Transport of thyroid hormone in brain

60 YEARS OF NEUROENDOCRINOLOGY: TRH, the first hypophysiotropic releasing hormone isolated: control of the pituitary-thyroid axis

This review presents the findings that led to the discovery of TRH and the understanding of the central mechanisms which control hypothalamus-pituitary-thyroid axis (HPT) activity. The earliest studies on thyroid physiology are now dated a century ago when basal metabolic rate was associated with thyroid status. It took over 50 years to identify the key elements involved in the HPT axis. Thyroid hormones (TH: T4 and T3) were characterized first, followed by the semi-purification of TSH whose later characterization paralleled that of TRH. Studies on the effects of TH became possible with the availability of synthetic hormones. DNA recombinant techniques facilitated the identification of all the elements involved in the HPT axis, including their mode of regulation. Hypophysiotropic TRH neurons, which control the pituitary-thyroid axis, were identified among other hypothalamic neurons which express TRH. Three different deiodinases were recognized in various tissues, as well as their involvement in cell-specific modulation of T3 concentration. The role of tanycytes in setting TRH levels due to the activity of deiodinase type 2 and the TRH-degrading ectoenzyme was unraveled. TH-feedback effects occur at different levels, including TRH and TSH synthesis and release, deiodinase activity, pituitary TRH-receptor and TRH degradation. The activity of TRH neurons is regulated by nutritional status through neurons of the arcuate nucleus, which sense metabolic signals such as circulating leptin levels. Trh expression and the HPT axis are activated by energy demanding situations, such as cold and exercise, whereas it is inhibited by negative energy balance situations such as fasting, inflammation or chronic stress. New approaches are being used to understand the activity of TRHergic neurons within metabolic circuits.

Effect of non-genomic actions of thyroid hormones on the anaesthetic effect of propofol

Hyperthyroidism is a common disease of the endocrine system and it is known that additional propofol anaesthesia is required during surgery for patients with hyperthyroidism compared with those with normal thyroid function. The aim of the present study was to determine the mechanism through which thyroid hormones (THs) inhibit the effect of propofol anaesthesia. Immunofluorescence techniques were used to verify the difference between the expression quantities of γ-aminobutyric acid type A (GABA_A) receptor subunits α2 and β2 in the dorsal root ganglions (DRGs) of rats with hyperthyroidism and those in normal rats. Perforated patch clamp recordings in the whole-cell mode were performed to detect the GABA-activated current in acutely isolated rat DRG neurons from rats with hyperthyroidism and normal rats. This method was also used to evaluate the change in the GABA-activated currents following the pre-perfusion of propofol with and without 3,3',5-L-triiodothyronine (T₃). Compared with normal rats, rats with hyperthyroidism expressed same quantities of GABA_A receptor α2 and β2 subunits in DRGs. In addition, no difference in GABA-activated currents in the acutely isolated DRG neurons from the two types of rat was observed (P>0.05). T₃ inhibits or minimises the augmentation effect of propofol on the GABA-activated currents (P<0.05). The inhibitory effect of T₃ on propofol was minimised by increasing the propofol concentration (P<0.05). The inhibitory effect of T₃ on the anaesthetic effect of propofol is achieved through the inhibition of the function of GABA_A receptors through the non-genomic actions of the THs, rather than by changing the number of GABA_A receptors. This inhibitory effect can be mitigated by increasing the propofol concentration. In conclusion, rats with hyperthyroidism require a larger dose of propofol to induce anaesthesia since the non-genomic actions of THs suppress GABA receptors, which in turn inhibits the anaesthetic action of propofol.

A Nonradioactive Uptake Assay for Rapid Analysis of Thyroid Hormone Transporter Function

Thyroid hormones (TH) are actively taken up into target cells via TH-transmembrane transporters (THTT). Their activity and expression patterns define a layer of endocrine regulation that is poorly understood. Therefore, THTT are potential targets for interfering agents (endocrine disruptors) as well as for pharmacological interventions. Inactivating mutations have been identified as the underlying cause of heritable diseases (monocarboxylate transporter 8-associated Allan-Herndon-Dudley syndrome) and might also define a class of subclinical TH insensitivity. As a basic tool to solve questions regarding THTT substrate specificity, activation or inactivation by compounds and functional changes from mutations, uptake assays with radiolabeled tracers are standard. Due to the need for radioactive isotopes, this technique is limited to screening of labelled substrates and disadvantageous regarding handling, setup, and regulatory issues. To overcome these hurdles, we developed an uptake assay protocol using nonradioactive ligands. In brief, uptake of nonradioactive iodine-containing substrate molecules was monitored via Sandell-Kolthoff reaction. The novel assay was designed to the common microtiter plate layout. As a prove-of-principle, we measured TH uptake by monocarboxylate transporter 8-transfected MDCK1 cells. Titrations with bromosulphthalein as an example for inhibitor screening setups and a side-by-side comparison with the radioactive method prove this assay to be reliable, sensitive, and convenient. Furthermore, the method was applicable on primary murine astrocytes, which enables high-throughput screening studies on in vitro model systems with physiological transporter regulation. Due to its design, it is applicable for high-throughput screening of modulatory compounds, but it is also a safe, inexpensive and an easily accessible method for functional testing of THTT in basic science.

Treatment of congenital thyroid dysfunction: Achievements and challenges

The active thyroid hormone tri-iodothyronine (T3) is essential for a normal development of children. Especially within the first years of life, thyroid hormone is pivotal in enabling maturation of complex brain function and somatic growth. The most compelling example for a life without thyroid hormone are those historical cases of children who came to birth without a thyroid gland - as shown in autopsy-studies- and who suffered from untreated hypothyroidism, at that time initially called "sporadic congenital hypothyroidism" (CH). In the last decades huge achievements resulted in a normal development of these children based on newborn screening programs that enable an early onset of a high dose LT4-treatment. Further progress will be necessary to further tailor an individualized thyroid hormone substitution approach and to identify those more complex patients with congenital hypothyroidism and associated defects, who will not benefit from an even optimized LT4 therapy. Besides the primary production of thyroid hormone a variety of further mechanisms are necessary to mediate the function of T3 on normal development that are located downstream of thyroid hormone production. Abnormalities of these mechanisms include the MCT8-transport defect, deiodinase-insufficiency and thyroid hormone receptor alpha-and beta defects. These thyroid hormone resistant diseases can not be treated with classical LT4 substitution alone. The development of new treatment options for those rare cases of thyroid hormone resistance is one of the most challenging tasks in the field of congenital thyroid diseases today.

Structural insights into thyroid hormone transport mechanisms of the L-type amino acid transporter 2

Thyroid hormones (THs) are transported across cell membranes by different transmembrane transporter proteins. In previous studies, we showed marked 3,3'-diiodothyronine (3,3'-T2) but moderate T3 uptake by the L-type amino acid transporter 2 (Lat2). We have now studied the structure-function relationships of this transporter and TH-like molecules. Our Lat2 homology model is based on 2 crystal structures of the homologous 12-transmembrane helix transporters arginine/agmatine antiporter and amino acid/polyamine/organocation transporter. Model-driven mutagenesis of residues lining an extracellular recognition site and a TH-traversing channel identified 9 sensitive residues. Using Xenopus laevis oocytes as expression system, we found that side chain shortening (N51S, N133S, N248S, and Y130A) expanded the channel and increased 3,3'-T2 transport. Side chain enlargements (T140F, Y130R, and I137M) decreased 3,3'-T2 uptake, indicating channel obstructions. The opposite results with mutations maintaining (F242W) or impairing (F242V) uptake suggest that F242 may have a gating function. Competitive inhibition studies of 14 TH-like compounds revealed that recognition by Lat2 requires amino and carboxylic acid groups. The size of the adjacent hydrophobic group is restricted. Bulky substituents in positions 3 and 5 of the tyrosine ring are allowed. The phenolic ring may be enlarged, provided that the whole molecule is flexible enough to fit into the distinctly shaped TH-traversing channel of Lat2. Taken together, the next Lat2 features were identified 1) TH recognition site; 2) TH-traversing channel in the center of Lat2; and 3) switch site that potentially facilitates intracellular substrate release. Together with identified substrate features, these data help to elucidate the molecular mechanisms and role of Lat2 in T2 transport.

Effects of isolated positive maternal thyroglobulin antibodies on brain development of offspring in an experimental autoimmune thyroiditis model

BACKGROUND

Autoimmune thyroiditis (AIT) is a very common endocrine disorder in pregnancy. However, the effect of maternal positive thyroglobulin antibodies (TgAb) on brain development of offspring remains unclear. This study used an experimental autoimmune thyroiditis model in CBA/J mice and determined whether isolated positive maternal TgAb directly affected learning and memory abilities of offspring.

METHODS

An experimental autoimmune thyroiditis model was established in CBA/J mice through immunization with murine thyroglobulin (mTg). Measuring thyroid function and serum TgAb titer confirmed the presence of isolated positive maternal TgAb. Offspring serum TgAb titer, MCT8, Reelin, RC3, and BNDF mRNA expression in the brain, and brain histology were measured on postnatal days 0, 10, and 40 (PND0, PND10, PND40), and nerve cell migration (BrdU labeling) at PND40. Morris water maze, long-term potentiation (LTP), and LTP-related factor ERK1/2 levels were measured at PND40 to determine offspring spatial learning and memory development.

RESULTS

Maternal serum TgAb titers increased and remained elevated through pregnancy compared to controls. Thyrotropin and thyroid hormone levels were normal. The T group offspring (Tg immunized) had higher TgAb titers than the control (C) group. However, antibody titers time-dependently decreased. MCT8, Reelin, RC3, and BDNF mRNA expression in the whole brain were similar in the T and C groups on PND0, PND10, and PND40. Neuronal distribution and BrdU from the cerebral cortex and hippocampus were similar in the T and C group offspring. Morris water maze tests, excitatory postsynaptic field potentials, and ERK1/2 levels were also similar between the T and C groups.

CONCLUSIONS

Isolated positive maternal TgAb did not clearly influence the learning ability and memory of offspring, or nerve cell migration, despite a transient increase in TgAb in immunized mice.

Transport of thyroid hormones via the choroid plexus into the brain: the roles of transthyretin and thyroid hormone transmembrane transporters

Thyroid hormones are key players in regulating brain development. Thus, transfer of appropriate quantities of thyroid hormones from the blood into the brain at specific stages of development is critical. The choroid plexus forms the blood-cerebrospinal fluid barrier. In reptiles, birds and mammals, the main protein synthesized and secreted by the choroid plexus is a thyroid hormone distributor protein: transthyretin. This transthyretin is secreted into the cerebrospinal fluid and moves thyroid hormones from the blood into the cerebrospinal fluid. Maximal transthyretin synthesis in the choroid plexus occurs just prior to the period of rapid brain growth, suggesting that choroid plexus-derived transthyretin moves thyroid hormones from blood into cerebrospinal fluid just prior to when thyroid hormones are required for rapid brain growth. The structure of transthyretin has been highly conserved, implying strong selection pressure and an important function. In mammals, transthyretin binds T4 (precursor form of thyroid hormone) with higher affinity than T3 (active form of thyroid hormone). In all other vertebrates, transthyretin binds T3 with higher affinity than T4. As mammals are the exception, we should not base our thinking about the role of transthyretin in the choroid plexus solely on mammalian data. Thyroid hormone transmembrane transporters are involved in moving thyroid hormones into and out of cells and have been identified in many tissues, including the choroid plexus. Thyroid hormones enter the choroid plexus via thyroid hormone transmembrane transporters and leave the choroid plexus to enter the cerebrospinal fluid via either thyroid hormone transmembrane transporters or via choroid plexus-derived transthyretin secreted into the cerebrospinal fluid. The quantitative contribution of each route during development remains to be elucidated. This is part of a review series on ontogeny and phylogeny of brain barrier mechanisms.

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.

Post-translational modifications of transthyretin affect the triiodonine-binding potential

Transthyretin (TTR) is a visceral protein, which facilitates the transport of thyroid hormones in blood and cerebrospinal fluid. The homotetrameric structure of TTR enables the simultaneous binding of two thyroid hormones per molecule. Each TTR subunit provides a single cysteine residue (Cys₁₀), which is frequently affected by oxidative post-translational modifications. As Cys₁₀ is part of the thyroid hormone-binding channel within the TTR molecule, PTM of Cys₁₀ may influence the binding of thyroid hormones. Therefore, we analysed the effects of Cys₁₀ modification with sulphonic acid, cysteine, cysteinylglycine and glutathione on binding of triiodothyronine (T3) by molecular modelling. Furthermore, we determined the PTM pattern of TTR in serum of patients with thyroid disease by immunoprecipitation and mass spectrometry to evaluate this association in vivo. The in silico assays demonstrated that oxidative PTM of TTR resulted in substantial reorganization of the intramolecular interactions and also affected the binding of T3 in a chemotype- and site-specific manner with S-glutathionylation as the most potent modulator of T3 binding. These findings were supported by the in vivo results, which indicated thyroid function-specific patterns of TTR with a substantial decrease in S-sulphonated, S-cysteinylglycinated and S-glutathionylated TTR in hypothyroid patients. In conclusion, this study provides evidence that oxidative modifications of Cys₁₀ seem to affect binding of T3 to TTR probably because of the introduction of a sterical hindrance and induction of conformational changes. As oxidative modifications can be dynamically regulated, this may represent a sensitive mechanism to adjust thyroid hormone availability.

An evo-devo approach to thyroid hormones in cerebral and cerebellar cortical development: etiological implications for autism

The morphological alterations of cortical lamination observed in mouse models of developmental hypothyroidism prompted the recognition that these experimental changes resembled the brain lesions of children with autism; this led to recent studies showing that maternal thyroid hormone deficiency increases fourfold the risk of autism spectrum disorders (ASD), offering for the first time the possibility of prevention of some forms of ASD. For ethical reasons, the role of thyroid hormones on brain development is currently studied using animal models, usually mice and rats. Although mammals have in common many basic developmental principles regulating brain development, as well as fundamental basic mechanisms that are controlled by similar metabolic pathway activated genes, there are also important differences. For instance, the rodent cerebral cortex is basically a primary cortex, whereas the primary sensory areas in humans account for a very small surface in the cerebral cortex when compared to the associative and frontal areas that are more extensive. Associative and frontal areas in humans are involved in many neurological disorders, including ASD, attention deficit-hyperactive disorder, and dyslexia, among others. Therefore, an evo-devo approach to neocortical evolution among species is fundamental to understand not only the role of thyroid hormones and environmental thyroid disruptors on evolution, development, and organization of the cerebral cortex in mammals but also their role in neurological diseases associated to thyroid dysfunction.