Myelin protein impact on membrane phase state, morphology and structure
Myelin is multibilayered, dominantly lipid sheath that enwraps axons and ensures proper transmission of neural impulses. One of the most highlighted compositional attributes of myelin is the domination of lipids (70-85 %) over proteins of which the most significant are proteolipid protein (PLP) and myelin basic protein (MBP) (15-30 %), with representative lipids being phospholipids (phosphatidylcholines (PCs), phosphatidylethanolamines (PEs), phosphatidylserines (PSs) and sphingomyelin (SM)) and cholesterol (chol). Disruption of spirally wrapped myelin sheath around axon axis is refferred as demyelination and it encompasses the effects such as the redundant unwrappping, vacuole formation and bilayer swelling due to water and ions leaking across the sheaths. Resulting with the impaired transmission of neural impulses, it produces clinical symptoms such as vision loss and muscle weakness which are common features of multiple sclerosis (MS). The studies of experimental autoimmune encephalomyelitis (EAE), as animal model of MS, demonstrated that the amount and ratios of representative myelin lipids were significantly modified in contrast with myelin of normal varieties and are further accompanied by reduced adhesive activity of myelin basic protein (MBP). Although extensively explored in vitro and in vivo models, a more detailed picture of the molecular level events that drive the demyelination in all of its forms remains unknown. This project aims to tackle these unknows by examination of model myelin membranes of different sizes and composition (with respect to the normal and modified myelin) in the presence of MBP and its simpler subunits (peptides) that make a direct
interaction with lipid membrane. Model myelin membranes will be prepared as large and giant uni- and multilamellar liposomes (LUVs/GUVs and MLVs/GMVs) and their response to the presence of peptides and MBP will be analyzed with microscopic and spectroscopic techniques, as well with the support provided by molecular dynamics simulations. Aside of understanding demyelination at the molecular level, obtained results will help in suggesting possible solutions in lipid composition regulation either by medications and appropriate nutrition or will provide the guidelines towards building-up the artificial myelin structure.
Model of demyelination on a molecular scale at physiological and pathological conditions
Myelin is multibilayered, dominantly lipid sheath that enwraps axons and ensures proper transmission of neural impulses. Loss of myelin integrity in terms of redundant unwrapping, vacuole formation and swelling of the bilayer sheaths is referred as demyelination and is related to multiple sclerosis. In vivo studies reported that the amounts of representative lipids, including phosphatidylethanolamines (PEs), are significantly changed in diseased compared to healthy animals, and are further accompanied by reduced adhesive activity of myelin basic protein (MBP). The impact of the amount of PE in mixed model lipid membranes on the transition temperature between lamellar (La) to inverse hexagonal (HII) phase, where the latter shares structural features with vacuole formation, is significant but rather elusive issue. The aim of this proposal is to establish a link between this temperature change and the size and arrangement of PE domains in mixed model lipid membranes in the presence and absence of MBP. Model myelin membranes will be prepared from representative myelin lipids according to their ratios found in normal and diseased species. La to HII phase transitions of PE domains will be studied by probing different surroundings of PE domains within the bilayer, along with the variations in the hydrating medium regarding the presence of MBP and ionic content. A detailed molecular picture of these events will be provided by combining temperature-dependent IR spectroscopy and computational chemistry; structural changes of PE domain during this phase transition will be identified together with the interactions between PE, MBP and neighboring domains. As a result, the parameters considered as the most critical in reduction of La to HII phase transition temperature will be revealed. Aside of understanding demyelination at the molecular level, obtained results will help in suggesting possible solutions in lipid composition regulation by medications and appropriate nutrition.