Design of stable biologically active recombinant lutropin analogs

Production of Recombinant Single-Chain Eel Luteinizing Hormone and Follicle-Stimulating Hormone Analogs in Chinese Hamster Ovary Suspension Cell Culture

We produced rec-single chain eel luteinizing (rec-eel LH) and follicle-stimulating (rec- eel FSH) hormones displaying high biological activity in Chinese hamster ovary suspension (CHO-S) cells. We constructed several mutants, in which a linker, including an O-linked glycosylated carboxyl-terminal peptide (CTP) of an equine chorionic gonadotropin (eCG) β-subunit, was attached between the β- and α-subunit (LH-M and FSH-M) or in the N-terminal (C-LH and C-FSH) or C-terminal (LH-C and FSH-C) regions. The plasmids were transfected into CHO-S cells, and culture supernatants were collected. The secretion of mutants from the CHO-S cells was faster than that of eel LHβ/α-wt and FSHβ/α-wt proteins. The molecular weight of eel LHβ/α-wt and eel FSHβ/α-wt was 32-34 and 34-36 kDa, respectively, and that of LH-M and FSH-M was 40-43 and 42-45 kDa, respectively. Peptide-N-glycanase F-treatment markedly decreased the molecular weight by approximately 8-10 kDa. The EC50 value and the maximal responsiveness of the eel LH-M and eel FSH-M increased compared with the wild-type proteins. These results show that the CTP region plays a pivotal role in early secretion and signal transduction. We suggest that novel rec-eel LH and FSH proteins, exhibiting potent activity, could be produced in large quantities using a stable CHO cell system.

Undissociable chemically cross-linked and single-chain gonadotropins

Undissociable gonadotropins can be obtained either by chemical cross-linking of the natural heterodimeric hormones or by expressing recombinant single-chain molecules through the fusion of their α and β polypeptide sequences. These undissociable hormones are not more active than their natural heterodimeric counterparts indicating that the β-subunit seatbelt embracing the α-subunit ensures the αβ heterodimer stability in physiological conditions. The main interests of single-chain gonadotropins are that 1/only one single plasmid is required to produce an active recombinant hormone, 2/the two subunits' domains are constantly present in equal amounts and 3/they remain in close proximity even at low concentration for forming the hormone bioactive 3D structure. These undissociable gonadotropins have been shown to exhibit excellent stability and activity but they have not yet been commercialized probably because of immunogenicity risks and cost of production. Nevertheless, they might be used as a basis for the development of chemically simpler and cheaper ligands of LH and FSH receptors.

The impact of male factors and their correct and early diagnosis in the infertile couple's pathway: 2021 perspectives

PURPOSE

The current clinical practice in reproductive medicine should pose the couple at the centre of the diagnostic-therapeutic management of infertility and requires intense collaboration between the andrologist, the gynaecologist and the embryologist. The andrologist, in particular, to adequately support the infertile couple, must undertake important biological, psychological, economical and ethical task. Thus, this paper aims to provide a comprehensive overview of the multifaceted role of the andrologist in the study of male factor infertility.

METHODS

A comprehensive Medline, Embase and Cochrane search was performed including publications between 1969 and 2021.

RESULTS

Available evidence indicates that a careful medical history and physical examination, followed by semen analysis, always represent the basic starting points of the diagnostic work up in male partner of an infertile couple. Regarding treatment, gonadotropins are an effective treatment in case of hypogonadotropic hypogonadism and FSH may be used in men with idiopathic infertility, while evidence supporting other hormonal and nonhormonal treatments is either limited or conflicting. In the future, pharmacogenomics of FSHR and FSHB as well as innovative compounds may be considered to develop new therapeutic strategies in the management of infertility.

CONCLUSION

To provide a high-level of care, the andrologist must face several critical diagnostical and therapeutical steps. Even though ART may be the final and decisive stage of this decisional network, neglecting to treat the male partner may ultimately increase the risks of negative outcome, as well as costs and psychological burden for the couple itself.

Follicle-stimulating hormone signaling in Sertoli cells: a licence to the early stages of spermatogenesis

Follicle-stimulating hormone signaling is essential for the initiation and early stages of spermatogenesis. Follicle-stimulating hormone receptor is exclusively expressed in Sertoli cells. As the only type of somatic cell in the seminiferous tubule, Sertoli cells regulate spermatogenesis not only by controlling their own number and function but also through paracrine actions to nourish germ cells surrounded by Sertoli cells. After follicle-stimulating hormone binds to its receptor and activates the follicle-stimulating hormone signaling pathway, follicle-stimulating hormone signaling will establish a normal Sertoli cell number and promote their differentiation. Spermatogonia pool maintenance, spermatogonia differentiation and their entry into meiosis are also positively regulated by follicle-stimulating hormone signaling. In addition, follicle-stimulating hormone signaling regulates germ cell survival and limits their apoptosis. Our review summarizes the aforementioned functions of follicle-stimulating hormone signaling in Sertoli cells. We also describe the clinical potential of follicle-stimulating hormone treatment in male patients with infertility. Furthermore, our review may be helpful for developing better therapies for treating patients with dysfunctional follicle-stimulating hormone signaling in Sertoli cells.

Roles of N-linked and O-linked glycosylation sites in the activity of equine chorionic gonadotropin in cells expressing rat luteinizing hormone/chorionic gonadotropin receptor and follicle-stimulating hormone receptor

Background

Equine chorionic gonadotropin (eCG), which comprises highly glycosylated α-subunit and β-subunit, is a unique member of the glycoprotein hormone family as it elicits both follicle-stimulating hormone (FSH)-like and luteinizing hormone (LH)-like responses in non-equid species. To examine the biological function of glycosylated sites in eCG, the following glycosylation site mutants were constructed: eCGβ/αΔ56, substitution of Asn⁵⁶ of α-subunit with Gln; eCGβ-D/α, deletion of the O-linked glycosylation site at the carboxyl-terminal peptide (CTP) region of the β-subunit; eCGβ-D/αΔ56, double mutant. The recombinant eCG (rec-eCG) mutants were expressed in Chinese hamster ovary suspension (CHO-S) cells. The FSH-like and LH-like activities of the mutants were examined using CHO-K1 cells expressing rat lutropin/CG receptor (rLH/CGR) and rat FSH receptor (rFSHR).

Results

Both rec-eCGβ/α and rec-eCGβ/αΔ56 were efficiently secreted into the CHO-S cell culture medium on day 1 post-transfection. However, the secretion of eCGβ-D/α and eCGβ-D/αΔ56, which lack approximately 12 O-linked glycosylation sites, was slightly delayed. The expression levels of all mutants were similar (200–250 mIU/mL) from days 3 to 7 post-transfection. The molecular weight of rec-eCGβ/α, rec-eCGβ/αΔ56 and rec-eCG β-D/α were in the ranges of 40–45, 37–42, and 34–36 kDa, respectively. Treatment with peptide-N-glycanase F markedly decreased the molecular weight to approximately 5–10 kDa. Rec-eCGβ/αΔ56 exhibited markedly downregulated LH-like activity. The signal transduction activity of both double mutants was completely impaired. This indicated that the glycosylation site at Asn⁵⁶ of the α-subunit plays a pivotal role in the LH-like activity of eCG. Similarly, the FSH-like activity of the mutants was markedly downregulated. eCGβ-D/α exhibited markedly downregulated LH-like and FSH-like activities.

Conclusions

Rec-eCGβ/α exhibits potent biological activity in cells expressing rLH/CGR and rFSHR. The findings of this study suggest that the LH-like and FSH-like activities of eCG are regulated by the N-linked glycosylation site at Asn⁵⁶ of the eCG α-subunit and/or by the O-linked glycosylation sites of the eCG β-subunit. These findings improved our understanding of the mechanisms underlying both LH-like and FSH-like activities of eCG.

Follicle-stimulating Hormone (FSH) Action on Spermatogenesis: A Focus on Physiological and Therapeutic Roles

BACKGROUND

Human reproduction is regulated by the combined action of the follicle-stimulating hormone (FSH) and the luteinizing hormone (LH) on the gonads. Although FSH is largely used in female reproduction, in particular in women attending assisted reproductive techniques to stimulate multi-follicular growth, its efficacy in men with idiopathic infertility is not clearly demonstrated. Indeed, whether FSH administration improves fertility in patients with hypogonadotropic hypogonadism, the therapeutic benefit in men presenting alterations in sperm production despite normal FSH serum levels is still unclear. In the present review, we evaluate the potential pharmacological benefits of FSH administration in clinical practice.

METHODS

This is a narrative review, describing the FSH physiological role in spermatogenesis and its potential therapeutic action in men.

RESULTS

The FSH role on male fertility is reviewed starting from the physiological control of spermatogenesis, throughout its mechanism of action in Sertoli cells, the genetic regulation of its action on spermatogenesis, until the therapeutic options available to improve sperm production.

CONCLUSION

FSH administration in infertile men has potential benefits, although its action should be considered by evaluating its synergic action with testosterone, and well-controlled, powerful trials are required. Prospective studies and new compounds could be developed in the near future.

Site-specific roles of N-linked oligosaccharides in recombinant eel follicle-stimulating hormone for secretion and signal transduction

Eel follicle-stimulating hormone (eelFSH) is composed of a common α-subunit and a hormone specific β-subunit, both of which contain two N-linked carbohydrate residues. We characterized the biologically active single chains by fusing the α-subunit to the carboxyl terminal region of the eelFSH β-subunit. Expression vectors were constructed and the biological activity of the recombinant hormones (rec-hormones) was characterized using Chinese hamster ovary (CHO) K1 cells expressing the eelFSH receptor gene. Mutagenesis of the individual and double glycosylated sites was performed to determine the functions of the oligosaccharide chains on signal transduction. The absence of the Asn²² (eelFSHβΔ22/α) and Asn^5.22 (eelFSHβΔ5.22/α) N-linked oligosaccharide chain in the eelFSH β-subunit completely reduced the secretion level in the medium and cell lysate of CHO-K1 cells. The expression levels of eelFSHβ/α wild-type in CHO suspension (CHO-S) cells was approximately 4-fold higher in CHO-k1 cells. The molecular weight of rec-eelFSHβ/α wild-type by western blotting analysis was found to be 34 kDa. Mutants (β/αΔ56, β/αΔ79, and βΔ5/α) lacking single oligosaccharide sites showed molecular weights that were reduced by approximately 10%. The digestion of N-linked oligosaccharides using PNGaseF treatment showed that the molecular weights of all mutants were reduced to 27-kDa. The oligosaccharide chains in rec-eelFSHβ/α wild-type were modified to a molecular weight of approximately 7-10 kDa in CHO-K1 and CHO-S cells. Oligosaccharide site deletions at positions Asn⁵⁶ and Asn⁷⁹ on the α-subunit and Asn⁵ on the β-subunit were found to play an essential role in cAMP signal transduction through the eelFSH receptor. The EC₅₀ values of Asn⁵⁶ and Asn⁵ resulted in a significant decrease in potency to 64% and 53% of the wild type, respectively. Specifically, the removal of the carbohydrates at Asn⁷⁹ of the α-subunit (β/αΔ79) was drastically reduced to 53.8% of the wild-type levels in maximum response. These results have allowed for the identification of the site-specific roles of carbohydrate residues in eel FSH. Our data suggest that N-linked oligosaccharide chains play a pivotal role in biological activity through the eelFSH receptor as suggested in similar studies of other mammalian FSH hormones.

Gonadotropins and Their Analogs: Current and Potential Clinical Applications

The gonadotropin receptors LH receptor and FSH receptor play a central role in governing reproductive competency/fertility. Gonadotropin hormone analogs have been used clinically for decades in assisted reproductive therapies and in the treatment of various infertility disorders. Though these treatments are effective, the clinical protocols demand multiple injections, and the hormone preparations can lack uniformity and stability. The past two decades have seen a drive to develop chimeric and modified peptide analogs with more desirable pharmacokinetic profiles, with some displaying clinical efficacy, such as corifollitropin alfa, which is now in clinical use. More recently, low-molecular-weight, orally active molecules with activity at gonadotropin receptors have been developed. Some have excellent characteristics in animals and in human studies but have not reached the market-largely as a result of acquisitions by large pharma. Nonetheless, such molecules have the potential to mitigate risks currently associated with gonadotropin-based fertility treatments, such as ovarian hyperstimulation syndrome and the demands of injection-based therapies. There is also scope for novel use beyond the current remit of gonadotropin analogs in fertility treatments, including application as novel contraceptives; in the treatment of polycystic ovary syndrome; in the restoration of function to inactivating mutations of gonadotropin receptors; in the treatment of ovarian and prostate cancers; and in the prevention of bone loss and weight gain in postmenopausal women. Here we review the properties and clinical application of current gonadotropin preparations and their analogs, as well as the development of novel orally active, small-molecule nonpeptide analogs.

Site specificity of eel luteinizing hormone N-linked oligosaccharides in signal transduction

Eel luteinizing hormone (eelLH) is composed of a common α-subunit and hormone specific β-subunit, both of which contain asparagine-linked carbohydrate residues, located at positions 56 and 79 on the α-subunit and position 10 on the β-subunit. The specific roles of the individual carbohydrate chains are poorly defined in eel. Thus, we characterized the biologically active single chains by fusing the α-subunit to the carboxyl terminal region of the eelLH β-subunit. Site-directed mutagenesis of the three N-linked glycosylation sites was performed to examine the function of individual glycosylation sites in secretion and signal transduction. The absence of the Asn⁷⁹N-linked sugar chain slightly reduced secretion in Chinese hamster ovary (CHO) cells. The expression of eelLHβ/α (wild-type) in CHO suspension cells was increased by approximately 2-fold higher than that of attached CHO cells. By western blotting analysis, the molecular weight of wild-type was found to be 32 kDa. Mutants (β/α△56, β/α△79, and β△10/α) of the oligosaccharide chain at a single site showed molecular weights that were reduced by approximately 10%. However, the double mutant (β/α△56.79) molecular weight was decreased by more than 20% compared to the wild-type. Enzymatic digestion of oligosaccharides using PNGaseF treatment showed that the molecular weights of all mutants, including wild-type, were reduced to 25 kDa. The results obtained in the absence of carbohydrates at Asn⁵⁶ of the α-subunit and at Asn¹⁰ of the β-subunit revealed their roles in signal transduction through the eelLH receptor. The EC₅₀ value of the cAMP response at Asn⁷⁹ of the α-subunit was increased by 5-fold, whereas the maximum response was dramatically reduced to 17.8% of wild-type levels. Specifically, removal of the carbohydrates at double mutant (β/α△56.79) is approximately 85% to wild-type levels in biopotency. These results revealed the site-specific roles of eelLH carbohydrate residues. Our data suggest that N-linked oligosaccharide chains play a pivotal role in biological activity through the eelLH receptor.

Toward developing recombinant gonadotropin-based hormone therapies for increasing fertility in the flatfish Senegalese sole

Captive flatfishes, such as the Senegalese sole, typically produce very low volumes of sperm. This situation is particularly prevalent in the first generation (F1) of reared sole males, which limits the development of artificial fertilization methods and the implementation of selective breeding programs. In this study, we investigated whether combined treatments with homologous recombinant follicle-stimulating (rFsh) and luteinizing (rLh) hormones, produced in a mammalian host system, could stimulate spermatogenesis and enhance sperm production in Senegalese sole F1 males. In an initial autumn/winter experiment, weekly intramuscular injections with increasing doses of rFsh over 9 weeks resulted in the stimulation of gonad weight, androgen release, germ cell proliferation and entry into meiosis, and the expression of different spermatogenesis-related genes, whereas a subsequent single rLh injection potentiated spermatozoa differentiation. In a second late winter/spring trial corresponding to the sole’s natural prespawning and spawning periods, we tested the effect of repeated rLh injections on the amount and quality of sperm produced by males previously treated with rFsh for 4, 6, 8 or 10 weeks. These latter results showed that the combination of rFsh and rLh treatments could increase sperm production up to 7 times, and slightly improve the motility of the spermatozoa, although a high variability in the response was found. However, sustained administration of rFsh during spawning markedly diminished Leydig cell survival and the steroidogenic potential of the testis. These data suggest that in vivo application of rFsh and rLh is effective at stimulating spermatogenesis and sperm production in Senegalese sole F1 males, setting the basis for the future establishment of recombinant gonadotropin-based hormone therapies to ameliorate reproductive dysfunctions of this species.

Effect of time and dose of recombinant follicle stimulating hormone agonist on the superovulatory response of sheep

The objective of this study was to determine the superovulatory potential of a single-chain analog of human FSH (Fcα) when administered to ewes either 3 days before, or coincident with, simulated luteolysis (pessary removal [PR]). A total of 40 animals were randomly assigned to receive Fcα at doses of 0.62, 1.25, or 2.5 IU/kg of body weight (bwt) 3 days before PR or 0.31, 0.62, 1.25, or 2.5 IU/kg of bwt at PR. Control ewes received protein without FSH activity. Blood samples were collected during the periovulatory period and ovarian tissue was collected 11 days after PR. Ovulation rate did not differ from the control group in ewes receiving the smallest doses of Fcα (0.31 and 0.62 IU/kg). However, a significant superovulatory response was noted in sheep receiving Fcα at doses of 1.25 and 2.5 IU/kg and this response was comparable in animals receiving the largest dose levels of Fcα at, or 3 days before, PR. The interval between PR and the LH surge was significantly extended and the LH surges were less synchronous in animals receiving Fcα at PR when compared with animals receiving the potent FSH agonist 3 days before PR. Taken together, these data indicate that the human single-chain gonadotropin with FSH activity promotes superovulation in ewe lambs in the breeding season. A single injection of the recombinant gonadotropin 3 days before luteolysis synchronizes the LH surge. The use of the single-chain analog of FSH in assisted reproduction for domestic animals is likely to be of practical significance as an alternative to conventional gonadotropins in superovulation protocols in livestock species.

Production of protein complexes via co-expression

Multi-protein complexes are involved in essentially all cellular processes. A protein's function is defined by a combination of its own properties, its interacting partners, and the stoichiometry of each. Depending on binding partners, a transcription factor can function as an activator in one instance and a repressor in another. The study of protein function or malfunction is best performed in the relevant context. While many protein complexes can be reconstituted from individual component proteins after being produced individually, many others require co-expression of their native partners in the host cells for proper folding, stability, and activity. Protein co-expression has led to the production of a variety of biological active complexes in sufficient quantities for biochemical, biophysical, structural studies, and high throughput screens. This article summarizes examples of such cases and discusses critical considerations in selecting co-expression partners, and strategies to achieve successful production of protein complexes.

Single chain human chorionic gonadotropin, hCGalphabeta: effects of mutations in the alpha subunit on structure and bioactivity

The strategy of translationally fusing the subunits of heterodimeric proteins into single chain molecules is often used to overcome the mutagenesis-induced defects in subunit interactions. The approach of fusing the alpha and beta subunits of human Chorionic Gonadotropin (hCG) to produce a single chain hormone (phCGalphabeta) was used to investigate roles of critical residues of the alpha subunit in hormone receptor interaction and biological activity. The alpha subunit was mutated using PCR-based site-directed mutagenesis, fused to the wild type beta subunit and the fusion protein was expressed using Pichia pastoris expression system. Following partial purification, the mutant proteins were extensively characterized using immunological probes, receptor assays, and in vitro bioassays. The mutation hCGalpha P38A, which disrupts subunit interaction in the heterodimeric molecule, produced a fusion molecule exhibiting altered subunit interactions as judged by the immunological criteria, but could bind to the receptor with lower affinity and elicit biological response. Mutation of hCGalpha T54A disrupting the glycosylation at Asparagine 52, believed to be important for bioactivity, also yielded a biologically active molecule suggesting that the glycosylation at this site is not as critical for bioactivity as it is in the case of the heterodimer. The fusion protein approach was also used to generate a superagonist of hormone action. Introduction of four lysine residues in the Loop 1 of the alpha subunit led to the generation of a mutant having higher affinity for the receptor and enhanced bioactivity. Immunological characterization of single chain molecules revealed that the interactions between the subunits were not identical to those seen in the heterodimeric hormone, and the subunits appeared to retain their isolated conformations, and also retained the ability to bind to the receptors and elicit response. These data suggest the plasticity of the hormone-receptor interactions.

Strategies for construction of luteinizing hormone beta subunit analogs with carboxyl terminal extensions in non-primate, non-equid mammalian species

Chorionic gonadotropins (CG) are unique because they have a carboxyl terminal peptide (CTP) extension on their beta subunits that prolongs circulatory survival. CGbeta genes from the human being and horse have evolved from ancestral luteinizing hormone (LH) beta genes by different pathways that involve deletions that change the reading frames and yield a CTP. Here we further review our previous analysis, aimed at determining whether LHbeta genes in non-primate, non-equid species inherently possess DNA sequences that encode CTP-like domains. In multiple mammalian species, simple frame-shift mutations using either the human or equine CGbeta gene as a model can be used to construct LHbeta analogs with putative CTP domains. Furthermore, DNA sequences from mammalian LHbeta genes can be aligned to maximize similarity with CGbeta genes in order to devise more refined strategies for construction of CTP-bearing LHbeta analogs as exemplified in the bovine case. Thus, mammalian LHbeta genes have DNA sequences that can be potentially expressed in order to construct CTP-bearing glycoprotein hormone analogs.

The steroidogenic effect of single-chain bovine LH analogs in cultured bovine follicular cells

Single-chain gonadotropin analogs had been constructed for the purpose of structure-function studies and analog design. Incorporation of a spacer derived from the carboxyl terminal peptide (CTP) of the choriogonadotropin (CG) beta subunit between the tethered subunit domains of the human gonadotropins is beneficial for the secretion of the single-chain variants without compromising biocactivity. Although the CGbeta subunit containing the CTP domain is expressed only in primates and equids, a CTP-like sequence exists in the untranslated region of the LHbeta gene of several mammalian species, including the bovine species. The CTP encrypted in the bovine LHbeta DNA (designated as 'boCTP') and the CTP derived from the human CGbeta subunit (denoted as 'huCTP') served as a linker sequence in the design of bovine single-chain luteinizing hormone (LH) analogs. The purpose of the present study was to evaluate steroidogenesis in cultured bovine theca cells following stimulation with these single-chain analogs. The concentration of the LHbetaboCTPalpha and LHbetahuCTPalpha analogs in the conditioned media of the expressing CHO cells was three- to six-fold higher than that of the "linkerless" LHbetaalpha and LHbeta111alpha variants. The four analogs induced androstenedione and progesterone secretion from the primary theca cells in a dose-dependent manner, but differences in the steroidogenic response were observed. The LHbetaboCTPalpha analog (10 ng/ml) effectively induced androstenedione and progesterone secretion over unstimulated levels (4.0- and 4.4-fold increase for androstenedione and progesterone, respectively). The response to the pituitary bovine LH standard (10 ng/ml) was less pronounced for both steroids (two- to three-fold increase over basal levels). The activities of LHbetahuCTPalpha, LHbetaalpha and LHbeta111alpha were comparable and sightly reduced relative to the LHbetaboCTPalpha activity. The data suggested that LHbetaboCTPalpha was ranked as the most potent and this was even more prominent when analogs were used at a lower dose (1 ng/ml). These data suggest that the design, including the huCTP or boCTP linker, is favorable for the production of single-chain bovine LH analogs. Furthermore, spacing of the tethered subunit domains with the cryptic boCTP sequence that originated from the bovine LHbeta gene appears advantageous for the purpose of stimulating steroid production in the species-specific bioassay. Thus, an effective strategy to produce bioactive single-chain LH analogs in non-primate, non-equid species would be the mutatation of the LHbeta genes with the aim of expressing the cryptic CTP sequence as a spacer derived from the DNA of the same organism.

Luteinizing Hormone and Follicle-Stimulating Hormone Exhibit Different Secretion Patterns from Cultured Madin-Darby Canine Kidney Cells

LH, FSH, and chorionic gonadotropin (CG) are comprised of a common alpha subunit and a hormone-specific beta subunit. Using Madin-Darby canine kidney (MDCK) epithelial cells to examine the polarized secretion of human CG/LH, we previously reported that CG and LH were detected in the apical and basolateral compartments, respectively, and the carboxyl terminal end of the CGbeta subunit contains a strong apical signal. Here we show that the carboxyl seven amino acids in the LHbeta subunit contribute to the basolateral secretion of LH, and an LH chimera bearing the CGbeta apical signal is redirected from the basolateral to the apical compartments. Because LH and FSH are synthesized in the same cell, we also compared the secretion polarity of LH with FSH. MDCK cells expressing the FSH dimer displayed an almost equal distribution of protein into the apical and basolateral compartments. Given that the LHbeta and CGbeta carboxy terminal sequences, which differ from that in the FSHbeta subunit, occupy a pivotal role in their polarized behavior, the results support the hypothesis that pituitary exit of LH and FSH occur via different secretion pathways, and are released spatially from the pituitary via different circulatory routes.

Yoked complexes of human choriogonadotropin and the lutropin receptor: evidence that monomeric individual subunits are inactive

Human choriogonadotropin (hCG) contains an alpha-subunit, common to other members of the glycoprotein hormone family, and a unique beta-subunit that determines hormone specificity. It is generally thought that heterodimer formation is obligatory for full hormonal activity, although other studies have indicated that individual subunits and homodimeric hCGbeta were capable of low affinity binding to the LH receptor (LHR) and subsequent activation. Previously, we constructed two yoked hormone (hCG)-LHR complexes, where the two hormone subunits and the heptahelical receptor were engineered to form single polypeptide chains, i.e. N-beta-alpha-LHR-C and N-alpha-beta-LHR-C. Expression of both complexes led to constitutive stimulation of cAMP production. In the present study, we investigated whether the human alpha-subunit and hCGbeta can act as functional agonists when covalently attached to or coexpressed with the LH receptor. Our initial results showed that hCGbeta, but not alpha, was able to activate LHR with an increase in intracellular cAMP in human embryonic kidney 293 cells but not in Chinese hamster ovary or COS-7 cells. Further examination of this apparent cell-specific agonist activity of hCGbeta revealed that low levels of endogenous alpha-subunit were expressed in human embryonic kidney 293 cells, thus enabling sufficient amounts of active heterodimer to form with the transfected hCGbeta to activate LHR. The studies in Chinese hamster ovary and COS-7 cells clearly demonstrate that, even under experimental conditions where hormone-receptor interactions are maximized, individual subunits of hCG can not act as functional agonists, at least in their monomeric form.

New developments in gonadotrophin pharmacology

This article reviews the past, present, and future of gonadotrophin therapy, including purification of gonadotrophins from animal or human urine sources and production of gonadotrophins through recombinant technology. With the advent of recombinant DNA methodologies combined with site-directed mutagenesis, a variety of structural modifications becomes possible.

Operation of the follicle-stimulating hormone (FSH)-inhibin B feedback loop in the control of primate spermatogenesis

This paper reviews our current understanding of the function and operation of the follicle-stimulating hormone (FSH)-inhibin feedback loop in the male rhesus monkey (Macaca mulatta). Inhibin B is the major testicular inhibin in the monkey, and the pattern of secretion of this hormone during postnatal development is temporally coupled to that of gonadotropin. Inhibin B secretion by the Sertoli cell is stimulated by FSH and inhibited by luteinizing hormone (LH), the latter presumably acting via Leydig cell production of testosterone (T). The dynamics of the FSH-inhibin B feedback loop in the adult monkey is revealed following unilateral orchidectomy (UO). Interestingly, a sustained, 50% deficit in inhibin B secretion occurs after UO and this persistent error signal, in turn, results in elevated concentrations of FSH in the circulation. The elevated secretion of FSH appears to be the principal drive for the increased sperm output by the remaining testis. Available data for the functioning of the FSH-inhibin B feedback loop in the human male are placed in perspective, and a model for the negative feedback regulation of sperm number in primates is proposed.

Novel recombinant gonadotropins

The gonadotropin hormones chorionic gonadotropin, luteinizing hormone and follicle-stimulating hormone are heterodimers that consist of a common alpha subunit noncovalently associated with a hormone-specific beta subunit. Site-directed mutagenesis and gene transfer techniques have been invaluable tools for elucidating structure-function determinants of these hormones. Here, we review how questions about the structural biology of these glycoprotein hormones have provided crucial information for creating analogs (agonists and antagonists) that can be used to treat infertility in the clinic. The ability to manipulate the protein structure of these hormones will enable the engineering of both long- and short-acting therapeutic agents.

Pulsatile stimulation with recombinant single chain human luteinizing hormone elicits precocious sertoli cell proliferation in the juvenile male rhesus monkey (Macaca mulatta)

In this study, we determined the relative role of LH and FSH in initiating the pubertal proliferation of Sertoli cells in primates. Sixteen juvenile male rhesus monkeys (Macaca mulatta) bearing venous catheters received intermittent intravenous infusions of single chain human LH (schLH) or recombinant human FSH (rhFSH) or a combination of both for 11 days. The schLH infusion elicited a physiological testosterone response. On Day 11, monkeys were castrated, and one-half of a testis was fixed in Bouin's fluid. Infusion of the gonadotropins, either alone or in combination, effected a significant increase in testicular weight, seminiferous cord diameter, and the number of Sertoli cells per testis (schLH, 295 +/- 46 x 10(6); rhFSH, 342 +/- 64 x 10(6); LH+FSH, 298 +/- 26 x 10(6) versus vehicle, 204 +/- 26 x 10(6)). The latter finding indicated that LH, in addition to FSH, plays a critical role in the initiation of the pubertal proliferation of Sertoli cells in primates. Moreover, combined gonadotropin treatment led to the appearance of germ cells as mature as early primary spermatocytes, indicating that initiation of spermatogenesis had been set in motion. Because the duration of hormone stimulation was only 11 days, the latter result suggests that Leydig and Sertoli cells of the juvenile monkey testis can immediately transduce a gonadotropin signal to the germ cell.

Genetic engineering of single-chain gonadotropins and hormone-receptor fusion proteins

The gonadotropin hormone family is distinguished by its heterodimeric structure in which the members share a common alpha subunit and a hormone-specific beta subunit. Since assembly of the heterodimer is often the rate-limiting step in production of functional hormone, single-chain hormones have been engineered by genetically linking the two subunits. The single-chain hormone can in turn be fused to its receptor to produce a functional single-chain hormone-receptor complex. These fusion constructs offer a valuable new approach in structure-function studies and in the generation of hormone analogs. In this article we describe the experimental design for the generation of single-chain human chorionic gonadotropin and single-chain hormone-receptor fusion complex and strategies for the expression of these fusion proteins.

Definition and measurement of follicle stimulating hormone

FSH has a key role in the development and function of the reproductive system and is widely used both diagnostically and therapeutically in developmental and reproductive medicine. The accurate measurement of FSH levels, in patients for diagnosis and monitoring and in therapeutic preparations for clinical use, is essential for safe and successful treatment. Historically, FSH was defined on the basis of classical in vivo endocrine activity, and early therapeutic preparations were calibrated using in vivo bioassays. There was early recognition that reference preparations were required for calibration if the results from different laboratories were to be comparable. In response to the perceived need, the World Health Organization established the first standard for such preparations in 1959. Subsequent developments in biotechnology have led to recognition that there is no single molecule that can be uniquely defined as FSH, and that FSH can induce a range of biological activities. Several highly purified standards for FSH are now available, but discontinuity and heterogeneity of estimates of FSH activity in terms of these standards made using in vitro assays and binding assays have been noted. It is thus essential that any measurement of FSH include specification both of the standard with which the measured FSH is compared and the assay method used for that comparison.

A biologically active single chain human chorionic gonadotropin analog with altered receptor binding properties

hCG is a heterodimer consisting of an alpha-subunit common among all members of the glycoprotein hormone family, LH, FSH, and TSH, and a unique beta-subunit responsible for receptor specificity. Biologically active single chain analogs of these hormones have been engineered in which the C-terminus of the beta-subunit was fused to the N-terminus of the alpha-subunit (N-beta-alpha-C) either with or without a linker such as the hCGbeta C-terminal peptide (CTP). This tandem order of subunits was chosen based on studies suggesting that the N-terminal region of hCGbeta and particularly the C-terminal region of the alpha-subunit are important in receptor binding and activation. Single chain hCG (YhCG1) can, in turn, be fused to the LH receptor to yield a hormone-receptor complex that is biologically active in transfected cells. Herein, we report the construction of a new single chain hCG analog (YhCG3) in which the C-terminus of the alpha-subunit is fused to the N-terminus of hCGbeta via a CTP (N-alpha-CTP-beta-C). Compared with YhCG1, this analog binds receptor with a 25- to 30-fold lower affinity, but, surprisingly, is capable of stimulating intracellular cAMP levels to the same extent. Furthermore, YhCG3 can be covalently linked to its receptor to produce a biologically active complex that results in elevated levels of basal cAMP in transfected cells. These results suggest that free N- and C-termini of hCGbeta and the alpha-subunit, respectively, are not essential for receptor binding and activation and that YhCG3 is in a more efficacious conformation for receptor activation than YhCG1.