P2RX4 and rheumatoid arthritis: Other GPCRs may also be associated with RA, including sphingosine 1-phosphate (S1P) receptor [32, 33], A3 adenosine receptor [34, 35] , A2A adenosine receptors [36], purinergic receptor P2X4 [37] and many chemokine receptors [38-41].