The role of aquaporins in human synovitis

Rheumatoid arthritis (RA) is an autoimmune disorder characterized by synovial proliferation (synovitis), articular cartilage and subchondral bone degradation and synovial inflammation. Joint swelling and oedema often accompany pannus formation and joint chronic inflammation in RA. Clinical evidence suggests that joint swelling and oedema frequently accompany the chronic inflammation observed in synovial joints of RA patients. Although joint swelling is understood to be a major problem in synovitis, very little is known about the molecular mechanisms responsible for the oedema fluid formation that is associated with joint inflammation. Recent studies from our laboratory have shown that articular chondrocytes and synoviocytes express aquaporin 1 (AQP1) water channels. Aquaporins are a family of small integral membrane proteins related to the major intrinsic protein (MIP or AQP0). In recent studies we have used immunohistochemistry to investigate whether the expression of the AQP1 water channel is altered in synovitis. Our data suggests that this membrane protein is upregulated in the synovium derived from RA and psoriatic arthritis patients. In this chapter these observations are discussed in the context of RA and psoriatic arthritis. AQP1 and other aquaporins may play an important role in joint swelling and the vasogenic oedema fluid formation and hydrarthrosis associated with synovial inflammation.