GPR56/ADGRG1 is a deorphanized brain Adhesion G protein Coupled Receptor (AGPCR) with a physiological ligand, collagen. We present new evidence that GPR56 is present on the surface of human and mouse platelets. This is very intriguing considering that autonomous platelet activation that leads to clot formation is mediated by platelet interactions with collagen that becomes exposed at sites of vessel injury. We found that pharmacological activation of platelet GPR56 is sufficient to elicit the physiological platelet activation program required for wound healing (hemostasis) and blood clotting (thrombosis). Moreover, Gpr56 knockout mice have a severe bleeding disorder and dramatic defects in clot formation in mouse vessel wall injury models. We hypothesize that GPR56 is the platelet collagen receptor that mediates the initial G protein 12/13-dependent platelet shape changes while platelets roll/translocate along exposed collagen to their site of stable adhesion at the lagging edges of blood vessel wounds. We will investigate platelet GPR56-mediated G protein signaling, platelet shape changes, in vitro platelet activation, and the role of GPR56 in mouse models of hemostasis and thrombosis. We found compelling preliminary evidence that platelets from Gpr56 knockout mice are dramatically defective in adhering to an immobilized collagen surface when mouse blood is flowed over the surface at high shear force. We propose that GPR56 is not directly involved in stable platelet adhesion, but as platelets tumble across the collagen surface, GPR56 is stimulated to cause the shape changes that allow platelets to then stably adhere via distinct non-GPCR collagen receptors. Before discovering GPR56 on platelets we proposed a general model of AGPCR activation that required anchoring of the receptor N-termini, and shear force-mediated receptor dissociation that leads to exposure of the AGPCR tethered agonists to induce G protein signaling. The conditions experienced by platelet-borne GPR56 align remarkably well with this theoretical model. Our work also includes conducting biochemical studies using newly developed photo-crosslinking agonist probes to define the agonist binding sites of the related AGPCRs, GPR56 and GPR114.
We propose that Adhesion G protein Coupled Receptors (AGPCRs) transmit signals from cell adhesive and shear force events to elicit change on the inside of the cell via G protein signaling. We found the collagen-responding AGPCR, GPR56 on the surface of human and mouse blood platelets and provide preliminary evidence showing that it initiates the platelet activation program to allow platelets to stick at the sites of blood vessel injury. GPR56 activation of platelets will be investigated in test-tube experiments and in animal models of hemostasis (wound healing) and thrombosis (clot formation), and the general molecular mechanism of AGPCR activation will be studied.
