Combined antisense knockdown of type 1 and type 2 iodothyronine deiodinases disrupts embryonic development in zebrafish (Danio rerio)

Thyroid hormones (THs) are important regulators of gene expression during vertebrate development. In teleosts, early embryos rely on the maternal TH deposit in the egg yolk, consisting predominantly of T4. Activation of T4 to T3 by iodothyronine deiodinases (Ds) may therefore be an important factor in determining T3-dependent development. In zebrafish, both Ds capable of T3 production, D1 and D2, are first expressed very early during embryonic development. We sought to determine their relative importance for zebrafish embryonic development by inhibiting their expression via antisense oligonucleotides against D1 and D2, and by a combined knockdown of both deiodinases. The impact of these treatments on the rate of embryonic development was estimated via three morphological indices: otic vesicle length, head-trunk angle and pigmentation index. Knockdown of D1 alone seemed not to affect developmental progression. In contrast, D2 knockdown resulted in a clear developmental delay in all parameters scored, suggesting that D2 is the major contributor to TH activation in developing zebrafish embryos. Importantly, combined knockdown of D1 and D2 caused not only a more pronounced developmental delay than D2 knockdown alone but also the appearance of dysmorphologies in a substantial minority of treated embryos. This shows that although D1 may not be essential in euthyroid conditions, it may be crucial under depleted thyroid status as is the case when T3 production by D2 is inhibited. These results indicate that zebrafish embryos are dependent on T4 uptake and its subsequent activation to T3, and suggest that substantial inhibition of embryonic T4 to T3 conversion reduces intracellular T3 availability below the threshold level necessary for normal development.