Low docosahexaenoic acid levels in rod outer segments of rats with P23H and S334ter rhodopsin mutations.

PURPOSE

Previous studies have shown that the level of docosahexaenoic acid (22:6n-3, DHA) is lower in the rod outer segment (ROS) membranes of dogs and mice with inherited retinal degeneration than in ROS from appropriate controls. In the present study, we analyzed the ROS fatty composition of several lines of transgenic rats with P23H and S334ter rhodopsin mutations. Lines were chosen that have different rates of retinal degeneration.

METHODS

At 21-22 days of age, animals were perfused and eyes fixed and sectioned for morphologic examination. Others were killed and retinas isolated for preparation of ROS by sucrose step-gradient centrifugation. Fatty acid composition of ROS phospholipids was determined by gas-liquid chromatography. Membrane purity was assessed by polyacrylamide gel electrophoresis.

RESULTS

Retinas of the slow degenerating lines were indistinguishable from controls, whereas there was a 15-20% and 50-60% loss of photoreceptor cell nuclei in intermediate and fast degenerating lines, respectively. Except for the slow P23H line, all mutant lines had lower levels of 22:6n-3 and total n-3 fatty acids in ROS phospholipids, compared to wild-type controls, and the level of 22:6n-3 was lowest in those lines with the fastest rate of degeneration. The relative levels of the other fatty acid families (saturated, monoenoic, and n-6) increased proportionately. The n-6/n-3 ratio increased in the more rapidly degenerating lines, but the phospholipid/protein ratios did not change. The low levels of 22:6n-3 in the ROS membranes were not compensated for by an increase in 22:5n-6, which always occurs in the retina of animals where 22:6n-3 levels are reduced by dietary manipulation.

CONCLUSIONS

Rats that express mutant rhodopsins have lower levels of 22:6n-3 in their ROS phospholipids than wild-type animals. We propose that photoreceptor-specific mutations provoke a metabolic stress in rod photoreceptor cells that generates an oxidant stress in these cells. The retina responds to this stress by reducing the level of substrate for lipid peroxidation (22:6n-3).