A Human Platelet Receptor Protein Microarray Identifies the High Affinity Immunoglobulin E Receptor Subunit (FcεR1) as an Activating Platelet Endothelium Aggregation Receptor 1 (PEAR1) Ligand

Genome-wide association studies to identify loci responsible for platelet function and cardiovascular disease susceptibility have repeatedly identified polymorphisms linked to a gene encoding platelet endothelium aggregation receptor 1 (PEAR1), an orphan cell surface receptor that is activated to stabilize platelet aggregates. To investigate how PEAR1 signaling is initiated, we sought to identify its extracellular ligand by creating a protein microarray representing the secretome and receptor repertoire of the human platelet. Using an avid soluble recombinant PEAR1 protein and a systematic screening assay designed to detect extracellular interactions, we identified the high affinity immunoglobulin E (IgE) receptor subunit (Fc epsilon R1) as a PEAR1 ligand. Fc epsilon R1 and PEAR1 directly interacted through their membrane-proximal Ig-like and 13th epidermal growth factor domains with a relatively strong affinity (K-D approximate to 30 nm). Precomplexing Fc epsilon R1 with IgE potently inhibited the Fc epsilon R1-PEAR1 interaction, and this was relieved by the anti-IgE therapeutic omalizumab. Oligomerized Fc epsilon R1 potentiated platelet aggregation and led to PEAR1 phosphorylation, an effect that was also inhibited by IgE. These findings demonstrate how a protein microarray resource can be used to gain important insight into the function of platelet receptors and provide a mechanistic basis for the initiation of PEAR1 signaling in platelet aggregation.