Inactivation of Thyroid Hormone Transporters Mct8/Oatp1c1 in Mouse Brain Endothelial Cells Causes Region-Specific Alterations in Central Thyroid Hormone Signaling.

Background: Mice lacking the thyroid hormone (TH) transporters monocarboxylate transporter 8 (Mct8) and organic anion transporter 1c1 (Oatp1c1) exhibit a strongly perturbed brain maturation, thereby replicating symptoms of patients with MCT8 deficiency. Mct8/Oatp1c1 double knockout (DKO) mice show a strongly diminished TH brain content, indicating a compromised TH passage across blood-brain barrier (BBB) endothelial cells that may represent a major pathogenic event causing CNS abnormalities. Here, we tested this hypothesis by generating mice that lack Mct8 and Oatp1c1 in endothelial cells only. Methods: Adult conditional Mct8/Oatp1c1 mice expressing a constitutively active Tek-driven Cre recombinase (Endo del mice) and control littermates were characterized regarding their hypothalamus-pituitary-thyroid axis, brain morphology as well as peripheral and central TH signaling. For comparison, age-matched DKO mice were included. Immunofluorescence (IF) studies were conducted to examine neural maturation. Fluorescence in situ hybridization (FISH) experiments and qPCR analysis were performed to determine transcript levels of TH-regulated genes in different brain regions. TH tissue content in dissected brain areas was quantified by LC-MS/MS analysis. Results: Analysis of different brain parameters by IF staining revealed a compromised maturation of cortical GABAergic interneurons and hypomyelination in Endo del mice, although the observed alterations were less profound than in DKO mice. TH content determination, FISH, and qPCR studies disclosed significantly reduced TH concentrations and, consequently, decreased TH signaling in several brain areas. Surprisingly, hippocampal T3 content and transcript levels of TH-regulated genes were found to be only mildly altered in Endo del mice compared with DKO animals. Conclusions: Deficiency of Mct8 and Oatp1c1 in endothelial cells results in reduced murine brain TH content and TH action, thereby underscoring the major function of BBB endothelial Mct8/Oatp1c1 in facilitating TH uptake into the CNS. Yet, the degree of central TH deficiency and neural impairments in Endo del mice are not as profound as in DKO mice. Particularly, unaltered hippocampal T3 signaling suggests a sufficient T3 supply of this brain area, possibly via the cerebrospinal fluid (CSF). These findings infer that apart from the BBB, additional Mct8/Oatp1c1-facilitated TH transmembrane passages (for instance, TH transport across the blood-CSF barrier) are required for proper mouse brain development and function.