Dynamin 2 (DNM2) and PACSIN2 Regulate Megakaryocyte Demarcation Membrane System Formation and Platelet Production in Concert

Blood platelets are produced in the bone marrow by megakaryocytes (MKs) in a process that requires extensive intracellular membrane rearrangements. These include the formation of the demarcation membrane system (DMS), the surface-connected membrane extension that invaginates into the cell body and further develops to provide membranes for future platelets. The precise molecular mechanisms responsible for these unique membrane rearrangements remain poorly understood. We have recently shown that Dnm2^fl/fl Pf4-Cre mice specifically lacking the large GTPase dynamin 2 (DNM2) in MKs develop severe macrothrombocytopenia due to impaired receptor-mediated endocytosis (RME) (Bender, Giannini et al. Blood. 2015;125(6):1014-1024). Specifically, Dnm2^fl/fl Pf4-Cre MKs accumulate arrested endocytic clathrin-coated vesicles that obstruct DMS formation. The actin nucleating factor Arp2/3 complex and polymerized actin clustered with clathrin at sites of impaired RME in Dnm2^fl/fl Pf4-Cre MKs. We hypothesized that a DNM2 partner recruits actin-regulatory proteins at sites of RME and investigated the contribution of the F-BAR protein PACSIN2, an internal component of the initiating DMS (Jurak Begonja, Pluthero et al. Blood. 2015;126(1):80-88), in DMS formation and platelet production, as PACSIN2 interacts with DNM2 and actin-regulatory proteins such as N-WASP and filamin A (FlnA). Pacsin2^–/– mice developed mild thrombocytopenia with slightly enlarged and shallow platelets. The DMS appeared less well defined and platelet territories were not readily visualized in Pacsin2^–/– MKs. Pacsin2^–/– Dnm2^fl/fl Pf4-Cre mice lacking both PACSIN2 and DNM2 in MKs were further generated to determine the contribution of PACSIN2 in clathrin and actin clustering in Dnm2^fl/fl Pf4-Cre MKs. Strikingly, PACSIN2 genetic deletion significantly improved the severe thrombocytopenia of Dnm2^fl/fl Pf4-Cre mice. Specifically, PACSIN2 deletion abrogated the accumulation of clathrin and actin clusters, thereby unclogging DMS formation, which appeared as elongated maze-like membrane tubules in Pacsin2^–/– Dnm2^fl/fl Pf4-Cre MKs. Our results show that DNM2 terminates PACSIN2-dependent actin polymerization that accompanies RME, thereby allowing membrane rearrangements required for DMS formation.