PEAR1 attenuates megakaryopoiesis via control of the PI3K/PTEN pathway

Platelet Endothelial Aggregation Receptor-1 (PEAR1) participates in platelet aggregation via sustaining αIIbβ3 activation. To investigate the role of PEAR1 in platelet formation, we monitored and manipulated PEAR1 expression in vitro in differentiating human CD34+ hematopoietic stem cells and in vivo in zebrafish embryos. PEAR1 expression rose during CD34+ cell differentiation up to megakaryocyte maturation. Two different lentiviral short hairpin knockdowns of PEAR1 didn't affect erythropoiesis in CD34+ cells, but increased CFU-MK cell numbers 2-fold vs. control in clonogenic assays, without substantially modifying MK maturation. The PEAR1 knockdown resulted in a 2-fold reduction of the PTEN phosphatase expression and modulated gene expression of several PI3K-Akt and Notch pathway genes. In zebrafish, Pear1 expression increased progressively during the first 3 days of embryo development. Both ATG and splice-blocking PEAR1 morpholinos enhanced thrombopoiesis, without affecting erythropoiesis. Western blots of 3-day-old Pear1 knockdown zebrafish revealed elevated Akt phosphorylation, coupled to transcriptional downregulation of the PTEN isoform Ptena. Neutralization by morpholinos of Ptena, but not of Ptenb, phenocopied the Pear1 zebrafish knockdown and triggered enhanced Akt phosphorylation and thrombocyte formation. In summary, this is the first demonstration that PEAR1 influences the PI3K/PTEN pathway, a critical determinant of Akt phosphorylation, itself controlling megakaryopoiesis and thrombopoiesis.