Nuclear Related Responses to Osmotic Challenge in Chondrocytes.

Abstract

The application of prolonged mechanical loading to cartilage alters the osmolality of the extracellular environment, with osmotic challenge known to alter the gene expression and the metabolic activity of chondrocytes. However, the mechanisms by which osmolality controls chondrocyte activity remain unclear. Previous study on various cell types, including chondrocytes, showed that hyper-osmotic challenge induces the condensation of chromatin, with highly condensed chromatin often associated with gene poor regions of DNA and gene silencing. The present study investigated the effect of osmotic challenge on chromatin organisation, genome wide gene-expression and the cellular and nuclear deformability of chondrocytes. In order to observe a broad effect of osmotic challenge on the nuclei, the chondrocytes were subjected to a range of hypo- and hyper-osmotic challenge and imaged by confocal microscopy. Chromatin condensation was quantified by the Sobel edge algorithm in MATLAB. Hyper-osmotic challenge on chondrocytes induced an increase in chromatin condensation. Interestingly, the most marked condensation occurred within the osmolality range of articular cartilage in vivo. The effect of osmotic challenge varied between the monolayer cultured and agarose seeded chondrocytes, which may be due to the differences in cytoskeleton organisation between the two culture conditions. Additionally, chromatin condensation induced by hyper-osmotic challenge was shown to be reversible. Marked differences were observed in the deformability of the cell and nucleus in chondrocytes post osmotic challenge, compared to the 300 mOsm/kg conditions typically used for in vitro isolated chondrocyte studies. From the microarray study, the application of 500 mOsm/kg for both 1 and 5 hours altered the gene expression, including the expression of histone related genes, with a higher number of genes affected by the 5 hours hyper-osmotic challenge. The findings of this study suggest that osmotically-induced alterations in nuclei morphology and chromatin structure may provide a direct biophysical mechanism that controls chondrocytes activity.