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

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.

Integrative model of the FSH receptor reveals the structural role of the flexible hinge region

The follicle-stimulating hormone receptor (FSHR) belongs to the glycoprotein hormone receptors, a subfamily of G-protein-coupled receptors (GPCRs). FSHR is involved in reproductive processes such as gonadal development and maturation. Structurally, the extensive extracellular domain, which contains the hormone-binding site and is linked to the transmembrane domain by the hinge region (HR), is characteristic for these receptors. How this HR is involved in hormone binding and signal transduction is still an open question. We combined in vitro and in situ chemical crosslinking, disulfide pattern analysis, and mutation data with molecular modeling to generate experimentally driven full-length models. These models provide insights into the interface, important side-chain interactions, and activation mechanism. The interface indicates a strong involvement of the connecting loop. A major rearrangement of the HR seems implausible due to the tight arrangement and fixation by disulfide bonds. The models are expected to allow for testable hypotheses about signal transduction and drug development for GPHRs.

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.

A Gq Biased Small Molecule Active at the TSH Receptor

G protein coupled receptors (GPCRs) can lead to G protein and non-G protein initiated signals. By virtue of its structural property, the TSH receptor (TSHR) has a unique ability to engage different G proteins making it highly amenable to selective signaling. In this study, we describe the identification and characterization of a novel small molecule agonist to the TSHR which induces primary engagement with G_αq/11. To identify allosteric modulators inducing selective signaling of the TSHR we used a transcriptional-based luciferase assay system with CHO-TSHR cells stably expressing response elements (CRE, NFAT, SRF, or SRE) that were capable of measuring signals emanating from the coupling of G_αs , G_αq/11, G_βγ, and G_α12/13, respectively. Using this system, TSH activated G_αs , G_αq/11, and G_α12/13 but not G_βγ. On screening a library of 50K molecules at 0.1,1.0 and 10 μM, we identified a novel G_q/11 agonist (named MSq1) which activated G_q/11 mediated NFAT-luciferase >4 fold above baseline and had an EC₅₀= 8.3 × 10^-9 M with only minor induction of G_αs and cAMP. Furthermore, MSq1 is chemically and structurally distinct from any of the previously reported TSHR agonist molecules. Docking studies using a TSHR transmembrane domain (TMD) model indicated that MSq1 had contact points on helices H1, H2, H3, and H7 in the hydrophobic pocket of the TMD and also with the extracellular loops. On co-treatment with TSH, MSq1 suppressed TSH-induced proliferation of thyrocytes in a dose-dependent manner but lacked the intrinsic ability to influence basal thyrocyte proliferation. This unexpected inhibitory property of MSq1 could be blocked in the presence of a PKC inhibitor resulting in derepressing TSH induced protein kinase A (PKA) signals and resulting in the induction of proliferation. Thus, the inhibitory effect of MSq1 on proliferation resided in its capacity to overtly activate protein kinase C (PKC) which in turn suppressed the proliferative signal induced by activation of the predomiant cAMP-PKA pathway of the TSHR. Treatment of rat thyroid cells (FRTL5) with MSq1 did not show any upregulation of gene expression of the key thyroid specific markers such as thyroglobulin(Tg), thyroid peroxidase (Tpo), sodium iodide symporter (Nis), and the TSH receptor (Tshr) further suggesting lack of involvement of MSq1 and G_αq/11 activation with cellular differentation. In summary, we identified and characterized a novel G_αq/11 agonist molecule acting at the TSHR and which showed a marked anti-proliferative ability. Hence, Gq biased activation of the TSHR is capable of ameliorating the proliferative signals from its orthosteric ligand and may offer a therapeutic option for thyroid growth modulation.

The cAMP/PKA pathway: steroidogenesis of the antral follicular stage

Pituitary gonadotropins, follicle-stimulating (FSH) and luteinizing hormone (LH) promote follicular recruitment and support antral follicle growth, maturation and selection, resulting in ovulation of the dominant follicle. FSH and LH biological functions are mediated by G protein-coupled receptors, FSHR and LHCGR, resulting in the activation of a number of signaling cascades, such as the cyclic AMP/protein kinase A (cAMP/PKA) pathway. Some in-vitro data are consistent with the dual, proliferative and pro-apoptotic role of cAMP, leaving unanswered questions on how cAMP/PKA signaling is linked to the follicle fate. Progression of the antral stage is characterized by the presence of dynamic serum gonadotropin and estrogen levels, accompanying proliferation and steroidogenesis of growing as well as apoptosis of atretic follicles. These events are parallel to changes of FSHR and LHCGR density at the cell surface occurring throughout the antral stage, reasonably modulating the cAMP/PKA activation pattern, cell metabolism and functions. Understanding whether gonadotropins and receptor expression levels impact on the steroidogenic pathway and play a role in determining the follicular fate, may put new light on molecular mechanisms regulating human reproduction. The aim of the present review is to update the role of major players modulating the cAMP/PKA pathway and regulating the balance between proliferative, differentiating and pro-apoptotic signals.

Functional characterization of two naturally occurring mutations V221G and T449N in the follicle stimulating hormone receptor

Naturally occurring mutations in follicle stimulating hormone receptor (FSHR) affect the receptor function. Here, we characterized two such previously reported mutations, V²²¹G and T⁴⁴⁹N, in the extracellular domain and transmembrane helix 3, of FSHR, respectively. Functional studies with the V²²¹G mutant demonstrated an impairment in FSH binding and signaling. Validation of X-ray crystallography data indicating the contribution of FSHR specific residues in the vicinity of V²²¹ to contribute to FSH-FSHR interaction was carried out. In vitro mutational studies showed that these residues are determinants of both FSH binding and FSH induced signaling. Analysis of the T⁴⁴⁹N mutation revealed that it results in an increase in FSH binding and high cAMP response at lower doses of FSH. A marginal hCG induced and no TSH induced cAMP production was also observed. These findings corroborated with the clinical manifestations of primary amenorrhea (V²²¹G) and spontaneous ovarian hyperstimulation syndrome (T⁴⁴⁹N) in women harbouring these mutations.

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.

Glycosylation pattern analysis of glycoprotein hormones and their receptors

We have studied glycosylation patterns in glycoprotein hormones (GPHs) and glycoprotein hormone receptor (GPHR) extracellular domains (ECD) from different species to identify areas not glycosylated that could be involved in intermolecular or intramolecular interactions. Comparative models of the structure of the TSHR ECD in complex with TSH and in complex with TSHR autoantibodies (M22, stimulating and K1-70, blocking) were obtained based on the crystal structures of the FSH-FSHR ECD, M22-TSHR leucine-rich repeat domain (LRD) and K1-70-TSHR LRD complexes. The glycosylation sites of the GPHRs and GPHs from all species studied were mapped on the model of the human TSH TSHR ECD complex. The areas on the surfaces of GPHs that are known to interact with their receptors are not glycosylated and two areas free from glycosylation, not involved in currently known interactions, have been identified. The concave faces of GPHRs leucine-rich repeats 3-7 are free from glycosylation, consistent with known interactions with the hormones. In addition, four other non-glycosylated areas have been identified, two located on the receptors' convex surfaces, one in the long loop of the hinge regions and one at the C-terminus of the extracellular domains. Experimental evidence suggests that the non-glycosylated areas identified on the hormones and receptors are likely to be involved in forming intramolecular or intermolecular interactions.

Dissecting the structural and functional features of the Luteinizing hormone receptor using receptor specific single chain fragment variables

The Luteinizing hormone receptor (LHR) has a large extracellular domain (amino acid residues, a.a.1-355) and a transmembrane domain (TMD; a.a. 356-699), essential for hormone binding and signaling, respectively. The LHR hinge region (a.a. 256-355) connects the two domains and acts as an activating switch for the receptor by an unknown mechanism. LHR hinge-specific Single chain fragment variables (ScFv) stimulated cAMP production by the stable and transiently transfected cell lines expressing LHR in a hormone-independent manner and the C-terminal region of LHR hinge (a.a. 313-349) was identified as the probable epitope for one agonistic ScFv. This epitope attained a helical conformation upon agonistic ScFv binding and the activity of the ScFv was dependent on Y331 sulfation. ScFv was also able to activate TMD mutants, D578Y and A593P, reemphasizing the role of TM helix VI in LHR activation.

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.

Follicle-stimulating hormone potentiates the steroidogenic activity of chorionic gonadotropin and the anti-apoptotic activity of luteinizing hormone in human granulosa-lutein cells in vitro

Luteinizing hormone (LH) and choriogonadotropin (hCG) are glycoprotein hormones regulating ovarian function and pregnancy, respectively. Since these molecules act on the same receptor (LHCGR), they were traditionally assumed as equivalent in assisted reproduction techniques (ART), although differences between LH and hCG were demonstrated at molecular and physiological level. In this study, we demonstrated for the first time that co-treatment with a follicle-stimulating hormone (FSH) dose in the ART therapeutic range potentiates different LH- and hCG-dependent responses in vitro, measured in terms of cAMP, phospho-CREB, -ERK1/2 and -AKT activation, gene expression, progesterone and estradiol production in human granulosa-lutein cells (hGLC). We show that in the presence of FSH, hCG biopotency is about 5-fold increased, in the presence of FSH, in terms of cAMP activation. Accordingly, CREB phosphorylation and steroid production is increased under hCG and FSH co-treatment. LH effects, evaluated as steroidogenic cAMP/PKA pathway activation, do not change in the presence of FSH, which, however, increases LH-dependent ERK1/2 and AKT, but not CREB phosphorylation, resulting in anti-apoptotic effects. The different modulatory activity of FSH on LH and hCG action in vitro corresponds to their different physiological functions, reflecting proliferative effects exerted by LH during the follicular phase and before trophoblast development, and the high steroidogenic potential of hCG requested to sustain pregnancy from the luteal phase onwards.

Mechanisms of Action of TSHR Autoantibodies

The availability of human monoclonal antibodies (MAbs) to the TSHR has enabled major advances in our understanding of how TSHR autoantibodies interact with the receptor. These advances include determination of the crystal structures of the TSHR LRD in complex with a stimulating autoantibody (M22) and with a blocking type autoantibody (K1-70). The high affinity of MAbs for the TSHR makes them particularly suitable for use as ligands in assays for patient serum TSHR autoantibodies. Also, M22 and K1-70 are effective at low concentrations in vivo as TSHR agonists and antagonists respectively. K1-70 has important potential in the treatment of the hyperthyroidism of Graves' disease and Graves' ophthalmopathy. Small molecule TSHR antagonists described to date do not appear to have the potency and/or specificity shown by K1-70. New models of the TSHR ECD in complex with various ligands have been built. These models suggest that initial binding of TSH to the TSHR causes a conformational change in the hormone. This opens a positively charged pocket in receptor-bound TSH which attracts the negatively charged sulphated tyrosine 385 on the hinge region of the receptor. The ensuing movement of the receptor's hinge region may then cause activation. Similar activation mechanisms seem to take place in the case of FSH and the FSHR and LH and the LHR. However, stimulating TSHR autoantibodies do not appear to activate the TSHR in the same way as TSH.

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.

New Frontier in Glycoprotein Hormones and Their Receptors Structure-Function

Last two decades of structure-function studies performed in numerous laboratories provided substantial progress in understanding basic science, physiological, pathophysiological, pharmacological, and comparative aspects of glycoprotein hormones (GPHs) and their cognate receptors. Multiple concepts and models developed based on experimental data in the past stood the test of time and have been, at least in part, confirmed and/or remained compatible with the new structures resolved at the atomic level. Major advances in understanding of the ligand-receptor relationships are heralding the dawn of a new era for GPHs and their receptors, although many basic questions still remain unanswered. This article examines retrospectively several basic science aspects of GPH super-agonists and related "biosuperiors" in a broader context of the advances in the ligand-receptor structure-function relationships and new mechanistic models generated based on the structure elucidation. Due to selective focus of my comments and perspectives in certain parts, the reader is directed to the most relevant publications and reviews in the field for more comprehensive analyses.