Profiling novel pharmacology of receptor complexes using Receptor-HIT

Synthesis and Pharmacological Characterization of Fluorescent Ligands Targeting the Angiotensin II Receptors Derived from Agonists, β-Arrestin-Biased Agonists, and Antagonists

Angiotensin II (AngII) regulates cerebral circulation and binds with a similar affinity to AT1 and AT2 receptors. Biased AT1 agonists, such as TRV027, which are able to selectively activate β-arrestin while blocking the Gq pathway, appear promising as new therapeutics. New pharmacological tools are needed to further explore the impact of biased AT1 agonists on cells or tissues, such as the cerebral vessels. We designed and synthesized new fluorescent derivatives based on AngII, TRV027, or the AT1 antagonist losartan. We conducted pharmacological characterization to determine their selectivity, potency, and ability to activate or not specific AT1 transduction pathways in cells and cerebral arteries. We report the first highly AT1-selective fluorescent ligand, based on losartan, that retains its antagonist activity with high affinity. Fluorescent derivatives of TRV027 become AT2-selective and lose their AT1 β-arrestin bias. These new ligands can be applied to trace AT1 or AT2 receptors in vitro and ex vivo.

Novel Pharmacology Following Heteromerization of the Angiotensin II Type 2 Receptor and the Bradykinin Type 2 Receptor

The angiotensin type 2 (AT₂) receptor and the bradykinin type 2 (B₂) receptor are G protein-coupled receptors (GPCRs) that have major roles in the cardiovascular system. The two receptors are known to functionally interact at various levels, and there is some evidence that the observed crosstalk may occur as a result of heteromerization. We investigated evidence for heteromerization of the AT₂ receptor and the B₂ receptor in HEK293FT cells using various bioluminescence resonance energy transfer (BRET)-proximity based assays, including the Receptor Heteromer Investigation Technology (Receptor-HIT) and the NanoBRET ligand-binding assay. The Receptor-HIT assay showed that Gα_q, GRK2 and β-arrestin2 recruitment proximal to AT₂ receptors only occurred upon B₂ receptor coexpression and activation, all of which is indicative of AT₂-B₂ receptor heteromerization. Additionally, we also observed specific coupling of the B₂ receptor with the Gα_z protein, and this was found only in cells coexpressing both receptors and stimulated with bradykinin. The recruitment of Gα_z, Gα_q, GRK2 and β-arrestin2 was inhibited by B₂ receptor but not AT₂ receptor antagonism, indicating the importance of B₂ receptor activation within AT₂-B₂ heteromers. The close proximity between the AT₂ receptor and B₂ receptor at the cell surface was also demonstrated with the NanoBRET ligand-binding assay. Together, our data demonstrate functional interaction between the AT₂ receptor and B₂ receptor in HEK293FT cells, resulting in novel pharmacology for both receptors with regard to Gα_q/GRK2/β-arrestin2 recruitment (AT₂ receptor) and Gα_z protein coupling (B₂ receptor). Our study has revealed a new mechanism for the enigmatic and poorly characterized AT₂ receptor to be functionally active within cells, further illustrating the role of heteromerization in the diversity of GPCR pharmacology and signaling.