Ectopic expression of the Aspergillus nidulans mitotic inducer, nimA kinase, in megakaryocytes: effect on polyploidization

Graded repression of PU.1/Sfpi1 gene transcription by GATA factors regulates hematopoietic cell fate

GATA-1 and PU.1 are essential hematopoietic transcription factors that control erythromegakaryocytic and myelolymphoid differentiation, respectively. These proteins antagonize each other through direct physical interaction to repress alternate lineage programs. We used immortalized Gata1(-) erythromegakaryocytic progenitor cells to study how PU.1/Sfpi1 expression is regulated by GATA-1 and GATA-2, a related factor that is normally expressed at earlier stages of hematopoiesis. Both GATA factors bind the PU.1/Sfpi1 gene at 2 highly conserved regions. In the absence of GATA-1, GATA-2 binding is associated with an undifferentiated state, intermediate level PU.1/Sfpi1 expression, and low-level expression of its downstream myeloid target genes. Restoration of GATA-1 function induces erythromegakaryocytic differentiation. Concomitantly, GATA-1 replaces GATA-2 at the PU.1/Sfpi1 locus and PU.1/Sfpi1 expression is extinguished. In contrast, when GATA-1 is not present, shRNA knockdown of GATA-2 increases PU.1/Sfpi1 expression by 3-fold and reprograms the cells to become macrophages. Our findings indicate that GATA factors act sequentially to regulate lineage determination during hematopoiesis, in part by exerting variable repressive effects at the PU.1/Sfpi1 locus.

A selective effect of Mpl ligand on mRNA stabilization during megakaryocyte differentiation

Megakaryocytes, the platelet precursors, are induced to differentiate in response to Mpl ligand. Here we report that stability of the megakaryocyte-specific platelet factor 4 (PF4) mRNA is substantially augmented in the presence of Mpl ligand. This stabilization requires protein synthesis, but the 3'-untranslated region of PF4 mRNA is not sufficient for granting the effect. This cytokine also significantly or mildly stabilizes Mpl receptor or GAPDH mRNAs, respectively, in contrast to a previously reported lack of effect on P2Y(1) receptor mRNA. Our study is the first to suggest that Mpl ligand-induced lineage specification is also determined by message stabilization.

BclxL overexpression in megakaryocytes leads to impaired platelet fragmentation

Fragmentation of polyploid megakaryocytes into platelets has great relevance for blood homeostasis. Apoptotic cell death is a highly regulated genetic program, which has been observed in mature megakaryocytes fragmenting into platelets. The antiapoptotic protein BclxL has been reported as up-regulated during megakaryocytic differentiation in vitro, but absent during late megakaryopoiesis. Our study focused on examining BclxL levels in megakaryocytes in vivo and in assessing the effect of its overexpression in transgenic mice (via the platelet factor 4 [PF4] promoter) on megakaryocyte development and platelet fragmentation. Interestingly, in the wild-type and less in PF4-driven transgenic mice, BclxL was not detected in a fraction of the large mature megakaryocytes, suggesting a regulation on the protein level. BclxL overexpression was associated with a moderate increase in megakaryocyte number, with no significant change in ploidy level or platelet counts. When the mice were challenged by induction of immune thrombocytopenia, the rate of platelet recovery was significantly slower in the transgenic mice as compared with controls. Moreover, proplatelet formation in vitro by transgenic megakaryocytes was limited. Transgenic megakaryocytes displayed poorly developed platelet demarcation membranes and cell margin extensions. Our study indicates that regulated expression of BclxL in megakaryocytes is important for the development of cells with a high potential to fragment into platelets.