Cholesterol metabolism and homeostasis in the brain

Cholesterol is an essential component for neuronal physiology not only during development stage but also in the adult life. Cholesterol metabolism in brain is independent from that in peripheral tissues due to bloodbrain barrier. The content of cholesterol in brain must be accurately maintained in order to keep brain function well. Defects in brain cholesterol metabolism has been shown to be implicated in neurodegenerative diseases, such as Alzheimer’s disease (AD), Huntington’s disease (HD), Parkinson’s disease (PD), and some cognitive deficits typical of the old age. The brain contains large amount of cholesterol, but the cholesterol metabolism and its complex homeostasis regulation are currently poorly understood. This review will seek to integrate current knowledge about the brain cholesterol metabolism with molecular mechanisms.     

Cholesterol and multilamellar bodies: Lysosomal dysfunction in GBA-Parkinson disease

Lipid and cholesterol metabolism might play a role in the pathogenesis of Parkinson disease (PD). However, the association between cholesterol and PD is not clearly established. Cholesterol accumulation is closely related to the expression of multilamellar bodies (MLBs). Also, cholesterol controls autophagosome transport. Thus, impaired cholesterol and autophagosome trafficking might lead to robust autophagic vacuole accumulation. Our recent work provides the first evidence that the presence of the N370S GBA mutation produces an accumulation of cholesterol, which alters autophagy-lysosome function with the appearance of MLBs, rendering the cell more vulnerable and sensitive to apoptosis.     

微生物细胞色素P450与胆固醇的生物代谢

细胞色素P450(CYP450)是一类含亚铁血红素的单加氧酶,广泛存在于各类生物体内,参与多种外源物质的代谢和内源物质的转化,如甾类激素、胆汁酸、胆固醇等的代谢。胆固醇是一种环戊烷多氢菲的衍生物,也是人类重要的脂类物质和许多特殊生物活性物质的前体之一,当其过量时会导致高胆固醇血症、动脉粥样硬化、静脉血栓生成等,对机体产生不利的影响。微生物CYP450酶可催化胆固醇的生物代谢,特别是其中的CYP125酶是胆固醇分解代谢起始的关键酶,可用作调节胆固醇代谢的药物靶标。     

Cholesterol as a causative factor in Alzheimer's disease: a debatable hypothesis

High serum/plasma cholesterol levels have been suggested as a risk factor for Alzheimer's disease (AD). Some reports, mostly retrospective epidemiological studies, have observed a decreased prevalence of AD in patients taking the cholesterol lowering drugs, statins. The strongest evidence causally linking cholesterol to AD is provided by experimental studies showing that adding/reducing cholesterol alters amyloid precursor protein (APP) and amyloid beta‐protein (Aβ) levels. However, there are problems with the cholesterol‐AD hypothesis. Cholesterol levels in serum/plasma and brain of AD patients do not support cholesterol as a causative factor in AD. Prospective studies on statins and AD have largely failed to show efficacy. Even the experimental data are open to interpretation given that it is well‐established that modification of cholesterol levels has effects on multiple proteins, not only amyloid precursor protein and Aβ. The purpose of this review, therefore, was to examine the above‐mentioned issues, discuss the pros and cons of the cholesterol‐AD hypothesis, involvement of other lipids in the mevalonate pathway, and consider that AD may impact cholesterol homeostasis.

     

Effects of Prolonged ACTH-stimulation on Adrenocortical Cholesterol Reserve and Apolipoprotein E Concentration in Young and Aged Fischer 344 Male Rats

Changes in the morphology of rat adrenal cortex with age include increased accumulations of lipid droplets and lipofuscin granules. Because glandular concentrations of cholesteryl esters (CE) and apolipoprotein (apo) E are also increased in parallel, the utilization or metabolism of lipid-droplet stored CE for steroidogenesis might be altered in aging cells. To explore this possibility, adrenocortical cholesterol storage and utilization were studied in 3-6 months-old (mo) (Y) rats and 20-23 mo (O) Fischer 344 male rats. Both groups received either adrenocorticotropin (ACTH1-39, Acthar gel) or gelatin alone daily for seven consecutive days.

We found that: (a) the CE concentration in O rats, but not Y animals, was diminished by ACTH. The depleted CE in stimulated-O rats was replenished within five days post stimulation. Failure to deplete CE in stimulated-Y rats was not associated with an insufficient dose of the hormone, since stimulation of Y animals with higher doses of ACTH actually increased the CE concentration. In contrast, adrenocortical free cholesterol concentration remained constant during stimulation regardless of age. (b) The depleted CE in stimulated-O rats was principally comprised of cholesteryl adrenate, cholesteryl arachidonate and cholesteryl cervonate. The accumulated CE in stimulated-Y animals was primarily comprised of cholesteryl adrenate, cholesteryl arachidonate and cholesteryl oleate. (c) Whereas in stimulated-Y rats adrenal apoE concentration declined, the concentration in stimulated O animals was well maintained. (d) In vitro, adrenal homogenate or cytosolic fraction from stimulated-O rats displayed a higher capacity to hydrolyze exogenous CE than its Y counterpart. However, cholesterol esterification with external fatty acid substrates in adrenal homogenate or microsomal fraction was comparable in the two age-groups.

Our findings revealed altered adrenocortical cholesterol reserve in O rats to cope with prolonged ACTH-stimulation. Changes in apoE levels and CE hydrolysis activity may be factors associated with this alteration. Depletion and accumulation of adrenocortical CE are reflected in parallel changes in cholesteryl adrenate and cholesteryl arachidonate, suggesting physiologic importance of these polyunsaturated fatty acids during sustained steroidogenesis.     

孤儿受体与胆固醇及胆汁酸的代谢调节

30多年前,已经发现体内胆固醇及胆汁酸在转录水平受反馈激活或反馈抑制的调节,其机理不清楚。最近,随着孤儿受体LXR基因的克隆及其功能的研究,逐步认识到包括LXR在内的几种孤儿受体作为体内胆固醇及胆汁酸的感受器,在转录水平调节体内胆固醇及胆汁酸的代谢平衡。这4类孤儿受体在胆固醇及其代谢产物与自身代谢平衡之间建立了直接的联系。综述了4类孤儿受体的研究进展,特别是它们和胆固醇及胆汁酸代谢平衡的关系。     

以芳香胺玻璃为载体的固定化的胆固醇脂酶和胆固醇氧化酶

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Plasma Markers of Cholesterol Homeostasis and Apolipoprotein B‐100 Kinetics in the Metabolic Syndrome

Objective: The metabolic syndrome is characterized by defective hepatic apolipoprotein B‐100 (apoB) metabolism. Hepato‐intestinal cholesterol metabolism may contribute to this abnormality. Research Methods and Procedures: We examined the association of cholesterol absorption and synthesis with the kinetics of apoB in 35 obese subjects with the metabolic syndrome. Plasma ratios of campesterol and lathosterol to cholesterol were used to estimate cholesterol absorption and synthesis, respectively. Very‐low‐density lipoprotein (VLDL), intermediate‐density lipoprotein (IDL), and low‐density lipoprotein apoB kinetics were studied using stable isotopy and mass spectrometry. Kinetic parameters were derived using multicompartmental modeling. Results: Compared with controls, the obese subjects had significantly lower plasma ratios of campesterol, but higher plasma ratios of lathosterol (p < 0.05 in both). This was associated with elevated VLDL‐apoB secretion rate (p < 0.05) and delayed fractional catabolism of IDL and low‐density lipoprotein‐apoB (p < 0.01). In the obese group, plasma ratios of campesterol correlated inversely with VLDL‐apoB secretion (r = ?0.359, p < 0.05), VLDL‐apoB (r = ?0.513, p < 0.01) and IDL‐apoB (r = ?0.511, p < 0.01) pool size, and plasma lathosterol ratio (r = ?0.366, p < 0.05). Subjects with low cholesterol absorption had significantly higher VLDL‐apoB secretion, VLDL‐apoB and IDL‐apoB pool size, and plasma lathosterol ratio (p < 0.05 in both) than those with high cholesterol absorption. Discussion: Subjects with the metabolic syndrome have oversecretion of VLDL‐apoB and decreased catabolism of apoB‐containing particles and low absorption and high synthesis rates of cholesterol. These changes in cholesterol homeostasis may contribute to the kinetic defects in apoB metabolism in the metabolic syndrome.     

Cholesterol esterification by ACAT2 is essential for efficient intestinal cholesterol absorption: evidence from thoracic lymph duct cannulation

The hypothesis tested in this study was that cholesterol esterification by ACAT2 would increase cholesterol absorption efficiency by providing cholesteryl ester (CE) for incorporation into chylomicrons. The assumption was that absorption would be proportional to Acat2 gene dosage. Male ACAT2^+/+, ACAT2^+/−, and ACAT2^−/− mice were fed a diet containing 20% of energy as palm oil with 0.2% (w/w) cholesterol. Cholesterol absorption efficiency was measured by fecal dual-isotope and thoracic lymph duct cannulation (TLDC) methods using [³H]sitosterol and [¹⁴C]cholesterol tracers. Excellent agreement among individual mice was found for cholesterol absorption measured by both techniques. Cholesterol absorption efficiency in ACAT2^−/− mice was 16% compared with 46–47% in ACAT2^+/+ and ACAT2^+/− mice. Chylomicrons from ACAT2^+/+ and ACAT2^+/− mice carried ∼80% of total sterol mass as CE, whereas ACAT2^−/− chylomicrons carried >90% of sterol mass in the unesterified form. The total percentage of chylomicron mass as CE was reduced from 12% in the presence of ACAT2 to ∼1% in ACAT2^−/− mice. Altogether, the data demonstrate that ACAT2 increases cholesterol absorption efficiency by providing CE for chylomicron transport, but one copy of the Acat2 gene, providing ∼50% of ACAT2 mRNA and enzyme activity, was as effective as two copies in promoting cholesterol absorption.     

Characterization of liver disease and lipid metabolism in the Niemann-Pick C1 mouse

Niemann-Pick type C1 (NPC1) disease is an autosomal-recessive cholesterol-storage disorder characterized by liver dysfunction, hepatosplenomegaly, and progressive neurodegeneration. The NPC1 gene is expressed in every tissue of the body, with liver expressing the highest amounts of NPC1 mRNA and protein. A number of studies have now indicated that the NPC1 protein regulates the transport of cholesterol from late endosomes/lysosomes to other cellular compartments involved in maintaining intracellular cholesterol homeostasis. The present study characterizes liver disease and lipid metabolism in NPC1 mice at 35 days of age before the development of weight loss and neurological symptoms. At this age, homozygous affected (NPC1(-/-)) mice were characterized with mild hepatomegaly, an elevation of liver enzymes, and an accumulation of liver cholesterol approximately four times that measured in normal (NPC1(+/+)) mice. In contrast, heterozygous (NPC1(+/-)) mice were without hepatomegaly and an elevation of liver enzymes, but the livers had a significant accumulation of triacylglycerol. With respect to apolipoprotein and lipoprotein metabolism, the results indicated only minor alterations in NPC1(-/-) mouse serum. Finally, compared to NPC1(+/+) mouse livers, the amount and processing of SREBP-1 and -2 proteins were significantly increased in NPC1(-/-) mouse livers, suggesting a relative deficiency of cholesterol at the metabolically active pool of cholesterol located at the endoplasmic reticulum. The results from this study further support the hypothesis that an accumulation of lipoprotein-derived cholesterol within late endosomes/lysosomes, in addition to altered intracellular cholesterol homeostasis, has a key role in the biochemical and cellular pathophysiology associated with NPC1 liver disease.     

Use of BODIPY‐Cholesterol (TF‐Chol) for Visualizing Lysosomal Cholesterol Accumulation

Dipyrromethene difluoride‐cholesterol (TopFluor‐Cholesterol, TF‐Chol) is a widely used cholesterol analogue due to its excellent fluorescence properties and considerable similarity with natural cholesterol in terms of membrane partitioning. However, the suitability of TF‐Chol for detecting lysosomal cholesterol deposition has recently been questioned. Here, we highlight the fact that the method of lipid delivery and the analysis of time‐point both affect the membrane distribution and labeling pattern of TF‐Chol, similarly as with radiolabeled cholesterol. Lysosomal sterol accumulation characteristic to a lysosomal storage disease is most readily detected when the probe is introduced via the physiological route, i.e. as a sterol fatty acid ester in low‐density lipoprotein particles. When administered to cells from solvent, lysosomal sterol sequestration becomes evident after an overnight equilibration between membranes.      

转运蛋白ABCG1/4 和ABCA1对脑胆固醇的调节作用

脑是富含胆固醇的器官,机体大约有25%的胆固醇集中在脑组织中.ATP结合盒超家族转运蛋白对脑组织中胆固醇的膜外转运和动态平衡起着重要的调节作用.研究发现,ATP结合盒超家族转运蛋白亚体ABCG1、ABCG4和ABCA1在成体脑组织中存在不同程度的表达,一种或多种亚体的缺失可以导致神经退行性病变.然而,ATP结合盒超家族转运蛋白亚体对脑发育过程中脑胆固醇动态变化的调节缺乏相关性的报道.在本研究中,从低胆固醇饮食喂养的C57BL/6J小鼠中获取出生后不同发育时期的脑组织,对ABCG1、ABCG4和ABCA1的mRNA与蛋白质表达水平进行测定,并对脑组织和血清中ATP结合盒超家族转运蛋白的表达水平与胆固醇水平的相关性进行研究.同时,使用ABCG1、ABCG4单一基因敲除鼠和ABCG1、ABCG4双基因敲除鼠,研究ATP结合盒超家族转运蛋白对与胆固醇合成的相关基因表达的影响以及对脑组织胆固醇代谢的调节作用.结果发现,ABCG1、ABCG4和ABCA1在机体多个器官中均有表达,但ABCG1和ABCG4在小鼠脑组织中表达量最高.在脑组织发育过程中,ABCG1和ABCG4mRNA水平呈现明显的表达时效性,小鼠于出生后42天达到峰值,而ABCA1 mRNA的表达水平无明显变化.血清和脑组织中中酯化型胆固醇水平呈双高峰分布,也于出生后42天达到最高.基因敲除鼠模型显示,单一敲除ABCG1或者ABCG4基因对脑组织胆固醇水平无明显影响,而ABCG1和ABCG4基因的同时缺失导致脑胆固醇水平显著升高,并明显降低胆固醇合成相关基因的表达水平.本研究表明,在脑发育成熟过程中,ATP结合盒超家族转运蛋白亚体ABCG1和ABCG4,而非ABCA1,以调节脑胆固醇的膜外转运;ABCG1和ABCG4互补调控脑胆固醇的动态平衡.     

Genetic risk for Alzheimer disease in children: Evidence from early‐life IQ and brain white‐matter microstructure

It remains unclear whether the genetic risk for late‐onset Alzheimer disease (AD) is linked to premorbid individual differences in general cognitive ability and brain structure. The objective of the present study was to determine whether the genetic risk of late‐onset AD is related to premorbid individual differences in intelligence quotient (IQ) and characteristics of the cerebral white‐matter in children. The study sample included children of the Generation R Study from Rotterdam, The Netherlands. IQ was measured using a well‐validated Dutch nonverbal IQ test (n = 1908) at ages 5 to 9 years. White‐matter microstructure was assessed by measuring fractional anisotropy (FA) of white‐matter tracts using diffusion tensor imaging (DTI) (n = 919) at ages 9 to 12 years. Genetic risk was quantified using three biologically defined genetic risk scores (GRSs) hypothesized to be related to the pathophysiology of late‐onset AD: immune response, cholesterol/lipid metabolism and endocytosis. Higher genetic risk for late‐onset AD that included genes associated with immune responsivity had a negative influence on cognition and cerebral white‐matter microstructure. For each unit increase in the immune response GRS, IQ decreased by 0.259 SD (95% CI [?0.500, ?0.017]). For each unit increase in the immune response GRS, global FA decreased by 0.373 SD (95% CI [?0.721, ?0.026]). Neither cholesterol/lipid metabolism nor endocytosis GRSs were associated with IQ or cerebral white‐matter microstructure. Our findings suggest that elevated genetic risk for late‐onset AD may in part be manifest during childhood neurodevelopment through alterations in immune responsivity.