Mechanical force induced activation of adhesion G protein-coupled receptor

Abstract

Among the various families of G protein-couple receptors (GPCR), the adhesion family of GPCRs is specialized by its expansive extracellular region, which facilitates the recruitment of various ligands. Previous hypothesis proposed that aGPCRs are activated by mechanical force, wherein a Stachel peptide is liberated from the GPCR autoproteolysis-inducing (GAIN) domain and subsequently binds to the transmembrane domain (7TM) upon activation. In this review, we summarize recent advancements in structural studies of aGPCRs, unveiling a conserved structural change of the Stachel peptide from the GAIN domain-embedded β-strand conformation to the 7TM-loaded α-helical conformation. Notably, using single-molecule studies, we directly observed the unfolding of GAIN domain and the release of Stachel peptide under physiological level of force, precisely supporting the mechanosensing mechanism for aGPCRs. We observed that the current complex structures of aGPCR adhesion domains with their respective ligands share a common pattern with the C-termini of each binding partner extending in opposite directions, suggesting a similar shearing stretch geometry for these aGPCRs to transmit the mechanical force generated in the circulating environment to the GAIN domain for its unfolding. Outstanding questions, including the relative orientations and interactions between 7TM and its preceding GAIN and adhesion domains of different aGPCRs, may require further structural and mechanical studies at the full-length receptor scale or cell-based level. Our analysis extends the current view of aGPCR structural organization and activation and offers valuable insights for the development of mechanosensor based on aGPCRs or discovery of mechanotherapy against aGPCRs.

Keywords: Adhesion GPCR; GAIN domain; Mechanosensing mechanism; Stachel peptide.

Graphical abstract

Fig. 1

Cartoon models for ligand recognition in different GPCR classes. For class A, the varied endogenous ligand is recognized by the orthosteric pocker in the 7TM domain. For class B1, the hormone peptide binds to both ECD and 7TM domains. For class B2, the receptor is pre-digested within the GAIN domain (marked with star) and a Stachel peptide is laterly released by mechanical force, the Stachel then functions like a tether agonst to activation the 7TM. For class C, the endogenous small molecules are recognized by orthosteric sites in the extracellular VFD. For class F, WNT protein binds the extracellular CRD of Frizzled receptors.

Fig. 2

Force-induced activation mode of adhesion GPCR. A. Scheme of activation model of aGPCRs. B–C. Representative inactive (PDB 8IKJ) and active (PDB 7YDM) structures of ADGRE5/CD97. In C, the close-up view of Stachel motif is framed and the key interacting residues are shown as sticks and labeled. In A–C, the tandem adhesion domains, GAIN A, GAIN B, and the Stachel motif are colored blue, pink, purple, and green, respectively. Inactive and active conformation of 7TM are also shown using different colors.

Fig. 3

Force magnitude and direction on aGPCR. A. Schematic of magnetic tweezers tension system. GAIN domain is tethered between a superparamagnetic bead which is subjected to an external force exerted by a pair of permanent magnets. B. Schematic of force induced unfolding of GAIN domain and dissociation of NTF/CTF. C. Force-bead height curves of representative tethers for GAIN domain of ADGRL1/LPHN1 during force loading until NTF/CTF dissociation. D. Superposition of aGPCR ECD structures on their GAIN domains. PDBs used for superposition are 8HC0 (GPR56), 8OEK (BAI2), 4DLO (BAI3), 4DLQ (LPHN1), 6V55 (GPR126), 5KVM (GPR56), 7QU8 (GPR97). The GAIN-A domains are emphasized on the right panel and the directions of each preceding adhesion domains are indicated by red arrows. E–H. The crystal structures of aGPCR adhesion domains in complex with each cognate ligand. PDBs used are 7DO4 (CD97-CD55), 7R86 (BAI1–RTN4R), 5CMN (LPHN3–FLRT3), and 6VHH (LPHN3–Teneurin). The directions to each transmembrane region within the complexes are indicated.