Differential Composition of Docosahexaenoic Acid and Very Long Chain Polyunsaturated Fatty Acids in Rod and Cone Photoreceptor Membranes

Abstract

Purpose: Polyunsaturated fatty acids (PUFA) 22:6n3 and 20:4n6 are highly enriched in the outer segment (OS) membranes of vertebrate retina, where they are further elongated to very long chain polyunsaturated fatty acids (C28-C40, VLC-PUFA). In photoreceptors, these PUFA are found only in phosphatidylcholine (PC). The cone-rich human macula has lower levels of 22:6n3 compared to rod-rich peripheral retina. To get a better understanding of the role of C20-C40 PUFA in the retina, we performed comprehensive lipidomic analyses of photoreceptor membrane preparations from rod- and cone-dominant retinas.

Methods: Total lipids were extracted from whole retinas and photoreceptor OS membrane preparations of rod-dominant [C57BL/6J (WT), Nrl+/- mice, Wistar rats] and cone-dominant [Nrl-/- mice, 13-lined ground squirrels (Ictidomys tridecemlineatus), Fox squirrel (Sciurus niger) and Tree Shrew (Tupaia belangeril)] retinas. Fatty acid methyl esters and whole lipids were analyzed by GC/MS and ESI/MS/MS, respectively.

Results: While there were no significant differences between glycerol phosphatidylcholine (GPC) lipids GPC-32:0 (16:0/16:0), GPC-34:01 (16:0/18:1), GPC-36:04 (16:0/20:4), GPC 36:01 (18:0/18.1) and GPC-38:04 (18:0/20:4n6) in rod- and cone-dominant retinas, there was a significant decrease in GPC-40:06 (18:0/22:6) and GPC-44:12 (22:6/22:6) in both whole retina and OS membranes isolated from cone-dominant retinas compared to rod dominant retinas. VLC-PUFA GPC-54:12 (22:6/32:6), GPC-56:12 (22:6/34:6), GPC-54:11 (22:6/32:5), GPC-54:10 (22:6/32:4), GPC-56:11 (22:6/34:5), and GPC-58:12 (22:6/36:6) were highly enriched in rod-dominant retinas, but were significantly reduced in the cone-dominant retina.

Conclusions: Compared to the well-characterized rod-dominant retinas, cone-dominant retinas have significantly different lipid composition in both whole retina and OS enriched membranes. Since PUFA are necessary for optimal G-protein-coupled receptor signaling in rods, these findings suggest that cones may not have the same lipid requirements as rods. This may account for the reduced 22:6n3 and VLC-PUFA levels found in the cone-rich macula compared to rod-rich peripheral retina.

Keywords: 695 retinal degenerations: cell biology • 688 retina • 494 degenerations/dystrophies