NPC (Niemann–Pick type C) disease is a rare lipidosis characterized by the accumulation of LDL (low-density lipoprotein)-derived non-esterified cholesterol in the E/L (endosomal/lysosomal) system. The gene products that are responsible for the two NPC complementation groups are distinct and dissimilar, yet their cellular and disease phenotypes are virtually indistinguishable. To investigate the relationship between NPC1 and NPC2 and their potential role in NPC disease pathogenesis, we have developed a method for the rapid and efficient isolation of late endocytic vesicles from mouse liver by magnetic chromatography. Late endosomes from Wt (wild-type) and NPC1 mice were found to differ not only in their cholesterol and sphingomyelin content, as expected, but also in their non-esterified (‘free’) fatty acid content, with NPC1 vesicles showing an approx. 7-fold increase in non-esterified fatty acid levels compared with Wt vesicles. Furthermore, we show that the NPC2 protein is in an incompletely deglycosylated form in NPC1 late endosomes by a mechanism that is specific to the NPC2 protein and not a global aberration of protein glycosylation/deglycosylation or trafficking, since NPC2 secreted from NPC1 cells is indistinguishable from that secreted from Wt cells. Also, a greater proportion of the normally soluble cellular NPC2 protein partitions with detergent-insoluble late endosomal internal membrane domains in NPC1 vesicles. In addition, we show that, although a small amount of the NPC2 protein associates with these membranes in Wt vesicles, this localization becomes much more pronounced in NPC1 vesicles. These results suggest that the function of the NPC2 protein may be compromised as well in NPC1 endosomes, which might explain the paradoxical phenotypic similarities of the two NPC disease complementation groups.
Human α-galactosidase A (EC 18.104.22.168; α-Gal A) is the homodimeric glycoprotein that hydrolyses the terminal α-galactosyl moieties from glycolipids and glycoproteins. The type, site occupancy and function of the N-linked oligosaccharide chains on this lysosomal hydrolase were determined. Endoglycosidase treatment of the purified recombinant enzyme and mutagenesis studies indicated that three (Asn-139, Asn-192 and Asn-215) of the four potential N-glycosylation consensus sequences were occupied by complex, high-mannose and hybrid-type oligosaccharides respectively. When expressed in COS-1 cells, glycoforms with glycosylation site 1 or 2 obliterated had more than 70% of wild-type activity, and both glycoforms were secreted. In contrast, the glycoform with only site 3 eliminated had decreased activity (less than 40%); little, if any, was secreted. Expressed mutant glycoforms in which site 3 and site 1 or 2 were obliterated had little, if any, intracellular or secreted enzymic activity, and immunofluorescence microscopy revealed that the expressed mutant glycoforms were retained in the endoplasmic reticulum, presumably where they were degraded. Thus glycosylation at site 3 was crucial to the formation of soluble, active enzyme, as well as transport to the lysosome. Absence of the site 3 hybrid-type oligosaccharide exposed an adjacent, normally protected, hydrophobic region, resulting in aggregation of the enzyme polypeptide in the endoplasmic reticulum. In support of this concept, endoglycosidase H-treated enzyme or mannose-terminated enzyme expressed in Autographa californica cells also aggregated when concentrated, emphasizing that site 3 occupancy by a hybrid-type oligosaccharide was required for enzyme solubility.