ABSTRACT
Rat chondrosarcoma (RCS) cells are unusual in that they display a stable chondrocyte phenotype in monolayer culture. This phenotype is reflected by a rounded cellular morphology with few actin-containing stress fibers and production of an extracellular matrix rich in sulfated proteoglycans, with high-level expression of aggrecan, COMP, Sox9, and collagens type II, IX, and XI. Additionally, these cells do not express collagen type I. Here it is shown that in the absence of any mechanical stimulation, treatment of RCS cells with gadolinium chloride (Gd3+), a stretch-activated cation channel blocker, caused the cells to undergo de-differentiation, adopting a flattened fibroblast phenotype with the marked appearance of actin stress fibers and vinculin-containing focal contacts. This change was accompanied by a dramatic reduction in the expression of aggrecan, Sox9, collagen types II, IX, and XI, with a corresponding increase in the expression of collagen type I and fibronectin. These effects were found to be reversible by simple removal of Gd3+ from the medium. Gd3+ also had a similar effect on expression of chondrocyte marker genes in freshly isolated human chondrocytes. These data suggest that mechanoreceptor signaling plays a key role in maintenance of the chondrocyte phenotype, even in the absence of mechanical stimulation. Further, treatment of RCS cells with Gd3+ provides a tractable system for assessing the molecular events underlying the reversible differentiation of chondrocytes.
