Reduced to Activation

Abstract

The transient receptor potential (TRP) channels constitute a large and diverse family of multimeric ion channels for which new mechanisms of activation and specific functions continue to emerge. The "canonical" TRP 5 (TRPC5) channel contains two cysteine residues in a predicted extracellular loop near the ion pore, leading Xu et al . to investigate the hypothesis that their reduction might modulate channel activity. The reducing agents dithiothreitol (DTT) and Tris (2-carboxyethyl) phosphine hydrochloride (TCEP) activated TRPC5 channels expressed in human embryonic kidney (HEK) 293 cells, whereas mutant channels in which alanine was substituted for cysteine were constitutively active. Moreover, the reduced form of the endogenous redox protein thioredoxin (rTRX) activated TRPC5 or a heteromultimer comprising TRPC5 and TRPC1 (in which the two cysteines are conserved). Noting that high concentrations of thioredoxin have been found in the serum and synovial fluid of individuals with rheumatoid arthritis, the authors showed that rTRX elicited a cation current in fibroblast-like synoviocytes (FLS cells) from people with rheumatoid arthritis. FLS cells expressed TRPC5 and TRPC1 mRNA and protein, and both the current-voltage relationship and pharmacological response of the current evoked by rTRX resembled that of the TRPC5-TRPC1 heteromultimer. Furthermore, both a dominant-negative TRPC5 mutant and antibodies directed against TRPC5 (T5E3) or TRPC1 (T1E3) suppressed rTRX-evoked current in FLS cells. Exposure to T5E3 or T1E3 enhanced matrix metalloproteinase (MMP) secretion by FLS cells (as did TRPC5 and TRPC1 knockdown) and blocked inhibition of MMP secretion by rTRX. Thus, extracellular rTRX appears to activate TRPC5 and the TRPC5-TRPC1 heteromultimer by breaking a disulphide bridge. S.-Z. Xu, P. Sukumar, F. Zeng, J. Li, A. Jairaman, A. English, J. Naylor, C. Ciurtin, Y. Majeed, C. J. Milligan, Y. M. Bahnasi, E. Al-Shawaf, K. E. Porter, L.-H. Jiang, P. Emery, A. Sivaprasadarao, D. J. Beech, TRPC channel activation by extracellular thioredoxin. Nature 451 , 69-72 (2008). [PubMed]