Expression profiles of RA synthases and catabolic enzymes in newly hatched and metamorphosing larvae of Japanese flounder, Paralichthys olivaceus

Retinoic acid (RA) plays various embryogenesis and post-embryogenesis roles in vertebrates. As exposure of metamorphosing flounder larvae to RA has teratogenic effects on skin color and vertebral column development, harmonized RA synthesis and catabolism are likely essential in metamorphic development. To approach understanding of the roles of RA in flounder metamorphic development, we here examined the tissue mRNA expression of RA synthases (aldh1a1, aldh1a2, aldh1a3) and catabolic enzymes (cyp26a1, cyp26b1, cyp26c1) in newly hatched and metamorphosing larvae, and three-month-old juveniles by in situ hybridization (ISH). No ISH signal was detected for any genes from the skin and vertebral column susceptible to the teratogenic effects by RA. Since the intestine expressed aldh1a2 at high level in larvae but not in juvenile, it is a possibility that the larval intestine serves as a source of RA, and RA catabolic enzymes function at the level below sensitivity of ISH at vertebral column and skin development. We found that aldh1a2 and aldh1a3 were expressed along the margin of the tectum and the neurohypophysis of pituitary, respectively, both in contact with the cerebrospinal fluid (CSF), and cyp26b1 at the posterior tectum and cerebellum. We hypothesize that RA is supplied from the tectum and pituitary via the CSF for brain growth and maintenance, and cyp26b1 locally regulates RA contents in the brain.