Site-specific mutagenesis defines the intracellular role of the asparagine-linked oligosaccharides of chorionic gonadotropin beta subunit

Characterization of beta subunit variants of recombinant human chorionic gonadotrophin

Human chorionic gonadotropin (hCG), an endogenous glycoprotein hormone, has been widely used for the treatment of infertility and corpus luteum defect in women. The biological specificity of hCG is essentially determined by its beta (β-) subunit, whereas the alpha (α-) subunit is a common subunit shared among the gonadotropin family. In development of a therapeutic recombinant hCG, the purity analysis showed that the beta (β-) subunit has two variants, β1 and β2. Structural characterization using a combination of analytical techniques has demonstrated that β1-subunit is derived from non-glycosylation at Asn 13, whereas β2-subunit is a normal species with complete N-glycosylation at both Asn 13 and Asn 30. In vivo Bioactivity evaluation of the r-hCG fractions with various ratios of β1-and β2-subunits showed that incomplete glycosylation at Asn 13 potentially reduced the biological activity of r-hCG to promote uterus growth. Although hCG has a long history of medicinal use, this is the first report to identify the structural difference of hCG β-subunit variants, as well as to preliminary establish the structure-activity relationship of this variation. The obtained results also suggest the importance of variant characterization and necessary quality control of product variants during the development of recombinant protein therapeutics.

Specific Biological Activity of Equine Chorionic Gonadotropin (eCG) Glycosylation Sites in Cells Expressing Equine Luteinizing Hormone/CG (eLH/CG) Receptor

Equine chorionic gonadotropin (eCG), produced by the endometrial cups of the placenta after the first trimester, is a specific glycoprotein that displays dual luteinizing hormone (LH)-like and follicle-stimulating hormone (FSH)-like effects in non-equid species. However, in equidaes, eCG exhibits only LH-like activity. To identify the specific biological functions of glycosylated sites in eCG, we constructed the following site mutants of N- and O-linked glycosylation: eCGβ/αΔ56, substitution of α-subunit⁵⁶ N-linked glycosylation site; eCGβ-D/α, deletion of the O-linked glycosylation sites at the β-subunit, and eCGβ-D/αΔ56, double mutant. We produced recombinant eCG (rec-eCG) proteins in Chinese hamster ovary suspension (CHO-S) cells. We examined the biological activity of rec-eCG proteins in CHO-K1 cells expressing the eLH/CG receptor and found that signal transduction activities of deglycosylated mutants remarkably decreased. The EC₅₀ levels of eCGβ/αΔ56, eCGβ-D/α, and eCGβ-D/αΔ56 mutants decreased by 2.1-, 5.6-, and 3.4-fold, respectively, compared to that of wild-type eCG. The Rmax values of the mutants were 56%-80% those of wild-type eCG (141.9 nmol/10⁴ cells). Our results indicate that the biological activity of eCG is greatly affected by the removal of N- and O-linked glycosylation sites in cells expressing eLH/CGR. These results provide important information on rec-eCG in the regulation of specific glycosylation sites and improve our understanding of the specific biological activity of rec-eCG glycosylation sites in equidaes.

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.

A Novel Follitropin Analog Inhibits Follitropin Activity In Vitro

Follitropin (FSH) is a heterodimeric protein composed of an α subunit that is shared with the glycoprotein hormone family, including lutropin (LH), thyrotropin (TSH), human choriogonadotropin (hCG), and a unique β specific subunit. Both α and FSHβ subunits contain two sites of N-linked oligosaccharides, which are important for its function. FSH has a crucial function in the reproductive process in mammals. However, there are some clinical conditions, such as menopausal osteoporosis or adiposity, associated with increased FSH activity. Moreover, in some cases, carcinogenesis is evidently associated with activation of FSH receptor. Therefore, developing a follitropin antagonist might be beneficial in the treatment of these conditions. Here, we describe a novel, engineered, non-glycosylated single-chain FSH variant, prepared by site-directed mutagenesis and fusion of the coding genes of the α and β subunits. The designed variant was expressed in Chinese hamster ovary (CHO) cells and successfully secreted into the culture medium. We found that the non-glycosylated single-chain FSH analog binds with high affinity to FSH receptor and efficiently inhibits FSH activity in vitro. This variant acts at the receptor level and has the potential to serve as a follitropin antagonist for clinical applications in the future.

Characterization of tethered equine chorionic gonadotropin and its deglycosylated mutants by ovulation stimulation in mice

Background

To directly assess the biological role of oligosaccharides in recombinant equine chorionic gonadotropin (rec-eCG) functioning, cDNA encoding the full-length eCGβ-subunit was fused with the mature protein part of the α-subunit, and we examined the expression levels of deglycosylated eCG mutants, the ovulation rate for deglycosylated mutants in C57BL/6 mice.

Results

The characterizations of heterodimeric and tethered mutants were studied following their respective secretions in culture medium, molecular weight and ovulation in vivo. Rec-eCG variants containing mutations at glycosylation sites at Asn82 of the α-subunit (eCGβ/αΔ82) and Asn13 of the β-subunit (eCGβΔ13/α) were not efficiently secreted into the culture medium from transfected cells. Western blot analysis revealed that the rec-eCGβ/α proteins have an approximate broad range of molecular weights of 40–46 kDa. Three rec-eCG mutants—a deglycosylated site at Asn56 of the α-subunit (eCGβ/αΔ56), a deletion of the C-terminal region of the β-subunit (eCGβ-D/α), and the double mutant (eCGβ-D/αΔ56)—turned out to have clearly lower (approximately 4–23 kDa) molecular weights. Protein N-glycosydase F (PNGase F) treatment markedly decreased the molecular weight to approximately 2–10 kDa. Normal oocytes were significantly more abundant in the natural eCG–treated group than in mutant rec-eCG–treated groups. In particular, numbers of nonfuntional oocytes were remarkably lower in all rec-eCG groups.

Conclusions

Our results indicate that the ovulation rates of oocytes are not affected by the deglycosylated rec-eCGβ/α mutant proteins. There are around 20% non-functional oocytes with natural eCG and only 2% with the rec-eCGs tested. These results provide insight into the molecular mechanisms underlying the production of rec-eCG hormones with excellent bioactivity in vivo.

Electronic supplementary material

The online version of this article (10.1186/s12896-019-0550-6) contains supplementary material, which is available to authorized users.

Follicle-Stimulating Hormone Glycobiology

FSH glycosylation varies in two functionally important aspects: microheterogeneity, resulting from oligosaccharide structure variation, and macroheterogeneity, arising from partial FSHβ subunit glycosylation. Although advances in mass spectrometry permit extensive characterization of FSH glycan populations, microheterogeneity remains difficult to illustrate, and comparisons between different studies are challenging because no standard format exists for rendering oligosaccharide structures. FSH microheterogeneity is illustrated using a consistent glycan diagram format to illustrate the large array of structures associated with one hormone. This is extended to commercially available recombinant FSH preparations, which exhibit greatly reduced microheterogeneity at three of four glycosylation sites. Macroheterogeneity is demonstrated by electrophoretic mobility shifts due to the absence of FSHβ glycans that can be assessed by Western blotting of immunopurified FSH. Initially, macroheterogeneity was hoped to matter more than microheterogeneity. However, it now appears that both forms of carbohydrate heterogeneity have to be taken into consideration. FSH glycosylation can reduce its apparent affinity for its cognate receptor by delaying initial interaction with the receptor and limiting access to all of the available binding sites. This is followed by impaired cellular signaling responses that may be related to reduced receptor occupancy or biased signaling. To resolve these alternatives, well-characterized FSH glycoform preparations are necessary.

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.

Clinical Applications of Gonadotropins in the Male

The pituitary gonadotropins, luteinizing hormone (LH), and follicle-stimulating hormone (FSH) play a pivotal role in reproduction. The synthesis and secretion of gonadotropins are regulated by complex interactions among several endocrine, paracrine, and autocrine factors of diverse chemical structure. In men, LH regulates the synthesis of androgens by the Leydig cells, whereas FSH promotes Sertoli cell function and thereby influences spermatogenesis. Gonadotropins are complex molecules composed of two subunits, the α- and β-subunit, that are noncovalently associated. Gonadotropins are decorated with glycans that regulate several functions of the protein including folding, heterodimerization, stability, transport, conformational maturation, efficiency of heterodimer secretion, metabolic fate, interaction with their cognate receptor, and selective activation of signaling pathways. A number of congenital and acquired abnormalities lead to gonadotropin deficiency and hypogonadotropic hypogonadism, a condition amenable to treatment with exogenous gonadotropins. Several natural and recombinant preparations of gonadotropins are currently available for therapeutic purposes. The difference between natural and the currently available recombinant preparations (which are massively produced in Chinese hamster ovary cells for commercial purposes) mainly lies in the abundance of some of the carbohydrates that conform the complex glycans attached to the protein core. Whereas administration of exogenous gonadotropins in patients with isolated congenital hypogonadotropic hypogonadism is a well recognized therapeutic approach, their role in treating men with normogonadotropic idiopathic infertility is still controversial. This chapter concentrates on the main structural and functional features of the gonadotropin hormones and how basic concepts have been translated into the clinical arena to guide therapy for gonadotropin deficit in males.

Novel pathways in gonadotropin receptor signaling and biased agonism

Gonadotropins play a central role in the control of male and female reproduction. Selective agonists and antagonists of gonadotropin receptors would be of great interest for the treatment of infertility or as non steroidal contraceptive. However, to date, only native hormones are being used in assisted reproduction technologies as there is no pharmacological agent available to manipulate gonadotropin receptors. Over the last decade, there has been a growing perception of the complexity associated with gonadotropin receptors' cellular signaling. It is now clear that the Gs/cAMP/PKA pathway is not the sole mechanism that must be taken into account in order to understand these hormones' biological actions. In parallel, consistent with the emerging paradigm of biased agonism, several examples of ligand-mediated selective signaling pathway activation by gonadotropin receptors have been reported. Small molecule ligands, modulating antibodies interacting with the hormones and glycosylation variants of the native glycoproteins have all demonstrated their potential to trigger such selective signaling. Altogether, the available data and emerging concepts give rise to intriguing opportunities towards a more efficient control of reproductive function and associated disorders.

Reproductive axis function and gonadotropin microheterogeneity in a male rat model of diet-induced obesity

Obesity causes complex metabolic and endocrine changes that may lead to adverse outcomes, including hypogonadism. We herein studied the reproductive axis function in male rats under a high-fat diet and analyzed the impact of changes in glycosylation of pituitary LH on the bioactivity of this gonadotropin. Rats were fed with a diet enriched in saturated fat (20% of total calories) and euthanized on days 90 or 180 of diet. Long-term (180 days), high-fat feeding rats exhibited a metabolic profile compatible with insulin resistance and metabolic syndrome; they concomitantly showed decreased intrapituitary and serum LH concentrations, low serum testosterone levels, and elevated serum 17beta-estradiol concentrations. A fall in biological to immunological ratio of intrapituitary LH was detected in 180 days control diet-treated rats but not in high-fat-fed animals, as assessed by a homologous in vitro bioassay. Chromatofocusing of pituitary extracts yielded multiple LH charge isoforms; a trend towards decreased abundance of more basic isoforms (pH 9.99-9.0) was apparent in rats fed with the control diet for 180 days but not in those that were fed the diet enriched in saturated fat. It is concluded that long-term high-fat feeding alters the function of the pituitary-testicular axis, resulting in hypogonadotropic hypogonadism. The alterations in LH function found in these animals might be subserved by changes in hypothalamic GnRH output and/or sustained gonadotrope exposure to an altered sex steroid hormone milieu, representing a distinctly different regulatory mechanism whereby the pituitary attempts to counterbalance the effects of long-term obesity on reproductive function.

Pituitary-testicular axis function, biological to immunological ratio and charge isoform distribution of pituitary LH in male rats with experimental diabetes

Men with insulinopenic diabetes mellitus frequently present hypogonadism and exhibit circulating luteinizing hormone (LH) molecules with increased biological activity. To further study this latter issue, we analyzed the pattern of isoform distribution and the impact of changes in terminal glycosylation of pituitary LH on the bioactivity of this gonadotropin in experimental diabetes. Adult male rats were treated with streptozotocin or vehicle and euthanized on days 30, 60, or 90 posttreatment. All diabetic groups exhibited a significant decrease in serum insulin and testosterone levels as well as in sperm count; serum gonadotropins and 17beta-estradiol decreased only after 90 days of insulinopenia. Both the immunoreactive concentrations and the biological to immunological ratio of intrapituitary LH significantly increased in all experimental groups, as assessed by an in vitro homologous bioassay in HEK-293 cells expressing a recombinant LH receptor. Chromatofocusing of pituitary extracts revealed the presence of multiple LH charge isoforms; the pH distribution profile of LH in diabetic and control rats was indistinguishable on days 30 and 60 posttreatment. By contrast, the abundance of more basic isoforms (pH 9.99-9.0) decreased and that of isoforms with pH values 8.99-8.0 increased in rats with long-standing diabetes compared to controls. It is concluded that experimental diabetes alters the function of the pituitary-testicular axis, resulting in reduced sex steroids levels and hypogonadotropism. Long-standing insulinopenia leads to a paradoxical accumulation of intrapituitary LH molecules enriched in bioactivity with altered terminal glycosylation, which are apparently subserved by distinct mechanisms involving altered hypothalamic and/or gonadal inputs on the gonadotrope.

Secretory trafficking signal encoded in the carboxyl-terminal region of the CGbeta-subunit

Although the LHbeta- and chorionic gonadotropin-beta- (CGbeta) subunits share a high degree of sequence identity (>85%) in the first 114 amino acids, there is considerable sequence divergence at their carboxy ends. The CGbeta-subunit terminates with a unique carboxyl-terminal extension (115-145; carboxyl-terminal peptide), which contains four O-linked oligosaccharides, whereas the LHbeta-subunit bears a hydrophobic heptapeptide (115-121) at its carboxy terminus. LH is released through the regulated pathway in the pituitary, whereas CG is secreted constitutively from the placenta. We previously demonstrated in rat somatotroph-derived GH(3) cells that the LH is associated primarily with a regulated routing, and although the majority of CG was released constitutively from the cells, there was a fraction that was segregated through the regulated pathway. Moreover, we showed that the LHbeta heptapeptide is a determinant for the regulated secretion of LH. Given that the primary evolutionary change between LHbeta and CGbeta occurred at the carboxy terminus, these data suggested that the presence of the CGbeta carboxyl-terminal peptide region is responsible for the constitutive secretion of CG. A CG114 mutant (CGDeltaT) was constructed and expressed in GH(3) cells. Steady-state labeling and pulse-chase experiments demonstrated that the CGDeltaT entered the regulated pathway resulting in over 4-fold increase in the intracellular pool. The secretagogue, forskolin, stimulated CGDeltaT release over 3-fold, which was accompanied by a parallel intracellular decrease, and only marginal stimulation of CG was seen. Immunofluorescence demonstrated a unique membrane pattern of staining for CGDeltaT compared with dispersed cytoplasmic puncta for CG. Stimulation with forskolin caused a significant reduction in the relative fluorescence of CGDeltaT cells compared with a minor reduction for CG. These data show that the CGDeltaT analog resembles LH in its intracellular trafficking, further supporting the hypothesis that determinants at the carboxyl-terminal end of the CGbeta-subunit evolved from the LHbeta-subunit primarily to overcome the slow release and intracellular storage of LH resulting in rapid secretion of CG from the placenta.

The role of O-linked and N-linked oligosaccharides on the structure-function of glycoprotein hormones: development of agonists and antagonists

Thyrotropin (TSH) and the gonadotropins; follitropin (FSH), lutropin (LH) and human chorionic gonadotropin (hCG) are a family of heterodimeric glycoprotein hormones. These hormones composed of two noncovalently linked subunits; a common alpha and a hormone specific beta subunits. Assembly of the subunits is vital to the function of these hormones. However, genetic fusion of the alpha and beta subunits of hFSH, hCG and hTSH resulted in active polypeptides. The glycoprotein hormone subunits contain one (TSH and LH) or two (alpha, FSHbeta and hCGbeta) asparagine-linked (N-linked) oligosaccharides. CGbeta subunit is distinguished among the beta subunits because of the presence of a carboxyl-terminal peptide (CTP) bearing four O-linked oligosaccharide chains. To examine the role of the oligosaccharide chains on the structure-function of glycoprotein hormones, chemical, enzymatic and site-directed mutagenesis were used. The results indicated that O-linked oligosaccharides play a minor role in receptor binding and signal transduction of the glycoprotein hormones. In contrast, the O-linked oligosaccharides are critical for in vivo half-life and bioactivity. Ligation of the CTP bearing four O-linked oligosaccharide sites to different proteins, resulted in enhancing the in vivo bioactivity and half-life of the proteins. The N-linked oligosaccharide chains have a minor role in receptor binding of glycoprotein hormones, but they are critical for bioactivity. Moreover, glycoprotein hormones lacking N-linked oligosaccharides behave as antagonists. In conclusion, the O-linked oligosaccharides are not important for in vitro bioactivity or receptor binding, but they play an important role in the in vivo bioactivity and half-life of the glycoprotein hormones. Addition of the O-linked oligosaccharide chains to the backbone of glycoprotein hormones could be an interesting strategy for designing long acting agonists of glycoprotein hormones. On the other hand, the N-linked oligosaccharides are not important for receptor binding, but they are critical for bioactivity of glycoprotein hormones. Deletion of the N-linked oligosaccharides resulted in the development of glycoprotein hormone antagonists. In the case of hTSH, development of an antagonist may offer a novel therapeutic strategy in the treatment of thyrotoxicosis caused by Graves' disease and TSH secreting pituitary adenoma.

Glycoprotein profiling by electrospray mass spectrometry

This work compares several different methods of site-specific analysis of glycoproteins using electrospray mass spectrometry. The glycoprotein, oLHalpha (ovine luteinizing hormone, alpha-subunit) was chosen as an appropriate example protein for these studies because of its biological relevance and extreme microheterogeneity. More than 20 unique glycoforms were detected for this glycoprotein at the Asn(56) site of oLHalpha. The carbohydrates present at this site affect receptor binding affinity, so understanding the great variety in the composition of these carbohydrates is important in studying ligand binding interactions. MS data was acquired on a quadrupole ion trap, a triple quadrupole, and a quadrupole time of flight mass spectrometer, and carbohydrate composition at the Asn(56) site of oLHalpha was determined using these instruments. Additionally, neutral loss and precursor ion scanning modes were also used to identify the glycoforms present, and these techniques were compared to the standard MS data. Of the three instruments compared in the study, the qTOF mass spectrometer achieved the lowest sample consumption, but all three instruments were useful in profiling the glycopeptide composition.

Cloning and sequence analysis of FSH and LH in the giant panda (Ailuropoda melanoleuca)

The giant panda (Ailuropoda melanoleuca) is an endangered species and indigenous to China. It has been proposed that it has a highly specialized reproductive pattern with low fecundity, but little is known about its basic reproductive biology at the molecular level. In this report the genes encoding gonadotropin subunits alpha, follicle-stimulating hormone (FSH) beta and luteinizing hormone (LH) beta of the giant panda were amplified for the first time by RT-PCR from pituitary total RNA, and were cloned, sequenced and analyzed. The results revealed that the open reading region (ORF) of gonadotropin subunits alpha, FSH beta and LH beta are 363, 390 and 426 bp long, respectively. They displayed a reasonably high degree (74-94, 85-93, 75-91%, for alpha, FSH beta and LH beta subunits, respectively) of identity when deduced amino acids were compared with homologous sequences from partial available mammals including human, cattle, sheep, pig, rat, mouse. Three distinct differences were found at the site of 59 aa of the alpha subunit and 55 aa, 68 aa of FSH beta subunit. Our results provide an insight into understanding the mechanism of reproduction regulation and genetic characteristics of giant panda which will make an actual contribution to its conservation. In addition they lay a foundation for a further study towards producing recombinant panda FSH and LH which can be used in artificial breeding aimed to increase its captive reproductive efficiency.

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.

Endocrine regulation of gonadotropin glycosylation

The pituitary gonadotropins--luteinizing hormone and follicle-stimulating hormone--as well as the placental choriogonadotropin belong to the family of glycoprotein hormones. These structurally related hormones, which regulate several major reproductive functions of the body, are heterodimers consisting of a common alpha-subunit noncovalently bound to a beta-subunit. The N- and O-linked oligosaccharide chains of these gonadotropins play an important role in intracellular folding, assembly, secretion, metabolic clearance, and biological activity of the hormone. Gonadotropin glycosylation is a highly complex process; within the gonadotropes it is modulated by a variety of extrapituitary factors of hypothalamic and gonadal origin. In particular, estrogens and androgens appear to regulate terminal sialylation and/or sulfation of the oligosaccharide attachments and hence some functional properties of the gonadotropin molecule determined by these residues, i.e., metabolic clearance and in vivo biopotency. Through these extrapituitary inputs, the anterior pituitary may not only regulate the quantity but also the quality of the gonadotropin signal delivered to the gonads in a given physiologic or pathologic condition.

Molecular architecture and biorecognition processes of the cystine knot protein superfamily: part I. The glycoprotein hormones

In this review article, the reader is introduced to recent advances in our knowledge on a subset of the cystine knot superfamily of homo- and hetero-dimeric proteins, from the perspective of the endocrine glycoprotein hormone family of proteins: follitropin (FSH), Iutropin (LH), thyrotropin. (TSH) and chorionic gonadotropin (CG). Subsequent papers will address the structure-function behaviour of other members of this increasingly significant family of proteins, including various members of the transforming growth factor-beta (TGF-beta) family of proteins, the activins, inhibins, bone morphogenic growth factor, platelet derived growth factor-beta, nerve growth factor and more than 35 other proteins with similar topological features. In the present review article, specific emphasis has been placed on advances with the glycoprotein hormones (GPHs) that have facilitated greater insight into their physiological functions, molecular structures and most importantly the basis of the molecular recognition events that lead to the formation of hetero-dimeric structures as well as their specific and selective recognition by their corresponding receptors and antibodies. Thus, this review article focuses on the structural motifs involved in receptor recognition and the current techniques available to identify these regions, including the role of immunological methodology, peptide fragment design and synthesis and mutagenesis to delineate their structure-function relationships and molecular recognition behaviour.

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.

Synthesis of multi-subunit domain gonadotropin complexes: a model for alpha/beta heterodimer formation

The human glycoprotein hormones chorionic gonadotropin (CG), thyrotropin (TSH), lutropin (LH), and follitropin (FSH) are heterodimers, composed of a common alpha subunit assembled to a hormone-specific beta subunit. The subunits combine noncovalently early in the secretory pathway and exist as heterodimers, but not as multimers. Little information is available regarding the steps associated with the assembly reaction. It is unclear if the initial alpha beta engagement results either in the formation of only mature heterodimer or if the nascent complex is reversible and can undergo an exchange of subunits or combine transiently with an additional subunit. This is relevant for the case of LH and FSH, because both are synthesized in the same cell (i.e., pituitary gonadotrophs) and several of the alpha subunit sequences required for association with either the LH beta or FSH beta subunits are different. Such features could favor the generation of short-lived, multi-subunit forms prior to completion of assembly. Previously, we showed that the CG beta or FSH beta subunit genes can be genetically fused to the alpha gene to produce biologically active single chains, CG beta alpha and F beta alpha, respectively. Studies using monoclonal antibodies sensitive to the conformation of the hCG subunits suggested that in contrast to the highly compact heterodimer, the interactions between the beta and alpha domains in the single chain are in a more relaxed configuration. That the tethered domains do not interact tightly predicts that they could combine with an additional subunit to form triple domain complexes. We tested this point by cotransfecting CHO cells with the genes encoding F beta alpha and the CG beta subunit or the CG beta alpha and FSH beta monomer. The CG beta subunit combined noncovalently with F beta alpha to form a F beta alpha/CG beta complex. Ternary complex formation was not restricted to a specific set of single chain/monomeric subunit, because a CG beta alpha/FSH beta complex was also detected implying that triple domain intermediates could be transiently generated along the secretory pathway. Monoclonal antibodies specific for the CG heterodimer recognized the F beta alpha/CG beta complex, which suggests that the epitopes unique for dimeric CG were established. In addition, media containing F beta alpha/CG beta displayed high-affinity binding to both CG and FSH receptors. The presence of CG activity is presumptive for the existence of a functional F beta alpha/CG beta complex, because neither F beta alpha nor the uncombined CG beta subunit binds to CG receptor. These data show that the alpha subunit of the tether, although covalently linked to the FSH beta domain, can functionally interact with a different beta subunit implying that the contacts in the nascent alpha beta dimer are reversible. The formation of a functional single chain/subunit complex was not restricted to the FSH single chain/CG beta subunit since CG single chain interacts with the monomeric FSH beta subunit and exhibits FSH activity. The presence of the triple domain configuration does not abolish bioactivity, suggesting that although the gonadotropins are heterodimers, the cognate receptor is capable of recognizing a larger ligand composed of three subunit domains.

A naturally occurring genetic variant in the human chorionic gonadotropin-beta gene 5 is assembly inefficient

The hCGbeta gene family is composed of six homologous genes linked in tandem repeat on chromosome 19; the order of the genes is 7, 8, 5, 1, 2, and 3. Previous studies have shown that hCGbeta gene 5 is highly expressed during the first trimester of pregnancy. The purpose of our study was to identify naturally occurring polymorphisms in hCGbeta gene 5 and determine whether these alterations affected hCG function. The data presented here show that hCGbeta gene 5 was highly conserved in the 334 asymptomatic individuals and 41 infertile patients examined for polymorphisms using PCR followed by single stranded conformational polymorphism analysis. Most of the polymorphisms detected were either silent or located in intron regions. However, one genetic variant identified in beta gene 5 exon 3 was a G to A transition that changed the naturally occurring valine residue to methionine in codon 79 (V79M) in 4.2% of the random population studied. The V79M polymorphism was always linked to a silent C to T transition in codon 82 (tyrosine). To determine whether betaV79M hCG had biological properties that differed from those of wild-type hCG, a beta-subunit containing the V79M substitution was created by site-directed mutagenesis and was coexpressed with the glycoprotein hormone alpha-subunit in Chinese hamster ovary cells and 293T cells. When we examined betaV79M hCG biosynthesis, we detected atypical betaV79M hCG folding intermediates, including a betaV79M conformational variant that resulted in a beta-subunit with impaired ability to assemble with the alpha-subunit. The inefficient assembly of betaV79M hCG appeared to be independent of beta-subunit glycosylation or of the cell type studied, but, rather, was due to the inability of the betaV79M subunit to fold correctly. The majority of the V79M beta-subunit synthesized was secreted as unassembled free beta. Although the amount of alphabeta hCG heterodimer formed and secreted by betaV79M-producing cells was less than that by wild-type beta-producing cells, the hCG that was secreted as alphabeta V79M heterodimer exhibited biological activity indistinguishable from that of wild-type hCG.

Protein glycosylation: implications for in vivo functions and therapeutic applications

The glycosylation machinery in eukaryotic cells is available to all proteins that enter the secretory pathway. There is a growing interest in diseases caused by defective glycosylation, and in therapeutic glycoproteins produced through recombinant DNA technology route. The choice of a bioprocess for commercial production of recombinant glycoprotein is determined by a variety of factors, such as intrinsic biological properties of the protein being expressed and the purpose for which it is intended, and also the economic target. This review summarizes recent development and understanding related to synthesis of glycans, their functions, diseases, and various expression systems and characterization of glycans. The second section covers processing of N- and O-glycans and the factors that regulate protein glycosylation. The third section deals with in vivo functions of protein glycosylation, which includes protein folding and stability, receptor functioning, cell adhesion and signal transduction. Malfunctioning of glycosylation machinery and the resultant diseases are the subject of the fourth section. The next section covers the various expression systems exploited for the glycoproteins: it includes yeasts, mammalian cells, insect cells, plants and an amoeboid organism. Biopharmaceutical properties of therapeutic proteins are discussed in the sixth section. In vitro protein glycosylation and the characterization of glycan structures are the subject matters for the last two sections, respectively.

Effect of individual N-glycosyl chains in the beta-subunit on the conformation of human choriogonadotropin

Human choriogonadotropin is a heterodimeric glycoprotein hormone comprised of noncovalently associated alpha- and beta-subunits. Each of the subunits has two N-glycosyl chains. In our previous communication, we investigated the role of individual carbohydrate chains in the alpha-subunit on the signal transduction function and conformation of the hormone. This paper deals with the effect of individual or both N-glycosyl chains in the beta-subunit on the function and conformation of the monomer as well as of the heterodimer. Three mutants each of hCGbeta and hCG lacking N-glycosyl chains at beta13Asn, beta30Asn and beta13,30Asn were prepared by site-directed mutagenesis by replacing Thr residues in the recognition triplet sequence at beta15 and beta32 positions with Gln. All mutant heterodimers had receptor binding and cAMP and progesterone stimulating activities comparable to wild type hCG. While the loss of carbohydrate at beta13Asn or beta13,30Asn in the case of hCGbeta monomer resulted in a 4-6%, decrease in the ordered structure, the loss of the glycosyl chain at beta 30Asn did not alter the conformation as compared with the wild type hCGbeta. Similarly, all carbohydrate deficient hCG heterodimers had a decrease of 6-8%) in the ordered structure as compared with hCG. Thus, while the individual N-glycosyl chains did not affect the function of the hormone, they did have marked effect on its conformation but the conformational changes were localized and did not perturb the receptor binding and signal transduction sites.

Identification of novel transmembrane gene sequence and its use for cell-surface targeting of beta subunit of human chorionic gonadotropin

We identified a 685-nucleotide gene fragment that codes for the transmembrane and cytoplasmic domains of glycoprotein of the LEP strain rabies virus and carried out experiments designed to express a novel fusion protein on the cell surface. The cDNA encoding the membrane anchor sequence was fused in the correct reading frame to the 3' end of the cDNA encoding the beta subunit of human chorionic gonadotropin (beta(h)CG), a secretory glycoprotein that is used as an antigen for a contraceptive vaccine being developed in our laboratory. The fusion gene cassette was placed under the control of a vaccinia virus early promoter and cloned in a host-restricted fowlpox viral vector. The recombinants, when used to infect mammalian cells that do not allow the replication of fowlpox virus, expressed the N-terminal 135 amino acid residues of beta(h)CG anchored in the cell membrane by the 75-amino acid C-terminal sequence derived from rabies virus glycoprotein. This hybrid protein is correctly processed post-translationally and transported efficiently to the plasma membrane of non-permissive cells such that the anchored beta(h)CG molecule retains the correctly folded native antigenic epitope(s).

The carboxyl-terminal region is a determinant for the intracellular behavior of the chorionic gonadotropin beta subunit: effects on the processing of the Asn-linked oligosaccharides

The placental hormone human CG (hCG) consists of two noncovalently linked alpha- and beta-subunits similar to the other glycoprotein hormones LH, FSH, and TSH. These heterodimers share a common alpha subunit but differ in their structurally distinct beta subunits. The CGbeta subunit is distinguished among the beta subunits by the presence of a C-terminal extension with four serine-linked oligosaccharides (carboxyl terminal peptide or CTP). In previous studies we observed that deleting this sequence decreased assembly of the truncated CGbeta subunit (CGbeta114) with the alpha-subunit and increased the heterogeneity of the secreted forms of the uncombined subunit synthesized in transfected Chinese hamster ovary (CHO) cells. The latter result was attributed to alterations in the processing of the two N-linked oligosaccharides. To examine at what step this heterogeneity occurs, the CGbeta and CGbeta114 genes were transfected into wild-type and mutant CHO cell lines that are defective in the late steps of the N-linked carbohydrate-processing pathway. We show here that removal of the CTP alters the processing of the core mannosyl unit of the subunit to complex forms at both glycosylation sites and that the oligosaccharides contain polylactosamine. Although it has been presumed that there is little intramolecular interaction between the CTP and the proximal domains of the subunit, our data suggest that the CTP sequence participates in the folding of the newly synthesized subunit, which is manifest by the posttranslational changes observed here.

Site-directed mutagenesis of the N-linked glycosylation site in platelet-derived growth factor B-chain results in diminished intracellular retention

Pulse-chase analysis of human platelet-derived growth factor (PDGF) B-chain was conducted in stably transfected Chinese hamster ovary cells to determine precisely the kinetics of processing, intracellular trafficking and secretion. Newly synthesized 31 kDa monomers of the B-chain (p31) dimerized rapidly via disulfide bonds to a p54 species (t1/2 < 30 min). The p54 dimer was processed to a group of intracellular, cell surface (suramin-releasable) and secreted forms whose rates of appearance and disappearance from the cell were measured over a 48 h period. The newly synthesized p31 species was quantitatively converted to p27 by treatment with endoglycosidase H, consistent with efficient N-glycosylation at a site in the N-terminal propeptide region (Asn63-Met64-Thr65). Interruption of B-chain glycosylation by oligodeoxynucleotide-directed mutagenesis resulted in a significant increase in suramin-releasable forms at the cell surface (p34-38) and a concomitant decrease in accumulation of an intracellular p24 species. The glycosylation-defective mutant exhibited slight increases in receptor binding and mitogenic activity. Our results suggest that N-linked glycosylation of the B-chain is not important for formation of mitogenically active protein, but that it plays a role in early intracellular sorting and proteolytic processing events.

Novel covalent chaperone complexes associated with human chorionic gonadotropin beta subunit folding intermediates

Molecular chaperones facilitate the folding of proteins in the endoplasmic reticulum (ER) of mammalian cells. The glycoprotein hormone chorionic gonadotropin beta subunit is a secretory protein whose folding in the ER has been demonstrated (Huth, J. R., Mountjoy, K., Perini, F., and Ruddon, R. W.(1992) J. Biol. Chem. 267, 8870-8879). Because folding of wild type hCG-beta subunit occurs in the ER with a t1/2 = 4-5 min, stable association of ER chaperones with hCG-beta have been difficult to detect probably because they have a short half-life. However, beta-chaperone complexes containing the ER chaperones BiP, ERp72, and ERp94 have been detected in slow folding mutants of hCG-beta subunit that lack both of the N-linked oligosaccharides (Feng, W., Matzuk, M. M., Mountjoy, K., Bedows, E., Ruddon, R. W., and Boime, I. (1995) J. Biol. Chem. 270, 11851-11859). The questions addressed here are 1) whether the detection of chaperone-containing complexes is related to the absence of carbohydrate or to the rate of hCG-beta subunit folding, 2) whether such complexes are dead-end or whether they lead to formation of a secreted, mature hCG-beta form, and 3) what the nature of the hCG-beta-chaperone binding is. The data obtained indicate that the amount of detectable hCG-beta-chaperone complexes correlates with the rate or extent of folding, that the complexes of hCG-beta with ER chaperones lead to the formation of secretable beta, and that the complexes of hCG-beta with chaperones involve the formation of intermolecular disulfide bonds.

Bacterial expression of human chorionic gonadotropin alpha subunit: studies on refolding, dimer assembly and interaction with two different beta subunits

Human chorionic gonadotropin (hCG) is a member of a family of heterodimeric glycoprotein hormones that have a common alpha subunit but differ in their hormone-specific beta subunit. The common alpha subunit contains two asparagine (N)-linked oligosaccharides. To study the function of carbohydrates on in vitro refolding of alpha subunit and dimer assembly, we generated recombinant non-glycosylated hCG alpha subunit (rNGl-hCGalpha) from E. coli. The expression vector was constructed by inserting hCGalpha cDNA coding for the mature form in-frame into a pQE-30 vector, which contains a 6 x His sequence immediately before the 5'-end of hCGalpha cDNA for subsequent purification of rNG-hCGalpha. The rNG-hCGalpha expressed in inclusion bodies was efficiently purified by immobilized metal chelate affinity chromatography on Ni-NTA resin. SDS-PAGE, solid-phase binding assay and immunoblotting demonstrated the expression of rNG-hCG. Its alpha molecular weight on SDS-PAGE was 14.7 kDa under reducing conditions and 15 kDa for a monomer accompanied with some higher molecular weight oligomer under non-reducing conditions. Reconstitution of rNG-hCGalpha with native hCGbeta and oFSHbeta occurred in very low yield under standard conditions. However, the oxidation-reduction system cystamine (1.34 mM) and cysteamine (7.3 mM) facilitated both the refolding of rNG-hCGalpha and reconstitution of rNG-hCGalpha with native hCGbeta to regain partially correct conformation. These were revealed by conformationally sensitive antibody and receptor binding assays. Cystamine and cysteamine were more effective in the recombination of rNG-hCGalpha with oFSHbeta as indicated by a 22-36-fold decrease in the amount required to cause a 50% competitive inhibition in radioreceptor assay. They have no effect on assembly of rNG-hCGalpha with oLHbeta. Our results suggest the carbohydrate moieties confer greater conformational flexibility to the backbone of the beta subunit and the relative rigidity of the beta subunit may serve as a conformational template of the alpha subunit. The present approach has made it possible to prepare the non-glycosylated gonadotropin alpha subunit in adequate amounts for further study on their biological and topographical features in complete absence of carbohydrate.

The 1994 Stevenson Award Lecture. Follicle-stimulating hormone and luteinizing hormone: a tale of two gonadotropins

Although most gonadotropes synthesize both luteinizing hormone and follicle-stimulating hormone, the transcription, content, and secretion rates of the two gonadotropins can be separated. The signals external to the gonadotropic cells that appear to be important in the differential regulation are gonadotropin-releasing hormone pulse frequency (high pulse frequency favors luteinizing hormone), steroid feedback (works on both but induces a more powerful negative feedback on luteinizing hormone), and gonadal peptide feedback (activin increases follicle-stimulating hormone; inhibin and follistatin decrease it). We know very little about the pathways within the gonadotropes that favor one gonadotropin rather than another. It is expected that the cloning of the genes for both gonadotropins and the use of specific cell lines and transfections will lead to elucidation of these pathways.

The asparagine-linked oligosaccharides of the human chorionic gonadotropin beta subunit facilitate correct disulfide bond pairing

The role of asparagine (N)-linked oligosaccharide chains in intracellular folding of the human chorionic gonadotropin (hCG)-beta subunit was determined by examining the kinetics of folding in Chinese hamster ovary (CHO) cells transfected with wild-type or mutant hCG-beta genes lacking one or both of the asparagine glycosylation sites. The half-time for folding of p beta 1 into p beta 2, the rate-determining step in beta folding, was 7 min for wild-type beta but 33 min for beta lacking both N-linked glycans. The p beta 1-->p beta 2 half-time was 7.5 min in CHO cells expressing the beta subunit missing the Asn13-linked glycan and 10 min for the beta subunit missing the Asn30-linked glycan. The inefficient folding of hCG-beta lacking both N-linked glycans correlated with the slow formation of the last three disulfide bonds (i.e. disulfides 23-72, 93-100, and 26-110) to form in the hCG-beta-folding pathway. Unglycosylated hCG-beta was slowly secreted from CHO cells, and beta subunit-folding intermediates retained in cells for more than 5 h were degraded into a hCG-beta core fragment-like protein. However, coexpression of the hCG-alpha gene enhanced folding and formation of disulfide bonds 23-72, 93-100, and 26-110 of hCG-beta lacking N-linked glycans. In addition, the molecular chaperones BiP, ERp72, and ERp94, but not calnexin, were found in a complex with unglycosylated, unfolded hCG-beta and may be involved in the folding of this beta form. These data indicate that N-linked oligosaccharides assist hCG-beta subunit folding by facilitating disulfide bond formation.

An investigation into the significance of the N-linked oligosaccharides of Leishmania gp63

Leishmania major promastigotes, when grown in the presence of tunicamycin (TM), produced a plasma membrane-bound, proteolytically active gp63 with a lower molecular weight than the native glycoprotein. However, this lower molecular weight form of gp63 continued to be recognized by concanavalin A (Con A), suggesting that inhibition of N-linked glycosylation was not complete. Metabolic labeling of gp63, using [35S]methionine, demonstrated that in the range of 5-10 micrograms ml-1 TM, only the lower molecular weight form was synthesized, suggesting that inhibition was complete and that lectin binding was likely due to the GPI anchored sugars. Removal of the oligosaccharides from L. major and L. mexicana amazonensis promastigotes using endoglycosidase F, caused the gp63 molecular weight to decrease to the same value observed in the presence of TM, once again without affecting the proteolytic activity. However, this deglycosylated enzyme continued to bind Con A until subsequently treated with periodate. The latter oxidation reaction resulted in complete loss of Con A binding without inhibiting the protease activity or the substrate specificity of gp63. Further investigations revealed that both glycosylated and deglycosylated gp63 were resistant to proteolytic digestion by either autolysis or cathepsin D. These findings indicate that the N-linked oligosaccharides of gp63 are not essential for folding, transport, maintenance of enzyme activity or resistance to proteolysis.

Structures and functions of the sugar chains of glycoproteins

Most proteins within living organisms contain sugar chains. Recent advancements in cell biology have revealed that many of these sugar chains play important roles as signals for cell-surface recognition phenomena in multi-cellular organisms. In order to elucidate the biological information included in the sugar chains and link them with biology, a novel scientific field called 'glycobiology' has been established. This review will give an outline of the analytical techniques for the structural study of the sugar chains of glycoproteins, the structural characteristics of the sugar chains and the biosynthetic mechanism to produce such characteristics. Based on this knowledge, functional aspects of the sugar chains of glycohormones and of those in the immune system will be described to help others understand this new scientific field.

Mutations of the human thyrotropin-beta subunit glycosylation site reduce thyrotropin synthesis independent of changes in glycosylation status

In recent studies, site-directed mutagenesis has been used to alter the tripeptide glycosylation recognition sequences of glycoprotein hormone subunits, thereby affecting their structure and function. However, it is not known whether these effects result from changes in glycosylation status, amino acid sequence, or both. We therefore studied the synthesis of wild-type and mutant recombinant human thyrotropins produced by transient transfection of a human cell line. Mutating the TSH-beta subunit glycosylation recognition sequence, Asn-Thr-Thr (codons 23-25), to either Gln-Thr-Thr or Asn-Thr-Tyr abolished subunit glycosylation, as demonstrated by the inability to incorporate 3H-carbohydrates. However, a third mutation (Asn-Thr-Ser) contained an intact glycosylation recognition sequence site, and was shown to retain glycosylation. The mutations that abolished TSH-beta subunit glycosylation resulted in greater than 90% decreases in TSH synthesis. However, the glycosylation recognition sequence mutant that retained beta subunit glycosylation exhibited a 70% decrease in TSH production. These decreases were not attributable to the intracellular accumulation of TSH or its free beta subunit. We also engineered two TSH-beta subunit mutations that did not alter the glycosylation recognition sequence. A glycine to arginine mutation adjacent to the glycosylation recognition sequence, in a region thought to be critical for heterodimer formation, abolished TSH production. In contrast, shortening the TSH-beta subunit carboxyterminus by six amino acids increased TSH synthesis.(ABSTRACT TRUNCATED AT 250 WORDS)

N-linked glycosylation affects the processing of mouse submaxillary gland prorenin in transfected AtT20 cells

Most mouse inbred strains carry two renin genes, Ren-1 and Ren-2, Renin-2, the product of the Ren-2 gene, is highly expressed in the submaxillary gland. It is a renin isoenzyme 96% similar to kidney renin-1, but unglycosylated. In order to investigate if glycosylation of prorenin affects its processing and/or secretion we have introduced two potential N-linked glycosylation sites into preprorenin-2 cDNA using site-directed mutagenesis. Expression plasmids were derived from wild-type and mutant renin-2 cDNA and were transfected into AtT20 cells. Both transfected cells, expressing glycosylated or unglycosylated forms, secreted prorenin and renin by the constitutive and regulated pathways, respectively. Prorenin was correctly processed to active renin but the second maturation site was not cleaved in AtT20 cells. The comparison of glycosylated and unglycosylated renin expression showed a diminished secretion of glycosylated active renin. Prevention of glycosylation with tunicamycin resulted in an improved secretion of active renin. Moreover, the efficiency of the trypsin activation in vitro was reduced for glycosylated prorenin and it was restored when the activation was performed on mutant renin secreted from tunicamycin-treated cells. It is proposed that the bulky carbohydrates attached to prorenin constitute a steric hindrance to proteolysis by maturation enzymes.