The Obligate Role of TAK1 in the Survival of Hematopoietic Stem Cells and Progenitors in Mice.

TGFβ1-activated kinase 1 (TAK1), a member of the MAPKKK family, is a key mediator of stress and proinflammatory signals. TAK1 can be activated by inflammation-mediating cytokines, including tumor necrosis factor-α (TNF-α and interleukin-1b (IL-1β), as well as by T- and B- cell receptors (TCR/BCR), and Toll-like receptors (TLRs) signals. Activated TAK1 induces the nuclear localization of NF-kB and the activation of JNK/AP1 by stimulating IKKβ and MKK3/MKK6 phosphorylation respectively. TAK1 has been found to play an important role in inflammation, immunity, T- and B-cell activation, and epithelial cell survival. The TAK1^{−/ −} phenotype is lethal in mice at the early embryonic stage. We found higher levels of TAK1 expression and activity in hematopoietic stem cells and progenitors (HSC/Ps), and reduced expression and activity in differentiated mature hematopoietic cells. To study the role of TAK1 in bone marrow hematopoiesis, we generated inducible-TAK1 knockout mice by crossing TAK1^loxp mice with Mx1Cre mice, the latter being an interferon-inducible Cre mouse line. After injection of polyI:C to induce the knockout, we found that all the TAK1 knockout mice died within 8 to 10 days after the first polyI:C injection, showing severe hematopoietic and other defects; heterozygotes were phenotypically comparable to wild-type control animals. The TAK1 deletion in these mice resulted in ablation of bone marrow hematopoiesis due to the loss of C-Kit⁺ HSC/Ps. Annexin-V staining showed a 3-fold increase in apoptosis in the C-Kit⁺ HSC/Ps from TAK1 mutant mice compared to those from littermate control mice. Almost all of the mutant animals showed intestinal bleeding as well as other hemorrhaging due to the significant reductions in platelet counts. In reciprocal bone marrow transplantation experiments, we found that the TAK1-mutant bone marrow microenvironment was able to support the growth and function of wild-type HSC/Ps, while HSC/Ps from TAK1^{−/ −} mice failed to grow within the wild-type bone marrow microenvironment. These observations suggest that the bone marrow ablation phenotype which develops in TAK1-mutant mice is the result of intrinsic defects in HSC/P’s. We propose that TAK1-mutant HSC/Ps might mediate a survival signal for HSC/Ps stimulated by hematopoietic growth factors and cytokines, such as stem cell factor (SCF). The details of possible mechanisms by which this phenomenon might occur is currently under active investigation by our group.