Characterization of the Milk-Derived Niemann-Pick C2 Protein

The NPC2 protein is a small soluble cholesterol binding glycoprotein, which binds the mannose-6-phosphate/IGF2-receptor. Several mammalian orthologs with highly conserved primary sequences have been described and found to be ubiquitous expressed, which indicates that the protein carries out essential functions in the organism. This has recently been confirmed by identification of the molecular basis for the fatal lysosomal cholesterol storage disease, Niemann-Pick type C2, in which NPC2 deficiency results in a defect in the intracellular trafficking of LDL-derived cholesterol. This leads to variable phenotypes including hepatosplenomegaly, lung emphysema, and progressive neurodegeneration.

We have identified and successfully purified NPC2 by a two-step procedure from the whey fraction of bovine milk, which somewhat surprisingly was found to contain relatively high amounts of the allegated lysosomal protein. The purified NPC2 contains mannose 6-phosphate (M6P), the hallmark of lysosomal proteins. Functional analysis using in vitro cell model systems provides evidence that extracellular administration of the bovine milk-derived NPC2 can substitute the missing protein in human skin fibroblasts derived from Niemann-Pick type C2 patients, thereby restoring cholesterol homeostasis. Moreover, our preliminary in vivo mouse experiments indicate that intact bovine NPC2 is recovered in the mouse blood and peripheral tissues following oral administration by gavage. A possible physiological implication of bioactive NPC2 in the milk is unknown.
Further investigations therefore aim to investigate the potential biological role of NPC2 in the offspring.