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

Abstract

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.

Effect of thrombopoietin receptor agonists on the apoptotic profile of platelets in patients with chronic immune thrombocytopenia

“…These could, in theory, resist apoptotic clearance for a period beyond that of normal platelets, achieving an extension of circulating life span and contributing to elevated platelet counts. In support of this notion, transgenic overexpression of BCL‐2 has recently been shown to increase platelet life span in mice . Surprisingly, after 14 days of treatment with eltrombopag, the sensitivity of patients' platelets to ABT‐737 returned to baseline.…”

Regulation of Platelet Production and Life Span: Role of Bcl-xL and Potential Implications for Human Platelet Diseases

“…The Mcl-1 protein has a short half-life in part as a result of fast proteosomal degradation. While Mcl-1 is critical for haematopoietic stem cells and multipotent haematopoietic progenitors [ 41 ] ( Figure 1 ), neither haematopoietic overexpression of Mcl-1 nor conditional deletion in the platelet lineage affected platelet counts or platelet life span in mice [ 42 , 43 , 44 ]. Mcl-1 protein is present in MKs, but absent in both human and murine platelets [ 19 , 20 , 22 , 28 , 42 , 43 ].…”

Regulation of platelet lifespan in the presence and absence of thrombopoietin signaling

“…Conversely, one might expect an absence of TPO signaling to result in less Bcl‐x L , resulting in compromised platelet survival. In mice, loss of Bak and Bax almost doubles the platelet lifespan , and overexpression of prosurvival BCL‐2 extends platelet survival . However, the effect of TPO receptor signaling on platelet survival is unresolved.…”