzyme activity was not detected in brain cytosol. The synthase activity was present in both mitochondrial and cytosolic fraction. The HMG-CoA synthesis by brain cytosol was optimal at pH 8.0 and did not require Mg2+ or exogenous acetoacetyl CoA. This indicates that brain cytosol can synthesize sufficient quantity of acetoacetyl CoA from acetyl CoA to be utilized for HMG-CoA synthesis. Our results also showed that the specific activity (nmol acetyl CoA incorporated/mg protein) of HMG-CoA synthase in brain cytosol was high (between 2-11 days of postnatal age) when the cholesterol content of brain is increasing rapidly, and the activity declined slowly thereafter. This suggests that in brain, cytosolic enzyme HMG-CoA synthase plays a role in the regulation of cholesterol synthesis.

Cholesterol esters and the ester-metabolizing enzymes in Jimpy and Quaking mouse brains

The concentration of free and esterified cholesterol and the activities of the cholesterol esterifying enzyme and cholesterol ester hydrolases in brains from Jimpy and Quaking mice and their normal littermates were determined. In Quaking affected mice these brain components did not differ significantly from those in brains of normal littermates. In the Jimpy affected mouse brain, however, the concentration of free cholesterol was reduced and the esterified cholesterol was increased. The increase in esterified cholesterol was accompanied by higher activity of cholesterol esterifying enzyme and a significant reduction in activity of hydrolases. In the absence of exogenous fatty acids, cholesterol esterification by cell-free preparations of brain from Jimpy affected mice was also significantly higher than the normal littermates. These data indicate that an increase in cholesterol ester concentrations in Jimpy affected brain may result either from an increase in esterification due to an increase in the availability of free fatty acid as substrate, or from deficiency in the activity of the hydrolases. In addition, the data indicate that in Jimpy mutants there is a significant reduction in brain cholesterol content, whereas in Quaking mutants the sterol content of developing brain is not significantly affected. This suggests that during brain development sterol synthesis in brain is reduced in Jimpy but not in Quaking mutants.

Elevated cholesterol in tissues of chicken embryos with hereditary myotonic muscular dystrophy

The cholesterol content of muscle, liver, serum, and brain was examined both in embryos of chickens with hereditary muscular dystrophy and in normal control embryos between 9 and 16 days in ovo. The superficial pectoral muscles, which are severely affected by muscular dystrophy later in development, have a significantly higher cholesterol content than those of normal embryos at all ages examined. In contrast, the cholesterol content of thigh muscles that are not severely affected by the dystrophic process is no different from that of control embryos. The cholesterol contents of liver and serum from dystrophic embryos are also increased, but the cholesterol content of the brain is not. The increased cholesterol contents of liver and serum are due to proportionate increases of both free cholesterol and cholesteryl esters. In contrast, in superficial pectoral muscles the ratio of cholesteryl esters to free cholesterol is significantly increased. These data indicate that, in addition to the elevated cholesterol in the liver and serum of dystrophic embryos, the regulation of cholesteryl ester content in the dystrophic pectoral muscles is abnormal.

Effect upon Brain Weight and Cholesterol Content of Maintaining Rats of Various Ages at Constant Weight

Rats weighing 400 g were maintained at constant weight for 500 days. Their diet consisted of 14 g/day for the first 150 days and 13 g/day thereafter. A second group of rats weighing 350 g was fed 11.5 g/day for 400 days; at the end of the experiment, these rats weighed 382 g. Under these conditions, the increase in brain weight and in quantity and concentration of cholesterol in the brain as a function of time was identical to that observed in the control rats fed ad libitum. Rats weighting 250 g fed 8.5 g/day for 200 days showed a body weight increase of 16 g. Up to the age of 115 days, the evolution of brain weight in terms of time did not differ from that observed in control rats. Rats weighting 100 g and fed 4.5 g/day showed an increase of 28 g after 300 days. The increase in brain weight and in brain cholesterol content as a function of time was less than that observed in the control rats. A curve deduced from these results has the practical interest of indicating the daily energy requirement for maintaining rats at a chosen constant weight. Expressed in terms of body surface area, the daily energy intake appears constant. It was also observed that, when conditions of minimal economy are imposed upon the adult rat, brain nutrition is not modified. But for young rats (100 g),brain development under these nutrtional conditions is affected.