Targeted disruption of the type 2 selenodeiodinase gene (D102) results in a phenotype of pituitary resistance to T-4

The type 2 deiodinase (D2), a selenoenzyme that catalyzes the conversion of T-4 to T-3 via 5'-deiodination, is expressed in the pituitary, brain, brow n adipose tissue (BAT), and the reproductive tract. To examine the physiolo gical role of this enzyme, a mouse strain lacking D2 activity was developed using homologous recombination. The targeting vector contained the Neo gen e in place of a 2.6-kb segment of the Dio2 gene. This segment comprises 72% of the coding region and includes the TGA codon that codes for the selenoc ysteine located at the active site of the enzyme. Mice homologous for the t argeted deletion [D2 knockout (D2KO)] had no gross phenotypic abnormalities , and development and reproductive function appeared normal, except for mil d growth retardation (9%) in males. No D2 activity was observed in any tiss ue in D2KO mice under basal conditions, or under those that normally induce this enzyme such as cold-exposure (BAT) or hypothyroidism (brain, BAT, and pituitary gland). Furthermore, no D2 activity was present in cultured astr ocytes, nor could it be induced by treatment of the cells with forskolin. A lthough D2 mRNA transcripts were detected in BAT RNA obtained from cold-exp osed wild-type (WT) mice, none was detected in BAT RNA from comparably-trea ted D2KO mice. Levels of D1 in the liver, thyroid, and pituitary were the s ame in WT and D2KO animals, whereas D3 activity in D2KO cerebrum was twice that in WT cerebrum. Serum T,(3)levels were comparable in adult WT and D2KO mice. However, serum T, and TSH levels were both elevated significantly (4 0% and 100%, respectively) in the D2KO mice, suggesting that the pituitary gland of the D2KO mouse is resistant to the feedback effect of plasma T-4. This view was substantiated by the finding that serum TSH levels in hypo-, thyroid WT mice were suppressed by administration of either T-4 or T-3, but only T-3 was effective in the D2KO mouse. The data also suggest that the c learance of T-4 from plasma was reduced in the D2KO mouse. In summary, targ eted inactivation of the selenodeiodinase Dio2 gene results in the complete loss of D2 activity in all tissues examined. The increased serum levels of T-4 and TSH observed in D2KO animals demonstrate that the D2 is of critica l importance in the feedback regulation of TSH secretion.