ORIGINAL ARTICLE A Transgenic Approach to Identify Thyroxine Transporter-Expressing Structures in Brain Development M.-F. Lang 1, *, S. Salinin* , D. A. Ridder* , J. Kleesiek*, J. Hroudova*, S. Bergerà, G. Schu ¨tzà and M. Schwaninger*  *Institute of Pharmacology, University of Heidelberg, Heidelberg, Germany.  Institute of Experimental and Clinical Pharmacology and Toxicology, University of Lu ¨beck, Lu ¨ beck, Germany. àGerman Cancer Research Center, Division Molecular Biology of the Cell I, Heidelberg, Germany. Thyroid hormones are well known for their importance in brain development. The active form of the hormone is 3,5,3¢-triiodothyro- nine (T3); however, studies have shown that maternal hypothyrox- inaemia, comprising reduced thyroxine (T4) levels, is a major cause of defects in foetal brain development (1). Early and transient maternal hypothyroxinaemia affects corticogenesis by altering neu- ronal migration (2–6). According to a prevailing model of thyroid hormone metabolism, plasma-derived T4 is a major source of brain T3 (7,8). T4 is trans- ported into the brain by the solute carrier organic anion transporter 1c1 (SLCO1C1, OATP14) through the blood brain–barrier (BBB). In astrocytes, the type 2 deiodinase (D2) converts T4 into T3 that exerts paracrine effects on neurones (9,10). Thus, T4 transport across the BBB plays an important role in determining brain T3 lev- els. SLCO1C1 is a member of the Na + -independent organic anion transporter family. Human SLCO1C1 has been detected in the brain and testis (11,12). The rodent SLCO1C1 was first cloned from rat brain endothelial cells using genomic approaches (13). Rodent SLCO1C1 has an 84% sequence homology to its human counterpart and has been detected in the luminal and abluminal membrane of brain capillary endothelial cells (14), as well as in the basolateral and apical membrane of choroid plexus epithelial cells (11,15). T4 and reverse T3 have the highest affinity for SLCO1C1 (14–16). Thus, Journal of Neuroendocrinology Correspondence to: M. Schwaninger, Institute of Experimental and Clinical Pharmacology and Toxicology, University of Lu ¨ beck, Ratzeburger Allee 160, 23538 Lu ¨beck, Germany (e-mail: markus.schwaninger@ pharma.uni-luebeck.de). 1 Present address: Department of Neurosciences, Beckman Research Institute of City of Hope, Duarte, CA, USA. Transporters are essential in thyroid hormone metabolism. Thyroxine (T4) is transported by solute carrier organic anion transporter 1c1 (SLCO1C1, OATP14) into the adult brain, where T4 is con- verted to 3,5,3¢-triiodothyronine (T3). In adults, SLCO1C1 expression is found in two brain barrier structures: the blood–brain barrier (BBB) and choroid plexus. However, little is known about how T4 is transported in the developing brain, when the BBB is not yet completely formed. We employed bacterial artificial chromosome recombineering to generate transgenic mice carrying Cre recombinase in the Slco1c1 locus (Slco1c1-Cre mice). In Slco1c1-Cre mice Cre was expressed at the sites that have been previously reported for SLCO1C1 in adults. To trace Cre expression during development, we crossed Slco1c1-Cre transgenic mice with Rosa26 reporter mice. b- galactosidase staining showed Cre activity in neurones of various brain structures, such as corti- cal layer 2 ⁄ 3 and the hippocampus, suggesting transient Slco1c1 expression during brain devel- opment. At embryonic day15, SLCO1C1 was expressed at the same site as TBR2, a marker of neuronal progenitors. Neurones that express SLCO1C1 during their development could be T4 sensitive. In support of this hypothesis, hypothyroxinaemia induced by propylthiouracil treatment of dams decreased the number of b-galactosidase-positive neurones in cortical layer 2 ⁄ 3 of newborn Slco1c1-Cre ⁄ Rosa26 mice. In conclusion, by generating Slco1c1-Cre transgenic mice, we demonstrated that SLCO1C1 is expressed in the neuronal cell lineage during brain develop- ment. Expression of SLCO1C1 may underlie the extraordinary sensitivity of specific neuronal populations to hypothyroxinaemia. Key words: solute carrier organic anion transporter 1c1, thyroxine transporter, thyroid hor- mones, hypothyroidism, brain development. doi: 10.1111/j.1365-2826.2011.02216.x Journal of Neuroendocrinology 23, 1194–1203 ª 2011 The Authors. Journal of Neuroendocrinology ª 2011 Blackwell Publishing Ltd Journal of Neuroendocrinology From Molecular to Translational Neurobiology