PB0387 - GPR56 is the platelet collagen receptor activator of G protein 13

Background: We recently identified the adhesion GPCR, GPR56/ADGRG1 as a platelet cell surface receptor that utilizes fibrillar collagen and blood flow-induced shear force to activate G protein 13. GPR56 is activated by a unique tethered-peptide-agonist mechanism. We propose that activation of GPR56 serves to initiate the actin cytoskeletal rearrangements that elicit platelet shape change and support primary platelet activation during hemostasis.

Aims: The goals of our current study are to:
1. Quantify GPR56 mRNA and cell surface receptor levels in human and mouse platelets.
2. Characterize the robust activation of G13 by GPR56 in the context of weaker G protein i signaling that GPR56 may also provide.
3. Use new transgenic mouse models to further understand the role of GPR56 in the regulation of hemostasis.

Methods: GPR56 expression and cell surface protein abundance will be measured by a combination of techniques including qPCR (directed and Taqman GPCR array), RNAseq, immunoblotting and flow cytometry. Ex vivo platelet signaling assays (Rho-GTP production, platelet spreading, and adenylyl cyclase activity) will be performed to understand the interplay of GPR56 mediated G13 and Gi signaling. In vivo hemostasis assays will be performed using new transgenic mouse strains.

Results: GPR56 is a highly abundant platelet GPCR. Preliminary results indicate that its abundance is similar to protease activated receptors (PAR) and purinergic receptors (P2Y). Human GPR56 activates G13 and weakly stimulates Gi, although mouse GPR56 seems to only activate G13. Global Gpr56 knockout mice exhibit hemostasis defects. New data may be presented to examine the effects of a platelet-specific Gpr56 deletion on these processes.

Conclusion(s): We identified GPR56 as a platelet collagen receptor that appears to work in concert with the repertoire of known platelet collagen receptors. We propose that GPR56 operates as transient collagen receptor to evoke platelet signaling and does not directly mediate stable platelet adhesion to collagen.