Pituitary glycoprotein hormone oligosaccharides: structure, synthesis and function of the asparagine-linked oligosaccharides on lutropin, follitropin and thyrotropin

A single nucleotide polymorphism in a miR-1302 binding site in CGA increases the risk of idiopathic male infertility

OBJECTIVE

To explore the possible association between single nucleotide polymorphisms (SNPs) in the miRNA-binding sites of spermatogenesis-related genes and idiopathic infertility in humans.

DESIGN

Prospective study.

SETTING

Research laboratory of a university hospital.

PATIENT(S)

A total of 494 patients with azoospermia or severe oligozoospermia and 357 fertile controls were included in our study.

INTERVENTION(S)

The 3' untranslated region sequences of 140 candidate genes for male infertility were analyzed using specialized algorithms including Pictar, miRanda, Targetscan, and RNAhybrid and 39 SNPs located at putative miRNA-binding sites were identified. The possible association of 6 putatively functional SNPs and male infertility was explored further with the use of case-control studies. The function of SNPs significantly associated with male infertility was analyzed by dual luciferase assay.

MAIN OUTCOME MEASURE(S)

Significantly associated SNPs and their influence on gene expression.

RESULT(S)

Two SNPs from two genes (rs6631 of CGA and rs2303846 of CPEB1) were found to be associated with idiopathic male infertility. Functionally, the substitution of A by T in rs6631 results in decreased binding affinity of miR-1302 and overexpression of CGA in vitro.

CONCLUSION(S)

Our results reveal for the first time that SNPs residing in miRNA-binding sites of CGA could influence expression of CGA and elevate the risk of spermatogenesis impairment.

N-glycan analysis of recombinant L-Selectin reveals sulfated GalNAc and GalNAc-GalNAc motifs

The leukocytic adhesion receptor L-selectin plays a crucial role in the first step of the adhesion cascade, enabling leukocytes to migrate into surrounding tissues during inflammation and immune surveillance. We analyzed the site-specific N-glycosylation of the lectin and EGF-like domain of L-selectin using recombinant variants ("LEHis"). The three glycosylation sites of LEHis were mutated to obtain singly glycosylated variants that were expressed in HEK293F cells. alpha1-Acid glycoprotein (AGP), expressed in the same system, was used to distinguish between cell type- and protein-specific glycosylation. Using mass spectrometry and exoglycosidase digestions, we established that LEHis was mostly bearing multifucosylated diantennary N-glycans with a major fraction terminating with GalNAc residues replacing the more common Gal. We could also show that parts of the GalNAc residues were sulfated. Furthermore, we identified novel diantennary glycan structures terminating with the motif GalNAc-GalNAc or SO(4)-GalNAc-GalNAc, which have not been described for N-glycans yet. Interestingly, none of these specific features were found in the N-glycan profile of AGP. This indicates that protein intrinsic information of L-selectin leads to decoration with specific N-glycans, which in turn may be related to L-selectin function.

Assessment of the quality and structural integrity of a complex glycoprotein mixture following extraction from the formulated biopharmaceutical drug product

Biological drugs represent an important and rapidly growing class of therapeutics useful in the treatment of a variety of disorders ranging from cancer to inflammation to infectious diseases. Unlike single chemical entities, the recombinant production of these drugs in living cells confers considerable structural and chemical heterogeneity to the biologically derived protein product that constitutes the active pharmaceutical ingredient (API). In mammalian based expression systems, much of this diversity is conferred through heterogeneous protein glycosylation. These post-translational modifications can have significant effects on the structure, biological function, and pharmacological properties of the API. In addition, the bulk proteins that comprise the API are further formulated through the use of multiple excipients designed to ensure product stability, solubility, and lot-to-lot consistency. Unfortunately, these matrices can interfere with commonly available analytical methods used in the thorough chemical characterization of the biological drug product. At the same time, a demonstration of the suitable extraction of the bulk drug substance in a manner and form that does not destabilize the active ingredient or introduce any structural bias with direct reference to the original drug product is both critical and necessary. Here, we use recombinant human follicle stimulating hormone (follitropin alpha for injection) from a pharmaceutical source as an example to illustrate a suitable purification strategy to effectively extract the bulk drug substance from the formulated drug product with high purity and yield. We assess the suitability of this extraction method in preserving the structural integrity and overall quality of the drug substance relative to the formulated drug product, placing a particular emphasis on glycosylation as a key product attribute. In so doing, we demonstrate that it is possible to effectively extract the active pharmaceutical ingredient from a formulated biological drug product in a manner that is consequently sufficient for its use in comparability studies.

Enrichment method of sulfated glycopeptides by a sulfate emerging and ion exchange chromatography

Sulfated glycoproteins are of growing importance for biomarker discovery, as well as for investigating molecular recognition processes. Mass spectrometry (MS) has become a powerful technique for the characterization of glycans and glycoproteins. However, characterization and detection of sulfated glycopeptides by MS is difficult because of the low abundance and low ionization efficiency of these molecules. To overcome this problem, we developed a novel enrichment procedure for sulfated glycopeptides. The procedure consists of anion exchange chromatography and a sulfate emerging (SE) method which controls the net charge of peptides by utilizing limited proteolyzes and modification with acetohydrazide. Using this procedure, we are able to enrich and characterize the sulfated glycopeptides of bovine luteinizing hormone (bLH). Furthermore, we demonstrate the enrichment and detection of sulfated glycopeptides from a complex mixture comprising human serum spiked with bLH at a concentration of 0.1%.

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.

Sequential enrichment of sulfated glycans by strong anion-exchange chromatography prior to mass spectrometric measurements

Structural characterization of sulfated glycans through mass spectrometry (MS) has been often limited by their low abundance in biological materials and inefficient ionization in the positive-ion mode. Here, we describe a microscale method for sequentially enriching sulfated glycans according to their degree of sulfation. This method is based on modifying the binding ability of strong anion-exchange material through the use of different sodium acetate concentrations, thus enabling fairly selective binding and a subsequent elution of different glycans according to their degree of sulfation. Before this enrichment, the negative charge on the sialic acid, which is commonly associated with such glycans, was eliminated through permethylation that is used to enhance the positive-ion mode matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-MS) signal for all glycans. This enrichment approach minimizes competitive ionization between sulfated and neutral glycans, as well as that between sulfated species with a different degree of sulfation. The described method was initially optimized using sulfated oligosaccharide standards, while its potential has been verified for the sulfated N-glycans originated from the bovine thyroid-stimulating hormone (bTSH), a glycoprotein possessing mono- and disulfated N-glycans. This enhancement of the MALDI-MS signal facilitates analysis of some otherwise undetected components.

Structural analysis of sulfated glycans by sequential double-permethylation using methyl iodide and deuteromethyl iodide

MALDI mass spectrometric characterization of sulfated glycans is often challenging due to their low ionization response in the positive ion mode. Here we demonstrate a new analytical approach, allowing the measurement of sulfated glycans by substituting the sulfate group with a deuteromethyl group. Sulfated glycan samples are initially permethylated before the methanolytic cleavage of their sulfate groups. Desulfated and permethylated glycans are then subjected to another permethylation step using deuteromethyl iodide to label the hydroxyl groups resulting from methanolysis. The number of attached sulfate groups is subsequently calculated from the mass-shift resulting from the chemical cleavage of these sulfate groups. The position of the sulfate substitution is then determined by collision-induced dissociation (CID) tandem mass spectrometry of permethylated and permethylated plus deuteromethylated samples. The described approach was initially optimized and validated using linear standard glycans, while its effectiveness has also been demonstrated here for the N-glycans derived from bovine thyroid-stimulating hormone (bTSH).

Expression of bovine follicle-stimulating hormone subunits in a Hansenula polymorpha expression system increases the secretion and bioactivity in vivo

Bovine follicle-stimulating hormone (bFSH), a pituitary gonadotropin, is a heterodimer hormone that consists of a common alpha-subunit non-covalently associated with the hormone-specific beta-subunit. Unfortunately, expression levels of recombinant bFSH or its subunits are invariably low. We report here the secretory expression of biologically active bFSHalpha and bFSHbeta subunit in the methylotrophic yeast Hansenula polymorpha. A slightly higher level of expression of recombinant bFSH subunits was achieved by using the Saccharomyces cerevisiae-derived calnexin (ScCne1) as a chaperone in engineered H. polymorpha strains. The preliminary data also suggested that bFSH subunits expressed in H. polymorpha appeared to be less-glycosylated. This isoform had been shown to be 80% increase in in vivo bioactivity compared with the hyperglycosylated Pichia pastoris-derived recombinant bFSHalpha/beta. More sophisticated applications of bFSH would profit from the assembled less-glycosylated heterodimer.

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.

Ablation of GalNAc-4-sulfotransferase-1 enhances reproduction by altering the carbohydrate structures of luteinizing hormone in mice

Luteinizing hormone (LH), produced in the anterior lobe of the pituitary, is a member of the hypothalamic-pituitary-gonad axis that is required for production of the sex hormones estradiol, progesterone, and testosterone. Perturbations in levels of hormones associated with this axis can result in defects in sexual development and maturity. LH bears unique N-linked carbohydrate units that terminate with a sulfated N-acetylgalactosamine structure (GalNAc-4-SO(4)) that mediates its clearance from the blood. To determine the significance of this terminal structure, we ablated the gene encoding the sulfotransferase responsible for sulfate addition to GalNAc on LH, GalNAc-4-sulfotransferase-1 (GalNAc-4-ST1) in mice. Mice lacking GalNAc-4-ST1 exhibited increased levels of circulating LH. In male mice, this resulted in elevated levels of testosterone and precocious maturation of testis and seminal vesicles. Female mice lacking GalNAc-4-ST1 demonstrated elevated estrogen levels and exhibited precocious sexual maturation and increased fecundity. Female mice remained in estrus for prolonged periods and produced almost 50% more litters per mouse than wild-type mice over the same period of time. Thus, sulfate modification of the terminal glycosylation of LH plays a central role in regulating the hypothalamic-pituitary-gonad axis in vivo.

Differential actions of FSH and LH during folliculogenesis

In the gonadotrophin-dependent stage of follicular development, FSH- and LH-signalling pathways play an obligatory role in follicle differentiation, selection and survival. Under the effect of LH the theca-interstitial cell layer acts as an androgen producer. Thus, androgen diffusing into the mural granulosa cell layer represents the substrate for FSH-induced aromatase for follicular oestradiol synthesis. This is the landmark 'two cell-two gonadotrophin' concept in the physiology of ovarian function in mammals. The increase in plasma FSH during luteo-follicular transition is the basis for follicle selection. The rise of FSH to the threshold concentration represents a critical condition for the growth of the most sensitive follicle in a given time frame of the last 14 days of the dominant follicle odyssey. The gonadotrophin-induced follicular oestradiol secretion inhibits pituitary secretion of FSH, which in turn causes the concentration of FSH in the developing cohort follicles to drop below threshold concentrations and the arrest of the development of the less FSH-sensitive follicle (FSH threshold and window concept). In the gonadotrophin-dependent phase of follicular development, LH also seems to acts within a critical window of the hormone concentration framed between the minimal threshold and a ceiling for the normal functions of the follicle unit.

Caprine luteinizing hormone isoforms during the follicular phase and anestrus

The relative proportion of the circulating luteinizing hormone isoforms in goats during follicular phase (pre-ovulatory peak; F) and anestrus (A) was investigated. Estrus was synchronized in six goats with a prostaglandin analogue. After estrus was detected, blood samples were taken at 1 h intervals for 24 h. Four anestrous goats received 100 microg i.v. of GnRH and blood samples were collected every 15 min for 5 h. Samples with the greatest LH concentration in follicular phase and after GnRH administration (anestrus) were analyzed by chromatofocusing and eluted with a pH gradient from 10.5 to 3.5. For quantification purposes eluted LH was grouped into basic (pH> or =7.5), neutral (pH 7.4-6.5) and acidic isoforms (pH< or =6.4) as well as by pH unit. In both physiological conditions (PC), basic and acidic isoforms were greater than the neutral. With this grouping criteria, there was an interaction between PC and pH group, with the proportion of neutral isoforms being greater (p<0.05) in A (12.0+/-0.8%) as compared with F (5+/-2%). Analysis by pH unit showed a very basic group of eluted isoforms (pH> or =10), which amounted to a percentage of 6.0+/-0.4% of the total observed during A, and 3+/-1% during F (p<0.05). Predominant isoforms in A eluted in the pH range 9.99-9.0 (42+/-3%) as compared to 7+/-3% (p<0.01) in that pH range in F. In contrast, the predominant isoforms in F eluted in the pH range 8.99-8.0, representing 55+/-8%, while in A the proportion was 11+/-2% (p<0.01). Isoforms eluted at the pH range 7.9-7 represented a significantly greater proportion during A (5.0+/-0.6%) as compared with F (3+/-1%). This is the first report on goat LH circulating isoforms. During A the LH isoforms secreted by the pituitary are more basic than during F.

Comparative glycomics of the glycoprotein follicle stimulating hormone: glycopeptide analysis of isolates from two mammalian species

Follicle stimulating hormone (FSH) is one of the important hormones that regulate gonadal functions. This hormone is glycosylated, and the glycans greatly influence the biological properties. In the present study the negatively charged glycopeptides of equine and human pituitary follicle stimulating hormone (eFSH and hFSH) have been characterized in a glycosylation site-specific manner using FT-ICR-MS and Edman sequencing. The characteristic pattern of glycan distribution at each glycosylation site has been deduced and compared between horse and human FSH preparations. The data suggest that site-specific differences exist between glycoforms of human and equine FSH. For instance, except for one site in the beta subunit (Asn7) of hFSH all other sites in both species have sulfated glycoforms. Also, glycoforms at Asn52 of hFSH are all complex type, whereas in eFSH, both complex and hybrid structures exist at this site. There is also a higher percentage of sulfated glycans in the latter site compared to the former. This is the first study that characterizes the glycans from this hormone in a glycosylation site-specific manner, and these data can be used to begin correlative studies between glycosylation structure and hormone function.

Conjugated estrogens and tibolone modify the gonadotrophin glycosylation pattern in postmenopausal women

OBJECTIVE

The purpose of this study was to evaluate the changes in the percentage of glycosylation of FSH and LH when using conjugated estrogens and tibolone as hormonal therapy (HT) in postmenopausal compared with regular menstrual cycles.

DESIGN

The study had three groups, with 10 participants in each group. The control group consisted of 10 women with normal menstrual cycles, a second group with 10 postmenopausal patients who received conjugated estrogens (Premarin 0.625 mg), and a third group had 10 patients who received tibolone (Livial 2.5 mg). All patients had hormonal determinations before and 6 months after the use of HT. Concavalin-A was used to separate the different glycosylated species of gonadotrophins, defined as unbound (UB: triantennary and bisecting oligosaccharides), weakly bound (WB: biantennary and truncated oligosaccharides), and firmly bound isoforms (FB: hybrid- and high-mannose-type oligosaccharides) in patients serum. Elusions containing the fractions with the isoforms were concentrated using Centriprep membranes (Amicon, Beverly, Mass., USA) and the different gonadotrophins isoforms were quantified by chemiluminescence using an automated system Immulite.

RESULTS

The pattern of distribution of gonadotrophins glycosylation in patients treated with conjugated estrogens and tibolone was similar to that observed in the control group with normal menstrual cycles, although with different glycosylation percentage. When the glycosylation percentages were compared for FSH, in the fraction UB the glycosylation did not have significant differences, but the fraction WB that has isoforms with high amounts of oligosaccharides showed a different percentage of glycosylation, where the recovery percentage was bigger with tibolone use (p < 0.05). The FB fraction had similar hormonal glycosylation with estrogens and tibolone use. When compared, LH glycosylation showed significant differences (p < 0.05) in the fraction UB with a bigger glycosylation degree, and in the fraction FB with a smaller glycosylation percentage, both in the tibolone group compared with conjugated estrogens use.

CONCLUSIONS

Our results show that there are different percentages of glycosylation of gonadotrophins depending on type of HT (conjugated estrogens vs tibolone), and this could be of utility in young women with ovarian problem since to know the isoform that more favors to cellular activity could help a better therapeutic.

Fractionation of follicle stimulating hormone charge isoforms in their native form by preparative electrophoresis technology

Complex glycoprotein biopharmaceuticals, such as follicle stimulating hormone (FSH), erythropoietin and tissue plasminogen activator consist of a range of charge isoforms due to the extent of sialic acid capping of the glycoprotein glycans. Sialic acid occupies the terminal position on the oligosaccharide chain, masking the penultimate sugar residue, galactose from recognition and uptake by the hepatocyte asialoglycoprotein receptor. It is therefore well established that the more acidic charge isoforms of glycoprotein biopharmaceuticals have higher in vivo potencies than those of less acidic isoforms due to their longer serum half-life. Current strategies for manipulating glycoprotein charge isoform profile involve cell engineering or altering bioprocesss parameters to optimise expression of more acidic or basic isoforms, rather than downstream separation of isoforms. A method for the purification of a discrete range of bioactive recombinant human FSH (rhFSH) charge isoforms based on Gradiflowtrade mark preparative electrophoresis technology is described. Gradiflowtrade mark electrophoresis is scaleable, and incorporation into glycoprotein biopharmaceutical production bioprocesses as a potential final step facilitates the production of biopharmaceutical preparations of improved in vivo potency.

Ovine serum and pituitary isoforms of luteinising hormone during the luteal phase

The relative abundance of the different isoforms of pituitary and circulating luteinising hormone (LH) in ewes, at different times after the administration of gonadotrophin-releasing hormone (GnRH), during the luteal phase of the oestrous cycle was investigated. Sixteen ewes on Day 9 of their cycle were divided into four groups (n = 4). The control group (T0) received saline solution; the remaining animals received 100 μg GnRH (i.m.) 30, 90 or 180 min (T30, T90 and T180, respectively) before serum and pituitary gland collection. Luteinising hormone polymorphism was analysed by chromatofocusing (pH 10.5–3.5). The LH eluted from each chromatofocusing was grouped on the basis of the following three criteria: (1) according to the pH of elution (pH ≥ 10–3.5); (2) as either a basic (pH ≥ 7.5), neutral (pH 7.4–6.5) and acidic (pH ≤ 6.4) elution of LH of serum and hypophyseal origin; and (3) on the basis of distinct isoforms, of which 10 (A–J) were identifiable in hypophyseal extracts and four (A–D) were found in the serum. In general, the most abundant forms of LH in both the pituitary and serum, at all times, were basic. However, that proportion was greater in hypophyseal extracts (84 ± 3%, 81 ± 4%, 82 ± 3% and 83 ± 2% at T0, T30, T90 and T180, respectively) than in serum (51 ± 5%, 48 ± 10% and 54 ± 6% at T30, T90 and T180, respectively). Neutral and acidic LH made up a larger proportion of the total LH in sera (neutral: 17 ± 4%, 20 ± 6% and 23 ± 3% at T30, T90 and T180, respectively; acidic: 32 ± 8%, 32 ± 11% and 23 ± 6% at T30, T90 and T180, respectively) than in the pituitary extracts (neutral: 4.0 ± 0.7%, 10 ± 4%, 7 ± 2% and 5.0 ± 0.5% at T0, T30, T90 and T180, respectively; acidic: 12 ± 3%, 11 ± 2%, 12 ± 2% and 12 ± 2% at T0, T30, T90 and T180, respectively) at all times. These data reveal that the relative composition of the LH present in the pituitary gland and the LH secreted into the circulation is different, with more neutral and acidic isoforms being secreted. The pattern of circulating LH isoforms changes between 30 and 180 min after GnRH peak induction, with a greater proportion of isoform C (eluting between pH 7.0 and 6.5) at T180 compared with T30 and T90.

Differential expression and enzymatic properties of GalNAc-4-sulfotransferase-1 and GalNAc-4-sulfotransferase-2

We have cloned two GalNAc-4-sulfotransferases, GalNAc-4-ST1 and GalNAc-4-ST2, that transfer sulfate to terminal beta1,4-linked GalNAc. In conjunction with the action of protein-specific beta1,4GalNAc-transferases, GalNAc-4-ST1 and GalNAc-4-ST2 account for the presence of terminal beta1,4-linked GalNAc-4-SO(4) on glycoproteins such as lutropin, thyrotropin (TSH), proopiomelanocortin (POMC), carbonic anhydratase-VI (CA-VI), and tenascin-R. GalNAc-4-ST1 and GalNAc-4-ST2 can be distinguished by their differing specificity for oligosaccharide acceptors and temperature lability. The differences in properties have been used to show that the levels of GalNAc-4-ST1 and GalNAc-4-ST2 activity are proportionate to the levels of their respective transcripts. Furthermore, we have found that both transcript and activity levels of GalNAc-4-ST1 and GalNAc-4-ST2 vary widely among different tissues indicating that the regulation of their expression differs. Differences in specificity and the regulation of expression may account for existence of two GalNAc-4-sulfotransferases in vivo. The highest levels of both GalNAc-4-ST1 and GalNAc-4-ST2 transcripts are present in the pituitary of the mouse with multiple cell types that produce glycoproteins terminating with GalNAc-4-SO(4). Genetic ablation of both GalNAc-4-ST1 and GalNAc-4-ST2 may be necessary to alter the pattern and/or extent of sulfate addition to terminal beta1,4GalNAc in tissues such as pituitary.

Molecular pharmacology of gonadotropins

Gonadotropins have been studied in biological systems for decades and many of their properties are well defined. These include pharmacological properties such as affinity, stability, and pharmacokinetics also used to characterize drugs. Technologies applied to research on gonadotropins have led to the creation of hormone analogs with alterations to one or more of these proper-ties. Some of these analogs have potential therapeutic applications. A challenge to realizing this potential is the accurate prediction of how these compounds will perform in humans. This could be facilitated by advances in biological models and the understanding of specific effects of the hormones on their receptors.

Novel Poly-GalNAcβ1–4GlcNAc (LacdiNAc) and Fucosylated Poly-LacdiNAc N-Glycans from Mammalian Cells Expressing β1,4-N-Acetylgalactosaminyltransferase and α1,3-Fucosyltransferase

Glycans containing the GalNAcbeta1-4GlcNAc (LacdiNAc or LDN) motif are expressed by many invertebrates, but this motif also occurs in vertebrates and is found on several mammalian glycoprotein hormones. This motif contrasts with the more commonly occurring Galbeta1-4GlcNAc (LacNAc or LN) motif. To better understand LDN biosynthesis and regulation, we stably expressed the cDNA encoding the Caenorhabditis elegans beta1,4-N-acetylgalactosaminyltransferase (GalNAcT), which generates LDN in vitro, in Chinese hamster ovary (CHO) Lec8 cells, to establish L8-GalNAcT CHO cells. The glycan structures from these cells were determined by mass spectrometry and linkage analysis. The L8-GalNAcT cell line produces complex-type N-glycans quantitatively bearing LDN structures on their antennae. Unexpectedly, most of these complex-type N-glycans contain novel "poly-LDN" structures consisting of repeating LDN motifs (-3GalNAcbeta1-4GlcNAcbeta1-)n. These novel structures are in contrast to the well known poly-LN structures consisting of repeating LN motifs (-3Galbeta1-4GlcNAcbeta1-)n. We also stably expressed human alpha1,3-fucosyltransferase IX in the L8-GalNAcT cells to establish a new cell line, L8-GalNAcT-FucT. These cells produce complex-type N-glycans with alpha1,3-fucosylated LDN (LDNF) GalNAcbeta1-4(Fucalpha1-3)GlcNAcbeta1-R as well as novel "poly-LDNF" structures (-3GalNAcbeta1-4(Fucalpha 1-3)GlcNAcbeta1-)n. The ability of these cell lines to generate glycoprotein hormones with LDN-containing N-glycans was studied by expressing a recombinant form of the common alpha-subunit in L8-GalNAcT cells. The alpha-subunit N-glycans carried LDN structures, which were further modified by co-expression of the human GalNAc 4-sulfotransferase I, which generates SO4-4GalNAcbeta1-4GlcNAc-R. Thus, the generation of these stable mammalian cells will facilitate future studies on the biological activities and properties of LDN-related structures in glycoproteins.

Pattern of circulating luteinizing hormone isoforms during the estrous and luteal phases in Holstein heifers

The pattern of distribution of circulating luteinizing hormone (LH) isoforms in cattle during estrus and the luteal phase was investigated. In each stage, the stage of the estrous cycle was synchronized in seven Holstein heifers with a prostaglandin analogue. After estrus was detected, blood samples were taken at 2-h intervals for 24h. In the luteal phase, animals received 250 microg i.v. of GnRH and blood samples were collected every 15 min for 5h. LH concentration in the samples was determined. Samples with the greatest LH concentration in estrus (pre-ovulatory peak) and those collected 60 min after GnRH administration (luteal phase) were analyzed by chromatofocusing, eluted with a pH gradient from 10.5 to 3.5. Eluted LH was grouped into basic (pH > or = 7.5), neutral (pH 7.4-6.5) and acidic isoforms (pH < or = 6.4) as well as by pH unit. In both phases, basic forms were the most abundant, and these were greater (P < 0.05) during the luteal phase (78.4 +/- 4.2%) as compared with during estrus (57.1 +/- 6.2%); the proportion of neutral and acidic isoforms in estrus (13.7 +/- 2.6%; 28.5 +/- 2.8%) was greater (P < 0.05) as compared with the luteal phase (3.0 +/- 0.7; 18.7 +/- 3.4). These results indicate that the relative proportion of LH isoforms secreted by the adenohypophysis differ by stage of estrous cycle. The addition of excess of NaCl to the column modifies the antigen-antibody binding in the RIA, and the proteins eluted are erroneously quantified as LH; this is an artifact of the technique.

Ethanol-induced alterations in Rab proteins: possible implications for pituitary dysfunction

Chronic exposure of pubertal male rats to ethanol results in a decline in serum testosterone, increased gonadotropins, pituitary luteinizing hormone (LH) and follicle stimulating hormone (FSH) content, and decreased or inappropriately normal serum LH and FSH levels, suggesting impaired secretory release of gonadotropins. The molecular mechanisms behind this disorder are undefined, but a disruption of vesicle-mediated secretory processes is possible because intracellular protein trafficking pathways are involved in secretion of glycoproteins such as FSH and LH. Because small GTP-binding proteins of Rab family have been implicated as key regulators of membrane and protein trafficking in mammalian cells, this study was designed to test if ethanol-impaired pituitary FSH and LH secretion is associated with changes in Rab proteins, particularly Rab1B, Rab3B, Rab6, and Rab11. Male Sprague-Dawley rats 35 days old were pair-fed a Lieber-DeCarli diet with ethanol or without ethanol for 5 to 60 days. After ethanol exposure, serum testosterone levels decreased while LH and FSH were inappropriately unchanged. Immunohistochemical staining showed decreased Rab1B, Rab3B, and Rab11 protein levels in ethanol-treated pituitaries. Immunoblotting showed that ethanol induced a transient reduction in Rab6 after 5 days of ethanol exposure, whereas Rab3B decreased after 20 days, Rab11 after 30 days, and Rab1B after 60 days. Despite these changes in Rab proteins, mRNA levels were unaffected by ethanol exposure. We concluded that reductions in key Rab proteins may lead to altered vesicle trafficking and may play a role in disruption of pituitary FSH and LH secretion caused by ethanol.

Batch-to-batch consistency of human-derived gonadotrophin preparations compared with recombinant preparations

Different gonadotrophin preparations derived from human urine or manufactured by recombinant technology are currently used in clinical practice for the treatment of infertility. It has been widely assumed that gonadotrophin products manufactured by recombinant technology have better batch-to-batch consistency compared with human-derived preparations and that this potentially will be shown to provide a more constant clinical response, but there is little evidence for either statement. This study compared the batch-to-batch consistency between urinary-derived and recombinant manufactured gonadotrophin preparations using standard analytical techniques, as well as a novel in-vitro follicle bioassay to evaluate the consistency of the biological response at the target organ. Oligosaccharide isoform profiling, immunoassay testing, size exclusion chromatography analysis and in-vitro bioassay testing of urinary derived gonadotrophin preparations (MENOPUR and BRAVELLE) confirm that these products display a high degree of batch-to-batch consistency, similar to recombinant FSH (GONAL-f) either filled by mass or bioassay. The data also suggest that the batch-to-batch variation is independent of the manufacturing procedure (filled-by-bioassay or filled-by-mass) for the recombinant preparation (Gonal-f), but that the total FSH bioactivity delivered from a single dose preparation after reconstitution differs between the two manufacturing procedures.

Factors influencing response to ovarian stimulation

Ovarian stimulation is an integral part of assisted reproduction treatments. Ovarian response to gonadotrophin treatment, besides other factors, determines the outcome of treatment, as the number and quality of oocytes retrieved are related to the chance of achieving a pregnancy. A number of factors have been identified that might predict ovarian response, such as age of the patient and antral follicle count. In addition, it has been shown that genetic factors such as the patient's FSH-receptor genotype also determine individual response to FSH treatment. Besides patient-related factors, the choice of drugs for ovarian stimulation plays a significant role. Until recently, biopotency of gonadotrophin preparations was tested by an in-vivo bioassay with an intrinsic variability up to 20%. Due to a superior manufacturing technique, follitropin alpha can now be filled by mass. This allows assessment of FSH with a precise SE-HPLC assay and variability of the FSH content between production lots has now been estimated at 1.6%. Results of recent studies indicate that treatment with follitropin alpha filled by mass results in consistent ovarian response, fewer treatment days and fewer cancelled cycles. This is an important step towards further minimizing drug-related variability of ovarian response to FSH treatment.

Use of SP-Sephadexto fractionate and obtain semi-pure preparation from whole pituitaries of Indian water buffaloes (Bubalus bubalis)

Different charge isoforms of native Luteinizing hormone (LH) (dimeric form) Ve/Vo = 1.49 can be fractionated on SP-Sephadex into four different charge isoforms (LHUB, LH25, LH50, and LH100) by stepwise elution using different molarities of Na2HPO4. LHUB was found to be difficult to purify, whereas LH50 and LH100 were found to be pure and highly immunoreactive against anti-bLHbeta serum as indicated by the results obtained from direct binding ELISA and Western blot analysis. SDS-PAGE of LH50 and LH100 showed two bands corresponding to the two subunits of LH. LH25 can be purified to homogeneity by rechromatography on S-300. Purification of buLH as a highly immunoreactive preparation has also been described using SP-Sephadex. This LH preparation (SP25B), which was obtained after slight modification in the pre-existing protocol, has been found to be highly immunoreactive against anti-bLHbeta serum in direct binding ELISA. Being a very simple and reproducible method, it can be used to obtain pure LH preparation, as a reference material, in a short period of time for various immunoassays and bioassays.

Multiple and overlapping combinatorial codes orchestrate hormonal responsiveness and dictate cell-specific expression of the genes encoding luteinizing hormone

Normal reproductive function in mammals requires precise control of LH synthesis and secretion by gonadotropes of the anterior pituitary. Synthesis of LH requires expression of two genes [alpha-glycoprotein subunit (alphaGSU) and LHbeta] located on different chromosomes. Hormones from the hypothalamus and gonads modulate transcription of both genes as well as secretion of the biologically active LH heterodimer. In males and females, the transcriptional tone of the genes encoding alphaGSU and LHbeta reflects dynamic integration of a positive signal provided by GnRH from hypothalamic neurons and negative signals emanating from gonadal steroids. Although alphaGSU and LHbeta genes respond transcriptionally in the same manner to changes in hormonal input, different combinations of regulatory elements orchestrate their response. These hormone-responsive regulatory elements are also integral members of much larger combinatorial codes responsible for targeting expression of alphaGSU and LHbeta genes to gonadotropes. In this review, we will profile the genomic landscape of the promoter-regulatory region of both genes, depicting elements and factors that contribute to gonadotrope-specific expression and hormonal regulation. Within this context, we will highlight the different combinatorial codes that control transcriptional responses, particularly those that mediate the opposing effects of GnRH and one of the sex steroids, androgens. We will use this framework to suggest that GnRH and androgens attain the same transcriptional endpoint through combinatorial codes unique to alphaGSU and LHbeta. This parallelism permits the dynamic and coordinate regulation of two genes that encode a single hormone.

The third chains of living organisms—a trail of glycobiology that started from the third floor of building 4 in NIH

Application of a finger-printing method to the analysis of human milk oligosaccharides led to the finding that several oligosaccharides were missing in the milk of non-secretor or Lewis-negative individuals. This finding helped us in opening the door of elucidating the enzymatic basis of blood types in human. Based on these successful studies, a strategy to establish reliable techniques to elucidate the structures and functions of the N-linked sugar chains of glycoproteins was devised. It was to contrive enzymatic and chemical means to release quantitatively the N-linked sugar chains as oligosaccharides, and finger-print them by using appropriate methods to demonstrate the sugar pattern of a glycoprotein. These methods enabled us to determine that the N-linked sugar chains of glycoproteins can be classified into three subgroups: high mannose-type, complex-type, and hybrid-type. By comparative studies of the sugar patterns of a glycoprotein produced by different organs and different animals, occurrences of organ- and species-specific glycosylation were found in many glycoproteins. By comparative studies of the glycosylation patterns of the subunits constructing human chorionic gonadotropin and other glycoproteins, occurrence of site-directed N-glycosylation was also found, indicating that the processing and maturation of the N-linked sugar chains of a glycoprotein might be controlled by the structure of polypeptide moiety. Furthermore, these methods enabled us to elucidate the structural alteration of the sugar chains of a glycoprotein induced by diseased state of the producing cells, such as rheumatoid arthritis and malignancy. Recent studies of glycoproteins in the brain-nervous system through aging revealed that N-glycosylation of P(0) in the rat spinal cord is induced by aging. Therefore, glycobiology is expanding tremendously into fields such as pathological and gerontological research.

Processing of O-linked glycosylation in the chimera consisting of alpha-subunit and carboxyl-terminal peptide of the human chorionic gonadotropin beta-subunit is affected by dimer formation with follicle-stimulating hormone beta-subunit

hCG, LH, FSH, and TSH are a family of heterodimeric glycoprotein hormones that contain a common alpha-subunit, but differ in their hormone-specific beta-subunits. hCGbeta is unique among beta-subunits due to a carboxyl-terminal peptide (CTP) bearing four O-linked oligosaccharides. We previously reported that there were differences in O-glycosylation between two chimeras consisting of alpha-subunit and CTP, i.e. a variant with CTP at the N-terminal region (Calpha) and another analog with CTP at the C-terminus (alphaC) of the alpha-subunit. To address whether O-glycosylation is influenced by the heterodimer formation, Calpha and alphaC were expressed alone or with FSHbeta-subunit in Chinese hamster ovary cells. The O-linked glycosylation was assessed by continuous labeling with [(35)S]methionine/cysteine, immunoprecipitation with anti-alpha or anti-FSH serum, serial digestion with endoglycosidase-F and neuraminidase, and sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The decrease in molecular weight of dimeric chimeras digested with endoglycosidase-F was greater in Calpha than that in alphaC after treatment with neuraminidase, revealing that both chimeras have different numbers of sialic acids on O-linked carbohydrates. By treating with endoglycosidase-F, the dimeric alphaC migrated faster than its free form, whereas the mobility difference between assembled and unassembled forms of Calpha was very little. These data indicate that processing of O-glycosylation is affected by the backbone polypeptide chain(s).

Neuronal-specific synthesis and glycosylation of tenascin-R

Tenascin-R (TN-R) is a member of the tenascin family of multidomain matrix glycoproteins that is expressed exclusively in the central nervous system by oligodendrocytes and small neurons during postnatal development and in the adult. TN-R contributes to the regulation of axon extension and regeneration, neurite formation and synaptogenesis, and neuronal growth and migration. TN-R can be modified with three distinct sulfated oligosaccharide structures: HNK-1 (SO(4)-3-GlcUAbeta1,3Galbeta1,4GlcNAc), GalNAc-4-SO(4), and chondroitin sulfate. We have determined that TN-R expressed in dendrite-rich regions of the rat cerebellum, hippocampus, and cerebral cortex is one of the major matrix glycoproteins that bears N-linked carbohydrates terminating with beta1,4-linked GalNAc-4-SO(4). The syntheses of these unique sulfated structures on TN-R are differentially regulated. Levels of HNK-1 on TN-R rise and fall in parallel to the levels of TN-R during postnatal development of the cerebellum. In contrast, levels of GalNAc-4-SO(4) are regulated independently from those of TN-R, rising late in cerebellar development and continuing into adulthood. As a result, the pattern of TN-R modification with distinct sulfated carbohydrate structures changes dramatically over the course of postnatal cerebellar development in the rat. Because TN-R interacts with a number of different matrix components and, depending on the circumstances, can either activate or inhibit neurite outgrowth, the highly regulated addition of these unique sulfated structures may modulate the adhesive properties of TN-R over the course of development and during synapse maintenance. In addition, the 160-kDa form of TN-R is particularly enriched for terminal GalNAc-4-SO(4) later in development and in the adult, suggesting additional levels of regulation.

Impact of carbohydrate heterogeneity in function of follicle-stimulating hormone: studies derived from in vitro and in vivo models

Carbohydrates attached to the protein core of glycoprotein hormones influence a number of intracellular and extracellular processes. As with other members of the glycoprotein hormone family, FSH is produced and released as an array of isoforms that differ from each other in the structure of their oligosaccharide attachments. In this review, we discuss how carbohydrate heterogeneity can impact on FSH action in different in vitro and in vivo systems. We present evidence for diverse effects of distinct charge isoforms at the target cell level, including differential and unique effects on various end responses, and discuss how the use of multiple cell-type assays has allowed identification of some specific effects of FSH isoforms on different cell populations and follicle compartments as well as oocyte maturation. Finally, we discuss recent information on the ability of naturally occurring and laboratory manufactured FSH isoforms to evoke particular effects on granulosa cell function and ovarian follicular maturation in vivo. Such studies have provided evidence that the type(s) of FSH signal delivered may in fact regulate distinct biological outcomes irrespective or in addition to outcomes dictated solely by clearance rate differences.

Prognostic significance of reduced expression of beta-N-acetylgalactosaminylated N-linked oligosaccharides in human breast cancer

Our previous studies showed that expression of the GalNAcbeta1-->4GlcNAc group on N-linked oligosaccharides is associated with functional differentiation of the bovine mammary gland. In the present study, the occurrence of the GalNAcbeta1-->4GlcNAc group was established in human milk proteins and membrane glycoproteins from a human breast cancer cell line, MRK-nu-1, by structural analysis of oligosaccharides released by hydrazinolysis. Whether the expression level of the disaccharide group is affected upon malignant transformation was examined in human breast cancer specimens using Wistaria floribunda agglutinin (WFA) which interacts with oligosaccharides with N-acetylgalactosamine at their termini. Lectin blot analysis of membrane glycoprotein samples from human breast cancer specimens showed that the number of protein bands reacting with WFA, as well as their intensities, are lower in samples from primary carcinoma lesions compared with samples from surrounding normal tissues. No lectin binding was observed when the blots were treated with jack bean beta-N-acetylhexosaminidase or N-glycanase, indicating that WFA-reactive oligosaccharides are N-linked. A histochemical study of tissue specimens from 92 patients with breast cancer revealed that the reduced WFA staining levels in primary carcinoma lesions correlate with advancing clinical stages and prognostic status (i.e., 58% of patients in a group showing reduced/negative staining died of disease recurrence, whereas more than 90% of those in the positive staining group survived for 5 years after surgery). These results indicate that reduced expression of beta-N-acetylgalactosaminylated N-linked oligosaccharides on primary carcinoma lesions predicts a poor prognosis for patients with breast cancer.

Biochemical and functional aspects of gonadotrophin-releasing hormone and gonadotrophins

Reproductive function in mammals is governed by the hypothalamic-pituitary-gonadal axis, which conforms a functional unit. Sexual maturation and the subsequent development of reproductive competence depend on the precise and coordinated function of this axis. The components of the reproductive axis communicate each other through endocrine signals. The hypothalamus synthesizes gonadotrophin-releasing hormone or GnRH, which in turn stimulates synthesis and secretion of the pituitary gonadotrophins, follicle-stimulating hormone (FSH) and luteinizing hormone (LH). The ovarian follicles and the interstitial and Sertoli cells of the testis are the targets for these pituitary signals. Under gonadotrophic stimulation, the gonads produce and secrete several steroid and non-steroid (polypeptide) factors, which in turn regulate in different ways the function of the hypothalamic-pituitary axis. An episodic and pulsatile mode of secretion of hormonal signals characterize (as in other endocrine systems) the function of the reproductive axis, particularly that of the hypothalamic-pituitary unit. The target cell response, and consequently the harmonic function of the corresponding gland, will depend on the adequate dynamics of this pulsatile secretion. The function of each component of the reproductive axis is strongly influenced by locally-produced signals acting either in a paracrine or autocrine manner; these particular signals represent fine-tuning regulation systems that eventually amplify or restrain the magnitude of response to a particular endocrine signal, providing additional mechanisms for tissue homeostasis and a better functional plasticity of the target gland. The design and rational use of novel therapeutic strategies for an optimal exogenously-controlled reproductive function largely depend on the detailed knowledge of the hypothalamic-pituitary-gonadal axis function and the structure and mechanism of action of those factors and signals involved in its regulation.

Targeted mutations in beta1,4-galactosyltransferase I reveal its multiple cellular functions

Beta1,4-galactosyltransferase I (GalT I) is one of the most extensively studied glycosyltransferases. It is localized in the trans-Golgi compartment of most eukaryotic cells, where it participates in the elongation of oligosaccharide chains on glycoproteins and glycolipids. GalT I has also been reported in non-Golgi locations, most notably the cell surface, where it has been suggested to function non-biosynthetically as a receptor for extracellular glycoside substrates. Cloning of the GalT I cDNAs revealed that the gene encodes two similar proteins that differ only in the length of their cytoplasmic domains. Whether these different GalT I proteins, or isoforms, have similar or different biological roles is a matter of active investigation. The functions of the GalT I proteins have been addressed by targeted mutations that eliminate either both GalT I isoforms or just the long GalT I isoform. Eliminating both GalT I proteins abolishes most, but not all, GalT activity, an observation that led to the realization that other GalT family members must exist. The loss of both GalT I isoforms leads to neonatal lethality due to a wide range of phenotypic abnormalities that are most likely the result of decreased galactosylation. When the long isoform of GalT I is eliminated, galactosylation proceeds grossly normal via the short GalT I isoform, but specific defects in cell interactions occur that are thought to depend upon a non-biosynthetic function of the long GalT I isoform.

Expression and characterization of functional recombinant bovine follicle-stimulating hormone (boFSHalpha/beta) produced in the milk of transgenic rabbits

Bovine follicle-stimulating hormone (boFSH) is a heterodimeric glycoprotein that belongs to the pituitary gonadotropins. Bioactive FSH is composed of alpha and beta subunits which require extensive N-glycosylation and sialylation. The mammary gland of transgenic livestock is an attractive source for the synthesis of post-translationally modified proteins. Two mammary gland-specific gene constructs with the cDNA for the boFSH alpha (boFSHalpha) and beta (boFSHbeta) subunits controlled by bovine alpha-s1 casein regulatory sequences were co-microinjected into fertilized rabbit oocytes. Two FSHalpha/FSHbeta double transgenic rabbit lines were established. The transgene expression was strictly lactation and mammary gland specific. Protein analysis revealed the presence of the boFSH heterodimer in the milk of transgenic rabbits showing a molecular weight similar to that of purified pituitary gland derived boFSH (boFSH-P). Subunit specific antibodies detected both polypeptides with the expected molecular sizes. Biochemical characterization demonstrated the expected isoelectric points of the recombinant boFSH. The presence of the post-translationally added terminal sialic acid residues was indicated by wheat germ agglutinin (WGA) lectin Western blotting. The biological activity of the recombinant mammary gland produced boFSH was determined using a FSH-dependent reporter cell line. The bioactivity of the recombinant boFSH was comparable to that of purified boFSH-P.

Physicochemical methods for predicting the biological potency of recombinant follicle stimulating hormone: an international collaborative study of isoelectric focusing and capillary zone electrophoresis

Two methods for predicting the specific in vivo bioactivity of recombinant follicle stimulating hormone (FSH), based on quantitative measures of isoform distribution by isoelectric focusing (IEF)(1) and by capillary zone electrophoresis (CZE)(2) respectively, have been subjected to an international collaborative study by six laboratories from six countries. Both methods were used to estimate the predicted bioactivities of four preparations of follitropin beta, coded FSH A-D, differing widely in their isoform compositions and specific bioactivities. The mean predicted estimate of potency by IEF and CZE for each FSH preparation by each laboratory was within 80-125% of its potency estimated by bioassay, except for the mean estimates by CZE of that for FSH A by one laboratory and of that of FSH D by another. Each of the six laboratories using the IEF method, and each of the five laboratories using the CZE method were able to rank these FSHs according to their bioactivities, namely FSH B>FSH C>FSH A>FSH D. All laboratories were able to use both IEF and CZE to discriminate between FSH A and C, with bioactivities within 76-132% of one another. Four of six laboratories were able to use IEF, and two of five laboratories were able to use CZE, to discriminate between FSH B and C, with bioactivities within 89-112% of one another. This suggests that the accuracy and precision of both these methods should be sufficient to discriminate between FSHs which would meet or fail European Pharmacopoeia requirements for this type of hormone, since these stipulate that estimates of potency should fall between 80-125% of its stated potency. Using in most cases duplicate estimates in two independent assays, and excluding Laboratory 4, the pooled intra-laboratory geometric coefficient of variation (GCV) was about 4% for both IEF and CZE, and the inter-laboratory GCV was about 7% for IEF and about 10% for CZE. The use of one FSH preparation as a standard, with its specific activity as an assigned value, reduced the inter-laboratory variability of estimates for the remaining FSHs by both methods. This increased the accuracy of the predicted estimates of bioactivity for these remaining FSHs in terms of their approximation to the values for their bioactivities estimated by bioassay. These data therefore suggest that both these methods, and particularly IEF, are sufficiently accurate, precise and robust to be used for predicting the bioactivity of batches of follitropin beta, and especially if used with a standard preparation.

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.

Thyroid-stimulating hormone and thyroid-stimulating hormone receptor structure-function relationships

This review focuses on recent advances in the structure-function relationships of thyroid-stimulating hormone (TSH) and its receptor. TSH is a member of the glycoprotein hormone family constituting a subset of the cystine-knot growth factor superfamily. TSH is produced by the pituitary thyrotrophs and released to the circulation in a pulsatile manner. It stimulates thyroid functions using specific membrane TSH receptor (TSHR) that belongs to the superfamily of G protein-coupled receptors (GPCRs). New insights into the structure-function relationships of TSH permitted better understanding of the role of specific protein and carbohydrate domains in the synthesis, bioactivity, and clearance of this hormone. Recent progress in studies on TSHR as well as studies on the other GPCRs provided new clues regarding the molecular mechanisms of receptor activation. Such advances are a result of extensive site-directed mutagenesis, peptide and antibody approaches, detailed sequence analyses, and molecular modeling as well as studies on naturally occurring gain- and loss-of-function mutations. This review integrates expanding information on TSH and TSHR structure-function relationships and summarizes current concepts on ligand-dependent and -independent TSHR activation. Special emphasis has been placed on TSH domains involved in receptor recognition, constitutive activity of TSHR, new insights into the evolution of TSH bioactivity, and the development of high-affinity TSH analogs. Such structural, physiological, pathophysiological, evolutionary, and therapeutic implications of TSH-TSHR structure-function studies are frequently discussed in relation to concomitant progress made in studies on gonadotropins and their receptors.

Assessment of the in vitro and in vivo biological activities of the human follicle-stimulating isohormones

Gonadotropins are synthesized and released in different molecular forms. In this article, we present evidence that the glycosylation variants of human pituitary FSH exhibit differential and divergent effects at the target cell level and that less sialylated, short-lived variants may exert significant effects in in vivo conditions. Less acidic/sialylated glycoforms (elution pH value 6.60-4.60 as disclosed by high resolution chromatofocusing of anterior glycoprotein extracts), induced higher cAMP release, estrogen production and tissue-type plasminogen activator (tPA) enzyme activity as well as cytochrome P450 aromatase and tPA mRNA expression in cultured rat granulosa cells than the more acidic analogs (pH<4.76). By contrast, the more acidic/sialylated glycoforms induced higher alpha-inhibin subunit mRNA expression than their less acidic counterparts. In cumulus enclosed oocytes isolated from mice ovaries, addition of less acidic isoforms induced resumption of meiosis more efficiently than the more acidic analogs. Interestingly, the least acidic isoform (pH>7.10) behave as a strong antagonist of several FSH-mediated effects. Assessment of the in vivo effects of the isoforms on granulosa cell proliferation in follicles from immature rats, revealed that short-lived isoforms were equally or even more efficient than their more acidic counterparts in maintaining granulosa cell proliferation when administered immediately after hypophysectomy. These results show that the naturally occurring human FSH isoforms may exhibit differential or even unique effects at the target cell level and that factors other than the metabolic clearance rate of the molecule (including receptor-binding affinity and capability of the ligand to activate its receptor and trigger intracellular signaling) also play an important role in determining the net in vivo effects of a particular FSH variant.

Regulation of lutropin circulatory half-life by the mannose/N-acetylgalactosamine-4-SO4 receptor is critical for implantation in vivo

Lutropin (LH) directs ovulation and implantation by regulating the production of estrogen and progesterone. We have shown that the circulatory half-life of LH is controlled by the Man/GalNAc-4-SO4 receptor, which binds GalNAc-4-SO4 on LH oligosaccharides. The short half-life in conjunction with episodic release of LH from the pituitary accounts for the pulsatile rise and fall in circulating LH. Complete genetic ablation of the Man/GalNAc-4-SO4 receptor results in death in utero. Heterozygous female mice clear LH from the circulation more slowly and have smaller litters due to a reduction in the rate of implantation. This reduction is fully correctable by exogenous progesterone and estrogen, indicating that the rate of LH clearance is critical for the production of sufficient progesterone and estrogen for implantation. Thus, the Man/GalNAc-4-SO4 receptor regulates the endocrinological status of the female and is essential for an early event in embryonic development.

Sugar profiling proves that human serum erythropoietin differs from recombinant human erythropoietin

Erythropoietin (EPO) from sera obtained from anemic patients was successfully isolated using magnetic beads coated with a human EPO (hEPO)-specific antibody. Human serum EPO emerged as a broad band after sodium dodecyl sulfate-polyacrylamide gel electrophoresis, with an apparent molecular weight slightly smaller than that of recombinant hEPO (rhEPO). The bandwidth corresponded with microheterogeneity because of extensive glycosylation. Two-dimensional gel electrophoresis revealing several different glycoforms confirmed the heterogeneity of circulating hEPO. The immobilized anti-hEPO antibody was capable of binding a representative selection of rhEPO glycoforms. This was shown by comparing normal-phase high-performance liquid chromatography profiles of oligosaccharides released from rhEPO with oligosaccharides released from rhEPO after isolation with hEPO-specific magnetic beads. Charge analysis demonstrated that human serum EPO contained only mono-, di-, and tri-acidic oligosaccharides and lacked the tetra-acidic structures present in the glycans from rhEPO. Determination of charge state after treatment of human serum EPO with Arthrobacter ureafaciens sialidase showed that the acidity of the oligosaccharide structures was caused by sialic acids. The sugar profiles of human serum EPO, describing both neutral and charged sugar, appeared significantly different from the profiles of rhEPO. The detection of glycan structural discrepancies between human serum EPO and rhEPO by sugar profiling may be significant for diagnosing pathologic conditions, maintaining pharmaceutical quality control, and establishing a direct method to detect the misuse of rhEPO in sports.

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.

Characterization of human FSH isoforms reveals a nonglycosylated beta-subunit in addition to the conventional glycosylated beta-subunit

Human FSH consists of a mixture of isoforms that can be separated on the basis of differences in negative charge conferred by variations in the numbers of sialic acid residues that terminate oligosaccharide branches. Western analysis of human FSH isoforms separated by chromatofocusing revealed the presence of two human FSHbeta isoforms that differed in size. A low mol wt human FSHbeta isoform was associated with all FSH isoform fractions. A high mol wt human FSHbeta isoform was associated with the more acidic fractions and increased in relative abundance as the pI decreased. Characterization of representative human FSHbeta isoforms by mass spectrometry and automated Edman degradation revealed a low mol wt isoform that was not glycosylated. A high mol wt isoform was N-glycosylated at Asn residues 7 and 24. These results indicate that pituitary human FSH consists of two classes of molecules: those that possess a nonglycosylated beta-subunit and those that possess a glycosylated beta-subunit. Glycoprotein hormones are known to be elliptical molecules, and the beta-subunit oligosaccharides project outward from the short diameter, thereby increasing it. It is interesting to speculate that this change in shape might affect ultrafiltration rates, leading to differences in delivery rates to target tissues and elimination by filtration in the kidney.

Secretion of biologically active glycoforms of bovine follicle stimulating hormone in plants

We chose the follicle stimulating hormone (FSH), a pituitary heterodimeric glycoprotein hormone, as a model to assess the ability of the plant cell to express a recombinant protein that requires extensive N-glycosylation for subunit folding and assembly, intracellular trafficking, signal transduction and circulatory stability. A tobacco mosaic virus (TMV) based transient expression system was used to express a single-chain (sc) version of bovine FSH in the tobacco related species Nicotiana benthamiana. Preparations of periplasmic proteins from plants infected with recombinant viral RNA contained high levels of sc-bFSH, up to 3% of total soluble proteins. Consistently, in situ indirect immunofluorescence revealed that the plant cell secreted the mammalian secretory protein to the extracellular compartment (EC). By mass spectrometric analysis of immunoaffinity purified sc-bFSH derived from EC fractions, we found two species of the plant paucimannosidic glycan type, truncated forms of complex-type N-glycans. Stimulation of cAMP production in a CHO cell line expressing the porcine FSH receptor acknowledged the native-like structure of sc-bFSH and a sufficient extent of N-glycosylation required for signal transduction. Furthermore, in superovulatory treatments of mice, sc-bFSH displayed significant in vivo bioactivity, although much lower than that of pregnant mare serum gonadotropin. We conclude that plants may have a broad utility as hosts for the recombinant expression of proteins even where glycosylation is essential for function.

Is there any physiological role for gonadotrophin oligosaccharide heterogeneity in humans? II. A biochemical point of view

Heterogeneity of gonadotrophin oligosaccharides caused either by pharmacological intervention or which occurs naturally during normal physiological changes is well documented. Recent advances in structure determination of oligosaccharides has to some extent led to a better appreciation of how oligosaccharide heterogeneity may affect protein folding, stability, measurement and modulation of receptor binding. Here it is discussed how carbohydrate structure can impact upon gonadotrophin structure and function. It is well documented that oligosaccharides can serve as a cognate site for protein binding. One functional aspect of gonadotrophin glycosylation heterogeneity is the modulation of receptor binding affinity, yielding partially agonistic glycoforms. Carbohydrate heterogeneity is problematic for a clinical chemist if immunochemical assays are sensitive to heterogeneity. However, even measurements made without such interference may not accurately reflect the biological activity that is a collective result of all isoforms in the circulation, and perhaps of the genotype of each individual. Moreover, oligosaccharide heterogeneity may affect heterodimer stability, therefore, biological activity and immunochemical activity, not to mention clearance. It seems reasonable to conclude that from a biochemical point of view, oligosaccharide heterogeneity is of considerable importance. However, accurate measurement of isoforms in blood, and appropriate in-vitro bioassays that are insensitive to matrix effects are needed to define the physiological significance of each glycoform, and thereby better define target therapeutics and interpret diagnostic results.

Is there any physiological role for gonadotrophin oligosaccharide heterogeneity in humans? I. Gondatrophins are synthesized and released in multiple molecular forms. A matter of fact

Carbohydrates attached to the protein core of all glycoprotein hormones play an essential role in the function of the molecule, influencing a number of intracellular and extracellular processes. As with other members of the glycoprotein hormone family, pituitary gonadotrophins are not produced as single or unique molecules but rather as arrays of isoforms that differ from each other mainly in the structure of their oligosaccharide attachments. In both experimental animals and in humans, the abundance of the different isoforms varies depending on the endocrine status of the donor present at the time of collection of the tissue or sample. Conditions characterized by an oestrogen-enriched hormonal milieu (eg. the preovulatory phase of the menstrual cycle) promote the formation and secretion of variants with relatively low sialic acid and/or sulphate content, whereas physiological deficiency of this sex steroid (as in the postmenopause) favours the production of highly sialylated, long-lived gonadotrophin variants. When tested individually, less sialylated isoforms exhibit higher receptor-binding and in-vitro biological activity but shorter plasma half-life than their more sialylated counterparts. Both the hormonal regulation and the functional differences among the naturally occurring isoforms strongly suggest that gonadotrophin heterogeneity represents a distinctly different mechanism through which the pituitary gland may regulate the intensity and duration of the gonadotrophic stimulus. Nevertheless, whereas the existence of the alternatively glycosylated variants of gonadotrophins in both the pituitary and in serum is currently without doubt, the physiological role of this phenomenon is still a controversial issue and a matter of debate.

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.