Cartilage-Specific Knockout of the Mechanosensory Ion Channel TRPV4 Decreases Age-Related Osteoarthritis

Abstract

Osteoarthritis (OA) is a progressive degenerative disease of articular cartilage and surrounding tissues, and is associated with both advanced age and joint injury. Biomechanical factors play a critical role in the onset and progression of OA, yet the mechanisms through which physiologic or pathologic mechanical signals are transduced into a cellular response are not well understood. Defining the role of mechanosensory pathways in cartilage during OA pathogenesis may yield novel strategies or targets for the treatment of OA. The transient receptor potential vanilloid 4 (TRPV4) ion channel transduces mechanical loading of articular cartilage via the generation of intracellular calcium ion transients. Using tissue-specific, inducible Trpv4 gene-targeted mice, we demonstrate that loss of TRPV4-mediated cartilage mechanotransduction in adulthood reduces the severity of aging-associated OA. However, loss of chondrocyte TRPV4 did not prevent OA development following destabilization of the medial meniscus (DMM). These results highlight potentially distinct roles of TRPV4-mediated cartilage mechanotransduction in age-related and post-traumatic OA, and point to a novel disease-modifying strategy to therapeutically target the TRPV4-mediated mechanotransduction pathway for the treatment of aging-associated OA.

Figure 1. Inducible loss of TRPV4-mediated Ca²⁺ signaling in articular cartilage.

(a) Immunofluorescence staining for TRPV4 two weeks following tamoxifen induction. Positive TRPV4 staining in WT mouse articular cartilage (i), absence of staining in the no primary control (ii) and cKO articular cartilage (iii), positive TRPV4 staining in cKO subchondral bone (iv). AC: articular cartilage; SB: Subchondral bone. (b) Representative confocal images of in situ chondrocyte intracellular Ca²⁺ signaling. (c) Percentage of chondrocytes responding to TRPV4 agonist GSK101 and hypo-osmotic stimulation. Ca²⁺ signaling in response to GSK101 and hypo-osmotic loading is present in WT chondrocytes and absent in cKO chondrocytes. Data not sharing a common superscript letter indicate a significant difference. p < 0.05, n = 3–9 (Bars do not have error bars because the percent responding metric does not have an error associated with it.).

Figure 2. Loss of chondrocyte TRPV4 in adulthood reduced aging-associated OA severity and total joint bone volume with age.

(a) Representative coronal sections of 1-year-old wild type (WT) and induced, cartilage-specific TRPV4 knockout (cKO) joints. (b) Representative histology of the joint synovial lining. (c) cKO mice exhibit significantly less OA severity, including Mankin and synovitis grading (p = 0.031 and p = 0.038, respectively). The difference in osteophyte scores is not significant (p = 0.153). (d) 3D microCT reconstructions show gross bony morphology of cKO joints is similar to that of WT mice. (e) cKO animals have significantly decreased total joint bone volume (p = 0.002). No significant differences in SCBT were seen between cKO and WT mice (MFC SCBT p = 0.158). (f) Total TGF-β1 levels in the synovial fluid of mouse hind limbs at sacrifice trends higher in cKO mice (p = 0.167). n = 12–13, mean + SEM. *p < 0.05.

Figure 3. Loss of chondrocyte TRPV4 did not alter OA pathogenesis following joint destabilization.

(a) Representative coronal sections of Control and DMM joints from wild type (WT) and induced, cartilage-specific TRPV4 knockout (cKO) joints. (i) WT Control, (ii) WT DMM, (iii) cKO Control, (iv) cKO DMM (b) Representative histology of the joint synovial lining (c) Representative histology of tibial plateau osteophytes (d) DMM causes significant osteoarthritic changes in both WT and cKO mice. (Control vs DMM comparisons: Mankin: WT p = 0.004, cKO p = 0.010; Synovitis: WT p = 0.005, cKO p = 0.023; Osteophyte: WT p = 0.001, cKO p < 0.001). n = 14–15, mean + SEM. *p < 0.05.

Figure 4. Loss of TRPV4 altered periarticular bone changes in the setting of joint destabilization.

(a) 3D microCT reconstructions of wild type (WT) and induced, cartilage-specific TRPV4 knockout (cKO) joints. (b) DMM caused a significant increase in TJBV in WT mice (p = 0.049), an effect that was not significant in cKO mice (p = 0.198). DMM also caused a significant increase in MFC SCBT in WT animals (p = 0.002), not seen in cKO mice (p = 0.197). (c) Total TGF-β1 levels in the synovial fluid of mouse hind limbs at sacrifice. n = 14–15, mean + SEM. *p < 0.05.