Defining thyrotropin-dependent and -independent steps of thyroid hormone synthesis by using thyrotropin receptor-null mice

Perfluorooctanoic acid disrupts thyroid hormone biosynthesis by altering glycosylation of Na+/I- symporter in larval zebrafish

Perfluorooctanoic acid (PFOA) is a well-known thyroid disruptor that has been found to induce hypothyroidism. However, the exact molecular mechanism by which PFOA reduces thyroid hormone levels remains unclear. In this study, we have discovered that PFOA disrupts the glycosylation process of the sodium/iodide symporter (NIS), which inhibits the translocation of NIS onto the plasma membrane of thyroid follicular cells. Our results also demonstrate that PFOA disrupts thyroid stimulating hormone (TSH)-dependent signaling pathways involved in cellular glycosylation, impairing NIS glycosylation and reducing the ability of iodine uptake. This leads to an insufficiency of iodine for thyroid hormone production inside the follicular cells of the thyroid, resulting in lower-than-normal thyroxine levels detected in zebrafish larvae. These findings are consistent with our previously published data, which showed that PFOA induces neural behavior changes during the early stages of neuronal development in zebrafish. This new discovery provides valuable insights into the molecular characteristics of endocrine-disrupting chemicals (EDCs) that are known to affect the thyroid. It may also contribute to a better understanding of how altered glycosylation could be a potential risk factor for the association between exposure to specific per- and polyfluoroalkyl substances (PFAS) and various health effects in humans.

Thyroid hormone deprival and TSH/TSHR signaling deficiency lead to central hypothyroidism-associated intestinal dysplasia

BACKGROUND

Central hypothyroidism (CH) is characterized by low T4 levels and reduced levels or bioactivity of circulating TSH. However, there is a lack of studies on CH-related intestinal maldevelopment. In particular, the roles of TH and TSH/TSHR signaling in CH-related intestinal maldevelopment are poorly understood. Herein, we utilized Tshr-/- mice as a congenital hypothyroidism model with TH deprival and absence of TSHR signaling.

METHODS

The morphological characteristics of intestines were determined by HE staining, periodic acid-shiff staining, and immunohistochemical staining. T4 was administrated into the offspring of homozygous mice from the fourth postnatal day through weaning or administrated after weaning. RT-PCR was used to evaluate the expression of markers of goblet cells and intestinal digestive enzymes. Single-cell RNA-sequencing analysis was used to explore the cell types and gene profiles of metabolic alternations in early-T4-injected Tshr-/- mice.

KEY FINDINGS

Tshr deletion caused significant growth retardation and intestinal maldevelopment, manifested as smaller and more slender small intestines due to reduced numbers of stem cells and differentiated epithelial cells. Thyroxin supplementation from the fourth postnatal day, but not from weaning, significantly rescued the abnormal intestinal structure and restored the decreased number of proliferating intestinal cells in crypts of Tshr-/- mice. Tshr-/- mice with early-life T4 injections had more early goblet cells and impaired metabolism compared to Tshr+/+ mice.

SIGNIFICANCE

TH deprival leads to major defects of CH-associated intestinal dysplasia while TSH/TSHR signaling deficiency promotes the differentiation of goblet cells and impairs nutrition metabolism.

Regulation of circulating thyroid hormone levels by hypothalamic tanycytes and hypophysial pars tuberalis-specific cells and their morphological and gene- and protein-expression changes under different photoperiods

Thyroid hormone in the hypothalamus acts as a key determinant of seasonal transitions. Thyroid hormone-levels in the brain are mainly regulated by the hypothalamic tanycytes and pituitary pars tuberalis (PT)-specific cells. TSHβ produced by the PT-specific cells stimulates Dio2 expression and decreases Dio3 expression of the tanycytes. Both tanycytes and PT-specific cells in photosensitive animals exhibit remarkable changes of morphological appearance and expressions of genes and proteins under different photoperiods. Long photoperiods induce increased gene- and protein-expressions and active features. Short photoperiods cause the decreased gene- and protein-expressions and inactive features. In the PT, expressions of TSHβ, common α-subunit of glycoprotein hormones (α-GSU), and MT1 receptor of melatonin receptors and eyes absent 3 change under different photoperiods. Diurnal rhythms of α-GSU mRNA expression are observed in the PT of Djungarian hamsters. Hes1, Nkx2.1, and LIM homeodomain gene 2 (Lhx2) are involved in the differentiation of PT. In the hypothalamic tanycytes, expressions of Dio2, Dio3, vimentin, serine/threonine kinase 33, GPR50, Nestin, Retinoid signaling genes (retinaldehyde dehydrogenase 1, cellular retinol binding protein 1, and Stra6), monocarboxylate transporter 8, and neural cell adhesion molecule change under different photoperiods. Rax, Lhx2, Nfia/b/x, and fibroblast growth factor 10 are involved in the differentiation of tanycytes.

Thyroid-stimulating hormone-thyroid hormone signaling contributes to circadian regulation through repressing clock2/npas2 in zebrafish

Thyroid-stimulating hormone (TSH) is important for the thyroid gland, development, growth, and metabolism. Defects in TSH production or the thyrotrope cells within the pituitary gland cause congenital hypothyroidism (CH), resulting in growth retardation and neurocognitive impairment. While human TSH is known to display rhythmicity, the molecular mechanisms underlying the circadian regulation of TSH and the effects of TSH-thyroid hormone (TH) signaling on the circadian clock remain elusive. Here we show that TSH, thyroxine (T4), triiodothyronine (T3), and tshba display rhythmicity in both larval and adult zebrafish and tshba is regulated directly by the circadian clock via both E'-box and D-box. Zebrafish tshba-/- mutants manifest congenital hypothyroidism, with the characteristics of low levels of T4 and T3 and growth retardation. Loss or overexpression of tshba alters the rhythmicity of locomotor activities and expression of core circadian clock genes and hypothalamic-pituitary-thyroid (HPT) axis-related genes. Furthermore, TSH-TH signaling regulates clock2/npas2 via the thyroid response element (TRE) in its promoter, and transcriptome analysis reveals extensive functions of Tshba in zebrafish. Together, our results demonstrate that zebrafish tshba is a direct target of the circadian clock and in turn plays critical roles in circadian regulation along with other functions.

Thyroid Stem Cell Speciation-a Major Role for PKC

Instructive signals that delineate the formation of thyroid follicles by thyrotropin (TSH) in stem cells are complex. Here, we have examined the role of protein kinase C (PKC) by using a unique Gαq/11 biased small molecule (MSq1) to develop thyroid progenitor cells. Mouse embryonic stem cells (mESCs) were differentiated into anterior endoderm cells and treated with either TSH or MSq1 in the presence or absence of PKC inhibitors. The transcriptional and translational response of key thyroid markers-sodium iodide symporter (NIS), thyroglobulin (TG), and thyrotropin receptor (TSHR) as well as potential signaling molecules-were then analyzed. The data confirmed that MSq1 is a potent Gαq/11 activator with a major increase in Gαq/11 signaling when compared to TSH. MSq1 activation resulted in an increase in thyroid-specific genes, demonstrating that enhanced PKC signaling was able to induce their expression. The specificity of the PKC signals over the protein kinase A (PKA) pathway in regulating thyroid gene expression was shown by using a specific PKC enzyme inhibitor. The data revealed that TG and NIS expression were suppressed in the presence of the PKC inhibition but, in contrast, were not influenced by PKA inhibition. This indicated that PKC activation was the dominant pathway in the inductive process for thyroid hormone production. Furthermore, by examining PKC isoforms we found that PKCξ was the predominant form in the ES cells that mediated the effects. Since PKCξ can lead to activation of transforming growth factor-β-activated kinase (pTAK1), and its downstream effector nuclear factor κB (NFκB) complex, this demonstrated the involvement of the TAK1/NFκB pathway in thyroid speciation.

Genetic variants of FGFR family associated with height, hypertension, and osteoporosis

BACKGROUND

Hypertension and osteoporosis are the most common types of health problems. A recent study suggested that the fibroblast growth factor receptor-like protein 1 (FGFRL1) gene in giraffes is the most promising candidate gene that may have direct effects on both the skeleton and the cardiovascular system.

AIM

Our study purposed to replicate the finding that the FGFR5 gene is related to giraffe-related characteristics (height, hypertension, and osteoporosis), and to assess the associations between genetic variants of the FGFR family and three phenotypes.

SUBJECTS AND METHODS

An association study was performed to confirm the connections between hypertension, osteoporosis, and height and the FGFR family proteins (FGFR1 to FGFR5).

RESULTS

We identified a total of 192 genetic variants in the FGFR family and found six SNVs in the FGFR2, FGFR3, and FGFR4 genes that were associated with two phenotypes simultaneously. Also, the FGFR family was found to be involved in calcium signalling, and three genetic variants of the FGFR3 gene showed significant signals in the pituitary and hypothalamus.

CONCLUSION

Taken together, these findings suggest that FGFR genes are associated with hypertension, height, and osteoporosis. In particular, the present study highlights the FGFR3 gene, which influences two fundamental regulators of bone remodelling.

The isl2a transcription factor regulates pituitary development in zebrafish

BACKGROUND

ISL LIM homeobox 2, also known as insulin gene enhancer protein ISL-2 (ISL2), is a transcription factor gene that participates in a wide range of developmental events. However, the role of ISL2 in the hypothalamus-pituitary-thyroid axis is largely unknown. In the present study, we characterized the expression patterns of ISL2 and revealed its regulative role during embryogenesis using zebrafish.

METHODS

We used the CRISPR/Cas9 system to successfully establish homozygous ISL2-orthologue (isl2a and isl2b) knockout zebrafish. Moreover, we utilized these knockout zebrafish to analyze the pituitary and thyroid phenotypes in vivo. For further molecular characterization, in situ hybridization and immunofluorescence were performed.

RESULTS

The isl2a mutant zebrafish presented with thyroid hypoplasia, reduced whole-body levels of thyroid hormones, increased early mortality, gender imbalance, and morphological retardation during maturity. Additionally, thyrotropes, a pituitary cell type, was notably decreased during development. Importantly, the transcriptional levels of pituitary-thyroid axis hormones-encoding genes, such as tshba, cga, and tg, were significantly decreased in isl2a mutants. Finally, the thyroid dysplasia in isl2a mutant larvae may be attributed to a reduction in proliferation rather than changes in apoptosis.

CONCLUSIONS

In summary, isl2a regulates the transcriptional levels of marker genes in hypothalamus-pituitary-thyroid axis, and isl2a knockout causing low thyroid hormone levels in zebrafish. Thus, isl2a identified by the present study, is a novel regulator for pituitary cell differentiation in zebrafish, resulting in thyroid gland hypoplasia and phenotypes of hypothyroidism.

Independent Skeletal Actions of Pituitary Hormones

Over the past years, pituitary hormones and their receptors have been shown to have non-traditional actions that allow them to bypass the hypothalamus-pituitary-effector glands axis. Bone cells-osteoblasts and osteoclasts-express receptors for growth hormone, follicle stimulating hormone (FSH), thyroid stimulating hormone (TSH), adrenocorticotrophic hormone (ACTH), prolactin, oxytocin, and vasopressin. Independent skeletal actions of pituitary hormones on bone have been studied using genetically modified mice with haploinsufficiency and by activating or inactivating the receptors pharmacologically, without altering systemic effector hormone levels. On another front, the discovery of a TSH variant (TSH-βv) in immune cells in the bone marrow and skeletal action of FSHβ through tumor necrosis factor α provides new insights underscoring the integrated physiology of bone-immune-endocrine axis. Here we discuss the interaction of each pituitary hormone with bone and the potential it holds in understanding bone physiology and as a therapeutic target.

Bovine models for human ovarian diseases

During early embryonic development, late fetal growth, puberty, adult reproductive years, and advanced aging, bovine and human ovaries closely share molecular pathways and hormonal signaling mechanisms. Other similarities between these species include the size of ovaries, length of gestation, ovarian follicular and luteal dynamics, and pathophysiology of ovarian diseases. As an economically important agriculture species, cattle are a foundational species in fertility research with decades of groundwork using physiologic, genetic, and therapeutic experimental techniques. Many technologies used in modern reproductive medicine, such as ovulation induction using hormonal therapy, were first used in cows before human trials. Human ovarian diseases with naturally occurring bovine correlates include premature ovary insufficiency (POI), polycystic ovarian syndrome (PCOS), and sex-cord stromal tumors (SCSTs). This article presents an overview of bovine ovary research related to causes of infertility, ovarian diseases, diagnostics, and therapeutics, emphasizing where the bovine model can offer advantages over other lab animals for translational applications.

Functions of the Thyroid-Stimulating Hormone on Key Developmental Features Revealed in a Series of Zebrafish Dyshormonogenesis Models

The hypothalamic–pituitary–thyroid (HPT) axis regulates many critical features in vertebrates. Utilizing TALENs and CRISPR/Cas9 techniques, thyroid-stimulating hormone subunit beta a (tshba), thyroglobulin (tg), and solute carrier family 16 member 2 (slc16a2) mutant zebrafish lines were generated. Among the three mutants, the earliest time point for the significantly altered T3 contents was observed in tshba mutants, which resulted in the most severe defects, including typical defects such as the retardation of inflated anterior swimming bladder (aSB), proper formation of fin ray and posterior squamation (SP), the larval-to-juvenile transition (LTJT) process, juvenile growth retardation, and mating failure. In tg mutants, which are actually compensated with an alternative splicing form, growth retardation was observed in the juvenile stage without LTJT and reproductive defects. The evident goiter phenotype was only observed in tg- and slc16a2 mutants, but not in tshba mutants. Other than goiters being observed, no other significant developmental defects were found in the slc16a2 mutants. Regarding the reproductive defects observed in tshba mutants, the defective formation of the secondary sex characteristics (SSCs) was observed, while no obvious alterations during gonad development were found. Based on our analyses, zebrafish at the 6–12 mm standard length or 16–35 days post-fertilization (dpf) should be considered to be in their LTJT phase. Using a series of zebrafish dyshormonogenesis models, this study demonstrated that the TSH function is critical for the proper promotion of zebrafish LTJT and SSC formation. In addition, the elevation of TSH levels appears to be essential for goiter appearance in zebrafish.

Activating mutations in BRAF disrupt the hypothalamo-pituitary axis leading to hypopituitarism in mice and humans

Germline mutations in BRAF and other components of the MAPK pathway are associated with the congenital syndromes collectively known as RASopathies. Here, we report the association of Septo-Optic Dysplasia (SOD) including hypopituitarism and Cardio-Facio-Cutaneous (CFC) syndrome in patients harbouring mutations in BRAF. Phosphoproteomic analyses demonstrate that these genetic variants are gain-of-function mutations leading to activation of the MAPK pathway. Activation of the MAPK pathway by conditional expression of the BrafV600E/+ allele, or the knock-in BrafQ241R/+ allele (corresponding to the most frequent human CFC-causing mutation, BRAF p.Q257R), leads to abnormal cell lineage determination and terminal differentiation of hormone-producing cells, causing hypopituitarism. Expression of the BrafV600E/+ allele in embryonic pituitary progenitors leads to an increased expression of cell cycle inhibitors, cell growth arrest and apoptosis, but not tumour formation. Our findings show a critical role of BRAF in hypothalamo-pituitary-axis development both in mouse and human and implicate mutations found in RASopathies as a cause of endocrine deficiencies in humans.

The Transcription Factor NF-κB Mediates Thyrotropin-Stimulated Expression of Thyroid Differentiation Markers

Background: The nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) transcription factor is a key regulator of cell survival, proliferation, and gene expression. Although activation of NF-κB signaling in thyroid follicular cells after thyrotropin (TSH) receptor (TSHR) engagement has been reported, the downstream signaling leading to NF-κB activation remains unexplored. Here, we sought to elucidate the mechanisms that regulate NF-κB signaling activation in response to TSH stimulation. Methods: Fisher rat-derived thyroid cell lines and primary cultures of NF-κB essential modulator (NEMO)-deficient mice thyrocytes were used as models. Signaling pathways leading to the activation of NF-κB were investigated by using chemical inhibitors and phospho-specific antibodies. Luciferase reporter gene assays and site-directed mutagenesis were used to monitor NF-κB-dependent gene transcriptional activity and the expression of thyroid differentiation markers was assessed by reverse transcription quantitative polymerase chain reaction and Western blot, respectively. Chromatin immunoprecipitation (ChIP) was carried out to investigate NF-κB subunit p65 DNA binding, and small interfering RNA (siRNA)-mediated gene knockdown approaches were used for studying gene function. Results: Using thyroid cell lines, we observed that TSH treatment leads to protein kinase C (PKC)-mediated canonical NF-κB p65 subunit nuclear expression. Moreover, TSH stimulation phosphorylated the kinase TAK-1, and its knockdown abolished TSH-induced NF-κB transcriptional activity. TSH induced the transcriptional activity of the NF-κB subunit p65 in a protein kinase A (PKA)-dependent phosphorylation at Ser-276. In addition, p65 phosphorylation at Ser-276 induced acetyl transferase p300 recruitment, leading to its acetylation on Lys-310 and thereby enhancing its transcriptional activity. Evaluation of the role played by NF-κB in thyroid physiology demonstrated that the canonical NF-κB inhibitor BAY 11-7082 reduced TSH-induced expression of thyroid differentiation markers. The involvement of NF-κB signaling in thyroid physiology was confirmed by assessing the TSH-induced gene expression in primary cultures of NEMO-deficient mice thyrocytes. ChIP and the knockdown experiments revealed that p65 is a nuclear effector of TSH actions, inducing the transcripcional expression of thyroid differentiation markers. Conclusions: Taken together, our results point to NF-κB being a pivotal mediator in the TSH-induced thyroid follicular cell differentiation, a relevant finding with potential physiological and pathophysiological implications.

Asynchrony of Apical Polarization, Luminogenesis, and Functional Differentiation in the Developing Thyroid Gland

Follicular thyroid tissue originates from progenitors derived from a midline endodermal primordium. Current understanding infers that folliculogenesis in the embryonic thyroid designates the latest morphogenetic event taking place after the final anatomical shape and position of the gland is established. However, this concept does not consider the fact that the thyroid isthmus develops chronologically before the lobes and also contains all progenitors required for lobulation. To elucidate whether cells committed to a thyroid fate might be triggered to differentiate asynchronously related to maturation and developmental stage, mouse embryonic thyroid tissues from E12.5-17.5 were subjected to immunofluorescent labeling of biomarkers (progenitors: NKX2-1; differentiation: thyroglobulin/TG); folliculogenesis: E-cadherin/CDH1; luminogenesis: mucin 1/MUC1; apical polarity: pericentrin/PCNT; basement membrane: laminin; growth: Ki67), quantitative RT-PCR analysis (Nkx2.1, Tg, Muc1) and transmission electron microscopy. Tg expression was detectable as early as E12.5 and gradually increased >1000-fold until E17.5. Muc1 and Nkx2.1 transcript levels increased in the same time interval. Prior to lobulation (E12.5-13.5), MUC1 and TG distinguished pre-follicular from progenitor cells in the developing isthmus characterized by intense cell proliferation. Luminogenesis comprised redistribution of MUC1+ vesicles or vacuoles, transiently associated with PCNT, to the apical cytoplasm and the subsequent formation of MUC1+ nascent lumens. Apical polarization of pre-follicular cells and lumen initiation involved submembraneous vesicular traffic, reorganization of adherens junctions and ciliogenesis. MUC1 did not co-localize with TG until a lumen with a MUC1+ apical membrane was established. MUC1 delineated the lumen of all newly formed follicles encountered in the developing lobes at E15.5-17.5. Folliculogenesis started before establishment of a complete follicular basal lamina. These observations indicate that embryonic thyroid differentiation is an asynchronous process consistent with the idea that progenitors attaining a stationary position in the connecting isthmus portion undergo apical polarization and generate follicles already at a primordial stage of thyroid development, i.e. foregoing growth of the lobes. Although the thyroid isthmus eventually comprises minute amounts of the total thyroid volume and contributes little to the overall hormone production, it is of principal interest that local cues related to the residence status of cells - independently of a prevailing high multiplication rate - govern the thyroid differentiation program.

Long Term Rescue of the TSH Receptor Knock-Out Mouse - Thyroid Stem Cell Transplantation Restores Thyroid Function

The synergistic activation of transcription factors can lead to thyroid progenitor cell speciation. We have previously shown in vitro that mouse or human stem cells, expressing the transcription factors NKx2-1 and Pax8, can differentiate into thyroid neo-follicular structures (TFS). We now show that syngeneic mouse TFS when implanted into hypothyroid TSH receptor knockout (TSHR-KO) mice can ameliorate the hypothyroid state for an extended period. ES cells derived from heterozygous TSHR-KO blastocysts were stably transfected with Nkx2-1-GFP and Pax8-mcherry constructs and purified into 91.8% double positive cells by flow cytometry. After 5 days of activin A treatment these double positive cells were then induced to differentiate into neo-follicles in Matrigel for 21 days in the presence of 500μU/mL of TSH. Differentiated TFS expressing thyroglobulin mRNA were implanted under the kidney capsule of 4-6 weeks old TSHR-KO mice (n=5) as well as hind limb muscle (n=2) and anterior chamber of one eye (n=2). Five of the mice tested after 4 weeks were all rendered euthyroid and all mice remained euthyroid at 20 weeks post implantation. The serum T4 fully recovered (pre-bleed 0.62 ± 0.03 to 8.40 ± 0.57 µg/dL) and the previously elevated TSH became normal or suppressed (pre-bleed 391 ± 7.6 to 4.34 ± 1.25 ng/dL) at the end of the 20 week observation period. The final histology obtained from the implanted kidney tissues showed only rudimentary thyroid follicular structures but which stained positive for thyroglobulin expression. The presence of only rudimentary structures at the site of implant on these extended animals suggested possible migration of cells from the site of implant or an inability of TFCs to maintain proper follicular morphology in these external sites for extended periods. However, there were no signs of tumor formation and no immune infiltration. These preliminary studies show that TSHR-KO mice are a useful model for orthotropic implantation of functional thyroid cells without the need for thyroidectomy, radioiodine ablation or anti thyroid drug control of thyroid function. This approach is also proof of principle that thyroid cells derived from mouse ES cells are capable of surviving as functional neo-follicles in vivo for an extended period of 20 weeks.

Unconventional Actions of Glycoprotein Hormone Subunits: A Comprehensive Review

The glycoprotein hormones (GPH) are heterodimers composed of a common α subunit and a specific β subunit. They act by activating specific leucine-rich repeat G protein-coupled receptors. However, individual subunits have been shown to elicit responses in cells devoid of the receptor for the dimeric hormones. The α subunit is involved in prolactin production from different tissues. The human chorionic gonadotropin β subunit (βhCG) plays determinant roles in placentation and in cancer development and metastasis. A truncated form of the thyrotropin (TSH) β subunit is also reported to have biological effects. The GPH α- and β subunits are derived from precursor genes (gpa and gpb, respectively), which are expressed in most invertebrate species and are still represented in vertebrates as GPH subunit paralogs (gpa2 and gpb5, respectively). No specific receptor has been found for the vertebrate GPA2 and GPB5 even if their heterodimeric form is able to activate the TSH receptor in mammals. Interestingly, GPA and GPB are phylogenetically and structurally related to cysteine-knot growth factors (CKGF) and particularly to a group of antagonists that act independently on any receptor. This review article summarizes the observed actions of individual GPH subunits and presents the current hypotheses of how these actions might be induced. New approaches are also proposed in light of the evolutionary relatedness with antagonists of the CKGF family of proteins.

Biased signaling in naturally occurring mutations of G protein-coupled receptors associated with diverse human diseases

G protein-coupled receptors (GPCRs) play critical roles in transmitting a variety of extracellular signals into the cells and regulate diverse physiological functions. Naturally occurring mutations that result in dysfunctions of GPCRs have been known as the causes of numerous diseases. Significant progresses have been made in elucidating the pathophysiology of diseases caused by mutations. The multiple intracellular signaling pathways, such as G protein-dependent and β-arrestin-dependent signaling, in conjunction with recent advances on biased agonism, have broadened the view on the molecular mechanism of disease pathogenesis. This review aims to briefly discuss biased agonism of GPCRs (biased ligands and biased receptors), summarize the naturally occurring GPCR mutations that cause biased signaling, and propose the potential pathophysiological relevance of biased mutant GPCRs associated with various endocrine diseases.

Unraveling the Complex Interplay Between Transcription Factors and Signaling Molecules in Thyroid Differentiation and Function, From Embryos to Adults

Thyroid differentiation of progenitor cells occurs during embryonic development and in the adult thyroid gland, and the molecular bases of these complex and finely regulated processes are becoming ever more clear. In this Review, we describe the most recent advances in the study of transcription factors, signaling molecules and regulatory pathways controlling thyroid differentiation and development in the mammalian embryo. We also discuss the maintenance of the adult differentiated phenotype to ensure the biosynthesis of thyroid hormones. We will focus on endoderm-derived thyroid epithelial cells, which are responsible for the formation of the thyroid follicle, the functional unit of the thyroid gland. The use of animal models and pluripotent stem cells has greatly aided in providing clues to the complicated puzzle of thyroid development and function in adults. The so-called thyroid transcription factors - Nkx2-1, Foxe1, Pax8 and Hhex - were the first pieces of the puzzle identified in mice. Other transcription factors, either acting upstream of or directly with the thyroid transcription factors, were subsequently identified to, almost, complete the puzzle. Among them, the transcription factors Glis3, Sox9 and the cofactor of the Hippo pathway Taz, have emerged as important players in thyroid differentiation and development. The involvement of signaling molecules increases the complexity of the puzzle. In this context, the importance of Bmps, Fgfs and Shh signaling at the onset of development, and of TSH, IGF1 and TGFβ both at the end of terminal differentiation in embryos and in the adult thyroid, are well recognized. All of these aspects are covered herein. Thus, readers will be able to visualize the puzzle of thyroid differentiation with most - if not all - of the pieces in place.

The paradoxical lean phenotype of hypothyroid mice is marked by increased adaptive thermogenesis in the skeletal muscle

Obesity is a major health problem worldwide, given its growing incidence and its association with a variety of comorbidities. Weight gain results from an increase in energy intake without a concomitant increase in energy expenditure. To combat the obesity epidemic, many studies have focused on the pathways underlying satiety and hunger signaling, while other studies have concentrated on the mechanisms involved in energy expenditure, most notably adaptive thermogenesis. Hypothyroidism in humans is typically associated with a decreased basal metabolic rate, lower energy expenditure, and weight gain. However, hypothyroid mouse models have been reported to have a leaner phenotype than euthyroid controls. To elucidate the mechanism underlying this phenomenon, we used a drug-free mouse model of hypothyroidism: mice lacking the sodium/iodide symporter (NIS), the plasma membrane protein that mediates active iodide uptake in the thyroid. In addition to being leaner than euthyroid mice, owing in part to reduced food intake, these hypothyroid mice show signs of compensatory up-regulation of the skeletal-muscle adaptive thermogenic marker sarcolipin, with an associated increase in fatty acid oxidation (FAO). Neither catecholamines nor thyroid-stimulating hormone (TSH) are responsible for sarcolipin expression or FAO stimulation; rather, thyroid hormones are likely to negatively regulate both processes in skeletal muscle. Our findings indicate that hypothyroidism in mice results in a variety of metabolic changes, which collectively lead to a leaner phenotype. A deeper understanding of these changes may make it possible to develop new strategies against obesity.

A Novel Antigonadotropic Role of Thyroid Stimulating Hormone on Leydig Cell-Derived Mouse Leydig Tumor Cells-1 Line

Subclinical hypothyroid men characterized by a rise in only thyroid stimulating hormone (TSH) levels and normal thyroid hormone levels showed a fall in their serum progesterone and testosterone levels. This suggested a role of TSH in regulating Leydig cell steroidogenesis. Therefore, we investigated the direct role of TSH on steroid production and secretion using a mouse Leydig tumour cell line, MLTC-1. MLTC-1 cells were treated with different doses of TSH isolated from porcine pituitary as well as recombinant TSH. Steroid secretion was measured by radioimmunoassay. The mRNA levels of steroidogenic enzymes were quantitated by real time PCR whereas the corresponding protein levels were determined by Western blot. In MLTC-1 cells, pituitary TSH as well as recombinant TSH inhibited progesterone and testosterone secretion in a dose dependent manner. The inhibitory action of TSH on steroid secretion was unique and not mimicked by other anterior pituitary hormones including FSH and ACTH. Recombinant TSH showed no effect on StAR and CYP11A1, the enzymes catalysing the non-steroidogenic and steroidogenic rate-limiting steps of steroid synthesis respectively. Recombinant TSH was shown to inhibit steroidogenesis in MLTC-1 cells by inhibiting the 3β hydroxy steroid dehydrogenase mRNA and protein levels, the enzyme that catalyses the conversion of pregnenolone to progesterone. This inhibitory effect of TSH is probably direct as both mRNA and protein of the TSH receptor were shown to be present in the MLTC-1 cells.

Thyroid Stimulating Hormone Triggers Hepatic Mitochondrial Stress through Cyclophilin D Acetylation

BACKGROUND & AIMS

Oxidative stress-related liver diseases were shown to be associated with elevated serum thyroid stimulating hormone (TSH) levels. Mitochondria are the main source of cellular reactive oxygen species. However, the relationship between TSH and hepatic mitochondrial stress/dysfunction and the underlying mechanisms are largely unknown. Here, we focused on exploring the effects and mechanism of TSH on hepatic mitochondrial stress.

METHODS

As the function of TSH is mediated through the TSH receptor (TSHR), Tshr -/- mice and liver-specific Tshr -/- mice and liver-specific Tshr -/- mice and liver-specific.

RESULTS

A relatively lower degree of mitochondrial stress was observed in the livers of Tshr -/- mice and liver-specific in vitro. Microarray and RT-PCR analyses showed that Tshr -/- mice and liver-specific.

CONCLUSIONS

TSH stimulates hepatic CypD acetylation through the lncRNA-AK044604/SIRT1/SIRT3 signaling pathway, indicating an essential role for TSH in mitochondrial stress in the liver.

A Stem Cell Surge During Thyroid Regeneration

BACKGROUND

Many tissues, including the thyroid, contain resident (adult) stem cells that are responsible for regeneration and repair after injury. The mechanisms of thyroid regeneration and the role of thyroid stem cells and thyroid progenitor cells in this process are not well understood. We have now used a new mouse thyroid injury model to gain insight into this phenomenon.

METHODS

Tamoxifen induced TPO-Cre mice (TPOCreER2) were crossed with inducible Diphtheria Toxin Receptor homozygous mice (ROSA26iDTR) to give rise to TPOCreER2/iDTR mice, allowing for the Cre-mediated expression of the DTR and rendering TPO expressing thyroid cells highly sensitive to diphtheria toxin (DT). This model of TPOCreER2/iDTR mice allowed us to study the repair/regeneration of thyroid follicles after diphtheria toxin induced thyroid damage by measuring serum thyroid hormones and cell fate.

RESULTS

In TPOCreER2/iDTR double transgenic mice we observed severe thyroid damage as early as 2 weeks after initiating intraperitoneal DT injections. There was marked thyroid tissue apoptosis and a ~50% drop in serum T4 levels (from 5.86 to 2.43 ug/dl) and a corresponding increase in serum TSH (from 0.18 to 8.39 ng/dl). In addition, there was a ~50% decrease in transcription of thyroid specific genes (thyroglobulin, TSH receptor, and sodium-iodide symporter). After suspending the DT administration, the thyroid rapidly recovered over a 4-week period during which we observed a transient surge in stem cell marker expression (including Oct4, Nanog, Sox2, and Rex1). In addition, cells immunostaining with stem cell markers Oct4 and Ssea-1 were found in clusters around new thyroid follicles in TPOCreER2/iDTR double transgenic mice. Furthermore, the presence of clusters of thyroid progenitor cells was also identified by Pax8 staining of thyroglobulin negative cells. This recovery of the injured gland was followed by a rapid and sequential restoration of thyroid function.

CONCLUSION

These data demonstrate that a new model of thyroid cell damage induced by DT can be used to study the mobilization of resident adult stem cells. Furthermore, the model clearly demonstrates the involvement of both stem and progenitor cells in the in vivo regeneration of the thyroid after severe destruction.

Molecular defects in thyroid dysgenesis

Congenital hypothyroidism (CH) is a neonatal endocrine disorder that might occur as itself or be associated to congenital extra-thyroidal defects. About 85% of affected subjects experience thyroid dysgenesis (TD), characterized by defect in thyroid gland development. In vivo experiments on null mice paved the way for the identification of genes involved thyroid morphogenesis and development, whose mutation has been strongly associated to TD. Most of them are thyroid-specific transcription factors expressed during early thyroid development. Despite the arduous effort in unraveling the genetics of TD in animal models, up to now these data have been discontinuously confirmed in humans and only 5% of TD have associated with known null mice-related mutations (mainly PAX8 and TSHR). Notwithstanding, the advance in genetic testing represented by the next-generation sequencing (NGS) approach is steadily increasing the list of genes whose highly penetrant mutation predisposes to TD. In this review we intend to outline the molecular bases of TD, summarizing the current knowledge on thyroid development in both mice and humans and delineating the genetic features of its monogenetic forms. We will also highlight current strategies to enhance the insight into the non-Mendelian mechanisms of abnormal thyroid development.

Thyrotropin aggravates atherosclerosis by promoting macrophage inflammation in plaques

The increased cardiovascular risk in subclinical hypothyroidism has traditionally been attributed to the associated metabolic disorders. This paper, however, revealed that TSH can aggravate atherosclerosis by promoting macrophage inflammation in the plaque, which deepens our understanding of the significance of TSH elevation in subclinical hypothyroidism.

Subclinical hypothyroidism is associated with cardiovascular diseases, yet the underlying mechanism remains largely unknown. Herein, in a common population (n = 1,103), TSH level was found to be independently correlated with both carotid plaque prevalence and intima-media thickness. Consistently, TSH receptor ablation in ApoE^−/− mice attenuated atherogenesis, accompanied by decreased vascular inflammation and macrophage burden in atherosclerotic plaques. These results were also observed in myeloid-specific Tshr-deficient ApoE^−/− mice, which indicated macrophages to be a critical target of the proinflammatory and atherogenic effects of TSH. In vitro experiments further revealed that TSH activated MAPKs (ERK1/2, p38α, and JNK) and IκB/p65 pathways in macrophages and increased inflammatory cytokine production and their recruitment of monocytes. Thus, the present study has elucidated the new mechanisms by which TSH, as an independent risk factor of atherosclerosis, aggravates vascular inflammation and contributes to atherogenesis.

Heterogeneity of Stem Cells in the Thyroid

Identification of thyroid stem cells in the past few years has made important contributions to our understanding of the cellular and molecular mechanisms that induce tissue regeneration and repair. Embryonic stem (ES) cells and induced-pluripotent stem cells have been used to establish reliable protocols to obtain mature thyrocytes and functional follicles for the treatment of thyroid diseases in mice. In addition, the discovery of resident thyroid progenitor cells, along with other sources of stem cells, has defined in detail the mechanisms responsible for tissue repair upon moderate or severe organ injury.In this chapter, we highlight in detail the current state of research on thyroid stem cells by focusing on (1) the description of the first experiments performed to obtain thyroid follicles from embryonic stem cells, (2) the identification of resident stem cells in the thyroid gland, and (3) the definition of the current translational in vivo and in vitro models used for thyroid tissue repair and regeneration.

Thyrocyte-specific deletion of insulin and IGF-1 receptors induces papillary thyroid carcinoma-like lesions through EGFR pathway activation

Insulin and insulin-like growth factor (IGF)-1 signaling in the thyroid are thought to be permissive for the coordinated regulation by thyroid-stimulating hormone (TSH) of thyrocyte proliferation and hormone production. However, the integrated role of insulin receptor (IR) and IGF-1 receptor (IGF-1R) in thyroid development and function has not been explored. Here, we generated thyrocyte-specific IR and IGF-1R double knockout (DTIRKO) mice to precisely evaluate the coordinated functions of these receptors in the thyroid of neonates and adults. Neonatal DTIRKO mice displayed smaller thyroids, paralleling defective folliculogenesis associated with repression of the thyroid-specific transcription factor Foxe1. By contrast, at postnatal day 14, absence of IR and IGF-1R paradoxically induced thyrocyte proliferation, which was mediated by mTOR-dependent signaling pathways. Furthermore, we found elevated production of TSH during the development of follicular hyperplasia at 8 weeks of age. By 50 weeks, all DTIRKO mice developed papillary thyroid carcinoma (PTC)-like lesions that correlated with induction of the ErbB pathway. Taken together, these data define a critical role for IR and IGF-1R in neonatal thyroid folliculogenesis. They also reveal an important reciprocal relationship between IR/IGF-1R and TSH/ErbB signaling in the pathogenesis of thyroid follicular hyperplasia and, possibly, of papillary carcinoma.

Pituitary Diseases and Bone

Neuroendocrinology of bone is a new area of research based on the evidence that pituitary hormones may directly modulate bone remodeling and metabolism. Skeletal fragility associated with high risk of fractures is a common complication of several pituitary diseases such as hypopituitarism, Cushing disease, acromegaly, and hyperprolactinemia. As in other forms of secondary osteoporosis, pituitary diseases generally affect bone quality more than bone quantity, and fractures may occur even in the presence of normal or low-normal bone mineral density as measured by dual-energy X-ray absorptiometry, making difficult the prediction of fractures in these clinical settings. Treatment of pituitary hormone excess and deficiency generally improves skeletal health, although some patients remain at high risk of fractures, and treatment with bone-active drugs may become mandatory. The aim of this review is to discuss the physiological, pathophysiological, and clinical insights of bone involvement in pituitary diseases.

Genetically modified mouse models to investigate thyroid development, function and growth

The thyroid gland produces thyroid hormones (TH), which are essential regulators for growth, development and metabolism. The thyroid is mainly controlled by the thyroid-stimulating hormone (TSH) that binds to its receptor (TSHR) on thyrocytes and mediates its action via different G protein-mediated signaling pathways. TSH primarily activates the G_s-pathway, and at higher concentrations also the G_q/11-pathway, leading to an increase of intracellular cAMP and Ca²⁺, respectively. To date, the physiological importance of other G protein-mediated signaling pathways in thyrocytes is unclear. Congenital hypothyroidism (CH) is defined as the lack of TH at birth. In familial cases, high-throughput sequencing methods have facilitated the identification of novel mutations. Nevertheless, the precise etiology of CH yet remains unraveled in a proportion of cases. Genetically modified mouse models can reveal new pathophysiological mechanisms of thyroid diseases. Here, we will present an overview of genetic mouse models for thyroid diseases, which have provided crucial insights into thyroid gland development, function, and growth with a special focus on TSHR and microRNA signaling.

Partial thyrocyte-specific Gαs deficiency leads to rapid-onset hypothyroidism, hyperplasia, and papillary thyroid carcinoma-like lesions in mice

Thyroid function is controlled by thyroid-stimulating hormone (TSH), which binds to its G protein-coupled receptor [thyroid-stimulating hormone receptor (TSHR)] on thyrocytes. TSHR can potentially couple to all G protein families, but it mainly activates the Gs- and Gq/11-mediated signaling cascades. To date, there is a knowledge gap concerning the role of the individual G protein cascades in thyroid pathophysiology. Here, we demonstrate that the thyrocyte-specific deletion of Gs-protein α subunit (Gαs) in adult mice [tamoxifen-inducible Gs protein α subunit deficient (iTGαsKO) mice] rapidly impairs thyrocyte function and leads to hypothyroidism. Consequently, iTGαsKO mice show reduced food intake and activity. However, body weight and the amount of white adipose tissue were decreased only in male iTGαsKO mice. Unexpectedly, hyperplastic follicles and papillary thyroid cancer-like tumor lesions with increased proliferation and slightly increased phospho-ERK1/2 staining were found in iTGαsKO mice at an older age. These tumors developed from nonrecombined thyrocytes still expressing Gαs in the presence of highly elevated serum TSH. In summary, we report that partial thyrocyte-specific Gαs deletion leads to hypothyroidism but also to tumor development in thyrocytes with remaining Gαs expression. Thus, these mice are a novel model to elucidate the pathophysiological consequences of hypothyroidism and TSHR/Gs/cAMP-mediated tumorigenesis.-Patyra, K., Jaeschke, H., Löf, C., Jännäri, M., Ruohonen, S. T., Undeutsch, H., Khalil, M., Kero, A., Poutanen, M., Toppari, J., Chen, M., Weinstein, L. S., Paschke, R., Kero, J. Partial thyrocyte-specific Gαs deficiency leads to rapid-onset hypothyroidism, hyperplasia, and papillary thyroid carcinoma-like lesions in mice.

Wnt Signaling in Thyroid Homeostasis and Carcinogenesis

The Wnt pathway is essential for stem cell maintenance, but little is known about its role in thyroid hormone signaling and thyroid stem cell survival and maintenance. In addition, the role of Wnt signaling in thyroid cancer progenitor cells is also unclear. Here, we present emerging evidence for the role of Wnt signaling in somatic thyroid stem cell and thyroid cancer stem cell function. An improved understanding of the role of Wnt signaling in thyroid physiology and carcinogenesis is essential for improving both thyroid disease diagnostics and therapeutics.

Generation and characterization of a hypothyroidism rat model with truncated thyroid stimulating hormone receptor

Thyroid stimulating hormone receptor (TSHR), a G-protein-coupled receptor, is important for thyroid development and growth. In several cases, frameshift and/or nonsense mutations in TSHR were found in the patients with congenital hypothyroidism (CH), however they have not been functionally studied in an animal model. In the present work, we generated a unique Tshr Df/Df rat model that recapitulates the phenotypes in TSHR Y444X patient by CRISPR/Cas genome editing technology. In this rat model, TSHR is truncated at the second transmembrane domain, leading to CH phenotypes as what was observed in the patients, including dwarf, thyroid aplasia, infertility, TSH resistant as well as low serum thyroid hormone levels. The phenotypes can be reversed, at least partially, by levothyroxine (L-T4) treatment after weaning. The thyroid development is severely impaired in the Tshr Df/Df rats due to the suppression of the thyroid specific genes, i.e., thyroperoxidase (Tpo), thyroglobulin (Tg) and sodium iodide symporter (Nis), at both mRNA and protein levels. In conclusion, the Tshr Df/Df rat serves as a brand new genetic model to study CH in human, and will greatly help to shed light into the development of terminal organs that are sensitive to thyroid hormones.

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.

GLIS3 is indispensable for TSH/TSHR-dependent thyroid hormone biosynthesis and follicular cell proliferation

Deficiency in Krüppel-like zinc finger transcription factor GLI-similar 3 (GLIS3) in humans is associated with the development of congenital hypothyroidism. However, the functions of GLIS3 in the thyroid gland and the mechanism by which GLIS3 dysfunction causes hypothyroidism are unknown. In the current study, we demonstrate that GLIS3 acts downstream of thyroid-stimulating hormone (TSH) and TSH receptor (TSHR) and is indispensable for TSH/TSHR-mediated proliferation of thyroid follicular cells and biosynthesis of thyroid hormone. Using ChIP-Seq and promoter analysis, we demonstrate that GLIS3 is critical for the transcriptional activation of several genes required for thyroid hormone biosynthesis, including the iodide transporters Nis and Pds, both of which showed enhanced GLIS3 binding at their promoters. The repression of cell proliferation of GLIS3-deficient thyroid follicular cells was due to the inhibition of TSH-mediated activation of the mTOR complex 1/ribosomal protein S6 (mTORC1/RPS6) pathway as well as the reduced expression of several cell division-related genes regulated directly by GLIS3. Consequently, GLIS3 deficiency in a murine model prevented the development of goiter as well as the induction of inflammatory and fibrotic genes during chronic elevation of circulating TSH. Our study identifies GLIS3 as a key regulator of TSH/TSHR-mediated thyroid hormone biosynthesis and proliferation of thyroid follicular cells and uncovers a mechanism by which GLIS3 deficiency causes neonatal hypothyroidism and prevents goiter development.

De novo triiodothyronine formation from thyrocytes activated by thyroid-stimulating hormone

The thyroid gland secretes primarily tetraiodothyronine (T₄), and some triiodothyronine (T₃). Under normal physiological circumstances, only one-fifth of circulating T₃ is directly released by the thyroid, but in states of hyperactivation of thyroid-stimulating hormone receptors (TSHRs), patients develop a syndrome of relative T₃ toxicosis. Thyroidal T₄ production results from iodination of thyroglobulin (TG) at residues Tyr⁵ and Tyr¹³⁰, whereas thyroidal T₃ production may originate in several different ways. In this study, the data demonstrate that within the carboxyl-terminal portion of mouse TG, T₃ is formed de novo independently of deiodination from T₄ We found that upon iodination in vitro, de novo T₃ formation in TG was decreased in mice lacking TSHRs. Conversely, de novo T₃ that can be formed upon iodination of TG secreted from PCCL3 (rat thyrocyte) cells was augmented from cells previously exposed to increased TSH, a TSHR agonist, a cAMP analog, or a TSHR-stimulating antibody. We present data suggesting that TSH-stimulated TG phosphorylation contributes to enhanced de novo T₃ formation. These effects were reversed within a few days after removal of the hyperstimulating conditions. Indeed, direct exposure of PCCL3 cells to human serum from two patients with Graves' disease, but not control sera, led to secretion of TG with an increased intrinsic ability to form T₃ upon in vitro iodination. Furthermore, TG secreted from human thyrocyte cultures hyperstimulated with TSH also showed an increased intrinsic ability to form T₃ Our data support the hypothesis that TG processing in the secretory pathway of TSHR-hyperstimulated thyrocytes alters the structure of the iodination substrate in a way that enhances de novo T₃ formation, contributing to the relative T₃ toxicosis of Graves' disease.

Deficiency in the manganese efflux transporter SLC30A10 induces severe hypothyroidism in mice

Manganese is an essential metal that becomes toxic at elevated levels. Loss-of-function mutations in SLC30A10, a cell-surface-localized manganese efflux transporter, cause a heritable manganese metabolism disorder resulting in elevated manganese levels and parkinsonian-like movement deficits. The underlying disease mechanisms are unclear; therefore, treatment is challenging. To understand the consequences of loss of SLC30A10 function at the organism level, we generated Slc30a10 knock-out mice. During early development, knock-outs were indistinguishable from controls. Surprisingly, however, after weaning and compared with controls, knock-out mice failed to gain weight, were smaller, and died prematurely (by ∼6-8 weeks of age). At 6 weeks, manganese levels in the brain, blood, and liver of the knock-outs were ∼20-60-fold higher than controls. Unexpectedly, histological analyses revealed that the brain and liver of the knock-outs were largely unaffected, but their thyroid exhibited extensive alterations. Because hypothyroidism leads to growth defects and premature death in mice, we assayed for changes in thyroid and pituitary hormones. At 6 weeks and compared with controls, the knock-outs had markedly reduced thyroxine levels (∼50-80%) and profoundly increased thyroid-stimulating hormone levels (∼800-1000-fold), indicating that Slc30a10 knock-out mice develop hypothyroidism. Importantly, a low-manganese diet produced lower tissue manganese levels in the knock-outs and rescued the phenotype, suggesting that manganese toxicity was the underlying cause. Our unanticipated discovery highlights the importance of determining the role of thyroid dysfunction in the onset and progression of manganese-induced disease and identifies Slc30a10 knock-out mice as a new model for studying thyroid biology.

Regenerative therapy for hypothyroidism: Mechanisms and possibilities

The ability to derive functional thyroid follicular cells from embryonic stem cells (ESCs) or induced pluripotent stem cells (iPSCs) would provide potential therapeutic benefit for patients with congenital or post-surgical hypothyroidism. Furthermore, understanding the process by which thyroid follicular cells develop will also provide great insight into the key steps that regulate the development of other tissues derived from endoderm. Here we review the advances in our understanding of the process of thyroid follicular cell development including the creation of two models that have allowed for the rescue of hypothyroid mouse recipients through the transplantation of thyroid follicular cells derived from mouse ESCs. Rapid progress in the field suggests that the same success should be achievable with human ESCs or iPSCs in the near future. Additionally, the availability of ESC or iPSC-derived thyroid follicular cell models will provide ideal systems to explore how genetic mutations, drugs or illness impact thyroid function in a cell-autonomous fashion.

Functional Thyroid Follicular Cells Differentiation from Human-Induced Pluripotent Stem Cells in Suspension Culture

The replacement of regenerated thyroid follicular cells (TFCs) is a promising therapeutic strategy for patients with hypothyroidism. Here, we have succeeded in inducing functional TFCs from human-induced pluripotent stem cells (iPSCs) in scalable suspension culture. Differentiation of iPSCs with Activin A treatment produced Sox17- and FoxA2-expressing definitive endodermal cells that also expressed thyroid transcription factors Pax8 and Nkx2-1. Further treatment with thyroid-stimulating hormone (TSH) induced TFCs expressing various types of thyroid proteins including TSH receptor, sodium-iodide symporter, thyroglobulin, and thyroid peroxidase. Interestingly, differentiated cells secreted free thyroxine in vitro. These results indicate successful differentiation of human iPSCs to functional TFCs that may enable us to fabricate thyroid tissues for regenerative medicine and disease models.

Expanding the Role of Thyroid-Stimulating Hormone in Skeletal Physiology

The dogma that thyroid-stimulating hormone (TSH) solely regulates the production of thyroid hormone from the thyroid gland has hampered research on its wider physiological roles. The action of pituitary TSH on the skeleton has now been well described; in particular, its action on osteoblasts and osteoclasts. It has also been recently discovered that the bone marrow microenvironment acts as an endocrine circuit with bone marrow-resident macrophages capable of producing a novel TSH-β subunit variant (TSH-βv), which may modulate skeletal physiology. Interestingly, the production of this TSH-βv is positively regulated by T3 accentuating such modulation in the presence of thyroid overactivity. Furthermore, a number of small molecule ligands acting as TSH agonists, which allosterically modulate the TSH receptor have been identified and may have similar modulatory influences on bone cells suggesting therapeutic potential. This review summarizes our current understanding of the role of TSH, TSH-β, TSH-βv, and small molecule agonists in bone physiology.

Insulin Resistance: Any Role in the Changing Epidemiology of Thyroid Cancer?

In the past few decades, the incidence of thyroid cancer (TC), namely of its papillary hystotype (PTC), has shown a steady increase worldwide, which has been attributed at least in part to the increasing diagnosis of early stage tumors. However, some evidence suggests that environmental and lifestyle factors can also play a role. Among the potential risk factors involved in the changing epidemiology of TC, particular attention has been drawn to insulin-resistance and related metabolic disorders, such as obesity, type 2 diabetes, and metabolic syndrome, which have been also rapidly increasing worldwide due to widespread dietary and lifestyle changes. In accordance with this possibility, various epidemiological studies have indeed gathered substantial evidence that insulin resistance-related metabolic disorders might be associated with an increased TC risk either through hyperinsulinemia or by affecting other TC risk factors including iodine deficiency, elevated thyroid stimulating hormone, estrogen-dependent signaling, chronic autoimmune thyroiditis, and others. This review summarizes the current literature evaluating the relationship between metabolic disorders characterized by insulin resistance and the risk for TC as well as the possible underlying mechanisms. The potential implications of such association in TC prevention and therapy are discussed.

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.

TSH increases synthesis of hepatic ATP-binding cassette subfamily A member 1 in hypercholesterolemia

Epidemiological evidence suggests that thyrotropin (TSH) levels are closely correlated with the severity of hypercholesterolemia. Reverse cholesterol transfer (RCT) plays an important role in regulating bloodcholesterol. However, the molecular mechanism of hypercholesterolemia in subclinical hypothyroidism (SCH) has not been fully clarified. The SCH mouse model, which is characterized by elevated serum TSH but not thyroid hormone levels, demonstrated a significant increase in plasma cholesterol compared with controls. Interestingly, Tshr KO mice, with normal thyroid hormone levels after thyroid hormone supplementation, showed lower plasma cholesterol levels compared with their wild-type littermates. ATP binding cassette subfamily A member 1(ABCA1) is a member of the ABC superfamily, which induces transfer of intracellular cholesterol to extracellular apolipoprotein. TSH upregulated hepatic ABCA1 to promote the efflux of intercellular cumulative cholesterol, resulting in increased plasma cholesterol. These data might partially explain the pathogenesis of hypercholesterolemia in SCH.

Long Noncoding RNA and mRNA Expression Profiles in the Thyroid Gland of Two Phenotypically Extreme Pig Breeds Using Ribo-Zero RNA Sequencing

The thyroid gland is an important endocrine organ modulating development, growth, and metabolism, mainly by controlling the synthesis and secretion of thyroid hormones (THs). However, little is known about the pig thyroid transcriptome. Long non-coding RNAs (lncRNAs) regulate gene expression and play critical roles in many cellular processes. Yorkshire pigs have a higher growth rate but lower fat deposition than that of Jinhua pigs, and thus, these species are ideal models for studying growth and lipid metabolism. This study revealed higher levels of THs in the serum of Yorkshire pigs than in the serum of Jinhua pigs. By using Ribo-zero RNA sequencing—which can capture both polyA and non-polyA transcripts—the thyroid transcriptome of both breeds were analyzed and 22,435 known mRNAs were found to be expressed in the pig thyroid. In addition, 1189 novel mRNAs and 1018 candidate lncRNA transcripts were detected. Multiple TH-synthesis-related genes were identified among the 455 differentially-expressed known mRNAs, 37 novel mRNAs, and 52 lncRNA transcripts. Bioinformatics analysis revealed that differentially-expressed genes were enriched in the microtubule-based process, which contributes to THs secretion. Moreover, integrating analysis predicted 13 potential lncRNA-mRNA gene pairs. These data expanded the repertoire of porcine lncRNAs and mRNAs and contribute to understanding the possible molecular mechanisms involved in animal growth and lipid metabolism.

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.

Disrupted TSH Receptor Expression in Female Mouse Lung Fibroblasts Alters Subcellular IGF-1 Receptor Distribution

A relationship between the actions of TSH and IGF-1 was first recognized several decades ago. The close physical and functional associations between their respective receptors (TSHR and IGF-1R) has been described more recently in thyroid epithelium and human orbital fibroblasts as has the noncanonical behavior of IGF-1R. Here we report studies conducted in lung fibroblasts from female wild-type C57/B6 (TSHR(+/+)) mice and their littermates in which TSHR has been knocked out (TSHR(-/-)). Flow cytometric analysis revealed that cell surface IGF-1R levels are substantially lower in TSHR(-/-) fibroblasts compared with TSHR(+/+) fibroblasts. Confocal immunofluorescence microscopy revealed similar divergence with regard to both cytoplasmic and nuclear IGF-1R. Western blot analysis demonstrated both intact IGF-1R and receptor fragments in both cellular compartments. In contrast, IGF-1R mRNA levels were similar in fibroblasts from mice without and with intact TSHR expression. IGF-1 treatment of TSHR(+/+) fibroblasts resulted in reduced nuclear and cytoplasmic staining for IGF-1Rα, whereas it enhanced the nuclear signal in TSHR(-/-) cells. In contrast, IGF-1 enhanced cytoplasmic IGF-1Rβ in TSHR(-/-) fibroblasts while increasing the nuclear signal in TSHR(+/+) cells. These findings indicate the intimate relationship between TSHR and IGF-1R found earlier in human orbital fibroblasts also exists in mouse lung fibroblasts. Furthermore, the presence of TSHR in these fibroblasts influenced not only the levels of IGF-1R protein but also its subcellular distribution and response to IGF-1. They suggest that the mouse might serve as a suitable model for delineating the molecular mechanisms overarching these two receptors.

Morphological and functional maturation of Leydig cells: from rodent models to primates

BACKGROUND

Leydig cells (LC) are the sites of testicular androgen production. Development of LC occurs in the testes of most mammalian species as two distinct growth phases, i.e. as fetal and pubertal/adult populations. In primates there are indications of a third neonatal growth phase. LC androgen production begins in embryonic life and is crucial for the intrauterine masculinization of the male fetal genital tract and brain, and continues until birth after which it rapidly declines. A short post-natal phase of LC activity in primates (including human) termed 'mini-puberty' precedes the period of juvenile quiescence. The adult population of LC evolves, depending on species, in mid- to late-prepuberty upon reawakening of the hypothalamic-pituitary-testicular axis, and these cells are responsible for testicular androgen production in adult life, which continues with a slight gradual decline until senescence. This review is an updated comparative analysis of the functional and morphological maturation of LC in model species with special reference to rodents and primates.

METHODS

Pubmed, Scopus, Web of Science and Google Scholar databases were searched between December 2012 and October 2014. Studies published in languages other than English or German were excluded, as were data in abstract form only. Studies available on primates were primarily examined and compared with available data from specific animal models with emphasis on rodents.

RESULTS

Expression of different marker genes in rodents provides evidence that at least two distinct progenitor lineages give rise to the fetal LC (FLC) population, one arising from the coelomic epithelium and the other from specialized vascular-associated cells along the gonad-mesonephros border. There is general agreement that the formation and functioning of the FLC population in rodents is gonadotrophin-responsive but not gonadotrophin-dependent. In contrast, although there is in primates some controversy on the role of gonadotrophins in the formation of the FLC population, there is consensus about the essential role of gonadotrophins in testosterone production. Like the FLC population, adult Leydig cells (ALC) in rodents arise from stem cells, which have their origin in the fetal testis. In contrast, in primates the ALC population is thought to originate from FLC, which undergo several cycles of regression and redifferentiation before giving rise to the mature ALC population, as well as from differentiation of stem cells/precursor cells. Despite this difference in origin, both in primates and rodents the formation of the mature and functionally active ALC population is critically dependent on the pituitary gonadotrophin, LH. From studies on rodents considerable knowledge has emerged on factors that are involved besides LH in the regulation of this developmental process. Whether the same factors also play a role in the development of the mature primate LC population awaits further investigation.

CONCLUSION

Distinct populations of LC develop along the life span of males, including fetal, neonatal (primates) and ALC. Despite differences in the LC lineages of rodents and primates, the end product is a mature population of LC with the main function to provide androgens necessary for the maintenance of spermatogenesis and extra-gonadal androgen actions.

Thyroid-stimulating hormone decreases HMG-CoA reductase phosphorylation via AMP-activated protein kinase in the liver

Cholesterol homeostasis is strictly regulated through the modulation of HMG-CoA reductase (HMGCR), the rate-limiting enzyme of cholesterol synthesis. Phosphorylation of HMGCR inactivates it and dephosphorylation activates it. AMP-activated protein kinase (AMPK) is the major kinase phosphorylating the enzyme. Our previous study found that thyroid-stimulating hormone (TSH) increased the hepatocytic HMGCR expression, but it was still unclear whether TSH affected hepatic HMGCR phosphorylation associated with AMPK. We used bovine TSH (bTSH) to treat the primary mouse hepatocytes and HepG2 cells with or without constitutively active (CA)-AMPK plasmid or protein kinase A inhibitor (H89), and set up the TSH receptor (Tshr)-KO mouse models. The p-HMGCR, p-AMPK, and related molecular expression were tested. The ratios of p-HMGCR/HMGCR and p-AMPK/AMPK decreased in the hepatocytes in a dose-dependent manner following bTSH stimulation. The changes above were inversed when the cells were treated with CA-AMPK plasmid or H89. In Tshr-KO mice, the ratios of liver p-HMGCR/HMGCR and p-AMPK/AMPK were increased relative to the littermate wild-type mice. Consistently, the phosphorylation of acetyl-CoA carboxylase, a downstream target molecule of AMPK, increased. All results suggested that TSH could regulate the phosphorylation of HMGCR via AMPK, which established a potential mechanism for hypercholesterolemia involved in a direct action of the TSH in the liver.

Thyroid-stimulating hormone inhibits adipose triglyceride lipase in 3T3-L1 adipocytes through the PKA pathway

Thyroid-stimulating hormone (TSH) has been shown to play an important role in the regulation of triglyceride (TG) metabolism in adipose tissue. Adipose triglyceride lipase (ATGL) is a rate-limiting enzyme controlling the hydrolysis of TG. Thus far, it is unclear whether TSH has a direct effect on the expression of ATGL. Because TSH function is mediated through the TSH receptor (TSHR), TSHR knockout mice (Tshr-/- mice) (supplemented with thyroxine) were used in this study to determine the effects of TSHR deletion on ATGL expression. These effects were verified in 3T3-L1 adipocytes and potential underlying mechanisms were explored. In the Tshr-/- mice, ATGL expression in epididymal adipose tissue was significantly increased compared with that in Tshr+/+ mice. ATGL expression was observed to increase with the differentiation process of 3T3-L1 preadipocytes. In mature 3T3-L1 adipocytes, TSH significantly suppressed ATGL expression at both the protein and mRNA levels in a dose-dependent manner. Forskolin, which is an activator of adenylate cyclase, suppressed the expression of ATGL in 3T3-L1 adipocytes. The inhibitory effects of TSH on ATGL expression were abolished by H89, which is a protein kinase A (PKA) inhibitor. These results indicate that TSH has an inhibitory effect on ATGL expression in mature adipocytes. The associated mechanism is related to PKA activation.

Thyrotropin increases hepatic triglyceride content through upregulation of SREBP-1c activity

BACKGROUND & AIMS

Hallmarks of non-alcoholic fatty liver disease (NAFLD) are increased triglyceride accumulation within hepatocytes. The prevalence of NAFLD increases steadily with increasing thyrotropin (TSH) levels. However, the underlying mechanisms are largely unknown. Here, we focused on exploring the effect and mechanism of TSH on the hepatic triglyceride content.

METHODS

As the function of TSH is mediated through the TSH receptor (TSHR), Tshr(-/-) mice (supplemented with thyroxine) were used. Liver steatosis and triglyceride content were analysed in Tshr(-/-) and Tshr(+/+) mice fed a high-fat or normal chow diet, as well as in Srebp-1c(-/-) and Tshr(-/-)Srebp-1c(-/-) mice. The expression levels of proteins and genes involved in liver triglyceride metabolism was measured.

RESULTS

Compared with control littermates, the high-fat diet induced a relatively low degree of liver steatosis in Tshr(-/-) mice. Even under chow diet, hepatic triglyceride content was decreased in Tshr(-/-) mice. TSH caused concentration- and time-dependent effects on intracellular triglyceride contents in hepatocytes in vitro. The activity of SREBP-1c, a key regulator involved in triglyceride metabolism and in the pathogenesis of NAFLD, was significantly lower in Tshr(-/-) mice. In Tshr(-/-)Srebp-1c(-/-) mice, the liver triglyceride content showed no significant difference compared with Tshr(+/+)Srebp-1c(-/-) mice. When mice were injected with forskolin (cAMP activator), H89 (inhibitor of PKA) or AICAR (AMPK activator), or HeG2 cells received MK886 (PPARα inhibitor), triglyceride contents presented in a manner dependent on SREBP-1c activity. The mechanism, underlying TSH-induced liver triglyceride accumulation, involved that TSH, through its receptor TSHR, triggered hepatic SREBP-1c activity via the cAMP/PKA/PPARα pathway associated with decreased AMPK, which further increased the expression of genes associated with lipogenesis.

CONCLUSIONS

TSH increased the hepatic triglyceride content, indicating an essential role for TSH in the pathogenesis of NAFLD.

Thyrotoropin receptor knockout changes monoaminergic neuronal system and produces methylphenidate-sensitive emotional and cognitive dysfunction

Attention deficit/hyperactivity disorder (ADHD) has been reported in association with resistance to thyroid hormone, a disease caused by a mutation in the thyroid hormone receptor β (TRβ) gene. TRβ is a key protein mediating down-regulation of thyrotropin (TSH) expression by 3,3',5-tri-iodothyronine (T3), an active form of thyroid hormone. Dysregulation of TSH and its receptor (TSHR) is implicated in the pathophysiology of ADHD but the role of TSHR remains elusive. Here, we clarified a novel role for TSHR in emotional and cognitive functions related to monoaminergic nervous systems. TSHR knockout mice showed phenotypes of ADHD such as hyperactivity, impulsiveness, a decrease in sociality and increase in aggression, and an impairment of short-term memory and object recognition memory. Administration of methylphenidate (1, 5 and 10mg/kg) reversed impulsiveness, aggression and object recognition memory impairment. In the knockout mice, monoaminergic changes including decrease in the ratio of 3-methoxy-4-hydroxyphenylglycol/noradrenaline and increase in the ratio of homovanillic acid/dopamine were observed in some brain regions, accompanied by increase in the expression of noradrenaline transporter in the frontal cortex. When TSH was completely suppressed by the supraphysiological administration of T3 to the adult mice, some behavioral and neurological changes in TSHR KO mice were also observed, suggesting that these changes were not due to developmental hypothyroidism induced by the inactivation of TSHR but to the loss of the TSH-TSHR pathway itself. Taken together, the present findings suggest a novel role for TSHR in behavioral and neurological phenotypes of ADHD.