Trafficking of cholesterol from cell bodies to distal axons in Niemann Pick C1-deficient neurons

Lipoprotein Particles in Cerebrospinal Fluid

The brain is the most lipid-rich organ in the body, and the intricate interplay between lipid metabolism and pathologies associated with neurodegenerative disorders is being increasingly recognized. The brain is bathed in cerebrospinal fluid (CSF), which, like plasma, contains lipid-protein complexes called lipoproteins that are responsible for extracellular lipid transport. Multiple CSF lipoprotein populations exist, some of which are produced de novo in the central nervous system and others that appear to be generated from protein constituents that are produced in the periphery. These CSF lipoproteins are thought to play key roles in maintaining lipid homeostasis in the central nervous system, while little else is known due to their limited accessibility and their low abundance in CSF. Recent work has provided new insights into the compositional complexity of CSF lipoprotein families and their metabolism in cerebral circulation. The purpose of this review is to summarize our current state of knowledge on the composition, origin, and metabolism of CSF lipoproteins.

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.

PTCHD1 Binds Cholesterol but Not Sonic Hedgehog, Suggesting a Distinct Cellular Function

Deleterious mutations in the X-linked Patched domain-containing 1 (PTCHD1) gene may account for up to 1% of autism cases. Despite this, the PTCHD1 protein remains poorly understood. Structural similarities to Patched family proteins point to a role in sterol transport, but this hypothesis has not been verified experimentally. Additionally, PTCHD1 has been suggested to be involved in Hedgehog signalling, but thus far, the experimental results have been conflicting. To enable a variety of biochemical and structural experiments, we developed a method for expressing PTCHD1 in Spodoptera frugiperda cells, solubilising it in glycol-diosgenin, and purifying it to homogeneity. In vitro and in silico experiments show that PTCHD1 function is not interchangeable with Patched 1 (PTCH1) in canonical Hedgehog signalling, since it does not repress Smoothened in Ptch1^-/- mouse embryonic fibroblasts and does not bind Sonic Hedgehog. However, we found that PTCHD1 binds cholesterol similarly to PTCH1. Furthermore, we identified 13 PTCHD1-specific protein interactors through co-immunoprecipitation and demonstrated a link to cell stress responses and RNA stress granule formation. Thus, our results support the notion that despite structural similarities to other Patched family proteins, PTCHD1 may have a distinct cellular function.

Efficacy and safety clinical trial with efavirenz in patients diagnosed with adult Niemann-pick type C with cognitive impairment

BACKGROUND

Niemann-Pick disease Type C (NPC) is a genetic, incurable, neurodegenerative disorder. This orphan disease is most frequently caused by mutations in the NPC1 protein, resulting in intralysossomal cholesterol accumulation. NPC1 is found in neuronal cell bodies, axon terminals and synaptosomes, suggesting it plays a role in lysosomal degradation pathway and in synaptic transmission. Neuronal function is especially vulnerable to NPC1 deficiency and synaptic changes seem a key element in disease development. Currently, Miglustat (Zavesca®) is the only approved treatment for NPC. However, preclinical evidence showed that low-dose Efavirenz reverted synaptic defects through pharmacological activation of the enzyme CYP46.

METHODS

This is a single-center, phase II clinical trial to evaluate the efficacy and safety of Efavirenz in addition to standard of care in patients diagnosed with adult or late juvenile-onset NPC with cognitive impairment. All enrolled patients will be treated orally with 25 mg/d of Efavirenz for 52 weeks (1 year). Secondary objectives include evaluating clinical (neurological and neuropsychological questionnaires) and biological (imaging and biochemical biomarkers) parameters.

DISCUSSION

NPC is still an unmet medical need. Although different therapeutic approaches are under study, this is the first clinical trial (to the best of our knowledge) studying the effects of Efavirenz in adult- and late-juvenile-onset NPC. Despite the small sample size and the single-arm design, we expect the results to show Efavirenz's capacity of activating the CYP46 enzyme to compensate for NPC1 deficiency and correct synaptic changes, therefore compensating cognitive and psychiatric changes in these patients. This study may provide direct benefit to enrolled patients in terms of slowing down the disease progression.

Force Field Parameterization for the Description of the Interactions between Hydroxypropyl-β-Cyclodextrin and Proteins

Cyclodextrins are cyclic oligosaccharides, widely used as drug carriers, solubilizers, and excipients. Among cyclodextrins, the functionalized derivative known as hydroxypropyl-β-cyclodextrin (HPβCD) offers several advantages due to its unique structural features. Its optimal use in pharmaceutical and medical applications would benefit from a molecular-level understanding of its behavior, as can be offered by molecular dynamics simulations. Here, we propose a set of parameters for all-atom simulations of HPβCD, based on the ADD force field for sugars developed in our group, and compare it to the original CHARMM36 description. Using Kirkwood-Buff integrals of binary HPβCD-water mixtures as target experimental data, we show that the ADD-based description results in a considerably improved prediction of HPβCD self-association and interaction with water. We then use the new set of parameters to characterize the behavior of HPβCD toward the different amino acids. We observe pronounced interactions of HPβCD with both polar and nonpolar moieties, with a special preference for the aromatic rings of tyrosine, phenylalanine, and tryptophan. Interestingly, our simulations further highlight a preferential orientation of HPβCD's hydrophobic cavity toward the backbone atoms of amino acids, which, coupled with a favorable interaction of HPβCD with the peptide backbone, suggest a propensity for HPβCD to denature proteins.

Modeling Brain Pathology of Niemann-Pick Disease Type C Using Patient-Derived Neurons

BACKGROUND

Niemann-Pick disease type C (NPC) is a rare autosomal-recessive lysosomal storage disease that is also associated with progressive neurodegeneration. NPC shares many pathological features with Alzheimer's disease, including neurofibrillary tangles, axonal spheroids, β-amyloid deposition, and dystrophic neurites. Here, we examined if these pathological features could be detected in induced pluripotent stem cell (iPSC)-derived neurons from NPC patients.

METHODS

Brain tissues from 8 NPC patients and 5 controls were analyzed for histopathological and biochemical markers of pathology. To model disease in culture, iPSCs from NPC patients and controls were differentiated into cortical neurons.

RESULTS

We found hyperphosphorylated tau, altered processing of amyloid precursor protein, and increased Aβ42 in NPC postmortem brains and in iPSC-derived cortical neurons from NPC patients.

CONCLUSION

Our findings demonstrated that the main pathogenic phenotypes typically found in NPC brains were also observed in patient-derived neurons, providing a useful model for further mechanistic and therapeutic studies of NPC. © 2021 International Parkinson and Movement Disorder Society.

NPC1 enables cholesterol mobilization during long-term potentiation that can be restored in Niemann-Pick disease type C by CYP46A1 activation

NPC is a neurodegenerative disorder characterized by cholesterol accumulation in endolysosomal compartments. It is caused by mutations in the gene encoding NPC1, an endolysosomal protein mediating intracellular cholesterol trafficking. Cognitive and psychiatric alterations are hallmarks in NPC patients pointing to synaptic defects. However, the role of NPC1 in synapses has not been explored. We show that NPC1 is present in the postsynaptic compartment and is locally translated during LTP. A mutation in a region of the NPC1 gene commonly altered in NPC patients reduces NPC1 levels at synapses due to enhanced NPC1 protein degradation. This leads to shorter postsynaptic densities, increased synaptic cholesterol and impaired LTP in NPC1nmf164 mice with cognitive deficits. NPC1 mediates cholesterol mobilization and enables surface delivery of CYP46A1 and GluA1 receptors necessary for LTP, which is defective in NPC1nmf164 mice. Pharmacological activation of CYP46A1 normalizes synaptic levels of cholesterol, LTP and cognitive abilities, and extends life span of NPC1nmf164 mice. Our results unveil NPC1 as a regulator of cholesterol dynamics in synapses contributing to synaptic plasticity, and provide a potential therapeutic strategy for NPC patients.

The NCATS Pharmaceutical Collection: a 10-year update

The National Center for Advancing Translational Sciences (NCATS) Pharmaceutical Collection (NPC), a comprehensive collection of clinically approved drugs, was made a public resource in 2011. Over the past decade, the NPC has been systematically profiled for activity across an array of pathways and disease models, generating an unparalleled amount of data. These data have not only enabled the identification of new repurposing candidates with several in clinical trials, but also uncovered new biological insights into drug targets and disease mechanisms. This retrospective provides an update on the NPC in terms of both successes and lessons learned. We also report our efforts in bringing the NPC up-to-date with drugs approved in recent years.

Model construction of Niemann-Pick type C disease in zebrafish

Niemann-Pick type C disease (NPC) is a rare human disease, with limited effective treatment options. Most cases of NPC disease are associated with inactivating mutations of the NPC1 gene. However, cellular and molecular mechanisms responsible for the NPC1 pathogenesis remain poorly defined. This is partly due to the lack of a suitable animal model to monitor the disease progression. In this study, we used CRISPR to construct an NPC1-/- zebrafish model, which faithfully reproduced the cardinal pathological features of this disease. In contrast to the wild type (WT), the deletion of NPC1 alone caused significant hepatosplenomegaly, ataxia, Purkinje cell death, increased lipid storage, infertility and reduced body length and life span. Most of the NPC1-/- zebrafish died within the first month post fertilization, while the remaining specimens developed slower than the WT and died before reaching 8 months of age. Filipin-stained hepatocytes of the NPC1-/- zebrafish were clear, indicating abnormal accumulation of unesterified cholesterol. Lipid profiling showed a significant difference between NPC1-/- and WT zebrafish. An obvious accumulation of seven sphingolipids was detected in livers of NPC1-/- zebrafish. In summary, our results provide a valuable model system that could identify promising therapeutic targets and treatments for the NPC disease.

Endosomal-Lysosomal Cholesterol Sequestration by U18666A Differentially Regulates Amyloid Precursor Protein (APP) Metabolism in Normal and APP-Overexpressing Cells

Amyloid β (Aβ) peptide, derived from amyloid precursor protein (APP), plays a critical role in the development of Alzheimer's disease. Current evidence indicates that altered levels or subcellular distribution of cholesterol can regulate Aβ production and clearance, but it remains unclear how cholesterol sequestration within the endosomal-lysosomal (EL) system can influence APP metabolism. Thus, we evaluated the effects of U18666A, which triggers cholesterol redistribution within the EL system, on mouse N2a cells expressing different levels of APP in the presence or absence of extracellular cholesterol and lipids provided by fetal bovine serum (FBS). Our results reveal that U18666A and FBS differentially increase the levels of APP and its cleaved products, the α-, β-, and η-C-terminal fragments, in N2a cells expressing normal levels of mouse APP (N2awt), higher levels of human wild-type APP (APPwt), or "Swedish" mutant APP (APPsw). The cellular levels of Aβ_1-40/Aβ_1-42 were markedly increased in U18666A-treated APPwt and APPsw cells. Our studies further demonstrate that APP and its cleaved products are partly accumulated in the lysosomes, possibly due to decreased clearance. Finally, we show that autophagy inhibition plays a role in mediating U18666A effects. Collectively, these results suggest that altered levels and distribution of cholesterol and lipids can differentially regulate APP metabolism depending on the nature of APP expression.

The adenosine A2A receptor agonist T1-11 ameliorates neurovisceral symptoms and extends the lifespan of a mouse model of Niemann-Pick type C disease

Niemann-Pick C is a fatal neurovisceral disorder caused, in 95% of cases, by mutation of NPC1 gene. Therapeutic options are extremely limited and new "druggable" targets are highly warranted. We previously demonstrated that the stimulation of the adenosine A_2A receptor (A_2AR) normalized the pathological phenotype of cellular models of NPC1. Since the validation of A_2ARs as a therapeutic target for NPC1 can be obtained only conducting studies in in vivo models of the disease, in the present paper, the effects of two agonists of A_2ARs were evaluated in the mouse model Balb/c Npc1^nih, hereafter indicated as NPC1-/-. The agonists CGS21680 (2.5 and 5mg/kg/day by intraperitoneal injection) and T1-11 (50mg/kg/day in drinking water) were administered at a presymptomatic stage of the disease of NPC1-/- mice (PN28 and PN30, respectively); the experimental groups were the following: vehicle-treated WT mice (N=16 for both CGS and T1-11 treatments); vehicle-treated NPC1-/- mice (N=14 for CGS and 12 for T1-11 treatment); CGS-treated NPC1-/- mice (N=7) and T1-11-treated NPC1-/- mice (N=11). The efficacy of the treatments was evaluated by comparing vehicle-treated and CGS or T1-11-treated NPC1-/- mice for their motor deficits (analyzed by both rotarod and footprint tests), hippocampal cognitive impairment (by Novel Object Recognition (NOR) test), cerebellar neurodegeneration (Purkinje neurons counting), and cholesterol and sphingomyelin accumulation in spleen and liver. Finally, the effect of both agonists on survival was evaluated by applying a humane late endpoint (weight loss >30% of peak weight, punched posture and reduced activity in the cage). The results demonstrated that, while CGS21680 only slightly attenuated cognitive deficits, T1-11 ameliorated motor coordination, significantly improved cognitive impairments, increased the survival of Purkinje neurons and reduced sphingomyelin accumulation in the liver. More importantly, it significantly prolonged the lifespan of NPC1-/- mice. In vitro experiments conducted in a neuronal model of NPC1 demonstrated that the ability of T1-11 to normalize cell phenotype was mediated by the selective activation of A_2ARs and modulation of intracellular calcium levels. In conclusion, our results fully confirm the validity of A_2ARs as a new target for NPC1 treatment. As soon as new ligands with improved pharmacokinetic characteristics (i.e. orally active, with brain bioavailability and metabolic stability) will be obtained, A_2AR agonists could represent a breakthrough in the treatment of NPC.

Aberrant activation of Cdc2/cyclin B1 is involved in initiation of cytoskeletal pathology in murine Niemann-Pick disease type C

Niemann-Pick disease type C (NPC) is a fatal, neurovisceral lipid storage disease, neuropathologically characterized by cytoplasmic sequestration of glycolipids in neurons, progressive neuronal loss, neurofibrillary tangles (NFTs) formation, and axonal spheroids (AS). Cytoskeletal pathology including accumulation of hyperphosphorylated cytoskeletal proteins is a neuropathological hallmark of the mouse model of NPC (npc mice). With a goal of elucidating the mechanisms underlying the lesion formation, we investigated the temporal and spatial characteristics of cytoskeletal lesions and the roles of cdc2, cdk4, and cdk5 in lesion formation in young npc mice. Cytoskeletal lesions were detectable in npc mice at three weeks of age. Importantly, concomitant activation of cdc2/cyclin B1 kinase and accumulation of a subsequently generated cohort of phospho-epitopes were detected. The activation of cdk4/cyclin D1 and cdk5/p25 kinases was observed during the fourth week of life in npc mice, and this activation contributed to the lesion formation. We concluded that the progression of cytoskeletal pathology in npc mice older than four weeks is accelerated by the cumulative effect of cdc2, cdk4, and cdk5 activation. Furthermore, cdc2/cyclin B1 may act as a key initial player one week earlier. Targeting cell cycle activation may be beneficial to slow down the NPC pathogenesis.

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.

Diversity of glycosphingolipid GM2 and cholesterol accumulation in NPC1 patient-specific iPSC-derived neurons

Niemann-Pick disease Type C1 (NPC1) is a rare progressive neurodegenerative disorder caused by mutations in the NPC1 gene. On the cellular level NPC1 mutations lead to an accumulation of cholesterol and gangliosides. As a thorough analysis of the severely affected neuronal cells is unfeasible in NPC1 patients, we recently described the cellular phenotype of neuronal cells derived from NPC1 patient iPSCs carrying the compound heterozygous mutation c.1836A>C/c.1628delC. Here we expanded the analysis to cell lines carrying the prevalent mutation c.3182T>C and the novel mutation c.1180T>C, as well as to the determination of GM2 and GM3 gangliosides in NPC1 patient-specific iPSC-derived neurons and glia cells. Immunocytochemical detection of GM2 revealed punctated staining pattern predominantly localized in neurons. Detection of cholesterol by filipin staining showed a comparable staining pattern, colocalized with GM2, indicating a deposit of GM2 and cholesterol in the same cellular compartments. Accumulations were not only restricted to cell bodies, but were also found in the neuronal extensions. A quantification of the GM2 amount by HPLC-MS/MS confirmed significantly higher amounts in neurons carrying a mutation. Additionally, these cells displayed a lowered activity of the catabolic enzyme Hex A, but not B4GALNT1. Molecular docking simulations indicated binding of cholesterol to Hex A, suggesting cholesterol influences the GM2 degradation pathway and, subsequently, leading to the accumulation of GM2. Taken together, this is the first study showing an accumulation of GM2 in neuronal derivatives of patient-specific iPSCs and thus proving further disease-specific hallmarks in this human in vitro model of NPC1.

Stressful learning paradigm precludes manifestation of cognitive ability in sphingomyelin synthase-2 knockout mice

Sphingomyelin synthases (SMSs) are enzymes converting ceramide to sphingomyelin. The behavioral phenotype attributed to their disruption has not been well described. We examined learning ability and hippocampal synaptic plasticity in mice deficient in SMS2 (SMS2 KO). In context-dependent fear learning and novel object recognition test, no difference in learning ability was detected in SMS2 KO and wild-type (WT) mice. By contrast, achievement of the Morris water maze (MWM) test was deteriorated in SMS2 KO mice. In the hippocampal CA1, while the basic synaptic transmission was normal, both short- and long-term synaptic plasticity was moderately suppressed. We interpret that the MWM test taking place in wet environment may represent learning paradigm under more stressful condition than those performed in dry conditions, and that the learning ability of SMS2 KO mice failed to manifest itself fully in stressful situations. In agreement, forced swimming induced depression-like behavior more easily in SMS2 KO mice. Mass spectrometry suggested a slightly altered species distribution of ceramide in the hippocampus of SMS2 KO mice. These findings support the proposal that altered synthesis of ceramide, which is the substrate of SMS2 and therefore expected to be modified in SMS2 KO mice, is associated with depression-like tendency in animal models and depressive disorder in humans.

Niemann-pick C1 is essential for ebolavirus replication and pathogenesis in vivo

Recent work demonstrated that the Niemann-Pick C1 (NPC1) protein is an essential entry receptor for filoviruses. While previous studies focused on filovirus entry requirements of NPC1 in vitro, its roles in filovirus replication and pathogenesis in vivo remain unclear. Here, we evaluated the importance of NPC1, and its partner in cholesterol transport, NPC2, by using a mouse model of Ebolavirus (EBOV) disease. We found that, whereas wild-type mice had high viral loads and succumbed to EBOV infection, Npc1^−/− mice were entirely free of viral replication and completely protected from EBOV disease. Interestingly, Npc1^+/− mice transiently developed high levels of viremia, but were nevertheless substantially protected from EBOV challenge. We also found Npc2^−/− mice to be fully susceptible to EBOV infection, while Npc1^−/− mice treated to deplete stored lysosomal cholesterol remained completely resistant to EBOV infection. These results provide mechanistic evidence that NPC1 is directly required for EBOV infection in vivo, with little or no role for NPC1/NPC2-dependent cholesterol transport. Finally, we assessed the in vivo antiviral efficacies of three compounds known to inhibit NPC1 function or NPC1-glycoprotein binding in vitro. Two compounds reduced viral titers in vivo and provided a modest, albeit not statistically significant, degree of protection. Taken together, our results show that NPC1 is critical for replication and pathogenesis in animals and is a bona fide target for development of antifilovirus therapeutics. Additionally, our findings with Npc1^+/− mice raise the possibility that individuals heterozygous for NPC1 may have a survival advantage in the face of EBOV infection.

Researchers have been searching for an essential filovirus receptor for decades, and numerous candidate receptors have been proposed. However, none of the proposed candidate receptors has proven essential in all in vitro scenarios, nor have they proven essential when evaluated using animal models. In this report, we provide the first example of a knockout mouse that is completely refractory to EBOV infection, replication, and disease. The findings detailed here provide the first critical in vivo data illustrating the absolute requirement of NPC1 for filovirus infection in mice. Our work establishes NPC1 as a legitimate target for the development of anti-EBOV therapeutics. However, the limited success of available NPC1 inhibitors to protect mice from EBOV challenge highlights the need for new molecules or approaches to target NPC1 in vivo.

Defects in the retina of Niemann-pick type C 1 mutant mice

Background

Niemann-Pick type C1 (NPC1) disease is an inherited lysosomal storage disease caused by mutation of the Npc1 gene, resulting in a progressive accumulation of unesterified cholesterol and glycolipids in lysosomes of multiple tissues and leading to neurodegeneration and other disease. In Npc1 mutant mice, retinal degeneration including impaired visual function, lipofuscin accumulation in the pigment epithelium and ganglion cells as well as photoreceptor defects has been found. However, the pathologies of other individual cell types of the retina in Npc1 mutant mice are still not fully clear. We hypothesized that horizontal cells, amacrine cells, bipolar cells and glial cells are also affected in the retina of Npc1 mutant mice.

Results

Immunohistochemistry and electron microscopy were used to investigate pathologies of ganglion cells, horizontal cells, amacrine cells, bipolar cells, and optic nerves as well as altered activity of glial cells in Npc1 mutant mice.

Electron microscopy reveals that electron-dense inclusions are generally accumulated in ganglion cells, bipolar cells, Müller cells, and in the optic nerve. Furthermore, abnormal arborisation and ectopic processes of horizontal and amacrine cells as well as defective bipolar cells are observed by immunohistochemistry for specific cellular markers. Furthermore, hyperactivity of glial cells, including astrocytes, microglial cells, and Müller cells, is also revealed.

Conclusions

Our data extend previous findings to show multiple defects in the retina of Npc1 mutant mice, suggesting an important role of Npc1 protein in the normal function of the retina.

Electronic supplementary material

The online version of this article (doi:10.1186/s12868-014-0126-2) contains supplementary material, which is available to authorized users.

Niemann-Pick C disease and mobilization of lysosomal cholesterol by cyclodextrin

Niemann-Pick type C (NPC) disease is a lysosomal storage disease in which endocytosed cholesterol becomes sequestered in late endosomes/lysosomes (LEs/Ls) because of mutations in either the NPC1 or NPC2 gene. Mutations in either of these genes can lead to impaired functions of the NPC1 or NPC2 proteins and progressive neurodegeneration as well as liver and lung disease. NPC1 is a polytopic protein of the LE/L limiting membrane, whereas NPC2 is a soluble protein in the LE/L lumen. These two proteins act in tandem and promote the export of cholesterol from LEs/Ls. Consequently, a defect in either NPC1 or NPC2 causes cholesterol accumulation in LEs/Ls. In this review, we summarize the molecular mechanisms leading to NPC disease, particularly in the CNS. Recent exciting data on the mechanism by which the cholesterol-sequestering agent cyclodextrin can bypass the functions of NPC1 and NPC2 in the LEs/Ls, and mobilize cholesterol from LEs/Ls, will be highlighted. Moreover, the possible use of cyclodextrin as a valuable therapeutic agent for treatment of NPC patients will be considered.

The neuropsychiatry of inborn errors of metabolism

A number of metabolic disorders that affect the central nervous system can present in childhood, adolescence or adulthood as a phenocopy of a major psychiatric syndrome such as psychosis, depression, anxiety or mania. An understanding and awareness of secondary syndromes in metabolic disorders is of great importance as it can lead to the early diagnosis of such disorders. Many of these metabolic disorders are progressive and may have illness-modifying treatments available. Earlier diagnosis may prevent or delay damage to the central nervous system and allow for the institution of appropriate treatment and family and genetic counselling. Metabolic disorders appear to result in neuropsychiatric illness either through disruption of late neurodevelopmental processes (metachromatic leukodystrophy, adrenoleukodystrophy, GM2 gangliosidosis, Niemann-Pick type C, cerebrotendinous xanthomatosis, neuronal ceroid lipofuscinosis, and alpha mannosidosis) or via chronic or acute disruption of excitatory/inhibitory or monoaminergic neurotransmitter systems (acute intermittent porphyria, maple syrup urine disease, urea cycle disorders, phenylketonuria and disorders of homocysteine metabolism). In this manuscript we review the evidence for neuropsychiatric illness in major metabolic disorders and discuss the possible models for how these disorders result in psychiatric symptoms. Treatment considerations are discussed, including treatment resistance, the increased propensity for side-effects and the possibility of some treatments worsening the underlying disorder.

Dysregulation of cholesterol balance in the brain: contribution to neurodegenerative diseases

Dysregulation of cholesterol homeostasis in the brain is increasingly being linked to chronic neurodegenerative disorders, including Alzheimer’s disease (AD), Huntington’s disease (HD), Parkinson’s disease (PD), Niemann-Pick type C (NPC) disease and Smith-Lemli Opitz syndrome (SLOS). However, the molecular mechanisms underlying the correlation between altered cholesterol metabolism and the neurological deficits are, for the most part, not clear. NPC disease and SLOS are caused by mutations in genes involved in the biosynthesis or intracellular trafficking of cholesterol, respectively. However, the types of neurological impairments, and the areas of the brain that are most affected, differ between these diseases. Some, but not all, studies indicate that high levels of plasma cholesterol correlate with increased risk of developing AD. Moreover, inheritance of the E4 isoform of apolipoprotein E (APOE), a cholesterol-carrying protein, markedly increases the risk of developing AD. Whether or not treatment of AD with statins is beneficial remains controversial, and any benefit of statin treatment might be due to anti-inflammatory properties of the drug. Cholesterol balance is also altered in HD and PD, although no causal link between dysregulated cholesterol homeostasis and neurodegeneration has been established. Some important considerations for treatment of neurodegenerative diseases are the impermeability of the blood-brain barrier to many therapeutic agents and difficulties in reversing brain damage that has already occurred. This article focuses on how cholesterol balance in the brain is altered in several neurodegenerative diseases, and discusses some commonalities and differences among the diseases.

β-amyloid inhibits protein prenylation and induces cholesterol sequestration by impairing SREBP-2 cleavage

Accumulation of β-amyloid (Aβ) inside brain neurons is an early and crucial event in Alzheimer's disease (AD). Studies in brains of AD patients and mice models of AD suggested that cholesterol homeostasis is altered in neurons that accumulate Aβ. Here we directly investigated the role of intracellular oligomeric Aβ(42) (oAβ(42)) in neuronal cholesterol homeostasis. We report that oAβ(42) induces cholesterol sequestration without increasing cellular cholesterol mass. Several features of AD, such as endosomal abnormalities, brain accumulation of Aβ and neurofibrillary tangles, and influence of apolipoprotein E genotype, are also present in Niemann-Pick type C, a disease characterized by impairment of intracellular cholesterol trafficking. These common features and data presented here suggest that a pathological mechanism involving abnormal cholesterol trafficking could take place in AD. Cholesterol sequestration in Aβ-treated neurons results from impairment of intracellular cholesterol trafficking secondary to inhibition of protein prenylation. oAβ(42) reduces sterol regulatory element-binding protein-2 (SREBP-2) cleavage, causing decrease of protein prenylation. Inhibition of protein prenylation represents a mechanism of oAβ(42)-induced neuronal death. Supply of the isoprenoid geranylgeranyl pyrophosphate to oAβ(42)-treated neurons recovers normal protein prenylation, reduces cholesterol sequestration, and prevents Aβ-induced neurotoxicity. Significant to AD, reduced levels of protein prenylation are present in the cerebral cortex of the TgCRND8 mouse model. In conclusion, we demonstrate a significant inhibitory effect of Aβ on protein prenylation and identify SREBP-2 as a target of oAβ(42), directly linking Aβ to cholesterol homeostasis impairment.

Disruption and therapeutic rescue of autophagy in a human neuronal model of Niemann Pick type C1

An unresolved issue about many neurodegenerative diseases is why neurons are particularly sensitive to defects in ubiquitous cellular processes. One example is Niemann Pick type C1, caused by defects in cholesterol trafficking in all cells, but where neurons are preferentially damaged. Understanding this selective failure is limited by the difficulty in obtaining live human neurons from affected patients. To solve this problem, we generated neurons with decreased function of NPC1 from human embryonic stem cells and used them to test the hypothesis that defective cholesterol handling leads to enhanced pathological phenotypes in neurons. We found that human NPC1 neurons have strong spontaneous activation of autophagy, and, contrary to previous reports in patient fibroblasts, a block of autophagic progression leading to defective mitochondrial clearance. Mitochondrial fragmentation is an exceptionally severe phenotype in NPC1 neurons compared with fibroblasts, causing abnormal accumulation of mitochondrial proteins. Contrary to expectation, these abnormal phenotypes were rescued by treatment with the autophagy inhibitor 3-methyladenine and by treatment with the potential therapeutic cyclodextrin, which mobilizes cholesterol from the lysosomal compartment. Our findings suggest that neurons are especially sensitive to lysosomal cholesterol accumulation because of autophagy disruption and accumulation of fragmented mitochondria, thus defining a new route to effective drug development for NPC1 disease.

Decreased expression of myelin gene regulatory factor in Niemann-Pick type C 1 mouse

Niemann-Pick type C 1 (NPC1) disease is an autosomal recessive cholesterol transport defect resulting in a neurodegenerative process in patients mainly at an early age, although some patients may start with manifestation in adult. Since loss of myelin is considered as a main pathogenetic factor, the precise mechanism inducing dysmylination in NPC1 disease is still unclear. In the present study, a quantitative evaluation on the myelin protein and its regulatory factors of oligodendrocytes, such as SRY-related HMG-box 10 (Sox10), Yin Yang 1 factor (YY1) and myelin gene regulatory factor (MRF), in different parts of the brain and spinal cord was performed in NPC1-mutant mice. The results showed that NPC1 protein was expressed in oligodendrocytes and the amount of myelin protein was generally decreased in all parts of the brain and spinal cord in NPC1-mutant mice. Compared to wild type, the amount of Sox10 and YY1 was not different in NPC1-mutant mice, but MRF was significantly decreased, suggesting a possible mechanism perturbing differentiation of oligodendrocytes and the myelination process in the NPC1-mutant mouse.

Lysosomal lipid storage diseases

Lysosomal lipid storage diseases, or lipidoses, are inherited metabolic disorders in which typically lipids accumulate in cells and tissues. Complex lipids, such as glycosphingolipids, are constitutively degraded within the endolysosomal system by soluble hydrolytic enzymes with the help of lipid binding proteins in a sequential manner. Because of a functionally impaired hydrolase or auxiliary protein, their lipid substrates cannot be degraded, accumulate in the lysosome, and slowly spread to other intracellular membranes. In Niemann-Pick type C disease, cholesterol transport is impaired and unesterified cholesterol accumulates in the late endosome. In most lysosomal lipid storage diseases, the accumulation of one or few lipids leads to the coprecipitation of other hydrophobic substances in the endolysosomal system, such as lipids and proteins, causing a "traffic jam." This can impair lysosomal function, such as delivery of nutrients through the endolysosomal system, leading to a state of cellular starvation. Therapeutic approaches are currently restricted to mild forms of diseases with significant residual catabolic activities and without brain involvement.

Altered levels and distribution of amyloid precursor protein and its processing enzymes in Niemann-Pick type C1-deficient mouse brains

Niemann-Pick type C (NPC) disease is an autosomal recessive neurodegenerative disorder characterized by intracellular accumulation of cholesterol and glycosphingolipids in many tissues including the brain. The disease is caused by mutations of either NPC1 or NPC2 gene and is accompanied by a severe loss of neurons in the cerebellum, but not in the hippocampus. NPC pathology exhibits some similarities with Alzheimer's disease, including increased levels of amyloid beta (Abeta)-related peptides in vulnerable brain regions, but very little is known about the expression of amyloid precursor protein (APP) or APP secretases in NPC disease. In this article, we evaluated age-related alterations in the level/distribution of APP and its processing enzymes, beta- and gamma-secretases, in the hippocampus and cerebellum of Npc1(-/-) mice, a well-established model of NPC pathology. Our results show that levels and expression of APP and beta-secretase are elevated in the cerebellum prior to changes in the hippocampus, whereas gamma-secretase components are enhanced in both brain regions at the same time in Npc1(-/-) mice. Interestingly, a subset of reactive astrocytes in Npc1(-/-) mouse brains expresses high levels of APP as well as beta- and gamma-secretase components. Additionally, the activity of beta-secretase is enhanced in both the hippocampus and cerebellum of Npc1(-/-) mice at all ages, while the level of C-terminal APP fragments is increased in the cerebellum of 10-week-old Npc1(-/-) mice. These results, taken together, suggest that increased level and processing of APP may be associated with the development of pathology and/or degenerative events observed in Npc1(-/-) mouse brains.

Exosome secretion ameliorates lysosomal storage of cholesterol in Niemann-Pick type C disease

Niemann-Pick type C1 disease is an autosomal-recessive lysosomal storage disorder. Loss of function of the npc1 gene leads to abnormal accumulation of free cholesterol and sphingolipids within the late endosomal and lysosomal compartments resulting in progressive neurodegeneration and dysmyelination. Here, we show that oligodendroglial cells secrete cholesterol by exosomes when challenged with cholesterol or U18666A, which induces late endosomal cholesterol accumulation. Up-regulation of exosomal cholesterol release was also observed after siRNA-mediated knockdown of NPC1 and in fibroblasts derived from NPC1 patients and could be reversed by expression of wild-type NPC1. We provide evidence that exosomal cholesterol secretion depends on the presence of flotillin. Our findings indicate that exosomal release of cholesterol may serve as a cellular mechanism to partially bypass the traffic block that results in the toxic lysosomal cholesterol accumulation in Niemann-Pick type C1 disease. Furthermore, we suggest that secretion of cholesterol by exosomes contributes to maintain cellular cholesterol homeostasis.

Pre-synaptic defects of NPC1-deficient hippocampal neurons are not directly related to plasma membrane cholesterol

Imbalances in brain cholesterol homeostasis have been observed in several neurodegenerative diseases. In Niemann-Pick Type C (NPC) disease, mutations in NPC1 or NPC2 lead to endosomal cholesterol accumulation, neuronal dysfunction and death. Cholesterol in synaptic plasma membranes influences membrane fluidity, curvature, and protein function, and its depletion may adversely affect synaptic vesicle cycling. We have investigated pre-synaptic function in primary hippocampal neurons with altered cholesterol distribution because of NPC1 deficiency or cyclodextrin treatment. In NPC1-deficient neurons grown in serum-free medium, plasma membrane cholesterol was reduced and total synaptic vesicle release during prolonged stimulation was attenuated. In NPC1-deficient neurons cultured in the presence of high-density lipoproteins, plasma membrane cholesterol markedly increased, but the defects in synaptic vesicle release in NPC1-deficient neurons were exacerbated. Treatment with 1 mM methyl-beta-cyclodextrin acutely depleted plasma membrane cholesterol in wild-type neurons to levels below those in NPC1 deficiency, but did not alter synaptic vesicle exo- or endocytosis. Defects only became apparent when higher methyl-beta-cyclodextrin concentrations were used. Our data indicate that synaptic vesicle release can tolerate some degree of plasma membrane cholesterol depletion and suggest that the pre-synaptic defects in NPC1-deficient neurons are not solely caused by a reduction of plasma membrane cholesterol.

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.

Defects of synaptic vesicle turnover at excitatory and inhibitory synapses in Niemann-Pick C1-deficient neurons

Niemann-Pick disease type C (NPC) is a progressive neurodegenerative disorder characterized by accumulation of free cholesterol in late endosomes/lysosomes. The pathological basis for the disease is poorly understood. In the present study, electrophysiological and fluorescent dye studies were applied to examine neuron-specific functions of Niemann-Pick disease type C1 (NPC1) and to determine whether excitatory and inhibitory synapses are differentially impaired by NPC1 deficiency. Densities of spines and postsynaptic receptor clusters were not affected by NPC1 deficiency over the period examined. However, drastic defects on exocytosis were found both in glutamatergic and GABAergic synapses. The defects were caused in part by a delay in the time required for replacement of excytosed vesicles with new fusion-competent ones. Moreover, we found that the delay of synaptic vesicle turnover was longer in inhibitory synapses (>3 s) than in excitatory synapses (<0.2 s). These defects may be early indicators, and could provide a potential explanation for key features of the disease, such as dystonia and seizures.

Abnormal gene expression in cerebellum of Npc1-/- mice during postnatal development

Niemann-Pick Type C (NPC) disease is an autosomal recessive neurodegenerative disorder with abnormal lipid storage as the major cellular pathologic hallmark. Genetic analyses have identified mutations in NPC1 gene in the great majority of cases, while mutations in NPC2 account for the remainders. Yet little is known regarding the cellular mechanisms responsible for NPC pathogenesis, especially for neurodegeneration, which is the usual cause of death. To identify critical steps that could account for the pathological manifestations of the disease in one of the most affected brain structures, we performed global gene expression analysis in the cerebellum from 3-week old Npc1+/+ and Npc1-/- mice with two different microarray platforms (Agilent and Illumina). Differentially expressed genes identified by both microarray platforms were then subjected to KEGG pathway analysis. Expression of genes in six pathways was significantly altered in Npc1-/- mice; functionally, these signaling pathways belong to the following three categories: (1) steroid and terpenoid biosynthesis, (2) immune response, and (3) cell adhesion/motility. In addition, the expression of several proteins involved in lipid transport was significantly altered in Npc1-/- mice. Our results provide novel molecular insight regarding the mechanisms of pathogenesis in NPC disease and reveal potential new therapeutic targets.

Lipids on trial: the search for the offending metabolite in Niemann-Pick type C disease

Niemann-Pick disease type C is a complex lysosomal storage disorder caused by mutations in either the NPC1 or NPC2 genes that is characterized at the cellular level by the storage of multiple lipids, defective lysosomal calcium homeostasis and unique trafficking defects. We review the potential role of each of the individual storage lipids in initiating the pathogenic cascade and propose a model of NPC1 and NPC2 function based on the current knowledge.

Increased activity and altered subcellular distribution of lysosomal enzymes determine neuronal vulnerability in Niemann-Pick type C1-deficient mice

Niemann-Pick disease type C (NPC), caused by mutations in the Npc1 or Npc2 genes, is a progressive neurodegenerative disorder characterized by intracellular accumulation/redistribution of cholesterol in a number of tissues including the brain. This is accompanied by a severe loss of neurons in selected brain regions. In this study, we evaluated the role of lysosomal enzymes, cathepsins B and D, in determining neuronal vulnerability in NPC1-deficient (Npc1(-/-)) mouse brains. Our results showed that Npc1(-/-) mice exhibit an age-dependent degeneration of neurons in the cerebellum but not in the hippocampus. The cellular level/expression and activity of cathepsins B and D are increased more predominantly in the cerebellum than in the hippocampus of Npc1(-/-) mice. In addition, the cytosolic levels of cathepsins, cytochrome c, and Bax2 are higher in the cerebellum than in the hippocampus of Npc1(-/-) mice, suggesting a role for these enzymes in the degeneration of neurons. This suggestion is supported by our observation that degeneration of cultured cortical neurons treated with U18666A, which induces an NPC1-like phenotype at the cellular level, can be attenuated by inhibition of cathepsin B or D enzyme activity. These results suggest that the increased level/activity and altered subcellular distribution of cathepsins may be associated with the underlying cause of neuronal vulnerability in Npc1(-/-) brains. Therefore, their inhibitors may have therapeutic potential in attenuating NPC pathology.

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.

Cholesterol synthesis inhibitor U18666A and the role of sterol metabolism and trafficking in numerous pathophysiological processes

The multiple actions of U18666A have enabled major discoveries in lipid research and contributed to understanding the pathophysiology of multiple diseases. This review describes these advances and the utility of U18666A as a tool in lipid research. Harry Rudney's recognition that U18666A inhibited oxidosqualene cyclase led him to discover a pathway for formation of polar sterols that he proved to be important regulators of 3-hydroxy-3-methyl-glutaryl coenzyme A reductase. Laura Liscum's recognition that U18666A inhibited the egress of cholesterol from late endosomes and lysosomes led to greatly improved perspective on the major pathways of intracellular cholesterol trafficking. The inhibition of cholesterol trafficking by U18666A mimicked the loss of functional Niemann-Pick type C protein responsible for NPC disease and thus provided a model for this disorder. U18666A subsequently became a tool for assessing the importance of molecular trafficking through the lysosomal pathway in other conditions such as atherosclerosis, Alzheimer's disease, and prion infections. U18666A also provided animal models for two important disorders: petite mal (absence) epilepsy and cataracts. This was the first chronic model of absence epilepsy. U18666A is also being used to address the role of oxidative stress in apoptosis. How can one molecule have so many effects? Perhaps because of its structure as an amphipathic cationic amine it can interact and inhibit diverse proteins. Restricting the availability of cholesterol for membrane formation through inhibition of cholesterol synthesis and intracellular trafficking could also be a mechanism for broadly affecting many processes. Another possibility is that through intercalation into membrane U18666A can alter membrane order and therefore the function of resident proteins. The similarity of the effects of natural and enantiomeric U18666A on cells and the capacity of intercalated U18666A to increase membrane order are arguments in favor of this possibility.

Mechanisms and consequences of impaired lipid trafficking in Niemann-Pick type C1-deficient mammalian cells

Niemann-Pick C disease is a fatal progressive neurodegenerative disorder caused in 95% of cases by mutations in the NPC1 gene; the remaining 5% of cases result from mutations in the NPC2 gene. The major biochemical manifestation of NPC1 deficiency is an abnormal sequestration of lipids, including cholesterol and glycosphingolipids, in late endosomes/lysosomes (LE/L) of all cells. In this review, we summarize the current knowledge of the NPC1 protein in mammalian cells with particular focus on how defects in NPC1 alter lipid trafficking and neuronal functions. NPC1 is a protein of LE/L and is predicted to contain thirteen transmembrane domains, five of which constitute a sterol-sensing domain. The precise function of NPC1, and the mechanism by which NPC1 and NPC2 (both cholesterol binding proteins) act together to promote the movement of cholesterol and other lipids out of the LE/L, have not yet been established. Recent evidence suggests that the sequestration of cholesterol in LE/L of cells of the brain (neurons and glial cells) contributes to the widespread death and dysfunction of neurons in the brain. Potential therapies include treatments that promote the removal of cholesterol and glycosphingolipids from LE/L. Currently, the most promising approach for extending life-span and improving the quality of life for NPC patients is a combination of several treatments each of which individually modestly slows disease progression.

Principles of lysosomal membrane degradation: Cellular topology and biochemistry of lysosomal lipid degradation

Cellular membranes enter the lysosomal compartment by endocytosis, phagocytosis, or autophagy. Within the lysosomal compartment, membrane components of complex structure are degraded into their building blocks. These are able to leave the lysosome and can then be utilized for the resynthesis of complex molecules or can be further degraded. Constitutive degradation of membranes occurs on the surface of intra-endosomal and intra-lysosomal membrane structures. Many integral membrane proteins are sorted to the inner membranes of endosomes and lysosome after ubiquitinylation. In the lysosome, proteins are degraded by proteolytic enzymes, the cathepsins. Phospholipids originating from lipoproteins or cellular membranes are degraded by phospholipases. Water-soluble glycosidases sequentially cleave off the terminal carbohydrate residues of glycoproteins, glycosaminoglycans, and glycosphingolipids. For glycosphingolipids with short oligosaccharide chains, the additional presence of membrane-active lysosomal lipid-binding proteins is required. The presence of lipid-binding proteins overcomes the phase problem of water soluble enzymes and lipid substrates by transferring the substrate to the degrading enzyme or by solubilizing the internal membranes. The lipid composition of intra-lysosomal vesicles differs from that of the plasma membrane. To allow at least glycosphingolipid degradation by hydrolases and activator proteins, the cholesterol content of these intraorganellar membranes decreases during endocytosis and the concentration of bis(monoacylglycero)phosphate, a stimulator of sphingolipid degradation, increases. A considerable part of our current knowledge about mechanism and biochemistry of lysosomal lipid degradation is derived from a class of human diseases, the sphingolipidoses, which are caused by inherited defects within sphingolipid and glycosphingolipid catabolism.

Therapeutic application of histone deacetylase inhibitors for central nervous system disorders

Histone deacetylases (HDACs)--enzymes that affect the acetylation status of histones and other important cellular proteins--have been recognized as potentially useful therapeutic targets for a broad range of human disorders. Pharmacological manipulations using small-molecule HDAC inhibitors--which may restore transcriptional balance to neurons, modulate cytoskeletal function, affect immune responses and enhance protein degradation pathways--have been beneficial in various experimental models of brain diseases. Although mounting data predict a therapeutic benefit for HDAC-based therapy, drug discovery and development of clinical candidates face significant challenges. Here, we summarize the current state of development of HDAC therapeutics and their application for the treatment of human brain disorders such as Rubinstein-Taybi syndrome, Rett syndrome, Friedreich's ataxia, Huntington's disease and multiple sclerosis.

The pathogenesis of Niemann-Pick type C disease: a role for autophagy?

Niemann-Pick type C disease (NPC) is a sphingolipid-storage disorder that results from inherited deficiencies of intracellular lipid-trafficking proteins, and is characterised by an accumulation of cholesterol and glycosphingolipids in late endosomes and lysosomes. Patients with this disorder develop progressive neurological impairment that often begins in childhood, is ultimately fatal and is currently untreatable. How impaired lipid trafficking leads to neurodegeneration is largely unknown. Here we review NPC clinical features and biochemical defects, and discuss model systems used to study this disorder. Recent studies have established that NPC is associated with an induction of autophagy, a regulated and evolutionarily conserved process by which cytoplasmic proteins are sequestered within autophagosomes and targeted for degradation. This pathway enables recycling of limited or damaged macromolecules to promote cell survival. However, in other instances, robust activation of autophagy leads to cell stress and programmed cell death. We summarise evidence showing that autophagy induction and flux are increased in NPC by signalling through a complex of the class III phosphoinositide 3-kinase and beclin-1. We propose that an imbalance between induction and flux through the autophagic pathway contributes to cell stress and neuronal loss in NPC and related sphingolipid-storage disorders, and discuss potential therapeutic strategies for modulating activity of this pathway.

Defining the importance of phosphatidylserine synthase-1 (PSS1): unexpected viability of PSS1-deficient mice

Phosphatidylserine (PS) is a quantitatively minor, but physiologically important, phospholipid in mammalian cells. PS is synthesized by two distinct base-exchange enzymes, PS synthase-1 (PSS1) and PS synthase-2 (PSS2), that are encoded by different genes. PSS1 exchanges serine for choline of phosphatidylcholine, whereas PSS2 exchanges ethanolamine of phosphatidylethanolamine for serine. We previously generated mice lacking PSS2 (Bergo, M. O., Gavino, B. J., Steenbergen, R., Sturbois, B., Parlow, A. F., Sanan, D. A., Skarnes, W. C., Vance, J. E., and Young, S. G. (2002) J. Biol. Chem. 277, 47701-47708) and found that PSS2 is not required for mouse viability. We have now generated PSS1-deficient mice. In light of the markedly impaired survival of Chinese hamster ovary cells lacking PSS1 we were surprised that PSS1-deficient mice were viable, fertile, and had a normal life span. Total serine-exchange activity (contributed by PSS1 and PSS2) in tissues of Pss1(-/-) mice was reduced by up to 85%, but except in liver, the PS content was unaltered. Despite the presumed importance of PS in the nervous system, the rate of axonal extension of PSS1-deficient neurons was normal. Intercrosses of Pss1(-/-) mice and Pss2(-/-) mice yielded mice with three disrupted Pss alleles but no double knockout mice. In Pss1(-/-)/Pss2(-/-) and Pss1(-/-)/Pss2(-/-) mice, serine-exchange activity was reduced by 65-91%, and the tissue content of PS and phosphatidylethanolamine was also decreased. We conclude that (i) elimination of either PSS1 or PSS2, but not both, is compatible with mouse viability, (ii) mice can tolerate as little as 10% of normal total serine-exchange activity, and (iii) mice survive with significantly reduced PS and phosphatidylethanolamine content.

Cholesterol level regulates endosome motility via Rab proteins

The role of cholesterol in the regulation of endosome motility was investigated by monitoring the intracellular trafficking of endocytosed folate receptors (FRs) labeled with fluorescent folate conjugates. Real-time fluorescence imaging of HeLa cells transfected with green fluorescent protein-tubulin revealed that FR-containing endosomes migrate along microtubules. Moreover, microinjection with antibodies that inhibit microtubule-associated motor proteins demonstrated that dynein and kinesin I participate in the delivery of FR-containing endosomes to the perinuclear area and plasma membrane, respectively. Further, single-particle tracking analysis revealed bidirectional motions of FR endosomes, mediated by dynein and kinesin motors associated with the same endosome. These experimental tools allowed us to use FR-containing endosomes to evaluate the impact of cholesterol on intracellular membrane trafficking. Lowering plasma membrane cholesterol by metabolic depletion or methyl-beta-cyclodextrin extraction was found to both increase FR-containing endosome motility and change endosome distribution from colocalization with Rab7 to colocalization with Rab4. These data provide evidence that cholesterol regulates intracellular membrane trafficking via modulation of the distribution of low molecular weight G-proteins that are adaptors for microtubule motors.

Lipid Homeostasis and Lipoprotein Secretion in Niemann-Pick C1-deficient Hepatocytes

Niemann-Pick C (NPC) disease is a fatal inherited disorder characterized by an accumulation of cholesterol and other lipids in late endosomes/lysosomes. Although this disease is considered to be primarily a neurodegenerative disorder, many NPC patients suffer from liver disease. We have investigated alterations that occur in hepatic lipid homeostasis using primary hepatocytes isolated from NPC1-deficient mice. The cholesterol content of Npc1(-/-) hepatocytes was 5-fold higher than that of Npc1(+/+) hepatocytes; phospholipids and cholesteryl esters also accumulated. In contrast, the triacylglycerol content of Npc1(-/-) hepatocytes was 50% lower than of Npc1(+/+) hepatocytes. We hypothesized that the cholesterol sequestration induced by NPC1 deficiency might inhibit very low density lipoprotein secretion. However, this process was enhanced by NPC1 deficiency and the secreted particles were enriched in cholesteryl esters. We investigated the mechanisms responsible for these changes. The synthesis of phosphatidylcholine, cholesteryl esters, and cholesterol in hepatocytes was increased by NPC1 deficiency and the amount of the mature form of sterol response element-binding protein-1 was also increased. These observations indicate that the enhanced secretion of lipoproteins from NPC1-deficient hepatocytes is due, at least in part, to increased lipid synthesis.

Cholesterol-dependent balance between evoked and spontaneous synaptic vesicle recycling

Cholesterol is a prominent component of nerve terminals. To examine cholesterol's role in central neurotransmission, we treated hippocampal cultures with methyl-beta-cyclodextrin, which reversibly binds cholesterol, or mevastatin, an inhibitor of cholesterol biosynthesis, to deplete cholesterol. We also used hippocampal cultures from Niemann-Pick type C1-deficient mice defective in intracellular cholesterol trafficking. These conditions revealed an augmentation in spontaneous neurotransmission detected electrically and an increase in spontaneous vesicle endocytosis judged by horseradish peroxidase uptake after cholesterol depletion by methyl-beta-cyclodextrin. In contrast, responses evoked by action potentials and hypertonicity were severely impaired after the same treatments. The increase in spontaneous vesicle recycling and the decrease in evoked neurotransmission were reversible upon cholesterol addition. Cholesterol removal did not impact on the low level of evoked neurotransmission seen in the absence of synaptic vesicle SNARE protein synaptobrevin-2 whereas the increase in spontaneous fusion remained. These results suggest that synaptic cholesterol balances evoked and spontaneous neurotransmission by hindering spontaneous synaptic vesicle turnover and sustaining evoked exo-endocytosis.

Lipid dynamics in neurons

Compared with other organs, the brain is highly enriched in cholesterol. Essentially all cholesterol in the brain is synthesized within the brain; the blood-brain barrier prevents the import of plasma lipoproteins into the brain. Consequently, the brain operates an independent lipoprotein transport system in which glial cells produce ApoE (apolipoprotein E)-containing lipoproteins that are thought to deliver cholesterol to neurons for axonal growth and repair. We have shown that ApoE-containing lipoproteins generated by glial cells stimulate axon extension. ApoE associated with lipoprotein particles, and a receptor of the low-density lipoprotein receptor family, are required for stimulation of axon growth. NPC (Niemann-Pick type C) disease is a severe neurological disorder caused by mutations in the NPC1 or NPC2 gene. A hallmark of this disease is impaired transport of cholesterol out of late endosomes/lysosomes and the accumulation of cholesterol in these organelles. Although cholesterol accumulates in cell bodies of neurons from NPC1-deficient mice, the cholesterol content of axons is reduced. The presence of NPC1 in endosomal structures in nerve terminals, and the finding of aberrant synaptic vesicles, suggest that defects in synaptic vesicle recycling contribute to neurological abnormalities characteristic of NPC disease. We have also shown that ApoE-containing lipoproteins produced by glial cells from NCP1-deficient mice are of normal composition and stimulate axon extension.

Lipid imbalance in the neurological disorder, Niemann-Pick C disease

Niemann-Pick C (NPC) disease is a progressive neurological disorder in which cholesterol, gangliosides and bis-monoacylglycerol phosphate accumulate in late endosomes/lysosomes. This disease is caused by mutations in either the NPC1 or NPC2 gene. NPC1 and NPC2 are involved in egress of lipids, particularly cholesterol, from late endosomes/lysosomes but the precise functions of these proteins are not clear. An important question regarding the function of NPC proteins is: why do mutations in these ubiquitously expressed proteins have such dire consequences in the brain? This review summarizes the roles of NPC proteins in lipid homeostasis particularly in the central nervous system.

Regulation of the cell swelling-activated chloride conductance by cholesterol-rich membrane domains

AIM

The role of high cholesterol-containing microdomains in the signal transduction cascade leading to the activation of volume-regulated anion channels (VRACs) was studied.

METHODS

Osmotic cell swelling-induced efflux of 125I- was determined in human epithelial Intestine 407 cells and in skin fibroblasts obtained from healthy controls or Niemann-Pick type C (NPC) patients. Cellular cholesterol content was modulated by pre-incubation with 2-hydroxypropyl-beta-cyclodextrin in the presence of acceptor lipid vesicles.

RESULTS

Osmotic cell swelling of human Intestine 407 cells leads to the rapid activation of a compensatory anion conductance. Treatment of the cells with cyclodextrin enhanced the response to submaximal hypotonic stimulation by approx. twofold, but did not further increase the efflux elicited by a saturating stimulus. In contrast, the volume-sensitive anion efflux was markedly inhibited when cholesterol-loaded cyclodextrin was used. Potentiation of the response by cholesterol depletion was maintained in caveolin-1 deficient Caco-2 colonocytes as well as in sphingomyelinase-treated Intestine 407 cells, indicating that cholesterol-rich microdomains are not crucially involved. However, treatment of the cells with progesterone, an inhibitor of NPC1-dependent endosomal cholesterol trafficking, not only markedly reduced the hypotonicity-provoked anion efflux, but also prevented its potentiation by cyclodextrin. In addition, the volume-sensitive anion efflux from human NPC skin fibroblasts was significantly smaller when compared with control fibroblasts.

CONCLUSIONS

The results support a model of regulatory volume decrease involving recruitment of volume-sensitive anion channels from intracellular compartments to the plasma membrane.