Sterol Transfer between Cyclodextrin and Membranes: Similar but Not Identical Mechanism to NPC2-Mediated Cholesterol Transfer

Abstract

Niemann–Pick C disease is an inherited disorder in which cholesterol and other lipids accumulate in the late endosomal/lysosomal compartment. Recently, cyclodextrins (CD) have been shown to reduce symptoms and extend lifespan in animal models of the disease. In the present studies we examined the mechanism of sterol transport by CD using in vitro model systems and fluorescence spectroscopy and NPC2-deficient fibroblasts. We demonstrate that cholesterol transport from the lysosomal cholesterol-binding protein NPC2 to CD occurs via aqueous diffusional transfer and is very slow; the rate-limiting step appears to be dissociation of cholesterol from NPC2, suggesting that specific interactions between NPC2 and CD do not occur. In contrast, the transfer rate of the fluorescent cholesterol analogue dehydroergosterol (DHE) from CD to phospholipid membranes is very rapid and is directly proportional to the acceptor membrane concentration, as is DHE transfer from membranes to CD. Moreover, CD dramatically increases the rate of sterol transfer between membranes, with rates that can approach those mediated by NPC2. The results suggest that sterol transfer from CD to membranes occurs by a collisional transfer mechanism involving direct interaction of CD with membranes, similar to that shown previously for NPC2. For CD, however, absolute rates are slower compared to NPC2 for a given concentration, and the lysosomal phospholipid lysobisphosphatidic acid (LBPA) does not stimulate rates of sterol transfer between membranes and CD. As expected from the apparent absence of interaction between CD and NPC2, the addition of CD to NPC2-deficient fibroblasts rapidly rescued the cholesterol accumulation phenotype. Thus, the recent observations of CD efficacy in mouse models of NPC disease are likely the result of CD enhancement of cholesterol transport between membranes, with rapid sterol transfer occurring during CD–membrane interactions.

Niemann-Pick C disease and mobilization of lysosomal cholesterol by cyclodextrin

“…Because CYCLO reduces cholesterol storage in LEs/Ls of either NPC1-or NPC2-defi cient cells ( 99,173 ), CYCLO can bypass the function of both proteins. In vitro studies have shown that CYCLO markedly accelerates cholesterol transfer between membranes via a collisional mechanism involving direct interaction between CYCLO and membranes ( 177 ). The mechanism by which CYCLO releases cholesterol from LEs/Ls was further investigated using two types of ␤ -cyclodextrin ( 167,176 ).…”

Cyclodextrins as Emerging Therapeutic Tools in the Treatment of Cholesterol-Associated Vascular and Neurodegenerative Diseases

“…The beneficial effects of HPβCD in NPC models remains unclear and have been studied in macrophages in which HPβCD exhibits its cholesterol extraction capacity for relatively high concentration of the CD (from >10 mM to 100 mM), whereas at much lower concentration (<1 mM) HPβCD rather behaves as a cholesterol transferring system between membranes of the LE/LY compartment [54,65], in a similar manner to NPC2 function [110]. Another study using another β-CD derivative supports the fact that β-CD family acts on unesterified cholesterol pool in NPC models.…”

Cyclodextrin mediates rapid changes in lipid balance in Npc1 mice without carrying cholesterol through the bloodstream

“…The ability of various cyclodextrins to interact, adsorb, and ultimately extract unesterifi ed cholesterol from lipid membranes into an aqueous environment has been extensively studied ( 2,16,(36)(37)(38)(39)(40). Single ␤ -CDs can accommodate an unesterifi ed cholesterol in their hydrophobic pocket, but about one-third of the sterol molecule is expected to protrude from the CD ( 36,40 ).…”

“…Notably, serial injections of HP-␤ -CD were shown to double the lifespan of the Npc1 Ϫ / Ϫ mouse ( 11,12 ), and continuous intrathecal administration was found to dramatically rescue Purkinje cell loss within the cerebellum ( 13 ). HP-␤ -CD does not require a functional NPC1 or NPC2 protein to work as suggested by studies using cell culture models (14)(15)(16) and confi rmed in mice when HP-␤ -CD was shown to signifi cantly decrease hepatic cholesterol synthesis rates in Npc1…”