Survivin Is Required for Proliferation, but Not Polyploidization of Megakaryocytes.

Abstract

Survivin is a member of chromosome passenger complex, which plays an important role in chromosome alignment, separation and cytokinesis. We recently reported that survivin is necessary for the proliferation and survival of hematopoietic stem and progenitor cells. Furthermore, we previously showed that reduced levels of survivin expression facilitates megakaryocyte development, whereas elevated levels of survivin inhibit their maturation and polyploidization. However, the extent to which survivin is necessary for polyploidization and terminal differentiation of committed megakaryocytes remains unclear. To determine whether survivin is required for megakaryocyte and platelet biogenesis, we mated mice with floxed alleles of survivin (sur fl/fl) to mice that express Cre recombinase under the control of the PF4 promoter. Compound mutant animals appeared grossly normal and harbored normal platelet counts. Furthermore, survivin deleted and control littermates displayed similar expression of CD41 and CD42, as well as similar DNA content within the CD41+ population. The only significant difference detected was an increase in annexin V staining of CD41+ cells within the bone marrow of the mice with survivin deletion. Analysis of DNA extracted from these bone marrows showed no evidence of the survivin deletion, indicating that the surviving cells all escaped excision. These in vivo findings are consistent with a requirement for survivin in the survival or proliferation of megakaryocyte progenitors. Next, to induce megakaryocyte development ex vivo, we cultured bone marrow from surv fl/fl mice in vitro in the presence of TPO. Using this approach, we were able to induce survivin deletion in 75% of the cells as evidenced by PCR. Despite the deletion of survivin, polyploidization of the ex vivo generated megakaryocytes was unaffected. Finally, we induced deletion of survivin by retroviral infection of surv fl/fl progenitors with MSCV-Cre and found that megakaryocyte polyploidization was actually increased in the excised population. Taken together, our results suggest that survivin is not required for polyploidization, but is necessary for proliferation of megakaryocyte progenitors.