Compensatory role of thyroid hormone receptor (TR) alpha 1 in resistance to thyroid hormone: study in mice with a targeted mutation in the TR beta gene and deficient in TR alpha 1

Low striatal T3 is implicated in inattention and memory impairment in an ADHD mouse model overexpressing thyroid hormone-responsive protein

Attention-deficit/hyperactivity disorder (ADHD) is a neurodevelopmental disorder, potentially with a biological basis; however, its exact cause remains unknown. Thyroid hormone (TH) abnormalities are more prevalent in patients with ADHD than in the general population, indicating a shared pathogenetic mechanism for these conditions. Previously, we identified that overexpression of thyroid hormone-responsive protein (THRSP), a gene highly responsive to TH status, induced inattention in male mice. Herein, we sought to explore whether TH function in THRSP-overexpressing (THRSP OE) mice influences ADHD-like (inattention) behavior. We now confirm that THRSP overexpression in male mice reproduces behavioral features of ADHD, including sustained inattention and memory impairment, accompanied by excessive theta waves that were found normal in both the THRSP-knockout and hetero groups. Physiological characterization revealed low striatal T3 levels in the THRSP OE mice due to reduced striatal T3-specific monocarboxylate transporter 8 (MCT8), indicating brain-specific hypothyroidism in this transgenic mouse strain. TH replacement for seven days rescued inattention and memory impairment and the normalization of theta waves. This study further supports the involvement of the upregulated THRSP gene in ADHD pathology and indicates that THRSP OE mice can serve as an animal model for the predominantly inattentive subtype of ADHD.

Homozygous thyroid hormone receptor β-gene mutations in resistance to thyroid hormone: three new cases and review of the literature

CONTEXT

The most common cause of resistance to thyroid hormone (RTH) is heterozygous thyroid hormone receptor β (THRB) gene mutations. Homozygous mutations in the THRB gene are a rare event.

OBJECTIVE

In this study, the clinical findings of three new patients (belonging to two families) homozygous for mutations in the THRB gene are compared to three other families in which affected individuals lack a normal TRβ.

METHODS

We conducted clinical studies and genetic analyses.

RESULTS

The clinical presentation in all three homozygous subjects was unusually severe; their phenotype was characterized by compromised intellectual development, tachycardia, goiter, growth retardation, and hearing loss. This was comparable with one other reported patient homozygous for mutant TRβ, but not in RTH due to THRB gene deletions.

CONCLUSION

We report three new subjects, from two families, in whom RTH was associated with homozygous mutations in the THRB gene. They represent an important addition to the single known patient homozygous for a mutant TRβ. The clinical and laboratory abnormalities indicate a strong dominant-negative effect and are in agreement with data obtained from mice expressing a mutant Thrb in both alleles. This report strengthens the concept that the mutated TRβ interferes with the function of the TRα1 in humans.

Molecular aspects of thyroid hormone actions

Cellular actions of thyroid hormone may be initiated within the cell nucleus, at the plasma membrane, in cytoplasm, and at the mitochondrion. Thyroid hormone nuclear receptors (TRs) mediate the biological activities of T(3) via transcriptional regulation. Two TR genes, alpha and beta, encode four T(3)-binding receptor isoforms (alpha1, beta1, beta2, and beta3). The transcriptional activity of TRs is regulated at multiple levels. Besides being regulated by T(3), transcriptional activity is regulated by the type of thyroid hormone response elements located on the promoters of T(3) target genes, by the developmental- and tissue-dependent expression of TR isoforms, and by a host of nuclear coregulatory proteins. These nuclear coregulatory proteins modulate the transcription activity of TRs in a T(3)-dependent manner. In the absence of T(3), corepressors act to repress the basal transcriptional activity, whereas in the presence of T(3), coactivators function to activate transcription. The critical role of TRs is evident in that mutations of the TRbeta gene cause resistance to thyroid hormones to exhibit an array of symptoms due to decreasing the sensitivity of target tissues to T(3). Genetically engineered knockin mouse models also reveal that mutations of the TRs could lead to other abnormalities beyond resistance to thyroid hormones, including thyroid cancer, pituitary tumors, dwarfism, and metabolic abnormalities. Thus, the deleterious effects of mutations of TRs are more severe than previously envisioned. These genetic-engineered mouse models provide valuable tools to ascertain further the molecular actions of unliganded TRs in vivo that could underlie the pathogenesis of hypothyroidism. Actions of thyroid hormone that are not initiated by liganding of the hormone to intranuclear TR are termed nongenomic. They may begin at the plasma membrane or in cytoplasm. Plasma membrane-initiated actions begin at a receptor on integrin alphavbeta3 that activates ERK1/2 and culminate in local membrane actions on ion transport systems, such as the Na(+)/H(+) exchanger, or complex cellular events such as cell proliferation. Concentration of the integrin on cells of the vasculature and on tumor cells explains recently described proangiogenic effects of iodothyronines and proliferative actions of thyroid hormone on certain cancer cells, including gliomas. Thus, hormonal events that begin nongenomically result in effects in DNA-dependent effects. l-T(4) is an agonist at the plasma membrane without conversion to T(3). Tetraiodothyroacetic acid is a T(4) analog that inhibits the actions of T(4) and T(3) at the integrin, including angiogenesis and tumor cell proliferation. T(3) can activate phosphatidylinositol 3-kinase by a mechanism that may be cytoplasmic in origin or may begin at integrin alphavbeta3. Downstream consequences of phosphatidylinositol 3-kinase activation by T(3) include specific gene transcription and insertion of Na, K-ATPase in the plasma membrane and modulation of the activity of the ATPase. Thyroid hormone, chiefly T(3) and diiodothyronine, has important effects on mitochondrial energetics and on the cytoskeleton. Modulation by the hormone of the basal proton leak in mitochondria accounts for heat production caused by iodothyronines and a substantial component of cellular oxygen consumption. Thyroid hormone also acts on the mitochondrial genome via imported isoforms of nuclear TRs to affect several mitochondrial transcription factors. Regulation of actin polymerization by T(4) and rT(3), but not T(3), is critical to cell migration. This effect has been prominently demonstrated in neurons and glial cells and is important to brain development. The actin-related effects in neurons include fostering neurite outgrowth. A truncated TRalpha1 isoform that resides in the extranuclear compartment mediates the action of thyroid hormone on the cytoskeleton.

Clinical features and genetic analysis of four Brazilian kindreds with resistance to thyroid hormone

OBJECTIVE

Resistance to thyroid hormone (RTH) is a dominantly inherited syndrome of reduced tissue responsiveness to thyroid hormone usually due to mutations located in the ligand-binding domain and adjacent hinge region of the thyroid hormone receptor beta (TRbeta). In the present report we describe the clinical and laboratory characteristics and the genetic analysis of patients with this rare disorder from a Brazilian population.

PATIENTS

Four unrelated Brazilian families with diagnosis of RTH were studied. Age at diagnosis varied from 14 months to 29 years.

RESULTS

All affected individuals were clinically euthyroid, except for one patient who presented immediately after birth with hyperthyroidism. All individuals had tachycardia and goitre, elevated concentrations of free thyroid hormones and reduced sensitivity to thyroid hormone. Direct sequencing analysis of the TRbeta gene revealed four previously reported mutations: c.949G-->A, c.1313G-->A, c.1357C-->A and c.1358dupC in families A, B, C and D, respectively.

CONCLUSION

The present report shows that the frequent mutations described in the thyroid hormone receptor worldwide are also present in the Brazilian population, which is characterized by a variable ethnic background.

Thyroid hormones, learning and memory

Thyroid hormones (THs), T3 and T4, have many physiological actions and are essential for normal behavioral, intellectual and neurological development. THs have a broad spectrum of effects on the developing brain and mediate important effects within the CNS throughout life. Insufficient maternal iodine intake during gestation and TH deficiency during human development are associated to pathological alterations such as cretinism and mental retardation. In adulthood, thyroid dysfunction is related to neurological and behavioral abnormalities, including memory impairment. Analysis of different experimental models suggests that most of the effects on cognition as a result of thyroid dysfunction rely on hippocampal modifications. Insufficiency of THs during development thus alters hippocampal synaptic function and impairs behavioral performance of hippocampal-dependent learning and memory tasks that persist in euthyroid adult animals. In the present review, we summarize the current knowledge obtained by clinical observations and experimental models that shows the importance of THs in learning and mnemonic processes.

PPARgamma insufficiency promotes follicular thyroid carcinogenesis via activation of the nuclear factor-kappaB signaling pathway

The molecular genetic events underlying thyroid carcinogenesis are poorly understood. Mice harboring a knock-in dominantly negative mutant thyroid hormone receptor beta (TRbetaPV/PV mouse) spontaneously develop follicular thyroid carcinoma similar to human thyroid cancer. Using this mutant mouse, we tested the hypothesis that the peroxisome proliferator-activated receptor gamma (PPARgamma) could function as a tumor suppressor in thyroid cancer in vivo. Using the offspring from the cross of TRbetaPV/+ and PPARgamma+/- mice, we found that thyroid carcinogenesis progressed significantly faster in TRbetaPV/PV mice with PPARgamma insufficiency from increased cell proliferation and reduced apoptosis. Reduced PPARgamma protein abundance led to the activation of the nuclear factor-kappaB signaling pathway, resulting in the activation of cyclin D1 and repression of critical genes involved in apoptosis. Treatment of TRbetaPV/PV mice with a PPARgamma agonist, rosiglitazone, delayed the progression of thyroid carcinogenesis by decreasing cell proliferation and activation of apoptosis. These results suggest that PPARgamma is a critical modifier in thyroid carcinogenesis and could be tested as a therapeutic target in thyroid follicular carcinoma.

Contrasting Skeletal Phenotypes in Mice with an Identical Mutation Targeted to Thyroid Hormone Receptor α1 or β

Thyroid hormone (T(3)) regulates bone turnover and mineralization in adults and is essential for skeletal development. Surprisingly, we identified a phenotype of skeletal thyrotoxicosis in T(3) receptor beta(PV) (TRbeta(PV)) mice in which a targeted frameshift mutation in TRbeta results in resistance to thyroid hormone. To characterize mechanisms underlying thyroid hormone action in bone, we analyzed skeletal development in TRalpha1(PV) mice in which the same PV mutation was targeted to TRalpha1. In contrast to TRbeta(PV) mice, TRalpha1(PV) mutants exhibited skeletal hypothyroidism with delayed endochondral and intramembranous ossification, severe postnatal growth retardation, diminished trabecular bone mineralization, reduced cortical bone deposition, and delayed closure of the skull sutures. Skeletal hypothyroidism in TRalpha1(PV) mutants was accompanied by impaired GH receptor and IGF-I receptor expression and signaling in the growth plate, whereas GH receptor and IGF-I receptor expression and signaling were increased in TRbeta(PV) mice. These data indicate that GH receptor and IGF-I receptor are physiological targets for T(3) action in bone in vivo. The divergent phenotypes observed in TRalpha1(PV) and TRbeta(PV) mice arise because the pituitary gland is a TRbeta-responsive tissue, whereas bone is TRalpha responsive. These studies provide a new understanding of the complex relationship between central and peripheral thyroid status.

Dual functions of the steroid hormone receptor coactivator 3 in modulating resistance to thyroid hormone

Mutations of the thyroid hormone receptor beta (TRbeta) gene cause resistance to thyroid hormone (RTH). RTH is characterized by increased serum thyroid hormone associated with nonsuppressible thyroid-stimulating hormone (TSH) and impaired growth. It is unclear how the actions of TRbeta mutants are modulated in vivo to affect the manifestation of RTH. Using a mouse model of RTH that harbors a knockin mutation of the TRbeta gene (TRbetaPV mouse), we investigated the effect of the steroid hormone receptor coactivator 3 (SRC-3) on RTH. In TRbetaPV mice deficient in SRC-3, dysfunction of the pituitary-thyroid axis and hypercholesterolemia was lessened, but growth impairment of RTH was worsened. The lessened dysfunction of the pituitary-thyroid axis was attributed to a significant decrease in growth of the thyroid and pituitary. Serum insulin-like growth factor 1 (IGF-1) was further reduced in TRbetaPV mice deficient in SRC-3. This effect led to reduced signaling of the IGF-1/phosphatidylinositol 3-kinase (PI3K)/AKT/mammalian target of rapamycin (mTOR) pathway that is known to mediate cell growth and proliferation. Thus, SRC-3 modulates RTH by at least two mechanisms, one via its role as a receptor coregulator and the other via its growth regulatory role through the IGF-1/PI3K/AKT/mTOR signaling.

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 resistance in the heart: role of the thyroid hormone receptor beta isoform

Several cardiac genes possess thyroid hormone (TH) response elements regulated by TH receptors. Mutation in TR-beta gene causes the human syndrome of resistance to TH, which is characterized by elevated serum concentration of T(4) and T(3) and variable degrees of insensitivity to TH. It is unclear, however, whether a mutant TR-beta could function as a dominant negative in the heart when expressed from the endogenous locus. A well-described resistance to TH (Delta337T) was either introduced into germline of mice (KI-mut) or expressed as a transgene in the heart using a cardiac-specific promoter (KS-mut). Mice were studied at baseline, after 5-propyl-2-thiouracil (PTU) or after PTU and T(3) treatment (PTU + T(3)). PTU + T(3) treatment significantly increased left ventricular mass in all groups compared with baseline measurements, although the increase in left ventricular mass was significantly less in KI-mut animals. Baseline heart rates (HRs) were similar in wild-type (WT) and KI-mut but were lower in KS-mut animals. After TH deprivation (PTU), HR decreased in WT and KI-mut animals; similarly, HR increased in WT and KI-mut after PTU + T(3). In contrast, HR in KS-mut animals did not change after either treatment. Except for cardiac hypertrophy, the presence of a germline TR-beta mutation had surprisingly little effect on cardiac function.