Caspase activation is involved in early megakaryocyte differentiation but not in platelet production from megakaryocytes

Kozuma, Yukinori; Yuki, Satoshi; Ninomiya, Haruhiko; Nagasawa, Toshiro; Kojima, Hiroshi

doi:10.1038/leu.2009.7

Cited by 23 publications

(28 citation statements)

References 23 publications

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“…We therefore tested the ability of megakaryocytes from WT and BCL-2 tg mice to produce pro-platelets in vitro . Consistent with a previous report, 30 Bcl-2 overexpression did not decrease proplatelet formation by fetal liver-derived megakaryocytes but, rather, caused a small but significant increase (Figure 4e). These data indicate that, at least in culture, BCL-2 tg megakaryocytes exhibit no obvious defects in maturation and proplatelet formation.…”

Section: Resultssupporting

confidence: 93%

Loss of Bak enhances lymphocytosis but does not ameliorate thrombocytopaenia in BCL-2 transgenic mice

et al. 2014

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Bax and Bak are critical effectors of apoptosis. Although both are widely expressed and usually functionally redundant, recent studies suggest that Bak has particular importance in certain cell types. Genetic and biochemical studies indicate that Bak activation is prevented primarily by Mcl-1 and Bcl-xL, whereas Bax is held in check by all pro-survival Bcl-2 homologues, including Bcl-2 itself. In this study, we have investigated whether loss of Bak or elevated Mcl-1 modulates haemopoietic abnormalities provoked by overexpression of Bcl-2. The Mcl-1 transgene had little impact, probably because the expression level was insufficient to effectively reduce Bak activation. However, loss of Bak enhanced lymphocytosis in vavP-BCL-2 transgenic mice and increased resistance of their thymocytes to some cytotoxic agents, implying that Bak-specific signals can be triggered in certain lymphoid populations. Nevertheless, lack of Bak had no significant impact on thymic abnormalities in vavP-BCL-2tg mice, which kinetic analysis suggested was due to accumulation of self-reactive thymocytes that resist deletion. Intriguingly, although Bak−/− mice have elevated platelet counts, Bak−/−vavP-BCL-2 mice, like vavP-BCL-2 littermates, were thrombocytopaenic. To clarify why, the vavP-BCL-2 platelet phenotype was scrutinised more closely. Platelet life span was found to be elevated in vavP-BCL-2 mice, which should have provoked thrombocytosis, as in Bak−/− mice. Analysis of bone marrow chimaeric mice suggested the low platelet phenotype was due principally to extrinsic factors. Following splenectomy, blood platelets remained lower in vavP-BCL-2 than wild-type mice. However, in Rag1−/− BCL-2tg mice, platelet levels were normal, implying that elevated lymphocytes are primarily responsible for BCL-2tg-induced thrombocytopaenia.

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Section: Resultssupporting

confidence: 93%

Loss of Bak enhances lymphocytosis but does not ameliorate thrombocytopaenia in BCL-2 transgenic mice

et al. 2014

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“…Conversely, disabling the pathway by knocking out Bak and Bax had no impact on thrombopoiesis. This is consistent with data showing that platelet counts in Bcl-2 transgenic mice are not significantly different to wild type when the spleen is removed, 26 and that Bim Ϫ/Ϫ mice on a C57BL/6 background are no longer thrombocytopenic. 27 Thus, it appears that Bcl-2 family-mediated apoptosis is not used by megakaryocytes to produce platelets.…”

Section: Introductionsupporting

confidence: 80%

Caspase-9 mediates the apoptotic death of megakaryocytes and platelets, but is dispensable for their generation and function

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Schoenwaelder

Josefsson

et al. 2012

Blood

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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 phospha- IntroductionApoptotic caspases are a family of aspartate-specific cysteinyl proteases that are activated during, and facilitate the execution of, programmed cell death. They cleave a range of cellular substrates, thereby causing the morphologic and biochemical signatures of apoptosis, such as DNA fragmentation, membrane blebbing, and phosphatidylserine (PS) externalization. In addition to their role in cell death, apoptotic caspases have been ascribed an increasing number of functions. These include critical roles in the differentiation of erythroid cells, 1 osteoblasts, 2 keratinocytes, 3 lens fiber cells, 4 skeletal muscle, 5 and embryonic stem (ES) cells. 6 Apoptotic caspases have also been suggested to regulate B-lymphocyte proliferation, 7 HSC quiescence, 8 activation of microglia, 9 and the reprogramming of fibroblasts into iPS (induced pluripotent stem cells). 10 There are 2 pathways to apoptosis, the intrinsic and the extrinsic. Both ultimately converge on the apoptotic effector caspases, caspase-3 and caspase-7. The intrinsic (or mitochondrial) apoptosis pathway is regulated by the interaction between Bcl-2 family proteins, which are divided into prosurvival and prodeath subsets. The critical mediators of the pathway are the prodeath proteins Bak and Bax, which, in viable cells are restrained by one or more of the 5 prosurvivals: Bcl-2, Bcl-x L , Mcl-1, Bcl-w, and A1. In response to apoptotic signals such as DNA damage or growth factor deprivation, a third group of Bcl-2 family members, the "BH3-only" proteins, trigger the activation of Bak and Bax. 11,12 Active Bak and Bax cause mitochondrial outer membrane permeabilization (MOMP), which allows apoptogenic factors, including cytochrome c, to enter the cytosol. 13 Cytochrome c interacts with the scaffolding protein Apaf-1. Subsequently, the initiator caspase, caspase-9, is recruited to the cytochrome c/Apaf-1 complex to form the apoptosome. 14 This leads to the activation of caspase-9 and triggering of the caspase cascade. The importance of the caspas...

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“…However, staurosporine does not stimulate thrombopoiesis, rather, it causes mitochondrial damage, failure of proplatelet formation and death. Taken together, our results and those of several recent studies 18,22,[28][29][30]56,57 leave little room for the notion that apoptosis, as it is currently defined 54 , is required for, or can augment, platelet production by megakaryocytes. It remains possible that one or more components of the apoptotic machinery may play a role in the process, but convincing evidence for this has yet to emerge.…”

Section: Discussionmentioning

confidence: 47%

Platelet production proceeds independently of the intrinsic and extrinsic apoptosis pathways

et al. 2014

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Caspase activation is involved in early megakaryocyte differentiation but not in platelet production from megakaryocytes

Cited by 23 publications

References 23 publications

Loss of Bak enhances lymphocytosis but does not ameliorate thrombocytopaenia in BCL-2 transgenic mice

Loss of Bak enhances lymphocytosis but does not ameliorate thrombocytopaenia in BCL-2 transgenic mice

Caspase-9 mediates the apoptotic death of megakaryocytes and platelets, but is dispensable for their generation and function

Platelet production proceeds independently of the intrinsic and extrinsic apoptosis pathways

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