APLNR
Apelin Receptor (APLNR) is a class A G protein-coupled receptor (GPCR) activated by the endogenous hormone apelin. It plays a crucial role in regulating the cardiovascular system, making it a promising therapeutic target for cardiovascular diseases, which remain the leading cause of mortality worldwide.
Even if many existing drugs are successfully used for treating heart failure, such as β-blockers and ACE inhibitors, none of these are able to enhance heart contractility. Addressing this unmet therapeutic need would actively improve heart function. The native ligand of APLNR, apelin, has shown cardioprotective effects, making APLNR-targeting agonist drugs attractive for novel medications.
However, developing such drugs has been challenging due to APLNR’s dual signaling bias. Apelin binding triggers both G-protein and β-arrestin downstream pathways, with opposing effects; while the G-protein pathway is associated with cardioprotective benefits, β-arrestin activation contributes to harmful cardiac hypertrophy. This signaling duality led to agonists that despite their intended benefits, still activate the β-arrestin pathway at low but significant levels, posing a risk of potential serious adverse effects.
A recent study by Wang et al. used cryo-EM structural analysis to design G-protein-biased peptide agonists that eliminate β-arrestin activation while preserving cardioprotective effects with promising results.
Apelin receptor in pharma and biotech pipelines
Pharmaceutical companies are actively exploring the Apelin receptor as a target for cardiovascular, metabolic, and pulmonary disease. Although BioAge Labs had high hopes for the APLNR agonist, Azelaprag (BGE-105), it was recently pushed back to preclinical trials due to high liver toxicity. Annapurna Therapeutics is currently evaluating ANPA-0073, a Phase 2 ready biased APLNR agonist, and finally, Apie Therapeutics is developing the small molecule drug APT-101 for the treatment of Pulmonary Fibrosis.
Key structural insights
Below is the video and written summary of key structural insights that led to the development of two synthetic peptides with enhanced cardioprotective effects.
A structural comparison of APLNR bound to three agonists (the native peptide apelin, its analog MM07; and the small molecule CMF-019), combined with mutagenesis analyses and functional assays, revealed that “twin hotspots” control APLNR’s signaling bias. Residues M11 and F13 in both apelin and MM07 peptides penetrate deeply into APLNR’s aromatic hydrophobic pocket. Where M11 interacts with the receptor’s residues T295 and Y299 on TM7 (the first “hotspot”), favoring G-protein activation, the residue F13, engaging with F110 on TM3 and M183 on ECL2 (the second “hotspot”), promotes β-arrestin recruitment. In addition to twin hotspots regulation, another important contribution to signaling bias is given by a hydrogen bond between N46 (TM1) and D75 (TM2), which stabilizes the receptor in a conformation favoring the β-arrestin pathway.
Based on these insights, researchers developed two synthetic peptides, WN353 and WN561, designed to eliminate β-arrestin activation while retaining G-protein signaling.
WN353 (PDB: 8XZJ, 3.0 Å): Replaces M11 with a bulky naphthyl group, stabilizing G-protein signaling.
WN561 (PDB: 8XZF, 3.0 Å): Replaces M11 with D-methionine and adds glycine, fully suppressing β-arrestin activation.
Cryo-EM structures of APLNR bound to these synthetic peptides revealed that the ligands adopt the anticipated binding poses, effectively engaging the receptor’s key residues to selectively activate G-protein signaling whilst avoiding β-arrestin recruitment.
Functional assays confirmed that WN353 and WN561 promote cardioprotective effects without causing cardiac hypertrophy, with WN561 demonstrating superior efficacy in reducing hypertrophic markers and greater stability in human plasma and cell culture.
Compared to previous APLNR agonists, both WN353 and WN561 seem to present a safer, more effective strategy and pave the way for next-generation APLNR agonists with improved safety and efficacy.
Signaling Bias
The preferential activation of one intracellular signaling pathway over another when a receptor (typically a G-protein coupled receptor) is stimulated by a ligand. Biased agonists are designed to selectively enhance one pathway, thus offering a more targeted approach.
Reference
Wang, W. W., Ji, S. Y., Zhang, W., Zhang, J., Cai, C., Hu, R., Zang, S. K., Miao, L., Xu, H., Chen, L. N., Yang, Z., Guo, J., Qin, J., Shen, D. D., Liang, P., Zhang, Y., & Zhang, Y. (2024). Structure-based design of non-hypertrophic apelin receptor modulator. Cell, 187(6), 1460–1475.e20. https://doi.org/10.1016/j.cell.2024.02.004
Kim, A. H., Jang, J. E., & Han, J. (2022). Current status on the therapeutic strategies for heart failure and diabetic cardiomyopathy. Biomedicine & Pharmacotherapy, 145, 112463. https://doi.org/10.1016/j.biopha.2021.112463
Zhou, Y., Zhang, Y., Zhao, D., Yu, X., Shen, X., Zhou, Y., Wang, S., Qiu, Y., Chen, Y., & Zhu, F. (2024). TTD: Therapeutic Target Database describing target druggability information. Nucleic Acids Research, 52(D1), D1465–D1477. https://doi.org/10.1093/nar/gkad751