Adhesion G protein-coupled receptors (aGPCRs) comprise the second largest yet least studied class of the GPCR superfamily. aGPCRs are involved in many developmental processes, immune and synaptic functions, but the mode of their signal transduction is unclear. Here, we show that a short peptide sequence (termed the Stachel sequence) within the ectodomain of two aGPCRs, GPR126 and GPR133, functions as a tethered agonist. Upon structural changes within the receptor ectodomain, this intramolecular agonist is exposed to the 7-transmembrane helix domain, which triggers G-protein activation. Our studies show high specificity of a given Stachel sequence for its receptor. Finally, the function of Gpr126 is abrogated in zebrafish with a mutated Stachel sequence, and signaling is restored in hypomorphic gpr126 zebrafish mutants upon exogenous Stachel peptide application. These findings illuminate a previously unknown mode of aGPCR activation, and can initiate the development of specific ligands for this currently untargeted GPCR family.

Adhesion GPCRs (aGPCRs) form the second largest, yet most enigmatic class of the GPCR superfamily. Although the physiologic importance of aGPCRs was demonstrated in several studies, the majority of these receptors is still orphan with respect to their agonists and signal transduction. Recent studies reported that aGPCRs are activated through a tethered peptide agonist, coined the Stachel sequence. The Stachel sequence is the most C-terminal part of the highly conserved GPCR autoproteolysis-inducing domain. Here, we used cell culture-based assays to investigate 2 natural splice variants within the Stachel sequence of the orphan G s coupling aGPCR GPR114/ADGRG5. There is 1 variant constitutively active in cAMP assays (∼25-fold over empty vector) and sensitive to mechano-activation. The other variant has low basal activity in cAMP assays (6-fold over empty vector) and is insensitive to mechano-activation. In-depth mutagenesis studies of these functional differences revealed that the N-terminal half of the Stachel sequence confers the agonistic activity, whereas the C-terminal part orientates the agonistic core sequence to the transmembrane domain. Sequence comparison and functional testing suggest that the proposed mechanism of Stachel-mediated activation is relevant not only to GPR114 but to aGPCRs in general.-Wilde, C., Fischer, L., Lede, V., Kirchberger, J., Rothemund, S., Schöneberg, T., Liebscher, I. The constitutive activity of the adhesion GPCR GPR114/ADGRG5 is mediated by its tethered agonist. FASEB J. 30, 666-673 (2016). www.fasebj.org

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BackgroundA large number of human inherited and acquired diseases and phenotypes are caused by mutations in G protein-coupled receptors (GPCR). Genome-wide association studies (GWAS) have shown that variations in the ADGRD1 (GPR133) locus are linked with differences in metabolism, human height and heart frequency. ADGRD1 is a Gs protein-coupled receptor belonging to the class of adhesion GPCRs.ResultsAnalysis of more than 1000 sequenced human genomes revealed approximately 9000 single nucleotide polymorphisms (SNPs) in the human ADGRD1 as listed in public data bases. Approximately 2.4 % of these SNPs are located in exons resulting in 129 non-synonymous SNPs (nsSNPs) at 119 positions of ADGRD1. However, the functional relevance of those variants is unknown. In-depth characterization of these amino acid changes revealed several nsSNPs (A448D, Q600stop, C632fs [frame shift], A761E, N795K) causing full or partial loss of receptor function, while one nsSNP (F383S) significantly increased basal activity of ADGRD1.ConclusionOur results show that a broad spectrum of functionally relevant ADGRD1 variants is present in the human population which may cause clinically relevant phenotypes, while being compatible with life when heterozygous.Electronic supplementary materialThe online version of this article (doi:10.1186/s12864-016-2937-2) contains supplementary material, which is available to authorized users.