Dominant negative effects of human follicle-stimulating hormone receptor expression-deficient mutants on wild-type receptor cell surface expression. Rescue of oligomerization-dependent defective receptor expression by using cognate decoys

Restoring function to inactivating G protein-coupled receptor variants in the hypothalamic-pituitary-gonadal axis1

G protein-coupled receptors (GPCRs) are central to the functioning of the hypothalamic-pituitary-gonadal axis (HPG axis) and include the rhodopsin-like GPCR family members, neurokinin 3 receptor, kappa-opioid receptor, kisspeptin 1 receptor, gonadotropin-releasing hormone receptor, and the gonadotropin receptors, luteinizing hormone/choriogonadotropin receptor and follicle-stimulating hormone receptor. Unsurprisingly, inactivating variants of these receptors have been implicated in a spectrum of reproductive phenotypes, including failure to undergo puberty, and infertility. Clinical induction of puberty in patients harbouring such variants is possible, but restoration of fertility is not always a realisable outcome, particularly for those patients suffering from primary hypogonadism. Thus, novel pharmaceuticals and/or a fundamental change in approach to treating these patients are required. The increasing wealth of data describing the effects of coding-region genetic variants on GPCR function has highlighted that the majority appear to be dysfunctional as a result of misfolding of the encoded receptor protein, which, in turn, results in impaired receptor trafficking through the secretory pathway to the cell surface. As such, these intracellularly retained receptors may be amenable to 'rescue' using a pharmacological chaperone (PC)-based approach. PCs are small, cell permeant molecules hypothesised to interact with misfolded intracellularly retained proteins, stabilising their folding and promoting their trafficking through the secretory pathway. In support of the use of this approach as a viable therapeutic option, it has been observed that many rescued variant GPCRs retain at least a degree of functionality when 'rescued' to the cell surface. In this review, we examine the GPCR PC research landscape, focussing on the rescue of inactivating variant GPCRs with important roles in the HPG axis, and describe what is known regarding the mechanisms by which PCs restore trafficking and function. We also discuss some of the merits and obstacles associated with taking this approach forward into a clinical setting.

A chromosome-level genome assembly enables the identification of the follicule stimulating hormone receptor as the master sex-determining gene in the flatfish Solea senegalensis

Sex determination (SD) shows huge variation among fish and a high evolutionary rate, as illustrated by the Pleuronectiformes (flatfishes). This order is characterized by its adaptation to demersal life, compact genomes and diversity of SD mechanisms. Here, we assembled the Solea senegalensis genome, a flatfish of great commercial value, into 82 contigs (614 Mb) combining long- and short-read sequencing, which were next scaffolded using a highly dense genetic map (28,838 markers, 21 linkage groups), representing 98.9% of the assembly. Further, we established the correspondence between the assembly and the 21 chromosomes by using BAC-FISH. Whole genome resequencing of six males and six females enabled the identification of 41 single nucleotide polymorphism variants in the follicle stimulating hormone receptor (fshr) consistent with an XX/XY SD system. The observed sex association was validated in a broader independent sample, providing a novel molecular sexing tool. The fshr gene displayed differential expression between male and female gonads from 86 days post-fertilization, when the gonad is still an undifferentiated primordium, concomitant with the activation of amh and cyp19a1a, testis and ovary marker genes, respectively, in males and females. The Y-linked fshr allele, which included 24 nonsynonymous variants and showed a highly divergent 3D protein structure, was overexpressed in males compared to the X-linked allele at all stages of gonadal differentiation. We hypothesize a mechanism hampering the action of the follicle stimulating hormone driving the undifferentiated gonad toward testis.

GPCR heteromers: An overview of their classification, function and physiological relevance

G protein-coupled receptors (GPCRs) are capable of interacting to form higher order structures such as homomers and heteromers. Heteromerisation in particular has implications for receptor function, with research showing receptors can attain unique expression, ligand binding, signalling and intracellular trafficking upon heteromerisation. As such, GPCR heteromers represent novel drug targets with extensive therapeutic potential. Changes to ligand affinity, efficacy and G protein coupling have all been described, with alterations to these pharmacological aspects now well accepted as common traits for heteromeric complexes. Changes in internalisation and trafficking kinetics, as well as β-arrestin interactions are also becoming more apparent, however, few studies to date have explicitly looked at the implications these factors have upon the signalling profile of a heteromer. Development of ligands to target GPCR heteromers both experimentally and therapeutically has been mostly concentrated on bivalent ligands due to difficulties in identifying and developing heteromer-specific ligands. Improving our understanding of the pharmacology and physiology of GPCR heteromers will enable further development of heteromer-specific ligands with potential to provide therapeutics with increased efficacy and decreased side effects.

Targeting trafficking as a therapeutic avenue for misfolded GPCRs leading to endocrine diseases

G protein-coupled receptors (GPCRs) are plasma membrane proteins associated with an array of functions. Mutations in these receptors lead to a number of genetic diseases, including diseases involving the endocrine system. A particular subset of loss-of-function mutant GPCRs are misfolded receptors unable to traffic to their site of function (i.e. the cell surface plasma membrane). Endocrine disorders in humans caused by GPCR misfolding include, among others, hypo- and hyper-gonadotropic hypogonadism, morbid obesity, familial hypocalciuric hypercalcemia and neonatal severe hyperparathyroidism, X-linked nephrogenic diabetes insipidus, congenital hypothyroidism, and familial glucocorticoid resistance. Several in vitro and in vivo experimental approaches have been employed to restore function of some misfolded GPCRs linked to endocrine disfunction. The most promising approach is by employing pharmacological chaperones or pharmacoperones, which assist abnormally and incompletely folded proteins to refold correctly and adopt a more stable configuration to pass the scrutiny of the cell's quality control system, thereby correcting misrouting. This review covers the most important aspects that regulate folding and traffic of newly synthesized proteins, as well as the experimental approaches targeted to overcome protein misfolding, with special focus on GPCRs involved in endocrine diseases.

fshr: a fish sex-determining locus shows variable incomplete penetrance across flathead grey mullet populations

Whole-genome sequencing data were produced from a single flathead grey mullet female and assembled into a draft genome sequence, whereas publicly available sequence data were used to obtain a male draft sequence. Two pools, each consisting of 60 unrelated individuals, respectively, of male and female fish were analyzed using Pool-Sequencing. Mapping and analysis of Pool-Seq data against the draft genome(s) revealed >30 loci potentially associated with sex, the most promising locus of which, encoding the follicle-stimulating hormone receptor (fshr) and harboring two missense variants, was genotyped on 245 fish from four Mediterranean populations. Genotype data showed that fshr represents a previously unknown sex-determining locus, although the incomplete association pattern between fshr genotype and sex-phenotype, the variability of such pattern across different populations, and the presence of other candidate loci reveal that a greater complexity underlies sex determination in the flathead grey mullet.

Tyrosine nitrations impaired intracellular trafficking of FSHR to the cell surface and FSH-induced Akt-FoxO3a signaling in human granulosa cells

Many infertile women suffered from poor ovarian response, and increased reactive oxygen species with age might mediate the poor ovarian response to FSH. In this study, we collected follicular fluids and isolated granulosa cells from female patients. Increased levels of peroxynitrite, tyrosine nitrations of FSH receptor (FSHR) and apoptosis were obviously detectable with decreased FSHR protein expressions in granulosa cells of the poor ovarian responders. In KGN (a human ovarian granulosa cell line) cells, exogenous peroxynitrite could sequester FSHR in the cytoplasm, and these dislocated FSHR might suffer from proteasome-mediated degradations. Here, we identified four peroxynitrite-mediated nitrated tyrosine residues of FSHR. Site-directed mutagenesis of FSHR revealed that Y626 was pivotal for intracellular trafficking of FSHR to the cell surface. Akt-induced inactivation of FoxO3a was required for the repression of FSH on granulosa cell apoptosis. However, peroxynitrite impaired FSH-induced Akt-FoxO3a signaling, while FSHR-Y626A mutant took similar effects. In addition, FoxO3a knockdown indeed impaired FSH-mediated cell survival, while FoxO3a-S253A mutant reversed that significantly.

Novel FSHR mutations in Han Chinese women with sporadic premature ovarian insufficiency

Premature ovarian insufficiency (POI) is characterized by amenorrhea and elevated levels of follicle-stimulating hormone (FSH, usually > 25 IU/L) before 40 years of age. To identify the relationship between FSHR mutations and sporadic POI patients of Han Chinese descent, we performed Sanger sequencing of FSHR gene in 192 sporadic POI patients and 192 matched controls of Han Chinese descent. Two heterozygous missense variants, c.793A > G (p.M265V) and c.1789C > A (p.L597I), were identified exclusively in POI patients. Functional studies showed that both mutants were expressed on the cell surface, while p.L597I showed decreased membrane localization compared with wild-type FSHR. Moreover, FSH-induced cAMP production and ERK1/2 phosphorylation were reduced in the cells transfected with p.L597I mutant, but not in the cells transfected with p.M265V mutant. In addition, two single-nucleotide polymorphisms (SNPs), rs1394205 (c.-29G > A) and rs140106399 (c.*111 T > C), were identified in both POI group and control group with significantly different genotypic and allelic distributions. These results indicated that dysfunctional FSHR due to mutation or SNPs might explain a fraction of sporadic POI cases in Han Chinese population.

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.

Pharmacoperones for Misfolded Gonadotropin Receptors

The gonadotropin receptors (luteinising hormone receptor; LHR and follicle-stimulating hormone receptor; FSHR) are G protein-coupled receptors (GPCRs) that play an important role in the endocrine control of reproduction. Thus genetic mutations that cause impaired function of these receptors have been implicated in a number of reproductive disorders. Disease-causing genetic mutations in GPCRs frequently result in intracellular retention and degradation of the nascent protein through misfolding and subsequent recognition by cellular quality control machinery. The discovery and development of novel compounds termed pharmacological chaperones (pharmacoperones) that can stabilise misfolded receptors and restore trafficking and plasma membrane expression are therefore of great interest clinically, and promising in vitro data describing the pharmacoperone rescue of a number of intracellularly retained mutant GPCRs has provided a platform for taking these compounds into in vivo trials. Thienopyrimidine small molecule allosteric gonadotropin receptor agonists (Org 42599 and Org 41841) have been demonstrated to have pharmacoperone activity. These compounds can rescue cell surface expression and in many cases, hormone responsiveness, of a range of retained mutant gonadotropin receptors. Should gonadotropin receptor selectivity of these compounds be improved, they could offer therapeutic benefit to subsets of patients suffering from reproductive disorders attributed to defective gonadotropin receptor trafficking.

A novel homozygous mutation in the FSHR gene is causative for primary ovarian insufficiency

OBJECTIVE

To identify the potential FSHR mutation in a Chinese woman with primary ovarian insufficiency (POI).

DESIGN

Genetic and functional studies.

SETTING

University-based reproductive medicine center.

PATIENT(S)

A POI patient, her family members, and another 192 control women with regular menstruation.

INTERVENTION(S)

Ovarian biopsy was performed in the patient. Sanger sequencing was carried out for the patient, her sister, and parents. The novel variant identified was further confirmed with the use of control subjects.

MAIN OUTCOME MEASURE(S)

Sanger sequencing and genotype analysis to identify the potential variant of the FSHR gene; hematoxylin and eosin staining of the ovarian section to observe the follicular development; Western blotting and immunofluorescence to detect FSH receptor (FSHR) expression; and cyclic adenosine monophosphate (cAMP) assay to monitor FSH-induced signaling.

RESULT(S)

Histologic examination of the ovaries in the patient revealed follicular development up to the early antral stage. Mutational screening and genotype analysis of the FSHR gene identified a novel homozygous mutation c.175C>T (p.R59X) in exon 2, which was inherited in the autosomal recessive mode from her heterozygous parents but was absent in her sister and the 192 control women. Functional studies demonstrated that in vitro the nonsense mutation caused the loss of full-length FSHR expression and that p.R59X mutant showed no response to FSH stimulation in the cAMP level.

CONCLUSION(S)

The mutation p.R59X in FSHR is causative for POI by means of arresting folliculogenesis.

Pharmacoperone rescue of vasopressin 2 receptor mutants reveals unexpected constitutive activity and coupling bias

Pharmacoperones are small molecules that diffuse into cells and rescue misfolded, mistrafficked protein mutants, restoring their function. These substances act with high target specificity, serving as templates to fold (or refold) receptors, enzymes, ion channels or other proteins and enable them to pass the scrutiny of the cellular quality control system ("rescue"). In the present study we demonstrate that a rescued mutant (L83Q) of the vasopressin type 2 receptor (V2R), shows a strong bias for Gs coupling unlike the WT V2 receptor, which couples to both Gs and Gq/11. Failure of the mutant to couple to Gq/11 was not due to a limiting quantity of G-proteins since other Gq/11-coupled receptors (WT V2R, histamine receptor and muscarinic receptor) responded appropriately to their ligands. Transfection with DNA encoding Gq enabled the V2 receptor mutant to couple to this G protein, but only modestly compared with the WT receptor. Fourteen V2R mutant pharmacoperones, of multiple chemical classes, obtained from a high throughput screen of a 660,000 structure library, and one V2R peptidomimetic antagonist rescues L83Q. The rescued mutant shows similar bias with all pharmacoperones identified, suggesting that the bias is intrinsic to the mutant protein's structure, rather than due to the chemical class of the pharmacoperone. In the case of V2R mutant Y128S, rescue with a pharmacoperone revealed constitutive activity, also with bias for Gs, although both IP and cAMP were produced in response to agonist. These results suggest that particular rescued receptor mutants show functional characteristics that differ from the WT receptor; a finding that may be important to consider as pharmacoperones are developed as therapeutic agents.

First mutation in the FSHR cytoplasmic tail identified in a non-pregnant woman with spontaneous ovarian hyperstimulation syndrome

Background

Spontaneous ovarian hyperstimulation syndrome (sOHSS) is a rare event occurring mostly during natural pregnancy. Among described etiologies, some activating mutations of FSH receptor (FSHR) have been identified.

Case presentation

We report hereby the case of a non-pregnant women with three episodes of sOHSS. Hormonal evaluation was normal and no pituitary adenoma was detected. However, genetic analysis identified a novel heterozygous FSHR mutation (c.1901 G > A). This R634H mutation is the first described in the cytoplasmic tail of the receptor. Functional analysis failed to reveal constitutive activity of the mutant but a decreased cAMP production in response to FSH. The weak activity of this mutant is correlated with a markedly reduced cell surface expression.

Conclusion

Pathophysiology of non gestationnal sOHSS is still ill established. The molecular characterization of this new mutant indicates that it might not be at play. Therefore, further investigations are needed to improve knowledge of the molecular mechanism of this syndrome.

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.

Single-molecule analyses of fully functional fluorescent protein-tagged follitropin receptor reveal homodimerization and specific heterodimerization with lutropin receptor

We have previously shown that the carboxyl terminus (cT) of human follicle-stimulating hormone (FSH, follitropin) receptor (FSHR) is clipped before insertion into the plasma membrane. Surprisingly, several different constructs of FSHR fluorescent fusion proteins (FSHR-FPs) failed to traffic to the plasma membrane. Subsequently, we discovered that substituting the extreme cT of luteinizing hormone (LH) receptor (LHR) to create an FSHR-LHRcT chimera has no effect on FSHR functionality. Therefore, we used this approach to create an FSHR-LHRcT-FP fusion. We found this chimeric FSHR-LHRcT-FP was expressed in HEK293 cells at levels similar to reported values for FSHR in human granulosa cells, bound FSH with high affinity, and transduced FSH binding to produce cAMP. Quantitative fluorescence resonance energy transfer (FRET) analysis of FSHR-LHRcT-YFP/FSHR-LHRcT-mCherry pairs revealed an average FRET efficiency of 12.9 ± 5.7. Advanced methods in single-molecule analyses were applied in order to ascertain the oligomerization state of the FSHR-LHRcT. Fluorescence correlation spectroscopy coupled with photon-counting histogram analyses demonstrated that the FSHR-LHRcT-FP fusion protein exists as a freely diffusing homodimer in the plasma membrane. A central question is whether LHR could oligomerize with FSHR, because both receptors are coexpressed in differentiated granulosa cells. Indeed, FRET analysis revealed an average FRET efficiency of 14.4 ± 7.5 when the FSHR-LHR cT-mCherry was coexpressed with LHR-YFP. In contrast, coexpression of a 5-HT2cVSV-YFP with FSHR-LHR cT-mCherry showed only 5.6 ± 3.2 average FRET efficiency, a value indistinguishable from the detection limit using intensity-based FRET methods. These data demonstrate that coexpression of FSHR and LHR can lead to heterodimerization, and we hypothesize that it is possible for this to occur during granulosa cell differentiation.

Identification of transmembrane domains that regulate spatial arrangements and activity of prokineticin receptor 2 dimers

The chemokine prokineticin 2 (PK2) activates its cognate G protein-coupled receptor (GPCR) PKR2 to elicit various downstream signaling pathways involved in diverse biological processes. Many GPCRs undergo dimerization that can modulate a number of functions including membrane delivery and signal transduction. The aim of this study was to elucidate the interface of PKR2 protomers within dimers by analyzing the ability of PKR2 transmembrane (TM) deletion mutants to associate with wild type (WT) PKR2 in yeast using co-immunoprecipitation and mammalian cells using bioluminescence resonance energy transfer. Deletion of TMs 5-7 resulted in a lack of detectable association with WT PKR2, but could associate with a truncated mutant lacking TMs 6-7 (TM1-5). Interestingly, TM1-5 modulated the distance, or organization, between protomers and positively regulated Gαs signaling and surface expression of WT PKR2. We propose that PKR2 protomers form type II dimers involving TMs 4 and 5, with a role for TM5 in modulation of PKR2 function.

Role of the Extracellular and Intracellular Loops of Follicle-Stimulating Hormone Receptor in Its Function

Follicle-stimulating hormone receptor (FSHR) is a leucine-rich repeat containing class A G-protein coupled receptor belonging to the subfamily of glycoprotein hormone receptors (GPHRs), which includes luteinizing hormone/choriogonadotropin receptor (LH/CGR) and thyroid-stimulating hormone receptor. Its cognate ligand, follicle-stimulating hormone binds to, and activates FSHR expressed on the surface of granulosa cells of the ovary, in females, and Sertoli cells of the testis, in males, to bring about folliculogenesis and spermatogenesis, respectively. FSHR contains a large extracellular domain (ECD) consisting of leucine-rich repeats at the N-terminal end and a hinge region at the C-terminus that connects the ECD to the membrane spanning transmembrane domain (TMD). The TMD consists of seven α-helices that are connected to each other by means of three extracellular loops (ELs) and three intracellular loops (ILs) and ends in a short-cytoplasmic tail. It is well established that the ECD is the primary hormone binding domain, whereas the TMD is the signal transducing domain. However, several studies on the ELs and ILs employing site directed mutagenesis, generation of chimeric receptors and in vitro characterization of naturally occurring mutations have proven their indispensable role in FSHR function. Their role in every phase of the life cycle of the receptor like post translational modifications, cell surface trafficking, hormone binding, activation of downstream signaling, receptor phosphorylation, hormone-receptor internalization, and recycling of hormone-receptor complex have been documented. Mutations in the loops causing dysregulation of these processes lead to pathophysiological conditions. In other GPHRs as well, the loops have been convincingly shown to contribute to various aspects of receptor function. This review article attempts to summarize the extensive contributions of FSHR loops and C-terminal tail to its function.

Follicle-stimulating hormone receptor (FSHR) alternative skipping of exon 2 or 3 affects ovarian response to FSH

Genes critical for fertility are highly conserved in mammals. Interspecies DNA sequence variation, resulting in amino acid substitutions and post-transcriptional modifications, including alternative splicing, are a result of evolution and speciation. The mammalian follicle-stimulating hormone receptor (FSHR) gene encodes distinct species-specific forms by alternative splicing. Skipping of exon 2 of the human FSHR was reported in women of North American origin and correlated with low response to ovarian stimulation with exogenous follicle-stimulating hormone (FSH). To determine whether this variant correlated with low response in women of different genetic backgrounds, we performed a blinded retrospective observational study in a Turkish cohort. Ovarian response was determined as low, intermediate or high according to retrieved oocyte numbers after classifying patients in four age groups (<35, 35-37, 38-40, >40). Cumulus cells collected from 96 women undergoing IVF/ICSI following controlled ovarian hyperstimulation revealed four alternatively spliced FSHR products in seven patients (8%): exon 2 deletion in four patients; exon 3 and exons 2 + 3 deletion in one patient each, and a retention of an intron 1 fragment in one patient. In all others (92%) splicing was intact. Alternative skipping of exons 2, 3 or 2 + 3 were exclusive to low responders and was independent of the use of agonist or antagonist. Interestingly, skipping of exon 3 occurs naturally in the ovaries of domestic cats--a good comparative model for human fertility. We tested the signaling potential of human and cat variants after transfection in HEK293 cells and FSH stimulation. None of the splicing variants initiated cAMP signaling despite high FSH doses, unlike full-length proteins. These data substantiate the occurrence of FSHR exon skipping in a subgroup of low responders and suggest that species-specific regulation of FSHR splicing plays diverse roles in mammalian ovarian function.

Mutations in G protein-coupled receptors that impact receptor trafficking and reproductive function

G protein coupled receptors (GPCRs) are a large superfamily of integral cell surface plasma membrane proteins that play key roles in transducing extracellular signals, including sensory stimuli, hormones, neurotransmitters, or paracrine factors into the intracellular environment through the activation of one or more heterotrimeric G proteins. Structural alterations provoked by mutations or variations in the genes coding for GPCRs may lead to misfolding, altered plasma membrane expression of the receptor protein and frequently to disease. A number of GPCRs regulate reproductive function at different levels; these receptors include the gonadotropin-releasing hormone receptor (GnRHR) and the gonadotropin receptors (follicle-stimulating hormone receptor and luteinizing hormone receptor), which regulate the function of the pituitary-gonadal axis. Loss-of-function mutations in these receptors may lead to hypogonadotropic or hypergonadotropic hypogonadism, which encompass a broad spectrum of clinical phenotypes. In this review we describe mutations that provoke misfolding and failure of these receptors to traffick from the endoplasmic reticulum to the plasma membrane. We also discuss some aspects related to the therapeutic potential of some target-specific drugs that selectively bind to and rescue function of misfolded mutant GnRHR and gonadotropin receptors, and that represent potentially valuable strategies to treat diseases caused by inactivating mutations of these receptors.

Transitioning pharmacoperones to therapeutic use: in vivo proof-of-principle and design of high throughput screens

A pharmacoperone (from "pharmacological chaperone") is a small molecule that enters cells and serves as molecular scaffolding in order to cause otherwise-misfolded mutant proteins to fold and route correctly within the cell. Pharmacoperones have broad therapeutic applicability since a large number of diseases have their genesis in the misfolding of proteins and resultant misrouting within the cell. Misrouting may result in loss-of-function and, potentially, the accumulation of defective mutants in cellular compartments. Most known pharmacoperones were initially derived from receptor antagonist screens and, for this reason, present a complex pharmacology, although these are highly target specific. In this summary, we describe efforts to produce high throughput screens that identify these molecules from chemical libraries as well as a mouse model which provides proof-of-principle for in vivo protein rescue using existing pharmacoperones.

Mutations and polymorphisms in FSH receptor: functional implications in human reproduction

FSH brings about its physiological actions by activating a specific receptor located on target cells. Normal functioning of the FSH receptor (FSHR) is crucial for follicular development and estradiol production in females and for the regulation of Sertoli cell function and spermatogenesis in males. In the last two decades, the number of inactivating and activating mutations, single nucleotide polymorphisms, and spliced variants of FSHR gene has been identified in selected infertile cases. Information on genotype-phenotype correlation and in vitro functional characterization of the mutants has helped in understanding the possible genetic cause for female infertility in affected individuals. The information is also being used to dissect various extracellular and intracellular events involved in hormone-receptor interaction by studying the differences in the properties of the mutant receptor when compared with WT receptor. Studies on polymorphisms in the FSHR gene have shown variability in clinical outcome among women treated with FSH. These observations are being explored to develop molecular markers to predict the optimum dose of FSH required for controlled ovarian hyperstimulation. Pharmacogenetics is an emerging field in this area that aims at designing individual treatment protocols for reproductive abnormalities based on FSHR gene polymorphisms. The present review discusses the current knowledge of various genetic alterations in FSHR and their impact on receptor function in the female reproductive system.

Trafficking of the follitropin receptor

The follitropin or follicle-stimulating hormone receptor (FSHR) belongs to a highly conserved subfamily of the G protein-coupled receptor (GPCR) superfamily and is mainly expressed in specific cells in the gonads. As any other GPCR, the newly synthesized FSHR has to be correctly folded and processed in order to traffic to the cell surface plasma membrane and interact with its cognate ligand. In this chapter, we describe in detail the conditions and procedures used to study outward trafficking of the FSHR from the endoplasmic reticulum to the plasma membrane. We also describe some methods to analyze phosphorylation, β-arrestin recruitment, internalization, and recycling of this particular receptor, which have proved useful in our hands for dissecting its downward trafficking and fate following agonist stimulation.

Pharmacoperones: a new therapeutic approach for diseases caused by misfolded G protein-coupled receptors

G Protein-coupled receptors (GPCRs) are cell membrane proteins that recognize specific chemical signals such as drugs and hormones and transduce these signals into cellular responses by activating G-proteins. As is the case for all newly synthesized proteins, GPCRs are subjected to conformational scrutiny at the endoplasmic reticulum prior to processing and trafficking to the cell surface membrane. Because of this stringent quality control screening mechanism, mutations that result in protein misfolding frequently lead to retention in the endoplasmic reticulum, aggregation or other misrouting and, eventually, to disease. This article reviews some patents and new therapeutic opportunities based on the misfolding and retention of otherwise functional GPCRs that represent promising approaches to correct conformational abnormalities leading to distinct disease states.

Regulation of GPCR signal networks via membrane trafficking

G-protein-coupled receptors (GPCRs) are a superfamily of cell surface signaling proteins that act as central molecular activators and integrators in all endocrine systems. Membrane trafficking of GPCRs is a fundamental process in shaping extensive signaling networks activated by these receptors. Mounting evidence has identified an increasingly complex network of pathways and protein interactions that a GPCR can traverse and associate with, indicating a multi-level system of regulation. This review will discuss the recent developments in how GPCRs are trafficked to the cell surface as newly synthesized receptors, their recruitment to the clathrin-mediated pathway for endocytosis, and their sorting to subsequent divergent post-endocytic fates, focusing primarily on hormone-activated GPCRs. Current models depicting the classic roles membrane trafficking plays in GPCR signaling have evolved to a highly regulated and complex system than previously appreciated. These developments impart key mechanistic information on how spatial and temporal aspects of GPCR signaling may be integrated and could provide pathway-specific targets to be exploited for therapeutic intervention.

Pharmacological chaperones for misfolded gonadotropin-releasing hormone receptors

Structural alterations provoked by mutations or genetic variations in the gene sequence of G protein-coupled receptors (GPCRs) may lead to abnormal function of the receptor molecule. Frequently, this leads to disease. While some mutations lead to changes in domains involved in agonist binding, receptor activation, or coupling to effectors, others may cause misfolding and lead to retention/degradation of the protein molecule by the quality control system of the cell. Several strategies, including genetic, chemical, and pharmacological approaches, have been shown to rescue function of trafficking-defective misfolded GPCRs. Among these, pharmacological strategies offer the most promising therapeutic tool to promote proper trafficking of misfolded proteins to the plasma membrane (PM). Pharmacological chaperones or "pharmacoperones" are small compounds that permeate the PM, enter cells, and bind selectively to misfolded proteins and correct folding allowing routing of the target protein to the PM, where the receptor may bind and respond to agonist stimulation. In this review, we describe new therapeutic opportunities based on mislocalization of otherwise functional human gonadotropin-releasing hormone receptors. This particular receptor is highly sensitive to single changes in chemical charge, and its intracellular traffic is delicately balanced between expression at the PM or retention/degradation in the endoplasmic reticulum; it is, therefore, a particularly instructive model to understand both the protein routing and the molecular mechanisms, whereby pharmacoperones rescue misfolded intermediates or conformationally defective receptors.

Emerging roles for the FSH receptor adapter protein APPL1 and overlap of a putative 14-3-3τ interaction domain with a canonical G-protein interaction site

The interaction of cytoplasmic proteins with intracellular domains of membrane receptors can occur at several opportunities, including: during biosynthesis, while in membrane residency and during internalization and recycling following ligand binding. Since the initial discovery that it interacts with the FSH receptor (FSHR) together with additional members of a potential signaling complex, APPL1 has been shown to interact with a variety of membrane receptors. Recent subcellular localizations of APPL1 place it in dynamic and varied venues in the cell, including at the cell membrane, the nucleus and the early endosomes. Another adapter protein family the 14-3-3 proteins, are largely recognized as binding to phosphorylation sites but recent work demonstrated that in the case of FSHR, the 14-3-3 site overlaps with the canonical G-protein binding site. G-proteins appear to sample the environment and exchange between the membrane and intracellular locales and this binding could be mediated by or modulated by receptor interactions at the 14-3-3 binding site. Observations that multiple proteins can interact with cytoplasmic domains of GPCRs leads to the inescapable conclusion that either the interactions occur via orderly replacement or exchange, or that receptors are simultaneously occupied by a variety of adapters and effectors or even that oligomers of dimeric GPCRs provide for platforms that can simultaneously interact with effectors and adaptors.