Fetal Hematopoietic Development in the Mouse Aorta-Gonad-Mesonephros Region Is Inhibited by Homeodomain-Interacting Protein Kinase 2.

Definitive hematopoiesis starts in the aorta-gonad-mesonephros (AGM) region of mouse embryo. Our previous studies showed that STAT3, a gp130 downstream transcription factor, is required for AGM hematopoiesis and that homeodomain-interacting protein kinase 2 (HIPK2) phosphorylates serine 727 of STAT3. HIPK2 is a serine/threonine kinase known to be involved in transcriptional repression and apoptosis. In the present study, we examined the role of HIPK2 in the hematopoietic development. HIPK2 transcripts were found in the fetal hematopoietic tissues such as the AGM region at embryonic day (E) 10.5 and E11.5 and the E14.5 fetal liver as detected by RT-PCR and whole mount in situ hybridization. The functional analysis of HIPK2 was done by introducing wild-type and its mutant constructs into the primary culture of the mouse AGM region. Retrovirus mediated forced expression of HIPK2 with green fluorescent protein (GFP) in the AGM culture remarkably inhibited the emergence of GFP⁺ nonadherent cells. In coculture of virus-infected GFP⁺ nonadherent cells with OP9 stromal cells, no cobblestone-like area-forming colonies were detected in the HIPK2-infected cells, while control GFP virus had no effect on the colony formation. Similarly, when GFP⁺ nonadherent cells were sorted and cultured in semisolid medium, colonies were not formed from the HIPK2-GFP virus-infected AGM culture. These data suggest that HIPK2 suppresses differentiation of adherent endothelial-like cells to colony-forming hematopoietic cells or, alternatively HIPK2 inhibits proliferation of newly developed hematopoietic progenitors. Development of cells positive for CD45 (an immature hematopoietic cell maker) cells was suppressed by forced expression of HIPK2 in the AGM culture. To determine the domain important for the inhibitory activity of HIPK2, we constructed a series of point and deletion mutants. Overexpression of HIPK2 mutants in the AGM culture showed that a certain structure of the kinase domain as well as its C-terminally located structure but not the kinase activity is essential for the HIPK2-mediated inhibition of hematopoiesis. When nonadherent cells in the AGM culture were fractionated for their expression levels of CD45 and c-Kit (a receptor for stem cell factor), CD45^lowc-Kit⁺ cells have a potential to form hematopoietic cell colonies. In the HIPK2-overexpressed AGM culture, emergence of CD45^lowc-Kit⁺ cells was significantly inhibited. These data indicated that HIPK2 plays a negative regulatory role in the development of AGM hematopoiesis in mouse embryo.