Novel insights into mouse models of ectopic proplatelet release.

Mature bone marrow megakaryocytes (MKs) produce platelets by extending proplatelets into sinusoidal blood vessels. Defects in this process can lead to thrombocytopenia with increased bleeding risk. Mice lacking the actin-regulatory proteins Profilin 1 (PFN1), Wiskott-Aldrich Syndrome protein (WASp), Actin Related Protein 2/3 complex (Arp2/3) or the adhesion and degranulation-promoting adapter protein (ADAP) display thrombocytopenia and ectopic release of (pro)platelet-like particles into the bone marrow compartment, pointing to an important axis of actin-mediated directional proplatelet formation. The mechanism underlying ectopic release in those mice is still incompletely understood, but we hypothesized that similar functional defects account for this observation. We analyzed WASp-, ADAP-, PFN1-, and ARPC2-knockout mice to determine the role of actin reorganization and integrin activation in directional proplatelet formation. ADAP-, ARPC2-, and PFN1-deficient megakaryocytes displayed reduced adhesion to collagen, defective F-actin organization, and diminished β1-integrin activation. WASp-deficient MKs showed the strongest reduction in the adhesion assay on collagen and altered F-actin organization with reduced podosome formation. Our results indicate that ADAP, PFN1, WASp and ARP2/3 are part of the same pathway, which regulates polarization processes in megakaryocytes and directional proplatelet formation into bone marrow sinusoids.