Human follicle stimulating hormone receptor trafficking and hormone binding sites in the amino terminus

Antibody fragments targeting the extracellular domain of follicular stimulating hormone receptor for contraception in male dogs and cats

The increased LH levels resulting from the absence of negative feedback after castration has been linked to long-term health issues. A need exists for an alternative contraceptive agent that functions without interfering the LH pathways. This study aimed to develop antibody fragments against the follicular-stimulating hormone receptor (anti-FSHr) using phage-display technology and evaluate its effects on Sertoli cell functions. Phage clones against the extracellular domain of dog and cat FSHr selected from an antibody fragment phagemid library were analyzed for binding kinetics by surface plasmon resonance. Sertoli cells were isolated from testes of adult animals (five dogs and five cats). Efficacy test was performed by treating Sertoli cell cultures (SCCs) with anti-FSHr antibody fragments compared with untreated in triplicates. Expressions of androgen binding protein (ABP), inhibin subunit beta B (IHBB) and vascular endothelial growth factor A (VEGFA) mRNA in SCCs were quantified by RT-qPCR. The results demonstrated that the molecular weight of the purified dog and cat anti-FSHr antibody fragment was 25 kDa and 15 kDa, respectively. Based on protein molecular weight, the antibody fragment of dogs and cats was therefore, so-called single-chain variable fragments (scFv) and nanobody (nb), respectively. The binding affinity with dissociation constant (KD) was 2.32 × 10-7 M and 2.83 × 10-9 M for dog and cat anti-FSHr antibody fragments, respectively. The cross-binding kinetic interactions between the dog anti-FSHr scFv and the cat ECD of FSHr could not be fitted to the curves to determine the binding kinetics. However, the cross-binding affinity KD between the cat anti-FSHr nb and the dog ECD FSHr was 1.75 × 10-4 M. The mRNA expression of ABP, IHBB and VEGFA in SCCs was less (P < 0.05) in both dogs (12.26, 4.07 and 5.11 folds, respectively) and cats (39.53, 14.07 and 20.29 folds, respectively) treated with anti-FSHr antibody fragments, indicating the Sertoli cell functions were suppressed. In conclusion, this study demonstrated the establishment of species-specific antibody fragments against FSHr in SCCs for dogs and cats. The fragment proteins illustrate potential to be developed as non-surgical contraceptive agent targeting FSHr in companion animals.

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.

Inhibition of follicle-stimulating hormone-induced preovulatory follicles in rats treated with a nonsteroidal negative allosteric modulator of follicle-stimulating hormone receptor

We previously described a negative allosteric modulator (NAM) of FSHR (ADX61623) that blocked FSH-induced cAMP and progesterone production but did not block estradiol production. That FSHR NAM did not affect FSH-induced preovulatory follicle development as evidenced by the lack of an effect on the number of FSH-dependent oocytes found in the ampullae following ovulation with hCG. A goal is the development of a nonsteroidal contraceptive. Toward this end, a high-throughput screen using human FSHR identified an additional nonsteroidal small molecule (ADX68692). Although ADX68692 behaved like ADX61623 in inhibiting production of cAMP and progesterone, it also inhibited FSH-induced estradiol in an in vitro rat granulosa primary cell culture bioassay. When immature, noncycling female rats were injected subcutaneously or by oral dosing prior to exogenous FSH administration, it was found that ADX68692 decreased the number of oocytes recovered from the ampullae. The estrous cycles of mature female rats were disrupted by administration by oral gavage of 25 mg/kg and 10 mg/kg ADX68692. In the highest dose tested (25 mg/kg), 55% of animals cohabited with mature males had implantation sites compared to 33% in the 10 mg/kg group and 77% in the control group. A surprising finding was that a structural analog ADX68693, while effectively blocking progesterone production with similar efficacy as ADX68692, did not block estrogen production and despite better oral availability did not decrease the number of oocytes found in the ampullae even when used at 100 mg/kg. These data demonstrate that because of biased antagonism of the FSHR, nonsteroidal contraception requires that both arms of the FSHR steroidogenic pathway must be effectively blocked, particularly estrogen biosynthesis. Thus, a corollary to these findings is that it seems reasonable to propose that the estrogen-dependent diseases such as endometriosis may benefit from inhibition of FSH action at the ovary using the FSHR NAM approach.

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.

The adapter protein APPL1 links FSH receptor to inositol 1,4,5-trisphosphate production and is implicated in intracellular Ca(2+) mobilization

FSH binds to its receptor (FSHR) on target cells in the ovary and testis, to regulate oogenesis and spermatogenesis, respectively. The signaling cascades activated after ligand binding are extremely complex and have been shown to include protein kinase A, mitogen-activated protein kinase, phosphatidylinositol 3-kinase/protein kinase B, and inositol 1,4,5-trisphosphate-mediated calcium signaling pathways. The adapter protein APPL1 (Adapter protein containing Pleckstrin homology domain, Phosphotyrosine binding domain and Leucine zipper motif), which has been linked to an assortment of other signaling proteins, was previously identified as an interacting protein with FSHR. Thus, alanine substitution mutations in the first intracellular loop of FSHR were generated to determine which residues are essential for FSHR-APPL1 interaction. Three amino acids were essential; when any one of them was altered, APPL1 association with FSHR mutants was abrogated. Two of the mutants (L377A and F382A) that displayed poor cell-surface expression were not studied further. Substitution of FSHR-K376A did not affect FSH binding or agonist-stimulated cAMP production in either transiently transfected human embryonic kidney cells or virally transduced human granulosa cells (KGN). In the KGN line, as well as primary cultures of rat granulosa cells transduced with wild type or mutant receptor, FSH-mediated progesterone or estradiol production was not affected by the mutation. However, in human embryonic kidney cells inositol 1,4,5-trisphosphate production was curtailed and KGN cells transduced with FSHR-K376A evidenced reduced Ca(2+) mobilization from intracellular stores after FSH treatment.

A negative allosteric modulator demonstrates biased antagonism of the follicle stimulating hormone receptor

High quality gamete production in males and females requires the pituitary gonadotropin follicle stimulating hormone (FSH). In this report a novel chemical class of small molecule inhibitors of FSH receptor (FSHR) is described. ADX61623, a negative allosteric modulator (NAM), increased the affinity of interaction between (125)I-hFSH and human FSHR (hFSHR) five fold. This form of FSHR occupied simultaneously by FSH and ADX61623 was inactive for cAMP and progesterone production in primary cultures of rat granulosa cells. In contrast, ADX61623 did not block estrogen production. This demonstrates for the first time, biased antagonism at the FSHR. To determine if ADX61623 blocked FSH induction of follicle development in vivo, a bioassay to measure follicular development and oocyte production in immature female rats was validated. ADX61623 was not completely effective in blocking FSH induced follicular development in vivo at doses up to 100mg/kg as oocyte production and ovarian weight gain were only moderately reduced. These data illustrate that FSHR couples to multiple signaling pathways in vivo. Suppression of one pool of FSHR uncouples Gαs and cAMP production, and decreases progesterone production. Occupancy of another pool of FSHR sensitizes granulosa cells to FSH induced estradiol production. Therefore, ADX61623 is a useful tool to investigate further the mechanism of the FSHR signaling dichotomy. This may lead to a greater understanding of the signaling infrastructure which enables estrogen biosynthesis and may prove useful in treating estrogen dependent disease.

Decreased degradation of internalized follicle-stimulating hormone caused by mutation of aspartic acid 6.30(550) in a protein kinase-CK2 consensus sequence in the third intracellular loop of human follicle-stimulating hormone receptor

A naturally occurring mutation in follicle-stimulating hormone receptor (FSHR) gene has been reported: an amino acid change to glycine occurs at a conserved aspartic acid 550 (D550, D567, D6.30(567)). This residue is contained in a protein kinase-CK2 consensus site present in human FSHR (hFSHR) intracellular loop 3 (iL3). Because CK2 has been reported to play a role in trafficking of some receptors, the potential roles for CK2 and D550 in FSHR function were evaluated by generating a D550A mutation in the hFSHR. The hFSHR-D550A binds hormone similarly to WT-hFSHR when expressed in HEK293T cells. Western blot analyses showed lower levels of mature hFSHR-D550A. Maximal cAMP production of both hFSHR-D550A as well as the naturally occurring mutation hFSHR-D550G was diminished, but constitutive activity was not observed. Unexpectedly, when (125)I-hFSH bound to hFSHR-D550A or hFSHR-D550G, intracellular accumulation of radiolabeled FSH was observed. Both sucrose and dominant-negative dynamin blocked internalization of radiolabeled FSH and its commensurate intracellular accumulation. Accumulation of radiolabeled FSH in cells transfected with hFSHR-D550A is due to a defect in degradation of hFSH as measured in pulse chase studies, and confocal microscopy imaging revealed that FSH accumulated in large intracellular structures. CK2 kinase activity is not required for proper degradation of internalized FSH because inhibition of CK2 kinase activity in cells expressing hFSHR did not uncouple degradation of internalized radiolabeled FSH. Additionally, the CK2 consensus site in FSHR iL3 is not required for binding because CK2alpha coimmunoprecipitated with hFSHR-D550A. Thus, mutation of D550 uncouples the link between internalization and degradation of hFSH.

Follicle-stimulating hormone receptor (FSHR)-derived peptide vaccine induced infertility in mice without pathological effect on reproductive organs

In a previous study it was found that priming with recombinant human follicle-stimulating hormone receptor (rhFSHR) protein (F140) and boosting with a peptide containing amino acids 32–44 from FSHR showed a specific immune response and fertility inhibition in adult male mice. However, this priming and boosting led to damage of the reproductive organs. Therefore, to eliminate this damage, the peptide prime–boost strategy was explored as a possible means of avoiding the pathological change while maintaining infertility. Immunisation with the peptide prime–boost strategy led to decreased fertility 10 weeks after vaccination, which is consistent with Balb/C mice treated with the protein prime–peptide boost regime. In contrast to the cellular swelling and spotty necrosis in spermatogonia observed in the protein-primed mice, the mice receiving peptide priming did not display pathological damage in seminiferous tubules and interstitial cells. Thus, the prime–boost immune regime with the FSHR-derived peptide potentially provides a much safer candidate for a contraceptive vaccine.

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.

Increased plasma membrane expression of human follicle-stimulating hormone receptor by a small molecule thienopyr(im)idine

A thienopyr(im)idine (Org41841) activates the luteinizing hormone (LH) receptor but does not compete with the natural ligand binding site and does not show agonistic action on the follicle-stimulating hormone receptor (hFSHR) at sub-millimolar concentrations. When this drug is preincubated at sub-micromolar concentrations with host cells expressing the hFSHR, and then washed out, binding analysis and assessment of receptor-effector coupling show that it increases plasma membrane expression of the hFSHR. Real-time PCR shows that this effect did not result from increased hFSHR mRNA accumulation. It is possible that Org41841 behaves as a pharmacoperone, a drug which increases the percentage of newly synthesized receptor routing to the membrane. Like pharmacoperones for other receptors, this drug was able to rescue a particular mutant hFSHR (A(189)V) associated with misrouting and endoplasmic reticulum retention, although other mutants could not be rescued. This is potentially the first member of the pharmacoperone drug class which binds at a site that is distinctive from the ligand binding site.

Identification of follicle-stimulating hormone-selective beta-strands in the N-terminal hormone-binding exodomain of human gonadotropin receptors

Glycoprotein hormone receptors contain large N-terminal extracellular domains (ECDs) that distinguish these receptors from most other G protein-coupled receptors. Each glycoprotein hormone receptor ECD consists of a curved leucine-rich repeat domain flanked by N- and C-terminal cysteine-rich regions. Selectivity of the different glycoprotein hormone receptors for their cognate hormones is exclusively determined by their ECDs and, in particular, their leucine-rich repeat domain. To identify human (h)FSH-selective determinants we used a gain-of-function mutagenesis strategy in which beta-strands of the hLH receptor (hLH-R) were substituted with their hFSH receptor (hFSH-R) counterparts. Introduction of hFSH-R beta-strand 1 into hLH-R conferred responsiveness to hFSH, whereas hLH-R mutants harboring one of the other hFSH-R beta-strands displayed none or very limited sensitivity to hFSH. However, combined substitution of hFSH-R beta-strand 1 and some of the other hFSH-R beta-strands further increased the sensitivity of the mutant hLH-R to hFSH. The apparent contribution of multiple hFSH-R beta-strands in providing a selective hormone binding interface corresponds well with their position in relation to hFSH as recently determined in the crystal structure of hFSH in complex with part of the hFSH-R ECD.

Identification of a novel epitope in the thyroid-stimulating hormone receptor ectodomain acting as intramolecular signaling interface

Glycoprotein hormone receptors (GPHRs) differ from the other seven transmembrane receptors mainly through a complex activation mechanism that requires the binding of a large hormone toward a large N-terminal ectodomain. The intramolecular mechanism of the signal transduction to the serpentine domain upon hormone binding at the ectodomain is not understood. To identify determinants at the GPHR ectodomain that may be involved in signal transduction, we first searched for homologous structural features. Based on high sequence similarity to the determined structures of the Nogo-receptor ectodomain and the intermolecular complex of the Interleukin-8 ligand (IL8) and the N-terminal peptide of the IL8 receptor (IL8RA), the hypothesis was developed that portions of the intramolecular components, Cysteine-box-2 and Cysteine-box-3, of the GPHR ectodomain interact and localize at the interface between ectodomain and serpentine domain. Indeed, point mutations within the D403EFN406 motif at Cysteine-box-3 of the thyrotropin receptor resulted in increased basal cAMP levels, suggesting that this motif may be important for transduction of the signal from the ectodomain to the transmembrane domain. New indications are provided about the tight spatial cooperation and relative location of the new epitope and other determinants at the thyrotropin receptor ectodomain, such as the leucine-rich repeat motif Ser281 and the cysteine boxes. According to the high sequence conservation, the results are of general relevance for the signal transduction mechanism of other glycoprotein hormone receptors such as choriogonadotrophic/luteinizing hormone receptor and follicle-stimulating hormone receptor.

Assessment of recombinant porcine follicle-stimulating hormone receptor using a novel polyclonal ectodomain antibody

Follicle-stimulating hormone (FSH) receptors (FSHR) are critically involved in mediating the responses of granulosa cells and Sertoli cells to FSH. The dynamic changes in cell surface FSH receptors (FSHR) in response to FSH remain unclear in part because of the heavy reliance on ligand-binding methodologies. This study was designed to determine the molecular and cellular properties of recombinant porcine FSHR using a novel, high-affinity purified polyclonal antibody to the ectodomain of the pFSHR. A full-length porcine FSHR cDNA was cloned and sequenced and recombinant pFSHR protein was stably expressed in a clonal cell line of Chinese hamster ovary cells (pFSHR-CHO). Recombinant receptor was stably expressed in an ovarian cell line with a density similar to that of porcine ovarian cells. A specific polyclonal antibody was generated in chickens to a 100-amino acid fragment of the pFSHR ectodomain. Immunoblotting, immunoprecipitation, indirect immunofluorescence cytochemistry and immunoelectron microscopy were performed using affinity-purified antibody to identify recombinant pFSHR in pFSHR-CHO cells. Immunoblotting of solubilized pFSHR-CHO proteins and immunoprecipitation of pFSHR-CHO protein metabolically labeled with 35S identified a single 74-kDa band in pFSHR-CHO cells; no bands were visualized in mock-transfected CHO cells. Indirect immunofluorescent labeling revealed the presence of pFSHR in pFSHR-CHO cells but not in mock-transfected CHO cells. Immunoelectron microscopy revealed the highest density of pFSHR associated with the plasma membrane and no pFSHR in mock-transfected CHO cells. The chicken anti-pFSHR antibody is a valuable tool for detecting and monitoring of FSHR using a variety of methodologies.

Induction of infertility in adult male bonnet monkeys by immunization with phage-expressed peptides of the extracellular domain of FSH receptor

Active immunization of proven fertile adult male bonnet monkeys (Macaca radiata) with phage-expressed follicle-stimulating hormone receptor (FSHR)-specific peptides from the extracellular domain resulted in a progressive drop in sperm count with all animals becoming azoospermic by day 100. However, serum testosterone concentrations were unaltered during the entire course of study and animals exhibited normal mating behaviour. Breeding studies with proven fertile female monkeys revealed that all the immunized males were infertile. Following interruption of immunization on day 225, sperm counts returned to normal with restoration of fertility. These results indicate that infertility can be induced in adult male monkeys by interfering with the action of FSH using specific peptides of the extracellular domain of FSHR as antigens, without the risk of producing cross-reacting antibodies to the other glycoprotein hormones.

Orientation of follicle-stimulating hormone (FSH) subunits complexed with the FSH receptor. Beta subunit toward the N terminus of exodomain and alpha subunit to exoloop 3

Follicle-stimulating hormone (FSH) comprises an alpha subunit and a beta subunit, whereas the FSH receptor consists of two halves with distinct functions: the N-terminal extracellular exodomain and C-terminal membrane-associated endodomain. FSH initially binds to exodomain, and the resulting FSH/exodomain complex modulates the endodomain and generates signal. However, it has been difficult to determine which subunit of FSH contacts the exodomain or endodomain and in what orientation FSH interacts with them. To address these crucial issues, the receptor was Ala-scanned and the hormone subunits were probed with photoaffinity labeling with receptor peptides corresponding to the N-terminal region of the exodomain and exoloop 3 of the endodomain. Our results show that both regions of the receptors are important for hormone binding and signal generation. In addition, the FSH beta subunit is specifically labeled with the N-terminal peptide, whereas the alpha subunit is labeled with the exoloop 3 peptide. These contrasting results show that the FSH beta subunit is close to the N-terminal region and that the alpha subunit is projected toward exoloop 3 in the endodomain. The results raise the fundamental question whether the alpha subunit, common among the glycoprotein hormones, plays a major role in generating the hormone signal common to all glycoprotein hormones.

Regulation of follitropin receptor cell surface residency by the ubiquitin-proteasome pathway

Little is known of the normal physiological processes that govern the cell surface residency of the human follitropin receptor (hFSHR), a G protein-coupled receptor expressed in the ovary and testis. In the hFSHR, the third intracellular (3i) loop is considered to be pivotal in attenuation of ligand activation, particularly internalization. To gain a better understanding of these processes, we used a yeast-based interaction trap to identify cytoplasmic proteins in a human ovarian cDNA library that interacted with the hFSHR 3i loop. Among the cDNA identified, four encoded isoforms of ubiquitin. Immunoprecipitated hFSHR probed with an antiubiquitin antibody revealed that the receptor is ubiquitinated, although not exclusively on the 3i loop. Cell-surface hFSHR levels increased when expressed at nonpermissive temperature in a temperature-sensitive, ubiquitination-defective cell line. Similarly, after treatment with proteasome inhibitors, HEK293 cells stably transfected with an hFSHR expression plasmid showed an increase in follitropin binding. Proteasome inhibitors did not affect the rate of FSH internalization when receptors were saturated before internalization was measured. In contrast, internalization decreased when binding experiments were performed under nonequilibrium conditions. A mutant hFSHR-K555R, which removes the only lysine in the 3i loop available for ubiquitination, was still ubiquitinated, illustrating that, although the third loop enables and interaction with ubiquitin, it is not the sole site of ubiquitination. These observations are consistent with a role for ubiquitination in the regulation of hFSHR cell surface residency. Additionally, it can be inferred that a sequence in the 3i loop is involved in regulating receptor ubiquitination and internalization.

Point mutations in follitropin receptor result in ER retention

Accumulating evidence suggests that the human follitropin receptor is unusually sensitive to mutation. Previous results (Mol. Cell. Endo. 166 (2000) 101) determined that scanning mutations in a.a. 12-14 and 22-30 neither bound follitropin nor were present on the cell surface, suggesting that these regions are involved in either hormone binding or trafficking. To distinguish between these hypotheses, single alanine substitutions in a.a. 12-14 and 22-30 were generated, all of which appeared to bind 125I-follitropin with an affinity constant similar to wild type (wt) follitropin receptor. However, the level of receptor on the cell surface varied widely, in some cases 100-fold lower than wt. Expression on the cell surface corresponded to expression of the mature 80 kD follitropin receptor. An accumulation of the ER-resident 62 kD band of follitropin receptor was observed in mutants that had low surface expression of receptor, suggesting that misfolded protein was trapped in the ER by a quality control mechanism.

Bacterial expression of a natively folded extracellular domain fusion protein of the hFSH receptor in the cytoplasm of Escherichia coli

We have expressed the extracellular domain of the hFSH receptor as a fusion protein with thioredoxin in the cytoplasm of an Escherichia coli strain that contains mutations in both the thioredoxin reductase and the glutathione reductase genes. The chimeric protein isolated following induction of expression was purified in a soluble form and binds hFSH with an affinity approximating that of native receptor. This truncated form of the receptor displays the same specificity as intact receptor and does not bind hCG. The protein is expressed at levels that exceed 5 mg/L in the bacterial cytoplasm. Expression of the properly folded extracellular domain of the hFSH receptor in the cytoplasm of E. coli allows the facile and economical purification of large quantities of material. This will facilitate the determination of the structure of the hormone-binding domain of this glycoprotein receptor as well as the production of epitope-specific antibodies.

A conserved Asn in TM7 of the thyrotropin receptor is a common requirement for activation by both mutations and its natural agonist

The wide spectrum of naturally occurring mutations able to activate the thyrotropin (TSH) receptor provides a useful tool to approach the structure of the active state(s) of the glycoprotein hormone receptors. Here we show that the side-chain of the highly conserved N7.49 (Asn 674) in TM7 is mandatory for activation of the TSH receptor, not only by TSH, but also by a panel of eight natural and two artificial activating mutations. Basal activity levels of the mutants were significantly decreased by suppression of the side-chain of N7.49 (N7.49A double mutants). In addition, comparative effects of the N7.49A substitution on the ten mutants demonstrate that basal activity and agonist- or mutation-stimulated activity might involve different structural changes.

Molecular, structural, and cellular biology of follitropin and follitropin receptor

Follitropin and the follitropin receptor are essential for normal gamete development in males and females. This review discusses the molecular genetics and structural and cellular biology of the follitropin/follitropin receptor system. Emphasis is placed on the human molecules when possible. The structure and regulation of the genes for the follitropin beta subunit and the follitropin receptor is discussed. Control of systemic and cellular protein levels is explained. The structural biology of each protein is described, including protein structure, motifs, and activity relationships. Finally, the follitropin/follitropin receptor signal transduction system is discussed.

Molecular approaches to contraceptive development

The next generation of contraceptives will be based on the identification of novel molecules essential for reproductive processes and will rely on the refinement of older as well as newer technologies. Functional analysis of naturally occurring reproductive genetic disorders and creation of mice null for specific genes would greatly assist in the choice of genetic targets for contraceptive development. Structure-based design of drugs as exemplified by the preparation of an orally active non-peptide gonadotropin releasing hormone (GnRH) would revolutionize drug formulation and delivery for a peptide analogue. This review examines some of the molecular targets that may change contraceptive choices in the future.

Structural biology of human follitropin and its receptor

In this review, the current understanding of structure-activity relationships of human follitropin and of the extracellular domain of its receptor is described. Comprehensive mutagenesis of human follitropin combined with the three-dimensional structure of human follitropin has ushered in a new era of understanding of how this complex hormone binds to and activates its receptor. Comparison of human choriogonadotropin and follitropin structures has proved invaluable in understanding how these human glycoprotein hormones have conserved primary sequence that enables high affinity binding while diverging in amino acids that provide specificity. Moreover, by comparison of the structures of deglycosylated and glycosylated human choriogonadotropin and glycosylated human follitropin, there appears to be no influence of oligosaccharides upon backbone conformation of human glycoprotein hormones. Extensive structure-activity relationships of human follitropin receptor have been studied, and new insights gained here as well. These studies indicate that follitropin binds to the central module of the extracellular domain of the follitropin receptor. Biophysical analyses of purified follitropin receptor extracellular domain further revealed conformational changes affected by hormone binding and by the solvent environment. Further, secondary structure analysis of the purified extracellular domain of follitropin receptor favors the leucine-rich repeat motif model of the glycoprotein hormone receptors. Together, the studies indicate that there are only a few residues that contribute to the overall energy of binding. Formation of a weak collisional complex between follitropin and its receptor likely involves complementation of compatible surfaces and steric hindrance by oligosaccharides, followed by conformational change and formation of active site residue salt bridges. In this regard and in light of these new data, current models of the glycoprotein hormone receptors may need to be re-evaluated.

Hormone-induced conformational change of the purified soluble hormone binding domain of follitropin receptor complexed with single chain follitropin

Human follicle-stimulating hormone receptor (hFSHR) belongs to family I of G protein-coupled receptors. FSHR extracellular domain (ECD) is predicted to have 8-9 alphabeta or leucine-rich repeat motif elements. The objective of this study was to identify elements of the FSHR ECD involved in ligand binding. Preincubation of recombinant hFSHR ECD with rabbit antisera raised against synthetic peptides of hFSHR ECD primary sequence abolished follitropin binding primarily in the region of amino acids 150-254. Accessibility of hFSHR ECD after hormone binding, captured by monoclonal antibodies against either ECD or FSH, was decreased for the region of amino acids 150-220 but additionally for amino acids 15-100. Thus, when hFSH bound first, accessibility of antibody binding was decreased to a much larger extent than if antibody was bound first. This suggestion of a conformational change upon binding was examined further. Circular dichroism spectra were recorded for purified single chain hFSH, hFSHR ECD, and hFSHR ECD-single chain hFSH complex. A spectral change indicated a small but consistent conformational change in the ECD.FSH complex after hormone binding. Taken together, these data demonstrate that FSH binding requires elements within the leucine-rich repeat motifs that form a central region of hFSHR ECD, and a conformational change occurs upon hormone binding.