The role of Tyro 3 subfamily receptors in the regulation of hemostasis and megakaryocytopoiesis.

BACKGROUND AND OBJECTIVES The molecular mechanisms regulating megakaryocytopoiesis and hemostasis remain largely unknown. The Tyro 3 subfamily of receptor tyrosine kinases (RTK), which is composed of three members (Tyro 3, Axl and Mer), plays important roles in various tissues, such as those in the nervous, immune and reproductive systems. Here, we investigate the roles of the Tyro 3 RTK subfamily in regulating megakaryocytopoiesis and hemostasis. DESIGN AND METHODS Single, double, and triple knock-out mice for the three Tyro 3 RTK were used in the study. Bleeding time, platelet count, megakaryocyte count, megakaryocyte ploidy, rate of proplatelet formation, platelet aggregation and ATP release were used as criteria to evaluate hemostasis, megakaryocytopoiesis and platelet function. RESULTS Mice lacking all three receptors had impaired hemostasis and mild thrombocytopenia, which may be due to platelet dysfunction and defective megakaryocytopoiesis. Mice lacking different combinations of two receptors of the Tyro 3 RTK subfamily had normal platelet counts in peripheral blood, but exhibited impaired hemostasis and platelet function. Although knock-out mice for any single receptor had normal hemostasis and megakaryocytopoiesis, they exhibited a mild platelet dysfunction. INTERPRETATION AND CONCLUSIONS The Tyro 3 RTK subfamily plays important roles in regulating hemostasis, megakaryocytopoiesis and platelet function.