Extracellular Matrix Stiffness Alters TRPV4 Regulation in Chondrocytes

During the progression of osteoarthritis (OA), degradation of the extracellular matrix alters the biomechanical properties of cartilage, especially the compressive modulus. The mechanosensitive ion channel transient receptor potential vanilloid 4 (TRPV4) is required for chondrocyte mechanotransduction However, how OA-mediated cartilage degradation influences TRPV4 signaling remains unknown. To determine if ATDC5 cells alter TRPV4-mediated calcium signaling and cell phenotype in response to softer substrates, we created PEGDA-RGDS hydrogels with Youngs moduli that simulated healthy (~350 kPa), OA (~175 kPa) and severe OA (~35 kPa) tissue. We found that softer substrates reduced the influx of calcium through TRPV4 when challenging chondrocytes with hypotonic swelling (HTS). Chondrocyte apoptosis also increased on the OA and severe OA gels due to elevated basal [Ca2+]i, which is attenuated with pharmacological agonism of TRPV4. Pharmacological agonism of TRPV4 rescued the expression of aggrecan and TRPV4 in chondrocytes cultured on OA gels and enhanced the type II collagen (col2) expression in cells on the normal and OA gels. These data suggest that the biomechanical properties of degraded cartilage alter TRPV4-mediated mechanotransduction in chondrocytes. Given that TRPV4 reduced apoptosis and improved the chondrogenic capacity of cells, TRPV4 stimulation could provide a potential therapeutic target in patients with early to moderate OA.