Quantitation of two pathways for cholesterol excretion from the brain in normal mice and mice with neurodegeneration

Although the pool of cholesterol in the adult central nervous system (CNS) is large and of constant size, little is known of the process(es) involved in regulation of sterol 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. Deletion of cholesterol 24-hydroxylase, an enzyme highly expressed in the CNS, did not alter brain growth or myelination, 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 reflect 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.     

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.     

Cholesterol homeostasis in neurons and glial cells

Cholesterol is highly enriched in the brain compared to other tissues. Essentially all cholesterol in the brain is synthesized endogenously since plasma lipoproteins are unable to cross the blood-brain barrier. Cholesterol is transported within the central nervous system in the form of apolipoprotein E-containing lipoprotein particles that are secreted mainly by glial cells. Cholesterol is excreted from the brain in the form of 24-hydroxycholesterol. Apolipoprotein E and cholesterol have been implicated in the formation of amyloid plaques in Alzheimer's disease. In addition, the progressive neurodegenerative disorder Niemann-Pick C disease is characterized by defects in intracellular trafficking of cholesterol.     

骨钙素的中枢调控作用及其生物学机制

骨钙素(OCN)能调节多种外周组织器官的生理结构与功能,也发挥重要的中枢调控作用,与个体的学习和记忆等高级认知功能密切相关。研究表明,OCN穿过血脑屏障进入大脑,并与神经元或神经胶质细胞膜上的G蛋白偶联受体(GPCR)家族成员GPR158和GPR37结合,激活或抑制细胞内相关信号通路,改变神经元或神经胶质细胞的生理活性。OCN在脑内的作用主要包括调节5-羟色胺、多巴胺、去甲肾上腺素和γ-氨基丁酸等神经递质合成与释放、增加脑源性神经营养因子表达、促进海马神经发生、增强海马神经元自噬及维持髓鞘稳态等。此外,OCN还能参与调控多种神经退行性疾病的病理生理学进程。在阿尔茨海默病(AD)中,OCN干预能够部分减少β-淀粉样蛋白(Aβ)沉积及Aβ诱发的细胞毒性等,改善学习和记忆能力缺陷;在帕金森氏病(PD)中,OCN干预能够部分抑制黑质和纹状体多巴胺能神经元丢失,增加酪氨酸羟化酶含量及降低神经炎症等,缓解运动功能障碍。本文通过解析GPR158和GPR37的结构与功能,分析OCN在脑内的作用及其生物学机制,探讨OCN对AD和PD等神经退行性疾病的影响,为进一步筛选促进脑健康的新型靶点提供依据。     

Neurosteroids: trophic effects in the nervous system]

Some steroids, named "neurostero?ds", can be synthesized from cholesterol within both the central and peripheral nervous systems. Thus, pregnenolone and progesterone persist in the brain and in peripheral nerves long after removal of the steroidogenic endocrine glands by castration and adrenalectomy. The role of neurosteroids during the development of the nervous system is not well known, although they are synthesized by glial cells and some populations of neurons already during embryonic life. Cell culture experiments suggest that neurosteroids may influence the survival and differentiation of neurons and glial cells. In the adult nervous system, neurosteroids play an important role during regeneration. Progesterone is indeed synthesized by Schwann cells in peripheral nerves, where it plays an important role in the formation of new myelin sheaths after lesion. This is the first demonstration of a vital role for a neurosteroid in the nervous system.     

NG2-glia as multipotent neural stem cells: fact or fantasy?

Cycling glial precursors-"NG2-glia"-are abundant in the developing and mature central nervous system (CNS). During development, they generate oligodendrocytes. In culture, they can revert to a multipotent state, suggesting that they might have latent stem cell potential that could be harnessed to treat neurodegenerative disease. This hope has been subdued recently by a series of fate-mapping studies that cast NG2-glia as dedicated oligodendrocyte precursors in the healthy adult CNS-though rare, neuron production in the piriform cortex remains a possibility. Following CNS damage, the repertoire of NG2-glia expands to include Schwann cells and possibly astrocytes-but so far not neurons. This reaffirms the central role of NG2-glia in myelin repair. The realization that oligodendrocyte generation continues throughout normal adulthood has seeded the idea that myelin genesis might also be involved in neural plasticity. We review these developments, highlighting areas of current interest, contention, and speculation.     

Hypothesis of an Energetic Function for Myelin

Nervous system is a great oxygen consumer, but the site of oxygen absorption has remained elusive. Four proteomic studies have shown that the respiratory complexes I to V may be expressed in isolated myelin. Myelin is an outgrowth of glial cells, surrounding many axons in multiple spires both in peripheral and central nervous system. Recent quantitative analyses strongly support the daring hypothesis that myelin is functional in aerobic ATP production, to supply the neuron with chemical energy. A vision of myelin sheath as a structure devoted to the oxygen absorbance for glucose combustion in nervous system thank to its enormous surface, would be also supported by an impressive series of characteristics and properties of myelin that do not presently find an explanation, all of which are herein examined.     

Theoretical analysis of the importance of recycling in measurements of protein turnover by constant infusion of a labelled amino acid

In studies of whole body protein turnover, recycling of tracer from the breakdown of labelled protein is usually neglected; this neglect may introduce a significant error. A three-pool model with fast and slowly turning over protein pools has been used to calculate recycling rates over a range of sizes and turnover rates of the protein pools. Complete and approximate solutions of the equations are given. The recycling rate of 1% per hour would fit the available data on the turnover rates of human tissue proteins.     

Turnover of brain mitochondrial membrane lipids

1. The turnover of lipids, of myelin and other brain subcellular particles has been studied in double-labelling experiments on intact rats. 2. Overall metabolism of brain mitochondrial lipids was three times slower than that of the liver. 3. Individual lipids of brain mitochondria and myelin were also separated and their metabolism was studied. 4. All myelin lipids examined undergo very slow turnover. Two pools of brain mitochondrial lipid were identified. The slowly metabolized lipids were cholesterol, cardiolipin plus phosphatidic acid and possibly sphingomyelin; the remaining phosphatides underwent more rapid turnover. 5. The possible significance of these results is discussed.     

Origin of cholesterol in myelin

We review some of the older literature concerning metabolic turnover of cholesterol in the nervous system. The overall picture is that incorporation of radioactive precursors into brain cholesterol is roughly proportional to the rate of myelination and that, once incorporated, radioactive cholesterol is relatively stable metabolically. We outline a strategy for demonstrating the source (local synthesis or uptake from the circulation) of cholesterol in brain. The experimental design involves determining the rate of accumulation of cholesterol this is calculated as the increasing amounts of sterol in brain at successive time intervals during development. The rate of appearance of newly synthesized cholesterol is determined from incorporation of radioactivity from³H₂O (injected i.p. several hours prior to sacrifice) into cholesterol. The radioactivity associated with the sterol fractions and the specific activity of body water determined from the serum can be used to calculate the absolute amount of sterol newly synthesized during the time when³H₂O was present. The results obtained demonstrated that all of the bulk cholesterol accumulating in brain can be accounted for by newly synthesized cholesterol. None of the radioactive cholesterol came from the circulation, since cholesterol feeding suppressed cholesterol biosynthesis in the liver and specific radioactivity of circulating cholesterol was negligible. Thus, almost all cholesterol accumulating in brain during development is locally synthesized. Special issue dedicated to Dr. Marion R. Smith.     

Scaling of brain metabolism with a fixed energy budget per neuron: implications for neuronal activity, plasticity and evolution

It is usually considered that larger brains have larger neurons, which consume more energy individually, and are therefore accompanied by a larger number of glial cells per neuron. These notions, however, have never been tested. Based on glucose and oxygen metabolic rates in awake animals and their recently determined numbers of neurons, here I show that, contrary to the expected, the estimated glucose use per neuron is remarkably constant, varying only by 40% across the six species of rodents and primates (including humans). The estimated average glucose use per neuron does not correlate with neuronal density in any structure. This suggests that the energy budget of the whole brain per neuron is fixed across species and brain sizes, such that total glucose use by the brain as a whole, by the cerebral cortex and also by the cerebellum alone are linear functions of the number of neurons in the structures across the species (although the average glucose consumption per neuron is at least 10× higher in the cerebral cortex than in the cerebellum). These results indicate that the apparently remarkable use in humans of 20% of the whole body energy budget by a brain that represents only 2% of body mass is explained simply by its large number of neurons. Because synaptic activity is considered the major determinant of metabolic cost, a conserved energy budget per neuron has several profound implications for synaptic homeostasis and the regulation of firing rates, synaptic plasticity, brain imaging, pathologies, and for brain scaling in evolution.     

Regulatory effects of synthetic liver X receptor- and peroxisome-proliferator activated receptor agonists on sterol transport pathways in polarized cerebrovascular endothelial cells

The blood-brain barrier contributes to maintain brain cholesterol metabolism and protects this uniquely balanced system from exchange with plasma lipoprotein cholesterol. Brain capillary endothelial cells, representing a physiological barrier to the central nervous system, express apolipoprotein A-I (apoA-I, the major high-density lipoprotein (HDL)-associated apolipoprotein), ATP-binding cassette transporter A1 (ABCA1), and scavenger receptor, class B, type I (SR-BI), proteins that promote cellular cholesterol mobilization. Liver X receptors (LXRs) and peroxisome-proliferator activated receptors (PPARs) are regulators of cholesterol transport, and activation of LXRs and PPARs has potential therapeutic implications for lipid-related neurodegenerative diseases. To clarify the functional impact of LXR/PPAR activation, sterol transport along the: (i) ABCA1/apoA-I and (ii) SR-BI/HDL pathway was investigated in primary, polarized brain capillary endothelial cells, an in vitro model of the blood-brain barrier. Activation of LXR (24(S)OH-cholesterol, TO901317), PPARalpha (bezafibrate, fenofibrate), and PPARgamma (troglitazone, pioglitazone) modulated expression of apoA-I, ABCA1, and SR-BI on mRNA and/or protein levels without compromising transendothelial electrical resistance or tight junction protein expression. LXR-agonists and troglitazone enhanced basolateral-to-apical cholesterol mobilization in the absence of exogenous sterol acceptors. Along with the induction of cell surface-located ABCA1, several agonists enhanced cholesterol mobilization in the presence of exogenous apoA-I, while efflux of 24(S)OH-cholesterol (the major brain cholesterol metabolite) in the presence of exogenous HDL remained unaffected. Summarizing, in cerebrovascular endothelial cells apoA-I, ABCA1, and SR-BI represent drug targets for LXR and PPAR-agonists to interfere with cholesterol homeostasis at the periphery of the central nervous system.     

Distribution of [1,2-3H]cholesterol in mouse brain after injection in the suckling period

Glutaraldehyde-carbohydrazide polymer (GACH) was used to embed olfactory tracts, trapezoid body, and sciatic nerves of 9-, 10-, and 49- day old mice 2 h, 24 h, and 6 wk (respectively) after the intraperitoneal administration of [1,2-3H]cholesterol. Greater than 94% of radioactive cholesterol was retained in the GACH-infiltrated brain 24 h or more after injection. The fine structural preservation of both central and peripheral nervous tissues was excellent. Quantitative analysis of electron microscope autoradiographs demonstrated that [1,2- 3H]cholesterol is limited to blood vessel walls and lumen within the central nervous system at 2 h and 6 wk postinjection, but neurons and neuropil also contain the labeled cholesterol. The thickest myelin sheaths in the adult mice appear to be uniformly labeled throughout their width. No relationship of the retained [1,2-3H]cholesterol to the node of Ranvier was found in the adult sciatic nerve.     

Deficiency of ABCA1 impairs apolipoprotein E metabolism in brain

ABCA1 is a cholesterol transporter that is widely expressed throughout the body. Outside the central nervous system (CNS), ABCA1 functions in the biogenesis of high-density lipoprotein (HDL), where it mediates the efflux of cholesterol and phospholipids to apolipoprotein (apo) A-I. Deficiency of ABCA1 results in lack of circulating HDL and greatly reduced levels of apoA-I. ABCA1 is also expressed in cells within the CNS, but its roles in brain lipid metabolism are not yet fully understood. In the brain, glia synthesize the apolipoproteins involved in CNS lipid metabolism. Here we demonstrate that glial ABCA1 is required for cholesterol efflux to apoA-I and plays a key role in facilitating cholesterol efflux to apoE, which is the major apolipoprotein in the brain. In both astrocytes and microglia, ABCA1 deficiency reduces lipid efflux to exogenous apoE. The impaired ability to efflux lipids in ABCA1-/- glia results in lipid accumulation in both astrocytes and microglia under normal culture conditions. Additionally, apoE secretion is compromised in ABCA1-/- astrocytes and microglia. In vivo, deficiency of ABCA1 results in a 65% decrease in apoE levels in whole brain, and a 75-80% decrease in apoE levels in hippocampus and striatum. Additionally, the effect of ABCA1 on apoE is selective, as apoJ levels are unchanged in brains of ABCA1-/- mice. Taken together, these results show that glial ABCA1 is a key influence on apoE metabolism in the CNS.     

Oligodendrocytes in neurodegenerative diseases

Oligodendrocyte is a highly specialized glial cell type in the vertebrate central nervous system, which guarantees the long-distance transmission of action potential by producing myelin sheath wrapping adjacent axons. Disrupted myelin and oligodendrocytes are hallmarks of some devastating neurological diseases, such as multiple sclerosis, although their contribution to neurodegeneration in a given disease is still controversial. However, accumulating evidence from clinical studies and genetic animal models implicates oligodendrocyte dysfunction as one of major events in the processes of initiation and progression of neurodegeneration. In this article, we will review recent progress in understanding non-traditional function of oligodendrocytes in neuronal support and protection independent of myelin sheath and its possible contribution to neurodegeneration. Oligodendrocytes play a pivotal role in neurodegenerative diseases among which special emphasis is given to multiple system atrophy and Alzheimer’s disease in this review.     

Cholesterol for Synthesis of Myelin Is Made Locally, Not Imported into Brain

Abstract: We examined whether cholesterol needed for myelin formation is locally synthesized or whether it comes from the circulation. The experimental design was to inject [³H]water and to use incorporation of label into brain cholesterol as a measure of the rate of accumulation of newly synthesized cholesterol in brain. The contribution of the circulation to this labeled cholesterol pool was minimized by repressing liver synthesis of cholesterol with a high cholesterol diet. The rate of accumulation of total cholesterol was calculated from the increasing amounts of sterol in brain regions at successive time intervals during development. Thus, accumulating cholesterol not explained as being newly synthesized (radioactive) could be assumed to have come from the circulation. Long-Evans rats, ranging in age from birth to 35 days, were injected intraperitoneally with [³H]water (0.3–1.0 mCi/g of body weight) and killed 2 h later. The brain was dissected into brainstem, cerebellum, and cerebral hemispheres, and total lipids were extracted. Cholesterol and its precursors were quantified by HPLC. The radioactivity associated with the sterol fractions and the specific activity of body water determined from serum were used to calculate the absolute amount of newly synthesized sterol. The rates of cholesterol synthesis were compared with the rates of accumulation of total cholesterol in each brain region. The rate of accumulation of total sterol (cholesterol and desmosterol) closely followed that of newly synthesized total sterol in all brain regions from the second through the fifth postnatal weeks. Thus, all sterol accumulation in brain during the period of rapid myelination can be explained by local synthesis; neither diet nor production of cholesterol by other organs plays a direct role in supplying cholesterol for myelination in brain.     

Glial cells

The nervous system is built from two broad categories of cells, neurones and glial cells. The glial cells outnumber the neurones and the two cell types occupy a comparable amount of space in nervous tissue. The main glial cell types are, in the central nervous system, astrocytes and oligodendrocytes and, in the peripheral nervous system, Schwann cells, enteric glial cells and satellite cells. In the embryo, glial cells form a cellular framework that permits the development of the rest of the nervous system, and regulate neuronal survival and differentiation. The best known function of glia in the adult is the formation of myelin sheaths around axons thus allowing the fast conduction of signalling essential for nervous system function. Glia also maintain appropriate concentrations of ions and neurotransmitters in the neuronal environment. Increasing body of evidence indicates that glial cells are essential regulators of the formation, maintenance and function of synapses, the key functional unit of the nervous system.     

Cholesterol in cerebrospinal fluid of brain tumor patients

A sensitive method for assay of total cholesterol (free plus esterified) in one ml of CSF is presented. Patients with brain tumors showed much higher levels of this sterol in CSF than those with neoplasia external to the central nervous system. Repeated assay of CSF cholesterol in post-surgical follow-up of brain tumor patients undergoing chemotherapy may provide a biochemical tool for detection of renewed tumor activity.     

A novel fluorescent probe that is brain permeable and selectively binds to myelin.

Myelin is a multilayered glial cell membrane that forms segmented sheaths around large-caliber axons of both the central nervous system (CNS) and peripheral nervous system (PNS). Myelin covering insures rapid and efficient transmission of nerve impulses. Direct visual assessment of local changes of myelin content in vivo could greatly facilitate diagnosis and therapeutic treatments of myelin-related diseases. Current histologic probes for the visualization of myelin are based on antibodies or charged histochemical reagents that do not enter the brain. We have developed a series of chemical compounds including (E,E)-1,4-bis(4'-aminostyryl)-2-dimethoxy-benzene termed BDB and the subject of this report, which readily penetrates the blood-brain barrier and selectively binds to the myelin sheath in brain. BDB selectively stains intact myelinated regions in wild-type mouse brain, which allows for delineation of cuprizone-induced demyelinating lesions in mouse brain. BDB can be injected IV into the brain and selectively detect demyelinating lesions in cuprizone-treated mice in situ. These studies justified further investigation of BDB as a potential myelin-imaging probe to monitor myelin pathology in vivo.