Placental rescue reveals a sole requirement for c-Myc in embryonic erythroblast survival and hematopoietic stem cell function

The c-Myc protein has been implicated in playing a pivotal role in regulating the expression of a large number of genes involved in many aspects of cellular function. Consistent with this view, embryos lacking the c-myc gene exhibit severe developmental defects and die before midgestation. Here, we show that Sox2Cre-mediated deletion of the conditional c-myc(flox) allele specifically in the epiblast (hence trophoectoderm and primitive endoderm structures are wild type) rescues the majority of developmental abnormalities previously characterized in c-myc knockout embryos, indicating that they are secondary defects and arise as a result of placental insufficiency. Epiblast-restricted c-Myc-null embryos appear morphologically normal and do not exhibit any obvious proliferation defects. Nonetheless, these embryos are severely anemic and die before E12. c-Myc-deficient embryos exhibit fetal liver hypoplasia, apoptosis of erythrocyte precursors and functionally defective definitive hematopoietic stem/progenitor cells. Specific deletion of c-myc(flox) in hemogenic or hepatocytic lineages validate the hematopoietic-specific requirement of c-Myc in the embryo proper and provide in vivo evidence to support a synergism between hematopoietic and liver development. Our results reveal for the first time that physiological levels of c-Myc are essential for cell survival and demonstrate that, in contrast to most other embryonic lineages, erythroblasts and hematopoietic stem/progenitor cells are particularly dependent on c-Myc function.

The role of TIF1 gamma during adult murine hematopoiesis

Konstantin Shakhbazov

Transforming Growth Factor beta (TGFβ) signaling plays an important role in a variety of cellular processes during embryonic development, adult tissue homeostasis and cancer. TGFβI ligand is a potent inhibitor of early hematopoietic progenitor [1] cells in vitro, however the exact role and mechanisms of this pathway during adult hematopoietic homeostasis remains poorly understood. The Transcriptional Intermediary Factor 1 gamma (Tif1γ) gene encodes a recently uncovered nuclear protein in the TGFβ transduction pathway. Tif1γ is defective in the zebrafish moonshine mutant, which exhibits primitive and definitive hematopoiesis defects [2]. In addition, Tif1γ has been shown to influence differentiation of human erythroid cells in vitro [3]. Recent work in our laboratory has provided genetic evidence that Tif1γ is required for the generation and survival of hematopoietic stem/progenitor cells during murine development (C.Adolphe, K.Shakhbazov et al., manuscript under submission). Given that Tif1γ function is required for embryonic development and null mutants are embryonic lethal [4] (R.Losson unpublished data), we took advantage of a conditional Tif1γ flox allele (R.Losson unpublished data) and combined it with the inducible MxCre line [5] to generate mice in which Tif1γ function can be specifically ablated during adult hematopoiesis. MxCre:Tif1γKO/flox mutants exhibit a decrease in erythroblasts and dramatic increase in granulocytes within the bone marrow. This phenotype is attributable to a massive increase in granulocyte-monocyte progenitors (GMPs) at the expense of common myeloid progenitors (CMPs) and megakaryocyte- erythrocyte progenitors (MEPs), which are severely decreased in Tif1γ mutant bone marrow. In addition, pre-B cells were dramatically decreased in numbers, attributable to an increase in cell death, indicating a pro-survival role for Tif1γ in B lymphocytes. In addition, we performed deletion of Tif1γ in non-competitive and competitive bone marrow transplantation settings, which indicated that only the CMP/MEP phenotype is a non-cell autonomous effect. Two distinct models have been reported suggesting how Tif1γ functions within the TGFβ pathway: one indicating that Tif1γ binds and acts through Smad2/3 and the other that Tif1γ functions as a Smad4 mono-ubiquitin ligase [3, 6, 7]. To genetically address this apparent discrepancy, we generated double Tif1γ/Smad4 hematopoietic specific mutant mice, whose phenotype was surprisingly similar to that observed in Tif1γ single mutants. These data suggest that Tif1γ function is independent of Smad4 in hematopoietic cells. To address the mechanism by which Tif1γ functions, we performed microarray profiling of normal and mutant progenitors, which revealed prostaglandin D2 synthase as a hub gene linking Tif1γ and TGFβ signaling. Finally, we were able to mimic the hematopoietic-Tif1γ null phenotype by providing 16,16-dimethyl prostaglandin D2 to wild-type hematopoietic cells in vitro. In summary, our data provide genetic evidence for a key role of Tif1γ during early myeloid development as well as pre-B cell survival. In addition, we have identified that the majority of Tif1γ function is Smad4 independent. Finally, we identified a novel link between Tif1γ function and prostaglandin metabolism/signaling as a likely mechanism by which Tif1γ regulates hematopoiesis.

EPFL2009

Dual Lineage-Specific Expression of Sox17 During Mouse Embryogenesis

Anne Grapin-Botton, Marine Marie Kraus, Laurence Anne E Lemaire

Sox17 is essential for both endoderm development and fetal hematopoietic stem cell (HSC) maintenance. While endoderm-derived organs are well known to originate from Sox17-expressing cells, it is less certain whether fetal HSCs also originate from Sox17-expressing cells. By generating a Sox17GFPCre allele and using it to assess the fate of Sox17-expressing cells during embryogenesis, we confirmed that both endodermal and a part of definitive hematopoietic cells are derived from Sox17-positive cells. Prior to E9.5, the expression of Sox17 is restricted to the endoderm lineage. However, at E9.5 Sox17 is expressed in the endothelial cells (ECs) at the para-aortic splanchnopleural region that contribute to the formation of HSCs at a later stage. The identification of two distinct progenitor cell populations that express Sox17 at E9.5 was confirmed using fluorescence-activated cell sorting together with RNA-Seq to determine the gene expression profiles of the two cell populations. Interestingly, this analysis revealed differences in the RNA processing of the Sox17 mRNA during embryogenesis. Taken together, these results indicate that Sox17 is expressed in progenitor cells derived from two different germ layers, further demonstrating the complex expression pattern of this gene and suggesting caution when using Sox17 as a lineage-specific marker. STEM Cells2012;30:22972308

Wiley-Blackwell2012

The role of TIF1 gamma during adult murine hematopoiesis

Konstantin Shakhbazov

EPFL2009