Gene expression of thyroid hormone receptors TR-alpha and TR-beta during early development in tilapia, Oreochromis mossambicus

Abstract

Thyroid hormones have an influence on growth, development, differentiation, metabolism and reproduction in teleosts. The actions of thyroid hormones are mediated by thyroid hormone receptors (TRs) derived from two distinct genes (TRa and TRb). To understand the role of thyroid hormones on tilapia development, TRa and TRb were cloned and sequenced. The gene expression of TRa1 and TRb1 was studied from 2 dpf (days post fertilization) to 20 dph (days post hatching) by semi-quantitative RT-PCR analysis. Both TRa1 and TRb1 were low at 2 dpf but increased during embryonic and larval development. TRa1 mRNA levels reached a peak on 13 dph and declined thereafter. TRb1 mRNA levels reached a peak on 15 dph, followed by a significant decline on 17 dph; however, TRb1 mRNA levels remained higher than those of TRa1 on 20 dph. Study of ontogenic expression of tilapia TRa1 and TRb1 was also carried out using 3 dpf to 6 dpf embryos (just prior to hatching) by in situ hybridization with digoxigenin labeled antisense probe. TRa1 and TRb1 transcripts were seen in the brain and the tail bud region in developing embryos from 3 dpf onwards. Semi-quantitative RT-PCR analysis of expression of TRa1 and TRb1 in adult tissues indicated that their levels of expression varied substantially in different tissues. The autoinduction of TR gene expression by thyroid hormone (T3) was also investigated. Treatment of newly hatched larvae and adult tissues with thyroid hormones upregulated the expression of TRs. Detection of TRa1 and TRb1 gene expression in tilapia before the maturation of the thyroid gland (around 7 dph) suggests a role for maternal thyroid hormones during early development. The present study suggests that both TRa1 and TRb1 receptors mediate the action of thyroid hormones in the early development of tilapia with a major role for TRb1 during late larval stages.