Regulation of low density lipoprotein receptor and 3-hydroxy-3-methyl-glutaryl-CoA reductase activities are differentially affected in Niemann-Pick type C and type D fibroblasts

Dynamin is involved in endolysosomal cholesterol delivery to the endoplasmic reticulum: role in cholesterol homeostasis

Cholesterol is one of the most essential membrane components in mammalian cells and plays a critical role in several cellular functions. It is now established that intracellular cholesterol transport contributes to the regulation of cellular cholesterol homeostasis by mechanisms that are yet poorly defined. In this study, we examined the role of clathrin- and dynamin-dependent trafficking on the regulatory machinery involved in cholesterol homeostasis. Thus, expression levels of three major sterol-sensitive genes, that is sterol-regulatory element binding protein 2 (SREBP-2), hydroxymethylglutaryl-coenzyme A (HMGCoA) reductase and low-density lipoprotein (LDL) receptor, were monitored to study the cell response to the addition of LDL-derived cholesterol. We found that inhibition of clathrin-dependent endocytosis had no effect on the intracellular distribution of cholesterol and the regulation of sterol-sensitive genes. In contrast, inhibition of dynamin activity resulted in the lack of regulation of SREBP-2, HMGCoA reductase and LDL receptor genes. Immunolocalization studies along with the measure of free and esterified cholesterol indicated that dynamin inactivation led to the accumulation of free cholesterol (FC) within the late endosomal (LE)/lysosomal compartment resulting in insufficient delivery of regulatory cholesterol to the endoplasmic reticulum (ER) where the transcriptional control of sterol-sensitive genes occurs. Our data therefore indicate that dynamin plays a critical role in the delivery of cholesterol from the LE/lysosomal network to the ER and highlight the importance of LE trafficking in cholesterol homeostasis.

Recent advances in elucidating Niemann-Pick C disease

Lysosomal sequestration of endocytosed LDL-derived cholesterol, premature and abnormal enrichment of cholesterol in trans Golgi cisternae and accompanying anomalies in intracellular sterol trafficking are the hallmark phenotypic features of the Niemann-Pick C (NPC) lesion. A variable severity of these alterations has been observed, with only partial correlation between clinical and biochemical phenotypes. NPC also affects the metabolism of sphingolipids, and other biochemical abnormalities have been reported. Occurrence of neurofibrillary tangles in the brain of patients with a slowly progressive course is a recent intriguing observation. Genetic heterogeneity was established by cell hybridization and linkage studies. The two complementation groups could not be distinguished from each other by clinical, cellular or biochemical criteria, suggesting that the two gene products may interact or function sequentially. The major (> 90% of patients) NPC1 gene was mapped to 18q11 and recently isolated by positional cloning. The cDNA sequence predicts a 1278-amino acid protein, with 13 to 16 possible transmembrane regions and a putative cholesterol-sensing domain. Two murine models of the disease involving the same gene are known. The murine cDNA and the npc(nih) mutation have been characterized. Described homologies of the NPC1 protein are in line with its putative involvement in cellular cholesterol traffic.

Cholesterol signaling at the endoplasmic reticulum occurs in npc1(-/-) but not in npc1(-/-), LDLR(-/-) mice

It remains controversial whether deficiency of the Niemann-Pick C1 (npc1) protein results in altered cholesterol signaling at the endoplasmic reticulum (ER). In this report, we have measured the processed, nuclear form of sterol regulatory element binding protein (SREBP)-1 in livers of npc1 wild-type, heterozygous, and homozygous deficient mice, alone, and in combination with deficiencies of the low density lipoprotein receptor (LDLR) or the multiple drug resistant (mdr)1a, P-glycoprotein. Cleavage of SREBPs to activated forms normally occurs when the ER is deficient in cholesterol. A large decrease in processed SREBP-1 was evident in fasted npc1(-/-) mice and npc1(-/-), mdr1a(-/-) mice, with no decrease evident in npc1(-/-), LDLR(-/-) mice. These results suggest that the increase in cellular cholesterol which occurs in npc1(-/-) and in npc1(-/-), mdr1a(-/-) mice includes the sites responsible for cholesterol signaling, while the similar increase in cholesterol found in npc1(-/-), LDLR(-/-) mice does not.

Niemann-Pick C variant detection by altered sphingolipid trafficking and correlation with mutations within a specific domain of NPC1

Niemann-Pick disease type C (NPC) is a fatal, autosomal recessive lipidosis characterized by lysosomal accumulation of unesterified cholesterol and multiple neurological symptoms, such as vertical supranuclear ophthalmoplegia, progressive ataxia, and dementia. More than 90% of cases of NPC are due to a defect in Niemann-Pick C1 (NPC1), a late endosomal, integral membrane protein that plays a role in cholesterol transport or homeostasis. Biochemical diagnosis of NPC has relied on the use of patient skin fibroblasts in an assay to demonstrate delayed low-density lipoprotein (LDL)-derived cholesterol esterification and a cytological technique-filipin staining-to demonstrate the intracellular accumulation of cholesterol. A small percentage of patients, referred to as "NPC variants," present with clinical symptoms of NPC but show near-normal results of these biochemical tests, making laboratory confirmation of NPC disease problematic. Here, we demonstrate that NPC-variant fibroblast samples can be detected as sphingolipid storage disease cells, using a fluorescent sphingolipid analog, BODIPY-lactosylceramide. This lipid accumulated in endosomes/lysosomes in variant cells preincubated with LDL cholesterol but targeted to the Golgi complex in normal cells under these conditions. The reproducibility of this technique was validated in a blinded study. In addition, we performed mutation analysis of the NPC1 gene in NPC variant and "classical" NPC cell samples and found a high incidence of specific mutations within the cysteine-rich region of NPC1 in variants. We also found that 5 of the 12 variant cell samples had no apparent defect in NPC1 but were otherwise indistinguishable from other variant cells. This is a surprising result, since, in general, approximately 90% of patients with NPC possess defects in NPC1. Our findings should be useful for the detection of NPC variants and also may provide significant new insight regarding NPC1 genotype/phenotype correlations.

Evidence for a Niemann-pick C (NPC) gene family: identification and characterization of NPC1L1

Niemann-Pick type C1 (NPC1) disease is caused by defects in the NPC1 protein, which result in perturbation of subcellular cholesterol transport. To identify related proteins that may be involved in subcellular cholesterol trafficking, the expressed sequence tag (EST) database was searched to find homologues of human NPC1. A short, weakly similar EST was identified and used to obtain a full-length human cDNA of about 5 kb and two alternatively spliced transcripts. The gene, named NPC1L1, was mapped to chromosome 7p13, contained 20 exons, including an unusually large 1526-bp exon 2, and spanned approximately 29 kb. In contrast to NPC1, the NPC1L1 putative promoter region contained a sterol-regulatory element. The predicted protein shared 42% identity and 51% similarity with NPC1. Interestingly, NPC1L1 contains the conserved amino-terminal "NPC1 domain" and the putative sterol-sensing domain, providing strong evidence that it is related to human NPC1 and suggesting that these may comprise a new family of NPC1-related proteins. However, the two differ with respect to their putative intracellular targeting signals. Collectively, these data suggest that NPC1L1 and NPC1 form part of a family of related proteins that may have similar functions at different subcellular locations, perhaps at sequential steps of the same cholesterol transport pathway.

Mutations in NPC1 highlight a conserved NPC1-specific cysteine-rich domain

Niemann-Pick type II disease is an autosomal recessive disorder characterized by a defect in intracellular trafficking of sterols. We have determined the intron/exon boundaries of eight exons from the conserved 3' portion of NPC1, the gene associated with most cases of the disease. SSCP analyses were designed for these exons and were used to identify the majority of mutations in 13 apparently unrelated families. Thirteen mutations were found, accounting for 19 of the 26 alleles. These mutations included eight different missense mutations (including one reported by Greer et al. [1998]), one 4-bp and two 2-bp deletions that generate premature stop codons, and two intronic mutations that are predicted to alter splicing. Two of the missense mutations were present in predicted transmembrane (TM) domains. Clustering of these and other reported NPC1 mutations in the carboxy-terminal third of the protein indicates that screening of these exons, by means of the SSCP analyses reported here, will detect most mutations. The carboxy-terminal half of the Npc1 protein shares amino acid similarity with the TM domains of the morphogen receptor Patched, with the largest stretch of unrelated sequence lying between two putative TM spans. Alignment of this portion of the human Npc1 protein sequence with Npc1-related sequences from mouse, yeast, nematode, and a plant, Arabidopsis, revealed conserved cysteine residues that may coordinate the structure of this domain. That 7 of a total of 13 NPC1 missense mutations are concentrated in this single Npc1-specific domain suggests that integrity of this region is particularly critical for normal functioning of the protein.

The Nova Scotia (type D) form of Niemann-Pick disease is caused by a G3097-->T transversion in NPC1

Niemann-Pick type D (NPD) disease is a progressive neurodegenerative disorder characterized by the accumulation of tissue cholesterol and sphingomyelin. This disorder is relatively common in southwestern Nova Scotia, because of a founder effect. Our previous studies, using classic linkage analysis of this large extended kindred, defined the critical gene region to a 13-cM chromosome segment between D18S40 and D18S66. A recently isolated gene from this region, NPC1, is mutated in the majority of patients with Niemann-Pick type C disease. We have identified a point mutation within this gene (G3097-->T; Gly992-->Trp) that shows complete linkage disequilibrium with NPD, confirming that NPD is an allelic variant of NPC1.

Increased expression of caveolin-1 in heterozygous Niemann-Pick type II human fibroblasts

Human Niemann-Pick type II fibroblasts, which encompass the panethnic type C (NPC) and Nova Scotia Acadian type D (NPD) variants, exhibit altered expression of caveolin-1 protein when examined by immunoblotting using an anti-caveolin-1 monoclonal antibody. Unexpectedly, caveolin-1 in heterozygous fibroblasts was significantly elevated as much as 10-fold compared to caveolin-1 in normal and homozygous affected fibroblasts. Homozygous NPC fibroblasts expressed caveolin-1 levels similar to those in normal fibroblasts, while the expression of caveolin-1 in homozygous NPD fibroblasts was slightly elevated. Northern analysis indicates that normal fibroblasts and NPC heterozygous fibroblasts have similar amounts of caveolin-1 mRNA, while NPC homozygous fibroblasts have significantly less caveolin-1 mRNA. In contrast, heterozygous and homozygous NPD fibroblasts exhibit increased levels of caveolin-1 mRNA. These novel findings suggest that caveolin-1 containing subcellular structures are involved in the pathophysiology of Niemann-Pick type II disease. Furthermore, altered caveolin-1 protein expression may serve as a useful marker for the diagnosis of carriers of NPC or NPD.

Linkage of Niemann-Pick disease type D to the same region of human chromosome 18 as Niemann-Pick disease type C

Niemann-Pick type II disease is a severe disorder characterized by accumulation of tissue cholesterol and sphingomyelin and by progressive degeneration of the nervous system. This disease has two clinically similar subtypes, type C (NPC) and type D (NPD). NPC is clinically variable and has been identified in many ethnic groups. NPD, on the other hand, has been reported only in descendants of an Acadian couple who lived in Nova Scotia in the early 18th century and has a more homogeneous expression resembling that of less severely affected NPC patients. Despite biochemical differences, it has not been established whether NPC and NPD are allelic variants of the same disease. We report here that NPD is tightly linked (recombination fraction .00; maximum LOD score 4.50) to a microsatellite marker, D18S480, from the centromeric region of chromosome 18q. Carstea et al. have reported that the NPC gene maps to this same site; therefore we suggest that NPC and NPD likely result from mutations in the same gene.