AQP1 and rheumatoid arthritis: In our previous study in other tissues, the joint edema showed in rheumatoid arthritis (RA) patients was caused by the osmotic change of the synovium, where it was addressed that not only AQP-1 but also a combined ion transporter such as Na+-K+-2Cl− co-transporter 1 (NKCC1) and osmotic regulatory protein oxidative stress-responsive kinase 1 were involved [127,128].