GM-CSF drives dysregulated hematopoietic stem cell activity and pathogenic extramedullary myelopoiesis in experimental spondyloarthritis

Dysregulated hematopoiesis occurs in several chronic inflammatory diseases, but it remains unclear how hematopoietic stem cells (HSCs) in the bone marrow (BM) sense peripheral inflammation and contribute to tissue damage in arthritis. Here, we show the HSC gene expression program is biased toward myelopoiesis and differentiation skewed toward granulocyte-monocyte progenitors (GMP) during joint and intestinal inflammation in experimental spondyloarthritis (SpA). GM-CSF-receptor is increased on HSCs and multipotent progenitors, favoring a striking increase in myelopoiesis at the earliest hematopoietic stages. GMP accumulate in the BM in SpA and, unexpectedly, at extramedullary sites: in the inflamed joints and spleen. Furthermore, we show that GM-CSF promotes extramedullary myelopoiesis, tissue-toxic neutrophil accumulation in target organs, and GM-CSF prophylactic or therapeutic blockade substantially decreases SpA severity. Surprisingly, besides CD4+ T cells and innate lymphoid cells, mast cells are a source of GM-CSF in this model, and its pathogenic production is promoted by the alarmin IL-33

Rôle de la sérotonine au cours du remodelage osseux

PARIS7-Bibliothèque centrale (751132105) / SudocSudocFranceF

Serotonin 2B receptor (5-HT2B R) signals through prostacyclin and PPAR-ß/δ in osteoblasts.

International audienceOsteoporosis 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

Decreased osteoclastogenesis in serotonin-deficient mice

International audienc

5-HT_2BR−/− osteoblasts phenotype is dependent on prostacyclin overexpression.

<p>A specific inhibitor of prostacyclin synthase (U51605) was used to evaluate the role of the prostacyclin overproduction. A: a cell aggregation assay was performed with this inhibitor on WT and 5-HT_2BR^−/− cells. U51605 had no effect on WT aggregation and restored 5-HT_2BR^−/− cell aggregation. B: mineralization assay: U51605 had no effect on WT cells, but did rescue 5-HT_2BR^−/− mineralization. C: The expressions of the alkaline phosphatase, osteopontin and cyclin D1 level markers modified by the absence of 5-HT_2BR were restored by adding U51605 to 5-HT_2BR−/− osteoblasts. Data are presented as means ± SEM of three independent experiments performed in triplicate. ** p<0.05 vs. WT, *** p<0.001 vs. WT.^$$$ p<0.005 vs 5-HT_2BR−/−.</p

Pharmacological inhibition of PPAR-ß/δ restores the phenotype of 5-HT2BR−/− osteoblasts.

<p>PGI₂ is an endogenous ligand of nuclear PPAR ß/δ, and also can act via the membrane IP receptor. We evaluated the involvement of these two potential prostacyclin targets in 5-HT_2BR^−/− cells. A: For the membrane IP receptor: the cAMP level and the binding parameters of the IP receptors were measured in both cell types. None of these parameters was modified by the absence of the 5-HT_2B receptor. B: PPAR-ß/δ and PPAR-γ expressions were analyzed by Western blotting, and found to be unchanged in both cell types. The involvement of PPAR ß/δ was determined using specific antagonist of this receptor. C: An aggregation assay was carried out in the presence of GSK0660 (a specific antagonist of PPAR-ß/δ) for 5-HT_2BR^−/− cultures. The GSK0660 treatment restored cell aggregation in 5-HT_2BR^−/− cultures. D: A mineralization assay (red alizarin staining) was performed under the various different conditions: GSK0660 treatment rescued the phenotype in 5-HT_2BR^−/− osteoblasts. E: Similar results were obtained for the transcriptional expressions of the alkaline phosphatase, osteopontin, and cyclin D1 level markers. Data are presented as means ± SEM of three independent experiments performed in triplicate. *** p<0.005 vs. WT and ^$$$ p<0.005 vs 5-HT_2BR−/−.</p