Brain glucose utilization in mice with a targeted mutation in the thyroid hormone alpha or beta receptor gene

Thyroid hormone action during GABAergic neuron maturation: The quest for mechanisms

Thyroid hormone (TH) signaling plays a major role in mammalian brain development. Data obtained in the past years in animal models have pinpointed GABAergic neurons as a major target of TH signaling during development, which opens up new perspectives to further investigate the mechanisms by which TH affects brain development. The aim of the present review is to gather the available information about the involvement of TH in the maturation of GABAergic neurons. After giving an overview of the kinds of neurological disorders that may arise from disruption of TH signaling during brain development in humans, we will take a historical perspective to show how rodent models of hypothyroidism have gradually pointed to GABAergic neurons as a main target of TH signaling during brain development. The third part of this review underscores the challenges that are encountered when conducting gene expression studies to investigate the molecular mechanisms that are at play downstream of TH receptors during brain development. Unravelling the mechanisms of action of TH in the developing brain should help make progress in the prevention and treatment of several neurological disorders, including autism and epilepsy.

Relationship between thyroid hormones and central nervous system metabolism in physiological and pathological conditions

Thyroid hormones (THs) play an important role in the regulation of energy metabolism. They also take part in processes associated with the central nervous system (CNS), including survival and differentiation of neurons and energy expenditure. It has been reported that a correlation exists between the functioning of the thyroid gland and the symptoms of CNS such as cognitive impairment, depression, and dementia. Literature data also indicate the influence of THs on the pathogenesis of CNS diseases, such as Alzheimer's disease, epilepsy, depression, and Parkinson's disease. This review describes the relationship between THs and metabolism in the CNS, the effect of THs on the pathological conditions of the CNS, and novel options for treating these conditions with TH derivatives.

Thyroid Hormone Deiodinases: Dynamic Switches in Developmental Transitions

Thyroid hormones exert pleiotropic, essential actions in mammalian, including human, development. These actions depend on provision of thyroid hormones in the circulation but also to a remarkable extent on deiodinase enzymes in target tissues that amplify or deplete the local concentration of the primary active form of the hormone T3 (3,5,3'-triiodothyronine), the high affinity ligand for thyroid hormone receptors. Genetic analyses in mice have revealed key roles for activating (DIO2) and inactivating (DIO3) deiodinases in cell differentiation fates and tissue maturation, ultimately promoting neonatal viability, growth, fertility, brain development, and behavior, as well as metabolic, endocrine, and sensory functions. An emerging paradigm is how the opposing activities of DIO2 and DIO3 are coordinated, providing a dynamic switch that controls the developmental timing of a tissue response, often during neonatal and maturational transitions. A second paradigm is how cell to cell communication within a tissue determines the response to T3. Deiodinases in specific cell types, often strategically located near to blood vessels that convey thyroid hormones into the tissue, can regulate neighboring cell types, suggesting a paracrine-like layer of control of T3 action. We discuss deiodinases as switches for developmental transitions and their potential to influence tissue dysfunction in human thyroid disorders.

The Role of Thyroid Hormone in Neuronal Protection

Thyroid hormone is essential for brain development and brain function in the adult. During development, thyroid hormone acts in a spatial and temporal-specific manner to regulate the expression of genes essential for normal neural cell differentiation, migration, and myelination. In the adult brain, thyroid hormone is important for maintaining normal brain function. Thyroid hormone excess, hyperthyroidism, and thyroid hormone deficiency, hypothyroidism, are associated with disordered brain function, including depression, memory loss, impaired cognitive function, irritability, and anxiety. Adequate thyroid hormone levels are required for normal brain function. Thyroid hormone acts through a cascade of signaling components: activation and inactivation by deiodinase enzymes, thyroid hormone membrane transporters, and nuclear thyroid hormone receptors. Additionally, the hypothalamic-pituitary-thyroid axis, with negative feedback of thyroid hormone on thyrotropin-releasing hormone (TRH) and thyroid-stimulating hormone (TSH) secretion, regulates serum thyroid hormone levels in a narrow range. Animal and human studies have shown both systemic and local reduction in thyroid hormone availability in neurologic disease and after brain trauma. Treatment with thyroid hormone and selective thyroid hormone analogs has resulted in a reduction in injury and improved recovery. This article will describe the thyroid hormone signal transduction pathway in the brain and the role of thyroid hormone in the aging brain, neurologic diseases, and the protective role when administered after traumatic brain injury. © 2021 American Physiological Society. Compr Physiol 11:1-21, 2021.

The Clinical Spectrum of Resistance to Thyroid Hormone Alpha in Children and Adults

Resistance to thyroid hormone alpha occurs due to pathogenic, heterozygous variants in THRA. The entity was first described in 2012 and to date only a small number of patients with varying severity have been reported. In this review, we summarize and interpret the heterogeneous clinical and laboratory features of all published cases, including ours. Many symptoms and findings are similar to those seen in primary hypothyroidism. However, thyroid-stimulating hormone levels are normal. Free triiodothyronine (T3) levels are in the upper half of normal range or frankly high and free thyroxine (T4) levels are low or in the lower half of normal range. Alterations in free T3 and free T4 may not be remarkable, particularly in adults, possibly contributing to underdiagnosis. In such patients, low reverse T3 levels, normo- or macrocytic anemia or, particularly in children, mildly elevated creatine kinase levels would warrant THRA sequencing. Treatment with L-thyroxine results in improvement of some clinical findings.

Temporal Pole Responds to Subtle Changes in Local Thyroid Hormone Signaling

To study thyroid hormone (TH) signaling in the human brain, we analyzed published microarray data sets of the temporal pole (Brodmann area 38) of 19 deceased donors. An index of TH signaling built on the expression of 19 well known TH-responsive genes in mouse brains (T3S+) varied from 0.92 to 1.1. After Factor analysis, T3S+ correlated independently with the expression of TH transporters (MCT8, LAT2), TH receptor (TR) beta and TR coregulators (CARM1, MED1, KAT2B, SRC2, SRC3, NCOR2a). Unexpectedly, no correlation was found between T3S+ vs DIO2, DIO3, SRC1, or TRα. An unbiased systematic analysis of the entire transcriptome identified a set of 1649 genes (set #1) with strong positive correlation with T3S+ (r > 0.75). Factor analysis of set #1 identified 2 sets of genes that correlated independently with T3S+, sets #2 (329 genes) and #3 (191 genes). When processed through the Molecular Signatures Data Base (MSigDB), both sets #2 and #3 were enriched with Gene Ontology (GO)-sets related to synaptic transmission and metabolic processes. Ranking individual human brain donors according to their T3S+ led us to identify 1262 genes (set #4) with >1.3-fold higher expression in the top half. The analysis of the overlapped genes between sets #1 and #4 resulted in 769 genes (set #5), which have a very similar MSigDB signature as sets #2 and #3. In conclusion, gene expression in the human temporal pole can be assessed through T3S+ and fluctuates with subtle variations in local TH signaling.

Clinical associations of maternal thyroid function with foetal brain development: Epidemiological interpretation and overview of available evidence

Thyroid hormone is an important regulator of early brain development, particularly during early stages of gestation during which foetal thyroid hormone availability depends on the maternal transfer of thyroid hormones. There is a wide range of experimental studies showing that low maternal thyroid hormone availability is associated with suboptimal brain development parameters. While few clinical studies have shown that overt maternal hypothyroidism is associated with lower child IQ, the question whether more subclinical changes in maternal thyroid function could also lead to suboptimal foetal brain development. In this review, we put the latter studies in perspective and discuss their interpretation from an epidemiological and clinical perspective. Furthermore, we extend this discussion to also include future perspective and identify important knowledge gaps in the field.

RNA SEQ Analysis Indicates that the AE3 Cl-/HCO3- Exchanger Contributes to Active Transport-Mediated CO2 Disposal in Heart

Loss of the AE3 Cl⁻/HCO₃⁻ exchanger (Slc4a3) in mice causes an impaired cardiac force-frequency response and heart failure under some conditions but the mechanisms are not known. To better understand the functions of AE3, we performed RNA Seq analysis of AE3-null and wild-type mouse hearts and evaluated the data with respect to three hypotheses (CO₂ disposal, facilitation of Na⁺-loading, and recovery from an alkaline load) that have been proposed for its physiological functions. Gene Ontology and PubMatrix analyses of differentially expressed genes revealed a hypoxia response and changes in vasodilation and angiogenesis genes that strongly support the CO₂ disposal hypothesis. Differential expression of energy metabolism genes, which indicated increased glucose utilization and decreased fatty acid utilization, were consistent with adaptive responses to perturbations of O₂/CO₂ balance in AE3-null myocytes. Given that the myocardium is an obligate aerobic tissue and consumes large amounts of O₂, the data suggest that loss of AE3, which has the potential to extrude CO₂ in the form of HCO₃⁻, impairs O₂/CO₂ balance in cardiac myocytes. These results support a model in which the AE3 Cl⁻/HCO₃⁻ exchanger, coupled with parallel Cl⁻ and H⁺-extrusion mechanisms and extracellular carbonic anhydrase, is responsible for active transport-mediated disposal of CO₂.

Thyroid Hormone Stimulation of Adult Brain Fatty Acid Oxidation

Thyroid hormone is a critical modulator of brain metabolism, and it is highly controlled in the central nervous system. Recent research has uncovered an important role of thyroid hormone in the regulation of fatty acid oxidation (FAO), an energetic process essential for neurodevelopment that continues to support brain metabolism during adulthood. Thyroid hormone stimulation of FAO has been shown to be protective in astrocytes and mouse models of brain injury, yet a clear mechanism of this relationship has not been elucidated. Thyroid hormone interacts with multiple receptors located in the nucleus and the mitochondria, initiating rapid and long-term effects via both genomic and nongenomic pathways. This has complicated efforts to isolate and study-specific interactions. This chapter presents the primary signaling pathways that have been identified to play a role in the thyroid hormone-mediated increase in FAO. Investigation of the impact of thyroid hormone on FAO in the adult brain has challenged classical models of brain metabolism and widened the window of potential neuroprotective strategies. A detailed understanding of these pathways is essential for any researchers aiming to expand the field of neuroenergetics.

Resistance to Thyroid Hormone due to Heterozygous Mutations in Thyroid Hormone Receptor Alpha

BACKGROUND

Thyroid hormone (TH) acts via nuclear thyroid hormone receptors (TRs). TR isoforms (TRα1, TRα2, TRβ1, TRβ2) are encoded by distinct genes (THRA and THRB) and show differing tissue distributions. Patients with mutations in THRB, exhibiting resistance within the hypothalamic-pituitary-thyroid axis with elevated TH and nonsuppressed thyroid-stimulating hormone (TSH) levels, were first described decades ago. In 2012, the first patients with mutations in THRA were identified. Scope of this review: This review describes the clinical and biochemical characteristics of patients with resistance to thyroid hormone alpha (RTHα) due to heterozygous mutations in THRA. The genetic basis and molecular pathogenesis of the disorder together with effects of levothyroxine treatment are discussed.

CONCLUSIONS

The severity of the clinical phenotype of RTHα patients seems to be associated with the location and type of mutation in THRA. The most frequent abnormalities observed include anemia, constipation, and growth and developmental delay. In addition, serum (F)T3 levels can be high-normal to high, (F)T4 and rT3 levels normal to low, while TSH is normal or mildly raised. Despite heterogeneous consequences of mutations in THRA, RTHα should be suspected in subjects with even mild clinical features of hypothyroidism together with high/high-normal (F)T3, low/low-normal (F)T4, and normal TSH.

In vivo Functional Consequences of Human THRA Variants Expressed in the Zebrafish

BACKGROUND

Heterozygous mutations in the thyroid hormone receptor alpha (THRA) gene cause resistance to thyroid hormone alpha (RTHα), a disease characterized by variable manifestations reminiscent of untreated congenital hypothyroidism but a raised triiodothyronine/thyroxine ratio and normal thyrotropin levels. It was recently described that zebrafish embryos expressing a dominant negative (DN) form of thraa recapitulate the key features of RTHα, and that zebrafish and human receptors are functionally interchangeable.

METHODS

This study expressed several human thyroid hormone receptor alpha (hTRα) variants in zebrafish embryos and analyzed the resulting phenotypes.

RESULTS

All hTRα-injected embryos showed variable defects, including cerebral and cardiac edema likely caused by an aberrant looping during heart development, anemia, and an incomplete formation of the vascular network. Moreover, the hTRα-injected embryos presented severe defects of motorneurons and craniofacial development, thus affecting their autonomous feeding and swimming behaviors. Surprisingly, expression of all hTRα mutants had no detectable effect on thyrotropin beta and thyrotropin-releasing hormone transcripts, indicating that their DN action is limited on the thyroid hormone reception beta 2 targets at the hypothalamic/pituitary level in vivo. As previously described in vitro, treatment with high triiodothyronine doses can efficiently revert the observed defects only in embryos injected with missense hTRα variants.

CONCLUSION

Injection of human THRA variants in zebrafish embryos causes tissue-specific defects recapitulating most of the RTHα clinical and biochemical manifestations. The described manipulation of zebrafish embryos represents a novel in vivo model to screen the functional consequences of THRA variants and the rescue potential of new therapeutic compounds.

Resistance to thyroid hormone due to defective thyroid receptor alpha

Thyroid hormones act via nuclear receptors (TRα1, TRβ1, TRβ2) with differing tissue distribution; the role of α2 protein, derived from the same gene locus as TRα1, is unclear. Resistance to thyroid hormone alpha (RTHα) is characterised by tissue-specific hypothyroidism associated with near-normal thyroid function tests. Clinical features include dysmorphic facies, skeletal dysplasia (macrocephaly, epiphyseal dysgenesis), growth retardation, constipation, dyspraxia and intellectual deficit. Biochemical abnormalities include low/low-normal T4 and high/high-normal T3 concentrations, a subnormal T4/T3 ratio, variably reduced reverse T3, raised muscle creatine kinase and mild anaemia. The disorder is mediated by heterozygous, loss-of-function, mutations involving either TRα1 alone or both TRα1 and α2, with no discernible phenotype attributable to defective α2. Whole exome sequencing and diagnostic biomarkers may enable greater ascertainment of RTHα, which is important as thyroxine therapy reverses some metabolic abnormalities and improves growth, constipation, dyspraxia and wellbeing. The genetic and phenotypic heterogeneity of RTHα and its optimal management remain to be elucidated.

Nuclear receptors and the Warburg effect in cancer

In 1927 Otto Warburg established that tumours derive energy primarily from the conversion of glucose to lactic acid and only partially through cellular respiration involving oxygen. In the 1950s he proposed that all causes of cancer reflected different mechanisms of disabling cellular respiration in favour of fermentation (now termed aerobic glycolysis). The role of aberrant glucose metabolism in cancer is now firmly established. The shift away from oxidative phosphorylation towards the metabolically expensive aerobic glycolysis is somewhat counter-intuitive given its wasteful nature. Multiple control processes are in place to maintain cellular efficiency and it is likely that these mechanisms are disrupted to facilitate the shift to the reliance on aerobic glycolysis. One such process of cell control is mediated by the nuclear receptor superfamily. This large family of transcription factors plays a significant role in sensing environmental cues and controlling decisions on proliferation, differentiation and cell death for example, to regulate glucose uptake and metabolism and to modulate the actions of oncogenes and tumour suppressors. In this review we highlight mechanisms by which nuclear receptors actions are altered during tumorigenic transformation and can serve to enhance the shift to aerobic glycolysis. At the simplest level, a basic alteration in NR behaviour can serve to enhance glycolytic flux thus providing a basis for enhanced survival within the tumour micro-environment. Ameliorating the enhanced NR activity in this context may help to sensitize cancer cells to Warburg targeted therapies and may provide future drug targets.

A temporary compendium of thyroid hormone target genes in brain

BACKGROUND

Thyroid hormone controls a number of developmental and physiological processes in the brain by directly acting on gene expression. Transcriptome analyses in rodent identified a number of thyroid hormone regulated genes in several brain areas at different stages. Genome wide analysis of chromatin occupancy in a neural cell line also identified a subset of genes which transcription is likely to be directly regulated by thyroid hormone receptors in neurons. However, the abundance of these data and apparent discrepancies between studies brought some confusion.

RESULTS

We present here a meta-analysis of available data to identify recurrent themes in thyroid hormone action in brain cells. This provides a curated list of 734 regulated genes in rodent brain, and highlights a small number of likely direct target genes. Some of these genes are also regulated in amphibians during metamorphosis. This article is part of a Special Issue entitled: Nuclear receptors in animal development.

Clinical Consequences of Mutations in Thyroid Hormone Receptor-α1

Thyroid hormone (TH) exerts its biological activity via the TH receptors TRα1 and TRβ1/2, which are encoded by the THRA and THRB genes. The first patients with mutations in THRB were identified decades ago. These patients had a clinical syndrome of resistance to TH associated with high serum TH and nonsuppressed thyroid-stimulating hormone levels. Until recently, no patients with mutations in THRA had been identified. In an attempt to predict the clinical phenotype of such patients, different TRα1 mutant mouse models have been generated. These mice have a variable phenotype depending on the location and severity of the mutation. Recently, the first humans with mutations in THRA were identified. Their phenotype consists of relatively low serum T4 and high serum T3 levels (and thus an elevated T3/T4 ratio), growth retardation, delayed mental and bone development, and constipation. While, in retrospect, certain features present in humans can also be found in mouse models, the first humans carrying a defect in TRα1 were not suspected of having a THRA gene mutation initially. The current review focuses on the clinical consequences of TRα1 mutations.

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

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

Making sense with thyroid hormone--the role of T(3) in auditory development

The senses are our window to the world, our interface with the habitat in which we live in and the basis for our communication with each other. Although sensory systems are not generally viewed as major targets of endocrine regulation, sensory development is profoundly influenced by thyroid hormone (T(3)) signalling. In this article, we discuss this developmental role of T(3) and highlight the auditory system as the best-studied example of the interplay between systemic and local tissue mechanisms by which T(3) stimulates the onset of sensory function. Several genes that mediate the action of T(3) are known to promote sensory development in mice, including genes that encode T(3) receptors and deiodinase enzymes that amplify or deplete levels of T(3). We also discuss the current knowledge of sensory defects in human genetic disorders in which T(3) signalling is impaired. As sensory input provides the only means of acquiring information from the environment, the stimulation of sensory development is one of the most fundamental functions of T(3) signalling.

Genome-wide analysis of thyroid hormone receptors shared and specific functions in neural cells

TRα1 and TRβ1, the two main thyroid hormone receptors in mammals, are transcription factors that share similar properties. However, their respective functions are very different. This functional divergence might be explained in two ways: it can reflect different expression patterns or result from different intrinsic properties of the receptors. We tested this second hypothesis by comparing the repertoires of 3,3',5-triiodo-L-thyronine (T3)-responsive genes of two neural cell lines, expressing either TRα1 or TRβ1. Using transcriptome analysis, we found that a substantial fraction of the T3 target genes display a marked preference for one of the two receptors. So when placed alone in identical situations, the two receptors have different repertoires of target genes. Chromatin occupancy analysis, performed at a genome-wide scale, revealed that TRα1 and TRβ1 cistromes were also different. However, receptor-selective regulation of T3 target genes did not result from receptor-selective chromatin occupancy of their promoter regions. We conclude that modification of TRα1 and TRβ1 intrinsic properties contributes in a large part to the divergent evolution of the receptors' function, at least during neurodevelopment.

Cerebellar abnormalities in mice lacking type 3 deiodinase and partial reversal of phenotype by deletion of thyroid hormone receptor α1

Thyroid hormone serves many functions throughout brain development, but the mechanisms that control the timing of its actions in specific brain regions are poorly understood. In the cerebellum, thyroid hormone controls formation of the transient external germinal layer, which contains proliferative granule cell precursors, subsequent granule cell migration, and cerebellar foliation. We report that the thyroid hormone-inactivating type 3 deiodinase (encoded by Dio3) is expressed in the mouse cerebellum at embryonic and neonatal stages, suggesting a need to protect cerebellar tissues from premature stimulation by thyroid hormone. Dio3(-/-) mice displayed reduced foliation, accelerated disappearance of the external germinal layer, and premature expansion of the molecular layer at juvenile ages. Furthermore, Dio3(-/-) mice exhibited locomotor behavioral abnormalities and impaired ability in descending a vertical pole. To ascertain that these phenotypes resulted from inappropriate exposure to thyroid hormone, thyroid hormone receptor α1 (TRα1) was removed from Dio3(-/-) mice, which substantially corrected the cerebellar and behavioral phenotypes. Deletion of TRα1 did not correct the previously reported small thyroid gland or deafness in Dio3(-/-) mice, indicating that Dio3 controls the activation of specific receptor isoforms in different tissues. These findings suggest that type 3 deiodinase constrains the timing of thyroid hormone action during cerebellar development.

Thyroid hormone receptors: the challenge of elucidating isotype-specific functions and cell-specific response

BACKGROUND

Thyroid hormone receptors TRα1, TRβ1 and TRβ2 are broadly expressed and exert a pleiotropic influence on many developmental and homeostatic processes. Extensive genetic studies in mice precisely defined their respective function.

SCOPE OF REVIEW

The purpose of the review is to discuss two puzzling issues:

MAJOR CONCLUSIONS

Mouse genetics support a balanced contribution of expression pattern and receptor intrinsic properties in defining the receptor respective functions. The molecular mechanisms sustaining cell specific response remain hypothetical and based on studies performed with other nuclear receptors.

GENERAL SIGNIFICANCE

The isoform-specificity and cell-specificity questions have many implications for clinical research, drug development, and endocrine disruptor studies. This article is part of a Special Issue entitled Thyroid hormone signalling.

Serum copper as a novel biomarker for resistance to thyroid hormone

Thyroid hormone action is mediated by the thyroid hormone receptors TRα1 and TRβ. Defects in TRβ lead to RTH (resistance to thyroid hormone) β, a syndrome characterized by high levels of thyroid hormone and non-suppressed TSH (thyroid-stimulating hormone). However, a correct diagnosis of RTHβ patients is difficult as the clinical picture varies. A biochemical serum marker indicative of defects in TRβ signalling is needed and could simplify the diagnosis of RTHβ, in particular the differentiation to TSH-secreting pituitary adenomas, which present with clinically similar symptoms. In the present paper we show that serum copper levels are regulated by thyroid hormone, which stimulates the synthesis and the export of the hepatic copper-transport protein ceruloplasmin into the serum. This is accompanied by a concerted reduction in the mRNA levels of other copper-containing proteins such as metallothioneins 1 and 2 or superoxide dismutase 1. The induction of serum copper is abolished in genetically hyperthyroid mice lacking TRβ and human RTHβ patients, demonstrating an important role of TRβ for this process. Together with a previously reported TRα1 specific regulation of serum selenium, we show that the ratio of serum copper and selenium, which is largely independent of thyroid hormone levels, volume changes or sample degradation, can constitute a valuable novel biomarker for RTHβ. Moreover, it could also provide a suitable large-scale screening parameter to identify RTHα patients, which have not been identified to date.

Genome-wide search reveals the existence of a limited number of thyroid hormone receptor alpha target genes in cerebellar neurons

Thyroid hormone (T3) has a major influence on cerebellum post-natal development. The major phenotypic landmark of exposure to low levels of T3 during development (hypothyroidism) in the cerebellum is the retarded inward migration of the most numerous cell type, granular neurons. In order to identify the direct genetic regulation exerted by T3 on cerebellar neurons and their precursors, we used microarray RNA hybridization to perform a time course analysis of T3 induced gene expression in primary cultures of cerebellar neuronal cell. These experiments suggest that we identified a small set of genes which are directly regulated, both in vivo and in vitro, during cerebellum post-natal development. These modest changes suggest that T3 does not acts directly on granular neurons and mainly indirectly influences the cellular interactions taking place during development.

Thyroid hormone action in metabolic regulation

Thyroid hormone plays pivotal roles in growth, differentiation, development and metabolic homeostasis via thyroid hormone receptors (TRs) by controlling the expression of TR target genes. The transcriptional activity of TRs is modulated by multiple factors including various TR isoforms, diverse thyroid hormone response elements, different heterodimeric partners, coregulators, and the cellular location of TRs. In the present review, we summarize recent advance in understanding the molecular mechanisms of thyroid hormone action obtained from human subject research, thyroid hormone mimetics application, TR isoform-specific knock-in mouse models, and mitochondrion study with highlights in metabolic regulations. Finally, as future perspectives, we share our thoughts about current challenges and possible approaches to promote our knowledge of thyroid hormone action in metabolism.

Differential recruitment of nuclear coregulators directs the isoform-dependent action of mutant thyroid hormone receptors

Studies using mice deficient in thyroid hormone receptors (TR) indicate that the two TR isoforms, TRα1 and TRβ1, in addition to mediating overlapping biological activities of the thyroid hormone, T3, also mediate distinct functions. Mice harboring an identical dominant negative mutation (denoted PV) at the C terminus of TRα1 (Thra1(PV) mice) or β1 (Thrb(PV) mice) also exhibit distinct phenotypes. These knockin mutant mice provide an opportunity to understand the molecular basis of isoform-dependent functions in vivo. Here we tested the hypothesis that the distinct functions of TR mutant isoforms are directed by a subset of nuclear regulatory proteins. Tandem-affinity chromatography of HeLa nuclear extracts showed that distinct 33 nuclear proteins including nuclear receptor corepressor (NCoR1) and six other proteins preferentially associated with TRα1PV or TRβ1PV, respectively. These results indicate that recruitment of nuclear regulatory proteins by TR mutants is subtype dependent. The involvement of NCoR1 in mediating the distinct liver phenotype of Thra1(PV) and Thrb(PV) mice was further explored. NCoR1 preferentially interacted with TRα1PV rather than with TRβ1PV. NCoR1 was recruited more avidly to the thyroid hormone response element-bound TRα1PV than to TRβ1PV in the promoter of the CCAAT/enhancer-binding protein α gene to repress its expression in the liver of Thra1(PV) mice, but not in Thrb(PV) mice. This preferential recruitment of NCoR1 by mutant isoforms could contribute, at least in part, to the distinct liver lipid phenotype of these mutant mice. The present study highlights a novel mechanism by which TR isoforms direct their selective functions via preferential recruitment of a subset of nuclear coregulatory proteins.

Thyroid hormone and cerebellar development

Thyroid hormone (TH) plays a key role in mammalian brain development. The developing brain is sensitive to both TH deficiency and excess. Brain development in the absence of TH results in motor skill deficiencies and reduced intellectual development. These functional abnormalities can be attributed to maldevelopment of specific cell types and regions of the brain including the cerebellum. TH functions at the molecular level by regulating gene transcription. Therefore, understanding how TH regulates cerebellar development requires identification of TH-regulated gene targets and the cells expressing these genes. Additionally, the process of TH-dependent regulation of gene expression is tightly controlled by mechanisms including regulation of TH transport, TH metabolism, toxicologic inhibition of TH signaling, and control of the nuclear TH response apparatus. This review will describe the functional, cellular, and molecular effects of TH deficit in the developing cerebellum and emphasize the most recent findings regarding TH action in this important brain region.

Severe psychomotor and metabolic damages caused by a mutant thyroid hormone receptor alpha 1 in mice: can patients with a similar mutation be found and treated?

Individuals suffering from the resistance to thyroid hormone syndrome (RTH) have a mutation in thyroid hormone receptor (TR) beta. Surprisingly, no patient with a mutation in TRalpha1 has been found. To facilitate their identification, animal models with a RTH-like mutation in TRalpha1 have been generated. The mutations introduced into the mouse decrease affinity to ligand, resulting in a 'receptor-mediated hypothyroidism' in tissues expressing the mutant receptor: brain, heart and bone. The mice present minor perturbances in thyroid hormone homeostasis, but show major aberrancies in postnatal development, psychomotor behaviour and metabolism. These parameters are akin to those seen in endemic cretinism and untreated congenital hypothyroidism. Treatment of the mice with high doses of triiodothyronine leads to normalization or amelioration of the dysfunctions when applied at adequate developmental periods.

CONCLUSION

Our studies on mice suggest the existence of a potentially debilitating disease caused by a mutant TRalpha1, and provide insights for identification and treatment of corresponding patients.

Physiological consequences of the TRalpha1 aporeceptor state

Many patients have been characterized harboring a mutation in thyroid hormone receptor (TR) beta. Surprisingly none has yet been identified carrying a mutation in TRalpha1. To facilitate the identification of such patients, several animal models with a mutant TRalpha1 have been generated. While some phenotypic characteristics, such as an adult euthyroidism, are similar in the mutant mice, other aspects such as metabolism are quite variable. This review summarizes the most important consequences of a mutation in TRalpha1 in mice focusing on the TRalpha1-R384C mutation, and projects the insights from the animal models to a putative phenotype of patients with a mutated TRalpha1.

Molecular cloning, tissue distribution and expression profiles of thyroid hormone receptors during embryogenesis in orange-spotted grouper (Epinephelus coioides)

Three distinct thyroid hormone receptor (TR) cDNAs (gTRalphaA, gTRalphaB and gTRbeta) were cloned and sequenced in orange-spotted grouper (Epinephelus coioides). The cDNA of gTRalphaA consisted of 1555-bp and encoded a putative protein of 416 amino acids, while the cDNA of gTRalphaB consisted of 1459-bp and encoded 398 amino acids. The cDNA of gTRbeta is 1470-bp in length and encoded 395 amino acids. The cDNAs of gTRalphaA and gTRalphaB had 68.7% identity in nucleotide sequence and 83.9% identity in deduced amino acid sequence, and shared 57.8%, 57.9% identity in nucleotide sequence and 76.5%, 76.6% in deduced amino acid sequence with gTRbeta, respectively. mRNA expression of gTR was determined by using real-time RT-PCR. The zygotic mRNA expression of gTRalphaA, gTRalphaB and gTRbeta was already presented at the blastula stage, and decreased by gastrulation and body segment appearance. Both gTRalphaA and gTRbeta mRNAs dramatically increased by brain vesicle appearance and lens formation stages, whereas the magnitude of the increase of gTRbeta was higher than that of gTRalphaA. However, gTRalphaB remained relatively constant. All three gTR mRNAs were detected in various tissues of adult fish, with considerable levels found in the pituitary and brain. The expression of gTRalphaA and gTRalphaB was ubiquitous, while the gTRbeta transcription in the pituitary was higher than other tissues. The results suggest that gTRs may be important in adult grouper brain function and that gTRalphaA and gTRalphaB may be important factors in metabolic regulation. In addition, gTRbeta may be the major TR isoform in the regulation of TSH activity by TH feedback in grouper.

Multigenic control of thyroid hormone functions in the nervous system

Thyroid hormone (TH) has a remarkable range of actions in the development and function of the nervous system. A multigenic picture is emerging of the mechanisms that specify these diverse functions in target tissues. Distinct responses are mediated by alpha and beta isoforms of TH receptor which act as ligand-regulated transcription factors. Receptor activity can be regulated at several levels including that of uptake of TH ligand and the activation or inactivation of ligand by deiodinase enzymes in target tissues. Processes under the control of TH range from learning and anxiety-like behaviour to sensory function. At the cellular level, TH controls events as diverse as axonal outgrowth, hippocampal synaptic activity and the patterning of opsin photopigments necessary for colour vision. Overall, TH coordinates this variety of events in both central and sensory systems to promote the function of the nervous system as a complete entity.

Neuroendocrine inhibition of glucose production and resistance to cancer in dwarf mice

Pit1 null (Snell dwarf) and Proph1 null (Ames dwarf) mutant mice lack GH, PRL and TSH. Snell and Ames dwarf mice also exhibit reduced IGF-I, resistance to cancer and a longer lifespan than control mice. Endogenous glucose production during fasting is reduced in Snell dwarf mice compared to fasting control mice. In view of cancer cell dependence on glucose for energy, low endogenous glucose production may provide Snell dwarf mice with resistance to cancer. We investigated whether endogenous glucose production is lower in Snell dwarf mice during feeding. Inhibition of endogenous glucose production by glucose injection was enhanced in 12 to 14 month-old female Snell dwarf mice. Thus, we hypothesize that lower endogenous glucose production during feeding and fasting reduces cancer cell glucose utilization providing Snell dwarf mice with resistance to cancer. The elevation of circulating adiponectin, a hormone produced by adipose tissue, may contribute to the suppression of endogenous glucose production in 12 to 14 month-old Snell dwarf mice. We compared the incidence of cancer at time of death between old Snell dwarf and control mice. Only 18% of old Snell dwarf mice had malignant lesions at the time of death compared to 82% of control mice. The median ages at death for old Snell dwarf and control mice were 33 and 26 months, respectively. By contrast, previous studies showed a high incidence of cancer in old Ames dwarf mice at the time of death. Hence, resistance to cancer in old Snell dwarf mice may be mediated by neuroendocrine factors that reduce glucose utilization besides elevated adiponectin, reduced IGF-I and a lack of GH, PRL and TSH, seen in both Snell and Ames dwarf mice. Proteomics analysis of pituitary secretions from Snell dwarf mice confirmed the absence of GH and PRL, the secretion of ACTH and elevated secretion of Chromogranin B and Secretogranin II. Radioimmune assays confirmed that circulating Chromogranin B and Secretogranin II were elevated in 12 to 14 month-old Snell dwarf mice. In summary, our results in Snell dwarf mice suggest that the pituitary gland and adipose tissue are part of a neuroendocrine loop that lowers the risk of cancer during aging by reducing the availability of glucose.

Supply and demand in cerebral energy metabolism: the role of nutrient transporters

Glucose is the obligate energetic fuel for the mammalian brain, and most studies of cerebral energy metabolism assume that the majority of cerebral glucose utilization fuels neuronal activity via oxidative metabolism, both in the basal and activated state. Glucose transporter (GLUT) proteins deliver glucose from the circulation to the brain: GLUT1 in the microvascular endothelial cells of the blood-brain barrier (BBB) and glia; GLUT3 in neurons. Lactate, the glycolytic product of glucose metabolism, is transported into and out of neural cells by the monocarboxylate transporters (MCT): MCT1 in the BBB and astrocytes and MCT2 in neurons. The proposal of the astrocyte-neuron lactate shuttle hypothesis suggested that astrocytes play the primary role in cerebral glucose utilization and generate lactate for neuronal energetics, especially during activation. Since the identification of the GLUTs and MCTs in brain, much has been learned about their transport properties, that is capacity and affinity for substrate, which must be considered in any model of cerebral glucose uptake and utilization. Using concentrations and kinetic parameters of GLUT1 and -3 in BBB endothelial cells, astrocytes, and neurons, along with the corresponding kinetic properties of the MCTs, we have successfully modeled brain glucose and lactate levels as well as lactate transients in response to neuronal stimulation. Simulations based on these parameters suggest that glucose readily diffuses through the basal lamina and interstitium to neurons, which are primarily responsible for glucose uptake, metabolism, and the generation of the lactate transients observed on neuronal activation.

Low Utilization of Circulating Glucose after Food Withdrawal in Snell Dwarf Mice

Glucose metabolism is altered in long-lived people and mice. Although it is clear that there is an association between altered glucose metabolism and longevity, it is not known whether this link is causal or not. Our current hypothesis is that decreased fasting glucose utilization may increase longevity by reducing oxygen radical production, a potential cause of aging. We observed that whole body fasting glucose utilization was lower in the Snell dwarf, a long-lived mutant mouse. Whole body fasting glucose utilization may be reduced by a decrease in the production of circulating glucose. Our isotope labeling analysis indicated both gluconeogenesis and glycogenolysis were suppressed in Snell dwarfs. Elevated circulating adiponectin may contribute to the reduction of glucose production in Snell dwarfs. Adiponectin lowered the appearance of glucose in the media over hepatoma cells by suppressing gluconeogenesis and glycogenolysis. The suppression of glucose production by adiponectin in vitro depended on AMP-activated protein kinase, a cell mediator of fatty acid oxidation. Elevated fatty acid oxidation was indicated in Snell dwarfs by increased utilization of circulating oleic acid, reduced intracellular triglyceride content, and increased phosphorylation of acetyl-CoA carboxylase. Finally, protein carbonyl content, a marker of oxygen radical damage, was decreased in Snell dwarfs. The correlation between high glucose utilization and elevated oxygen radical production was also observed in vitro by altering the concentrations of glucose and fatty acids in the media or pharmacologic inhibition of glucose and fatty acid oxidation with 4-hydroxycyanocinnamic acid and etomoxir, respectively.

General background on the hypothalamic-pituitary-thyroid (HPT) axis

This article reviews the thyroid system, mainly from a mammalian standpoint. However, the thyroid system is highly conserved among vertebrate species, so the general information on thyroid hormone production and feedback through the hypothalamic-pituitary-thyroid (HPT) axis should be considered for all vertebrates, while species-specific differences are highlighted in the individual articles. This background article begins by outlining the HPT axis with its components and functions. For example, it describes the thyroid gland, its structure and development, how thyroid hormones are synthesized and regulated, the role of iodine in thyroid hormone synthesis, and finally how the thyroid hormones are released from the thyroid gland. It then progresses to detail areas within the thyroid system where disruption could occur or is already known to occur. It describes how thyroid hormone is transported in the serum and into the tissues on a cellular level, and how thyroid hormone is metabolized. There is an in-depth description of the alpha and beta thyroid hormone receptors and their functions, including how they are regulated, and what has been learned from the receptor knockout mouse models. The nongenomic actions of thyroid hormone are also described, such as in glucose uptake, mitochondrial effects, and its role in actin polymerization and vesicular recycling. The article discusses the concept of compensation within the HPT axis and how this fits into the paradigms that exist in thyroid toxicology/endocrinology. There is a section on thyroid hormone and its role in mammalian development: specifically, how it affects brain development when there is disruption to the maternal, the fetal, the newborn (congenital), or the infant thyroid system. Thyroid function during pregnancy is critical to normal development of the fetus, and several spontaneous mutant mouse lines are described that provide research tools to understand the mechanisms of thyroid hormone during mammalian brain development. Overall this article provides a basic understanding of the thyroid system and its components. The complexity of the thyroid system is clearly demonstrated, as are new areas of research on thyroid hormone physiology and thyroid hormone action developing within the field of thyroid endocrinology. This review provides the background necessary to review the current assays and endpoints described in the following articles for rodents, fishes, amphibians, and birds.

Thyroid hormone receptors in brain development and function

Thyroid hormones are important during development of the mammalian brain, acting on migration and differentiation of neural cells, synaptogenesis, and myelination. The actions of thyroid hormones are mediated through nuclear thyroid hormone receptors (TRs) and regulation of gene expression. The purpose of this article is to review the role of TRs in brain maturation. In developing humans maternal and fetal thyroid glands provide thyroid hormones to the fetal brain, but the timing of receptor ontogeny agrees with clinical data on the importance of the maternal thyroid gland before midgestation. Several TR isoforms, which are encoded by the THRA and THRB genes, are expressed in the brain, with the most common being TRalpha1. Deletion of TRalpha1 in rodents is not, however, equivalent to hormone deprivation and, paradoxically, even prevents the effects of hypothyroidism. Unliganded receptor activity is, therefore, probably an important factor in causing the harmful effects of hypothyroidism. Accordingly, expression of a mutant receptor with impaired triiodothyronine (T(3)) binding and dominant negative activity affected cerebellar development and motor performance. TRs are also involved in adult brain function. TRalpha1 deletion, or expression of a dominant negative mutant receptor, induces consistent behavioral changes in adult mice, leading to severe anxiety and morphological changes in the hippocampus.

Thyroid hormone receptor mutations and disease: insights from knock-in mouse models

Thyroid hormone nuclear receptors (TRs) mediate thyroid hormone's activities in growth, differentiation, and development. Two TR genes (α and β ) encode four thyroid hormone-binding receptors that regulate target gene expression. Mutations of the TRβ gene cause the genetic syndrome of resistance to thyroid hormone. Studies indicate a close association between TRβ mutations and several human cancers, suggesting their oncogenic role. A TRβ gene knock-in mutant mouse (TRβ^PV/PV mouse) that spontaneously develops thyroid cancer allows elucidation of the oncogenic functions in vivo. TRβPV is a potent dominant negative mutant identified in a resistance to thyroid hormone patient. Molecular studies indicate that the PV mutant mediates its oncogenic activities via nucleus-initiated transcription and novel extranuclear actions. Thus, the deleterious effects of the gene mutations go beyond resistance to thyroid hormone and are more severe and extensive than previously envisioned. This newly identified oncogene exerts its tumorigenic effects via multiple signaling mechanisms.

Thyroid hormone receptor mutations and disease: beyond thyroid hormone resistance

Thyroid hormone receptors (TRs) are ligand-dependent transcription factors that mediate the biological activities of thyroid hormone (T3). Two THR genes (A and B), located on different chromosomes, yield four T3-binding isoforms with highly conserved sequences in the DNA- and ligand-binding domains. Mutations of THRB cause a human genetic disease, thyroid hormone resistance syndrome (RTH). Comprehensive genomic profiling unveiled the contribution of novel change-of-function mutations of TRbeta to the pathogenesis of RTH. In addition, abnormalities associated with mutations of the THRA gene have been uncovered recently. The phenotypic manifestations of mutated THRB and THRA genes are distinct, indicating isoform-dependent actions of TR mutants in vivo. Therefore, mutant TRs provide a new paradigm to understand the molecular basis of receptor disease.

Thyroid hormone-dependent gene expression in differentiated embryonic stem cells and embryonal carcinoma cells: identification of novel thyroid hormone target genes by deoxyribonucleic acid microarray analysis

T3 is required for normal early development, but relatively few T3-responsive target genes have been identified. In general, in vitro stem cell differentiation techniques stimulate a wide range of developmental programs, including thyroid hormone receptor (TR) pathways. We developed several in vitro stem cell models to more specifically identify TR-mediated gene expression in early development. We found that embryonic carcinoma (EC) cells have reduced T3 nuclear binding capacity and only modestly express the known T3 target genes, neurogranin (RC3) and Ca2+/calmodulin-dependent protein kinase IV (CaMKIV), in response to T3. Full T3 induction in transient transfection of EC cells was restored with cotransfection of a TR expression vector. We, therefore, performed gene expression profiles in wild-type embryonic stem (ES) cells compared with expression in cells with deficient (EC) or mutant TR (TRalpha P398H mutant ES cells), to identify T3 target genes. T3 stimulation of wild-type ES cells altered mRNA expression of 610 known genes (26% of those studied), although only approximately 60 genes (1%) met criteria for direct T3 stimulation based on the magnitude of induction and requirement for the presence of TR. We selected five candidate T3 target genes, neurexophilin 2, spermatid perinuclear RNA-binding protein (SPNR), kallikrein-binding protein (KBP), prostate-specific membrane antigen (PSMA), and synaptotagmin II, for more detailed study. T3 responsiveness of these genes was evaluated in both in vitro endogenous gene expression and in vivo mouse model systems. These genes identified in a novel stem cell system, including those induced and repressed in response to T3, may mediate thyroid hormone actions in early development.

Thyroid hormones and brain development

The action of thyroid hormones (thyroxine, T4; triiodothyronine, T3) on brain development and function is gaining renewed interest. It has been known for many years that thyroid hormones are very important in mammalian brain maturation, influencing many aspects related to neural cell migration, differentiation, and signaling. In the last 10 years, genes regulated by thyroid hormones have been identified in the rodent brain, and understanding of the role of thyroid hormone nuclear receptors has been facilitated with the analysis of the phenotype of mutant mice for the different receptor isoforms. The general picture that emerges is that T4 and T3 may enter the brain through specific transporters. T4 is converted to the active hormone, T3, in glial cells, astrocytes, and tanycytes, although the main target cells are neurons and maturing oligodendrocytes. T3, acting through the nuclear receptors, controls the expression of genes involved in myelination, cell differentiation, migration, and signaling. In addition to transducing the T3 signal, the nuclear receptors also have activity in the unliganded state (i.e., as aporeceptors), mainly as repressors of transcription. The physiological meaning of aporreceptor action is not known, but they may play a role in the genesis of the hypothyroid phenotype. Among the questions that remain to be explored in more detail is the role of thyroid hormones and the T3 receptors, both liganded and unliganded, in the fetal brain, especially before onset of fetal thyroid gland function. These questions are relevant for human health and the management of thyroid diseases during pregnancy.

Cardiac glucose utilization in mice with mutated alpha- and beta-thyroid hormone receptors

Abnormal thyroid function is usually associated with altered cardiac function. Mutations in the thyroid hormone (TH)-binding region of the TH beta-receptor (TRbeta) that eliminate its TH-binding ability lead to the thyroid hormone resistance syndrome (RTH) in humans, which is characterized by high blood TH levels, goiter, hyperactivity, and tachycardia. Mice with "knock-in" mutations in the TH alpha-receptor (TRalpha) or TRbeta that remove their TH-binding ability have been developed, and those with the mutated TRbeta (TRbeta(PV/PV)) appear to provide a model for RTH. These two types of mutants show different effects on cerebral energy metabolism, e.g., negligible change in glucose utilization (CMR(Glc)) in TRbeta(PV/PV) mice and markedly reduced CMR(Glc), like that found in cretinous rats, in the mice (TRalpha(PV/+)) with the knock-in mutation of the TRalpha gene. Studies in knockout mice have indicated that the TRalpha may also influence heart rate. Because mutations in both receptor genes appear to affect some parameters of cardiac function and because cardiac functional activity and energy metabolism are linked, we measured heart glucose utilization (HMR(Glc)) in both the TRbeta(PV/PV) and TRalpha(PV/+) mutants. Compared with values in normal wild-type mice, HMR(Glc) was reduced (-77 to -95%) in TRalpha(PV/+) mutants and increased (87 to 340%) in TRbeta(PV/PV) mutants, the degree depending on the region of the heart. Thus the TRalpha(PV/+) and TRbeta(PV/PV) mutations lead, respectively, to opposite effects on energy metabolism in the heart that are consistent with the bradycardia seen in hypothyroidism and the tachycardia associated with hyperthyroidism and RTH.

Expression of thyroid hormone receptor isoforms in the oligodendrocyte lineage

Thyroid hormone (T3) regulates brain development and function and in particular ensures normal myelination. Animal models and in vitro systems have been employed to demonstrate the effects of T3, which acts via nuclear hormone receptors. T3 receptors (TRs) are transcription factors that activate or suppress target gene expression, such as myelin basic protein (MBP), in a hormone-dependent or -independent fashion. Two distinct genes, TR alpha and TR beta, encode several receptor isoforms with specific functions. This overview summarizes current knowledge on the cellular expression and the role of these isoforms and also examines the action of T3 on oligodendrocyte lineage cell types at defined developmental stages. Re-expression of TRs and also that of other transcription factors in oligodendrocytes may constitute some of the metabolic changes required for succesfull remyelination in the adult central nervous system after demyelinating lesions.

The thyroid hormone receptor beta gene: structure and functions in the brain and sensory systems

Thyroid hormone profoundly influences the development of the vertebrate nervous system. The thyroid hormone receptor beta gene (Thrb) is a key mediator of many of these actions. The Thrb gene is complex, spanning up to 400 kb in mammals, and differentially expresses distinct receptor subtypes through independent tissue-specific promoters and alternative splicing. These receptors serve a range of functions in the brain as well as particularly sensitive functions in the auditory and visual sensory systems. The Thrb gene illustrates how versatility in neurodevelopmental control can be achieved at the receptor level.

Control of thyroid hormone action in the developing rat brain

Thyroid hormones play important roles in brain development. The physiologic function of thyroid hormones in the developing brain is to provide a timing signal that leads to the induction of differentiation and maturation programs during precise stages of development. Inappropriate initiation of these timing events leads to asynchrony in developmental processes and a deleterious outcome. The developing brain is protected from premature thyroid hormone signaling through a variety of measures. Firstly, local brain levels of both thyroxine and triiodothyronine are controlled by ontogenically regulated patterns of production and metabolism. Secondly, developmentally regulated expression of nuclear proteins involved with the nuclear TH response apparatus control the temporal response of brain genes to thyroid hormone. Finally, developmental regulation of TH action modulating transcription factor expression also controls TH action in the developing brain. Together these molecular mechanisms cooperatively act to temporally control TH action during brain development. A description of these controlling mechanisms is the subject of this review.

A thyroid hormone receptor alpha gene mutation (P398H) is associated with visceral adiposity and impaired catecholamine-stimulated lipolysis in mice

Thyroid hormone has profound effects on metabolic homeostasis, regulating both lipogenesis and lipolysis, primarily by modulating adrenergic activity. We generated mice with a point mutation in the thyroid hormone receptor alpha (TRalpha) gene producing a dominant-negative TRalpha mutant receptor with a proline to histidine substitution (P398H). The heterozygous P398H mutant mice had a 3.4-fold (p < 0.02) increase in serum thyrotropin (TSH) levels. Serum triiodothyronine (T3) and thyroxine (T4) concentrations were slightly elevated compared with wild-type mice. The P398H mice had a 4.4-fold increase in body fat (as a fraction of total body weight) (p < 0.001) and a 5-fold increase in serum leptin levels (p < 0.005) compared with wild-type mice. A 3-fold increase in serum fasting insulin levels (p < 0.002) and a 55% increase in fasting glucose levels (p < 0.01) were observed in P398H compared with wild-type mice. There was a marked reduction in norepinephrine-induced lipolysis, as reflected in reduced glycerol release from white adipose tissue isolated from P398H mice. Heart rate and cold-induced adaptive thermogenesis, mediated by thyroid hormone-catecholamine interaction, were also reduced in P398H mice. In conclusion, the TRalpha P398H mutation is associated with visceral adiposity and insulin resistance primarily due to a marked reduction in catecholamine-stimulated lipolysis. The observed phenotype in the TRalpha P398H mouse is likely due to interference with TRalpha action as well as influence on other metabolic signaling pathways. The physiologic significance of these findings will ultimately depend on understanding the full range of actions of this mutation.

Molecular basis of resistance to thyroid hormone

Resistance to thyroid hormone (RTH) is a syndrome in which patients have raised serum thyroid hormone (TH) levels and raised or inappropriately normal thyrotropin (TSH) levels. In general, patients exhibit TH resistance in the pituitary and peripheral tissues. Novel techniques and genetically engineered mouse model systems have increased our understanding of thyroid hormone receptor (TR) action, and shed new light on the underlying molecular mechanisms for RTH. In particular, we are learning how mutant TRs from RTH patients can block wild-type TR function, with consequent effects in various tissues and cells. This dominant-negative activity has important implications for other hormone-resistant conditions and in hormone-sensitive tumors. This article examines the molecular basis of RTH.

Functional activation of cerebral metabolism in mice with mutated thyroid hormone nuclear receptors

Neonatal hypothyroidism impairs structural maturation in the brain and results in diminished electrical activities and energy metabolism. We recently found that glucose utilization (CMR(glc)) is markedly depressed throughout the brain in mice with targeted mutations in thyroid hormone receptor alpha1 (TR alpha 1), but not TR beta. Previous studies had shown that CMR(glc) increases linearly with spike frequency in the afferent pathways to synapse-rich regions in neuropil, but not in neuronal cell bodies. To determine whether the decreased CMR(glc) in mutant TR alpha 1(PV/+) mice reflected lesser synaptic density or reduced functional activity in existing synapses, we stimulated vibrissae unilaterally and measured CMR(glc) bilaterally in four stations of the whisker-to-barrel cortex pathway. Baseline CMR(glc) (unstimulated side) was markedly lower in all four stations in the TR alpha 1(PV/+) mutants than in wild-type controls, even though Northern blot and immunohistochemical examinations showed normal Na(+),K(+)-adenosine triphosphatase expression and neuronal differentiation. Despite the lower baseline CMR(glc), however, vibrissal stimulation evoked percent increases in CMR(glc) in the TR alpha 1(PV/+) mutants that were as great as those in wild-type mice. These results indicate that in the TR alpha 1(PV/+) mutants there it is a reduction in synaptic density that is responsible for the decrease in CMR(glc), but functionality of existing synapses is retained.

Thyroid hormone receptors: lessons from knockout and knock-in mutant mice

The genes encoding thyroid hormone receptor alpha and beta (TRalpha and TRbeta) encode four thyroid hormone receptors and four variant isoforms with antagonistic properties. Because of this complexity, numerous models of TR mutation have been developed to understand the functions of specific receptors. In total, 13 mutant strains are now available. Phenotype analysis has shown that the two genes serve distinct functions: TRalpha is crucial for postnatal development and cardiac function, whereas TRbeta mainly controls inner ear and retina development, liver metabolism and thyroid hormone levels. These mouse mutant strains also provide us with the unique opportunity to address the respective contribution of each receptor isoform and isotype in vivo and highlight the in vivo importance of the ligand-independent function of the TR gene products.