Function of thyroid hormone transporters in the central nervous system.

BACKGROUND: Iodothyronines are charged amino acid derivatives that cannot passively cross a phospholipid bilayer. Transport of thyroid hormones across plasma membranes is mediated by integral membrane proteins belonging to several gene families. These transporters therefore allow or limit access of thyroid hormones into brain. Since thyroid hormones are essential for brain development and cell differentiation, it is expected that genetic deficiency of such transporters would result in neurodevelopmental derangements. SCOPE OF REVIEW: We introduce concepts of thyroid hormone transport into the brain and into brain cells. Important thyroid hormone transmembrane transporters are presented along with their expression patterns in different brain cell types. A focus is placed on monocarboxylate transporter 8 (MCT8) which has been identified as an essential thyroid hormone transporter in humans. Mutations in MCT8 underlie one of the first described X-linked mental retardation syndromes, the Allan-Herndon-Dudley syndrome. MAJOR
CONCLUSIONS: Thyroid hormone transporter molecules are expressed in a developmental and cell type-specific pattern. Any thyroid hormone molecule has to cross consecutively the luminal and abluminal membranes of the capillary endothelium, enter astrocytic foot processes, and leave the astrocyte through the plasma membrane to finally cross another plasma membrane on its way towards its target nucleus. GENERAL SIGNIFICANCE: We can expect more transporters being involved in or contributing to in neurodevelopmental or neuropsychiatric disease. Due to their expression in cellular components regulating the hypothalamus-pituitary-thyroid axis, mutations and polymorphisms are expected to impact on negative feedback regulation and hormonal setpoints. This article is part of a Special Issue entitled Thyroid hormone signalling.