Ordered Membrane Domain-Forming Properties of the Lipids of Borrelia burgdorferi

Abstract

Co-existing disordered and ordered (raft) membrane domains exist in Borrelia burgdorferi, the causative agent of Lyme disease. However, although B. burgdorferi contains cholesterol lipids, it lacks sphingolipids-a crucial component of rafts in eukaryotes. To define the principles of ordered lipid domain formation in Borrelia, the domain forming properties of vesicles composed of its three major lipids, acylated cholesteryl galactoside (ACGal), monogalactosyl diacyglycerol (MGalD), and phosphatidylcholine (PC) and/or their mixtures were studied. Anisotropy and fluorescence resonance energy transfer measurements were used to assay membrane order and ordered-domain formation. ACGal had the highest potential to form ordered domains. Interestingly, mixtures of ACGal with B. burgdorferi PC formed ordered domains more readily than mixtures of ACGal with MGalD. This appears to reflect the relatively high level of saturation observed for B. burgdorferi PC, as vesicles containing ACGal and PC, but in which the unsaturated lipid dioleoyl PC was substituted for Borrelia PC, failed to form ordered domains. In addition, the properties of ACGal were compared to those of cholesterol. Depending on what other lipids were present, ordered-domain formation in the presence of ACGal was greater than or equal to that in the presence of cholesterol. Giant unilamellar vesicles formed from ACGal-containing mixtures showed rounded domain shapes similar to those in analogous vesicles containing cholesterol, indicative of liquid-ordered state rather than solid-like gel-state domain formation. Over all, principles of ordered-domain formation in B. burgdorferi appear to be very similar to those in eukaryotes, with saturated PC taking the place of sphingolipids, but with ACGal being the main lipid component inducing ordered-domain formation.

Lipid rafts can form in the inner and outer membranes of Borrelia burgdorferi and have different properties and associated proteins

“…Previous studies have shown the presence of cholesterol‐lipid containing microdomains in B.burgdorferi in vitro and in vivo (LaRocca et al ., ; LaRocca et al ., ), and have evaluated the contribution of different Borrelia lipid components to domain formation (Huang et al ., ). The finding that there are raft‐associated proteins as well as cholesterol and cholesterol glycolipids in the IM led us to study whether rafts can also be formed from their lipids.…”

“…Lipids having unsaturated acyl chains (acyl chains containing double bonds) have a decreased ability to form ordered domains relative to those with saturated acyl chains (London, ). The saturation of acyl chains is important for the formation of tightly packed ordered domains in eukaryotic cells (London, ), and in B. burgdorferi (Huang et al ., ). Therefore, using 1 H NMR we measured the number of double bonds in the three major Borrelia lipid species: PC, ACGal and MGalD.…”

“…Similarly, vesicles from the OM lipids had a higher degree of segregation into domains as assayed by FRET than those from the IM lipids, indicating that lipids from the OM have a higher ability to promote raft formation. This could be due, at least in part, to the higher quantity of ACGal, in the OM, because it appears to be the main lipid component inducing ordered‐domain formation in B. burgdorferi (Huang et al ., ). The higher degree of saturation level of the acyl chains of lipids from the OM could also contribute to their ability to support raft formation to a greater degree than IM lipids, since tight acyl chain packing promoted by lipid saturation is a key feature of lipid raft organization (Ahmed et al ., ; Brown and London, ).…”

“…Sphingolipids, which are key components of lipid rafts in eukaryotes, have largely saturated acyl chains that allow them to pack tightly together. In B. burgdorferi it has been shown that PC, at least partly, takes the place of sphingolipids in lipid rafts (Huang et al ., ), probably due to its relative high level of acyl chain saturation (Hossain et al ., ; Huang et al ., ). Thus, the higher level of PC saturation in the outer membrane could contribute to the higher ability of outer membrane lipids to form lipid rafts.…”

“…B. burgdorferi and Helicobacter pylori incorporate host cholesterol (Crowley et al ., ; Correia et al ., ) and through the action of a galactose and a glucose transferase, respectively (Lebrun et al ., ; Ostberg et al ., ), make cholesterol glycolipids (Hirai et al ., ). In B. burgdorferi , one of these cholesterol glycolipids, cholesteryl 6‐O‐palmitoyl‐β‐ d ‐galactopyranoside (ACGal), is essential in the formation of lipid rafts (Huang et al ., ), and a recent study detected a flotillin homolog enriched in detergent resistant membranes (DRM), suggesting the presence of lipid rafts in H. pylori (Hutton et al ., ).…”