Molecular Imaging Mass Spectrometry, Lipidomics, and Functional Assessment of DHA Mechanisms of Retina Uptake and Retention

Abstract

Purpose : Photoreceptor cells (PRC) contain the highest DHA concentration in the body. DHA is the precursor of docosanoid and elovanoid mediators, including Neuroprotectin D1 (NPD1) and the elovanoids ELV-N32 and -34, respectively, stress-injury response mediators in retinal pigment epithelium (RPE), PRC, and other neural cells. DHA uptake mechanisms from choriocapillaris to RPE and retention by PRC are not fully defined. Recently, adiponectin receptor 1 (AdipoR1) was shown to be necessary for this function (Rice et al., 2015). Both AdipoR1 and MFRP KO (rd6) mice display flecked retinas, and progressive PRC loss. Here we report that MFRP is also necessary for uptake and retention of DHA, and plays a role in PRC homeostasis. We hypothesize that MFRP works in conjunction with AdipoR1 for DHA uptake/retention/distribution to the PRC.

Methods : Retinal function was assessed by ERGs and photoreceptor layer thickness was determined by OCT and histology for MFRP^rd6 and WT mice. Lipids were extracted from eyes of mutant and controls, and phosphatidylcholine (PC) molecular species identified, quantified, and localized by LC-MS/MS-based lipidomic analysis and MALDI imaging mass spectroscopy.

Results : OCT, histology, and ERGs revealed progressive retinal degeneration in MFRP^rd6 mice (1-9 months); PRC thickness significantly declined from 3 to 9 months. PCs with >40C were greatly reduced in MFRP^rd6 retinas. Comparison of PC molecular species of MFRP^rd6 and AdipoR1 KO revealed a similar profile, with 34C and 36C PCs increasing and DHA-containing PCs (38:6 and 40:6) declining. PC(44:12), an n-3 marker (DHA esterified at sn-1 and 2), was greatly reduced in both mutants; while PC(34:8), an n-6 marker (with arachidonic acid), was increased. MALDI showed abundant VLC-PUFAs (elovanoid precursors) derived from DHA, localized to the outer PRC layer in WT but was absent in both mutants.

Conclusions : MFRP^rd6 retinas displayed drastic reduction in the biosynthesis of PCs, particularly those containing DHA and DHA-elongation products, potentially causative of retinal degeneration. Here we propose that MFRP acts with AdipoR1 for DHA uptake/retention, which, in turn, switches on availability of precursors for pro-homeostatic lipid mediators that sustain RPE and PRC integrity.

This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.