The Fibronectin-Mediated Inhibition of Chondrogenic Marker Sox9 Expression in Mesenchymal Stem Cells: A First Characterization

Abstract

Endochondral bone formation is a highly regulated process that begins with the crucial condensation of mesenchymal stem cells (MSCs), followed by the differentiation and proliferation of chondrocytes, or cartilage cells. The transcription factor Sox9 is a master regulator of chondrogenesis and a reliable marker in vitro for the induction of chondrogenic differentiation, but it is expressed in undifferentiated MSCs as well as early and proliferative chondrocytes. Fibronectin is a modular protein in the multi-functional extracellular matrix (ECM) and is deposited rapidly during MSC condensation and early chondrogenesis, but, similarly to Sox9, it is also present in undifferentiated MSCs. The ECM and its components are known to be capable of inducing chondrogenic, osteogenic, and adipogenic gene expression in MSCs, but are also implicated in the maintenance of stem cell pluripotency and self-renewal. Here, using real-time polymerase chain reactions to assess relative mRNA expression levels, we show that peptide-mediated inhibition of fibronectin matrix assembly may have an effect on Sox9 expression in human MSCs, though results are inconclusive. We also demonstrate that growing MSCs on fibronectin substrate significantly down-regulates Sox9 expression in a dose-dependent manner for at least up to two days and may also facilitate cell spreading. Plating cells on collagen type I, which is also up-regulated during MSC condensation, led to a similar down-regulation of Sox9 expression. Fibronectin and collagen type I may thus be involved in the maintenance of the stem cell state through the repression of Sox9 expression in MSCs.