ORCID Profile
0000-0001-9553-2701
Current Organisations
University of Bordeaux
,
University Hospital
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Publisher: American Society of Hematology
Date: 14-06-2012
DOI: 10.1182/BLOOD-2011-12-398834
Abstract: Mature megakaryocytes depend on the function of Bcl-xL, a member of the Bcl-2 family of prosurvival proteins, to proceed safely through the process of platelet shedding. Despite this, loss of Bcl-xL does not prevent the growth and maturation of megakaryocytes, suggesting redundancy with other prosurvival proteins. We therefore generated mice with a megakaryocyte-specific deletion of Mcl-1, which is known to be expressed in megakaryocytes. Megakaryopoiesis, platelet production, and platelet lifespan were unperturbed in Mcl-1Pf4Δ/Pf4Δ animals. However, treatment with ABT-737, a BH3 mimetic compound that inhibits the prosurvival proteins Bcl-2, Bcl-xL, and Bcl-w resulted in the complete ablation of megakaryocytes and platelets. Genetic deletion of both Mcl-1 and Bcl-xL in megakaryocytes resulted in preweaning lethality. Megakaryopoiesis in Bcl-xPf4Δ/Pf4ΔMcl-1Pf4Δ/Pf4Δ embryos was severely compromised, and these animals exhibited ectopic bleeding. Our studies indicate that the combination of Bcl-xL and Mcl-1 is essential for the viability of the megakaryocyte lineage.
Publisher: Elsevier BV
Date: 12-2010
Publisher: Rockefeller University Press
Date: 12-09-2011
DOI: 10.1084/JEM.20110750
Abstract: It is believed that megakaryocytes undergo a specialized form of apoptosis to shed platelets. Conversely, a range of pathophysiological insults, including chemotherapy, are thought to cause thrombocytopenia by inducing the apoptotic death of megakaryocytes and their progenitors. To resolve this paradox, we generated mice with hematopoietic- or megakaryocyte-specific deletions of the essential mediators of apoptosis, Bak and Bax. We found that platelet production was unperturbed. In stark contrast, deletion of the prosurvival protein Bcl-xL resulted in megakaryocyte apoptosis and a failure of platelet shedding. This could be rescued by deletion of Bak and Bax. We examined the effect on megakaryocytes of three agents that activate the intrinsic apoptosis pathway in other cell types: etoposide, staurosporine, and the BH3 mimetic ABT-737. All three triggered mitochondrial damage, caspase activation, and cell death. Deletion of Bak and Bax rendered megakaryocytes resistant to etoposide and ABT-737. In vivo, mice with a Bak−/− Bax−/− hematopoietic system were protected against thrombocytopenia induced by the chemotherapeutic agent carboplatin. Thus, megakaryocytes do not activate the intrinsic pathway to generate platelets rather, the opposite is true: they must restrain it to survive and progress safely through proplatelet formation and platelet shedding.
Publisher: Springer Science and Business Media LLC
Date: 24-01-2014
DOI: 10.1038/CDD.2013.201
Publisher: American Society of Hematology
Date: 31-03-2011
DOI: 10.1182/BLOOD-2010-11-318055
Abstract: Histone deacetylase inhibitor (HDACI)–induced thrombocytopenia (TCP) is a major dose-limiting toxicity of this new class of drugs. Using preclinical models to study the molecular and biologic events that underpin this effect of HDACI, we found that C57BL/6 mice treated with both the HDAC1/2-selective HDACI romidepsin and the pan-HDACI panobinostat developed significant TCP. HDACI-induced TCP was not due to myelosuppression or reduced platelet lifespan, but to decreased platelet release from megakaryocytes. Cultured primary murine megakaryocytes showed reductions in proplatelet extensions after HDACI exposure and a dose-dependent increase in the phosphorylation of myosin light chain 2 (MLC2). Phosphorylation of MLC to phospho-MLC (pMLC) and subsequent proplatelet formation in megakaryocytes is regulated by the Rho-GTPase proteins Rac1, CDC42, and RhoA. Primary mouse megakaryocytes and the human megakaryoblastic cell line Meg-01 showed reductions in Rac1, CDC42, and RhoA protein levels after treatment with HDACIs. We were able to overcome HDACI-induced TCP by administering the mouse-specific thrombopoietin (TPO) mimetic AMP-4, which improved platelet numbers to levels similar to untreated controls. Our report provides the first detailed account of the molecular and biologic processes involved in HDACI-mediated TCP. Moreover, our preclinical studies provide evidence that dose-limiting TCP induced by HDACIs may be circumvented using a TPO mimetic.
Publisher: American Society of Hematology
Date: 03-05-2012
DOI: 10.1182/BLOOD-2011-11-394858
Abstract: Apoptotic caspases, including caspase-9, are thought to facilitate platelet shedding by megakaryocytes. They are known to be activated during platelet apoptosis, and have also been implicated in platelet hemostatic responses. However, the precise requirement for, and the regulation of, apoptotic caspases have never been defined in either megakaryocytes or platelets. To establish the role of caspases in platelet production and function, we generated mice lacking caspase-9 in their hematopoietic system. We demonstrate that both megakaryocytes and platelets possess a functional apoptotic caspase cascade downstream of Bcl-2 family-mediated mitochondrial damage. Caspase-9 is the initiator caspase, and its loss blocks effector caspase activation. Surprisingly, steady-state thrombopoiesis is unperturbed in the absence of caspase-9, indicating that the apoptotic caspase cascade is not required for platelet production. In platelets, loss of caspase-9 confers resistance to the BH3 mimetic ABT-737, blocking phosphatidylserine (PS) exposure and delaying ABT-737–induced thrombocytopenia in vivo. Despite this, steady-state platelet lifespan is normal. Casp9−/− platelets are fully capable of physiologic hemostatic responses and functional regulation of adhesive integrins in response to agonist. These studies demonstrate that the apoptotic caspase cascade is required for the efficient death of megakaryocytes and platelets, but is dispensable for their generation and function.
No related grants have been discovered for Chloe James.