Fibronectin matrix assembly is essential for cell condensation during chondrogenesis

Abstract

Mesenchymal cell condensation is the initiating event in endochondral bone formation. Cell condensation is followed by differentiation into chondrocytes accompanied by induction of chondrogenic gene expression. Gene mutations involved in chondrogenesis cause chondrodysplasias and other skeletal defects. Using mesenchymal stem cells (MSCs) in an in vitro chondrogenesis assay, we found that knockdown of the diastrophic dysplasia sulfate transporter (DTDST), which is required for normal cartilage development, blocked cell condensation and caused a significant reduction in fibronectin matrix. Knockdown of fibronectin with siRNAs also blocked condensation. Fibrillar fibronectin matrix is detected prior to cell condensation and levels increased during and after condensation. Inhibition of fibronectin matrix assembly by the functional upstream domain (FUD) prevented cell condensation by MSCs and also by the chondrogenic cell line ATDC5. Our data show that cell condensation and induction of chondrogenesis depend on fibronectin matrix assembly and DTDST and indicate that this transporter is required earlier in chondrogenesis than previously appreciated. They also raise the possibility that certain of the skeletal defects in DTD patients might derive from the link between DTDST, fibronectin matrix, and condensation.