Expression of the thyroid hormone transporters monocarboxylate transporter-8 (SLC16A2) and organic ion transporter-14 (SLCO1C1) at the blood-brain barrier.

Thyroid hormones require transport across cell membranes to carry out their biological functions. The importance of transport for thyroid hormone signaling was highlighted by the discovery that inactivating mutations in the human monocarboxylate transporter-8 (MCT8) (SLC16A2) cause severe psychomotor retardation due to thyroid hormone deficiency in the central nervous system. It has been reported that Mct8 expression in the mouse brain is restricted to neurons, leading to the model that organic ion transporter polypeptide-14 (OATP14, also known as OATP1C1/SLCO1C1) is the primary thyroid hormone transporter at the blood-brain barrier, whereas MCT8 mediates thyroid hormone uptake into neurons. In contrast to these reports, we report here that in addition to neuronal expression, MCT8 mRNA and protein are expressed in cerebral microvessels in human, mouse, and rat. In addition, OATP14 mRNA and protein are strongly enriched in mouse and rat cerebral microvessels but not in human microvessels. In rat, Mct8 and Oatp14 proteins localize to both the luminal and abluminal microvessel membranes. In human and rodent choroid plexus epithelial cells, MCT8 is concentrated on the epithelial cell apical surface and OATP14 localizes primarily to the basal-lateral surface. Mct8 and Oatp14 expression was also observed in mouse and rat tanycytes, which are thought to form a barrier between hypothalamic blood vessels and brain. These results raise the possibility that reduced thyroid hormone transport across the blood-brain barrier contributes to the neurological deficits observed in affected patients with MCT8 mutations. The high microvessel expression of OATP14 in rodent compared with human brain may contribute to the relatively mild phenotype observed in Mct8-null mice, in contrast to humans lacking functional MCT8.

Synthetic compound libraries displayed on the surface of encoded bacteriophage.

We describe a technology for attaching libraries of synthetic compounds to coat proteins of bacteriophage particles such that the identity of the chemical structure is encoded in the genome of the phage, analogous to peptides displayed on phage surfaces by conventional phage-display techniques. This format allows a library of synthetic compounds to be screened very efficiently as a single pool. Encoded phage serve as extremely robust reporters of the presence of each compound, providing exquisite sensitivity for identification of active compounds engaged in complex biological processes such as receptor-mediated endocytosis and transcytosis. To evaluate this approach, we constructed a library of 980 analogs of folic acid displayed on T7 phage, and demonstrated rapid identification of compounds that bind to folate receptor and direct endocytosis of associated phage particles into cells that express the targeted receptor.