Dynamin 2‐Dependent Endocytosis is Required For Normal Megakaryocyte Development

Abstract

Dynamins are large and highly conserved GTPases involved in a wide range of cellular functions, including endocytosis and vesicle trafficking, and mutations in the ubiquitous dynamin 2 (DNM2) have been associated with thrombocytopenia in humans. To determine the role of DNM2 in megakaryopoiesis we generated Dnm2fl/fl Pf4-Cre mice specifically lacking DNM2 in the megakaryocyte (MK) lineage. Dnm2fl/fl Pf4-Cre mice had severe macrothrombocytopenia with moderately accelerated platelet clearance. Dnm2-null bone marrow MKs had altered DMS, appearing at times as a compact, narrow twisting membrane system of clathrin-coated vesicles. The endocytic pathway was disrupted in Dnm2-null MKs, as evidenced by reduced early endosome EEA1 and APPL1 staining. MK-specific DNM2 deletion severely disrupted bone marrow homeostasis, as indicated by massive MK hyperplasia, myelofibrosis, and consequent extramedullary hematopoiesis and splenomegaly. 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. However, additional Mpl genetic deletion failed to rescue the severe splenomegaly of Dnm2fl/fl Pf4-Cre mice, and Mpl-/- Dnm2fl/fl Pf4-Cre mice died at 4-5 weeks of age. Taken together, our data establishes a previously unrecognized role for DNM2-dependent endocytosis in megakaryopoiesis, thrombopoiesis and bone marrow homeostasis.