STAT3 Deletion In Hematopoietic Tissue Causes Mitochondrial Dysfunction with Increased ROS Leading to LT-Repopulating Stem Cell Depletion, Erythroid Dysplasia, Peripheral Neutrophilia, and a Rapid Aging-Like Phenotype.

Abstract 2621

The essential transcription factor, Stat3, has been extensively studied in the context of cytokine signal transduction. Tissue-specific targeted deletion of STAT3 has been studied in heart, liver, and other tissues. In mouse hematopoietic tissue, STAT3 deletion (STAT3-KO mice) results in uniform post-natal death about 6–8 weeks after birth, although we show here that the pups are born with normal peripheral lymphoid and myeloid blood cell counts and pups appear normal until 4–5 weeks old. Then an aging-like shift in the lymphoid/myeloid ratio quickly occurs and the animals die within about 2 weeks of this shift. These animals suffer from intestinal inflammation, leukocyte infiltration in various tissues/organs, osteoporosis, dysfunctional innate immunity, myeloid cell overproduction and dendritic cell depletion. These findings are believed to be due, primarily, to abnormal cytokine signaling, however, the exact cause of death remains unknown. Stat3 has recently been implicated as a positive regulator of mitochondrial respiration and function. Mitochondrial function has been linked to aging and aging-related diseases like myelodysplastic syndromes (MDS), anemia, and erythroid dysplasia. It is therefore possible that Stat3 deletion in hematopoietic tissue results in disruption of mitochondrial regulation, which could also contribute to the observed phenotype of STAT3-KO mice. We have now investigated the function of mitochondria in the bone marrow stem/progenitor compartment of these mice as well as hematopoietic stem cell (HSC) repopulating ability and other cytokine responses. We show, for the first time, that STAT3 deletion causes pronounced erythroid dysplasia and anemia with erythrocyte hypochromasia and codocytes (target cells), aging-like lymphoid/myeloid ratio shift, hypersegmented neutrophilia, pronounced bone marrow accumulation of immature myeloid cells (bands), erythroblast depletion, and splenic hypertrophy and enlargement that appears to be due to erythroid and myeloid cell infiltration suggesting a shift in the primary site of erythropoiesis from the bone marrow to the spleen. Colony formation from bone marrow myeloid progenitors was unaffected when stimulated with single cytokines, but synergistic cytokine combinations failed to demonstrate proliferative synergy. HSC competitive repopulating ability was significantly diminished, especially in female STAT3 KO donors, but secondary transplants show that HSC self-renewal potential may not be affected, thus suggesting a reduced HSC number/function in STAT3 KO mouse bone marrow. This is consistent with the finding of significantly decreased numbers of CD34⁻SLAM⁺LSK cells. Mitochondrial mass was significantly increased in CD34⁺LSK subpopulation, but mitochondrial function (membrane potential, ΔΨm) was significantly diminished in STAT3 KO mouse LSK cells, which is consistent with the proposed positive role of Stat3 in mitochondrial function and also consistent with the finding of significantly increased ROS levels in bone marrow LSK subpopulations. Graded loss of Sca-1 surface marker (differentiation) in these cells was associated a with a graded increase in ΔΨm (activation); STAT3 KO mouse LSK cells displayed premature mitochondrial activation during differentiation to Sca-1⁻ progenitors. These data show, for the first time, that in-vivo deletion of STAT3 in mouse hematopoietic stem/progenitor cells results in defective mitochondrial function with increased ROS production along with disrupted differentiation-driven mitochondrial biogenesis and activation in these cells. This likely contributes to an aging-like phenotype of the hematopoietic system with lymphoid/myeloid shift, anemia, non-sideroblastic erythroid dysplasia, myeloproliferative dysfunction, and diminished HSC number/function and raises the possibility that aberrant Stat3/mitochondria regulation could contribute to aging-associated diseases like MDS and progressive anemias.