Type II iodothyronine deiodinase is preferentially expressed in rainbow trout (Oncorhynchus mykiss) liver and gonads

It is well admitted that thyroid hormones (TH) play a role in the developme nt of vertebrates. The major secretory product of the thyroid is a pro-horm one, T-4, which is activated in peripheral tissues by outer ring deiodinati on to T-3. We have isolated from rainbow trout testis, a full length cDNA e ncoding type II iodothyronine deiodinase (rtD2). The cDNA was 2410 nucleoti des long and coded for a polypeptide of 264 amino acids including a selenoc ysteine residue. The predicted molecular weight of rtD2 was 29.3 kDa and th e isoelectric point 8.71. The deduced amino acids sequence showed 80% ident ity with Fundulus heteroclitus D2 (fhD2) but only 68-69% identity with rat, mouse, and human D2. The 3' UTR contained a putative selenocysteine insert ion sequence (SECIS) similar to that described in human cDNA. The rtD2 gene was isolated and the gene structure was similar to that described in human with two exons separated by a large intron. We studied rtD2 gene expressio n by Northern blot analysis using total RNA extracted from testis, ovary, a nd other tissues. We found a high expression of a 3 kb transcript in liver and in gonads. A lower expression was also detected in posterior kidney. In testis, rtD2 mRNA expression was dependent on spermatogenic stages: it inc reased at the onset of spermatogenesis. Our results show that the structura l characteristics of the D2 protein and gene have been highly conserved dur ing evolution. The rtD2 mRNA expression in the gonads suggests that rtD2 ma y be a key factor regulating local supply of active T-3 during rainbow trou t gametogenesis. (C) 2001 Wiley-Liss, Inc.