The apelin receptor (APJ) and the opioid-related nociceptin receptor 1 (ORL1) are family A G protein-coupled receptors that participate in a variety of physiological processes. The distribution and function of APJ and ORL1 in the nervous system and peripheral tissues are similar; however, the detailed mechanism of how these two receptors modulate signaling and physiological effects remains unclear. Here, we examined whether APJ and ORL1 form dimers, and investigated signal transduction pathways. The endogenous co-expression of APJ and ORL1 in SH-SY5Y cells was confirmed by western blotting and RT-PCR. Bioluminescence and fluorescence resonance energy transfer assays, as well as a proximity ligation assay and co-immunoprecipitation experiments, demonstrated that APJ and ORL1 heterodimerize in HEK293 cells. We found that the APJ-ORL1 heterodimer is selectively activated by apelin-13, which causes the dimer to couple to Gαi proteins and reduce the recruitment of GRKs and β-arrestins to the dimer. We showed that the APJ-ORL1 dimer exhibits biased signaling, in which G protein-dependent signaling pathways override β-arrestin-dependent signaling pathways. Our results demonstrate that the structural interface of the APJ-ORL1 dimer switches from transmembrane domain TM1/TM2 in the inactive state to TM5 in the active state. We used mutational analysis and BRET assays to identify key residues in TM5 (APJ L2185.55, APJ I2245.61, and ORL1 L2295.52) responsible for the receptor-receptor interaction. These results provide important information on the APJ-ORL1 heterodimer and may assist the design of new drugs targeting biased signaling pathways for treatment of pain and cardiovascular and metabolic diseases.
Keywords: Apelin receptor (APJ); Biased signal transduction; Bioluminescence resonance energy transfer (BRET); Heterodimers; Opioid-related nociceptin receptor 1 (ORL1); Transmembrane domain (TM).
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