Expression of aquaporin 1 (AQP1) in human synovitis

Rheumatoid arthritis (RA) is an autoimmune disorder characterized by synovial proliferation (synovitis), articular cartilage and subchondral bone degradation as well as joint swelling. Joint swelling and edema often accompany pannus formation and chronic joint inflammation in RA. We have recently shown that human chondrocytes and synoviocytes express aquaporin 1 (AQP1) water channels and that AQP1 is upregulated in RA cartilage. Clinical evidence suggests that joint swelling and edema accompany the chronic inflammation observed in synovial joints of RA patients. Therefore we hypothesized that AQP1 water channels may be involved in joint swelling and synovial edema formation. To test this hypothesis, we performed immunostaining of normal and human synovitis tissue microarrays (TMAs) to investigate whether the expression of AQP1 water channels is altered in the synovium in synovitis. Immunohistochemistry revealed that AQP1 is expressed in synovial micro-vessels and synoviocytes from normal joints (n=20 normal subjects). Semi-quantitative histomorphometric analysis of AQP1 expression in the TMAs revealed upregulation of the membrane protein in the synovium derived from RA (n=10) and psoriatic arthritis (n=8) patients. These results indicate a potential role for synovial AQP1 and other aquaporins in joint swelling and the vasogenic edema fluid formation and hydrarthrosis associated with synovial inflammation. Future experiments will need to determine whether the expression of other aquaporins is altered in synovitis.