RESUMO
CONTEXT: T(3) action in neurons is essential for brain development. Recent evidence indicates that monocarboxylate transporter 8 (MCT8) is important for neuronal T(3) uptake. Hemizygous mutations have been identified in the X-linked MCT8 gene in boys with severe psychomotor retardation and elevated serum T(3) levels. OBJECTIVE: The objective of this study was to determine the functional consequences of MCT8 mutations regarding transport of T(3). DESIGN: MCT8 function was studied in wild-type or mutant MCT8-transfected JEG3 cells by analyzing: 1) T(3) uptake, 2) T(3) metabolism in cells cotransfected with human type 3 deiodinase, 3) immunoblotting, and 4) immunocytochemistry. RESULTS: The mutations identified in MCT8 comprise four deletions (24.5 kb, 2.4 kb, 14 bp, and 3 bp), three missense mutations (Ala224Val, Arg271His, and Leu471Pro), a nonsense mutation (Arg245stop), and a splice site mutation (94 amino acid deletion). All tested mutants were inactive in uptake and metabolism assays, except MCT8 Arg271His, which showed approximately 20% activity vs. wild-type MCT8. CONCLUSION: These findings support the hypothesis that the severe psychomotor retardation and elevated serum T(3) levels in these patients are caused by inactivation of the MCT8 transporter, preventing action and metabolism of T(3) in central neurons.
Assuntos
Doenças Genéticas Ligadas ao Cromossomo X/genética , Deficiência Intelectual/genética , Transportadores de Ácidos Monocarboxílicos/genética , Transtornos Psicomotores/genética , Tri-Iodotironina/sangue , Linhagem Celular Tumoral , Códon sem Sentido , Deleção de Genes , Doenças Genéticas Ligadas ao Cromossomo X/metabolismo , Humanos , Deficiência Intelectual/metabolismo , Neurônios/metabolismo , Mutação Puntual , Transtornos Psicomotores/metabolismo , Sítios de Splice de RNA , Simportadores , Transfecção , Tri-Iodotironina/farmacocinéticaRESUMO
Cellular entry is required for conversion of thyroid hormone by the intracellular deiodinases and for binding of 3,3',5-triiodothyronine (T(3)) to its nuclear receptors. Recently, several transporters capable of thyroid hormone transport have been identified. Functional expression studies using Xenopus laevis oocytes have demonstrated that organic anion transporters (e.g., OATPs), and L-type amino acid transporters (LATs) facilitate thyroid hormone uptake. Among these, OATP1C1 has a high affinity and specificity for thyroxine (T(4)). OATP1C1 is expressed in capillaries throughout the brain, suggesting it is critical for transport of T(4) over the blood-brain barrier. We have also characterized a member of the monocarboxylate transporter family, MCT8, as a very active and specific thyroid hormone transporter. Human MCT8 shows preference for T(3) as the ligand. MCT8 is highly expressed in liver and brain but is also widely distributed in other tissues. The MCT8 gene is located on the X chromosome. Recently, mutations in MCT8 have been found to be associated with severe X-linked psychomotor retardation and strongly elevated serum T(3) levels.
Assuntos
Proteínas de Membrana/biossíntese , Transportadores de Ânions Orgânicos/biossíntese , Tri-Iodotironina/metabolismo , Sequência de Aminoácidos , Animais , Transporte Biológico , Barreira Hematoencefálica , Encéfalo/metabolismo , Humanos , Fígado/metabolismo , Proteínas de Membrana/fisiologia , Camundongos , Modelos Biológicos , Dados de Sequência Molecular , Transportadores de Ácidos Monocarboxílicos/genética , Transportadores de Ácidos Monocarboxílicos/metabolismo , Mutação , Oócitos/metabolismo , Transportadores de Ânions Orgânicos/fisiologia , Filogenia , Ratos , Homologia de Sequência de Aminoácidos , Simportadores , Xenopus laevisRESUMO
Thyroid hormone is essential for the development of the brain and the nervous system. Cellular entry is required for conversion of thyroid hormones by the intracellular deiodinases and for binding of T(3) to its nuclear receptors. Several transporters capable of thyroid hormone transport have been identified. Functional expression studies using Xenopus laevis oocytes have so far identified two categories of transporters involved in thyroid hormone uptake (i.e., organic anion transporters and amino acid transporters). Among the organic anion transporters, both Na(+) taurocholate cotransporting polypeptide (NTCP) and various members of the organic anion transporting polypeptide (OATP) family mediate transport of iodothyronines. Because iodothyronines are a particular class of amino acids derived from tyrosine residues, it is no surprise that some amino acid transporters have been shown to be involved in thyroid hormone transport. We have characterized monocarboxylate transporter 8 (MCT8) as a very active and specific thyroid hormone transporter, the gene of which is located on the X chromosome. MCT8 is highly expressed in liver and brain but is also widely distributed in other tissues. MCT8 shows 50% amino acid identity with a system T amino acid transporter 1 (TAT1). TAT1, also called MCT10, has been characterized to transport aromatic amino acids but no iodothyronines. We have also found that mutations in MCT8 are associated with severe X-linked psychomotor retardation and strongly elevated serum T(3) levels in young boys.
