Structure of follicle-stimulating hormone in complex with the entire ectodomain of its receptor

Abstract

FSH, a glycoprotein hormone, and the FSH receptor (FSHR), a G protein-coupled receptor, play central roles in human reproduction. We report the crystal structure of FSH in complex with the entire extracellular domain of FSHR (FSHR ED ), including the enigmatic hinge region that is responsible for signal specificity. Surprisingly, the hinge region does not form a separate structural unit as widely anticipated but is part of the integral structure of FSHR ED . In addition to the known hormone-binding site, FSHR ED provides interaction sites with the hormone: a sulfotyrosine (sTyr) site in the hinge region consistent with previous studies and a potential exosite resulting from putative receptor trimerization. Our structure, in comparison to others, suggests FSHR interacts with its ligand in two steps: ligand recruitment followed by sTyr recognition. FSH first binds to the high-affinity hormone-binding subdomain of FSHR and reshapes the ligand conformation to form a sTyr-binding pocket. FSHR then inserts its sTyr (i.e., sulfated Tyr335) into the FSH nascent pocket, eventually leading to receptor activation.F SH is a gonadotropin that stimulates steroidogenesis and gametogenesis in the gonads. Secreted by the anterior pituitary gland, FSH regulates the menstrual cycle and ovarian follicular maturation in women and supports sperm production in men. FSH acts by binding to the FSH receptor (FSHR) on the granulosa cell surface in ovaries and the Sertoli cell surface in testes. The stimulated receptor leads to the dissociation of α-and βγ-subunits of G protein heterotrimer inside the cell. The α-subunit activates adenylyl cyclase, resulting in an increase of cAMP levels, and ultimately leads to the increased steroid production that is necessary for follicular growth and ovulation in women. The free βγ dimers recruit G protein-coupled receptor (GPCR) kinases to the receptor, which, in turn, lead to the recruitment of β-arrestin to the receptor (1). FSH is used clinically for controlled ovarian stimulation in women treated with assisted reproductive technologies and also for the treatment of anovulatory infertility in women and hypogonadotropic hypogonadism in men. The central role of FSH in human reproduction makes its receptor a unique pharmaceutical target in the field of fertility regulation (2-4).The glycoprotein hormone (GPH) family has four members: FSH; two other pituitary hormones, luteinizing hormone and thyroid-stimulating hormone (TSH); and one placental hormone, chorionic gonadotropin. The four members are homologous in sequence, structure, and function. Each member is a heterodimer composed of a common α-subunit and a hormone-specific β-subunit. The crystal structures of FSH and human CG (hCG) revealed that both α-and β-subunits adopt similar folds of cystineknot architecture (5-7). The assembled α-and β-heterodimers bind to their respective receptors with high affinity and hormone specificity, resulting in similar signaling pathways but distinct biological responses.

Epitope-specific monoclonal antibodies to FSHβ increase bone mass

“…For comparative modeling, we used the crystal structure of the human FSH in complex with the entire ectodomain of the human FSHR as the template (PDB id 4AY9) (14). Sequence of mouse Fshr was obtained from UniProt (id Q9QWV8) and homology construction was achieved using Modeller (34), with stereochemical parameters being checked using PROCHECK (35) and PROSA (36).…”

“…The crystal structure of the human FSHR-FSH complex indicates that the FSHβ subunit binds to specific amino acids in the FSHR binding groove (14). By modeling the Fshr-Fsh interaction in mouse, we developed a polyclonal antibody to a 13-amino acid sequence in the binding domain of Fshβ (12,15,16).…”

Mutations and polymorphisms in FSH receptor: functional implications in human reproduction

“…The ICD is predominantly coupled to a Gs protein that is responsible for initiating a cascade of intracellular events leading to specific biological effects of the ligand. The X-ray crystal structures of FSH complexed with the ECD of FSHR (Fan & Hendrickson 2005, Jiang et al 2012 have immensely contributed to understanding the interaction of FSH with its receptor at the molecular level. However, it does not provide information on ligandinduced activation of the receptor leading to steroidogenesis.…”

Assessment of the immunogenicity of gonadotrophins during controlled ovarian stimulation

“…The alpha subunits of LH, FSH, TSH and hCG are almost identical and contain around 96 amino acids, while the beta subunits are specific to each hormone. FSH has a beta subunit of 111 amino acids (FSH β), which confers its specific biologic action 14. LH and hCG have beta subunits of 120 and 145 amino acids, respectively, that confer its specific biologic action Due to the presence of a common chain and the homology between FSH, LH and hCG, the putative antibodies directed against one hormone could cross‐react with the other hormones.…”