Articular chondrocyte response to osmotic stress: The involvement of calcium and the cytoskeleton in cellular volume adaptation and regulation

The biophysical environment of the chondrocyte plays an important role in the health, turnover, and homeostasis of articular cartilage. Under normal physiologic loading, chondrocytes are exposed to a complex and diverse array of biophysical signals, including mechanical, electrochemical, osmotic stresses, fluid flow and pressures. Due to the charged and hydrated nature of the extracellular matrix, mechanical compression causes exudation of interstitial fluid in cartilage, which alters the osmotic environment of the chondrocytes. These osmotic changes have been shown to affect both chondrocyte structure and function in both an explant as well as an isolated cell model. The research in this report characterized intracellular signaling events that are initiated in response to an altered osmotic environment and elucidated the biophysical mechanism that initiated the signaling event.; The majority of the experiments in this report were performed on isolated primary articular chondrocytes. Chondrocytes were exposed to both hypo- and hyper-osmotic stress and intracellular calcium concentrations [Ca2+ ]_i within the cells were monitored. Cells responded to both hypo- and hyper- osmotic stress with a Ca2+ transient, however their mechanism appeared to be different. In other experiments, the cytoskeleton was observed using immunofluorescence and also chemically disrupted for the purpose of examining its adaptation and involvement in regards to osmotic loading. It was found that the cytoskeleton not only reorganized itself in response to osmotic loading but also played a role in the recovery events following altered volume change. In additions, inositol trisphosphate (IP ₃) and gelsolin, an actin binding and severing protein were analyzed following osmotic challenge. Lastly, there was an attempt to isolate the actual biophysical phenomena responsible for the elicitation of the Ca2+ transient. Upon exposure to an altered osmotic environment, the cell swells or shrinks. Subsequent to the application of osmotic stress is membrane deformation and volume change. Experiments were performed to isolate these different biophysical phenomena. It was found through these experiments that an altered osmotic environment was responsible for the elicitation of the Ca2+ transient and not necessarily the volume change or membrane deformation.; In summary, this set of experiments examined various intracellular signaling events that occur following exposure to an altered osmotic environment. Consequences of these initial signaling events may be altered mechanical properties of the cells or altered gene transcription and translation patterns, all of which are thought to be involved in the initiation of the joint disease of osteoarthritis. A deeper understanding of these mechanisms will lead to a broader knowledge of the etiology of cartilage degradation and potentially aid in the discovery of novel therapeutic intervention.