Submitted March 31, 2003
Accepted July 31, 2003
A role for Rab27b in NF-E2-dependent pathways of platelet formation
Sanjay Tiwari, Joseph E Italiano, Duarte C Barral, Emilie H Mules, Edward K Novak, Richard T Swank, Miguel C Seabra, and Ramesh A Shivdasani*
Departments of Medical Oncology and Cancer Biology, Dana-Farber Cancer Institute, Boston, MA, USA; Department of Medicine, Brigham & Women's Hospital, Boston, MA, USA; Department of Medicine, Harvard Medical School, Boston, MA, USA
Division of Biomedical Sciences, Imperial College, London, United Kingdom
Roswell Park Cancer Institute, Buffalo, NY, USA
* Corresponding author; email: ramesh_shivdasani{at}dfci.harvard.edu.
Megakaryocytes release platelets by reorganizing the cytoplasm into proplatelet extensions. Fundamental to this process is the need to coordinate transport of products and organelles in the appropriate abundance to nascent platelets. The importance of the Rab family of small GTPases in platelet biogenesis is revealed in gunmetal (gm/gm) mice, which show deficient Rab isoprenylation and macrothrombocytopenia with few granules and abnormal megakaryocyte morphology. Although some Rab proteins are implicated in vesicle and organelle transport along microtubules or actin, the role of any Rab protein in platelet biogenesis is unknown. The limited number of Rab proteins with defective membrane association in gm/gm megakaryocytes prominently includes Rab27a and Rab27b. Normal expression of Rab27b is especially increased with terminal megakaryocyte differentiation and dependent on NF-E2, a transcription factor required for thrombopoiesis. Chromatin immunoprecipitation demonstrates recruitment of NF-E2 to the putative Rab27B promoter. Inhibition of endogenous Rab27 function in primary megakaryocytes causes severe quantitative and qualitative defects in proplatelet formation that mimic findings in gm/gm cells. Rab27b localizes to alpha and dense granules in megakaryocytes. These results establish a role for Rab27 in proplatelet formation and suggest that Rab27b in particular may coordinate proplatelet formation with granule transport, possibly by recruiting specific effector pathways.
