Vladimir Y. Butnev

All-or-none N-glycosylation in primate follicle-stimulating hormone β-subunits

Abstract Human FSH exists as two major glycoforms designated, tetra-glycosylated and di-glycosylated hFSH. The former possesses both α- and β-subunit carbohydrates while the latter possesses only α-subunit carbohydrate. Western blotting differentiated the glycosylated, 24,000 M r hFSHβ band from the non-glycosylated 21,000 M r FSHβ band. Postmenopausal urinary hFSH preparations possessed 75–95% 24,000 M r hFSHβ, while pituitary hFSH immunopurified from 21- to 43-year-old females and 21–43-year-old males possessed only 35–40% 24,000 M r hFSHβ. The pituitary hFSH from a postmenopausal woman on estrogen replacement was 75% 21,000 M r hFSHβ. Other immunopurified postmenopausal pituitary hFSH preparations possessed 50–60% 21,000 M r hFSHβ. Gel filtration removed predominantly 21,000 M r free hFSHβ and reduced its abundance to 13–22% in postmenopausal pituitary hFSH heterodimer preparations. A major regulatory mechanism for FSH glycosylation involves control of β-subunit N -glycosylation, possibly by inhibition of oligosaccharyl transferase. Two primate species exhibited the same all-or-none pattern of pituitary FSHβ glycosylation.

Partially Deglycosylated Equine LH Preferentially Activates β-Arrestin-Dependent Signaling at the Follicle-Stimulating Hormone Receptor

Deglycosylated FSH is known to trigger poor Galphas coupling while efficiently binding its receptor. In the present study, we tested the possibility that a deglycosylated equine LH (eLHdg) might be able to selectively activate beta-arrestin-dependent signaling. We compared native eLH to an eLH derivative [i.e. truncated eLHbeta (Delta121-149) combined with asparagine56-deglycosylated eLHalpha (eLHdg)] previously reported as an antagonist of cAMP accumulation at the FSH receptor (FSH-R). We confirmed that, when used in conjunction with FSH, eLHdg acted as an antagonist for cAMP accumulation in HEK-293 cells stably expressing the FSH-R. Furthermore, when used alone at concentrations up to 1 nM, eLHdg had no detectable agonistic activity on cAMP accumulation, protein kinase A activity or cAMP-responsive element-dependent transcriptional activity. At higher concentrations, however, a weak agonistic action was observed with eLHdg, whereas eLH led to robust responses whatever the concentration. Both eLH and eLHdg triggered receptor internalization and led to beta-arrestin recruitment. Both eLH and eLHdg triggered ERK and ribosomal protein (rp) S6 phosphorylation at 1 nM. The depletion of endogenous beta-arrestins had only a partial effect on eLH-induced ERK and rpS6 phosphorylation. In contrast, ERK and rpS6 phosphorylation was completely abolished at all time points in beta-arrestin-depleted cells. Together, these results show that eLHdg has the ability to preferentially activate beta-arrestin-dependent signaling at the FSH-R. This finding provides a new conceptual and experimental framework to revisit the physiological meaning of gonadotropin structural heterogeneity. Importantly, it also opens a field of possibilities for the development of selective modulators of gonadotropin receptors.

Hypo-glycosylated human follicle-stimulating hormone (hFSH21/18) is much more active in vitro than fully-glycosylated hFSH (hFSH24)

Hypo-glycosylated hFSH(21/18) (possesses FSHβ(21) and FSHβ(18)bands) was isolated from hLH preparations by immunoaffinity chromatography followed by gel filtration. Fully-glycosylated hFSH(24) was prepared by combining the fully-glycosylated FSHβ(24) variant with hCGα and isolating the heterodimer. The hFSH(21/18) glycoform preparation was significantly smaller than the hFSH(24) preparation and possessed 60% oligomannose glycans, which is unusual for hFSH. Hypo-glycosylated hFSH(21/18) was 9- to 26-fold more active than fully-glycosylated hFSH(24) in FSH radioligand assays. Significantly greater binding of (125)I-hFSH(21/18) tracer than hFSH(24) tracer was observed in all competitive binding assays. In addition, higher binding of hFSH(21/18) was noted in association and saturation binding assays, in which twice as much hFSH(21/18) was bound as hFSH(24). This suggests that more ligand binding sites are available to hFSH(21/18) in FSHR than to hFSH(24). Hypo-glycosylated hFSH(21/18) also bound rat FSHRs more rapidly, exhibiting almost no lag in binding, whereas hFSH(24) specific binding proceeded very slowly for almost the first hour of incubation.

Identification of Twelve O-Glycosylation Sites in Equine Chorionic Gonadotropin β and Equine Luteinizing Hormone β by Solid-Phase Edman Degradation

Abstract The O-glycosylation sites for equine LHβ (eLHβ) and eCGβ were identified by solid-phase Edman degradation of four glycopeptides derived from the C-terminal region. Both subunits were O-glycosylated at the same 12 positions, rather than the 4–6 sites anticipated. These sites were partially glycosylated, with carbohydrate attachment ranging from 20% to 100% for eCGβ and from 10% to 100% for eLHβ. When the C-terminal peptide containing all but one of the O-linked oligosaccharides was removed by mild acid hydrolysis of either eLHβ or eCGβ, hybrid hormones could be obtained by reassociating eLHα,eFSHα, or eCGα with the truncated β subunit derivatives. These hybrid hormones were identical in LH receptor-binding activity when des(121-149)eLHβ or des(121-149)eCGβ were combined with the same α subunit preparation. Thus, O-glycosylation appears to be responsible for the β subunit contribution to the substantial difference in LH receptor-binding activity between eLH and eCG. Comparison of the equid LH/CGβ sequences with those available for the primate CGβ subunits indicated a greater conservation of glycosylation patterns in the former.

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.

Production, purification, and characterization of recombinant hFSH glycoforms for functional studies

Previously, our laboratory demonstrated the existence of a β-subunit glycosylation-deficient human FSH glycoform, hFSH(21). A third variant, hFSH(18), has recently been detected in FSH glycoforms isolated from purified pituitary hLH preparations. Human FSH(21) abundance in individual female pituitaries progressively decreased with increasing age. Hypo-glycosylated glycoform preparations are significantly more active than fully-glycosylated hFSH preparations. The purpose of this study was to produce, purify and chemically characterize both glycoform variants expressed by a mammalian cell line. Recombinant hFSH was expressed in a stable GH3 cell line and isolated from serum-free cell culture medium by sequential, hydrophobic and immunoaffinity chromatography. FSH glycoform fractions were separated by Superdex 75 gel-filtration. Western blot analysis revealed the presence of both hFSH(18) and hFSH(21) glycoforms in the low molecular weight fraction, however, their electrophoretic mobilities differed from those associated with the corresponding pituitary hFSH variants. Edman degradation of FSH(21/18)-derived β-subunit before and after peptide-N-glycanase F digestion confirmed that it possessed a mixture of both mono-glycosylated FSHβ subunits, as both Asn(7) and Asn(24) were partially glycosylated. FSH receptor-binding assays confirmed our previous observations that hFSH(21/18) exhibits greater receptor-binding affinity and occupies more FSH binding sites when compared to fully-glycosylated hFSH(24). Thus, the age-related reduction in hypo-glycosylated hFSH significantly reduces circulating levels of FSH biological activity that may further compromise reproductive function. Taken together, the ability to express and isolate recombinant hFSH glycoforms opens the way to study functional differences between them both in vivo and in vitro.

Comparison of Follicle-Stimulating Hormone Glycosylation Microheterogenity by Quantitative Negative Mode Nano-Electrospray Mass Spectrometry of Peptide-N-Glycanase-Released Oligosaccharides

Glycans from six highly purified hFSH preparations were released by peptide-N-glycanase digestion and analyzed by negative mode nano-ESI mass spectrometry before and after neuraminidase digestion. Pituitary glycan structures were mainly high-mannose, di-, tri-, and tetra-antennary, and their abundance largely paralleled that reported by other investigators using different approaches. For most of the FSH preparations, the differences in glycosylation appeared to be restricted to relative abundances of the major glycan families, as defined by their neutral core oligosaccharide structures. Qualitative differences between glycan populations were largely relegated to those species that were lowest in abundance. Significant qualitative differences were noted in two cases. Recombinant GH3-hFSH triantennary glycans appeared to have the third antenna exclusively on the mannose6-branch, in contrast to all pituitary and urinary hFSH triantennary glycans, in which this antenna was exclusively attached to the mannose3-branch. The hypo-glycosylated hFSH preparation isolated from purified hLH was decorated with high mannose glycans that accounted for over 40% of the total in this population. As this preparation was found to be consistently 20-fold more active than hFSH24 in FSH receptor-binding assays, it appears that both macroheterogeneity and microheterogeneity in FSH preparations need to be taken into account.

Macro- and Micro-heterogeneity in Pituitary and Urinary Follicle-Stimulating Hormone Glycosylation

FSH glycosylation macroheterogeneity in pituitary and urinary hFSH samples was evaluated by Western blotting. Microheterogeneity in two highly purified urinary and pituitary hFSH preparations was evaluated by nano-electrospray mass spectrometry of peptide-N-glycanase-released oligosaccharides. An age-related loss of hypo-glycosylated hFSH in individual female pituitaries was indicated by progressively reduced abundance of hFSH21 relative to hFSH24. Urinary hFSH was evaluated as a potentially non-invasive indicator of glycoform abundance, as the only way for pituitary FSH to reach the urine is through the blood. Both highly purified and crude postmenopausal urinary hFSH preparations possessed the same amount of hFSH21 as postmenopausal pituitary gland FSH. Considerable microheterogeneity was encountered in both pituitary and urinary hFSH glycan populations, as 84 pituitary hFSH glycan ions were observed as compared with 68 urinary hFSH glycans. The biggest quantitative differences between the two populations were reduced abundance of bisecting GlcNAc-containing and fucosylated glycans, along with sulfated glycans in the urinary hFSH glycan population. The relative abundance of sialic acid and glycan antenna did not rationalize the retarded electrophoretic mobilities of the urinary hFSHβ21- and α-subunit bands relative to the corresponding pituitary hFSH bands, as the most abundant glycans in the former possessed only 2 more branches and the same sialic content as in the latter. Site-specific glycosylation information will probably be necessary.

A human FSHB transgene encoding the double N-glycosylation mutant (Asn7Δ Asn24Δ) FSHβ subunit fails to rescue Fshb null mice

Follicle-stimulating hormone (FSH) is a gonadotrope-derived heterodimeric glycoprotein. Both the common α- and hormone-specific β subunits contain Asn-linked N-glycan chains. Recently, macroheterogeneous FSH glycoforms consisting of β-subunits that differ in N-glycan number were identified in pituitaries of several species and subsequently the recombinant human FSH glycoforms biochemically characterized. Although chemical modification and in vitro site-directed mutagenesis studies defined the roles of N-glycans on gonadotropin subunits, in vivo functional analyses in a whole-animal setting are lacking. Here, we have generated transgenic mice with gonadotrope-specific expression of either an HFSHB(WT) transgene that encodes human FSHβ WT subunit or an HFSHB(dgc) transgene that encodes a human FSHβ(Asn7Δ 24Δ) double N-glycosylation site mutant subunit, and separately introduced these transgenes onto Fshb null background using a genetic rescue strategy. We demonstrate that the human FSHβ(Asn7Δ 24Δ) double N-glycosylation site mutant subunit, unlike human FSHβ WT subunit, inefficiently combines with the mouse α-subunit in pituitaries of Fshb null mice. FSH dimer containing this mutant FSHβ subunit is inefficiently secreted with very low levels detectable in serum. Fshb null male mice expressing HFSHB(dgc) transgene are fertile and exhibit testis tubule size and sperm number similar to those of Fshb null mice. Fshb null female mice expressing the mutant, but not WT human FSHβ subunit-containing FSH dimer are infertile, demonstrate no evidence of estrus cycles, and many of the FSH-responsive genes remain suppressed in their ovaries. Thus, HFSHB(dgc) unlike HFSHB(WT) transgene does not rescue Fshb null mice. Our genetic approach provides direct in vivo evidence that N-linked glycans on FSHβ subunit are essential for its efficient assembly with the α-subunit to form FSH heterodimer in pituitary. Our studies also reveal that N-glycans on FSHβ subunit are essential for FSH secretion and FSH in vivo bioactivity to regulate gonadal growth and physiology.

Inositol phosphate stimulation by LH requires the entire α Asn56 oligosaccharide

Abstract Lentil lectin-bound, fucose-enriched hTSH was reported to stimulate both cAMP and inositol phosphate (IP) intracellular signalling pathways, whereas fucose-depleted hTSH stimulated only the cAMP pathway. Gonadotropins activate the cAMP pathway and in several studies higher concentrations activate the IP pathway. Since only the 10% of α subunit Asn 56 oligosaccharides (Asn 52 in humans) are fucosylated, the higher glycoprotein hormone concentrations required for IP pathway activation might be related to the abundance of competent hormone isoforms. Lentil lectin-fractionated equine (e)LHα and eFSHα preparations were combined with a truncated, des(121–149)eLHβ preparation. All four hybrid hormone preparations induced IP accumulation in porcine theca cells, suggesting that activation of the IP pathway was not dependent on fucosylation at α subunit Asn 56 . However, the presence of Asn 56 carbohydrate was necessary for increased IP accumulation. Intact, rather than Asn 56 -deglycosylated eLH preparations provoked a biphasic steroidogenic response by rat testis Leydig cells, suggesting that Gα i stimulation was also sensitive to loss of Asn 56 carbohydrate. While rat granulosa cells responded to human FSH preparations in a biphasic manner, a classical sigmoidal response was obtained to eFSH and Asn 56 -deglycosylated eFSH, suggesting that the equine preparations did not activate Gα i . Purified oLHα Asn 56 oligosaccharides inhibited FSH-stimulated steroidogenesis in rat granulosa cell cultures indicating a direct role for carbohydrate in FSH action. The same carbohydrate preparation inhibited hCG-stimulated fluorescence energy transfer suggesting oligosaccharide involvement in activated LH receptor self-association.