Follicle-stimulating Hormone Isoforms Research Articles

O-279 The type of gonadotrophin: Does it improve ART outcome?

Abstract Ovarian stimulation is a vital clinical component of an IVF cycle, with gonadotrophins being the main stimulation drugs. Follicle stimulating hormone (FSH), the key driver of follicle recruitment during an ovarian cycle is a complex glycosylated protein comprised of an α and β subunit with four glycosylation sites. Differences in glycosylation lead to heterogeneity with the FSH isoforms varying in their receptor binding and biological activity. The cyclic physiological variation of FSH isoforms and the FSH threshold/ window concept lead to the selection of a single dominant follicle during a menstrual cycle. Surpassing the FSH threshold and extension of the FSH window with ovarian stimulation using gonadotrophins salvages follicles with higher FSH sensitivity leading to multi-follicular development and retrieval of multiple oocytes. The first IVF birth in 1978 was from a natural cycle and was preceded by several unsuccessful attempts at oocyte retrieval. Ovarian stimulation was therefore incorporated into IVF treatment to overcome the inefficiencies encountered with a natural cycle. The objective of ovarian stimulation during IVF treatment is to obtain several mature oocytes, leading to good quality embryos for immediate replacement and cryopreservation for future use. An ideal stimulation wherein gonadotrophins are the mainstay is one that achieves maximal efficacy, safety and cost-effectiveness. Several gonadotrophin preparations have been available for clinical use over the years since the first use of pregnant mare serum gonadotrophin (PMSG) in 1940 and the first successful birth using human pituitary gonadotrophin (hPG) in 1961. These initial preparations were soon abandoned due to concerns with antigenicity and safety. Subsequent important milestones in the development gonadotrophins for clinical use include the urinary derived products, recombinant FSH preparations, long acting FSH and the biosimilars. Given the selection of gonadotrophins currently available, the apparent question is if they differ? Circulating FSH isoforms change during different stages of the menstrual cycle and through the reproductive stages. More acidic isoforms are predominant during the early follicular phase, with less acidic or basic isoforms predominant in the late follicular phase of the menstrual cycle, and more acidic isoforms during the menopausal stage. Similarly, the different FSH products vary in their properties owing to differences in their production and purification. Urinary derived products are more acidic compared to recombinant preparations and the different recombinant products vary in degree of acidity due to differences in the extent and pattern of glycosylation. The acidic and basic properties of gonadotrophin types influence metabolic stability, half-life and their biological activity. The question then remains if these differences in bioactivity affect clinical outcomes? Clinical effectiveness of the various gonadotrophins types should be addressed in the context of the different pituitary suppression regimens, ovarian response categories and patient variables given the implementation of individualised stimulation protocols. It is also imperative to clearly define successful clinical outcomes incorporating both efficacy and safety. Cumulative live birth rate (CLBR) following fresh and frozen embryo transfers resulting from one episode of ovarian stimulation should be adopted as the measure of the overall efficiency along with OHSS as a measure of immediate safety. There are several studies and systematic reviews comparing the different gonadotrophin types and products for various outcomes including surrogate endpoints, with inconsistent results attributed to differences in the inclusion/ exclusion criteria, heterogeneity across studies and varied quality of the primary studies. Furthermore, there have been sparse studies reporting on CLBR and there is a continuing debate on what would be considered as a relevant clinical difference in efficacy. In addition to efficacy and safety, the ultimate choice of gonadotrophin should be based on regional availability, patient acceptability and cost-effectiveness for a live birth.

Follicle-stimulating isohormones: regulation and biological significance

Follicle-stimulating hormone (FSH) is a key hormone in the regulation of follicular development. Although the existence of FSH heterogeneity is well established, the physiological significance of this pleomorphism remains unknown. Observed changes in circulating FSH heterogeneity during critical reproductive events such as puberty and reproductive cyclicity suggest that different combinations of FSH isoforms reach the target sites during different physiological states to influence a variety of biological end points such as cellular growth, development, steroidogenesis and protein synthesis. Considering that these FSH isoforms have different physicochemical properties and potential to bind not only their cognate receptors but also structurally related, non-FSH receptors with various affinities, the regulatory implications of FSH heterogeneity in modulating the various FSH-induced functions are enormous. However, assigning functional significance to FSH heterogeneity has been hampered because of (1) difficulties associated with procurement of highly purified, naturally occurring, circulating FSH isoforms; (2) absence of reference standards that contain the entire repertoire of FSH isoforms present in biological fluids; and (3) specificity issues inherent to the detection systems used. If particular FSH isoforms do possess selective biological functions, specific combinations of FSH isoforms could be generated to regulate fertility in farm animals and humans.

Метод капиллярного зонного электрофореза для анализа гликозилированного рекомбинантного фолликулостимулирующего гормона

Conditions for analyzing sialylated isoforms of recombinant follicle-stimulating hormone (FSH) by the method of capillary zone electrophoresis have been selected. Follicle-stimulating hormone is a glycosylated heterodimer with the large number of sialic acids that affect its basic biological and physicochemical properties. The optimum protein separation into 6 isoforms was achieved using as a background electrolyte of a solution with pH 4.3 containing 0.01 M trycin, 0.01 sodium chloride, 7 M urea and 2.5 mM 1,4-diaminobutane. follicle-stimulating hormone isoforms, capillary zone electrophoresis, sialic acids, follicle-stimulating hormone.

What is the Clinical Relevance of Follicle-Stimulating Hormone Isoforms in Fertility Treatment?

Follicle-stimulating hormone, both naturally synthesized and as commercial preparations, exists as different isoforms. Variation in the process of glycosylation, particularly in the number of terminal sialic-acid residues, gives rise to isoforms of varying acidic profiles with differences in half-life and bioactivity. Based on the known follicle-stimulating hormone isoform variation across the reproductive cycle, it is possible that the follicle-stimulating hormone isoform profile used in controlled ovarian stimulation may impact follicular recruitment and clinical treatment outcomes. In light of the uncertainty regarding the clinical relevance of follicle-stimulating hormone isoforms in fertility treatment, published studies exploring this topic are reviewed.

Occurrence of postmenopausal-like acidic follicle-stimulating hormone (FSH) isoforms precedes the rise of FSH before menopause

To assess the glycoform distribution patterns of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) during the menstrual cycle at different ages and FSH levels, after menopause, and with premature ovarian failure (POF). Controlled clinical study. Healthy volunteers in an academic research environment. Women aged 20 to 25 years with normal early follicular (EF) serum FSH (<10 IU/L), women aged 40 to 45 years with normal or increased EF serum FSH, postmenopausal women, and women with POF. None. FSH and LH glycoform distributions as assessed by chromatofocusing. In both postmenopausal and in women with POF, more acidic FSH glycoforms were found compared with young cyclic premenopausal women. In women aged 40 to 45 years with normal FSH levels, these acidic glycoform profiles already showed a statistically significant difference from the younger women. This difference was to attributable to the early follicular and luteal cycle phases. Overall, during aging and after ovarian failure, FSH becomes more acidic, a difference that is already statistically significantly detectable in premenopausal, older women before FSH rises. This is not seen for LH glycoforms. The occurrence of postmenopausal-like acidic FSH isoforms precedes the rise of FSH before menopause.

Follicle-stimulating hormone isoforms and plasma concentrations of estradiol and inhibin A in dairy cows with ovulatory and non-ovulatory follicles during the first postpartum follicle wave

Following parturition, all cows display a wave of ovarian follicular growth, but a large proportion fail to generate a preovulatory rise in estradiol, and hence fail to ovulate. Follicle-stimulating hormone (FSH) exists as multiple isoforms in the circulation depending on the type and extent of glycosylation, and this has pronounced effects on its biological properties. This study examined differences in plasma FSH, estradiol, and inhibin A concentrations, and the distribution of FSH isoforms in cows with ovulatory or atretic dominant follicles during the first postpartum follicle wave. Plasma FSH isoform distribution was examined in both groups during the period of final development of the dominant follicle by liquid phase isoelectric focusing. Cows with an ovulatory follicle had higher circulating estradiol and inhibin A concentrations, and lower plasma FSH concentrations. The distribution of FSH isoforms displayed a marked shift toward the less acidic isoforms in cows with ovulatory follicles. A higher proportion of the FSH isoforms had a p I > 5.0 in cows with ovulatory follicles compared to those with atretic follicles. In addition, cows with ovulatory follicles had greater dry matter intake, superior energy balance, elevated circulating concentrations of insulin and insulin-like growth factor-I, and lower plasma nonesterified fatty acids. The shift in FSH isoforms toward a greater abundance of the less acidic isoforms appears to be a key component in determining the capability for producing a preovulatory rise in estradiol, and this shift in FSH isoforms was associated with more favorable bioenergetic and metabolic status.

A Longitudinal study of the expression of Follicle Stimulating Hormone in Prepubertal Mice

Follicle‐stimulating hormone (FSH) is a glycoprotein secreted from the anterior pituitary. Like other glycoproteins, FSH exists as a mixture of isoforms that vary in the number and type of sugar groups. Observed changes in FSH isoforms during critical reproductive events, such as puberty onset, suggest that different combinations of FSH isoforms influence reproduction differently. It has also been suggested that all FSH isoforms may not equally bind to antibody based assays. In order to establish a working model, a study was performed using pre‐pubertal mice to determine at what age FSH mRNA expression begins. Gene expression was determined for the alpha, FSH beta, LH beta, and TSH beta subunits in five‐, ten‐, fifteen‐, twenty‐, thirty‐, and sixty‐day‐old mice. All subunits were expressed in all age groups indicating that FSH expression begins very early in development. However, despite the expression of mRNA, morphological studies do not reflect an active form of FSH is initiating gonad development, hormone production, or gametogenesis. Early expression of FSH may be critical to establish a hormonal milieu that leads to maturation of the gonads. Another possibility is that different isoforms of FSH determine its physiological significance during different metabolic states.

Chromatofocusing Fails To Separate hFSH Isoforms on the Basis of Glycan Structure

Follicle-stimulating hormone (FSH) glycosylation is regulated by feedback from the gonads, resulting in an array of glycans associated with FSH preparations derived from pools of pituitary or urine extracts. FSH glycosylation varies due to inhibition of FSHbeta N-glycosylation, elaboration of 1-4 branches possessed by mature N-glycans, and the number and linkage of terminal sialic acid residues. To characterize FSH glycosylation, FSH isoforms in pituitary gland extracts and a variety of physiological fluids are commonly separated by chromatofocusing. Variations in the ratios of immunological and biological activities in the resulting FSH isoform preparations are generally attributed to changes in glycosylation, which are most often defined in terms of sialic acid content. Using Western blotting to assess human FSHbeta glycosylation inhibition revealed 30-47% nonglycosylated hFSHbeta associated with four of six hFSH isoform preparations derived by chromatofocusing. Glycopeptide mass spectrometry assessment of glycan branching in these isoforms extensively characterized two N-glycosylation sites, one at alphaAsn52, the critical glycan for FSH function, and the other at betaAsn24. With two to four N-glycans per FSH molecule, many combinations of charges distributed over these sites can provide the same isoelectric point. Indeed, several glycans were common to all isoform fractions that were analyzed. There was no trend showing predominantly monoantennary glycans associated with the high-pI fractions, nor were predominantly tri- and tetra-antennary glycans associated with low-pI fractions. Thus, differences in receptor binding activity could not be associated with any specific glycan type or location in the hormone. FSH aggregation was associated with reduced receptor binding activity but did not affect immunological activity. However, as gel filtration indicated sufficient heterodimer was present in each isoform preparation to generate complete inhibition curves, the near total loss of receptor binding activity in several preparations could not be explained by aggregation alone, and the mechanism remains unknown.

Assessment of ovarian reserve by using the follicle-stimulating hormone isoform distribution pattern to predict the outcome of in vitro fertilization

Changes in FSH isoforms occur at temporally specific times during the menstrual cycle and reproductive lifespan. Changes in ratio in FSH isoforms may provide a sensitive test of ovarian reserve before IVF.

Follicle-stimulating hormone isoforms are not useful as pretreatment predictors of outcome in in vitro fertilization: a pilot study

The objective of this study was to determine the predictive value of FSH isoforms for the outcome of IVF treatment. Although this pilot study comprises only a small number of patients, we conclude that because no statistical differences could be found in the isoform-composition between poor and good responders, it is not likely that FSH isoforms predict treatment outcome after IVF.

Urofollitropin and Ovulation Induction

Anovulation is a common cause of female infertility. Treatment for women with anovulation is aimed at induction of ovulation. Ovulation induction with follicle-stimulating hormone (FSH) is indicated in women with WHO type II anovulation in whom treatment with clomifene citrate (clomifene) has failed. The majority of these women have polycystic ovary syndrome. The major disadvantages of ovulation induction with FSH are the risk of ovarian hyperstimulation syndrome and the risk of higher order multiple pregnancies. To reduce the rate of complications due to multiple follicular development, FSH should be administered using a chronic low-dose protocol with small dose increments. In women with WHO type I anovulation, an exogenous supply of luteinizing hormone (LH) is required to achieve an adequate follicular response to FSH treatment. Thus, ovulation induction with FSH is not the treatment of choice in these women. FSH is a hormone that stimulates follicle growth and oocyte maturation. Endogenous FSH is produced by the pituitary gland and exists as a family of isohormones exhibiting distinct oligosaccharide structures. FSH for exogenous administration is derived from urine or is produced as recombinant FSH. The commercially available FSH products all contain different mixtures of FSH isoforms. To determine the effectiveness of urofollitropin (urinary-derived FSH), a comparison with the other available gonadotropins was made (i.e. recombinant FSH and human menopausal gonadotropin). Urofollitropin and recombinant FSH appear to be equally effective and well tolerated for ovulation induction. Human menopausal gonadotropin is comparably effective to urofollitropin in terms of pregnancy outcomes. It remains unclear whether human menopausal gonadotropins have a higher risk of overstimulation and ovarian hyperstimulation syndrome compared to urofollitropin in women with polycystic ovary syndrome. In practice, recombinant products are more convenient to use but are also more expensive. Therefore, if availability is not an issue but costs are, there is still a place for the use of urofollitropins for ovulation induction.

Assessment of ovarian reserve using follicle stimulating hormone (FSH) isoforms to predict outcome with in vitro fertilization (IVF)

FSH has a critical role in folliculogenesis and is secreted in a variety of molecular forms. The glycosylation variants are influenced by hormonal milieu and are age and cycle sensitive. We hypothesized that FSH isoforms could reliably assess ovarian reserve and identify patients with a low likelihood of success with IVF prior to any observed elevations in serum concentrations of FSH or estradiol on cycle day 3. To study this hypothesis, we compared FSH isoform profiles and content in successful and failed IVF cycles after normal ovarian reserve testing.

Effect of Dihydrotestosterone on Pituitary Follicle-Stimulating Hormone Isoforms in Adult Male Rats Treated with a Gonadotropin-Releasing Hormone Antagonist

The effect of androgens on the oligosaccharide structure of follicle-stimulating hormone (FSH) isoforms was studied in adult male rats treated with a potent gonadotropin-releasing hormone (GnRH) antagonist. Animals were castrated (Cx), and 24 h later were treated with physiological doses of either testosterone propionate (T) or dihydrotestosterone propionate (DHT) (0.25 mg/rat in corn oil, daily, s.c.) for 7 days. The antiandrogen flutamide (F; 5 mg/day/rat, twice a day, s.c.) was administered to either Cx-T or Cx-DHT rats to block androgen action. The GnRH antagonist Org 30276 (Ant; 1 mg/kg/day, s.c.) was injected to both Cx- and Cx-DHT treated rats. FSH serum levels reached normal values in Cx rats treated with either T or DHT, whereas those treated with F retained Cx conditions. Both Cx-Ant and Cx-Ant-DHT treated animals presented normal serum FSH levels. Concanavalin A affinity chromatography was used to isolate pituitary FSH isoforms according to their carbohydrate inner structure. An increased proportion of FSH isoforms bearing complex-type oligosaccharides was observed in Cx rats treated with T or DHT, whereas the proportion of these isoforms was reduced in Cx and Cx-T-F or Cx-DHT-F rats. These results demonstrate that functional androgens are needed to complete the oligosaccharide processing of FSH. In addition, the proportion of the different pituitary FSH isoforms after DHT replacement was equivalent to that found when GnRH action was blocked, indicating that androgens are involved in the regulation of carbohydrate incorporation to the FSH molecule by acting mainly at the pituitary level, independently of GnRH action.

Is there a place for different isoforms of FSH in clinical medicine? IV. The clinician's point of view.

Follicle stimulating hormone (FSH) plays an important role in gametogenesis in both men and women. In men it is required for the establishment and probably maintenance of spermatogenesis. In women it stimulates the growth of ovarian follicles and fluctuations in its secretion are involved in the selection of the ovulatory follicle. It is now possible by recombinant DNA technology to synthesize a range of isoforms of FSH with differing biological action and half-life which would provide a greater degree of flexibility in the treatment of women with anovulatory infertility. Longer acting isoforms of FSH would find clinical application in the treatment of hypogonadotrophic men.

Effect of different FSH isoforms on cyclic-AMP production by mouse cumulus-oocyte-complexes: a time course study.

The ability of different isoforms of follicle stimulating hormone (FSH) to induce accumulation of cAMP in cultured mouse cumulus-oocyte-complexes (COC) was evaluated in a time course study. Using isoform fractions representing less acidic (pI 6.43-5.69), mid-acidic (pI 5.62-4.96) and acidic (pI 4.69-3.75) isoforms, the accumulation of cAMP was monitored after an exposure time of 0, 5, 10, 15, 30, 60, 120 and 180 min. In addition, cAMP production was monitored for 0, 5, 10, 15 and 30 min following a 5 min exposure to FSH isoform fractions. Based on FSH measurements using radioimmunoassays, the less and mid-acidic isoforms caused almost twice as much cAMP to be accumulated than the acidic isoform fraction, thereby confirming an enhanced biological activity of FSH isoforms with a isoelectric point (pI) of >5.0. For all isoform fractions, maximal accumulation of cAMP was achieved after 30 min of exposure, after which the production declined to background levels. After a 5 min exposure to isoform fractions, levels of cAMP were significantly higher in the less acidic isoform fractions, but after isoform removal, the decline in cAMP production to background levels followed a similar time course. The results demonstrate that FSH isoforms with a pI of >5.0 induced significant biological responses within a period of 30 min and that prolonged exposure caused attenuated signal transduction. The present results, set in the context of the pulsatile characteristics of FSH release from the pituitary and the reported half-life of less acidic isoforms of approximately 35 min, make it conceivable that isoforms with a pI >5.0 actually possess important physiological functions during the periovulatory period.

Androgen Regulation of Immunological and Biological Activities of Pituitary Follicle-Stimulating Hormone Isoforms inMale Rats

Follicle-stimulating hormone (FSH) is involved in the regulation and maintenance of gametogenesis. It exists in multiple molecular forms with different oligosaccharide structures which in turn are influenced by the hormonal milieu. Previous studies from our laboratory demonstrated that antiandrogen administration to immature male rats altered the biological activity and the distribution profile of pituitary FSH isoforms. The aim of this study was to examine possible modifications in pituitary FSH polymorphism throughout sexual development (10-, 32- and 75-day-old rats). In addition, the effect of androgen deprivation by castration (32-day-old rats) and its replacement with a nonaromatizable androgen – dihydrotestosterone – on pituitary FSH polymorphism was determined. Concanavalin A affinity chromatography was used to isolate groups of FSH isoforms according to their carbohydrate inner structure. Radioimmunoassay and Sertoli cell bioassay were used to evaluate FSH immuno- and bioactivities. Androgen rise in serum was accompanied by a marked increase in pituitary bio- and immuno-FSH content in 32- and 75-day-old rats. However, FSH pituitary content did not vary despite the significant increment observed in serum FSH levels after castration and decrease to control levels after androgen replacement. The distribution profile of immuno- and bioactive FSH changed throughout sexual maturation. The proportion of pituitary FSH isoforms bearing complex oligosaccharide structures (triantennary, bisecting, complete and truncated biantennary) increased with age, with a concomitant decrease in the proportion of isoforms bearing incomplete carbohydrate chains. The distribution profile observed in castrated 32-day-old rats was similar to that determined in 10-day-old animals. Androgen replacement restored the distribution profile to normal. These results suggest that androgens regulate the incorporation of sugar residues to the carbohydrate chains of pituitary FSH favoring the biosynthesis of complex-type oligosaccharide structures.

Biological characterization of recombinant human follicle stimulating hormone isoforms.

It has been established that follicle stimulating hormone (FSH) circulates in the bloodstream as a heterogeneous population of molecules. Individual FSH isoforms, while displaying identical amino acid sequences, differ in their extent of post-translational modification. As a result of these variations, the FSH isoforms exhibit differences in overall charge, degree of sialic acid or sulphate incorporation, receptor binding affinity and plasma half-life. Taking advantage of the fact that these forms can be separated from each other on the basis of their charge, we have evaluated in rats the metabolic clearance rates of the acidic [with an isoelectric point (pI) </=ISOdia</= 4.8] and the less acidic (pI > 4.8) isoforms of recombinant human FSH (rhFSH) obtained after chromatofocusing. The less acidic isoform group was found to have a faster clearance from the circulation in rats as compared with the acidic isoform group. This finding is in agreement with the lower bioactivity in vivo (as determined by the Steelman-Pohley assay) of the less acidic isoform group, compared with the acidic one. The mass spectra of the two groups of isoforms showed a difference in the sialic acid content thus highlighting the importance of these residues on the in-vivo activity of FSH. Conversely, when the two groups of isoforms were tested in vitro by using the Y1 human FSH receptor (Y1 hFSHR) assay and a reporter gene assay, no significant differences in the biological activities between these preparations were detected when test concentrations were based on mass.

Effects of acute exposure to PCBs 126 and 153 on anterior pituitary and thyroid hormones and FSH isoforms in adult Sprague Dawley male rats

3,3'4,4',5-Pentachlorobiphenyl (PCB 126) and 2,2',4,4',5,5'-hexachlorobiphenyl (PCB 153) were administered to adult male rats in order to identify sensitive indicators of endocrine disruption. We tested the hypothesis that PCB exposure modifies follicle-stimulating hormone (FSH) pituitary isoforms, as well as the pituitary and serum concentrations of FSH, luteinizing hormone (LH), growth hormone, prolactin, and thyroid-stimulating hormone (TSH). Effects on serum levels of thyroxine (T4) and testosterone (T), and prostate androgen receptor content, were also tested. In one experiment, 5 groups of 8 rats each received two i.p. injections, one day apart, of either corn oil or 6.25, 25, 100 or 400 micrograms/kg/day of PCB 126. Decreases (p < 0.05) in the serum concentrations of T4 and LH started at doses of 25 and 100 micrograms/kg/day, respectively. Serum FSH concentrations were reduced (p = 0.07) in the highest dose group. In contrast, pituitary content of FSH and LH increased with PCB-126 doses (p = 0.004, p = 0.002, respectively). Despite changes in reproductive hormones, PCB-126 had no effect on the androgen receptor content of the prostate. The effect of PCB-126 was tested in the hemicastrated rat, and suggested adverse effects on testosterone secretion. To test the effects of PCB exposure on FSH pituitary isoforms, 4 groups of 10 male rats received two i.p. injections, one day apart, of either corn oil, PCB 153 (25 mg/kg/day), estradiol-17 beta (E2; 20 micrograms/kg/day), or PCB 126 (0.1 mg/kg/day). Serum T4 levels were higher (p < 0.01) in the E2 and PCB 153 groups, and slightly reduced in the PCB 126-treated groups, compared to controls. Simultaneous purification of pituitary FSH and TSH isoforms was performed by HPLC, using two chromatofocusing columns in series. In contrast to TSH isoforms, the distribution of FSH isoforms over the chromatography run differed slightly between treatment groups; the amounts of FSH isoform eluted during the pH gradient were lower (p < 0.05) in E2 and PCB 153-treated rats than in control or PCB 126-treated rats. The similarity between the effects of E2 and PCB 153 on T4 and FSH isoforms supports the contention that PCB 153 possesses estrogenic properties. Serum LH and T4 concentrations were the most sensitive and practical endocrine indicators of PCBs 126 and 153 exposure in male rats.

Mutational analysis of the follicle-stimulating hormone (FSH) receptor in normal and infertile men: identification and characterization of two discrete FSH receptor isoforms.

In a search for pathophysiological causes of idiopathic male infertility we investigated the occurrence of mutations of the FSH receptor in 48 men with this disorder. The entire FSH receptor gene was analyzed by single stranded conformation polymorphism analysis (SSCP). A heterozygous point mutation without functional consequences, exchanging Val to Ala in codon 341, was found in one patient. SSCP analysis led to the identification of 2 polymorphisms in exon 10 associated in 2 discrete FSH receptor allelic variants, i.e. Thr307-Asn680 and Ala307-Ser680. The frequency and distribution of the two allelic variants was further analyzed in 86 proven fathers and 75 infertile men by SSCP (codon 307) and restriction fragment length polymorphism (codon 680). The 2 receptor isoforms showed similar Mendelian distribution in proven fathers and in infertile men. Serum FSH, inhibin B, and combined testicular volume did not differ between subjects with different receptor isoforms. Binding studies in transiently transfected COS-7 cells showed similar binding affinity for the two receptor variants. Moreover, the Ala307-Ser680 and the Thr307-Asn680 FSH receptors responded in vitro to FSH with comparable cAMP production. These data suggest that different isoforms of the FSH receptor with similar functional properties exist in normal and infertile men. We conclude that mutations of the FSH receptor or the FSH receptor genotype do not play a pathogenic role in male idiopathic infertility. The possibility that different FSH isoforms might interact differently with the 2 receptor variants remains to be investigated.

Receptor binding activity and in vitro biological activity of the human FSH charge isoforms as disclosed by heterologous and homologous assay systems: implications for the structure-function relationship of the FSH variants.

Follicle-stimulating hormone (FSH) is produced and secreted in multiple molecular forms. These isoforms differ in their oligosaccharide structures, which determine the particular behavior of a given variant in in vitro and in vivo systems. Employing heterologous cell assay systems, this and other laboratories have shown that highly sialylated human FSH variants exhibit lower receptor binding/immunoactivity as well as in vitro bioactivity/immunoactivity relationships than their less sialylated counterparts. It is not known, however, whether this characteristic behavior of the FSH isoforms is reproduced by homologous assay systems, in which unique variants of the receptor are presumptively expressed. To gain further insights into the structure-activity relationship of the various FSH isoforms, we analyzed the capacity of nine charge isoforms obtained after high-resolution chromatofocusing (pH window, 7.10 to <3.80) of anterior pituitary glycoprotein extracts to bind and activate their cognate receptor expressed by naturally occurring heterologous cell systems (rat granulosa cells and seminiferous tubule homogenates) as well as by human embryonic kidney-derived 293 (HEK-293) cells transfected with the human FSH (FSH-R) receptor cDNA. In both (heterologous and homologous) receptor assay systems, the isoforms displaced 125I-labeled FSH from the receptor in a dose-response manner; however, whereas in the heterologous systems, the receptor binding activity varied according to the elution pH value/sialic content of the isoforms, with the less acidic variants exhibiting higher receptor binding activity (r = 0.851 and 0.495 [p < 0.01 and p < 0.05] for the granulosa cell and testicular homogenate receptor assay systems, respectively) than the more acidic/sialylated analogs, in the homologous assay, this relationship was practically absent (r = 0.372, p N.S.). The capacity of the isoforms to induce androgen aromatization by rat granulosa cells followed the same trend shown by its corresponding receptor assay system (r = 0.864, p < 0.01). Interestingly and in contrast to the results observed in the homologous receptor binding assay, the ability of the isoforms to induce cAMP production by HEK-293 cells varied according to their elution pH value, with the more sialylated isoforms exhibiting lower potency than their less acidic counterparts (r = 0.852, p < 0.01). The results yielded by the heterologous assays suggest that the different potency of the isoforms to elicit a biological effect in a naturally occurring receptor system depends primarily on the particular affinity of the receptor molecule for each isoform. The existence of a clear dissociation between receptor binding and signal transduction in the homologous system indicate that this later function is rather related to the different ability of the FSH glycosylation variants to induce and/or stabilize distinct receptor conformations that may permit preferential or different degrees of activation/inhibition of a given signal transduction pathway. Thus, the human FSH receptor-transducer system apparently possesses sufficient versatility to respond in a different manner to glycosylation-dependent diverse FSH signals.