Hematopoietic TAK1 is Required for Maintaining Lysosomal Integrity and Cell Survival in Resident Macrophages

Transforming growth factor‐β activated kinase 1 (TAK1) is an intracellular signaling molecule regulating apoptosis and necrosis (necroptosis) in several cell types, including epithelial and endothelial cells. Aberrant cell death of hematopoietic cells, both in excess and in deficiency, is known to cause a number of diseased conditions, but the role of TAK1 in hematopoietic cells is not yet fully determined. This study aimed to determine the role of TAK1 in the hematopoietic system, specifically during development, where hematopoietic cells are involved not only in immunity but also in morphogenesis. Here, we report that hematopoietic TAK1 plays a critical role predominantly in tissue‐resident macrophage maintenance and survival. Hematopoietic lineage‐specific deletion of Tak1 gene (Tak1 HKO ) showed diminished thymic and lung resident macrophages, accumulation of cellular debris in the embryonic thymus, and impaired alveolar expansion in perinatal mice, which were accompanied by animal mortality. However, thymocytes, splenic lymphocytes, and myeloid cells developed normally in Tak1 HKO mice, suggesting that TAK1 is required selectively for the maintenance of macrophages in the embryonic hematopoietic system. Resident macrophages are known to play a vital role during embryonic development, and depletion or alterations of macrophage function during organogenesis can lead to developmental defects or embryonic lethality. Thus, loss of macrophage in Tak1 HKO mice is likely the cause of accumulation of dead cell debris and impaired lung development. To investigate the role of TAK1 in macrophages, we utilized bone marrow‐derived macrophages (BMDMs). We found that Tak1 gene deletion induced profound cell death in BMDMs without exogenous stimulation. Since macrophages express cell death‐inducing receptors ( e.g. TNF receptor (TNFR) and pattern recognition receptors), autocrine signaling may be involved in the cell death. However, the ablations of toll‐like receptor (TLR) signaling intermediates Trif or Myd88 did not block cell death, while Tnfr1 ‐deletion only partially restored cell viability in Tak1 ‐deficient BMDMs. This suggests the involvement of TNF/TLR‐independent mechanisms of cell death, potentially associated with cell intrinsic mechanisms. Tak1 ‐deficient BMDMs exhibited marginal activation of caspase‐3, suggesting non‐apoptotic types of cell death. We found that Tak1 ‐deficient BMDMs exhibited aberrant lysosomal structures, which can lead to the leakage of lysosomal proteins. Consistently, inhibition of a lysosomal protease, cathepsin B, reduced Tak1 ‐deficient BMDM death. Furthermore, we found that ROS was highly upregulated in Tak1 ‐deficient BMDMs, and ROS scavenging effectively prevented cell death. Since active phagocytosis in macrophages is associated with lysosomal ROS production, Tak1 deficiency may result in the failure to eliminate lysosomal ROS. Such accumulation of lysosomal ROS may be the cause of the lysosome rupture‐induced cell death in Tak1 ‐deficient macrophages. Our results collectively suggest a novel function of TAK1 in maintaining lysosome integrity in macrophages, which is essential for resident macrophage survival and organ development. Support or Funding Information R01GM068812 R01GM112986Working model: TAK1 prevents macrophage death by preventing ROS‐induced lysosomal rupture and subsequent cathepsin release.