Expression pattern of thyroid hormone transporters in the postnatal mouse brain

Abstract

For a comprehensive description of the tissue-specific thyroidal state under normal as well as under pathophysiological conditions it is of utmost importance to include thyroid hormone (TH) transporters in the analysis as well. The current knowledge of the cell-specific repertoire of TH transporters, however, is still rather limited, although several TH transporting proteins have been identified. Here, we describe the temporal and spatial distribution pattern of the most prominent TH transporters in the postnatal mouse brain. For that purpose, we performed radioactive in situ hybridization studies in order to analyze the cellular mRNA expression pattern of the monocarboxylate transporters Mct8 and Mct10, the L-type amino acid transporters Lat1 and Lat2 as well as the organic anion transporting peptide Oatp1c1 at different postnatal time points. Highest TH transporter expression levels in the CNS were observed at postnatal day 6 and 12, while hybridization signal intensities visibly declined after the second postnatal week. The only exception was Mct10 for which the strongest signals could be observed in white matter regions at postnatal day 21 indicating that this transporter is preferentially expressed in mature oligodendrocytes. Whereas Mct8 and Lat2 showed an overlapping neuronal mRNA expression pattern in the cerebral cortex, hippocampus, and in the hypothalamus, Oatp1c1 and Lat1 specific signals were most prominent in capillary endothelial cells throughout the CNS. In the choroid plexus, expression of three transporters (Mct8, Lat2, and Oatp1c1) could be detected, whereas in other brain areas (e.g., striatum, thalamus, and brain stem nuclei) only one of the transporter candidates appeared to be present. Overall, our study revealed a distinct mRNA distribution pattern for each of the TH transporter candidates. Further studies will reveal to which extent these transporters contribute to the cell-specific TH uptake and efflux in the mouse CNS.

Keywords: CNS; Lat1; Lat2; Mct10; Mct8; Oatp1c1; T3; T4.

Figure 1

Illustration of TH transporter mRNA expression in the murine forebrain. Coronal cryosections of mouse brains collected at different postnatal days were hybridized with radioactively labeled riboprobes specific for Mct8 (A1–A4), Lat2 (B1–B4), Oatp1c1 (C1–C4), Lat1 (D1–D4), and Mct10 (E1–E4). Dark-field autoradiograms illustrate the mRNA expression patterns at the level of the striatum (CPu, caudate–putamen) around Bregma 0.5 mm. VI, layer 6 of the cerebral cortex; cc, corpus callosum; Cg, cingulate cortex, LV, lateral ventricle; Pir, piriform cortex; SP, subplate neurons (cerebral cortex).

Figure 2

mRNA distribution patterns of TH transporters in the murine hippocampus and diencephalon. Mct8 (A1–A4) and Lat2 (B1–B4) exhibited an overlapping mRNA expression in the cerebral cortex, hippocampus (Hip), amygdala (Amyg), hypothalamic paraventricular nucleus (PVN), and choroid plexus (ChP). Mct8 but not Lat2 is highly expressed in tanycytes (Tan) lining the third ventricle (3V). In contrast, Lat2 but not Mct8 is strongly expressed in thalamic nuclei (Thal). Elongated hybridization signals scattered throughout the CNS indicate a capillary localization of Oatp1c1 (C1–C4) and Lat1 (D1–D4). In addition, Lat1 is present in the hippocampal CA3 neurons at postnatal day P6. Mct10 (E1–E4) is also temporarily expressed in hippocampal and cortical neurons. At P21, however, highest hybridization signal intensities are observed in white matter areas [e.g., in the cerebral peduncle (cp) or the hippocampal fimbria (fi)]. DG, dentate gyrus; DM, dorsomedial hypothalamic nucleus.

Figure 3

Differential expression of TH transporters in midbrain regions. Whereas Mct8 mRNA expression is close to the detection limit in the midbrain area (A1–A4), Lat2 specific hybridization signals (B1–B4) are observed in distinct nuclei such as the paratrochlear nucleus (Pa4) and the pontine nuclei (Pn). Weak signals for Lat2 are also found in the superior colliculi (Sc). Oatp1c1 (C1–C4) and Lat1 (D1–D5) exhibit a capillary expression pattern whereas Mct10 (E1–E4) is strongly expressed in the white matter and dentate gyrus neurons (DG). Aq, aqueduct.

Figure 4

Cerebellar and brainstem expression pattern of TH transporters. In the developing cerebellum, Mct8 specific hybridization signals can be found in the external granule cell layer (EGL) and the Purkinje cell layer (PCL) (A1,A2) whereas after weaning Mct8 mRNA expression is largely restricted to choroid plexus structures of the fourth ventricle (4V) (A3,A4). In comparison, Lat2 mRNA levels are high in the internal granule cell layer (IGL) indicating that mature granule cells express this transporter (B1–B4). Lat2 specific signals could also be found in various brainstem nuclei such as the facial nucleus (7), the raphe obscurus nucleus (ROb), the spinal 5 nucleus (Sp5), and the spinal vestibular nucleus (SpVe). Oatp1c1 is highly expressed in choroid plexus as well as in endothelial cells (C1–C4) where also Lat1 staining can be observed. In addition, Lat1 expression is found in the EGL and PCL of the cerebellar cortex at P6 (D1–D4). Mct10 expression pattern indicates a pronounced localization in mature oligodendrocytes (E1–E4).