Ca 2+ -Dependent Endoplasmic Reticulum Stress Regulates Mechanical Stress–Mediated Cartilage Thinning

Abstract

Our previous study identified that endoplasmic reticulum stress (ERS) plays a critical role in chondrocyte apoptosis and mandibular cartilage thinning in response to compressive mechanical force, although the underlying mechanisms remain elusive. Because the endoplasmic reticulum (ER) is a primary site of intracellular Ca²⁺ storage, we hypothesized that Ca²⁺-dependent ERS might be involved in mechanical stress–mediated mandibular cartilage thinning. In this study, we used in vitro and in vivo models to determine Ca²⁺ concentrations, histological changes, subcellular changes, apoptosis, and the expression of ERS markers in mandibular cartilage and chondrocytes. The results showed that in chondrocytes, cytosolic Ca²⁺ ([Ca²⁺]i) was dramatically increased by compressive mechanical force. Interestingly, the inhibition of Ca²⁺ channels by ryanodine and 2-aminoethoxydiphenyl borate, inhibitors of ryanodine receptors and inositol trisphosphate receptors, respectively, partially rescued mechanical force–mediated mandibular cartilage thinning. Furthermore, chondrocyte apoptosis was also compromised by inhibiting the increase in [Ca²⁺]i that occurred in response to compressive mechanical force. Mechanistically, the ERS induced by compressive mechanical force was also repressed by [Ca²⁺]i inhibition, as demonstrated by a decrease in the expression of the ER stress markers 78 kDa glucose-regulated protein (GRP78) and 94 kDa glucose-regulated protein (GRP94) at both the mRNA and protein levels. Collectively, these data identified [Ca²⁺]i as a critical mediator of the pathological changes that occur in mandibular cartilage under compressive mechanical force and shed light on the treatment of mechanical stress–mediated cartilage degradation.