Biased PAR1 Agonism in Sickle Cell Disease

PROJECT SUMMARY Title: Biased PAR1 Agonism in Sickle Cell Disease Sickle cell disease (SCD) is caused by a single nucleotide mutation in the β-globin gene, resulting in altered red cell physiology that drives chronic hemolytic anemia and painful vaso-occlusive crisis (VOC) triggered by microvascular occlusion/stasis. VOC is the leading cause of hospitalizations of sickle cell

patients. Activation of the main thrombin receptor, protease activated receptor 1 (PAR1), enhances the interactions between endothelial cells and sickle RBCs. In my recently published study, I demonstrated that PAR1 deficiency on nonhematopoietic cells or inhibition of PAR1 with vorapaxar attenuates

microvascular stasis in a mouse model of SCD. In addition to thrombin, PAR1 is also activated by activated protein C (APC). The APC-mediated activation of PAR1 is referred to as “biased agonism” because it activates a different signaling pathway than thrombin and ultimately induces cytoprotective

and anti-inflammatory effects. My central hypothesis is that canonical thrombin/PAR1 signaling contributes to microvascular stasis whereas non-canonical APC/PAR1 signaling reduces microvascular stasis and thromboinflammation. I hypothesize that inducing beneficial PAR1 biased signaling will be advantageous compared to complete PAR1 inhibition, which blocks the deleterious thrombin-

dependent signaling as well as beneficial APC signaling. In the first aim I will compare the roles of thrombin/PAR1 and APC/PAR1 signaling on coagulation, inflammation, and microvascular stasis in sickle cell mice using mice with PAR1 point mutations that select for activation by either thrombin or APC. In the second aim, I will compare the therapeutic potential of a signaling-selective form of APC,

3K3A-APC, to two inhibitors of PAR1, parmodulin 2 and vorapaxar, on inflammation, microvascular stasis, and acute chest syndrome. Finally, in the third aim, I will investigate the effects of biased PAR1 agonism on mortality and end-organ damage in sickle mice. These studies will investigate the role of

biased PAR1 signaling in the pathology of SCD.