Microthrombocytopenia caused by impaired microtubule stability in RhoB-deficient mice

Megakaryocytes are large cells in the bone marrow, which give rise to blood platelets. Platelet biogenesis involves megakaryocyte maturation, the localization of mature cells in close proximity to bone marrow sinusoids and the formation of protrusions, which are shed into the circulation. Rho GTPases play important roles in platelet biogenesis and function. RhoA-deficient mice display macrothrombocytopenia and a striking mislocalization of megakaryocytes into bone marrow sinusoids and a specific defect in G-protein signaling in platelets. However, the role of the closely related protein RhoB in megakaryocytes or platelets remains unknown. In this study, we show that, in contrast to RhoA deficiency, genetic ablation of RhoB in mice results in microthrombocytopenia (decreased platelet count and size). RhoB-deficient platelets displayed mild functional defects predominantly upon induction of the collagen/glycoprotein VI pathway. Megakaryocyte maturation and localization within the bone marrow, as well as actin dynamics were not affected in the absence of RhoB. However, in vitro generated proplatelets revealed pronouncedly impaired microtubule organization. Furthermore, RhoB-deficient platelets and megakaryocytes displayed selective defects in microtubule dynamics/stability, correlating with pronouncedly reduced levels of acetylated α-tubulin. Our findings imply that absence of this tubulin posttranslational modification results in decreased microtubule stability leading to microthrombocytopenia in RhoB-deficient mice. Our data thus points to specifically impaired microtubule - but not actin - dynamics as a general mechanism underlying the manifestation of microthrombocytopenia in vivo . We furthermore demonstrate that RhoA and RhoB have specific, non-redundant functions in the megakaryocyte lineage. KEY POINTS ### Competing Interest Statement The authors have declared no competing interest.