The menopausal transition, an unavoidable aging-related phenomenon in women, is accompanied by increased abdominal adiposity and the concomitant incidence of adipose-related comorbidities, but the underlying mechanisms remain uncertain. Project 3 of the University of Colorado Specialized Center of Research Excellence on Sex Differences (SCORE) is based on the premise that alterations in the cellular composition of adipose tissue are responsible for the shift in body fat distribution and worsening metabolic health observed after menopause. We previously discovered a novel lineage of adipocytes in the major white adipose depots of mice and humans generated from hematopoietic stem cells rather than conventional mesenchymal precursors. In mice, these hematopoietic stem cell-derived adipocytes (HSCDAs) were detected in greater numbers in abdominal fat depots, suggesting a critical role in influencing metabolic health. Furthermore, estrogen receptor alpha (ERa) knockdown or ovariectomy significantly increased HSCDA production. Recently, two laboratories have presented evidence that diet-induced obesity requires the interaction between the metabolic intermediate, kynurenine (Kyn) and the aryl hydrocarbon receptor (AhR). Preliminary data from the Klemm laboratory shows that the enzyme that catalyzes kynurenine production, indoleamine dioxygenase 1 (IDO1) in adipose tissue, and the AhR are expressed at significantly higher levels in HSCDAs than other adipocyte populations. Thus, we hypothesize that the loss of estrogen signaling via ER⍺ promotes the production of HSCDAs and their subsequent production of Kyn (via IDO1) and AhR, which promotes weight gain, increased adiposity, and related metabolic dysfunction. Three specific aims will address this hypothesis: Aim 1: test whether HSCDA-targeted deletion of IDO1 or AhR will suppress increases in body weight, adiposity, and other adipose tissue-related endpoints due to knockdown of ERa in adipocytes (collaboration with Project 2); Aim 2: test whether targeted ablation of HSCDAs decreases Kyn, IDO1 and AhR and prevents changes in body weight, adiposity and other parameters induced by ERa knockdown (collaboration with Project 2); and Aim 3: test whether suppression of estrogen production in premenopausal women stimulates HSDCA production and their production of IDO1 and AhR, and is prevented by estrogen replacement (collaboration with Project 1). Successful completion of these aims will demonstrate that HSCDAs are a primary intermediate in the regulation of body weight and adiposity influenced by ERa signaling. Additionally, these studies will demonstrate the potential benefits of targeting kynurenine metabolism in the context of gonadal aging. Since HSCDAs are produced from hematopoietic rather than mesenchymal progenitor cells, they offer the opportunity to modulate not only body-wide metabolism, but also adipocyte metabolic phenotypes associated with changes in sex hormone production.