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.