The tsA58 simian virus 40 large tumor antigen disrupts megakaryocyte differentiation in transgenic mice

Conditional overexpression of transgenes in megakaryocytes and platelets in vivo

Megakaryocyte (MK)-specific transgene expression has proved valuable in studying thrombotic and hemostatic processes. Constitutive expression of genes, however, could result in altered phenotypes due to compensatory mechanisms or lethality. To circumvent these limitations, we used the tetracycline/doxycycline (Tet)-off system to conditionally over-express genes in megakaryocytes and platelets in vivo. We generated 3 transactivator transgenic lines expressing the Tet transactivator element (tTA), under the control of the MK-specific platelet factor 4 promoter (PF4-tTA-VP16). Responder lines were simultaneously generated, each with a bidirectional minimal cytomegalovirus (CMV)-tTA responsive promoter driving prokaryotic beta-galactosidase gene, as a cellular reporter, and a gene of interest (in this case, the mitotic regulator Aurora-B). A transactivator founder line that strongly expressed PF4-driven tTA-viral protein 16 (VP16) was crossbred to a responder line. The homozygous double-transgenic mouse line exhibited doxycycline-dependent transgene overexpression in MKs and platelets. Using this line, platelets were conveniently indicated at sites of induced stress by beta-galactosidase staining. In addition, we confirmed our earlier report on effects of constitutive expression of Aurora-B, indicating a tight regulation at protein level and a modest effect on MK ploidy. Hence, we generated a new line, PF4-tTA-VP16, that is available for conditionally overexpressing genes of interest in the MK/platelet lineage in vivo.

−245 bp of 5′-Flanking Region From the Human Platelet Factor 4 Gene Is Sufficient to Drive Megakaryocyte-Specific Expression In Vivo

Platelet factor 4 (PF4) serves as a lineage-specific marker of megakaryocyte development. We previously identified two positively acting sequences in the human platelet factor 4 (hPF4) gene promoter that synergized to drive high-level luciferase reporter gene expression in vitro. Using portions of the hPF4 5′-flanking region linked to the lacZ reporter gene, we observed in this investigation that constructs with −245 bp of 5′-flanking region were more active than constructs with −2 kb of 5′-flanking region in vitro. We created two independent transgenic mouse lines with a −245-bp hPF4/lacZ construct. Cells from these mice were tested for β-galactosidase (β-gal) expression at the mRNA level by Northern blot and semiquantitative reverse transcription polymerase chain reaction (RT-PCR) and at the protein level by immunohistochemistry assay. Mice from one line showed β-gal expression specifically in all megakaryocytes of all ploidy classes from bone marrow and in platelets. Expression level was comparable to that driven by the 1.1-kb rat PF4 promoter in other transgenic mouse lines. Those in the second line showed no β-gal expression in megakaryocytes, platelets, or any of the eight organs tested. The −245-bp hPF4 promoter is capable of driving reporter gene expression in a megakaryocyte-specific manner in transgenic mice. The small size of this megakaryocyte-specific promoter is compatible with that required in some viral vectors and may provide a model for targeting gene expression to megakaryocytes.

−245 bp of 5′-Flanking Region From the Human Platelet Factor 4 Gene Is Sufficient to Drive Megakaryocyte-Specific Expression In Vivo

Platelet factor 4 (PF4) serves as a lineage-specific marker of megakaryocyte development. We previously identified two positively acting sequences in the human platelet factor 4 (hPF4) gene promoter that synergized to drive high-level luciferase reporter gene expression in vitro. Using portions of the hPF4 5′-flanking region linked to the lacZ reporter gene, we observed in this investigation that constructs with −245 bp of 5′-flanking region were more active than constructs with −2 kb of 5′-flanking region in vitro. We created two independent transgenic mouse lines with a −245-bp hPF4/lacZ construct. Cells from these mice were tested for β-galactosidase (β-gal) expression at the mRNA level by Northern blot and semiquantitative reverse transcription polymerase chain reaction (RT-PCR) and at the protein level by immunohistochemistry assay. Mice from one line showed β-gal expression specifically in all megakaryocytes of all ploidy classes from bone marrow and in platelets. Expression level was comparable to that driven by the 1.1-kb rat PF4 promoter in other transgenic mouse lines. Those in the second line showed no β-gal expression in megakaryocytes, platelets, or any of the eight organs tested. The −245-bp hPF4 promoter is capable of driving reporter gene expression in a megakaryocyte-specific manner in transgenic mice. The small size of this megakaryocyte-specific promoter is compatible with that required in some viral vectors and may provide a model for targeting gene expression to megakaryocytes.

The cell cycle in polyploid megakaryocytes is associated with reduced activity of cyclin B1-dependent cdc2 kinase

The platelet precursor, the megakaryocyte, matures to a polyploid cell as a result of DNA replication in the absence of mitosis (endomitosis). The factors controlling endomitosis are accessible to analysis in our megakaryocytic cell line, MegT, generated by targeted expression of temperature-sensitive simian virus 40 large T antigen to megakaryocytes of transgenic mice. We aimed to define whether endomitosis consists of a continuous phase of DNA synthesis (S) or of S phases interrupted by gaps. Analysis of the cell cycle in MegT cells revealed that, upon inactivation of large T antigen, the cells shifted from a mitotic cell cycle to an endomitotic cell cycle consisting of S/Gap phases. The level of the G1/S cyclin, cyclin A, as well as of the G1 phase cyclin, cyclin D3, were elevated at the onset of DNA synthesis, either in MegT cells undergoing a mitotic cell cycle or during endomitosis. In contrast, the level of the mitotic cyclin, cyclin B1, cycled in cells displaying a mitotic cell cycle while not detectable during endomitosis. Comparable levels of the mitotic kinase protein, Cdc2, were detected during the mitotic cell cycle or during endomitosis; however, cyclin B1-dependent Cdc2 kinase activity was largely abolished in the polyploid cells. Fibroblasts immortalized with the same heat-labile oncogene do not display reduced levels of cyclin B1 upon shifting to high temperature nor do they become polyploid, indicating that reduced levels of cyclin B1 is a property of megakaryocytes and not of the T-antigen mutant. We conclude that cellular programming during endoreduplication in megakaryocytes is associated with reduced levels of cyclin B1.

E2F-1 blocks terminal differentiation and causes proliferation in transgenic megakaryocytes

The transcription factor E2F-1 plays a central role in the cell cycle through its ability to activate genes involved in cell division. E2F-1 activity is regulated by a number of proteins, including the retinoblastoma susceptibility gene product, cyclin-dependent kinases, and their inhibitors, proteins that have been implicated in the control of certain developmental processes. To investigate a potential role of E2F-1 in differentiation, we assayed the ability of megakaryocytes to form platelets in an in vivo transgenic model. E2F-1 expression in megakaryocytes blocked differentiation during maturation, resulting in severe thrombocytopenia. Ultrastructural analysis of megakaryocytes revealed abnormal development characterized by hyperdemarcation of cytoplasmic membranes and reduced numbers of alpha granules. Administration of megakaryocyte growth and development factor or interleukin 6 could not overcome the differentiation block. Additionally, E2F-1 caused massive megakaryocyte accumulation in both normal and ectopic sites, first evident in E15 embryonic liver. Furthermore, significant apoptosis was observed in transgenic megakaryocytes. These data indicate that E2F-1 can prevent terminal differentiation, probably through its cell cycle-stimulatory activity.