Thyroid hormone transporters in the brain

The Role of Thyroid Hormone in the Regulation of Cerebellar Development

The proper organized expression of specific genes in time and space is responsible for the organogenesis of the central nervous system including the cerebellum. The epigenetic regulation of gene expression is tightly regulated by an intrinsic intracellular genetic program, local stimuli such as synaptic inputs and trophic factors, and peripheral stimuli from outside of the brain including hormones. Some hormone receptors are expressed in the cerebellum. Thyroid hormones (THs), among numerous circulating hormones, are well-known major regulators of cerebellar development. In both rodents and human, hypothyroidism during the postnatal developmental period results in abnormal morphogenesis or altered function. THs bind to the thyroid hormone receptors (TRs) in the nuclei and with the help of transcriptional cofactors regulate the transcription of target genes. Gene regulation by TR induces cell proliferation, migration, and differentiation, which are necessary for brain development and plasticity. Thus, the lack of TH action mediators may directly cause aberrant cerebellar development. Various kinds of animal models have been established in a bid to study the mechanism of TH action in the cerebellum. Interestingly, the phenotypes differ greatly depending on the models. Herein we summarize the actions of TH and TR particularly in the developing cerebellum.

New Thyrointegrin αvβ3 Antagonist with a Scalable Synthesis, Brain Penetration, and Potent Activity against Glioblastoma Multiforme

We have previously reported that the α_vβ₃ inhibitor P-bi-TAT, a bifunctional version of the thyroid hormone metabolite tetraiodothyroacetic acid (tetrac) conjugated to polyethylene glycol (PEG) MW 4000, has excellent efficacy in a glioblastoma multiforme (GBM) mouse model. However, bioanalysis problems due to PEG polydispersity and large-scale synthesis issues led to a search for new molecules, culminating in the discovery of fb-PMT, a conjugate of tetrac and monodisperse PEG36, with a lipophilic 4-fluorobenzyl group at the opposite end of the PEG chain. fb-PMT reduces GBM tumor growth and viability by up to 98%, is suitable for large-scale synthesis, and is amenable to bioanalysis using mass spectrometry-based detection. We also showed that changes in lipophilicity at the opposite end of the PEG chain from the active tetrac component affected the proton NMR chemical shift of the tetrac moiety in D₂0 and brain levels of the compound after subcutaneous dosing.

Low CSF/serum ratio of free T4 is associated with decreased quality of life in mild hypothyroidism - A pilot study

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General health, according to the Likert scale, was considerable affected even in mild hypothyroidism.

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The level of T4 in the brain, expressed as the CSF/serum f-T4 ratio, was associated with decreased general health.

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Depressive symptoms, according to the MADRS scale, correlated with the CSF/serum f-T4 ratio.

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T4 might have a direct effect in the brain, and not only as a storage hormone for the more active T3.

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Further studies on pharmacokinetics of CSF-thyroxine might be of benefit especially in patients not feeling well.

Background & Objective

Patients with mild hypothyroidism often are depressed and have impaired quality of life despite serum free-T4 and T3 within reference values. Therefore, we investigated whether their symptoms were dependent on the concentrations of free -T4 and T3 in the circulation and cerebrospinal fluid (CSF).

Methods

Twenty-five newly diagnosed, untreated hypothyroid subjects and as many age- and sex-matched healthy controls were investigated. Blood and CSF sampling was performed in the morning after an overnight fast. Quality of life (QoL) was assessed by a Likert scale. In the hypothyroid subjects, the MADRS rating scale was also used to evaluate symptoms of depression. Furthermore, the results obtained by the questionnaires were related to serum and CSF levels of free- T4 and T3 as well as the ratios between them in CSF and in serum.

Results

Self-reported health was considerably lower in hypothyroid subjects. MADRS was considerably higher than the normal range for healthy individuals. Low CSF/serum free-T4 ratio was correlated with an increased depressed state according to MADRS (p < 0.01), and in addition, CSF/serum free-T4 ratio correlated positively with the self-reported general health Likert scale (p < 0.05). Concentrations of TSH, or free-T3 in serum or CSF, were not associated with an increased depressed state or self-reported general health.

Conclusions

Low CSF/serum ratio of free-T4 was correlated with impaired general health and mood, in contrast to serum measurements not showing any correlations. These findings might partly explain why some patients with hypothyroidism suffer from mental symptoms, despite adequate serum levels of free-T4. However, the findings need to be confirmed in further and larger studies.

Melatonin release by exocytosis in the rat parotid gland

Several beneficial effects on oral health are ascribed to melatonin. Due to its lipophilic nature, non-protein-bound circulating melatonin is usually thought to enter the saliva by passive diffusion through salivary acinar gland cells. Recently, however, using transmission electron microscopy (TEM), melatonin was found in acinar secretory granules of human salivary glands. To test the hypothesis that granular located melatonin is actively discharged into the saliva by exocytosis, i.e. contrary to the general belief, the β-adrenergic receptor agonist isoprenaline, which causes the degranulation of acinar parotid serous cells, was administered to anaesthetised rats. Sixty minutes after an intravenous bolus injection of isoprenaline (5 mg kg-1 ), the right parotid gland was removed; pre-administration, the left control gland had been removed. Samples were processed to demonstrate melatonin reactivity using the immunogold staining method. Morphometric assessment was made using TEM. Gold particles labelling melatonin appeared to be preferentially associated with secretory granules, occurring in their matrix and at membrane level but, notably, it was also associated with vesicles, mitochondria and nuclei. Twenty-six per cent of the total granular population (per 100 μm2 per cell area) displayed melatonin labelling in the matrix; three-quarters of this fraction disappeared (P < 0.01) in response to isoprenaline, and melatonin reactivity appeared in dilated lumina. Thus, evidence is provided of an alternative route for melatonin to reach the gland lumen and the oral cavity by active release through exocytosis, a process which is under the influence of parasympathetic and sympathetic nervous activity and is the final event along the so-called regulated secretory pathway. During its stay in granules, anti-oxidant melatonin may protect their protein/peptide constituents from damage.

The placenta as a window to the brain: A review on the role of placental markers in prenatal programming of neurodevelopment

BACKGROUND

During development, the placenta can be said to be the most important organ, however, the most poorly researched. There is currently a broader understanding of how specific insults during development affect the fetal brain, and also the importance of placental signaling in neurodevelopmental programming. Epigenetic responses to maternal and fetal signals are an obvious candidate for transforming early life inputs into long-term programmatic outcomes. As a mediator of maternal and environmental signals to the developing fetus, epigenetic processes within the placenta are particularly powerful such that alterations of placental gene expression, downstream function, and signalling during foetal development have the potential for dramatic changes in developmental programming.

SUMMARY

In this article, we reviewed emerging evidence for a placental role in prenatal neurodevelopmental programming with a specific focus on nutrient and prenatal stress signals integration into chromatin changes; this new understanding, we hope will provide the means for lowering developmentally based disorder risk, and new therapeutic targets for treatment in adulthood.

KEY MESSAGES

Based on this review, the placenta is a potent micro-environmental player in neurodevelopment as it orchestrates a series of complex maternal-foetal interactions. Maternal insults to this microenvironment will impair these processes and disrupt foetal brain development resulting in the prenatal programming of neurodevelopmental disorders. These findings should inspire advance animal model and human research drive to appraise gene-environment impacts during pregnancy that will target the developmental cause of adult-onset mental disorders.

Thyroxine (T4) Transfer from Blood to Cerebrospinal Fluid in Sheep Isolated Perfused Choroid Plexus: Role of Multidrug Resistance-Associated Proteins and Organic Anion Transporting Polypeptides

Thyroxine (T₄) enters the brain either directly across the blood–brain barrier (BBB) or indirectly via the choroid plexus (CP), which forms the blood–cerebrospinal fluid barrier (B-CSF-B). In this study, using isolated perfused CP of the sheep by single-circulation paired tracer and steady-state techniques, T4 transport mechanisms from blood into lateral ventricle CP has been characterized as the first step in the transfer across the B-CSF-B. After removal of sheep brain, the CPs were perfused with ¹²⁵I-T₄ and ¹⁴C-mannitol. Unlabeled T₄ was applied during single tracer technique to assess the mode of maximum uptake (U_max) and the net uptake (U_net) on the blood side of the CP. On the other hand, in order to characterize T₄ protein transporters, steady-state extraction of ¹²⁵I-T₄ was measured in presence of different inhibitors such as probenecid, verapamil, BCH, or indomethacin. Increasing the concentration of unlabeled-T₄ resulted in a significant reduction in U_max%, which was reflected by a complete inhibition of T₄ uptake into CP. In fact, the obtained U_net% decreased as the concentration of unlabeled-T₄ increased. The addition of probenecid caused a significant inhibition of T₄ transport, in comparison to control, reflecting the presence of a carrier mediated process at the basolateral side of the CP and the involvement of multidrug resistance-associated proteins (MRPs: MRP1 and MRP4) and organic anion transporting polypeptides (Oatp1, Oatp2, and Oatp14). Moreover, verapamil, the P-glycoprotein (P-gp) substrate, resulted in ~34% decrease in the net extraction of T₄, indicating that MDR1 contributes to T₄ entry into CSF. Finally, inhibition in the net extraction of T₄ caused by BCH or indomethacin suggests, respectively, a role for amino acid “L” system and MRP1/Oatp1 in mediating T₄ transfer. The presence of a carrier-mediated transport mechanism for cellular uptake on the basolateral membrane of the CP, mainly P-gp and Oatp2, would account for the efficient T₄ transport from blood to CSF. The current study highlights a carrier-mediated transport mechanism for T4 movement from blood to brain at the basolateral side of B-CSF-B/CP, as an alternative route to BBB.

Disruption of type 2 iodothyronine deiodinase activity in cultured human glial cells by polybrominated diphenyl ethers

Polybrominated diphenyl ether (PBDE) flame retardants are endocrine disruptors and suspected neurodevelopmental toxicants. While the direct mechanisms of neurodevelopmental toxicity have not been fully elucidated, it is conceivable that alterations in thyroid hormone levels in the developing brain may contribute to these effects. Cells within the brain locally convert thyroxine (T4) to the biologically active triiodothyronine (T3) through the action of the selenodeiodinase type 2 iodothyronine deiodinase (DIO2). Previous studies have demonstrated that PBDEs can alter hepatic deiodinase activity both in vitro and in vivo; however, the effects of PBDEs on the deiodinase isoforms expressed in the brain are not well understood. Here, we studied the effects of several individual PBDEs and hydroxylated metabolites (OH-BDEs) on DIO2 activity in astrocytes, a specialized glial cell responsible for production of more than 50% of the T3 required by the brain. Primary human astrocytes and H4 glioma cells were exposed to individual PBDEs or OH-BDEs at concentrations up to 5 μM. BDE-99 decreased DIO2 activity by 50% in primary astrocyte cells and by up to 80% in the H4 cells at doses of ≥500 nM. 3-OH-BDE-47, 6-OH-BDE-47, and 5'-OH-BDE-99 also decreased DIO2 activity in cultured H4 glioma cells by 45-80% at doses of approximately 1-5 μM. Multiple mechanisms appear to contribute to the decreased DIO2 activity, including weakened expression of DIO2 mRNA, competitive inhibition of DIO2, and enhanced post-translational degradation of DIO2. We conclude that decreases in DIO2 activity caused by exposure to PBDEs may play a role in the neurodevelopmental deficits caused by these toxicants.

Thyroid hormone and the developing hypothalamus

Thyroid hormone (TH) plays an essential role in normal brain development and function. Both TH excess and insufficiency during development lead to structural brain abnormalities. Proper TH signaling is dependent on active transport of the prohormone thyroxine (T4) across the blood-brain-barrier and into brain cells. In the brain T4 undergoes local deiodination into the more active 3,3′,5-triiodothyronine (T3), which binds to nuclear TH receptors (TRs). TRs are already expressed during the first trimester of pregnancy, even before the fetal thyroid becomes functional. Throughout pregnancy, the fetus is largely dependent on the maternal TH supply. Recent studies in mice have shown that normal hypothalamic development requires intact TH signaling. In addition, the development of the human lateral hypothalamic zone coincides with a strong increase in T3 and TR mRNA concentrations in the brain. During this time the fetal hypothalamus already shows evidence for TH signaling. Expression of components crucial for central TH signaling show a specific developmental timing in the human hypothalamus. A coordinated expression of deiodinases in combination with TH transporters suggests that TH concentrations are regulated to prevent untimely maturation of brain cells. Even though the fetus depends on the maternal TH supply, there is evidence suggesting a role for the fetal hypothalamus in the regulation of TH serum concentrations. A decrease in expression of proteins involved in TH signaling towards the end of pregnancy may indicate a lower fetal TH demand. This may be relevant for the thyrotropin (TSH) surge that is usually observed after birth, and supports a role for the hypothalamus in the regulation of TH concentrations during the fetal period anticipating birth.

Propylthiouracil-induced hypothyroidism during lactation alters leucine and mannose metabolism in rat cerebellar slices

Thyroid hormone deficiency during perinatal development results in significant alterations in neurological functions. The relationship between such events and brain metabolism is not completely understood. The aim of this study was to investigate the effects of hypothyroidism on leucine, mannose, glucose and lactate metabolism in rat cerebellar slices. Experimental hypothyroidism was induced by exposing mothers and pups to propylthiouracil (PTU) until weaning - postnatal day 21. Metabolic analyses were performed in postnatal day 10 (PND10) and 21 (PND21) animals. A matching group of animals received the same oral treatment also after weaning until adulthood PND60 with T3 supplement during lactation (P1-P21). In PND21 animals, PTU treatment significantly increased the rate of leucine oxidation to CO2, although glucose and lactate oxidations were not affected. PTU treatment also increased the oxidation of leucine to CO2 at PND60 (adult animals). PND10 hypothyroidism animals showed a decrease in conversion of mannose to glycolipids and glycoprotein compared with the control group. However, PTU treatment increased the conversion of mannose to glycolipids and glycoprotein in PND21 animals. The replacement of T3 normalized mannose and leucine metabolism in adult rats. These results indicate that deficits in thyroid hormones during lactation could delay or alter brain development and metabolism.

Thyroid hormone transporters and deiodinases in the developing human hypothalamus

OBJECTIVE

Thyroid hormone (TH) signaling in brain cells is dependent on transport of TH across the plasma membrane followed by intracellular deiodination and binding to the nuclear TH receptors. The aim of this study is to investigate the expression of the specific TH transporters monocarboxylate transporter 8 (MCT8 (SLC16A2)), MCT10, organic anion transporting polypeptide 1C1 (OATP1C1 (SLCO1C1)), and the types 2 and 3 deiodinases (D2 and D3) in the developing human hypothalamus.

DESIGN

Fifteen postmortem brain samples of fetuses and young children ranging between 17 weeks of gestation and 29 months of postnatal age including one child (28 months) with central congenital hypothyroidism were studied.

METHODS

Sections of the different hypothalami were stained with polyclonal rabbit antisera against MCT8, MCT10, OATP1C1, D2, and D3.

RESULTS

We found MCT8 and D3 but not D2 protein expression to be present in our earliest sample of 17 weeks of gestation, indicating triiodothyronine degradation, but not production at this time of development. At term, expression of TH transporters and D2 decreased and D3 expression increased, suggesting decreased TH signaling just before birth. The child with central congenital hypothyroidism showed higher MCT8 and D2 expression compared with the other children of similar age.

CONCLUSIONS

This study reports the developmental timing of expression of components crucial for central TH signaling in the human hypothalamus. In general, during fetal hypothalamic development, the coordinated expression of D2 and D3 in combination with the different TH transporters suggests that proper TH concentrations are regulated to prevent untimely maturation of brain cells.

Expression Profiling of Solute Carrier Gene Families at the Blood-CSF Barrier

The choroid plexus (CP) is a highly vascularized tissue in the brain ventricles and acts as the blood-cerebrospinal fluid (CSF) barrier (BCSFB). A main function of the CP is to secrete CSF, which is accomplished by active transport of small ions and water from the blood side to the CSF side. The CP also supplies the brain with certain nutrients, hormones, and metal ions, while removing metabolites and xenobiotics from the CSF. Numerous membrane transporters are expressed in the CP in order to facilitate the solute exchange between the blood and the CSF. The solute carrier (SLC) superfamily represents a major class of transporters in the CP that constitutes the molecular mechanisms for CP function. Recently, we systematically and quantitatively examined Slc gene expression in 20 anatomically comprehensive brain areas in the adult mouse brain using high-quality in situ hybridization data generated by the Allen Brain Atlas. Here we focus our analysis on Slc gene expression at the BCSFB using previously obtained data. Of the 252 Slc genes present in the mouse brain, 202 Slc genes were found at detectable levels in the CP. Unsupervised hierarchical cluster analysis showed that the CP Slc gene expression pattern is substantially different from the other 19 analyzed brain regions. The majority of the Slc genes in the CP are expressed at low to moderate levels, whereas 28 Slc genes are present in the CP at the highest levels. These highly expressed Slc genes encode transporters involved in CSF secretion, energy production, and transport of nutrients, hormones, neurotransmitters, sulfate, and metal ions. In this review, the functional characteristics and potential importance of these Slc transporters in the CP are discussed, with particular emphasis on their localization and physiological functions at the BCSFB.

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.

Classification of inhibitors of hepatic organic anion transporting polypeptides (OATPs): influence of protein expression on drug-drug interactions

The hepatic organic anion transporting polypeptides (OATPs) influence the pharmacokinetics of several drug classes and are involved in many clinical drug–drug interactions. Predicting potential interactions with OATPs is, therefore, of value. Here, we developed in vitro and in silico models for identification and prediction of specific and general inhibitors of OATP1B1, OATP1B3, and OATP2B1. The maximal transport activity (MTA) of each OATP in human liver was predicted from transport kinetics and protein quantification. We then used MTA to predict the effects of a subset of inhibitors on atorvastatin uptake in vivo. Using a data set of 225 drug-like compounds, 91 OATP inhibitors were identified. In silico models indicated that lipophilicity and polar surface area are key molecular features of OATP inhibition. MTA predictions identified OATP1B1 and OATP1B3 as major determinants of atorvastatin uptake in vivo. The relative contributions to overall hepatic uptake varied with isoform specificities of the inhibitors.

The Link between Thyroid Function and Depression

The relation between thyroid function and depression has long been recognized. Patients with thyroid disorders are more prone to develop depressive symptoms and conversely depression may be accompanied by various subtle thyroid abnormalities. Traditionally, the most commonly documented abnormalities are elevated T4 levels, low T3, elevated rT3, a blunted TSH response to TRH, positive antithyroid antibodies, and elevated CSF TRH concentrations. In addition, thyroid hormone supplements appear to accelerate and enhance the clinical response to antidepressant drugs. However, the mechanisms underlying the interaction between thyroid function and depression remain to be further clarified. Recently, advances in biochemical, genetic, and neuroimaging fields have provided new insights into the thyroid-depression relationship.

Expression of thyroid hormone transporters in the human hypothalamus

CONTEXT

Transport of thyroid hormone across the plasma membrane is required for proper thyroid hormone action and metabolism. Several specific thyroid hormone transporters have been identified capable of facilitating uptake and/or efflux of thyroid hormones. Monocarboxylate transporter (MCT)-8, MCT10, and organic anion transporting polypeptide 1C1 (OATP1C1) are the best-characterized specific thyroid hormone transporters to date.

OBJECTIVE

Our earlier studies in the human hypothalamus have shown that MCT8 is present in neurons of the hypothalamic paraventricular nucleus (PVN) and infundibular nucleus (IFN) and in tanycytes. We hypothesized that also MCT10 and OATP1C1 are present in specific areas of the human hypothalamus.

DESIGN

We studied postmortem brain samples of patients with known serum thyroid hormone levels using immunocytochemistry to investigate the distribution of MCT10 and OATP1C1 in the hypothalamus.

RESULTS

We found strong neuronal MCT10 immunocytochemical staining in a number of hypothalamic nuclei, including the PVN, IFN, and supraoptic nucleus. Intense staining was also observed in neurons of the lateral hypothalamus including the perifornical area. OATP1C1 immunoreactivity was present in glial cells throughout the hypothalamus. In addition, staining was present in capillary walls and in neurons of the PVN, IFN, and supraoptic nucleus.

CONCLUSION

The strong expression of MCT10 and OATP1C1 in the human hypothalamus indicates a possible role in the regulation of the hypothalamus-pituitary-thyroid axis.

Longitudinal comparison of thyroxine pharmacokinetics between pregnant and nonpregnant women: a stable isotope study

The treatment of maternal hypothyroidism presents clinicians with a unique challenge, because dosing regimens previously developed and validated for nonpregnant women cannot be easily extrapolated to dosing in pregnancy. Thyroid hormone requirement increases by 20% to 40% early during pregnancy, persisting throughout gestation. Accordingly, women with treated hypothyroidism need to increase their levothyroxine dose to prevent maternal hypothyroidism and the associated impaired cognitive development and increased fetal mortality. We investigated the pharmacokinetic properties of levothyroxine during pregnancy through the use of a novel, traceable form of levothyroxine. The objective was to conduct a longitudinal study to determine whether levothyroxine pharmacokinetics differ in the pregnant versus nonpregnant state. We used a unique C-levothyroxine-tracer method to distinguish between endogenous and exogenous levothyroxine and studied the pharmacokinetics of a single oral dose of levothyroxine using tandem mass spectrometry. Moreover, we were able to detect single dose amounts of the drug, in picogram/mL concentrations. The area under the curve was 23.0 ng*h/mL in pregnancy and 14.8 ng*h/mL in nonpregnant women (P < 0.03) with median serum half-lives of 32.1 hours and 24.1 hours, respectively (P < 0.04). Further research involves the measurement of free thyroxine on these samples using tandem mass spectrometry. Future work should focus on the mechanisms responsible for the gestational differences in pharmacokinetics and whether these should necessitate dose schedule changes in pregnancy.

Thyroid disease and mental disorders: cause and effect or only comorbidity?

PURPOSE OF REVIEW

To discuss the effects of thyroid dysfunction and thyroid autoimmunity on mental symptoms and disorders in patients with thyroid disease with reference to recent epidemiological, clinical, and genetic findings.

RECENT FINDINGS

During brain development, iodine deficiency, maternal thyroid dysfunction, and neonatal thyroid malformations together with genetic factors contribute to neurological deficit. Most adults with thyroid dysfunction will develop mental symptoms. In hyperthyroidism, adrenergic hyperactivity is a major cause of psychiatric symptoms, and beta-adrenergic antagonists are effective treatment. Most patients with severe hypothyroidism will also demonstrate mental symptoms; however, causality is not so evident as in hyperthyroidism. Polymorphism in deiodinase genes and in transporter genes appears to make an important contribution to the presentation of mental symptoms as well as to the outcome of treatment of hypothyroidism. A thyroid autoimmunity process may by itself contribute to mental symptoms in vulnerable patients. Data from epidemiological studies provide conflicting evidence as to associations between thyroid disorders and mental symptoms.

SUMMARY

In the adult brain, compared with the developing brain, brain-thyroid relationships are less apparent but still important. Adrenergic hyperactivity is a major cause of psychiatric symptoms in hyperthyroidism. Genetic factors contribute to the development and treatment outcome of mental disorder in hypothyroidism.

Thyroid disorders in mental patients

PURPOSE OF REVIEW

Thyroid hormones play important roles in brain development and function. Recent findings concerning thyroid hormones secretion, transport, and metabolism in the brain have provided a better understanding of the role of thyroid hormones in mental disorders.

RECENT FINDINGS

The intracellular actions of thyroid hormones in brain are determined by a complex of factors, including circulating concentrations of thyroid hormones, availability of free hormone, activity of thyroid hormone transporters and deiodinase enzymes, and activity of thyroid hormone receptors. Individual genetic variations and mutations of thyroid-axis-related proteins influence thyroid hormone activity in the brain and contribute to the presentation of mental disorders, as well as to response to psychiatric treatments.

SUMMARY

Consideration of molecular mechanism related to genetic alterations in thyroid hormone transport into the neuron, intracellular thyroid hormone metabolism in the brain, as well as polymorphism in thyroid hormone receptors, opens new venues for better understanding of thyroid hormone effects in the brain as well as for finding genetic markers and new targets for the treatment of mental disorders.

Prevalence and functional analysis of the S107P polymorphism (rs6647476) of the monocarboxylate transporter 8 (SLC16A2) gene in the male population of north-west Spain (Galicia)

OBJECTIVE

Mutations in SLC16A2, the gene encoding the thyroid hormone (TH)-specific transporter monocarboxylate transporter 8 (MCT8), result in a thyroid phenotype and severe mental retardation caused by neuronal TH deficiency. These mutational effects raise the question of whether polymorphic variation in SLC16A2 may also be associated with differences in serum levels of TH and/or TSH.

DESIGN

This is the first major study of the frequency of the SLC16A2 rs6647476 single nucleotide polymorphism (SNP) (amino acid change Ser107Pro). We also studied the relationships of SLC16A2 genetic variants with serum levels of TSH, T4 and T3, with their mRNA expression and with expression of the TH-responsive genes ZAKI-4 and BTEB in white blood cells. Experiments in cultured fibroblasts were carried out to ascertain the dynamics of the T3 response.

METHODS

A total of 276 men were studied. Genotyping of the S107P SNP was carried out using polymerase chain reaction restriction fragment length polymorphism (PCR-RFLP); serum hormone levels were determined by chemiluminescence; expression of mRNA was quantified by real-time PCR.

RESULTS

The SLC16A2 S107P SNP was found in 36% of Galician males. With the present sample size we did not find any association of this polymorphism with variability in serum levels of TSH, free T4 (fT4) or fT3, or with basal expression of mRNA for SLC16A2 or the two TH-responsive genes ZAKI-4 and BTEB, either in white blood cells or in cultured human fibroblasts from either Ser107 or Pro107 genotypes under T3 stimulation.

CONCLUSIONS

The S107P change in MCT8 is frequent in the male population in Galicia. In the population studied in this report an association with a thyroid phenotype was not demonstrated, even though the S107P SNP causes an important amino acid change.