Mechanisms of adhesion G protein-coupled receptor activation

Abstract

Adhesion G protein-coupled receptors (AGPCRs) are a thirty-three-member subfamily of Class B GPCRs that control a wide array of physiological processes and are implicated in disease. AGPCRs uniquely contain large, self-proteolyzing extracellular regions that range from hundreds to thousands of residues in length. AGPCR autoproteolysis occurs within the extracellular GPCR autoproteolysis-inducing (GAIN) domain that is proximal to the N terminus of the G protein-coupling seven-transmembrane-spanning bundle. GAIN domain-mediated self-cleavage is constitutive and produces two-fragment holoreceptors that remain bound at the cell surface. It has been of recent interest to understand how AGPCRs are activated in relation to their two-fragment topologies. Dissociation of the AGPCR fragments stimulates G protein signaling through the action of the tethered-peptide agonist stalk that is occluded within the GAIN domain in the holoreceptor form. AGPCRs can also signal independently of fragment dissociation, and a few receptors possess GAIN domains incapable of self-proteolysis. This has resulted in complex theories as to how these receptors are activated in vivo, complicating pharmacological advances. Currently, there is no existing structure of an activated AGPCR to support any of the theories. Further confounding AGPCR research is that many of the receptors remain orphans and lack identified activating ligands. In this review, we provide a detailed layout of the current theorized modes of AGPCR activation with discussion of potential parallels to mechanisms used by other GPCR classes. We provide a classification means for the ligands that have been identified and discuss how these ligands may activate AGPCRs in physiological contexts.

Keywords: ADGR; G protein; G protein-coupled receptor (GPCR); GAIN domain; adhesion; allostery; autoproteolysis; extracellular matrix protein; family B2 GPCRs; protease; shear force; tethered-peptide agonist.

Figure 1.

Structural topology of adhesion GPCRs. A, adhesion GPCRs are two fragment receptors that arise from a constitutive autoproteolytic cleavage event at the conserved GPS within the central core of the membrane-proximal, ∼320-amino acid GAIN domain. In the holoreceptor form, the two AGPCR fragments are noncovalently bound. In the dissociated form, the NTF or ECR is released extracellularly, whereas the freed CTF or GPCR domain remains in the plasma membrane. The ∼20-residue stalk (orange) that is exposed following NTF/CTF dissociation is termed the tethered-peptide agonist. B, the GAIN domain is a fully self-sufficient protease that constitutively cleaves the internal His Leu/Thr consensus site (GPS) via a nucleophilic attack mechanism, as shown for ADGRL1 (latrophilin-1) (5). C, ribbon representation of the β-strand 12–GPS proteolyzed loop–β-strand 13 orientation within the GAIN domain in the holoreceptor state. D and E, space-filled models of the ADGRG1 (GPR56) NTF (PDB entry 5KVM) with stabilizer antibody and the GAIN domain plus adjacent HormR domain from ADGRL1 (PDB entry 4DLQ). The residues of the tethered-peptide agonists and remainder of the stalks are colored orange and depict the degree of concealment within the interior core of the GAIN domain in the holoreceptor state.

Figure 2.

Models of adhesion GPCR activation. A, adhesion GPCRs, like other GPCRs, occupy a range of activated and inhibited states. Consequently, AGPCRs possess varying levels of basal G protein signaling. The adhesion GPCR N-terminal subdomains (dark green, yellow, and brown modules) are portrayed to reflect the potential variety within adhesion GPCR ECRs. B, in the orthosteric agonism model of activation, NTF/CTF dissociation via an anchored ligand (depicted by the green star) results in exposure of the tethered-peptide agonist (orange), allowing it to bind to an orthosteric site that is predicted to lie within the 7TM helical bundle. Orthosteric agonism is proposed to be a threshold response (all or none) due to forced NTF dissociation, which results in stabilization of highly active states of the receptors and maximal signaling. C and D, in allosteric modes of AGPCR regulation, ligands (indicated with a blue or red star) can interact with various AGPCR N-terminal adhesive motifs to stabilize active (activation, C) or inactive states (inhibition, D), respectively. Allosteric activation and inhibition mechanisms are unknown but may be mediated by GAIN-7TM interactions that favor stabilization of specific receptor conformations. E, relative signaling strength outputs in response to stimulus for each of the receptor modulation modes.

Figure 3.

Parallels of adhesion GPCR allosteric activation to Class B1 and Class C GPCRs. A, family B1 GPCRs are receptors for soluble peptide agonists and are closely related to adhesion GPCRs. B1 receptors possess ECDs that inhibit signaling of the 7TM domain in the absence of a ligand via interactions with ECLs. In the active state of the receptor, the N terminus of the peptide agonist (orange) binds within the 7TM bundle, whereas the C terminus binds to the ECD. Shown is the structure of active glucagon-like peptide-1 receptor (GLP1R) (gray and blue, ribbon) bound to glucagon-like peptide-1 (GLP1) peptide (orange, space-filled, PDB entry 5VAI). B, Class C GPCRs are distantly related to adhesion GPCRs but also possess large ECDs. Class C receptors are obligate dimers that possess an extracellular VFT domain (blue) and a CRD (yellow) N-terminal to the 7TM bundle. Binding of ligands to the VFT domain results in activation of the 7TM. Long-range transmission of the bound ligand signal from the VFT involves conformational changes of the CRD that impart changes to the transmembrane helices of the 7TM bundle to favor an active-state conformation. This is aided, in part, by a critical interaction with ECL2 of the 7TM domain that acts as a rigid linker for a subtle rotational shift in the 7TMs. C, it is not certain how AGPCR ligands allosterically modulate adhesion GPCR signaling in the absence of NTF/CTF dissociation. In the absence of ligand, the GAIN domain may interact with the ECLs of the 7TM bundle to repress active conformations (left), similarly to the ECDs of family B1 receptors. In the presence of an allosteric ligand, the activation signal may be transmitted through the GAIN domain to the 7TM (right). This could activate the receptor by relieving repression conferred by the NTF or by utilizing the CTF stalk as a fulcrum in a manner akin to class C receptors that use a rigid body to impart activating conformational changes to the 7TM.

Figure 4.

Trans-synaptic adhesion GPCRs. ADGRL and ADGRB receptors are enriched in the central and peripheral nervous systems, and their activation is thought to induce synaptogenesis and axonal growth. ADGRB receptors are enriched in the post-synapse and bind secreted C1q-like proteins and an unknown trans-synaptic ligand (possibly the peripheral membrane-associated RTN4R) to regulate synapse formation and maintenance via the TSR domains (46, 98, 142). ADGRL is also enriched in the post-synapse and interacts with both FLRT and teneurin (TEN) single-pass receptors simultaneously to form trans-synaptic links that promote synaptogenesis (78–80). FLRT binds to the ADGRL2 N-terminal Olf domain, whereas teneurin-2 binds the ADGRL2 Lec domain. In a fragment dissociation-independent model, a force-dependent signal may be transduced through the trans-synaptic junction to ADGRL and ADGRB receptors via allosteric modulation to induce G_12/13 signaling necessary for Rho activation and downstream cytoskeletal remodeling (left) (40). G_12/13 is coupled to both subclasses of receptors, but ADGRL may also signal via G_i/o and G_q/11 pathways (128, 130, 131). ADGRL and ADGRB NTFs and CTFs may be dissociated when synaptic terminals retract, allowing for tethered agonist orthosteric activation of the receptors to induce synaptic remodeling (right).