Thyrotropin and homologous glycoprotein hormone receptors: structural and functional aspects of extracellular signaling mechanisms

The full-length TSH receptor is stabilized by TSH ligand

The receptor for thyroid stimulating hormone (TSHR), a GPCR, is the primary antigen in autoimmune hyperthyroidism (Graves' disease) caused by stimulating TSHR antibodies. While we have previously published a full length model of the TSHR, including its leucine rich domain (LRD), linker region (LR) and transmembrane domain (TMD), to date, only a partial LRD (aa 21-261) stabilized with TSHR autoantibodies has been crystallized. Recently, however, cryo-EM structures of the full-length TSHR have been published but they include only an incomplete LR. We have now utilized the cryo-EM models, added disulfide bonds to the LR and performed longer (3000 ns) molecular dynamic (MD) simulations to update our previous model of the entire full-length TSHR, with and without the presence of TSH ligand. As in our earlier work, the new model was embedded in a lipid membrane and was solvated with water and counterions. We found that the 3000 ns Molecular Dynamic simulations showed that the structure of the LRD and TMD were remarkably constant while the LR, known more commonly as the "hinge region", again showed significant flexibility, forming several transient secondary structural elements. Analysis of the new simulations permitted a detailed examination of the effect of TSH binding on the structure of the TSHR. We found a structure-stabilizing effect of TSH, including increased stability of the LR, which was clearly demonstrated by analyzing several intrinsic receptor properties including hydrogen bonding, fluctuation of the LRD orientation, and radius of gyration. In conclusion, we were able to quantify the flexibility of the TSHR and show its increased stability after TSH binding. These data indicated the important role of ligands in directing the signaling structure of a receptor.

Pituitary crosstalk with bone, adipose tissue and brain

Traditional textbook physiology has ascribed unitary functions to hormones from the anterior and posterior pituitary gland, mainly in the regulation of effector hormone secretion from endocrine organs. However, the evolutionary biology of pituitary hormones and their receptors provides evidence for a broad range of functions in vertebrate physiology. Over the past decade, we and others have discovered that thyroid-stimulating hormone, follicle-stimulating hormone, adrenocorticotropic hormone, prolactin, oxytocin and arginine vasopressin act directly on somatic organs, including bone, adipose tissue and liver. New evidence also indicates that pituitary hormone receptors are expressed in brain regions, nuclei and subnuclei. These studies have prompted us to attribute the pathophysiology of certain human diseases, including osteoporosis, obesity and neurodegeneration, at least in part, to changes in pituitary hormone levels. This new information has identified actionable therapeutic targets for drug discovery.

Intramolecular activity regulation of adhesion GPCRs in light of recent structural and evolutionary information

The class B2 of GPCRs known as adhesion G protein-coupled receptors (aGPCRs) has come under increasing academic and nonacademic research focus over the past decade due to their physiological importance as mechano-sensors in cell-cell and cell-matrix contexts. A major advance in understanding signal transduction of aGPCRs was achieved by the identification of the so-called Stachel sequence, which acts as an intramolecular agonist at the interface between the N terminus (Nt) and the seven-transmembrane helix domain (7TMD). Distinct extracellular signals received by the Nt are integrated at the Stachel into structural changes of the 7TMD towards an active state conformation. Until recently, little information was available on how the activation process of aGPCRs is realized at the molecular level. In the past three years several structures of the 7TMD plus the Stachel in complex with G proteins have been determined, which provide new insights into the architecture and molecular function of this receptor class. Herein, we review this structural information to extract common and distinct aGPCR features with particular focus on the Stachel binding site within the 7TMD. Our analysis extends the current view of aGPCR activation and exposes similarities and differences not only between diverse aGPCR members, but also compared to other GPCR classes.

Menge W, Martens G, op het Veld SJ, Th.H. van Eupen J, Demon M, van Achterberg TA, Arisse-Thijssen MJ, Santegoeds-Lenssen EW, van der Lee MM, Ubink R, Arends RJ, Sesink A, Blomenröhr M, Marco Timmers C. Discovery of SYD5115, a novel orally active small molecule TSH-R antagonist

The thyrotropin receptor (TSH-R) regulates the thyroid gland and is normally activated by thyrotropin. In patients with Graves' disease, TSH-R is also stimulated by stimulatory TSH-R autoantibodies leading to hyperthyroidism. In this paper, we describe the discovery of SYD5115 (67), a novel small molecule TSH-R antagonist with nanomolar potency. SYD5115 also blocks stimulating antibody induced synthesis of the thyroid hormone thyroxine (T₄) in vivo, after a single oral dose. During optimization, several issues had to be addressed such as the low metabolic stability and the potential mutagenicity of our first series of compounds.

Allosteric Regulation of G-Protein-Coupled Receptors: From Diversity of Molecular Mechanisms to Multiple Allosteric Sites and Their Ligands

Allosteric regulation is critical for the functioning of G protein-coupled receptors (GPCRs) and their signaling pathways. Endogenous allosteric regulators of GPCRs are simple ions, various biomolecules, and protein components of GPCR signaling (G proteins and β-arrestins). The stability and functional activity of GPCR complexes is also due to multicenter allosteric interactions between protomers. The complexity of allosteric effects caused by numerous regulators differing in structure, availability, and mechanisms of action predetermines the multiplicity and different topology of allosteric sites in GPCRs. These sites can be localized in extracellular loops; inside the transmembrane tunnel and in its upper and lower vestibules; in cytoplasmic loops; and on the outer, membrane-contacting surface of the transmembrane domain. They are involved in the regulation of basal and orthosteric agonist-stimulated receptor activity, biased agonism, GPCR-complex formation, and endocytosis. They are targets for a large number of synthetic allosteric regulators and modulators, including those constructed using molecular docking. The review is devoted to the principles and mechanisms of GPCRs allosteric regulation, the multiplicity of allosteric sites and their topology, and the endogenous and synthetic allosteric regulators, including autoantibodies and pepducins. The allosteric regulation of chemokine receptors, proteinase-activated receptors, thyroid-stimulating and luteinizing hormone receptors, and beta-adrenergic receptors are described in more detail.

Mechanism of hormone and allosteric agonist mediated activation of follicle stimulating hormone receptor

Follicle stimulating hormone (FSH) is an essential glycoprotein hormone for human reproduction, which functions are mediated by a G protein-coupled receptor, FSHR. Aberrant FSH-FSHR signaling causes infertility and ovarian hyperstimulation syndrome. Here we report cryo-EM structures of FSHR in both inactive and active states, with the active structure bound to FSH and an allosteric agonist compound 21 f. The structures of FSHR are similar to other glycoprotein hormone receptors, highlighting a conserved activation mechanism of hormone-induced receptor activation. Compound 21 f formed extensive interactions with the TMD to directly activate FSHR. Importantly, the unique residue H615^7.42 in FSHR plays an essential role in determining FSHR selectivity for various allosteric agonists. Together, our structures provide a molecular basis of FSH and small allosteric agonist-mediated FSHR activation, which could inspire the design of FSHR-targeted drugs for the treatment of infertility and controlled ovarian stimulation for in vitro fertilization.

Thyroid-Stimulating Hormone Receptor: the Role in the Development of Thyroid Pathology and Its Correction

Abstract

One of the key elements responsible for the thyroid response to thyroid-stimulating hormone (TSH) is the TSH receptor (TSHR), which belongs to the G protein-coupled receptor superfamily. Binding of TSH or stimulatory autoantibodies to the TSHR extracellular domain triggers multiple signaling pathways in target cells that are mediated through various types of G proteins and β-arrestins. Inhibitory autoantibodies, in contrast, suppress TSHR activity, inducing hypothyroid states. Activating mutations lead to constitutively active TSHR forms and can trigger cancer. Therefore, the TSHR is one of the key targets for the regulation of thyroid function and thyroid status, as well as correction of diseases caused by changes in TSHR activity (autoimmune hyper- and hypothyroidism, Graves’ ophthalmopathy, thyroid cancer). TSH preparations are extremely rarely used in medicine due to their immunogenicity and severe side effects. Most promising is the development of low-molecular allosteric TSHR regulators with an activity of full and inverse agonists and neutral antagonists, which are able to penetrate into the allosteric site located in the TSHR transmembrane domain and specifically bind to it, thus controlling the ability of the receptor to interact with G proteins and β-arrestins. Allosteric regulators do not affect the binding of TSH and autoantibodies to the receptor, which enables mild and selective regulation of thyroid function, while avoiding critical changes in TSH and thyroid hormone levels. The present review addresses the current state of the problem of regulating TSHR activity, including the possibility of using ligands of its allosteric sites.

Hormone- and antibody-mediated activation of the thyrotropin receptor

Thyroid stimulating hormone (TSH), through activation of its G protein-coupled thyrotropin receptor (TSHR), controls the synthesis of thyroid hormone (TH), an essential metabolic hormone^1-3. Aberrant signaling of TSHR by autoantibodies causes Graves' disease and hypothyroidism that affect millions of patients worldwide⁴. Here we report the active structures of TSHR with TSH and an activating autoantibody M22⁵, both bound to an allosteric agonist ML-109⁶, as well as an inactivated TSHR structure with inhibitory antibody K1-70⁷. Both TSH and M22 push the extracellular domain (ECD) of TSHR into the upright active conformation. In contrast, K1-70 blocks TSH binding and is incapable of pushing the ECD to the upright conformation. Comparisons of the active and inactivated structures of TSHR with those of the luteinizing hormone-choriogonadotropin receptor (LHCGR) reveal a universal activation mechanism of glycoprotein hormone receptors, in which a conserved 10-residue fragment (P10) from the hinge C-terminal loop mediates ECD interactions with the TSHR transmembrane domain⁸. One surprisingly feature is that there are over 15 cholesterols surrounding TSHR, supporting its preferential location in lipid rafts⁹. These structures also highlight a similar ECD-push mechanism for TSH and autoantibody M22 to activate TSHR, thus providing the molecular basis for Graves' disease.

Expression and Characterization of Relaxin Family Peptide Receptor 1 Variants

G-protein coupled receptors (GPCR) transduce extracellular stimuli into the cell interior and are thus centrally involved in almost all physiological-neuronal processes. This essential function and association with many diseases or pathological conditions explain why GPCRs are one of the priority targets in medical and pharmacological research, including structure determination. Despite enormous experimental efforts over the last decade, both the expression and purification of these membrane proteins remain elusive. This is attributable to specificities of each GPCR subtype and the finding of necessary experimental in vitro conditions, such as expression in heterologous cell systems or with accessory proteins. One of these specific GPCRs is the leucine-rich repeat domain (LRRD) containing GPCR 7 (LGR7), also termed relaxin family peptide receptor 1 (RXFP1). This receptor is characterized by a large extracellular region of around 400 amino acids constituted by several domains, a rare feature among rhodopsin-like (class A) GPCRs. In the present study, we describe the expression and purification of RXFP1, including the design of various constructs suitable for functional/biophysical studies and structure determination. Based on available sequence information, homology models, and modern biochemical and genetic tools, several receptor variations with different purification tags and fusion proteins were prepared and expressed in Sf9 cells (small-scale), followed by an analytic fluorescence-detection size-exclusion chromatography (F-SEC) to evaluate the constructs. The most promising candidates were expressed and purified on a large-scale, accompanied by ligand binding studies using surface plasmon resonance spectroscopy (SPR) and by determination of signaling capacities. The results may support extended studies on RXFP1 receptor constructs serving as targets for small molecule ligand screening or structural elucidation by protein X-ray crystallography or cryo-electron microscopy.

Expression and Purification of the Human Thyroid-Stimulating Hormone Receptor

The thyroid-stimulating hormone receptor (TSHR) is a Class A G protein-coupled receptor (GPCR) that mediates signalling through the hypothalamic-pituitary-thyroid axis. Inappropriate activation of TSHR by autoantibodies or mutations, results in human disease such as Grave's disease and Hashimito's thyroiditis. Therefore, there is a need to develop novel therapeutics targeting the TSHR. Understanding the structure and mechanism of activation of this receptor would help elucidate the pathogenesis of disease and aid drug development. Here, we describe a method for the expression of the human TSHR in a mammalian cell line generated through a lentiviral expression system. The receptor is then purified by affinity chromatography in the ligand-free state and is suitable for structure determination by single-particle electron cryo-microscopy (cryo-EM).

Misfolded G Protein-Coupled Receptors and Endocrine Disease. Molecular Mechanisms and Therapeutic Prospects

Misfolding of G protein-coupled receptors (GPCRs) caused by mutations frequently leads to disease due to intracellular trapping of the conformationally abnormal receptor. Several endocrine diseases due to inactivating mutations in GPCRs have been described, including X-linked nephrogenic diabetes insipidus, thyroid disorders, familial hypocalciuric hypercalcemia, obesity, familial glucocorticoid deficiency [melanocortin-2 receptor, MC2R (also known as adrenocorticotropin receptor, ACTHR), and reproductive disorders. In these mutant receptors, misfolding leads to endoplasmic reticulum retention, increased intracellular degradation, and deficient trafficking of the abnormal receptor to the cell surface plasma membrane, causing inability of the receptor to interact with agonists and trigger intracellular signaling. In this review, we discuss the mechanisms whereby mutations in GPCRs involved in endocrine function in humans lead to misfolding, decreased plasma membrane expression of the receptor protein, and loss-of-function diseases, and also describe several experimental approaches employed to rescue trafficking and function of the misfolded receptors. Special attention is given to misfolded GPCRs that regulate reproductive function, given the key role played by these particular membrane receptors in sexual development and fertility, and recent reports on promising therapeutic interventions targeting trafficking of these defective proteins to rescue completely or partially their normal function.

Obese patients with higher TSH levels had an obvious metabolic improvement after bariatric surgery

OBJECTIVE

Bariatric surgery has become the most effective treatment for morbid obesity. Increasing evidence showed that bariatric surgery can alleviate insulin resistance and influence thyroid function. This study aimed to investigate the relationship between changes in thyroid function and adipose tissue insulin resistance (adipo-IR) after bariatric surgery.

METHODS

A total of 287 non-diabetic participants with regular thyroid function were recruited and divided into the lean, overweight and obese groups. Among them, 50 morbidly obese patients submitted to bariatric surgery.

RESULTS

The obese group had a higher level of adipo-IR, thyroid-stimulating hormone (TSH), free triiodothyronine (FT3), FT3/free thyroxine (FT4) and metabolism disorders than the lean and overweight groups. BMI was correlated with TSH, FT3, FT3/FT4 and adipo-IR (r = 0.309, 0.315, 0.322 and 0.651, respectively, all P < 0.001). Adipo-IR was significantly correlated with TSH (r = 0.402, P < 0.001), FT3 (r = 0.309, P < 0.001), and FT3/FT4 (r = 0.228, P < 0.05). Bariatric surgery resulted in a sharp decline in BMI, adipo-IR, TSH, FT3 and FT3/FT4 levels, meanwhile, metabolic disorders improved. The decrease in BMI after bariatric surgery was significantly correlated with reductions in adipo-IR (r = 0.577, P < 0.001) and TSH (r = 0.401, P = 0.005). Interestingly, the fasting blood glucose, fasting insulin, adipo-IR and TSH in the higher TSH group decreased more remarkably than in the lower TSH group.

CONCLUSION

Obese individuals with higher TSH levels had an obvious metabolic improvement after bariatric surgery.

Functional characterization of naturally-occurring constitutively activating/inactivating mutations in equine follicle-stimulating hormone receptor (eFSHR)

Objective

Follicle-stimulating hormone (FSH) is the central hormone involved in mammalian reproduction, maturation at puberty, and gamete production that mediates its function by control of follicle growth and function. The present study investigated the mutations involved in the regulation of FSH receptor (FSHR) activation.

Methods

We analyzed seven naturally-occurring mutations that were previously reported in human FSHR (hFSHR), in the context of equine FSHR (eFSHR); these include one constitutively activation variant, one allelic variant, and five inactivating variants. These mutations were introduced into wild-type eFSHR (eFSHR-wt) sequence to generate mutants that were designated as eFSHR-D566G, -A306T, -A189V, -N191I, -R572C, -A574V, and -R633H. Mutants were transfected into PathHunter EA-parental CHO-K1 cells expressing β-arrestin. The biological function of mutants was analyzed by quantitating cAMP accumulation in cells incubated with increasing concentrations of FSH.

Results

Cells expressing eFSHR-D566G exhibited an 8.6-fold increase in basal cAMP response, as compared to that in eFSHR-wt. The allelic variation mutant eFSHR-A306T was not found to affect the basal cAMP response or EC50 levels. On the other hand, eFSHR-D566G and eFSHR-A306T displayed a 1.5- and 1.4-fold increase in the maximal response, respectively. Signal transduction was found to be completely impaired in case of the inactivating mutants eFSHR-A189V, -R572C, and -A574V. When compared with eFSHR-wt, eFSHR-N191I displayed a 5.4-fold decrease in the EC50 levels (3910 ng/mL) and a 2.3-fold decrease in the maximal response. In contrast, cells expressing eFSHR-R633H displayed in a similar manner to that of the cells expressing the eFSHR-wt on signal transduction and maximal response.

Conclusion

The activating mutant eFSHR-D566G greatly enhanced the signal transduction in response to FSH, in the absence of agonist treatment. We suggest that the state of activation of the eFSHR can modulate its basal cAMP accumulation.

Teprotumumab: Interpreting the Clinical Trials in the Context of Thyroid Eye Disease Pathogenesis and Current Therapies

Teprotumumab, a monoclonal antibody targeted against the insulin-like growth factor 1 (IGF-1) receptor, was recently approved by the United States Food and Drug Administration for the treatment of thyroid eye disease (TED). Phase 1 studies of teprotumumab for the treatment of malignancies demonstrated an acceptable safety profile but limited effectiveness. Basic research implicating the IGF-1 receptor on the CD-34+ orbital fibrocyte in the pathogenesis of TED renewed interest in the drug. Two multicenter, randomized, double-masked, clinical trials (phase 2 and 3) evaluated the efficacy of 8 infusions of teprotumumab every 3 weeks versus placebo in 170 patients with recent-onset active TED, as defined by a clinical activity score (CAS) of at least 4. Teprotumumab was superior to placebo for the primary efficacy end points in both studies: overall responder rate as defined by a reduction of 2 or more CAS points and a reduction of 2 mm or more in proptosis (69% vs. 20%; P < 0.001; phase 2 study) and proptosis responder rate as defined by a reduction of 2 mm or more in proptosis (83% vs. 10%; P < 0.001; phase 3 study). In both studies, treatment with teprotumumab compared with placebo achieved a significant mean reduction of proptosis (-3.0 mm vs. -0.3 mm, phase 2 study; -3.32 mm vs. -0.53 mm, phase 3 study) and CAS (-4.0 vs. -2.5, phase 2 study; -3.7 vs. -2.0, phase 3 study). Teprotumumab also resulted in a greater proportion of patients with a final CAS of 0 or 1, higher diplopia responder rate, and a larger improvement in the Graves' Ophthalmopathy Quality of Life overall score. More than half of patients (62%, phase 2 trial; 56%, phase 3 trial) who were primary end point responders maintained this response at 51 weeks after the last dose of therapy. The most common adverse events reported with teprotumumab included muscle spasms (25%), nausea (17%), alopecia (13%), diarrhea (13%), fatigue (10%), hearing impairment (10%), and hyperglycemia (8%). Teprotumumab is contraindicated for those with inflammatory bowel disease and who are pregnant. Although the current dosing regimen has proven effective for TED, dose-ranging studies including variable concentrations, infusion frequencies, and durations of teprotumumab therapy in the setting of TED have not been performed.

Targeting TSH and IGF-1 Receptors to Treat Thyroid Eye Disease

Graves' disease (GD) is an autoimmune disease caused in part by thyroid-stimulating antibodies (TSAbs) that activate the thyroid-stimulating hormone receptor (TSHR). In Graves' hyperthyroidism (GH), TSAbs cause persistent stimulation of thyroid cells leading to continuous thyroid hormone synthesis and secretion. Thyroid eye disease (TED), also called Graves' orbitopathy, is an orbital manifestation of GD. We review the important roles of the TSHR and the insulin-like growth factor 1 receptor (IGF-1R) in the pathogenesis of TED and discuss a model of TSHR/IGF-1R crosstalk that considers two pathways initiated by TSAb activation of TSHR in the eye, an IGF-1R-independent and an IGF-1R-dependent signaling pathway leading to hyaluronan (HA) secretion in orbital fibroblasts. We discuss current and future therapeutic approaches targeting the IGF-1R and TSHR. Teprotumumab, a human monoclonal anti-IGF-1R-blocking antibody, has been approved as an effective treatment in patients with TED. However, as the TSHR seems to be the primary target for TSAbs in patients with GD, future therapeutic interventions directly targeting the TSHR, e.g. blocking antibodies and small molecule antagonists, are being developed and have the advantage to inhibit the IGF-1R-independent as well as the IGF-1R-dependent component of TSAb-induced HA secretion. Antigen-specific immunotherapies using TSHR peptides to reduce serum TSHR antibodies are being developed also. These TSHR-targeted strategies also have the potential to treat both GH and TED with the same drug. We propose that combination therapy targeting TSHR and IGF-1R may be an effective and better tolerated treatment strategy for TED.

The intramolecular agonist is obligate for activation of glycoprotein hormone receptors

In contrast to most rhodopsin-like G protein-coupled receptors, the glycoprotein hormone receptors (GPHR) have a large extracellular N-terminus for hormone binding. The hormones do not directly activate the transmembrane domain but mediate their action via a, thus, far only partially known Tethered Agonistic LIgand (TALI). The existence of such an intramolecular agonist was initially indicated by site-directed mutation studies and activating peptides derived from the extracellular hinge region. It is still unknown precisely how TALI is involved in intramolecular signal transmission. We combined systematic mutagenesis studies at the luteinizing hormone receptor and the thyroid-stimulating hormone receptor (TSHR), stimulation with a drug-like agonist (E2) of the TSHR, and structural homology modeling to unravel the functional and structural properties defining the TALI region. Here, we report that TALI (a) is predisposed to constitutively activate GPHR, (b) can by itself rearrange GPHR into a fully active conformation, (c) stabilizes active GPHR conformation, and (d) is not involved in activation of the TSHR by E2. In the active state conformation, TALI forms specific interactions between the N-terminus and the transmembrane domain. We show that stabilization of an active state is dependent on TALI, including activation by hormones and constitutively activating mutations.

Graves' disease

Graves' disease (GD) is an autoimmune disease that primarily affects the thyroid gland. It is the most common cause of hyperthyroidism and occurs at all ages but especially in women of reproductive age. Graves' hyperthyroidism is caused by autoantibodies to the thyroid-stimulating hormone receptor (TSHR) that act as agonists and induce excessive thyroid hormone secretion, releasing the thyroid gland from pituitary control. TSHR autoantibodies also underlie Graves' orbitopathy (GO) and pretibial myxoedema. Additionally, the pathophysiology of GO (and likely pretibial myxoedema) involves the synergism of insulin-like growth factor 1 receptor (IGF1R) with TSHR autoantibodies, causing retro-orbital tissue expansion and inflammation. Although the aetiology of GD remains unknown, evidence indicates a strong genetic component combined with random potential environmental insults in an immunologically susceptible individual. The treatment of GD has not changed substantially for many years and remains a choice between antithyroid drugs, radioiodine or surgery. However, antithyroid drug use can cause drug-induced embryopathy in pregnancy, radioiodine therapy can exacerbate GO and surgery can result in hypoparathyroidism or laryngeal nerve damage. Therefore, future studies should focus on improved drug management, and a number of important advances are on the horizon.

The rs1991517 polymorphism is a genetic risk factor for congenital hypothyroidism

The objective of the current study is to explore the association of thyroid-stimulating hormone receptor (TSHR) rs1991517 polymorphism (c.2337 C > G, p.D727E) with congenital hypothyroidism (CH) through a case-control study followed by a meta-analysis. The case-control study was based on 45 CH subjects and 700 healthy controls. Meta-analysis comprised of seven published studies and our current findings (1044 CH cases and 1649 healthy controls). The allele contrast model showed that the presence of G- allele increased CH risk by 45% (OR: 1.45, 95% CI 1.20-1.76) and 41% (OR: 1.41, 95% CI 1.03-1.93) in fixed effect and random effect models, respectively. The GG- genotype is associated with 2.3-fold (95% CI 1.32-3.99) increased risk for CH in the fixed-effect model. I ² (0.58) and Cochran's Q test (Q: 16.72, p = 0.02) revealed evidence of heterogeneity in the association. No publication bias was observed by Egger's test (p = 0.70). Sensitivity analysis revealed that even after excluding any study this polymorphism is associated with risk for CH. The rs1991517 mutation alters the binding affinity to cAMP (ΔG of 727D vs.727E: - 7.27 vs. - 7.34 kcal/mol). In conclusion, rs1991517 is a genetic risk factor for CH and exerts its impact by altering cAMP-mediated signal transduction.

Slit2 May Underlie Divergent Induction by Thyrotropin of IL-23 and IL-12 in Human Fibrocytes

IL-23 and IL-12, two structurally related heterodimeric cytokines sharing a common subunit, divergently promote Th cell development and expansion. Both cytokines have been implicated in the pathogenesis of thyroid-associated ophthalmopathy (TAO), an autoimmune component of Graves disease. In TAO, CD34+ fibrocytes, putatively derived from bone marrow, can be identified in the orbit. There they masquerade as CD34+ orbital fibroblasts (OF) (CD34+ OF) and cohabitate with CD34- OF in a mixed fibroblast population (GD-OF). Slit2, a neural axon repellent, is expressed and released by CD34- OF and dampens the inflammatory phenotype of fibrocytes and CD34+ OF. In this study we report that thyrotropin (TSH) and the pathogenic, GD-specific monoclonal autoantibody, M22, robustly induce IL-23 in human fibrocytes; however, IL-12 expression is essentially undetectable in these cells under basal conditions or following TSH-stimulation. In contrast, IL-12 is considerably more inducible in GD-OF, cells failing to express IL-23. This divergent expression and induction of cytokines appears to result from cell type-specific regulation of both gene transcription and mRNA stabilities. It appears that the JNK pathway activity divergently attenuates IL-23p19 expression while enhancing that of IL-12p35. The shift from IL-23p19 expression in fibrocytes to that of IL-12p35 in their derivative CD34+ OF results from the actions of Slit2. Thus, Slit2 might represent a molecular determinant of balance between IL-23 and IL-12 expression, potentially governing immune responses in TAO.

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.

Expression of BANCR promotes papillary thyroid cancer by targeting thyroid stimulating hormone receptor

Papillary thyroid carcinoma (PTC) is the most common form of non-medullary thyroid cancer, accounting for ~80% of all cases of thyroid cancer. The aim of the present study was to explore the role of BRAF-activated long noncoding RNA (BANCR) in the development of PTC. Using reverse transcription-quantitative polymerase chain reaction (RT-qPCR), the mRNA expression levels of BANCR, thyroid-stimulating hormone receptor (TSHR) and cyclin D1 between PTC and benign control thyroid nodule tissue samples from 60 patients were determined. Using RT-qPCR and western blot analysis, the expression levels of TSHR and cyclin D1 mRNA and protein were determined in cells transfected with BANCR-small interfering (si)RNA. An MTT assay and flow cytometry were used to analyze the effect of BANCR knockdown on the proliferation and cell cycle distribution of IHH-4 PTC cells. The expression of BANCR, TSHR and cyclin D1 was increased in the PTC group compared with the control group based on the RT-qPCR data. The transfection of IHH-4 cells with BANCR-siRNA induced the inhibition of TSHR and cyclin D1 expression compared with a transfection control. In addition, the proliferation of the IHH-4 cells transfected with BANCR-siRNA was suppressed, relative to the transfection control, and cells arrested in the G0/G1 phase, potentially due to the inhibition of the expression of cyclin D1. The data suggested that the expression of BANCR may promote the development of malignant thyroid nodules via the modulation of TSHR expression and its downstream effector, cyclin D1.

Intracellular Trafficking of Gonadotropin Receptors in Health and Disease

Gonadotropin receptors belong to the highly conserved subfamily of the G protein-coupled receptor (GPCR) superfamily, the so-called Rhodopsin-like family (class A), which is the largest class of GPCRs and currently a major drug target. Both the follicle-stimulating hormone receptor (FSHR) and the luteinizing hormone/chorionic gonadotropin hormone receptor (LHCGR) are mainly located in the gonads where they play key functions associated to essential reproductive functions. As any other protein, gonadotropin receptors must be properly folded into a mature tertiary conformation compatible with quaternary assembly and endoplasmic reticulum export to the cell surface plasma membrane. Several primary and secondary structural features, including presence of particular amino acid residues and short motifs and in addition, posttranslational modifications, regulate intracellular trafficking of gonadotropin receptors to the plasma membrane as well as internalization and recycling of the receptor back to the cell surface after activation by agonist. Inactivating mutations of gonadotropin receptors may derive from receptor misfolding and lead to absent or reduced plasma membrane expression of the altered receptor, thereby manifesting an array of phenotypical abnormalities mostly characterized by reproductive failure and/or abnormal or absence of development of secondary sex characteristics. In this chapter we review the structural requirements necessary for intracellular trafficking of the gonadotropin receptors, and describe how mutations in these receptors may lead to receptor misfolding and disease in humans.

Structure-Function Relationships of the Follicle-Stimulating Hormone Receptor

The follicle-stimulating hormone receptor (FSHR) plays a crucial role in reproduction. This structurally complex receptor is a member of the G-protein coupled receptor (GPCR) superfamily of membrane receptors. As with the other structurally similar glycoprotein hormone receptors (the thyroid-stimulating hormone and luteinizing hormone-chorionic gonadotropin hormone receptors), the FSHR is characterized by an extensive extracellular domain, where binding to FSH occurs, linked to the signal specificity subdomain or hinge region. This region is involved in ligand-stimulated receptor activation whereas the seven transmembrane domain is associated with receptor activation and transmission of the activation process to the intracellular loops comprised of amino acid sequences, which predicate coupling to effectors, interaction with adapter proteins, and triggering of downstream intracellular signaling. In this review, we describe the most important structural features of the FSHR intimately involved in regulation of FSHR function, including trafficking, dimerization, and oligomerization, ligand binding, agonist-stimulated activation, and signal transduction.

[Pharmacological approaches for correction of thyroid dysfunctions in diabetes mellitus]

Thyroid diseases are closely associated with the development of types 1 and 2 diabetes mellitus (DM), and as a consequence, the development of effective approaches for their treatment is one of the urgent problems of endocrinology. Traditionally, thyroid hormones (TH) are used to correct functions of the thyroid system. However, they are characterized by many side effects, such as their negative effect on the cardiovascular system as well as the ability of TH to enhance insulin resistance and to disturb insulin-producing function of pancreas, exacerbating thereby diabetic pathology. Therefore, the analogues of TH, selective for certain types of TH receptors, that do not have these side effects, are being developed. The peptide and low-molecular weight regulators of thyroid-stimulating hormone receptor, which regulate the activity of the thyroid axis at the stage of TH synthesis and secretion in thyrocytes, are being created. Systemic and intranasal administration of insulin, metformin therapy and drugs with antioxidant activity are effective for the treatment of thyroid pathology in types 1 and 2 DM. In the review, the literature data and the results of own investigations on pharmacological approaches for the treatment and prevention of thyroid diseases in patients with types 1 and 2 DM are summarized and analyzed.

TSHR as a therapeutic target in Graves' disease

INTRODUCTION

Graves' disease (GD) and thyroid-associated ophthalmopathy (TAO) are thought to result from actions of pathogenic antibodies mediated through the thyrotropin receptor (TSHR). This leads to the unregulated consequences of the antibody-mediated receptor activity in the thyroid and connective tissues of the orbit. Recent studies reveal antibodies that appear to be directed against the insulin-like growth factor-I receptor (IGF-IR). Areas covered: In this brief article, I attempt to review the fundamental characteristics of the TSHR, its role in GD and TAO, and its relationship to IGF-IR. Strong evidence supports the concept that the two receptors form a physical and functional complex and that IGF-IR activity is required for some of the down-stream signaling initiated through TSHR. Recently developed small molecules and monoclonal antibodies that block TSHR and IGF-IR signaling are also reviewed in the narrow context of their potential utility as therapeutics in GD and TAO. The Pubmed database was searched from its inception for relevant publications. Expert opinion: Those agents that can interrupt the TSHR and IGF-IR pathways possess the potential for offering more specific and better tolerated treatments of both hyperthyroidism and TAO. This would spare patients exposure to toxic drugs, ionizing radiation and potentially hazardous surgeries.

Structural-Functional Features of the Thyrotropin Receptor: A Class A G-Protein-Coupled Receptor at Work

The thyroid-stimulating hormone receptor (TSHR) is a member of the glycoprotein hormone receptors, a sub-group of class A G-protein-coupled receptors (GPCRs). TSHR and its endogenous ligand thyrotropin (TSH) are of essential importance for growth and function of the thyroid gland and proper function of the TSH/TSHR system is pivotal for production and release of thyroid hormones. This receptor is also important with respect to pathophysiology, such as autoimmune (including ophthalmopathy) or non-autoimmune thyroid dysfunctions and cancer development. Pharmacological interventions directly targeting the TSHR should provide benefits to disease treatment compared to currently available therapies of dysfunctions associated with the TSHR or the thyroid gland. Upon TSHR activation, the molecular events conveying conformational changes from the extra- to the intracellular side of the cell across the membrane comprise reception, conversion, and amplification of the signal. These steps are highly dependent on structural features of this receptor and its intermolecular interaction partners, e.g., TSH, antibodies, small molecules, G-proteins, or arrestin. For better understanding of signal transduction, pathogenic mechanisms such as autoantibody action and mutational modifications or for developing new pharmacological strategies, it is essential to combine available structural data with functional information to generate homology models of the entire receptor. Although so far these insights are fragmental, in the past few decades essential contributions have been made to investigate in-depth the involved determinants, such as by structure determination via X-ray crystallography. This review summarizes available knowledge (as of December 2016) concerning the TSHR protein structure, associated functional aspects, and based on these insights we suggest several receptor complex models. Moreover, distinct TSHR properties will be highlighted in comparison to other class A GPCRs to understand the molecular activation mechanisms of this receptor comprehensively. Finally, limitations of current knowledge and lack of information are discussed highlighting the need for intensified efforts toward TSHR structure elucidation.

Minireview: Insights Into the Structural and Molecular Consequences of the TSH-β Mutation C105Vfs114X

Naturally occurring thyrotropin (TSH) mutations are rare, which is also the case for the homologous heterodimeric glycoprotein hormones (GPHs) follitropin (FSH), lutropin (LH), and choriogonadotropin (CG). Patients with TSH-inactivating mutations present with central congenital hypothyroidism. Here, we summarize insights into the most frequent loss-of-function β-subunit of TSH mutation C105Vfs114X, which is associated with isolated TSH deficiency. This review will address the following question. What is currently known on the molecular background of this TSH variant on a protein level? It has not yet been clarified how C105Vfs114X causes early symptoms in affected patients, which are comparably severe to those observed in newborns lacking any functional thyroid tissue (athyreosis). To better understand the mechanisms of this mutant, we have summarized published reports and complemented this information with a structural perspective on GPHs. By including the ancestral TSH receptor agonist thyrostimulin and pathogenic mutations reported for FSH, LH, and choriogonadotropin in the analysis, insightful structure function and evolutionary restrictions become apparent. However, comparisons of immunogenicity and bioactivity of different GPH variants is hindered by a lack of consensus for functional analysis and the diversity of used GPH assays. Accordingly, relevant gaps of knowledge concerning details of GPH mutation-related effects are identified and highlighted in this review. These issues are of general importance as several previous and recent studies point towards the high impact of GPH variants in differential signaling regulation at GPH receptors (GPHRs), both endogenously and under diseased conditions. Further improvement in this area is of decisive importance for the development of novel targeted therapies.

Oligomerization of GPCRs involved in endocrine regulation

More than 800 different human membrane-spanning G-protein-coupled receptors (GPCRs) serve as signal transducers at biological barriers. These receptors are activated by a wide variety of ligands such as peptides, ions and hormones, and are able to activate a diverse set of intracellular signaling pathways. GPCRs are of central importance in endocrine regulation, which underpins the significance of comprehensively studying these receptors and interrelated systems. During the last decade, the capacity for multimerization of GPCRs was found to be a common and functionally relevant property. The interaction between GPCR monomers results in higher order complexes such as homomers (identical receptor subtype) or heteromers (different receptor subtypes), which may be present in a specific and dynamic monomer/oligomer equilibrium. It is widely accepted that the oligomerization of GPCRs is a mechanism for determining the fine-tuning and expansion of cellular processes by modification of ligand action, expression levels, and related signaling outcome. Accordingly, oligomerization provides exciting opportunities to optimize pharmacological treatment with respect to receptor target and tissue selectivity or for the development of diagnostic tools. On the other hand, GPCR heteromerization may be a potential reason for the undesired side effects of pharmacological interventions, faced with numerous and common mutual signaling modifications in heteromeric constellations. Finally, detailed deciphering of the physiological occurrence and relevance of specific GPCR/GPCR-ligand interactions poses a future challenge. This review will tackle the aspects of GPCR oligomerization with specific emphasis on family A GPCRs involved in endocrine regulation, whereby only a subset of these receptors will be discussed in detail.

Rearrangement of the Extracellular Domain/Extracellular Loop 1 Interface Is Critical for Thyrotropin Receptor Activation

The thyroid stimulating hormone receptor (TSHR) is a G protein-coupled receptor (GPCR) with a characteristic large extracellular domain (ECD). TSHR activation is initiated by binding of the hormone ligand TSH to the ECD. How the extracellular binding event triggers the conformational changes in the transmembrane domain (TMD) necessary for intracellular G protein activation is poorly understood. To gain insight in this process, the knowledge on the relative positioning of ECD and TMD and the conformation of the linker region at the interface of ECD and TMD are of particular importance. To generate a structural model for the TSHR we applied an integrated structural biology approach combining computational techniques with experimental data. Chemical cross-linking followed by mass spectrometry yielded 17 unique distance restraints within the ECD of the TSHR, its ligand TSH, and the hormone-receptor complex. These structural restraints generally confirm the expected binding mode of TSH to the ECD as well as the general fold of the domains and were used to guide homology modeling of the ECD. Functional characterization of TSHR mutants confirms the previously suggested close proximity of Ser-281 and Ile-486 within the TSHR. Rigidifying this contact permanently with a disulfide bridge disrupts ligand-induced receptor activation and indicates that rearrangement of the ECD/extracellular loop 1 (ECL1) interface is a critical step in receptor activation. The experimentally verified contact of Ser-281 (ECD) and Ile-486 (TMD) was subsequently utilized in docking homology models of the ECD and the TMD to create a full-length model of a glycoprotein hormone receptor.

Building the Case for Insulin-Like Growth Factor Receptor-I Involvement in Thyroid-Associated Ophthalmopathy

The pathogenesis of orbital Graves' disease (GD), a process known as thyroid-associated ophthalmopathy (TAO), remains incompletely understood. The thyrotropin receptor (TSHR) represents the central autoantigen involved in GD and has been proposed as the thyroid antigen shared with the orbit that could explain the infiltration of immune cells into tissues surrounding the eye. Another cell surface protein, insulin-like growth factor-I receptor (IGF-IR), has recently been proposed as a second antigen that participates in TAO by virtue of its interactions with anti-IGF-IR antibodies generated in GD, its apparent physical and functional complex formation with TSHR, and its necessary involvement in TSHR post-receptor signaling. The proposal that IGF-IR is involved in TAO has provoked substantial debate. Furthermore, several studies from different laboratory groups, each using different experimental models, have yielded conflicting results. In this article, we attempt to summarize the biological characteristics of IGF-IR and TSHR. We also review the evidence supporting and refuting the postulate that IGF-IR is a self-antigen in GD and that it plays a potentially important role in TAO. The putative involvement of IGF-IR in disease pathogenesis carries substantial clinical implications. Specifically, blocking this receptor with monoclonal antibodies can dramatically attenuate the induction by TSH and pathogenic antibodies generated in GD of proinflammatory genes in cultured orbital fibroblasts and fibrocytes. These cell types appear critical to the development of TAO. These observations have led to the conduct of a now-completed multicenter therapeutic trial of a fully human monoclonal anti-IGF-IR blocking antibody in moderate to severe, active TAO.

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.

FSH receptor-specific residues L501 and I505 in extracellular loop 2 are essential for its function

The extracellular loop 2 (EL2) of FSH receptor (FSHR) plays a pivotal role in various events downstream of FSH stimulation. Because swapping the six FSHR-specific residues in EL2 (chimeric EL2M) with those from LH/choriogonadotropin receptor resulted in impaired internalization of FSH-FSHR complex and low FSH-induced cAMP production, six substitution mutants of EL2 were generated to ascertain the contribution of individual amino acids to the effects shown by chimeric EL2M. Results revealed that L(501)F mainly and I(505)V to a lesser extent contribute to the diminished receptor function in chimeric EL2M. HEK293 cells stably expressing WT and chimeric EL2M FSHR were generated to track the fate of the receptors post FSH induction. The chimeric EL2M FSHR stable clone showed weak internalization and cAMP response similar to transiently transfected cells. Furthermore, reduced FSH-induced ERK phosphorylation was also observed. The interaction of activated chimeric EL2M and L(501)F FSHR with β-arrestins was weak compared with WT FSHR, thus explaining the impaired internalization of chimeric EL2M and corroborating the indispensable role of EL2 in receptor function.

A case of ovarian torsion in a patient carrier of a FSH receptor gene mutation previously affected by spontaneous ovarian hyperstimulation syndrome

We here report a case of ovarian torsion in a patient with an history of two previous episodes of spontaneous ovarian hyperstimulation syndrome during her two pregnancies. A mutation of follicle-stimulating hormone receptor (FSHr) gene was identified in this patient and in other members of the family. Two years after her successful second pregnancy, the patient showed signs of severe thyroiditis during administration of oral contraceptive, with suppressed TSH and increased thyreoglobulin, in the absence of any abnormalities of the auto-antibodies. In few days, she developed severe pelvic pain and ultrasonographic evidence of increased ovarian volume. She underwent laparoscopy with unilateral adnexectomy for ovarian ischemic necrosis due to adnexal torsion. Our experience suggests that patients' carrier of a mutation of FSHr gene are at risk of ovarian pathologies also when non-pregnant and in the presence of low TSH levels. Further investigations are needed for an appropriate knowledge of typical and atypical manifestations of spontaneous ovarian hyperstimulation syndrome.

Differences in Signal Activation by LH and hCG are Mediated by the LH/CG Receptor's Extracellular Hinge Region

The human lutropin (hLH)/choriogonadotropin (hCG) receptor (LHCGR) can be activated by binding two slightly different gonadotropic glycoprotein hormones, choriogonadotropin (CG) - secreted by the placenta, and lutropin (LH) - produced by the pituitary. They induce different signaling profiles at the LHCGR. This cannot be explained by binding to the receptor's leucine-rich-repeat domain (LRRD), as this binding is similar for the two hormones. We therefore speculate that there are previously unknown differences in the hormone/receptor interaction at the extracellular hinge region, which might help to understand functional differences between the two hormones. We have therefore performed a detailed study of the binding and action of LH and CG at the LHCGR hinge region. We focused on a primate-specific additional exon in the hinge region, which is located between LRRD and the serpentine domain. The segment of the hinge region encoded by exon10 was previously reported to be only relevant to hLH signaling, as the exon10-deletion receptor exhibits decreased hLH signaling, but unchanged hCG signaling. We designed an advanced homology model of the hormone/LHCGR complex, followed by experimental characterization of relevant fragments in the hinge region. In addition, we examined predictions of a helical exon10-encoded conformation by block-wise polyalanine (helix supporting) mutations. These helix preserving modifications showed no effect on hormone-induced signaling. However, introduction of a structure-disturbing double-proline mutant LHCGR-Q303P/E305P within the exon10-helix has, in contrast to exon10-deletion, no impact on hLH, but only on hCG signaling. This opposite effect on signaling by hLH and hCG can be explained by distinct sites of hormone interaction in the hinge region. In conclusion, our analysis provides details of the differences between hLH- and hCG-induced signaling that are mainly determined in the L2-beta loop of the hormones and in the hinge region of the receptor.

Discovery and Development of Small Molecule Allosteric Modulators of Glycoprotein Hormone Receptors

Glycoprotein hormones, follicle-stimulating hormone (FSH), luteinizing hormone (LH), and thyroid-stimulating hormone (TSH) are heterodimeric proteins with a common α-subunit and hormone-specific β-subunit. These hormones are dominant regulators of reproduction and metabolic processes. Receptors for the glycoprotein hormones belong to the family of G protein-coupled receptors. FSH receptor (FSHR) and LH receptor are primarily expressed in somatic cells in ovary and testis to promote egg and sperm production in women and men, respectively. TSH receptor is expressed in thyroid cells and regulates the secretion of T3 and T4. Glycoprotein hormones bind to the large extracellular domain of the receptor and cause a conformational change in the receptor that leads to activation of more than one intracellular signaling pathway. Several small molecules have been described to activate/inhibit glycoprotein hormone receptors through allosteric sites of the receptor. Small molecule allosteric modulators have the potential to be administered orally to patients, thus improving the convenience of treatment. It has been a challenge to develop a small molecule allosteric agonist for glycoprotein hormones that can mimic the agonistic effects of the large natural ligand to activate similar signaling pathways. However, in the past few years, there have been several promising reports describing distinct chemical series with improved potency in preclinical models. In parallel, proposal of new structural model for FSHR and in silico docking studies of small molecule ligands to glycoprotein hormone receptors provide a giant leap on the understanding of the mechanism of action of the natural ligands and new chemical entities on the receptors. This review will focus on the current status of small molecule allosteric modulators of glycoprotein hormone receptors, their effects on common signaling pathways in cells, their utility for clinical application as demonstrated in preclinical models, and use of these molecules as novel tools to dissect the molecular signaling pathways of these receptors.

Influence of the hinge region and its adjacent domains on binding and signaling patterns of the thyrotropin and follitropin receptor

Glycoprotein hormone receptors (GPHR) have a large extracellular domain (ECD) divided into the leucine rich repeat (LRR) domain for binding of the glycoprotein hormones and the hinge region (HinR), which connects the LRR domain with the transmembrane domain (TMD). Understanding of the activation mechanism of GPHRs is hindered by the unknown interaction of the ECD with the TMD and the structural changes upon ligand binding responsible for receptor activation. Recently, our group showed that the HinR of the thyrotropin receptor (TSHR) can be replaced by those of the follitropin (FSHR) and lutropin receptor (LHCGR) without effects on surface expression and hTSH signaling. However, differences in binding characteristics for bovine TSH at the various HinRs were obvious. To gain further insights into the interplay between LRR domain, HinR and TMD we generated chimeras between the TSHR and FSHR. Our results obtained by the determination of cell surface expression, ligand binding and G protein activation confirm the similar characteristics of GPHR HinRs but they also demonstrate an involvement of the HinR in ligand selectivity indicated by the observed promiscuity of some chimeras. While the TSHR HinR contributes to specific binding of TSH and its variants, no such contribution is observed for FSH and its analog TR4401 at the HinR of the FSHR. Furthermore, the charge distribution at the poorly characterized LRR domain/HinR transition affected ligand binding and signaling even though this area is not in direct contact with the ligand. In addition our results also demonstrate the importance of the TMD/HinR interface. Especially the combination of the TSHR HinR with the FSHR-TMD resulted in a loss of cell surface expression of the respective chimeras. In conclusion, the HinRs of GPHRs do not only share similar characteristics but also behave as ligand specific structural and functional entities.

An essential splice site mutation (c.317+1G>A) in the TSHR gene leads to severe thyroid dysgenesis

Congenital hypothyroidism (CH) is the most common neonatal endocrine disorder and 2% of cases have familial origin. Our aim in this study was to determine the genetic alterations in two siblings with CH coming from a consanguineous family. Because CH is often inherited in autosomal recessive manner in consanguineous/multicase-families, we first performed genetic linkage studies to all known causative CH loci followed by conventional sequencing of the linked gene. The family showed potential linkage to the TSHR locus, and we detected an essential splice site mutation (c.317+1G>A) in both siblings. RT-PCR analysis confirmed the functionality of the mutation. The mutation was homozygous in the cases whereas heterozygous in carrier parents and an unaffected sibling. Here we conclude that thyroid agenesis in both siblings in this study originates from c.317+1G>A splice site mutation in the TSHR gene, and this study underlines the importance of detailed molecular genetic studies in the definitive diagnosis and classification of CH.

A deletion including exon 2 of the TSHR gene is associated with thyroid dysgenesis and severe congenital hypothyroidism

Congenital hypothyroidism (CH) is the most common neonatal endocrine disorder and 2% of cases have a familial origin. Our aim in this study was to determine the genetic alterations in two siblings with CH coming from a consanguineous family. As CH is often inherited in an autosomal recessive manner in consanguineous/multi case-families, we first performed genetic linkage studies to all known causative CH loci followed by conventional sequencing of the linked gene. The family showed potential linkage to the TSHR locus and our attempts to amplify and sequence exon 2 of the TSHR gene continuously failed. Subsequent RT-PCR analysis using mRNA and corresponding cDNA showed a large deletion including the exon 2 of the gene. The deletion was homozygous in affected cases whilst heterozygous in carrier parents. Here we conclude that CH in both siblings of this study originates from a large deletion including the exon 2 of the TSHR gene. This study demonstrates that full sequence analysis in a candidate CH gene might not always be enough to detect genetic alterations, and additional analyses such as RT-PCR and MLPA might be necessary to describe putative genetic causes of the disease in some cases. It also underlines the importance of detailed molecular genetic studies in the definitive diagnosis and classification of CH.

A Japanese family with nonautoimmune hyperthyroidism caused by a novel heterozygous thyrotropin receptor gene mutation

BACKGROUND

Hyperthyroidism caused by activating mutations of the thyrotropin receptor gene (TSHR) is rare in the pediatric population.

METHODS

We found a Japanese family with hyperthyroidism without autoantibody. DNA sequence analysis of TSHR was undertaken in this family. The functional consequences for the Gs-adenylyl cyclase and Gq/11-phospholipase C signaling pathways and cell surface expression of receptors were determined in vitro using transiently transfected human embryonic kidney 293 cells.

RESULTS

We identified a heterozygous mutation (M453R) in exon 10 of TSHR. In this family, this mutation was found in all individuals who exhibited hyperthyroidism. The results showed that this mutation resulted in constitutive activation of the Gs-adenylyl cyclase system. However, this mutation also caused a reduction in the activation capacity of the Gq/11-phospholipase C pathway, compared with the wild type.

CONCLUSION

We demonstrate that the M453R mutation is the cause of nonautoimmune hyperthyroidism.

Differences between lutropin-mediated and choriogonadotropin-mediated receptor activation

The human lutropin/choriogonadotropin receptor (hLHR) for the gonadotropic hormones human luteinizing hormone (hLH; lutropin) and human choriogonadotropin (hCG) is crucial for normal sexual development and fertility. We aimed to unravel differences between the two hLHR hormones in molecular activation mechanisms at hLHR. We utilized a specific hLHR variant that lacks exon 10 (hLHR-delExon10), which maintains full cAMP signaling by hCG, but decreases hLH-induced receptor signaling, resulting in a pathogenic phenotype. Exon 10 encodes 27 amino acids within the hinge region, which is an extracellular segment that is important for signaling and hormone interaction. Initially, we assumed that the lack of exon 10 might disturb intermolecular trans-activation of hLH, a mechanism that has been reported for hCG at hLHR. Coexpression of signaling-deficient hLHR and binding-deficient hLHR can be used to examine the mechanisms of receptor signaling, in particular intermolecular cooperation and intramolecular cis-activation. Therefore, hLHR-delExon10 was combined with the hLHR Lys605→Glu mutant, in which signaling is abolished, and the hLHR mutant Cys131→Arg, in which binding is deficient. We found that hCG signaling was partially rescued, indicating trans-activation. However, the hLH signal could not be restored via forced trans-activation with any construct. Fluorescence cross-correlation spectroscopy detected oligomerization in all combinations, indicating that these functional differences cannot be explained by monomerization of hLHR-delExon10. Thus, our data demonstrate not only that the different behavior of hLH at hLHR-delExon10 is unlikely to be related to modified intermolecular receptor activation, but also that hLH may exclusively stimulate the targeted hLHR by cis-activation, whereas hCG is also capable of inducing trans-activation.

Structural biology of glycoprotein hormones and their receptors: insights to signaling

This article reviews the progress made in the field of glycoprotein hormones (GPH) and their receptors (GPHR) by several groups of structural biologists including ourselves aiming to gain insight into GPH signaling mechanisms. The GPH family consists of four members, with follicle-stimulating hormone (FSH) being the prototypic member. GPH members belong to the cystine-knot growth factor superfamily, and their receptors (GPHR), possessing unusually large N-terminal ectodomains, belong to the G-protein coupled receptor Family A. GPHR ectodomains can be divided into two subdomains: a high-affinity hormone binding subdomain primarily centered on the N-terminus, and a second subdomain that is located on the C-terminal region of the ectodomain that is involved in signal specificity. The two subdomains unexpectedly form an integral structure comprised of leucine-rich repeats (LRRs). Following the structure determination of hCG in 1994, the field of FSH structural biology has progressively advanced. Initially, the FSH structure was determined in partially glycosylated free form in 2001, followed by a structure of FSH bound to a truncated FSHR ectodomain in 2005, and the structure of FSH bound to the entire ectodomain in 2012. Comparisons of the structures in three forms led a proposal of a two-step monomeric receptor activation mechanism. First, binding of FSH to the FSHR high-affinity hormone-binding subdomain induces a conformational change in the hormone to form a binding pocket that is specific for a sulfated-tyrosine found as sTyr 335 in FSHR. Subsequently, the sTyr is drawn into the newly formed binding pocket, producing a lever effect on a helical pivot whereby the docking sTyr provides as the 'pull & lift' force. The pivot helix is flanked by rigid LRRs and locked by two disulfide bonds on both sides: the hormone-binding subdomain on one side and the last short loop before the first transmembrane helix on the other side. The lift of the sTyr loop frees the tethered extracellular loops of the 7TM domain, thereby releasing a putative inhibitory influence of the ectodomain, ultimately leading to the activating conformation of the 7TM domain. Moreover, the data lead us to propose that FSHR exists as a trimer and to present an FSHR activation mechanism consistent with the observed trimeric crystal form. A trimeric receptor provides resolution of the enigmatic, but important, biological roles played by GPH residues that are removed from the primary FSH-binding site, as well as several important GPCR phenomena, including negative cooperativity and asymmetric activation. Further reflection pursuant to this review process revealed additional novel structural characteristics such as the identification of a 'seat' sequence in GPH. Together with the 'seatbelt', the 'seat' enables a common heteodimeric mode of association of the common α subunit non-covalently and non-specifically with each of the three different β subunits. Moreover, it was possible to establish a dimensional order that can be used to estimate LRR curvatures. A potential binding pocket for small molecular allosteric modulators in the FSHR 7TM domain has also been identified.

Constitutive activity in gonadotropin receptors

Constitutively active mutants (CAMs) of gonadotropin receptors are, in general, rare conditions. Luteinizing hormone-choriogonadotropin receptor (LHCGR) CAMs provoke the dramatic phenotype of familial gonadotropin-independent isosexual male-limited precocious puberty, whereas in females, there is not yet any identified phenotype. Only one isolated follicle-stimulating hormone receptor (FSHR) CAM (Asp567Gly) has so far been detected in a single male patient, besides other FSHR weak CAMs linked to pregnancy-associated ovarian hyperstimulation syndrome or to impaired desensitization and internalization. Several animal models have been developed for studying enhanced gonadotropin action; in addition to unraveling valuable new information about the possible phenotypes of isolated FSHR and LHCGR CAMs in women, the information obtained from these mouse models has served multiple translational goals, including the development of new diagnostic and therapeutic targets as well as the prediction of phenotypes for mutations not yet identified in humans. Mutagenesis and computational studies have shed important information on the physiopathogenic mechanisms leading to constitutive activity of gonadotropin receptors; a common feature in these receptor CAMs is the release of stabilizing interhelical interactions between transmembrane domains (TMDs) 3 and 6 leading to an increase, with respect to the wild-type receptor, in the solvent accessibility at the cytosolic extension of TMDs 3, 5, and 6, which involves the highly conserved Glu/Asp-Arg-Tyr/Trp sequence. In this chapter, we summarize the structural features, functional consequences, and mechanisms that lead to constitutive activation of gonadotropin receptor CAMs and provide information on pharmacological approaches that might potentially modulate gonadotropin receptor CAM function.

A selective TSH receptor antagonist inhibits stimulation of thyroid function in female mice

Because the TSH receptor (TSHR) plays an important role in the pathogenesis of thyroid disease, a TSHR antagonist could be a novel treatment. We attempted to develop a small molecule, drug-like antagonist of TSHR signaling that is selective and active in vivo. We synthesized NCGC00242364 (ANTAG3) by chemical modification of a previously reported TSHR antagonist. We tested its potency, efficacy, and selectivity in a model cell system in vitro by measuring its activity to inhibit stimulation of cAMP production stimulated by TSH, LH, or FSH. We tested the in vivo activity of ANTAG3 by measuring its effects to lower serum free T4 and thyroid gene expression in female BALB/c mice continuously treated with ANTAG3 for 3 days and given low doses of TRH continuously or stimulated by a single administration of a monoclonal thyroid-stimulating antibody M22. ANTAG3 was selective for TSHR inhibition; half-maximal inhibitory doses were 2.1 μM for TSHR and greater than 30 μM for LH and FSH receptors. In mice treated with TRH, ANTAG3 lowered serum free T4 by 44% and lowered mRNAs for sodium-iodide cotransporter and thyroperoxidase by 75% and 83%, respectively. In mice given M22, ANTAG3 lowered serum free T4 by 38% and lowered mRNAs for sodium-iodide cotransporter and thyroperoxidase by 73% and 40%, respectively. In conclusion, we developed a selective TSHR antagonist that is effective in vivo in mice. This is the first report of a small-molecule TSHR antagonist active in vivo and may lead to a drug to treat Graves' disease.

Constitutive activities in the thyrotropin receptor: regulation and significance

The thyroid-stimulating hormone receptor (TSHR, or thyrotropin receptor) is a family A G protein-coupled receptor. It not only binds thyroid-stimulating hormone (TSH, or thyrotropin) but also interacts with autoantibodies under pathological conditions. The TSHR and TSH are essential for thyroid growth and function and thus for all thyroid hormone-associated physiological superordinated processes, including metabolism and development of the central nervous system. In vitro studies have found that the TSHR permanently stimulates ligand-independent (constitutive) activation of Gs, which ultimately leads to intracellular cAMP accumulation. Furthermore, a vast variety of constitutively activating mutations of TSHR-at more than 50 different amino acid positions-have been reported to enhance basal signaling. These lead in vivo to a "gain-of-function" phenotype of nonautoimmune hyperthyroidism or toxic adenomas. Moreover, many naturally occurring inactivating mutations are known to cause a "loss-of-function" phenotype, resulting in resistance to thyroid hormone or hyperthyrotropinemia. Several of these mutations are also characterized by impaired basal signaling, and these are designated here as "constitutively inactivating mutations" (CIMs). More than 30 amino acid positions with CIMs have been identified so far. Moreover, the permanent TSHR signaling capacity can also be blocked by inverse agonistic antibodies or small drug-like molecules, which both have a potential for clinical usage. In this chapter, information on constitutive activity in the TSHR is described, including up- and downregulation, linked protein conformations, physiological and pathophysiological conditions, and related intracellular signaling.

Novel insights on thyroid-stimulating hormone receptor signal transduction

The TSH receptor (TSHR) is a member of the glycoprotein hormone receptors, a subfamily of family A G protein-coupled receptors. The TSHR is of great importance for the growth and function of the thyroid gland. The TSHR and its endogenous ligand TSH are pivotal proteins with respect to a variety of physiological functions and malfunctions. The molecular events of TSHR regulation can be summarized as a process of signal transduction, including signal reception, conversion, and amplification. The steps during signal transduction from the extra- to the intracellular sites of the cell are not yet comprehensively understood. However, essential new insights have been achieved in recent years on the interrelated mechanisms at the extracellular region, the transmembrane domain, and intracellular components. This review contains a critical summary of available knowledge of the molecular mechanisms of signal transduction at the TSHR, for example, the key amino acids involved in hormone binding or in the structural conformational changes that lead to G protein activation or signaling regulation. Aspects of TSHR oligomerization, signaling promiscuity, signaling selectivity, phenotypes of genetic variations, and potential extrathyroidal receptor activity are also considered, because these are relevant to an understanding of the overall function of the TSHR, including physiological, pathophysiological, and pharmacological perspectives. Directions for future research are discussed.

Molecular sampling of the allosteric binding pocket of the TSH receptor provides discriminative pharmacophores for antagonist and agonists

The TSHR (thyrotropin receptor) is activated endogenously by the large hormone thyrotropin and activated pathologically by auto-antibodies. Both activate and bind at the extracellular domain. Recently, SMLs (small-molecule ligands) have been identified, which bind in an allosteric binding pocket within the transmembrane domain. Modelling driven site-directed mutagenesis of amino acids lining this pocket led to the delineation of activation and inactivation sensitive residues. Modified residues showing CAMs (constitutively activating mutations) indicate signalling-sensitive positions and mark potential trigger points for agonists. Silencing mutations lead to an impairment of basal activity and mark contact points for antagonists. Mapping these residues on to a structural model of TSHR indicates locations where an SML may switch the receptor to an inactive or active conformation. In the present article, we report the effects of SMLs on these signalling-sensitive amino acids at the TSHR. Surprisingly, the antagonistic effect of SML compound 52 was reversed to an agonistic effect, when tested at the CAM Y667A. Switching agonism to antagonism and the reverse by changing either SMLs or residues covering the binding pocket provides detailed knowledge about discriminative pharmacophores. It prepares the basis for rational optimization of new high-affinity antagonists to interfere with the pathogenic activation of the TSHR.

Acute ovarian torsion and primary hypothyroidism

A 12-year-old girl presented with acute abdominal pain due to an acute ovarian torsion. She required an oophorectomy. Clinical and laboratory assessment confirmed severe primary hypothyroidism. In this report, we review this rare complication of untreated primary hypothyroidism and the physiologic mechanisms proposed to explain this phenomenon.