Intracellular calcium oscillations in articular chondrocytes induced by basic calcium phosphate crystals lead to cartilage degradation

SummaryObjectiveBasic calcium phosphate (BCP) crystals, including octacalcium phosphate (OCP), carbonated-apatite (CA) and hydroxyapatite (HA) crystals are associated with destructive forms of osteoarthritis. Mechanisms of BCP-induced cartilage breakdown remain incompletely understood. We assessed the ability of BCP to induce changes in intracellular calcium (iCa2+) content and oscillations and the role of iCa2+ in BCP-induced cartilage degradation.MethodsBovine articular chondrocytes (BACs) and bovine cartilage explants (BCEs) were stimulated with BCP or monosodium urate (MSU) crystals. iCa2+ levels were determined by spectrofluorimetry and oscillations by confocal microscopy. mRNA expression of matrix metalloproteinase 3 (MMP-3), a disintegrin and metalloprotease with thrombospondin-like motifs 4 (ADAMTS-4) and ADAMTS-5 was assessed by quantitative real-time PCR. Glycosaminoglycan (GAG) release was measured in the supernatants of BCE cultures.ResultsAll three BCP crystals significantly increased iCa2+ content. OCP also induced iCa2+ oscillations. Rate of BACs displaying iCa2+ oscillations increased over time, with a peak after 20 min of stimulation. OCP-induced iCa2+ oscillations involved both extracellular Ca2+ (eCa2+) influx and iCa2+ stores. Indeed, OCP-induced iCa2+ oscillations decreased rapidly in Ca2+-free medium. Both voltage- and non-voltage-dependent Ca2+ channels were involved in eCa2+ influx. BCP crystal-induced variation in iCa2+ content was associated with BCP crystal-induced cartilage matrix degradation. However, iCa²+ was not associated with OCP crystal-induced mRNA expression of MMP-3, ADAMTS-4 or ADAMTS-5.ConclusionBCP crystals can induce variation in iCa2+ content and oscillations in articular chondrocytes. Furthermore, BCP crystal-induced changes in iCa2+ content play a pivotal role in BCP catabolic effects on articular cartilage.