Deletion of Bak and Bax, the effectors of mitochondrial apoptosis, does not affect platelet production, however, loss of prosurvival Bcl-xL results in megakaryocyte apoptosis and failure of platelet shedding.

Mature megakaryocytes depend on the function of Bcl-x L , a member of the Bcl-2 family of prosurvival proteins, to proceed safely through the process of platelet shedding. Despite this, loss of Bcl-x L 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 IntroductionMegakaryocytes are large polyploid cells responsible for the production of blood platelets. They develop primarily in the BM and spleen. On reaching maturity, megakaryocytes extend long pseudopodial projections called proplatelets into the circulation, and it is from these structures that platelets are released. [1][2][3] Once born, platelets have a brief lifespan in the circulation: 10 days in humans, 5 days in mice. 4,5 In recent years, it has become apparent that this finite existence is governed by the interplay between members of the Bcl-2 protein family, the critical regulators of the "intrinsic" or "mitochondrial" apoptosis pathway. 6 Platelets depend on the prosurvival protein Bcl-x L to maintain their viability. 7 Mutations in murine Bcl-x L cause dose-dependent cell-intrinsic reductions in platelet lifespan. 7-9 Pharmacologic blockade of Bcl-x L with the BH3 mimetic drugs ABT-737 10 or ABT-263 11 triggers platelet death 7,12-15 and thrombocytopenia in mice, 7,16 dogs, 15 and humans. 17,18 The function of Bcl-x L is to restrain the prodeath proteins Bak and Bax. When activated, Bak and Bax induce mitochondrial outer membrane permeabilization (MOMP), resulting in platelet apoptosis, which, at least in vitro, is characterized by cytochrome c release, caspase activation, and phosphatidylserine exposure. [12][13][14][15] At steady state in vivo, aged platelets that have escaped hemostatic consumption undergo Bak-mediated apoptosis and clearance from the circulation. Genetic deletion of Bak and Bax almost doubles platelet lifespan, 9 rescues the thrombocytopenia caused by loss of Bcl-x L , 9 and renders platelets refractory to the effects of ABT-737. 13 We recently demonstrated that Bcl-x L is also essential for mature megakaryocytes to proceed safely through the process of platelet shedding. 9 In vitro, loss of Bcl-x L triggers Bak-and Bax-mediated mitochondrial damage, caspase activation, and failure of proplatelet formation. In vivo, mature, shedding megakaryocytes lacking Bcl-x L exhibit severe dysmorphology and produce grossly abnormal platelets that survive only hours in the circulation. Bcl-x L is, however, dispensable for the growth and development of megakaryocytes, suggesting that other Bcl-2 family prosurvival proteins are necessary to promote survival during this process. Of the 5 prosurvival members of the Bcl-2 family, 6 3 are known to be expressed in megakaryocytes: Bcl-2, Bcl-x L , and Mcl-1. 9 Given its critical role in hematopoietic progenitors and lymphocy...