Megakaryocytes possess a functional intrinsic apoptosis pathway that must be restrained to survive and produce platelets

Abstract

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.

Mcl-1 and Bcl-xL regulate Bak/Bax-dependent apoptosis of the megakaryocytic lineage at multistages

“…Ploidy analysis revealed an increase in large polyploid megakaryocytes (32N, 64N) of the knockout mice ( Figure 1n) and their serum TPO levels were significantly lower than those of the control littermates ( Figure 1o); both findings were consistent with previous reports. 16 These findings suggested that Bcl-xL was involved in preventing mature megakaryocytes from apoptosis but was not essential for their growth and development.…”

“…Nevertheless, platelets possess Bcl-xL alone, unlike megakaryocytes in which two functional anti-apoptotic proteins, Bcl-xL and Mcl-1, are present. 11,12,16 We thus studied the effect of post-transcriptional degradation of Mcl-1 in platelets in vivo. At 1 or 2 days after the administration to wild-type mice of MG-132, a proteasome inhibitor, western blot revealed that the Mcl-1 protein was observed in platelets isolated from the MG-132-treated mice in contrast to platelets from the vehicle-treated mice (Figure 6a).…”

“…10,31 This agrees with our in-vitro study showing moderate apoptosis of megakaryoblastic cells upon ABT-737 treatment (Figures 5b-e). On the other hand, in in-vivo settings, Josefsson et al 16 very recently analyzed this in detail and reported that Bcl-xL played an important role in preventing mature megakaryocytes from apoptosis at the proplatelet formation stage but was not required for their growth and development. In agreement with these findings, in our in-vivo study, although apoptosis of mature megakaryocytes was observed in the Bcl-xL knockout mice, they showed an increased number of mature megakaryocytes, containing many large polyploid cells (Figures 1k-n).…”

Platelet production proceeds independently of the intrinsic and extrinsic apoptosis pathways

“…In platelets, BCL-X L is the primary prosurvival protein that restrains BAK and BAX [18][19][20][21] . Accordingly, mutations in murine BCL-X L cause dose-dependent, cell-intrinsic reductions in platelet lifespan 18,19,22 . Moreover, pharmacological blockade of BCL-X L with the BH3 mimetic drugs ABT-737 (ref.…”

“…In megakaryocytes, MCL-1 and BCL-X L are both required to keep BAK and BAX in check 18,28,29 . Their simultaneous deletion leads to a failure of megakaryopoiesis, haemorrhage and embryonic lethality.…”