GPR56 is a Novel Platelet Collagen Receptor that Mediates Platelet Activation

Abstract

The adhesion G protein‐coupled receptor (AGPCR) GPR56 (ADGRG1) is required for brain development and nerve myelination. GPR56 is expressed in other tissues, suggesting additional physiological roles. We pharmacologically identified GPR56 in human and mouse platelets, corroborated its presence by immunoblotting, and showed that stimulation of GPR56 with a synthetic peptide‐ or small molecule‐agonist activates platelets. Currently there is an enigma regarding the complete repertoire of collagen receptors that mediate initial platelet interaction with the vessel wound‐exposed collagen sub‐endothelium that fully accounts for platelet activation. GPR56 is a known collagen receptor. We hypothesize that GPR56‐mediated platelet activation is dependent on blood flow shear forced dissociation of the non‐covalently bound GPR56 N‐terminal fragment (NTF) and the C‐terminal fragment (CTF) to expose the GPR56 tethered‐peptide‐agonist. As previously demonstrated, operative dissociation of these GPR56 fragments led to enhanced G13 and Gi activation. Thus, shear force‐induced shedding of the NTF onto collagen is a consequence of GPR56 activation during platelet translocation that leads to platelet shape change and activation via G13 and/or Gi activity. To establish the functional role of GPR56 in platelet activation, human or GPCR deficent (Gpr56−/−) and wildtype (WT) mouse platelets were stimulated with varying concentrations of synthetic GPR56 peptides or 3‐a‐DOG agonist and platelet activation was assessed via a number of tests. Both GPR56 peptide‐ and small molecule‐agonists potently induced platelet aggregation and spreading comparably to the established platelet agonist PAR4 AP. Additionally, dihydromunduletone (DHM), a GPR56 antagonist, inhibited platelet activation mediated by the GPR56 agonists, but not by PAR4 AP. In the case of hemostasis, high shear blood flow rate over collagen coated slides were performed with WT and Gpr56−/−. Gpr56−/− showed decreased platelet adhesion on collagen compared to WT. Gpr56−/− mice also had prolonged hemostatic tail bleeding times compared to the WT. Overall, these functional assays establish GPR56 as an additional collagen receptor that is critical for platelet adhesion and activation in order to maintain hemostasis.Support or Funding InformationNIH RO1 GM120110 (G.G.T), RO1 NS103946 (G.G.T), T32‐HL007853 (CVC), and Michigan Life Sciences FellowshipThis abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.