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Jana Fischer, Gunnar Kleinau, Anne Müller, Peter Kühnen, Denise Zwanziger, Anita Kinne, Maren Rehders, Lars C Moeller, Dagmar Führer, Annette Grüters, Heiko Krude, Klaudia Brix, and Heike Biebermann

The monocarboxylate transporter 8 (MCT8) is a member of the major facilitator superfamily (MFS). These membrane-spanning proteins facilitate translocation of a variety of substrates, MCT8 specifically transports iodothyronines. Mutations in MCT8 are the underlying cause of severe X-linked psychomotor retardation. At the molecular level, such mutations led to deficiencies in substrate translocation due to reduced cell-surface expression, impaired substrate binding, or decreased substrate translocation capabilities. However, the causal relationships between genotypes, molecular features of mutated MCT8, and patient characteristics have not yet been comprehensively deciphered. We investigated the relationship between pathogenic mutants of MCT8 and their capacity to form dimers (presumably oligomeric structures) as a potential regulatory parameter of the transport function of MCT8. Fourteen pathogenic variants of MCT8 were investigated in vitro with respect to their capacity to form oligomers. Particular mutations close to the substrate translocation channel (S194F, A224T, L434W, and R445C) were found to inhibit dimerization of MCT8. This finding is in contrast to those for other transporters or transmembrane proteins, in which substitutions predominantly at the outer-surface inhibit oligomerization. Moreover, specific mutations of MCT8 located in transmembrane helix 2 (del230F, V235M, and ins236V) increased the capacity of MCT8 variants to dimerize. We analyzed the localization of MCT8 dimers in a cellular context, demonstrating differences in MCT8 dimer formation and distribution. In summary, our results add a new link between the functions (substrate transport) and protein organization (dimerization) of MCT8, and might be of relevance for other members of the MFS. Finally, the findings are discussed in relationship to functional data combined with structural–mechanistical insights into MCT8.

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Helena Kerp, Kostja Renko, Georg Sebastian Hönes, Klaudia Brix, Josef Köhrle, Lars Christian Moeller, and Dagmar Führer

Thyroid hormone (TH) metabolism and cellular TH action are influenced by ageing. To investigate the response to thyroxine (T4) overtreatment, a kinetic study was conducted in young and aged mice with chronic hyperthyroidism and hormone withdrawal. Five and 22 months old male mice were treated with T4 or PBS over 5 weeks, followed by observation for up to 12 days. Serial analysis was performed for thyroid function parameters, transcript levels of TH target genes, deiodinase type 1 (DIO1) activity as well as serum lipids at 12, 24, 72, 144, 216, and 288 h after cessation of T4 administration. Higher FT3 concentrations and higher renal DIO1 activities were noted in aged mice 12 h after T4 withdrawal and marked thyroid-stimulating hormone elevation was found in aged mice after 12 days compared to respective controls. A biphasic expression pattern occurred for TH target genes in all organs and a hypothyroid organ state was observed at the end of the study, despite normalization of TH serum concentrations after 72 h. In line with this, mirror-image kinetics were detected for serum cholesterol and triglycerides in aged and young mice. Recovery from TH overtreatment in mice involves short- and medium-term adaption of TH metabolism on systemic and organ levels. Increased renal DIO1 activity may contribute to higher T3 concentrations and prolonged thyrotoxicosis followed by hypothyroidism in an aged-mouse organism. Translation of these findings in the clinical setting seems warranted and may lead to better management of hyperthyroidism and prevention of T4 overtreatment in aged patients.

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Vaishnavi Venugopalan, Maren Rehders, Jonas Weber, Lisa Rodermund, Alaa Al-Hashimi, Tonia Bargmann, Janine Golchert, Vivien Reinecke, Georg Homuth, Uwe Völker, Francois Verrey, Janine Kirstein, Heike Heuer, Ulrich Schweizer, Doreen Braun, Eva K Wirth, and Klaudia Brix

Proteolytic cleavage of thyroglobulin (Tg) for thyroid hormone (TH) liberation is followed by TH release from thyroid follicles into the circulation, enabled by TH transporters. The existence of a functional link between Tg-processing cathepsin proteases and TH transporters has been shown to be independent of the hypothalamus–pituitary–thyroid axis. Thus, lack of cathepsin K, combined with genetic defects in the TH transporters Mct8 and Mct10, that is the Ctsk−/−/Mct8−/y/Mct10−/− genotype, results in persistent Tg proteolysis due to autophagy induction. Because amino acid transport by L-type amino acid transporter 2 (Lat2) has been described to regulate autophagy, we asked whether Lat2 availability is affected in Ctsk−/−/Mct8−/y/Mct10−/− thyroid glands. Our data revealed that while mRNA amounts and subcellular localization of Lat2 remained unaltered in thyroid tissue of Ctsk−/−/Mct8−/y/Mct10−/− mice in comparison to WT controls, the Lat2 protein amounts were significantly reduced. These data suggest a direct link between Lat2 function and autophagy induction in Ctsk−/−/Mct8−/y/Mct10−/− mice. Indeed, thyroid tissue of Lat2−/− mice showed enhanced endo-lysosomal cathepsin activities, increased autophagosome formation, and enhanced autophagic flux. Collectively, these results suggest a mechanistic link between insufficient Lat2 protein function and autophagy induction in the thyroid gland of male mice.