Association/dissociation of gonadotropin subunits involves disulfide bridge disruption which is influenced by carbohydrate moiety

Characterization of gonadotropins and their receptors in a chondrichthyan, Scyliorhinus canicula, fills a gap in the understanding of their coevolution

In Gnathostomes, reproduction is mainly controlled by the hypothalamic-pituitary-gonadal (HPG) axis, with the involvement of the pituitary gonadotropic hormones (GTH), follicle-stimulating hormone (FSH) and luteinizing hormone (LH), which activate their cognate receptors, FSHR and LHR, expressed in gonads. Each GTH consists of a common α subunit and of a specific FSHβ or LHβ subunit. Chondrichthyes (holocephalans and elasmobranchs) is a sister group of bony vertebrates. This position is highly favorable for the understanding of the evolution of endocrine regulations of reproduction among gnathostomes. Surprisingly, the characterization of gonadotropins and their receptors is still limited in chondrichthyes. In the present study, GTH and GTHR sequences have been identified from several chondrichthyan genomes, and their primary structures were analyzed relative to human orthologs. 3D models of GTH/GTHR interaction were built, highlighting the importance of the receptor hinge region for ligand recognition. Functional hormone-receptor interactions have been studied in HEK cells using the small-spotted catshark (Scyliorhinus canicula) recombinant proteins and showed that LHR was specifically activated by LH whereas FSHR was activated by both FSH and LH. Expression profiles of GTHs and their receptors were explored by real-time PCR, in situ hybridization and immunohistochemistry during spermatogenesis, along the male genital tract and other tissues, as well as in some female tissues for comparison. Tissue-expression analyses showed that the highest levels were observed for fshr transcripts in testis and ovary and for lhr in specific extragonadal tissues. The two receptors were expressed at all stages of spermatogenesis by both germ cells and somatic cells, including undifferentiated spermatogonia, spermatocytes, spermatids, somatic precursors and Sertoli cells; differentiated Leydig cells being absent in the testis of S. canicula. Receptors were also expressed by the lymphomyeloid epigonal tissue and the testicular tubules. These results, suggest a wide range of gonadotropin-regulated functions in Elasmobranchs, as well as functional redundancy during spermatogenesis. These extended functions are discussed in an evolutionary context in which the specificity of gonadotropin signaling must have contributed to the evolution of gonadal cells' morphology and function.

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.

Ancestral glycoprotein hormone and its cognate receptor present in primitive chordate ascidian: Molecular identification and functional characterization

The glycoprotein hormone (GPH) system is fundamentally significant in regulating the physiology of chordates, such as thyroid activity and gonadal function. However, the knowledge of the GPH system in the primitive chordate ascidian species is largely lacking. Here, we reported an ancestral GPH system in the ascidian (Styela clava), which consists of GPH α subunit (Sc-GPA2), GPH β subunit (Sc-GPB5), and the cognate leucine-rich repeat-containing G protein-coupled receptor (Sc-GPHR). Comparative structure analysis revealed that distinct from vertebrate GPH β subunits, Sc-GPB5 was less conserved, showing an atypical N-terminal sequence with a type II transmembrane domain instead of a typical signal peptide. By investigating the presence of recombinant Sc-GPA2 and Sc-GPB5 in cell lysates and culture media of HEK293T cells, we confirmed that these two subunits could be secreted out of the cells via distinct secretory pathways. The deglycosylation experiments demonstrated that N-linked glycosylation only occurred on the conserved cysteine residue (N78) of Sc-GPA2, whereas Sc-GPB5 was non-glycosylated. Although Sc-GPB5 exhibited distinct topology and biochemical properties in contrast to its chordate counterparts, it could still interact with Sc-GPA2 to form a heterodimer. The Sc-GPHR was then confirmed to be activated by tethered Sc-GPA2/GPB5 heterodimer on the Gs-cAMP pathway, suggesting that Sc-GPA2/GPB5 heterodimer-initiated Gs-cAMP signaling pathway is evolutionarily conserved in chordates. Furthermore, in situ hybridization and RT-PCR results revealed the co-expression patterns of Sc-GPA2 and Sc-GPB5 with Sc-GPHR transcripts, respectively in ascidian larvae and adults, highlighting the potential functions of Sc-GPA2/GPB5 heterodimer as an autocrine/paracrine neurohormone in regulating metamorphosis of larvae and physiological functions of adults. Our study systematically investigated the GPA2/GPB5-GPHR system in ascidian for the first time, which offers insights into understanding the function and evolution of the GPH system within the chordate lineage.

Development and characterization of in vitro self-assembled recombinant human follicle stimulating hormone originated from goat mammary epithelial cells

Follicle stimulating hormone (FSH), composed of FSHα and FSHβ subunits, is essential for female follicle development and male spermatogenesis. The recombinant human FSH (rhFSH) products on the market are mainly generated from mammalian cells and are expensive. Large animal mammary gland bioreactors are urgently needed to produce large amounts of rhFSH. However, there are currently no effective methods to prepare rhFSH by large animals mainly due to the fact that excessive accumulation of FSH might cause many adverse effects in animals. We herein report the development and characterization of functional self-assembled rhFSH produced in goat mammary epithelial cells (GMECs). FSHα and FSHβ stably expressed in Chinese hamster ovary (CHO) cell lines were secreted into culture medium and well glycosylated. Importantly, FSHα and FSHβ expressed apart were able to assemble into functional FSH. We next inserted human FSHα or FSHβ gene separately into goat β-Lactoglobulin locus in GMECs by CRISPR/Cas9. Inactive FSHα and FSHβ subunits expressed from GMECs assembled into rhFSH as analyzed by His-tag pull down assay. Functional assessment of rhFSH by cAMP induction assay, mouse ovulation induction and rat ovarian weight gain experiments showed that the bioactivity of self-assembled rhFSH expressed by GMECs was comparable to that of Gonal-F both in vitro and in vivo. Our study demonstrated that FSHα and FSHβ can be separately expressed and assembled into functional rhFSH, and provided the basis for future preparing FSH by goat mammary gland bioreactor with less health problems on the producing animals.

Effects of Equine Chorionic Gonadotropin on Ovulatory and Luteal Characteristics of Mares Submitted to an P4-Based Protocol of Ovulation Induction With hCG

The aim of this study was to evaluate the effect of equine chorionic gonadotropin (eCG) at the end of progesterone (P4) treatment on follicular and luteal characteristics during transition period (TP) and reproductive breeding season (RP). A total of 13 crossbred mares were distributed in two experimental groups in the spring and summer (n = 26). The animals received intravaginal P4 (1.9 g) releasing device from D0 to D10. On removal of P4 device, the mares received 400 IU of eCG (eCG group) or saline solution (control group). Human chorionic gonadotropin (hCG; 1.750 IU) was administered (DhCG) as soon as ovulatory follicle (OF) ≥35 mm was detected. Ovarian ultrasonography was performed from D0 until 15 days after ovulation. Blood samples were collected on D0, D5, D10, DhCG, 9 days after ovulation (CL9D), and 13 days after ovulation (CL13D). P4 and estradiol concentrations were assessed by chemiluminescence. Data were compared by Tukey test at P < .05. Ovulation rate was similar (P = .096) between seasons (RP = 100%; TP = 70%) but occurred earlier (P = .015) in RP (34.8 ± 10.1 hours) compared with TP (42.0 ± 10.4 hours). Interactions between season and treatment were observed for OF diameter (mm) (RP/control = 36.2 ± 1.8ab; RP/eCG = 32.9 ± 2.8 b; TP/control = 32.2 ± 1.2 b; TP/eCG = 37.2 ± 1.9a; P = .004) and for corpus luteum (CL) diameter (mm) on CL13D (RP/control = 25.4 ± 3.5a; RP/eCG = 22.5 ± 1.8ab; TP/control = 21.6 ± 4.9 b; TP/eCG = 27.4 ± 4.3a; P = .023), although no differences were observed for serum P4 on CL13D (RP/control = 6.0 ± 3.1 ng/mL; RP/eCG = 5.8 ± 0.9 ng/mL; TP/control = 3.6 ± 2.7 ng/mL; TP/eCG = 5.1 ± 2.3 ng/mL; P = .429) or for day of structural CL regression (RP/control = 12.8 ± 1.9; RP/eCG = 12.1 ± 1.1; TP/control = 11.0 ± 1.7; TP/eCG = 13.2 ± 2.0; P = .102). The application of eCG at the moment of P4 implant removal seemed to increase the capacity of luteal maintenance during spring TP. However, eCG treatment was worthless during the breeding season.

Expression Profiling, Downstream Signaling, and Inter-subunit Interactions of GPA2/GPB5 in the Adult Mosquito Aedes aegypti

GPA2/GPB5 and its receptor constitute a glycoprotein hormone-signaling system native to the genomes of most vertebrate and invertebrate organisms. Unlike the well-studied gonadotropins and thyrotropin, the exact function of GPA2/GPB5 remains elusive, and whether it elicits its functions as heterodimers, homodimers or as independent monomers remains unclear. Here, the glycoprotein hormone signaling system was investigated in adult mosquitoes, where GPA2 and GPB5 subunit expression was mapped and modes of its signaling were characterized. In adult Aedes aegypti mosquitoes, GPA2 and GPB5 transcripts co-localized to bilateral pairs of neuroendocrine cells, positioned within the first five abdominal ganglia of the central nervous system. Unlike GPA2/GPB5 homologs in human and fly, GPA2/GPB5 subunits in A. aegypti lacked evidence of heterodimerization. Rather, cross-linking analysis to determine subunit interactions revealed A. aegypti GPA2 and GPB5 subunits may form homodimers, although treatments with independent subunits did not demonstrate receptor activity. Since mosquito GPA2/GPB5 heterodimers were not evident by heterologous expression, a tethered fusion construct was generated for expression of the subunits as a single polypeptide chain to mimic heterodimer formation. Our findings revealed A. aegypti LGR1 elicited constitutive activity with elevated levels of cAMP. However, upon treatment with recombinant tethered GPA2/GPB5, an inhibitory G protein (Gi/o) signaling cascade is initiated and forskolin-induced cAMP production is inhibited. These results further support the notion that heterodimerization is a requirement for glycoprotein hormone receptor activation and provide novel insight to how signaling is achieved for GPA2/GPB5, an evolutionary ancient neurohormone.

The role of thyrostimulin and its potential clinical significance

Thyrostimulin is a glycoprotein heterodimer of GPA2 and GPB5, first described in 2002. It is involved in the physiological function of several tissues. Moreover, evidence points towards the ability of thyrostimulin's individual monomers to induce a biological effect, which could denote the circulatory/systemic effects of the molecule when found in higher concentrations. From the evolutionary point of view, thyrostimulin shares a binding epitope with the thyroid-stimulating hormone for the thyroid stimulating hormone receptor, whilst possessing affinity for another unique binding site on the same receptor. Although thyrostimulin can be involved in the hypothalamicpituitary- thyroid axis, its presence in various tissues in an eclectic array of different species renders it multifunctional. From weight loss via increasing metabolic rate to progression of cancer in human ovaries, it is certainly not a signaling molecule to overlook. Furthermore, thyrostimulin has been implicated in bone metabolism, acute illness, and reproductive function. In summary, to our knowledge, this is the first review dealing with the physiological role of thyrostimulin and its potential applications in the clinical practice.

Differential thermal stability of human, bovine and ovine Follicle-Stimulating Hormone (FSH) and Luteinizing Hormone (LH) quaternary structures

Quaternary structure of human, bovine and ovine Follicle-Stimulating Hormones (hFSH, bFSH and oFSH) and Luteinizing Hormone was assessed in sandwich ELISAs using monoclonal anti-oFSHβ or anti-oLHβ antibodies, respectively, for capture and a biotinylated anti-hFSHα (α4 epitope) for detection. Neither free subunit gave any signal in this assay so that it was possible to measure the residual heterodimeric fraction after thermal treatment of the gonadotropins under study. The hormones were subjected to 5-min heating between 37 and 90 °C before rapid cooling in melting ice before ELISA. The data show half-dissociation of natural and recombinant human and ovine FSH preparations between 68 and 74 °C whereas bovine FSH preparations exhibited lower stability in these conditions with half-dissociation between 61 and 64 °C. Moreover, whereas all human and bovine as well as most ovine FSH preparations were fully dissociated at temperatures above 80 °C, one natural oFSH and one recombinant hLH preparations contained an important fraction that resisted dissociation even at 93 °C and retained in vitro bioactivity. This suggests the existence of gonadotropin αβ heterodimer with covalently linked subunits. Similarly, about 20% of the recombinant hLH preparation was also found withstand heat denaturation and also probably to have cross-linked subunits. The origin and chemical nature of these inter-subunit bonds remain to be determined.

Structure-function relationships of glycoprotein hormones and their subunits' ancestors

Glycoprotein hormones (GPHs) are the most complex molecules with hormonal activity. They exist only in vertebrates but the genes encoding their subunits' ancestors are found in most vertebrate and invertebrate species although their roles are still unknown. In the present report, we review the available structural and functional data concerning GPHs and their subunits' ancestors.

Comparative structure analyses of cystine knot-containing molecules with eight aminoacyl ring including glycoprotein hormones (GPH) alpha and beta subunits and GPH-related A2 (GPA2) and B5 (GPB5) molecules

BACKGROUND

Cystine-knot (cys-knot) structure is found in a rather large number of secreted proteins and glycoproteins belonging to the TGFbeta and glycoprotein hormone (GPH) superfamilies, many of which are involved in endocrine control of reproduction. In these molecules, the cys-knot is formed by a disulfide (SS) bridge penetrating a ring formed by 8, 9 or 10 amino-acid residues among which four are cysteine residues forming two SS bridges. The glycoprotein hormones Follicle-Stimulating Hormone (FSH), Luteinizing Hormone (LH), Thyroid-Stimulating Hormone (TSH) and Chorionic Gonadotropin (CG) are heterodimers consisting of non-covalently associated alpha and beta subunits that possess cys-knots with 8-amino-acyl (8aa) rings. In order to get better insight in the structural evolution of glycoprotein hormones, we examined the number and organization of SS bridges in the sequences of human 8-aa-ring cys-knot proteins having 7 (gremlins), 9 (cerberus, DAN), 10 (GPA2, GPB5, GPHalpha) and 12 (GPHbeta) cysteine residues in their sequence.

DISCUSSION

The comparison indicated that the common GPH-alpha subunit exhibits a SS bridge organization resembling that of DAN and GPA2 but possesses a unique bridge linking an additional cysteine inside the ring to the most N-terminal cysteine residue. The specific GPHbeta subunits also exhibit a SS bridge organization close to that of DAN but it has two additional C-terminal cysteine residues which are involved in the formation of the "seat belt" fastened by a SS "buckle" that ensures the stability of the heterodimeric structure of GPHs. GPA2 and GPB5 exhibit no cys residue potentially involved in interchain SS bridge and GPB5 does not possess a sequence homologous to that of the seatbelt in GPH beta-subunits. GPA2 and GPB5 are thus not expected to form a stable heterodimer at low concentration in circulation.

SUMMARY

The 8-aa cys-knot proteins GPA2 and GPB5 are expected to form a heterodimer only at concentrations above 0.1 microM: this would be consistent with a short-term paracrine role but not with an endocrine role after dilution in circulation. Consequently, GPA2 and GPB5 could exert separate endocrine roles either during development and/or during adult life of both vertebrates and invertebrates.

Efficient isolation of the subunits of recombinant and pituitary glycoprotein hormones

Complete dissociation into subunits was attained by incubating Chinese hamster ovary (CHO)-derived or native human thyrotropin, follitropin and lutropin overnight at 37 degrees C in acetic acid. The alpha-and beta-subunits of the pituitary glycoprotein hormones were rapidly and quantitatively isolated by reversed-phase high-performance liquid chromatography (RP-HPLC). A dissociation efficiency of > 98% was obtained on the basis of mass determinations of the heterodimers and subunits carried out via mass spectrometry. CHO-derived or native subunits were isolated on a C4 column (80-90% total recovery) and characterized comparatively for purity, hydrophobicity, molecular mass and charge distribution by HPLC, mass spectrometry, sodium dodecylsulfate-polyacrylamide gel electrophoresis and isoelectric focusing. Thyrotropin was used as a model for showing that, after subunit reassociation, the in vivo bioactivity of the hormone was completely restored. The method described is mild, practical, flexible, and can be adapted to dissociate microgram amounts of native or recombinant glycoprotein hormones, allowing characterization of each subunit.

Nitro-thiocyanobenzoic acid (NTCB) reactivity of cysteines beta100 and beta110 in porcine luteinizing hormone: metastability and hypothetical isomerization of the two disulfide bridges of its beta-subunit seatbelt

Luteinizing hormone (LH) like all other glycoprotein hormones is composed of two dissimilar subunits, alpha and beta, that are non-covalently associated. The heterodimer is stabilized by a region of the beta-subunit called the "seatbelt" because it wraps around the alpha-subunit and it is fastened by a disulfide bridge between cysteines beta26 and beta110. Although all 22 cysteines of porcine LH (pLH) are engaged in disulfide bridges, we previously showed that the free cysteine-specific reagent NTCB could react with pLH: it slowly cyanylated two cysteines in pLH and there was a close relationship between NTCB reaction with pLH and association/dissociation kinetics of its subunits. Therefore, cysteines beta26 and beta110 were considered as the best candidates for NTCB reaction. In order to identify the NTCB-reactive cysteines in pLH we have performed a mass spectroscopic analysis of the peptides released after mild basic hydrolysis of S-cyanylated pLH and its subunits. Only cysteines beta100 and beta110 were found to react with NTCB. Since these residues are not linked by a disulfide bridge in the crystallographic 3D structure of gonadotropins, it is proposed that their respective counterparts (Cysbeta93 and beta26) do not react with NTCB either because they are shielded from solvent or because they form a transient bridge. In the first hypothesis, both seatbelt bridges would be independently metastable; in the second one, a fast reversible isomerization between bridges beta26-beta110 and beta93-beta100 would occur. Such a reaction could be catalyzed by the previously recognized intrinsic protein disulfide isomerase (PDI) activity of gonadotropins.