The role of docosahexaenoic acid containing phospholipids in modulating G protein-coupled signaling pathways: visual transduction.

In order to understand the role of the high levels of docosahexaenoic acid (DHA) in neuronal and retinal tissue, a study of the effect of membrane lipid composition on the visual pathway, a G protein-coupled system, was undertaken. The level of metarhodopsin II (MII) formation was determined to be a function of phospholipid acyl-chain unsaturation, with the highest levels seen in DHA-containing bilayers. Similarly, the rate of coupling of MII to the retinal G protein, Gt, to form a MII-Gt complex, was enhanced in DHA bilayers relative to less unsaturated phospholipids. Complex formation initiates the first stage of amplification in the visual pathway. The activation of the cGMP phosphodiesterase (PDE), the effector enzyme, represents the integrated pathway function. DHA-containing bilayers were found to support PDE levels comparable to those of the rod outer segment (ROS) disk membranes. Inclusion of 30 mol% cholesterol in the reconstituted bilayers had an inhibitory effect on each step in the visual pathway studied. Inclusion of cholesterol reduced MII formation and PDE activity and increased the lag time between the appearance of MII and the formation of the MII-Gt complex. However, signaling in DHA bilayers was far less affected by the addition of cholesterol than in bilayers containing less unsaturated phospholipids. These studies point up the importance of DHA acyl chains in promoting optimal function in G protein-coupled signaling pathways. The results reported here suggest that visual and cognitive deficits observed in n-3 deficiency may result from decreased efficiency in related neurotransmitter and visual signaling pathways in the absence of DHA.