Sphingomyelin Acyl Chains Influence the Formation of Sphingomyelin- and Cholesterol-Enriched Domains

Abstract

The segregation of lipids into lateral membrane domains has been extensively studied. It is well established that the structural differences between phospholipids play an important role in lateral membrane organization. When a high enough cholesterol concentration is present in the bilayer, liquid-ordered (L o ) domains, which are enriched in cholesterol and saturated phospholipids such as sphingomyelin (SM), may form. We have recently shown that such a formation of domains can be facilitated by the affinity differences of cholesterol for the saturated and unsaturated phospholipids present in the bilayer. In mammalian membranes, the saturated phospholipids are usually SMs with different acyl chains, the abundance of which vary with cell type. In this study, we investigated how the acyl chain structure of SMs affects the formation of SM-and cholesterol-enriched domains. From the analysis of trans-parinaric acid fluorescence emission lifetimes, we could determine that cholesterol facilitated lateral segregation most with the SMs that had 16 carbon-long acyl chains. Using differential scanning calorimetry and Fö rster resonance energy transfer techniques, we observed that the SM-and cholesterol-enriched domains with 16 carbonlong SMs were most thermally stabilized by cholesterol. The Fö rster resonance energy transfer technique also suggested that the same SMs also form the largest L o domains. In agreement with our previously published data, the extent of influence that cholesterol had on the propensity of lateral segregation and the properties of L o domains correlated with the relative affinity of cholesterol for the phospholipids present in the bilayers. Therefore, the specific SM species present in the membranes, together with unsaturated phospholipids and cholesterol, can be used by the cell to fine-tune the lateral structure of the membranes.

The role of plasmalogens, Forssman lipids, and sphingolipid hydroxylation in modulating the biophysical properties of the epithelial plasma membrane

“…Domain formation by sphingomyelin is consistent with its key role in ordered domain formation in a variety of membranes. 89,90 A well-mixed distribution of PI lipids contrasts with previous observations of PI/PI clustering in the average and neuronal plasma membranes. 45…”

The Origin of Lipid Rafts

“…A large body of evidence now exists that shows that cholesterol favors association with high-melting lipids over low-melting lipids (i.e., lipids having T m values less than 37 °C). This evidence can be traced back to early monolayer studies that were carried out at the air/water interface where discrete cholesterol–phospholipid complexes were hypothesized. , Subsequent studies of analogous bilayers via fluorescence lifetime, differential scanning calorimetry, 2 H NMR, and Forster resonance energy transfer measurements have yielded additional support for cholesterol favoring association with high-melting lipids. − Related experiments that we have carried out ourselves support a “hydrophobic contact mechanism” as a basis for this affinity. , Specifically, our results support a model in which the flexible, saturated acyl chains of high-melting lipids establish a large number of hydrocarbon contacts with the flat and rigid cholesterol molecule, thereby producing strong attractive van der Waals interactions . In contrast, low-melting phospholipids, which bear one or more “permanent kinks” (i.e., cis -double bonds), have a weaker association with cholesterol because they are unable to create as many hydrocarbon contacts …”

Depth-Dependent Segmental Melting of the Sphingomyelin Alkyl Chain in Lipid Bilayers

“…Biological membranes comprise lipid bilayers, which not only act as an insulating film to physically separate cells from the external environment but also play important roles in the selective transportation of small molecules and ions as well as cell–cell communication across membranes. − Since the physical properties of biological membranes largely depend on the structure, composition, and distribution of lipids in bilayers, , a considerable number of studies have been conducted on model lipid bilayers largely based on physicochemical methodologies and have led to elucidation of the functions of lipid structural components, such as an ionic head group, hydrophobic chains, and their linking moiety. − Particularly, the temperature-dependent behavior of the acyl and other hydrocarbon chains has been investigated from the aspect of chain length, degree of unsaturation, and methyl substitution. − Except for the pioneering studies by Seelig’s group and several groups, − chain packing and melting of model bilayers generally focus on the intermolecular interactions of the entire alkyl chains. The local mobility of the chain segment is often overlooked for the following reasons: (1) the hydrocarbon chains have a simple structure and flexible nature, making segmental analysis along the depth of a bilayer difficult, particularly in the gel (L β ) or ripple (P β ) phase, and (2) the hydrocarbon chains of lipids largely comprise simple repeating methylene units, which hampers the observation of a specific segment using spectroscopic methods.…”