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Cells acquire cholesterol either by de novo synthesis in the endoplasmic reticulum or by internalization of cholesterol-containing lipoproteins, particularly low density lipoprotein (LDL), via receptor-mediated endocytosis. The inherited disorder Niemann-Pick type C (NPC), in which abnormal LDL-cholesterol trafficking from the endo/lysosomal compartment leads to substantial cholesterol and glycolipid accumulation in lysosomes, is caused by defects in either of two genes that encode for proteins designated as NPC1 and NPC2. NPC2 is a small intralysosomal protein that has been characterized biochemically as a cholesterol binding protein. We determined the rate and mechanism by which NPC2 delivers cholesterol to model phospholipid membranes. A fluorescence dequenching assay was used to monitor the kinetics of cholesterol transfer from the protein to membranes. The endogenous tryptophan fluorescence of the NPC2 was quenched upon binding of cholesterol, and the subsequent addition of acceptor vesicles resulted in dequenching of the tryptophan signal, enabling the monitoring of cholesterol transfer to membranes. The rates of cholesterol transfer were evaluated as a function of acceptor vesicle concentration, acceptor vesicle phospholipid headgroup composition, and aqueous phase properties. The results suggest that NPC2 rapidly transports cholesterol to phospholipid vesicles via a collisional mechanism which involves a direct interaction with the acceptor membrane. Transfer of cholesterol to membranes is faster in an acidic environment and is greatly enhanced by the presence of the unique lysosomal/late endosomal phospholipid lyso-bisphosphatidic acid (LBPA) (also known as bismonoacylglycerol phosphate). Finally, we found that the rate of transfer of cholesterol from vesicles to NPC2 was dramatically increased by the presence of lyso-bisphosphatidic acid in the donor vesicles. These results support a role for the NPC2 protein in the egress of LDL derived cholesterol out of the endosomal/lysosomal compartment.

Cellular cholesterol trafficking includes numerous individual processes, as cholesterol cannot only be obtained endogenously or exogenously, but also has many possible fates within the cell, including incorporation into membranes, metabolic conversion to a number of different products, covalent modification of specific proteins, regulation of gene expression, and efflux from the cell. Distinct trafficking pathways are thought to exist for de novo synthesized cholesterol and cholesterol obtained via the receptor-mediated endocytosis of low density lipoproteins. Exogenously obtained cholesterol must pass through the endosomal/lysosomal compartment prior to its trafficking to other subcellular sites, such as the endoplasmic reticulum, the Golgi apparatus, and the plasma membrane. The inherited disorder Niemann-Pick type C, in which abnormal cholesterol trafficking from the endo-lysosomal compartment leads to substantial cholesterol and glycolipid accumulatioin in lysosomes, is caused by defects in either of two genes that encode for proteins designated as NPC1 and NPC2. NPC1 is a multiple membrane spanning domain protein containing a sterol sensing domain similar to those found in several proteins involved in cholesterol homeostasis. NPC2 is a small intralysosomal protein that has been characterized biochemically as a cholesterol binding and transport protein. While there is abundant evidence suggesting a role for the NPC proteins in late endosomal/lysosomal trafficking of cholesterol, their precise functions and mechanisms of action remain to be discovered.

Insulin resistance and altered maternal metabolism in gestational diabetes mellitus (GDM) may impair fetal arachidonic acid (AA) and docosahexaenoic acid (DHA) status. The objectives were to test the hypothesis that fetal polyunsaturated fatty acids would be altered with GDM and identify factors related to fetal phospholipid (PL) AA and DHA. Maternal and cord vein erythrocyte PL fatty acids were determined in GDM (n = 13) and healthy pregnant women (controls, n = 12). Cord vein erythrocyte PL AA and DHA concentrations were significantly lower in GDM vs. controls. Maternal blood hemoglobin A1C was inversely correlated to fetal erythrocyte PL DHA and AA in controls and GDM (n = 25). Pregravid body mass index was negatively associated with fetal PL DHA. The data support the hypothesis that there is impairment in fetal accretion of DHA and AA in GDM.