Organic Anion Transporting Polypeptide 1C1 Research Articles

Thyroid Hormone Transporters MCT8 and OATP1C1 Are Expressed in Projection Neurons and Interneurons of Basal Ganglia and Motor Thalamus in the Adult Human and Macaque Brains.

Monocarboxylate transporter 8 (MCT8) and organic anion-transporting polypeptide 1C1 (OATP1C1) are thyroid hormone (TH) transmembrane transporters relevant for the availability of TH in neural cells, crucial for their proper development and function. Mutations in MCT8 or OATP1C1 result in severe disorders with dramatic movement disability related to alterations in basal ganglia motor circuits. Mapping the expression of MCT8/OATP1C1 in those circuits is necessary to explain their involvement in motor control. We studied the distribution of both transporters in the neuronal subpopulations that configure the direct and indirect basal ganglia motor circuits using immunohistochemistry and double/multiple labeling immunofluorescence for TH transporters and neuronal biomarkers. We found their expression in the medium-sized spiny neurons of the striatum (the receptor neurons of the corticostriatal pathway) and in various types of its local microcircuitry interneurons, including the cholinergic. We also demonstrate the presence of both transporters in projection neurons of intrinsic and output nuclei of the basal ganglia, motor thalamus and nucleus basalis of Meynert, suggesting an important role of MCT8/OATP1C1 for modulating the motor system. Our findings suggest that a lack of function of these transporters in the basal ganglia circuits would significantly impact motor system modulation, leading to clinically severe movement impairment.

Tsh Induces Agrp1 Neuron Proliferation in Oatp1c1-Deficient Zebrafish.

Thyroid hormones (THs), thyroxine (T4), and triiodothyronine (T3), regulate growth, metabolism, and neurodevelopment. THs secretion is controlled by the pituitary thyroid-stimulating hormone (TSH) and the hypothalamic-pituitary-thyroid (HPT) axis. The organic anion-transporting polypeptide 1C1 (OATP1C1/SLCO1C1) and the monocarboxylate transporter 8 (MCT8/SLC16A2) actively transport THs, which bind to their nuclear receptors and induce gene expression. A mutation in OATP1C1 is associated with brain hypometabolism, gradual neurodegeneration, and impaired cognitive and motor functioning in adolescent patients. To understand the role of Oatp1c1 and the mechanisms of the disease, we profiled the transcriptome of oatp1c1 mutant (oatp1c1 -/-) and mct8 -/- xoatp1c1 -/- adult male and female zebrafish brains. Among dozens of differentially expressed genes, agouti-related neuropeptide 1 (agrp1) expression increased in oatp1c1 -/- adult brains. Imaging in the hypothalamus revealed enhanced proliferation of Agrp1 neurons in oatp1c1 -/- larvae and adults, and increased food consumption in oatp1c1 -/- larvae. Similarly, feeding and the number of Agrp1 neurons increased in thyroid gland-ablated zebrafish. Pharmacological treatments showed that the T3 analog TRIAC (3,3',5-tri-iodothyroacetic acid), but not T4, normalized the number of Agrp1 neurons in oatp1c1 -/- zebrafish. Since the HPT axis is hyperactive in the oatp1c1 -/- brain, we used the CRISPR-Cas9 system to knockdown tsh in oatp1c1 -/- larvae, and inducibly enhanced the HPT axis in wild-type larvae. These manipulations showed that Tsh promotes proliferation of Agrp1 neurons and increases food consumption in zebrafish. The results revealed upregulation of both the HPT axis-Agrp1 circuitry and feeding in a zebrafish model for OATP1C1 deficiency.SIGNIFICANCE STATEMENT Mutation in the thyroid hormone (TH) transporter OATP1C1 is associated with cognitive and motor functioning disturbances in humans. Here, we used an oatp1c1 -/- zebrafish to understand the role of organic anion-transporting polypeptide 1C1 (Oatp1c1), and the characteristics of OATP1C1 deficiency. Transcriptome profiling identified upregulation of agrp1 expression in the oatp1c1 -/- brain. The oatp1c1 -/- larvae showed increased thyroid-stimulating hormone (tsh) levels, proliferation of Agrp1 neurons and food consumption. Genetic manipulations of the hypothalamic-pituitary-thyroid (HPT) axis showed that Tsh increases the number of Agrp1 neurons and food consumption. The T3 analog TRIAC (3,3',5-tri-iodothyroacetic acid) normalizes the number of Agrp1 neurons and may have potential for the treatment of Oatp1c1 deficiency. The findings demonstrate a functional interaction between the thyroid and feeding systems in the brain of zebrafish and suggest a neuroendocrinological mechanism for OATP1C1 deficiency.

Distinct Actions of the Thyroid Hormone Transporters Mct8 and Oatp1c1 in Murine Adult Hippocampal Neurogenesis.

Inactivating mutations in the thyroid hormone (TH) transporter monocarboxylate transporter 8 (MCT8) result in Allan-Herndon-Dudley Syndrome, a severe form of psychomotor retardation, while inactivating mutations in another TH transporter, organic anion transporting polypeptide 1c1 (OATP1C1), are linked to juvenile neurodegeneration. These diseases point to essential roles for TH transporters in CNS function. We recently defined the presence of Mct8 in adult hippocampal progenitors and mature granule cell neurons and unraveled cell-autonomous and indirect requirements for Mct8 in adult hippocampal neurogenesis. Here, we investigated whether Oatp1c1 is involved in the hippocampal neurogenic process in concert with Mct8. We detected Oatp1c1 gene expression activity and transcripts in subsets of progenitors, neurons and niche cells in the dentate gyrus. Absence of Oatp1c1 resulted in increased neuroblast and reduced immature neuron numbers in 6-month-old Oatp1c1ko and Mct8/Oatp1c1 double knockout (M/Odko) mice. Reduced EdU-label retention in Mct8ko and M/Odko mice confirmed the impact of Mct8 on neuron formation. In contrast, no significant effect of Oatp1c1 loss on granule cell neuron production and anxiety-like behavior in the open field arena were seen. Together, our results reinforce that distinct actions of each TH transporter are required at multiple stages to ensure proper adult hippocampal neurogenesis.

Absence of Both Thyroid Hormone Transporters MCT8 and OATP1C1 Impairs Neural Stem Cell Fate in the Adult Mouse Subventricular Zone.

SummaryAdult neural stem cell (NSC) generation in vertebrate brains requires thyroid hormones (THs). How THs enter the NSC population is unknown, although TH availability determines proliferation and neuronal versus glial progenitor determination in murine subventricular zone (SVZ) NSCs. Mice display neurological signs of the severely disabling human disease, Allan-Herndon-Dudley syndrome, if they lack both MCT8 and OATP1C1 transporters, or MCT8 and deiodinase type 2. We analyzed the distribution of MCT8 and OATP1C1 in adult mouse SVZ. Both are strongly expressed in NSCs and at a lower level in neuronal cell precursors but not in oligodendrocyte progenitors. Next, we analyzed Mct8/Oatp1c1 double-knockout mice, where brain uptake of THs is strongly reduced. NSC proliferation and determination to neuronal fates were severely affected, but not SVZ-oligodendroglial progenitor generation. This work highlights how tight control of TH availability determines NSC function and glial-neuron cell-fate choice in adult brains.

Central Hypothyroidism Impairs Heart Rate Stability and Prevents Thyroid Hormone-Induced Cardiac Hypertrophy and Pyrexia.

Background: Tachycardia, cardiac hypertrophy, and elevated body temperature are major signs of systemic hyperthyroidism, which are considered to reflect the excessive thyroid hormone (TH) action in the respective peripheral tissues. However, recent observations indicate that the central actions of TH also contribute substantially to cardiovascular regulation and thermogenesis. Methods: In this study, we dissect the individual contributions of peripheral TH action versus the central effects in body temperature regulation and cardiovascular functions by taking advantage of mice lacking the TH transporters monocarboxylate transporter 8 (MCT8) and organic anion transporting polypeptide 1C1 (OATP1C1) (M/O double knock-out [dko]), which exhibit elevated serum triiodothyronine (T3) levels while their brain is in a profoundly hypothyroid state. We compared these animals with wild-type (WT) mice that were treated orally with T3 to achieve similarly elevated serum T3 levels, but are centrally hyperthyroid. For the studies, we used radiotelemetry, infrared thermography, gene expression profiling, Western blot analyses, and enzyme linked immunosorbent assays (ELISA) assays. Results: Our analyses revealed mild hyperthermia and cardiac hypertrophy in T3-treated WT mice but not in M/O dko animals, suggesting that central actions of TH are required for these hyperthyroid phenotypes. Although the average heart rate was unaffected in either model, the M/O dko exhibited an altered heart rate frequency distribution with tachycardic bursts in active periods and bradycardic episodes during resting time, demonstrating that the stabilization of heart rate by the autonomic nervous system can be impaired in centrally hypothyroid animals. Conclusions: Our studies unravel distinct phenotypical traits of hyperthyroidism that depend on an intact central nervous system, and provide valuable insight into the cardiovascular pathology of the Allan-Herndon-Dudley syndrome, a condition caused by the lack of MCT8 in humans.

Age effect on thyroid hormone brain response in male mice

Thyroid hormones (TH) are important for brain development and central nervous system (CNS) function. Disturbances of thyroid function occur with higher prevalence in the ageing population and may negatively impact brain function. We investigated the age impact on behavior in young adult and old male mice (5 vs. 20 months) with chronic hypo- or hyper-thyroidism as well as in sham-treated controls. Expression of TH transporters and TH responsive genes was studied in CNS and pituitary by in situ hybridization and qRT-PCR, whereas TH serum concentrations were determined by immunoassay. Serum TH levels were lower in old compared with young hyperthyroid mice, suggesting a milder hyperthyroid phenotype in the aged group. Likewise, elevated plus maze activity was reduced in old hyperthyroid animals. Under hypothyroid conditions, thyroxine serum concentrations did not differ in young and old mice. Both groups showed a comparable decline in activity and elevated anxiety levels. However, an attenuated increase in hypothalamic thyrotropin releasing hormone and pituitary thyroid stimulating hormone transcript expression was found in old hypothyroid mice. Brain expression of monocarboxylate transporter 8 and organic anion transporting polypeptide 1c1 was not affected by age or TH status. In summary, ageing attenuates neurological phenotypes in hyperthyroid but not hypothyroid mice, which fits with age effects on TH serum levels in the animals. In contrast no changes in TH transporter expression were found in aged mouse brains with hyper- or hypo-thyroid state.

Thyroid hormone availability in the human fetal brain: novel entry pathways and role of radial glia

Thyroid hormones (TH) are crucial for brain development; their deficiency during neurodevelopment impairs neural cell differentiation and causes irreversible neurological alterations. Understanding TH action, and in particular the mechanisms regulating TH availability in the prenatal human brain is essential to design therapeutic strategies for neurological diseases due to impaired TH signaling during neurodevelopment. We aimed at the identification of cells involved in the regulation of TH availability in the human brain at fetal stages. To this end, we studied the distribution of the TH transporters monocarboxylate transporter 8 (MCT8) and organic anion-transporting polypeptide 1C1 (OATP1C1), as well as the TH-metabolizing enzymes types 2 and 3 deiodinases (DIO2 and DIO3). Paraffin-embedded human brain sections obtained from necropsies of thirteen fetuses from 14 to 38 gestational weeks were analyzed by immunohistochemistry and in situ hybridization. We found these proteins localized along radial glial cells, in brain barriers, in Cajal-Retzius cells, in migrating fibers of the brainstem and in some neurons and glial cells with particular and complex spatiotemporal patterns. Our findings point to an important role of radial glia in controlling TH delivery and metabolism and suggest two additional novel pathways for TH availability in the prenatal human brain: the outer, and the inner cerebrospinal fluid-brain barriers. Based on our data we propose a model of TH availability for neural cells in the human prenatal brain in which several cell types have the ability to autonomously control the required TH content.

Deiodinases, Organic Anion Transporter Polypeptide Polymorphisms, and Thyroid Hormones in Patients with Myocardial Infarction.

To investigate the association among deiodinases (DIO), organic anion-transporting polypeptide 1C1 (OATP1C1) gene polymorphisms, and thyroid hormones (THs) in patients with acute myocardial infarction (AMI). In summary, 290 patients with AMI were evaluated for sociodemographic and clinical characteristics, coronary artery disease (CAD) risk factors, and comorbidities, as well as circulating thyroid-stimulating hormone and TH (triiodothyronine [T3], thyroxine [T4], free T3, free T4, and reverse T3) levels. Ten single nucleotide polymorphisms for thyroid axis related genes: DIO1 (rs11206244-C/T, rs12095080-A/G, rs2235544-A/C), DIO2 (rs225014-T/C, rs225015-G/A), DIO3 (rs945006-T/G), and OATP1C1 (rs10444412-T/C, rs10770704-C/T, rs1515777-A/G, rs974453-G/A) were genotyped. Marginal associations were observed between the DIO1, DIO2, and OATP1C1 gene polymorphisms and almost all analyzed THs (p's < 0.05). After controlling for potential confounders, the OATP1C1 rs1515777-A/G minor allele homozygous genotype (G/G) was associated with a decrease in circulating free T3 and free T3/free T4. In the AMI cohort, associations between: DIO1 rs12095080 and hypertension; DIO2 rs225015 and diabetes mellitus; and the OATP1C1 rs974453 genotype, and AMI type were established. DIO1 and DIO2 gene polymorphisms are mainly associated with T3, free T4, free T3/free T4, and [natural-log transformed (ln)] reverse T3 levels, while the OATP1C1 minor allele homozygous genotype is associated with free T3 and free T3/free T4 in CAD patients after AMI.

Experimental study on DEHP affect the neurodevelopment through interfering with placental thyroid hormones transport

Objective: The present study was represented by di-(2-ethylhexyl) phthalate (DEHP), to explore the role of thyroid hormones (THs) disruption in the connection of placenta and neurodevelopmental toxicity. Methods: During fetal mice neural tube closed (pregnancy 9.5 days, E9.5d) to begin synthesis of THs (E15.5 d), all pregnant mice were administered with different concentration of DEHP (0、10、50、200 mg/kg) by gavage once a day(10 mice per group). All pregnant mice were conducted with BrdU administration in E14d by subcutaneous injection. Seven pregnant mice from each group were scarified after anesthesia in E15.5 d, serum and amniotic fluid were collected to determinate the levels of THs(T(3), T(4), FT(3) and FT(4)) by the automatic biochemical analyzer, detecting fetal mice placental protein expression of monocarboxylate transporter 8 (MCT8), organic anion transporting polypeptide 1C1 (OATP1C1) and deiodinaseⅡ&Ⅲ (DIO(2), DIO(3)) by Western blot. Each group of the remaining three pregnant mices were killed after anesthesia in E18d, take the male fetal brain, BrdU immunohistochemistry was used to detect the proliferation and migration of fetal brain cortical neurons. Results: There was no abnormalities in diet, water intake, body weight and general activity of pregnant mice in each treatment group, and there were no difference in the general physiolo. Results There was no abnormalities in diet, water intake, body weight and general activity of pregnant mice in each treatment group, and there were no difference in the general physiological development status of body weight, brain weight, brain body ratio between the mice of each group. There was no statistically significant differences in serum T(3), T(4), FT(3), FT(4) and amniotic fluid FT(4) in pregnant mice of each group (P>0.05), Compared with the control group, the FT(3) levels in the amniotic fluid of the DEHP 50 and 200 mg/kg groups were significantly decreased(P<0.05). Compared with the control group, the placental MCT8 and DIO(2) protein levels of male fetal mice in the DEHP 50 and 200 mg/kg group decreased, and the level of OATP1C1 protein in 200 mg/kg group decreased(P<0.05), and there was no statistically significant difference in DIO(3) protein levels among all groups (P>0.05). Compared with the control group, the number of BrdU positive cells in the cerebral cortex of male mice in DEHP 200 mg/kg group decreased, 56.5% was distributed in VZ-SVZ layer, and the percentage of BrdU positive cells in the IZ layer of 50 mg/kg group increased (P<0.05). Conclusion: DEHP 50, 200 mg/kg may affect the proliferation and migration of neural cells in the developing brain, which may be related to its interference with thyroid hormone by placental transport.

Chapter 23 - Endocrine disorders and the cerebellum: from neurodevelopmental injury to late-onset ataxia

Hormonal disorders are a source of cerebellar ataxia in both children and adults. Normal development of the cerebellum is critically dependent on thyroid hormone, which crosses both the blood-brain barrier and the blood-cerebrospinal fluid barrier thanks to specific transporters, including monocarboxylate transporter 8 and the organic anion-transporting polypeptide 1C1. In particular, growth and dendritic arborization of Purkinje neurons, synaptogenesis, and myelination are dependent on thyroid hormone. Disturbances of thyroid hormone may also impact on cerebellar ataxias of other origin, decompensating or aggravating the pre-existing ataxia manifesting with motor ataxia, oculomotor ataxia, and/or Schmahmann syndrome. Parathyroid disorders are associated with a genuine cerebellar syndrome, but symptoms may be subtle. The main conditions combining diabetes and cerebellar ataxia are Friedreich ataxia, ataxia associated with anti-GAD antibodies, autoimmune polyglandular syndromes, aceruloplasminemia, and cerebellar ataxia associated with hypogonadism (especially Holmes ataxia/Boucher-Neuhäuser syndrome). The general workup of cerebellar disorders should include the evaluation of hormonal status, including thyroid-stimulating hormone and free thyroxine levels, and hormonal replacement should be considered depending on the laboratory results. Cerebellar deficits may be reversible in some cases.

Functional Characterization of Xenopus Thyroid Hormone Transporters mct8 and oatp1c1.

Xenopus is an excellent model for studying thyroid hormone signaling as it undergoes thyroid hormone-dependent metamorphosis. Despite the fact that receptors and deiodinases have been described in Xenopus, membrane transporters for these hormones are yet to be characterized. We cloned Xenopus monocarboxylate transporter 8 (mct8) and organic anion-transporting polypeptide 1C1 (oatpc1c1), focusing on these two transporters given their importance for vertebrate brain development. Protein alignment and bootstrap analysis showed that Xenopus mct8 and oatp1c1 are closer to their mammalian orthologs than their teleost counterparts. We functionally characterized the two transporters using a radiolabeled hormones in vitro uptake assay in COS-1 cells. Xenopus mct8 was found to actively transport both T3 and T4 bidirectionally. As to the thyroid precursor molecules, diiodotyrosine (DIT) and monoiodotyrosine (MIT), both human and Xenopus mct8, showed active efflux, but no influx. Again similar to humans, Xenopus oatp1c1 transported T4 but not T3, MIT, or DIT. We used reverse transcription quantitative polymerase chain reaction and in situ hybridization to characterize the temporal and spatial expression of mct8 and oatp1c1 in Xenopus. Specific expression of the transporter was observed in the brain, with increasingly strong expression as development progressed. In conclusion, these results show that Xenopus thyroid hormone transporters are functional and display marked spatiotemporal expression patterns. These features make them interesting targets to elucidate their roles in determining thyroid hormone availability during embryonic development.

Zebrafish – An emerging model to explore thyroid hormone transporters and psychomotor retardation

Thyroid hormones (THs) regulate a variety of fundamental physiological processes, including the development and maintenance of the brain. For decades, it was thought that THs enter the cells by passive diffusion. However, it is now clear that TH transport across the cell membrane requires specific transporter proteins that facilitate the uptake and efflux of THs. Several thyroid hormone transmembrane transporters (THTTs) have been identified, including monocarboxylate transporter 8 (MCT8), MCT10, and organic anion transporting polypeptide 1C1 (OATP1C1). The critical role of THTTs in regulating metabolism and brain function is demonstrated in the Allan-Herndon-Dudley syndrome (AHDS), an X-linked psychomotor retardation associated with mutations in the MCT8/SLC16A2 gene. In addition to traditional research on humans, cell-lines, and rodents, the zebrafish has recently emerged as an attractive model to study THTTs and neuroendocrinological-related disorders. In this review, we describe the unique contribution of zebrafish studies to the understanding of the functional role of THTTs in live animals, and how this transparent vertebrate model can be used for translational studies on TH-related disorders.

Characterization of Chicken Thyroid Hormone Transporters.

Thyroid hormone (TH) transmembrane transporters are key regulators of TH availability in target cells where correct TH signaling is essential for normal development. Although the chicken embryo is a valuable model for developmental studies, the only functionally characterized chicken TH transporter so far is the organic anion transporting polypeptide 1C1 (OATP1C1). We therefore cloned the chicken L-type amino acid transporter 1 (LAT1) and the monocarboxylate transporters 8 (MCT8) and 10 (MCT10), and functionally characterized them, together with OATP1C1, in JEG3, COS1, and DF-1 cells. In addition, we used in situ hybridization to study their mRNA expression pattern during development. MCT8 and OATP1C1 are both high affinity transporters for the prohormone T4, whereas receptor-active T3 is preferably transported by MCT8 and MCT10. The latter one shows lower affinity but has a high Vmax and seems to be especially good at T3 export. Also, LAT1 has a lower affinity for its preferred substrate 3,3'-diiodothyronine. Reverse T3 is transported by all 4 TH transporters and is a good export product for OATP1C1. TH transporters are strongly expressed in eye (LAT1, MCT8, MCT10), pancreas (LAT1, MCT10), kidney, and testis (MCT8). Their extensive expression in the central nervous system, especially at the brain barriers, indicates an important role in brain development. In conclusion, we show TH transport by chicken MCT8, MCT10, and LAT1. Together with OATP1C1, these transporters have functional characteristics similar to their mammalian orthologs and are interesting target genes to further elucidate the role of THs during embryonic development.

Mosaic Expression of Thyroid Hormone Regulatory Genes Defines Cell Type-Specific Dependency in the Developing Chicken Cerebellum.

The cerebellum is a morphologically unique brain structure that requires thyroid hormones (THs) for the correct coordination of key cellular events driving its development. Unravelling the interplay between the multiple factors that can regulate intracellular TH levels is a key step to understanding their role in the regulation of these cellular processes. We therefore investigated the regional/cell-specific expression pattern of TH transporters and deiodinases in the cerebellum using the chicken embryo as a model. In situ hybridisation revealed expression of the TH transporters monocarboxylate transporter 8 (MCT8) and 10 (MCT10), L-type amino acid transporter 1 (LAT1) and organic anion transporting polypeptide 1C1 (OATP1C1) as well as the inactivating type 3 deiodinase (D3) in the fourth ventricle choroid plexus, suggesting a possible contribution of the resulting proteins to TH exchange and subsequent inactivation of excess hormone at the blood-cerebrospinal fluid barrier. Exclusive expression of LAT1 and the activating type 2 deiodinase (D2) mRNA was found at the level of the blood-brain barrier, suggesting a concerted function for LAT1 and D2 in the direct access of active T3 to the developing cerebellum via the capillary endothelial cells. The presence of MCT8 mRNA in Purkinje cells and cerebellar nuclei during the first 2weeks of embryonic development points to a potential role of this transporter in the uptake of T3 in central neurons. At later stages, together with MCT10, detection of MCT8 signal in close association with the Purkinje cell dendritic tree suggests a role of both transporters in TH signalling during Purkinje cell synaptogenesis. MCT10 was also expressed in late-born cells in the rhombic lip lineage with a clear hybridisation signal in the outer external granular layer, indicating a potential role for MCT10 in the proliferation of granule cell precursors. By contrast, expression of D3 in the first-born rhombic lip-derived population may serve as a buffering mechanism against high T3 levels during early embryonic development, a hypothesis supported by the pattern of expression of a fluorescent TH reporter in this lineage. Overall, this study builds a picture of the TH dependency in multiple cerebellar cell types starting from early embryonic development.

Endotoxin-induced inflammation down-regulates L-type amino acid transporter 1 (LAT1) expression at the blood-brain barrier of male rats and mice.

BackgroundWe recently reported that bacterial lipopolysaccharide (LPS)-induced inflammation decreases the expression of the primary thyroid hormone transporters at the blood–brain barrier, organic anion-transporting polypeptide 1c1 (OATP1c1) and monocarboxylate transporter 8 (MCT8). l-type amino acid transporters 1 and 2 (LAT1 & LAT2) are regarded as secondary thyroid hormone transporters, and are expressed in cells of the blood–brain or blood-cerebrospinal fluid barrier and by neurons. The purpose of this study was to examine the effect of LPS-induced inflammation on the expression of LAT1 and LAT2, as these may compensate for the downregulation of OATP1c1 and MCT8.MethodsLPS (2.5 mg/kg body weight) was injected intraperitoneally to adult, male, Sprague–Dawley rats and C57Bl/6 mice, which were euthanized 2, 4, 9, 24 or 48 h later. LAT1 and LAT2 mRNA expression were studied on forebrain sections using semiquantitative radioactive in situ hybridization. LAT1 protein levels in brain vessels were studied using LAT1 immunofluorescence. Statistical comparisons were made by the non-parametric Kruskal–Wallis and Dunn’s tests.ResultsIn both species, LAT1 mRNA decreased in brain blood vessels as soon as 2 h after LPS injection and was virtually undetectable at 4 h and 9 h. During recovery from endotoxemia, 48 h after LPS injection, LAT1 mRNA in brain vessels increased above control levels. A modest but significant decrease in LAT1 protein levels was detected in the brain vessels of mice at 24 h following LPS injection. LPS did not affect LAT1 and LAT2 mRNA expression in neurons and choroid plexus epithelial cells.ConclusionsThe results demonstrate that LPS-induced inflammation rapidly decreases LAT1 mRNA expression at the blood–brain barrier in a very similar manner to primary thyroid hormone transporters, while changes in LAT1 protein level follow a slower kinetics. The data raise the possibility that inflammation may similarly down-regulate other blood–brain barrier transport systems at the transcriptional level. Future studies are required to examine this possibility and the potential pathophysiological consequences of inflammation-induced changes in blood–brain barrier transport functions.Electronic supplementary materialThe online version of this article (doi:10.1186/s12987-015-0016-8) contains supplementary material, which is available to authorized users.

Expression of thyroid hormone transporters and deiodinases at the brain barriers in the embryonic chicken: Insights into the regulation of thyroid hormone availability during neurodevelopment

Thyroid hormones (THs) are key regulators in the development of the vertebrate brain. Therefore, TH access to the developing brain needs to be strictly regulated. The brain barriers separate the central nervous system from the rest of the body and impose specific transport mechanisms on the exchange of molecules between the general circulation and the nervous system. As such they form ideal structures for regulating TH exchange between the blood and the brain. To investigate the mechanism by which the developing brain regulates TH availability, we investigated the ontogenetic expression profiles of TH transporters, deiodinases and the TH distributor protein transthyretin (TTR) at the brain barriers during embryonic and early postnatal development using the chicken as a model. In situ hybridisation revealed expression of the TH transporters monocarboxylate transporter 8 (MCT8) and 10 (MCT10), organic anion transporting polypeptide 1C1 (OATP1C1) and L-type amino acid transporter 1 (LAT1) and the inactivating type 3 deiodinase (D3) in the choroid plexus which forms the blood–cerebrospinal fluid barrier. This was confirmed by quantitative PCR which additionally indicated strongly increasing expression of TTR as well as detectable expression of the activating type 2 deiodinase (D2) and the (in)activating type 1 deiodinase (D1). In the brain capillaries forming the blood–brain barrier in situ hybridisation showed exclusive expression of LAT1 and D2. The combined presence of LAT1 and D2 in brain capillaries suggests that the blood–brain barrier forms the main route for receptor-active T3 uptake into the embryonic chicken brain. Expression of multiple transporters, deiodinases and TTR in the choroid plexus indicates that the blood–cerebrospinal fluid barrier is also important in regulating early TH availability. The impact of these barrier systems can be deduced from the clear difference in T3 and T4 levels as well as the T3/T4 ratio between the developing brain and the general circulation. We conclude that the tight regulation of TH exchange at the brain barriers from early embryonic stages is one of the factors needed to allow the brain to develop within a relative microenvironment.

Parallel regulation of thyroid hormone transporters OATP1c1 and MCT8 during and after endotoxemia at the blood-brain barrier of male rodents.

There is increasing evidence that local thyroid hormone (TH) availability changes profoundly in inflammatory conditions due to altered expression of deiodinases that metabolize TH. It is largely unknown, however, how inflammation affects TH availability via the expression of TH transporters. In this study we examined the effect of bacterial lipopolysaccharide (LPS) administration on two TH transporters that are critically important for brain TH homeostasis, organic anion-transporting polypeptide 1c1 (OATP1c1), and monocarboxylate transporter 8 (MCT8). MRNA levels were studied by in situ hybridization and qPCR as well as protein levels by immunofluorescence in both the rat and mouse forebrain. The mRNA of both transporters decreased robustly in the first 9 hours after LPS injection, specifically in brain blood vessels; OATP1c1 mRNA in astrocytes and MCT8 mRNA in neurons remained unchanged. At 24 and/or 48 hours after LPS administration, OATP1c1 and MCT8 mRNAs increased markedly above control levels in brain vessels. OATP1c1 protein decreased markedly in vessels by 24 hours whereas MCT8 protein levels did not decrease significantly. These changes were highly similar in mice and rats. The data demonstrate that OATP1c1 and MCT8 expression are regulated in a parallel manner during inflammation at the blood-brain barrier of rodents. Given the indispensable role of both transporters in allowing TH access to the brain, the results suggest reduced brain TH uptake during systemic inflammation.

Expression of Organic Anion Transporting Polypeptide 1c1 and Monocarboxylate Transporter 8 in the Rat Placental Barrier and the Compensatory Response to Thyroid Dysfunction

Thyroid hormones (THs) must pass from mother to fetus for normal fetal development and require the expression of placental TH transporters. We investigate the compensatory effect of placental organic anion transporting polypeptide 1c1 (Oatp1c1) and monocarboxylate transporter 8 (Mct8) on maternal thyroid dysfunction. We describe the expressions of these two transporters in placental barriers and trophoblastic cell populations in euthyroidism and thyroid dysfunction resulting from differential iodine nutrition at gestation day (GD) 16 and 20, that is, before and after the onset of fetal thyroid function. Immunohistochemistry revealed that in the blood-placenta barrier, these two TH transporters were strongly expressed in the villous interstitial substance and were weakly expressed in trophoblast cells. Levels of Oatp1c1 protein obviously increased in the placental fetal portion during maternal thyroid deficiency at GD16. Under maternal thyroid deficiency after the production of endogenous fetal TH, quantitative PCR analysis revealed down-regulation of Oatp1c1 occurred along with up-regulation of Mct8 in trophoblast cell populations isolated by laser capture microdissection (LCM); this was consistent with the protein levels in the fetal portion of the placenta. In addition, decreased D3 mRNA at GD16 and increased D2 mRNA on two gestational days were observed in trophoblast cells with thyroid dysfunction. However, levels of Oatp1c1 mRNA at GD16 and D3 mRNA at GD20 were too low to be detectable in trophoblast cells. In conclusion, placental Oatp1c1 plays an essential compensatory role when the transplacental passage of maternal THs is insufficient at the stage before the fetal TH production. In addition, the coordinated effects of Oatp1c1, Mct8, D2 and D3 in the placental barrier may regulate both transplacental TH passage and the development of trophoblast cells during thyroid dysfunction throughout the pregnancy.

Inherited defects of thyroid hormone-cell-membrane transport

This review summarizes the most significant findings over the last year regarding human and animal models deficient in thyroid hormone cell-membrane transporters (THCMTs). Although several THCMTs have been modelled in genetically engineered mice, the only THCMT defect known in humans is that caused by mutations in the monocarboxylate transporter 8 (MCT8) gene. The importance of several amino acid residues has been assessed in vitro to further our understanding on the structure-function of the MCT8. The administration of the thyromimetic compound, diiodothyropropionic acid, has been tested in patients with MCT8 gene mutations, following studies of its use in mice. Another thyroid hormone analogue, 3,3',5,5'-tetraiodothyroacetic acid, was tested in Mct8-deficient mice. The phenotypes of L-type aminoacid transporter 2 and organic anion transporting polypeptide 1C1 deficiencies have been studied in mouse models. Mct8/organic anion transporting polypeptide 1C1 double knockout mice have been shown to manifest neurodevelopmental deficits. Zebrafish is emerging as another vertebrate model that may be useful to study the role of Mct8 in brain development. Studies on the pathogenesis and therapy of MCT8 deficiency are in progress, and new vertebrate models that are suitable to study the neurological consequences of the syndrome are being explored.

Boucher–Neuhäuser syndrome

Hormonal disorders are a source of cerebellar ataxia in both children and adults. Normal development of the cerebellum is critically dependent on thyroid hormone, which crosses both the blood–brain barrier and the blood–cerebrospinal fluid barrier thanks to specific transporters, including monocarboxylate transporter 8 and the organic anion-transporting polypeptide 1C1. In particular, growth and dendritic arborization of Purkinje neurons, synaptogenesis, and myelination are dependent on thyroid hormone. Disturbances of thyroid hormone may also impact on cerebellar ataxias of other origin, decompensating or aggravating the pre-existing ataxia manifesting with motor ataxia, oculomotor ataxia, and/or Schmahmann syndrome. Parathyroid disorders are associated with a genuine cerebellar syndrome, but symptoms may be subtle. The main conditions combining diabetes and cerebellar ataxia are Friedreich ataxia, ataxia associated with anti-GAD antibodies, autoimmune polyglandular syndromes, aceruloplasminemia, and cerebellar ataxia associated with hypogonadism (especially Holmes ataxia/Boucher–Neuhäuser syndrome). The general workup of cerebellar disorders should include the evaluation of hormonal status, including thyroid-stimulating hormone and free thyroxine levels, and hormonal replacement should be considered depending on the laboratory results. Cerebellar deficits may be reversible in some cases.