Oligodendrocyte progenitor cell maturation is dependent on dual function of MCT8 in the transport of thyroid hormone across brain barriers and the plasma membrane

Inactivating mutations in the thyroid hormone (TH) transporter monocarboxylate transporter 8 (MCT8) causes a rare and debilitating form of X-linked psychomotor disability known as Allan Herndon Dudley syndrome (AHDS). One of the most prominent pathophysiological symptoms of MCT8-deficiency is hypomyelination. Here, patient-derived induced pluripotent stem cells (iPSCs) were used to study the role of MCT8 and TH on the maturation of oligodendrocytes. Interestingly, neither MCT8 mutations nor reduced TH affected the in vitro differentiation of control or MCT8-deficient iPSCs into oligodendrocytes. To assess whether patient-derived iPSC-derived oligodendrocyte progenitor cells (iOPCs) could provide myelinating oligodendrocytes in vivo, cells were transplanted into the shiverer mouse corpus callosum where they survived, migrated, and matured into myelinating oligodendrocytes, though the myelination efficiency was reduced compared with control cells. When MCT8-deficient and healthy control iOPCs were transplanted into a novel hypothyroid immunodeficient triple knockout mouse (tKO, mct8^-/- ; oatp1c1^-/- ; rag2^-/- ), they failed to provide behavioral recovery and did not mature into oligodendrocytes in the hypothyroid corpus callosum, demonstrating the critical role of TH transport across brain barriers in oligodendrocyte maturation. We conclude that MCT8 plays a cell autonomous role in oligodendrocyte maturation and that functional TH transport into the central nervous system will be required for developing an effective treatment for MCT8-deficient patients.