Mesenchymal Stem Cells Regulate Oxidative Stress-Induced Hematopoiesis Via Peroxisomal Metabolism

The role of peroxisomes in hematopoiesis and the bone marrow microenvironment remains poorly understood. Peroxisomes are small, membrane-bound organelles found in the cytoplasm of virtually all eukaryotic cells. They play a key role in various metabolic processes, primarily involving the breakdown of fatty acids, regulation of oxidative stress, the detoxification of hydrogen peroxide (H₂O₂), and the synthesis of plasmalogens. Past efforts in understanding the biology of peroxisomes has relied on knockout mice, but has been limited due to the effects on hematopoiesis and embryonically lethality. The novel PEX1-Gly844Asp knock-in hypomorphic mouse lacks peroxisome formation and is peroxisome deficient, but has survival into adulthood. We observed that peroxisome deficient animals had up to 50% greater numbers of peripheral lymphocytes, neutrophils, and platelets which contained 2-fold greater reactive oxygen species (ROS, p = 0.0002) compared to wild-type (WT) animals. Assessment of the bone marrow showed 2-fold greater numbers of total cells and colony forming units compared to WT (CFU, p = 0.0009 and < 0.0001, respectively). We found expansion (up to 3-fold) in the hematopoietic stem and progenitor cell (HSPC) compartment compared to that of WT demonstrated by in vivo enumeration of LSK (p < 0.0001) and LSK-SLAM (p < 0.0001). Importantly, through competitive bone marrow transplant experiments we show that peroxisome deficient cells outcompete WT by 50% (6 months after primary and secondary transplant). Interestingly, we further demonstrate that peroxisome deficient HSPC harbor very high levels of intrinsic ROS which are attenuated after repopulation. Isolation of mesenchymal stem cells (MSC) isolated from peroxisome deficient mice also showed 2-fold elevated levels of ROS compared to WT MSC. Finally, we found elevated levels of stem cell factor (SCF aka cKIT ligand) in the plasma of peroxisome deficient mice and peroxisome deficient MSC expressed 2-fold more SCF compared to WT. LSK expanded in the absence on SCF on peroxisome deficient MSC >10-fold greater than on WT MSC (p < 0.0001) in in vitro coculture assays. In conclusion, the increase in HSPC number we observed in peroxisome deficient animals is, in part, driven by a response to ROS in the microenvironment leading to increased SCF secretion. These data add new insight into the role of peroxisomes and oxidative stress in the bone marrow microenvironment and hematopoiesis.