Naturally Occurring Follicle-Stimulating Hormone Glycosylation Variants

Abstract

Follicle-stimulating hormone (FSH) is a member of the glycoprotein hormone family, which is a subfamily of the cystine knot growth factor superfamily [1,2]. The glycoprotein hormones are composed of heterodimeric glycoprotein subunits, a common α-subunit, and a hormone-specific β-subunit. While the α-subunit primary structure is identical for all glycoprotein hormones within the same species, the oligosaccharide populations differ in a hormone-specific manner [3–6]. Characterizing the oligosaccharides released from an α-subunit preparation can identify the hormone from which the subunit was derived [7]. There are 3 to 4 β-subunits in vertebrates, which combine with α-subunit to create either FSH, luteinizing hormone (LH), thyroid-stimulating hormone (TSH), or in primates and equids, chorionic gonadotropin (CG) [8]. As both glycoprotein hormone subunits are cystine knot proteins [9–11] the protein backbone is folded into a series of three loops, two relatively rigid hairpin loops on one side of the knot, designated L1 and L3, and a single, flexible loop on the other side [12], designated L2. Oligosaccharides are attached to all 3 loops in a subunit-specific pattern (Figure 1). FSH subunits possess two potential N-glycosylation sites on each subunit and all four are of the Asn-Xaa-Thr type, which exhibit very efficient carbohydrate attachment [13]. Indeed, the α-subunit is always glycosylated at both sites in all known glycoprotein hormones. Because FSH α and β subunits co-migrate during electrophoresis, it is difficult to detect missing N-glycans in this hormone. FSHβ-specific Western blots have revealed partial glycosylation in equine FSHβ, human FSHβ (hFSH β), rhesus FSH β, and Japanese macaque FSHβ [14–16]. During the past few years, we have studied partially glycosylated hFSH isolated from pituitary extracts, postmenopausal urine, and conditioned tissue culture medium containing recombinant hFSH. Each glycosylation site in hFSH is decorated with a population of N-glycans. When total glycans are removed from reduced, carboxy-methylated FSH subunits, 39–130 glycans are found in mass spectra. We have data from only one glycosylation site, αAsn52, which possessed 29 neutral core ions, and when decorated with various patterns of sialic acid grew to 109 unique glycan structures. Micro heterogeneity can affect electrophoretic mobility, for example, placental hCGα with hybrid and biantennary glycans migrated faster than pituitary hFSHα, with triantennary, biantennary and tetraantennary glycans, which complicated sorting out the hFSH variants that resulted from loss of one or more N-glycans [17].