Myosin-II repression favors pre/proplatelets but shear activation generates platelets and fails in macrothrombocytopenia.
Megakaryocyte ploidy and the generation of pre/proplatelets are both increased in culture by pharmacologic inhibition of myosin-II, but nonmuscle myosin-IIA (MIIA) mutations paradoxically cause MYH9-related diseases (MYH9-RD) that adversely affect platelets. In marrow, megakaryocytes extend projections into the microcirculation, where shear facilitates fragmentation to large pre/proplatelets, suggesting that fluid stresses and myosin-II activity somehow couple in platelet biogenesis. Here, in bulk shear, plateletlike particles generated from megakaryocytes are maximized at a shear stress typical of that in the microcirculation and after treatment with a myosin-II inhibitor. MIIA activity in static cells is naturally repressed through phosphorylation at Serine-1943, but shear decreases phosphorylation, consistent with MIIA activation and localization to platelet cortex. Micropipette aspiration of cells shows myosin-II accumulates at stressed sites, but its inhibition prevents such mechanoactivation and facilitates generation of CD41(+) fragments similar in size to pre/proplatelets. MYH9-RD mutants phenocopy inhibition, revealing a dominant negative effect. MIIA is diffuse in the large platelets of a MYH9-RD patient with macrothrombocytopenia and is also diffuse in normal pre/proplatelets treated with inhibitor that blocks in vitro division to small platelets. The findings explain the large platelets in MYH9-RD and the near-normal thrombocrit of patients. Myosin-II regulation thus controls platelet size and number.
Molecular & Cell Biophysics Laboratory and.
Kyle R
KR
Spinler
Molecular & Cell Biophysics Laboratory and Graduate Group in Pharmacological Science, University of Pennsylvania, Philadelphia, PA;
Jae-Won
JW
Shin
Divison of Hematology, The Children's Hospital of Philadelphia, Philadelphia, PA; and Department of Pediatrics, Perelman School of Medicine at the University of Pennsylvania, Philadelphia, PA.
Michele P
MP
Lambert
Molecular & Cell Biophysics Laboratory and Graduate Group in Pharmacological Science, University of Pennsylvania, Philadelphia, PA;
Dennis E
DE
Discher
D001792:Blood Platelets / Q000378:metabolism / Q000473:pathology*; D005434:Flow Cytometry; D005455:Fluorescent Antibody Technique; D005799:Genes, Dominant; D006801:Humans; D008533:Megakaryocytes / Q000378:metabolism / Q000473:pathology*; D020409:Molecular Motor Proteins / Q000235:genetics / Q000378:metabolism*; D009154:Mutation / Q000235:genetics; D018995:Myosin Heavy Chains / Q000235:genetics / Q000378:metabolism*; D010766:Phosphorylation; D033081:Shear Strength; D013314:Stress, Mechanical; D013921:Thrombocytopenia / Q000235:genetics / Q000378:metabolism / Q000473:pathology*
C103814:MYH9 protein, human; D020409:Molecular Motor Proteins; D018995:Myosin Heavy Chains