Relationship between abnormal cholesterol synthesis and retarded learning in rats

Chemical Inhibition of Sterol Biosynthesis

Cholesterol is an essential molecule of life, and its synthesis can be inhibited by both genetic and nongenetic mechanisms. Hundreds of chemicals that we are exposed to in our daily lives can alter sterol biosynthesis. These also encompass various classes of FDA-approved medications, including (but not limited to) commonly used antipsychotic, antidepressant, antifungal, and cardiovascular medications. These medications can interfere with various enzymes of the post-lanosterol biosynthetic pathway, giving rise to complex biochemical changes throughout the body. The consequences of these short- and long-term homeostatic disruptions are mostly unknown. We performed a comprehensive review of the literature and built a catalogue of chemical agents capable of inhibiting post-lanosterol biosynthesis. This process identified significant gaps in existing knowledge, which fall into two main areas: mechanisms by which sterol biosynthesis is altered and consequences that arise from the inhibitions of the different steps in the sterol biosynthesis pathway. The outcome of our review also reinforced that sterol inhibition is an often-overlooked mechanism that can result in adverse consequences and that there is a need to develop new safety guidelines for the use of (novel and already approved) medications with sterol biosynthesis inhibiting side effects, especially during pregnancy.

Disposition of Proteins and Lipids in Synaptic Membrane Compartments Is Altered in Q175/Q7 Huntington's Disease Mouse Striatum

Dysfunction at synapses is thought to be an early change contributing to cognitive, psychiatric and motor disturbances in Huntington's disease (HD). In neurons, mutant Huntingtin collects in aggregates and distributes to the same sites as wild-type Huntingtin including on membranes and in synapses. In this study, we investigated the biochemical integrity of synapses in HD mouse striatum. We performed subcellular fractionation of striatal tissue from 2 and 6-month old knock-in Q175/Q7 HD and Q7/Q7 mice. Compared to striata of Q7/Q7 mice, proteins including GLUT3, Na+/K+ ATPase, NMDAR 2b, PSD95, and VGLUT1 had altered distribution in Q175/Q7 HD striata of 6-month old mice but not 2-month old mice. These proteins are found on plasma membranes and pre- and postsynaptic membranes supporting hypotheses that functional changes at synapses contribute to cognitive and behavioral symptoms of HD. Lipidomic analysis of mouse fractions indicated that compared to those of wild-type, fractions 1 and 2 of 6 months Q175/Q7 HD had altered levels of two species of PIP2, a phospholipid involved in synaptic signaling, increased levels of cholesterol ester and decreased cardiolipin species. At 2 months, increased levels of species of acylcarnitine, phosphatidic acid and sphingomyelin were measured. EM analysis showed that the contents of fractions 1 and 2 of Q7/Q7 and Q175/Q7 HD striata had a mix of isolated synaptic vesicles, vesicle filled axon terminals singly or in clusters, and ER and endosome-like membranes. However, those of Q175/Q7 striata contained significantly fewer and larger clumps of particles compared to those of Q7/Q7. Human HD postmortem putamen showed differences from control putamen in subcellular distribution of two proteins (Calnexin and GLUT3). Our biochemical, lipidomic and EM analysis show that the presence of the HD mutation conferred age dependent disruption of localization of synaptic proteins and lipids important for synaptic function. Our data demonstrate concrete biochemical changes suggesting altered integrity of synaptic compartments in HD mice that may mirror changes in HD patients and presage cognitive and psychiatric changes that occur in premanifest HD.

Neuronal Cholesterol Accumulation Induced by Cyp46a1 Down-Regulation in Mouse Hippocampus Disrupts Brain Lipid Homeostasis

Impairment in cholesterol metabolism is associated with many neurodegenerative disorders including Alzheimer's disease (AD). However, the lipid alterations underlying neurodegeneration and the connection between altered cholesterol levels and AD remains not fully understood. We recently showed that cholesterol accumulation in hippocampal neurons, induced by silencing Cyp46a1 gene expression, leads to neurodegeneration with a progressive neuronal loss associated with AD-like phenotype in wild-type mice. We used a targeted and non-targeted lipidomics approach by liquid chromatography coupled to high-resolution mass spectrometry to further characterize lipid modifications associated to neurodegeneration and cholesterol accumulation induced by CYP46A1 inhibition. Hippocampus lipidome of normal mice was profiled 4 weeks after cholesterol accumulation due to Cyp46a1 gene expression down-regulation at the onset of neurodegeneration. We showed that major membrane lipids, sphingolipids and specific enzymes involved in phosphatidylcholine and sphingolipid metabolism, were rapidly increased in the hippocampus of AAV-shCYP46A1 injected mice. This lipid accumulation was associated with alterations in the lysosomal cargoe, accumulation of phagolysosomes and impairment of endosome-lysosome trafficking. Altogether, we demonstrated that inhibition of cholesterol 24-hydroxylase, key enzyme of cholesterol metabolism leads to a complex dysregulation of lipid homeostasis. Our results contribute to dissect the potential role of lipids in severe neurodegenerative diseases like AD.

Dietary cholesterol degrades rabbit long term memory for discrimination learning but facilitates acquisition of discrimination reversal

We have shown previously that feeding dietary cholesterol before learning can improve acquisition whereas feeding cholesterol after learning can degrade long term memory. To examine these different findings within a single paradigm, we fed groups of rabbits 2% cholesterol or normal chow with or without 0.12 ppm copper added to the drinking water following two-tone discrimination learning of the nictitating membrane response in which a 8-kHz tone (conditioned stimulus, CS+) was followed by air puff and a 1-kHz tone (CS-) was not. After eight weeks on the diet, we assessed the rabbits' conditioned responding during testing and retraining. We then reversed the two-tone discrimination and assessed responding to the 1-kHz tone CS+ and the 8-kHz CS-. During testing, rabbits given cholesterol without copper had lower levels of responding to CS+ than rabbits in the other groups suggesting they did not retain the discrimination as well. However, during a brief discrimination retraining session, their response levels to the CS+ returned to the level of the other groups, demonstrating a return of the memory of the original discrimination. At the end of discrimination reversal, these same rabbits exhibited superior discrimination indexed by lower response levels to CS- but similar levels to CS+, suggesting they were better able to acquire the new relationship between the two tones by inhibiting CS- responses. These results add to our previous data by showing cholesterol diet-induced degradation of an old memory and facilitation of a new memory can both be demonstrated within a discrimination reversal paradigm. Given discrimination reversal is a hippocampally-dependent form of learning, the data support the role of cholesterol in modifying hippocampal function as we have shown previously with in vitro brain slice recordings.

Cholesterol and copper affect learning and memory in the rabbit

A rabbit model of Alzheimer's disease based on feeding a cholesterol diet for eight weeks shows sixteen hallmarks of the disease including beta amyloid accumulation and learning and memory changes. Although we have shown that feeding 2% cholesterol and adding copper to the drinking water can retard learning, other studies have shown that feeding dietary cholesterol before learning can improve acquisition and feeding cholesterol after learning can degrade long-term memory. We explore the development of this model, the issues surrounding the role of copper, and the particular contributions of the late D. Larry Sparks.

The molecular basis of cognitive deficits in pervasive developmental disorders

Persons with pervasive developmental disorders (PDD) exhibit a range of cognitive deficits that hamper their quality of life, including difficulties involving communication, sociability, and perspective-taking. In recent years, a variety of studies in mice that model genetic syndromes with a high risk of PDD have provided insights into the underlying molecular mechanisms associated with these disorders. What is less appreciated is how the molecular anomalies affect neuronal and circuit function to give rise to the cognitive deficits associated with PDD. In this review, we describe genetic mutations that cause PDD and discuss how they alter fundamental social and cognitive processes. We then describe efforts to correct cognitive impairments associated with these disorders and identify areas of further inquiry in the search for molecular targets for therapeutics for PDD.

Dietary Cholesterol Concentration and Duration Degrade Long-Term Memory of Classical Conditioning of the Rabbit's Nictitating Membrane Response

A rabbit model of Alzheimer's disease based on feeding a cholesterol diet for eight weeks shows sixteen hallmarks of the disease, including learning and memory changes. Although we have shown 2% cholesterol and copper in water can retard learning, other studies show feeding dietary cholesterol before learning can improve acquisition whereas feeding cholesterol after learning can degrade long-term memory. We explored this issue by manipulating cholesterol concentration and duration following classical trace conditioning of the rabbit's nictitating membrane response and assessed conditioned responding after eight weeks on cholesterol. First, rabbits given trace classical conditioning followed by 0.5%, 1%, or 2% cholesterol for eight weeks showed body weight and serum cholesterol levels that were a function of dietary cholesterol. Although all concentrations of cholesterol showed some sign of retarding long-term memory, the level of memory retardation was correlated with serum cholesterol levels. Second, rabbits given trace conditioning followed by different durations of a 2% cholesterol diet combined with different durations of a 0% control diet for 8 weeks showed duration and timing of a 2% cholesterol diet were important in affecting recall. The data support the idea that dietary cholesterol may retard long-term memory.

The effects of cholesterol on learning and memory

Cholesterol is vital to normal brain function including learning and memory but that involvement is as complex as the synthesis, metabolism and excretion of cholesterol itself. Dietary cholesterol influences learning tasks from water maze to fear conditioning even though cholesterol does not cross the blood brain barrier. Excess cholesterol has many consequences including peripheral pathology that can signal brain via cholesterol metabolites, pro-inflammatory mediators and antioxidant processes. Manipulations of cholesterol within the central nervous system through genetic, pharmacological, or metabolic means circumvent the blood brain barrier and affect learning and memory but often in animals already otherwise compromised. The human literature is no less complex. Cholesterol reduction using statins improves memory in some cases but not others. There is also controversy over statin use to alleviate memory problems in Alzheimer's disease. Correlations of cholesterol and cognitive function are mixed and association studies find some genetic polymorphisms are related to cognitive function but others are not. In sum, the field is in flux with a number of seemingly contradictory results and many complexities. Nevertheless, understanding cholesterol effects on learning and memory is too important to ignore.

Dietary cholesterol impairs memory and memory increases brain cholesterol and sulfatide levels

Cholesterol and sulfatides play many important roles in learning and memory. To date, our observations about the effects of cholesterol on learning have been assessed during response acquisition; that is, the learning of a new memory. Here, we report for the first time to our knowledge, on the effect of a cholesterol diet on a previously formed memory. Rabbits were given trace conditioning of the nictitating membrane response for 10 days, then fed a 2% cholesterol diet for 8 weeks, and then assessed for memory recall of the initially learned task. We show that dietary cholesterol had an adverse effect on memory recall. Second, we investigated whether dietary cholesterol caused an increase in brain cholesterol and sulfatide levels in four major brain structures (hippocampus, frontal lobe, brainstem, and cerebellum) using a technique for analyzing myelin and myelin-free fractions separately. Although our data confirm previous findings that dietary cholesterol does not directly affect cholesterol and establish that it does not affect sulfatide levels in the brain, these levels did increase rather significantly in the hippocampus and frontal lobe as a function of learning and memory.

Rapid, direct analysis of cholesterol by charge labeling in reactive desorption electrospray ionization

Direct and rapid analysis of cholesterol was accomplished in the ambient environment using reactive desorption electrospray ionization (DESI) mass spectrometry. This was achieved by electrospraying reagent solutions in the form of high velocity charged droplets at surfaces such as dried serum samples and animal tissue sections. Betaine aldehyde, incorporated into the spray solvent, reacts selectively and rapidly with the alcohol group of cholesterol by nucleophilic addition, forming a hemiacetal salt. Limits of detection for pure cholesterol and related compounds were approximately 1 ng when a solution of cholesterol of 1 microg/mL was spotted onto the surface. Quantitative analysis of free cholesterol in serum using reactive DESI was demonstrated using cholesterol-d7 as internal standard. High throughput analysis of small volumes of serum spotted onto a suitable substrate was achieved at an analysis rate of approximately 14 s per sample, with a relative standard deviation (RSD) of ca. 6%. Use of reactive DESI in the imaging mode allowed 2D spatial distributions of phospholipids and cholesterol to be recorded simultaneously in rat brain tissues.

The effects of coadministration palmitic acid and oleic acid (omega 9) on spatial learning and motor activity in adult male rat

In this study, the effects of oleic acid (10%) and palmitic acid (10%) on spatial learning and motor activity have been investigated. So, adult male Wister rats divided into four groups (n=10). Control group was fed with ordinary diet. other groups were fed with a diet containing oleic acid (10%) for 4 weeks or palmitic acid (10%) for 1 week. Also, coadministration group was fed with a diet containing oleic acid (10%) for 4 weeks plus palmitic acid (10%) for 1 week. In next step, rats were trained for spatial learning task by using T-maze at subsequently 9 days. Moreover, rats were tested for motor activity task by using Rota rod based on standard method. The results showed that the spatial learning and motor activity were significantly (p < 0.01, p < 0.001) increased in rats fed with oleic acid (10%) for 4 weeks. Also Spatial learning and motor activity were significantly (p < 0.001, p < 0.001) increased due to the diet containing palmitic acid 10% for 1 week. Also the Spatial learning and motor activity were significantly (p < 0.01, p < 0.001) increased in coadministration group diet containing oleic acid (10%) for 4 weeks and palmitic acid (10%) for 1 week but statistical test between three diet groups were not significant. A consensus has emerged from recent research that it is important the balance of the different type of fats. Because the type of fatty acids in the diet and ratio of these fatty acids determines the type of fatty acids that is available to the composition of cell membranes spatially in nerves system.

High dietary cholesterol facilitates classical conditioning of the rabbit's nictitating membrane response

Studies have shown that modifying dietary cholesterol may improve learning and that serum cholesterol levels can be positively correlated with cognitive performance. Rabbits fed a 0, 0.5, 1 or 2% cholesterol diet for eight weeks and 0.12 ppm copper added to their drinking water received trace and then delay classical conditioning pairing tone with corneal air puff during which movement of the nictitating membrane (NM) across the eye was monitored. We found that the level of classical conditioning and conditioning-specific reflex modification (CRM) as well as the number of beta amyloid-labeled neurons in the cortex and hippocampus were a function of the concentration of cholesterol in the diet. The data provide support for the idea that dietary cholesterol may facilitate learning and memory.

Cholesterol enhances classical conditioning of the rabbit heart rate response

The cholesterol-fed rabbit is a model of atherosclerosis and has been proposed as an animal model of Alzheimer's disease. Feeding rabbits cholesterol has been shown to increase the number of beta amyloid immunoreactive neurons in the cortex. Addition of copper to the drinking water of cholesterol-fed rabbits can increase this number still further and may lead to plaque-like structures. Classical conditioning of the nictitating membrane response in cholesterol-fed rabbits is retarded in the presence of these plaque-like structures but may be facilitated in their absence. In a factorial design, rabbits fed 2% cholesterol or a normal diet (0% cholesterol) for 8 weeks with or without copper added to the drinking water were given trace classical conditioning using a tone and periorbital electrodermal stimulation to study the effects of cholesterol and copper on classical conditioning of heart rate and the nictitating membrane response. Cholesterol-fed rabbits showed significant facilitation of heart rate conditioning and conditioning-specific modification of heart rate relative to normal diet controls. Consistent with previous research, cholesterol had minimal effects on classical conditioning of the nictitating membrane response when periorbital electrodermal stimulation was used as the unconditioned stimulus. Immunohistochemical analysis showed a significant increase in the number of beta amyloid positive neurons in the cortex, hippocampus and amygdala of the cholesterol-fed rabbits. Supplementation of drinking water with copper increased the number of beta amyloid positive neurons in the cortex of cholesterol-fed rabbits but did not produce plaque-like structures or have a significant effect on heart rate conditioning. The data provide additional support for our finding that, in the absence of plaques, dietary cholesterol may facilitate learning and memory.

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.

Intestinal absorption of cholesterol by patients with Smith-Lemli-Opitz syndrome

The Smith-Lemli-Opitz syndrome (SLOS) is a disorder of impaired cholesterol biosynthesis because of a deficiency of the enzyme 7-dehydrocholesterol-Delta(7)-reductase, in the last step in cholesterol biosynthesis. Dietary cholesterol has been proposed as a potential therapy for SLOS and is being tested currently. Because there is no information on cholesterol absorption in SLOS, we recruited 12 SLOS patients into the General Clinical Research Center for 1-wk periods for administration of test meals and for blood and stool collections. A test breakfast that contained tracer cholesterol-4-C(14) with egg yolk or with crystalline cholesterol in suspension was given subsequently. Twenty-four and 48-h blood and 1-wk stool samples then were collected. The radioactivities in these samples were analyzed to determine the absorption of cholesterol by these patients. In 11 patients who were given egg yolk cholesterol, cholesterol absorption was 27.3 +/- 6.7%. The absorption was slightly less at 20.5 +/- 10.3% but not significantly different for the six patients who were given crystalline cholesterol. There was a positive correlation between the absorption of isotopic cholesterol as measured by determination of radioactive cholesterol in stool and the amount of isotopic cholesterol in the plasma at 24 and 48 h after the meal. Our data indicated that SLOS patients absorb cholesterol from the diet. However, the percentage of absorption is lower than reported values for normal adults and for hypercholesterolemic children. The absorption of crystalline cholesterol in suspension was slightly lower than the absorption of cholesterol in egg yolk cholesterol by these patients. The absorption of cholesterol may ameliorate some of the biochemical and developmental deficits in SLOS patients.

Thematic review series: brain Lipids. Cholesterol metabolism in the central nervous system during early development and in the mature animal

Unesterified cholesterol is an essential structural component of the plasma membrane of every cell. During evolution, this membrane came to play an additional, highly specialized role in the central nervous system (CNS) as the major architectural component of compact myelin. As a consequence, in the human the mean concentration of unesterified cholesterol in the CNS is higher than in any other tissue (approximately 23 mg/g). Furthermore, even though the CNS accounts for only 2.1% of body weight, it contains 23% of the sterol present in the whole body pool. In all animals, most growth and differentiation of the CNS occurs in the first few weeks or years after birth, and the cholesterol required for this growth apparently comes exclusively from de novo synthesis. Currently, there is no evidence for the net transfer of sterol from the blood into the brain or spinal cord. In adults, the rate of synthesis exceeds the need for new structural sterol, so that net movement of cholesterol out of the CNS must take place. At least two pathways are used for this excretory process, one of which involves the formation of 24(S)-hydroxycholesterol. Whether or not changes in the plasma cholesterol concentration alter sterol metabolism in the CNS or whether such changes affect cognitive function in the brain or the incidence of dementia remain uncertain at this time.

Effects of dietary cholesterol on plasma lipoproteins in Smith-Lemli-Opitz syndrome

Smith-Lemli-Opitz syndrome is a condition of impaired cholesterol synthesis that is caused by mutations in DHCR7 encoding 7-dehydrocholesterol-Delta7 reductase. Birth defects and mental retardation are characteristic. Deficient plasma and tissue cholesterol and excess cholesterol precursors 7 and 8 dehydrocholesterol (7DHC and 8DHC) contribute to the pathogenesis. Cholesterol is transported to tissues via lipoproteins. We measured the effect of dietary cholesterol (egg yolk) on plasma lipoproteins to evaluate this potential treatment. We used the enzymatic method to measure total sterols in lipoproteins (n=12) and plasma (n=16). In addition, we analyzed individual plasma sterols by a gas chromatographic method. Samples were evaluated after 3 wk of a cholesterol-free diet and after 6-19 mo of dietary cholesterol. We also analyzed the distribution of sterols in lipoproteins and the apolipoprotein E genotype. Dietary cholesterol significantly increased the total sterols in plasma (2.22 +/- 0.13 to 3.10 +/- 0.22; mean +/- SEM; p < 0.002), in LDL (0.98 +/- 0.13 to 1.52 +/- 0.17 mM), and in HDL (0.72 +/- 0.04 to 0.92 +/- 0.07). Plasma cholesterol increased (1.78 +/- 0.16 to 2.67 +/- 0.25 mM; p < 0.007) and plasma 7DHC decreased in 10 children, but the mean decrease was not significant. The distribution of individual sterols in each lipoprotein fraction was similar to the distribution in plasma. The baseline cholesterol and the response to dietary cholesterol was the same in children with 3/3 and 3/4 apolipoprotein E genotypes. Dietary cholesterol increased total sterols in plasma, LDL, and HDL in children with Smith-Lemli-Opitz syndrome. These favorable increases in the lipoproteins are potentially therapeutic for this condition.

Cholesterol supplementation does not improve developmental progress in Smith-Lemli-Opitz syndrome

OBJECTIVE

Smith-Lemli-Opitz syndrome (SLOS) results in multiple malformations, growth deficiency, and mental retardation. Cholesterol supplementation has been used for several years to treat symptoms of SLOS. We assessed the developmental progress of children and adolescents with SLOS over a 6-year period.

STUDY DESIGN

Patients with SLOS (n=14) received continuous cholesterol supplementation as part of a longitudinal study. Assessment of their developmental progress in the areas of cognitive, motor, and adaptive skills occurred every 6 to 12 months. The progress of each subject over time and the progress of the group as a whole were analyzed by using a repeated-measures design and multiple t tests.

RESULTS

Developmental quotients did not improve over time for children with SLOS receiving cholesterol. In addition, baseline cholesterol levels, rather than age when supplementation began or increase in cholesterol levels, best predicted developmental outcome.

CONCLUSIONS

These results suggest that cholesterol supplementation in its current form does not improve the developmental progress of children and adolescents with SLOS.

Cholesterol modifies classical conditioning of the rabbit (Oryctolagus cuniculus) nictitating membrane response

Cholesterol plays an important role in synapse formation, receptor function, and synaptic plasticity, and animal studies show that modifying cholesterol may improve learning and memory. Other data show that feeding animals cholesterol can induce beta amyloid accumulation. Rabbits (Oryctolagus cuniculus) fed 2% cholesterol for 8 weeks were given trace conditioning of the nictitating membrane response using a 100-ms tone, a 700-ms trace, and periorbital electrical stimulation or airpuff. Rabbits fed cholesterol showed significant facilitation of trace conditioning to airpuff and conditioning-specific reflex modification to periorbital electrical stimulation and airpuff. The cholesterol-fed rabbits had beta amyloid accumulation in the cortex, but little in the hippocampus. The data suggest cholesterol had facilitative effects that outweighed potential amnesic effects of cortical beta amyloid.

Knockout of the cholesterol 24-hydroxylase gene in mice reveals a brain-specific mechanism of cholesterol turnover

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 approximately 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.

Molecular characterization of the detergent-insoluble cholesterol-rich membrane microdomain (raft) of the central nervous system

Many fundamental neurological issues such as neuronal polarity, the formation and remodeling of synapses, synaptic transmission, and the pathogenesis of the neuronal cell death are closely related to the membrane dynamics. The elucidation of functional roles of a detergent-insoluble cholesterol-rich domain (raft) could therefore provide good clues to the molecular understanding of these important phenomena, for the participation of the raft in the fundamental cell functions, such as signal transduction and selective transport of lipids and proteins, has been elucidated in nonneural cells. Interestingly, the brain is rich in raft and the brain-derived raft differs in its lipid and protein components from other tissue-derived rafts. Since many excellent reviews are written on the membrane lipid dynamics of this microdomain, signal transduction, and neuronal glycolipids, we review on the characterization of the raft proteins recovered in the detergent-insoluble low-density fraction from rat brain. Special focus is addressed on the biochemical characterization of a neuronal enriched protein, NAP-22, for the lipid organizing activity of this protein has become increasingly clear.

Developmental sensitivity of associative learning to cholesterol synthesis inhibitors

Patients with Smith-Lemli-Opitz syndrome, a genetic disorder associated with severe mental retardation, are unable to convert 7-dehydrocholesterol to cholesterol. Treatment of rats with agents that block cholesterol synthesis produces a sterol profile reminiscent of Smith-Lemli-Opitz patients i.e., low levels of cholesterol accompanied by the appearance of its immediate precursor 7-dehydrocholesterol. In previous work, chronic inhibition of cholesterol synthesis in just-weaned rats impaired acquisition of the classically conditioned eyeblink response. The present study had two primary goals--(1) to determine whether the learning impairment depended on the age in which treatment was initiated; and (2) to determine whether the deficit was associative or due to performance factors. Consistent with earlier work, acquisition of the eyeblink conditioned response was impaired when the 30-day treatment was initiated on postnatal day (PND) 21. Reactivity to acoustic stimuli and to eyelid stimulation were normal, suggesting that the learning impairment was associative in nature. The learning impairment was transitory; acquisition was normal when evaluated 30 days after the cessation of treatment. When treatment was initiated 30 days after weaning (PND 51), acquisition of the eyeblink response was normal. However, brain sterols of young adult rats were less affected than those of just-weaned rats. Thus, there is a developmental sensitivity to cholesterol synthesis blocking agents both in terms of their effects on brain sterols and new motor learning.

A cholesterol-lowering drug reduces beta-amyloid pathology in a transgenic mouse model of Alzheimer's disease

Clinical, epidemiological, and laboratory studies suggest that cholesterol may play a role in the pathogenesis of Alzheimer's disease (AD). Transgenic mice exhibiting an Alzheimer's beta-amyloid phenotype were treated with the cholesterol-lowering drug BM15.766 and tested for modulation of beta-amyloid levels. BM15.766 treatment reduced plasma cholesterol, brain Abeta peptides, and beta-amyloid load by greater than twofold. A strong, positive correlation between the amount of plasma cholesterol and Abeta was observed. Furthermore, drug treatment reduced the amyloidogenic processing of the amyloid precursor protein, suggesting alterations in processing in response to cholesterol modulation. This study demonstrates that hypocholesterolemia is associated with reduced Abeta accumulation suggesting that lowering cholesterol by pharmacological means may be an effective approach for reducing the risk of developing AD.

Cholesterol supplementation with egg yolk increases plasma cholesterol and decreases plasma 7-dehydrocholesterol in Smith-Lemli-Opitz syndrome

Smith-Lemli-Opitz syndrome (SLOS), an autosomal recessive condition comprising multiple malformations, mental retardation, and growth failure, results from reduced activity of the final enzyme in cholesterol biosynthesis, 7-dehydrocholesterol Delta(7)-reductase (DHCR7). Reduced plasma and tissue cholesterol concentrations and accumulation of cholesterol precursors including 7-dehydrocholesterol (7-DHC) are characteristic biochemical abnormalities. While it is still unclear what role these potentially toxic precursors have in the pathogenesis of this disorder, the accumulation of 7-DHC in the brain has been associated with impaired learning in rats and oxidized 7-DHC has been shown to induce growth retardation in cultured rat embryos. We hypothesized that supplemental dietary cholesterol would increase plasma cholesterol levels and suppress synthesis of 7-DHC and other abnormal sterols in individuals with SLOS. After baseline sterol levels were obtained, patients were provided supplemental cholesterol as egg yolk. Plasma sterols were analyzed by capillary-column gas chromatography over time in four children with SLOS. When evaluated at 4-8 weeks after the initiation of cholesterol supplementation, there was a marked increase in mean plasma cholesterol, from 53 mg/dl to 82 mg/dl. While the percent of total sterols as 7-DHC decreased from 15% to 10%, there was no change in total plasma 7-DHC levels. However, when evaluated 35-90 weeks after the institution of cholesterol supplementation, mean plasma 7-DHC decreased, from 11.3 mg/dl to 3.5 mg/dl (-67%, P < 0.05), along with an increase in mean plasma cholesterol from 53 mg/dl to 114 mg/dl (+116%, P < 0.05). These results support the hypothesis that over time dietary cholesterol supplementation from egg yolk increases the plasma cholesterol levels and decreases levels of 7-DHC which may be toxic. These data have important therapeutic implications in the management of SLOS.

Hypercholesterolemia accelerates the Alzheimer's amyloid pathology in a transgenic mouse model

Recent data suggest that cholesterol metabolism is linked to susceptibility to Alzheimer's disease (AD). However, no direct evidence has been reported linking cholesterol metabolism and the pathogenesis of AD. To test the hypothesis that amyloid beta-peptide (Abeta) deposition can be modulated by diet-induced hypercholesterolemia, we used a transgenic-mouse model for AD amyloidosis and examined the effects of a high-fat/high-cholesterol diet on central nervous system (CNS) Abeta accumulation. Our data showed that diet-induced hypercholesterolemia resulted in significantly increased levels of formic acid-extractable Abeta peptides in the CNS. Furthermore, the levels of total Abeta were strongly correlated with the levels of both plasma and CNS total cholesterol. Biochemical analysis revealed that, compared with control, the hypercholesterolemic mice had significantly decreased levels of sAPPalpha and increased levels of C-terminal fragments (beta-CTFs), suggesting alterations in amyloid precursor protein processing in response to hypercholesterolemia. Neuropathological analysis indicated that the hypercholesterolemic diet significantly increased beta-amyloid load by increasing both deposit number and size. These data demonstrate that high dietary cholesterol increases Abeta accumulation and accelerates the AD-related pathology observed in this animal model. Thus, we propose that diet can be used to modulate the risk of developing AD.

Blocking cholesterol synthesis impairs acquisition of the classically conditioned eyeblink response

Smith-Lemli-Opitz (SLO) syndrome is a congenital disorder characterized by severe mental retardation. Patients with SLO lack 7-dehydrocholesterol (7 dH) reductase, which catalyzes the last step of cholesterol synthesis. Administration of an agent that blocks 7 dH cholesterol reductase, BM 15.766 (BM), leads to a biochemical profile which resembles that of SLO patients, i.e., lower plasma, liver, and brain cholesterol levels accompanied by the appearance of the precursors 7 dH and 8 dH cholesterol. In this article we address the functional consequences of chronic BM treatment on new motor learning by assessing acquisition of the classically conditioned eyeblink response. Just-weaned rats were fed BM by gavage for four months, with half of these rats given exogenous cholesterol during the last two months of BM treatment. Acquisition of the eyeblink response was impaired in BM-treated rats. Impaired acquisition of the eyeblink response was not accompanied by alterations in responsiveness to either the conditioned or unconditioned stimulus. Exogenous cholesterol, a clinically relevant countertreatment, failed to correct for the learning impairment produced by BM treatment. Chronic treatment with a cholesterol synthesis-blocking agent impaired associative learning in just-weaned rats.

Cholesterol biosynthesis from lanosterol. Molecular cloning, tissue distribution, expression, chromosomal localization, and regulation of rat 7-dehydrocholesterol reductase, a Smith-Lemli-Opitz syndrome-related protein

The cDNA encoding the 471-amino acid rat 7-dehydrocholesterol reductase (DHCR), an enzyme that has been implicated in both cholesterol biosynthesis and developmental abnormalities (e.g. Smith-Lemli-Opitz syndrome) in mammals, has been cloned and sequenced, and the primary structure of the enzyme has been deduced. The DHCR gene was mapped to chromosome 8q2.1 by fluorescence in situ hybridization. Rat DHCR, calculated molecular mass of 54.15-kDa polypeptide, shares a close amino acid identity with mouse and human DHCRs (96 and 87%, respectively) as compared with its other related proteins (e.g. fungal sterol Delta14-reductase) and exhibits high hydrophobicity (>68%) with 9 transmembrane domains. Five putative sterol-sensing domains were predicted to be localized in transmembrane domains 4-8, which are highly homologous to those found in 3-hydroxymethylglutaryl-CoA reductase, sterol regulatory element-binding protein cleavage-activating protein, and patched protein. The polypeptide encoded by DHCR cDNA was expressed in yeast as a 55.45-kDa myc-tagged fusion protein, which was recognized with anti-myc monoclonal antibody 9E10 and shown to possess full DHCR activity with respect to dependence on NADPH and sensitivity to DHCR inhibitors. Northern blot analysis indicates that the highest expression of DHCR mRNA was detected in liver, followed by kidney and brain. In rat brains, the highest level of mRNA encoding DHCR was detected in the midbrain, followed by the spinal cord and medulla. Feeding rats 5% cholestyramine plus 0.1% lovastatin in chow resulted in both approximately a 3-fold induction of DHCR mRNA and a 5-fold increase of the enzymic activity in the liver. When rats were fed 0.1% (w/w) AY-9944 (in chow) for 14-days, a complete inhibition of DHCR activity and a significant reduction in serum total cholesterol level were observed. However, the level of hepatic DHCR mRNA fell only slightly, suggesting that AY-9944 may act more rapidly at the protein level than at the level of transcription of the DHCR gene under these conditions.

Molecular genetics of the Smith-Lemli-Opitz syndrome and postsqualene sterol metabolism

The Smith-Lemli-Opitz syndrome is a disorder of morphogenesis resulting from an enzymatic defect in the last step of cholesterol metabolism (reduction of 7-dehydrocholesterol). Analysis of the defective gene and identification of mutations therein have paved the way for the study of the molecular genetics of the disorder which is caused by numerous different mutations. Future efforts should identify a postulated intracellular signalling activity of sterol intermediates, isolate proteins that govern the sterol traffic between intracellular compartments, structurally characterize the enzyme delta 7-sterol reductase defective in the Smith-Lemli-Opitz syndrome and investigate the pathomechanism of sterol depletion-induced dysmorphogenesis.