New Human Follitropin Preparations: How Glycan Structural Differences May Affect Biochemical and Biological Function and Clinical Effect

Abstract

It is well accepted that pituitary follitropin is secreted into the circulation as a mixture of variants, which differ not in primary structure but rather at the level of glycosylation. These glycosidic forms vary in the number of glycosylation sites filled, complexity of glycosidic chains, and sialylation and sulfation. It is generally agreed that high sialylation, 2,3 sialic acid capping of terminal N-acetyl galactosamine or galactose leads to longer circulating half-life, by blocking binding of asialoglycoprotein receptor (ASGPR) in the liver. In contrast, 2,6 sialic acid found in humans does not prevent recognition of galactose and N-acetyl galactosamine by ASGPR. Few studies on clinical outcomes comparing differences in sialylation of follitropin found in commercially available preparations are available. Thus, there is a clear need for a consortium of open data to address this unmet need. Recently, FSH glycosylation, primarily on the β-subunit, which varies as women age, has emerged as a key modifier of follitropin action, with profound biological effects in vivo in animal models. To date, limited information of recombinant follitropin hormone preparations is available. Thus, most of the studies with FSH that is well characterized biochemically have been done in vitro, with engineered non gonadal host cells bearing recombinant receptors or in animal models. Since limited studies in human granulosa cells are available, a question is whether structural differences in glycosylation in commercially available follitropin affects biological function and clinical effect in humans. The presence of fucose, for example, has not been studied greatly even though, in the case of antibody therapy it has been shown to have a large effect on antibody targeting. This review on glycosidic variability of follitropin from the biochemical/structural point of view reflects on this question and presents an assessment in the context of available published data. If clinical differences are to be expected or not, the readers will have a better understanding of the evidence for and limitations of such expectations.

Strategies for Glycoengineering Therapeutic Proteins

“…This hormone is produced in the anterior pituitary and travels through the circulation to gonodal cells where it interacts with FSH receptors (FSHRs) to promote follicle development in women and spermatogenesis in men ( Daya 2004 ; Davis et al, 2014 ; Ulloa-Aguirre et al, 2018 ). Therapeutically, recombinant FSH or follitropins can substitute for naturally-occurring FSH deficiencies to treat infertility ( Dias & Ulloa-Aguirre 2021 ).…”

“…Endogenous FSH consists of an alpha and beta subunit; both have two putative sites of N-glycosylation. The alpha subunit is consistently fully glycosylated with the beta subunit occupied with zero, one, or two N-glycans ( Davis et al, 2014 ; Ulloa-Aguirre et al, 2018 ; Dias & Ulloa-Aguirre 2021 ). The alpha subunit of FSH plays a pivotal role in receptor interactions by engaging the receptor-ligand interface ( Ulloa-Aguirre et al, 2018 ; Dias & Ulloa-Aguirre 2021 ).…”

“…The alpha subunit is consistently fully glycosylated with the beta subunit occupied with zero, one, or two N-glycans ( Davis et al, 2014 ; Ulloa-Aguirre et al, 2018 ; Dias & Ulloa-Aguirre 2021 ). The alpha subunit of FSH plays a pivotal role in receptor interactions by engaging the receptor-ligand interface ( Ulloa-Aguirre et al, 2018 ; Dias & Ulloa-Aguirre 2021 ). The importance of the glycosylation of the alpha subunit is illustrated by the deletion of one glycosite (at Asn78), which increases FSHR binding, while the removal of its other N-glycan (Asn52) decreases efficacy ( Ulloa-Aguirre et al, 2018 ; Dias & Ulloa-Aguirre 2021 ).…”

“…The alpha subunit of FSH plays a pivotal role in receptor interactions by engaging the receptor-ligand interface ( Ulloa-Aguirre et al, 2018 ; Dias & Ulloa-Aguirre 2021 ). The importance of the glycosylation of the alpha subunit is illustrated by the deletion of one glycosite (at Asn78), which increases FSHR binding, while the removal of its other N-glycan (Asn52) decreases efficacy ( Ulloa-Aguirre et al, 2018 ; Dias & Ulloa-Aguirre 2021 ). Similarly, removal of the glycosylation sites on the beta subunit of FSH yielded significantly greater bioactivity ( Dias & Ulloa-Aguirre 2021 ).…”

Interplay Between mTOR and Hippo Signaling in the Ovary: Clinical Choice Guidance Between Different Gonadotropin Preparations for Better IVF

“…FSH is a glycoprotein, and it was recently shown that the hypo-glycosylated forms might be more efficient in promoting follicular growth and supporting granulosa cell survival in vivo , possibly by increasing serum estradiol levels ( 22 ). Interestingly, young women express partially glycosylated FSH whereas postmenopausal women express mainly the fully glycosylated form ( 23 , 24 ), and this might influence both the biochemical properties and the efficacy of the various FSH preparations ( 25 ). This issue has been thoroughly discussed in a Delphi Consensus study recently ( 26 ).…”