(179n) Reduced Cholesterol Levels Alter the Biophysical Properties of Repaired Myelin

The myelin sheath is a multilayered membrane primarily composed of different phospholipids (L-α-phosphatidylcholine, L-α-phosphatidylethanolamine, and L-α-phosphatidylserine) and cholesterol, which wraps around axons to promote efficient neuronal signal transmission. During neurodegenerative disease progression, such as multiple sclerosis (MS), demyelination followed by remyelination may occur. One hallmark of remyelination is a thinner myelin membrane with increased signal leaking. In this study, we investigated the structure-function relationship of model myelin membranes. Specifically, using biochemical and biophysical techniques, we explore how changes in myelin lipid composition can impact membrane stability and structure in a mouse model of remyelination. We started our studies by using mass spectroscopy to evaluate changes in the lipid profiles of brain extracts obtained from healthy controls and repaired myelin from a genetically modified mouse model that undergoes the demyelination and remyelination process (diseased mice). Complementary, we used a Langmuir trough to assess changes in the surface activity of monolayers formed from the brain lipid extracts. Our lipidomic results showed reduced cholesterol levels in diseased mice extracts, and our surface activity results showed that those extracts produced lipid films with decreased stiffness. This suggests that repaired myelin membranes have lower lateral molecular packing. Furthermore, synthetic membranes were prepared based on the major phospholipid and cholesterol contents in healthy and diseased mice. Our results showed that reduced cholesterol levels (but not small changes in the phospholipid content) after remyelination have a primary impact on altering biophysical properties of the model myelin membranes. Supplementing the diseased lipid extracts with cholesterol increased membrane surface pressures and stiffness. Overall, our results suggest that high cholesterol levels are necessary for maintaining myelin membrane stability. In turn, reduced cholesterol levels can worsen myelin membrane’s stability, impacting how myelin leaflets wrap and pack around axons after remyelination.