Expression of extracellular domain peptides of the FSH receptor and their effect on receptor-ligand interactions in vitro

The antigen binding sites of various hCG monoclonal antibodies show homology to different domains of LH receptor

The common feature of receptors and antibodies against the ligand is that both display very specific, high affinity binding towards the ligand. Therefore, it can be hypothesized that the paratope of antibodies may exhibit homology with distinct domains of the receptor. By locating the hormone epitopes and determining the structure of the paratopes, it should be possible to identify the contact points between the ligand and the receptor. This hypothesis has been tested using hCG monoclonal antibodies (MAbs) recognizing different epitopes and having different effects on hormone binding and response. The beta subunit and heterodimer specific antibodies inhibited both hormone binding and response, while the alpha subunit specific antibodies inhibited response without affecting binding. The single chain fragment variables (ScFvs) produced from these antibodies also retained the properties of the parent antibodies. The amino acid sequences of the ScFvs exhibited homology to different regions of the receptor; the beta subunit specific antibody being homologous to the concave surface of the leucine rich repeats (LRR) of the receptor, particularly the concave surface of the LRRs, while the heterodimer specific antibody showed homology to the hinge region. The alpha subunit specific antibody showed homology to the transmembrane domain of the receptor. The exact locations of the epitopes of the monoclonal antibodies in the hormone molecule have also been identified. The data presented here also support the model of glycoprotein hormone-receptor interaction in which the hormone binds to the extracellular domain through the beta subunit and then the alpha subunit is brought in contact with the transmembrane domain leading to signal transduction.

Demonstration of follicle-stimulating hormone receptor in cauda epididymis of rat

FSH receptor has been shown to be specifically expressed only in the Sertoli cells in males. In one of our studies that consisted of deprival of endogenous FSH in immature rats and adult bonnet monkeys, atrophy of the epididymis was observed, cauda region being the most affected. Although epididymis is an androgen-dependent tissue, the changes in histology of the cauda region were observed without any associated change in the levels of testosterone in FSH-deprived animals. Considering this, it was of interest to evaluate the possibility of epididymis being a direct target for FSH action. In the present study, we have examined the expression of FSH receptor in the epididymis of rat and monkey. In the cauda region of rat epididymis, FSH receptor expression was demonstrated by RT-PCR and Northern and Western blot analyses. FSH receptor was found to be functional as observed by its ability to bind 125IoFSH, by an increase in cAMP production, and by BrdU incorporation following addition of FSH under in vitro conditions. These results suggest the possibility of a role for FSH in regulating the growth of the epididymis.

Delineation of Regions in the Extracellular Domain of Follicle-Stimulating Hormone Receptor Involved in Hormone Binding and Signal Transduction

PROBLEM

To use antipeptide antibodies to potential surface-oriented regions of the extracellular domain (ECD) of the human follicle-stimulating hormone receptor (hFSHR) to delineate regions involved in FSH binding and FSH-induced signal transduction.

METHOD OF STUDY

We developed and characterized antipeptide antibodies to different, potentially surface-oriented regions of the ECD of hFSHR. The ability of these antibodies to recognize the receptor was studied by sodium dodecyl sulfate-polyacrylamide gel electrophoresis followed by Western blotting. The ability to modulate FSH binding and cAMP generation was studied by the radioreceptor assay and in vitro FSH bioassay respectively.

RESULTS

Antipeptide antibodies to regions 15-31, 216-235, 285-300 and 327-341 hFSHR inhibited both FSH binding and cAMP production. Regions 15-31 and 216-235 were accessible to their cognate antipeptide antibodies both before and after FSH binding, while regions 285-300 and 327-341 hFSHR were accessible only prior to FSH binding.

CONCLUSIONS

Based on the observations made with respect to accessibility to antipeptide antibodies, ability of antibodies to inhibit FSH binding and the subsequent cAMP generation and kinetics of antibody binding, regions 285-300 and 327-341 hFSHR appear to be the chief FSH-binding sites, while regions 15-31 and 216-235 hFSHR serve as ancillary FSH-binding sites.

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.

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.

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.

Deletion of follicle-stimulating hormone (FSH) receptor residues encoded by exon one decreases FSH binding and signaling in the rat

The rat FSH receptor (rFSHR) shares considerable homology with the rat LH receptor (rLHR), yet binds human FSH (hFSH) with high fidelity, suggesting that the binding determinant encoded by the rFSHR gene shares no homology with the analogous rLHR primary sequence, thereby affording specificity of ligand binding. Two such regions of primary sequence have been previously identified and studied by peptide challenge tests and immunoneutralization studies. We therefore implemented site-directed mutagenesis to delete the regions S9-N30 and D300-F315 of the mature rFSHR sequence. The mutant receptor (DeltarFSHR) cDNAs were expressed in insect cells. The large deletion DeltarFSHRS9-N30 and a smaller deletion, DeltarFSHRS9-S18, did not bind (125)I-hFSH. However, DeltarFSHRK19-R29 and DeltarFSHRD300-F315 bound (125)I-hFSH with an affinity indistinguishable from wild-type rFSHR. The deletion mutants DeltarFSHR S9-N30 or DeltarFSHRS9-S18 were not detectable on the cell surface by flow cytometry unless cells were sheared. Although (125)I-hFSH binding to DeltarFSHRK19-R29 was normal, this form of the receptor was defective for signal transduction whereas DeltarFSHRD300-F315 was not. Furthermore, neither region seems to be a specificity determinant, since their removal did not result in high-affinity binding of hCG to DeltarFSHR.

In vitro inhibition of the bioactivity of follicle-stimulating hormone by antisera against a peptide representing part of the FSH-receptor

The aim of the present work was to define an FSH receptor (FSHR) peptide that can induce antibodies that will inhibit the bioactivity of FSH. Therefore, the hFSHR sequence was aligned with that of all other known G-protein coupled receptors. An area with increased sequence homology was identified between the FSH-, LH-, TSH receptors, the C5a receptor and the IL8 receptor. The similarity consists of a richness in acidic (D and E) and hydrophobic (Y and F) residues. In hFSHR the sequence is EDNESSYSRGFDMTYTEFDYDLCNEVVD (amino acid 299-326). Research on both the C5a- and IL8-receptor has indicated that this part is responsible for hormone binding but not for signal transduction. Protamine. an antagonist for both the C5a- and IL8 receptor also inhibited the bioactivities of FSH and LH when tested in a bioassay. This suggests that in the hFSHR this region might also be involved in hormone binding. Specificity of this region towards the diverse ligands all binding to the C5a or to the IL8 receptor might be attributed to differences in the profile of alternating basic and hydrophobic residues. Therefore, the hypothesis was tested as to whether antisera raised against peptides of this FSHR-domain would inhibit FSH-bioactivity but not LH-bioactivity. Indeed antisera were found (anti-hFSHR 309-322) that inhibited the biological activity of FSH in a bioassay. These antisera proved to be specific since they did not inhibit the bioactivity of LH. These data suggest that the core sequence (hFSHR 309-322) of the aligned domain of the hFSHR, in analogy to the IL8- and C5a receptors, is involved in hormone binding and ligand specificity. This domain therefore forms a valuable tool in FSH- and FSHR research for scientific and medical purposes.

Expression of follicle stimulating hormone-receptor mRNA during gonadal development

Receptors for follicle-stimulating hormone (FSH) are found only in the gonads and have been localised to the Sertoli cells of the testis and the granulosa cells of the ovary. During gonadal development, functional signal transduction systems are present before gonadotrophin receptors appear indicating the expression of the receptors is the crucial step in development of gonadal responsiveness to gonadotrophins. The FSH receptor gene contains a single large exon which encodes the transmembrane and intracellular domains and nine smaller exons which encode most of the extracellular domain. In all species studied so far the FSH-receptor primary transcript has been shown to undergo alternate splicing. The function of these alternate transcripts is unclear but changes in alternate splicing appear to be associated with development of receptor mRNA expression. In the rat transcripts encoding only the extracellular domain of the receptor are detectable 2 days before transcripts encoding the full length receptor. In the mouse ovary FSH-receptor mRNA levels and alternate splicing has been measured during development. Results show that FSH-receptor mRNA is detectable in day 1 ovaries which contain only primordial follicles. At this stage mRNA levels are low but a significant increase in FSH-receptor mRNA is seen around day 5 when primary follicles first appear. This correlates with in situ hybridisation studies which first detect FSH-receptor transcripts in primary follicles. At all stages of development the level of transcripts encoding the extracellular domain was significantly greater than that encoding for the transmembrane and intracellular regions suggesting that significant levels of shortened transcripts are produced. In the hypogonadal (hpg) mouse which lacks circulating gonadotrophins levels of FSH-receptor mRNA appeared normal up to 15 days. This shows that gonadotrophins ar not require for development of FSH-receptor mRNA levels. Studies on FSH-receptor mRNA levels during granulosa cell luteinization show that there is complete loss of full-length transcripts soon after luteinization. Transcripts encoding the extracellular domain remain present, however, up to at least mid-cycle. Thus, changes in receptor transcript splicing during loss of FSH-receptors appear to mimic, in reverse, changes occurring during development. It may be that the FSH-receptor gene is constitutively expressed in follicular (pre-granulosa) cells, granulosa cells and granulosa-luteal cells but that control of RNA splicing regulates levels of full-length FSH-receptor transcript.

Expression of follicle-stimulating hormone-receptor mRNA alternate transcripts in bovine granulosa cells during luteinization in vivo and in vitro

In cattle, as in many other species, formation of the corpus luteum is associated with loss of sensitivity to FSH. To determine whether this is associated with changes in FSH-receptor mRNA levels or alternate splicing of the primary transcript, reverse transcription and the polymerase chain reaction (RT-PCR) were used to examine transcripts during granulosa cell luteinization in vivo and in vitro. Using RT-PCR and Southern blotting, three alternate transcripts of the FSH-receptor were found consistently in bovine granulosa cells. The largest transcript encoded the full-length receptor while the other transcripts lacked either exon 9 or exons 4, 5 and 9. One day after luteinization in vivo, full-length FSH-receptor mRNA was detectable at low levels in the newly-formed corpus luteum. By day 3, however, no full-length transcripts were detectable in the corpus luteum. In contrast, when primers were used which amplify only the extracellular domain, FSH-receptor transcripts were detectable in all corpora lutea tested up to mid-cycle. In granulosa cells, luteinized in vitro, there was a similar loss of full-length FSH-receptor transcripts after day 1 but continued expression of transcripts encoding the extracellular domain. Results show that granulosa cell luteinization in cattle is associated with a change in splicing of the FSH-receptor primary transcript such that after luteinization only shortened transcripts coding for the extracellular domain are detectable. This process resembles, in reverse, changes in FSH-receptor transcript splicing during development of the gonads.