Rapid and selective uptake, metabolism, and cellular distribution of docosahexaenoic acid among rod and cone photoreceptor cells in the frog retina

Abstract

The uptake, metabolism, and cellular distribution of 3H-docosahexaenoic acid (3H-22:6) in the frog retina during in vitro incubation were studied. An initial diffuse labeling throughout the retina was detected by autoradiography and was followed by an active steady increase in labeled photoreceptor cells. After 6 hr of incubation, 92% of the label was concentrated in photoreceptor cells. Among these cells, 435 rods (green rods) labeled heavily and showed two to three times higher uptake than the 502~rods (red rods). Cone uptake labeling was the lowest, showing negligible labeling throughout the cytoplasm. However, oil droplets of the 575 cones actively concentrated 22:6. The high uptake of 3H-22:6 by photoreceptor cells was followed by its rapid esterification into phospholipids. After 6 hr of labeling, only 5% of the radioactivity in the retina was free 22:6, whereas 66% was esterified into phospholipids. The remaining 22:6 was distributed equally in triacylglycerols (TAGS) and diacylglycerols. When 3H-22:6 (0.11 PM) of high specific activity was used, early incubation times showed phosphatidylinositol (PI) labeling to be of the same order of magnitude or greater than that of phosphatidylcholine (PC) or phosphatidylethanolamine (PE). Although the amount of endogenous 22:6 esterified into PI accounted for less than 2% of the 22:6 in retinal phospholipids, 27% of 3H-22:6 labeling was recovered in this phospholipid. When 'C-22:6 at a final concentration of 70 PM was used, a different profile of lipid labeling was observed. Forty percent of the labeling remained in the free fatty acid pool, followed by TAG (24%), PC (14%), and PE (12%). PI showed the smallest increase in picomoles of 14C-22:6 incorporated, when compared with 3H-22:6. In conclusion, a selective and differential uptake of 3H-22:6 by photoreceptor cells is coupled to its active utilization for phospholipid biosynthesis, mainly that of PC, PE, and PI. The differential uptake of 3H-22:6 among photoreceptor cells may reflect involvement of this fatty acid in cell-specific functions.

Alterations in rabbit retina lipid metabolism induced by detachment

“…Since phospholipids enriched in DHA are major components of photoreceptor membranes, our results suggest an as-yet-unrecognized effect of detachment on these important constituents necessary for retinal function. The present results also extend to the rabbit our previous observations regarding [3H]DHA metabolism [18,19].…”

“…DHA, arriving at the retina through the choriocapillaris, is selectively taken up by retinal pigment epithelial cells (RPE) [12][13][14] and then released into the interphotoreceptor matrix for uptake by photoreceptors [11,12,15]. Analysis of [3H]DHA-labeled retinas by light and electron microscope autoradiography, both under in vivo [16,17] and in vitro [ 18,19] conditions, reveals preferential uptake by photoreceptor cells and accumulation in rod outer segments and synaptic terminals.…”

Syntaxin 3 and SNAP-25 pairing, regulated by omega-3 docosahexaenoic acid, controls the delivery of rhodopsin for the biogenesis of cilia-derived sensory organelles, the rod outer segments

“…However, in photoreceptors, the total amount of unesterified DHA is small because of its rapid acylation into phospholipids. Only when retinas are incubated with high concentrations of DHA, similar to the concentrations used in this study, does the majority of DHA remain unesterified (Rodriguez de Turco et al, 1991). Even so, the physiological uptake of free DHA from the RPE by the photoreceptors (Rodriguez de Turco et al, 1994) leads to preferential incorporation into the RIS PM domains proximal to ROS that we now show are enriched in syntaxin 3.…”

Fish and Shellfish Allergens

“…These fatty acids have been implicated in preventing age -related macular degeneration [23] . High levels of docosahexaenoic acid (DHA) are found in the retina, specifi cally in the disk membrane of the outer segments of the receptor cells, suggesting that DHA plays an important role in the retina function [24] . In animals, dietary deprivation of DHA results in abnormal electroretinograms and visual impairment accompanied by lower retinal DHA levels, further suggesting that DHA may be benefi cial in reducing the risk of eye diseases that are associated with vascular changes of the retina or choroids [23] .…”