Dominant Role of Thyrotropin-releasing Hormone in the Hypothalamic-Pituitary-Thyroid Axis

Regulation of Thyroid Hormone Levels by Hypothalamic Thyrotropin-Releasing Hormone Neurons

Background: Thyrotropin-releasing hormone (TRH) neurons in the paraventricular nucleus of the hypothalamus (PVN) have been identified as direct regulators of thyrotropin (TSH) and thyroid hormone (TH) levels. They play a significant role in context of negative feedback by TH at the level of TRH gene expression and during fasting when TH levels fall due, in part, to suppression of TRH gene expression. Methods: To test these functions directly for the first time, we used a chemogenetic approach and activated PVN TRH neurons in both fed and fasted mice. Next, to demonstrate the signals that regulate the fasting response in TRH neurons, we activated or inhibited agouti-related protein (AgRP)/neuropeptide Y (NPY) neurons in the arcuate nucleus of the hypothalamus of fed or fasted mice, respectively. To determine if the same TRH neurons responsive to melanocortin signaling mediate negative feedback by TH, we disrupted the thyroid hormone receptor beta (TRβ) in all melanocortin 4 receptor (MC4R) neurons in the PVN. Results: Activation of TRH neurons led to increased TSH and TH levels within 2 hours demonstrating the specific role of PVN TRH neurons in the regulation of the hypothalamic-pituitary-thyroid (HPT) axis. Moreover, activation of PVN TRH neurons prevented the fall in TH levels in fasting mice. Stimulation of AgRP/NPY neurons led to a fall in TH levels despite increasing feeding. Inhibition of these same neurons prevented the fall in TH levels during a fast presumably via their ability to directly regulate PVN TRH neurons via, in part, the MC4R. Surprisingly, TH-mediated feedback was not impaired in mice lacking TRβ in MC4R neurons. Conclusions: TRH neurons are major regulators of the HPT axis and the fasting-induced suppression of TH levels. The latter relies, at least in part, on the activation of AgRP/NPY neurons in the arcuate nucleus. Interestingly, present data do not support an important role for TRβ signaling in regulating MC4R neurons in the PVN. Thus, it remains possible that different subsets of TRH neurons in the PVN mediate responses to energy balance and to TH feedback.

Evaluating the Hypothalamic-Pituitary-Thyroid (HPT) Axis in Mice

As the genome of experimental animals has become easier to manipulate, a number of mouse models have been developed to understand in vivo thyroid hormone action. A major site of thyroid hormone action is the HPT axis. While several methods are available that provide a detailed understanding of the HPT axis in mice, many authors choose to include only cursory data about this axis, which can lead to erroneous conclusions about in vivo thyroid hormone action. A standard protocol is proposed to evaluate the HPT axis in mice.

Brain to bone: What is the contribution of the brain to skeletal homeostasis?

The brain, which governs most, if not all, physiological functions in the body, from the complexities of cognition, learning and memory, to the regulation of basal body temperature, heart rate and breathing, has long been known to affect skeletal health. In particular, the hypothalamus - located at the base of the brain in close proximity to the medial eminence, where the blood-brain-barrier is not as tight as in other regions of the brain but rather "leaky", due to fenestrated capillaries - is exposed to a variety of circulating body cues, such as nutrients (glucose, fatty acids, amino acids), and hormones (insulin, glucagon, leptin, adiponectin) [1-3].Information collected from the body via these peripheral cues is integrated by hypothalamic sensing neurons and glial cells [4-7], which express receptors for these nutrients and hormones, transforming these cues into physiological outputs. Interestingly, many of the same molecules, including leptin, adiponectin and insulin, regulate both energy and skeletal homeostasis. Moreover, they act on a common set of hypothalamic nuclei and their residing neurons, activating endocrine and neuronal systems, which ultimately fine-tune the body to new physiological states. This review will focus exclusively on the brain-to-bone pathway, highlighting the most important anatomical sites within the brain, which are known to affect bone, but not covering the input pathways and molecules informing the brain of the energy and bone metabolic status, covered elsewhere [8-10]. The discussion in each section will present side by side the metabolic and bone-related functions of hypothalamic nuclei, in an attempt to answer some of the long-standing questions of whether energy is affected by bone remodeling and homeostasis and vice versa.

Integrating Thyroid Hormone Signaling in Hypothalamic Control of Metabolism: Crosstalk Between Nuclear Receptors

The obesity epidemic is well recognized as a significant global health issue. A better understanding of the energy homeostasis mechanisms could help to identify promising anti-obesity therapeutic strategies. It is well established that the hypothalamus plays a pivotal role governing energy balance. The hypothalamus consists of tightly interconnected and specialized neurons that permit the sensing and integration of several peripheral inputs, including metabolic and hormonal signals for an appropriate physiological response. Current evidence shows that thyroid hormones (THs) constitute one of the key endocrine factors governing the regulation and the integration of metabolic homeostasis at the hypothalamic level. THs modulate numerous genes involved in the central control of metabolism, as TRH (Thyrotropin-Releasing Hormone) and MC4R (Melanocortin 4 Receptor). THs act through their interaction with thyroid hormone receptors (TRs). Interestingly, TH signaling, especially regarding metabolic regulations, involves TRs crosstalk with other metabolically linked nuclear receptors (NRs) including PPAR (Peroxisome proliferator-activated receptor) and LXR (Liver X receptor). In this review, we will summarize current knowledge on the important role of THs integration of metabolic pathways in the central regulation of metabolism. Particularly, we will shed light on the crosstalk between TRs and other NRs in controlling energy homeostasis. This could be an important track for the development of attractive therapeutic compounds.

Thyroid Hormone Signaling in the Development of the Endochondral Skeleton

Thyroid hormone (TH) is an established regulator of skeletal growth and maintenance both in clinical studies and in laboratory models. The clinical consequences of altered thyroid status on the skeleton during development and in adulthood are well known, and genetic mouse models in which elements of the TH signaling axis have been manipulated illuminate the mechanisms which underlie TH regulation of the skeleton. TH is involved in the regulation of the balance between proliferation and differentiation in several skeletal cell types including chondrocytes, osteoblasts, and osteoclasts. The effects of TH are mediated primarily via the thyroid hormone receptors (TRs) α and β, ligand-inducible nuclear receptors which act as transcription factors to regulate target gene expression. Both TRα and TRβ signaling are important for different stages of skeletal development. The molecular mechanisms of TH action in bone are complex and include interaction with a number of growth factor signaling pathways. This review provides an overview of the regulation and mechanisms of TH action in bone, focusing particularly on the role of TH in endochondral bone formation during postnatal growth.

Central hypothyroidism - a neglected thyroid disorder

Central hypothyroidism is a rare and heterogeneous disorder that is characterized by a defect in thyroid hormone secretion in an otherwise normal thyroid gland due to insufficient stimulation by TSH. The disease results from the abnormal function of the pituitary gland, the hypothalamus, or both. Moreover, central hypothyroidism can be isolated or combined with other pituitary hormone deficiencies, which are mostly acquired and are rarely congenital. The clinical manifestations of central hypothyroidism are usually milder than those observed in primary hypothyroidism. Obtaining a positive diagnosis for central hypothyroidism can be difficult from both a clinical and a biochemical perspective. The diagnosis of central hypothyroidism is based on low circulating levels of free T₄ in the presence of low to normal TSH concentrations. The correct diagnosis of both acquired (also termed sporadic) and congenital (also termed genetic) central hypothyroidism can be hindered by methodological interference in free T₄ or TSH measurements; routine utilization of total T₄ or T₃ measurements; concurrent systemic illness that is characterized by low levels of free T₄ and normal TSH concentrations; the use of the sole TSH-reflex strategy, which is the measurement of the sole level of TSH, without free T₄, if levels of TSH are in the normal range; and the diagnosis of congenital hypothyroidism based on TSH analysis without the concomitant measurement of serum levels of T₄. In this Review, we discuss current knowledge of the causes of central hypothyroidism, emphasizing possible pitfalls in the diagnosis and treatment of this disorder.

Gene expression of T3-regulated genes in a mouse model of the human thyroid hormone resistance

AIMS

To understand how thyroid hormone (TH) regulates tissue-specific gene expression in patients with the syndrome of resistance to TH (RTHβ), we used a mouse model that replicates the human RTHβ, specifically the ∆337T mutation in the thyroid hormone receptor β (THRβ).

MAIN METHODS

We investigated the expression of key TH target genes in the pituitary and liver of TRβ^∆337T and wild type THRβ mice by qPCR before and after a T₃ suppression test consisting of the administration of increasing concentrations of T₃ to hypothyroid mice.

KEY FINDINGS

Pituitary Tshb and Cga expression decreased and Gh expression increased in TRβ^∆337T mice after T₃ suppression. The stimulation of positively regulated TH genes was heterogeneous in the liver. Levels of liver Me1 and Thsrp were elevated in TRβ^∆337T mice after T₃ administration. Slc16a2 and Gpd2 did not respond to T₃ stimulation in the liver of TRβ^∆337T mice whereas Dio1 response was lower than that observed in WT mice. Moreover, although Chdh and Upd1 genes were negatively regulated in the liver, the expression of these genes was elevated after T₃ suppression. We did not observe significant changes in THRα expression in the liver and pituitary, while THRβ levels were diminished in the pituitary and increased in the liver.

SIGNIFICANCE

Using a model expressing a THRβ unable to bind T3, we showed the expression pattern of liver negative and positive regulated genes by T3.

Sox21 deletion in mice causes postnatal growth deficiency without physiological disruption of hypothalamic-pituitary endocrine axes

The hypothalamic-pituitary axes are the coordinating centers for multiple endocrine gland functions and physiological processes. Defects in the hypothalamus or pituitary gland can cause reduced growth and severe short stature, affecting approximately 1 in 4000 children, and a large percentage of cases of pituitary hormone deficiencies do not have an identified genetic cause. SOX21 is a protein that regulates hair, neural, and trophoblast stem cell differentiation. Mice lacking Sox21 have reduced growth, but the etiology of this growth defect has not been described. We studied the expression of Sox21 in hypothalamic-pituitary development and examined multiple endocrine axes in these mice. We find no evidence of reduced intrauterine growth, food intake, or physical activity, but there is evidence for increased energy expenditure in mutants. In addition, despite changes in pituitary hormone expression, hypothalamic-pituitary axes appear to be functional. Therefore, SOX21 variants may be a cause of non-endocrine short stature in humans.

Negative feedback regulation of thyrotropin subunits and pituitary deiodinases in red drum, Sciaenops ocellatus

Thyroxine (T₄) undergoes dynamic daily cycles in the perciform fish the red drum, Sciaenops ocellatus, that are inversely timed to cycles of thyrotropin (TSH) subunit mRNA expression in the pituitary gland. We have proposed that these daily cycles are regulated by negative feedback of circulating T₄ on expression of pituitary thyroid hormone deiodinase type 3 (Dio3), such that elevated circulating T₄ results in diminished pituitary thyroid hormone catabolism and consequent increased negative feedback on expression of TSH subunits during the day. To determine whether thyroid hormones function to modulate expression of pituitary deiodinase enzymes we developed an immersion technique to administer physiological doses of T₃ and T₄in vivo. Immersion in T₄ or T₃ significantly inhibited the mRNA expression of the TSH α and β subunits from 4 to 66h of immersion. Pituitary Dio3 expression was significantly diminished by T₃ and T₄ at 22h. These results indicate that both T₄ and T₃ are capable of negative feedback regulation of TSH subunit expression in red drum at physiological concentrations and on a time scale consistent with the T₄ daily cycle. Furthermore, thyroid hormones negatively regulate Dio3 expression in the pituitary in a manner suggesting that negative thyroxine feedback on Dio3 promotes the release of TSH subunits from TH inhibition and may be an important mechanism for generating daily thyroid hormone cycles. These results highlight a potentially important role for D3 in mediating thyroid hormone feedback on TSH expression, not previously described in other species.

Hypothalamus-Pituitary-Thyroid Axis

The hypothalamus-pituitary-thyroid (HPT) axis determines the set point of thyroid hormone (TH) production. Hypothalamic thyrotropin-releasing hormone (TRH) stimulates the synthesis and secretion of pituitary thyrotropin (thyroid-stimulating hormone, TSH), which acts at the thyroid to stimulate all steps of TH biosynthesis and secretion. The THs thyroxine (T4) and triiodothyronine (T3) control the secretion of TRH and TSH by negative feedback to maintain physiological levels of the main hormones of the HPT axis. Reduction of circulating TH levels due to primary thyroid failure results in increased TRH and TSH production, whereas the opposite occurs when circulating THs are in excess. Other neural, humoral, and local factors modulate the HPT axis and, in specific situations, determine alterations in the physiological function of the axis. The roles of THs are vital to nervous system development, linear growth, energetic metabolism, and thermogenesis. THs also regulate the hepatic metabolism of nutrients, fluid balance and the cardiovascular system. In cells, TH actions are mediated mainly by nuclear TH receptors (210), which modify gene expression. T3 is the preferred ligand of THR, whereas T4, the serum concentration of which is 100-fold higher than that of T3, undergoes extra-thyroidal conversion to T3. This conversion is catalyzed by 5'-deiodinases (D1 and D2), which are TH-activating enzymes. T4 can also be inactivated by conversion to reverse T3, which has very low affinity for THR, by 5-deiodinase (D3). The regulation of deiodinases, particularly D2, and TH transporters at the cell membrane control T3 availability, which is fundamental for TH action. © 2016 American Physiological Society. Compr Physiol 6:1387-1428, 2016.

Role of Thyroid Hormones in Skeletal Development and Bone Maintenance

The skeleton is an exquisitely sensitive and archetypal T3-target tissue that demonstrates the critical role for thyroid hormones during development, linear growth, and adult bone turnover and maintenance. Thyrotoxicosis is an established cause of secondary osteoporosis, and abnormal thyroid hormone signaling has recently been identified as a novel risk factor for osteoarthritis. Skeletal phenotypes in genetically modified mice have faithfully reproduced genetic disorders in humans, revealing the complex physiological relationship between centrally regulated thyroid status and the peripheral actions of thyroid hormones. Studies in mutant mice also established the paradigm that T3 exerts anabolic actions during growth and catabolic effects on adult bone. Thus, the skeleton represents an ideal physiological system in which to characterize thyroid hormone transport, metabolism, and action during development and adulthood and in response to injury. Future analysis of T3 action in individual skeletal cell lineages will provide new insights into cell-specific molecular mechanisms and may ultimately identify novel therapeutic targets for chronic degenerative diseases such as osteoporosis and osteoarthritis. This review provides a comprehensive analysis of the current state of the art.

Penetration of thyroliberin in the blood and brain regions at intranasal or intravenous administration

The maximum amounts of the thyroliberin in the blood and brain of rats at intranasal and intravenous administration were determined. It is found that rat hippocampal, cortical, and cerebellar membranes contain two types of specific binding sites (high- and low-affinity) for the labeled ligand. It was shown that, at intranasal and intravenous administration, maximum amounts of the thyroliberin were detected in the cerebellum and then in the cortex and hippocampus. The degradation of the thyroliberin in the rat brain and its regions at intranasal and intravenous administration was studied. It is shown that the degree of degradation and the formation of proteolytic products of the thyroliberin is different in different regions of the rat brain.

60 YEARS OF NEUROENDOCRINOLOGY: TRH, the first hypophysiotropic releasing hormone isolated: control of the pituitary-thyroid axis

This review presents the findings that led to the discovery of TRH and the understanding of the central mechanisms which control hypothalamus-pituitary-thyroid axis (HPT) activity. The earliest studies on thyroid physiology are now dated a century ago when basal metabolic rate was associated with thyroid status. It took over 50 years to identify the key elements involved in the HPT axis. Thyroid hormones (TH: T4 and T3) were characterized first, followed by the semi-purification of TSH whose later characterization paralleled that of TRH. Studies on the effects of TH became possible with the availability of synthetic hormones. DNA recombinant techniques facilitated the identification of all the elements involved in the HPT axis, including their mode of regulation. Hypophysiotropic TRH neurons, which control the pituitary-thyroid axis, were identified among other hypothalamic neurons which express TRH. Three different deiodinases were recognized in various tissues, as well as their involvement in cell-specific modulation of T3 concentration. The role of tanycytes in setting TRH levels due to the activity of deiodinase type 2 and the TRH-degrading ectoenzyme was unraveled. TH-feedback effects occur at different levels, including TRH and TSH synthesis and release, deiodinase activity, pituitary TRH-receptor and TRH degradation. The activity of TRH neurons is regulated by nutritional status through neurons of the arcuate nucleus, which sense metabolic signals such as circulating leptin levels. Trh expression and the HPT axis are activated by energy demanding situations, such as cold and exercise, whereas it is inhibited by negative energy balance situations such as fasting, inflammation or chronic stress. New approaches are being used to understand the activity of TRHergic neurons within metabolic circuits.

The selective loss of the type 2 iodothyronine deiodinase in mouse thyrotrophs increases basal TSH but blunts the thyrotropin response to hypothyroidism

The type 2 iodothyronine deiodinase (D2) is essential for feedback regulation of TSH by T4. We genetically inactivated in vivo D2 in thyrotrophs using a mouse model of Cga-driven cre recombinase. Pituitary D2 activity was reduced 90% in the Cga-cre D2 knockout (KO) mice compared with control Dio2(fl/fl) mice. There was no growth or reproductive phenotype. Basal TSH levels were increased 1.5- to 1.8-fold, but serum T4 and T3 were not different from the controls in adult mice. In hypothyroid adult mice, suppression of TSH by T4, but not T3, was impaired. Despite mild basal TSH elevation, the TSH increase in response to hypothyroidism was 4-fold reduced in the Cga-cre D2KO compared with control mice despite an identical level of pituitary TSH α- and β-subunit mRNAs. In neonatal Cga-cre D2KO mice, TSH was also 2-fold higher than in the controls, but serum T4 was elevated. Despite a constant TSH, serum T4 increased 2-3-fold between postnatal day (P) 5 and P15 in both genotypes. The pituitary, but not cerebrocortical, D2 activity was markedly elevated in P5 mice decreasing towards adult levels by P17. In conclusion, a congenital severe reduction of thyrotroph D2 causes a major impairment of the TSH response to hypothyroidism. This would be deleterious to the compensatory adaptation of the thyroid gland to iodine deficiency.

Homeostatic Control of the Thyroid-Pituitary Axis: Perspectives for Diagnosis and Treatment

The long-held concept of a proportional negative feedback control between the thyroid and pituitary glands requires reconsideration in the light of more recent studies. Homeostatic equilibria depend on dynamic inter-relationships between thyroid hormones and pituitary thyrotropin (TSH). They display a high degree of individuality, thyroid-state-related hierarchy, and adaptive conditionality. Molecular mechanisms involve multiple feedback loops on several levels of organization, different time scales, and varying conditions of their optimum operation, including a proposed feedforward motif. This supports the concept of a dampened response and multistep regulation, making the interactions between TSH, FT4, and FT3 situational and mathematically more complex. As a homeostatically integrated parameter, TSH becomes neither normatively fixed nor a precise marker of euthyroidism. This is exemplified by the therapeutic situation with l-thyroxine (l-T4) where TSH levels defined for optimum health may not apply equivalently during treatment. In particular, an FT3-FT4 dissociation, discernible FT3-TSH disjoint, and conversion inefficiency have been recognized in l-T4-treated athyreotic patients. In addition to regulating T4 production, TSH appears to play an essential role in maintaining T3 homeostasis by directly controlling deiodinase activity. While still allowing for tissue-specific variation, this questions the currently assumed independence of the local T3 supply. Rather it integrates peripheral and central elements into an overarching control system. On l-T4 treatment, altered equilibria have been shown to give rise to lower circulating FT3 concentrations in the presence of normal serum TSH. While data on T3 in tissues are largely lacking in humans, rodent models suggest that the disequilibria may reflect widespread T3 deficiencies at the tissue level in various organs. As a consequence, the use of TSH, valuable though it is in many situations, should be scaled back to a supporting role that is more representative of its conditional interplay with peripheral thyroid hormones. This reopens the debate on the measurement of free thyroid hormones and encourages the identification of suitable biomarkers. Homeostatic principles conjoin all thyroid parameters into an adaptive context, demanding a more flexible interpretation in the accurate diagnosis and treatment of thyroid dysfunction.

Hypothalamic-pituitary thyroid axis alterations in female mice with deletion of the neuromedin B receptor gene

Neuromedin B, a peptide highly expressed at the pituitary, has been shown to act as autocrine/paracrine inhibitor of thyrotropin (TSH) release. Here we studied the thyroid axis of adult female mice lacking neuromedin B receptor (NBR-KO), compared to wild type (WT) littermates. They exhibited slight increase in serum TSH (18%), with normal pituitary expression of mRNA coding for α-glycoprotein subunit (Cga), but reduced TSH β-subunit mRNA (Tshb, 41%), lower intra-pituitary TSH content (24%) and increased thyroid hormone transporter MCT-8 (Slc16a2, 44%) and thyroid hormone receptor β mRNA expression (Thrb, 39%). NBR-KO mice exhibited normal thyroxine (T4) and reduced triiodothyronine (T3) (30%), with no alterations in the intra-thyroidal content of T4 and T3 or thyroid morphological changes. Hypothalamic thyrotropin-releasing hormone (TRH) mRNA (Trh) was increased (68%), concomitant with a reduction in type 2 deiodinase mRNA (Dio2, 30%) and no changes in MCT-8 and thyroid hormone receptor mRNA expression. NBR-KO mice exhibited a 56% higher increase in serum TSH in response to an acute single intraperitoneal injection of TRH concomitant with a non-significant increase in pituitary TRH receptor (Trhr) mRNA at basal state. The phenotype of female NBR-KO mice at the hypothalamus-pituitary axis revealed alterations in pituitary and hypothalamic gene expression, associated with reduced serum T3, and higher TSH response to TRH, with apparently normal thyroid morphology and hormonal production. Thus, results confirm that neuromedin B pathways are importantly involved in secretory pathways of TSH and revealed its participation in the in vivo regulation of gene expression of TSH β-subunit and pituitary MCT8 and Thrb and hypothalamic TRH and type 2 deiodinase.

Thyroid hormone receptors and resistance to thyroid hormone disorders

Thyroid hormone action is predominantly mediated by thyroid hormone receptors (THRs), which are encoded by the thyroid hormone receptor α (THRA) and thyroid hormone receptor β (THRB) genes. Patients with mutations in THRB present with resistance to thyroid hormone β (RTHβ), which is a disorder characterized by elevated levels of thyroid hormone, normal or elevated levels of TSH and goitre. Mechanistic insights about the contributions of THRβ to various processes, including colour vision, development of the cochlea and the cerebellum, and normal functioning of the adult liver and heart, have been obtained by either introducing human THRB mutations into mice or by deletion of the mouse Thrb gene. The introduction of the same mutations that mimic human THRβ alterations into the mouse Thra and Thrb genes resulted in distinct phenotypes, which suggests that THRA and THRB might have non-overlapping functions in human physiology. These studies also suggested that THRA mutations might not be lethal. Seven patients with mutations in THRα have since been described. These patients have RTHα and presented with major abnormalities in growth and gastrointestinal function. The hypothalamic-pituitary-thyroid axis in these individuals is minimally affected, which suggests that the central T3 feedback loop is not impaired in patients with RTHα, in stark contrast to patients with RTHβ.

Rats with altered behaviour following nerve injury show evidence of centrally altered thyroid regulation

The co-morbidity of mood disturbance, in a proportion of patients, is now described across a wide range of chronic disease states. Similarly, a 'Low Thyroid Syndrome' is also reported in a proportion of individuals with chronic diseases. Here, we report on central changes in an animal model of inflammatory stress in which altered social behaviour, representing social disability, persists in a sub-group of rats following injury. We showed in an earlier study that rats with social disability following injury have significantly decreased peripheral thyroid hormones, with no increase in Thyroid Stimulating Hormone (TSH). Only rats identified by behavioural change showed changes in hypothalamic gene expression. In whole hypothalamus extracted RNA, relative expression of mRNA for Thyrotrophin-releasing hormone (TRH) was significantly down-regulated in disabled rats (p=0.039) and deiodinase 3 up-regulated (p=0.006) compared to controls. Specifically in the paraventricular nucleus (PVN), numbers of immunoreactive cells for deiodinase 3-like and thyroid hormone receptor beta-like proteins were decreased in the sub-group with disability compared to the control group (p=0.031 and p=0.011 respectively). In rats with behavioural change post-injury, down-regulation of TRH provides an explanation for the failure of the hypothalamo-pituitary-thyroid (HPT) axis to respond to the post-injury decrease in thyroxine. Decreased local expression of deiodinase 3 protein, resulting in a local increase in T3, offers an explanation for down regulation of TRH in the hypophysiotrophic TRH neurons. It is possible that, in a sub-group of animals identified behaviourally, a mechanism resulting in hypothalamic down-regulation of the HPT axis persists following inflammatory injury.

Thyroid hormone signaling in vivo requires a balance between coactivators and corepressors

Resistance to thyroid hormone (RTH), a human syndrome, is characterized by high thyroid hormone (TH) and thyroid-stimulating hormone (TSH) levels. Mice with mutations in the thyroid hormone receptor beta (TRβ) gene that cannot bind steroid receptor coactivator 1 (SRC-1) and Src-1(-/-) mice both have phenotypes similar to that of RTH. Conversely, mice expressing a mutant nuclear corepressor 1 (Ncor1) allele that cannot interact with TRβ, termed NCoRΔID, have low TH levels and normal TSH. We hypothesized that Src-1(-/-) mice have RTH due to unopposed corepressor action. To test this, we crossed NCoRΔID and Src-1(-/-) mice to create mice deficient for coregulator action in all cell types. Remarkably, NCoR(ΔID/ΔID) Src-1(-/-) mice have normal TH and TSH levels and are triiodothryonine (T(3)) sensitive at the level of the pituitary. Although absence of SRC-1 prevented T(3) activation of key hepatic gene targets, NCoR(ΔID/ΔID) Src-1(-/-) mice reacquired hepatic T(3) sensitivity. Using in vivo chromatin immunoprecipitation assays (ChIP) for the related coactivator SRC-2, we found enhanced SRC-2 recruitment to TR-binding regions of genes in NCoR(ΔID/ΔID) Src-1(-/-) mice, suggesting that SRC-2 is responsible for T(3) sensitivity in the absence of NCoR1 and SRC-1. Thus, T(3) targets require a critical balance between NCoR1 and SRC-1. Furthermore, replacement of NCoR1 with NCoRΔID corrects RTH in Src-1(-/-) mice through increased SRC-2 recruitment to T(3) target genes.

Cyclic mRNA expression of thyrotropin subunits and deiodinases in red drum, Sciaenops ocellatus

The role of thyrotropin (thyroid-stimulating hormone, TSH) in driving peripheral thyroid function in non-mammalian species is still poorly understood. Thyroxine (T₄), the principal hormone released from the thyroid gland in response to TSH stimulation, circulates with a robust daily rhythm in the teleost fish the red drum. Previous research suggests that the red drum T₄ cycle is circadian in nature, driven by TSH secretion in the early photophase and inhibited by T₄ feedback in the early scotophase. To determine whether TSH is produced in a pattern consistent with feedback inhibition by this T₄ cycle, we used quantitative real time PCR (qPCR) to quantify the daily cycle of expression of the pituitary TSH subunits GSUα, and TSHβ. We found that TSH expression cycled inversely to, and 6-12 h out of phase with, the T₄ cycle, consistent with the hypothesis that TSH secretion drives the T₄ cycle. To examine the potential role of deiodinases in negative feedback regulation of this TSH cycle, we also utilized qPCR to assess the pituitary expression patterns of the TH activating enzyme outer-ring deiodinase (Dio2) and the TH deactivating enzyme inner-ring deiodinase (Dio3). Dio2 was not expressed with an obvious daily cycle, whereas Dio3 expression mirrored the expression of TSH. These results are consistent with circulating T₄ providing the negative feedback signal controlling both TSH production and Dio3 expression in the pituitary, and suggest that TH inactivation by inner ring deiodination is an important component of TSH negative feedback control.

Thyrotropin secretion patterns in health and disease

Thyroid hormones are extremely important for metabolism, development, and growth during the lifetime. The hypothalamo-pituitary-thyroid axis is precisely regulated for these purposes. Much of our knowledge of this hormonal axis is derived from experiments in animals and mutations in man. This review examines the hypothalamo-pituitary-thyroid axis particularly in relation to the regulated 24-hour serum TSH concentration profiles in physiological and pathophysiological conditions, including obesity, primary hypothyroidism, pituitary diseases, psychiatric disorders, and selected neurological diseases. Diurnal TSH rhythms can be analyzed with novel and precise techniques, eg, operator-independent deconvolution and approximate entropy. These approaches provide indirect insight in the regulatory components in pathophysiological conditions.

Clinical review: Central hypothyroidism: pathogenic, diagnostic, and therapeutic challenges

CONTEXT

Central hypothyroidism (CH) is a particular hypothyroid condition due to an insufficient stimulation by TSH of an otherwise normal thyroid gland. This condition raises several challenges for clinicians; therefore, a review of the most relevant findings on CH epidemiology, pathogenesis, and clinical management has been performed.

METHODOLOGY

The relevant papers were selected by a PubMed search using appropriate key words.

MAIN FINDINGS

CH can be the consequence of various disorders affecting either the pituitary gland or the hypothalamus, but most frequently affecting both of them. CH is about 1000-fold rarer than primary hypothyroidism. Except for the neonatal CH due to biallelic TSHβ mutations, the thyroid hormone defect is rarely as profound as can be observed in some primary forms. In contrast with primary hypothyroidism, CH is most frequently characterized by low/normal TSH levels, and adequate thyroid hormone replacement is associated with the suppression of residual TSH secretion. Thus, CH often represents a clinical challenge because physicians cannot rely on the systematic use of the "reflex TSH strategy." The clinical management of CH is further complicated by the frequent combination with other pituitary deficiencies and their substitution.

Minireview: The neural regulation of the hypothalamic-pituitary-thyroid axis

Thyroid hormone (TH) signaling plays an important role in development and adult life. Many organisms may have evolved under selective pressure of exogenous TH, suggesting that thyroid hormone signaling is phylogenetically older than the systems that regulate their synthesis. Therefore, the negative feedback system by TH itself was probably the first mechanism of regulation of circulating TH levels. In humans and other vertebrates, it is well known that TH negatively regulates its own production through central actions that modulate the hypothalamic-pituitary-thyroid (HPT) axis. Indeed, primary hypothyroidism leads to the up-regulation of the genes encoding many key players in the HPT axis, such as TRH, type 2 deiodinase (dio2), pyroglutamyl peptidase II (PPII), TRH receptor 1 (TRHR1), and the TSH α- and β-subunits. However, in many physiological circumstances, the activity of the HPT axis is not always a function of circulating TH concentrations. Indeed, circadian changes in the HPT axis activity are not a consequence of oscillation in circulating TH levels. Similarly, during reduced food availability, several components of the HPT axis are down-regulated even in the presence of lower circulating TH levels, suggesting the presence of a regulatory pathway hierarchically higher than the feedback system. This minireview discusses the neural regulation of the HPT axis, focusing on both TH-dependent and -independent pathways and their potential integration.

NR4A1 (Nur77) mediates thyrotropin-releasing hormone-induced stimulation of transcription of the thyrotropin β gene: analysis of TRH knockout mice

Thyrotropin-releasing hormone (TRH) is a major stimulator of thyrotropin-stimulating hormone (TSH) synthesis in the anterior pituitary, though precisely how TRH stimulates the TSHβ gene remains unclear. Analysis of TRH-deficient mice differing in thyroid hormone status demonstrated that TRH was critical for the basal activity and responsiveness to thyroid hormone of the TSHβ gene. cDNA microarray and K-means cluster analyses with pituitaries from wild-type mice, TRH-deficient mice and TRH-deficient mice with thyroid hormone replacement revealed that the largest and most consistent decrease in expression in the absence of TRH and on supplementation with thyroid hormone was shown by the TSHβ gene, and the NR4A1 gene belonged to the same cluster as and showed a similar expression profile to the TSHβ gene. Immunohistochemical analysis demonstrated that NR4A1 was expressed not only in ACTH- and FSH- producing cells but also in thyrotrophs and the expression was remarkably reduced in TRH-deficient pituitary. Furthermore, experiments in vitro demonstrated that incubation with TRH in GH4C1 cells increased the endogenous NR4A1 mRNA level by approximately 50-fold within one hour, and this stimulation was inhibited by inhibitors for PKC and ERK1/2. Western blot analysis confirmed that TRH increased NR4A1 expression within 2 h. A series of deletions of the promoter demonstrated that the region between bp -138 and +37 of the TSHβ gene was responsible for the TRH-induced stimulation, and Chip analysis revealed that NR4A1 was recruited to this region. Conversely, knockdown of NR4A1 by siRNA led to a significant reduction in TRH-induced TSHβ promoter activity. Furthermore, TRH stimulated NR4A1 promoter activity through the TRH receptor. These findings demonstrated that 1) TRH is a highly specific regulator of the TSHβ gene, and 2) TRH mediated induction of the TSHβ gene, at least in part by sequential stimulation of the NR4A1-TSHβ genes through a PKC and ERK1/2 pathway.

Fundamentally distinct roles of thyroid hormone receptor isoforms in a thyrotroph cell line are due to differential DNA binding

Thyroid hormones have a profound influence on human development and disease. The hypothalamic-pituitary-thyroid axis involves finely tuned feedback mechanisms to maintain thyroid hormone (TH) levels. Despite the important role of TH-negative feedback in regulating this axis, the mechanism by which this occurs is not clearly defined. Previous in vivo studies suggest separate roles for the two thyroid hormone receptor isoforms, THRA and THRB, in this axis. We performed studies using a unique pituitary thyrotroph cell line (TαT1.1) to determine the relative roles of THRA and THRB in the regulation of Tshb. Using chromatin immunoprecipitation assays, we found that THRB, not THRA, bound to the Tshb promoter. By selectively depleting THRB, THRA, or both THRA and THRB in TαT1.1 cells, we found that simultaneous knockdown of both THRB and THRA abolished T(3)-mediated down-regulation of Tshb at concentrations as high as 100 nm T(3). In contrast, THRA knockdown alone had no effect on T(3)-negative regulation, whereas THRB knockdown alone abolished T(3)-mediated down-regulation of Tshb mRNA levels at 10 nm but not 100 nm T(3) concentrations. Interestingly, chromatin immunoprecipitation assays showed that THRA becomes enriched on the Tshb promoter after knockdown of THRB. Thus, a likely mechanism for the differential effects of THR isoforms on Tshb may be based on their differential DNA-binding affinity to the promoter.

NPY and MC4R signaling regulate thyroid hormone levels during fasting through both central and peripheral pathways

Fasting-induced suppression of the hypothalamic-pituitary-thyroid (HPT) axis is an adaptive response to decrease energy expenditure during food deprivation. Previous studies demonstrate that leptin communicates nutritional status to the HPT axis through thyrotropin-releasing hormone (TRH) in the paraventricular nucleus (PVN) of the hypothalamus. Leptin targets TRH neurons either directly or indirectly via the arcuate nucleus through pro-opiomelanocortin (POMC) and agouti-related peptide/neuropeptide Y (AgRP/NPY) neurons. To evaluate the role of these pathways in vivo, we developed double knockout mice that lack both the melanocortin 4 receptor (MC4R) and NPY. We show that NPY is required for fasting-induced suppression of Trh expression in the PVN. However, both MC4R and NPY are required for activation of hepatic pathways that metabolize T(4) during the fasting response. Thus, these signaling pathways play a key role in the communication of fasting signals to reduce thyroid hormone levels both centrally and through a peripheral hepatic circuit.

Thyroxine-induced expression of pyroglutamyl peptidase II and inhibition of TSH release precedes suppression of TRH mRNA and requires type 2 deiodinase

Suppression of TSH release from the hypothyroid thyrotrophs is one of the most rapid effects of 3,3',5'-triiodothyronine (T(3)) or thyroxine (T(4)). It is initiated within an hour, precedes the decrease in TSHβ mRNA inhibition and is blocked by inhibitors of mRNA or protein synthesis. TSH elevation in primary hypothyroidism requires both the loss of feedback inhibition by thyroid hormone in the thyrotrophs and the positive effects of TRH. Another event in this feedback regulation may be the thyroid hormone-mediated induction of the TRH-inactivating pyroglutamyl peptidase II (PPII) in the hypothalamic tanycytes. This study compared the chronology of the acute effects of T(3) or T(4) on TSH suppression, TRH mRNA in the hypothalamic paraventricular nucleus (PVN), and the induction of tanycyte PPII. In wild-type mice, T(3) or T(4) caused a 50% decrease in serum TSH in hypothyroid mice by 5  h. There was no change in TRH mRNA in PVN over this interval, but there was a significant increase in PPII mRNA in the tanycytes. In mice with genetic inactivation of the type 2 iodothyronine deiodinase, T(3) decreased serum TSH and increased PPII mRNA levels, while T(4)-treatment was ineffective. We conclude that the rapid suppression of TSH in the hypothyroid mouse by T(3) occurs prior to a decrease in TRH mRNA though TRH inactivation may be occurring in the median eminence through the rapid induction of tanycyte PPII. The effect of T(4), but not T(3), requires the type 2 iodothyronine deiodinase.

GATA2 mediates thyrotropin-releasing hormone-induced transcriptional activation of the thyrotropin β gene

Thyrotropin-releasing hormone (TRH) activates not only the secretion of thyrotropin (TSH) but also the transcription of TSHβ and α-glycoprotein (αGSU) subunit genes. TSHβ expression is maintained by two transcription factors, Pit1 and GATA2, and is negatively regulated by thyroid hormone (T3). Our prior studies suggest that the main activator of the TSHβ gene is GATA2, not Pit1 or unliganded T3 receptor (TR). In previous studies on the mechanism of TRH-induced activation of the TSHβ gene, the involvements of Pit1 and TR have been investigated, but the role of GATA2 has not been clarified. Using kidney-derived CV1 cells and pituitary-derived GH3 and TαT1 cells, we demonstrate here that TRH signaling enhances GATA2-dependent activation of the TSHβ promoter and that TRH-induced activity is abolished by amino acid substitution in the GATA2-Zn finger domain or mutation of GATA-responsive element in the TSHβ gene. In CV1 cells transfected with TRH receptor expression plasmid, GATA2-dependent transactivation of αGSU and endothelin-1 promoters was enhanced by TRH. In the gel shift assay, TRH signal potentiated the DNA-binding capacity of GATA2. While inhibition by T3 is dominant over TRH-induced activation, unliganded TR or the putative negative T3-responsive element are not required for TRH-induced stimulation. Studies using GH3 cells showed that TRH-induced activity of the TSHβ promoter depends on protein kinase C but not the mitogen-activated protein kinase, suggesting that the signaling pathway is different from that in the prolactin gene. These results indicate that GATA2 is the principal mediator of the TRH signaling pathway in TSHβ expression.

The obesogen hypothesis: a shift of focus from the periphery to the hypothalamus

The obesogen concept proposes that environmental contaminants may be contributing to the epidemic of obesity and its related pathology, metabolic disorder. The first references to such a notion appeared at the beginning of the current decade, with the hypothesis that the correlation between increasing incidence of obesity and enhanced industrial chemical production was not simply coincidental, but potentially causally related. The next event was the introduction of the term "obesogen" as representing an environmental pollutant that adversely affects various aspects of adipose tissue functions. More recently, the concept was extended to include substances that may modify metabolic balance at the central, hypothalamic level. The actions of two prime candidate obesogens, tributyltin (TBT) and tetrabromobisphenol A (TBBPA), acting at the central level are the main focus of this review. Having discussed the evidence for contaminant accumulation in the environment and in human tissues and the potential mechanisms of action, data are provided showing that these two widespread pollutants modify hypothalamic gene regulations. Our studies are based on maternal exposure and measurement of effects in the progeny, mainly based on in vivo gene reporter assays. Such models are obviously pertinent to testing current hypotheses that propose that early exposure might exert effects on later development and physiological functions. The potential molecular mechanisms involved are discussed, as are the broader physiological consequences of these hypothalamic dysregulations.

SIRT1 Regulates Thyroid-Stimulating Hormone Release by Enhancing PIP5Kgamma Activity through Deacetylation of Specific Lysine Residues in Mammals

BACKGROUND

SIRT1, a NAD-dependent deacetylase, has diverse roles in a variety of organs such as regulation of endocrine function and metabolism. However, it remains to be addressed how it regulates hormone release there.

METHODOLOGY/PRINCIPAL FINDINGS

Here, we report that SIRT1 is abundantly expressed in pituitary thyrotropes and regulates thyroid hormone secretion. Manipulation of SIRT1 level revealed that SIRT1 positively regulated the exocytosis of TSH-containing granules. Using LC/MS-based interactomics, phosphatidylinositol-4-phosphate 5-kinase (PIP5K)gamma was identified as a SIRT1 binding partner and deacetylation substrate. SIRT1 deacetylated two specific lysine residues (K265/K268) in PIP5Kgamma and enhanced PIP5Kgamma enzyme activity. SIRT1-mediated TSH secretion was abolished by PIP5Kgamma knockdown. SIRT1 knockdown decreased the levels of deacetylated PIP5Kgamma, PI(4,5)P(2), and reduced the secretion of TSH from pituitary cells. These results were also observed in SIRT1-knockout mice.

CONCLUSIONS/SIGNIFICANCE

Our findings indicated that the control of TSH release by the SIRT1-PIP5Kgamma pathway is important for regulating the metabolism of the whole body.

Molecular genetics of the developing neuroendocrine hypothalamus

Formation of the mammalian endocrine system and neuroendocrine organs involves complex regulatory networks resulting in a highly specialized cell system able to secrete a diverse array of peptide hormones. The hypothalamus is located in the mediobasal region of the brain and acts as a gateway between the endocrine and nervous systems. From an endocrinology perspective, the parvicellular neurons of the hypothalamus are of particular interest as they function as a control centre for several critical physiological processes including growth, metabolism and reproduction by regulating hormonal signaling from target cognate cell types in the anterior pituitary. Delineating the genetic program that controls hypothalamic development is essential for complete understanding of parvicellular neuronal function and the etiology of congenital disorders that result from hypothalamic-pituitary axis dysfunction. In recent years, studies have shed light on the interactions between signaling molecules and activation of transcription factors that regulate hypothalamic cell fate commitment and terminal differentiation. The aim of this review is to summarize the recent molecular and genetic findings that have advanced our understanding of the emergence of the known important hypophysiotropic signaling molecules in the hypothalamus. We have focused on reviewing the literature that provides evidence of the dependence on expression of specific genes for the normal development and function of the cells that secrete these neuroendocrine factors, as well as studies of the elaboration of the spatial or temporal patterns of changes in gene expression that drive this development.

Thyroid hormone receptor {alpha} modulates lipopolysaccharide-induced changes in peripheral thyroid hormone metabolism

Acute inflammation is characterized by low serum T(3) and T(4) levels accompanied by changes in liver type 1 deiodinase (D1), liver D3, muscle D2, and muscle D3 expression. It is unknown at present whether thyroid hormone receptor alpha (TRalpha) plays a role in altered peripheral thyroid hormone metabolism during acute illness in vivo. We induced acute illness in TRalpha-deficient (TRalpha(0/0)) mice by administration of a sublethal dose of LPS. Compared with wild-type, TRalpha(0/0) mice have lower basal serum T(4) and lower liver D1 activity and muscle D3 mRNA expression, whereas liver D3 activity is higher. These changes are gender specific. The inflammatory response to LPS was similar in WT and TRalpha(0/0) mice. The decrease in serum thyroid hormones and liver D1 was attenuated in TRalpha(0/0) mice, whereas the LPS induced fall in liver D3 mRNA was more pronounced in TRalpha(0/0) mice. Muscle D2 mRNA increased similarly in both strains, whereas muscle D3 mRNA decreased less pronounced in TRalpha(0/0) mice. We conclude that alterations in peripheral thyroid hormone metabolism induced by LPS administration are partly regulated via TRalpha.

Thyrotropin releasing hormone (TRH): a new player in human hair-growth control

Thyrotropin-releasing hormone (TRH) is the most proximal component of the hypothalamic-pituitary-thyroid axis that regulates thyroid hormone synthesis. Since transcripts for members of this axis were detected in cultured normal human skin cells and since human hair follicles (HFs) respond to stimulation with thyrotropin, we now have studied whether human HF functions are also modulated by TRH. Here we report that the epithelium of normal human scalp HFs expresses not only TRH receptors (TRH-R) but also TRH itself at the gene and protein level. Stimulation of microdissected, organ-cultured HFs with TRH promotes hair-shaft elongation, prolongs the hair cycle growth phase (anagen), and antagonizes its termination by TGF-beta2. It also increases proliferation and inhibits apoptosis of hair matrix keratinocytes. These TRH effects may be mediated in part by reducing the ATM/Atr-dependent phosphorylation of p53. By microarray analysis, several differentially up- or down-regulated TRH-target genes were detected (e.g., selected keratins). Thus, human scalp HFs are both a source and a target of TRH, which operates as a potent hair-growth stimulator. Human HFs provide an excellent discovery tool for identifying and dissecting nonclassical functions of TRH and TRH-mediated signaling in situ, which emerge as novel players in human epithelial biology.

Analysis of hypertrophic thyrotrophs in pituitaries of athyroid Pax8-/- mice

Thyroid hormone is important for pituitary development and maintenance. We previously reported that in the Pax8(-/-) mouse model of congenital hypothyroidism, lactotrophs are almost undetectable, whereas the thyrotrophs exhibit hyperplasia and hypertrophy. Because the latter might be caused by an overstimulation of thyrotrophs with TRH, we analyzed TRH-R1(-/-)Pax8(-/-) double-knockout mice, which miss a functional thyroid gland and the TRH transducing receptor-1 at pituitary target sites. Interestingly, in these double mutants, the hypertrophy and hyperplasia of the thyrotrophs still persist, suggesting that the phenotype is rather a direct consequence of the athyroidism of the animals. The increased expression of TSH in the Pax8(-/-) mice was paralleled by a strongly up-regulated expression of deiodinase type 2 (Dio2) in thyrotrophic cells. Moreover, coexpression of TSH and Dio2 could also be demonstrated in the pituitary of wild-type mice, underlining the important role of this enzyme in the negative feedback regulation of TSH by thyroid hormone. As another consequence of the athyroidism in the mutant mice, tyrosine hydroxylase mRNA expression was found to be also highly up-regulated in thyrotrophic cells of the pituitaries from Pax8(-/-) mice, whereas the transcript levels in the hypothalamus were not affected. Accordingly, tyrosine hydroxylase protein levels, enzyme activities, and ultimately dopamine concentrations were found to be strongly increased in the pituitaries of Pax8(-/-) mice compared with wild-type animals. These findings may explain in part the reduced number of lactotrophs found in the pituitary of athyroid Pax8(-/-) mice and suggest a novel paracrine regulatory mechanism of lactotroph activity.

Study on correlation of signal molecule genes and their receptor-associated genes with rat liver regeneration

To investigate the effect of signal molecules and their receptor-associated genes on rat liver regeneration (LR) at the transcriptional level, the associated genes were originally obtained by retrieving the databases and related scientific publications; their expression profiles in rat LR were then checked using the Rat Genome 230 2.0 microarray. The LR-associated genes were identified by comparing gene expression difference between partial hepatectomy groups and operation-control groups. A total of 454 genes were proved to be LR related. The genes associated with the seven kinds of signal molecules (steroid hormones, fatty acid derivatives, protein and polypeptide hormones, amino acids and their derivatives, choline, cytokines, and gas signal molecules) were detected to be enriched in a cluster characterized by upregulated expression in LR. The number of genes related to the seven kinds of signal molecules was, in sequence, 63, 27, 100, 102, 16, 166, and 18. The 1027 frequencies of upregulation and 823 frequencies of downregulation in total as well as 42 types of different expression patterns suggest the complex and diverse gene expression changes in LR. It is presumed that signal molecules played an important role in metabolism, inflammation, cell proliferation, growth and differentiation, etc., during rat LR.

Thyroid hormone receptor β mediates acute illness-induced alterations in central thyroid hormone metabolism

Acute illness in mice profoundly affects thyroid hormone metabolism in the hypothalamus and pituitary gland. It remains unknown whether the thyroid hormone receptor (TR)-β is involved in these changes. In the present study, we investigated central thyroid hormone metabolism during lipopolysaccharide (LPS)-induced illness in TRβ(-/-) mice compared to wild-type (WT) mice. We administered a sublethal dose of LPS or saline to TRβ(-/-) and WT mice. TRβ(-/-) mice displayed higher basal levels of serum triiodothyronine (T(3)) and thyroxine (T(4)) compared to WT, reflecting thyroid hormone resistance. In the periventricular area of the hypothalamus, we observed a marked decrease in thyrotrophin-releasing hormone (TRH) mRNA expression in TRβ(-/-) and WT mice at t = 4 h, coinciding with the peak in plasma corticosterone. The decrease in TRH mRNA persisted in WT, but not in TRβ(-/-) mice at t = 24 h. By contrast, the increase of type 2 deiodinase (D2) mRNA already present at 4 h after LPS remained significant at 24 h in TRβ(-/-), but not in WT mice. LPS decreased pituitary thyroid-stimulating hormone β mRNA expression in WT at 24 h but not in TRβ(-/-) mice. The peak in pituitary D2 expression at t = 4 h in WT was absent in TRβ(-/-) mice. The relative decrease in plasma T(3) and T(4) upon LPS treatment was similar in both strains, although, at t = 24 h, plasma T(3) tended to be restored in TRβ(-/-) mice. Our results suggest that TRβ is involved in suppression of the central component of the hypothalamic-pituitary-thyroid axis in acute illness.

Reversal of physiological deficits caused by diminished levels of peptidylglycine alpha-amidating monooxygenase by dietary copper

Amidated peptides are critically involved in many physiological functions. Genetic deletion of peptidylglycine alpha-amidating monooxygenase (PAM), the only enzyme that can synthesize these peptides, is embryonically lethal. The goal of the present study was the identification of physiological functions impaired by haploinsufficiency of PAM. Regulation of the hypothalamic-pituitary-thyroid axis and body temperature, functions requiring contributions from multiple amidated peptides, were selected for evaluation. Based on serum T(4) and pituitary TSH-beta mRNA levels, mice heterozygous for PAM (PAM(+/-)) were euthyroid at baseline. Feedback within the hypothalamic-pituitary-thyroid axis was impaired in PAM(+/-) mice made hypothyroid using a low iodine/propylthiouracil diet. Despite their normal endocrine response to cold, PAM(+/-) mice were unable to maintain body temperature as well as wild-type littermates when kept in a 4 C environment. When provided with additional dietary copper, PAM(+/-) mice maintained body temperature as well as wild-type mice. Pharmacological activation of vasoconstriction or shivering also allowed PAM(+/-) mice to maintain body temperature. Cold-induced vasoconstriction was deficient in PAM(+/-) mice. This deficit was eliminated in PAM(+/-) mice receiving a diet with supplemental copper. These results suggest that dietary deficiency of copper, coupled with genetic deficits in PAM, could result in physiological deficits in humans.

Minireview: Thyrotropin-releasing hormone and the thyroid hormone feedback mechanism

Thyroid hormone (TH) plays a critical role in development, growth, and cellular metabolism. TH production is controlled by a complex mechanism of positive and negative regulation. Hypothalamic TSH-releasing hormone (TRH) stimulates TSH secretion from the anterior pituitary. TSH then initiates TH synthesis and release from the thyroid gland. The synthesis of TRH and TSH subunit genes is inhibited at the transcriptional level by TH, which also inhibits posttranslational modification and release of TSH. Although opposing TRH and TH inputs regulate the hypothalamic-pituitary-thyroid axis, TH negative feedback at the pituitary was thought to be the primary regulator of serum TSH levels. However, study of transgenic animals showed an unexpected, dominant role for TRH in regulating the hypothalamic-pituitary-thyroid axis and an unanticipated involvement of the thyroid hormone receptor ligand-dependent activation function (AF-2) domain in TH negative regulation. These results are summarized in the review.

Implications of Research on Assays to Characterize Thyroid Toxicants

Many aspects of thyroid endocrinology are very well conserved across vertebrate taxa. These aspects include thyroid hormone chemistry, the mechanism of its synthesis, and the proteins involved in these processes. In addition, the system by which the hormone is delived from the thyroid gland to target cells, including transport and regulation within the hypothalamic-pituitary-thyroid (HPT) axis, and the proteins that regulate the different components of this delivery system appear to be highly conserved across the vertebrates. Finally, the receptors that mediate thyroid hormone action and the roles thyroid hormone plays are very similar among the vertebrates. Thus, the goal of this chapter is to provide a brief synopsis of the literature supporting existing screening and testing strategies in different vertebrate taxa, and to provide insight into the strengths, weaknesses, and likely changes over time. It was determined during this review that, because of the complexity of the thyroid system, it is unlikely that current in vitro assays for thyroid toxicity will be able to sufficiently replace in vivo assays for thyroid toxicants. However, the in vitro assays serve an important purpose in providing mode of action information and could provide potential screening tools, and should continue to be developed for use. Moreover, because in vivo assays are added on to preexisting reproductive or developmental screens and tests, there are no additional animals required for the in vivo assays. Specific in vitro assays were identified for development, including the thyroid receptor binding and activation assays, and in vitro assays to evaluate thyroid hormone action. Some in vivo endpoints suggested for further research included neuronal differentiation and migration, measures of histogenesis, and measures for thyroid gland thyroid hormone content, which may be more sensitive indicators of TSH stimulation. The most commonly used endpoints currently used to monitor thyroid function are thyroid hormone levels (T3 and T4), TSH, thyroid gland weight, and thyroid histology. Thyroid endocrinology is rapidly advancing and new discoveries will certainly warrant incorporation into future assays. The development of additional endpoints that measure thyroid hormone's actions peripheral to the HPT axis and the development of new reagents for nonmammalian vertebrate species will significantly improve the ability of today's assays to detect chemicals that disrupt the thyroid system in multiple vertebrate species. It is our hope that this series of thyroid articles will provide regulators and research scientists the information needed for each individual to identify the assays and endpoints most suited for their specific purposes.

Critical role of the hypothalamic-pituitary-thyroid axis in bone

Studies in genetically modified mice have highlighted the importance of the hypothalamic-pituitary-thyroid (HPT) axis during skeletal development and the maintenance of adult bone. Recently, the conventional view that skeletal responses to abnormal thyroid status result solely from altered T3 action in bone has been complicated by studies proposing TSH as a negative regulator of bone turnover. Although skeletal consequences of thyrotoxicosis may result from thyroid hormone excess or TSH deficiency, the two alternatives are not necessarily mutually exclusive and cannot easily be differentiated because the HPT axis maintains them in a physiological reciprocal relationship. By contrast, situations in which this inverse relationship is disrupted have the potential to resolve the roles of T3 and TSH in the skeleton. We discuss these situations and the relative importance of T3 and TSH in skeletal homeostasis.

The role of the thyrotropin-releasing hormone (TRH) neuron as a metabolic sensor

Thyroid hormone (TH) plays a critical role in mediating changes in development and metabolism in humans. Thus, circulating TH levels are regulated by a number of distinct mechanisms to allow them to remain at physiologic levels. The central regulation of the thyroid axis by thyrotropin-releasing hormone (TRH) neurons in the paraventricular nucleus of the hypothalamus (PVH) is absolutely required for normal function of the axis. Remarkably, the TRH neurons in the PVH are regulated by multiple pathways that allow for the set point of TRH production to be determined. The following review will focus on how the TRH neuron is regulated by TH as well as key pathways that regulate energy expenditure. By integrating these inputs, the TRH neuron is able to set the thyroid axis at the appropriate level given the physiologic demands present.

Impaired serum thyrotropin response to hypothyroidism in mice with disruption of neuromedin B receptor

Neuromedin B (NB), a neuropeptide highly concentrated in pituitary, has been proposed to be an inhibitor of thyrotropin (TSH) secretion. Previous study showed that mice with disruption of neuromedin B receptor (NBR-KO) have higher TSH release in response to thyrotropin-releasing hormone (TRH), although TSH seems to have decreased bioactivity. Here we examined in NBR-KO mice the response of TSH to thyroid hormone (TH) deprivation, obtained by methimazole treatment, or excess, obtained by acute and chronic TH administration. In response to hypothyroidism NBR-KO mice exhibited a lower magnitude increase in serum TSH compared to wild-type (WT) mice (1.7 vs. 3.3-times increase compared to euthyroid values, respectively, P<0.001). One hour after a single T4 injection (0.4 microg/100 g BW), WT and NBR-KO hypothyroid mice presented similar degree of serum TSH reduction (54%, P<0.05). However, 3 h after T4 administration, WT mice presented serum TSH similar to hypothyroid baseline, while NBR-KO mice still had decreased serum TSH (30% reduced in comparison to hypothyroid baseline P<0.05). T3 treatment of euthyroid mice for 21 days, with progressively increasing doses, significantly reduced serum TSH similarly in WT and NBR-KO mice. Also, serum T4 exhibited the same degree of suppression in WT and NBR-KO. In conclusion, disruption of neuromedin B receptor did not interfere with the sensitivity of thyroid hormone-mediated suppression of TSH release, but impaired the ability of thyrotroph to increase serum TSH in hypothyroidism, which highlights the importance of NB in modulating the set point of the hypothalamus-pituitary-thyroid axis at hypothyroidism.

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.

Integrating basic research on thyroid hormone action into screening and testing programs for thyroid disruptors

Thyroid hormone signaling is highly conserved among all the vertebrates, and appears to be present in some invertebrates. Both the components that comprise the system and its general role in development and physiology are evolutionarily conserved, although specific events regulated by thyroid hormones, such as amphibian metamorphosis, may differ among taxonomic groups. The articles in this issue review the thyroid systems of mammals (specifically humans and rodents), fish, amphibians, and birds, and the states of the assays and endpoints used to detect disruption of the thyroid system within a toxicological paradigm. It must be noted that while reptiles represent an enormously important group, they were excluded because there was not enough information in the literature on thyroid toxicology in reptiles at the time that this series of reviews was drafted. Each review highlights the best assays for current regulatory use and those that may be considered for development for future use and research. However, it is important to remember that thyroid research is moving ahead at a fast pace. New thyroid research will impact the design of future thyroid assays used for the detection of thyroid system disruption in ways that may not be anticipated at the time of this writing. Several new areas of exploration are discussed that reveal potential sites of disruption in the thyroid system, including (1) the importance of the neural drive for TSH upregulation, (2) thyroid hormone transport, including cellular transporters like monocarboxylate anion transporter 8 (MCT8) that can regulate thyroid hormone action at the cellular level, and thyroid hormone-binding proteins in the serum that have been shown to differentially bind to environmental chemicals (e.g., certain PCB congeners), and (3) the deiodinases as a target for disruption of thyroid hormone activity in the peripheral thyroid system. The review papers in this issue represent the current state of thyroid assays and endpoints for detection of chemicals that disrupt the thyroid system.