Thyroid hormone receptor α mutation causes a severe and thyroxine-resistant skeletal dysplasia in female mice

TRα1 mutant suppresses KLF9 to cause endometrial metaplasia with ectopic IL-33 expression leading to uterine fibrosis and infertility

Thyroid hormone receptors (TRs) mediate the genomic actions of thyroid hormone. Mutations of THRA gene cause a human disease known as resistance to thyroid hormone (RTHα). We created a mouse model expressing a dominant negative mutated TRα1 (Thra1PV/+ mice) that exhibits growth retardation, bone abnormalities, constipation, and anemia, as found in RTHα patients. In addition, female Thra1PV/+ mice exhibit decreased fertility. In the present study, we aimed to characterize the molecular events leading to infertility. Histologically, there was progressive uterine atrophy in Thra1PV/+ mutant mice, characterized by squamous metaplasia of the endometrial mucosa and endometrial fibrosis. RNA-seq analysis of laser-captured micro-dissected endometrium and spatial transcriptomics revealed a key role for Krüppel-like factor (Klf9), a directly-regulated TR target gene, in normal endometrial differentiation. Klf9 was suppressed in the endometrium of mice harboring mutated TRα1 and pathway analysis revealed that deficient Klf9 signaling was associated with squamous differentiation, consistent with the endometrial metaplasia observed histologically. Further, we showed that this metaplastic endometrial mucosa was the source of ectopic IL-33, which was associated with increased T-cell infiltrates, destruction of glands, and endometrial fibrosis. Our studies provide new insights to understand uterine epithelial morphogenesis and how thyroid dysfunction could lead to female infertility.

International Union of Basic and Clinical Pharmacology CXIII: Nuclear Receptor Superfamily-Update 2023

The NR superfamily comprises 48 transcription factors in humans that control a plethora of gene network programs involved in a wide range of physiologic processes. This review will summarize and discuss recent progress in NR biology and drug development derived from integrating various approaches, including biophysical techniques, structural studies, and translational investigation. We also highlight how defective NR signaling results in various diseases and disorders and how NRs can be targeted for therapeutic intervention via modulation via binding to synthetic lipophilic ligands. Furthermore, we also review recent studies that improved our understanding of NR structure and signaling. SIGNIFICANCE STATEMENT: Nuclear receptors (NRs) are ligand-regulated transcription factors that are critical regulators of myriad physiological processes. NRs serve as receptors for an array of drugs, and in this review, we provide an update on recent research into the roles of these drug targets.

Long-term follow-up results and treatment outcomes of children and adults with resistance to thyroid hormone alpha

PURPOSE

Resistance to thyroid hormone alpha (RTHα) is a rare entity and has no specific treatment. To date, mostly levothyroxine has been used, but there is a lack of knowledge about the long-term outcomes of this treatment. We aimed to evaluate the long-term follow-up results and treatment outcomes of children and their parents diagnosed with RTHα.

METHODS

Four children [the median (minimum-maximum) age at diagnosis, 4.5 (1.4-9.5) years] and three adults [age at diagnosis, 31.7 (28.0-35.3) years] from two families were included in the study, who had RTHα and followed up between 2014 and 2021.

RESULTS

The median duration of treatment was 6.7 (5.9-8.0) years, and the levothyroxine dose at the final visit was 1.4 (1.2-2.2) and 1.9 (1.2-2.4) mcg/kg/day for adults and pediatric patients, respectively. Treatment ameliorated constipation in all patients with this complaint (n = 5). Normal mental functions were achieved and IQ scores improved in most children except one (age at diagnosis, 9.5 years). At the final visit, creatine kinase levels relative to the reference upper limit were significantly lower compared to the pre-treatment ratios [0.9 (0.2-1.3) vs. 1.3 (0.5-1.6), p = 0.028]. Anemia was present in five patients at diagnosis, which resolved in one adult patient but occurred in one child despite treatment (p = 0.999). A minimal pericardial effusion persisted in one pediatric patient.

CONCLUSIONS

We demonstrated that constipation was ameliorated, neuromotor development of some children was improved, and creatine kinase levels were diminished with levothyroxine treatment in patients with RTHα, while some features including anemia did not resolve.

Sfrp4 expression in thyroxine treated calvarial cells

AIMS

Evidence suggests alterations of thyroid hormone levels can disrupt normal bone development. Most data suggest the major targets of thyroid hormones to be the Htra1/Igf1 pathway. Recent discovery by our group suggests involvement of targets WNT pathway, specifically overexpression of antagonist Sfrp4 in the presence of exogenous thyroid hormone.

MAIN METHODS

Here we aimed to model these interactions in vitro using primary and isotype cell lines to determine if thyroid hormone drives increased Sfrp4 expression in cells relevant to craniofacial development. Transcriptional profiling, bioinformatics interrogation, protein and function analyses were used.

KEY FINDINGS

Affymetrix transcriptional profiling found Sfrp4 overexpression in primary cranial suture derived cells stimulated with thyroxine in vitro. Interrogation of the SFRP4 promoter identified multiple putative binding sites for thyroid hormone receptors. Experimentation with several cell lines demonstrated that thyroxine treatment induced Sfrp4 expression, demonstrating that Sfrp4 mRNA and protein levels are not tightly coupled. Transcriptional and protein analyses demonstrate thyroid hormone receptor binding to the proximal promoter of the target gene Sfrp4 in murine calvarial pre-osteoblasts. Functional analysis after thyroxine hormone stimulation for alkaline phosphatase activity shows that pre-osteoblasts increase alkaline phosphatase activity compared to other cell types, suggesting cell type susceptibility. Finally, we added recombinant SFRP4 to pre-osteoblasts in combination with thyroxine treatment and observed a significant decrease in alkaline phosphatase positivity.

SIGNIFICANCE

Taken together, these results suggest SFRP4 may be a key regulatory molecule that prevents thyroxine driven osteogenesis. These data corroborate clinical findings indicating a potential for SFRP4 as a diagnostic or therapeutic target for hyperostotic craniofacial disorders.

Human pluripotent stem cell-derived cartilaginous organoids promote scaffold-free healing of critical size long bone defects

Background

Bones have a remarkable capacity to heal upon fracture. Yet, in large defects or compromised conditions healing processes become impaired, resulting in delayed or non-union. Current therapeutic approaches often utilize autologous or allogeneic bone grafts for bone augmentation. However, limited availability of these tissues and lack of predictive biological response result in limitations for clinical demands. Tissue engineering using viable cell-based implants is a strategic approach to address these unmet medical needs.

Methods

Herein, the in vitro and in vivo cartilage and bone tissue formation potencies of human pluripotent stem cells were investigated. The induced pluripotent stem cells were specified towards the mesodermal lineage and differentiated towards chondrocytes, which subsequently self-assembled into cartilaginous organoids. The tissue formation capacity of these organoids was then challenged in an ectopic and orthotopic bone formation model.

Results

The derived chondrocytes expressed similar levels of collagen type II as primary human articular chondrocytes and produced stable cartilage when implanted ectopically in vivo. Upon targeted promotion towards hypertrophy and priming with a proinflammatory mediator, the organoids mediated successful bridging of critical size long bone defects in immunocompromised mice.

Conclusions

These results highlight the promise of induced pluripotent stem cell technology for the creation of functional cartilage tissue intermediates that can be explored for novel bone healing strategies.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-021-02580-7.

Thyroid Hormone Receptor α Regulates Autophagy, Mitochondrial Biogenesis, and Fatty Acid Use in Skeletal Muscle

Skeletal muscle (SM) weakness occurs in hypothyroidism and resistance to thyroid hormone α (RTHα) syndrome. However, the cell signaling and molecular mechanism(s) underlying muscle weakness under these conditions is not well understood. We thus examined the role of thyroid hormone receptor α (TRα), the predominant TR isoform in SM, on autophagy, mitochondrial biogenesis, and metabolism to demonstrate the molecular mechanism(s) underlying muscle weakness in these two conditions. Two genetic mouse models were used in this study: TRα1PV/+ mice, which express the mutant Thra1PV gene ubiquitously, and SM-TRα1L400R/+ mice, which express TRα1L400R in a muscle-specific manner. Gastrocnemius muscle from TRα1PV/+, SM-TRα1L400R/+, and their control mice was harvested for analyses. We demonstrated that loss of TRα1 signaling in gastrocnemius muscle from both the genetic mouse models led to decreased autophagy as evidenced by accumulation of p62 and decreased expression of lysosomal markers (lysosomal-associated membrane protein [LAMP]-1 and LAMP-2) and lysosomal proteases (cathepsin B and cathepsin D). The expression of peroxisome proliferator-activated receptor γ coactivator 1α (PGC1α), mitochondrial transcription factor A (TFAM), and estrogen-related receptor α (ERRα), key factors contributing to mitochondrial biogenesis as well as mitochondrial proteins, were decreased, suggesting that there was reduced mitochondrial biogenesis due to the expression of mutant TRα1. Transcriptomic and metabolomic analyses of SM suggested that lipid catabolism was impaired and was associated with decreased acylcarnitines and tricarboxylic acid cycle intermediates in the SM from the mouse line expressing SM-specific mutant TRα1. Our results provide new insight into TRα1-mediated cell signaling, molecular, and metabolic changes that occur in SM when TR action is impaired.

Generation and Characterization of a New Resistance to Thyroid Hormone Mouse Model with Thyroid Hormone Receptor Alpha Gene Mutation

Background: In humans, resistance to thyroid hormone (RTH) caused by mutations in the thyroid hormone receptor alpha (THRA) gene, RTHα, manifests as tissue-specific hypothyroidism and circulating thyroid hormone levels exhibit hypothyroid-like clinical features. Before the identification of patients with RTHα, several Thrα1 knock-in mouse models were generated to clarify the function of TRα1. However, the phenotypes of these mice were not consistent with the clinical presentation of RTHα in humans. For the present study, we generated an RTHα mouse model that carries the Thra1E403X mutation found in human RTHα patients. Here, we report the gross phenotypes of this mouse RTHα model. Methods: Traditional homologous recombination gene targeting techniques were used to introduce a mutation (Thra1E403X) in the mouse Thra gene. The phenotypes of the resulting mice were studied and compared with clinical features observed for RTHα with THRAE403X. Results: Thrα1E403X/E403X homozygous mice exhibited severe neurological phenotypes, such as spasticity and motor ataxia, which were similar to those observed in endemic cretinism. Thrα1E403X/+ heterozygous mice reproduced most clinical manifestations of patient with RTHα, such as a normal survival rate and male fertility, as well as delayed postnatal growth and development, neurological and motor coordination deficits, and anemia. The mice had typical thyroid function with a modest increase in serum triiodothyronine (T3) levels, a low thyroxine (T4)/T3 ratio, and low reverse T3 (rT3) levels. Conclusions: The Thrα1E403X/+ mice faithfully recapitulate the clinical features of human RTHα and thus can provide a useful tool to dissect the role of TRα1 in development and to determine the pathological mechanisms of RTHα.

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

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

A trans-eQTL network regulates osteoclast multinucleation and bone mass

Functional characterisation of cell-type-specific regulatory networks is key to establish a causal link between genetic variation and phenotype. The osteoclast offers a unique model for interrogating the contribution of co-regulated genes to in vivo phenotype as its multinucleation and resorption activities determine quantifiable skeletal traits. Here we took advantage of a trans-regulated gene network (MMnet, macrophage multinucleation network) which we found to be significantly enriched for GWAS variants associated with bone-related phenotypes. We found that the network hub gene Bcat1 and seven other co-regulated MMnet genes out of 13, regulate bone function. Specifically, global (Pik3cb^-/-, Atp8b2^+/-, Igsf8^-/-, Eml1^-/-, Appl2^-/-, Deptor^-/-) and myeloid-specific Slc40a1 knockout mice displayed abnormal bone phenotypes. We report opposing effects of MMnet genes on bone mass in mice and osteoclast multinucleation/resorption in humans with strong correlation between the two. These results identify MMnet as a functionally conserved network that regulates osteoclast multinucleation and bone mass.

Noncanonical Action of Thyroid Hormone Receptors α and β

Thyroid hormone (TH) is essential for the regulation of many physiological processes, especially growth, organ development, energy metabolism and cardiovascular effects. TH acts via the TH receptors (TR) α and β. By binding to thyroid hormone responsive elements (TREs) on the DNA, TRs regulate expression of TH target genes. Thus, TRs are mainly characterized as ligand dependent transcription factors and regulation of gene expression and protein synthesis is considered the canonical mode of TH/TR action. The demonstration that the ligand-bound TRs α and β also mediate activation of the phosphatidylinositol-3-kinase (PI3K) pathway established noncanonical TH/TR action as an additional mode of TH signaling. Recently, TR mutant mouse models allowed to determine the underlying mode of TH/TR action, either canonical or noncanonical TH/TR signaling, for several physiological TH effects in vivo: Regulation of the hypothalamic-pituitary-thyroid axis requires DNA-binding of TRβ, whereas hepatic triglyceride content appears to be regulated by noncanonical TRβ signaling. TRα mediated effects in bone development are dependent on DNA-binding, whereas several cardiovascular TRα effects are rapid and independent from DNA-binding. Therefore, noncanonical TH/TR action contributes to the overall effects of TH in physiology.

Role of thyroid hormones in craniofacial development

The development of the craniofacial skeleton relies on complex temporospatial organization of diverse cell types by key signalling molecules. Even minor disruptions to these processes can result in deleterious consequences for the structure and function of the skull. Thyroid hormone deficiency causes delayed craniofacial and tooth development, dysplastic facial features and delayed development of the ossicles in the middle ear. Thyroid hormone excess, by contrast, accelerates development of the skull and, in severe cases, might lead to craniosynostosis with neurological sequelae and facial hypoplasia. The pathogenesis of these important abnormalities remains poorly understood and underinvestigated. The orchestration of craniofacial development and regulation of suture and synchondrosis growth is dependent on several critical signalling pathways. The underlying mechanisms by which these key pathways regulate craniofacial growth and maturation are largely unclear, but studies of single-gene disorders resulting in craniofacial malformations have identified a number of critical signalling molecules and receptors. The craniofacial consequences resulting from gain-of-function and loss-of-function mutations affecting insulin-like growth factor 1, fibroblast growth factor receptor and WNT signalling are similar to the effects of altered thyroid status and mutations affecting thyroid hormone action, suggesting that these critical pathways interact in the regulation of craniofacial development.

Generation of Novel Genetic Models to Dissect Resistance to Thyroid Hormone Receptor α in Zebrafish

Background: Patients with mutations of the thyroid hormone receptor alpha (THRA) gene show resistance to thyroid hormone alpha (RTHα). No amendable mouse models are currently available to elucidate deleterious effects of TRα1 mutants during early development. Zebrafish with transient suppressed expression by morpholino knockdown and ectopic expression of TRα1 mutants in the embryos have been reported. However, zebrafish with germline transmittable mutations have not been reported. The stable expression of thra mutants from embryos to adulthood facilitated the study of molecular actions of TRα1 mutants during development. Methods: In contrast to human and mice, the thra gene is duplicated in zebrafish, thraa, and thrab. Using CRISPR/Cas9-mediated targeted mutagenesis, we created dominant negative mutations in the two duplicated thra genes. We comprehensively analyzed the molecular and phenotypic characteristics of mutant fish during development. Results: Adult and juvenile homozygous thrab 1-bp ins (m/m) mutants exhibited severe growth retardation, but adult homozygous thraa 8-bp ins (m/m) mutants had very mild growth impairment. Expression of the growth hormone (gh1) and insulin-like growth factor 1 was markedly suppressed in homozygous thrab 1-bp ins (m/m) mutants. Decreased messenger RNA and protein levels of triiodothyronine-regulated keratin genes and inhibited keratinocyte proliferation resulted in hypoplasia of the epidermis in adult and juvenile homozygous thrab 1-bp ins (m/m) mutants, but not homozygous thraa 8-bp ins (m/m) mutants. RNA-seq analysis showed that homozygous thrab 1-bp ins (m/m) mutation had global impact on the functions of the adult pituitary. However, no morphological defects nor any changes in the expression of gh1 and keratin genes were observed in the embryos and early larvae. Thus, mutations of either the thraa or thrab gene did not affect initiation of embryogenesis. But the mutation of the thrab gene, but not the thraa gene, is detrimental in postlarval growth and skin development. Conclusions: The thra duplicated genes are essential to control temporal coordination in postlarval growth and development in a tissue-specific manner. We uncovered novel functions of the duplicated thra genes in zebrafish in development. These mutant zebrafish could be used as a model for further analysis of TRα1 mutant actions and for rapid screening of therapeutics for RTHα.

Andersen-Tawil Syndrome and Hypothyroidism: A Case Report with an Unusual Association

Andersen-Tawil syndrome (ATS) is an autosomal dominant disorder, characterized by the triad of muscular paralysis, skeletal, and craniofacial anomalies and prolonged QT interval on echocardiogram with a tendency toward malignant ventricular arrhythmia. Although the patient may express one or two of the three components of triad, hypothyroidism is an endocrine disorder resulting in the delayed eruption of teeth, defective mineralization of bone and teeth, and speech and hearing deformity. Here, we report a case of ATS with hypothyroidism. To the best of authors' knowledge, no such association has been reported in the literature.

Genetic and Pharmacological Targeting of Transcriptional Repression in Resistance to Thyroid Hormone Alpha

Background: Thyroid hormones act in bone and cartilage via thyroid hormone receptor alpha (TRα). In the absence of triiodothyronine (T3), TRα interacts with co-repressors, including nuclear receptor co-repressor-1 (NCoR1), which recruit histone deacetylases (HDACs) and mediate transcriptional repression. Dominant-negative mutations of TRα cause resistance to thyroid hormone alpha (RTHα; OMIM 614450), characterized by excessive repression of T3 target genes leading to delayed skeletal development, growth retardation, and bone dysplasia. Treatment with thyroxine has been of limited benefit, even in mildly affected individuals, and there is a need for new therapeutic strategies. It was hypothesized that (i) the skeletal manifestations of RTHα are mediated by the persistent TRα/NCoR1/HDAC repressor complex containing mutant TRα, and (ii) treatment with the HDAC inhibitor suberoylanilide hydroxamic acid (SAHA) would ameliorate these manifestations. Methods: The skeletal phenotypes of (i) Thra1^PV/+ mice, a well characterized model of RTHα; (ii) Ncor1^ΔID/ΔID mice, which express an NCoR1 mutant that fails to interact with TRα; and (iii) Thra1^PV/+Ncor1^ΔID/ΔID double-mutant adult mice were determined. Wild-type, Thra1^PV/+, Ncor1^ΔID/ΔID, and Thra1^PV/+Ncor1^ΔID/ΔID double-mutant mice were also treated with SAHA to determine whether HDAC inhibition results in amelioration of skeletal abnormalities. Results:Thra1^PV/+ mice had a severe skeletal dysplasia, characterized by short stature, abnormal bone morphology, and increased bone mineral content. Despite normal bone length, Ncor1^ΔID/ΔID mice displayed increased cortical bone mass, mineralization, and strength. Thra1^PV/+Ncor1^ΔID/ΔID double-mutant mice displayed only a small improvement of skeletal abnormalities compared to Thra1^PV/+ mice. Treatment with SAHA to inhibit histone deacetylation had no beneficial or detrimental effects on bone structure, mineralization, or strength in wild-type or mutant mice. Conclusions: These studies indicate treatment with SAHA is unlikely to improve the skeletal manifestations of RTHα. Nevertheless, the findings (i) confirm that TRα1 has a critical role in the regulation of skeletal development and adult bone mass, (ii) suggest a physiological role for alternative co-repressors that interact with TR in skeletal cells, and (iii) demonstrate a novel role for NCoR1 in the regulation of adult bone mass and strength.

Thyroid Hormone Receptor Alpha Mutations Lead to Epithelial Defects in the Adult Intestine in a Mouse Model of Resistance to Thyroid Hormone

BACKGROUND

The thyroid hormone triiodothyronine (T3) is critical for vertebrate development and affects the function of many adult tissues and organs. Its genomic effects are mediated by thyroid hormone nuclear receptors (TRs) present in all vertebrates. The discovery of patients with resistance to thyroid hormone (RTHβ) >50 years ago and subsequent identification of genetic mutations in only the THRB gene in these patients suggest that mutations in the THRA gene may have different pathological manifestations in humans. Indeed, the recent discovery of a number of human patients carrying heterozygous mutations in the THRA gene (RTHα) revealed a distinct phenotype that was not observed in RTH patients with THRB gene mutations (RTHβ). That is, RTHα patients have constipation, implicating intestinal defects caused by THRA gene mutations.

METHODS

To determine how TRα1 mutations affect the intestine, this study analyzed a mutant mouse expressing a strong dominantly negative TRα1 mutant (denoted TRα1PV; Thra1^PV mice). This mutant mouse faithfully reproduces RTHα phenotypes observed in patients.

RESULTS

In adult Thra1^PV/+ mice, constipation was observed just like in patients with TRα mutations. Importantly, significant intestinal defects were discovered, including shorter villi and increased differentiated cells in the crypt, accompanied by reduced stem-cell proliferation in the intestine.

CONCLUSIONS

The findings suggest that further analysis of this mouse model should help to reveal the molecular and physiological defects in the intestine caused by TRα mutations and to determine the underlying mechanisms.

Analysis of Physiological Responses to Thyroid Hormones and Their Receptors in Bone

Thyroid hormone has profound effects on skeletal development and adult bone maintenance. Here, we review the current literature concerning thyroid hormone action in bone and cartilage in relation to human disease and animal models. We describe state-of-the-art imaging and biomechanical methods used to determine structural and functional parameters in the skeletal phenotyping of mouse models.

Thyroid Hormone and Skeletal Development

Thyroid hormone (TH) is essential for skeletal development from the late fetal life to the onset of puberty. During this large window of actions, TH has key roles in endochondral and intramembranous ossifications and in the longitudinal bone growth. There is evidence that TH acts directly in skeletal cells but also indirectly, specially via the growth hormone/insulin-like growth factor-1 axis, to control the linear skeletal growth and maturation. The presence of receptors, plasma membrane transporters, and activating and inactivating enzymes of TH in skeletal cells suggests that direct actions of TH in these cells are crucial for skeletal development, which has been confirmed by several in vitro and in vivo studies, including mouse genetic studies, and clinical studies in patients with resistance to thyroid hormone due to dominant-negative mutations in TH receptors. This review examines progress made on understanding the mechanisms by which TH regulates the skeletal development.

Thyroid diseases and bone health

Thyroid hormones are essential for skeletal development and are important regulators of bone maintenance in adults. Childhood hypothyroidism causes delayed skeletal development, retarded linear growth and impaired bone mineral accrual. Epiphyseal dysgenesis is evidenced by classic features of stippled epiphyses on X-ray. In severe cases, post-natal growth arrest results in a complex skeletal dysplasia. Thyroid hormone replacement stimulates catch-up growth and bone maturation, but recovery may be incomplete dependent on the duration and severity of hypothyroidism prior to treatment. A severe phenotype characteristic of hypothyroidism occurs in children with resistance to thyroid hormone due to mutations affecting THRA encoding thyroid hormone receptor α (TRα). Discovery of this rare condition recapitulated animal studies demonstrating that TRα mediates thyroid hormone action in the skeleton. In adults, thyrotoxicosis is well known to cause severe osteoporosis and fracture, but cases are rare because of prompt diagnosis and treatment. Recent data, however, indicate that subclinical hyperthyroidism is associated with low bone mineral density (BMD) and an increased risk of fracture. Population studies have also shown that variation in thyroid status within the reference range in post-menopausal women is associated with altered BMD and fracture risk. Thus, thyroid status at the upper end of the euthyroid reference range is associated with low BMD and increased risk of osteoporotic fragility fracture. Overall, extensive data demonstrate that euthyroid status is required for normal post-natal growth and bone mineral accrual, and is fundamental for maintenance of adult bone structure and strength.

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.

NCOR1 modulates erythroid disorders caused by mutations of thyroid hormone receptor α1

Thyroid hormone receptor α (THRA) gene mutations, via dominant negative mode, cause erythroid abnormalities in patients. Using mice expressing a dominant negative TRα1 mutant (TRα1PV; Thra1^PV/+ mice), we showed that TRα1PV acted directly to suppress the expression of key erythroid genes, causing erythroid defects. The nuclear receptor corepressor 1 (NCOR1) was reported to mediate the dominant negative effects of mutated TRα1. However, how NCOR1 could regulate TRα1 mutants in erythroid defects in vivo is not known. In the present study, we crossed Thra1^PV/+ mice with mice expressing a mutant Ncor1 allele (NCOR1ΔID; Ncor1^ΔID mice). TRα1PV mutant cannot bind to NCOR1ΔID. The expression of NCOR1ΔID ameliorated abnormalities in the peripheral blood indices, and corrected the defective differentiation potential of progenitors in the erythroid lineage. The defective terminal erythropoiesis of lineage-negative bone marrow cells of Thra1^PV/+ mice was rescued by the expression of NCOR1ΔID. De-repression of key erythroid genes in Thra1^PV/+Ncor1^ΔID/ΔID mice led to partial rescue of terminal erythroid differentiation. These results indicate that the inability of TRα1PV to recruit NCOR1ΔID to form a repressor complex relieved the deleterious actions of TRα1 mutants in vivo. NCOR1 is a critical novel regulator underpining the pathogenesis of erythroid abnormalities caused by TRα1 mutants.

Noncanonical thyroid hormone signaling mediates cardiometabolic effects in vivo

Thyroid hormone (TH) and TH receptors (TRs) α and β act by binding to TH response elements (TREs) in regulatory regions of target genes. This nuclear signaling is established as the canonical or type 1 pathway for TH action. Nevertheless, TRs also rapidly activate intracellular second-messenger signaling pathways independently of gene expression (noncanonical or type 3 TR signaling). To test the physiological relevance of noncanonical TR signaling, we generated knockin mice with a mutation in the TR DNA-binding domain that abrogates binding to DNA and leads to complete loss of canonical TH action. We show that several important physiological TH effects are preserved despite the disruption of DNA binding of TRα and TRβ, most notably heart rate, body temperature, blood glucose, and triglyceride concentration, all of which were regulated by noncanonical TR signaling. Additionally, we confirm that TRE-binding-defective TRβ leads to disruption of the hypothalamic-pituitary-thyroid axis with resistance to TH, while mutation of TRα causes a severe delay in skeletal development, thus demonstrating tissue- and TR isoform-specific canonical signaling. These findings provide in vivo evidence that noncanonical TR signaling exerts physiologically important cardiometabolic effects that are distinct from canonical actions. These data challenge the current paradigm that in vivo physiological TH action is mediated exclusively via regulation of gene transcription at the nuclear level.

Increased circulating interleukin-8 in patients with resistance to thyroid hormone receptor α

Innate immune cells have recently been identified as novel thyroid hormone (TH) target cells in which intracellular TH levels appear to play an important functional role. The possible involvement of TH receptor alpha (TRα), which is the predominant TR in these cells, has not been studied to date. Studies in TRα^0/0 mice suggest a role for this receptor in innate immune function. The aim of this study was to determine whether TRα affects the human innate immune response. We assessed circulating interleukin-8 concentrations in a cohort of 8 patients with resistance to TH due to a mutation of TRα (RTHα) and compared these results to healthy controls. In addition, we measured neutrophil and macrophage function in one of these RTHα patients (mutation D211G). Circulating interleukin-8 levels were elevated in 7 out of 8 RTHα patients compared to controls. These patients harbor different mutations, suggesting that this is a general feature of the syndrome of RTHα. Neutrophil spontaneous apoptosis, bacterial killing, NAPDH oxidase activity and chemotaxis were unaltered in cells derived from the RTHαD211G patient. RTHα macrophage phagocytosis and cytokine induction after LPS treatment were similar to results from control cells. The D211G mutation did not result in clinically relevant impairment of neutrophil or pro-inflammatory macrophage function. As elevated circulating IL-8 is also observed in hyperthyroidism, this observation could be due to the high-normal to high levels of circulating T₃ found in patients with RTHα.

Defective erythropoiesis caused by mutations of the thyroid hormone receptor α gene

Patients with mutations of the THRA gene exhibit classical features of hypothyroidism, including erythroid disorders. We previously created a mutant mouse expressing a mutated TRα1 (denoted as PV; Thra1^PV/+ mouse) that faithfully reproduces the classical hypothyroidism seen in patients. Using Thra1^PV/+ mice, we explored how the TRα1PV mutant acted to cause abnormalities in erythropoiesis. Thra1^PV/+ mice exhibited abnormal red blood cell indices similarly as reported for patients. The total bone marrow cells and erythrocytic progenitors were markedly reduced in the bone marrow of Thra1^PV/+ mice. In vitro terminal differentiation assays showed a significant reduction of mature erythrocytes in Thra1^PV/+ mice. In wild-type mice, the clonogenic potential of progenitors in the erythrocytic lineage was stimulated by thyroid hormone (T3), suggesting that T3 could directly accelerate the differentiation of progenitors to mature erythrocytes. Analysis of gene expression profiles showed that the key regulator of erythropoiesis, the Gata-1 gene, and its regulated genes, such as the Klf1, β-globin, dematin genes, CAII, band3 and eALAS genes, involved in the maturation of erythrocytes, was decreased in the bone marrow cells of Thra1^PV/+ mice. We further elucidated that the Gata-1 gene was a T3-directly regulated gene and that TRα1PV could impair erythropoiesis via repression of the Gata-1 gene and its regulated genes. These results provide new insights into how TRα1 mutants acted to cause erythroid abnormalities in patients with mutations of the THRA gene. Importantly, the Thra1^PV/+ mouse could serve as a preclinical mouse model to identify novel molecular targets for treatment of erythroid disorders.

Patients with mutations of the THRA gene exhibit erythroid disorders. The molecular pathogenesis underlying erythroid abnormalities is poorly understood. In Thra1^PV/+ mice expressing a dominant negative mutant TRα1PV, we found abnormal red blood cell indices similar to patients. Total bone marrow cells, the clonogenic potential of erythrocytic progenitors, and terminal differentiation of erythrocytes were markedly decreased in Thra1^PV/+ mice. We elucidated that Gata-1, a key erythroid gene, was directly positively regulated by TRα1. The erythroid defects in Thra1^PV/+ mice were due, at least partly, to the TRα1PV-mediated suppression of the Gata-1 gene and its down-stream target genes. Over-expression of Gata-1 rescued impaired terminal differentiation. Our studies elucidated molecular mechanisms by which TRα1 mutants caused erythroid disorders in patients. The present study suggests that therapies aimed at GATA1 could be tested as a potential target in treating erythroid abnormalities in patients.

An Essential Physiological Role for MCT8 in Bone in Male Mice

T3 is an important regulator of skeletal development and adult bone maintenance. Thyroid hormone action requires efficient transport of T4 and T3 into target cells. We hypothesized that monocarboxylate transporter (MCT) 8, encoded by Mct8 on the X-chromosome, is an essential thyroid hormone transporter in bone. To test this hypothesis, we determined the juvenile and adult skeletal phenotypes of male Mct8 knockout mice (Mct8KO) and Mct8D1D2KO compound mutants, which additionally lack the ability to convert the prohormone T4 to the active hormone T3. Prenatal skeletal development was normal in both Mct8KO and Mct8D1D2KO mice, whereas postnatal endochondral ossification and linear growth were delayed in both Mct8KO and Mct8D1D2KO mice. Furthermore, bone mass and mineralization were decreased in adult Mct8KO and Mct8D1D2KO mice, and compound mutants also had reduced bone strength. Delayed bone development and maturation in Mct8KO and Mct8D1D2KO mice is consistent with decreased thyroid hormone action in growth plate chondrocytes despite elevated serum T3 concentrations, whereas low bone mass and osteoporosis reflects increased thyroid hormone action in adult bone due to elevated systemic T3 levels. These studies identify an essential physiological requirement for MCT8 in chondrocytes, and demonstrate a role for additional transporters in other skeletal cells during adult bone maintenance.

Thyroid Function during Early Life and Dental Development

Children with low levels of thyroid hormones (hypothyroidism) have delayed tooth eruption, enamel hypoplasia, micrognathia, and anterior open bite, whereas children with hyperthyroidism may suffer from accelerated tooth eruption, maxillary, and mandibular osteoporosis. However, it is still unknown whether thyroid function variations within the normal or subclinical range also have an impact on hard dental tissues in healthy children. The objective of this study was, therefore, to investigate the association between thyroid function from the fetal period until early childhood and dental development at school age. This study is embedded in the Generation R Study, a population-based cohort study established in Rotterdam, the Netherlands. Maternal thyroid function (thyroid-stimulating hormone [TSH], free thyroxine [FT4], and thyroid peroxidase antibody [TPOAb] concentrations) was measured during early pregnancy, and thyroid function of the offspring (TSH and FT4) was measured in cord blood at birth and in early childhood (6 y). Dental development was assessed from panoramic radiographs of children of school-going age (9 y). In total, 2,387 to 2,706 subjects were available for the multivariable linear regression analysis, depending on the point in time of thyroid function measurement. There was an inverse association between cord blood and early childhood TSH concentrations with dental development, with a -0.06 lower standard deviation (SD) per 1 mU/L of TSH (95% confidence interval [CI], -0.11 to -0.01) and a -0.06 lower SD per 1 mU/L of TSH (95% CI, -0.11 to 0.00), respectively. There was no association between the maternal thyroid function during pregnancy and the dental development score of the child. However, TPOAb-positive mothers had children with a -0.20 SD (adjusted 95% CI, -0.35 to -0.04) lower dental development score compared with TPOAb-negative mothers. The findings of this study suggest that the thyroid hormone is involved in the maturation of teeth from the early stages of life onward.

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

BACKGROUND

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

CONCLUSIONS

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

REMODELING OF THE BONE TISSUE OF RATS OF DIFFERENT LINES AFTER MELATONIN EFFECT

The markers of remodeling of bone tissue (BT) were studied after the impact of pharmacologic dose of melatonin (5 mg/kg of the animal body weight) during 28 days for 3-months rats-males of Wistar and SHR lines. The studies were carried out in autumn. The aim of our work was to study the indices of bone tissue remodeling of rats of Wistar and SHR lines after introduction of pharmacologic dose of exogenous melatonin. Biochemical and immune-enzyme methods of analysis were used in the study. In rats of Wistar line was revealed a reliable increase of activity of alkaline phosphatase (ALP) by 43,9% (Р<0,05), hyaluronidase activity (HA) by 15,4% and free thyroxin concentration (fТ4) by 30%. There was also registered the decrease of pyridinoline (PYD) concentration by 48% and glycosaminoglycan (GAG) by 46,7%. In rats of SHR line under the same conditions the results essentially differed. The activity of acid phosphatase (ACP) has a tendency to increase by 11,6%, and PYD concentration decreased by 25%. The concentrations of free triiodothyronine (fТ3) and fТ4 reliably increased by 51,3% and 31,1% respectively. In the result of research we revealed that melatonin plays the main role among several main factors of regulation of bone tissue remodeling. It has stimulating influence on bone tissue and hormones of thyroid gland.

T3 Regulates a Human Macrophage-Derived TSH-β Splice Variant: Implications for Human Bone Biology

TSH and thyroid hormones (T3 and T4) are intimately involved in bone biology. We have previously reported the presence of a murine TSH-β splice variant (TSH-βv) expressed specifically in bone marrow-derived macrophages and that exerted an osteoprotective effect by inducing osteoblastogenesis. To extend this observation and its relevance to human bone biology, we set out to identify and characterize a TSH-β variant in human macrophages. Real-time PCR analyses using human TSH-β-specific primers identified a 364-bp product in macrophages, bone marrow, and peripheral blood mononuclear cells that was sequence verified and was homologous to a human TSH-βv previously reported. We then examined TSH-βv regulation using the THP-1 human monocyte cell line matured into macrophages. After 4 days, 46.1% of the THP-1 cells expressed the macrophage markers CD-14 and macrophage colony-stimulating factor and exhibited typical morphological characteristics of macrophages. Real-time PCR analyses of these cells treated in a dose-dependent manner with T3 showed a 14-fold induction of human TSH-βv mRNA and variant protein. Furthermore, these human TSH-βv-positive cells, induced by T3 exposure, had categorized into both M1 and M2 macrophage phenotypes as evidenced by the expression of macrophage colony-stimulating factor for M1 and CCL-22 for M2. These data indicate that in hyperthyroidism, bone marrow resident macrophages have the potential to exert enhanced osteoprotective effects by oversecreting human TSH-βv, which may exert its local osteoprotective role via osteoblast and osteoclast TSH receptors.

Emerging role of thyroid hormone metabolites

Thyroid hormones (THs) are essential for the regulation of development and metabolism in key organs. THs produce biological effects both by directly affecting gene expression through the interaction with nuclear receptors (genomic effects) and by activating protein kinases and/or ion channels (short-term effects). Such activations can be either direct, in the case of ion channels, or mediated by membrane or cytoplasmic receptors. Short-term-activated signalling pathways often play a role in the regulation of genomic effects. Several TH intermediate metabolites, which were previously considered without biological activity, have now been associated with a broad range of actions, mostly attributable to short-term effects. Here, we give an overview of the physiological roles and mechanisms of action of THs, focusing on the emerging position that TH metabolites are acquiring as important regulators of physiology and metabolism.

Resistance to thyroid hormone α, revelation of basic study to clinical consequences

In the past 3 years, 15 patients with resistance to thyroid hormone α (RTHα), nine THRA gene mutations have been reported, reforming classification of RTH. RTHα exhibits distinguished clinical manifestations from RTHβ, including growth retardation, skeletal dysplasia, impaired neurodevelopment, cardiovascular dysfunction, constipation and specific thyroid axis type. This review focuses on possible pathogenesis by revelatory basic science of RTHα animal models in vivo, and patients' mutant thyroid hormone receptor α (TRα) in vitro. Clinical manifestations and L-T4 effects are summarized, showing strong correlation to the severity of mutation mostly within the domain which dominated TR interaction with T3 and its corepressors/coactivators. In particular, we propose the diagnosis clues and promising treatment for clinicians.

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.

Resistance to Thyroid Hormone Alpha in an 18-Month-Old Girl: Clinical, Therapeutic, and Molecular Characteristics

BACKGROUND

Recently, the first patients with resistance to thyroid hormone alpha (RTHα) due to inactivating mutations in the thyroid hormone receptor alpha (TRα) were identified. These patients are characterized by growth retardation, variable motor and cognitive defects, macrocephaly, and abnormal thyroid function tests. The objective was to characterize a young girl (18 months old) with a mutation in both TRα1 and TRα2, and to study the effects of early levothyroxine (LT4) treatment.

METHODS

The patient was assessed clinically and biochemically before and during 12 months of LT4 treatment. In addition, the consequences of the mutation for TRα1/2 receptor function were studied in vitro.

RESULTS

At 18 months of age, the patient presented with axial hypotonia, delayed motor development, severe growth retardation, and abnormally elevated triiodothyronine (T3)/thyroxine (T4) ratios. RTHα was suspected, and concomitantly a c.632A>G/p.D211G missense mutation was identified, affecting both the TRα1 and TRα2 proteins. This mutation was also found in the girl's father. LT4 treatment was started, resulting in a marked improvement of her hypotonia, motor skills, and growth. Functionally, the missense mutation led to decreased transcriptional activity of TRα1, which could be overcome by higher T3 levels in vitro. The mutant TRα1 showed a moderate dominant negative activity on wild type (WT) TRα1. In contrast, WT TRα2 and mutant TRα2 had negligible transcriptional activity and showed no dominant-negative effect over TRα1.

CONCLUSIONS

This report describes the phenotype of a young RTHα patient with a mild TRα mutation before and during early LT4 treatment. Treatment had beneficial effects on her muscle tone, motor development, and growth.

TRα receptor mutations extend the spectrum of syndromes of reduced sensitivity to thyroid hormone

Since 2012, eight different abnormalities have been described in the THRA gene (encoding the TRα1 thyroid hormone receptor) of 14 patients from 9 families. These mutations induce a clinical phenotype (resistance to thyroid hormone type α) associating symptoms of untreated mild congenital hypothyroidism and a near-normal range of free and total thyroid hormones and TSH (the T4/T3 ratio is nevertheless usually low). The phenotype can diversely include short stature (due to growth retardation), dysmorphic syndrome (face and limb extremities), psychoneuromotor disorders, constipation and bradycardia. The identified genetic abnormalities are located within the ligand-binding domain and result in defective T3 binding, an abnormally strong interaction with corepressors and a dominant negative activity against still functional receptors. The identification of patients with consistent phenotypes and the underlying mutations are warranted to better delineate the spectrum of the syndromes of reduced sensitivity to thyroid hormone.

Resistance to thyroid hormone due to defective thyroid receptor alpha

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