Angiotensin II cyclic analogs as tools to investigate AT 1 R biased signaling mechanisms.

G protein coupled receptors (GPCRs) produce pleiotropic effects by their capacity to engage numerous signaling pathways once activated. Functional selectivity (also called biased signaling), where specific compounds can bring GPCRs to adopt conformations that enable selective receptor coupling to distinct signaling pathways, continues to be significantly investigated. However, an important but often overlooked aspect of functional selectivity is the capability of ligands such as angiotensin II (AngII) to adopt specific conformations that may preferentially bind to selective GPCRs structures. Understanding both receptor and ligand conformation is of the utmost importance for the design of new drugs targeting GPCRs. In this study, we examined the properties of AngII cyclic analogs to impart biased agonism on the angiotensin type 1 receptor (AT1 R). Positions 3 and 5 of AngII were substituted for cysteine and homocysteine residues ([Sar1 Hcy3,5 ]AngII, [Sar1 Cys3 Hcy5 ]AngII and [Sar1 Cys3,5 ]AngII) and the resulting analogs were evaluated for their capacity to activate the Gq/11, G12, Gi2, Gi3, Gz, ERK and β-arrestin (βarr) signaling pathways via AT1 R. Interestingly, [Sar1 Hcy3,5 ]AngII exhibited potency and full efficacy on all pathways tested with the exception of the Gq pathway. Molecular dynamic simulations showed that the energy barrier associated with the insertion of residue Phe8 of AngII within the hydrophobic core of AT1 R, associated with Gq/11 activation, is increased with [Sar1 Hcy3,5 ]AngII. These results suggest that constraining the movements of molecular determinants within a given ligand by introducing cyclic structures may lead to the generation of novel ligands providing more efficient biased agonism.