Enzymatic perturbations of fatty acids synthesis in Mfrp and Adipor1-associated retinal degenerations

Abstract

Purpose : The ω-3 essential fatty acid (FA) docosahexaenoic acid (DHA, 22:6) and its Very Long Chain PolyUnsaturated Fatty Acid derivatives (VLC-PUFAs, ≥28 carbons), are concentrated in photoreceptors (PRC) and necessary for vision. These FAs are the products of elongases and desaturases. Mutations in Adipor1 and Mfrp lead to a selective reduction of DHA- and VLC-PUFAs-containing phospholipids in retinas and retinal pigment epithelium (RPE), resulting in impaired retinal function and progressive loss of PRC. Using Mfrp^rd6 and Adipor1^-/- mice, we hypothesized that the expression and activity of Elongation of Very Long chain FA (Elovl) enzymes are perturbed, leading to the observed decrease of VLC-PUFA abundance in these animals.

Methods : Retina and RPE-eyecups of 4-week-old Mfrp^rd6, Adipor1^-/- and wild-type (C57BL6/J) male and female mice were harvested. Lipid abundance of ω-3 and ω-6 fatty acid intermediates was determined by LC-MS/MS. Elovl2 and Elovl4 protein expressions were assessed by Western blot.

Results : Globally, female mice had lower Elovl2 expression compared to males. Elovl2 and Elovl4 protein expressions were downregulated in both mutant eyes but more noticeably in female Mfrp^rd6 mice (Elovl2). LC/MS-MS analysis showed that Mfrp^rd6 retina had a significantly higher content of FA20:5n3 to 24:5n3, compared to Adipor1^-/- and control retinas. Interestingly, FA24:6n3, a product of the Fatty Acid Desaturase 2 (FADS2, also known as Delta 6 desaturase) using 24:5n3 substrate, was significantly downregulated in Mfrp^rd6, while FA24:6n3 reduction in Adipor1^-/- more likely resulted from the already low FA24:5n3. Additionally, 20:4n6, 22:4n6, and 24:4n6 were mostly increased in Mfrp^rd6 retina, as well as 24:5n6, suggesting that the altered FADS2 enzymatic activity is specific to the ω-3 synthesis pathway in Mfrp^rd6.

Conclusions : Our results show that although both Mfrp^rd6 and Adipor1^-/- have depleted abundance of VLC-PUFAs from 24:6n3 onwards, the synthesis pathway to get to this point differs from one mutant to another. We found that Elovl4 and Elovl2 expression levels are diminished in both mutants, with the females being more affected. This suggests that there is a sexual dimorphism in elongases’ expression and may be a factor in the onset and progression of retinal degenerations.

This abstract was presented at the 2024 ARVO Annual Meeting, held in Seattle, WA, May 5-9, 2024.