Functional characterization of TRPV4 as an osmotically sensitive ion channel in porcine articular chondrocytes

Objective. Transient receptor potential vanilloid 4 (TRPV4) is a Ca ²⁺-permeable channel that can be gated by tonicity (osmolarity) and mechanical stimuli. Chondrocytes, the cells in cartilage, respond to their osmotic and mechanical environments; however, the molecular basis of this signal transduction is not fully understood. This study was undertaken to demonstrate the presence and functionality of TRPV4 in chondrocytes. Methods. TRPV4 protein expression was measured by immunolabeling and Western blotting. In response to TRPV4 agonist/antagonists, osmotic stress, and interleukin-1 (IL-1), changes in Ca²⁺ signaling, cell volume, and prostaglandin E₂ (PGE₂) production were measured in porcine chondrocytes using fluorescence microscopy, light microscopy, or immunoassay, respectively. Results. TRPV4 was expressed abundantly at the RNA and protein levels. Exposure to 4α-phorbol 12,13-didecanoate (4αPDD), a TRPV4 activator, caused Ca²⁺ signaling in chondrocytes, which was blocked by the selective TRPV4 antagonist, GSK205. Blocking TRPV4 diminished the chondrocytes' response to hypo-osmotic stress, reducing the fraction of Ca²⁺ responsive cells, the regulatory volume decrease, and PGE₂ production. Ca ²⁺ signaling was inhibited by removal of extracellular Ca ²⁺ or depletion of intracellular stores. Specific activation of TRPV4 restored the defective regulatory volume decrease caused by IL-1. Chemical disruption of the primary cilium eliminated Ca²⁺ signaling in response to either 4αPDD or hypo-osmotic stress. Conclusion. Our findings indicate that TRPV4 is present in articular chondrocytes, and chondrocyte response to hypo-osmotic stress is mediated by this channel, which involves both an extracellular Ca²⁺ and intracellular Ca²⁺ release. TRPV4 may also be involved in modulating the production or influence of proinflammatory molecules in response to osmotic stress.