Conclusion
SLS function is reduced in the knee joints of aged mice due to decreased VEGF-C/VEGFR-3 signaling. VEGF-C treatment attenuates OA joint damage and improves synovial lymphatic drainage in aged mice. The SLS and VEGF-C/VEGFR-3 signaling represent novel physiopathologic mechanisms that could potentially be used as therapeutic targets for age-related OA.
Methods
SLS function was assessed in the knees of young (3-month-old) and aged (19-24-month-old) male and female C57BL/6J mice via a newly established in vivo IVIS-dextran imaging approach, which, in addition to histology, was used to assess the effects of VEGF-C treatment on SLS function and OA pathology in aged mice. RNA-sequencing of synovial tissue was performed to explore molecular mechanisms of the disease in the mouse knee joints.
Objective
The synovial lymphatic system (SLS) removes catabolic factors from the joint. Vascular endothelial growth factor C (VEGF-C) and its receptor, VEGFR-3, are crucial for lymphangiogenesis. However, their involvement in age-related osteoarthritis (OA) is unknown. This study was undertaken to determine whether the SLS and the VEGF-C/VEGFR-3 pathway contribute to the development and progression of age-related OA, using a murine model of naturally occurring joint disease.
Results
Results showed that aged mice had impaired SLS function, including decreases in joint clearance (mean T1/2 of signal intensity clearance, 2.8 hours in aged mice versus 0.5 hours in young mice; P < 0.0001), synovial influx (mean ± SD 1.7 ± 0.8% in aged mice versus 4.1 ± 1.9% in young mice; P = 0.0004), and lymph node draining capacity (mean ± SD epifluorescence total radiant intensity ([photons/second]/[μW/cm2 ]) 1.4 ± 0.8 in aged mice versus 3.7 ± 1.2 in young mice; P < 0.0001). RNA-sequencing of the synovial tissue showed that Vegf-c and Vegfr3 signaling genes were decreased in the synovium of aged mice. VEGF-C treatment resulted in improvements in SLS function in aged mice, including increased percentage of signal intensity joint clearance (mean ± SD 63 ± 9% in VEGF-C-treated aged mice versus 52 ± 15% in vehicle-treated aged mice; P = 0.012), increased total articular cartilage cross-sectional area (mean ± SD 0.38 ± 0.07 mm2 in VEGF-C-treated aged mice versus 0.26 ± 0.07 mm2 in vehicle-treated aged mice; P < 0.0001), and decreased percentage of matrix metallopeptidase 13-positive staining area within total synovial area in 22-month-old VEGF-C-treated mice versus 22-month-old vehicle-treated mice (mean ± SD decrease 7 ± 2% versus 4 ± 1%; P = 0.0004).