MMP9 and rheumatoid arthritis: ASIC1a-RNAi and PCTX-1 have been reported to decrease the extracellular acidification-induced invasion and migration of RA-FLSs and the expression of MMP2, MMP9, and p-FAK, which are upregulated by ASIC1a (19), suggesting that these proteins may be downstream signaling molecules of ASIC1a that are involved in synovial migration and invasion.