The research proposed in this application is carried out with the objective of obtaining basic information about the effects of sphingomyelin on biological membrane structure and properties and how these effects might be related to atherosclerosis in blood vessels and aging. The work outlined will utilize liposomes (multi and single lamellar) and biological membranes in inact enveloped viruses, mycoplasma, red blood cells, and rat heart myocytes in culture. The liposomes and each one of these biological systems will be used to answer specific questions related to: a) compositional asymmetry and interdigitation between the two opposite bilayer faces; b) compositional domains and dynamics of membrane components within the membrane plane. Studies of bilayer permeability, availability of membrane lipids as substrates for enzymes, lipid-protein interactions as functions of composition and temperature will be undertaken in systems exhibiting compositional domains. The information already obtained from our previous work on model systems and biological membranes, suggests that liposomes do serve as a good model for investigating the impact of cholesterol content, and sphingomyelin/phosphatidylcholine mole ratio on membrane dynamics and organization. Our work also established that the content of the above three membrane lipids in biological membranes can be controlled by exchange and/or net transfer using suitable donors or acceptors (liposomes or modified lipoproteins). It was found that the membrane lipid composition determined the lateral organization and dynamics of membrane components (lipids and proteins) and that these changes in lipid composition affect the physiological and biochemical membrane activities. This best exemplified by rat heart myocytes, which, in culture, undergo age-dependent increases in sphingomyelin and cholesterol levels. These alterations are prevented or reversed by prolonged incubation with egg phosphatidylcholine liposomes, which serve as a donor of phosphatidylcholine and acceptor of cholesterol. Changes in the spontaneous beating rate, muscarinic receptor, enzymatic activities, and protein kinase C activity are correlated with these changes in membrane lipid composition. Special attention will be given to determining whether physical properties such as the existence of compositional domains, cholesterol-sphingomyelin interactions and sphingomyelin acyl chain composition play a role in maintaining the age-related alterations in membrane lipid composition.
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