Identification of monocarboxylate transporter 8 as a specific thyroid hormone transporter

3,3',5-tri-iodothyroacetic acid (TRIAC) transporters

Introduction Thyroid hormone transporters are essential for thyroid hormones to enter target cells. Monocarboxylate transporter MCT8 is a key transporter and expressed at the blood-brain barrier, in neural cells and many other tissues. Patients with MCT8 deficiency have severe neurodevelopmental delays due to cerebral hypothyroidism and chronic sequelae of peripheral thyrotoxicosis. The T3 analog 3,3,5-triiodothyroacetic acid (TRIAC) rescued neurodevelopmental features in animal models mimicking MCT8 deficiency and improved key metabolic features in patients with MCT8 deficiency. However, the identity of the transporter(s) that facilitate TRIAC transport are unknown. Here, we screened candidate transporters that are expressed at the human blood-brain barrier and/or brain-cerebrospinal fluid barrier and known thyroid hormone transporters for TRIAC transport. Materials and methods Plasma membrane expression was determined by cell surface biotinylation assays. Intracellular accumulation of 1 nM TRIAC was assessed in COS-1 cells expressing candidate transporters in Dulbecco's phosphate buffered saline (DPBS)/0.1% glucose or Dulbecco's modified Eagle's medium (DMEM) with or without 0.1% bovine serum albumin (BSA). Expression of Slc22a8 was determined by fluorescent in situ hybridization (FISH) in brain sections from wild-type and Mct8/Oatp1c1 knock-out mice at postnatal day 12, 21 and 120. Results Fifty-nine plasma membrane transporters were selected for screening of TRIAC accumulation (n=40 based on expression at the human blood-brain barrier and/or brain-cerebrospinal fluid barrier and having small organic molecules as substrates; n=19 known thyroid hormone transporters). Screening of the selected transporter panel showed that 18 transporters facilitated significant intracellular accumulation of TRIAC in DPBS/0.1% glucose or DMEM in the absence of BSA. In the presence of BSA, substantial transport was noted for SLCO1B1 and SLC22A8 (in DPBS/0.1% glucose and DMEM) and SLC10A1, SLC22A6 and SLC22A24 (in DMEM). The zebrafish and mouse orthologues of these transporters similarly facilitated intracellular accumulation of TRIAC. Highest Slc22a8 mRNA expression was detected in mouse brain capillary endothelial cells and choroid plexus epithelial cells at early postnatal time points, but wasreduced at P120. Conclusions Human SLC10A1, SLCO1B1, SLC22A6, SLC22A8 and SLC22A24 as well as their mouse and zebrafish orthologues are efficient TRIAC transporters. These findings contribute to the understanding of TRIAC treatment in patients with MCT8 deficiency and animal models thereof.

Are resting metabolic rate and clinical symptoms affected by variation of serum thyroid stimulating hormone levels within the normal range in healthy and women with hypothyroidism? A case-control study

BACKGROUND

It is unclear whether variation in thyroid stimulating hormone (TSH) levels within the reference range affect energy expenditure and clinical symptoms and even within the normal range of TSH levels, resting energy expenditure may alter. The aim of the present study was to determine whether treated hypothyroid subjects and healthy subjects with a low-normal TSH range (0.3-2.3 mIU/L) have better clinical outcomes and increased energy expenditure than those with a high-normal TSH range (2.3-4.3 mIU/L).

METHODS

This was a case-control study of 160 overweight/obese women with TSH levels across the reference range of 0.3-4.3 mU/l. Subjects were paired in four groups: healthy subjects with low-normal target TSH (n = 40), healthy subjects with high-normal target TSH (n = 40), subjects with treated hypothyroidism with low-normal target TSH (n = 40), and subjects with treated hypothyroidism with high-normal target TSH (n = 40). Resting energy expenditure (RMR), dietary intake, body composition, physical activity, and biochemical markers were assessed.

RESULTS

Subjects with low-normal (≤2.3 mU/L) and high-normal (>2.3 mU/L) TSH levels did not differ in terms of RMR, serum T3 levels, and clinical symptoms except fatigue (P = 0.013). However, serum fT4 levels were found to be significantly different between the study groups (P = 0.002). Serum fT4 concentration was the highest in subjects with treated hypothyroidism with low-normal target TSH.

CONCLUSION

Variation in serum TSH levels within the reference range did not significantly affect REE and clinical symptoms except fatigue in healthy and women with hypothyroidism.

Identification of human TRIAC transmembrane transporters

BACKGROUND

TRIAC (3,5,3'-triiodothyroacetic acid) is a T3-receptor agonist pharmacologically used in patients to mitigate T3 resistance. It is additionally explored to treat some symptoms of patients with inactivating mutations in the thyroid hormone (TH) transporter MCT8 (SLC16A2). MCT8 is expressed along the blood-brain-barrier, on neurons, astrocytes, and oligodendrocytes. Hence, pathogenic variants in MCT8 limit the access of TH into and their functions within the brain. TRIAC was shown to enter the brain independently of MCT8 and to modulate expression of TH-dependent genes. The aim of the study was to identify transporters that facilitate TRIAC uptake into cells.

METHODS

We performed a whole-genome RNAi screen in HepG2 cells stably expressing a T3-receptor-dependent luciferase reporter gene. Validation of hits from the primary and confirmatory secondary screen involved a counter screen with siRNAs and compared the cellular response to TRIAC to the effect of T3, in order to exclude siRNAs targeting the gene expression machinery. MDCK1 cells were stably transfected with cDNA encoding C-terminally myc-tagged versions of the identified TRIAC-preferring transporters. Several individual clones were selected after immunocytochemical characterization for biochemical characterization of their 125I-TRIAC transport activities.

RESULTS

We identified SLC22A9 and SLC29A2 as transporters mediating cellular uptake of TRIAC. SLC22A9 encodes the organic anion transporting polypeptide 7 (OAT7), a sodium-independent organic anion transporter expressed in the plasma membrane in brain, pituitary, liver and other organs. Competition with the SLC22A9/OAT7 substrate estrone-3-sulfate reduced 125I-TRIAC uptake. SLC29A2 encodes the equilibrative nucleoside transporter 2 (ENT2), which is ubiquitously expressed including pituitary and brain. Co-incubation with the SLC29A2/ENT2 inhibitor nitrobenzyl-6-thioinosine reduced 125I-TRIAC uptake. Moreover, ABCD1, an ATP-dependent peroxisomal pump, was identified as a 125I-TRIAC exporter in transfected MDCK1 cells.

CONCLUSIONS

Knowledge of TRIAC transporter expression patterns, also during brain development, may thus in the future help to interpret observations on TRIAC effects as well as understand why TRIAC may not show a desirable effect on cells or organs not expressing appropriate transporters. The identification of ABCD1 highlights the sensitivity of our established screening assay, but it may not hold significant relevance for patients undergoing TRIAC treatment.

c-fos induction in the choroid plexus, tanycytes and pars tuberalis is an early indicator of spontaneous arousal from torpor in a deep hibernator

Hibernation is an extreme state of seasonal energy conservation, reducing metabolic rate to as little as 1% of the active state. During the hibernation season, many species of hibernating mammals cycle repeatedly between the active (aroused) and hibernating (torpid) states (T-A cycling), using brown adipose tissue (BAT) to drive cyclical rewarming. The regulatory mechanisms controlling this process remain undefined but are presumed to involve thermoregulatory centres in the hypothalamus. Here, we used the golden hamster (Mesocricetus auratus), and high-resolution monitoring of BAT, core body temperature and ventilation rate, to sample at precisely defined phases of the T-A cycle. Using c-fos as a marker of cellular activity, we show that although the dorsomedial hypothalamus is active during torpor entry, neither it nor the pre-optic area shows any significant changes during the earliest stages of spontaneous arousal. Contrastingly, in three non-neuronal sites previously linked to control of metabolic physiology over seasonal and daily time scales - the choroid plexus, pars tuberalis and third ventricle tanycytes - peak c-fos expression is seen at arousal initiation. We suggest that through their sensitivity to factors in the blood or cerebrospinal fluid, these sites may mediate metabolic feedback-based initiation of the spontaneous arousal process.

Thyroid hormone analogues: Promising therapeutic avenues to improve the neurodevelopmental outcomes of intrauterine growth restriction

Intrauterine growth restriction (IUGR) is a pregnancy complication impairing fetal growth and development. The compromised development is often attributed to disruptions of oxygen and nutrient supply from the placenta, resulting in a number of unfavourable physiological outcomes with impaired brain and organ growth. IUGR is associated with compromised development of both grey and white matter, predisposing the infant to adverse neurodevelopmental outcomes, including long-lasting cognitive and motor difficulties. Cerebral thyroid hormone (TH) signalling, which plays a crucial role in regulating white and grey matter development, is dysregulated in IUGR, potentially contributing to the neurodevelopmental delays associated with this condition. Notably, one of the major TH transporters, monocarboxylate transporter-8 (MCT8), is deficient in the fetal IUGR brain. Currently, no effective treatment to prevent or reverse IUGR exists. Management strategies involve close antenatal monitoring, management of maternal risk factors if present and early delivery if IUGR is found to be severe or worsening in utero. The overall goal is to determine the most appropriate time for delivery, balancing the risks of preterm birth with further fetal compromise due to IUGR. Drug candidates have shown either adverse effects or little to no benefits in this vulnerable population, urging further preclinical and clinical investigation to establish effective therapies. In this review, we discuss the major neuropathology of IUGR driven by uteroplacental insufficiency and the concomitant long-term neurobehavioural impairments in individuals born IUGR. Importantly, we review the existing clinical and preclinical literature on cerebral TH signalling deficits, particularly the impaired expression of MCT8 and their correlation with IUGR. Lastly, we discuss the current evidence on MCT8-independent TH analogues which mimic the brain actions of THs by being metabolised in a similar manner as promising, albeit underappreciated approaches to promote grey and white matter development and improve the neurobehavioural outcomes following IUGR.

Identification of Iodotyrosines as Novel Substrates for the Thyroid Hormone Transporter MCT8

Background: Monocarboxylate transporter 8 (MCT8) is the most specific thyroid hormone transporter identified to date, deficiency of which has been associated with severe intellectual and motor disability and abnormal serum thyroid function tests. However, it is presently unknown if MCT8, similar to other thyroid hormone transporters, also accepts additional substrates, and if disruption of their transport may contribute to the observed phenotype. Methods: In this study, we aimed to identify such substrates by applying liquid chromatography-mass spectrometry-based metabolome analysis in lysates of control and MCT8-overexpressing Xenopus oocytes. A subset of identified candidate substrates were validated by direct transport studies in transiently transfected COS-1 cells and human fibroblasts, which endogenously express MCT8. Moreover, transport characteristics were determined, including transport saturation and cis-inhibition potency of thyroid hormone transport. Results: Metabolome analysis identified 21 m/z ratios, corresponding to 87 candidate metabolites, with a 2.0-times differential abundance in MCT8-injected oocytes compared with controls. These metabolites included 3,5-diiodotyrosine (DIT) and several amino acids, including glutamate and glutamine. In accordance, MCT8-expressing COS-1 cells had 2.2-times lower intracellular accumulation of [125I]-DIT compared with control cells. This effect was largely blocked in the presence of 3,3',5-triiodothyronine (T3) (IC50: 2.5 ± 1.5 µM) or thyroxine (T4) (IC50: 5.8 ± 1.3 µM). Conversely, increasing concentrations of DIT enhanced the accumulation of T3 and T4. The MCT8-specific inhibitor silychristin increased the intracellular accumulation of DIT in human fibroblasts. COS-1 cells expressing MCT8 also exhibited a 50% reduction in intracellular accumulation of [125I]-3-monoiodotyrosine (MIT). In contrast, COS-1 cells expressing MCT8 did not alter the intracellular accumulation of [3H]-glutamate or [3H]-glutamine. However, studies in human fibroblasts showed a 1.5-1.9 times higher glutamate uptake in control fibroblasts compared with fibroblasts derived from patients with MCT8 deficiency, which was not affected in the presence of silychristin. Conclusions: Taken together, our results suggest that the iodotyrosines DIT and MIT can be exported by MCT8. MIT and DIT interfere with MCT8-mediated transport of thyroid hormone in vitro and vice versa. Future studies should elucidate if MCT8, being highly expressed in thyroidal follicular cells, also transports iodotyrosines in vivo.

Spatially Dependent Tissue Distribution of Thyroid Hormones by Plasma Thyroid Hormone Binding Proteins

Plasma thyroid hormone (TH) binding proteins (THBPs), including thyroxine-binding globulin (TBG), transthyretin (TTR), and albumin (ALB), carry THs to extrathyroidal sites, where THs are unloaded locally and then taken up via membrane transporters into the tissue proper. The respective roles of THBPs in supplying THs for tissue uptake are not completely understood. To investigate this, we developed a spatial human physiologically based kinetic (PBK) model of THs, which produces several novel findings. (1) Contrary to postulations that TTR and/or ALB are the major local T4 contributors, the three THBPs may unload comparable amounts of T4 in Liver, a rapidly perfused organ; however, their contributions in slowly perfused tissues follow the order of abundances of T4TBG, T4TTR, and T4ALB. The T3 amounts unloaded from or loaded onto THBPs in a tissue acting as a T3 sink or source respectively follow the order of abundance of T3TBG, T3ALB, and T3TTR regardless of perfusion rate. (2) Any THBP alone is sufficient to maintain spatially uniform TH tissue distributions. (3) The TH amounts unloaded by each THBP species are spatially dependent and nonlinear in a tissue, with ALB being the dominant contributor near the arterial end but conceding to TBG near the venous end. (4) Spatial gradients of TH transporters and metabolic enzymes may modulate these contributions, producing spatially invariant or heterogeneous TH tissue concentrations depending on whether the blood-tissue TH exchange operates in near-equilibrium mode. In summary, our modeling provides novel insights into the differential roles of THBPs in local TH tissue distribution.

Transporters, Ion Channels, and Junctional Proteins in Choroid Plexus Epithelial Cells

The choroid plexus (CP) plays significant roles in secreting cerebrospinal fluid (CSF) and forming circadian rhythms. A monolayer of epithelial cells with tight and adherens junctions of CP forms the blood-CSF barrier to control the movement of substances between the blood and ventricles, as microvessels in the stroma of CP have fenestrations in endothelial cells. CP epithelial cells are equipped with several kinds of transporters and ion channels to transport nutrient substances and secrete CSF. In addition, junctional components also contribute to CSF production as well as blood-CSF barrier formation. However, it remains unclear how junctional components as well as transporters and ion channels contribute to the pathogenesis of neurodegenerative disorders. In this manuscript, recent findings regarding the distribution and significance of transporters, ion channels, and junctional proteins in CP epithelial cells are introduced, and how changes in expression of their epithelial proteins contribute to the pathophysiology of brain disorders are reviewed.

Impaired T3 uptake and action in MCT8-deficient cerebral organoids underlie Allan-Herndon-Dudley syndrome

Patients with mutations in the thyroid hormone (TH) cell transporter monocarboxylate transporter 8 (MCT8) gene develop severe neuropsychomotor retardation known as Allan-Herndon-Dudley syndrome (AHDS). It is assumed that this is caused by a reduction in TH signaling in the developing brain during both intrauterine and postnatal developmental stages, and treatment remains understandably challenging. Given species differences in brain TH transporters and the limitations of studies in mice, we generated cerebral organoids (COs) using human induced pluripotent stem cells (iPSCs) from MCT8-deficient patients. MCT8-deficient COs exhibited (i) altered early neurodevelopment, resulting in smaller neural rosettes with thinner cortical units, (ii) impaired triiodothyronine (T3) transport in developing neural cells, as assessed through deiodinase-3-mediated T3 catabolism, (iii) reduced expression of genes involved in cerebral cortex development, and (iv) reduced T3 inducibility of TH-regulated genes. In contrast, the TH analogs 3,5-diiodothyropropionic acid and 3,3',5-triiodothyroacetic acid triggered normal responses (induction/repression of T3-responsive genes) in MCT8-deficient COs, constituting proof of concept that lack of T3 transport underlies the pathophysiology of AHDS and demonstrating the clinical potential for TH analogs to be used in treating patients with AHDS. MCT8-deficient COs represent a species-specific relevant preclinical model that can be utilized to screen drugs with potential benefits as personalized therapeutics for patients with AHDS.

Factors and Mechanisms of Thyroid Hormone Activity in the Brain: Possible Role in Recovery and Protection

Thyroid hormones (THs) are essential in normal brain development, and cognitive and emotional functions. THs act through a cascade of events including uptake by the target cells by specific cell membrane transporters, activation or inactivation by deiodinase enzymes, and interaction with nuclear thyroid hormone receptors. Several thyroid responsive genes have been described in the developing and in the adult brain and many studies have demonstrated a systemic or local reduction in TH availability in neurologic disease and after brain injury. In this review, the main factors and mechanisms associated with the THs in the normal and damaged brain will be evaluated in different regions and cellular contexts. Furthermore, the most common animal models used to study the role of THs in brain damage and cognitive impairment will be described and the use of THs as a potential recovery strategy from neuropathological conditions will be evaluated. Finally, particular attention will be given to the link observed between TH alterations and increased risk of Alzheimer's Disease (AD), the most prevalent neurodegenerative and dementing condition worldwide.

Optimized Mass Spectrometry Detection of Thyroid Hormones and Polar Metabolites in Rodent Cerebrospinal Fluid

Thyroid hormones (TH) are required for brain development and function. Cerebrospinal fluid (CSF), which bathes the brain and spinal cord, contains TH as free hormones or as bound to transthyretin (TTR). Tight TH level regulation in the central nervous system is essential for developmental gene expression, which governs neurogenesis, myelination, and synaptogenesis. This integrated function of TH highlights the importance of developing precise and reliable methods for assessing TH levels in CSF. We report an optimized liquid chromatography-mass spectrometry (LC-MS)-based method to measure TH in rodent CSF and serum, applicable to both fresh and frozen samples. Using this new method, we find distinct differences in CSF TH in pregnant dams vs. non-pregnant adults and in embryonic vs. adult CSF. Further, targeted LC-MS metabolic profiling uncovers distinct central carbon metabolism in the CSF of these populations. TH detection and metabolite profiling of related metabolic pathways open new avenues of rigorous research into CSF TH and will inform future studies on metabolic alterations in CSF during normal development.

Optimized Mass Spectrometry Detection of Thyroid Hormones and Polar Metabolites in Rodent Cerebrospinal Fluid

BACKGROUND

Thyroid hormones (TH) are required for brain development and function. Cerebrospinal fluid (CSF), which bathes the brain and spinal cord, contains TH as free or transthyretin (TTR)-bound. Tight thyroid hormone level regulation in the central nervous system is essential for developmental gene expression that governs neurogenesis, myelination, and synaptogenesis. This integrated function of TH highlights the importance of developing precise and reliable methods for assessing TH levels in CSF.

METHODS

we report an optimized LC-MS based method to measure thyroid hormones in rodent CSF and serum, applicable to both fresh and frozen samples.

RESULTS

We find distinct differences in CSF thyroid hormone in pregnant dams vs. non-pregnant adults and in embryonic vs. adult CSF. Further, targeted LC-MS metabolic profiling uncovers distinct central carbon metabolism in the CSF of these populations.

CONCLUSIONS

TH detection and metabolite profiling of related metabolic pathways open new avenues of rigorous research into CSF thyroid hormone and will inform future studies on metabolic alterations in CSF during normal development.

Repeated toluene and cyclohexane inhalation produces differential effects on HPA and HPT axes in adolescent male rats

Misused volatile solvents typically contain toluene (TOL) as the main psychoactive ingredient. Cyclohexane (CHX) can also be present and is considered a safer alternative. Solvent misuse often occurs at early stages of life, leading to permanent neurobehavioral impairment and growth retardation. However, a comprehensive examination of the effects of TOL and CHX on stress regulation and energy balance is lacking. Here, we compared the effect of a binge-pattern exposure to TOL or CHX (4,000 or 8,000 ppm) on body weight, food intake, the hypothalamus-pituitary-adrenal (HPA) and hypothalamus-pituitary-thyroid (HPT) axes in male adolescent Wistar rats. At 8,000 ppm, TOL decreased body weight gain without affecting food intake. In addition, TOL and CHX altered the HPA and HPT axes' function in a solvent- and concentration-dependent manner. The highest TOL concentration produced HPA axis hyperactivation in animals not subjected to stress, which was evidenced by increased corticotropin-releasing-factor (CRF) release from the median eminence (ME), elevated adrenocorticotropin hormone (ACTH) and corticosterone serum levels, and decreased CRF mRNA levels in the hypothalamic paraventricular nucleus (PVN). TOL (8,000 ppm) also increased triiodothyronine (T3) serum levels, decreased pro-thyrotropin-releasing-hormone (pro-TRH) mRNA transcription in the PVN, pro-TRH content in the ME, and serum thyroid stimulating hormone (TSH) levels. CHX did not affect the HPA axis. We propose that the increased HPT axis activity induced by TOL can be related to the impaired body weight gain associated with inhalant misuse. These findings may contribute to a better understanding of the effects of the misused solvents TOL and CHX.

Allan-Herndon-Dudley syndrome in Hong Kong: Implication for newborn screening

BACKGROUND

Allan-Herndon-Dudley syndrome (MCT 8 deficiency) is an X-linked recessive condition caused by hemizygous pathogenic variants in SLC16A2 encoding the monocarboxylate transporter 8 (MCT8). Patients present with global developmental delay and neurological impairment, and abnormal serum thyroid function tests. The drug, 3,3',5 triiodothyroacetic acid (TRIAC), was recently demonstrated to improve the endocrinological profile. Improvement in diagnostic approach is key to earlier start of treatment.

PATIENT FINDINGS

We described four Chinese patients with MCT8 deficiency undergoing different diagnostic odysseys. Their initial presentation included global developmental delay and dystonia. Patient 2 also had epilepsy. Patients 1 and 2 presented with two novel variants: (1)hemizygous NM_006517.4(SLC16A2):c.1170 + 2 T > A; p.(?), and (2)hemizygous NM_006517.4(SLC16A2):c.305dupT; p.(Val103GlyfsTer17) respectively. Patients 3 and 4 were biological brothers harboring hemizygous NM_006517.4(SLC16A2):c.305dupT; p.(Val103GlyfsTer17), which was first reported in 2004. We obtained the measurement of triiodothyronine (T3) and reverse T3 (rT3) from dried blood spot samples collected on Day 1 of life from Patient 1 and studied the biomarkers (rT3 and T3/rT3 ratio) proposed by Iwayama et al. for the detection of MCT8 deficiency at birth. Our data verified the significantly reduced rT3 level in Patient 1, compared with healthy newborns, although low T3 level and comparable T3/rT3 ratio with controls were detected.

SUMMARY

Patients with MCT8 deficiency often undergo diagnostic odysseys. An early diagnosis could be missed by a normal newborn thyroid function screening result based on biochemical measurement of TSH and/or T4/fT4. Early detection of rT3 is key to improving current diagnostic approach.

CONCLUSION

We recommend that full thyroid function profile (TSH, T4/fT4, T3/fT3, rT3) be considered early for all pediatric patients presenting with unexplained developmental delay and/or dystonia. The potential inclusion of rT3 measurement in newborn screening may prove promising.

Establishing Patient-Centered Outcomes for MCT8 Deficiency: Stakeholder Engagement and Systematic Literature Review

Introduction

The SCL16A2 gene encodes the thyroid hormone (TH) transporter MCT8. Pathogenic variants result in a reduced TH uptake into the CNS despite high serum T3 concentrations. Patients suffer from severe neurodevelopmental delay and require multidisciplinary care. Since a first compassionate use study in 2008, the development of therapies has recently gained momentum. Treatment strategies range from symptom-based approaches, supplementation with TH or TH-analogs, to gene therapy. All these studies have mainly used surrogate endpoints and clinical outcomes. However, the EMA and FDA strongly encourage researchers to involve patients and their advocacy groups in the design of clinical trials. This should strengthen the patients' perspective and identify clinical endpoints that are clinically relevant to their daily life.

Methods

We involved patient families to define patient-relevant outcomes for MCT8 deficiency. In close collaboration with patient families, we designed a questionnaire asking for their five most preferred therapeutic goals, which, if achieved at least, make a difference in their lives. In addition, we performed a systematic review according to Cochrane recommendations of the published treatment trials.

Results

We obtained results from 15 families with completed questionnaires from 14 mothers and 8 fathers. Improvement in development, especially in gross motor skills, was most important to the parents. 59% wished for head control and 50% for sitting ability. Another 36% wished for weight gain, 32% for improvement of expressive language skills, and 18% for a reduction of dystonia/spasticity, less dysphagia, and reflux. Paraclinical aspects were least important (5-9%). In a treatment trial (n=46) and compassionate use cases (n=83), the results were mainly inconclusive, partly due to a lack of predefined patient-centered clinical endpoints.

Discussion

We recommend that future trials should define a relevant improvement in "development" and/or other patient-relevant outcomes compared to natural history as treatment goals.

Proteome Analysis of Thyroid Hormone Transporter Mct8/Oatp1c1-Deficient Mice Reveals Novel Dysregulated Target Molecules Involved in Locomotor Function

Thyroid hormone (TH) transporter MCT8 deficiency causes severe locomotor disabilities likely due to insufficient TH transport across brain barriers and, consequently, compromised neural TH action. As an established animal model for this disease, Mct8/Oatp1c1 double knockout (DKO) mice exhibit strong central TH deprivation, locomotor impairments and similar histo-morphological features as seen in MCT8 patients. The pathways that cause these neuro-motor symptoms are poorly understood. In this paper, we performed proteome analysis of brain sections comprising cortical and striatal areas of 21-day-old WT and DKO mice. We detected over 2900 proteins by liquid chromatography mass spectrometry, 67 of which were significantly different between the genotypes. The comparison of the proteomic and published RNA-sequencing data showed a significant overlap between alterations in both datasets. In line with previous observations, DKO animals exhibited decreased myelin-associated protein expression and altered protein levels of well-established neuronal TH-regulated targets. As one intriguing new candidate, we unraveled and confirmed the reduced protein and mRNA expression of Pde10a, a striatal enzyme critically involved in dopamine receptor signaling, in DKO mice. As altered PDE10A activities are linked to dystonia, reduced basal ganglia PDE10A expression may represent a key pathogenic pathway underlying human MCT8 deficiency.

A Historical Reflection on Scientific Advances in Understanding Thyroid Hormone Action

Background: Thyroid hormone (TH) has actions in every tissue of the body and is essential for normal development, as well as having important actions in the adult. The earliest markers of TH action that were identified and monitored clinically, even before TH could be measured in serum, included oxygen consumption, basal metabolic rate, serum cholesterol, and deep tendon reflex time. Cellular, rodent, amphibian, zebrafish, and human models have been used to study TH action. Summary: Early studies of the mechanism of TH action focused on saturable-specific triiodothyronine (T3) nuclear binding and direct actions of T3 that altered protein expression. Additional effects of TH were recognized on mitochondria, stimulation of ion transport, especially the sodium potassium ATPase, augmentation of adrenergic signaling, role as a neurotransmitter, and direct plasma membrane effects. The cloning of the thyroid hormone receptor (THR) genes in 1986 and report of the THR crystal structure in 1995 produced rapid progress in understanding the mechanism of TH nuclear action, as well as the development of modified THR ligands. These findings revealed nuances of TH signaling, including the role of nuclear receptor coactivators and corepressors, repression of positively stimulated genes by the unliganded receptor, THR isoform-specific actions of TRα (THRA) and TRβ (THRB), and THR binding DNA as a heterodimer with retinoid-x-receptor (RXR) for genes positively regulated by TH. The identification of genetic disorders of TH transport and signaling, especially Resistance to Thyroid Hormone (RTH) and monocarboxylate transporter 8 (Mct8) defects, has been highly informative with respect to the mechanism of TH action. Conclusions: The impact of THR isoform, post-translational modifications, receptor cofactors, DNA response element, and selective TH tissue uptake, on TH action, have clinical implications for diagnosing and treating thyroid disease. Additionally, these findings have led to the development of novel TH and TH analogue therapies for metabolic, neurological, and cardiovascular diseases.

Distribution of Monocarboxylate Transporters in Brain and Choroid Plexus Epithelium

The choroid plexus (CP) plays central roles in regulating the microenvironment of the central nervous system by secreting the majority of cerebrospinal fluid (CSF) and controlling its composition. A monolayer of epithelial cells of CP plays a significant role in forming the blood-CSF barrier to restrict the movement of substances between the blood and ventricles. CP epithelial cells are equipped with transporters for glucose and lactate that are used as energy sources. There are many review papers on glucose transporters in CP epithelial cells. On the other hand, distribution of monocarboxylate transporters (MCTs) in CP epithelial cells has received less attention compared with glucose transporters. Some MCTs are known to transport lactate, pyruvate, and ketone bodies, whereas others transport thyroid hormones. Since CP epithelial cells have significant carrier functions as well as the barrier function, a decline in the expression and function of these transporters leads to a poor supply of thyroid hormones as well as lactate and can contribute to the process of age-associated brain impairment and pathophysiology of neurodegenerative diseases. In this review paper, recent findings regarding the distribution and significance of MCTs in the brain, especially in CP epithelial cells, are summarized.

Thyroid Hormone Transporters MCT8 and OATP1C1 Are Expressed in Projection Neurons and Interneurons of Basal Ganglia and Motor Thalamus in the Adult Human and Macaque Brains

Monocarboxylate transporter 8 (MCT8) and organic anion-transporting polypeptide 1C1 (OATP1C1) are thyroid hormone (TH) transmembrane transporters relevant for the availability of TH in neural cells, crucial for their proper development and function. Mutations in MCT8 or OATP1C1 result in severe disorders with dramatic movement disability related to alterations in basal ganglia motor circuits. Mapping the expression of MCT8/OATP1C1 in those circuits is necessary to explain their involvement in motor control. We studied the distribution of both transporters in the neuronal subpopulations that configure the direct and indirect basal ganglia motor circuits using immunohistochemistry and double/multiple labeling immunofluorescence for TH transporters and neuronal biomarkers. We found their expression in the medium-sized spiny neurons of the striatum (the receptor neurons of the corticostriatal pathway) and in various types of its local microcircuitry interneurons, including the cholinergic. We also demonstrate the presence of both transporters in projection neurons of intrinsic and output nuclei of the basal ganglia, motor thalamus and nucleus basalis of Meynert, suggesting an important role of MCT8/OATP1C1 for modulating the motor system. Our findings suggest that a lack of function of these transporters in the basal ganglia circuits would significantly impact motor system modulation, leading to clinically severe movement impairment.

Iodine and Thyroid Maternal and Fetal Metabolism during Pregnancy

Thyroid hormones and iodine are required to increase basal metabolic rate and to regulate protein synthesis, long bone growth and neuronal maturation. They are also essential for protein, fat and carbohydrate metabolism regulation. Imbalances in thyroid and iodine metabolism can negatively affect these vital functions. Pregnant women are at risk of hypo or hyperthyroidism, in relation to or regardless of their medical history, with potential dramatic outcomes. Fetal development highly relies on thyroid and iodine metabolism and can be compromised if they malfunction. As the interface between the fetus and the mother, the placenta plays a crucial role in thyroid and iodine metabolism during pregnancy. This narrative review aims to provide an update on current knowledge of thyroid and iodine metabolism in normal and pathological pregnancies. After a brief description of general thyroid and iodine metabolism, their main modifications during normal pregnancies and the placental molecular actors are described. We then discuss the most frequent pathologies to illustrate the upmost importance of iodine and thyroid for both the mother and the fetus.

In a zebrafish biomedical model of human Allan-Herndon-Dudley syndrome impaired MTH signaling leads to decreased neural cell diversity

Background

Maternally derived thyroid hormone (T3) is a fundamental factor for vertebrate neurodevelopment. In humans, mutations on the thyroid hormones (TH) exclusive transporter monocarboxylic acid transporter 8 (MCT8) lead to the Allan-Herndon-Dudley syndrome (AHDS). Patients with AHDS present severe underdevelopment of the central nervous system, with profound cognitive and locomotor consequences. Functional impairment of zebrafish T3 exclusive membrane transporter Mct8 phenocopies many symptoms observed in patients with AHDS, thus providing an outstanding animal model to study this human condition. In addition, it was previously shown in the zebrafish mct8 KD model that maternal T3 (MTH) acts as an integrator of different key developmental pathways during zebrafish development.

Methods

Using a zebrafish Mct8 knockdown model, with consequent inhibition of maternal thyroid hormones (MTH) uptake to the target cells, we analyzed genes modulated by MTH by qPCR in a temporal series from the start of segmentation through hatching. Survival (TUNEL) and proliferation (PH3) of neural progenitor cells (dla, her2) were determined, and the cellular distribution of neural MTH-target genes in the spinal cord during development was characterized. In addition, in-vivo live imaging was performed to access NOTCH overexpression action on cell division in this AHDS model. We determined the developmental time window when MTH is required for appropriate CNS development in the zebrafish; MTH is not involved in neuroectoderm specification but is fundamental in the early stages of neurogenesis by promoting the maintenance of specific neural progenitor populations. MTH signaling is required for developing different neural cell types and maintaining spinal cord cytoarchitecture, and modulation of NOTCH signaling in a non-autonomous cell manner is involved in this process.

Discussion

The findings show that MTH allows the enrichment of neural progenitor pools, regulating the cell diversity output observed by the end of embryogenesis and that Mct8 impairment restricts CNS development. This work contributes to the understanding of the cellular mechanisms underlying human AHDS.

Comparative transcriptome analysis of Peromyscus leucopus and C3H mice infected with the Lyme disease pathogen

Lyme disease (LD), the most prevalent tick-borne disease of humans in the Northern Hemisphere, is caused by the spirochetal bacterium of Borreliella burgdorferi (Bb) sensu lato complex. In nature, Bb spirochetes are continuously transmitted between Ixodes ticks and mammalian or avian reservoir hosts. Peromyscus leucopus mice are considered the primary mammalian reservoir of Bb in the United States. Earlier studies demonstrated that experimentally infected P. leucopus mice do not develop disease. In contrast, C3H mice, a widely used laboratory strain of Mus musculus in the LD field, develop severe Lyme arthritis. To date, the exact tolerance mechanism of P. leucopus mice to Bb-induced infection remains unknown. To address this knowledge gap, the present study has compared spleen transcriptomes of P. leucopus and C3H/HeJ mice infected with Bb strain 297 with those of their respective uninfected controls. Overall, the data showed that the spleen transcriptome of Bb-infected P. leucopus mice was much more quiescent compared to that of the infected C3H mice. To date, the current investigation is one of the few that have examined the transcriptome response of natural reservoir hosts to Borreliella infection. Although the experimental design of this study significantly differed from those of two previous investigations, the collective results of the current and published studies have consistently demonstrated very limited transcriptomic responses of different reservoir hosts to the persistent infection of LD pathogens.

Importance

The bacterium Borreliella burgdorferi (Bb) causes Lyme disease, which is one of the emerging and highly debilitating human diseases in countries of the Northern Hemisphere. In nature, Bb spirochetes are maintained between hard ticks of Ixodes spp. and mammals or birds. In the United States, the white-footed mouse, Peromyscus leucopus, is one of the main Bb reservoirs. In contrast to humans and laboratory mice (e.g., C3H mice), white-footed mice rarely develop clinical signs (disease) despite being (persistently) infected with Bb. How the white-footed mouse tolerates Bb infection is the question that the present study has attempted to address. Comparisons of genetic responses between Bb-infected and uninfected mice demonstrated that, during a long-term Bb infection, C3H mice reacted much stronger, whereas P. leucopus mice were relatively unresponsive.

The importance of thyroid hormone signaling during early development: Lessons from the zebrafish model

The zebrafish is an optimal experimental model to study thyroid hormone (TH) involvement in vertebrate development. The use of state-of-the-art zebrafish genetic tools available for the study of the effect of gene silencing, cell fate decisions and cell lineage differentiation have contributed to a more insightful comprehension of molecular, cellular, and tissue-specific TH actions. In contrast to intrauterine development, extrauterine embryogenesis observed in zebrafish has facilitated a more detailed study of the development of the hypothalamic-pituitary-thyroid axis. This model has also enabled a more insightful analysis of TH molecular actions upon the organization and function of the brain, the retina, the heart, and the immune system. Consequently, zebrafish has become a trendy model to address paradigms of TH-related functional and biomedical importance. We here compilate the available knowledge regarding zebrafish developmental events for which specific components of TH signaling are essential.

Maternal Administration of the CNS-Selective Sobetirome Prodrug Sob-AM2 Exerts Thyromimetic Effects in Murine MCT8-Deficient Fetuses

Background: Monocarboxylate transporter 8 (MCT8) deficiency is a rare X-linked disease where patients exhibit peripheral hyperthyroidism and cerebral hypothyroidism, which results in severe neurological impairments. These brain defects arise from a lack of thyroid hormones (TH) during critical stages of human brain development. Treatment options for MCT8-deficient patients are limited and none have been able to prevent or ameliorate effectively the neurological impairments. This study explored the effects of the TH agonist sobetirome and its CNS-selective amide prodrug, Sob-AM2, in the treatment of pregnant dams carrying fetuses lacking Mct8 and deiodinase type 2 (Mct8/Dio2 KO), as a murine model for MCT8 deficiency. Methods: Pregnant dams carrying Mct8/Dio2 KO fetuses were treated with 1 mg of sobetirome/kg body weight/day, or 0.3 mg of Sob-AM2/kg body weight/day for 7 days, starting at embryonic day 12.5 (E12.5). As controls, pregnant dams carrying wild-type and pregnant dams carrying Mct8/Dio2 KO fetuses were treated with daily subcutaneous injections of vehicle. Dams TH levels were measured by enzyme-linked immunosorbent assay (ELISA). Samples were extracted at E18.5 and the effect of treatments on the expression of triiodothyronine (T3)-dependent genes was measured in the placenta, fetal liver, and fetal cerebral cortex by real-time polymerase chain reaction. Results: Maternal sobetirome treatment led to spontaneous abortions. Sob-AM2 treatment, however, was able to cross the placental as well as the brain barriers and exert thyromimetic effects in Mct8/Dio2 KO fetal tissues. Sob-AM2 treatment did not affect the expression of the T3-target genes analyzed in the placenta, but it mediated thyromimetic effects in the fetal liver by increasing the expression of Dio1 and Dio3 genes. Interestingly, Sob-AM2 treatment increased the expression of several T3-dependent genes in the brain such as Hr, Shh, Dio3, Kcnj10, Klf9, and Faah in Mct8/Dio2 KO fetuses. Conclusions: Maternal administration of Sob-AM2 can cross the placental barrier and access the fetal tissues, including the brain, in the absence of MCT8, to exert thyromimetic actions by modulating the expression of T3-dependent genes. Therefore, Sob-AM2 has the potential to address the cerebral hypothyroidism characteristic of MCT8 deficiency from fetal stages and to prevent neurodevelopmental alterations in the MCT8-deficient fetal brain.

Hepatic Energy Metabolism under the Local Control of the Thyroid Hormone System

The energy homeostasis of the organism is orchestrated by a complex interplay of energy substrate shuttling, breakdown, storage, and distribution. Many of these processes are interconnected via the liver. Thyroid hormones (TH) are well known to provide signals for the regulation of energy homeostasis through direct gene regulation via their nuclear receptors acting as transcription factors. In this comprehensive review, we summarize the effects of nutritional intervention like fasting and diets on the TH system. In parallel, we detail direct effects of TH in liver metabolic pathways with regards to glucose, lipid, and cholesterol metabolism. This overview on hepatic effects of TH provides the basis for understanding the complex regulatory network and its translational potential with regards to currently discussed treatment options of non-alcoholic fatty liver disease (NAFLD) and non-alcoholic steatohepatitis (NASH) involving TH mimetics.

Thyroid Hormone Transporters MCT8 and OATP1C1 Are Expressed in Pyramidal Neurons and Interneurons in the Adult Motor Cortex of Human and Macaque Brain

Monocarboxylate transporter 8 (MCT8) and organic anion transporter polypeptide 1C1 (OATP1C1) are thyroid hormone (TH) transmembrane transporters that play an important role in the availability of TH for neural cells, allowing their proper development and function. It is important to define which cortical cellular subpopulations express those transporters to explain why MCT8 and OATP1C1 deficiency in humans leads to dramatic alterations in the motor system. By means of immunohistochemistry and double/multiple labeling immunofluorescence in adult human and monkey motor cortices, we demonstrate the presence of both transporters in long-projection pyramidal neurons and in several types of short-projection GABAergic interneurons in both species, suggesting a critical position of these transporters for modulating the efferent motor system. MCT8 is present at the neurovascular unit, but OATP1C1 is only present in some of the large vessels. Both transporters are expressed in astrocytes. OATP1C1 was unexpectedly found, only in the human motor cortex, inside the Corpora amylacea complexes, aggregates linked to substance evacuation towards the subpial system. On the basis of our findings, we propose an etiopathogenic model that emphasizes these transporters' role in controlling excitatory/inhibitory motor cortex circuits in order to understand some of the severe motor disturbances observed in TH transporter deficiency syndromes.

The Emerging Role of Epigenetics in Metabolism and Endocrinology

Each cell in a multicellular organism has its own phenotype despite sharing the same genome. Epigenetics is a somatic, heritable pattern of gene expression or cellular phenotype mediated by structural changes in chromatin that occur without altering the DNA sequence. Epigenetic modification is an important factor in determining the level and timing of gene expression in response to endogenous and exogenous stimuli. There is also growing evidence concerning the interaction between epigenetics and metabolism. Accordingly, several enzymes that consume vital metabolites as substrates or cofactors are used during the catalysis of epigenetic modification. Therefore, altered metabolism might lead to diseases and pathogenesis, including endocrine disorders and cancer. In addition, it has been demonstrated that epigenetic modification influences the endocrine system and immune response-related pathways. In this regard, epigenetic modification may impact the levels of hormones that are important in regulating growth, development, reproduction, energy balance, and metabolism. Altering the function of the endocrine system has negative health consequences. Furthermore, endocrine disruptors (EDC) have a significant impact on the endocrine system, causing the abnormal functioning of hormones and their receptors, resulting in various diseases and disorders. Overall, this review focuses on the impact of epigenetics on the endocrine system and its interaction with metabolism.

The ATP-binding cassette proteins ABCB1 and ABCC1 as modulators of glucocorticoid action

Responses to hormones that act through nuclear receptors are controlled by modulating hormone concentrations not only in the circulation but also within target tissues. The role of enzymes that amplify or reduce local hormone concentrations is well established for glucocorticoid and other lipophilic hormones; moreover, transmembrane transporters have proven critical in determining tissue responses to thyroid hormones. However, there has been less consideration of the role of transmembrane transport for steroid hormones. ATP-binding cassette (ABC) proteins were first shown to influence the accumulation of glucocorticoids in cells almost three decades ago, but observations over the past 10 years suggest that differential transport propensities of both exogenous and endogenous glucocorticoids by ABCB1 and ABCC1 transporters provide a mechanism whereby different tissues are preferentially sensitive to different steroids. This Review summarizes this evidence and the new insights provided for the physiology and pharmacology of glucocorticoid action, including new approaches to glucocorticoid replacement.

Intracerebroventricular High Doses of 3,3',5-Triiodothyroacetic Acid at Juvenile Stages Improve Peripheral Hyperthyroidism and Mediate Thyromimetic Effects in Limited Brain Regions in a Mouse Model of Monocarboxylate Transporter 8 Deficiency

Introduction: Patients lacking functional monocarboxylate transporter 8 (MCT8), a highly specific thyroid hormone (TH) transporter, present severe psychomotor disabilities. MCT8 deficiency leads to peripheral hyperthyroidism and brain hypothyroidism, the latter due to impaired transport of TH across brain barriers. Available treatments for patients are limited and aim to overcome the limited TH transport across brain barriers. The use of TH analogues such as 3,3',5-triiodothyroacetic acid (TRIAC) that do not require MCT8 to cross the cellular membranes is considered a potential therapy for MCT8 deficiency. Previous studies have shown that systemic administration of TRIAC at therapeutic doses does not increase TRIAC content in the brain, while intracerebroventricular (ICV) administration of therapeutic doses of TRIAC increases TRIAC content in the brain but does not mediate thyromimetic effects. In view of this, we hypothesize that ICV administration of high doses of TRIAC can mediate thyromimetic effects in the brain without worsening the brain hypothyroidism or peripheral hyperthyroidism of patients. Methods: We administered 400 ng/g of body weight per day of ICV TRIAC in a mouse model of MCT8 deficiency: Mct8^-/y and deiodinase 2 (Dio2)^-/- double knockout mice. The effects of this treatment on TH and TRIAC levels/content in blood and tissues were determined by radioimmunoassay and effects on TH-regulated genes were assessed by real time-quantitative polymerase chain reaction in peripheral and central tissues. Results: ICV administration of high doses of TRIAC ameliorated the peripheral hyperthyroidism. In the brain, this treatment did not further aggravate brain hypothyroidism and increased TRIAC content in several brain regions; however, only moderate thyromimetic activity was observed in restricted brain areas. Conclusion: Administration of high doses of TRIAC by ICV delivery at juvenile stages in a mouse model of MCT8 deficiency is effective in normalizing peripheral hyperthyroidism but exerts minimal thyromimetic activity in the brain.

TRIAC Treatment Improves Impaired Brain Network Function and White Matter Loss in Thyroid Hormone Transporter Mct8/Oatp1c1 Deficient Mice

Dysfunctions of the thyroid hormone (TH) transporting monocarboxylate transporter MCT8 lead to a complex X-linked syndrome with abnormal serum TH concentrations and prominent neuropsychiatric symptoms (Allan-Herndon-Dudley syndrome, AHDS). The key features of AHDS are replicated in double knockout mice lacking MCT8 and organic anion transporting protein OATP1C1 (Mct8/Oatp1c1 DKO). In this study, we characterize impairments of brain structure and function in Mct8/Oatp1c1 DKO mice using multimodal magnetic resonance imaging (MRI) and assess the potential of the TH analogue 3,3',5-triiodothyroacetic acid (TRIAC) to rescue this phenotype. Structural and functional MRI were performed in 11-weeks-old male Mct8/Oatp1c1 DKO mice (N = 10), wild type controls (N = 7) and Mct8/Oatp1c1 DKO mice (N = 13) that were injected with TRIAC (400 ng/g bw s.c.) daily during the first three postnatal weeks. Grey and white matter volume were broadly reduced in Mct8/Oatp1c1 DKO mice. TRIAC treatment could significantly improve white matter thinning but did not affect grey matter loss. Network-based statistic showed a wide-spread increase of functional connectivity, while graph analysis revealed an impairment of small-worldness and whole-brain segregation in Mct8/Oatp1c1 DKO mice. Both functional deficits could be substantially ameliorated by TRIAC treatment. Our study demonstrates prominent structural and functional brain alterations in Mct8/Oatp1c1 DKO mice that may underlie the psychomotor deficiencies in AHDS. Additionally, we provide preclinical evidence that early-life TRIAC treatment improves white matter loss and brain network dysfunctions associated with TH transporter deficiency.

Thyroid Hormone Transporters in Pregnancy and Fetal Development

Thyroid hormone is essential for fetal (brain) development. Plasma membrane transporters control the intracellular bioavailability of thyroid hormone. In the past few decades, 15 human thyroid hormone transporters have been identified, and among them, mutations in monocarboxylate transporter (MCT)8 and organic anion transporting peptide (OATP)1C1 are associated with clinical phenotypes. Different animal and human models have been employed to unravel the (patho)-physiological role of thyroid hormone transporters. However, most studies on thyroid hormone transporters focus on postnatal development. This review summarizes the research on the thyroid hormone transporters in pregnancy and fetal development, including their substrate preference, expression and tissue distribution, and physiological and pathophysiological role in thyroid homeostasis and clinical disorders. As the fetus depends on the maternal thyroid hormone supply, especially during the first half of pregnancy, the review also elaborates on thyroid hormone transport across the human placental barrier. Future studies may reveal how the different transporters contribute to thyroid hormone homeostasis in fetal tissues to properly facilitate development. Employing state-of-the-art human models will enable a better understanding of their roles in thyroid hormone homeostasis.

Tsh Induces Agrp1 Neuron Proliferation in Oatp1c1-Deficient Zebrafish

Thyroid hormones (THs), thyroxine (T4), and triiodothyronine (T3), regulate growth, metabolism, and neurodevelopment. THs secretion is controlled by the pituitary thyroid-stimulating hormone (TSH) and the hypothalamic-pituitary-thyroid (HPT) axis. The organic anion-transporting polypeptide 1C1 (OATP1C1/SLCO1C1) and the monocarboxylate transporter 8 (MCT8/SLC16A2) actively transport THs, which bind to their nuclear receptors and induce gene expression. A mutation in OATP1C1 is associated with brain hypometabolism, gradual neurodegeneration, and impaired cognitive and motor functioning in adolescent patients. To understand the role of Oatp1c1 and the mechanisms of the disease, we profiled the transcriptome of oatp1c1 mutant (oatp1c1 -/-) and mct8 -/- xoatp1c1 -/- adult male and female zebrafish brains. Among dozens of differentially expressed genes, agouti-related neuropeptide 1 (agrp1) expression increased in oatp1c1 -/- adult brains. Imaging in the hypothalamus revealed enhanced proliferation of Agrp1 neurons in oatp1c1 -/- larvae and adults, and increased food consumption in oatp1c1 -/- larvae. Similarly, feeding and the number of Agrp1 neurons increased in thyroid gland-ablated zebrafish. Pharmacological treatments showed that the T3 analog TRIAC (3,3',5-tri-iodothyroacetic acid), but not T4, normalized the number of Agrp1 neurons in oatp1c1 -/- zebrafish. Since the HPT axis is hyperactive in the oatp1c1 -/- brain, we used the CRISPR-Cas9 system to knockdown tsh in oatp1c1 -/- larvae, and inducibly enhanced the HPT axis in wild-type larvae. These manipulations showed that Tsh promotes proliferation of Agrp1 neurons and increases food consumption in zebrafish. The results revealed upregulation of both the HPT axis-Agrp1 circuitry and feeding in a zebrafish model for OATP1C1 deficiency.SIGNIFICANCE STATEMENT Mutation in the thyroid hormone (TH) transporter OATP1C1 is associated with cognitive and motor functioning disturbances in humans. Here, we used an oatp1c1 -/- zebrafish to understand the role of organic anion-transporting polypeptide 1C1 (Oatp1c1), and the characteristics of OATP1C1 deficiency. Transcriptome profiling identified upregulation of agrp1 expression in the oatp1c1 -/- brain. The oatp1c1 -/- larvae showed increased thyroid-stimulating hormone (tsh) levels, proliferation of Agrp1 neurons and food consumption. Genetic manipulations of the hypothalamic-pituitary-thyroid (HPT) axis showed that Tsh increases the number of Agrp1 neurons and food consumption. The T3 analog TRIAC (3,3',5-tri-iodothyroacetic acid) normalizes the number of Agrp1 neurons and may have potential for the treatment of Oatp1c1 deficiency. The findings demonstrate a functional interaction between the thyroid and feeding systems in the brain of zebrafish and suggest a neuroendocrinological mechanism for OATP1C1 deficiency.

Thyroid stimulating hormone suppresses the expression and activity of cytosolic sulfotransferase 1a1 in thyrocytes

Sulfonation is an important step in the metabolism of dopamine, estrogens, dehydroepiandrosterone, as well as thyroid hormones. However, the regulation of cytosolic sulfotransferases in the thyroid is not well understood. In a DNA microarray analysis of rat thyroid FRTL-5 cells, we found that the mRNA expression of 10 of 48 sulfotransferases was significantly altered by thyroid stimulating hormone (TSH), with that of sulfotransferase family 1A member 1 (SULT1A1) being the most significantly affected. Real-time PCR and Western blot analyses revealed that TSH, forskolin and dibutyryl cyclic AMP significantly suppressed SULT1A1 mRNA and protein levels in a time- and concentration-dependent manner. Moreover, immunofluorescence staining of FRTL-5 cells showed that SULT1A1 is localized in the perinuclear area in the absence of TSH but is spread throughout the cytoplasm with reduced fluorescence intensity in the presence of TSH. Sulfotransferase activity in FRTL-5 cells, measured using 3'-phosphoadenosine-5'-phosphosulfate as a donner and p-nitrophenol as an acceptor substrate, was significantly reduced by TSH. These findings suggest that the expression and activity of SULT1A1 are modulated by TSH in thyrocytes.

Genetic disorders of thyroid development, hormone biosynthesis and signalling

Development and differentiation of the thyroid gland is directed by expression of specific transcription factors in the thyroid follicular cell which mediates hormone biosynthesis. Membrane transporters are rate-limiting for cellular entry of thyroid hormones (TH) (T4 and T3) into some tissues, with selenocysteine-containing, deiodinase enzymes (DIO1 and DIO2) converting T4 to the biologically active hormone T3. TH regulate expression of target genes via hormone-inducible nuclear receptors (TRα and TRβ) to exert their physiological effects. Primary congenital hypothyroidism (CH) due to thyroid dysgenesis may be mediated by defects in thyroid transcription factors or impaired thyroid stimulating hormone receptor function. Dyshormonogenic CH is usually due to mutations in genes mediating thyroidal iodide transport, organification or iodotyrosine synthesis and recycling. Disorders of TH signalling encompass conditions due to defects in membrane TH transporters, impaired hormone metabolism due to deficiency of deiodinases and syndromes of Resistance to thyroid hormone due to pathogenic variants in either TRα or TRβ. Here, we review the genetic basis, pathogenesis and clinical features of congenital, dysgenetic or dyshormonogenic hypothyroidism and disorders of TH transport, metabolism and action.

Zebrafish as an emerging tool for drug discovery and development for thyroid diseases

Zebrafish is a useful model for understanding human genetics and diseases and has evolved into a prominent scientific research model. The genetic structure of zebrafish is 70% identical to that of humans. Its small size, low cost, and transparent embryo make it a valuable tool in experimentation. Zebrafish and mammals possess the same molecular mechanism of thyroid organogenesis and development. Thus, thyroid hormone signaling, embryonic development, thyroid-related disorders, and novel genes involved in early thyroid development can all be studied using zebrafish as a model. Here in this review, we emphasize the evolving role of zebrafish as a possible tool for studying the thyroid gland in the context of physiology and pathology. The transcription factors nkx2.1a, pax2a, and hhex which contribute a pivotal role in the differentiation of thyroid primordium are discussed. Further, we have described the role of zebrafish as a model for thyroid cancer, evaluation of defects in thyroid hormone transport, thyroid hormone (TH) metabolism, and as a screening tool to study thyrotoxins. Hence, the present review highlights the role of zebrafish as a novel approach to understand thyroid development and organogenesis.

Thyroid hormone-dependent oligodendroglial cell lineage genomic and non-genomic signaling through integrin receptors

Multiple sclerosis (MS) is a heterogeneous autoimmune disease whereby the pathological sequelae evolve from oligodendrocytes (OLs) within the central nervous system and are targeted by the immune system, which causes widespread white matter pathology and results in neuronal dysfunction and neurological impairment. The progression of this disease is facilitated by a failure in remyelination following chronic demyelination. One mediator of remyelination is thyroid hormone (TH), whose reliance on monocarboxylate transporter 8 (MCT8) was recently defined. MCT8 facilitates the entry of THs into oligodendrocyte progenitor cell (OPC) and pre-myelinating oligodendrocytes (pre-OLs). Patients with MS may exhibit downregulated MCT8 near inflammatory lesions, which emphasizes an inhibition of TH signaling and subsequent downstream targeted pathways such as phosphoinositide 3-kinase (PI3K)-Akt. However, the role of the closely related mammalian target of rapamycin (mTOR) in pre-OLs during neuroinflammation may also be central to the remyelination process and is governed by various growth promoting signals. Recent research indicates that this may be reliant on TH-dependent signaling through β1-integrins. This review identifies genomic and non-genomic signaling that is regulated through mTOR in TH-responsive pre-OLs and mature OLs in mouse models of MS. This review critiques data that implicates non-genomic Akt and mTOR signaling in response to TH-dependent integrin receptor activation in pre-OLs. We have also examined whether this can drive remyelination in the context of neuroinflammation and associated sequelae. Importantly, we outline how novel therapeutic small molecules are being designed to target integrin receptors on oligodendroglial lineage cells and whether these are viable therapeutic options for future use in clinical trials for MS.

Role of thyroid hormones-induced oxidative stress on cardiovascular physiology

Thyroid hormones (THs) play an essential role in the maintenance of cardiovascular homeostasis and are involved in the modulation of cardiac contractility, heart rate, diastolic function, systemic vascular resistance, and vasodilation. THs have actions on cardiovascular physiology through the activation or repression of target genes or the activation of intracellular signals through non-genomic mechanisms. Hyperthyroidism alters certain intracellular pathways involved in the preservation of the structure and functionality of the heart, causing relevant cardiovascular disorders. Reactive oxygen species (ROS) play an important role in the cardiovascular system, but the exacerbated increase in ROS caused by chronic hyperthyroidism together with regulation on the antioxidant system have been associated with the development of cardiovascular dysfunction. In this review, we analyze the role of THs-induced oxidative stress in the cellular and molecular changes that lead to cardiac dysfunction, as well as the effectiveness of antioxidant treatments in attenuating cardiac abnormalities developed during hyperthyroidism.

Thyroid Hormone Transporters in a Human Placental Cell Model

Background: Fetal brain development in the first half of pregnancy is dependent on maternal thyroid hormone (TH), highlighting the importance of trans-placental TH transport. It is yet unclear which transporters are involved in this process. We aimed to identify the major TH transporters in a human placental cell model (BeWo cells). Methods: Messenger RNA expression of the known TH transporters (the monocarboxylate transporter [MCT]8, MCT10, the L-type amino acid transporter [LAT]1, LAT2, the organic anion transporting peptide [OATP]1A2 and OATP4A1) in BeWo cells and human placenta were determined by quantitative PCR. To determine the specificity and efficacy of transporter inhibitors, we first determined TH uptake at different inhibitor concentrations in African green monkey kidney fibroblast-like cells (COS1 cells) overexpressing TH transporters. We then tested TH uptake in BeWo cells in the presence or absence of the optimal inhibitor concentrations. Results: All tested TH transporters were expressed in human term placentas, whereas MCT8 was absent in BeWo cells. Both 2-amino-2-norbornanecarboxylic acid (BCH) and L-tryptophan at 1 mM inhibited LATs, whereas at the highest concentration (10 mM) L-tryptophan also inhibited MCT10. Verapamil inhibited OATP1A2 and less efficiently both MCTs, but not LATs. Both rifampicin and naringin reduced OATP1A2 activity. Finally, silychristin inhibited MCT8 at submicromolar concentrations and OATP1A2 partially only at the highest concentration tested (10 μM). In BeWo cells, verapamil reduced triiodothyronine (T3) uptake by 24%, BCH by 31%, and 1 mM L-tryptophan by 41%. The combination of BCH and verapamil additively decreased T3 uptake by 53% and the combination of BCH and 10 mM L-tryptophan by 60%, suggesting a major role for MCT10 and LATs in placental T3 uptake. Indeed, transfection of BeWo cells with MCT10-specific small interfering RNA significantly reduced T3 uptake. Only the combination of BCH and verapamil significantly reduced thyroxine (T4) uptake in BeWo cells, by 32%. Conclusions: Using pharmacological inhibitors, we show that MCT10 and LATs play a major role in T3 uptake in BeWo cells. T4 uptake appears independent of known TH transporters, suggesting the presence of, currently unknown, alternative transporter(s).

IGSF1 Deficiency Leads to Reduced TSH Production Independent of Alterations in Thyroid Hormone Action in Male Mice

Loss of function mutations in IGSF1/Igsf1 cause central hypothyroidism. Igsf1 knockout mice have reduced pituitary thyrotropin-releasing hormone receptor, Trhr, expression, perhaps contributing to the phenotype. Because thyroid hormones negatively regulate Trhr, we hypothesized that IGSF1 might affect thyroid hormone availability in pituitary thyrotropes. Consistent with this idea, IGSF1 coimmunoprecipitated with the thyroid hormone transporter monocarboxylate transporter 8 (MCT8) in transfected cells. This association was impaired with IGSF1 bearing patient-derived mutations. Wild-type IGSF1 did not, however, alter MCT8-mediated thyroid hormone import into heterologous cells. IGSF1 and MCT8 are both expressed in the apical membrane of the choroid plexus. However, MCT8 protein levels and localization in the choroid plexus were unaltered in Igsf1 knockout mice, ruling out a necessary chaperone function for IGSF1. MCT8 expression was low in the pituitary and was similarly unaffected in Igsf1 knockouts. We next assessed whether IGSF1 affects thyroid hormone transport or action, by MCT8 or otherwise, in vivo. To this end, we treated hypothyroid wild-type and Igsf1 knockout mice with exogenous thyroid hormones. T4 and T3 inhibited TSH release and regulated pituitary and forebrain gene expression similarly in both genotypes. Interestingly, pituitary TSH beta subunit (Tshb) expression was consistently reduced in Igsf1 knockouts relative to wild-type regardless of experimental condition, whereas Trhr was more variably affected. Although IGSF1 and MCT8 can interact in heterologous cells, the physiological relevance of their association is not clear. Nevertheless, the results suggest that IGSF1 loss can impair TSH production independently of alterations in TRHR levels or thyroid hormone action.

Thyroid hormone resistance: Mechanisms and therapeutic development

As an essential primary hormone, thyroid hormone (TH) is indispensable for human growth, development and metabolism. Impairment of TH function in several aspects, including TH synthesis, activation, transportation and receptor-dependent transactivation, can eventually lead to thyroid hormone resistance syndrome (RTH). RTH is a rare syndrome that manifests as a reduced target cell response to TH signaling. The majority of RTH cases are related to thyroid hormone receptor β (TRβ) mutations, and only a few RTH cases are associated with thyroid hormone receptor α (TRα) mutations or other causes. Patients with RTH suffer from goiter, mental retardation, short stature and bradycardia or tachycardia. To date, approximately 170 mutated TRβ variants and more than 20 mutated TRα variants at the amino acid level have been reported in RTH patients. In addition to these mutated proteins, some TR isoforms can also reduce TH function by competing with primary TRs for TRE and RXR binding. Fortunately, different treatments for RTH have been explored with structure-activity relationship (SAR) studies and drug design, and among these treatments. With thyromimetic potency but biochemical properties that differ from those of primary TH (T3 and T4), these TH analogs can bypass specific defective transporters or reactive mutant TRs. However, these compounds must be carefully applied to avoid over activating TRα, which is associated with more severe heart impairment. The structural mechanisms of mutation-induced RTH in the TR ligand-binding domain are summarized in this review. Furthermore, strategies to overcome this resistance for therapeutic development are also discussed.

AAV9-MCT8 Delivery at Juvenile Stage Ameliorates Neurological and Behavioral Deficits in a Mouse Model of MCT8-Deficiency

Background: Allan-Herndon-Dudley syndrome (AHDS) is a severe psychomotor disability disorder that also manifests characteristic abnormal thyroid hormone (TH) levels. AHDS is caused by inactivating mutations in monocarboxylate transporter 8 (MCT8), a specific TH plasma membrane transporter widely expressed in the central nervous system (CNS). MCT8 mutations cause impaired transport of TH across brain barriers, leading to insufficient neural TH supply. There is currently no successful therapy for the neurological symptoms. Earlier work has shown that intravenous (IV), but not intracerebroventricular adeno-associated virus serotype 9 (AAV9) -based gene therapy given to newborn Mct8 knockout (Mct8^-/y) male mice increased triiodothyronine (T₃) brain content and partially rescued TH-dependent gene expression, suggesting a promising approach to treat this neurological disorder. Methods: The potential of IV delivery of AAV9 carrying human MCT8 was tested in the well-established Mct8^-/y/Organic anion-transporting polypeptide 1c1 (Oatp1c1)^{-/ -} double knockout (dKO) mouse model of AHDS, which, unlike Mct8^-/y mice, displays both neurological and TH phenotype. Further, as the condition is usually diagnosed during childhood, treatment was given intravenously to P30 mice and psychomotor tests were carried out blindly at P120-P140 after which tissues were collected and analyzed. Results: Systemic IV delivery of AAV9-MCT8 at a juvenile stage led to improved locomotor and cognitive functions at P120-P140, which was accompanied by a near normalization of T₃ content and an increased response of positively regulated TH-dependent gene expression in different brain regions examined (thalamus, hippocampus, and parietal cortex). The effects on serum TH concentrations and peripheral tissues were less pronounced, showing only improvement in the serum T₃/reverse T₃ (rT₃) ratio and in liver deiodinase 1 expression. Conclusion: IV administration of AAV9, carrying the human MCT8, to juvenile dKO mice manifesting AHDS has long-term beneficial effects, predominantly on the CNS. This preclinical study indicates that this gene therapy has the potential to ameliorate the devastating neurological symptoms in patients with AHDS.

Sodium Phenylbutyrate Rescues Thyroid Hormone Transport in Brain Endothelial-Like Cells

Background: Monocarboxylate transporter 8 (MCT8) deficiency is a rare genetic disease leading to a severe developmental delay due to a lack of thyroid hormones (THs) during critical stages of human brain development. Some MCT8-deficient patients are not as severely affected as others. Previously, we hypothesized that these patients' mutations do not affect the functionality but destabilize the MCT8 protein, leading to a diminished number of functional MCT8 molecules at the cell surface. Methods: We have already demonstrated that the chemical chaperone sodium phenylbutyrate (NaPB) rescues the function of these mutants by stabilizing their protein expression in an overexpressing cell system. Here, we expanded our previous work and used iPSC (induced pluripotent stem cell)-derived brain microvascular endothelial-like cells (iBMECs) as a physiologically relevant cell model of human origin to test for NaPB responsiveness. The effects on mutant MCT8 expression and function were tested by Western blotting and radioactive uptake assays. Results: We found that NaPB rescues decreased mutant MCT8 expression and restores transport function in iBMECs carrying patient's mutation MCT8-P321L. Further, we identified MCT10 as an alternative TH transporter in iBMECs that contributes to triiodothyronine uptake, the biological active TH. Our results indicate an upregulation of MCT10 after NaPB treatment. In addition, we detected an increase in thyroxine (T4) uptake after NaPB treatment that was not mediated by rescued MCT8 but an unidentified T4 transporter. Conclusions: We demonstrate that NaPB is suitable to stabilize a pathogenic missense mutation in a human-derived cell model. Further, it activates TH transport independent of MCT8. Both options fuel future studies to investigate repurposing the Food and Drug Administration-approved drug NaPB in selected cases of MCT8 deficiency.

A Historical Review of Brain Drug Delivery

The history of brain drug delivery is reviewed beginning with the first demonstration, in 1914, that a drug for syphilis, salvarsan, did not enter the brain, due to the presence of a blood-brain barrier (BBB). Owing to restricted transport across the BBB, FDA-approved drugs for the CNS have been generally limited to lipid-soluble small molecules. Drugs that do not cross the BBB can be re-engineered for transport on endogenous BBB carrier-mediated transport and receptor-mediated transport systems, which were identified during the 1970s-1980s. By the 1990s, a multitude of brain drug delivery technologies emerged, including trans-cranial delivery, CSF delivery, BBB disruption, lipid carriers, prodrugs, stem cells, exosomes, nanoparticles, gene therapy, and biologics. The advantages and limitations of each of these brain drug delivery technologies are critically reviewed.

Increased/Targeted Brain (Pro)Drug Delivery via Utilization of Solute Carriers (SLCs)

Membrane transporters have a crucial role in compounds’ brain drug delivery. They allow not only the penetration of a wide variety of different compounds to cross the endothelial cells of the blood–brain barrier (BBB), but also the accumulation of them into the brain parenchymal cells. Solute carriers (SLCs), with nearly 500 family members, are the largest group of membrane transporters. Unfortunately, not all SLCs are fully characterized and used in rational drug design. However, if the structural features for transporter interactions (binding and translocation) are known, a prodrug approach can be utilized to temporarily change the pharmacokinetics and brain delivery properties of almost any compound. In this review, main transporter subtypes that are participating in brain drug disposition or have been used to improve brain drug delivery across the BBB via the prodrug approach, are introduced. Moreover, the ability of selected transporters to be utilized in intrabrain drug delivery is discussed. Thus, this comprehensive review will give insights into the methods, such as computational drug design, that should be utilized more effectively to understand the detailed transport mechanisms. Moreover, factors, such as transporter expression modulation pathways in diseases that should be taken into account in rational (pro)drug development, are considered to achieve successful clinical applications in the future.

A novel frameshift mutation in Allan-Herndon-Dudley syndrome

Allan-Herndon-Dudley syndrome (AHDS) is a very rare, X-linked psychomotor disability syndrome with delayed myelination, almost exclusively affecting boys. We present a case of a 4-year-old boy with AHDS who was found cyanotic, with intermittent vomiting and paroxysmal convulsions about 4 h after his parents went out, and was then taken to the hospital, where he eventually died the next day. The autopsy revealed foreign bodies in the tiny bronchi and alveoli of the deceased, congestion, and punctate hemorrhage in multiple organs, consistent with the diagnosis of asphyxia. Compared with a normally developing 4-year-old boy, the deceased showed cerebral atrophy and cerebral edema, and Luxol Fast Blue (LFB) stain indicated delayed cerebellar, hippocampal, and basal ganglia development and myelination. A novel frameshift mutation c.584delG in the SLC16A2 gene was detected. Family lineage investigation showed that the mutation was also detected in the deceased's 8-year-old brother and biological mother. The present work enriches the profile mutations in SLC16A2 related to AHDS and emphasizes the importance of autopsy and postmortem genetic analysis in such cases.

Screening for drugs potentially interfering with MCT8-mediated T3 transport in vitro identifies dexamethasone and some commonly used drugs as inhibitors of MCT8 activity

BACKGROUND

Monocarboxylate transporter 8 (MCT8) is the first thyroid hormone transporter that has been linked to a human disease. Besides genetic alterations other factors might impair MCT8 activity.

AIM

This study aimed at investigating whether some common drugs having a structural similarity with TH and/or whose treatment is associated with thyroid function test abnormalities, or which behave as antagonists of TH action can inhibit MCT8-mediated T₃ transport.

METHODS

[¹²⁵I]T₃ uptake and efflux were measured in COS-7 cells transiently transfected with hMCT8 before and after exposure to increasing concentrations of hydrocortisone, dexamethasone, prednisone, prednisolone, amiodarone, desethylamiodarone, dronedarone, buspirone, carbamazepine, valproic acid, and L-carnitine. The mode of inhibition was also determined.

RESULTS

Dexamethasone significantly inhibited T₃ uptake at 10 μM; hydrocortisone reduced T₃ uptake only at high concentrations, i.e. at 500 and 1000 μM; prednisone and prednisolone were devoid of inhibitory potential. Amiodarone caused a reduction of T₃ uptake by MCT8 only at the highest concentrations used (44% at 50 μM and 68% at 100 μM), and this effect was weaker than that produced by desethylamiodarone and dronedarone; buspirone resulted a potent inhibitor, reducing T₃ uptake at 0.1-10 μM. L-Carnitine inhibited T₃ uptake only at 500 mM and 1 M. Kinetic experiments revealed a noncompetitive mode of inhibition for all compounds. All drugs inhibiting T₃ uptake did not affect T₃ release.

CONCLUSION

This study shows a novel effect of some common drugs, which is inhibition of T₃ transport mediated by MCT8. Specifically, dexamethasone, buspirone, desethylamiodarone, and dronedarone behave as potent inhibitors of MCT8.

The interplay of thyroid hormones and the immune system - where we stand and why we need to know about it

Over the past few years, growing evidence suggests direct crosstalk between thyroid hormones (THs) and the immune system. Components of the immune system were proposed to interfere with the central regulation of systemic TH levels. Conversely, THs regulate innate and adaptive immune responses as immune cells are direct target cells of THs. Accordingly, they express different components of local TH action, such as TH transporters or receptors, but our picture of the interplay between THs and the immune system is still incomplete. This review provides a critical overview of current knowledge regarding the interaction of THs and the immune system with the main focus on local TH action within major innate and adaptive immune cell subsets. Thereby, this review aims to highlight open issues which might help to infer the clinical relevance of THs in host defence in the context of different types of diseases such as infection, ischemic organ injury or cancer.

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.