BIOLOGICAL FUNCTIONS OF ESSENTIAL FATTY ACIDS

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The objective of the proposed research is to investigate the molecular basis for essential fatty acid deficiency in the retina, which is part of the brain and comprises a valuable and uniquely accessible model for the mammalian nervous system. Current knowledge indicates that long chain polyunsaturated fatty acids derived from essential omega3 fatty acids may play an important role in retinal and brain development. Studies of both rats and rhesus monkeys indicate a marked alteration of the electrical response functions of the eye due to dietary alteration of the content of membrane phospholipids containing docosahexaenoic acid (22:6omega3, abbreviated DHA). These studies have stimulated discussion regarding the question of whether long-chain polyunsaturated fatty acids such as DHA should be added to infant formulas. Moreover there is increased concern regarding the ratios of omega3 / omega6 essential fatty acids in the western diet. The process of vision in retinal rods is triggered by a conformational change of the visual protein rhodopsin embedded within the membranes of the rod outer segment (ROS). We will test the hypothesis that the properties of the retinal rod disk membrane lipid and protein constituents govern visual function through their influences on the MI-MII conformational transition of rhodopsin, the triggering event in visual excitation. The retinal ROS disk membranes are extraordinarily abundant in phospholipids, containing highly polyunsaturated fatty acidS, including DHA and arachidonic acid (20:4omega6). We propose that interactions of rhodopsin with polyunsaturated membrane lipids modulate the free energies of the MI and MII conformational states, thus altering key amplification steps involving the signal transducing G protein (transducin) and cGMP phosphodiesterase. These in turn affect closure of the cGMP-gated plasma membrane sodium channels, and subsequently the generation of a visual nerve impulse. Changes in the content of polyunsaturated membrane lipids due to diet or disease shifts the MI-MII equilibrium of photolyzed rhodopsin, and influence the electrical response of the retina. Flash photolysis methods will be applied and further developed to monitor the influences of polyunsaturated lipids on the MI-MII transition of rhodopsin in membrane recombinants. Particular emphasis will be placed upon investigating the role of both the polyunsaturated acyl chain composition as well as the polar head group composition of the membrane phospholipids. The influences of polyunsaturated membrane phospholipids on later amplification stages of the visual photoresponse will be investigated including the binding and activation of the G protein (transducin) to photolyzed rhodopsin, and subsequent activation of cGMP phosphodiesterase. Finally, nuclear magnetic resonance (NMR) spectroscopy will yield complementary knowledge of the physicochemical properties of polyunsaturated phospholipids and their interactions with visual proteins in membranes. A comprehensive picture of the role of polyunsaturated phospholipids in the visual process will be provided in relation to the results of dietary investigations of essential omega3 fatty acid deficiency in animals and humans.