Dual-mode tethered intramolecular agonist-dependent mechanoactivation of the adhesion GPCR ADGRG1/GPR56

Mechanical stimuli instruct cardinal cellular decisions pertaining to cell fate, proliferation, morphology, and movement1,2. How adhesion G protein-coupled receptors (aGPCRs), a large family of mechanosensors with more than 30 members in humans3-5, transduce mechanical cues into metabotropic commands has been a matter of debate due to the lack of suitable approaches to analyze receptor activation during mechanotransduction in live cells6. A central question is whether aGPCR activation strictly requires mechanical dissociation of the autoproteolyzed N- and C-terminal fragment (NTF and CTF) complex to expose the tethered intramolecular agonist (TIA; 'Stachel'). Here, we investigated human ADGRG1/GPR56 (G1), an aGPCR with roles in brain development7, skeletal muscle8, and platelet function9, to study the events during aGPCR mechanotransduction by monitoring receptor dissociation and G protein recruitment in real time during cell migration and substrate deformation. Magnetic-tweezer measurements at a physiologically relevant loading rate showed that isolated G1 GAIN domains can partially unfold and refold at forces below those required for NTF-CTF dissociation. In cells, G1 dissociation occurred at retracting fibers during cell migration10 over an adhesive substrate through its native extracellular matrix ligand, tissue transglutaminase 2 (TG2), and was enhanced by cell stretching. Simultaneous live recording of G protein recruitment and pharmacological assays during mechanical stimulation of G1 reveal a dual-mode activation mechanism: mechanical force elicits robust metabotropic signaling without requiring NTF dissociation and generates an intracellular response commensurate with the fully dissociated receptor. Both signaling modes depend on an intact Stachel element11,12 of the receptor under our experimental conditions. Our findings determine mechanotransduction principles governing aGPCR activation at the cell-ECM interface. We reveal that G1 does not function as a simple binary switch, but rather as a highly sophisticated mechanosensor that dynamically encodes localized extracellular tension into distinct, highly scalable metabotropic outputs.