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M.-P. G. Agbaga, R. S. Brsuh, D. K. Vaughan, K. R. Henry, S. M. Conley, M. I. Naash, A. Swaroop, R. E. Anderson; Fatty Acid Composition of Photoreceptor Membranes of Nrl-/- Mice and Thirteen-Lined Ground Squirrel (S. tridecemlineatus) Retinas. Invest. Ophthalmol. Vis. Sci. 2007;48(13):3054.
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Vertebrate retinas are highly enriched in long-chain polyunsaturated fatty acids (PUFA), especially docosahexaenoic acid (DHA or 22:6n3) with largest amounts found in the rod outer segments (ROS). Because most studies on retinal fatty acid composition come from rod-dominant retinas, and there is very limited data on the lipid constituents of cones. In this study, we compared the fatty acid composition of total retina and enriched outer segment (OS) preparations sampled from cone-dominant Nrl-/- mice and Thirteen-lined ground squirrels (Spermophilus tridecemlineatus) to that of rod-dominant Nrl+/- mice, wild type (WT) C57/BL6 mice, and Sprague Dawley (SD) rats.
Cone outer segments (COS) and ROS were prepared by discontinuous sucrose gradient centrifugation. An aliquot of protein from each preparation was taken for Western blotting for rod and cone opsins. Total lipids were extracted from COS, ROS, band II and pellets from the sucrose gradient centrifugation, and from whole retinas. The total lipid extracts were converted to fatty acid methyl esters (FAMEs) and analyzed by gas-liquid chromatography.
As predicted, Nrl-/- and squirrel OS preparations contained M and S cone-opsins without detectable levels of rod-opsin. Total fatty acid analyses of both OS and whole retina preparations from Nrl+/- mice, WT mice, and SD rats yielded 30-40 molar % of 22:6n-3, as previously reported. In contrast, comparable preparations from the Nrl-/- mice and squirrel retinas yielded only 18-20 molar % of 22:6n-3. Compared to the rod-dominant models, both cone-dominant models yielded slightly more 18:0 and 20:4n-6, but the cone membranes were not enriched in 22:5n-6, which occurs in n-3 fatty acid deficiency.
Compared to the well-characterized rod-dominant retinas, the cone-dominant retinas have significantly less 22:6n-3, in both whole retinas and within OS membranes. Since 22:6n-3 is necessary for optimal G-protein-coupled receptor signaling in rods, these findings suggest that cones may not have the same requirement.
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