Minimal requirements for ubiquitination mediated regulation of thyroid hormone activation

Activation of thyroxine by outer ring deiodination is the crucial first step of thyroid hormone action. Substrate-induced ubiquitination of type 2 deiodinase (D2) is the most rapid and sensitive mechanism known to regulate thyroid hormone activation. While the molecular machinery responsible for D2 ubiquitination has been extensively studied, the combination of molecular features sufficient and required to allow D2 ubiquitination remained to be determined. To address this question we constructed chimeric deiodinases by introducing different combinations of D2-specific elements into type 1 deiodinase (D1), another member of the deiodinase enzyme family, which however does not undergo ubiquitination in its native form. Studies on the chimeric proteins expressed transiently in HEK-293T cells revealed that combined insertion of the D2-specific instability loop and the K237/K244 D2 ubiquitin-carrier lysines into the corresponding positions of D1 could-not ubiquitinate D1 unless the chimera was directed to the endoplasmic reticulum (ER). Fluorescence resonance energy transfer measurements demonstrated that the C-terminal globular domain of the ER-directed chimera was able to interact with the E3 ligase subunit WSB1. However, this interaction did not occur between the chimera and the TEB4 E3 ligase although a native D2 could readily interact with the N-terminus of TEB4. In conclusion, insertion of the instability loop and ubiquitin-carrier lysines in combination with direction to the ER are sufficient and required to govern WSB1-mediated ubiquitination of an activating deiodinase enzyme

Thyroid Hormone and the Neuroglia: Both Source and Target

Thyroid hormone plays a crucial role in the development and function of the nervous system. In order to bind to its nuclear receptor and regulate gene transcription thyroxine needs to be activated in the brain. This activation occurs via conversion of thyroxine to T3, which is catalyzed by the type 2 iodothyronine deiodinase (D2) in glial cells, in astrocytes, and tanycytes in the mediobasal hypothalamus. We discuss how thyroid hormone affects glial cell function followed by an overview on the fine-tuned regulation of T3 generation by D2 in different glial subtypes. Recent evidence on the direct paracrine impact of glial D2 on neuronal gene expression underlines the importance of glial-neuronal interaction in thyroid hormone regulation as a major regulatory pathway in the brain in health and disease

A központi idegrendszeri pajzsmirigyhormon aktiváció szabályozásának molekuláris biológiája = Molecular biology of the regulation of thyroid hormone activation in the central nervous system

A pajzsmirigyhormonok számos biológiai folyamat, így a fejlődés, a növekedés és az anyagcsere alapvető fontosságú tényezői. A tiroxin (T4) pro-hormon, melynek a pajzsmirigyhormon aktiváció során dejodációval T3-á kell alakulnia ahhoz, hogy a pajzsmirigyhormon magreceptorhoz kötődve biológiai hatásait kifejthesse. A T4 aktivációt az agyban a kettes-típusú dejodáz enzim (D2) katalizálja. A pályázat célkitűzései a központi idegrendszeri pajzsmirigyhormon aktiváció celluláris és molekuláris vonatkozásainak vizsgálatához kapcsolódtak. Az alacsony T3 szindróma molekuláris pathogenezisének vizsgálata során feltártuk az emberi dio2 gén NF-kB érzékenységét és leírtuk annak molekuláris hátterét. Jellemeztük a D2 fehérje ubikvitinációjában kulcsszerepet játszó fehérjék idegrendszeri eloszlását, továbbá a D2 enzim poszt-transzlációs szabályozásának szubcelluláris és konformációs feltételeit. Vizsgálataink során tanulmányoztuk a T3 képződés mechanizmusát a fejlődő idegrendszerben és kimutattuk a D2 expresszió megjelenését a fejlődő hipotalamusz tanicitáiban. Munkánk az idegrendszeri T3 függő génexpressziós profilok kialakulásért felelős szabályozó mechanizmusok feltérképezése révén járul hozzá a fejlődő és kifejlett agy működését befolyásoló tényezők jobb megértéséhez. | Thyroid hormone is a crucial factor of development, growth and metabolism. The pro-hormone thyroxine (T4) has to be converted to T3 via thyroid hormone activation in order to bind the thyroid hormone receptor to modulate thyroid hormone dependent pathways. Local T3 generation in the adult and developing brain is catalyzed by the type 2 deiodinase selenoenzyme (D2). The aim of the project was to better understand the cellular and molecular events underlying thyroid hormone activation in the central nervous system. We studied the molecular pathogenesis of the low T3 syndrome and described the molecular components ensuring NF-kB mediated induction of the human dio2 gene. We characterized the distribution of key proteins of D2 ubiquitination in the brain and determined the subcellular and conformational requirements of post-translational regulation of the D2 enzyme. We studied the mechanisms of T3 generation in the developing brain and determined the developmental expression of D2 in hypothalamic tanycytes. Our data contribute to the better understanding of factors regulating the function of the developing and adult brain via providing a deeper insight into mechanisms generating T3 dependent gene expression profiles

Scope and limitations of iodothyronine deiodinases in hypothyroidism.

The coordinated expression and activity of the iodothyronine deiodinases regulate thyroid hormone levels in hypothyroidism. Once heralded as the pathway underpinning adequate thyroid-hormone replacement therapy with levothyroxine, the role of these enzymes has come into question as they have been implicated in both an inability to normalize serum levels of tri-iodothyronine (T3) and the incomplete resolution of hypothyroid symptoms. These observations, some of which were validated in animal models of levothyroxine monotherapy, challenge the paradigm that tissue levels of T3 and thyroid-hormone signalling can be fully restored by administration of levothyroxine alone. The low serum levels of T3 observed among patients receiving levothyroxine monotherapy occur as a consequence of type 2 iodothyronine deiodinase (DIO2) in the hypothalamus being fairly insensitive to ubiquitination. In addition, residual symptoms of hypothyroidism have been linked to a prevalent polymorphism in the DIO2 gene that might be a risk factor for neurodegenerative disease. Here, we discuss how these novel findings underscore the clinical importance of iodothyronine deiodinases in hypothyroidism and how an improved understanding of these enzymes might translate to therapeutic advances in the care of millions of patients with this condition

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). Messenger RNA levels were studied by in situ hybridization and quantitative PCR, and protein levels by immunofluorescence in both the rat and mouse forebrain. The mRNA of both transporters decreased robustly in the first 9h after LPS injection, selectively in brain blood vessels; OATP1c1 mRNA in astrocytes and MCT8 mRNA in neurons remained unchanged. At 24 and/or 48h after LPS administration, OATP1c1 and MCT8 mRNAs increased markedly above control levels in brain vessels. OATP1c1 protein decreased markedly in vessels by 24h, 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

Prevalent Polymorphism in Thyroid Hormone-Activating Enzyme Leaves a Genetic Fingerprint that Underlies Associated Clinical Syndromes

Context: A common polymorphism in the gene encoding the activating deiodinase (Thr92Ala-D2) is known to be associated with quality of life in millions of patients with hypothyroidism and with several organ-specific conditions. This polymorphism results in a single amino acid change within the D2 molecule where its susceptibility to ubiquitination and proteasomal degradation is regulated. Objective: To define the molecular mechanisms underlying associated conditions in carriers of the Thr92Ala-D2 polymorphism. Design, Setting, Patients: Microarray analyses of nineteen postmortem human cerebral cortex samples were performed to establish a foundation for molecular studies via a cell model of HEK-293 cells stably expressing Thr92 or Ala92 D2. Results: The cerebral cortex of Thr92Ala-D2 carriers exhibits a transcriptional fingerprint that includes sets of genes involved in CNS diseases, ubiquitin, mitochondrial dysfunction (chromosomal genes encoding mitochondrial proteins), inflammation, apoptosis, DNA repair and growth factor signaling. Similar findings were made in Ala92-D2-expressing HEK-293 cells and in both cases there was no evidence that thyroid hormone signaling was affected, i.e. the expression level of T3-responsive genes was unchanged, but that several other genes were differentially regulated. The combined microarray analyses (brain/cells) led to the development of an 81-gene classifier that correctly predicts the genotype of homozygous brain samples. In contrast to Thr92-D2, Ala92-D2 exhibits longer half-life and was consistently found in the Golgi. A number of Golgi-related genes were down-regulated in Ala92-D2-expressing cells but were normalized after 24h-treatment with the antioxidant N-acetylecysteine. Conclusions: Ala92-D2 accumulates in the Golgi, where its presence and/or ensuing oxidative stress disrupts basic cellular functions and increases pre-apoptosis. These findings are reminiscent to disease mechanisms observed in other neurodegenerative disorders such as Huntington's disease, and could contribute to the unresolved neurocognitive symptoms of affected carriers

Thyrotropin-Releasing-Hormone-Synthesizing Neurons of the Hypothalamic Paraventricular Nucleus are Inhibited by Glycinergic Inputs

Glycine is a classical neurotransmitter that has role in both inhibitory and excitatory synapses. To understand whether glycinergic inputs are involved in the regulation of the hypophysiotropic thyrotropin-releasing hormone (TRH) neurons, the central controllers of the hypothalamic-pituitary-thyroid axis, the glycinergic innervation of the TRH neurons was studied in the hypothalamic paraventricular nucleus (PVN). Methods: Double-labeling immunocytochemistry and patch-clamp electrophysiology were used to determine the role of glycinergic neurons in the regulation of TRH neurons in the PVN. Anterograde and retrograde tracing methods were used to determine the sources of the glycinergic input of TRH neurons. Results: Glycine transporter-2 (GLYT2), a marker of glycinergic neurons, containing axons were found to establish symmetric type of synapses on TRH neurons in the PVN. Furthermore, glycine receptor immunoreactivity was observed in these TRH neurons. The raphe magnus (RMg) and the ventrolateral periaqueductal gray (VLPAG) were found to be the exclusive sources of the glycinergic innervation of the TRH neurons within the PVN. Patch-clamp electrophysiology using sections of TRH-IRES-tdTomato mice showed that glycine hyperpolarized the TRH neurons and completely blocked the firing of these neurons. Glycine also markedly hyperpolarized the TRH neurons in the presence of tetrodotoxin demonstrating the direct effect of glycine. In more than 60% of the TRH neurons, spontaneous inhibitory postsynaptic currents (sIPSCs) were observed, even after the pharmacological inhibition of glutamatergic and GABAergic neuronal transmission. The glycine antagonist, strychnine, almost completely abolished these sIPSCs, demonstrating the inhibitory nature of the glycinergic input of TRH neurons. Conclusions: These data demonstrate that TRH neurons in the PVN receive glycinergic inputs from the RMg and the VLPAG. The symmetric type of synaptic connection and the results of the electrophysiological experiments demonstrate the inhibitory nature of these inputs