Formation of 7-dehydrocholesterol-containing membrane rafts in vitro and in vivo, with relevance to the Smith-Lemli-Opitz syndrome

Abstract

Smith-Lemli-Opitz syndrome (SLOS) is a recessive disease typified by 7-dehydrocholesterol (7DHC) accumulation and depletion of cholesterol. Because cholesterol is a primary component of detergent-resistant membrane domains ("rafts"), we examined the compatibility of 7DHC with raft formation. Liposomes containing bovine brain phosphatidylcholine, sphingomyelin, cerebrosides, and either cholesterol, 7DHC, or coprostanol (the latter being incompatible with raft formation) were prepared. 7DHC was indistinguishable from cholesterol in its ability to become incorporated into membrane rafts, as judged by physical and chemical criteria, whereas coprostanol did not form rafts. The in vivo compatibility of 7DHC with raft formation was evaluated in brains of rats treated with trans-1,4-bis (2-dichlorobenzylamino-ethyl)cyclohexane dihydrochloride (AY9944), which mimics the SLOS biochemical defect. 7DHC/cholesterol ratios in rafts and whole brains from AY9944-treated rats were similar, indicating comparable efficiency of 7DHC and cholesterol incorporation into brain rafts. In contrast, dolichol (a nonsterol isoprenoid incompatible with raft formation) was greatly depleted in brain rafts relative to whole brain. Although brain raft fractions prepared from AY9944-treated and control rats yielded similar sterol-protein ratios, their gel electrophoresis profiles exhibited multiple differences, suggesting that altered raft sterol composition perturbs raft protein content. These results are discussed in the context of the SLOS phenotype, particularly with regard to the associated central nervous system defects. -Kennedy, R. K., T. P. Arnold, and S. J. Fliesler.

7DHC-induced changes of Kv1.3 operation contributes to modified T cell function in Smith-Lemli-Opitz syndrome

“…Nevertheless, 7DHC is the prominent sterol in the SLOS membrane and the biophysical parameters of Kv1.3 gating were similarly affected in SLOS and in T cells treated with MβCD/7DHC complex. This suggests that acute loading of T cells with 7DHC mimics adequately the effect of the lipid milieu existing in SLOS on the operation of Kv1.3 [11,29,30,54,68].…”

“…The effect of 7DHC on the biophysical properties of the cell membrane is contradictory. For example, it has been demonstrated that 7DHC destabilizes cholesterol rich domains, increases the membrane fluidity and alters the protein pattern of rafts [11,30,61]. On the contrary, according to Liu et al, the difference between CHOL and 7DHC from the perspective of membrane ordering and condensation is essentially imperceptible [35].…”

“…Moreover, accumulation of the 7DHC in the plasma membrane can affect the physicalchemical behavior of the lipid bilayer, and also influence the function of membrane proteins by disrupting/modifying the raft structure and/or interacting directly with the proteins themselves [11,30,61]. The single-channel conductance of the BK Ca channel, which colocalizes with caveolin in CHOLrich membrane domains, is reduced and the voltage dependence of the open probability is right-shifted in SLOS fibroblasts [48].…”

Lipid rafts, cholesterol, and the brain

“…The possibility that protein-protein or protein-lipid interactions stabilize small transient domains and/or induce the formation of larger, longer-lived entities has been proposed by a number of groups (Kusumi et al 2004;Kusumi and Suzuki 2005;Hancock 2006;Jacobson et al 2007). Finally, the possibility that cell membranes exist close to a phase boundary and thus small changes in lipid composition could drive raft assembly or disassembly has also been suggested (Keller et al 2004;Veatch and Keller 2005). …”

“…This elevation in 7-dehydrocholesterol inhibits the activity of Hmgcr, thus further exacerbating the cholesterol deficit (Honda et al 1998;Fitzky et al 2001). Importantly, as 7-dehydrocholesterol can incorporate into lipid rafts and alters the protein composition within it (Keller et al 2004), at least some of the disturbances seen in SLOS can be due to the altered function of the lipid rafts. The greatly increased level of 7-dehydrocholesterol leads to increased membrane fluidity and decreased intermolecular packing of phospholipids fatty acyl chains (Vainio et al 2006;Rog et al 2007;Samuli Ollila et al 2007).…”