Technology Insight: modern methods to monitor protein-protein interactions reveal functional TSH receptor oligomerization

Imaging strategies for receptor tyrosine kinase dimers in living cells

Receptor tyrosine kinases (RTKs) are the essential regulators of cell signal transduction pathways and play important roles in biological processes. RTK dimerization is generally considered the first step in receptor activation and cell communication. And the abnormal expression of RTK dimers is closely related to the occurrence and development of many diseases. Therefore, the visualization of RTK dimerization is of great significance for monitoring physiological processes. The genetic and nongenetic imaging strategies have attracted widespread attention due to their high efficiency and high sensitivity. In this review, the RTKs and their dimers as well as the advances in strategies for imaging RTK dimers are introduced. Furthermore, we analyze the limitations of existing imaging strategies and put forward suggestions for the future development of imaging probes. We expect that this review will inspire more in-depth investigation of RTK dimers, which will also broaden the application of strategies of RTK dimers in biomedical areas.

Proximity ligation assay to study TSH receptor homodimerization and crosstalk with IGF-1 receptors in human thyroid cells

Proximity ligation assay (PLA) is a methodology that permits detection of protein-protein closeness, that is, proteins that are within 40 nanometers of each other, in cells or tissues at endogenous protein levels or after exogenous overexpression. It detects the protein(s) with high sensitivity and specificity because it employs a DNA hybridization step followed by DNA amplification. PLA has been used successfully with many types of proteins. In this methods paper, we will describe the workings of PLA and provide examples of its use to study TSH/IGF-1 receptor crosstalk in Graves' orbital fibroblasts (GOFs) and TSH receptor homodimerization in primary cultures of human thyrocytes.

Determining the Stoichiometry of Small Protein Oligomers Using Steady-State Fluorescence Anisotropy

A large fraction of soluble and membrane-bound proteins exists as non-covalent dimers, trimers, and higher-order oligomers. The experimental determination of the oligomeric state or stoichiometry of proteins remains a nontrivial challenge. In one approach, the protein of interest is genetically fused to green fluorescent protein (GFP). If a fusion protein assembles into a non-covalent oligomeric complex, exciting their GFP moiety with polarized fluorescent light elicits homotypic Förster resonance energy transfer (homo-FRET), in which the emitted radiation is partially depolarized. Fluorescence depolarization is associated with a decrease in fluorescence anisotropy that can be exploited to calculate the oligomeric state. In a classical approach, several parameters obtained through time-resolved and steady-state anisotropy measurements are required for determining the stoichiometry of the oligomers. Here, we examined novel approaches in which time-resolved measurements of reference proteins provide the parameters that can be used to interpret the less expensive steady-state anisotropy data of candidates. In one approach, we find that using average homo-FRET rates (k_FRET), average fluorescence lifetimes (τ), and average anisotropies of those fluorophores that are indirectly excited by homo-FRET (r_ET) do not compromise the accuracy of calculated stoichiometries. In the other approach, fractional photobleaching of reference oligomers provides a novel parameter a whose dependence on stoichiometry allows one to quantitatively interpret the increase of fluorescence anisotropy seen after photobleaching the candidates. These methods can at least reliably distinguish monomers from dimers and trimers.

TSH Receptor Homodimerization in Regulation of cAMP Production in Human Thyrocytes in vitro

Thyrotropin hormone (TSH) was reported to exhibit biphasic regulation of cAMP production in human thyroid slices; specifically, upregulation at low TSH doses transitioning to inhibition at high doses. We observed this phenomenon in HEK293 cells overexpressing TSH receptors (TSHRs) but in only 25% of human thyrocytes (hThyros) in vitro. Because TSHR expression in hThyros in vitro was low, we tested the hypothesis that high, in situ levels of TSHRs were needed for biphasic cAMP regulation. We increased expression of TSHRs by infecting hThyros with adenoviruses expressing human TSHR (AdhTSHR), measured TSH-stimulated cAMP production and TSHR homodimerization. TSHR mRNA levels in hThyros in vitro were 100-fold lower than in human thyroid tissue. AdhTSHR infection increased TSHR mRNA expression to levels found in thyroid tissue and flow cytometry showed that cell-surface TSHRs increased more than 15-fold. Most uninfected hThyro preparations exhibited monotonic cAMP production. In contrast, most hThyro preparations infected with AdhTSHR expressing TSHR at in vivo levels exhibited biphasic TSH dose responses. Treatment of AdhTSHR-infected hThyros with pertussis toxin resulted in monotonic dose response curves demonstrating that lower levels of cAMP production at high TSH doses were mediated by Gi/Go proteins. Proximity ligation assays confirmed that AdhTSHR infection markedly increased the number of TSHR homodimers. We conclude that in situ levels of TSHRs as homodimers are needed for hThyros to exhibit biphasic TSH regulation of cAMP production.

Selective and Wash-Resistant Fluorescent Dihydrocodeinone Derivatives Allow Single-Molecule Imaging of μ-Opioid Receptor Dimerization

μ-Opioid receptors (μ-ORs) play a critical role in the modulation of pain and mediate the effects of the most powerful analgesic drugs. Despite extensive efforts, it remains insufficiently understood how μ-ORs produce specific effects in living cells. We developed new fluorescent ligands based on the μ-OR antagonist E-p-nitrocinnamoylamino-dihydrocodeinone (CACO), that display high affinity, long residence time and pronounced selectivity. Using these ligands, we achieved single-molecule imaging of μ-ORs on the surface of living cells at physiological expression levels. Our results reveal a high heterogeneity in the diffusion of μ-ORs, with a relevant immobile fraction. Using a pair of fluorescent ligands of different color, we provide evidence that μ-ORs interact with each other to form short-lived homodimers on the plasma membrane. This approach provides a new strategy to investigate μ-OR pharmacology at single-molecule level.

Thyrotropin receptor, still much to be learned from the patients

In the absence of crystal available for the full-length thyrotropin receptor, knowledge of its structure and functioning has benefitted from the identification and characterization of mutations in patients with various thyroid dysfunctions. The characterization of activating mutations has contributed to the elaboration of a model involving the extracellular domain of the receptor as an inverse tethered agonist which, upon binding of the ligand, relieves the transmembrane domain from an inhibiting interaction and activates it. The models derived from comparisons with other receptors, enriched with the information provided by the study of mutations, have proven useful for the design of small-molecule agonists and antagonists that may be used in the future to treat thyroid dysfunctions. In this review, extrathyroidal expression of the thyrotropin receptor is described, the role of which is still poorly defined.

Nongenetic Approach for Imaging Protein Dimerization by Aptamer Recognition and Proximity-Induced DNA Assembly

Herein, we report a nongenetic and real-time approach for imaging protein dimerization on living cell surfaces by aptamer recognition and proximity-induced DNA assembly. We use the aptamer specific for the receptor monomer as a recognition probe. When receptor dimerization occurs, the dimeric receptors bring two aptamer probes into close proximity, thereby triggering dynamic DNA assembly. The proposed approach was successfully applied to visualize dimerization of Met receptor and transforming growth factor-β type II receptor. This approach allows us to image the two states (monomer/dimer) of a receptor protein on living cell surfaces in real time, opening a universal method for further investigation of protein dimerization and the corresponding activation processes in signal transduction.

The proximity-labeling technique BioID identifies sorting nexin 6 as a member of the insulin-like growth factor 1 (IGF1)-IGF1 receptor pathway

The insulin-like growth factor 1 receptor (IGF1R) is a receptor tyrosine kinase with critical roles in various biological processes. Recent results from clinical trials targeting IGF1R indicate that IGF1R signaling pathways are more complex than previously thought. Moreover, it has become increasingly clear that the function of many proteins can be understood only in the context of a network of interactions. To that end, we sought to profile IGF1R-protein interactions with the proximity-labeling technique BioID. We applied BioID by generating a HEK293A cell line that stably expressed the BirA* biotin ligase fused to the IGF1R. Following stimulation by IGF1, biotinylated proteins were analyzed by MS. This screen identified both known and previously unknown interactors of IGF1R. One of the novel interactors was sorting nexin 6 (SNX6), a protein that forms part of the retromer complex, which is involved in intracellular protein sorting. Using co-immunoprecipitation, we confirmed that IGF1R and SNX6 physically interact. SNX6 knockdown resulted in a dramatic diminution of IGF1-mediated ERK1/2 phosphorylation, but did not affect IGF1R internalization. Bioluminescence resonance energy transfer experiments indicated that the SNX6 knockdown perturbed the association between IGF1R and the key adaptor proteins insulin receptor substrate 1 (IRS1) and SHC adaptor protein 1 (SHC1). Intriguingly, even in the absence of stimuli, SNX6 overexpression significantly increased Akt phosphorylation. Our study confirms the utility of proximity-labeling methods, such as BioID, to screen for interactors of cell-surface receptors and has uncovered a role of one of these interactors, SNX6, in the IGF1R signaling cascade.

Resistance to thyrotropin

Resistance to thyrotropin (RTSH) is broadly defined as reduced sensitivity of thyroid follicle cells to stimulation by biologically active TSH due to genetic defects. Affected individuals have elevated serum TSH in the absence of goiter, with the severity ranging from nongoitrous isolated hyperthyrotropinemia to severe congenital hypothyroidism with thyroid hypoplasia. Conceptually, defects leading to RTSH impair both aspects of TSH-mediated action, namely thyroid hormone synthesis and gland growth. These include inactivating mutations in the genes encoding the TSH receptor and the PAX8 transcription factor. A common third cause has been genetically mapped to a locus on chromosome 15, but the underlying pathophysiology has not yet been elucidated. This review provides a succinct overview of currently defined causes of nonsyndromic RTSH, their differential diagnoses (autoimmune; partial iodine organification defects; syndromic forms of RTSH) and implications for the clinical approach to patients with RTSH.

Single-Molecule Fluorescence Methods to Study Plant Hormone Signal Transduction Pathways

Plant-hormone-initiated signaling pathways are extremely vital for plant growth, differentiation, development, and adaptation to environmental stresses. Hormonal perception by receptors induces downstream signal transduction mechanisms that lead to plant responses. However, conventional techniques-such as genetics, biochemistry, and physiology methods-that are applied to elucidate these signaling pathways can only provide qualitative or ensemble-averaged quantitative results, and the intrinsic molecular mechanisms remain unclear. The present study developed novel methodologies based on in vitro single-molecule fluorescence assays to elucidate the complete and detailed mechanisms of plant hormone signal transduction pathways. The proposed methods are based on multicolor total internal reflection fluorescence microscopy and a flow cell model for gas environment control. The methods validate the effectiveness of single-molecule approaches for the extraction of abundant information, including oligomerization, specific gas dependence, and the interaction kinetics of different components.

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.

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.

A novel humanized GLP-1 receptor model enables both affinity purification and Cre-LoxP deletion of the receptor

Class B G protein-coupled receptors (GPCRs) are important regulators of endocrine physiology, and peptide-based therapeutics targeting some of these receptors have proven effective at treating disorders such as hypercalcemia, osteoporosis, and type 2 diabetes mellitus (T2DM). As next generation efforts attempt to develop novel non-peptide, orally available molecules for these GPCRs, new animal models expressing human receptor orthologs may be required because small molecule ligands make fewer receptor contacts, and thus, the impact of amino acid differences across species may be substantially greater. The objective of this report was to generate and characterize a new mouse model of the human glucagon-like peptide-1 receptor (hGLP-1R), a class B GPCR for which established peptide therapeutics exist for the treatment of T2DM. hGLP-1R knock-in mice express the receptor from the murine Glp-1r locus. Glucose tolerance tests and gastric emptying studies show hGLP-1R mice and their wild-type littermates display similar physiological responses for glucose metabolism, insulin secretion, and gastric transit, and treatment with the GLP-1R agonist, exendin-4, elicits similar responses in both groups. Further, ex vivo assays show insulin secretion from humanized islets is glucose-dependent and enhanced by GLP-1R agonists. To enable additional utility, the targeting construct of the knock-in line was engineered to contain both flanking LoxP sites and a C-terminal FLAG epitope. Anti-FLAG affinity purification shows strong expression of hGLP-1R in islets, lung, and stomach. We crossed the hGLP-1R line with Rosa26Cre mice and generated global Glp-1r-/- animals. Immunohistochemistry of pancreas from humanized and knock-out mice identified a human GLP-1R-specific antibody that detects the GLP-1R in human pancreas as well as in the pancreas of hGLP-1r knock-in mice. This new hGLP-1R model will allow tissue-specific deletion of the GLP-1R, purification of potential GLP-1R partner proteins, and testing of novel therapeutic agents targeting the hGLP-1R.

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.

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.

Modern methods to investigate the oligomerization of glycoprotein hormone receptors (TSHR, LHR, FSHR)

As for other GPCRs, the oligomerization of glycoprotein hormone receptors (GPHRs) appears as critical event for receptor function. By means of modern techniques based on the BRET or FRET principle, GPHR oligomerization has been reported to explain several physiological and pathological conditions. In particular, the presence of oligomers was demonstrated not only in in vitro heterologous systems but also in in vivo tissues, and GPHR homodimerization appears associated with strong negative cooperativity, thus suggesting that one hormone molecule may be sufficient for receptor dimer stimulation. In addition, oligomerization has been reported to occur early during the posttranslational maturation process and to be involved in the dominant negative effect exerted by loss-of-function TSH receptor (TSHR) mutants, that are prevalently retained inside the cell, on the surface expression of wild-type receptors. This molecular mechanism thus explains the dominant inheritance of certain forms of TSH resistance. Here, we provide the description of the methods used in the original BRET, FRET, and HTRF-RET experiments.

New pathogenic thyrotropin receptor mutations decipher differentiated activity switching at a conserved helix 6 motif of family A GPCR

CONTEXT

In this paper we report two new TSH receptor (TSHR) mutations. One mutation (Pro639(6.50)Leu) was identified in two siblings with congenital hypothyroidism, and a second mutation (Cys636(6.47)Arg) was found in a patient suffering from nonautoimmune hyperthyroidism. Both mutations are located in transmembrane helix (TMH) 6 at the conserved Cys(6.47)-Trp(Met)(6.48)-Leu(Ala)(6.49)-Pro(6.50) motif of family A G protein-coupled receptors (GPCR).

OBJECTIVE

To study the pathogenic mechanisms, we tested patients' mutations and further side chain variations regarding their effects on TSHR signaling.

RESULTS

Substitution Pro639Leu fully inactivates the promiscuous TSHR for cAMP (Gs) and IP (Gq) signaling. In contrast, Cys636Arg leads to constitutive activation of Gs. Organization of TSHR in oligomers was not modified by mutations at position 636. Interestingly, it is known from crystal structures of GPCR that Pro(6.50) is located at a TMH6 kink-distortion, which is a pivot during activation-related helical movements. However, the cell surface expressions of all mutants at position 639 were comparable to wild type, indicating a helical conformation like wild type.

CONCLUSION

Until now, only naturally occurring constitutively activating mutations in TSHR TMH6 have been reported, but here we present the first pathogenic inactivating mutation (Pro639Leu). Our data are indicative of differentiated regulation of Gs and Gq signaling at particular TMH6 positions, but without any effects on TSHR oligomer constellation. Details of signaling modulation by each mutant at positions 636(6.47) and 639(6.50) help us to understand high conservation of these amino acids in family A GPCR. Described molecular (pathogenic) mechanisms are likely not unique for TSHR.

From molecular details of the interplay between transmembrane helices of the thyrotropin receptor to general aspects of signal transduction in family a G-protein-coupled receptors (GPCRs)

Transmembrane helices (TMHs) 5 and 6 are known to be important for signal transduction by G-protein-coupled receptors (GPCRs). Our aim was to characterize the interface between TMH5 and TMH6 of the thyrotropin receptor (TSHR) to gain molecular insights into aspects of signal transduction and regulation. A proline at TMH5 position 5.50 is highly conserved in family A GPCRs and causes a twist in the helix structure. Mutation of the TSHR-specific alanine (Ala-593⁵·⁵⁰) at this position to proline resulted in a 20-fold reduction of cell surface expression. This indicates that TMH5 in the TSHR might have a conformation different from most other family A GPCRs by forming a regular α-helix. Furthermore, linking our own and previous data from directed mutagenesis with structural information led to suggestions of distinct pairs of interacting residues between TMH5 and TMH6 that are responsible for stabilizing either the basal or the active state. Our insights suggest that the inactive state conformation is constrained by a core set of polar interactions among TMHs 2, 3, 6, and 7 and in contrast that the active state conformation is stabilized mainly by non-polar interactions between TMHs 5 and 6. Our findings might be relevant for all family A GPCRs as supported by a statistical analysis of residue properties between the TMHs of a vast number of GPCR sequences.

Syndromes of resistance to TSH

The resistance to TSH action is a genetic disease characterized by molecular defects hampering the adequate transmission of TSH stimulatory signal into thyroid cells. In principle the defect may affect every step along the cascade of events following the binding of TSH to its receptor (TSHR) on thyroid cell membranes. The phenotypic expressivity of TSH resistance is highly variable going from severe congenital hypothyroidism (CH) with thyroid hypoplasia to mild hyperthyrotropinemia (hyperTSH) associated with an apparent euthyroid state. More severe forms follow a recessive pattern of inheritance and occur in patients with biallelic mutations both causing a severe loss of TSHR function. Differential diagnosis in these cases includes the exclusions of other causes of isolated thyroid dysgenesis. Mildest forms may instead occur in patients with monoallelic TSHR defects following a dominant mode of inheritance. In these cases we described the dominant negative effect exerted by some mutants on the activity of the receptor encoded by the wild type allele. In these cases, differential diagnosis involves the exclusion of autoimmune thyroid disease or pseudohypoparathyroidism associated with defects at the GNAS locus. This review will focus on the variable clinical expression of this disease.

Thyroid-stimulating hormone receptor activity after internalization

The thyroid-stimulating hormone (TSH) receptor (TSHR) belongs to the large family of G-protein-coupled receptors (GPCRs) and is predominantly coupled to G(s). Thus, the effects of TSH are largely mediated by the stimulation of adenylyl cyclase and the ensuing rise of intracellular cyclic AMP (cAMP) concentrations. Like for other GPCRs, a prolonged stimulation of the TSHR leads to its internalization into endosomes followed by its recycling to the cell surface. Until recently, GPCRs were believed to activate "classical" G-protein-dependent pathways only when located on the cell surface and to cease doing so upon agonist-induced internalization. However, our recent findings on the TSHR and similar ones on the parathyroid hormone and sphingosine receptors suggest that internalized GPCRs can continue to signal through G(s)-cAMP in an intracellular compartment. Interestingly, this type of intracellular cAMP signaling differs from that occurring on the cell surface, as it is persistent and apparently leads to specific signaling outcomes. Although further studies are needed to investigate the possible physiological and pathophysiological consequences of GPCR-cAMP signaling in the endocytic compartment, endosomes should no longer be viewed as passive carriers for receptors en route to degradation but rather as specialized intracellular platforms for GPCR signaling.

The R450H mutation and D727E polymorphism of the thyrotropin receptor gene in a Chinese child with congenital hypothyroidism

BACKGROUND

Congenital hypothyroidism (CH) is the most prevalent congenital endocrine disorder. The molecular cause of CH in the majority of newborns is unknown. The aim of this study was to investigate the mutation of thyrotropin receptor (TSHR) gene in Chinese children with congenital hypothyroidism (CH). and the hereditary characteristic.

METHODS

Eighteen Chinese children with CH were enrolled for molecular analysis of the TSHR gene and 105 normal controls were evaluated. The exons 1-9, and 10 of TSHR gene were detected by PCR-SSCP (single-stranded conformation polymorphism) and sequenced.

RESULTS

A slower and a faster mobility SSCP shift showed in a 12-year old child with hypoplasic gland. Sequencing of TSHR gene revealed a homozygous mutation (CGC --> CAC, Arg450His) and a polymorphism (GAC --> GAG, Asp727Glu). The controls revealed no variants. The 12 relatives of the proband were enrolled and investigated. Six relatives, including his mother and father, were heterozygous for R450H mutation and D727E polymorphism of the TSHR gene. Thyroid hormone levels were normal except for circulating TSH (5.96-6.92 mU/L) level slightly elevated in six heterozygous family members.

CONCLUSIONS

Homozygous mutation R450H of the TSHR gene led to CH. Heterozygous mutation R450H was the cause of subclinical hypothyroidism.

Trafficking of G-protein-coupled receptors to the plasma membrane: insights for pharmacoperone drugs

G protein-coupled receptors (GPCRs) are among the most common potential targets for pharmacological design. Synthesized in the endoplasmic reticulum, they interact with endogenous chaperones that assist in folding (or can retain incorrectly folded proteins) and are transferred to the plasma membrane where they exert their physiological functions. We summarize trafficking of the gonadotropin-releasing hormone receptor (GnRHR) to the plasma membrane. The trafficking of GnRHR is among the best characterized due in part to its small size and the consequent ease of making mutant proteins. Human mutations that cause disease through the misrouting of GPCRs including GnRHR are also reviewed. Special emphasis is placed on therapeutic opportunities presented by pharmacological chaperone drugs, or pharmacoperones, that allow misrouted mutants to be routed correctly and restored to function.

A tyrosine residue on the TSH receptor stabilizes multimer formation

BACKGROUND

The thyrotropin stimulating hormone receptor (TSHR) is a G protein coupled receptor (GPCR) with a large ectodomain. The ligand, TSH, acting via this receptor regulates thyroid growth and thyroid hormone production and secretion. The TSH receptor (TSHR) undergoes complex post-translational modifications including intramolecular cleavage and receptor multimerization. Since monomeric and multimeric receptors coexist in cells, understanding the functional role of just the TSHR multimers is difficult. Therefore, to help understand the physiological significance of receptor multimerization, it will be necessary to abrogate multimer formation, which requires identifying the ectodomain and endodomain interaction sites on the TSHR. Here, we have examined the contribution of the ectodomain to constitutive multimerization of the TSHR and determined the possible residue(s) that may be involved in this interaction.

METHODOLOGY/PRINCIPAL FINDINGS

We studied ectodomain multimer formation by expressing the extracellular domain of the TSHR linked to a glycophosphotidyl (GPI) anchor in both stable and transient expression systems. Using co-immunoprecipitation and FRET of tagged receptors, we established that the TSH receptor ectodomain was capable of multimerization even when totally devoid of the transmembrane domain. Further, we studied the effect of two residues that likely made critical contact points in this interaction. We showed that a conserved tyrosine residue (Y116) on the convex surface of the LRR3 was a critical residue in ectodomain multimer formation since mutation of this residue to serine totally abrogated ectodomain multimers. This abrogation was not seen with the mutation of cysteine 176 on the inner side of the LRR5, demonstrating that inter-receptor disulfide bonding was not involved in ectodomain multimer formation. Additionally, the Y116 mutation in the intact wild type receptor enhanced receptor degradation.

CONCLUSIONS/SIGNIFICANCE

These data establish the TSH receptor ectodomain as one site of multimerization, independent of the transmembrane region, and that this interaction was primarily via a conserved tyrosine residue in LRR3.

The thyroid-stimulating hormone receptor: impact of thyroid-stimulating hormone and thyroid-stimulating hormone receptor antibodies on multimerization, cleavage, and signaling

The thyroid-stimulating hormone receptor (TSHR) has a central role in thyrocyte function and is also one of the major autoantigens for the autoimmune thyroid diseases. We review the post-translational processing, multimerization, and intramolecular cleavage of TSHR, all of which may modulate its signal transduction. The recent characterization of monoclonal antibodies to the TSHR, including stimulating, blocking, and neutral antibodies, have also revealed unique biologic insights into receptor activation and the variety of these TSHR antibodies may help explain the multiple clinical phenotypes seen in autoimmune thyroid diseases. Knowledge of the structure/function relationship of the TSHR is beginning to provide a greater understanding of thyroid physiology and thyroid autoimmunity.

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

The TSH receptor (TSHR) together with the homologous lutropin/choriogonadotropin receptor and the follitropin receptor are glycoprotein hormone receptors (GPHRs). They constitute a subfamily of the rhodopsin-like G protein-coupled receptors with seven transmembrane helices. GPHRs and their corresponding hormones are pivotal proteins with respect to a variety of physiological functions. The identification and characterization of intra- and intermolecular signaling determinants as well as signaling mechanisms are prerequisites to gaining molecular insights into functions and (pathogenic) dysfunctions of GPHRs. Knowledge about activation mechanisms is fragmentary, and the specific aspects have still not been understood in their entirety. Therefore, here we critically review the data available for these receptors and bring together structural and functional findings with a focus on the important large extracellular portion of the TSHR. One main focus is the particular function of structural determinants in the initial steps of the activation such as: 1) hormone binding at the extracellular site; 2) hormone interaction at a second binding site in the hinge region; 3) signal regulation via sequence motifs in the hinge region; and 4) synergistic signal amplification by cooperative effects of the extracellular loops toward the transmembrane region. Comparison and consolidation of data from the homologous glycoprotein hormone receptors TSHR, follitropin receptor, and lutropin/choriogonadotropin receptor provide an overview of extracellular mechanisms of signal initiation, conduction, and regulation at the TSHR and homologous receptors. Finally, we address the issue of structural implications and suggest a refined scenario for the initial signaling process on GPHRs.

Pharmacological onomastics: what's in a name?

Drugs are named for their primary receptor target and overt action (agonism, antagonism) but the observation of multiple or collateral efficacies emanating from drugs activating a single receptor target is posing a challenge for drug classification and nomenclature. With increasing abilities to detect alteration in cellular function has come the identification of efficacies that are not necessarily manifest in obvious changes in cell response. Specifically, some agonists selectively activate cellular pathways, demonstrate phenotypic behaviour associated with cell type and some antagonists actively induce receptor internalization without activation. In addition, the effects of allosteric modulators can be linked to the nature of the co-binding ligand posing a similar complication in classification and naming. Thus, accurate labels for this new generation of selective drugs may require identification of receptor partners (G-protein type, beta-arrestin) or pathway or, in the case of allosteric modulators, identification of co-binding ligands. The association of distinct phenotypic behaviours with molecules opens the opportunity to better associate clinical effects with distinct pharmacological properties.