title: Megakaryocytes possess a functional intrinsic apoptosis pathway that must be restrained to survive and produce platelets.
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-x(L) 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.
authors.affiliation: Molecular Medicine Division, Cancer and Hematology Division, The Walter and Eliza Hall Institute of Medical Research, Parkville 3052, Australia; Department of Medical Biology, The University of Melbourne, Parkville 3010, Australia.
authors.forename: Emma C
authors.initials: EC
authors.lastname: Josefsson
authors.forename: Chloé
authors.initials: C
authors.lastname: James
authors.forename: Katya J
authors.initials: KJ
authors.lastname: Henley
authors.forename: Marlyse A
authors.initials: MA
authors.lastname: Debrincat
authors.forename: Kelly L
authors.initials: KL
authors.lastname: Rogers
authors.forename: Mark R
authors.initials: MR
authors.lastname: Dowling
authors.forename: Michael J
authors.initials: MJ
authors.lastname: White
authors.forename: Elizabeth A
authors.initials: EA
authors.lastname: Kruse
authors.forename: Rachael M
authors.initials: RM
authors.lastname: Lane
authors.forename: Sarah
authors.initials: S
authors.lastname: Ellis
authors.forename: Paquita
authors.initials: P
authors.lastname: Nurden
authors.forename: Kylie D
authors.initials: KD
authors.lastname: Mason
authors.forename: Lorraine A
authors.initials: LA
authors.lastname: O'Reilly
authors.forename: Andrew W
authors.initials: AW
authors.lastname: Roberts
authors.forename: Donald
authors.initials: D
authors.lastname: Metcalf
authors.forename: David C S
authors.initials: DC
authors.lastname: Huang
authors.forename: Benjamin T
authors.initials: BT
authors.lastname: Kile
mesh_terms: D000818:Animals; D017209:Apoptosis / Q000502:physiology*; D001792:Blood Platelets / Q000378:metabolism*; D002470:Cell Survival; D008533:Megakaryocytes / Q000166:cytology* / Q000502:physiology*; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D050998:bcl-2 Homologous Antagonist-Killer Protein / Q000235:genetics / Q000378:metabolism; D051028:bcl-2-Associated X Protein / Q000235:genetics / Q000378:metabolism; D051020:bcl-X Protein / Q000235:genetics / Q000378:metabolism
chemical_list: D050998:bcl-2 Homologous Antagonist-Killer Protein; D051028:bcl-2-Associated X Protein; D051020:bcl-X Protein