Cholesterol homeostasis is maintained by a plethora of intracellular lipid sensing and trafficking mechanisms, in combination with tight control of its synthesis and catabolism. Many of the processes that influence this homeostasis are poorly understood and these are the focus of my research. To gain insight into key mechanisms of cholesterol homeostasis, initial studies have investigated the functions of two proteins, Niemann-Pick C (NPC)1 and NPC2 that are defective in the human, autosomal recessive disorder, NPC disease. This disorder is characterized by a defect in LDL-derived free cholesterol efflux from the endosomal/lysosomal system, which results in cholesterol accumulation in these intracellular compartments and a progressively debilitating and incurable disease phenotype. Understanding how these proteins function will not only enhance our knowledge regarding the mechanisms that control cholesterol homeostasis but will also aid in the development of treatments for this disorder.

Since the majority of NPC disease patients (95%) have a defect in NPC1, this protein has been the major focus of my studies. We have determined the subcellular localization of NPC1, mapped the topology of this multipolytopic protein and identified key targeting signals within the protein. Further research revealed that NPC1 facilitates the transport of fatty acids across cell membranes, utilizing a proton motive force. These studies indicated that NPC1 was a mammalian member of the well-studied and distantly-related prokaryotic resistance-nodulation-division permeases, the first mammalian protein shown to operate similarly to this evolutionarily ancient superfamily of proteins. Further studies of the precise lipid ligands transported by NPC1 are currently in progress.

Studies to identify homologues of NPC1 have lead to our identification of the NPC1-Like 1 (L1) protein and have expanded our interest in proteins that influence cholesterol homeostasis. Since identifying three alternatively spliced forms of the human form of the protein we have generated an NPC1L1 gene knockout mouse that has alterations in lipid trafficking, including cholesterol transport. In contrast to defects in NPC1 that result in a severe disease phenotype, defects in NPC1L1 actually appear to be beneficial. The NPC1L1 knockout mice appear to be phenotypically normal, although when fed a high-cholesterol diet these mice are clearly resistant to the hypercholesterolemia that occurs in wild-type mice. We have also investigated the tissue expression of NPC1L1 and shown that it is expressed predominantly in the mouse intestine, where it is known to facilitate cholesterol absorption. Interestingly, in humans the highest expression is observed in the liver, although intestinal expression is also higher than in other tissues. Intracellularly, we have localized this protein to Rab 5-positive vesicles. Currently, we are actively investigating the precise role of NPC1L1 in facilitating the transport of cholesterol and other lipids in both humans and mice. This knowledge will aid in the development of treatments for hypercholesterolemia and related disorders.