Expression of genes associated with retinoid metabolism in the trout ovarian follicle

In vertebrates producing oviparous eggs, retinoids and their precursor molecules need to be deposited in oocytes during vitellogenesis. While most studies focus on the transport of retinoids and carotenoids formed outside the fish ovary and their deposition within the developing oocyte, recent investigations in mammalian species suggest the ovary is an important site for retinoid and carotenoid metabolism. Therefore, we investigated the expression of six genes (bcmo1, bcdo2, rbp1, lrat, rbp4, and stra6) associated with retinoids and carotenoids in juvenile and adult trout ovaries. Except for bcdo2, these genes were expressed in the ovary. Expression of stra6 was detected in the ovary but not in the liver. Gene expression levels of bcmo1 and stra6 were significantly higher in juvenile ovaries, in contrast to those of rbp1, rbp4, and lrat, which were similar in all tested ovarian stages. The mean values of the relative mRNA levels of the tested genes differed between the ovary and the liver. Gene transcripts of rbp4 and bcmo1 were identified by in situ hybridization in the theca layer, and all five genes were expressed in the granulosa, stromal cells, and only the early vitellogenic oocyte. The occurrence of retinol-binding protein in the theca and granulosa cells and within oocytes at all developmental stages was revealed by immunocytochemistry. These results indicate that ovarian cells express genes putatively associated with cleavage of beta-carotene, storage and mobilization of retinyl-esters, and of retinol-binding protein synthesis, suggesting a novel pathway for providing retinoids and carotenoids to developing fish ovarian follicles.