Project Summary/Abstract: Our group has demonstrated that increasing levels of the oxidized tryptophan metabolite kynurenine (Kyn), and downstream signaling through its activation of the aryl hydrocarbon receptor (AhR), contribute to age-related musculoskeletal dysfunction. Surprisingly, however, Kyn exerts more deleterious effects on the female musculoskeletal system as compared to its effects in males. The molecular mechanism behind this dimorphic phenomenon is not yet known, but our results suggest strong relevance to the pathophysiologies of osteoporosis and frailty, which are more prevalent in women than in men. Our working hypothesis is that the sexually dimorphic effects of Kyn in the musculoskeletal system may be attributed to crosstalk between Kyn/AhR and signaling downstream of ligand-regulated DNA-binding transcription factors in the nuclear receptor superfamily including the estrogen receptor (ER alpha/beta) and the glucocorticoid receptor (GR). Glucocorticoids (GC) are key mediators of stress responses that regulate skeletal homeostasis. During aging, endogenous GC levels increase, lose proper circadian regulation, and show amplified local activation from increased expression of GC-activating enzymes (e.g., Hsd11b1) with age in bone. Despite the known inhibitory effects of exogenous GC in bone, our data suggest that endogenous GC/GR-mediated signaling in osteoblasts is protective for aging bone, as loss of GR signaling in osteoblasts promoted low bone mass and increased bone marrow adipose tissue (BMAT). As we saw with Kyn, these effects from impaired GC/GR signaling in bone are more severe in female than in male mice, and preliminary data suggest that the deleterious effects of Kyn on osteoblasts are linked to impaired GC signaling. Therefore, the objective of Project 4 is to understand AhR function with age, and determine the molecular mechanisms by which Kyn, and crosstalk between Kyn/AhR and the nuclear receptors described above, drives aging-related phenotypes in bone and its resident cell populations, with the ultimate goal of identifying novel targets to mitigate these effects