1H magnetic resonance spectroscopy in monocarboxylate transporter 8 gene deficiency

Mapping Thyroid Hormone Action in the Human Brain

Background: Normal brain development, mood, and cognitive functions depend on thyroid hormone (TH) action. However, little is known about how TH mediates its actions in the human brain. This is due to limited access to human brains deprived of TH during fetal and early postnatal life, as well as from adults with altered thyroid status. One way to partially bypass these limitations is by using magnetic resonance imaging and spectroscopy, two neuroimaging techniques that provide detailed, noninvasive information on human brain structure and function. Another way is using human-induced pluripotent stem cell (hiPSCs)-derived three-dimensional in vitro systems, known as brain organoids, which allow for the study of fundamental aspects of the early stages of human brain development. Summary: This narrative review focuses on neuroimaging and brain organoid studies. Neuroimaging of human brains performed in individuals with different thyroid conditions provides information on the volume, myelination, blood flow, neural activity, and connectivity of different areas. Such studies show that suboptimal thyroid status can impact human brain development and its normal function throughout life. This is true not only for patients with sporadic congenital hypothyroidism, during pregnancy or early after birth, but also for adult patients with hypo- or hyperthyroidism, patients carrying mutations that manifest as impaired sensitivity to TH, and even for normal individuals during aging. Studies using brain organoids generated from hiPSCs of healthy individuals or patients with thyroid genetic conditions provide insights into how TH can impact the early development of the human cerebral cortex. Conclusions: The developmental alterations in children born to mothers with different degrees of gestational hypothyroidism or who developed hypothyroidism early in life are remarkable, affecting multiple brain regions and pathways, including the cerebral cortex, hippocampus, cerebellum, interhemispheric and corticospinal tracts, and associative nuclei. The data connecting such changes to poor neurological outcomes in adult patients with hypothyroidism represent an objective link between thyroid-specific functional brain alterations and behavior. Growing brain organoids require TH, which is critical for human neurogenesis and oligodendrogenesis. These models have proven useful in screening drugs with potential therapeutic effects for patients with genetic thyroid diseases.

Disease characteristics of MCT8 deficiency: an international, retrospective, multicentre cohort study

BACKGROUND

Disordered thyroid hormone transport, due to mutations in the SLC16A2 gene encoding monocarboxylate transporter 8 (MCT8), is characterised by intellectual and motor disability resulting from cerebral hypothyroidism and chronic peripheral thyrotoxicosis. We sought to systematically assess the phenotypic characteristics and natural history of patients with MCT8 deficiency.

METHODS

We did an international, multicentre, cohort study, analysing retrospective data from Jan 1, 2003, to Dec 31, 2019, from patients with MCT8 deficiency followed up in 47 hospitals in 22 countries globally. The key inclusion criterion was genetically confirmed MCT8 deficiency. There were no exclusion criteria. Our primary objective was to analyse the overall survival of patients with MCT8 deficiency and document causes of death. We also compared survival between patients who did or did not attain full head control by age 1·5 years and between patients who were or were not underweight by age 1-3 years (defined as a bodyweight-for-age Z score <-2 SDs or <5th percentile according to WHO definition). Other objectives were to assess neurocognitive function and outcomes, and clinical parameters including anthropometric characteristics, biochemical markers, and neuroimaging findings.

FINDINGS

Between Oct 14, 2014, and Jan 17, 2020, we enrolled 151 patients with 73 different MCT8 (SLC16A2) mutations. Median age at diagnosis was 24·0 months (IQR 12·0-60·0, range 0·0-744·0). 32 (21%) of 151 patients died; the main causes of mortality in these patients were pulmonary infection (six [19%]) and sudden death (six [19%]). Median overall survival was 35·0 years (95% CI 8·3-61·7). Individuals who did not attain head control by age 1·5 years had an increased risk of death compared with patients who did attain head control (hazard ratio [HR] 3·46, 95% CI 1·76-8·34; log-rank test p=0·0041). Patients who were underweight during age 1-3 years had an increased risk for death compared with patients who were of normal bodyweight at this age (HR 4·71, 95% CI 1·26-17·58, p=0·021). The few motor and cognitive abilities of patients did not improve with age, as evidenced by the absence of significant correlations between biological age and scores on the Gross Motor Function Measure-88 and Bayley Scales of Infant Development III. Tri-iodothyronine concentrations were above the age-specific upper limit in 96 (95%) of 101 patients and free thyroxine concentrations were below the age-specific lower limit in 94 (89%) of 106 patients. 59 (71%) of 83 patients were underweight. 25 (53%) of 47 patients had elevated systolic blood pressure above the 90th percentile, 34 (76%) of 45 patients had premature atrial contractions, and 20 (31%) of 64 had resting tachycardia. The most consistent MRI finding was a global delay in myelination, which occurred in 13 (100%) of 13 patients.

INTERPRETATION

Our description of characteristics of MCT8 deficiency in a large patient cohort reveals poor survival with a high prevalence of treatable underlying risk factors, and provides knowledge that might inform clinical management and future evaluation of therapies.

FUNDING

Netherlands Organisation for Health Research and Development, and the Sherman Foundation.

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).

Monocarboxylate Transporter 8 Deficiency: Delayed or Permanent Hypomyelination?

Monocarboxylate transporter 8 (MCT8) deficiency or the Allan-Herndon-Dudley Syndrome (AHDS) is an X-linked psychomotor disability syndrome with around 320 clinical cases described worldwide. SLC16A2 gene mutations, encoding the thyroid hormone (TH) transporter MCT8, result in intellectual disability due to impaired TH uptake in the developing brain. MCT8 deficiency is a multi-organ affecting disease with a predominant neuronal cell-based pathology, with the glial component inadequately investigated. However, deficiency in myelin, a key component of white matter (WM) enabling fast nerve conduction, is a TH-dependent hallmark of the disease. Nevertheless, analysis of the myelin status in AHDS patients has led to conflicting interpretations. The majority of individual case studies reported delayed myelination, that was restored later in life. In contrast, post-mortem studies and high-resolution MRIs detected WM (micro-) abnormalities throughout adolescence, suggesting permanent hypomyelination. Thus, interpretations vary depending on methodology to investigate WM microstructure. Further, it is unknown whether the mutation within the MCT8 is linked to the severity of the myelin deficiency. Consequently, terminology is inconsistent among reports, and AHDS is occasionally misdiagnosed as another WM disorder. The evolutionary conserved TH signaling pathway that promotes the generation of myelinating oligodendrocytes enabled deciphering how the lack of MCT8 might affect myelinogenesis. Linking patient findings on myelination to those obtained from models of MCT8 deficiency revealed underlying pathophysiological mechanisms, but knowledge gaps remain, notably how myelination progresses both spatially and temporally in MCT8 deficiency. This limits predicting how myelin integrity might benefit therapeutically, and when to initiate. A recurrent observation in clinical trials is the absence of neurological improvement. Testing MCT8-independent thyromimetics in models, and evaluating treatments used in other demyelinating diseases, despite different etiologies, is crucial to propose new therapeutic strategies combatting this devastating disease.

Expanding the phenotypic spectrum of Allan-Herndon-Dudley syndrome in patients with SLC16A2 mutations

The aim of the study was to redefine the phenotype of Allan-Herndon-Dudley syndrome (AHDS), which is caused by mutations in the SLC16A2 gene that encodes the brain transporter of thyroid hormones. Clinical phenotypes, brain imaging, thyroid hormone profiles, and genetic data were compared to the existing literature. Twenty-four males aged 11 months to 29 years had a mutation in SLC16A2, including 12 novel mutations and five previously described mutations. Sixteen patients presented with profound developmental delay, three had severe intellectual disability with poor language and walking with an aid, four had moderate intellectual disability with language and walking abilities, and one had mild intellectual disability with hypotonia. Overall, eight had learned to walk, all had hypotonia, 17 had spasticity, 18 had dystonia, 12 had choreoathetosis, 19 had hypomyelination, and 10 had brain atrophy. Kyphoscoliosis (n=12), seizures (n=7), and pneumopathies (n=5) were the most severe complications. This study extends the phenotypic spectrum of AHDS to a mild intellectual disability with hypotonia. Developmental delay, hypotonia, hypomyelination, and thyroid hormone profile help to diagnose patients. Clinical course depends on initial severity, with stable acquisition after infancy; this may be adversely affected by neuro-orthopaedic, pulmonary, and epileptic complications. WHAT THIS PAPER ADDS: Mild intellectual disability is associated with SLC16A2 mutations. A thyroid hormone profile with a free T₃ /T₄ ratio higher than 0.75 can help diagnose patients. Patients with SLC16A2 mutations present a broad spectrum of neurological phenotypes that are also observed in other hypomyelinating disorders. Axial hypotonia is a consistent feature of Allan-Herndon-Dudley syndrome and leads to specific complications.

A SLC16A1 Mutation in an Infant With Ketoacidosis and Neuroimaging Assessment: Expanding the Clinical Spectrum of MCT1 Deficiency

The solute carrier family 16 member 1 (SLC16A1) gene encodes for monocarboxylate transporter 1 (MCT1) that mediates the movement of monocarboxylates, such as lactate and pyruvate across cell membranes. Inactivating recessive homozygous or heterozygous mutations in the SLC16A1 gene were described in patients with recurrent ketoacidosis and hypoglycemia, a potentially lethal condition. In the brain where MCT1 is highly localized around axons and oligodendrocytes, glucose is the most crucial energy substrate while lactate is an alternative substrate. MCT1 mutation or reduced expression leads to neuronal loss due to axonal degeneration in an animal model. Herein, we describe a 28 months old female patient who presented with the first hypoglycemic attack associated with ketoacidosis starting at the age of 3 days old. Whole exome sequencing (WES) performed at 6 months of age revealed a c.218delG mutation in exon 3 in the SLC16A1 gene. The variant is expected to result in loss of normal MCT1 function. Our patient is amongst the youngest presenting with MCT1 deficiency. A detailed neuroimaging assessment performed at 18 months of age revealed a complex white and gray matter disease, with heterotopia. The threshold of blood glucose to circumvent neurological sequelae cannot be set because it is patient-specific, nevertheless, neurodevelopmental follow up is recommended in this patient. Further functional studies will be required to understand the role of the MCT1 in key tissues such as the central nervous system (CNS), liver, muscle and ketone body metabolism. Our case suggests possible neurological sequelae that could be associated with MCT1 deficiency, an observation that could facilitate the initiation of appropriate neurodevelopmental follow up in such patients.

Thyroid hormone transporters--functions and clinical implications

The cellular influx and efflux of thyroid hormones are facilitated by transmembrane protein transporters. Of these transporters, monocarboxylate transporter 8 (MCT8) is the only one specific for the transport of thyroid hormones and some of their derivatives. Mutations in SLC16A2, the gene that encodes MCT8, lead to an X-linked syndrome with severe neurological impairment and altered concentrations of thyroid hormones. Histopathological analysis of brain tissue from patients who have impaired MCT8 function indicates that brain lesions start prenatally, and are most probably the result of cerebral hypothyroidism. A Slc16a2 knockout mouse model has revealed that Mct8 is an important mediator of thyroid hormone transport, especially T3, through the blood-brain barrier. However, unlike humans with an MCT8 deficiency, these mice do not have neurological impairment. One explanation for this discrepancy could be differences in expression of the T4 transporter OATP1C1 in the blood-brain barrier; OATP1C1 is more abundant in rodents than in primates and permits the passage of T4 in the absence of T3 transport, thus preventing full cerebral hypothyroidism. In this Review, we discuss the relevance of thyroid hormone transporters in health and disease, with a particular focus on the pathophysiology of MCT8 mutations.

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.

Altered behavioral performance and live imaging of circuit-specific neural deficiencies in a zebrafish model for psychomotor retardation

The mechanisms and treatment of psychomotor retardation, which includes motor and cognitive impairment, are indefinite. The Allan-Herndon-Dudley syndrome (AHDS) is an X-linked psychomotor retardation characterized by delayed development, severe intellectual disability, muscle hypotonia, and spastic paraplegia, in combination with disturbed thyroid hormone (TH) parameters. AHDS has been associated with mutations in the monocarboxylate transporter 8 (mct8/slc16a2) gene, which is a TH transporter. In order to determine the pathophysiological mechanisms of AHDS, MCT8 knockout mice were intensively studied. Although these mice faithfully replicated the abnormal serum TH levels, they failed to exhibit the neurological and behavioral symptoms of AHDS patients. Here, we generated an mct8 mutant (mct8−/−) zebrafish using zinc-finger nuclease (ZFN)-mediated targeted gene editing system. The elimination of MCT8 decreased the expression levels of TH receptors; however, it did not affect the expression of other TH-related genes. Similar to human patients, mct8−/− larvae exhibited neurological and behavioral deficiencies. High-throughput behavioral assays demonstrated that mct8−/− larvae exhibited reduced locomotor activity, altered response to external light and dark transitions and an increase in sleep time. These deficiencies in behavioral performance were associated with altered expression of myelin-related genes and neuron-specific deficiencies in circuit formation. Time-lapse imaging of single-axon arbors and synapses in live mct8−/− larvae revealed a reduction in filopodia dynamics and axon branching in sensory neurons and decreased synaptic density in motor neurons. These phenotypes enable assessment of the therapeutic potential of three TH analogs that can enter the cells in the absence of MCT8. The TH analogs restored the myelin and axon outgrowth deficiencies in mct8−/− larvae. These findings suggest a mechanism by which MCT8 regulates neural circuit assembly, ultimately mediating sensory and motor control of behavioral performance. We also propose that the administration of TH analogs early during embryo development can specifically reduce neurological damage in AHDS patients.

In a wide range of brain disorders, mutations in specific genes cause alterations in the development and function of neural circuits that ultimately affect behavior. A major challenge is to uncover the mechanism and provide treatment which is capable of preventing brain damage. Allan-Herndon-Dudley syndrome (AHDS) is a severe psychomotor retardation characterized by intellectual disabilities, neurological impairment and abnormal thyroid hormone (TH) levels. Mutations in the TH transporter MCT8 are associated with AHDS. Mice that lack the MCT8 protein exhibited impaired TH levels, as is the case in human patients; however, they lack neurological defects. Here, we generated an mct8 mutant (mct8−/−) zebrafish, which exhibited neurological and behavioral deficiencies and mimics pathological conditions of AHDS patients. The zebrafish is a simple transparent vertebrate and its nervous system is conserved with mammals. Time-lapse live imaging of single axons and synapses, and video-tracking of behavior revealed deficiencies in neural circuit assembly, which are associated with disturbed sleep and altered locomotor activity. In addition, since the mct8−/− larvae provides a highthroughput platform for testing therapeutic drugs, we showed that TH analogs can recover neurological deficiencies in an animal model for psychomotor retardation.

Delayed myelination is not a constant feature of Allan-Herndon-Dudley syndrome: report of a new case and review of the literature

INTRODUCTION

Allan-Herndon-Dudley syndrome is an X-linked condition caused by mutations of the monocarboxylate transporter 8 gene. This syndrome is characterized by axial hypotonia, severe mental retardation, dysarthria, athetoid movements, spastic paraplegia, and a typical thyroid hormone profile. In most of the cases reported so far, brain magnetic resonance imaging showed delayed myelination of the central white matter and this finding greatly affects the diagnosis of the syndrome.

CASE REPORT

We present a new case studied with magnetic resonance imaging and spectroscopy and we reviewed all the articles published between 2004 and 2012 containing information on brain neuroimaging in this syndrome. An Italian boy, showing a classical phenotype of the syndrome, was diagnosed at 17months of age. Genetic analysis revealed a new frameshift mutation of the monocarboxylate transporter 8 gene. His brain magnetic resonance imaging and spectroscopy, performed at the age of 14months, were normal.

DISCUSSION

Among the 33 cases reported in the literature, 3 cases had normal neuroimaging and in 7 of 14 cases, having a longitudinal follow-up, the initial finding of delayed myelination gradually improved. Our case and the review of the pertinent literature suggest that Allan-Herndon-Dudley syndrome should be suspected in males with the typical neurological and thyroid profile, even in cases with normal brain myelination.

Genetic disorders of thyroid metabolism and brain development

Normal thyroid metabolism is essential for human development, including the formation and functioning of the central and peripheral nervous system. Disorders of thyroid metabolism are increasingly recognized within the spectrum of paediatric neurological disorders. Both hypothyroid and hyperthyroid disease states (resulting from genetic and acquired aetiologies) can lead to characteristic neurological syndromes, with cognitive delay, extrapyramidal movement disorders, neuropsychiatric symptoms, and neuromuscular manifestations. In this review, the neurological manifestations of genetic disorders of thyroid metabolism are outlined, with particular focus on Allan-Herndon-Dudley syndrome and benign hereditary chorea. We report in detail the clinical features, major neurological and neuropsychiatric manifestations, molecular genetic findings, disease mechanisms, and therapeutic strategies for these emerging genetic 'brain-thyroid' disorders.

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.

MCT8 deficiency: extrapyramidal symptoms and delayed myelination as prominent features

Monocarboxylate transporter 8 (MCT8) deficiency is an X-linked disorder resulting from an impairment of the transcellular transportation of thyroid hormones. Within the central nervous system thyroid hormone transport is normally mediated by MCT8. Patients are described as affected by a static or slowly progressive clinical picture which consists of variable degrees of mental retardation, hypotonia, spasticity, ataxia and involuntary movements, occasionally paroxysmal. The authors describe the clinical and neuroradiological picture of 3 males patients with marked delayed brain myelination and in which the clinical picture was dominated by early onset nonparoxysmal extrapyramidal symptoms. In one subject a novel mutation is described.

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.

The syndromes of reduced sensitivity to thyroid hormone

BACKGROUND

Six known steps are required for the circulating thyroid hormone (TH) to exert its action on target tissues. For three of these steps, human mutations and distinct phenotypes have been identified.

SCOPE OF REVIEW

The clinical, laboratory, genetic and molecular characteristics of these three defects of TH action are the subject of this review. The first defect, recognized 45years ago, produces resistance to TH and carries the acronym, RTH. In the majority of cases it is caused by TH receptor β gene mutations. It has been found in over 3000 individuals belonging to approximately 1000 families. Two relatively novel syndromes presenting reduced sensitivity to TH involve membrane transport and metabolism of TH. One of them, caused by mutations in the TH cell-membrane transporter MCT8, produces severe psychomotor defects. It has been identified in more than 170 males from 90 families. A defect of the intracellular metabolism of TH in 10 individuals from 8 families is caused by mutations in the SECISBP2 gene required for the synthesis of selenoproteins, including TH deiodinases.

MAJOR CONCLUSIONS

Defects at different steps along the pathway leading to TH action at cellular level can manifest as reduced sensitivity to TH.

GENERAL SIGNIFICANCE

Knowledge of the molecular mechanisms involved in TH action allows the recognition of the phenotypes caused by defects of TH action. Once previously known defects have been ruled out, new molecular defects could be sought, thus opening the avenue for novel insights in thyroid physiology. This article is part of a Special Issue entitled Thyroid hormone signaling.

Function of thyroid hormone transporters in the central nervous system

BACKGROUND

Iodothyronines are charged amino acid derivatives that cannot passively cross a phospholipid bilayer. Transport of thyroid hormones across plasma membranes is mediated by integral membrane proteins belonging to several gene families. These transporters therefore allow or limit access of thyroid hormones into brain. Since thyroid hormones are essential for brain development and cell differentiation, it is expected that genetic deficiency of such transporters would result in neurodevelopmental derangements.

SCOPE OF REVIEW

We introduce concepts of thyroid hormone transport into the brain and into brain cells. Important thyroid hormone transmembrane transporters are presented along with their expression patterns in different brain cell types. A focus is placed on monocarboxylate transporter 8 (MCT8) which has been identified as an essential thyroid hormone transporter in humans. Mutations in MCT8 underlie one of the first described X-linked mental retardation syndromes, the Allan-Herndon-Dudley syndrome.

MAJOR CONCLUSIONS

Thyroid hormone transporter molecules are expressed in a developmental and cell type-specific pattern. Any thyroid hormone molecule has to cross consecutively the luminal and abluminal membranes of the capillary endothelium, enter astrocytic foot processes, and leave the astrocyte through the plasma membrane to finally cross another plasma membrane on its way towards its target nucleus.

GENERAL SIGNIFICANCE

We can expect more transporters being involved in or contributing to in neurodevelopmental or neuropsychiatric disease. Due to their expression in cellular components regulating the hypothalamus-pituitary-thyroid axis, mutations and polymorphisms are expected to impact on negative feedback regulation and hormonal setpoints. This article is part of a Special Issue entitled Thyroid hormone signalling.

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.

Hypothyroidism and mood disorders: integrating novel insights from brain imaging techniques

Abstract

Thyroid hormones play a critical role in brain development but also in the adult human brain by modulating metabolic activity. Hypothyroid states are associated with both functional and structural brain alterations also seen in patients with major depression. Recent animal experimental and preclinical data indicate subtle changes in myelination, microvascular density, local neurogenesis, and functional networks. The translational validity of such studies is obviously limited. Clinical evidence for neurobiological correlates of different stages and severities of hypothyroidism and effects of pharmacological intervention is lacking but may be achieved using advanced imaging techniques, e.g. functional and quantitative MRI techniques applied to patients with hypothyroidism before and after hormone replacement therapy.

[Thyroid hormone resistance syndromes]

Thyroid hormone resistance syndromes are a group of genetic conditions characterized by decreased tissue sensitivity to thyroid hormones. Three syndromes, in which resistance to hormone action is respectively due to mutations in the gene encoding for thyroid hormone receptor TRβ, impaired T4 and T3 transport, and impaired conversion of T4 to T3 mediated by deiodinases. An updated review of each of these forms of resistance is provided, and their pathogenetic mechanisms and clinical approaches are discussed.

Minireview: thyroid hormone transporters: the knowns and the unknowns

The effects of thyroid hormone (TH) on development and metabolism are exerted at the cellular level. Metabolism and action of TH take place intracellularly, which require transport of the hormone across the plasma membrane. This process is mediated by TH transporter proteins. Many TH transporters have been identified at the molecular level, although a few are classified as specific TH transporters, including monocarboxylate transporter (MCT)8, MCT10, and organic anion-transporting polypeptide 1C1. The importance of TH transporters for physiology has been illustrated dramatically by the causative role of MCT8 mutations in males with psychomotor retardation and abnormal serum TH concentrations. Although Mct8 knockout animals have provided insight in the mechanisms underlying parts of the endocrine phenotype, they lack obvious neurological abnormalities. Thus, the pathogenesis of the neurological abnormalities in males with MCT8 mutations is not fully understood. The prospects of identifying other transporters and transporter-based syndromes promise an exciting future in the TH transporter field.

White matter abnormalities and dystonic motor disorder associated with mutations in the SLC16A2 gene

AIM

Mutations in the SLC16A2 gene have been implicated in Allan-Herndon-Dudley syndrome (AHDS), an X-linked learning disability* syndrome associated with thyroid function test (TFT) abnormalities. Delayed myelination is a non-specific finding in individuals with learning disability whose genetic basis is often uncertain. The aim of this study was to describe neuroimaging findings and neurological features in males with SLC16A2 gene mutations.

METHOD

We reviewed brain magnetic resonance imaging (MRI) findings and neurological features in a cohort of five males aged between 1 year 6 months and 6 years (median 4y) from four families harbouring SLC16A2 gene mutations.

RESULTS

The participants presented aged between 4 and 9 months with initial hypotonia and subsequent spastic paraparesis with dystonic posturing and superimposed paroxysmal dyskinesias. Dystonic cerebral palsy was the most common initial clinical diagnosis, and AHDS was suspected only retrospectively, considering the characteristically abnormal thyroid function tests, with high serum tri-iodothyronine (T(3)), as the most consistent finding. Brain MRI showed absent or markedly delayed myelination in all five participants, prompting the suspicion of Pelizaeus-Merzbacher disease in one patient.

INTERPRETATION

Our findings indicate a consistent association between defective neuronal T(3) uptake and delayed myelination. SLC16A2 involvement should be considered in males with learning disability, an associated motor or movement disorder, and evidence of delayed myelination on brain MRI. Although dysmorphic features suggestive of AHDS are not always present, T(3) measurement is a reliable screening test.

Leukodystrophies and other genetic metabolic leukoencephalopathies in children and adults

Abnormalities of CNS white matter are frequently detected in patients with neurological disorders when MRI studies are performed. Among the many causes of such abnormalities, a large group of rare genetic diseases poses considerable diagnostic problems. Here we present a compilation of genetic leukoencephalopathies to consider when one is confronted with white matter disease of possibly genetic origin. The table contains essentials such as age at onset of symptoms, clinical and MRI characteristics, basic defect, and useful diagnostic studies. The table serves as a diagnostic check list.

Thyroid disorders in mental patients

PURPOSE OF REVIEW

Thyroid hormones play important roles in brain development and function. Recent findings concerning thyroid hormones secretion, transport, and metabolism in the brain have provided a better understanding of the role of thyroid hormones in mental disorders.

RECENT FINDINGS

The intracellular actions of thyroid hormones in brain are determined by a complex of factors, including circulating concentrations of thyroid hormones, availability of free hormone, activity of thyroid hormone transporters and deiodinase enzymes, and activity of thyroid hormone receptors. Individual genetic variations and mutations of thyroid-axis-related proteins influence thyroid hormone activity in the brain and contribute to the presentation of mental disorders, as well as to response to psychiatric treatments.

SUMMARY

Consideration of molecular mechanism related to genetic alterations in thyroid hormone transport into the neuron, intracellular thyroid hormone metabolism in the brain, as well as polymorphism in thyroid hormone receptors, opens new venues for better understanding of thyroid hormone effects in the brain as well as for finding genetic markers and new targets for the treatment of mental disorders.