From molecule to neuron : thyroid hormone action in the brain

The importance of thyroid hormone (TH) for mental health has been known for decades. With the identification of the thyroid hormone receptors α and β (TRα, TRβ), TR dependent mechanisms involved in brain development were investigated, identifying TRα1 as the main TR isoform in the brain. However, the lack of reliable antibodies has hampered the identification of TRα1 expressing cell types and TRα1 target genes. Here, we used two mouse lines to unravel TRα1 action in the developing brain. Firstly, using the TRα1-GFP mice circumvented the need for specific TRα1 antibodies in the study of TRα1 expression and target genes. Secondly, the TRα1+m mice which express a dominant negative TRα1 allowed the study of a receptor mediated hypothyroidism that furthermore can be ameliorated by supraphysiological doses of TH. These mice exhibit a delayed differentiation of parvalbumin expressing (PV+) GABAergic interneurons in the neocortex. In paper I we describe the expression of TRα1 in the developing and mature brain of TRα1-GFP mice. TRα1 expression occurred in postmitotic neurons, which persisted in most mature neurons and in the glial tanycytes in the hypothalamus; the postnatal cerebellum however showed a transient expression in Purkinje cells. This mouse model was further utilized in paper II to identify TRα1 target genes in the TRα1-GFP mice by establishing a ChIP assay. This enabled us to enrich specifically for TRα1-bound DNA with GFP antibodies and to use wildtype chromatin as perfect background control. In paper III and IV this method led to identification of further target genes. Paper III we revealed a tissue specific regulation of carbonic anhydrase 4 (Car4) expression by TH. In brain and liver, Car4 was downregulated in TRα1+m mice, which could not be normalized by TH treatment, indicating the importance of proper TRα1 signaling during development for establishing the Car4 expression level in the adult. In contrast, renal Car4 expression was unaltered in the mutants but downregulated in response to TH treatment, revealing a suppressive function of renal TH. In paper IV, we investigated the role of TRα1 in developing PV+ cells and showed that TRα1 does not interfere with proliferation or migration of these cells but instead postpones their final step of differentiation. This was accompanied by a reduced expression of the neurotrophin NT-3. In summary, we described the temporal and spatial expression of TRα1 in the brain and established a method to reliable identify TRα1 target genes which is crucial to further understand the mechanism involved in TH dependent brain development