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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 Dnm2fl/flPf4-Cremice 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, Dnm2fl/flPf4-CreMKs 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 Dnm2fl/flPf4-CreMKs. 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-/-Dnm2fl/flPf4-Cremice lacking both PACSIN2 and DNM2 in MKs were further generated to determine the contribution of PACSIN2 in clathrin and actin clustering in Dnm2fl/flPf4-CreMKs. Strikingly, PACSIN2 genetic deletion significantly improved the severe thrombocytopenia of Dnm2fl/flPf4-Cremice. 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-/-Dnm2fl/flPf4-CreMKs. Our results show that DNM2 terminates PACSIN2-dependent actin polymerization that accompanies RME, thereby allowing membrane rearrangements required for DMS formation.

No relevant conflicts of interest to declare.

Proteins of the Bin-amphiphysin-Rvs (BAR) and Fes-CIP4 homology BAR (F-BAR) families bind and deform membranes, promoting in cells tubular invaginations reminiscent of the platelet open canalicular system (OCS) and megakaryocyte (MK) demarcation membrane system (DMS). Here we investigated the role of the F-BAR protein PACSIN2 in platelets and MKs. Spinning disk laser fluorescence confocal microscopy showed that PACSIN2 co-localized with the von Willebrand factor receptor subunit GPIbα at the openings of the OCS in platelets. Endogenous and over-expressed PACSIN2 associated with anastomosing tubular structures reminiscent of the pre-DMS in mouse bone marrow MKs. Pacsin2-null mice had mild thrombocytopenia and polycythemia, MK hyperplasia and splenomegaly. Pacsin2-null mice had a bleeding disorder, as evidenced by significantly prolonged tail bleeding and delayed in vivothrombus formation following FeCl3vascular injury, which was normalized by injection of WT platelets. However, Pacsin2-null platelets expressed P-selectin and activated the integrin αIIbβ3 normally in response to stimulation with thrombin and the GPVI-specific collagen-related peptide. In contrast, Pacsin2-null platelets had abnormal flattened morphology, with more and narrower OCS channels on their surface, compared to WT platelets. Together, the data indicate that the F-BAR protein PACSIN2 orchestrates the intracellular membrane architecture of platelets, thereby regulating their in vivohemostatic functions.

No relevant conflicts of interest to declare.

Abstract 3261