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Yoshikazu Imanishi; Noninvasive two-photon imaging reveals retinyl ester storage structures in the eye. Journal of Vision 2006;6(13):38. doi: https://doi.org/10.1167/6.13.38.
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Metabolic transformations of retinoids within the retinal photoreceptor and retinal pigment epithelial (RPE) cells are responsible for the production of the visual chromophore, 11-cis-retinal, which is essential for vision. Within RPE cells, all-trans-retinol (vitamin A), a product of photoisomerised and reduced 11-cis-retinal, is esterified with fatty acids in a reaction catalyzed by lecithin:retinol acyl transferase (LRAT). While a number of retinoid metabolites are generated during regeneration of 11-cis-retinal, only retinol and retinyl esters show a weak intrinsic fluorescence (excitation l ex at ∼320 nm). Two-photon excitation laser scanning microscopy was applied as a noninvasive imaging technique to investigate the light-dependent distribution of fluorescent all-trans retinol and all-trans retinyl ester in the dissected mouse eye. Using the intrinsic fluorescence of all-trans-retinyl esters, noninvasive two-photon microscopy in combination with HPLC analysis revealed previously uncharacterized structures distinct from other cellular organelles, termed the retinyl ester storage particles or retinosomes. These structures form autonomous all-trans-retinyl ester-rich intracellular compartments distinct from other organelles and colocalize with adipose differentiation-related protein (ADRP). As demonstrated by in vivo experiments using wild-type mice, the retinosomes participate in 11-cis-retinal formation. Retinosomes accumulate in Rpe65-/- mice incapable of carrying out the enzymatic isomerization, and correspondingly, are absent in the eyes of Lrat-/- mice deficient in retinyl ester synthesis. These results indicate that retinosomes located close to the RPE lateral plasma membrane are essential components in 11-cis-retinal production.
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