Cryo-EM Reveals How Two Approved GH-Boosting Drugs Lock Into Their Target
High-resolution cryo-EM structures of GHSR bound to Macimorelin and Anamorelin expose the molecular blueprint for designing superior growth hormone therapies.
Summary
Researchers used cryo-electron microscopy to capture atomic-level snapshots of the growth hormone secretagogue receptor (GHSR) bound to two approved drugs: Macimorelin, used to diagnose adult growth hormone deficiency, and Anamorelin, approved in Japan for cancer cachexia. Both drugs docked into a split binding pocket defined by a conserved salt bridge between two receptor amino acids. The team resolved structures at 2.63 Å and 2.52 Å resolution, then used targeted mutations to pinpoint why Anamorelin binds more tightly than Macimorelin. The findings clarify how GHSR selects among G protein partners and provide a structural foundation for engineering next-generation agonists with greater potency and selectivity.
Detailed Summary
The growth hormone secretagogue receptor (GHSR) is a Class A G protein-coupled receptor that serves as the primary target for ghrelin, the so-called 'hunger hormone.' Beyond regulating appetite, GHSR governs growth hormone (GH) release, energy homeostasis, and muscle preservation — processes directly relevant to aging, sarcopenia, and cancer cachexia. Two synthetic GHSR agonists have reached clinical approval: Macimorelin, used diagnostically to provoke GH release in adults suspected of GH deficiency, and Anamorelin, approved in Japan to counteract muscle wasting in cancer patients. Despite their clinical utility, the precise atomic interactions governing how each drug engages GHSR remained incompletely understood, limiting rational drug design efforts.
To fill this gap, Wang, Sun, Liu, Guo, and colleagues at the Shanghai Institute of Materia Medica and partner institutions employed single-particle cryo-electron microscopy (cryo-EM) to determine structures of GHSR in complex with Gq protein, bound separately to Macimorelin and Anamorelin. The Macimorelin-GHSR-Gq complex was resolved at 2.63 Å and the Anamorelin-GHSR-Gq complex at 2.52 Å — resolutions sufficient to resolve individual side-chain conformations and water-mediated interactions with high confidence. These high-resolution maps allowed the team to build detailed atomic models of each drug nestled within the receptor's orthosteric binding site.
A central finding was that both drugs occupy a bifurcated binding pocket divided by a conserved intramolecular salt bridge between glutamate 124 (E124³·³³) and arginine 283 (R283⁶·⁵⁵). This structural 'divider' creates two sub-pockets that each drug engages differently, explaining why structurally similar molecules can have meaningfully different affinities and pharmacological profiles. Systematic alanine-scanning mutagenesis combined with functional assays (cAMP accumulation and calcium mobilization) identified the specific residues responsible for Anamorelin's higher binding affinity relative to Macimorelin. Key interaction differences centered on hydrophobic contacts and hydrogen bonding patterns in the deeper sub-pocket, consistent with Anamorelin's superior clinical potency data.
The study also compared GHSR structures across different G protein subtypes (Gq versus others documented in prior literature), illuminating the structural determinants of G protein selectivity. Subtle conformational differences in the receptor's intracellular face — particularly in transmembrane helices 5 and 6 and the third intracellular loop — appear to choreograph preferential coupling to Gq over Gi or Gs. This selectivity is pharmacologically significant because different G protein pathways mediate distinct physiological outcomes, including GH secretion versus metabolic regulation.
For clinicians and drug developers, these structures offer a precise template for structure-based design of next-generation GHSR agonists. The atomic detail reveals which molecular features drive potency and which could be modified to tune receptor-G protein selectivity, potentially separating the GH-releasing effect from appetite stimulation or metabolic side effects. This is particularly relevant for applications in age-related GH decline, sarcopenia, frailty, and cancer cachexia. A key caveat is that the study is entirely structural and biochemical — no in vivo efficacy or safety data are reported, and translation to clinical improvements remains to be demonstrated.
Key Findings
- Cryo-EM structures of GHSR–Gq complexes resolved at 2.63 Å (Macimorelin) and 2.52 Å (Anamorelin), among the highest resolutions reported for this receptor class
- Both drugs bind a bifurcated orthosteric pocket divided by a conserved E124³·³³–R283⁶·⁵⁵ salt bridge, a previously underappreciated structural feature
- Systematic mutagenesis identified specific residues explaining Anamorelin's higher binding affinity versus Macimorelin, with differences localized to hydrophobic and hydrogen-bonding contacts in the deeper sub-pocket
- Structural comparison across G protein subtypes revealed transmembrane helix 5/6 and intracellular loop 3 conformations as determinants of Gq-preferential coupling
- Functional assays (cAMP and calcium mobilization) validated mutagenesis findings, confirming that identified residues are necessary for full agonist activity
- Results provide a structural blueprint that could guide design of GHSR agonists with separated GH-releasing versus appetite-stimulating pharmacology
Methodology
The team expressed human GHSR with Gq protein heterotrimer in insect cells, purified the complexes, and performed single-particle cryo-EM data collection and processing to achieve 2.63 Å and 2.52 Å resolution maps. Atomic models were built into the density maps using established refinement protocols. Mutagenesis studies introduced alanine substitutions at candidate binding-pocket residues, and functional consequences were assessed via cAMP accumulation (Gs/Gq readout) and calcium mobilization assays in transfected cell lines, with concentration-response curves used to derive EC50 values for comparison between wild-type and mutant receptors.
Study Limitations
The study is purely structural and biochemical, with no in vivo animal or human data presented, so clinical translation of the structural insights remains speculative at this stage. The cryo-EM structures represent a single, ligand-bound, Gq-coupled state and may not capture the full conformational ensemble relevant to biased signaling or receptor internalization. The authors do not explicitly disclose conflicts of interest in the available text, though the work was conducted at a Chinese Academy of Sciences institution with government funding.
Enjoyed this summary?
Get the latest longevity research delivered to your inbox every week.
Enter your email to subscribe: