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International audience; Osteoporosis is due to an imbalance between decreased bone formation by osteoblasts and increased resorption by osteoclasts. Deciphering factors controlling bone formation is therefore of utmost importance for the understanding and the treatment of osteoporosis. Our previous in vivo results showed that bone formation is reduced in the absence of the serotonin receptor 5-HT2B, causing impaired osteoblast proliferation, recruitment, and matrix mineralization. In this study, we investigated the signaling pathways responsible for the osteoblast defect in 5-HT2BR(-/-) mice. Notably, we investigated the phospholipase A2 pathway and synthesis of eicosanoids in 5-HT2BR(-/-) compared to wild type (WT) osteoblasts. Compared to control osteoblasts, the lack of 5-HT2B receptors was only associated with a 10-fold over-production of prostacyclin (PGI2). Also, a specific prostacyclin synthase inhibitor (U51605) rescued totally osteoblast aggregation and matrix mineralization in the 5-HT2BR(-/-) osteoblasts without having any effect on WT osteoblasts. Prostacyclin is the endogenous ligand of the nuclear peroxisome proliferator activated receptor ß/δ (PPAR-ß/δ), and its inhibition in 5-HT2BR(-/-) cells rescued totally the alkaline phosphatase and osteopontin mRNA levels, cell-cell adhesion, and matrix mineralization. We conclude that the absence of 5-HT2B receptors leads to the overproduction of prostacyclin, inducing reduced osteoblast differentiation due to PPAR-ß/δ -dependent target regulation and defective cell-cell adhesion and matrix mineralization. This study thus reveals a previously unrecognized cell autonomous osteoblast defect in the absence of 5-HT2BR and highlights a new pathway linking 5-HT2B receptors and nuclear PPAR- ß/δ via prostacyclin.