Follicle-stimulating hormone macroheterogeneity and its effect on receptor conformation

Abstract

Follicle-stimulating hormone (FSH) is a glycoprotein hormone that plays a significant role in gonadal development and fertility. FSH is a heterodimeric glycoprotein composed of a common $\beta$-subunit non-covalently associated with a hormone-specific FSH$\beta$-subunit. Three major glycoforms of FSH have been found in humans based on Asn-glycosylation of the FSH$\beta$ subunits. FSH24 possesses all four potential glycans with a 24kDa-FSH$\beta$ subunit. Two, more active, hypo-glycosylated FSH variants possess either singly glycosylated 21kDa- or 18kDa-FSH$\beta$ subunits that lack either $N^{-24}$ or $N^7$ glycans, respectively, thereby possessing only three glycans. In females, hypo-glycosylated FSH abundance has been shown to decrease with age; we hypothesized that the same trend occurred in males. FSH was immunopurified from human male pituitaries along a spectrum of ages. FSH extracted from these pituitaries was purified by immunoaffinity chromatography and heterodimer separated from free subunit by gel filtration. Glycoform abundance was determined by automated Western blot. We found that males show no significant decrease in hypo-glycosylated FSH with age, as the proportion of FSH21, with one exception, was approximately 70% for all ages evaluated. Recent studies have revealed that FSH glycoforms affect follicle-stimulating hormone receptor (FSHR) conformation and function, but the specific structural differences have not yet been elucidated in the full receptor. We hypothesize that glycoforms induce different conformational changes in the receptor, leading to biased signal output. Chinese Hamster Ovarian (CHO) cells expressing FSHR were used for purification with a membrane-spanning protein. Purified FSHR will be evaluated for suitability in structural studies using cryogenic electron microscopy.