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In 1977, the Nobel Prize in Â鶹ÒùÔºiology 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 is a critical target for treating reproductive disorders such as infertility and prostate cancer. Leuprolide, a peptide analog of GnRH targeting GnRHR, has dominated the market for therapies against prostate and breast cancers. However, the receptor's low expression levels and instability have long hindered efforts to resolve its structure, leaving questions about how GnRH activates GnRHR unanswered.

In a study in the Proceedings of the National Academy of Sciences, 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, and analyzed nearly 10 approved drugs targeting this receptor, shedding light on the conserved recognition of GnRH and the molecular basis of receptor activation.

The selected two species are Xenopus laevis (African clawed frog) and Sus scrofa (a pig whose GnRHR closely resembles that of humans), each in complex with endogenous mammalian GnRH and Gq protein. The researchers determined cryo-EM structures at 2.67 Ã… and 3.18 Ã… resolution for the frog and pig GnRHR-Gq complexes, respectively.

GnRH was found to adopt a unique, conserved inverted "U-shaped" conformation, inserting both its N- and C-termini deep into the receptor's orthosteric pocket. These termini interacted with key residues in transmembrane regions and extracellular loops. Comparative sequence analysis highlighted the high conservation of residues surrounding the binding pockets, including K^3.32, Y^6.51, and Y^6.52 that stabilize GnRH through hydrogen bonds and π-π interactions.

Functional experiments confirmed that mutations in these residues markedly impaired GnRH signaling. Besides, it was found that GnRH binding triggered conformational changes: the receptor's N-terminus flips outward, and TM6 shifts to form the Gq binding interface.

By comparing active structures with inactive GnRHR bound to antagonists, the researchers identified key molecular switches driving receptor activation. They also revealed the structure-activity relationships (SAR) by modeling and structural analyses of nine marketed GnRH analogs such as triptorelin and nafarelin.

Importantly, the researchers found that the substitution of glycine at position 6 with D-amino acids (e.g., D-Trp) promoted a type II' β-turn, pre-organizing the peptide for receptor binding and enhancing activity, and this substitution also reduced the impact of mutations in pocket residues, offering clues for designing more robust peptide drugs.

The findings of this study complete the structural map of key receptors along the hypothalamic-pituitary-gonadal axis, providing templates for rational drug design targeting reproductive disorders and related cancers.