Abstract
Levels of many hydrophobic cellular substances are tightly regulated because of their potential cytotoxicity. These compounds tend to self-aggregate in cytoplasmic storage depots termed lipid droplets/bodies that have well defined structures that contain additional components, including cholesterol and various proteins. Hydrophobic substances in these structures become mobilized in a specific and regulated manner as dictated by cellular requirements. Retinal pigmented epithelial cells in the eye produce retinyl ester-containing lipid droplets named retinosomes. These esters are mobilized to replenish the visual chromophore, 11-cis-retinal, and their storage ensures proper visual function despite fluctuations in dietary vitamin A intake. But it remains unclear whether retinosomes are structures specific to the eye or similar to lipid droplets in other organs/tissues that contain substances other than retinyl esters. Thus, we initially investigated the production of these lipid droplets in experimental cell lines expressing lecithin:retinol acyltransferase, a key enzyme involved in formation of retinyl ester-containing retinosomes from all-trans-retinol. We found that retinosomes and oleate-derived lipid droplets form and co-localize concomitantly, indicating their intrinsic structural similarities. Next, we isolated native retinosomes from bovine retinal pigmented epithelium and found that their protein and hydrophobic small molecular constituents were similar to those of lipid droplets reported for other experimental cell lines and tissues. These unexpected findings suggest a common mechanism for lipid droplet formation that exhibits broad chemical specificity for the hydrophobic substances being stored.
Highlights
In vivo imaging of the RPE in mice revealed the presence of retinyl ester storage particles or retinosomes [15, 16]
We first investigated the formation of lipid droplets in five experimental cell lines, i.e. transformed human RPE cells called ARPE19 cells, human embryonic kidney HEK293 cells, mouse fibroblast NIH3T3 cells engineered to express lecithin: retinol acyltransferase (LRAT) (NIH/L) [42, 49], and NIH3T3 cells engineered to express both LRAT and RPE-specific 65-kDa protein (RPE65) (NIH/RL) [42, 49]
We repeated these experiments with native bovine RPE cells and analyzed the derived retinosomes to determine how their composition compared with that published for lipid droplets derived from other cell types
Summary
In vivo imaging of the RPE in mice revealed the presence of retinyl ester storage particles or retinosomes [15, 16]. Lipid Droplets in the Retina microscopy (TPM) revealed aberrant trafficking of all-transretinyl esters in the RPE of these mice, a problem caused by prolonged maintenance of retinosomes in the dark-adapted state. These findings suggest that PLIN2 plays a unique role in vision by maintaining proper storage and trafficking of retinoids within the eye. Especially prevalent in adipocytes, liver hepatocytes, and stellate cells, they are found in virtually every cell type (29 –31) These droplets are surrounded by phospholipid monolayer membranes [32] and contain large amounts of proteins such as PLIN1, PLIN2, and PLIN3 [31]. Our observations provide evidence for a close relationship between retinosomes in the RPE and lipid droplets in other types of cells
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