Gut microbiota-derived indole-3-Acetic Acid attenuates skeletal fluorosis via AHR-mediated suppression of Wnt/β-Catenin signaling

Chronic fluoride exposure causes skeletal fluorosis (SF), a debilitating bone disease, but the lack of a comprehensive understanding of its pathogenesis has hindered the development of effective therapies. Although fluoride is known to stimulate osteoblasts, the role of gut-bone crosstalk in SF remains uninvestigated. In this study, using a rat SF model and in vitro osteoblast studies, we found that fluoride disrupted gut microbial tryptophan metabolism, resulting in decreased serum levels of indole-3-acetic acid (3-IAA) in both SF model rats and human SF patients. This deficiency in 3-IAA impaired the activation of aryl hydrocarbon receptor (AHR), leading to Wnt/β-Catenin pathway hyperactivation, excessive osteoblast differentiation/mineralization, and pathological bone formation. Restoring 3-IAA levels, either through direct supplementation or an intermittent high-tryptophan diet-reactivated AHR, suppressed Wnt/β-Catenin signaling, and alleviated bone damage in vivo. Crucially, knockdown or inhibition of AHR abolished the protective effect of 3-IAA, confirming that the 3-IAA-AHR axis is essential for mitigating SF. In conclusion, we identify a gut microbiota-3-IAA-AHR-Wnt/β-Catenin axis that drives the pathogenesis of SF, and our findings suggest that intermittent tryptophan supplementation could serve as a novel therapeutic strategy to reduce fluoride-induced bone damage by restoring microbial metabolite signaling.