Proplatelet formation deficit and megakaryocyte death contribute to thrombocytopenia in Myh9 knockout mice

Background: Inactivation of the mouse Myh9 gene (Myh9 Delta) or its mutation in MYH9-related diseases leads to macrothrombocytopenia. Paradoxically, previous studies using in vitro differentiated megakaryocytes showed an increased capacity for proplatelet formation when myosin was absent or inhibited. Methods: To explore the origin of the thrombocytopenia induced by myosin deficiency, we studied proplatelet formation using bone marrow explants of wild-type (WT) and Myh9 Delta mouse where megakaryocytes have matured in their native environment. Results and discussion: A dramatic decrease in the number and complexity of proplatelets was observed in megakaryocytes from Myh9 Delta mice, while inhibition of myosin activity by blebbistatin increased proplatelet formation from WT mature megakaryocytes. Moreover, Myh9 Delta megakaryocytes had a smaller size than the WT cells. These data indicate that myosin deficiency acts negatively on proplatelet formation, probably by impairing in situ megakaryocyte maturation, while myosin activity is dispensable at the latest stage of proplatelet formation. In addition, ultrastructural examination of Myh9 Delta bone marrow revealed an increased proportion of megakaryocytes exhibiting signs of non-apoptotic cell death as compared with the WT mice. Conclusion: These data indicate that thrombocytopenia in Myh9 Delta mice results from defective development of megakaryocyte size, impaired proplatelet formation and increased cell death.