Elevated Serum T3 Levels Research Articles

Towards a treatment for thyroid hormone transporter MCT8 deficiency: achievements and challenges.

Patients with an inactive thyroid hormone (TH) transporter MCT8 (Allan-Herndon-Dudley Syndrome, AHDS) display severe neurological impairments and motor disabilities indicating an indispensable function of MCT8 in facilitating TH access to the human brain. As a consequence, the CNS of AHDS patients appears to be in a TH deficient state which greatly compromises proper neural development and function. As another hallmark of this disease, patients exhibit elevated serum T3 levels leading to a hyperthyroid situation in peripheral tissues. Several treatment strategies have been developed and evaluated in preclinical mouse models as well as in patients. Here, we discuss these different therapeutic approaches to overcome MCT8 deficiency and summarize the current achievements and challenges in improving brain maturation in the absence of MCT8.

Abstract 127: Pituitary resistance to thyroid hormone due to a novel mutation in the NCOR2 gene: The first case report

Background: Pituitary resistance to thyroid hormone-β (RTH-β) presents with the symptoms of hyperthyroidism with inappropriate secretion of thyroid stimulating hormone (TSH). Mutation in the coding region of the TR-β gene is not observed in 10% of cases, which are termed as non-TRβ RTH. Till date, the specific contributions of co-repressors and co-activators in pituitary resistance to thyroid hormone remain unknown.Materials and Methods: We present a case of 44-year male with pituitary RTH, showing features suggestive of thyrotoxicosis, atrial fibrillation and goiter. Clinical investigations included estimation of serum FT4, T3, TSH, TRab, TPO antibodies, Tc-thyroid scan, MRI for sella. To understand the genetic cause of the disorder, we undertook the whole genome sequencing (WES) in the proband and family members, which was followed by Sanger's sequencing for confirmation. To validate the possibility of RTH due to the novel gene discovered, we undertook in-vitro functional studies using NCOR2 siRNA.Results: A 44 year old male presented with atrial fibrillation with thyrotoxicosis and elevated serum T4, FT4, and T3, and nonsuppresed TSH levels (serum total T4 24.50 ug/dl, total T3 5.22 nmol/l and TSH 5.50 uIU/ml. On treatment with PTU, he had serum TSH 16.24 uIU/ml, FT4 50 nmol/l, and total T3 6.61 nmol/l. Further clinical workup by 99mTc pertechnetate thyroid scan showed bilateral diffusely increased uptake, negative serum TRAb and TPO antibody. A differential diagnosis of RTH or TSH secreting pituitary adenoma was made. Gadolinium enhanced MRI of sella was normal. We observed loss of inverse correlation of serum T3 and TSH levels, signifying the loss of negative feedback mechanism. WES identified two novel mutations in the NCOR2 gene (p.Ser2304Gly, p.Ala2489Thr) in the proband. Both the mutation (p.Ala2489Thr) were confirmed in the proband and one of his children. Bioinformatics analysis suggested p.Ala2489Thr to be pathogenic. Functional studies in GH3 cell lines using NCOR2 siRNA revealed NCOR2 dependent regulation of TSH levels, further strengthening that NCOR2 mutations could cause RTH.Conclusion: We hereby describe the first case of pituitary RTH due to a novel mutation in the NCOR2 gene in a family with thyrotoxicosis.

Heterogeneous Clinical Characteristics of Allan-Herndon-Dudley Syndrome with SLC16A2 Mutations

Purpose: The purpose of this study was to expand our understanding of phenotypic and genetic variation in Allan-Herndon-Dudley syndrome (AHDS), which is a rare X-linked mental retardation syndrome characterized by hypotonia, generalized spasticity, and moderate-to-severe psychomotor retardation. AHDS is caused by a mutation of solute carrier family 16 member 2 (SLC16A2), which encodes monocarboxylate transporter 8 (MCT8), the transporter of triiodothyronine (T3) into neurons. Methods: We enrolled nine patients with AHDS from unrelated families, except for two patients who were cousins, through a retrospective chart review. Clinical features, brain imaging, electroencephalograms, thyroid hormone profiles, and genetic data were reviewed retrospectively and compared with previously reported cases. Results: We found three novel and five previously reported pathogenic variants in nine patients from eight families. All patients presented with hypotonia, spasticity, severe developmental delay, and elevated serum T3 levels. Cataplexy, which is a previously unreported phenotype, was found in two patients with the same mutation. In our cohort, seizures were uncommon (n=1) but intractable. Conclusion: This study broadens the known phenotypic variations of AHDS, ranging from relatively mild global developmental delay to a severe form of encephalopathy with hypotonia, spasticity, and no acquisition of independent sitting. The syndromic classification or genetic etiology of global developmental delay is extremely heterogeneous; therefore, early clinical suspicion is challenging for clinicians. However, severe mental retardation with hypotonia, spasticity, and elevated serum T3 levels in male patients is a highly suspicious clinical clue for the early diagnosis of AHDS.

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.

Tissue-Specific Function of Thyroid Hormone Transporters: New Insights from Mouse Models.

Thyroid hormone (TH) transporters are required for cellular transmembrane passage of TH and are thus mandatory for proper TH metabolism and action. Consequently, inactivating mutations in TH transporters such as MCT8 or OATP1C1 can cause tissue- specific changes in TH homeostasis. As the most prominent example, patients with MCT8 mutations exhibit elevated serum T3 levels, whereas their CNS appear to be in a TH deficient state. Here, we will briefly summarize recent studies of mice lacking Mct8 alone or in combination with the TH transporters Mct10 or Oatp1c1 that shed light on many aspects and pathogenic events underlying global MCT8 deficiency and also underscore the contribution of Mct10 and Oatp1c1 in tissue-specific TH transport processes. Moreover, development of conditional knock-out mice that allow a cell-specific inactivation of TH transporters in distinct tissues, disclosed cell-specific changes in TH signaling, thereby highlighting the pathophysiological significance of local control of TH action.

Functional analysis of monocarboxylate transporter 8 mutations in Japanese Allan-Herndon-Dudley syndrome patients

Monocarboxylate transporter 8 (MCT8) facilitates T3 uptake into cells. Mutations in MCT8 lead to Allan-Herndon-Dudley syndrome (AHDS), which is characterized by severe psychomotor retardation and abnormal thyroid hormone profile. Nine uncharacterized MCT8 mutations in Japanese patients with severe neurocognitive impairment and elevated serum T3 levels were studied regarding the transport of T3. Human MCT8 (hMCT8) function was studied in wild-type (WT) or mutant hMCT8-transfected human placental choriocarcinoma cells (JEG3) by visualizing the locations of the proteins in the cells, detecting specific proteins, and measuring T3 uptake. We identified 6 missense (p.Arg445Ser, p.Asp498Asn, p.Gly276Arg, p.Gly196Glu, p.Gly401Arg, and p.Gly312Arg), 2 frameshift (p.Arg355Profs*64 and p.Tyr550Serfs*17), and 1 deletion (p.Pro561del) mutation(s) in the hMCT8 gene. All patients exhibited clinical characteristics of AHDS with high free T3, low-normal free T4, and normal-elevated TSH levels. All tested mutants were expressed at the protein level, except p.Arg355Profs*64 and p.Tyr550Serfs*17, which were truncated, and were inactive in T3 uptake, excluding p.Arg445Ser and p.Pro561del mutants, compared with WT-hMCT8. Immunocytochemistry revealed plasma membrane localization of p.Arg445Ser and p.Pro561del mutants similar with WT-hMCT8. The other mutants failed to localize in significant amount(s) in the plasma membrane and instead localized in the cytoplasm. These data indicate that p.Arg445Ser and p.Pro561del mutants preserve residual function, whereas p.Asp498Asn, p.Gly276Arg, p.Gly196Glu, p.Gly401Arg, p.Gly312Arg, p.Arg355Profs*64, and p.Tyr550Serfs*17 mutants lack function. These findings suggest that the mutations in MCT8 cause loss of function by reducing protein expression, impairing trafficking of protein to plasma membrane, and disrupting substrate channel.

In vitro and mouse studies support therapeutic utility of triiodothyroacetic acid in MCT8 deficiency

MCT8 transports thyroid hormone (TH) across the plasma membrane. Mutations in MCT8 result in the Allan-Herndon-Dudley syndrome (AHDS), comprising severe psychomotor retardation and elevated serum T3 levels. As the neurological symptoms are most likely caused by a lack of TH transport into the CNS, the administration of a TH analogue which does not require MCT8 for cellular uptake may represent a therapeutic strategy. Here, we investigated the therapeutic potential of the biologically active T3 metabolite Triac (TA3) by studying TA3 transport, metabolism and action both in vitro and in vivo. Incubation of SH-SY5Y neuroblastoma cells and MO3.13 oligodendrocytes with labeled substrates showed a time-dependent uptake of T3 and TA3. In intact SH-SY5Y cells, both T3 and TA3 were degraded by endogenous type 3 deiodinase, and they influenced gene expression to a similar extent. Fibroblasts from MCT8 patients showed an impaired T3 uptake compared to controls, whereas TA3 uptake was similar in patient and control fibroblasts. In transfected cells, TA3 did not show significant transport by MCT8. Most importantly, treatment of athyroid Pax8 knockout mice and Mct8/Oatp1c1 double knockout mice between postnatal day 1 and 12 with TA3 restored T3-dependent neural differentiation in the cerebral and cerebellar cortex indicating that TA3 can replace T3 in promoting brain development. In conclusion, we demonstrated uptake of TA3 in neuronal cells and in fibroblasts of MCT8 patients, and similar gene responses to T3 and TA3. This indicates that TA3 bypasses MCT8 and may be used to improve the neural status of MCT8 patients.

In vitro and mouse studies supporting therapeutic utility of triiodothyroacetic acid in MCT8 deficiency.

Monocarboxylate transporter 8 (MCT8) transports thyroid hormone (TH) across the plasma membrane. Mutations in MCT8 result in the Allan-Herndon-Dudley syndrome, comprising severe psychomotor retardation and elevated serum T3 levels. Because the neurological symptoms are most likely caused by a lack of TH transport into the central nervous system, the administration of a TH analog that does not require MCT8 for cellular uptake may represent a therapeutic strategy. Here, we investigated the therapeutic potential of the biologically active T3 metabolite Triac (TA3) by studying TA3 transport, metabolism, and action both in vitro and in vivo. Incubation of SH-SY5Y neuroblastoma cells and MO3.13 oligodendrocytes with labeled substrates showed a time-dependent uptake of T3 and TA3. In intact SH-SY5Y cells, both T3 and TA3 were degraded by endogenous type 3 deiodinase, and they influenced gene expression to a similar extent. Fibroblasts from MCT8 patients showed an impaired T3 uptake compared with controls, whereas TA3 uptake was similar in patient and control fibroblasts. In transfected cells, TA3 did not show significant transport by MCT8. Most importantly, treatment of athyroid Pax8-knockout mice and Mct8/Oatp1c1-double knockout mice between postnatal days 1 and 12 with TA3 restored T3-dependent neural differentiation in the cerebral and cerebellar cortex, indicating that TA3 can replace T3 in promoting brain development. In conclusion, we demonstrated uptake of TA3 in neuronal cells and in fibroblasts of MCT8 patients and similar gene responses to T3 and TA3. This indicates that TA3 bypasses MCT8 and may be used to improve the neural status of MCT8 patients.

Status of Oral Ulcerative Mucositis and Biomarkers to Monitor Posttraumatic Stress Disorder Effects in Breast Cancer Patients

This study was designed to assess oral ulcerative mucositis, C-reactive protein, blood pressure, heart rate and thyroid function in breast cancer patients in relation to the occurrence of posttraumatic stress disorder (PTSD). A total of 120 female breast cancer patients and women 100 healthy subjects were enrolled in this study. PTSD status was assessed by questionnaire. Before and after treatment (modified radical mastectomy and chemotherapy), serum samples were collected and measured for levels of triiodothyronine (T3), thyroxine (T4), thyroid stimulating hormone (TSH) and high-sensitivity C-reactive protein (hs-CRP) by ELISA. Oral ulcerative mucositis was evaluated by the number and duration of oral ulcers and the degree of pain. Breast cancer patients experienced long-term PTSD and had elevated serum T3 and T4 levels. Patients experienced more severe pain and longer duration of oral ulcers compared with the healthy group. Oral ulcers were significantly associated with PTSD score in terms of the number of ulcers (p=0.0025), the degree of pain (p<0.0001) and the duration of ulcers (p<0.0001). These findings support that thyroid function is altered in breast cancer patients with PTSD. Elevation of T3 and T4 and oral ulcerative mucositis might be indicative of the emotional status of breast cancer patients.

Type 3 Deiodinase Deficiency Causes Spatial and Temporal Alterations in Brain T3 Signaling that Are Dissociated from Serum Thyroid Hormone Levels

The type 3 deiodinase (D3) is an enzyme that inactivates thyroid hormones (TH) and is highly expressed during development and in the central nervous system. D3-deficient (D3KO) mice develop markedly elevated serum T(3) level in the perinatal period. In adulthood, circulating T(4) and T(3) levels are reduced due to functional deficits in the thyroid axis and peripheral tissues (i.e. liver) show evidence of decreased TH action. Given the importance of TH for brain development, we aimed to assess TH action in the brain of D3KO mice at different developmental stages and determine to what extent it correlates with serum TH parameters. We used a transgenic mouse model (FINDT3) that expresses the reporter gene β-galactosidase (β-gal) in the central nervous system as a readout of local TH availability. Together with experiments determining expression levels of TH-regulated genes, our results show that after a state of thyrotoxicosis in early development, most regions of the D3KO brain show evidence of decreased TH action at weaning age. However, later in adulthood and in old age, the brain again manifests a thyrotoxic state, despite reduced serum TH levels. These region-specific changes in brain TH status during the life span of the animal provide novel insight into the important role of the D3 in the developing and adult brain. Our results suggest that, even if serum concentrations of TH are normal or low, impaired D3 activity may result in excessive TH action in multiple brain regions, with potential consequences of altered neural function that may be of clinical relevance to neurological and neuroendocrine disorders.

Elevated serum triiodothyronine and intellectual and motor disability with paroxysmal dyskinesia caused by a monocarboxylate transporter 8 gene mutation

Monocarboxylate transporter 8 (MCT8 or SLC16A2) is important for the neuronal uptake of triiodothyronine (T3) in its function as a specific and active transporter of thyroid hormones across the cell membrane, thus being essential for human brain development. We report on a German male with Allan-Herndon-Dudley syndrome presenting with severe intellectual and motor disability, paroxysmal dyskinesia combined with truncal muscular hypotonia, and peripheral muscular hypertonia at his current age of 9 years. Additionally, the patient has a lesion in the left putamen region revealed by magnetic resonance imaging and elevated serum T3 levels. The male appeared to have a hemizygous mutation (R271H) in the MCT8 gene that was sequenced directly from genomic DNA and occurred de novo in the maternal germline, as both his mother and his sister were not carriers of the mutation. Ruling out a common polymorphism, 50 normal individuals of the same ethnic background did not harbour the mutation. The identified MCT8 gene mutation (R271H) is very likely to be the genetic cause for neuronal hypothyroidism despite elevated serum T3 levels.

Pathophysiological Importance of Thyroid Hormone Transporters

Thyroid hormone metabolism and action are largely intracellular events that require transport of iodothyronines across the plasma membrane. It has been assumed for a long time that this occurs by passive diffusion, but it has become increasingly clear that cellular uptake and efflux of thyroid hormone is mediated by transporter proteins. Recently, several active and specific thyroid hormone transporters have been identified, including monocarboxylate transporter 8 (MCT8), MCT10, and organic anion transporting polypeptide 1C1 (OATP1C1). The latter is expressed predominantly in brain capillaries and transports preferentially T(4), whereas MCT8 and MCT10 are expressed in multiple tissues and are capable of transporting different iodothyronines. The pathophysiological importance of thyroid hormone transporters has been established by the demonstration of MCT8 mutations in patients with severe psychomotor retardation and elevated serum T(3) levels. MCT8 appears to play an important role in the transport of thyroid hormone in the brain, which is essential for the crucial action of the hormone during brain development. It is expected that more specific thyroid hormone transporters will be discovered in the near future, which will lead to a better understanding of the tissue-specific regulation of thyroid hormone bioavailability.

ATHEROSCLEROSIS RISK IN SUBCLINIC HYPOTHYROIDISM

Thyroid hormone (TH) is essential for the proper development of numerous tissues, notably the brain. TH acts mostly intracellularly, which requires transport by TH transporters across the plasma membrane. Although several transporter families have been identified, only monocarboxylate transporter (MCT)8, MCT10 and organic anion-transporting polypeptide (OATP)1C1 demonstrate a high degree of specificity towards TH. Recently, the biological importance of MCT8 has been elucidated. Mutations in MCT8 are associated with elevated serum T3 levels and severe psychomotor retardation, indicating a pivotal role for MCT8 in brain development. MCT8 knockout mice lack neurological damage, but mimic TH abnormalities of MCT8 patients. The exact pathophysiological mechanisms in MCT8 patients remain to be elucidated fully. Future research will probably identify novel TH transporters and disorders based on TH transporter defects.

MCT8 mutation analysis and identification of the first female with Allan–Herndon–Dudley syndrome due to loss of MCT8 expression

Mutations in the thyroid monocarboxylate transporter 8 gene (MCT8/SLC16A2) have been reported to result in X-linked mental retardation (XLMR) in patients with clinical features of the Allan-Herndon-Dudley syndrome (AHDS). We performed MCT8 mutation analysis including 13 XLMR families with LOD scores >2.0, 401 male MR sibships and 47 sporadic male patients with AHDS-like clinical features. One nonsense mutation (c.629insA) and two missense changes (c.1A>T and c.1673G>A) were identified. Consistent with previous reports on MCT8 missense changes, the patient with c.1673G>A showed elevated serum T3 level. The c.1A>T change in another patient affects a putative translation start codon, but the same change was present in his healthy brother. In addition normal serum T3 levels were present, suggesting that the c.1A>T (NM_006517) variation is not responsible for the MR phenotype but indicates that MCT8 translation likely starts with a methionine at position p.75. Moreover, we characterized a de novo translocation t(X;9)(q13.2;p24) in a female patient with full blown AHDS clinical features including elevated serum T3 levels. The MCT8 gene was disrupted at the X-breakpoint. A complete loss of MCT8 expression was observed in a fibroblast cell-line derived from this patient because of unfavorable nonrandom X-inactivation. Taken together, these data indicate that MCT8 mutations are not common in non-AHDS MR patients yet they support that elevated serum T3 levels can be indicative for AHDS and that AHDS clinical features can be present in female MCT8 mutation carriers whenever there is unfavorable nonrandom X-inactivation.

Thyroid hormone transporters in the brain

Thyroid hormone plays an essential role in proper mammalian development of the central nervous system and peripheral tissues. Lack of sufficient thyroid hormone results in abnormal development of virtually all organ systems, a syndrome termed cretinism. In particular, hypothyroidism in the neonatal period causes serious damage to neural cells and leads to mental retardation. Although thyroxine is the major product secreted by the thyroid follicular cells, the action of thyroid hormone is mediated mainly through the deiodination of T(4) to the biologically active form 3,3', 5-triiodo-L-thyronine, followed by the binding of T(3) to a specific nuclear receptor. Before reaching the intracellular targets, thyroid hormone must cross the plasma membrane. Because of the lipophilic nature of thyroid hormone, it was thought that they traversed the plasma membrane by simple diffusion. However, in the past decade, a membrane transport system for thyroid hormone has been postulated to exist in various tissues. Several classes of transporters, organic anion transporter polypeptide (oatp) family, Na(+)/Taurocholate cotransporting polypeptide (ntcp) and amino acid transporters have been reported to transport thyroid hormones. Monocarboxylate transporter8 (MCT8) has recently been identified as an active and specific thyroid hormone transporter. Mutations in MCT8 are associated with severe X-linked psycomotor retardation and strongly elevated serum T3 levels in young male patients. Several other molecules should be contributed to exert the role of thyroid hormone in the central nervous system.

Genotype-Phenotype Relationship in Patients with Mutations in Thyroid Hormone Transporter MCT8

Loss-of-function mutations in thyroid hormone transporter monocarboxylate transporter 8 (MCT8) lead to severe X-linked psychomotor retardation and elevated serum T(3) levels. Most patients, for example those with mutations V235M, S448X, insI189, or delF230, cannot stand, walk, or speak. Patients with mutations L434W, L568P, and S194F, however, walk independently and/or develop some dysarthric speech. To study the relationship between mutation and phenotype, we transfected JEG3 and COS1 cells with wild-type or mutant MCT8. Expression and function of the transporter were studied by analyzing T(3) and T(4) uptake, T(3) metabolism (by cotransfected type 3 deiodinase), Western blotting, affinity labeling with N-bromoacetyl-T(3), immunocytochemistry, and quantitative RT-PCR. Wild-type MCT8 increased T(3) uptake and metabolism about 5-fold compared with empty vector controls. Mutants V235M, S448X, insI189, and delF230 did not significantly increase transport. However, S194F, L568P, and L434W showed about 20, 23, and 37% of wild-type activity. RT-PCR did not show significant differences in mRNA expression between wild-type and mutant MCT8. Immunocytochemistry detected the nonfunctional mutants V235M, insI189, and delF230 mostly in the cytoplasm, whereas mutants with residual function were expressed at the plasma membrane. Mutants S194F and L434W showed high protein expression but low affinity for N-bromoacetyl-T(3); L568P was detected in low amounts but showed relatively high affinity. Mutations in MCT8 cause loss of function through reduced protein expression, impaired trafficking to the plasma membrane, or reduced substrate affinity. Mutants L434W, L568P, and S194F showed significant residual transport capacity, which may underlie the more advanced psychomotor development observed in patients with these mutations.

Functional Analysis of Monocarboxylate Transporter 8 Mutations Identified in Patients with X-Linked Psychomotor Retardation and Elevated Serum Triiodothyronine

T(3) action in neurons is essential for brain development. Recent evidence indicates that monocarboxylate transporter 8 (MCT8) is important for neuronal T(3) uptake. Hemizygous mutations have been identified in the X-linked MCT8 gene in boys with severe psychomotor retardation and elevated serum T(3) levels. The objective of this study was to determine the functional consequences of MCT8 mutations regarding transport of T(3). MCT8 function was studied in wild-type or mutant MCT8-transfected JEG3 cells by analyzing: 1) T(3) uptake, 2) T(3) metabolism in cells cotransfected with human type 3 deiodinase, 3) immunoblotting, and 4) immunocytochemistry. The mutations identified in MCT8 comprise four deletions (24.5 kb, 2.4 kb, 14 bp, and 3 bp), three missense mutations (Ala224Val, Arg271His, and Leu471Pro), a nonsense mutation (Arg245stop), and a splice site mutation (94 amino acid deletion). All tested mutants were inactive in uptake and metabolism assays, except MCT8 Arg271His, which showed approximately 20% activity vs. wild-type MCT8. These findings support the hypothesis that the severe psychomotor retardation and elevated serum T(3) levels in these patients are caused by inactivation of the MCT8 transporter, preventing action and metabolism of T(3) in central neurons.

Type 3 deiodinase is critical for the maturation and function of the thyroid axis

Developmental exposure to appropriate levels of thyroid hormones (THs) in a timely manner is critical to normal development in vertebrates. Among the factors potentially affecting perinatal exposure of tissues to THs is type 3 deiodinase (D3). This enzyme degrades THs and is highly expressed in the pregnant uterus, placenta, and fetal and neonatal tissues. To determine the physiological role of D3, we have generated a mouse D3 knockout model (D3KO) by a targeted inactivating mutation of the Dio3 gene in mouse ES cells. Early in life, D3KO mice exhibit delayed 3,5,3'-triiodothyronine (T3) clearance, a markedly elevated serum T3 level, and overexpression of T3-inducible genes in the brain. From postnatal day 15 to adulthood, D3KO mice demonstrate central hypothyroidism, with low serum levels of 3,5,3',5'-tetraiodothyronine (T4) and T3, and modest or no increase in thyroid-stimulating hormone (TSH) concentration. Peripheral tissues are also hypothyroid. Hypothalamic T3 content is decreased while thyrotropin-releasing hormone (TRH) expression is elevated. Our results demonstrate that the lack of D3 function results in neonatal thyrotoxicosis followed later by central hypothyroidism that persists throughout life. These mice provide a new model of central hypothyroidism and reveal a critical role for D3 in the maturation and function of the thyroid axis.

Thyroid hormone transporters

Thyroid hormone is important for development of various tissues, in particular brain, and for regulation of metabolic processes throughout life. The follicular cells of the thyroid gland produce predominantly T4 (thyroxine), but the biological activity of thyroid hormone is largely exerted by T3 (3,3',5-tri-iodothyronine). The deiodinases involved in T4-to-T3 conversion or T4 and T3 degradation, as well as the T3 receptors, are located intracellularly. Therefore the action and metabolism of thyroid hormone require transport of iodothyronines across the cell membrane via specific transporters. Recently, a number of transporters capable of cellular uptake of iodothyronines have been identified. The most specific transporters identified so far are OATP1C1 and MCT8, which appear to be involved in T4 transport across the blood-brain barrier, and in T3 transport into brain neurons, respectively. The MCT8 gene is located on human chromosome Xq13, and mutations in MCT8 are associated with X-linked severe psychomotor retardation and elevated serum T3 levels.

Type 1 Iodothyronine Deiodinase Is a Sensitive Marker of Peripheral Thyroid Status in the Mouse

Mice with one thyroid hormone receptor (TR) alpha-1 allele encoding a dominant negative mutant receptor (TR alpha1(PV/+)) have persistently elevated serum T3 levels (1.9-fold above normal). They also have markedly increased hepatic type 1 iodothyronine deiodinase (D1) mRNA and enzyme activity (4- to 5-fold), whereas other hepatic T3-responsive genes, such as Spot14 and mitochondrial alpha-glycerol phosphate dehydrogenase (alpha-GPD), are only 0.7-fold and 1.7-fold that of wild-type littermates (TR alpha1+/+). To determine the cause of the disproportionate elevation of D1, TR alpha1+/+ and TR alpha1(PV/+) mice were rendered hypothyroid and then treated with T3. Hypothyroidism decreased hepatic D1, Spot14, and alpha-GPD mRNA to similar levels in TR alpha1+/+ and TR alpha1(PV/+) mice, whereas T3 administration caused an approximately 175-fold elevation of D1 mRNA but only a 3- to 6-fold increases in Spot14 and alpha-GPD mRNAs. Interestingly, the hypothyroidism-induced increase in cerebrocortical type 2 iodothyronine deiodinase activity was 3 times greater in the TR alpha1(PV/+) mice, and these mice had no T3-dependent induction of type 3 iodothyronine deiodinase. Thus, the marked responsiveness of hepatic D1 to T3 relative to other genes, such as Spot14 and alpha-GPD, explains the relatively large effect of the modest increase in serum T3 in the TR alpha1(PV/+) mice, and TR alpha plays a key role in T3-dependent positive and negative regulation of the deiodinases in the cerebral cortex.