Transient Receptor Potential Vanilloid 5 Mediates Ca2+ Influx and Inhibits Chondrocyte Autophagy in a Rat Osteoarthritis Model

Abstract

Background: Autophagy, a self-protective mechanism of chondrocytes, has become a promising target to impede the progress of osteoarthritis (OA). Autophagy is regulated by cytosolic Ca²⁺ activity and may thus be modified by the Ca²⁺ permeable transient receptor potential channel vanilloid 5 (TRPV5). Therefore, we investigated the potential role of TRPV5 in mediating Ca²⁺ influx and in inhibiting chondrocyte autophagy in a rat OA model. Methods: The rat OA model was assessed by macroscopic and histological analyses. light chain 3B (LC3B) immunolocalization was detected by immunohistochemistry. TRPV5, LC3B and calmodulin in OA articular cartilage were assessed by real time polymerase chain reaction (RT-PCR) and western blotting. TRPV5 small interfering RNA (TRPV5 siRNA) were transfected into rat primary chondrocyte then the calmodulin and LC3B was detected by immunofluorescence. The functionality of the TRPV5 was assessed by Ca²⁺ influx. Western blot was used to measure autophagy-related proteins. Results: We constructed a monosodium iodoacetate (MIA) -induced rat OA model and found that ruthenium red (TRPV5 inhibitor) slowed the progression of joint destruction. We found that the TRPV5 and calmodulin were up-regulated but LC3B was down-regulated in articular cartilage following prolonged progression of OA. Furthermore, the up-regulated TRPV5 channel caused an increase in the Ca²⁺ influx in chondrocytes. The up-regulation of TRPV5 stimulated Ca²⁺ influx, which inhibited autophagy by increasing the production of calmodulin, phosphorylation of calmodulin dependent protein kinases II (p-CAMK II), phosphorylation of Beclin1 (p-Beclin1), and protein of B-cell lymphoma-2 (Bcl-2), and attenuating ratio of LC3-II/ LC3-. Conclusion: Up-regulated TRPV5 as an initiating factor inhibited chondrocyte autophagy via the mediation of Ca²⁺ influx.