Obesity can be considered as a chronic organismal stress negatively impacting multiple systemic and tissue-specific functions. Notably obesity promotes type-2 diabetes, cardiovascular and liver diseases as well as a multitude of cancers. In the hematopoietic system, obesity has a global impact on immunity, with increased susceptibility to infection and paradoxically sensitivity to autoimmune diseases. Obesity is also associated with hematological malignancies such as myelodysplastic syndromes, myeloproliferative neoplasms, and leukemia. Obesity not only triggers higher incidence of these pathologies but also directly affects disease severity, treatment efficacy and therefore overall outcome. However, despite their clinical relevance, mechanisms by which obesity affects the normal hematopoiesis and contribute to its pathological dysregulation remain elusive. Notably the impact of obesity on the fitness of hematopoietic stem cell (HSC) compartment has not been fully described. The HSC compartment at the apex of the hematopoietic system and is the cornerstone of an efficient, tightly regulated, and lifelong blood cell production. To perform thei homeostatic functions, HSCs rely on short-range interactions in the bone marrow (BM) as well as a complex network of systemic signals, all parameters profoundly affected by obesity and its numerous sequelae. Here we propose to conduct an HSC-centric study to determine how obesity impacts normal HSCs and how these cells develop specific adaptive mechanisms to respond to the obese environment. In Aim1, we will investigate at the single cell level how obesity impacts on cell fate decisions in normal HSCs. Notably we will determine whether the obese environment dysregulates the balance between self-renewal and differentiation in HSCs. Mechanistically, we will determine whether the obese environment contributes to the clonal selection of HSC subtypes with specific functional and molecular characteristics. Notably analyses will include state-of-the-art single cell gene expression analyses and in vivo cellular barcoding for clonal analysis of the hematopoietic system in the context of obesity. In Aim2, we will decipher specific HSC transcriptional networks and signaling pathways that might be affected in obesity. Based on our preliminary results, we will focus on the role of the master transcriptional regulator Gfi1 and the Akt signaling pathway in the HSC response to the obese environment. We will then investigate the mechanisms by which HSCs cope with the aberrant environmental signals triggered by obesity. Altogether, these studies will improve our fundamental understanding of the interactions between the obese environment and the HSC compartment. As such they will lay the foundation for the identification of specific molecular targets that can be manipulated to mitigate the negative effects of the obesity on the fitness of HSC compartment and prevent the emergence of hematological pathologies in this context.