A novel cholesterol stain reveals early neuronal cholesterol accumulation in the Niemann-Pick type C1 mouse brain

Cholesterol redistribution triggered by CYP46A1 gene therapy improves major hallmarks of Niemann-Pick type C disease but is not sufficient to halt neurodegeneration

Cholesterol 24-hydroxylase (CYP46A1) is an exclusively neuronal cytochrome P450 enzyme responsible for converting cholesterol into 24S-hydroxycholesterol, which serves as the primary pathway for eliminating cholesterol in the brain. We and others have shown that increased activity of CYP46A1 leads to reduced levels of cholesterol and has a positive effect on cognition. Therefore, we hypothesized that CYP46A1 could be a potential therapeutic target in Niemann-Pick type C (NPC) disease, a rare and fatal neurodegenerative disorder, characterized by cholesterol accumulation in endolysosomal compartments. Herein, we show that CYP46A1 ectopic expression, in cellular models of NPC and in Npc1tm(I1061T) mice by adeno-associated virus-mediated gene therapy improved NPC disease phenotype. Amelioration in functional, biochemical, molecular and neuropathological hallmarks of NPC disease were characterized. In vivo, CYP46A1 expression partially prevented weight loss and hepatomegaly, corrected the expression levels of genes involved in cholesterol homeostasis, and promoted a redistribution of brain cholesterol accumulated in late endosomes/lysosomes. Moreover, concomitant with the amelioration of cholesterol metabolism dysregulation, CYP46A1 attenuated microgliosis and lysosomal dysfunction in mouse cerebellum, favoring a pro-resolving phenotype. In vivo CYP46A1 ectopic expression improves important features of NPC disease and may represent a valid therapeutic approach to be used concomitantly with other drugs. However, promoting cholesterol redistribution does not appear to be enough to prevent Purkinje neuronal death in the cerebellum. This indicates that cholesterol buildup in neurons might not be the main cause of neurodegeneration in this human lipidosis.

The Niemann-Pick type diseases – A synopsis of inborn errors in sphingolipid and cholesterol metabolism

Disturbances of lipid homeostasis in cells provoke human diseases. The elucidation of the underlying mechanisms and the development of efficient therapies represent formidable challenges for biomedical research. Exemplary cases are two rare, autosomal recessive, and ultimately fatal lysosomal diseases historically named "Niemann-Pick" honoring the physicians, whose pioneering observations led to their discovery. Acid sphingomyelinase deficiency (ASMD) and Niemann-Pick type C disease (NPCD) are caused by specific variants of the sphingomyelin phosphodiesterase 1 (SMPD1) and NPC intracellular cholesterol transporter 1 (NPC1) or NPC intracellular cholesterol transporter 2 (NPC2) genes that perturb homeostasis of two key membrane components, sphingomyelin and cholesterol, respectively. Patients with severe forms of these diseases present visceral and neurologic symptoms and succumb to premature death. This synopsis traces the tortuous discovery of the Niemann-Pick diseases, highlights important advances with respect to genetic culprits and cellular mechanisms, and exposes efforts to improve diagnosis and to explore new therapeutic approaches.

Organ Weights in NPC1 Mutant Mice Partly Normalized by Various Pharmacological Treatment Approaches

Niemann-Pick Type C1 (NPC1, MIM 257220) is a rare, progressive, lethal, inherited autosomal-recessive endolysosomal storage disease caused by mutations in the NPC1 leading to intracellular lipid storage. We analyzed mostly not jet known alterations of the weights of 14 different organs in the BALB/cNctr-Npc1^m1N/-J Jackson Npc1 mice in female and male Npc1^+/+ and Npc1^-/- mice under various treatment strategies. Mice were treated with (i) no therapy, (ii) vehicle injection, (iii) a combination of miglustat, allopregnanolone, and 2-hydroxypropyl-ß-cyclodextrin (HPßCD), (iv) miglustat, and (v) HPßCD alone starting at P7 and repeated weekly throughout life. The 12 respective male and female wild-type mice groups were evaluated in parallel. In total, 351 mice (176 Npc1^+/+, 175 Npc1^-/-) were dissected at P65. In both sexes, the body weights of None and Sham Npc1^-/- mice were lower than those of respective Npc1^+/+ mice. The influence of the Npc1 mutation and/or sex on the weights of various organs, however, differed considerably. In males, Npc1^+/+ and Npc1^-/- mice had comparable absolute weights of lungs, spleen, and adrenal glands. In Npc1^-/- mice, smaller weights of hearts, livers, kidneys, testes, vesicular, and scent glands were found. In female Npc1^-/- mice, ovaries, and uteri were significantly smaller. In Npc1^-/- mice, relative organ weights, i.e., normalized with body weights, were sex-specifically altered to different extents by the different therapies. The combination of miglustat, allopregnanolone, and the sterol chelator HPßCD partly normalized the weights of more organs than miglustat or HPßCD mono-therapies.

Glial contribution to cyclodextrin-mediated reversal of cholesterol accumulation in murine NPC1-deficient neurons in vivo

Niemann-Pick type C disease is a rare and fatal lysosomal storage disorder presenting severe neurovisceral symptoms. Disease-causing mutations in genes encoding either NPC1 or NPC2 protein provoke accumulation of cholesterol and other lipids in specific structures of the endosomal-lysosomal system and degeneration of specific cells, notably neurons in the central nervous system (CNS). 2-hydroxypropyl-beta-cyclodextrin (CD) emerged as potential therapeutic approach based on animal studies and clinical data, but the mechanism of action in neurons has remained unclear. To address this topic in vivo, we took advantage of the retina as highly accessible part of the CNS and intravitreal injections as mode of drug administration. Coupling CD to gold nanoparticles allowed us to trace its intracellular location. We report that CD enters the endosomal-lysosomal system of neurons in vivo and enables the release of lipid-laden lamellar inclusions, which are then removed from the extracellular space by specific types of glial cells. Our data suggest that CD induces a concerted action of neurons and glial cells to restore lipid homeostasis in the central nervous system.

Role of Microglia in Regulating Cholesterol and Tau Pathology in Alzheimer's Disease

Cholesterol, a principal constituent of the cell membrane, plays a crucial role in the brain by regulating the synaptic transmission, neuronal signaling, as well as neurodegenerative diseases. Defects in the cholesterol trafficking are associated with enhanced generation of hyperphosphorylated Tau and Amyloid-β protein. Tau, a major microtubule-associated protein in the brain, is the key regulator of the mature neuron. Abnormally hyperphosphorylated Tau hampers the major functions related to microtubule assembly by promoting neurofibrillary tangles of paired helical filaments, twisted ribbons, and straight filaments. The observed pathological changes due to impaired cholesterol and Tau protein accumulation cause Alzheimer's disease. Thus, in order to regulate the pathogenesis of Alzheimer's disease, regulation of cholesterol metabolism, as well as Tau phosphorylation, is essential. The current review provides an overview of (1) cholesterol synthesis in the brain, neurons, astrocytes, and microglia; (2) the mechanism involved in modulating cholesterol concentration between the astrocytes and brain; (3) major mechanisms involved in the hyperphosphorylation of Tau and amyloid-β protein; and (4) microglial involvement in its regulation. Thus, the answering key questions will provide an in-depth information on microglia involvement in managing the pathogenesis of cholesterol-modulated hyperphosphorylated Tau protein.

Understanding and Treating Niemann-Pick Type C Disease: Models Matter

Biomedical research aims to understand the molecular mechanisms causing human diseases and to develop curative therapies. So far, these goals have been achieved for a small fraction of diseases, limiting factors being the availability, validity, and use of experimental models. Niemann–Pick type C (NPC) is a prime example for a disease that lacks a curative therapy despite substantial breakthroughs. This rare, fatal, and autosomal-recessive disorder is caused by defects in NPC1 or NPC2. These ubiquitously expressed proteins help cholesterol exit from the endosomal–lysosomal system. The dysfunction of either causes an aberrant accumulation of lipids with patients presenting a large range of disease onset, neurovisceral symptoms, and life span. Here, we note general aspects of experimental models, we describe the line-up used for NPC-related research and therapy development, and we provide an outlook on future topics.

Single Cell Transcriptome Analysis of Niemann-Pick Disease, Type C1 Cerebella

Niemann-Pick disease, type C1 (NPC1) is a lysosomal disease characterized by endolysosomal storage of unesterified cholesterol and decreased cellular cholesterol bioavailability. A cardinal symptom of NPC1 is cerebellar ataxia due to Purkinje neuron loss. To gain an understanding of the cerebellar neuropathology we obtained single cell transcriptome data from control (Npc1^+/+) and both three-week-old presymptomatic and seven-week-old symptomatic mutant (Npc1^-/-) mice. In seven-week-old Npc1^-/- mice, differential expression data was obtained for neuronal, glial, vascular, and myeloid cells. As anticipated, we observed microglial activation and increased expression of innate immunity genes. We also observed increased expression of innate immunity genes by other cerebellar cell types, including Purkinje neurons. Whereas neuroinflammation mediated by microglia may have both neuroprotective and neurotoxic components, the contribution of increased expression of these genes by non-immune cells to NPC1 pathology is not known. It is possible that dysregulated expression of innate immunity genes by non-immune cells is neurotoxic. We did not anticipate a general lack of transcriptomic changes in cells other than microglia from presymptomatic three-week-old Npc1^-/- mice. This observation suggests that microglia activation precedes neuronal dysfunction. The data presented in this paper will be useful for generating testable hypotheses related to disease progression and Purkinje neurons loss as well as providing insight into potential novel therapeutic interventions.

Loss of Calreticulin Uncovers a Critical Role for Calcium in Regulating Cellular Lipid Homeostasis

A direct link between Ca²⁺ and lipid homeostasis has not been definitively demonstrated. In this study, we show that manipulation of ER Ca²⁺ causes the re-distribution of a portion of the intracellular unesterified cholesterol to a pool that is not available to the SCAP-SREBP complex. The SREBP processing pathway in ER Ca²⁺ depleted cells remained fully functional and responsive to changes in cellular cholesterol status but differed unexpectedly in basal activity. These findings establish the role of Ca²⁺ in determining the reference set-point for controlling cellular lipid homeostasis. We propose that ER Ca²⁺ status is an important determinant of the basal sensitivity of the sterol sensing mechanism inherent to the SREBP processing pathway.

Reversal of Pathologic Lipid Accumulation in NPC1-Deficient Neurons by Drug-Promoted Release of LAMP1-Coated Lamellar Inclusions

Aging and pathologic conditions cause intracellular aggregation of macromolecules and the dysfunction and degeneration of neurons, but the mechanisms are largely unknown. Prime examples are lysosomal storage disorders such as Niemann-Pick type C (NPC) disease, where defects in the endosomal-lysosomal protein NPC1 or NPC2 cause intracellular accumulation of unesterified cholesterol and other lipids leading to neurodegeneration and fatal neurovisceral symptoms. Here, we investigated the impact of NPC1 deficiency on rodent neurons using pharmacologic and genetic models of the disease. Improved ultrastructural detection of lipids and correlative light and electron microscopy identified lamellar inclusions as the subcellular site of cholesterol accumulation in neurons with impaired NPC1 activity. Immunogold labeling combined with transmission electron microscopy revealed the presence of CD63 on internal lamellae and of LAMP1 on the membrane surrounding the inclusions, indicating their origins from intraluminal vesicles of late endosomes and of a lysosomal compartment, respectively. Lamellar inclusions contained cell-intrinsic cholesterol and surface-labeled GM1, indicating the incorporation of plasma membrane components. Scanning electron microscopy revealed that the therapeutic drug candidate β-cyclodextrin induces the subplasmalemmal location of lamellar inclusions and their subsequent release to the extracellular space. In parallel, β-cyclodextrin mediated the NPC1-independent redistribution of cholesterol within neurons and thereby abolished a deleterious cycle of enhanced cholesterol synthesis and its intracellular accumulation, which was indicated by neuron-specific transcript analysis. Our study provides new mechanistic insight into the pathologic aggregation of macromolecules in neurons and suggests exocytosis as cellular target for its therapeutic reversal.

SIGNIFICANCE STATEMENT

Many neurodegenerative diseases involve pathologic accumulation of molecules within neurons, but the subcellular location and the cellular impact are often unknown and therapeutic approaches lacking. We investigated these questions in the lysosomal storage disorder Niemann-Pick type C (NPC), where a defect in intracellular cholesterol transport causes loss of neurons and fatal neurovisceral symptoms. Here, we identify lamellar inclusions as the subcellular site of lipid accumulation in neurons, we uncover a vicious cycle of cholesterol synthesis and accretion, which may cause gradual neurodegeneration, and we reveal how β-cyclodextrin, a potential therapeutic drug, reverts these changes. Our study provides new mechanistic insight in NPC disease and uncovers new targets for therapeutic approaches.

The factors affecting lipid profile in adult patients with Mucopolysaccharidosis

Background

Mucopolysaccharidoses (MPS) are a group of rare inherited disorders characterized by abnormal accumulation of glycosaminoglycans (GAGs) within the myocytes and coronary arteries. Little is known about hyperlipidaemia as a potential cardiovascular risk factor in these patients. Baseline cholesterol data in adults are scarce. Therefore, the aim of this study was to analyse factors affecting lipid profile in different types of MPSs to determine if abnormalities in lipid profile contribute to the overall risk of cardiovascular disease.

Methods

Adult patients (above the age of 16) with MPS type I, II, III, IV and VI attending clinics in two Inherited Metabolic Disorders centres were included. Their lipid profile, lipoprotein (a), HbA1c, Glucose Tolerance Test (GTT), BMI and treatment type were extracted. Analysis included descriptive statistics and Student t-test.

Results

Eighty two patients with five MPS types (I, II, III, IV and VI) were included in the study; 29 were females (35%) and 53 were males (65%). BMI above 25 kg/m² in all MPS types indicated that some patients were overweight for their height. Only one patient post-HSCT had diabetes. In 3 cases insulin was analysed during GTT and showed no insulin resistance despite raised BMI. Mean total cholesterol and LDL-cholesterol were below 5 mmol/L and 3 mmol/L, respectively, in five individual MPS types. Lipoprotein (a) was available for 6 MPS IV patients and was not significantly raised.

Conclusions

MPS disorders are not associated with significant hypercholesterolaemia or diabetes mellitus despite increased BMI. Total cholesterol and LDL-cholesterol were within the targets for primary prevention for non-MPS population. Lipoprotein (a) is not a useful marker of cardiovascular disease in a small group of adult MPS IV patients irrespectively of treatment option. Whether long-term cardiovascular risk is dependent on lipid profile, diabetes, obesity or GAGs deposition within the organ system remains unanswered.

Cellular cholesterol homeostasis and Alzheimer's disease

Alzheimer's disease (AD) is the most common form of dementia in older adults. Currently, there is no cure for AD. The hallmark of AD is the accumulation of extracellular amyloid plaques composed of amyloid-β (Aβ) peptides (especially Aβ1-42) and neurofibrillary tangles, composed of hyperphosphorylated tau and accompanied by chronic neuroinflammation. Aβ peptides are derived from the amyloid precursor protein (APP). The oligomeric form of Aβ peptides is probably the most neurotoxic species; its accumulation eventually forms the insoluble and aggregated amyloid plaques. ApoE is the major apolipoprotein of the lipoprotein(s) present in the CNS. ApoE has three alleles, of which the Apoe4 allele constitutes the major risk factor for late-onset AD. Here we describe the complex relationship between ApoE4, oligomeric Aβ peptides, and cholesterol homeostasis. The review consists of four parts: 1) key elements involved in cellular cholesterol metabolism and regulation; 2) key elements involved in intracellular cholesterol trafficking; 3) links between ApoE4, Aβ peptides, and disturbance of cholesterol homeostasis in the CNS; 4) potential lipid-based therapeutic targets to treat AD. At the end, we recommend several research topics that we believe would help in better understanding the connection between cholesterol and AD for further investigations.

The new obesity-associated protein, neuronal growth regulator 1 (NEGR1), is implicated in Niemann-Pick disease Type C (NPC2)-mediated cholesterol trafficking

Neuronal growth regulator 1 (NEGR1) is a newly identified raft-associated protein, which has recently been spotlighted as a new locus related to human obesity. Niemann-Pick disease Type C2 (NPC2) protein functions as a key player in the intracellular cholesterol trafficking, and its defect is linked to a fatal human neurodegenerative disease, NPC. In this study, we identified that NEGR1 interacts with NPC2 and increases its protein stability. Ectopically expressed NEGR1 proteins relieved an abnormal cholesterol accumulation in endosomal compartments. Importantly, NEGR1-defective mouse embryonic fibroblast cells exhibit increased cholesterol levels and triglyceride contents. These findings provide the first insight into the role of NEGR1 in intracellular cholesterol homeostasis, possibly explaining the missing link between NEGR1 with human obesity.

Glymphatic distribution of CSF-derived apoE into brain is isoform specific and suppressed during sleep deprivation

Background

Apolipoprotein E (apoE) is a major carrier of cholesterol and essential for synaptic plasticity. In brain, it’s expressed by many cells but highly expressed by the choroid plexus and the predominant apolipoprotein in cerebrospinal fluid (CSF). The role of apoE in the CSF is unclear. Recently, the glymphatic system was described as a clearance system whereby CSF and ISF (interstitial fluid) is exchanged via the peri-arterial space and convective flow of ISF clearance is mediated by aquaporin 4 (AQP4), a water channel. We reasoned that this system also serves to distribute essential molecules in CSF into brain. The aim was to establish whether apoE in CSF, secreted by the choroid plexus, is distributed into brain, and whether this distribution pattern was altered by sleep deprivation.

Methods

We used fluorescently labeled lipidated apoE isoforms, lenti-apoE3 delivered to the choroid plexus, immunohistochemistry to map apoE brain distribution, immunolabeled cells and proteins in brain, Western blot analysis and ELISA to determine apoE levels and radiolabeled molecules to quantify CSF inflow into brain and brain clearance in mice. Data were statistically analyzed using ANOVA or Student’s t- test.

Results

We show that the glymphatic fluid transporting system contributes to the delivery of choroid plexus/CSF-derived human apoE to neurons. CSF-delivered human apoE entered brain via the perivascular space of penetrating arteries and flows radially around arteries, but not veins, in an isoform specific manner (apoE2 > apoE3 > apoE4). Flow of apoE around arteries was facilitated by AQP4, a characteristic feature of the glymphatic system. ApoE3, delivered by lentivirus to the choroid plexus and ependymal layer but not to the parenchymal cells, was present in the CSF, penetrating arteries and neurons. The inflow of CSF, which contains apoE, into brain and its clearance from the interstitium were severely suppressed by sleep deprivation compared to the sleep state.

Conclusions

Thus, choroid plexus/CSF provides an additional source of apoE and the glymphatic fluid transporting system delivers it to brain via the periarterial space. By implication, failure in this essential physiological role of the glymphatic fluid flow and ISF clearance may also contribute to apoE isoform-specific disorders in the long term.

Electronic supplementary material

The online version of this article (doi:10.1186/s13024-016-0138-8) contains supplementary material, which is available to authorized users.

A murine Niemann-Pick C1 I1061T knock-in model recapitulates the pathological features of the most prevalent human disease allele

Niemann-Pick Type C1 (NPC1) disease is a rare neurovisceral, cholesterol-sphingolipid lysosomal storage disorder characterized by ataxia, motor impairment, progressive intellectual decline, and dementia. The most prevalent mutation, NPC1(I1061T), encodes a misfolded protein with a reduced half-life caused by ER-associated degradation. Therapies directed at stabilization of the mutant NPC1 protein reduce cholesterol storage in fibroblasts but have not been tested in vivo because of lack of a suitable animal model. Whereas the prominent features of human NPC1 disease are replicated in the null Npc1(-/-) mouse, this model is not amenable to examining proteostatic therapies. The objective of the present study was to develop an NPC1 I1061T knock-in mouse in which to test proteostatic therapies. Compared with the Npc1(-/-) mouse, this Npc1(tm(I1061T)Dso) model displays a less severe, delayed form of NPC1 disease with respect to weight loss, decreased motor coordination, Purkinje cell death, lipid storage, and premature death. The murine NPC1(I1061T) protein has a reduced half-life in vivo, consistent with protein misfolding and rapid ER-associated degradation, and can be stabilized by histone deacetylase inhibition. This novel mouse model faithfully recapitulates human NPC1 disease and provides a powerful tool for preclinical evaluation of therapies targeting NPC1 protein variants with compromised stability.

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 blood-brain 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.

Characterisation of detergent-insoluble membranes in pollen tubes of Nicotiana tabacum (L.)

Pollen tubes are the vehicle for sperm cell delivery to the embryo sac during fertilisation of Angiosperms. They provide an intriguing model for unravelling mechanisms of growing to extremes. The asymmetric distribution of lipids and proteins in the pollen tube plasma membrane modulates ion fluxes and actin dynamics and is maintained by a delicate equilibrium between exocytosis and endocytosis. The structural constraints regulating polarised secretion and asymmetric protein distribution on the plasma membrane are mostly unknown. To address this problem, we investigated whether ordered membrane microdomains, namely membrane rafts, might contribute to sperm cell delivery. Detergent insoluble membranes, rich in sterols and sphingolipids, were isolated from tobacco pollen tubes. MALDI TOF/MS analysis revealed that actin, prohibitins and proteins involved in methylation reactions and in phosphoinositide pattern regulation are specifically present in pollen tube detergent insoluble membranes. Tubulins, voltage-dependent anion channels and proteins involved in membrane trafficking and signalling were also present. This paper reports the first evidence of membrane rafts in Angiosperm pollen tubes, opening new perspectives on the coordination of signal transduction, cytoskeleton dynamics and polarised secretion.

Assessing cholesterol storage in live cells and C. elegans by stimulated Raman scattering imaging of phenyl-Diyne cholesterol

We report a cholesterol imaging method using rationally synthesized phenyl-diyne cholesterol (PhDY-Chol) and stimulated Raman scattering (SRS) microscope. The phenyl-diyne group is biologically inert and provides a Raman scattering cross section that is 88 times larger than the endogenous C = O stretching mode. SRS microscopy offers an imaging speed that is faster than spontaneous Raman microscopy by three orders of magnitude, and a detection sensitivity of 31 μM PhDY-Chol (~1,800 molecules in the excitation volume). Inside living CHO cells, PhDY-Chol mimics the behavior of cholesterol, including membrane incorporation and esterification. In a cellular model of Niemann-Pick type C disease, PhDY-Chol reflects the lysosomal accumulation of cholesterol, and shows relocation to lipid droplets after HPβCD treatment. In live C. elegans, PhDY-Chol mimics cholesterol uptake by intestinal cells and reflects cholesterol storage. Together, our work demonstrates an enabling platform for study of cholesterol storage and trafficking in living cells and vital organisms.

Complex lipid trafficking in Niemann-Pick disease type C

Niemann-Pick disease type C (NPC) is an atypical lysosomal storage disease resulting from mutations in one of two genes, either NPC1 or NPC2. Although a neurovisceral disorder, it is above all a neurodegenerative disease in the vast majority of patients. Not an enzyme deficiency, it is currently conceived as a lipid trafficking disorder. Impaired egress of cholesterol from the late endosomal/lysosomal (LE/L) compartment is a specific and key element of the pathogenesis, but other lipids, more specially sphingolipids, are also involved, and there are indications for further abnormalities. The full function of the NPC1 and NPC2 proteins is still unclear. This review provides a reappraisal of lipid storage and lysosomal enzymes activities in tissues/cells from NPC patients and animal models. It summarizes the current knowledge on the NPC1 and NPC2 proteins and their function in transport of cholesterol within the late endosomal-lysosomal compartment, with emphasis on differences between systemic organs and the brain; it also discusses regulation by membrane lipids of the NPC2-mediated cholesterol trafficking, interplay between cholesterol and sphingomyelin, the metabolic origin of glycosphingolipids stored in brain, and the putative role of free sphingoid bases in pathogenesis. Brief mention is finally made of diseases affecting other genes that were very recently shown to impact the "NPC pathway".

Cholesterol homeostatic responses provide biomarkers for monitoring treatment for the neurodegenerative disease Niemann-Pick C1 (NPC1)

Niemann-Pick C1 (NPC1) disease is a rare, neurodegenerative lysosomal cholesterol storage disorder, typified by progressive cognitive and motor function impairment. Affected individuals usually succumb to the disease in adolescence. 2-Hydroxypropyl-β-cyclodextrin (HP-β-CD) has emerged as a promising intervention that reduces lipid storage and prolongs survival in NPC1 disease animal models. A barrier to the development of HP-β-CD and other treatments for NPC disease has been the lack of validated biochemical measures to evaluate efficacy. Here we explored whether cholesterol homeostatic responses resulting from HP-β-CD-mediated redistribution of sequestered lysosomal cholesterol could provide biomarkers to monitor treatment. Upon direct CNS delivery of HP-β-CD, we found increases in plasma 24(S)-HC in two independent NPC1 disease animal models, findings that were confirmed in human NPC1 subjects receiving HP-β-CD. Since circulating 24(S)-HC is almost exclusively CNS-derived, the increase in plasma 24(S)-HC provides a peripheral, non-invasive measure of the CNS effect of HP-β-CD. Our findings suggest that plasma 24(S)-HC, along with the other cholesterol-derived markers examined in this study, can serve as biomarkers that will accelerate development of therapeutics for NPC1 disease.

A marked paucity of granule cells in the developing cerebellum of the Npc1(-/-) mouse is corrected by a single injection of hydroxypropyl-β-cyclodextrin

In this study we show that postnatal development of cerebellar granule neurons (GNs) is defective in Npc1^−/− mice. Compared to age-matched wild-type littermates, there is an accelerated disappearance of the external granule layer (EGL) in these mice. This is due to a premature exit from the cell cycle of GN precursors residing at the level of the EGL. As a consequence, the size of cerebellar lobules of these mice displays a 20%–25% reduction compared to that of age-matched wild-type mice. This size reduction is detectable at post-natal day 28 (PN28), when cerebellar GN development is completed while signs of neuronal atrophy are not yet apparent.

Based on the analysis of EGL thickness and the determination of proliferating GN fractions at increasing developmental times (PN8–PN14), we trace the onset of this GN developmental defect during the second postnatal week. We also show that during this developmental time Shh transcripts undergo a significant reduction in Npc1^−/− mice compared to age-matched wild-type mice. In light of the mitogenic activity of Shh on GNs, this observation further supports the presence of defective GN proliferation in Npc1^−/− mice. A single injection of hydroxypropyl-β-cyclodextrin at PN7 rescues this defect, restoring the normal patterns of granule neuron proliferation and cerebellar lobule size.

To our knowledge, these findings identify a novel developmental defect that was underappreciated in previous studies. This defect was probably overlooked because Npc1 loss-of-function does not affect cerebellar foliation and causes the internal granule layer and molecular layer to decrease proportionally, giving rise to a normally appearing, yet harmoniously smaller, cerebellum.

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Cerebellar lobules of adult Npc1^−/− mice display a 20–25% reduction in size compared to wild-type age-matched mice.

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The proliferation of granule neuron (GN) precursors in the developing cerebellum of Npc1^−/− mice is defective.

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Npc1^−/− GN precursors of the external granule layer (EGL) undergo a premature exit from the cell cycle.

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The EGL of Npc1^−/− mice is thinner and persists for a shorter time.

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A single injection of hydroxypropyl-β-cyclodextrin at PN7 rescues these defects.

Visualization of cholesterol deposits in lysosomes of Niemann-Pick type C fibroblasts using recombinant perfringolysin O

BACKGROUND

Niemann-Pick disease type C (NPC) is caused by defects in cholesterol efflux from lysosomes due to mutations of genes coding for NPC1 and NPC2 proteins. As a result, massive accumulation of unesterified cholesterol in late endosomes/lysosomes is observed. At the level of the organism these cholesterol metabolism disorders are manifested by progressive neurodegeneration and hepatosplenomegaly. Until now filipin staining of cholesterol deposits in cells has been widely used for NPC diagnostics. In this report we present an alternative method for cholesterol visualization and estimation using a cholesterol-binding bacterial toxin, perfringolysin O.

METHODS

To detect cholesterol deposits, a recombinant probe, perfringolysin O fused with glutathione S-transferase (GST-PFO) was prepared. GST-PFO followed by labeled antibodies or streptavidin was applied for immunofluorescence and immunoelectron microscopy to analyze cholesterol distribution in cells derived from NPC patients. The identity of GST-PFO-positive structures was revealed by a quantitative analysis of their colocalization with several organelle markers. Cellular ELISA using GST-PFO was developed to estimate the level of unesterified cholesterol in NPC cells.

RESULTS

GST-PFO recognized cholesterol with high sensitivity and selectivity, as demonstrated by a protein/lipid overlay assay and surface plasmon resonance analysis. When applied to stain NPC cells, GST-PFO decorated abundant deposits of cholesterol in intracellular vesicles that colocalized with filipin-positive structures. These cholesterol deposits were resistant to 0.05%-0.2% Triton X-100 used for cells permeabilization in the staining procedure. GST-PFO-stained organelles were identified as late endosomes/lysosomes based on their colocalization with LAMP-1 and lysobisphosphatidic acid. On the other hand, GST-PFO did not colocalize with markers of the Golgi apparatus, endoplasmic reticulum, peroxisomes or with actin filaments. Only negligible GST-PFO staining was seen in fibroblasts of healthy individuals. When applied to cellular ELISA, GST-PFO followed by anti-GST-peroxidase allowed a semiquantitative analysis of cholesterol level in cells of NPC patients. Binding of GST-PFO to NPC cells was nearly abolished after extraction of cholesterol with methyl-β-cyclodextrin.

CONCLUSIONS

Our data indicate that a recombinant protein GST-PFO can be used to detect cholesterol accumulated in NPC cells by immunofluorescence and cellular ELISA. GST-PFO can be a convenient and reliable probe for revealing cholesterol deposits in cells and can be useful in diagnostics of NPC disease.

Trypanosoma cruzi infection results in an increase in intracellular cholesterol

Chagasic cardiomyopathy caused by Trypanosoma cruzi is a major health concern in Latin America and among immigrant populations in non-endemic areas. T. cruzi has a high affinity for host lipoproteins and uses the low density lipoprotein receptor (LDLr) for invasion. Herein, we report that T. cruzi infection is associated with an accumulation of LDL and cholesterol in tissues in both acute and chronic murine Chagas disease. Similar findings were observed in tissue samples from a human case of Chagasic cardiomyopathy. T. cruzi infection of cultured cells displayed increased invasion with increasing cholesterol levels in the medium. Studies of infected host cells demonstrated alterations in their cholesterol regulation. T. cruzi invasion/infection via LDLr appears to be involved in changes in intracellular cholesterol homeostasis. The observed changes in intracellular lipids and associated oxidative stress due to these elevated lipids may contribute to the development of Chagasic cardiomyopathy.

Perfringolysin O structure and mechanism of pore formation as a paradigm for cholesterol-dependent cytolysins

Cholesterol-dependent cytolysins (CDCs) constitute a family of pore forming toxins secreted by Gram-positive bacteria. These toxins form transmembrane pores by inserting a large β-barrel into cholesterol-containing membrane bilayers. Binding of water-soluble CDCs to the membrane triggers the formation of oligomers containing 35-50 monomers. The coordinated insertion of more than seventy β-hairpins into the membrane requires multiple structural conformational changes. Perfringolysin O (PFO), secreted by Clostridium perfringens, has become the prototype for the CDCs. In this chapter, we will describe current knowledge on the mechanism of PFO cytolysis, with special focus on cholesterol recognition, oligomerization, and the conformational changes involved in pore formation.

Pre-symptomatic activation of antioxidant responses and alterations in glucose and pyruvate metabolism in Niemann-Pick Type C1-deficient murine brain

Niemann-Pick Type C (NPC) disease is an autosomal recessive neurodegenerative disorder caused in most cases by mutations in the NPC1 gene. NPC1-deficiency is characterized by late endosomal accumulation of cholesterol, impaired cholesterol homeostasis, and a broad range of other cellular abnormalities. Although neuronal abnormalities and glial activation are observed in nearly all areas of the brain, the most severe consequence of NPC1-deficiency is a near complete loss of Purkinje neurons in the cerebellum. The link between cholesterol trafficking and NPC pathogenesis is not yet clear; however, increased oxidative stress in symptomatic NPC disease, increases in mitochondrial cholesterol, and alterations in autophagy/mitophagy suggest that mitochondria play a role in NPC disease pathology. Alterations in mitochondrial function affect energy and neurotransmitter metabolism, and are particularly harmful to the central nervous system. To investigate early metabolic alterations that could affect NPC disease progression, we performed metabolomics analyses of different brain regions from age-matched wildtype and Npc1^-/- mice at pre-symptomatic, early symptomatic and late stage disease by ¹H-NMR spectroscopy. Metabolic profiling revealed markedly increased lactate and decreased acetate/acetyl-CoA levels in Npc1^-/- cerebellum and cerebral cortex at all ages. Protein and gene expression analyses indicated a pre-symptomatic deficiency in the oxidative decarboxylation of pyruvate to acetyl-CoA, and an upregulation of glycolytic gene expression at the early symptomatic stage. We also observed a pre-symptomatic increase in several indicators of oxidative stress and antioxidant response systems in Npc1^-/- cerebellum. Our findings suggest that energy metabolism and oxidative stress may present additional therapeutic targets in NPC disease, especially if intervention can be started at an early stage of the disease.

Identification of Niemann-Pick C1 disease biomarkers through sphingolipid profiling

Niemann-Pick type C (NPC)1 is a rare neurodegenerative disease for which treatment options are limited. A major barrier to development of effective treatments has been the lack of validated biomarkers to monitor disease progression or serve as outcome measures in clinical trials. Using targeted metabolomics to exploit the complex lipid storage phenotype that is the hallmark of NPC1 disease, we broadly surveyed Npc1(-/-) mouse tissues and identified elevated species across multiple sphingolipid classes that increased with disease progression. There was a striking accumulation of sphingoid bases, monohexosylceramides (MCs), and GM2 gangliosides in liver, and sphingoid bases and GM2 and GM3 gangliosides in brain. These lipids were modestly decreased following miglustat treatment, but markedly decreased in response to treatment with 2-hydroxypropyl-β-cyclodextrin (HP-β-CD), two drugs that have shown efficacy in NPC1 animal models. Extending these studies to human subjects led to identification of sphingolipid classes that were significantly altered in the plasma of NPC1 patients. Plasma MCs and ceramides were elevated, whereas sphingoid bases were reduced in NPC1 subjects. Intervention with miglustat in NPC1 patients was accompanied by striking alterations in plasma (reductions in GM1 and GM3 gangliosides) and cerebrospinal fluid (CSF) (increased MCs) sphingolipids. Similar alterations were observed in the CSF from the NPC1 feline model following HP-β-CD treatment. Our findings suggest that these lipid biomarkers may prove useful as outcome measures for monitoring efficacy of therapy in clinical trials.

Lipid metabolism and neuroinflammation in Alzheimer's disease: a role for liver X receptors

Liver X receptors (LXR) are nuclear receptors that have emerged as key regulators of lipid metabolism. In addition to their functions as cholesterol sensors, LXR have also been found to regulate inflammatory responses in macrophages. Alzheimer's disease (AD) is a neurodegenerative disease characterized by a progressive cognitive decline associated with inflammation. Evidence indicates that the initiation and progression of AD is linked to aberrant cholesterol metabolism and inflammation. Activation of LXR can regulate neuroinflammation and decrease amyloid-β peptide accumulation. Here, we highlight the role of LXR in orchestrating lipid homeostasis and neuroinflammation in the brain. In addition, diabetes mellitus is also briefly discussed as a significant risk factor for AD because of the appearing beneficial effects of LXR on glucose homeostasis. The ability of LXR to attenuate AD pathology makes them potential therapeutic targets for this neurodegenerative disease.

A novel mouse model of Niemann-Pick type C disease carrying a D1005G-Npc1 mutation comparable to commonly observed human mutations

We have identified a point mutation in Npc1 that creates a novel mouse model (Npc1(nmf164)) of Niemann-Pick type C1 (NPC) disease: a single nucleotide change (A to G at cDNA bp 3163) that results in an aspartate to glycine change at position 1005 (D1005G). This change is in the cysteine-rich luminal loop of the NPC1 protein and is highly similar to commonly occurring human mutations. Genetic and molecular biological analyses, including sequencing the Npc1(spm) allele and identifying a truncating mutation, confirm that the mutation in Npc1(nmf164) mice is distinct from those in other existing mouse models of NPC disease (Npc1(nih), Npc1(spm)). Analyses of lifespan, body and spleen weight, gait and other motor activities, as well as acoustic startle responses all reveal a more slowly developing phenotype in Npc1(nmf164) mutant mice than in mice with the null mutations (Npc1(nih), Npc1(spm)). Although Npc1 mRNA levels appear relatively normal, Npc1(nmf164) brain and liver display dramatic reductions in Npc1 protein, as well as abnormal cholesterol metabolism and altered glycolipid expression. Furthermore, histological analyses of liver, spleen, hippocampus, cortex and cerebellum reveal abnormal cholesterol accumulation, glial activation and Purkinje cell loss at a slower rate than in the Npc1(nih) mouse model. Magnetic resonance imaging studies also reveal significantly less demyelination/dysmyelination than in the null alleles. Thus, although prior mouse models may correspond to the severe infantile onset forms of NPC disease, Npc1(nmf164) mice offer many advantages as a model for the late-onset, more slowly progressing forms of NPC disease that comprise the large majority of human cases.

Generation of a human neuronal stable cell model for niemann-pick C disease by RNA interference

Niemann-Pick type C (NPC) disease is a fatal autosomal recessive neurodegenerative disorder caused, most commonly, by mutations in the NPC1 gene. At the cellular level, the disease is characterized by the storage of multiple lipids in the endosomal-lysosomal system, including free cholesterol, glycosphingolipids, sphingomyelin and the catabolic product of sphingolipids, sphingosine. Therapeutic options for NPC disease are relatively limited. One drawback for the development of novel therapies is the lack of suitable human neuronal cell models. In this work, a stable SH-SY5Y cell model for NPC disease was generated using short hairpin RNAs. An inhibition of the NPC1 expression of around 90% was obtained at the RNA level. The NPC1 knockdown was confirmed at the protein level. To characterize the stable cell line generated, cholesterol levels were analyzed in the NPC1-knockdown SH-SY5Y cells by filipin staining and gas chromatography-mass spectrometry. A characteristic NPC pattern and a twofold increase of the free cholesterol levels, related to intact SH-SY5Y cells, were found. Moreover, sphingolipids were analyzed by liquid chromatography-mass spectrometry and an increase in ganglioside GM2 levels was observed. The stable NPC1-knockdown SH-SY5Y cell line generated in the present study provides a human neuronal cell model for this lethal disease that could be a valuable tool for the study of future therapeutic approaches.

Cholesterol metabolism in Huntington disease

The CNS is rich in cholesterol, which is essential for neuronal development and survival, synapse maturation, and optimal synaptic activity. Alterations in brain cholesterol homeostasis are linked to neurodegeneration. Studies have demonstrated that Huntington disease (HD), a progressive and fatal neurodegenerative disorder resulting from polyglutamine expansion in the huntingtin protein, is associated with changes in cellular cholesterol metabolism. Emerging evidence from human and animal studies indicates that attenuated brain sterol synthesis and accumulation of cholesterol in neuronal membranes represent two distinct mechanisms occurring in the presence of mutant huntingtin that influence neuronal survival. Increased knowledge of how changes in intraneuronal cholesterol metabolism influence the pathogenesis of HD will provide insights into the potential application of brain cholesterol regulation as a therapeutic strategy for this devastating disease.

Cholesterol metabolism in neurons and astrocytes

Cells in the mammalian body must accurately maintain their content of cholesterol, which is an essential membrane component and precursor for vital signalling molecules. Outside the brain, cholesterol homeostasis is guaranteed by a lipoprotein shuttle between the liver, intestine and other organs via the blood circulation. Cells inside the brain are cut off from this circuit by the blood-brain barrier and must regulate their cholesterol content in a different manner. Here, we review how this is accomplished by neurons and astrocytes, two cell types of the central nervous system, whose cooperation is essential for normal brain development and function. The key observation is a remarkable cell-specific distribution of proteins that mediate different steps of cholesterol metabolism. This form of metabolic compartmentalization identifies astrocytes as net producers of cholesterol and neurons as consumers with unique means to prevent cholesterol overload. The idea that cholesterol turnover in neurons depends on close cooperation with astrocytes raises new questions that need to be addressed by new experimental approaches to monitor and manipulate cholesterol homeostasis in a cell-specific manner. We conclude that an understanding of cholesterol metabolism in the brain and its role in disease requires a close look at individual cell types.

Endosomal/lysosomal processing of gangliosides affects neuronal cholesterol sequestration in Niemann-Pick disease type C

Niemann-Pick disease type C (NPC) is a severe neurovisceral lysosomal storage disorder caused by defects in NPC1 or NPC2 proteins. Although numerous studies support the primacy of cholesterol storage, neurons of double-mutant mice lacking both NPC1 and an enzyme required for synthesis of all complex gangliosides (β1,4GalNAc transferase) have been reported to exhibit dramatically reduced cholesterol sequestration. Here we show that NPC2-deficient mice lacking this enzyme also exhibit reduced cholesterol, but that genetically restricting synthesis to only a-series gangliosides fully restores neuronal cholesterol storage to typical disease levels. Examining the subcellular locations of sequestered compounds in neurons lacking NPC1 or NPC2 by confocal microscopy revealed that cholesterol and the two principal storage gangliosides (GM2 and GM3) were not consistently co-localized within the same intracellular vesicles. To determine whether the lack of GM2 and GM3 co-localization was due to differences in synthetic versus degradative pathway expression, we generated mice lacking both NPC1 and lysosomal β-galactosidase, and therefore unable to generate GM2 and GM3 in lysosomes. Double mutants lacked both gangliosides, indicating that each is the product of endosomal/lysosomal processing. Unexpectedly, GM1 accumulation in double mutants increased compared to single mutants consistent with a direct role for NPC1 in ganglioside salvage. These studies provide further evidence that NPC1 and NPC2 proteins participate in endosomal/lysosomal processing of both sphingolipids and cholesterol.

Attenuation of the lysosomal death pathway by lysosomal cholesterol accumulation

In the past decade, lysosomal membrane permeabilization (LMP) has emerged as a significant component of cell death signaling. The mechanisms by which lysosomal stability is regulated are not yet fully understood, but changes in the lysosomal membrane lipid composition have been suggested to be involved. Our aim was to investigate the importance of cholesterol in the regulation of lysosomal membrane permeability and its potential impact on apoptosis. Treatment of normal human fibroblasts with U18666A, an amphiphilic drug that inhibits cholesterol transport and causes accumulation of cholesterol in lysosomes, rescued cells from lysosome-dependent cell death induced by the lysosomotropic detergent O-methyl-serine dodecylamide hydrochloride (MSDH), staurosporine (STS), or cisplatin. LMP was decreased by pretreating cells with U18666A, and there was a linear relationship between the cholesterol content of lysosomes and their resistance to permeabilization induced by MSDH. U18666A did not induce changes in expression or localization of 70-kDa heat shock proteins (Hsp70) or antiapoptotic Bcl-2 proteins known to protect the lysosomal membrane. Induction of autophagy also was excluded as a contributor to the protective mechanism. By using Chinese hamster ovary (CHO) cells with lysosomal cholesterol overload due to a mutation in the cholesterol transporting protein Niemann-Pick type C1 (NPC1), the relationship between lysosomal cholesterol accumulation and protection from lysosome-dependent cell death was confirmed. Cholesterol accumulation in lysosomes attenuates apoptosis by increasing lysosomal membrane stability.

Probes for studying cholesterol binding and cell biology

Cholesterol is a multifunctional lipid in eukaryotic cells. It regulates the physical state of the phospholipid bilayer, is crucially involved in the formation of membrane microdomains, affects the activity of many membrane proteins, and is the precursor for steroid hormones and bile acids. Thus, cholesterol plays a profound role in the physiology and pathophysiology of eukaryotic cells. The cholesterol molecule has achieved evolutionary perfection to fulfill its different functions in membrane organization. Here, we review basic approaches to explore the interaction of cholesterol with proteins, with a particular focus on the high diversity of fluorescent and photoreactive cholesterol probes available today.

Plasticity in membrane cholesterol contributes toward electrical maturation of hearing

Advances in refining the "fluid mosaic" model of the plasma membrane have revealed that it is wrought with an ordered lipid composition that undergoes remarkable plasticity during cell development. Despite the evidence that specific signaling proteins and ion channels gravitate toward these lipid microdomains, identification of their functional impact remains a formidable challenge. We report that in contrast to matured auditory hair cells, depletion of membrane cholesterol in developing hair cells produced marked potentiation of voltage-gated K(+) currents (I(Kv)). The enhanced magnitude of I(Kv) in developing hair cells was in keeping with the reduced cholesterol-rich microdomains in matured hair cells. Remarkably, potentiation of the cholesterol-sensitive current was sufficient to abolish spontaneous activity, a functional blueprint of developing and regenerating hair cells. Collectively, these findings provide evidence that developmental plasticity of lipid microdomains and the ensuing changes in K(+) currents are important determinants of one of the hallmarks in the maturation of hearing.

Cholesterol oxidation products are sensitive and specific blood-based biomarkers for Niemann-Pick C1 disease

Niemann-Pick type C1 (NPC1) disease is a rare progressive neurodegenerative disorder characterized by accumulation of cholesterol in the endolysosomes. Previous studies implicating oxidative stress in NPC1 disease pathogenesis raised the possibility that nonenzymatic formation of cholesterol oxidation products could serve as disease biomarkers. We measured these metabolites in the plasma and tissues of the Npc1(-/-) mouse model and found several cholesterol oxidation products that were elevated in Npc1(-/-) mice, were detectable before the onset of symptoms, and were associated with disease progression. Nonenzymatically formed cholesterol oxidation products were similarly increased in the plasma of all human NPC1 subjects studied and delineated an oxysterol profile specific for NPC1 disease. This oxysterol profile also correlated with the age of disease onset and disease severity. We further show that the plasma oxysterol markers decreased in response to an established therapeutic intervention in the NPC1 feline model. These cholesterol oxidation products are robust blood-based biochemical markers for NPC1 disease that may prove transformative for diagnosis and treatment of this disorder, and as outcome measures to monitor response to therapy.

Niemann-Pick disease type C

Niemann-Pick C disease (NP-C) is a neurovisceral atypical lysosomal lipid storage disorder with an estimated minimal incidence of 1/120 000 live births. The broad clinical spectrum ranges from a neonatal rapidly fatal disorder to an adult-onset chronic neurodegenerative disease. The neurological involvement defines the disease severity in most patients but is typically preceded by systemic signs (cholestatic jaundice in the neonatal period or isolated spleno- or hepatosplenomegaly in infancy or childhood). The first neurological symptoms vary with age of onset: delay in developmental motor milestones (early infantile period), gait problems, falls, clumsiness, cataplexy, school problems (late infantile and juvenile period), and ataxia not unfrequently following initial psychiatric disturbances (adult form). The most characteristic sign is vertical supranuclear gaze palsy. The neurological disorder consists mainly of cerebellar ataxia, dysarthria, dysphagia, and progressive dementia. Cataplexy, seizures and dystonia are other common features. NP-C is transmitted in an autosomal recessive manner and is caused by mutations of either the NPC1 (95% of families) or the NPC2 genes. The exact functions of the NPC1 and NPC2 proteins are still unclear. NP-C is currently described as a cellular cholesterol trafficking defect but in the brain, the prominently stored lipids are gangliosides. Clinical examination should include comprehensive neurological and ophthalmological evaluations. The primary laboratory diagnosis requires living skin fibroblasts to demonstrate accumulation of unesterified cholesterol in perinuclear vesicles (lysosomes) after staining with filipin. Pronounced abnormalities are observed in about 80% of the cases, mild to moderate alterations in the remainder ("variant" biochemical phenotype). Genotyping of patients is useful to confirm the diagnosis in the latter patients and essential for future prenatal diagnosis. The differential diagnosis may include other lipidoses; idiopathic neonatal hepatitis and other causes of cholestatic icterus should be considered in neonates, and conditions with cerebellar ataxia, dystonia, cataplexy and supranuclear gaze palsy in older children and adults. Symptomatic management of patients is crucial. A first product, miglustat, has been granted marketing authorization in Europe and several other countries for specific treatment of the neurological manifestations. The prognosis largely correlates with the age at onset of the neurological manifestations.

Improvement in lipid and protein trafficking in Niemann-Pick C1 cells by correction of a secondary enzyme defect

Different primary lysosomal trafficking defects lead to common alterations in lipid trafficking, suggesting cooperative interactions among lysosomal lipids. However, cellular analysis of the functional consequences of this phenomenon is lacking. As a test case, we studied cells with defective Niemann-Pick C1 (NPC1) protein, a cholesterol trafficking protein whose defect gives rise to lysosomal accumulation of cholesterol and other lipids, leading to NPC disease. NPC1 cells also develop a secondary defect in acid sphingomyelinase (SMase) activity despite a normal acid SMase gene (SMPD1). When acid SMase activity was restored to normal levels in NPC1-deficient CHO cells through SMPD1 transfection, there was a dramatic reduction in lysosomal cholesterol. Two other defects, excess lysosomal bis-(monoacylglycerol) phosphate (BMP) and defective transferrin receptor (TfR) recycling, were also markedly improved. To test its relevance in human cells, the acid SMase activity defect in fibroblasts from NPC1 patients was corrected by SMPD1 transfection or acid SMase enzyme replacement. Both treatments resulted in a dramatic reduction in lysosomal cholesterol. These data show that correcting one aspect of a complex lysosomal lipid storage disease can reduce the cellular consequences even if the primary genetic defect is not corrected.

Defective cholesterol trafficking in Niemann-Pick C-deficient cells

Pathways of intracellular cholesterol trafficking are poorly understood at the molecular level. Mutations in Niemann-Pick C (NPC) proteins, NPC1 and NPC2, however, have led to insights into the mechanism by which endocytosed cholesterol is exported from late endosomes/lysosomes (LE/L). Mutations in NPC1, a multi-spanning membrane protein of LE/L, or mutations in NPC2, a soluble luminal protein of LE/L, cause the neurodegenerative disorder NPC disease. This review focuses on data supporting a model in which movement of cholesterol out of LE/L is mediated by the sequential action of the two NPC proteins. We also discuss potential therapies for NPC disease, including evidence that treatment of NPC-deficient mice with the cholesterol-binding compound, cyclodextrin, markedly attenuates neurodegeneration, and increases life-span, of NPC1-deficient mice.

Multi-system disorders of glycosphingolipid and ganglioside metabolism

Glycosphingolipids (GSLs) and gangliosides are a group of bioactive glycolipids that include cerebrosides, globosides, and gangliosides. These lipids play major roles in signal transduction, cell adhesion, modulating growth factor/hormone receptor, antigen recognition, and protein trafficking. Specific genetic defects in lysosomal hydrolases disrupt normal GSL and ganglioside metabolism leading to their excess accumulation in cellular compartments, particularly in the lysosome, i.e., lysosomal storage diseases (LSDs). The storage diseases of GSLs and gangliosides affect all organ systems, but the central nervous system (CNS) is primarily involved in many. Current treatments can attenuate the visceral disease, but the management of CNS involvement remains an unmet medical need. Early interventions that alter the CNS disease have shown promise in delaying neurologic involvement in several CNS LSDs. Consequently, effective treatment for such devastating inherited diseases requires an understanding of the early developmental and pathological mechanisms of GSL and ganglioside flux (synthesis and degradation) that underlie the CNS diseases. These are the focus of this review.

Cholesterol-protein interaction: methods and cholesterol reporter molecules

Cholesterol is a major constituent of the plasma membrane in eukaryotic cells. It regulates the physical state of the phospholipid bilayer and is crucially involved in the formation of membrane microdomains. Cholesterol also affects the activity of several membrane proteins, and is the precursor for steroid hormones and bile acids. Here, methods are described that are used to explore the binding and/or interaction of proteins to cholesterol. For this purpose, a variety of cholesterol probes bearing radio-, spin-, photoaffinity- or fluorescent labels are currently available. Examples of proven cholesterol binding molecules are polyene compounds, cholesterol-dependent cytolysins, enzymes accepting cholesterol as substrate, and proteins with cholesterol binding motifs. Main topics of this report are the localization of candidate membrane proteins in cholesterol-rich microdomains, the issue of specificity of cholesterol- protein interactions, and applications of the various cholesterol probes for these studies.

Cholesterol in Alzheimer's disease and other amyloidogenic disorders

The complex association of cholesterol metabolism and Alzheimer's disease is presented in depth, including the possible benefits to be gained from cholesterol-lowering statin therapy. Then follows a survey of the role of neuronal membrane cholesterol in Abeta pore formation and Abeta fibrillogenesis, together with the link with membrane raft domains and gangliosides. The contribution of structural studies to Abeta fibrillogenesis, using TEM and AFM, is given some emphasis. The role of apolipoprotein E and its isoforms, in particular ApoE4, in cholesterol and Abeta binding is presented, in relation to genetic risk factors for Alzheimer's disease. Increasing evidence suggests that cholesterol oxidation products are of importance in generation of Alzheimer's disease, possibly induced by Abeta-produced hydrogen peroxide. The body of evidence for a link between cholesterol in atherosclerosis and Alzheimer's disease is increasing, along with an associated inflammatory response. The possible role of cholesterol in tau fibrillization, tauopathies and in some other non-Abeta amyloidogenic disorders is surveyed.

Lipid oxidation and peroxidation in CNS health and disease: from molecular mechanisms to therapeutic opportunities

Reactive oxygen species (ROS) are produced at low levels in mammalian cells by various metabolic processes, such as oxidative phosphorylation by the mitochondrial respiratory chain, NAD(P)H oxidases, and arachidonic acid oxidative metabolism. To maintain physiological redox balance, cells have endogenous antioxidant defenses regulated at the transcriptional level by Nrf2/ARE. Oxidative stress results when ROS production exceeds the cell's ability to detoxify ROS. Overproduction of ROS damages cellular components, including lipids, leading to decline in physiological function and cell death. Reaction of ROS with lipids produces oxidized phospholipids, which give rise to 4-hydroxynonenal, 4-oxo-2-nonenal, and acrolein. The brain is susceptible to oxidative damage due to its high lipid content and oxygen consumption. Neurodegenerative diseases (AD, ALS, bipolar disorder, epilepsy, Friedreich's ataxia, HD, MS, NBIA, NPC, PD, peroxisomal disorders, schizophrenia, Wallerian degeneration, Zellweger syndrome) and CNS traumas (stroke, TBI, SCI) are problems of vast clinical importance. Free iron can react with H(2)O(2) via the Fenton reaction, a primary cause of lipid peroxidation, and may be of particular importance for these CNS injuries and disorders. Cholesterol is an important regulator of lipid organization and the precursor for neurosteroid biosynthesis. Atherosclerosis, the major risk factor for ischemic stroke, involves accumulation of oxidized LDL in the arteries, leading to foam cell formation and plaque development. This review will discuss the role of lipid oxidation/peroxidation in various CNS injuries/disorders.

Cholesterol in Niemann-Pick Type C disease

Niemann-Pick Type C (NPC) disease is associated with accumulation of cholesterol and other lipids in late endosomes/lysosomes in virtually every organ; however, neurodegeneration represents the fatal cause for the disease. Genetic analysis has identified loss-of-function mutations in NPC1 and NPC2 genes as the molecular triggers for the disease. Although the precise function of these proteins has not yet been clarified, recent research suggests that they orchestrate cholesterol efflux from late endosomes/lysosomes. NPC protein deficits result in impairment in intracellular cholesterol trafficking and dysregulation of cholesterol biosynthesis. Disruption of cholesterol homeostasis is also associated with deregulation of autophagic activity and early-onset neuroinflammation, which may contribute to the pathogenesis of NPC disease. This chapter reviews recent achievements in the investigation of disruption of cholesterol homeostasis-induced neurodegeneration in NPC disease, and provides new insight for developing a potential therapeutic strategy for this disorder.

Cholesterol-binding toxins and anti-cholesterol antibodies as structural probes for cholesterol localization

Cholesterol is one of the major constituents of mammalian cell membranes. It plays an indispensable role in regulating the structure and function of cell membranes and affects the pathology of various diseases. In recent decades much attention has been paid to the existence of membrane microdomains, generally termed lipid "rafts", and cholesterol, along with sphingolipids, is thought to play a critical role in raft structural organization and function. Cholesterol-binding probes are likely to provide useful tools for analyzing the distribution and dynamics of membrane cholesterol, as a structural element of raft microdomains, and elsewhere within the cell. Among the probes, non-toxic derivatives of perfringolysin O, a cholesterol-binding cytolysin, bind cholesterol in a concentration-dependent fashion with a strict threshold. They selectively recognize cholesterol in cholesterol-enriched membranes, and have been used in many studies to detect microdomains in plasma and intracellular membranes. Anti-cholesterol antibodies that recognize cholesterol in domain structures have been developed in recent years. In this chapter, we describe the characteristics of these cholesterol-binding proteins and their applications to studies on membrane cholesterol localization.

Niemann-Pick Disease Type C: from molecule to clinic

1. Niemann-Pick Type C disease (NPC) is an incurable cholesterol-storage disorder that stems from inherited deficiencies of lysosomal proteins involved in intracellular lipid-trafficking proteins. The condition manifests as progressive neurological impairment and leads to death at an early age. 2. To improve clinical recognization and investigate therapeutic strategies, recent studies using molecular and genetic approaches have led to significant advances in the creation of animal models of NPC, as well as in the understanding of the cellular and molecular mechanisms underlying the pathogenesis of NPC. 3. Patients with NPC are divided into four groups based on age at presentation, whereas the clinical features of NPC can be divided into five categories based on the severity of the disease. Progressive neuronal loss, especially of cerebellar Purkinje cells, is a hallmark of NPC. Ballooned neurons, axonal abnormalities and astroglyosis are among the pathological changes seen. Severe demyelination is also present in the mouse model of NPC. 4. Mutations in the NPC1 gene cause approximately 95% of cases of NPC, whereas mutations in the NPC2 gene account for the remainder of cases. NPC1 is a transmembrane protein and NPC2 is a soluble protein involved in lipid trafficking in lysosomes. Loss-of-function mutations in the NPC1 gene lead to a failure of the calcium-mediated fusion of endosomes with lysosomes, resulting in the accumulation of cholesterol and other lipids in late endosomes and lysosomes. 5. The present review updates the disorders of NPC from clinical features to animal models and molecular mechanisms.