Intercellular Ca^{2+} waves in mechanically stimulated articular chondrocytes

Article type: Research Article

Authors: D’Andrea, Paola | Calabrese, Alessandra | Capozzi, Ilaria | Grandolfo, Micaela | Tonon, Rossana | Vittur, Franco

Affiliations: Dipartimento Biochimica, Biofisica e Chimica delle Macromolecole, Università di Trieste, Italy

Abstract: Articular cartilage is a tissue designed to withstand compression during joint movement and, in vivo, is subjected to a wide range of mechanical loading forces. Mechanosensitivity has been demonstrated to influence chondrocyte metabolism and cartilage homeostasis, but the mechanisms underlying mechanotransduction in these cells are poorly understood. In many cell types mechanical stimulation induces increases of the cytosolic Ca^{2+} concentration that propagates from cell to cell as an intercellular Ca^{2+} wave. Cell‐to‐cell communication through gap junctions underlies tissue co‐ordination of metabolism and sensitivity to extracellular stimuli: gap junctional permeability to intracellular second messengers allows signal transduction pathways to be shared among several cells, ultimately resulting in co‐ordinated tissue responses. Mechanically‐induced Ca^{2+} signalling was investigated with digital fluorescence video imaging in primary cultures of rabbit articular chondrocytes. Mechanical stimulation of a single cell, obtained by briefly distorting the plasmamembrane with a micropipette, induced a wave of increased Ca^{2+} that was communicated to surrounding cells. Intercellular Ca^{2+} spreading was inhibited by 18α‐glycyrrhetinic acid, suggesting the involvement of gap junctions in signal propagation. The functional expression of gap junctions was assessed, in confluent chondrocyte cultures, by the intercellular transfer of Lucifer yellow dye in microinjection experiments while the expression of connexin 43 could be detected in Western blots. A series of pharmacological tools known to interfere with the cell calcium handling capacity were employed to investigate the mechanism of mechanically‐induced Ca^{2+} signalling. In the absence of extracellular Ca^{2+} mechanical stimulation induced communicated Ca^{2+} waves similar to controls. Mechanical stress induced Ca^{2+} influx both in the stimulated chondrocyte but not in the adjacent cells, as assessed by the Mn^{2+} quenching technique. Cells treatment with thapsigargin and with the phospholipase C inhibitor U73122 blocked mechanically‐induced signal propagation. These results provide evidence that in chondrocytes mechanical stimulation activates phospholipase C, thus leading to an increase of intracellular inositol 1,4,5‐trisphosphate. The second messenger, by permeating gap junctions, stimulates intracellular Ca^{2+} release in neighbouring cells. Intercellular Ca^{2+} waves may provide a mechanism to co‐ordinate tissue responses in cartilage physiology.

Journal: Biorheology, vol. 37, no. 1-2, pp. 75-83, 2000