NCR-1 and NCR-2, theC. eleganshomologs of the human Niemann-Pick type C1 disease protein, function upstream of DAF-9 in the dauer formation pathways

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Abstract
Mutations in the human NPC1 gene cause most cases of Niemann-Pick type C(NP-C) disease, a fatal autosomal recessive neurodegenerative disorder. NPC1 is implicated in intracellular trafficking of cholesterol and glycolipids, but its exact function remains unclear. The C. elegans genome contains two homologs of NPC1, ncr-1 and ncr-2, and an ncr-2;ncr-1 double deletion mutant forms dauer larvae constitutively (Daf-c). We have analyzed the phenotypes of ncr single and double mutants in detail, and determined the ncr gene expression patterns. We find that the ncr genes function in a hormonal branch of the dauer formation pathway upstream of daf-9 and daf-12, which encode a cytochrome P450 enzyme and a nuclear hormone receptor, respectively. ncr-1 is expressed broadly in tissues with high levels of cholesterol, whereas expression of ncr-2 is restricted to a few cells. Both Ncr genes are expressed in the XXX cells, which are implicated in regulating dauer formation via the daf-9 pathway. Only the ncr-1 mutant is hypersensitive to cholesterol deprivation and to progesterone, an inhibitor of intracellular cholesterol trafficking. Our results support the hypothesis that ncr-1 and ncr-2 are involved in intracellular cholesterol processing in C. elegans, and that a sterol-signaling defect is responsible for the Daf-c phenotype of the ncr-2; ncr-1 mutant.