Eugene D. Carstea

The Niemann-Pick C1 Protein Resides in a Vesicular Compartment Linked to Retrograde Transport of Multiple Lysosomal Cargo

Niemann-Pick C disease (NP-C) is a neurovisceral lysosomal storage disorder. A variety of studies have highlighted defective sterol trafficking from lysosomes in NP-C cells. However, the heterogeneous nature of additional accumulating metabolites suggests that the cellular lesion may involve a more generalized block in retrograde lysosomal trafficking. Immunocytochemical studies in fibroblasts reveal that theNPC1 gene product resides in a novel set of lysosome-associated membrane protein-2 (LAMP2)(+)/mannose 6-phosphate receptor(−) vesicles that can be distinguished from cholesterol-enriched LAMP2(+) lysosomes. Drugs that block sterol transport out of lysosomes also redistribute NPC1 to cholesterol-laden lysosomes. Sterol relocation from lysosomes in cultured human fibroblasts can be blocked at 21 °C, consistent with vesicle-mediated transfer. These findings suggest that NPC1(+) vesicles may transiently interact with lysosomes to facilitate sterol relocation. Independent of defective sterol trafficking, NP-C fibroblasts are also deficient in vesicle-mediated clearance of endocytosed [14C]sucrose. Compartmental modeling of the observed [14C]sucrose clearance data targets the trafficking defect caused by mutations in NPC1 to an endocytic compartment proximal to lysosomes. Low density lipoprotein uptake by normal cells retards retrograde transport of [14C]sucrose through this same kinetic compartment, further suggesting that it may contain the sterol-sensing NPC1 protein. We conclude that a distinctive organelle containing NPC1 mediates retrograde lysosomal transport of endocytosed cargo that is not restricted to sterol.

Neurofibrillary tangles in Niemann-Pick disease type C

Niemann-Pick disease type C (NPC) is an autosomal recessive disease, belonging to a clinically heterogeneous group of lipid storage diseases, distinguished by a unique error in cellular trafficking of exogenous cholesterol, associated with lysosomal accumulation of unesterified cholesterol. Unlike Niemann-Pick disease types A and B, there is no primary genetic defect in sphingomyelinase in NPC. During the routine neuropathological study of NPC patients, we found neurofibrillary tangles (NFT) in a series of cases with a slowly progressive chronic course. These were not associated with β-amyloid deposits. The NFT were most frequent in the orbital gyrus, cingulate gyrus and entorhinal region of the cerebral cortex, but were also frequently found in the basal ganglia, thalamus and hypothalamus. In one of the most severely affected case, the NFT were even found in the neurons in the inferior olivary nucleus and in the spinal cord. The NFT were immunostained with Alz 50, and cosisted of paired helical filaments. The distribution of the neurons bearing the NFT was generally similar to that of the swollen storage neurons, and storage neurons often contained NFT in their perikarya and/or in the meganeurites. However, neurons with NFT could be noted without swollen perikarya. The coexistence of neuronal storage and NFT in NPC without amyloid deposits suggests that perturbed cholesterol metabolism and/or lysosomal membrane trafficking may play a role in the formation of NFT, and that amyloid deposits are not necessarily the prerequisite for NFT formation. The results of our study also suggest that NFT formation may be a rather nonspecific cellular reaction of neurons to certain slowly progressive metabolic perturbations of an as yet undefined nature.

Mutagenesis of the putative sterol-sensing domain of yeast Niemann Pick C–related protein reveals a primordial role in subcellular sphingolipid distribution

Lipid movement between organelles is a critical component of eukaryotic membrane homeostasis. Niemann Pick type C (NP-C) disease is a fatal neurodegenerative disorder typified by lysosomal accumulation of cholesterol and sphingolipids. Expression of yeast NP-C–related gene 1 (NCR1), the orthologue of the human NP-C gene 1 (NPC1) defective in the disease, in Chinese hamster ovary NPC1 mutant cells suppressed lipid accumulation. Deletion of NCR1, encoding a transmembrane glycoprotein predominantly residing in the vacuole of normal yeast, gave no phenotype. However, a dominant mutation in the putative sterol-sensing domain of Ncr1p conferred temperature and polyene antibiotic sensitivity without changes in sterol metabolism. Instead, the mutant cells were resistant to inhibitors of sphingolipid biosynthesis and super sensitive to sphingosine and C2-ceramide. Moreover, plasma membrane sphingolipids accumulated and redistributed to the vacuole and other subcellular membranes of the mutant cells. We propose that the primordial function of these proteins is to recycle sphingolipids and that defects in this process in higher eukaryotes secondarily result in cholesterol accumulation.

Mutation analysis of feline Niemann-Pick C1 disease.

Niemann-Pick C (NPC) disease is an autosomal recessive neurovisceral lysosomal storage disorder that results in defective intracellular transport of cholesterol. The major form of human NPC (NPC1) has been mapped to chromosome 18, the NPC1 gene (NPC1) has been sequenced and several mutations have been identified in NPC1 patients. A feline model of NPC has been characterized and is phenotypically, morphologically, and biochemically similar to human NPC1. Complementation studies using cultured fibroblasts from NPC affected cats and NPC1 affected humans support that the gene responsible for the NPC phenotype in this colony of cats is orthologous to human NPC1. Using human-based PCR primers, initial fragments of the feline NPC cDNA were amplified and sequenced. From these sequences, feline-specific PCR primers were generated and designed to amplify six overlapping bands that span the entire feline NPC1 open reading frame. A single base substitution (2864G-C) was identified in NPC1 affected cats. Obligate carriers are heterozygous at the same allele and a PCR-based assay was developed to identify the geneotype of all cats in the colony. The mutation results in an amino acid change from cysteine to serine (C955S). Several of the mutations identified in people occur in the same region. Marked similarity exists between the human and feline NPC1 cDNA sequences, and is greater than that between the human and murine NPC1 sequences. The human cDNA sequence predicts a 1278aa protein with a lysosomal targeting sequence, several trans-membrane domains and extensive homology with other known mediators of cholesterol homeostasis.