Proplatelet formation is regulated by the Rho/ROCK pathway

• Published in: Blood Vol. 109; no. 10; pp. 4229 - 4236
• Main Authors: Chang, Yunhua, Auradé, Frédéric, Larbret, Frédéric, Zhang, Yanyan, Le Couedic, Jean-Pierre, Momeux, Laurence, Larghero, Jerôme, Bertoglio, Jacques, Louache, Fawzia, Cramer, Elisabeth, Vainchenker, William, Debili, Najet
• Format: Journal Article
• Language: English
• Published: Washington, DC Elsevier Inc 15-05-2007; The Americain Society of Hematology
• Subjects: Biological and medical sciences; Blood Platelets - cytology; Cardiac Myosins - metabolism; Cell Differentiation - genetics; Cell differentiation, maturation, development, hematopoiesis; Cell physiology; Cells, Cultured; Enzyme Activation - drug effects; Fundamental and applied biological sciences. Psychology; Gene Expression Regulation; Hematologic and hematopoietic diseases; Hematopoiesis - drug effects; Humans; Intracellular Signaling Peptides and Proteins - antagonists & inhibitors; Intracellular Signaling Peptides and Proteins - genetics; Intracellular Signaling Peptides and Proteins - physiology; Medical sciences; Megakaryocytes - cytology; Molecular and cellular biology; Myosin Light Chains - metabolism; Myosin-Light-Chain Kinase - antagonists & inhibitors; Myosin-Light-Chain Kinase - metabolism; Phosphorylation - drug effects; Protein-Serine-Threonine Kinases - antagonists & inhibitors; Protein-Serine-Threonine Kinases - genetics; Protein-Serine-Threonine Kinases - physiology; rho GTP-Binding Proteins - antagonists & inhibitors; rho GTP-Binding Proteins - genetics; rho GTP-Binding Proteins - metabolism; rho GTP-Binding Proteins - physiology; rho-Associated Kinases; Signal Transduction; Thrombopoietin - pharmacology; Thrombopoiesis; Hematology; Enzyme; Hematopoiesis; Myosin; Proplatelet; Contractile protein; Light peptide chain
• ISSN: 0006-4971; 1528-0020
• DOI: 10.1182/blood-2006-04-020024

Platelets are released by megakaryocytes (MKs) via cytoplasmic extensions called proplatelets, which require profound changes in the microtubule and actin organization. Here, we provide evidence that the Rho/ROCK pathway, a well-known regulator of actin cytoskeleton, acts as a negative regulator of proplatelet formation (PPF). Rho is expressed at a high level during the entire MK differentiation including human CD34+ cells. Thrombopoietin stimulates its activity but at a higher extent in immature than in mature MKs. Overexpression of a dominant-negative or a spontaneously active RhoA leads to an increase or a decrease in PPF indicating that Rho activation inhibits PPF. This inhibitory effect is mediated through the main Rho effector, Rho kinase (ROCK), the inhibition of which also increases PPF. Furthermore, inhibition of Rho or ROCK in MKs leads to a decrease in myosin light chain 2 (MLC2) phosphorylation, which is required for myosin contractility. Interestingly, inhibition of the MLC kinase also decreases MLC2 phosphorylation while increasing PPF. Taken together, our results suggest that MLC2 phosphorylation is regulated by both ROCK and MLC kinase and plays an important role in platelet biogenesis by controlling PPF and fragmentation.