Mechanisms of disease: psychomotor retardation and high T3 levels caused by mutations in monocarboxylate transporter 8

Evidence for thyroid hormone regulation of amygdala-dependent fear-relevant memory and plasticity

The amygdala is an established site for fear memory formation, and clinical studies suggest involvement of hormone signaling cascades in development of trauma-related disorders. While an association of thyroid hormone (TH) status and mood disorders is established, the related brain-based mechanisms and the role of TH in anxiety disorders are unknown. Here we examine the role that TH receptor (TR, a nuclear transcriptional repressor when unbound and a transcriptional activator when bound to TH) may have in mediating the initial formation of fear memories in the amygdala. We identified mRNA levels of TR and other TH pathway regulatory genes, including thyrotropin-releasing hormone (Trh), transthyretin (Ttr), thyrotropin-releasing hormone receptor (Trhr), type 2 iodothyronine deiodinase (Dio2), mediator complex subunit 12 (Med12/Trap230) and retinoid X receptor gamma (Rxrg) to be altered in the amygdala following Pavlovian fear conditioning. Using TH agonist and antagonist infusion into the amygdala, we demonstrated that this pathway is both necessary and sufficient for fear memory consolidation. Inhibition of TH signaling with the TR antagonist 1-850 decreased fear memory consolidation; while activation of TR with T3 (triiodothyronine) resulted in increased memory formation. Using a systemic hypothyroid mouse model, we found that intra-amygdala infusions of T3 were sufficient to rescue deficits in fear memory. Finally, we demonstrated that T3 was sufficient to activate TR-specific gene pathways in the amygdala. These findings on the role of activity-dependent TR modulation support a model in which local TH is a critical regulator of fear memory-related plasticity in the amygdala.

Identifying elevated plasma free triiodothyronine levels: age-adapted reference intervals for pediatrics

OBJECTIVES

Elevated free T3 (FT3) is an important feature for the early diagnosis of several diseases among which Grave's disease or Allan-Hernon-Dudley syndrome. However, there is a lack of age-adapted reference intervals for plasma thyroid hormones in children. We conducted a study to define reference values of peripheral FT3 in children using a commonly used automated immunoassay.

METHODS

All thyroid function test (TFT) results from our lab collected during 9 months were extracted anonymously, and reference intervals establishment followed recommendations validated by International Federation of Clinical Chemistry (IFCC).

RESULTS

We defined five reference intervals covering the whole pediatric period. Overall, 26.1% of peripheral FT3 measured in children with normal TSH are out of the adult reference range, and 22.2% are upper it leading to misinterpretation. In a 9-month old patient with severe neurodevelopmental disorders, a pathological elevated FT3 has been securely interpreted using the newly established interval.

CONCLUSIONS

The study highlights the poor relevance of adult intervals in pediatric cares, as it confirms that plasmatic FT3 is higher during the whole pediatric period. This work reports useful age-adapted reference intervals for free T3 in pediatrics using a widely used electrochemiluminescent Immunoassay (ECLIA) kit.

A novel variant in SLC16A2 associated with typical Allan-Herndon-Dudley syndrome: a case report

Background

Allan-Herndon-Dudley syndrome (AHDS) is an X-linked recessive neurodegenerative disorder caused by mutations in the SLC16A2 gene that encodes thyroid hormone transporter. AHDS has been rarely reported in China.

Case presentation

This study reported a novel splicing mutation in the SLC16A2 gene in an 18-month-old male patient with AHDS. The patient was born to non-consanguineous, healthy parents of Chinese origin. He passed new-born screening for hypothyroidism, but failed to reach developmental milestones. He presented with hypotonia, severe mental retardation, dysarthria and ataxia. Genetic analysis identified a novel splicing mutation, NM_006517.4: c.431-2 A > G, in the SLC16A2 gene inherited from his mother. The patient received Triac treatment, (triiodothyroacetic acid), a thyroid hormone analogue for 3 months. Triac treatment effectively reduced serum TSH concentrations and normalized serum T3 concentrations in the patient.

Conclusions

This study reported the first case of AHDS treated by Triac in China. And the study expanded the mutational spectrum of the SLC16A2 gene in AHDS patients.

Supplementary information

The online version contains supplementary material available at 10.1186/s12887-022-03259-5.

Functional Characterization of the Novel and Specific Thyroid Hormone Transporter SLC17A4

Background: A recent genome-wide association study identified the SLC17A4 locus associated with circulating free thyroxine (T4) concentrations. Human SLC17A4, being widely expressed in the gastrointestinal tract, was characterized as a novel triiodothyronine (T3) and T4 transporter. However, apart from the cellular uptake of T3 and T4, transporter characteristics are currently unknown. In this study, we delineated basic transporter characteristics of this novel thyroid hormone (TH) transporter. Methods: We performed a broad range of well-established TH transport studies in COS-1 cells transiently overexpressing SLC17A4. We studied cellular TH uptake in various incubation buffers, TH efflux, and the inhibitory effects of different TH metabolites and known inhibitors of other TH transporters on SLC17A4-mediated TH transport. Finally, we determined the effect of tunicamycin, a pharmacological inhibitor of N-linked glycosylation, and targeted mutations in Asn residues on SLC17A4 function. Results: SLC17A4 induced the cellular uptake of T3 and T4 by ∼4 times, and of reverse (r)T3 by 1.5 times over control cells. The uptake of T4 by SLC17A4 was Na⁺ and Cl^- independent, stimulated by low extracellular pH, and reduced by various iodothyronines and metabolites thereof, particularly those that contain at least three iodine moieties irrespective of the presence of modification at the alanine side chain. None of the classical TH transporter inhibitors studied attenuated SLC17A4-mediated TH transport. SLC17A4 also facilitates the efflux of T3 and T4, and to a lesser extent of 3,3'-diiodothyronine (T2). Immunoblot studies on lysates of transfected cells cultured in absence or presence of tunicamycin indicated that SLC17A4 is subject to N-linked glycosylation. Complementary mutational studies identified Asn66, Asn75, and Asn90, which are located in extracellular loop 1, as primary targets. Conclusions: Our studies show that SLC17A4 facilitates the transport of T3 and T4, and less efficiently rT3 and 3,3'-T2. Further studies should reveal the physiological role of SLC17A4 in TH regulation.

Genetic and Neurological Deficiencies in the Visual System of mct8 Mutant Zebrafish

Thyroid hormones (THs; T3 and T4) enter cells using specific transporters and regulate development and metabolism. Mutation in the TH transporter monocarboxylate transporter 8 (MCT8, SLC16A2) is associated with brain hypothyroidism and neurological impairment. We established mct8 mutant (mct8-/-) zebrafish as a model for MCT8 deficiency, which causes endocrinological, neurological, and behavioral alterations. Here, we profiled the transcriptome of mct8-/- larvae. Among hundreds of differentially expressed genes, the expression of a cluster of vision-related genes was distinct. Specifically, the expression of the opsin 1 medium wave sensitive 2 (opn1mw2) decreased in two mct8 mutants: mct8-/- and mct8^-25bp-/- larvae, and under pharmacological inhibition of TH production. Optokinetic reflex (OKR) assays showed a reduction in the number of conjugated eye movements, and live imaging of genetically encoded Ca²⁺ indicator revealed altered neuronal activity in the pretectum area of mct8^-25bp-/- larvae. These results imply that MCT8 and THs regulate the development of the visual system and suggest a mechanism to the deficiencies observed in the visual system of MCT8-deficiency patients.

Characteristics of Allan-Herndon-Dudley Syndrome in Chinese children: Identification of two novel pathogenic variants of the SLC16A2 gene

OBJECTIVE

The aim of this study was to identify causative variants associated with Allan-Herndon-Dudley syndrome (AHDS) in two unrelated Chinese families, and to determine their potential pathogenicity. We also summarized the core clinical symptoms of AHDS by reviewing the related literature.

METHODS

Genomic DNA was isolated from the peripheral blood of AHDS patients and their family members. Whole exome sequencing (WES) was performed on the proband from each family to identify the candidate variants. Subsequently, Sanger sequencing was used to verify the identified candidate variants and to assess co-segregation among the available family members. In silico prediction combined with 3D protein modeling was conducted to predict the functional effects of the variants on the encoded protein.

RESULTS

Two novel hemizygous variants of SLC16A2, c.1111_1112insGTCTTGT (Gly375fs*6) and c.942delA (Val315fs*28), were detected in two patients. We compared the clinical symptoms of the patients with all patients with AHDS reported in China and those reported in the literature. While both our patients presented symptoms mostly consistent with AHDS, Patient 1 had no abnormal brain structure and thyroid function, and yet showed other symptoms including lactic aciduria, conjunctival hyperemia, vomiting, laryngeal stridor, low immunoglobulin and iron levels.

CONCLUSIONS

This study expands the mutation spectrum of AHDS and has clinical value for variant-based prenatal and postnatal screening for this condition. Doctors often have difficulty identifying AHDS by using clinical symptoms. WES can help to identify specific disorder when diagnosis cannot be made based on symptoms alone.

Role of thyroid hormones in normal and abnormal central nervous system myelination in humans and rodents

Thyroid hormones (THs) are instrumental in promoting the molecular mechanisms which underlie the complex nature of neural development and function within the central nervous system (CNS) in vertebrates. The key neurodevelopmental process of myelination is conserved between humans and rodents, of which both experience peak fetal TH concentrations concomitant with onset of myelination. The importance of supplying adequate levels of THs to the myelin producing cells, the oligodendrocytes, for promoting their maturation is crucial for proper neural function. In this review we examine the key TH distributor and transport proteins, including transthyretin (TTR) and monocarboxylate transporter 8 (MCT8), essential for supporting proper oligodendrocyte and myelin health; and discuss disorders with impaired TH signalling in relation to abnormal CNS myelination in humans and rodents. Furthermore, we explore the importance of using novel TH analogues in the treatment of myelination disorders associated with abnormal TH signalling.

Monocarboxylate Transporter 8 Deficiency: From Pathophysiological Understanding to Therapy Development

Genetic defects in the thyroid hormone transporter monocarboxylate transporter 8 (MCT8) result in MCT8 deficiency. This disorder is characterized by a combination of severe intellectual and motor disability, caused by decreased cerebral thyroid hormone signalling, and a chronic thyrotoxic state in peripheral tissues, caused by exposure to elevated serum T3 concentrations. In particular, MCT8 plays a crucial role in the transport of thyroid hormone across the blood-brain-barrier. The life expectancy of patients with MCT8 deficiency is strongly impaired. Absence of head control and being underweight at a young age, which are considered proxies of the severity of the neurocognitive and peripheral phenotype, respectively, are associated with higher mortality rate. The thyroid hormone analogue triiodothyroacetic acid is able to effectively and safely ameliorate the peripheral thyrotoxicosis; its effect on the neurocognitive phenotype is currently under investigation. Other possible therapies are at a pre-clinical stage. This review provides an overview of the current understanding of the physiological role of MCT8 and the pathophysiology, key clinical characteristics and developing treatment options for MCT8 deficiency.

Congenital Hypothyroidism

Congenital hypothyroidism (CH) is the commonest preventable cause of mental retardation in human species. It is so important for clinician to know its etiology epidemiology, clinical manifestation and treatment strategies. Since it is one of the rare serious diseases that should not be diagnosed clinically because late clinical features corresponds to advanced mental retardation, the neonatal screening detection is the best and preferable way of early diagnosis of this congenital disease. Confirmatory laboratory and radiological diagnostic tests should be performed immediately after the positive neonatal screening test. In order to prevent mental defects and to maintain long term clinical as well as biochemical euthyroidism in affected children its diagnosis approach, medical treatment and follow-up should be well established knowledge to all pediatricians during the childhood period and later on to general practitioners when these individuals grow up as adults. Congenital hypothyroidism is a potentially serious disease that we need to emphasize on early detection, using proper diagnostic tools and early and planned therapeutic approach.

Thyroid Hormone Transporters

Thyroid hormone transporters at the plasma membrane govern intracellular bioavailability of thyroid hormone. Monocarboxylate transporter (MCT) 8 and MCT10, organic anion transporting polypeptide (OATP) 1C1, and SLC17A4 are currently known as transporters displaying the highest specificity toward thyroid hormones. Structure-function studies using homology modeling and mutational screens have led to better understanding of the molecular basis of thyroid hormone transport. Mutations in MCT8 and in OATP1C1 have been associated with clinical disorders. Different animal models have provided insight into the functional role of thyroid hormone transporters, in particular MCT8. Different treatment strategies for MCT8 deficiency have been explored, of which thyroid hormone analogue therapy is currently applied in patients. Future studies may reveal the identity of as-yet-undiscovered thyroid hormone transporters. Complementary studies employing animal and human models will provide further insight into the role of transporters in health and disease. (Endocrine Reviews 41: 1 - 55, 2020).

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.

Insights from zebrafish deficiency models to understand the impact of local thyroid hormone regulator action on early development

Thyroid hormones (THs) stimulate and coordinate a wide range of processes to ensure normal development, mainly by binding of the most active TH 3,5,3'-triiodothyronine (T₃) to nuclear receptors resulting in changes in gene transcription. Local TH action is monitored at three distinct levels by different types of regulators: transmembrane transporters (TH influx and efflux), deiodinases (TH activation and inactivation) and nuclear receptors (TH signalling). Since TH regulators are strongly conserved among vertebrate species, the externally and rapidly developing zebrafish (Danio rerio) has become one of the favourite models to study their role in TH-dependent development. Most regulators are expressed in zebrafish from early stages in development in a dynamic and tissue-specific pattern. Transient or permanent disruption of a given regulator severely perturbs development of multiple organs. These zebrafish deficiency models help to explain why, next to overall hypo-/hyperthyroidism, inactivating mutations in the genes encoding TH regulators such as MCT8 and THRA/B have irreversible adverse effects on human development. Zebrafish are also increasingly used as a high-throughput model to assess the toxicity of various xenobiotics and their impact on development. While adverse effects on TH metabolism and gene expression have been shown, information on direct interaction with TH regulators is scarce, albeit essential to fully understand their mechanism of action. For the future, the combination of novel gene silencing tools, fluorescent reporter lines and (single-cell) transcriptomics holds promise for new zebrafish models to further elucidate the role of each TH regulator in vertebrate development.

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.

From zebrafish to human: A comparative approach to elucidate the role of the thyroid hormone transporter MCT8 during brain development

Monocarboxylate transporter 8 (MCT8) facilitates transmembrane transport of thyroid hormones (THs) ensuring their action on gene expression during vertebrate neurodevelopment. A loss of MCT8 in humans results in severe psychomotor deficits associated with the Allan-Herndon-Dudley Syndrome (AHDS). However, where and when exactly a lack of MCT8 causes the neurological manifestations remains unclear because of the varying expression pattern of MCT8 between specific brain regions and cells. Here, we elaborate on the animal models that have been generated to elucidate the mechanisms underlying MCT8-deficient brain development. The absence of a clear neurological phenotype in Mct8 knockout mice made it clear that a single species would not suffice. The evolutionary conservation of TH action on neurodevelopment as well as the components regulating TH signalling however offers the opportunity to answer different aspects of MCT8 function in brain development using different vertebrate species. Moreover, the plethora of tools for genome editing available today facilitates gene silencing in these animals as well. Studies in the recently generated mct8-deficient zebrafish and Mct8/Oatp1c1 double knockout mice have put forward the current paradigm of impaired TH uptake at the level of the blood-brain barrier during peri- and postnatal development as being the main pathophysiological mechanism of AHDS. RNAi vector-based, cell-specific induction of MCT8 knockdown in the chicken embryo points to an additional function of MCT8 at the level of the neural progenitors during early brain development. Future studies including also additional in vivo models like Xenopus or in vitro approaches such as induced pluripotent stem cells will continue to help unravelling the exact role of MCT8 in developmental events. In the end, this multispecies approach will lead to a unifying thesis regarding the cellular and molecular mechanisms responsible for the neurological phenotype in AHDS patients.

Regulatory aspects of the human hypothalamus-pituitary-thyroid axis

Thyroid hormones are essential for growth, differentiation and metabolism during prenatal and postnatal life. The hypothalamus-pituitary-thyroid (HPT)-axis is optimized for these actions. Knowledge of this hormonal axis is derived from decades of experiments in animals and man, and more recently from spontaneous mutations in man and constructed mutations in mice. This review examines the HPT-axis in relation to 24 h TSH profiles in men in various physiological and pathophysiological conditions, including obesity, age, longevity, and primary as well as central hypothyroidism. Hormone rhythms can be analyzed by quantitative methods, e.g. operator-independent deconvolution, approximate entropy and fitting the 24-h component by Cosinor analysis or related procedures. These approaches have identified some of the regulatory components in (patho)physiological conditions.

The transcriptome of the Didelphis virginiana opossum kidney OK proximal tubule cell line

The OK cell line derived from the kidney of a female opossum Didelphis virginiana has proven to be a useful model in which to investigate the unique regulation of ion transport and membrane trafficking mechanisms in the proximal tubule (PT). Sequence data and comparison of the transcriptome of this cell line to eutherian mammal PTs would further broaden the utility of this culture model. However, the genomic sequence for D. virginiana is not available and although a draft genome sequence for the opossum Monodelphis domestica (sequenced in 2012 by the Broad Institute) exists, transcripts sequenced from both species show significant divergence. The M. domestica sequence is not highly annotated, and the majority of transcripts are predicted rather than experimentally validated. Using deep RNA sequencing of the D. virginiana OK cell line, we characterized its transcriptome via de novo transcriptome assembly and alignment to the M. domestica genome. The quality of the de novo assembled transcriptome was assessed by the extent of homology to sequences in nucleotide and protein databases. Gene expression levels in the OK cell line, from both the de novo transcriptome and genes aligned to the M. domestica genome, were compared with publicly available rat kidney nephron segment expression data. Our studies demonstrate the expression in OK cells of numerous PT-specific ion transporters and other key proteins relevant for rodent and human PT function. Additionally, the sequence and expression data reported here provide an important resource for genetic manipulation and other studies on PT cell function using these cells.

Effects of thyroid hormone transporters MCT8 and MCT10 on nuclear activity of T3

Transport of thyroid hormone (TH) across the plasma membrane is necessary for the genomic action of T3 mediated by its nuclear T3 receptor. MCT8 and MCT10 have been identified as important TH transporters. Mutations in MCT8 result in severe psychomotor retardation. In addition to TH transport into the cell, MCT8 and MCT10 also facilitate TH efflux from cells. Therefore, the aim of this study was to examine if MCT8 and MCT10 increase the availability of T3 for its nuclear receptor rather than generate a rapid equilibrium between cellular and serum T3. T3 action was investigated in JEG3 cells co-transfected with TRβ1 and a T3 response element-driven luciferase construct, and T3 metabolism was analyzed in cells transfected with type 3 deiodinase (D3). In addition, cells were transfected with MCT8 or MCT10 and/or the cytoplasmic T3-binding protein mu-crystallin (CRYM). Luciferase signal was markedly stimulated by incubating cells for 24 h with 1 nM T3, but this response was not augmented by MCT8 or MCT10 expression. Limiting the time of T3 exposure to 1-6 h and co-transfection with CRYM allowed for a modest increase in luciferase response to T3. In contrast, T3 metabolism by D3 was potently stimulated by MCT8 or MCT10 expression, but it was not affected by expression of CRYM. These results suggest that MCT8 and MCT10 by virtue of their bidirectional T3 transport have less effect on steady-state nuclear T3 levels than on T3 levels at the cell periphery where D3 is located. CRYM alters the dynamics of cellular TH transport but its exact function in the cellular distribution of TH remains to be determined.

The long N-terminus of the human monocarboxylate transporter 8 is a target of ubiquitin-dependent proteasomal degradation which regulates protein expression and oligomerization capacity

Monocarboxylate transporter 8 (MCT8) equilibrates thyroid hormones between the extra- and the intracellular sides. MCT8 exists either with a short or a long N-terminus, but potential functional differences between both variants are yet not known. We, therefore, generated MCT8 constructs which are different in N-terminal length: MCT8(1-613), MCT8(25-613), MCT8(49-613) and MCT8(75-613). The M75G substitution prevents translation of MCT8(75-613) and ensures expression of full-length MCT8 protein. The K56G substitution was made to prevent ubiquitinylation. Cell-surface expression, localization and proteasomal degradation were investigated using C-terminally GFP-tagged MCT8 constructs (HEK293 and MDCK1 cells) and oligomerization capacity was determined using N-terminally HA- and C-terminally FLAG-tagged MCT8 constructs (COS7 cells). MCT8(1-613)-GFP showed a lower protein expression than the shorter MCT8(75-613)-GFP protein. The proteasome inhibitor lactacystin increased MCT8(1-613)-GFP protein amount, suggesting proteasomal degradation of MCT8 with the long N-terminus. Ubiquitin conjugation of MCT8(1-613)-GFP was found by immuno-precipitation. A diminished ubiquitin conjugation caused by K56G substitution resulted in increased MCT8(1-613)-GFP protein expression. Sandwich ELISA was performed to investigate if the bands at higher molecular weight observed in Western blot analysis are due to MCT8 oligomerization, which was indeed shown. Our data imply a role of the long N-terminus of MCT8 as target of ubiquitin-dependent proteasomal degradation affecting MCT8 amount and subsequently oligomerization capacity.

Further Insights into the Allan-Herndon-Dudley Syndrome: Clinical and Functional Characterization of a Novel MCT8 Mutation

Background

Mutations in the thyroid hormone (TH) transporter MCT8 have been identified as the cause for Allan-Herndon-Dudley Syndrome (AHDS), characterized by severe psychomotor retardation and altered TH serum levels. Here we report a novel MCT8 mutation identified in 4 generations of one family, and its functional characterization.

Methods

Proband and family members were screened for 60 genes involved in X-linked cognitive impairment and the MCT8 mutation was confirmed. Functional consequences of MCT8 mutations were studied by analysis of [¹²⁵I]TH transport in fibroblasts and transiently transfected JEG3 and COS1 cells, and by subcellular localization of the transporter.

Results

The proband and a male cousin demonstrated clinical findings characteristic of AHDS. Serum analysis showed high T3, low rT3, and normal T4 and TSH levels in the proband. A MCT8 mutation (c.869C>T; p.S290F) was identified in the proband, his cousin, and several female carriers. Functional analysis of the S290F mutant showed decreased TH transport, metabolism and protein expression in the three cell types, whereas the S290A mutation had no effect. Interestingly, both uptake and efflux of T3 and T4 was impaired in fibroblasts of the proband, compared to his healthy brother. However, no effect of the S290F mutation was observed on TH efflux from COS1 and JEG3 cells. Immunocytochemistry showed plasma membrane localization of wild-type MCT8 and the S290A and S290F mutants in JEG3 cells.

Conclusions

We describe a novel MCT8 mutation (S290F) in 4 generations of a family with Allan-Herndon-Dudley Syndrome. Functional analysis demonstrates loss-of-function of the MCT8 transporter. Furthermore, our results indicate that the function of the S290F mutant is dependent on cell context. Comparison of the S290F and S290A mutants indicates that it is not the loss of Ser but its substitution with Phe, which leads to S290F dysfunction.

Psychomotor retardation caused by a defective thyroid hormone transporter: report of two families with different MCT8 mutations

BACKGROUND/AIMS

Monocarboxylate transporter 8 (MCT8) is essential for thyroid hormone (TH) transport in the brain. Mutations in MCT8 are associated with the Allan-Herndon-Dudley syndrome (AHDS), characterized by severe psychomotor retardation and altered serum thyroid parameters. Here we report two novel mutations in MCT8 and discuss the clinical findings.

CASE REPORT AND RESULTS

We describe 4 males with AHDS from two unrelated families varying in age from 1.5 to 11 years. All 4 patients presented with typical clinical signs of AHDS, including severe psychomotor retardation, axial hypotonia, lack of speech, diminished muscle mass, increased muscle tone, hyperreflexia, myopathic facies, high T3, low T4 and rT3, and normal/mildly elevated TSH levels. Comparison of patients at different ages suggests the progressive nature of AHDS. Genetic analyses identified a novel missense MCT8 mutation (p.G495A) in family 1 and a 2.8-kb deletion comprising exons 3 and 4 in family 2. Functional analysis of p.G495A revealed impaired TH transport varying from 20 to 85% depending on the cell context.

CONCLUSION

Here we report 4 AHDS patients in unrelated Turkish families harboring novel MCT8 mutations. Despite the widely different mutations, the clinical phenotypes are very similar and findings support the progressive nature of AHDS.

Deletion of exon 1 of the SLC16A2 gene: a common occurrence in patients with Allan-Herndon-Dudley syndrome

BACKGROUND

Allan-Herndon-Dudley syndrome (AHDS) is an X-linked type of mental retardation resulting from hindered thyroid hormone access to neurons. Clustered nonrecurrent deletions of SLC16A2 exon 1 have been described in three patients with AHDS. We report a fourth patient with such a deletion and discuss possible mechanisms leading to these rearrangements.

CASE PRESENTATION

A three-and-a-half-year-old male with clinical and biochemical AHDS phenotype and a history of normal neonatal screening for hypothyroidism underwent SLC16A2 molecular analysis. Unexpectedly, he showed skeletal signs of hypothyroidism.

METHODS AND RESULTS

The exons of the SLC16A2 (MCT8) gene and the sequences surrounding exon 1 were amplified using PCR. The patient had a 36-kb deletion affecting exon 1 of SLC16A2. The deletion junction was subjected to bioinformatic analyses, along with two other reported exon 1 deletion junctions, identifying possible sequence features and mechanisms responsible for such genomic rearrangements.

DISCUSSION/CONCLUSION

This patient had a classic AHDS phenotype with an unexpectedly large anterior fontanel and delayed bone age and dentition. Bioinformatic analyses suggested that exon 1 deletions in patients with AHDS are caused by microhomology-mediated replicative-based and nonhomologous end-joining mechanisms. Rearrangement susceptibility may be due to the size of intron 1 and the percentage of repeat sequences.

Modulation of monocarboxylate transporter 8 oligomerization by specific pathogenic mutations

The monocarboxylate transporter 8 (MCT8) is a member of the major facilitator superfamily (MFS). These membrane-spanning proteins facilitate translocation of a variety of substrates, MCT8 specifically transports iodothyronines. Mutations in MCT8 are the underlying cause of severe X-linked psychomotor retardation. At the molecular level, such mutations led to deficiencies in substrate translocation due to reduced cell-surface expression, impaired substrate binding, or decreased substrate translocation capabilities. However, the causal relationships between genotypes, molecular features of mutated MCT8, and patient characteristics have not yet been comprehensively deciphered. We investigated the relationship between pathogenic mutants of MCT8 and their capacity to form dimers (presumably oligomeric structures) as a potential regulatory parameter of the transport function of MCT8. Fourteen pathogenic variants of MCT8 were investigated in vitro with respect to their capacity to form oligomers. Particular mutations close to the substrate translocation channel (S194F, A224T, L434W, and R445C) were found to inhibit dimerization of MCT8. This finding is in contrast to those for other transporters or transmembrane proteins, in which substitutions predominantly at the outer-surface inhibit oligomerization. Moreover, specific mutations of MCT8 located in transmembrane helix 2 (del230F, V235M, and ins236V) increased the capacity of MCT8 variants to dimerize. We analyzed the localization of MCT8 dimers in a cellular context, demonstrating differences in MCT8 dimer formation and distribution. In summary, our results add a new link between the functions (substrate transport) and protein organization (dimerization) of MCT8, and might be of relevance for other members of the MFS. Finally, the findings are discussed in relationship to functional data combined with structural-mechanistical insights into MCT8.

Tissue-specific expression of monocarboxylate transporters during fasting in mice

Monocarboxylates such as pyruvate, lactate and ketone bodies are crucial for energy supply of all tissues, especially during energy restriction. The transport of monocarboxylates across the plasma membrane of cells is mediated by monocarboxylate transporters (MCTs). Out of 14 known mammalian MCTs, six isoforms have been functionally characterized to transport monocarboxylates and short chain fatty acids (MCT1-4), thyroid hormones (MCT8, -10) and aromatic amino acids (MCT10). Knowledge on the regulation of the different MCT isoforms is rare. In an attempt to get more insights in regulation of MCT expression upon energy deprivation, we carried out a comprehensive analysis of tissue specific expression of five MCT isoforms upon 48 h of fasting in mice. Due to the crucial role of peroxisome proliferator-activated receptor (PPAR)-α as a central regulator of energy metabolism and as known regulator of MCT1 expression, we included both wildtype (WT) and PPARα knockout (KO) mice in our study. Liver, kidney, heart, small intestine, hypothalamus, pituitary gland and thyroid gland of the mice were analyzed. Here we show that the expression of all examined MCT isoforms was markedly altered by fasting compared to feeding. Expression of MCT1, MCT2 and MCT10 was either increased or decreased by fasting dependent on the analyzed tissue. MCT4 and MCT8 were down-regulated by fasting in all examined tissues. However, PPARα appeared to have a minor impact on MCT isoform regulation. Due to the fundamental role of MCTs in transport of energy providing metabolites and hormones involved in the regulation of energy homeostasis, we assumed that the observed fasting-induced adaptations of MCT expression seem to ensure an adequate energy supply of tissues during the fasting state. Since, MCT isoforms 1–4 are also necessary for the cellular uptake of drugs, the fasting-induced modifications of MCT expression have to be considered in future clinical care algorithms.

Structure and function of thyroid hormone plasma membrane transporters

Thyroid hormones (TH) cross the plasma membrane with the help of transporter proteins. As charged amino acid derivatives, TH cannot simply diffuse across a lipid bilayer membrane, despite their notorious hydrophobicity. The identification of monocarboxylate transporter 8 (MCT8, SLC16A2) as a specific and very active TH transporter paved the way to the finding that mutations in the MCT8 gene cause a syndrome of psychomotor retardation in humans. The purpose of this review is to introduce the current model of transmembrane transport and highlight the diversity of TH transmembrane transporters. The interactions of TH with plasma transfer proteins, T3 receptors, and deiodinase are summarized. It is shown that proteins may bind TH owing to their hydrophobic character in hydrophobic cavities and/or by specific polar interaction with the phenolic hydroxyl, the aminopropionic acid moiety, and by weak polar interactions with the iodine atoms. These findings are compared with our understanding of how TH transporters interact with substrate. The presumed effects of mutations in MCT8 on protein folding and transport function are explained in light of the available homology model.

Knockdown of monocarboxylate transporter 8 (mct8) disturbs brain development and locomotion in zebrafish

Allan-Herndon-Dudley syndrome (AHDS) is an inherited disorder of brain development characterized by severe psychomotor retardation. This X-linked disease is caused by mutations in the monocarboxylate transporter 8 (MCT8), an important thyroid hormone transporter in brain neurons. MCT8-knockout mice lack the 2 major neurological symptoms of AHDS, namely locomotor problems and cognitive impairment. The pathological mechanism explaining the symptoms is still obscure, and no cure for this condition is known. The development of an animal model that carries MCT8-related neurological symptoms is warranted. We have employed morpholino-based gene knockdown to create zebrafish deficient for mct8. Knockdown of mct8 results in specific symptoms in the thyroid axis and brain. The mct8-morphants showed impaired locomotor behavior and brain development. More specifically, we observed maldevelopment of the cerebellum and mid-hindbrain boundary and apoptotic clusters in the zebrafish brain. The mRNA expression of zebrafish orthologs of mammalian TSH, thyroid hormone transporters, and deiodinases was altered in mct8 morphants. In particular, deiodinase type 3 gene expression was consistently up-regulated in zebrafish mct8 morphants. The thyroid hormone metabolite tetrac, but not T3, partly ameliorated the affected phenotype and locomotion disability of morphant larvae. Our results show that mct8 knockdown in zebrafish larvae results in disturbances in the thyroid axis, brain, and locomotion behavior, which is congruent with the clinical aspect of impaired locomotion and cognition in patients with AHDS. Taken together, the zebrafish is a suitable animal model for the study of the pathophysiology of AHDS.

Transporters MCT8 and OATP1C1 maintain murine brain thyroid hormone homeostasis

Allan-Herndon-Dudley syndrome (AHDS), a severe form of psychomotor retardation with abnormal thyroid hormone (TH) parameters, is linked to mutations in the TH-specific monocarboxylate transporter MCT8. In mice, deletion of Mct8 (Mct8 KO) faithfully replicates AHDS-associated endocrine abnormalities; however, unlike patients, these animals do not exhibit neurological impairments. While transport of the active form of TH (T3) across the blood-brain barrier is strongly diminished in Mct8 KO animals, prohormone (T4) can still enter the brain, possibly due to the presence of T4-selective organic anion transporting polypeptide (OATP1C1). Here, we characterized mice deficient for both TH transporters, MCT8 and OATP1C1 (Mct8/Oatp1c1 DKO). Mct8/Oatp1c1 DKO mice exhibited alterations in peripheral TH homeostasis that were similar to those in Mct8 KO mice; however, uptake of both T3 and T4 into the brains of Mct8/Oatp1c1 DKO mice was strongly reduced. Evidence of TH deprivation in the CNS of Mct8/Oatp1c1 DKO mice included highly decreased brain TH content as well as altered deiodinase activities and TH target gene expression. Consistent with delayed cerebellar development and reduced myelination, Mct8/Oatp1c1 DKO mice displayed pronounced locomotor abnormalities. Intriguingly, differentiation of GABAergic interneurons in the cerebral cortex was highly compromised. Our findings underscore the importance of TH transporters for proper brain development and provide a basis to study the pathogenic mechanisms underlying AHDS.

Tissue-specific alterations in thyroid hormone homeostasis in combined Mct10 and Mct8 deficiency

The monocarboxylate transporter Mct10 (Slc16a10; T-type amino acid transporter) facilitates the cellular transport of thyroid hormone (TH) and shows an overlapping expression with the well-established TH transporter Mct8. Because Mct8 deficiency is associated with distinct tissue-specific alterations in TH transport and metabolism, we speculated that Mct10 inactivation may compromise the tissue-specific TH homeostasis as well. However, analysis of Mct10 knockout (ko) mice revealed normal serum TH levels and tissue TH content in contrast to Mct8 ko mice that are characterized by high serum T3, low serum T4, decreased brain TH content, and increased tissue TH concentrations in the liver, kidneys, and thyroid gland. Surprisingly, mice deficient in both TH transporters (Mct10/Mct8 double knockout [dko] mice) showed normal serum T4 levels in the presence of elevated serum T3, indicating that the additional inactivation of Mct10 partially rescues the phenotype of Mct8 ko mice. As a consequence of the normal serum T4, brain T4 content and hypothalamic TRH expression were found to be normalized in the Mct10/Mct8 dko mice. In contrast, the hyperthyroid situation in liver, kidneys, and thyroid gland of Mct8 ko mice was even more severe in Mct10/Mct8 dko animals, suggesting that in these organs, both transporters contribute to the TH efflux. In summary, our data indicate that Mct10 indeed participates in tissue-specific TH transport and also contributes to the generation of the unusual serum TH profile characteristic for Mct8 deficiency.

Thyroid hormone role on cerebellar development and maintenance: a perspective based on transgenic mouse models

Cerebellum development is sensitive to thyroid hormone (TH) levels, as THs regulate neuronal migration, differentiation, and myelination. Most effects of THs are mediated by the thyroid hormone receptor (TR) isoforms TRβ1, TRβ2, and TRα1. Studies aimed at identifying TH target genes during cerebellum development have only achieved partial success, as some of these genes do not possess classical TH-responsive elements, and those that do are likely to be temporally and spatially regulated by THs. THs may also affect neurodevelopment by regulating transcription factors that control particular groups of genes. Furthermore, TH action can also be affected by TH transport, which is mediated mainly by monocarboxylate transporter family members. Studies involving transgenic animal models and genome-wide expression analyses have helped to address the unanswered questions regarding the role of TH in cerebellar development. Recently, a growing body of evidence has begun to clarify the molecular, cellular, and functional aspects of THs in the developing cerebellum. This review describes the current findings concerning the effects of THs on cerebellar development and maintenance as well as advances in the genetic animal models used in this field.

Expression pattern of thyroid hormone transporters in the postnatal mouse brain

For a comprehensive description of the tissue-specific thyroidal state under normal as well as under pathophysiological conditions it is of utmost importance to include thyroid hormone (TH) transporters in the analysis as well. The current knowledge of the cell-specific repertoire of TH transporters, however, is still rather limited, although several TH transporting proteins have been identified. Here, we describe the temporal and spatial distribution pattern of the most prominent TH transporters in the postnatal mouse brain. For that purpose, we performed radioactive in situ hybridization studies in order to analyze the cellular mRNA expression pattern of the monocarboxylate transporters Mct8 and Mct10, the L-type amino acid transporters Lat1 and Lat2 as well as the organic anion transporting peptide Oatp1c1 at different postnatal time points. Highest TH transporter expression levels in the CNS were observed at postnatal day 6 and 12, while hybridization signal intensities visibly declined after the second postnatal week. The only exception was Mct10 for which the strongest signals could be observed in white matter regions at postnatal day 21 indicating that this transporter is preferentially expressed in mature oligodendrocytes. Whereas Mct8 and Lat2 showed an overlapping neuronal mRNA expression pattern in the cerebral cortex, hippocampus, and in the hypothalamus, Oatp1c1 and Lat1 specific signals were most prominent in capillary endothelial cells throughout the CNS. In the choroid plexus, expression of three transporters (Mct8, Lat2, and Oatp1c1) could be detected, whereas in other brain areas (e.g., striatum, thalamus, and brain stem nuclei) only one of the transporter candidates appeared to be present. Overall, our study revealed a distinct mRNA distribution pattern for each of the TH transporter candidates. Further studies will reveal to which extent these transporters contribute to the cell-specific TH uptake and efflux in the mouse CNS.

Different causes of reduced sensitivity to thyroid hormone: diagnosis and clinical management

Normal thyroid hormone (TH) metabolism and action require adequate cellular TH signalling. This entails proper function of TH transporters in the plasma membrane, intracellular deiodination of TH and action of the bioactive hormone T3 at its nuclear receptors (TRs). The present review summarizes the discoveries of different syndromes with reduced sensitivity at the cellular level. Mutations in the TH transporter MCT8 cause psychomotor retardation and abnormal thyroid parameters. Mutations in the SBP2 protein, which is required for normal deiodination, give rise to a multisystem disorder including abnormal thyroid function tests. Mutations in TRβ1 are a well-known cause of resistance to TH with mostly a mild phenotype, while only recently, patients with mutations in TRα1 were identified. The latter patients have slightly abnormal TH levels, growth retardation and cognitive defects. This review will describe the mechanisms of disease, clinical phenotype, diagnostic testing and suggestions for treatment strategies for each of these syndromes.

Increased oxidative metabolism and neurotransmitter cycling in the brain of mice lacking the thyroid hormone transporter SLC16A2 (MCT8)

Mutations of the monocarboxylate transporter 8 (MCT8) cause a severe X-linked intellectual deficit and neurological impairment. MCT8 is a specific thyroid hormone (T4 and T3) transporter and the patients also present unusual abnormalities in the serum profile of thyroid hormone concentrations due to altered secretion and metabolism of T4 and T3. Given the role of thyroid hormones in brain development, it is thought that the neurological impairment is due to restricted transport of thyroid hormones to the target neurons. In this work we have investigated cerebral metabolism in mice with Mct8 deficiency. Adult male mice were infused for 30 minutes with (1-(13)C) glucose and brain extracts prepared and analyzed by (13)C nuclear magnetic resonance spectroscopy. Genetic inactivation of Mct8 resulted in increased oxidative metabolism as reflected by increased glutamate C4 enrichment, and of glutamatergic and GABAergic neurotransmissions as observed by the increases in glutamine C4 and GABA C2 enrichments, respectively. These changes were distinct to those produced by hypothyroidism or hyperthyroidism. Similar increments in glutamate C4 enrichment and GABAergic neurotransmission were observed in the combined inactivation of Mct8 and D2, indicating that the increased neurotransmission and metabolic activity were not due to increased production of cerebral T3 by the D2-encoded type 2 deiodinase. In conclusion, Mct8 deficiency has important metabolic consequences in the brain that could not be correlated with deficiency or excess of thyroid hormone supply to the brain during adulthood.

Importance of His192 in the human thyroid hormone transporter MCT8 for substrate recognition

Monocarboxylate transporter 8 (MCT8) facilitates cellular uptake and efflux of thyroid hormone (TH). So far, functional domains within MCT8 are not well defined. Mutations in MCT8 result in severe psychomotor retardation due to impaired neuronal differentiation. One such mutation concerns His192 (H192R), located at the border of transmembrane domain (TMD) 1 and extracellular loop (ECL) 1, suggesting that this His residue is important for efficient TH transport. Here, we studied the role of different His residues, predicted within TMDs or ECLs of MCT8, in substrate recognition and translocation. Therefore, we analyzed the effects of the His-modifying reagent diethylpyrocarbonate (DEPC) and of site-directed mutagenesis of several His residues on TH transport by MCT8. Reaction of MCT8 with DEPC inhibited subsequent uptake of T(3) and T(4), whereas T(3) and T(4) efflux were not inhibited. The inhibitory effect of DEPC on TH uptake was prevented in the presence of T(3) or T(4), suggesting that TH blocks access to DEPC-sensitive residues. Three putative DEPC target His residues were replaced by Ala: H192A, H260A, and H450A. The H260A and H450A mutants showed similar TH transport and DEPC sensitivity as wild-type MCT8. However, the H192A mutant showed a significant reduction in TH uptake and was insensitive to DEPC. Taken together, these results indicate that His192 is sensitive to modification by DEPC and may be located close to a putative substrate recognition site within the MCT8 protein, important for efficient TH uptake.

Mutations in MCT8 in patients with Allan-Herndon-Dudley-syndrome affecting its cellular distribution

Monocarboxylate transporter 8 (MCT8) is a thyroid hormone (TH)-specific transporter. Mutations in the MCT8 gene are associated with Allan-Herndon-Dudley Syndrome (AHDS), consisting of severe psychomotor retardation and disturbed TH parameters. To study the functional consequences of different MCT8 mutations in detail, we combined functional analysis in different cell types with live-cell imaging of the cellular distribution of seven mutations that we identified in patients with AHDS. We used two cell models to study the mutations in vitro: 1) transiently transfected COS1 and JEG3 cells, and 2) stably transfected Flp-in 293 cells expressing a MCT8-cyan fluorescent protein construct. All seven mutants were expressed at the protein level and showed a defect in T3 and T4 transport in uptake and metabolism studies. Three mutants (G282C, P537L, and G558D) had residual uptake activity in Flp-in 293 and COS1 cells, but not in JEG3 cells. Four mutants (G221R, P321L, D453V, P537L) were expressed at the plasma membrane. The mobility in the plasma membrane of P537L was similar to WT, but the mobility of P321L was altered. The other mutants studied (insV236, G282C, G558D) were predominantly localized in the endoplasmic reticulum. In essence, loss of function by MCT8 mutations can be divided in two groups: mutations that result in partial or complete loss of transport activity (G221R, P321L, D453V, P537L) and mutations that mainly disturb protein expression and trafficking (insV236, G282C, G558D). The cell type-dependent results suggest that MCT8 mutations in AHDS patients may have tissue-specific effects on TH transport probably caused by tissue-specific expression of yet unknown MCT8-interacting proteins.

Mechanism-based testing strategy using in vitro approaches for identification of thyroid hormone disrupting chemicals

The thyroid hormone (TH) system is involved in several important physiological processes, including regulation of energy metabolism, growth and differentiation, development and maintenance of brain function, thermo-regulation, osmo-regulation, and axis of regulation of other endocrine systems, sexual behaviour and fertility and cardiovascular function. Therefore, concern about TH disruption (THD) has resulted in strategies being developed to identify THD chemicals (THDCs). Information on potential of chemicals causing THD is typically derived from animal studies. For the majority of chemicals, however, this information is either limited or unavailable. It is also unlikely that animal experiments will be performed for all THD relevant chemicals in the near future for ethical, financial and practical reasons. In addition, typical animal experiments often do not provide information on the mechanism of action of THDC, making it harder to extrapolate results across species. Relevant effects may not be identified in animal studies when the effects are delayed, life stage specific, not assessed by the experimental paradigm (e.g., behaviour) or only occur when an organism has to adapt to environmental factors by modulating TH levels. Therefore, in vitro and in silico alternatives to identify THDC and quantify their potency are needed. THDC have many potential mechanisms of action, including altered hormone production, transport, metabolism, receptor activation and disruption of several feed-back mechanisms. In vitro assays are available for many of these endpoints, and the application of modern '-omics' technologies, applicable for in vivo studies can help to reveal relevant and possibly new endpoints for inclusion in a targeted THDC in vitro test battery. Within the framework of the ASAT initiative (Assuring Safety without Animal Testing), an international group consisting of experts in the areas of thyroid endocrinology, toxicology of endocrine disruption, neurotoxicology, high-throughput screening, computational biology, and regulatory affairs has reviewed the state of science for (1) known mechanisms for THD plus examples of THDC; (2) in vitro THD tests currently available or under development related to these mechanisms; and (3) in silico methods for estimating the blood levels of THDC. Based on this scientific review, the panel has recommended a battery of test methods to be able to classify chemicals as of less or high concern for further hazard and risk assessment for THD. In addition, research gaps and needs are identified to be able to optimize and validate the targeted THD in vitro test battery for a mechanism-based strategy for a decision to opt out or to proceed with further testing for THD.

Identification, functional analysis, prevalence and treatment of monocarboxylate transporter 8 (MCT8) mutations in a cohort of adult patients with mental retardation

OBJECTIVE

Monocarboxylate transporter 8 (MCT8) is an essential thyroid hormone (TH) transporter as humans with MCT8 mutations have severe neurological and endocrine abnormalities. The objectives are (i) to identify novel MCT8 mutations and (ii) to assess their functional relevance; (iii) to describe the effects of block-and-replace treatment in an MCT8 patient.

DESIGN

The TOP-R study is a cross-sectional nation-wide multicentre study.

PATIENTS

Subjects with unexplained mental retardation (MR) were screened for MCT8 mutations.

RESULTS

We identified three mutations: p.F501del (previously described), p.L492P and p.T162T. The F501del and L492P mutants, but not the T162T mutant, showed diminished T3, T4 and rT3 transport in transfected cells. TH transport in T162T fibroblasts was also not affected. One patient was treated with block-and-replace therapy to normalize serum TH levels. The results indicated a slow onset of the decrease in serum T4 and T3 by successive treatment with methimazole and PTU, and eventually their complete normalization by administration of LT4 with PTU but not with methimazole. The frequency of MCT8 mutations in males with X-linked MR approximately 3·9%.

CONCLUSIONS

We identified several MCT8 mutations in a cohort of subjects with unexplained MR. We demonstrated the pathogenicity of two missense mutations. The synonymous variant did not affect TH transport. Block-and-replace therapy of one patient reversed the TH abnormalities. Our data suggest a decreased TH secretion rate and an increased T4 to T3 conversion by the type I deiodinase in patients with MCT8 mutations. Our study indicates that MCT8 mutations are a relatively frequent cause of X-linked MR.

Tetrac can replace thyroid hormone during brain development in mouse mutants deficient in the thyroid hormone transporter mct8

The monocarboxylate transporter 8 (MCT8) plays a critical role in mediating the uptake of thyroid hormones (THs) into the brain. In patients, inactivating mutations in the MCT8 gene are associated with a severe form of psychomotor retardation and abnormal serum TH levels. Here, we evaluate the therapeutic potential of the TH analog 3,5,3',5'-tetraiodothyroacetic acid (tetrac) as a replacement for T(4) in brain development. Using COS1 cells transfected with TH transporter and deiodinase constructs, we could show that tetrac, albeit not being transported by MCT8, can be metabolized to the TH receptor active compound 3,3',5-triiodothyroacetic acid (triac) by type 2 deiodinase and inactivated by type 3 deiodinase. Triac in turn is capable of replacing T(3) in primary murine cerebellar cultures where it potently stimulates Purkinje cell development. In vivo effects of tetrac were assessed in congenital hypothyroid Pax8-knockout (KO) and Mct8/Pax8 double-KO mice as well as in Mct8-KO and wild-type animals after daily injection of tetrac (400 ng/g body weight) during the first postnatal weeks. This treatment was sufficient to promote TH-dependent neuronal differentiation in the cerebellum, cerebral cortex, and striatum but was ineffective in suppressing hypothalamic TRH expression. In contrast, TSH transcript levels in the pituitary were strongly down-regulated in response to tetrac. Based on our findings we propose that tetrac administration offers the opportunity to provide neurons during the postnatal stage with a potent TH receptor agonist, thereby eventually reducing the neurological damage in patients with MCT8 mutations without deteriorating the thyrotoxic situation in peripheral tissues.

Understanding the hypothalamus-pituitary-thyroid axis in mct8 deficiency

Thyroid hormone (TH) metabolism and action via binding to nuclear receptors are intracellular events that require the passage of TH across the plasma membrane. This process is mediated by specific TH transporters of which the monocarboxylate transporter 8 (Mct8) has received major attention. Mct8 is highly expressed in different tissues such as liver, kidney, thyroid, pituitary and brain. In humans, inactivating mutations of the MCT8 gene (SLC16A2) are associated with severe forms of psychomotor retardation and abnormal TH serum levels (Allan-Herndon-Dudley syndrome). Surprisingly, Mct8 knockout (ko) mice do not exhibit overt neurological symptoms but fully replicate the unusual serum TH profile with highly increased serum T3 in the presence of low serum T4. In order to evaluate the underlying mechanisms for these abnormalities, TH transport and metabolism have been intensively studied in different tissues of Mct8 ko mice. Here, we summarize the observed changes within the hypothalamus-pituitary-thyroid axis that result in altered TH production and secretion. Although analysis of Mct8 ko mice has greatly expanded our knowledge, many open questions still remain to be addressed in order to define the tissue- and cell-specific role of this important TH transporter.

The pathophysiological consequences of thyroid hormone transporter deficiencies: Insights from mouse models

BACKGROUND

As a prerequisite for thyroid hormone (TH) metabolism and action TH has to be transported into cells where TH deiodinases and receptors are located. The trans-membrane passage of TH is facilitated by TH transporters of which the monocarboxylate transporter MCT8 has been most intensively studied. Inactivating mutations in the gene encoding MCT8 are associated with a severe form of psychomotor retardation and abnormal serum TH levels (Allan-Herndon-Dudley syndrome). In order to define the underlying pathogenic mechanisms, Mct8 knockout mice have been generated and intensively studied. Most surprisingly, Mct8 ko mice do not show any neurological symptoms but fully replicate the abnormal serum thyroid state.

SCOPE OF REVIEW

We will summarize the findings of these mouse studies that shed light on various aspects of Mct8 deficiency and unambiguously demonstrated the pivotal role of Mct8 in mediating TH transport in various tissues. These studies have also revealed the presence of the complex interplay between different pathogenic mechanisms that contribute to the generation of the abnormal TH serum profile.

MAJOR CONCLUSIONS

Most importantly, studies of Mct8 ko mice indicated the presence of additional TH transporters that act in concert with Mct8. Interesting candidates for such a function are the L-type amino acid transporters Lat1 and Lat2 as well as the organic anion transporting polypeptide Oatp1c1.

GENERAL SIGNIFICANCE

Overall, the analysis of Mct8 deficient mice has greatly expanded our knowledge about the (patho-) physiological function of this transporter and established a sound basis for the characterization of additional TH transporter candidates. This article is part of a Special Issue entitled Thyroid hormone signalling.

Thyroid hormone metabolism in skeletal development and adult bone maintenance

Metabolism of thyroid hormones by the type 2 and type 3 iodothyronine deiodinases (D2, D3) in T3-responsive target cells is a sophisticated mechanism that helps to maintain local T3 concentrations and facilitates T3 action in a cell-specific manner that is independent of circulating thyroid hormone concentrations. Recent findings have demonstrated an essential physiological role for the thyroid hormone-activating enzyme D2 in the optimization of bone mineralization and strength. Emerging population studies have also identified the genes encoding D2 and the thyroid hormone-inactivating enzyme D3 as susceptibility loci for osteoarthritis. These new data reveal an essential role for the local control of T3 availability in osteoblasts and chondrocytes during maintenance and repair of bone and cartilage.

Impact of Oatp1c1 deficiency on thyroid hormone metabolism and action in the mouse brain

Organic anion-transporting polypeptide 1c1 (Oatp1c1) (also known as Slco1c1 and Oatp14) belongs to the family of Oatp and has been shown to facilitate the transport of T(4). In the rodent brain, Oatp1c1 is highly enriched in capillary endothelial cells and choroid plexus structures where it may mediate the entry of T(4) into the central nervous system. Here, we describe the generation and first analysis of Oatp1c1-deficient mice. Oatp1c1 knockout (KO) mice were born with the expected frequency, were not growth retarded, and developed without any overt neurological abnormalities. Serum T(3) and T(4) concentrations as well as renal and hepatic deiodinase type 1 expression levels were indistinguishable between Oatp1c1 KO mice and control animals. Hypothalamic TRH and pituitary TSH mRNA levels were not affected, but brain T(4) and T(3) content was decreased in Oatp1c1-deficient animals. Moreover, increased type 2 and decreased type 3 deiodinase activities indicate a mild hypothyroid situation in the brain of Oatp1c1 KO mice. Consequently, mRNA expression levels of gene products positively regulated by T(3) in the brain were down-regulated. This central nervous system-specific hypothyroidism is presumably caused by an impaired passage of T(4) across the blood-brain barrier and indicates a unique function of Oatp1c1 in facilitating T(4) transport despite the presence of other thyroid hormone transporters such as Mct8.

MCT8: from gene to disease and therapeutic approach

Thyroid hormone metabolism and action are largely intracellular processes that require transport of the hormone across the plasma membrane by different transporters. Two of these, MCT8 and MCT10, are close members of the monocarboxylate transporter family. MCT8 is expressed in a variety of tissues, including liver, kidney, thyroid and brain. The MCT8 gene is located on the X chromosome, and mutations in MCT8 result in severe psychomotor retardation and low serum T4 and high T3 levels in affected males. The psychomotor retardation is thought to be caused by impaired neuronal T3 uptake during brain development. The abnormal thyroid hormone levels appear to result from an increased T4 to T3 conversion in the kidney as well as altered hormone secretion from the thyroid gland. Options for therapy aim at early treatment with T3 analogues, neuronal uptake of which does not require MCT8.

Impact of monocarboxylate transporter-8 deficiency on the hypothalamus-pituitary-thyroid axis in mice

In patients, inactivating mutations in the gene encoding the thyroid hormone-transporting monocarboxylate transporter 8 (Mct8) are associated with severe mental and neurological deficits and disturbed thyroid hormone levels. The latter phenotype characterized by high T3 and low T4 serum concentrations is replicated in Mct8 knockout (ko) mice, indicating that MCT8 deficiency interferes with thyroid hormone production and/or metabolism. Our studies of Mct8 ko mice indeed revealed increased thyroidal T3 and T4 concentrations without overt signs of a hyperactive thyroid gland. However, upon TSH stimulation Mct8 ko mice showed decreased T4 and increased T3 secretion compared with wild-type littermates. Moreover, similar changes in the thyroid hormone secretion pattern were observed in Mct8/Trhr1 double-ko mice, which are characterized by normal serum T3 levels and normal hepatic and renal D1 expression in the presence of very low T4 serum concentrations. These data strongly indicate that absence of Mct8 in the thyroid gland affects thyroid hormone efflux by shifting the ratio of the secreted hormones toward T3. To test this hypothesis, we generated Mct8/Pax8 double-mutant mice, which in addition to Mct8 lack a functional thyroid gland and are therefore completely athyroid. Following the injection of these animals with either T4 or T3, serum analysis revealed T3 concentrations similar to those observed in Pax8 ko mice under thyroid hormone replacement, indicating that indeed increased thyroidal T3 secretion in Mct8 ko mice represents an important pathogenic mechanism leading to the high serum T3 levels.

Prenatal thyroxine treatment disparately affects peripheral and amygdala thyroid hormone levels

A prenatal hypothyroid state is associated with behavioral abnormalities in adulthood. Wistar Kyoto (WKY) rats exhibit hypothyroidism and increased depressive and anxiety-like behaviors. Thus, the WKY could illuminate the mechanisms by which the reversal of developmental hypothyroidism in humans and animals results in adult behavioral improvement. We examined the outcome of maternal thyroxine (T4) treatment on thyroid hormone-regulated functions and adult behavior of the WKY offspring. Pregnant WKY dams completed gestation with and without T4 administration and their adult male offspring were tested. Measures included depressive and anxiety-like behaviors, and thyroid hormone (TH) concentrations in both plasma and specific brain regions. In addition, the expression of two proteins affecting thyroid hormone trafficking and metabolism, monocarboxylate transporter 8 (MCT-8) and iodothyronine deiodinase type III (Dio3), and of several behavior-altering molecules, glucocorticoid receptor (GR), prepro-thyrotropin releasing hormone (prepro-TRH) and corticotrophin releasing hormone (CRH), were determined in the hippocampus and amygdala of the offspring. Prenatal T4 treatment of WKYs did not affect adult depressive behavior but increased anxiety-like behavior and decreased plasma levels of THs. In the hippocampus of males treated with T4 in utero, Dio3 and MCT-8 protein levels were increased, while in the amygdala, there were increases of free T4, MCT-8, GR, prepro-TRH protein and CRH mRNA levels. These results show that T4 administration in utero programs adult peripheral and amygdalar thyroid hormone levels divergently, and that the resulting upregulation of anxiety-related genes in the amygdala could be responsible for the exacerbated anxiety-like behavior seen in WKYs after prenatal T4 treatment.

Congenital hypothyroidism

Congenital hypothyroidism (CH) occurs in approximately 1:2,000 to 1:4,000 newborns. The clinical manifestations are often subtle or not present at birth. This likely is due to trans-placental passage of some maternal thyroid hormone, while many infants have some thyroid production of their own. Common symptoms include decreased activity and increased sleep, feeding difficulty, constipation, and prolonged jaundice. On examination, common signs include myxedematous facies, large fontanels, macroglossia, a distended abdomen with umbilical hernia, and hypotonia. CH is classified into permanent and transient forms, which in turn can be divided into primary, secondary, or peripheral etiologies. Thyroid dysgenesis accounts for 85% of permanent, primary CH, while inborn errors of thyroid hormone biosynthesis (dyshormonogeneses) account for 10-15% of cases. Secondary or central CH may occur with isolated TSH deficiency, but more commonly it is associated with congenital hypopitiutarism. Transient CH most commonly occurs in preterm infants born in areas of endemic iodine deficiency. In countries with newborn screening programs in place, infants with CH are diagnosed after detection by screening tests. The diagnosis should be confirmed by finding an elevated serum TSH and low T4 or free T4 level. Other diagnostic tests, such as thyroid radionuclide uptake and scan, thyroid sonography, or serum thyroglobulin determination may help pinpoint the underlying etiology, although treatment may be started without these tests. Levothyroxine is the treatment of choice; the recommended starting dose is 10 to 15 mcg/kg/day. The immediate goals of treatment are to rapidly raise the serum T4 above 130 nmol/L (10 ug/dL) and normalize serum TSH levels. Frequent laboratory monitoring in infancy is essential to ensure optimal neurocognitive outcome. Serum TSH and free T4 should be measured every 1-2 months in the first 6 months of life and every 3-4 months thereafter. In general, the prognosis of infants detected by screening and started on treatment early is excellent, with IQs similar to sibling or classmate controls. Studies show that a lower neurocognitive outcome may occur in those infants started at a later age (> 30 days of age), on lower l-thyroxine doses than currently recommended, and in those infants with more severe hypothyroidism.

The nature of the compensatory response to low thyroid hormone in the developing brain

Thyroid hormone is essential for normal brain development, although the degree to which the developing brain is sensitive to small perturbations in serum thyroxin is not clear. An important concept related to this is that the developing brain possesses potent mechanisms to compensate for low serum thyroid hormone, and this concept is routinely employed in discussions concerning clinical treatments or public health. However, experimental studies have not directly tested whether (or the degree to which) putative compensatory mechanisms can ameliorate the consequences of small reductions in serum thyroxin (T(4)). To formally test this concept, we employed a model of graded T(4) reductions using doses of propylthiouracil (PTU) that were 200- to 67-fold lower than the dose traditionally used to produce hypothyroidism in rats. PTU produced a stepwise decrease in serum total T(4), and a stepwise increase in serum thyroid-stimulating hormone (TSH), in type 2 deiodinase mRNA expression and enzyme activity in the brain, and in the expression of the mRNA encoding the tri-iodothyronine (T(3)) transporter MCT8 in the postnatal day (P) 15 cortex. However, the mRNA encoding RC3/neurogranin, a direct target of T(3) action, exhibited a strong negative linear correlation with serum total T(4) despite these adaptive responses. In addition, single-cell analysis of RC3 mRNA levels in cortical neurones demonstrated that the co-expression of MCT8 did not alter the relationship between RC3 mRNA and serum T(4). These findings do not support the currently envisioned concept of the developing brain being capable of compensating for low T(4).

Thyroid hormone action during brain development: more questions than answers

Thyroid hormone is essential for proper brain development since it acts on processes such as neuronal migration and differentiation, myelination and synaptogenesis. In this review, we summarize the consequences of thyroid hormone deficiency for brain development with special focus on the cerebellum, an important target of thyroid action. In addition, we discuss the role of iodothyronine deiodinases and thyroid hormone transporters in regulating local thyroid hormone concentrations as well as current knowledge about the function of thyroid hormone receptors and their target genes during brain maturation. Despite considerable progress in recent years in deciphering thyroid hormone signaling pathways we still know very little on the molecular level by which mode of action thyroid hormone exerts its cell-specific effects. Hence, we will particularly address the open questions that remain to be addressed in order to better understand the role of thyroid hormone in brain development.

Consequences of monocarboxylate transporter 8 deficiency for renal transport and metabolism of thyroid hormones in mice

Patients carrying inactivating mutations in the gene encoding the thyroid hormone transporting monocarboxylate transporter (MCT)-8 suffer from a severe form of psychomotor retardation and exhibit abnormal serum thyroid hormone levels. The thyroidal phenotype characterized by high-serum T(3) and low-serum T(4) levels is also found in mice mutants deficient in MCT8 although the cause of these abnormalities is still unknown. Here we describe the consequences of MCT8 deficiency for renal thyroid hormone transport, metabolism, and function by studying MCT8 null mice and wild-type littermates. Whereas serum and urinary parameters do not indicate a strongly altered renal function, a pronounced induction of iodothyronine deiodinase type 1 expression together with increased renal T(3) and T(4) content point to a general hyperthyroid state of the kidneys in the absence of MCT8. Surprisingly, accumulation of peripherally injected T(4) and T(3) into the kidneys was found to be enhanced in the absence of MCT8, indicating that MCT8 deficiency either directly interferes with the renal efflux of thyroid hormones or activates indirectly other renal thyroid hormone transporters that preferentially mediate the renal uptake of thyroid hormones. Our findings indicate that the enhanced uptake and accumulation of T(4) in the kidneys of MCT8 null mice together with the increased renal conversion of T(4) into T(3) by increased renal deiodinase type 1 activities contributes to the generation of the low-serum T(4) and the increase in circulating T(3) levels, a hallmark of MCT8 deficiency.