Significance of Sterol Structural Specificity

Abstract

Desmosterol is an immediate precursor of cholesterol in the Bloch pathway of sterol synthesis and an abundant membrane lipid in specific cell types. The significance of the difference between the two sterols, an additional double bond at position C24 in the tail of desmosterol, is not known. Here, we provide evidence that the biophysical and functional characteristics of the two sterols differ and that this is because the double bond at C24 significantly weakens the sterol ordering potential. In model membranes, desmosterol was significantly weaker than cholesterol in promoting the formation or stability of ordered domains, and in mammalian cell membranes, desmosterol associated less avidly than cholesterol with detergentresistant membranes. Atomic scale molecular dynamics simulations showed that the double bond gives rise to additional stress in the tail, creating a rigid structure between C24 and C27 and favoring tilting of desmosterol distinct from cholesterol. Functional effects of desmosterol in cell membranes were assessed upon acutely exchanging ϳ70% of cholesterol to desmosterol. This led to impaired raft-dependent signaling via the insulin receptor, whereas non-raft-dependent protein secretion was not affected. We suggest that the choice of cholesterol synthesis route may provide a physiological mechanism to modulate raft-dependent functions in cells.In model membranes, cholesterol associates preferentially with long, saturated acyl chains, such as those in sphingolipids, thus reducing the area per lipid molecule (1, 2). There is substantial evidence to suggest that ordered lipid domains (rafts) composed of sterol and saturated lipids also exist in eukaryotic cell membranes and play important roles in numerous biological processes (3, 4). Lipid rafts are considered to exist in a liquid-ordered (L o ) 2 state characterized by tight ordering but relatively high lateral mobility of lipids and operationally often defined as detergent-resistant membranes (DRMs) (5, 6). Instead, unsaturated phospholipids are loosely packed, forming a liquid-disordered (L d ) membrane that is solubilized upon the addition of mild detergents. At least in model membranes, cholesterol is able to promote the separation of L o and L d domains (7-9). In cells, cholesterol levels influence the domain partitioning and biological activity of proteins that co-isolate in detergent-resistant membranes (DRMs) (10, 11).Taking the postulated critical role for cholesterol in raft formation and the diversity of sterols in biological materials, the sterol structural requirements for promoting ordered domain formation are highly relevant. Until now, the effects of sterol/steroid structure have mostly been addressed in model membranes. Slight modifications of the cholesterol structure (e.g. a shift of the double bond in the sterol ring or alteration of the 3-OH group) change the domain-forming properties of the molecule (12-14). Among the structurally closest relatives of cholesterol are its immediate biosynthetic precursors. The only difference ...

Cholesterol Substitution Increases the Structural Heterogeneity of Caveolae

“…This would require large scale preparation of immunoisolated caveolae from 20,25-DAC-treated versus untreated animals, for example. The C24 double bond gives rise to additional stress in the sterol tail, favors tilting of the molecule in the bilayer (13), and may thereby sterically hinder interactions with Cav1. The reduced sterol affinity may, in turn, explain the reduced association of Cav1 with DRMs as well as the decreased stability of caveolin oligomers.…”

“…This resulted in the accumulation of desmosterol, which differs from cholesterol by an additional double bond between carbon atoms 24 and 25 in the sterol tail. We chose this strategy as data on DHCR24-deficient mice (11, 12) and on cultured cells containing desmosterol (13,14) suggested that desmosterol accumulation may specifically interfere with the function of proteins enriched in lipid rafts/caveolae. Our results show that the minor structural difference between cholesterol and desmosterol weakens sterol-caveolin interaction, results in distorted caveolae with an altered number of Cav1 molecules per caveola, and affects the function of caveolae as endocytic vehicles.…”

“…13. For Triton-Optiprep TM fractionation, the cells were collected by centrifugation at 1000 ϫ g for 5 min and incubated for 10 min on ice in 240 l of lysis buffer (25 mM Tris-HCl, pH 7.5, 150 mM NaCl, 5 mM EDTA, 1 mM dithiothreitol, 10% sucrose, and 1% Triton X-100, CLAP).…”

Coarse-grained model for phospholipid/cholesterol bilayer employing inverse Monte Carlo with thermodynamic constraints

“…The rafts have been suggested to participate in a wide range of cellular processes such as trafficking and sorting of proteins. Recent atomistic simulations of lipid rafts 8,9 and other lipid bilayer systems including cholesterol and other sterols 10,[14][15][16] have provided insight into the atomicand molecular-scale interaction mechanisms of the raftassociated lipids. However, to reach the length and time scales necessary to study large-scale phenomena, one has to resort to coarse-grained models that employ effective interactions and simplify the description of the underlying system.…”

Molecular Requirement for Sterols in Herpes Simplex Virus Entry and Infectivity

“…Cholesterol is oriented in the bilipid membrane with its hydrocarbon tail at the interface of the outer and inner leaflets. The double bond in des- mosterol causes greater tilt in the sterol structure that results in decreased membrane lipid packing and ordering, thus altering membrane properties such as fluidity (27). Desmosterol's effect on membrane architecture may adversely influence the membrane flexibility necessary for membrane fusion.…”