P23H and S334ter opsin mutations: Increasing photoreceptor outer segment n-3 fatty acid content does not affect the course of retinal degeneration

PURPOSE

The n-3 polyunsaturated fatty acids (PUFA) facilitate retinal development and function. Rats carrying transgenes with P23H and S334ter rhodopsin mutations lose their photoreceptors and have lower levels of 22:6n-3 in rod photoreceptor outer segments (ROS) than wild type (WT) animals. We tested the hypothesis that the rate of retinal degeneration in these mutant animals could be sensitive to the n-3 fatty acid content of retina.

METHODS

Beginning embryonic day 15, WT and heterozygous transgenic rats with P23H and S344ter rhodopsin mutations were fed semi-synthetic diets enriched in n-6 (safflower oil, SO) or n-3 (flaxseed oil, FO) PUFA. At 35 and 55 days of age, electroretinographic (ERG) response, outer nuclear layer (ONL) thickness, and fatty acid composition of plasma and ROS were determined. Student's t-tests and multivariate analysis of variance with post hoc tests determined statistical differences.

RESULTS

Rats fed FO or SO diets had different n-6/n-3 PUFA ratios in plasma (1.3 and 62) and ROS (0.2 and 1.1, respectively). Although there were profound effects of the diets on the plasma fatty acid composition, there were only minor differences between WT and transgenic animals within each dietary regime. The ROS of FO fed rats had 70% more 22:6n-3 than those fed SO, and the WT had higher concentrations of 22:6n-3 than the transgenic animals (WT>P23H>S334ter). In contrast, there was no difference in 22:6n-3 levels in ROS of WT and transgenic rats fed the SO diet. At P55, both transgenic lines had diminished ERGs and ONL thickness relative to the WT. There was no detectable effect of ROS fatty acid enrichment on the rate of retinal degeneration in the transgenic animals. However, the FO-diet provided a modest protection of function (b-wave) in S334ter animals.

CONCLUSIONS

Feeding n-3 fatty acids to rats with mutant rhodopsin transgenes significantly increased the levels of 22:6n-3 in ROS membranes, but had no effect on the rate of retinal degeneration. Therefore, the degeneration is not the result of low (or high) 22:6n-3 in ROS and supplementation with 18:3n-3 will not rescue dying photoreceptor cells in these animal models of inherited retinal degenerations.