Dynamin 2-Dependent Endocytosis Regulates Megakaryopoiesis

Dynamins are large and highly conserved GTPases involved in endocytosis and vesicle trafficking. Mutations K562E/del in the ubiquitous dynamin 2 (DNM2) have been associated with thrombocytopenia in humans. To determine the role of DNM2 in megakaryopoiesis we generated Dnm2^fl/fl Pf4-Cre mice specifically lacking DNM2 in the megakaryocyte (MK) lineage. Dnm2^fl/fl Pf4-Cre mice were viable, but had severe macrothrombocytopenia with moderately accelerated platelet clearance and prolonged bleeding due to poorly functional platelets. Dnm2-null bone marrow MKs had altered demarcation membrane system, appearing at times as a compact, narrow twisting membrane system of clathrin-coated vesicles. Fetal liver cell derived Dnm2-null MKs formed proplatelets poorly in vitro, showing that DNM2 plays a major role in MK membrane formation and thrombopoiesis. Both endogenous DNM2 and overexpressed DNM2 WT, but not DNM2 K562E/del mutants localized with the early endosome in bone marrow MKs. The endocytic pathway was disrupted in Dnm2-null MKs, as evidenced by severely reduced early endosome EEA1 and APPL1 staining and weak LysoTracker internalization. Endocytosis of the thrombopoietin (TPO) receptor Mpl was impaired in Dnm2-null platelets, causing constitutive phosphorylation of the tyrosine kinase JAK2 and elevated circulating TPO levels. MK-specific DNM2 deletion severely disrupted bone marrow homeostasis, as reflected by massive MK hyperplasia and myelofibrosis, and consequent extramedullary hematopoiesis and splenomegaly. However, additional Mpl genetic deletion failed to rescue the severe splenomegaly of Dnm2^fl/fl Pf4-Cre mice, and Mpl^-/- Dnm2^fl/fl Pf4-Cre mice instead died at 4-5 weeks of age. Taken together, our data demonstrates that unrestrained MK growth and proliferation results in rapid myelofibrosis independently of Mpl expression and other bone marrow cell types, and establishes a previously unrecognized role for DNM2-dependent endocytosis in megakaryopoiesis, thrombopoiesis and bone marrow homeostasis.