Lipid-Protein and Lipid-Lipid Interactions in the Myelin Sheath

Abstract

The myelin sheath, which is produced by oligodendrocytes in the central nervous system (CNS), enwraps the axons to provide fast conduction of the action potential and to ensure axonal integrity. For the myelin membrane, the most abundant lipids are cholesterol and galactolipids such as galactosylceramide (GalC) and sulfatide(SGC). The major CNS myelin proteins are proteolipid protein (PLP) and myelin basic protein (MBP); their interaction with lipids and their diffusional behavior are pivotal for myelin formation and maintenance. Any alteration in the spatio-temporal architecture of myelin leads to severe neurological diseases such as multiple sclerosis. Myelin's molecular organization, as driven by lipid-lipid and lipid-protein interactions, was investigated by confocal microscopy. We also applied Fluorescence Correlation Spectroscopy (FCS) to study the lateral diffusion of myelin proteins. For this purpose, we employed primary oligodendrocytes in various differentiation stages, GFP-PLP transfected oligodendrocyte cells, OLN-93, and Giant Unilamellar Vesicles, (GUVs) prepared from myelin lipids. Here, we show the different distribution pattern of specific proteins, such as PLP and MBP, as well as lipids, such as GalC and SGC, in OLN-93 cells and primary oligodendrocytes. We also mapped the diffusion behavior of GFP-PLP in plasma membrane and subcellular compartments of OLN 93 cells by FCS. Furthermore, the spatial distribution of MBP, PLP, GalC and SGC varies along the different stages of differentiation of primary oligodendrocytes. In GUVs, we show the effect of GalC and SGC on lipid organization and domain assembly. By integrating confocal microscopy and FCS data from live cells and model membranes, we shed some light on the relation between myelin lipid-lipid and lipid-protein interactions and myelin assembly and function, in order to better understand neurological diseases.