In 1977, the Nobel Prize in Physiology or Medicine was awarded to Roger Guillemin and Andrew Schally for their discovery and synthesis of gonadotropin-releasing hormone (GnRH), a key regulator of reproductive function. Today, the GnRH receptor (GnRHR) remains at the forefront of biomedical research.
GnRHR plays a central role in the treatment of reproductive disorders such as infertility and hormone-dependent cancers such as prostate and breast cancer. Leuprolide, a peptide analogue of GnRH that targets GnRHR, has become the leading therapy against these two cancers. However, efforts to determine the receptor's structure have been hampered for decades by its low expression levels and inherent instability, leaving unresolved questions about how GnRH activates GnRHR.
In a study published in PNAS , a team led by DUAN Jia and XU Huaqiang (H. Eric XU) from the Shanghai Institute of Materia Medica of the Chinese Academy of Sciences reported high-resolution cryo-electron microscopy (cryo-EM) structures of GnRHR from two species. The researchers also analyzed nearly ten approved drugs targeting this receptor, shedding light on the conserved recognition of GnRH and the molecular mechanisms underlying receptor activation.
The two species involved in this study are Xenopus laevis (African clawed frog) and Sus scrofa (domestic and wild pig, whose GnRHR closely resembles that of humans). In both cases, the receptor was studied in complex with endogenous mammalian GnRH and the Gq protein. The researchers determined cryo-EM structures for the frog and pig GnRHR–Gq complexes at resolutions of 2.67 Å and 3.18 Å, respectively.
GnRH was found to adopt a unique and conserved inverted "U-shaped" conformation, inserting both its N- and C-termini deep into the receptor's orthosteric pocket. These termini interact with key residues located in the transmembrane regions and extracellular loops of the receptor. Comparative sequence analysis revealed a high degree of conservation among residues lining the binding pocket, including K3.32, Y6.51, and Y6.52, which stabilize GnRH through hydrogen bonding and π–π interactions.
Functional experiments confirmed that mutations in these residues markedly impaired GnRH signaling. In addition, GnRH binding was found to induce conformational changes, including outward movement of the receptor's N-terminus and a shift in transmembrane helix 6 (TM6) to form the Gq protein binding interface.
By comparing active structures with inactive GnRHR bound to antagonists, the researchers identified key molecular switches driving receptor activation. They also elucidated structure–activity relationships (SAR) through modeling and structural analyses of nine clinically approved GnRH analogues, including triptorelin and nafarelin.
Importantly, the researchers found that substituting glycine at position 6 with D-amino acids (e.g., D-Trp) promoted a type II' β-turn, which pre-organized the peptide for receptor binding and enhanced its activity. This modification also reduced the impact of mutations in pocket residues, providing valuable insights for designing more robust peptide drugs.
This study completes the structural map of key receptors along the hypothalamic–pituitary–gonadal axis, providing templates for the rational design of drugs targeting reproductive disorders and related cancers.
