Publications

• Essential molecular determinants for thyroid hormone transport and first structural implications for monocarboxylate transporter 8.

  Kinne A., Kleinau G., Hoefig C.S., Grueters A., Koehrle J., Krause G., Schweizer U.

  Monocarboxylate transporter 8 (MCT8, SLC16A2) is a thyroid hormone (TH) transmembrane transport protein mutated in Allan-Herndon-Dudley syndrome, a severe X-linked psychomotor retardation. The neurological and endocrine phenotypes of patients deficient in MCT8 function underscore the physiological ... >> More

  Monocarboxylate transporter 8 (MCT8, SLC16A2) is a thyroid hormone (TH) transmembrane transport protein mutated in Allan-Herndon-Dudley syndrome, a severe X-linked psychomotor retardation. The neurological and endocrine phenotypes of patients deficient in MCT8 function underscore the physiological significance of carrier-mediated TH transmembrane transport. MCT8 belongs to the major facilitator superfamily of 12 transmembrane-spanning proteins and mediates energy-independent bidirectional transport of iodothyronines across the plasma membrane. Structural information is lacking for all TH transmembrane transporters. To gain insight into structure-function relations in TH transport, we chose human MCT8 as a paradigm. We systematically performed conventional and liquid chromatography-tandem mass spectrometry-based uptake measurements into MCT8-transfected cells using a large number of compounds structurally related to iodothyronines. We found that human MCT8 is specific for L-iodothyronines and requires at least one iodine atom per aromatic ring. Neither thyronamines, decarboxylated metabolites of iodothyronines, nor triiodothyroacetic acid and tetraiodothyroacetic acid, TH derivatives lacking both chiral center and amino group, are substrates for MCT8. The polyphenolic flavonoids naringenin and F21388, potent competitors for TH binding at transthyretin, did not inhibit T(3) transport, suggesting that MCT8 can discriminate its ligand better than transthyretin. Bioinformatic studies and a first molecular homology model of MCT8 suggested amino acids potentially involved in substrate interaction. Indeed, alanine mutation of either Arg(445) (helix 8) or Asp(498) (helix 10) abrogated T(3) transport activity of MCT8, supporting their predicted role in substrate recognition. The MCT8 model allows us to rationalize potential interactions of amino acids including those mutated in patients with Allan-Herndon-Dudley syndrome. << Less

  J. Biol. Chem. 285:28054-28063(2010) [PubMed] [EuropePMC]

  This publication is cited by 3 other entries.

• Identification of monocarboxylate transporter 8 as a specific thyroid hormone transporter.

  Friesema E.C.H., Ganguly S., Abdalla A., Manning Fox J.E., Halestrap A.P., Visser T.J.

  Transport of thyroid hormone across the cell membrane is required for its action and metabolism. Recently, a T-type amino acid transporter was cloned which transports aromatic amino acids but not iodothyronines. This transporter belongs to the monocarboxylate transporter (MCT) family and is most h ... >> More

  Transport of thyroid hormone across the cell membrane is required for its action and metabolism. Recently, a T-type amino acid transporter was cloned which transports aromatic amino acids but not iodothyronines. This transporter belongs to the monocarboxylate transporter (MCT) family and is most homologous with MCT8 (SLC16A2). Therefore, we cloned rat MCT8 and tested it for thyroid hormone transport in Xenopus laevis oocytes. Oocytes were injected with rat MCT8 cRNA, and after 3 days immunofluorescence microscopy demonstrated expression of the protein at the plasma membrane. MCT8 cRNA induced an approximately 10-fold increase in uptake of 10 nM 125I-labeled thyroxine (T4), 3,3',5-triiodothyronine (T3), 3,3',5'-triiodothyronine (rT3) and 3,3'-diiodothyronine. Because of the rapid uptake of the ligands, transport was only linear with time for <4 min. MCT8 did not transport Leu, Phe, Trp, or Tyr. [125I]T4 transport was strongly inhibited by L-T4, D-T4, L-T3, D-T3, 3,3',5-triiodothyroacetic acid, N-bromoacetyl-T3, and bromosulfophthalein. T3 transport was less affected by these inhibitors. Iodothyronine uptake in uninjected oocytes was reduced by albumin, but the stimulation induced by MCT8 was markedly increased. Saturation analysis provided apparent Km values of 2-5 microM for T4, T3, and rT3. Immunohistochemistry showed high expression in liver, kidney, brain, and heart. In conclusion, we have identified MCT8 as a very active and specific thyroid hormone transporter. << Less

  J. Biol. Chem. 278:40128-40135(2003) [PubMed] [EuropePMC]

  This publication is cited by 3 other entries.

• In Vitro Characterization of Human, Mouse, and Zebrafish MCT8 Orthologues.

  Groeneweg S., Kersseboom S., van den Berge A., Dolcetta-Capuzzo A., van Geest F.S., van Heerebeek R.E.A., Arjona F.J., Meima M.E., Peeters R.P., Visser W.E., Visser T.J.

  <b><i>Background:</i></b> Mutations in the thyroid hormone (TH) transporter monocarboxylate transporter 8 (MCT8) cause MCT8 deficiency, characterized by severe intellectual and motor disability and abnormal serum thyroid function tests. Various <i>Mct8</i> knock-out mouse models as well as <i>mct8 ... >> More

  <b><i>Background:</i></b> Mutations in the thyroid hormone (TH) transporter monocarboxylate transporter 8 (MCT8) cause MCT8 deficiency, characterized by severe intellectual and motor disability and abnormal serum thyroid function tests. Various <i>Mct8</i> knock-out mouse models as well as <i>mct8</i> knock-out and knockdown zebrafish models are used as a disease model for MCT8 deficiency. Although important for model eligibility, little is known about the functional characteristics of the MCT8 orthologues in these species. Therefore, we here compared the functional characteristics of mouse (mm) MCT8 and zebrafish (dr) Mct8 to human (hs) MCT8. <b><i>Methods:</i></b> We performed extensive transport studies in COS-1 and JEG-3 cells transiently transfected with hsMCT8, drMct8, and mmMCT8. Protein expression levels and subcellular localization were assessed by immunoblotting, surface biotinylation, and immunocytochemistry. Sequence alignment and structural modeling were used to interpret functional differences between the orthologues. <b><i>Results:</i></b> hsMCT8, drMct8, and mmMCT8 all facilitated the uptake and efflux of 3,3'-diiodothyronine (3,3'-T2), rT3, triiodothyronine (T3), and thyroxine (T4), although the initial uptake rates of drMct8 were 1.5-4.0-fold higher than for hsMCT8 and mmMCT8. drMct8 exhibited 3<b>-</b>50-fold lower apparent IC₅₀ values than hsMCT8 and mmMCT8 for all tested substrates, and substrate preference of drMct8 (3,3'-T2, T3 > T4 > rT3) differed from hsMCT8 and mmMCT8 (T3 > T4 > rT3, 3,3'-T2). Compared with hsMCT8 and mmMCT8, <i>cis</i>-inhibition studies showed that T3 uptake by drMct8 was inhibited at a lower concentration and by a broader spectrum of TH metabolites. Total and cell surface expression levels of drMct8 and hsMCT8 were equal and both significantly exceeded those of mmMCT8. Structural modeling located most non-conserved residues outside the substrate pore, except for H192 in hsMCT8, which is replaced by a glutamine in drMct8. However, a H192Q substituent of hsMCT8 did not alter its transporter characteristics. <b><i>Conclusion:</i></b> Our studies substantiate the eligibility of mice and zebrafish models for human MCT8 deficiency. However, differences in the intrinsic transporter properties of MCT8 orthologues may exist, which should be realized when comparing MCT8 deficiency in different <i>in vivo</i> models. Moreover, our findings may indicate that the protein domains outside the substrate channel may play a role in substrate selection and protein stability. << Less

  Thyroid 29:1499-1510(2019) [PubMed] [EuropePMC]

  This publication is cited by 3 other entries.