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Journal of Neuroscience, Vol 10, 2190-2202, Copyright © 1990 by Society for Neuroscience
Docosahexaenoic acid utilization during rod photoreceptor cell renewal
WC Gordon and NG Bazan
Louisiana State University, Eye Center, New Orleans 70112.
The supply of docosahexaenoic acid (22:6) to the frog retina, and its
subsequent use by retinal cells, was studied by autoradiography and
biochemical methods. Different delivery routes of 3H-22:6 were evaluated.
Predominant uptake by the neural retina, mainly in ganglion cell axons,
outer synaptic layer, and Muller cells, was observed when the radiolabeled
fatty acid was given intravitreally or by short-term incubations of
eyecups. In short-term eyecup incubations, Muller cells preferentially
labeled, suggesting their involvement as a transient storage site. After
intravenous or dorsal lymph sac injections of 3H- 22:6, most of the retinal
label was seen in rod photoreceptor cells. Two different labeling patterns
were found in rod outer segments (ROS) as a function of postinjection time:
an overall diffuse labeling pattern, as well as a dense-label region at the
ROS base. This dense- label region expanded until it reached the apex of
the ROS after about 30 d. HPLC analysis of fatty acid methyl esters from
retinal lipid extracts showed that 3H-22:6 comprised essentially all of the
label until after day 46, indicating lack of metabolic recycling of this
molecule. Lipid-extracted retinal residue was devoid of radioactivity,
demonstrating that protein did not contain significant covalently bound
label. 3H-22:6 acylated to phospholipids in photoreceptor membranes moved
apically, as evidenced by the expanding labeled region from the base of the
ROS. Oil droplets in both the pigment epithelium and the cone
photoreceptors labeled heavily, suggesting that 22:6 may be transiently
stored. ROS tips that were phagocytosed by the pigment epithelium contained
label similar in density to that of the outer segments, demonstrating that
22:6-phospholipids, at least in part, cycle through the pigment epithelial
cells during visual cell renewal. In parallel experiments in frogs injected
with 3H-leucine and maintained under the same experimental conditions,
well-define, narrow protein bands were observed. Since the leading edge of
the 3H-leucine- labeled band (rhodopsin), and that of the dense-label
region of 3H-22:6 migrated at the same rate, reaching the rod tips at the
same time, we suggest that the 3H-22:6-labeled phospholipids giving this
profile are a unique molecular species noncovalently associated with
rhodopsin.
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