Transcriptional and Genomic Regulation of Pituitary Function by Thyroid Hormone Receptor Beta

Background: The pituitary is a key target for thyroid hormone but underlying transcriptional mechanisms are poorly understood. Thyroid hormone modifies expression of hormones, including growth hormone (GH) and thyroid-stimulating hormone (TSH, thyrotropin). Wider transcriptome responses are undefined. Thyroid hormone receptor beta (TRb) encoded by THRB are expressed in the anterior pituitary and THRB mutations cause human resistance to thyroid hormone. Method: To investigate genomic genomic regulation by TRb, we derived Thrb-HAB knockin mice that express TRb protein with a tag that is biotinylated in vivo in presence of an R26-BirA allele. Specific, sensitive streptavidin pull-down facilitated Chromatin-Affinity-Purification-sequencing (ChAPseq) to identify genomic TRβ binding sites in pituitary of male mice. Hypo- and hyperthyroidism were produced using methimazole (MMI) in drinking water for 4 weeks with/without added thyroid hormone (T3) for the 4th week. Pituitaries from wild type and Thrb-KO mice were also isolated for RNA-sequencing (RNA-seq). Selected expression changes were confirmed by quantitative PCR. Epigenetic changes were determined by ChIPseq for histone acetylation and methylation and open chromatin analysis (ATAC-seq). Results: Transcriptome analysis revealed genes with statistically different expression induced by T3, including known response genes such as Tshb, Hr and Gh. Responses were impaired in Thrb-KO mice. T3 induced recruitment of TRb binding, chromatin opening and specific histone acetylation marks. Conclusion: Most T3 response genes in pituitary depend to some extent upon TRb. T3-dependent chromatin modifications indicate properties of TRb-dependent enhancer regions and a critical role for TRb in transcriptional regulation of pituitary function.