Endocrine Abstracts

Mice with combined deficiency in the thyroid hormone transporters Mct8 and Oatp1c1 (Mct8/Oatp1c1 dko mice) display a strongly diminished TH brain content and, consequently, a disturbed neuronal maturation and myelination as well as locomotor abnormalities while serum T3 levels are highly elevated. This phenotype can be explained by an impaired transport of T4 and T3 into the CNS in the absence of both transporters. Yet, the exact cell-specific function of Mct8 and Oatp1c1 in brain barrier cells remains to be investigated. To address this question, we generated mice lacking Mct8 and/or Oatp1c1 specifically in endothelial cells (= Endo del mice) by crossing conditional Mct8 flox and Oatp1c1 flox animals with mice expressing cre-recombinase under the constitutively active Tie2 promoter. In order to eliminate Mct8/Oatp1c1 specifically in astrocytes in the adult CNS (=Astro del mice), we took advantage of a Tamoxifen-inducible Aldh1l1 CreERT2 line and crossed them with conditional Mct8/Oatp1c1 flox mice. Both Mct8/Oatp1c1 Astro del as well as Endo del mice were phenotypically indistinguishable from their wildtype littermates. Monitoring Cre recombinase activity with a YFP reporter construct confirmed the cell-specific cre-mediated recombination in astrocytes and endothelial cells, respectively. Likewise, Mct8 protein expression was not affected in choroid plexus epithelial cells as well as in tanycytes excluding off-target effects. Mct8 and Oatp1c1 mRNA expression further was studied by fluorescence in situ hybridisation (FISH). Indeed, in both mouse models, a decreased mRNA expression for both TH transporters could be observed. To evaluate the functional outcome of the respective genetic modification, Mct8/Oatp1c1 Astro del mice were subjected to locomotor tests and revealed only mild coordination deficiencies in beam walk test whereas Mct8 Endo del mice showed a normal locomotor performance. To assess the cell-specific TH status in different brain cells, a first set of FISH experiments was conducted and already revealed normal TRH transcript levels in hypothalamic PVN neurons of Mct8/Oatp1c1 Astro del mice whereas TRH expression was found to be highly elevated in Endo del mice. Currently, further FISH studies are ongoing that include a variety of different TH-regulated target genes in different neural cell types and are expected to provide further information regarding the cell- and brain area-specific TH status. By this approach, we ultimately aim to clarify the function of Mct8 and/or Oatp1c1 in astrocytic vs endothelial TH transport.