MOLECULAR REPRODUCTION AND DEVELOPMENT 60:338±350 (2001) Type II Iodothyronine Deiodinase Is Preferentially Expressed in Rainbow Trout (Oncorhynchus mykiss) Liver and Gonads ELISABETH SAMBRONI, STE  PHANIE GUTIERES, CHANTAL CAUTY, YANN GUIGUEN, BERNARD BRETON, AND JEAN-JACQUES LAREYRE* Equipe SexualiteÂet Reproduction des Poissons, Station Commune de Recherche en Ichtyophysiologie, BiodiversiteÂet Environnement (SCRIBE), Institut National de la Recherche Agronomique (INRA), Campus de Beaulieu, Rennes Cedex, France ABSTRACT It is well admitted that thyroid hormones (TH) play a role in the development of vertebrates. The major secretory product of the thyroid is a pro-hormone, T 4 , which is activated in peripheral tissues by outer ring deiodination to T 3 . We have isolated from rainbow trout testis, a full length cDNA encoding type II iodothyronine deiodinase (rtD2). The cDNA was 2410 nucleotides long and coded for a polypeptide of 264 amino acids including a seleno- cysteine residue. The predicted molecular weight of rtD2 was 29.3 kDa and the isoelectric point 8.71. The deduced amino acids sequence showed 80% identity with Fundulus heteroclitus D2 (fhD2) but only 68 ± 69% identity with rat, mouse, and human D2. The 3 0 UTR contained a putative selenocysteine insertion 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 expression by Northern blot analysis using total RNA extracted from testis, ovary, and 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 increased at the onset of spermatogenesis. Our results show that the structural characteristics of the D2 protein and gene have been highly conserved during evolution. The rtD2 mRNA expression in the gonads suggests that rtD2 may be a key factor regulating local supply of active T 3 during rainbow trout gametogenesis. Mol. Reprod. Dev. 60: 338±350, 2001. ß 2001 Wiley-Liss, Inc. Key Words: spermatogenesis; T 3 ;T 4 ; thyroxine; ovary; testis INTRODUCTION It is well known that thyroid hormones (TH) are required for normal development and growth in verteb- rates (Barrington, 1975; Eales, 1985; Leatherland, 1987). In addition, speci®c receptors for iodothyronines have been identi®ed in nuclei of testicular and ovarian cells in ®sh and in mammals including humans suggesting that TH control the reproductive function (Jannini et al., 1990; Jana and Bhattacharya, 1993). Mammalian and ®sh thyroid hormone receptors are highly homologous and show a similar preferential af®nity for T 3 as compared to other iodothyronines (Eales, 1985). However, the major secretion product of the thyroid follicles is not the biologically active T 3 but rather a prohormone termed thyroxine T 4 (Eales, 1985). This implies that circulating T 4 is activated by 5 0 -deiodination to T 3 in peripheral target tissues. Three iodothyronine deiodinases that differ in their catalytic properties are involved in the deiodination processes (St. Germain and Galton, 1997). Only two of them are able to activate T 4 in T 3 . The ®rst one, termed Type I iodothyronine deiodinase (D1), has both outer ring and inner ring deiodination activities with rT 3 and sulfated iodothyronines as preferential substrate. The second enzyme is termed type II iodothyronine deiodinase (D2) and only catalyzes outer ring deiodina- tion with T 4 as preferred substrate. The third enzyme termed type III iodothyronine deiodinase (D3) is likely involved in the catabolism of the iodothyronines since it has only inner ring deiodinase activity which converts T 3 and T 4 to the inactive metabolites 3,3 0 -diiodothyr- onine (T 2 ) and rT 3 , respectively. The three deiodinases are highly homologous and belong to the family of selenoproteins (St. Germain and Galton, 1997). The incorporation of an essential ß 2001 WILEY-LISS, INC. Abbreviations used: D2, type II iodothyronine deiodinase; fhD2, Fundulus Heteroclitus D2; rtD2, rainbow trout D2; TH, thyroid hormones The nucleodide sequences reported in this paper have been submitted to the GenBank/EMBL Data Bank with accession numbers AF312395, AF312396, and AF207900. *Correspondence to: Jean-Jacques Lareyre, Equipe Sexualite  et Reproduction des Poissons, Station Commune de Recherche en Ichtyophysiologie, Biodiversite  et Environnement (SCRIBE), Institut National de la Recherche Agronomique (INRA), Ba Ãtiment 16, Campus de Beaulieu, 35042 Rennes Cedex, France. E-mail: lareyre@beaulieu.rennes.inra.fr Received 16 March 2001; Accepted 8 June 2001