Monocyte chemotactic protein-1 regulates the chondrogenic activity of synovial mesenchymal progenitor cells

Abstract

Purpose: Osteoarthritis (OA) is a multifactorial, often progressive, painful disease that often progresses with the apparently irreversible loss of articular cartilage, exposing underlying bone, resulting in pain and loss of mobility. This cartilage loss is thought to be permanent due to ineffective repair, and apparent lack of stem/progenitor cells in that tissue. However, the adjacent synovial lining and synovial fluid are abundant with mesenchymal progenitor/stem cells (sMPCs) capable of differentiating into cartilage both in vitro and in vivo, but it remains unknown if these cells play an active role in cartilage repair. Previous studies have demonstrated that MPCs can home to factors such as monocyte chemotactic protein 1 (MCP-1/CCL2) expressed after injury. However, the role of MCP-1 in the recruitment and retention of stem/progenitor cells to a site of injury or chronic disease (such as OA) is not fully understood which makes the elucidation of its roles in OA and stem cell biology in general particularly interesting. In the current study, we present evidence suggesting that MCP-1 is up regulated in the synovial fluid of patients with OA and exposure to this molecule triggers changes in sMPC transcriptional regulation, leading to a loss of chondrogenic potential in this cell population. Methods: Knee synovial fluid was aspirated from normal individuals (n1⁄410) and patients with OA (n1⁄410). The inflammatory profile of the fluid was analyzed using Luminex Multiplex Assays for 65 distinct cytokines and chemokines. sMPCs were derived from fresh synovial fluid (normal and OA) by aspiration and centrifugation. After 7-14 days in culture, colonies were observed and the cells were sorted using FACS for the CD105+, CD90+, CD73+, CD45-, CD11b-population. Chondrogenesis of sMPCs was evaluated in each of the treatment groups by quantifying the relative gene and protein expression of the chondrogenic markers Sox9, Collagen2, and Aggrecan using quantitative polymerase chain reaction (qPCR) and Western blot analysis. Microarray analysis was performed using the Affymetrix GeneChip Human Gene 1.0 ST and the data was examined using GeneSpring software. Results: MCP-1 is up-regulated in OA synovial fluid compared to normal samples and exposure to physiological (OA knee joint synovial fluid) levels of MCP-1 activates sMPCs (increasing telomerase activity), while concurrently inhibiting these cells from undergoing chondrogenesis (at the gene, protein, and primitive tissue levels). Furthermore, exposure to physiological levels of MCP-1 triggers changes in the transcriptome of sMPCs (Figure 1) and prolonged exposure to the chemokine induces the expression of MCP-1 in sMPCs, resulting in a positive feedback loop from which sMPCs cannot apparently escape (Figure 2). Conclusions: We have demonstrated that MCP-1 can significantly regulate the chondrogenic potential and telomerase activity of human synovial MPCs possibly through modification of the transcriptional network within the cells. Understanding the mechanism by which MCP-1 ‘re-programs’ these sMPCs may lead to novel pharmaceutical targets and interventions in OA possibly through ‘short circuiting’ the vicious cycle that appears to be activated by this chemokine within the joint environment after the onset of injury/OA.