Cardiovascular phenotype and temperature control in mice lacking thyroid hormone receptor-beta or both alpha1 and beta

Thyroid Hormone Receptors in Control of Heart Rate

Thyroid hormone has profound effects on cardiovascular functions, including heart rate. These effects can be mediated directly, for example, by changing the expression of target genes in the heart through nuclear thyroid hormone receptors, or indirectly by altering the autonomic nervous systems output of the brain. The underlying molecular mechanisms as well as the cellular substrates, however, are far from being understood. In this review, we summarize the recent key findings on the individual contributions of the two thyroid hormone receptor isoforms on the regulation of heart rate, challenging the role of the pacemaker channel genes Hcn2 and Hcn4 as sole mediators of the hormone's effect. Furthermore, we discuss the possible actions of thyroid hormone on the autonomic nervous system affecting heart rate distribution, and highlight the possibility of permanent alterations in heart and brain by impaired thyroid hormone action during development as important factors to consider when analyzing or designing experiments.

Partial Resistance to Thyroid Hormone-Induced Tachycardia and Cardiac Hypertrophy in Mice Lacking Thyroid Hormone Receptor β

Background: Thyroid hormones regulate cardiac functions mainly through direct actions in the heart and by binding to the thyroid hormone receptor (TR) isoforms α1 and β. While the role of the most abundantly expressed isoform, TRα1, is widely studied and well characterized, the role of TRβ in regulating heart functions is still poorly understood, primarily due to the accompanying elevation of circulating thyroid hormone in TRβ knockout mice (TRβ-KO). However, their hyperthyroidism is ameliorated at thermoneutrality, which allows studying the role of TRβ without this confounding factor. Methods: Here, we noninvasively monitored heart rate in TRβ-KO mice over several days using radiotelemetry at different housing temperatures (22°C and 30°C) and upon 3,3',5-triiodothyronine (T3) administration in comparison to wild-type animals. Results: TRβ-KO mice displayed normal average heart rate at both 22°C and 30°C with only minor changes in heart rate frequency distribution, which was confirmed by independent electrocardiogram recordings in freely-moving conscious mice. Parasympathetic nerve activity was, however, impaired in TRβ-KO mice at 22°C, and only partly rescued at 30°C. As expected, oral treatment with pharmacological doses of T3 at 30°C led to tachycardia in wild-types, accompanied by broader heart rate frequency distribution and increased heart weight. The TRβ-KO mice, in contrast, showed blunted tachycardia, as well as resistance to changes in heart rate frequency distribution and heart weight. At the molecular level, these observations were paralleled by a blunted cardiac mRNA induction of several important genes, including the pacemaker channels Hcn2 and Hcn4, as well as Kcna7. Conclusions: The phenotyping of TRβ-KO mice conducted at thermoneutrality allows novel insights on the role of TRβ in cardiac functions in the absence of the usual confounding hyperthyroidism. Even though TRβ is expressed at lower levels than TRα1 in the heart, our findings demonstrate an important role for this isoform in the cardiac response to thyroid hormones.

Transcriptional Regulation of Postnatal Cardiomyocyte Maturation and Regeneration

During the postnatal period, mammalian cardiomyocytes undergo numerous maturational changes associated with increased cardiac function and output, including hypertrophic growth, cell cycle exit, sarcomeric protein isoform switching, and mitochondrial maturation. These changes come at the expense of loss of regenerative capacity of the heart, contributing to heart failure after cardiac injury in adults. While most studies focus on the transcriptional regulation of embryonic or adult cardiomyocytes, the transcriptional changes that occur during the postnatal period are relatively unknown. In this review, we focus on the transcriptional regulators responsible for these aspects of cardiomyocyte maturation during the postnatal period in mammals. By specifically highlighting this transitional period, we draw attention to critical processes in cardiomyocyte maturation with potential therapeutic implications in cardiovascular disease.

Thermogenesis in Adipose Tissue Activated by Thyroid Hormone

Thermogenesis is the production of heat that occurs in all warm-blooded animals. During cold exposure, there is obligatory thermogenesis derived from body metabolism as well as adaptive thermogenesis through shivering and non-shivering mechanisms. The latter mainly occurs in brown adipose tissue (BAT) and muscle; however, white adipose tissue (WAT) also can undergo browning via adrenergic stimulation to acquire thermogenic potential. Thyroid hormone (TH) also exerts profound effects on thermoregulation, as decreased body temperature and increased body temperature occur during hypothyroidism and hyperthyroidism, respectively. We have termed the TH-mediated thermogenesis under thermoneutral conditions “activated” thermogenesis. TH acts on the brown and/or white adipose tissues to induce uncoupled respiration through the induction of the uncoupling protein (Ucp1) to generate heat. TH acts centrally to activate the BAT and browning through the sympathetic nervous system. However, recent studies also show that TH acts peripherally on the BAT to directly stimulate Ucp1 expression and thermogenesis through an autophagy-dependent mechanism. Additionally, THs can exert Ucp1-independent effects on thermogenesis, most likely through activation of exothermic metabolic pathways. This review summarizes thermogenic effects of THs on adipose tissues.

Sobetirome and its Amide Prodrug Sob-AM2 Exert Thyromimetic Actions in Mct8-Deficient Brain

BACKGROUND

Loss of function mutations in the thyroid hormone (TH)-specific cell membrane transporter, the monocarboxylate transporter 8 (MCT8), lead to profound psychomotor retardation and abnormal TH serum levels, with low thyroxine (T4) and high triiodothyronine (T3). Several studies point to impaired TH transport across brain barriers as a crucial pathophysiological mechanism resulting in cerebral hypothyroidism. Treatment options for MCT8-deficient patients are limited and are focused on overcoming the brain barriers. The aim of this study was to evaluate the ability of the TH analog sobetirome and its prodrug Sob-AM2 to access the brain and exert thyromimetic actions in the absence of Mct8.

METHODS

Juvenile wild-type (Wt) mice and mice lacking Mct8 and deiodinase type 2 (Mct8/Dio2KO) were treated systemically with daily injections of vehicle, 1 mg of sobetirome/kg body weight/day, or 0.3 mg of Sob-AM2/kg body weight/day for seven days. Sobetirome content was measured using liquid chromatography-tandem mass spectrometry, and T4 and T3 levels by specific radioimmunoassays. The effect of sobetirome treatment in the expression of T3-dependent genes was measured in the heart, liver, and cerebral cortex by real-time polymerase chain reaction.

RESULTS

Sob-AM2 treatment in Mct8/Dio2KO animals led to 1.8-fold more sobetirome content in the brain and 2.5-fold less in plasma in comparison to the treatment with the parent drug sobetirome. Both sobetirome and Sob-AM2 treatments in Mct8/Dio2KO mice greatly decreased plasma T4 and T3 levels. Dio1 and Ucp2 gene expression was altered in the liver of Mct8/Dio2KO mice and was not affected by the treatments. In the heart, Hcn2 but not Atp2a2 expression was increased after treatment with the analogs. Interestingly, both sobetirome and Sob-AM2 treatments increased the expression of several T3-dependent genes in the brain such as Hr, Abcd2, Mme, and Flywch2 in Mct8/Dio2KO mice.

CONCLUSIONS

Sobetirome and its amide prodrug Sob-AM2 can access the brain in the absence of Mct8 and exert thyromimetic actions modulating the expression of T3-dependent genes. At the peripheral level, the administration of these TH analogs results in the depletion of circulating T4 and T3. Therefore, sobetirome and Sob-AM2 have the potential to address the cerebral hypothyroidism and the peripheral hyperthyroidism characteristic of MCT8 deficiency.

microRNA and thyroid hormone signaling in cardiac and skeletal muscle

Thyroid hormone (TH) signaling plays critical roles in the differentiation, growth, metabolism, and physiological function of all organs or tissues, including heart and skeletal muscle. Due to the significant progress in our understanding of the molecular mechanisms that underlie TH action, it's widely accepted that TH signaling is regulated at multiple levels. A growing number of discoveries suggest that microRNAs (miRNAs) act as fine-tune regulators of gene expression and adds sophisticated regulatory tiers to signaling pathways. Recently, some pioneering studies in cardiac and skeletal muscle demonstrating the interplay between miRNAs and TH signaling suggest that miRNAs might mediate and/or modulate TH signaling. This review presents recent advances involving the crosstalk between miRNAs and TH signaling and current evidence showing the importance of miRNA in TH signaling with particular emphasis on the study of muscle-specific miRNAs (myomiRs) in cardiac and skeletal muscle. Although the research of the reciprocal regulation of miRNAs and TH signaling is only at the beginning stage, it has already contributed to our current understanding of both TH action and miRNA biology. We also encourage further investigations to address the relative contributions of miRNAs in TH signaling under physiological and pathological conditions and how a group of miRNAs are coordinated to integrate into the complex hierarchical regulatory network of TH.

Thyroid hormones and the metabolic syndrome

BACKGROUND

Clustering of various metabolic parameters including abdominal obesity, hyperglycaemia, low high-density lipoprotein cholesterol, elevated triglycerides and hypertension have been used worldwide as metabolic syndrome to predict cardiometabolic risk. Thyroid dysfunction impacts on various levels of these components.

OBJECTIVES

The purpose of the present review is to summarize available data on thyroid hormone-dependent action on components of the metabolic syndrome.

METHODS

A PubMed search for any combination of hyperthyroidism, thyrotoxicosis or hypothyroidism and metabolic syndrome, blood pressure, hypertension, hyperlipidaemia, cholesterol, high-density lipoprotein cholesterol, glucose, diabetes mellitus, body weight or visceral fat was performed. We included papers and reviews published between 2000 and today but accepted also frequently cited papers before 2000.

RESULTS

There is convincing evidence for a major impact of thyroid function on all components of the metabolic syndrome, reflecting profound alterations of energy homeostasis at many levels.

CONCLUSION

Even though the interactions shown in animal models and man are complex, it is evident that insulin sensitivity is highest and adverse thyroid effects on the metabolic system are lowest in euthyroid conditions.

Distinct behavioral phenotypes in male mice lacking the thyroid hormone receptor α1 or β isoforms

Thyroid hormones influence both neuronal development and anxiety via the thyroid hormone receptors (TRs). The TRs are encoded by two different genes, TRα and TRβ. The loss of TRα1 is implicated in increased anxiety in males, possibly via a hippocampal increase in GABAergic activity. We compared both social behaviors and two underlying and related non-social behaviors, state anxiety and responses to acoustic and tactile startle in the gonadally intact TRα1 knockout (α1KO) and TRβ (βKO) male mice to their wild-type counterparts. For the first time, we show an opposing effect of the two TR isoforms, TRα1 and TRβ, in the regulation of state anxiety, with α1 knockout animals (α1KO) showing higher levels of anxiety and βKO males showing less anxiety compared to respective wild-type mice. At odds with the increased anxiety in non-social environments, α1KO males also show lower levels of responsiveness to acoustic and tactile startle stimuli. Consistent with the data that T4 is inhibitory to lordosis in female mice, we show subtly increased sex behavior in α1KO male mice. These behaviors support the idea that TRα1 could be inhibitory to ERα driven transcription that ultimately impacts ERα driven behaviors such as lordosis. The behavioral phenotypes point to novel roles for the TRs, particularly in non-social behaviors such as state anxiety and startle.

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.

Thyroid hormone receptor-α and vascular function

Thyroid hormone (TH) treatment exerts beneficial effects on the cardiovascular system: it lowers cholesterol and LDL levels and enhances cardiac contractile function. However, little is known about the effect of TH on vascular function or the functional role of TH receptors (TRs) in the regulation of vascular tone. We have investigated the contribution of TRs to vascular contractility in the heart. Among different TR subtype-specific knockout (KO) mice, vascular contraction was significantly enhanced in coronary arteries isolated from TRα KO compared with wild-type mice, while chronic TH treatment significantly attenuated coronary vascular contraction. We found that TRα is the predominant TR in mouse coronary smooth muscle cells (SMCs). Coronary SMCs isolated from TRα KO mice exhibited a significant decrease in K(+) channel activity, whereas TH treatment increased K(+) channel activity in a dose-dependent manner. These data suggest that TRα in SMCs has prominent effects on regulation of vascular tone and TH treatment helps decrease coronary vascular tone by increasing K(+) channel activity through TRα in SMCs.

In vivo activity of the thyroid hormone receptor beta- and α-selective agonists GC-24 and CO23 on rat liver, heart, and brain

Thyroid hormone analogs with selective actions through specific thyroid hormone receptor (TR) subtypes are of great interest. They might offer the possibility of mimicking physiological actions of thyroid hormone with receptor subtype or tissue specificity with therapeutic aims. They are also pharmacological tools to dissect biochemical pathways mediated by specific receptor subtypes, in a complementary way to mouse genetic modifications. In this work, we studied the in vivo activity in developing rats of two thyroid hormone agonists, the TRβ-selective GC-24 and the TRα-selective CO23. Our principal goal was to check whether these compounds were active in the rat brain. Analog activity was assessed by measuring the expression of thyroid hormone target genes in liver, heart, and brain, after administration to hypothyroid rats. GC-24 was very selective for TRβ and lacked activity on the brain. On the other hand, CO23 was active in liver, heart, and brain on genes regulated by either TRα or TRβ. This compound, previously shown to be TRα-selective in tadpoles, displayed no selectivity in the rat in vivo.

Resistance to thyroid hormone is associated with raised energy expenditure, muscle mitochondrial uncoupling, and hyperphagia

Resistance to thyroid hormone (RTH), a dominantly inherited disorder usually associated with mutations in thyroid hormone receptor beta (THRB), is characterized by elevated levels of circulating thyroid hormones (including thyroxine), failure of feedback suppression of thyrotropin, and variable tissue refractoriness to thyroid hormone action. Raised energy expenditure and hyperphagia are recognized features of hyperthyroidism, but the effects of comparable hyperthyroxinemia in RTH patients are unknown. Here, we show that resting energy expenditure (REE) was substantially increased in adults and children with THRB mutations. Energy intake in RTH subjects was increased by 40%, with marked hyperphagia particularly evident in children. Rates of muscle TCA cycle flux were increased by 75% in adults with RTH, whereas rates of ATP synthesis were unchanged, as determined by 13C/31P magnetic resonance spectroscopy. Mitochondrial coupling index between ATP synthesis and mitochondrial rates of oxidation (as estimated by the ratio of ATP synthesis to TCA cycle flux) was significantly decreased in RTH patients. These data demonstrate that basal mitochondrial substrate oxidation is increased and energy production in the form of ATP synthesis is decreased in the muscle of RTH patients and that resting oxidative phosphorylation is uncoupled in this disorder. Furthermore, these observations suggest that mitochondrial uncoupling in skeletal muscle is a major contributor to increased REE in patients with RTH, due to tissue selective retention of thyroid hormone receptor alpha sensitivity to elevated thyroid hormone levels.

Distinct dysregulation of lipid metabolism by unliganded thyroid hormone receptor isoforms

Thyroid hormone receptors (TRs) play critical roles in energy homeostasis. To understand the role of TRs in lipid homeostasis in vivo, we adopted the loss-of-function approach by creating knock-in mutant mice with targeted mutation in the TRalpha gene (TRalpha1PV mouse) or TRbeta gene (TRbetaPV mouse). The PV mutation, identified in a patient with resistance to thyroid hormone, exhibits potent dominant-negative activity. Here we show that in contrast to TRalpha1PV mouse, TRbetaPV mice exhibited no significant reduction in WAT but had significant increases in serum free fatty acids and total triglycerides. Moreover, the liver of TRbetaPV mice was markedly increased (33%) with excess lipid accumulation, but the liver mass of TRalpha1PV mouse was decreased (23%) with paucity of lipids. These results indicate that apo-TRbeta and apo-TRalpha1 exerted distinct abnormalities in lipid metabolism. Further biochemical analyses indicate that increased lipogenic enzyme expression, activated peroxisome proliferator-activated receptor gamma (Ppargamma) signaling, and decreased fatty acid beta-oxidation activity contributed to the adipogenic steatosis and lipid accumulation in the liver of TRbetaPV mice. In contrast, the expression of lipogenic enzymes and Ppargamma was decreased in the liver of TRalpha1PV mice. These results suggest that the regulation of genes critical for lipid metabolism by TRs in the liver is isoform dependent. These results indicate that apo-TRbeta and apo-TRalpha1 had different effects on lipid metabolism and that both TR isoforms contribute to the pathogenesis of lipid metabolism in hypothyroidism.

Cardiac hypertrophy and thyroid hormone signaling

Thyroid hormone exerts a large number of influences on the cardiovascular system. Increased thyroid hormone action increases the force and speed of systolic contraction and the speed of diastolic relaxation and these are largely beneficial effects. Furthermore, thyroid hormone has marked electrophysiological effects increasing heart rate and the propensity for atrial fibrillation and these effects are largely mal-adaptive. In addition, thyroid hormone markedly increases cardiac angiogenesis and decreases vascular tone. These multiple thyroid hormone effects are largely mediated by the action of nuclear based thyroid hormone receptors (TR) the thyroid hormone receptor alpha and beta. TRα is the predominant isoform in the heart. Rapid nongenomic thyroid hormone effects also occur, which can be clearly demonstrated in ex-vivo experiments. Some of the most marked thyroid hormone effects in cardiac myocytes involve influences on calcium flux, with thyroid hormone promoting expression of the gene encoding the calcium pump of the sarcoplasmic reticulum (SERCa2). In contrast, in hypothyroid animals phospholamban levels, which inhibit the SERCa2 pump, are increased. In addition, marked effects are exerted on the calcium channel of the sarcoplasmic reticulum the ryanodine channel. Related to myofibrillar proteins, myosin heavy chain alpha is increased by T3 and MHC beta is decreased. Complex and interesting interactions occur between cardiac hypertrophy induced by excess thyroid hormone action and cardiac hypertrophy occurring with heart failure. The thyroid hormone mediated cardiac hypertrophy in its initial phases presents a physiological hypertrophy with increases in SERCa2 levels and decreased expression of MHC beta. In contrast, pressure overload induced heart failure leads to a “pathological” cardiac hypertrophy which is largely mediated by activation of the calcineurin system and the MAPkinases signaling system. Recent evidence indicates that heart failure can lead to a downregulation of the thyroid hormone signaling system in the heart. In the failing heart, decreases of thyroid hormone receptor levels occur. In addition, serum levels of T4 and T3 are decreased with heart failure in the frame of the non-thyroidal illness syndrome. The decrease in T3 serves as an indicator for a bad prognosis in the heart failure patient being linked to increased mortality. In animal models, it can be shown that in pressure overload-induced cardiac hypertrophy a decrease of thyroid hormone receptor levels occurs. Cardiac function can be improved by increasing expression of thyroid hormone receptors mediated by adeno-associated virus based gene transfer. The failing heart may develop a “hypothyroid” status contributing to diminished cardiac contractile function.

Hypermetabolism in mice caused by the central action of an unliganded thyroid hormone receptor alpha1

Thyroid hormone, via its nuclear receptors TRalpha and TRbeta, controls metabolism by acting locally in peripheral tissues and centrally by regulating sympathetic signaling. We have defined aporeceptor regulation of metabolism by using mice heterozygous for a mutant TRalpha1 with low affinity to T3. The animals were hypermetabolic, showing strongly reduced fat depots, hyperphagia and resistance to diet-induced obesity accompanied by induction of genes involved in glucose handling and fatty acid metabolism in liver and adipose tissues. Increased lipid mobilization and beta-oxidation occurred in adipose tissues, whereas blockade of sympathetic signaling to brown adipose tissue normalized the metabolic phenotype despite a continued perturbed hormone signaling in this cell type. The results define a novel and important role for the TRalpha1 aporeceptor in governing metabolic homeostasis. Furthermore, the data demonstrate that a nuclear hormone receptor affecting sympathetic signaling can override its autonomous effects in peripheral tissues.

Postnatal development has a marked effect on ventricular repolarization in mice

To better understand the mechanisms that underlie cardiac repolarization abnormalities in the immature heart, this study characterized and compared K(+) currents in mouse ventricular myocytes from day 1, day 7, day 20, and adult CD1 mice to determine the effects of postnatal development on ventricular repolarization. Current- and patch-clamp techniques were used to examine action potentials and the K(+) currents underlying repolarization in isolated myocytes. RT-PCR was used to quantify mRNA expression for the K(+) channels of interest. This study found that action potential duration (APD) decreased as age increased, with the shortest APDs observed in adult myocytes. This study also showed that K(+) currents and the mRNA relative abundance for the various K(+) channels were significantly greater in adult myocytes compared with day 1 myocytes. Examination of the individual components of total K(+) current revealed that the inward rectifier K(+) current (I(K1)) developed by day 7, both the Ca(2+)-independent transient outward current (I(to)) and the steady-state outward K(+) current (I(ss)) developed by day 20, and the ultrarapid delayed rectifier K(+) current (I(Kur)) did not fully develop until the mouse reached maturity. Interestingly, the increase in I(Kur) was not associated with a decrease in APD. Comparison of atrial and ventricular K(+) currents showed that I(to) and I(Kur) density were significantly greater in day 7, day 20, and adult myocytes compared with age-matched atrial cells. Overall, it appears that, in mouse ventricle, developmental changes in APD are likely attributable to increases in I(to), I(ss), and I(K1), whereas the role of I(Kur) during postnatal development appears to be less critical to APD.

The rat thyroid hormone receptor (TR) Deltabeta3 displays cell-, TR isoform-, and thyroid hormone response element-specific actions

The THRB gene encodes the well-described thyroid hormone (T3) receptor (TR) isoforms TRbeta1 and TRbeta2 and two additional variants, TRbeta3 and TRDeltabeta3, of unknown physiological significance. TRbeta1, TRbeta2, and TRbeta3 are bona fide T3 receptors that bind DNA and T3 and regulate expression of T3-responsive target genes. TRDeltabeta3 retains T3 binding activity but lacks a DNA binding domain and does not activate target gene transcription. TRDeltabeta3 can be translated from a specific TRDeltabeta3 mRNA or is coexpressed with TRbeta3 from a single transcript that contains an internal TRDeltabeta3 translation start site. In these studies, we provide evidence that the TRbeta3/Deltabeta3 locus is present in rat but not in other vertebrates, including humans. We compared the activity of TRbeta3 with other TR isoforms and investigated mechanisms of action of TRDeltabeta3 at specific thyroid hormone response elements (TREs) in two cell types. TRbeta3 was the most potent isoform, but TR potency was TRE dependent. TRDeltabeta3 acted as a cell-specific and TRE-dependent modulator of TRbeta3 when coexpressed at low concentrations. At higher concentrations, TRDeltabeta3 was a TRE-selective and cell-specific antagonist of TRalpha1, -beta1, and -beta3. Both TRbeta3 and TRDeltabeta3 were expressed in the nucleus in the absence and presence of hormone, and their actions were determined by cell type and TRE structure, whereas TRDeltabeta3 actions were also dependent on the TR isoform with which it interacted. Analysis of these complex responses implicates a range of nuclear corepressors and coactivators as cell-, TR isoform-, and TRE-specific modulators of T3 action.

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.

Thyroid hormone receptor alpha is a molecular switch of cardiac function between fetal and postnatal life

Thyroid hormones are involved in the regulation of many physiological processes and regulate gene transcription by binding to their nuclear receptors TRalpha and TRbeta. In the absence of triiodothyronine (T3), the unliganded receptors (aporeceptors) do bind DNA and repress the transcription of target genes. The role of thyroid hormone aporeceptors as repressors was observed in hypothyroid adult mice, but its physiological relevance in nonpathological hypothyroid conditions remained to be determined. Here we show that, in the normal mouse fetus, TRalpha aporeceptors repress heart rate as well as the expression of TRbeta and several genes encoding ion channels involved in cardiac contractile activity. Right after birth, when T3 concentration sharply increases, liganded TRalpha (holoreceptors) turn on the expression of some of these same genes concomitantly with heart rate increase. These data describe a physiological situation under which conversion of TRalpha from apo-receptors into holo-receptors, upon changes in T3 availability, plays a determinant role in a developmental process.

Mice lacking melanin-concentrating hormone receptor 1 demonstrate increased heart rate associated with altered autonomic activity

Melanin-concentrating hormone (MCH) plays an important role in energy balance. The current studies were carried out on a new line of mice lacking the rodent MCH receptor (MCHR1(-/-) mice). These mice confirmed the previously reported lean phenotype characterized by increased energy expenditure and modestly increased caloric intake. Because MCH is expressed in the lateral hypothalamic area, which also has an important role in the regulation of the autonomic nervous system, heart rate and blood pressure were measured by a telemetric method to investigate whether the increased energy expenditure in these mice might be due to altered autonomic nervous system activity. Male MCHR1(-/-) mice demonstrated a significantly increased heart rate [24-h period: wild type 495 +/- 4 vs. MCHR1(-/-) 561 +/- 8 beats/min (P < 0.001); dark phase: wild type 506 +/- 8 vs. MCHR1(-/-) 582 +/- 9 beats/min (P < 0.001); light phase: wild type 484 +/- 13 vs. MCHR1(-/-) 539 +/- 9 beats/min (P < 0.005)] with no significant difference in mean arterial pressure [wild type 110 +/- 0.3 vs. MCHR1(-/-) 113 +/- 0.4 mmHg (P > 0.05)]. Locomotor activity and core body temperature were higher in the MCHR1(-/-) mice during the dark phase only and thus temporally dissociated from heart rate differences. On fasting, wild-type animals rapidly downregulated body temperature and heart rate. MCHR1(-/-) mice displayed a distinct delay in the onset of this downregulation. To investigate the mechanism underlying these differences, autonomic blockade experiments were carried out. Administration of the adrenergic antagonist metoprolol completely reversed the tachycardia seen in MCHR1(-/-) mice, suggesting an increased sympathetic tone.

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.

Noninvasive cardiovascular phenotyping in mice

With the growth of genetic engineering, mice have become common as models of human diseases, which in turn has stimulated the development of techniques to monitor and image the murine cardiovascular system. Invasive methods are often more quantitative, but noninvasive methods are preferred when measurements must be repeated serially on living animals during development or in response to pharmacological or surgical interventions. Because of the small size and high heart rates in mice, high spatial and temporal resolutions are required to preserve signal fidelity. Monitoring of body temperature and the electrocardiogram is essential when animals must be anesthetized for a measurement or other procedure. Several other groups have developed cardiovascular imaging modalities suitable for murine applications, and ultrasound is the most widely used. Our group has developed and applied high-resolution Doppler probes and signal processing for measuring blood velocity in the heart and peripheral vessels of anesthetized mice noninvasively. We can measure cardiac filling and ejection velocities as indices of systolic and diastolic ventricular function and for timing of cardiac events; velocity pulse arrival times for determining pulse-wave velocity and arterial stiffness; peripheral velocity waveforms as indices of arterial resistance, compliance, and wave reflections; stenotic velocities for estimation of pressure drop and detection of vorticity; and tail artery velocity for determining systolic and diastolic blood pressure using a pressure cuff. These noninvasive methods are convenient and easy to apply and have been used to detect and evaluate numerous cardiovascular phenotypes in mutant mice.

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.

Effects of GABA agonists on body temperature regulation in GABA(B(1))-/- mice

1. Activation of GABA(B) receptors evokes hypothermia in wildtype (GABA(B(1))+/+) but not in GABA(B) receptor knockout (GABA(B(1))-/-) mice. The aim of the present study was to determine the hypothermic and behavioural effects of the putative GABA(B) receptor agonist gamma-hydroxybutyrate (GHB), and of the GABA(A) receptor agonist muscimol. In addition, basal body temperature was determined in GABA(B(1))+/+, GABA(B(1))+/- and GABA(B(1))-/- mice. 2. GABA(B(1))-/- mice were generated by homologous recombination in embryonic stem cells. Correct gene targeting was assessed by Southern blotting, PCR and Western blotting. GABA(B) receptor-binding sites were quantified with radioligand binding. Measurement of body temperature was done using subcutaneous temperature-sensitive chips, and behavioural changes after drug administration were scored according to a semiquantitative scale. 3. GABA(B(1))-/- mice had a short lifespan, probably caused by generalised seizure activity. No histopathological or blood chemistry changes were seen, but the expression of GABA(B(2)) receptor protein was below the detection limit in brains from GABA(B(1))-/- mice, in the absence of changes in mRNA levels. 4. GABA(B) receptor-binding sites were absent in brain membranes from GABA(B(1))-/- mice. 5. GABA(B(1))-/- mice were hypothermic by approximately 1 degrees C compared to GABA(B(1))+/+ and GABA(B(1))+/- mice. 6. Injection of baclofen (9.6 mg kg-1) produced a large reduction in body temperature and behavioural effects in GABA(B(1))+/+ and in GABA(B(1))+/- mice, but GABA(B(1))-/- mice were unaffected. The same pattern was seen after administration of GHB (400 mg kg-1). The GABA(A) receptor agonist muscimol (2 mg kg-1), on the other hand, produced a more pronounced hypothermia in GABA(B(1))-/-mice. In GABA(B(1))+/+ and GABA(B(1))+/- mice, muscimol induced sedation and reduced locomotor activity. However, when given to GABA(B(1))-/- mice, muscimol triggered periods of intense jumping and wild running. 7. It is concluded that hypothermia should be added to the characteristics of the GABAB(1)-/-phenotype. Using this model, GHB was shown to be a selective GABAB receptor agonist. In addition, GABAB(1)-/- mice are hypersensitive to GABAA receptor stimulation, indicating that GABAB tone normally balances GABAA-mediated effects.

Evaluation and applications of radiotelemetry in small laboratory animals

Radiotelemetry is the "state of the art" for monitoring physiological functions in awake and freely moving laboratory animals, while minimizing stress artifacts. For researchers, especially those in the fields of pharmacology and toxicology, the technique provides a valuable tool for defining the physiological and pathophysiological consequences derived from advances molecular, cellular, and tissue biology and in predicting the effectiveness and safety of new compounds in humans. There is ample evidence that radiotelemetry systems for measuring physiological functions has been sufficiently validated. Today, the technology is an important tool for collection of a growing number of physiological parameters, for contributing to animal welfare (reduction and refinement alternatives), and for reducing overall animal research costs.

Isometric force and endurance in skeletal muscle of mice devoid of all known thyroid hormone receptors

The importance of thyroid hormone receptors for isometric force, endurance and content of specific muscle enzymes was studied in isolated slow-twitch soleus and fast-twitch extensor digitorum longus (EDL) muscles in mice deficient in all known subtypes of thyroid hormone receptors (i.e. TR alpha1, beta1, beta2 and beta3). The weights of soleus and EDL muscles were lower in TR-deficient (TRalpha1-/-beta-/-) mice than in wild-type controls. The force per cross-sectional area was not significantly different between TRalpha1-/-beta-/- and wild-type muscles. Soleus muscles of TRalpha1-/-beta-/- mice showed increased contraction and relaxation times and the force-frequency relationship was shifted to the left. Soleus muscles of TRalpha1-/-beta-/- mice were more fatigue resistant than wild-type controls. Protein analysis of TRalpha1-/-beta-/- soleus muscles showed a marked increase in expression of the slow isoform of the sarcoplasmic reticulum Ca2+ pump (SERCa2), whilst expression of the fast type (SERCa1) was decreased. There was also a major decrease in the alpha2-subunit of the Na+-K+ pump in TRalpha1-/-beta-/- soleus muscles. EDL muscles from TRalpha1-/-beta-/- and wild-type mice showed no significant difference in contraction and relaxation times, fatigue resistance and protein expression. In conclusion, the present data show changes in contractile characteristics of skeletal muscles of TRalpha1-/-beta-/- mice similar to those seen in hypothyroidism. We have previously shown that muscles of mice deficient in TRalpha1 or TRbeta display modest changes in muscle function. Thus, in skeletal muscle there seems to be functional overlap between TRalpha1 and TRbeta, so that the lack of one of the receptors to some extent can be compensated for by the presence of the other.

Specificity of thyroid hormone receptor subtype and steroid receptor coactivator-1 on thyroid hormone action

Isoforms of the thyroid hormone receptor (TR)alpha and TRbeta genes mediate thyroid hormone action. How TR isoforms modulate tissue-specific thyroid hormone (TH) action remains largely unknown. The steroid receptor coactivator-1 (SRC-1) is among a group of transcriptional coactivator proteins that bind to TRs, along with other members of the nuclear receptor superfamily, and modulate the activity of genes regulated by TH. Mice deficient in SRC-1 possess decreased tissue responsiveness to TH and many steroid hormones; however, it is not known whether or not SRC-1-mediated activation of TH-regulated gene transcription in peripheral tissues, such as heart and liver, is TR isoform specific. We have generated mice deficient in TRalpha and SRC-1, as well as in TRbeta and SRC-1, and investigated thyroid function tests and effects of TH deprivation and TH treatment compared with wild-type (WT) mice or those deficient in either TR or SRC-1 alone. The data show that 1) in the absence of TRalpha or TRbeta, SRC-1 is important for normal growth; 2) SRC-1 modulates TRalpha and TRbeta effects on heart rate; 3) two new TRbeta-dependent markers of TH action in the liver have been identified, osteopontin (upregulated) and glutathione S-transferase (downregulated); and 4) SRC-1 may mediate the hypersensitivity to TH seen in liver of TRalpha-deficient mice.

Estrogen and thyroid hormone receptor interactions: physiological flexibility by molecular specificity

The influence of thyroid hormone on estrogen actions has been demonstrated both in vivo and in vitro. In transient transfection assays, the effects of liganded thyroid hormone receptors (TR) on transcriptional facilitation by estrogens bound to estrogen receptors (ER) display specificity according to the following: 1) ER isoform, 2) TR isoform, 3) the promoter through which transcriptional facilitation occurs, and 4) cell type. Some of these molecular phenomena may be related to thyroid hormone signaling of seasonal limitations upon reproduction. The various combinations of these molecular interactions provide multiple and flexible opportunities for relations between two major hormonal systems important for neuroendocrine feedbacks and reproductive behaviors.

Retardation of post-natal development caused by a negatively acting thyroid hormone receptor alpha1

Most patients with the syndrome resistance to thyroid hormone (RTH) express a mutant thyroid hormone receptor beta (TRbeta) with transdominant negative transcriptional effects. Since no patient with a mutant TRalpha has been identified, we introduced a point mutation into the mouse thyroid hormone receptor (TRalpha1) locus originally found in the TRbeta gene, that reduces ligand binding 10-fold. Heterozygous 2- to 3-week- old mice exhibit a severe retardation of post-natal development and growth, but only a minor reduction in serum thyroxine levels. Homozygous mice died before 3 weeks of age. Adult heterozygotes overcome most of these defects except for cardiac function abnormalities, suggesting that other factors compensate for the receptor defect. However, the additional deletion of the TRbeta gene in this mouse strain caused a 10-fold increase in serum thyroxine, restored hormonal regulation of target genes for TRs, and rescued the growth retardation. The data demonstrate a novel array of effects mediated by a dominant negative TRalpha1, and may provide important clues for identification of a potentially unrecognized human disorder and its treatment.

Thyroid hormone and cardiac function in mice deficient in thyroid hormone receptor-alpha or -beta: an echocardiograph study

We investigated the effect of thyroid hormone (TH) receptor (TR)alpha and -beta isoforms in TH action in the heart. Noninvasive echocardiographic measurements were made in mice homozygous for disruption of TRalpha (TRalpha(0/0)) or TRbeta (TRbeta(-/-)). Mice were studied at baseline, 4 wk after TH deprivation (using a low-iodine diet containing propylthiouracil), and after 4-wk treatment with TH. Baseline heart rates (HR) were similar in wild-type (WT) and TRalpha(0/0) mice but were greater in TRbeta(-/-) mice. With TH deprivation, HR decreased 49% in WT and 37% in TRbeta(-/-) mice and decreased only 5% in TRalpha(0/0) mice from baseline, whereas HR increased in all genotypes with TH treatment. Cardiac output (CO) and cardiac index (CI) in WT mice decreased (-31 and -32%, respectively) with TH deprivation and increased (+69 and +35%, respectively) with TH treatment. The effects of CO and CI were blunted with TH withdrawal in both TRalpha(0/0) (+8 and -2%, respectively) and TRbeta(-/-) mice (-17 and -18%, respectively). Treatment with TH resulted in a 64% increase in LV mass in WT and a 44% increase in TRalpha(0/0) mice but only a 6% increase in TRbeta(-/-) mice (ANOVA P < 0.05). Taken together, these data suggest that TRalpha and TRbeta play different roles in the physiology of TH action on the heart.

Autonomic control of blood pressure in mice: basic physiology and effects of genetic modification

Control of blood pressure and of blood flow is essential for maintenance of homeostasis. The hemodynamic state is adjusted by intrinsic, neural, and hormonal mechanisms to optimize adaptation to internal and environmental challenges. In the last decade, many studies showed that modification of the mouse genome may alter the capacity of cardiovascular control systems to respond to homeostatic challenges or even bring about a permanent pathophysiological state. This review discusses the progress that has been made in understanding of autonomic cardiovascular control mechanisms from studies in genetically modified mice. First, from a physiological perspective, we describe how basic hemodynamic function can be measured in conscious conditions in mice. Second, we focus on the integrative role of autonomic nerves in control of blood pressure in the mouse, and finally, we depict the opportunities and insights provided by genetic modification in this area.

A practical guide to evaluating cardiovascular, renal, and pulmonary function in mice

The development and widespread use of genetically altered mice to study the role of various proteins in biological control systems have led to a renewed interest in methodologies and approaches for evaluating physiological phenotypes. As a result, cross-disciplinary approaches have become essential for fully realizing the potential of these new and powerful animal models. The combination of classical physiological approaches and modern innovative technology has given rise to an impressive arsenal for evaluating the functional results of genetic manipulation in the mouse. This review attempts to summarize some of the techniques currently being used for measuring cardiovascular, renal, and pulmonary variables in the intact mouse, with specific attention to practical considerations useful for their successful implementation.

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

Brain glucose utilization is markedly depressed in adult rats made cretinous after birth. To ascertain which subtype of thyroid hormone (TH) receptors, TRalpha1 or TRbeta, is involved in the regulation of glucose utilization during brain development, we used the 2-[(14)C]deoxyglucose method in mice with a mutation in either their TRalpha or TRbeta gene. A C insertion produced a frameshift mutation in their carboxyl terminus. These mutants lacked TH binding and transactivation activities and exhibited potent dominant negative activity. Glucose utilization in the homozygous TRbetaPV mutant mice and their wild-type siblings was almost identical in 19 brain regions, whereas it was markedly reduced in all brain regions of the heterozygous TRalpha1PV mice. These suggest that the alpha1 receptor mediates the TH effects in brain. Inasmuch as local cerebral glucose utilization is closely related to local synaptic activity, we also examined which thyroid hormone receptor is involved in the expression of synaptotagmin-related gene 1 (Srg1), a TH-positively regulated gene involved in the formation and function of synapses [Thompson, C. C. (1996) J. Neurosci. 16, 7832-7840]. Northern analysis showed that Srg1 expression was markedly reduced in the cerebellum of TRalpha(PV/+) mice but not TRbeta(PV/PV) mice. These results show that the same receptor, TRalpha1, is involved in the regulation by TH of both glucose utilization and Srg1 expression.

Physiological and molecular basis of thyroid hormone action

Thyroid hormones (THs) play critical roles in the differentiation, growth, metabolism, and physiological function of virtually all tissues. TH binds to receptors that are ligand-regulatable transcription factors belonging to the nuclear hormone receptor superfamily. Tremendous progress has been made recently in our understanding of the molecular mechanisms that underlie TH action. In this review, we present the major advances in our knowledge of the molecular mechanisms of TH action and their implications for TH action in specific tissues, resistance to thyroid hormone syndrome, and genetically engineered mouse models.

Thyroid hormone--sympathetic interaction and adaptive thermogenesis are thyroid hormone receptor isoform--specific

In newborns and small mammals, cold-induced adaptive (or nonshivering) thermogenesis is produced primarily in brown adipose tissue (BAT). Heat production is stimulated by the sympathetic nervous system, but it has an absolute requirement for thyroid hormone. We used the thyroid hormone receptor-beta--selective (TR-beta--selective) ligand, GC-1, to determine by a pharmacological approach whether adaptive thermogenesis was TR isoform--specific. Hypothyroid mice were treated for 10 days with varying doses of T3 or GC-1. The level of uncoupling protein 1 (UCP1), the key thermogenic protein in BAT, was restored by either T3 or GC-1 treatment. However, whereas interscapular BAT in T3-treated mice showed a 3.0 degrees C elevation upon infusion of norepinephrine, indicating normal thermogenesis, the temperature did not increase (<0.5 degrees C) in GC-1--treated mice. When exposed to cold (4 degrees C), GC-1--treated mice also failed to maintain core body temperature and had reduced stimulation of BAT UCP1 mRNA, indicating impaired adrenergic responsiveness. Brown adipocytes isolated from hypothyroid mice replaced with T3, but not from those replaced with GC-1, had normal cAMP production in response to adrenergic stimulation in vitro. We conclude that two distinct thyroid-dependent pathways, stimulation of UCP1 and augmentation of adrenergic responsiveness, are mediated by different TR isoforms in the same tissue.

Genetic analysis reveals different functions for the products of the thyroid hormone receptor alpha locus

Thyroid hormone receptors are encoded by the TRalpha (NR1A1) and TRbeta (NR1A2) loci. These genes are transcribed into multiple variants whose functions are unclear. Analysis by gene inactivation in mice has provided new insights into the functional complexity of these products. Different strategies designed to modify the TRalpha locus have led to strikingly different phenotypes. In order to analyze the molecular basis for these alterations, we generated mice devoid of all known isoforms produced from the TRalpha locus (TRalpha(0/0)). These mice are viable and exhibit reduced linear growth, bone maturation delay, moderate hypothermia, and reduced thickness of the intestinal mucosa. Compounding TRalpha(0) and TRbeta(-) mutations produces viable TRalpha(0/0)beta(-/-) mice, which display a more severe linear growth reduction and a more profound hypothermia as well as impaired hearing. A striking phenotypic difference is observed between TRalpha(0/0) and the previously described TRalpha(-/-) mice, which retain truncated TRDeltaalpha isoforms arising from a newly described promoter in intron 7. The lethality and severe impairment of the intestinal maturation in TRalpha(-/-) mice are rescued in TRalpha(0/0) animals. We demonstrate that the TRDeltaalpha protein isoforms, which are natural products of the TRalpha locus, are the key determinants of these phenotypical differences. These data reveal the functional importance of the non-T3-binding variants encoded by the TRalpha locus in vertebrate postnatal development and homeostasis.

Developmental expression analysis of thyroid hormone receptor isoforms reveals new insights into their essential functions in cardiac and skeletal muscles

Nuclear thyroid hormone (TH) receptors (TR) play a critical role in mediating the diverse actions of TH in development, differentiation, and metabolism of most tissues, but the role of TR isoforms in muscle development and function is unclear. Therefore, we have undertaken a comprehensive expression analysis of TRalpha 1, TRbeta 1, TRbeta 2 (TH binding), and TRalpha 2 (non-TH binding) in functionally distinct porcine muscles during prenatal and postnatal development. Use of a novel and highly sensitive RNase protection assay revealed striking muscle-specific developmental profiles of all four TR isoform mRNAs in cardiac, longissimus, soleus, rhomboideus, and diaphragm. Distribution of TR isoforms varied markedly between muscles; TRalpha expression was considerably greater than TRbeta and there were significant differences in the ratios TRalpha 1:TRalpha 2, and TRbeta 1:TRbeta 2. Together with immunohistochemistry of myosin heavy chain isoforms and data on myogenesis and maturation of the TH axis, these findings provide new evidence that highlights central roles for 1) TRalpha isoforms in fetal myogenesis, 2) the ratio TRalpha 1:TRalpha 2 in determining cardiac and skeletal muscle phenotype and function; 3) TRbeta in maintaining a basal level of cellular response to TH throughout development and a specific maturational function around birth. These findings suggest that events disrupting normal developmental profiles of TR isoforms may impair optimal function of cardiac and skeletal muscles.

Increased sensitivity to thyroid hormone in mice with complete deficiency of thyroid hormone receptor

Only three of the four thyroid hormone receptor (TR) isoforms, alpha1, beta1, and beta2, bind thyroid hormone (TH) and are considered to be true TRs. TRalpha2 binds to TH response elements on DNA, but its role in vivo is still unknown. We produced mice completely deficient in TRalpha (TRalpha(o/o)) that maintain normal serum thyroid-stimulating hormone (TSH) concentration despite low serum thyroxine (T(4)), suggesting increased sensitivity to TH. We therefore examined the effects of TH (L-3,3',5-triiodothyronine, L-T3) given to TH-deprived and to intact TRalpha(o/o) mice. Controls were wild-type (WT) mice of the same strain and mice resistant to TH due to deficiency in TRbeta (TRbeta(-/-)). In liver, T3 produced significantly greater responses in TRalpha(o/o) and smaller responses in TRbeta(-/-) as compared with WT mice. In contrast, cardiac responses to L-T3 were absent or reduced in TRalpha(o/o), whereas they were similar in WT and TRbeta(-/-) mice, supporting the notion that TRalpha1 is the dominant TH-dependent TR isoform in heart. 5-Triiodothyronine (L-T3) given to intact mice produced a greater suppression of serum T(4) in TRalpha(o/o) than it did in WT mice and reduced by a greater amount the TSH response to TSH-releasing hormone. This is an in vivo demonstration that a TR deficiency can enhance sensitivity to TH. This effect is likely due to the abrogation of the constitutive "silencing" effect of TRalpha2 in tissues expressing the TRbeta isoforms.

Increased sensitivity to thyroid hormone in mice with complete deficiency of thyroid hormone receptor alpha

Only three of the four thyroid hormone receptor (TR) isoforms, alpha1, beta1, and beta2, bind thyroid hormone (TH) and are considered to be true TRs. TRalpha2 binds to TH response elements on DNA, but its role in vivo is still unknown. We produced mice completely deficient in TRalpha (TRalpha(o/o)) that maintain normal serum thyroid-stimulating hormone (TSH) concentration despite low serum thyroxine (T(4)), suggesting increased sensitivity to TH. We therefore examined the effects of TH (L-3,3',5-triiodothyronine, L-T3) given to TH-deprived and to intact TRalpha(o/o) mice. Controls were wild-type (WT) mice of the same strain and mice resistant to TH due to deficiency in TRbeta (TRbeta(-/-)). In liver, T3 produced significantly greater responses in TRalpha(o/o) and smaller responses in TRbeta(-/-) as compared with WT mice. In contrast, cardiac responses to L-T3 were absent or reduced in TRalpha(o/o), whereas they were similar in WT and TRbeta(-/-) mice, supporting the notion that TRalpha1 is the dominant TH-dependent TR isoform in heart. 5-Triiodothyronine (L-T3) given to intact mice produced a greater suppression of serum T(4) in TRalpha(o/o) than it did in WT mice and reduced by a greater amount the TSH response to TSH-releasing hormone. This is an in vivo demonstration that a TR deficiency can enhance sensitivity to TH. This effect is likely due to the abrogation of the constitutive "silencing" effect of TRalpha2 in tissues expressing the TRbeta isoforms.

Cloning and characterization of two novel thyroid hormone receptor beta isoforms

Thyroid hormone (T(3)) activates nuclear receptor transcription factors, encoded by the TRalpha (NR1A1) and TRbeta (NR1A2) genes, to regulate target gene expression. Several TR isoforms exist, and studies of null mice have identified some unique functions for individual TR variants, although considerable redundancy occurs, raising questions about the specificity of T(3) action. Thus, it is not known how diverse T(3) actions are regulated in target tissues that express multiple receptor variants. I have identified two novel TRbeta isoforms that are expressed widely and result from alternative mRNA splicing. TRbeta3 is a 44.6-kDa protein that contains an unique 23-amino-acid N terminus and acts as a functional receptor. TRDeltabeta3 is a 32.8-kDa protein that lacks a DNA binding domain but retains ligand binding activity and is a potent dominant-negative antagonist. The relative concentrations of beta3 and Deltabeta3 mRNAs vary between tissues and with changes in thyroid status, indicating that alternative splicing is tissue specific and T(3) regulated. These data provide novel insights into the mechanisms of T(3) action and define a new level of specificity that may regulate thyroid status in tissue.

Analysis of organ physiology in transgenic mice

The increasing availability of transgenic mouse models of gene deletion and human disease has mandated the development of creative approaches to characterize mouse phenotype. The mouse presents unique challenges to phenotype analysis because of its small size, habits, and inability to verbalize clinical symptoms. This review describes strategies to study mouse organ physiology, focusing on the cardiovascular, pulmonary, renal, gastrointestinal, and neurobehavioral systems. General concerns about evaluating mouse phenotype studies are discussed. Monitoring and anesthesia methods are reviewed, with emphasis on the feasibility and limitations of noninvasive and invasive procedures to monitor physiological parameters, do cannulations, and perform surgical procedures. Examples of phenotype studies are cited to demonstrate the practical applications and limitations of the measurement methods. The repertoire of phenotype analysis methods reviewed here should be useful to investigators involved in or contemplating the use of mouse models.

Functions of thyroid hormone receptors in mice

Thyroid hormone receptors (TRs) play a central role in mediating the actions of thyroid hormone in development and homeostasis in vertebrate species. The TRs are nuclear receptors that act as ligand-regulated transcription factors. There are two TR genes (TRalpha and TRbeta), each capable of generating different variant products, suggesting a potentially complex array of TR pathways. Targeted mutagenesis in the mouse has indicated that there are specific individual functions for the TR genes in vivo. The deletion of combinations of TRalpha and TRbeta variants has revealed that additional functions are convergently regulated by both TR genes and indicates that control of an extended range of functions is facilitated by a network of specific and common TR pathways. The TR-deficient mouse models have allowed investigation of the TR pathways underlying many functions of thyroid hormone and provide a unique perspective on receptor-mediated mechanisms of biological control.

Tissue-specific actions of thyroid hormone: insights from animal models

The major developmental targets for thyroid hormone are the brain, small intestine, and bone. Clear defects in gene regulation and tissue function as a consequence of TR gene inactivation can additionally be shown in the pituitary, hypothalamus, heart, and liver. TR gene knockout models show a clear distinction between thyroid hormone requirements for development and those that are required for functions in the adult animal. T3-mediated gene repression appears especially important in a number of tissues including brain, pituitary, and the heart. Preliminary evaluation of the combined TR knockout models suggests that hypothyroidism is associated with more significant abnormalities than receptor deficiency, indicating that the repressive action of the unliganded receptor may have physiological relevance. These various animal models should be very useful to design and test thyroid hormone analogues to selectively stimulate desired thyroid hormone actions.

Mice devoid of all known thyroid hormone receptors are viable but exhibit disorders of the pituitary-thyroid axis, growth, and bone maturation

Thyroid hormone (T3) has widespread functions in development and homeostasis, although the receptor pathways by which this diversity arises are unclear. Deletion of the T3 receptors TRalpha1 or TRbeta individually reveals only a small proportion of the phenotypes that arise in hypothyroidism, implying that additional pathways must exist. Here, we demonstrate that mice lacking both TRalpha1 and TRbeta (TRalpha1(-/-)beta-/-) display a novel array of phenotypes not found in single receptor-deficient mice, including an extremely hyperactive pituitary-thyroid axis, poor female fertility and retarded growth and bone maturation. These results establish that major T3 actions are mediated by common pathways in which TRalpha1 and TRbeta cooperate with or substitute for each other. Thus, varying the balance of use of TRalpha1 and TRbeta individually or in combination facilitates control of an extended spectrum of T3 actions. There was no evidence for any previously unidentified T3 receptors in TRalpha1(-/-)beta-/- mouse tissues. Compared to the debilitating symptoms of severe hypothyroidism, the milder overall phenotype of TRalpha1(-/-)beta-/- mice, lacking all known T3 receptors, indicates divergent consequences for hormone versus receptor deficiency. These distinctions suggest that T3-independent actions of T3 receptors, demonstrated previously in vitro, may be a significant function in vivo.