SOX genes comprise a family of transcription factors characterised by a conserved HMG-box domain that confer pleiotropic effects on cell fate and differentiation through binding to the minor groove of DNA. Paracrine regulation and contact-dependant Notch signalling has been suggested to modulate the induction of SOX gene expression. The objective of this study is to investigate the crosstalk between and preconditioning of mesenchymal stem cells (MSCs) with chondrocytes through comparing SOX gene expression in their co-culture and respective monocultures.

Our study adopted an in vitro autologous co-culture of p0 adipose-derived MSCs (AMSCs) and articular chondrocytes derived from Kellgren-Lawrence Grade III/IV osteoarthritic knee joints (n=7). Samples were handled according to the 2004 UK Human Tissue Act. Cells were purified and co-cultured with one AMSC for every chondrocyte at 5000 cells/cm². The AMSCs were characterised by a panel of MSC surface markers in flow cytometry and were allowed to undergo trilineage differentiation for subsequent histological investigation. SOX5, SOX6, and SOX9 expression of co-cultures and monoculture controls were quantified by TaqMan quantitative real-time PCR. Experiments were performed in triplicate.

AMSC phenotype was evidenced by the expression of CD105, CD73, CD90 & heterogenous CD34 but not CD45, CD14, CD19 & HLA-DR in flow cytometry, and also differentiation into chondrogenic, osteogenic and adipogenic lineages with positive Alcian blue, Alizarin Red and Oil Red O staining. The expression of SOX5, SOX6, and SOX9 were greater in observed co-cultures than would be expected from an expression profile modelled from monocultures.

The findings provides evidence for the upregulation of SOX family transcription factors expression during the co-culture of MSCs and chondrocytes, suggesting an active induction of chondrogenic differentiation and change of cell fate amidst a microenvironment that facilitates cell-contact and paracrine secretion. This provides insight into the chondrogenic potential and therapeutic effects of MSCs preconditioned by the chondrocyte secretome (or potentially chondrocytes reinvigorated by the MSC secretome), and ultimately, cartilage repair.