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Mesenchymal stem/stromal cells (MSCs) are considered a disease-modifying treatment for osteoarthritis (OA). However, the precise molecular mechanisms of actions under which MSCs exert their therapeutic effect have not yet been identified in OA. Since MSCs actively interact with their environment, most likely the inflammatory OA milieu will stimulate their response. To identify these mechanisms, we retrieved GFP+ bone marrow-derived MSCs after intra-articular (IA) delivery in a murine collagenase induced osteoarthritis (CIOA) model. The transcriptome of retrieved cells and control in vitro licensed GFP+ BM-MSCs were analyzed to identify the predicted secretome and potential novel therapeutic factors activated by the OA microenvironment.
CIOA was induced in C57BL/6 mice (n=8) and 2x105 mouse syngeneic GFP+ bone marrow derived MSCs (BM-MSCs) were IA injected at D14 and D56 representing early acute and late OA. For cell retrieval, knee joints were digested and isolated by FACS-coupled for RNA sequencing. Samples were compared to GFP+ BM-MSCs retrieved from SHAM joints, where the knee joints were injected with saline solution. BM-MSCs licensed in vitro with a single dose of interleukin 6 (IL-6) and a combination of IL-6, monocyte chemoattractant protein-1 (MCP-1) and interferon gamma (IFN- γ) were also analysed. After 72 hours, cells were processed for RNA sequencing. Validation of BMP/retinoic acid-inducible neural-specific protein 3 (BRINP3) as a new MSC marker was performed using indirect immunofluorescence staining in healthy and OA murine cartilage, mouse embryonic limb and in vitro chondrogenic differentiation in human MSCs and articular progenitor cells (ACPs).
BRINP3 was identified as a common element between the four groups and as a novel protein associated with MSC modulation. BRINP3 protein expression was validated, identifying positive signal in meniscal cells in healthy murine cartilage, and was also detected on the meniscal and articular cartilage surfaces in mouse models of OA and expressed in joint-forming locations and in the periosteal sleeve in the developing mouse limb. We further investigated BRINP3 expression during in vitro chondrogenic differentiation of MSCs and ACPs. Positive expression was identified in the cytoplasm of MSCs and at later stages of chondrogenic differentiation external to the cells suggesting active secretion. However, ACP signal was confined solely in the cell cytoplasm throughout all stages of cell differentiation.
We generated a database of predicted secreted genes that can be a valuable resource for identification of small molecules with potential therapeutic efficacy for OA treatment. Furthermore, the data provided insights of the therapeutic mechanisms of action of MSCs in the context of OA. Among secreted genes, we identified for the first time BRINP3 as a new protein expressed during embryonic limb development, in vitro chondrogenic differentiation and on the meniscal and articular cartilage surface in vivo where chondroprogenitor cells are located. The data also highlights a mechanism of action of with surviving MSCs taking on a chondroprotective role and future studies are needed to validate the potential of BRINP3 as a local treatment of OA.
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