Macrophage Protease-activated Receptor 2 Regulates Fetal Liver Erythropoiesis in Mice

Deficiencies in many coagulation factors and protease-activated receptors (PARs) affect embryonic development. We describe a defect in definitive erythropoiesis in PAR2-deficient mice. Embryonic PAR2 deficiency increases embryonic death associated with variably severe anemia in comparison with PAR2-expressing embryos. PAR2-deficient fetal livers display reduced macrophage densities, erythroblastic island areas, and messenger RNA expression levels of markers for erythropoiesis and macrophages. Coagulation factor synthesis in the liver coincides with expanding fetal liver hematopoiesis during midgestation, and embryonic factor VII (FVII) deficiency impairs liver macrophage development. Cleavage-insensitive PAR2-mutant mice recapitulate the hematopoiesis defect of PAR2-deficient embryos, and macrophage-expressed PAR2 directly supports erythroblastic island function and the differentiation of red blood cells in the fetal liver. Conditional deletion of PAR2 in macrophages impairs erythropoiesis, as well as increases inflammatory stress, as evidenced by upregulation of interferon-regulated hepcidin antimicrobial peptide. In contrast, postnatal macrophage PAR2 deficiency does not have any effect on steady-state Kupffer cells, bone marrow macrophage numbers, or erythropoiesis, but erythropoiesis in macrophages from PAR2-deficient mice is impaired following hemolysis. These data identify a novel function for macrophage PAR2 signaling in adapting to rapid increases in blood demand during gestational development and postnatal erythropoiesis under stress conditions.