Structural insights into brain thyroid hormone transport via MCT8 and OATP1C1

Adequate delivery of thyroid hormones to the brain is crucial for normal neurological development. MCT8 and OATP1C1, two solute carrier (SLC) transporters, mediate the passage of thyroid hormones across the blood-brain barrier and into the central nervous system. Mutations in MCT8 result in Allan-Herndon-Dudley syndrome (AHDS), an X-linked birth defect characterized by neurodevelopmental impairments and peripheral hyperthyroidism, whereas OATP1C1 deficiency is linked to brain hypometabolism and progressive neurodegeneration. Here, we report cryoelectron microscopy (cryo-EM) structures of MCT8 and OATP1C1 bound with the active thyroid hormone triiodothyronine (T3) and the prohormone thyroxine (T4) at 2.9 and 2.3 Å resolutions, respectively. Combined with functional studies, we elucidate their distinct thyroid hormone recognition and transport mechanisms and explain disease mutations. Although extracellular allosteric sites are not a common feature of SLC transporters, we identify one in OATP1C1. Collectively, these findings illuminate key aspects of thyroid hormone transport, a fundamental process in development and disease.