TNFα-Induced Apoptosis of Hematopoietic Progenitors Is the Major Cause of Bone Marrow Failure Observed In TAK-1 Knockout Mice.

Abstract 1557

In bone marrow (BM) hematopoietic stem cells/progenitors (HSC/Ps), the apoptotic machinery is tightly controlled by a complex interplay between intrinsic signals and stimuli from the surrounding microenvironment, inducing a dynamically balanced network between pro-apoptotic and anti-apoptotic influences. Disruption of this balance can result in hematopoietic disorders such as various BM failure syndromes. Studies have suggested that tumor necrosis factor-α (TNF-α) and Fas-ligand induce programmed cell death and/or differentiation of HSC/Ps, thus exercising negative regulation over hematopoiesis. However, whether the apoptotic cell death induced by these factors plays a role in BM failure syndromes remains ambiguous.

We have reported that transforming growth factor beta-activated kinase-1 (TAK-1) plays an essential role in the survival of HSC/Ps. Mice with TAK-1 deletion in BM hematopoietic cells develop BM failure due to the apoptotic death of mutant HSC/Ps. We have taken advantage of the dramatic phenotypes of this mutant mouse line to closely examine TNF-α and Fas signaling in order to understand which of these is related to the induction of apoptosis of HSC/Ps, and to what degree. To do so, we generated TAK-1/TNFR1a, TAK-1/TNFR1b and TAK-1/Fas compound-mutant mice in order to evaluate which signaling system, when inactivated, permitted the rescue the BM failure defects observed in TAK-1 knockout mice, and the degree to which it did so, using both in vivo and in vitro studies.

We found that, as was the case with TAK-1 knockout mice, TAK-1/TNFR1b and TAK-1/Fas compound-mutant mice died within 8 to 10 days after induction of TAK-1 deletion with exactly the same type of BM failure observed in TAK-1 knockout mice. In vitro studies indicated that neither TNFR1b nor Fas deletion was able to protect cells from apoptosis, nor could they rescue the colony-forming ability of TAK-1 mutant HSC/Ps. However, TAK-1/TNFR1a-mutant mice appeared to be healthy one month after induction of TAK1 deletion. By careful analysis, we found that TNFR1a deletion partially rescued the BM failure phenotype of TAK-1 knockout mice. The total numbers of nucleated BM and splenic cells in TAK-1/TNFR1a- mutant mice are approximately 54.7% and 83.8% (respectively) of those of their wild-type littermate controls. These percentages represent significant increases comparing to their littermates with TAK-1 deletion only (7.5% and 17% of WT control). In vitro studies demonstrated that TNFR1a deletion restored colony-forming ability in 20–30% of TAK1-knockout HSC/Ps. Currently, we are in the process of analyzing the hematopoietic phenotypes of TAK-1/TNFR1a/1b triple-mutant mice in order to determine whether the complete inactivation of TNF-α signaling further reverses the hematopoietic defects seen in TAK1-knockout mice.

Our study demonstrated that TNF-α, via its receptor 1a-induced apoptosis, contributes substantially to the loss of HSC/Ps in TAK-1 knockout mice. Our results also suggest that pro-apoptotic signaling other than TNF-α is also involved in the BM failure syndrome observed in TAK-1 mutant mice.