BIOLOGICAL FUNCTIONS OF ESSENTIAL FATTY ACIDS

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.