Importance of Sox2 in maintenance of cell proliferation and multipotency of mesenchymal stem cells in low-density culture

Objectives: This study has aimed to repopulate ‘primitive’ cells from late‐passage mesenchymal stem cells (MSCs) of poor multipotentiality and low cell proliferation rate, by simply altering plating density. Materials and methods: Effects of low density culture compared t high density culture on late‐passage bone marrow (BM)‐derived MSCs and pluripotency markers of multipotentiality were investigated. Cell proliferation, gene expression, RNA interference and differentiation potential were assayed. Results and conclusions: We repopulated ‘primitive’ cells by replating late‐passage MSCs at low density (17 cells/cm²) regardless of donor age. Repopulated MSCs from low‐density culture were smaller cells with spindle shaped morphology compared to MSCs from high‐density culture. The latter had enhanced colony‐forming ability, proliferation rate, and adipogenic and chondrogenic potential. Strong expression of osteogenic‐related genes (Cbfa1, Dlx5, alkaline phosphatase and type Ι collagen) in late‐passage MSCs was reduced by replating at low density, whereas expression of three pluripotency markers (Sox2, Nanog and Oct‐4), Osterix and Msx2 reverted to levels of early‐passage MSCs. Knockdown of Sox2 and Msx2 but not Nanog, using RNA interference, showed significant decrease in colony‐forming ability. Specifically, knockdown of Sox2 significantly inhibited multipotentiality and cell proliferation. Our data suggest that plating density should be considered to be a critical factor for enrichment of ‘primitive’ cells from heterogeneous BM and that replicative senescence and multipotentiality of MSCs during in vitro expansion may be predominantly regulated through Sox2.