Insulin-like Growth Factor-1 Regulates the Mechanosensitivity of Chondrocytes by Modulating TRPV4

Both mechanical and IGF-1 stimulation are required for normal articular cartilage development and maintenance of the extracellular matrix. While much effort has been made to define the signaling pathways associated with these anabolic stimuli, we focused on how these pathways interact to regulate chondrocyte function. The Transient Receptor Potential Vanilloid 4 (TRPV4) channel is central to chondrocyte mechanotransduction and regulation of cartilage homeostasis. However, the mechanism by which TRPV4 is mechanically gated or regulated is not clear. In this study we propose that insulin-like growth factor 1 (IGF-1), which is important in regulating matrix production during mechanical load, modulates TRPV4 channel activity. Our studies indicate that IGF-1 reduces hypotonic-induced TRPV4 currents, and intracellular calcium flux by increasing stress fiber formation and apparent cell stiffness. Disruption of F-actin following IFG-1 treatment results in the return of the intracellular calcium response to hypotonic swelling. Furthermore, we highlight that IGF-1 suppresses TRPV4 mediated calcium flux through the MAP7 binding domain (aa. 798-809), where actin binds to the TRPV4 channel. IGF-1 treatment differentially influences the intracellular calcium flux of HEK 293 cells stably expressing either wild-type or mutant (P799L or G800D) TRPV4 during hypotonic challenge. A key down-stream response to mechanical stimulation of chondrocytes is ATP release. Data here indicate that activation of TRPV4 through hypotonic swelling induces ATP release, but this release is greatly reduced with IGF-1 treatment. Taken together this study indicates that IGF-1 modulates TRPV4 channel response to mechanical stimulation by increasing cell stiffness. As chondrocyte response to mechanical stimulation is greatly altered during OA progression, IGF-1 presents as a promising candidate for prevention and treatment of articular cartilage damage.