Regulatory mechanisms of hematopoietic stem cell functions post-transplant

Abstract Hematopoietic stem cells (HSC) sustain the production of all blood and immune cells throughout life by differentiating into all blood lineages and regenerate long-lived HSC, ie self-renew. However, although HSC have high regenerative potential, the actual capacity of ‘self-renewal’, ie regenerating a daughter cell that has identical properties’ is limited. HSC sustain injury after bone marrow transplantation and become less functional. A clear understanding of the mechanism responsible for HSC functional decline with HSC transplantation is still lacking. This lack of knowledge has hampered our ability to maintain HSC functions through divisions and improve HSCT outcomes. The overall goal of this grant application is to understand the mechanisms behind HSC functional decline after bone marrow transplantation. We have discovered that once HSCs get activated, mitochondria irreversibly remodel and do not return to homeostatic conditions. HSCs accumulate dysfunctional mitochondria due to a progressive decline in mitochondrial quality control mechanisms, including reduced mitochondrial dynamism such that HSCs carry mitochondria have that are different in shape and functions. Mechanistically, HSC lose mitochondrial fission activity [ie, loss of the fission regulator Drp1 activity], which causes a decrease in HSC regenerative potential. In addition, we found that HSC with high repopulation activity in vivo have high levels of mitophagy. In contrast mitophagy is drastically reduced in HSC after transplantation. We hypothesize that mitochondrial defects drive HSC functional decline after transplantation. The main objectives of this project are to understand why abnormal mitochondria in HSC are not removed after bone marrow transplantation and the impact it has on HSC functions (Aim1). Aim2 will investigate the impact of abnormal mitochondria on HSC metabolism with a focus the mitochondrial retrograde signaling ATF4-on carbon/folate pathway. We will test effect of metabolite supplementation in ameliorating HSC functions in vivo during bone marrow transplantation, using both murine bone marrow transplantation and xenotransplant model of human CD34+ into immunodeficient recipients. The proposed studies provide a unique opportunity to examine the specific contribution of abnormal mitochondrial functions to HSC functional decline during and after bone marrow transplantation. It will investigate the novel concept that HSCs accumulate dysfunctional mitochondria that reprogram their metabolism driving their functional decline, which may lead to the identification of novel approaches for pharmacological intervention to maintain HSC functions through divisions.