Speaker
Description
Sphingolipids constitute a significant fraction of cellular plasma membrane lipid content. Among sphingolipids, ceramide levels are usually very low. However, in some cell processes like apoptosis, cell membrane ceramide levels increase markedly due to activation of enzymes like sphingomyelinase. This increase can alter membrane fluidity. Increased molecular order and solid ordered (So) phase domain formation has been observed in a previous 2H NMR study on membranes consisting of palmitoyl sphingomyelin (PSM) and palmitoyl ceramide (PCer). Cholesterol (Chol), which is also present at high concentrations in mammalian plasma membranes, has a favorable interaction with sphingomyelin (SM). Together they form domains liquid ordered (Lo) phase domains in model membranes. There are reports that Chol is able to displace ceramide (Cer) in SM bilayers and abolish the So phase domains formed by SM:Cer. This ability of Chol appears to be concentration dependent; in membranes with low Chol and high Cer contents, So phase domains hypothesized to be rich in Cer coexist with the continuous fluid phase of the membrane. Here, we present studies on the effect of increasing PCer concentration in PSM:Chol bilayers, using 2H NMR. Chol:PCer mol ratios were 3:1, 3:2 and 3:3, at a fixed 7:3 PSM:cholesterol mol ratio. Both PSM and PCer were monitored, in separate samples, for changes in their physical state by introducing a perdeuterated palmitoyl chain in either molecule. Second, we investigate the effect of replacing PSM with DPPC to test the influence on membrane phase behavior of replacing sphingosine with a palmitoylated glycerol backbone. We found that PCer induces highly stable So phase domains in both PSM:Chol, DPPC:Chol bilayers. This effect is most pronounced in bilayers with Chol:PCer 1:1 molar ratios, and below 40 oC. PCer is more effective in ordering PSM:Chol bilayers than analogous bilayers composed of DPPC:Chol.
