Carotenoid and retinoid transport to fish oocytes and eggs: what is the role of retinol binding protein?

Fish eggs contain carotenoids, retinals (retinal and dehydroretinal) and retinols (retinol, dehydroretinol and retinyl-esters) that are utilized during embryonic development, after fertilization. The carotenoids (mainly astaxanthins) are transported in the plasma by the low density lipoproteins, high density lipoproteins, and very high density lipoproteins (VHDL) and were found to be associated also with serum albumin. Retinals were found to be associated vitellogenin (VTG), a component of the plasma VHDL fraction that is internalized by oocytes during vitellogenesis. However, the transport of retinols and retinyl-esters that were located in the oil droplet fraction of homogenized eggs, has yet to be elucidated. Retinols are more abundant in freshwater fish eggs than in eggs of marine fish species. Since retinol is transported in the plasma of vertebrates in association with retinol binding protein (RBP), recent studies on the molecular characterization and expression sites of RBP, could contribute to determining the involvement of RBP in transporting retinol to developing oocytes in vertebrates.Recently, results from our laboratory show that RBP mRNA levels in the liver and RBP plasma levels did not significantly change with the onset and during vitellogenesis in the Rainbow trout. These results were in contrast with a dramatic elevation in the mRNA levels of VTG in the liver and an increase in VTG plasma levels that was observed in the same females. Moreover, 17beta-estradiol treatment of immature fish, resulted in relatively lower mRNA levels of RBP in the liver, concomitantly with an increase in the level of VTG transcripts and the appearance of VTG in the plasma of treated fish. In addition, RBP was localized in the cytosol of ovulated oocytes. These results for Rainbow trout are similar to those reported for the chicken but differ from those of Xenopus, where an increase in RBP mRNA was reported in the liver and higher levels of retinal and retinol were found in the plasma of 17beta-estradiol treated animals. The results, reported here for the first time in Rainbow trout, showing RBP transcripts in the ovary, oviduct (the ovarian tissue adjacent to the gonopore) and oocytes, suggest a modulating role for RBP in follicular development, as has been suggested for the bovine ovary.